Фотофизика одиночных центров окраски в алмазе с узкополосной люминесценцией (Photophysics of single color centers in diamond with narrowband luminescence) тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Нелюбов Артур Юрьевич

• Нелюбов Артур Юрьевич
• кандидат наук
• 2025

Оглавление диссертации кандидат наук Нелюбов Артур Юрьевич

Table of Contents

Introduction

Chapter 1. Methods and samples

1.1 Fluorescent spectra-microscopy. Room temperature

1.1.1 Main approach

1.1.2 Imaging and spectroscopy

1.1.3 Photon statistics analysis. Second order correlation function

1.2 Fluorescent spectra-microscopy. Low temperature

1.3 Scanning electron microscopy and lithography

1.4 Diamonds

1.5 Chapter summary

Chapter 2. Spectral inhomogeneity of group-IV vacancy centers at low temperature

2.1 Color centers in diamonds

2.2 Investigation of diamond films with GeV- centers

2.2.1 Sample GDD-1 with relatively high concentration of GeV- centers

2.2.2 Annealing effect

2.2.3 Sample GDD-2 with ultra-low concentration of GeV- centers

2.3 Investigation of microdiamonds with GeV- and SnV- centers

2.3.1 GeV- centers

2.3.2 SnV- centers

2.4 Chapter summary

Chapter 3. Characterization of a new class of impurity centers in microdiamonds (LX centers)

3.1 Unidentified spectral lines in HPHT microdiamonds

3.2 Characteristic properties of single LX-centers

3.2.1 PL and PLE spectra

3.2.2 Brightness and photostability

3.2.3 Polarization

3.2.4 Temperature sensitivity

3.2.5 Spectral stability

3.3Unknown nature of LX-centers

3.4 Chapter summary

Chapter 4. Photophysics of single LX centers

4.1 Second order cross-correlation function of a blinking emitter

4.2 Intensity-dependent correlation function of single LX-centers

4.3 Deviations from the simple case

4.4 Chapter summary

Conclusion

Acknowledgements

Glossary

References

List of figures

Введение диссертации (часть автореферата) на тему «Фотофизика одиночных центров окраски в алмазе с узкополосной люминесценцией (Photophysics of single color centers in diamond with narrowband luminescence)»

Introduction

Work relevance. One of the most informative approaches to non-invasive study of living organisms is based on optical imaging, that allows real-time tracking of biochemical reactions with high spatial and temporal resolution. This method relies on the use of various luminescent probe objects among which are organic fluorophores, semiconductor quantum dots, carbon nanomaterials, metal nanoparticles, etc. Desirable markers must be bright, non-toxic, easy to produce and possess no photo-bleaching which is hard to satisfy at the same time for the listed chromophores. This is one of the application areas for which diamonds with color centers are of considerable interest [1].

Diamond is a widegap semiconductor that is transparent in the visible range. Impurity atoms embedded in the crystal lattice of diamond form color centers which introduce electronic states into the wide band gap, resulting in transitions that absorb or emit light. Diamond being the hardest material has almost no interaction with the environment that provides high biocompability and no toxicity [2-4], together with long term stability of physical and optical properties of color centers. Modern synthesis methods make it possible to produce diamonds of high quality, with fine control over the size of crystals and the composition of introduced elements [5-8]. The most famous and well-studied color center in diamond to date is Nitrogen-Vacancy center (NV) [9]. NV centers are quite easily embedded even in nanoscale diamonds, have bright fluorescence, and do not bleach, which altogether make them a powerful luminescent probe object. Besides, the unique energy level structure of negatively charged NV centers paves the way to a variety of sensing possibilities including the measurement of local fields, temperature, pressure, and others [10-15]. This has important relevance for life-sciences, where real-time, all-optical sensing in biological structures such as living cells can shed light on the processes involved [16]. However, practical applications of NV centers are yet limited due to a number of inherent drawbacks, such as insignificant spectral brightness with a zero-phonon line (ZPL) corresponding to only 3% of the total

photoluminescence emission [9], or spontaneous transitions between charge states that alter the photophysical properties [17].

