Yaoguo Fang

Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations

mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW þ lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral VO DFEs from the DFT calculations along the ZnO [0001] and [10 � directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [10 � directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations. V C 2013 Author(s). All article

First-principles investigation of the size-dependent structural stability and electronic properties of O-vacancies at the ZnO polar and non-polar surfaces

In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (101¯0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction, and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termi...

Large-scale highly ordered Sb nanorod array anodes with high capacity and rate capability for sodium-ion batteries

Na-ion batteries are a potential substitute to Li-ion batteries for energy storage devices. However, their poor electrochemical performance, especially capacity and rate capability, is the major bottleneck to future development. Here we propose a performance-oriented electrode structure, which is 1D nanostructure arrays with large-scale high ordering, good vertical alignment, and large interval spacing. Benefiting from these structural merits, a great enhancement in electrochemical performance could be achieved. Taking Sb as an example, we firstly report large-scale highly ordered Sb nanorod arrays with uniform large interval spacing (190 nm). In return for this electrode design, high ion accessibility, fast electron transport, and strong electrode integrity are presented here. Used as additive- and binder-free anodes for Na-ion batteries, Sb nanorod arrays showed a high capacity of 620 mA h g−1 at the 100th cycle with a retention of 84% up to 250 cycles at 0.2 A g−1, and a superior rate capability for delivering reversible capacities of 579.7 and 557.7 mA h g−1 at 10 and 20 A g−1, respectively. A full cell coupled by a P2-Na2/3Ni1/3Mn2/3O2 cathode and a Sb nanorod array anode was also constructed, which showed good cycle performance up to 250 cycles, high rate capability up to 20 A g−1, and large energy density up to 130 Wh kg−1. These excellent electrochemical performances shall pave the way for developing more applications of Sb nanorod arrays in energy storage devices.

Extended π-Conjugated System for Fast-Charge and -Discharge Sodium-Ion Batteries

Organic sodium-ion batteries (SIBs) are potential alternatives of current commercial inorganic lithium-ion batteries for portable electronics (especially wearable electronics) because of their low cost and flexibility, making them possible to meet the future flexible and large-scale requirements. However, only a few organic SIBs have been reported so far, and most of them either were tested in a very slow rate or suffered significant performance degradation when cycled under high rate. Here, we are focusing on the molecular design for improving the battery performance and addressing the current challenge of fast-charge and -discharge. Through reasonable molecular design strategy, we demonstrate that the extension of the π-conjugated system is an efficient way to improve the high rate performance, leading to much enhanced capacity and cyclability with full recovery even after cycled under current density as high as 10 A g(-1).

Cost-effective atomic layer deposition synthesis of Pt nanotube arrays: application for high performance supercapacitor.

Due to the unique advantages of Pt, it plays an important role in fuel cells and microelectronics. Considering the fact that Pt is an expensive metal, a major challenging point nowadays is how to realize efficient utilization of Pt. In this paper, a cost-effective atomic layer deposition (ALD) process with a low N2 filling step is introduced for realizing well-defined Pt nanotube arrays in anodic alumina nano-porous templates. Compared to the conventional ALD growth of Pt, much fewer ALD cycles and a shorter precursor pulsing time are required, which originates from the low N2 filling step. To achieve similar Pt nanotubes, about half cycles and 10% Pt precursor pulsing time is needed using our ALD process. Meanwhile, the Pt nanotube array is explored as a current collector for supercapacitors based on core/shell Pt/MnO2 nanotubes. This nanotube-based electrode exhibits high gravimetric and areal specific capacitance (810 Fg(-1) and 75 mF cm(-2) at a scan rate of 5 mV s(-1) ) as well as an excellent rate capability (68% capacitance retention from 2 to 100 Ag(-1) ). Additionally, a negligible capacitance loss is observed after 8000 cycles of random charging-discharging from 2 to 100 Ag(-1) .

Fabrication and characterization of well-aligned, high density ZnO nanowire arrays and their realizations in Schottky device applications using a two-step approach

We report a two-step general and viable approach for preparing a large area of high density and horizontally well-aligned arrays of zinc oxide nanowires (ZnO NWs) for the realization of Schottky device applications on inexpensive, flexible polymer substrate. A modified chemical vapor deposition (CVD) process is initially used for synthesizing the highly efficient and rapid growth of vertical ZnO NW arrays along their [0001] direction, which is perpendicular to the donor substrate surface without using any metal catalyze. This is followed by transferring the NWs to a receiver substrate by a dry contact printing method. Utilizing the ZnO NWs synthesized by our method, a fully controllable and relatively large separation between the adjacent rows of silver (Ag) electrodes for the electrical contact with the NWs can be obtained using a photolithographic process. The printed ZnO NWs are well aligned along their c-axis, resulting in a spontaneous polarization which leads to a potential gradient along the length of the individual NW. This coupled with the effect of the surface states in the ZnO NWs result in the formation of a Schottky contact at the Ag/ZnO NW interface. Hence, virtually all of the ZnO NW arrays are functional as Schottky diodes which display non-linear current–voltage characteristics with good rectifying diode-like behaviour.

