IN SITU MEASUREMENT OF SURFACE PLASMON ENERGIES ON METAL NANOPARTICLES AS A FUNCTION OF ENVIRONMENT

Pin Ann Lin, Jonathan Winterstein, Henri Lezec, and Renu Sharma

         Metal nanoparticles are being used in nano-sensing and optical devices due to their surface plasmon resonance (SPR) property.  The SPR peak positions and intensities depend both on the size and shape of the particles as well as the local dielectric surroundings, making them sensitive to environmental changes.  We have employed an environmental scanning transmission electron microscope (ESTEM) equipped with a monochromator to measure the SPR energies of gold nanoparticles using electron energy-loss spectroscopy (EELS).  Detection of the dark modes and weak dipoles for localized SPRs with high spatial and energy resolution is further improved by using the ESTEM in scanning transmission electron microscope (STEM) mode.  The STEM-EELS data were obtained with a beam size on the order of 1 nm and with 0.12 eV energy resolution from individual gold nanoparticles.  We have utilized this technique to map the SPR intensity of gold nanoparticles with various shapes in vacuum.  In addition to measuring the effects of particle geometry, we also measure the effect of substrates with different refractive index (n) values on the localized SPR energies of nanoparticles.  The ability to measure environmental effects is demonstrated by introducing hydrogen at various partial pressures and recording the SPR peak positions (energy shift) simultaneously.  A specific SPR mode of gold nanoparticles on TiO2 substrate shows a red-shift with hydrogen introduction.   Changes in the position and intensity of red-shift as observed on edges of the gold particle on substrates with different dielectric constants and in gaseous environment will be presented.