Conventional Scanning Tunneling Microscopy Research Articles

Atom-resolved imaging with a silicon tip integrated into an on-chip scanning tunneling microscope.

Limited throughput is a shortcoming of the Scanning Tunneling Microscope (STM), particularly when used for atomically precise lithography. To address this issue, we have developed an on-chip STM based on Microelectromechanical-Systems (MEMS) technology. The device reported here has one degree of freedom, replacing the Z axis in a conventional STM. The small footprint of the on-chip STM provides a great opportunity to increase STM throughput by incorporating a number of on-chip STMs in an array to realize parallel STM. The tip methodology adopted for the on-chip STM presented here, which is a batch-fabricated Si tip, makes our design conducive to this goal. In this work, we investigate the capability of this on-chip STM with an integrated Si tip for STM imaging. We integrate the on-chip STM into a commercial ultrahigh-vacuum STM system and perform imaging with atomic resolution on par with conventional STMs but at higher scan speeds due to the higher sensitivity of the MEMS actuator relative to a piezotube. The results attest that it is possible to achieve a parallel and high-throughput STM platform, which is a fully batch-fabricated MEMS STM nanopositioner capable of performing atomic-resolution STM imaging.

Constructing Dual-Molecule Junctions to Probe Intermolecular Crosstalk.

Understanding and controlling charge transport across multiple parallel molecules are fundamental to the creation of innovative functional electronic components, as future molecular devices will likely be multimolecular. The smallest possible molecular ensemble to address this challenge is a dual-molecule junction device, which has potential to unravel the effects of intermolecular crosstalk on electronic transport at the molecular level that cannot be elucidated using either conventional single-molecule or self-assembled monolayer (SAM) techniques. Herein, we demonstrate the fabrication of a scanning tunneling microscopy (STM) dual-molecule junction device, which utilizes noncovalent interactions and allows for direct comparison to the conventional STM single-molecule device. STM-break junction (BJ) measurements reveal a decrease in conductance of 10% per molecule from the dual-molecule to the single-molecule junction device. Quantum transport simulations indicate that this decrease is attributable to intermolecular crosstalk (i.e., intermolecular π-π interactions), with possible contributions from substrate-mediated coupling (i.e., molecule-electrode). This study provides the first experimental evidence to interpret intermolecular crosstalk in electronic transport at the STM-BJ level and translates the experimental observations into meaningful molecular information to enhance our fundamental knowledge of this subject matter. This approach is pertinent to the design and development of future multimolecular electronic components and also to other dual-molecular systems where such crosstalk is mediated by various noncovalent intermolecular interactions (e.g., electrostatic and hydrogen bonding).

Recent Progress with In Situ Characterization of Interfacial Structures under a Solid-Gas Atmosphere by HP-STM and AP-XPS.

Surface science is an interdisciplinary field involving various subjects such as physics, chemistry, materials, biology and so on, and it plays an increasingly momentous role in both fundamental research and industrial applications. Despite the encouraging progress in characterizing surface/interface nanostructures with atomic and orbital precision under ultra-high-vacuum (UHV) conditions, investigating in situ reactions/processes occurring at the surface/interface under operando conditions becomes a crucial challenge in the field of surface catalysis and surface electrochemistry. Promoted by such pressing demands, high-pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS), for example, have been designed to conduct measurements under operando conditions on the basis of conventional scanning tunneling microscopy (STM) and photoemission spectroscopy, which are proving to become powerful techniques to study various heterogeneous catalytic reactions on the surface. This report reviews the development of HP-STM and AP-XPS facilities and the application of HP-STM and AP-XPS on fine investigations of heterogeneous catalytic reactions via evolutions of both surface morphology and electronic structures, including dehydrogenation, CO oxidation on metal-based substrates, and so on. In the end, a perspective is also given regarding the combination of in situ X-ray photoelectron spectroscopy (XPS) and STM towards the identification of the structure–performance relationship.

