DOCTORAL THESIS STATEMENT

Transcription

1 CZECH TECHNICAL UNIVERSITY IN PRAGUE DOCTORAL THESIS STATEMENT

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3 Czech Technical University in Prague Faculty of Nuclear and Physical Engineering Department of Solid State Engineering Mgr. Alexander Kovalenko THE STUDY OF BIOACTIVE AND BIOCOMPATIBLE SURFACES AND NOVEL NANOSTRUCTURAL COMPOSITES Doctoral study program: Physical Engineering Branch of study: Solid State Engineering Doctoral thesis statement for obtaining the academic title of Doctor, abbreviated to Ph.D. Prague 2012

4 The doctoral thesis was produced in full-time manner Ph.D. study at the department of Solid State Engineering of the Faculty of Faculty of Nuclear and Physical Engineering of the CTU in Prague Candidate: Mgr. Alexander Kovalenko Establishment Faculty of Nuclear and Physical Engineering Address Břehová 7, Prague 1 Supervisor: Prof. RNDr. Ivo Kraus, DrSc Department Department of Solid State Engineering, Faculty of Nuclear and Physical Engineering Address Trojanova 13, Prague 2 Supervisor specialist: Doc. Ing. Irena Kratochvílová, Ph.D. Department Department of Solid State Engineering Faculty of Nuclear and Physical Engineering Address Trojanova 13, Prague 2 Opponents: Ing. Stanislav Záliš, CSc. Doc. RNDr. Jiří Pavluch, CSc Prof. RNDr. Tomáš Polívka, Ph.D. The doctoral thesis statement was distributed on: The defence of the doctoral thesis will be held on at a.m./p.m. before the Board for the defence of the Doctoral Thesis in the branch of study Solid State Engineering in the meeting room No. of the Faculty of Nuclear and Physical Engineering of the CTU in Prague. Those interested may get acquainted with the doctoral thesis concerned at the Dean Office of the Faculty of Nuclear and Physical Engineering of the CTU in Prague, at the Department for Science and Research, Břehová 7, Prague 1. prof. Ing. Stanislav Vratislav, CSc. Chairman of the Board for the Defence of the Doctoral Thesis in the branch of study Solid State Engineering Faculty of Nuclear and Physical Engineering of the CTU in Prague Trojanova 13, Prague 2.

5 TABLE OF CONTENT 1 CURRENT SITUATION OF THE STUDIED PROBLEM 2 2 GOALS OF THE DOCTORAL THESIS 5 3 METHODOLOGY 7 4 SELECTED RESULTS 9 5 CONCLUSIONS AND RESEARCH CONTRIBUTION 20 REFERENCES 22 LIST OF CANDIDATE S WORKS 25 SUMMARY 27 RÉSUMÉ 28 1

6 1 CURRENT SITUATION OF THE STUDIED PROBLEM Recently nanodiamonds, like many other nanomaterials have attracted particular interest of researchers. Nanocrystalline (ND) and ultrananocrystalline (UND) diamond nanoparticles [1] in addition to the properties of bulk diamond have new qualities which make them a unique and attractive material. Another unique interesting physical feature of specifically modified nanodiamonds is their ability of an extremely intense fluorescence [2] [3-12]. Inherent photoluminescence is emitted from structural defects and impurities. Of particular importance for the intrinsic photoluminescence properties of diamond are defects containing complexes of substitutional impurity atoms and vacancies. This property could be important for future development of more accurate methods of bio-labeling used for diagnostic monitoring of complex biological processes. In recent years a diversity of embodiments for single photon sources has been investigated. Among them are schemes based on single molecules or atoms [2-5], single ions trapped in cavities [6], color centers in diamonds [7], [8], quantum dots [9], [10] and parametric down conversion (PDC) [11], [12]. These sources differ in the wavelength and purity of the emitted photons, their repetition rate and whether they produce a photon on demand or heralded, i.e, announced by an event. Many luminescent defects in diamond possess a single-photon behavior, thus can be successfully used for quantum applications. Due to their nanometer sizes, crystallites of nanodiamonds have a colossal specific surface area. The surface of nanodiamond crystal has due to production and purification a number of defects. ND, surface structure is different from the regular diamond, the surface carbon atoms have uncompensated dangling bonds. This makes an ultra-high surface activity of nanodiamonds [1]. NDs have unique properties and can be successfully used not only in technology and production, but also as an effective drug delivery system in biomedicine. 1.1 NV centers Aggregations of nitrogen admixtures, which are the dominant impurities in natural and synthetic diamonds, create with vacancy thermally stable structures possessing high quantum efficiency up to temperatures above 500 K. NV color centers are either neutral (NV 0 ) or negatively charged (NV - ). Both of these centers are photostable and, irradiation, which is emitted from the fluorescence nanodiamonds, can be easily identified at the individual level. Nitrogen vacancy (NV) centers can be produces from synthetic bulk diamond of diamond nanoparticles containing nitrogen impurities in the form of single substitutions (called C or P1 centers in diamond literature) by irradiation with a high energy beam with further by annealing at temperatures above 700 C for several hours. The post-irradiation annealing process allows carbon atoms neighboring a vacancy to hop into a vacant place and leave an empty position in the diamond lattice; by this process a vacancy can migrate through the diamond, forming complex lattice compounds with other vacancies, interstitial atoms, (forming Frenkel pairs), or Schottky defects [13] i.e. NV centers. The newly-formed compound defects are optically active; the precise optical properties depend on the annealing time and type of initial defects and impurities present. [14] A wide range of high-energy particle beams can be used for a primary irradiation, including electrons, protons, neutrons, ions, and gamma photons. Irradiation produces lattice defects such as vacancies, dispositions etc. Those defects are immobile at room temperature; however, at higher temperatures defects can migrate in the diamond lattice. Single substitutional nitrogen creates strain in the diamond lattice; [15] which are capable to capture moving vacancies, [16] creating the NV centers. Due to the presence of relatively large lattice strain in diamonds, optical transitions are split and shifted from individual centers by these strains from individual centers resulting in broad lines in the ensembles of centers. [14] Negatively charged NV color centers emit bright red light which can be easily excited by visible light sources, such as argon or krypton lasers, frequency doubled Nd:YAG lasers, dye lasers, or 2

