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PhD Defense: Optoelectronic properties in heterostructures of 2-dimensional materials

Friday, July 20, 2018 - 16:00
CFM Auditorium
Nieves Morquillas, Nanoimaging Group, nanoGUNE
Source Name: 

Transition metal dichalcogenides (TMD) MX2 (M=Mo, W, Nb; X= S, Se, Te) are promising materials for optoelectronic applications due to their exceptionally tunable properties. Monolayers (1L) of some TMDs present different optical and electrical properties than bulk. In the single-layer limit, MoS2 and WSe2 behave as a direct-gap-like semiconductor, whereas bulk MoS2 and WSe2 are indirect-gap-like semiconductor. This difference, which comes from a change of the electronic band structure with thickness, gives to higher photoluminescence quantum efficiency in the single-layer regime. 
Another interesting property of these materials appears as a consequence of the inversion symmetry breaking for an odd number of layers. The broken symmetry and the strong spin orbit coupling (SOC) in the single layer limit make the gaps in K and K′ point distinguishable. Therefore, the electrons are described not only by their charge and spin but also by the extra degree of freedom called "valley". This gave raise to a new field with applications in information encoding and processing, called "valleytronics". Most of the research lines efforts are devoted to obtain valleytronic devices able to produce a current coming exclusively from one of the valleys. 
The properties of single layer TMDCs can be also highly affected by the interaction with the substrate where they are deposited. Here, we study the optical and electrical properties of 1L-MoS2 and 1L-WSe2 on top of a superconductor and an insulating ferrimagnetic material respectively. In the first case, we aim to study the properties of a superconductor semiconductor junction. Our structures are formed by 1L-MoS2 deposited on top of a bulk like NbSe2, all in a SiO2 substrate. NbSe2 is a superconductor with Tc=7 K. Above this temperature, NbSe2 behaves as a normal metal and the hybrid presents a typical Schottcky barrier. 
However, below the critical temperature the resistance of NbSe2 drops to zero and we can study the formation of a super Schottky barrier. In the second case, we explore the properties of 1L and few layer flakes (MoS2 and WSe2) on top of a ferrimagnetic substrate (Yttrium Iron Garnet-YIG). In these samples, we study the effect of the inhomogeneous magnetic texture of the substrate on the valley exciton transitions of the MoS2. 
We study the optoelectronic properties by combined luminescence and electronic transport measurements, at room and low temperatures. Since the substrate could modify not only the electron but also the phonon properties of MoS2, heterostructures are also studied by Raman Spectroscopy under different excitation wavelengths and temperatures. To determine the role of the substrate, results are compared to those of samples deposited on SiO2 substrate.

Supervisor: J.I. Pascual

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