Utilization of coupled eigenmodes in Akiyama atomic force microscopy probes for bimodal multifrequency sensing.

Akiyama atomic force microscopy probes represent a unique means of combining several of the desirable properties of tuning fork and cantilever probe designs. As a hybridized mechanical resonator, the vibrational characteristics of Akiyama probes result from a complex coupling between the intrinsic vibrational eigenmodes of its constituent tuning fork and bridging cantilever components. Through a combination of finite element analysis modeling and experimental measurements of the thermal vibrations of Akiyama probes we identify a complex series of vibrational eigenmodes and measure their frequencies, quality factors, and spring constants. We then demonstrate the viability of Akiyama probes to perform bimodal multi-frequency force sensing by performing a multimodal measurement of a surface's nanoscale photothermal response using photo-induced force microscopy imaging techniques. Further performing a parametric search over alternative Akiyama probe geometries, we propose two modified probe designs to enhance the capability of Akiyama probes to perform sensitive bimodal multifrequency force sensing measurements.

Temperature sensitivity of scattering-type near-field nanoscopic imaging in the visible range.

Due to its superb imaging spatial resolution and spectroscopic viability, scattering-type scanning near-field optical microscopy (s-SNOM) has proven to be widely applicable for nanoscale surface imaging and characterization. However, limited works have investigated the sensitivity of the s-SNOM signal to sample temperature. This paper reports the sample temperature effect on the non-interferometric (self-homodyne) s-SNOM scheme at a visible wavelength (λ=638 nm). Our s-SNOM measurements for an arrayed vanadium/quartz sample demonstrate a monotonic decrease in signal intensity as sample temperature increases. As a result, s-SNOM imaging cannot distinguish quartz or vanadium when the sample is heated to ∼309 K: all signals are close to the root-mean-square noise of the detection scheme used for this study (i.e., 19 µV-rms). While further studies are required to better understand the underlying physics of such temperature dependence, the obtained results suggest that s-SNOM measurements should be carefully conducted to meet a constant sample temperature condition, particularly when a visible-spectrum laser is to be used as the light source.

Electronically driven fragmentation of silver nanocrystals revealed by ultrafast electron crystallography.

We report an ultrafast electron diffraction study of silver nanocrystals under surface plasmon resonance excitation, leading to a concerted fragmentation. By examining simultaneously transient structural, thermal, and Coulombic signatures of the prefragmented state, an electronically driven nonthermal fragmentation scenario is proposed.

The development and applications of ultrafast electron nanocrystallography.

We review the development of ultrafast electron nanocrystallography as a method for investigating structural dynamics for nanoscale materials and interfaces. Its sensitivity and resolution are demonstrated in the studies of surface melting of gold nanocrystals, nonequilibrium transformation of graphite into reversible diamond-like intermediates, and molecular scale charge dynamics, showing a versatility for not only determining the structures, but also the charge and energy redistribution at interfaces. A quantitative scheme for 3D retrieval of atomic structures is demonstrated with few-particle (<1,000) sensitivity, establishing this nanocrystallographic method as a tool for directly visualizing dynamics within isolated nanomaterials with atomic scale spatio-temporal resolution.

Dynamics of size-selected gold nanoparticles studied by ultrafast electron nanocrystallography.

We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined with subpicosecond and picometer accuracies. In an ultrafast photoinduced melting, the nanoparticles are driven to a nonequilibrium transformation, characterized by the initial lattice deformations, nonequilibrium electron-phonon coupling, and, upon melting, the collective bonding and debonding, transforming nanocrystals into shelled nanoliquids. The displasive structural excitation at premelting and the coherent transformation with crystal/liquid coexistence during photomelting differ from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle.

The effect of the cut surface during electrical stimulation of a cardiac wedge preparation.

Optical mapping from the cut surface of a "wedge preparation" allows observation inside the heart wall, below the epicardium or endocardium. We use numerical simulations based on the bidomain model to illustrate how the transmembrane potential is influenced by the cut surface. The distribution of transmembrane potential around a unipolar cathode depends on the fiber angle. For intermediate angles, hyperpolarization appears on only one side of the electrode, and is large and widespread.