Research

Charge Motion at the Nanoscale

We are interested in better understanding the structure and function of materials at the nanometer scale. We are particularly interested in soft, thin-film materials, which remain a challenge to image. Recent advances in electron microscopy have led to near-routine imaging of individual heavy atoms in cross-sectional slices of hard materials. The resulting images have transformed our understanding of oxide interfaces, unconventional superconductors, and nanomagnets. Radiation damage makes it impossible to visualize individual light atoms this way in soft materials, however, frustrating the study of soft materials ranging from organic solar cells and batteries to biomolecular complexes and membrane proteins.

Organic semiconductors are an especially interesting class of soft material. In these materials it remains poorly understood how free charge is generated from light and moves across interfaces. To gain a better microscopic understanding of these processes, we develop new scanned probe microscopes capable of imaging at nanometer resolution the transient voltages, capacitances, and electric field fluctuations present near the surface of a thin-film material while it is under electrical bias and exposed to light. Subprojects include studies of

• Interfacial charge transport via high-sensitivity measurements of local electric fields; charge trapping in n-channel organic semiconductors
• Charge transport and fluctuations in organic semiconductors; charge generation in organic solar cell materials
• Development of substrates with tunable work function via electrochemical synthesis
• Localized measurements of the microsecond and nanosecond dynamics of photo-generated charge in semiconductors
• Localized measurements charge conduction in confined-but-connected films of nanocrystalline semiconductors

Representative publications

• Lekkala, S.; Marohn, J. A. & Loring, R. F.“Electric force microscopy of semiconductors: Cantilever frequency fluctuations and noncontact friction”, J. Chem. Phys., 2013, 139, 184702 [10.1063/1.4828862].
• Smieska, L. M.; Pozdin, V. A.; Luria, J. L.; Hennig, R. G.; Hines, M. A.; Lewis, C. A. & Marohn, J. A.“Following Chemical Charge Trapping in Pentacene Thin Films by Selective Impurity Doping and Wavelength-resolved Electric Force Microscopy”, Adv. Funct. Mater., 2012, 22, 5096 - 5106 [10.1002/adfm.201200595].
• Luria, J. L.; Hoepker, N.; Bruce, R.; Jacobs, A. R.; Groves, C. & Marohn, J. A.“Spectroscopic Imaging of Photopotentials and Photoinduced Potential Fluctuations in a Bulk Heterojunction Solar Cell Film”, ACS Nano, 2012, 6, 9392 - 9401 [10.1021/nn300941f].
• Lekkala, S.; Hoepker, N.; Marohn, J. A. & Loring, R. F.“Charge carrier dynamics and interactions in electric force microscopy”, J. Chem. Phys., 2012, 137, 124701 [10.1063/1.4754602].
• O’Dea, J. R.; Brown, L. M.; Hoepker, N.; Marohn, J. A. & Sadewasser, S. “Scanned Probe Microscopy of Solar Cells: From Inorganic Thin Films to Organic Photovoltaics”, Mater. Res. Soc. Bulletin, 2012, 37, 642 - 650 [10.1557/mrs.2012.143].
• Choi, J. J.; Luria, J.; Hyun, B.-R.; Bartnik, A. C.; Sun, L.; Lim, Y.-F.; Marohn, J. A.; Wise, F. W. & Hanrath, T. “Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies”, Nano Lett., 2010, 10, 1805 - 1811 [10.1021/nl100498e].

Funding

• “Scanned-probe characterization of degradation and charge generation in organic semiconductors” (J. A. Marohn; Cornell University; National Science Foundation, grant number DMR-1006633; 08/01/13 – 07/31/16).
• “Graduate student fellowship” (S. R. Nathan and J. A. Marohn; Cornell University; National Science Foundation; 09/01/13 – 08/31/16).
• “NSF GK12: Grass roots graduate fellowship” (R. Dwyer, J. A. Marohn, and P. Clancy; Cornell University; National Science Foundation; 06/01/13 – 05/31/14).

Nano-MRI

In a complimentary project, we are working to apply magnetic resonance imaging (MRI) — the premier tool for characterizing soft materials at the millimeter scale — to study the structure and function of devices, materials, and molecular complexes at the nanometer scale. Our approach is to use silicon cantilevers and a custom microscope to detect magnetic resonance at unprecedented sensitivity and carry out nanoscale magnetic resonance imaging (nano-MRI) experiments.

