Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N38: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods |
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Sponsoring Units: GIMS Chair: Andreas Heinrich, IBM-Almaden Room: Colorado Convention Center 501 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N38.00001: AFM/STM with sub-Angstrom modulation. Invited Speaker: Atomic manipulation of single atoms and molecules by scanning probe microscopy enables the assembly of structures at the single-atom scale - the ultimate lower size limit. However, it has been difficult to answer the simple question: How much force does it take to manipulate atoms and molecules on surfaces? To address this question, we combine scanning tunneling microscopy and frequency modulated atomic force microscopy. To enable simultaneous detection of the tunneling current and frequency shift we utilize the q-plus sensor design, in which a metallic STM tip is mounted on a cantilever made from a quartz tuning fork. The instrument operates in ultra-high vacuum at liquid helium temperature. High mechanical stability together with a stiff cantilever design, which avoids snap to contact between sample and tip, allows us to use very small modulation amplitudes of 25 pm normal to the surface. To detect such a small amplitude with a piezoelectric cantilever requires a low-temperature preamplifier stage. Mapping the frequency shift at different heights above the sample surface allows us to calculate the vertical forces acting between tip and surface. This data is then used to determine the full 3D interaction potential between the tip and a single adsorbate on a clean metallic surface by integrating the forces normal to the surface. A small amplitude is essential to achieve 10 pm resolution in all spatial directions necessary to discriminate between long range and short rage forces. With this method we are able to determine the vertical and lateral forces that are required to move individual cobalt (Co) atoms and carbon monoxide (CO) molecules across a copper (111) surface. The lateral forces, which are responsible for moving the adsorbates, are one to two orders of magnitudes smaller than the forces that act in conventional atomic force microscopy with atomic resolution. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N38.00002: Atomically-resolved surface imaging by low temperature atomic force microscopy using a quartz resonator Yukio Hasegawa, Toshu An, Takahiro Nishio, Toyoaki Eguchi, M. Ono, Kotone Akiyama We have developed a frequency-modulation atomic force microscope (FM-AFM) using a length-extension quartz resonator as a force sensor. Atomically-resolved images of the Si(111) 7x7 surface were obtained with the AFM in UHV both at room temperature [1] and 5 K. The high resonance frequency ($\sim $1 MHz) of the resonator improves the sensitivity to its deflection. Its self-sensing property eliminates the cumbersome optical alignment, which is usually required in conventional AFMs, and thus it can be easily installed into a low temperature system. The high stiffness of the resonator enables us to operate with a very small oscillation amplitude; less than 0.1nm, and thus to detect a short-range force effectively, such as a covalent bonding force, which is crucial for the highly resolved imaging. For the probe tip, a tungsten wire was attached at the end of the resonator and sharpened by focused ion beam. The native oxide layer covering the tip was removed by \textit{in-situ} field ion microscopy. [1] T. An, T. Eguchi, K. Akiyama and Y. Hasegawa, APL \textbf{87}, 133114 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N38.00003: Imaging of electronic defect states in SiO2 and HfSiOx films with sub-nanometer spatial resolution by two-way Single Electron Tunneling Force Mircroscopy J.P. Johnson, N. Zheng, C.C. Williams Electronic defects in dielectric materials are currently in sharp focus, for nano-technology and quantum information processing. A novel technique has been developed for imaging these states with sub-nanometer spatial resolution. It can be applied to completely non-conducting dielectric films, in contrast to the STM. The method is based on force detected single electron tunneling events to and from the defect states [1-3]. The exponential dependence of the tunneling rate on tip-sample gap provides the same spatial resolution as STM. An oscillating AFM tip is scanned at constant height above the sample surface. A voltage waveform, synchronous with the tip motion is applied. When the tip is above an accessible state, individual electrons shuttle between tip and state with the applied voltage (300 Hz). The two-way tunneling causes a detectable change in tip resonance. Images of SiO2 and HfSiOx films show a repeatable, random array of individual ``point-like'' defect states, some with sub-nanometer width. Spectroscopic measurements of the defect energy are also performed by this approach. The new method and an analysis of the defects in SiO2 and HfSiOx will be presented. [1] E Bussman et al., Appl. Phys. Lett. 85, 2538 (2004) [2] E Bussman and CC Williams, Appl. Phys. Lett. 88, 263108 (2006) [3] E Bussman et al., Nano Lett. 6, 2577 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N38.00004: Detection of Embedded nanostructures by Electrostatic Force Microscopy Zonghai Hu, Yuanzhen Chen, Michael Fischbein, Robin Havener, Marija Drndic Non-destructive imaging of embedded structures