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A new beamline, XTIP, has been constructed at the Advanced Photon Source to deliver monochromatic soft X-rays of between 400 and 1900 eV for the emerging synchrotron X-ray scanning tunneling microscopy technique., In recent years, there have been numerous efforts worldwide to develop the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique. Here, the inauguration of XTIP, the world’s first beamline fully dedicated to SX-STM, is reported. The XTIP beamline is located at Sector 4 of the Advanced Photon Source at Argonne National Laboratory. It features an insertion device that can provide left- or right-circular as well as horizontal- and vertical-linear polarization. XTIP delivers monochromatic soft X-rays of between 400 and 1900 eV focused into an environmental enclosure that houses the endstation instrument. This article discusses the beamline system design and its performance.

Two-dimensional (2D) materials provide a plethora of novel condensed matter physics and are the new playground in materials science, offering potentially vast applications. One of the critical hurdles for many 2D systems is the synthesis of these low-dimensional systems as well as the prediction and identification of new candidates. Herein, a self-assembly of a monolayer tellurene by bonding CdTe wafers is demonstrated for the first time. The conventional applications of wafer-bonding range from the production of microelectromechanical systems to the synthesis of lattice-mismatched multi-junction photovoltaics. Due to the heterogeneous materials that are typically employed, the bond-interface usually contains a thin amorphous layer or arrays of dislocations. Such an interface is thus itself inactive and in many cases has detrimental effects on the device. The new material phase stabilized in this work consists of an undulating monolayer of tellurium atoms covalently bonded to {111} Cd-terminated CdTe wafer surfaces. First-principles calculations and experimentally observed changes in the localized plasmon excitation energy indicate the clear rearrangement of the underlying band-structure suggesting a metallic character, bands showing linear dispersion, and a significant asymmetric spin-band splitting. The I–V characteristics show the presence of a highly conductive pathway that lowers the resistivity by three orders of magnitude, as compared to bulk CdTe, which can be attributed to the tellurium monolayer. The findings indicate that suitably chosen crystallographic wafer surfaces can act as structural templates allowing the production of exotic phases. The presently stabilized monolayer is an addition to the family of tellurene variants, providing new insights into the fundamental properties of this and other emerging 2D materials, while attracting attention to the unusual side of the wafer-bonding technology exemplified in this study.

The 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.

High-speed beam choppers are a crucial part of time-resolved x-ray studies as well as a necessary component to enable elemental contrast in synchrotron x-ray scanning tunneling microscopy (SX-STM). However, many chopper systems are not capable of operation in vacuum, which restricts their application to x-ray studies with high photon energies, where air absorption does not present a significant problem. To overcome this limitation, we present a fully ultra-high vacuum (UHV) compatible chopper system capable of operating at variable chopping frequencies up to 4 kHz. The lightweight aluminum chopper disk is coated with Ti and Au films to provide the required beam attenuation for soft and hard x-rays with photon energies up to about 12 keV. The chopper is used for lock-in detection of x-ray enhanced signals in SX-STM.

Structural details of epitaxially grown silver nanorods on a ‚-MoTe2 surface have been investigated by scanning tunneling microscopy. Atomicscale surface bending and surface reconstruction are directly observed at nanorod surfaces and edges. In agreement with a theoretical prediction, a stress fluctuation is observed in the two topmost atomic layers of nanorods. Due to a tensile stress the surface layers are occasionally bent while the subsurface layer experiences a compressive stress resulting in a novel surface reconstruction.

We report on a fast simulation method to investigate the movement of an atom induced by the tip during lateral manipulation with a scanning tunneling microscope. The simulation is based on a model assuming the atom moving in the combined potential of tip and surface. The pathway of the tip is subdivided in small steps, and the atomic position for each step is calculated by an iterative algorithm searching for the closest energetic minimum. The method is demonstrated for manipulation on the (1 1 1) surface of an fcc metal. Our model calculations predict which energetic minima of the surface are attained by the atom during manipulation. The details of the modelled manipulation curves allow a precise description of the atomic pathway in dependence on manipulation direction and positioning of the tip relative to the atom. Furthermore, the simulation predicts a transition from the so-called pulling to sliding manipulation mode upon reducing tip–surface distance, well in agreement with general experimental observations. To test our algorithm we present experimental results for the manipulation of iodine on Cu(1 1 1) along the ½2 � �� direction and compare them to simulated manipulation curves. The comparison allows for a complete understanding of all details in atomic movements during manipulation along a complicated path. 2001 Published by Elsevier Science B.V.

By using synchrotron X-rays as a probe and a nanofabricated smart tip of a tunneling microscope as a detector, we have achieved chemical fingerprinting of individual nickel clusters on a Cu(111) surface at 2 nm lateral resolution, and at the ultimate single-atomic height sensitivity. Moreover, by varying the photon energy, we have succeeded to locally measure photoionization cross sections of just a single Ni nanocluster, which opens new exciting opportunities for chemical imaging of nanoscale materials.

Molecules self-assemble to form tiny oscillating cantilevers that could be used to sense single atoms or molecules.

In order to achieve elemental and chemical sensitivity in scanning tunneling microscopy (STM), synchrotron x-rays have been applied to excite core-level electrons during tunneling. The x-ray photo-excitations result in tip currents that are superimposed onto conventional tunneling currents. While carrying important physical information, the varying x-ray induced currents can destabilize the feedback loop causing it to be unable to maintain a constant tunneling current, sometimes even causing the tip to retract fully or crash. In this paper, we report on an easy-to-implement filter circuit that can separate the x-ray induced currents from conventional tunneling currents, thereby allowing simultaneous measurements of topography and chemical contrasts. The filter and the schematic presented here can also be applied to other variants of light-assisted STM such as laser STM.

The effect of a local external electric field on the barrier potential of a tunneling gap is studied utilizing an emerging technique, synchrotron x-ray scanning tunneling microscopy. Here, we demonstrate that the shape of the potential barrier in the tunneling gap can be altered by a localized external electric field, generated by voltages placed on the metallic outer shield of a nanofabricated coaxial metal-insulator-metal tip, resulting in a controlled linear modulation of the tunneling current. Experiments at hard and soft x-ray synchrotron beamlines reveal that both the chemical contrast and magnetic contrast signals measured by the tip can be drastically enhanced, resulting in improved local detection of chemistry and magnetization at the surface.

An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) capable of single atom/molecule manipulation on molecular beam epitaxy (MBE) grown semiconductor surfaces has been designed and constructed. The STM scanner design is based on a modified Besoke-Beetle type and the thermal drift of the system is less than 0.1 nm/hr, which allows to perform I-V, dI/dV and vibrational tunneling spectroscopy measurements at single atom level. The sample holding stage is designed to have electrical contact at the surface layer of the sample. This permits tunneling into wide band-gap MBE grown semiconductor surfaces at low substrate temperatures. As a demonstration, the first low temperature STM image of GaN(0001) surface at 5 K and an atom-by-atom deposition process using vertical atom-manipulation on this are also presented.

An automation scheme for atom/molecule manipulations with the scanning tunneling microscope tip is developed. The program recognizes the type of particles to be manipulated such as atoms, molecules, or nanoparticles based on their sizes and shapes. It also determines the obstacles to be avoided during manipulation such as defects, steps, and clusters. The shortest manipulation paths are then computed by the program and finally, the manipulations are performed according to the computed paths. This method allows automated construction of nanostructures using single atoms or molecules as basic building blocks.

The conductivity of a single graphene nanoribbon can be measured by lifting the nanoribbon off a surface with the tip of a scanning tunnelling microscope.