Measured Valence Band Research Articles

Optical properties from photoelectron energy-loss spectroscopy of low-temperature aqueous chemically synthesized ZnO nanorods grown on Si

The optical properties of zinc oxide (ZnO) nanorods (NRs) synthesized by the low-temperature aqueous chemical method on top of silicon (Si) substrate have been investigated by means of photoelectron energy loss spectroscopy (PEELS). The ZnO NRs were obtained by the low temperature aqueous chemical synthesis on top of Si substrate. The measured valence band, the dynamical dielectric functions and optical absorption of the material show a reasonable agreement when the trending and shape of the theoretical calculations are considered. A first-principle calculation based on density functional theory (DFT) was performed using the partially self-consistent GW approximation (scGW0) and compared to the experimental results. The application of these two techniques brings a new analysis of the electronic properties of this material. The experimental results regarding the density of states (DOS) obtained for the valence band using x-ray photoelectron spectroscopy (XPS) was found to be consistent with the theoretical calculated value. Due to this consistency, the same wavefunctions was then employed to calculate the dielectric function of the ZnO NRs. The experimentally extracted dielectric function was also consistent with the calculated values.

DFT calculations of energy dependent XPS valence band spectra

In the past few years it became regularly possible to measure valence band X-ray photoelectron spectra (XPS) using variable excitation energies. This ranges from UV-light to conventional X-ray sources (like Al Kα) all the way to synchrotron radiation with energies of several keV. In order to explain the observed variations in intensity with respect to the excitation energy, we performed XPS calculations using the WIEN2k code. The new PES module computes the XPS spectra using a combination of partial density of states times excitation-energy-dependent atomic-orbital cross sections. It considers as additional correction the charge fraction of the corresponding orbital located inside the atomic spheres. The resulting XPS spectra are compared with experimental data for SiO2, PbO2, CeVO4, In2O3 and ZnO at different excitation energies and in general good agreement between the simulated and experimental spectra has been achieved. In some cases significant unexpected contributions like Pb-6d in PbO2 or Zn-4p in ZnO appear and explain some features in the experimental spectra which previously have not been identified.

Laser writing of nanostructured silicon arrays for the SERS detection of biomolecules with inhibited oxidation.

The present work reports the processing of laser irradiated Si arrays (LISi) and underlines their surface enhanced Raman scattering (SERS) functionality. A nanostructured Si/SiOx surface forms providing additional fluidic and photoprotective properties. Because of their optical and surface characteristics, the arrays exhibit a SERS analytical enhancing factor of 500, without any noble metals such as gold or silver. Micro-Raman maps allowed studying LISi properties, identifying maximum amplification in nanostructured areas characterized by the presence of 7 nm Si nanocrystals. These structures are confined by a SiOx layer as illustrated by XPS valence band measurements. The highly hydrophilic LISi areas allow a pre-concentration of target molecules prior to SERS analysis. A relevant application of LISi was found in the detection of apomorphine (APO), a drug used for the treatment of Parkinson's disease. In contrast with what is obtained by using gold SERS substrates, LISi allows the detection of APO with no sign of oxidation. This invites for the use of the Si/SiOx SERS detection in future systems for the personalized delivery of APO.

Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide

The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.

Van der Waals epitaxy of two-dimensional single-layer h-BN on graphite by molecular beam epitaxy: Electronic properties and band structure

We report on the controlled growth of h-BN/graphite by means of molecular beam epitaxy. X-Ray photoelectron spectroscopy suggests the presence of an interface without any reaction or intermixing, while the angle resolved photoemission spectroscopy (ARPES) measurements show that the h-BN layers are epitaxially aligned with graphite. A well-defined band structure is revealed by ARPES measurements, reflecting the high quality of the h-BN films. The measured valence band maximum located at 2.8 eV below the Fermi level reveals the presence of undoped h-BN films (band gap ∼ 6 eV). These results demonstrate that, although only weak van der Waals interactions are present between h-BN and graphite, a long range ordering of h-BN can be obtained even on polycrystalline graphite via van der Waals epitaxy, offering the prospect of large area, single layer h-BN.

