Yeung Yu Hui

Exceptional tunability of band energy in a compressively strained trilayer MoS2 sheet

Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

Layer-Dependent Nonlinear Optical Properties and Stability of Non-Centrosymmetric Modification in Few-Layer GaSe Sheets†

Gallium selenide, an important second-order nonlinear semiconductor, has received much scientific interest. However, the nonlinear properties in its two-dimensional (2D) form are still unknown. A strong second harmonic generation (SHG) in bilayer and multilayer GaSe sheets is reported. This is also the first observation of SHG on 2D GaSe thin layers. The SHG of multilayer GaSe above five layers shows a quadratic dependence on the thickness; while that of a sheet thinner than five layers shows a cubic dependence. The discrepancy between the two SHG responses is attributed to the weakened stability of non-centrosymmetric GaSe in the atomically thin flakes where a layer-layer stacking order tends to favor centrosymmetric modification. Importantly, two-photon excited fluorescence has also been observed in the GaSe sheets. Our free-energy calculations based on first-principles methods support the observed nonlinear optical phenomena of the atomically thin layers.

Highly impermeable and transparent graphene as an ultra-thin protection barrier for Ag thin films

Ag thin films have a wide variety of applications in optics. However, Ag is chemically unstable under atmospheric conditions, which significantly degrades its optical properties and hinders its practical applications. Conventional protective coatings retard or inhibit the corrosion of Ag, but also alter the optical properties of Ag substantially. In this work, we transfer highly impermeable and transparent monolayer graphene onto the surface of Ag thin films as an ultra-thin protection barrier. We comparatively study the morphological and spectroscopic characteristics of the Ag thin films with and without the graphene protective barrier, revealing the high corrosion-resistance of monolayer graphene to gases and liquids. The Tafel analysis shows that the corrosion rate of the Ag thin film is reduced by about 66 times by the use of a graphene protection barrier. We further demonstrate that the graphene coated Ag thin films can be used for optical applications, including optical mirrors and surface enhanced Raman spectroscopy substrates. Our results show that monolayer graphene as a protective barrier simultaneously maintains the high stability and unique optical properties of Ag thin films.

Effects of controllable biaxial strain on the Raman spectra of monolayer graphene prepared by chemical vapor deposition

Controllable biaxial strain is delivered to monolayer graphene prepared by chemical vapor deposition via applying an electric field to the underlying piezoelectric [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 substrate. The effects of tunable strain on the Raman spectra of graphene are investigated in reversible and real-time manners. Such strain can result in a blue shift in 2D band of graphene. The calculations based on the Gruneisen parameter identify the actual biaxial strain to graphene, leading to a continuous 2D band shift, which is detected during the retention of bias voltage. The physical mechanism behind this unique Raman behavior is discussed.

A deep ultraviolet to near-infrared photoresponse from glucose-derived graphene oxide

Graphene oxide (GO) was synthesized by a hydrothermal method using glucose solution as the sole reagent. The wavelength-dependent photoresponse of GO was investigated by fabricating metal–GO–metal photodetectors. The devices demonstrated a broadband photoresponse from 290 to 1610 nm covering deep ultraviolet (UV) to near-infrared (NIR), which is the broadest spectral range yet demonstrated on GO. The response times of the photodetectors in the UV and visible range are about 100 ms, which are at least one order of magnitude faster than photodetectors based solely on GO reported previously. The responsivity of the photodetector can be as high as 23.6 mA W−1 in the visible range. The wavelength-dependent photoresponse is closely related to the absorption characteristics of GO. Potential for a self-powered GO based photodetector is first demonstrated, and the device shows a prominent photoresponse at zero bias. The GO based photodetectors pave the way for developing low-cost, broadband, self-powered as well as spectrally tuneable photodetectors.

n- and p-Type modulation of ZnO nanomesh coated graphene field effect transistors.

Periodic zinc oxide (ZnO) nanomeshes of different thicknesses were deposited on single-layer graphene to form back-gated field effect transistors (GFETs). The GFETs exhibit tunable electronic properties, featuring n- and p-type characteristics by merely controlling the thickness of the ZnO nanomesh layer. Furthermore, the effect of thermal strain on the GFETs from the substrate is suppressed by the ZnO nanomesh, which improves the thermal stability of the GFETs. This nanopatterning technique could modulate the electronic properties of the GFETs effectively.

Solution-processable graphene oxide as an insulator layer for metal–insulator–semiconductor silicon solar cells

Development of a high quality but low cost insulating layer is important for its application in photovoltaic cells. In this work, solution processable graphene oxide (GO) thin films were first utilized as an insulating layer to construct MIS silicon solar cells. The efficient water-soluble GO nano-sheets with controlled thickness produced a good contact with the hydrophilic silicon surfaces. The average open circuit voltage (VOC) of the GO incorporated MIS silicon solar cell was increased by 0.2 V, while their power conversion efficiency (PCE) was demonstrated as 88% higher than that of the corresponding Schottky solar cells. The improvement of the device performance in the GO-based MIS silicon solar cell is attributed to the increased built-in potential as well as the reduced interface defect, resulting in reduced carrier recombination. The ability to establish a low-cost and solution-processable insulating layer will open the door for wide application in photovoltaic and other optoelectronic devices.

Electroluminescence from AlN nanowires grown on p-SiC substrate

Aluminum nitride (AlN) nanowires were prepared by the carbothermal reduction method. A heterojunction light-emitting diode (LED) was fabricated by depositing randomly aligned AlN nanowires onto p-type 4H–SiC substrate. When a forward bias voltage greater than 8 V was applied to the LED, a broad band emission peaked at 417 nm could be observed. The peak deconvolution revealed four emission peaks at ∼400, 420, 468, and 525 nm. These emission peaks may be attributed to the radiative recombination between electrons from trap-level states and holes from the valence band of the AlN nanowires.

Application of a graphene buffer layer for the growth of high quality SnS films on GaAs(100) substrate

Tin mono-sulfide (SnS) thin films have been grown by molecular beam epitaxy (MBE) on two different substrates, GaAs (100) and soda lime glass at 400°C. High resolution X-ray Diffraction (HXRD) and Scanning Electron Microscopy (SEM) are used to characterize the structural properties of the as grown SnS films. By introducing a graphene buffer layer between the SnS thin film and the substrate, the XRD rocking curves full width at half maximum (FWHM) of the SnS film grown on GaAs (100) and soda lime glass decrease from 2.92° to 0.37° and from 6.58° to 2.04° respectively, indicating a significant improvement of SnS thin films.