29 research outputs found
Exploring condensed phases in engineered semiconducting nanocrystals
No full textThe Jain lab employs a topotactic method called cation exchange to produce semiconductor nanocrystals (NCs) in novel morphologies, compositions, and crystallographic phases. My dissertation research focuses on the understanding of the physical properties and phase transitions of these new nanomaterials prepared by cation exchange. In Chapter 1, I describe the countless possibilities of the exploration of physicochemical properties and applications of molecularly precise semiconductor nanoclusters, a class of materials that we were able to expand with the help of cation exchange. In Chapter 2, I discuss how ultrasmall copper selenide (Cu2-xSe) NCs prepared by cation exchange of cadmium selenide NCs exhibit a disordered cationic sub-lattice under ambient conditions. This behavior is quite unlike larger NCs or the bulk, suggesting an interesting effect of crystallite size and strain on the stability of super-ionic phases. In Chapter 3, I describe my investigations of Li-doping of Cu2-xSe NCs and how this doping influences the crystal structure and consequently the phase transition behavior. A close-to-ambient-temperature transition from the non-superionic to superionic phase transition also appears to be present in the final lithium selenide (Li2Se) NCs formed from this doping reaction. In Chapter 4, I explain on the basis of optical spectra measurements and density functional theory (DFT) calculations how HgSe NCs, prepared using cation exchange in a novel wurtzite phase, differ from their natural zinc-blende counterparts. The latter is a semi-metal, whereas the newer phase obtained from cation exchange is found to have an inverted band structure along with a finite band-gap, making it a potential 3D topological insulator. In Chapter 5, I extend the understanding of ion exchange reactions to an “anion exchange” process in zinc oxide (ZnO) NCs. As a detour from the central thesis of my dissertation, in Chapter 6, I present my work on electrodynamic simulations of optical properties of nanostructures, which helped demonstrate that localized surface plasmons can be imaged in real space with nanometer resolution using a scanning tunneling microscope (STM) coupled to a laser.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2020-12-01The student, Progna Banerjee, accepted the attached license on 2018-11-23 at 13:07.The student, Progna Banerjee, submitted this Dissertation for approval on 2018-11-23 at 13:29.This Dissertation was approved for publication on 2018-11-27 at 08:38.DSpace SAF Submission Ingestion Package generated from Vireo submission #13088 on 2019-02-08 at 11:38:51Made available in DSpace on 2019-02-08T18:39:46Z (GMT). No. of bitstreams: 4
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Previous issue date: 2018-11-27Embargo set by: Seth Robbins for item 109937
Lift date: 2021-02-08T18:40:00Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 109937
Lift date: 2021-02-08T18:42:23Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 109937
Lift date: 2021-02-08T18:43:54Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 109937
Lift date: 2021-02-08T18:44:50Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemLimited Restriction Lifted for Item 109937 on 2021-02-09T10:15:34Z
Exploring condensed phases in engineered semiconducting nanocrystals
No full textThe Jain lab employs a topotactic method called cation exchange to produce semiconductor nanocrystals (NCs) in novel morphologies, compositions, and crystallographic phases. My dissertation research focuses on the understanding of the physical properties and phase transitions of these new nanomaterials prepared by cation exchange. In Chapter 1, I describe the countless possibilities of the exploration of physicochemical properties and applications of molecularly precise semiconductor nanoclusters, a class of materials that we were able to expand with the help of cation exchange. In Chapter 2, I discuss how ultrasmall copper selenide (Cu2-xSe) NCs prepared by cation exchange of cadmium selenide NCs exhibit a disordered cationic sub-lattice under ambient conditions. This behavior is quite unlike larger NCs or the bulk, suggesting an interesting effect of crystallite size and strain on the stability of super-ionic phases. In Chapter 3, I describe my investigations of Li-doping of Cu2-xSe NCs and how this doping influences the crystal structure and consequently the phase transition behavior. A close-to-ambient-temperature transition from the non-superionic to superionic phase transition also appears to be present in the final lithium selenide (Li2Se) NCs formed from this doping reaction. In Chapter 4, I explain on the basis of optical spectra measurements and density functional theory (DFT) calculations how HgSe NCs, prepared