Master of Science (MS), Wright State University, 2007, Physics

Mitchell, William D. M.S., Department of Physics, Wright State University, 2007. Polarization reversal in both hydrothermal and flux grown potassium titanyl phosphate was studied using square pulses at room temperature from 1600 V/mm to 5000 V/mm. Maximum switching current and inverse switching time data is compared to the ferroelectric polarization reversal model developed by Fatuzzo and Merz in the last century. Room temperature calculation of spontaneous polarization is reported and compared to that of potassium titanyl phosphate in the literature.

Committee: Lok Lew Yan Voon (Advisor) Subjects: Physics, Condensed Matter

Master of Science (MS), Wright State University, 2007, Physics

ZnO micro-structures and nano-structures have been grown on two types of substrate by reacting condensed Zn vapor with oxygen. The source material was either pure zinc powder or zinc acetate which was either evaporated or decomposed. This was done in the temperature range 500 C to 650 C, in a flowing Ar plus oxygen ambient at atmospheric pressure. Variations in the carrier gas composition, gas flow rate and the position of the substrate in the furnace were found to control the formation and the morphology of the nanostructures. Scanning electron microscopy images of samples grown from a Zn powder source show forested needles approximately 100 nm in diameter by 1 micrometers long, and faceted rods from 500 nm to 700 nm thick. Samples grown from Zn acetate show the formation of nano crystals (from ~100nm to ~300nm) dispersed across the substrates. Photoluminescence measurements at 4.2K show a dominant line at ~3.36 eV with additional features at 3.32 and 3.37 eV. The line widths are ~3.5 meV indicating good quality material. The usual green-band emission is also observed. Hall measurements and CV profiling were attempted but they were unsuccessful due to the inability to make good contacts.

Committee: Gary Farlow (Advisor) Subjects: Physics, Condensed Matter

Master of Science (MS), Wright State University, 2006, Physics

We have developed a new unifying tight-binding theory that can account for the electronic properties of recently proposed Si-based nanostructures, namely, Si graphene-like sheets and Si nanotubes. We considered the sp2s* and sp3 models up to first- and second-nearest neighbors, respectively. Our results show that the corresponding Si nanotubes follow the so-called Hamada's rule. Comparison to a recent ab initio calculation is made.

Committee: Lok Lew Yan Voon (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2001, Arts and Sciences : Physics

The transition metal oxide Vanadium Oxide evokes wide ranging research interest due to diverse and complex physical phenomenon it exhibits. The material has a rich phase diagram with four phases that are characterized by remarkably different properties. In recent years, the unique properties and phenomenon observed in the insulating regime of Vanadium Oxide have received a lot of attention. In addition to the long standing issue of the unusual magnetic ordering in the magnetically ordered insulating phase, recent experimental studies including neutron and resonant x-ray scattering, and x-ray absorption studies have raised new, interesting questions of the complex physics of the disordered and magnetically ordered insulating phases and the transition between these phases. In this work we have tried to gain a comprehensive physical understanding of insulating Vanadium Oxide by developing a microscopic S=2 bond model that is based on spin and orbital degrees of freedom, and is consistent with the parameters and phenomenology of this system. We have used this model to study insulating Vanadium Oxide in different temperature and parameter regimes, and thereby tried to elucidate the role played by spin and orbital physics in governing the interesting behavior observed in this system.We find that using the S=2 bond model with spin and orbital degrees of freedom, we can satisfactorily explain not only the anomalous magnetic ordering, but also all the other properties of the disordered and magnetically ordered insulating phases observed by experimental studies. The model also shows a phase transition between these phases which is completely consistent with the phenomenology of the magnetic transition in insulating Vanadium Oxide. In addition, the S=2 model predicts changes in the phase transition phenomenology for the AFI transition in the presence of magnetic field, and the possibility of an additional phase transition at lower temperatures. In this work, we have explored a (open full item for complete abstract)

Committee: Fu-Chun Zhang (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2001, Arts and Sciences : Physics

