Electronic Structure Of Thin Films Research Articles

Influence of thickness scaling on the electronic structure and optical properties of oxygen deficient BaBiO3-δ thin films grown on SrTiO3-buffered Si(001) substrate

BaBiO3 has attracted a lot of research attention since it was discovered as the parent compound for the high-Tc superconducting BaPbxBi1–xO3 and Ba1–xKxBiO3. In its pure state, BaBiO3 is an insulator due to the presence of a breathing distortion of the BiO6 octahedra. The distortion is attributed to the valency of Bi in the compound being charge-ordered in the form of Bi3+ and Bi5+ along the lattice, resulting in alternating expanded or contracted BiO6 octahedra. The interaction between the electronic properties and the thickness of the thin film is crucial to study. We conducted a thorough study to investigate the effect of the thickness reduction of BaBiO3-δ grown on SrTiO3-buffered Si substrates on the optical properties as well as the Bi electronic structure of the thin films. We conclude that modifications in the valency of Bi in the ultra-thin film regime result in an optically conducting layer.

Electronic Structure of Colloidal 2H-MoS2 Mono and Bilayers Determined by Spectroelectrochemistry.

The electronic structure of mono and bilayers of colloidal 2H-MoS2 nanosheets synthesized by wet-chemistry using potential-modulated absorption spectroscopy (EMAS), differential pulse voltammetry, and electrochemical gating measurements is investigated. The energetic positions of the conduction and valence band edges of the direct and indirect bandgap are reported and observe strong bandgap renormalization effects, charge screening of the exciton, as well as intrinsic n-doping of the as-synthesized material. Two distinct transitions in the spectral regime associated with the C exciton are found, which overlap into a broad signal upon filling the conduction band. In contrast to oxidation, the reduction of the nanosheets is largely reversible, enabling potential applications for reductive electrocatalysis. This work demonstrates that EMAS is a highly sensitive tool for determining the electronic structure of thin films with a few nanometer thicknesses and that colloidal chemistry affords high-quality transition metal dichalcogenide nanosheets with an electronic structure comparable to that of exfoliated samples.

Manganese Oxide as an Inorganic Catalyst for the Oxygen Evolution Reaction Studied by X‐Ray Photoelectron and Operando Raman Spectroscopy

AbstractManganese oxide (MnOx) is considered a promising material for the oxygen evolution reaction (OER) to replace noble metal catalysts in water splitting. The improvement of MnOx requires mechanistic and kinetic knowledge of the four‐electron transfer steps of the OER. X‐ray photoelectron spectroscopy, a widely used tool to characterize the electronic structure of thin films, is used in combination with surface‐enhanced Raman spectroscopy to gain a deeper knowledge of the different mixed MnOx types and their respective change in chemical composition. Using Raman spectroscopy during electrochemical measurements, all samples were found to reveal Birnessite‐type MnO2 motifs in alkaline media at an applied potential. Their activity correlates with two shifting Raman active modes, one of them being assigned to the formation of MnIII species, and one to the expansion of layers of MnO6 octahedra. A special activation treatment leads independent of the starting material to a highly amorphous mixed‐valence oxide, which shows the highest OER activity.

Seebeck coefficients in CuFeS2 thin films by first-principles calculations

We analyze electronic contributions to thermoelectric properties of CuFeS2 thin films. The Seebeck coefficients, electronic structures, and electrical conductances are obtained using the first-principles calculations. All calculations are based on density functional theory, and nonequilibrirum Green’s function formalism within the linear response regime for the electronic transport calculations. Electronic structures of thin films oriented to [001] and [100] in three different magnetic phases; non-, ferro-, and antiferro- are examined. All the calculated systems are metallic, and antiferromagnetic electronic states are most stable. Calculated electrical conductances reach to twice larger than that in the bulk, however, the calculated Seebeck coefficients in thin films are suppressed by the metallicity, remain maximally to about a half of the results in the bulk.

