Влияние химического состава нанокристаллов тройных соединений AgInS2 и AgBiS2 на их энергетическую структуру тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Карамышева Софья Павловна

• Карамышева Софья Павловна
• кандидат наук
• 2025

Оглавление диссертации кандидат наук Карамышева Софья Павловна

ОГЛАВЛЕНИЕ

РЕФЕРАТ

SYNOPSIS

ВВЕДЕНИЕ 52 ГЛАВА 1. ПОЛУПРОВОДНИКОВЫЕ НК ТРОЙНЫХ СОЕДИНЕНИЙ И ИХ

СВОЙСТВА

1.1 Синтез тройных НК на основе серебра и меди

1.2 Синтез анизотропных тройных и четвертных НК

1.3 Структура энергетических уровней НК тройных халъкогенидов

1.4 Хироптические свойства тройных НК 71 Выводы по главе

ГЛАВА 2. МАТЕРИАЛЫ И МЕТОДЫ

2.1 Материалы

2.2 Методы синтеза AIS и ABS НК

2.2.1 Водный синтез НК AIS и AIS/ZnS с нагревом 76 Синтез серии образцов AIS/ZnS НК с различной толщиной оболочки ZnS 77 Замена лиганд НК в водной фазе для перевода в органическую фазу

2.2.2 Водный синтез тройных AIS НК

2.2.3 Водный синтез тройных ABS НК

2.2.4 Метод «горячего впрыска» для синтеза AIS и AIS/ZnS НК

2.2.5 Метод «горячего впрыска» для синтеза ABS НК

2.2.6 Методики синтеза НК тройных соединений, легированных редкоземельными металлами

Синтез AIS НК, легированных иттербием

2.3 Методы исследования морфологии и оптических свойств AIS и ABS НК 81 ГЛАВА 3. ВЛИЯНИЕ СТРУКТУРЫ И ХИМИЧЕСКОГО СОСТАВА НК ТРОЙНЫХ СОЕДИНЕНИЙ НА ИХ ОПТИЧЕСКИЕ СВОЙСТВА

3.1 Оптические свойства и морфология AIS и AIS/ZnS НК

3.2 Изучение оптических свойств ABS НК в зависимости от условий синтеза

Изучение оптических свойств ABS НК в зависимости от соотношения прекурсоров на стадии синтеза

Изучение оптических свойств ABS НК в зависимости от времени синтеза 106 3.3 Выводы по главе

ГЛАВА 4. ИССЛЕДОВАНИЕ ОПТИЧЕСКИХ СВОЙСТВ И МОРФОЛОГИИ AIS И ABS НК, ЛЕГИРОВАННЫХ YB-ИОНОМ

4.2 Морфология и оптические свойства ABS НК, легированных Yb-ионом

4.3 Выводы для главы 4 123 ГЛАВА 5. АНИЗОТРОПИЯ ОПТИЧЕСКИХ СИГНАЛОВ НК ТРОЙНЫХ СОЕДИНЕНИЙ

5.1 Разработка синтеза НК тройных соединений с хиралъным лигандом на поверхности

5.2 Морфология и оптические свойства AIS НК с хиралъными лигандами на поверхности

5.3 Выводы по главе 5 137 ЗАКЛЮЧЕНИЕ 139 СПИСОК СОКРАЩЕНИЙ И УСЛОВНЫХ ОБОЗНАЧЕНИЙ 142 СПИСОК ИСПОЛЬЗОВАННЫХ ИСТОЧНИКОВ 144 СПИСОК ИЛЛЮСТРАТИВНОГО МАТЕРИАЛА 157 ТЕКСТЫ ПУБЛИКАЦИЙ

РЕФЕРАТ

Общая характеристика диссертации

Введение диссертации (часть автореферата) на тему «Влияние химического состава нанокристаллов тройных соединений AgInS2 и AgBiS2 на их энергетическую структуру»

Актуальность

Прогресс технологий в различных областях тесно связан с достижениями материаловедения, которое включает в себя разработку различных видов наноматериалов. С этой целью в последние несколько десятилетий активно развивались подходы к изготовлению широкого спектра коллоидных полупроводниковых нанокристаллов (НК), постепенно переходя от «классических» квантовых точек халькогенидов кадмия и свинца к НК различного химического состава и формы [1]. Одно из важных направлений этого развития связано с полупроводниковыми НК с оптическими переходами в красной и ближней инфракрасной (ИК) областях спектра, в состав которых не входят такие токсичные элементы, как Cd, РЬ и К настоящему времени представлено несколько типов таких материалов, а именно бинарные (П-У1, III-V), тройные (I-Ш-УЬ, ЫУ-УЪ, 1з-У-У14, П-ПЬ-УЦ) и четвертные (ЫЫУ-УЦ, ЫП-Щ-УЦ) соединения и их сплавы, и достигнут значительный прогресс в их синтезе в виде НК различных размеров и форм [2-4]. Среди большого разнообразия составов НК тройных соединений большинство исследований было сосредоточено на НК на основе меди и серебра. Такие НК перспективны для различных применений, таких как фотовольтаические и оптоэлектронные устройства и эффективные фотокатализаторы.

Следует отметить, что для повышения производительности и расширения функционала оптоэлектронных устройств без применения сложных оптических схем в качестве активных материалов могут быть использованы НК с анизотропными свойствами [5,6]. Такие полупроводниковые НК тройных соединений можно получать методом химического синтеза, контролируя тип и молярное соотношение органических лигандов [7,8], используя методы роста на зародышах [9] или катионного обмена [10]. Помимо развития синтеза НК с анизотропной формой, получение поляризованного излучения также привлекает интерес для дальнейшего применения в оптоэлектронных устройствах и

биоприложений. Чувствительность материала и изменение его оптического отклика на циркулярно поляризованный свет может быть достигнута в хиральных НК, методы синтеза которых на данный момент активно разрабатываются учёными [11,12].

Целью диссертационной работы является установление зависимости энергетической структуры электронных уровней и оптических свойств НК AgInS2 (AIS) и AgBiS2 (ABS) от их химического состава.

Для достижения данной цели в рамках диссертации были поставлены и решены следующие задачи:

Задача 1 - Определение влияния структуры и химического состава ABS и AIS НК на их оптические свойства, а именно при изменении состава путём изменения соотношения прекурсоров и времени синтеза, а также при наращивании полупроводниковой оболочки;

Задача 2 - Исследование оптических свойств ABS и AIS НК при легировании их редкоземельными металлами, такими как иттербий;

Задача 3 - Установление взаимосвязи оптических свойств ABS и AIS НК при их функционализации хиральными лигандами;

Задача 4 - Установление зависимости фотокаталитических свойств AIS НК в зависимости от их химического состава и оптимизации с помощью хирального лиганда.

Методы исследования. В диссертации применялись следующие методы исследования: спектрофотометрия, спектрофлуориметрия, люминесцентная конфокальная микроскопия с разрешением по времени, спектроскопия кругового дихроизма, атомно-силовая микроскопия, динамическое рассеяние света, просвечивающая электронная микроскопия, рентгеновская фотоэлектронная спектроскопия, ультрафиолетовая фотоэлектронная спектроскопия.

Основные положения, выносимые на защиту: 1. Наращивание оболочки ZnS на ядрах AgInS2 в водных дисперсиях нанокристаллов в течении 30 минут приводит к формированию аллоидной структуры ZnAgInS, что приводит к синему сдвигу полосы

фотолюминесценции на 130 нм с сохранением ее ширины, а также к увеличению средневзвешенного времени затухания фотолюминесценции на 17% с 416 до 487 нс.

2. Изменение химического состава нанокристаллов Ag-Bi-S путем изменения соотношения содержаний металла и серы, а также времени синтеза открывает возможность управления энергетической структуры нанокристалла. Так уменьшение содержания серы от 48 до 32% и увеличение содержания серебра от 49 до 63% в AgBiS2 нанокристаллах приводит к смещению валентной зоны с -7,05 до -6,37 эВ при незначительном изменении ширины запрещенной зоны в диапазоне от 1,79 до 1,83 эВ.

3. Легирование нанокристаллов AgInS2 ионами иттербия приводит к различному изменению оптических свойств нанокристаллов в зависимости от способа внедрения ионов: одностадийное легирование AgInS2 нанокристаллов ионами Yb3+ приводит к уменьшению средневзвешенного времени затухания фотолюминесценции с 452 до 361 нс с уменьшением квантового выхода фотолюминесценции с 5,4 до 1,7%, в то время как легирование при двухстадийном синтезе, а именно формировании оболочки на нанокристаллах AgInS2, приводит к более резкому уменьшению средневзвешенного времени затухания фотолюминесценции до 314 нс с сохранением квантового выхода фотолюминесценции на уровне 5,7%.

4. Функционализация поверхности нанокристаллов AgInS2 и AgBiS2 хиральными лигандами приводит к появлению хирального отклика в поглощении нанокристаллов, причем использование тиол-содержащих лигандов (Ь-цистеин, L-глутатион) приводит к более выраженному сигналу в спектрах кругового дихроизма в сравнении с аминокислотами без тиольной группы ^-триптофан, L-фенилглицин) благодаря созданию координационных связей между тиольными группами лигандов и поверхностью нанокристаллов. Максимальные значения фактора диссимметрии составили для AgInS2-L-глутатион 3,0-10~з (на 215 нм) и для AgBiS2-L-цистеин 3,2 104 (на 210 нм).

Научная новизна диссертации отражена в следующих пунктах: Научная новизна 1 - Разработка новых научно-обоснованных подходов к целенаправленному синтезу и модификации нанокристаллов тройных халькогенидов серебра (AgInS2, AgBiS2) с управляемыми оптическими и электронными свойствами для оптоэлектроники;

Научная новизна 2 - Установление ранее не описанных и противоречивых фундаментальных закономерностей взаимодействия легирующих ионов редкоземельных металлов с матрицами полупроводников тройных соединений на основе хальконегидов серебра;

Научная новизна 3 - Экспериментальное доказательство и установление механизмов взаимосвязи между химическим составом хирального лиганда, типом его координации с поверхностью нанокристалла и возникающим хироптическим откликом.

Научно-техническая задача, решаемая в диссертации, заключается в создании научно-обоснованной методики направленного синтеза и модификации коллоидных нанокристаллов тройных халькогенидов серебра (AgInS2, AgBiS2) с управляемыми оптико-электронными и хироптическими свойствами для перспективных применений в оптоэлектронике и фотонике.

Объектом исследования являются нанокристаллы тройных соединений состава Л§-1п-Б и Л§-Бь8.

Предметом исследования является зависимость оптических и электронных свойств (спектров поглощения и люминесценции, времени жизни возбужденного состояния, положения энергетических уровней) и хироптического отклика данных нанокристаллов от их химического состава, стехиометрии, размера, морфологии, типа поверхностных лигандов и условий синтеза.

Теоретическая значимость результатов диссертационной' работы состоит в следующем:

1. Установлены новые количественные закономерности влияния стехиометрии на положение валентной зоны в нанокристаллах Ag-Bi-S, углубляющие теорию зонной структуры нестехиометрических наноматериалов;

2. Выявлен новый механизм влияния легирования Yb3+ в зависимости от типа матрицы, расширяющий понимание релаксации возбуждения в гибридных системах "полупроводник-лантаноид";

3. Доказана ключевая роль координационной связи через серу в эффективности индуцирования хиральности, что вносит вклад в теорию хирально-индуцированных явлений в коллоидных нанообъектах;

4. Установлена взаимосвязь между кинетикой роста оболочки и рекомбинационной динамикой, дополняющая теоретические модели для гетероструктур состава ядро-оболочка.

Практическая значимость р езультатов диссертационной' работы состоит в следующем:

1. Разработаны протоколы синтеза нанокристаллов с управляемыми свойствами, рекомендуемые к использованию для получения перспективных материалов;

2. Предложен новый метод управления энергетической структурой нанокристаллов через контроль стехиометрии для применения в солнечных элементах и фотоприемниках;

3. Доказана эффективность стратегии индукции хироптического отклика тиол-содержащими лигандами для создания материалов хиральной нанофотоники. Достоверность полученных результатов обеспечивается применением

комплекса взаимодополняющих современных экспериментальных методов исследования, результаты которых находятся в согласии друг с другом. Использование таких методов, как рентгеновская фотоэлектронная спектроскопия и энергодисперсионная спектроскопия, позволило точно определить элементный состав и химическое состояние элементов в исследуемых нанокристаллах. Морфология и размеры нанокристаллов были охарактеризованы методами просвечивающей и сканирующей электронной микроскопии. Оптические свойства, включая спектры поглощения, фотолюминесценции и кинетику затухания люминесценции, были исследованы с использованием сертифицированного спектроскопического оборудования. Важным аспектом

обеспечения достоверности является соответствие полученных экспериментальных данных известным литературным результатам для аналогичных систем, а также наблюдение ожидаемых физических эффектов в предельных случаях. Все применяемые методики являются стандартными и общепризнанными в научном сообществе, а используемое оборудование прошло регулярную метрологическую аттестацию. Воспроизводимость результатов в сериях последовательно синтезированных образцов подтверждается следованию выявленным закономерностям. Теоретические интерпретации

экспериментальных данных основаны на общепринятых теоретических моделях и концепциях современной химии коллоидных нанокристаллов и физики полупроводников. Выводы и теоретические положения, изложенные в диссертации, подтверждаются экспериментальными данными, представленными в виде графиков, схем и аналитических зависимостей. Выдвинутые гипотезы основаны на установленных научных фактах и подтверждены экспериментальными результатами, согласующимися с данными предыдущих исследований. Дополнительным подтверждением достоверности работы служит апробация результатов на научных конференциях и их публикация в рецензируемых изданиях.

