ICMCTF2016 Session F1-1: Nanomaterials and Nanofabrication

Zinc oxide (ZnO) is a II-VI n-type semiconductor with a wide band gap of 3.3eV, higher exciton binding energy of 60 meV, and can be used in ultraviolet or visible optoelectronic devices, photocatalyst, gas sensors, and one of the candidate transparent conductive oxides (TCOs). In this study, thermally evaporated pure and Sb-doped ZnO nanowires was prepared in a tubular furnace at 850°C wherein a mechanism of VLS with the catalyst Au operated and triggered the nucleation and growth of the ZnO nanowires on silicon wafers. High quality powder Sb₂O₃ was used to be doped into the ZnO lattice with 2.1 at.%, 1.1 at.%, and 0.37 at% Sb. As a few of Sb³⁺ substituted the Zn²⁺ sites in ZnO, the electron concentration would increase; resulting in an increase of the electrical current from 0.2×10^-8 A (0 at.% Sb) to 4.1×10^-8 A (1.1 at.%Sb) in forward bias at 4 V.

XRD patterns showed that there were five main peaks at (100), (002), (101), (102), and (110) planes indicating the wurtzite crystal structure. The peak intensity, however, would decrease as the Sb content increased and a less crystalline nature would result. For instance, the 2θangle of (101) would decrease from 36.35° to 36.27°; a reduction of 2θ 0.22% as the doped Sb increased to 2.1at.% due to the difference of radius between Sb³⁺ (0.76Å) and Zn²⁺ (0.72Å). The TEM images and SAD patterns indicated that the Sb-doped ZnO nanowires kept the same crystal wurtzite structure with a growth direction of [0001].

Nanoporous carbon-based materials with large surface areas and pore volumes have attracted significant attention as potential adsorbents for storing highly-dense energy carriers such as hydrogen . Physical adsorption of hydrogen gas is considered an appealing storage method due to the fully reversible nature of the process and the fast adsorption/desorption kinetics. In the present work, a series of carbon materials with enhanced surface areas (400-1000 m²/g) including few-layer graphene powders, reduced graphene oxide sponges, ordered mesoporous carbons (OMC), metal-doped OMC analogues with Pt and Ru additives and activated carbon cloths, were investigated for their hydrogen adsorption performance at cryogenic (i.e. 77 K) and room temperatures (i.e. 298 K) using high-pressures up to 100 bar. The surface morphology and porous structure was studied by scanning and transmission electron microscopy. Textural and porosity properties, including Brunauer-Emmet-Teller (BET) surface area, micropore volume and pore size distribution were calculated by the nitrogen gas adsorption/desorption data obtained at 77 K and pressures between 0 and 1 bar. Maximum gravimetric capacities in excess of 2.5 wt.% H₂ were recorded herein while the adsorption behavior was correlated with specific textural/porosity features (i.e. BET surface area, micropore volume, etc) of these materials.

Ultra-thin two-dimensional (2D) semiconducting materials possess a combination of large, tunable electronic bandgaps, optical transparency, and mechanical flexibility, and will likely revolutionize electronic devices such as wearable sensors and flexible displays. A primary step in the development of such devices with integrated 2D materials is the development of scalable, transfer-free synthesis over large areas. By depositing electrically insulating amorphous films at room temperature on polymer substrates, and subsequent illumination with light, the material can be crystallized in selected areas. Focused laser light with a power density of ~1 kW cm²is suitable for writing micron scale features in semiconducting transition metal dichalcogenides on polymer substrates. Broad band illumination from a xenon lamp can also be used over the large substrate areas (> 100 cm²), or passed through a physical mask to print features only in desired locations. The semiconducting properties of 2D transition metal dichalcogenide (TMD) MoS₂ and WS₂ materials synthesized in this way have been characterized using conductive atomic force microscopy, and other techniques to observe the expected temperature dependence on electrical conductivity. Structure and composition of the materials can be controlled by altering the incident fluence as well as by controlling the ambient environment during illumination, as verified by Raman spectroscopy, X-ray photoelectron spectroscopy, cross-sectional transmission electron spectroscopy and other techniques. Multiple layers of 2D materials can also be treated in this way. For example, both layers in a MoS₂/WS₂ heterostructure with a total thickness of 10 nm on a polymer (PDMS) substrate were crystallized upon laser illumination. Diverse 2D architectures and devices built from this pioneering technique will be highlighted.

