Single-nanostructure Bandgap Modulation And Nanophotonic Devices

The unique physical and chemical properties of nanostructures are not only helpfulfor miniaturization and integration of semiconductor devices but also possesspromising applications in novel nano-optoelectronic components. Bandgap is one ofthe most important intrinsic parameters for semiconductor-based optoelectronicdevices and it fundamentally decides the basic spectral-response feature of the devices.Unfortunately, it is well know that the bandgaps available in nature semiconductorsconsist of a very finite discrete set. In order to address this limitation, bandgapmodulation was developed by viarous means, like alloy structures and quantumconfinement effect, which significantly enriches the available bandgaps ofsemiconductors, even integration of a series of graded bandgaps along single substrate.Bandgap modulation within single-nanostructures is increasingly important for therapid development of nanotechnology considering the facts that it can enrich bandgapsdown to nanometer scale and provide brand new material platform for R&D ofnano-optoelectronic components.With these ideas in mind, in this dissertation, we well designed and improved thetraditional thermal evaporation growth method of nanostructures and accordinglyvarious special nanostructures were fabricated. Ultimately, bandgap modulation wassuccessfully achieved within single-nanostructures via variation of alloy composition.More importantly, several kinds of novel proof-of-concept nanophotonic devices wereconstructed using these special nanostructures and their performances were evaluated.The main achievements are summarized as follows.(1) We firstly succeeded in exchange of solid-state source reagents and movement ofcollection substrate during the growth of nanostructures at high temperature byintegrating pulling system of magnetic force into traditional thermal evaporationgrowth system. Based on these improved-strategy, CdSxSe1-x(x=0~1) andZnxCd1-xSySe1-y(x, y=0~1) nanowires with a completely compositional gradient alongaxial direction of the nanowire were fabricated, and accordingly graded bandgapmodulation along axial direction of single-nanowire was achieved. Both of them arehigh-quality integrated nanoscale light sources with abundant wavelengths. Especiallyfor those ZnxCd1-xSySe1-ynanowires, since their emission wavelengths cover almost theentire visible spectrum region and their nanometer scale footprints are beyond theresolution of human naked eyes, they can be directly used as high-quality nanoscale white light source.(2) We theoretically predicted the asymmetric light propagation alongcompositionally-graded semiconductor nanowires and further demonstratedasymmetric light propagation in semiconductor nanowires features simple structure,low cost, tunable work wavelength, ultra-low operation power, and nanoscale footprintbased on compositionally-graded CdSxSe1-xnanowires. Our results show that thepropagation optical loss along compositionally-graded semiconductor nanowires ishighly dependent on the guided direction. The propagation loss alongbandgap-increasing direction is dominated by the disorder-induced band-tailabsorption and non-perfect geometry-caused leakage. In comparison, the total lossalong bandgap-decreasing direction would be significantly increased, coming from thehuge nonradiative loss involved in each band-to-band re-absorption and re-emittingprocess, except for the same propagation loss along the opposite direction. It is thishuge propagation direction-dependent loss contrast that provides the physical base ofthe asymmetric light propagation along semiconductor nanowires.(3) We comprehensively investigated the optical waveguide in compositionally-graded CdSxSe1-xnanowires and presented the first wavelength conversion waveguidealong the bandgap-decreasing direction of these nanowires. Study revealed that whenthe narrow bandgap end of the wire is excited with a focused laser beam, the nanowireitself acts as a passive cavity for the emitting light of the nanowire, hence it can beguided passively along the bandgap-increasing direction of the nanowire by totalinternal reflection, keeping the photonic energy of the guided light almost unchangedduring the whole propagation. In comparison, when the wide bandgap end of thenanowire is excited, the nanowire itself acts as an active cavity for the emitting light ofthe nanowire, and it can be guided actively through incessantly repeated band-to-bandre-absorption and re-emitting processes along the bandgap-decreasing direction,resulting in a gradual wavelength conversion during propagation. In this case, thenanowire can work as optical wavelength converter. On the basis of thiswavelength-converted waveguide, a concept of nanoscale wavelength splitter isdemonstrated by assembling a graded nanowire with several compositionally-uniformnanowires into branched nanowire structure.(4) We reported the first controllable growth of sandwitch-like nanoribbon lateralheterostructures made of a CdSxSe1-xcentral region and two epitaxial CdS lateral sidesusing a multistep thermal evaporation route with moving sources, accordingly abruptbandgap modulation along the width direction of single-nanoribbons was achieved. Under excitation of an ultrviolet laser, the emission of these ribbons indicates finesandwich-like structures along width direction, with characteristic red emission in thecenter and green emission at both edges. Under pump of a pulse laser, a roomtemperature dual-color lasing with tunable wavelengths was demonstrated based onthese single-nanoribbon heterostructures for the first time. Moreover, the wavelengthspacing of this dual-color lasing is adjustable by viaring the composition of the centralCdSxSe1-xalloy.(5) We successfully controlled the doping concentration of Sn in CdS nanowiresby the improved thermal evaporation route, and accordingly high-quality dilutetin-doped CdS nanowires with controlled trap-state emission were synthesized. Usingthese nanowires, the active guiding loss of CdS nanowires was significantly reducedand the guiding efficiency was considerably increased. In addition, the bandedgeemission of CdS will be totally absorbed after a certain transport distance due to theexistence of trap-state emission, which could be discounted as a bandedge emissionfilter. This study is the first example of using dopants to shift emission away fromsemiconductor bandgap absorption for the purpose of enhanced waveguide. Moreimportantly, low threshold trap-state whispering gellory modes semiconductornanolaser with work wavelength spanning from540nm to750nm was reported usingthese high-quality doping nanowires for the first time.