Andrew Thye Shen Wee

Raman spectroscopy of epitaxial graphene on a SiC substrate

may speed up the application of graphene for future electronic devices. The interaction of EG and the SiC substrate is critical to its electronic and physical properties. In this work, the Raman spectroscopy was used to study the structure of EG and its interaction with SiC substrate. All the Raman bands of EG blueshift from that of bulk graphite and graphene made by micromechanical cleavage, which was attributed to the compressive strain induced by the substrate. A model containing 13 13 honeycomb lattice cells of graphene on carbon nanomesh was constructed to explain the origin of strain. The lattice mismatch between graphene layer and substrate causes the compressive stress of 2.27 GPa on graphene. We also demonstrate that the electronic structures of EG grown on Si- and C-terminated SiC substrates are quite different. Our experimental results shed light on the interaction between graphene and SiC substrate, which are critical to the future applications of EG.

Surface-Energy Engineering of Graphene

Contact angle goniometry is conducted for epitaxial graphene on SiC. Although only a single layer of epitaxial graphene exists on SiC, the contact angle drastically changes from 69 degrees on SiC substrates to 92 degrees on graphene. It is found that there is no thickness dependence of the contact angle from the measurements of single-, bi-, and multilayer graphene and highly ordered pyrolytic graphite (HOPG). After graphene is treated with oxygen plasma, the level of damage is investigated by Raman spectroscopy and the correlation between the level of disorder and wettability is reported. By using a low-power oxygen plasma treatment, the wettability of graphene is improved without additional damage, which can solve the adhesion issues involved in the fabrication of graphene devices.

Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection

Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.

An XPS investigation of the oxidation/corrosion of melt-spun Mg

Abstract The oxide films formed on the surfaces of melt-spun Mg exposed to air, immersed in distilled water or 3% NaCl solution saturated with Mg(OH)2 have been investigated by X-ray photoelectron spectroscopy (XPS). High resolution XPS spectra revealed two distinct oxygen species on the surface films: one assigned to O2− in MgO, the other to OH− in Mg(OH)2. Depth profiling revealed that the two species had different depth distributions in the films. The oxide film formed in air comprised a contamination outer layer and a relatively thick (5–6 nm) predominantly MgO inner layer. The film formed in distilled water or 3% NaCl solution saturated with Mg(OH)2 was mainly a mixture of Mg(OH)2 and MgO. Mg(OH)2 was predominant at the top layer and decreased gradually with depth while MgO exhibited the opposite behavior. The corrosion product formed in 3% NaCl solution was more hydrated and much thicker that the films formed in the other two conditions. Cl− ion was incorporated in the oxide film formed in 3% NaCl solution. There exists both partial and complete dissociation of adsorbed water when melt-spun pure Mg ribbons are immersed in distilled water or 3% NaCl solution saturated with Mg(OH)2.

Bandgap tunability at single-layer molybdenum disulphide grain boundaries

Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 ± 0.05 eV for single-layer, 2.10 ± 0.05 eV for bilayer and 1.75 ± 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 ± 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

Bottom-up Growth of Epitaxial Graphene on 6H-SiC(0001)

We use in situ low temperature scanning tunneling microscopy (STM) to investigate the growth mechanism of epitaxial graphene (EG) thermally grown on Si-terminated 6H-SiC(0001). Our detailed study of the transition from monolayer EG to trilayer EG reveals that EG adopts a bottom-up growth mechanism. The thermal decomposition of one single SiC bilayer underneath the EG layers causes the accumulation of carbon atoms to form a new graphene buffer layer at the EG/SiC interface. Atomically resolved STM images show that the top EG layer is physically continuous across the boundaries between the monolayer and bilayer EG regions and between the bilayer and trilayer EG regions.

Surface transfer doping induced effective modulation on ambipolar characteristics of few-layer black phosphorus

Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm(2) V(-1) s(-1) after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.

Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene

In situ low-temperature scanning tunneling microscopy is used to study the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on epitaxial graphene (EG) on 6H-SiC(0001), as well as on HOPG for comparison. PTCDA adopts a layer-by-layer growth mode, with its molecular plane lying flat on both surfaces. The PTCDA films grow continuously over the EG step edges, but not on HOPG. STS performed on single-layer PTCDA on monolayer EG shows a wide band gap larger than 3.3 eV, consistent with pristine PTCDA films. Synchrotron-based high-resolution photoemission spectroscopy reveals weak charge transfer between PTCDA and EG. This suggests weak electronic coupling between PTCDA and the underlying EG layer.

Electron-Doping-Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate

We report effective and stable electron doping of monolayer molybdenum disulfide (MoS2) by cesium carbonate (Cs2CO3) surface functionalization. The electron charge carrier concentration in exfoliated monolayer MoS2 can be increased by about 9 times after Cs2CO3 functionalization. The n-type doping effect was evaluated by in situ transport measurements of MoS2 field-effect transistors (FETs) and further corroborated by in situ ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering measurements. The electron doping enhances the formation of negative trions (i.e., a quasiparticle comprising two electrons and one hole) in monolayer MoS2 under light irradiation and significantly reduces the charge recombination of photoexcited electron-hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS2 FETs.

Surface transfer hole doping of epitaxial graphene using MoO3 thin film

Synchrotron-based in situ photoelectron spectroscopy investigations demonstrate effective surface transfer p-type doping of epitaxial graphene (EG) thermally grown on 4H–SiC(0001) via the deposition of MoO3 thin film on top. The large work function difference between EG and MoO3 facilitates electron transfer from EG to the MoO3 thin film. This leads to hole accumulation in the EG layer with an areal hole density of about 1.0×1013 cm−2, and places the Fermi level 0.38 eV below the graphene Dirac point.