ABSTRACT
To completely displace the carboxylate surface ligands from cadmium selenide nanocrystals, oleic acid impurities are first removed using dimethylcadmium or diethylzinc. In addition to metal carboxylate and methane coproducts, reactions with CdMe2 produce surface bound methyl groups (δ = 0.4 ppm, 0.04-0.22 nm-2) that photolytically dissociate to methyl radicals and n-doped nanocrystals. Without oleic acid impurities, cadmium carboxylate can be completely displaced from the surface using n-alkylamines (NH2R', R' = n-butyl, n-hexyl, n-octyl) (≤0.01 carboxylates nm-2). Colloidal dispersions of amine bound nanocrystals (CdSe-NH2R') are indefinitely stable at amine concentrations of 0.1 M or higher and slowly aggregate at lower concentrations. Dissociation and evaporation of the amine ligands in 4-ethylpyridine, tri-n-butylphosphine, or molten tri-n-octylphosphine oxide solution results in nanocrystal aggregation. CdSe-NH2R' reacts with oleic acid, n-octadecylphosphonic acid, or carbon dioxide to form surface bound n-alkylammonium oleate, phosphonate, and carbamate ion pairs that bind with greater affinity than primary n-alkylamines. The results indicate that nanocrystal dispersions solely stabilized by neutral donor ligands are relatively unstable compared to those stabilized by adsorbed metal carboxylate or phosphonate complexes or by ion pairs. The challenge of differentiating between the neutral ligand bound form and adsorbed ion pairs is discussed.
Subject(s)
Cadmium Compounds/chemistry , Carbamates/chemistry , Carboxylic Acids/chemistry , Nanoparticles/chemistry , Organophosphonates/chemistry , Selenium Compounds/chemistry , Anions/chemistry , Binding Sites , Molecular StructureABSTRACT
We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.
ABSTRACT
We report the structure of methylammonium lead(II) iodide perovskite in mesoporous TiO2, as used in high-performance solar cells. Pair distribution function analysis of X-ray scattering reveals a two component nanostructure: one component with medium range crystalline order (30 atom %) and another with only local structural coherence (70 atom %). The nanostructuring correlates with a blueshift of the absorption onset and increases the photoluminescence. Our findings underscore the importance of fully characterizing and controlling the structure for improved solar cell efficiency.
