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Energy Band Alignment of a Monolayer MoS <sub/>2</sub> with SiO <sub/>2</sub> and Al <sub/>2</sub> O <sub/>3</sub> Insulators from Internal Photoemission
摘要： Internal photoemission of electrons (IPE) from large area one monolayer 2H-MoS2 films synthesized on top of amorphous (a-) SiO2 or Al2O3 is used to determine the energy of the semiconductor valence band (VB) relative to the reference level of the insulator conduction band (CB). This allows us to compare the VB top energy in MoS2 to that of the (100)Si substrate crystal at the interface with the same insulator. Despite the CB in a–Al2O3 is found to be ~1 eV below that in SiO2 as measured relative to the Si VB edge, the authors observe nearly no shift of the spectral threshold in the case of IPE from the MoS2 VB. This observation indicates violation of electroneutrality at the MoS2/a–Al2O3 interface causing an increase in barrier by ~1 eV. This conclusion is supported by the much weaker field dependence of the IPE threshold at the MoS2/a–Al2O3 interface compared to the MoS2/a–SiO2 one, suggesting the presence of negative charges and/or interface dipoles. Therefore, the commonly accepted electron affinity rule (EAR) appears to be not appropriate to describe the band alignment at 2D/insulator interfaces.
关键词： internal photoemission spectroscopy,MoS2,electron affinity,electron barrier,band alignment
更新于2025-09-19 17:15:36
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Epitaxial growth and determination of the band alignment for NixMg1-xO/MgO interface by laser molecular beam epitaxy
摘要： By laser molecular beam epitaxy, single crystalline NixMg1-xO films have been successfully synthesized on MgO(100) surface. The in situ reflection high-energy electron diffraction patterns show that the induced O2 background gas with at least 1.0 × 10?3 Pa is necessary to the epitaxial growth of NixMg1-xO films. The X-ray diffraction patterns reveal the single-phase growth along (200) direction. The energy band alignment at the interface is investigated by employing in situ X-ray photoelectron spectroscopy. The valence band offsets are determined to be from 1.47 eV to 1.50 eV with the decrease of Ni content (0.39 / 0.35). Furthermore, the work function is evaluated by using in situ ultraviolet photoemission spectroscopy, indicating the values from 4.33 eV to 4.64 eV. This study provides a promising guidance for the solar-blind device design and fabrication.
关键词： Work function,X-ray photoelectron spectroscopy,Band alignment,Ultraviolet photoemission spectroscopy,Laser molecular beam epitaxy
更新于2025-09-19 17:13:59
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Defects controlled doping and electrical transport in TiS <sub/>2</sub> single crystals
摘要： TiS2 has been intensively studied as an electrode material and a thermoelectric material for energy storage and conversion applications due to its high electrical conductivity. Understanding the influence of defects on electrical transport is of importance not only to resolve the long-standing question concerning the nature of TiS2, but also for the rational design of TiS2 based devices for energy scavenging applications. In this study, we integrate photoemission spectroscopy, Raman spectroscopy, and electrical transport measurements to determine the chemical compositions dominated by defects and their influence on the doping and electrical properties. Our results demonstrate that TiS2 is a heavily self-doped semiconductor with the Fermi level close to the conduction band, which serves as the conclusive experimental evidence regarding the semiconducting nature of TiS2. The doping effect is sensitive to the (subtle) changes in the chemical composition. The electron donation from the Ti interstitials (Tii) to the TiS2 host explains the high carrier concentration. The Ti Frenkel pair (TiF) acting as the acceptor is responsible for the decrease in the electron carrier concentration and electrical conductivity. High conductivity maintains upon partial oxidization, indicating the oxidization-tolerance in terms of the electronic structure. Our results provide valuable insight into the evolution of electronic properties modulated by defects that reveal unambiguously the self-doped semiconducting nature of TiS2 and chemical- and environment-tolerance of TiS2 as an advanced energy scavenging material.
关键词： TiS2,doping,defects,electrical transport,energy storage,energy conversion,photoemission spectroscopy,thermoelectric material,Raman spectroscopy
更新于2025-09-19 17:13:59
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In-Depth Spectroscopy and New Heights for Organic Solar Cells
摘要： In this issue of Joule, Lami et al. describe a method that enables UV photoemission spectroscopy (UPS) in the transverse dimension of polymeric semiconductor layers with nanometer-scale resolution. The approach is based on the use of Argon gas cluster ion beam (GCIB) etching instead of monoatomic ion beam bombardment. The use of GCIB reduces surface damage, enabling in depth UPS. The method is applied to the study of critical electronic levels and photovoltage in organic solar cells.