Growth and coverage dependent electronic structure of MgO on Ag(001)

Growth and coverage dependent electronic structure of MgO films on Ag(001) have been studied using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopic (ARPES) techniques. The appearance of mosaic structure in the LEED pattern has been observed around 10 ML coverage due to misfit dislocations in the metal-oxide interface. The XPS core level spectra show a substantial amount of binding energy shift relative to their bulk values, indicating the reduction of the on-site Coulomb (U) and change transfer (Δ) energies in oxide layers. This binding energy shift is mainly associated with the image charge screening effect of the highly polarizable metallic substrate. The effect of image charge screening is also reflected in the valence band electronic structure as the bands are shifted to lower binding energies with decreasing film coverage maintaining the bulk MgO band gap till 5 ML. However, for the film coverages below 5 ML, the band gap is reduced substantially in presence of O 2p-Ag 5sp hybridized bands with the finite size effects in proximity to the Ag(001) substrate. Furthermore, our measured valence band electronic structure has been compared with the theoretical calculations and a good agreement has been observed between them for the bulk MgO, however, a more detailed calculation is necessary to explain the monolayer MgO band structure properly.

Magnetic moments and exchange splitting in Mn3s and Mn2p core levels of magnetocaloric Mn1.1Fe0.9P0.6As0.4 and Mn1.1Fe0.9P0.5As0.4Si0.1 compounds

The magnetocaloric Mn1.1Fe0.9P0.6As0.4 and Mn1.1Fe0.9P0.5As0.4Si0.1 compounds were prepared via spark plasma sintering method. Prepared structural studies including X-ray diffraction measurements have shown the unit cell expanding for silicon substituted sample. The silicon substitution leads also to increase of the adiabatic temperature change from 2.4 K to 3.7 K through the phase transition at 258 K and 268 K in comparison to Mn1.1Fe0.9P0.6As0.4. The magnetic entropy curve showed a complex structure at the field of 5 T. The total saturation of magnetic moments derived from magnetic measurements corresponded to 3.9 μB/f.u. at low temperature. The electronic structure was tested by X-ray photoemission spectroscopy at room temperature and below the phase transition at 180 K. The observed exchange splitting of 2p states of manganese indicated existence of magnetic moments of d electrons and correlate with changes in the valence band measurements. The specific magnetic moments of iron and manganese were deduced from the obtained values of splitting within the 3s levels.

Electronic structure and photoluminescence properties of Zn-ion implanted silica glass before and after thermal annealing

The results of XPS core-level and valence band measurements, photoluminescence spectra of a-SiO2 implanted by Zn-ions (E=30keV, D=1·1017cm−2) and Density Functional Theory calculations of electronic structure as well as formation energies of structural defects in silica glass induced by Zn-ion implantation are presented. Both theory and experiment show that it is energetically more favorable for implanted zinc ions to occupy the interstitial positions instead of cation substitution. As a result, the Zn-ions embedded to interstitials, form chemical bonds with the surrounding oxygen atoms, fabrication ZnO-like nanoparticles and oxygen-deficient SiOx matrix. The subsequent thermal annealing at 900°C (1h) strongly reduces the amount of ZnO nanoparticles and induces the formation of secondary α-Zn2SiO4 phase which markedly enhances the green emission.

Observation and Origin of Extraordinary Atomic Mobility at Metal-Semiconductor Interfaces at Low Temperatures.

Extraordinarily high mobility of Si and Ge atoms at semiconductor (Si, Ge)-metal (Al) interfaces is observed at temperatures as low as 80K during thin metal film deposition. Insitu x-ray photoemission spectroscopic valence-band measurements reveal a changed chemical bonding nature of the semiconductor atoms, from localized covalentlike to delocalized metalliclike, at the interface with the Al metal. The resulting delocalized bonding nature of the interfacial semiconductor atoms brings about the observed extreme enhancement of their mobility. The finding opens avenues for tailoring reaction kinetics and phase transformations in nanostructured materials, as functional thin-film systems, at ultralow temperatures by dedicated interfacial design.

Electronic structure of the heavy fermion superconductor Ce2PdIn8: Experiment and calculations

The electronic structure of a heavy-fermion superconductor Ce2PdIn8 was investigated by means of X-ray photoelectron spectroscopy (XPS) and ab initio density functional band structure calculations. The Ce 3d core-level XPS spectra point to stable trivalent configuration of Ce atoms that is also reproduced in the band structure calculations within the generalized gradient approximation GGA+U approach. Analysis of the 3d9f2 weight in the 3d XPS spectra within the Gunnarsson-Schönhammer model suggests that the onsite hybridization energy between Ce 4f and the conduction band states, Δfs, is ∼120 meV, which is about 30 meV larger than Δfs in isostructural Ce2TIn8 compounds with T = Co, Rh, and Ir. Taking into account a Coulomb repulsion U on both the Ce 4f and Pd 4d states in electronic band structure calculations, a satisfactory agreement was found between the calculated density of states (DOS) and the measured valence band XPS spectra.

