Light scattering and organic electronics research groupResearch
CURRENT RESEARCH PROJECTS - ORGANIC AND HYBRID ELECTRONICS
We acknowledge the National Science Foundation (Grant nos. ECCS-1707588, 1305642, 0823563, 0523656, IIA-1339011, DMR-0413601), Research Corporation, ACS-Petroleum Research Fund, Dept. of Army, and the University of Missouri Research Board for funding our research.
A major focus of our research in the past few years has been on improving the performance of organic field-effect transistors (FETs) by controlling the polymer-dielectric interface and understanding the mechanism of carrier transport. We have used Matrix-assisted pulsed laser evaporation (MAPLE), a derivative of pulsed laser deposition, as an alternative method of depositing polymer and biomaterial films allowing homogenous film coverage of high molecular weight organic materials for a layer-by-layer growth without any laser induced damage. The MAPLE technique is a viable alternative for fabricating device quality organic metal-insulator-semiconductor (MIS) diodes and FETs, and overcoming the solvent selectivity, inherent swelling, and dissolution problems associated with polymer dielectrics. See Refs.
S. Guha, N. B. Ukah, D. Adil, R. K. Gupta, and K. Ghosh, Applied Physics A 105, 547 (2011).
Our current work focuses on understanding charge transport mechanisms in FETs using polymer ferroelectric dielectrics. Ferroelectric dielectrics, where the dielectric constant can be tuned by an order of magnitude with temperature, are ideal for extracting information on polaron hopping lengths and barrier heights. By choosing small-molecule organic semiconductors with distinct differences in their bulk transport properties, the connection between the nature of traps and transport in ferroelectric based FETs is elucidated. A weak temperature dependence of the FET charge carrier mobility in the ferroelectric phase of the polymer ferroelectric is attributed to polarization fluctuation driven transport. The temperature-dependent mobility shows a negative coefficient when the semiconductor shows discrete-trap space-charge-limited conduction.
Carrier mobilities of organic semiconductors in FETs are strongly impacted by device geometry, physical/chemical attributes of the organic semiconductor, and the various interfaces: metal-semiconductor and semiconductor-insulator. We are currently developing transient electric field-induced second-harmonic generation methods, based on the third-order susceptibility for probing carrier motion in organic FETs. This technique will be a powerful methodology for visualizing transport and pave the way for predicting accurate carrier mobilities, free from contact resistance issues and device geometrical factors.
An additional facet includes vibrational spectroscopy from organic FETs and light-emitting diodes under applied electric fields. These studies elucidate the role of conformational changes in the molecules/polymers along with insights into the role of polarons/bipolarons in charge-transport. We have developed a novel technique of generating Raman maps across the polymer-Au interface in FETs, providing a powerful visualization tool for correlating the device performance under bias stress to the structural changes of the molecule/polymer.
Hybrid interfaces using nanopatterned ZnO and donor-acceptor conjugated polymers are being developed for near-IR photodetectors and solar cells. These architectures pave the way for non-fullerene based hybrid photodetectors. Diketopyrrolopyrrole (DPP) copolymers with peak absorption above 700 nm range with distinct differences in their backbone conformation have been utilized in hybrid photodetectors. To improve organic solar cell efficiencies and to understand the mechanism of the photovoltaic process, we probe the role of electronic states such as charge-transfer complexes in solar cells and photovoltaics using modulation spectroscopic and photocurrent techniques.
The performance of molecular electronics as biomimetic devices is mainly determined by the supramolecular organization of functionalized nanoscale biocompatible materials. The nanostructures obtained from biomolecules are attractive due to their biocompatibility, ability for molecular recognition, and ease of chemical modification. Peptide-based nanostructures (PNS) derived from natural amino acids are superior building blocks for biocompatible devices as they can be used in a bottom-up process without any need for expensive lithography. Based on self-assembly and mimicking the strategies occurring in nature, peptide materials play a unique role in a new generation of hybrid materials.
The self-assembly process in FF (L,L-phenylalanine), a short dipeptide made from the covalent link of two aromatic phenylalanine units, begins at the molecular level with the association of six FF monomers self-associating into macrocycle structures where ammonium and carboxylate groups constitute the inner core of the cycle. The macrocycles stack into a columnar phase forming narrow tubes with diameter ~1 nm. These channels self-associate to make a hexagonal packing pattern and give rise to pleated sheets. The formation of micro/nanotubes occurs by the closure of the sheets along the axes of the channels, leading to tubular arrays at the nanoscale. The FF-based peptide nanostructures possess a P61 non-centrosymmetric space group. This odd-tensor space group is the basis for the existence of several physical properties such as ferroelectricity and piezoelectricity, and thus serves as a potential platform for nonlinear optics. We have recently shown strong second harmonic generation (SHG) properties from individual FF nano/microtubes and correlate the non-linear optical properties to the tube dimensions.
Charge modulated FETs for the development of biosensors
S. Khanra, K. Ghosh, F. F. Ferreira, W. A. Alves, F. Punzo. P. Yu, and S. Guha, Phys. Chem. Chem. Phys. 19, 3084 (2017).
Application of hydrostatic pressure increases the intermolecular interactions and changes the molecular geometry without a change in the chemical make-up of compounds. At the simplest level higher intermolecular interactions in conjugated polymers result in a decrease of the luminescence quantum yield, which is clearly seen as one goes from a solution state to the solid-state. A common feature seen in all conjugated polymers is a red-shift of the photoluminescence (PL) spectrum with increasing pressure (shown in the figure below), indicative of an increasing degree of conjugation.
More recently we have focused our efforts on the structure-property relationship in polyfluorenes (PF), a class of blue-emitting polymers. Di-octyl substituted PF (PF8), in particular, has different backbone conformations, which may be induced by thermal/solvent treatment. The as-is polymer has some fraction of the planar Cbeta conformation (the backbone torsional angle ~180°) ; hence, the PL is always dominated by this low-energy phase. Thermal annealing of PF8 eliminates the low-energy Cbeta conformation making the polymer more non-planar (Calpha /Cgamma ). The figure below shows the position of the 0-1 PL peak in as-is and thermally annealed PF8 samples. The inset shows the calculated energy gap of a fluorene dimer as a function of pressure for three different backbone conformations. Since the annealed sample is mainly in the Calpha and Cgamma conformation, pressure produces two effects: increased effective conjugation similar to the as-is sample and planarization of the backbone driving it toward a Cbeta conformation.
Our recent efforts entail combining high pressure optical studies of conjugated polymers with x-ray diffraction under high pressure.
See our latest review paper:
M. Knaapila and S. Guha, Rep. Prog. Phys. 79, 066601 (2016).