Effect of Copper Doping on Electronic Structure and Optical Absorption of Cd33Se33 Quantum Dots

Abstract

The photophysical properties of Cu-doped CdSe quantum dots (QDs) can be affected by the oxidation state of Cu impurity, but disagreement still exists on the Cu oxidation state (+1 or +2) in these QDs, which is debated and poorly understood for many years. In this work, by using density functional theory (DFT)-based calculations with the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid functional, we clearly demonstrate that the incorporation of Cu dopants into the surface of the magic sized Cd33Se33 QD leads to non-magnetic Cu 3d orbitals distribution and Cu+1 oxidation state, while doping Cu atoms in the core region of QDs can lead to both Cu+1 and Cu+2 oxidation states, depending on the local environment of Cu atoms in the QDs. In addition, it is found that the optical absorption of the Cu-doped Cd33Se33 QD in the visible region is mainly affected by Cu concentration, while the absorption in the infrared regime is closely related to the oxidation state of Cu. The present results enable us to use the doping of Cu impurity in CdSe QDs to achieve special photophysical properties for their applications in high-efficiency photovoltaic devices. The methods used here to resolve the electronic and optical properties of Cu-doped CdSe QDs can be extended to other II-VI semiconductor QDs incorporating transition-metal ions with variable valence.

Insights into the Oxidation State of Cu Dopants in II–VI Semiconductor Nanocrystals

“…Figure d shows the percentage of Cu 2+ present in each of the Cu-doped ZnSe samples that traces the journey of the Cu ion and its oxidation state from surface doping to internal doping and finally surface expulsion. When the overcoating temperature is 190 °C, Cu 2+ is negligible, which suggests that the dopant ions were loosely adsorbed onto the NC surfaces (surface doped) and reduced to their surface stable state Cu 1+ consistent with earlier literature . However, when the dopant ions are properly adsorbed and uniformly diffused inside the NC lattice (internally doped) at relatively higher overcoating temperatures (200 and 210 °C), most of the dopant ions are present in +2 oxidation state.…”

“…Additionally, it has been shown using DFT studies that copper is more stable in its +1 oxidation state when it is adsorbed on the surface of the nanocrystal while it prefers the +2 oxidation state when present in the core of the nanocrystal. 45 Hence, the atoms on the surface would most likely be in the Cu 1+ state while the satellite peaks might not be detected in the XPS spectrum because of their weak/undetectable intensities. On the other hand, X-ray absorption spectroscopy has been used to extract relevant information like valence state and the local environment of the elements present in any metal containing nanomaterials.…”

“…When the overcoating temperature is 190 °C, Cu 2+ is negligible, which suggests that the dopant ions were loosely adsorbed onto the NC surfaces (surface doped) and reduced to their surface stable state Cu 1+ consistent with earlier literature. 45 However, when the dopant ions are properly adsorbed and uniformly diffused inside the NC lattice (internally doped) at relatively higher overcoating temperatures (200 and 210 °C), most of the dopant ions are present in +2 oxidation state. If the overcoating temperature is further raised beyond 210 °C, the dopant ions are expected to be expelled out of the NCs, leading to the formation of Cu 2 Se as discussed in earlier literature, 1,22 although some of the Cu ions remain doped inside the NCs (internal and surface doped).…”

“…Since Cu 1+ is a soft metal ion, it binds better to soft donor sites like sulfur or selenium unlike Cu 2+ (borderline acceptor) and is more stable as compared to Cu 2+ when present in such similar anionic environment. Additionally, it has been shown using DFT studies that copper is more stable in its +1 oxidation state when it is adsorbed on the surface of the nanocrystal while it prefers the +2 oxidation state when present in the core of the nanocrystal . Hence, the atoms on the surface would most likely be in the Cu 1+ state while the satellite peaks might not be detected in the XPS spectrum because of their weak/undetectable intensities.…”

Light Absorption Enhancement and Laser-Induced Damage Ability Improvement of Aluminum Alloy 6061 with Non-Porous Alumina/CdSe@Al2O3/SiO2 Functional Gradient Films

“…Semiconductor nanocrystals (SNCs) such as cadmium selenide (CdSe) quantum dots offer wide applications in the fields of photovoltaics, solar energy harvesting, nanophotonics, imaging, sensing and other fields [ 20 , 21 , 22 , 23 , 24 , 25 ]. Since incident radiation causes excitation of free electrons, metal nanoparticles (MNPs) can generate intense electric fields in their vicinity [ 26 ].…”