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: A review of the physical properties of group 14 clathrate-II materials with emphasis placed on structure-property relationships, in particular with regard to the electronic and transport properties is given in this article.
Abstract: Crystalline open-framework and nanoporous materials have long attracted the attention of chemists, physicists, and materials scientists. The intriguing structures such materials exhibit are often intimately related to the unique physical properties they possess. This article reviews recent developments in the preparation and characterization of one such class of inorganic materials, those comprised of group 14 elements crystallizing with the clathrate-II crystal structure. These materials correspond to expanded forms and, in some cases, metastable allotropes of Si, Ge, and Sn. Just as interesting as the structure and properties that group 14 clathrate-II materials display is the diverse range of synthetic techniques used to prepare them and these techniques are discussed. We review the work to date characterizing the physical properties of group 14 clathrate-II materials with emphasis placed on structure–property relationships, in particular with regard to the electronic and transport properties.
164 citations
TL;DR: The main aims is to identify and describe key design features needed for nanoscale vehicles to achieve effective delivery of siRNA‐mediated gene silencing agents in vivo.
Abstract: With the recent FDA approval of the first siRNA-derived therapeutic, RNA interference (RNAi)-mediated gene therapy is undergoing a transition from research to the clinical space. The primary obstacle to realization of RNAi therapy has been the delivery of oligonucleotide payloads. Therefore, the main aims is to identify and describe key design features needed for nanoscale vehicles to achieve effective delivery of siRNA-mediated gene silencing agents in vivo. The problem is broken into three elements: 1) protection of siRNA from degradation and clearance; 2) selective homing to target cell types; and 3) cytoplasmic release of the siRNA payload by escaping or bypassing endocytic uptake. The in vitro and in vivo gene silencing efficiency values that have been reported in publications over the past decade are quantitatively summarized by material type (lipid, polymer, metal, mesoporous silica, and porous silicon), and the overall trends in research publication and in clinical translation are discussed to reflect on the direction of the RNAi therapeutics field.
163 citations
TL;DR: A tetragonal allotrope (12 atoms/cell, named T12) commonly found in C, Si, and Ge is computationally discovered through a particle swarm structural search, which results in favorable thermodynamic conditions compared with most other experimentally or theoretically known sp(3) species of group 14 elements.
Abstract: Group 14 elements (C, Si, and Ge) exist as various stable and metastable allotropes, some of which have been widely applied in industry. The discovery of new allotropes of these elements has long attracted considerable attention; however, the search is far from complete. Here we computationally discovered a tetragonal allotrope (12 atoms/cell, named T12) commonly found in C, Si, and Ge through a particle swarm structural search. The T12 structure employs sp3 bonding and contains extended helical six-membered rings interconnected by pairs of five- and seven-membered rings. This arrangement results in favorable thermodynamic conditions compared with most other experimentally or theoretically known sp3 species of group 14 elements. The T12 polymorph naturally accounts for the experimental d spacings and Raman spectra of synthesized metastable Ge and Si-XIII phases with long-puzzling unknown structures, respectively. We rationalized an alternative experimental route for the synthesis of the T12 phase via deco...
162 citations
TL;DR: In this article, a photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless etching of p−-Si in a 1:2:1 solution of HF, H2O2, and methanol.
Abstract: Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless etching of p−-Si (100) in a 1:2:1 solution of HF, H2O2, and methanol. The peak emission wavelength of the PSi could be tuned in the range 500 nm⩽λ⩽600 nm simply by changing the time of etching. The luminescence is sufficiently intense at all wavelengths to be visible by eye. Furthermore, by patterning the metal areas on the surface prior to etching, the luminescence can be controlled spatially. To investigate the relationship among processing variables — principally etch time and spatial proximity to Pt — and morphology, scanning electron microscopy (SEM), true color fluorescence microscopy, and spatially resolved phonon line shape studies were undertaken. SEM images show nanocrystalline features in the region where the luminescence originates, a region which shifts spatially as a function of etch time, as indicated by fluorescence microscopy. Raman scattering measurements of the shift and broadening of the longitudinal optical ...
161 citations
TL;DR: In this article, the authors demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon, where the key to this achievement is a considerable reduction of nonradiative carrier recombination due to impurities and core defects by impurity gettering and hydrogen passivation, respectively.
Abstract: We demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon. The key to this achievement is a considerable reduction of nonradiative carrier recombination at dislocations due to impurities and core defects by impurity gettering and hydrogen passivation, respectively, which is shown by means of deep-level transient spectroscopy. Time-resolved EL measurements reveal a response time below 1.8 μs, which is much faster, compared to the band-to-band luminescence of bulk silicon.
161 citations
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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