Session H36: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods

Multi-dimensional Scanning Probe Microscopy

Since the first demonstration of true atomic resolution with a scanning force microscope in 1995 a large variety of samples have been imaged with atomic resolution. In 2000 Giessibl obtained atomic resolution with a (macroscopic) tuning fork operated with small oscillation amplitudes that correspond to the length of the inter-atomic interaction potentials. However, tuning fork sensors have a series of disadvantages that can all be overcome by optimized micro-fabricated cantilevers operated at similarly small or even smaller oscillation amplitudes. With their small mass and high mechanical quality factor, particularly after appropriate annealing in UHV force sensitivities that are orders of magnitude better than that of tuning forks and also far better than that of the best NEMS sensors is obtained. Recently Kawai et al and Sugimoto et al obtained atomic resolution with cantilevers with a operated in higher oscillation modes with sub-nanometer amplitudes. In order to make use of the excellent force sensitivity we developed a Fabry-Perot type interferometer optical sensor that maps flexural and torsional cantilever oscillations with fm/sqrt(Hz) deflection sensitivity. This deflection sensor allows the use of sub-{\AA} cantilever oscillation amplitudes and thus the direct measurement of atomic interaction force gradients and, -- in principle, the use of advanced tunnelling spectroscopy techniques that are well-established in the field of scanning tunnelling microscopy. The simultaneous mapping of flexural and torsional oscillation modes further allows the measurement of vertical and lateral tip-sample interaction forces and corresponding atomic scale energy loss processes.   

A Low Temperature Scanning Force Microscope with a Vertical Cantilever and Interferometric Detection Scheme

We have developed a fiber-optic interferometry system with a vertical cantilever for scanning force microscopy. A lens, mounted on a Pan-type walker, was used to collect the interference signal in the cavity between the cantilever and the single mode fiber. ~This vertical geometry has several advantages: (1) it is directly sensitive to lateral forces; (2) low spring constant vertical cantilevers may allow increased force sensitivity by solving the ``snap-in'' problem that ~occurs with soft horizontal cantilevers. ~We have sharpened vertical cantilevers by focused ion beam (FIB), achieving a tip radius of 20 nm. ~We will show test results of a magnetic force microscope (MFM) with this vertical cantilever system.   

New Interpretation of Non-contact Atomic Force Microscopy Images of Dihydride Si(001) Surface Based on Simulation

Hydride Si(001) surfaces have attracted attention as a substrate of organic semiconductor devices. In non-contact atomic force microscopy (NC-AFM) observation of dihydride Si(001) surface, 1x1 and 2x1 images were observed depending on the preset frequency shift value [1]. For both images, bright spots were assigned to hydrogen atoms. Recently, we have developed a simulator on the basis of the density-functional based tight-binding method, and have simulated NC-AFM image of the dihydride Si(001) surface. As a result, we obtained frequency shift images with the 1x1 and 2x1 periodicities, which agree well with the experiments. Surprisingly, we have found that the bright spots of the 2x1 image do not correspond to hydrogen atoms. Each spot corresponds to the bridge site, where the sum of the attractive forces from two nearby hydrogen atoms becomes large. \newline [1] S. Morita, Y. Sugawara, Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 4857.   

Real-time detection and reduction of probe-loss in atomic force microscopy

In this presentation, a real-time methodology is developed to determine regions of dynamic atomic force microscopy based image where the cantilever fails to be an effective probe of the sample. Probe-loss is more pronounced during high speed imaging operations. A quantitative measure called reliability index is proposed as diagnostic measure for determining probe- loss. It is experimentally demonstrated that probe-loss affected portion of the image can be unambiguously identified by a signal termed the reliability index, that can be determined in real-time. The reliability index signal, apart from indicating the probe-loss affected regions, can be used to minimize such regions of the image. A PI controller with adjustable gains has been implemented on FPGA (Field programmable gate array), which uses reliability index signal to switch. It is experimentally demonstrated that by using such a scheme, probe-loss areas can be reduced by a factor of 4, suggesting a possible increase in imaging bandwidth by the same factor. Improvement in on-sample performance has also been observed.   

Evaluation of sensitivity and selectivity of piezoresistive cantilever-array sensors

Microfabricated cantilever-array sensors have attracted much attention in recent years due to their real-time detection of low concentration of molecules. Since the piezoresistive cantilever-array sensors do not require a bulky and expensive optical read-out system, they possess many advantages compared with optical read-out cantilever-array sensors. They can be miniaturized and integrated into a match-box sized device. In this study, we present the piezoresistive cantilever-array sensor system and evaluate its sensitivity and selectivity using various vapors of molecules, including alkane molecules with different chain length from 5 (n-pentane) to 12 (n-dodecane). Piezoresistive cantilevers were coated with different polymers (PVP, PAAM, PEI, and PVA) using an inkjet spotter. Each cantilever has a reference cantilever, constituting a Wheatstone-bridge. Each vapor was mixed with a constant nitrogen gas flow and introduced into the measurement chamber. According to the principle component analysis of data obtained, each molecule can be clearly distinguished from others. We also confirmed that this piezoresistive cantilever-array sensor system has sub-ppm sensitivity.   

Observation of locally excited ferromagnetic resonance via magnetic resonance force microscopy

Magnetic resonance force microscopy spectra of a 50 nm thick permalloy film were measured as a function of the probe-sample distance and the angle between the film plane and the direction of the externally applied magnetic field. At larger angles the multiple resonance modes were observed at small probe-sample distances. Micromagnetic simulations which include the inhomogeneous magnetic field of the probe tip reveal the \textit{localized} nature of the exited resonance modes, opening a way to spatially resolved ferromagnetic resonance measurements in a continuous ferromagnetic media.   

