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We report on the dynamics of coherent phonons in semimetal 1T′- MoTe 2 using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond timescale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature T d − MoTe 2 phase. Unlike the other optical phonon modes, the shear phonon mode was found to strongly couple with photoexcited carriers. Moreover, the amplitude of the shear mode surprisingly decreased with increasing excitation density, a phenomenon that can be attributed to be a consequence of the lattice temperature increasing after excitation. These results provide useful physical information on ultrafast lattice symmetry switching between the normal semimetal 1T′ and the lattice inversion symmetry breaking Type-II Weyl semimetal T d phases.

The photogating effect is a photocurrent generation mechanism that leads to marked responsivity in two-dimensional (2D) semiconductor-based devices. A key step to promote the photogating effect in a 2D semiconductor is to integrate it with a high density of charge traps. Here, we show that self-limiting surface oxides on atomically thin WSe2 can serve as effective electron traps to facilitate p-type photogating. By examining the gate-bias-induced threshold voltage shift of a p-type transistor based on single-layer WSe2 with surface oxide, the electron trap density and the trap rate of the oxide are determined to be >1012 cm−2 and >1010 cm−2 s−1, respectively. White-light illumination on an oxide-covered 4-layer WSe2 transistor leads to the generation of photocurrent, the magnitude of which increases with the hole mobility. During illumination, the photocurrent evolves on a timescale of seconds, and a portion of the current persists even after illumination. These observations indicate that the photogenerated electrons are trapped deeply in the surface oxide and effectively gate the underlying WSe2. Owing to the pronounced photogating effect, the responsivity of the oxide-covered WSe2 transistor is observed to exceed 3000 A/W at an incident optical power of 1.1 nW, suggesting the effectiveness of surface oxidation in facilitating the photogating effect in 2D semiconductors.

The perpendicular magnetic anisotropy (PMA) of a Co (0.5 nm)/Pt (2.4 nm) bilayer film was electrostatically tuned using a high-κ dielectric, SrTiO3, with a back-gating configuration. The Co film was directly deposited onto a SrTiO3 (001) substrate and capped by a Pt layer. The coercivity was enhanced (reduced) by applying a positive (negative) gate bias. The efficiency of the PMA energy modulation by the electric field was determined to be 1300–23 100 fJ/V m, which is 1–3 orders of magnitude higher than the efficiencies obtained in previous studies. The superior efficiency of this system is most likely attributable to the large dielectric constant of the SrTiO3 gate insulator and could be evidence that the PMA energy modulation is induced by charge accumulation.

Nanostructures consisting of C_ molecules were fabricated on a GaSe/MoS_2 heterostructure. A submonolayer film of a lamellar compound semiconductor GaSe was grown on a MoS_2 substrate to form nanoscale holes or grooves surrounded by monolayer steps. Atomic force microscope (AFM) observation indicates that C_ molecules grow only in the bare MoS2 nanoregions at a substrate temperature of 180 degrees C. C_ molecules fill up those holes and grooves, and nanoscale C_ domains with polygonal shapes can be formed. This selective growth method can be combined with nanoscale patterning made by a scanning-tunneling microscope or AFM to produce nanostructures of molecular crystals with designed shapes. (c) 1997 American Institute of Physics.

Epilayers of transition metal dichalcogenides (TMDs) with two‐dimensional structures can be grown with molecular beam epitaxy onto other TMDs substrates without regard to lattice matching. Although there is no strong bonding between the epilayer and the substrate, the van der Waals interaction between the two hexagonally closest packed lattices results in a periodic distortion which, due to electronic effects, is prominently imaged with the scanning tunneling microscope.

Layered transition metal dichalcogenides (MoSe_2, NbSe_2) have been heteroepitaxially grown on (NH_4)_2 Sx (x~2) treated GaAs( 111 )Ga, GaAs( TIT)As surfaces in spite of the large difference in their crystal structures. The in situ observation of reflection high-energy electron diffraction has shown that the grown film hasits own lattice constant even from the first layer. The lattice matching condition, which is severely restricting in the usual heteroepitaxial growth case, is greatly relaxed in the present system because only weak van der Waals forces exist between the grown film and the substrate. This results from the fact that sulfur atoms regularly terminate dangling bonds on the GaAs surface after the (NH_4)_2S_x treatment.

