Luminescent semiconducting quantum dots (QDs) are central to emerging technologies that range from tissue imaging to solid-state lighting. However, existing samples are heterogeneous, which has prevented atomic-resolution determination of their structures and obscured the relationship between their atomic and electronic structures. Here we report the synthesis, isolation, and structural characterization of three cadmium selenide QDs with uniform compositions (Cd35Se20(X)30(L)30, Cd56Se35(X)42(L)42, Cd84Se56(X)56(L)56; X = O2CPh, L = H2N-C4H9). Their UV-absorption spectra show a lowest energy electronic transition that decreases in energy (3.54 eV, 3.26 eV, 3.04 eV) and sharpens as the size of the QD increases (fwhm = 207 meV, 145 meV, 115 meV). The photoluminescence spectra of all three QDs are broad with large Stokes shifts characteristic of trap-luminescence. Using a combination of single-crystal X-ray diffraction and atomic pair distribution function analysis, we determine the structures of their inorganic cores, revealing a series of pyramidal nanostuctures with cadmium terminated {111} facets. Theoretical and experimental studies on these materials will open the door to a deeper fundamental understanding of structure-property relationships in quantum-confined semiconductors.

Lead halide perovskites such as methylammonium lead triiodide (CH 3 NH 3 PbI 3 ) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. Previous

We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 μM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.

Organic-inorganic coupling in the hybrid lead-halide perovskite is a central issue in rationalizing the outstanding photovoltaic performance of these emerging materials. Here we compare and contrast the evolution of structure and dynamics of the hybrid CH3NH3PbBr3 and the inorganic CsPbBr3 lead-halide perovskites with temperature, using Raman spectroscopy and single-crystal Xray diffraction. Results reveal a stark contrast between their order-disorder transitions, abrupt for the hybrid whereas smooth for the inorganic perovskite. X-ray diffraction observes an intermediate incommensurate phase between the ordered and the disordered phases in CH3NH3PbBr3. Lowfrequency Raman scattering captures the appearance of a sharp soft mode in the incommensurate phase, ascribed to the theoretically predicted amplitudon mode. Our work highlights the interaction between the structural dynamics of organic cation CH3NH + 3 and the lead-halide framework, and unravels the competition between tendencies of the organic and inorganic moieties to minimize energy in the incommensurate phase of the hybrid perovskite structure.Organic-inorganic hybrid lead-halide perovskites have emerged as a promising class of new generation photovoltaic materials [1][2][3], showing excellent electronic properties and outstanding power conversion efficiencies [4]. Fundamental insight of the structural dynamics holds the key to understanding their unique electronic properties, and is the subject of intensive theoretical and experimental investigations.[5-10] Methylammonium cation (CH 3 NH + 3 , MA), the most frequently incorporated A site cation in recent photovoltaic applications, possesses a large dipole and exhibits dynamic orientational disorder at room temperature. [11,12] The orientational degrees of freedom of the anisotropic organic cation have been suggested to be responsible for the excellent electronic properties of the hybrid perovskites. [13][14][15]. Yet the inorganic lead-halide framework is the actual optically and electronically active component, on which charge carriers reside. A gap of knowledge exists on how the structural dynamics of MA and lead-halide framework couple with each other despite intensive investigations.In this Letter, we focus on unravelling the interplay between organic MA cations and the inorganic leadhalide framework, and highlight the unique signatures of such coupling. Here we report drastically different phase transformation behaviors between hybrid and inorganic lead-halide perovskites. Using low-frequency Raman scattering, we observe that there is an intermediate structural phase in CH 3 NH 3 PbBr 3 between the orthorhombic and tetragonal phases. With single-crystal X-ray diffraction, we conclude that the intermediate state to be an incommensurate phase, and we follow the evolution of the incommensurate modulation wave vector as * mpimenta@fisica.ufmg.br a function of temperature. A newly activated Raman mode of the incommensurate phase is assigned to the vibration in the amplitude of the structural modulation (...

We use micro-Raman spectroscopy to measure the vibrational structure of the atomically precise cadmium selenide quantum dots CdSeXL, CdSeXL, and CdSeXL. These quantum dots have benzoate (X) and n-butylamine (L) ligands and tetrahedral (T) shape with edges that range from 1.7 to 2.6 nm in length. Investigating this previously unexplored size regime allows us to identify the transition from molecular vibrations to bulk phonons in cadmium selenide quantum dots for the first time. Room-temperature Raman spectra have broad CdSe peaks at 175 and 200 cm. Density functional theory calculations assign these peaks to molecular surface and interior vibrational modes, respectively, and show that the interior, surface, and ligand atom motion is strongly coupled. The interior peak intensity increases relative to the surface peak as the cluster size increases due to the relative increase in the polarizability of interior modes with quantum dot size. The Raman spectra do not change with temperature for molecular CdSeXL, while the interior peak narrows and shifts to higher energy as temperature decreases for CdSeXL, a spectral evolution typical of a phonon. This result shows that the single bulk unit cell contained within CdSeXL is sufficient to apply a phonon confinement model, and that CdSeXL, with its 2.1 nm edge length, marks the boundary between molecular vibrations and phonons.