Tuesday, September 27, 2016

Single-Crystal Graphene Films Grown More Than 100 Times as Fast as Previously Possible

The adaptation of chemical vapor deposition (CVD) production of graphene so that it’s compatible with roll-to-roll processing is transforming graphene manufacturing. That effort is being led by companies like Graphene Frontiers, based in Philadelphia.
However, the production of single-crystal graphene on copper foils in a CVD process remains a fairly time consuming procedure. Fabrication of centimeter-size single crystals of graphene still takes as much as a day.
Now researchers at Hong Kong Polytechnic University and Peking University have developed a technique that accelerates the process so that the growth happens at 60 micrometers per second—far faster than the typical 0.4 µm per second. The key to this 150-fold speed increase was adding a little oxygen directly to the copper foils.
In the research, which is described in the journal Nature Nanotechnology, the China-based researchers placed an oxide substrate 15 micrometers below the copper foil. The result: a continuous supply of oxygen that lowers the energy barrier to the decomposition of the carbon feedstock, thereby increasing the graphene growth rate.
The expectations were that the oxide substrate would release the oxygen at the high temperatures inside the CVD surface (over 800 degrees Celsius). The researchers confirmed this through the use of electron spectroscopy. While the measurements indicated that oxygen was indeed being released, the amount was still fairly minimal. Nevertheless, this minuscule amount of oxygen proved sufficient for their purposes because the very small space between the oxide substrate and the copper foil created a trapping effect that multiplied the effect of the oxygen.
In their experiments, the researchers were able to successfully produce single-crystal graphene materials as large as 0.3 millimeter in just five seconds. That, according to the researchers, is more than two orders of magnitude faster than other methods in which graphene is grown on copper foils.
The researchers believe that this ultrafast synthesis of graphene makes possible a new era of scalable production of high-quality, single-crystal graphene films by combining this process with roll-to-roll methods.
Counterintuitively, speeding up the process of producing single-crystal graphene films may not automatically lead to wider adoption of graphene in various devices. Just a few years ago, graphene production was stuck at around a 25-percent utilization rate, and there is no reason to believe that demand has increased enough to have dramatically changed those figures. (Graphene producers will tell you that if demand for CVD-produced graphene suddenly spiked, volume could be doubled nearly overnight.)
Nonetheless, speed in manufacturing is always an attractive option for any product. It just might not offer a change to the graphene landscape as much as a few “killer apps” might.
Keywords: chemical vapor deposition (CVD); Graphene Frontiers; single-crystal graphene; Hong Kong Polytechnic University; Peking University;  graphene;

 Source:  www.ieee.org
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Monday, September 19, 2016

Dielectric and piezoelectric properties of high curie temperature 0.63Bi(Mg1/2Ti1/2)O3–0.37PbTiO3 single crystals with morphotropic phase boundary composition

Highlights

•Novel perovskite ferroelectric 0.63BMT–0.37PT single crystals near the morphotropic phase boundary were successfully grown by a self-flux method.

•The 0.63BMT–0.37PT single crystals presented high Curie temperature (Tc) about 460 °C and a rhombohedral to tetragonal phase transition temperature (Tr-t) 249 °C.

•The d33 of 0.63BMT–0.37PT single crystals is 320pC/N, much higher than that of ceramics with the same composition (220pC/N).

Novel perovskite ferroelectric 0.63Bi(Mg1/2Ti1/2)O3–0.37PbTiO3 (0.63BMT–0.37PT) single crystals with a composition around the morphotropic phase boundary (MPB) were successfully grown by a self-flux method. They presented Curie temperature (Tc) about 460 °C and a rhombohedral to tetragonal phase transition temperature (Tr-t) 249 °C. The piezoelectric constant d33 for (001) oriented 0.63BMT–0.37PT single crystals reached 320pC/N, which was higher than that (208pC/N) in the tetragonal phase 0.38BMT–0.62PT crystals. The large piezoelectric constant d33 and high Tc make BMT–PT single crystals promising candidates used for the next generation of high performance and high temperature actuators and transducers.