Внедрение результатов работы

Результаты диссертационной работы используются при выполнении гранта при реализации проекта «Разработка композита на основе полипропилена и нанокристаллов тройного состава с высокой фотостабильностью для применения в агрофотонике» в рамках практико-ориентированных НИОКТР при поддержке Университета ИТМО. Материалы диссертационной работы могут быть использованы в учебном процессе в таких дисциплинах, как «Спектроскопия атомов и молекул», «Оптическая активность наноструктур», входящих в бакалаврскую программу обучения Университета ИТМО в образовательной программе «Физика наноструктур» направления 12.03.04 «Фотоника и оптоинформатика», а также в рамках предметов «Физические механизмы функционирования гибридных наноструктур», «Оптическая спектроскопия и

люминесценция многоатомных систем», входящих в магистерскую программу обучения Университета ИТМО по направлению подготовки 12.03.04 «Фотоника и оптоинформатика».

Апробация результатов работы. Основные результаты работы докладывались и обсуждались на следующих конференциях:

1. XII International Conference on Photonics and Information Optics 14.05.2024 -17.05.2024

2. XI Конгресс молодых учёных ИТМО 04.04.2022-06.04.2022

3. XII Конгресс молодых учёных ИТМО 03.04.2023-06.04.2023

4. XII Международная конференция по фотонике и информационной оптике

14.05.2024-17.05.2024

5. LIV научная и учебно-методическая конференция Университета ИТМО 2025

27.01.2025-31.01.2025 Личный вклад автора.

Личный вклад автора в представленную диссертационную работу является определяющим и заключается в непосредственном выполнении и активном участии на всех ключевых этапах исследования. Автором лично проведен анализ современного состояния проблемы и формулировка цели и задач исследования, осуществленные совместно с научным руководителем.

Соискатель принимал непосредственное участие в планировании и проведении всего комплекса экспериментальных работ, включая синтез нанокристаллов тройных соединений AgInS2 и AgBiS2 с варьированием параметров синтеза, их модификацию путем легирования ионами цинка и иттербия, функционализацию поверхности хиральными лигандами, а также наращивание оболочек ZnS.

Автор лично проводил основные спектроскопические исследования: регистрировал и интерпретировал спектры оптического поглощения и фотолюминесценции, измерял кинетику затухания люминесценции. Большая часть обработки и интерпретации полученных экспериментальных данных,

установление корреляций между составом, структурой и свойствами нанокристаллов выполнены лично автором.

Соискателем подготовлены тексты и графические материалы для всех основных публикаций по работе, а также лично представлены результаты исследования на международных и всероссийских научных конференциях. Весь полученный научный результат является плодом непосредственной экспериментальной и аналитической работы автора.

Структура и объем диссертации.

Диссертация состоит из введения, пяти глав, заключения, списка используемых источников и приложения, где представлены оттиски статей. Общий объем диссертации составил 210 страницу, включая 53 рисунка и 5 таблиц. Список цитированной литературы включает 128 источников.

Во введении описаны актуальность работы, сформулированы цели и задачи, научная новизна исследований, практическая значимость полученных результатов, представлены выносимые на защиту научные положения и определена структура диссертации.

В первой главе представлен всесторонний анализ современных исследований в области полупроводниковых нанокристаллов (НК) тройных соединений на основе серебра и меди. Основное внимание уделено фундаментальным аспектам, включая методы синтеза, особенности кристаллической структуры, морфологические характеристики и ключевые оптические свойства этих перспективных наноматериалов. Рассматриваются соединения со структурой типа АВС2, где А = Ag+ или Си+, В = 1п3+, Biз+, Ga3+, а С = $е2~ или Те2 . Особый акцент сделан на материалах, не содержащих токсичные элементы (Cd, РЬ, что определяет их потенциальную безопасность для биомедицинских применений и экологической совместимости.

Глава начинается с рассмотрения общих принципов синтеза тройных халькогенидных НК. Подробно описаны различные подходы, включая метод горячего впрыска прекурсоров, термолиз в высококипящих растворителях и синтез в водной фазе. Особое внимание уделено роли лигандов (додекантиол, олеиновая

кислота, олеиламин) и источников серы в контроле реакционной способности катионов и формировании монодисперсных НК. Отмечается, что значительная разница в заряде и "мягкости" (поляризуемости) между катионами А (Си+, Ag+) и В (1п3+, ВР+) требует тонкого балансирования условий синтеза для получения стехиометрически контролируемых продуктов. Особый раздел посвящен синтезу в водной среде с использованием биосовместимых лигандов (тиогликолевая кислота, глутатион), что важно для последующих биомедицинских применений, хотя такие методы часто сталкиваются с проблемами низкой кристалличности и квантового выхода фотолюминесценции (КВ ФЛ).

Значительное внимание уделено особенностям кристаллической структуры НК. Выделены три основных полиморфных типа: халькопиритная (ХП), сфалеритная (СФ) и вюрцитная (ВЦ) фазы (рисунок 1).

СИа1соруп1е гтс-Ыепс1е УУиггКе

Рисунок 1 - Общие кристаллографические структуры тройных НК АВС2: халькопирит (ХП), сфалерит (СФ) и вюрцит (ВЦ) [13]

Подчеркивается, что кристаллическая фаза не только определяет симметрию решетки, но и непосредственно влияет на возможность получения анизотропных морфологий. ВЦ-структура, в отличие от кубических ХП и СФ, обладает пониженной симметрией и ярко выраженной анизотропией, что делает ее ключевой для синтеза одномерных наностержней и других анизотропных форм. Фаза продукта сильно зависит от используемых реагентов: например, присутствие аминогрупп или фторид-ионов может инициировать фазовый переход из аморфного состояния в кристаллическое с определенной решеткой.

В разделе, посвященном морфологическому разнообразию, систематизированы данные о широком спектре форм НК, полученных различными методами. Прямой синтез, выращивание на затравках и катионный обмен позволяют получать наностержни, нанопластины, нанодиски, наноцветы, нанотетраподы и гетероструктуры типа "ядро-оболочка" или "Янус". Подчеркивается, что анизотропия формы придает НК уникальные свойства, такие как линейно-поляризованное излучение, повышенная поверхностная плотность лигандов и усиленные хироптические эффекты, что открывает дополнительные возможности для их применения в оптоэлектронике и сенсорике.

Центральное место в главе занимает анализ оптических свойств и структуры энергетических уровней. Подробно рассмотрена зависимость ширины запрещенной зоны от химического состава, размера НК и типа кристаллической фазы. Для объемных материалов ширина запрещённой зоны варьируется от 1,05 эВ (CuInS2 (CIS)) до 1,98 эВ (орторомбический AIS), а для НК дополнительно проявляется квантово-размерный эффект. Спектры ФЛ тройных НК характеризуются широкой полосой излучения с шириной на полувысоте порядка 70-100 нм и значительным стоксовым сдвигом (200-500 мэВ), что связано с рекомбинацией носителей заряда с вовлечением внутризонных дефектных состояний. Детально проанализированы четыре основные модели, объясняющие механизм люминесценции: рекомбинация донорно-акцепторных пар (ДАП), свободно-связанный переход, модель самозахвата экситона (СЗЭ) и влияние тонкой структуры. Особое внимание уделено модели свободно-связанного перехода, которая предполагает рекомбинацию делокализованного электрона в зоне проводимости с локализованной на ионе меди (Cu+) дыркой, что подтверждается современными экспериментальными данными. Отмечается ключевая роль дефектов, таких как вакансии меди (V_Cu) или антисайт In_Cu, в формировании оптических свойств этих материалов.

Большой раздел посвящен стратегиям пассивации поверхности НК для улучшения их оптических характеристик. Наиболее эффективным методом признано наращивание оболочки из широкозонного полупроводника (ZnS), что

позволяет повысить КВ ФЛ с 10-15% до 60% и более за счет подавления безызлучательной рекомбинации носителей заряда на поверхностных ловушках. Альтернативой является использование органических лигандов (олеиламин, тиолы), которые стабилизируют поверхность и предотвращают окисление. Подчеркивается, что процесс пассивации часто сопровождается катионным обменом между ядром и оболочкой, приводя к образованию четвертных соединений (например, ZnAgInS) с градиентным составом и новыми свойствами.

В заключительной части главы подробно обсуждаются хироптические свойства и практические применения тройных НК. Показано, что хиральность может быть индуцирована на ахиральных НК путем функционализации поверхности хиральными лигандами (Ь/О-цистеин, гистидин) или включения их в хиральные супрамолекулярные матрицы (гидрогели). Высокие значения фактора диссимметрии ^^аСюг до 10 ^ - 10~з), наблюдаемые в сигналах кругового дихроизма (КД) в полосе собственного поглощения НК и циркулярно поляризованной ФЛ, свидетельствуют об эффективном процессе индуцирования хиральности лигандом на неорганическое ядро НК.

Перспективы применения охватывают широкий спектр областей: от солнечных элементов и фотокатализа (разложение красителей, восстановление тяжелых металлов, генерация водорода) до биомедицины (фотодинамическая терапия рака, биоимиджинг).

Глава завершается выводом о том, что, несмотря на значительный прогресс, сохраняются ключевые проблемные стороны данного материала, требующие дальнейших исследований. К ним относятся необходимость точного контроля стехиометрии и уменьшения неоднородности состава для сужения полос в спектрах ФЛ, разработка более эффективных и универсальных методов пассивации поверхности, углубленное изучение взаимосвязи между анизотропией формы и хироптическими свойствами, а также проведение доклинических и клинических испытаний для биомедицинских применений. Сочетание уникальных оптических свойств, низкой предполагаемой токсичности и возможности многофункциональной функционализации делает тройные и четвертные НК на

основе меди и серебра чрезвычайно перспективной платформой для создания нового поколения оптоэлектронных устройств и тераностических платформ.

Во второй главе была описана экспериментальная часть работы, которая направлена на разработку методов синтеза, модификации и комплексное изучение свойств НК тройных соединений на основе серебра: AIS и ABS, включая их легированные иттербием формы и структуры типа «ядро-оболочка» AIS/ZnS. Исследования базировались на применении реактивов высокой степени чистоты от ведущих мировых производителей», что обеспечивало воспроизводимость и надежность получаемых результатов.

В работе был успешно применен и адаптирован ряд методов синтеза. Основными подходами выступили низкотемпературный коллоидный синтез в водной среде и метод «горячего впрыска» в органических растворителях. Водный синтез НК AIS/ZnS проводился по методике, основанной на соосаждении прекурсоров в присутствии меркаптоуксусной кислоты (МУК) в качестве стабилизатора. Процесс включал формирование ядра AIS при температуре 90-95 °C с последующим наращиванием оболочки ZnS из комплекса Znn-MyK для пассивации поверхностных дефектов и повышения интенсивности ФЛ. Была получена серия образцов с варьируемой толщиной оболочки ZnS путем отбора аликвот на разных стадиях роста. Для проведения исследований в неполярных средах был оптимизирован протокол перевода гидрофильных НК в органическую фазу путем замены исходных лигандов на олеиновую кислоту (ОК) и олеиламин (ОАм). На основе данного метода также был разработан водный синтез хиральных НК AIS с использованием L- и D-изомеров цистеина в качестве хиральных лигандов и источника серы. Успешная адаптация этой методики для синтеза НК ABS путем замены прекурсора индия на висмут позволила получить новый класс наноматериалов с отличными оптическими характеристиками.

Альтернативным путем синтеза высококачественных НК выступил метод «горячего впрыска», проводившийся в инертной атмосфере с использованием ОК, ОАм и додекантиола (ДДТ). Для синтеза НК AIS/ZnS прекурсоры серебра и индия в смеси с ДДТ и ОК нагревались до 185°C, после чего температура повышалась до

210°C для роста первой оболочки. Аллоидная оболочка ZnS формировалась путем многократных инъекций прекурсора цинка на основе стеарата цинка. Синтез НК ABS по методу «горячего впрыска» осуществлялся путем быстрого впрыска раствора элементарной серы в ОАм в нагретую до 100°C смесь ацетатов серебра и висмута в ОК с последующей выдержкой в течение 5 минут.

Важным направлением работы стало легирование НК ионами редкоземельного элемента иттербия с целью модификации их оптических свойств. На основе водного синтеза AIS НК были разработаны одно- и двухстадийные методики введения прекурсора Yb, что позволило предположительно варьировать локализацию легирующих ионов в структуре нанокристалла. Для систем на основе ABS легирование Yb проводилось методом «горячего впрыска» путем совместного растворения ацетатов серебра, висмута и иттербия в ОК с последующим впрыском серы.

Характеризация полученных наноматериалов проводилась с помощью комплекса современных физико-химических методов. Оптические свойства, включая спектры поглощения и ФЛ, КВ ФЛ и время жизни возбужденного состояния, исследовались на спектрофотометре UV-probe 3600, спектрофлуориметрах Cary Eclipse и Jasco V630, а также на флуоресцентном микроскопе MicroTime100 и специализированном комплексе для ближнего ИК-диапазона. Хиральные свойства НК изучались с помощью спектрометра кругового дихроизма JASCO J-1500. Для анализа электронной структуры применялись ИК-Фурье спектроскопия и метод Тауца для определения ширины запрещенной зоны по спектрам поглощения. Морфология, размеры и агрегативное состояние НК исследовались методами атомно-силовой микроскопии (АСМ), динамического рассеяния света (ДРС) и просвечивающей электронной микроскопии (ПЭМ). Элементный состав и химическое состояние элементов на поверхности НК определялись методами рентгеновской фотоэлектронной спектроскопии (РФЭС) и ультрафиолетовой фотоэлектронной спектроскопии (УФЭС).