Electronic structure of thin films of hydrocarbon molecules under an external electric field

Using the density functional theory combined with the effective screening medium method, we studied the electronic structures of thin films of naphthalene under the external electric field in terms of their molecular conformation with respect to the electrode. Our calculations showed that the carrier accumulations and depletions strongly depend on the molecular conformation to the electrode. Carriers are primarily distributed on the molecules situated near the electrode. Furthermore, asymmetric carrier accumulation with respect to the carrier species occurs for the thin films where the molecules have tilting arrangements to the electrode. For the slabs consisting of six molecular layers, injected carriers are distributed throughout the slab irrespective of the carrier species. On the other hand, the carrier penetration is highly suppressed for the slabs where the molecules possess tilting conformation with respect to the electrode.

Time-dependent density functional theory for interaction of ultrashort light pulse with thin materials

We present a comprehensive theoretical description for an irradiation of an ultrashort light pulse normally on thin materials based on first-principles time-dependent density functional theory. As the most elaborate scheme, we develop a microscopic description solving Maxwell equations for light electromagnetic fields and the time-dependent Kohn-Sham equation for electron dynamics simultaneously in the time domain using a common spatial grid. We call it the microscopic Maxwell-TDDFT scheme. We test this scheme for silicon thin films of various thickness, from a few atomic layers to a few tens of nm. We show that the microscopic Maxwell-TDDFT scheme provides a satisfactory description incorporating the electronic structure of thin films in the first-principles level, multiple reflections of the electromagnetic fields at the surfaces, and nonlinear light-matter interaction when the incident light pulse is strong. However, the calculation becomes expensive as the thickness increases. We then consider two limiting cases of extremely thin and sufficiently thick films and develop approximate schemes. For the extremely thin case including two-dimensional atomic-layered materials, a two-dimensional macroscopic electromagnetism is developed: a two-dimensional susceptibility is introduced for a weak field, while time evolution equation is derived for an intense field. For sufficiently thick films, the microscopic Maxwell-TDDFT scheme is expected to coincide with a description utilizing ordinary macroscopic electromagnetism. We numerically confirm it comparing the calculated results: For a weak field, a comparison is made with a description using the bulk dielectric susceptibility. For a strong field, a comparison is made with a multiscale Maxwell-TDDFT scheme which the authors' group developed previously.

Optical and Electrical Properties of ECR-PECVD Grown SiCN Thin Films

Recent progress in scaling down the metal-oxide semiconductor (MOS) devices accompanied limitations arising from quantum effects, which leads to a search for new materials. Silicon oxide (SiO2) thin films have been a favourable dielectric for more than 4 decades. Their usage have become impractical due to the power dissipation and delay in interconnects. To increase the electrical performance, low extinction coefficient (low-k) dielectric materials are being developed as a substitute to SiO2. Those materials draw considerable attraction due to their advances in the integrated circuit technology to increase the transistor density. Among numerous materials, silicon carbonitride (SiCN), which is an intermediate compound between silicon nitride (SiN) and silicon carbide (SiC), has become noteworthy with its unique properties as low-k, robustness, high thermal stability, and wide bandgap of 2.2eV- 5eV1. In this work we present the optical, structural, and electrical analysis of SiCN thin films grown using electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD). The influence of the precursors (SiH4, C2H2, and N2 gases) on the composition and electronic structure of thin films which were characterized by Rutherford backscattering spectroscopy (RBS) and Fourier transmission infrared spectroscopy (FTIR). Furthermore, current-voltage characteristics were studied in dark and illuminated environment at room temperature. We studied the variation of photodiode characteristics with the thin film compositions. These results were also compared with the SiCN thin films growth using identical parameters except for the carbon source (CH4 gas) which has been reported in earlier studies2. Our findings showed that the hydrogen content influenced the optical coefficients of these two set of samples. Index of refraction and extinction coefficients were characterized by variable angle spectroscopic ellipsometry (VASE). In addition to our works on developing low-k matrix to enhance the performance of the integrated circuits, the wide gap feature of SiCN thin films enable us to consider the ultraviolet (UV) photo responsivity effect which is studied for photodetector device application.

