Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) thin films on indium tin oxide and glass substrates have been fabricated and subjected to a non-adiabatic annealing process. The films showed subtle changes in their structure and optical properties as well as an increase in conductivity due to the effects of rapid thermal annealing. Through a combination of Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy studies in conjunction with electrical characterization, and four-point probe measurements, material enrichment of conductive PEDOT domains at the polymer-metal interface have been demonstrated, which well explains the surface conductivity improvement of a thin film of PEDOT:PSS after annealing.
The conjugated polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is subjected to non-adiabatic rapid thermal processing and exhibits an increase in conductivity through the film. Electrical measurements on an ITO/PEDOT:PSS/Al diode structure display a current-voltage relationship that correlates to space charge limited conduction with the presence of an exponential trap distribution, which is commonly seen in other organic media. With careful application of this current transport theory to the obtained experimental results, the root cause of the conductivity enhancement can be attributed solely to an increase in the charge mobility of carriers in the PEDOT:PSS film. In comparison to an untreated PEDOT:PSS film, processing at 200 °C for 30 s results in a 35% increase in carrier mobility to 0.0128 cm2 V−1 s−1. Values for other material characteristics of PEDOT:PSS can also be extracted from this electrical analysis, and additionally are found to be unchanged with processing. Hole concentration, effective density of states, and total trap density are found to be 7.4 × 1014 cm−3, 1.5 × 1018 cm−3, and 3.7 × 1017 cm−3, respectively.
Oligodeoxyribonucleotides (ODNs) that have four repeats of the human telomeric sequence d(TTAGGG)(n) can assume multiple monomolecular G-quadruplex topologies. These are determined by the cation species present, the bases at the 5' or 3' end, and the sample preparation technique. In this work, we report our studies of the concentration dependence of the circular dichroism (CD) and the vibrational modes probed by Raman scattering of three previously characterized monomolecular G-quadruplexes: H-Tel, d[5'-A(GGGTTA)(3)GGG-3']; hybrid-1, d[5'-AAA(GGGTTA)(3)GGGAA-3']; and hybrid-2, d[5'-TTA(GGGTTA)(3)GGGTT-3']. At high (millimolar) ODN concentrations, we observed a transformation of the CD spectrum of H-Tel, with a relaxation time on the order of 10 h. Analysis of the kinetics of this process is consistent with the formation of an aggregated complex of folded H-Tel monomers. Upon dilution, the aggregates dissociate rapidly, yielding spectra identical to those of monomeric H-Tel. Both hybrid sequences undergo a similar transition under high-salt (1 M) conditions. The measurements suggest that for these ODN concentrations, which are typically used in high-resolution spectroscopies, the monomolecular G-quadruplex structures undergo a transition to multimolecular structures at room temperature. Guided by our findings, we propose that the terminal bases of the hybrid-1 and hybrid-2 ODNs impede the formation of these aggregates; however, in solutions containing 1 M salt, the hybrid oligonucleotides aggregate.
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