Results in 2019
Color centers in nanodiamond. — Diamond nanostructures containing optically active point defects or so called “color centers” have attracted attention of many researchers in last decades. These unique nanoobjects with excellent physical and optical properties are very promising candidates for novel photonic and biological applications, including but not limited to room temperature solid state single-photon source, extremely sensitive magnetometry, nanoscopy, biocompatible labeling or drug delivery systems. Most of the mentioned applications are based on the superb photoluminescence (PL) properties of this diamond-based nanosystem among which the zero-phonon line (ZPL) peak postion, intensity and spectral shape are the most important characteristics.
Figure 1. SEM micrographs of nanocrystalline diamond thin films with average grain size of (a) 120, (b) 70 and (c) 30 nm (top) and the differences in their ZPL line shape revealed by applying laser excitations of 405 (3.06 eV), 488 (2.54 eV), 532 (2.33 eV) and 635 (1.95 eV) nm (bottom). The full ZPL emission spectra are shown in the inset. All PL spectra were recorded at room temperature.
The asymmetric line shape in form of an elongated tail on the low energy side of the ZPL was studied in detail for silicon-vacancy (SiV) center being a very strong candidate for above mentioned applications. By applying multi-wavelength excitation PL spectroscopy, significant differences in the spectral line shape were observed in case of the SiV center containing chemical vapor deposited (CVD) nanocrystalline diamond thin films having different grain sizes (Fig. 1). The PL measurements revealed that the SiV center related 1.68 eV PL line shape sensitive to the excitation energy, and the asymmetric tail on the low-energy side of ZPL becomes more pronounced with decreasing of the excitation energy. The asymmetry of the 1.68 eV PL line was found to be stronger in the samples containing larger diamond nanocrystals and it was attributed to the luminescence contribution of the simplest point defect in the diamond lattice, which builds up by a vacant single carbon site, namely to the GR1 center, being simultaneously presented in the structure. The varying degree of the ZPL asymmetry with the average grain size of the nanodiamond films was explained by the preferential environmental condition for the GR1 defect formation process .
Figure 2. Calculated distributions of near-field intensity (E) inside an inverse pyramid with 2 microns base entrapping a gold nanosphere of 200 nm diameter.
Plasmonic amplification in hierarchically combined micro- and nanostructures. — Being a non-contact, fast and relatively easy material characterization technique requiring no sample preparation, Raman spectroscopy is finding many applications in biology, life sciences and other areas. However, the effect is relatively week, but it can be enhanced with different techniques, like surface enhanced Raman scattering (SERS), where the incident or scattered light is enhanced by the interaction with localized electron oscillations on metallic surfaces (plasmons). In SERS the enhancement of the plasmonic surface has two origins: one – usually weaker – comes from electrochemical phenomenon, the other – from electromagnetic interaction. The material, shape, symmetry, orientation, surrounding media are the most important factors affecting the amplification. In case of core-shell structures the inner diameter and layer thickness ratio are also crucial. In addition, surface functionalization can be used to achieve selective enhancement of specific target molecules.
Lithography and nanotechnology allow to fabricate hierarchically combined micro- and nanostructures with advanced SERS properties . In cooperation with the MEMS lab of the Institute of Technical Physics and Materials Science, Centre for Energy Research, novel SERS substrates have been developed capable of entrapping and Raman characterization of different molecules . Functional groups of human blood and parvovirus were identified with this technique . In addition, giant SERS enhancement was discovered in SERS substrates consisting of inverse pyramid entrapping gold nanoparticle. The results of modelling performed with finite-difference time-domain method showed that the near-field intensity distribution has extreme maximum in the small voids around the contact points of the nanosphere and pyramid surface (Fig. 2) .
Nano-gold catalyzed synthesis of novel As-S crystallites. — Due to the advantageous combination of infrared transparency, optical activity, structural photosensitivity and high level of third-order optical non-linearity amorphous and crystalline chalcogenides attract significant attention in modern optoelectronics and photonics. They offer wide possibilities for modern applications as a media for optical data storage, optical signal processing, photolithography, thermal imaging, molecular bio-chemical sensing, solar cells, solid electrolytes, bioscience and medical applications, etc. Most of the physico-chemical properties of chalcogenides are determined by their nanoscale structure , . Among different compositions in the As-S system, several compounds such as duranusite (As4S), α- and β-dimorphites (As4S3), α-, β-realgar and pararealgar (As4S4), uzonite (As4S5), alacranite (As8S9) and orpiment (As2S3) are known as minerals.