The effect of inserting an ultrathin layer of ferroelectric (FE) poly(vinylidene fluoride-tetrafluoroethylene) P(VDF-TeFE) at the crystalline (c-)Si/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) interface of a c-Si/PEDOT:PSS Schottky junction solar cell is demonstrated. P(VDF-TeFE) is a highly resistive material that exhibits a large, permanent, internal polarization electric field by poling of molecular dipole among the polymer chains. Because of these properties, performance can be enhanced by adjusting the thickness of the FE layer and subsequent poling process. Inserting a 3-nm-thick FE layer increases the power conversion efficiency η from 10.2% to 11.4% with a short-circuit current density Jsc of 28.85 mA/cm2, an open-circuit voltage Voc of 0.57 V, and a fill factor FF of 0.692. Subsequent poling of the FE layer under a reverse DC bias stress increased η up to 12.3% with a Jsc of 29.7 mA/cm2, a Voc of 0.58 V, and an FF of 0.71. The obtained results confirm that the spontaneous...

An investigation was carried out into the effect of uniaxial optical anisotropy in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) on the photovoltaic performance of crystalline Si/PEDOT:PSS heterojunction solar cells fabricated by spin coating using either a methanol (MeOH) solvent alone or using MeOH and ethylene glycol (EG) as cosolvents. Spectroscopic ellipsometry revealed that the extraordinary index of refraction increased by the use of the cosolvents. In contrast, the ordinary index of refraction indicated metallic properties and was almost independent of the concentration of MeOH or EG. The highest conductivity was found for a (PEDOT:PSS):(MeOH):(EG) weight ratio of 1:1:0.1, and this sample exhibited a relatively high power conversion efficiency of 11.23%. These findings suggest that the increase in the extraordinary index of refraction leads to an enhancement of the hole mobility in PEDOT:PSS, resulting in improved photovoltaic performance.

The effects of MoO3 molecular doping in poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS) on the chemical structure and, in turn, on the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells are demonstrated. Scanning electron microscopy revealed that the hydrophilic PSS polymer chain was intercalated into the interlayer van der Waals gap of MoO3 flake sheets, which modified the chemical structure of PEDOT:PSS. MoO3 exhibited intense photoluminescence in the 350–550 nm region, which enhanced the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells with no significant changes. These findings suggest that the intense photoluminescence of MoO3 and its light wavelength conversion contribute to the increased carrier collection efficiency.

We demonstrate a highly efficient hybrid crystalline silicon (c-Si) based photovoltaic devices with hole-transporting transparent conductive poly-(3,4-ethlenedioxythiophene):poly(styrenesufonic acid) (PEDOT:PSS) films, incorporating a Zonyl fluorosurfactant as an additive, compared to non additive devices. The usage of a 0.1% Zonly treated PEDOT:PSS improved the adhesion of precursor solution on hydrophobic c-Si wafer without any oxidation process. The average power conversion efficiency η value was 10.8%-11.3%, which was superior to those of non-treated devices. Consequently, c-Si/Zonyl-treated PEDOT:PSS heterojunction devices exhibited the highest η of 11.34%. The Zonyl-treated soluble PEDOT:PSS composite is promising as a hole-transporting transparent conducting layer for c-Si/organic photovoltaic applications.

Ferromagnetism at room temperature in (Ti,Co)O2 − δ was controlled by changing its electron density via chemical doping, where the oxygen vacancy δ served as an electron donor. With increasing the electron density, the ferromagnetic anomalous Hall effect and magnetization emerged from a paramagnetic state, while undergoing an insulator to metal transition. This result supports that the electron carriers mediated the ferromagnetic exchange interaction and is consistent with the electric field effect study on the ferromagnetism recently reported [Y. Yamada, K. Ueno, T. Fukumura, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, and M. Kawasaki, Science 332, 1065 (2011)].

