Controlling and Characterizing Molecular Ordering of Noncovalently Functionalized Graphene Via PM-IRRAS: Toward Templated Crystallization of Complex Organic Molecules

Shane R Russell, Purdue University

Abstract

The fabrication and functionalization of layered materials is a key feature in the development of high performance, next generation devices [1–11]. Device applications, such as nanoscale electronics, biosensing, energy conversion, drug delivery and study of proteopathic diseases require interfacial structures that express chemically orthogonal patterns at sub-10-nm scales. For instance, in many semiconducting polymer materials for organic photovoltaics (OPVs), the exciton diffusion length is approximately 5-10 nm [12], meaning that poor pattern resolution between n-type and p-type semiconductors in a bulk heterojunction can substantially reduce charge transfer. Conversely, n-type and p-type domains assembled with a high resolution, sub-10-nm periodicity would produce relatively greater charge transfer. Therefore, the ability to generate alternating patterns at the sub-10-nm scale with high fidelity is crucial in optimizing device performance, and scalability requires that the process be low-cost, highly reproducible and easily screened. Many techniques have been developed to generate high resolution patterns at the nanoscale. Photolithography [13–15] is a commonly employed process for fabricating conductive/nonconductive semiconductor patterns at the nanoscale, but experiences poor cost effectiveness for sub-10-nm nodes. Soft lithography [16, 17], such as microcontact printing with PDMS stamps, and mechanical lithography, such as dip-pen nanolithography] [18], are more cost effective than photolithography at small scales, and can apply high fidelity patterns at the sub-µm scale, but often must compromise between high resolution patterns and scalability. Bottom-up assembly strategies, such as block copolymers [19, 20], have been employed as highly modifiable building blocks for generating chemically distinct regions, though often lack the spatial control and pattern resolution required for next generation devices. In other studies, self-assembled monolayers (SAMs) of alkanethiols on Au(111) have been employed as a facile, low-cost way to control interfacial assembly of diverse structures including mineral crystals as well as soft matter at the sub µm scale [21,22]. However, expressing sub-10-nm pattern resolution of chemical functionality at the fidelity and scale required for next generation nanoelectronic and OPV devices remains challenging. Graphene has been studied extensively as a 2D material template for device fabrication [2, 3, 8, 9, 23]. This is primarily due to graphene’s technologically attractive properties, such as its high surface area, tensile strength [24], exceptional electronic conductivity [25] and biocompatibility [26]. Graphene can also be readily functionalized both covalently and noncovalently to dramatically modulate its physical and chemical properties [1–3, 6]. Modified graphene has been explored as a drug delivery platform for aromatic anti-tumor agents [27], highly selective sensors [28] and as a model platform for interfacial self-assembly of peptide nanostructures [6,29–31]. The noncovalent modifications to graphene are especially useful, as they preserve most of the intrinsic properties of the interface, while providing a route towards spatial modulation of surface chemistry [1]. Monolayers of diynoic fatty acids, such as 10-12 pentacosadiynoic acid (PCDA), self-assemble noncovalently in a head-to-head lying-down phase on graphene, stabilized both by the epitaxial match between the alkyl zig-zag and the hexagonal graphene lattice, as well the formation of hydrogen bonded dimers between adjacent rows of head groups [7, 32–34]. In this manner, stripes of polar head groups are arranged in a lamellar pattern with a 6 nm pitch. The diyne moiety can be polymerized in these ordered monolayers to form an ene-yne polymer backbone [32,33], providing a means to stabilize the interface towards solvothermal processing steps [35].

Degree

Ph.D.

Advisors

Claridge, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Polymer chemistry

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