Spectroscopic Metrics for Alkyl Chain Ordering in Lying-Down Noncovalent Monolayers of Diynoic Acids on Graphene

Noncovalent monolayer chemistries are widely used to control physical properties of 2D materials. For many applications (e.g., energy conversion, sensing), molecular ordering across a range of length scales is important in determining the physical properties of the interface. Scanning probe microscopy can resolve details of molecular packing and orientation over nanoscopic areas of graphene, graphite, and other 2D materials; however, evaluating molecular ordering over larger scales is also key. Such ordering is especially challenging to characterize at large scales for lying-down phases (thickness <0.5 nm) on topographically rougher materials such as graphene (vs flatter graphite). Here, we combine scanning electron microscopy and polarization modulated IR reflection absorption spectroscopy to evaluate alkyl chain ordering in lying-down monolayers of diynoic acids on few-layer graphene and graphite substrates with areas similar to 1 cm(2). The ability to assess ordering in this widely used class of molecules reinforces the potential utility of spectroscopic metrics for evaluating structure in noncovalently functionalized 2D materials at micro- and macroscopic scales.