A few approaches have been employed to tune the band gap of colloidal organic-inorganic trihalide perovskites (OTPs) nanocrystals by changing the halide anion. However, to date, there is no report of electronic structure tuning of perovskite NCs upon changing the organic cation. We report here, for the first time, the room temperature colloidal synthesis of (EA)x(MA)1-xPbBr3 nanocrystals (NCs) (where, x varies between 0 and 1) to tune the band gap of hybrid organic-inorganic lead perovskite NCs from 2.38 to 2.94 eV by varying the ratio of ethylammonium (EA) and methylammonium (MA) cations. The tuning of band gap is confirmed by electronic structure calculations within density functional theory, which explains the increase in the band gap upon going toward larger "A" site cations in APbBr3 NCs. The photoluminescence quantum yield (PLQY) of these NCs lies between 5% to 85% and the average lifetime falls in the range 1.4 to 215 ns. A mixture of MA cations and its higher analog EA cations provide a versatile tool to tune the structural as well as optoelectronic properties of perovskite NCs.

The growth was monitored by in-situ RHEED (reflection high energy electron diffraction). Fig. S1(a) shows the time dependence of intensity of specular reflection (0,0), recorded during the growth of NdNiO 3 (NNO) film on NdGaO 3 (NGO) substrate and layer-by-layer growth has been confirmed by the sharp drops during ablation and gradual recovery within next few seconds to the same level of intensity after the deposition of each unit cell. Inset of Fig. S1(b) shows RHEED pattern for the NNO film, recorded after cooling to room temperature. The streak patterns of specular and off-specular: (0 1), (0-1) reflections (in pseudo cubic (p.c.) notation) confirm the desired two-dimensional surface morphology. X-ray diffraction: Success of epitaxial growth along [0 0 1] p.c. has been further confirmed by 2θ-ω scan in X-ray diffraction (Fig. S1(b)). Each diffraction pattern consists of a sharp substrate peak, a broad film peak (indicated by solid triangle in Fig. S1(b)) and thickness fringes, arises due to the finite thickness of film. Out-of plane lattice constant (c p.c. of NNO films are found to be 3.75Å (STO), 3.78Å (NGO), 3.83Å (SPGO), 3.84Å (SLAO) and 3.86Å (YAO) and these follow the expected tetragonal distortion relation for the cube on cube growth.

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Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe2, a trigonal 3×3 structural modulation is coupled to a charge modulation. Recent experiments reported that a trigonal-stripe transition occurred at the surface, possibly due to local strain and/or accidental doping. We employ ab-initio calculations to investigate the structural instabilities arising from strain in a pristine single layer and analyze the energy hierarchy of the structural and charge modulations. Our ab-initio calculations support the observation of phase separation between trigonal and stripe phases in NbSe2 single layers and surface in the clean limit, reproducing it with the observed wavelength. *

The century-old Vegard's law has been remarkably accurate in describing the evolution of the lattice parameters of almost all solid solutions. Contractions or expansions of lattice parameters of such systems depend on the size of the guest atom being smaller or larger than the host atom it replaces to form the solid solution. This has given rise to the concept of "chemical pressure" in analogy to the physical pressure. We have investigated using EXAFS the evolution of the local structure in terms of atom-pair distances extending up to the third-nearest neighbors in the family of compounds, ZnSe x S 1−x as an example of an anionic solid solution, in contrast to all previous studies focusing on cationic solid solutions. Our results establish several common features between these two types of solid solutions, while strongly suggesting that the concept of a chemical pressure is inaccurate and misleading. Most interestingly, we also find a qualitative difference between the cationic solid solutions, reported earlier, and the anionic solid solution.