Synthesis Of Single-crystalline Research Articles

Anchoring Catalytic Metal Nodes within a Single-Crystalline Pyrazolate Metal-Organic Framework for Efficient Heterogeneous Catalysis.

The synthesis of single-crystalline and robust pyrazolate metal-organic frameworks (Pz-MOFs) capable of facilitating challenging organic transformations is fundamentally significant in catalysis. Here we demonstrate a metal-node-based catalytic site anchoring strategy by synthesizing a single-crystalline and robust Pz-MOF (PCN-1004). PCN-1004 features one-dimensional (1D) copper-Pz chains interconnected by well-organized ligands, forming a porous three-dimensional (3D) network with two types of 1D open channels. Notably, PCN-1004 displays exceptional stability in aqueous solutions across a broad pH range (1 to 14), attributed to the robust copper-Pz coordination bonds. Significantly, PCN-1004 functions as an outstanding catalyst in cross dehydrogenative coupling reactions for constructing C-O/C-S bonds, even in the absence of directing groups, achieving yields of up to ~99%, with long cycle lives and high substrate compatibility. PCN-1004 outperforms all previously reported porphyrin-based homogeneous and heterogeneous catalysts. Control experiments and computations elucidate the pivotal catalytic role of the copper-Pz chains and reveal a free radical pathway for the reaction. This work not only demonstrates the successful implementation of a metal-node-based catalytic site anchoring strategy for the efficient catalysis of challenging organic transformations but also highlights the synergistic effect of a robust framework, 1D open channels, and active sites in enhancing catalytic efficiency within MOFs.

Nonconfinement growth of edge-curved molecular crystals for self-focused microlasers.

Synthesis of single-crystalline micro/nanostructures with curved shapes is essential for developing extraordinary types of optoelectronic devices. Here, we use the strategy of liquid-phase nonconfinement growth to controllably synthesize edge-curved molecular microcrystals on a large scale. By varying the molecular substituents on linear organic conjugated molecules, it is found that the steric hindrance effect could minimize the intrinsic anisotropy of molecular stacking, allowing for the exposure of high-index crystal planes. The growth rate of high-index crystal planes can be further regulated by increasing the molecular supersaturation, which is conducive to the cogrowth of these crystal planes to form continuously curved-shape microcrystals. Assisted by nonrotationally symmetric geometry and optically smooth curvature, edge-curved microcrystals can support low-threshold lasing, and self-focusing directional emission. These results contribute to gaining an insightful understanding of the design and growth of functional molecular crystals and promoting the applications of organic active materials in integrated photonic devices and circuits.

Epitaxial registry and crystallinity of MoS2 via molecular beam and metalorganic vapor phase van der Waals epitaxy

Two-dimensional transition metal dichalcogenide (TMD) semiconductors have risen as an important material class for novel nanoelectronic applications. Molybdenum disulfide (MoS2) is the most representative TMD compound due to its superior stability and attractive properties for (opto-) electronic devices. However, the synthesis of single-crystalline and functional MoS2 across large-area substrates remains crucial for its successful integration in semiconductor industry platforms. Therefore, this work focuses on the study of MoS2 epitaxy via two well-established industry-compatible synthesis methods, promising for the large-area and single-crystalline integration of van der Waals (vdW) materials. These methods are molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) and have studied MoS2 quasi-vdW heteroepitaxy on reconstructed sapphire substrates and MoS2 vdW homoepitaxy on exfoliated MoS2 flakes. By examining the MoS2 structural properties using diffraction and spectroscopy techniques, the epitaxial relation and crystal quality are assessed, which reveals insights into the prevalence of inter- and intragrain defects such as grain boundaries and sulfur vacancies. The MBE method yields superior epitaxial MoS2 registry on both sapphire and MoS2 surfaces as compared to MOVPE, although inferior defectivity arises from the typical lower MBE growth temperature and chalcogen partial pressure. Moreover, both synthesis methods generate high densities of twinned MoS2 grain boundaries, which hamper defect-free integration. As a result, this challenging integration might become an important bottleneck for industrial TMD-based applications with a low tolerance for material defects.

Synthesis of single-crystalline, porous TaON microspheres toward visible-light photocatalytic conversion of CO2 into liquid hydrocarbon fuels

The porous spherical architecture of TaON plays a significant role in photoreduction of CO2, including the more reaction sites, easier trapping of photon, and shorter charge transfer distance from interior to surface to expedite charge separation.

Germanium nanowire synthesis using solid precursors

We report on the synthesis of single crystalline, high aspect ratio germanium (Ge) nanowires (NWs) by vapor transport method using three different solid powder precursors. Investigated precursors were either powder like germanium or powder mixtures like germanium dioxide with carbon and germanium iodide with germanium. As-grown NWs were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) to obtain structural information. The effect of temperature and pressure on the diameter and morphology of the NWs were determined. Both pressure and temperature were found to increase the diameter of the NWs independent of precursor type. Growth direction of the NWs was found to be the same while clear differences in the morphology and surrounding oxide layer thickness were observed with different precursors. Oxide layer removal via hydrobromic acid treatment was also realized. Results provided in this paper allow the basis for optimizing the synthesis of Ge NWs using solid precursors.

