Local Work Function Changes Research Articles

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy.

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

Comparison of thermionic filament and carbon nanotube field emitter-based electron ionization sources in cycloidal coded aperture mass analyzers

This work compares the coded aperture imaging performance of thermionic filament and carbon nanotube (CNT) field emitter-based electron sources in cycloidal-coded aperture mass spectrometers. The use of spatially coded apertures in mass spectrometry enables miniaturization by improving throughput without sacrificing resolution. CNT-based electron ionization sources for mass spectrometers provide several potential benefits over conventional thermionic emitters, including low voltage and low power consumption, room temperature operation, long lifetime, and ability to emit electrons in a pulsed mode. However, spatiotemporal variation in electron emission from CNTs is a major disadvantage. In this study, electron emission stability and spatiotemporal stability of the coded aperture image were compared for coded aperture cycloidal mass analyzers with either a CNT-based ion source or a thermionic filament-based ion source. We found that the thermionic filament-based ion source produced a significantly more stable coded aperture image than the CNT based ion source. The aperture image fluctuations in the CNT-based source are likely a result of adsorption and desorption of molecules on the CNT surface that cause local work function changes and induce spatiotemporal variation in electron emission and subsequent ion generation.

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance.

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno[3,2-b]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do. In both cases, the consequence of depositing fluorinated fullerene (C60F48) as a molecular p-dopant is the formation of C60F48 crystalline islands decorating the step edges of the underlying semiconductor film surface. We demonstrate that local work function changes along with a peculiar nanomorphology lead to the double beneficial effect of lowering the contact resistance and providing long-term and enhanced thermal stability of the devices.

Probing site-dependent decoupling of hexagonal boron nitride with molecular frontier orbitals

Hexagonal boron nitride (h-BN) was grown on Pt(111) and Ru(0001) to serve as a buffer layer for molecular adsorbates. Hydrocarbon lander molecule C64H36 does not exhibit preference for specific h-BN adsorption sites on Pt(111), while on Ru(0001), wire and pore sites of the two-dimensional mesh are favored. The spectroscopic signatures of C64H36 frontier orbitals show a strong dependence on the adsorption site. For h-BN on Pt(111), C64H36 frontier orbital energies exhibit a common shift that leaves the gap between the orbitals invariant and reflects local work function changes of the h-BN lattice the molecule is weakly coupled to. In contrast, h-BN on Ru(0001) leads to a nonuniform behavior of the frontier orbital energies, which is tentatively attributed to additional charge transfer processes between the molecule and the surface.

Field-Emission Resonances on Graphene on Insulators

Field-emission resonances (FERs) can be used to map the work function of a conducting surface with a high spatial resolution. While FERs have been measured on different graphene systems, measurements on graphene transferred on insulating substrates have not been carried out previously. Here we have measured FERs on graphene transferred on hexagonal boron nitride (h-BN) and silicon dioxide (SiO2) substrates. We have also developed a simple 1D model that can be used to directly interpret the constant current spectroscopy experiments. On both surfaces, FERs can be used to map the local work function changes.

Correction: Corrigendum: Assessment of Scanning Tunneling Spectroscopy Modes Inspecting Electron Confinement in Surface-Confined Supramolecular Networks

Scanning tunneling spectroscopy (STS) enables the local, energy-resolved investigation of a samples surface density of states (DOS) by measuring the differential conductance (dI/dV) being approximately proportional to the DOS. It is popular to examine the electronic structure of elementary samples by acquiring dI/dV maps under constant current conditions. Here we demonstrate the intricacy of STS mapping of samples exhibiting a strong corrugation originating from electronic density and local work function changes. The confinement of the Ag(111) surface state by a porous organic network is studied with maps obtained under constant-current (CC) as well as open-feedback-loop (OFL) conditions. We show how the CC maps deviate markedly from the physically more meaningful OFL maps. By applying a renormalization procedure to the OFL data we can mimic the spurious effects of the CC mode and thereby rationalize the physical effects evoking the artefacts in the CC maps.

Backscattered electron imaging at low emerging angles: A physical approach to contrast in LVSEM

Due to the influence of refraction effects on the escape probability of the Back-Scattered Electrons (BSE), an expression of the fraction of these BSE is given as a function of the beam energy, E°, and emission angle (with respect to the normal) α. It has been shown that these effects are very sensitive to a local change of the work function in particular for low emerging angles. This sensitivity suggests a new type of contrast in Low Voltage Scanning Electron Microscopy (LVSEM for E°<2 keV): the work function contrast. Involving the change of φ with crystalline orientation, this possibility is supported by a new interpretation of a few published images. Some other correlated contrasts are also suggested. These are topographical contrasts or contrasts due to subsurface particles and cracks. Practical considerations of the detection system and its optimization are indicated.

