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Molybdenum disulfide (MoS2) is one of the two-dimensional layered semiconductor transition metal dichalcogenides (TMDCs) with great potential in electronics, optoelectronics, and spintronic devices. Sulfur vacancies in MoS2 are the most prevalent defects. However, the effect of sulfur vacancies on the electronic structure of MoS2 is still in dispute. Here we experimentally and theoretically investigated the effect of sulfur vacancies in MoS2. The vacancies were intentionally introduced by thermal annealing of MoS2 crystals in a vacuum environment. Angle-resolved photoemission spectroscopy (ARPES) was used directly to observe the electronic structure of the MoS2 single crystals. The experimental result distinctly revealed the appearance of an occupied defect state just above the valence band maximum (VBM) and an upward shift of the VBM after creating sulfur vacancies. In addition, density functional theory (DFT) calculations also confirmed the existence of the occupied defect state close to the VBM as well as two deep unoccupied states induced by the sulfur vacancies. Our results provide evidence to contradict that sulfur vacancies indicate the origin of n-type behaviour in MoS2. This work provides a rational strategy for tuning the electronic structures of MoS2.

The electronic transport and photoconduction properties of the molybdenum diselenide (MoSe2 ) nanosheets were investigated. The transfer length method analysis suggests the electric current following an anomalously two-dimensional (2D) flow rather than the general 3D mode. The 2D current implies a higher electron concentration and downward band bending at the surface. A broad spectral photoresponse from ultraviolet to near-infrared wavelengths was also observed for the MoSe 2 nanosheets. The optimal responsivity and photoconductive gain respectively at 58 A/W and 179 under the wavelengths at 405 nm were achieved. A surface-controlled photoconduction mechanism based on the surface band bending and electron accumulation is proposed to explain the long carrier lifetime and its dependence of light intensity and environment in MoSe 2 nanostructures .

Ultrathin bilayers (BLs) of bismuth have been predicated to be a two-dimensional (2D) topological insulator. Here we report on a new route to manufacture the high-quality Bi bilayers from a 3D topological insulator, a top-down approach to prepare large-area and well-ordered Bi(111) BL with deliberate hydrogen etching on epitaxial Bi2Se3 films. With scanning tunneling microscopy (STM) and X-ray photoelectron spectra (XPS) in situ, we confirm that the removal of Se from the top of a quintuple layer (QL) is the key factor, leading to a uniform formation of Bi(111) BL in the van der Waals gap between the first and second QL of Bi2Se3. The angle resolved photoemission spectroscopy (ARPES) in situ and complementary density functional theory (DFT) calculations show a giant Rashba splitting with a coupling constant of 4.5 eV A in the Bi(111) BL on Bi2Se3. Moreover, the thickness of Bi BLs can be tuned by the amount of hydrogen exposure. Our ARPES and DFT study indicated that the Bi hole-like bands increase with i...

Compound semiconductors doped with magnetically active transition metals (TM) are actively pursued for semiconductor spintronics. Herein we report that delta doping (δ-doping) has been used to incorporate manganese (Mn) into the gallium nitride (GaN) matrix resulting in phase pure highly crystalline heteroepitaxial thin films of (Ga,Mn)N on c-plane sapphire (Al2O3) by plasma-assisted molecular beam epitaxy. Magnetotransport measurements reveal the anomalous Hall effect (AHE) while magnetic measurements show the existence of ferromagnetic ordering. Density functional theory calculations show that δ-doping of Mn atom enables exchange interactions between the Mn 3d states and N 2p states thereby resulting in spin polarized holes in the valence band giving rise to observed AHE and ferromagnetism. This work opens new avenues for incorporating spin polarized carriers in III-nitrides based semiconductors by creating magnetically active Mn layers in heterostructures for spintronics applications.

An exfoliated Kish graphite sample on a heavily n-doped Si substrate covered with native oxide was prepared with a conventional micromechanical cleavage method. From angle-resolved photoemission spectra (ARPES), we measured the band structure of graphite over photon energies from 28 eV to 116 eV. The inner potential V0 = 17.25 eV is determined with a period from the band dispersion in the KH direction. A set of parameters in the tight-binding method and the SWMcC model for graphite is extracted from the fitted results. A comparison of constant-energy mapping results at large binding energy indicates the reliability of the tight-binding parameters extracted from the ARPES results.

The determination of the band structure along k parallel to the chain direction demonstrates significant electronic delocalization. The small effective mass [m* = 0.106mo ] is consistent with the high measured mobility.

To explore the possibility of employing thermally generated H atoms to determine the chemical composition of a metallic surface, we investigated the adsorption and desorption of H atoms generated with a thermal cracker on surfaces Au(111) and Ag/Au(111). Angle-resolved photoemission spectra showed that the noble-metallic surfaces deteriorated upon exposure to a flux of H atoms at ~ 100 K. Upon subsequent annealing, the order of the surfaces was mostly recovered when H atoms on the surfaces desorbed, but the recovery was incomplete even with annealing at a temperature much higher than that at which H atoms desorb. X-ray photoemission spectra showed that O-containing contaminants existed on the surfaces after the H dosing. The evolution of O 1s during annealing indicated that the O-containing contaminants were H 2 O and its moieties generated during thermal cracking; the disturbances of the surfaces remaining above the desorption temperature of H atoms were likely caused by chemisorbed O. Our results show that it is possible to employ thermally generated H atoms to determine the chemical composition of a metallic surface, but a small proportion of H 2 O in a H 2 gas line might be unavoidable; precautions against possible O contamination are required when a thermal cracker is employed.

Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials., Comment: 8 pages, 7 figures

Reference EPFL-ARTICLE-195580doi:10.1002/adom.201300221View record in Web of Science Record created on 2014-01-09, modified on 2017-05-12

The surface photovoltage (SPV) effect is key to the development of opto-electronic devices such as solar-cells and photo-detectors. For the prototypical transition metal dichalcogenide WSe2, core level and valence band photoemission measurements show that the surface band bending of pristine cleaved surfaces can be readily modified by adsorption with K (an electron donor) or C60 (an electron acceptor). Time-resolved pump-probe photoemission measurements reveal that the SPV for pristine cleaved surfaces is enhanced by K adsorption, but suppressed by C60 adsorption, and yet the SPV relaxation time is substantially shortened in both cases. Evidently, adsorbate-induced electronic states act as electron-hole recombination centers that shorten the carrier lifetime.

A topological insulating Bi2Te3 single crystal was successfully grown with good uniformity using a home-made resistance-heated floated zone furnace. The temperature-dependent resistance and Hall voltage confirm that the transport is metallic and the overall carriersareholes. The angle and temperature dependence of the quantum Shubnikov–de Haas oscillation period amplitude suggests that the transport comes from the carriers of surface states. The Berry phase, determined from Landau level diagram, also reveals that the transport carriers are Dirac fermions. In contrast with many previous publications, the transport parameters relating to the surface carriers derived from the relationship of the Lifshitz–Kosevich (LK) theory are consistent with angle resolved photoemission spectroscopy results.

We report a mechanism for rapid deterioration of metallic surfaces during photoemission. The surface of interest is Cu(111), but a Cu(111) surface partially covered with Ag was chosen for testing. The surface of Ag/Cu(111) is characterized with its Shockley states, of which the binding energy and line width are very sensitive to surface order and composition; the evolution of the surface can be characterized by monitoring the change of its Shockley states. We observed a rapid degradation of the Shockley state of Cu(111) during photoemission at temperatures less than 220 K. A different evolution of the Shockley states during annealing and cooling was also discovered. All these observations favor a mechanism based on photonstimulated chemisorption of hydrogen on the surface. We suggest that this mechanism of surface deterioration affects all investigations of a surface involving intense radiation. Our experiments were performed on Beamline 21B1. All measurements were performed with photons of energy 21 eV; the beam flux and position are presumed constant. The base pressure of the system was better than 6×10 torr. The sample was prepared on depositing 0.5 monolayer (ML) of Ag on Cu(111) and subsequently annealing to 450 K. After annealing, clean Cu(111) and Cu(111) covered by Ag at 1 ML, denoted “1-ML Ag/Cu(111)”, coexist on the sample surface. Figure 1 illustrates how the Shockley states evolve with prolonged exposure to radiation at various temperatures. After exposures at 220 K and 150 K, the sample was moved ~1.5 mm vertically; a further spectrum was recorded and is plotted at the bottom of the stacked plot. Each spectrum in Fig. 1 comprises two peaks, labeled SCu and S1 pertaining to the Shockley states of Cu(111) and 1-ML Ag/Cu(111), respectively. Dissimilarity in the evolution of spectra with temperature is evident: the spectra remain essentially unchanged at 300 K, whereas SCu degrades noticeably at 220 and 150 K. Although showing a slight broadening, the bottom spectra in Figs. 1(b) and 1(c) resemble the top spectra measured at the beginning of the experiments, which is a strong indication that rapid degradation of SCu exists only in the area under irradiation. Figure 2 is a stacked plot of photoemission spectra measured at various temperatures. The measurement was completed with an annealing and a cooling cycle. As observed in the figure, the width of SCu is noticeably wider during annealing. The dependence of the line widths of the Shockley states on temperature is determined from fitting the spectra in Fig. 2. The results from the fitting are plotted in Fig. 3. The line widths of the Shockley states of Cu(111) (SCu) and 1-ML Ag/Cu(111) (S1) are shown as triangles and diamonds, respectively. The line widths measured during annealing and cooling are labeled with filled and open symbols, respectively. The results show noticeably the difference in the temperature-dependent variation of the line widths between SCu and S1. Furthermore, the widths of both SCu and S1 vary differently between annealing and cooling. The widths of the Shockley states underwent a large increase and then decreased to the values for a clean ordered surface; our measurement can be considered to constitute temperature-programmed desorption (TPD). The result indicates that the desorption of the contaminants from Cu(111) and Ag/Cu(111) is complete at ~350 K and ~180 K, respectively; these numbers match satisfactorily the temperatures for chemisorbed hydrogen atoms to desorb from Cu(111) and Ag(111). The results suggest that chemisorbed hydrogen atoms are likely a principal contaminant involved in the rapid deterioration of metallic surfaces observed in our experiments. We believe that all photoemission measurements on surfaces are subject to the mechanism of surface deterioration that we here report. The surface deterioration according to the proposed mechanism is likely to become a major obstacle to the success of future experiments on surfaces utilizing micro-focused beams.

Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing-wave formation governed by a momentum-dependent phase-shift function. No evidence is found for in-plane electron localization within the confined geometry.

The epitaxial growth of Cu on Ag(111) was explored on monitoring the evolution of its surface states with angle-resolved photoemission spectroscopy; the chemical element on the top of Cu islands on Ag(111) with a (9×9) reconstruction was determined to be not Cu but Ag. Our result demonstrates a technique with which the chemical property associated with a surface state is explorable and an identification of the chemical element in the topmost layer of the surface becomes feasible.