YANG ET AL . VOL. 8 ’ NO. 8 ’ 7639–7647 ’ 2014 www.acsnano.org 7639 June 23, 2014 C 2014 American Chemical Society Hetero-oligomer Nanoparticle Arrays for Plasmon-Enhanced Hydrogen Sensing Ankun Yang, † Mark D. Huntington, † M. Fernanda Cardinal, ‡ Sicelo S. Masango, ‡ Richard P. Van Duyne, ‡ and Teri W. Odom †,‡, * † Department of Materials Science & Engineering and ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States M etallic nanoparticles (NPs) can spatially confine light on the nano- meter scale by trapping energy into localized surface plasmon (LSP) modes. 1À3 The LSP resonance of a single NP depends on the material, size, and shape as well as the local dielectric environment. 4 This localized mode can also be tuned by placing another metal NP in close proximity (<50 nm), 5 where the LSP of one particle can couple to the other through dipolar interactions. 6 Furthermore, plasmonic NPs arranged into assemblies (three or more NPs) can manipulate near-field profiles 7À9 and modify far-field characteristics compared to a single NP. 8 For example, Au NP trimers can enhance the intensity of dark plasmon modes, 8,10 and Au NP tetramers can preserve the polarization state of incident light while enhancing the local field. 11 The intense loca- lized fields confined within the gaps of NP assemblies have been exploited for appli- cations such as surface-enhanced Raman scattering 12 and plasmon-enhanced biological and chemical sensing. 13 Plasmonic assem- blies made from different materials (e.g., AuÀAg NP dimers) can show otherwise forbidden optical signatures because of composition asymmetry. 14,15 Plasmonic het- eroparticle assemblies can also boost the function of weak or nonplasmonic materials in plasmon-enhanced chemistries. 16,17 For example, AuÀPd NP dimers and Au@Pd coreÀshell nanocrystals have been tested as H 2 gas sensors 16,18 because Pd NPs can rapidly incorporate hydrogen gas due to short diffusion lengths for H atoms, while the enhanced near-fields of Au NPs sensi- tively detect and report changes in Pd NPs as far-field spectral shifts. Bottom-up methods that can organize metallic NPs into well-defined assemblies typically involve molecular linkers with spe- cific recognition properties. For example, the hybridization of complementary DNA strands can facilitate the assembly of Au NPs into extended structures 19 with control over NP separation down to 0.5 nm; 20 however, * Address correspondence to todom@northwestern.edu. Received for review May 7, 2014 and accepted June 23, 2014. Published online 10.1021/nn502502r ABSTRACT This paper describes how the ability to tune each nanoparticle in a plasmonic hetero-oligomer can optimize architec- tures for plasmon-enhanced applications. We demonstrate how a large-area nanofabrication approach, reconstructable mask litho- graphy (RML), can achieve independent control over the size, position, and material of up to four nanoparticles within a subwavelength unit. We show how arrays of plasmonic hetero- oligomers consisting of strong plasmonic materials (Au) and reactant-specific elements (Pd) provide a unique platform for enhanced hydrogen gas sensing. Using finite-difference time-domain simulations, we modeled different configurations of AuÀPd hetero-oligomers and compared their hydrogen gas sensing capabilities. In agreement with calculations, we found that AuÀPd nanoparticle dimers showed a red-shift and that AuÀPd trimers with touching Au and Pd nanoparticles showed a blue-shift upon exposure to both high and low concentrations of hydrogen gas. Both AuÀPd hetero-oligomer sensors displayed high sensitivity, fast response times, and excellent recovery. KEYWORDS: plasmonic assemblies . heterogeneous oligomers . nanofabrication . hydrogen sensing . AuÀPd nanoparticle dimers . AuÀPd nanoparticle trimers ARTICLE