Abstract
Oriented assemblies of small crystals forming larger structuresare common in nature and crucial for forthcoming technologies asthey circumvent the difficulties of structural manipulation atmicroscopic scale. We have discovered two distinctive concentricassemblies of zinc oxide rods, wherein each rod has an intrinsicallypositive and a negative polar end induced by the noncentrosymmetricarrangement of Zn and O atoms. All the rods in a single assemblyemanate out of a central core maintaining a single polar direction.Due to growth along the two polar surfaces with different atomicarrangements, these assemblies are distinct in their intrinsicproperties and exhibit strong UV luminescence in the exterior ofZn-polar assemblies, unlike the O-polar assemblies. We have reportedsurface-enhanced Raman scattering (SERS) from Ag and Au nanoparticlesdecorated on thin carbon nanowalls (CNWs) grown by microwave plasmachemical vapor deposition. The Ag morphology is controlled byexposing the CNWs to oxygen plasma and through the electrodepositionprocess by varying the number of deposition cycles. The SERSsubstrates are capable of detecting low concentrations of rhodamine6G and bovine serum albumin, showing much higher Raman enhancementthan ordinary planar HOPG with Ag decoration. The major factorscontributing to this behavior include: high density of Agnanoparticles, large surface area, high surface roughness, and theunderlying presence of vertically oriented CNWs. The relativelysimple procedure of substrate preparation and nanoparticledecoration suggests that this is a promising approach forfabricating ultrasensitive SERS substrates for biological andchemical detection at the single-molecule level, while alsoenabling the study of fundamental SERS phenomena.
