Chunxiao Cong

Hysteresis of Electronic Transport in Graphene Transistors

Graphene field effect transistors commonly comprise graphene flakes lying on SiO(2) surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in a helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has a much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene transistors. These findings improved our understanding of the electrical response of graphene to its surroundings. The unique sensitivity to environment and related phenomena in graphene deserve further studies on nonvolatile memory, electrostatic detection, and chemically driven applications.

Mechanical Exfoliation and Characterization of Single‐ and Few‐Layer Nanosheets of WSe2, TaS2, and TaSe2

Single- and few-layer transition-metal dichalcogenide nanosheets, such as WSe₂ , TaS₂, and TaSe₂, are prepared by mechanical exfoliation. A Raman microscope is employed to characterize the single-layer (1L) to quinary-layer (5L) WSe₂ nanosheets and WSe₂ single crystals with a laser excitation power ranging from 20 μW to 5.1 mW. Typical first-order together with some second-order and combinational Raman modes are observed. A new peak at around 308 cm⁻¹ is observed in WSe₂ except for the 1L WSe₂, which might arise from interlayer interactions. Red shifting of the A(1g) mode and the Raman peak around 308 cm⁻¹ is observed from 1L to 5L WSe₂. Interestingly, hexagonal- and monoclinic-structured WO₃ thin films are obtained during the local oxidation of thinner (1L-3L) and thicker (4L and 5L) WSe₂ nanosheets, while laser-burned holes are found during the local oxidation of the WSe₂ single crystal. In addition, the characterization of TaS₂ and TaSe₂ thin layers is also conducted.

A general strategy toward graphene@metal oxide core–shell nanostructures for high-performance lithium storage

We demonstrate a simple, efficient, yet versatile method for the realization of core–shell assembly of graphene around various metal oxide (MO) nanostructures, including nanowires (NWs) and nanoparticles (NPs). The process is driven by (i) the ring-opening reaction between the epoxy groups and amine groups in graphene oxide (GO) platelets and amine-modified MO nanostructures, respectively, and (ii) electrostatic interaction between these two components. Nearly every single NW or NP is observed to be wrapped by graphene. To the best of our knowledge, this is the first report that substrate-supported MO NWs are fully coated with a graphene shell. As an example of the functional properties of these compound materials, the graphene@α-Fe2O3 core–shell NPs are investigated as the lithium-ion battery (LIB) electrode, which show a high reversible capacity, improved cycling stability, and excellent rate capability with respect to the pristine α-Fe2O3. The superior performance of the composite electrode is presumably attributed to the effectiveness of the graphene shell in preventing the aggregation, buffering the volume change, maintaining the integrity of NPs, as well as improving the conductivity of the electrode.

Fabrication of Co3O4-reduced graphene oxide scrolls for high-performance supercapacitor electrodes

A new type of scrolled structure of Co(3)O(4)/reduced graphene oxide (r-GO) is facilely prepared through a two-step surfactant-assisted method. This assembly enables almost every single Co(3)O(4) scroll to connect with the r-GO platelets, thus leading to remarkable electrochemical performances in terms of high specific capacitance and good rate capability.

Nonblinking, Intense Two-Dimensional Light Emitter: Monolayer WS2 Triangles

Monolayer WS2 (1L-WS2), with a direct band gap, provides an ideal platform to investigate unique properties of two-dimensional semiconductors. In this work, light emission of a 1L-WS2 triangle has been studied by using steady-state, time-resolved, and temperature-dependent photoluminescence (PL) spectroscopy. Two groups of 1L-WS2 triangles have been grown by chemical vapor deposition, which exhibit nonuniform and uniform PL, respectively. Observed nonuniform PL features, i.e., quenching and blue-shift in certain areas, are caused by structural imperfection and n-doping induced by charged defects. Uniform PL is found to be intrinsic, intense, and nonblinking, which are attributed to high crystalline quality. The binding energy of the A-exciton is extracted experimentally, which gives direct evidence for the large excitonic effect in 1L-WS2. These superior photon emission features make 1L-WS2 an appealing material for optoelectronic applications such as novel light-emitting and biosensing devices.

