In this Letter, we introduce scanning tunneling spectroscopy (STS) to quantify the Urbach energy (E U) in disordered semiconductors. The technique enabled us to gain precise information on the extending component of conduction and valence band-edges responsible for Urbach tailing, individually; such information has been obtained from the width of band-energy-histograms drawn from STS studies at many different points. STS, as a probing method at the microscopic scale to derive E U, is in contrast to commonly employed optical spectroscopy studies which provide information at the macroscopic scale. A comparison between Urbach energy values from optical studies and distribution of band-edges obtained from STS revealed the inherent inaccuracies involved in the optical characterization process. We have considered copper oxide (Cu x O) thin films in this regard; we show that through STS and the associated density of state (DOS) spectra, we can derive accurate information on the band-edges’ distribution leading to E U in different phases of the binary oxide thin films.

Bandgap bowing has recently been emerged as an effective strategy toward band-engineering in metal halide perovskites. In this work, we report extensive studies on the bowing phenomenon in Cs2NaBiCl6 double perovskite upon alloying with silver at the sodium site. Through optical spectroscopy, composition-dependent bandgap in Cs2Na1−x Ag x BiCl6 (0 ⩽ x ⩽ 1) evidenced bandgap bowing with an upward-concave nature. Further from the quadratic fit, the bowing coefficient (b = 0.74 eV) turned out to be independent of composition and is close to the theoretically predicted value. From scanning tunneling spectroscopy and associated studies on band-energies, we have observed that the bandgap-lowering originated from a significant change of the valence-band position. The behavior of band-energies has been explained through the orbital-contributions of the constituent elements responsible in forming the two bands. Formation of an internal type-I band-alignment between the two end-members, namely Cs2NaBiCl6 and Cs2AgBiCl6 could be visualized. Based on experimental evidences, we could infer that the bow-like evolution of bandgap in Cs2Na1−x Ag x BiCl6 alloys is principally dominated by structural distortions in the Cs2NaBiCl6 host-lattice upon incorporation of silver.

We report the formation of CuIn (1−x) Ga x S 2 (CIGS) thin films through a solution approach, namely, successive ionic layer adsorption and reaction (SILAR) technique. The obtained films possessed a high degree of crystallinity indicating the efficacy of the deposition process in forming CIGS films. A series of alloys have evidenced band gap bowing, that is, the optical band gap does not follow a linear relationship with the composition; the band gap of an intermediate compound is higher than that is interpolated from a linear relationship or Vegard's law. The composition-dependent band gap followed a quadratic relationship evidencing reverse band gap bowing, manifesting an upward convex behavior. With scanning tunneling spectroscopy (STS) and thereby the density of states of the disordered semiconductors, we have observed a bowing behavior in the transport gap as well and identified the roles of the conduction and valence bands in yielding the bowing phenomenon. The results are explained in terms of the anionic and cationic orbitals involved in forming the two bands. The STS studies have been analyzed further to derive the Urbach energy of the disordered semiconductors. When STS studies are combined with Kelvin probe force microscopy, which in effect provides the Fermi energy of the alloyed semiconductors, we could derive the band edges and the Fermi energy of the whole series in an absolute energy scale during the manifestation of the bowing phenomenon.

The active sites of the electrocatalyst play a crucial role in material designing and mechanistic exploration of an electrocatalytic reaction. The defect-tailored heteroatom-doped carbon-based electrocatalysts for oxygen reduction reaction (ORR)...

No abstract