Resume

• 2009

  PhD

  Mechanical Engineering

• 2008

  MS

  Mechanical Engineering

• 2004

  BS

  Mechanical Engineering

• Nanotechnology

  COMSOL

  Matlab

  Microscopy

  Characterization

  ANSYS

  FIB

  Thin Films

  Finite Element Analysis

  Labview

  Scanning Electron Microscopy

  Metrology

  Heat Transfer

  Design of Experiments

  MEMS

  AFM

  Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes

  Among the remarkable variety of semiconducting nanomaterials that have been discovered over the past two decades

  single-walled carbon nanotubes remain uniquely well suited for applications in high-performance electronics

  sensors and other technologies. The most advanced opportunities demand the ability to form perfectly aligned

  horizontal arrays of purely semiconducting

  chemically pristine carbon nanotubes. Here

  we present strategies that offer this capability. Nanoscale thermocapillary flows in thin-film organic coatings followed by reactive ion etching serve as highly efficient means for selectively removing metallic carbon nanotubes from electronically heterogeneous aligned arrays grown on quartz substrates. The low temperatures and unusual physics associated with this process enable robust

  scalable operation

  with clear potential for practical use. We carry out detailed experimental and theoretical studies to reveal all of the essential attributes of the underlying thermophysical phenomena. We demonstrate use of the purified arrays in transistors that achieve mobilities exceeding 1

  000 cm2 V−1 s−1 and on/off switching ratios of ~10

  000 with current outputs in the milliamp range. Simple logic gates built using such devices represent the first steps toward integration into more complex circuits.

  Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes

  Atomic force microscope infrared spectroscopy (AFM-IR) can perform IR spectroscopic chemical identification with sub-100 nm spatial resolution

  but is relatively slow due to its low signal-to-noise ratio (SNR). In AFM-IR

  tunable IR laser light is incident upon a sample

  which results in a rise in temperature and thermomechanical expansion of the sample. An AFM tip in contact with the sample senses this nanometer-scale photothermal expansion. The tip motion induces cantilever vibrations

  which are measured either in terms of the peak-to-peak amplitude of time-domain data or the integrated magnitude of frequency-domain data. Using a continuous Morlet wavelet transform to the cantilever dynamic response

  we show that the cantilever dynamics during AFM-IR vary as a function of both time and frequency. Based on the observed cantilever response

  we tailor a time–frequency-domain filter to identify the region of highest vibrational energy. This approach can increase the SNR of the AFM cantilever signal

  such that the throughput is increased 32-fold compared to state-of-the art procedures. We further demonstrate significant increases in AFM-IR imaging speed and chemical identification of nanometer-scale domains in polymer films.

  Improved Atomic Force Microscope Infrared Spectroscopy for Rapid Nanometer-Scale Chemical Identification

  We describe an atomic force microscope cantilever design for which the second flexural mode frequency can be tailored relative to the first mode frequency

  for operation in contact with a substrate. A freely resonating paddle internal to the cantilever reduces the stiffness of the second flexural mode relative to the first while nearly maintaining the mass of the original cantilever. Finite element analysis is used to predict the performance of various cantilever designs and several cantilevers are fabricated and tested. This strategy allows the ratio of the first two resonant modes f2/f1 to be controlled over the range 1.6–4.5. The ability to vary f2/f1 could improve a variety of dynamic contact-mode measurements.

  Mechanical design for tailoring the resonance harmonics of an atomic force microscope cantilever during tip–surface contact

  We report a wear-resistant ultrananocrystalline (UNCD) diamond tip integrated onto a heated atomic force microscope (AFM) cantilever and UNCD tips integrated into arrays of heated AFM cantilevers. The UNCD tips are batch-fabricated and have apex radii of approximately 10 nm and heights up to 7 μm. The solid-state heater can reach temperatures above 600 °C and is also a resistive temperature sensor. The tips were shown to be wear resistant throughout 1.2 m of scanning on a single-crystal silicon grating at a force of 200 nN and a speed of 10 μm s−1. Under the same conditions

  a silicon tip was completely blunted. We demonstrate the use of these heated cantilevers for thermal imaging in both contact mode and intermittent contact mode

  with a vertical imaging resolution of 1.9 nm. The potential application to nanolithography was also demonstrated

  as the tip wrote hundreds of polyethylene nanostructures.

