Optical absorption microscopy and spectroscopy with nanometre resolution

Abstract

SEVERAL variants of the scanning tunnelling microscope (STM) have been developed since its invention1. Here we present the first demonstration of a new type of microscope, based on the STM, that is capable of recording optical absorption spectra with nanometre (that is, molecular) resolution. Our starting point is conventional photothermal spectroscopy, in which the temperature rise owing to the absorption of tunable monochromatic light is measured, giving a direct measurement of the absorption spectrum of a sample as a function of illuminating wavelength. The spatial resolution of such measurements is determined by the sizes of the pump beam or detector, whichever is smaller. We use the STM to measure the 'thermoelectric' tunnelling voltage due to absorption. Conventional photothermal measurements2,3 have been limited to a resolution of ∼1 μm, although we have recently developed a thermal sensor with a spatial resolution of ∼50 nm (ref. 4), and suggested its use for spectroscopy on that scale5. The STM, however, permits resolution of about 1 nm, and as the image is controlled by the thermal properties of the sample, our technique can image variations in the thickness of surface films, and changes in sub-surface characteristics, as well as chemical variations. © 1989 Nature Publishing Group.