Spontaneous Chemical Vapor Growth of NiSi Nanowires and Their Metallic Properties

One-dimensional metallic nanostructures offer challenging opportunities to investigate their possible applications as interconnects in electronic circuitry and as field emitters in field emission displays. In particular, metallic silicide nanowires are promising candidates, because their growth can be easily integrated with silicon processing technology. Indeed the synthesis of single-crystalline, transition metal silicide nanowires, such as TaSi2, [6] e-FeSi, and CoSi, has been recently reported using diverse methods. The synthetic methods employed involve relatively high temperature processes, and may not be compatible with the low temperature processes required for their practical applications into electronic and display devices. In contrast, single-crystalline Ni-silicide nanowires, which have been experimentally verified to possess intrinsically low resistivity at low dimensions compared with the others, can be processed at relatively low temperatures, thus can provide practical advantages. For example, Wu et al. reported highly metallic properties of single-crystalline NiSi nanowires fabricated by a solid-state reaction at 550 °C between individual Si nanowires and Ni overshells. Here a simpler and controlled synthesis at low temperatures (∼400 °C) is reported, where single-crystalline NiSi nanowires spontaneously grow by SiH4 chemical vapor deposition (CVD) on Ni thin films predeposited on various substrates, such as SiO2/Si, quartz, and indium tin oxide (ITO) substrates. In fact, the Ni-catalyzed vapor phase thin film deposition has been previously investigated for applications into Si microelectronics and photovoltaics. Specifically, the dimensionality of the Ni-silicides have been reproducibly directed from thin films to single-crystalline nanowires, as well as their phases, by fine-tuning of growth parameters during CVD of a SiH4 gas precursor on predeposited Ni thin films. It is noted that Decker et al. and Kim et al. recently reported similar Ni-silicide nanowire growth by CVD and sputtering, however, the detailed growth mechanism, which is the main focus of this study, is unavailable to date. Here, by providing close observations of the morphological and phase evolution of Ni-silicide nanowires under optimized growth conditions, the spontaneous nanowire growth mechanism is discussed based on one-dimensional nucleation and growth of NiSi at a low supersaturation limit along with one-dimensional Ni diffusion during the chemical vapor reaction. Singlecrystalline NiSi nanowires in this study exhibit typical metallic behaviors, and show promising field-emission properties. It is suggested that our simple method to spontaneously grow NiSi nanowires at low temperatures can provide a practical strategy to fabricate metallic nanostructures based on bottom-up synthetic approaches. Our synthetic approach employs the Ni-catalyzed decomposition of SiH4, [14] which can occur well below the thermal decomposition temperature of SiH4 at above 600 °C. Thermally evaporated Ni thin films of 60–80 nm in thickness are used. During the CVD of SiH4 within the wide range of precursor partial pressures and growth temperatures of 10–100 torr and 250–600 °C, the reaction produces either Ni-silicide thin films or nanowires of various phases. Figure 1 illustrates the evolution of the morphology and the phases of Ni-silicides, grown at 250, 400, and 600 °C at 50 torr of SiH4, with the corresponding scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and element depth profiling by Auger spectrometry. Within the entire growth range, Ni-silicide thin films have been commonly formed on the substrates; however, the surface morphology of the Ni-silicide thin films has been modified from planar sheets to nanowires, as well as phases in Ni-to-Si stoichiometry. The observations from Figure 1 are summarized in the following within the framework of SiH4 vapor–Ni solid reactions, where SiH4 decomposition and solid Ni diffusion are both thermally activated. First, at 250 °C, the catalyzed decomposition of SiH4 and the Ni out-diffusion from the pre-deposited Ni thin films are not sufficient to turn the Ni film into a Ni-silicide thin film during the reaction, and C O M M U N IC A IO N

Single‐Crystalline and Near‐Monodispersed NaMF3 (M: Mn, Co, Ni, Mg) and LiMAlF6 (M: Ca, Sr) Nanocrystals from Cothermolysis of Multiple Trifluoroacetates in Solution.

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Optical band gap in the system ZnO1 – x S x . An experimental and quantum chemical study

The synthesis of single crystalline and polycrystalline material is reported in the system ZnO1 – x S x . Substitution of oxygen by sulphur yields hexagonal wurtzite-type mixed crystals in the range 0 ≤ x ≤ 0.05. The cubic zincblende structure is observed for oxygen incorporation into ZnS for 0.96 ≤ x ≤ 1. At intermediate compositions, the system is two-phase. For both types of anion substitution, the band gap energies, as determined from optical transmission and remission experiments, are found to decrease from the pure end members. Quantum chemical calculations at density functional level identify local structural changes as the main factor responsible for the reduction of band gap energies.