Electronic structure of Fe1.08Te bulk crystals and epitaxial FeTe thin films on Bi2Te3

The electronic structure of thin films of FeTe grown on Bi2Te3 is investigated using angle-resolved photoemission spectroscopy, scanning tunneling microscopy and first principles calculations. As a comparison, data from cleaved bulk Fe1.08Te taken under the same experimental conditions is also presented. Due to the substrate and thin film symmetry, FeTe thin films grow on Bi2Te3 in three domains, rotated by 0°, 120°, and 240°. This results in a superposition of photoemission intensity from the domains, complicating the analysis. However, by combining bulk and thin film data, it is possible to partly disentangle the contributions from three domains. We find a close similarity between thin film and bulk electronic structure and an overall good agreement with first principles calculations, assuming a p-doping shift of 65 meV for the bulk and a renormalization factor of around two. By tracking the change of substrate electronic structure upon film growth, we find indications of an electron transfer from the FeTe film to the substrate. No significant change of the film’s electronic structure or doping is observed when alkali atoms are dosed onto the surface. This is ascribed to the film’s high density of states at the Fermi energy. This behavior is also supported by the ab initio calculations.

Energetics and electronic structures of thin films and heterostructures of a hexagonal GaN sheet

Using the density functional theory (DFT) combined with van der Waals correction and effective screening medium methods, we study the geometric and electronic structures of GaN thin films, each atomic layer of which exhibits a hexagonally bonded two-dimensional (2D) network. Our DFT calculations containing the van der Walls correction showed that the hexagonal GaN (h-GaN) sheets in the thin films are tightly bound to each other owing to the small interlayer spacing, such that their electronic structures are sensitive to the number of layers. We also investigate the energetics and electronic structures of hybrid structures of h-GaN with other layered materials, graphene and h-BN: For both hybrids, the optimum interlayer spacing is 3.4 Å, indicating that the h-GaN sheet is bound to graphene or h-BN via a weak van der Waal interaction. Owing to the weak interlayer interaction, graphene and h-BN retain their characteristic electronic structures. We further found that GaN thin films with a wurtzite structure undergo a structural phase transition into the layered structure of h-GaN when a biaxial tensile strain is applied.

Polarization-Controlled Two-Dimensional White-Light Spectroscopy of Semiconducting Carbon Nanotube Thin Films

Polarized two-dimensional white-light (2D-WL) spectra are reported for thin films of semiconducting carbon nanotubes. The orientational responses for 4-point correlation functions are derived for samples that are isotropic in two dimensions. Spectra measured using ⟨−45°,+45°,0°,90°⟩ polarizations eliminate the diagonal peaks in the spectra arising from S1 transitions to uncover cross peaks to a weaker transition that is assigned to radial breathing modes. In nanotubes purified by unwrapping PFO-BPY polymer using metal chelation, an absorption at 1160 nm is observed that is assigned to hole doping that forms trions. The trion peak may have a transition dipole nonparallel to the S1 transitions, and so its cross peak is prominent in polarized 2D-WL spectra. Energy transfer of photoexcitons to the trion peak occurs within 1 ps. Identifying and understanding the effects of purification on the electronic structure of thin films of semiconducting carbon nanotubes is important for learning how the inherent photop...

Strain-induced metal-semiconductor transition in monolayers and bilayers of gray arsenic: A computational study

We study the equilibrium geometry and electronic structure of thin films of layered gray arsenic using ab initio density functional theory. In contrast to bulk gray As that is semimetallic, thin films display a significant band gap that depends sensitively on the number of layers, in-layer strain, layer stacking, and interlayer spacing. We find that metallic character can be introduced by increasing the number of layers beyond two or by subjecting semiconducting monolayers and bilayers to moderate tensile strain. The strain-induced metal-semiconductor transition is triggered by changes in the band ordering near the top of the valence band that causes an abrupt change from $\ensuremath{\sigma}$ to $\ensuremath{\pi}$ character of the frontier states.

