This article describes recent progress in organic semiconductor research with ultraviolet photoelectron spectroscopy (UPS). Origins of the energy level alignment at organic-conductor/dielectrics interfaces and the charge mobility in organic semiconductor systems are the most important subjects to be elucidated. Direct experimental detection of very low electronic density-of-states in the band gap is necessary to unravel mysteries of the energy level alignment. Experimental measurements of the energy band dispersion give information about the intermolecular interaction and the effective mass of transporting hole, and experimental detection of the highest occupied molecular orbital (HOMO) holemolecular vibration coupling is indispensable to comprehend impacts of the coupling on the hole transport. In this article, we summarize our recent challenges on UPS measurements of organic semiconductors, which give the band dispersion of the HOMO, the HOMO hole-vibration coupling and the band-gap states appearing near the interface.
In this article, we review recent progress in theoretical studies on the electronic properties of organic/metal interfaces, especially, on the origin of the interface dipoles. We first discuss the effect of the interface dipole on the charge injection barriers at organic/metal interfaces. Then, we observe the importance of the interface structure, especially, the organic-metal distances in physisorption systems. In this case, the experimentally observed substrate dependence of the interface dipole can be attributed mainly to the difference in the organic-metal distance. In the case of chemisorption systems, the induced density of interface states tends to pin the Fermi level relative to the HOMO and LUMO levels of molecules. We also discuss the importance of the alignment of molecular permanent dipoles in the case of Alq3/metal interfaces, which are relevant to organic light emitting devices.
This paper introduces two examples of organic thin film growth on in plane heterogeneous substrates. One is growth of pentacene films on SiO2 with metal electrodes, in which nucleation and initial growth process was observed real time by photoemission electron microscopy. Another is growth of sexithiophene (6T) films on SiO2 with periodic grooves (graphoepitaxy). Elongated 6T grains could be obtained along the groove under an optimal growth condition. The mechanism of these growth processes could be basically understood according to a conventional thin film growth theory.
The characteristic of an organic field effect transistor (OFET) depends strongly on both properties of an organic film and a carrier injection phenomenon. We have investigated the influence of carrier injection at the interface between the source electrode and the organic film on the characteristics of an OFET using a rubrene single crystal. The mobility estimated from the transfer characteristic of the OFET depended strongly on the channel length and the thickness of the rubrene single crystal although the mobility is intrinsically independent of the dimensions of an OFET. On the other hand, the temperature dependence of the saturation drain current was in good agreement with the thermal-field emission theory. These suggest that OFETs are controlled by not only the carrier accumulation at the channel but also the carrier injection.
Field-effect carrier doping to organic molecular solids is discussed from view of organic transistor application and modification of physical properties. The organic field-effect transistors with high k gate dielectric and electric double layer capacitor (EDL) are investigated, and the low-voltage operation is realized at present stage. Furthermore, the trap states are fully discussed because carriers accumulated by electric-field first occupy the trap states. In this report, we show the future prospective for modification of physical properties of organic solids.
This paper describes a band-filling-controlled Mott transition at an interface of an organic field effect transistor. Conductance and Hall coefficient of a thin crystal of an organic Mott-insulator laminated onto a SiO2/Si substrate were measured under various gate electric fields at low temperature. The maximum field effect mobility of this device reached 94 cm2/Vs. The carrier density at finite positive gate voltages corresponds to that of a metallic state of this material, rather than that of a Mott-insulating state. This observation indicates the electric-field induced Mott transition in the interface.
Common principles which govern the incorporation of impurity molecules into the lattice are discussed after a short overview on representative simple organic crystals, their cohesive energies and crystal structures. Representative means for realizing high purity organic crystals such as chromatography, recrystallization form solution, vacuum sublimation and zone-refining are discussed with comments on their advantages and limitations.
Surface plasmon resonance (SPR) bio-sensors comprise an attenuated total reflection (ATR) system known as the Kretschmann geometry. The sensing surface comprises flat Au thin films deposited on a high index flat glass substrate (LAL-10, n = 1.72). This paper describes the effects of the 3D nano-textured Au thin films on the sensing properties are discussed. The nano-textured Au thin films were fabricated by the deposition of the uniform Au thin films on the (0001) vicinal sapphire substrates. The vicinal substrates comprise periodic atomic step lines and terraces. The step height is chiefly half lattice spacing of the sapphire (0.22 nm). The terrace width is governed by the off-cut angles. The stepped surface structure were reconstructed by the annealing at 900 to 1400oC (1 to 3 h) in air. The annealed stepped surfaces comprised stepped heights of 5 to 12 nm with the terrace width of 2.7 to 140 nm for the off-cut angles at 5 degrees. The laser diodes (635 nm) were used for the optical light source. It is found that the detection limit of the SPR bio-sensors using the flat Au thin films on the (0001) sapphire substrates is 1.6 ng/ml for the tumor marker (AFP, α-fetoprotein) by antibody/antigen reaction, which is almost the same to the performance of the flat Au thin films on the conventional high index glass substrates as expected by the simulation Winspal 2. The SPR sensitivities using the nano-textured Au thin films with the step height of 12 nm and the terrace width of 140 nm were one order of magnitude smaller than those of Au thin films on the flat (0001) sapphire, 20 ng/ml, probably due to the scattering of the surface plasmon and/or evanescent wave. However, well-designed nano-textured Au thin films will enhance the SPR sensitivity. The sapphire substrates could integrate optical devices and will provide novel optical-bio MEMS.