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Register a new userChemical reaction directed oriented attachment: from precursor particles to new substances
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The oriented attachment (OA) of nanoparticles is an important mechanism for the synthesis of the crystals of inorganic functional materials, and the formation of natural minerals. For years it has been generally acknowledged that OA is a physical process, i.e., particle alignments and interface fusion via mass diffusion, not involving the formation of new substances. Hence, the obtained crystals maintain identical crystallographic structures and chemical constituents to those of the precursor particles. Here we report a chemical reaction directed OA growth, through which Y2(CO3)3.2H2O nanoparticles are converted to single-crystalline double-carbonates (e.g., NaY(CO3)2.6H2O). The dominant role of OA growth is supported by our first-principles calculations. Such a new OA mechanism enriches the aggregation-based crystal growth theory.
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Oriented attachment (OA) has become a well-recognized mechanism for the growth of metal, ceramic, and biomineral crystals. While many computational and experimental studies of OA have shown that particles can attach with some misorientation then rotate to remove adjoining grain boundaries, the underlying atomistic pathways for this Imperfect OA process remain the subject of debate. In this study, molecular dynamics and in situ TEM were used to probe the crystallographic evolution of up to 30 gold and copper nanoparticles during aggregation. It was found that Imperfect OA occurs because (1) grain boundaries become quantized when their size is comparable to the separation between constituent dislocations and (2) kinetic barriers associated with the glide of grain boundary dislocations are small. In support of these findings, TEM experiments show the formation of a single crystal aggregate after annealing 9 initially misoriented, agglomerated particles with evidence of dislocation slip and twin formation during particle/grain alignment. These observations motivate future work on assembled nanocrystals with tailored defects and call for a revision of Read-Shockley models for grain boundary energies in nanocrystalline materials.
Photoluminescence spectra of YVO$_{4}$:Eu$^{3+}$ nanoparticles are presented,with and without the attachment of of organic linker molecules that are proposed for linking to biomolecules. YVO$_{4}$:Eu$^{3+}$ nanoparticles with 5% dopant concentration were synthesized by wet chemical synthesis. X-ray diffraction and transmission electron microscopy show the expected wakefieldite structure of tetragonal particles with an average size of 17 nm. Fourier-transform infrared spectroscopy determines that metal-carboxylate coordination is successful in replacing the native metal-hydroxyl bonds with three organic linkers, namely benzoic acid, 3-nitro 4-chloro-benzoic acid and 3,4-dihydroxybenzoic acid, in separate treatments. UV-excitation photoluminescence spectra show that the position and intensity of dominant $^{5}D_{0}-^{7}F_{2}$ electric-dipole transition at 619 nm is unaffected by the benzoic acid and 3-nitro 4-chloro-benzoic acid treatments. Attachment of the 3,4-dihydroxybenzoic acid produces an order-of-magnitude quenching of the photoluminescence, due to the presence of high-frequency modes in the linker. Ratios of the dominant electric- and magnetic-dipole transitions confirm infrared measurements, which indicate that the bulk crystal of the nanoparticle is unchanged by all three treatments.
The ability to synthesis well-ordered two-dimensional materials under ultra-high vacuum and directly characterize them by other techniques in-situ can greatly advance our current understanding on their physical and chemical properties. In this paper, we demonstrate that iso-oriented {alpha}-MoO3 films with as low as single monolayer thickness can be reproducibly grown on SrTiO3(001) (STO) substrates by molecular beam epitaxy ( (010)MoO3 || (001)STO, [100]MoO3 || [100]STO or [010]STO) through a self-limiting process. While one in-plane lattice parameter of the MoO3 is very close to that of the SrTiO3 (aMoO3 = 3.96 {AA}, aSTO = 3.905 {AA}), the lattice mismatch along other direction is large (~5%, cMoO3 = 3.70 {AA}), which leads to relaxation as clearly observed from the splitting of streaks in reflection high-energy electron diffraction (RHEED) patterns. A narrow range in the growth temperature is found to be optimal for the growth of monolayer {alpha}-MoO3 films. Increasing deposition time will not lead to further increase in thickness, which is explained by a balance between deposition and thermal desorption due to the weak van der Waals force between {alpha}-MoO3 layers. Lowering growth temperature after the initial iso-oriented {alpha}-MoO3 monolayer leads to thicker {alpha}-MoO3(010) films with excellent crystallinity.
We perform a systematic first-principles study of phosphorene in the presence of typical monovalent (hydrogen, fluorine) and divalent (oxygen) impurities. The results of our modeling suggest a decomposition of phosphorene into weakly bonded one-dimensional (1D) chains upon single- and double-side hydrogenation and fluorination. In spite of a sizable quasiparticle band gap (2.29 eV), fully hydrogenated phosphorene found to be dynamically unstable. In contrast, full fluorination of phosphorene gives rise to a stable structure, being an indirect gap semiconductor with the band gap of 2.27 eV. We also show that fluorination of phosphorene from the gas phase is significantly more likely than hydrogenation due to the relatively low energy barrier for the dissociative adsorption of F2 (0.19 eV) compared to H2 (2.54 eV). At low concentrations, monovalent impurities tend to form regular atomic rows phosphorene, though such patterns do not seem to be easily achievable due to high migration barriers (1.09 and 2.81 eV for H2 and F2, respectively). Oxidation of phosphorene is shown to be a qualitatively different process. Particularly, we observe instability of phosphorene upon oxidation, leading to the formation of disordered amorphous-like structures at high concentrations of impurities.
Cobalt carbide nanoparticles were processed using polyol reduction chemistry that offers high product yields in a cost effective single-step process. Particles are shown to be acicular in morphology and typically assembled as clusters with room temperature coercivities greater than 4 kOe and maximum energy products greater than 20 KJ/m3. Consisting of Co3C and Co2C phases, the ratio of phase volume, particle size, and particle morphology all play important roles in determining permanent magnet properties. Further, the acicular particle shape provides an enhancement to the coercivity via dipolar anisotropy energy as well as offering potential for particle alignment in nanocomposite cores. While Curie temperatures are near 510K at temperatures approaching 700 K the carbide powders experience an irreversible dissociation to metallic cobalt and carbon thus limiting operational temperatures to near room temperature.
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