262 Matching Results

Search Results

Advanced search parameters have been applied.

Metallic Nanodroplet Induced Coulomb Catalysis for Off-Resonant Plasmonic Enhancement of Photoemission in Semiconductors

Description: This article reports a new mechanism based on electrostatic interactions of carriers and their image charges in metals to generate more photons from optical sources at frequencies that are off-resonant to the localized plasmon frequency.
Date: April 8, 2016
Creator: Neogi, Arup; Grycznski, Karol G.; Llopis, A.; Lin, Jie; Main, Kyle; Shimada, Ryoko et al.
Partner: UNT College of Arts and Sciences

Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures

Description: III-V nitrides have been put to use in a variety of applications including laser diodes for modern DVD devices and for solid-state white lighting. Plasmonics has come to the foreground over the past decade as a means for increasing the internal quantum efficiency (IQE) of devices through resonant interaction with surface plasmons which exist at metal/dielectric interfaces. Increases in emission intensity of an order of magnitude have been previously reported using silver thin-films on InGaN/GaN MQWs. the dependence on resonant interaction between the plasmons and the light emitter limits the applications of plasmonics for light emission. This dissertation presents a new non-resonant mechanism based on electrostatic interaction of carriers with induced image charges in a nearby metallic nanoparticle. Enhancement similar in strength to that of plasmonics is observed, without the restrictions imposed upon resonant interactions. in this work we demonstrate several key features of this new interaction, including intensity-dependent saturation, increase in the radiative recombination lifetime, and strongly inhomogeneous light emission. We also present a model for the interaction based on the aforementioned image charge interactions. Also discussed are results of work done in the course of this research resulting in the development of a novel technique for strain measurement in light-emitting structures. This technique makes use of a spectral fitting model to extract information about electron-phonon interactions in the sample which can then be related to strain using theoretical modeling.
Date: May 2012
Creator: Llopis, Antonio
Partner: UNT Libraries

Synthesis and Characterization of Two and Three Coordinate Gold (I) Conjugated and Rigid Metallodendrimers

Description: This dissertation is a study of two major topics that involve synthetic strategies for new classes of phosphorescent gold(I)-based metallodendrimers. The phosphorescence of organic and inorganic luminophores originates from spin-orbit coupling owing to internal or external heavy atom effects as well as metal-centered emissions. Previous work in the Omary group entailed systematically designed small molecules, metallopolymers, and unconjugated metallodendrimers that contain d10 and d8 metals, whereas this dissertation aims in part to expand such strategies to the conjugated metallodendrimer regime. In one approach novel synthetic strategies were used to make first-generation phenyl acetylene dendrimers and phosphine derivatives thereof. The phosphine dendrimers are made by tethering one of the phosphines to an unsaturated dendrimer, as such phosphine dendrimers are better chromophores and luminophores due to their structural rigidity and extended conjugation. In another approach, 2- and 3-coordinate Au(I) dendritic complexes are synthesized from these phosphine dendrimers. This study is further extended to study metallodendritic complexes with different cores, for example triphenylene-based metallodendritic complexes with six acetylene branches. The physical properties of the metallodendrimers can be modulated upon proceeding to further dendrimer generations or by using solubilizing groups on the peripheral phosphines, thus allowing better processability for thin-film fabrication as required for molecular electronic devices and higher chance for crystal growth toward accurate structural characterization. Other data produced in this project suggested that some structural alterations led to porous solids that render them suitable for realized and potential applications in energy storage and carbon capture. The interesting luminescence properties of the metallodendrimers and porous extended solids produced in this dissertation are significant toward utilizing such materials for optoelectronic applications such as energy-saving organic light-emitting diodes and optical sensors for environmental pollutants.
Date: August 2012
Creator: Kaipa, Ushasree
Partner: UNT Libraries

