Core-level Electronic Structure Research Articles

Ion implantation as an approach for tuning of electronic structure, optical, morphological and electrical transport properties of sputtered molybdenum disulfide thin films

Molybdenum disulfide (MoS2) thin films have acquired increasing consideration in the field of low-dimensional electronics as device-active layers and also in hydrogen evolution reaction (HER) as catalysts in distinct electrochemical processes. Thus, for distinct applications, extensive scalable fabrication techniques with tunable MoS2 properties are significant. To possess tunable efficiency of various device applications at nanoscale it is significant to acquire control over the density of defects, although attaining with direct growth techniques remains a significant issue. Here we illustrate a robust and effective approach to tailor the density of defects using Au ion implantation. In the present work, we illustrate the scalable technique i.e. magnetron sputtering for the fabrication of MoS2 thin films, subsequently, ion implantation with 80 keV Au ions at a distinct fluence of 1 × 1015 and 5 × 1015 ions/cm2 was carried out. Striking deviations were remarked in nanostructure and depth distributions of the resultant thin films of MoS2. Pristine and ion implanted thin films were characterized to comprehend the morphological, electrical transport and electronic structure properties. The surface micrographs’ results revealed noteworthy alterations in the thin films’ surface roughness. The modifications induced by low energy Au ion implantation in core-level electronic structure and atomic bonding was probed by XPS and Raman spectroscopy technique. The electrical properties of these films were studied to demonstrate the scattering and conduction mechanisms. The present work contributes to the significant passage of tailoring 2D MoS2 thin films’ electronic structure, optical and morphological properties through doping and defects formation by low energy ion beam implantation to extend their practical device applications.

Probing the molecular structure of aqueous triiodide via X-ray photoelectron spectroscopy and correlated electron phenomena

Liquid-microjet-based X-ray photoelectron spectroscopy was applied to aqueous I3− anions to investigate their valence- and core-level electronic structure, ionization dynamics, associated electron-correlation effects, and nuclear geometric structure.

Spin-Orbit Coupling Effects in Au 4f Core-Level Electronic Structures in Supported Low-Dimensional Gold Nanoparticles.

Despite their many advantages, issues remain unresolved over the variability in catalytic activities in supported gold nanoparticle (AuNP)-based catalysts, which requires precise characterization to unravel the presence of any fine features. Herein, upon analyzing the Au 4f core-level spin-orbit components in many as-synthesized AuNP-based catalysts, we observed that like deviations in the Au 4f7/2 binding energy positions, both the Au 4f7/2-to-Au 4f5/2 peak intensity and linewidth ratios varied largely from the standard statistical bulk reference values. These deviations were observed in all the as-synthesized supported AuNPs irrespective of different synthesis conditions, variations in size, shape or morphology of the gold nanoparticles, and different support materials. On the other hand, the spin-orbit-splitting values remained almost unchanged and did not show any appreciable deviations from the atomic or bulk standard gold values. These deviations could originate due to alterations in the electronic band structures in the supported AuNPs and might be present in other NP-based catalyst systems as well, which could be the subject of future research interest.

The Diagram of p–n Junction Formed on the n-GaAs Surface by 1.5 keV Ar+ Ion Beam

The core-level and valence band electronic structure of the n-GaAs (100) has been studied by synchrotron-based high-resolution photoelectron spectroscopy after irradiation by an Ar+ ion beam with energy Ei = 1500 eV and fluence Q = 1 × 1015 ions/cm2. Conversion of the conductivity type of the surface layer and formation of a p–n structure have been observed. The p-surface layer thickness (d ~ 5.0 nm) and band structure were experimentally determined from the Ga3d photoelectron spectrum by separation and analysis of the low intense n-type bulk contribution from deeper layers. A band diagram of the p–n junction formed on the n-GaAs surface by Ar+ ion bombardment was reconstructed. The p–n junction proved to be unexpectedly narrow compared to the extended tail of the implanted ion depth distribution.

