8 research outputs found
Step-Edge-Guided Nucleation and Growth of Aligned WSe<sub>2</sub> on Sapphire <i>via</i> a Layer-over-Layer Growth Mode
Two-dimensional (2D) materials beyond graphene have drawn a lot of attention recently. Among the large family of 2D materials, transitional metal dichalcogenides (TMDCs), for example, molybdenum disulfides (MoS<sub>2</sub>) and tungsten diselenides (WSe<sub>2</sub>), have been demonstrated to be good candidates for advanced electronics, optoelectronics, and other applications. Growth of large single-crystalline domains and continuous films of monolayer TMDCs has been achieved recently. Usually, these TMDC flakes nucleate randomly on substrates, and their orientation cannot be controlled. Nucleation control and orientation control are important steps in 2D material growth, because randomly nucleated and orientated flakes will form grain boundaries when adjacent flakes merge together, and the formation of grain boundaries may degrade mechanical and electrical properties of as-grown materials. The use of single crystalline substrates enables the alignment of as-grown TMDC flakes via a substrate–flake epitaxial interaction, as demonstrated recently. Here we report a step-edge-guided nucleation and growth approach for the aligned growth of 2D WSe<sub>2</sub> by a chemical vapor deposition method using C-plane sapphire as substrates. We found that at temperatures above 950 °C the growth is strongly guided by the atomic steps on the sapphire surface, which leads to the aligned growth of WSe<sub>2</sub> along the step edges on the sapphire substrate. In addition, such atomic steps facilitate a layer-over-layer overlapping process to form few-layer WSe<sub>2</sub> structures, which is different from the classical layer-by-layer mode for thin-film growth. This work introduces an efficient way to achieve oriented growth of 2D WSe<sub>2</sub> and adds fresh knowledge on the growth mechanism of WSe<sub>2</sub> and potentially other 2D materials
High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS<sub>2</sub> Transistors
Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS<sub>2</sub> as high-performance room temperature chemical sensors. The Schottky-contacted MoS<sub>2</sub> transistors show current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO<sub>2</sub> and NH<sub>3</sub>. Specifically, the MoS<sub>2</sub> sensors show clear detection of NO<sub>2</sub> and NH<sub>3</sub> down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS<sub>2</sub> sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors
Screw-Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid-like WSe<sub>2</sub> by Sulfur-Assisted Chemical Vapor Deposition
Two-dimensional (2D) layered tungsten diselenides (WSe<sub>2</sub>) material has recently drawn a lot of attention due to its unique optoelectronic properties and ambipolar transport behavior. However, direct chemical vapor deposition (CVD) synthesis of 2D WSe<sub>2</sub> is not as straightforward as other 2D materials due to the low reactivity between reactants in WSe<sub>2</sub> synthesis. In addition, the growth mechanism of WSe<sub>2</sub> in such CVD process remains unclear. Here we report the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe<sub>2</sub> flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe<sub>2</sub> flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO<sub>3</sub>, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herringbone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe<sub>2</sub>. The generation and propagation mechanisms of screw dislocations during the growth of WSe<sub>2</sub> were discussed. Back-gated field-effect transistors were made on these 2D WSe<sub>2</sub> materials, which show on/off current ratios of 10<sup>6</sup> and mobility up to 44 cm<sup>2</sup>/(V·s)
Patterning, Characterization, and Chemical Sensing Applications of Graphene Nanoribbon Arrays Down to 5 nm Using Helium Ion Beam Lithography
Bandgap engineering of graphene is an essential step toward employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of <10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization, and superior chemical sensing of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO<sub>2</sub> gas sensitivity of the 5 nm GNR array devices down to parts per billion (ppb) levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications
Black Phosphorus Gas Sensors
The utilization of black phosphorus and its monolayer (phosphorene) and few-layers in field-effect transistors has attracted a lot of attention to this elemental two-dimensional material. Various studies on optimization of black phosphorus field-effect transistors, PN junctions, photodetectors, and other applications have been demonstrated. Although chemical sensing based on black phosphorus devices was theoretically predicted, there is still no experimental verification of such an important study of this material. In this article, we report on chemical sensing of nitrogen dioxide (NO<sub>2</sub>) using field-effect transistors based on multilayer black phosphorus. Black phosphorus sensors exhibited increased conduction upon NO<sub>2</sub> exposure and excellent sensitivity for detection of NO<sub>2</sub> down to 5 ppb. Moreover, when the multilayer black phosphorus field-effect transistor was exposed to NO<sub>2</sub> concentrations of 5, 10, 20, and 40 ppb, its relative conduction change followed the Langmuir isotherm for molecules adsorbed on a surface. Additionally, on the basis of an exponential conductance change, the rate constants for adsorption and desorption of NO<sub>2</sub> on black phosphorus were extracted for different NO<sub>2</sub> concentrations, and they were in the range of 130–840 s. These results shed light on important electronic and sensing characteristics of black phosphorus, which can be utilized in future studies and applications
High-Performance WSe<sub>2</sub> Field-Effect Transistors <i>via</i> Controlled Formation of In-Plane Heterojunctions
Monolayer
WSe<sub>2</sub> is a two-dimensional (2D) semiconductor with a direct
band gap, and it has been recently explored as a promising material
for electronics and optoelectronics. Low field-effect mobility is
the main constraint preventing WSe<sub>2</sub> from becoming one of
the competing channel materials for field-effect transistors (FETs).
