Yao Guo

Unravelling Orientation Distribution and Merging Behavior of Monolayer MoS2 Domains on Sapphire

Monolayer MoS2 prepared by chemical vapor deposition (CVD) has a highly polycrystalline nature largely because of the coalescence of misoriented domains, which severely hinders its future applications. Identifying and even controlling the orientations of individual domains and understanding their merging behavior therefore hold fundamental significance. In this work, by using single-crystalline sapphire (0001) substrates, we designed the CVD growth of monolayer MoS2 triangles and their polycrystalline aggregates for such purposes. The obtained triangular MoS2 domains on sapphire were found to distributively align in two directions, which, as supported by density functional theory calculations, should be attributed to the relatively small fluctuations of the interface binding energy around the two primary orientations. Using dark-field transmission electron microscopy, we further imaged the grain boundaries of the aggregating domains and determined their prevalent armchair crystallographic orientations with respect to the adjacent MoS2 lattice. The coalescence of individual triangular flakes governed by unique kinetic processes is proposed for the polycrystal formation. These findings are expected to shed light on the controlled MoS2 growth toward predefined domain orientation and large domain size, thus enabling its versatile applications in next-generation nanoelectronics and optoelectronics.

Charge trapping at the MoS2-SiO2 interface and its effects on the characteristics of MoS2 metal-oxide-semiconductor field effect transistors

The field effect transistors (FETs) based on thin layer MoS2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxide-semiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO2-MoS2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS2 based metal-oxide-semiconductor FETs is adjusted from 4u2009V to −45u2009V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS2 sheet.

Electrical transport properties of individual WS2 nanotubes and their dependence on water and oxygen absorption

The electrical properties of WS2 nanotubes (NTs) were studied through measuring 59 devices. Important electrical parameters, such as the carrier concentration, mobility, and effective barrier height at the contacts, were obtained through fitting experimental non-linear I-V curves using a metal-semiconductor-metal model. The carrier mobility was found to be several orders of magnitude higher than that have been reported previously for WS2 NTs. Water absorption was found to decrease the conductivity and carrier mobility of the NTs, and could be removed when the sample was dried. Oxygen absorption also slightly decreased the conductivity of WS2 NTs.

Transversally and axially tunable carbon nanotube resonators in situ fabricated and studied inside a scanning electron microscope.

We report a new design of carbon nanotube (CNT) resonator, whose resonance frequency can be tuned not only transversally by a gate voltage, but also by the axial strain applied through directly pulling the CNT. The resonators are fabricated from individual suspended single-walled CNT (SWCNT) in situ inside a scanning electron microscope. The resonance frequency of a SWCNT resonator can be tuned by more than 20 times with an increase of quality factor when the axial strain of the SWCNT is only increased from nearly zero to 2% at room temperature. The transversal gate-tuning ability is found to be weaker than the axial-tuning ability and decrease with increasing the axial strain. The gate voltage can hardly tune the resonance frequency when the initial axial strain is larger than 0.35% and the CNT acts like a tied string. The relationship among resonance frequency, gate voltage, and initial axial strain of the CNT obtained presently will allow for the designs of CNT resonators with high frequency and large tuning range. The present resonator also shows ultrahigh sensitivity in displacement and force detection, with a resolution being better than 2.4 pm and 0.55 pN, respectively.

Distinctive in-Plane Cleavage Behaviors of Two-Dimensional Layered Materials

Mechanical exfoliation from bulk layered crystal is widely used for preparing two-dimensional (2D) layered materials, which involves not only out-of-plane interlayer cleavage but also in-plane fracture. Through a statistical analysis on the exfoliated 2D flakes, we reveal the in-plane cleavage behaviors of six representative layered materials, including graphene, h-BN, 2H phase MoS2, 1T phase PtS2, FePS3, and black phosphorus. In addition to the well-known interlayer cleavage, these 2D layered materials show a distinctive tendency to fracture along certain in-plane crystallography orientations. With theoretical modeling and analysis, these distinct in-plane cleavage behaviors can be understood as a result of the competition between the release of the elastic energy and the increase of the surface energy during the fracture process. More importantly, these in-plane cleavage behaviors provide a fast and noninvasive method using optical microscopy to identify the lattice direction of mechanical exfoliated 2D layered materials.

Contact properties of field-effect transistors based on indium arsenide nanowires thinner than 16 nm.

With the scaling down of field effect transistors (FETs) to improve performance, the contact between the electrodes and the channel becomes more and more important. Contact properties of FETs based on ultrathin InAs NWs (with the diameter ranging from sub-7 nm to 16 nm) are investigated here. Chromium (Cr) and nickel (Ni) are proven to form ohmic contact with the ultrathin InAs NWs, in contrast to a recent report (Razavieh A et al ACS Nano 8 6281). Furthermore, the contact resistance is found to depend on the NW diameter and the contact metals, which between Cr and InAs NWs increases more rapidly than that between Ni and InAs NWs when the NW diameter decreases. The origins of the contact resistance difference for the two kinds of metals are studied and NixInAs is believed to play an important role. Based on our results, it is advantageous to use Ni as contact metal for ultrathin NWs. We also observe that the FETs are still working in the diffusive regime even when the channel length is scaled down to 50 nm.

In situ multiproperty measurements of individual nanomaterials in SEM and correlation with their atomic structures.

The relationship between property and structure is one of the most important fundamental questions in the field of nanomaterials and nanodevices. Understanding the multiproperties of a given nano-object also aids in the development of novel nanomaterials and nanodevices. In this paper, we develop for the first time a comprehensive platform for in situ multiproperty measurements of individual nanomaterials using a scanning electron microscope (SEM). Mechanical, electrical, electromechanical, optical, and photoelectronic properties of individual nanomaterials, with lengths that range from less than 200 nm to 20 μm, can be measured in situ with an SEM on the platform under precisely controlled single-axial strain and environment. An individual single-walled carbon nanotube (SWCNT) was measured on the platform. Three-terminal electronic measurements in a field effect transistor structure showed that the SWCNT was semiconducting and agreed with the structure characterization by transmission electron microscopy after the in situ measurements. Importantly, we observed a bandgap increase of this SWCNT with increasing axial strain, and for the first time, the experimental results quantitatively agree with theoretical predictions calculated using the chirality of the SWCNT. The vibration performance of the SWCNT, a double-walled CNT, and a triple-walled CNT were also studied as a function of axial strain, and were proved to be in good agreement with classical beam theory, although the CNTs only have one, two, or three atomic layers, respectively. Our platform has wide applications in correlating multiproperties of the same individual nanostructures with their atomic structures.