Wai Leong Chow

Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe2

Recently, two-dimensional layers of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, and WSe2, have attracted much attention for their potential applications in electronic and optoelectronic devices. The selenide analogues of MoS2 and WS2 have smaller band gaps and higher electron mobilities, making them more appropriate for practical devices. However, reports on scalable growth of high quality transition metal diselenide layers and studies of their properties have been limited. Here, we demonstrate the chemical vapor deposition (CVD) growth of uniform MoSe2 monolayers under ambient pressure, resulting in large single crystalline islands. The photoluminescence intensity and peak position indicates a direct band gap of 1.5 eV for the MoSe2 monolayers. A back-gated field effect transistor based on MoSe2 monolayer shows n-type channel behavior with average mobility of 50 cm(2) V(-1) s(-1), a value much higher than the 4-20 cm(2) V(-1) s(-1) reported for vapor phase grown MoS2.

Paper-based all-solid-state flexible micro-supercapacitors with ultra-high rate and rapid frequency response capabilities

Paper-based flexible supercapacitors (SCs) have attracted great attention as they enable the realization of next-generation bendable, light-weight, and environmentally-friendly portable electronics. However, conventional paper-based SCs adopt a sandwich-like structure suffering from poor rate performance, slow frequency response and difficulty in direct integration with other micro-devices. We report here for the first time paper-based all-solid-state flexible planar micro-supercapacitors (MSCs) using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-CNT/Ag as the electrode material by the inkjet printing technique. The as-fabricated paper-based all-solid-state flexible MSCs deliver the best rate capability among all reported paper-based MSCs/SCs (up to 10 000 mV s−1), fast frequency response (relaxation time constant τ0 = 8.5 ms), high volumetric specific capacitance (23.6 F cm−3) and long cycle stability (92% capacitance retention after 10 000 cycles), which shows a strong dependence on the film thickness and the interdigitated spacing between neighbouring fingers, respectively. Furthermore, the series and parallel connections reveal that the as-prepared paper-based MSCs obey the basic theorem of series and parallel connections of capacitors, respectively. The combination of the simple fabrication technology and excellent performances presented here not only make paper-based all-solid-state flexible MSCs an attractive candidate for powering future flexible portable electronics, but also provide important references for the design and fabrication of other high-performance flexible energy storage devices.

Band gap effects of hexagonal boron nitride using oxygen plasma

Tuning of band gap of hexagonal boron nitride (h-BN) has been a challenging problem due to its inherent chemical stability and inertness. In this work, we report the changes in band gaps in a few layers of chemical vapor deposition processed as-grown h-BN using a simple oxygen plasma treatment. Optical absorption spectra show a trend of band gap narrowing monotonically from 6 eV of pristine h-BN to 4.31 eV when exposed to oxygen plasma for 12 s. The narrowing of band gap causes the reduction in electrical resistance by ∼100 fold. The x-ray photoelectron spectroscopy results of plasma treated hexagonal boron nitride surface show the predominant doping of oxygen for the nitrogen vacancy. Energy sub-band formations inside the band gap of h-BN, due to the incorporation of oxygen dopants, cause a red shift in absorption edge corresponding to the band gap narrowing.

Direct growth of nanocrystalline hexagonal boron nitride films on dielectric substrates

Atomically thin hexagonal-boron nitride (h-BN) films are primarily synthesized through chemical vapor deposition (CVD) on various catalytic transition metal substrates. In this work, a single-step metal-catalyst-free approach to obtain few- to multi-layer nanocrystalline h-BN (NCBN) directly on amorphous SiO2/Si and quartz substrates is demonstrated. The as-grown thin films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited substrate as inspected using optical and atomic force microscopy. The starting layers of NCBN orient itself parallel to the substrate, initiating the growth of the textured thin film. Formation of NCBN is due to the random and uncontrolled nucleation of h-BN on the dielectric substrate surface with no epitaxial relation, unlike on metal surfaces. The crystallite size is ∼25 nm as determined by Raman spectroscopy. Transmission electron microscopy shows that the NCBN formed sheets of multi-stacked layers with controllable thickness from ∼2 to 25...

Re-ordering Chaotic Carbon: Origins and Application of Textured Carbon

Formation of nanocrystals with preferred orientation within the amorphous carbon matrix has attracted lots of theoretical and experimental attentions recently. Interesting properties of this films, easy fabrication methods and practical problems associated with the growth of other carbon nanomaterials such as carbon nanotubes (CNTs) and graphene gives this new class of carbon nanostructure a potential to be considered as a replacement for some applications such as thermal management at nanoscale and interconnects. In this short review paper, the fabrication techniques and associated formation mechanisms of these nanostructured films have been discussed. Besides, electrical and thermal properties of these nanostructured films have been compared with CNTs and graphene.

High Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe2 ), a noble-transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field-effect transistors (FETs) based on ultrathin PdSe2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe2 to exhibit electron-dominated transport with high mobility (µe (max) = 216 cm2 V-1 s-1 ) and on/off ratio up to 103 . Hole-dominated-transport PdSe2 can be obtained by molecular doping using F4 -TCNQ. This pioneer work on PdSe2 will spark interests in the less explored regime of noble-TMDCs.

Identifying the mechanisms of p-to-n conversion in unipolar graphene field-effect transistors

The mechanisms of p-to-n conversion and vice versa in unipolar graphene field-effect transistors (GFETs) were systematically studied using Raman spectroscopy. Unipolar p-type GFETs are achieved by decorating the graphene surface with a thin layer of titanium (Ti) film, resulting in a Raman D peak. The D peak is observed to recover by annealing the GFET in nitrogen ambient followed by silicon nitride (Si3N4) deposition, suggesting that the Ti adatoms are being partially removed. Furthermore, unipolar n-type GFETs are obtained after the passivation on p-type GFETs. The threshold voltage of the n-type GFET is dependent on the thickness of the Si3N4 layer, which increases as the thickness decreases. A comparison between the Si3N4 and SiO2 passivation layers shows that SiO2 passivation does not convert the GFET into n-type graphene, which identifies the significance of ammonia (NH3) for the formation of the n-type GFETs. This study provides an insight into the mechanism of controlling the conduction behavior of unipolar GFETs.