Monitoring surface ion mobility on aluminum oxide: Effect of chemical pretreatments

Despite of its crucial role in many electrochemical processes, a clear surface chemistry-correlated understanding of interfacial ion mobility is missing. This study presents an improved application of a previously introduced method making use of the scanning Kelvin probe (SKP) technique on aluminum oxide surfaces. The local work function changes, induced by the displacement of the mobile surface ions in an electric field applied across the surface, are monitored as a function of surface chemistry modified by different surface pretreatments and ambient atmosphere. Complementary quartz crystal microbalance (QCM) and surface current measurements reveal how the modified surface chemistry affects the thickness of the adsorbed water layer and the surface conductivity, respectively. Besides the usefulness of the SKP technique for mobility studies, we demonstrate a selective control of the superficial ion mobility by engineering the surface fixed charges.

Assessment of Scanning Tunneling Spectroscopy Modes Inspecting Electron Confinement in Surface-Confined Supramolecular Networks

Scanning tunneling spectroscopy (STS) enables the local, energy-resolved investigation of a samples surface density of states (DOS) by measuring the differential conductance (dI/dV) being approximately proportional to the DOS. It is popular to examine the electronic structure of elementary samples by acquiring dI/dV maps under constant current conditions. Here we demonstrate the intricacy of STS mapping of samples exhibiting a strong corrugation originating from electronic density and local work function changes. The confinement of the Ag(111) surface state by a porous organic network is studied with maps obtained under constant-current (CC) as well as open-feedback-loop (OFL) conditions. We show how the CC maps deviate markedly from the physically more meaningful OFL maps. By applying a renormalization procedure to the OFL data we can mimic the spurious effects of the CC mode and thereby rationalize the physical effects evoking the artefacts in the CC maps.

Mixed self-assembled monolayers of azobenzene photoswitches with trifluoromethyl and cyano end groups

Mixed self-assembled monolayers (SAMs) of alkanethiolates carrying azobenzene chromophores with either a trifluoromethyl or a cyano substituent have been studied. High-resolution x-ray photoelectron spectroscopy proves that the ratio of adsorbed molecules can be arbitrarily adjusted via the molar fractions in solution. As a function of these molar fractions core level shifts are observed which are attributed to local work-function changes. By simulating the electric dipole field distribution, the continuous core level shifts are ascribed to a homogeneous mixture of molecules with different end groups adsorbed on adjacent lattice sites. Near-edge x-ray absorption fine structure measurements reveal formation of well-ordered SAMs. Despite the difference in dipole moment of the end groups, the molecular tilt and twist angles are identical for both single-component SAMs and a 1:1 mixed SAM.

Catalysis on microstructured bimetallic surfaces.

Microstructured bimetallic Pt/Rh and Pt/Ti surfaces have been employed to study the dynamics of catalytic NO reduction and the O(2)+H(2) reaction at low pressure (p<10(-3) mbar). Photoelectron emission microscopy and scanning photoelectron microscopy were used as spatially resolved in situ methods to image the local work function changes and to identify chemical changes in the substrate and in the adsorbate layer. It is shown that diffusional coupling leads to dynamic effects which are dependent on the macroscopic size (&mgr;m range). With alkali metals on the surface, stationary patterns form whose mechanism of formation has been studied in detail. (c) 2002 American Institute of Physics.

Pattern formation during the CO-oxidation involving subsurface oxygen.

This paper focuses on subsurface oxygen and its influence on pattern formation during CO-oxidation on platinum surfaces. For the observation of spatiotemporal pattern formation during catalytic reactions the photoelectron emission microscope (PEEM) has proven to be an excellent real-time imaging instrument, capable of tracking local work function changes. The existence of subsurface oxygen on platinumlike surfaces has been extensively discussed and for palladium its presence has been clearly established during rate oscillations. Subsurface oxygen is defined at this point as an atomic O species located directly underneath the uppermost metal crystal layer; its dipole moment therefore considerably lowers the work function of the surface. Here we review some of the investigations involving subsurface oxygen, focusing on the role subsurface oxygen might play in pattern formation during CO-oxidation on platinum. We will also present some new results, where this species clearly interacts with chemisorbed oxygen under restrictions by boundary conditions on the Pt(110) single crystal. These previously (through microlithography) constructed domain boundaries on the surface are made out of Rh or Pd, and they are acting as an additional source of CO molecules for the Pt surface. (c) 2002 American Institute of Physics.

Toggling the local surface work function by pinning individual promoter atoms

Atom by atom control over the local work function of a surface has been performed by using the tip of a scanning tunneling microscope to move and pin individual promoter atoms at predetermined atomic sites on the surface. Both positive and negative local work-function changes could be achieved, depending on the exact binding configuration of the pinned promoter atoms. This manipulation of individual promoter atoms has been exploited for testing the fabrication of atomic-scale chemical ``seeds.''

