Basics: Scanning Tunneling Microscopy

Scanning probe microscopy has revolutionized the study of the structure, growth, morphology, and electronic structure of surfaces, thin films and nanostructures. We use an Omicron VT-SPM to study the growth of ultrathin metal overlayers on metal surface that exhibit quantum size effects, metal overlayer morphology, and the adsorption of organics molecules on a variety of surfaces. We also study the spatially resolved electronic properties of these systems. The instrument has a separate sample preparation with a loadlock for rapid sample and tip introduction. The preparation chamber includes metal and organic molecule deposition capabilities, as well as a range of surface characterization techniques such as LEED, Auger spectroscopy and XPS.

Sample Results: Scanning Tunneling Microscopy

The stoichiometric rutile TiO2(110) surface (30nm x 30nm) and (6nm x 6nm): SBy an appropriate sequence of sputtering and annealing in ultrahigh vacuum, an atomically clean and well ordered TiO2(110) surface can be prepared and imaged in UHV.  The bright rows are assigned to the bridging oxygen ions in what is essentially a bulk-truncated geometry. This has been our starting surface for studies of organic dyes on oxide surfaces relevant to dye sensitized solar cells.

Cu islands on the ZnO(0001) surface (100nm x 30nm):  Step edges on the Zn(001) surface form a characteristic pattern reflecting the 3-fold symmetry of the basal plane of the wurtzite structure.  Triangular islands or vacancy pits of sequential atomic layers are rotated by 180.  Cu neither wets the surface nor disrupts the surface morphology.  Rather, 3-dimensional Cu islands, typically 30 A in diameter and 8 A tall,  form.

Metallic Quantum Well Systems (300nm x 300nm): A multilayer system comprised of a Cu(100) surface covered by a metastable ~ 5  monolayer fccFe(100) film on which is deposited an fcc Cu(100) overlayer exhibits quantum confined electrons in Cu overlayer.  The electronic subbands that arise from this quantization change energy with Cu overlayer thickness and give rise to novel materials properties when they pass through the Fermi level of the system.  This image  of the ~ 4.5ML Cu/ 5ML Fe/Cu(100) system illustrates the large terraces obtained for these films, and the layer-by-layer growth mode these systems exhibit.      

       More Details: STM                                                                       (back to top)

The basic principles and implementation of STM are illustrated at the right. A sharp tip is placed in close proximity of the sample surface. A small bias is applied to the tip (either + or - ) and when the tip is within a few Angstrom of the surface, a tunneling current will flow. In contrast to the image at the right, a tip is rarely sharpened to atomic dimensions in a controlled manner. Typically a tip is chemically etched to a radius of several nanometers, and it is from the small asperities that provide the ability for atomic resolution. The tip is affixed to a piezoelectric tube with electrodes. Applying a high voltage across opposite electrodes cause a small distortion of the piezoelectric tube and thus effects the in-plane motion of the tip needed for scanning (i.e., x-y motion) and distortion along the axis of the tube (i.e. z-motion). While the tip is scanned, the tunneling current is monitored and fed into a feedback loop that controls the voltage applied to the z-piezo. Images are usually obtained in either of two modes. In the constant current mode, illustrated in the figure at the right, a fixed voltage is applied between the tip and the sample while the feedback loop adjusts the z-piezo so as to maintain constant tunneling current as the tip is scanned across the surface. As the tunneling current depends exponentially on the tip-sample distance, the tip is maintained at a constant height above the surface in this mode. Therefore, by monitoring the displacement of the Omicron VT-STM z-piezo during scanning, a topographic image of the surface is produced. The other common mode is the constant-z mode. In this mode, the z-position of the tip is fixed in space as the tip is scanned across the sample with a constant tunneling bias. As features of the surface are encountered, the tunneling current will vary with the tip-sample distance and can be recorded to form an image of the surface. This second mode emphasizes that the STM is sensitive to the electronic structure of the surface and therefore images do not necessarily correspond to true surface morphology. However, this sensitivity to electronic structure can be used to obtain real-space images of the surface charge density, as well as to aide in the identification of different chemical species on the surface.

Read more: Direct determination of HOMO and LUMO band alignment for N3 dye and isonicotinic acid on TiO2(110) and ZnO(11-20), S. Rangan, E.J. Bersch, J.-P. Theisen, and R.A. Bartynski, Science, (submitted)

       Recent Publications                                                                       (back to top)