Novel 2D materials: tuning electronic properties on the atomic scale
University of Bremen, Germany
Recently, there has been enormous progress in the production of atomic crystals that are strictly two dimensional (2D) and can be viewed as individual planes of atomic-scale thickness pulled out of bulk crystals like graphite, h-BN, several transition metal dichalcogenides (TMD), and complex oxides. By stacking various atomic crystals on top of each other, it is possible to create multilayer heterostructures which hold the promise for novel devices with designed electronic properties. For instance, concepts for novel transistors based on lateral and vertical transport as well as optoelectronics have been proven.
While obviously promising for applications, these new materials are very different from usual three dimensional bulk materials, as now all atoms are very close to a surface or some interface and vertical quantum confinement or stacking control properties like electronic gaps. Reduced dimensionality is generally expected to affect heat and charge transport in these novel materials. However, a coherent understanding of how to control the electronic properties of new 2D materials and, particularly, a unifying “standard model” of these novel hybrid materials, remains to be built.
Therefore, this proposed workshop shall bring together hitherto rather split communities working on materials like graphene, hBN and alternative layered materials, most importantly TMDs. The aim is to develop a unified understanding of electronic properties in this new class of materials with a focus on transport (heat and charge carrier), doping, stacking effects, quantum confinement, and electronic contacts.
Research on low dimensional materials is driven both by fundamental scientific questions and the perspective of applications. For instance, electrons in graphene – an atomically think sheet of carbon – behave like massless Dirac particles and exhibit remarkably high mobilities [1, 2] even though graphene is an all surface material and electrons are directly exposed to perturbations from its environment. These properties are prospective for applications like, e.g., ultra-high frequency transistors [3, 4] and novel “vertical” electron tunnelling transistors . Similarly, a single layer MoS2 proved very promising for transistor applications [6, 7].
Today, it is possible to produce atomically thin crystals from various materials like graphene, h-BN, several TMDs, and complex oxides by exfoliation techniques , in particular using ultrasonication techniques [7-10] or epitaxial growth . Most recently, the field has expanded beyond studying isolated 2D crystals and moved towards heterostructures made from a combination of alternating layers of graphene, hexagonal boron-nitride (h-BN), MoS2, etc. Such heterostructures can provide a higher electronic quality than bare devices [12, 13] and, also, allow a conceptually new degree of flexibility in designing electronic, optoelectronic and other devices [5, 14, 15]. In addition, new fundamental physics not present for individual 2D crystals is widely expected to emerge in heterostructures [14, 16].
The freedom for tuning electronic properties of 2D crystals at the atomic scale is huge: The roughness of the 2D crystals [17, 18] and the amount defect scattering can be controlled [12, 13] through interfacing with other 2D materials. The dispersion of electronic quasiparticles can be completely changed – also through interfacing – as the examples of multilayer graphene [19-22] as well as graphene boron nitride heterostructures  demonstrate: Dirac particles can become massive (bilayer graphene) or novel Dirac particles (graphene on hBN) can emerge. In the world of 2D materials interfacing and (vertical) quantum confinement are largely intertwined and control electronic properties of these materials, in general. For instance, also TMDs change their electronic structure from indirect (bulk) to direct (monolayer) band gaps , i.e. upon quantum confinement or equivalently upon interfacing with themselves. However, the emergence of novel electronic properties in interfaced / quantum confined hybrid materials frequently poses challenging multi-scale problems, which are controversially debated today as in the case of graphene/hBN heterostructures [24-26].
TMDs offer the possibility to tune their properties chemically. The choice of the transition metal may determine if a material is metallic (e.g. NbX2 or ReX2, X=S, Se, Te) or semiconducting (e.g. MoX2, WX2), and various band gap values can be selected upon the choice of the chalcogenide. That means that the intrinsic band gap of the TMD layers can be defined by stoichiometry. Similarly, chemical functionalization can induce a Dirac material to an insulator transition in graphene [28-30]. Under tensile stress a change of semiconducting TMD monolayers to metallic phases has been observed . Closely related, in graphene, the coupling of electrons to external strain fields leads might not only control the conductivity  but also trigger peculiar quantum effects like the formation of pseudo Landau levels [33-36].
Particularly important for transistor applications and optoelectronics in any 2D material is the issue of charge doping. For graphene/hBN/metal hybrid structures the importance of work function changes and related doping has been demonstrated theoretically . Intercalation and the interaction with molecular adsorbates provide additional routes for chemical doping which are unique to low dimensional materials [38-40]. Yet, microscopic doping mechanisms remained so far often unresolved and controversial as the example of graphene interacting with water demonstrates [41-43]. In more general terms, microscopic mechanisms responsible for device operation often remain to be understood. Open problems include the discrepancy of contact resistance between theoretical predictions on ideal systems and real device implementations [6, 44] and issues of momentum conservation arising both in TMD FETs  as well as vertical graphene based tunnelling transistors .
The treatment of physical phenomena in graphene, h-BN and TMD at various levels of theory will be essential to fully understand, and eventually to exploit, the properties these 2D materials offer. We believe that the examples discussed in the preceding paragraphs clearly show that many phenomena and related scientific challenges arise very generally and similarly the context of various novel hybrid materials. This calls for a unified yet realistic theory to be developed and presents the central motivation for this CECAM workshop.
Thomas Frauenheim (University of Bremen) - Organiser
Thomas Heine (University of Leipzig) - Organiser
Tim O. Wehling (University of Bremen) - Organiser
Mikhail I. Katsnelson (Radboud University Nijmegen) - Organiser