Recent results have demonstrated that chemical treatments can modify
the electrical properties of transition metal dichalcogenides (TMDCs),
including MoS<sub>2</sub> and WSe<sub>2</sub>. Here, we report that
controlled heating in air significantly improves device performance
of WSe<sub>2</sub> FETs in terms of on-state currents and field-effect
mobilities. Specifically, after being heated at optimized conditions,
chemical vapor deposition grown monolayer WSe<sub>2</sub> FETs showed
an average FET mobility of 31 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> and on/off current ratios up to 5 ×
10<sup>8</sup>. For few-layer WSe<sub>2</sub> FETs, after the same
treatment applied, we achieved a high mobility up to 92 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. These values
are significantly higher than FETs fabricated using as-grown WSe<sub>2</sub> flakes without heating treatment, demonstrating the effectiveness
of air heating on the performance improvements of WSe<sub>2</sub> FETs.
The underlying chemical processes involved during air heating and
the formation of in-plane heterojunctions of WSe<sub>2</sub> and WO<sub>3–<i>x</i></sub>, which is believed to be the reason
for the improved FET performance, were studied by spectroscopy and
transmission electron microscopy. We further demonstrated that, by
combining the air heating method developed in this work with supporting
2D materials on the BN substrate, we achieved a noteworthy field-effect
mobility of 83 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> for monolayer WSe<sub>2</sub> FETs. This work is
a step toward controlled modification of the properties of WSe<sub>2</sub> and potentially other TMDCs and may greatly improve device
performance for future applications of 2D materials in electronics
and optoelectronics
Highly Sensitive and Wearable In<sub>2</sub>O<sub>3</sub> Nanoribbon Transistor Biosensors with Integrated On-Chip Gate for Glucose Monitoring in Body Fluids
Nanoribbon-
and nanowire-based field-effect transistor (FET) biosensors
have stimulated a lot of interest. However, most FET biosensors were
achieved by using bulky Ag/AgCl electrodes or metal wire gates, which
have prevented the biosensors from becoming truly wearable. Here,
we demonstrate highly sensitive and conformal In<sub>2</sub>O<sub>3</sub> nanoribbon FET biosensors with a fully integrated on-chip
gold side gate, which have been laminated onto various surfaces, such
as artificial arms and watches, and have enabled glucose detection
in various body fluids, such as sweat and saliva. The shadow-mask-fabricated
devices show good electrical performance with gate voltage applied
using a gold side gate electrode and through an aqueous electrolyte.
The resulting transistors show mobilities of ∼22 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> in 0.1× phosphate-buffered
saline, a high on–off ratio (10<sup>5</sup>), and good mechanical
robustness. With the electrodes functionalized with glucose oxidase,
chitosan, and single-walled carbon nanotubes, the glucose sensors
show a very wide detection range spanning at least 5 orders of magnitude
and a detection limit down to 10 nM. Therefore, our high-performance
In<sub>2</sub>O<sub>3</sub> nanoribbon sensing platform has great
potential to work as indispensable components for wearable healthcare
electronics
Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe<sub>2</sub> and Applications for Devices
Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe<sub>2</sub> are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe<sub>2</sub> as active channel materials. Recently, a phase engineering concept of 2D MoS<sub>2</sub> materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe<sub>2</sub> and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe<sub>2</sub>. We have also shown that metallic phase WSe<sub>2</sub> can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe<sub>2</sub> flakes with phase-engineered metallic 1T-WSe<sub>2</sub> as contact regions and intact semiconducting 2H-WSe<sub>2</sub> as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10<sup>7</sup> and mobilities up to 66 cm<sup>2</sup>/V·s for monolayer WSe<sub>2</sub>. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials