Size-Dependent Extinction Coefficients and Transition Energies of Near-Infrared β‑Ag₂Se Colloidal Quantum Dots

Our investigations of silver selenide colloidal quantum dots, emitting in the biologically important near-infrared region, demonstrate the size-dependence of their optical properties. Ag₂Se nanocrystals were prepared in the orthorhombic phase with their average radius varying from 0.95 to 4.7 nm as observed by transmission electron microscopy. The high purity of the samples, established by energy-dispersive X-ray spectroscopy and X-ray diffraction, allowed for the accurate determination of the Ag₂Se content of colloidal suspensions by a thermogravimetric method. The energy of the first observed transition is found to decrease asymptotically with colloidal quantum dot size, tending toward a value of 1.1 eV, a value significantly above the β-Ag₂Se bulk bandgap. Furthermore, the molar extinction coefficient of this absorption is proportional to r₀^2.7±0.2, where r₀ is the cQD radius. At higher energies, the extinction coefficient eventually follows the classically predicted cubic power law with r₀.