Secondary electron emission and photoemission from a negative electron affinity semiconductor with large mean escape depth of excited electrons

The formulae for parameters of a negative electron affinity semiconductor(NEAS)with large mean escape depth of secondary electrons A(NEASLD)are deduced.The methods for obtaining parameters such asλ,B,E_(pom)and the maximumδandδat 100.0 keV≥E_(po)≥1.0 keV of a NEASLD with the deduced formulae are presented(B is the probability that an internal secondary electron escapes into the vacuum upon reaching the emission surface of the emitter,δis the secondary electron yield,E_(po)is the incident energy of primary electrons and E_(pom)is the E_(po)corresponding to the maximumδ).The parameters obtained here are analyzed,and it can be concluded that several parameters of NEASLDs obtained by the methods presented here agree with those obtained by other authors.The relation between the secondary electron emission and photoemission from a NEAS with large mean escape depth of excited electrons is investigated,and it is concluded that the presented method of obtaining A is more accurate than that of obtaining the corresponding parameter for a NEAS with largeλ_(ph)(λ_(ph)being the mean escape depth of photoelectrons),and that the presented method of calculating B at E_(po)>10.0 keV is more widely applicable for obtaining the corresponding parameters for a NEAS with largeλ_(ph).

DFT Calculations of the Ionization Potential and Electron Affinity of Alaninamide

Adiabatic and vertical ionization potentials （IPs） mad valence electron affmities （EAs） of alaninamide in gas phase have been detennined using density functional theory （BLYP, B3LYP, B3P86） methods with 6-311＋＋G（d, p） basis set, respectively. IPs and EAs of alaninamide in solutions have been calculated at the B3LYP/6-311＋＋G（d, p） level. Five possible conformers of alaninamide and their charged states have been optimized employing density functional theory B3LYP method with 6-311＋＋（d, p） basis set, respectively.

Observation of positive and small electron affinity of Si-doped AlN films grown by metalorganic chemical vapor deposition on n-type 6H–SiC

We have investigated the electron affinity of Si-doped AlN films（N_（Si）= 1.0 × 10^（18）–1.0 × 10_（19）cm^（-3）） with thicknesses of 50, 200, and 400 nm, synthesized by metalorganic chemical vapor deposition（MOCVD） under low pressure on the ntype（001）6H–SiC substrates. The positive and small electron affinity of AlN films was observed through the ultraviolet photoelectron spectroscopy（UPS） analysis, where an increase in electron affinity appears with the thickness of AlN films increasing, i.e., 0.36 eV for the 50-nm-thick one, 0.58 eV for the 200-nm-thick one, and 0.97 e V for the 400-nm-thick one.Accompanying the x-ray photoelectron spectroscopy（XPS） analysis on the surface contaminations, it suggests that the difference of electron affinity between our three samples may result from the discrepancy of surface impurity contaminations.

Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoe- mission behaviour, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes have been investigated using a multi-information measurement system. The activation exper- iment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs-O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.

Accurate electron affinity of atomic cerium and excited states of its anion

Electron affinities(EA)of most lanthanide elements still remain unknown owing to their relatively lower EA values and the fairly complicated electronic structures.In the present work,we report the high-resolution photoelectron spectra of atomic cerium anion Ce−using the slow electron velocity-map imaging method in combination with a cold ion trap.The electron affinity of Ce is determined to be 4840.62(21)cm^-1 or 0.600160(26)eV.Moreover,several excited states of Ce(^4H9/2,^4I9/2,^2H9/2,^2G9/2,^2G7/2,^4H13/2,^2F5/2,and ^4I13/2)are observed.

Work Function and Electron Affinity of Semiconductors:Doping Effect and Complication due to Fermi Level Pinning

Semiconductors are a major category of functional materials essential to various applications to sustain the modern society.Most applied materials or devices utilizing semiconductors are enabled by interfaces or junctions,such as solar cells,electronic/photonic devices,environmental sensors,and redox hetero-catalysts.Herein,the author provides a critical commentary on photoemission measurement of the work function and,more importantly,the electron affinity of semiconductors essential for energy band diagram of heterojunctions.Particular effort is made towards addressing complications associated with Fermi level pinning due to surficial states of doped semiconductors.

