Crystals

14 pages, 7740 KiB

Open AccessArticle

Selective Area Growth and Structural Characterization of GaN Nanostructures on Si(111) Substrates

by Alexana Roshko, Matt Brubaker, Paul Blanchard, Todd Harvey and Kris A. Bertness

Crystals 2018, 8(9), 366; https://doi.org/10.3390/cryst8090366 - 16 Sep 2018

Cited by 20 | Viewed by 5571

Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. [...] Read more.

Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. Defects related to Al–Si eutectic formation were observed in all samples, irrespective of lattice mismatch and buffer layer polarity. Eutectic related defects in the Si surface caused voids in N-polar samples, but not in metal-polar samples. Likewise, inversion domains were present in N-polar, but not metal-polar samples. The morphology of Ga-polar GaN SAG on nitride buffered Si(111) was similar to that of homoepitaxial GaN SAG. Full article

(This article belongs to the Special Issue Growth, and Structural Characterization of Self-Nucleated Nanowires)

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Figure 1
Plane-view field emission scanning electron microscopy (FESEM) images of selective area growth (SAG) nanowires on different buffers: (a) and (b) N–Al (AlN/Si(111) substrate; Δao = 2.369%), (c) and (d) N-AlGa (N-polar GaN/AlN/Si(111) substrate; Δao = −0.213%), (e) and (f) N-AlGaSL-1 (N-polar GaN + SL/AlN/Si(111) substrate; Δao = 0.028%), (g) and (h) N–GaN (commercial N-polar GaN substrate. The diameters of the patterned holes were 300 nm for all images. Full article ">Figure 2
STEM ABF images of NWs on sample N–AlGaSL-1 showing: (a) inversion domains, some of which propagate from the GaN buffer and some of which initiate at the regrowth interface; (b) threading dislocations propagating from the GaN buffer into the NW and annihilating near the NW base. After the NW growth, an AlGaN layer was deposited on this sample at low a temperature and can be seen on the SiNx buffer in both images. Full article ">Figure 3
FESEM images of SAG lines: (a), (b), and (c) on a commercial N-polar GaN substrate (N-GaN), 300 nm wide; (d) and (e), on a PAMBE grown N-polar GaN + SL/AlN/Si(111) substrate (N–AlGaSl-2), 250 nm wide. Complete filling of the lines is achieved on the N-polar GaN substrate, but not on the PAMBE substrate. Image (f) is of a bare PAMBE grown N-polar GaN + SL/AlN/Si(111), taken prior to processing for SAG. The scale of all the images is the same (white lines are 1 µm long), except (c) which is a magnified view of the area outlined in (b) and shows m-plane faceting of sidewalls patterned along the a-plane. Full article ">Figure 4
(a) Schematic of the STEM lamellae position on sample N–AlGaSL-1; the arrow indicates the STEM imaging direction. (b) Atomic resolution STEM ABF image showing the zone axis is [<math display="inline"><semantics> <mrow> <mn>11</mn> <mover accent="true"> <mn>2</mn> <mo>¯</mo> </mover> <mn>0</mn> </mrow> </semantics></math>] and, therefore, the lamella has {<math display="inline"><semantics> <mrow> <mn>11</mn> <mover accent="true"> <mn>2</mn> <mo>¯</mo> </mover> <mn>0</mn> </mrow> </semantics></math>} surfaces and the NW facets are [<math display="inline"><semantics> <mrow> <mn>1</mn> <mover accent="true"> <mn>1</mn> <mo>¯</mo> </mover> <mn>00</mn> </mrow> </semantics></math>] The atomic resolution image also shows the nitrogen polarity of the NWs. Full article ">Figure 5
(a) through (c) STEM ABF images of the N–AlGaSL-2 buffer: (a) lower magnification image showing the columnar structure associated with the granularity observed in the SEM (<a href="#crystals-08-00366-f002" class="html-fig">Figure 2</a>d,e), the dashed white line marks the interface between the GaN buffer and the SAG GaN; (b) lower magnification composite image showing columnar voids in the GaN buffer beneath the SiNx mask; (c) high magnification image of a void which initiates in the Si buffer (left box in (b)); and (d) atomic scale STEM HAADF image at the base of another void (right box in (b)) where the Si protrudes above the substrate plane and generates a defect in the AlN buffer. Full article ">Figure 6
(a) and (b) STEM ABF images of the Ga-polar buffer for Ga–AlGaSL: (a) lower magnification composite image showing threading dislocations and mosaic structure in the Al-polar AlN, which propagate into the GaN buffer; (b) atomic resolution STEM ABF image showing Al-polarity in the AlN buffer (top box in (a)); and (c) atomic scale STEM HAADF image of an Si hillock above the substrate plane at the Si/AlN interface (bottom box in (a)), similar to that in <a href="#crystals-08-00366-f004" class="html-fig">Figure 4</a>d. Full article ">

68 pages, 5563 KiB

Open AccessReview

Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes

by Andreas Løken, Sandrine Ricote and Sebastian Wachowski

Crystals 2018, 8(9), 365; https://doi.org/10.3390/cryst8090365 - 14 Sep 2018

Cited by 126 | Viewed by 11472

Abstract

This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while [...] Read more.

This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry. Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr_1−xY_xO_3−δ) and oxidation/reduction of La_1−xSr_xCo_0.2Fe_0.8O_3−δ. While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally. This was motivated by the discrepancy of some values in literature. The conclusions are that the heating rate has little to no effect on the obtained values. Models for the expansion coefficients of a composite material are presented and include the effect of porosity. A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors. Finally, the methods of mitigation of the thermal mismatch problem are discussed. Full article

(This article belongs to the Special Issue Ceramic Conductors)

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12 pages, 949 KiB

Open AccessArticle

Temperature Effects on the Elastic Constants, Stacking Fault Energy and Twinnability of Ni₃Si and Ni₃Ge: A First-Principles Study

by Lili Liu, Liwan Chen, Youchang Jiang, Chenglin He, Gang Xu and Yufeng Wen

Crystals 2018, 8(9), 364; https://doi.org/10.3390/cryst8090364 - 14 Sep 2018

Cited by 3 | Viewed by 3396

Abstract

The volume versus temperature relations for Ni

_{3}

Si and Ni

_{3}

Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized [...] Read more.

The volume versus temperature relations for Ni

_{3}

Si and Ni

_{3}

Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized stacking fault energies and generalized planar fault energies of Ni

_{3}

Si and Ni

_{3}

Ge are investigated by first principles calculations. The elastic constants, antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies and twinning energy decrease with increasing temperature. The twinnability of Ni

_{3}

Si and Ni

_{3}

Ge are examined using the twinnability criteria. It is found that their twinnability decrease with increasing temperature. Furthermore, Ni

_{3}

Si has better twinnability than Ni

_{3}

Ge at different temperatures. Full article

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10 pages, 1767 KiB

Open AccessArticle

Mosaicity of Spin-Crossover Crystals

by Sabine Lakhloufi, Elodie Tailleur, Wenbin Guo, Frédéric Le Gac, Mathieu Marchivie, Marie-Hélène Lemée-Cailleau, Guillaume Chastanet and Philippe Guionneau

Crystals 2018, 8(9), 363; https://doi.org/10.3390/cryst8090363 - 13 Sep 2018

Cited by 9 | Viewed by 4422

Abstract

Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state [...] Read more.

Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state and others in the low spin (LS) state. Since the volume of HS and LS crystal packings are known to be very different, the assembly of domains within the crystal, i.e., its mosaicity, may be modified at the spin crossover. With little data available in the literature we propose an investigation into the temperature dependence of mosaicity in certain spin-crossover crystals. The study was preceded by the examination of instrumental factors, in order to establish a protocol for the measurement of mosaicity. The results show that crystal mosaicity appears to be strongly modified by thermal spin-crossover; however, the nature of the changes are probably sample dependent and driven, or masked, in most cases by the characteristics of the crystal (disorder, morphology …). No general relationship could be established between mosaicity and crystal properties. If, however, mosaicity studies in spin-crossover crystals are conducted and interpreted with great care, they could help to elucidate crucial crystal characteristics such as mechanical fatigability, and more generally to investigate systems where phase transition is associated with large volume changes. Full article

(This article belongs to the Special Issue Synthesis and Applications of New Spin Crossover Compounds)

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Temperature dependence of the mosaicity for a small crystal of [Fe(PM-AzA)2(NCS)2] in the cooling mode. Full article ">Figure 2
Temperature dependence of the mosaicity for a crystal of [Fe(PM-TeA)2(NCS)2]·0.5MeOH in the cooling (black squares) and warming (grey triangles) modes. Full article ">Figure 3
Temperature dependence of the mosaicity for a crystal of [Fe(PM-TheA)2(NCS)2]-II in the cooling (black squares) and warming (light-grey triangles) modes. Full article ">Figure 4
Temperature dependence of the mosaicity of [Fe(PM-PeA)2(NCSe)2] in the cooling mode (black squares), including the return values (light grey) after warming at room temperature. Full article ">Figure 5
Temperature dependence of the mosaicity of [Fe(PM-BrA)2(NCS)2] in the cooling mode, with diffraction images at 120 K (LS), 205 K (mixture HS/LS) and 260 K (HS). Full squares mark the cooling mode and the grey triangle the value when returned to room temperature. Full article ">Scheme 1
schematic views of the molecular iron(II) complexes of the [FeII(PM-L)2(NCX)2] series studied in this paper, labelled 1 to 5. Full article ">

12 pages, 3610 KiB

Open AccessArticle

Inducing Crystallinity of Metal Thin Films with Weak Magnetic Fields without Thermal Annealing

by Stefan S. Ručman, Winita Punyodom, Jaroon Jakmunee and Pisith Singjai

Crystals 2018, 8(9), 362; https://doi.org/10.3390/cryst8090362 - 13 Sep 2018

Cited by 9 | Viewed by 3764

Abstract

Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic [...] Read more.

Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic field at ambient conditions under ultra-pure nitrogen flow to replace the annealing of thin films, and thus designed an environmentally friendly and energy-efficient thin film fabrication method. We employed grazing incidence X-Ray Diffraction spectroscopy to characterize crystallinity of Iron, Nickel, Copper and Tungsten thin films prepared by a sparking discharge process in the presence of 0.4 T magnetic field at an ambient temperature of 25 °C. Control experiment was conducted by sparking without a magnetic field present and using ultra-pure nitrogen flow and ambient air containing oxygen. The Iron thin film prepared in ultra-pure nitrogen flow preserved crystallinity even after one year of ageing. Nickel exhibited higher crystallinity when sparked in nitrogen gas flow than when sparked in atmospheric air and was the only element to crystalize under atmospheric air. Tungsten successfully crystalized after just 40 min of sparking and aluminium failed to crystalize at all, even after 12 h of sparking under nitrogen flow. Full article

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12 pages, 3300 KiB

Open AccessArticle

Molecular Dynamics Investigation of Graphene Nanoplate Diffusion Behavior in Poly-α-Olefin Lubricating Oil

by Lupeng Wu, Baoyu Song, Leon M. Keer and Le Gu

Crystals 2018, 8(9), 361; https://doi.org/10.3390/cryst8090361 - 13 Sep 2018

Cited by 16 | Viewed by 4393

Abstract

Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method [...] Read more.

Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method to improve the dispersibility of graphene in lubricants. In this study, the equilibrium molecular dynamics (EMD) simulations were carried out to investigate the diffusion behavior of graphene nanoplates in poly-α-olefin (PAO) lubricating oil. The effects of graphene-size, edge-functionalization, temperature, and pressure on the diffusion coefficient were studied. In order to understand the influence of edge-functionalization, three different functional groups were grafted to the edge of graphene nanoplates: COOH, COON(CH₃)₂, CONH(CH₂)₈CH₃ (termed GO, MG, and AG, respectively). The EMD simulations results demonstrated that the relationships between diffusion coefficient and graphene-size and number of functional groups were linear while the temperature and pressure had a nonlinear influence on the diffusion coefficient. It was found that the larger dimension and more functional groups provided the lower diffusion coefficient. AG with eight CONH(CH₂)₈CH₃ groups exhibited the lowest diffusion coefficient. Furthermore, the experimental results and radial distribution function for graphene-PAO illustrated that the diffusion coefficient reflected the dispersibility of nanoparticles in nanofluids to some degree. To our best knowledge, this study is the first time the diffusion behavior of graphene in PAO lubricating oil was investigated using EMD simulations. Full article

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12 pages, 1940 KiB

Open AccessArticle

Tracing Phase Transformation and Lattice Evolution in a TRIP Sheet Steel under High-Temperature Annealing by Real-Time In Situ Neutron Diffraction

by Dunji Yu, Yan Chen, Lu Huang and Ke An

Crystals 2018, 8(9), 360; https://doi.org/10.3390/cryst8090360 - 11 Sep 2018

Cited by 11 | Viewed by 5029

Abstract

Real-time in situ neutron diffraction was used to characterize the crystal structure evolution in a transformation-induced plasticity (TRIP) sheet steel during annealing up to 1000 °C and then cooling to 60 °C. Based on the results of full-pattern Rietveld refinement, critical temperature regions [...] Read more.

Real-time in situ neutron diffraction was used to characterize the crystal structure evolution in a transformation-induced plasticity (TRIP) sheet steel during annealing up to 1000 °C and then cooling to 60 °C. Based on the results of full-pattern Rietveld refinement, critical temperature regions were determined in which the transformations of retained austenite to ferrite and ferrite to high-temperature austenite during heating and the transformation of austenite to ferrite during cooling occurred, respectively. The phase-specific lattice variation with temperature was further analyzed to comprehensively understand the role of carbon diffusion in accordance with phase transformation, which also shed light on the determination of internal stress in retained austenite. These results prove the technique of real-time in situ neutron diffraction as a powerful tool for heat treatment design of novel metallic materials. Full article

(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)

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Schematic configuration of the real-time in situ neutron diffraction on the sample under heating by induction coils. The upper right inset shows the camera view of the heating set-up. Full article ">Figure 2
Camera views of the specimen at different temperatures: (a) 60 °C in Cycle 1 before heating up, (b) isothermal holding at 850 °C in Cycle 1, (c) isothermal holding at 1000 °C in Cycle 2 and (d) 60 °C in Cycle 2 after cooling down. (e) Annealing profiles of two cycles. Full article ">Figure 3
(a) Contour plots of diffraction peak intensity as a function of time/temperature (upper image: Cycle 1, lower image: Cycle 2). The color indicates the scale of intensity with purple as minimum (background) and yellow as maximum. (b) Selected 1-min diffraction patterns during annealing. (c) Examples of full-pattern Rietveld refinement via GSAS. Full article ">Figure 4
Weight fraction evolutions of constituent phases as a function of temperature/time. The three subsets from left to right correspond to heating, isothermal holding and cooling stages, respectively. Full article ">Figure 5
Evolution of lattice parameter as a function of temperature: (a) BCC and (b) FCC. The dash lines are the linear fittings by Equation (1) in different temperature regions: (a) L1 for 60~300 °C, L2 for 600~700 °C, and L3 for 900~1000 °C; (b) L1 for 60~300 °C, L2 for 300~60 °C, and L3 for 1000~900 °C. The reference temperature was set to 0 °C for comparison convenience. Full article ">

12 pages, 1895 KiB

Open AccessArticle

DFT Calculations and Mesophase Study of Coumarin Esters and Its Azoesters

by M. Hagar, H.A. Ahmed and O.A. Alhaddadd

Crystals 2018, 8(9), 359; https://doi.org/10.3390/cryst8090359 - 8 Sep 2018

Cited by 42 | Viewed by 4267

Abstract

Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), I_n, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), II_n, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length [...] Read more.

Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), I_n, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), II_n, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length of the mesogenic part. Within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10, 12 and 16 carbons. Mesophase behavior was investigated by differential scanning calorimetry (DSC) and identified by polarized light microscopy (PLM). Density functional theory (DFT) calculations for coumarin derivatives were discussed. The results revealed that the incorporation of azo group incorporated in the mesogenic core decreases the energy differences, increases the dipole moments and stabilities of coumarin azoesters series more than coumarin esters. Full article

(This article belongs to the Section Liquid Crystals)

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Figure 1
Prepared compounds In and IIn. Full article ">Figure 2
DSC thermograms of some representative compounds recorded from heating and cooling at a rate of 10 °C/min for: (a) I8 and (b) II10. Full article ">Figure 3
Effect of alkoxy-chain length on the mesophase behavior of the individual homologous series of (a) In and (b) IIn. Full article ">Figure 4
The optimized structures of I6 and II6. Full article ">Figure 5
Molecular Electrostatic potentials (MEP) mapped on the electron density surface calculated by the DFT/B3LYP method. Full article ">Figure 6
The ground state isodensity surface plots for the frontier molecular orbitals. Full article ">Scheme 1
Synthesis of coumarin derivatives In and IIn. Full article ">

9 pages, 1463 KiB

Open AccessArticle

Investigation of Inverted Perovskite Solar Cells for Viscosity of PEDOT:PSS Solution

by Pao-Hsun Huang, Yeong-Her Wang, Chien-Wu Huang, Wen-Ray Chen and Chien-Jung Huang

Crystals 2018, 8(9), 358; https://doi.org/10.3390/cryst8090358 - 6 Sep 2018

Cited by 7 | Viewed by 7097

Abstract

In this paper, we demonstrate that the inverted CH₃NH₃PbI₃ (perovskite) solar cells (PSCs) based on fullerene (C₆₀) as an acceptor is fabricated by applying an improved poly(3,4-ethlyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution as a hole transport layer (HTL). The [...] Read more.

