Measured Valence Band Research Articles

WSe2/n-GaN and WSe2/p-GaN heterojunction band alignment

Abstract In this work, we measured the band alignment of 2D/3D heterostructures of WSe2 on n-doped, pdoped, and intrinsic GaN by X-ray photoelectron spectroscopy (XPS). The WSe2/n-GaN heterojunction exhibits type-II band alignment, with measured valence band offset (VBO) and conduction band offset (CBO) values of 2.28±0.15 eV and 0.96±0.15 eV, respectively. On the other hand, the WSe2/p-GaN heterojunction shows type-I band alignment, with measured VBO and CBO values of 0.1±0.15 eV and 1.22±0.15 eV, respectively. The results show that doping has a significant effect on the arrangement of the energy bands of the heterostructure, and provides a reference for device applications based on heterojunctions.

An insight into a novel calixarene-sensitized Calix@Nb2CTx/g-C3N4 MXene-based photocatalytic heterostructure: Fabrication, physicochemical, optoelectronic, and photoelectrochemical properties

The development of highly efficient and cost-effective multifunctional photocatalysts is of current global interest because these photocatalysts have the potential to address the energy and water crisis. Herein, an efficient calixarene-based [email protected]2CTx/g-C3N4 (Cx-Nb-CN) photocatalyst was prepared through the formation of covalent bonds between the calixarene (Cx-COCl), g-C3N4 (CN), and Nb2CTx MXene. Enhanced optoelectronic and photoelectrochemical (PEC) properties were observed upon introducing Cx-COCl calixarene and Nb2CTx complexes to the g-C3N4 (CN) photocatalyst. The XPS valence band measurements demonstrated the narrowing of the energy band gap for the composites due to the downshifting and upshifting of the CB and VB, correspondingly. Due to the sensitization effect, the Cx-CN presented superior photocatalytic properties relative to the pristine CN. Moreover, reduced charge transfer resistance (Rct = 110.7 Ω.cm2) and the highest photocurrent density (Jp = 7.95 mA/cm2) were observed for the Cx-Nb-CN-5 heterostructure. The Schottky-heterostructures Cx-Nb-CN-1, Cx-Nb-CN-3, and Cx-Nb-CN-5 presented high linear sweep current densities (JLSV) of 8.61, 12.39, 14.04 mA/cm2 signifying excellent light utilization and efficient separation of charge carriers, respectively. The fabricated photocatalyst exhibits excellent physicochemical and photocatalytic properties with the potential to facilitate host-guest complexation towards environmental detoxification and energy conversions.

Band Alignments, Electronic Structure, and Core-Level Spectra of Bulk Molybdenum Dichalcogenides (MoS2, MoSe2, and MoTe2).

A comprehensive study of bulk molybdenum dichalcogenides is presented with the use of soft and hard X-ray photoelectron (SXPS and HAXPES) spectroscopy combined with hybrid density functional theory (DFT). The main core levels of MoS2, MoSe2, and MoTe2 are explored. Laboratory-based X-ray photoelectron spectroscopy (XPS) is used to determine the ionization potential (IP) values of the MoX2 series as 5.86, 5.40, and 5.00 eV for MoSe2, MoSe2, and MoTe2, respectively, enabling the band alignment of the series to be established. Finally, the valence band measurements are compared with the calculated density of states which shows the role of p-d hybridization in these materials. Down the group, an increase in the p-d hybridization from the sulfide to the telluride is observed, explained by the configuration energy of the chalcogen p orbitals becoming closer to that of the valence Mo 4d orbitals. This pushes the valence band maximum closer to the vacuum level, explaining the decreasing IP down the series. High-resolution SXPS and HAXPES core-level spectra address the shortcomings of the XPS analysis in the literature. Furthermore, the experimentally determined band alignment can be used to inform future device work.

