Browsing by Subject "Photoluminescence"
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Item Influence of surface passivation on the photoluminescence from silicon nanocrystals(2010-08) Salivati, Navneethakrishnan; Ekerdt, John G.; Downer, Michael C.; Mullins, C. B.; Korgel, Brian A.; Hwang, Gyeong S.Although silicon (Si) nanostructures exhibit size dependent light emission, which can be attributed to quantum confinement, the role of surface passivation is not yet fully understood. This understanding is central to the development of nanocrystal-based detectors. This study investigated the growth, surface chemistry, passivation with deuterium (D2), ammonia (ND3) and diborane (B2D6) and the resulting optical properties of Si nanostructures. Si nanocrystals less than 6 nm in diameter are grown on SiO2 surfaces in an ultra high vacuum chamber using hot-wire chemical vapor deposition and the as grown surfaces are exposed to atomic deuterium. Temperature programmed desorption (TPD) spectra show that that the nanocrystals surfaces are covered by a mix of monodeuteride, dideuteride and trideuteride species. The manner of filling of the deuteride states on nanocrystals differs from that for extended surfaces as the formation of the dideuteride and trideuteride species is facilitated by the curvature of the nanocrystal. No photoluminescence (PL) is observed from the as grown unpassivated nanocrystals. As the deuterium dose is increased, the PL intensity also begins to increase. This can be associated with increasing amounts of mono-, di- and trideuteride species on the nanocrystal surface, which results in better passivation of the dangling bonds and relaxing of the reconstructed surface. At high deuterium doses, the surface structure breaks down and amorphization of the top layer of the nanocrystal takes place. Amorphization reduces the PL intensity. Finally, as the nanocrystal size is varied, the PL peak shifts, which is characteristic of quantum confinement. The dangling bonds and the reconstructed bonds at the NC surface are also passivated and transformed with D and NDx by using deuterated ammonia (ND3), which is predissociated over a hot tungsten filament prior to adsorption. At low hot wire ND3 doses PL emission is observed at 1000 nm corresponding to reconstructed surface bonds capped by predominantly monodeuteride and Si-ND2 species. As the hot wire ND3 dose is increased, di- and trideuteride species form and intense PL is observed around 800 nm that does not shift with NC size and is associated with defect levels resulting from NDx insertion into the strained Si-Si bonds forming Si2=ND. The PL intensity at 800 nm increases as the ND3 dose is increased and the intensity increase is correlated to increasing concentrations of deuterides. At extremely high ND3 doses PL intensity decreases due to amorphization of the NC surface. In separate experiments, Si NCs were subjected to dissociative (thermal) exposures of ammonia followed by exposures to atomic deuterium. These NCs exhibited size dependent PL and this can be attributed to the prevention of the formation of Si2=ND species. Finally, deuterium-passivated Si NCs are exposed to BDx radicals formed by dissociating deuterated diborane (B2D6) over a hot tungsten filament and photoluminescence quenching is observed. Temperature programmed desorption spectra reveal the presence of low temperature peaks, which can be attributed to deuterium desorption from surface Si atoms bonded to subsurface boron atoms. The subsurface boron likely enhances nonradiative Auger recombination.Item Latent Fingerprint and trace explosives detection by photoluminescence and time-resolved imaging(Texas Tech University, 2003-05) Bouldin, Kimberly KayLatent fingerprint detection by photoluminescence is a well-developed field. Many development techniques exist and are currently being employed in forensic laboratories to detect fingerprints by making them luminescent. However, in forensic science, timeresolved imaging techniques, designed to suppress background fluorescence that interferes with fingerprint detectability, are to date not used outside of the research laboratory, and the chemistry necessary to use time-resolved imaging for fingerprint detection is somewhat limited. For this reason, the first section of this dissertation deals with fingerprint detection methods that have direct application to time-resolved imaging techniques. Trace explosive detection field methods based on chemical reactions have until recently utilized only colorimetric products. To increase the sensitivity of such detection, a field explosive test kit which produces a product that is both colorimetric and luminescent is studied. Detection sensitivity can be gained by taking advantage of the luminescence of these products, something that has not been done to date. When the appropriate chemistry is chosen for explosive detection, time-resolved imaging techniques may again be applicable. This dissertation thus looks at possibilities of taking trace explosives detection to this next level.Item