Polyamorphism in silicon nanocrystals under pressure

Many works have been devoted to describing mechanisms of pressure-induced polyamorphism. This phenomenon is apparent in the phase transition between low- and high-density amorphous states (LDA and HDA) upon the application of pressure, resulting in substantial changes in the structure and physical properties of the amorphous state. The HDA–LDA transition in Si nanocrystals is observed when recording Raman spectra in situ during decompression at 6.68 GPa.     

Electron-density distribution in diamond,silicon and germanium under high pressure

The valence-electron densities of C, Si, and Ge under high pressure are studied with the full-potential linearized APW method. For all three materials, the forbidden x-ray structure factor F(222) is stable under pressure and varies less than 3% under volume changes as big as ± 10%. The 30% drop of F(222) recently measured in Si just before the transition to the β - Sn structure is interpreted as an effect of the coexistence of the diamond and β - Sn phases over a 10 Kbar pressure range centered at the transition pressure.     

Elastic properties of diamond,silicon, and germanium nanocrystals as functions of their size and shape

The dependence of elastic modulus B on the size (number of atoms (N)) and the shape of a nanocrystal of a simple monoatomic substance is studied when the nanocrystal is considered as a rectangular parallelepiped with a varied surface shape. The elastic modulus is shown to decrease during an isomorphic-isothermal decrease of the nanocrystal size. At low temperatures, the B(N) dependence is less pronounced, and the case where the B(N) function increases during an isomorphic-isothermal decrease of the nanocrystal size is possible here. The size dependences of the elastic modulus, Poisson ratio μ, Young’s modulus Y, shear modulus G, and lattice parameter compression are calculated for diamond, Si, and Ge. It is shown that B, Y, and G decrease and μ increases during an isomorphic-isothermal decrease of the nanocrystal size. The surface pressure compresses a nanocrystal at low temperatures and expands it at high temperatures. The larger the deviation of the nanocrystal shape from the most energetically favorable shape, the more pronounced the changes of these functions during an isothermal decrease of the nanocrystal size.     

Surface control of optical properties in silicon nanocrystals produced by laser pyrolysis

Macroscopic quantities (g/h) of Si nanoparticles were prepared by laser pyrolysis of silane and showed photoluminescence (PL) emission in the range 700-1050 nm after oxidation in air at a temperature T ≥ 700 °C. Two different strategies were followed to reduce as-produced particle agglomeration which hinders most of the applications, namely etching with either acid or alkaline solutions. Well isolated single particles were detected after acid etching in HF. Disaggregation was also achieved by the combined effect of the high power sonication and alkaline etching by tetra-methyl ammonium hydroxide (TMAH), which leaves OH terminated surfaces. However, in both cases re-aggregation was observed within a few hours after oxide removal. Stable dispersions of Si nanoparticles in different solvents were obtained by treatments of H-terminated surfaces with the surfactant TOPO (C₂₄H₅₁PO, trioctylphospine oxide) and by treatment of OH-terminated surfaces with Na₃PO₄.     

Doping in silicon nanocrystals

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B-P co-doping results to be easier than simple B- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcome. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects.     

On self-diffusion and surface energy under compression of diamond, silicon, and germanium

The dependences of activation (vacancy formation and self-diffusion) parameters, specific surface energy σ, and its isochoric temperature derivative versus relative volume V/V ₀ are calculated for diamond, silicon, and germanium along two isotherms at 300 and 3000 K. Here, V ₀ is the crystal volume under pressure P = 0 at temperature T = 0 K. It is shown that under a compression to V/V ₀ > (V/V ₀)_min, activation processes are suppressed during isothermal compression and enhanced during isochoric heating. However, for V/V ₀ = (V/V ₀)_min, the self-diffusion coefficient attains its minimal value. And for V/V ₀ < (V/V ₀)_min, self-diffusion is intensified; in this case, the self-diffusion coefficient is independent of temperature. This is due to the quantum effect: under superstrong compression, the atomic spacing becomes comparable with the amplitude of atomic vibrations, which leads to the tunnel transport of atoms over the crystal. It is shown that upon an isothermal decrease in V/V ₀, the surface energy, which attains is maximal value at (V/V ₀)_max, sharply decreases upon a further compression. For V/V ₀ ≤ (V/V ₀)_fr, the surface energy becomes negative (σ(V/V ₀)_fr = 0), which must stimulate fragmentation of the crystal, i.e., an increase in the surface (per atom) intercrystallite surface. It is shown that (V/V ₀)_fr ? (V/V ₀)_min.     

