Secondary Electron Emission Spectra Research Articles

Nanoscale spectroscopic mapping by scanning probe electron energy spectroscopy

Scanning probe electron energy spectroscopy (SPEES) is a developing technique capable of both topographic and spectroscopic mapping of the surface. Here we report a SPEES study for Ag nanostructures on graphite with a microelectrode-shielding tip. The spatial distributions of electron energy loss spectra as well as secondary-electron emission spectra in the same region on the surface are measured. The spatial resolution is determined to be about 17 nm. This demonstrates the nanoscale spectroscopic mapping ability of the SPEES and a promising tool for investigating highly-localized object on surface and related phenomena, such as plasmonic substrates, nonlinear electron scattering, etc.

Revealing Localised Mechanochemistry of Biomaterials Using In Situ Multiscale Chemical Analysis.

The study of mechanical and chemical phenomena arising within a material that is being subjected to external stress is termed mechanochemistry (MC). Recent advances in MC have revealed the prospect not only to enable a greener route to chemical transformations but also to offer previously unobtainable opportunities in the production and screening of biomaterials. To date, the field of MC has been constrained by the inability of current characterisation techniques to provide essential localised multiscale chemically mapping information. A potential method to overcome this is secondary electron hyperspectral imaging (SEHI). SEHI is a multiscale material characterisation technique applied within a scanning electron microscope (SEM). Based on the collection of secondary electron (SE) emission spectra at low primary beam energies, SEHI is applicable to the chemical assessment of uncoated polymer surfaces. Here, we demonstrate that SEHI can provide in situ MC information using poly(glycerol sebacate)-methacrylate (PGS-M) as an example biomaterial of interest. This study brings the use of a bespoke in situ SEM holder together with the application of SEHI to provide, for the first time, enhanced biomaterial mechanochemical characterisation.

Angle-resolved secondary photoelectron emission from graphene interfaces

Angle-resolved secondary electron emission spectra were analyzed for studying the electron transmission properties of graphene when it is tightly bound to a metallic substrate and when it is decoupled from the latter. As respective model systems, graphene/Ni(111) and graphene/Au/Ni(111) were considered. We discovered two types of spectral structures linked with the transmission properties of graphene: bands related to scattering resonances and to umklapp reflection. These findings are interpreted based on an ab initio theory of the scattering of the outgoing electrons by the graphene overlayers. The experimental secondary electron emission maps are in good agreement with the calculations, indicating that such approach can be successfully applied for studying the electron transmission properties of a variety of two-dimensional materials.

Secondary electron emission and yield spectra of metals from Monte Carlo simulations and experiments

In this work, we present a computational method, based on the Monte Carlo statistical approach, for calculating electron energy emission and yield spectra of metals, such as copper, silver and gold. The calculation of these observables proceeds via the Mott theory with a Dirac–Hartree–Fock spherical potential to deal with the elastic scattering processes, and by using the Ritchie dielectric approach to model the electron inelastic scattering events. In the latter case, the dielectric function, which represents the starting point for the evaluation of the energy loss, is obtained from experimental reflection electron energy loss spectra. The generation of secondary electrons upon ionization of the samples is also implemented in the calculation. A remarkable agreement is obtained between both theoretical and experimental electron emission spectra and yield curves.

Ion-induced secondary electron emission from metal surfaces

Using a helium ion hitting various metal surfaces as a model system, we describe a general quantum-kinetic approach for calculating ion-induced secondary electron emission spectra at impact energies where the emission is driven by the internal potential energy of the ion. It is based on an effective model of the Anderson−Newns-type for the subset of electronic states of the ion−surface system most strongly affected by the collision. Central to our approach is a pseudo-particle representation for the electronic configurations of the projectile which enables us, by combining it with two additional auxiliary bosons, to describe in a single Hamiltonian emission channels involving electronic configurations with different internal potential energies. It is thus possible to treat Auger neutralization of the ion on an equal footing with Auger de-excitation of temporarily formed radicals and/or negative ions. From the Dyson equations for the projectile propagators and an approximate evaluation of the self-energies, rate equations are obtained for the probabilities with which the projectile configurations occur and an electron is emitted in the course of the collision. Encouraging numerical results, especially for the helium−tungsten system, indicate the potential of our approach.

Time-dependent first-principles study of angle-resolved secondary electron emission from atomic sheets

Angle-resolved secondary electron emission (ARSEE) spectra of atomic sheets have been considered to reflect the unoccupied energy bands of the targets. However, far less is known about the dynamics of electron excitation from the valence bands to the unoccupied bands upon electron impact, leading to emission into the vacuum. Here, the authors keep track of the electrons excited to the unoccupied bands and emitted to the vacuum in real time, position space, and k-space simultaneously by a time-dependent density-functional theory simulation.

