Energy Hν Research Articles

Enhancing the accuracy of the Fowler method for monitoring non-constant work functions.

The Fowler method is a prominent non-invasive technique to determine the absolute work function of a surface based on the photoelectric effect. The evaluation procedure relies on the correlation of the photocurrent with the incident photon energy hν which is mainly dependent on the surface work function χ. Applying Fowler's theory of the photocurrent, the measurements can be fitted by the theoretical curve near the threshold hν⪆χ yielding the work function χ and a parameter A. The straightforward experimental implementation of the Fowler method is to use several particular photon energies, e.g. via interference filters. However, with a realization like that the restriction hν ≈ χ can easily be violated, especially when the work function of the material is decreasing during the measurements as, for instance, with coating or adsorption processes. This can lead to an overestimation of the evaluated work function value of typically some 0.1 eV, reaching up to more than 0.5 eV in an unfavorable case. A detailed analysis of the Fowler theory now reveals the background of that effect and shows that the fit-parameter A can be used to assess the accuracy of the determined value of χ conveniently during the measurements. Moreover, a scheme is introduced to quantify a potential overestimation and to perform a correction to χ to a certain extent. The issues are demonstrated exemplarily at the monitoring of the work function reduction of a stainless steel sample surface due to caesiation.

Study of Opto-Electronic Properties of Copper Sulphide Thin Film Grown by Chemical Bath Deposition Technique for Electronic Device Application

Cu2S Thin Film have been synthesized by simple and economic solution growth technique onto silica glass substrate with bath temperature at 40°C for an hour. Optical, properties of the prepared Thin Films were investigated by UV-Visible Spectrophotometer and Electrical properties of the prepared Thin Films were investigated by I-V Measurement System. The optical band gap was calculated by plotting Energy (hν) versus (αhν)2 and it was found to be 2.315 eV which is in very good agreement with the reported values. The semiconducting nature of the film has been confirmed from I-V Measurement curve which has shown ohmic nature in dark condition. Surface morphology have been studied by Scanning Electron Microscopy which revealed entire substrate surface was covered with uniform deposition which contains irregular shaped grains are uniformly distributed.

Optical properties of PbS thin films

The complex dielectric function of PbS thin films is studied by spectroscopic ellipsometry in the spectral range from 0.74 to 6.45 eV at a temperature of 293 K. The critical energies are determined to be E 1 = 3.53 eV and E 2 = 4.57 eV. For both energy regions, the best fit is attained at the critical point 2D (m = 0). In addition, the Raman spectra and the optical-absorption spectra of PbS thin films are studied. From the dependence of the quantity (αhν)2 on the photon energy hν, the band gap is established at E g = 0.37 eV.

Effect of localized modes in the optical absorption spectra of CuGaSe2 and CuGa3Se5

The optical absorption coefficient α as a function of the incident photon energy hν at different temperatures between 50 and 300K and the infrared spectra between 80 and 375meV have been studied in the ternary compound CuGaSe2 and the ordered defect compound CuGa3Se5. It is found that the phonon energy hνp involved in the formation of Urbach tail is much higher than the highest lattice vibrational modes in these compounds. A combined analysis of the Raman modes and infrared spectra shows, after discarding the possible donor/acceptor transitions, replicas of phonon modes at higher energies in both Raman and infrared spectra, and the peaks originating from the spectrophotometer, that the average phonon energy hνp∗ of the remaining lattice vibrational and localized modes are in good agreement with the phonon energy hνp obtained from the study of Urbach tail. This confirms that in the formation of Urbach tail, in addition to the contribution of lattice vibrational modes, other localized modes caused by structural defects and enhanced electronic distortion, originating from ordered defects, are also involved. It is suggested that this type of study can be a useful tool in determining the quality of the samples and the ingots, important in device applications.

A revisit to ultrathin NiO(0 0 1) film: LEED and valence band photoemission studies

LEED and photoemission measurements have been performed on ultrathin NiO films to reinvestigate its surface quality and valence electronic structures, respectively. On Ag(001) substrate, the best epitaxial order was observed for high temperature deposition with sufficient oxygen flux associated with a post-deposition oxygen annealing. The effect of the substrate vicinity on valence band electronic structure, in case of interfacial NiO layers, has been explained. The variation of Ni 3d to O 2p photoemission cross-section with photon energy (hν) has been demonstrated in this work.