Local work-function changes of Pt(111) studied by STM and IRAS: coadsorption of Cl − with H 3O +, NO, and CO molecules

The coadsorption of chloride anion (Cl −) with hydronium cation (H 3O +), nitrogen monoxide (NO), and carbon monoxide (CO) on Pt(111) was studied in an ultra-high-vacuum system using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and low-energy electron diffraction (LEED). HCl molecules adsorbed on Pt(111) at 100 K form a (3×3) structure ( θ Cl − =0.44). Water adsorption on the 3×3 structure produces c(4×2)-(Cl −+H 3O +) coadsorption structures. The hydronium cation adsorbs through oxygen lone pair, and hydrogen bonding (OH…Cl) extends on the surface with these structures. Stretching absorption bands of NO (or CO) adsorbed on the 3×3-Cl − and c(4×2)-(Cl −+H 3O +) adlayers on Pt(111) show remarkably higher and lower frequency shifts, respectively. The frequency shifts can be explained by the local charge density states of platinum atoms derived from electron withdrawal or supply from the coadsorbates (Cl − or H 3O +) to platinum atoms.

Peculiarities of imaging one- and two-dimensional structures in an emission electron microscope. 1. theory

Local changes in work function cause deviations of the electrical microfield near a sample surface as a result of the uniform accelerating field distribution between the sample (cathode) and the extractor electrode (anode). This results in a change in the electron trajectories. As a consequence, the microscope image shows remarkable changes in position, size, intensity and lateral resolution of distinct details, which can be quantitatively described by the calculations presented here. Analysing these effects in the image gives an opportunity to determine the real lateral size of the observed structures and the distribution of local contact potentials.

Threshold Energy Effects in Secondary Electron Emission

Abstract In large bandgap semiconductors and insulators, the threshold energies for e–h pair production and ionization damage can lie above the vacuum level. For low energy imaging, a window is then opened whose width is potentially sensitive to local changes in work function, doping level, or acidity. Recent progress and future opportunities for damage-free imaging of these properties using low energy electrons are discussed in the light of the underlying physics, as well as of recent instrumental developments in low energy electron microscopy (LEEM), environmental scanning electron microscopy (ESEM), photoelectron emission microscopy (PEEM), scanned probe microscopy (SPM), and projection electron microscopy.

Threshold Energy Effects in Secondary Electron Emission

AbstractIn large bandgap semiconductors and insulators, the threshold energies for e–h pair production and ionization damage can lie above the vacuum level. For low energy imaging, a window is then opened whose width is potentially sensitive to local changes in work function, doping level, or acidity. Recent progress and future opportunities for damage-free imaging of these properties using low energy electrons are discussed in the light of the underlying physics, as well as of recent instrumental developments in low energy electron microscopy (LEEM), environmental scanning electron microscopy (ESEM), photoelectron emission microscopy (PEEM), scanned probe microscopy (SPM), and projection electron microscopy.

Threshold Energy Effects in Secondary Electron Emission

The work function ϕ, the bandgap Eg, the threshold energy level Et, for the inelastic scattering of excited electrons and the threshold energy transfer Ed for the onset of structural ionisation damage are clearly of major significance in various actively developing forms of hot carrier imaging. Two exciting examples here are the ability to image small and dynamic local changes in work function by PEEM and as well as mapping the variations in electronic structure between p and n-type regions of a semiconductor by SE imaging in the SEM. More recently still there have been indications that the SE signal in the ESEM might be sensitive to local changes in bandgap and suggestions that it might be even possible to image spatial variations in pH. It is increasingly clear that if these attractive opportunities are to be efficiently explored and developed, systematic and preferably quantitative observations are needed. Such work requires specimens whose atomic and electronic structure is either fully known beforehand or can be deduced from other signals available in a situation where the physical processes in the microscope are well understood.

Interaction of hydrogen with ultrathin titanium layers on tungsten

Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiH x at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H 2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H 2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β + adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.

Mirror electron microscopy applied to the continuous local measurement of work-function variations

In surface studies, the mirror electron microscope (MEM) can give a picture of electrical and geometrical roughness and simultaneously a continuous measurement of local work-function changes Delta phi ; however, no critical study has been done on the accuracy of such a measurement. Using the MEM, the authors developed a new method to obtain the total electron reflection coefficient R in the very low energy range which allows them to determine accuracy and exhibit some correlations between Delta phi measurements and R variations near zero energy. By using a local deconvolution treatment for our experimental results on O/W (100), O and CO/Ni(111) and O and CO/Cu(100) the authors determined the injection threshold for monokinetic electrons. The results lead the authors to present a new sequential operating method giving R which is of great interest in the low energy range and a Delta phi measurement with controlled accuracy.