Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers
TL;DR: In this paper, free standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography using electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers.
Abstract: Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under <1 mW unfocused (<0.1 W cm−2) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.
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TL;DR: In this paper, a hexagonal two-dimensional photonic crystal (PhC) microcavities with Ge self-assembled quantum dots were fabricated on silicon-on-insulator substrates.
Abstract: Freestanding hexagonal two-dimensional photonic crystal (PhC) microcavities with Ge self-assembled quantum dots were fabricated on silicon-on-insulator substrates. Strong photoluminescence associated with Ge quantum dots was observed in the wavelength region of 1.2–1.6μm at room temperature. Sharp peaks dominated the spectrum, showing strong optical resonance inside the cavity. A resonant peak with a quality factor of 560 was observed at 1.58μm along with a significant enhancement of the luminescence in the microphotoluminescence spectrum. The peaks were also observed to reasonably shift when the structural parameter of PhC was changed.
82 citations
TL;DR: In this paper, a detailed study of photoluminescence excitation spectra in a wide range of excitation photon energies (270-420nm) reveals specific behavior of the Stokes shift of the fast ultraviolet-blue photoluminance (PL) band that agrees well with theoretical calculation of optical transitions in small silicon nanocrystals.
Abstract: Colloidal suspensions of small silicon nanoparticles (diameter around 2nm) with fast and efficient ultraviolet-blue photoluminescence (PL) band are fabricated by enhanced electrochemical etching of Si wafers. The detailed study of photoluminescence excitation spectra in a wide range of excitation photon energies (270-420nm) reveals specific behavior of the Stokes shift of the fast PL band that agrees well with theoretical calculation of optical transitions in small silicon nanocrystals and is distinct from emission of silicon dioxide defects.
82 citations
TL;DR: Porous silicon (PSi) films displaying both photoluminescence (PL) and interference fringes were etched, stabilized by anodic oxidation, and mounted in a specially designed chamber connected to the gas flow system of an electronic artificial nose (FOX 3000 from Alpha MOS) as mentioned in this paper.
Abstract: Porous silicon (PSi) films displaying both photoluminescence (PL) and interference fringes were etched, stabilized by anodic oxidation, and mounted in a specially designed chamber connected to the gas flow system of an electronic artificial nose (FOX 3000 from Alpha MOS) The changes in reflectivity and PL spectra of experimental PSi chips were recorded during the injection of analyte, and compared to the response of commercial chemiresistive metal oxide sensors A series of solvent vapors, ethyl esters, and perfumes were investigated and discrimination indices (DI) obtained with PSi sensors have been found to be as good as those obtained with metal oxide sensors The PL and reflectivity signals from PSi chips are reversible and reproducible Moreover, the recovery to baseline for the PSi chips takes 30 s while the metal oxide sensors under similar conditions require 15 min
82 citations
TL;DR: High-aspectratioTiNTs are used to enhance the detection level of fluorescence-labeled proteins in an immunoassay concept to lowering the detection threshold, ultimately reaching singlecell and single-molecule diagnostics and reducing the probe volume to aminimum.
Abstract: Since their discovery by Iijima in 1991, carbon nanotubes have attracted tremendous scientific and technological interest due to a combination of unique inherent properties and high expectations regarding their applications. Apart from carbon, over the past fewyears a number of ‘‘inorganic’’ nanotubular or nanoporous systems have also been reported that can be grown by a self-organizing electrochemical anodization process on various metallic or semiconductor substrates. A particular advantage is that distinct tubular features can be formed in regular arrays perpendicular to the substrate surface. This arrangement makes such structures ideal for use as nano test tubes for capturing, concentrating, releasing load, or probing formolecules, and such features have been reported for silica and alumina. Herein, we show how to use titania-nanotube (TiNT) arrays as a small-volume, high-sensitivity immunoassay detection system. Due to the unique photocatalytic properties of TiO2 these arrays have self-cleaning features, which makes themmost attractive for reusable devices. TiNTarrays canbe tuned in geometry (diameter, aspect ratio), ‘‘crystal’’ structure (amorphous, anatase, rutile), electronic and biomedical properties, and even freestanding membranes can be fabricated, and therefore a very versatile nanoscale architecture can be built. Up to now an unexploited path is the direct use of such nanotube arrays as reusable immunoassay platforms. In general, one of the key goals in immunoassay research is lowering the detection threshold, ultimately reaching singlecell and single-molecule diagnostics and reducing the probe volume to aminimum. In the present work, we use high-aspectratioTiNTs (whichprovidea longobservation length combined with a small volume) to enhance the detection level of fluorescence-labeled proteins in an immunoassay concept. Self-organized TiNT arrays were grown by anodization of Ti foils in ethylene glycol electrolytes containing NH4F, as described in the Experimental Section. The resulting TiNT layers are shown in the scanning electron microscopy (SEM)
82 citations
TL;DR: This work examined the optical properties, optical absorption and photoluminescence of hydrogels that were self-assembled from Fmoc-FF building blocks, and followed the formation of a quantum confined structure within the hydrogel nanotubes.