Li Vs. Na Batteries - Investigation of the SEI and Conversion Mechanisms of Nanostructured Fe2O3 Composites Electrodes Via Soft X-Ray Photoelectron Spectroscopy

The properties of the SEI layers formed on composite nanostructured iron oxide electrodes upon cycling in Li- and Na-half cells[1] containing analogous electrolytes (i.e. LiClO4, NaClO4) have been investigated by an advanced non-destructive analysis based on Soft X-ray Photoelectron Spectroscopy (PES). Using synchrotron radiation, this approach has enabled probing the corresponding SEI layers at different depths by varying the photon energy hν, in the range of 100-1200 eV. [2,3] This methodology has granted access to a careful study of both the intimate structure and the compositions of these passivating films formed upon initial lithiation/sodiation of the electrode.[4] In this respect, understanding the SEI features without causing any interference or damage during the PES measurement processes has proved crucial to obtain a deeper insight into the specific mechanisms that lead to the formation of such films. The latter have a crucial technological relevance, since they directly affect the electrochemical performances of the active materials undergoing a particular electrochemical reaction, e.g. in a Li or Na cell. The results of this study will be presented highlighting the differences and the similarities that exist between these two electrochemical systems. In particular, it will be shown that such step-by-step analysis conducted on various electrodes left in a defined state of charge upon reduction (i.e. Li+ or Na+ uptake) and oxidation (i.e. Li+ or Na+ release) has revealed interesting differences for the structure of their respective SEI layers, as well as different covering properties in both cases. Finally, the mechanisms of nanostructured iron oxide upon reaction with Li+ and Na+ ions will be discussed. Additional information will be provided on the conversion reaction occurring at low potentials vs. Li+/Li and Na+/Na when the thickness of the grown SEI layer was not critical and has allowed probing the underlying active material, its oxidation state and surrounding chemical environment. References M. Valvo, F. Lindgren, U. Lafont, F. Björefors, K. Edström, J. Power Sources 245 (2014) 967.B. Philippe, R. Dedryvère, J. Allouche, F. Lindgren, M. Gorgoi, H. Rensmo, D. Gonbeau, K. Edström, Chem. Mater. 24 (2012) 1107.S. Malmgren, K. Ciosek, M. Hahlin, T. Gustafsson, M. Gorgoi, H. Rensmo, K. Edström, Electroch. Acta 97 (2013) 23.B. Philippe, M. Valvo, F. Lindgren, H. Rensmo, K. Edström, Chem. Mater., 26 (2014) 5028

From photoelectron detachment spectra of BrHBr(-), BrDBr(-) and IHI(-), IDI(-) to vibrational bonding of BrMuBr and IMuI.

Photoelectron detachment XLX(-)(00(0)0) + hν → XLX(vib) + e(-) + KER (X = Br or I, L = H or D) at sufficiently low temperatures photoionizes linear dihalogen anions XLX(-) in the vibrational ground state (v1v2 (l)v3 = 00(0)0) and prepares the neutral radicals XLX(vib) in vibrational states (vib). At the same time, part of the photon energy (hν) is converted into kinetic energy release (KER) of the electron [R. B. Metz, S. E. Bradforth, and D. M. Neumark, Adv. Chem. Phys. 81, 1 (1992)]. The process may be described approximately in terms of a Franck-Condon type transfer of the vibrational wavefunction representing XLX(-)(00(0)0) from the domain close to the minimum of its potential energy surface (PES) to the domain close to the linear transition state of the PES of the neutral XLX. As a consequence, prominent peaks of the photoelectron detachment spectra (pds) correlate with the vibrational energies EXLX,vib of states XLX(vib) which are centered at linear transition state. The corresponding vibrational quantum numbers may be labeled vib = (v1v2 (l)v3) = (00(0)v3). Accordingly, the related most prominent peaks in the pds are labeled v3. We construct a model PES which mimics the "true" PES in the domain of transition state such that it supports vibrational states with energies EXLX,pds,00(0)v3 close to the peaks of the pds labeled v3 = 0, 2, and 4. Subsequently, the same model PES is also used to calculate approximate values of the energies EXMuX,00(0)0 of the isotopomers XMuX(00(0)0). For the heavy isotopomers XHX and XDX, it turns out that all energies EXLX,00(0)v3 are above the threshold for dissociation, which means that all heavy XLX(00(0)v3) with wavefunctions centered at the transition state are unstable resonances with finite lifetimes. Turning the table, bound states of the heavy XLX are van der Waals (vdW) bonded. In contrast, the energies EXMuX,00(0)0 of the light isotopomers XMuX(00(0)0) are below the threshold for dissociation, with wavefunctions centered at the transition state. This means that XMuX(00(0)0) are vibrationally bonded. This implies a fundamental change of the nature of chemical bonding, from vdW bonding of the heavy XHX, XDX to vibrational bonding of XMuX. For BrMuBr, the present results derived from experimental pds of BrHBr(-) and BrDBr(-) confirm the recent discovery of vibrational bonding based on quantum chemical ab initio calculations [D. G. Fleming, J. Manz, K. Sato, and T. Takayanagi, Angew. Chem., Int. Ed. 53, 13706 (2014)]. The extension from BrLBr to ILI means the discovery of a new example of vibrational bonding. These empirical results for the vibrational bonding of IMuI, derived from the photoelectron spectra of IHI(-) and IDI(-), are supported by ab initio simulations of the spectra and of the wavefunction representing vibrational bonding of IMuI.

