AVS2001 Session SS1-WeA: New Opportunities in Surface Microscopy

Wednesday, October 31, 2001 2:00 PM in Room 120

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS1-WeA-1 Imaging with Helium Atoms: Developments in Scanning Atom Microscopy D.A. MacLaren, W. Allison (University of Cambridge, U.K.) We report on the preparation and production of ultra-smooth helium atom mirrors and on recent advances in the development of a Scanning Atom Microscope (SAM). A bent Si(111)-(1x1)H crystal is an ideal mirror for helium atoms and can be used as the focusing element of a SAM. ¹ Based upon a focused microprobe of thermal helium atoms, a SAM provides the opportunity for sub-micron, non-destructive and surface-sensitive imaging and could have a profound impact on surface science, particularly in studies of delicate organic systems. A low aberration, high intensity atom mirror requires control over both the macroscopic and microscopic surface properties. Our approach is to bend an ultrasmooth single crystal into the optimum macroscopic profile by application of precise electrostatic fields; we have demonstrated that aberration-free focusing is possible using this method.² The atom mirror must also have a high helium reflectivity, which requires careful manipulation of the surface microstructure. Here, we discuss refinements to the ex-situ preparation of Si(111)-(1x1)H. Atomic Force Microscopy is used to study the kinetics of the etching mechanism used to produce the mirror surface. We show that small changes in miscut angle can alter the kinetic steady state to promote the formation of deep etch pits and stable self-aligned 'etch hillocks' on the micron scale. Our study has led to the production of surfaces that are homogeneous over tens of microns and which have substantially improved atom reflectivity. The results are a significant improvement in silicon preparation and are a crucial step in the development of a scanning atom microscope. ¹ B. Holst and W. Allison, Nature (London) 286, (1997), 244. ² D.A. MacLaren, W. Allison and B. Holst, Rev. Sci. Instrum. 71, (2000), 2625.
2:20 PM		SS1-WeA-2 Scanning Near-Field Infrared Microscopy E.S. Gillman (Jefferson Lab) Nanoscale chemical identification of objects below the diffraction limit is possible using a scanned probe technique, the Scanning Near-Field Infrared Microscope (SNFIM). In most cases vibrational spectroscopy in the infrared region is restricted due to the limitations of beam focusing to samples of macroscopic dimensions, on the order of one to several microns. The scale of the measured area is completely determined by the diffraction limit of the incident radiation. With a scanned probe technique resolution of chemical features on the order of λ/20 or ~100 nm can be achieved. An overview of previous experimental results using a free electron laser (FEL)¹^,², and more conventional infrared sources³^,⁴, will be discussed. A description of the experiment and recent results from the SNFIM at the Jefferson Lab Free Electron Laser facility will be presented. This work was supported by U.S. DOE Contract No. DE-AC05-84-ER40150, ONR Contract No. N00014-99-1-09B, the Commonwealth of Virginia and the Laser Processing Consortium. ¹ A. Piednoir, C. Licope, and F. Creuzet, Opt. Commun. 129, 414 (1996). ² M. K. Hong, A. G. Jeung, N. V. Dokholyan, T. I. Smith, H. A. Schwettman, P. Huie and S. Erramilli, Nucl. Instrum. Methods Phys. Res. B 144, 246 (1998). ³ B. Knoll and F. Keilmann, Nature 399, 134 (1999). ⁴ C. A. Michaels, S. J. Stranick, L. J. Richter, and R. R. Cavanaugh, J. Appl. Phys. 88, 4832 (2000).
2:40 PM	Invited	SS1-WeA-3 Spectroscopy, Microscopy, and Chemistry at the Spatial Limit W. Ho (University of California, Irvine); M. Persson (Chalmers University, Sweden) The combination of vibrational spectroscopy and microscopy with the imaging, manipulation, and chemical modification capabilities of the scanning tunneling microscope (STM) has made it possible to probe surface chemistry with sub-Angstrom resolution. Direct visualization of the nature of the chemical bonds and their transformations at the single molecule level not only provides convincing evidences but also fundamental understandings of chemical processes. The STM junction is effectively a nanoreator in which the metallic tip and substrate work together to induce chemical transformations of individual molecules adsorbed either on the substrate or the tip. Many aspects of chemistry can be probed by the STM, including the rotational, vibrational, and translational motions, the conformational changes, the energy transfer, the electrical conductivity, the coupling of electrons to the nuclear motions, and the bond breaking and formation of individual molecules.
3:40 PM		SS1-WeA-6 Focused Inelasticity in Scanning Tunneling Spectroscopy J.W. Gadzuk (National Institute of Standards and Technology); M. Plihal (KLA-Tencor) Scanning tunneling microscopy/spectroscopy of magnetic atoms adsorbed upon non-magnetic metal surfaces (possibly Kondo systems) has provided intriguing visual images and spectroscopic signatures in the form of Fano lineshapes in which the asymmetry depends in a diagnostically-useful way on tip location with respect to the adsorbate. Additional STM studies have demonstrated the ability of suitable two-dimensional nanostructures (such as "quantum corrals") to influence the surface electron transport that is part of the total elastic STM process. In a well known paradigm, an elliptical arrangement of Co atoms on Cu(111) gives rise to an apparent enhanced electronic communication between points on the surface which are near the two elliptical foci, showing some behavior consistent with classical ray tracing. The spectroscopic signature of a Kondo atom adsorbed at one focus shows an identical signature (though diminished in intensity) when the STM tip is placed over the vacant focus, suggestive of a mirage or "phantom atom". We report here on similar remote sensing for STM procedures involving inelastic tunneling in which adsorbate-surface or intra-molecular vibrational excitation occurs. We have extended our nonequilibrium theory of scanning tunneling spectroscopyfootnote1 to include the additional processes, inelastic adsorbate vibrational excitation and elastic surface nanostructure scattering/focusing and the general theory has been applied to an elliptic corral realization. The characteristics of such "focused inelasticity" will be presented within the context of both Kondo systems such as Co/Cu(111) and also simple molecular systems. ¹ M. Plihal and J. W. Gadzuk, Phys.Rev.B63(2001)085404.