We use photoluminescence spectroscopy to investigate the optical properties of two different II-VI semiconductor nanostructure systems: ZnMnSe/ZnSe multiple quantum wells and self-assembled CdSe quantum dots. The behavior of excitons in ZnMnSe quantum wells is examined using polarized magneto-photoluminescence, while (temperature-dependent) time-resolved photoluminescence is used to study the dynamics of excitons confined to quantum dot structures. When Zn0.86Mn0.14Se/ZnSe multiple quantum wells are placed in an external magnetic fields, the spin-down holes become confined to the ZnMnSe “barriers,” while the electrons remain in the ZnSe “wells.” This spatial separation of electrons and holes results in weak electron-hole overlap, which should result in a large increase of the radiative lifetime of the spin-down exciton. As a result, we see the formation of exciton magnetic polarons (EMP) which lower their energy by spontaneously aligning the magnetic impurities in the exciton Bohr radius. We find that the EMP polarization approaches 100% in fields as small as 200 mT in these spatially indirect EMP, which is consistent with their extremely long recombination lifetime (~10 ns). This contrasts with previous measurements on other magnetic quantum well systems for which the polarization of short-lived spatially direct EMP never saturates. Time-resolved photoluminescence measurements of excitons confined to CdSe self-assembled quantum dots (SAQDs) reveal the existence of two different exciton decay times: a short 450 ps lifetime and a much longer ( > 4 ns) lifetime. While the emission resulting from the short lifetime excitons persists to room temperatures, the longlifetime component decreases in intensity with increasing temperature and is nearly completely gone by 60 K. Time-resolved spectra further reveal that the long lifetime component arises from spectrally sharp features (~200 µeV), while the rapid decay results from an underlying broad emission (~60 meV). Further (open full item for complete abstract)

Committee: Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2000, Arts and Sciences : Physics

We use time-resolved magneto-photoluminescence spectroscopy to study spin relaxation of excitons in a series of strained and unstrained ZnMnSe-based heterostructures. In an unstrained ZnMnSe epilayer, we find rapid spin relaxation, with a spin relaxation time of less than five picoseconds. We attribute this rapid spin relaxation to the complicated band structure: the various exciton spin bands intersect each other numerous times. The excitons can freely scatter between the spin bands, resulting in rapid spin relaxation. In contrast, we find extremely slow spin relaxation in a strained ZnMnSe/ZnFeSe multiple quantum well, with a spin relaxation time of greater than one nanosecond. Once excitons cool to the bottom of the band, very little spin relaxation occurs, and an extremely non-thermal exciton spin distribution persists throughout the lifetime of the exciton. In addition, we show that the dominant spin relaxation mechanism in this structure is LO-phonon emission during the momentum relaxation process, which occurs within 1 ps of the exciting laser pulse. We find similar results in two additional strained structures. For a strained ZnMnSe epilayer and a strained ZnMnSe/ZnSe multiple quantum well, we also see very slow spin relaxation, with spin relaxation times of greater than 1 ns. We conclude that this effect is due to the removal of the light hole - heavy hole valence band degeneracy by the lattice strain. This eliminates the band-mixing effects that lead to rapid spin relaxation in unstrained ZnMnSe-based heterostructures, thus resulting in extremely slow spin relaxation. We also find that the addition of a small fraction of cadmium to a strained quantum well strongly increases the spin relaxation rate. In a strained ZnCdMnSe/ZnSe quantum well, we see rapid spin relaxation, with a spin relaxation time of less than 5 ps, similar to that of the unstrained ZnMnSe epilayer. However, as in the other strained structures, the excitons spins never fully thermalize wit (open full item for complete abstract)

Committee: Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2000, Arts and Sciences : Physics

