Lewis J. Rothberg

New phases of C60 synthesized at high pressure

The fullerene C60 can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C60 on reheating to 300�C at ambient pressure. For synthesis temperatures between 300� and 400�C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter ao = 13.6 angstroms. When treated at 500� to 800�C at the same pressure, C60 transforms into a rhombohedral structure with hexagonal lattice parameters of ao = 9.22 angstroms and co = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C60 molecules are linked.

Design and application of electron-transporting organic materials

Operating lifetime is the main problem that complicates the use of polymeric light-emitting diodes (LEDs). A class of electron transport (ET) polymers [poly(aryl acrylate) and poly(aryl ether)s] is reported in which moieties with high electron affinities are covalently attached to stable polymer backbones. Devices based on poly(p-phenylenevinylene) (PPV) prepared with these materials exhibited a 30-fold improvement in stability and, in one case, dramatically lower (10 volts versus about 30 volts) operating voltage relative to those having conventional ET layers. The current-carrying capacity of indium tin oxide-PPV-polymeric ET layer-aluminum LEDs was also increased by a factor of 30. These improvements lead to an enhancement in power efficiency of nearly an order of magnitude. Choosing polymers with high glass transition temperatures increases device lifetime.

Thermal and Photochemical Origin of Carbonyl Group Defects in Poly-(P-Phenylenevinylene)

Abstract The thermal stability of poly(p-phenylenevinylene) (PPV), along with its ability to form flexible devices, has sparked a considerable amount of research in the area of polymer light emitting diodes (LEDs). The presence of carbonyl groups as defects in the PPV structure has been reported elsewhere.1 An increase in carbonyl groups correlates well with a dramatic reduction in the photoluminescence and electroluminescence of this polymer. We have discovered that these carbonyl defects can be introduced thermally as well as photochemically. This paper reports the mechanisms for the thermal and photochemical oxidation of PPV. Although PPV is thermally quite stable, its hydroxy containing defects present in the precursor polymer are very susceptible to reaction with traces of oxygen at the elevated temperatures used in thermal conversion, resulting in the formation of ketone based carbonyl impurities. In addition, pristine PPV is very unstable in the presence of light and oxygen. The photooxidation of P...

Tunable femtosecond radiation in the mid-infrared for time-resolved absorption in semiconductors

We describe the generation of 350-fs pulses tunable between 2.3 and 5.5 microm by seeded optical parametric amplification. These pulses are synchronized with visible pulses for time-resolved absorption and reflectivity in semiconductors with 500-fs resolution.

Intrinsic and extrinsic constraints on phenylenevinylene polymer electroluminescence

Studies of phenylenevinylene polymer photophysics show that luminescence yield is reduced by interchain charge transfer quenching of excitons. We present evidence that non-emissive interchain excitons also play a role in electroluminescent devices and describe one synthetic strategy to reduce their importance. Addition of oxidation defects can further reduce luminescence yield and the mechanism for this is clarified by studies of model oligomers.

Fundamental and practical issues for phenylenevinylene-based polymer light-emitting diodes

An overview of photophysical measurements in phenylenevinylene polymers is presented and evaluated in light of the implications for organic electroluminescent materials design. Intrinsic limitations placed on device efficiency are discussed and we show that the polymer synthesis is critical to obtaining morphologies which exhibit reduced aggregation quenching of luminescence. We also document that oxidation of the polymer caused during the synthesis, or by subsequent excited state reactions, leads to luminescence quenching defects which can dramatically reduce fluorescence efficiency. We conclude with a brief review of other considerations in engineering efficient, long-lived polymer light-emitting diodes (LEDs).

Dynamics of photogenerated solitons in trans-polyacetylene

Time-resolved absorption using pole wavelengths of 2.5-5.5 mu m has been used to study photoexcitations in the quasi-one-dimensional semiconductor, trans-polyacetylene. Present apparatus has 0.5-ps resolution, which is sufficient to observe the decay of the photogenerated charged soliton pairs which form after intrachain excitation of electron-hole pairs. The tunability of the apparatus permits the unambiguous identification of solitons and observation of spectral relaxation during their formation. >

Vibrational energy transfer to metal surfaces probed by sum generation: CO/Cu(100) and CH3S/Ag(111)

Abstract Vibrational energy relaxation of molecules adsorbed at single crystal metal surfaces is measured by populating excited vibrational levels with picosecond infrared pump pulses and probing the population by infrared-visible sum frequency generation. Energy relaxation of the v=1 levels of the C-O stretching mode in CO/Cu(100) and of the symmetric C-H stretching mode of CH3S/Ag(111) are discussed.

Picosecond time-resolved absorption in the mid-infrared by seeded optical parametric generation

An apparatus to probe picosecond photoinduced absorption in semiconductors between 2 and 5 microm with picosecond resolution is described. The IR source is based on difference frequency mixing an amplified picosecond dye laser pulse with a synchronized white light continuum pulse. The choice of liquid for continuum generation is discussed, and applications to transient absorption in germanium and trans-polyacetylene are presented.

Picosecond time-resolved vibrational spectroscopy using a regenerative amplifier.

We demonstrate and characterize an apparatus for transient infrared spectroscopy with a 5-ps resolution. A visible pump and narrow-band mid-infrared probe are both derived from a source that is based on a pulsed Nd:YAG regenerative amplifier that operates at 540 Hz. We measure transient spectra in the range 1770-2250 cm(-1) with a 1-cm(-1) resolution and sensitivity to absorbance changes less than or equal to 1 x 10(-3). As an example, we present results from experiments to study ligand photodissociation from carboxymyoglobin in D(2)0.