Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

We investigate the energy relaxation of excitons during the real-space transport in ZnSe quantum wells by using microphotoluminescence with spatial resolution enhanced by a solid immersion lens. The spatial evolution of the LO-phonon sideband, originating from the LO-phonon assisted recombination of hot excitons, is measured directly. By calculating the LO-phonon assisted recombination probability, we obtain the nonthermal energy distribution of excitons and observe directly the energy relaxation of hot excitons during their transport. We find the excitons remain hot during their transport on a length scale of several micrometers. Thus, the excitonic transport on this scale cannot be described by classical diffusion.Comment: 4 pages, 4 figure

Hot exciton transport in ZnSe quantum wells

The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Froehlich coupling, the initial kinetic energy of the excitons is well controlled by choosing the excess energy of the excitation laser. When increasing the laser excess energy, we find a general trend of increasing transport length and more importantly a pronounced periodic quenching of the transport length when the excess energy corresponds to multiples of the LO-phonon energy. Such features show the dominant role of the kinetic energy of excitons in the transport process. Together with the excitation intensity dependence of the transport length, we distinguish the phonon wind driven transport of cold excitons and defect-limited hot exciton transport.Comment: 4 pages, 4 figure

A study of the Lenoir Limestone in the Knoxville-Friendsville belt, Knox and Blount counties, Tennessee.

The stratigraphy, subdivision, and correlation of the rocks of the Chickamauga Group or their equivalents have long been the subject of controversy. In the east central belts of the Valley and Ridge province, interpretation of the Lenoir Limestone is a major issue involved in the controversy. The Lenoir proper or Lenoir-type lithologies have been widely recognized in the southern Appalachians, from Virginia to Alabama. The type locality and distinctive outcropping belts of this formation are located in central East Tennessee

Comparing Models of Demographic Subpopulations

Understanding specific multi-dimensional demographics of populations in the United States at high resolutions is made difficult by the restriction of data released by the Census Bureau because of privacy concerns. Efforts to model these subpopulations have been increasing in recent years. These modeled populations have applications in decision making at all levels of government as well as in academia and the private sector. Two models have shown promising techniques for incorporating multiple levels of data to model sub populations in a meaningful way. These models, the Copula Model by Kao et al. (2012) and the Penalized Maximum Entropy Model by Nagle et al. (2014), have been applied in different study areas using different attributes. This paper provides a direct comparison which is needed to understand the strengths and weakness of each model as well as to assess the possibility of expanding their application nationally

Exploring the Relationship Between Myers-Briggs Type and Instructional Perspectives among College Faculty across Academic Units

Education has the opportunity to play an integral role in sustaining the health of our economy in an increasingly competitive, global market. A review of the issues and trends impacting higher education reveals growing pressure placed on faculty to advance instructional outcomes among more diverse populations. Imbedded is the challenge to create new knowledge about how to improve instruction. As diversity among college students in terms of age, gender, and ethnicity continues to increase, psychological type provides a means for examining important differences in choice of academic discipline(s), persistence, learning style, and teaching style preferences. Faculty members face increasing pressure to be critically reflective in their instructional practices. Research investigating the link between the psychological type and instructional perspectives offers insight for examining differences and promoting dialogue on ways higher education institutions can become more responsive to the needs of students of all types. This research investigated the relationship of psychological type, as measured by the MBTI and instructional perspective, as measured by the Modified Instructional Perspectives Inventory (IPI) among faculty across academic disciplines at four campuses of a public land-grant university. This study also examined variations in instructional perspectives among faculty of similar type teaching in the same academic disciplines and whether these variations are related to exposure to adult learning theories, methods, and/or instructional strategies. Research found a significant relationship between the Myers-Briggs Type Indicator and the Modified Instructional Perspectives Inventory. Findings provide evidence that variations in instructional perspectives among faculty members of similar MBTI types teaching in same academic disciplines do exist and that exposure to adult learning theories, methods, and/or instructional strategies accounts for a significant proportion of the variation

Correlated Photon Emission from a Single II-VI Quantum Dot

We report correlation and cross-correlation measurements of photons emitted under continuous wave excitation by a single II-VI quantum dot (QD) grown by molecular-beam epitaxy. A standard technique of microphotoluminescence combined with an ultrafast photon correlation set-up allowed us to see an antibunching effect on photons emitted by excitons recombining in a single CdTe/ZnTe QD, as well as cross-correlation within the biexciton ($X_{2}$)-exciton ($X$) radiative cascade from the same dot. Fast microchannel plate photomultipliers and a time-correlated single photon module gave us an overall temporal resolution of 140 ps better than the typical exciton lifetime in II-VI QDs of about 250ps.Comment: 4 pages, 3 figures, to appear in Appl. Phys. Let

Ionic polarization-induced current-voltage hysteresis in ch3nh3pbx3 perovskite solar cells

CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current–voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MAþ rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells

Atomic layer deposition of HfO2 on graphene from HfCl4 and H20

Atomic layer deposition of ultrathin HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180 degrees C. HfO2 was deposited also in a two-step temperature process where the initial growth of about 1 nm at 170 degrees C was continued up to 10-30 nm at 300 degrees C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10-12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30-40% far away from it.Comment: 4 figures, accepted by CEJ

Operando deconvolution of photovoltaic and electrocatalytic performance in ALD TiO2 protected water splitting photocathodes

The dual-working-electrode technique enables the deconvolution of the intrinsic properties of the buried p–n junction and the electrocatalyst on the surface for water splitting photocathodes