PHOTOEMISSION STABILITY OF MULTIALKALI PHOTOCATHODES WITH NEGATIVE ELECTRON AFFINITY

In this paper a negative electron affinity （NEA） multialkali photocathode of （Na2KSb-Cs）-O-Cs structure is fabricated by new technology. It is found that its emission stability is much better than that of the NEA GaAs photocathode, but is inferior to that of the conventional Na2KSb（Cs）. After 70 hour performance in a pumping vacuum system, the emission sensitivity of the NEA （Na2KSb-Cs）-O-Cs photocathode drops only by 2.5%. The emission stability is closely related to the states of the activation cesium and oxygen during activation, best results being obtained with cesium ions and excited oxygen. Furthermore, better photoemission sensitivity and emission stability may be obtained if the cathode is illuminated by intense white light during the activation process. The performance of the NEA （Na2KSb-Cs）-O-Cs cathode which has not been illuminated by intense white light during activation may be improved by the illumination even during operation intermission.

Theoretical Studies on Structures and Electron Affinities of 5-and 6-Halouracils

The molecular structures and electron affinities of 5- and 6-halouracils（XU, X=F, Cl, Br） were determined by means of five different density functional methods. The basis set used in this work is of double-（plus polariza- tion（DZP） quality with additional diffuse s- and p-type functions, denoted DZP＋＋. The geometries were fully optimized with each density functional theory（DFT） method, and discussed. Three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity（AEA）, the vertical electron affinity（VEA）, and the vertical detachment energy（VDE）. An excellent agreement between our B3LYP results and those obtained by other calculations regarding the structural modifications and electron affinities of neutral and anion 5XU was found. The differences in charge distribution between uracil and halogenated uracils were indicated, thus the ability to form the hydrogen bonds of halogenated uracils was discussed. It shows the applicability of the DFT/DZP＋＋ method to predict the similar molecular models at a highly economical computational cost.

Formula for average energy required to produce a secondary electron in an insulator

Based on a simple classical model specifying that the primary electrons interact with the electrons of a lattice through the Coulomb force and a conclusion that the lattice scattering can be ignored, the formula for the average energy required to produce a secondary electron （ε） is obtained. On the basis of the energy band of an insulator and the formula for e, the formula for the average energy required to produce a secondary electron in an insulator （εi） is deduced as a function of the width of the forbidden band （Eg） and electron affinity X. Experimental values and the εi values calculated with the formula are compared, and the results validate the theory that explains the relationships among Eg, X, and ei and suggest that the formula for εi is universal on the condition that the primary electrons at any energy hit the insulator.

Study of Structures and Electronic Properties of Pd_(n–1)Pb and Pd_n (n≤8) Clusters

Geometric and electronic properties of Pdn–1Pb and Pdn （n≤8） clusters have been studied by using density functional theory with effective core potentials, focusing on the differences between mono- and bimetallic clusters. The average bond length of Pdn–1Pb （n≤8） bimetallic clusters is longer than that of pure palladium clusters except for n = 2 and 3. The most stable structure of Pdn–1Pb （n≤7） is the singlet where there is at least a Pd or Pb atom on its excited state. The energy gaps of Pd–Pb binary clusters are narrower than those of Pdn clusters, and then the chemical activity is strengthened when Pdn clusters are doped with Pb.

