The elementary reactions for incorporation into crystals.

The growth rates of crystals are largely dictated by the chemical reaction between solute and kinks, in which a solute molecule severs its bonds with the solvent and establishes new bonds with the kink. Details on this sequence of bond breaking and rebuilding remain poorly understood. To elucidate the reaction at the kinks we employ four solvents with distinct functionalities as reporters on the microscopic structures and their dynamics along the pathway into a kink. We combine time-resolved in situ atomic force microscopy and x-ray and optical methods with molecular dynamics simulations. We demonstrate that in all four solvents the solute, etioporphyrin I, molecules reach the steps directly from the solution; this finding identifies the measured rate constant for step growth as the rate constant of the reaction between a solute molecule and a kink. We show that the binding of a solute molecule to a kink divides into two elementary reactions. First, the incoming solute molecule sheds a fraction of its solvent shell and attaches to molecules from the kink by bonds distinct from those in its fully incorporated state. In the second step, the solute breaks these initial bonds and relocates to the kink. The strength of the preliminary bonds with the kink determines the free energy barrier for incorporation into a kink. The presence of an intermediate state, whose stability is controlled by solvents and additives, may illuminate how minor solution components guide the construction of elaborate crystal architectures in nature and the search for solution compositions that suppress undesirable or accelerate favored crystallization in industry.

Integration of Highly Luminescent Lead Halide Perovskite Nanocrystals on Transparent Lead Halide Nanowire Waveguides through Morphological Transformation and Spontaneous Growth in Water.

The integration of highly luminescent CsPbBr3 quantum dots on nanowire waveguides has enormous potential applications in nanophotonics, optical sensing, and quantum communications. On the other hand, CsPb2 Br5 nanowires have also attracted a lot of attention due to their unique water stability and controversial luminescent property. Here, the growth of CsPbBr3 nanocrystals on CsPb2 Br5 nanowires is reported first by simply immersing CsPbBr3 powder into pure water, CsPbBr3- γ Xγ (X = Cl, I) nanocrystals on CsPb2 Br5 -γ Xγ nanowires are then synthesized for tunable light sources. Systematic structure and morphology studies, including in situ monitoring, reveal that CsPbBr3 powder is first converted to CsPb2 Br5 microplatelets in water, followed by morphological transformation from CsPb2 Br5 microplatelets to nanowires, which is a kinetic dissolution-recrystallization process controlled by electrolytic dissociation and supersaturation of CsPb2 Br5 . CsPbBr3 nanocrystals are spontaneously formed on CsPb2 Br5 nanowires when nanowires are collected from the aqueous solution. Raman spectroscopy, combined photoluminescence, and SEM imaging confirm that the bright emission originates from CsPbBr3 -γ Xγ nanocrystals while CsPb2 Br5 -γ Xγ nanowires are transparent waveguides. The intimate integration of nanoscale light sources with a nanowire waveguide is demonstrated through the observation of the wave guiding of light from nanocrystals and Fabry-Perot interference modes of the nanowire cavity.

Precrystallization solute assemblies and crystal symmetry.

Solution crystallization is a part of the synthesis of materials ranging from geological and biological minerals to pharmaceuticals, fine chemicals, and advanced electronic components. Attempts to predict the structure, growth rates and properties of emerging crystals have been frustrated, in part, by the poor understanding of the correlations between the oligomeric state of the solute, the growth unit, and the crystal symmetry. To explore how a solute monomer or oligomer is selected as the unit that incorporates into kinks and how crystal symmetry impacts this selection, we combine scanning probe microscopy, optical spectroscopy, and all-atom molecular simulations using as examples two organic materials, olanzapine (OZPN) and etioporphyrin I (EtpI). The dominance of dimeric structures in OZPN crystals has spurred speculation that the dimers preform in the solution, where they capture the majority of the solute, and then assemble into crystals. By contrast, EtpI in crystals aligns in parallel stacks of flat EtpI monomers unrelated by point symmetry. Raman and absorption spectroscopies show that solute monomers are the majority solute species in solutions of both compounds. Surprisingly, the kinetics of incorporation of OZPN into kinks is bimolecular, indicating that the growth unit is a solute dimer, a minority solution component. The disconnection between the dominant solute species, the growth unit, and the crystal symmetry is even stronger with EtpI, for which the (010) face grows by incorporating monomers, whereas the growth unit of the (001) face is a dimer. Collectively, the crystallization kinetics results with OZPN and EtpI establish that the structures of the dominant solute species and of the incorporating solute complex do not correlate with the symmetry of the crystal lattice. In a broader context, these findings illuminate the immense complexity of crystallization scenarios that need to be explored on the road to the understanding and control of crystallization.

Chemical vapor deposition of thin crystals of layered semiconductor SnS2 for fast photodetection application.

Layered two-dimensional (2D) semiconductors, such as MoS(2) and SnS(2), have been receiving intensive attention due to their technological importance for the next-generation electronic/photonic applications. We report a novel approach to the controlled synthesis of thin crystal arrays of SnS(2) at predefined locations on chip by chemical vapor deposition with seed engineering and have demonstrated their application as fast photodetectors with photocurrent response time ∼ 5 µs. This opens a pathway for the large-scale production of layered 2D semiconductor devices, important for applications in integrated nanoelectronic/photonic systems.

Four-fold Raman enhancement of 2D band in twisted bilayer graphene: evidence for a doubly degenerate Dirac band and quantum interference.

