Session Q36: Focus Session: Advances in Scanned Probe Microscopy III: Force Methods

Three-Dimensional Force Imaging and Quantification with Atomic Resolution

Atomic resolution images in noncontact atomic force microscopy (NC-AFM) reflect planes of constant frequency shift. To draw conclusions on the chemical activity at specific surface sites, however, the force acting between tip and sample should be known locally rather than the frequency shift. This is not an easy translation due to the non-linear nature of the relationship between the two. To overcome this problem, several groups have developed an extension to NC-AFM, \textit{dynamic force spectroscopy}, which allows the precise, distance-dependent measurement of tip-sample forces. The forces are determined from frequency shift versus distance curves by mathematical analysis. Even though this approach had some success, prior attempts resulted either only in two-dimensional atomic resolution force \textit{xz}-maps or in data sets of relatively low resolution, as long-term drift stability has been a problem. Using our recently completed home-built low temperature, ultrahigh vacuum NC-AFM, we were able to map the complete three-dimensional (3D) force field over a surface. Simultaneously, the tip-sample interaction potential and the energy dissipation of the oscillation process were recorded. As a test material, we used highly oriented pyrolytic graphite (HOPG) in order to study the atomic-scale origins of its qualities as a solid lubricant. Individual data points have been acquired over a surface area comprising several unit cells in a 3D grid with less than 6 pm grid size in all directions. From this data set, representations of cuts in any direction can be produced. While constant height images show atomic resolution with pN force resolution, vertical cuts visualize how the attractive force fields of the atoms extend into the vacuum space. We expect that the technique will find applications in fields of science where a local knowledge of interaction forces is beneficial, such as catalysis, chemical imaging, thin film growth, device fabrication, and tribology.   

Tuning the instability in Static Mode Atomic Force Spectroscopy by applying electric field

We study the force-distance (f-d) curves in the absence and presence of a dc bias between the cantilever tip and sample using Atomic Force Microscope (AFM). We find a new kind of bistability in the f-d curves obtained from Atomic Force Spectroscopy. The experimental signatures for this bistability point to a hysteresis like phenomenon when the f-d curves are cycled through the approach and retract paths. Interestingly, it is also observed that on application of a dc bias between the cantilever tip and sample, this bistability in the f-d curves can be tuned. This means that the ``jump-into-contact'' and ``jump-off-contact'' positions in the f-d curves change with the applied dc bias while keeping the other parameters constant. We simulate a simple model for AFM and show that this bistability is a characteristic feature of the experimental procedure and it can be controlled by applying a bias externally between the tip and sample.   

Theoretical simulation of tapping mode AFM in water

For the AFM of bio-molecules as proteins, the measurement in water is essential, since properties of bio-systems in vacuum are different from those in water. We developed a basic simulation method for the tapping mode AFM in water and applied to protein samples. First, the cantilever elastic body oscillation in water very close to the sample is analyzed, by solving the fluid dynamics of water simultaneously by a specially designed finite element method. The calculated resonance curve showed strong nonlinear features, as well as the reduction of resonant frequency and lose of the sharpness of the resonance. This method is useful for designing cantilever shapes. Next, the tapping process of the tip by the sample is simulated by a visco-elastic model of bio-samples obtained coarse graining the atomistic model. Sticking and detaching of the tip to the sample, which causes large disturbance of the cantilever motion, are also analyzed. With including these processes altogether, the frequency shift, dissipating energy, phase delay of the oscillation are obtained and used to calculate tapping mode images of proteins.   

Functional probes for scanning probe microscopy

Inspite of importance of the probe in scanning probe microscopy (SPM), little attention was paid for the SPM probes for most of the measurements of SPM. We developed sharp metal-tip cantilevers with a typical curvature radius better than 5nm using focused ion beam (FIB) suitable for Kelvin probe force microscopy (KFM)$^{1}$. We obtained atomically resolved KFM images with an energy resolution less than 3meV with the probe$^{2}$. We also developed a glass-coated tungsten tip for synchrotron radiation-scanning tunneling microscopy with the FIB method$^{3}$ and obtained elementally resolved images in a resolution less than 20nm$^{4}$. We are now developing a precise atomic force microscope (AFM) lithography$^{5}$ with the FIB-milled tip attached to a quartz tuning fork controlled by noncontact AFM. We will present recent results of our AFM lithography, such as an Au line with a width of 20$\sim $30 nm and characters drawn with Au nano dots on a Si surface. 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\textbf{ 89}, 243119 (2006) 5 K. Akiyama\textit{ et al.}, JP \textbf{61}, 22 (2007).   

Fabrication of a NEMS Resonator Over-shield for Mass Sensing

The frequency shift of a resonating cantilever or bridge due to mass loading is dependent on the position of the loaded mass on the resonator. Therefore, for the purpose of accurate mass sensing, it is increasingly important to know the exact position of an added mass on a resonating mass sensor. Discussed is a novel technique to build over-shield structures on top of NEMS resonating devices to physically limit the position in which a loading mass can be deposited on a mass sensor. The over-shield is composed of a PECVD silicon nitride film which is supported by a sacrificial aluminum layer. Essentially, this MEMS over NEMS device acts as an integrated shadow mask for the resonator. With this over-shield device the effect of the position of added mass on a resonator is also examined.   

Ferromagnetic Resonance Investigation of an Individual Permalloy Dot Using Magnetic Resonance Force Microscopy

We report Ferromagnetic Resonance (FMR) investigations of individual 5.3 micron diameter permalloy dots using low temperature (4 K) Magnetic Resonance Force Microscopy (MRFM). The dot magnetization is saturated in the external magnetic field perpendicular to the plane of the sample. The evolution of the MRFM signal as probe-sample separation and the lateral probe position are varied reveals the shape of the magnetostatic modes excited in the dot in the presence of the strongly inhomogeneous magnetic field of the MRFM probe magnet. The experimental data agree excellently with micromagnetic modeling which suggests that localized FMR modes are excited in the sample. This effect opens the way for spatially resolved studies of ferromagnetic systems.   