Structures,Electron Affinities and Vibrational Frequencies of SF_5OX/SF_5OX^-(X=F,Cl,Br)

The molecular structures,electron affinities,vibrational frequencies and IR intensities of the SF5OX/SF5OX-（X=F,Cl,Br）species have been determined by four different density functional theory（DFT）methods（B3LYP,BHLYP,BP86,BLYP）in conjunction with DZP＋＋ basis set.The BHLYP method predicts the best geometry parameters.The adiabatic electron affinities（EAad）predicted by the B3LYP/DZP＋＋ method are 4.36 eV（SF5OF）,4.13 eV（SF5OCl）,4.12 eV（SF5OBr）,respectively.Large electron affinity implies the corresponding anions are stable.The total intensities in the near IR ＂window＂ of the anions SF5OX-（X=F,Cl,Br）at B3LYP level are 1 602 km/mol（SF5OF-）,1 868 km/mol（SF5OCl-）and 1 916 km/mol（SF5OBr-）,respectively,larger than those of the corresponding neutrals.It suggests that SF5OX/SF5OX-（X=F,Cl,Br）may be used to ＂warm＂ Mars.

Ionization Potentials and Electron Affinities of Cu_n Atomic Clusters

Ionization potentials and electron affinities of Cux (n = 2-7) atomic clusters with the optimal geom etries have been calculated by use of SC F-Xa-SW method and Slater's transition state theory. Theo retical calcuIations show that the ionization potentiaIs and electron affinities of Cu. (n = 2-7) atom ic clusters have a sharp even / odd alternation with increasing their sizes, which are related to the electronic structure of Cun atomic clusters. The theoretical results are consistent with the related ex perimental ones.

DFT Study of Electron Affinities of o-, m-,p-Halobenzyl Radicals

The electron affinities of the isomer XC6H4CH2/ XC6H4CH2- (X=F, Cl, Br) species have been determined using seven density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. Three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The most reliable adiabatic electron affinities are obtained at the DZP++ BPW91, BP86, and B3LYP level of theory. The BPW91 methods are the closest to the experiment values; The BHLYP method predicts the smallest EAad and B3P86 method predicts the largest EAad, which are the worst reliable methods. In addition, for a given halogen substituent, the meta isomer has the largest electron affinity and the para isomer has the smallest.

Prediction of the Atomic Electron Affinities Using a Wavelet Neural Network

Wavelet Neural Network (WNN) was employed in a rapid calculation of the atomic electron affinities, with atomic ionization potentials and atomic electron configurations chosen as input parameters. Many unmeasured values were obtained, and the estimates appeared to be sufficiently accurate for potential applications.

Air-stable ambipolar organic field effect transistors with heterojunction of pentacene and N,N'-bis(4-trifluoromethylben-zyl) perylene-3,4,9,10-tetracarboxylic diimide

Fabrication of ambipolar organic field-effect transistors （OFETs） is essential for the achievement of an organic complementary logic circuit. Ambipolar transports in OFETs with heterojunction structures are realized.We select pentacene as a P-type material and N,N＇-bis（4-trifluoromethylben-zyl）perylene-3,4,9,10-tetracarboxylic diimide （PTCDI-TFB） as a n-type material in the active layer of the OFETs.The field-effect transistor shows highly air-stable ambipolar characteristics with a field-effect hole mobility of 0.18 cm^2/（V·s） and field-effect electron mobility of 0.031 cm^2/（V·s）.Furthermore the mobility only slightly decreases after being exposed to air and remains stable even for exposure to air for more than 60 days.The high electron affinity of PTCDI-TFB and the octadecyltrichlorosilane （OTS） self-assembly monolayer between the SiO2 gate dielectric and the organic active layer result in the observed air-stable characteristics of OFETs with high mobility.The results demonstrate that using the OTS as a modified gate insulator layer and using high electron affinity semiconductor materials are two effective methods to fabricate OFETs with air-stable characteristics and high mobility.

Energy level engineering of charge selective contact and halide perovskite by modulating band offset:Mechanistic insights