Construction of a gigahertz-bandwidth radio-frequency scanning tunneling microscope based on a commercial low-temperature system

The highest frequency of the electric signal that a conventional scanning tunneling microscope (STM) can process typically lies in the kilohertz regime, imposing a limitation on its temporal resolution to the submillisecond regime. When extracting (feeding) the high frequency, or radio-frequency (RF), signal out of (into) the tunnel junction, the most challenging part is that the tunnel junction has a very high impedance, causing significant reflections. Here, we present a systematic solution on the construction of RF-STM with high sensitivity. To minimize radiation loss, using coaxial cables as conducting wires, we designed an active impedance matching network (IMN) based on a field-effect transistor, which can provide impedance matching over a wide frequency range and can bridge the enormous impedance difference associated with the tunnel junction. To shorten the signal cable before amplification, the STM probe itself was directly mounted on the IMN as the input pin, which is an unprecedented attempt to minimize the undesired parasitic capacitances. Furthermore, we employed a two-stage cryogenic SiGe low noise amplifier and a high-end spectrum analyzer to amplify and subsequently analyze the RF signal of interest. After this systematic engineering, the bandwidth of our STM has been improved to the gigahertz regime, implying a six orders of magnitude improvement. The sensitivity level of our newly built RF-STM is measured to be better than 1.0 pA/√Hz at 200 MHz at 78 K. The RF-STM also finds its application in nanoscale thermometry. Our efforts in its instrumentation should contribute to the development of high frequency scanning tunneling microscopy.

Amplifier for scanning tunneling microscopy at MHz frequencies

Conventional scanning tunneling microscopy (STM) is limited to a bandwidth of a few kHz around DC. Here, we develop, build, and test a novel amplifier circuit capable of measuring the tunneling current in the MHz regime while simultaneously performing conventional STM measurements. This is achieved with an amplifier circuit including a LC tank with a quality factor exceeding 600 and a home-built, low-noise high electron mobility transistor. The amplifier circuit functions while simultaneously scanning with atomic resolution in the tunneling regime, i.e., at junction resistances in the range of giga-ohms, and down towards point contact spectroscopy. To enable high signal-to-noise ratios and meet all technical requirements for the inclusion in a commercial low temperature, ultra-high vacuum STM, we use superconducting cross-wound inductors and choose materials and circuit elements with low heat load. We demonstrate the high performance of the amplifier by spatially mapping the Poissonian noise of tunneling electrons on an atomically clean Au(111) surface. We also show differential conductance spectroscopy measurements at 3 MHz, demonstrating superior performance over conventional spectroscopy techniques. Further, our technology could be used to perform impedance matched spin resonance and distinguish Majorana modes from more conventional edge states.

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics.

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

Postprocessing Algorithm for Driving Conventional Scanning Tunneling Microscope at Fast Scan Rates

We present an image postprocessing framework for Scanning Tunneling Microscope (STM) to reduce the strong spurious oscillations and scan line noise at fast scan rates and preserve the features, allowing an order of magnitude increase in the scan rate without upgrading the hardware. The proposed method consists of two steps for large scale images and four steps for atomic scale images. For large scale images, we first apply for each line an image registration method to align the forward and backward scans of the same line. In the second step we apply a “rubber band” model which is solved by a novel Constrained Adaptive and Iterative Filtering Algorithm (CIAFA). The numerical results on measurement from copper(111) surface indicate the processed images are comparable in accuracy to data obtained with a slow scan rate, but are free of the scan drift error commonly seen in slow scan data. For atomic scale images, an additional first step to remove line-by-line strong background fluctuations and a fourth step of replacing the postprocessed image by its ranking map as the final atomic resolution image are required. The resulting image restores the lattice image that is nearly undetectable in the original fast scan data.

Local imaging of high mobility two-dimensional electron systems with virtual scanning tunneling microscopy

Correlated electron states in high mobility two-dimensional electron systems (2DESs), including charge density waves and microemulsion phases intermediate between a Fermi liquid and Wigner crystal, are predicted to exhibit complex local charge order. Existing experimental studies, however, have mainly probed these systems at micron to millimeter scales rather than directly mapping spatial organization. Scanning probes should be well-suited to study the spatial structure of these states, but high mobility 2DESs are found at buried semiconductor interfaces, beyond the reach of conventional scanning tunneling microscopy. Scanning techniques based on electrostatic coupling to the 2DES deliver important insights, but generally with resolution limited by the depth of the 2DES. In this letter, we present our progress in developing a technique called “virtual scanning tunneling microscopy” that allows local tunneling into a high mobility 2DES. Using a specially designed bilayer GaAs/AlGaAs heterostructure where the tunnel coupling between two separate 2DESs is tunable via electrostatic gating, combined with a scanning gate, we show that the local tunneling can be controlled with sub-250 nm resolution.

X-ray STM: Nanoscale elemental analysis & Observation of atomic track.