Detecting Few Electron Spins by Magnetic Resonance Force Microscopy with Potential Application for Donor Mapping in Semiconductor

We report measurements of the statistical polarization of small electron spin ensembles by Magnetic Resonance Force Microscopy (MRFM). The experiments were conducted at T = 4 K using an IBM-style ultrasoft micromechanical cantilever outfitted with a high coercivity micromagnetic probe. Magnetic resonance signals from tens of electron spins with spin-correlation time of $\sim $400 ms are obtained. In order to apply MRFM for spatial mapping of donor electrons in semiconductors, non-contact friction between the cantilever and the silicon surface is investigated. We found the combination of hydrogen passivation, gold coating, and shielding the sample surface from stray laser light reduces the non-contact friction by almost a factor of 100.   

Force detected electron spin resonance from N@C$_{60}$ thin films

We report on force-detected electron spin resonance studies of thin films of endohedral fullerene N@C$_{60}$. The electron spin associated with the nitrogen atom exhibits long spin-lattice relaxation times ($T_1$) at low temperatures. By combining microwave pulses with periodic adiabatic spin inversions in large gradients, we are able to selectively manipulate and detect the spins in submicron volumes. We also discuss our progress in detecting {\em statistical fluctuations} of the spin magnetization in this system using ultrasensitive force detection techniques.   

Development of a $^3$He Nuclear Magnetic Resonance Force Microscope*

We report on construction of a $^3$He Nuclear Magnetic Resonance Force Microscopy (NMRFM) probe for nanoscale scanning and relaxation-time applications. Dual 3-axis piezo-driven stages yielded nanoscale positioning precision across several millimeters. We performed measurements on $^1$H nuclei in single crystal (NH$_4$)$_2$SO$_4$ in a sample-on-oscillator configuration at room temperature. A 0.25-mm-diameter permalloy magnet provided a field gradient of $\sim$500 T/m. The magnet position was scanned to achieve resonance; the RF frequency was also independently varied to verify the NMR nature of the force-detected signal. These first tests used a commercial AFM cantilever with a loaded resonance frequency of 2.0 kHz and spring constant of $\sim$0.03 N/m; motion was detected with a laser interferometer (1310 nm). Using cyclic adiabatic inversion (CAI), we detected a nuclear moment of 1.9 $\times 10^{-16}$ J/T with SNR $\approx 6$. By preceding the CAI sequence with a short, variable-length pulse, a spin nutation signal was observed over several cycles of period 17 $\mu$s, implying a rotating RF field of 14 G. Using a $(\pi/2) $-$\tau$-$\pi$-$t$-$\pi/2$-CAI sequence, a spin-echo was mapped out, with a FWHM of 8 $\mu$s. We also discuss plans to extend measurements towards the base temperature of the probe, 0.3 K. *This work was supported by NSF Grant Nos. DMR-0605828 and DGE- 0549417.   

NMR Force Microscopy on Co/Cu interface

We present our recent NMR Force Microscopy experiments, where we demonstrate the first detection of 63Cu and 65Cu NMR using Magnetic Resonance Force Microscopy (MRFM). The signals were detected at T = 5 K using a commercial Si3N4 cantilever with a spherical NdFeB probe magnet. We demonstrate MRFM detection sensitivity of 1.0e5 nuclear spins. We report measurements of the relaxation time, signal lifetime, and the results of nutation experiments. We also discuss the application of NMRFM for spatially resolved mapping of the local hyperfine field variation in the vicinity of a buried Co/Cu interface arising from the RKKY interaction.   

Micromagnetic Modeling of Localized Ferromagnetic Resonance Detected with Magnetic Resonance Force Microscopy

Magnetic Resonance Force Microscopy (MRFM) is a novel scanned probe technique based on mechanical detection of magnetic resonance. Its extreme sensitivity originates partially from the high magnetic field gradient of MRFM probe micromagnet which couples the MRFM probe to the magnetic moments in the sample. We report micromagnetic modeling of Ferromagnetic Resonance (FMR) performed in the local field of the micromagnetic MRFM probe: its strongly inhomogeneous field enables the excitation of localized FMR modes in the sample. This unusual effect provides a mechanism for spatially resolved FMR investigations of ferromagnetic systems. We discuss spatial resolution and results for both quasi 2D and 1D systems.   

Magnetic Resonance Force Microscopy System Design for the Study of Organic Materials

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. With the system we have obtained an MRFM signal on a sample of GaAs with known characteristics. We will present the most recent results of our program to study organic materials with MRFM.   

Applications of scanning Kelvin probe microscopy in the characterization of photovoltaic materials and devices

We have in recent years developed scanning Kelvin probe microscopy (SKPM), and applied this nanometer resolution technique to the characterization of III-V-, II-VI-, and thin film Si-based single- and multi-junction solar cell devices. In this presentation, we will report our improvements of the SKPM technique and show three examples of the potential measurements. We will first show a Bi-incorporation-induced junction movement in a MBE-grown single-junction GaInNAs cell. This junction movement caused significant device degradation, especially in the short wavelength range. We then present potential distributions among the top and bottom junctions in a GaInP$_{2}$/GaAs tandem-junction cell. A light-induced potential flattening in the top junction and a potential accumulation in the bottom junction was clearly measured. Lastly, we will show a non-uniform distribution of the electric field across an a-Si:H $n-i-p$ junction, and this electric field was significantly improved by depositing buffer layers at the $n/i$ and $i/p$ interfaces.