The first part of my dissertation work involved the design, construction, and operation of a scanning microscope that uses a superconducting quantum interference device (SQUID) as a probe. This system can produce two-dimensional (2D) images of the magnetic flux above samples as large as 1cm by 1cm. The microscope has a spatial resolution on the order of 40µm and a flux resolution on the order of a micro flux quantum. Next, I used this scanning SQUID microscope system to study the distribution of currents in 2D arrays of SNS Josephson junctions and attempt to estimate the penetration depth for perpendicular magnetic fields, λ⊥. λ⊥ is an important parameter in that it is a determining factor for the possibility of a Kosterlitz-Thouless phase transition in arrays. Raster scanned images of the flux above an array were produced for various temperatures and currents. The first method I used to determine λ⊥ was transforming the 2D flux images into images of current distribution. Intrinsic problems with the transforation algorithm led to a second approach of fitting a single flux scan to the Biot-Savart law. For the samples studied, λ⊥ was determined to be on the order of the array lattice constant. Finally, we investigated the usefulness of Fisher, Fisher, and Huse dynamical scaling to determine the occurrence of a Kosterlitz-Thouless transition in 2D systems. We simulated current-voltage (IV) curves for a 2D Josephson junction array using appropriate parameters and expressions above and below the Kosterlitz-Thouless-Berezinskii transition temperature (TKT). We also included a contribution arising fromfinite-size induced free vortices. The curves were scaled for different voltage cutoffs to simulate the minimum sensitivity of a voltmeter. We found that the value of the dynamical scaling exponent, z, for the best scaling fit and the optimal value of the transition temperature, depended upon the voltage cutoff level chosen; in effect the fit depended upon how much of the (open full item for complete abstract)

Committee: Richard Newrock (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

This dissertation reports the first experimental observation of spontaneous spin polarization due to lateral spin orbit coupling (LSOC) in side- gated (SG) quantum point contacts (QPCs). The QPC devices are fabricated on InAs/InGaAs quantum well structures using e-beam lithography. The low band gap InAs semiconductor was chosen because of its large intrinsic spin-orbit coupling. The side gates are realized by wet etching technique which is optimized to pattern the QPC devices. The width of the QPC is varied from 200 nm to 500 nm, while the length of the QPC is kept in the range 150-200 nm. The gradient in the lateral potential confinement in a side gated (SG) quantum point contact (QPC) causes a spin-orbit coupling (SOC). This LSOC induces a spontaneous spin polarization of opposite nature at the two edges of the QPC in the absence of any applied magnetic field. We have observed an anomalous conductance plateau at G @ 0.5 (2e2/h) (0.5 structure) in the SG QPCs fabricated on InAs/InGaAs QW structures. The 0.5 structure moves up in perpendicular magnetic field and approaches the normal conduction quantization at G = (2e2/h) in high magnetic field, whereas in-plane magnetic field has no effect on it. The evolution in magnetic field clearly indicates LSOC is responsible for the 0.5 structure. We believe it is the asymmetry in the confining potential of the QPC that leads to a net spin polarization giving the 0.5 structure. By electrically modulating the asymmetry of the QPC confinement, we have succeeded in making this structure appear and disappear. Such a QPC can conceivably be used as a spin polarizer or detector on demand by tuning the gate voltages. We also have proposed a dual-QPC device to experimentally validate the spin polarization by electrical means.

Committee: Philippe Debray (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

We have measured Coulomb drag (CD) between two spatially separated and electrically isolated one-dimensional (1D) wires to study the Luttinger liquid (LL) state in 1D systems. We have fabricated dual-wire CD devices with long quantum wires (≥ 1 µm) and short quantum wires (≤ 500 nm) with respect to the thermal lengths. The devices are made from high-mobility (≅106 cm2/Vs) two-dimensional electron gas (2DEG) in AlGaAs/GaAs heterostructures, using high-resolution e-beam lithography, combined with metal deposition by e-beam evaporation to form surface Schottky gates. Peak in drag voltage occurs when the subband bottoms of the lowest energy subbands of the drive and the drag wires line up with each other and the Fermi level. We have observed drag on 1 µm device at 22 mK temperature which is found to be reminiscent of the drag observed earlier on a 2 µm device. An extensive reanalysis of the drag results obtained on the 2 µm device indicates a power-law temperature dependence of drag for both identical and non-identical wires. Also drag is found to decay exponentially with the mismatch between the wires. These properties indicate the existence of Luttinger liquid (LL) state in the long wire device. We have observed positive and negative drags on short wire devices. The observed temperature dependence of drag resistance, for both positive and negative drags, shows first an increase, followed by a constant plateau and finally a decrease as the temperature is increased. This is in line with the predictions of the Fermi–Luttinger liquid (FLL) forward momentum transfer theory. This is the first experimental observation of 1D Coulomb drag due to forward momentum transfer between wires. A negative drag between same type of carriers (holes or electrons) may conceivably result from forward momentum transfer or forward scattering if the band curvature of the drag wire at or near the Fermi point is negative. Negative band curvature may result from asymmetry in the wire confining (open full item for complete abstract)

