NEAR-FIELD SPECTROSCOPY OF SINGLE MOLECULES AT ROOM-TEMPERATURE

THE ability to observe the optical spectrum of a single molecule can a fford insights into the interactions that distinguish one molecular en vironment from another. Such sensitivity has recently been achieved at liquid-helium temperatures(1-5). Here we show that the near-field sca nning optical microscope(6,7) can be used to obtain the time-dependent emission spectrum of a single molecule in air at room temperature, wi th a spatial resolution of about 100 nm. We have examined single molec ules of -dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (diI) dispe rsed on polymethylmethacrylate. The spectra of individual molecules ex hibit shifts of +/-8 nm relative to the average spectrum, and are typi cally narrower, as is expected for spectral lines broadened inhomogene ously (that is, by a distribution of molecular environments). The spec tra also vary in width by up to 8 nm, some being as broad as the far-f ield many-molecule spectrum. The emission spectra of some individual m olecules exhibit time-dependent shifts of up to 10 nm. This variety in spectral position, width, shape and time dependence can be understood within a model of inhomogeneous broadening in which there is a distri bution of barrier heights to rearrangement of the molecular environmen t.