A novel method has been developed to prepare As-S chalcogenide crystals by gold-catalyzed thermally initiated chemical vapor deposition. Different sizes (5, 20, 40 and 60 nm) of gold nanoparticles were used in order to activate the catalytic reaction. Crucial differences were observed between these structures and those prepared by the well-known thermal deposition of As-S chalcogenide films. The latter were found to contain large concentrations of photosensitive realgar-like As4S4 and As4S5 inclusions in comparison with the structure of bulk As2S3 glass. In contrast, the growth of molecular nanocrystals was observed on the surface of films obtained with the gold-catalyst assisted method. The size and shape of crystallites on the surface of As-S films weredefined by scanning electron microscopy. Depending on synthesis conditions, these crystallites are built from different cage-like molecules. The surface enhanced Raman spectra of crystallites together with the results of density functional theory calculations let us identify As4S5 and realgar-type As4S4 cage-like molecules forming tetra-arsenic pentasulfide (uzonite) and tetra-arsenic tetra-sulfide (realgar) crystals, respectively. The uzonite structure was found to be insensitive to laser irradiation while photo-structural transformation was observed for the latter in a result of laser exposure (Fig. 3).
Figure 3. Raman spectrum of As4S5 crystallites prepared with nano-gold catalyst (left). In-situ photostructural transformation (realgar-pararealgar transition) of As4S4 (realgar type) crystallites prepared with nano-gold catalyst, measured by Raman spectroscopy (right).
Results in 2018
Gold-catalyzed synthesis and structural characterization of a new type of As-S nanocrystallites. — Non-crystalline chalcogenides with high infrared transparency have stood out as materials of choice for infrared optics. Detailed studies of the physical properties of these materials revealed their unique and remarkable structural, electronic, optical properties and large functionality, and has attracted significant attention, representing an important scientific and technological challenge as well. They offer wide possibilities in domains such as information technologies (optical data storage, ultrafast optical transmission and information processing), photolithography, renewable energy technologies (high efficiency solar cells, solid electrolytes), medicine, thermal imaging, sensing and biosensing etc. Recent progress in photonics shows that amorphous chalcogenides are among the best candidates as active optical media for ultrafast all-optical processing systems.
Crucial differences were observed between the structure and properties of As-S chalcogenide thin films prepared by normal and gold-catalyzed thermally initiated vapor deposition from the same As2S3 glass precursor. The as-deposited As2S3 film prepared by thermal evaporation contains large concentrations of photosensitive realgar-like As4S4 inclusions compared to the structure of bulk As2S3 glass. The irradiation of this film by near- or over-bandgap coherent light initiates significant structural transformations connected mainly with induced transformation of realgar As4S4 molecules into their pararealgar polymorph. In contrast, formation of novel molecular nanocrystals was observed on the surface of As2S3 films synthesized with gold-catalysis. The size and shape of the crystallites was characterized by electron microscopy and their structure was found to be not photosensitive. Surface-enhanced Raman spectra of crystallites were interpreted with DFT calculations and showed that the crystallites are built from As4S5 cage-like molecules forming tetra-arsenic pentasulfide (Fig. 1).
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Figure 1. (I) Surface morphology and distribution of As4S5 microcrystallites on As2S3 films synthesized by gold-catalyzed vapor deposition on 5-60 nm Au nanoparticles (A-F); (II) SEM images of As2S3 nanolayers synthesized with spherical gold nanoparticles of different size: 5 nm Au-np (A,B), 40 nm Au-np (C,D), and 60 nm Au-np (E,F); (III) FT-Raman spectrum of glassy As2S3 (1) and surface-enhanced Raman spectra of As-S thin films prepared by thermal deposition (2) and gold-catalyzed vapor deposition (3). The main cluster constituents of the films are also indicated on the Figure.
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Color centers in nanodiamond. — A gold-coated array of flow-through inverse pyramids applicable as substrate for entrapment and immobilization of microobjects and for their subsequent surface enhanced Raman spectroscopic (SERS) characterization was fabricated using bulk micromachining techniques from silicon. The inverse pyramids have 2.2x2.2 microns sized base on the top (being flush with the silicon surface) and a 0.5x0.5 micron-size opening on the bottom (Fig. 2). The perforated periodic 3D structure was demonstrated for parallel particle trapping and sensitive detection of molecules by entrapment and SERS characterization of polymeric microspheres. It was found that the periodic array has efficient near-field enhancement in the 650-850 nm region. Raman intensity maps recorded over the entrapped microspheres indicated efficient SERS enhancement inside the inverse pyramids.