7 He-Ne lasers. Laser enlightenment, however, also leads conversion NV - into NV 0 centers. [17] Emission is relatively quick (relaxation time 10 ns). [18, 19] At room temperature, no sharp peaks are observed owing to the thermal broadening. Nevertheless, cooling the NV - centers with liquid nitrogen or by liquid helium dramatically narrows the lines down. A significant feature of the luminescence from individual NV centers is its high temporal stability. While bleaching occurs after the emission of photons in organic dyes [20] and < 10 5 photons in autofluorescent proteins [21], no bleaching is observed for the NV centers at room temperature. [2, 22, 23] Clearly, photoluminescent NDs are attractive alternative to molecular dyes as biological markers and traceable molecular carriers, in the function of stable options of fluorescent markers for special applications. As distinct from molecular dyes, photoluminescent NDs do not photobleach. This results in a fluorescent signal of greater intensity and stability. A further benefit over molecular dyes is that a wider diversity of biomolecules is able to be attached to nanodiamond surface. Photoluminescence emitted by nitrogen-vacancy defects can be altered by many parameters - applying a magnetic field, electric field, microwave radiation and also by modifying a diamond surface with a various molecular groups. In consequence of that fact the present work is based on a research of the connection between the nanodiamond cluster surface termination and its fluorescence. In the experimental part of the work experiments which were made in a purpose to prove the possibility to use nanodiamond particles for in-vitro labeling are described. Also, preparation of fluorescent ND particles by a high-energy proton beam implantation specific luminescence nanodiamond particles with NV - & NV 0 color centers and methods of surface terminations are described. 1.2 SiV centers Intensive, narrow-band single photon emission from the silicon vacancy (SiV) complex defect in diamond, which can be fabricated by Si ion irradiation has been reported [24-27]. Single SiV centers are photostable and have a spectrum consisting of a sharp zero phonon line (FWHM is about 5 nm) at 738 nm exposing even at room temperature very weak vibronic sidebands. The short luminescence lifetime of 2.7 ns (300K) enables a proficient generation of single photons. All this features make the SiV centers promising candidate for building an efficient single photon source. Silicon-vacancy centers emit strong narrow-line luminescence with emission energy in the far red region ev (738 nm) [24]. This source of luminescence has been reported in films grown by a variety of chemical vapor deposition (CVD) methods including hot-filament [25], oxygenacetylene combustion [26] and microwave-plasma [27]. The most interesting and unique property of such centers, which makes them perfect candidate for spintronics application is that, they have a very low vibronic structure (phonon wing) both in luminescence and absorption, and no symmetry with a respect to the zero-phonon line [28]. Even at room temperatures luminescent peak is very sharp, showing FWHM of about 6nm with the majority of the emission concentrated in the zero phonon line (ZPL). In comparison with widely investigated NV centers, factor showing the ratio of the intensity of the one-phonon peak to the intensity of the ZPL (Huang-Rhys factor), is about 15 times lower for the SiV centers (3.21 and 0.24 respectively) [29]. SiV centers show extremely short luminescence lifetime, which is often ascribed to the existence of non-radiative transitions, which is confirmed by the observation that the intensity of the ZPL of SiV centers drops with increasing temperature [30]. The emitting lifetime was measured to be 4 ns at 5 K and 2.7 ns at room temperature in the homoepitaxial CVD diamond film and about 1 ns almost independent of temperature in a polycrystalline CVD diamond film [31] (as an example ev center with 29 ns) [32]. SiV luminescent centers charge state mainly considered to be neutral, due to the fact, that SiV centers show similar behavior in photochromism measurements as the neutral NV centers. The 3

