Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W46: Focus Session: Advances in Scanned Probe Microscopy III: Novel Approaches and Ultrasensitive Detection |
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Sponsoring Units: GIMS Chair: Eric Hudson, Pennsylvania State University Room: Hilton Baltimore Holiday Ballroom 5 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W46.00001: Probing single molecules with the STM in the frequency and time domains Hikari Kimura, Weicai Cao, Calvin Patel, Wilson Ho We have constructed a scanning tunneling microscope (STM) and combined it with a tunable femtosecond laser (210 nm to 1040 nm) to probe single molecules with simultaneous spatial and temporal resolutions. Employing the RF lock-in amplifier to measure the laser-induced tunneling current that is directly synchronized with the high repetition rate of the laser ($\sim$80 MHz), time resolved measurement of single molecules with atomic scale resolution can be achieved by varying the time delay between pairs of laser pulses in the two-pulse correlation or two-color pump-probe configuration. A femtosecond laser system with widely tunable wavelength enables resonant excitation of single molecules that are partially decoupled electronically from the underlying metallic substrate by a thin oxide or additionally atomic or molecular layers. The experimental arrangement allows measurement of molecular lifetimes by two-photon photoemission spectroscopy and microscopy. [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W46.00002: High Resolution Single Molecule Vibrational Spectroscopy with the STM Chen Xu, Chilun Jiang, Yanning Zhang, Ruqian Wu, Wilson Ho Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) has been regarded as the ultimate tool to identify and characterize single molecules adsorbed on solid surfaces with atomic spatial resolution. With the improvement of energy resolution obtained at $\sim$ 600 mK, STM-IETS is able to resolve the lowest vibrational energies and reveal subtle interactions between the molecule and its environment which were previously not possible at higher temperatures. Here we demonstrate the capability of sub-Kelvin STM on detecting the influence of the tip as well as the anisotropy of the reconstructed Au(110) surface on the low energy hindered vibrational motions of single adsorbed CO molecule. Single molecule vibrational spectroscopy at $\sim$ 600 mK with atomic scale spatial resolution opens new possibilities to probe molecular interactions with high spectral sub-THz resolution. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W46.00003: Measuring infrared absorption of molecular adsorbates at the submonolayer level by scanning tunneling microscopy-based IR spectroscopy (IR-STM) Ivan V. Pechenezhskiy, Giang D. Nguyen, Xiaoping Hong, Jeremy E. P. Dahl, Feng Wang, Michael F. Crommie Here we present a simple, effective technique whereby a scanning tunneling microscope (STM) can achieve vibrational spectroscopy of molecular adsorbates at the submonolayer level through the use of a tunable infrared (IR) laser source. By using the STM as a detector to probe the IR molecular response, the technique takes advantage of the high spectral resolution inherent to IR measurements while avoiding the typical difficulties related to optical detection. This technique also allows sub-nm scale spatial mapping of surface structure under the same experimental conditions that the STM-IR absorption spectra are acquired (sub-nm spatial resolution for specific IR spectral features has not yet been achieved). Using this technique we have obtained IR absorption spectra of higher diamondoid molecules, specifically [121]tetramantane and [123]tetramantane, deposited on a Au(111) surface. The significant differences between the IR-STM spectra obtained for these two molecular isomers show the power of this new technique to differentiate chemical structures. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W46.00004: Imaging the Electron-Phonon Interaction on the Atomic Scale Igor Altfeder, Konstantin Matveev, Andrey Voevodin New STM-based spectroscopic imaging technique, direct real-space imaging of electron-phonon interaction parameter $\lambda $, was demonstrated using the combination of STM and inelastic electron tunneling spectroscopy (IETS) for thin Pb islands epitaxially grown on 7x7 reconstructed Si(111). We found that $\lambda $ increases when the electron scattering at the Pb/Si(111) interface is diffuse and decreases when the electron scattering becomes specular. We show that the effect is driven by transverse redistribution of the electron density inside a quantum well. Reference: Igor Altfeder, K. A. Matveev, A. A. Voevodin, ``Imaging the Electron-Phonon Interaction on the Atomic Scale'', Physical Review Letters 109, 166402 (2012). [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W46.00005: Vibrational and electronic properties of small molecules on metal surfaces Yanning Zhang, Chen Xu, Chi-Lun Jiang, Wilson Ho, Ruqian Wu Research of manipulating chemical bonds in a single molecule has been extremely active in recent years. Using a newly built milli-Kelvin scanning tunneling microscope, we can now resolve vibrational spectroscopic features down to a few tenths meV. Synergistic density functional calculations allow correct interpretation for each vibrational mode and provide links between experimental observations to the change of individual chemical bonds. In particular, we explored the effect of tunneling gap distance on different vibrational energies, by moving the tip toward the molecules, so as to shed some light for selective bond dissociation and formation. Here we discuss our results of the atomic structure, vibrational and electronic properties of several small molecules such as CO on the anisotropic Au(110) surface and C2H2 on the Cu(001) square lattice. Calculated vibrational frequencies, using the generalized gradient approximation or the non-local van der Waals density functional, are in good agreement with experimental results. \textbf{Acknowledgement. }Work was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE. [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W46.00006: Design and Implementation of a 4K Cryocooler-Based Scanning Tunneling Microscope Ramya Vishnubhotla, Neal Harrington, Bill Dusch, Carrie Geng, Riju Bannerjee, Lavish Pabbi, Eric W. Hudson Low temperature, ultra-high vacuum scanning tunneling microscopes (STMs) have proved to be excellent tools for the study of electronic properties of complex materials. Unfortunately, with the continuing increase in liquid helium prices, already a dominant cost for operating these systems, their use is becoming exceedingly expensive. Here we describe the design and implementation of a STM cooled by a Cryomech PT407 Remote Motor Cryorefrigerator, allowing us to reach helium temperatures using a closed thermodynamic cycle with zero cryogen waste. Unfortunately, this refrigeration technique is not ultra-high vacuum (UHV) compatible and introduces vibrations. To tackle these problems, we separately house the cryocooler in a high-vacuum (HV) chamber. This provides both a UHV environment for the STM and mechanical isolation to minimize vibrations reaching the instrument. However, it makes for more challenging thermal connections. This last difficulty we solve by introducing a novel coaxial thermal feedthrough between the HV and UHV chambers. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 4:18PM |
W46.00007: Spin dynamics of atoms and magnetic nanostructures on surfaces Invited Speaker: Andreas Heinrich Scanning tunneling microscopy is a powerful tool for studying the electronic and magnetic properties of magnetic nanostructures on surfaces. Over the last decade, inelastic tunneling spectroscopy has been used to probe discrete energy levels of quantum spin systems. These states can often be described as solutions of simple spin Hamiltonians. In spin excitation spectroscopy, a spin system is kicked from the ground into excited spin states at discrete energy increments. In this talk we will focus on the dynamics of quantum spin systems on surfaces. STM can measure tunnel currents in the range of pico amps with millisecond time resolution. This time resolution is well matched to observing transition between spin states of artificial magnetic nanostructures on surfaces that can be built and measured with STM. We will highlight an example of extended, artificial antiferromagnets on a Cu2N surface (Science 2012). Smaller magnetic clusters relax much faster but their dynamics can be measured with pump probe techniques. A pump voltage pulse drives the spin system into excited states and a subsequent probe pulse measures the resulting population of spin states. An exponential decay back to the ground state is observed when averaging over many pump-probe cycles (Science 2010). We will show results down to nanosecond time resolution with an ultimate limit set by modern electronics at about 100 pico seconds. Individual atoms on Cu2N relax their spin states even faster. Hence, another technique is employed to determine spin relaxation times: small tunnel currents always leave the spin system in the ground state while high currents can create non-equilibrium distributions of spin states. This approach relies on some modeling but allows time domain measurements down to about 1 pico second (Nature Physics 2010). Transition metal atoms on metal surfaces relax even faster, on time scales of about 100 femtoseconds. This fast relaxation manifests itself as a measurable lifetime broadening of spin excitation spectra. Combining these approaches allows measurements of spin relaxation times over about 16 orders