Influence of the doping level on the porosity of silicon nanowires prepared by metal-assisted chemical etching

A systematic method to control the porosity of silicon nanowires is presented. This method is based on metal-assisted chemical etching (MACE) and takes advantage of an HF/H2O2 etching solution and a silver catalyst in the form of a thin patterned film deposited on a doped silicon wafer. It is found that the porosity of the etched nanowires can be controlled by the doping level of the wafer. For low doping concentrations, the wires are primarily crystalline and surrounded by only a very thin layer of porous silicon (pSi) layer, while for highly doped silicon, they are porous in their entire volume. We performed a series of controlled experiments to conclude that there exists a well-defined critical doping concentration separating the crystalline and porous regimes. Furthermore, transmission electron microscopy investigations showed that the pSi has also a crystalline morphology on a length scale smaller than the pore size, determined from positron annihilation lifetime spectroscopy to be mesoscopic. Based on the experimental evidence, we devise a theoretical model of the pSi formation during MACE and apply it for better control of the nanowire morphology.

Enhanced dielectric properties of sol–gel-BaTiO3/P(VDF-HFP) composite films without surface functionalization

BaTiO₃–P(VDF-HFP) composite films without surface functionalization show a tenfold increase of the relative permittivity compared to the pure polymer.

Nanoscaled surface patterns influence adhesion and growth of human dermal fibroblasts

In general, there is a need for passivation of nanopatterned biomaterial surfaces if cells are intended to interact only with a feature of interest. For this reason self-assembled monolayers (SAM), varying in chain length, are used; they are highly effective in preventing protein adsorption or cell adhesion. In addition, a simple and cost-effective technique to design nanopatterns of various sizes and distances, the so-called nanosphere lithography (NSL), is discussed, which allows the control of cell adhesion and growth depending on the feature dimensions. Combining both techniques results in highly selective nanostructured surfaces, showing that single proteins selectively adsorb on activated nanopatterns. Additionally, adhesion and growth of normal human dermal fibroblasts (NHDF) is strongly affected by the nanostructure dimensions, and it is proven that fibronectin (FN) matrix formation of these cells is influenced, too. Moreover, the FN fibrils are linked to the hexagonally close-packed nanopatterns. As a result, the system presented here can be applied in tissue engineering and implant design due to the fact that the nanopattern dimensions give rise to further modifications and allow the introduction of chemical heterogeneity to guide stem cell differentiation in the future.

Ag-mediated charge transport during metal-assisted chemical etching of silicon nanowires

The charge transport mechanism during metal-assisted chemical etching of Si nanowires with contiguous metal films has been investigated. The experiments give a better insight how the charges and reaction products can penetrate to the etching front. The formation of a layer of porous Si between the metal film and the bulk Si is a prerequisite for the etching process. The electronic holes (positive charges) necessary for the etching of porous Si are generated at the surface of the metal in contact with the oxidative agent. Because of the insulating character of the thin walls of the porous Si, the transport of the electronic holes through this layer is not possible. Instead, it is found that the transport of electronic holes proceeds primarily by means of the Ag/Ag(+) redox pair circulating in the electrolyte and diffusing through the etched pores in the Si. The charge transport occurs without the ionic contribution at the positions where the metal is in direct contact with the Si. Here, an electropolishing process takes place, leading to an extensive removal of the Si and sinking in of the film into the Si substrate.

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Sub-20 nm Si/Ge superlattice nanowires by metal-assisted etching

An effective and low-cost method to fabricate hexagonally patterned, vertically aligned Si/Ge superlattice nanowires with diameters below 20 nm is presented. By combining the growth of Si/Ge superlattices by molecular beam epitaxy, prepatterning the substrate by anodic aluminum oxide masks, and finally metal-assisted chemical wet etching, this method generates highly ordered hexagonally patterned nanowires. This technique allows the fabrication of nanowires with a high area density of 10(10) wires/cm(2), including the control of their diameter and length.

Silicon nanocolumns on nanosphere lithography templated substrates: effects of sphere size and substrate temperature

Glancing angle ion beam sputter deposition was used to grow regular arrays of Si nanocolumns with a nominal height of 650 nm at room temperature on polystyrene nanospheres with sphere diameters between 260 nm and 3550 nm, and at elevated temperatures on SiO2 nanospheres with a sphere diameter of 360 nm. Top view and cross sectional scanning electron microscopy reveals that the Si nanocolumns resemble cylinder-like structures, terminated by a hemispherical cap. Diameter, height and inter-column-spacing are found to depend linearly on the nanosphere diameter, thus giving the possibility to grow arrays of vertical Si columns with distinct porosities. For the growth at elevated temperatures, it was found that while on non-patterned substrates diffusion effects lead to broadening and finally merging of initially separated nanocolumns, on nanosphere patterned substrates this broadening effect is only moderate. No merging of columns is observable in this case, but a decrease of the column height due to a temperature-driven inter-column densification.

Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy

Because of their importance in fundamental research and possible applications in nanotechnology and nanoelectronics, semiconductor nanowires have attracted much interest. In addition to the growth itself, the control of the size and location is an essential problem. Here we show the growth of ordered arrays of vertically aligned silicon nanowires by molecular beam epitaxy using prepatterned arrays of gold droplets on Si(111) substrates. The ordered arrays of gold particles were produced by nanosphere lithography.

Raman microscopic investigations of BaTiO3 precursors with core?shell structure

Due to their outstanding dielectric and ferroelectric properties, barium titanate (BaTiO(3))-based ceramics have found many applications in electronic devices. To optimise the final quality of such ceramics, a detailed knowledge of the complex processes involved in the formation of BaTiO(3) is required. The phase formation process in ordered structures of the BaCO(3)/TiO(2) system was analysed by X-ray diffraction and by Raman spectral imaging (RSI) as a function of the annealing temperature. RSI was used for the first time as a locally resolving method for phase analysis, and proved to be a useful tool in examining the formation process of BaTiO(3) starting from spherical, core-shell structured precursors of the type TiO(2) core/BaCO(3) shell. The Raman spectra of different BaO-TiO(2) phases appearing as intermediate phases during the formation of BaTiO(3) were recorded for separately-prepared pure substances. Using these spectra as fingerprints, and choosing phase filters by setting wave number windows, "phase landscape pictures" of the samples at different temperatures during the genesis of BaTiO(3) could be created with a lateral resolution of up to 200 nm. These pictures confirm shell-like formation of the different barium titanate phases according to the diffusion of barium and oxygen ions from the Ba-rich shell into the TiO(2) core. At an intermediate state of the phase formation process, the phase sequence Ba(2)TiO(4), BaTiO(3), BaTi(2)O(5), BaTi(4)O(9) and BaTi(5)O(11) to TiO(2) was detected from the outer to the inner parts of the core-shell structures.