Jian Zhen Ou

Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Electrochemical Control of Photoluminescence in Two-Dimensional MoS2 Nanoflakes

Two-dimensional (2D) transition metal dichalcogenide semiconductors offer unique electronic and optical properties, which are significantly different from their bulk counterparts. It is known that the electronic structure of 2D MoS2, which is the most popular member of the family, depends on the number of layers. Its electronic structure alters dramatically at near atomically thin morphologies, producing strong photoluminescence (PL). Developing processes for controlling the 2D MoS2 PL is essential to efficiently harness many of its optical capabilities. So far, it has been shown that this PL can be electrically or mechanically gated. Here, we introduce an electrochemical approach to actively control the PL of liquid-phase-exfoliated 2D MoS2 nanoflakes by manipulating the amount of intercalated ions including Li(+), Na(+), and K(+) into and out of the 2D crystal structure. These ions are selected as they are crucial components in many bioprocesses. We show that this controlled intercalation allows for large PL modulations. The introduced electrochemically controlled PL will find significant applications in future chemical and bio-optical sensors as well as optical modulators/switches.

Ion-Driven Photoluminescence Modulation of Quasi-Two-Dimensional MoS2 Nanoflakes for Applications in Biological Systems

Quasi-two-dimensional (quasi-2D) molybdenum disulfide (MoS2) is a photoluminescence (PL) material with unique properties. The recent demonstration of its PL, controlled by the intercalation of positive ions, can lead to many opportunities for employing this quasi-2D material in ion-related biological applications. Here, we present two representative models of biological systems that incorporate the ion-controlled PL of quasi-2D MoS2 nanoflakes. The ion exchange behaviors of these two models are investigated to reveal enzymatic activities and cell viabilities. While the ion intercalation of MoS2 in enzymatic activities is enabled via an external applied voltage, the intercalation of ions in cell viability investigations occurs in the presence of the intrinsic cell membrane potential.

Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

The anodized crystalline WO3 nanoporous network with enhanced electrochromic properties

We demonstrate that a three dimensional (3D) crystalline tungsten trioxide (WO(3)) nanoporous network, directly grown on a transparent conductive oxide (TCO) substrate, is a suitable working electrode material for high performance electrochromic devices. This nanostructure, with achievable thicknesses of up to 2 μm, is prepared at room temperature by the electrochemical anodization of a RF-sputtered tungsten film deposited on a fluoride doped tin oxide (FTO) conductive glass, under low applied anodic voltages and mild chemical dissolution conditions. For the crystalline nanoporous network with thicknesses ranging from 0.6 to 1 μm, impressive coloration efficiencies of up to 141.5 cm(2) C(-1) are achieved by applying a low coloration voltage of -0.25 V. It is also observed that there is no significant degradation of the electrochromic properties of the porous film after 2000 continuous coloration-bleaching cycles. The remarkable electrochromic characteristics of this crystalline and nanoporous WO(3) are mainly ascribed to the combination of a large surface area, facilitating increased intercalation of protons, as well as excellent continuous and directional paths for charge transfer and proton migration in the highly crystalline material.

Elevated temperature anodized Nb2O5: A photoanode material with exceptionally large photoconversion efficiencies

Here, we demonstrate that niobium pentoxide (Nb(2)O(5)) is an ideal candidate for increasing the efficiencies of dye-sensitized solar cells (DSSCs). The key lies in developing a Nb(2)O(5) crisscross nanoporous network, using our unique elevated temperature anodization process. For the same thicknesses of ∼4 μm, the DSSC based on the Nb(2)O(5) layer has a significantly higher efficiency (∼4.1%) when compared to that which incorporates a titanium dioxide nanotubular layer (∼2.7%). This is the highest efficiency among all of the reported photoanodes for such a thickness when utilizing back-side illumination. We ascribe this to a combination of reduced electron scattering, greater surface area, wider band gap, and higher conduction band edge, as well as longer effective electron lifetimes.

Characterization of metal contacts for two-dimensional MoS2 nanoflakes

While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts. This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS2. It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contacts.

Field Effect Biosensing Platform Based on 2D α-MoO3

Electrical-based biosensing platforms offer ease of fabrication and simple sensing solutions. Recently, two-dimensional (2D) semiconductors have been proven to be excellent for the fabrication of field effect transistors (FETs) due to their large transconductance, which can be efficiently used for developing sensitive bioplatforms. We present a 2D molybdenum trioxide (MoO3) FET based biosensing platform, using bovine serum albumin as a model protein. The conduction channel is a nanostructured film made of 2D α-MoO3 nanoflakes, with the majority of nanoflake thicknesses being equal to or less than 2.8 nm. The response time is impressively low (less than 10 s), which is due to the high permittivity of the 2D α-MoO3 nanoflakes. The system offers a competitive solution for future biosensing applications.

Electronic Tuning of 2D MoS2 through Surface Functionalization

The electronic properties of thiol-functionalized 2D MoS2 nanosheets are investigated. Shifts in the valence and conduction bands and Fermi levels are observed while bandgaps remain unaffected. These findings allow the tuning of energy barriers between 2D MoS2 and other materials, which can lead to improved control over 2D MoS2 -based electronic and optical devices and catalysts.

Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals

A variety of deposition methods for two-dimensional crystals have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we introduce a technique for depositing and patterning of wafer-scale two-dimensional metal chalcogenide compounds by transforming the native interfacial metal oxide layer of low melting point metal precursors (group III and IV) in liquid form. In an oxygen-containing atmosphere, these metals establish an atomically thin oxide layer in a self-limiting reaction. The layer increases the wettability of the liquid metal placed on oxygen-terminated substrates, leaving the thin oxide layer behind. In the case of liquid gallium, the oxide skin attaches exclusively to a substrate and is then sulfurized via a relatively low temperature process. By controlling the surface chemistry of the substrate, we produce large area two-dimensional semiconducting GaS of unit cell thickness (∼1.5 nm). The presented deposition and patterning method offers great commercial potential for wafer-scale processes.