Nanomaterial electronic structure investigation by valence electron energy loss spectroscopy - an example of doped ZnO nanowires

The effects of doping (by ion implantation) on the electronic structure of ZnO nanowires, particularly on the defect states generation in the band gap of ZnO, are investigated using valence electron energy loss spectroscopy (VEELS) performed in a transmission electron microscope (TEM). The improved spectrum energy resolution via the introduction of a gun monochromator, together with the reduced intensity in the zero loss peak tail as realized by spectrum acquisition at non-zero momentum transfer, enable us to extract such electronic structure information from the very low loss region of the EEL spectra. We have compared the doping effects of several dopant elements, i.e., Er, Yb, and Co, and found that generation of the band tail states ( approximately 2-3.3eV) is a common consequence of the ion implantation process. On the other hand, specific mid-gap state(s) in the lower energy range are created only in the rare earth element doped ZnO nanowires, suggesting the dopant-sensitive nature of such state.     

Probing the photonic local density of states with electron energy loss spectroscopy

Electron energy loss spectroscopy performed in transmission electron microscopes is shown to directly render the photonic local density of states with unprecedented spatial resolution, currently below the nanometer. Two special cases are discussed in detail: (i) 2D photonic structures with the electrons moving along the translational axis of symmetry and (ii) quasiplanar plasmonic structures under normal incidence. Nanophotonics in general and plasmonics, in particular, should benefit from these results connecting the unmatched spatial resolution of electron energy loss spectroscopy with its ability to probe basic optical properties such as the photonic local density of states.     

Band transitions in wurtzite GaN and InN determined by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2 eV) and for wurtzite GaN at (3.3±0.2 eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal.     

An electron energy loss spectroscopy study of the conductivity of polar ZnO surfaces

The surface conductivity of the polar ZnO faces was changed by exposure to H and O₂, by Ar ion bombardment and by heat treatments. Simultaneous studies of the corresponding Electron Energy Loss Spectra in the range 3–20 eV surprisingly indicated that the lowest energy state, at 3 eV, was not involved in these conductivity changes. The only states that could be involved were at 5.3 and 14.3 eV.     

Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy

Electron energy-loss spectroscopy (EELS) and ab initio band structure calculations have been performed to determine the optical properties of wurtzite structured zinc oxide (ZnO) nanowires. Compared with other techniques, EELS significantly extends the energy range and is a useful technique for analysis of the dielectric properties on a microstructure level. The first-principles calculations allow deep insight into the experimental results. Furthermore, the polarization dependencies of optical properties have been discussed. Our results give some reference to the thorough understanding of optical properties of ZnO.     

Photoelectron spectroscopy of the valence electronic structure of polymers

Photoelectron spectroscopy has been highly developed as an investigative tool of the bulk and surface chemical and electronic structure of condensed matter during the past 2 decades. The use of soft X-ray sources led to the development of electroq spectroscopy for chemical applications (ESCA),^1,2 which is also known to the physicst as X-ray photoelectron spectroscopy (XPS). These developments were followed by the use of ultraviolet sources for the study of gases³ and the bulk and surface electronic structure of solids.^4,5 Recently, the use of the continuous spectral distribution of synchrotron radiation has had a major impact upon the study of solid surfaces^6,7 and has made the largely historical terminology of XPS (or ESCA) and ultraviolet photoelectron spectroscopy (UPS) somewhat less meaningful. Angle-resolved UPS and XPS (ARUPS and ARXPS) can provide additional information about electronic band structure, surface states, and adsorbates, but are typically applied to the study of the surfaces of ideal single crystals, where crystallographic high-symmetry directions as well as the polarization of the light source can be used to high advantage.⁷ Since polymers are almost never single crystals, the angle-resolved issue will not be addressed here.     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of nanostructured ZnO

The dielectric properties of nanostructured wurtzite-type ZnO are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. Characteristic peaks at about 12 and 32 eV in the imaginary part of the dielectric function shift to lower energies as particle size decreases. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA), GGA+U and modified Becke-Johnson (mBJ) is presented. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states of Zn and O.     

K-shell excitation of HCN by electron energy loss spectroscopy

Small angle inelastic scattering of fast electrons has been used to study carbon and nitrogen K-shell excitation and ionization of HCN. The K→π* transitions in HCN have been investigated with high resolution.     

