Tailoring Nanoscopic and Macroscopic Noncovalent Chemical Patterns on Layered Materials at Sub-10 nm Scales

Jae Jin Bang, Purdue University

Abstract

The unprecedented properties of 2D materials such as graphene and MoS2 have been researched extensively [1,2] for a range of applications including nanoscale electronic and optoelectronic devices [3–6]. Their unique physical and electronic properties promise them as the next generation materials for electrodes and other functional units in nanostructured devices. However, successful incorporation of 2D materials into devices entails development of high resolution patterning techniques that are applicable to 2D materials. Patterning at the sub-10 nm scale is particularly of great interest as the next technology nodes require patterning of (semi)conductors and insulators at 7 nm and 5 nm scales for nanoelectronics. It will also benefit organic photovoltaic cells as phase segregation of p/n-type semiconducting polymers on 2D electrodes at length scales smaller than the typical exciton diffusion length (10 nm) is expected to improve the charge separation efficiency [7]. However, traditional photolithography faces challenges at high resolution due to the diffraction limit [8,9]; designing high NA optics for shorter wavelengths is accompanied by prohibitively increasing cost. Other top-down methods such as electron beam lithography and dip-pen lithography offer high resolution but have limited throughput. Therefore, to construct 2D devices with reproducible properties, it is necessary to develop novel strategies to control interfacial chemistry of 2D materials with high precision—ideally at sub-10 nm scales. One preferred approach for locally functionalizing 2D materials is via non-covalent assembly of molecular monolayers to avoid disrupting the in-plane bonding network of the 2D materials (e.g. delocalized π-electrons in graphene) that give rise to their unique properties. Self-assembly is an inexpensive bottom-up process in which molecules organize themselves into defined structures without external stimuli. It presents a feasible and scalable alternative to current lithographic techniques. Such non-covalent assemblies are particularly useful at interfaces with 2D materials, which exhibit high sensitivity of their physical/chemical properties to the interfacial environment [10, 11]. Self-assembled monolayers (SAMs) on 2D materials have been reported for various classes of organic molecules (e.g. alkanes and their derivatives, polymers and polycyclic aromatics such as porphyrins, and phthalocyanines) [10, 12]. In order to compensate for the relative weak molecule-substrate interactions in noncovalent SAMs, the constituent molecules often include long alkyl chains and/or aromatic cores which result in large numbers of atoms experiencing van der Waals interactions with the substrate [11–13]. Especially, the long alkyl chain groups assist with molecular packing via intermolecular van der Waals interactions, improving the ordering and the stability of the monolayer. We noted that the cross-section of cell membranes, consisting of amphiphiles, have length scales that closely match with the sub-10 nm patterning scale mentioned above. Along with other amphiphiles, fatty acids have been widely reported to assemble into striped lying-down phases on highly oriented pyrolytic graphite (HOPG) with head-to-head interactions between carboxylic groups [12, 14]. In particular, 2D assemblies of 10,12-pentacosadiynoic acid (PCDA) have been studied by many groups for their ability to form conductive ene-yne molecular wires upon photopolymerization [14–17].

Degree

Ph.D.

Advisors

Claridge, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Electromagnetics|Nanotechnology|Physics

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