Subpicosecond Exciton Dynamics and Biexcitonic Feature in Colloidal CuInS₂ Nanocrystals: Role of In–Cu Antisite Defects

Abstract

Charge carrier dynamics of multinary quantum dots like CuInS2 (CIS) nanocrystals (NCs) is not clearly understood, especially in ultrafast time scales. Herein we have synthesized colloidal CIS NCs that show defect-induced emission between donor (antisite) and acceptor (internal/surface) states as indicated from steady-state and time-resolved photoluminescence (PL) measurements. Subpicosecond transient absorption (TA) spectra of CIS NCs reveal a gradient of electronic states that exists above the conduction band edge. The electron cooling rate has been determined to be ∼0.1-0.15 eV/ps. The cascade of electron cooling dynamics was monitored after following the TA kinetics at different electronic states. Interestingly, the kinetics at the antisite state unveil a biexcitonic feature, which has been enlightened through a probe-induced biexciton mechanism. With progressively higher fluence (⟨N⟩), the biexciton binding energy increases, and the electron cooling to the antisite state considerably slows down. Extra energy released during Auger recombination of bi/multiexcitons are used to re-excite the electron to a further high energy level, resulting in longer electron cooling time to the antisite states.

High performance of Mn-doped CdSe quantum dot sensitized solar cells based on the vertical ZnO nanorod arrays

“…By comparing the integrated areas of Mn 2p and Cd 3d peaks, the intensity ratio is 4.79:11.45. This result indicates that surface-bound Mn 2þ takes up a large portion, which is in line with the result in the literature that most of the dopant ions exist as isolated Mn 2þ ions on the surface of QD [35,36]. The result likely exclude the formation of MnSe 2 (or similar) compound.…”

“…3(a)). Similiar results were reported for Mn 2þ doped CdSe nanocrystals previously [33,35]. It is well-known that the orange emission located at about 590 nm (~2.1 eV) under normal conditions is attributable to the 4 T 1 e 6 A 1 ligand-field transition of Mn 2þ -doped nanocrystals, which is marked by the vertical dashed line in Fig.…”

“…Beaulac and co-workers [45] found that the PL of large Mn 2þ -doped CdSe QDs (d>3.3 nm) is dominated by excitonic recombination, and energy transfer to Mn 2 þ does not occur. Ghosh et al suggested that the 4 T 1 states of Mn are overlapped with conduction band (CB) states in CdSe QDs with surfaceaccumulated Mn-dopant [35]. In the present investigation, we found that 4 T 1 state of Mn lies above the CB edge of CdSe QD, and the excitonic emission of MneCdSe appeared at~650 nm.…”

Prospects of Colloidal Copper Chalcogenide Nanocrystals

“…Despite a decade of research, the origin of these intriguing optical properties is not yet understood and is still under debate. The most often used model is that the PL of CIS NCs has the same origin as that of bulk CIS and is thus due to a donor–acceptor pair recombination involving native point defects such as Cu In , V In , or V Cu (Figure a) . This model is consistent with the observation that the PL band position depends on the CIS NC composition (PL band shifts to shorter wavelengths with decrease in the Cu/In ratio) and is based on the assumption that the defect structure of CIS does not change at the nanoscale.…”

Rational Design and Synthesis of Highly Luminescent Multinary Cu‐In‐Zn‐S Semiconductor Nanocrystals with Tailored Nanostructures

“…In particular, CuInS 2 is one of the most intensively studied NC, and a lot of literatures have been reported on synthetic recipes and potential applications . Nevertheless, the intrinsic surface traps or internal defects facilitate nonradiative recombination, resulting in a lower photoluminescence quantum yield (PLQY) . Learning from the case of Cd‐based NCs, overcoating an inorganic shell on the core material to form core/shell structures is an important and effective strategy to improve the PL emission.…”