Crystal structure, Mössbauer and X-ray photoelectron spectroscopies, and magnetic properties of Sr2FeMo1-xNbxO6 compounds

A systematic experimental study of the crystal structure, Mössbauer and X-ray photoelectron spectroscopies as well as magnetic measurements were carried out to study the effect of Mo substitution by Nb in the Sr2FeMo1-xNbxO6 compounds with x ​= ​0.5, 0.6, 0.7, 0.8 and 1.0. Rietveld analysis made to the crystal structure indicates that the tetragonal structure is preserved for the compounds with a transition phase from I4/m for x ​= ​0.5, 0.6 and 0.7, to an I4/mcm for x ​= ​0.8 and 1.0 by disorder in the Fe and Mo/Nb cations. Mössbauer spectroscopy showed three different environments around iron. XPS results reveal a mixed oxidation states for Mo6+/5+ ions and a trend to Mo5+ as Nb amount increases. XPS valence band measurements indicate a reduction of the charge carries suggesting a more insulating behavior as Nb content increases. This is reflected in magnetic measurements which show a Ms reduction as consequence of Nb content increases, and a non-reversible behavior between ZFC and FC at low temperatures suggest a spin-glass behavior according to previous works.

Confined Monolayer Ag As a Large Gap 2D Semiconductor and Its Momentum Resolved Excited States.

2D materials have intriguing quantum phenomena that are distinctively different from their bulk counterparts. Recently, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are attracting attention not only for their potential applications but also for exotic quantum effects such as superconductivity. By mapping momentum-resolved electronic states using time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal that monolayer Ag confined between bilayer graphene and SiC is a large gap (>1 eV) 2D semiconductor, consistent with ab initio GW calculations. The measured valence band dispersion matches the GW quasiparticle band structure. However, the conduction band dispersion shows an anomalously large effective mass of 2.4 m0. Possible mechanisms for this large enhancement in the "apparent mass" are discussed.

Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG.

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O2 during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V5+ and V4+. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.

Effect of Band Bending in Photoactive MOF-Based Heterojunctions.

Semiconductor/metal–organic framework (MOF) heterojunctions have demonstrated promising performance for the photoconversion of CO2 into value-added chemicals. To further improve performance, we must understand better the factors which govern charge transfer across the heterojunction interface. However, the effects of interfacial electric fields, which can drive or hinder electron flow, are not commonly investigated in MOF-based heterojunctions. In this study, we highlight the importance of interfacial band bending using two carbon nitride/MOF heterojunctions with either Co-ZIF-L or Ti-MIL-125-NH2. Direct measurement of the electronic structures using X-ray photoelectron spectroscopy (XPS), work function, valence band, and band gap measurements led to the construction of a simple band model at the heterojunction interface. This model, based on the heterojunction components and band bending, enabled us to rationalize the photocatalytic enhancements and losses observed in MOF-based heterojunctions. Using the insight gained from a promising band bending diagram, we developed a Type II carbon nitride/MOF heterojunction with a 2-fold enhanced CO2 photoreduction activity compared to the physical mixture.

Tailorable properties of Nd-doped ZnO epitaxial thin films for optoelectronic and plasmonic devices

Transparent conductive oxides with both low resistivity and high transparency in the visible and near-infrared wavelengths are required for solar cells and plasmonic applications. In this work we demonstrate that highly conducting and transparent Nd:ZnO thin films have been grown on c-cut sapphire single crystal substrates by pulsed electron beam deposition under argon (10−2 mbar). At 2% at. Nd doping, these films crystallize in the wurtzite structure at T > 300 °C, with well-defined epitaxial relationships with the c-cut sapphire as a function of the substrate temperature. The improvement of the film structural quality with increasing growth temperature led to the lowest resistivity value of 5.12 × 10−4 Ωcm and highest transparency that were maintained more than 36 months under ambient conditions. X-ray photoelectron valence band measurements revealed that the dominant Zn-polar face of Nd:ZnO thin films explains the improved electrical properties. The dielectric functions of films were determined and show that these films are suitable as transparent contact electrodes for solar cell devices or tunable epsilon-near-zero materials for plasmonics.

