Variable Temperature STM

This variable temperature scanning tunneling microscope (VT-STM) system operates in ultra-high vacuum (UHV). It detects structural, electronic, and spin textures, and measures thermal voltages of surfaces and thin films.

Science Overview

The CNMS VT-STM system consists of one vacuum chamber for two different functions of in situ growth and characterizations.

For sample growth, routine surface science techniques like Argon sputtering, PBN or direct heating are incorporated to clean samples or substrates in vacuum. High quality thin films can be grown by molecular-beam epitaxy (MBE) with three electron beam evaporators. It is equipped with Reflection High-Energy Electron Diffraction (RHEED) to verify the surface crystallinity of surfaces and thin films.

The Omicron variable temperature STM has Q-plus sensors for atomic force microscope (AFM). Both STM and AFM can resolve atomic structures of surfaces. The microscope has been augmented with tip functionalization for spin polarized STM (SP-STM) and scanning tunneling thermovoltage microscopy (STVthM) functions for studies of magnetic and thermoelectric materials.

Applications

The atomic resolution STM and scanning tunneling spectroscopy (STS) enables imaging of local electronic states confined by defects and boundaries. It allowed us to reveal a 2D heteroepitaxial growth from the 1D edge of a 2D seed crystal. Monolayer crystalline h-BN grew from fresh edges of monolayer graphene with atomic lattice coherence, forming an abrupt one-dimensional interface, or boundary. The spatial and energetic distributions of the boundary states were mapped with STS, which are found to be strongly associated with boundary terminations, asymmetric screening, and a polarity-induced electric field. Science, 343, 163-167(2014); Nature Communications 5, 5403 (2014).

The STV_thM enables mapping of thermoelectric power on the surface down to the atomic resolution. Using the STV_thM function, we spatially resolved thermovoltage on epitaxial graphene with direct correspondence to graphene atomic structures. A thermovoltage arises from a temperature gradient between the STM tip and the sample, and variations of thermovoltage are distinguished at defects and boundaries with atomic resolution. The thermoelectric power, the electronic structure, the carrier concentration, and their interplay can thus be analyzed on the level of individual defects and boundaries in graphene. Nano Letters, 13 (7), 3269–3273 (2013).

SP-STM is a powerful technique to probe and control the spin states of the surface and interface, which is key to spintronic applications of magnetic materials. Using a ferromagnetic tip in a VT STM, we studied the evolution of surface magnetism of Co nanoislands on Cu(111) upon hydrogen adsorption and desorption with the hope of realizing reversible control of spin-dependent tunneling. Nano Letters, 17 (1), 292–298 (2017). It further allowed us to reveal and manipulate magnetic domains in a quasi-2D crystal Fe₃GeTe₂. The evolutions of these domains in response to varying temperature and STM-tip manipulations are investigated. Physical Review B 97, 014425 (2018). 

Specifications

General

• Samples must be UHV compatible, able to be mounted on a metal transfer plate and relatively flat.
• Sample size up to 10×10 mm²
• In general, samples must be cleaned in situ to provide meaningful measurements.
• Inserted sample surfaces can be prepared by cleaving, inert gas sputtering, or annealing.

Growth

• Ar⁺ sputtering for sample cleaning
• Reflection High Energy Electron Diffraction (RHEED)
• Three electron beam evaporators

STM/AFM

• Omicron VT-STM/STS/Q-plus AFM
• Temperatures from 30-450 K
• STM requires a conductive path to ground
• Tip functionalization by metallic deposition, for example for spin detection
•  Specific positioning of the AFM/STM tip on the sample (down to ~10µm)