Abstract: Adv. Mater. 2010, 22, 2311–2315 2010 WILEY-VCH Verlag G T IO N Hydrogels can be composed of natural or synthetic polymers. They form a three-dimensional (3D) scaffold that can absorb a large quantity of water (>99% by volume). They can mimic the extracellular matrix, having good biocompatible and biodegradable qualities, which enables them to support the growth of cultured cells. Among the various polymers for forming hydrogels, short peptides are an important group. It has been shown that several peptides can undergo gelation and form the 3D structure of a hydrogel, such as peptides that influence the selective differentiation of neural cells, short alternating charged amino acid peptides, and peptide clusters from the nuclear pore complex. Moreover, the addition of the protective group N-fluorenylmethoxycarbonyl (Fmoc) to the short peptides may improve the hydrogel formation process. Naturally self-assembled nanostructures of protein fibrils are associated with neurodegenerative conditions such as Alzheimer’s disease, where the fibril structure is made of amyloid-b (Ab) peptides. It has been found that the minimal core recognition motif of Ab peptides is a diphenylalanine element. The chemically synthesized dipeptide NH2Phe-Phe-COOH (FF) can self-assemble into well-ordered peptide nanotubes (PNTs). By using the FFmotif connected to an Fmoc moiety (Fmoc-FF), we have shown the formation of a peptidebased hydrogel made from a PNT network. In the work presented here, we have examined the optical properties, optical absorption and photoluminescence (PL), of hydrogels that were self-assembled from Fmoc-FF building blocks. From the optical properties we were able to follow the formation of a quantum confined structure within the hydrogel nanotubes. The most common example of quantum confinement (QC) is a 2D-QC—also called a quantum well (QW)—system of GaAs. This system contains various structures, the most noticeable of which is GaAs/AlGaAs, along with GaAs/InGaAs and InGaN/GaN. These structures have a double heterostructure consisting of a thin layer of GaAs ca. 10 nm thick, whose bandgap is smaller than that of the surrounding AlGaAs bulk. Another QC system, which was first revealed by Canham in 1990, is found in mesoporous silicon layers. Canham showed that by increasing the pore size, hence decreasing the size of the bulk silicon skeleton between the pores, a QC effect can occur. In order to observe the QC effects, the bulk silicon dimensions need to be less than the dimensions of the free exciton Bohr radius of bulk silicon of ca. 50 Å. Recently QC has been demonstrated in inorganic nanotubes, such as nanowires of ZnO or InGaN/ GaN multiple QWs. QW structures can produce remarkable changes in the optical and electrical properties of semiconductor structures; hence QW structures are most commonly characterized by spectroscopy measurements. We have very recently demonstrated the QC phenomenon in several peptide nanostructures. We showed a QW structure within PNTs, which were formed by vapor deposition of the FF building blocks. Furthermore, we showed a quantum dot (QD) structure within self-assembled peptide spheres, made from an analogue of the FF building block. By understanding the phenomenon, we were able to demonstrate a unique blue luminescence from the bioinspired PNTs. The hydrogel’s self-assembled PNT network is composed of Fmoc-FF building blocks (Fig. 1A) with a final diameter of 7–15 nm (Figs. 1B,C). When the environment of the solution is not suitable for their self-assembly, the Fmoc-FF molecules form aggregates in the solution. The optical absorption of the hydrogel and the aggregates is presented in Figure 2. In Figure 2A in the curves of the high-concentration samples, characterized by PNT network formation, pronounced step-like behavior of the absorption spectra is observed. The spectrum indicates a major step in the UV region with a peak at its ‘‘red’’ edge with wavelength
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TL;DR: In this article, the properties of electrolyte-semiconductor barriers are described, with emphasis on germanium, and the use of these barriers in localizing electrolytic etching is discussed.
Abstract: Properties of electrolyte-semiconductor barriers are described, with emphasis on germanium. The use of these barriers in localizing electrolytic etching is discussed. Other localization techniques are mentioned. Electrolytes for etching germanium and silicon are given.
1,039 citations
TL;DR: It is found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive, which has implications for the ultimate efficiency of silicon solar cells.
Abstract: We have found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive. With preparation in this manner, the surface-recombination velocity on Si111g is only 0.25 cm/sec, which is the lowest value ever reported for any semiconductor. Multiple-internal-reflection infrared spectroscopy shows that the surface appears to be covered by covalent Si-H bonds, leaving virtually no surface dangling bonds to act as recombinatiuon centers. These results have implications for the ultimate efficiency of silicon solar cells.
910 citations
TL;DR: In this paper, multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces, and these very inert surfaces are found to be almost completely covered by atomic hydrogen.
Abstract: Multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces. These very inert surfaces are found to be almost completely covered by atomic hydrogen. Results using polarized radiation on both flat and stepped Si(111) and Si(100) surfaces reveal the presence of many chemisorption sites (hydrides) that indicate that the surfaces are microscopically rough, although locally ordered. In particular, the HF‐prepared Si(100) surface appears to have little in common with the smooth H‐saturated Si(100) surface prepared in ultrahigh vacuum.
588 citations
TL;DR: In this article, the authors measured hydrogen desorption from monohydride and dihydride species on crystalline-silicon surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy.
Abstract: Hydrogen desorption kinetics from monohydride and dihydride species on crystalline-silicon surfaces were measured using transmission Fourier-transform infrared (FTIR) spectroscopy. The FTIR desorption measurements were performed in situ in an ultrahigh-vacuum chamber using high-surface-area porous-silicon samples. The kinetics for hydrogen desorption from the monohydride and dihydride species was monitored using the SiH stretch mode at 2102 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the ${\mathrm{SiH}}_{2}$ scissors mode at 910 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Annealing studies revealed that hydrogen from the ${\mathrm{SiH}}_{2}$ species desorbed between 640 and 700 K, whereas hydrogen from the SiH species desorbed between 720 and 800 K. Isothermal studies revealed second-order hydrogen desorption kinetics for both the monohydride and dihydride surface species. Desorption activation barriers of 65 kcal/mol (2.82 eV) and 43 kcal/mol (1.86 eV) were measured for the monohydride and dihydride species, respectively. These desorption activation barriers yield upper limits of 84.6 kcal/mol (3.67 eV) and 73.6 kcal/mol (3.19 eV) for the Si-H chemical bond energies of the SiH and ${\mathrm{SiH}}_{2}$ surface species.
479 citations