Growth control of AgTCNQ nanowire arrays by using a template-assisted electro-deposition method

The growth control of AgTCNQ nanowire arrays is achieved by using a template-assisted electro-deposition method. We find that the diffusion of the electrolyte into the nanopores plays an important role in the electro-deposition process, and an equilibrium between the reduction and diffusion is necessary to achieve continuous AgTCNQ nanowire arrays.

Selective growth and piezoelectric properties of highly ordered arrays of vertical ZnO nanowires on ultrathin alumina membranes

A well controlled and cost effective method of fabricating highly ordered arrays of vertical zinc oxide (ZnO) nanowires or nanopores is demonstrated where an ultrathin alumina membrane (UTAM) itself is utilized as a substrate for the selective growth of the ordered arrays. A thin film of gold was thermally evaporated on the UTAM followed by the growth of highly regular ZnO nanowires using chemical vapor deposition (CVD). Alternatively, highly ordered ZnO nanopores arrays were also grown by CVD on the bare UTAM. Additionally, piezoelectric currents were generated from the ZnO nanowires during the conductive atomic force microscopy probe tip scan across the array.

Synthesis and field emission properties of different ZnO nanostructure arrays

In this article, zinc oxide (ZnO) nanostructures of different shapes were fabricated on silicon substrate. Well-aligned and long ZnO nanowire (NW) arrays, as well as leaf-like ZnO nanostructures (which consist of modulated and single-phase structures), were fabricated by a chemical vapor deposition (CVD) method without the assistance of a catalyst. On the other hand, needle-like ZnO NW arrays were first fabricated with the CVD process followed by chemical etching of the NW arrays. The use of chemical etching provides a low-cost and convenient method of obtaining the needle-like arrays. In addition, the field emission properties of the different ZnO NW arrays were also investigated where some differences in the turn-on field and the field-enhancement factors were observed for the ZnO nanostructures of different lengths and shapes. It was experimentally observed that the leaf-like ZnO nanostructure is most suitable for field emission due to its lowest turn-on and threshold field as well as its high field-enhancement factor among the different synthesized nanostructures.

Preparation and enhanced microwave absorption properties of Ni-Co attached single-walled carbon nanotubes and CoFe2O4 nanocomposites

The electromagnetism and microwave absorption properties were investigated in the frequency range of 2–18 GHz for the nanocomposites NiCo-SWCNTs/CoFe2O4 consisting of Ni-Co attached single-walled carbon nanotubes (NiCo-SWCNTs) and CoFe2O4 nanocrystals with different ingredient weight ratios. NiCo-SWCNTs were mass-produced by a direct current arc discharge in helium and CoFe2O4 was synthesized by a sol-gel method. Premium microwave absorption properties (mainly in Ku-band, i.e., 12–18 GHz) were obtained due to the appropriate combination of the complex permeability and permittivity resulting from the magnetic nanocrystals and high-crystalline NiCo-SWCNTs. The NiCo-SWCNTs/CoFe2O4 nanocomposites with 15 wt. % NiCo-SWCNTs exhibited the best microwave absorption property, whose reflection loss (RL) value reached −47.9 dB at 14.7 GHz and the absorption bandwidth ( RL < − 10 dB) was up to 7.1 GHz (from 10.5 to 17.6 GHz) with a matching thickness of only 1.8 mm. Our results indicate that the studied nanocomposite could be used as a promising candidate for lightweight microwave absorption materials.The electromagnetism and microwave absorption properties were investigated in the frequency range of 2–18 GHz for the nanocomposites NiCo-SWCNTs/CoFe2O4 consisting of Ni-Co attached single-walled carbon nanotubes (NiCo-SWCNTs) and CoFe2O4 nanocrystals with different ingredient weight ratios. NiCo-SWCNTs were mass-produced by a direct current arc discharge in helium and CoFe2O4 was synthesized by a sol-gel method. Premium microwave absorption properties (mainly in Ku-band, i.e., 12–18 GHz) were obtained due to the appropriate combination of the complex permeability and permittivity resulting from the magnetic nanocrystals and high-crystalline NiCo-SWCNTs. The NiCo-SWCNTs/CoFe2O4 nanocomposites with 15 wt. % NiCo-SWCNTs exhibited the best microwave absorption property, whose reflection loss (RL) value reached −47.9 dB at 14.7 GHz and the absorption bandwidth ( RL < − 10 dB) was up to 7.1 GHz (from 10.5 to 17.6 GHz) with a matching thickness of only 1.8 mm. Our results indicate that the studied nanocomposit...