Role of random magnetic anisotropy on the valence, magnetocaloric and resistivity properties in a hexagonal Sm2Ni0.87Si2.87 compound

In this work, we report the effect of random magnetic anisotropy (RMA) on the valence, magnetocaloric and resistivity properties in a glassy intermetallic material Sm2Ni0.87Si2.87. On the basis of detailed studies on the valence band and core level electronic structure, we have established that both the Sm3+ and Sm2+ ions are present in the system, suggesting the compound to be of mixed valence in nature. The significant observation of positive magnetic entropy change in zero-field cooled measurement has been argued due to the presence of RMA that develops due to local electronic environmental variations between the rare-earth ions in the system. The quantum interference effect caused by the elastic electron–electron interaction is responsible for the resistivity upturn at low-temperature for this disordered metallic conductor.

Experimental observation of structural phase transition in CsBr clusters

Formation and growth of CsBr clusters embedded in unsupported Ar clusters was studied using synchrotron radiation photoelectron spectroscopy. The development of the core-level electronic structure for cluster sizes between a few and a few hundred atoms contained information about the local coordination of the constituent particles. The experimental results indicate that a gradual structural phase transition from NaCl structure to CsCl structure for CsBr clusters takes place at around 160 atoms per cluster.

The appearance of Ti3+ states in solution-processed TiOx buffer layers in inverted organic photovoltaics

We study the low-temperature solution processed TiOx films and device structures using core level and valence X-ray photoelectron spectroscopy (XPS) and electronic structure calculations. We are able to correlate the fraction of Ti3+ present as obtained from Ti 2p core level XPS with the intensity of the defect states that appear within the band gap as observed with our valence XPS. Constructing an operating inverted organic photovoltaic (OPV) using the TiOx film as an electron selective contact may increase the fraction of Ti3+ present. We provide evidence that the number of charge carriers in TiOx can be significantly varied and this might influence the performance of inverted OPVs.

Li induced effects in the core level and π-band electronic structure of graphene grown on C-face SiC

Studies of the effects induced in the electronic structure after Li deposition, and subsequent heating, on graphene samples prepared on C-face SiC are reported. The as prepared graphene samples are essentially undoped, but after Li deposition, the Dirac point shifts down to 1.2 eV below the Fermi level due to electron doping. The shape of the C 1s level also indicates a doping concentration of around 1014 cm−2 after Li deposition, when compared with recent calculated results of core level spectra of graphene. The C 1s, Si 2p, and Li 1s core level results show little intercalation directly after deposition but that most of the Li has intercalated after heating at 280 °C. Heating at higher temperatures leads to desorption of Li from the sample, and at 1030 °C, Li can no longer be detected on the sample. The single π-band observable from multilayer C-face graphene samples in conventional angle resolved photoelectron spectroscopy is reasonably sharp both on the initially prepared sample and after Li deposition. After heating at 280 °C, the π-band appears more diffuse and possibly split. The Dirac point becomes located at 0.4 eV below the Fermi level, which indicates occurrence of a significant reduction in the electron doping concentration. Constant energy photoelectron distribution patterns extracted from the as prepared graphene C-face sample and also after Li deposition and heating at 280 °C look very similar to earlier calculated distribution patterns for monolayer graphene.

Laboratory hard X-ray photoelectron spectroscopy of La1−xSrxMnO3

A laboratory hard X-ray photoelectron spectroscopy (HXPS) system equipped with a monochromatic Cr Kα (hν = 5414.7 eV) X-ray source was applied to an investigation of the core-level electronic structure of La1−xSrxMnO3. No appreciable high binding-energy shoulder in the O 1s HXPS spectra were observed while an enhanced low binding-energy shoulder structure in the Mn 2p3/2 HXPS spectra were observed, both of which are manifestation of high bulk sensitivity. Such high bulk sensitivity enabled us to track the Mn 2p3/2 shoulder structure in the full range of x, giving us a new insight into the binding-energy shift of the Mn 2p3/2 core level. Comparisons with the results using the conventional laboratory XPS (hν = 1486.6 eV) as well as those using a synchrotron radiation source (hν = 7939.9 eV) demonstrate that HXPS is a powerful and convenient tool to analyze the bulk electronic structure of a host of different compounds.

Correlation effects driven by reduced dimensionality in magnetic surface alloys

The evolution of electronic properties and correlation effects in manganese-based two- dimensional magnetic surface alloys are discussed. Enhanced correlations resulting from the reduced dimensionality of the surface alloys lead to the modification of the core level and valence band electronic structures resulting in the appearance of distinct satellite features. Apart from this, surface alloying-induced strong modifications in the substrate surface states arising from charge reorganization and electron transfer to the surface states as well as band-gap openings are also discussed.