4:00 PM		SS1-WeA-7 Calculations of Elastic and Vibrational Inelastic Electron Tunneling Images M. Persson, F. Olsson (Chalmers University, Sweden); N. Lorente (IRSAMC, Univ. P. Sabatier Touluose, France) The inelastic electron tunneling spectroscopy (IETS) in the STM is capable of mapping the vibrational excitation of single molecules in real space with sub-Å spatial resolution and meV spectral resolution.¹ Despite the obvious promise of a spectroscopy with these unique capabilities, STM-IETS raises several issues that we need to address by theory to fully exploit this spectroscopy. These issues include; (1) why are so few modes detected; (2) what can be learnt from IET images; (3) what is the nature of the coupling between the tunneling electron and the vibration; (4) what determines the vibrational lineshape ? To this end we have studied the excitation mechanism in STM-IETS using a generalization of Tersoff-Hamann theory to IET combined with density functional calculations.^2,3 We have shown that this many-electron theory give quantitative agreement with experiments and have identified several general effects: (1) elastic and inelastic contributions to the IET tend to cancel; (2) a symmetry selection rule connecting the symmetries of the IET images, adsorbate-induced states at the Fermi level and the vibrational mode; (3) a Fano-like lineshape for the second derative of the tunneling current with bias. These effects will be illustrated from a comparison of results of calculated vibrational inelastic images from several systems with experiments, in particular, oxygen adsorbed on silver surface, which provides a typical example of inelastic coupling through a single resonance level.⁴ We will also illustrate the applicability of the Tersoff-Hamann approximation for the calculations of elastic STM images. ¹ B.C. Stipe, M.A. Rezaei and W. Ho, Science 208, 1732 (1998); Phys. Rev. Lett. 82, 1724 (1999). ² N. Lorente and M. Persson, Phys. Rev. Lett. 85, 2997 (2000). ³ N. Lorente, M. Persson, L. Lauhon, and W. Ho, Phys. Rev. Lett. 86, 2593 (2001). ⁴ J.R. Hahn and W. Ho, Phys. Rev. Lett. 85, 1914 (2000).
4:40 PM		SS1-WeA-9 Progress in Dynamic Force Microscopy: From High-Resolution Imaging of Insulators to the Measurement of Dissipative Interaction Forces U.D. Schwarz, H. Hölscher, W. Allers, S. Langkat (University of Hamburg, Germany); B. Gotsmann, H. Fuchs (University of Münster, Germany); R. Wiesendanger (University of Hamburg, Germany) Recent progress in dynamic force microscopy (DFM) operated in ultrahigh vacuum, often also called non-contact atomic force microscopy (NC-AFM), enabled the imaging of the atomic structure of surfaces including the observation of point defects independent from the sample's conductivity. However, only few results on insulators have been published so far, possibly due to difficulties in preparing suitable sample surfaces for NC-AFM, e.g., electrostatic charging of the surfaces in vacuum. In order to illustrate the high-resolution capabilities of DFM on insulators, we present the first part of our talk results obtained on NiO(001) at low temperatures. Transition metal oxides are a class of magnetic insulators, which have been of great interest for several decades due to their electronic and magnetic properties. On this material, monatomic defects and atomic resolution across step edges could be observed, achieving a vertical resolution of 1.5 pm. In a second part, the spectroscopic potential of DFM based on a self-driven oscillator set-up is analysed. Introducing a very general tip-sample force law, we show that one of the two quantities measured, the frequency shift, is determined by the mean tip-sample force, while the other quantity, the gain factor (or excitation amplitude), is directly related to dissipative processes like hysteresis or viscous damping. This insight into the measurement principle can be used to examine the contrast mechanism in more detail. The application to non-reactive surfaces like graphite(0001) and xenon(111) allows us to simulate complete DFM images. A comparison between experiment and simulation shows that on xenon, atoms are imaged as maxima, whereas on graphite, the atomic positions of carbon atoms appear as minima and the hollow sites as maxima, in contrast to a simple interpretation of the experimental images.
5:00 PM		SS1-WeA-10 Characterization of Structure Transition in Ion-Implanted Amorphous Silicon J.-Y. Cheng (University of Illinois at Urbana-Champaign); J.M. Gibson, P.M. Baldo (Argonne National Laboratory) We use fluctuation electron microscopy to characterize disordered structures in silicon. In fluctuation electron microscopy, variance of dark-field image intensity contains the information of high-order atomic correlations, primarily in medium-range order length scale (1-3nm). In this study, amorphous silicon is produced by self-ion implantation of silicon at liquid nitrogen temprature, followed by annealing processes. As-implanted and annealed structures have been identified as paracrystalline structures and a continuous random network. However, the connection of structure transition to free energy release has not yet been fully understood. We will present new results from materials prepared by post-anneal He irradiation and post-He-bombardment annealing, and discuss effects of He implantation as the system evolves in consecutive treatments.