Для количественного анализа данных была применена строгая математическая обработка. Кинетика затухания ФЛ аппроксимировалась

трёхэкспоненциальной функцией с расчетом средневзвешенного времени затухания ФЛ. Относительный КВ ФЛ определялся сравнительным методом с использованием стандарта (Родамин 6Ж). Для описания связи между временами затухания ФЛ и КВ ФЛ использовались фундаментальные соотношения между излучательными и безызлучательными константами скорости. На основе данных УФЭС проводился расчет энергии высшей занятой и низшей свободной молекулярных орбиталей.

Список литературы диссертационного исследования кандидат наук Карамышева Софья Павловна, 2025 год

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СПИСОК ИЛЛЮСТРАТИВНОГО МАТЕРИАЛА

Рисунок 1.1 - (a) Иллюстрация синтеза тройных НК на примере АВС2 методом нагревания, при этом додекантиол (ДДТ) выступает как в качестве лиганда, так и в качестве источника серы. (b) Общие кристаллографические структуры тройных НК АВС2: халькопирит (ХП), сфалерит (СФ) и вюрцит (ВЦ) 63 Рисунок 1.2 - Некоторые представленные в статьях тройные и четвертные анизотропные НК различных форм: (a) CIS НС [62], (b) ZnCuInS2 НС [55], (c) CIS/CdS ядро/оболочка НС [63], (d) CuInSe2/CuInS2 «точка-в-стержне» ГНС [64], (e) Cu2-xS/CIS ГНС Януса [65], (f) Ag-AIS НК в виде головастика [66], (g) AIS НК в виде семечки [66], (h) CIS-ZnS НК в виде факела [67]. На вставках показан схематический рисунок для каждой конкретной формы. Масштаб на (a)-(e): 50 нм, в (f), (g): 100 нм, в (h): 25 нм 66

Рисунок 1.3 - Схемы структур энергетических уровней электронных состояний и путей рекомбинации носителей заряда, которые могут возникать в тройных НК: (a) модель ДАП; (b, c) модель свободносвязанного перехода, показанная для двух случаев - локализованного электрона (b) и локализованной дырки (c); (d) модель СЗЭ. 69

Рисунок 1.4 - Левая панель: (a) Схема формирования гибридного композита на основе CISe/ZnS НК, внедренных в L/D-гидрогель. (b) КД-спектры CISe/ZnS НК (обозначены как «QD»), L-геля, D-геля, КТ@L-геля и КТ@D-геля, и (с) Cпектры кругового дихроизма ФЛ этих 4 образцов, возбужденных лазером с длиной волны 808 нм. Правая панель: (d) Схема обмена лигандов на ABS НК на хиральные молекулы цистеина (Цис). (e) Спектры поглощения УФ-видимого диапазона и (f) КД-спектры НК L-AIS (синий) и D-AIS (красный). (g) Спектры поглощения УФ-видимого диапазона и (h) КД спектры ABS НК, модифицированных L-цистеином и D-цистеином 73

Рисунок 2.1 - Схема водного синтеза AIS и ABS НК 78 Рисунок 2.2 - Схема метода «горячего впрыска» для синтеза ABS НК 80 Рисунок 3.1 - Распределение гидродинамического диаметра AIS и AIS/ZnS НК в воде (а) и в хлороформе (б) 87

Рисунок 3.2 - Спектры поглощения серий образцов НК в воде (а) и хлороформе (в) и их вторые производные (б,г), соответственно. Время наращивания оболочки указано в легенде. 88

Рисунок 3.3 - Положение фундаментального перехода НК AIS и AIS/ZnS в воде (синий цвет) и хлороформе (красный цвет) 89

Рисунок 3.5 - Положение максимумов полос ФЛ НК AIS с различным временем наращивания оболочки в воде (синий цвет) и хлороформе (красный цвет). 91 Рисунок 3.6 - Кривые затухания ФЛ НК AIS и AIS/ZnS в воде (а) и в хлороформе (б) 93

Рисунок 3.7 - Зависимость средневзвешенных времён затухания ФЛ от времени наращивания оболочки ZnS на НК AIS 96

Рисунок 3.8 - ПЭМ-изображения ABS НК с различным соотношением прекурсоров металлов к сере в масштабе 140x140 нм (а) и 54x54 нм (б). Изображение кристаллической решётки НК ABS с различным соотношением прекурсоров металлов к сере с указанием межплоскостного расстояния (в) 98 Рисунок 3.9 - Спектры поглощения (а) и диаграмма Тауца (б) ABS НК с различным соотношением прекурсоров металлов к сере 99

Рисунок 3.10 - Спектры РФЭС НК ABS с различным соотношением прекурсоров металлов к сере 100

Рисунок 3.11 - Процентное содержание по элементам для ABS НК с различным соотношением прекурсоров металлов к сере 101

Рисунок 3.12 - Спектры РФЭС ABS НК с различным соотношением прекурсоров металлов к сере 102

Рисунок 3.13 - Спектры РФЭС высокого разрешения полосы висмута для НК ABS с различным соотношением прекурсоров металлов к сере 104 Рисунок 3.14 - Энергетическая структура НК ABS с различным соотношением прекурсоров металлов к сере. Синими линиями показаны уровни ВЗМО, красными - НСМО 105

Рисунок 3.15 - Зависимость ширины запрещенной зоны ABS НК от соотношения прекурсоров металлов к сере 106

Рисунок 3.16 - Зависимость свойств электронной структуры НК ABS от их размера: уровни ширины запрещенной зоны (а), ВЗМО (б) и НСМО (в). 106 Рисунок 3.17 - ПЭМ-изображения НК, выращиваемых от 1 до 5 минут, в масштабе 140x140 нм (а) и 54x54 нм (б). Изображение кристаллической решётки НК ABS, выращиваемых от 1 до 5 минут, с указанием межплоскостного расстояния (в) 108 Рисунок 3.18 - Спектры поглощения (а) и диаграмма Тауца (б) НК ABS, выращиваемых от 1 до 5 минут 109

Рисунок 3.19 - Спектры РФЭС НК ABS, выращиваемых от 1 до 5 минут 110 Рисунок 3.20 - Процентное содержание по элементам для ABS НК, синтезируемых от 1 до 5 минут 111

Рисунок 3.21 - Энергетическая структура НК ABS, выращиваемых от 1 до 5 минут. Синим цветом обозначены НСМО, а красным - ВЗМО 112 Рисунок 3.22 - Зависимость свойств электронной структуры НК ABS, выращиваемых от 1 до 5 минут, от времени выращивания: запрещённой зоны (а), ВЗМО (б) и НСМО (в) 113

Рисунок 3.23 - Зависимость свойств электронной структуры НК ABS, синтезируемых от 1 до 5 минут, от их размера: ширины запрещённой зоны (а), ВЗМО (б) и НСМО (в) 113

Рисунок 4.1 - Схема синтезов НК тройных соединений, легированных Yb-ионами. Кристаллические решетки AIS (а) и ABS (е). Схема синтеза НК 1-S AIS:Yb (б), 2-S AIS:Yb (г), ABS:Yb (ж). Фотографии полученных коллоидных растворов AIS:Yb (в), 2-S AIS:Yb (д), ABS:Yb (з) Ошибка! Закладка не определена. Рисунок 4.2 - АСМ изображения образцов AIS (а), 1-S AIS:Yb (в) и 2-S AIS:Yb (д)и соответсвующие распределение их высот (б, г, е). Гидродинамический диаметр НК AIS, 1-S AIS:Yb и 2-S AIS:Yb (ж). Сравнение средних величин высот и гидродинамического диаметра для образцов НК AIS, 1-S AIS:Yb и 2-S AIS:Yb (з) 119

Рисунок 4.3 - Спектральные характеристики AIS НК (красный), 1-S AIS:Yb НК (желтый) и 2-S AIS:Yb НК (бордовый). Спектры поглощения (а) и ФЛ (длина волны

возбуждения - 380 нм) (б). Кривые затухания ФЛ (в) с параметрами аппроксимации триэкспоненциальной функцией (формула 1) 120

Рисунок 4.4 - АСМ изображения образцов ABS и ABS:Yb (а, в), распределение их высот (б, г). Гидродинамический диаметр ABS и ABS:Yb НК (д). Сравнение средних величин высот и гидродинамического диаметра для образцов ABS и ABS:Yb НК (е) 122

Рисунок 5.1 - Используемые в синтезе хиральные лиганды (а). Процедура синтеза НК ABS-ХЛ (б) и AIS-ХЛ (в) с использованием различных хиральных лигандов 126

Рисунок 5.2 - Процедура синтеза ABS-МУК-ХЛ (а) и AIS-МУК-ХЛ НК (б) 127 Рисунок 5.3 - ИК-Фурье-спектры НК ABS, функционализированные МУК и L-Cys 128

Рисунок 5.4 - Спектры поглощения, КД и МКД свободных молекул L-Cys, L-Gluth, L-Try и L-PhG 129

Рисунок 5.5 - Спектры поглощения, КД и МКД НК ABS без хиральных лиганд (с МУК) (а), ABS-L-Cys (б) и ABS-L-Gluth (в) 132

Рисунок 5.6 - Спектры поглощения, КД и МКД НК ABS-МУК-Тгу (а) и ABS-МУК-PhG (б) 134

Рисунок 5.7 - Спектр поглощения (а) и спектры ФЛ (Хвозб - 480 нм) НК AIS, функционализированных МУК и хиральными лигандами 135 Рисунок 5.8 - Зависимость максимума полосы ФЛ (звёзды) и ширины на полувысоте полосы ФЛ (ромбы) для AIS НК, функционализированных различными хиральными лигандами 136

Рисунок 5.9 - Спектры поглощения (a) и КД (б) НК водорастворимых AIS-ХЛ 137

ТЕКСТЫ ПУБЛИКАЦИЙ

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Cite this: Nanoscale, 2025,17,16193

Interplay between the shape anisotropy and optical properties of Cu- and Ag-based ternary and quaternary chalcogenide nanocrystals

Arsenii S. Portniagin,ta Sofia P. Karamysheva,tb Kirill V. Bogdanov,b Elena V. Ushakova *a b and Andrey L. Rogach *a c

Received 4th April 2025, Accepted 7th June 2025

DOI: 10.1039/d5nr01376c rsc.li/nanoscale

The availability of nanomaterials with optical transitions in the near-infrared spectral range is of great importance for the development of solar cells and photodetectors, as well as for (bio)sensing and biola-belling. One attractive class of such materials comprises colloidal nanocrystals of ternary semiconductor materials, namely I — III —compounds which eventually can be doped with Zn(ii) to become quaternary (I-II-III-VI) compounds. Recently, anisotropic shapes of these nanocrystals have been reported, with some specific areas of potential applications related to anisotropy in their optical responses, such as chir-optical responses in absorption and emission. In this review, we summarize the state-of-the-art approaches for the synthesis of ternary and quaternary (Zn-doped) Cu- and Ag-based chalcogenide nanocrystals with a special focus on their anisotropic shapes (such as nanorods and nanowires). We consider their energy level structure and spectral characteristics, including chiroptical properties, and provide perspectives in a view of their potential applications.

aDepartment of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR 999077, P. R. China. E-mail: [e-mail], [e-mail] bPhysNano Department, ITMO University, 197101 Saint Petersburg, Russia cIT4Innovations, VSB - Technical University of Ostrava, Ostrava-Poruba 70800, Czech Republic

t These authors contributed equally to this work.

Introduction

The progress of technologies in various areas is strongly related to materials science advances, which includes the development of different kinds of nanomaterial. For this purpose, approaches for the fabrication of a broad variety of colloidal semiconductor nanocrystals (NCs) developed rapidly over the last few decades, gradually moving from "classical" cadmium chalcogenide and lead chalcogenide quantum dots

Arsenii S. Portniagin received his Ph.D. in Physical Chemistry (2019) from the Institute of Chemistry FEB RAS, and Ph.D. in Materials Science (2023) from the City University of Hong Kong under supervision of A. L. Rogach. He is currently working as a postdoctoral researcher at the Material Science and Engineering Department of the City University of Hong Kong. His research focuses on the synthesis and ligand chemistry of anisotropic semiconductor nanocrystals, and their application for infrared photodetection.

Arsenii S. Portniagin

Sofia P. Karamysheva

Sofia P. Karamysheva received her M.Sc. in Chemical Nanoengineering (2020) from ITMO University (St-Petersburg, Russia), studying carbon materials at the Scientific Center of Solution Chemistry of Advanced Materials and Technologies. She is currently working on her Ph.D. in Optics at ITMO University under the supervision of E. V. Ushakova. Her research is on synthesis methods of silver-based ternary nanocrystals with a focus on chiral properties.