In situ engineering and characterization on the artificial heterostructures of correlated materials with integrated OMBE–ARPES

The combination of oxide molecular beam epitaxy (OMBE) and angle resolved photoemission spectroscopy (ARPES) opens a new field for studying correlated electron systems. The in situ growth of single crystalline films by OMBE increases the variety of materials available for ARPES study. Investigating the electronic structure of thin films by ARPES deepens our understanding on the interaction of different degrees of freedom at thin films and interfaces. In this paper, we review the key elements of the integrated OMBE–ARPES technique and some representative works, including the in situ ARPES investigations on thin films with cubic perovskite structure, ultra-thin films with thickness dependent metal–insulator transition, the composition and strain dependence of interfacial superconductivity, and correlated behavior in superlattices. These examples illustrate the new opportunities brought by integrating OMBE and ARPES on exploring novel physical states and functional materials.

Electronic structure and stability of fluorophore-nitroxide radicals from ultrahigh vacuum to air exposure.

Thin film processes of organic radicals remain widely unknown, although these materials may have a significant technological potential. In aiming at their use in applications, we explore the electronic structure of thin films of a nitronyl nitroxide radical attached to a fluorophore core. According to our findings, this molecule maintains its radical function and, consequently, its sensing capabilities in the thin films. The films are characterized by a high structural degree of the molecular arrangement, coupled to strong vacuum and air stability that make this fluorophore-nitroxide radical an extremely promising candidate for application in electronics. Our work also identifies a quantitative correlation between the results obtained by the simultaneous use of X-ray photoemission and electron paramagnetic resonance spectroscopy. This result can be used as a standard diagnostic tool in order to link the (in situ-measured) electronic structure with classical ex situ paramagnetic investigations.

Theoretical study of the thermodynamic stability and electronic structure of thin films of 3C, 2H, and 2D silicon carbides

Silicon carbide is among the most common materials used in semiconductor engineering. Silicon carbide thin films are attractive from the standpoint of designing devices based on heterojunctions. This is due to a characteristic feature of this compound, such as polytypism, leading to the difference in the physical properties and also hampering the preparation of high-quality material samples. In this work, the thermodynamic stability and electronic structure of thin films based on the polytypes 3C, 2H, and 2D with a thickness of a few nanometers have been studied.

Determination of the individual atomic site contribution to the electronic structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA)

We have studied the element and orbital-specific electronic structure of thin films of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) using a combination of synchrotron radiation-exited resonant x-ray emission spectroscopy, x-ray absorption spectroscopy, x-ray photoelectron spectroscopy, as well as density functional theory calculations. Resonant and non-resonant x-ray emission spectroscopies were used to measure the C and O 2p partial densities of state in PTCDA. Furthermore, resonant x-ray emission at the C and O K-edges is shown to be able to measure the partial densities of states associated with individual atomic sites. The flat molecular orientation of PTCDA on various substrates is explained in terms of the carbonyl O atom acting as a hydrogen-bond acceptor leading to multiple in-plane intermolecular C=O···H-C hydrogen bonding between carbonyl groups and the perylene core of the neighboring PTCDA molecules. We support this conclusion by comparison of our calculations to measurements of the electronic structure using element-, site-, and orbital-selective C and O K-edge resonant x-ray emission spectroscopy, and photoemission spectroscopy.

Investigation of thickness dependence on electronic structures of iron and nickel thin films by L-edge X-ray absorption spectroscopy

We have studied the effect of the film thickness on the electronic structure of pure nickel and iron thin films. Series of the thin films were evaporated by e-beam evaporation on SiN substrates. The electronic structure of the thin films was investigated using X-ray absorption near edge structure (XANES) spectroscopy. We have showed the thickness dependent variation of the experimental branching ratio (BR) and full-width at half-maximum (FWHM) at the L3 and L2 edges for both thin films. A strong thickness dependence of the L2,3 BR and FWHM was found. We have also focused on the deviation of L3 to L2 ratio from its statistical value. The average L3/L2 white-line intensity ratio was calculated to be 3.4 and 3.0 from peak height and integrated area under each L3 and L2 peaks, respectively for iron. Additionally, a theoretical L2,3 edge calculation for nickel was presented. The obtained results were consistent with the general view of the L2,3 BR and FWHM of iron and nickel transition metals.

