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.

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.

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Single-layer graphene was transferred onto (1 − x)[Pb(Mg 1/3 Nb 2/3 )O 3 ]−x[PbTiO 3 ] 0.3 (PMN-PT) substrate to investigate the transport properties of graphene-based field effect transistors (FETs) by ferroelectric gating. The graphene/PMN-PT FET exhibited p-type characteristics with a large memory window and an on/off current ratio of about 5.5 in air ambient conditions at room temperature. By prepoling the PMN-PT substrate, the FET showed a reduction in p-doping for the graphene/PMN-PT FET, implying the pre-polarization and the polarization reversal played an important part in the behaviors of graphene on PMN-PT. The observation of simultaneous rise in gate current with the dramatic transition in drain current suggested that the transport properties of graphene mainly stemmed from the coupling of the ferroelectric polarization to the charge carriers in graphene. The field effect mobility and the excess hole concentration were calculated to be about 4.52 × 10 3 cm 2 V −1 s −1 and 6.74 × 10 12 cm −2 , respectively. Furthermore, the sheet resistance showed high dependence on temperature and gate voltage, indicating metallic behaviors of graphene on PMN-PT. Additionally, the sheet resistance of graphene on the PMN-PT was much smaller than that on SiO 2 .

Articles you may be interested inComplete band offset characterization of the HfO 2 / SiO 2 / Si stack using charge corrected x-ray photoelectron spectroscopy Structural and electronic properties of boron nitride thin films containing siliconAlthough an increasing volume of x-ray photoemission spectroscopic ͑XPS͒ data has been accumulated on boron and boron-rich compounds because of their unusual properties, including a unique three-center, two-electron bonding configuration, their common nonmetallic nature has been overlooked. Typically, the measured energy-state data are not clarified by surface Fermi level positions of these nonmetallic samples, which compromises the scientific contents of the data. In the present study, we revisited the XPS studies of sputter-cleaned ␤-rhombohedral boron (␤ r -B), the oxidized surface of ␤ r -B, B 6 O pellet, and polished B 2 O 3 , to illustrate the impact and resolution of this scientific issue. These samples were chosen because ␤ r -B is the most thermodynamically stable polytype of pure boron, B 2 O 3 is its fully oxidized form, and B 6 O is the best known superhard family member of boron-rich compounds. From our XPS measurements, including those from a sputter-cleaned gold as a metal reference, we deduced that our ␤ r -B had a surface Fermi level located at 0.7Ϯ0.1 eV from its valence-band maximum ͑VBM͒ ͑referred as E FL ) and a binding energy for its B 1s core level at 187.2 eV from VBM (E b,VBM ). The latter attribute, unlike typical XPS binding energy data that are referenced to a sample-dependent Fermi level (E b,FL ), is immune from any uncertainties and variations arising from sample doping and surface charging. For bulk B 2 O 3 , we found an E b,VBM for its B 1s core level at 190.5 eV and an E b,FL at 193.6 eV. For our ␤ r -B subjected to a surface oxidation treatment, an overlayer structure of ϳ1.2 nm B 2 O 3 / ϳ2 nm B 2 O/B was found. By comparing the data from this sample and those from ␤ r -B and bulk B 2 O 3 , we infer that the oxide overlayer carried some negative fixed charge and this induced on the semiconducting ␤ r -B sample an upward surface band bending of ϳ0.6 eV. As for our B 6 O sample, we found an E FL of ϳ1.7 eV and two different chemical states having E b,VBM of 185.4 and 187.2 eV, with the former belonging to boron with no oxygen neighbor and the latter to boron with an oxygen neighbor. The methodology in this work is universally applicable to all nonmetallic samples.

We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2–3 μm is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, α, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of α of the order of 104 cm−1. Sharp cutoff in the values of α, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concentration of localized states in the bandgap. Cu-doping is achieved by co-evaporation technique. It is demonstrated that the hole concentration of the films can be controlled between 1016 cm−3 and 5 × 1017cm−3 by varying the temperature of the Cu K-cell. Hole mobility as high as 81 cm2V−1s−1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the surface for both SnS and the graphene buffer layer. Consequently, the interaction between the SnS thin films and the graphene buffer layer is dominated by van der Waals force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.

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 Grüneisen 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.

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.