关键词： Argon gas cluster ion beam,photovoltage,UV photoemission spectroscopy,electronic levels,organic solar cells
更新于2025-09-19 17:13:59
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revealed by spatially and angle-resolved photoemission spectroscopy
摘要： Nematicity, where rotational symmetry is broken while translational symmetry is conserved, is prevalent in high-temperature superconductors. In particular, nematic quantum critical point has been universally found near the optimum doping of the superconducting dome of several iron-based superconductor families. In such a regime, evidence for strong nematic ?uctuations have been observed. As the precursor to this order, nematic ?uctuations emerge before nematicity, providing favorable ground to study how nematic order modi?es the electronic structure in the absence of structural distortion. Here we use spatially resolved angle-resolved photoemission spectroscopy to investigate the correlation between the onset of nematic ?uctuations and electronic structure in an optimally doped BaFe2(As1?xPx )2 (x ～ 0.3) superconductor. We reveal a strong spatially varying anisotropy of the Fermi surface on a length scale of tens of microns with strong correlation between the changes in the hole and electron Fermi pockets, consistent with the variations expected in the presence of ?uctuating nematic order. These results provide direct evidence for spatial nematic ?uctuations in the optimal doping regime of iron-based superconductors.
关键词： iron-based superconductors,high-temperature superconductors,angle-resolved photoemission spectroscopy,nematicity,Fermi surface
更新于2025-09-19 17:13:59
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Ultra-thin films of barium fluoride with low work function for thermionic-thermophotovoltaic applications
摘要： The deposition of barium fluoride thin and ultra-thin films on gallium arsenide substrates was performed by electron beam evaporation for analyzing the influence of film thickness and chemical composition on the work function of the resulting heterostructure. X-ray photoemission spectroscopy combined with ultraviolet photoemission spectroscopy measurements reveals that films of 2 nm nominal thickness and Ba/F ? 1.0 stoichiometry ratio induce the achievement of a significantly low work function of 2.1 eV to the BaFx/GaAs heterostructure. The significant reduction of the work function at least down to 3.0 eV is confirmed by a test thermionic converter operating at a cathode temperature of 1385 C, where the heterostructure was applied as anode. The low work function, together with a negligible optical absorption, makes feasible the practical application of barium fluoride coatings on GaAs within hybrid thermionic-thermophotovoltaic devices.
关键词： Ultraviolet photoemission spectroscopy,Work function,Barium fluoride,Thermionic-thermophotovoltaic energy conversion
更新于2025-09-19 17:13:59
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultrafast Extreme-Ultraviolet Photoemission Spectroscopy at 18.4 MHz
摘要： In the past two decades, attosecond science has led to important insights into ultrafast electron dynamics. Whilst extreme-ultraviolet (XUV) photon flux attained from high harmonic generation (HHG) has increased significantly over this time, space-charge effects limit the maximum allowable number of photons per pulse for many attosecond experiments, especially in multidimensional – i.e., either angularly (ARPES) or spatially (PEEM) resolved – photoelectron spectroscopy (PES) on solids [1,2]. To allow for higher average flux without increasing space-charge impairments, HHG sources at higher repetition rates are needed. We combine a fibre-based ytterbium amplifier system with 4.5-μJ, 40-fs and 1030-nm pulses at 18.4 MHz repetition rate with intra-cavity HHG with an enhancement of 35 (Fig. 1a). Geometric output coupling through a pierced mirror affords high photon energies of 25-60 eV at a flux of 9×1012 XUV photons per second [3], which is unprecedented at this photon energy and MHz repetition rates. The HHG beam is focused on a tungsten target and the generated photoelectrons are detected by an angle-resolving time-of-flight (ToF) spectrometer. Simultaneous stabilization of cavity length and oscillator repetition rate result in excellent intensity stability of the system and enable long-term measurements with three orders of magnitude less integration time at equal space charge conditions in comparison to state-of-the-art kHz HHG sources. An evaluation of the photoelectron statistics of the setup by observing the temporal evolution of the relative standard deviation σ of the photoelectron spectrum proves the aptitude of our setup for high-precision, long-term PES experiments, with a statistical behaviour for up to 3×109 shots (see Fig. 1b). By measuring photoelectron spectra at different XUV flux, we show that no space charge within the instrumental resolution of 0.3 eV can be observed in a 10-μm-diameter focus spot. Our experiments reveal the XUV-IR-delay dependence of sidebands in the photoelectron spectrum generated by laser-assisted photoemission. This represents the first measurement with attosecond precision at MHz repetition rate.
