Thomas Heine, University of Leipzig (Germany)
Yandong Ma, Jacobs University Bremen (Germany)
Tim Wehling, University of Bremen (Germany)
Young-Hee Lee (Institute for Basic Science, Suwon (Korea)
Thomas Frauenheim, University of Bremen (Germany)
University of Bremen, Germany, 27th June until 1st of July 2016
The workshop “Tailor-made 2D-materials and functional devices” was held at the University of Bremen, Germany from June 27th to 1st 2016. In total, 76 participants from Belgium, Brazil, Finland, Ireland, Luxemburg, The Netherlands, Portugal, Slovenia, Spain, Sweden, Switzerland, Denmark, France, Germany, UK and US attended the workshop.
The programme consisted of 30 invited lectures, one poster session presenting 37 posters and many events (reception / conference dinner) to allow for informal exchange. The lectures were scheduled to last 40 min, including 5-10 min discussion time. In addition to this extended time for discussion, the chairpersons were instructed to introduce the subject of the session and to actively participate in the discussion. This “Gordon-conference-style” was essential to guarantee a vivid discussion. The organizers ensured that well-established scientists acted as invited speakers and chairpersons.
Concerning the poster session, we accepted only 37 posters to allow for an intense exchange of ideas at each single poster. Here, we encouraged in particular the young scientists to ask questions. The participation of PhD students was supported by partly covering local accommodation costs.
Due to the compact organization and accommodation in one hotel only all participants stayed together for the whole time of the conference, which additionally enforced the scientific discussion which was mandatory since scientists from various separated fields, i.e. advanced quantum chemistry, many-body perturbation theory, DFT and beyond, scanning probe techniques, optical spectroscopy, ARPES, etc. were attending the meeting to merge ideas and formulate a common goal for future directions and collaborations of theory groups with experimental groups.
Financial support from the DFG, Psi-k Network, and the German CECAM node multi-scale modelling from first principles, cecam-mm1p.de and the University Bremen is gratefully acknowledged.
II. Scientific content, main outcome of key presentations, selected discussions
Various sessions have been related to the topics (a) Carbon and other hexagonal nanostructures (b) topology and superconductivity, (c) TMDCs: optical properties and carrier dynamics, (d) heterostructures and devices and (e) TMDCs: electronic properties. These topics are commonly related to each other. So, extensive attentions and discussions have been stimulated between researchers from different fields.
On the experimental side, progress on several ways to fabricate atomically thin crystals and particularly heterostructures thereof has been reported. Current options include exfoliation, ultrasonication and epitaxial growth. The on-demand production of graphene-hBN heterostructures as well as graphene transition metal dichalcogenide (TMDC) heterostructures by means of exfoliation, subsequent transfer and annealing techniques has been reported by Heejun Yang, Hyun Seok Lee and Young-Hee Lee (Sungkyunkwan University, Suwon, Korea. It has been shown by transmission electron microscopy (TEM) that these heterostructure are surprisingly clean: contaminations from the production process are squeezed out of the heterostructures most likely by van-der-Waals interactions. The presentation by Hannu Pekka Komsa (Aalto University, Helsinki) showed that the combination of TEM with first-principles DFT simulations can reveal e.g. atomically precise defect geometries of e.g. single vacancies in graphene as well as their evolution, particularly the clustering of defects, under the electron beam. The experimental progress reported at this workshop it makes clear that heterostructures (both lateral and vertical) of 2d materials open a large space which is now available for materials engineering. Among possible interesting future directions is an idea reported by David Abergel (Nordita Sweden) that graphene nanoribbons with boron nitride ‘cladding’ can support topologically protected conducting states, even in the presence of significant crystalline disorder at the interfaces.
