2,561 research outputs found
Current-Voltage Characteristics of Polymer Light-Emitting Diodes
Conduction in pristine conjugated polymers (other than polyacetylene) is by
polaron hopping between sites corresponding to conjugation lengths. The strong
increase of current with voltage observed for both emission-limited and
ohmic contacts is due in large part to mobility increase as increasing field
makes it more possible to overcome internal barriers, such as energy
differences between sites. For emission-limited contacts an additional source
of nonlinear increase of with increasing is greater ability to escpe
return to the injecting electrode due to the image force. For ohmic contacts
additional nonlinearity comes from space charge effects. We are able to fit
vs. for electron or hole conduction in some poly(-phenylene vinylene),
PPV, derivatives with ohmic contacts for reasonable values of the parameters
involved.Comment: 9 pages, REVTeX, 1 figure is aviable upon request, to be published in
SPIE pro
Effect of interchain coupling on conducting polymer luminescence: excimers in derivatives of poly(phenylene vinylene)
Optical excitation of a chain in a polymer film may result in formation of an
excimer, a superposition of on-chain excitons and charge-transfer excitons on
the originally excited chain and a neighboring chain. The excimer emission is
red-shifted compared to that of an on-chain exciton by an amount depending on
the interchain coupling . Setting up the excimer wavefunction and
calculating the red shift, we determine average values, referred to a
monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy -phenylene
cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4
p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the
effect of interchain distance on the emission.Comment: 10 pages, RevTeX, 1 PS figure, replaced version of cond-mat/9707095,
accepted for publication in Phys. Rev. B, Rapid Communicatio
Contact Injection into Polymer Light-Emitting Diodes
The variation of current I with voltage V for poly(phenylene vinylene) and
other polymer light-emitting diodes has been attributed to carriers tunneling
into broad conduction and valence bands. In actuality the electrons and holes
tunnel into polaron levels and transport is by hopping among these levels. We
show that for small injection the I-V characteristic is determined mainly by
the image force, for large injection by space charge effects, but in both cases
the strong variation of mobility with field due to disorder plays an important
role.Comment: 9 pages, two Postscript figures are aviable upon reques
Semiconductor saturable absorbers for ultrafast THz signals
We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP
and Ge in THz frequency range at room temperature using nonlinear THz
spectroscopy. The saturation mechanism is based on a decrease in electron
conductivity of semiconductors at high electron momentum states, due to
conduction band nonparabolicity and scattering into satellite valleys in strong
THz fields. Saturable absorber parameters, such as linear and non-saturable
transmission, and saturation fluence, are extracted by fits to a classic
saturable absorber model. Further, we observe THz pulse shortening, and an
increase of the group refractive index of the samples at higher THz pulse peak
fields.Comment: Submitted to Appl. Phys. Lett
Correlated-electron description of the photophysics of thin films of -conjugated polymers
We extend Mulliken's theory of ground state charge transfer in a
donor-acceptor complex to excited state charge transfer between pairs of
identical -conjugated oligomers, one of which is in the optically excited
state and the other in the ground state, leading to the formation of a
charge-transfer exciton. Within our theory, optical absorptions from the
charge-transfer exciton should include a low energy intermolecular
charge-transfer excitation, as well as distinct intramolecular excitations from
both the neutral delocalized exciton component and the Coulombically bound
polaron-pair component of the charge-transfer exciton. We report high order
configuration-interaction calculations for pairs of oligomers of
poly-paraphenylenevinylene (PPV) that go beyond our previous single
configuration-interaction calculation and find all five excited state
absorptions predicted using heuristic arguments based on the Mulliken concept.
Our calculated excited state absorption spectrum exhibits strong qualitative
agreement with the complete wavelength-dependent ultrafast photoinduced
absorption in films of PPV derivatives, suggesting that a significant fraction
of the photoinduced absorption here is from the charge-transfer exciton. We
make detailed comparisons to experiments, and a testable experimental
prediction
The Telomere and Telomerase Activity
In lieu of an abstract, below is the article\u27s first paragraph.
In the past few years, a new area of genetics has emerged that has scientists in many other fields intrigued. The discovery of telomeres, the end segments on our chromosomes, and the enzyme that controls them, has evoked an interest in professionals from a wide range of specialties. Articles on the subject have appeared in journals of cell and molecular biology, microbiology, genetics, gerontology, and have even surfaced in the popular media. There is already a wealth of information to be found on the topic, yet there is many times more still to be discovered, in several different fields. Because of the relatedness of this topic to so many areas, telomere biology has almost emerged as a branch of biology in and of itself
Interdisciplinary public relations: a curriculum for undergraduate study at Oakwood College
The purpose of this project was to create a public relations curriculum for undergraduate study to prepare students as effective practitioners and communicators in the 21st century.
Public relations education has changed over the years to meet the needs and trends of the industry, and today is no exception. The continued rise of Integrated Marketing Communications, and the desire to strengthen the counseling and management functions of public relations make it necessary to evaluate undergraduate curricula to ensure students are being properly prepared to enter the workforce.
The undergraduate curricula of 11 schools were analyzed and compared for subject matter and skills promoted, as related to the concerns and needs expressed in related literature by current public relations practitioners and educators. The schools were chosen through a survey of recognized experts in the public relations industry, as schools having the top undergraduate public relations programs in the country.
The resulting curriculum reflects the need for students to have a social science grounding, a working understanding of other disciplines, such as advertising and marketing, and better skills in writing, business management, and strategic thinking
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