Band Alignment of Al2O3 on α-(AlxGa1-x)2O3

X Ray Photoelectron Spectroscopy was used to measure valence band offsets for Al2O3 deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3 alloys over a wide range of Al contents, x, from 0.26–0.74, corresponding to a bandgap range from 5.8–7 eV. These alloys were grown by Pulsed Laser Deposition. The band alignments were type I (nested) at x <0.5, with valence band offsets 0.13 eV for x = 0.26 and x = 0.46. At higher Al contents, the band alignment was a staggered alignment, with valence band offsets of − 0.07 eV for x = 0.58 and −0.17 for x = 0.74, ie. negative valence band offsets in both cases. The conduction band offsets are also small at these high Al contents, being only 0.07 eV at x = 0.74. The wide bandgap of the α-(AlxGa1-x)2O3 alloys makes it difficult to find dielectrics with nested band alignments over the entire composition range.

Facile Processing of p-Type Oxides and Oxide-Based p-n Heterojunction Applications

Oxide electronics have gained prominence in recent years and have been established as one of the most promising new technologies for various electronic and optoelectronic applications including next generation displays, solar cells, and photodetectors. The high potential use seen in oxide electronics is due primarily to their high carrier mobilities and their ability to be fabricated at low temperatures.However, the vast majority of oxide semiconductors are n-type oxides, which limits the current applications to unipolar devices and ultimately stunts the development of oxide-based bipolar device applications such as p-n diodes and complementary metal–oxide–semiconductors.The goal of this research is to resolve the scientific questions which prevent the realization of low-temperature processed p-type oxides and oxide-based p-n heterojunctions showcasing high rectification behavior, low saturation current, and a small turn-on voltage.The current study is unique in that:(1) Unlike several previous reports on oxide p-n junctions fabricated exploiting a thin film epitaxial growth technique (known as molecular beam epitaxy, MBE) or a high-powered laser beam process (known as pulsed laser deposition, PLD) that requires ultra-high vacuum conditions, a large amount of power, and is limited for large area processing, we demonstrate oxide-based heterojunction p-n diodes that consists of sputter-synthesized p-SnOx and n-IGZO of which the manufacturing routes are in-line with current manufacturing requirements.(2) The synthesized p-SnOx films are devoid of metallic Sn phases (i.e., Sn0 state) with carrier density tuneability and high carrier mobility (> 2 cm2/Vs).(3) The charge blocking performance of the metallurgical junction is significantly enhanced by the engineering of trap/defect density of n-IGZO, which is identified using photoelectron microscopy and valence band measurements.(4) The resulting oxide-based p-n heterojunction exhibits a high rectification ratio greater than 103 at ±3 V (highest among the sputter-processed oxide junctions), a low saturation current of ~2×10-10 A, and a small turn-on voltage of ~0.5 V.The outcomes of the current study are expected to contribute to the development of p-type oxides and their industrial device applications such as p-n diodes of which the manufacturing routes are in-line with the current processing requirements.REFERENCES Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004, 432 (7016), 488-492.Kawazoe, H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H., P-type electrical conduction in transparent thin films of CuAlO2. Nature 1997, 389 (6654), 939-942.Kawazoe, H.; Yanagi, H.; Ueda, K.; Hosono, H., Transparent p-Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions. MRS Bull. 2000, 25 (08), 28-36.Lee, S.; Paine, D. C., Metallization selection and the performance of amorphous In-Zn-O thin film transistors. Applied Physics Letters 2014, 104 (25), 252103.Liu, M.; Kim, H.; Wang, X.; Song, H. W.; No, K.; Lee, S., Carrier Density-Tunable Work Function Buffer at the Channel/Metallization Interface for Amorphous Oxide Thin-Film Transistors. ACS Applied Electronic Materials 2021, 3 (6), 2703-2711. ACKNOWLEDGMENTThis work was supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088. S.L. and H.W.S. acknowledge the support from the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.