Several speakers addressed the question of how peculiar quantum states of matter arise 2d materials driven by electronic interactions, spin-orbit coupling / topology quantum and their interplay. Two speakers (Yong Xu, Tsinghua University and Jinfeng Jia, Shanghai Jiao Tong University) reported the successful growth of stanene, a tin based analog of graphene which is predicted to be a 2d topological insulator which could potentially host topological superconductivity. While is latter topic remained an open issue, several speakers addressed how superconductivity can be realistically described in 2d materials. Elena Roxana Margine (Binghampton University) reported on a material realistic implementation of the anisotropic Eliashberg theory using Wannier-Fourier electron-phonon interpolation and first applications of this novel computational approach to superconductivity graphene based systems. Also within the framework of Eliashberg theory and its combination with ab initio techniques Gunnar Schönhoff (University of Bremen) showed how the influence of Coulomb interactions on superconductivity in 2d materials can be realistically modelled. He showed that substrates and external screening can affect superconductivity in 2d materials but only if the internal screening of the material is not too strong. While progress on the experimental and the theoretical side has been made it remains an open issue, how the materials design freedom in 2d materials and heterostructures can be used to push superconducting critical temperatures and to realize in experiment exotic quantum phases like topological superconductivity.
Progress in understanding and controlling the optical properties of 2d materials, particularly TMDCs like MoS2 or WS2 etc. has been reported by several participants from experiment and theory.
While our general understanding of interaction effects in 2d materials is clearly still quite at the beginning, the area of optical properties and carrier dynamics in 2d materials has been very rapidly advancing during the last year as also several talks at the workshop showed. Alexey Chernikov (University of Regensburg) explained in his talk how excitons in 2d materials can be controlled by material thickness and screening. Johnathan Finley (TU Munich) showed recent studies in which he employ strong DC-electric fields to control the linear and non-linear optical response of mono- and few-layer 2H-stacked MoS2 crystals, where the electric fields could effectively break and restore inversion symmetry. On the theory side several develops towards the realistic description of correlation effects and their influence of optical as well as electronic quasiparticle properties of 2d materials have been reported. This includes developments of the “GdW”-method by Michael Rohlfing (U. Münster) as well as combinations of ab initio and model based approaches by Alexander Steinhoff (U. Bremen). Realistic simulations of ground and excited state optical properties are now possible for 2d materials and even complex heterostructures.
III. Assessment of the results and impact on future direction of the field
The foremost objective of the proposed workshop was to bring together leading and active researchers who work on novel 2-dimensional materials in both experiment and theory and to discuss about possible solutions to improve the quality and tailor the materials based upon computational approaches from physics and chemistry. Aiming for a focused meeting with 70-80 participants we have stimulated exchange, awareness of challenges, approaches and achievements in the respective fields. Moreover, we emphasize to strengthen the links between experimental and computational groups in this fast-developing field and to interconnect current model-type and first-principles descriptions.
The workshop became a forum to discuss about possible solutions of improving the quality of novel 2D materials and correlating experiment and theory on a highly predictive level. We have been able to achieve the following key objectives:
- In the discussions we have identified the major problems in our current understanding of novel layered materials with a focus on lateral and multi-layer stacking effects, quantum confinement, doping and importantly electronic interaction effects. To this end we brought together experiment and different theory communities seeking for predictive power and general understanding of electronic properties of novel 2D materials. Invited overview talks by highly recognised experimentalists from different parts of the field (organic and inorganic layered materials, topological insulators, optical spectroscopies) and related computational talks have contributed to the general understanding.
- We have summarised the major achievements from communities working on different 2D materials (semiconducting vs. metallic vs. topological), and have identified common problems. The workshop has stimulated knowledge exchange across the boundaries of formerly rather separate communities.
- In the discussions the main advantages and shortcomings of currently available theoretical techniques to model and understand the electronic, optical and dynamic of novel 2D materials have been specified. The techniques considered and discussed comprise (but not be limited to): density functional theory (LDA, GGA, LDA+U, etc), TD-DFT, GW and BSE quasi-particle methods, quantum transport techniques (Landauer Büttiker, Kubo formula), multi-scale approaches, quantum lattice models, many-body theory and quanrum optics.
- During the period of the workshop possible solutions in optimizing the quality and properties of 2D topological insulators and fabricating new devices have been outlined. The mutual exchange between researchers from both experiments and computational materials science helped a lot to better understand current problems in synthesis and application of topological materials, and determined the priority target of questions to be addressed by state of-the-art first-principle methods.
- The workshop stimulated the interconnection of phenomenological model-type and computational first-principles approaches. Phenomenological theorists and computational materials scientists will foster mutual interactions not only to supply practical parameter inputs for theoretical models, but also to realize interesting proposals using explicit materials.
September 27th 2016