In-situ nanochemistry for optoelectronics
Kim, Won Jin
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This thesis describes recent results on simple methods to arrange nanosize objects such as semiconductor nanocrystals, noble metal nanoparticles, and upconversion nanophosphors by means of top-down processes. Specific focus is directed towards approaches to produce predefined patterns of various nanostructure materials using optical lithography for direct writing of films for optoelectronic and electronic devices. To obtain photo-patternability, the nanostructure materials [for example semiconductor nanocrystals (CdSe, CdTe, PbSe), metallic nanoparticles (Ag), upconversion nanophosphors (Er 3+ /Yb 3+ or Tm 3+ /Yb 3+ co-doped NaYF 4 ), and transparent conducting oxide nanoparticles (ITO, ZnO)] were functionalized by incorporation of the functional ligand t -butoxycarbonyl ( t -BOC) which has an acid-labile moiety. The t -BOC group undergoes a cleavage, when subjected to UV irradiation in the presence of a photo acid generator (PAG) to releases isobutene and carbon dioxide. Depending on the need of the application, either the exposed regions (negative pattern) or the non-exposed regions (positive pattern) could be developed from the exposed films by appropriate solvent selection. The photo exposed regions of the film are rendered hydrophilic due to the degradation of the t -BOC, the un-exposed regions remain hydrophobic. This solubility change in the QDs is the basis of their patternablity. The un-exposed regions can be removed to obtain the negative pattern by washing with hydrophobic solvents, whereas the exposed regions can be selectively removed to obtain positive pattern by washing with hydrophilic solvents. This change in the surface chemistry results in the ability to photo-pattern the various nanostructure materials where desired for a number of optoelectronic device geometries. We demonstrate that the ultimate resolution (linewidth and spacing) of this technique is below submicron. Details on technological aspects concerning nanoparticle patterning as well as practical examples in photolithgrapy are given. Furthermore, practical applications in optoelectronic devices and technological features of the resulting optoelectronic devices are provided. The types of devices studied involve photodetectors, solar cell devices, field effect transistors, security devices, and photorefractive devices.