Empty Electronic States Research Articles

Morphology and Electronic Properties of Hybrid Organic-Inorganic System: Ag Nanoparticles Embedded into CuPc Matrix

Materials with a high on-off resistance ratio could become the basis for resistive random-access memory (RRAM). It is assumed that one of RRAM types can be based on hybrid organic-inorganic systems, while particular attention is focused on hybrid systems consisting of metal nanoparticles (NP) embedded in organic matrix (OM). In this investigation we created and studied the hybrid organic-inorganic systems made of metal (Ag) nanoparticles embedded in organic semiconductor material CuPc. The LEED patterns and NEXAFS data demonstrate that the CuPc films deposited on Au(001) substrate are highly ordered and molecular planes lie parallel to the gold surface. The metal atoms were deposited on the outer surface of the organic molecular film and self-assembled into nanoparticles due to surface and bulk diffusion. The properties of nano-composite materials seem to be significantly dependent on the microstructure, i.e. the size, concentration, bulk- and size-distribution of nanoparticles; therefore we have studied by high resolution transmission electron microscopy the evolution of morphology of nano-composite films as a function of nominal metal deposition. The filled and empty electronic states of the hybrid organic-inorganic systems, energy level alignment at interfaces formed between metal nanoparticles and the organic semiconductor CuPc as well as the chemical interaction at the NP/OM interface were studied by UPS, XPS and NEXAFS methods.

A rate equation approach towards surface ionisation gas detection

Surface ionisation (SI) is a form of gas response that can be observed on noble and refractory metals as well as on metal oxide surfaces. In SI gas detection, adsorbed analyte molecules are ionised by a transfer of valence electrons to empty electron states inside the adsorbent solid. The so-formed ionic species are extracted from the surface by an oppositely biased counter electrode, positioned at a small distance from the emitting surface. In the present paper, we present a rate equation approach, which describes how analyte gas molecules proceed through the sequence of partial processes of neutral-state adsorption, molecular decomposition and surface ionisation to form analyte ions. Under steady-state conditions, this rate equation approach describes the temperature and the gas concentration dependences of the SI response as well as the current transport across the air gap. The predictions of this theory are compared with experimental data which had been obtained on platinum (Pt) and tin dioxide (SnO 2) flat-plate emitters.

Ultraviolet spectroscopy study of empty electron states of Cu nanoclusters

Empty image-potential electron states for Cu nanoclusters on SiO2 have been observed using ultraviolet (UV) light in the energy region of 3.1 eV–6.5 eV. Cu nanoclusters have been formed on the silicon oxide surface, and their size was about 500 nm. In addition to the photoelectron emission from the occupied Shockley surface state, other features have been found in the UV electron spectra. These features are attributed to the direct transition into the image potential states n = 1and 2 from the Shockley surface state and then to the escape from these states into vacuum.

Control of Electron Injection Barrier by Electron Doping of Metal Phthalocyanines

Metal phthalocyanine (MPc, with M = Fe, Co, Ni, Cu, Zn) thin-films grown on Au(110) have been electron-doped by exposition to alkali metal (potassium). Progressive occupation of the former lowest unoccupied molecular orbital (LUMO) up to the filling of the 2-fold degenerate LUMO with 2eg symmetry has been observed in photoemission spectra for the NiPc, CuPc, and ZnPc films. The evolution of the electron filling induces a different process for FePc and CoPc, whose empty electronic states localized on the central metal atoms are involved in the electron donation process by the alkali metal. Electron filling is accompanied by a sudden decrease of the work function mainly due to a variation of the surface dipole. The molecular films remain semiconducting, with the hole injection barrier strongly reduced at saturation of the electron filling. The selective influence of the electron donation process on the molecular states (ligands, metal-related) is discussed.

Effect of local doping on the electronic properties of epitaxial graphene on SiC

AbstractGraphene grown on silicon carbide (SiC) is a promising material for high speed electronic devices. However, for possible future applications it is important to understand the electron properties of this material and how it is affected by the interaction with the SiC interface. Here we report an atomically resolved scanning tunneling microscopy and spectroscopy study of local structural and electronic properties of epitaxial graphene. Sharp localized states from the graphene/SiC(0001) interface have been found to strongly influence the electronic properties of the first graphene layer, causing local doping of graphene layer. The disordered high electron density states have originated from the underlying carbon‐rich interface layer whose structure is discussed.magnified image Scanning tunneling microscopy images of a first graphene layer on SiC(0001) taken at bias voltage −500 and 500 mV (5 × 5 nm2). Red circles and blue crosses indicate that localized states in the filled and empty electron states belonging to the underlying interface layer are located at different positions.

