A scanning probe microscope (SPM) in its simplest form uses a fine probe tip in proximity to a sample surface to measure a particular physical property. SPMs achieve atomic or nanometer scale resolution using probe tips that have dimensions in this range, and typically measure physical properties by scanning the sample with tip-sample separations of one to a few nanometers using piezoelectric actuators. The advent of scanning probe microscopy in the 1980s led to the birth of the current disciplines of nanoscale science and technology, which focus on measuring and controlling the properties of materials and fabricating new structures at nanometer length scales. The goal of this project is to develop and construct state-of-the-art SPM instruments to advance the state of the art in nanoscale measurement.
SPM is a general acronym for various probe instruments. The “P” in SPM stands for various types of probe measurements, such as capacitance (C), force (F), tunneling (T), etc. The scanning tunneling microscope (STM), including custom designs at the CNST, uses the quantum mechanical principle of tunneling between a probe tip and a conducting surface. Force measurements are also currently under development. In STM operation, a fine probe tip is brought to within a fraction of a nanometer from a surface to establish a tunneling current between the probe tip and the surface. The tip is rastered across the surface and the tunneling current is used in a feedback loop to servo the tip position. The measurement of the tip position is recorded as the tip-sample distance and is adjusted to maintain a constant tunneling current. To first order, this raster image yields the surface topography. In addition to topographic features, the STM is inherently sensitive to surface electronic properties due to the dependence of the tunneling process on the availability of electron states. We exploit this sensitivity to measure the surface electron density of states with high spatial resolution, notably for recent measurements on graphene. In certain cases, STM also allows one to selectively image different chemical species. Moreover, we use the STM to fabricate perfect nanostructures on an atom-by-atom basis. Here, an atom is manipulated using the interactions in the tip/sample junction, and moved to a desired location under the control of the STM tip. Our experimental efforts emphasize the custom design of the instrumentation with which we strive to push the frontiers of measurement at the nanometer scale. Within the CNST, three custom instruments have been developed for this purpose, briefly described below.
4 K Ultra-high Vacuum Vector High Magnetic Field Scanning Tunneling Microscope
An ultra-high vacuum (UHV) cryogenic STM was custom designed and built as part of the Atomic Scale Characterization and Manipulation Laboratory. The main features of this microscope are operation down to temperatures of 2.5 K inside a superconducting vector magnet cryostat. The vector magnet has a vertical solenoid capable of producing a 10 T field and a horizontal split pair magnet producing up to 1.5 T. Vector operation is computer controlled and can rotate a 1.5 T field in a two-dimensional plane. The microscope system is housed inside an acoustic and electrical shielded room with three stages of vibration isolation. The UHV systems include an MBE system for the growth of materials, a field-ion microscope for tip preparation, and a UHV transfer system to move samples and probe tips through the UHV systems (see Atomic Scale Characterization and Manipulation Laboratory).
A unique feature of this microscope design is a transportable STM module (Fig. 1a). The STM module contains a three-axis coarse positioning system of the tip-sample junction incorporating 18 shear piezo motor stacks lFig. 1(b)]. The sample is mounted on a vertical piezo translator while the tip is mounted on a two-dimensional, horizontal translator stage that is used to position the probe tip over a specific area of the sample surface. The overall STM module was constructed out of a solid piece of molybdenum to achieve a stiff structural base with good thermal conductivity. The conical shape of the molybdenum module ensures good thermal contact when pressed against a Cu mating cone inside the UHV insert. With this design, we achieve a temperature of 4.3 K. Lower temperature operation down to 2.5 K can be achieved with the aid of a "lambda-refrigerator" inside the cryostat.
Fig. 1 (a) Photograph of a transportable STM module. (b) 3D CAD model cross section of the 3three-axis piezo motor system inside the STM module. (c) Photograph of the UHV insert being loaded inside the superconducting magnet cryostat.
Lead Organizational Unit:cnst
Seoul National University
University of Regensburg
Joseph Stroscio, Phone 301-975-3716