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Composite electrodes composed of silicon nanowires synthesized using the supercritical fluid-liquid-solid (SFLS) method mixed with amorphous carbon or carbon nanotubes were evaluated as Li-ion battery anodes. Carbon coating of the silicon nanowires using the pyrolysis of sugar was found to be crucial for making good electronic contact to the material. Using multiwalled carbon nanotubes as the conducting additive was found to be more effective for obtaining good cycling behavior than using amorphous carbon. Reversible capacities of 1500 mAh/g were observed for 30 cycles.
ACS Nano 2010 Mar 23
PMID:Solution-grown silicon nanowires for lithium-ion battery anodes. 2020 47

Silicon one-dimensional (Si 1D) materials are of particular relevance due to their prospect as versatile building materials for nanoelectronic devices. We report the growth of Si 1D structures from quasi-hexagonally ordered gold (Au) nanoparticle (NP) arrays on borosilicate glass (BSG) and SiOx/Si substrates. Using hydrogen instead of oxygen plasma during NP preparation enhances the catalytic activity of AuNPs (diameters of 10-20 nm), enabling Si 1D growth at temperatures as low as 320 degrees C. On BSG, Si nanowires (SiNWs) are identified and reasonable vertical alignment is achieved at 420 degrees C. On SiOx/Si, only Si nanotubes (SiNTs) are obtained right up to 420 degrees C. A mixture of SiNTs and SiNWs is observed at 450 degrees C and only SiNWs grow at 480 degrees C.
ACS Nano 2010 Apr 27
PMID:Low-temperature growth of silicon nanotubes and nanowires on amorphous substrates. 2021 67

Carbon nanotubes show great potential in developing solar cells with enhanced power conversion efficiency, yet the device stability has not been thoroughly studied. Here, we show how the interaction between components in a nanotube-based hybrid solar cell could cause a significant change in output voltage and fill factor, resulting in photoinduced degradation in device performance. We functionalized carbon nanotubes with CdS nanoparticles to make hybrid films and deposited these films onto silicon substrates to construct solar cells with efficiencies up to 1.4%. The I-V characteristics show reversible change in response to light illumination, suggesting potential applications as visible-light sensors. The fill factor and open-circuit voltage gradually decrease under continuous illumination, inversely proportional to the incident light energy within a considerable range up to 60 J. The unique photoresponse is attributed to a charge-transfer process between nanotubes and nanoparticles under excitation and to change in series resistance in the solar cells.
ACS Nano 2010 Apr 27
PMID:Solar cells and light sensors based on nanoparticle-grafted carbon nanotube films. 2022 75

We demonstrate here two-terminal, charge-based memory from C60 films inside vertical 7 nm silicon nanogap devices. This testbed structure eliminated the possibility of metal migration in the nanostructure because the two electrodes are made solely of silicon; hence, the often troublesome and confusing possibility of filamentary metal formation is obviated. Saturated solutions of C60 in toluene, mesitylene, and 1-methylnaphthalene were each used to deposit these films at elevated temperatures. Electrical I-V measurements reveal a high yield (67%) of devices demonstrating bipolar, switchable hysteresis from both the mesitylene- and 1-methylnaphthalene-deposited devices, while the toluene-grafted devices display no such behavior. Pulse-based memory measurements of switching devices indicate high ON/OFF ratios (maximum approximately 1500), good stability (>100 cycles without device degradation) for molecular devices, and low operating currents (approximately 10(-11) A) in room temperature testing.
ACS Nano 2010 Apr 27
PMID:Two-terminal molecular memories from solution-deposited C60 films in vertical silicon nanogaps. 2023 44

We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), Delta, and two inequivalent sub-band ladders, Delta(2) and Delta(4), form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between Delta(2) and Delta(4) and their shift with respect to the bulk value directly from the 2p(3/2)-->Delta transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110).
ACS Nano 2010 Apr 27
PMID:Quantum confinement, surface roughness, and the conduction band structure of ultrathin silicon membranes. 2030 37

Medicinal surface modification of silicon nanowires (SiNWs) with selected bisphosphonates, such as the known antiosteoporotic drug alendronate, is described. In terms of specific assays relevant to orthopedic applications, the impact of selected bisphosphonate attachment on acellular calcification in simulated plasma is reported. To further investigate biocompatibility, proliferation assays of these modified nanowires were carried out using an orthopedically relevant cell line: mesenchymal stem cells derived from mouse stroma. It is found that the identity of the bisphosphonate ligand strongly and sensitively impacts its resultant cytotoxicity.
ACS Appl Mater Interfaces 2009 Feb
PMID:Medicinal surface modification of silicon nanowires: impact on calcification and stromal cell proliferation. 2030 99

