Session H27: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods

Temperature Dependence of Single-Asperity Friction for Diamond on Diamond and DLC Interfaces

A variable temperature, ultrahigh vacuum atomic force microscope with a diamond-coated probe was used to characterize interfacial friction over a temperature range of 30 to 300 Kelvin. A vertical scan was used to measure tip to surface adhesion and contact normal forces. Friction (lateral) force measurements were taken by dragging the tip along the surface. Calibration was done in situ using substrates with known dimensions and angles. Measurements were made on diamond-like carbon surface and a single crystallite in a micro crystalline diamond film. Results were analyzed by fitting into the DMT continuum model. Comparison of friction versus load showed approximately a factor of two increase in the friction at cryogenic temperatures compared to room temperature. Results are qualitatively consistent with MD simulations but are not well described by models of thermally activated friction. Problems associated with temperature gradients at the tip- surface interface will be discussed.   

Small amplitude vibrations of curved atomic force microscope cantilevers: Theory

The shifts in resonance frequencies of cantilevers are used to infer tip--sample interactions in tapping--mode atomic force microscopy as well as in a wide variety of cantilever based sensors. In this work, we investigate theoretically as well as experimentally the effect of curvature on the vibration dynamics of micro--cantilevers to which a micro--sphere is attached at the free end. We show that resonance frequencies of cantilevers to which a tip mass is attached can be altered by controlling the curvature of the cantilever. This control over the resonance frequency spectrum is independent of other causes of resonance frequency variation, such as adsorbed mass on cantilever or variation of material properties due to change in temperature. In the case when the cantilever is a bi--material cantilever, this shift in resonance frequency can be used as to detect changes in the thermal environment of the cantilever.   

Intermodulation Atomic Force Microscopy and Spectroscopy

We present a powerful new method of dynamic AFM, which allows to gain far more information about the tip-surface interaction than standard amplitude or phase imaging, while scanning at comparable speed. Our method, called intermodulation atomic force microscopy (ImAFM), employs the manifestly nonlinear phenomenon of intermodulation to extract information about tip-surface forces. ImAFM uses one eigenmode of a mechanical resonator, the latter driven at two frequencies to produce many spectral peaks near its resonace, where sensitivity is highest [1]. We furthermore present a protocol for decoding the combined information encoded in the spectrum of intermodulation peaks. Our theoretical framework suggests methods to enhance the gained information by using a different parameter regime as compared to Ref. [1]. We also discuss strategies for solving the inverse problem, i.e., for extracting the nonlinear tip-surface interaction from the response, also naming limitations of our theoretical analysis. We will further report on latest progress to experimentally employ our new protocol.\\[3pt] [1] D. Platz, E. A. Tholen, D. Pesen, and D. B. Haviland, Appl. Phys. Lett. 92, 153106 (2008).   

Independent determination of depth and energy of electronic trap states in dielectric films by Dynamic Tunneling Force Microscopy

Dynamic Tunneling Force Microscopy (DTFM) is a new scanning probe technique that images electronic states in completely non-conducting films with sub-nanometer spatial resolution$^{1}$. In DTFM, electrons are shuttled via quantum tunneling between a metallic tip and localized electronic states in an insulating dielectric film, while a lock-in amplifier detects an electrostatic force signal that is proportional to the shuttled charge. The DTFM signal provides a map of the available electronic states within tunneling range of the surface. These states are not observable by STM. The depth of the states can be estimated from the dependence of tunneling rate on the tip/sample gap$^{2}$ and also inferred from their apparent lateral size. Images show states below the surface that drop out of the image when the tip/sample gap is increased. A methodology is introduced to independently determine state energy and depth, potentially on a sub-nanometer scale. This work was supported by AFOSR and SRC. [1] J P Johnson and C C Williams, \textit{Nanotechnology} (accepted) [2] N Zheng, et al., \textit{Journ. App. Phys.} \textbf{101}, 093702   