c-plane and a-plane AlN films have been grown on single-crystal ZnO substrates by pulsed laser deposition at room temperature. The electronic structures of c-plane and a-plane AlN/ZnO heterojunctions have been characterized by synchrotron radiation photoemission spectroscopy. Based on the binding energies of core-levels and valence-band maximum values, the valence-band offsets have been found to be 0.4±0.1 and 0.1±0.1 eV for the c-plane and a-plane AlN/ZnO heterojunctions, respectively. Both heterojunctions show type-II band configurations with conduction band offsets of 3.0±0.1 and 2.7±0.1 eV, respectively. The potential on the ZnO side bends downward toward the interface for the a-plane AlN/ZnO heterojunction. However, that bends upward toward the interface for the c-plane AlN/ZnO heterojunction. This phenomenon is explained well by the effect of spontaneous polarization in AlN and ZnO.

A ZnO/Mg0.2Zn0.8O heterostructure was characterized at T=2 K through capacitance measurements with using a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as a Schottky contact. The Nyquist diagram, which is the trajectory curve of the complex impedance vector, appeared to be an excellent semicircular shape, implying that the PEDOT:PSS/ZnO/Mg0.2Zn0.8O junction can be described with an equivalent single RC parallel circuit. Capacitance-voltage characteristics elucidate the existence of a two-dimensional electron gas, where 1019 cm-3 electrons are confined within 5 nm at the ZnO/Mg0.2Zn0.8O heterointerface.

Room-temperature ferromagnetic oxide semiconductor Co-doped TiO2 films are grown on glass substrates by sputtering method. Conducting films are ferromagnetic at room temperature that is consistent with the carrier-mediated nature of the ferromagnetism. Nearly full-polarized magnetization, large magneto-optical effect, and anomalous Hall effect are observed at room temperature. The magneto-optical effect shows nearly fourfold enhancement in a one-dimensional magnetophotonic crystal structure with a standard dielectric multilayer (SiO2/TiO2).

Pentacene field-effect transistors(FETs) were fabricated on a poly(vinyl alcohol) (PVA)-coated Ta 2 O 5 gate dielectric using Au source/drain electrodes. Their device characteristics were examined before and after annealing the FETs in a vacuum. Before annealing, the pentacene FET showed an ambipolar operation with hole mobility of 0.1 cm 2 V − 1 s − 1 and electron mobility of 0.005 cm 2 V − 1 s − 1 . After annealing at 50 ° C , however, the p -type operation characteristics almost diminished and an enhanced n -type operation was observed. Na + ions in the PVA are supposed to change the character of the injection barrier at the Au/pentacene junction and enable the n -type operation.

We developed a nanoscale orientation template for quasisingle crystalline epitaxial growth of thin film phase pentacene. By using alpha-3x3 Bi termination of surface dangling bonds on step-bunched vicinal surface of Si(111), thin film phase epitaxial pentacene was grown with the crystal axes aligned to the surface steps. Alignment occurred when the step height was higher than the molecular height. The mechanism of the alignment was examined by calculating the energy of the crystal edge.

Anomalous Hall effect (AHE) of a ferromagnetic semiconductor anatase Ti1−xCoxO2−δ thin film is studied from 10to300K. Magnetic field dependence of anomalous Hall resistance is coincident with that of magnetization, while the anomalous Hall resistance decreases at low temperature in spite of nearly temperature-independent magnetization. Anomalous Hall conductivity σAHE is found to be proportional to the square of Hall mobility, suggesting that charge scattering strongly affects the AHE in this system. The anatase Ti1−xCoxO2−δ also follows a scaling relationship to conductivity σxx as σAHE∝σxx1.6, which was observed for another polymorph rutile Ti1−xCoxO2−δ, suggesting an identical mechanism of their AHE.

The epitaxial growth of pentacene on hydrogen-terminated Si(111) is reported. Reflection high energy electron diffraction (RHEED) revealed that the crystal packing resembles that in the bulk crystal even at a monolayer thickness, which was maintained in multilayers. A ripening effect was clearly observed by atomic force microscopy (AFM). These results are important to obtain oriented crystalline films of pentacene combined with silicon microdevices with reduced defect densities. (c) 2005 American Institute of Physics.