Complete Report - University of New South Wales

Dispersion curves for InAs QDs ina simple hexagonal SL. The matrixis In 0.5Ga 0.5-xAl xAs with (x = 0.4).There is a large phonon bandgappresent in the system due to thepresence of Al. The gap seemsvery tolerant to Al location. (Farleft): , (Centre): , (Left): directions.Figure 4.5.58Densities of states (DOS) for thedispersion curve immediately to theleft. Note small changes in the DOScould be due to pseudo-randomlocations of the Ga and Al particlesor to differences in crystallinedirection. (Far left): ,(Centre): , (Left): directions respectively.Figure 4.5.59of each atomic mass) was very similar as the lightelement dominates in this case.On this iteration structures with an In 0.5Ga 0.5-xAl xAsmatrix (with x=0.4) and InAs QDs were grown.Significant Al content was introduced into thesestructures with the expectation that this lightelement, segregated to the matrix material,might produce appreciable phonon bandgaps. Aschematic of the structure is shown in Fig. 4.5.56.Some images derived from characterisation ofthe structure are presented in Fig. 4.5.57. Thesuperlattice of QDs has a simple hexagonalstructure. Extraordinary periodic out-of-planestacking is achievable and largely defect freestructures can be grown on the order of microns.Force constant modelling of this structure predictsan appreciable phonon bandgap, as shown inFig. 4.5.58 and Fig. 4.5.59. This bandgap is duealmost entirely to the presence of the Al in thesystem. A small bandgap is present due to themass difference between In, Ga and As, but it isless than a quarter of the size shown in Fig. 4.5.58and Fig. 4.5.59. Due to computational constraintsthe size of the QDs is quite small, only about ananometre in diameter. While actual sizes for thesestructures are too computationally intensive tomodel without extreme effort, recent modellingwith gradually increasing size suggests that thedispersion relations scale linearly with size. Thatis, once the discrete distances (bond lengths) aresmall relative to the superlattice unit cell dimension,the dispersion should look exactly the samewhen scaled such that the relative dimensions arepreserved. This will be investigated in greater detailin further work.4.5.3.3.3.2 Fabrication andcharacterisation of highly orderednanoparticle arrays for HotCarrier absorberResearchers:Lara Treiber, Shujuan Huang,Gavin ConibeerAs demonstrated by the work onmodelling phonon modulation,periodic core-shell QD arrays offera way to significantly change thephonon modulation in a superlatticebecause the core and shell can be of materials ofvery different force constant, directly leading to astrong phonon confinement. Deposition methodsare being investigated to fabricate such highlyordered QD arrays.The Langmuir-Blodgett (LB) technique fabricatesthin films from colloidal dispersions of quantumdots (QDs). The LB technique allows for controllable,uniform film formation and subsequent transferonto a solid substrate, such as quartz [4.5.69].Through surface passivation of the QDs,interparticle spacing can also be controlled. Figure4.5.60 illustrates the set-up of the LB apparatus andthe functionalized QDs.In 2010 work has focused on fabrication of periodicthin films of Si nanoparticles (NPs) provided bythe University of Minnesota. The Si NPs underinvestigation are surface passivated with dodecane(C 12H 26), a long chain hydrocarbon rendering thesurface hydrophobic. The particles were dispersedin chloroform and investigated using TEM, UV-visspectroscopy and photoluminescence (PL). UsingTEM imaging the average diameter was found to be3.0 ±0.5 nm. Fig. 4.5.61 shows a droplet of solution(dispersed in chloroform) and the inset highlightsthe lattice fringes confirming the presence of silicon- a 3.14 Å lattice spacing corresponding to the{111} planes.Langmuir-Blodgett set-up forfilm fabrication with surfacefunctionalized Si NPs.Figure 4.5.60(Upper) TEM images of Si NPs driedon a grid. (Lower) TEM image ofSi NP monolayer deposited at 50mN/m of SP.Figure 4.5.6189