8 fact that the intensity of the ZPL of SiV centers can be increased up to ten times by UV illumination indicates the existence of a positively charged state. However, no narrow-line absorption, which could be ascribed to that state, could be found in the range ev [33]. The procedure for the fabrication of SiV centers via ion implantation had been shown by C. Wang in [34]. Diamond surface was implanted with 10 MeV Si 2+ ions at room temperature. Post-implantation annealing was carried out at 1000 C for 5 min in vacuum. Samples were washed in a solution of K 2 Cr 2 O 7 :H 2 SO 4 at 180 C for 2 min to remove the graphite layer formed during the annealing process. The narrow emission bandwidth of about 5 nm at room temperature, the high photostability and the short lifetime of about 2.7 ns, make SiV centers interesting as a single photon source in practical quantum cryptography. 1.3 Metal originated luminescent centers Transition metals, especially nickel, are common impurities in synthetic high pressure high temperature (HPHT) diamonds [35, 36, 37]. Throughout the HPHT process, metal atoms are exclusively incorporated into the {1 1 1} growth sectors and their concentration can vary within the same growth sector [38]. Nickel related centers can play the role of single photon emitters, due to its sharp luminescent peak observed at room temperature. Chromium-based luminescent centers is a new addition to the group of diamond-based single-photon emitters. Such emitters were initially discovered by growing nanodiamond crystals on a sapphire substrate [39]. Chromium is a regular impurity in sapphire, which can be integrated into a diamond crystal during the CVD growth [40]. Chromium originated centers are for the most part attractive owing to their narrow bandwidth emission observed even at room temperature. It has been shown that Cr emitters can be successfully fabricated by ion implantation of chromium with oxygen, sulfur or boron atoms [41-43]. Cr-containing luminescence centers have very short emission lifetimes from 1 to 4 ns, whereas NV centers have emission lifetimes an order of magnitude longer: NV 0 29 ns, NV - 13 ns [44]. Also in this work it has been demonstrated the formation of new single-photon emitters containing silicon and nickel atoms (Si+Ni centers), first produced by co-implantation of Ni and Si atoms into the diamond. The formed centers are not previously known NE8 but show surprising and potentially important properties including an estimated near unity quantum efficiency, a near infrared emission 768 nm, and a 2 ns radiative lifetime. A further co-implantation of Ni and Si atoms into pure bulk diamond indicates that the defect center belongs to a new class of nickel-silicon composite centers [37]. The simple fabrication method and the extended optical properties make this defect a very attractive candidate for integration with external microstructures. The coimplantation of Ni/Si is likely to spur substantial research into the physical structure and properties of such composites. 4

9 2 GOALS OF THE DOCTORAL THESIS In regard to NV centers one of the main object of the research is to provide a deep study of the connection between the nanodiamond cluster surface terminations and its properties, such as lattice geometry, molecular orbitals, charge distribution, and, especially, absorption and emitting processes. To get qualitatively better understanding of the complicated and specific process of nanodiamond particles luminescence, computer simulation based on the density functional theory, was used; model processes and states influencing the luminescence of variously terminated ND particles. Clusters containing from 35 to 900 atoms of carbon, with the impurities in a form of NV color centers with two different charge states (NV - & NV 0 ) terminated by carbonyl, carboxyl, hydroxyl, fluorine and amine groups were modeled. Explored systems were studied by DFT based calculations using Gaussian 09 program package. Main goals and objectives can be summarized as: - Definition of the theoretical model and methods for the simulation of nanodiamond clusters; measure of accuracy was the correlation of experimental and theoretical bandgap and excitation energy; - Optimization of the structure of nanodiamond particles to its minima, finding of the lattice geometry; bond lengths and types; - The study of the size dependence of nanodiamond particles (i.e. quantum dimensional effect); of particular interest are influences on excitation energy for luminescent particles (i.e. adsorption spectra) and bandgap of clusters with no defects in the lattice; - The study of the influence of surface oxygen groups on the luminescent conditions; according to the experimental research surface oxygen passivation was modeled in the form of carbonyl, hydroxyl and bridge-type bonding; - The study of the influence of other surface groups on the luminescent conditions; particularly amine, fluorine and carboxyl groups; - Definition of the density of states distribution, in particular electron density distribution in the ground and excited states; - The study of possible influence between NV centers and other crystallographic defects in terms of positively charged single substituted nitrogen; As respects silicon and nickel related centers, surface chemistry was not of particular importance in this case, due to the fact that ND particles are not highly desired as single photon sources and size is no the matter. However computational demand does not allow us to calculate macrostructures, nevertheless we deal with point defects witch allows to reach high accuracy on the small models. According to this clusters containing about 100 atoms of carbon with silicon and nickel related defects were modeled. All structures were examined with a variety of charges and multiplicities in order to find out energetically preferred system. Explored systems were studied by DFT based calculations using Gaussian 09 program package. Main goals and objectives can be summarized as: - Definition of the theoretical model and methods for the simulation of nanodiamond clusters; measure of accuracy was the correlation of experimental and theoretical bandgap and excitation energy; - Analysis of the bonding nature of Si and Ni diamond lattice defects in the semi-divacancy site remained unclear, thus finding of the lattice geometry; bond lengths, types and orders was an important task; - TD-DFT calculations in order to define excitation energies of the structures under study; Cr and Ni+Si related luminescent centers are newly discovered and there are still many open questions about their structure, bonding, charges and spin states. Using DFT based calculations using Gaussian 09 program package we tried to approach to the experimental results. Clusters containing 5

10 chromium along with oxygen, sulfur and nitrogen in the variety of charges and multiplicities were modeled. Goals and objectives can be defined as: - Definition of the lattice geometry and components; the structure of Cr related centers remained unclear so far; - Definition of the theoretical model and methods for the simulation of nanodiamond clusters; measure of accuracy was the correlation of experimental and theoretical bandgap and excitation energy; - Analysis of the bonding nature of Cr and Si+Ni related diamond lattice defects as far as structure remained unclear, thus finding of the lattice geometry; bond lengths, types and orders as far as charges and multiplicities was an important task; - TD-DFT calculations in order to define excitation energies of the structures under study; 6