Hard X-ray photoelectron spectroscopy of LixNi1-xO epitaxial thin films with a high lithium content.

The core-level and valence-band electronic structures of LixNi1-xO epitaxial thin films with x = 0, 0.27, and 0.48 were studied by hard X-ray photoelectron spectroscopy. A double peak structure, consisting of a main peak and a shoulder peak, and a satellite structure were observed in the Ni 2p3/2 core-level spectra. The intensity ratio of the shoulder to main peak in this double peak structure increased with increasing lithium content in LixNi1-xO. This lithium doping dependence of the Ni 2p3/2 core-level spectra was investigated using an extended cluster model, which included the Zhang-Rice (ZR) doublet bound states arising from a competition between O 2p - Ni 3d hybridization and the Ni on-site Coulomb interaction. The results indicated that the change in the intensity ratio in the main peak is because of a reduction in the ZR doublet bound states from lithium substitutions. This strongly suggests that holes compensating Li doping in LixNi1-xO are of primarily ZR character.

Core level photoemission studies on conducting polypyrrole polymer nanotubes showing switching transitions

Conducting polypyrrole (PPY) nanotubes is a classical model system for strongly correlated disordered materials showing intriguing switching transitions from low to high conductivity states at low temperature. This switching behaviour can be tuned by incorporating gold nanoclusters to form composite nanotubes (AuPPY). Here, we present core level electronic structure studies on PPY and AuPPY nanotubes with different diameters using hard X-ray photoemission spectroscopy at room temperature. The spectroscopic data provide information on the role of diameter as well as metal cluster incorporation for a modification of the electronic structure of this important class of nanotubes. Furthermore, electrical transport measurements were performed at low temperature to extract the change in localization length and doping level in these nanotubes. Based on these results, we explain the observed electrical transport behavior of these nanotubes as the interplay of disorder and carrier concentration.

Hidden relationship between the electrical conductivity and the Mn 2p core-level photoemission spectra in La1−xSrxMnO3

Core-level electronic structure of La1−xSrxMnO3 has been studied by x-ray photoemission spectroscopy (XPS). We first report, by the conventional XPS, the well-screened shoulder structure in Mn 2p3∕2 peak, which had been observed only by hard x-ray photoemission spectroscopy so far. Multiple-peak analysis revealed that the Mn4+ spectral weight was not proportional to the nominal hole concentration x, indicating that a simple Mn3+/Mn4+ intensity ratio analysis may result in a wrong quantitative elemental analysis. Considerable weight of the shoulder at x = 0.0 and the fact that the shoulder weight was even slightly going down from x = 0.2 to 0.4 were not compatible with the idea that this weight simply represents the metallic behavior. Further analysis found that the whole Mn 2p3∕2 peak can be decomposed into four portions, the Mn4+, the (nominal) Mn3+, the shoulder, and the other spectral weight located almost at the Mn3+ location. We concluded that this weight represents the well-screened final state at Mn4+ sites, whereas the shoulder is known as that of the Mn3+ states. We found that the sum of these two spectral weight has an empirical relationship to the conductivity evolution with x.

An experimental investigation on the poor hydrogen sorption properties of nano-structured LaNi 5 prepared by ball-milling

Nano-structured LaNi 5 hydrogen storage materials prepared by ball-milling is analysed using differential scanning calorimetry (DSC) and x-ray photoelectron spectroscopy (XPS). DSC results indicate a partial elimination of defects at 500 °C in a more efficient way for the short-time ball-milled powders compared to the long-time ball-milled ones. XPS results show almost no change in the core-level electronic structure for La and Ni of LaNi 5 in the bulk and the nano-structured forms, but gives an indication that the self-restoring mechanism of the active surface observed in the bulk sample (Siegmann et al. Phys. Rev. Lett. 40, 972) may not be occurring in the nano-powders. Results from the X-ray diffraction and the local structural studies together with the above observations suggest that the reduced unit-cell volume and the enhanced atomic disorder in the nano-structured LaNi 5 cause a larger energy barrier for the hydrogen sorption reactions of the long-time ball-milled samples.

