"The main goal of the project was to synthesize and characterize new architectures formed by the combination of graphene - a single layer of carbon atoms - and hexagonal boron nitride (hBN) - a single layer of alternating boron and nitrogen atoms. Notably, the interest was focused on structures where graphene and hBN are integrated within a one-atom-thick sheet. The technological motivation behind the project is the realization of a novel semiconducting ultra-thin channel that would be used for new-generation field-effect transistors.
At this aim, we have fabricated ordered arrays of graphene nano-domains (dots), epitaxially embedded in a two-dimensional (2D) boron–carbon–nitrogen (BCN) alloy. The growth was performed in a ultra-high-vacuum chamber, where the molecular precursors of graphene and hBN were mixed at a controlled pressure. On the hot surface of an iridium crystal, the gas mixture reacts to give rise to a one-atom-thick sheet where graphene dots are embedded in a BCN matrix. These dots exhibit a strikingly uniform size of 1.6 ± 0.2 nm and strong ordering, and the array periodicity can be tuned by adjusting the growth conditions.
Several experimental techniques have been used to characterize in detail the structure of this novel material. Scanning tunneling microscopy has been used to assess structural and electronic properties. this was executed in the laboratory of Prof. L. Hornekær at Aarhus University during the secondment phase of the project. Experiments of low-energy electron microscopy were carried out in the Center for Functional Nanomaterials at Brookhaven National Laboratory (NY, USA), in collaboration with Dr. J. Sadowski. The scope of this particular experiment was to gain more insight into the growth dynamics, by looking in real time at the growth of the 2D sheet. Additionally, low-energy electron diffraction technique was used to characterize the long range order (that is, in the micron-scale range) of the graphene dots. Ultimately, X-ray photoemission spectroscopy, performed at ASTRID II synchrotron radiation facility at Aarhus (Denmark), was employed to gain details about the chemical composition of the 2D sheet.
From a theoretical point of view, we have developed a model in collaboration with Dr. J. Tersoff at IBM (NY, USA) to explain the observed behavior. The model takes into account dot-boundary energy, a moiré-modulated substrate interaction and a long-range repulsion between dots.
The results were published as an Open Access article in Nature Communications (CC-BY license). The raw data have been stored in the Zenodo repository and are available to anyone with Internet access.
Furthermore, the data have been presented at International conferences like NanoSEA 2016, ECOSS 32, MRS 2016 Fall Meeting, Graphene 2017, Carbonhagen 2017 and 2nd World Congress and Expo on Graphene and 2D Materials. Ultimately, I have presented my project at the ""Danish Science Festival 2017"" (Forskningens døgn), an event organized by the Danish Ministry of Higher Education and Science with the aim of establishing a meeting place and enhance the relationship between researchers and the general public, showcasing how research and innovation help solve social challenges and support public engagement in research.
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