V. Lysenko

Application of 3C-SiC quantum dots for living cell imaging

Highly luminescent, stable, and biocompatible 3C-SiC quantum dots (QDs) with no protective shells have been applied for fluorescence imaging of biological living cells. Structural and luminescent properties of the 3C-SiC QDs are described. Marking of the living cells with such QDs highlights the penetration, accumulation, and heterogeneous distribution of the QDs inside the intracellular space.

Measurement of porous silicon thermal conductivity by micro-Raman scattering

We present a noncontact and nondestructive method to measure thermal conductivity in layered materials using micro-Raman scattering. This method was successfully applied to monocrystalline silicon whose thermal conductivity was found to be 63 W/m K at about 550 °C and then applied to porous silicon layers. For a 50 μm thick layer with 50% porosity, we found a thermal conductivity of 1 W/m K confirming the thermal insulating properties of this material.

Thermal conductivity of thick meso-porous silicon layers by micro-Raman scattering

We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-porous silicon layers. The model is in good agreement with experimental measurements performed by micro-Raman scattering on the layers surface. For the first time, thermal conductivity inhomogeneity along the porous layer thickness of 100 μm is studied. Direct correlation between the thermal conductivity and morphology variations along the layer thickness is brought to the fore. A new approach to estimate local porosity of the porous layers based on thermal conductivity and Si nanocrystallite size measurements is also proposed.

Influence of the interfacial chemical environment on the luminescence of 3CSiC nanoparticles

Surface chemistry of as-prepared 3CSiC nanoparticles obtained by electrochemical etching of bulk 3CSiC substrates was studied. Chemical environment was found to influence strongly the photoinduced electronic transitions in the 3CSiC nanoparticles. The influence of different interfacial chemical environments of the 3CSiC nanoparticles, such as surface chemistry, solvent nature, and surface charges on the photoinduced absorption and luminescence of the nanoparticles at room temperature, is described and discussed in detail. For example, oxidation induced passivation of the radiative band gap states allows visualization of the transitions between energy levels in the nanoparticles in which photogenerated charge carriers are quantumly confined. Electrostatic screening of the radiative band gap states by highly polar solvent media leads to a blueshift and a decrease in the width at half maximum of the photoluminescence spectra of the nanoparticles. As for the surface charges, they govern band bending slope...

Bragg surface wave device based on porous silicon and its application for sensing

Results concerning a Bragg surface wave device based on porous silicon and intended for sensing application are reported. Existence of optical surface waves on Bragg structures is experimentally shown. Such device is expected to be very sensitive to the grafting of biological molecules. The authors demonstrate this sensing effect by grafting of amine chemical groups. The optical characterization using m-line spectroscopy shows that the increase of the coupling angle is about 20° after the amine grafting. The authors show that porosity is essential for reaching this high sensitivity.Results concerning a Bragg surface wave device based on porous silicon and intended for sensing application are reported. Existence of optical surface waves on Bragg structures is experimentally shown. Such device is expected to be very sensitive to the grafting of biological molecules. The authors demonstrate this sensing effect by grafting of amine chemical groups. The optical characterization using m-line spectroscopy shows that the increase of the coupling angle is about 20° after the amine grafting. The authors show that porosity is essential for reaching this high sensitivity.

On mechanical properties of nanostructured meso-porous silicon

Mechanical properties of meso-porous silicon are studied using topographic measurements and finite element simulations. Our approach is based on an original analysis of the strain at the free surface of porous silicon tub embedded in bulk Si regions allowing the determination of the Young’s modulus of the porous layers. In particular, the internal stress in the porous Si region is evaluated from the corresponding deformation of the monocrystalline Si adjacent region which mechanical parameters are well known. Moreover, a mechanical anisotropy of the columnar nanostructured porous Si is brought to the fore from the characteristic shape of the strained porous layer profile. Moderately oxidized, 70% in porosity, porous silicon patterns were investigated. Correlation of our measurements with x-ray data reported early in literature shows the macroscopic strain being close to the silicon lattice relative increase revealing an elastic deformation regime. The porous layers exhibit an unexpected low and strongly a...

