Multiprobe Scanning Tunneling Microscope Research Articles

Improving the intrinsic conductance of selective area grown in-plane InAs nanowires with a GaSb shell

The nanoscale intrinsic electrical properties of in-plane InAs nanowires grown by selective area epitaxy are investigated using a process-free method involving a multi-probe scanning tunneling microscope. The resistance of oxide-free InAs nanowires grown on an InP(111)B substrate and the resistance of InAs/GaSb core–shell nanowires grown on an InP(001) substrate are measured using a collinear four-point probe arrangement in ultrahigh vacuum. They are compared with the resistance of two-dimensional electron gas reference samples measured using the same method and with the Van der Pauw geometry for validation. A significant improvement of the conductance is achieved when the InAs nanowires are fully embedded in GaSb, exhibiting an intrinsic sheet conductance close to the one of the quantum well counterpart.

Resistivity of Surface Steps in Bulk-Insulating Topological Insulators

Electron transport in topological insulators usually involves both topologically protected surface states and trivial electronic states in the bulk material. The surface transport is particularly interesting; however, it is also susceptible to atomic defects on the surfaces, such as vacancies, impurities, and step edges. Experimental determination of scattering effects of these surface defects requires both nanoscale spatial resolution and the ability to decipher surface transport from bulk transport. Here we directly measure the resistivity of individual surface steps in the surface dominating transport process of topological insulator Bi2Te2Se. A variable probe-spacing transport spectroscopy with a multiprobe scanning tunneling microscope is used to differentiate the surface conductance from bulk conductance, allowing the identification of a surface dominant transport regime. The technique also reveals a deviation from ideal 2D transport at atomic steps. Then, a multi-probe scanning tunneling potentiometry is employed to visualize the electrochemical potentials across individual step edges. A quantitative analysis of the potential distributions enables us to acquire a resistivity of 0.530 mΩ · cm for the one quintuple-layer atomic step. The result indicates that atomic defects, despite preserving the time-reversal symmetry, can still significantly affect the transport in topological insulators.

Ohmic Contact to Two-Dimensional Nanofabricated Silicon Structures with a Two-Probe Scanning Tunneling Microscope.

We used multiprobe scanning tunneling microscope (STM) to fabricate and electrically characterize nanostructures on Si surfaces. We overcame resistive contacts by using field evaporation to clean tip apexes in order to create Ohmic contact with the Si surface states on a Si substrate. A two-probe (2P-) STM with Ohmic contact allowed for measurement at very low bias, limiting conduction through space-charge layer and bulk states. The Ohmic 2P-STM measurement clarified the surface conductivity of the Si(111)-(7 × 7) surface. We also confirmed that Ohmic 2P-STM can be replaced with more convenient Ohmic one-probe STM for the conductance measurements on the Si surface. We prepared nanostructures using STM lithography to define electronically isolated two-dimensional (2D) regions with various aspect ratios. Their surface conduction properties are described well by the conventional sheet model, proving the diffusive 2D conduction on the Si surface. Constrictions and breaks in 2D structures were also evaluated. Ohmic 2P-STM will be helpful for the investigation of exploratory atomic-scale circuitry or cutting-edge materials sciences.

A time-shared switching scheme designed for multi-probe scanning tunneling microscope.

We report the designof a time-shared switching scheme, aiming to realize the manipulation and working modes (imaging mode and transport measurement mode) switching between multiple scanning tunneling microscope (STM) probes one by one with a shared STM control system (STM CS) and an electrical transport characterization system. This scheme comprises three types of switch units, switchable preamplifiers (SWPAs), high voltage amplifiers, and a main control unit. Together with the home-made software kit providing the graphical user interface, this scheme achieves a seamless switching process between different STM probes. Compared with the conventional scheme using multiple independent STM CSs, this scheme possesses more compatibility, flexibility, and expansibility for lower cost. The overall architecture and technique issues are discussed in detail. The performances of the system are demonstrated, including the millimeter scale moving range and atomic scale resolution of a single STM probe, safely approached multiple STM probes beyond the resolution of the optical microscope (1.1 µm), qualified STM imaging, and accurate electrical transport characterization. The combinational technique of imaging and transport characterization is also shown, which is supported by SWPA switches with ultra-high open circuit resistance (909TΩ). These successful experiments prove the effectiveness and the usefulness of the scheme. In addition, the scheme can be easily upgraded with more different functions and numbers of probe arrays, thus opening a new way to build an extremely integrated and high throughput characterization platform.

Investigation of charge carrier depletion in freestanding nanowires by a multi-probe scanning tunneling microscope

Profiling of the electrical properties of nanowires (NWs) and NW heterocontacts with high spatial resolution is a challenge for any application and advanced NW device development. For appropriate NW analysis, we have established a four-point prober, which is combined in vacuo with a state-of-the-art vapor-liquid-solid preparation, enabling contamination-free NW characterization with high spatial resolution. With this ultrahigh-vacuum-based multi-tip scanning tunneling microscopy (MT-STM), we obtained the resistance and doping profiles of freestanding NWs, along with surface-sensitive information. Our in-system 4-probe STM approach decreased the detection limit for low dopant concentrations to the depleted case in upright standing NWs, while increasing the spatial resolution and considering radial depletion regions, which may originate from surface changes. Accordingly, the surface potential of oxide-free GaAs NW {112} facets has been estimated to be lower than 20 mV, indicating a NW surface with very low surface state density.