Our Cornell team was the first to demonstrate scanned-probe detection of nuclear magnetic resonance, using a new physical effect in which the sample spins change the frequency of a magnet-tipped cantilever. This innovation opens up the technique of magnetic resonance force microscopy to a much wider array of samples than was previously possible, including paramagnetic spin labels such as nitroxides widely used to study proteins and nucleic acids. The overall goal of this project is to develop a way to image individual molecules and thin-film devices at nanometer or sub-nanometer resolution. Subprojects include studies of

• Dynamic nuclear polarization in magnetic resonance force microscopy; microscope development; microwave waveguide simulation and testing
• Measurements and theory of non-contact friction; cantilever and nanomanget fabrication; microwave waveguide fabrication; magnetization fluctuations in individual nanomagnets
• Image encoding and reconstruction in nanoscale magnetic resonance imaging
• Studies of thin-film semiconductor devices by magnetic resonance force microscopy

Representative Publications

• Chen, L.; Longenecker, J. G.; Moore, E. W. & Marohn, J. A.“Long-Lived Frequency Shifts Observed in a Magnetic Resonance Force Microscope Experiment Following Microwave Irradiation of a Nitroxide Spin Probe”, Appl. Phys. Lett., 2013, 102, 132404 [10.1063/1.4795018][PMCID:PMC3631243].
• Longenecker, J. G.; Mamin, H. J.; Senko, A. W.; Chen, L.; Rettner, C. T.; Rugar, D. & Marohn, J. A.“High-Gradient Nanomagnets on Cantilevers for Sensitive Detection of Nuclear Magnetic Resonance”, ACS Nano, 2012, 6, 9637 - 9645 [10.1021/nn3030628][PMCID:PMC3535834].
• Moore, E. W.; Lee, S.-G.; Hickman, S. A.; Harrell, L. E. & Marohn, J. A.“Evading Surface and Detector Frequency Noise in Harmonic Oscillator Measurements of Force Gradients”, Appl. Phys. Lett., 2010, 97, 044105 [10.1063/1.3465906][PMCID:PMC2924902].
• Moore, E. W.; Lee, S.-G.; Hickman, S. A.; Wright, S. J.; Harrell, L. E.; Borbat, P. P.; Freed, J. H. & Marohn, J. A.“Scanned-Probe Detection of Electron Spin Resonance from a Nitroxide Spin Probe”, Proc. Natl. Acad. Sci. U.S.A., 2009, 106, 22251 - 22256 [10.1073/pnas.0908120106][PMCID:PMC2799694].
• Garner, S. R.; Kuehn, S.; Dawlaty, J. M.; Jenkins, N. E. & Marohn, J. A.“Force-Gradient Detected Nuclear Magnetic Resonance”, Appl. Phys. Lett., 2004, 84, 5091 - 5093 [10.1063/1.1762700].

Funding

• “Microwave-enhanced nanoscale magnetic resonance imaging of individual biomacromolecules” (E. Afshari and J. A. Marohn; Cornell University; Army Research Office, grant number W911NF-14-1-0674; 10/1/14 – 9/30/17).
• “NSF GK12: Grass roots graduate fellowship” (C. Isaac, J. A. Marohn, and P. Clancy; Cornell University; National Science Foundation; 06/01/14 – 05/31/15).
• “Nanoscale magnetic resonance imaging and characterization of organic electronic materials” (J. A. Marohn; Cornell University; Army Research Office, Division of Materials Research, grant number W911NF-12-1-0221; 06/01/2012 - 03/31/2015).
• “Cantilever magnetic resonance of biomolecules” (J. A. Marohn; Cornell University; National Institutes of Health, National Institute of General Medical Sciences, grant number R01GM070012-07; 06/01/09 – 05/31/14).

Fuel Cell Materials

A final project involves devising a recipe for making transition metal nitrides that both conduct electricity and are stable to degradation and dissolution under the extreme environmental conditions present in a fuel cell. This project is a collaboration between five Cornell groups in three different departments. The Marohn group’s role in this project is to use conducting-probe atomic-force microscopy to measure the transport of charge through a few-nanometer oxide layer at the surface of the thin-film transition metal oxy-nitride samples.

Representative Publications

• O’Dea, J. R.; Holtz, M. E.; Legard, A. E.; Young, S. D.; Burns, R. G.; Van Wassen, A. R.; Muller, D. A.; Abruña, H. D.; DiSalvo, F. J.; van Dover, R. B. & Marohn, J. A.“Conductivity and Microstructure of Combinatorially Sputter-Deposited Nitride Thin Films”, Chem. of Mater. (in press), 2015 [10.1021/cm504599s].
• Van Wassen, A. R.; Legard, A. E.; O’Dea, J. R.; Young, S. D.; Burns, R. G.; DiSalvo, F. J.; Marohn, J. A.; van Dover, R. B. & Abruña, H. D.“Electrochemical Characterization of Ta-Ti-Al Nitride Thin Films”, submitted 07/01/15, 2015.

Funding

• “Energy Frontier Research Center: Nanostructured interfaces for energy generation, conversion and storage” (H. D. Abruña, T. A. Arias, L. A. Archer, J. D. Brock, F. J. DiSalvo, T. Hanrath, J. A. Marohn, and D. A. Muller; Cornell University; Department of Energy, Division of Basic Energy Sciences, grant number DE-SC0001086; 08/01/09 – 07/31/14).