Reflection electron energy loss spectroscopy of aluminum

Reflection electron energy loss spectra (REELS) of Al(111) single crystal and of the aluminum polycrystalline (poly Al) film were measured at 200 eV and 1000 eV electron energies for a variety of experimental geometries and were mutually compared. No anisotropy was found for the poly Al, as expected. Polar intensity plots evaluated from the elastic (no loss) and inelastic first surface plasmon- and first bulk plasmon-loss intensities of the Al(111) surface show clearly discernable peaks for both considered electron energies. Their positions on the angular axis are the same for the elastic as well as for the inelastic, surface and bulk plasmon-loss peaks. The polar plots of intensities of the elastically and inelastically reflected electrons were compared to calculated intensities of photoelectrons emitted from the Al 2s core level to the same kinetic energy. Peak positions in the theoretically determined polar plots of electron intensities agree with those obtained experimentally in REELS.     

Image simulation for electron energy loss spectroscopy

Aberration correction of the probe forming optics of the scanning transmission electron microscope has allowed the probe-forming aperture to be increased in size, resulting in probes of the order of 1 A in diameter. The next generation of correctors promise even smaller probes. Improved spectrometer optics also offers the possibility of larger electron energy loss spectrometry detectors. The localization of images based on core-loss electron energy loss spectroscopy is examined as function of both probe-forming aperture and detector size. The effective ionization is nonlocal in nature, and two common local approximations are compared to full nonlocal calculations. The affect of the channelling of the electron probe within the sample is also discussed.     

X-ray and electron spectroscopy investigation of the core-shell nanowires of ZnO:Mn

ZnO/ZnO:Mn core-shell nanowires were studied by means of X-ray absorption spectroscopy of the Mn K- and L_2,3-edges and electron energy loss spectroscopy of the O K-edge. The combination of conventional X-ray and nanofocused electron spectroscopies together with advanced theoretical analysis turned out to be fruitful for the clear identification of the Mn phase in the volume of the core-shell structures. Theoretical simulations of spectra, performed using the full-potential linear augmented plane wave approach, confirm that the shell of the nanowires, grown by the pulsed laser deposition method, is a real dilute magnetic semiconductor with Mn²⁺ atoms at the Zn sites, while the core is pure ZnO.     

Background subtraction in electron spectroscopy by use of reflection electron energy loss spectra

The use of reflected electron energy loss spectra (REELS) in deconvoluting the inelastic background signal from XPS and AES spectra from homogeneous samples is studied. It is demonstrated that under certain assumptions, the cross section for inelastic electron scattering can be extracted from a REELS spectrum. This cross section is applied to deconvolute an experimental XPS spectrum of aluminium. The method, its limitations and its relation to other methods are discussed.     

Comment on "Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy"

Recently, Zhang et al. have published a paper [Zhang, Z.H., Qi, X.Y., Jian, J.K., Duan, X.F., 2006. Micron 37, 229–233] in which – among others – the determination of the optical properties of a semiconductor by use of electron energy-loss spectroscopy (EELS) is performed with 200 keV electrons and a collection angle of only 0.3 mrad. The authors do not take into account relativistic effects such as Čerenkov losses (CL) before performing Kramers–Kronig Analysis (KKA) on the EELS spectra obtaining erroneous results. Although the positions of features within the optical properties are consistant with the simulated ones, the relative hights or absolute values differ a lot.     

Study of He bubbles in Al by electron energy loss spectroscopy

High energy electron energy loss spectroscopy (ELS) has been used to study He bubbles in Al, which were obtained by irradiation of He ions or α-particles of energy ranging from 500 eV to 8 keV and fluences 1x10²⁰m^?2 and 5x10²⁰m^?2. ELS reveals surface plasmon losses of the Al cavities as well as pressure shifts of the He-resonance lines as large as 1 eV. This is viewed as evidence for the existence of a so-called super-dense He in the bubbles. ELS is therefore a promising tool for obtaining information on the He pressure within the bubbles.     

High-detective-quantum-efficiency fast-electron detector for electron energy loss spectroscopy

In order to increase the sensitivity of the parallel electron detector used in electron energy loss spectroscopy (EELS), we have developed a direct electron exposed detector, based on a photodiode array (PDA). This work investigates the performance of this detector at 100 keV incident electrons in terms of the detective quantum efficiency (DQE), the modulation transfer function (MTF) and radiation damage.     

