Threshold Energy Of Photoemission Research Articles

Ionization potential and electron attenuation length of titanium dioxide deposited by atomic layer deposition determined by photoelectron spectroscopy in air

Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission threshold energy (also referred to as the “ionization potential”) of TiO2 thin films, avoiding inherent artifacts due to photoelectron emission from the substrate, which supplies misleading results. This has been achieved using PESA measurements performed as a function of TiO2 film thickness on two substrates with different work functions. We find that proper measurements of the photoemission threshold energy (including work function and ionization potential) of thin films by PESA require the use of films much thicker than their electron attenuation length (EAL). A photoemission threshold energy of 5.0±0.2eV is obtained for TiO2 and has been attributed to a trap level due to oxygen vacancies, which lie within the band gap of the TiO2. The analysis of the photoemission decay with film thickness also provides a method for determining a “practical” (or effective) EAL at excitation energy slightly above the photoemission threshold energy of the material. We extract an EAL for the deposited TiO2 of 0.65±0.02nm (at 0.5eV). The procedure can also be adopted for determining the thickness of extremely thin films, provided their thickness is smaller than their EAL.

(Invited) Synergistic Use of Electrochemical Impedance Spectroscopy and Photoelectrochemical Measurements for Studying Solid State Properties of Anodic HfO2

Hafnium oxide and Nb doped HfO2 were grown by anodizing sputtering-deposited Hf and Hf-4at.%Nb. Photoelectrochemical characterization was carried out in order to estimate solid state properties such as band gap, flat band potential and electrons internal photoemission threshold energy as a function of thickness and composition of anodic oxides. Impedance measurements were performed in a wide range of electrode potentials in order to study the response of the anodic oxides under anodic and cathodic polarization and to estimate their dielectric constants.

The Effect of Nb Incorporation on the Electronic Properties of Anodic HfO2

Hafnium oxide and Nb doped HfO2 were grown by anodizing sputtering-deposited Hf and Hf-4at.%Nb. Photoelectrochemical characterization was carried out in order to estimate solid state properties such as band gap, flat band potential and electrons internal photoemission threshold energy as a function of thickness and composition of anodic oxides. Optical transitions at energy lower than the band gap value of the investigated anodic films were evidenced, and they are attributed to optical transitions involving localized states inside the band gap. Such states were located at 3.6 eV and 3.9 eV below the conduction band edge for the Nb free and Nb containing hafnium oxide, respectively. Impedance measurements were performed in a wide range of electrode potentials under anodic and cathodic polarization with respect to the flat band potential, confirming the formation of insulating oxides with high dielectric constant. The latter increases from 20 to 41 due to incorporation of Nb into the oxide.

Tuning Work Function of Multi-Walled Carbon Nanotubes By Defects Creation

Carbon nanotube(CNT) is a very attractive material to various electronic devices and optoelectronic applications owing to its promising structural and electronic properties comparing with other carbon materials.1CNT has been successfully applied on field emission displays, light emitting diodes, solar cells, etc. To these electronic or optoelectronic devices which require modification of the junction interface to achieve a well-defined electronic energy level leading to a desired performance, work function is a critical parameter. It is of importance to develop a systematic methodology to manipulate CNT’s work function to match diverse energy structures. Impacts of multi-walled carbon nanotube(MWCNT) surface functional groups on its work function has been poposed,2 which provides a potential strategy of tailoring an appropriate work function for different applications. In our previous study,3we found that creating defects on MWCNT benefits planting abundant functional groups on it, this would contribute to control work function in a wide range. In this work, a surface functional group decoration methods associated with coating, defects creation, surface functional groups decoration and doping on MWCNT for tuning work function has been designed and carried out. Inductively coupled plasma mass spectrometry(ICP-MS) are employed to distinguish the impact of impure metal elements, since the raw CNT always contains some metal impurities which may have negative effect on investigating the origin of work function changes. Annealing MWCNT samples in different temperatures to remove certain functional group is conducted to identify which certain functional group responsible for the work function tuning. Photo-electron spectroscopy in air (PESA)* and ultraviolet photoelectron spectroscopy (UPS) are conducted to measure modified CNT’s work function directly and also provide information of density of state (DOS) near Femi level. Temperature programed desorption(TPD) and X-ray photoelectron spectroscopy(XPS) analysis provides estimation of different functional group amount and compelling evidence of doping and coating. Field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM) are adopted to observe the morphology of modified MWCNTs. Structure disorder information is supplied by Raman spectroscopy. It has been revealed that a 0.55 eV work function difference between different modified WMCNTs is achieved at current stage (Figure 1). The further modification parameters will be presented on the conference. PESA*: It is an instrument for Photoelectron Spectroscopy at atmospheric pressure. UV photons emitted from a deuterium lamp are monochromated by a grating spectrometer and focused on a sample. Then, photoelectrons emitted from the sample are counted by an open counter. In terms of metal, when the photon energy is smaller than work function, there is no electron reach counter. If the photon energy is larger than work function, the electron from sample starts to be counted by open counter which indicates the exact value of work function. After plotting data in a graph, the crossing point of the back ground and the yield line is a photoemission threshold energy, also called the work function or the ionization potential. Acknowledgement: This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co., Ltd. Japan. and Riken Keiki Co., Ltd. Their contribution is greatly appreciated. Reference: 1 S. Iijima, Nature, 1991, 354, 561–58. 2 H. Ago, T. Kugler, F. Cacialli, W. R. Salaneck, M. Shaffer, A. H. Windle and R. H. Friend, J. Mater. Chem. B, 1999, 103, 8116–8121. 3 K, Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada, Energy Environ. Sci., 2014,7, 1950-1958 Figure 1

