All-inorganic nanocrystals (NCs) are of great importance in a range of electronic devices. However, current all-inorganic NCs suffer from limitations in their optical properties, such as low fluorescence efficiencies. Here, we develop a general surface treatment strategy to obtain intensely luminescent all-inorganic NCs (ILANs) by using designed metal salts with noncoordinating anions that play a dual role in the surface treatment process: (i) removing the original organic ligands and (ii) binding to unpassivated Lewis basic sites to preserve the photoluminescent (PL) properties of the NCs. The absolute photoluminescence quantum yields (PLQYs) of red-emitting CdSe/ZnS NCs, green-emitting CdSe/CdZnSeS/ZnS NCs and blue-emitting CdZnS/ZnS NCs in polar solvents are 97%, 80% and 72%, respectively. Further study reveals that the passivated Lewis basic sites of ILANs by metal cations boost the efficiency of radiative recombination of electron-hole pairs. While the passivation of Lewis basic sites leads to a high PLQY of ILANs, the exposed Lewis acidic sites provide the possibility for in situ tuning of the functions of NCs, creating opportunities for direct optical patterning of functional NCs with high resolution.
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Colloidal CdSe nanocrystals are inherently defective
Colloidal CdSe nanocrystals (NCs) have shown promise in applications ranging from LED displays to medical imaging. Their unique photophysics depend sensitively on the presence or absence of surface defects. Using simulations, we show that CdSe NCs are inherently defective; even for stoichiometric NCs with perfect ligand passivation and no vacancies or defects, we still observe that the low energy spectrum is dominated by dark, surface-associated excitations, which are more numerous in larger NCs. Surface structure analysis shows that the majority of these states involve holes that are localized on two-coordinate Se atoms. As chalcogenide atoms are not passivated by any Lewis base ligand, varying the ligand should not dramatically change the number of dark states, which we confirm by simulating three passivation schemes. Our results have significant implications for understanding CdSe NC photophysics, and suggest that photochemistry and short-range photoinduced charge transfer should be much more facile than previously anticipated.
more » « less- Award ID(s):
- 1905164
- NSF-PAR ID:
- 10360766
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Background Research into perovskite nanocrystals (PNCs) has uncovered interesting properties compared to their bulk counterparts, including tunable optical properties due to size‐dependent quantum confinement effect (QCE). More recently, smaller PNCs with even stronger QCE have been discovered, such as perovskite magic sized clusters (PMSCs) and ligand passivated PbX2metal halide molecular clusters (MHMCs) analogous to perovskites.
Objective This review aims to present recent data comparing and contrasting the optical and structural properties of PQDs, PMSCs, and MHMCs, where CsPbBr3PQDs have first excitonic absorption around 520 nm, the corresponding PMSCS have absorption around 420 nm, and ligand passivated MHMCs absorb around 400 nm.
Results Compared to normal perovskite quantum dots (PQDs), these clusters exhibit both a much bluer optical absorption and emission and larger surface‐to‐volume (S/V) ratio. Due to their larger S/V ratio, the clusters tend to have more surface defects that require more effective passivation for stability.
Conclusion Recent study of novel clusters has led to better understanding of their properties. The sharper optical bands of clusters indicate relatively narrow or single size distribution, which, in conjunction with their blue absorption and emission, makes them potentially attractive for applications in fields such as blue single photon emission.
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Spatial confinement of charge carriers in nanosize semiconductor quantum dots (QDs) results in highly tunable, size-dependent optoelectronic properties that can be utilized in various commercial applications. Although in such nanostructures, non-stoichiometry is frequently encountered using conventional synthesis techniques, it is not often addressed or considered. Here, we perform ab initio molecular dynamics simulations on non-stoichiometric CdSe clusters to study the phonon-mediated charge carrier relaxation dynamics. We model cation-rich and anion-rich QDs passivated with monocharged neutralizing ligands of different sizes. Our studies confirm the presence of localized trap states at the valence band edge in only anion-rich QDs due to the presence of undercoordinated exposed surface Se atoms. Noteworthily, these localized states disappear when using bulkier ligands. Calculations reveal that the size of the ligands controls the crystal vibrations and electron–phonon coupling, while ligand coordination number affects the electronic structure. For a particular non-stoichiometric CdSe QD, a change of a ligand can either increase or decrease the total electron relaxation time compared to that of stoichiometric QDs. Our results emphasize the importance of ligand engineering in non-stoichiometric QDs for photoinduced dynamics and guide future work for the implementation of improved materials for optoelectronic devices.more » « less
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Abstract Plasmonic hot‐electron‐assisted control of emission intensities and dynamics of CdSe/ZnS and infrared PbS quantum dots are studied. This is done by exploring the impact of Au/Si and Ag/Si Schottky junctions on the decay rates of such quantum dots when these junctions are placed in close vicinity of a Si/Al oxide charge barrier, forming metal‐oxide plasmonic metafilms. Such structures are used to investigate how metal‐dependent distributions of hot electrons and their capture via Schottky junctions can lead to suppression of the defect environments of quantum dots, offering a novel platform wherein off‐resonant (non‐Purcell) plasmonic processes are used to control exciton dynamics. These results show that Ag metafilms can enhance the emission of CdSe/ZnS quantum dots and elongate their lifetimes more than Au metafilms. This highlights the more efficient nature of Ag/Si Schottky junctions for hot electron excitation and capture. These results also show that such junctions can significantly suppress the nonradiative decay rates of PbS quantum dots at frequencies far from the plasmon resonances. These results demonstrate a field‐effect passivation of quantum dot defects via entrapment of hot electrons and control of emission intensities and dynamics of quantum dots via the nearly frequency‐independent electrostatic field of such electrons.
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Abstract Semiconducting molecules have been employed to passivate traps extant in the perovskite film for enhancement of perovskite solar cells (PSCs) efficiency and stability. A molecular design strategy to passivate the defects both on the surface and interior of the CH3NH3PbI3perovskite layer, using two phthalocyanine (Pc) molecules (
NP ‐SC6‐ZnPc andNP ‐SC6‐TiOPc) is demonstrated. The presence of lone electron pairs on S, N, and O atoms of the Pc molecular structures provides the opportunity for Lewis acid–base interactions with under‐coordinated Pb2+sites, leading to efficient defect passivation of the perovskite layer. The tendency of bothNP ‐SC6‐ZnPc andNP ‐SC6‐TiOPc to relax on the PbI2terminated surface of the perovskite layer is also studied using density functional theory (DFT) calculations. The morphology of the perovskite layer is improved due to employing the Pc passivation strategy, resulting in high‐quality thin films with a dense and compact structure and lower surface roughness. UsingNP ‐SC6‐ZnPc andNP ‐SC6‐TiOPc as passivating agents, it is observed considerably enhanced power conversion efficiencies (PCEs), from 17.67% for the PSCs based on the pristine perovskite film to 19.39% forNP ‐SC6‐TiOPc passivated devices. Moreover, PSCs fabricated based on the Pc passivation method present a remarkable stability under conditions of high moisture and temperature levels.
