DOI: 10.1002/adma.200701442 Controlled Loading of Nanoparticles into Submicrometer Holes** By Nikolay A. Mirin, Mel Hainey, Jr., and Naomi J. Halas* A wide variety of metallic nanoparticles and nanostructured metals that support localized electromagnetic resonances or propagating surface waves, known as surface plasmons, are becoming extremely important structures for subwavelength optics. This rapidly emerging field, known as plasmonics, of- fers new ways to manipulate electromagnetic radiation, often at deep subwavelength dimensions. The burgeoning list of applications in this area include chemical and biological sensing, [1–5] plasmonic waveguiding, [6] sub-diffraction-limited focusing for imaging applications, [7] and the development of negative index materials. [8] Both chemical techniques and planar fabrication cleanroom-based techniques have been successfully used to create plasmonic structures, and novel protocols have been developed that combine both strategies to fabricate new plasmonic architectures unattainable by either approach alone. [9–11] A particularly promising plasmonic architecture is a “nano- hole,” a hole with a submicrometer diameter fabricated in an otherwise continuous metallic film. Interest in continuous films with nanoscale holes was initially stimulated by reports of extraordinary optical transmission through subwavelength hole arrays, [12] which have led to elegant examinations of the origin of their unique and complex electromagnetic response. [13] A recent series of investigations has focused on individual nanoholes and their plasmonic properties. [14,15] In- dividual nanoholes are fabricated using a hybrid method where chemically synthesized nanoparticles are utilized as sacrificial masks, placed on a substrate prior to metal film growth, and then subsequently removed. [14] The resulting geometry of randomly dispersed, noninteracting individual holes is particularly compelling for sensing and plasmonic applications, since the holes could be filled with various media or analytes of interest. Strong interactions between the hole plasmon and plasmonic nanoparticles placed inside the hole should result in new geometries in which hybridized plasmons may be observed, [16] and filling the holes with other materials will provide opportunities for studying plasmonic coupling to excitations in other types of materials. [17,18] To date, only a single study has reported the incorporation of nanoscale objects within these structures, with considerable variations in yield of filled holes and in filling factor. [19] Here we describe a simple and easily controlled method for trapping and loading plasmonic nanoparticles into submicron holes in continuous metallic films. We show that holes with positively charged floors fabricated in an Au film with a hydrophobic molecular coating provide an attractive trap potential for nanoparticles with negative surface charges. This “electrostatic funneling” mechanism has recently been shown to be useful in other geometries for controlled deposition of nanoparticles into specific regions of fabricated structures. [20] While nanoparticle trapping proceeds by electrostatic attrac- tion between hole and particles, the number of nanoparticles that can be loaded into a hole trap is controlled by adjusting the ionic strength of the solution in which the nanoparticles are suspended. Once inside the trap’s attractive potential, in- teractions, such as Van der Waals forces, mediate the particle- particle interactions, resulting in a reduction in interparticle spacings and the formation of compact nanoparticle aggre- gates in the center of the trap. Once trapped, the nanoparti- cles remain stably bound to the floors of the nanoholes. The samples of submicron holes in Au films have been fab- ricated using a modified nanoparticle lithography procedure previously described. [14] The fabrication procedure is outlined in Figure 1. The polymer coating on the glass substrate was chosen to be an ionic polymer with high positive charge den- sity so that both the negatively charged latex particles and the Au colloidal particles will strongly adhere. Poly(diallyldi- methylammonium chloride) (PDDA) forms a dense flat monolayer on glass surfaces, [21] and supports the deposition of high quality metal films on its surface. Each monomer unit of PDDA carries a positive charge with an attached negative counterion. In aqueous media exposed regions of the polymer release counterions, building up a significant positive surface charge inside the holes. Detachment of the charged PDDA chains from the exposed regions within the holes does not ap- pear to occur. It is likely that the charged PDDA interacts electrostatically with the glass surface. Since the average chain length of the PDDA (2500–3000 nm) is nominally 10 times larger than the hole diameter (60–300 nm), the PDDA chains COMMUNICATION Adv. Mater. 2008, 20, 535–538 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 535 – [*] Prof. N. J. Halas Department of Electrical and Computer Engineering William Marshall Rice University 6100 Main Street MS-366 Houston TX 77005, USA E-mail: halas@rice.edu N. A. Mirin, M. Hainey, Jr. Department of Chemistry William Marshall Rice University 6100 Main Street MS-60 Houston TX 77005 (USA) [**] The authors wish to thank Prof. E. Zubarev and Prof. P. Nordlander for useful discussions. This work is supported by the Air Force Office of Scientific Research Grant F49620-03-C-0068, the National Science Foundation (NSF) Grant EEC-0304097 and ECS-0421108, the Texas Institute for Bio-Nano Materials and Structures for Aero- space Vehicles funded by NASA Cooperative Agreement No. NCC-1- 02038, the Robert A. Welch Foundation Grant C-1220, and the Multidisciplinary University Research Initiative (MURI) Grant W911NF-04-01-0203.