Manipulating fluorescence dynamics in semiconductor quantum dots and metal nanostructures(2011-12) Ratchford, Daniel Cole; Li, Elaine; Chelikowsky, James; Florin, Ernst-Ludwid; Shvets, Gennady; Vanden Bout, DavidRecent scientific progress has resulted in the development of sophisticated hybrid nanostructures composed of semiconductor and metal nanoparticles. These hybrid structures promise to produce a new generation of nanoscale optoelectronic devices that combine the best attributes of each component material. The optical response of metal nanostructures is dominated by surface plasmon resonances which create large local electromagnetic field enhancements. When coupled to surrounding semiconductor components, the enhanced local fields result in strong absorption/emission, optical gain, and nonlinear effects. Although hybrid nanostructures are poised to be utilized in a variety of applications, serious hurdles for the design of new devices remain. These difficulties largely result from a poor understanding of how the structural components interact at the nanoscale. The interactions strongly depend on the exact composition and geometry of the structure, and therefore, a quantitative comparison between theory and experiment is often difficult to achieve. Colloidal semiconductor quantum dots are strong candidates for integration with metal nanostructures because they have a variety of desirable optical properties, such as tunable emission and long term photostability. However, one potential drawback of colloidal quantum dots is the intermittency in their fluorescence (commonly referred to as “blinking”). Blinking was first observed over a decade ago, yet there is still no complete theory to explain why it occurs. In spite of the lack of a full theoretical explanation, multiple methods have been used to reduce blinking behavior, including modifying quantum dot interfaces and coupling quantum dots with metal nanostructures. This thesis focuses on studying the coupling between colloidal quantum dots and metal nanoparticles in simple model systems. Atomic force microscopy nanomanipulation is used to assemble the hybrid structures with a controlled geometry. The experimental studies report for the first time the modified fluorescence decay, emission intensity, and blinking of a single quantum dot coupled to a single Au nanoparticle. Since the geometry of the structure is known, these studies provide reliable information on the interparticle coupling, and quantitative experimental results are shown to be consistent with classical electrodynamic theories.Item Methods development and measurements for understanding morphological effects on electronic and optical properties in solution processable photovoltaic materials(2012-12) Ostrowski, David Paul; Vanden Bout, David A.; Rossky, Peter J; Holliday, Bradley J; Korgel, Brian A; Dodabalapur, Ananth JThe effects of morphology on electronic and optical properties in solution processable photovoltaic (PV) materials have been studied through two different approaches. One approach, scanning photocurrent (PC) and photoluminescence (PL) microscopy, involved mapping PC generation and PL in functional PV devices on the length scale of around 250-500 nm. Additionally, local diode characteristics were studied from regions of interest in the PV through local voltage-dependent photocurrent (LVPC) measurements. In a PV made from a Copper Indium Gallium Selenide (CIGS) nanocrystal (NC) "ink", two morphological features were found to cause the spatial heterogeneity in PC generation. Cadmium Sulfide (CdS) aggregates lowered PC generation by blocking incident light to the photoactive layer, and cracks in the CIGS-NC film enhanced PC generation through improved charge carrier extraction. LVPC measurements showed all regions to have similar diode characteristics with the main difference being the PC generated at zero bias voltage. For another PV made from a donor/acceptor blend of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl- 1,4-phenylenediamine (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), two incident laser wavelengths were used to selectively illuminate only one or both polymers. The results showed that when F8BT is illuminated, the PFB-rich regions produced the most PC and when both polymers are illuminated (but mostly PFB), the F8BT-rich regions produce the most PC; showing PC generation is more affective when less absorber material is present in the morphology. The other approach to study morphological effects on PV properties was to fabricate particles that mimicked morphological variations known to occur in solution-processable PVs. Through solution processing of an oligothiophene molecule, a range of weakly coupled H-aggregate particles were made. These particles, identifiable by shape, were shown to have a varying degree of energetic disorder (as gauged by the 0-0 vibronic band intensity in the emission spectrum), despite all particles showing a similarly high degree of molecular order from fluorescence dichroism (FD) measurements. A trend was observed