Electrochromic effect in Q-carbon

Abstract

Here, we report the electrochromic effect in undoped Q-carbon. This unique phase of carbon, Q-carbon, is formed by pulsed laser melting of amorphous carbon and subsequent ultrafast quenching process. The excess amount of unpaired electrons near the Fermi energy level in the Q-carbon causes a 48% increase in the optical absorption at 265 nm with an applied electric field of 10 V. The Kelvin probe force microscopy (KPFM) also indicates that the Fermi level of Q-carbon is situated ∼40 meV higher than that in the neighboring diamond-like carbon region. It is also observed that, with an increase in the tip voltage from 0 to 10 V in KPFM, there occurs an increase in the image phase contrast thereby indicating an increase in the concentration of electrons. This causes an increase in optical absorbance with the application of an electric field in Q-carbon. The direct and indirect optical band gaps in Q-carbon are calculated to be 3.82 and 2.93 eV, respectively, using the Tauc analysis. High-resolution scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction pattern depict the formation and amorphous nature of Q-carbon. Raman and electron energy-loss spectroscopy of Q-carbon reveal over 75% sp3-bonded carbon (rest sp2) and excess electrons near the Fermi level. This discovery of electrochromic effect in Q-carbon shows that highly non-equilibrium synthesis can be elegantly used to fabricate smart windows with immense energy-saving applications.