Characterization of Catechins in Water by Photoemission Yield Spectroscopy in Air.

Photoemission yield spectroscopy in air (PYSA) was applied for the characterization of catechins in water in ambient conditions. According to the results of measurements on aqueous solutions of epigallocatechin gallate (EGCg) of various concentrations, the photoemission yield is almost proportional to the concentration of EGCg. Contrarily, the threshold energy of photoemission, EPET, is almost constant at 5.46 ± 0.02 eV. Moreover, we measured aqueous solutions of epicatechin (EC), epigallocatechin (EGC), and epicatechin gallate (ECg). The values of EPET of EC, EGC, ECg were estimated to be 5.72 ± 0.02, 5.68 ± 0.01, and 5.45 ± 0.02 eV, respectively, and a dependence on the molecular structure was found. Furthermore, changes in the photoemission yield spectra of heated EGCg were well explained by molecular orbital calculations on the basis of an assumption of epimerization.

Shift of the photoemission threshold in III–V and II–VI semiconductors

The reasons why the photoemission threshold energy of semiconductors is lower than the ionization energy of constituent atoms have been investigated. It has been indicated that the previously proposed interpretation of this phenomenon based on the inclusion of an additional intra-atomic Coulomb interaction between the valence electrons is insufficient. It has been shown that the calculation of the electronic energy structure of semiconductors, in particular, of the photoemission threshold, requires taking into account a change in the localization region of the valence electrons when a free atom is embedded into a crystal. A way of taking this change into account in the tight-binding theory has been demonstrated. Corrections to the tight-binding Hamiltonian have been found. The photoemission thresholds of III–V and II–VI semiconductors have been calculated with the inclusion of these corrections. Comparison of the results with the experimental data has been performed.

Electronic structure analysis of Mn- and Fe-codoped In2O3 by photoemission yield measurements

The valence band electronic structures of Mn- and/or Fe-doped In2O3, i.e., In2O3:Mn, In2O3:Fe, and In2O3:(Mn, Fe), are investigated by photoemission yield measurements. Significant changes are observed in the threshold energy of photoemission, depending on the doped magnetic ions, which indicates that an additional occupied band appears above the top of the valence band of In2O3 owing to doping with Mn and/or Fe ions. It is confirmed that the order of the threshold energies of photoemission, EPET, is EPET(In2O3:Mn)<EPET(In2O3:(Mn, Fe))<EPET(In2O3:Fe)<EPET(In2O3). To gain a better understanding of these results, first-principles molecular orbital calculations are also carried out, which successfully explain the observed changes in the photoemission threshold energies.

Reversible switching in self-assembled monolayers of azobenzene thiolates on Au (111) probed by threshold photoemission

The reversible photo- and thermally activated isomerization of the molecular switch 3-(4-(4-Hexyl-phenylazo)-phenoxy)-propane-1-thiol (ABT, short for AzoBenzeneThiol) deposited by self-assembly from solution on Au (111) was studied using laser-based photoelectron spectroscopy. Differences in the molecular dipole moment characteristic for the trans and the cis isomer of ABT were monitored via changes in the sample work function, accessible by detection of the threshold energy for photoemission. A quantitative analysis of our data shows that the fraction of molecules within the densely packed monolayer that undergoes a switching process is of the order of 1%. This result indicates the relevance of substrate and film defects required to overcome the steric or electronic hindrance of the isomerization reaction in a densely packed monolayer.

