"The main results related to the objectives above, described in more detail:
1. The Researcher performed electric measurements on several PTMC based devices. They were all contacted with graphene, and encapsulated in hexagonal boron nitride (hBN) for protection against environmental contamination or oxidation. GaSe devices showed insulating behaviour, probably due to low quality contacts. Therefore, the InSe devices had top gates over the graphene-InSe overlap (see the attached image for illustration), to locally tune the areal density of electrons and thus improve contact resistances. These samples were conductive, but contact resistances were inconsistent, scattered over several orders of magnitude. Applying a top gate voltage at the contacts produced varying responses.
2. In spite of the moderate yield of working contacts and high-quality samples, it was possible to fabricate a few devices where electrons were electrostatically trapped into narrow conducting channels, or into islands: quantum dots (QD). This was accomplished by applying a voltage to local top gates, illustrated on the attached image. These results were evaluated by the Researcher and have been published in the journal Nano Letters, in 2018, with the title ""Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures"". Measurements on the channels demonstrated conductance quantization. Single quantum dot measurements showed that, though the confinement was weak, electrons were indeed isolated to the small area of the dot. These are the first demonstration of one and zero dimensional confinement, as well as conductance quantization, in InSe based nanodevices.
3. Several InSe heterostructures were fabricated, consisting of graphene, hBN, InSe, hBN and graphene stacked on each other. When voltage was applied between the outermost (graphene) layers, these devices produced electroluminescence, i.e. they worked as light emitting diodes (LEDs). Their current-voltage characteristics were compared with theoretical simulations developed by the Researcher. These results have been accepted to Nature Communications for publication. The characterization methods described in the manuscript could be exploited in the future to probe the finer features in a 2D device’s band structure. Furthermore, the appearance of electroluminescence demonstrated that InSe can be used in the fabrication of infrared LEDs, where the output wavelength varies with the number of layers.
4. Due to the moderate yield of working contacts and high quality devices, and the limited effectiveness of top gates to induce confinement, the fabrication procedure had to be further improved. A promising solution to these issues is to prepare each layer and stack them in an ultra-high vacuum (UHV) chamber, to avoid contamination and corrosion. The Researcher successfully adapted a layer stacking technique used in atmosphere, and fabricated the first 2D samples in UHV. In general, these devices showed less contamination, while their electronic qualities were comparable to the best samples prepared in a glovebox, the cleanest environment before this approach.
Besides the publications mentioned above, these results have been widely disseminated in the form of lectures or poster presentations at several seminars, workshops, and international conferences, including Graphene Week 2018, or the Basel Workshop on 2D Materials 2018."
