Laser Fusion Research Center

"Strain engineering" in functional materials has been widely explored to tailor the physical properties of electronic materials and improve their electrical and/or optical properties. Here, we exploit both in plane and out of plane uniaxial tensile strains in MoS2 to modulate its band gap and engineer its optical properties. We utilize X-ray diffraction and cross-sectional transmission electron microscopy to quantify the strains in the as-synthesized MoS2 nanosheets and apply measured shifts of Raman-active modes to confirm lattice strain modification of both the out-of-plane and in-plane phonon vibrations of the MoS2 nanosheets. The induced band gap evolution due to in-plane and out-of-plane tensile stresses is validated by photoluminescence (PL) measurements, promising a potential route for unprecedented manipulation of the physical, electrical and optical properties of MoS2.

Computed Tomography (CT) is a powerful method for non-destructive testing (NDT) and metrology awakes with expanding application fields. To improve the spatial resolution of high energy CT, a micro-spot gamma-ray source based on bremsstrahlung from a laser wakefield accelerator was developed. A high energy CT using the source was performed, which shows that the resolution of reconstruction can reach 100 μm at 10% contrast. Our proof-of-principle demonstration indicates that laser driven micro-spot gamma-ray sources provide a prospective way to increase the spatial resolution and toward to high energy micro CT. Due to the advantage in spatial resolution, laser based high energy CT represents a large potential for many NDT applications.

Co3O4 is an attractive earth-abundant catalyst for CO oxidation, and its high catalytic activity has been attributed to Co3+ cations surrounded by Co2+ ions. Hence, the majority of efforts for enhancing the activity of Co3O4 have been focused on exposing more Co3+ cations on the surface. Herein, we enhance the catalytic activity of Co3O4 by replacing the Co2+ ions in the lattice with Cu2+. Polycrystalline Co3O4 nanowires for which Co2+ is substituted with Cu2+ are synthesized using a modified hydrothermal method. The Cu-substituted Co3O4_Cux polycrystalline nanowires exhibit much higher catalytic activity for CO oxidation than pure Co3O4 polycrystalline nanowires and catalytic activity similar to those single crystalline Co3O4 nanobelts with predominantly exposed most active {110} planes. Our computational simulations reveal that Cu2+ substitution for Co2+ is preferred over Co3+ both in the Co3O4 bulk and at the surface. The presence of Cu dopants changes the CO adsorption on the Co3+ surface sites only slightly, but the oxygen vacancy is more favorably formed in the bonding of Co3+–O–Cu2+ than in Co3+–O–Co2+. This study provides a general approach for rational optimization of nanostructured metal oxide catalysts by substituting inactive cations near the active sites and thereby increasing the overall activity of the exposed surfaces.

The structures and stabilities of gold clusters with up to 14 atoms have been determined by density-functional theory. The structure optimizations and frequency analysis are performed with the Perdew-Wang 1991 gradient-corrected functional combined with the effective core potential and corresponding valence basis set (LANL2DZ). The turnover point from two-dimensional to three-dimensional geometry for gold clusters occurs at Au12. The energetic and electronic properties of the small gold clusters are strongly dependent on sizes and structures, which are in good agreement with experiment and other theoretical calculations. The even-odd oscillation in cluster stability and electronic properties predicted that the clusters with even numbers of atoms were more stable than the neighboring clusters with odd numbers of atoms. The stability and electronic structure properties of gold clusters are also characterized by the maximum hardness principle of chemical reactivity and minimum polarizability principle.

Terahertz waves provide a better contrast in imaging soft biomedical tissues than X-rays, and unlike X-rays, they cause no ionisation damage, making them a good option for biomedical imaging. Terahertz absorption imaging has conventionally been used for cancer diagnosis. However, the absorption properties of a cancerous sample are influenced by two opposing factors: an increase in absorption due to a higher degree of hydration and a decrease in absorption due to structural changes. It is therefore difficult to diagnose cancer from an absorption image. Phase imaging can thus be critical for diagnostics. We demonstrate imaging of the absorption and phase-shift distributions of 3.2 mm × 2.3 mm × 30-μm-thick human hepatocellular carcinoma tissue by continuous-wave terahertz digital in-line holography. The acquisition time of a few seconds for a single in-line hologram is much shorter than that of other terahertz diagnostic techniques, and future detectors will allow acquisition of meaningful holograms without sample dehydration. The resolution of the reconstructions was enhanced by sub-pixel shifting and extrapolation. Another advantage of this technique is its relaxed minimal sample size limitation. The fibrosis indicated in the phase distribution demonstrates the potential of terahertz holographic imaging to obtain a more objective, early diagnosis of cancer.

