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Engineered two-dimensional hetero-architectures for nanoelectronics

Periodic Reporting for period 1 - 2D Hetero-architecture (Engineered two-dimensional hetero-architectures for nanoelectronics)

Periodo di rendicontazione: 2016-03-24 al 2018-03-23

Graphene – a single layer of carbon atoms – exhibits a unique combination of superior properties, which makes it a credible starting point for new disruptive technologies in a wide range of fields.. This research project will tackle the challenge by incorporating narrow boron nitride or boron carbon nitride barriers in graphene domains. These lateral hetero-structures will be used as channel elements in field-effect transistors, whose electron transport properties will be evaluated by a combination of high- and low-temperature electrical measurements. The synthesis of the hetero-structures will be extensively investigated with combining optical, electronic, probe microscopy and spectroscopy analyses. This approach is designed to take a holistic view of the synthesis process, not only hunting for the highest apparent quality of the final devices, but also gaining an understanding for the underlying atomistic growth mechanism, and thus enable engineering of these structures.The project will advance European scientific competitiveness in the field of synthesis of 2D hetero-structures, currently a prerogative of a select few US and Korean groups, and will provide new platforms which might represent a viable option to obtain graphene-based transistors suitable for digital logic.The multidisciplinary aspect of the project will allow me as experienced researcher to gather knowledge in topics as different as synthesis of nanomaterials, electronic device manufacturing, and low temperature characterization. It will also allow me to re-integrate the European scientific network, after my postdoc experience in the US, forming a perfect basis of my goal of establishing an independent research group at a European university.
"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.
To date, the absence of an energy gap in its electronic band structure prevents graphene´s use in digital logic devices. Thus, from a technological point of view, this project was driven by the interest in fabricating a two-dimensional (2D) structure able to surpass graphene, that is, a structure exhibiting both high carrier mobility and a fundamental band gap in its electronic band structure.The potential impact of such novel material comprises therefore further development of new-generation electronics based on 2D materials.
Within this project we managed to synthesize and characterize uniform-size graphene nanodots that form periodic arrays within a 2D boron–carbon–nitrogen alloy. This new 2D material, which theory predicts to be an ordered composite of uniform-size semiconducting graphene quantum dots laterally integrated within a larger-bandgap matrix, holds promise for novel electronic and optoelectronic properties, with a variety of potential device applications. In this sense, the project successfully went beyong the state of the art.
However, this material requires a metal substrate to be grown on. The presence of this metal substrate has hampered so far the use of this 2D material for electronic or onto-electronic applications. At this aim, the 2D material needs to be transferred from the growth substrate to a suitable insulating one. This step has not been achieved with success so far, and further studies will be needed.
Graphene nanodots self-assemble on an iridium substrate