Committee: Prof. Richard Newrock (Committee Co-Chair); Prof. Philippe Debray (Committee Co-Chair) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

In this dissertation, I review pertinent laboratory, computational, and analytical methods which have developed a picture of selective-membrane-based heavy-metal-ion separation. Experimental methods to apply an electrochemical gradient across the selective membrane to enhance the ion flux are presented. Poisson-Nernst-Planck theoretical model predictions of the ion fluxes are validated against the experimentally observed results for our electrode-membrane-electrolyte system. Molecular dynamics simulations to model and elucidate the structure, functionality, and solute partitioning of the selectivity layer are also presented. These large scale Potential of Mean Force (PMF) simulations are the first studies of solute transfer across this selectivity layer, which is an example of a liquid-liquid interface system.

Committee: Dr. Thomas Beck (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

We study the dynamics of excitons in bimodal CdSe quantum dots. The effect of exciton localization is investigated by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe an exciton emission from low energy (QDs1) and weaker emission from high energy (QDs2) at low excitation levels at 10 K. Temperature-dependent photoluminescence (PL) studies reveal a thermally activated exciton transfer from QDs1 to QDs2. Time-resolved PL estimate the characteristic radiative and nonradiative decay rates as well as the trapping rate from the QD-precursor layer. The observed PL is reasonably reproduced using a coupled rate equation model. We investigate 10 nm Zn0.94Mg0.06Se/ZnSe quantum well (QW) with two-beam four-wave mixing (FWM) using 90 fs pulses. At high intensity the signal is dominated by χ(3) FWM processes and at low intensity it reveals an exciton resonant phase coherent photorefractive (PCP) effect that is attributed to the formation of an electron grating within the QW by the interference of coherent QW excitons. The observed traces and spectra are reproduced by the model based on a 15-level system and a phenomenological PCP model. The dynamical properties of the electron grating responsible for the PCP is further studied reducing the pulse repetition at 45, 55 and 65 K. The PCP diffraction reveals a nearly constant efficiency up to 1 μs that implies a constant average equilibrium electron density. With increasing temperature, the efficiency decreases due to QW electron escape back to the substrate reducing the grating lifetime. The observed PCP efficiency is studied with a model that considers the equilibrium density dynamics in the QW. We further report on PCP effect in Zn0.9Mg0.1Se/ZnSe QW by performing intensity dependent and polarization dependent two-beam FWM experiments using 30 fs pulses at 2.79 and 2.84 eV. The PCP effect is attributed to the formation of an electron grating within the QW by the interference of exc (open full item for complete abstract)

Committee: Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells (QWs). The interband and interexciton coherences has been investigated in three-pulse FWM experiments using 100 fs pulses in a 10 nm ZnSe QWs. The FWM signals obtained in three-beam FWM experiment are explained by optical Bloch equations (OBEs) for a ten-level model including excitation induced dephasing (EID). Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs. The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization of third beam with both an exciton grating and an eh-pair grating. Model calculations using the OBEs of a two level system that includes EID at both types of density grating provide information about exciton formation processes involved. We observe coherent exciton-LO-phonon polarons in the FWM spectrum of a 3 nm ZnSe single quantum well using 30 fs pulses. The formation of the new quasi-particles is attributed to a strong phonon coupling that is caused by strong Frohlich interaction, a huge exciton binding that exceeds the LO-phonon energy and a close resonance between the 2s exciton state and LO-phonon energy. The rapid decay of these coherent quasiparticles is attributed to the disintegration into zero-phonon excitons and free LO-phonons as well as to the inhomogeneous broadening of LO-phonon energies due to disorder and k-space dispersion A novel phase coherent photorefractive (PCP) effect has been observed in ZnSe single QWs using ultra-short pulses that do not overlap in time and its spectral and thermal dependence have been investigated. The observed PCP effec (open full item for complete abstract)