In this paper, we demonstrate that the inverted CH₃NH₃PbI₃ (perovskite) solar cells (PSCs) based on fullerene (C₆₀) as an acceptor is fabricated by applying an improved poly(3,4-ethlyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution as a hole transport layer (HTL). The power conversion efficiency (PCE) of inverted PSCs is increased by 37.5% with stable values of open-circuit voltage (V_OC) and fill factor (FF) because we enhance the viscosity of the PEDOT:PSS solution, indicating the perfect effect on both external quantum efficiency (EQE) and surface grain size. The characteristics of the PEDOT:PSS solution, which is being improved through facile methods of obtaining excellent growth of PEDOT:PSS thin film, have a considerable impact on carrier transport. A series of further processing fabrications, including reliable and feasible heating and stirring techniques before the formation of the PEDOT:PSS thin film via spin-coating, not only evaporate the excess moisture but also obviously increase the conductivity. The raised collection of holes become the reason for the enhanced PCE of 3.0%—therefore, the stable performance of FF and V_OC are attributed to lower series resistance of devices and the high-quality film crystallization of perovskite and organic acceptors, respectively. Full article

(This article belongs to the Special Issue Selected Papers from Taiwan Association for Academic Innovation, TAAI 2018)

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30 pages, 6272 KiB

Open AccessReview

A Review of Current Development of Graphene Mechanics

by Qiang Cao, Xiao Geng, Huaipeng Wang, Pengjie Wang, Aaron Liu, Yucheng Lan and Qing Peng

Crystals 2018, 8(9), 357; https://doi.org/10.3390/cryst8090357 - 6 Sep 2018

Cited by 72 | Viewed by 10585

Abstract

Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship [...] Read more.

Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship between the strain and stress plays an essential role in graphene mechanics. Strain can dramatically influence the electronic and optical properties, and could be utilized to engineering those properties. Furthermore, graphene with specific kinds of defects exhibit mechanical enhancements and thus the electronic enhancements. In this short review, we focus on the current development of graphene mechanics, including tension and compression, fracture, shearing, bending, friction, and dynamics properties of graphene from both experiments and numerical simulations. We also touch graphene derivatives, including graphane, graphone, graphyne, fluorographene, and graphene oxide, which carve some fancy mechanical properties out from graphene. Our review summarizes the current achievements of graphene mechanics, and then shows the future prospects. Full article

(This article belongs to the Special Issue Graphene Mechanics)

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10 pages, 4771 KiB

Open AccessArticle

Microstructure and Mechanical Properties of Mg–6Al–1Sn–0.3Mn Alloy Sheet Fabricated through Extrusion Combined with Rolling

by Peng Peng, Aitao Tang, Xianhua Chen, Jia She, Shibo Zhou, Jiangfeng Song and Fusheng Pan

Crystals 2018, 8(9), 356; https://doi.org/10.3390/cryst8090356 - 5 Sep 2018

Cited by 6 | Viewed by 3345

Abstract

Hot rolling was carried out in this study to modify the microstructures of an extruded Mg–6Al–1Sn–0.3Mn alloy sheet and investigate its effects on mechanical properties. After hot rolling, the grains and second phase of the extruded alloy sheet were remarkably refined, and the [...] Read more.

Hot rolling was carried out in this study to modify the microstructures of an extruded Mg–6Al–1Sn–0.3Mn alloy sheet and investigate its effects on mechanical properties. After hot rolling, the grains and second phase of the extruded alloy sheet were remarkably refined, and the c-axis of a few grains was parallel to the transverse direction. The strength improvement was mainly attributed to the grain and Mg₁₇Al₁₂ particle refinement due to the Hall–Petch effect and the Orowan mechanism. The random orientation of the fine grains resulted in improving ductility and anisotropy. Full article

(This article belongs to the Special Issue Microstructure and Physical Properties of Magnesium Alloys and Composites)

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27 pages, 9835 KiB

Open AccessReview

Microwave Liquid Crystal Technology

by Holger Maune, Matthias Jost, Roland Reese, Ersin Polat, Matthias Nickel and Rolf Jakoby

Crystals 2018, 8(9), 355; https://doi.org/10.3390/cryst8090355 - 5 Sep 2018

Cited by 81 | Viewed by 13232

Abstract

Tunable Liquid Crystal (LC)-based microwave components are of increasing interest in academia and industry. Based on these components, numerous applications can be targeted such as tunable microwave filters and beam-steering antenna systems. With the commercialization of first LC-steered antennas for K_u-band [...] Read more.

Tunable Liquid Crystal (LC)-based microwave components are of increasing interest in academia and industry. Based on these components, numerous applications can be targeted such as tunable microwave filters and beam-steering antenna systems. With the commercialization of first LC-steered antennas for K_u-band e.g., by Kymeta and Alcan Systems, LC-based microwave components left early research stages behind. With the introduction of terrestrial 5G communications systems, moving to millimeter-wave communication, these systems can benefit from the unique properties of LC in terms of material quality. In this paper, we show recent developments in millimeter wave phase shifters for antenna arrays. The limits of classical high-performance metallic rectangular waveguides are clearly identified. A new implementation with dielectric waveguides is presented and compared to classic approaches. Full article

(This article belongs to the Section Liquid Crystals)

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10 pages, 2669 KiB

Open AccessArticle

Highly Efficient Nano-Porous Polysilicon Solar Absorption Films Prepared by Silver-Induced Etching

by Yeu-Long Jiang and Hsiao-Ku Shih

Crystals 2018, 8(9), 354; https://doi.org/10.3390/cryst8090354 - 4 Sep 2018

Cited by 1 | Viewed by 4954

Abstract

Nano-porous polysilicon high-temperature resistant solar absorption films were prepared by a thin layer of silver nanoparticles catalytic chemical etching. The polysilicon films with average tiny grain size of approximately 30 nm were obtained by high-temperature 800 °C furnace annealing of hydrogenated amorphous silicon [...] Read more.

Nano-porous polysilicon high-temperature resistant solar absorption films were prepared by a thin layer of silver nanoparticles catalytic chemical etching. The polysilicon films with average tiny grain size of approximately 30 nm were obtained by high-temperature 800 °C furnace annealing of hydrogenated amorphous silicon films that were deposited on stainless substrate by plasma-enhanced chemical vapor deposition. The uniformly distributed 19 nm sized silver nanoparticles with 8 nm interspacing deposited on poly-Si film, were controlled by thin 4 nm thickness and very slow deposition rate 0.4 nm/min of thermal evaporation. Small silver nanoparticles with short spacing catalyzes the detouring etching process inducing the nano-porous textured surface with deep threaded pores. The etching follows the trail of the grain boundaries, and takes a highly curved thread like structure. The etching stops after reaching a depth of around 1100 nm, and the rest of the bulk thickness of the film remains mostly unaffected. The structure consists of three crystal orientations (111), (220), and (331) close to the surface. This crystalline nature diminishes gradually in the bulk of the film. High absorbance of 95% was obtained due to efficient light-trapping. Hence, preparation of nano-porous polysilicon films by this simple method can effectively increase solar absorption for the receiver of the solar thermal electricity Stirling Engine. Full article

(This article belongs to the Special Issue Selected Papers from Taiwan Association for Academic Innovation, TAAI 2018)

