A LIPID TALE: ALKYL TAIL IMPURITIES IN TECHNICAL-GRADE OLEYLAMINE REGULATE THE GROWTH AND ASSEMBLY OF ULTRANARROW GOLD NANOWIRES AT CHEMICALLY PATTERNED INTERFACES

  

A staggering number of problems in materials chemistry relate to controlling the assembly of matter at <10 nm scales, including those with applications in nanoelectronics, energy harvesting, and biomedical device design. It is difficult to achieve precise chemical patterning at the short length scales required for such applications using traditional top-down fabrication methods (e.g., lithographic techniques). On the other hand, biological systems create high-resolution chemical patterns with remarkable efficiency, by assembling simple molecular building blocks with nm-scale features (e.g., nucleotides, amino acids, lipids) into structurally complex motifs capable of carrying out the diverse functions required for life. 

Drawing inspiration from the diverse structures and functions of lipids in biological membranes, this work uses lipids to create high-resolution chemical patterns at interfaces, control the growth and self-assembly of nanocrystals, and to facilitate interactions that precisely template nanocrystals at chemically patterned surfaces.

Functional alkanes assemble into striped phase monolayers on highly oriented pyrolytic graphite (HOPG), in which the alkyl chains are oriented parallel to the substrate, expressing both the polar and nonpolar regions of the amphiphile at the environmental interface. The same is true for diyne phosphoethanolamine (dPE), a phospholipid with a zwitterionic headgroup. When assembled into striped phases on HOPG, the headgroup zwitterions of dPE are confined in 1-nm-wide rows of functional groups with a pitch of ~7 nm, resulting in ordered arrays of orientable dipoles at the HOPG surface. The chemistry of dimensionally confined functional groups is distinct from bulk solution phase chemistry, and in this case enables powerful directing effects which can be used to template the adsorption of ultranarrow gold nanowires (AuNWs) in precise alignment with the template stripes. 

Technical grade oleylamine (cis-9-octadecen-1-amine, OLAm, 70% purity) serves as the capping ligand for the AuNWs used in this work, and additionally plays an important role in the assembly of AuNWs at dPE/HOPG surfaces. While technical-grade reagents enable cost-effective and scalable production of materials, variation in the composition of impurities between different batches have significant impacts on nanocrystal morphology and assembly. We show that thermal transitions of alkyl chain impurities (trans and saturated chains) in AuNW ligand shells can be used to regulate AuNW assembly at chemically patterned interfaces. 

Characterization of OLAm reagents by 1H NMR and mass spectrometry reveals significant and highly variable fractions elaidylamine (ELAm, trans-9-octadecen-1-amine) and octadecylamine (ODAm) between different batches of OLAm. To understand the phase behavior of mixtures of the C18 alkylamines commonly found in technical grade OLAm, we synthesize isomerically pure OLAm and its trans isomer, elaidylamine (ELAm), to generate binary and ternary mixtures with (ODAm), which is commercially available in high purity. Differential scanning calorimetry reveals limited miscibility of the C18 chains, and demonstrates the significant impact of chain composition on the physical properties of mixtures of alkyl chains (e.g., tech. grade OLAm). Finally, we examine the impacts of trans and saturated alkyl chains on AuNW synthesis. We find that inclusion of ODAm and ELAm in the ligand blend used for AuNW synthesis each result in shorter AuNWs than those synthesized with pure OLAm. We also observe enhanced stability of surface adsorbed AuNWs conferred by trans and saturated chains.