Stephen Wayne Howell

Nonlinear tapping dynamics of multi-walled carbon nanotube tipped atomic force microcantilevers

The nonlinear dynamics of an atomic force microcantilever (AFM) with an attached multi-walled carbon nanotube (MWCNT) tip is investigated experimentally and theoretically. We present the experimental nonlinear frequency response of a MWCNT tipped microcantilever in the tapping mode. Several unusual features in the response distinguish it from those traditionally observed for conventional tips. The MWCNT tipped AFM probe is apparently immune to conventional imaging instabilities related to the coexistence of attractive and repulsive tapping regimes. A theoretical interaction model for the system using an Euler elastica MWCNT model is developed and found to predict several unusual features of the measured nonlinear response.

Frequency Domain Identification of Tip-sample van der Waals Interactions in Resonant Atomic Force Microcantilevers

Hamaker constants are characteristic material properties that determine the magnitude of the nonlinear van der Waals force between atoms, molecules and nanoscale aggregates of atoms. This paper explores the novel possibility of using Harmonic Balance based nonlinear system identification methods to extract from the nonlinear vibration spectrum of resonant atomic force silicon microcantilevers, the Hamaker constants between a few atoms at the tip of the microcantilever and graphite, gold and silicon carbide samples. First, the nonlinear dynamics of a diving board microcantilever coupled to the samples through van der Waals force potentials are investigated through a discretized model of the system. Next, the feasibility of using Harmonic Balance based nonlinear system identification techniques are demonstrated using simulations of the discretized model. Finally the method is implemented on an AFM system. The results indicate that the proposed method provides a novel alternative way to measure Hamaker constants and the measured results are within the range of known experimental data.

Humidity-dependent open-circuit photovoltage from a bacteriorhodopsin/indium tin oxide bioelectronic heterostructure

Non-contact electrostatic force microscopy techniques were used to study the open-circuit photovoltage of purple membrane multilayers electrodeposited on indium tin oxide. A humidity-dependent photovoltage response under illumination by a 635 nm photodiode was observed. Peak photovoltages in excess of 2 V at ~36 W m−2 were obtained for ~15% relative humidity.

Self-Heating and Failure in Scalable Graphene Devices

Self-heating induced failure of graphene devices synthesized from both chemical vapor deposition (CVD) and epitaxial means is compared using a combination of infrared thermography and Raman imaging. Despite a larger thermal resistance, CVD devices dissipate >3x the amount of power before failure than their epitaxial counterparts. The discrepancy arises due to morphological irregularities implicit to the graphene synthesis method that induce localized heating. Morphology, rather than thermal resistance, therefore dictates power handling limits in graphene devices.

Testing gravity in space and at ultrashort distances

In this paper we discuss experiments testing gravity in space and at ultrashort distances. We show that the proposed STEP experiment has sufficient sensitivity to test how gravity couples to neutrinos and to higher-order weak interactions. Then, after briefly reviewing the recent interest in ultrashort distance gravity experiments, we describe a preliminary round of atomic force microscope (AFM) experiments which utilize the ‘iso-electronic effect’. Our experimental results set new limits on proposed gravity-like forces over distance ranges ∼1–4 nm.

Complex Dynamics of Carbon Nanotube Probe Tips

Carbon nanotube (CNT) tips in tapping mode atomic force microscopy (AFM) enable very high-resolution imaging, measurements, and manipulation at the nanoscale. We present recent results based on experimental analysis that yield new insights into the dynamics of CNT probe tips in tapping mode AFM. Experimental measurements are presented of the static response, the frequency response and dynamic amplitude-distance data of a high-aspect-ratio multi-walled (MW) CNT tip to demonstrate the nonlinear features including tip amplitude saturation preceding the dynamic buckling of the MWCNT. The differences between the nonlinear tapping mode response of CNT tips are compared with previously known results on the nonlinear response of conventional tips. Surface scanning is performed using a MWCNT tip on a SiO2 grating to verify the imaging instabilities associated with MWCNT buckling when used with normal control schemes in the tapping mode. Lastly, the choice of optimal setpoints for tapping mode control using CNT tip are discussed using the experimental results.© 2003 ASME