Appearance Potential Spectroscopy (APS): Old Method, but Applicable to Study of Nano-structures

An old surface analytical method, appearance potential spectroscopy (APS), which can probe empty electronic states, is briefly reviewed, since it has an ability to be applied to studying nanostructures. Although the theoretical calculation of the APS spectra including a two-electron process is not much simpler than that of x-ray absorption spectroscopy (XAS) spectra with a one-electron process, measurements of the APS spectra are simpler than those of the XAS ones. A careful interpretation of the spectra would lead to a characteristic and simple surface chemical analysis.

Synthesis, growth mechanism and optical properties of (K,Na)NbO3 nanostructures

(K1−xNax)NbO3 (KNN) nanomaterials with various morphologies, including nanorods, step-like structures etc. were synthesized through a hydrothermal route. For KNN nanomaterials, the phase structures changed from orthorhombic to monoclinic as the x value increased from 0.27 to 0.66. Oriented attachment and aggregation are the major formation mechanism of the nanorods and the step-like structures, respectively. The compositional change in KNN induced a series of shifts of the Raman bands, of which the mechanisms are briefly discussed. A UV/vis absorption study shows that, for K0.50Na0.50NbO3 nanorods, two interband transitions from O2p electron states to empty Nb4d electron states and two intraband transitions in relation to Nb5+ cations are revealed, and the band gap is about 3.09 eV.

Optical properties of β-Sn films

Optical properties of white tin (β-Sn) have been investigated using spectroscopic ellipsometry in the photon-energy range between 0.6 and 6.5 eV at room temperature. The β-Sn films are deposited by vacuum evaporation on Si(001) substrates. The structural properties of the films are evaluated by x-ray diffraction and ex situ atomic force microscopy. The measured ε(E) spectra reveal distinct structures at several interband critical points in the Brillouin zone of β-Sn. These spectra are analyzed on the basis of a simplified model of the interband transitions, including the free-carrier absorption between the filled and empty electronic states. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of bulk β-Sn films are also presented.

Core and valence exciton formation in x-ray absorption, x-ray emission andx-ray excited optical luminescence from passivated Si nanocrystals at the SiL2,3 edge

Resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopyand x-ray excited optical luminescence (XEOL) have been used tomeasure element specific filled and empty electronic states over the SiL2,3 edge of passivated Si nanocrystals of narrow size distribution (diameter2.2 ± 0.4 nm). These techniques have been employed to directly measure absorption and luminescencespecific to the local Si nanocrystal core. Profound changes occur in the absorptionspectrum of the nanocrystals compared with bulk Si, and new features are observed in thenanocrystal RIXS. Clear signatures of core and valence band exciton formation, promotedby the spatial confinement of electrons and holes within the nanocrystals, areobserved, together with band narrowing due to quantum confinement. XEOL at 12 Kshows an extremely sharp feature at the threshold of orange luminescence (i.e., at∼1.56 eV (792 nm)) which we attribute to recombination of valence excitons, providing a lowerlimit to the nanocrystal band gap.

Band offsets between Si and epitaxial rare earth sesquioxides (RE2O3, RE=La,Nd,Gd,Lu): Effect of 4f-shell occupancy

Internal photoemission of electrons and holes into cubic Nd2O3 epitaxially grown on (100)Si reveals a significant contribution of Nd 4f states to the spectrum of the oxide gap states. In contrast to oxides of other rare earth (RE) elements (Gd, Lu) epitaxially grown in the same cubic polymorph, to hexagonal LaLuO3, and to polycrystalline HfO2, the occupied Nd 4f states produce an additional filled band 0.8eV above the O 2p derived valence band. The unoccupied portion of the Nd 4f shell leads to empty electron states in the energy range of 1eV below the RE 5d derived oxide conduction band. The exposed Nd 4f states suggest the possibility to use this metal and, possibly, other REs with low f-shell occupancy to control the interface band offsets by selective interface doping.

Dynamics of photoinduced electron transfer from adsorbed molecules into solids

Ultrafast interfacial electron transfer from the donor orbital of organic chromophores into empty electronic acceptor states of a semiconductor and of a metal was investigated by two-photon photoemission spectroscopy (2PPE). Experimental tools and procedures have been developed for carrying out wet-chemistry preparation of the molecule/solid interface. The organic chromophore perylene was investigated with several different bridge/anchor groups on TiO2(110). One perylene compound was investigated for comparison on Ag(110). Angle and polarization dependent 2PPE measurements revealed the orientation of the perylene chromophore on the surface as controlled by the adsorption geometry of the respective anchor group on TiO2. UPS measurements gave the position of the HOMO level of the chromophore with respect to the Fermi level of the solid. The donor level of each molecule was found high enough to fulfill the “wide band limit” of heterogeneous electron transfer dynamics. Time constants for heterogeneous electron transfer were extracted from 2PPE transients. A difference by a factor of four was found, 13 fs against 47 fs, when a conjugated bond was exchanged for a saturated bond in the otherwise identical bridge group. The two different contributions to the 2PPE transients arising firstly from the excited state of the chromophore and secondly from the injected electrons were separated by measuring the latter contribution separately in the case of instantaneous interfacial electron transfer realized with catechol as adsorbate. The time scales measured for the electron transfer step and for the subsequent electron escape process from the surface into the bulk of TiO2 showed both good agreement with recent theoretical predictions of other groups for these systems.

