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Browsing by Author "Dolai, Sukanta"
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Item Investigation of Photophysical and Electrochemical Properties of Magic-Sized CdS Nanocrystals(Office of the Vice Chancellor for Research, 2013-04-05) Lawrence, Katie N.; Dolai, Sukanta; Irving, CharlesColloidal semiconductor nanocrystals (NCs) have been the interest of many studies over the past two decades due to their applications in device fabrication, electrocatalysts, and medical diagnostics. Recent discovery of thermodynamically stable ultra-small nanocrystals (“magic-sized”) has provided the opportunity to understand their different properties at the molecular level. Herein we present the synthesis and purification of poly(ethylene glycol) thiolate-capped magic-sized CdS nanocrystals with distinct photophysical properties. These CdS NCs overcame solubility restraints by directly transferring from aqueous to organic mediums and also showed significant increased in peak sharpness when analyzed by high-resolution MALDI-TOF-MS, which confirmed formation of (CdS)33,34 nanocrystals. The electrochemical properties of dissolved CdS nanocrystals were investigated in organic solvent/electrolyte medium by different voltammetric techniques. The nanocrystals displayed molecule-like HOMO-LUMO energy gap. The electrochemical features are strongly temperature, solvent, and capping-ligand thickness dependent. We also developed a working model of the energy level structure of the PEG-thiolate-capped (CdS)33,34 nanocrystals.Item Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors(ACS, 2015-01) Dolai, Sukanta; Dutta, Poulami; Muhoberac, Barry B.; Irving, Charles D.; Sardar, Rajesh; Department of Chemistry & Chemical Biology, IU School of ScienceWe have designed a new nonphosphinated reaction pathway, which includes synthesis of a new, highly reactive Se-bridged organic species (chalcogenide precursor), to produce bright white light-emitting ultrasmall CdSe nanocrystals of high quality under mild reaction conditions. The detailed characterization of structural properties of the selenium precursor through combined 77Se NMR and laser desorption ionization–mass spectrometry (LDI-MS) provided valuable insights into Se release and delineated the nanocrystal formation mechanism at the molecular level. The 1H NMR study showed that the rate of disappearance of Se precursor maintained a single-exponential decay with a rate constant of 2.3 × 10–4 s–1 at room temperature. Furthermore, the combination of LDI-MS and optical spectroscopy was used for the first time to deconvolute the formation mechanism of our bright white light-emitting nanocrystals, which demonstrated initial formation of a smaller key nanocrystal intermediate (CdSe)19. Application of thermal driving force for destabilization resulted in (CdSe)n nanocrystal generation with n = 29–36 through continuous dissolution and addition of monomer onto existing nanocrystals while maintaining a living-polymerization type growth mode. Importantly, our ultrasmall CdSe nanocrystals displayed an unprecedentedly large fluorescence quantum yield of ∼27% for this size regime (<2.0 nm diameter). These mixed oleylamine and cadmium benzoate ligand-coated CdSe nanocrystals showed a fluorescence lifetime of ∼90 ns, a significantly large value for such small nanocrystals, which was due to delocalization of the exciton wave function into the ligand monolayer. We believe our findings will be relevant to formation of other metal chalcogenide nanocrystals through expansion of the understanding and manipulation of surface ligand chemistry, which together will allow the preparation of “artificial solids” with high charge conductivity and mobility for advanced solid-state device applications.Item Size-Dependent Optical and Electrochemical Energy Gaps Comparison of CdSe Nanolusters(Office of the Vice Chancellor for Research, 2013-04-05) Teunis, Meghan B.; Lawrence, Katie N.; Dolai, SukantaThe size-dependent optical and electronic properties of semiconductor nanocrystals have made them the focus of much research including the designing of photovoltaic devices and photocatalysts. These properties occur as a result of the phenomenon called quantum confinement. To improve the device efficiency it is important to have a better understanding of their size dependent electrochemical properties. Herein we demonstrate for the first time, a comparison of the size dependent optical properties and electrochemical energy gaps of poly(ethylene glycol) thiolate-protected ultra-small CdSe nanoclusters. The electrochemical energy gaps for various sized nanoclusters were determined from cyclic and differential pulse voltammetry in organic solvent/electrolyte medium, where large, moleculelike HOMO-LUMO energy gaps were observed. It was also found that a significant amount of charging energy is involved in the electrochemical energy gap. The effect of the thickness of the surface-pasivating ligands on the HOMO-LUMO energy gap is demonstrated and a quantized double layer (QDL) charging