Energy transfer in ZnO-anthracene hybrid structure

Description: This article investigates the origin of the modification of the emission properties of the hybrid structure using temperature dependent and time-dependent photoluminescence spectroscopy.
Date: April 4, 2012
Creator: Shimada, Ryoko; Urban, Ben E.; Sharma, Mamta; Singh, Akhilesh; Avrutin, Vitaliy; Morkoç, Hadis et al.
Partner: UNT College of Arts and Sciences

Laser Ablated Carbon Nanodots for Light Emission

Description: This article reports the synthesis of fluorescent carbon dots-like nanostructures obtained through the laser ablation of a carbon solid target in liquid environment is reported.
Date: September 22, 2016
Creator: Reyes, Delfino; Camacho, Marco; Camacho, Miguel; Mayorga, Miguel; Weathers, Duncan L.; Salamo, Greg et al.
Partner: UNT College of Arts and Sciences

Electrostatic Effects in III-V Semiconductor Based Metal-optical Nanostructures

Description: The modification of the band edge or emission energy of semiconductor quantum well light emitters due to image charge induced phenomenon is an emerging field of study. This effect observed in quantum well light emitters is critical for all metal-optics based light emitters including plasmonics, or nanometallic electrode based light emitters. This dissertation presents, for the first time, a systematic study of the image charge effect on semiconductor–metal systems. the necessity of introducing the image charge interactions is demonstrated by experiments and mathematical methods for semiconductor-metal image charge interactions are introduced and developed.
Date: May 2012
Creator: Gryczynski, Karol Grzegorz
Partner: UNT Libraries

Electric Field Enhanced Photoluminescence of CdTe Quantum Dots Encapsulated in Poly (N-Isopropylacrylamide) Nano-Spheres

Description: This article shows an electric field dependent collapse of the poly(N-isopropylacramide) nanospheres and employs the electric field dependence as a means to adjust the photoluminescence process of the quantum dots in a hybrid material.
Date: November 10, 2008
Creator: Garner, Brett W.; Cai, Tong; Hu, Zhibing & Neogi, Arup
Partner: UNT College of Arts and Sciences

Strain-Dependent Photoluminescence Behavior of CdSe/CdS Nanocrystals with Spherical, Linear, and Branched Topologies

Description: The photoluminescence of CdSe/CdS core/shell quantum dots, nanorods, and tetrapods is investigated as a function of applied hydrostatic and non-hydrostatic pressure. The optoelectronic properties of all three nanocrystal morphologies are affected by strain. Furthermore, it is demonstrated that the unique morphology of a tetrapod is highly sensitive to non-isotropic stress environments. Seeded tetrapods can thereby serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications ranging from sensitive optomechanical devices to investigations of biomechanical processes.
Date: August 13, 2009
Creator: Choi, Charina L.; Koski, Kristie J.; Sivasankar, Sanjeevi & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Mg-induced increase of bandgap in Zn1-xMgxO nanorods revealed by x-ray absorption and emission spectroscopy

Description: X-ray absorption near-edge structure (XANES) and x-ray emission spectroscopy (XES) measurements were used to investigate the effect of Mg doping in ZnO nanorods. The intensities of the features in the O K-edge XANES spectra of Zn{sub 1-x}Mg{sub x}O nanorods are lower than those of pure ZnO nanorods, suggesting that Mg doping increases the negative effective charge of O ions. XES and XANES spectra of O 2p states indicate that Mg doping raises (lowers) the conduction-band-minimum (valence-band-maximum) and increases the bandgap. The bandgap is found to increase linearly with the Mg content, as revealed by photoluminescence and combined XANES and XES measurements.
Date: July 10, 2008
Creator: Pong, Way-Faung; Chiou, J. W.; Tsai, H. M.; Pao, C. W.; Chien, F. Z.; Pong, W. F. et al.
Partner: UNT Libraries Government Documents Department

Nonlinear broadband photoluminescence of graphene induced by femtosecond laser irradiation

