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Dr. Daniel A. Higgins



B.A., St. Olaf College (1988)
Ph.D., University of Wisconsin-Madison (1993)
Postdoctoral Research Fellow, University of Minnesota (1993-1996)

Email: higgins@ksu.edu
Office Phone: 785-532-6371
Lab Phone: 785-532-6079
Fax: 785-532-6666

Higgins Group

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Research Overview

Research in the Higgins laboratory involves the implementation of novel optical microscopic techniques for characterization of mesostructured thin film materials. The techniques used include near-field scanning optical microscopy (NSOM), single molecule spectroscopy (SMS), and multiphoton-excited fluorescence microscopy (MPEFM). We are presently using these methods to characterize semiconducting organic thin films and sol-gel derived silicate thin films. We have also devoted significant effort to the understanding of polymer-disersed liquid crystals and related photorefractive thin films. The main goal in all our research projects is to obtain a better understanding of the micron-to-nanometer-scale properties of these materials.

Near-Field Scanning Optical Microscopy

NSOM provides high resolution optical and topographic images of our samples, while also allowing for sample characterization by spectroscopic means. Developed primarily in the 1980's and '90's (although its origins were much earlier), NSOM is a form of scanned-probe microscopy that uses visible light to produce optical images with 50-100 nm resolution. A tapered, metal-coated optical fiber is used as the NSOM probe. The end diameter of the probe is typically less than 100 nm, forming a similarly sized light source. The size of the probe, and the ability to hold the sample within its "near field" with standard scanned-probe techniques allows for high-resolution images to be obtained. Conventional optics and detectors collect and measure the intensity, polarization, and wavelength (in luminescence imaging) of the light from the near field. We have recently helped develope a new NSOM method in which we use the metallized NSOM probe as an electrode. In this mode, we apply concentrated electric fields to local sample regions for studies of field-induced dynamics (i.e. molecular reorientation, charge generation, etc.). NSOM methods are used to detect the field-induced processes, allowing for spatial variations in the local rates of dynamical processes to be mapped.

Single Molecule Spectroscopy

While NSOM provides resolution of about 100 nm, chemical and/or physical variations in materials properties also occur on molecular length scales. We use SMS to selectively probe thin film properties on this scale. Dyes are selected or synthesized for use as chemical probes of specific phenomena and doped into the films at low concentration. The dye Nile Red is known to be highly sensitive to the "polarity" and "rigidity" of the environment in which it resides and is used extensively. More recently, we have begun to study diffusion (mass transport) phenomena in thin films, at the single molecule level. We have also used pH-sensitive dyes for characterizing the pH of local environments using single molecule methods in sol-gel derived silicates. The experiments performed involve confocal imaging of samples with well-dispersed single molecules, these molecules are then individually selected for spectroscopic interrogation. Both single molecule fluorescence spectra and the time-dependence of the spectrally-integrated fluorescence are used to better understand local environmental properties. These studies focus primarily on the characterization of silicate glass films through a collaborative effort with Prof. Maryanne Collinson's group.

Multiphoton-Excited Fluorescence Microscopy

Nonlinear optical microscopies provide additional high-resolution information. We used femtosecond pulses of light from a Ti:sapphire laser to induce multiphoton transitions in our samples. Each chromophore absorbs two or more photons from a single laser pulse. The sample then reemits a fluorescence photon at a wavelength shorter than that of the incident light. Because multiphoton absorption has a nonlinear dependence on the incident intensity, the sample volume excited is inherently confined in three dimensions. These features allow for background-free fluorescence images to be obtained. In addition, time-resolved measurements of the local fluorescence lifetime and the rates of local electric-field-induced phenomena can be measured. Most importantly, our multiphoton imaging methods allow us to probe sample properties deep within optically thick films, such as the polymer dispersed liquid crystals studied previously in our group. More recently, we have been using this powerful method to probe deep within p-n heterojunction photovoltaic devices prepared from organic semiconducting materials.

Selected Publications

1. Fangmao Ye, Maryanne M. Collinson and Daniel A. Higgins "What Can Be Learned from Single Molecule Spectroscopy? Applications to Sol-Gel-Derived Silica Materials", Phys. Chem. Chem. Phys., 2009, 11, 66.

2. Fangmao Ye, Chenchen Cui, Alec Kirkeminde, Dong Dong, Maryanne M. Collinson and Daniel A. Higgins "Fluorescence Spectroscopy Studies of Silica Film Polarity Gradients Prepared by Infusion-Withdrawal Dip-Coating", Chem. Mater., 2010, 22, 2970.

3. Jeffrey J. Lange, Maryanne M. Collinson, Christopher T. Culbertson and Daniel A. Higgins "Single Molecule Studies of Oligomer Extraction and Uptake of Dyes in Poly(dimethylsiloxane) Films", Anal. Chem., 2009, 81, 10089.

4. Thomas A. Everett and Daniel A. Higgins "Electrostatic Self-Assembly of Ordered Perylene-Diimide/Polyelectrolyte Nanofibers in Fluidic Devices: from Nematic Domains to Macroscopic Alignment", Langmuir, 2009, 25, 13045.

5. Corey R. Weitzel, Thomas A. Everett and Daniel A. Higgins "Aggregation and its Influence on Macroscopic In-Plane Organization in Thin Films of Electrostatically Self-Assembled Perylene-Diimide/Polyelectrolyte Nanofibers", Langmuir, 2009, 25, 1188.

6. Xiao Yao, Takashi Ito and Daniel A. Higgins "Grayscale Patterning of Polymer Thin Films with Nanometer Precision by Direct-Write Multiphoton Photolithography", Langmuir, 2008, 24, 8939-8943.

7. Fangmao Ye, Maryanne M. Collinson and Daniel A. Higgins "Molecular Orientation and its Influence on Autocorrelation Amplitudes in Single Molecule Imaging Experiments", Anal. Chem., 2007, 79, 6465.

8. Fangmao Ye, Daniel A. Higgins and Maryanne M. Collinson "Probing Chemical Interactions at the Single Molecule Level in Mesoporous Silica Thin Films ", J. Phys. Chem. C, 2007, 111, 6772.

9. Thomas A. Everett, Amy A. Twite, Aifang Xie, Srinivas K. Battina, Duy H. Hua and Daniel A. Higgins "Preparation and Characterization of Nanofibrous Perylene-Diimide-Polyelectrolyte Composite Thin Films ", Chem. Mater., 2006, 18, 5937.

10. Aifang Xie, Takashi Ito and Daniel A. Higgins "Fabrication and Characterization of Polymer/Liquid-Crystal Composite Diffractive Optics by Multiphoton Methods", Adv. Funct. Mater., 2007, 17, 1515.