Thick oxidised porous silicon layers for the design of a biomedical thermal conductivity microsensor

Abstract Porous silicon (PS) offers new possibilities to be applied as thermal insulating material for microsensor design due to its low thermal conductivity (TC) value compared with TC of SiO2. A biomedical TC microsensor based on differential thermoelectric measurements has been designed using a PS substrate. In order to ensure an efficient thermal isolation in the microsensor, main thermal and geometrical characteristics of the PS layers as well as of the whole microsensor have been numerically simulated. PS layers with low TC have to be thick and mechanically stable under further processing. To form thick (50–200 μm) and stable PS layers, a new approach based on progressive changing of anodisation current density (from 100 to 25 mA/cm2) during PS formation has been elaborated. To find a suitable compromise between low TC and mechanical stability of thick PS layers, an adapted thermal oxidation recipe at moderate temperatures (500–600°C) in dry oxygen atmosphere has been applied. It leads to 20–50% oxidation fraction in PS layers (measured by Energy Dispersive Spectroscopy) corresponding to SiO2 TC value. A test device has been realised and characterised. A Seebeck coefficient of 400 μV/°C per junction has been measured for a Poly-Si/Al thermopile deposited on the PS layer.

Technology and micro-Raman characterization of thick meso-porous silicon layers for thermal effect microsystems

Thermal effect microsystems (TEMS) need a highly thermally insulated substrate. Porous silicon (PS) offers promising applications for insulation of thermal transducers from silicon wafers as its thermal conductivity is close to that of silicon oxide. A thorough investigation of PS thermal conductivity has been carried out regarding its technological parameters, i.e., porosity, thickness and oxidation temperature, by means of micro-Raman spectroscopy which yielded thermal conductivity values less than 2 W/m K as predicted by theoretical considerations. For TEMS, a 100-μm-thick meso-PS layer with a porosity of about 50% and oxidized at a moderate temperature (300°C) presents the best attributes to ensure both an efficient thermal insulation, as its thermal conductivity value was found to be 0.6 W/m K, and a good mechanical strength.

Photoluminescence of 6H–SiC nanostructures fabricated by electrochemical etching

Photoluminescence (PL) spectra of 6H–SiC nanostructures (nanoporous layers and nanopowder) fabricated by electrochemical etching of bulk wafers consist of broad subgap emission bands as well as above-gap tails. These features are explained, respectively, in terms of radiative recombinations via N–Al donor-acceptor electronic levels and surface states as well as quantum confinement phenomenon in small nanocrystallites. An excitation power dependent PL study allowed differentiation of the radiative channels mentioned above. The particular role of surface states on the room temperature PL of the SiC nanopowder is highlighted. A concentration dependent PL study on aqueous suspensions of the nanopowder points out the quenching of PL emission coming from recombination of quantum-confined excitons in small SiC nanoparticles interacting with the larger ones.

Amorphization and reduction of thermal conductivity in porous silicon by irradiation with swift heavy ions

In this article, we demonstrate that the thermal conductivity of nanostructured porous silicon is reduced by amorphization and also that this amorphous phase in porous silicon can be created by swift (high-energy) heavy ion irradiation. Porous silicon samples with 41%-75% porosity are irradiated with 110 MeV uranium ions at six different fluences. Structural characterisation by micro-Raman spectroscopy and SEM imaging show that swift heavy ion irradiation causes the creation of an amorphous phase in porous Si but without suppressing its porous structure. We demonstrate that the amorphization of porous silicon is caused by electronic-regime interactions, which is the first time such an effect is obtained in crystalline silicon with single-ion species. Furthermore, the impact on the thermal conductivity of porous silicon is studied by micro-Raman spectroscopy and scanning thermal microscopy. The creation of an amorphous phase in porous silicon leads to a reduction of its thermal conductivity, up to a factor of 3 compared to the non-irradiated sample. Therefore, this technique could be used to enhance the thermal insulation properties of porous Si. Finally, we show that this treatment can be combined with pre-oxidation at 300 °C, which is known to lower the thermal conductivity of porous Si, in order to obtain an even greater reduction.