关键词： High Harmonic Generation,Extreme-Ultraviolet,Ultrafast,Photoemission Spectroscopy,MHz repetition rates
更新于2025-09-16 10:30:52
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Broadening of van Hove Singularities Measured by Photoemission Spectroscopy of Single and Mixed Chirality Single-Walled Carbon Nanotubes
摘要： The occupied valence electronic states of single walled carbon nanotubes (SWCNTs) are responsible for their optoelectronic properties and are unique for each SWCNT chirality. Photoemission spectroscopy (PES) is one of the few methods capable of directly measuring the electron density in the valence states of materials but there are only few reports which have observed the valence states of SWCNTs and no examples for single-chirality SWCNTs. Here we prepare single and mixed chirality SWCNT films and characterise their valence states using PES. Chirality pure SWCNTs were isolated using both gel permeation chromatography and ssDNA facilitated aqueous two phase extraction from starting materials consisting of mixed chirality species. Chirality separation and purity was confirmed with UV-Vis-nIR absorption spectroscopy. SWCNT films were prepared for the single chirality species (10,3), (7,6), (7,3), (6,5), (8,3), (9,1) along with SWCNT chirality mixture of metallic and semiconducting SWCNTs, and as-synthesised mixtures possessing a range of SWCNT diameter. PES using synchrotron radiation was completed for all samples with survey and C 1s core level spectra obtained to confirm SWCNT coverage, defect level and purity. Valence band PES was obtained to characterise the valence electronic states and showed significant broadening of the signal, in comparison to calculated density of states, which could not be accounted for by instrument resolution. An inverse diameter dependence of the broadening was observed with greater broadening for smaller diameter SWCNTs. The broadening is hypothesised to be related to the photohole lifetime which was found to be significantly longer for wide diameter SWCNTs. The diameter dependence of the broadening and photohole lifetimes is discussed in terms of both Tomonaga-Luttinger and Landau theory of Fermi liquids.
关键词： chirality,valence electronic states,single-walled carbon nanotubes,van Hove singularities,photoemission spectroscopy
更新于2025-09-16 10:30:52
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Electronic Structure of Sr <sub/>1?</sub><i> <sub/>y</sub></i> Ca <i> <sub/>y</sub></i> Fe <sub/>2</sub> (As <sub/>1?</sub><i> <sub/>x</sub></i> P <i> <sub/>x</sub></i> ) <sub/>2</sub> ( <i>x</i> = 0.25, <i>y</i> = 0.08) Revealed by Angle-Resolved Photoemission Spectroscopy
摘要： We have investigated the electronic structure of Sr1?yCayFe2(As1?xPx)2 (x = 0.25, y = 0.08) by means of angle-resolved photoemission spectroscopy. From the comparison with the results of BaFe2(As1?xPx)2, the effects of smaller structural anisotropy (c/a) on the Fermi surfaces (FSs) and the gap structures are discussed. The observed FSs have three dimensional shapes. One of the hole FSs is strongly warped between the Γ and Z points, and the innermost FS observed at the Z point disappears at the Γ point, which is similar to the FS features of SrFe2(As1?xPx)2 (x = 0.35). In the superconducting state, the node like gap-minimum is present for the dxy electron FS near the X point, while the gaps around the other high symmetry points are isotropic. Several theoretical models based on the spin and/or the orbital fluctuation are examined to explain all these experimental results.
关键词： spin fluctuation,orbital fluctuation,electronic structure,Fermi surfaces,angle-resolved photoemission spectroscopy,superconducting gap
更新于2025-09-16 10:30:52
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Growth of InGaN films on hardness-controlled bulk GaN substrates
摘要： We carried out an evaluation of the crystalline quality of bulk GaN substrates and the properties of InGaN ?lms grown on them. The Urbach energy estimated by photothermal de?ection spectroscopy and the tail states near the valence band maximum determined by hard x-ray photoemission spectroscopy were larger for hardness-controlled bulk GaN (hard GaN) than those for conventional bulk GaN (conventional GaN). However, InGaN on hard GaN grows in a step-?ow-like mode, while InGaN grown on conventional GaN exhibits spiral-like growth. The photoluminescence decay at room temperature for InGaN grown on the hard GaN was 470 ps, compared with 50 ps for that grown on the conventional GaN. This can be attributed to the suppression of spiral-like growth due to the resistance to deformation of the hard GaN. These results indicate that substrate hardness is one of the most important factors for III–V nitride growth on the bulk GaN substrate.
关键词： hard x-ray photoemission spectroscopy,photoluminescence,bulk GaN substrates,InGaN films,photothermal de?ection spectroscopy,crystalline quality
更新于2025-09-11 14:15:04