Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L21: Focus Session: Imaging and Modifying Materials at the Limits of Space and Time Resolution II |
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Sponsoring Units: DMP GIMS DCP Chair: Richard Haglund, Vanderbilt University Room: D161 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L21.00001: Nanoscale phase transitions within single ion tracks. William Weber, Ram Devanathan, Pedro Moreira The dynamics of track development due to the passage of energetic ions through solids is a long-standing issue relevant to nuclear materials, age-dating of minerals, space exploration, and nanoscale fabrication of novel devices. We have integrated computer simulation and experimental approaches to investigate nanoscale phase transitions under the extreme conditions created within single tracks of energetic ions in the Gd$_{2}$Zr$_{2-x}$Ti$_{x}$O$_{7}$ system and ZrSiO$_{4}$. Based on the inelastic thermal spike model, we have used molecular dynamics simulations to follow the time evolution of the structure of individual tracks and to reveal the phase transition pathways to experimentally observed concentric track structures. The molecular dynamics simulations clearly demonstrate the dependence of track evolution on composition, deposited energy density, and the complex competition among melting, disordering and recrystallization processes. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L21.00002: Exploring electron beam induced heat and mass transport at the atomic scale Christian Kisielowski In recent years the performance of mid-voltage electron microscopes was significantly boosted to reach deep sub-{\AA}ngstrom resolution around 0.5 {\AA} at 300 kV in broad beam (TEM) and focused probe (STEM) modes. Atomic resolution microscopy at voltages as low as 50 kV (and possibly below) was fostered. As a result the detection of single atoms across the Periodic Table of Elements is now possible even if light atoms are considered. After decades of striving for resolution enhancement, electron microscopy has now reached a limit that is given at a fundamental level by the Coulomb scattering process itself and by beam-sample interactions, which set a maximum dose limit that can be easily reached for soft and hard materials with the developed high-brightness electron guns. Consequently, new frontiers for electron microscopy emerge and this contribution addresses dynamic processes at the single atom level that can now be captured in time series of images at frequencies below 1 Hz reaching towards kHz. In this frequency range much of the observed atom dynamics is electron beam induced and the control of beam-sample interaction imposes constraints as well as opportunities. In this contribution it is shown that it seems feasible to exploit beam sample interactions to gain better insight into heat and mass transport in soft and hard matter at atomic resolution. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L21.00003: Gold nanoislands for sensitivity enhancement in organic and imaging mass spectrometries (LDIMS, keV- and MeV-SIMS) Arnaud Delcorte, Oscar Restrepo, Aneesh Prabhakaran Gold nanoparticles condensed on the surface of organic materials induce large ion yield enhancements in secondary ion mass spectrometry, using atomic projectiles. Here, we first show that the interest of surface metallization extends to MeV-SIMS and to UV laser desorption/ionization, in which the energy of the primary beam is deposited through the electronic subsystems (but not to keV-cluster-SIMS). For the three methods, gold nanoislands induce at least a ten-fold increase of the characteristic fragment and molecular ion yields, making surface metallization an interesting approach for imaging MS of organic surfaces. In the second part of this report, we discuss the underlying physics. For instance, using molecular dynamics simulations, we explain why 10 keV atomic projectiles interacting with metallized organic surfaces desorb more molecules, and why it is not the case with cluster projectiles such as C$_{60}$ and Au$_{400}$. For the other regimes of irradiation, arguments involving photon absorption and electronic effects are proposed. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L21.00004: Surface characterization at the spatial resolution limit with Individual Cluster Impacts Francisco Fernandez-Lima, Michael J. Eller, John D. DeBord, Stanislav V. Verkhoturov, Serge Della-Negra, Emile A. Schweikert The use of cluster bombardment (e.g. C$_{60}$ and Au$_{400})$ for surface analysis and characterization has shown significant advantages due to enhanced emission of molecular ions, low damage cross section, and reduced molecular fragmentation. At temporally and spatially discrete cluster impacts, the small impacted volume (10$^{3}$ nm$^{3})$ and ionized ejecta are ideal candidates for surface molecule interrogation. In the present talk, recent measurements of co-emitted photons, electrons and secondary ions from individual cluster impacts for several projectile-target combinations will be presented. Inspection of the photon and electron emissions show that the emission profiles are correlated with the target structure/composition at the nanometer level, with the particularity that co-emitted photons, electrons and secondary ion pairs can be used as indicators of the surface content and homogeneity. Examples of surface mapping of intact molecules via electron emission microscopy combined with secondary ion detection will be shown. