Para-phenylene type molecules are efficient photoluminescence emitters in the ultraviolet-blue-green spectral range. They are used in light emitting diodes (LEDs) and photopumped lasers. Photoexcited para-phenylene type molecules give rise to strong emission from singlet excitons, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons. Since the latter two processes represent absorption of the emitted light of singlet excitons, the presence of polarons and triplet excitons might be a fundamental problem for laser diodes made from para-phenylene type molecules. In our experiments we modify the molecular geometry by the application of hydrostatic pressures up to 80 kbar in a temperature range of 10 to 300 K. In particular we show how triplet and polaron states, which are present in LEDs under operation, react to the induced geometric changes. The spectra of ground state absorption, excited state emission, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons are significantly broadened and shifted in energy. In order to explain the observed behavior we have performed three-dimensional bandstructure calculations within density functional theory for the planar poly(para-phenylene). By varying the intermolecular distances and the length of the polymer repeat unit pressure effects can be simulated.
The concept of polarization and its most basic consequence, Malus' Law, is usually not taught in the elementary or middle grades because of conceptual difficulties. We introduce the concept of polarization using sunglasses to understand the consequences of parallel and crossed polarizers. We then expand the concept with four puzzles. The puzzles are cut out of sheets of linear polarizers and are viewed through a (hand held) spinning polarizer. The first puzzle is constructed out of wedge shaped pieces of linear polarizer so that the wheel appears to rotate when viewed through the spinning polarizer. The second puzzle consists of concentric circles that appear to radiate outward. The third and fourth puzzles are four- and twelve piece wedges that are manipulated to produce different symmetric designs. We have tested these activities on fifth and sixth graders, and find that they enjoy the manipulative as well as the problem solving aspects of the puzzles. They are also able to understand that when light is polarized, 'whatever it is that waves' (the electric field) is oriented in one direction. The materials are inexpensive and can be easily made by teachers for classroom learning.
Conventional nanosynthesis involves film growth followed by direct-write nanolithography. The last step has two major shortcomings in that (a) it causes material damage to the nanostructures and (b) it is always serial in nature whereby each wafer has to be patterned one at a time. The latter makes it impractical for large-scale commercial applications. To overcome these drawbacks, we have developed a novel and `gentle' electrochemical process for fabricating quantum dot arrays that allows parallel processing of millions of wafers. It causes minimal damage, is much cheaper than conventional nanolithography, and yet has the spatial resolution (approximately 1 nm) of state-of-the-art techniques. Semiconductor quantum dot arrays produced by this process show strong signatures of quantum confinement in their photoluminescence spectra. Superconducting quantum dots show a significant transition- temperature shift arising from an interplay of superconductivity with quantum confinement, while ferromagnetic quantum dots give rise to a novel giant magnetoresistance effect caused by remote spin-dependent scattering of electrons. These structures have also been characterized by a variety of analytical techniques--all of which attest to their high quality.
Superlattices of alternating layers of semiconductors were first proposed1 in 1970, and since then a variety
of structures have been grown. Their technological importance has spurred considerable experimental and
theoretical work. The unique feature of quantum confinement of carriers has made possible unusual
devices. By combining various semiconductors and alloys of ffl-V, 11-TV and group IV materials, unusual
band lineups between neighboring layers have been obtained. Both lattice matched and strained layer
structures have been grown.
In this article we will focus on the electronic structure of the quantum well heterostructures under the
external perturbation of hydrostatic pressure. Pressure has been used extensively to investigate materials
in regions of phase space not otherwise accessib1. lu the study of quantum well structures, it has also
been used to move band edges in a controlled fashion, and alter band lineups, allowing the determination
of band offsets with an accuracy that was not possible without the use of pressure. As in bulk
semiconductors, optical techniques provide powerful tools in studying the electronic states in quantum
well heterostructures (QWH). Photoluminescence (PL) spectroscopy is only sensitive to spectral features
associated with energy states close to the bottom of the well due to rapid thermalization of carriers.
Photoluminescence excitation (PLE) is often limited by the availability of tunable lasers. Photoreflectance
(PR), on the other hand, can provide a rich structure due to both symmetry allowed and forbidden
transitions encompassing the entire quantum well. This sensitivity is due to the derivative nature of the
spectroscopy. Experiments can be carried out easily at different temperatures and over wide spectral
This article is organized as follows. In section 2 we will review some of the theoretical calculations of
electronic bands in quantum wells and discuss the changes expected under pressure. In Sec. 3, we
discuss the experimental details, including descriptions of the optical techniques used. Section 4 will deal
with studies of the quantized transitions in GaAs/GaixAlxAs and GaSb/A1Sb QWH under pressure,using
PR and PL. The examples are illustrative of the comparative merits of the two techniques.