Optically Transparent Au{111} Substrates: Flat Gold Nanoparticle Platforms for High-Resolution Scanning Tunneling Microscopy Daminda H. Dahanayaka, Jane X. Wang, Sohrab Hossain, and Lloyd A. Bumm* Center for Semiconductor Physics in Nanostructures, Homer L. Dodge Department of Physics and Astronomy, The UniVersity of Oklahoma, 440 West Brooks Street, Norman, Oklahoma 73019 Received February 5, 2006; E-mail: bumm@nhn.ou.edu In the last two decades, Au{111} substrates have become widely used for surface modification, 1 nanolithography, 2,3 and single molecule studies 4,5 because of the broad applicability of alkanethiol self-assembled monolayer (SAM) chemistry. Typically, theses substrates are Au{111} oriented thin films on mica 6 or Au{111} surfaces of bulk single crystals, which are both commercially available. Because these substrates are opaque, optical access is restricted to the front side, which complicates simultaneous electrical and optical investigation on Au{111} substrates. We recently developed a new type of Au{111} substrate which is transparent (allows optical access from front and back) 7 and is suitable for high- resolution STM. This substrate consists of solution-grown flat gold nanoparticles (FGNPs) that are deposited on indium tin oxide (ITO)- coated glass. The transparent ITO support allows optical access to the FGNP substrates, which exhibit plasmon resonance modes extending well into the NIR region. These FGNP/ITO substrates have the benefits of low cost, simple preparation, and broad applicability. We also anticipate that the plasmon resonances of the FGNPs will facilitate optical coupling to the adsorbed molecules. We propose that these substrates can be generally applied as atomically flat Au{111} substrates for scanning probe microscopy and that the FGNPs will find application as nanometer-scale platforms. High-resolution STM imaging imposes several stringent criteria on the substrates. The substrate must be atomically flat, or be composed of atomically flat terraces large enough for imaging. This is necessary, otherwise the surface roughness will swamp out the molecular corrugation of the SAM and also can lead to disorder of the SAM. This practically limits our choice to single crystals. The substrate must also be in electrical contact to the STM electronics and, for our purposes, also optically transparent. ITO-coated glass is a readily available supporting substrate for the FGNPs, which is both optically transparent and electrically conducting. Although the ITO coating is rough at the nanometer scale, the FGNPs ride on the ITO and provide the required atomically flat surfaces. The FGNPs are prepared by adding 1.2 mL of 48 mM citric acid solution to 95 mL of 0.24 mM HAuCl 4 solution at 4 °C (refrigerator). The particles grow over a period of 3-4 days. The resulting sol is polydisperse, consisting of large flat triangles, hexagons, and intermediate shapes along with spheroidal particles. The flat particles are single crystals which range in lateral size from tens to thousands of nanometers but are only 15-20 nm thick. Our procedure is a variant of the standard citrate sol described by Turkevich, 8 reducing HAuCl 4 with citric acid. 9 The FGNPs are deposited on ITO-coated glass substrates by centrifugation. Freshly cleaned ITO substrates were placed ITO side up at the bottom of a test tube filled with the Au sol containing the FGNPs, and centrifuged in a swinging-bucket rotor for 10 min at 600-1500g (min/max g values at the inner/outer radius). Centrifu- gation deposits all the particles onto the ITO surface. The resulting reddish brown layer of nanoparticles is visible by eye. Figure 1A is an SEM image of this very dense layer containing FGNPs, Au spheroids, and aggregates. Ultrasonication in deionized water for a few minutes removes the spheres and aggregates leaving the FGNPs (Figure 1B). Longer ultrasonication times do not dramati- cally affect the results. The ultrasonication step is so efficient that our initial efforts to separate the FGNPs from the spheroids proved unnecessary. We hypothesize that the FGNPs are more strongly bound than the spheres due to their larger contact area. These substrates constitute our measurement-ready samples. Extinction spectra of FGNPs on glass show that the plasmon resonances of the FGNPs extend well into the NIR. The polydis- persity of the FGNPs produces the broad extinction band beginning above 600 nm and extending out past 1600 nm. This region includes the dipole, quadrupole, and higher modes. 10,11 STM images of the FGNPs typically show that only 3-4 atomic layers are exposed at the surface (Figure 2A). The topmost exposed layers are islands, and the lowest exposed layers are vacancy islands. In all cases we have observed, the island step edges are meandering steps. We only observe low index step edges at the edges of the FGNP. Furthermore, the islands are distributed uniformly across the FGNP surface, indicating that the thickness is uniform across the particle. Note that the ITO surface is very rough at the nanometer scale, which is typical for sputtered polycrystalline films. Despite the roughness of the ITO supporting substrate, the FGNPs remain flat for high-quality STM imaging. Figure 1. (A) SEM image before ultrasonication of the crude FGNPs on ITO. (B) SEM image of the measurement-ready substrate after 5 min of ultrasonication; the surface has been cleaned of spheres, aggregates, and poorly contacted FGNPs. (C) Vis-NIR extinction spectrum of the FGNP/ glass substrate prepared as in B. The glitch at 800 nm is an artifact due to a grating change. The fine structure above 1000 nm is due to optical interference from multiple reflections within the glass substrate. The peak at 540 nm is due to residual spheroidal particles. Published on Web 04/19/2006 6052 9 J. AM. CHEM. SOC. 2006, 128, 6052-6053 10.1021/ja060862l CCC: $33.50 © 2006 American Chemical Society