Two‐Dimensional Fullerene Assembly from an Exfoliated van der Waals Template

Abstract

Two-dimensional (2D) materials are commonly prepared by exfoliating bulk layered van der Waals crystals. The creation of synthetic 2D materials from bottom-up methods is an important challenge as their structural flexibility will enable chemists to tune the materials properties.A2D material was assembled using C 60 as ap olymerizable monomer.The C 60 building blocks are first assembled into alayered solid using am olecular cluster as structure director.T he resulting hierarchical crystal is used as atemplate to polymerizei ts C 60 monolayers,w hichc an be exfoliated down to 2D crystalline nanosheets.D erived from the parent template,t he 2D structure is composed of al ayer of inorganic cluster, sandwiched between two monolayers of polymerizedC 60 .The nanosheets can be transferred onto solid substrates and depolymerized by heating.Electronic absorption spectroscopy reveals an optical gap of 0.25 eV,narrower than that of the bulk parent crystalline solid.The discovery of graphene [1] has catalyzed acomprehensive search for two-dimensional (2D) materials,and has led to the development of 2D transition metal dichalcogenides,h exagonal boron nitride,a nd phosphorene. [2][3][4] These materials serve not only as excellent model systems for exploring 2D physics,b ut also as potential candidates for av ariety of electronic and optoelectronic devices.T raditional atomic 2D materials are commonly isolated from layered van der Waals solids exhibiting strong in-plane bonding and comparatively weak interactions between the layers.A ssembling molecular building blocks into synthetic 2D materials offers exciting opportunities for tuning the material properties through chemical design and for discovering new behaviors. [5][6][7] The assembly of such material, however, is am ajor synthetic challenge,w ith only af ew reported examples,w hich include 2D covalent organic frameworks [8][9][10][11] and metal-organic frameworks. [12,13] Herein, we describe an ew 2D material assembled from C 60 building blocks that are covalently linked. Then ano-material is formed by polymerizing al ayered van der Waals crystal assembled from C 60 and the molecular cluster Co 6 Se 8 -(PEt 2 phen) 6 (phen = 9-ethynylphenanthrene), and thereafter dissolving the supporting template (Figure 1a). Theresulting 2D structure is composed of al ayer of inorganic cluster, sandwiched between two monolayers of polymerized C 60 . Conceptually,t his structure can be visualized as as tructural analogue of 2D transition metal chalcogenides or halides in which the atomic building blocks are pseudo spherical molecular clusters rather than simply atoms. C 60 is an attractive building block for creating synthetic 2D structures owing to its remarkable physical and chemical properties.W hile crystalline C 60 has ab and gap of 2.3 eV, [14] theoretical studies on both polymeric and van der Waals C 60 monolayers predict band gaps ranging from 0.58 to 1.01 eV, [15][16][17][18] which is lower than that of any experimentally isolated monolayer semiconducting materials. [19] Additi...