Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A21: Focus Session: Advances in Scanned Probe Microscopy I: Novel Approaches and Ultrasenstive Detection |
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Sponsoring Units: GIMS Chair: Nikolai Zhitenev, National Institute of Standards and Technology Room: 201 |
Monday, March 2, 2015 8:00AM - 8:12AM |
A21.00001: Cryogen-free low temperature STM/AFM based on a closed cycle cryostat Byoung Choi, Stefen Ulrich, Ryan Murdick Closed cycle cryogenic scanning tunneling microscope (CCC-STM) and atomic force microscope (AFM) will be presented. By using He heat exchange gas, thermally linked and mechanically decoupled CCC-STM/AFM enables atomically resolved microscopy and spectroscopy on various surfaces. We will present the noise measurement of the tunneling current and the thermal drift analysis. Temperature as low as 14K on the sample and the tip-sample distance fluctuation as low as 2 picometer have been achieved from 9K cryostat after 8h of cooling time. Low thermal drift in a lateral direction (\textless 0.1nm/hr) enables to get a scanning tunneling spectroscopy grid with more than 64x64 pixels which typically takes over 10 hrs. We will also present the high stability and reproducibility of the CCC-STM/AFM with the atom resolved imaging of Si, Pt, Au and KBr surfaces in STM and AFM mode. These results demonstrate that the developed CCC-STM/AFM is a~versatile~instrument enabling experiments on a variety of materials and surfaces at picometer resolution without using any liquid cryogen. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A21.00002: Construction of a $^3$He Magnetic Force Microscope with a Vector Magnet Jinho Yang, Yunwon Kim, Ilkyu Yang, Juyoung Jeong, Dongwoo Shin, Dirk Wulferding, Yoonhee Jeong, Hanwoong Yeom, Jeehoon Kim We have built a $^3$He magnetic force microscope (MFM) with a base temperature of 300 mK, operating in a vector magnet with the field of Z=9 T, X=2 T, Y=2 T for each axis. We employed a fiber interferometer system for detection of a cantilever motion that includes two auttocube types of walkers for alignment between the fiber end and a cantilever tip. We apply our novel microscope to investigate unconventional magnetic materials and superconductors such as centrosymmetric skyrmion crystals and Heavy Fermion Superconductors. We will show some preliminary MFM images in these systems [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A21.00003: Mapping surface charge density with a scanning nanopipette Lasse Hyldgaard Klausen, Thomas Fuhs, Flemming Besenbacher, Mingdong Dong Characterisation of the surface charge density (SCD) is important in interface and colloid science, and especially local variations in SCD of biological samples are of keen interest. The surface charge of lipid bilayers governs the uptake of charged particles and guides cell-cell interactions. As the electrostatic potential is screened by high physiological salt concentrations, direct probing of the potential can only be performed at a sub nanometer distance; therefore it was impossible to directly measure the SCD under physiological conditions. Yet the charged surface attracts counter ions leading to an enhanced ionic concentration near the surface, creating a measurable surface conductivity. In this study we measure SCD using a scanning ion-conductance microscope (SICM) setup, where the electrolyte current through a nanopipette is monitored as the pipette is positioned in the vicinity of the sample. We investigate the current dependency of SCD and pipette potential using numerical solutions to Poisson and Nernst-Planck equations and characterise a complex system governed by a multitude of factors such as pipette size, geometry and charge. We then propose an imaging method and prove its feasibility by mapping the surface charge density of phase separated lipid bilayers. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A21.00004: The SQCRAMscope: Probing exotic materials with quantum gases Shenglan Qiao, Richard Turner, Jack DiSciacca, Benjamin Lev Microscopy techniques co-opted from nonlinear optics and high energy physics have complemented solid-state probes in elucidating exotic order manifest in condensed matter materials.~ Up until now, however, no attempts have been made to use modern techniques of ultracold atomic physics to directly explore properties of strongly correlated or topologically protected materials.~ Our talk will present the SQCRAMscope, a novel Scanning Quantum CRyogenic Atom Microscope technique for imaging magnetic and electric fields near cryogenically cooled materials.~ With our SQCRAMscope, we aim to image inhomogeneous transport and domain percolation in technologically relevant materials whose order has evaded elucidation. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A21.00005: Simulated imaging of intermolecular bonds using high throughput real-space density functional calculations Alex Lee, Minjung Kim, James Chelikowsky Recent experimental noncontact atomic force microscopy (AFM) studies on 8-hydroxyquinoline (8-hq) assemblies have imaged distinct lines between molecules that are thought to represent intermolecular bonding. To aid the interpretation of these images, we calculate simulated AFM images of an 8-hq dimer with a CO functionalized tip using a real-space pseudopotential formalism. We examine the effects of Pauli repulsion and tip probe relaxation as explanations for the enhanced resolution that resolves these intermolecular force lines. Our study aims to compute \emph{ab initio} real-space images of intermolecular interactions. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A21.00006: \textit{Ab initio} simulations of subatomic resolution images in noncontact atomic force microscopy Minjung Kim, James R. Chelikowsky Direct imaging of polycyclic aromatic molecules with a subatomic resolution has recently been achieved with noncontact atomic force microscopy (nc-AFM). Specifically, nc-AFM employing a CO functionalized tip has provided details of the chemical bond in aromatic molecules, including the discrimination of bond order. However, the underlying physics of such high resolution imaging remains problematic. By employing new, efficient algorithms based on real space pseudopotentials, we calculate the forces between the nc-AFM tip and specimen. We simulate images of planar organic molecules with two different approaches: 1) with a chemically inert tip and 2) with a CO functionalized tip. We find dramatic differences in the resulting images, which are consistent with recent experimental work. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:48AM |
A21.00007: Absorption Spectroscopy and Imaging from the Visible through Mid-IR with 20 nm Resolution Using AFM probes Invited Speaker: Andrea Centrone Correlated nanoscale composition and optical property maps are important to engineer nanomaterials in applications ranging from photovoltaics to sensing and therapeutics. Wavelengths ($\lambda $s) from the visible to near-IR probe electronic transitions in materials, providing information regarding band gap and defects while light in mid-IR probes vibrational transitions and provide chemical composition. However, light diffraction limits the lateral resolution of conventional micro-spectroscopic techniques to approximately $\lambda $/2, which is insufficient to image nanomaterials. Additionally, the $\lambda $-dependent resolution impedes direct comparison of spectral maps from different spectral ranges. Photo Thermal Induced Resonance (PTIR) is a novel technique that circumvents light diffraction by employing an AFM tip as a local detector for measuring light absorption with $\lambda $-independent nanoscale resolution. Our PTIR setup combines an AFM microscope with three lasers providing $\lambda $-tunability from 500 nm to 16000 nm continuously. The AFM tip transduces locally the sample thermal expansion induced by light absorption into large cantilever oscillations. Local absorption spectra (electronic or vibrational) and maps are obtained recording the amplitude of the tip deflection as a function of $\lambda $ and position, respectively. The working principles of the PTIR technique will be described first, and nano-patterned polymer samples will be used to evaluate its lateral resolution, sensitivity and linearity. Results show that the PTIR signal intensity is proportional to the local absorbed energy suggesting applicability of this technique for quantitative chemical analysis at nanoscale, at least for thin (less than 1000 nm thick) samples. Additionally, a $\lambda $-independent resolution as high as 20 nm is demonstrated across the whole spectral range. In the second part of the talk, PTIR will be applied to image the dark plasmonic resonance of gold Asymmetric Split Ring Resonators (A-SRRs) in the mid-IR. Additionally, the chemically-specific PTIR signal will be used to map the near-field absorption enhancement of PMMA coated A-SRRs, revealing hot-spots with enhancement factors up to $\approx $ 30. PTIR has broad applicability; recent examples from my lab include the characterization of chemically heterogeneous domains in metal-organic frameworks crystals and solar cells materials. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A21.00008: NC-AFM identification of different aluminum atoms on Al$_{2}$O$_{3}$/NiAl(110) surface Ivan Stich, Jan Brndiar, Yan Jun Li, Yasuhiro Sugawara Ultrathin alumina film formed by oxidation of NiAl(110) is widely used as a system for technologically important oxide-supported catalysts. Using small amplitude NC-AFM we have obtained images of this system with unprecedented resolution, significantly surpassing the previous STM and NC-AFM images. In particular, we are able to resolve aluminum atoms with different coordination, such as five-, and four-fold coordinated Al atoms. Experiments are supported by extensive density functional theory modeling. Starting from the previous atomic model [1], we have been able to describe the gross image features such as the dark oxygen sites. We find that the system is strongly ionic with the oxygen sites strongly negatively charged and aluminum sites positively charged. Hence, the NC-AFM images can reliably be understood from electrostatic potentials. These finding also suggest an oxygen terminated apex. Resolving finer contrast features of the differently coordinated Al atoms required construction of better and more realistic approximants to the ultra-thin incommensurable alumina interface. [1] G. Kresse et al., Science \textbf{308}, 1440 (2005). [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A21.00009: Improving Nano-MRI Spatial Resolution with Phase Multiplexing Brad Moores, Alex Eichler, Christian Degen Magnetic resonance force microscopy (MRFM) is a scanning probe technique that allows measuring nuclear spin densities with resolution better than 10nm. Detecting such small volumes of spins (less than (10nm)$^{\mathrm{3}}$ corresponds to approximately 20,000 spins) requires long averaging of signals from statistically polarized nuclei. For instance, previous work demonstrated that imaging a single isotope ($^{\mathrm{1}}$H) of a Tobacco Mosaic Virus required averaging for 2 weeks, and therefore the chemical contrast abilities of MRFM had to be forfeited to enable higher spatial resolution. In order to reconcile the chemical selectivity of MRFM along with the proven high spatial resolution, we have developed a phase multiplexing technique capable of simultaneously acquiring spin signals from multiple isotopes and from up to six spatial locations. We have demonstrated this method using a nanowire test sample, and have achieved one-dimensional imaging resolution of less than 5 nm and subnanometer positional accuracy. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A21.00010: Direct visualization of concerted proton tunneling in a water nanocluster Xiangzhi Meng, Jing Guo, Jinbo Peng, Ji Chen, Zhichang Wang, Jun-Ren Shi, Xin-Zheng Li, En-Ge Wang, Ying Jiang Proton transfer through hydrogen bonds is of great importance to many aspects of physics, chemistry and biology, such as phase transition, signal transduction, topological organic ferroelectrics, photosynthesis, and enzyme catalysis. The proton dynamics is susceptible to nuclear quantum effect in terms of proton tunneling, which tends to involve many hydrogen bonds simultaneously, leading to correlated many-body tunneling. In contrast to the well-studied incoherent single particle tunneling, our understanding of the many-body tunneling, especially the effect of local environment on the tunneling process, is still in its infancy. Here we report the real-space observation of concerted proton tunneling within a hydrogen-bonded water tetramer using a cryogenic scanning tunneling microscope (STM). This is achieved by monitoring in real time the reversible interconversion of the hydrogen-bonding chirality of the cyclic water tetramer with a chlorine-terminated STM tip. Interestingly, we found that the presence of the Cl anion at the tip apex may either enhance or suppress the concerted tunneling process depending on the details of coupling symmetry between the Cl anion and the protons. This work opens up the possibility of controlling the quantum states of protons with atomic-scale precision. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A21.00011: Connecting the Tips of a Millikelvin Dual-Tip STM Wan-Ting Liao, Michael Dreyer, Christopher Lobb, Frederick Wellstood, Robert Anderson We have built a dual tip scanning tunneling microscope (STM) [1] with two niobium tips connected by niobium foil made by laser cutting. Both tips could be Josephson-coupled to a superconducting surface sample to form an asymmetric SQUID loop. Our scheme involves holding one of the tips fixed as a reference junction while the other tip is scanned to image the gauge-invariant phase of superconductors at the atomic scale [2]. The microscope has worked at millikelvin temperature with two independent tips and we are testing the connected-tips setup at room temperature. We will modulate the z voltage of each tip and use two lock-in amplifiers to distinguish the current contribution of each junction allowing us to independently scan two regions of the sample. [1] A. Roychowdhury, et al., Rev. Sci. Inst. \textbf{85}, 04.706(2014) [2] ``Asymmetric superconducting quantum interference devices for suppression of phase diffusion in small Josephson junctions'', D. F. Sullivan, et al., J. Appl. Phys. \textbf{113}, 183905 (2013) [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A21.00012: Kelvin probe force microscopy: imaging open-circuit voltage in optoelectronic devices Elizabeth Tennyson, Joseph Garrett, Jesse Frantz, Jason Myers, Robel Bekele, Jasbinder Sanghera, Jeremy Munday, Marina Leite Scanning probe microscopy has been successfully implemented to probe the electrical characteristics of optoelectronic devices. Currently, a method that directly correlates measured signals to device performance is missing. We implement illuminated Kelvin probe force microscopy (KPFM) to spatially resolve the open-circuit voltage of optoelectronics with nanoscale resolution, 5 orders of magnitude better than previous methods. In illuminated-KPFM, the surface photovoltage, is the difference between the contact potential difference under illumination and in the dark, and proportional to the Fermi level splitting. We apply our imaging method to a variety of solar cells and find that the open-circuit voltage in some materials varies locally by \textgreater 0.2 V, suggesting the spatial variation of non-radiative recombination strongly affects performance. A detailed examination of possible topography pick-up was excluded by measuring samples with modified surface morphology and considering the tip-sample separation dependence of the signal. This novel metrology enables new insights into the loss mechanisms that hinder solar cells and provides a new platform to image device performance with nanoscale resolution. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A21.00013: A method for correcting out-of-plane, fast time scale positional drift in atomic force microscopy John P. Wheeler, Pardeep S. Banwait, Isaac D. Rohrer, Greg A. Hamilton, Kelly A. Shaw, Michael C. Leopold, Matthew L. Trawick We describe a method for correcting atomic force microscopy images that have been affected by random fluctuations (fast time scale positional drift) in measurements along the axis perpendicular to the sample plane. These fluctuations, typically manifested as random horizontal streaks or bands across the image, have several sources, including electrostatic charging and the transfer of material between the sample and the tip. Our correction method involves scanning a second, partial image after each full image scan, and applying an offset correction to each individual scan line in both images in order to minimize the statistical discrepancy between them. This method supersedes the widely used ``flattening'' algorithm, which can destroy valid height information and can create additional image artifacts. [Preview Abstract] |
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