Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U38: Focus Session: Advances in Scanned Probe Microscopy III: High Frequency and Probe Characterization |
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Sponsoring Units: GIMS Chair: Eric Hudson, Massachusetts Institute of Technology Room: Colorado Convention Center 501 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U38.00001: Radio-frequency Scanning Tunneling Microscopy (rf-STM) Utku Kemiktarak, Tchefor Ndukum, Keith Schwab, Kamil Ekinci Scanning Tunneling Microscopy (STM) possesses unprecedented spatial resolution, extending down to sub-Angstroms. There is a serious obstacle, however, in front of realizing the full potential of the STM technique: insufficient temporal resolution. Electronic bandwidths in typical STM experiments extend only up to $\sim $10 kHz. Here, a novel approach to attain time resolution in STM is demonstrated. The impedance of the tip-sample tunnel junction in the M$\Omega $ range is matched close to 50 $\Omega $ by using a LC tank circuit, which allows for bandwidths up to 20 MHz. To demonstrate broadband rf-STM operation, several experiments have been performed. First, radio-frequency reflectometry has been used to image surfaces, resulting in spatial resolution comparable to that available in conventional STM. Second, shot noise in the tunnel current has been measured at high frequencies. Third, high frequency displacements of surfaces have been detected using the STM tip as a nanomechanical displacement detector. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U38.00002: Very high frequency (VHF) self-sensing nanoscale cantilevers and their mass sensing applications in ambient conditions. Mo Li, Hong Tang, Michael Roukes We report the development a new class of self-sensing, nanometer-scale cantilevers with fundamental-mode mechanical resonances up to very high frequencies (VHF). The sensors employ integrated piezoresistive displacement transducers; we show that, at the nanoscale, these are optimally realized using thin, metallic-density films. Our approach enables detection of VHF cantilever vibrations, to date as high as 127 MHz, at the thermomechanical noise limit. Displacement sensitivity of 39 fm/Hz$^{1/2}$ and extremely low 1/f noise are attained. Our smallest devices have lateral dimensions approaching the mean free path at atmospheric pressure; hence their high quality factors are preserved in ambient. Measurements of molecular chemisorption onto polymer-coated nanocantilevers yield unprecedented mass resolution below 1 attogram (10$^{-18}$ g) level at atmospheric pressure and room temperature. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U38.00003: High Frequency Cantilevers for Magnetic Resonance Force Microscopy Christian Degen, Martino Poggio, Ben Chui, John Mamin, Dan Rugar We are exploring the possibility of using high frequency cantilevers for detection of magnetic resonance spin signals, possibly at the Larmor frequency of nuclear spins. For this purpose we have fabricated smaller, 20 micron long cantilevers that resonate at frequencies near 1 MHz. Operating at 4K, these levers can have surprisingly high Q values, over 300,000, and can achieve force noise levels in the few attonewton range, despite their rather high stiffness of about 0.1 Newton per meter. We discuss some experimental challenges that will be increasingly important for future generations of cantilevers with even smaller dimensions. Finally we look ahead into what we might expect when such high sensitivity, nanomechanical resonators become tightly coupled to small ensembles of nuclear spins. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U38.00004: Construction of a Low Temperature Capable Frequency-Modulation Magnetic Resonance Force Microscope SangGap Lee, Sun Ho Won, Seung-Bo Saun, Soonchil Lee We constructed a low temperature capable frequency-modulation magnetic resonance force microscope (FM-MRFM) and applied to detecting electron spin resonance signals from tiny DPPH particles with ease in control and thereby improved signal-to-noise ratio. The electronics was composed of a spin-polarization modulator, a cantilever-oscillation feedback gain controller, and a phase-locked loop (PLL) FM demodulator. The controller kept the cantilever oscillating at its resonance frequency by positively feeding back to a piezo-actuator the gain-controlled drive signal phase shifted by 90 degrees with respect to the cantilever oscillation in the PLL circuit. The modulator generated waveforms to modulate the strength or frequency of radio-frequency field in phase with the drive signal. The whole setup enabled to sense as readily as spins resonate by tracking the cantilever resonance frequency without breakdown in fastidious conditions. We will discuss the details and features of our microscope and furthermore ongoing MRFM results. