Luminescence Quantum Yield of Single Gold Nanorods Mustafa Yorulmaz, Saumyakanti Khatua, Peter Zijlstra, † Alexander Gaiduk, and Michel Orrit* Institute of Physics, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands * S Supporting Information ABSTRACT: We study the luminescence quantum yield (QY) of single gold nanorods with different aspect ratios and volumes. Compared to gold nanospheres, we observe an increase of QY by about an order of magnitude for particles with a plasmon resonance >650 nm. The observed trend in QY is further confirmed by controlled reshaping of a single gold nanorod to a spherelike shape. Moreover, we identify two spectral compo- nents, one around 500 nm originating from a combination of interband transitions and the transverse plasmon and one coinciding with the longitudinal plasmon band. These components are analyzed by correlating scattering and luminescence spectra of single nanorods and performing polarization sensitive measurements. Our study contributes to the understanding of luminescence from gold nanorods. The enhanced QY we report can benefit applications in biological and soft matter studies. KEYWORDS: Gold nanorods, photothermal, luminescence, QY, reshaping, interband O ptical probes that provide good contrast and are small enough not to perturb the system under investigation are essential to obtain structural and dynamical information on the nanoscale. For this purpose, single fluorescent molecules are widely used as probes of, for instance, soft matter systems 1−4 and biological mechanisms 5,6 since they are small and have a high fluorescence quantum yield. However, single molecules suffer from blinking 7 and bleaching, 8 which limits their observation time. Gold nanoparticles do not blink nor bleach and their nontoxicity and biocompatibility make them attractive for biological applications. 9,10 There has been a considerable effort to investigate their size- and shape-dependent optical proper- ties, both experimentally 11,12 and theoretically. 13,14 Their scattering and absorption properties have been widely investigated and characterized, which has led to many new applications. 15 In recent studies, the photoluminescence from single gold nanoparticles has proven to be a complementary property to absorption and scattering for imaging and sensing purpo- ses. 16−20 Although the luminescence quantum yield (QY) of gold nanoparticles is several orders of magnitude lower than the QY of fluorescent labels such as organic dyes or semiconductor nanoparticles, their large absorption cross section compensates for their low QY, making them high-contrast imaging agents. The first observation of photoluminescence of gold dates back to 1969, when Mooradian studied bulk gold 21 and observed a broad luminescence spectrum with a QY of about 10 −10 . Photoluminescence from bulk gold originates from radiative transitions of conduction electrons toward empty electron states, which can be either holes in the d-band (electron−hole interband recombination), 21−24 or empty electron states or holes within the sp-conduction band (intraband transitions). 25 Later, the effect of surface roughness on the photo- luminescence of gold was studied by Boyd et al. who showed that the QY could be enhanced by several orders of magnitude compared to a smooth film. 23 The enhancement was attributed to the concentration of fields at tips of surface protrusions (lightning-rod effect) and to the presence of localized surface plasmons. 23,26 Since this first observation of plasmon-enhanced emission, the effect of localized surface plasmons has been investigated in solutions of nanoparticles with different sizes and shapes. Early efforts to understand the emission from gold nanoparticles have relied on ensemble measurements. A QY of ∼10 −5 to 10 −4 was reported for ensembles of 5 nm diameter gold nanospheres by Wilcoxon et al. 27 Mohammed et al. 28 have measured the luminescence from ensembles of gold nanorods with aspect ratios ranging from 2.4 to 5.4 and reported QYs of 10 −4 −10 −3 . They have explained the observed luminescence by the radiative recombination of electron−hole pairs that is enhanced by local fields associated with the particle plasmon resonance. In a recent ensemble study by Dulkeith et al., the QY of gold nanospheres with diameters ranging from 2 to 60 nm was reported to be ∼10 −6 and independent of size. 29 These authors argued that the moderate field enhancement of about 10 was too weak to explain this enhancement by 4 orders of magnitude compared with bulk gold. Instead of the radiative recombination of e−h pairs, the authors proposed a process in which d-band holes recombine non-radiatively with sp Received: June 11, 2012 Revised: July 6, 2012 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl302196a | Nano Lett. XXXX, XXX, XXX−XXX