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(QDs) to a variety of NCs of different chemical compositions and shapes. One important direction of this development is related to infrared-emissive semiconductor NCs which do not include toxic elements such as Cd, Pb, and Hg. To date, several types of such material, namely binary (II-VI, III-V), ternary (I-III-VI2, I2-IV-VI3, I3-V-VI4, II-III2-VI4), and quaternary (I2-II-IV-VI4, I-III-II2-VI4) compounds and their alloys, have been demonstrated,1'2 and significant progress has been achieved in their synthesis as NCs of different sizes and shapes.3^ Among that large variety of compositions, the majority of studies were focused on Cu-based NCs, whose optical and electrical properties are the most studied.5

Overall, ternary NCs are promising for different applications, such as photovoltaic devices and as efficient photoca-talysts.6 To advance the performance of optoelectronic devices with a view to controlling their light characteristics without applying complicated optical schemes, there is a demand for optical materials with intrinsic anisotropic properties. This can be achieved by employing anisotropic semiconductor NCs,7 which can be produced by wet-chemical syntheses while controlling the nature and molar ratio of organic Iigands, using seeded growth techniques, and applying cation exchange. Interested readers are referred to the recent review by Jia and co-workers,8 where syntheses of anisotropic nano-rods (NRs), nanowires (NWs), nanotubes, nanoplatelets, nanosheets, nanocubes, and branched structures have been considered for InP, Zn-, Ag-, CuIn-, ZnIn-, and AgIn-chalco-genides in general. For the Cu-based ternary and quaternary anisotropic NCs, a review by Bai et al.9 highlighted the demand for controIIed and reproducibIe preparation approaches for ternary and quaternary NCs, which can be uti-Iized in photonic and photovoItaic devices. Apart from the advancement of syntheses of NCs with an anisotropic shape, their poIarized emission is a particuIarIy usefuI property, extending the range of their potential optoelectronic10 and bio-applications.11 Sensitivity of the optical responses to circu-

IarIy poIarized Iight can be achieved in chiraI NCs, whose synthetic methods were outlined in the review by Kuznetsova et al.12 The present review focuses on Cu- and Ag-based NCs with (mostly) ternary and to some extent also quaternary (when they also include Zn cations) compositions, which exhibit either an anisotropic shape and/or optical responses. We discuss their synthesis procedures, energy IeveI structure and anisotropic opticaI properties, and briefIy cover their potential applications followed by future perspectives.

Synthesis of ternary and quaternary Cu- and Ag-based NCs

Reactivity gap between mono- and polyvalent cations and strategies for the synthesis of ternary sulfides

Ternary chalcogenide NCs have a general formula ABC2, where A is Ag+ or Cu+ cation, B is In3+, Bi3+ or other trivalent cations, and C is S2- or Se2- anion. Let us start to consider their synthesis based on mutual affinities of the constituting elements on the example of ternary sulfide NCs (Fig. 1a). Formation of ternary suIfides requires simuItaneous interaction of either Cu+ or Ag+ with In3+ and S2-, which resuIts in a CuInS2 (often abbreviated as CIS) and AgInS2 (often abbreviated as AIS) composition, respectiveIy. The kinetics of interaction between those three participating chemicaI entities governs the size and shape of the resuIting ternary NCs, which in turn determines their optical properties. There is a large difference in the charge state of the cations involved (+3 vs. +1), which resuIts in a Iarge difference in the reactivity towards suIfur between In3+ and Cu+/Ag+. In3+ cation is a rather hard Lewis acid, which has a much lower tendency to react with a soft base S2- anion, which is different for the soft, singIe-charged Cu+ and Ag+ cations.13 For that reason, a lot of synthetic efforts were devoted to baIancing the reactivity between A-type and B-type cations.

Kirill V. Bogdanov received his Ph.D. from ITMO University (St-Petersburg, Russia) in 2014 under the supervision of Professor A. V. Baranov on the topic "Raman spectroscopy of nanocarbon materials". After working as a postdoctoral fellow until 2022 in the same division, he now heads the recently created laboratory "Raman spectroscopy of semiconductor and dielectric nanostructures" at ITMO University. His research focuses on a wide range of nanomaterials, including quantum dots and plasmonic, carbon, and diamond nanoparticles.

Kirill V. Bogdanov

Elena V. Ushakova received her Ph.D. in Optics from ITMO University (St-Petersburg,

Russia) in 2013. She is now a Head of the "Light-emitting carbon quantum nanostructures" Laboratory at ITMO University, and since 2019 has also served as a Visiting Associate Professor at the Material Science and Engineering Department of the City University of Hong Kong. Her research focuses on the synthesis-property correlations of semiconductor quantum dots, metal nanoparticles, perovskite nanocrystals, carbon dots, and self-assembled nanostructured materials.

Elena V. Ushakova

Review

Fig. 1 (a) Illustration of synthesis of ternary NCs on an example of ABS2 by the heat-up approach, with DDT acting both as a ligand and a sulfur source. (b) Common crystallographic structures of ternary ABC2 NCs: chalcopyrite (CP), zinc blende (ZB), and wurtzite (WZ).

One of the first hot-injection syntheses of monodisperse CIS NCs was demonstrated by Peng et al., who introduced the concept of cation reactivity tuning.14 In that study, the balance in reactivity between Cu+ and In3+ cations was achieved by using thiols which can bind to Cu+, forming thiolates, followed by the appearance of the Cu2-xS phase at a considerable rate only at temperatures higher than 230 °C.14 This strategy allowed the involvement of both Cu+ and In3+ cations into the reaction with thiolates, providing better control over nuclea-tion and growth of the resulting CIS NCs. Further advancement was reported by Zhong and Scholes, who used CuI and In(OAc)3 together with dodecanthiol (DDT), and applied the so-called heat-up approach to let them react.15 Using iodide as a soft Lewis base further reduced Cu+ reactivity towards DDT, while the non-injection (heat-up) approach allowed for the easier scalability of this synthesis. Most of the further recipes

Andrey L. Rogach received his Ph.D. in 1995 from the Belarusian State University in Minsk (Belarus), and completed his Habilitation in Experimental Physics at the University of Munich (Germany) in 2009. He is now a Chair Professor and a Founding Director of the Centre for Functional Photonics at the City University of Hong Kong. His research focuses on the synthesis, assembly and optical spectroscopy of semiconductor and metal nanocrystals, and their use for optoelectronic and energy-related applications.

Andrey L. Rogach

for CIS NCs relied on this precursor combination, while alteration of the ratio between Cu+ and In3+ cations could be conveniently used to tune the bandgap and hence the photoluminescence (PL) peak position of the resulting CIS NCs due to their stoichiometry variation, with a lower Cu-to-In ratio leading to the blue-shift of both the absorption edge and PL peak.16

As mentioned above, owing to the high reactivity of Cu+ towards S2-, nucleation and growth of Cu2-xS NCs occur at first, which then serves as a template for later reaction stages,17 turning into CIS NCs as a result of partial Cu-to-In cation exchange; this occurs through a slight rearrangement of the anion sublattice.18 Later on, it was suggested19 that the formation of CIS NCs is preceded by 2D lamellar structures which appear first at 100 °C, with the largest fragment corresponding to the formula [L7Cu3In3I6]-, where L is SC12H25-, which then convert into CIS NCs at 230 °C (Fig. 1a). The appearance of a Cu2-xS phase was not observed by synchrotron X-ray diffraction in that study,19 although the low crystallinity of copper chalco-genides may make the determination of such phases by diffraction methods quite difficult. It was also reported that a two-step reaction which temporally separates the formation of Cu2-xS and CIS NCs not only results in a broadening of the size distribution of the latter, but also in a large inhomogen-eity of composition, with Cu and In contents different from particle to particle within the ensemble.15 Such broad distributions in terms of both size and composition are responsible for the broadening of the absorption and PL spectra of CIS NCs, which will be discussed below in the section devoted to their optical properties.15

The formation of AIS NCs also follows the abovementioned processes of nucleation and growth. Tian et al.20 synthesized AIS NCs in a metastable orthorhombic phase by implementing a hot-injection method in the presence of oleic acid (OA) and DDT. Ogawa et al.21 found out that the sulfur source and the

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presence of amine Iigands are important for AIS NC formation, in contrast to their CIS counterparts. Commonly, metal thio-Iates polymerize and then decompose to form monodisperse sulfide NCs as depicted in Fig. 1a, while in the case of Ag-thio-Iates they decompose to disulfides (RS-SR) and Ag0, and then the Iatter reacts with In-thioIates, forming dendrites at the NC surface. To synthesize monodisperse AIS NCs, the authors proposed to use a combination of S-dodecanethiol and S-dodecylamine, in which case Ag2S seeds were formed first, followed by cation exchange with In3+. The synthetic routes towards Ag-Bi-based analogues, such as AgBiS2, are usually similar to AIS NCs, except that the indium precursor is replaced by the Bi-containing equivalent. For instance, a combination of Bi- and Ag-oleates were used as metal precursors for AgBiS2 NC formation, while hexamethyldisilathiane22 or oIeyIamine@suIfur (OLA-S) complexes23 were used as the suIfur precursor.

The synthetic approach described above, where DDT acts both as a Iigand and a suIfur source (Fig. 1a), commonIy Ieads to the formation of tetrahedraIIy shaped CIS and AIS NCs which beIong to chaIcopyrite phase (CP), a sub-cIass of cubic structures (Fig. 1b).5'24 Apart from the CP phase, zinc bIende (ZB) is yet another cubic poIymorph of ABC2 materiaIs in which copper/siIver and indium/bismuth ions are interchange-abIe in the Iattice, whiIe in CP phase they exist in a perfect order (Fig. 1b). Cubic phases, however, cannot be utiIized for the synthesis of anisotropic (rod-shaped) NCs, as there is no distinct direction in the crystaI owing to the high cubic symmetry. That is why ABC2 NCs with an anisotropic shape exhibit yet another, wurtzite (WZ) structure (Fig. 1b), for exampIe those synthesized by cation exchange starting from Cd-based NRs with intrinsic WZ structure, which we wiII consider in detaiI in the next section. It is worth mentioning that the crystaI phase of ternary NCs aIso depends on the chemicaIs used during their synthesis, as was recentIy shown for AIS NCs whose phase transition from amorphous to orthorhombic was triggered by the presence of amino-groups or F- anions.25

The chemicaI composition of ternary chaIcogenide NCs can aIso be tuned in a wide range: as aIready mentioned above, instead of In3+ cations, Bi3+ or even Ga3+ cations can be used,26 whiIe chaIcogenide anions are not onIy S2- but aIso Se2- and Te2-. Moreover, those ternary NCs can aIso be turned into so-caIIed core-sheII nanoparticIes by depositing other semiconductors with a Iarger bandgap (such as CdS or ZnS) as a sheII for improving their opticaI properties. WhiIe the ZnS sheII was deposited on AIS NCs,27 this can aIso initiate the cation exchange between Zn2+ cations and the metaI cations present in the NC core, which resuIts in the formation of ternary-quaternary or aIIoyed NCs.28 This wouId significantIy aIter their energy IeveI structure and opticaI properties, as we wiII discuss Iater on.

The abovementioned exampIes of ternary NCs were synthesized in non-poIar organic soIvents at rather high temperatures, which resuIts in their good crystaIIinity and high PL quantum yieId (QY). At the same time, the aqueous phase synthesis is regarded as a safer and more environmentaIIy friendIy

approach, and water-soIubIe NCs can be directIy appIied in bioIogicaI studies.29-33 However, the synthesis of high-quaIity ternary NCs in water presents a chaIIenge, as the typicaI water-soIubIe thioI Iigands used for coIIoidaI stabiIization, such as thiogIycoIic acid, l-gIutathione, and 3-mercaptopropionic acid, can easiIy form compIexes with Cu+, Ag+, In3+, and Zn2+ cations, and this compIexation Ieads to a deIicate baIance that must be maintained for the optimaI quaIity of ternary NCs.8'33-36 The ratio of thioI-to-In3+ is yet another cruciaI parameter that must be carefuIIy baIanced to optimize the synthesis of ternary NCs in aqueous media, because it determines the reIative strength of the thioI Iigand binding and has a significant impact on the finaI reaction products.37'38 The pH vaIue of the aqueous soIution aIso pIays a significant roIe, as it controIs the protonation state of the thioI Iigands and the formation of metaI hydroxides, which can infIuence the overaII synthesis.39 The effective concentration of free In3+ ions in the reaction mixture is highIy dependent on the pH, as the hydroxide ion is a typicaI hard base. RecentIy, it was reported that both AIS and the core-sheII AIS/ZnS NCs can be synthesized in water in a continuous flow reactor at rather Iow temperatures (100-120 °C).27

Water-soIubIe core-sheII CIS/ZnS NCs can be synthesized using In3+ and Cu2+ as cations, sodium suIfide as a suIfur source, and gIutathione as a Iigand.40 As this synthesis can be accompIished under microwave heating, it enabIes ease of impIementation, scaIabiIity, and high product yieId. However, the finaI materiaI showed a very Iow PL QY, most IikeIy due to the Iow synthesis temperature and therefore Iow crystaIIinity of both core and sheII constituents.40 To battIe this probIem, hot injection of Ag+ ions was proposed as a way to enhance their PL QY via the formation of a AgInS2 interIayer,3 which indeed improved it to over 26%.

Synthesis of quaternary Cu- or Ag-chalcogenides

Other Cu- and Ag-chaIcogenide-based quaternary materiaIs, apart from the aIready considered Zn-doped AIS, CIS and ABS,41,42 incIude Ga, TI, Ni, Fe, Sn and some other eIements resuIting in NCs with chemicaI compositions such as Cu-Ga-In-S, Cu-In-TI-S,43 Cu-Ni-Sn-S,44 Cu-Fe-Sn-Se,45 Cu-In-Sn-S,46 Cu-Ga-Zn-S,47 Ag-Zn-Sn-S,48 and Ag-Sb-Bi-Se.49 AIso, quinternary NCs with a composition of Ag-In-Ga-Zn-S have been reported.50 The synthesis of those NCs mostIy invoIves hot-injection methods with additionaI precursors for the Zn, Ga, TI, Sn, Ni etc. eIements being introduced. The guiding principIes for the formation of those quaternary NCs are very simiIar to their ternary counterparts, which we discussed above. Using different moIar ratios of the precursors, one can achieve a different stoichiometry of these materiaIs, which then affects their properties, as we wiII discuss Iater on.