of magnitude for spins on surfaces -- while maintaining the atomic scale spatial resolution of STM! [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W46.00008: A versatile variable field module for Asylum Cypher scanning probe system Hongxue Liu, Ryan Comes, Jiwei Lu, Stuart Wolf, Jim Hodgson, Maarten Rutgers Atomic force microscopy (AFM) has become one of the most widely used techniques for measuring and manipulating various characteristics of materials at the nanoscale. However, there are very limited option for the characterization of field dependence properties. In this work, we demonstrate a versatile variable field module (VFM) with magnetic field up to 1800 Oe for the Asylum Research Cypher system. The magnetic field is changed by adjusting the distance between a rare earth magnet and the AFM probe. A built-in Hall sensor makes it possible to perform in-situ measurements of the field. Rotating the magnet makes it possible to do angular field dependent measurements. The capability of the VFM system is demonstrated by degaussing a floppy disk media with increasing magnetic field. The written bits are erased at about 800 Oe. Angular dependence measurements clearly show the evolution of magnetic domain structures. A completely reversible magnetic force microscopy (MFM) phase contrast is observed when the magnetic field is rotated by 180$^{\circ}$. Further demonstration of successful magnetic switching of CoFe$_{2}$O$_{4}$ pillars in CoFe$_{2}$O$_{4}$-BiFeO$_{3}$ nanocomposites will be presented and field dependent MFM and piezoresponse force microscopy (PFM) will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W46.00009: Magnetoelectric Force Microscopy for visualizing cross-coupled domains Yanan Geng, Weida Wu Intensive studies have been focused on magnetoelectric (ME) effect ever since Dzyaloshinskii and Astrov's seminal works on linear ME effect in Cr$_{2}$O$_{3}$. The measurements of the components of ME tensor are of great importance in technical applications and in fundamental science (e.g. determining magnetic point groups). For bulk ME measurements, it is necessary to obtain a single domain state by the ME annealing (i.e. applying magnetic and electric fields simultaneously) of the specimen through its transition temperature. However, the ME domain structure has never been directly observed due to the weakness of the ME effect in most magnetoelectrics. To address this critical issue, we have developed a nanoscale imaging technique, namely, the Magnetoelectric Force Microscopy (MeFM), to directly detect local ME response based on magnetic force microscopy with \textit{in-situ} high voltages. Preliminary results of visualizing ME domains will be presented to demonstrate the feasibility of the MeFM technique. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W46.00010: Background-free Piezoresponse Force Microscopy with high sensitivity Wenbo Wang, Yanan Geng, Weida Wu Piezoresponse Force Microscopy (PFM) detects small mechanical deformation of a specimen by applying an AC voltage between a conductive AFM tip (as a top electrode) and the bottom electrode. It is widely used for visualizing ferroelectric domain patterns with high lateral resolution. In nominal or commercial setups, the PFM signal is contaminated by the so-called ``system-inherent background'' with a complex frequency spectrum which consists of many cross-talk resonances with peak amplitude over 10 pm/V [1]. The presence of the system-inherent background will severely distort the PFM contrast (especially the phase signal) and the domain pattern in PFM images of ferroelectrics with weak piezoelectric response ($<$1 pm/V). Although the system-inherent background can be subtracted out by proper calibration using a non-piezoelectric material (e.g. glass slides), it is highly desirable to eliminate it directly from PFM setup for background-free measurements. Here we demonstrate that the system-inherent background can be eliminated using carefully designed electric wiring of PFM setup. Results of background-free PFM detection with excellent sensitivity($\le$0.1 pm/V) will be presented. \\[4pt] [1] Jungk et al, Appl. Phys. Lett. 89 163507 (2006). [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W46.00011: On-line Scanned Probe Microscopy Transparently Integrated with DualBeam SEM/FIB Systems Andrey Ignatov, Anatoly Komissar, Aaron Lewis A multifunctional scanning probe microscope (SPM) will be described that transparently integrates with a DualBeam SEM/FIB System. This is done without perturbing any of the capabilities of the Dual Beam in terms of detectors, gas injectors, analyzers etc while allowing for a completely exposed probe tip to be imaged online even with immersion objectives at working distances as short as 4 mm. In addition, the completely free motion of the rotation axis of the stage is maintained