  Ultrananocrystalline diamond tip integrated onto a heated atomic force microscope cantilever

  We report exceptional nanoscale wear and fouling resistance of ultrananocrystalline diamond (UNCD) tips integrated with doped silicon atomic force microscope (AFM) cantilevers. The resistively heated probe can reach temperatures above 600 °C. The batch fabrication process produces UNCD tips with radii as small as 15 nm

  with average radius 50 nm across the entire wafer. Wear tests were performed on substrates of quartz

  silicon carbide

  silicon

  or UNCD. Tips were scanned for more than 1 m at a scan speed of 25 μm s−1 at temperatures ranging from 25 to 400 °C under loads up to 200 nN. Under these conditions

  silicon tips are partially or completely destroyed

  while the UNCD tips exhibit little or no wear

  no signs of delamination

  and exceptional fouling resistance. We demonstrate nanomanufacturing of more than 5000 polymer nanostructures with no deterioration in the tip.

  Wear-Resistant Diamond Nanoprobe Tips with Integrated Silicon Heater for Tip-Based Nanomanufacturing

  We investigate the nanometer-scale flow of molten polyethylene from a heated atomic force microscope (AFM) cantilever tip during thermal dip-pen nanolithography (tDPN). Polymer nanostructures were written for cantilever tip temperatures and substrate temperatures controlled over the range 100–260 °C and while the tip was either moving with speed 0.5–2.0 µm s−1 or stationary and heated for 0.1–100 s. We find that polymer flow depends on surface capillary forces and not on shear between tip and substrate. The polymer mass flow rate is sensitive to the temperature-dependent polymer viscosity. The polymer flow is governed by thermal Marangoni forces and non-equilibrium wetting dynamics caused by a solidification front within the feature.

  Nanometer-scale flow of molten polyethylene from a heated atomic force microscope tip

  We investigate the control of tip temperature on feature size during dip-pen nanolithography (DPN) of mercaptohexadecanoic acid (MHA) on Au. Heated atomic force microscopy (AFM) probes operated between 25 °C and 50 °C wrote nanostructures of MHA for various dwell times and tip speeds. The feature size exhibited an exponential dependence on tip temperature with an apparent activation barrier of 165 kJ/mol. Analysis of the ink transfer process shows that

  while ∼1/3 of the barrier is from ink dissolution into the meniscus

  the rest reflects the barrier to adsorption onto the growing feature

  a process that has been ignored in previous DPN models.

  Temperature-dependence of ink transport during thermal dip-pen nanolithography

  We measure the infrared spectra of polyethylene nanostructures of height 15 nm using atomic force microscope infrared spectroscopy (AFM-IR)

  which is about an order of magnitude improvement over state of the art. In AFM-IR

  infrared light incident upon a sample induces photothermal expansion

  which is measured by an AFM tip. The thermomechanical response of the sample-tip-cantilever system results in cantilever vibrations that vary in time and frequency. A time-frequency domain analysis of the cantilever vibration signal reveals how sample thermomechanical response and cantilever dynamics affect the AFM-IR signal. By appropriately filtering the cantilever vibration signal in both the time domain and the frequency domain

  it is possible to measure infrared absorption spectra on polyethylene nanostructures as small as 15 nm.

  Atomic Force Microscope Infrared Spectroscopy on 15 Nanometer Scale Polymer Nanostructures

  Infrared (IR) spectroscopy is one of the most widely used techniques for identifying and characterizing materials

  but is diffraction limited to a spatial resolution of no smaller than several micrometers. This paper reports IR spectroscopy with 100 nm spatial resolution

  using a tunable laser whose absorption in an organic layer is measured via atomic force microscopy. Wavelength-dependent absorption in the sample results in local thermomechanical deformation

  which is sensed using the sharp tip of a resonant atomic force microscope cantilever. We introduce a cantilever and system design capable of 100 nm spatial resolution and a 6 × sensitivity improvement over previous approaches.

  High-sensitivity nanometer-scale infrared spectroscopy using a contact mode microcantilever with an internal resonator paddle

  We report measurements of near-field absorption in heavily silicon-doped indium arsenide microparticles using atomic force microscope infrared spectroscopy (AFM-IR). The microparticles exhibit an infrared absorption peak at 5.75 μm

  which corresponds to a localized surface plasmon resonance within the microparticles. The near-field absorption measurements agree with far-field measurements of transmission and reflection

  and with results of numerical solutions of Maxwell equations. AFM-IR measurements of a single microparticle show the temperature increase expected from Ohmic heating within the particle

  highlighting the potential for high resolution infrared imaging of plasmonic and metamaterial structures.

