Session B15: Focus Session: Advances in Scanned Probe Microscopy I: Novel AFM, MRFM, and Acoustic Microscopy

Probing bias-strain coupling on the nanoscale by Piezoresponse Force Microscopy: from ferroelectric and multiferroics to energy storage materials

Coupling between the electric fields and strains is ubiquitous on the nanoscale, ranging from piezoelectricity and electrostriction in piezo- and ferroelectrics to complex phenomena mediated by the changes in mobile ion concentration in intercalation compounds and collective Jahn-Teller distortions coupled to oxidation states in correlated oxides. In this presentation, I will summarize recent advances in Piezoresponse Force Microscopy applied for studies of bias-induced phase transitions in ferroelectrics and multiferroics, and demonstrate potential for mapping polarization switching on a single defect level. Phase-field modeling allows the corresponding mesoscopic mechanisms to be deciphered, and further suggests strategies for (symmetry forbidden) manipulation of in-plane polarization component. Controlled creation of ferroelectric closure domains is demonstrated. In the second part of the talk, I will demonstrate applications of spectroscopic PFM for mapping Li-ion dynamics and diffusion in energy storage materials and devices, potentially extending PFM for studies of nanoscale phenomena in an extremely broad range of materials beyond ferroelectrics.   

Towards ultrasensitive scanning probe force detection with silicon nanowire mechanical resonators

Silicon nanowires have emerged as promising force sensors due to their low intrinsic mechanical dissipation. At room temperature, the nanowires we study possess a mechanical dissipation in the range of 2$\times $10$^{-15}$-~2$\times $10$^{-14}$~kg/s, corresponding to a force sensitivity of 6-18 $\times $10$^{-18 }$N/$\surd $Hz. Force sensitivities below 10$^{-18 }$N/$\surd $Hz should be possible by cooling the nanowires to low temperatures. This is an encouraging prospect for applications such as magnetic resonance force microscopy. To this end, we describe our progress toward the use of silicon nanowires as scanning probe force sensors at 4K. We also discuss a novel form of active feedback to cancel the native cubic nonlinearity of silicon nanowire resonators.   

Characterization of an MRFM probe in the SPAM geometry

This talk will describe a new MRFM probe built by the author that uses in the SPAM geometry, operates in vacuum at 4 K, up to 9 T, has 3D sample stage motion, uses Cornell cantilevers with spring constants of 0.1 mN/m with 7 um diameter nickel spheres mounted on the cantilever, and spring based vibration isolation that results in Brownian motion limited behavior at 4 K. The talk will describe the probe's Brownian motion and frequency deviations noise behavior vs. operating conditions, the cantilever's frequency and Q dependence vs. background magnetic field, power density spectra of cantilever fluctuations both far from and near a gold coated GaAs surface, the importance of cantilever control for noise abatement, and NMR line shapes of Ga69.   

High sensitivity electron spin magnetic resonance force microscopy for labeled biological samples

Magnetic resonance force microscopy is a promising route to 3-dimensional nanoscale imaging of organic materials due to its high sensitivity and isotopic specificity. Labeling of proteins, DNA and biomolecular assemblies with free radical labels for inductive detection are well established techniques, although many of these radical's relaxation times are too short to support previously demonstrated techniques for single electron detection by magnetic resonance force microscopy. We report on our efforts toward sub-single electron sensitivity on organic radicals using batch fabricated 100 nm nickel nanorod tipped ultrasensitive cantilevers.   

Non-degenerate parametric amplification used for surface noise evasion in scanned probe microscopy

A straightforward way to enhance sensitivity and spatial resolution of magnetic resonance force microscopy is approaching an attonewton-sensitivity cantilever having a 100-nm diameter magnetic tip to closer than 50 nm proximity of spin samples. When one detects magnetic resonance via cantilever frequency-shift measurements, cantilevers experience a drastic increase of surface frequency noise at small tip-sample separations. Even along with lessening contribution of conducting tip charge to the noise, surface frequency noise remains as a remarkable obstacle. On the other hand, surface force noise was found to remain surprisingly unchanged up until about 10 nm with custom-fabricated overhanging magnetic nanorod tips. We thus developed a novel protocol, reading out a force-gradient (frequency-shift) spin signal as a force (amplitude change), harnessing spin-driven parametric amplification to evade surface noise and detector noise in force-gradient detected scanned probe magnetic resonance, presenting a demonstration on ESR from nitroxide spin probe in a thin film.   

Whispering-Gallery Acoustic-Sensing of Shear-forces

A novel Whispering-Gallery Acoustic Sensing (WGAS) modality has been integrated into a tuning-fork (TF) based scanning probe microscope (TF-SPM) for monitoring, via acoustic means, the distance-dependent shear-force interactions between the laterally oscillating microscope's probe and the analyzed sample. The novelty in the WGAS consist of exploiting the microscope's head-stage itself as an acoustic resonant cavity for efficiently and sensitively detecting the acoustic waves generated by the TF (in physical contact with the cavity), whose mechanical motion is modulated by the probe-sample interaction. The WGAS is able to sensitively detect the probe-sample distance dependence of the shear forces, which allows to implement an acoustic feedback control distance. Images obtained based on the WGAS mechanism will be presented.   

The Ultrasonic-Coupled Near-Field Microscope

A novel, compact and versatile imaging technique is presented. Fully operated by acoustic sensors, the Ultrasonically-Coupled Near-field Microscope (UCNM) is capable of sensitively monitoring the acoustic waves generated by mesoscopic fluid-like films trapped between two periodically-sliding solid boundaries. Another novelty includes an acoustic-based feedback for controlling the UCNM probe's vertical position, which is exploited for topographic imaging characterization. The UCNM constitutes an alternative way for characterizing surface phenomena (nanotribology, adhesion, wetting) at the nanoscale and presents potential capabilities for imaging sub-surface materials properties.   

