Abstract
Halide ions cap and stabilize colloidal semiconductor nanocrystal (NC) surfaces allowing for NCs surface interactions that may improve the performance of NC thin film devices such as photo-detectors and/or solar cells. Current ways to introduce halide anions as ligands on surfaces of NCs produced by the hot injection method are based on post-synthetic treatments. In this work we explore the possibility to introduce Cl in the NC ligand shell in situ during the NCs synthesis. With this aim, the effect of 1,2-dichloroethane (DCE) in the synthesis of CdSe rod-like NCs produced under different Cd/Se precursor molar ratios has been studied. We report a double role of DCE depending on the Cd/Se precursor molar ratio (either under excess of cadmium or selenium precursor). According to mass spectrometry (ESI-TOF) and nuclear magnetic resonance ((1)H NMR), under excess of Se precursor (Se dissolved in trioctylphosphine, TOP) conditions at 265 °C ethane-1,2-diylbis(trioctylphosphonium)dichloride is released as a product of the reaction between DCE and TOP. According to XPS studies chlorine gets incorporated into the CdSe ligand shell, promoting re-shaping of rod-like NCs into pyramidal ones. In contrast, under excess Cd precursor (CdO) conditions, DCE reacts with the Cd complex releasing chlorine-containing non-active species which do not trigger NCs re-shaping. The amount of chlorine incorporated into the ligand shell can thus be controlled by properly tuning the Cd/Se precursor molar ratio.
Highlights
Colloidal semiconductor nanocrystals (NCs) show tunable and photostable luminescence properties along with broadband absorption and narrow emission, which makes them suitable for optoelectronic and photovoltaic applications.[1,2] For these purposes, ne control over the synthetic procedure to obtain monodisperse, robust and highly emissive semiconductor NCs is required
Shaped CdSe NCs synthesized in the presence of the chlorinated precursor DCE can be interpreted as a ngerprint of the chlorine incorporation into the ligand shell.[27]
To survey the correlation between Cd, Se and Cl precursors in the chlorine incorporation into the NC ligand shell, we developed a series of experiments in which the Cd/Se precursor molar ratios were varied from 4 to 0.25, as stated in Table S1
Summary
Colloidal semiconductor nanocrystals (NCs) show tunable and photostable luminescence properties along with broadband absorption and narrow emission, which makes them suitable for optoelectronic and photovoltaic applications.[1,2] For these purposes, ne control over the synthetic procedure to obtain monodisperse, robust and highly emissive semiconductor NCs is required. The development of the hot-injection method[3] provides high control over size, shape, shape distribution and NCs surface composition for semiconductor materials such as CdSe,[4,5] PbSe6,7 or PbS.[8,9] This method allows ne control over the NCs growth, which is achieved by the use of long alkyl chain surfactants acting as both metal complexing agents and ligands These ligands form an insulating layer between NCs, restricting the charge transport between them and, as a consequence, devices containing such NCs (solar cells and photodetectors10,11) may show low conversion efficiencies. The passivating role of halide anions in PbS systems has been reported[11] and other reports have evidenced the passivation effect of chlorine in CdSe NCs, highlighting the possibility of using halogenated molecules to efficiently displace both L (coordinative) and/or X type ligands.[18,19] Post-reaction treatments with chlorine-containing solutions seem to provide some bene ts to certain colloidal NCs, such as more robust optical
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