Towards designing the form and function of 2D molecular systems

The accelerating pace of development of new technologies for applications such as solar cells, organic electronics, and nanoelectronics has created an increasing demand for functional materials that can be implemented at the nanoscale. Materials design will be crucial for successful realization and commercialization of next-generation miniaturized devices. Since such appliances will be assembled from the ‘bottom up,’ using individual molecules on a supporting surface, they can be considered 2D systems.

The widespread adoption of scanning-probe microscopy (SPM) techniques for materials research has created a new paradigm in developing and engineering advanced functional materials. SPM, and in particular scanning-tunneling microscopy (STM), has enabled imaging of low-dimensional structures at surfaces with submolecular resolution, providing unprecedented access to structural and electronic information about isolated or aperiodic 2D systems.

Molecular self-assembly, defined by Whitesides as the spontaneous association of molecules through noncovalent bonding in equilibrium conditions,¹ has been used widely to design and realize 2D surface structures. Molecular building blocks are carefully selected for their potential to associate with one another within the constraints of a specific geometry, such as the hexagonal mesh typically formed by molecules with threefold-symmetric bonding sites. If molecule-substrate interactions are minimized by using a relatively inert sample surface like graphite or gold, the equilibrium geometry of the molecular structure will be dictated by self-assembly.

Figure 1. Scanning-tunneling microscope (STM) image of C₆₀(carbon) guest molecules (purple), partially populating coexisting regions of the chicken-wire (left) and flower (right) polymorphs of self-assembled trimesic acid (TMA). The inset models show the position of the fullerenes (purple) within the TMA meshes (brown).

Of particular interest for materials applications are porous self-assembled networks, which incorporate voids as part of their unit cell. These voids, which occur within a periodic array, can be used to capture and confine a secondary molecular species in a ‘host-guest’ relationship.^2–4 An example of this is the confinement of fullerene molecules by the porous hydrogen (H)-bonded 1,3,5-benzenetricarboxylic-acid (trimesic-acid or TMA) network,⁵ which can incorporate a single C₆₀ (carbon) molecule in each of its pores.⁶ We showed that fullerenes adsorb in the pores of different polymorphs of the TMA network⁷ (see Figure 1), allowing tuning of the relative areal ratio of TMA to fullerene and adjustment of the spacing between the fullerenes. In a forthcoming publication, we describe the insertion of various numbers of fullerene guests in a porous network of self-assembled oligothiophene molecules. Tuning the fullerene occupation of this organic photovoltaic material might allow for customization of its electronic properties, possibly leading to more efficient solar-energy conversion.

Novel self-assembled systems like these have emerged as the result of many years of widespread study of supramolecular surface chemistry. Conversely, there have been relatively few publications describing networks joined by covalent (conjugated) bonding, despite the century-old interest in the synthesis of bulk (non-2D) conjugated materials. Only in the last few years have conjugated systems been assembled at surfaces.⁸ Here, the chemical reaction leading to covalent bonding was activated either electrochemically,⁹ photochemically,¹⁰ by heating,¹¹ or by local field pulses from an SPM tip.¹² We recently demonstrated an elegant method to initiate assembly through catalysis, using the supporting substrate as a key component of the chemical reaction.¹³

Figure 2. Polyphenylene synthesis from iodobenzene precursors on Cu(110).¹⁰(a) Para- and (b) meta-diiodobenzene molecules are dosed onto the surface and subsequently dehalogenate to form polyphenylene chains when the surface is heated. The STM data each contains properly scaled subimages in the lower right, indicating the resulting structures in the absence of heating. In both panels (a) and (b), the chains are surrounded by ‘satellites’ of iodine atoms. The schematic in the upper right of each STM image illustrates the respective molecular structures.

Our technique exploits the Ullmann reaction—i.e., the production of aromatic dimers and copper iodide—by heating an iodoaromatic compound with a copper powder. This is the oldest-known method for carbon-carbon coupling of aromatic halides. The reaction cleaves the carbon-iodine bond, leaving aromatic radicals free to dimerize with their nearest neighbors. The logical progression is to use diiodoaromatics as precursors to form long-chain conjugated organic polymers. We have demonstrated that one can use the surface atoms of a copper substrate to dehalogenate para and meta phases of diiodobenzene, which produce poly(para)phenylene and poly(meta)phenylene at the surface, respectively. In both cases, the polymers produced are aligned with preferred high-symmetry directions of the substrate and exhibit geometries as expected for polymers produced in the bulk (see Figure 2).

The possibilities for fabricating the desired 2D molecular structures for next-generation devices cover a variety of approaches. By exploiting the self-assembly of carefully chosen molecules, we have created porous H-bonded networks that can subsequently be ‘doped’ with guests. We have also successfully demonstrated the surface-confined synthesis of conjugated organic polymers, using the supporting substrate as a catalyst. These approaches illustrate possible avenues available for the synthesis and control of 2D organic functional materials. Our ongoing and future work focuses on using Ullmann dehalogenation to create surface-confined polymers from high-carrier-mobility conducting polythiophenes and to investigate the viability of this technique for creating 2D conjugated polymers.