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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - EMERGE (Tuning Emergent Phases in 2D Materials)

Teaser

Summary

The primary aim of this project was to study the electronic phases which emerge when layered materials are thinned to the two-dimensional limit and interfaced with other crystalline surfaces. For the past 15 years, researchers have had the experimental ability to isolate monolayers of crystalline solids on surfaces; this initial surge of research has revealed that the environment surrounding such two-dimensional crystals can play a large role in the observed properties of the materialsâ€™ themselves. In certain contexts, it is even possible for interfacial effects to produce entirely new or emergent properties, which are not present in the isolated two-dimensional material. The range of possible emergent properties in two-dimensional materials is extremely broad, which means these properties can be used in applications from computation to optoelectronics to energy transmission and importantly, help society deal with large scale problems related to the energy used in computation and data storage, the efficient transmission of energy over large distances, and the efficient conversion of light to electricity. As all emergent properties of two-dimensional materials is a global scale research effort, this project has chosen to focus on the specific properties of a unique class of two-dimensional materials â€“ three atom thick metallic sheets. The objectives were to study how these sheets behave and are modified by their interaction with the surfaces which support them. To achieve this primary goal, we developed a new procedure to synthesize these new two-dimensional materials (in this case vanadium sulfide) under extremely clean conditions on atomically pristine single crystal metal substrates (gold). Using ultra-low temperature atomic scale microscopy to study the resulting 2D material, we were able to successfully characterize the emergent electronic phase in the vanadium sulfide. We were able to show that a geometric effect (termed Moire effect), which is observed between two periodic surfaces, drove an electronic phase transition to a charge density wave ground state. We could prove that these Moire effects were actually the cause of this electronic phase transition by observed the way these electronic changes behaved when the Moire structure was modified (for instance by changing the angle of the vanadium sulfide monolayer). This direct observation of correlated electronic ordering and surface geometric effects shows that not only do electronic phases emerge in this particular 2D material, but implies that we can also control them via their interfaces with other materials. Ultimately, dynamic control of this electrical state could be used to store information in next-generation memory devices.

Work performed

During the course of this project, we designed, implemented, and commissioned an experimental set-up used to grow atomically thin 2D materials. The set-up was directly integrated into one of our low-temperature microscopes, which we used to confirm the successful growth of vanadium sulfide. We subsequently characterized the electronic properties and phases of the resulting vanadium sulfide material at temperatures ranging from 1-7K. We found that a charge density wave emerges in this material within this temperature range. Significantly, we further found that the real-space structure of the charge ordering is dictated by the Moire effect between the vanadium sulfide and underlying gold surface. We have shown, in great detail, that this Moire mediated charge density wave can be modified by changing the Moire pattern itself (for example, by rotating the two lattices with respect to one another). We further performed spin-polarized and magnetic field dependent measurements to confirm that the atomically thin layer lacks long-range magnetic order. The work has recently been concluded and preparations for dissemination are ongoing.

Alternatively, we performed a related project in parallel to this work to probe the emergent electronic properties of a single magnetic atom on the surface of a layered semiconducting material. Using Co atoms on the surface of black phosphorus, we successfully demonstrated that black phosphorus stabilizes two distinct valencies in the cobalt atoms. In effect, the same single atom could be modified, with a metallic probe, from a 4s13d8 to a 4s03d9 electronic configuration. As the transition was stable in time and could be both read and written with the metallic probe, it constituted the first experimental demonstration of so-called orbital atomic memory. The work was disseminated through an open-access publication in the journal Nature Communications and was subsequently broadcast throughout both scientific and popular news media outlets. I also gave an interview with the Dutch BNR news radio station about the outcome.

Final results

In the course of completing the work outlined above, we have made several significant scientific steps beyond the current state of the art.
First, we have successfully integrated a materials synthesis set-up with a running ultra-low temperature scanning tunneling microscope capable of measuring within magnetic fields up to 9 T. This set-up enables the growth and in-situ characterization of new atomically thin magnetic materials and their characterization at the atomic scale with a world-leading instrument.
Second, we have demonstrated the demonstrated the control of a many-body state of electronic matter using the geometric effects occurring at crystalline interfaces.
Finally, we have shown that single atoms on particular layered materials can exhibit bistable valencies, which can be used to store information at the atomic scale.
While the majority of the work done throughout this proposal has been driven by fundamental science, the results have clear potential for impact in future technologies. In particular, both electronic phases in 2D monolayers and single atoms on layered materials could be used to greatly reduce the energy necessary to store information. The current amount of energy being used to store information is nearly 5% of the total energy budget, a number which will certainly grow as data proliferates. These results present possible alternatives to current technologies, which would help us address these problems.