Keywords:  A. BMT-PT single crystals;  B. Crystal growth;  D. Dielectric properties;  D. Piezoelectric properties

Source: Sciencedirect

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Tuesday, September 6, 2016

Growth of single crystals of BaFe12O19 by solid state crystal growth

Highlights

•Single crystals of BaFe12O19 are grown by solid state crystal growth.

•A single crystal up to~130μm thick (c-axis direction) grows on the seed crystal.

•The single crystal and surrounding ceramic matrix have similar composition.
•Micro-Raman scattering shows the single crystal has the BaFe12O19 structure.

Single crystals of BaFe12O19 are grown for the first time by solid state crystal growth. Seed crystals of BaFe12O19 are buried in BaFe12O19+1 wt% BaCO3 powder, which are then pressed into pellets containing 
the seed crystals. During sintering, single crystals of BaFe12O19 up to ~130μm thick in the c-axis direction grow on the seed crystals by consuming grains from the surrounding polycrystalline matrix. Scanning electron microscopy-energy dispersive spectroscopy analysis shows that the single crystal and the surrounding polycrystalline matrix have the same chemical composition. Micro-Raman scattering shows the single crystal to have the BaFe12O19 structure. The optimum growth temperature is found to be 1200 °C. The single crystal growth behavior is explained using the mixed control theory of grain growth.
 
Keywords:  BaFe12O19;  Single crystal;  Scanning electron microscopy;  Raman scattering;  Grain growth

Source: Sciencedirect

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Growth and characterization of WSe2 single crystals using TeCl4 as transport agent

Highlights

•WSe2 single crystals (25–100 mm2) grown by the chemical vapor transport method.

•High quality single crystals obtained from slowly cooled polycrystalline powders.

•WSe2 single crystals with a photocurrent plateau at 75 mA cm−2.

The growth of WSe2 single crystals, using TeCl4 as transport agent was performed successfully from slowly cooled polycrystalline powders as precursors. The resulting single crystals were characterized by X-ray diffraction (XRD) and their surfaces examined by scanning electron microscopy (SEM) presented fewer defects than single crystals prepared from air-quenched polycrystalline powders. Energy Dispersive X-ray microanalysis (EDX), inductively coupled plasma (ICP) spectroscopy and X-ray photoelectron spectroscopy (XPS) revealed that the single crystals are homogeneous and stoichiometric. Electrical conductivity and photocurrent measurements have confirmed the semiconducting character of the single crystals and a photocurrent of 75 mA cm−2 has been reached. In addition, single crystals with areas in the 25–100 mm2 range can be obtained under the reported growth conditions.

Keywords:  A1. Characterization;  A1. Crystal morphology;  A2. Single crystal growth;  B1. Inorganic compounds;  B2. Semiconducting materials; B3. Solar cells

Source: Sciencedirect

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Friday, July 29, 2016

Synthesis of large single crystals of layered titanosilicate JDF-L1 with orthogonally lamellar architecture

Highlights

•
Synthesis of JDF-L1 large single crystals with orthogonally lamellar architecture.
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New observations on phase transformation between ETS-10 and JDF-L1.
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Different effects of K+ and F− ions on crystal phase and growth orientation.
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Square sheets with 3-dimensions of ca. 15, 15 and 1.5 µm in orthogonal orientations.

Abstract

Facile procedures were reported for synthesis of sizable lamellar JDF-L1 crystals. Phase transformation between ETS-10 and JDF-L1 was experimentally demonstrated founding on the role of K+, Na+ and F− ions in crystal growth. Factually, K+ ions were confirmed not to participate in the crystal growth of JDF-L1. High concentrations of only Na+ and F− ions in initial gels facilitated the formation of lamellar JDF-L1 differing from normal JDF-L1 in crystal morphology and unit-cell dimensions.

Graphical abstract

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Keywords

  • Sol–gel preparation; 
  • Crystal growth; 
  • Intergrowth; 
  • Large single crystal; 
  • Lamellar texture
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Tuesday, July 26, 2016

Extreme anisotropy of electromigration: Nickel in single-crystal tin

Highlights

•
A new approach is developed to fabricate single-crystal Sn solder joints.
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Grain orientation of solder joints can be artificially controlled.
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Ni3Sn4 forms on the surfaces of single-crystal Sn with regularity after electromigration (EM) test.
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Experimental results unequivocally confirm the existence of extremely anisotropic EM system: Ni in single-crystal Sn.