Synthesis of ternary and quaternary rod-like NCs

To date, ternary and quaternary NCs have been synthesized in a Iarge variety of shapes. The most common of them incIude NRs51-55 and NWs,56,57 but aIso tadpoIe-Iike and heIiotrope

Review

Fig. 2 Some reported examples of ternary and quaternary anisotropic NCs of mostly rod-like shapes: shapes: (a) CIS NRs, (b) ZnCuInS2 NRs, (c) core/shell CIS/CdS dot-in-rod NRs, (d) CuInSe2/CuInS2 dot-in-rod hetero-NRs, (e) Janus-type Cu2_xS/CIS hetero-NRs, (f) Ag-AIS tadpole-like NCs, (g) AIS heliotrope seed-like NCs, (h) CIS-ZnS torch-like NC. Insets show a schematic cartoon for each particular shape. Scale bar in (a)-(e): 50 nm, in (f), (g): 100 nm, in (h): 25 nm. (a) Reproduced with permission from ref. 51; Copyright 2010 American Chemical Society; (b) reproduced under terms of the CC-BY license from ref. 79. Copyright 2019 MDPI Publishing. (c) Reproduced with permission from ref. 71; Copyright 2022 John Wiley and Sons; (d) reproduced under terms of the CC-BY license from ref. 75; Copyright 2015 American Chemical Society; (e) reproduced under terms of the CC-BY license from ref. 60; Copyright 2010 American Chemical Society; (f and g) reproduced from ref. 58; Copyright 2010 Royal Society of Chemistry 2010; (h) reproduced with permission from ref. 59; Copyright 2013 Royal Society of Chemistry.

seed-like shapes,58 and torch-like NCs59 were reported, as illustrated in Fig. 2.

NRs synthesized by a direct synthesis often demonstrate tapering towards one of the ends for both ternary (Fig. 2a) and quaternary (Fig. 2b) materials. NRs can also be synthesized by means of the seeded growth approach, leading to dot-in-rod heterostructures of core/shell CIS/CdS (Fig. 2c), or CISe/CIS heterostructures when the cation exchange approach is implemented (Fig. 2d), while Janus-type hetero-NRs can be synthesized by a combination of the two approaches (Fig. 2e). For the latter synthesis, Xia et al. suggested a two-stage approach starting from Cu2_xS hexagonal nanoplatelets as seeds, following by injecting In precursor that converted one of the facets of the seed into CIS via partial Cu+-to-In3+ exchange.60 A similar approach was applied to produce tadpole (Fig. 2f), heliotrope (Fig. 2g), and torch-like (Fig. 2h) NC morphologies of different compositions. In the following sections, we will consider the formation of CIS and AIS NRs by the three following approaches: direct synthesis, seeded growth, and cation exchange.

Direct synthesis of ternary and quaternary NRs

In general terms, synthesis of NRs is often designed in a way which allows us to set the reaction into a kinetic mode, when less stable but faster-forming products appear first. One such synthesis of CIS NRs was demonstrated by Kolny-Olesiak et al. ,51 where trioctylphosphine oxide (TOPO) was used alongside oleylamine and DDT. The role of TOPO was not only to induce the formation of hexagonal Cu2S NCs at the intermediate stage, but also to slow down their conversion into CIS by reducing the reactivity of the In3+ precursors and therefore maintaining a relatively high concentration of this cation

throughout the synthesis. The NR growth occurred at the Cu2_xS/CIS interface61 and then, depending on the reaction kinetics, proceeded either in two directions (fast growth), or in one direction (slow growth). The Cu2_xS phase underwent rearrangement during the synthesis, forming disk-shaped nanoparticles which offered a larger interface for the anisotropic growth. Disk shape formation for Cu2_xS NCs was also reported in another study on CIS NRs,54 where intermediate Cu2_xS/CIS heterostructures were identified. Since the CIS NR growth occurred on a time scale of about an hour in the presence of TOPO, CIS NCs were also found to grow by homoepi-taxy of Cu, In, and S elements, in addition to Cu2_xS ^ CIS conversion.60 Such prolonged reaction times can contribute to the increase of size of the resulting NRs, which somewhat complicates the formation of narrow, quantum-confined nano-structures. Thus, TOPO-free synthesis of CIS NRs was proposed, based on an OA/oleylamine mixture as surfactants and t-DDT/n-DDT as a sulfur source.61 The scheme of this direct synthesis is illustrated in Fig. 3a. It was found that the growth of CIS NRs started from the formation of Cu2_xS seeds; intrigu-ingly, two different growth mechanisms of the CIS NRs were observed in that case, which were dependent upon the size of the Cu2_xS seeds, controlled by the amount of OA. Smaller Cu2_xS seeds (~4 nm) were gradually converted into CIS NRs upon incorporation of In, while larger Cu2_xS seeds (8 nm) served as a template for the nucleation of CIS, which resulted in the formation of an intermediate Cu2_xS-CIS hybrid nano-structure, followed by its transformation towards CIS NRs

(Fig. 3b).61

Not only the use of proper Iigands but also alloying with Zn element could be used as a tool to induce shape anisotropy in CIS NCs, which then turn into quarternary Cu-In-Zn-S (abbre-

Nanoscale

Fig. 3 Examples of three main synthetic approaches leading to anisotropic ternary NCs. (a) Schematics of a direct synthesis and (b) TEM image of CIS NRs. Reproduced under terms of the CC-BY license from ref. 61; Copyright 2014 American Chemical Society. (c) Schematics of seeded growth and (d) electron tomography reconstruction of core-shell CIS/ZnS NRs. Reproduced under terms of the CC-BY license from ref. 68; Copyright 2018 American Chemical Society. (e) Schematics of the cation exchange performed on Cu2S/ZnS NRs and (f and g) HAADF-STEM images of the intermediate product Cu2-xS NRs (f) and the final product CIS NRs (g). Reproduced under terms of the CC-BY license from ref. 66; Copyright 2020 American Chemical Society. (h) Schematics of the cation exchange performed on CdSe/CdS dot-in-rods to produce ABSe2/ABS2 NRs and (i) TEM images of the resulting core-shell CISe/CIS NRs. Reproduced under terms of the CC-BY license from ref. 68; Copyright 2024 John Wiley and Sons.

viated as CIZS) NRs, in this case. One of the first syntheses of this kind was performed by a heat-up approach and employed a mixture of diethyldithiocarbamate salts of indium, copper, and zinc in combination with DDT and OA as ligands, resulting in the formation of WZ-phase CIZS NRs (Fig. 2b).62 It was found that the use of the higher amount of Zn2+ increased the length of the resulting NRs, while also blue-shifting their absorption band-edge because of an increasing bandgap. Another synthesis of CIZS NRs was performed using TOPO as a ligand and t-DDT as a sulfur source,63 similarly to the cases of CIS NRs described above.51 The resulting CIZS NRs were highly monodisperse (<5%), which allowed them to form well-ordered assemblies, both in-plane and perpendicular to the substrate. At the same time, the diameters of the CIZS NRs were in the range of 8.0-9.5 nm, smaller than in the case of CIS NRs,51 because Zn2+ cations were incorporated into Cu2-xS at the earlier stage, due to Zn having a higher affinity towards S than In. Both for CIS and CIZS NR growth, Cu2-xS seeds served as a catalyst similarly to the solution-liquid-solid mechanism,64 and at later stages became sacrificed. The occurrence of the solution-liquid-solid mechanism was further proved for the synthesis of heterostructured Cu2S/ZnS NRs, where Cu2S seeds became elongated into NRs by depositing a

ZnS shell.65 The same kind of heterostructure was produced by separating the Cu2S seed formation step and injecting them into the hot S/TOPO mixture to synthesize Cu2S/ZnS NRs, which were then used as templates for CIS NRs.66

For the synthesis of AIS NRs, Torimoto et al.67 developed a two-step heat-up process using metal precursors Ag(CH3COO), In(CH3COO) 3, and Zn(CH3COO)2, and as sulfur precursors, elemental sulfur and 1,3-dibutylthiourea. At the first heating step (150 °C), isotropic AIS NCs were formed, while at the second step, when the temperature was raised to 250 °C, aniso-tropic growth has occurred and resulted in the formation of AIS NRs with the same diameter as the initial NCs. Use of an additional Zn2+ precursor resulted in the formation of alloyed ZnS-AIS NCs, while the increase of the reaction time led to formation of another, nanorice-like shape.

Seeded growth synthesis of ternary and quaternary NRs and NWs

Homogeneous nucleation followed by anisotropic growth may result in a rather broad distribution of NR diameters, which can significantly affect their optical properties. One of the ways to narrow down the diameter distribution of NRs is to inject seeds together with the anion precursor into a solution of the

Review

cation precursor (Fig. 3c), which results in a heterogeneous nucleation and growth of the NRs from the seeds, while the size of the seeds would determine the diameter of those NRs. If Cu2-xS are used as seeds, a temporal separation between Cu2-xS seed nucleation and In3+ incorporation does not change the overall course of the reaction, which still occurs through a Cu2-xS/CIS interface, as discussed above. Growth of CIS NRs from Cu2-xS seeds was demonstrated by Xia et al. ,60 where the formation of Janus-type Cu2-xS/CIS heterostructures (Fig. 2e) was observed at the intermediate stage of the reaction, while later on the Cu2-xS tips disappeared, alongside with formation of purely CIS NRs. It was also revealed that the In3+ precursor participated in the reaction in the form of indium-thio-late complexes, which allowed for cation exchange owing to a higher affinity of Cu towards S in comparison with In.

Apart from the seeded growth mechanism, Zhang et al.56 reported the formation of AIS NWs occurring by self-assembly. Ag2S seeds were formed at 130 °C in DDT, and then a mixture of In(OAc)3 and OA was quickly injected and further heated up to 210 °C. The dissolving In3+ precursor was involved in cation exchange, which resulted in the formation of AIS NCs followed by their assembly into NWs in the presence of In-S species serving as inorganic ligands and controlling the oriented attachment of AIS NCs. As a result of this two-step process, AIS NWs with a small diameter of 1.2 ± 0.2 nm and a length of 24.0 ± 3.7 nm were formed.

Formation of an anisotropic shell of a wide-bandgap semiconductor material such as CdS or ZnS over pre-synthesized spherical NCs using a seeded growth approach is yet another way to attain the final NR shape. In this case, the reaction conditions discussed above in relation to the direct anisotropic growth remain similar, namely relatively large monomer concentrations to drive the reaction into 1D kinetic mode, and stabilization of the WZ structure of the shell material. Moreover, seeds have to adopt the WZ phase as well, in order not only to provide conditions for epitaxial growth of the shell material, but also to avoid the formation of any by-products of other, undesired shapes. For example, the use of CP CIS cores resulted in CIS/CdS tetrapods, as each of the facets of the CIS tetrahedra, which form the (111) family of facets, acted as a support surface for the precipitation of the shell material.55 In order to produce the NR shape exclusively, one has to use WZ CIS cores as seeds, which ensures epitaxial anisotropic growth of the WZ shell material. One of the first anisotropic syntheses utilizing this strategy was demonstrated by de Mello Donega et al. ,68 who fabricated WZ CIS cores by cation exchange from Cu2-xS seeds, which were injected into the hot solution containing zinc oleate, hexadecylamine, and sulfur dissolved in ODE, yielding core-shell CIS/ZnS NRs with a high aspect ratio (Fig. 3d). Early injection of the sulfur precursor was necessary to build up a high concentration of ZnS monomers in order to drive the reaction into 1D growth mode. Other commonly used ligands, namely DDT, trioctylamine or OA, provided rather poor shape control. If TOPO was used instead of amine, the produced NRs showed no PL signal, while the presence of TOP completely suppressed the growth. It is noteworthy that, in

these nanostructures, whose growth rate exceeded 20 nm min-1, the CIS core was positioned much closer to one end (Fig. 3d), differently to the conventional synthesis of CdSe/CdS core/shell NRs where the core was situated at the one-third position of the NR length.69 In the case of CIS/ZnS NRs, the use of a long-chain amine (hexadecylamine) together with OA allowed a complete block of the cation-rich (002) facet, while rendering the (002) facet the most reactive one. Synthesis of CdSe/CdS NRs, on the other hand, involved hexylphosphonic acid with only six carbon atoms, which left the (002) facet partially reactive; in that case the ratio between hexylphosphonic and octadecylphosphonic acids determined the core position within the NR.70 In addition, CIS cores after the ZnS shell growth underwent partial etching and alloying, leading to a significant blue-shift of the absorption edge by ~185 nm,68 which was opposite to the red-shift occurring during the CdS anisotropic growth over CdSe.69 At the same time, a strong improvement of PL QY was achieved, from 0.2% for CIS cores to 20% for CIS/ZnS NRs. Even though an anisotropic shape was attained for CIS/ZnS NRs, the NRs' dimensions were shown to be difficult to control, and the PL position was hardly tunable within this synthesis route. Anisotropic CdS shell growth over CIS cores allowed one to overcome those issues, providing an exquisite level of control over both NR size (Fig. 2c) and PL peak position.71 A mixture of phosphonic acids was used to render the polar (002) and (002) facets of the WZ structure in the CIS core more reactive in order to create suitable conditions for anisotropic growth. The length of the resulting CIS/CdS NRs was tuned from 13 to 30 nm by changing the injection temperature, while their diameter was determined by the size of the CIS seeds. Since the reaction temperatures were rather high (350-380 °C), and phosphonic acids were present in the reaction mixture alongside with TOP, the CIS cores underwent some etching and alloying, and the smaller the core, the higher the degree of etching/alloying attained. The resulting CIS/CdS NRs showed PL spectra tunable in the range of 700-900 nm, with longer wavelengths achieved when larger CIS cores were used. Apart from controlling the size, alloying of the CIS cores with Zn was also efficient in tuning the PL wavelength of the resulting CIZS/ CdS, whose NIR emission was enhanced at the same time. The PL QY of CIS/CdS and CIZS/CdS NRs reached 30 and 40%, respectively, which rendered them the brightest NIR aniso-tropic emitters reported at that time.71 Even though the presence of Cd hindered their further widespread implementation, this material platform can be used as a template for cation exchange, as will be discussed in the next section.

