Investigation of Copper(I) Oxide Quantum Dots by Near-Edge X-ray Absorption Fine Structure Spectroscopy

Copper(I) oxide quantum dots (OQDs) were grown in various thicknesses on different SrTiO 3 (001) surfaces and were investigated by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experimental growth conditions for the OQDs were optimized to obtain Cu 2 O as the major phase. The OQDs grown on clean SrTiO 3 (001) surfaces at 825 K and higher with p (O 2 ) of 9.0 × 10 - 7 Torr or greater contain mostly CuO, contrasting to OQDs grown at 800 K with p (O 2 ) of ∼7.0 × 10 - 7 Torr that contain primarily Cu 2 O. Furthermore, there is a strong interaction between the SrTiO 3 (001) surface and the first few monolayers of the OQDs, which induces the formation of Cu(II). However, this interaction is mitigated with increasing thickness of OQDs, resulting in the exclusive formation of Cu 2 O in the topmost layers. The influence of the SrTiO 3 (001) substrate on the formation of OQDs can be minimized by modifying the substrate surface using chemical treatment and/or energetic Au 2+ ion-beam irradiation. Examination of the photochemical properties of these OQDs shows that prolonged soft X-ray irradiation under vacuum reduces Cu(II), which is present as a minor impurity in the Cu(I) OQDs.