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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. Daniel A. Higgins, Jeffrey E. Hall and Aifang Xie, "Optical Microscopy Studies of Dynamics within Individual Polymer-Dispersed Liquid Crystal Droplets", Acc. Chem. Res., 2005, 38, 137.
2. Daniel A. Higgins and Maryanne M. Collinson, "Gaining Insight into the Nanoscale Properties of Sol-Gel-Derived Silciate Thin Films by Single Molecule Spectroscopy", Langmuir, 2005, 21, 9023.
3. Aifang Xie, Bei Liu, Jeffrey E. Hall, Sarah L. Barron and Daniel A. Higgins, "Self-Assembled Photoactive Polyelectrolyte/Perylene-Diimide Composites", Langmuir, 2005, 21, 4149.
4. Skylar A. Martin-Brown, Yi Fu, Ginagunta Saroja, Maryanne M. Collinson and Daniel A. Higgins, "Single Molecule Studies of Diffusion by Oligomer-Bound Dyes in Organically-Modified Sol-Gel-Derived Silicate Films", Anal. Chem., 2005, 77, 486.
5. Jeffrey E. Hall and Daniel A. Higgins, "Enhanced Photorefractivity from Ion-Doped Polymer-Dispersed Liquid Crystals", J. Phys. Chem. B, 2004, 108, 16050.
6. Yi Fu, Maryanne M. Collinson and Daniel A. Higgins, "Single Molecule Spectroscopy Studies of Microenvironmental Acidity in Silicate Thin Films", J. Am. Chem. Soc., 2004, 126, 13838.
7. Aifang Xie and Daniel A. Higgins, "Electric-Field-Induced Dynamics in Radial Liquid Crystal Droplets Studied by Multiphoton-Excited Fluorescence Microscopy: Interfacial Effects and Metastable Configurations", Appl. Phys. Lett., 2004, 84, 4014.
8. Jeffrey E. Hall and Daniel A. Higgins, "Exploring the Photorefractive Effect in Polymer-Dispersed Liquid Crystals using Near-Field Scanning Optical Microscopy", J. Phys. Chem. B, 2003, 107, 14211.
9. Daniel A. Higgins and Brenda J. Luther, "Watching Molecules Reorient in Liquid Crystal Droplets with Multiphoton-Excited Fluorescence Microscopy", J. Chem. Phys., 2003, 119, 3935.
10. Yanwen Hou and Daniel A. Higgins, “Single Molecule Studies of Dynamics in Polymer Films and at Surfaces: Effect of Ambient Relative Humidity", J. Phys. Chem. B, 2002, 106, 10306.
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