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The fabrication process flow of the nano-porous poly-Si solar absorption films. Full article ">Figure 2
The θ/2θ XRD patterns of the plasma-enhanced chemical vapor deposition (PECVD) deposited a-Si:H film and the poly-Si film converted from annealing of the a-Si:H film. Full article ">Figure 3
The scanning electron microscopy (SEM) images of the surface morphology of the silver nano-particles (Ag-NPS) layer on the poly-Si films after (a) 5, (b) 7.5, and (c) 10 min thermal evaporation with very low deposition rate of 0.4 nm/min. The Ag-NPS size increases with longer deposition time, and the particle surface density reduces, in the Ag-NPS layer. Full article ">Figure 4
Ag-NPS average size (□) and the surface density (○) with respect to thermal evaporation deposition time. Full article ">Figure 5
SEM plane-view of the nano-porous poly-Si film by Ag-NPS etched of 1500 s. The irregular nano-porous texture surface and deep pores are clearly observed. Full article ">Figure 6
The TEM cross-section images of nano-porous poly-Si absorption films, (a) entire thickness (b) red region, and (c) green region, after 1500 s Ag-NPS induced etching. Full article ">Figure 7
The high angle annular dark field (HAADF) and selected area diffraction (SAD) images of three different regions (from top to bottom) of the film cross-section after 1500 sec Ag-NPS induced etching.HAADF images show the (a) red, (b) yellow, and (c) green regions. The SAD images show the (d) red, (e) yellow, and (f) green circles. Full article ">Figure 8
The reflectance (R(λ)) from 250 nm to 2500 nm of the reference stainless steel (SS) substrate and the nano-porous poly-Si film on SS substrate. The average reflectance (65%) of the reference SS sample is much higher than the 4.3% of the nano-porous poly-Si film sample. Full article ">

10 pages, 1712 KiB

Open AccessArticle

New Topologically Unique Metal-Organic Architectures Driven by a Pyridine-Tricarboxylate Building Block

by Jinzhong Gu, Yan Cai, Min Wen, Zhijun Ge and Alexander M. Kirillov

Crystals 2018, 8(9), 353; https://doi.org/10.3390/cryst8090353 - 3 Sep 2018

Cited by 6 | Viewed by 3007

Abstract

Two new three-dimensional (3D) coordination compounds, namely a lead(II) coordination polymer (CP) {[Pb₃(µ₅-cpta)(µ₆-cpta)(phen)₂]·2H₂O}_n (1) and a zinc(II) metal-organic framework (MOF) {[Zn₂(µ₄-cpta)(µ-OH)(µ-4,4′-bipy)]·6H₂O}_n (2 [...] Read more.

Two new three-dimensional (3D) coordination compounds, namely a lead(II) coordination polymer (CP) {[Pb₃(µ₅-cpta)(µ₆-cpta)(phen)₂]·2H₂O}_n (1) and a zinc(II) metal-organic framework (MOF) {[Zn₂(µ₄-cpta)(µ-OH)(µ-4,4′-bipy)]·6H₂O}_n (2), were hydrothermally assembled from 2-(5-carboxypyridin-2-yl)terephthalic acid (H₃cpta) as an unexplored principal building block and aromatic N,N-donors as crystallization mediators. Both products were isolated as air-stable microcrystalline solids and were fully characterized by IR spectroscopy, elemental and thermogravimetric analysis, and powder and single-crystal X-ray diffraction. Structural and topological features of CP 1 and MOF 2 were analyzed in detail, allowing to identify a topologically unique 4,5,5,6-connected net in 1 or a very rare 4,4-connected net with the isx topology in 2. Thermal stability and solid-state luminescent behavior of 1 and 2 were also investigated. Apart from revealing a notable topological novelty, both compounds also represent the first structurally characterized Pb(II) and Zn(II) derivatives assembled from H₃cpta, thus opening up the application of this unexplored pyridine-tricarboxylate block in the design of new metal-organic architectures. Full article

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Graphical abstract

Graphical abstract
Full article ">Figure 1
Structural fragments of 1. (a) Coordination environment around the Pb(II) atoms; H atoms are omitted for clarity. Symmetry code: i = x + 1/2, −y, z + 1/2; ii = x, y + 1, z; iii = x + 1/2, −y − 1, z + 1/2; iv = x, y − 1, z; v = x − 1/2, −y, z + 1/2. (b) 3D coordination polymer seen along the b axis. (c,d) Topological representation of an underlying tetranodal 4,5,5,6-connected net with the unique topology; views along the c (c) and b (d) axis. Color codes: 2-, 4-, and 5-connected Pb centers (turquoise balls; Pb3, Pb1, and Pb2, respectively; partial labelling scheme is shown), centroids of 5- and 6-connected cpta3− blocks (gray). Full article ">Figure 1 Cont.
Structural fragments of 1. (a) Coordination environment around the Pb(II) atoms; H atoms are omitted for clarity. Symmetry code: i = x + 1/2, −y, z + 1/2; ii = x, y + 1, z; iii = x + 1/2, −y − 1, z + 1/2; iv = x, y − 1, z; v = x − 1/2, −y, z + 1/2. (b) 3D coordination polymer seen along the b axis. (c,d) Topological representation of an underlying tetranodal 4,5,5,6-connected net with the unique topology; views along the c (c) and b (d) axis. Color codes: 2-, 4-, and 5-connected Pb centers (turquoise balls; Pb3, Pb1, and Pb2, respectively; partial labelling scheme is shown), centroids of 5- and 6-connected cpta3− blocks (gray). Full article ">Figure 2
Structural fragments of 2. (a) Coordination environment around the Zn(II) atoms; H atoms are omitted for clarity except one of OH− group. Symmetry code: i = −x, −y, −z; ii = −x + 1/2, y + 1/2, −z + 1/2; iii = x, −y, z + 1/2; iv = −x + 1/2, y − 1/2, −z + 1/2. (b) 3D metal-organic framework seen along the c axis. (c) Topological representation of an underlying binodal 4,4-connected framework with the isx topology; view along the c axis. Color codes: 4-connected Zn centers (cyan balls), centroids of 4-connected cpta3− blocks (gray), centroids of 2-connected μ-OH− (red) and µ-4,4′-bipy (blue) linkers. Full article ">Figure 3
Solid-state emission spectra of H3cpta, CP 1 and MOF 2 at room temperature; λex is 320 (H3cpta), 318 (1), or 348 nm (2)). Full article ">Scheme 1
Various coordination modes of cpta3− blocks in 1 (modes I, II) and 2 (mode III). Full article ">

10 pages, 3108 KiB

Open AccessArticle

Improved Solubility of Vortioxetine Using C2-C4 Straight-Chain Dicarboxylic Acid Salt Hydrates

by Lei Gao, Xian-Rui Zhang, Shao-Ping Yang, Juan-Juan Liu and Chao-Jie Chen

Crystals 2018, 8(9), 352; https://doi.org/10.3390/cryst8090352 - 2 Sep 2018

Cited by 6 | Viewed by 3917

Abstract

The purpose of this study was to improve the solubility of vortioxetine by crystal engineering principles. In this paper, three C2-C4 straight-chain dicarboxylic acid salt hydrates of vortioxetine (VOT-OA, VOT-MA-H₂O, and VOT-SUA-H₂O, VOT = vortioxetine, OA = Oxalic acid, [...] Read more.

The purpose of this study was to improve the solubility of vortioxetine by crystal engineering principles. In this paper, three C2-C4 straight-chain dicarboxylic acid salt hydrates of vortioxetine (VOT-OA, VOT-MA-H₂O, and VOT-SUA-H₂O, VOT = vortioxetine, OA = Oxalic acid, MA = malonic acid, SUA = succinic acid) were synthesized and characterized by single X-ray diffraction, powder X-ray diffraction, and differential scanning calorimetry. The single crystal structure of three salts reveals that vortioxetine has torsional flexibility, which can encourage VOT to allow combination with aliphatic dicarboxylic acids through N⁺-H···O hydrogen bonds. The solubility of all salts exhibits a dramatic increase in distilled water, especially for VOT-MA-H₂O salt, where it shows the highest solubility, by 96-fold higher compared with pure vortioxetine. Full article

