The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

https://science.sciencemag.org/content/sci/291/5510/1947.full.pdf

Nanobelts of Semiconducting Oxides

The surface and materials science of tin oxide

Zinc oxide can be called a multifunctional material thanks to its unique physical and chemical properties. The first part of this paper presents the most important methods of preparation of ZnO divided into metallurgical and chemical methods. The mechanochemical process, controlled precipitation, sol-gel method, solvothermal and hydrothermal method, method using emulsion and microemulsion enviroment and other methods of obtaining zinc oxide were classified as chemical methods. In the next part of this review, the modification methods of ZnO were characterized. The modification with organic (carboxylic acid, silanes) and inroganic (metal oxides) compounds, and polymer matrices were mainly described. Finally, we present possible applications in various branches of industry: rubber, pharmaceutical, cosmetics, textile, electronic and electrotechnology, photocatalysis were introduced. This review provides useful information for specialist dealings with zinc oxide.

Zinc Oxide-From Synthesis to Application: A Review.

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

ZnO nanowires were mass produced using a physical vapor deposition approach. The ZnO nanowire monocrystallites have an average diameter around 60 nm and length up to a few micrometers. The unidirectional growth of the ZnO nanowires was controlled by the conventional vapor-liquid-solid mechanism. Intensive UV light emission peaked around 3.27 eV was observed at room temperature, which was assigned to emission from free exciton under low excitation intensity. The observed room temperature UV emission was ascribed to the decrease in structure defects as compared to bulk ZnO materials, and in particularly to the size effect in the ZnO wires.

Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach

Ga2O3 nanowires prepared by physical evaporation

Heat assisted magnetic recording (HAMR) is a possible solution to further increase hard drive recording density. In a HAMR system, an external light source is employed to locally heat up magnetic media. To achieve a realistic HAMR system, a near-field transducer (NFT) is necessary to focus optical fields down to 10-20 nm in size. Different approaches to excite NFTs have been investigated, including focusing of external light sources and coupling from dielectric waveguides. Here we demonstrate evanescent coupling between a series of single mode dielectric waveguides and adjacent plasmonic waveguides with relatively large cross section (200 nm×300 nm). We envision for these large plasmonic guides could then be used, in turn, to efficiently drive NFT's with smaller cross sectional area. A series of samples with various overlap length of the dielectric c and plasmonic waveguides demonstrates that coupling in these waveguides can be designed and understood using mode indices calculated from the eigenmodes of the waveguide cross sections. Coupling strength which oscillates with the length of the overlap section of the waveguides is shown according to standard coupled mode theory.

Evanescent Coupling Between Dielectric and Plasmonic Waveguides for HAMR Applications

The wider deployment of commercial quantum key distribution (QKD) may benefit from an integrated system with reduced cost, small form-of-factor and high robutness. Silicon photonic circuits are good candidates while their performance stability in some contexts remains a challenge. We demonstrate a silicon photonic QKD transceiver based on time-bin protocol. The stability of the transceiver is investigated and a feedback function is proposed to improve the temperature-dependent performance of the transceiver. With the help of a faster data-processing ability, such scheme can facilitate more application scenarios, therefore achieving wider implementation of QKD in the future.

Stable quantum key distribution using a silicon photonic transceiver.

Integrated single-photon sources are a key component for photonic quantum technology but are generally limited to low single-photon rates. For sources based on photon pair generation by four-wave mixing, increasing the repetition rate of pump laser pulses is a straightforward way to enhance the single-photon rate, but the benefits and practical limitations have not yet been demonstrated and analyzed in a CMOS-compatible platform. In this work, we demonstrate correlated photon pair generation in integrated silicon nanowires and systematically analyze the count rate and coincidence to accidental ratio as the pump rate is varied between 156.25 MHz and 10 GHz. We show that the highest useful pump rate is limited by the timing resolution of the single-photon detection system, and that in this regime, the nonlinear loss of the silicon nanowire does not have a significant effect on the single-photon generation.

Yunchuan Kong

Papers

Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach

Ga2O3 nanowires prepared by physical evaporation

Evanescent Coupling Between Dielectric and Plasmonic Waveguides for HAMR Applications

Stable quantum key distribution using a silicon photonic transceiver.

High repetition rate correlated photon pair generation in integrated silicon nanowires.