Crystal 6H Research Articles

2H‐Au Nanosheet‐Templated Growth of PdFe for Electrocatalytic Methanol Oxidation

AbstractPd‐based alloy nanomaterials normally crystallize in the conventional face‐centered cubic (fcc) crystal phase. Here, 2H‐Au nanosheets (NSs), possessing 2H crystal phase (2H: hexagonal close‐packed with a stacking sequence of “AB”), are used as templates for the growth of PdFe, during which the 2H‐to‐fcc phase transformation in Au NSs happens, leading to the formation of 2H/fcc Au@PdFe core–shell NSs. By changing the Pd/Fe atomic ratio, the 2H/fcc phase ratio in 2H/fcc Au@PdFe NSs can be tuned accordingly. As a proof‐of‐concept application, the as‐synthesized 2H/fcc Au@Pd0.66Fe0.34 NSs are used as an electrocatalyst for methanol oxidation reaction (MOR) in alkaline media, exhibiting a remarkable mass activity of 4.39 A mg−1Pd, which is 1.7, 5.4 and 12.9 times that of 2H/fcc Au@Pd0.56Fe0.44 NSs (2.57 A mg−1Pd), 2H/fcc Au@Pd0.40Fe0.60 NSs (0.81 A mg−1Pd), and Pd black (0.34 A mg−1Pd), respectively, placing it among the best of reported Pd‐based MOR electrocatalysts. This strategy, based on phase engineering of nanomaterials (PEN), paves an efficient way to the rational design and controlled synthesis of noble multimetallic nanostructures with unconventional phases for exploring the phase‐dependent properties and applications.

AgFeO2 Delafosit Malzemelerin Sentez Koşulları, Kristal Yapısı, Morfolojisi ve OER Aktivitesi Arasındaki İlişki

In this current study, delafossite type AgFeO2 materials were synthesized using the hydrothermal method. Depending on synthesis temperature (115, 155 or 180 oC) and NaOH mineralizer mass (1.0, 1.6, 2.0 or 2.4 g) amount, the delafossites were composed of either mixed-phase 3R/2H-AgFeO2 or single-phase 3R-AgFeO2 crystals. Higher temperatures and NaOH mass were helpful to the growth of µm-sized 3R (space group of R-3m) crystals while lower temperatures and mineralizer mass produced nm-sized 2H (space group of P63/mmc) crystals. The comparison of different delafossite samples revealed that the oxygen evolution reaction (OER) current was larger in mixed-phase materials. Also, the transition from two-phase system to single-phase system was accompanied with a sudden drop in OER current. On the other hand, the difference between the electrochemical behavior of 3R and 2H phase delafossite is discussed in detail by considering similar studies in the literature.

Controllable growth of high-density tapered N-doped SiC nanowires arrays

Tapered nitrogen-doped SiC nanowires (N-doped SiCNWs) arrays were vertically grown on the carbon cloth utilizing Si, SiO2, C, and dicyandiamide powders as raw materials via a simple Fe(NO3)3 assisted chemical vapor deposition process. High-density tapered N-doped SiCNWs possessed homogeneous morphology. The diameter of the synthesized nanowire increased from ∼120 nm at the tail to ∼1.30 μm at the tip, while the length could be up to tens of micrometers. Moreover, the as-prepared N-doped SiCNWs were examined to be 2H/3C–SiC mixed crystals. The appearance of 2H crystal might be responsible for the high density defects in 3C–SiC. And since the interesting tapered shape was significantly affected by the catalyst droplet, the growth of N-doped SiCNWs may be governed by Fe-assisted VLS mechanism. In addition, the main emission peak of PL spectrum of N-doped SiCNWs was located at 474 nm, exhibiting a certain blue shift, which was mainly attributed to the special tapered shape, defects, 2H/3C–SiC heterostructure and the quantum confinement effect of the N-doped SiCNWs.

Tuning the 1T'/2H phases in WxMo1-xSe2 nanosheets.

Controlling materials' morphology, crystal phase and chemical composition at the atomic scale has become central in materials research. Wet chemistry approaches have great potential in directing the material crystallisation process to achieve tuneable chemical compositions as well as to target specific crystal phases. Herein, we report the compositional and crystal phase tuneability achieved in the quasi-binary WxMo1-xSe2 system with chemical and crystal phase mixing down to the atomic level. A series of WxMo1-xSe2 solid solutions in the form of nanoflowers with atomically thin petals were obtained via a direct colloidal reaction by systematically varying the ratios of transition metal precursors. We investigate the effect of selenium precursor on the morphology of the WxMo1-xSe2 material and show how using elemental selenium can enable the formation of larger and distinct nanoflowers. While the synthesised materials are compositionally homogeneous, they exhibit crystal phase heterogeneity with the co-existing domains of the 1T' and 2H crystal phases, and with evidence of MoSe2 in the metastable 1T' phase. We show at single atom level of resolution, that tungsten and molybdenum can be found in both the 1T' and 2H lattices. The formation of heterophase 1T'/2H WxMo1-xSe2 electrocatalysts allowed for a considerable improvement in the activity for the acidic hydrogen evolution reaction (HER) compared to pristine, 1T'-dominated, WSe2. This work can pave the way towards engineered functional nanomaterials where properties, such as electronic and catalytic, have to be controlled at the atomic scale.

Fabrication of 2H/3C-SiC heterophase junction nanocages for enhancing photocatalytic CO2 reduction

The morphology and structure of photocatalyst play an important role in photocatalytic activity. SiC semiconductor is considered as a promising material for the photocatalytic CO2 reduction due to its negative conduction band position. Herein, SiC nanocages is creatively synthesized by simple low-temperature molten-salt-mediated magnesiothermic reduction method with using SiO2 as template. The morphology and phase composition of SiC nanocages can be controlled by magnesium dosage and reaction temperature. The 2H and 3C crystal phase in SiC nanocage can form heterophase junctions uniformly to effectively accelerate the photogenerated electron transfer, and plays a key role in improving the photocatalytic activity of 2H/3C-SiC samples. The optimal SiC nanocage sample possesses a CO generation rate of 4.68 μmol g-1h−1 for photocatalytic CO2 reduction, which is 3.25 times higher than that of commercial SiC.

Controllable growth of multilayered XSe2 (X = W and Mo) for nonlinear optical and optoelectronic applications

The layered transition metal dichalcogenides (TMDs) exhibit the intriguing physical properties and potential application in novel electronic devices. However, controllable growth of multilayer TMDs remains challenging. Herein, large-scale and high-quality multilayer prototype TMDs of W(Mo)Se2 were synthesized via chemical vapor deposition. For Raman and photoluminescence measurements, 2H and 3R multilayer WSe2 crystals displayed significant layer-dependent peak position and intensity feature. Besides, different from the oscillatory relationship of second harmonic generation (SHG) intensity for odd–even layer numbers in 2H-stacked multilayer WSe2, the SHG intensity of 3R-stacked ones parabolically increased with the thickness due to the absence of inversion symmetry. For device application, photodetectors based on WSe2 with increasing thickness exhibited p-type (bilayer), ambipolar (trilayer), and n-type (four layers) semiconductor behaviors, respectively. Furthermore, photodetectors based on the as-synthesized 3R-stacked WSe2 flakes displayed an excellent responsivity of 7.8 × 103 mA W−1, high specific detectivity (Da*) of 1.7 × 1014 Jones, outstanding external quantum efficiency of 8.6 × 102%, and fast response time (τ Rise = 57 ms and τ Fall = 53 ms) under 532 nm illumination with bias voltage of V ds = 5 V. Similar results have also been achieved in multilayer MoSe2 crystals. All these findings indicate great potential of 3R-stacked TMDs in two-dimensional optoelectronic applications.

X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1)

Single crystals of 2H and 3R niobium diselenide were grown by a chemical transport reaction. The current determinations by single crystals X-ray diffraction reveal a non-stoichiometric composition. The structures are built from Se—Nb—Se slabs with Nb in trigonal prismatic coordination whereas the extra or additional Nb atoms are located in the octahedral holes between the slabs giving rise to the formula 2H and 3R-Nb1+xSe2 with 0.07 < x < 0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe2. By increasing the number of additional Nb atoms in the pure 2H-NbSe2, a transition 2H to 3R polytype should occur in order to minimize the Nblayer—Nbextra—Nblayer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb1+xSe2 are thermodynamically stable in the range 0 < x < 0.1.

2H and 4H silver colloidal suspension synthesis, as a new potential drug carrier

In this paper diffusion impact on the silver nanoparticles synthesis and their crystal structure is shown. For this purpose, comparative studies between continuous flow two-phase microdroplets systems vs. two-phase batch reactor were performed. Such two-phase configuration of the investigated system makes it possible to study the impact of mass transfer phenomenon on the nanoparticles formation process. In the case of two-phase microdroplets system, diffusion is replaced by convection, accelerated by interphase friction between water droplets and a continuous organic phase. It was concluded that under diffusion control it was possible to obtain silver nanoparticles with a hexagonal crystal structure. Moreover, for the first time using a chemical method, and diffusion rate-limiting step, the hexagonal structure 4H and 2H of Ag nanoparticles were synthesized. We suggest, that such hexagonal crystals can be applied as a smart drug delivery system, where the drug releases can be controlled by short light impulse. In effects, thermodynamically unstable 4H and 2H crystals transform to more dense-packed FCC structure.

Magnetotransport in Fe-intercalated TS2 : Comparison between T=Ti and Ta

Sharp magnetization switching and large magnetoresistance (MR) were previously discovered in single crystals of 2H-${\mathrm{Fe}}_{x}{\mathrm{TaS}}_{2}$ and attributed to the Fe superstructure and its defects. We report similar sharp switching and large MR in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ ($0.086\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}\phantom{\rule{0.28em}{0ex}}x\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}0.703$) while providing a side-by-side comparison of the only two such ferromagnetic transition-metal dichalcogenides. The switching field ${H}_{s}$ and MR values are similar in both 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ and 2H-${\mathrm{Fe}}_{x}{\mathrm{TaS}}_{2}$, with a larger than expected bowtie $\ensuremath{\rho}(H)$ and a sharper hysteresis loop for $H\ensuremath{\parallel}c$ in the former. The Curie and Weiss temperatures remain roughly constant below $x\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}1/3$ in the $T=\text{Ti}$ single crystals before increasing monotonically for higher $x$, while ${H}_{s}$ and MR reach maxima where defects in the superstructure exist, or a minimum near superstructure compositions, and they remain constant above $x\ensuremath{\sim}0.4$. Despite previous reports, electron diffraction shows only the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superstructure in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$. Glassy behavior is shown to coexist within the ferromagnetic state in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ for compositions between 0.1 and 0.703. A simple theoretical model considering first-, second- and third-neighbor interactions yields a phase diagram that accounts for both spin glass behavior and different superstructures.

MoS2 edges and heterophase interfaces: energy, structure and phase engineering

The transition metal dichalcogenides exhibit polymorphism; i.e. both 2H and 1T′ crystal structures, each with unique electronic properties. These two phases can coexist within the same monolayer microstructure, producing 2H/1T′ interfaces. Here we report a systematic investigation of the energetics of the experimentally most important MoS2 heterophase interfaces and edges. The stable interface and edge structures change with chemical potential (these edges/interfaces are usually non-stoichiometric). Stable edges tend to be those of highest atomic density and the stable interfaces correspond to those with local atomic structure very similar to the 2H crystal. The interfacial energies are lower than those of the edges, and the 1T′ edges have lower energy than the 2H edges. Because the 1T′ edges have much lower energy than the 2H edges, a sufficiently narrow 1T′ ribbon will be more stable than the corresponding 2H ribbon (this critical width is much larger in MoTe2 than in MoS2). Similarly, a large 2H flake have an equilibrium strip of 1T′ along its edge (again this effect is much larger in MoTe2 than in MoS2). Application of tensile strains can increase the width of the stable 1T′ strip or the critical thickness below which a ribbon favors the 1T′ structure. These effects provide a means to phase engineer transition metal dichalcogenide microstructures.

Ferromagnetism-dependent polytypism: CrAs versus MnAs

Abstract Density spin-polarized functional theory using pseudopotential plane wave method is used here to explore the structural and magnetic properties of 3C, 4H, 6H and 2H polytypes of transition metal arsenides: CrAs and MnAs. The results reveal that CrAs manifest weak dependence of the lattice parameter a and the c/a ratio versus hexagonality, but for MnAs, the lattice parameters display strong dependence on crystal polytype. Most importantly, our results show that the different crystal 3C, 4H, 6H and 2H polytypes exhibit significant distinct magnetism in CrAs and MnAs. While the total spin moments induced in CrAs is strongly independent of the crystal structure adopted, the ferromagnetism in MnAs is found sensitive to polytypism.

