Preparation and characterisation of silver quantum dot superlattice using self-assembled monolayers of pentanedithiol Sushama Pethkar, a M. Aslam, a I. S. Mulla, a P. Ganeshan b and K. Vijayamohanan* a a Physical/Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India. E-mail: viji@ems.ncl.res.in; Fax: 0091-020-5893044 b Low Temperature Lab., Inter University Consortium for DAE Facilities, Indore 452 001, India Received 22nd November 2000, Accepted 13th March 2001 First published as an Advance Article on the web 9th April 2001 A superlattice of silver nanoclusters was prepared using sequential self-assembly of 1,5-pentanedithiol on an Au(111) substrate. The formation of highly ordered silver nanocluster arrays was confirmed using AFM and also by the longitudinal periodicity observed in the low angle X-ray diffraction pattern. In contrast to the behaviour of dithiol self-assembled monolayers (SAM) on gold substrates, which give only blocking behaviour, the superlattice exhibits interesting electrochemical properties in terms of redox accessibility of silver nanoclusters with respect to potential cycling. The room temperature emission spectrum suggests the formation of minibands in the superlattice structure as evidenced by three distinct peaks at 320, 400 and 620 nm. I. Introduction Different methods of organising quantum dot superlattices of metals and semiconductors have received significant interest in recent times as the tailoring of particle sizes and interparticle separation can, in principle, cause unique magnetic, optical and electronic behaviour. 1–5 For example, several nanocluster assemblies organised in different length scales have been found to be promising due to their potential applications in many diverse areas such as optoelectronic devices, single electron transistors and chemical sensors. 4–8 One of the principal objectives is to accomplish the design of broadly applicable synthetic schemes that produce superlattices of nanoparticles where the particle size as well as the interparticle coupling can be controlled. 9 Self-assembled monolayer (SAM) formation offers a simple and flexible method for organising nanoclusters on noble metal surfaces and there have been several attempts recently to obtain a systematic arrangement of metal and semiconductor nanoparticles in different dimensions. Self-organised 2-D nanoparticle superlattices of semiconductors 10,11 and metals 12–15 as well as 3-D assemblies of nanoparticles 16,17 have been constructed and analysed. One of the important advantages of this approach is that the tailoring of band structure can be accomplished by selecting appropriate organic molecules to protect nanoclusters. Since many metallic and semiconducting clusters (Au, Ag, CdS, CdSe etc.) have high affinity for amine and thiol moieties, both the size and the interparticle separation can be controlled by changing the organic spacer. While it has been generally proved that both Ag and Au nanoclusters can be organised on a SAM surface, the sequential extension of this organisation to demonstrate novel optical and electrochemical properties is rather unknown. More significantly, the presence of organic ligands on the cluster surface can lead to considerable disorder in the superlattice formation and it is important to know if a sequential approach can be adopted to extend the organisation of superlattices of different cell parameters. Two-dimensional organisation of these clusters requires the control of the interparticle coupling and in comparison with semiconductors, metal clusters have strong dispersion interactions. 18 The nature of these interactions is indicative of the dependence of superlattice formation on particle size and also the degree of interparticle coupling through the bridging ligand. In the present study, we have fabricated layer-by-layer self- assembly of ordered uncapped silver nanoclusters using a 1,5- pentanedithiol SAM as a building block on a gold substrate. More specifically, the role of the protective monolayer coating in controlling both the particle–particle interaction and the electron transfer properties of Ag clusters is investigated using X-ray diffraction (XRD), photoluminescence and electrochem- istry after repeated SAM formation and cluster organisation. Although superlattices of alkanethiolate passivated silver nanoclusters have been reported earlier, 18,19 this is the first report of a repeated multilayer superlattice with photolumines- cence and electrochemical characterisation. II. Experimental Vacuum deposited 200 nm Au (purity 99.99%) film was prepared using the procedure described previously. 20 The preferred orientation was found to be (111) by XRD. 1,5- Pentanedithiol, AgNO 3 and NaBH 4 obtained from Aldrich were used as received. In all the experiments deionized water from the Milli-Q system was used. The layer-by-layer formation of a silver quantum dot superlattice on a SAM surface was carried out in the following manner. A SAM of 1,5-pentanedithiol was formed on an Au surface from a 1 mM ethanolic solution of the respective compounds for 24 h, followed by washing with ethanol and drying in a jet of argon. The details of the preparation and characterisation of these dithiol monolayers have been reported earlier. 21 The SAM covered gold surface was then introduced into a dilute aqueous dispersion of uncapped silver nanoclusters, instanta- neously prepared by the controlled reduction of 10 24 M aqueous AgNO 3 solution using the desired amount of NaBH 4 by following reported procedures. 22 After the adsorp- tion of silver particles (typically for 2 h), the substrate was washed with ethanol and dried in air. The second layer of dithiol SAM was formed by immersing this substrate in an 1710 J. Mater. Chem., 2001, 11, 1710–1714 DOI: 10.1039/b009372f This journal is # The Royal Society of Chemistry 2001