Characterization of the High Coercivity Magnetic Probe Tips for Magnetic Resonance Force Microscopy

Magnetic Resonance Force Microscopy on ferromagnetic systems calls for high coercivity probe magnets needed for exciting localized Ferromagnetic Resonance (FMR) modes in the sample under investigations. We have characterized high coercivity Sm2Co17 MRFM probes fabricated by Focused Ion Beam (FIB) micro machining and mounted on a commercial Si cantilever (characteristic dimension 1$\mu$m x 1$\mu$m x 1$\mu$m). We report vibrating cantilever magnetometry measurements of probe coercivity. Low temperature (4 K) probe coercivity as high as 1 T has been observed. Probe characteristics have also been deduced from deconvolution of MFM data obtained on 5.3 4$\mu$m diameter permalloy dots. We also discuss energy dissipation in the micromechanical cantilever when the probe magnet approaches a ferromagnetic sample.   

Fabrication Challenges in Producing Magnet-tipped Cantilevers for Magnetic Resonance Force Microscopy

Magnetic resonance force microscopy (MRFM) is a technique that may allow MR imaging of single molecules -- an extremely exciting prospect. To date we have demonstrated MRFM sensitivity of $\sim$10$^5$ proton spins. By making improved magnetic tips and increasing force sensitivity, it may be possible to achieve single-proton sensitivity necessary for molecular imaging. In MRFM the force exerted on the cantilever, per spin, is proportional to the field gradient from the cantilever's magnetic tip. Achieving single proton sensitivity thus requires dramatically reducing magnet size. We have developed an e-beam lithography process for batch fabricating nanoscale magnets on silicon cantilevers. With these sized magnets we will still require attonewton force sensitivity. Research by our group has shown that surface induced dissipation is a major noise source. We believe this can be minimized by producing magnets overhanging the cantilever end. As proof of concept, we will show a 50-nm overhanging cobalt magnet made by a process involving KOH etching, as well as preliminary work on making overhanging magnets by dry fabrication methods. Our current challenge appears to be preventing the formation of metal silicides.   

Fabricating overhanging magnets for use in magnetic resonance force microscopy using a XeF2 isotropic etch.

Pushing magnetic resonance force microscopy towards single proton sensitivity demands meeting the nanofabrication challenge of producing an attonewton-sensitivity cantilever with a magnetic tip whose diameter is 50 nm or less. At the same time, the cantilever should also experience low force noise (and force gradient noise) near the surface of technologically interesting samples. Ideally then, the magnetic tip would overhang the leading edge of the cantilever -- to increase the signal created by the magnet while simultaneously minimizing the noise created by the rest of the cantilever interacting with the surface. We will show that the isotropic etchant XeF$_{2}$ can be used to underetch a single crystal silicon cantilever to create an overhanging magnet. This etch is a controllable etch process with high selectivity to metals that can be used not only to produce magnetic resonance force microscopy cantilevers, but other overhanging metallic structures as well.   

Bimodal AFM imaging of individual protein molecules with sub-pico Newton force sensitivity.

The capability of atomic force microscopes (AFM) to generate atomic or nanoscale resolution images of surfaces has deeply transformed the study of materials. However, high resolution imaging of biological systems has proved more difficult than obtaining atomic resolution images of crystalline surfaces. In many cases, the forces exerted by the tip on the molecules (1-10 nN) either displace them laterally or break the noncovalent bonds that hold the biomolecules together. Here, we apply a force microscope concept based on the simultaneous excitation of the first two flexural modes of the cantilever (bimodal excitation). The coupling of the modes generated by the tip-molecule forces enables imaging under the application of forces (around 35 pN) which are smaller than those needed to break non-covalent bonds. With this instrument we have resolved the intramolecular structure of antibodies in monomer and pentameric forms. Furthermore, the instrument has a force sensitivity of 0.2 pN which enables the identification of compositional changes along the protein fragments.   

Rheological Measurements by AFM of the Formation of Polymer Nanofibers

Polymer fiber can be formed by pulling a thread of polymeric liquid if the fiber solidifies before it breaks up by capillary thinning. Fiber diameter is well correlated with a processing parameter that is a simple function of viscosity, surface tension and evaporation rate. The fundamental material parameters can also be determined with the same AFM setup. The usual problem with tapered AFM tips, of liquids wetting unstably up the tapered AFM tip and even onto the cantilever, is resolved by the use of long cylindrical tips of constant diameter. We recently demonstrated a method of growing Ag-Ga nanowires onto AFM tips at room temperature. These constant diameter nanowires are shown to give clearly measurable force-distance curves when inserted through the surface of a liquid, which provides clean measurements of surface tension, contact angle, and evaporation rate, while shear viscosity is determined through cantilever Q-damping as a function of insertion distance into the liquid.   

Micromechanical force detectors for measuring magnetization at high magnetic fields and the magnetic~ response of Ba3Cr2O8.

We report magnetization measurements of Ba3Cr2O8 using micromechanical faraday balance magnetometers. The magnetometers consist of a movable polysilicon plate (500 by 500 micrometers) supported~ by four springs 2.75 micrometers above a fixed electrode. When small samples of the magnetic material are placed at the center of the movable plate, the natural gradient of the field creates a force on the sample that changes the capacitance between the plate and electrode, while the response to magnetic torque is minimized. The absolute magnetization of the sample can be determined provided that the magnetic field gradient is known. The device is used to measure the magnetization of a small sample of Ba3Cr2O8 with mass of 1 microgram. At high fields, our measurements reveal an asymmetric dome like structure in the temperature-magnetic field phase diagram, possibly related to the Bose-Einstein condensation of spin triplet degrees of freedom.