using cation exchange in a novel wurtzite phase, differ from their natural zinc-blende counterparts. The latter is a semi-metal, whereas the newer phase obtained from cation exchange is found to have an inverted band structure along with a finite band-gap, making it a potential 3D topological insulator. In Chapter 5, I extend the understanding of ion exchange reactions to an “anion exchange” process in zinc oxide (ZnO) NCs. As a detour from the central thesis of my dissertation, in Chapter 6, I present my work on electrodynamic simulations of optical properties of nanostructures, which helped demonstrate that localized surface plasmons can be imaged in real space with nanometer resolution using a scanning tunneling microscope (STM) coupled to a laser.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste
Liquid-like cationic sub-lattice in copper selenide clusters
No full textAbstractSuper-ionic solids, which exhibit ion mobilities as high as those in liquids or molten salts, have been employed as solid-state electrolytes in batteries, improved thermoelectrics and fast-ion conductors in super-capacitors and fuel cells. Fast-ion transport in many of these solids is supported by a disordered, ‘liquid-like’ sub-lattice of cations mobile within a rigid anionic sub-lattice, often achieved at high temperatures or pressures via a phase transition. Here we show that ultrasmall clusters of copper selenide exhibit a disordered cationic sub-lattice under ambient conditions unlike larger nanocrystals, where Cu+ ions and vacancies form an ordered super-structure similar to the bulk solid. The clusters exhibit an unusual cationic sub-lattice arrangement wherein octahedral sites, which serve as bridges for cation migration, are stabilized by compressive strain. The room-temperature liquid-like nature of the Cu+ sub-lattice combined with the actively tunable plasmonic properties of the Cu2Se clusters make them suitable as fast electro-optic switches.</jats:p
In-situ electron microscopy mapping of an order-disorder transition in a superionic conductor
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In-situ electron microscopy mapping of an order-disorder transition in a superionic conductor
No full textSolid-solid phase transitions are processes ripe for the discovery of correlated atomic motion in crystals. Here, we monitor an order-disorder transition in real-time in nanoparticles of the super-ionic solid, Cu2−xSe. The use of in-situ high-resolution transmission electron microscopy allows the spatiotemporal evolution of the phase transition within a single nanoparticle to be monitored at the atomic level. The high spatial resolution reveals that cation disorder is nucleated at low co-ordination, high energy sites of the nanoparticle where cationic vacancy layers intersect with surface facets. Time-dependent evolution of the reciprocal lattice of individual nanoparticles shows that the initiation of cation disorder is accompanied by a ~3% compression of the anionic lattice, establishing a correlation between these two structural features of the lattice. The spatiotemporal insights gained here advance understanding of order-disorder transitions, ionic structure and transport, and the role of nanoparticle surfaces in phase transitions
Optical anisotropy of CsPbBr3 perovskite nanoplatelets
No full textAbstract The two-dimensional CsPbBr3 nanoplatelets have a quantum well electronic structure with a band gap tunable with sample thicknesses in discreet steps based upon the number of monolayers. The polarized optical properties of CsPbBr3 nanoplatelets are studied using fluorescence anisotropy and polarized transient absorption spectroscopies. Polarized spectroscopy shows that they have absorption and emission transitions which are strongly plane-polarized. In particular, photoluminescence excitation and transient absorption measurements reveal a band-edge polarization approaching 0.1, the limit of isotropic two-dimensional ensembles. The degree of anisotropy is found to depend on the thickness of the nanoplatelets: multiple measurements show a progressive decrease in optical anisotropy from 2 to 5 monolayer thick nanoplatelets. In turn, larger cuboidal CsPbBr3 nanocrystals, are found to have consistently positive anisotropy which may be attributed to symmetry breaking from ideal perovskite cubes. Optical measurements of anisotropy are described with respect to the theoretical framework developed to describe exciton fine structure in these materials. The observed planar absorption and emission are close to predicted values at thinner nanoplatelet sizes and follow the predicted trend in anisotropy with thickness, but with larger anisotropy than theoretical predictions. Dominant planar emission, albeit confined to the thinnest nanoplatelets, is a valuable attribute for enhanced efficiency of light-emitting devices