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Figure 2. SEM view of the periodic perforated SERS substrate applicable for flow-through experiments and particle and cell entrapment (left). Polymeric microparticle entrapped in an inverse pyramid of the SERS substrate (top right). Comparison of the SERS spectra recorded on the clean SERS surface and entrapped microparticle (ST) and normal Raman spectrum of the latter on silicon (bottom right).
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Results in 2017
Color centers in nanodiamond. — Among the numerous optically active defects (color centers), studied in nanosized diamond (ND), the silicon-vacancy (SiV) center is a promising candidate for utilization in different fields like quantum computing and cryptography, nanoscopy, medicine or cell biology. Most applications are based on the intensive and narrow zero-phonon emission line (ZPL) of the mentioned color center, which can be detected in near infrared wavelength region, around 1.68 eV (738 nm). However, the asymmetric lineshape of the SiV ZPL may restrict the spectral parameters important for different applications and prohibit the determination of the real ZPL characteristics by traditional spectroscopic techniques.
Micro-photoluminescence measurements performed on a high number of CVD nanodiamond films containing SiV centers showed that the undesirable asymmetric tail on the low-energy side of the ZPL is related to another optically active defect (so called GR1 center) being present in the nanodiamond structures as well. Regions with relatively high GR1 content and with well-distinguishable zero-phonon lines related to different optically active defect structures can be localized by mapping of the ND film with an appropriate excitation wavelength (Fig. 1).
Figure 1. Fine-structured emission spectrum of CVD nanodiamond film around the SiV center ZPL region excited by 635 nm and recorded at room temperature. The deconvoluted peaks correspond to SiV center ZPL (1.680 eV) and the double ZPL of the GR1 defect (1.664 eV and the weak shoulder at 1.672 eV).
Preparation of new tetragonal silicon polymorphs by ultrashort laser pulses. — Tetragonal polymorphs of silicon were created successfully by irradiation of microcrystalline silicon powder with femtosecond laser pulses (800 nm center wavelength with 1 kHz repetition rate and 42 fs pulse duration) in air at room temperature. Surface enhanced Raman spectroscopy and, in collaboration with the Research Institute for Materials Science, Centre for Energy Research, HAS and the Research Centre for Natural Sciences, HAS, transmission electron microscopy (TEM) measurements were carried out to prove the presence of bt8 (Fig. 2) and t32* (Fig. 3) Si phases.
Figure 2. TEM image of bt8 Si polymorph. In addition to ordinary cubic Si, weak extra reflections occur with the following d spacings: 4.70 (1), 3.37 (2), 2.60 (3), 2.38 (4), 2.10 (5) Å. These extra reflections are consistent with {101} (1), {200} (2), {211} (3), {220} (4) and {301} (5) reflections of bt8 Si.
Figure 3. TEM image of t32* Si polymorph. In addition to ordinary cubic Si, there are two sets of strong reflections with 6.60 (1) and 3.20 (2) Å. These strong reflections and their measured angles are consistent with {110} and {0-12} of t32* Si grain. The additional weak reflections with 3.63 (3), 2.65 (4), 2.16 (5) Å can be interpreted with {211}, {311} and {013} of t32* Si.
Results in 2016
Silicon-vacancy center in nanodiamonds. – Due to their excellent light emission properties, the investigation of the negatively charged silicon–vacancy (SiV−) centers in diamond nanostructures has attracted much interest during last decades. The SiV− center consisting of one interstitial silicon atom in a split–vacancy configuration with D3d symmetry has a bright, stable and narrow zero-phonon line (ZPL) at 1.681 eV. It has a weak phonon sideband since more than 70% of the emitted light is concentrated into the ZPL even at room temperature. These advantageous properties of the optical transition make the SiV– center a promising candidate for solid-state single-photon emitter that can be used to realize numerous novel applications in quantum computing and cryptography or nanoscopy and cell biology, etc. For quantum information and quantum processing purposes, indistinguishable single photons from distinct SiV− center emitters are required. Variation of spectral parameters of the mentioned center observed by different groups strongly restricts its implementation to real applications. Novel methods have to be developed for the creation of uniformly emitting silicon–vacancy centers in diamond nanostructures on large scale, and the origin of the processes responsible for the variation of the ZPL of the SiV– centers has to be resolved.
 Figure 1. Spectral distribution of SiV– center emission line in photoluminescence spectra measured at room temperature in nanocrystalline diamond layers grown at substrate temperatures and methane concentrations in the precursor Ar + CH4 mixture of (1) T=700 oC and 2.0% CH4 (2) T=750 oC and 0.2% CH4 (3) T=750 oC and 3.0% CH4 (4) T=800 oC and 1.0% CH4 (5) T=850 oC and 0.2% CH4 (6) T=850 oC and 3.0% CH4. The emission was excited by the 2.54 eV line of an Ar-ion laser.
