Timothy T. Gorman

Time-of-flight studies of hole mobilities in DNA-CTMA films fabricated and passivated in a dry environment

Abstract. Salmon DNA-based films, including as-received DNA (molecular weight, MW>2000  kDa) and sonicated DNA of MW ∼200  kDa, both complexed with hexacetyltrimethyl-ammonium chloride (CTMA) surfactant, were studied. The DNA solutions were spin-coated on indium tin oxide (ITO)-coated quartz slides with vacuum deposited gold charge-collecting electrodes. The films were fabricated entirely at ∼0% humidity (0 to 86 ppmv of water) in a nitrogen-purged glove box and coated with 500 to 600 nm passivating layers of conformal urethane before exposure to room air. A quadrupled, 20 ns pulsed Nd:YAG laser with output at 266 nm was used for charge injection. The room temperature photoconductive transients were dispersive with hole mobilities in DNA films ranging between 7E-3 to 5E-5  cm2/Vs for fields ranging from 10 to 380  kV/cm. No electron response was observed in these films. The mobilities were determined from the transient curves at the intersections of initial and final tangent lines that defined the shoulders in the log-log plots. The results of these hole mobility studies, which appear to support predictions of high hole mobility in DNA, are the first on DNA-based films that were fabricated in a dry environment and passivated for measurements in room air.

Studies of charge transport in DNA films using the time-of-flight (TOF) technique

Measurements were carried out on salmon DNA-based films, including as-received DNA (molecular weight, MW>2000 kDa) without and with hexacetyltrimethl-ammonium chloride (CTMA) surfactant, and sonicated DNA of MW~200 kDa with CTMA. The test specimens were spin-coated or drop-cast films on ITO-coated quartz slides with a gold charge-collecting electrode. To protect the films from atmospheric influences, the TOF devices were coated with a 200-400 nm polyurethane passivation layer. A quadrupled 20 ns, pulsed Nd:YAG laser with output at 266 nm was used for charge injection. The room temperature photoconductive transients were dispersive to varying degrees with hole mobilities in DNA materials films ranging between 2E-5 to 6E-3 cm2/Vs for fields ranging from 8 to 58 kV/cm. Only hole response was observed in DNA. The dispersive data were analyzed using a simple, quasi-empirical equation for the photocurrent transient data.