We report the observation of a strong 2D band Raman in twisted bilayer graphene (tBLG) with large rotation angles under 638 nm and 532 nm visible laser excitations. The 2D band Raman intensity increased four-fold as opposed to the two-fold increase observed in single-layer graphene. The same tBLG samples also exhibited rotation-dependent G-line resonances and folded phonons under 364 nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive interference between two double-resonance Raman pathways, which were enabled by a nearly degenerate Dirac band in the tBLG Moiré superlattices.

Photoluminescence and Raman Spectra of One-Dimensional Lead-free Perovskite CsCu2I3 Single-Crystal Wires.

Lead-free highly luminescent CsCu2I3 perovskite has attracted much attention recently, but agreements on basic optical properties have remained unsettled. By correlating X-ray diffraction with the photoluminescence (PL) of CsCu2I3 single-crystal wires, we first show that blue PL at 420 nm originates from CuI. We then exclude defect states as a source for the broadband emission centered at 570 nm from the lack of defect absorption, PL under sub-bandgap photoexcitation, observations of a linear dependence of PL intensity on excitation laser power, and a strong spectral blueshift under mild hydrostatic pressure. Finally, using a model of the self-trapped exciton and the associated coordinate configuration diagram, we explain pressure evolutions of PL energy, intensity, and lifetime. Single-crystal wires also enable us to obtain polarization-dependent Raman spectra down to 10 cm-1 and confirm their respective ambient crystal structure of orthorhombic Cmcm and phase transition to Pbnm at ∼5 GPa.

Origin of Luminescent Centers and Edge States in Low-Dimensional Lead Halide Perovskites: Controversies, Challenges and Instructive Approaches.

With only a few deep-level defect states having a high formation energy and dominance of shallow carrier non-trapping defects, the defect-tolerant electronic and optical properties of lead halide perovskites have made them appealing materials for high-efficiency, low-cost, solar cells and light-emitting devices. As such, recent observations of apparently deep-level and highly luminescent states in low-dimensional perovskites have attracted enormous attention as well as intensive debates. The observed green emission in 2D CsPb2Br5 and 0D Cs4PbBr6 poses an enigma over whether it is originated from intrinsic point defects or simply from highly luminescent CsPbBr3 nanocrystals embedded in the otherwise transparent wide band gap semiconductors. The nature of deep-level edge emission in 2D Ruddlesden-Popper perovskites is also not well understood. In this mini review, the experimental evidences that support the opposing interpretations are analyzed, and challenges and root causes for the controversy are discussed. Shortcomings in the current density functional theory approaches to modeling of properties and intrinsic point defects in lead halide perovskites are also noted. Selected experimental approaches are suggested to better correlate property with structure of a material and help resolve the controversies. Understanding and identification of the origin of luminescent centers will help design and engineer perovskites for wide device applications.

New Way to Synthesize Robust and Porous Ni1-xFex Layered Double Hydroxide for Efficient Electrocatalytic Oxygen Evolution.

Traditional catalysts are usually synthesized by sputtering, electrochemical deposition, or hydrothermal methods, and they also need to be combined with substrates to obtain the working electrodes. Here we introduce a new route to produce an efficient catalyst for oxygen evolution reaction (OER) that is made by ball milling and sintering. By using Se as a grinding aid, the bulk electrode Ni1-xFexSe1.15 is obtained with high porosity and robust mechanical strength after sintering. Active Ni1-xFex layered double hydroxide (LDH) nanosheets are subsequently produced on the surface of the Ni1-xFexSe1.15 by in situ electrochemical oxidation. Compared with traditional synthesis methods, the new process displays superior advantages, such as producing an electrode that is substrate-free and exhibits robust mechanical strength as well as being cost-effective for mass production. Additionally, V- and Mn-doped Ni0.75Fe0.25-LDH exhibit comparable and competent OER performance in 1 M KOH solution. Ni0.71V0.04Fe0.25-LDH achieves current densities of 100 and 1000 mA cm-2 at overpotentials of 244 and 300 mV, respectively. This work demonstrates a promising way to synthesize highly efficient and robust electrocatalysts for water oxidation.

Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides.

Since the first report of the green emission of 2D all-inorganic CsPb2 Br5 , its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb2 Br5 , a two-step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same-spot Raman-PL as a static property-structure probe reveals that CsPbBr3 nanocrystals are contributing to the green emission of CsPb2 Br5 ; pressure-dependent Raman-PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb2 Br5 is 0.3-0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full-fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr3- x Xx (X = Cl or I) is responsible for the strong visible PL in CsPb2 Br5- x Xx . This experimental approach is applicable to all PL-active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all-inorganic lead halides.

AB-stacked multilayer graphene synthesized via chemical vapor deposition: a characterization by hot carrier transport.

We report the synthesis of AB-stacked multilayer graphene via ambient pressure chemical vapor deposition on Cu foils and demonstrate a method to construct suspended multilayer graphene devices. In four-terminal geometry, such devices were characterized by hot carrier transport at temperatures down to 240 mK and in magnetic fields up to 14 T. The differential conductance (dI/dV) shows a characteristic dip at longitudinal voltage bias V = 0 at low temperatures, indicating the presence of hot electron effect due to a weak electron-phonon coupling. Under magnetic fields, the magnitude of the dI/dV dip diminishes through the enhanced intra-Landau level cyclotron phonon scattering. Our results provide new perspectives in obtaining and understanding AB-stacked multilayer graphene, important for future graphene-based applications.