Posted July 16, 2014 by Rino Rakhmata Mukti, Dr. rer.nat in Guest Seminar

Can we measure the photoconductivity of organic semiconductors using local probe techniques?

Dr. Arramel, NIMS, Japan,  16 July 2014, 15.00-16.00

Oganic semiconductor nanostructures are offering an opportunity as versatile building block for
the integration of optoelectronic and nanophotonic devices.[1-5] The enhancement of
photoconductivity or photocurrent stimulation by light is expected in the perylene-core
molecules due to their π-π electron interaction, high electron affinity and good chemical
stability.[1-3] In this work, we report on the comparative topographic and triggered-photocurrent
investigations of the n-type and p-type of perylene–based family using non-contact atomic force
microscopy (NC-AFM) and scanning tunneling spectroscopy (STS) at room temperature.
It turns out that the Lowest Unoccupied Molecular Orbital (LUMO) position of PTCDI-C7
molecule is few meV lower than that of PTCDI molecule before photo-irradiation probed by
scanning tunneling spectroscopy (STS) technique. This finding indicates an efficiently charge
carrier mobility is mediated by the alkyl side chains.[6] In contrast to its homolog n-type
perylene molecules, we successfully extracted both of the outmost frontier energy levels by
measuring the I-V characteristics of PTCDA molecule in a tunneling resistance from 4 to 100
GΩ at room temperature. We found that visible light has a dramatic impact on the frontier
molecular orbital energy position. For example, the extracted transport gap of PTCDA molecule
was decreased from 2.77 eV before irradiation to 2.28 eV during light irradiation (both were
measured with a comparable tunneling resistance ~100 GΩ).[6] In summary, an externally lighttriggered
photoconducting molecules study using STS technique shed some light of the
photophysics phenomena between photon and dye-organic molecule at the nanoscale.

Figure. Schematic illustration of the experimental testbed.

[1] S. Tatemichi et al., Appl. Phys. Lett. 89, 112108 (2006).
[2] S. Günes et al., Chem. Rev. 107, 1324 (2007).
[3] T. N. Krauss et al., J. Phys. Chem. C 113, 4502 (2009).
[4] W.-Y. Chou et al., Phys. Chem. Chem. Phys. 14, 5284 (2012).
[5] T. Hino et al., Small 6, 1745 (2010).
[6] Arramel et al., in preparation (2014).

Dr. Arramel has h-index of 4 as of April 2017.