Committee: Dr. Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

Resonant Raman scattering (RRS) is a sensitive means to probe electronic and vibrational states in semiconductor nanostructures, even when such states are not accessible either through photoluminescence or transport techniques. In this thesis, resonant Raman scattering is used to probe the electronic and vibronic states of CdTe/ZnTe quantum dots and CdS nanowires at room temperature. In case of the CdTe/ZnTe quantum dots, the sensitivity of the ZnTe longitudinal optical phonon (LO) resonance is used to probe the excited state in the CdTe quantum dots. In addition, this technique is utilized to study the energy shifts due to post growth rapid thermal annealing. Due to annealing, both the ground and excited state energies of the quantum dots move to a higher value. This is consistent with the interdiffusion of Zn and Cd which makes the confining potential shallower and the average diameter of the quantum dots larger. In the case of the CdS nanowires, the 1-LO and 2-LO resonance is readily observed above the broad photoluminescence (PL) background. The lineshape of the integrated intensity of the 1-LO and 2-LO resembles the PL spectrum suggesting that the RRS probes the electronic states in both the ensemble and in single nanowires. In addition, the energy separation between the peaks of the 1-LO and 2-LO resonance of the ensemble CdS nanowires was found to be 34 meV which is comparable to the 37 meV value measured in bulk samples. We also show that spatially-resolved Raman scattering at resonance probes the structural uniformity in a single CdS nanowire and is able to identify the layers of the CdTe/ZnTe quantum dots sample. We found RRS intensity of single CdS nanowire strongly depends on the morphology of the wire. In conclusion, we have shown that RRS at room temperature is a sensitive and nondestructive tool to probe electronic and vibration sates as well as structural uniformity in semiconductor nanostructures.

Committee: Dr. Howard Jackson (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

We study the magnetic and optical properties of diluted magnetic semiconductor CdMnTe quantum dots (QDs) using various photoluminescence (PL) techniques. We use spatially resolved photoluminescence imaging to study single magnetic QDs. A solid immersion lens is combined with the confocal microscopy to achieve a high spatial resolution (~ 200nm) for PL imaging. We observe formation of exciton magnetic polarons (EMPs) in magnetic QDs for non-resonant excitation at B = 0T and T = 7K. However, the spin alignments of individual EMPs are distributed randomly resulting in zero global magnetization. Moreover, the spin alignment in the magnetic QD persists as long as the excitation is continued. This persistent magnetization is because of a long spin relaxation time of Mn ions. We estimate the relaxation time to be of the order of 1ms in CdMnTe QDs. For resonant excitations, CdMnTe QDs exhibit a strong PL polarization (40%) at B = 0T and T = 7K. The measurements show predominant &sigma +(&sigma -) –polarized PL emission for &sigma +(&sigma -) –polarized excitations that create spin polarized excitons. This suggests that one can control the spin alignment of magnetic impurities in magnetic CdMnTe QDs optically by using selectively polarized excitations. The magnetization created by such spin polarized excitons persists up to 170K. We attribute this robust persistent magnetization to strong three dimensional confinements of excitons in smaller CdMnTe QDs. In a low Manganese density QD sample, we observe a mixture of PL emission lines of magnetic and non-magnetic QDs. The results demonstrate that the Zeeman splitting of such non-magnetic QDs depends on the polarization of excitation. We believe that this excitation dependent Zeeman splitting is due to coupling between magnetic and non-magnetic QDs through the exchange interaction.

Committee: Dr. Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

Three different approaches to correlated electron systems are presented. (i) The single impurity Anderson model is solved by a semi-analytical approach based on the flow equation method. (ii) A numerically implementable Multi-scale Many-Body approach to strongly correlated electron systems is introduced. An extension to quantum cluster methods, it approximates correlations on any given length-scale commensurate with the strength of the correlations on the respective scale. (iii) The spectral properties of the one-dimensional Holstein and breathing polaron models using the self-consistent Born approximation are presented.

