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The mechanism of photoconductivity in a crystalline photoconductor synthesized from 1:1 ratio of meso-tetra(4-pyridyl)porphyrin (TPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) ionic tectons was examined. The rod-like crystals of TPyP:TSPP insulate in the dark but become photoconducting on illumination and a portion of the photoinduced current persists after the laser light is turned off. This persistent photoconductivity (PPC) is investigated as a function of laser illumination wavelength, laser power, and sample temperature. The primary charge carriers in the TPyP:TSPP upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. The number of electrons contributing to the photocurrent is directly proportional to the number of photons absorbed thus, the mechanisms of the photoconductivity resulting from excitations within the Soret band and the Q-band are the same. The PPC is interpreted to be the result of the formation of photoinduced metastable defects that allow for Miller–Abrahams-like hopping conductivity. The TPyP:TSPP has an incommensurately modulated crystal lattice and its proposed model structure is based on both ionic and neutral porphyrin tectons. The thermogravimetric analysis shows that the porphyrin crystals undergo dehydration on heating (˜50 ∘C) by losing water molecules located in the crystalline channels. Temperature dependent XRD indicates that dehydration causes irreversible changes to the crystal structure. The loss of crystallinity observed with heating the TPyP:TSPP crystals above 90 ∘C causes approximately 25% loss in photoconductivity but has little effect on the lifetime associated with the persistent photoconductivity.

We present a low-noise, high modulation-bandwidth design for a laser current driver with excellent long-term stability. The driver improves upon the commonly used Hall-Libbrecht design. The current driver can be operated remotely by way of a microprocessing unit, which controls the current set point digitally. This allows precise repeatability and improved accuracy and stability. It also allows the driver to be placed near the laser for reduced noise and for lower phase lag when using the modulation input. We present the theory of operation for our driver in detail, and give a thorough characterization of its stability, noise, set-point accuracy and repeatability, temperature dependence, transient response, and modulation bandwidth.

Persistent conductivity is observed following Ar + bombardment of meso-tetra(4-pyridyl)porphyrin:mesotetra(4-sulfonatophenyl)porphyrin (TPyP:TSPP) nanorods. The lifetime of the persistent conductivity in ultrahigh vacuum (UHV) is exceptionally long at room temperature, between 10 6 and 10 7 s. Ion beam currents can be used to both increase and decrease the level of persistent conductivity in these nanorods. Initial Ar + bombardment of a sample causes an increase in the persistent current. Subsequent bombardment with low-energy Ar + can cause a rapid decrease in the persistent current. A model is presented which presumes that persistent conductivity is carried by metastable defects with rates of excitation and relaxation following the Arrhenius relationship. Energy conservation suggests that ion bombardment introduces a thermal gradient across the nanorod which quickly quenches when ion bombardment ceases. This quick quenching results in a population of metastable defects which decay very slowly at room temperature.

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