Photoluminescence spectra of ultradisperse diamond

The first study of materials based on ultradisperse diamond produced by detonation is reported. A luminescence band in the visible has been observed, and some of its structural features have been interpreted by analogy with the known luminescence bands of centers in synthetic and natural diamonds. A comparison of the spectra obtained from ultradisperse diamond samples with the surface modified by different chemical treatments suggests that their pattern is governed to a considerable extent by the presence of a graphitic layer on the grain surface. Fiz. Tverd. Tela (St. Petersburg) 39, 2156–2158 (December 1997)     

Luminescence of Eu<Superscript>3+</Superscript> ions in ultradisperse diamond powders

The luminescence of Eu³⁺ ions implanted in ultradisperse diamond powders, activated by impregnating with a solution of Eu(NO₃)₃·6H₂O and heat-treated at various temperatures, is studied. A multiple increase in the efficiency of excitation in the charge-transfer band is observed for the ⁵D₀ state of Eu³⁺ ions as compared to europium nitrate heat-treated similarly. This effect is explained by an increase in the degree of Eu-O bond covalency and a change in the activator coordination polyhedron due to the formation of chemical bonds Eu-O-C.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

Diamond-graphite phase transition in ultradisperse-diamond clusters

A systematic study of the diamond-graphite structural phase transition in ultradisperse-diamond clusters obtained by the detonation technique is reported. Samples of two types, differing in the kinetics of detonation-product cooling, were investigated. The phase transition was achieved under heating in an inert atmosphere in the temperature range 720–1400 K. The transition was identified by Raman scattering and x-ray diffraction data. Raman and x-ray characterization showed the ultradisperse diamond, irrespective of the cooling rate used, to be cluster material possessing diamond structure with a characteristic nanocrystal size of 43 Å. The diamond-graphite phase transition in ultradisperse diamond is shown to start from the cluster surface inwards at T _pt≈1200 K, i.e. at substantially lower temperatures than is the case with bulk diamond single crystals.     

Photoacoustic spectroscopy of diamond powders and polycrystalline films

Photoacoustic spectroscopy is used to study optical absorption in diamond powders and polycrystalline films. The photoacoustic spectra of diamond powders with crystallite sizes in the range from ∼100 μm to 4 nm and diamond films grown by chemical vapor deposition (CVD) had a number of general characteristic features corresponding to the fundamental absorption edge for light with photon energies exceeding the width of the diamond band gap (∼5.4 eV) and to absorption in the visible and infrared by crystal-structure defects and the presence of non-diamond carbon. For samples of thin (∼10 μm) diamond films on silicon, the photoacoustic spectra revealed peculiarities associated with absorption in the silicon substrate of light transmitted by the diamond film. The shape of the spectral dependence of the amplitude of the photoacoustic signal in the ultraviolet indicates considerable scattering of light specularly reflected from the randomly distributed faces of the diamond crystallites both in the polycrystalline films and in the powders. The dependence of the shape of the photoacoustic spectra on the light modulation frequency allows one to estimate the thermal conductivity of the diamond films, which turns out to be significantly lower than the thermal conductivity of single-crystal diamond. Fiz. Tverd. Tela (St. Petersburg) 39, 1787–1791 (October 1997)     

Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

Electron-Microscopic Study of Morphology and Phase Composition of Lithium-Titanium Ferrites

In the present paper, the phase composition and morphology of lithium-titanium ferrospinel, synthesized by ceramic technology and subjected to thermal (T) and radiation-thermal (RT) annealing at a temperature of 1373 K, are studied. RT annealing of samples was performed by heating radiation of high-power electron beam pulses with energy 1.8 MeV. Bulk-density powders and powders compacted under a pressure of 1200 kg/cm² were used as samples. The results obtained indicate the polyphase composition of initial powders. Annealing decreases the relative content of the secondary LiFeO₂-based phase and increases the relative content of the master LiFe₅O₈-based phase. The efficiency of this process depends on the powder compactness and the heating regime. RT annealing significantly increases the rate of dissolution of the LiFeO₂-based phase. An impact diffraction analysis has shown that the secondary phases have polycrystalline structures, whereas the LiFe₅O₈-based phase is formed by monocrystalline particles. Annealing of the compacted powder by an electron beam completely eliminates polycrystalline aggregates with ultradisperse grained structure.     