Cation exchange synthesis of ternary NRs

Contrary to direct synthetic methods, cation exchange approaches allow for the fabrication of anisotropic shapes of ternary NCs using pre-synthesized templates, which allows flexibility in attaining the desired shape and size while at the same time ensuring tunable optical properties of those materials and at the same time eventual elimination of toxic Cd.72 During the cation exchange, new cations from solution

Nanoscale

replace (partially or completely) the existing cations within the NCs' volume, while leaving the anion framework and the NC morphology largely unaltered (Fig. 3e and h). The capability of the cation exchange was demonstrated using the example of CIS synthesis following the route Cu2-xSspheres Cu2-xS/ ZnSNRs ^ Cu2-xSNRs ^ CISNRs, whereas Cu2-x/ZnS NRs were prepared by the seed growth method and then converted into CIS NRs by two-step cation-exchange (Fig. 3f and g).66 However, those CIS NRs showed no PL signal due to the lack of a passivating shell and abundant Cu and In vacancies, which cause the occurrence of a localized surface plasmon resonance band in the near infrared part of the absorption spectrum.

Since the synthetic chemistry of Cd chalcogenide NCs has been perfected over the last few decades, also in terms of their anisotropic growth (NRs), these materials are frequently employed as templates for cation exchange transformations.73'74 Van der Stam et al. synthesized core-shell CISe/CIS NRs starting from CdSe/CdS NRs by means of two-step cation exchange, which is shown schematically in Fig. 3h.75 During the first Cd-to-Cu exchange step, CdSe/CdS NRs in toluene were converted into Cu2-xSe/Cu2-xS NRs by adding a methanol solution of [Cu(CH3CN)4]PF6, as the solubility products of the respective Cu chalcogenides are several tens of magnitudes lower than those of Cd chalcogenides.72 During the next Cu-to-In exchange step, Cu2-xSe/Cu2-xS NRs were subjected to partial cation exchange with In3+ to yield CISe/CIS NRs, which retained both the shape and the size distribution of the starting Cd-based NRs. The key feature of that study was the use of equimolar amounts of InCl3 and TOP in order to reduce the number of defects originating from the Kirkendall effect. Still, the PL QY of such prepared CISe/CIS NRs was very low, <1%. In a follow-up study, Portniagin et al. used In(OAc)3 and DDT during the cation exchange, which improved the PL QY of the resulting CISe/CIS NRs to 20% (Fig. 3i).76 Still, this kind of Cd-to-Cu cation exchange yielded Cu2-xSe/Cu2-xS NRs which contained up to 5 at% of toxic Cd, which was shown to be retained by ligands bound to CdSe/CdS NRs. In contrast, ligand exchange in the presence of octyla-mine and TOPO ensured removal of Cd down to a residual content of <1 at%, while the addition of indium phosphonates as Z-type ligands led to improved colloidal stability of the resulting CISe/CIS NRs. Another example was demonstrated for Cu2-x/CuIn(Sb, Sn)S2 heterostructures derived from Cu2-xS NRs by means of partial cation exchange with In, Sb, or Sn.77 It was found that the presence of In and Sn led to the formation of heterostructures along the long NR axis, while Sb caused formation of a CuSbS2 shell.

Cation exchange approaches have also been applied for Ag-based ternary NRs. Bi2S3 NRs were converted into AgBiS2 NRs with a width of 13 nm and a length of 130 nm through partial cation exchange between Bi3+ and Ag+ ions.78 It was shown that cation exchange began at the two ends of the [001]-grown Bi2S3 NRs, and proceeded through the formation of intermediate heterostructured AgBiS2-Bi2S3-AgBiS2 NRs with preservation of the width of the initial Bi2S3 NRs. Another relevant

research study on binary-ternary heterostructured anisotropic NCs was reported by Prusty et al. ,26 who used Ag2Se NCs as sacrificial seeds with catalytic functions for the formation of II-VI and I-III-VI coupled semiconductor NCs resulting in CdSe-AgInSe2, CdSe-AgGaSe2, ZnSe-AgInSe2, and ZnSe-AgGaSe2 dot-in-rod heterostructures.

Other types of anisotropic ternary and quaternary chalcogenide NC

Considering other types of anisotropic NC, whose shapes are different from rod-like, one can mention 2D NCs with a ternary composition such as CIS nanoplatelets (Fig. 4a),79 CIS triangular nanosheets,80 quaternary triangular-shape CuZnInSe NCs (Fig. 4b),81 CuGaZnS nanobelts (Fig. 4c),82 and Zn-CuInGaS2 nanodisks.83 More sophisticated shapes, which are better classified as 3D nanostructures, have been demonstrated as well, represented by tetrapods and nanoflowers: CuInS2 (core)/CdS (arms) (Fig. 4d)55 and Cu2SnSe3 nanotetra-pods,84 CuZnSnSe nanotetrapods (Fig. 4e and f),85 CuZnSnSe nanorods/nanotetrapods,86 AIS,57 ABS (Fig. 4g)87 CIS,79 Cu3BiS3 88 nanoflowers, and berry-shaped NCs.32

Optical properties of ternary and quaternary chalcogenides

Energy level structure of ternary chalcogenide NCs

Before discussing the optical properties of anisotropic NCs, let us briefly consider the energy level structure of ternary NCs in general. Their bandgap depends both on the chemical composition and crystallographic structure (CP, WZ, etc.), as summarized for the most common bulk materials in Table 1. For the related NCs, the bandgap shifts towards larger energies with the decrease of size, as was shown by Xia et al.89 with the example of CIS NCs. A strong dependence of the bandgap on the chemical composition, and in particular on the ratio between A and B cations in the ABC2 structure, is related to the fact that the energy levels at the edges of the valence band (VB) and conduction band (CB) are affected by hybridized states of Cu (Ag) and chalcogen orbitals. Thus, for both the ternary and quaternary NCs it may be rather difficult to distinguish between the effects of the quantum confinement (decreasing sizes) and the chemical stoichiometry. The presence of the structural defects/vacancies results in the appearance of localized states within the bandgap, which is observed as an Urbach tail in the absorption spectra of ternary NCs.90 In the core-shell ternary NCs, excitons should become confined in the core of lower-bandgap materials, but it was also observed that deposition of the ZnS shell on ternary NCs may result in formation of alloys, which affected their bandgaps.91^92 Through the variation of size and chemical composition of ternary NCs, it was possible to obtain materials with optical absorption edges in visible and NIR spectral ranges ranging from 300 to 1100 nm (ref. 93) and even 1300 nm (ref. 94) for AgBiS2 NCs, from 300 to 650 nm to AIS

NCs,9

and from 300-800 nm for CIS NCs.7

Review

Fig. 4 Examples of ternary and quaternary anisotropic NCs of 2D shapes (upper panel) and 3D shapes (lower panel): (a) TEM image of CIS nanopla-telets. (b) TEM image of 8.7 nm CuZnInSe triangular NCs. (c) TEM images of CuGaZnS nanobelts. (d) TEM image of CIS(core)/CdS(arm)-heterostruc-tured nanotetrapods. (e) Annular dark field and (f) high-resolution TEM images of a single CZTSe nanocrystal with ZB- and WZ-derived structures; the bottom inset in (e) shows EDX elemental mappings, with Cu in red, Zn in yellow, Sn in green, and Se in cyan; (g) SEM image of ABS nanoflowers. Scale bars in (a), (b), and (d): 50 nm, (c): 200 nm, (f): 20 nm, (g): 2 ^m. (a) Reproduced under terms of the CC-BY license from ref. 79; Copyright 2019 MDPI. (b) Reproduced under terms of the CC-BY license from ref. 81; Copyright 20254 Royal Society of Chemistry. (c) Reproduced with permission from ref. 82; Copyright 2024 John Wiley and Sons. (d) Reproduced with permission from ref. 55. Copyright 2018 American Chemical Society. (e and f) Reproduced with permission from ref. 85. Copyright 2024 American Chemical Society. (g) Reproduced with permission from ref. 87. Copyright 2024 Elsevier.

Table 1 Chemical composition, crystallographic structure, and band-gaps of Cu- and Ag-based ternary chalcogenides (for bulk materials)

Composition Crystal phase Bandgap, eV Ref.

Cu-based

CIS WZ 1.28 89

Polymorph 1.555° 97

CP 1.535 98

CISe CP 1.05 97

Ag-based

AIS CP 1.87 24 and 97

Tetragonal 1.8 99

Orthorhombic 1.98 56

AgInSe2 CP 1.2 97

AgBiS2 Face-centered cubic 1.2 93

Cubic 1.3 22

° Measured at 4.2 K.

The PL profiles of ternary and quaternary NCs are broad in both visible and NIR spectral ranges, with substantial Stokes shifts. Furthermore, the contribution of defects is pronounced in such materials, thereby rendering elucidation of the PL mechanism rather sophisticated. Fig. 5 shows the typical optical properties of ternary ABC2 nanomaterials on an example of AIS NCs. Compared with the bulk AIS bandgap of 1.87 eV, the bandgap of 1.7 nm AIS NCs increased to 2.58 eV, and further increased to 2.63 eV for 2.4 nm core-shell AIS/ZnS NCs27 (Fig. 5a). The broad PL spectral profiles for AIS and AIS/

Fig. 5 Typical optical properties of ternary NCs. (a) Absorption and PL spectra of AIS (termed "core") and AIS/ZnS (termed "core/shell" on this graph) NCs. Reproduced with permission from ref. 27; Copyright 2022 American Chemical Society. (b) Normalized PL spectra of CP AIS NCs produced by slow injection and orthorhombic AIS NCs produced by fast injection. Reproduced with permission from ref. 33; Copyright 2020 American Chemical Society.

Nanoscale

ZnS NCs (Fig. 5a) shifted towards larger energies when the size of the AIS NCs increases, which was accompanied by an increase of PL QY from 32 to 77% upon deposition of the ZnS shell.27 Another trend is exemplified by Fig. 5b for AIS NCs: the PL band shifted from 710 to 760 nm and PL QY decreases from 36 to 8% when their crystal structure changes from CP (NCs produced by slow injection) to orthorhombic (NCs produced by fast injection).33

A comprehensive study of the dependence of optical properties on the chemical composition and stoichiometry was recently done on small quaternary Cu-Zn-In-Se NCs,41 where both bandgap and emission peak energies shifted towards lower values upon increase of the Cu to (Cu + In + Zn) atomic ratio (Fig. 6a and b). In contrast, there was no such trend for the PL QY, which mostly depended on the crystal lattice quality and therefore demonstrated the highest values for the specific stoichiometry (Fig. 6c). For AIS NCs, an introduction of Zn and Ga into the crystal lattice resulted in NCs with controllable optical responses in the 470-614 nm spectral range.50 There are several mechanisms (illustrated in Fig. 7) which were proposed to explain the origin of emission of ternary compounds at the nanoscale, namely (i) donor-acceptor pair (DAP) recombination, (ii) free-to-bound recombination, (iii) self-trapped exciton (STE) model, and (iv) fine-structure splitting of the lowest-energy exciton due to crystal field asymmetry. In the DAP model, the terms donor (D) and acceptor (A) denote intragap states located near the CB and VB, respectively, and radiative recombination occurs as a transition from the D level to A level (Fig. 7a). Because the defect concentration in CIS is often high owing to the low defect formation energy (<2 eV for VCu and InCu),100 inhomogeneity of their composition may result in a wide distribution of D and A pairs, which rationalizes the broad PL bandwidth and multiexponen-tial PL lifetimes observed for CIS NCs using the DAP model. However, the size-dependence of the PL peak as well as ground state bleaching observed by transient absorption spectroscopy could not be properly described in terms of this model for CIS101 and AIS24 NCs.

In the recent paper by Szymura et al.102 it was experimentally shown that emission from CIS NCs is governed by the so-called free-to-bound mechanism, which implies recombination involving a free and a localized charge carrier. As illustrated in Fig. 7b and 7c, either electrons or holes can be localized, respectively; however, the electron localization in the case of CIS NCs was largely excluded based on the power-dependent ground state bleach dynamics, corresponding to a 2-fold degenerate state.103 As the top of the VB state is far more degenerate,104 it should be the bottom of the CB which is responsible for the ground state bleaching, thus proving the electron was delocalized, while the hole was localized. After excitation, the electron delocalizes over the CB, while the hole becomes captured by either a Cu-related defect or Cu+ ion, thus turning Cu+ into Cu2+, so that the recombination pathway encompasses the transition CB-Cu2+ (Fig. 7c). The appearance of Cu2+ during excitation was proposed to explain the right-hand side and left-hand side circular polarized light intensity dependence on temperature and magnetic field.105'106 One of the debatable points here is whether Cu2+ is present in the CIS NCs prior to excitation, when they are still in the ground state as emission-ready centers. Spectro-electrochemical data showed a reduced PL intensity of CIS NCs under negative (highly reductive) electrode potentials, which could be interpreted as a conversion of the majority of Cu2+ into Cu1+.107 However, highly negative potentials could also lead to a partial reduction of In3+, leading to the detachment of ligands and formation of surface traps which can reduce the PL intensity as well. Moreover, Cu2+ was not found in the ground state of CIS NCs by means of X-ray photoelectron spectroscopy, extended X-ray absorption fine structure and X-ray absorption near-edge structure studies; however, Cu2+ was detected under laser excitation of CIS NCs using the latter technique.108 Thus, the hole localization may occur on the Cu+ cation, resulting in the appearance of Cu2+ only in the excited state of CIS NCs, and the presence of Cu2+ prior to the excitation still remains unclear.