Photoelectron spectroscopy in a wide hν region from 6 eV to 8 keV with full momentum and spin resolution

High resolution photoelectron spectroscopy is recognized to be a very powerful approach to study surface and bulk electronic structures of various solids by employing different photon energies (hν). In particular, angle resolved photoelectron spectroscopy (ARPES) has progressed dramatically in the last few decades providing useful information on Fermi surface (FS) topology and band dispersions.The information of the electron spin is often decisive to fully understand the electronic properties of many material classes. However, spin-resolved studies by photoelectron spectroscopy were strongly hindered by the low detection efficiency of spin detectors. In the case of surface electronic structures, possible surface degradation with time is a serious problem to discuss intrinsic electronic effects. Therefore rather fast and high efficiency detection is required in the case of surface sensitive spin-resolved ARPES.Two-dimensional (2D) detection is nowadays widely employed in ARPES. In the use of a conventional hemispherical deflection analyzer (HDA), one direction on the 2D detector corresponds to the binding energy EB and the other direction to the emission angle. The novel concept of momentum microscopy, however, directly provides 2D (kx,ky) maps of the photoemission intensities. The reciprocal space image directly represents the cross section through the valence band structure of the sample at a selected energy. By scanning EB, very high resolution three-dimensional EB(kx,ky) maps of the band-dispersion can be obtained with high efficiency.If combined with a 2D imaging spin filter, based on electron reflection at a Au/Ir(001) surface, spin detection efficiency with orders of magnitude higher than other spin detectors is realized, opening a breakthrough in spin-resolved ARPES (SP-ARPES).

Investigation of the Electrode/Electrolyte Interface of Fe2O3 Composite Electrodes: Li vs Na Batteries

We have investigated the properties of the electrode/electrolyte interfaces of composite electrodes based on nanostructured iron oxide cycled in Li- and Na-half cells containing analogous electrolytes (i.e., LiClO4 or NaClO4 in ethylene carbonate:diethyl carbonate (EC:DEC)). A meticulous nondestructive step-by-step analysis of the first discharge/charge cycle has been conducted via soft X-ray photoelectron spectroscopy using synchrotron radiation. In this way, different depths were probed by varying the photon energy (hν) for both electrochemical systems. The results of this thorough study clearly highlight the differences and the similarities of their respective solid electrolyte interface (SEI) layers in terms of formation, composition, structure, or thickness, as well as their conversion mechanisms. We specifically point out that the SEI coverage is more pronounced, and a homogeneous distribution rich in inorganic species exists in the case of Na, compared to the organic/inorganic layered structure obs...

Spectroscopy of Deep Gap States in High-k Insulators

Spectroscopy of Deep Gap States in High-k Insulators

Optical characterization by variable angle spectroscopic ellipsometry of nitrogen-doped MgxZn1 − xO thin films prepared by the plasma-assisted reactive evaporation method

Non-doped MgxZn1−xO films (MgxZn1−xO films) and nitrogen-doped MgxZn1−xO films (MgxZn1−xO:N films) were grown epitaxially on Zn faces of ZnO single crystal substrates by the plasma-assisted reactive evaporation (PARE) method using ZnMg alloys. Optical parameters, refractive index n, extinction coefficient k, and absorption coefficient α of these films and ZnO substrate were easily estimated by variable angle spectroscopic ellipsometry. The k values of these films abruptly increased with increase in photon energy near the absorption edge region. Dispersions of n of these films were sharper with larger peak values than that of a ZnO substrate. MgxZn1−xO films and MgxZn1−xO:N films grown on a ZnO substrate by the PARE method are of high quality with less defects than those of a ZnO substrate. Maximum value of α of these films calculated from k was about 2×105cm−1. The values of intercepts on photon energy hν axis of (αhν)2‐hν plots for these films and the ZnO substrate agree with peak energy of PL spectra, the origin of which is free excitons. These results indicate that absorption in those materials is dominated by exciton absorption.

Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

LaAlO3 and NdGaO3 thin films of different thicknesses have been grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide.

On the nature and temperature dependence of the fundamental band gap of In2O3

The onset of optical absorption in In2O3 at about 2.7 eV is investigated by transmission spectroscopy of single crystals grown from the melt. This absorption is not defect related but is due to the fundamental band gap of In2O3. The corresponding spectral dependence of the absorption coefficient is determined up to α = 2500 cm−1 at a photon energy hν = 3.05 eV at room temperature without indication of saturation. A detailed analysis of the hν dependence of α including low‐temperature absorption data shows that the absorption process can be well approximated by indirect allowed transitions. It is suggested that the fundamental band gap of In2O3 is of indirect nature. The temperature dependence of the fundamental band gap is measured over a wide range from 9 to 1273 K and can be well fitted by a single‐oscillator model. Compared to other semiconductors the reduction of the gap with increasing temperature is exceptionally strong in In2O3.

X-rays from a microsecond X-pinch

The characteristics of x-rays emitted by X-pinches driven by discharging a current of ∼320 kA with a quarter period of 1 μs in crossed 25 μm wires have been investigated. The x-ray emissions are studied using filtered silicon photodiodes, diamond radiation detectors, and pinhole cameras. The results show that predominantly x-rays from the microsecond X-pinch tend to be emitted in two distinct sets of bursts. The first is predominantly “soft,” i.e., with photon energy hν < 5 keV, followed by a second set of bursts beginning up to 100 ns following the initial bursts, and usually consisting of higher photon energies. Our results show, however, that the x-ray emissions do not contain a significant component with hν > 10 keV as might be expected from electron beam activity within the plasma or from the X-pinch diode. High-resolution images obtained with the observed x-rays suggest a well-defined small source of soft x-rays that demonstrates the potential of the microsecond X-pinch.

Strong Near-Infrared Photoluminescence Emission of (003)-Oriented MgTiO3 Thin Films

In this study, ilmenite-MgTiO3 films were sputtered on p-type Si(111) substrates and the extrinsic effects, such as grain size, crystallinity, and orientation of photoluminescence (PL) properties of the films are discussed. To reduce the effect of oxygen vacancies (act as shallow defects) on PL emissions in the films, oxygen (O2) was introduced as the sputtering gas and the excitation light source (λ = 532 nm) which has a corresponding energy (hν = 2.33 eV) below the shallow defect states was used. In this study, intense near-infrared (NIR) PL emission centered at 810.1 nm at room temperature can be observed when the MgTiO3 thin films exhibit the preferred (003)-orientation and accompanied by the presence of hexagon-shaped grains. In this study, the experiment results reveal that the NIR emission intensity of MgTiO3 films highly depend on crystal orientation and/or grain morphology.

Experimental Study on Surface-Orientation/Strain Dependence of Phonon Confinement Effects and Band Structure Modulation in Two-Dimensional Si Layers

We have experimentally studied the surface orientation/strain effects on quantum mechanical confinement (QMC) in two-dimensional (2D) Si layers with thicknesses less than the Si lattice constant for future metal–oxide–semiconductor (MOS) devices. By UV–Raman spectroscopy, we have demonstrated that the quantum phonon confinement effects (PCEs) rapidly increase with decreasing 2D Si thickness TS, but is almost independent of surface orientation and strain. Thus, electron saturation velocity of the 2D Si is degraded by the reduced phonon energy owing to the PCEs. On the other hand, photoluminescence (PL) emitted from the only (100)-surface 2D Si layers, depends on the excitation photon energy hν (2.33≤hν≤3.81 eV), and PL intensity increases with decreasing TS. The PL data can be explained by simple PL models considering the electron/hole pair recombination mechanism. Consequently, it is necessary to reconstruct the device design for future Si devices, considering the TS dependence of the 2D Si properties.

