The multi-probe scanning tunneling microscopy (STM) laboratory hosts two independent state of the art 4-probe STM instruments. The facility is dedicated for determination of electronic transport properties in variety of system at scales ranging from several micrometers down to single atom level. We welcome both applied projects including nano- and micro-scale devices characterization as well as fundamental research applications aiming in atomic-scale understanding of processes bridging complex material morphology and electronic structure with the resulting transport properties.
Science Overview
The CNMS offers users two ultra-high vacuum (UHV) systems with 4-probe STM instruments. A cryogenic 4-probe microscope system from RHK/Unisoku is located at the CNMS and a LT-Nanoprobe from Scienta-Omicron, managed by the ORNL Materials Characterization program, can be accessed through the CNMS user program.
The RHK/Unisoku 4-probe STM is the first cryogenic multi-probe STM in the US. Installed in 2006, it has four independent probes that can move and scan separately. In situ sample preparation in UHV chambers and the ability to reach cryogenic temperature with both tip and sample enables to study the surface of samples in atomic scale. A JEOL scanning electron microscope (SEM) column with 20 nm nominal resolution is attached to the STM, which enables precise positioning of tips and samples. E-beam and thermal evaporators are attached to the preparation chamber, which allows deposition of various materials on the sample. The instrument can be used in single probe mode for conventional STM, scanning tunneling spectroscopy (STS), or 2- and 4-probe mode for in situ transport measurements. Scanning tunneling potentiometry (STP) has also been developed to visualize potential change in nanometer resolution.
The LT-Nanoprobe system is a state-of-the-art system, installed in 2018. It combines four independent low-temperature STM scanners on the same stage which can reach 4.5K. The probes are initially observed and navigated by a SEM Gemini column then can be operated in STM mode at separation distances down to below 100 nm. This configuration opens novel possibilities for multi-probe research, which can be performed with precision so far accessible only by single-probe STM/STS experiments.
Applications
The multi-probe STM instruments are suitable for measuring transport properties of individual nanostructures, including carbon and boron nitride nanotubes (Advanced Materials 25, 4544-4548 (2013)), metal nanowires (Nano Letters 12, 938-942 (2012)), and graphene nanoribbons (Nature 506, 349-354 (2014)). Resistivity of atomic size defects in the nanostructures are also detectable, for example, the measurement in copper nanowires revealed the resistivity of individual grain boundaries (Nano Letters 10, 3096-3100 (2010)).
Scanning tunneling potentiometry (STP) is uniquely implemented in the multiprobe STM, in which a current is applied to the sample with source probes and another probe scans the local potential and topography simultaneously in nanometer resolution. Potentiometry has enabled measurement of the conductance of the nanoscale features in the sample. For example, conductance of domain and grain boundaries in graphene has been determined with the technique (ACS Nano 7, 7956-7966 (2013); Physical Review X 4, 011021 (2014)).
In situ transport measurements allow us to see the intrinsic properties of various quantum materials. For example, topological insulators have surface conductance that is often intertwined with the bulk conductance. We cleaved the material inside the UHV chamber to expose a clean surface and then performed multiple transport measurements at varying probe configuration and distance. By using a variable probe-spacing spectroscopy method we have differentiated and quantified the bulk and surface conductance (Nano Letters 16, 2213–2220 (2016)).
It is possible to probe spin transport in materials by combining spin-polarized STM with four-probe transport measurements. Ferromagnetic tips were employed to detect the spin-dependent potential on topological insulators. CNMS researchers detected the spin-polarized current induced by a spin-momentum locking and quantified the degree of spin polarization (Phys. Rev. Lett. 119, 137202 (2017)).
Specifications
General
- Samples must be UHV compatible, able to be mounted on a metal transfer plate and relatively flat
- In most cases, samples must be prepared in situ to provide a clean surface
- Inserted sample surfaces can be prepared by cleaving, inert gas sputtering and/or heated in vacuum
RHK/UNISOKU 4-probe
- Four completely independent STM probes with nanoscopic resolution
- In situ sample preparation in UHV chamber (~10-10 torr)
- Temperatures from 10-300 K
- Sample size up to (l×w×h) 10×2×1.5 mm3
- JEOL SEM column (20 nm nominal resolution)
- MBE sources to deposit various materials on the surface
- Single probe STM/STS & standard 2- and 4-probe transport capabilities
- Scanning tunneling potentiometry (STP) capability
- Magnetic tips for spin polarization
ScientaOmicron LT-Nanoprobe
- Four completely independent STM probes with atomic resolution (sub-Angstrom stability at 4.5K)
- Non-contact force feedback (nc-AFM) tuning fork option for single chosen scanner
- In situ sample preparation in UHV chamber (~10-10 torr)
- Temperatures from 4.5-300 K (cryogenic temperatures are recommended)
- Long liquid helium holding time (up to ~40 hrs)
- Magnetic field up to 20 mT (at 4.5 K)
- Magnetic tips for spin polarization
- Sample size up to (l×w×h) 10×6×4 mm3 (resistive heating)
- Sample size up to (l×w×h) 10×2×1.5 mm3 (direct current heating), l>8mm
- 2×2mm2 operating area (at 4.5K)
- Up to 4 additional sample contacts at microscope sample stage
- SEM Gemini column (20nm nominal resolution)
- Standard 2- and 4-probe transport experimental setups
- Scanning Tunneling Potentiometry (STP) capabilities
- Focused Ion Beam (FIB) prepared tips with exact apex diameters lower than 50nm
Additional analytical techniques
- Low energy electron diffraction (LEED)
Surface Preparation
- Temperatures from 120-1200K (resistive heating)
- Temperature measurement by optical pyrometer/thermocouples
- Sputtering Ion Gun
- Controlled gas exposures
- Effusion cells for organic/inorganic materials deposition
