Reverse bandgap-bowing in nickel-cadmium sulfide alloys (Ni_1-CdS) and its origin

We present evolution of band energies in α-NiS when alloyed with a cationic doping through isovalent cadmium (Cd²⁺). Optical bandgap of nickel-cadmium sulfide (Ni_1-CdS) alloys, as a deviation from the linear relationship or Vegard's law, have exhibited a reverse bandgap-bowing in the form of downward-concave dependence. Such a phenomenon, which manifests as a negative value of bowing coefficient (b), is uncommon in chalcogenide alloys. In this work, we have deliberated on the origin of reverse bandgap-bowing in nickel-cadmium alloys and identified the band responsible for the bowing phenomenon. While thin-films of the alloys were formed through successive ionic layer adsorption and reaction method, tunnel conductance and thereby density of states of the materials were derived from scanning tunneling spectroscopy. The spectroscopy provided the variation of conduction and valence band-edges (CB and VB, respectively) with respect to the cadmium-content in Ni_1-CdS. The CB-edge of the alloys could be seen to remain mostly unaffected with increasing cadmium-content, since the band is composed of only the S 2porbitals; the VB-energy, on the other hand, which forms due to an effective coupling between the metaldand the anionporbitals, could be seen to be affected due to ap-drepulsion. Based on our experimental findings, we inferred that an antagonism between volume deformation and structural relaxation had resulted in the reverse bandgap-bowing in Ni_1-CdS alloys.