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.
Highlights
Doping of semiconductor nanocrystals (NCs) or quantum dots (QDs) with transitionmetal ions has attracted significant interest in the applications of lasers [1,2], biolabeling [3,4,5,6], light-emitting diodes [7,8,9], and optoelectronics devices [10,11,12,13,14]
New energy levels could be introduced into the bandgap of the host NCs, which can exchange charges with the valence band or the conduction band, thereby significantly influence their electronic and optical properties
The average Se-Cd bond length in the relaxed pristine QD is determined to be 2.667 Å located between 2.678 Å in the core region and 2.657 Å in the surface region, which agree well with the value of 2.685 Å in other density functional theory (DFT) calculations [39]
Summary
Doping of semiconductor nanocrystals (NCs) or quantum dots (QDs) with transitionmetal ions has attracted significant interest in the applications of lasers [1,2], biolabeling [3,4,5,6], light-emitting diodes [7,8,9], and optoelectronics devices [10,11,12,13,14]. The active charge can be permanently introduced into the NC host lattice by incorporating a transition-metal ion having a variable valence [17,18] By such doping, new energy levels could be introduced into the bandgap of the host NCs, which can exchange charges with the valence band or the conduction band, thereby significantly influence their electronic and optical properties. Nanomaterials 2021, 11, 2531 the electronic and chemical structures of Cu ions dispersed in CdSe QDs and concluded that Cu ions had a +1 oxidation state [29]. The different positions of Cu 3d energy levels within the forbidden band will lead to very different recombination mechanisms of Cu impurity emission [32,35] Far, it is unknown how the position of Cu 3d states varies with the dopant location in QDs either. The present work provides a theoretical perspective on how to control Cu dopants in CdSe NCs to achieve desired optoelectronic properties for their applications in high-efficiency photovoltaic devices
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