In order to have detailed overview on the spectral properties, SiV– center ensembles were investigated in a large number of nanodiamond films by fluorescence spectroscopy (Fig. 1). Conditions of SiV– center formation were varied systematically in microwave plasma-assisted chemical vapor deposition (MW CVD) process and spectral parameters of the zero-phonon line (ZPL) were obtained by fitting procedure from experimentally measured spectra. The average size of nanodiamond grains determined from scanning electron microscopic (SEM) micrographs and residual stress of diamond layer calculated from diamond Raman peak position were used as sample parameters. The SiV– centers ZPL peak positions were found to vary from 1.677 to 1.681 eV, while their line broadening between 6.5 and 18.1 meV. The smallest linewidth was observed in a diamond layer of 30 nm average grain size and it was comparable with line broadening values reported for individual SiV– centers.
A significant blue shift and line narrowing of the ZPL peak position was observed with decreasing average grain size of the SiV– containing diamond thin films. The residual stress, being dependent on the grain size, was identified as the major cause of the variation of the ZPL parameters. It was found that the increase of the residual stress from 0.64 GPa tensile to 2.25 GPa compressive one correlates well with the changes in ZPL peak parameters. The blue shift and line narrowing of SiV– centers were explained by the suppression of the orbital relaxation processes, involving ground and excited electronic levels, initiated by the different local strain fields in the vicinity of the SiV– centers. Acoustic phonon mode confinement due to small diamond grain size also contributes to the suppression of relaxation process by electron–phonon transitions. Our results indicate decisive influence of the diamond grain size and internal residual stress of the layer on the spectral parameters of the zero phonon line of the SiV– centers.
 Nickel-silicon (Ni-Si) related complex color center in nanodiamond. — Ni-Si related complex color center was successfully created in nanocrystalline diamond grains through CVD deposition process, which emits highly intensive narrow-bandwidth ZPL at 865 nm (1.433 eV) with 2 nm (3 meV) full width at half maximum. This color center is highly significant in the field of biological and medical applications since its excitation and emission wavelength range is lying in the near-infrared window of biological tissues. Variation of ZPL peak position and line width have been detected in nanodiamond grains prepared under different conditions (Fig. 2). Experimental results on the residual stress determined from the position of diamond Raman peak measured on different nanodiamond grains exhibit different values in the range of -0.963 to +0.284 GPa, and can be of compressive or tensile type. In contrast to the SiV– color center, direct relation between the ZPL position and local stress has not been established until now. The relatively large size of the complex center and the lack of vacancy in the center could be the possible reasons of this behavior. (Fig. 2)
Figure 2. Dependence of the ZPL spectral shape of Ni-Si related complex impurity center in diamond nanograins on deposition conditions. The spectra were recorded by 488 nm laser excitation at room temperature.
Preparation of nanocrystalline diamond and nanocrystalline silicon carbide by ultrashort laser pulses
Favorable properties of color centers formed in nanodiamond for various applications in quantum informatics, medical imaging and biolabeling generate a need to develop fast, reliable and widespread technology for their fabrication. Therefore a new concept was introduced for the creation of one-photon emitter centers in nanodiamond by using ultrashort laser pulses (800 nm center wavelength with 1 kHz repetition rate and 42 fs pulse duration). Nanodiamond crystals were produced with ultrashort laser pulses by using different carbon- and silicon-based materials as source material. Surface-enhanced Raman spectroscopy measurements were performed to prove the presence of diamond nanocrystals (Fig. 3, left). The incorporation of silicon foreign atoms into the nanodiamond structure under laser irradiation was also demonstrated.
Figure 3.(Left) Surface-enhanced Raman scattering spectrum of nanocrystalline diamond grains. (Right) Micro-Raman spectrum of the silicon carbide agglomerate excited by 514 nm probing wavelength.
Silicon carbide (SiC) nanocrystals were created successfully by the irradiation of carbon- and silicon-based materials with femtosecond laser pulses (800 nm center wavelength with 1 kHz repetition rate and 42 fs pulse duration) in air at room temperature. Micro-Raman spectroscopic and scanning electron microscopy measurements were carried out to prove the presence of SiC nanocrystals and to analyze the bonding structure of formed nanostructures. Detailed analysis of the transversal (TO) and longitudinal (LO) optical modes support the formation of cubic and hexagonal SiC nanocrystals (3C, 4H and 6H) with average grain size of 100-500 nm (Fig. 3, right).
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