The search for new types of color centers in diamond is stimulated in order to achieve emitters with preferable photophysical parameters such as brightness, emission wavelengths, photoconversion processes, and the sensitivity to the parameters of local environment. The stable sources of spectrally narrow emission are of particular interest. In this sense, an interesting set of properties is possessed by negatively charged split vacancy centers formed by elements of the fourth group of the periodic table [18,19] such as Silicon-Vacancy (SiV-) [20], Germanium-Vacancy (GeV-) [21], Tin-Vacancy (SnV-) [22] that appeared not so long ago. The color centers of this type have the same physical structure and possess similar photophysical properties [19]. The interaction with the phonon modes of diamond for such emitters is not as strong as in the case of NV centers. As a result, the most part of the signal in the photoluminescence (PL) spectrum is attributed to the narrow ZPL (characteristic full-width at half maximum FWHM is 5-7nm), while the intensity of the phonon sideband (PSB) is small. The parameters of ZPL are temperature sensitive, and allow temperature sensing with higher accuracy, compared to NV- centers [23-25].

Our studies of microdiamonds have uncovered some very intriguing sources of emission. We discovered multiple sharp spectral lines in the luminescence of different microdiamonds including presumably pure ones. These spectral lines are observed to be in average 5 time narrower than in case of group-IV Vacancy centers, and for some microdiamonds only single spectral line appears in the PL spectrum, while the broad phonon sideband, characteristic for other impurity centers, is absent. The observed emission is not described in any known literature and presumably correspond to the emission of as yet unknown impurity centers in diamonds.

Besides imaging and optical sensing, the sources of spectrally narrow emission are of interest for quantum information transfer protocols that requires reliable sources of single photons [26]. Such sources should have stable emission with high count rate and high spectral stability. Most of the current realizations of such protocols operate with attenuated laser pulses, although they do not meet all of the imposed requirements. In that

sense the group-IV vacancy centers cooled down to the liquid helium temperature are perspective candidates for that role [27-29]. Newly discovered emitters in diamond show extremely attractive properties for applications in that area as well, with presumably higher operating temperature.

However, when introducing new impurity centers, a broad range of questions arises about the nature of the photophysical processes in these materials, in particular regarding fine energy level structure, spectral properties and their relationship with morphological properties.

Aim and objectives.Considering the high potential for practical applications of color centers in diamonds with narrow-band emission in quantum information transfer protocols, as well as in biological imaging and sensing, the aim of this work is to provide complex investigation of photophysical properties of color centers with narrowband emission on the level of small ensembles down to single emitters. In order to achieve the goal of this thesis, the tasks are the following:

1. Experimentally investigate the spectral inhomogeneity and its relationship with morphological properties of diamond films, synthesized by chemically vapor deposition (CVD) method, with high and low concentration of GeV- centers at liquid helium temperature by means of photoluminescence excitation (PLE) spectra analysis.

2. Perform PL spectra investigation of microdiamonds with small ensembles of SnV-and GeV- color centers synthesized by CVD method at liquid helium temperature.

3. Develop a method for correlative scanning electron microscopy (SEM) and fluorescent microscopy for comprehensive investigation and characterization of micro and nano diamonds with color centers.

4. Perform complex analysis of photophysical properties of the discovered unidentified narrowband emission sources (LX-centers) in PL spectra of pure high pressure-high temperature (HPHT) microdiamonds.

5. Investigate the photodynamics of single LX centers luminescence by means of the second order cross-correlation function analysis.

