We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors like sterics and chelation, though also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd:Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (~60 %) and fully reversed at elevated temperature. A model is proposed where electron accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.

Recent discoveries of highly efficient solar cells based on lead iodide perovskites have led to a surge in research activity on understanding photo carrier generation in these materials, but little is known about trap states that may be detrimental to solar cell performance. Here we provide direct evidence for hole traps on the surfaces of three-dimensional (3D) CH3NH3PbI3 perovskite thin films and excitonic traps below the optical gaps in these materials. The excitonic traps possess weak optical transition strengths, can be populated from the relaxation of above gap excitations, and become more significant as dimensionality decreases from 3D CH3NH3PbI3 to two-dimensional (2D) (C4H9NH3I)2(CH3NH3I) n−1(PbI2) n (n = 1, 2, 3) perovskites and, within the 2D family, as n decreases from 3 to 1. We also show that the density of excitonic traps in CH3NH3PbI3 perovskite thin films grown in the presence of chloride is at least one-order of magnitude lower than that grown in the absence of chloride, thus explaining a widely known mystery on the much better solar cell performance of the former. The trap states are likely caused by electron–phonon coupling and are enhanced at surfaces/interfaces where the perovskite crystal structure is most susceptible to deformation.

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

The molecular mechanism of precursor evolution in the synthesis of colloidal group II-VI semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M-OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd-ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd-OA and Cd-ODPA can be fit to a single-exponential decay (kobs = (1.30 +/- 0.08) x 10-3 s-1, Cd-ODPA, 260 degrees C, and kobs = (1.51 +/- 0.04) x 10-3 s-1, Cd-OA, 117 degrees C). The reaction approaches completion at 70-80% conversion of TOPSe under anhydrous conditions and 100% conversion in the presence of added water. Activation parameters for the reaction between TBPSe and Cd-OA in n-nonane-d20 were determined from the temperature dependence of the TBPSe decay over the range of 358-400 K (deltaH++ = 62.0 +/- 2.8 kJ.mol-1, deltaS++ = -145 +/- 8 J.mol-1.K-1). A reaction mechanism is proposed where trialkylphsophine chalcogenides deoxygenate the oleic acid or phosphonic acid surfactant to generate trialkylphosphine oxide and oleic or phosphonic acid anhydride products. Results from kinetics experiments suggest that cleavage of the phosphorus chalcogenide double bond (TOP=E) proceeds by the nucleophilic attack of phosphonate or oleate on a (TOP=E)-M complex, generating the initial M-E bond.

Controlling the size of colloidal nanocrystals is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods control size by terminating the growth, an approach that limits the reaction yield and causes batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales.