Vladimir Švrček

Blue luminescent silicon nanocrystals prepared by ns pulsed laser ablation in water

Here the authors report on a simple and cost-effective procedure to prepare blue luminescent colloidal silicon nanocrystals (Si-ncs) in de-ionized water at room temperature and atmospheric pressure by nanosecond pulsed laser ablation. At low laser fluences well-separated and spherical Si-ncs aggregates are formed. The fluence increase leads to fragmentation of spherical aggregates and the generation of finer Si-ncs with quantum confinement size effect. After aging in de-ionized water, such irregular Si-ncs fragments get stabilized into quite regular spheres and ovals. During the aging process the increase of blue room temperature photoluminescence centered around 2.9eV is recorded.

Self-organized nanostructures on atmospheric microplasma exposed surfaces

We present here the observation of self-organized nanostructures on atmospheric microplasma exposed surfaces. In particular, we present the formation of self-aligned Mo-oxide nanoparticles, as well as the production of self-organized carbon-based connections between silicon nanocrystal micrograins and between Ag nanoparticles. The strong electromagnetic fields that are present at the processing surface play an important role in the self-organization process. This result represents an interesting phenomenon and suggests possible routes to promote and exploit self-organization for the production of nanostructured materials.

Ex situ prepared Si nanocrystals embedded in silica glass: Formation and characterization

In this article we present an alternative approach for the fabrication of silicon nanocrystals (Si–nc) prepared ex situ of the silicon dioxide (SiO2) host matrix. The Si–nc are scratched from porous silicon layers and incorporated into a host spin-on-glass SiO2 based matrix. High-resolution transmission electron microscopy and Raman spectroscopy revealed Si–nc of 2–5 nm size. These nanocrystallites exhibit visible room temperature photoluminescence (PL) with a maximum at about 700 nm. The presence of the dopant in the host matrix is shown to induce a blueshift of the PL maxima due to modified surface states of the Si–nc. This approach allows the fabrication of self-supporting samples with very high Si–nc concentrations. A bright photoluminescence at room temperature is obtained on such materials. Finally, strong indication of optical gain at room temperature is shown for samples with high Si–nc concentrations in a phosphorus doped sol gel host matrix.

Optical gain in porous silicon grains embedded in sol-gel derived SiO2 matrix under femtosecond excitation

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm−1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission.

Basic features of transport in microcrystalline silicon

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.

Ambient-stable blue luminescent silicon nanocrystals prepared by nanosecond-pulsed laser ablation in water

Even with intensive research, air-stable blue light emission from silicon nanocrystals (Si-ncs) at room temperature still remains a challenge. We show that stable and blue-luminescent Si-ncs can be produced by laser-generated plasma (nanosecond-pulsed excimer laser) confined in water. These Si-ncs exhibit quantum confinement effect due to their size and are produced with an environmentally compatible process. The effect of aging for several weeks in water and air on blue Si-ncs emission properties is compared. The oxide shell around the nanocrystalline core formed during laser processing in water offers the required conditions for the confinement of excitons that allow for stable (in either air or water) blue photoluminescence at room temperature.

Microplasma-induced surface engineering of silicon nanocrystals in colloidal dispersion

We report on an atmospheric-pressure dc microplasma that can be used to passivate silicon nanocrystals (SiNCs) in ethanol and that stabilizes their optoelectronic properties. We show that microplasma processing enhances the SiNCs photoluminescence intensity by factor of more than ten times and ∼80 nm redshift of its maximum. The microplasma induces the replacement of hydrogen terminations with hydroxyl-/organic-based bonds. The resulting surface characteristics are responsible for the formation of conductive and stable SiNCs self-organized assemblies extending over 0.5 mm after dewetting on a substrate.

Model of transport in microcrystalline silicon

Abstract Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

A hybrid heterojunction based on fullerenes and surfactant-free, self-assembled, closely packed silicon nanocrystals

We demonstrate that nanosecond-pulsed laser chemistry in water leads to closely packed and stable luminescent assemblies of silicon nanocrystals (SiNCs) that can be electronically coupled with fullerenes (C60) without any additional surfactant or catalyst. We show that the fragmentation time in water determines the photoluminescence (PL) intensity (>40%) and redshifts the PL maxima (45nm) of the SiNCs. Heterojunction solar cells made out of these laser-produced self-assemblies of SiNCs and C60 show photovoltaic action with increased quantum efficiency in the region where the absorption of SiNCs appears. (Some figures in this article are in colour only in the electronic version)

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.