Soft X-ray Photoelectron Spectroscopy of Heusler-type Thermoelectric Alloys Fe2-x-yIryV1+xAl

The valence-band and core-level electronic structures of Heusler-type alloys Fe2-x-yIryV1+xAl have been investigated by soft X-ray photoelectron spectroscopy, which shows that the valence-band structure near the Fermi level EF is drastically altered by the off-stoichiometric concentration change x, while the position of EF is moderately tuned by the Ir substitution y in a rigid-band-like manner. These changes in the electronic structure lead to the remarkable enhancement of their thermoelectric power. For x > 0, the V 2p core level spectra also show the appearance of new chemically shifted components in the low binding energy side of the main line, which may be attributed mainly to the excess V occupying the Fe site (the anti-site V) as well as V in the surface layer. The present results suggest that the drastic and moderate changes in the electronic structure may be caused by the emergence of such anti-site defects in the off-stoichiometric Fe and V concentrations and by the common band formation between the Fe 3d bands and the substituted Ir 5d states, respectively.

Core-level X-ray photoemission spectral shift through the successive phase transitions in layered TlInS 2

Temperature-dependent change in core-level electronic structures of the layered semiconductor-ferroelectric TlInS 2 with incommensurate phase has been investigated by means of X-ray photoemission spectroscopy. The temperature dependence of the relative peak position for each core level (Tl 4f, In 3d and S 2p) is found to differ very much in the regions bordering each other at the normal-incommensurate phase transition point of 218 K. The obtained data suggest that the charge distribution in TlInS 2 dramatically changes upon passing from the normal phase ( T > 218 K) to the spatially modulated incommensurate phase ( T < 218 K).

Core-level electronic structure of solid-phase glycine, glycyl-glycine, diglycyl-glycine, and polyglycine: X-ray photoemission analysis and Hartree–Fock calculations of their zwitterions

X-ray photoelectron spectroscopy (XPS) has been used to investigate the core-level electronic structures of glycine (G) and its peptides, including glycyl-glycine (GG), diglycyl-glycine (GGG), and polyglycine (poly-G), in their powder forms. Increasing the number of G units in the peptides does not change the locations of the respective C 1s, N 1s, and O 1s features corresponding to different functional groups: -COO(-), -NH(3)(+), >CH(2), and -CONH-. The electronic structures of the zwitterions of these molecules have been calculated as isolated molecules and as molecules in an aqueous environment under the periodic boundary conditions by quantum-mechanical and molecular mechanics methods. In the case of glycine zwitterion, the binding energies of the C 1s, N 1s, and O 1s XPS features are found to be in reasonable accord with the respective orbital energies obtained by Hartree-Fock self-consistent-field calculations, within the context of Koopmans' approximation. However, considerably worse agreement in the binding energies is found for the larger zwitterions (with the specific conformations considered in this work), indicating the need for higher-level calculations. The present work shows that optimizing the zwitterion in an aqueous environment under the periodic boundary conditions by molecular mechanics could be a very cost-effective approach for calculating the electronic structures of large, complex biomolecular systems.

Revisiting the Valence-Band and Core-Level Photoemission Spectra of NiO

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.

Core-level electronic structure evolution of heavily boron-doped superconducting diamond studied with hard x-ray photoemission spectroscopy

T. Yokoya,1,2 E. Ikenaga,2 M. Kobata,2 H. Okazaki,1 K. Kobayashi,2 A. Takeuchi,2 A. Awaji,2 Y. Takano,3 M. Nagao,3 I. Sakaguchi,3 T. Takenouchi,4 K. Kobayashi,4 H. Kawarada,4 and T. Oguchi5 1The Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan 2Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan 3National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 4School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan 5Department of Quantum Matter, Graduate School of Advanced Sciences of Matter (ADSM), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Received 7 November 2006; revised manuscript received 12 April 2007; published 21 May 2007

STM and LEED studies of atomically ordered terraced Si(557) surfaces

We report results of LEED, STM, and photoemission studies of the atomic and electronic structure of atomically ordered terraced Si(hhm) surfaces. LEED and STM data demonstrate the possibility of fabricating atomically accurate terraced structures based on Si(557) with different periodicities depending on the thermal treatment procedure. Atomically resolved STM images reveal (7 × 7) terrace ordering and triple step structure. Comparative photoemission studies of the valence band and Si 2p core level electronic structure have been done on clean stepped surfaces with different terrace widths (namely, Si(557), Si(556)) and flat Si(111)-(7 × 7).