11 3 METHODOLOGY Numerous properties of molecules and crystals can be simulated using classical methods in due form of molecular mechanics and dynamics. The classical force field is based on empirical results, which are consequence of averaging over a large number of molecules. Because of this widespread averaging, the results can be good for typical systems, but there are many important questions in chemistry which can not be estimated at all by methods based on the empirical approach. If it is required to know more than just structure or other properties that are derived only from the potential energy surface, in particular properties that depend directly on the electron density distribution, it is necessary to resort to a more fundamental and general approach: quantum chemistry. The same holds for all non-standard cases for which molecular mechanics is simply not applicable. In quantum chemistry, the system is described by a wavefunction which can be found by solving the Schrödinger equation. This equation relates the stationary states of the system and their energies to the Hamiltonian operator, which can be viewed as the way to obtain the energy associated with a wavefunction describing the positions of the nuclei and electrons in the system. In practice the Schrodinger equation cannot be solved exactly and approximations have to be made using the ab-initio approach using no empirical information, except for the fundamental constants of nature such as the mass of the electron, Planck's constant etc. that are required to arrive at numerical predictions. In spite of the necessary approximations, ab-initio theory has the conceptual advantage of generality, and the practical advantage that its successes and failures are more or less predictable. The major drawback of ab-initio quantum chemistry calculations is a big demand on computation technique power. Consequently, such approximations have been applied for a long time with a good correlation which allows introducing the empirical parameters into the theoretical model. This has led to a number of semi-empirical quantum chemical methods, which can be applied to larger systems, and give reasonable electronic wavefunctions so that electronic properties can be predicted. Compared with ab-initio calculations their correlation is less and their applications are restricted by the condition for parameters, just like in molecular mechanics. In general, ab-initio quantum chemistry calculations is preferred for relatively small systems, which are need to be explored at a very high level, when electronic properties are required and for structures, for which valid molecular mechanics parameters are not obtainable. Electronic structures of all systems examined were calculated by density functional theory (DFT) methods using the Gaussian 09 [42] program package. DFT is a formally exact method for investigation of many-body system electronic structure. The DFT methods attain considerably greater accuracy than Hartree- Fock theory at only a modest augment in the calculation time. This effect is obtained by specific including of electron correlation. However, for the simulation of relatively complicated systems, sophisticated hybrid exchange-correlation functionals are needed [43-46]. In our case Becke s three-parameter hybrid functional (B3LYP) [47] correlation functional was used. This functional include a mixture of exact exchange from Hartree-Fock theory with the Lee, Yang, and Parr exchange - correlation functional. Open shell systems were treated by the unrestricted Kohn Sham (UKS) procedure. The geometry optimizations and spectral calculations were carried out without any symmetry restriction. For geometry optimization of smaller nanodiamond clusters (up to 200 C atoms) 6-31G(d) Pople split-valence polarized double-ζ basis sets for H, C, N and O atoms were used. Relatively large nanodiamond particles (No. of C atoms up to 700) were optimized by the DFTB method. Electronic excitations were calculated by time dependent DFT (TD-DFT) at optimized geometries. TD-DFT is an extension of DFT determined to investigate excited states and non-equilibrium properties of manybody systems in the presence of time-dependent potentials. This method enables the analysis of the character and localization of individual excited states [23-25]. A. S. Zyubin et al. [48] used TD-DFT for the analysis of optical properties of approximately 100 atoms containing ND particles with neutral and negatively charged vacancy-related ND point defects (N 2 V 0, N 2 V -, and N 3 V 0 ). DFT methods have 7

12 been successfully used for calculations of the optical gap, absorption spectrum and luminescence of small Si nanocrystals, with hydrogen and oxygen at the surface [49, 50]. For clusters containing more than 82 carbon atoms (more than 1 nm size), the changes of the parameters studied here with cluster size were negligible and the lowest lying excitation energies were close to the experimental data. Gaussian is a connected system of programs for performing ab-initio and semi-empirical molecular orbital (MO) quantum chemical calculations. It has been designed with the needs of the user in mind. Thus, all the standard input is free-format and mnemonic. Gaussian 09 provides an analysis of molecular potential energy surfaces to determine molecular structures and spectral properties of stable and unstable species. It provides a wide number of ab-initio models and some semi-empirical models. Program package provides calculations of one- and two-electron integrals over s, p, d and f contracted Gaussian functions. The basis set can be either Cartesian Gaussians or pure angular momentum functions, and a variety of basis sets are stored in the program and can be requested by name. Integrals can be stored externally or recomputed as needed via the direct self-consistent field procedures. Variety of methods can be used, self-consistent field calculations for restricted closed shell, unrestricted open-shell, and open shell restricted Hartree-Fock wavefunctions as multiconfigurational wavefunctions that fall within the generalized valence bond-perfect pairing formalism. Automated geometry optimization to either minima, or saddle points, numerical differentiation to produce force constants, polarizabilities and dipole derivatives and reaction path following is also one of the Gaussian capabilities. 8

