Surface Decides the Photoluminescence of Colloidal CdSe Nanoplatelets Based Core/Shell Heterostructures

Abstract

The photophysical properties of core/shell semiconductor nanocrystals are influenced by the shell thickness as well by the surface, whether it is cationic or anionic. In this work, we have investigated the effect of thickness of shell as well as the surface terminating layeranionic and cationicon the optical properties in CdSe/CdS which is a quasi-type-II system and CdSe/ZnS, a type-I heterostructured core/shell nanoplatelets (NPLs). The results reveal that no matter which cation is on the surface – Zn or Cd–the photoluminescence (PL) is always high compared to the surface being anion terminated. An alternating behavior in the PL intensity is observed upon successively terminating the surface with cations and anions, which has been achieved using the colloidal atomic layer deposition (cALD) technique. Not only the PL intensity but also the PL lifetimes and the emission peak widths too follow this similar alternating trend. All of these can be simply explained on the basis of the trap states that are created on the surface depending upon cation or anion termination.

Highly Stable, Near‐Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero‐Nanoplatelets Enabled by ZnS‐Shell Hot‐Injection Growth

“…Such a large band‐offset can be achieved with the growth of ZnS shell, which can substantially reduce trap sites. The previous studies on the ZnS shell growth commonly relied on the so‐called colloidal atomic layer deposition (c‐ALD) and these CdSe/ZnS core/shell NPLs possess very low QY levels in the range of 1–4% . In addition to the lower QY, conventional c‐ALD approach is time‐consuming and a large amount of precursor is wasted during its tedious synthesis protocol.…”

“…This approach generally helps to reduce the surface nonradiative recombination sites by passivating surface traps, leading to increased quantum yield and decreased emission blinking at a single particle level . Among the shell structures used for CdSe‐core NPLs, the most notable ones demonstrated thus far include CdS and ZnS, which results in quasi‐type‐II and ‐type‐I band alignment structures, respectively. In the past few years, research on the core/shell NPLs has concentrated mostly on the colloidal synthesis of CdSe/CdS core/shell NPLs owing to the smaller lattice mismatch between these materials and the resulting quasi‐type II nature of electronic structure enabling highly tunable excitonic features .…”

“…Among the shell structures used for CdSe‐core NPLs, the most notable ones demonstrated thus far include CdS and ZnS, which results in quasi‐type‐II and ‐type‐I band alignment structures, respectively. In the past few years, research on the core/shell NPLs has concentrated mostly on the colloidal synthesis of CdSe/CdS core/shell NPLs owing to the smaller lattice mismatch between these materials and the resulting quasi‐type II nature of electronic structure enabling highly tunable excitonic features . For example, Rossinelli et al have recently reported the uniform growth CdS shell on CdSe NPLs at higher temperatures resulting in narrower emission linewidth (≈20 nm) with suppressed blinking and moderately high photoluminescence quantum yield (QY) of 55–60% .…”

On the use of CdSe scintillating nanoplatelets as time taggers for high-energy gamma detection

“…13,19 However, their implementation as radiation detectors is technologically challenging owing to intrinsic short-comings such as rather low stopping power of individual nanoparticles, 20-23 high self-absorption owing to small Stokes-shift of emission [24][25][26] and the necessary presence of organic ligands needed for surface passivation and shaping the luminescence and spatial distribution of nanoplatelets. [27][28][29][30][31] In this contribution, we focus on understanding and demonstrating the timing potential capabilities of these types of materials under ionizing radiation in section 2.1 and 2.2. The section 2.3 has the purpose of showing how CdSe/CdS nanoplatelets could be used in a real detector geometry for medical applications and what are the scintillating features of this type of sample.…”

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…At the same time, the inorganic shells could help to suppress the rapid nonradiative Auger recombination (AR) of excitons with lower Auger rates and much reduced emission intermittency at single particle level, thereby enabling to realize low‐threshold (≈6 µJ cm −2 ) amplified spontaneous emission (ASE) and obtain high threshold for gain saturation . Generally, the epitaxial growth of semiconductor shells in core/shell hetero‐nanoplatelets is performed via the colloidal atomic layer deposition (c‐ALD) method . However, as c‐ALD usually takes place at room temperature, such synthetic approach allows for CdSe/CdS NPLs with only limited QYs (≈20–40%) and stability .…”