Charge transport behavior of N-(2-Mercaptopropionyl glycine)-protected gold clusters with temperature

N-(2-Mercaptopropionyl glycine)-passivated gold clusters (Au-MPG) represent a unique networked system of monolayer-protected clusters (MPCs) due to inter- and/or intramolecular hydrogen bonding between the MPC units thanks to the terminal carboxylic acid groups of the monolayers. We have investigated the size-dependent electric charge transport in well-dried thin films of Au-MPG MPCs by a four-probe methodology as a function of temperature. The MPCs display a novel behavior of metallic-like-to-semiconductor crossover with increase of temperature. The MPCs having average cluster core sizes of approximately 1.6, approximately 3, and approximately 4 nm displayed a metallic-like nature (a linear increase of resistance of the films with increase of temperature) at low temperatures ( approximately 20-60 K), which crosses over to semiconductor behavior (decrease of resistance with increase of temperature) at high temperatures.