Band structure measurement and analysis of the Bi2Te3/CdTe (111)B heterojunction

The band alignments of the Bi2Te3/CdTe (111)B (Te-terminated) heterojunction were investigated using high-resolution x-ray and ultraviolet photoemission spectroscopy. The measured valence band offset and the conduction band offset of the Bi2Te3/CdTe were 0.22 ± 0.05 and 1.12 ± 0.05 eV, respectively, and indicated that the Bi2Te3/CdTe heterojunction had a type I band structure.

Electronic structure of C and N co-doped TiO2: A combined hard x-ray photoemission spectroscopy and density functional theory study

We have studied the electronic structure of C and N co-doped TiO2 using hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal overlap of the 2p states of O, N, and C in the system which shifts the valence band maximum towards the Fermi level. Combined with optical data we show that co-doping is an effective route for band gap reduction in TiO2. Comparison of the measured valence band with theoretical photoemission density of states reveals the possibility of C on Ti and N on O site.

Atomic and electronic structure of the ferroelectric BaTiO3/Ge(001) interface

In this study, we demonstrate the epitaxial growth of BaTiO3 on Ge(001) by molecular beam epitaxy using a thin Zintl template buffer layer. A combination of density functional theory, atomic-resolution electron microscopy and in situ photoemission spectroscopy is used to investigate the electronic properties and atomic structure of the BaTiO3/Ge interface. Aberration-corrected scanning transmission electron micrographs reveal that the Ge(001) 2 × 1 surface reconstruction remains intact during the subsequent BaTiO3 growth, thereby enabling a choice to be made between several theoretically predicted interface structures. The measured valence band offset of 2.7 eV matches well with the theoretical value of 2.5 eV based on the model structure for an in-plane-polarized interface. The agreement between the calculated and measured band offsets, which are highly sensitive to the detailed atomic arrangement, indicates that the most likely BaTiO3/Ge(001) interface structure has been identified.

Optical and electronic properties of nanosized BiTaO4 and BiNbO4 photocatalysts: Experiment and theory

Nanosized BiTaO4 and BiNbO4 were prepared by the citrate method. The electronic and optical properties of BiTaO4 and BiNbO4 have been investigated by means of photo‐acoustic spectroscopy (PAS), X‐ray photo‐electron spectroscopy (XPS), and first‐principles calculations based on density functional theory. The measured valence band (from XPS) of both materials agreed well with the theoretical findings. It was also found that the calculated optical properties such as dynamical dielectric function and optical absorption spectra are in good agreement with the experimental findings. According to the absorption spectra, the absorption edges of BiNbO4 and BiTaO4 are located at 370 and 330 nm, respectively. Both phases have the ability to harvest UV light and relatively high surface area to volume ratio and can be used as UV/visible light‐driven photocatalysts.

Oxygen vacancy formation and the induced defect states in HfO2 and Hf-silicates – A first principles hybrid functional study

O vacancy formation and the associated electronic defect states in Hf-based oxides have been investigated over a wide range of compositions using first-principles hybrid density functional calculations. Based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device.

Correlation between optical absorption redshift and carrier density in phase change materials

Here, we report an optical absorption redshift map for GeTe-Sb2Te3 pseudo-binary alloys. We found that, with phase change from amorphous to crystalline, the observed redshift increases with Ge concentration along pseudo-line of compositions, which directly reflects the enhanced electron delocalization/resonant bonding and increased carrier concentrations in the respective crystal compounds. The measured valence band maximum shift towards the Fermi energy from amorphous to crystalline phase supports the observed similar trend in redshift and carrier density. We show that the correlation between optical redshift and carrier density, attributed to the resonant bonding, can be rationalized by calculating the valence electron concentration, the ionicity, and hybridization.