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Figure 1
Chemical structures of VOT and coformers. Full article ">Figure 2
(a) Inversion related VOT-OA salt molecules connected by N+-H···O hydrogen bonds in a tetrameric <math display="inline"><semantics> <msubsup> <mi mathvariant="normal">R</mi> <mn>4</mn> <mn>4</mn> </msubsup> </semantics></math>(12) motif; (b) The sandwich layer structure is connected by O-H···O hydrogen bonds. Full article ">Figure 3
(a) Two MA anions and two water molecules form a tetrameric <math display="inline"><semantics> <msubsup> <mi mathvariant="normal">R</mi> <mn>4</mn> <mn>4</mn> </msubsup> </semantics></math>(16) motif through O5-H5A···O1 and O5-H5B···O2 hydrogen bonds; (b) The two-dimensional structure is connected by N1+-H1A···O5 and N1+-H1B···O2 hydrogen bonds. Full article ">Figure 4
(a) Two SUA anions and one water molecules form a sheet-like structure through O5-H5A···O4 and O2-H2···O3 hydrogen bonds; (b) The <math display="inline"><semantics> <msubsup> <mi mathvariant="normal">R</mi> <mn>6</mn> <mn>6</mn> </msubsup> </semantics></math>(22) motif structure are connected by N1+-H1A···O1 and N1+-H1B···O5 hydrogen bonds. Full article ">Figure 5
Overlay of VOT molecules extracted from crystal structures. Color codes: blue = VOT; black = VOT-OA (1:1); red = VOT-MA-H2O (1:1:1); green = VOT-SUA-H2O (1:1:0.5). Full article ">Figure 6
Experimental (black) and simulated (red) powder X-ray diffraction patterns for VOT, VOT-OA, VOT-MA-H2O, and VOT-SUA-H2O salt. Full article ">Figure 7
DSC thermograms of VOT, VOT-OA, VOT-MA-H2O, and VOT-SUA-H2O salt. Full article ">Figure 8
Powder dissolution profiles of VOT, VOT-OA, VOT-MA-H2O, and VOT-SUA-H2O salts at different time points in pure water at 37 °C. Full article ">

32 pages, 11096 KiB

Open AccessArticle

Band Structures Analysis of Elastic Waves Propagating along Thickness Direction in Periodically Laminated Piezoelectric Composites

by Qiangqiang Li, Yongqiang Guo, Jingya Wang and Wei Chen

Crystals 2018, 8(9), 351; https://doi.org/10.3390/cryst8090351 - 1 Sep 2018

Cited by 7 | Viewed by 3318

Abstract

Existing studies on elastic waves in periodically laminated piezoelectric structures mainly concerned the passive band properties, since the electrical boundaries in the considered structures cannot vary. This paper investigates the tuning of band properties of uncoupled primary and shear (P- and S-) waves [...] Read more.

Existing studies on elastic waves in periodically laminated piezoelectric structures mainly concerned the passive band properties, since the electrical boundaries in the considered structures cannot vary. This paper investigates the tuning of band properties of uncoupled primary and shear (P- and S-) waves along the thickness direction by actively varying the electrical field in periodically multilayered piezoelectric structures consisting of orthotropic materials. The alteration of the electrical field is realized in the multilayered unit cell here by either applying or switching four kinds of electrical boundary conditions, including the electric-open, applied electric capacitance, electric-short, and applied feedback voltage, to the constituent piezoelectric layer via the constituent electrode layers covering both its surfaces. First, the state space formalism is introduced to obtain the partial wave solution of any constituent orthotropic layer in the unit cell. Second, the traditional transfer matrix method is adopted to derive the dispersion equation of general, periodically laminated piezoelectric composites with unit cells consisting of an arbitrary number of piezoelectric layers with various boundaries and of elastic layers. Third, numerical examples are provided to verify the proposed analysis method, and to study the influences of electrode thickness as well as four electrical boundaries on the band structures. All the frequency-related dispersion curves are also illustrated by numerical examples to summarize the general dispersion characteristics of uncoupled P- and S-waves in periodically laminated piezoelectric composites. The main finding is that the innovative dispersion characteristic resulting from the negative capacitance may also be achieved via feedback control. Full article

(This article belongs to the Special Issue Phononics)

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5 pages, 1830 KiB

Open AccessArticle

A Screen Method for the Mass-Production of the Small-Size and Side-View LED Package

by Wen-Ray Chen and You-Sheng Luo

Crystals 2018, 8(9), 350; https://doi.org/10.3390/cryst8090350 - 31 Aug 2018

Cited by 1 | Viewed by 2515

Abstract

In this study, the small-size and side-view LED packages used in the backlight modules were examined by performing the electrostatic discharge (ESD) process. The high voltages used for the ESD process were 2 kV, 3 kV, and 4 kV. After ESD, the current–voltage [...] Read more.

In this study, the small-size and side-view LED packages used in the backlight modules were examined by performing the electrostatic discharge (ESD) process. The high voltages used for the ESD process were 2 kV, 3 kV, and 4 kV. After ESD, the current–voltage curves were scanned from −7 V to 3 V. It was found that the significant leakage currents were in the reverse bias of 4 V~7 V and also in the forward bias of 1 V~2.5 V for ESD-damaged LED chips. However, the phenomenon of a slight increase in current was not found for the non-damaged samples. In our study case, the screening conditions could be set at a bias of −7 V or 2.3 V and the current of 0.1 µA. Full article

(This article belongs to the Special Issue Selected Papers from Taiwan Association for Academic Innovation, TAAI 2018)

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12 pages, 547 KiB

Open AccessArticle

A Revisit to the Notation of Martensitic Crystallography

by Yipeng Gao

Crystals 2018, 8(9), 349; https://doi.org/10.3390/cryst8090349 - 30 Aug 2018

Cited by 5 | Viewed by 3523

Abstract

As one of the most successful crystallographic theories for phase transformations, martensitic crystallography has been widely applied in understanding and predicting the microstructural features associated with structural phase transformations. In a narrow sense, it was initially developed based on the concepts of lattice [...] Read more.

As one of the most successful crystallographic theories for phase transformations, martensitic crystallography has been widely applied in understanding and predicting the microstructural features associated with structural phase transformations. In a narrow sense, it was initially developed based on the concepts of lattice correspondence and invariant plane strain condition, which is formulated in a continuum form through linear algebra. However, the scope of martensitic crystallography has since been extended; for example, group theory and graph theory have been introduced to capture the crystallographic phenomena originating from lattice discreteness. In order to establish a general and rigorous theoretical framework, we suggest a new notation system for martensitic crystallography. The new notation system combines the original formulation of martensitic crystallography and Dirac notation, which provides a concise and flexible way to understand the crystallographic nature of martensitic transformations with a potential extensionality. A number of key results in martensitic crystallography are reexamined and generalized through the new notation. Full article

(This article belongs to the Special Issue Microstructures and Properties of Martensitic Materials)

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12 pages, 3643 KiB

Open AccessArticle

Bending Behavior of a Wrought Magnesium Alloy Investigated by the In Situ Pinhole Neutron Diffraction Method

by Wei Wu, Alexandru D. Stoica, Dunji Yu, Matthew J. Frost, Harley D. Skorpenske and Ke An

Crystals 2018, 8(9), 348; https://doi.org/10.3390/cryst8090348 - 30 Aug 2018

Cited by 8 | Viewed by 4234

Abstract

The tensile twinning and detwinning behaviors of a wrought magnesium alloy have been investigated during in situ four-point bending using the state-of-the-art high spatial resolution pinhole neutron diffraction (PIND) method. The PIND method allowed us to resolve the tensile twinning/detwinning and lattice strain [...] Read more.

The tensile twinning and detwinning behaviors of a wrought magnesium alloy have been investigated during in situ four-point bending using the state-of-the-art high spatial resolution pinhole neutron diffraction (PIND) method. The PIND method allowed us to resolve the tensile twinning/detwinning and lattice strain distributions across the bending sample during a loading-unloading sequence with a 0.5 mm step size. It was found that the extensive tensile twinning and detwinning occurred near the compression surface, while no tensile twinning behavior was observed in the middle layer and tension side of the bending sample. During the bending, the neutral plane shifted from the compression side to the tension side. Compared with the traditional neutron diffraction mapping method, the PIND method provides more detailed information inside the bending sample due to a higher spatial resolution. Full article

(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)