Electron emission from a semiconductor quantum ring under normally incidentradiation

In this paper we investigate the current density due to electrons emitted during normalirradiation of a toroidal quantum ring of a strongly degenerate wide-gap semiconductor.The computed results show that the current density increases in a step-like manner withincrease in the incident photon energy. Increased current density can be obtained byreducing the dimensions of the ring. The threshold energy for photoemission becomes anoscillatory function of the cross-sectional radius, and can be used as an important tool formonitoring parameters such as the cross-sectional radius and doping density.The threshold energy is, however, independent of the circumference of the ring.

Photoemission properties and surface structures of homoepitaxially grown CVD diamond(1 0 0) surfaces

In the present study, the surface properties of homoepitaxially grown chemical vapor deposition (CVD) diamond have been investigated using total photoyield measurements, scanning tunneling microscope (STM) observations and scanning tunneling spectroscopy (STS). The STM results show that as-grown CVD diamond(100) surfaces are characterized mainly by two types of regions with 2×1/1×2 structures and much less structures. These well-ordered surface structures can be significantly clearer after a low-temperature annealing at 150–200°C in ultra high vacuum. On the other hand, the STS results indicate the presence of band gap states, which agrees well with the fact that the observed photoemission threshold energy is located at 5eV, well below the band gap energy of 5.5eV, for the as-grown CVD diamond surface. The absolute value measurements of the photoemission total yields using a calibrated Si photodetector demonstrate substantially low photoemission efficiencies even for negative electron affinity (NEA) diamond surfaces, being consistent with the STM/STS data.

Dependence of the electron affinity of homoepitaxially grown CVD diamond on the amount of surface oxygen

Photoemission threshold energies have been measured for boron-doped homoepitaxially grown chemical-vapor-deposited (CVD) diamond with various degrees of the surface oxidation below 0.5 diamond (100) monolayer, or 7.8×10 14 O atoms/cm 2. In the case of the smooth surfaces, the observed threshold energy almost linearly increases with increasing amounts of chemisorbed oxygen atoms at low oxygen coverages below 0.1 monolayer and is followed by a saturation phenomenon. This behavior can be explained in terms of the Topping model, where the dipole–dipole interaction is taken into account and becomes more important at high dipole densities to reduce the increases in the work function, therefore, in the electron affinity of the diamond. On the other hand, in the case of substantial nonuniform oxidation due to the presence of significant amounts of particles abnormally grown, the threshold energy is almost unchanged at ∼5.4 eV for oxygen coverages below 0.3 monolayer, indicating the threshold energy is determined by surface area with low work functions or low electron affinities. The dipole-induced surface potential is discussed in relation to the changes in the electron affinity.

Spectral Distribution of Photoemission Quantum Efficiency for Gex(As2Se3)1-x Glasses

The photoemission threshold energy for Gex(As0.4Se0.6)1-x(x = 0, 0.1, 0.2, 0.3) glasses has been derived from the spectral distribution of the photoemission quantum efficiency in the near-threshold region. Glass densities and optical band gaps have been measured. The top of the valence band has been calculated by the linear combination of atomic orbitals method and is in quantitative agreement with photoemission threshold value for the glasses under investigation. The changes in the optical band gap have been found to be related to the changes in the top of the valence band. The peculiarities of the concentrational dependences for the top of the valence band have been considered.

Characteristics of Photoemission of High Polymers

Measurements of photoemission of various high polymers were carried out in the wavelength region between 160 and 780 nm. The number of photoelectrons is changeable during the measurement depending on the wavelength of incident light because of surface charge-up, photoexcitation of electrons and other effects. The threshold energy of photoemission depends on the light intensity used for illuminating the sample. It is found that high-intensity illumination is necessary to obtain precise threshold energy values for high-polymer samples. For all polymer samples examined, the threshold energies of photoemission were from 3 to 3.5 eV, which are considerably smaller than the values corresponding to the depths of HOMO levels.

VUV light-induced electron emission from organic liquids

Electron emission from low vapor pressure liquids induced by vacuum ultravoilet light was measured as a function of photon energy between 6 eV and 10.8 eV. Absolute quantum yields were determined for 17 organic compounds comprising hydrocarbons, phthalates, phosphates, dicarboxylic esters, amines and polysiloxane fluids. The quantum yields rise from 10 −6 to 10 −1 with increasing photon energy. Systematic changes of the yields with the molecular structure of the liquids were observed. The photoemission threshold energies of 11 liquids have been determined in this work. The electron energy levels and the photoemission process are discussed by taking into account the electronic properties of the isolated molecules, the influence of charge polarization, and the transport of hot electrons through the liquid/vapor interface.

