Mercaptoalkanoic Acid-Induced Band Gap Attenuation of Copper Selenide Quantum Dot

Copper selenide quantum dot (CuSeQD) materials functionalised with mercaptoalkanoic acids {3-mercaptopropionic acid (3-MPA), 6-mercaptohexanoic acid (6-MHA) and mercaptosuccinic acid (MSA)} were synthesized by a reproducible aqueous colloidal technique at room temperature. The impact of the capping agents on the size and the crystallinity of the CuSeQD materials, were investigated by small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) spectroscopic techniques, respectively. SAXS results confirmed that 6-MHA-CuSeQD had the smallest average particle core size when dried, whereas MSA-CuSeQD had the smallest size in aqueous solution, though with a tendency to aggregate. Dynamic light scattering (DLS) measurements indicated strong bonding of the capping agents to CuSe particles, with MSA being the weakest binding agent, as confirmed by comparatively, low Zeta potential(zeta=-31.1 mV) and high polydispersity index (0.469) values. UV-Vis absorption studies confirmed a large blue shift of the band gap for the QD compared to the bulk material, with characteristic absorption band (lambda) and direct band gap (E-gd) values being (lambda=435 nm, E-gd=8.0 eV), (lambda=400 nm, E-gd=5.6 eV) and (lambda=340 nm, E-gd=4.0 eV), for 6-MHA-CuSeQD, 3-MPA-CuSeQD and MSA-CuSeQD, respectively. As supported by the formal potential values for 6-MHA-CuSeQD (E-0 'approximate to 120 mV), 3-MPA-CuSeQD (E-0 'approximate to 159 mV) and MSA-CuSeQD (E-0 'approximate to 183 mV), the smaller the particle size, the lower the potential required for the application of the quantum dots in an electron transfer process.