Photocatalytically Reducing CO2 to Methyl Formate in Methanol Over Ag Loaded SrTiO3 Nanocrystal Catalysts

Ag–SrTiO3 nanocrystal photocatalysts were prepared by hydrothermal synthesis method for reducing CO2 to methyl formate (MF) in methanol via ultraviolet irradiation. CO2 was reduced to MF by electrons on conduction band of the photocatalyst. In order to obtain a high rate of MF evolution, we researched and optimized the preparation procedure of Ag–SrTiO3 by changing the Ag dosage, hydrothermal temperature and time. The as synthesized photocatalysts were characterized with X-ray diffraction, UV–Vis absorption spectra, transmission electron microscope, N2 sorption analysis at 77 K and activity evaluation. A catalyst of 7 wt% Ag on SrTiO3 with hydrothermal synthesis at 150 °C and 22 h was found to exhibit the highest photocatalytic activity in MF formation rate of 3,006 μmol/(h g cat), which was more remarkable than that of pristine SrTiO3. When the SrTiO3 is irradiated by UV light of suitable wavelength with sufficient photonic energy (hν), photo-generated electron–hole pairs were created, part of the excited electrons and holes recombined together and radiated out heat, while rest part would react with the reactants near the SrTiO3 surface. It was certain electrons transmitting to the surface of Ag particle that avoided their recombination with holes, resulting in effective extension of the life-span of the electron–hole pairs.

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(001), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.

Nonequilibrium Molecular Dynamics of Trp Zwitterion in Water: Picosecond Fluorescence Measurements Versus Computer Simulations

This is an experimental test of MD simulations on the picosecond timescale. Tryptophan zwitterion in TIP3P water at 278°K was simulated using CHARMM22 forcefield with the excited-state Trp atomic charges from [Toptygin et al., J. Phys. Chem. B 2010, 114, 11323]. Six stable excited-state rotamers of Trp sidechain were found with the population density peaks near (χ1,χ2) = (67°,80°), (−170°,57°), (−65°,115°), (65°,-85°), (−165°,-112°), (−65°,-80°). Curved boundaries between the rotamers on the (χ1,χ2) map were drawn along the troughs of the population density. Population density distribution within the boundaries of one rotamer reaches equilibrium in less than 20ps; equilibration between different rotamers takes much longer. At t>20ps rotamer populations can be described by a system of six first-order homogeneous linear differential equations. The solution is a sum of six terms Vmnexp(-t/τn). Population decay of each rotamer is not monoexponential and τn is not a lifetime. The same set of τn applies to all rotamers, but a different set of Vmn corresponds to each rotamer. The rotamers have slightly different fluorescence emission spectra, therefore fluorescence intensity is a sum of six terms αn(ν)exp(-t/τn), where αn vary with the photon energy hν. We have determined τn and αn(ν) in the global analysis of spectrally- and time-resolved fluorescence data (time resolution 65ps FWHM). Only four exponential terms could be resolved from the experimental data in H2O at 5°C (τ1 = 4780ps, τ2 = 2500ps, τ3 = 867ps, τ4 = 411ps); according to MD simulations the fifth term (τ5 = 241ps) has a very small amplitude, and the sixth (τ6 = 22ps) is faster than the time resolution. For a precise agreement between the experimental and simulated values of τn it is necessary to lower all potential barriers between rotamers by 0.178kcal/mol. This shows that fluorescence spectroscopy can be used to fine-tune torsional parameters.

X-ray luminescence spectra of graded-gap AlxGa1−xAs structures irradiated by alpha particle

The influence of 241 Am alpha particle irradiation on X-ray luminescence spectra of the graded-gap Al x Ga 1− x As structures of different thicknesses is investigated. It is observed that the integral X-ray luminescence intensity of nonirradiated thin (15 μm) structure is 1.4 times less than that in the thick (32 μm) structure, and this difference increases to 3 times after 3×10 10 cm −2 dose of irradiation by alpha particle. The X-ray luminescence intensity of the energy hν <1.5 eV of thin nonirradiated structure is about 7 times less than that in thick one. The internal graded-gap electric field F gg is responsible of that large difference, because it shifts the X-ray generated carriers to the narrow-gap surface with great nonradiative surface recombination rate. The alpha particle irradiation increases nonradiative recombination rate and causes a decrease of the X-ray luminescence intensity of all spectra lines in the thin (15 μm) detector. The most significant drop in X-ray luminescence efficiency is observed from the region at narrow-gap surface after the initial stage (10 9 cm −2 dose) of alpha particle irradiation. In the 32 μm thick detector, the luminescence intensity of the energy hν =1.8 eV does not change up to 2×10 10 cm −2 of alpha particle irradiation dose. That means the high irradiation hardness of the thick graded-gap X-ray detector with optical response.