Wenqiang Lu

Catalyst-Free, Selective Growth of ZnO Nanowires on SiO2 by Chemical Vapor Deposition for Transfer-Free Fabrication of UV Photodetectors

Catalyst-free, selective growth of ZnO nanowires directly on the commonly used dielectric SiO2 layer is of both scientific significance and application importance, yet it is still a challenge. Here, we report a facile method to grow single-crystal ZnO nanowires on a large scale directly on SiO2/Si substrate through vapor-solid mechanism without using any predeposited metal catalyst or seed layer. We found that a rough SiO2/Si substrate surface created by the reactive ion etching is critical for ZnO growth without using catalyst. ZnO nanowire array exclusively grows in area etched by the reactive ion etching method. The advantages of this method include facile and safe roughness-assisted catalyst-free growth of ZnO nanowires on SiO2/Si substrate and the subsequent transfer-free fabrication of electronic or optoelectronic devices. The ZnO nanowire UV photodetector fabricated by a transfer-free process presented high performance in responsivity, quantum efficiency and response speed, even without any post-treatments. The strategy shown here would greatly reduce the complexity in nanodevice fabrication and give an impetus to the application of ZnO nanowires in nanoelectronics and optoelectronics.

Three-dimensional hierarchical nickel–cobalt–sulfide nanostructures for high performance electrochemical energy storage electrodes

To meet the ever-growing global demand for highly efficient and reliable energy storage systems, novel three-dimensional (3D) hierarchical porous cobalt–nickel–sulfide, H-(Co, Ni)3S2, nanostructures were designed and fabricated. The electrodes, based on a 3D hierarchical, porous nanoarchitecture, exhibit outstanding comprehensive performance with ultra-high specific capacitance of 4840 F g−1 (7.3 F cm−2) at current density of 1 A g−1, excellent rate capability of 3984 F g−1 (6.0 F cm−2) even at 20 A g−1, and superior cycling stability with as high as 93% capacitance retention after 5000 cycles at 10 A g−1. The performance greatly exceeds most previously reported faradaic electrodes for supercapacitors, due to the hierarchical, porous nanostructures, large active ion accessible surface area, varied and efficient faradic redox reactions, as well as strong mechanical stability and robust adhesion to the conductive matrix. Supercapacitors based on the 3D hierarchical H-(Co, Ni)3S2 nanostructured electrodes possess not only outstanding power and life performance, but also competitive energy densities, compared to the current, popular batteries.

Controllable growth of laterally aligned zinc oxide nanorod arrays on a selected surface of the silicon substrate by a catalyst-free vapor solid process – a technique for growing nanocircuits

We report a simple and effective vapor deposition method for directly growing ultra-long, laterally aligned, zinc oxide (ZnO) nanorod arrays only on the side edges of a bare silicon (Si) substrate without using any catalysts and precursors. The growth on the top surface of the substrate is restrained by controlling the flow of source vapor in a tube furnace through the chemical vapor solid process. The optimized growth parameters have been thoroughly investigated and identified. Direct growth of laterally aligned ZnO nanowire arrays on the desired surface of the substrate is successfully achieved. A vapor solid mechanism with source vapor flow rate control has been proposed to explain the synthesis: ZnO nanodots first form on the bare Si substrate side edges due to the local large binding energy and high zinc (Zn) vapor concentration, and then nanorods epitaxially grow from the nanodots. In addition, the lateral, ultra-long ZnO nanorods grown on orthogonal silicon microelectrodes are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation nanoelectronics.

Shear modulus property characterization of nanorods.

We demonstrate an innovative technique for the direct measurement on the shear modulus of an individual nanorod. This measurement is based on atomic force microscopy (AFM) and microfabrication techniques. A nanorod is first aligned along the edge of a small trench in a silicon substrate, and then one end of the nanorod is fixed on the substrate. When an AFM tip scans over the nanorod in contact mode, the nanorod will be twisted by the comprehensive action from the force of the AFM tip, confinement from the trench edge and the fixing end. The shear deformation and the corresponding force that caused the deformation can be retrieved from topography and lateral force image, respectively. By small-angle approximation, the shear modulus of the ZnO NR, which has a radius of 166 nm and a length of 4 μm, is measured to be 8.1 ± 1.9 GPa. This method can be applied directly to characterize the shear modulus of any nanowire/nanorod that possesses a polygon cross section.

Simulation of Nanoscale Multifunctional Interferometric Logic Gates Based on Coupled Metal Gap Waveguides

A novel design of ultrasmall multifunctional Boolean logic gates is proposed in this letter. This interferometric logic device is based on the coupled metal waveguides. This structure is a theoretical analysis, and logic performances are proved by using the finite-difference time-domain method. The single device can perform individually four different kinds of basic functions: and, or, xor, and not operations. The device with an extremely small feature size is an attractive candidate for high density nano-photonic integrated circuits.