Propositions for the defence:

1. Strong spatially inhomogeneous stress/strain of crystal lattice, occurring during chemical vapor deposition synthesis of diamond, influence the energy structure of color centers such as GeV- centers and SnV- centers so that even within a diffraction-limited region of the diamond there may be single color centers or small ensembles that experience different local lattice stresses or strains, which influences the electron transition energy of different emitter sites, and manifests itself as a deviation from a characteristic four-peak ZPL to a single or multiple narrower spectral lines in PL and PLE spectra at cryogenic temperature. The annealing of diamonds in inert atmosphere allows to mitigate the inhomogeneity of stresses in crystal structure of diamond that leads to homogenization of the local conditions of color centers and restoration of 4-peak ZPL structure.

2. A new class of defect centers naturally occurring in HPHT microdiamonds synthesized from adamantane - LX centers - at the level of single emitter are characterized by a linearly polarized narrow (~1 nm) ZPL, negligible phonon sideband, and several small features on the distance 180-200meV from the ZPL in PL and PLE spectra. The ZPL position of individual LX centers vary from 600 to 650 nm. The linewidth and the position of ZPL are temperature sensitive with FWHM temperature susceptibility sT = 0.0065nm/K.

3. The emission of single LX centers is intermittent at the microsecond time scale that follows from presence of photon bunching together with anti-bunching in second-order cross correlation function. The intensity dependences of radiative and non-radiative state durations obtained from the parameters of bunching corresponds to the energy level structure of LX center consisting of ground, excited, and two metastable states.

Scientific novelty:

1. It is shown for the first time that the CVD diamond films with low ensembles of GeV- centers show high spatial and spectral inhomogeneity on macro and micro scale, which appears as the variation of the ZPL structure in PLE spectra at low temperature. For the first time it is shown that annealing of diamond films in Ar

atmosphere at 1200°C leads to the equalization of crystal lattice stresses, as a result of which the inhomogeneous broadening decreases and the structure of the ZPL of GeV- centers ensembles approaches the characteristic quartet-like one.

2. For the first time it is found that microdiamonds with low concentration of GeV-and SnV- centers synthesized by CVD method show no characteristic ZPL quartet structure but single narrow lines instead in PL spectra at cryo temperature due to tangible stresses of crystal lattice.

3. Numerous unidentified narrow (FWHM ~ 1nm) lines in the 600-650nm region in the PL spectra of presumably pure microdiamonds that were not intentionally doped with additional elements during HPHT synthesis from adamantane have been studied for the first time. It was shown that the emission corresponding to those lines originate from the previously unknown emitters. They have been comprehensively characterized at the single emitter level and named LX centers.

4. The photodynamic processes associated with the luminescence of single LX-centers have been investigated for the first time which included intensity-dependent luminescence intermittency analysis at microsecond scale. Fluctuations of the photophysical properties, manifested as stochastic simultaneous changes in luminescence intensity and cross-correlation function parameters at constant excitation, were observed for the first time.

Theoretical and practical significance of the results is the investigation of photophysical properties of color centers in diamonds - GeV-, SnV- and newly discovered LX-centers. The obtained results demonstrate that group-IV color centers at liquid Helium-4 temperature act as perspective candidates for indistinguishable single photons sources for quantum information transfer protocols. The discovered color centers named as LX-centers have shown record-narrow luminescence at room temperature that is of high interest for biological imaging and sensing, multicolor and super-resolution microscopy and other applications.

Part of the research devoted to the low temperature spectroscopic investigation of diamonds with color centers on the level of small ensembles was carried out within the

state assignment of The Ministry of Education of The Russian Federation "Physics of nanostructured materials and highly sensitive sensorics: synthesis, fundamental research and applications in photonics, life sciences, quantum and nanotechnology" (theme No. -124031100005-5), at the Moscow Pedagogical State University.

Validity and reliability of the results and conclusions. The results presented in this thesis are obtained by means of modern experimental methods of research with the use of well-tuned world-class equipment. The experimental data obtained during the experiments are in agreement with those reported by other internationally respected teams and are consistent with the theoretical rationale provided in this study.