Synthesis, characterization, and anticancer activities of lipophilic pyridinecarboxaldimine platinum(II) complexes

We have prepared eleven pyridinecarboxaldimines (N-N’R) from the condensation of 2-pyridinecarboxaldehyde and the corresponding lipophilic primary amines. Addition of these ligands to [PtCl2(η2-coe)]2 (coe=cis-cyclooctene) gave complexes of the type cis-PtCl2(N-N’R) (2a-k) in moderate to high yields. The molecular structure of the N-butylphenyl derivative (2h) has been confirmed by an X-ray diffraction study. Crystals of 2h were monoclinic with a=8.5414(9)Å, b=11.6774(12)Å, c=16.1235(16)Å, β=92.7610(10)° in the space group P2(1)/c. All platinum complexes were examined for their in vitro cytotoxic properties against two human glioblastoma multiforme cell lines LN18 and LN405. The DNA binding properties of one of the most cytotoxic compounds was compared with that of cisplatin, but no effect was observed under the conditions tested.

Depth profiling by Raman spectroscopy of high-energy ion irradiated silicon carbide

Single crystals of 6H–SiC were irradiated at room temperature with 20MeV carbon ions at fluences of 1.5×1015 and 6.0×1015cm−2. Raman measurements were performed to study irradiation induced damage and the in-depth damage profile of SiC. A clear change of damage from the surface down to the stopping region of carbon ions as simulated by SRIM is exhibited. The affected area as detected by Raman is in good agreement with SRIM predictions while a little shallower dpa profile is observed. The partial disorder defined in the present work as a function of depth is demonstrated. A shift of the position of the TO peak towards lower wavenumbers with in-depth damage and then to higher wavenumbers beyond the most damaged region indicates that tensile strain due to defects has a backward V-curve distribution. The damaged layer is subjected to a compressive in-plane stress associated with the out-of-plane strain and the magnitude of this stress also has a backward V-curve depth profile. The evolution of line width of the TO peak with depth clearly shows the density of defects reaches the higher level at the most damaged region. The Raman spectroscopy scanning technique is proved to be a powerful tool for profiling of crystal damage induced by high-energy ion implantation.

Recrystallization of amorphous ion-implanted silicon carbide after thermal annealing

Single crystals of 6H–SiC were implanted at room temperature with 4-MeV Au ions to a fluence of 1015 cm−2. Raman spectra showed that full amorphization was achieved. The recrystallization process was studied by micro-Raman spectrometry after isochronal thermal annealing between 700 and 1500°C. The spectra permitted the evolution upon annealing of Si–C bonds, and also of Si–Si and C–C bonds, to be followed. Amorphous phase relaxation takes place below 700°C; then recrystallization of the 6H polytype sets in at 700°C. At 900°C crystallites with different crystalline states are formed. Moreover, Raman spectra provide evidence of graphitic nanocluster formation at 1500°C.

Theoretical hardness calculated from crystallo-chemical data for MoS2 and WS2 crystals and nanostructures

The calculation of the hardness of Mo and W disulfides using a crystallo-chemical model provides a unique opportunity to obtain separate quantitative information on the maximum hardness H(max) governed by strong intra-layer covalent bonds acting within the (0001) plane versus the minimum hardness H(min) governed by weak inter-layer van der Waals bonds acting along the c-axis of the hexagonal lattice. The penetration hardness derived from fundamental crystallo-chemical data (confirmed by experimental determinations) proved to be far lower in MS(2) (M = Mo, W) than in graphite and hexagonal BN, both for H(max) (H(graph)/H(MoS2) = 3.85; H(graph)/H(WS2) = 3.60; H(hBN)/H(MoS2) = 2.54; H(hBN)/H(WS2) = 2.37) as well as for H(min) (H(graph)/H(MoS2) = 6.22; H(graph)/H(WS2) = 5.87; H(hBN)/H(MoS2) = 4.72; H(hBN)/H(WS2) = 4.46). However, the gap between H(max) and H(min) is considerably larger in MS(2) (M = Mo,W), as indicated by H(max)/H(min) being 279 in 2H-MoS(2), 282 in 2H-WS(2), 173 in graphite and 150 in hBN. The gap was found to be even larger in MS(2) (M = Mo, W) nanostructures. These findings help to explain the excellent properties of MS(2) (M = Mo, W) as solid lubricants in high tech fields, either as bulk 2H crystals (inter-layer shear and peeling off lubricating mechanisms), or especially as onion-like fullerene nanoparticles (rolling/sliding mechanisms).