Description: Upon femtosecond laser irradiation, a bright, broadband photoluminescence is observed from graphene at frequencies well above the excitation frequency. Analyses show that it arises from radiative recombination of a broad distribution of nonequilibrium electrons and holes, generated by rapid scattering between photoexcited carriers within tens of femtoseconds after the optical excitation. Its highly unusual characteristics come from the unique electronic and structural properties of graphene.
Date: July 1, 2010
Creator: Liu, Wei-Tao; Wu, S.W.; Schuck, P.J.; Salmeron, Miquel; Shen, Y.R. & Wang, F.
Partner: UNT Libraries Government Documents Department

Nature of room-temperature photoluminescence in ZnO

Description: The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.
Date: November 11, 2004
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Yuan, H.B.; Xin, H.P. et al.
Partner: UNT Libraries Government Documents Department

X-ray absorption spectroscopy from H-passivated porous Si and oxidized Si nanocrystals

Description: Quantum confinement in nanoscale Si structures is widely believed to be responsible for the visible luminescence observed from anodically etched porous silicon (por-Si), but little is known about the actual size or shape of these structures. Extended x-ray absorption fine structure data from a wide variety of por-Si samples show significantly reduced average Si coordination numbers due to the sizable contribution of surface-coordinated H. (The IUSI ratios, as large as 1.2, were independently confirmed by ir-absorption and {alpha}-recoil measurements.) The Si coordinations imply very large surface/volume ratios, enabling the average Si structures to be identified as crystalline particles (not wires) whose dimensions are typically <15 {Angstrom}. Comparison of the size-dependent peak luminescence energies with those of oxidized Si nanocrystals, whose shapes are known, shows remarkable agreement. Furthermore, near-edge x-ray absorption fine structure measurements of the nanocrystals shows the outer oxide and interfacial suboxide layers to be constant over a wide range of nanocrystal sizes. The combination of these results effectively rules out surface species as being responsible for the observed visible luminescence in por-Si, and strongly supports quantum confinement as the dominant mechanism occurring in Si particles which are substantially smaller than previously reported or proposed.
Date: November 1, 1994
Creator: Schuppler, S.; Marcus, M.A. & Friedman, S.L.
Partner: UNT Libraries Government Documents Department

Optically detected magnetic resonance studies on {pi}-conjugate polymers and novel carbon allotropes

Description: This report describes the following: introduction to photoluminescence detected magnetic resonance (PLDMR); introduction to {pi}-conjugated systems; PLDMR measurements on poly(p-phenylene)-type ladder polymers; PLMDR measurements on poly(p-phenylene ethylene); and PLDMR measurements on C{sub 70}, polythiophene, poly(p-phenylene vinylene) and Dan-40. Appendices to this report describe: Operation of ODMR (optically detected magnetic resonance) spectrometer; ODMR system parameters; and Special purpose circuitry.
Date: February 12, 1999
Creator: Partee, J.
Partner: UNT Libraries Government Documents Department

Photoluminescence Investigations of InGaAsN Alloys Lattice-Matched to GaAs

Description: InGaAsN is a semiconductor alloy system with the property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In this paper, we have measured the conduction-band mass measurements by three different techniques for 2% nitrogen in InGaAsN lattice matched to GaAs. Additionally, we also report pressure dependent measurements of the conduction-band mass between ambient and 40 kbar. Based on our results, we suggest that the observed changes in masses are a result of {Lambda}-X mixing.
Date: June 1, 1999
Creator: Jones, E.D.; Modine, N.R.; Allerman, A.A.; Fritz, I.J.; Kurtz, S.R.; Wright, A.F. et al.
Partner: UNT Libraries Government Documents Department

Effects of chemical modifications on photophysics and exciton dynamics on {pi}-conjugation attenuated and metal-chelated photoconducting polymers

Description: Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylenevinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2{prime}-bipyridyl-5-vinylene units (bpy V) in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared to those of the homopolymer, poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in {pi}-conjugation throughout the polymer backbone. The attenuation in {pi}-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.
Date: March 11, 2000
Creator: Chen, L. X.; Jager, W. J. H.; Gosztola, D. J.; Niemczyk, M. P. & Wasielewski, M. R.
Partner: UNT Libraries Government Documents Department

