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Assistant Professor in Chemistry
Postdoctoral Fellow in Electrical Engineering & Computer Science at Massachusetts Institute of Technology; Ph.D. 2011, Massachusetts Institute of Technology; B.S. 2005, University of Washington
Principal Research Interests:
Trisha recently won a 3M Nontenured Faculty award and was named on the "Forbes 30 under 30 in energy" list by Forbes magazine (link).
We have ongoing research endeavors in: (1) creating solar textiles and other monolithically-integrated wearable technologies using vapor phase organic chemistry; (2) effecting subdiffraction optical lithography using organic photochromes; and (3) synthesizing organic analogs of diluted magnetic semiconductors and characterizing spin-dependent charge and exciton transport in these materials.
(1) Monolithically-Integrated Wearable Electronics
Our lab builds textile electronics using a vapor deposition process called oxidative chemical vapor deposition (oCVD), which allows monolithic integration of electronic circuits onto flexible textile substrates. oCVD enables the fabrication of chemically well-defined thin films of selected conjugated polymers, such as p-doped PEDOT and proDOT, neutral poly(thiophene)s, and conductive polyradicals onto arbitrary substrates with micro- and nano-scale features. In oCVD, films are formed directly on the substrate of interest as vapors of an oxidant and precursor (or monomer) are introduced into an evacuated reactor chamber simultaneously, whereupon a polymer is formed in situ that subsequently coats arbitrary substrates placed in the polymerization region. This method allows for conformal coating of rough surfaces, with features resolvable down to 100-200 nm. The modularity of this technique ensures that careful monomer choice will lead to the in situ film growth of a host of conductive polymers displaying varied optoelectronic properties.
We plan to expand our repertoire of vapor-phase organic chemical reactions and deposit various films of conjugated polymers and conductive polyradicals onto textile substrates. Vapor phase chemical deposition of both metallic and semiconducting conjugated polymer and polyradical films will be explored. Proposed elaborated textile devices include: (1) metal-free conductive textiles that can serve as circuit elements in wearable technologies; (2) solar harvesting fabrics containing a monolithically-integrated photodiode array on the textile substrate; and (3) polyradical coated textiles for charge storage and, potentially, thermoelectric devices.
(2) Subdiffraction Optical Nanopatterning Using Photoswitchable Molecules
Currently nanomanufacturing is achieved via fast pattern-replication (nanoimprint lithography, optical-projection lithography, etc.) and is stymied by extremely slow pattern generation (scanning-electron-beam lithography). In other words, the time it takes to generate a new pattern is orders of magnitude longer than what it takes to simply replicate one. Light offers significant advantages over charged particles for pattern generation. However, it suffers from one Achilles heel - diffraction. In the far field, the smallest focused spot that can be generated with light is limited to approximately half the wavelength. This, so called far-field diffraction limit or the Abbé limit, effectively prevents the use of long-wavelength photons (>300nm) from patterning nanostructures (<100nm).
We have shown that optics, when combined with novel photochemistry, can result in deep sub-wavelength patterning with speeds that are far higher than with conventional approaches. Our idea is to record the nanoscale pattern in an ultra-thin or monolayer film comprised of photochromic molecules. These molecules undergo photoswitching between two isomeric forms, A and B. When isomer A absorbs a photon of wavelength l1, it turns into isomer B. When B absorbs photon of wavelength l2, it turns back to A. The binary nature of the switching process ensures that sub-diffraction-limited regions of B interspersed in A are formed. In addition, we design the molecules such that B can be selectively converted in an irreversible manner to form C via a “locking” step, which allows us to create 3D patterns. These advances in pattern generation, when combined with continuous replication technologies such as roll-to-roll nanoimprint lithography can enable a new paradigm in high throughput top-down nanomanufacturing.
(3) Organic Diluted Magnetic Semiconductors
Organic magnets, defined as primarily carbon-containing molecules, contain magnetic moments originating from the p molecular orbitals of each particular molecule. There are possibilities in organic magnets that are absent in the inorganic systems, such as flexibility, transparency, thin-film-forming ability, and low density, which allow unmatched control over processing conditions and make possible nontraditional device architectures on arbitrary substrates. Furthermore, the bulk optical, electrical and magnetic properties of organic materials display a remarkable sensitivity to molecular structure and intermolecular organization, allowing an informed researcher to rationally tune desired properties with amazing precision using chemical knowledge. In concert, all of these properties allow one to envision that electronic and spintronic devices on inexpensive and arbitrary substrates can, ultimately, be fabricated with high throughput using high-spin organic materials, leading to a suite of revolutionary nanostructured technologies that take advantage of both the charge and spin of an electron.
We are currently working to chemically-control and understand the magnetic properties of organic crystals, to integrate high-spin organic materials with inorganic semiconductors, and to fabricate transistors and spin valves using organic magnets.
First, we are working on generating a small library of novel organic diluted magnetic semiconductors. We prepared a new class of poly(thiophene)-based high-spin organic semiconductors and a new small-molecule bridging/linker moiety, tetrasubstituted indenofluorene, that demonstrate long-range spin polarization (as characterized by EPR) and maintain a rigidly-defined close packing of molecules, respectively. We are currently characterizing the emerging physical properties arising from spin-polaron and spin-exciton interactions in these materials.
Our major goal for the immediate future is to fabricate magnetic tunnel junction devices with our radical-containing molecules and to characterize magnetic field-dependent charge transport in these systems. Ongoing work involves investigation of various deposition techniques (physical vapor deposition, chemical vapor deposition, spincasting and self-assembled monolayers) to yield ordered thin films of organic radicals. We are also exploring various spectroscopic techniques to characterize the spin coherence lengths of our radical films.