Silicon nanowires have great potential for applications in electronics, photovoltaics and biomedical applications, but as of yet, silicon nanowires are not used in any commercial application. More characterization is needed to better understand and control their surfaces and electronic properties. Here, electron energy loss spectroscopy (EELS) is used to characterize silicon nanowires made by the supercritical-fluid-liquid-solid process. EELS is able to analyze the elemental composition of core and shell structures with high spatial resolution. Additionally, the biocompatibility and antibacterial properties of silicon nanowires are assessed for potential bioapplications. Preliminary investigations suggest that nanowires have an anti-proliferative effect on E. coli and discourage adhesion of mammalian cells. Future investigations may prove that silicon nanowires are a promising material for biomedical implant coatings to prevent biofouling. / text

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No / Nanotechnologies for intracellular delivery are of great value in clinical and biological research. Diamond nanoneedle arrays are a novel and attractive platform to facilitate drug delivery with minimal cytotoxicity. Using our technique, the cellular membranes can be temporarily disrupted for enhanced diffusion of drug molecules to cytoplasm. Herein we show that this technique is applicable to deliver different types of anticancer drugs into a variety of cell lines, although the membrane of each cell line possesses varied rigidity and hardness and each drug has its own unique properties and targets. When anticancer drugs and nanoneedle arrays are collaboratively used to treat cancer cells, the cell viability dramatically decreases by up to 40 % in comparison with the cells treated with drugs only. Attractively, therapeutic molecules can be efficiently delivered to drug-resistant cells with the aid of nanoneedle arrays. The combination of diamond nanoneedle arrays and anticancer drug cisplatin can decrease the viability of A549 cisplatin-resistant cells to about 60 %, while the cells only treated with the same concentration of drug are essentially not affected due to their drug resistance. These results indicate that dense nanoneedle arrays represent an effective approach to enhance the delivery of biological molecules to different types of cells. Such approach will certainly be beneficial to microbiological research and clinical applications in the future.

Thesis advisor: Kian L. Tan / Thesis advisor: Dunwei Wang / Silicon nanowires are utilized to harvest the energy from visible light. The introduction of a nickel pre-catalyst, 1, allows for this energy to be stored in chemical bonds, which are subsequently used in the carboxylation of 4-octyne. / Thesis (MS) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

There is a growing interest for relying on cleaner and more sustainable energy sources due to the negative side-effects of the dominant fossil-fuel based energy storage and conversion systems. Cleaner, electrochemical energy storage through lithium-ion batteries has gained considerable interest and market value for applications such as electric vehicles and renewable energy storage. However, capacity and rate (power) limitations of current lithium-ion battery technology hinder its ability to meet the high energy demands in a competitive and reliable fashion. Silicon is an element with very high capacity to Li-ion storage although commercially impractical due to its poor stability and rate capabilities. Nevertheless, it has been heavily researched with more novel electrode nanostructures to improve its stability and rate capability. It was found that silicon nanomaterials such as silicon nanowires have inherently higher stability due to mitigation of cracking and higher rate capability due to the short Li-ion diffusion distance. However, electrode compositions based only on silicon nanowires without additional structural features and a high conductive support do not have enough stability and rate capability for successful commercialization. One structural and conductive support of silicon materials studied in literature is graphene. Graphene-based electrodes have been reported as material capable of rapid electron transport enabling new strides in rate capabilities for Li ion batteries. This thesis presents a novel electrode nanostructure with a simple, inexpensive, scalable method of silicon nanwire synthesis on graphene nanosheets via nickel catalyst. The research herein shows the different electrode compositions and variables studied to yield the highest achievable capacity, stability and rate capability performance. The carbon coating methodology in addition to enhancing the 3D conductivity of the electrode by replacing typical binders with pyrolyzed polyacrylonitrile provided the highest performance results.

>Magister Scientiae - MSc / In this thesis we report on the deposition of silicon nanostructures using a 3-zone thermal chemical vapour deposition process at atmospheric pressure. Nickel and gold thin films, deposited by DC sputtering on crystalline silicon substrates, were used as the catalyst material required for vapour-solid-liquid growth mechanism of the Si nanostructures. The core of this work is centred around the effect of catalyst type, substrate temperature and the source-to-substrate distance on the structural and optical properties of the resultant Si nanostructures, using argon as the carrier gas and Si powder as the source. The morphology and internal structure of the Si nanostructures was probed by using high resolution scanning and transmission electron microscopy, respectively. The crystallinity was measured by x-ray diffraction and the high resolution transmission electron microscopy. For composition and elemental analysis, Fourier transform infrared spectroscopy was used to quantify the bonding configuration, while electron energy-loss spectroscopy in conjunction with electron dispersion spectroscopy reveals the composition. Photoluminescence and UV-visible spectroscopy was used to extract the emission and reflection properties of the synthesized nanostructures.

