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.

The nonlinear optical response of CdSe and CdSe/CdS core/shell nanoplatelets (NPLs) in toluene suspension was determined by using Z-scan and optical Kerr gate (OKG) techniques at 800 nm in the femtosecond regime. First, an analysis of the nonlinearities based on the Z-scan resulted in n 2 = 1.4 × 10 −16 cm 2 /W and n 2 = 1.3 × 10 −16 cm 2 /W for CdSe and CdSe/CdS core/shell NPLs, respectively. Fifth-order nonlinear absorption behavior was predominant, resulting in α 4 = 4.0 × 10 −2 cm/GW for CdSe NPLs and α 4 = 9.0 × 10 −3 cm/GW for CdSe/CdS core/shell NPLs for an optical intensity of 60.97 and 122 GW/cm 2 , respectively. The nonlinear refractive index obtained from Z-scan is in good agreement with those obtained from the OKG, and the response time obtained from OKG showed a decay of less than 1 ps for all studied materials, about 5 times slower than the pulse duration. For optical intensities in the range between 30.45 and 182.8 GW/cm 2 , NLA was not observed for toluene but was observed for CdSe and CdSe/CdS core/shell NPLs, revealing that the presence of the NPLs decreases the threshold of the absorptive nonlinearity. The results are discussed and explained based on the well-known materials' electronic structure.Article pubs.acs.org/JPCC

Colloidal two-dimensional (2D) CdSe nanoplatelets (NPLs) with strong quantum confinement in the vertical direction are being explored extensively, because of their potential optoelectronic and photonic applications. Here, we have synthesized CdSeS alloy NPLs by incorporating S atoms in the CdSe NPLs to tune their optical properties and investigated the charge transfer dynamics of alloy NPLs by preparing a hybrid with a 4-amino thiophenol (ATP) molecule. Hole transfer from alloy NPLs to the ATP molecule is evident from femtosecond transient absorption (TA) spectroscopy study in a time scale of 3.1 ps. Efficient charge separation from alloy NPLs to ATP molecules is beneficial for photon-to-current conversion and optoelectronic applications. Photodetector devices based on hybrid alloy NPLs exhibit high photocurrent, fast response (∼114 ms), and high detectivity of 2.5 × 10 10 Jones.