David B. Geohegan

Highly Responsive Ultrathin GaS Nanosheet Photodetectors on Rigid and Flexible Substrates

The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Highly crystalline, exfoliated GaS nanosheets are promising for optoelectronics due to strong absorption in the UV-visible wavelength region. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photoresponsivity at 254 nm up to 4.2 AW(-1) and 19.2 AW(-1), respectively, which exceeds that of graphene, MoS2, or other 2D material-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7 dB and 78.73 dB, respectively. Both surpass that of currently exploited InGaAs photodetectors (66 dB). Theoretical modeling of the electronic structures indicates that the reduction of the effective mass at the valence band maximum (VBM) with decreasing sheet thickness enhances the carrier mobility of the GaS nanosheets, contributing to the high photocurrents. Double-peak VBMs are theoretically predicted for ultrathin GaS nanosheets (thickness less than five monolayers), which is found to promote photon absorption. These theoretical and experimental results show that GaS nanosheets are promising materials for high-performance photodetectors on both conventional silicon and flexible substrates.

Fast intensified-CCD photography of YBa2Cu3O7-x laser ablation in vacuum and ambient oxygen

The propagation of laser ablation plumes from 248 nm laser‐irradiated YBCO into vacuum and 100 mTorr ambient oxygen has been photographed with a gated, intensified CCD array camera system. The thermalization of the laser plasma and onset of shock structures due to collisions with the background gas are investigated from two‐dimensional digitized images of the visible plume emission.

Time-resolved imaging of gas phase nanoparticle synthesis by laser ablation

The dynamics of nanoparticle formation, transport, and deposition by pulsed laser ablation of c-Si into 1–10 Torr He and Ar gases are revealed by imaging laser-induced photoluminescence and Rayleigh-scattered light from gas-suspended 1–10 nm SiOx particles. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles, while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. Nanoparticles unambiguously formed in the gas phase were collected on transmission electron microscope grids for Z-contrast imaging and electron energy loss spectroscopy analysis. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described.

DYNAMICS OF LASER ABLATION PLUME PENETRATION THROUGH LOW PRESSURE BACKGROUND GASES

The dynamics of laser‐ablated yttrium plume propagation through background argon have been investigated with fast time‐ and spatially‐resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low‐pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y+. These are correlated with intensified‐CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume‐splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial,...

Equally Efficient Interlayer Exciton Relaxation and Improved Absorption in Epitaxial and Nonepitaxial MoS2/WS2 Heterostructures

Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2). Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

Perovskite Solar Cells with Near 100% Internal Quantum Efficiency Based on Large Single Crystalline Grains and Vertical Bulk Heterojunctions

Imperfections in organometal halide perovskite films such as grain boundaries (GBs), defects, and traps detrimentally cause significant nonradiative recombination energy loss and decreased power conversion efficiency (PCE) in solar cells. Here, a simple layer-by-layer fabrication process based on air exposure followed by thermal annealing is reported to grow perovskite films with large, single-crystal grains and vertically oriented GBs. The hole-transport medium Spiro-OMeTAD is then infiltrated into the GBs to form vertically aligned bulk heterojunctions. Due to the space-charge regions in the vicinity of GBs, the nonradiative recombination in GBs is significantly suppressed. The GBs become active carrier collection channels. Thus, the internal quantum efficiencies of the devices approach 100% in the visible spectrum range. The optimized cells yield an average PCE of 16.3 ± 0.9%, comparable to the best solution-processed perovskite devices, establishing them as important alternatives to growing ideal single crystal thin films in the pursuit toward theoretical maximum PCE with industrially realistic processing techniques.

Physics and diagnostics of laser ablation plume propagation for high-Tc superconductor film growth

The formation, composition and propagation of laser-produced plasmas used for pulsed laser deposition (PLD) of Y1Ba2Cu3O7−x have been studied under film growth conditions. Four complementary spatially and temporally resolved in situ diagnostic techniques are applied to characterize the expansion of the laser plume into both vacuum and ambient gases: optical emission and absorption spectroscopy, fast ion probe measurements, and fast photography with a gated, image-intensified charge-coupled detector-array (ICCD) camera system. Transient optical absorption spectroscopy reveals large densities of ground state atoms, ions, and molecules in the plume as well as a slower component to the plume transport than is indicated by the plasma fluorescence and ion current. Ablation into background gases results in scattering and attenuation of the laser plume. The exponential attenuation of the positive ion flux transmitted through 50–300 mTorr background oxygen is measured and used to define an overall ion-oxygen reaction cross-section σi−O2 = 2.3 × 10−16 cm2 under the described film growth conditions. The slowing of the laser plasma and formation of shock structures due to collisions with the ambient gas are described using ion probe measurements and ICCD photographic comparisons of expansion into vacuum and background oxygen. At the pressures used for PLD, distance-time R−t plots derived from the photographs and ion probe waveforms indicate that the higher pressure plume initially expands through the ambient gas in accordance with a drag model (where R = xf[1 − exp( − βt)]), experiencing little slowing until a visible shock structure forms. Following a transition period, in which the plume appears to have two components, a single-component shock structure propagates in better agreement with a shock, or blast wave (R = ξ0(E/ϱ0)15t25) model.

Fast and highly anisotropic thermal transport through vertically aligned carbon nanotube arrays

This letter reports on fast and highly anisotropic thermal transport through millimeter-tall, vertically aligned carbon nanotube arrays (VANTAs) synthesized by chemical vapor deposition on Si substrates. Thermal diffusivity measurements were performed for both longitudinal and transverse to the nanotube alignment direction, with longitudinal values as large as 2.1±0.2cm2∕s and anisotropy ratios as large as 72. Longitudinal thermal conductivities of 15.3±1.8W∕(mK) for porous 8±1vol% VANTAs in air and 5.5±0.7W∕(mK) for epoxy-infiltrated VANTAs already exceed those of phase-changing thermal interface materials used in microelectronics. Data suggest that further improvements are possible through optimization of density and defects in the arrays.

Characterization of ground‐state neutral and ion transport during laser ablation of Y1Ba2Cu3O7−x using transient optical absorption spectroscopy

Transient optical absorption spectroscopy has been utilized for the first time to study the transport of ground‐state Y, Ba, Cu, and Ba+ following excimer laser ablation of Y1Ba2Cu3O7−x pellets. Spectral broadening of the atomic lines monitored in both absorption and emission is reported, indicating the existence of gas phase collisions in the plume of ejected material. Time‐of‐flight velocity distributions of the nonemitting neutrals and ions determined by the absorption technique are broadened and shifted to lower velocities than the velocity distributions inferred from excited‐state fluorescence in the plume. Absorption by ground‐state Y+, YO, BaO, and CuO also has been observed with this technique. The absorption technique, and its application as an in situ monitor of neutral and ion transport during deposition of superconducting thin films, is described.

Growth behavior of carbon nanotubes on multilayered metal catalyst film in chemical vapor deposition

Abstract The temperature and time dependences of carbon nanotube (CNT) growth by chemical vapor deposition are studied using a multilayered Al/Fe/Mo catalyst on silicon substrates. Within the 600–1100 °C temperature range of these studies, narrower temperature ranges were determined for the growth of distinct types of aligned multi-walled CNTs and single-walled CNTs by using high-resolution transmission electron microscopy and Raman spectroscopy. At 900 °C, in contrast to earlier work, double-walled CNTs are found more abundant than single-walled CNTs. Defects also are found to accumulate faster than the ordered graphitic structure if the growth of CNTs is extended to long durations.