Optical properties of colloidal germanium nanocrystals

Description: Highly crystalline germanium (Ge) nanocrystals in the size range 2--10 nm were grown in inverse micelles and purified and size-separated by high pressure liquid chromatography with on-line optical and electrical diagnostics. The nanocrystals retain the diamond structure of bulk Ge down to at least 2.0 nm (containing about 150 Ge atoms). The background- and impurity-free extinction and photoluminescence (PL) spectra of these nanocrystals revealed rich structure which was interpreted in terms of the bandstructure of Ge shifted to higher energies by quantum confinement. The shifts ranged from {minus}0.1 eV to over 1 eV for the various transitions. PL in the range 350--700 nm was observed from nanocrystals 2--5 nm in size. The 2.0 nm nanocrystals yielded the most intense PL (at 420 nm) which is believed to be intrinsic and attributed to direct recombination at {Gamma}. Excitation at high energy (250 nm) populates most of the conduction bands resulting in competing recombination channels and the observed broad PL spectra.
Date: May 1, 2000
Partner: UNT Libraries Government Documents Department

Universal bandgap bowing in group III nitride alloys

Description: The energy gaps of MBE-grown wurtzite-structure In{sub 1-x}Al{sub x}N alloys with x {le} 0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of {approx}0.8 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and AlGaN, these results show a universal relationship between the bandgap variations of group III nitride alloys and their compositions.
Date: August 6, 2002
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Ager III, J.W.; Li, S.X.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department

Raman spectroscopy and time-resolved photoluminescence of BN and BxCyNz nanotubes

Description: We report Raman and time-resolved photoluminescence spectroscopic studies of multiwalled BN and B{sub x}C{sub y}N{sub z} nanotubes. The Raman spectroscopy shows that the as-grown B{sub x}C{sub y}N{sub z} charge recombination, respectively. Comparison of the photoluminescence of BN nanotubes to that decay process is characterized by two time constants that are attributed to intra- and inter-BN sheet nanotubes as predicted by theory. nanotubes are radially phase separated into BN shells and carbon shells. The photoluminescence of hexagonal BN is consistent with the existence of a spatially indirect band gap in multi-walled BN.
Date: January 21, 2004
Creator: Wu, J.; Han, Wei-Qiang; Walukiewicz, W.; Ager III, J.W.; Shan, W.; Haller,E.E. et al.
Partner: UNT Libraries Government Documents Department

Effect of oxygen on the electronic band structure in ZnO{sub x}Se{sub 1-x} alloys

Description: The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnO{sub x}Se{sub 1-x} epitaxial films (0 {<=} x {<=} 1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.
Date: March 14, 2003
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Wu, J.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department

Comparison of Photoluminescence Imaging on Starting Multi-Crystalline Silicon Wafers to Finished Cell Performance: Preprint

Description: Photoluminescence (PL) imaging techniques can be applied to multicrystalline silicon wafers throughout the manufacturing process. Both band-to-band PL and defect-band emissions, which are longer-wavelength emissions from sub-bandgap transitions, are used to characterize wafer quality and defect content on starting multicrystalline silicon wafers and neighboring wafers processed at each step through completion of finished cells. Both PL imaging techniques spatially highlight defect regions that represent dislocations and defect clusters. The relative intensities of these imaged defect regions change with processing. Band-to-band PL on wafers in the later steps of processing shows good correlation to cell quality and performance. The defect band images show regions that change relative intensity through processing, and better correlation to cell efficiency and reverse-bias breakdown is more evident at the starting wafer stage as opposed to later process steps. We show that thermal processing in the 200 degrees - 400 degrees C range causes impurities to diffuse to different defect regions, changing their relative defect band emissions.
Date: June 1, 2012
Creator: Johnston, S.; Yan, F.; Dorn, D.; Zaunbrecher, K.; Al-Jassim, M.; Sidelkheir, O. et al.
Partner: UNT Libraries Government Documents Department