Santiago D. Solares

Electromechanical properties of graphene drumheads

Straining Suspended Graphene The electronic properties of graphene are best displayed by suspended sheets free from contact with an underlying substrate. Klimov et al. (p. 1557) probed how deformation of suspended graphene sheets could lead to further tuning of its electronic properties with a scanning tunneling microscope; the graphene sheets could also be deformed via an electric field from an underlying electrode. Spectroscopic studies reveal that the induced strain led to charge-carrier localization into spatially confined quantum dots, an effect consistent with the formation of strain-induced pseudomagnetic fields. Mechanical straining of suspended graphene films leads to confinement of charge carriers into quantum dots. We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip–induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.

Simulations of High-Pressure Phases in RDX

Using a fully flexible molecular potential in equilibrium molecular dynamics simulations, we study the α- and γ-polymorphs of the energetic molecular crystal hexahydro-1,3,5-trinitro-s-triazine (RDX), their respective properties, and the conditions that contribute to the stress-induced γ → α solid-solid phase transition mechanisms. We find the pressure-dependent atomic structure, mechanical properties, and transition behavior to be described reasonably well. Uniaxial deformation of α-RDX along the crystal axes is shown to result in three different crystal responses where compression of the c-axis results in the α → γ transition, compression of the b-axis causes a transition with resulting structure similar to stacking faults observed by Cawkwell et al. [ J. Appl. Phys. 2010, 107, 063512], and no transitions are observed for compression of the a-axis.

Nanoscale Interfacial Friction and Adhesion on Supported versus Suspended Monolayer and Multilayer Graphene

Using atomic force microscopy (AFM), supported by semicontinuum numerical simulations, we determine the effect of tip-subsurface van der Waals interactions on nanoscale friction and adhesion for suspended and silicon dioxide supported graphene of varying thickness. While pull-off force measurements reveal no layer number dependence for supported graphene, suspended graphene exhibits an increase in pull-off force with thickness. Further, at low applied loads, friction increases with increasing number of layers for suspended graphene, in contrast to reported trends for supported graphene. We attribute these results to a competition between local forces that determine the deformation of the surface layer, the profile of the membrane as a whole, and van der Waals forces between the AFM tip and subsurface layers. We find that friction on supported monolayer graphene can be fit using generalized continuum mechanics models, from which we extract the work of adhesion and interfacial shear strength. In addition, we show that tip-sample adhesive forces depend on interactions with subsurface material and increase in the presence of a supporting substrate or additional graphene layers.

Triple-frequency intermittent contact atomic force microscopy characterization: Simultaneous topographical, phase, and frequency shift contrast in ambient air

We present computational simulation and experimental results of ambient air atomic force microscopy (AFM) characterization with simultaneous excitation and control of three eigenmodes of a rectangular microcantilever beam. Trimodal characterization combining amplitude and frequency modulation is an enhancement of the capabilities of the AFM technique, which could allow the rapid acquisition of topographical, phase, and frequency shift contrast with a single surface scan at normal scan rates. The results suggest that, in general, the phase and frequency shift contrast are affected similarly but in opposite directions by the tip-sample interactions, although deviations from this trend are often observed in the experiments, such that all available sources of contrast could provide complementary information on surface properties.

Frequency response of higher cantilever eigenmodes in bimodal and trimodal tapping mode atomic force microscopy

We have recently implemented a trimodal tapping mode atomic force microscopy (AFM) imaging scheme for ambient air with which it is possible to simultaneously acquire topographical, phase and frequency shift contrast images. In implementing this method we have identified conditions, such as very low fundamental amplitude setpoints and very low oscillation amplitudes for the higher eigenmodes, for which the stability of the imaging process can be compromised. In this work we use numerical simulations to gain insight into the changes in the frequency response of the higher eigenmodes in bimodal and trimodal operation for different levels of sample stiffness, tip–sample dissipative forces, oscillation amplitudes for each of the eigenmodes and cantilever rest positions above the surface. Although we do not attempt to convey a complete dynamics picture of the system, the results provide general guidelines for the selection of conditions that lead to stable imaging as well as insight into the observed phase and frequency shift contrast, highlighting a few potential imaging artifacts. Our simulation results are in general agreement with our experimental observations.

