The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8 eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4×10^{7} V m^{-1}, are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.
We should like to thank the NSRRC for providing access to the VUV High-Flux beam line BL03A for this work. We acknowledge the support of the UK Science and Technology Facilities Council (STFC, ST/M001075/1), UK Engineering and Physical Science Research Council (EPSRC, EP/D506158/1), and the European Community FP7-ITN Marie-Curie Programme (LASSIE project, grant agreement No. 238258). A. R. F. thanks HWU for a James Watt Scholarship. G. M. C. also acknowledges support of funding under MINECO (YA2014-60585-P and AYA2015- 71975-REDT). A. C. would like to acknowledge the support of the Grant No. MOST106-2112-M-008-019.
