Multiphoton Scanning Photoionization Imaging Microscopy for Single-Particle Studies of Plasmonic Metal Nanostructures

Photoionization studies of single Au and Ag metal nanostructures are presented, using a scanning multiphoton photoionization microscope (SPIM) with single-electron detection capability. Four-photon photoemission following ultrafast excitation at around 840 nm and two-photon photoemission following excitation at 420 nm yield high signal-to-noise 2D photoelectron images for a variety of sample materials. By way of a test demonstration of the technique, we present results obtained from SPIM imaging photolithographically patterned gold nanostructures, as well as chemically prepared crystalline gold nanorods and polycrystalline silver nanospheres. For both chemically prepared samples, striking differences in the photoemissive properties of individual nanoparticles are observed that have gone unnoticed in bulk studies. Under 840 nm excitation, for example, each Au nanorod on a Pt substrate exhibits a clear cos8(θ−θ0) dependence of photoemission strength on the angle between laser polarization (θ) and the rod axis (θ0), suggesting that four-photon photoemission is initiated by excitation of the long-axis dipolar plasmon resonance. Surprisingly, strongly polarization-dependent photoelectron signals are also observed for nominally spherical Ag nanoparticles, albeit with varying degrees of anisotropy for different particles. AFM images of identically prepared samples reveal coverages that are consistent with those observed in SPIM images, suggesting that particle aggregation is at least not a predominant effect. One possibility consistent with the data is that localized regions of concentrated electric fields (i.e., “hot spots”) or local variations of the emission propensity in these polycrystalline particles may be responsible for the polarization anisotropy, as well as dramatic temporal variations in the electron emission intensities. In summary, the studies presented here establish the SPIM technique as a new approach to exploring local electronic properties of individual metallic nanostructures.