Guoan Cheng

Vapor–Solid Growth of Few-Layer Graphene Using Radio Frequency Sputtering Deposition and Its Application on Field Emission

The carbon nanotube (CNT) and graphene hybrid is an attractive candidate for field emission (FE) because of its unique properties, such as high conductivity, large aspect ratio of CNT, and numerous sharp edges of graphene. We report here a vapor-solid growth of few-layer graphene (FLG, less than 10 layers) on CNTs (FLG/CNT) and Si wafers using a radio frequency sputtering deposition system. Based on SEM, TEM, and Raman spectrum analyses, a defect nucleation mechanism of the FLG growth was proposed. The FE measurements indicate that the FLG/CNT hybrids have low turn-on (0.956 V/μm) and threshold fields (1.497 V/μm), large field enhancement factor (∼4398), and good stability. Excellent FE properties of the FLG/CNT hybrids make them attractive candidates as high-performance field emitters.

Surface Morphology-Dependent Photoelectrochemical Properties of One-Dimensional Si Nanostructure Arrays Prepared by Chemical Etching

Maximizing the optical absorption of one-dimensional Si nanostructure arrays (1DSiNSAs) is desirable for excellent performance of 1DSiNSA-based optoelectronic devices. However, a quite large surface-to-volume ratio and enhanced surface roughness are usually produced by modulation of the morphology of the 1DSiNSAs prepared in a top-down method to improve their optical absorption. Surface recombination is mainly determined by the surface characteristics and significantly affects the photogenerated carrier collection. In this paper, we systematically investigated the photoelectrochemical characteristics of 1DSiNSAs with various morphologies prepared by the metal-assisted chemical etching of Si wafers. Our results show that the saturation photocurrent density and photoresponsivity of 1DSiNSAs first increased and then gradually decreased with an increasing etching time, while the reflection spectrum was gradually suppressed to the measurable minimum. To identify the behaviors of the photoresponsivity and optical absorption of the various 1DSiNSAs, we analyzed the morphology, structure, and minority-carrier lifetime. Additionally, device physics simulations were used to confirm the significance of surface recombination. We proposed that future directions for the design of nanostructure-based optoelectronic devices should include not only strong optical absorption but also low surface carrier recombination. High-performance devices could be obtained only by balancing the requirements for light absorption and photogenerated carrier collection.

Room Temperature Electrical and Thermal Switching CNT/Hexadecane Composites

A large contrast in the electrical and thermal conductivities via a first order phase transition in surface-functionalized carbon nanotube(CNT)/hexadecane composites is reported. Surface modification of the CNTs improves the electrical conductivity contrast and the stability of the composites. We demonstrate that, with these composites, the electrical conductivity changes above 10(5) times and the thermal conductivity varies up to 3 times at 18 °C.

Fabrication and Properties of Ag-nanoparticles Embedded Amorphous Carbon Nanowire/CNT Heterostructures

Carbon nanotubes were subjected to doping with an energetic Ag ion beam, and the carbon nanotubes on the top of the array were transformed into amorphous carbon nanowires with embedded Ag-nanoparticles. The field emission characteristics of these nanowires were investigated. The minimum turn-on and threshold fields were 0.68 and 1.09 V/μm, respectively, which were lower than those of the as-grown carbon nanotubes. This was probably because Ag-nanoparticles embedded in the carbon nanowires reduced the effective work function from 4.59 to 4.23 eV. Large doping amounts produced serious structural damage at the top of the nanowires and impaired the field emission characteristics.

Field emission enhancement of Au-Si nano-particle-decorated silicon nanowires

Au-Si nano-particle-decorated silicon nanowire arrays have been fabricated by Au film deposition on silicon nanowire array substrates and then post-thermal annealing under hydrogen atmosphere. Field emission measurements illustrated that the turn-on fields of the non-annealed Au-coated SiNWs were 6.02 to 7.51 V/μm, higher than that of the as-grown silicon nanowires, which is about 5.01 V/μm. Meanwhile, after being annealed above 650°C, Au-Si nano-particles were synthesized on the top surface of the silicon nanowire arrays and the one-dimensional Au-Si nano-particle-decorated SiNWs had a much lower turn-on field, 1.95 V/μm. The results demonstrated that annealed composite silicon nanowire array-based electron field emitters may have great advantages over many other emitters.

