Vladimir Sivakov

Optical properties of silicon nanowire arrays formed by metal-assisted chemical etching: evidences for light localization effect

We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with a mean diameter of approximately 100 nm and length of about 1–25 μm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infrared spectral region, the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity, approximately 75%), while a strong light scattering for wavelength of 0.3 ÷ 1 μm results in a decrease of the total reflectance of 1%-5%, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at approximately 1 μm increases strongly in the sample with SiNWs in comparison with that in c-Si substrate. This effect is related to an increase of the light-matter interaction time due to the strong scattering of the excitation light in SiNW array. The prepared SiNWs are discussed as a kind of ‘black silicon’, which can be formed in a large scale and can be used for photonic applications as well as in molecular sensing.

Tunable nanoporous silicon oxide templates by swift heavy ion tracks technology

Nanoporous silicon oxide templates formed by swift heavy ion tracks technology have been investigated. The influence of the heavy ion characteristics, such as type of ion, energy, stopping power and irradiation fluence on the pore properties of the silicon oxide templates, has been studied. Furthermore, the process of pore formation by chemical etching with hydrofluoric acid has been thoroughly investigated by assessing the effect of etchant concentration and etching time. The outcome of this investigation enables us to have precise control over the resulting geometry of nanopores arrays. As a result, guidelines for the creation of a-SiO2/Si templates with tunable parameters and general recommendations for their further application are presented.

Studies of silicon nanoparticles uptake and biodegradation in cancer cells by Raman spectroscopy

In-vitro Raman micro-spectroscopy was used for diagnostics of the processes of uptake and biodegradation of porous silicon nanoparticles (SiNPs) in breast cancer cells (MCF-7 cell line). Two types of nanoparticles, with and without photoluminescence in the visible spectral range, were investigated. The spatial distribution of photoluminescent SiNPs within the cells obtained by Raman imaging was verified by high-resolution structured-illumination optical microscopy. Nearly complete biodegradation of SiNPs inside the living cells was observed after 13days of the incubation. The results reveal new prospects of multi-modal visualization of SiNPs inside cancer cells for theranostic applications.

On the morphology of Si/SiO 2 /Ni nanostructures with swift heavy ion tracks in silicon oxide

Si/SiO2/Ni nanostructures are fabricated by the irradiation of an oxidized Si surface with swift heavy ions, nanopore etching in the SiO2 layer, and the electrochemical deposition of nickel in the nanopores with their partial (∼50%) or complete filling. Studies of the morphology of the metal in the nanopores shows that the nickel-cluster structure is rather homogeneous and formed by crystallites ∼30–50 nm in size. The effects of deposition modes and structure morphology on current transport are analyzed with the use of test temperature dependences of the magnetoresistance. The reproducibility and stability of the magnetoresistance values for the case of homogeneous structure and pore filling with nickel make the Si/SiO2/Ni system promising for application as the base element for high-sensitivity low-temperature magnetic field sensors.

Effects of light localization in photoluminescence and Raman scattering in silicon nanostructures

The effect of light localization in photoluminescence (PL) and Raman scattering (RS) in silicon nanowires with diameter of 100 nm was investigated. The optical excitation was done by CW radiation of a YAG:Nd laser at 1.064 μm. The PL an RS intensities were found to increase strongly for the samples with Si nanowires in comparison with corresponding values of c-Si substrate. The effect is explained by an increase of the lifetime of photons in silicon nanowire structures.

Linear and Non-Linear Optical Imaging of Cancer Cells with Silicon Nanoparticles

New approaches for visualisation of silicon nanoparticles (SiNPs) in cancer cells are realised by means of the linear and nonlinear optics in vitro. Aqueous colloidal solutions of SiNPs with sizes of about 10–40 nm obtained by ultrasound grinding of silicon nanowires were introduced into breast cancer cells (MCF-7 cell line). Further, the time-varying nanoparticles enclosed in cell structures were visualised by high-resolution structured illumination microscopy (HR-SIM) and micro-Raman spectroscopy. Additionally, the nonlinear optical methods of two-photon excited fluorescence (TPEF) and coherent anti-Stokes Raman scattering (CARS) with infrared laser excitation were applied to study the localisation of SiNPs in cells. Advantages of the nonlinear methods, such as rapid imaging, which prevents cells from overheating and larger penetration depth compared to the single-photon excited HR-SIM, are discussed. The obtained results reveal new perspectives of the multimodal visualisation and precise detection of the uptake of biodegradable non-toxic SiNPs by cancer cells and they are discussed in view of future applications for the optical diagnostics of cancer tumours.

Dependence of the efficiency of Raman scattering in silicon nanowire arrays on the excitation wavelength

The features of Raman scattering in layers of silicon nanowires from 50 to 350 nm in diameter, obtained by the chemical etching of crystalline silicon (c-Si) wafers with preliminarily deposited silver nanoparticles in fluoric acid solutions are studied. c-Si wafers with various crystallographic orientations and doping levels are used, which is conditioned by the different sizes and degrees of ordering of the formed nanostructures. It is found that the radiation of the Raman scattering of samples is depolarized, and its efficiency depends strongly on the excitation wavelength. Upon excitation by light with a wavelength of 1064 nm, the ratio of Raman-scattering intensities of silicon nanowire samples and c-Si is 2 to 5; as the wavelength decreases, this ratio increases for structures with larger silicon-nanowire diameters and higher degrees of ordering and decreases for less ordered structures. The results obtained are explained by the effect of partial light localization in silicon nanowire arrays.

Optical Properties of Nanowire Structures Produced by the Metal-Assisted Chemical Etching of Lightly Doped Silicon Crystal Wafers

Layers of Si nanowires produced by the metal-assisted chemical etching of (100)-oriented single-crystal p-Si wafers with a resistivity of 1–20 Ω · cm are studied by reflectance spectroscopy, Raman spectros-copy, and photoluminescence measurements. The nanowire diameters are 20–200 nm. The wafers are supplied by three manufacturing companies and distinguished by their different lifetimes of photoexcited charge carriers. It is established that the Raman intensity for nanowires longer than 1 μm is 3–5 times higher than that for the substrates. The interband photoluminescence intensity of nanowires at the wavelength 1.12 μm is substantially higher than that of the substrates and reaches a maximum for samples with the longest bulk lifetime, suggesting a low nonradiative recombination rate at the nanowire surfaces.

Influence of surface pre-treatment on the electronic levels in silicon MaWCE nanowires

Deep level transient spectroscopy (DLTS) was performed on n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with gold as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible dependence of the level scheme on the processing temperature, DLTS measurements were performed on the nanowires grown on a non-treated Au/Si surface and on a thermally pre-treated Au/Si surface. A noticeable modification of the configuration of the energy levels was observed, induced by the annealing process. Based on our results on these MaWCE nanowires and on literature data about deep levels in bulk silicon, some hypotheses were advanced regarding the identification of the defects responsible of the energy levels revealed.

Electronic levels in silicon MaWCE nanowires: evidence of a limited diffusion of Ag

Deep level transient spectroscopy (DLTS) was performed on lowly n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with silver as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible diffusion of atoms into the bulk, DLTS investigation was also performed on the samples after removing the nanowires. Two of the four energy levels observed in the nanowires were also detected inside the substrate. Based on these results and on literature data about deep levels in bulk silicon, some hypotheses are advanced regarding the identification of the defects responsible for the energy levels revealed.