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Figure 1
(a,b) The conventional and the pinhole neutron diffraction (PIND) setup respectively; (c) The schematics of the stainless-tungsten layered sample and diffraction pattern from PIND measurement; (d) The analytical results of the stainless-tungsten layered sample for a vertical single channel slit configuration. Full article ">Figure 1 Cont.
(a,b) The conventional and the pinhole neutron diffraction (PIND) setup respectively; (c) The schematics of the stainless-tungsten layered sample and diffraction pattern from PIND measurement; (d) The analytical results of the stainless-tungsten layered sample for a vertical single channel slit configuration. Full article ">Figure 2
(a) The schematics of the high spatial resolution PIND method; (b) The neutron diffraction patterns before and after the bending test; (c) The schematics of the in situ four-point bending experiment using PIND method; (d) The schematics of traditional neutron diffraction mapping setup. Full article ">Figure 3
(a,b) The diffraction peak intensity variations of (11.0) and (00.2) grain in the εx direction during in situ loading-unloading, respectively; (c,d) The lattice strain evolution of (11.0) and (00.2) grains in the εx and εy directions, respectively. Full article ">Figure 4
(a,b) The (11.0) peak intensity and lattice strain distributions in the εx direction using the traditional neutron diffraction mapping method, respectively; (c,d) Comparison of the traditional neutron diffraction mapping and the PIND method for the (11.0) peak intensity and residual lattice strain, respectively. Full article ">

21 pages, 4910 KiB

Open AccessReview

GaAs Nanowires Grown by Catalyst Epitaxy for High Performance Photovoltaics

by Ying Wang, Xinyuan Zhou, Zaixing Yang, Fengyun Wang, Ning Han, Yunfa Chen and Johnny C. Ho

Crystals 2018, 8(9), 347; https://doi.org/10.3390/cryst8090347 - 29 Aug 2018

Cited by 8 | Viewed by 5172

Abstract

Photovoltaics (PVs) based on nanostructured III/V semiconductors can potentially reduce the material usage and increase the light-to-electricity conversion efficiency, which are anticipated to make a significant impact on the next-generation solar cells. In particular, GaAs nanowire (NW) is one of the most promising [...] Read more.

Photovoltaics (PVs) based on nanostructured III/V semiconductors can potentially reduce the material usage and increase the light-to-electricity conversion efficiency, which are anticipated to make a significant impact on the next-generation solar cells. In particular, GaAs nanowire (NW) is one of the most promising III/V nanomaterials for PVs due to its ideal bandgap and excellent light absorption efficiency. In order to achieve large-scale practical PV applications, further controllability in the NW growth and device fabrication is still needed for the efficiency improvement. This article reviews the recent development in GaAs NW-based PVs with an emphasis on cost-effectively synthesis of GaAs NWs, device design and corresponding performance measurement. We first discuss the available manipulated growth methods of GaAs NWs, such as the catalytic vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) epitaxial growth, followed by the catalyst-controlled engineering process, and typical crystal structure and orientation of resulted NWs. The structure-property relationships are also discussed for achieving the optimal PV performance. At the same time, important device issues are as well summarized, including the light absorption, tunnel junctions and contact configuration. Towards the end, we survey the reported performance data and make some remarks on the challenges for current nanostructured PVs. These results not only lay the ground to considerably achieve the higher efficiencies in GaAs NW-based PVs but also open up great opportunities for the future low-cost smart solar energy harvesting devices. Full article

(This article belongs to the Special Issue III-V Heteroepitaxy for Solar Energy Conversion)

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7 pages, 7133 KiB

Open AccessArticle

Formation of Dislocations in the Growth of Silicon along Different Crystallographic Directions—A Molecular Dynamics Study

by Naigen Zhou, Xiuqin Wei and Lang Zhou

Crystals 2018, 8(9), 346; https://doi.org/10.3390/cryst8090346 - 29 Aug 2018

Cited by 12 | Viewed by 3297

Abstract

Molecular dynamics simulations of the seeded solidification of silicon along <100>, <110>, <111> and <112> directions have been carried out. The Tersoff potential is adopted for computing atomic interaction. The control of uniaxial strains in the seed crystals is enabled in the simulations. [...] Read more.

Molecular dynamics simulations of the seeded solidification of silicon along <100>, <110>, <111> and <112> directions have been carried out. The Tersoff potential is adopted for computing atomic interaction. The control of uniaxial strains in the seed crystals is enabled in the simulations. The results show that the dislocation forms stochastically at the crystal/melt interface, with the highest probability of the formation in <111> growth, which agrees with the prediction from a previously proposed twinning-associated dislocation formation mechanism. Applications of the strains within a certain range are found to inhibit the {111}-twinning-associated dislocation formation, while beyond this range they are found to induce dislocation formation by different mechanisms. Full article

(This article belongs to the Special Issue Growth and Evaluation of Multicrystalline Silicon)

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Figure 1
A snapshot of the simulation cell in running. Full article ">Figure 2
Growth rates of the simulated growths of silicon from melt along the different crystallographic directions. Full article ">Figure 3
A snapshot of an example of dislocation formation in a strain-free <112> growth, at 1000 ps. The arrows indicate two locations of the dislocations formed. Full article ">Figure 4
Probability of dislocation formation in the growth of silicon along <100>, <110>, <111> and <112>, respectively, under different strain conditions. Full article ">Figure 5
A snapshot of the <112> growth with a tensile strain of 0.02, at 700 ps. Full article ">Figure 6
Snapshots of highly strained growths along <100>. Full article ">Figure 7
Sequential snapshots of the <100> growth with ε = −0.06. Only half of the simulation cell with two (001) atomic planes is displayed for visual convenience. Full article ">

8 pages, 3837 KiB

Open AccessArticle

Microstructure and Phase Transformation Analysis of Ni_50−xTi₅₀La_x Shape Memory Alloys

by Weiya Li and Chunwang Zhao

Crystals 2018, 8(9), 345; https://doi.org/10.3390/cryst8090345 - 29 Aug 2018

Cited by 8 | Viewed by 3362

Abstract

The microstructure and martensitic transformation behavior of Ni_50−xTi₅₀La_x (x = 0.1, 0.3, 0.5, 0.7) shape memory alloys were investigated experimentally. Results show that the microstructure of Ni_50−xTi₅₀La_x alloys consists of [...] Read more.

The microstructure and martensitic transformation behavior of Ni_50−xTi₅₀La_x (x = 0.1, 0.3, 0.5, 0.7) shape memory alloys were investigated experimentally. Results show that the microstructure of Ni_50−xTi₅₀La_x alloys consists of a near-equiatomic TiNi matrix, LaNi precipitates, and Ti₂Ni precipitates. With increasing La content, the amounts of LaNi and Ti₂Ni precipitates demonstrate an increasing tendency. The martensitic transformation start temperature increases gradually with increasing La content. The Ni content is mainly responsible for the change in martensite transformation behavior in Ni_50−xTi₅₀La_x alloys. Full article

(This article belongs to the Special Issue Microstructures and Properties of Martensitic Materials)

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Back-scattering SEM images of Ni50−xTi50Lax alloys: (a) Ni49.9Ti50La0.1; (b) Ni49.7Ti50La0.3; (c) Ni49.5Ti50La0.5; (d) Ni49.3Ti50La0.7. Full article ">Figure 2
Normalized pixel amounts of precipitates in SEM images of Ni50−xTi50Lax alloys. Full article ">Figure 3
EDS spectra of phases in Ni49.3Ti50La0.7 alloy: (a) Ti2Ni; (b) TiNi; (c) LaNi. Full article ">Figure 3 Cont.
EDS spectra of phases in Ni49.3Ti50La0.7 alloy: (a) Ti2Ni; (b) TiNi; (c) LaNi. Full article ">Figure 4
XRD patterns of Ni50−xTi50Lax alloys: (a) All alloys; (b) phases corresponding to diffraction peaks of Ni49.5Ti50La0.5. Full article ">Figure 5
DSC results of Ni50−xTi50Lax alloys: (a) DSC curves; (b) Ms vs. La content. Full article ">

8 pages, 854 KiB

Open AccessArticle

Temperature Dependence of the Vacancy Formation Energy in Solid ⁴He

by Riccardo Rota and Jordi Boronat

Crystals 2018, 8(9), 344; https://doi.org/10.3390/cryst8090344 - 28 Aug 2018

Cited by 2 | Viewed by 5130

Abstract

We studied the thermal effects on the behavior of incommensurate solid

^{4}

He at low temperatures using the path integral Monte Carlo method. Below a certain temperature, depending on the density and the structure of the crystal, the vacancies delocalize and a finite [...] Read more.