correlating a decrease in energetic disorder with an increase in the local contact potential (LCP) difference as measured with Kelvin probe force microscopy (KPFM). The LCP difference was found to range by 70 mV between particles of moderate to low energetic disorder.Item Photoluminescence studies in hydrogenated amorphous silicon and its alloys(Texas Tech University, 1993-08) Palsule, Chintamani P.Not availableItem Photoluminescence studies of <100>-and <111>-grown indium gallium arsenide strained single quantum wells under hydrostatic pressure(Texas Tech University, 1998-05) Sauncy, Toni DInGaAs/GaAs strained quantum wells are of technological importance and current scientific interest. Most of this interest stems from variations achieved in the electronic band structure due to the combined effects of alloying, strain and quantum confinement. When these heterostructures are grown along non-[001] directions, a strain-induced piezoelectric field is present within the coherently strained layer. This interesting effect significantly complicates the optical transitions. While these novelly-grown-strained quantum wells have already been successfully fabricated into optoelectronic devices, the effects of the piezoelectric field on fundamental recombination processes is not well understood. Further, the alignment in (lll)-grown. In GaAs/GaAs heterostructures has not been established. In this work, photoluminescence spectroscopy is combined with hydrostatic pressure to examine band edge radiative recombination from simultaneously grown single quantum well pairs with different growth axis orientations: (100) and (111)A. The (l00)-grown quantum well is used as a basis for comparing the optical response and pressure dependence of the (111)-grown well. .Ml photoluminescence measurements were done with the samples at 77K. Ambient pressure work established the magnitude of the strain induced piezoelectric fic4d within the (111)-grown quantum well. The experimentally determined value of 72 kV/cm agrees with the value computed using first-order elastic theory. Pressure studies precisely establish a valence band discontinuity for the 100 grown well of 61 meV which corresponds to an unstrained bandgap off-set ratio of ÄEc : ÄEv = 0.75 : 0.25. By carefully monitoring the nonlinear optical response of the photoluminescence from the (lll)-grown quantum well under pressure, reliable identification of high pressure photoluminescence emission processes was accomplished. The first time observation of the above crossover indirect emission from the (HI)- grown quantum well allowed for the establishment of limits on the valence band discontinuity of !^100 to 130 meV.Item Recombination and transport in hydrogenated amorphous silicon(Texas Tech University, 1996-05) Yi, Seung-HoThis dissertation is the study of properties of a-Si:H using photoluminescence (PL) and photoconductivity (PC) measurements to shed light on understanding of the recombination and transport process of photogenerated carriers. Based on the field dependent PL measurement, we developed a model to explain the field quenching in PL at low temperatures (below 100 K). Our model showed that most radiative recombination occurs between geminate pairs. We determined the distribution of separations of geminate pairs from results of the field quenching in PL. The distribution does not change much with excitation energy. Results of simultaneous PC measurement also indicated the geminate pair recombination, even though it is not so apparent due to, probably, the existence of a fast non-radiative recombination process. Our results imply that there is no significant change in the distance between an electron and a hole in a geminate pair in bandtail states due to thermalization, which is contradictory to a generally accepted model. Field dependent PC measurements at various temperatures confirmed the validity of the concept of effective temperature' invented for explaining field dependent PC in terms of temperature. Results from the measurements showed the existence of safe hole traps (SHTs), which might relate to light degradation. The intensity dependent PC measurements supported this conclusion. We also investigated phosphorous doped a-Si:H with above gap and sub gap excitation in order to study the effect of defects in a-Si:H. The results allow us to develop a possible recombination mechanism for defect related PL.Item Self-assembled quantum dots in advanced structures(2012-05) Creasey, Megan Elizabeth; Li, XiaoqinAdvances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs.Item Structural phases of disordered carbon materials(Texas Tech University, 1996-08) Dallas, Timothy E. J.The purpose of this study is to characterize the structural and optical properties of a number of disordered carbon materials using the non-destructive optical techniques of Raman and photoluminescence spectroscopies. The samples were produced with many different growth techniques and included microcrystalline graphites, amorphous carbons, and synthetic diamond films. A new CCD-based, multichannel