with the probe tip at the eucentric point, this makes it possible to orient the sample in any direction on any structure The X and Y scan range of the atomic force microscopic (AFM) imaging achieves 35 microns with rough motion over 10 millimeters. This permits the SPM to tilt into position perpendicular to the SEM or FIB or under an angle for rapid and accurate placement of the probe tip at or on structures such as biopolymeric materials that are nanometric in X, Y and Z extent. Thus, not only can a structure's nanometric height be accurately profiled but this can be accomplished with the on-line excellence of SEM for X, Y metrology. Furthermore, electron and ion beam sensitive samples can be imaged and characterized by AFM at high resolution. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W46.00012: Massively Multiplexed Cantilever-free Scanning Probe Lithography Keith A. Brown, Daniel J. Eichelsdoerfer, Wooyoung Shim, Radha Boya, Abrin L. Schmucker, Guoliang Liu, Chad A. Mirkin Cantilever-free scanning probe lithography has emerged as a low-cost technique for rapidly patterning nanoscale materials. In this architecture, an array of probes is fabricated on a soft backing layer that provides mechanical compliance to each probe while an underlying hard surface maintains the structural integrity of the array. One drawback of this technique is that each probe in the array acts simultaneously and thus generates a copy of the same pattern. Here, we discuss recent efforts to incorporate heaters into these probe arrays so that when a given heater is activated, the thermal expansion of the elastomer actuates a single tip. We find thermal actuation to be powerful enough to actuate individual tips over 4 $\mu $m with minimal crosstalk, fast enough to actuate on relevant time scales (20 ms), and scalable by virtue of being electrically addressable. Furthermore, tuning the individual heaters allows for variability in the arrays to be compensated for precisely, resulting in high quality nanopatterning. The addition of tunable actuators transforms cantilever-free scanning probe lithography into a technique capable of true desktop nanofabrication. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W46.00013: Tuning the Spring Constant of Cantilever-free Probe Arrays Daniel J. Eichelsdoerfer, Keith A. Brown, Radha Boya, Wooyoung Shim, Chad A. Mirkin The versatility of atomic force microscope (AFM) based techniques such as scanning probe lithography is due in part to the utilization of a cantilever that can be fabricated to match a desired application. In contrast, cantilever-free scanning probe lithography utilizes a low cost array of probes on a compliant backing layer that allows for high throughput nanofabrication but lacks the tailorability afforded by the cantilever in traditional AFM. Here, we present a method to measure and tune the spring constant of probes in a cantilever-free array by adjusting the mechanical properties of the underlying elastomeric layer. Using this technique, we are able to fabricate large-area silicon probe arrays with spring constants that can be tuned in the range from 7 to 150 N/m. This technique offers an advantage in that the spring constant depends linearly on the geometry of the probe, which is in contrast to traditional cantilever-based lithography where the spring constant varies as the cube of the beam width and thickness. To illustrate the benefit of utilizing a probe array with a lower spring constant, we pattern a block copolymer on a delicate 50 nm thick silicon nitride window. [Preview Abstract] |
Thursday, March 21, 2013 5:30PM - 5:42PM |
W46.00014: Debye screening length of electrolytic solutions from capacitive force measurements using atomic force microscopy Bharat Kumar, Scott R. Crittenden We present a method to obtain the Debye screening length of a dilute electrolytic solution by measuring the capacitve force using atomic force microscopy (AFM). A small AC bias voltage of frequency $\omega$ was applied between an AFM cantilever and conducting substrate in an electrolytic solution and the resulting capacitive force between them was measured from the cantilever oscillations. The $2\omega$ component of the oscillating force was used to obtain the capacitance gradient between the AFM cantilever tip and substrate as a function of tip-sample distance $z$. An analytic expression relating tip-sample distance $z$ and capacitance gradient between AFM tip and conducting substrate in an electrolytic solution was derived using the solution of the linearized Poisson-Boltzmann equation. We find that the analytic expression fits well with the experimental data for dilute KCl-water solutions. The fit parameters were further used to calculate the Debye screening length of the electrolytic solution. [Preview Abstract] |
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