NEAR-FIELD SPECTROSCOPY OF SINGLE MOLECULES AT ROOM-TEMPERATURE

THE ability to observe the optical spectrum of a single molecule can afford insights into the interactions that distinguish one molecular environment from another. Such sensitivity has recently been achieved at liquid-helium temperatures(1-5). Here we show that the near-field scanning optical microscope(6,7) can be used to obtain the time-dependent emission spectrum of a single molecule in air at room temperature, with a spatial resolution of about 100 nm. We have examined single molecules of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (diI) dispersed on polymethylmethacrylate. The spectra of individual molecules exhibit shifts of +/-8 nm relative to the average spectrum, and are typically narrower, as is expected for spectral lines broadened inhomogeneously (that is, by a distribution of molecular environments). The spectra also vary in width by up to 8 nm, some being as broad as the far-field many-molecule spectrum. The emission spectra of some individual molecules 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 distribution of barrier heights to rearrangement of the molecular environment.