Applications of AFM for atomic manipulation and spectroscopy

Since the first demonstration of atom-by-atom assembly [1], atomic manipulation with scanning tunneling microscopy has yielded stunning realizations in nanoscience. A new exciting panorama has been recently opened with the possibility of manipulating atoms at surfaces using atomic force microscopy (AFM) [2-5]. In this talk, we will present two different approaches that enable patterning structures at semiconductor surfaces by manipulating individual atoms with AFM and at room temperature [2, 3]. We will discuss the physics behind each protocol through the analysis of the measured forces associated with these manipulations [3-5]. Another challenging issue in scanning probe microscopy is the ability to disclose the local chemical composition of a multi-element system at atomic level. Here, we will introduce a single-atom chemical identification method, which is based on detecting the forces between the outermost atom of the AFM tip and the atoms at a surface [6]. We demonstrate this identification procedure on a particularly challenging system, where any discrimination attempt based solely on topographic measurements would be impossible to achieve. \\[4pt] \textbf{References}: \\[0pt] [1] D. M. Eigler and E. K. Schweizer, \textit{Nature} \textbf{344}, 524 (1990); \\[0pt] [2] Y. Sugimoto, M. Abe, S. Hirayama, N. Oyabu, O. Custance and S. Morita, \textit{Nature Materials} \textbf{4}, 156 (2005); \\[0pt] [3] Y. Sugimoto, P. Pou, O. Custance, P. Jelinek, M. Abe, R. Perez and S. Morita, \textit{Science} \textbf{322}, 413 (2008); \\[0pt] [4] Y. Sugimoto, P. Jelinek, P. Pou, M. Abe, S. Morita, R. Perez and O. Custance, Phys. Rev. Lett. 98, 106104 (2007); \\[0pt] [5] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl and A. J. Heinrich, \textit{Science} \textbf{319}, 1066 (2008); \\[0pt] [6] Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Perez, S. Morita, and O. Custance, \textit{Nature} \textbf{446}, 64 (2007)   

Design of a variable temperature scanning force microscope

We have developed the variable temperature scanning force microscope capable of performing both magnetic resonance force microscopy (MRFM) and magnetic force microscopy (MFM) measurements in the temperature range between 5 and 300 K. Modular design, large scanning area, and interferometric detection of the cantilever deflection make it a sensitive, easy to operate and reliable instrument suitable for studies of the dynamic and static magnetization in various systems. We have verified the performance of the microscope by imaging microfabricated permalloy dots and vortices in Nb thin film in the MFM mode of operation. MRFM spectra in a diphenyl-picryl-hydrazyl film were recorded to evaluate the MRFM mode of operation.   

Bimodal atomic force microscopy imaging of isolated antibodies in air and liquids

We develop a dynamic atomic force microscopy (AFM) method based on the simultaneous excitation of the first two flexural modes of the cantilever. The instrument, called bimodal AFM, opens up additional channels (amplitude and phase of the 2$^{nd}$ mode) which can be used for imaging with enhanced lateral resolution with respect to amplitude modulation AFM (AM-AFM). Bimodal AFM allows us to resolve the structural components of antibodies in both monomer and pentameric forms. The instrument operates in both high and low quality factor environments, i.e., air an liquids, so that the imaging of biomolecules can be carried out in their natural media. Bimodal AFM is studied in great detail by means of theoretical and numerical methods. Our model allows us to study the material contrast sensitivity of the two additional channels (amplitude and phase of the 2$^{nd}$ mode) that can be used for imaging. The theoretical approach also allows us to estimate the forces applied on the sample during bimodal AFM operation. The calculated forces lie below 120 pN, an essential fact when imaging proteins. This is due to the enhanced sensitivity of 2$^{nd}$ mode phase to detect changes while the cantilever is far away from the sample.   

High-Resolution Magnetic Resonance Force Microscopy using Iron Filled Carbon Nanotubes

Magnetic Resonance Force Microscopy is able to probe below surfaces to map out spins in a non-destructive manner by measuring the force from the dipolar coupling of a magnetic probe to spins in the sample. We have used low force constant cantilevers with low intrinsic dissipation to obtain 2 spin sensitivity. To obtain better sensitivity one avenue of improvement is to increase the magnetic field gradient from the magnetic probe. ~Iron-filled carbon nanotubes provide a promising route for very high magnetic field gradient micromagnetic probes; we have successfully attached these iron nanowires to IBM style ultrasoft silicon cantilevers. The smaller size of the tip (15 to 25 nm) allows gradients an order of magnitude greater than micron-sized rare-earth magnets. In addition, iron filled carbon nanotubes have the possibility to lower the non-contact friction by reducing the surface area of the probe close to the sample. Iron filled carbon nanotubes also exhibit high anisotropy fields, a result of the shape anisotropy. This work was supported by The Army Research Office under W911NF-07-1-0305 and the National Science Foundation under DMR-0807093.   

