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 article, a modified Penn model, taking into account the quantum confinement induced discrete energy states, was applied to a sphere and to a wire, and the calculated size dependent e is consistent with the wave-vector-dependent e(q).
Abstract: As the physical size approaches several nanometers, reduction in the static dielectric constant e becomes significant. A modified Penn model, taking into account the quantum confinement induced discrete energy states, was applied to a sphere and to a wire. The calculated size dependent e is consistent with the wave-vector-dependent e(q). However, this form of e is more amenable for calculations of donor and exciton binding energies in a finite quantum confined nanoparticle when a full electrostatic boundary value problem must be tackled. The results of our model compare favorably with other, far more sophisticated, calculations.
112 citations
TL;DR: In this paper, the first "naked" silicon clusters larger than four atoms, Si9, were discovered and structurally characterized in the compound Rb12Si17 by using a metathesis reaction (comproportionation) between negatively charged silicon or germanium in the solids KSi or NaGe, respectively, and the corresponding positively charged ions in the tetrachlorides.
Abstract: We have discovered and structurally characterized the first “naked” silicon clusters larger than four atoms, Si9, in the compound Rb12Si17. Silicon is of unparalleled importance for many electronic applications, and the research associated with it spans over many different areas of science: chemistry, physics, surface science, materials processing, etc. In addition to the extensively studied silicon clusters in the gas phase,1 much interest has been focused lately on silicon nanoparticles and porous silicon due to their valuable optical properties.2 Recently, the first synthesis of such Si and Ge nanoparticles in liquid medium was reported.3 This approach utilizes a metathesis reaction (comproportionation) between the negatively charged silicon or germanium in the solids KSi or NaGe, respectively, and the corresponding positively charged ions in the tetrachlorides. Perhaps a better approach to the synthesis of such nanoparticles is to use premade silicon clusters in the solid state, extract them intact into a solution, and then use a metathesis reaction. Unfortunately, the only known such species, Si4 tetrahedra in A4Si4 (A ) alkali metal)4 and some lithium-stabilized planar silicon formations,5 cannot be extracted, apparently due to the large charge per atom ratio.6 The same is true for the tetrahedra of the other tetrels (tetrel ) a group 14 element), Ge4, Sn4, and Pb4. On the other hand, nine-atom clusters of germanium, tin, and lead have been identified in solutions made by dissolving the corresponding alkali-metal tetrelides in ethylenediamine or liquid ammonia.7 Such clusters with different shapes and charges have also been crystallized from the corresponding solutions and have been structurally characterized.6 It should be pointed out that in all reports the tetrelide precursors for such solutions have been labeled either “alloys” or “melts”, i.e., substances with no particular structure and without defined cluster formations. Hence, the current understanding on the formations of the clusters in solution is that they do not exist in the precursors but are rather assembled somehow during the process of dissolution.6 However, our recent studies of the systems alkali-metal-tetrel show that clusters of Ge9 and Pb9 exist in the compounds Cs4Ge9 and K4Pb9, respectively.8 Similarly, recent Raman studies of these and other alkali-metal-tetrel systems also suggest the existence of such clusters.9 The same clusters most likely existed in the precursors for the solution studies. The new compound, Rb12Si17, contains isolated nine-atom silicon clusters, Si9, and is a potential candidate for a precursor for silicon clusters in solution. The compound is made at 900 °C (kept for 1 h at that temperature and then slowly cooled with a rate of 5°/h) by direct synthesis from the pure elements sealed in niobium containers and jacketed in evacuated ampules of fused-silica. The same approach also yields the isostructural K12Sn17 (the phase which most likely “produces” the Sn9 in solution) and (KxRb1-x)12Si17. Furthermore, phases with the same or very similar structures and stoichiometries seem to exist in the systems Cs-Si, Cs-Sn, RbSn, Cs-Pb, and Rb-Pb, according to their X-ray diffraction powder patterns.
112 citations
TL;DR: In this paper, the use of silicon as an anode for Li-ion batteries is reviewed, where factors such as film thickness, doping, alloying, and their response to reversible lithiation processes are summarized and discussed with respect to battery cell performance.
Abstract: This review outlines the developments and recent progress in metal-assisted chemical etching of silicon, summarizing a variety of fundamental and innovative processes and etching methods that form a wide range of nanoscale silicon structures. The use of silicon as an anode for Li-ion batteries is also reviewed, where factors such as film thickness, doping, alloying, and their response to reversible lithiation processes are summarized and discussed with respect to battery cell performance. Recent advances in improving the performance of silicon-based anodes in Li-ion batteries are also discussed. The use of a variety of nanostructured silicon structures formed by many different methods as Li-ion battery anodes is outlined, focusing in particular on the influence of mass loading, core-shell structure, conductive additives, and other parameters. The influence of porosity, dopant type, and doping level on the electrochemical response and cell performance of the silicon anodes are detailed based on recent findings. Perspectives on the future of silicon and related materials, and their compositional and structural modifications for energy storage via several electrochemical mechanisms, are also provided.
112 citations
Cites background or methods from "Silicon quantum wire array fabricat..."
...The discovery of light-emitting nanoporous Si[8, 22, 23] propelled investigations of pore formation in III−V[24-30] and other group IV semiconductors[31, 32] and II-VI materials [33, 34]....
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...The controlled formation of porosity and roughness in Si[6-8] has attracted substantial attention, and nanoscale Si in the form of nanocrystals[9], NWs, and mesoporous analogues have been successfully applied[10] to LIBs[11], photovoltaics[12], sensing[13] and optoelectronics....
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TL;DR: In this article, electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon, and the presence of oxygen-related centers (nonbridging oxygen-hole center clusters), which consist of similar core structures in as-made and oxidised porous silicon (PSi) samples, has been found.
Abstract: Electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon. Results indicate the presence of oxygen‐related centers (nonbridging oxygen‐hole center clusters), which consist of similar core structures in as‐made and oxidized porous silicon (PSi) samples. A direct correlation exists between the presence of these centers and a red photoluminescence observed in both freshly anodized and oxidized PSi, suggesting that this emission process is the result of optical transitions in the oxygen‐hole centers.
111 citations
TL;DR: The analysis of the nanoindentation data, including the specific problem linked with porous materials, is presented in this paper, showing that the Young's modulus values obtained appear to be drastically dependent on the porosity and on the doping level.
Abstract: Young’s modulus of porous silicon samples, with porosity ranging from 36% to 90%, is measured by the nanoindentation technique. The analysis of the nanoindentation data, including the specific problem linked with porous materials, is presented. The Young’s modulus values Ep thus obtained appear to be drastically dependent on the porosity and on the doping level (p or p+ type). The dependence of Ep versus the relative density (for a series of p+ type samples) is quadratic, in good agreement with the model of Gibson and Ashby developed for cellular materials. This also shows that highly porous silicon layers exhibit very low Young’s modulus (for a porosity of 90% it is about two orders of magnitude smaller than that of the nonporous material).
111 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