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L21.00005: Electronic response of dielectric covered metal surfaces to highly charged ions R.E. Lake, J.M. Pomeroy, C.E. Sosolik The strong Coulombic perturbation on a solid target from a highly charged ion (HCI) initiates a complex many-body response from target electrons that can produce novel effects such as potential energy sputtering, nanofeature formation and huge secondary electron yields. Far above the surface, HCIs reach a critical electron capture distance and neutralization proceeds via resonant charge transfer over the vacuum barrier [1]. Motivated by recent experiments [2], we detail the onset of charge transfer between a HCI and a metal covered with a dielectric thin film (Co with 1.5 nm Al$_{2}$O$_{3}$) to determine the film's effect on the critical distance. Surprisingly, we find that the first captured electrons are pulled through the exposed dielectric and come from the underlying metal. Additionally, the Al$_{2}$O$_{3}$ film lowers the effective work function of the target and extends the critical distance compared to a clean metal. I will discuss how the experimental parameters (thin film material/thickness and ion charge state/velocity) can be tuned to allow the ion to interact with electrons in either the metal or thin film.\\[4pt] [1] Phys. Rev. A \textbf{44}, 5674 (1991).\\[0pt] [2] J. Phys.: Condens. Matter \textbf{22}, 084008 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L21.00006: Extraordinary sensitivity of nanoscale infrared spectroscopy demonstrated on Graphene and thin SiO$_2$ Greg Andreev, Z. Fei, W. Bao, Z. Zhao, C.N. Lau, L.M. Zhang, M. Fogler, G. Dominguez, M. Thiemens, F. Keilmann, D. Basov Infrared Spectroscopy is a powerful tool for characterizing materials by their vibrational mode fingerprint and/or electron conductivity. Its application to nanoscale resolved studies is highly desirable but remained challenging mainly for two reasons: a suitable source of intense, broadband infrared illumination was not widely available and the spatial resolution of conventional microscopes was limited by diffraction. We have resolved both issues by utilizing tunable External Cavity Quantum Cascade Lasers (EC-QCLs) as an intense illumination source for a scattering Scanning Near Field Optical Microscope (s-SNOM), capable of $<$10nm spatial resolution. With this combination of EC-QCLs + s-SNOM we demonstrate $<$10nm resolution imaging and spectroscopy of extremely thin materials: Silicon oxide layers (SiO$_2$) as thin as 2nm and even single atomic layers of Carbon (Graphene). The spectra register contrasts for volumes as small as 20x20x1nm$^3$ = 400 yoktoliters of SiO$_2$, and about 70 yl of Graphene over a broad spectral range: 1065-2250cm$^{-1}$. We explain the origins of this extraordinary sensitivity with an improved theoretical framework for calculating the near field response of a multilayer system. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L21.00007: Mesoscopic metal-insulator transitions at twin domain walls in improper ferroelastic VO$_2$ Invited Speaker: Appearance of unusual phenomena at interfaces of different materials due to symmetry breaking and atomic, electronic, or spin reconstructions is well established area of intensive research. Domain walls in ferroic materials also can show unusual behavior due to symmetry discontinuities. VO$_2$ is a strongly-correlated-electron material, which exhibits a metal-insulator phase transition with a structural, lattice symmetry-lowering transformation making this material an improper ferroelastic. We observe mesoscopic metal-insulator transitions at the ferroelastic domain walls in the lower-symmetry phase of VO$_2$ that occur at temperatures as much as 10-12 $^{\circ}$C below the bulk transition, resulting in the formation of metallic channels in the semiconducting material. The experiments are made using AFM-based scanning near-field microwave microscopy, which allows simultaneous accurate imaging of topography and the low-frequency dielectric function with a special resolution as high as 50 nm. The latter is possible due to a relatively high frequency used (in a few GHz range), when the sample-probe capacitive coupling becomes sufficiently strong and the electric current path is complete by displacement currents between the sample, probe tip, and the probe shield electrode. Density functional calculations indicate that ferroelastic domain walls of this type possess metallic character at low temperatures, which can be ascribed to elevated structural symmetry at the domain walls. The observed behavior, linked as well to the strain inhomogeneity inherent to ferroelastic materials, is generally relevant to symmetry-lowering phase transitions in other material systems. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L21.00008: Simulation of Non-contact Atomic Force Microscopy for Structural Analysis James Chelikowsky, Tzu-Liang Chan, C.Z. Wang, Kai-Ming Ho A powerful probe of materials centers on the use of atomic force microscopy (AFM). However, an analysis of AFM images can be complex and problematic. We will present an efficient scheme to simulate non-contact AFM images by employing a first-principles self-consistent potential from the sample as the essential input. This scheme does not require an explicit modeling of the AFM tip. Our method will be illustrated by applying it to various types of semiconductor surfaces including Si(111) (7x7), TiO2 (110) (1x1), Ag/Si(111)- ($\sqrt{3} \times \sqrt{3}$) R30$^o$ and Ge/Si(105) (1x2) surfaces. We obtain good agreement with experimental results and previous theoretical studies by using this scheme. The method can quickly and efficiently aid in identifying different models for surface structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L21.00009: Synchrotron X-ray Enhanced Scanning Tunneling Microscopy Volker Rose, John Freeland Proper understanding of complex phenomena occurring in nanostructures requires tools with both the ability to resolve the nanometer scale as well as provide detailed information about chemical, electronic, and magnetic structure. Scanning tunneling microscopy (STM) achieves the requisite high spatial resolution; however, direct elemental determination is not easily accomplished. X-ray microscopies, on the other hand, provide elemental selectivity, but currently have spatial resolution only of tens of nanometers. We present a novel and radically different concept that employs detection of local synchrotron x-ray interactions utilizing a STM that provides spatial resolution, and x-ray absorption directly yields chemical, electronic, and magnetic sensitivity. If during tunneling the sample is simultaneously illuminated with monochromatic x-rays, characteristic absorption will arise. Electrons that are excited into unoccupied levels close to the Fermi level modulate the tunneling current giving rise to elemental contrast. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L21.00010: Frequency comb generation in a tunneling junction by intermode mixing of ultrafast laser pulses Mark Hagmann, Dzmitry Yarotski, Anatoly Efimov, Antoinette Taylor Nonlinear interaction of electromagnetic radiation with tunneling electrons results in a number of peculiar physical phenomena, such as frequency mixing and imaging of insulating surfaces with scanning tunneling microscopy (STM). Arguably, the most promising among them is coupling of femtosecond laser pulses to the STM for material dynamics observation at nm/ps scales. However, the underlying physics is still poorly understood and the majority of existing studies of nonlinear mixing have been restricted to the use of CW lasers in a narrow range. Here, we present a new method for the hyper-spectral characterization of the nonlinear effects in tunneling junction. We use a 10-fs laser pulses at a nominal repetition rate of 74.25 MHz to generate a frequency comb in the tunneling current with frequencies up to 1 GHz. The typical output power at the fundamental (repetition) frequency is -120 dBm, and decreases for higher harmonics. The observed magnitude and square-law dependence of the signal power on the tunneling current and incident laser power are in good agreement with theoretical predictions. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L21.00011: First principles computation of dynamical structure factor in real and momentum space in cuprates Yung Jui Wang, B. Barbiellini, Hsin Lin, Tanmoy Das, Susmita Basak, P. E. Mijnarends, S. Kaprzyk, R. S. Markiewicz, A. Bansil We present a method for efficient, accurate first-principles calculations of the dynamical structure factor $S(\textbf{q},\omega)$ in periodic systems, using products of real space Green functions and fast Fourier transforms (FFT). We further invert $S(\textbf{q},\omega)$ via Fourier transformation [1] to reconstruct the propagator of electron density $X(\textbf{x},t)$ in real space and time domain, thereby visualising spatially the dynamics of an electron doped cuprate system in real time. The present method is useful for many-body perturbation theories of excitations based on Density Function Theory (DFT) and modeling of various highly resolved spectroscopies going beyond the standard LDA [2-5]. Some illustrative examples will be presented. Work supported by the US DOE.\\[4pt] [1] P. Abbamonte \textit{et al.}, Phys. Rev. Lett. {\bf 92}, 237401 (2004).\\[0pt] [2] Susmita Basak \textit{et al.}, Phys. Rev. B {\bf 80}, 214520 (2009).\\[0pt] [3] J. Nieminen \textit{et al.}, Phys. Rev. B {\bf 80}, 134509 (2009). \\[0pt] [4] R. S. Markiewicz \textit{et al.}, Phys. Rev. B {\bf 77}, 094518 (2008).\\[0pt] [5] G. Stutz \textit{et al.}, Phys. Rev. B {\bf 60}, 7099 (1999). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L21.00012: Beam self-focusing in the near field emission scanning electron microscopy Fuxiang Li, Artem Abanov Recent experiment on the near field emission scanning electron microscopy shows an unexpectedly high lateral and vertical resolution. We show that these effects can be explained by the beams self-focusing. We derive the equations for the beam propagation and solve them numerically. Our results are in a very good agreement with the experiment. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L21.00013: Enhancing the spatial resolution in PEEM beyond 30nm using diamondoid surface coating Hitoshi Ishiwata, Hendrik Ohldag, Zhi-Xun Shen, Nick Melosh, Andreas Scholl The spatial resolution in Photoemission Electron Microscopy typically does not allow imaging features smaller than 30nm. PEEM resolution is limited by chromatic and spherical aberrations of the electrostatic lenses in the microscope column in combination with a wide angular and energy distribution of the secondary electrons to make these aberrations significant. Diamondoids have recently been shown to act as a monochromator for secondary electrons, thus reducing chromatic aberration in PEEM. In addition to improving the resolution of the microscope the diamondoid coating will also enhance the image intensity since now more secondary electrons will be accepted by the aperture. At 10kV the spatial resolution of PEEM3 is of the order of 150-200nm so that the magnetic domains can hardly be recognized anymore without the diamondoid coating. However, they become visible on the sample that was coated with diamondoids, indicating that the coating improved the spatial resolution by monochromatizing the secondary electrons. We also find that the image intensity is enhanced by a factor of 2-3 with the diamondoid coating. These initial findings on samples with relatively large domains of 150nm are very encouraging and we are therefore convinced that we can push the resolution limit below 30nm studying samples with smaller domains at higher acceleration voltages of 20kV. [Preview Abstract] |
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