Tailoring ZnSe–CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals

Abstract

We report a study of Zn(2+) by Cd(2+) cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd(2+) ions originally in solution, Cdsol, that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cdsol/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 °C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 °C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at T ≥ 240 °C. Remarkably, sequential heating (150 °C followed by 220 °C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250 °C converts the ZnSe/CdSe core/shell HNCs into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…The elemental distribution profile of hetero-NCs can go from a core/shell geometry with a sharp heterointerface to a homogenous alloy QD, via gradient alloy NCs of increasing homogeneity which seamlessly connect these two extremes [80][81][82]. In this way, the optoelectronic properties can be continuously tuned from those of core/shell hetero-NCs (type-I, type-II or type-I 1/2 ) to those of homogeneous alloy NCs, with preservation of the total volume and composition of the NC [80][81][82][83]. Moreover, core/shell hetero-NCs with a gradient alloy heterointerface have been shown to possess unique properties, such as reduced Auger recombination rates and lower threshold for amplified spontaneous emission [84].…”

“…Moreover, core/shell hetero-NCs with a gradient alloy heterointerface have been shown to possess unique properties, such as reduced Auger recombination rates and lower threshold for amplified spontaneous emission [84]. Alloy QDs and graded interface core/shell hetero-NCs can also be directly synthesized and have attracted increasing interest in the last few years, leading to the investigation of several II-VI and IV-VI compositions [viz., Cd(Te,Se), Cd(S,Se), Pb(S,Se), (Cd,Zn)Se, (Cd,Zn)S, (Cd,Zn)(S,Se)] [80][81][82][83][84][85][86][87][88][89].…”

“…Another important consequence of the large contribution of surface atoms to the properties of NCs is the enhancement of the solid-state diffusion rates. This has made it possible to use nanoscale cation exchange and/or controlled interdiffusion as post-synthetic strategies to tailor the properties of NCs and hetero-NCs while preserving their size, shape, and heterostructure, by tuning their composition and/or elemental distribution profile [82,[104][105][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122]. These techniques have also been recently used to achieve doping of semiconductor NCs [96,97,100].…”

“…2.2 above) or with suitable ligands that form strong bonds with the surface atoms, thereby shifting the energies of the surface states away from the HOMO-LUMO gap of the NC [16,123]. Other ligands may in fact generate localized interfacial states or mid-gap states that trap one of the carriers and induce PL quenching (e.g., hole trapping by alkanethiols on CdSe QDs [82,123,124]), or directly shift the NC electronic states due to electrostatic or orbital mixing effects [64,123,[125][126][127]. The capping ligand shell can be viewed as a self-assembled monolayer (SAM) at the surface of the NC [16,123].…”

Formation of ZnSe/Bi2Se3 QDs by surface cation exchange and high photothermal conversion

“…18 CE technology has exhibited various advantages in the field of colloidal nanocrystals synthesis, such as tuning heterostructure type of CS structure QDs, tailoring the composition, and even regulating free carrier density by forming the interstitial or vacancy defects in QDs. [19][20][21][22][23][24] We have prepared the CS structure CdSe/Bi 2 Se 3 by CE technology recently, despite their apparent photothermal conversion advantages, the intrinsic toxicity of cadmium has cast a doubtful future for this promising field. 25 In this study, we investigate the forming mechanism of ZnSe QDs coated with In a typical experiment, 0.0632g of Se, 0.3025g of Na 2 SO 3 and 20mL of water are added into a 100mL round-bottomed flask and stirring vigorously at 100 • C under N 2 atmosphere for 3h to prepare Na 2 SeSO 3 solution.…”