Scanning tunneling microscopy (STM) combined with brilliant X-rays from synchrotron radiation (SR) can provide various possibilities of original and important applications, such as the elemental analysis on solid surfaces at an atomic scale. The principle of the elemental analysis is based on the inner-shell excitation of an element-specific energy level "under STM observation". A key to obtain an atomic locality is to extract the element-specific modulation of the local tunneling current (not emission that can damage the spatial resolution), which is derived from the inner-shell excitation [1]. On this purpose, we developed a special SR-STM system and smart tip. To surmount a tiny core-excitation efficiency by hard X-rays, we focused two-dimensionally an incident beam having the highest photon density at the SPring-8.After successes in the elemental analyses by SR-STM [1,2] on a semiconductor hetero-interface (Ge on Si) and metal-semiconductor interface (Cu on Ge), we succeeded in obtaining the elemental contrast between Co nano-islands and Au substrate. The results on the metallic substrate suggest the generality of the method and give some important implications on the principle of contrast. For all cases of three samples, the spatial resolution of the analysis was estimated to be ∼1 nm or less, and it is worth noting that the measured surface domains had a deposition thickness of less than one atomic layer (Fig. 1, left and center).jmicro;63/suppl_1/i14-a/DFU045F1F1DFU045F1Fig. 1.(left) Topographic image and (center) beam-induced tip current image of Ge(111)-Cu (-2V, 0.2 nA). (right) X-ray- induced atomic motion tracks on Ge(111) that were newly imaged by the Xray-STM. On the other hand, we found that the "X-ray induced atomic motion" can be observed directly with atomic scale using the SR-STM system effectively under the incident photon density of ∼2 x10(15) photon/sec/mm(2) [3]. SR-STM visualized successfully the track of the atomic motion (Fig. 1, right), which enabled the further analysis on the mechanism of the atomic motion. It is worth comparing our results with past conventional thermal STM observations on the same surface [4], where the atomic motion was found to occur in the 2-dimensional domain. However, our results show the atomic track having a local chain distribution [3].The above mentioned results will allow us to investigate the chemical analysis and control of the local reaction with the spatial resolution of STM, giving hope of wide applications.

Molecular Tips for Scanning Tunneling Microscopy: Intermolecular Electron Tunneling for Single-molecule Recognition and Electronics

This paper reviews the development of molecular tips for scanning tunneling microscopy (STM). Molecular tips offer many advantages: first is their ability to perform chemically selective imaging because of chemical interactions between the sample and the molecular tip, thus improving a major drawback of conventional STM. Rational design of the molecular tip allows sophisticated chemical recognition; e.g., chiral recognition and selective visualization of atomic defects in carbon nanotubes. Another advantage is that they provide a unique method to quantify electron transfer between single molecules. Understanding such electron transfer is mandatory for the realization of molecular electronics.

Hidden surface states at non-polar GaN (101¯) facets: Intrinsic pinning of nanowires

We investigate the electronic structure of the GaN(101¯0) prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean (101¯0) sidewall facets are intrinsically pinned.

Synchrotron X‐Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation

AbstractThe combination of the high spatial resolution of scanning tunneling microscopy with the chemical and magnetic contrast provided by synchrotron X‐rays has the potential to allow a unique characterization of advanced functional materials. While the scanning probe provides the high spatial resolution, synchrotron X‐rays that produce photo‐excitations of core electrons add chemical and magnetic contrast. However, in order to realize the method's full potential it is essential to maintain tunneling conditions, even while high brilliance X‐rays irradiate the sample surface. Different from conventional scanning tunneling microscopy, X‐rays can cause a transition of the tip out of the tunneling regime. Monitoring the reaction of the z‐piezo (the element that controls the tip to sample separation) alone is not sufficient, because a continuous tip current is obtained. As a solution, an unambiguous and direct way of fingerprinting such near to far field transitions of the tip that relies on the simultaneous analysis of the X‐ray‐induced tip and sample current is presented. This result is of considerable importance because it opens the path to the ultimate resolution in X‐ray enhanced scanning tunneling microscopy.

Imaging of Ti0.87O2 nanosheets using scanning tunneling spectroscopy

We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements on single layer Ti0.87O2 nanosheets deposited on Pt(200nm)/Si substrates. These nanosheets have a band gap of 3.8eV and are virtually invisible in conventional STM images. However, an impressive contrast is observed in the di/dz map, which allows for a clear identification of the nanosheets. The di/dz signal is obtained by adding a high frequency, small amplitude modulation to the z-piezo using a lock-in amplifier.