ABSTRACT In this paper, the electronic structures of NbO 2 and Nb 2 O 5 are theoretically and experimentally analyzed. The oxides in the samples are mainly consisted of NbO 2 and NbO, whereas the outmost layer of the samples is NbO 2 . After exposure to air, the outermost layer on all niobium samples is Nb 2 O 5 . The photoelectrons from the first 2–4 Å contribute to the spectra, so the valence band structure of NbO 2 and Nb 2 O 5 can be confirmed from ultraviolet photoelectron spectroscopy (UPS). By comparing the UPS with density of state results, the electronic structure of NbO 2 and Nb 2 O 5 can be distinguished from each other, and then the electronic structure was deconvoluted into several electronic states. The agreement between experimental result and theory is, in the best case, satisfactory. Copyright © 2013 John Wiley & Sons, Ltd.

We report here on terahertz (THz) digital holography on a biological specimen. A continuous-wave (CW) THz in-line holographic setup was built based on a 2.52 THz CO(2) pumped THz laser and a pyroelectric array detector. We introduced novel statistical method of obtaining true intensity values for the pyroelectric array detector's pixels. Absorption and phase-shifting images of a dragonfly's hindwing were reconstructed simultaneously from single in-line hologram. Furthermore, we applied phase retrieval routines to eliminate twin image and enhanced the resolution of the reconstructions by hologram extrapolation beyond the detector area. The finest observed features are 35 μm width cross veins.

The development of two-dimensional (2D) intrinsic ferromagnetic semiconductors is urgent in the spintronic field. Motivated by the recent experiments on the successful synthetization of monolayer (ML) Janus transition-metal dichalcogenides (MoSSe) and ferromagnetic (FM) VSe2, a highly stable ML Janus 2H-VSeTe is fabricated by density functional theory and confirmed by a global minimum search. The Janus VSeTe shows a large valley polarization of 158 meV as the space- and time-reversal symmetry is broken. The VSeTe shows FM order with Curie temperature (Tc) of 350 K and a sizable magnetocrystalline anisotropy (MCA) of -8.54 erg cm-2. The high Tc and large valley polarization suggest the 2D Janus VSeTe is a promising magnetic material for potential applications in electronics, spintronics, and valleytronics.

for 1000 cycles. The individual contribution of nanowires and nanoparticles to the enhanced capacitance has been discussed, and the enhanced capacitive properties can be ascribed to the hybrid structure better for charge transport during the electrochemical process. More importantly, this route can be extended to incorporate nanowires of other metal oxides into mesoporous carbon, and enhanced capacitive properties can be expected.

As a promising high performance FET, the graphene/PtSe₂ vdW heterojunction has a band gap of 0.264 eV.

Superior photovoltaic performance in organic–inorganic hybrid perovskite is based on the unique properties of each moiety contined within it. Identifying the role of metal atoms in the perovskite is of great importance to explore the low‐toxicity lead‐free perovskite solar cells. By using the first‐principle calculations, four types of AMX 3 (A = CH 3 NH 3 , M = Pb, Sn, Ge, Sr, X = I) perovskite materials are investigated and an attempt is made to understand the structural and electronic influences of the metal atoms on the properties of perovskites. Then, the solutions to the replacement of Pb are discussed. It is found that for the small radius metal atoms as compared with Pb, the strong geometry distortion will result in a less p–p electron transition and larger carrier effective mass. The outer ns 2 electrons of the metal ions play critical roles on the modulation of the optical and electronic properties for perovskite materials. These findings suggest that the solutions to the Pb replacement might be metal or metallic clusters that have effective ionic radius and outer ns 2 electrons configuration on the metal ions with low ionization energy similar to Pb 2+ . Based on this, lead‐free perovskite solar cells are expected to be realized in the near future.

Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene's plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectronic devices. Here, we use two counter propagating frequency combs with temporally synchronized pulses to demonstrate deterministic all-optical generation and electrical control of multiple plasmon polaritons, excited via difference frequency generation (DFG). Electrical tuning of a hybrid graphene-fibre device offers a precise control over the DFG phase-matching, leading to tunable responses of the graphene's plasmons at different frequencies across a broadband (0 ~ 50 THz) and provides a powerful tool for high-speed logic operations. Our results offer insights for plasmonics on hybrid photonic devices based on layered materials and pave the way to high-speed integrated optoelectronic computing circuits.