AFM Study of Charge Transfer Between Metals Due to the Oxygen Redox Couple in Water

The oxygen redox couple in an adsorbed water film can pin the Fermi level at the surfaces of diamond, GaN and ZnO.\footnote{V. Chakrapani, C. Pendyala, K. Kash, A. B. Anderson, M. K. Sunkara and J. C. Angus, \textit{J. Am. Chem. Soc.} \textbf{130} (2008) 12944-12952, and ref. 6 therein.} We report here preliminary observations of the same phenomenon at metal surfaces. A Pt-coated atomic force microscope (AFM) tip was used to take force-distance measurements on Au, Ag, and Pt surfaces placed in pH-controlled water. The work functions of these surfaces vary over $\sim $2eV and span the electrochemical potential range of the oxygen redox couple, which varies with pH according to the Nernst equation. Adjusting the pH of the water from 4 to 12 allowed us to change the redox potential energy from -5.42eV to -4.95eV, changing the surface charge and the associated screening charge and modulating the pull-off force. This work has relevance to AFM of many materials in air, and to contact electrification, mechanical friction, and nanoscale corona discharges.   

Towards Obtaining Ultimate Resolution with Atomic Force Microscopy

To increase the resolution of surface microscopy is one of the most significant goals of surface science. The resolution of atomic force microscopy (AFM) is critically defined and scaled by the radius of the AFM tip. Ultimate resolution can be achieved by functionalizing the tip with a molecule like carbon monoxide with the tip molecule significantly contributing to the measured force. The force and therefore the resolution crucially depends on the chemical nature of the terminating tip molecule. In this work molecules such as pentacene are imaged with unprecedented resolution by employing such functionalized tips. The interactions of the tip molecule with imaged molecule are studied with ab initio density functional theory (DFT) calculations. The calculations show that Pauli repulsion is the source of the high resolution, whereas van-der-Waals and electrostatic forces only add a diffuse attractive background. This enhancement of the resolution is also observed experimentally and compares very well with the theoretical findings.   

Spatial Resolution of Electrical Measurements Performed with Scanning Probe Microscopes as a Function of Tip Shape

Scanning capacitance microscopy and Scanning Kelvin force microscopy are used to measure complex dopant profiles in semiconductors and surface potential/work function images respectively. Both techniques measure a signal related to the capacitance between the tip and sample. The spatial resolution and accuracy of both techniques depends on the shape of the terminal tip and the stray capacitance between the sample and cantilever. Tip diameter and the aspect ratio (length/width) of the tip are important parameters. We have measured the SCM and SKFM response using three different styles of SPM tip: conventional metal-coated silicon cantilevered tips, solid Pt wire tips, and a conventional silicon tip which was terminated by a single carbon nanotube. Samples measured included ultra shallow ion implanted dopant profiles, which were annealed at four different temperatures to produce junction depths between 10 nm and 40 nm; a work function test structure with alternating lines of Al, Cr, and Au; and a dopant concentration calibration structure. The ability of the three types of SPM tips to resolve the four slightly different dopant profiles and the width of the transitions between different materials was characterized.   

Micromolding fabrication of SiC SPM probes

Micromolding techniques have been employed to form SiC cantilevers on silicon substrates for use in SPM. A pre-ceramic polymer is molded into the desired probe shape and then converted to SiC via pyrolisys. Due to SiC's much higher softening temperature compared to silicon, this approach may enable AFM scanning of very high temperature surfaces. Additionally, the use of polymer molding techniques can lead to the creation of cantilever cross sections with unconventional geometries allowing for higher resonant frequencies while maintaining a length $>$ 100$\mu$m.   

Atomic-scale Nanoprobes Fabricated by Localized Ion Flux Reduction and Field-Directed Sputter Sharpening

The Field-Directed Sputter Sharpening (FDSS) method for fabrication of atomic-scale metallic probes is explained as a localized reduction in ion flux at the probe apex, leading to a reduction in apex atom migration by surface diffusion and a corresponding decline in equilibrium radius of curvature. The resulting apices are found to exhibit reproducible, regenerable, stable and superior behavior as scanning tunneling microscopy tips, and to facilitate resolution-enhanced imaging and patterning. Here we describe and demonstrate the distinction between FDSS and conventional sputter erosion techniques by simulated and experimental sputter erosion sharpening, and demonstrate the capabilities of FDSS-sharpened probes for atomically-precise patterning of the Si(100) surface.   

Elastic property characterization of oxidized Si nanowires by contact-resonance atomic force microscopy

The synthesis and processing of materials into nanostructures opens new avenues for advancement and diversification of current electronic, optoelectronic, and sensor applications. Among these structures, Si NWs are distinctly remarkable as they bring the previous decades knowledge of silicon technology into nanoscale applications. From this perspective, the characterization and understanding of the mechanical properties of nonplanar Si-SiO2 interfaces are of significant utility in developing Si nanostructures for Si-based integrated circuits. To investigate the elastic properties of as-grown and oxidized Si NWs we have extended and specifically tailored the applicability of contact-resonance atomic force microscopy (CR- AFM). From such CR-AFM measurements, the effect of the compressive stress at the Si-SiO2 interface was revealed in a diameter dependence of the elastic modulus of oxidized Si NWs. A modified core-shell model that includes the interface stress developed during oxidation captures the observed dependence. The values of strain and stress as well as the width of the stressed transition region at the Si-SiO2 interface agree with those reported from simulations and other experiments. This novel approach advances CR-AFM applicability in investigating structure-mechanical property relationships at the nanoscale.