Electronic structure of poly(tetrafluoroethylene) studied by UPS, VUV absorption, and band calculations

The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

UV photoelectron spectroscopy of several one- and two-component organic photoconductors

Solid state photoelectron spectra have been measured at several UV wavelengths for vapor-deposited films of the organic photoconductors phenothiazine, anthracene, and carbazole and for the 1:1 donor:acceptor complexes of phenothiazine, carbazole, and acridine with the common acceptor pyromellitic dianhydride (PMDA). Differences in photoemission threshold energies of the one-component photoconductors are used to estimate hole trap depth energies for systems where one of the compounds is doped with another one. Further, the photoemission threshold values are used for predicting the energy differences of charge transfer transitions in donor:acceptor crystals with a common acceptor. The reliability of this kind of estimation for the above mentioned substances is tested. In addition, by comparing the threshold values obtained for the D:A complexes with those of the neat donors, it is found (in confirmation of an earlier result) that complex formation with PMDA tends to increase the ionization threshold of the respective donor by 0.2–0.3 eV.

Vacuum–ultraviolet photoelectron spectroscopy of hexatriacontane (n-C36H74) polycrystal: A model compound of polyethylene

Photoelectron spectra of hexatriacontane (n-C36H74) films evaporated in situ under vacuum were measured at 12 photon energies between 11.4 eV and 21.2 eV. The escape depth of the photoelectrons was found to be much less than 10 nm. Although five valence band features on the photoelectron spectra are expected in this region from the findings by the XPS method, only one or two valence band features were observed; four features from the conduction band were found to dominate the spectra. The short escape depth and the smearing out of the valence band structure are attributed to scattering, which includes the scattering by phonons, and the appearance of the conduction band structure is attributed to accumulation of the secondary electrons to the high density of states parts associated with the conduction bands. The threshold energy of photoemission was found to be 9.1±0.5 eV. The finding for the evaporated films of polyethylene, despite some degree of decomposition, gave essentially the same result. The results for the conduction bands were compared with the theoretical band calculation on an isolated polyethylene chain. The lack of reliable theoretical calculations of the conduction band of solid polyethylene is pointed out.

Solvated electrons in hexamethylphosphoramide. Part 2.—Density measurements, electrical properties of frozen solutions, electron emission from surfaces of solutions

The dissolution of alkali metals in hexamethylphosphoramide (HMPA) causes a decrease in density and a value of approximately 80 cm3 for the apparent molal volume of the solvated electron has been deduced. The change of electrical conductance of alkali metal solutions in HMPA on freezing has been studied. Surprisingly, while the conductance of sodium solutions falls almost to zero on freezing, that of lithium solutions increases by a factor of about one hundred. The solid lithium solutions have a peak conductance at about –19°C; possible reasons for this behaviour are discussed. Thermionic- and photo-emission of electrons have also been measured. The dependence of thermionic current on concentration is unexpected; for both lithium and sodium solutions there is a pronounced maximum at concentrations of about 10–3 mol dm–3 and the current decreases to about 1/40 of the peak value at a concentration of 0.4 mol dm–3. The thermionic emission energy has been calculated from the temperature dependence of the thermionic current, and is about 0.74 eV at c∼ 10–3 mol dm–3, and 0.98 eV at c∼ 0.4 mol dm–3. The threshold energy for photo-emission calculated from the long wavelength limit for photo-emission from a 0.4 mol dm–3 solution is 1.05 eV.

Theory of Internal Photoemission

Fowler's hypothesis commonly used to interpret results of photoemission into the vacuum has been extended to deal with the problem of internal photoemission. This leads to the empirical relation $Y\ensuremath{\propto}{(E\ensuremath{-}{E}_{t})}^{2}$ between the quantum efficiency $Y$ and the photon energy $E$, currently used to interpret hole photoemission experiments. ${E}_{t}$ is the threshold energy for photoemission. Various cases of internal electron and hole photoemission between metals and semiconductors are discussed.

Photoemission of Electrons from Silicon into Silicon Dioxide. Effects of Ion Migration in the Oxide

Photoemission of electrons from silicon into SiO2 has been used as a tool to study changes at the silicon-SiO2 interface which result from the application of high electric fields at elevated temperature. Before treatment the photoemission threshold energy for electrons from silicon into SiO2 is 4.22 eV. After applying a dc voltage across the oxide at elevated temperature, with polarity such that the silicon is negative, there are significant changes in the photoemission spectrum. A new low-energy branch appears which gives a threshold energy around 3.05 eV. This is believed to be due to the formation of a degenerate n-type surface. Light absorption by electrons in the conduction band becomes important, and there is a corresponding contribution to the photoemission current. Semiquantitative arguments are advanced to show what the charge distribution in the oxide must be to account for the experimental results.