Maxim Shkunov

High ambipolar and balanced carrier mobility in regioregular poly(3-hexylthiophene)

The carrier transport of carefully purified regioregular poly(3-hexylthiophene) films has been studied using time-of-flight photocurrent measurements. We find balanced ambipolar transport with a room-temperature mobility for holes of 3×10−4cm2V−1s−1 and for electrons of 1.5×10−4cm2V−1s−1 at electric fields ⩾105V∕cm. The transport is relatively field independent and weakly temperature dependent, pointing to a high degree of chemical regioregularity and purity. These factors make poly(3-hexylthiophene) attractive for use in a range of electronic applications.

Tunable, Gap-State Lasing in Switchable Directions for Opal Photonic Crystals

A photonic crystal laser that is tunable throughout the visible in three-dimensionally switchable directions is demonstrated. This photo-pumped laser utilizes a dye-infiltrated, single-crystal SiO2 opal having incomplete bandgaps. Our results support a gap-state-enhanced distributed feedback mechanism for lasing. Three different types of wavelength tunability are demonstrated, each applicable over a different frequency range and involving either single or multiple bandgaps. The many independent laser cavities that exist in one photonic crystal are demonstrated by simultaneously obtaining lasing in various colors and directions from an opal crystal. The observation of characteristic laser emission lines provides a new spectroscopy for characterizing intra-gap photonic states, which may be useful for developing the photonic crystal analogues of electronic circuitry.

Optical signature of delocalized polarons in conjugated polymers

Quantum-chemical calculations, incorporating both electron–phonon and electron–electron effects, are performed on two-chain model aggregates to assess the influence of interchain interactions on the nature of the singly charged species (polarons) in organic conjugated polymers. In symmetric cofacial dimers with fixed intermolecular distances, our theoretical approach leads to polarons fully delocalized over the two conjugated chains. Such a delocalization strongly affects the geometric and electronic relaxation phenomena induced by charge injection, which in turn leads to a dramatic spectral redistribution of the linear absorption cross section. Optical signatures for delocalized polarons in conjugated materials are identified from our theoretical description and compared to experimental data from optical charge modulation spectroscopy; these are fingerprints for the degree of interchain order, a decisive factor in the achievement of high charge carrier mobilities in polymer-based field-effect transistors.

Spray-Deposited Li-Doped ZnO Transistors with Electron Mobility Exceeding 50 cm(2)/Vs

Ambient spray pyrolysis is used for the deposition of high quality polycrystalline ZnO films utilizing blends of precursor solutions based on Zinc and Lithium acetates and the demonstration of n-channel thin-film transistors with electron mobility exceeding 50 cm(2)/Vs (see figure).

Polymerisable liquid crystalline organic semiconductors and their fabrication in organic field effect transistors

The performance of the semiconducting component in organic field effect transistors (OFETs) is a key parameter in the advancement of organic electronic devices. New semiconductors are required, which can be solution processed, possess high mobility and current modulation, and are stable in ambient conditions. This work provides the first demonstration of working field effect transistor devices fabricated from novel solution processible, polymerisable, small molecule liquid crystalline semiconductors, referred to as reactive mesogens. The design, synthesis, and performance of these materials in transistor devices are reported. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology was employed to control the liquid crystalline morphology, in an attempt to optimise organisation and packing. Alignment of the molecules in large homeotropic domains was achieved through surface treatment techniques, and the highly ordered mesophase was preserved by polymerisation of the reactive end groups, creating a crosslinked network.

Lasing and stimulated emission in /spl pi/-conjugated polymers

Recent studies of lasing and stimulated emission in luminescent /spl pi/-conjugated polymers performed by our group are presented. Optical properties of cylindrical high-Q polymer microcavities are discussed. The emission spectra of plastic microring and microdisk lasers are measured and analyzed. Light-emitting polymer microdiodes are demonstrated as possible candidates for electrically driven plastic lasers. In addition, two unusual regimes of stimulated emission characterized by narrow laser-like spectral lines are demonstrated in thin waveguiding polymer films. These regimes may be associated with random optical feedback introduced by light scattering inside the polymer films and amplified Raman scattering, respectively.

Excitons, polarons, and laser action in poly(p-phenylene vinylene) films

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)−E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogenerati...

Photonic versus random lasing in opal single crystals

We demonstrate a photopumped photonic crystal laser based on synthetic opal single crystal infiltrated with various laser dye solutions. The optical feedback in these lasers is provided via Bragg scattering of light from crystallographic planes in the opal. Localization of the laser modes within the stop band is assigned to defect states caused by crystal structure faults. We distinguish this photonic lasing mechanism from another regime of stimulated emission associated with random lasing what could be also observed in dye-infiltrated opals, but at higher excitation intensities.

Laser action in polydialkylfluorene films: Influence of low-temperature thermal treatment

We have used a variety of optical probes to investigate the changes occurring upon low-temperature thermal treatment to the emissive properties of dialkyl substituted polyfluorene thin films. We found that the low-temperature-driven morphological changes involving aggregates formation, which are observed in the absorption and photoluminescence spectra at low excitation intensities, give rise at high excitation intensities to laser action in the blue spectral range. In some cases, switching between two stimulated emission bands was also observed.

High mobility ambipolar charge transport in a cross-linked reactive mesogen at room temperature

By photochemically cross linking the molecules of a reactive end group functionalized calamitic liquid crystal (bis(heptylphenyl)-bithiophene-oxetane) in the smectic mesophase, an insoluble rigid molecular network is formed which remains structurally unchanged on cooling to room temperature. The charge transport is investigated by the time-of-flight method and the resulting material shows long-range (∼10μm) ambipolar transport with nondispersive field independent hole mobility, μhole=0.016cm2∕Vs and dispersive field dependent electron mobility, typically μelectron=0.028cm2∕Vs (both at 4×104Vcm−1 and at 15 °C). These desirable charge transport properties make this system a promising candidate for organic electronic applications, such as charge transport layers in organic light-emitting diodes and field effect transistors.