13 4 SELECTED RESULTS 4.1 NV centers It has been reported [2], that for ND particles containing NV - centers close to the hydrogen terminated surface (resulting in a positive electrostatic potential of the surface layer), the luminescence probability is significantly reduced or/and the NV - centers in hydrogen terminated ND particles are converted into NV 0 centers. In the case of oxygen containing a ND termination, the layer with an excess of electrons is created (resulting in negative electrostatic potential of the surface layer). Previously, it has been reported according to the experimental results [2] that ND particles NV 0 centers luminescence is not practically changed by surface states (terminations). Thus in the present work deep study of the surrounding/particle surface impact on nanodiamond NV - centers luminescence is presented. There are many parameters affecting NV - centers luminescence process in ND particles and some of them have been discussed in previously published works [2, 48-50]. Here it is shown how specific orbitals in NV - containing NDs occupations affect luminescence. It can be said that all lowest lying excitation transitions are between β (spin down) orbitals. With the change of particles size or NV center position in particle degenerated triplet state splits - which was shown experimentally [51], and simulated theoretically [52]. Pinto et al. [52] described models of the diamond vacuum slab, resulting 0.2 ev split of two empty spin-down orbitals when NV center was closer than 1 nm to the surface. In the present work excitation energies, absorption lines split and absorption spectra broadening were calculated. Simulated absorption spectra (black) and calculated transitions (red vertical lines) of the variously terminated and sized ND particles are shown on Figure 1. Lowest two lines correspond to triplet-triplet transitions between frontier β orbitals. For 1.2 nm hydrogen terminated ND the difference (split) between two lowest electronic transitions equals to 0.18 ev. For OH terminated particles the lowest lying electron transition shifts from ~730 to 850 nm, which corresponds to larger energy levels split ev. Similar situation is observed for fluorine terminations: 0.42 ev is the difference between lowest transitions (Figure 1). Surfaces of hydrogenated particles were 100 % covered by hydrogen, in case of oxygenation/fluorination 10% of hydrogen was substituted by carbonyl groups/fluorine. Also, several carbonyl/fluorine rations were studied (2%, 4%, 6%, 8%, and 10%); in all cases the tendency of βhomo to βlumo transition energy withdraws remained the same. As it has been explained βhomo - βlumo+1 transition energy shows value ( 1.9 ev) which is in a good correspondence with the experimental results, in case of smaller particles (1.2 nm), or oxygenated/fluorinated surface βhomo to βlumo transition energy is lower than measured adsorption/emission energy of NV -. This is consistent with the fact that LUMO orbital of cluster (C 696 H 300 N 1 ) is located on the surface (i.e. electron can be pulled back from its initial position); however HOMO and LUMO+1 are on the NV center s immediate vicinity (see Figure 2). Thus, electron which dropped down from the LUMO+1 to LUMO is dislocated from its initial position and with high probability relaxes to the ground state possessing no standard luminescence. Moreover electron can be trapped by vicinity defects or surface terminations. For fluorinated and oxidized NDs the βlumo and βlumo+1 energy difference and k-space position difference is larger (comparing to H- terminated systems) and the probability of the electron relaxation to βlumo decreases, thus βlumo+1 to βhomo relaxation, which with high probability possess zero phonon line transition is more preferable. In the case of close arrangement of βlumo and βlumo+1 (H termination) electron can transit to the ground state non-radiatively. Therefore, fluoridating or oxygenating of the ND particles preserve conditions for standard zero phonon line emission. 9

14 ε [arb. un.] ε [arb. un.] a) b) Wavelength [nm] 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0,00 Oscillator strength ε [arb. un.] c) d) ,07 0,06 0,05 0,04 0,03 0,02 0,01 Oscillator strength ε [arb. un.] Wavelength [nm] 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0,00 0,06 0,05 0,04 0,03 0,02 0,01 Oscillator Strength Oscillator Strength Wavelength [nm] 0, Wavelenght [nm] 0,00 Figure 1: Simulated absorption spectra and calculated transitions (vertical lines) of the 100% hydrogenated 1.7 nm (a), 100% hydrogenated 1.2 nm (b), 10% fluorinated 1.7 nm (c) and 10% fluorinated 1.2 nm (d) ND particles. a) b) c) Figure 2: βhomo (a), βlumo (b) and βlumo+1 (c) orbitals of cluster (C 696 H 300 N 1 ). In the first and third cases (a) and (c), electron density located in the immediate vicinity to the NV center, in the second case (b) electron density is shifted to the surface. Due to the fact that for small particles degenerated 3 E state splits into two levels there are two different transitions - βhomo to βlumo and βhomo to βlumo+1. In case of hydrogenated ND particles excited electron relaxation to βlumo causes it s delocalization from the NV center. Electron can be trapped by ND s surface or vicinity defects. For O/F terminated ND the energy difference between βlumo and βlumo+1 is higher (compared to H-terminated NDs) and βlumo+1 to βhomo relaxation with no shelving, is more preferable. 10

15 4.2 Silicon related centers Silicon in diamond has been identified with a Silicon-Vacancy complex in the semi-divacancy site. Theoretical approach shows the distance between Si and the nearest carbon is about 2.05 Å which is more than regular carbon-silicon bond. The bond orders between Si and C atoms were found in the range of Thus, silicon atom is located in the cavity between two carbon vacancies. Calculated excitation energy using TD-DFT method for neutrally charged Si atom educes that the most probable irradiative electronic transition has energy equal to ev (656 nm, experimental 738 nm) and oscillator strength f= (see Table 1). Table 1: TD-DFT calculated lowest energies excitations parameters for neutrally charged Si containing defects: excitation energy and oscillator strengths. Defect type Orbitals involved Excitation energy, ev Oscillator strength SiV 0 singlet HOMO -> LUMO HOMO-2 -> LUMO HOMO-1 -> LUMO SiV 0 triplet βhomo -> βlumo βhomo-1->βlumo βhomo -> βlumo a) b) ε [arb. un.] ,08 0,06 0,04 0,02 Oscillator strength ε [arb. un.] ,08 0,06 0,04 0,02 Osillator strength , ,00 Wavelength [nm] Wavelength [nm] Figure 3: Simulated absorption spectra and calculated transitions (vertical lines) of the neutrally charged Si related luminescent centers in the singlet (a) and triplet (b) spin states. As it can be in Figure 3, neutrally charged Si related centers have similar absorption spectra in the singlet and triplet spin states, however singlet state is 0.97 ev lower. Thus, singlet state is more energetically favorable, but triplet close enough to the singlet ground state to be excited by laser forming triplet state, consequently two spin states can emit luminescence at the similar wavelengths, which can cause overlapping of emission. Negatively (-1) charged Si-related center shows strong absorption lines in the range of 2.21 and 3.58 (see Table 2) possessing splitting of the orbitals due to the low symmetry of studied cluster, however similar splitting into four lines was reported in [53, 54] Simulated absorption spectrum of negatively charged Si related center depicted in Figure 4. 11

