Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
Control of the energy level alignment in semiconductor heterostructures determines localization of carriers within the structure, and is required for many optoelectronic devices. In heterostructures composed of bulk semiconductors, the energy difference between the bands is fixed for a given material combination. Instead, in semiconductor Quantum Dots (QDs) the position of the energy levels depends on the QD size, which can be controlled and tuned during synthesis1, as well as on the surface composition2-3. Thus there are multiple handles for controlling the energy-level alignment between two QD materials.
We report evidence of PbSe-CdSe QD heterostructure films showing a type-II band-alignment, obtained through size tuning and ligand-exchange. Heterostructure QD films were fabricated via layer-by-layer deposition and ligand exchange of alternating PbSe and CdSe QD layers, allowing functionalization of the two material components with different ligands. Ligand treatments are selected from the ligand library characterized by Brown et al.,2with particular focus on the combination with the highest predicted shift between the two materials (tetrabutylammonium chloride on CdSe QDs, benzenethiol on PbSe QDs).
Spectroelectrochemical measurements are performed, showing the relative energy position of the conduction bands of the two QDs materials can be inverted by suitable ligand treatments, while transient absorption measurements reveal the direction of electron transfer across the heterojunction. This demonstrates the possibility to switch the energy-alignment of a heterostructure between type-I and type-II via the use of appropriate ligand-treatments, thus increasing the set of material combinations available to achieve the desired energy-alignment in QD devices.
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