Pulsed laser deposition with simultaneous in situ real-time monitoring of optical spectroscopic ellipsometry and reflection high-energy electron diffraction

We present a pulsed laser deposition system that can monitor growth by simultaneously using in situ optical spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The RHEED precisely monitors the number of thin-film layers and surface structure during the deposition, and the SE measures the optical spectra of the samples simultaneously. The thin-film thickness information obtained from RHEED facilitates the SE modeling process, which allows extracting the in situ optical spectra, i.e., the dielectric functions of thin-films during growth. The in situ dielectric functions contain indispensable information about the electronic structure of thin-films. We demonstrate the performance of this system by growing LaMnO3+δ (LMO) thin-films on SrTiO3 (001) substrates. By using in situ SE and RHEED simultaneously, we show that real-time thickness and dielectric functions of the LMO thin-films can be effectively extracted. The simultaneous monitoring of both optical SE and RHEED offers important clues to understand the growth mechanism of atomic-scale thin-films.

Electronic structures of silicon monoxide film probed by X-ray absorption spectroscopy

Electronic structures of thin films of silicon monoxides (SiO) deposited on a solid surface have been in-situ investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS). As a substrate, a highly oriented pyrolytic graphite (HOPG) was used because the hybridization between molecules and substrate can be excluded due to the inertness of the surface. For thin films with less than monolayer, XPS spectrum showed that the binding energy of the Si 1s is located just between those of the elemental silicon (Si0) and SiO2 (Si4+). The result indicates that the stable divalent silicon surely exists in the deposited SiO layer. For the Si K-edge XAFS spectrum of the SiO thin film, the energy of the core-to-valence resonance peak is also located between those of the elemental silicon (Si0) and SiO2 (Si4+). The polarization dependence of the Si K-edge XAFS spectra for the SiO film revealed that the SiO molecules are well-ordered and almost perpendicularly oriented on HOPG surface. The XAFS results also support the existence of the silicon divalent states in the deposited SiO films. The obtained well-ordered SiO films with divalent silicon will become an excellent starting material for the synthesis of low-dimensional SiOx films.

Phenanthro[1,2-b : 8,7-b’]dithiophene: a new picene-type molecule for transistor applications

A new picene-type molecule, phenanthro[1,2-b : 8,7-b’]dithiophene, has been synthesized for use in organic field-effect transistors (OFETs). The molecule consists of a phenanthrene core with two thiophene rings fused on the ends. This molecule can be recognized as a picene analogue. The electronic structure of the molecule was determined by its optical absorption spectrum together with a theoretical calculation based on density functional theory (DFT). The topological and electronic structures of thin films produced by direct thermal evaporation of the compounds and by deposition from a solution were characterized by optical imaging, X-ray diffraction, and atomic force microscopy. FET devices were fabricated with these thin films, and showed field-effect mobility as high as 10−1 cm2 V−1 s−1.

Study on Lattice and Electronic Structures at the Surface of BaTiO<sub>3</sub> Thin Films by DFT Method

In this paper, the density functional theory (DFT) study on nano-thin films of the most typical ferroelectric compound BaTiO$_3$ was presented. The results showed that the convergence of the film model using for DFT calculation were obtained for the thickness of the vacuum slabs as large as ten times of the BaTiO$_3$ slabs within the total energy error $\bsim$ 0.01 eV. The lattice contraction was observed in the surface region of the material. The difference in atomic layer termination, between BaO terminated and TiO$_2$ terminated films, gives rise to the difference in lattice reconstruction at the surface region which, in turn, leads to the difference in surface electric dipoles. The surface effect acts on the electronic structure of thin films in the manner of the asymmetry of atomic sites and the structural reconstruction due to the surface relaxation. Our results indicated that the termination also decides how and how much these two manners take effects on the electronic structure of the material.