Mapping of conservative and dissipative interactions in bimodal atomic force microscopy using open-loop and phase-locked-loop control of the higher eigenmode

We compare the ability of higher cantilever eigenmodes to map conservative and dissipative tip-sample interactions in bimodal atomic force microscopy under three different control schemes, namely, open-loop (OL), constant-excitation phase-locked-loop (CE-PLL), and constant-amplitude phase-locked-loop (CA-PLL). We perform a direct comparison of these schemes by applying analytical expressions of the virial and dissipated power to imaging and spectroscopy experiments conducted on a two-component polymer sample in air. We find that OL and CE-PLL provide similar information, while CA-PLL explores a broader range of interactions, especially for softer samples, due to its constant sensitivity to tip-sample forces.

Characterization of deep nanoscale surface trenches with AFM using thin carbon nanotube probes in amplitude-modulation and frequency-force-modulation modes

The characterization of deep surface trenches with atomic force microscopy (AFM) presents significant challenges due to the sharp step edges that disturb the instrument and prevent it from faithfully reproducing the sample topography. Previous authors have developed AFM methodologies to successfully characterize semiconductor surface trenches with dimensions on the order of tens of nanometers. However, the study of imaging fidelity for features with dimensions smaller than 10 nm has not yet received sufficient attention. Such a study is necessary because small features in some cases lead to apparently high-quality images that are distorted due to tip and sample mechanical deformation. This paper presents multi-scale simulations, illustrating common artifacts affecting images of nanoscale trenches taken with fine carbon nanotube probes within amplitude-modulation and frequency-force-modulation AFM (AM-AFM and FFM-AFM, respectively). It also describes a methodology combining FFM-AFM with a step-in/step-out algorithm analogous to that developed by other groups for larger trenches, which can eliminate the observed artifacts. Finally, an overview of the AFM simulation methods is provided. These methods, based on atomistic and continuum simulation, have been previously used to study a variety of samples including silicon surfaces, carbon nanotubes and biomolecules.

Nanomechanical stimulus accelerates and directs the self-assembly of silk-elastin-like nanofibers.

One-dimensional nanostructures are ideal building blocks for functional nanoscale assembly. Peptide-based nanofibers have great potential in building smart hierarchical structures due to their tunable structures at the single residue level and their ability to reconfigure themselves in response to environmental stimuli. We observed that pre-adsorbed silk-elastin-based protein polymers self-assemble into nanofibers through conformational changes on a mica substrate. Furthermore, we demonstrate that the rate of self-assembly was significantly enhanced by applying a nanomechanical stimulus using atomic force microscopy. The orientation of the newly grown nanofibers was mostly perpendicular to the scanning direction, implying that the new fiber assembly was locally activated with directional control. Our method provides a novel way to prepare nanofiber patterned substrates using a bottom-up approach.

Numerical analysis of dynamic force spectroscopy using the torsional harmonic cantilever

A spectral analysis method has been recently introduced by Stark et al (2002 Proc. Natl Acad. Sci. USA 99 8473-8) and implemented by Sahin et al (2007 Nat. Nanotechnol. 2 507-14) using a T-shaped cantilever design, the torsional harmonic cantilever (THC), which is capable of performing simultaneous tapping-mode atomic force microscopy imaging and force spectroscopy. Here we report on numerical simulations of the THC system using a simple dual-mass flexural-torsional model, which is applied in combination with Fourier data processing software to illustrate the spectroscopy process for quality factors corresponding to liquid, air and vacuum environments. We also illustrate the acquisition of enhanced topographical images and deformed surface contours under the application of uniform forces, and compare the results to those obtained with a previously reported linear dual-spring-mass model.

Amplitude modulation dynamic force microscopy imaging in liquids with atomic resolution: comparison of phase contrasts in single and dual mode operation

We present a systematic analysis of the atomic-scale imaging capabilities for mineral surfaces in a liquid environment in single and dual mode amplitude modulation dynamic force microscopy. To study the difference in sensitivity between the first and second eigenmode phase signals we investigate the observed atomic-scale contrasts of the mica-water interface under varying imaging conditions. For this purpose, we systematically change the main imaging parameters including the setpoint amplitude of the imaging feedback, the free oscillation amplitudes of the first and second flexural eigenmodes, and their ratio. This allows for an in-depth analysis of the sensitivities of the first and second eigenmode phase signals to draw conclusions regarding the underlying physical mechanisms and the interpretation of the contrast in the multi-frequency technique.