Contrast In STM Images Research Articles

Probing Magnetic Exchange Interactions with Helium.

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Computational insight into the origin of unexpected contrast in chiral markers as revealed by STM.

Internal substituents can serve the double purpose of generating stereogenic centers and (potentially) being identifiable with Scanning Tunneling Microscopy (STM) in 2D self-assembled molecular layers. We investigate computationally the origin of stark contrast variations in STM images of chirally substituted self-assembled organic films. STM images of alkyl derivatives with secondary -CH3 and -OH groups have been simulated. Density functional theory calculations reveal bias-dependent contrast reversals in the substituent regions: a lack of local density of states in the relevant energy regime results in 'dark spots' in the simulated STM images, which turn bright upon increasing the bias voltage.

Topological and Electron-Transfer Properties of Glucose Oxidase Adsorbed on Highly Oriented Pyrolytic Graphite Electrodes

The topological and electron-transfer properties of the redox enzyme glucose oxidase (GOD) adsorbed on highly oriented pyrolytic graphite (HOPG) were studied at a single molecule level with in situ electrochemical scanning tunneling microscopy (EC-STM). In situ STM and cyclic voltammetry were performed to understand the potential-dependent tunneling process in the presence of adsorbed GOD. Potential-dependent apparent changes in the GOD morphology on HOPG were observed in STM images and are discussed. For example, an issue in the STM scanning of adsorbed GOD was the influence of EC currents on the measured tip current. These Faradaic currents might be due to involved redox species that react at the enzyme-modified electrode surface and subsequently diffuse to the STM tip. Such effects of local enzyme activity on the contrast in STM images have been studied and are evaluated within a proposed model.

Dynamics of Porphyrin Electron‐Transfer Reactions at the Electrode–Electrolyte Interface at the Molecular Level

Electron transfer is an essential step in many important processes and has been the subject of intensive research, with relevance to areas such as molecular electronics, electrochemistry, biology, catalysis, information storage, and solarenergy conversion. Therefore, understanding interfacial electron transfer between molecules and electrodes, and their dynamics at the molecular level, is important for fundamental science as well as for technological applications. However, the present theoretical and experimental treatment of interfacial electron transfer between molecules and electrodes mainly relies on ensemble-averaged optical spectroscopic and electrochemical measurements. Thus, our understanding of the interfacial electron transfer dynamics at the molecular level is very limited. For example, we do not know how local defects or adsorption sites on a heterogeneous electrode surface affect the dynamics of redox reactions at the molecular level. Nevertheless, it is frequently assumed that reactions principally occur at steps or defects (“active sites”). On a homogeneous surface, is the redox reaction of adsorbed molecules homogeneous? How does the electrode potential affect the spatial distribution of an interfacial redox reaction at the molecular level? How is charge transferred laterally between molecules? Answers to these questions are critical for progress in our understanding. Therefore, there is a tremendous need to probe the dynamics of interfacial electron transfer at the molecular level. In the experiments reported here, a potential-pulse perturbation was employed to control the electrochemical oxidation of a simple porphyrin (5,10,15,20-tetra(4-pyridyl)21H,23H-porphine (TPyP); Scheme 1) at the Au(111)/0.1m H2SO4 interface, and scanning tunneling microscopy was employed to provide an insight into the electrochemical oxidation dynamics at the molecular level. TPyP on Au(111) was chosen as a model system for the following reasons: 1) The TPyP molecules can form an ordered monolayer at the Au(111)/0.1m H2SO4 interface, depending on the electrode potential. 2) The redox states of adsorbed porphyrins on an electrode surface can be distinguished by their different contrast in STM images. 3) The adsorption of TPyP onto the Au(111) electrode has a dramatic effect its electrochemical activity. For example, the reduction of pre-adsorbed oxidized TPyP can be very slow, taking as long as tens of minutes at 0.05 V. However, the oxidation of adsorbed TPyP at 0.2 V is much faster, occurring in seconds. (The potential for the onset of oxidation of adsorbed TPyP is about 0.1 V.)

The bonding geometry of alkylsilanes on gold: Relation to surface pattern development and STM image contrast

The adsorption of methylsilane (MeSiH 3) on Au(1 1 1) was investigated using density functional theory (DFT) within a generalized gradient approximation (GGA). Two preferred chemisorption sites are identified: the hollow site and an atop site with an ejected gold substrate atom. Both of these configurations result in a tetracoordinate Si with a distorted tetrahedral geometry. The energy of adsorption is calculated allowing an analysis of the preferred binding geometry as a function of coverage. The relation of the supermolecular length scale pattern that emerges from a spinodal decomposition of the two phases arising from the two binding geometries is discussed. The pattern observed in the STM images is shown to be consistent with the local density of states calculated for the two different binding geometries.

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff–Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.

Origin of contrast in STM images of oxygen on Pd(111) and its dependence on tip structure and tunneling parameters

The dependence of the contrast and symmetry of scanning tunneling microscope images of $\mathrm{O}∕\mathrm{Pd}(111)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}2$ on the structure of the tunneling tip and on tunneling parameters is explained using first-principles density functional theory. Experimentally, the contrast changes in different ways when a metal-terminated tip over hcp and top sites changes its bias and tip-sample distance. These changes are also reflected in the symmetry of the image. A detailed analysis of the tunneling contributions indicates that for the metallic tips, the $\mathrm{Pd}\phantom{\rule{0.2em}{0ex}}d$ orbitals are determining the image symmetry at close range and low bias, while at larger separations and high bias the $\mathrm{Pd}\phantom{\rule{0.2em}{0ex}}{p}_{z}$ orbitals are the ones that control the image contrast. For oxygen-terminated tips, we predict a positive image contrast, associated with the tip oxygen bonds, as opposed to the negative contrast images obtained with metallic tips.