16 Table 2: TD-DFT calculated lowest energies excitations parameters for negatively charged Si containing defects: excitation energy and oscillator strengths. Defect type Orbitals involved Excitation energy, ev Oscillator strength SiV - singlet βhomo-1 -> βlumo βhomo-2 -> βlumo βhomo-3 -> βlumo βhomo-4 -> βlumo ε [arb. un.] wavelength [nm] 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0,00 Oscillator strength Figure 4: Simulated absorption spectrum and calculated transitions (vertical lines) of the negatively charged Si related luminescent centers in the doublet state. 4.3 Chromium and nickel related centers Geometry optimizations were performed for neutral and doubly charged Ni centers both in singlet and triplet states in C 101 H 72 ND clusters. Calculations on model clusters indicate that the single neutrally charged nickel should be placed in divacancy microcavity of the diamond lattice. In this case, the triplet state lies 0.22 ev lower than the singlet state. For the doubly positively charged Ni-V center, the singlet state has the lowest energy while the triplet state energy is about 0.80 ev higher. In the case of neutral Ni, the calculated C-Ni bond lengths are in the range of Å (bond orders ). The octahedral nickel atom is weakly bonded to the surrounding carbons. For the doubly positively charged Ni-V center, the Ni-C bond orders (0.36) and bond lengths (from 2.07 to 2.11 Å) were close to these parameters of neutral nickel containing centers. In both cases, the Ni-C bonds are weak. The oscillator strengths represent the probability of a transition from the ground state to the excited state. The excitation energy calculated for a system with a neutral Ni center ( 3 A state) is 2.34 ev, which is close to the experimental 2.51 ev ZPL [47]. For a Ni 2+ luminescence center ( 1 A state), was calculated an excitation energy value of 1.73 ev, which is also close to the experimental value of ev (see Figure 5) [45]. 12

17 a) b) ε [arb. un.] , ,030 0,025 0,020 0,015 0,010 0,005 Oscillator strength ε [arb. un.] ,030 0,025 0,020 0,015 0,010 0,005 Oscillator strength Wavelength [nm] 0, Wavelength [nm] 0,000 Figure 5: Simulated absorption spectra and calculated transitions (vertical lines) of 1.25 nm sized ND particles containing NiV centers (a) in a neutral triplet state, (b) in a double positively charged singlet state. Cr-based color centers in ND particles were as it has been mentioned previously engineered by Cr implantation followed by the implantation of oxygen, sulfur or nitrogen. Subsequent annealing causes the formation of luminescent centers [37, 39 41]. Such centers can be leading candidates for future quantum devices thanks to their room temperature operation, photostability, narrow bandwidth and short lifetime. To model such center, Cr atom was placed in the semi-divacancy site, due to the fact, that standard Cr-C bond length is longer than a C-C bond, and thus substitutional chromium atom leads large lattice stresses in the diamond, during the annealing (such treatment was also applied in the process of creating Cr luminescent defects [41]) chromium atom placed in the lattice site can push away carbon atom forming octahedral coordination semi-divacancy complex. Moreover, it also has been proved by calculations that Cr-V complex is energetically more favorable than substitutional single chromium atom placed in the diamond lattice. The experiments performed have shown that the presence of nitrogen is a crucial condition for creating chromium-based luminescent centers. Thus, nitrogen atom was placed in the immediate vicinity to Cr atom. Further irradiation with either sulfur, oxygen or boron ions allows the replacement of nitrogen or carbon atoms Cr s nearest neighbors which causes an augmentation of the single photon emitters. a) b) Figure 6: The optimized geometries of Cr-based centers, a chromium atom placed in the cavity of two carbon atoms and bonded to five adjacent carbons and one nitrogen atom- (a), or nitrogen substituted by sulfur - (b). The DFT calculations were done for C 98 H 72 Cr 1 N 1 clusters containing triply charged Cr centers for 1 A, 3 A and 5 A states (Figures 6 (a), (b)). Such structures allow the placement of atoms with minimal lattice stresses corresponding to energetically advantageous positions. Thus, the octahedral triply charged chromium atom in a semi-divacancy site (the Cr-C bond was found to be longer than the C-C bond ( Å)) and the adjacent carbon was substituted by nitrogen or sulfur was modeled. Figure 6 (a) shows a model of a ND cluster containing an imbedded Cr atom in a semidivacancy site with a bonded nitrogen substitution. Geometry optimizations indicate 3 A state as the most stable. For the triply charged Cr-related center between C and Cr, two shorter bonds (1.93 Å, Wiberg index 0.64) and four longer bonds (in the range from 2 to 2.51 Å and Wiberg index , respectively) were found. The Cr-N bond 13

