Two-dimensional (2D) semiconductor colloidal nanoplatelets (NPLs) have shown great potential as light-harvesting materials due to their advanced optical properties. Here, we designed hybrid nanostructures of 2D CdSe nanoplatelets with phenothiazine (PTZ) for high-performance photodetector with varying thickness of CdSe NPLs by controlling the charge transfer process. Significant photoluminescence quenching and the shortening of the average decay time of CdSe NPLs in the presence of PTZ reveal the charge transfer process. Transient absorption spectroscopic analysis reveals the hot carrier cooling dynamics varies with changing the thickness of monolayers (ML) of CdSe NPLs. Furthermore, the slow exciton recombination of CdSe NPLs in the presence of PTZ indicates the efficient charge separation. The optimized CdSe NPLs-PTZ hybrid exhibits a significant enhancement of photocurrent (∼(4.7 × 103)-fold photo-to-dark current ratio) as compared to pure 3 ML CdSe NPLs (∼10 fold) at the applied voltage of 1.5 V. The measured external quantum efficiency, maximum detectivity, and response time for the optimized hybrid are found to be ∼40%, 4 × 1011 Jones, and 107 ms, with the responsivity value of 160 mA/W. These highly efficient measured parameters clearly suggest that CdSe NPLs-PTZ hybrid systems are a promising alternate for ultrasensitive photodetector.
The electronic interactions between colloidal two-dimensional (2D) semiconductor nanoplatelets (NPLs) and Au nanoclusters (NCs) remain unexplored, which are decisive for optoelectronic applications. Here, we report the synthesis of heterostructures based on colloidal 2D CdSe NPLs and Au 25 NCs and investigate their electronic interactions using density functional theory (DFT) calculations. The steady state, time-resolved photoluminescence, and transient absorption (TA) spectroscopic studies are carried out to understand the charge-transfer dynamics. The replacement of CdSe bands by Au bands in the valence band edge in CdSe NPLs−Au NCs heterostructures attests the charge transfer from the conduction band of CdSe to Au. Ultrafast TA spectroscopy further confirms the electron transfer in the heterostructures, and the faster bleach recovery kinetics is also observed in CdSe NPLs−Au NCs heterostructures. The observed charge transfer from the conduction band of CdSe NPLs to Au NCs has been corroborated by the difference in the orbital composition of the valence band edges between CdSe and Au NCs, as calculated by DFT. Photodetectors fabricated with these heterostructures feature high enhancement in photocurrent (∼350-fold), fast photoresponse (∼200 ms), and high detectivity (∼2.5 × 10 11 Jones), which hold promise for the future design of 2D NPL-based materials for optoelectronic applications.
The strategy of a new generation of light-harvesting systems based on two-dimensional (2D) colloidal semiconductor nanoplatelets (NPLs) is an emerging field of research owing to the strong confinement in one direction, narrow emission, large exciton binding energy, and large absorption cross-section. In this Perspective, we provide fundamental insights into the ultrafast dynamics and luminescent properties of 2D semiconductor NPLs and their heterostructures. The variation of hot carrier cooling dynamics with changing the thickness of monolayers of CdSe NPLs is evident. Slow exciton recombination of 2D NPLs is reported in the presence of electron and hole acceptor molecules, indicating efficient charge separation which eventually controls the device performance. We envision the possibility of designing 2D NPLs heterostructures in combination with metal clusters, perovskite nanocrystals, and graphene oxide for photodetector, light-emitting diodes, photocatalysis, electroluminescence, and photovoltaics applications by controlling their charge transfer dynamics. The future perspective of this research field with promising areas is discussed.
Atomically precise two-dimensional (2D) cadmium chalcogenide nanoplatelets (NPLs) have increased interest in optoelectronic applications. In this work, we highlight the influence of the reaction growth time on the structural and photophysical properties, carrier dynamics, and photodetection properties of NPLs. The coexistence of the wurtzite (WZ) and zinc blende (ZB) polymorphs in ∼5:3 ratio is found in CdSe/CdS core/shell (CS) NPLs from the Rietveld analysis of X-ray diffraction (XRD) patterns. The tuning of photoluminescence emission from green to red and 12 times enhancement of the decay time in CdSe/CdS CS NPLs have been achieved by increasing the growth time. Femtosecond transient absorption spectroscopic analysis reveals the increase in rise time from 500 to 900 fs, and the overall bleach recovery dynamics get slower with the growth time. In the CdSe/CdS CS NPLs-based photodetector device, the photo-to-dark current intensity ratio is ∼600 with a fast photoresponse time of ∼100 ms. The maximum photoresponsivity in the visible region is around ∼113 mA/W with a very high detectivity of ∼2.1 × 1013 Jones. Analysis reveals that these solution-processed CdSe/CdS CS NPLs-based photodetectors are promising for next-generation optoelectronic applications.
Doped II–VI semiconductor two-dimensional (2D) nanoplatelets (NPLs) are emergent optoelectronic materials due to one-dimensional strong quantum confinement. Here, we report the impact of Cu2+ dopant and post-synthetic heat treatment on structural modification and the ultrafast carrier relaxation of CdSe NPLs. Rietveld’s analysis reveals the isostructural atomic arrangements of Cu-doped CdSe NPLs, where dopant Cu atoms substitutionally replace Cd atoms. Significant photoluminescence quenching and the shortening of the decay time of CdSe NPLs are found in the presence of the Cu dopant. The influence of the dopant and post-synthetic heat treatment on the ultrafast relaxation processes and trap-mediated relaxation times is confirmed by the femtosecond transient absorption spectroscopy (fs-TAS) study because of the substitutional incorporation of Cu2+ ions in the CdSe lattice. These findings will pave the way to design doped semiconductor NPLs-based optoelectronic devices.
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