Committee: Dr. Mark Jarrell (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

This dissertation focuses on the study of ferromagnetism in Dilute Magnetic Semiconductors and Heavy Fermion Systems. To study these strongly correlated electronic systems, two non-perturbative techniques are used: the Dynamical Mean Field Approximation (DMFA) and the Dynamical Cluster Approximation (DCA). The model used for Dilute Magnetic Semiconductors incorporates the strong spin-orbit couplings of the carrier holes as found in most III-V semiconductors doped with manganese such as Ga1-xMnxAs. Calculated within the DMFA, the spin-orbit coupling effects give rise to various interesting physics, primarily the anisotropic behavior of the impurity band that affect the charge transport properties in the ferromagnetic phase. Heavy Fermion Systems are studied using the Periodic Anderson Model (PAM). In order to investigate the effects of spatial correlations, both the DCA and the DMFA techniques are used. Two different ferromagnetic mechanisms are found in these systems, one is driven by the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interactions, and the other by the precursors of charge density wave (CDW) formation of the conduction electrons. We provide evidence for the existence of charge density wave (CDW) precursors within one- and two-particle levels, and the argument to explain how these CDW precursors lead the system to ferromagnetic phase.

Committee: Dr. Mark Jarrell (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

In this dissertation, we use different optical and imaging spectroscopy techniques to study electronic structure and optical properties of CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots (SAQDs). We perform single dot photoluminescence excitation experiments to identify carrier excitation mechanisms in CdTe/ZnTe QDs. The first mechanism is direct excitation into the QD excited states followed by relaxation to the ground state and the second mechanism is direct excitation into the QD ground states through LO phonon-assisted absorption. We then execute resonant PL measurements for both CdTe and CdSe QD ensembles to study the dependence of exciton-LO phonon coupling on QD size in these II-VI SAQDs. We shown that the strength of exciton-LO phonon coupling increases significantly for QDs with lateral sizes smaller than the exciton Bohr radius (e.g. as-grown CdTe QDs) while for larger QDs (e.g. CdSe or CdTe annealed) it is almost independent of the QD emission energy, and therefore presumably of the QD size. In order to study electronic coupling between SAQDs, we setup imaging experiments with the use of a hemisphere solid immersion lens. While the PLE imaging measurements show the existence two-dimensional platelets with a typical size of about 500 nm which provide spatially extended but strong localized states through which different QDs could be populated simultaneously, the spatially resolved imaging data demonstrates a complete 2D map of those platelets. These results are further supported by computational calculations based on finite element analysis. Low temperature exciton spin relaxation in symmetric CdTe SAQDs has been thoroughly studied by means of cw polarized magneto-PL and polarized time-resolved PL spectroscopies. We find that the degeneracy of exciton energy levels has a strong influence on the spin transition. When the exciton spin states in QDs are degenerate, the spin relaxation time is much shorter than the exciton recombination time. In contrast, (open full item for complete abstract)

Committee: Dr. Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

In this work, optical properties of organic nanostructures are investigated. In temperature dependent (10 – 300 K) absorption studies of PTCDA and PTCDA/Alq3 multilayers a red shift and a line narrowing of vibrational Frenkel exciton bands is observed when the temperature is decreased. The reduced transition energy is explained by a thermal contraction along the PTCDA molecular stacks that cause an increased inter-molecular overlap between PTCDA molecules leading to an enhanced environmental shift. The reduction of the inhomogeneous broadening of the bands is explained by a reduced population of internal and external vibrational levels of the electronic ground state. The reduced temperature shift in multilayer is attributed to a reduced thermal contraction in the PTCDA crystallites due to adjacent Alq3 interlayers that possess a smaller thermal contraction than PTCDA. The exciton emission in PTCDA thin films, PTCDA/Alq3 multilayers and co-deposited PTCDA/Alq3 layers is studied by temperature dependent (10 – 300 K) PL measurements. The different recombination channels arising from Frenkel excitons and self-trapped excitons (charge-transfer excitons CT1-nr, CT1, CT2 and excimers) that were observed earlier in PTCDA single crystals also appear in films, multilayers and in co-deposited layers. In PTCDA/Alq3 multilayers, an unknown low energy line dominates the emission spectrum up to 200 K. In accordance to investigations using X-ray diffraction, FTIR absorption and strain dependent PL measurements the new channel is attributed to a modified CT2 transition due the compressive strain between stacked molecules. Temperature dependent (10 – 300 K) PL measurements of Alq3 layers are performed. An exciton trapping model which includes the formation of self-trapped excitons is proposed to explain the observed temperature dependent PL intensity enhancement and the spectral red-shift of the PL spectrum (at ~ 180 K). Alq3 based OLED structures are fabricated and electro-optical m (open full item for complete abstract)