18 value on the Wiberg index is Thus, in a chromium-related center, the Cr 3+ is strongly bonded to the two adjacent carbons while the other bonds are weak. The calculated triplet triplet electronic transition (from αhomo to αlumo, Cr 3+ ) found at 747 nm (see Figure 8 (b)) corresponds well to the measured excitation energy of a Cr-containing center in a diamond. As in this luminescence center the chromium atom is strongly bonded to the two adjacent carbons and weakly to the other carbons and nitrogen and the N-C bond is weak; the nitrogen atom can be again easily substituted by implanted ions. Ulterior irradiation with sulfur atoms can cause the substitution of either nitrogen or carbon in the center s immediate vicinity, and new configurations expand the excitation/emission frequency scale. If nitrogen is substituted by sulfur, it then shows the sulfur is bonded to the adjacent Cr 3+ more strongly than the initial nitrogen (Wiberg index 0.54, bond length 2.16 Å). The chromium atom is in turn strongly bonded to the two carbon atoms (Wiberg indexes 0.73, bond lengths 1.98 Å) and relatively weakly to other carbons (Wiberg indexes 0.28, bond lengths Å). It can thus be suggested, that the sulfur-containing center is more stable. Bonding lengths for Cr-based centers containing one atom of chromium bonded either with nitrogen of sulfur are depicted in figures 6 (a) and 6 (b) respectively. a) b) Figure 7: Bond lengths of optimized Cr-based centers, a chromium atom placed in the cavity of two carbon atoms and bonded to five adjacent carbons and one nitrogen atom- (a), or nitrogen substituted by sulfur - (b). a) b) ,010 ε [arb. un.] Wavelength [nm] 0,008 0,006 0,004 0,002 0,000 Oscillator strength Figure 8: (a) Lattice geometry of Cr related center in a semi-divacancy site with a bonded nitrogen substitution and simulated absorption spectrum of Cr-N center. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Cr-N center. 14

19 a) b) ε [arb. un.] Wavelength [nm] 0,012 0,010 0,008 0,006 0,004 0,002 0,000 Oscillator strength Figure 9: (a) Lattice geometry of the Cr-N-B center. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Cr-N-B center. Further implantation with boron can cause substitution of carbon in the CR-N center. Geometry optimizations indicate positively doubly charged 3 A state as the most stable in case of Cr- N-B center (Figure 9 (a)). For the triply charged Cr-related center between C and Cr, two strong bonds with lengths 1.93 Å and 1.94 Å, and Wiberg indexes 0.67 and 0.70 respectively and three longer Cr-C bonds in the range from 1.99 to 2.49 Å and Wiberg index , respectively. The Cr-N bond value on the Wiberg index is 0.33 and bond order of the Cr-B bond is equal to In the Figure 9 (b) it can be seen, that Cr-N-B centers has strong absorption with the wavelength about 745 nm, which is in a good correspondence with the experimental ZPL emission. Total calculated Wiberg index for chromium is a) b) ε [arb. un.] Wavelength [nm] 0,018 0,016 0,014 0,012 0,010 0,008 0,006 0,004 0,002 0,000 Oscillator strength Figure 10: (a) Lattice geometry of the Cr-N-O center. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Cr-N-O center. The triply charged Cr-N-O center has two shorter bonds 1.91 and 1.98 Å, Wiberg indexes are 0.72 and 0.65 respectively, three longer bonds (in the range from 2.11 to 2.36 Å and Wiberg index , respectively). The Cr-N bond value on the Wiberg index is 0.29, Cr-O bond is very weak with a length 2.40 Å and Wiberg index 0.07 Thus, in a Cr-N-O center, the Cr 3+ is strongly bonded to the two adjacent carbons while the other bonds are weak. Cr atom is placed in the double vacancy cavity, its total Wiberg index equals to Simulated absorption spectrum of the Cr-N-O center shows intensive absorption at 760 nm, which shows possibility of existence such type of luminescence centers (Figure 10). 15

20 a) b) ε [arb. un.] ,035 0,030 0,025 0,020 0,015 0,010 0,005 Oscillator strength Wavelength [nm] 0,000 Figure 11: (a) Lattice geometry of the Cr-N-S center. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Cr-N-S center. For the triply charged Cr-N-S (Figure 11 (a)) center there are three strong bindings between C and Cr, with a length in the range from 1.91 to 2.07 Å, and bond orders respectively and one longer Cr-C bond with the length 2.37 Å and Wiberg index The Cr-N bond value on the Wiberg index is Thus, in a chromium-related center, the Cr 3+ is strongly bonded to the two adjacent carbons while the other bonds are weak. Calculated absorption spectrum for Cr-N-S center showed the strongest and the sharpest absorption line (figure 11 (b)) among all Cr-related centers considered. In the case of sulfur containing center total Wiberg index of chromium atom has the highest value As regards to Cr and Ni containing centers, their excitations were of 3d origin and the number of states lying between ground and excited state is relatively large. This great variety of excited states can interact via spin-orbit coupling and lead to shorter lifetimes and deactivation. Significant advance of the present research is the discovery of possible structure of the chromium related centers with a good correlation with the experiment performed by Arahonovich, Muller et al. [40-43]. It has been found that single chromium atom placed in the semi-divacancy site is bonded to the nitrogen atom in its initial state. Ulterior irradiation with oxygen/sulfur/boron can cause substitution of adjacent carbon with implanted atoms increasing number of emitters. Geometry optimizations indicate 3 A state as the most stable. For the triply charged Cr-related center between C and Cr, two shorter bonds (Wiberg index 0.64) and four longer bonds (Wiberg index ) were found. The Cr-N bond value on the Wiberg index is Thus, in a chromiumrelated center, the Cr 3+ is strongly bonded to the two adjacent carbons while the other bonds are weak. The calculated triplet triplet electronic transition found in the range of 750 nm corresponds well to the measured excitation energy of a Cr-containing center in a diamond. Centers with one carbon atoms substituted by oxygen, boron and sulfur showed similar spectra, which means overlapping of luminescence originated from different defects. I case of oxygen and sulfur substitution defects were found to be triply charged in the triplet state, defect containing boron was found to be doubly charged (triplet). The specificity of bonds strongly influences the excitation process. 4.4 Ni+Si luminescent centers Very new and interesting color centers in diamond are containing complexes with Ni [39, 40, 41]. A very short lifetime of only 2 ns, a total high count rate (from two APDs) of 280 khz and operation at room temperature makes some of these systems perfect solid state candidate to be used in integrated quantum optics [55, 56] i.e. various optical light guide cavities which couple individual photons with minimum loss. In preceding papers it has been reported various emission lines characteristic of nickel impurities in diamond [57]; however, an emission around 770 nm has not been observed or assigned to any particular nickel related centre up to now. Silicon, which is the most common diamond impurity besides N, is favorably incorporated within the diamond lattice 16

