Wei Wang

Novel Planar-Structure Electrochemical Devices for Highly Flexible Semitransparent Power Generation/Storage Sources

Flexible and transparent power sources are highly desirable in realizing next-generation all-in-one bendable, implantable, and wearable electronic systems. The developed power sources are either flexible but opaque or semitransparent but lack of flexibility. Therefore, there is increasing recognition of the need for a new concept of electrochemical device structure design that allows both high flexibility and transparency. In this paper, we present a new concept for electrochemical device design--a two-dimensional planar comb-teeth architecture on PET substrate--to achieve both high mechanical flexibility and light transparency. Two types of prototypes--dye-sensitized solar cells and supercapacitors--have been fabricated as planar devices and demonstrated excellent device performance, such as good light transparency, excellent flexibility, outstanding multiple large bending tolerance, light weight, effective prevention of short circuits during bending, and high device integration with up-date microelectronics, compared to conventional sandwich structure devices. Our planar design provides an attractive strategy toward the development of flexible, semitransparent electrochemical devices for fully all-in-one elegant and wearable energy management.

Measuring the Refractive Index of Highly Crystalline Monolayer MoS2 with High Confidence

Monolayer molybdenum disulphide (MoS2) has attracted much attention, due to its attractive properties, such as two-dimensional properties, direct bandgap, valley-selective circular dichroism, and valley Hall effect. However, some of its fundamental physical parameters, e.g. refractive index, have not been studied in detail because of measurement difficulties. In this work, we have synthesized highly crystalline monolayer MoS2 on SiO2/Si substrates via chemical vapor deposition (CVD) method and devised a method to measure their optical contrast spectra. Using these contrast spectra, we extracted the complex refractive index spectrum of monolayer MoS2 in the wavelength range of 400u2005nm to 750u2005nm. We have analyzed the pronounced difference between the obtained complex refractive index spectrum and that of bulk MoS2. The method presented here is effective for two-dimensional materials of small size. Furthermore, we have calculated the color contour plots of the contrast as a function of both SiO2 thickness and incident light wavelength for monolayer MoS2 using the obtained refractive index spectrum. These plots are useful for both fundamental study and device application.

Studies on Parylene C-caulked PDMS (pcPDMS) for low permeability required microfluidics applications

This note introduced a complete fabrication strategy of Parylene C-caulked PDMS (pcPDMS) for low permeability required microfluidics applications. The bonding issue enrolled in the pcPDMS fabrication was solved based on careful surface analyses of the Parylene C caulked status in the PDMS matrix.

Enhanced field emission from large scale uniform monolayer graphene supported by well-aligned ZnO nanowire arrays

Large scale, uniform single-layer graphene was transferred onto well-aligned ZnO nanowire arrays to produce high density nanoscale protrusions within graphene for efficient field emission. Polymethyl methacrylate (PMMA) was used as a supporting layer to provide a quasi-flat surface for graphene transfer. Highly efficient (maximum current density of ∼500u2009μA/cm2) and stable field emission with low turn-on fields (5.4u2009V/μm) was observed due to highly localized electric field, which is much better than those without using PMMA. F-N plot showed an unique up-bending feature of single-layer graphene. Our approach provides an efficient way to produce high quality single-layer graphene field emitters.

Nanofluidic electrokinetics in nanoparticle crystal

We scrutinize experimentally and theoretically the electrokinetics of a nanochannel network in a nanoparticle crystal. Conductance of the nanochannel network, a function of concentrations of the loaded electrolyte solutions, exhibits typical electrokinetic properties of individual nanochannel but with a much larger electrical signal. A widely accepted electrokinetic equation is used to elucidate the origin of the high throughput from a top-down perspective. Using a bottom-up analysis, a stepwise modeling process from individual nanochannel to highly ordered nanochannel network is proposed to render an in-depth view of the nanofluidics network.

Nanofluidic diode in a suspended nanoparticle crystal

This work demonstrates a nanofluidic diode in a suspended nanoparticle crystal (S-NPC) constructed by sequentially packing hydroxyl-modified and amino-modified nanoparticles into a microfabricated silicon micropore. Current rectification in this nanofluidic diode comes from the asymmetric surface charge polarities along the nanochannel network inside the nanoparticle crystal. The maximum current rectification ratio was about 48 for the 173 nm S-NPC nanofluidic diode and the maximum forward current was larger than 700 nA at 3 V bias. Since it is inexpensive, easy to manufacture, and the surface charge properties are easily formed, having excellent electrical performance, this S-NPC nanofluidic diode holds application for biosensors.

Origin of Improved Optical Quality of Monolayer Molybdenum Disulfide Grown on Hexagonal Boron Nitride Substrate

Monolayer MoS2 is synthesized on hexagonal boron nitride (h-BN) flakes with a simple, high-yield method. Monolayer MoS2 on h-BN exhibits improved optical quality. Combining the theoretical and experimental analysis, it is concluded that the enhanced photoluminescence and Raman intensities of monolayer MoS2 probably originate from the relatively weak doping effect from the h-BN substrate rather than the optical interference effect.

Interference effect on optical signals of monolayer MoS2

Both Raman spectroscopy and the photoluminescence (PL) spectroscopy are intensively used in studying the characteristics of monolayer MoS2. However, the substrate-related interference effect will influence the optical signal intensities collected from monolayer MoS2. In this work, we investigated the influence of multilayer film interference on the optical signal intensity of monolayer MoS2 on SiO2/Si substrate. Based on our results, the most commonly used substrate for monolayer MoS2, SiO2/Si with SiO2 thickness around 280–300u2009nm, is not the optimized choice. By considering the interference effect caused by the Air-MoS2-SiO2 multilayers, we can now enhance the optical signal intensity of monolayer MoS2 greatly by selecting proper SiO2 thickness for any chosen incident light wavelength.

A unique strategy for improving top contact in Si/ZnO hierarchical nanoheterostructure photodetectors

Three-dimensional p-Si nanopillar/n-ZnO nanowire-array hierarchical nanoheterostructure photodetectors were achieved via a highly accessible and controllable fabrication process. By introducing PMMA to provide a flat, continuous and uniform surface, unique high transparency, low resistance top contact has been fabricated. The improved front electrode showed very small sheet resistivity of 0.002 Ω cm, two orders of magnitude lower than that of direct deposition of ITO onto ZnO (0.58 Ω cm).

Multi-component continuous separation chip composed of micropillar arrays in a split-level spiral channel

The separation and concentration of blood components or biological functional microparticles, have become indispensable in various biomedical and environmental applications. In this paper, a chip with two rows of micropillar arrays along a spiral-channel, in which plasma and various blood cells with different sizes can be separated or enriched simultaneously, has been achieved. The proposed method combines crossflow, centrifugation and size-selective separation approaches. Specially, a 40 μm-high step, which can increase the pressure difference, is designed between the inner and middle channel to enhance the separation efficiency and purity. In the chip with split-level channels, the number of red blood cells (RBCs) gathered from the inner outlet decreased significantly, and the collected plasma volume increased, compared to those in the plain one.