We report on the experimental demonstration of a new type of nanoelectromechanical resonator based on black phosphorus crystals. Facilitated by a highly efficient dry transfer technique, crystalline black phosphorus flakes are harnessed to enable drumhead resonators vibrating at high and very high frequencies (HF and VHF bands, up to ∼100 MHz). We investigate the resonant vibrational responses from the black phosphorus crystals by devising both electrical and optical excitation schemes, in addition to measuring the undriven thermomechanical motions in these suspended nanostructures.

We report on the initial explorations of engineering atomically-thin semiconducting crystals into a new class of two-dimensional nanoelectromechanical systems (2D NEMS) that are attractive for realizing ultimately thin 2D transducers for embedding in both planar and curved systems. We describe the first resonant NEMS operating at radio frequencies (RF), based on MoS2, a hallmark of 2D semiconducting crystals derived from layered materials in transition metal dichalcogenides (TMDCs).

We report on experimental measurement of air damping effects in high frequency nanomembrane resonators made of atomically thin molybdenum disulfide (MoS2) drumhead structures. Circular MoS2 nanomembranes with thickness of monolayer, few-layer, and multi-layer up to ∼70 nm (∼100 layers) exhibit intriguing pressure dependence of resonance characteristics. In completely covered drumheads, where there is no immediate equilibrium between the drum cavity and environment, resonance frequencies and quality (Q) factors strongly depend on environmental pressure due to bulging of the nanomembranes.

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model.

The optical properties of graphene are strongly affected by electron-electron (e-e) and electron-hole (e-h) interactions. Here we tune these many-body interactions through varying the density of free charge carriers. Measurements from the infrared to the ultraviolet reveal significant changes in the optical conductivity of graphene for both electron and hole doping. The shift, broadening, and modification in shape of the saddle-point exciton resonance reflect strong screening of the many-body interactions by the carriers, as well as changes in quasiparticle lifetimes.

Most approaches to high-capacity 3D optical data storage (ODS) require confinement of the writing action to a specified depth in the writing medium. This is achieved by a nonlinear photoresponse, usually two-photon absorption, which requires a pulsed long-wavelength source. Fluorescence photobleaching of a dye/polymer composite can be used at a short wavelength to store data at the diffraction limit in a layered storage medium.

Mechanical exfoliation is a convenient and effective approach to deriving two-dimensional (2D) nanodevices from layered materials; but it is also generally perceived as unpreferred as it often yields devices with structural irregularities and nonidealities. Here we show that such nonidealities can lead to new and engineerable features that should be embraced and exploited.

Silicon is widely used for solar energy harvesting applications. Here we investigate the dynamics and transport properties of photoexcited carriers in silicon nanowires by ultrafast terahertz spectroscopy. The carrier lifetime was observed to approach 0.7 ns, and the carrier mobility to be ∼1000 cm 2/(Vs). We found that Silicon nanowire arrays fabricated by the metal-assisted chemical etching is better for solar cell application. © 2013 IEEE.

We report the synthesis of a direct gap semiconductor, ZnSnN2, by a plasma-assisted vapor-liquid-solid technique. Powder X-ray diffraction measurements of polycrystalline material yielded lattice parameters in good agreement with predicted values. Photoluminescence efficiency at room temperature was observed to be independent of excitation intensity between 103 and 108 W/cm2. The band gap was measured by photoluminescence excitation spectroscopy to be 1.7 ± 0.1 eV.

Upon femtosecond laser pumping of a topological insulator Bi2Se3, we observed efficient THz generation from the surface electrons. By performing polarization-resolved studies on the emitted THz spectrum, two emission mechanisms are identified. THz emission spectroscopy provides a valuable spectroscopic tool for studies of the dynamics of the surface electrons in centrosymmetric topological insulators. © 2013 IEEE.