X-ray Absorption Spectroscopy of Strained-Si Nanomembranes

We use x-ray absorption (XAS) total electron yield measurements to investigate strain-induced splitting of the conduction band minimum in very thin Si layers and membranes. We use synchro-tron generated X rays with variable energy to excite electrons from the 2p core level of strained Si to empty electronic states in the conduction band minima. The Auger electrons emitted in the re-laxation process make the method extremely surface sensitive and reflect the position and density of states of the conduction band. We determine these parameters for elastically strained Si nanomembranes and strained-Si-on-insulator. We demonstrate sig-nificant lateral nonuniformity in the strain in strained-Si-on-insulator, while the strain is uniform on strain sharing nanomem-branes. LEEM measurements support this conclusion.

Role of a pore network for band energy configuration in mesostructured materials

This work is focused on the charge transfer process of mesoporous amorphous titania to build a band energy diagram by spectro- and photoelectrochemical characterization. The surface topology of mesoporous titania is completely different from a nanocrystalline film, as transmission electron microscopy confirmed. Mesoporous titania consists of an amorphous framework of titania walls where cylindrical pores are ordered in a hexagonal arrangement. Two features have been attributed to the surface topology of mesoporous titania during electrochemical characterization: (i) dominance of capacitive surface-confined electrochemical processes due to the huge surface area of amorphous titania walls showing a metallic behavior; (ii) a band energy denominated “mesoscopic” band which intermediated charge transfer from the substrate into the surface states and defect sites (Ti4+∕Ti3+) resulting in a cathodic current when mesoporous titania acted as photovoltaic solar cells. The spectroelectrochemical characterization confirmed that mesostructured titania has a different band energy diagram determined by analysis of the filling of empty electronic states during a lithium intercalation process. A surface model for mesostructured materials is introduced in this work where quantum sized particles are surrounded by hollow titania particles, modifying their optical and electrical properties. These hollow particles contain surface states and defect sites (Ti4+∕Ti3+) ordered in a hexagonal arrangement due to a porous network of mesoporous titania and, consequently, a mesoscopic band appears. This conception of band energy can give a different insight to build functional devices like solar cells, electrochromical windows and batteries where mesostructured materials can act as a cathode transporting holes through their pore network.

Role of Molecular Anchor Groups in Molecule-to-Semiconductor Electron Transfer

The dynamics of heterogeneous electron transfer (ET) from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum. Data from ultraviolet photoelectron spectroscopy and from linear absorption spectroscopy showed that the donor state of the chromophore was located around 900 meV above the lower edge of the conduction band. With the wide band limit fulfilled the rate of the heterogeneous ET reaction was only controlled by the strength of the electronic coupling and not reduced by Franck-Condon factors. Two different time constants for the electron transfer, i.e., 13 and 28 fs, were measured with carboxylic acid and phosphonic acid as the respective anchor groups. The difference in the ET time constants was explained with the different extension of the donor orbital onto the respective anchor group to reach the empty electronic states of the semiconductor. The time constants were extracted by means of a simple rate equation model. The validity of applying this model on this ultrafast time scale was verified by comparing the rate equation model with an optical Bloch equation model.

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.

Empty electronic states in low-energy absorbed current spectroscopy of NbSe 2(0 0 0 1) surface

The fine structure of the experimental absorbed current spectra along the normal to a clean NbSe 2(0 0 0 1) surface is interpreted theoretically. It is shown that the fine structure of absorbed current spectra is mainly due to the electronic structure of unoccupied high-level electronic states (above the vacuum level), which become occupied by electrons entering a solid. The predominant role of the bulk energy-band structure effects in the spectra formation is shown. In addition, a comparison to existing theoretical and experimental data is given.

Interaction of slow electrons with surface: Empty electronic states and inelastic scattering effects

The fine structure of experimental true secondary-electron emission spectra along the normal to a clean W(1 1 0) surface, target (or total) current spectra, and low-energy-electron-transmission spectra along the normal to a clean 2 H-MoS 2(0 0 0 1) surface are interpreted theoretically. It is shown that the fine structure of these spectra is mainly due to the electronic structure of unoccupied high-level electronic states (above the vacuum level), which become occupied by electrons entering the solid. In addition, a comparison to existing theoretical and experimental data is given.