with high lateral resolution is of great technological interest. Scanning probe microscopy is generally thought to be sensitive only to surfaces. We report that electrostatic force microscopy (EFM) can be used to study electrostatic inhomogeneities hundreds of nanometers below a uniform sample surface with sub-micron lateral resolution. The sub-surface material can be in liquid phase. Our experimental and simulation results show that the EFM signal depends on many factors such as the distance between the tip and the sample, the depth, dielectric constants, and the carrier density of the embedded inhomogeneities. Potential applications of this technique will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N38.00005: Sinc or Sine? The Band Excitation Method and Energy Dissipation Measurements by SPM Stephen Jesse, Sergei Kalinin Quantitative energy dissipation measurements in force-based SPM is the key to understanding fundamental mechanisms of energy transformations on the nanoscale, molecular, and atomic levels. To date, these measurements are invariably based on either phase and amplitude detection in constant frequency mode, or as amplitude detection in frequency-tracking mode. The analysis in both cases implicitly assumes that amplitude is inversely proportional to the Q-factor and is not applicable when the driving force is position dependent, as is the case for virtually all SPM measurements. All current SPM methods sample only a single frequency in the Fourier domain of the system. Thus, only two out of three parameters (amplitude, resonance, and Q) can be determined independently. Here, we developed and implemented a new approach for SPM detection based on the excitation and detection of a signal having a finite amplitude over a selected region in the Fourier domain and allows simultaneous determination of all three parameters. This band excitation method allows acquisition of the local spectral response at a 10ms/pixel rate, compatible with fast imaging, and is illustrated for electromechanical and mechanical imaging and force-distance spectroscopy. The BE method thus represents a new paradigm in SPM, beyond traditional single-frequency excitation. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N38.00006: Spring constant calibration of AFM cantilevers with a piezolever transfer standard D. Hurley, E. Langlois, G. Shaw, J. Kramar, J. Pratt Accurate determination of forces in the AFM requires knowledge of the cantilever spring constant $k_{c}$. We describe a method to measure $k_{c}$ traceable to SI units. The transfer standard was a commercial piezoresistive cantilever (``piezolever'') calibrated by the NIST electrostatic force balance (EFB). The active piezolever device eliminates the need to measure the optical lever sensitivity. The method does not depend on cantilever geometry and determines $k_{c}$ under loading conditions. The calibrated piezolever was used to measure cantilevers with nominal values of $k_{c}$ from 0.2 to 40 N/m. Measured values differed by as much as 300 \% from the nominal values. Values of $k_{c}$ were also obtained with four other methods: thermal noise, geometric (Sader), nanoindentation loading, and direct EFB loading. Differences between the direct EFB and piezolever results ranged from 15-20 \% for the stiffest cantilevers to $<$1 \% for the most compliant. Experimental issues critical to accurate measurements with each method will be discussed. Methods will also be compared in terms of implementation in other laboratories. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N38.00007: High Speed Scanning Property Measurements David Shuman, Ramesh Nath, Ramamoorthy Ramesh, Bryan Huey Atomic Force Microscopy (AFM) is a ubiquitous surface science tool, but the slow speed of standard equipment remains a continuing limitation for widespread application. A novel AFM variation is reported here for High-Speed Scanning Property Mapping (HS-SPM), uniquely allowing full-frame nanoscale-resolution image acquisition in $<$3 seconds with tip speeds $>$1 cm/sec. Using off-the-shelf commercial equipment, the method combines acoustic and AFM concepts: the sensitivity of AFM-cantilever contact resonances to materials properties, and conversely the insensitivity of these resonances to contact force variations due to rapidly raster scanning an AFM probe. The method is applicable to a broad range of materials and properties, as demonstrated by mechanical property maps of bacterial membrane fragments and integrated circuits; magnetic property maps of domains in magnetic hard drives; and movies of ferroelectric domain reading and writing with sub-second frame rates for dynamic domain nucleation and growth studies. HS-SPM thereby provides a novel yet off-the-shelf solution for both significantly enhanced throughput in nanoscale materials property mapping, as well as dynamic surface studies with previously inaccessible time constants. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N38.00008: Atomic Force Microscope Tip for Dielectrophoresis J.A. Aguilar, T.P. Hunt, A.C. Bleszynski, R.M. Westervelt Bottom-up fabrication of nanoscale structures has long been an aspiration of the nanotechnology community. We have designed and built an AFM tip with coaxial electrodes that produces a very high field gradient. Dielectrophoresis (DEP) in a strong localized, RF electric field is useful for manipulating nanoparticles in a fluid, performing electric force microscopy, and reading and writing data on ferroelectric materials. The capabilities of an AFM allow imaging the sample before and after manipulation with DEP, and the sharpness of the AFM tip gives high spatial resolution. The inner electrode is formed by a doped Si AFM tip, which is insulated from the grounded outer shield by a thin thermal Si oxide layer. The field lines escape through a small hole in the outer shield cut at the tip's point by a focused ion beam. Currently, the tip diameter is about 300 nm; this can easily be made smaller. This sets the stage for experimentation on the actual manipulation of nanoscale particles with coaxial AFM tips. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N38.00009: Cantilever mean deflection: average tip-sample force in tapping mode spectroscopy F. Michael Serry In tapping mode AFM spectroscopy, tip-sample interaction is nearly always studied in cantilever amplitude and phase. Theory shows that the \textit{mean} deflection is another quantity that carries a wealth of information about the interaction (1). However, mean deflection remains largely unexplored in experiments. One historic reason is tapping mode was invented to avoid relying on static (mean) deflection of cantilever in contact mode AFM. Mean deflection is easier to measure with softer cantilevers, and becomes more important with smaller amplitudes. We present mean deflection data which often contain features with no readily decipherable counterparts in amplitude or phase; validate some theoretical results; and possibly contradict others. The data (vs. mean tip-sample separation) provide a direct, intuitive, experimental proof that phase follows the polarity of average tip-sample force (2). However, the slope of this data does not always follow that of the phase. Average force often plateaus as mean separation reduces and even approaches zero, which may help explain why similarly high-quality images are frequently possible across a range of amplitude setpoint values. (1) A. San Paulo, R. Garcia, Phys Rev B (64), p193411, 2001. (2) R. Garcia, A. San Paulo, Phys Rev B (60), p4961, 1999. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N38.00010: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N38.00011: Monotonic and fatigue tests of amorphous silicon nanostructures using atomic force microscope Churamani Gaire, D.-X. Ye, T.-M. Lu, G.-C. Wang, C. R. Picu The plastic deformation and the failure properties of a-Si slanted nanostructures (one- and two-armed) fixed at one end to the substrate, grown by oblique angle physical vapor deposition, have been studied with the use of AFM. Monotonic loading/unloading tests were carried out to determine the elastic and plastic failure properties. We also developed the fatigue test methodology suitable for nanoscale specimens with the use of AFM. The AFM was used for imaging (to locate) as well as for loading the structures in monotonic bending and force (stress) controlled cyclic loading/unloading mode until the specimen failed completely. A novel way was used to identify the failure of the specimens during the fatigue test. The post-test analysis of the failure surface was done through SEM imaging. The possible inhibition of the brittleness of the a-Si samples with the reduction of the size and damage evolution during fatigue test on the nanoscale specimens will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N38.00012: Enhanced compositional sensitivity in atomic force microscopy by the excitation of the first two flexural modes Ricardo Garcia, Nicolas F. Martinez, Shivprasad Patil, Jose R. Lozano We demonstrate that the compositional sensitivity of an atomic force microscope is enhanced by the simultaneous excitation of its first two normal eigenmodes$^{1-2}$. The coupling of those modes by the non-linear probe-surface interactions enables to map compositional changes in several conjugated molecular materials with a phase shift sensitivity that is about two orders of magnitude higher than the one achieved in amplitude modulation atomic force microscopy. \begin{enumerate} \item T.R. Rodriguez and R. Garcia, Appl. Phys. Lett. 84, 449 (2004) \item N.F. Martinez, S. Patil, J.R. Lozano and R. Garcia, Appl. Phys. Lett. 89, 153115 (2006) \end{enumerate} [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N38.00013: Systematic Variations in Apparent Topographic Height as Measured by Non-contact Atomic Force Microscopy Deng-Sung Lin, T.-C. Chiang, K.M. Yang, J.Y. Chung, M.F. Hsieh, S.S. Ferng A flat Si(100) surface is prepared with neighboring n- and p- doped regions. The contact potential difference between the tip and the two well-defined regions of similar material is utilized to examine the effects and interplay of essential tip- sample forces in atomic force microscopy. Measurements with a frequency-modulated non-contact atomic force microscope (nc- AFM) show large apparent topographic height variations across the differently doped regions. The height differences depend on the bias polarity, bias voltage, radius, and conducting state of the tip. The functional relationships are well explained by integrated model calculations. These findings provide a coherence scenario of nc-AFM operation under these essential forces and facilitate quantitative understanding of the systematic errors in surface topographic height measurement commonly performed in nanoscience. [Preview Abstract] |
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