Nanostructured molybdenum disulfide (MoS2) has emerged as a promising catalytic alternative to the widely used Pt in the hydrogen evolution reaction from water because it is inexpensive and earth-abundant. The central prerequisite in realizing its potential is to enhance the surface activities by increasing the concentration of metallic edge sites. However, MoS2 thermodynamics favors the presence of a two-dimensional basal plane, and therefore, the one-dimensional edge sites surrounding the basal plane are very limited. Herein, we report the first synthesis of single-crystal MoS2 nanobelts with the top surface fully covered by edge sites. The nanobelt structure comprises parallel stacked atomic layers with the basal plane vertical to the substrate, and these layer edges form the top surface of the nanobelt. The surface is highly active: it optically quenches all of the indirect band gap excitons and chemically leads to a high electrocatalytic hydrogen evolution efficiency (a low onset overpotential of 170 mV for an electrocatalytic current density of 20 mA/cm(2) and a Tafel slope of 70 mV/decade).

Hexagonal WO₃ with well-ordered mesopores was achieved using a one-pot hydrothermal method, and it exhibited excellent capacitive performance.

Recently, two-dimensional (2D) layered transition metal dichalcogenides (LTMDs) have attracted great scientific interest for ion battery applications.

In this letter, we investigated the behaviors of surface- and buffer-induced current collapse in AlGaN/GaN high-electron mobility transistors (HEMTs) using a soft-switched pulsed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(I-V\) </tex-math></inline-formula> measurement with different quiescent bias points. It is found that the surface- and buffer-related current collapse have different relationship with the gate and drain biases ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V<inline-formula> <tex-math notation="TeX">\(_{\mathrm { {\!GS0,}}}\) </tex-math></inline-formula>V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(_{\mathrm { {\!DS0}}}\) </tex-math></inline-formula> ) during quiescent bias stress. The surface-induced current collapse in devices without passivation monotonically increases with the negative <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(_{\mathrm { {\!GS0}}}\) </tex-math></inline-formula> , suggesting that an electron injection to the surface from gate leakage is the dominant mechanism and the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> passivation could effectively eliminate such current collapse. The buffer-induced current collapse in devices with intentionally carbon-doped buffer layer exhibits a different relationship with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(_{\mathrm { {\!GS0}}}\) </tex-math></inline-formula> after surface passivation. The buffer-related current collapse shows a bell-shaped behavior with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(_{\mathrm { {\!GS0}}}\) </tex-math></inline-formula> , suggesting that a hot electron trapping in the buffer is the dominant mechanism. The soft-switched pulsed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(I-V\) </tex-math></inline-formula> measurement provides an effective method to distinguish between the surface- and buffer-related current collapse in group III-nitride HEMTs.

We first used carbon aerogel microspheres prepared <italic>via</italic> emulsion polymerization and ambient drying techniques as electrode material for capacitive deionization.

We show that a high-energy electron bunch can be used to capture the instantaneous longitudinal and transverse field structures of the highly transient, microscopic, laser-excited relativistic wake with femtosecond resolution. The spatiotemporal evolution of wakefields in a plasma density up ramp is measured and the reversal of the plasma wake, where the wake wavelength at a particular point in space increases until the wake disappears completely only to reappear at a later time but propagating in the opposite direction, is observed for the first time by using this new technique.

Solitons in microresonators have spurred intriguing nonlinear optical physics and photonic applications. Here, by combining Kerr and Brillouin nonlinearities in an over-modal microcavity, we demonstrate spatial multiplexing of soliton microcombs under a single external laser pumping operation. This demonstration offers an ideal scheme to realize highly coherent dual-comb sources in a compact, low-cost and energy-efficient manner, with uniquely low beating noise. Moreover, by selecting the dual-comb modes, the repetition rate difference of a dual-comb pair could be flexibly switched, ranging from 8.5 to 212 MHz. Beyond dual-comb, the high-density mode geometry allows the cascaded Brillouin lasers, driving the co-generation of up to 5 space-multiplexing frequency combs in distinct mode families. This Letter offers a novel physics paradigm for comb interferometry and provides a widely appropriate tool for versatile applications such as comb metrology, spectroscopy, and ranging.

Herein, density functional theory (DFT) calculations were performed to investigate the adsorption of a H₂S molecule on the surface of hydrogenated graphene (graphane).