Influence of Oxygen Ion Migration from Substrates on Photochemical Degradation of CH3NH3PbI3 Hybrid Perovskite

Measurements of XPS survey, core levels (N 1s, O 1s, Pb 4f, I 3d), and valence band (VB) spectra of CH3NH3PbI3 (MAPbI3) hybrid perovskite prepared on different substrates (glass, indium tin oxide (ITO), and TiO2) aged under different light-soaking conditions at room temperature are presented. The results reveal that the photochemical stability of MAPbI3 depends on the type of substrate and gradually decreases when glass is replaced by ITO and TiO2. Also, the degradation upon exposure to visible light is accompanied by the formation of MAI, PbI2, and Pb0 products as shown by XPS core levels spectra. According to XPS O 1s and VB spectra measurements, this degradation process is superimposed on the partial oxidation of lead atoms in ITO/MAPbI3 and TiO2/MAPbI3, for which Pb–O bonds are formed due to the diffusion of the oxygen ions from the substrates. This unexpected interaction leads to additional photochemical degradation.

Study of band alignment at MoS2/SiO2 interfaces grown by pulsed laser deposition method

We report on the large-area and high-quality growth of single- to few-monolayer thick MoS2 thin films on oxidized Si (100) substrates via the pulsed laser deposition method. Our Raman, x-ray photoelectron spectroscopic, and FE-SEM measurements confirmed that atomically thin MoS2 layers are highly uniform and are stoichiometric. We found a type-I band alignment at the MoS2/SiO2 heterointerfaces through photoemission spectroscopic valence-band measurements. The valence- and conduction band offset (VBO and CBO) at the 1L MoS2/SiO2 interface was observed to be 3.91 and 2.96 ± 0.05 eV, respectively. The values of VBO and CBO increase up to 4.15 and 3.56 ± 0.05 eV, respectively, with an increase in the MoS2 layer number. This observation can be attributed to the shift of the Mo-4dz2 orbitals due to interlayer coupling for thicker MoS2 films, reducing its bandgap, resulting in an increment in VBO and CBO values.

Tailoring the d-band center of N-doped carbon nanotube arrays with Co4N nanoparticles and single-atom Co for a superior hydrogen evolution reaction

A 3D self-supported integrated electrode, consisting of heteroatomic nitrogen-doped carbon nanotube arrays on carbon cloth with confined ultrafine Co4N nanoparticles and a distribution of anchored single-atom Co, is fabricated via a cobalt-catalyzed growth strategy using dicyandiamide as the nitrogen and carbon source and a layered cobalt hydroxide-nitrate salt as the precursor. The abundance of exposed active sites, namely, the Co4N nanoparticles, single-atom Co, and heteroatomic N-doped carbon nanotubes, and multiple synergistic effects among these components provide suitable tailoring of the d-band center for facilitating vectorial electron transfer and efficient electrocatalysis. Benefiting from the merits of its structural features and electronic configuration, the prepared electrode exhibits robust performance toward the hydrogen evolution reaction with overpotentials of only 78 and 86 mV at 10 mA cm−2 in acidic and basic electrolytes, respectively. Density functional theory calculations and X-ray photoelectron spectroscopy valence band measurements reveal that the effective tailoring of the d-band center by Co4N nanoparticles plays a crucial role in optimizing the hydrogen adsorption free energy to a more thermoneutral value for efficient electrocatalysis.