Mixed cation and anion based perovskites solar cells exhibited enhanced stability under outdoor conditions,however,it yielded limited power conversion efficiency when TiO_(2) and Spiro-OMeTAD were employed as electron and hole transport layer(ETL/HTL)respectively.The inevitable interfacial recombination of charge carriers at ETL/perovskite and perovskite/HTL interface diminished the efficiency in planar(n-i-p)perovskite solar cells.By employing computational approach for uni-dimensional device simulator,the effect of band offset on charge recombination at both interfaces was investigated.We noted that it acquired cliff structure when the conduction band minimum of the ETL was lower than that of the perovskite,and thus maximized interfacial recombination.However,if the conduction band minimum of ETL is higher than perovskite,a spike structure is formed,which improve the performance of solar cell.An optimum value of conduction band offset allows to reach performance of 25.21%,with an open circuit voltage(VOC)of 1231 mV,a current density JSC of 24.57 mA/cm^(2) and a fill factor of 83.28%.Additionally,we found that beyond the optimum offset value,large spike structure could decrease the performance.With an optimized energy level of Spiro-OMeTAD and the thickness of mixed-perovskite layer performance of 26.56% can be attained.Our results demonstrate a detailed understanding about the energy level tuning between the charge selective layers and perovskite and how the improvement in PV performance can be achieved by adjusting the energy level offset.

Thin film micro-scaled cold cathode structures of undoped and Si-doped AlN grown on SiC substrate with low turn-on voltage

The field emission characteristics of the AlN thin films with micro-scaled cold cathode structures are tested in the high vacuum system. The aluminum nitride （A1N） thin films with a thickness of about 100 nm are prepared on the n-type 6H-SiC （0001） substrate at 1100℃ by metal organic chemical vapor deposition （MOCVD） under low pressure. The I-V curves and surface micro-images of undoped and Si-doped AlN films are investigated. From the I-V and Fowler-Nordheim plots, it can be seen that the Si-doped AlN shows better field emission characteristics compared with the undoped AlN sample. The obtained turn-on field is 6.7 V/μm and the maximum emission current density is 154 mA/cm2 at 69.3 V for the Si- doped AlN film cathode after proper surface treatment. It is proposed that the relatively low electric resistivity of Si-doped AlN films is significant for electron migration to the surface region, and their rougher surface morphology is beneficial to a higher local electric field enhancement for the field emission.

Influence of cesium on the stability of a GaAs photocathode

The stability of a reflection-mode GaAs photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature. We observe the photocurrent of the cathode decaying with time in the vacuum system under the action of Cs current, and find that the Cs atoms residing in the vacuum system are helpful in prolonging the life of the cathode. We examine the evolution and analyse the influence of the barrier on the spectral response of the cathode. Our results support the double dipolar mode] for the explanation of the negative electron affinity effect.

PROCESS AND PROPERTIES OF THE MODERN MULTIALKALI PHOTOCATHODE USED IN A CASCADE IMAGE INTENSIFIER

A new processing technology and the properties of multialkali antimonide photocathodesare described.The technique requires an initial antimony layer to be deposited on a fiber-optic window at asuitable temperature.After that.the layer is activated by multialkali and antimonide.Thus.the multialkaliphotocathodes named“The Modern Multialkali Photocathode”are obtained.It has been found that aphotocathode thus processed gives a sensitivity of 200 to 300μA/Im for white light at a color temperature of2854K.This paper shows the variation of photosensitivity during processing of a Modern MultialkaliPhotocathode and the variatioon of spectral sensitivity at different wavelengths.The author discusses theresults.gives a supposition of“photoemission center”,and discusses the role of Cs in multialkaliphotocathodes.

Theoretical Calculations on Amphoteric Adsorptions of the Hydrogenated C(100)-2×1 Surface

In this work we study the adsorptions of some small molecules or group on the hydrogenated C（100）-2×1 surface using density functional theory method. The calculated results show that the ionization potential （IP） of the hydrogenated C（100）-2×1 surfaces after adsorption has amphoteric characteristics. From the weak basic NH3 molecule with small IP and negative electron affinity （EA）, through the neutral H2O molecule, to the weak acid HF molecule and the OH group with large EA and IP, the IP values of the adsorbed diamond surfaces vary from decrease, through invariability, to slight increase for HF and obvious increase for OH. In all adsorption species, only the OH group makes the hydrogenated C（100）-2×1 surface change to the metal from the semiconductor with a wide-band gap, while the others only introduce impurity states into the electronic structures of the hydrogenated C（100）-2×1 surfaces.