Secondary Electron Emission of Pt: Experimental Study and Comparison With Models in the Multipactor Energy Range

Experimental data of secondary emission yield (SEY) and electron emission spectra of Pt under electron irradiation for normal incidence and primary energies lower than 1 keV are presented. Several relevant magnitudes, as total SEY, elastic backscattering probability, secondary emission spectrum, and backscattering coefficient, are given for different primary energies. These magnitudes are compared with theoretical or semiempirical formulas commonly used in the related literature.

Helium ion microscopy and energy selective scanning electron microscopy – two advanced microscopy techniques with complementary applications

Both scanning electron microscopes (SEM) and helium ion microscopes (HeIM) are based on the same principle of a charged particle beam scanning across the surface and generating secondary electrons (SEs) to form images. However, there is a pronounced difference in the energy spectra of the emitted secondary electrons emitted as result of electron or helium ion impact. We have previously presented evidence that this also translates to differences in the information depth through the analysis of dopant contrast in doped silicon structures in both SEM and HeIM. Here, it is now shown how secondary electron emission spectra (SES) and their relation to depth of origin of SE can be experimentally exploited through the use of energy filtering (EF) in low voltage SEM (LV-SEM) to access bulk information from surfaces covered by damage or contamination layers. From the current understanding of the SES in HeIM it is not expected that EF will be as effective in HeIM but an alternative that can be used for some materials to access bulk information is presented.

Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N2+ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N2+ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N2+ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of SixNy on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N2+ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.

FORMATION OF ONE-DIMENSIONAL Ga STRUCTURES ON Si(5 5 12)-2 × 1 TRENCHED TEMPLATES

In the advent of GaN photonic devices, there is renewed interest in looking at the growth of nanostructures of III-Nitrides that can yield novel properties. To grow a high density of self-assembled GaN nanostructures on a large scale, there is a need to use templates with well-defined nanometer scale features. The Si (5 5 12)-2 × 1 surface is a naturally faceted stable surface that has several nanotrenches of (225) and (337) with a large unit cell of 5.35 × 0.77 nm2. In this work we present here, the first report on the X-ray photoelectron spectroscopy study of the adsorption kinetics of Ga on Si (5 5 12)-2 × 1 reconstructed surface. The uptake curve which is the plot of the ratio of Ga (2 p ) to Si (2 p ) intensity with deposition time, shows that Ga grows in a Stranski–Krastnov growth mode on Si (5 5 12) surface. The core level spectra of Si (2 p ) and Ga (2 p ) have been deconvoluted to see the evolution of the interface during the adsorption process of Ga . The valence-band spectra show the modification of the electronic structure and the secondary electron emission spectra yield the change in work function during Ga adsorption. Correlating the electronic and structural aspects of the interface formation results the study demonstrates the possibility of integrating Si and group III-nitride technologies while exploiting the novel properties due to reduced dimensions.

Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.

Hole Injection Layer of Thermally Evaporated Copper Oxide for Top Emitting Organic Light Emitting Diodes

We report the enhancement of the electroluminescent property of top emitting organic light emitting diodes (TEOLEDs) using thermally evaporated copper oxide as a hole injection layer (HIL) between UV ozone-treated silver (Ag) anodes and -bis[-(1-naphthyl)--phenylamino]biphenyl . The operation voltage of TEOLEDs at decreased from 6.2 to 5.0 V as a 2 nm thick layer was used as the HIL. layers were separately in situ deposited on both Ag and thermally evaporated -coated Ag and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of is higher by 0.53 eV than that of Ag. The corresponding interface dipole energies were for Ag and for . As a result, plays a role in reducing the hole injection barrier from 1.63 to 0.96 eV, resulting in a decrease in the turn-on voltage of TEOLEDs.

Secondary electron emission spectra from clean and cesiated Al surfaces: the role ofplasmon decay and data analysis for applications

We report measurements of energy spectra of secondary electrons emitted from clean andcesiated aluminum surfaces under the impact of 130 eV electrons. Measurements show thatthe decay of bulk and surface plasmons dominates the electron emission. In contrast withtheoretical calculations, our experiments indicate that the electron collision cascade insidethe solid produced by electrons excited by plasmon decay do not contribute significantly toelectron emission.A simple analysis of electron energy distributions measured as a function of Cs surfacecoverage allows separation of rediffused incident electrons from the continuumbackground of true secondary electrons. The result shows that yields of rediffusedelectrons used in several applications may have been significantly overestimated.

