Haiyan Nan

Hopping transport through defect-induced localized states in molybdenum disulphide

Molybdenum disulphide is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. Here we report a study of transport in few-layer molybdenum disulphide, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulphur vacancies exist in molybdenum disulphide, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbour hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of molybdenum disulphide.

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

AbstractThere have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.

Layer-by-Layer Thinning of MoS2 by Plasma

The electronic structures of two-dimensional materials are strongly dependent on their thicknesses; for example, there is an indirect to direct band gap transition from multilayer to single-layer MoS2. A simple, efficient, and nondestructive way to control the thickness of MoS2 is highly desirable for the study of thickness-dependent properties as well as for applications. Here, we present layer-by-layer thinning of MoS2 nanosheets down to monolayer by using Ar(+) plasma. Atomic force microscopy, high-resolution transmission electron microscopy, optical contrast, Raman, and photoluminescence spectra suggest that the top layer MoS2 is totally removed by plasma while the bottom layer remains almost unaffected. The evolution of Raman and photoluminescence spectra of MoS2 with thickness change is also investigated. Finally, we demonstrate that this method can be used to prepare two-dimensional heterostructures with periodical single-layer and bilayer MoS2. The plasma thinning of MoS2 is very reliable (with almost 100% success rate), can be easily scaled up, and is compatible with standard semiconductor process to generate heterostructures/patterns at nanometer scale, which may bring out interesting properties and new physics.

Two-dimensional quasi-freestanding molecular crystals for high-performance organic field-effect transistors

Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically smooth and effectively decoupled from the substrate due to weak van der Waals interactions, affording a pristine interface for high-performance organic transistors. As a result, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10 cm(2) V(-1) s(-1) and aggressively scaled saturation voltage ~1 V. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.

High‐Performance Monolayer WS2 Field‐Effect Transistors on High‐κ Dielectrics

The combination of high-quality Al2 O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm(2) V(-1) s(-1) (337 cm(2) V(-1) s(-1) ) is reached at room temperature (low temperature) for monolayer WS2 . A theoretical model for electron transport is also developed.

Epitaxial Ultrathin Organic Crystals on Graphene for High-Efficiency Phototransistors

Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W(-1) and a photoconductive gain over 10(8) . The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy.

A van der Waals pn heterojunction with organic/inorganic semiconductors

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/optical properties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate the hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C8-BTBT) and n-type MoS2. We find that few-layer C8-BTBT molecular crystals can be grown on monolayer MoS2 by vdW epitaxy, with pristine interface and controllable thickness down to monolayer. The operation of the C8-BTBT/MoS2 vertical heterojunction devices is highly tunable by bias and gate voltages between three different regimes: interfacial recombination, tunneling, and blocking. The pn junction shows diode-like behavior with rectifying ratio up to 105 at the room temperature. Our devices also exhibit photovoltaic responses with a power conversion efficiency of 0.31% and a photoresponsivity of 22 mA/W. With wide material combinations, such hybrid 2D structures will offer possibilities for opto-electronic devices that are not possible from individual constituents.

Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions

Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.

Graphene surface plasmon induced optical field confinement and lasing enhancement in ZnO whispering-gallery microcavity.

Fundamental physics under the surface plasmon (SP) of graphene and the functional application beyond ultraviolet (UV) lasing of ZnO are both fascinating research areas. Herein, the optical field confinement induced by graphene SP was simulated theoretically in a graphene-coated ZnO microrod, which acted as a whispering-gallery microcavity for lasing resonance. Distinct optical field confinement and photoluminescence (PL) enhancement were observed experimentally. Stable and transient spectra were employed to analyze the PL enhancement and the coupling dynamics between graphene SP and ZnO interband emission. As a functional application, the graphene-coated ZnO microcavities presented the obviously improved whispering-gallery mode (WGM) lasing performance. These results would be valuable for designing novel optical and photoelectronic devices based on SP coupling in graphene-semiconductor hybrid materials.