TY - THES

T1 - On the chemical and crystalline structure of colloidal CdSe/CdS core-shell nanocrystals

AU - Ludescher, Lukas

N1 - embargoed until null

PY - 2016

Y1 - 2016

N2 - Cdse/cds core-shell nanocrystals are, at the time this thesis is being written, one of the most researched systems in the field of semiconducting nanocrystals. These kind of materials have the big advantage of highly tuneable physical properties by varying parameters such as the size of the particles or the changing the relation between radius of by core and shell-thickness. Because these parameters are all adjusted through the synthesis of the particles, it is very important to understand the influence of the process itself on the particle resulting geometry, crystal structure and phase composition. This is why we used X-ray anomalous small angle scattering and wide angle scattering (ASAXS/WAXS) to retrieve information about these properties from samples synthesised by our partners from the ETH Zürich using a novel approach. This approach is based on the so called “hot injection” methods, in which the growth is stimulated by increased temperatures around 300°C. The particles investigated exhibit Wurtzite structure and are expected to yield higher optical performance than traditionally synthesised batches via a technique called cation exchange at lower temperatures. The investigated samples are all taken during the “hot injection” synthesis of the shell on three different core sizes at different times, hence exhibiting different relations between core size and shell thickness. It becomes apparent that with increasing core diameter, the nanocrystals exhibit an increasingly elliptical shape. This result is retrieved from the peak width for select Bragg peaks and from the solutions via shape retrieval of the SAXS data. From the ASAXS analysis, no evidence for interdiffusion between core and shell has been extracted from the anomalous scattering data, hence a sharp interface between core and shell is implied. In addition, we could detect an increasing amount of Zincblende crystal phase present within the core, which so far cannot be directly related to the elliptical shape. We propose a simple mechanism of martensitic phase transformation between the original Wurtzite and the Zincblende structure, which is due to condensation of planar growth faults in the particles. All of these findings may help to explain differences in the optical performance of the particles measured by our partners at the ETH and help further design of future material systems for colloidal core-shell nanocrystals.

AB - Cdse/cds core-shell nanocrystals are, at the time this thesis is being written, one of the most researched systems in the field of semiconducting nanocrystals. These kind of materials have the big advantage of highly tuneable physical properties by varying parameters such as the size of the particles or the changing the relation between radius of by core and shell-thickness. Because these parameters are all adjusted through the synthesis of the particles, it is very important to understand the influence of the process itself on the particle resulting geometry, crystal structure and phase composition. This is why we used X-ray anomalous small angle scattering and wide angle scattering (ASAXS/WAXS) to retrieve information about these properties from samples synthesised by our partners from the ETH Zürich using a novel approach. This approach is based on the so called “hot injection” methods, in which the growth is stimulated by increased temperatures around 300°C. The particles investigated exhibit Wurtzite structure and are expected to yield higher optical performance than traditionally synthesised batches via a technique called cation exchange at lower temperatures. The investigated samples are all taken during the “hot injection” synthesis of the shell on three different core sizes at different times, hence exhibiting different relations between core size and shell thickness. It becomes apparent that with increasing core diameter, the nanocrystals exhibit an increasingly elliptical shape. This result is retrieved from the peak width for select Bragg peaks and from the solutions via shape retrieval of the SAXS data. From the ASAXS analysis, no evidence for interdiffusion between core and shell has been extracted from the anomalous scattering data, hence a sharp interface between core and shell is implied. In addition, we could detect an increasing amount of Zincblende crystal phase present within the core, which so far cannot be directly related to the elliptical shape. We propose a simple mechanism of martensitic phase transformation between the original Wurtzite and the Zincblende structure, which is due to condensation of planar growth faults in the particles. All of these findings may help to explain differences in the optical performance of the particles measured by our partners at the ETH and help further design of future material systems for colloidal core-shell nanocrystals.

KW - anomale-streuung

KW - nanokristalle

KW - saxs

KW - waxs

KW - anomalous-scattering

KW - nanocrystals

KW - semiconductor

KW - saxs

KW - waxs

M3 - Diploma Thesis

ER -