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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. Dipak Giri, Chelsea N. Hanks, Maryanne M. Collinson and Daniel A. Higgins "Single Molecule Spectroscopic Imaging Studies of Polarity Gradients Prepared by Infusion-Withdrawal Dip-Coating", J. Phys. Chem. C, 2014, 118, 6423.

2. Daniel A. Higgins, Khanh-Hoa Tran-Ba and Takashi Ito "Following Single Molecules to a Better Understanding of Self-Assembled One-Dimensional Nanostructures", J. Phys. Chem. Lett., 2013, 4, 3095.

3. Rajib Pramanik, Takashi Ito and Daniel A. Higgins "Molecular Length Dependence of Single Molecule Wobbling within Surfactant and Solvent Filled Silica Mesopores", J. Phys. Chem. C, 2013, 117, 15438.

4. Qin Li, Chenchen Cui, Daniel A. Higgins and Jun Li "Fluorescence Quenching Studies of Potential-Dependent DNA Reorientation Dynamics at Glassy Carbon Electrode Surfaces", J. Am. Chem. Soc., 2012, 134, 14467.

5. Khanh-Hoa Tran-Ba, Jason J. Finley, Daniel A. Higgins and Takashi Ito "Single Molecule Tracking Studies of Millimeter-Scale Cylindrical Domain Alignment in Polystyrene-Poly(ethylene oxide) Diblock Copolymer Films Induced by Solvent Vapor Penentration",J. Phys. Chem. Lett., 2012, 3, 1968.

6. Alec W. Kirkeminde, Travis Torres, Takashi Ito and Daniel A. Higgins "Multiple Diffusion Pathways in Pluronic F127 Mesophases Revealed by Single Molecule Tracking and Fluorescence Correlation Spectroscopy", J. Phys. Chem. B, 2011, 115, 12736.

7. Chenchen Cui, Alec Kirkeminde, Balamurali Kannan, Maryanne M. Collinson and Daniel A. Higgins "Spatiotemporal Evolution of Fixed and Mobile Dopant Populations in Silica Thin-Film Gradients as Revealed by Single Molecule Tracking", J. Phys. Chem. C, 2011, 115, 728.

8. Khanh Hoa Tran Ba, Thomas A. Everett, Takashi Ito and Daniel A. Higgins "Trajectory Angle Determination in One Dimensional Single Molecule Tracking Data by Orthogonal Regression Analysis ", Phys. Chem. Chem. Phys., 2011, 13, 1827.

9. 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.

10. 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.