Yuting Shen

Towards intrinsic charge transport in monolayer molybdenum disulfide by defect and interface engineering

Molybdenum disulfide is considered as one of the most promising two-dimensional semiconductors for electronic and optoelectronic device applications. So far, the charge transport in monolayer molybdenum disulfide is dominated by extrinsic factors such as charged impurities, structural defects and traps, leading to much lower mobility than the intrinsic limit. Here we develop a facile low-temperature thiol chemistry route to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps. High mobility >80u2009cm(2)u2009V(-1)u2009s(-1) is achieved in backgated monolayer molybdenum disulfide field-effect transistors at room temperature. Furthermore, we develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps. Our combined experimental and theoretical study provides a clear path towards intrinsic charge transport in two-dimensional dichalcogenides for future high-performance device applications.

Enhanced photocatalytic activity of hierarchical ZnO nanoplate-nanowire architecture as environmentally safe and facilely recyclable photocatalyst

A hierarchical ZnO nanoplate-nanowire (ZNP-ZNW) architecture immobilized onto a substrate via a facile two-step synthesis strategy was used as an environmentally safe and recyclable photocatalyst. It showed greatly enhanced photocatalytic activity compared with a monomorphological ZnO nanoplate structure in the degradation of methyl orange (MO). The higher content of surface oxygen defects, which can capture the photogenerated electrons and holes separately and make them available for decomposing organic contaminants, is considered to play an important role in the degradation of MO and makes a major contribution to the enhanced photocatalysis. Increasing the surface-to-volume (S/V) ratio without limit cannot benefit the photocatalytic activity significantly if there are not enough defects to separate additional photogenerated charges caused by a larger S/V ratio. A detailed photocatalytic mechanism related to surface defects of the hierarchical architecture was clearly demonstrated. The present study provides a new paradigm for further understanding the photocatalytic mechanism and suggests a new direction to design high-efficiency photocatalysts based on increasing the number of surface defects of nanostructures.

Elemental superdoping of graphene and carbon nanotubes.

Doping of low-dimensional graphitic materials, including graphene, graphene quantum dots and single-wall carbon nanotubes with nitrogen, sulfur or boron can significantly change their properties. We report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphitic materials with a high level of N, S or B. The superdoping results in the following doping levels: (i) for graphene, 29.82, 17.55 and 10.79u2009at% for N-, S- and B-doping, respectively; (ii) for graphene quantum dots, 36.38u2009at% for N-doping; and (iii) for single-wall carbon nanotubes, 7.79 and 10.66u2009at% for N- and S-doping, respectively. As an example, the N-superdoping of graphene can greatly increase the capacitive energy storage, increase the efficiency of the oxygen reduction reaction and induce ferromagnetism. Furthermore, by changing the degree of fluorination, the doping level can be tuned over a wide range, which is important for optimizing the performance of doped low-dimensional graphitic materials.

Tuning the Catalytic Activity of Graphene Nanosheets for Oxygen Reduction Reaction via Size and Thickness Reduction

Currently, the fundamental factors that control the oxygen reduction reaction (ORR) activity of graphene itself, in particular, the dependence of the ORR activity on the number of exposed edge sites remain elusive, mainly due to limited synthesis routes of achieving small size graphene. In this work, the synthesis of low oxygen content (<2.5±0.2 at. %), few layer graphene nanosheets with lateral dimensions smaller than a few hundred nanometers were achieved using a combination of ionic liquid assisted grinding of high purity graphite coupled with sequential centrifugation. We show for the first time that the graphene nanosheets possessing a plethora of edges exhibited considerably higher electron transfer numbers compared to the thicker graphene nanoplatelets. This enhanced ORR activity was accomplished by successfully exploiting the plethora of edges of the nanosized graphene as well as the efficient electron communication between the active edge sites and the electrode substrate. The graphene nanosheets were characterized by an onset potential of -0.13 V vs Ag/AgCl and a current density of -3.85 mA/cm2 at -1 V, which represent the best ORR performance ever achieved from an undoped carbon based catalyst. This work demonstrates how low oxygen content nanosized graphene synthesized by a simple route can considerably impact the ORR catalytic activity and hence it is of significance in designing and optimizing advanced metal-free ORR electrocatalysts.

Defects as a factor limiting carrier mobility in WSe2: A spectroscopic investigation

The electrical performance of two-dimensional transition metal dichalcogenides (TMDs) is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence (PL) spectra of electron-beam-lithographyprocessed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2·V–1·s–1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

Atomically thin lateral p-n junction photodetector with large effective detection area

The widely used photodetector design based on atomically thin transition metal dichalcogenides (TMDs) has a lateral metal-TMD-metal junction with a fairly small, line shape photoresponsive active area at the TMD-electrode interface. Here, we report a highly efficient photodetector with extremely large photoresponsive active area based on a lateral junction of monolayer-bilayer WSe2. Impressively, the separation of the electron–hole pairs (excitons) extends onto the whole 1L–2L WSe2 junction surface. The responsivity of the WSe2 junction photodetector is over 3200 times higher than that of a monolayer WSe2 device and leads to a highest external quantum efficiency of 256% due to the efficient carrier extraction. Unlike the TMD p–n junctions modulated by dual gates or localized doping, which require complex fabrication procedures, our study establishes a simple, controllable, and scalable method to improve the photodetection performance by maximizing the active area for current generation.

Electron Beam Etching of CaO Crystals Observed Atom by Atom

With the rapid development of nanoscale structuring technology, the precision in the etching reaches the sub-10 nm scale today. However, with the ongoing development of nanofabrication the etching mechanisms with atomic precision still have to be understood in detail and improved. Here we observe, atom by atom, how preferential facets form in CaO crystals that are etched by an electron beam in an in situ high-resolution transmission electron microscope (HRTEM). An etching mechanism under electron beam irradiation is observed that is surprisingly similar to chemical etching and results in the formation of nanofacets. The observations also explain the dynamics of surface roughening. Our findings show how electron beam etching technology can be developed to ultimately realize tailoring of the facets of various crystalline materials with atomic precision.

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS2 ) and bismuth telluride (Bi2 Te3 ) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high-reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS2 and Bi2 Te3 is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS2 and Bi2 Te3 crystals with unprecedented resolution. Real-time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single-crystalline free-standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.

Formation of graphene oxide/graphene membrane on solid-state substrates via Langmuir-Blodgett self-assembly

Graphene, an important two-dimensional nanomaterial, is drawing unprecedented, worldwide attention, especially in electronics, energy storage, environmental science and biological engineering. We herein report a convenient protocol for forming a high-quality graphene/graphene oxide (GO) thin membrane on solid-state substrate via the Langmuir-Blodgett self-assembly technique. The process includes layer formation at the liquid-solid-air interface, pulling solid-state substrate vertically from the interface and chemical reduction of the membrane (selective process). Parameters which may alter the properties of membrane have been investigated. The pH was considered the most crucial factor since it affected the smoothness and continuity of the membrane. Pulling cycles had pronounced influence on the thickness of the membrane. Direct formation of the graphene-based membrane on solid-state substrates may eliminate concerns about metal particle residue from the graphene transfer process, incomplete removal of the organic chemicals, and assure the intactness and continuity of the graphene/GO layer. It is therefore believed that the procedure may shed light on the fabrication of high-quality graphene nanodevices.