Another possible explanation for the recombination mechanism of spectral properties of ternary NCs is the STE model

Fig. 6 Compositional trends for the optical properties of 3.3 nm Cu-Zn-In-Se NCs. (a) Optical bandgap (Eg); (b) emission peak energies for the two PL channels (low energy emission, EPL1, and high energy emission, EPL2); and (c) PL QY as a function of the Cu cation fraction. The data for CuInSe2, ZnSe, and Cu-doped ZnSe NCs of the same size are also provided for comparison. Reproduced with permission from ref. 41. Copyright 2024 John Wiley and Sons.

Review

Fig. 7 Schematics of energy level structures and emission pathways which may occur in ternary NCs. (a) Donor (D)-acceptor (A) pair (DAP) model; (b and c) free-to-bound model, shown for the two cases of localized electrons (b) and localized holes (c); (d) self-trapped exciton (STE) model. Reproduced from ref. 145. Copyright 2023 Royal Society of Chemistry; (e) valence band structure model. Reproduced under terms of the CC-BY license from ref. 101; Copyright 2019 American Chemical Society.

(Fig. 7d), which was proposed for AIS NCs in ref. 109. The generation of STEs can be caused by two key factors: a large exciton binding energy and a strong electron-phonon coupling of the distorted exciton and the host lattice in ternary NCs. Consequently, the PL band energy can be estimated as a sum of bandgap energy, exciton binding energy, energy of STE, and energy of lattice deformation, and their collective contribution determines the large Stokes shift observed for ternary and quaternary NCs. Finally, and differently to the abovementioned models which involve the presence of defects, the valence band structure model (illustrated in Fig. 7e) proposes that interactions between the exciton and the crystal lattice phonons may result in a broadband emission, even in the case of a single nanoparticle.110'111 This model suggests that the Stokes shift increases with the number of emitted phonons. Recently, scientists also attempted to combine several different mechanisms, such as defect-based and electron-phonon interaction models, to better explain the complex optical properties observed in ternary NCs. The model presented in ref. 112 for AIS NCs is a comprehensive approach that integrates several key features of previously proposed models: the presence of DAP recombination, strong electron-phonon interaction, and the non-localized character of electrons in CB.

Effect of anisotropic shape on the optical properties of ternary and quaternary chalcogenides

As discussed above, optical transitions of ternary and quaternary NCs depend on their size, chemical composition (influenced both by stoichiometry and eventually alloying), and the presence of the shell in the case of core-shell NCs, which is also true for their anisotropic counterparts. Alloying with elements forming wide-bandgap sulfides such as Zn and Cd can increase the bandgap of the resulting quaternary NCs in comparison with the respective ternary nanostructures, as was reported for CIS,113'114 CISe,81 and AIS67 NCs.

As demonstrated by Portniagin et al.,71 the change in the core size of the core-shell CIS/CDs NR from 3.7 down to 2.5 and then to 1.9 nm resulted in the PL shift from 896 to 824 and 766 nm, whereas the growth of the CdS shell led to a blue-

shift of the PL band,68 which was more pronounced for CIS seeds with a smaller diameter (Fig. 8a-c). While both the absorption and emission peaks of CIS NCs can be blue-shifted towards the visible spectral range by alloying with Zn,115,116 as a result of the CdS shell growth the PL red-shifted back to NIR because all the Zn was replaced by Cd, leading to the bandgap reduction (Fig. 8d).71'117 Furthermore, their PL band position depended on the stoichiometry of the core, which could be changed by alloying with Zn, leading to formation of CIZS cores (Fig. 8d), and on the aspect ratio of NRs as summarized in Fig. 8e. A similar approach was applied for ZnAgInS2 (abbreviated as ZAIS) rod-shaped nanoparticles,67 where the energy of bandgap increased with an increasing Zn amount.

A high defect concentration and the ability to adopt a variety of compositions by ternary NCs often result in their featureless absorption profiles, while the PL profile is typically broad owing to the recombination with one charge carrier being localized, as we discussed above. In fact, for CIS NRs, most of the papers lack the PL data (see Table 2), basically due to a very low PL intensity of the produced materials.51'63'66'117 In addition, many direct syntheses yield ternary NRs with rather broad size distributions, which result in broad and non-symmetrical PL spectra.66 Passivation of the surface defects by deposition of an anisotropic shell of either a ZnS68 or CdS71 wide-bandgap semiconductor could improve the PL QY of CIS/ ZnS and CIS(CIZS)/CdS NRs to the values of ~20% and ~30-40%, respectively. In both cases, a blue-shift of the PL bands occurred as compared with the core NCs, which was especially strong for the ZnS shell (~200 nm) and less strong for the CdS shell (20-80 nm). CISe/CIS NRs also featured an intense PL in the NIR spectral range (850-1000 nm) owing to their core/shell structure, with PL QY up to 57%.76 Removing Cd impurities by ligand exchange resulted in a much narrower emission line (172 meV vs. 255 meV before removal), owing to the smaller number of interfacial defects originating from the presence of Cd.76

NCs with anisotropic shapes can form polytypic crystal structures, as was recently shown for CuaZnpSnYSe6 NCs with shapes ranging from platelet-like and rod-like to tetrahedra

Nanoscale

Fig. 8 Absorption and PL spectra of (a-c) CIS and (d) CIZS cores, and of the CIS/CdS and CIZS/CdS NRs produced thereof. The yellow-shaded area indicates the position of CdS excitonic transition. (e and f) TEM images of the CIS/CdS NRs with a core size of 1.9 nm (e) and 3.7 nm (f). (g) Elemental ratios between (Cu + In) and Cd cations in CIS/CdS and CIZS/CdS NRs. Reproduced with permission from ref. 71. Copyright 2022 John Wiley and Sons.

Table 2 Optical characteristics of some reported Cu- and Ag-based ternary/quaternary anisotropic NCs of different shapes

Composition and shape Crystal phase Absorption range, nm PL range, nm PL QY, % Ref.

CIS NRs WZ 400- -1050 NA NA 66

WZ 500- -900 NA NA 120

CIS nanoplatelets WZ 300- -1100 NA NA 79

CIS/CdS nanotetrapods WZ 400- 650 650-1050 Up to 52 55

ZCISe nanosheets N/A 450- 1100 527-960 70 80

CISe/CIS NRs WZ 600- 950 750-1200 20-57 76

AIS nanorice Orthorhombic 300- 590 NA NA 121

AIS, tadpole-shaped Orthorhombic 350- -800 NA NA 58

ZAIS NRs WZ 300- 550 450-800 6 67

ZAIS nanorice-shaped WZ 350- -740 500-850 2 67

Cu„ZnpSnTSe5 nanoplates WZ N/A ~640 N/A 86

CuaZnpSnySe5 tetrahedra ZB N/A ~530 N/A 86

CuZnInSe triangular nanoplates Tetragonal 300- -1200 1082-1218 1-40 81

and tetrapods with different crystal types - WZ and ZB.86 For hexagonal nanoplatelets with a WZ crystal structure, a bright PL band was centered around 1.95 eV, while for tetrahedron NCs with a ZB crystal structure it was observed at a higher energy of 2.34 eV. Other anisotropic NCs (rod-like and tetra-pod-like) demonstrated a combination of WZ and ZB phases, with more contribution of the ZB emission for rod-like NCs and the WZ emission for tetrapods.86

Optical properties of the non-spherical NCs are severely altered by the effects imposed by their anisotropy on the density of states (DOS). As compared with spherical NCs which are confined in all 3 dimensions, whose DOS resemble discrete lines of atomic spectra, 1D (NRs) and 2D (nanoplatelets) nano-structures exhibit Van Hove singularities and step-like DOS, respectively.118 In addition, a dielectric confinement effect119 leads to stronger electron-hole interactions and thus the formation of strongly bound excitons, which manifests in the

sharp excitonic features observed in the optical spectra of NRs and nanoplatelets, if their size distribution is narrow. For example, the absorption spectrum of CISe/CIS NRs derived from CdSe/CdS NRs showed much sharper peaks corresponding to the absorption of the CIS shell at 700 nm due to the rod-like shape and nearly monodisperse size distri-bution,76 as compared with the usually "featureless" spectra of either CISe or CIS spherical NCs. Another example of how an-isotropy of NCs affects the PL parameters was provided by Bora et al.81 The authors reported quaternary triangular CuZnInSe nanosheets whose synthesis involved the formation of In2Se3 seeds further doped with Cu and Zn atoms, followed by ZnS shell formation (Fig. 9a). The PL band shifted towards longer wavelengths with the increase of the lateral NC size, while their narrow PL band was attributed to the absence of compositional inhomogeneities and low thickness of the triangular nanosheets (Fig. 9b-d).81

Review

Fig. 9 (a) Absorption spectra of In2Se3 seeds, CIS seeds, and In2Se3 nanosheets. Absorption and PL spectra of CISe, CZISe, and core/shell CZISe/ZnS NCs of three different sizes (small, medium, and large CZISe NCs - 8.7 nm, 16.2 nm, and 20.5 nm, respectively; small, medium, and large CZISe/ZnS NCs - 10.9 nm, 18.3 nm, and 22.9 nm, respectively), whose PL maxima are located at (b) 1082 nm, (c) 1175 nm, and (d) 1218 nm. Corresponding PL QYand FWHM values are provided on these 3 frames as well. Reproduced under terms of the CC-BY license from ref. 81; Copyright 2025 Royal Society of Chemistry.

These and several other examples of the optical characteristics of ternary/quaternary NCs with anisotropic shapes are provided in Table 2.

Chiroptical properties of ternary chalcogenide NCs

Several recent studies were devoted to the chirality of Cu- or Ag-based ternary NCs, which was induced either by chiral Iigands95'122 or by the environment of NCs which exhibited a chiral structure.123 Gao et al.123 functionalized achiral CISe/ ZnS NCs with chiral amino acid Iigands (l/d-histidine and N-(9-fIuorenyImethoxy-carbonyI)-protected glutamic acid), and the resuIting NCs were assembIed into a supramoIecuIar hydrogel (Fig. 10a). This chiral composite exhibited a circular dichroism (CD) signal in the range of 300-900 nm with dissymmetry factors (gabs) as large as 1.3 x 10-2 and 0.3 x 10-2 observed at 380 nm and 808 nm, respectively (Fig. 10b), and circular polarized luminescence (CPL) appeared at 1050 nm with a dissymmetry factor gIum equal to 3.4 x 10-3 (Fig. 10c). Another way to induce chiraIity at the nanoscaIe is to use a Iigand exchange with chiral Iigands (Fig. 10d), which is well-developed for cIassicaI Cd-based NCs.12 This approach was appIied to AIS NCs by Branzi et al.,95 who showed that chiraI signaIs originated from the eIectronic or structuraI conformation of the cysteine-nanocrystaI chiraI interface. The absorption spectra of L- and D-AIS NCs were aImost identicaI

(Fig. 10e), whereas the CD spectra were a mirror-image of each other (Fig. 10f). The CD spectrum of L-AIS NCs showed a Cotton effect with a negative band Iocated at 420 nm and a positive one at 375 nm with a gabs of -0.92 x 10-4 and 0.63 x 10-4, respectiveIy (Fig. 10f). In a recent study by Ding et al.,122 a simiIar mechanism was demonstrated for AgBiS2 NCs. The absorption spectra were aImost the same for l- or d-Cys AgBiS2 NCs, with an absorption band observed at 300-350 nm (Fig. 10g). The authors showed that the binding of the chiraI Iigand l/d-cysteine at the NC surface occurred in a bidentate mode, resuIting in a significant crystaI Iattice distortion. Charge transfer between AgBiS2 NCs and cysteine Iigands resuIted in a strong CD signaI in the range of 200-400 nm, with a Iarge gabs of 1 x 10-2 (Fig. 10h).

Applications of Cu- and Ag-based ternary chalcogenide NCs

A tunabIe energy IeveI structure, aIongside the anisotropic opticaI responses, render Cu- and Ag-based ternary/quaternary NCs attractive for a Iarge variety of appIications. For instance, NCs with bright PL can be used as emitters in LEDs, both uti-Iizing down-conversion and charge-injection.124 Xie et al.50 demonstrated AgInGaZnS NCs with a narrow PL band ranging from 470 to 614 nm, which were used for the fabrication of bIue, green, and red NC-based LEDs with a maximum Iumi-nance of 405, 184, and 196 cd m-2, respectiveIy.50 Moreover, the abiIity of such NCs to emit Iight over a broad spectraI range can be used for the signaI coding under resonant conditions, i.e. utiIizing whispering gaIIery modes, as was recentIy shown in ref. 125. Furthermore, the absorption and PL of Cu-and Ag-based ternary NCs in the deep-red and NIR spectraI regions make them attractive for use in soIar ceIIs, Iumines-cent soIar concentrators, and photodetectors.126,127 Since they do not contain toxic Cd, Pb, or Hg eIements, these materiaIs may be particuIarIy suitabIe for biomedicaI appIications, as Iuminescent tags within the bioIogicaI window of transparency, and sensors.128

Solar cells, luminescent solar concentrators, and photodetectors

Broad absorption bands of Cu- and Ag-based ternary/quaternary NCs extending towards the NIR spectraI region, combined with rather Iarge extinction coefficients, make them suitabIe for appIication as active Iayers in soIar ceIIs and photodetectors.126,127 Stoichiometry controI in the case of Ag-based ternary NCs was shown to improve the absorption in the deep-red and NIR spectraI regions. Choi et al.129 showed that thin AgBiS2 NC fiIms post-anneaIed at 200 °C exhibited the most favorabIe microstructure with high crystaIIinity and thus couId be used in soIar ceIIs. Nanoinks based on AgBiS2 NCs were empIoyed to fabricate soIar ceIIs, whose absorption efficiency and hence power conversion efficiency (PCE) was affected by the moIar ratio between Ag, Bi and S in those NCs.130 SoIar ceIIs based on AgBiS2 NCs with the moIar ratio of

Nanoscale

Fig. 10 Left panel: (a) Schematics of formation of a hybrid composite based on CISe/ZnS NCs embedded in a L/D-hydrogel. (b) CD spectra of CISe/ ZnS NCs (denoted as "QDs"), L-Gel, D-Gel, QDs@L-Gel, and QDs@D-Gel, and (c) CPL spectra of those 4 samples excited by an 808 nm laser. Reproduced with permission from ref. 123. Copyright 2024 Elsevier Ltd. Right panel: (d) Schematics of a solution-phase ligand exchange performed on AgBiS2 NCs using a chiral ligand cysteine (Cys). Reproduced under terms of the CC-BY license from ref. 122; Copyright 2024 John Wiley and Sons. (e) UV-vis absorption and (f) CD spectra of L-AIS (blue) and D-AIS (red) NCs. Reproduced under terms of the CC-BY license from ref. 68; Copyright 2022 Royal Society of Chemistry. (g) UV-vis absorption and (h) CD spectra of AgBiS2 NCs modified with L-cysteine and D-cysteine. Reproduced under terms of the CC-BY license from ref. 122; Copyright 2024 John Wiley and Sons.