Bridging Electrical and Structural Interface Properties: a Combined DFT-GW Approach

The selection of a proper metal for replacement of polycrystalline silicon as the metal gate in future generation transistors has been hampered by pinning of the metal Fermi level at the metal/dielectric interface. Using monoclinic hafnia and zirconia as the gate dielectric we compare three different metal gate/gate dielectric interface structures where the oxygen affinity of the metal gate varies from low to high under normal processing conditions. For each of the metal gate/gate dielectric combination we considered a number of interface stoichiometries and tried to identify the most likely interface composition by comparing the calculated and measured valence band offsets (VBO). Because density functional theory (DFT) underestimates the dielectric band gap, it also underestimates the VBO thus requiring a correction to the band edges, which we accomplished using GW for cubic and monoclinic hafnia. Our GW shift value for monoclinic hafnia (0.3 eV) indicates mostly reduced interfaces in all metal/dielectric combinations considered.

Plasma-Induced Defects Enhance the Visible-Light Photocatalytic Activity of MIL-125(Ti)-NH2 for Overall Water Splitting.

Defect engineering in metal-organic frameworks is commonly performed by using thermal or chemical treatments. Herein we report that oxygen plasma treatment generates structural defects on MIL-125(Ti)-NH2 , leading to an increase in its photocatalytic activity. Characterization data indicate that plasma-treated materials retain most of their initial crystallinity, while exhibiting somewhat lower surface area and pore volume. XPS and FT-IR spectroscopy reveal that oxygen plasma induces MIL-125(Ti)-NH2 partial terephthalate decarboxylation and an increase in the Ti-OH population. Thermogravimetric analyses confirm the generation of structural defects by oxygen plasma and allowed an estimation of the resulting experimental formula of the treated MIL-125(Ti)-NH2 solids. SEM analyses show that oxygen plasma treatment of MIL-125(Ti)-NH2 gradually decreases its particle size. Importantly, diffuse reflectance UV/Vis spectroscopy and valence band measurements demonstrate that oxygen plasma treatment alters the MIL-125(Ti)-NH2 band gap and, more significantly, the alignment of highest occupied and lowest unoccupied crystal orbitals. An optimal oxygen plasma treatment to achieve the highest efficiency in water splitting with or without methanol as sacrificial electron donor under UV/Vis or simulated sunlight was determined. The optimized plasma-treated MIL-125(Ti)-NH2 photocatalyst acts as a truly heterogeneous photocatalyst and retains most of its initial photoactivity and crystallinity upon reuse.

Large positive magnetoresistance and Dzyaloshinskii\u2013Moriya interaction in CrSi driven by Cr 3d localization

Spin chiral systems with Dzyaloshinskii–Moriya (DM) interaction due to broken inversion symmetry are extensively studied for their technological applications in spintronics and thermoelectrics. Here, we report an experimental study on the magnetization, magnetoresistance (MR) and electronic structure of a non-centrosymmetric compound CrSi with B20 crystal structure. Both magnetization and MR shows competing ferromagnetic (FM) and antiferromagnetic (AFM) correlations with the FM correlations being comparatively weaker indicating the presence of DM interaction in CrSi. A large positive MRsim ,25% obtained at 5 K and 5 T magnetic field arises due to the stronger AFM correlations. Resonant photoemission shows both localized and itinerant nature of Cr 3d electrons to be present in CrSi and this is supported by the temperature dependence of magnetic susceptibility. Drastic variation in the density of states along with valence band broadening at low temperature indicates the increase in hybridization between Cr 3d and Si 3s–3p states which enhances the localization effects. Spin polarized itinerant Cr 3d electrons give rise to AFM spin density wave in CrSi. Magnetic interaction between the localized and itinerant Cr 3d electrons are found to be crucial for realizing DM interaction in this system. Spectral density of states derived from high resolution valence band measurements provides evidence of electronic topological transition in CrSi. Large density of polarized itinerant electrons which varies with temperature and the large positive MR with AFM correlations suggests CrSi as a potential candidate for both the thermoelectric and spintronics applications.

Optical Properties and First-Principles Study of CH3NH3PbBr3 Perovskite Structures.