Paper/Carbon Nanotube-Based Wearable Pressure Sensor for Physiological Signal Acquisition and Soft Robotic Skin

A wearable and flexible pressure sensor is essential to the realization of personalized medicine through continuously monitoring an individuals state of health and also the development of a highly intelligent robot. A flexible, wearable pressure sensor is fabricated based on novel single-wall carbon nanotube /tissue paper through a low-cost and scalable approach. The flexible, wearable sensor showed superior performance with concurrence of several merits, including high sensitivity for a broad pressure range and an ultralow energy consumption level of 10-6 W. Benefited from the excellent performance and the ultraconformal contact of the sensor with an uneven surface, vital human physiological signals (such as radial arterial pulse and muscle activity at various positions) can be monitored in real time and in situ. In addition, the pressure sensors could also be integrated onto robots as the artificial skin that could sense the force/pressure and also the distribution of force/pressure on the artificial skin.

Ultrafast growth of large-area monolayer MoS2 film via gold foil assistant CVD for a highly sensitive photodetector

Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetectors owing to its tunable band gap and high absorption coefficient. However, controlled synthesis of high-quality, large-area monolayer molybdenum disulfide (MoS2) is still a challenge in practical application. In this work, we report a gold foil assistant chemical vapor deposition method for the synthesisxa0of large-size (>400 μm) single-crystal MoS2 film on axa0silicon dioxide (SiO2) substrate. The influence of Au foil in enlarging the size of single-crystal MoS2xa0isxa0investigated systemically using thermal simulation in Ansys workbench 16.0, including thermal conductivity, temperature difference and thermal relaxation time of the interface of SiO2 substrate and Au foil, which indicate that Au foil canxa0increase the temperature of the SiO2 substrate rapidly and decrease the temperature difference between the oven and substrate. Finally, the properties of the monolayer MoS2 film arexa0further confirmed using back-gated field-effect transistors: a high photoresponse of 15.6 A W-1 and a fast photoresponse time of 100 ms. The growth techniques described in this study could be beneficial for the development of other atomically thin two-dimensional transition metal dichalcogenidexa0materials.Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetector owing to its tunable band gap and high absorption coefficient. However, controlled synthesis of high quality, large area monolayer molybdenum disulfide (MoS2) is still a challenge in practical application. In this work, we report a gold foil assistant chemical vapor deposition (CVD) method of large size (>400 μm) single crystal MoS2 film on silicon dioxide (SiO2) substrate. The influence of Au foil in enlarging the size of single crystal MoS2 were investigated systemically using thermal simulation in Ansys workbench 16.0, including thermal conductivity, temperature difference and thermal relaxation time of the interface of SiO2 substrate and Au foil, which indicated that Au foil could increase the temperature of the SiO2 substrate rapidly and decrease the temperature difference between the oven and substrate. At last, the property of the monolayer MoS2 film was further confirmed by the back-gated field effect transistors (FETs), a high photo-response of 15.6 A/W and a fast photo-response time of 100 ms was obtained. The growth techniques described in this study could be benefit for the development of other atomically thin two-dimensional transition metal dichalcogenides (TMD) materials.

Observation of surface-plasmon-polariton transmission through a silver film sputtered on a photorefractive substrate

The diffraction of holographic gratings in a photorefractive iron-doped lithium niobate (LiNbO3:Fe) crystal, on which surface a silver film was sputtered, was experimentally investigated. Besides the Bragg diffraction, an additional diffraction was observed. The experimental results present evidence of surface-plasmon-polariton (SPP) transmission through the silver film on the photorefractive substrate. The excitation of SPPs is speculated to be due to the corrugations of the silver film, which are caused by the photorefractive and the converse piezoelectric effect in the LiNbO3:Fe sample.

High-efficiency piezoelectric micro harvester for collecting low-frequency mechanical energy

A single-layer zinc oxide (ZnO) nanorod array-based micro energy harvester was designed and integrated with a piezoelectric metacapacitor. The device presents outstanding low-frequency (1-10 Hz) mechanical energy harvesting capabilities. When compared with conventional pristine ZnO nanostructured piezoelectric harvesters or generators, both open-circuit potential and short-circuit current are significantly enhanced (up to 3.1 V and 124 nA cm-2) for a single mechanical knock (∼34 kPa). Higher electromechanical conversion efficiency (1.3 pC/Pa) is also observed. The results indicate that the integration of the piezoelectric metacapacitor is a crucial factor for improving the low-frequency energy harvesting performance. A double piezoelectric-driven mechanism is proposed to explain current higher output power, in which the metacapacitor plays the multiple roles of charge pumping, storing and transferring. An as-fabricated prototype device for lighting an LED demonstrates high power transference capability, with over 95% transference efficiency to the external load.

Ultrafast UV response detectors based on multi-channel ZnO nanowire networks

An UV detector based on multi-channel three dimensional ZnO nanowire networks was fabricated via a catalyst-free CVD method. The ultrafast response time of 20 ms for UV detection was tested in detail. The result revealed that the formation of multi-channels on lateral growth ZnO nanowire arrays can construct a transmission path for UV excited electrons, which is essential for gaining outstanding UV detecting performance. This work not only reports a new way to fabricate in situ nanowire network UV sensors on a chip with large-scale periodic microstructures and a single optical-lithography step via a catalyst free and well controlled synthesis method, but it also confirms a novel mechanism for achieving ultrafast UV detection.