Metastable carbon allotropes in picosecond-laser-modified diamond

In this paper, we report on the bulk modifications of type IIa single-crystal diamond with visible 10-ps pulses (at λ = 532 nm) and microstructural changes characterized by the appearance of several ‘unidentifiable’ vibrational modes in the frequency range of 1000–1400 cm^?1 in the Raman spectra of laser-modified diamond. It is found that the new Raman modes are strongly pronounced in the spectra of high-stress regions in immediate proximity to the bulk microstructures in the absence of the G mode at ~1580 cm^?1 characteristic of the sp² phase. The high internal stresses are determined from the splitting of the triply degenerate diamond Raman line. The revealed structure transformation is localized within a narrow bulk layer near the bulk microstructures formed, and the stress relaxation is found to result in disappearance of the detected vibrational modes in the spectra. It is suggested that the formation of bulk regions with a sp³ carbon structure consisting of Z-carbon and hexagonal diamond is responsible for the appearance of new Raman modes in the spectra of laser-modified diamond. These findings evidence that the stress-assisted formation of novel metastable carbon phases or defect structures occur in the course of bulk modification of diamond with ps-laser pulses. In addition, we report the results of simulations of internal stresses in the system ‘graphitized cylinder-in-diamond’ to show (1) the effect of the mechanical properties of laser-modified diamond on the resulting stresses and (2) formation of bulk microscopic regions with high stresses of >10 GPa, i.e., the conditions at which various sp³ carbon allotropes and defect structures become more stable than graphite.     

Copper-induced diamond formation in amorphous carbon

Diamond formation in hydrogenated amorphous-carbon films containing ultradisperse copper has been studied by measuring IR absorption at two-phonon diamond frequencies. The anomalously high two-phonon absorption observed in the experiments has permitted improving the sensitivity of the method. Mechanisms of the two-phonon absorption enhancement are discussed in terms of the theory of a medium containing nanosized copper-doped graphite fragments. It is shown that the observed enhancement of two-phonon absorption is caused by electrical induction of the local fields induced by irradiation in diamond nanocrystals acting on adjoining copper-doped graphite fragments. Fiz. Tverd. Tela (St. Petersburg) 41, 1863–1866 (October 1999)     

氮氢共掺杂金刚石中氢的典型红外特征峰的表征

所有天然Ia型金刚石红外光谱中都存在3107 cm-1特征峰,而在金属触媒直接合成的金刚石红外光谱中没有检测出3107 cm-1特征峰.本文在6.3 GPa,1500?C条件下,通过Fe70Ni30触媒中添加P3N5直接合成出具有3107 cm-1特征峰的氮氢共掺杂的金刚石.红外光谱分析表明,合成的金刚石中氢有两种存在形式:一种对应着乙烯基团C=CH2中C—H键的伸缩振动(3107 cm-1)和弯曲振动(1450 cm-1)的吸收峰,另一种对应着sp3杂化C—H键的对称伸缩振动(2850 cm-1)和反对称伸缩振动(2920 cm-1)的吸收峰.通过分析发现,3107 cm-1吸收峰与金刚石中聚集态的氮原子有关,当金刚石中没有聚集态的氮元素时,即使氮含量高也不会出现3107 cm-1峰;并且2850和2920 cm-1附近的吸收峰比3107 cm-1附近的吸收峰更为普遍存在.这说明sp3杂化C—H键比乙烯基团的C—H键更广泛存在于金刚石中,从两者的峰值看,天然金刚石中的氢杂质主要以乙烯基团C=CH2存在.3107 cm-1吸收峰与聚集态的氮原子的这种存在关系为天然金刚石形成机制的研究提供了一种新思路,同时较低的合成条件也可能为氢与其他元素共掺杂合成具有n型半导体特性的金刚石提供一个较理想的合成环境.     

Effect of modifiers on mechanical properties of polymer-based composite materials

In this paper, we present the experimental results on the study of mechanical properties of polymer-based nanocomposite materials with carbon nanotube or ultradisperse diamond inclusions. Tests are performed by nanoindentation methods. The results obtained for nanocomposites and a polymer used as a matrix in nanocomposites are compared.     

掺杂CVD金刚石薄膜的应力分析

在不同实验条件下，用微波等离子体化学气相沉积(MPCVD)技术在Si基体上制备了S掺杂和B-S共掺杂CVD金刚石薄膜，利用X射线衍射仪和拉曼光谱仪研究掺杂对CVD金刚石薄膜的应力影响.研究结果发现，随着S掺杂浓度的增加，薄膜中sp²杂化碳含量和缺陷增多，CVD金刚石薄膜压应力增加；小尺寸的B原子与大尺寸的S原子共掺杂时，微量B的加入改变了CVD金刚石薄膜的应力状态，共掺杂形成B-S复合体进入金刚石晶体后降低金刚石晶体的晶格畸变程度，减少S原子在晶界上偏聚数量和晶体中非金刚石结构相含量，降低由于杂质、缺陷及sp²杂化碳含量产生的晶格畸变和薄膜压应力，提高晶格完整性. 关键词： 金刚石薄膜 掺杂 应力     

On the thermal expansion of nanoparticles

Magnetite nanoparticles with an average particle size of 7.5 nm were studied by Mössbauer spectroscopy in the 100–300 K temperature range. Using experimental data on the temperature variation of isomer shifts in the partial spectra of ⁵⁷Fe nuclei in the A and B crystal sublattices, the linear thermal expansion coefficient of the ultradisperse magnetite particles in the temperature range studied was estimated within the framework of the molecular orbital formalism: α=(1.2±0.2)×10^?4K^?1.     

