Hydrogenated Silicon Nanocrystals Research Articles

Kinetics of Hydrogen Generation from Oxidation of Hydrogenated Silicon Nanocrystals in Aqueous Solutions.

Hydrogen generation rate is one of the most important parameters which must be considered for the development of engineering solutions in the field of hydrogen energy applications. In this paper, the kinetics of hydrogen generation from oxidation of hydrogenated porous silicon nanopowders in water are analyzed in detail. The splitting of the Si-H bonds of the nanopowders and water molecules during the oxidation reaction results in powerful hydrogen generation. The described technology is shown to be perfectly tunable and allows us to manage the kinetics by: (i) varying size distribution and porosity of silicon nanoparticles; (ii) chemical composition of oxidizing solutions; (iii) ambient temperature. In particular, hydrogen release below 0 °C is one of the significant advantages of such a technological way of performing hydrogen generation.

Molecular hydrogen‐induced nucleation of hydrogenated silicon nanocrystals at low temperature

Highly crystallized hydrogenated silicon layers were obtained via the treatment of hydrogenated polymorphous silicon films in a molecular hydrogen ambient. This contrasts other postdeposition studies that obtained nanocrystalline silicon films but necessitated either a plasma activation or high‐temperature annealing. The structure of the samples was analyzed by Raman spectroscopy to determine the crystallite volume fraction, which was found to increase up to 80% within 1 hour of treatment. Atomic force microscopy (AFM) showed that the roughness of the surfaces was found to increase after the H2 treatment. Optical transmission and spectroscopic ellipsometry revealed the pronounced porosity of the films characterized by a static refractive index that is below three, which is a low value for hydrogenated silicon films and a void fraction that is around 15% in the bulk of the films. The effect of the hydrogen molecules on the structure of the films was discussed in terms of the compressive stress exerted by the molecules, trapped in structural inhomogeneities, on the amorphous tissue. It is suggested that for this process to take effect, the films need to be porous and that the amorphous network needs to be in a “relaxed” state.

Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory

We present an implementation in a linear-scaling density-functional theory code of an electronic enthalpy method, which has been found to be natural and efficient for the ab initio calculation of finite systems under hydrostatic pressure. Based on a definition of the system volume as that enclosed within an electronic density isosurface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett. 94, 145501 (2005)], it supports both geometry optimizations and molecular dynamics simulations. We introduce an approach for calibrating the parameters defining the volume in the context of geometry optimizations and discuss their significance. Results in good agreement with simulations using explicit solvents are obtained, validating our approach. Size-dependent pressure-induced structural transformations and variations in the energy gap of hydrogenated silicon nanocrystals are investigated, including one comparable in size to recent experiments. A detailed analysis of the polyamorphic transformations reveals three types of amorphous structures and their persistence on depressurization is assessed.

Electronic and Optical Properties of Chlorinated Silicon Nanoparticles

First-principles calculations are used to investigate the structure, electronic and optical properties of silicon nanocystals with chlorine-passivated surface. The nanocrystals considered were approximately spherical, with diameters between 1.5 and 3.0 nm. We show that the nanocrystals with chlorinated surface have a smaller bandgap, lower optical absorption threshold, and greater ionization energy and electron affinity than hydrogenated silicon nanocrystals of the same size.

Theoretical investigation of structural stability and electronic properties of hydrogenated silicon nanocrystals: Size, shape, and surface reconstruction

By model analysis and the first-principles calculations, we have performed a systematic investigation of hydrogenated silicon nanocrystals (H-SiNCs) with various size, shape, and surface reconstruction. We demonstrated that H-SiNCs with the smallest H/Si ratio are enclosed by (111) facets before reconstruction, and two typical surface reconstructions will dramatically decreases the H/Si ratio, involving dimers and steps on (100) and (111) facets, respectively. With the first-principles calculations, we determined the stable H-SiNCs (up to 200 Si atoms) and found that the surface morphologies vary with the size and hydrogen chemical potential. In addition, we also studied the electronic properties of H-SiNCs focused on the gap variations and charge distributions as a function of size, shape, and surface reconstruction. The dimer and step reconstructions not only bring significant decrement of the gap values and the H/Si ratio, but also modulate the charge distributions and induce the spatial separation of near-gap levels.