Optical properties and electronic transitions of SnO2 thin films by reflection electron energy loss spectroscopy

Optical properties of SnO₂ thin films in the 4–60 eV energy range are determined by reflection electron energy loss spectroscopy. Bulk and surface electron loss functions, real and imaginary parts of the dielectric function, refraction index, extinction and absorption coefficients are obtained from the analysis of the electron energy loss spectra. Electronic transitions are identified through the interpretation of the optical data. The samples (250–500 nm thick) were produced by ion beam-induced chemical vapor deposition. It is found that the compacity of the SnO₂ thin films affects their optical properties and therefore the relative intensity of the observed electronic transitions. The advantages of this method to determine optical properties of thin films are discussed. Inelastic mean free paths (6.2, 17 and 41 Å for electrons traveling in SnO₂ with kinetic energies of 300, 800 and 2000 eV, respectively) are obtained from the corresponding inelastic electron scattering cross-sections.     

The electronic structure of from high energy spectroscopy

We report the results of a detailed study of the occupied and unoccupied electronic structure of dimers of the new heterofullerene by means of photoemission and electron energy-loss spectroscopy. A close similarity is found between the electronic structures of pristine and with an additional broadening of the spectra in the former due to the distortion of the fullerene cage caused both by dimerization and the chemical substitution. Both the occupied and unoccupied electronic states, as well as the interband transitions between them, attest to the high degree of molecular character retained in the solid state. Comparison of the shake-up structures in the and X-ray photoemission spectra confirm that the highest lying occupied states in the heterofullerene have a strong degree of N character, whereas the lowest lying unoccupied states have mainly C character. We also present the optical conductivity of the heterofullerene (derived from the loss function), which shows an optical gap of 1.4 eV, some 0.4 eV smaller than that of . Received: 25 August 1997 / Revised: 22 September 1997 / Accepted: 16 October 1997     

The adsorption of oxygen and carbon monoxide on cleaved polar and nonpolar ZnO surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) with primary energies e₀ ? 80 eV has been performed on ultrahigh vacuum (UHV) cleaved nonpolar (11?00) and polar zinc (0001) and oxygen (0001?) surfaces of ZnO to study the adsorption of oxygen and carbon monoxide. Except for CO on the nonpolar surface where no spectral changes in ELS are observed a surface transition near 11.5 eV is strongly affected at 300 K on all surfaces by CO and O₂. At 300 K clear evidence for new adsorbate characteristic transitions is found for oxygen adsorbed on the Zn polar surface near 7 and 11 eV. At 100 K on all three surfaces both CO and O₂ adsorb in thick layers and produce loss spectra very similar to the gas phase, thus indicating a physisorbed state.     

Role of cross section on the stability and electronic structure of Ag-doped ZnO nanowires

Semiconductor nanowires (NWs) exhibit tunable physical properties intrinsically related to their reduced dimensionality, quantum size effect, morphology, and surface effects. By using density functional theory, we investigated the cross-sectional effect on the electronic structure of Ag-doped ZnO NWs. Three types of NWs have been considered: hexagonal cross-sectional ZnO NWs with zigzag and armchair surfaces, respectively, and triangular cross-sectional ZnO NW with zigzag surface. The results show that Ag prefers to substitute surface Zn atoms and induces typical p-type characteristic for all kinds of NWs. Moreover, single Ag doping could create a much shallower acceptor with a smaller hole effective mass in triangular ZnO NW than in the two hexagonal ZnO NWs. With the increase of Ag concentration, the p-type doping becomes much less effective overall. However, double Ag substituting in the zigzag surface of triangular ZnO NW improves the p-type properties, while substituting in the angle site seriously damage the p-type conduction. As the triangular ZnO NWs and prismatic ZnO nanoparticles have been synthesized recently, on the basis of our results, we expect that effective p-type could be achieved via incorporating Ag in triangular ZnO NWs experimentally.     

Local electronic structure of phosphorus-doped ZnO films investigated by X-ray absorption near-edge spectroscopy

Using X-ray absorption near-edge structure spectroscopy (XANES), we investigate the local electronic structure of phosphorus (P) and its chemical valence in laser-ablated n-type (as-grown), and p-type (annealed) P-doped ZnO thin films. Both the P L ₁- and P L _2,3-edge XANES spectra reveal that the valence state of P is 3− (P³⁻) in the p-type as well as in the n-type P-doped ZnO. However, the peak intensity is stronger in the former than that in the latter, suggesting that P replaces O (O²⁻ sites with the P³⁻) after rapid thermal annealing. The Zn and O K-edges XANES spectra consistently demonstrate that, in the p-type state, P ions substitutionally occupy O sites in the ZnO lattice.