Educational Background:

PhD, California Institute of Technology, 1996 BS, University of Rochester, 1989 BA, University of Rochester, 1989

Awards:

• NSF Career Award 2002 • National Research Council / U. S. Army Research Laboratory Postdoctoral Fellow • W. R. Grace and Company Graduate Fellowship

Research Description:

Our research involves investigations of mesoscale and nanoscale materials by novel scanned-probe microscopies.

The study and manipulation of matter at the mesoscale and nanoscale continues to be hampered by a lack of informative nondestructive probes of matter at these scales. Of the chemists' "workhorse" spectroscopies - magnetic resonance, infrared spectroscopy, and mass spectroscopy among them - only mass spectroscopy routinely approaches single-atom sensitivity, but at the expense of destroying the sample. Although scanned-probe microscopies have stepped in to provide non-destructive maps of surface quantities such as conductivity and atomic forces at the nanoscale - even collecting single molecule optical spectra over mesoscale volumes is now possible - the level of atomic-scale chemical information provided by these techniques is low compared to bulk studies by the workhorse spectroscopies.

Our research projects capitalize on recent breakthroughs in scanned-probe microscopy technology to formulate new techniques more capable of answering both fundamental and applied questions in mesoscale and nanoscale materials. In one such breakthrough, it has been shown recently that spin magnetization can be measured as a force between a sample and the magnetic field gradient produced by a micron-sized magnetic particle. In magnetic resonance force microscopy (MRFM), either the sample or the magnetic particle is attached to a microcantilever, and magnetic resonance is recorded as a force by measuring cantilever deflection. MRFM is noteworthy as a unique marriage of a ubiquitous "workhorse" bulk spectroscopic tool (magnetic resonance) with an ultrasensitive scanned-probe technology. And because magnetic forces are long range, MRFM is a type of scanned probe microscopy that can probe below a surface (to study a solid-solid interface, for example).

A host of technical innovations is advancing MRFM beyond the current state-of-the-art micron-scale resolution towards the single-spin limit (whether this limit is attainable is another research area). One such innovation, involving fabrication of "ultra-floppy" microcantilevers, can be adapted for electric-force microscopy (EFM). Electric forces are also long-range, and can in principle probe below a surface; another project seeks to dramatically improve the sensitivity of EFM in imaging charges near a surface.

We use these tools study advanced thin-film electronic and magnetic materials, probing both charge (with electric forces) and spin (with magnetic forces). One especially interesting class of materials is organic conductors and semiconductors; demonstration of field-effect transistors and light-emitting diodes built from easily-processable films of organic semiconductors has garnered much recent interest. While examples of conducting and semiconducting organic compounds (both polymers and molecular crystals) have long been known, advances in synthesis and fabrication have only recently yielded solution-processable materials with mobility and/or luminescence efficiency high enough for consideration as device components. We expect that our novel scanned probe microscopies can answer both fundamental and applied questions in these materials.

This work will be accomplished on modified commercial and homebuilt instrumentation and will be flavored with much thinking from first principles. We expect to bring to bear techniques and tools from spectroscopy and quantum mechanics, from chemical physics, chemistry, materials science, and from a host of cutting-edge technologies (including, for example, silicon "MEMS" fabrication, fiber optics, cryogenics, vacuum science, analog and digital rf electronics, and computer interfacing and experiment design).

Selected Publications:

Ng, T.N.; Silveira, W.R., Marohn, J.A. The Dependence of Charge Injection on Temperature, Electric Field, and Energetic Disorder in an Organic Semiconductor. Physical Review Letters, 2007, 98, 066101.

Kuehn, S.; Loring, R.F., Marohn, J.A. Dielectric Fluctuations and the Origins of Non-Contact Friction. Physical Review Letters, 2006, 96, 156103.

Muller, E.M. and Marohn, J.A. Microscopic evidence for spatially inhomogeneous charge trapping in pentacene. Advanced Materials, 2005, 17 (11), p. 1410-1414.

Garner, S.R., Kuehn, S.; Dawlaty, J.M.; Jenkins, N.E.; Marohn, J.A. Force-gradient detected nuclear magnetic resonance. Applied Physics Letters, 2004, 84, 5091.

Silveira, W.R., Marohn, J.A. Microscopic View of Charge Injection in an Organic Semiconductor. Physical Review Letters, 2004, 93 (11), 116104.

A full listing of publications can be found here.