model presented.Item Solvent-like ligand-coated ultrasmall cadmium selenide nanocrystals: Strong electronic coupling in a self-organized assembly(RSC, 2015-07) Lawrence, Katie N.; Johnson, Merrell A.; Dolai, Sukanta; Kumbhar, Amar; Sardar, Rajesh; Department of Chemistry & Chemical Biology, School of ScienceStrong inter-nanocrystal electronic coupling is a prerequisite for delocalization of exciton wave functions and high conductivity. We report 170 meV electronic coupling energy of short chain poly(ethylene glycol) thiolate-coated ultrasmall (<2.5 nm in diameter) CdSe semiconductor nanocrystals (SNCs) in solution. Cryo-transmission electron microscopy analysis showed the formation of a pearl-necklace assembly of nanocrystals in solution with regular inter-nanocrystal spacing. The electronic coupling was studied as a function of CdSe nanocrystal size where the smallest nanocrystals exhibited the largest coupling energy. The electronic coupling in spin-cast thin-film (<200 nm in thickness) of poly(ethylene glycol) thiolate-coated CdSe SNCs was studied as a function of annealing temperature, where an unprecedentedly large, ∼400 meV coupling energy was observed for 1.6 nm diameter SNCs, which were coated with a thin layer of poly(ethylene glycol) thiolates. Small-angle X-ray scattering measurements showed that CdSe SNCs maintained an order array inside the films. The strong electronic coupling of SNCs in a self-organized film could facilitate the large-scale production of highly efficient electronic materials for advanced optoelectronic device application.Item Synthesis of PEG-Thiolate Monolayer Protected CdSe Nanoclusters with Unique Solubility Properties(Office of the Vice Chancellor for Research, 2012-04-13) Lawrence, Katie N.; Dolai, Sukanta; Sardar, RajeshLigands protected metal chalcogenides have shown potential applications in bionanotechnology and device fabrication due to their unique optical properties. However, most metal chalcogenides suffer from solubility problems, which hinders their applications. To overcome the solubility issue of metal chalcogenide nanoclusters, we have demonstrated the aqueous phase synthesis of polyethylene glycol thiolate (PEG-S-) protected CdSe nanoclusters for the first time. The CdSe nanoclusters displayed a first absorption peak ~430 nm, which indicated formation of magic-sized nanoclusters with possible composition of (CdSe)33,34. The PEG-thiolate protected CdSe nanoclusters demonstrated unique solubility properties. The resulting nanoclusters can easily be transferred to organic solvents from an aqueous medium by a simple solvent extraction method. The organic-phase extracted CdSe nanoclusters can readily be redispersed in a wide array of organic solvents such as CH3CN, CH2Cl2, DMF, THF, and CH3Cl. Most importantly, the CdSe nanoclusters, soluble in organic solvents, can also be redispersed in aqueous medium as well. We investigated different chain length PEGn-thiols, e.g., PEG4-SH, PEG6-SH, PEG12-SH, and PEG18-SH and found that the PEG-chain length significantly influenced the aqueous to organic phase transfer properties. Successful transfers were accomplished for PEGn-SH (n = 6, 12, 18). Future studies will be performed on the synthesis of PEG-SH stabilized various metal chalcogenide nanoclusters (CdS, CdTe, ZnS, ZnSe, and CdSe/ZnS nanoclusters).Item Unraveling the Mechanism Underlying Surface Ligand Passivation of Colloidal Semiconductor Nanocrystals: A Route for Preparing Advanced Hybrid Nanomaterials(ACS, 2017-09) Teunis, Meghan B.; Liyanage, Thakshila; Dolai, Sukanta; Muhoberac, Barry B.; Sardar, Rajesh; Agarwal, Mangilal; Chemistry and Chemical Biology, School of ScienceOptically bright colloidal semiconductor nanocrystals (CSNCs) are important nanomaterials because of their potential applications such as cellular imaging and solid-state lighting. The optoelectronic properties of CSNCs are strongly controlled by the chemical nature of the surface passivating ligands that are introduced during their synthesis. However, the existing LaMer growth model does not provide a clear understanding of the stage when ligands become attached onto the CSNC surface. Herein, apart from the three stage formation mechanism of CSNCs (supersaturation, nucleation, and growth), an entirely new stage—solely involving surface ligand attachment onto fully grown CSNCs—is now reported that controls their photoluminescence properties. Furthermore, we also demonstrate a fundamentally new surface modification approach using partially passivated CSNCs to introduce a variety of functional groups (azide, alkene, and siloxane), including photoisomerizable molecular machines (e.g., azobenzene), without the use of “state-of-the art” ligand exchange chemistry. Knowledge of the ligand adsorption phenomena and resulting adsorption time dependence expands our fundamental understanding of structure–property relationships while allowing us to engineer novel hybrid functional nanomaterials with both previously unknown optoelectronic properties and supermolecular assembly options for various applications.