功能纳米与软物质研究院学术报告:Electronic materials interfaces: Role of defects and dimensionality

报告人:Prof. Norbert Koch (Institut für Physik & IRIS Adlershof, Humboldt-Universitat zu Berlin, 12489 Berlin, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany; norbert.koch@physik.hu-berlin.de )

报告题目:Electronic materials interfaces: Role of defects and dimensionality

报告时间39日上午10:00

报告地点:909-B

 

摘要

The alignment of energy levels at interfaces within electronic and optoelectronic devices determines their function and efficiency. Consistent guidelines for predicting the level alignment for a given material combination and methods to optimize it are needed to facilitate efficient engineering approaches. These are sufficiently understood for established electronic materials, but for emerging materials, this is work in progress. To modify the level alignment at interfaces, the control of the local charge density distribution is a most versatile approach. Molecular electron acceptors and donors are potent agents that enable tuning the energy levels at interfaces with electronic materials. Example show that the work function of electrodes can be extended to ultra-low and ultra-high values and that tuning of the energy level alignment at hybrid organic/inorganic semiconductor heterojunctions becomes possible. Particularly relevant for perovskites is the ability of molecular agents to reduce the density of surface states, which are detrimental for efficient photovoltaic cells.

Two dimensional (2D) transition metal dichalcogenides (TMDCs) stand out by their strong light-matter coupling, particularly in the form of a single layer with a direct band gap. However, these materials are strongly excitonic, with the exciton binding energy reaching magnitudes known for organic semiconductors. While the exciton binding energies for MoS2 and WSe2, determined by angle-resolved direct/inverse photoelectron spectroscopy at the K-point and reflectivity measurements, vary notably for different dielectric environments (i.e., insulator vs. metal substrate), the exciton transition energies change significantly less, due to different screening of single charges as compared to correlated electron-hole pairs. The electronic properties of a one-dimensional (1D) intrinsic/p-type WSe2 junction are investigated at an interface between bare and molecular acceptor-covered areas, allowing to retrieve the intrinsic Thomas-Fermi screening lengths of the semiconductor on the nm scale. By comparing measurements with high spatial resolution (scanning tunneling spectroscopy) and in an area-averaging mode (photoemission spectroscopy), one can assess a reliable quantitative understanding of the electronic structure of these complex structures.

 


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