Anisotropy and temperature dependence of the first critical field in 2H–NbS2

We report on μ Hall probe measurements on single crystals of 2H–NbS2. This compound is the only superconducting 2H-dichalcogenide which does not develop a charge density wave. At low temperature and low magnetic field, a Bean profile is observed, allowing to evaluate the critical current. Moreover, the anisotropy and temperature dependence of the first critical field in 2H–NbS2 was measured down to 1.2K. A linear temperature dependence of the first penetration field is clearly observed. The absolute magnetic penetration depth is found to be 83nm which is slightly reduced compared to the iso-structural compound 2H–NbSe2.

The Effect of Process Parameters on 4H-SiC Single Crystal Grown by a PVT Method

Extensive study of various process parameters to influence on the growth of 4H-SiC crystal has been carried out using the transformation of the 6H-SiC seed by a PVT method. The axial temperature gradients were increased throughout increasing the crucible length along growth direction in order to enhance the growth rate and transformed crystal yield. The N2/Ar gas ratio used during the crystal growth related with carrier concentration/mobility of grown crystal. In the initial stage of growth, foreign polytypes such as 6H/15R were observed on 6H-SiC seed crystal but 4H crystals were entirely grown after the process optimization. While the typical absorption spectrum of SiC seed crystal indicated that the SiC polytype was the 6H-SiC with fundamental absorption energy of about 3.02eV, absorption spectrum of grown SiC crystal exhibited 4H-SiC with fundamental absorption energy of about 3.26eV. The entirely transformed SiC region exhibited lower micropipe density than 6H/4H transition region.

Ultra-Precision Machining of Stainless Steel and Nickel with Single Crystal 4H and 6H Boule SiC

Ultra-precision machining is dominated by single-crystal diamond cutting tools, and is typically applied to a narrow range of materials, particularly aluminum and copper. Single-crystal SiC can be comparable to some diamonds in hardness and thermal conductivity, while potentially having superior chemical and thermal stability, yet it has not been explored as a cutting tool for ultra-precision machining. We made two cutting tools with single-crystal SiC, one with sharp corners and one with a large circular radius, and used them to cut flat surfaces on two materials, 316 stainless steel and nickel. These materials generally cause unacceptably rapid diamond tool wear. We report the average roughness of the resulting surfaces cut with single-crystal 4H and 6H SiC tools.

Structural and optical properties of 6H–SiC helium-implanted at 600 K

Abstract Single crystals of 6H–SiC were implanted at 600 K with 100 keV He ions to three successively fluences and subsequently annealed at different temperatures ranging from 873 to 1473 K in vacuum. The recovery of lattice damage was investigated by different techniques including Rutherford backscattering spectrometry in channeling geometry, Raman spectroscopy and Fourier transform infrared spectroscopy. All three techniques showed that the damage induced by helium ion implantation in the lattice is closely related to the fluence. Rutherford backscattering spectrometry/channeling data on high temperature implantations suggest that for a fluence of 3 × 1016 He+/cm2, extended defects are created by thermal annealing to 1473 K. Apart from a well-known intensity decrease of scattering peaks in Raman spectroscopy it was found that the absorbance peak in Fourier transform infrared spectroscopy due to the stretching vibration of Si–C bond shifted to smaller wave numbers with increasing fluence, shifting back to larger wave numbers with increasing annealing temperature. These phenomena are attributed to different lattice damage behavior induced by the hot implantation process, in which simultaneous recovery was prevailing.

Application of 6H to 4H Polytype Conversion to Effective Reduction of Micropipes in 4H SiC Crystals

Bulk crystals of 6H and 4H silicon carbide have been grown by PVT method. 6H-SiC were obtained in optimized near-to-equilibrium growth conditions in order to improve the crystal quality and to provide the 6H seeds for 6H to 4H-SiC conversion. In experiments of 6H to 4H polytype transformation a set of invariable growth conditions was applied: C-face seed, C-rich atmosphere, on-axis seed orientation, pre-heating of the source material, slightly convex crystallization front and optimized geometry of the growth system. Other growth parameters were varied to optimize the polytype conversion, e.g.: structural quality of the seed, intentionally added impurity (N and/or Sc), initial growth stage recipe, argon pressure and temperature gradient - resulting in variety of growth rates and temperatures of the seed. Special attention was paid to seed passivation and a scheme of temperature and inert gas pressure changes during growth. Crystals were characterized by KOH etching, X-ray diffraction, optical and AFM microscopy. A reproducible method of 75% efficient conversion was elaborated. A large central surface free of micropipes was observed with characteristic six symmetrical ridges as well as the increased concentration of nitrogen. The parasitic 15R-SiC polytype was nucleated on the vicinal part of the crystallization front of 6H-SiC and 4H-SiC crystals.