We analyze a toy model for p-n junction photovoltaic devices by simulating photoexcited state dynamics in silicon nanowires. One nanowire is approximately circular in cross section with a diameter of d = 1.17 nm. The other has an approximately rhombic cross-section with d1 = 1.16 nm and d2 = 1.71 nm. Both nanowires have been doped with aluminum and phosphorus atoms and capped with gold leads. We use Boltzmann transport equation (BE) that includes phonon emission, carrier multiplication (CM), and exciton transfer. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on Density Functional Theory (DFT) simulations, including excitonic effects from Bethe-Salpeter Equation. We compute total charge transfer amount generated from the initial photoexcitation and find an enhancement when CM is included. In particular, we see between 78% and 79% enhancement in the smaller wire, while we see 116% enhancement in the larger nanowire

The objective of this work is to develop methods to investigate the thermoelectric (TE) transport in semiconducting nanowires (NWs). The thermal conductivity of degenerately doped electrochemically-etched (EE) silicon NWs was measured to be lower than silicon NWs synthesized by a vapor-liquid-solid (VLS) method without showing a clear dependence on the NW diameter. The thermoelectric figure of merit (ZT) at near room temperature obtained from the three measured TE properties on the same EE Si NW was found to be between 0.01 of a very rough NW and 0.08 of a relatively smooth NW, the latter of which is about four times higher than that reported for bulk p-type Si at the optimum doping concentration. In addition, the NW samples could be contaminated or oxidized during the device processing. Based on the TEM characterization, they have relatively thick oxide layer and small surface roughness, and are apparently different from the EE Si NWs that a Berkeley team reported. Typical rough NWs reported by the Berkeley team have thin oxide layer and are free of major structural defects. Hence, given the significant structural differences in the samples, it would be scientifically inappropriate to compare the transport properties obtained from the two studies. In addition, a five to ten fold reduction in thermal conductivity was observed in wurtzite InAs NWs compared to bulk InAs of zinc blend phase, and is mainly attributed to diffuse surface scattering of phonons. Moreover, InSb NWs have been synthesized at three different base pressures. The NWs were found to be zinc-blende structure with <110> growth direction. The ZT of the two NWs is about 10 times lower than the bulk values mainly because of the much higher doping levels in NWs than the bulk as well as mobility suppression in the NWs. The ZT of one NW grown at a high vacuum base pressure is higher than another NW grown at low vacuum. These results show that it is necessary to better control the impurity doping in order to increase the ZT of the InSb NWs. / text

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>Magister Scientiae - MSc / One-dimensional silicon nanowires (SiNWs) are promising building blocks for solar cells as they provide a controlled, vectorial transport route for photo-generated charge carriers in the device as well as providing anti-reflection for incoming light. Two major approaches are followed to synthesise SiNWs, namely the bottom-up approach during vapour-liquid-solid mechanism which employs chemical vapour deposition techniques. The other method is the top-down approach via metal assisted chemical etching (MaCE). MaCE provides a simple, inexpensive and repeatable process that yields radially and vertically aligned SiNWs in which the structure is easily controlled by changing the etching time or chemical concentrations. During MaCE synthesis, a crystalline silicon (c-Si) substrate covered with metal nanoparticles (catalyst) is etched in a diluted hydrofluoric acid solution containing oxidising agents. Since the first report on SiNWs synthesised via MaCE, various publications have described the growth during the MaCE process. However lingering questions around the role of the catalyst during formation, dispersion and the eventual diameter of the nanowires remain. In addition, very little information pertaining to the changes in crystallinity and atomic bonding properties of the nanowires post synthesis is known. As such, this study investigates the evolution of vertical SiNWs from deposited silver nanoparticles by means of in-depth electron microscopy analyses. Changes in crystallinity during synthesis of the nanowires are probed using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Deviations in the optical properties are quantified using optical reflectivity measurements by employing ultraviolet-visible (UV-Vis) spectroscopy, whereas the bonding configurations of the nanowires are probed by Raman and Fourier transforms infrared spectroscopy. Diameters of 50 – 200 nm vertical SiNWs were obtained from scanning electron micrographs and nanowires lengths linearly increased with etching time duration from about 130 nm after 30 seconds to over 15 μm after 80 minutes. No diameter modulations along nanowires axial direction and rough nanowires apexes were observed for nanowires obtained at longer etching times. These SiNWs remained crystalline as their bulk single crystalline Si wafers but had a thin amorphous layer on the surface, findings confirmed by TEM, XRD and Raman analysis. Nanowires were found to be partially passivated with oxygen with small traces of hydrogen termination, confirmed with infrared absorption studies. Finally, low optical reflection of less than 10% over visible range compared to an average of 30% for bulk Si were measured depicting an antireflective ability required in silicon solar cells.