"van der Waals heterostructures" refer to vertical stacks of different two-dimensional (2D) crystals, such as boron nitride, graphene, etc. that are held together by weak van der Waals forces. These advanced multilayer structures can serve as a versatile platform for studying a range of phenomena at the nanoscale. Particularly, mechanical superimposition of the 2D crystals creates 2D periodic potentials, which impart unusual chemical and physical properties to the system.
Using a bottom-up supramolecular approach, a group of European researchers created self-assembled organic molecular lattices with an atomic precision and controlled geometry on the graphene surface, and induced 1D periodic potentials in the ensuing organic-inorganic hybrid heterostructures.
For that purpose, the researchers carefully synthesized and designed molecular building blocks, which were equipped with (i) a long aliphatic tail that directs the periodicity and self-assembly of the potential, and (ii) a photoreactive diazirine head group; the surface potential of the underlying graphene sheet is modulated by this head group.
Before depositing the diazirine moiety on graphene, it is irradiated with ultraviolet (UV) light and cleaved, which leads to the formation of a reactive carbene species. The latter tends to react with solvent molecules, resulting in a mixture of new compounds having different functionalities.
The nanoscale arrangement of the supramolecular lattices, formed on graphene and graphite surfaces, was characterized using scanning tunneling microscope (STM) imaging. Graphite and graphene surfaces determine the geometry and periodicity of the induced potentials. Subsequently, electrical characterization was carried out on graphene-based field-effect devices to evaluate the effect of the various self-assembled organic layers on the electrical properties of the 2D material.
The interactions between the molecular assembly and graphene were revealed by computational simulations, and a hypothetical analysis further established that the origin of the doping effects can be due to the orientation of electrical dipoles in the head groups. Lastly, a periodic potential with a different intensity but the same geometry could be created from a supramolecular lattice, which was prepared by irradiating the molecular building block with UV light in a different solvent.
In this manner, the research team was able to show that organic supramolecular lattices can be used for creating controllable 1D periodic potentials on the graphene surface. Interestingly, a careful molecular design can be used to pre-program and adjust the amplitude, periodicity and sign of the induced potentials. This bottom-up supramolecular method can also be used on transition metal dichalcogenides and other similar inorganic 2D materials. This could help develop more intricate multilayer van der Waals heterostructures.
The study findings are very important to obtain organic-inorganic hybrid materials with controllable electronic and structural characteristics featuring unparalleled optical, magnetic, electrical and piezoelectric functionalities.