M. E. Gershenson

Intrinsic charge transport on the surface of organic semiconductors.

The air-gap field-effect technique enabled realization of the intrinsic (not limited by static disorder) polaronic transport on the surface of rubrene (C42H28) crystals over a wide temperature range. The signatures of this intrinsic transport are the anisotropy of the carrier mobility, mu, and the growth of mu with cooling. Anisotropy of mu vanishes in the activation regime at low temperatures, where the transport is dominated by shallow traps. The deep traps, introduced by x-ray radiation, increase the field-effect threshold without affecting mu, an indication that the filled traps do not scatter polarons.

Organic single-crystal field-effect transistors

Organic electronics constitute an innovative field, with interesting applications complementary to the silicon semiconductor technology. From a scientific perspective, there is large interest in the fundamental understanding of electrical transport in organic semiconductors. However, a well-developed microscopic description is still lacking, due to the complicated character of the many-body polaronic-type of charge carriers in molecular compounds. In this Thesis, we have experimentally studied the intrinsic charge transport properties of organic semiconductors by using organic single-crystal field-effect transistors. The electric field-effect has been frequently used to investigate thin films of organic compounds. Unfortunately, thin-film transistors are not suitable for the study of intrinsic electronic properties of organic conductors, because their characteristics are often strongly affected by imperfections of the film structure and by insufficient purity of organic materials. Thus, for a higher degree of molecular ordering and an improved quality of the FET, we fabricate devices on the surface of a free-standing single crystal of organic molecules. In short, in this work we have achieved successful fabrication of high-quality single-crystal FETs, exhibiting high mobilities and signs of intrinsic transport. Herewith, we have identified new aspects that influence charge transport in organic semiconductor FETs, and we have performed exploratory measurements in the charge density regime approaching one carrier per molecule.

Field-effect transistors on rubrene single crystals with parylene gate insulator

We report on the fabrication and characterization of the organic field-effect transistors (OFETs) on the surface of single crystals of rubrene. The parylene polymer film has been used as the gate insulator. At room temperature, these OFETs exhibit the p-type conductivity with the field-effect mobility 0.1–1 cm2/V s and the on/off ratio⩾104. The temperature dependence of the mobility is discussed.

Single-crystal organic field effect transistors with the hole mobility ∼8 cm2/V s

We report on the fabrication and characterization of single-crystal organic p-type field-effect transistors (OFETs) with the field-effect mobility μ∼8 cm2/V s, substantially higher than that observed in thin-film OFETs. The single-crystal devices compare favorably with thin-film OFETs not only in this respect: the mobility for the single-crystal devices is nearly independent of the gate voltage and the field effect onset is very sharp. The subthreshold slope as small as S=0.85 V/decade has been observed for a gate insulator capacitance Ci=2±0.2 nF/cm2. This corresponds to the intrinsic subthreshold slope Si≡SCi at least one order of magnitude smaller than that for the best thin-film OFETs and amorphous hydrogenated silicon (α-Si:H) devices.

High-mobility field-effect transistors based on transition metal dichalcogenides

We report on fabrication of field-effect transistors (FETs) based on transition metal dichalcogenides. The unique structure of single crystals of these layered inorganic semiconductors enables fabrication of FETs with intrinsically low field-effect threshold and high charge carrier mobility, comparable to that in the best single-crystal Si FETs (up to 500 cm2/V s for the p-type conductivity in the WSe2-based FETs at room temperature). These FETs demonstrate ambipolar operation. Owing to mechanical flexibility, they hold potential for applications in “flexible” electronics.

Hall Effect in the Accumulation Layers on the Surface of Organic Semiconductors

We have observed the Hall effect in the field-induced accumulation layer on the surface of single-crystal samples of a small-molecule organic semiconductor rubrene. The Hall mobility muH increases with decreasing temperature in both the intrinsic (high-temperature) and trap-dominated (low-temperature) conduction regimes. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, nH, coincides with the density n calculated using the gate-channel capacitance and becomes smaller than n in the trap-dominated regime. The Hall data are consistent with the diffusive bandlike motion of field-induced charge carriers between trapping events.

Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers

Self-assembled monolayers (SAMs) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of the surface conductivity of organic materials, which can be very large for SAMs with a strong electron-withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10(-5) S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity shows sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications.

Ultrasensitive hot-electron nanobolometers for terahertz astrophysics

The submillimetre or terahertz region of the electromagnetic spectrum contains approximately half of the total luminosity of the Universe and 98% of all the photons emitted since the Big Bang. This radiation is strongly absorbed in the Earths atmosphere, so space-based terahertz telescopes are crucial for exploring the evolution of the Universe. Thermal emission from the primary mirrors in these telescopes can be reduced below the level of the cosmic background by active cooling, which expands the range of faint objects that can be observed. However, it will also be necessary to develop bolometers-devices for measuring the energy of electromagnetic radiation-with sensitivities that are at least two orders of magnitude better than the present state of the art. To achieve this sensitivity without sacrificing operating speed, two conditions are required. First, the bolometer should be exceptionally well thermally isolated from the environment; second, its heat capacity should be sufficiently small. Here we demonstrate that these goals can be achieved by building a superconducting hot-electron nanobolometer. Its design eliminates the energy exchange between hot electrons and the leads by blocking electron outdiffusion and photon emission. The thermal conductance between hot electrons and the thermal bath, controlled by electron-phonon interactions, becomes very small at low temperatures ( approximately 1 x 10-16 W K-1 at 40 mK). These devices, with a heat capacity of approximately 1 x 10-19 J K-1, are sufficiently sensitive to detect single terahertz photons in submillimetre astronomy and other applications based on quantum calorimetry and photon counting.

Martensitic accommodation strain and the metal-insulator transition in manganites

In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance

Photoinduced charge transfer across the interface between organic molecular crystals and polymers.

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