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U38.00005: MRFM System Design for the Study of Organic Materials Doran Smith, David Kim We will present an overview of our program to develop an MRFM system specialized for the study of organic materials at 4 K. The system uses the SPAM geometry and the CERMIT protocol and is predicted to be capable of imaging organic materials in 3D. The MRFM probe head design will be overviewed and progress toward system completion will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U38.00006: Progress on Magnetic Resonance Force Microscopy Detection of Statistical Polarization of Electron Spins K.C. Fong, I.H. Lee, P. Banerjee, Yu. Obukhov, D. Pelekhov, P.C. Hammel Here we report our experimental progress on detecting statistical polarization of electron spins. In the condition of low external magnetic field and high temperature, polarizati on due to Boltzman factor could be small, i.e. $\mu B/k_BT \ll 1 $. The $\sqrt{N}$ statistical polarization can dominate the Boltzmann polariation when the spins ensemble is suffi ciently small. With its unprecedented force sensitivity, Magnetic Resonance Force Microscopy (MRFM) has demonstrated the capability to observe this self-polarizing nature of spins via the i-OSCAR detection protocol\footnote {H.J. Mamin, R. Budakian, B.W. Chui and D. Rugar, Phys. Rev. Lett. \textbf{91}, 207604 (2003)}. Our efforts to use MRFM to detect this statistical polarization will be presented. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U38.00007: Cross Polarization Imaging with Magnetic Resonance Force Microscopy Kai W. Eberhardt, Qiong Lin, Andreas Hunkeler, Urban Meier, Beat H. Meier Cross Polarization (CP) is a standard method in Nuclear Magnetic Resonance Spectroscopy (NMR) for signal enhancement of nuclei with a low gyromagnetic ratio and was recently applied in MRFM [1]. We demonstrate two techniques based on CP. In the first we perform frequency-sweept CP to enhance the polarization of low-$\gamma$ S-spins. In the second method the S spins are used as a polarization sink for the high-$\gamma$ I-spins coupled to them. The I-spins can be completely depolarized by adding phase-jumps to the frequency-sweep of the S-spin channel, allowing that their presence is detected indirectly via the high-$\gamma$, often more abundant I-spins with improved SNR. 1D images with $\mu$m-resolution of a $KPF_{6}$ - $CaF_{2}$ sample are presented for both techniques. In the example the depolarization scheme allowed for an order of magnitude signal-to-noise ratio enhancement over direct detection. [1] Q. Lin et al., Phys. Rev. Lett. 2006, 96, 137604. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U38.00008: Near-field scanning microwave microscope with separated excitation and sensing probes Keji Lai, M.B. Ji, N. Leindecker, M. Kelly, Z.X. Shen We present here the design and experimental results of a near- field scanning microwave microscope (NSMM), currently working at a frequency of 1GHz. The coplanar waveguides were patterned onto the silicon nitride cantilever interchangeable with AFM tips, which are robust for high speed scanning. Our microscope is unique in that the sensing probe is separated from the excitation electrode to significantly suppress the common mode signal. The reflected signal, at the same time, can be used for the feedback of height control in a non-contact mode. In the contact mode which we are currently using, the contrast comes from both the sample topography and the difference of the complex dielectric constant. Our NSMM shows the ability to achieve high resolution microwave images on nano-particles, nano-wires, and biological samples with mostly topographical contrast, as well as buried structures with mostly electrical contrast. Numerical analysis of the tip-sample interaction was also performed and will be discussed in detail. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U38.00009: Ferromagnetic resonance force microscopy of a permalloy film E. Nazaretski, I. Martin, J.D. Thompson, R. Movshovich, D.V. Pelekhov, P.C. Hammel, P. Wigen, M. Zalalutdinov, T. Mewes, J. Baldwin, B. Houston We describe Ferromagnetic Resonance Force Microscopy (FMRFM) experiments performed on a 50 nm thick permalloy film. We have studied the evolution of the FMRFM force spectra as a function of temperature. The temperature-dependent studies show a decrease of the ferromagnetic resonance field with increasing temperature which we attribute to the temperature-dependent changes of the saturation magnetization. The experiments demonstrate the potential of FMRFM to study temperature dependent ferromagnetic resonance phenomena. We analyzed the FMRFM force spectra evolution as a function of the probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-sample interaction force. Excellent agreement between the experimental data and the simulation results provides the new insight into the mechanism of the FMR mode excitation in an FMRFM experiment. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U38.00010: Magnetic particle imaging with a cantilever torque magnetometer John Moreland, Jason Eckstein, Yushun Lin, Sy-Hwang Liou, Steven Ruggiero We have demonstrated magnetic particle imaging$^{1 }$with a cantilever torque magnetometer. Imaging is based on measuring the harmonic content of the magnetic moment of a particle driven to saturation by an applied ac magnetic field while adjusting the zero point of the field gradient with a slowly sweeping dc magnetic field. Large field gradients ($>$ 100 T/m) necessary for high resolution imaging can be generated by opposing electromagnets with ferrite cores and thus there is the potential for submicrometer image resolution. Results on an array of 50 $\mu $m Permalloy dots patterned on microcantilevers will be reported. $^{1}$B. Gleich and J. Weizenecker, Nature 435, 1214 (2005). [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U38.00011: Resonant Measurement of Coupling Forces Between Two Microcantilevers Onur Basarir, Kamil L. Ekinci Here we studied the nature of nonlinear coupling forces between two microcantilevers. We employed a resonant measurement technique similar to that used in non-contact atomic force microscopy (NC-AFM). A stiff cantilever, which was driven at its resonance at a constant amplitude, was brought to the close vicinity of a second cantilever excited by thermal noise. A spectral analysis of the displacement signal of the driven cantilever revealed the effects of the coupling forces at the sum and difference frequencies of the resonances of two microcantilevers. From this, the resonance frequency and the quality factor of the thermally excited cantilever were extracted. As the nominal distance between the two cantilevers was reduced, we observed an increase in the dissipation as well as a shift in the resonance frequencies. We shall discuss how these observations may lead to a better understanding of the coupling forces. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U38.00012: Functional Probes for Scanning Probe Microscopy Kotone Akiyama, Toyoaki Eguchi, Toshu An, Yasunori Fujikawa, Yukio Hasegawa, Toshio Sakurai For superior performance of scanning probe microscopy, we are working to fabricate functional probes. For Kelvin probe force microscopy, we fabricated a metal-tip cantilever by attaching a thin metal wire to a regular Si cantilever and milling it by focused ion beam (FIB)$^{1}$. By using the W tip with a curvature radius of 3.5 nm, we obtained the potential profile of Ge/Si(105) surface in atomic resolution with the energy resolution better than 3 meV$^{2}$. For synchrotron-radiation-light-irradiated scanning tunneling microscopy which aims at atomically resolved elemental analysis, we fabricated a glass-coated W tip using FIB$^{3}$. It is found that the glass coating blocks the unwanted secondary electrons, which come from large area of the sample, by a factor of 40 with respect to the case no coating. Using the tip to detect the electrons emitted just below the tip, we obtained element specific images with a spatial resolution better than 20 nm under the photo irradiation whose energy is just above the adsorption edge of the element$^{4}$. 1 K. Akiyama \textit{et al.}, RSI \textbf{76}, 033705 (2005) 2 T. Eguchi, K. Akiyama \textit{et al.}, PRL \textbf{93}, 266102 (2004) 3 K. Akiyama \textit{et al.}, RSI \textbf{76}, 083711 (2005) 4 T. Eguchi, K. Akiyama\textit{ et al.}, APL, in press [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U38.00013: Determination of the cross-sectional area of the indenter in nano indentation tests Weidian Shen, Jeremy McMinis, Rene Crombez, Eva Montalvo To measure the hardness and elastic modulus of a composite system in the nano scale the precise determination of the cross- section area of the indenter tip at different heights is a key. A method of using scanning probing microscopy to image the tip, and then using its analysis software, Histogram program and Bearing curve, combined with the information of the area of each pixel, to determine the cross section area at different heights is introduced in this presentation. Compared with other techniques, it is simple, straightforward, and readily provides a precise relationship between the cross-sectional area and the height. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U38.00014: In-situ observation of the transformation process into the nano-contact of single shell carbon fullerene using TEM-STM Makoto Yoshida, Yoshihiko Kurui, Yoshifumi Oshima, Kunio Takayanagi In this study, we proposed the simple fabrication method of the single shell carbon fullerene (SSF) from an amorphous carbon between two gold electrodes by applying the bias voltage. The STM system which was combined with TEM system was employed as applying the bias voltage and observing the fabrication process in high vacuum condition (about 10$^{-6}$ [Pa]) at room temperature. As the applied bias voltage was increased, the transformation into the SSF via the glassy carbon was caused. It was found that transformation into the SSF occurred above 0.6V. In this method, we obtained the SSF which was range from C$_{60}$ to C$_{620}$. This proposal method is very easier than the previous techniques for obtaining the SSF between two metallic electrodes, for example the combination of the synthesis and evaporation of the SSF, and very useful for researching the electrical conductance property such as the structure effect, the effect of the connection way between both metallic electrodes. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U38.00015: Quantitative imaging of lateral stiffness using sub-{\AA}ngstrom oscillation amplitude nc-AFM Mehrdad Atabak, Sevil Ozer, H. Ozgur Ozer, Ahmet Oral The specially designed and constructed a nc-AFM, capable of measuring lateral stiffness simultaneously with tunneling current will be discussed. In our technique a sensitive fiber interferometer is aligned at the side of a home--made tungsten cantilever with typical stiffness of about 150 N/m. To improve the sensitivity, a RF circuit is designed to inject RF current into the laser diode. The frequency and the amplitude of the RF current can be adjusted to optimize the noise reduction. Using this technique a noise level of $\sim $1$\times $10$^{-4 }${\AA}/$\sqrt {Hz} $ is obtained. The cantilever is dithered in lateral direction respect to the sample with sub-{\AA}ngstrom oscillation amplitudes (A$_{0}$ =0.25 {\AA}) at a frequency, well below the resonance frequency and the changes in lateral oscillation amplitudes. The amplitude at the tip, which is altered by the tip-surface, is detected from the interferometer output using a lock-in amplifier. We present the performance of our microscope and lateral stiffness images as a function of tunnel current (relative tip-sample distance) on HOPG surface. [Preview Abstract] |
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