Nanomanipulation and nanofabrication with multi-probe scanning tunneling microscope: From individual atoms to nanowires

The wide variety of nanoscale structures and devices demands novel tools for handling, assembly, and fabrication at nanoscopic positioning precision. The manipulation tools should allow for in situ characterization and testing of fundamental building blocks, such as nanotubes and nanowires, as they are built into functional devices. In this paper, a bottom-up technique for nanomanipulation and nanofabrication is reported by using a 4-probe scanning tunneling microscope (STM) combined with a scanning electron microscope (SEM). The applications of this technique are demonstrated in a variety of nanosystems, from manipulating individual atoms to bending, cutting, breaking carbon nanofibers, and constructing nanodevices for electrical characterizations. The combination of the wide field of view of SEM, the atomic position resolution of STM, and the flexibility of multiple scanning probes is expected to be a valuable tool for rapid prototyping in the nanoscience and nanotechnology.

Nanochannel-Directed Growth of Multi-Segment Nanowire Heterojunctions of Metallic Au1–xGex and Semiconducting Ge

We report on the synthesis of multi-segment nanowire (NW) junctions of Au(1-x)Ge(x) and Ge inside the nanochannels of porous anodic aluminum oxide template. The one-dimensional heterostructures are grown with a low-temperature chemical vapor deposition process, assisted by electrodeposited Au nanowires (AuNWs). The Au-catalyzed vapor-liquid-solid growth process occurs simultaneously in multiple locations along the nanochannel, which leads to multi-segment Au(1-x)Ge(x)/Ge heterojunctions. The structures of the as-grown hybrid NWs, analyzed by using transmission electron microscopy and energy-dispersive X-ray spectroscopy elemental mapping, show clear compositional modulation with variable modulation period and controllable junction numbers. Remarkably, both GeNW and Au(1-x)Ge(x)NW segments are single crystalline with abrupt interfaces and good crystallographic coherences. The electronic and transport properties of individual NW junctions are measured by using a multi-probe scanning tunneling microscope, which confirms the semiconducting nature of Ge segments and the metallic behavior of Au(1-x)Ge(x) segments, respectively. The high yield of multiple segment NW junctions of a metal-semiconductor can facilitate the applications in nanoelectronics and optoelectronics that harness multiple functionalities of heterointerfaces.

NANOSCALE CHARACTERIZATION BY SCANNING TUNNELING MICROSCOPY

Nanoscale characterization is a key field in nanoscience and technology as it provides fundamental understanding of the properties and functionalities of materials down to the atomic and molecular scale. In this article, we review the development and application of scanning tunneling microscope (STM) techniques in nanoscale characterization. We will discuss the working principle, experimental setup, operational modes, and tip preparation methods of scanning tunneling microscope. Selected examples are provided to illustrate the application of STM in the nanocharacterization of semiconductors. In addition, new developments in STM techniques including spin-polarized STM (SP-STM) and multi-probe STM (MP-STM) are discussed in comparison with conventional non-magnetic and single tip STM methods.

Soft Control of Scanning Probe Microscope with High Flexibility

Most commercial scanning probe microscopes have multiple embedded digital microprocessors and utilize complex software for system control, which is not easily obtained or modified by researchers wishing to perform novel and special applications. In this paper, we present a simple and flexible control solution that just depends on software running on a single-processor personal computer with real-time Linux operating system to carry out all the control tasks including negative feedback, tip moving, data processing and user interface. In this way, we fully exploit the potential of a personal computer in calculating and programming, enabling us to manipulate the scanning probe as required without any special digital control circuits and related technical know-how. This solution has been successfully applied to a homemade ultrahigh vacuum scanning tunneling microscope and a multiprobe scanning tunneling microscope.

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.

Exploiting Metal Coating of Carbon Nanotubes for Scanning Tunneling Microscopy Probes

By exploiting the metal coating of carbon nanotube (CNT) tips for a scanning tunneling microscope (STM), we demonstrated atomic imaging/spectroscopy and showed their potential for electrical nanoprobes. A CNT glued to a W tip was uniformly coated with a thin W layer 3–6 nm thick. Using this tip, stable atomic imaging and spectroscopy were carried out on clean Si(111)-7×7 and Si(100)-2×1 surfaces. The mechanical flexibility of the coated CNT was maintained by virtue of the thin-layer coating, enabling repeated direct contact to the sample surface. Two W-coated CNT tips were brought together within a distance of approximately 50 nm. These results indicate that the tips are useful for electronic transport measurements on a nanometer scale after installation into a multiprobe STM.