Electronic properties, doping, and defects in chlorinated silicon nanocrystals

Silicon nanocrystals with diameters between 1 and 3 nm and surfaces passivated by chlorine or a mixture of chlorine and hydrogen were modeled using density functional theory, and their properties compared with those of fully hydrogenated nanocrystals. It is found that fully and partially chlorinated nanocrystals are stable, and have higher electron affinity, higher ionization energy and lower optical absorption energy threshold. As the hydrogenated silicon nanocrystals, chlorinated silicon nanocrystals doped with phosphorus or boron require a high activation energy to transfer an electron or hole, respectively, to undoped silicon nanocrystals. The electronic levels of surface dangling bonds are similar for both types of surface passivation, although in the chlorinated silicon nanocrystals some fall outside the narrower energy gap.

Ultrastable Silicon Nanocrystals due to Electron Delocalization

We report a new nanocrystalline form of silicon that gives birth to pure hydrogenated silicon nanocrystals that absorb light in the ultraviolet, visible, and infrared spectral region despite their small size of only 1 nm and without the need for expensive or toxic metal atoms. On the basis of first-principles calculations, we demonstrate that those pure, but overcoordinated silicon nanocrystals are more stable than any other known silicon nanocrystals due to electron delocalization and that they form spontaneously via self-assembly. Therefore, we predict their immediate application in fields ranging from photovoltaic and light-emitting devices to photothermal cancer treatment.

Magnetic interaction in pairwise Mn-doped Si nanocrystals

The electronic-structure and magnetic properties of hydrogenated silicon nanocrystals doped with pairs of manganese atoms are investigated using spin-density-functional theory. Formation energies and total magnetic moments sensitively depend on the two sites occupied by manganese. Usually pairs at interstitial and substitutional sites with small total moment are energetically favored. Pairs at sites with the same character tend to ferromagnetic spin arrangements which are, however, significantly influenced by their noncollinearity. The resulting magnetic ordering is clearly related to the impurity levels and their occupation. The magnetic coupling is distance dependent, antiferromagnetic for small distances, and almost ferromagnetic for larger Mn-Mn distances. A Rudderman-Kittel-Kasuya-Yoshida-type exchange mechanism may describe the distance dependence but simultaneously not its magnitude.

Role of surface passivation and doping in silicon nanocrystals

The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the atomic structure and on the optical spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, (i) different Si/O bonding at the cluster surface and (ii) different doping configurations have been considered. We have found that the presence of a Si-O-Si bridge bond at the nanocrystal surface gives rise to significant excitonic luminescence features in the near-visible range that are in fair agreement with photoluminescence (PL) measurements on oxidized and SiO$_{2}$ embedded nanocrystals. The study of the structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have shown that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbors sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in agreement with the experiment.

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.

Controlling the electronic band structures in hydrogenated silicon nanocrystals by shallow impurity doping

By the aid of magnetic-field-dependent Hall effect measurements, we have extracted the electron mobility and concentration in hydrogenated Si nanocrystals grown on crystalline silicon substrates within the framework of mobility spectrum analysis. A unified model based on diffusive and ballistic transport mechanisms has been employed to explain the observed electron mobility in Si nanocrystals with different doping levels, as well as the mobility edge in low-doping Si nanocrystals. Both the theoretical and experimental results clearly demonstrate the control of the electronic band structures by shallow impurity phosphorus doping in Si nanocrystals, which provide an experimental basis for further nanoelectronic device design using Si nanocrystals.

Electronic, structural and optical properties of hydrogenated silicon nanocrystals: the role of the excited states

In this paper we report on a first-principle calculation of the electronic and structural properties of hydrogenated silicon nanocrystals both in the ground- and in an excited-state configuration. The presence of an electron-hole pair created under excitation is taken into account and its effects on both the electronic spectrum and the cluster geometry are pointed out. The interpretation of the results is done within a four-level model, which also allows the explanation of the experimentally observed Stokes shift. Size-related aspects are also analysed and discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Structural properties of hydrogenated silicon nanocrystals and nanoclusters

The structures of fully and partially hydrogenated Si nanocrystals and nanoclusters are studied by geometric optimizations and molecular dynamics simulations based on an empirical tight-binding approach. It is shown that the structural properties of the hydrogen saturated Si nanocrystals have little size effect, contrary to their electronic properties. The surface relaxation is quite small in the fully hydrogen saturated Si nanocrystals, with a lattice contraction of 0.01 to 0.02 Å residing in the outermost two or three layers. Inside the hydrogenated Si nanocrystals, there is only very small strain (lattice expansion) of the order 10−4 to 10−3, in agreement with the x-ray diffraction measurement. The fully hydrogenated Si nanocrystals are the most stable structures compared to those partially hydrogenated. For the smaller SimHx (m⩽151) nanocrystals, removing up to 50% of the surface terminating H atoms only causes distortions to the crystal structure, with the basic tetrahedral structural features still retained. However, removal of more than 70%–80% of the surface terminating H atoms will lead to a change to more compact structures.