Molecular Control of the Nanoscale: Effect of Phosphine–Chalcogenide Reactivity on CdS–CdSe Nanocrystal Composition and Morphology

Abstract

We demonstrate molecular control of nanoscale composition, alloying, and morphology (aspect ratio) in CdS-CdSe nanocrystal dots and rods by modulating the chemical reactivity of phosphine-chalcogenide precursors. Specific molecular precursors studied were sulfides and selenides of triphenylphosphite (TPP), diphenylpropylphosphine (DPP), tributylphosphine (TBP), trioctylphosphine (TOP), and hexaethylphosphorustriamide (HPT). Computational (DFT), NMR ( 31 P and 77 Se), and high-temperature crossover studies unambiguously confirm a chemical bonding interaction between phosphorus and chalcogen atoms in all precursors. Phosphine-chalcogenide precursor reactivity increases in the order: TPPE < DPPE < TBPE < TOPE 1-x Se x quantum dots were synthesized via single injection of a R 3 PS-R 3 PSe mixture to cadmium oleate at 250 degree C. X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV/Vis and PL optical spectroscopy reveal that relative R 3 PS and R 3 PSe reactivity dictates CdS 1 -xSe x dot chalcogen content and the extent of radial alloying (alloys vs core/shells). CdS, CdSe, and CdS 1 -x Se x quantum rods were synthesized by injection of a single R 3 PE (E = S or Se) precursor or a R 3 PS-R 3 PSe mixture to cadmium-phosphonate at 320 or 250 degree C. XRD and TEM reveal that the length-to-diameter aspect ratio of CdS and CdSe nanorods is inversely proportional to R 3PE precursor reactivity. Purposely matching or mismatching R 3 PS-R 3 PSe precursor reactivity leads to CdS 1 -x Se x nanorods without or with axial composition gradients, respectively. We expect these observations will lead to scalable and highly predictable "bottom-up" programmed syntheses of finely heterostructured nanomaterials with well-defined architectures and properties that are tailored for precise applications. P reparative nanotechnology or "nanomanufacturing" is rapidly evolving toward fabrication of ever more complex materials with precise structure and properties. Tuning composition, relative configuration, and spatial arrangement of heterostructured nanomaterials can impact our ability to engineer and direct energy flows at the nanoscale. In the case of II 3 VI and IV 3 VI semiconductors, composition control has been demonstrated for homogeneously alloyed CdS 1Àx Se x , 1À4 CdS 1Àx Te x , 5 CdSe 1Àx Te x , 6 PbS x Se 1Àx , PbS x Te 1Àx , and PbSe x Te 1Àx 7 nanocrystals with size-and composition-tunable band gaps. 4,8,9 In some cases, a nonlinear relationship between composition and absorption/emission energies, called optical bowing, resulted in new properties not obtainable from the parent binary systems. 3 For example, CdS x Te 1Àx nanocrystals displayed small absorptionÀ emission spectral overlap, up to 150 nm Stokes shifts, and significantly red-shifted PL with respect to CdS and CdTe nanocrystals. 5 Controlling nanocrystal morphology is key to controlling nanocrystal properties. 10À14 A common technique to produce nanorods, for example, is to perform slow and/or subsequent reactant injections. 15À17 In i...

Bright Gradient-Alloyed CdSexS1–x Quantum Dots Exhibiting Cyan-Blue Emission

“…Our alloyed CdSeS QDs were synthesized from Cd(OA) 2 , TOPSe, TOPS, and HPPh 2 at 180°C in ODE via a non-hotinjection approach. The key innovation of our synthetic strategy is the use of HPPh 2 combined with high Cd/E and high S/Se feed molar ratios, for the formation of the gradientalloyed structure comprising of a Se-rich inner core, a Se−S equivalent middle layer, and a S-rich outer region (as shown by Scheme S1).…”

“…Basically, the activation at low temperature by HPPh 2 proceeds through chalcogen exchange leading to more reactive EPPh 2 H, as illustrated by eq 1. High Cd/(Se + S) feed molar ratios move the equilibrium effectively to the right via the coordination of TOP to Cd(OA) 2 . 30 …”

“…1−3,19−25 Table S1 (Supporting Information) lists the main approaches to CdSeS QDs together with their optical properties. These approaches were usually based on the addition of a mixture of tri-n-octylphosphine selenide (TOPSe) and tri-n-octylphosphine sulfide (TOPS) to a solution of cadmium carboxylate (Cd(OOCR) 2 )i n1 -octadecene (ODE) at high temperature. After the quick injection of the chalcogenide mixture, the reaction temperature around 315°C was quickly decreased to ∼275°C for the growth of alloyed QDs.…”

“…45 The reaction mechanism for the formation of the phosphorus-containing compounds A, B, and C has been documented for the reaction of HPPh 2 + Cd(OA) 2 + TOPSe, in which SePPh 2 H reacts with Cd(OA) 2 instead of TOPSe. 30,45 Our high Cd(OA) 2 to TOPSe feed molar ratios facilitate TOPSe + HPPh 2 ⇔ TOP + SePPh 2 H Chem. Mater.…”

“…(1) Figure 1 shows the optical property of our CdSeS QDs synthesized at 180°C for 90 min from batch 4HPPh 2 Figure S1A-2). To the best of our knowledge, this absolute PLQY is the highest for cyan-blue CdSeS QDs ever reported.…”