Wenjing Jie

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.

Wafer-Scale Synthesis of High-Quality Semiconducting Two-Dimensional Layered InSe with Broadband Photoresponse

Large-scale synthesis of two-dimensional (2D) materials is one of the significant issues for fabricating layered materials into practical devices. As one of the typical III-VI semiconductors, InSe has attracted much attention due to its outstanding electrical transport property, attractive quantum physics characteristics, and dramatic photoresponse when it is reduced to atomic scale. However, scalable synthesis of single phase 2D InSe has not yet been achieved so far, greatly hindering further fundamental studies and device applications. Here, we demonstrate the direct growth of wafer-scale layered InSe nanosheets by pulsed laser deposition (PLD). The obtained InSe layers exhibit good uniformity, high crystallinity with macro texture feature, and stoichiometric growth by in situ precise control. The characterization of optical properties indicates that PLD grown InSe nanosheets have a wide range tunable band gap (1.26-2.20 eV) among the large-scale 2D crystals. The device demonstration of field-effect transistor shows the n-type channel feature with high mobility of 10 cm2 V-1 s-1. Upon illumination, InSe-based phototransistors show a broad photoresponse to the wavelengths from ultraviolet to near-infrared. The maximum photoresponsivity attains 27 A/W, plus a response time of 0.5 s for the rise and 1.7 s for the decay, demonstrating the strong and fast photodetection ability. Our findings suggest that the PLD grown InSe would be a promising choice for future device applications in the 2D limit.

Graphene/gallium arsenide-based Schottky junction solar cells

Chemical-vapor-deposited single- and bi-layer graphene sheets have been transferred onto n-type GaAs substrates. The rectifying characteristics and photovoltaic behaviors of graphene/GaAs junctions have been systematically investigated. The graphene sheets can be combined with the underlying n-type GaAs substrates to form Schottky junctions. For bilayer graphene, the Schottky junction shows photovoltaic effects with the open-circuit voltage of 0.65 V and the short-circuit current density of 10.03 mA/cm2, yielding a power conversion efficiency of 1.95%, which are superior to single-layer one. Such performance parameters are comparable to those of other pristine graphene/semiconductor junction-based devices.

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.

Colossal permittivity properties of Zn,Nb co-doped TiO2 with different phase structures

Colossal permittivity properties were studied in Zn,Nb co-doped TiO2 with different phase structures. The (Zn1/3Nb2/3)0.05Ti0.95O2 rutile ceramics were prepared by the solid state sintering technique, while the amorphous and anatase films were respectively fabricated by a pulsed laser deposition method and a subsequent rapid thermal annealing. The ceramics showed a frequency (102–106 Hz) independent dielectric response with a colossal dielectric permittivity (∼30 000), and a relatively low dielectric loss (∼0.05) at room temperature. The excellent colossal permittivity properties are comparable to those of the previously reported rutile TiO2 ceramics by co-doping trivalent and pentavalent elements. For amorphous films, the dielectric permittivity decreased, and the dielectric loss increased slightly compared to those of the ceramics. Compared with the amorphous thin films, the annealed anatase ones exhibited a simultaneous increase in both dielectric permittivity and loss at low frequency while kept almost unchanged at high frequency. These results suggest that co-doping of bivalent elements with Nb into TiO2 with various phase structures can yield colossal permittivity effects, including ultra-high dielectric permittivity, relatively low dielectric loss. Furthermore, the colossal permittivity properties may be mainly attributed to the effect of the electron-pinned defect-dipoles in Zn,Nb co-doped TiO2 with different phase structures rather than the grain boundary capacitance effect. Besides, the frequency and bias dependent dielectric properties were also investigated in thin film forms, which could be affected by the electrode-film interface and mobile ions. Our results are helpful for not only investigating the new class of colossal permittivity materials, but also developing dielectric thin film device applications.

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.

Observation of Room-Temperature Magnetoresistance in Monolayer MoS2 by Ferromagnetic Gating

Room-temperature magnetoresistance (MR) effect is observed in heterostructures of wafer-scale MoS2 layers and ferromagnetic dielectric CoFe2O4 (CFO) thin films. Through the ferromagnetic gating, an MR ratio of -12.7% is experimentally achieved in monolayer MoS2 under 90 kOe magnetic field at room temperature (RT). The observed MR ratio is much higher than that in previously reported nonmagnetic metal coupled with ferromagnetic insulator, which generally exhibited MR ratio of less than 1%. The enhanced MR is attributed to the spin accumulation at the heterostructure interface and spin injection to the MoS2 layers by the strong spin-orbit coupling effect. The injected spin can contribute to the spin current and give rise to the MR by changing the resistance of MoS2 layers. Furthermore, the MR effect decreases as the thickness of MoS2 increases, and the MR ratio becomes negligible in MoS2 with thickness more than 10 layers. Besides, it is interesting to find a magnetic field direction dependent spin Hall magnetoresistance that stems from a combination of the spin Hall and the inverse spin Hall effects. Our research provides an insight into exploring RT MR in monolayer materials, which should be helpful for developing ultrathin magnetic storage devices in the atomically thin limit.

Temperature dependence of broadband near-infrared luminescence from Ni2+-doped Ba0.5Sr0.5TiO3

The dielectric and photoluminescence properties of Ni2+-doped Ba0.5Sr0.5TiO3 (BST) were studied at different temperatures. Under 350 nm excitation, the NIR luminescence band from 1200 nm to >1600 nm covers the optical communication window (O-L bands), with a typical bandwidth exceeding 200 nm. The crystal structure of Ni2+-doped BST evolves from rhombohedral to cubic when the temperature increases from 100 to 300 K. The luminescence properties are tightly correlated with the crystal structure of the host BST. The luminescence variations are mainly affected by phase transition induced crystal field change and nonradiative relaxation.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.