Voltage tuning of reflectance from a strongly coupled metasurface-semiconductor hybrid structure (Conference Presentation)

Metasurfaces have been investigated for various applications ranging from beam steering, focusing, to polarization conversion. Along with passive metasurfaces, significant efforts are also being made to design metasurfaces with tunable optical response. Among various approaches, voltage tuning is of particular interest because it creates the possibility of integration with electronics. In this work, we demonstrate voltage tuning of reflectance from a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode in an ultrathin semiconductor layer. Our approach involves electrically controlling the carrier concentration of the ENZ layer to modulate the polaritonic coupling between the dipole resonances of the metasurface and the ENZ mode for modulating the reflectance of the metasurface. The hybrid structure we fabricate is similar to MOSCAP configuration where the complementary metasurface offers a continuous gold top layer for biasing and positive/negative bias to the metasurface leads to accumulation/depletion of carriers in the ENZ layer beneath it. We optimized our structure by using InGaAs as the ENZ material because of its high mobility and low effective mass. This allowed us to reduce the doping requirement and thereby reduce the ionized impurity scattering as well as the reverse bias required to deplete the ENZ layer. For low leakage and efficient modulation of carrier density, we used Hafnia as the gate dielectric. We further added a reflecting backplane below the ENZ layer to enhance the interaction and by applying bias, we achieved spectral shifts of 500 nm and amplitude modulation of 11% of one of the polariton branches at 14 µm.

Visibility of dielectrically passivated graphene films

The visibility of monolayer graphene is dependent on its surrounding dielectric environment and the presence of any contamination associated with 2D layer transfer. Here, the optical contrast of residually contaminated monolayer graphene encased within a range of dielectric stacks characteristic of realistic devices is examined, highlighting the utility of optical microscopy for a graphene assessment, both during and after lithographic processing. Practically, chemical vapor deposited graphene is encapsulated in dielectric stacks of varying thicknesses of SiO2. Optical contrast is then measured and compared to predictions of a multilayer model. Experimentally measured contrast is in close agreement with simulation only when contamination is included.

Exploring graphene field effect transistor devices to improve spectral resolution of semiconductor radiation detectors

Graphene, a planar, atomically thin form of carbon, has unique electrical and material properties that could enable new high performance semiconductor devices. Graphene could be of specific interest in the development of room-temperature, high-resolution semiconductor radiation spectrometers. Incorporating graphene into a field-effect transistor architecture could provide an extremely high sensitivity readout mechanism for sensing charge carriers in a semiconductor detector, thus enabling the fabrication of a sensitive radiation sensor. In addition, the field effect transistor architecture allows us to sense only a single charge carrier type, such as electrons. This is an advantage for room-temperature semiconductor radiation detectors, which often suffer from significant hole trapping. Here we report on initial efforts towards device fabrication and proof-of-concept testing. This work investigates the use of graphene transferred onto silicon and silicon carbide, and the response of these fabricated graphene field effect transistor devices to stimuli such as light and alpha radiation.

Enabling Graphene Nanoelectronics

Recent work has shown that graphene, a 2D electronic material amenable to the planar semiconductor fabrication processing, possesses tunable electronic material properties potentially far superior to metals and other standard semiconductors. Despite its phenomenal electronic properties, focused research is still required to develop techniques for depositing and synthesizing graphene over large areas, thereby enabling the reproducible mass-fabrication of graphene-based devices. To address these issues, we combined an array of growth approaches and characterization resources to investigate several innovative and synergistic approaches for the synthesis of high quality graphene films on technologically relevant substrate (SiC and metals). Our work focused on developing the fundamental scientific understanding necessary to generate large-area graphene films that exhibit highly uniform electronic properties and record carrier mobility, as well as developing techniques to transfer graphene onto other substrates.