Electron Spin Magnetic Resonance Force Microscopy of Nitroxide Spin Labels

Nitroxide spin labels are widely used in electron spin resonance studies of biological and polymeric systems. Magnetic resonance force microscopy (MRFM) is a magnetic resonance technique that couples the high spatial resolution of a scanning probe microscope with the species selectivity of magnetic resonance. We report on our investigations of 4-amino TEMPO, a nitroxide spin label, by force-gradient MRFM. Our microscope operates at high vacuum in liquid helium, using a custom fabricated ultra-soft silicon cantilever in the magnet-on-cantilever geometry. An 18 GHz gap coupled microstripline resonator supplies the transverse field.   

A Compact, Wide Temperature Range (300mK-300K) Magnetic Force Microscope using High Resolution Fibre Interferometer and Alignment-Free Cantilevers

We describe a design of a Low Temperature Magnetic Force Microscope (LT-MFM) for variable temperatures between milli-Kelvin temperatures to 300 K. The design of LT-MFM is very compact, 23.6mm ODx200mm, flexible and is compatible with almost any cryostat (included PPMS of Quantum Design Inc), even He3 systems or DR, provided that there is enough space. The sensor is mounted on a scan piezo tube which has five electrodes: four quadrants are used for scanning, the fifth electrode is used for dithering the cantilever by means of a digital Phase Lock Loop (PLL) with 5mHz frequency resolution. We employed a fibre interferometer deflection measurement for our LT-MFM. A special alignment holder is designed for this purpose. A 225$\mu $m length MFM cantilever is placed on an Alignment-Free AFM Cantilever holder chip from NanoSensors. Our design can sustain cantilever-fiber alignment down to 300mK without any signal loss. An improved fiber interferometer with $\sim $1x10$^{-3}$ A/$\surd $Hz noise level is designed and used to detect cantilever deflection. LT-MFM also enables us to work under high external magnetic fields.   

A novel method to measure 3 components of magnetic fields with submicron resolution using Scanning Hall Probe Microscopy/Gradiometry

We present the development of a new 4-lead hall gradiometer and a novel method to measure 3 components( Bx, By {\&} Bz) of magnetic fields on specimen surfaces with submicron resolution using Scanning Hall probe Microscope[1] and gradiometer. We used a 1$\mu $m size P-HEMT Hall sensor, operated in gradiometer configuration to image Bx, By and Bz distribution of a hard disk sample surface at 77K. The SHPM was used in Quartz Crystal AFM tracking mode[2]. This simple and quick novel method shows $\sim $40 better spatial resolution compared to previously developed techniques[3] and can be improved even further, down to sub 50nm resolution. 1. Chang, A.M., et al., \textit{Scanning Hall Probe Microscopy.} Applied Physics Letters, 1992. \textbf{61}(16): p. 1974-1976. 2. Dede, M., et al., \textit{Scanning Hall Probe Microscopy (SHPM) using quartz crystal AFM feedback.} Journal of Nanoscience and Nanotechnology, 2008. \textbf{8}(2): p. 619-622. 3. Gregusova, D., et al., \textit{Fabrication of a vector Hall sensor for magnetic microscopy.} Applied Physics Letters, 2003. \textbf{82}(21): p. 3704-3706.   

Development of Superconducting Quantum Interference Device DC Magnetometer for High Magnetic Field and Dilution Refrigerator Applications

A commercially available SQUID (Superconducting Quantum Interference Device) DC magnetometer is often limited by its relatively high temperature ($\ge $ 1.9 K) and low magnetic field ($\le $ 7 T) operating environment. The need for the lower temperature and higher field DC magnetization measurements keeps growing as more materials show interesting physical phenomena whose energy scales are relevant to low temperatures ($\sim $ mK). To meet these needs we have developed a probe for a top loading dilution refrigerator in which all the DC magnetometer components including SQUID electronics, detection coil, and sample motion shaft are placed together. The probe was tested in a top loading dilution refrigerator and the results show that the base temperature at 25 mK increased $\sim $ 1.6 {\%} when the sample displacement was 3.2 cm with a speed of 3 cm/min. The moment of the test sample was successfully detected down to 50 mK. Improvement in coil balancing and shielding of the detection coil are planned.