Solution-processed organic–inorganic hybrid perovskites have attracted attention as light-harvesting materials for solar cells and photonic applications. The present study focuses on cubic single crystals and microstructures of CH3NH3PbBr3 perovskite fabricated by a one-step solution-based self-assembly method. It is seen that, in addition to the nucleation from the precursor solution, crystallization occurs when the solution is supersaturated, followed by the formation of a small nucleus of CH3NH3PbBr3 that self-assembles into bigger hollow cubes. A three-dimensional (3D) fluorescence microscopy investigation of hollow cubes confirmed the formation of hollow plates on the bottom; then, the growth starts from the perimeter and propagates to the center of the cube. Furthermore, the growth in the (001) direction follows a layer-by-layer growth model to form a complete cube, confirmed by scanning electronic microscopy (SEM) observations. Two-dimensional (2D)–3D fluorescence microscopy and photoluminescence (PL) measurements confirm a peak emission at 535 nm. To get more insights into the structural and optical properties, density functional theory (DFT) simulations were conducted. The electronic and optical properties calculated by DFT are in agreement with the obtained experimental values. The density-of-state (DOS) calculations revealed that the valence band maximum (VBM) consists of states contributed by Br and Pb, which agrees with the X-ray photoelectron spectroscopy valence band (XPS VB) measurements.

Growth and characterization of nickel oxide ultra-thin films

The oxidation of the Ni(111) surface under ultrahigh-vacuum conditions is studied experimentally with low-energy electron diffraction and high-resolution X-ray photoelectron spectroscopy. Exposure of the clean Ni(111) surface to molecular oxygen at room temperature followed by annealing at 400 K leads to the formation of two different structures (2×2) and (33×33)R30∘, prior to the formation of the NiO(111) monolayer. The O 1s core levels indicate that the obtained oxide is terminated by oxygen atoms while the valence band measurements clearly reveal the band gap of NiO. The energy difference between the Fermi level and the maximum of the valance band is extracted and is found to be 0.47 eV.

Imaging microscopic electronic contrasts at the interface of single-layer WS2 with oxide and boron nitride substrates

The electronic properties of devices based on two-dimensional materials are significantly influenced by interactions with the substrate and electrode materials. Here, we use photoemission electron microscopy to investigate the real- and momentum-space electronic structures of electrically contacted single-layer WS2 stacked on hBN, SiO2, and TiO2 substrates. Using work function and X-ray absorption imaging, we single-out clean microscopic regions of each interface type and collect the valence band dispersion. We infer the alignments of the electronic bandgaps and electron affinities from the measured valence band offsets of WS2 and the three substrate materials using a simple electron affinity rule and discuss the implications for vertical band structure engineering using mixed three- and two-dimensional materials.

Band alignment determination of bulk h-BN and graphene/h-BN laminates using photoelectron emission microscopy

Because graphene stacked on hexagonal boron nitride (h-BN) exhibits high electron mobility, it is expected to be applied to next-generation high-speed transistors and electron emitters. To further improve the performance of graphene/h-BN devices, it is necessary to determine the band alignment of graphene/h-BN laminates. However, because mechanically peeled h-BN single crystals transferred onto Si substrates are small, pinpoint observation of h-BN with ordinary photoelectron spectroscopy is difficult. In this study, the electric structure of a graphene/h-BN laminate was identified by photoemission electron microscopy and local measurements of valence band and secondary electron spectra using micro-ultraviolet photoelectron spectroscopy were performed. From these measurements, we determined the band alignment of a graphene/h-BN laminate with a crystal size of a few tens of micrometers. The work function and electron affinity measured by photoelectron spectroscopy of single-crystal h-BN were 4.6 and −0.5 eV, respectively. Laminating graphene on h-BN caused the Fermi level of h-BN to rise 0.85 eV above that of nonlaminated h-BN. In addition, it was found that graphene on h-BN displayed weak n-type conductivity. The results obtained in this research are expected to be widely applied in the field of electronics such as electron emitters using h-BN with negative electron affinity.