Raman system was set up and used to measure spectra with very high signal-to-noise in a very short period of time. Micro-Raman spectroscopy was used to determine the bonding structure of a pulse-laser annealed graphite sample. The extent of crystalline structure was quantitatively determined in both the planar and stacking directions. A follow-up study on a series of graphitic amorphous carbon (GAC) samples confirmed models by showing evidence that GAC contained very small hexagonal clusters interconnected by odd-membered rings. Annealing the GAC increased the size of the hexagonal clusters and reduced the amount of disorder. A shift in the energy position of the main phonon band of graphite has been attributed to a finite-size effect. Disorder induced vibrational modes in carbons can vary in intensity and peak position depending upon the excitation wavelength used for Raman scattering. This resonance Raman phenomena was used to help detect difference in the Raman spectra of GAC, amorphous carbon, and disordered synthetic diamond. Continuous wave (CWPL) and time-resolved (TRPL) photoluminescence spectroscopies were used to study low level point defects and structures in synthetic polycrystalline diamond films. A series of sharp peaks observed in the CWPL spectrum of a film produced by Arc-Jet chemical vapor deposition have been attributed to tungsten atoms incorporated during growth. Broadband PL in the red wavelength region was determined to be a combination of emissions from vibronics and amorphous carbon structure.Item Studies of optical properties of single CdS nanorods(2003) Kulik, Dmitri; Shih, Chih-KangThis work describes low-temperature studies of the optical properties of single CdS nanorods in relation to the nanorods’ morphology. Keeping track of the same nanorods in the photoluminescence (PL) study and during the scanning electron microscope (SEM) imaging we were able to distinguish intrinsic spectral features of the nanorods from those of nanocluster aggregates. We observed strong photoluminescence polarization in the direction perpendicular to the nanorods’ axis. The sharp peaks in the “blue” region of the photoluminescence spectra are red-shifted relative to those of the bulk CdS and of the CdS microparticles in the glass matrix. Finally, we observe a very interesting “spectral diffusion” of the main emission peaks with the following characteristics: (a) energy-energy correlation of the two main peaks; (b) Correlation and anticorrelation between energy and intensity for the two peaks. Possible mechanisms behind this phenomenon are discussed.Item Time-correlated single photon studies of hydrogenated amorphous silicon carbon(Texas Tech University, 1993-05) Pleil, Matthias W.A time correlated single photon counting system has been developed to probe very fast photoluminescence decay properties associated with excitonic recombination in hydrogenated amorphous silicon carbon. A detailed description of this system includes a unique unbiased distribution analysis technique capable of resolving up to five, nanosecond and subnanosecond component lifetimes. A resolution limit of 20 ps is demonstrated using synthetic data and down to 87 ps on a sample of known decay time. This is the first time that time correlated single photon counting has been apphed to probe the subnanosecond regime of the photoluminescence decay in hydrogenated amorphous silicon carbon thin films. Five samples of carbon to silicon atomic concentrations from 0.50 to 0.82 were deposited using an electron cyclotron resoucince plasma system with a liquid source of diethylsilane. Studies on absorption and emission spectra and luminescence decay acquired over the visible spectrum show that as the relative carbon concentration increases, the defect density, and bandtail broadening also increase as the photoluminescence peak emission decreases. The room temperature photoluminescence decay comprises of three and four distinct lifetime distributions which remain constant in time over the emission wavelengths studied and between samples; only the relative contribution of these individual components vary. Application of the bound exciton theory reveals that these four resolved distributions centered at 30 ps, 300 ps, 2 ns and 6-8 ns correspond to the recombination of the electrons with four possible hole states with respective wavefunction radii of 4.0A,2.9A,2.0A eind 1.6A. The relative probabihty of each transition depends on the density of the four hole states within each sample. The higher carbon concentration samples have shorter overall decay times, lower band gap energy, broader bcind tails and, hence a higher density of the most localized hole states. The percent contribution of the shorter lifetime components increases with carbon concentration and are attributed to increasing structural disorder. The sample of 0.50 Ccirbon concentration hcis the best overall photoluminescence properties, thus, establishing the capability of the electron cyclotron system to produce hydrogenated amorphous sihcon carbon thin films of good quality.