Superlattices of Metal and Metal-Semiconductor Quantum Dots Obtained by Layer-by-Layer Deposition of Nanoparticle Arrays K. Vijaya Sarathy, P. John Thomas, G. U. Kulkarni, and C. N. R. Rao* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur, Bangalore-560 064, India ReceiVed: September 23, 1998; In Final Form: NoVember 12, 1998 Superlattices formed by arrays of Pt or Au nanoparticles have been obtained by layer-by-layer deposition by using dithiols as cross-linkers. The superlattices have been characterized by X-ray diffraction, photoelectron spectroscopy, and scanning tunneling microscopy. The core-level intensities of the metal and of the dithiol in the X-ray photoelectron spectra show the expected increase with successive depositions. The formation of such structures has been confirmed by depositing Pt and Au layers alternatively. Layers of metal and CdS nanoparticles have been deposited alternatively to obtain heterostructures. The quest for nanoscale architectures has provided much impetus to investigate the self-assembly of metal and semicon- ductor particles as well as of other materials. A major motivation to construct such structures has been to exploit the novel properties that would manifest from size quantization which may allow band-gap engineering and to design nanoelectronic devices, sensors, and the like. Thus, Colvin et al. 1 have made use of a self-assembly of semiconductor nanocrystals to construct an optoelectronic device. Murray et al. 2 have dem- onstrated the self-organization of CdSe nanocrystallites into a three-dimensional superlattice. Multilayers of semiconductor CdS nanoparticles have been deposited on a gold substrate using the self-assembly of dithiol molecules. 3,4 There has been some effort to obtain regular arrangements of metal nanoparticles in different dimensions. A linear arrangement of Au clusters stabilized by phosphine ligands has been obtained by binding them to single-stranded DNA, 5 while ordered channels of porous alumina membranes have been filled with Au particles to obtain wires. 6 Two-dimensional arrays of metal nanoparticles are readily prepared by using alkanethiols. 7-9 Whetten et al. 9 have described gold nanocrystals stabilized by thiols. Although there are indications that multilayers of metal particles can be formed by use of dithiols, 4 there is no definitive evidence, based on spectroscopic, microscopic, and diffraction studies, for such superstructures. It was our interest to prepare superlattices of well-characterized metal quantum dots by use of spacer molecules. In this letter, we report the layer-by-layer deposition of nanocrystalline arrays of quantum dots of a single metal or of two metals by employing dithiol molecules to construct superlattices. Such lattices with alternate layers of metal and semiconductor nanoparticles can also be prepared. The procedure employed to obtain metal quantum dot superlattices is simple. A clean polycrystalline metal substrate of gold or silver, prepared by the resistive evaporation of the metal onto a freshly cleaved mica substrate at 500 °C in a vacuum, was immersed in a 50 mM toluene solution of 1,10- decanedithiol. After 2 h, it was washed with toluene and dried in air. The dithiol-covered substrate was then immersed in a dilute dispersion of metal nanoparticles of the desired size (obtained by the controlled reduction of metal ions complexed with tetra-n-octylammonium bromide in toluene using NaBH 4 ) in toluene for 12 h. After the formation of a particulate layer by this means, it was washed with toluene and dried. The above steps employed for depositing the first layer of nanoparticles were repeated to obtain multilayer superlattices. This is shown schematically in Figure 1. After each deposition, the nanostruc- tures were characterized by X-ray and UV photoelectron spectroscopy (XPS and UPS), scanning tunneling microscopy * Corresponding author. Figure 1. Schematic drawing depicting the layer-by-layer deposition of Pt nanoparticles onto a Au substrate, the layers being separated by dithiol molecules. Also shown is the formation of a heterostructure consisting of alternate layers of semiconductor and metal nanoparticles. 399 J. Phys. Chem. B 1999, 103, 399-401 10.1021/jp983836l CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999