  Near-Field Infrared Absorption of Plasmonic Semiconductor Microparticles Studied Using Atomic Force Microscope Infrared Spectroscopy

  We demonstrate measurement and control of single-asperity friction by using cantilever probes featuring an in situ solid-state heater. The heater temperature was varied between 25 and 650 °C (tip temperatures from 25 ± 2 to 120 ± 20 °C). Heating caused friction to increase by a factor of 4 in air at 30% relative humidity

  but in dry nitrogen friction decreased by 40%. Higher velocity reduced friction in ambient with no effect in dry nitrogen. These trends are attributed to thermally assisted formation of capillary bridges between the tip and substrate in air

  and thermally assisted sliding in dry nitrogen. Real-time friction measurements while modulating the tip temperature revealed an energy barrier for capillary condensation of 0.40 ± 0.04 eV but with slower kinetics compared to isothermal measurements that we attribute to the distinct thermal environment that occurs when heating in real time. Controlling the presence of this nanoscale capillary and the associated control of friction and adhesion offers new opportunities for tip-based nanomanufacturing.

  Local Nanoscale Heating Modulates Single-Asperity Friction

  There is a significant need for chemical identification and chemical imaging of nanofabricated structures and devices

  especially for multiple materials integrated at the nanometer scale. Here we present nanofabrication

  chemical identification

  and nanometer-scale chemical imaging of polymer nanostructures with better than 100 nm spatial resolution. Polymer nanostructures of polyethylene

  polystyrene

  and poly(3-dodecylthiophene-2

  5-diyl) were fabricated by tip-based nanofabrication. Nanometer-scale infrared measurements using atomic force microscopy infrared spectroscopy (AFM-IR) obtained quantitative chemical spectra of these nanostructures. We show chemical imaging of intersecting patterns of nanometer-scale polymer lines of different chemical compositions. The results indicate that for closely packed heterogeneous nanostructures

  the spatial resolution of AFM-IR is not limited by nanometer-scale thermal diffusion

  but is instead limited by the cantilever sensitivity and the signal-to-noise ratio of the AFM-IR system.

  Nanometer Scale Infrared Spectroscopy of Heterogeneous Polymer Nanostructures Fabricated by Tip-Based Nanofabrication

  We demonstrate measurement and control of nanoscale single-asperity friction by using cantilever probes featuring an in situ solid-state heater in contact with silicon oxide substrates. The heater temperature was varied between 25 and 790 °C. By using a low thermal conductivity sample

  silicon oxide

  we are able to vary tip temperatures over a broad range from 25 ± 2 to 255 ± 25 °C. In ambient atmosphere with 30% relative humidity

  the control of friction forces was achieved through the formation of a capillary bridge whose characteristics exhibit a strong dependence on temperature and sliding speed. The capillary condensation is observed to be a thermally activated process

  such that heating in ambient air caused friction to increase due to the capillary bridge nucleating and growing. Above tip temperatures of 100 ± 10 °C

  friction decreased drastically

  which we attribute to controllably evaporating water from the contact at the nanoscale. In contrast

  in a dry nitrogen atmosphere

  friction was not affected appreciably by temperature changes. In the presence of a capillary

  friction decreases at higher sliding speeds due to disruption of the capillary; otherwise

  friction increases in accordance with the predictions of a thermally assisted sliding model. In ambient atmospheres

  the rate of increase of friction with sliding speed at room temperature is sufficiently strong that the friction force changes from being smaller than the response at 76 ± 8 °C to being larger. Thus

  an appropriate change in temperature can cause friction to increase at one sliding speed

  while it decreases at another speed.

  Controlling Nanoscale Friction through the Competition between Capillary Adsorption and Thermally-Activated Sliding

  Heated Atomic Force Microscope Cantilevers and their Applications

  Jonathan

  Felts

  General Electric

  General Electric

  University of Illinois at Urbana-Champaign

  Texas A&M University

  Naval Research Laboratory

  National Research Council Fellow

  Washington D.C. Metro Area

  Naval Research Laboratory

  Assistant Professor

  Bryan/College Station

  Texas Area

  Texas A&M University

  AFM based Techniques for nanoscale manufacturing and metrology.

  University of Illinois at Urbana-Champaign

  Co-op

  Worked in Range New Product Introduction developing burn safety tests for radiant cooktops as well as redesigning electronics drop box to be safer and easier to service.

  General Electric

  Co-op

  Worked in Cleaning Products Cost Take Out administrating a transition to overseas-friendly shipping containers for Chinese manufactured dishwasher racks. Also conducted work on optimizing top-loaded washing machine suspension performance.

  General Electric

  Pi Tau Sigma

  Member

  Member

  Tau Beta Pi

  University of Illinois

  National Academy of Science National Research Council Postdoctoral Fellowship

  National Academy of Sciences

  Department of Energy Office of Science Graduate Fellow

  Department of Energy