We studied the thermal effects on the behavior of incommensurate solid

^{4}

He at low temperatures using the path integral Monte Carlo method. Below a certain temperature, depending on the density and the structure of the crystal, the vacancies delocalize and a finite condensate fraction appears. We calculated the vacancy formation energy as a function of the temperature and observed a behavior compatible with a two-step structure, with a gap of few K appearing at the onset temperature of off-diagonal long-range order. Estimation of the energy cost of creating two vacancies seems to indicate an effective attractive interaction among the vacancies but the large error inherent to its numerical estimation precludes a definitive statement. Full article

(This article belongs to the Special Issue Quantum Crystals)

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15 pages, 4960 KiB

Open AccessArticle

Investigation of Approaches to Control the Compositions of Zn(Se,OH) Buffers Prepared by Chemical Bath Deposition Process for Cu(In,Ga)Se₂ (CIGS) Solar Cells

by Chia-Hua Huang, Yueh-Lin Jan, Wen-Jie Chuang and Po-Tsung Lu

Crystals 2018, 8(9), 343; https://doi.org/10.3390/cryst8090343 - 26 Aug 2018

Cited by 11 | Viewed by 3719

Abstract

We deposited zinc-based films with various ammonia (ammonium hydroxide; NH₄OH) and selenourea concentrations, at the bath temperature of 80 °C, on soda-lime glass substrates using the chemical bath deposition (CBD) process. We analyzed the results using X-ray photoelectron spectroscopy (XPS), which [...] Read more.

We deposited zinc-based films with various ammonia (ammonium hydroxide; NH₄OH) and selenourea concentrations, at the bath temperature of 80 °C, on soda-lime glass substrates using the chemical bath deposition (CBD) process. We analyzed the results using X-ray photoelectron spectroscopy (XPS), which showed binding energies of zinc, selenium, and oxygen. The as-deposited films, containing zinc selenide, zinc oxide, and zinc hydroxide, were also verified. The films prepared in this investigation can be referred to a zinc compound, characterized as Zn(Se,OH). A conformal coverage of the Zn(Se,OH) films, with the smooth surface morphologies, was obtained by optimizing the ammonia or selenourea concentrations in the deposition solutions. The Zn(Se,OH) films had a preferred (111) orientation, corresponding to a cubic crystal structure. The bandgap energies of the as-deposited Zn(Se,OH) films were determined from the optical absorption data, suggesting a dependence of the bandgap energies on the atomic percentages of ZnSe, Zn(OH)₂ and ZnO in the films. The same variation tendency of the compositions and the bandgap energies for the films, deposited with an increment in the ammonia or selenourea concentrations was achieved, attributing to the facilitation of ZnSe formation. These results show that the compositions, and therefore the bandgap energies, can be controlled by the ammonia concentrations, or selenourea concentrations. Full article

(This article belongs to the Special Issue Selected Papers from Taiwan Association for Academic Innovation, TAAI 2018)

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8 pages, 2639 KiB

Open AccessArticle

Simulation and Analysis of Single-Mode Microring Resonators in Lithium Niobate Thin Films

by Huangpu Han, Bingxi Xiang and Jiali Zhang

Crystals 2018, 8(9), 342; https://doi.org/10.3390/cryst8090342 - 24 Aug 2018

Cited by 17 | Viewed by 5981

Abstract

The single-mode microring resonators on lithium niobate thin films were designed and simulated using 2.5-D variational finite difference time domain mode simulations from Lumerical mode Solutions. The single-mode conditions and the propagation losses of lithium niobate planar waveguide with different SiO₂ cladding [...] Read more.

The single-mode microring resonators on lithium niobate thin films were designed and simulated using 2.5-D variational finite difference time domain mode simulations from Lumerical mode Solutions. The single-mode conditions and the propagation losses of lithium niobate planar waveguide with different SiO₂ cladding layer thicknesses were studied and compared systematically. The optimization of design parameters such as radii of microrings and gap sizes between channel and ring waveguides were determined. The key issues affecting the resonator design such as free spectral range and Quality Factor were discussed. The microring resonators had radius R = 20 μm, and their transmission spectrum had been tuned using the electro-optical effect. Full article

(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)

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Figure 1
A schematic of the waveguide-coupled microring resonator on LNOI. Full article ">Figure 2
Effective index of the TE (solid lines) and TM (dashed lines) modes in LN waveguides as a function of the film thickness for a waveguide with 0.7 μm width; the modes were calculated at λ = 1.55 μm. Full article ">Figure 3
Effective index of the TE (solid lines) and TM (dashed lines) modes in LN waveguides as a function of the width for a 0.5 μm thick film. The modes were calculated at λ = 1.55 μm. Full article ">Figure 4
The propagation losses of LN planar waveguide with the different SiO2 layer thicknesses. Full article ">Figure 5
(a) Q-factor of microring resonator as different ring radii for different gap sizes, (b) Bending loss variation as bending radius. The modes were calculated at λ = around 1.55 μm. Full article ">Figure 6
FSR of microring resonators as different ring radii. The modes were calculated at λ = around 1.55 μm. Full article ">Figure 7
The LN microring resonator was embedded in the middle SiO2 layer, and the electrodes were placed over below and above the SiO2 layer. Full article ">Figure 8
Transmission spectra of wavelength shift due to different electric field intensities in the Z-direction. The calculations referred to the TM mode at the microring radius R = 20 μm. Full article ">

15 pages, 4491 KiB

Open AccessArticle

Investigation of the Grain Boundary Character and Dislocation Density of Different Types of High Performance Multicrystalline Silicon

by Gaute Stokkan, Adolphus Song and Birgit Ryningen

Crystals 2018, 8(9), 341; https://doi.org/10.3390/cryst8090341 - 24 Aug 2018

Cited by 25 | Viewed by 5414

Abstract

Wafers from three heights and two different lateral positions (corner and centre) of four industrial multicrystalline silicon ingots were analysed with respect to their grain structure and dislocation density. Three of the ingots were non-seeded and one ingot was seeded. It was found [...] Read more.

Wafers from three heights and two different lateral positions (corner and centre) of four industrial multicrystalline silicon ingots were analysed with respect to their grain structure and dislocation density. Three of the ingots were non-seeded and one ingot was seeded. It was found that there is a strong correlation between the ratio of the densities of (coincidence site lattice) CSL grain boundaries and high angle grain boundaries in the bottom of a block and the dislocation cluster density higher in the block. In general, the seeded blocks, both the corner and centre block, have a lower dislocation cluster density than in the non-seeded blocks, which displayed a large variation. The density of the random angle boundaries in the corner blocks of the non-seeded ingots was similar to the density in the seeded ingots, while the density in the centre blocks was lower. However, the density of CSL boundaries was higher in all the non-seeded than in the seeded ingots. It appears that both of these grain boundary densities influence the presence of dislocation clusters, and we propose they act as dislocation sinks and sources, respectively. The ability to generate small grain size material without seeding appears to be correlated to the morphology of the coating, which is generally rougher in the corner positions than in the middle. Furthermore, the density of twins and CSL boundaries depends on the growth mode during initial growth and thus on the degree of supercooling. Controlling both these properties is important in order to be able to successfully produce uniform quality high-performance multicrystalline silicon by the advantageous non-seeding method. Full article

(This article belongs to the Special Issue Growth and Evaluation of Multicrystalline Silicon)

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Orientation image micrographs (OIM) and individual grain images (IGI) of selected samples: Bottom, middle and top positions of corner and centre blocks from one of the reference non-seeded ingots (VH1) and corner block from a seeded ingot. IGIs include twin boundaries in the leftmost column but disregard these in the three other columns. The bottom, middle and top positions refer to wafer number 1, 300 and 600 for non-seeded ingots and 1, 300 and 400 for seeded ingots, respectively. Full article ">Figure 2
(a) Total density of random angle grain boundaries vs. block height (the numbers may contain a very small fraction of non Σ3n boundaries, which however are so uncommon in silicon such as not to disturb the general trend. Sub grain boundaries with misorientation <5° are not included; (b) The total density of Σ3 boundaries vs. block height; (c) The total density of Σ27 (a,b) grain boundaries vs. wafer number. Full article ">Figure 3
Fraction of different grain boundary types for the samples in the study. Full article ">Figure 4
(a) Dislocation density maps of wafer number 001, 301 and 601 of a corner and centre block of a non-seeded ingot (VH1) and the dislocation density maps of wafers 009, 309 and 409 from a corner block of the seeded ingot; (b) Density of dislocation clusters as function of height in ingot. Full article ">Figure 5
Measurements of coating morphology using confocal microscope at 5× magnification. (a) Corner sample; (b) centre sample. Full article ">Figure 6
(a) Dependence of dislocation cluster density on density of Σ27 boundaries. The linear fit of bottom, middle and top wafers is shown along with the coefficient of dependence (R2) for the three series; (b) Scatter plot of Σ27 (source) boundaries vs. random angle (sink) boundaries. Full article ">Figure 7
The ratio of dislocation “source” grain boundaries over “sink” grain boundaries in the bottom wafers versus the dislocation cluster density in the top wafers. Full article ">

13 pages, 2520 KiB

Open AccessArticle

Novel Devices for Transporting Protein Crystals to the Synchrotron Facilities and Thermal Protection of Protein Crystals

by Andrea Flores-Ibarra, Camila Campos-Escamilla, Yasel Guerra, Enrique Rudiño-Piñera, Nicola Demitri, Maurizio Polentarutti, Mayra Cuéllar-Cruz and Abel Moreno

Crystals 2018, 8(9), 340; https://doi.org/10.3390/cryst8090340 - 23 Aug 2018

Cited by 3 | Viewed by 4687

Abstract

In this article, we use novel and non-conventional devices, based on polyolefins that help to increase the thermal protection of protein crystals in their crystallization conditions for crystallographic applications. The present contribution deals with the application of some ad hoc devices designed for [...] Read more.