Bulk electronic structure studied by hard X-ray photoelectron spectroscopy of the valence band: The case of intermetallic compounds

Photoelectron spectroscopy (PES) has evolved into the most relevant, powerful, and nondestructive method for investigating atoms, molecules, and solids. In particular, hard X-ray photoelectron spectroscopy (HAXPES) has emerged as a powerful tool for investigating the bulk electronic structure of materials in a variety of applied fields such as chemistry, physics, and materials science. In addition, PES was used for investigating the symmetries of various materials’ electronic structures. However, thus far, such studies have been restricted to atoms, molecules, adsorbates, and surfaces because low-energy (<1keV) electrons have limited probing depths. This is disadvantageous because three-dimensional (3D) bulk states cannot be studied. The present work demonstrates that this drawback can be eliminated by using hard X-rays with variable polarization for excitation. In the current study, this issue was investigated using several Heusler compounds, which have been attracting increasing levels of interest. There are more than 2000 Heusler compounds in total. Owing to their tunable electronic structures, Heusler compounds exhibit multifarious properties useful for spintronic, optoelectronic, shape memory, and thermoelectric applications. Herein, we report the results of bulk-sensitive, high energy photoelectron spectroscopy of the valence bands of several Heusler compounds for various applications. It is shown that the measured valence band spectra are clearly resolved and are in good agreement with the first-principles calculations of the compounds’ electronic structures.

Band offsets for mismatched interfaces: The special case of ZnO on CdTe (001)

High-quality planar interfaces between ZnO and CdTe would be useful in optoelectronic applications. Although CdTe is zinc blende with cubic lattice constant a = 6.482 Å while ZnO is hexagonal wurtzite with a = 3.253 Å and c = 5.213 Å, (001)-oriented cubic zinc blende ZnO films could be stabilized epitaxially on a CdTe (001) surface in an √2 × √2 R45° configuration with a lattice mismatch of &amp;lt;0.5%. Modeling such a configuration allows density-functional total-energy electronic-structure calculations to be performed on several interface arrangements (varying terminations and in-plane fractional translations) to identify the most likely form of the interface, and to predict valence-band offsets between CdTe and ZnO in each case. Growth of ZnO on Te-terminated CdTe(001) is predicted to produce small or even negative (CdTe below ZnO) valence band offsets, resulting in a Type I band alignment. Growth on Cd-terminated CdTe is predicted to produce large positive offsets for a Type II alignment as needed, for example, in solar cells. To corroborate some of these predictions, thin layers of ZnO were deposited on CdTe(001) by pulsed laser deposition, and the band alignments of the resulting heterojunctions were determined from x-ray photoelectron spectroscopy measurements. Although zinc blende ZnO could not be confirmed, the measured valence band offset (2.0–2.2 eV) matched well with the predicted value.

Band offsets in transition-metal oxide heterostructures

We measured valence band offsets in Ta2O5–WO3, Ta2O5–Nb2O5 and WO3–Nb2O5 heterostructure couples by in situ x-ray photoelectron spectroscopy, immediately following the bi-layer growth in ultra-high vacuum. Conduction band offsets were estimated using the measured valence band offsets in conjunction with the literature values for the respective band gaps. The offsets between Ta2O5 and WO3 and between Ta2O5 and Nb2O5 layers were strongly asymmetric, with 0.8–1.1 eV (0.1–0.2 eV) barriers for the conduction (valence) bands, depending on the particular couple and the stacking sequence. Such asymmetry can be very useful in switching devices.

Comparative Study of Atomic-Layer-Deposited Stacked (HfO2/Al2O3) and Nanolaminated (HfAlOx) Dielectrics on In0.53Ga0.47As

The high-k gate dielectric structures in stacked (HfO2/Al2O3) and nanolaminated (HfAlOx) forms with a similar apparent accumulation capacitance were atomic-layer-deposited on n-type In0.53Ga0.47As substrates, and their electrical properties were investigated in comparison with a single-layered HfO2 film. Al-oxide interface passivation in both forms proved to be effective in preventing a significant In incorporation in the high-k film and reducing the interface state density. The measured valence band spectra in combination with the reflection electron energy loss spectra were used to extract the energy band parameters of various dielectric structures on In0.53Ga0.47As. A further decrease in the interface state density was achieved in the stacked structure than in the nanolaminated structure. However, in terms of the other electrical properties, the nanolaminated sample exhibited better characteristics than the stacked sample, with a smaller border trap density and lower leakage current under substrate injection conditions with and without voltage stressing.