Applications for two-dimensional (2D) materials and their heterostructures are currently limited by the absence of direct and repeatable synthesis methods for cost effective device fabrication. In particular, these next-generation nanoelectronics based on two-dimensional materials are exceptionally exciting as a possibility in high speed, high frequency, flexible electronic devices. While conducting (e.g. graphene, TaS₂), and semiconducting (e.g. MoS₂, WS₂) 2D materials are being rapidly advanced for these applications, ultra-thin and high dielectric strength materials are relatively less-developed; primarily a result of the fundamental challenge in synthesis of ultra-thin insulating materials at moderate temperatures (< 900 ^oC) needed for direct growth over large lateral dimensions. Ultra-thin amorphous boron of thicknesses from 1-15 nm produced by pulsed laser deposition, may offer this possibility, as the dielectric is demonstrated to be universal in structure and stoichiometric chemistry when directly grown on numerous substrates including flexible PDMS, insulating substrates such as SiO₂ and Al₂O₃, other 2D materials including graphene, 2D MoS₂, and conducting metals and metal foils. The versatile, wafer-scale and conformal pulsed laser deposition growth technique is performed at temperatures less than 200°C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. Excellent electrical properties are measured in the ultra-thin a-BN, as a dielectric constant near 6, dielectric breakdown strength of 8.0 ± 0.35 MV/cm, and an optical bandgap of 4.5 rivals that of high temperature processed CVD h-BN of comparable thickness, and are approaching values for single crystal h-BN. In this work, plasma diagnostic procedures to determine critical parameters for optimal BN growth, chemical and structural properties of the as-deposited films, as well as prospects for a-BN integration in nanoelectronic systems and devices will be discussed.

We present a hybrid plasma spray-like deposition technique, based on geometrically-confined, supersonic microplasma jets, which can create a wide range of metal, metal oxide/sulfide, and semiconductor nanoparticles and nanostructured thin film materials (e.g., CuO/CuS, ZnO, SnO₂, NiO/NiFe₂O₄, Co_xO_y) on virtually any surface (e.g., conductors, insulators, plastics, fibers, and patterned substrates). The talk will highlight the effects of microplasma operation, deposition conditions, surface charging, and substrate type and patterning on film morphology. Applications of microplasma-deposited films to be discussed include solar cell electrodes, photo(electro)catalysts, and biphasic nanogranular films for magnetic exchange bias studies.

Carbon nanotubes (CNTs) possessing unique structure and properties are attractive building blocks for novel materials and devices of important practical interest. Particularly, Multi wall carbon nanotubes (MWCNTs) have attracted extensive attention in sensing application owing to their unique one-dimensional carbon nanostructure and electrical properties. Generally, carbon nanotubes are very sensitive to many types of target molecules showing an apparent lack of selectivity. This drawback of carbon nanotube sensors can be overcome through functionalization of nanotube surface in order to provide molecular specificity or unbalanced promoted sensitivity to a class of chemical species. Introduction of functional groups, such as carboxyl and amino groups, not only can improve CNTs solubility in various solvents, but also are useful for the further chemical link with other compounds. Phthalocyanine (Pc), as an excellent sensing material, has been extensively studied based on its high sensitivities, excellent thermal and chemical stability. Substituting functional groups on phthalocyanine molecules make them soluble in various organic solvents and thus enable them for low cost solution processing techniques such as spin coating and self-assembly etc Therefore, we expect that combining the nanoscale CNTs with gas sensing active Pc would feature not only the intrinsic properties of CNTs and Pc arising from the mutual interaction between CNTs and Pc but also enhance the gas sensing behaviour of CNTs.

In this work, We have prepared a hybrid material of MWCNTs-COOH and substituted copper phthalocyanine (CuPcOC₈). The formation of CuPcOC₈/MWCNTs-COOH (S₁) hybrid was confirmed by UV-Visible, Raman and FT-IR spectroscopy. SEM , TEM and AFM studies revealed that CuPcOC₈ molecules were successfully anchored on the surface of MWCNTs-COOH through π-π stacking interaction. Subsequently, a chemi-resistive sensor have been fabricated by drop casting CuPcOC₈/MWCNTs-COOH hybrid onto alumina substrate. The gas sensing potential of the fabricated hybrid materials has been tested upon exposure to different hazardous gases like NO₂, NO, Cl₂ and NH₃ at different operating temperatures. It has been demonstrated that CuPcOC₈/MWCNTs-COOH hybrid is highly selective towards Cl₂ with minimum detection limit of 100 ppb. The response of sensor increases linearly with increase in Cl₂ concentration. The results obtained emphasize on the application of CuPcOC₈/MWCNTs-COOH hybrid material in Cl₂ sensing applications.