Committee: Dr. Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2005, Arts and Sciences : Physics

We study the excitonic structure and optical properties of single quantum dots in three different II-VI semiconductor structure systems: as-grown CdSe/ZnSe, as-grown CdTe/ZnTe, and annealed CdTe/ZnTe self-assembled quantum dots(QD) using photoluminescence(PL), micro-PL, micro-PL obtained using a sub-wavelength aperture, and PL images obtained using a Solid Immersion Lens (SIL). These experiments are performed both at zero field and magnetic field up to 3 Tesla. While in zero field, linear polarized PL are used to find exchange splitting and QD asymmetry direction of the three QD samples, in a 3 Tesla magnetic field, circular polarized PL is used to determine the g-factor and diamagnetic shift. With a solid immersion lens (SIL), we are able to directly image a large number of single QD at 6K with 400 nm spatial resolution. Using zero field measurements, we find that while the some of the QDs are symmetric, and emit completely unpolarized light, over half of the QDs are antisymmetric which are typically aligned along one of the in-plane crystallographic directions, and so emit linear polarized PL. Since polarization properties as well as spin properties such as exchange splitting, g-factor and diamagnetic coefficient are different from dot to dot due to their size and asymmetry variations, we have carried out a statistical study of these properties in order to understand what changes occur in the QDs between these three samples. The major result from this work shows that thermal annealing of CdTe dots significantly reduces the statistical variation of exchange splittings for asymmetric dots as well as the g-factors for symmetric dots. We infer that these changes result from enlargement of the CdTe dots due to interdiffusion of Zn between the barriers and the dots. This enlargement is reflected in the increase of the median distributions of the diamagnetic shifts. Moreover, in CdSe dots, where the dot size is comparable or slightly larger than the exciton Bohr diameter (open full item for complete abstract)

Committee: Dr. Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2004, Arts and Sciences : Physics

Large molecules and clusters figure prominently in biophysics and nanoscience. With the advent of large computing platforms and novel algorithms, it is becoming feasible to simulate these systems at an accurate ab initio level. In this context, ab initio implies solving for the electronic wavefunction or density with a fixed configuration of nuclei, and perhaps updating the nuclear positions utilizing forces obtained from the electron density. In this way, highly accurate results can be obtained for systems with hundreds or even thousands of electrons. The predominant theoretical framework for such large calculations is currently density functional theory, since the Kohn-Sham method provides for efficient solution while including some degree of electron correlation. This dissertation is directed at the development of novel multiscale algorithms for making these electronic structure calculations more efficient. Recently it has been shown that the higher-order real-space methods utilizing pseudopotentials can produce results in electronic structure calculations comparable to those of plane-wave methods. Multiscale methods provide efficient and robust algorithms for large scale electronic structure calculations. In this dissertation, I discuss multiscale methods to solve self-consistent eigenvalue problems for non-periodic systems such as molecules with pseudopotentials. The two most expensive operations on the fine grid are the Gram-Schmidt orthogonalization and the Ritz projection. It has been shown that, for systems with few wavefunctions or well defined cluster structures (degeneracies), these two operations can be brought to coarser levels. But the algorithm stalls in its original form when applied to realistic systems such as large molecules having tens of wavefunctions. I found a new method which is called Ritz projection performed on clusters along with GRBR to solve this problem. The main advantage of the new method is that it scales as N e 2 for modest-sized (open full item for complete abstract)

Committee: Dr. Thomas Beck (Advisor) Subjects: Physics, Condensed Matter