Significant reduction of thermal conductivity in silicon nanowire arrays

Vertically aligned single-crystal silicon nanowire arrays (SiNWs) with various lengths, surface roughnesses and porosities were fabricated with the metal-assisted chemical etching method. Using the laser flash technique and differential scanning calorimetry, we characterized the thermal conductivities of bulk SiNWs/Si/SiNWs sandwich-structured composites (SSCs) at room temperature (300 K). The results demonstrate that the thermal conductivities of SSCs notably decrease with increases in the length, surface roughness and porosity of SiNWs. Furthermore, based on the series thermal-resistance model, we calculated the thermal conductivity of porous SiNWs to be as low as 1.68 W m(-1) K(-1) at 300 K. Considering the remarkable phonon scattering from the diameter, surface roughness and porosity of SiNWs, leading to a significant reduction of the thermal conductivity, SSCs and SiNWs could be applied to high-performance thermoelectric devices.

Irradiation damage determined field emission of ion irradiated carbon nanotubes.

Figuring out the underlying relationship between the field emission (FE) properties and the ion irradiation induced structural change of carbon nanotubes (CNTs) is of great importance in developing high-performance field emitters. We report here the FE properties of Si and C ion irradiated CNTs with different irradiation doses. It is found that the FE performance of the ion irradiated CNTs ameliorates before and deteriorates after an irradiation-ion-species related dose. The improved FE properties are ascribed to the increased amount of defects, while the degraded FE performance is attributed to the great shape change of CNTs. These two structural changes are further characterized by a structural damage related parameter: dpa (displacement per atom), and the FE performance of the ion irradiated CNTs is surprisingly found to be mainly dependent on the dpa. The optimal dpa for FE of the ion irradiated CNTs is ∼0.60. We ascribe this to the low irradiation doses and the low substrate temperature that make the ion irradiation play a more important role in producing defects rather than element doping. Furthermore, the ion irradiated CNTs exhibit excellent FE stability, showing promising prospects in practical applications.

One-step synthesis of hollow Cr(OH)3 micro/nano-hexagonal pellets and the catalytic properties of hollow Cr2O3 structures

Novel Cr(OH)3 hollow hexagonal pellets are synthesized at room temperature based on the chemical reaction between CrCl3 and NaBH4 in aqueous solution without any templates and surfactants. The evolution of the products obtained at different reaction times revealed that the hollow structure was formed via an orientation self-assembly and selective-etching process. The size and shape of the Cr(OH)3 hollow hexagonal pellets can be adjusted by changing the reaction conditions. Annealing these Cr(OH)3 hollow pellets at high temperatures convert them into Cr2O3 hollow structures, which are demonstrated in this paper to exhibit better catalytic performance than conventional Cr2O3 nanoparticles in the dehydrogenation of isobutane. This template-free and surfactant-free method enables low cost and high yield synthesis of hollow micro/nanostructures.

High current density and longtime stable field electron transfer from large-area densely arrayed graphene nanosheet-carbon nanotube hybrids.

Achieving high current and longtime stable field emission from large area (larger than 1 mm(2)), densely arrayed emitters is of great importance in applications for vacuum electron sources. We report here the preparation of graphene nanosheet-carbon nanotube (GNS-CNT) hybrids by following a process of iron ion prebombardment on Si wafers, catalyst-free growth of GNSs on CNTs, and high-temperature annealing. Structural observations indicate that the iron ion prebombardment influences the growth of CNTs quite limitedly, and the self-assembled GNSs sparsely distributed on the tips of CNTs with their sharp edges unfolded outside. The field emission study indicates that the maximum emission current density (Jmax) is gradually promoted after these treatments, and the composition with GNSs is helpful for decreasing the operation fields of CNTs. An optimal Jmax up to 85.10 mA/cm(2) is achieved from a 4.65 mm(2) GNS-CNT sample, far larger than 7.41 mA/cm(2) for the as-grown CNTs. This great increase of Jmax is ascribed to the reinforced adhesion of GNS-CNT hybrids to substrates. We propose a rough calculation and find that this adhesion is promoted by 7.37 times after the three-step processing. We consider that both the ion prebombardment produced rough surface and the wrapping of CNT foot by catalyst residuals during thermal processing are responsible for this enhanced adhesion. Furthermore, the three-step prepared GNS-CNT hybrids present excellent field emission stability at high emission current densities (larger than 20 mA/cm(2)) after being perfectly aged.

Novel Au/Cu2O multi-shelled porous heterostructures for enhanced efficiency of photoelectrochemical water splitting

In this study, we report novel Au/Cu2O multi-shelled porous heterostructures (MSPHs). The results of photoelectrochemical (PEC) examination indicate that the photocurrent density of the as-prepared Au/Cu2O MSPHs electrode reaches 150 μA cm−2, which is almost 7.5 times higher than 20 μA cm−2 of pure Cu2O MSP at a 0 V bias potential versus Ag/AgCl. The enhanced PEC efficiency of the Au/MSPHs is ascribed to the Schottky barrier at the Au–MSP NP interface and the surface plasmon resonance (SPR) effect of Au. We also found that Au nanoparticles deposited on the surface of Cu2O MSP could effectively adjust their band structure.