Physics Commons^™

Self-Assembled In0.5Ga0.5As Quantum Dots On Gap, Yuncheng Song, Paul J. Simmonds, Minjoo Larry Lee

Paul J. Simmonds

We demonstrate the growth and luminescence of coherently strained In_0.5Ga_0.5As self-assembled quantum dots on GaP. Cross-sectional and planar-view transmission electron microscopy confirmed the dislocation-free nature of the In_0.5Ga_0.5As quantum dots and GaP cap layers. Intense photoluminescence from the quantum dots was measured at 80 K and was visible to the unaided eye in ambient lighting. The photoluminescence results show that emission energy can be controlled by varying the In_0.5Ga_0.5As deposition thickness. In combination with recent advances in the growth of GaP on Si, the In_0.5Ga_{0.5 …}

Radical Constructivism Has An Answer - But This Answer Is Not An Easy One, Dewey I. Dykstra

Physics Faculty Publications and Presentations

>Context • In spite of its advantages and its ability to make valid responses to objections, radical constructivism is not mainstream.

>Problem • Extolling the virtues of radical constructivism and responding logically to the objections does not work. We know this from the evidence of many attempts. Our theoretical stance, radical constructivism, also suggests this approach is not likely to have much influence on realists. We cannot transmit understanding in the signals with which we attempt to communicate. How can we in radical constructivism enable those outside of RC to understand our explanation of human knowing?

>Method • Examine our …

What Can We Learn From The Misunderstandings Of Radical Constructivism?: Commentary On Slezak’S “Radical Constructivism: Epistemology, Education And Dynamite”, Dewey I. Dykstra

Physics Faculty Publications and Presentations

>Problem • What alternative strategies from our experiences using a Piaget-based radical constructivist pedagogy might have more and better results than the current practice of responding in debate form, each side trying to prove the other wrong?

>Method • Use of Slezak’s paper to illuminate the point that the central problem with the interpretation of RC generally used in such writing is that the authors seem not to be able to operate from the central tenet of RC, which is the opposite of that used in realism. Description of how this failure to use the central tenet of RC results …

Tensile Strained Island Growth At Step-Edges On Gaas(110), Paul J. Simmonds, M. L. Lee

Paul J. Simmonds

We report the growth of tensile strained GaP islands on a GaAs(110) surface. Three-dimensional island formation proceeds via a step-edge nucleation process. To explain the dislocation-free nature of these islands, we consider the kinetics of strain relief within the context of a model for dislocation glide as a function of surface orientation and sign of strain.

Ferroelectricity In Non-Stoichiometric Srtio3 Films Studied By Ultraviolet Raman Spectroscopy, Dmitri A. Tenne, A. K. Farrar

Physics Faculty Publications and Presentations

Homoepitaxial Sr_1+xTiO_3+δ films with -0.2 ≤ x ≤ 0.25 grown by reactive molecular-beam epitaxy on SrTiO₃ (001) substrates have been studied by ultraviolet Raman spectroscopy. Non-stoichiometry for strontium- deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of a spontaneous polarization with a paraelectric-to-ferroelectric transition temperature above room temperature. Strontium-rich samples also show a strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution …

Physical Basis Of The Inducer-Dependent Cooperativity Of The Central Glycolytic Genes Repressor/Dna Complex, Denis Chaix, Matthew L. Ferguson, Cedric Atmanene, Alain Van Dorsselaer, Sarah Sanglier-Cianférani, Catherine A. Royer, Nathalie Declerck

Matthew L. Ferguson

The Central glycolytic genes Repressor (CggR) from Bacillus subtilis belongs to the SorC family of transcription factors that control major carbohydrate metabolic pathways. Recent studies have shown that CggR binds as a tetramer to its tandem operator DNA sequences and that the inducer metabolite, fructose 1,6-bisphosphate (FBP), reduces the binding cooperativity of the CggR/DNA complex. Here, we have determined the effect of FBP on the size, shape and stoichiometry of CggR complexes with full-length and half-site operator sequence by small-angle X-ray scattering, size-exclusion chromatography, fluorescence cross-correlation spectroscopy and noncovalent mass spectrometry (MS). Our results show that CggR forms a compact …

Transition Metal Dopants Essential For Producing Ferromagnetism In Metal Oxide Nanoparticles, Lydia M. Johnson, Aaron Thurber, Joshua Anghel, Maryam Sabetian, Mark H. Engelhard, Dmitri A. Tenne, Charles B. Hanna, Alex Punnoose

Physics Faculty Publications and Presentations

Recent claims that ferromagnetism can be produced in nanoparticles of metal oxides without the presence of transition metal dopants have been challenged in this work by investigating 62 high quality well-characterized nanoparticle samples of both undoped and Fe doped (0-10% Fe) ZnO. The undoped ZnO nanoparticles showed zero or negligible magnetization, without any dependence on the nanoparticle size. However, chemically synthesized Zn_1-xFe_xO nanoparticles showed clear ferromagnetism, varying systematically with Fe concentration. Furthermore, the magnetic properties of Zn_1-xFe_xO nanoparticles showed strong dependence on the reaction media used to prepare the samples. The zeta …

Multiferroic Pb(Fe1/2Nb1/2)O3 Single Crystals: A Raman Scattering Study, A. F. García-Flores, Dmitri A. Tenne, W. J. Ren, X. X. Xi., S.-W. Cheong

Physics Faculty Publications and Presentations

No abstract provided.

Tensile Strained Iii-V Self-Assembled Nanostructures On A (110) Surface, Minjoo L. Lee, Paul J. Simmonds

Paul J. Simmonds

The vast majority of research on epitaxial quantum dots use compressive strain as the driving force for self-assembly on the (001) surface, with InAs/GaAs(001) and Ge/Si(001) being the best-known examples. In this talk, I will discuss our work on determining the feasibility of growing coherent, tensile-strained III-V nanostructures on a (110) surface. GaP on GaAs(110) was chosen as an initial test system. It is hoped that our efforts on self-assembled, tensile-strained dots on a (110) surface will lead the way to new devices exploiting the fundamental differences between the (110) and (001) surfaces. Furthermore it is anticipated that this work …

Ferroelectricity In Strain-Free Srtio3 Thin Films, Dmitri A. Tenne

Physics Faculty Publications and Presentations

Biaxial strain is known to induce ferroelectricity in thin films of nominally non-ferroelectric materials such as SrTiO₃. However, even strain-free SrTiO₃ films and the paraelectric phase of strained films exhibit bulk frequency-dependent polarization hysteresis loops on the nanoscale at room temperature, and stable switchable domains at 50 K. By a direct comparison of the strained and strain-free SrTiO₃ films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that SrTiO₃ films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions, …

Growth Of Metamorphic Ingap For Wide-Bandgap Photovoltaic Junction By Mbe, John Simon, Stephanie Tomasulo, Paul J. Simmonds, Manuel J. Romero, Minjoo Larry Lee

Paul J. Simmonds

Metamorphic triple-junction solar cells can currently attain efficiencies as high as 41.1%. Using additional junctions could lead to efficiencies above 50%, but require the development of a wide bandgap (2.0-2.2eV) material to act as the top layer. In this work we demonstrate wide bandgap In_yGa_1-yP grown on GaAs_xP_1-x via solid source molecular beam epitaxy. Unoptimized tensile GaAs_xP_1-x buffers grown on GaAs exhibit asymmetric strain relaxation, along with formation of faceted trenches 100-300 nm deep in the [01-1] direction. Smaller grading step size and higher substrate temperatures minimizes the facet trench …