Approbation of the work and publications. The results of the work have been presented at 3 international conferences:

A1. A. Neliubov, I. Eremchev, V. Drachev, S. Kosolobov, E. Ekimov, A. Naumov. Enigmatic color centers in diamonds with bright, stable, and narrow-band fluorescence. The 31st International Conference Advanced Laser Technologies (ALT). Vladivostok, Russia. 23-27 September 2024. A2. A. Neliubov, I. Eremchev, V. Drachev, S. Kosolobov, E. Ekimov, A. Naumov. Enigmatic color centers in diamonds with bright, stable, and narrow-band fluorescence. Photon Echo Coherent Spectroscopy (PhECS). Svetlogorsk, Russia. 22-27 September 2023. A3. Ivan Yu. Eremchev, Arthur Yu. Neliubov, Kirill N. Boldyrev, Victor G. Ralchenko, Vadim S. Sedov, Andrey V. Naumov, Lothar Kador. Low temperature study of photoluminescence excitation spectra of CVD diamond films with GeV-centers. XVIII International Feofilov Symposium (IFS). Moscow, Russia. 22-2 August 2022

The results, presented in the thesis have been published in 4 peer-reviewed Q1 journals (according to Scimago Journal Rating) indexed by Web of Science and Scopus data bases and 1 peer-reviewed journal indexed in Scopus:

B1. Eremchev I.Y., Neliubov A.Y., Boldyrev K.N., Ralchenko V.G., Sedov V.S., Kador L., Naumov A.V. Microscopic insight into the inhomogeneous broadening of zero-phonon lines of GeV-color centers in chemical vapor deposition diamond films synthesized from gaseous Germane // The Journal of Physical Chemistry C -2021 - Vol. 125, No. 32 - pp. 17774-17785.

B2. Sedov V., Martyanov A., Neliubov A., Tiazhelov I., Savin S., Eremchev I., Eremchev M., Pavlenko M., Mandal S., Ralchenko V., Naumov A. Narrowband photoluminescence of Tin-Vacancy colour centres in Sn-doped chemical vapour deposition diamond microcrystals // Philosophical Transactions of the Royal Society A - 2024 - Vol. 382, No. 2265 - Article 20230167.

B3. Neliubov A.Y., Eremchev I.Y., Drachev V.P., Kosolobov S.S., Ekimov E.A., Arzhanov A.I., Tarasevich A.O., Naumov A.V. Enigmatic color centers in microdiamonds with bright, stable, and narrow-band fluorescence // Physical Review B - 2023 - Vol. 107, No. 8 - Article L081406.

B4. Neliubov A.Y., Tarasevich A.O., Pavlenko M.I., Ekimov E.A., Naumov A.V., Eremchev I.Y. Photophysics of single LX centers in high pressure-high temperature microdiamonds // Physical Review B - 2025 - Vol. 111, No. 15 -Article 155420.

B5. Pavlenko M.I., Neliubov A.Y., Eremchev I.Y., Sedov V.S., Tiazhelov I.A., Martyanov A.K. Low-temperature spectroscopy of single CVD microdiamonds with GeV color centers // Bulletin of the Russian Academy of Sciences: Physics -2025 - Vol. 88 - pp. S658-S663.

Personal contribution. The author personally participated in the development of experimental techniques, modification and adjustment of the experimental setups, acquisition and processing of the experimental data, discussion and conceptualization of the results obtained, and writing the original (B1, B3, B4) and final (B1-B5) versions of the manuscripts.

Structure and volume of the dissertation. The dissertation contains an introduction, 4 chapters, and conclusion. It is written on 117 pages of typewritten text and includes 47 figures and 1 table. The list of references includes 97 titles.

Chapter 1 presents a detailed description of the experimental methods used to obtain the experimental data in this study. Additionally, it gives a brief overview of diamond samples synthesis.

