An ultrathin layer of PbS was prepared by using a two-dimensional matrix composed of a hydrophobic alpha-helical tetracosapeptide having a crown ether unit at the C-terminal. The peptide was spread on a subphase containing Pb(NO3)(2), and the peptide monolayer was transferred onto a substrate together with Pb2+ ion in complex with the crown ether unit. The helical peptides took horizontal orientation on the substrate. After conversion of Pb2+ to PbS by using an H2S gas, which was confirmed by X-ray photoelectron spectroscopy, formation of a two-dimensional PbS network nanostructure on the substrate was observed by scanning tunneling microscopy. Notably, scanning tunneling spectroscopy revealed that the PbS nanostructure showed a semiconducting property with a wider band gap than the bulk PbS due to the quantum size effect. (c) 2005 Elsevier B.V. All rights reserved.