Ag : Bi : S of 0.72 : 0.9 :1 showed the highest PCE of 3.3%; this value could be further improved to 7.35% when implementing AgBiS2 NCs synthesized by cation exchange.131 Stoichiometry control was further explored by Wang et al.,94 who increased the absorption coefficients of AgBiS2 NCs by 5-10 times across a wide spectral range from 400 to 1000 nm making use of controlled cation disorder. Using those NCs as ink, solar cells with a PCE of 9.17% were demonstrated. Yet another direction for improving the PCE of solar cells was proposed by Hayes et al.,132 who focused on the ligand engineering on CuInS2 NCs. The use of N-methyI-2-pyrroIidone as a ligand resulted in a larger average grain size of the NC films, and the respective solar cells reached a PCE of 11%. Similarly, in a recent paper by Wu et al.,133 Iigand engineering was shown to be an effective approach for increasing the PCE of the luminescent solar concentrators based on core-sheII CuInS2/ZnS NCs. The Stokes shift of these NCs reached 585 meV after Iigand exchange with off-stoichiometric thioI-ene poIymers, and the PL QY was 85% in the 300-800 nm spectraI range, which resuIted in an increased PCE of 1.36% of the Iuminescent soIar concentrators.

In the area of photodetectors, Sharma et al.134 used AgBr and AgI to passivate the surface of AgBiS2 NCs, which were

then used as nanoinks to fabricate NIR photodetectors with a dark current of 6.01 x 10-7 A cm-2 and a high specific detectivity of 1.8 x 1012 Jones at 800 nm. Anisotropic Cu- and Ag-based ternary NCs have been considered as perspective materiaIs for photodetectors of poIarized Iight in the NIR spectraI range, in par-ticuIar with a view to using them as a Iight signaI receiver in an encrypted opticaI communication system (Fig. 11a). In the reIated study by Chen et al.,135 CIS nanofIakes were deposited onto SnO2 nanopiIIars to fabricate seIf-powered photodetectors with detectivities of 1.19 x 1010, 6.35 x 1010, and 1.02 x 1010 Jones under 254 nm soIar-bIind uItravioIet Iight, 475 nm visibIe Iight, and 940 nm NIR Iight, respectiveIy. A simiIar approach was impIemented by Li et al.,136 where ternary CIS nanofIakes were deposited on TiO2 nanorods, and the resuIting photodetectors exhibited detectivity in 365-650 nm spectraI range with a maximaI vaIue of 4.6 x 1012 Jones at 365 nm. Portniagin et al.76 used CISe/CIS NRs synthesized by two-step cation exchange and Iigand repIacement to fabricate fieId-effect transistor photo-detectors for the red-NIR spectraI range, as iIIustrated in Fig. 11b. Depending on the remaining Cd-content in those NRs, nameIy Cd-0 (<1 at%), Cd-5 (2-5 at%), and Cd-26 (14-26 at%), the respon-sivity of the photodetectors reached a maximum vaIue of 0.2 A W-1 and specific detectivity of 108 Jones at 685 nm (Fig. 11c).76

Review

Fig. 11 (a) Schematics of the CIS/SnO2 photodetector used for encrypted optical communication utilizing incident light with different wavelengths as the input signal, and transient current (J) curves together with binary codes as outputs. Reproduced with permission from ref. 135; Copyright 2024 American Chemical Society. (b) Schematics of the field-effect transistor photodetector based on CISe/CIS NRs, and (c) specific detectivity of such devices based on three kinds of CISe/CIS NR with different Cd-content (0, 5, and 26 at%) under 685 nm laser illumination and low bias of -1V. (b) and (c) Reproduced under terms of the CC-BY license from ref. 76; Copyright 2024 John Wiley and Sons. (d) I-V curves for D-Cys-AgBiS2 NCs incorporated into a spin filter shown in the inset, and (e) spin polarization of D-Cys AgBiS2 NCs. Reproduced under terms of the CC-BY license from ref. 122; Copyright 2024 John Wiley and Sons.

Considering chiral ternary NCs, Ding et al.122 used chiral AgBiS2 NCs in a spin-filter device, whose schematics are illustrated in the inset of Fig. 11d. Films formed on ITO substrates using l-cysteine capped and d-cysteine capped AgBiS2 NCs showed averaged spin polarizations of 83.7 ± 4.4% and -86.5 ± 6.3% (Fig. 11e), respectively.

Biolabelling and sensing

In a recent review by Aladesuyi et al.137 the main directions in contemporary biomedicine were highlighted as follows: drug and gene delivery, biosensing and multimodal molecular imaging, and pharmacokinetics. Since the optical responses of Cu- and Ag-based ternary NCs are in the deep-red/NIR spectral range, and they do not contain heavy metals such as Cd, Pb and Hg as their toxic II-VI NC analogues, these materials can become particularly useful for bioimaging and sensing. Flower-shaped AgBiS2 nanostructures were implemented in the photoelectrochemical sensor for the detection of bisphenol A, which is an endocrine-disrupting substance,138 achieving a detection limit as low as 0.18 nM. Similar nanostructures were utilized for photoelectrochemical sensing of S. aureus under NIR light.139 Bi3+-doped Ag2S NCs with a PL peak at 1230 nm and PL QY of 13% were used for glioma imaging in mice models with the skull/scalp remaining intact after intravenous injection,140 providing more efficient emission in the so-called NIR-II range and a deeper penetration of the light as compared with the commonly used organic dye indocyanine green. Mn2+-doped Zn-Cu-In-Se/ZnS NCs were used for multi-

mode imaging based on their PL and MRI (magnetic resonance imaging) responses.141 Moreover, these NCs were able to produce reactive oxygen species for photodynamic

therapy.141

Considering specific interactions of chiral NCs with bio-objects, ternary NCs can be functionalized with chiral moieties or DNA. Biocompatible AIS/ZnS NCs covered with 5-6 strands of hexylthiol-modified DNA showed a PL peak at 700 nm with a PL QY of 55% and a long PL lifetime of 900 ns.33 Surface plasmon resonance imaging was implemented to confirm that the DNA strands were successfully bound to AIS/ZnS NCs and maintained their bioactivity by response amplification by a factor of 3 of monitored reflectivity for DNA-covered AIS/ZnS NCs with respect to free DNA (Fig. 12a). In another study, CISe/ ZnS@chiral gel was used to affect cell viabilities, which depended on excitation with light of different polarization: at 300 mL-1 of NCs in the L-gel, the viability of cells was 12%, 65%, and 47% after exposure to left circular polarized (LCP), right circular polarized (RCP) light, and linear polarized (LP), respectively.123 Moreover, as shown in Fig. 12b and c, CISe/ZnS NCs embedded in the chiral gel could increase the temperature of the tumor upon illumination with light of different polarization (LP, LCP, and Ctrl - control, natural unpolarized light). As a result, the tumor growth was significantly suppressed after illumination with 808 nm LCP light as compared with illumination with Ctrl or LP light (Fig. 12c). These findings illustrated the possibility to enhance the efficiency of phototherapy for tumors using chiral NIR-emissive ternary NCs.

Nanoscale

Fig. 12 (a) Sensorgrams illustrating variation of reflectivity (expressed as A reflectivity, %) versus time, obtained upon injection of AIS/ZnS NCs covered with DNA (designated as csQD-DNA, blue curve) as compared with modified DNA (designated as ssDNAQD, red curve), AIS/ZnS NCs with glutathione (designated as csQDs, orange curve), and AIS/ZnS NCs covered with DNA control sample (designated as csQD-DNA control, green curve). The inset shows the schematics for the hybridization process of AIS/ZnS NCs covered with DNA on sites on a biochip. Reproduced with permission from ref. 33; Copyright 2020 American Chemical Society. (b) In vivo infrared thermal images MCF-7 breast tumor-bearing mice treated by CISe/ZnS NCs embedded in chiral gel (designated as QDs@L-Gel) upon natural (Ctrl), left circular polarized (LCP), and linear polarized (LP) light. (c) Trends of corresponding volumes of tumors with CISe/ZnS NCs embedded in chiral gel after the treatments with LCP and LP light. Reproduced with permission from ref. 123. Copyright 2024 Elsevier Ltd.

Summary and outlook

In recent years, ternary and quaternary Cu- and Ag-based chal-cogenide NCs have been gradually developed into a diverse material platform, offering less-toxic light-emissive alternatives to Cd- and Pb-based chalcogenide NCs. Several approaches have been explored to induce the shape anisotropy in such NCs, either by direct synthesis methods or by cation exchange using cadmium chalcogenides as a starting material. In particular, cation exchange approaches matured into an effective tool to produce high-quality ternary and quaternary NRs. Still, Cd removal strategies can be further advanced to allow us to preserve the morphology of the initial templates intact, while the PL QYs of the resulting NRs can be further improved by developing shell passivation strategies. Optical chirality and its applications for bioimaging and phototherapy are emerging topics, where the ternary NC compounds can be explored as well. Still, there are several challenges that have to be addressed in order to attain a higher level of technological maturity of ternary and quaternary Cu- and Ag-based chalco-genide NCs.

Control of stoichiometry of ternary and quaternary NCs to achieve a precise tuning of their bandgap. High concentrations of defects in ternary and quaternary chalcogenides result in a broad variation of compositions within an ensemble of NCs, which in turn leads to inhomogeneous broadening of their absorption and PL spectra. Thus, precise control of the types and numbers of defects and stoichiometry in ternary and quaternary NCs is required, which could eventually be attained by developing new single-source precursors, containing all 3 or 4 constituting elements and providing steady chemical conversion kinetics during their nucleation and growth.

Development of synthetic approaches resulting in reliable formation of ternary and quaternary anisotropic NCs with specific

shapes, including NRs, nanoplatelets, nanotetrapods, etc. As we discussed in this review, shape anisotropy in one or two dimensions offers several advantages as compared with spherically shaped, isotropic NCs, such as linearly polarized emission or higher surface Iigand density.142'143 Novel synthetic methods allowing us to precisely tune the diameters of such NRs or nanotetrapods within the strong confinement regime are in great demand in order to exploit their full potential. In particular, ternary and quaternary nanoplatelets still lack comparable optical characteristics as opposed to their cadmium counterparts due to Iack of atomic-IeveI controI over the thickness, which remains yet another chaIIenge to be addressed.

Development of reliable approaches for inducing chirality of ternary and quaternary NCs, which can be achieved by shape or Iigand engineering, and assembly into chiral superstructures. Ligand exchange has already been proved to be efficient to anchor chiral molecules to the surface of ternary/quaternary NCs, whiIe different chiraI Iigands can be depIoyed during the NC synthesis. The complex structure of energy levels, which is a characteristic of ternary and quaternary NCs, can be utiIized to aIign the energy IeveIs of NCs with chiraI Iigands, thereby enhancing the efficiency of the Cotton effect and achieving Iarger vaIues of dissymmetry factors. An additionaI approach for enhancing circular dichroism signals involves the use of anisotropic NCs, which have been demonstrated to affect the dissymmetry factors of NRs in comparison with nanocubes.58 Given the established biocompatibility of ternary and quaternary NCs, development of chiral NCs holds considerable promise for bio-applications and sensing of chiral bio-objects.

Defect management and device applications of ternary/quaternary NCs. Recently, there were reports emphasizing the ability to achieve a low defect concentration either by lowering cation impurities such as in CISe/CIS NRs76 or by structural ordering with the help of Zn cations in quaternary Cu-In-Zn-Se41 and

Review

Ag-In-Zn-Se/ZnSe144 systems. Those ternary and quaternary anisotropic NCs with a defect-free structure have to be tested on a device level in order to get more insights into how their synthetic conditions affect the respective optoelectronic performance.

Author contributions

Arsenii S. Portniagin - conceptualization, validation, writing original draft; Sofia P. Karamysheva - visualization, writing -original draft; Kirill V. Bogdanov - resources, validation; Elena V. Ushakova - conceptualization, visualization, resources, writing - review and editing; Andrey L. Rogach - project administration, resources, writing - review and editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Priority 2030 Federal Academic Leadership Program, the Innovation and Technology Commission of the Hong Kong Special Administrative Region, China (project ITS/027/22MX), the Research Grant Council of the Hong Kong Special Administrative Region, China (project CityU SRFS2324-1S04), and the Russian Science Foundation (agreement 23-72-10010).

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