21 forming common Si-V centers. Therefore, in [37] it has been assumed that the new centre is related to both nickel and silicon complexes. To check these assumptions, a reference experiment by implanting both silicon and nickel into pure synthetic, type IIa, diamond was performed. Nickel ions were implanted with an energy of 37.5 kev while silicon ions were implanted using 25 kev. A similar PL line centered at 766 nm was observed (figure 12) whereas when only nickel was implanted into the same crystal did not result in the formation of this specific emission line. Doublet at around 883/885 nm associated with an interstitial nickel defect in diamond [41, 57] appeared in both performed nickel implantations. These observations provide solid evidence that the new emission line at around 770 nm is due to a complex containing both nickel and silicon. Those two atoms create a large distortion in the diamond crystal. Hence, it is possible that upon annealing a vacancy is combined with those two atoms and the actual structure consists of Si and Ni impurities associated to a vacancy. Figure 12: PL spectra from Ni only (black) and a co implantation of Ni/Si into type IIa e6 CVD diamond. The measurement is taken at 77K under 514 nm excitation. The red curve is recorded from the non implanted region. [37] a) b) ε [arb. un.] Wavelength [nm] 0,010 0,008 0,006 0,004 0,002 0,000 Oscillator Strength Figure 13: (a) Lattice geometry of the Si+Ni center. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Si+Ni center. In order to find optimal state of the defect geometry optimizations were performed simulations for single and triplet stated for charges 0, and +2; doublet and quadruplet states were used with charges +1 and +3. Calculations on model clusters indicate that the nickel and silicon should be placed in close vicinity to each other in the divacancy sites of the diamond lattice (Fig. 13). For the mentioned structure intensive electron transition at 1000 nm (oscillator strength 0.01) was found in the case of +1 charged system in a quadruplet spin state. In this case quadruplet state lays 0.22 ev lower than the doublet state. In this case Ni-C bond lengths are in the range of Å, Wiberg indexes were 17

22 found in the range of respectively. Si-C bonds were slightly longer than Ni-C bonds and were found to be in the range of Å, bond orders defined by Wiberg indexes were in the range of respectively. Simulated bond between Si and Ni atoms was equal to 2.34 Å, bond order Total Wiberg indexes for Ni and Si were 3.37 and 3.01 respectively. a) b) ε [arb. un.] ,012 0,010 0,008 0,006 0,004 0,002 Oscillator strength Wavelength [nm] 0,000 Figure 14: (a) Lattice geometry of the Si+Ni center with a nitrogen bonded to nickel. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Si+Ni center. However, addition of nitrogen bonded to nickel atom causes the appearance of strong electron transition near 700 nm with oscillator strength 0.012, which is closer correspondence with the experimental results showing luminescence at 766 nm. In this case, the bond between Si and Ni atoms remains almost the same (2.33 Å, Wiberg index 0.20), bonds Ni-C are also in the range of Å, with the bond orders , Si-C bonds were found to be slightly longer than in the case of system without nitrogen: bong lengths Å, bond orders respectively. The lengths of the Ni-N bond were found to be equal to 2.01 Å with the bond order Total Wiberg indexes by atoms were: Si 3.0, Ni 3.4, N 3.2. a) b) ε [arb. un.] ,05 0,04 0,03 0,02 0,01 Oscillator Strength Wavelength [nm] 0,00 Figure 13. (a) Lattice geometry of the Si+Ni center with a nitrogen bonded to nickel and silicon. (b) Simulated absorption spectra and calculated transitions (vertical lines) of the Si+Ni center. Further analysis of the studied system showed that addition of nitrogen bonded to both silicon and nickel atoms causes intensification of the 700 nm line and totally diminishes 1000 nm line. Oscillator strength in this case is 0.05, which corresponds to the most intensive transition studied in the present work. In case of +1 charged, triplet state, system with the nitrogen bonded to Si and Ni atoms simultaneously showed, the expansion of Si-Ni bond length (from 2.34 Å to 2.41 Å, Wiberg index 0.23), Si-C bonds were found to be in the range of Å, Wiberg indexes: Ni- C bonds had lengths in the range of Å, which is also longer than in previously studied systems, bond orders were in the range of respectively. Ni-N and Si-N bond were found to 18