Chapter 2 is devoted to the spectra-microscopy of small ensembles of GeV- and SnV- centers in CVD microdiamonds and diamond films at low temperature. The main emphasis of the chapter is on the study of spectral and morphological inhomogeneity of CVD diamonds with color centers.

Chapter 3 focuses on the results of the study of unidentified spectrally narrow emission sources. It provides a detailed characterization of the photophysical properties of a new class of emitters in diamonds, named LX centers.

Chapter 4 is aimed at the investigation of photodynamics processes associated with the luminescence of single LX-centers. It includes analysis of intensity-dependent intermittency and spontaneous fluctuations of photophysical properties.

Conclusion provides the summary of the results obtained in the scope of the presented thesis, their potential areas of use and a direction of future research.

Заключение диссертации по теме «Другие cпециальности», Нелюбов Артур Юрьевич

Conclusion

Color centers in diamonds proved to be highly attractive for broad range of applications including fields of imaging, sensing, and quantum information. The search for new types of color centers in diamond is stimulated by the interest in obtaining full control of various photophysical parameters such as luminescence quantum yield, emission wavelengths, photoconversion processes, and the sensitivity to temperature, pressure, and external fields. Emitters with narrowband luminescence are of special interest. When introducing defect centers, a broad range of questions arises about the nature of the photophysical processes in the new materials, in particular regarding the relationship between spectral and morphological properties.

We provide results of the microscopic investigation of the spectral properties of diamonds with GeV- and SnV- centers, synthesized by CVD method. The low temperature studies showed significant spectral inhomogeneity of investigated samples, attributed to lattice strains/stresses. The observed deviations of PLE and PL spectra from the expected ones are in favour of the fact that the distribution of color centers in diamond is not uniform as well as its response to a lattice strains/stresses. Annealing was shown to be a useful tool to eliminate the imperfections of lattice.

Thorough investigation of the sources of unidentified emission, discovered in this research, LX centers, showed that emitters of this kind have extremely narrowband and intensive luminescence and in many cases are superior to the well-known color centers. Photon statistics of single LX-centers investigation showed the intensity-dependent intermittency of luminescence on the microsecond scale, which is then explained within a four-level energy model through analytical and numerical studies.

The discovered emitters have high potential for applications in bio imaging, sensing, and quantum optics. However, on demand synthesis of diamonds with LX centers is not achievable, because the nature of such defect centers is yet to be clarified. For this purpose, it may be useful to study the effect of annealing on the spectral

properties as well as low-temperature spectroscopic study of these defects, which will be our next line of research.

Main results of the Dissertation:

1) Low temperature spectroscopic investigation of diamond films with GeV- centers and microdiamonds with GeV- and SnV- centers showed spatial and spectral inhomogeneity. Prominent non-uniform lattice strains/stresses affect single emitters and small ensembles so that the typical fine structure degenerates and instead of expected 4-peak ZPL structure we observe multiple or single narrow spectral lines in the corresponding spectral region.

2) The homogeneous widths of the discovered spectral lines are shown to be significantly smaller than the widths of peaks within quartet-like ZPL reported elsewhere. In some cases, Fourier-transform limited ones were observed.

3) It is shown that annealing of diamond films leads to the equalization of crystal lattice stresses, as a result of which the inhomogeneous broadening decreases and the structure of the ZPL of GeV- centers ensembles approaches the characteristic quartet-like one.

4) A new class of defect centers in HPHT microdiamonds - LX centers was discovered and thoroughly investigated. It was shown that at the level of single emitter LX-centers are characterized by a linearly polarized narrow (~1 nm, with minimum of 0.52nm), temperature sensitive ZPL, negligible phonon sideband, and several small features on the distance 180-200meV from the ZPL in PL and PLE spectra.

5) Investigation of the photodynamic processes associated with the luminescence of single LX centers showed intensity-dependent luminescence intermittency at the microsecond level. The parameters of observed intensity-dependent intermittency are in agreement with the calculations and numerical simulations given within the framework of the four-level energy system model.

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