Self-assembly of CdSe/CdS quantum dots by hydrogen bonding on Au surfaces for photoreception Jing Tang, a Henrik Birkedal, b Eric W. McFarland* c and Galen D. Stucky* ab a Materials Department, University of California, Santa Barbara, CA, 93106, USA b Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, 93106, USA. E-mail: stucky@chem.ucsb.edu c Department of Chemical Engineering, University of California, Santa Barbara, CA, 93106, USA. E-mail: mcfar@engr.ucsb.edu Received (in West Lafayette, IN) 17th June 2003, Accepted 21st July 2003 First published as an Advance Article on the web 5th August 2003 CdSe/CdS core-shell quantum dots have been self-assembled onto thiolcarboxylic acid functionalized gold surfaces by hydrogen bonding; control of the pH during deposition allows producing a high coverage photoactive surface for use in a surface sensitized Schottky barrier photovoltaic struc- ture. The self-assembly of semiconductor quantum dots (QDs) onto conducting substrates has been intensively studied due to the potential role for such assemblies in the fabrication of optoelectronic devices and chemical sensors. 1–3 QDs may be used as an alternative to the organic dyes used as photoreceptors on surface sensitized Schottky barrier photovoltaic structures. 2 The unique tunable optical properties of QDs and their high resistance to photo-bleaching will offer a QD-based device additional advantages over the corresponding dye-system. The QD device structures are typically fabricated utilizing a bifunctional self-assembled monolayer (SAM) on the conduct- ing surface as a molecular linker. By selecting appropriate linker functional groups, semiconductor QDs can be deposited onto the substrate either covalently or by non-covalent inter- actions such as hydrogen bonding or electrostatic attractions. 4 We report here a self-assembling system based on water-soluble citrate-capped CdSe/CdS core-shell QDs which assemble through hydrogen bonding onto a gold surface prefunction- alized by a thiolcarboxylic acid SAM. The procedure is found to be applicable for large-area device fabrication with a homoge- neous surface coverage. Citrate stabilized CdSe/CdS QDs (4–10 nm in diameter) were synthesized using the method of Rogach et al. 5 and sized by transmission electron microscopy. Surface modification of the Au substrate† with SAM molecular linkers was performed in a 2 mM ethanolic solution of 3-mercaptopropionic acid (MPA) under argon for 1 hour, followed by thorough rinsing with ethanol and drying in a stream of argon. The assembly of QDs onto the modified substrate was performed in an aqueous solution containing CdSe/CdS QDs for 4 hours under argon at room temperature. The pH of the freshly made QD solution was approximately 7.2 and was adjusted by dropwise addition of 0.1 M HCl. Fig. 1 (a–e) shows AFM (tapping mode) images of several samples prepared under different conditions. The spherical particles appearing white in the images are CdSe/CdS QDs which are also indicated in the AFM cross sectional profile accompanying each image. Fig. 1b shows that few QDs were attached to the substrate without the molecular linker due to the weak interaction between the citrate capping agent and the bare Au surface. However, when the pH is reduced from 7.0 to 6.0, as shown in Fig. 1c and Fig. 1d, respectively, the observed coverage of QDs increased significantly in the presence of the molecular linkers. At pH 6, Fig. 1d, the coverage was approximately 77%. Lowering the pH to 5.0, Fig. 1e, causes aggregation of the QDs. X-Ray Photoelectron Spectroscopy (XPS) was used to confirm the surface composition. The spectra in Fig. 1f were recorded from the same sample shown in Fig. 1d. Comparison of the XPS spectra obtained before (dotted line) and after addition of QDs (solid line) shows that attachment leads to a small shift toward lower binding energy for the 162 eV sulfur 2p peak. The sulfur emission measured before QD attachment is due to sulfur atoms bound only to Au. The weaker binding energy between sulfur and cadmium in the QD’s shell results in the small shift. Note that the original signal intensity from Au–S bond near the substrate vanished because of the electron signal attenuation by the high QD coverage. 6 We propose that the interaction between the QDs and the MPA SAM is one of hydrogen bonding. The effectiveness of this interaction depends strongly on whether the participating carboxylic acid moieties are protonated or not; which in turn is a function of pH and the respective pK a values of the acids. The Fig. 1 (a) AFM image of Au substrate; (b) image of quantum dots attached to the Au surface without molecular linkers at pH = 7; (c–e) images of quantum dots attached to the Au surface with molecular linkers at pH = 7, pH = 6 and pH = 5, respectively; all scale bars in AFM images represent 200 nm; (f) XPS spectra from sample (d) before (dotted) and after (solid) quantum-dot attachment; data calibrated to Au 4f(7/2) = 84.0 eV. This journal is © The Royal Society of Chemistry 2003 2278 CHEM. COMMUN. , 2003, 2278–2279 DOI: 10.1039/b306888a