Efficient Top-Emitting Organic Light Emitting Diode with Surface Modified Silver Anode

The enhancement of quantum efficiency using a suface modified Ag anode in top-emitting organic light emitting diodes (TEOLEDs) is reported. The operation voltage at the current density of 1 mA/cm 2 of TEOLEDs decreased from 9.3 V to 4.3 V as the surface of anode coated with CuOx layer. The work function of these structures were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of the Ag/CuOx structure is higher by 0.6 eV than that of Ag. Thus, the CuOx structure acts as a role in reducing the hole injection barrier by about 0.6 eV, resulting in a decrease of the turn-on voltage of top-emitting light emitting diodes.

Local secondary-electron emission spectra of graphite and gold surfaces obtained using theScanning Probe Energy Loss Spectrometer (SPELS)

Secondary-electron emission (SEE) spectra have been obtained with the ScanningProbe Energy Loss Spectrometer at a tip–sample distance of only 50 nm. Suchshort working distances are required for the best theoretical spatial resolution (<10 nm). The SEE spectra of graphite, obtained as a function of tip bias voltage, are shown tocorrespond to unoccupied states in the electronic band structure. The SEE spectra of thingold films demonstrate the capability of identifying (carbonaceous) surface contaminationwith this technique.

Simulation of secondary electron emission spectra from thin metal foils after proton impact

We have derived inelastic interaction cross sections for electrons and protons with Al, Cu, and Au within the framework of a plane-wave Born approximation and an optical data model. We have implemented a transport model for protons and electrons in Cu into the Monte Carlo track structure code PARTRAC. We have simulated secondary electron yields in Cu and compared them with measurements from the literature. We find that simulated yields in general follow the measurements but clearly overestimate them at energies below 50 eV.

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C–SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar + ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2–4 keV energy and fluence ranging from 3 × 10 13 ions/cm 2 to 1 × 10 18 ions/cm 2. Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N 2 + ions and 7.2 × 10 17 ions/cm 2 are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar + ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.

Effect of O2-Plasma Treatment of Mo on the Crystal Growth Mode of Pentacene of Organic Thin-Film Transistors

We report the change in the crystallinity of pentacene film on -plasma-treated molybdenum (Mo) and the electrical properties of pentacene thin-film transistors using Mo as source–drain electrodes. The surface energy of the Mo film was increased with -plasma treatment. Synchrotron radiation photoemission spectroscopy results revealed that the valence band split from the metallic Fermi level as the surface of the Mo was treated with plasma. This provides evidence that the surface of the Mo film changed to Mo oxide . Grazing-angle incidence X-ray diffraction results showed that pentacene molecules on -plasma-treated Mo has well-aligned features in the vertical direction as well as in the lateral direction, leading to a high crystalline film growth. The growth mode of pentacene changed from a tilted bulklike structure to a thin-film phase type by the treatment. This change in the growth mode is attributed to the reduction in interaction energy between pentacene and Mo via the formation of oxide at the Mo/pentacene interface. Secondary electron emission spectra showed that the work function increased by 0.55 eV after -plasma treatment on the Mo surface. Thus, the Mo oxides lowered the potential barrier for the hole injection from Mo to pentacene, increasing the drain current of bottom-contact organic thin-film transistors.

In Situ Analysis of Hole Injection Barrier of Molybdenum-Oxide-Coated Anode with Organic Materials Using Synchrotron Radiation Photoemission Spectroscopy

We report the enhancement of the electroluminescent property of organic light-emitting diodes (OLEDs) using as a hole injection layer between indium tin oxide (ITO) anodes and -bis[-(1-naphthyl)--phenyl-amino]biphenyl . The operation voltage for of OLEDs with a 10 Å thick layer decreased from 24.5 to 14.7 V, and the luminescence value at increased from 1280 to . was deposited in situ on -plasma-treated ITO anodes and anodes coated with an -plasma-treated layer , and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra after the deposition of on anodes revealed that the work function of is 0.55 eV higher than that of . The corresponding interface dipole energies were −0.20 eV for and −0.50 eV for . The hole injection barrier at the interface was reduced from 1.15 to 0.85 eV by the deposition of , resulting in an enhancement of the electroluminescence property of OLEDs.

Electrochemical properties of composite materials based on platinum modified with molybdenum compounds

The composite coating Pt-MoOx is produced by an electrochemical technique under potentiodynamic conditions on the surface of a preliminarily prepared electrode of glassy carbon. The inclusion of molybdenum into the composition of the obtained electrode deposit is confirmed by the data of cyclic voltammetry and the secondary-electron emission spectra. In the cyclic voltammograms that are obtained in a 2 M solution of sulfuric acid one can distinguish a pair of peaks at potentials equal to 0.46 V (anodic run) and 0.3 V (cathodic run), which are connected with the redox transitions experienced by molybdenum compounds. It is discovered that the obtained deposit possesses catalytic properties with respect to the oxygen reduction reaction. The number of electrons that are corresponding to the redox transitions experienced by molybdenum compounds is calculated. It amounts to 0.27 electrons per molybdenum atom.