In this article, we use novel and non-conventional devices, based on polyolefins that help to increase the thermal protection of protein crystals in their crystallization conditions for crystallographic applications. The present contribution deals with the application of some ad hoc devices designed for transporting protein crystals to the synchrotron facilities. These new devices help transporting proteins without cryo-cooling them, therefore replacing the conventional dry Dewars. We crystallized four model proteins, using the classic sitting-drop vapor diffusion crystallization setups. The model proteins lysozyme, glucose isomerase, xylanase, and ferritin were used to obtain suitable crystals for high-resolution X-ray crystallographic research. Additionally, we evaluated the crystallization of apo-transferrin, which is involved in neurodegenerative diseases. As apo-transferrin is extremely sensitive to the changes in the crystallization temperature, we used it as a thermal sensor to prove the efficiency of these thermal protection devices when transporting proteins to the synchrotron facilities. Full article

(This article belongs to the Section Biomolecular Crystals)

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This device can support all types of the conventional plates for vapor diffusion set up in hanging and sitting-drops techniques: (a) An overview of the device and dimensions; (b) crystallization plate (96 wells) and (c) insertion of the crystallization plate. Full article ">Figure 2
This shows the dimensions and experimental details of the (a) top and front view of the plastic disposable inserts used in the TG40 apparatus and (b) the general overview and dimensions of the thermal protection device. Full article ">Figure 3
This shows the design of the thermal protection device, when using the Qiagen plates for protein crystallization. On the top left hand-side the cover slip with screw system contains the protein drop mixed with precipitating agent. On the right hand-side the thermal protection device with its dimensions. Full article ">Figure 4
This shows the crystal shape and crystal structure as well as the high-resolution electron density map (2F0–Fc) for (a) lysozyme, (b) glucose isomerase, (c) xylanase and (d) ferritin respectively. Contour level was 2.5–2.3 σ due to the high resolution reached. Full article ">Figure 5
(a) Optimal temperature and crystallization conditions for apo-transferrin at 18 °C, (b) crystals grown at 19 °C were slightly dissolved. Crystal conservation over time: (c) Crystals grown during three days after crystallization; and (d) crystal observed after transportation (trip) to the synchrotron facility 2.5 months later. The crystals were preserved inside these devices and worked as thermal sensors. Full article ">Figure 6
It shows the crystal structure of apo-transferrin; crystals diffracted up to 2.9 Å at the best temperature, keeping the optimal pH and appropriate cryo-protection conditions as described in the experimental part. Full article ">

8 pages, 2677 KiB

Open AccessArticle

Effects on the Surface and Luminescence Properties of GaAs by SF₆ Plasma Passivation

by Yumeng Xu, Xin Gao, Xiaolei Zhang, Zhongliang Qiao, Jing Zhang, Lu Zhou and Baoxue Bo

Crystals 2018, 8(9), 339; https://doi.org/10.3390/cryst8090339 - 23 Aug 2018

Cited by 5 | Viewed by 3471

Abstract

The passivation effects of the SF₆ plasma on a GaAs surface has been investigated by using the radio frequency (RF) plasma method. The RF’s power, chamber pressure, and plasma treatment time are optimized by photoluminescence (PL), atomic force microscopy (AFM), and X-ray [...] Read more.

The passivation effects of the SF₆ plasma on a GaAs surface has been investigated by using the radio frequency (RF) plasma method. The RF’s power, chamber pressure, and plasma treatment time are optimized by photoluminescence (PL), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The PL intensity of passivated GaAs samples is about 1.8 times higher than those which are untreated. The oxide traps and As-As dimers can be removed effectively by using SF₆ plasma treatment, and Ga-F can form on the surface of GaAs. It has also been found that the stability of the passivated GaAs surface can be enhanced by depositing SiO₂ films onto the GaAs surface. These indicate that the passivation of GaAs surfaces can be achieved by using SF₆ plasma treatment. Full article

(This article belongs to the Section Crystal Engineering)

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32 pages, 17582 KiB

Open AccessReview

Luminescent Metal–Organic Framework Thin Films: From Preparation to Biomedical Sensing Applications

by Zhengluan Liao, Tifeng Xia, Enyan Yu and Yuanjing Cui

Crystals 2018, 8(9), 338; https://doi.org/10.3390/cryst8090338 - 23 Aug 2018

Cited by 31 | Viewed by 8637

Abstract

Metal-organic framework (MOF) thin films are receiving increasing attention in a number of different application fields, such as optoelectronics, gas separation, catalysis electronic devices, and biomedicine. In particular, their tunable composition and structure, accessible metal sites and potential for post-synthetic modification for molecular [...] Read more.

Metal-organic framework (MOF) thin films are receiving increasing attention in a number of different application fields, such as optoelectronics, gas separation, catalysis electronic devices, and biomedicine. In particular, their tunable composition and structure, accessible metal sites and potential for post-synthetic modification for molecular recognition make MOF thin films promising candidates for biosensing applications. Compared with solution-based powdery probes, film-based probes have distinct advantages of good stability and portability, tunable shape and size, real-time detection, non-invasion, extensive suitability in gas/vapor sensing, and recycling. In this review, we summarize the recent advances in luminescent MOF thin films, including the fabrication methods and origins of luminescence. Specifically, luminescent MOF thin films as biosensors for temperature, ions, gases and biomolecules are highlighted. Full article

(This article belongs to the Special Issue MOF-Based Membranes)

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20 pages, 10327 KiB

Open AccessArticle

Aging Behavior of Aluminum Alloy 6082 Subjected to Friction Stir Processing

by Khaled Al-Fadhalah and Fahad Asi

Crystals 2018, 8(9), 337; https://doi.org/10.3390/cryst8090337 - 22 Aug 2018

Cited by 8 | Viewed by 4686

Abstract

The present work examined the effect of artificial aging on the microstructure, texture, and hardness homogeneity in aluminum alloy AA6082 subjected to friction stir processing (FSP). Aging was applied to FSP samples at three different temperatures (150 °C, 175 °C, and 200 °C) [...] Read more.

The present work examined the effect of artificial aging on the microstructure, texture, and hardness homogeneity in aluminum alloy AA6082 subjected to friction stir processing (FSP). Aging was applied to FSP samples at three different temperatures (150 °C, 175 °C, and 200 °C) for a period of 1 h, 6 h, and 12 h. Microstructure analysis using optical Microscopy (OM) and Electron Back-Scattered Diffraction (EBSD) indicated that FSP produced fine equiaxed grains, with an average grain size of 6.5 μm, in the stir zone (SZ) due to dynamic recrystallization. Aging was shown to result in additional grain refinement in the SZ due to the occurrence of recovery and recrystallization with either increasing aging temperature and/or aging time. An optimum average grain size of 3–4 μm was obtained in the SZ by applying aging at 175 °C. This was accompanied by an increase in the fraction of high-angle grain boundaries. FSP provided a simple shear texture with a major component of B fiber. Increasing aging temperature and/or time resulted in the formation of recrystallization texture of a Cube orientation. In addition, Vickers microhardness was evaluated for the FSP sample, indicating a softening in the SZ due to the dissolution of the hardening precipitates. Compared to other aging temperatures, aging at 175 °C resulted in maximum hardness recovery (90 Hv) to the initial value of base metal (92.5 Hv). The hardness recovery is most likely attributed to the uniform distribution of fine hardening precipitates in the SZ when increasing the aging time to 12 h. Full article

(This article belongs to the Special Issue Recrystallization of Metallic Materials )

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