Spin‐polarized XANES: theoretical analysis of the Ni K‐edge of NiF2

AbstractTheoretical interpretations of spin‐dependent X‐ray absorption near edge structure (XANES) spectra measured by selectively monitoring of the Ni K‐beta emission while scanning the excitation energy through the Ni K absorption edge have been performed. Analysis is based on a combination of self‐consistent spin‐polarized calculation of muffin‐tin potential and a full multiple scattering theory of X‐ray absorption. This approach allows us to separate the influence of dipole transition matrix elements and the density of empty electronic states on spin‐dependent XANES. It is found that the matrix elements affect splitting between spin‐up and spin‐down spectra only near the absorption threshold, while differences in densities of states slightly shift the spectra in the region 25–35 eV above the main edge. The effects of the multielectron‐term‐dependent broadening of spin‐dependent XANES and mixing of purely spin‐polarized spectra were taken into account. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Local Electronic Structure ofMgB2by X-Ray Raman Scattering at the BoronKEdge

We have investigated the electronic structure of ${\mathrm{MgB}}_{2}$ using x-ray Raman scattering at the boron $K$ edge in a single crystal sample. Using both the direction and the magnitude dependence of the momentum transfer we can identify the spatial orientation of empty electronic states with different symmetries. The local symmetry-resolved electronic structure of ${\mathrm{MgB}}_{2}$ is analyzed using two independent first-principles computational approaches. We show that near the Fermi level the density of $s$ symmetry states is vanishingly small. We also identify and separate the contributions from boron ${p}_{xy}(\ensuremath{\sigma})$ and ${p}_{z}(\ensuremath{\pi})$ states, which play an important role in determining the superconducting properties of ${\mathrm{MgB}}_{2}$.

Low-lying unoccupied electronic states in3dtransition-metal fluorides probed by NEXAFS at theF1sthreshold

The near-edge x-ray absorption fine structure (NEXAFS) at the $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, $\mathrm{Ti}{\mathrm{F}}_{4}$, $\mathrm{V}{\mathrm{F}}_{4}$, $\mathrm{V}{\mathrm{F}}_{3}$, $\mathrm{Cr}{\mathrm{F}}_{3}$, $\mathrm{Cr}{\mathrm{F}}_{2}$, $\mathrm{Mn}{\mathrm{F}}_{3}$, $\mathrm{Mn}{\mathrm{F}}_{2}$, $\mathrm{Fe}{\mathrm{F}}_{3}$, $\mathrm{Fe}{\mathrm{F}}_{2}$, $\mathrm{Co}{\mathrm{F}}_{2}$, $\mathrm{Ni}{\mathrm{F}}_{2}$, $\mathrm{Cu}{\mathrm{F}}_{2}$, and $\mathrm{Zn}{\mathrm{F}}_{2}$ as well as for $\mathrm{S}{\mathrm{F}}_{6}$ in the gas phase, and for the $\mathrm{P}{\mathrm{F}}_{6}^{\ensuremath{-}}$ and $\mathrm{Ti}{\mathrm{F}}_{6}^{2\ensuremath{-}}$ molecular anions of the solid compounds $\mathrm{K}\mathrm{P}{\mathrm{F}}_{6}$ and ${\mathrm{K}}_{2}\mathrm{Ti}{\mathrm{F}}_{6}$. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and $\mathrm{Cu}{\mathrm{F}}_{2}$ were taken under comparable experimental conditions at the D1011 beamline at MAX-lab. The spectra of the solid samples were recorded via the total electron yield. The NEXAFS spectra were taken with the aim to elucidate the role of covalent bonding and its manifestation in x-ray absorption spectra as well as to gain information on the electronic structure of the conduction band along the whole series of $3d$ transition-metal (TM) fluorides. The spectra of these most ionic compounds of the $3d$ TM's have been analyzed in a comparative way considering also the $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ NEXAFS spectrum of the molecular $\mathrm{Ti}{\mathrm{F}}_{6}^{2\ensuremath{-}}$ anion in solid ${\mathrm{K}}_{2}\mathrm{Ti}{\mathrm{F}}_{6}$. In its turn, the latter spectrum has been interpreted by comparing with the $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ NEXAFS spectrum of the molecular $\mathrm{P}{\mathrm{F}}_{6}^{\ensuremath{-}}$ anion in $\mathrm{K}\mathrm{P}{\mathrm{F}}_{6}$ and that of $\mathrm{S}{\mathrm{F}}_{6}$ in the gas phase. In this way, the low-lying empty electronic states of the $3d$ TM fluorides are shown to be formed by covalent mixing of the TM $3d$ with the fluorine $2p$ electronic states. It is further found that the number of low-lying empty electronic states with TM $3d$\ensuremath{-}fluorine $2p$ hybridized character decreases gradually along the series of $3d$ TM fluorides, and is essentially zero in the case of $\mathrm{Zn}{\mathrm{F}}_{2}$.