We have designed and constructed a low temperature, ultrahigh vacuum scanning tunneling microscope ͑STM͒, taking extreme measures to isolate the microscope from acoustic, vibrational, and electronic noise. We combined a 4 K STM with line-of-sight dosing to enable one to position the crystal surface in front of an impinging molecular beam as in scattering experiments. Due to the mechanical stability of the instrument and the minimal thermal drift associated with working at 4 K we are able to locate and to image repeatedly isolated adsorbates and atomic-scale structures, such as step edges, for extended periods days. The instrument has been designed for the topographic and spectroscopic characterization of atoms and molecules on metal and semiconductor surfaces, for the investigation of the mechanism by which the STM images adsorbates on surfaces, and for inelastic electron tunneling spectroscopy of single molecules.

Two-dimensional hexagonal arrays of micrometer-sized spheroidal cavities were fabricated from poly(vinyl alcohol) films on microscope cover glasses. The patterned surfaces in contact with solution and another regular cover glass were used to contain single molecules. Bursts of fluorescence from a series of single molecules entering and leaving the beam focus were observed. The molecules were all rotating rapidly compared with the fastest binning time of 100 µs. The fluorescence anisotropy decayed on the nanosecond time scale. The fluorescence spectra of mixtures of dyes confirmed that the bursts separated by time intervals of several to tens of seconds are from different molecules, while those bursts spanning intervals of several to tens of milliseconds are from the same molecule continually reentering the focus. The autocorrelation function of the time-resolved fluorescence intensity suggests a translational diffusion coefficient of 1.7 × 10 -8 cm 2 s -1 for 6-carboxyrhodamine 6G hydrochloride molecules near the pattern, which is ∼260 times smaller than that in free solution. The mechanism of slowing the transverse diffusion process of single molecules near the pattern was further elucidated by total internal reflection microscopy, from which the molecules were observed to be avoiding the cavities.

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