Two-photon-excited luminescence spectra in diamond nanocrystals

Two-photon-excited luminescence (TEL) spectra have been recorded in the blue (400–500 nm) and near-ultraviolet (300–400 nm) ranges for diamond particles with 4 nm average size, which were obtained by detonation synthesis from explosives. The observed TEL bands are attributed, by comparing the obtained spectra with the impurity luminescence spectra in large diamond crystals, to N2 and N3 defects associated with the presence of nitrogen impurities in diamond. The TEL spectra presented are found to have certain distinguishing features: short-wavelength shift of the maximum and changes in the shape and width of the spectral bands for ultradispersed diamond compared with the spectrum in bulk crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1110–1112 (June 1999)     

Influence of the dielectric matrix on photoluminescence and energy exchange in ensembles of silicon nanocrystals

The results of a numerical simulation of photoluminescence in ensembles of Si nanocrystals incorporated into SiO₂ and ZrO₂ matrices are presented. It is shown that, in the ZrO₂ matrix, which produces a lower potential barrier for electrons and holes in nanocrystals, the photoluminescence intensity decreases significantly and the spectral peak shifts towards lower energies.     

Electronic state calculations of spherical diamond nanocrystals

Electronic-state calculations of diamond nanocrystals simulated by ultrasmall quantum spheres of diamond passivated by hydrogen are performed by the extended Hückel-type nonorthogonal tight-binding method. Two kinds of surface configuration (ideal and dimerized ones) are studied. Special attention has been paid to surface as well as quantum-confinement effects. The calculated results have demonstrated that, while the HOMO (highest occupied molecular orbital) energies are independent of the surface configuration and depend clearly on the size of the diamond spheres, the LUMO (lowest unoccupied molecular orbital) energies of the diamond spheres with one or two dimers on the surface are rather insensitive to the size, in agreement with experiment. The latter is found to be ascribed to the occurrence of surfacelike states associated with the backbonds of the dimer. It is shown that calculated lifetimes across the energy gap are less than 100 microseconds, suggesting that the diamond nanocrystals are promising light-emitting materials.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.     

Energy transfer between silicon nanocrystals

It is shown that the energy migration between silicon nanocrystals embedded into a silicon dioxide host is caused by the “nonresonant” dipole-dipole interaction. This process is efficient only for a part of small nanocrystals among the whole ensemble of nanocrystals. The nonresonant dipole-dipole energy transfer has such a feature as the emission of two optical phonons at each step of the process. The time of the excitation transfer has been experimentally determined for nanocrystals 1.5 nm in size existing in the ensemble of nanocrystals with a density of 10¹⁸ cm⁻³ and the size distribution with a standard deviation of 20%. A value of 30 μs obtained for this time is in good agreement with the performed theoretical estimation.     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Optical losses and absorption cross-section of silicon nanocrystals

 Si-rich silicon oxide and SiO₂ (SRSO)/SiO₂ multilayer (ML) samples were grown by reactive magnetron sputtering and then annealed at high temperature to induce the formation of Si-nc with mean size of 3-4 nm and density of about 3.5×10¹⁸ cm⁻³ as deduced from high resolution TEM micrographs. Refractive index and thickness have been determined by m-line measurements, which have shown a birefringence of about 1.5% due to the ML structure. Rib-loaded waveguides have been fabricated to measure propagation losses in the visible-infrared range. The analysis of the different contributions to optical losses such as Mie scattering and scattering due to waveguide roughness has allowed us to isolate the contribution due to the absorption losses and thus to extract the absorption cross-section at different wavelengths. Values of about 3.5×10⁻¹⁸ cm² have been found at 830 nm, increasing with decreasing of the wavelength.     

Quantum confinement in phosphorus-doped silicon nanocrystals

Electronic properties of phosphorus donors in hydrogenated silicon nanocrystals are investigated using a real-space ab initio pseudopotential method for systems with up to 500 atoms. We present calculations for the ionization energy, binding energy, and electron density associated with the doped nanocrystal. We find that the ionization energy for the nanocrystal is virtually independent of size. This behavior may be attributed to localization of the electron around the impurity site owing to a large electron-impurity interaction within confined systems. In contrast to this result, the calculated hyperfine splitting exhibits a strong size dependence. For small nanocrystals it greatly exceeds the bulk value. This finding agrees with recent experimental measurements.     

Gas-phase growth of silicon-doped luminescent diamond films and isolated nanocrystals

A method of controlled diamond doping, consisting in introducing a solid-state silicon source into a CVD reactor chamber is proposed and implemented. Such an approach is tested during diamond film and isolated nanocrystallite growth on silicon, molybdenum, sapphire, copper, and quartz substrates. The approach to nanodiamond doping with silicon during CVD synthesis, developed in this paper, is promising for developing stable highly efficient luminescent nanodiamonds.     

Transfection and imaging of diamond nanocrystals as scattering optical labels

We report on the first demonstration of nanodiamond (ND) as a scattering optical label in a biological environment. NDs were efficiently transfected into cells using cationic liposomes, and imaged using differential interference and Hoffman modulation ‘space’ contrast microscopy techniques. We have shown that 55 nm NDs are biologically inert and produce a bright signal compared to the cell background. ND as a scattering label presents the possibility for extended biological imaging with relatively little thermal or biochemical perturbations due to the optical transparency and biologically inert nature of diamond.