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Molecular and organic nano-electronics

 

Organic electronics is currently experiencing a surge of activities worldwide prompted in part by recent advances in achieving high electronic mobility, light emission over a broad range of frequencies, demonstration of spin valve operation with giant magneto-resistance and other effects. Despite these encouraging premises, there are many roadblocks that hinder a broader proliferation of “plastic electronics” in contemporary technology. Arguably, the most significant of them is the challenge to achieve a comprehensive understanding of the fundamentals of charge injection and charge transport in polymers. Recently, in collaboration with A.J. Heeger (UCSB), Vitaly Podzorov and Michael Gershenson (Rutgers), we have investigated charge injection effect in the field effect transistor (FET) based on organic molecular crystals [1] and polymers [2]. Combined with IR microscopy these new experimental capabilities promise to deliver a comprehensive understanding of energy, length and time scales involved in the injection processes in the FET structures. This work is currently supported using funding from the NSF “Organics” program and from the Petroleum Research fund for this research area.

(a): Molecular crystal FET's

  

Left panel: A photograph of a FET device based on single crystal rubrene. Middle panel: A schematic of the device structure of rubrene FETs. Right panel: the optical conductivity of the accumulation layer in rubrene FETs at a gate voltage of -280V at 300K, with the E vector of the IR light along the a and b axis of rubrene. Our data indicate that transport in rubrene is governed by light quasiparticles in molecular orbital bands, and the polaronic effects in rubrene FETs are weaker at room temperature than previously thought [1].

(b): Polymer FET's

  

IR imaging of charge injection in organic FETs based on poly(3-hexylthiophene) (P3HT) [2]. The top left panel displays a fragment of the cross-section of a “grid-electrode” organic FET; a photograph of an actual device with dimensions 10x14 mm2 is depicted in the bottom left panel. Right panel: the gate-voltage-induced absorption of the device. Employing infrared spectroscopy, we have been able to directly probe the electronic excitations associated with the injected carriers in a functional organic FET device: IR active vibrational modes (sharp resonances in the 1,000-1,500 cm-1) and polaron (broad absorption band centered at 3500 cm-1).

By focusing an IR beam in a spot with a diameter of 50-100 microns on the FET, it becomes possible to examine the decay of the injected charge density in the vicinity of the injection electrodes. In these experiments we detected the variation of carrier density away from the injection contacts (middle panel) in the area shown by the square in the bottom left panel by spatially monitoring spectroscopic fingerprints of the injected charges (right pannel).

Recent Publications:

1: Z. Q. Li, V. Podzorov, N. Sai, M.C. Martin, M.E. Gershenson, M. Di Ventra and D.N. Basov, “Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors”, Phys. Rev. Lett. 99, 016403 (2007). 

2: Z. Q. Li, G. M. Wang, N. Sai, D. Moses, M. C. Martin, M. Di Ventra, A. J. Heeger, and D. N. Basov, “Infrared Imaging of the Nanometer-Thick Accumulation Layer in Organic Field-Effect Transistors”, Nano Letters 6 (2), 224 (2006).

3: Z.Q. Li, G.M. Wang, K.J. Mikolaitis, D.Moses, A. J. Heeger, and D.N. Basov, “An infrared probe of tunable dielectrics in metal-oxide-semiconductor structures”, Appl. Phys. Lett 86, 223506 (2005).