Field Emission from an Individual Freestanding Graphene Edge

A freestanding graphene field emitter is prepared by quenching hot HOPG in a cold suspension, then manually loaded graphene on a metal probe. This single-layer graphene edge emits electrons from few atoms. Hence, this graphene probe (GP) is suitable for scanning tunneling microscope (STM). The GP (left) performs higher resolution than that of a conventional STM probe (right).

Theory of nonequilibrium single-electron dynamics in STM imaging of dangling bonds on a hydrogenated silicon surface

During fabrication and scanning-tunneling-microscope (STM) imaging of dangling bonds (DBs) on a hydrogenated silicon surface, we consistently observed halo-like features around isolated DBs for specific imaging conditions. These surround individual or small groups of DBs, have abnormally sharp edges, and cannot be explained by conventional STM theory. Here we investigate the nature of these features by a comprehensive 3-dimensional model of elastic and inelastic charge transfer in the vicinity of a DB. Our essential finding is that non-equilibrium current through the localized electronic state of a DB determines the charging state of the DB. This localized charge distorts the electronic bands of the silicon sample, which in turn affects the STM current in that vicinity causing the halo effect. The influence of various imaging conditions and characteristics of the sample on STM images of DBs is also investigated.

Measurement of shear strength for HOPG with scanning tunneling microscopy by thermal excitation method

An experimental observation of force interactions in scanning tunneling microscopy (STM) is presented. A technique for measuring force interactions between a conventional STM probe and a sample by spectra analysis of its thermal fluctuations from tunneling current in STM is developed theoretically and experimentally. Thermally excited fluctuation of the STM probe is exactly discerned in air and then force gradient is determined from its corresponding eigen-frequency with a formula similar to that for a small-amplitude atomic force microscopy (AFM). The observed force interactions are consistent with forces in dynamic AFM. Shear strength of 7 GPa for highly oriented pyrolytic graphite (HOPG) under compressive stress is obtained from the experiment and using the elastic theory. We believe that this technique is of scientific significance as it enables accurate measurement of short-range force interactions at atomic scale under true STM conditions.

Direct Imaging of Room Temperature Optical Absorption with Subnanometer Spatial Resolution

Optical absorption can detect individual molecules and nanostructures even in dissipative or strongly quenching environments where fluorescence signals are weak. Here we image optical absorption of individual carbon nanotubes with subnanometer resolution. We show that we can discriminate adjacent nanotubes on a length scale far below the diffraction limit. Then we compare optical absorption imaging of a defect in a single carbon nanotube (CNT) with conventional scanning tunneling microscopy (STM) and conventional current-voltage scan (I-V) bandgap profiles. We directly visualize the penetration depth σ' = 0.9 ± 0.3 nm of the CNT exciton state into the smaller bandgap region of the defect and derive a size σ = 1.8 ± 0.6 nm for the exciton state. Optical absorption provides a spectroscopic map of molecules simultaneously with conventional STM.

Simultaneous AFM and STM measurements on theSi(111)−(7×7)surface

We perform simultaneous atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements on the $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. AFM/STM constant height images are obtained at various tip-surface distances. Force/current distance spectroscopy using the same tip apex allows us to estimate the relative tip-surface distance for each image as well as the short/long-range force and the tunneling current. We demonstrate that the tunneling current at tip-surface distances where AFM clearly resolves atoms is much larger than the typical values in conventional STM. In addition, at the tip-surface distances for conventional STM, the short-range force is too small to provide atomic contrast in AFM. We show that the differences in the signal-to-noise ratio of the constant height images between AFM and STM produce different optimal imaging distances. In addition, we note that the different imaging distance also influences images obtained using double tips.

Study of dynamic processes on semiconductor surfaces using time-resolved scanningtunneling microscopy

The time resolution of a conventional scanning tunneling microscope can be improved bymany orders of magnitude by recording open feedback loop current–time traces.The enhanced time resolution comes, however, at the expense of the ability toobtain spatial information. In this paper, we first consider the Ge(111)-c(2 × 8) surface as an example of how surface dynamics can show up in conventional STM images.After a brief introduction to the time-resolved scanning tunneling microscopy technique, itscapabilities will be demonstrated by addressing the dynamics of a dimer pair of a Ptmodified Ge(001).