Professor John Marohn

Professor John Marohn
Date
Mon October 27th 2014, 4:15pm
Location
Braun Lecture Hall
S.G. Mudd Building
Stanford University

"Nanoscale Functional Imaging of Organic Materials"

About the Seminar

Using cantilevers to detect magnetic resonance, spectroscopically probe electronic energy levels, image charge generation, and study charge diffusion at the nanoscale Imaging the structure and function of organic materials at nanometer resolution represents a major challenge.

To address this challenge, we are pushing magnetic resonance imaging (MRI) to nanometer resolution.   I will describe the development of attonewton-sensivity cantilevers with integrated nanomagnet tips.  We have used these cantilevers to detect magnetic resonance from a few hundred protons in a polymer film at cryogenic temperatures.  This sensitivity breakthrough indicates the possibility of using magnetic-tipped cantilevers to characterize thin-film devices using magnetic resonance – an exciting possibility even at cryogenic temperatures because nuclear spins are exquisite probes of atomic-scale magnetic and electric fields.  Nanoscale MRI thus offers exciting possibilities for imaging material properties such as triplet yield, current, photocurrent, and internal electric fields.

The second half of my talk will describe nanoscale studies of charges in organic semiconductors. I will show that scanning Kelvin probe microscopy (SKPM) – when equipped with variable-wavelength sample illumination – can be used to infer the electronic energy levels of charged defect states in organic thin-film transistors.  I will show movies of surface voltage versus wavelength collected over an organic bulk heterojunction solar cell; from this data we can obtain localized charge generation spectra for the different material phases present in the solar cell. Both of these studies give new microscopic insight into how to improve organic semiconductor materials.  Finally, I will show that cantilever frequency noise in an SKPM experiment is a sensitive probe of both thermal atomic motions and charge fluctuations in an organic film.  Comparisons between theory and experiment indicate that this capability can give fresh insight into the effect of inter-carrier Coulomb repulsion on charge mobility in organic semiconductors.

About the Speaker

John Marohn earned a B.S. in Chemistry and a B.A. in Physics from the University of Rochester and carried out his Ph.D. work with Daniel P. Weitekamp at the California Institute of Technology with the thesis entitled “Multiple-Pulse Radio-Frequency Gradient Nuclear Magnetic Resonance Imaging of Solids and II. Optical Nuclear Magnetic Resonance of Epitaxial Gallium Arsenide Structures.” He performed postdoctoral work as a U.S. National Research Council Postdoctoral Associate of the U.S. Army Research Laboratory. In 1999, John joined the Cornell University department of Chemistry and Chemical Biology faculty, was promoted to Associate Professor in 2005, and has received an NSF career award. He is a member of the Cornell Center for Nanoscale Systems, the Cornell Center for Materials Research, and the Kavli Institute at Cornell for Nanoscale Science. He serves on the Executive Committees of the Cornell Center for Materials Research and the Cornell NanoScale Science and Technology Facility.

John Marohn’s research consists of using scanned probe microscopy to understand the chemical behaviour of systems on the nanoscale. His lab uses custom-built scanned probe microscopes to record high-resolution maps of electrostatic potential, capacitance, photo-generation of charge, and electric field fluctuations in working organic electronic devices for applications including organic photovoltaics. The second thrust of his group’s research is developing approaches for imaging single molecules such as, for example, an individual membrane protein. Here the approach is to push magnetic resonance imaging to nanometer resolution using tiny silicon cantilevers as mechanical detectors of magnetic resonance.