热灯丝CVD金刚石膜硼掺杂效应研究

利用Ｒａｍａｎ谱和Ｘ射线衍射微分析研究了金刚石膜硼掺杂效应．研究发现，在硼浓度较低时，金刚石膜中非晶碳（Ｓｐ²键合）含量减少，而随着硼浓度增加，金刚石膜的Ｒａｍａｎ特征峰（Ｓｐ^３）向低频方向漂移，并且展宽和出现不对称．这是由于硼掺杂引起晶格变化及Ｆａｎｏ互作用所致 关键词：     

Structure of ultradisperse diamonds

The structure of crystallites (regions of coherent scattering — RCS), which comprise the grains of ultradisperse diamond (UDD) powder, was investigated by x-ray crystallography and coordination-sphere diffractometry. It was established that the RCS have an average size $\bar L = 42 \pm 2{\AA}$ and that these regions are themselves UDD grains with the structure of a diamond lattice and a negligible content of clusters of carbon atoms distributed throughout the volume that do not form a lattice and are mainly located at interatomic distances ～1.5 Å. The clusters of carbon atoms, for which the distribution of interatomic distances is close to Gaussian, which may indicate an amorphous structure of the boundary layers, lie along grain boundaries.     

锌添加对大尺寸金刚石生长的影响

利用温度梯度法,在6.2—6.4 GPa,1270—1400℃条件下,通过在NiMnCo-C体系中添加不同比例的锌粉成功合成出3 mm左右的大尺寸金刚石单晶.研究了锌添加对金刚石颜色、形貌、内部氮杂质以及晶体结晶度的影响.结果表明:随着锌添加量逐渐增加,晶体的颜色逐渐变浅,晶体的透光性增强;当锌添加比例达到3 wt.%时,晶体表面出现大量不规则的凹坑;晶体内氮杂质主要以C心形式存在,随着锌添加量的增多晶体内氮含量逐渐降低,基于锌的除氮能力总结出两种可能的除氮机制;拉曼光谱测试结果表明,在锌添加量小于3.0 wt.%的研究范围内,锌的添加有利于提高晶体的结晶度.本研究不仅有助于天然金刚石形成机制的探究,而且对丰富金刚石的种类以及扩展人工合成金刚石的应用领域都有着重要意义.     

Kinetic peculiarities of diamond crystallization in K-Na-Mg-Ca-Carbonate-Carbon melt-solution

The kinetic peculiarities of diamond crystallization in multicomponent K-Na-Mg-Ca-carbonate-carbon system have been studied in conditions of diamond stability at 1500–1800°C and 7.5–8.5 GPa. It has been established that the diamond phase nucleation density at a fixed temperature of 1600°C decreases from 1.3 × 10⁵ nuclei/mm³ at 8.5 GPa to 3.7 × 10³ nuclei/mm³ at 7.5 GPa. The fluorescence spectra of obtained diamond crystals contain peaks at 504 nm (H3-defect), 575 nm (NV-center), and 638 nm (NV-defect), caused by the presence of nitrogen impurity. In the cathodoluminescence spectra, an A-band with the maximum at 470 nm is present. The obtained data make it possible to assign the synthesized diamonds in the carbonate-carbon system to the mixed Ia + Ib type.     

Study of synthetic diamonds by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Four I_b-type synthetic diamond crystals were studied by dynamic nuclear polarization (DNP)-enhanced high resolution solid state¹³C nuclear magnetic resonance (NMR) spectroscopy. The home built DNP magic-angle-spinning (MAS) NMR spectrometer operates at a field strength of 1.9 T and the highest DNP enhancement factor of synthetic diamonds came near to 10³. Comparing with I_b-type natural diamonds, the¹³C NMR linewidths of synthetic diamonds in static spectra are broader. The¹³C spin-lattice relaxation time and DNP polarization time of synthetic diamond are shorter than those of I_b-type natural diamond. From the hyperfine structure of the DNP enhancement curve, four kinds of nitrogen-centred free radicals could be identified in synthetic diamond.     

Effect of hydrogen on the structure of ultradisperse diamond

The paper reports on a study of the effect of annealing in hydrogen on the structural phase transition in clusters of ultradisperse diamond (UDD) obtained by the detonation method. The samples studied were of two types, namely, prepared by the “dry” and “wet” techniques, which differ in the cooling rate of the detonation products and, accordingly, in the structure of the diamond nanocluster shell. It is shown that, irrespective of the type of synthesis, the relative content of the diamond (sp3) phase increases within the anneal temperature range of 450 to 750°C, the increase being more pronounced in the samples prepared by “dry” synthesis. A model accounting for the observed structural transformation processes is discussed. A hypothesis of the possibility of compacting UDD clusters into bulk single crystals is put forward.