New fluoropolyurethane hybrids containing fluorinated polyhedral oligomeric silsesquioxane were synthesized for thin film applications using fluoro(13) disilanol isobutyl-POSS (FluoroPOSS) and a short chain fluorodiol and diisocyanate. The kinetics of the urethane reaction was monitored using Fourier transform infrared spectroscopy (FTIR) and the formation of urethane was confirmed using (29)Si Nuclear magnetic resonance spectroscopy (NMR). The effect of addition of FluoroPOSS either in the I step or II step of the two step polymerization reaction is evaluated using various spectroscopic, thermal, microscopic, and diffraction techniques. In general, the major shortcoming of the lack of flexibility of fluoropolyurethane from short chain diol and diisocyanate has been overcome by the use of tethered FluoroPOSS. X-ray photoelectron spectroscopy (XPS), atomic force microscpopy (AFM), and contact angle measurements on the hybrid thin films on silicon wafer demonstrate the migration of FluoroPOSS segment to the air-thin film interface when FluoroPOSS is used in I stage reaction, and it resides at the interface when used as a chain extender. However, in both cases, the formed thin film exhibits ultrahydrophobicity with water contact angle of approximately 107 degrees and low contact angle hysteresis and solvent resistance, which are preferable for protective thin film applications.
ACS Appl Mater Interfaces 2009 Feb
PMID:Fluoro-silsesquioxane-urethane hybrid for thin film applications. 2035 21

In this report, we studied the interactions between biological cells and vertically aligned silicon nanowire (SiNW) arrays and focused on how SiNW arrays affected cellular behaviors such as cell adhesion and spreading. We observed that SiNW arrays could support cell adhesion and growth and guide cell adhesion and spreading behaviors. The results also showed that SiNW arrays could not only enhance the cell-substrate adhesion force but also restrict cell spreading. Combining the results from scanning electron microscopy images of cell morphology and the expression analysis of genes and proteins related to cell adhesion and spreading process, we proposed a mechanism on how cell adhesion and spreading was controlled by arrayed SiNWs. The effects of SiNW arrays in guiding cell adhesion and spreading behavior might be useful in the development of cell microarrays, tissue engineering scaffolds, and molecule delivery vehicles in which strong cell-substrate adhesion and reduced cell-cell communication were beneficial.
ACS Appl Mater Interfaces 2009 Jan
PMID:Cell adhesion and spreading behavior on vertically aligned silicon nanowire arrays. 2035 48

Uniform high-quality iron oxide thin films can be formed from the spin coating of iron oxide/hydroxide sol-gels on a silicon substrate. Thermal processing of the films at temperatures of approximately 300 degrees C results in the transformation of films into a ternary layered structure with iron oxide, Fe(2)O(3), at the surface, characterized by Mossbauer spectroscopy, and reduced, metallic iron characterized by depth profiling of the surface by X-ray photoelectron spectroscopy as a function of Ar(+) etching. Imaging of the etched surface by scanning electron microscopy reveals two distinct regions at the interface, nanoparticles that are very iron-rich separated by an unstructured region that is somewhat less iron-rich. The results demonstrate a synthetic protocol for the spontaneous formaton of a ternary layered structure from a simple one-step preparation.
ACS Appl Mater Interfaces 2009 Sep
PMID:Sol-gel-derived iron oxide thin films on silicon: surface properties and interfacial chemistry. 2035 2

Carbon dioxide (CO2) is a sustainable solvent because it is nonflammable, exhibits a relatively low toxicity, and is naturally abundant. As a selective, nonpolar solvent, supercritical CO2 (scCO2) is an ideal fit for the development of low-surface-energy polymers. The development of directly patterned poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) brushes in scCO2 was investigated. PTFEMA, in particular, was selected over other fluorinated polymers because of its very high electron-beam (e-beam) sensitivity. PTFEMA brushes were grown on silicon substrates via controlled surface-initiated atom-transfer radical polymerization of TFEMA. Surface analysis techniques including ellipsometry, contact-angle goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy were used to characterize the thickness, hydrophilicity, roughness, and chemical composition of the polymer brushes. PTFEMA brushes were directly patterned in a single step using e-beam lithography and were processed in an environmentally benign scCO2 solvent. Tapping-mode AFM imaging confirmed the successful e-beam patterning and development of these brushes. The sensitivity of PTFEMA brushes toward direct patterning with the e-beam, followed by scCO2 development, was studied and compared to development in tetrahydrofuran solvent. Using this direct-patterning method, followed by dry development in scCO2, highly resolved nanostructured polymer brush lines down to 78 nm could be prepared. This method can be generalized to prepare fluorinated low-surface-energy polymer brush surfaces in a single step for various applications.
ACS Appl Mater Interfaces 2009 Sep
PMID:Development of a directly patterned low-surface-energy polymer brush in supercritical carbon dioxide. 2035 27


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