Raman Spectroscopy Study of Lattice Vibration and Crystallographic Orientation of Monolayer MoS2 under Uniaxial Strain

The false-color (3D type) image of the intensity of the Raman spectra of monolayer MoS2 versus both peak positions and polar angles is plotted. It shows that the strongest E2g (1+) and E2g (1-) peaks appear at different angles, reflected as the alternation of the maxima of the intensity within the frequency range of the E2g (1) mode, which is the consequence of the crystallographic orientation relevant to the strain direction as predicted by theoretical analysis.

Observation of Excitonic Fine Structure in a 2D Transition-Metal Dichalcogenide Semiconductor

Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenide monolayers (TMD 1Ls), have attracted increasing attention owing to the underlying fundamental physics (e.g., many body effects) and the promising optoelectronic applications such as light-emitting diodes. Though much progress has been made, intrinsic excitonic states of TMD 1Ls are still highly debated in theory, which thirsts for direct experimental determination. Here, we report unconventional emission and excitonic fine structure in 1L WS2 revealed by electrical doping and photoexcitation, which reflects the interplay of exciton, trion, and other excitonic states. Tunable excitonic emission has been realized in a controllable manner via electrical and/or optical injection of charge carriers. Remarkably enough, the superlinear (i.e., quadratic) emission is unambiguously observed which is attributed to biexciton states, indicating the strong Coulomb interactions in such a 2D material. In a nearly neutral 1L WS2, trions and biexcitons possess large binding energies of ∼ 10-15 and 45 meV, respectively. Moreover, our finding of electrically induced robust emission opens up a possibility to boost the luminous efficiency of emerging 1L TMD light emitting diodes.

Giant enhancement of top emission from ZnO thin film by nanopatterned Pt

The authors report the enhancement of the bandgap emission from ZnO thin films by surface modification and surface plasmon cross-coupling. 12-fold and twofold enhancements of bandgap emission from the metal side of ZnO film were observed by sputtering Pt nanopattern and Pt film onto ZnO film, respectively. Time-resolved photoluminescence indicates that the decay time is slowed down by Pt capping, contrary to common observations. The “abnormal” phenomena are interpreted by considering both the surface modification and surface plasmon coupling.

Stacking-Dependent Optical Conductivity of Bilayer Graphene

The optical conductivities of graphene layers are strongly dependent on their stacking orders. Our first-principle calculations show that, while the optical conductivities of single-layer graphene (SLG) and bilayer graphene (BLG) with Bernal stacking are almost frequency-independent in the visible region, the optical conductivity of twisted bilayer graphene (TBG) is frequency-dependent, giving rise to additional absorption features due to the band folding effect. Experimentally, we obtain from contrast spectra the optical conductivity profiles of BLG with different stacking geometries. Some TBG samples show additional features in their conductivity spectra, in full agreement with our calculation results, while a few samples give universal conductivity values similar to that of SLG. We propose that those variations of optical conductivity spectra of TBG samples originate from the difference between the commensurate and incommensurate stackings. Our results reveal that the optical conductivity measurements of graphene layers indeed provide an efficient way to select graphene films with desirable electronic and optical properties, which would greatly help the future application of those large-scale misoriented graphene films in photonic devices.

Thermal conductivity determination of suspended mono- and bilayer WS2 by Raman spectroscopy

We report the thermal conductivities of monolayer (1L) and bilayer (2L) WS2 grown by chemical vapor deposition (CVD), which are determined by use of temperature and excitation dependences of E2g1 and A1g Raman modes. The first-order temperature coefficients of E2g1 and A1g modes in both supported and suspended WS2 layers were extracted. The frequency shift of the A1g mode with temperature is larger than that of the E2g1 mode for 1L-WS2, which is attributed to stronger electron-phonon coupling for the A1g mode than that for the E2g1 mode. Moreover, by use of the shift of the phonon mode induced by laser heating, the thermal conductivities at room temperature were estimated to be 32 and 53 W/(m·K) for 1L- and 2L-WS2, respectively. Our results provide fundamental information about the thermal properties of WS2 layers, which is crucial for developing applications of atomically-thin WS2 devices.