Electronic versus geometric contrast in cross-sectional STM images of III-V semiconductor heterostructures

We report first-principles calculations of the electronic and geometric structure of the (110) cross-sectional surfaces on InAs/GaSb superlattices, and compare the results with scanning tunneling microscopy images of filled electronic states. In both the predicted and measured images the InAs surfaces appear lower than GaSb, a height difference we show is caused primarily by differences in the electronic structure of the two materials. In contrast, local variations in the apparent height of surface atoms at InSb- or GaAs-like interfaces arise primarily from geometric distortions associated with local differences in bond length. Our calculated energies for atomic intermixing indicate that anion exchanges are exothermic at GaAs interfaces but endothermic at InSb interfaces. This difference may explain why GaAs interfaces are typically more disordered than InSb interfaces in these heterostructures.

Scanning tunneling microscopy with chemically modified gold tips: in situ reestablishment of chemical contrast.

A method was developed for the reestablishment of chemical contrast in STM images obtained with chemically modified gold tips. Such tips display selective chemical contrast, which allows the selective imaging of specific species on the sample surface. Chemically modified STM tips can be fabricated by forming a self-assembled monolayer (SAM) on an electrochemically etched gold tip. One difficulty with this method thus far has been the relatively short lifetime of SAM-treated tips. The method described here utilizes the brief application of a high bias voltage between the sample and the tip to cause SAM molecules to reoccupy the tip apex, thereby allowing the tips to display selective chemical contrast in imaging. These treatments consist of applying a +1.9-V sample bias for 0.5-10 min under tunneling conditions. The usable lifetime of SAM-modified tips could be increased by more than 2 orders of magnitude, from hours to at least a month, dramatically increasing the efficiency of using SAM-modified gold tips. SAM molecules can also be removed from the tip apex by application of a negative sample bias (-2.0 V for 0.5-10 min) making it possible to alternate between conventional STM images and STM images with chemically enhanced contrasts.

Scanning Tunneling Microscopy Study of Alanes/Al(111): Contrasting Neat and Molecularly-Terminated W Tips

Scanning tunneling microscopy has proven to be a very powerful method for real space imaging of molecules on surfaces. Here we demonstrate how the morphological and chemical structure of the tunneling tip alters the apparent topography of STM images. When imaged with conventional W tips, isolated aluminum hydride (alane) oligomers appear as 2.8 Å protrusions on Al(111). When imaged with alane-terminated tips, STM image contrast is both enhanced and inverted, with alane oligomers appearing as depressions. We attribute this image reversal to the electronic levels of tip-adsorbed alanes, whose distance from the surface Fermi levels reduces the coherence in the tunneling junction. The absence of a bias dependence in these images suggests predominant tunneling through tip-alane LUMO states.

Tip-induced surface polarization: a new mechanism for contrast in the scanning tunnelling microscope

We propose a new mechanism influencing the contrast in STM images. It involves the polarization of the surface by the electric field from the tip; this affects the contrast when different parts of the surface have different polarizabilities. We demonstrate its importance for the case of ethene (C2H4) molecules adsorbed on the Si(001) surface; for this system we are able to obtain qualitative agreement between experiments and ab initio calculations by including the tip-induced field.

Distance-dependent STM-study of the W(110)/C-R(15×3) surface

The W(110)/C-R(15×3) reconstructed surface has been studied by STM at variable tip-surface separation controlled by the tunneling gap resistance. A pronounced dependence of the STM image contrast as a function of tip height has been observed which is explained by the suppression of higher Fourier components, i.e. small wavelength features, with increasing tip height and an additional spatial dependence of the decay length of the surface wavefunction. As an important implication of our study we have found that STM images of non-trivial surface structures can depend critically on the tunneling gap resistance.

STM investigations of C6Br6 on HOPG and MoS2

Monolayers of the organic molecule hexabromobenzene on HOPG and MoS2 have been imaged with scanning tunneling microscopy. The flat lying molecules form an ordered close-packed array with unit cell parameters corresponding to their van der Waals radii. In both cases the overlayers are incommensurable to the substrate. In the images with submolecular resolution, the molecules appear different on the two substrates. On HOPG the observed STM image contrast shows a detailed submolecular pattern depending on the tunneling voltage, whereas on MoS2 the molecules appear with a less pronounced inner structure. Neither on HOPG nor on MoS2 the observed submolecular contrast depends on the adsorption site.

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Monolayers of the organic molecules perylene- 3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and -di- imide (PTCDI) on graphite and MoS 2 have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the inter- molecular interaction but nearly independent of the sub- strate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aro- matic perylene system whereas the polar oxygen and ni- trogen groups are invisible. The correlation of the ob- served inner structure of the molecules to their molecular structure allows us to compare our results with theoreti- cal considerations.

The role of electronic interferences in determining the appearance of STM images: application to the S(2 × 2)/Re(0001) system

Abstract The ESQC theoretical approach to calculate STM image contrast is applied to the case of S chemisorbed on the Re(0001) surface. The method explicitly includes the interference of various electron pathways between tip and surface atoms by exact calculation of a scattering matrix of electrons through the tunneling gap. We show that due to these electronic interferences, the image is sensitive both to the tip structure and S position over the surface.

The role of electronic interferences in determining the appearance of STM images: application to the S(2 × 2)/Re(0001) system

The ESQC theoretical approach to calculate STM image contrast is applied to the case of S chemisorbed on the Re(0001) surface. The method explicitly includes the interference of various electron pathways between tip and surface atoms by exact calculation of a scattering matrix of electrons through the tunneling gap. We show that due to these electronic interferences, the image is sensitive both to the tip structure and S position over the surface.