Abstract

A new approach is developed to fabricate single-crystal Sn solder joints with a line-type structure. The primary purpose is to investigate the diffusion characteristics of Ni in single-crystal Sn with four different grain orientations during electromigration. An interesting new experimental phenomenon that Ni3Sn4 forms on the surfaces of single-crystal Sn with regularity occurs which should be attributed to the extremely anisotropic diffusion property of Ni in single-crystal Sn. Besides, the diffusion velocity of Ni in single-crystal Sn during electromigration is ranked as followed: (001)> (101)> (301)> (100). Experimental observations are in good agreement with kinetic analysis.

Graphical abstract

Image for unlabelled figure

Keywords

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Wednesday, July 20, 2016

Structure and superconductivity of (Li1−x Fe x )OHFeSe single crystals grown using A x Fe2−y Se2 (A  =  K, Rb, and Cs) as precursors

Abstract

We present results on the hydrothermal growth of ($\text{L}{{\text{i}}_{1-x}}\text{F}{{\text{e}}_{x}}$ )OHFeSe single crystals using floating-zone-grown ${{A}_{x}}\text{F}{{\text{e}}_{2-y}}\text{S}{{\text{e}}_{2}}$  (A  =  K, Rb, and Cs) as precursors. The growth proceeds by the hydrothermal ion exchange of Li/Fe–O–H for K, Rb, and Cs, resulting in a stacking layer of ($\text{L}{{\text{i}}_{1-x}}\text{F}{{\text{e}}_{x}}$ )OH sandwiched between the FeSe layers. Optimal growth parameters are achieved using high quality A 0.80Fe1.81Se2 single crystals added to the mixtures of LiOH, H2O, Fe and C(NH2)2Se in an autoclave and subsequently heated to 120 °C for 2 d, to obtain highest quality single crystals. The obtained crystals have lateral dimensions up to centimeters, with the final size related to that of the precursor crystal used. All ($\text{L}{{\text{i}}_{1-x}}\text{F}{{\text{e}}_{x}}$ )OHFeSe single crystals show a superconducting transition temperature T c  >  42 K, regardless of the phase of the precursor such as superconducting K0.80Fe1.81Se2 (T c  =  29.31 K) or non-superconducting Rb0.80Fe1.81Se2 or poor-superconducting Cs0.80Fe1.81Se2 (T c  =  28.67 K). The T c and transition width ΔT vary in the obtained single crystals, due to the inhomogeneity of the ionic exchange. X-ray diffraction analysis demonstrates that the 245 insulating phase is absent in the ion-exchanged single crystals, while it is observed in the ${{A}_{x}}\text{F}{{\text{e}}_{2-y}}\text{S}{{\text{e}}_{2}}$  precursors. Comparative studies of the structure, magnetization, and superconductivity on the parent A 0.80Fe1.81Se2 and the ion-exchanged ($\text{L}{{\text{i}}_{1-x}}\text{F}{{\text{e}}_{x}}$ )OHFeSe crystals are discussed. A phase diagram including antiferromagnetic spin density wave and superconducting phases is also proposed.

Keywords

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Tuesday, July 12, 2016

Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric interface control and very thin single crystal

Abstract
A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a 'seed-controlled' pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm2 V−1 s−1—more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices.

Keywords

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Friday, July 8, 2016

Single crystal growth of type I Na–Si clathrate by using Na–Sn flux

Highlights

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Single crystals of type I Na–Si clathrate, Na8Si46, were synthesized by using Na–Sn flux.
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Preparation conditions of the single crystals were cleared.
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The details of the crystal growth process were investigated.

Abstract

Single crystals of type I Na–Si clathrate, Na8Si46, were synthesized by heating Na, Na4Si4, and Na15Sn4 at 723 K under an Ar gas pressure of 104 Pa for 12 h. The single crystals having {110} habit planes grew up to 1.5 mm in size due to Na evaporation from a Na–Si–Sn melt with a starting compositional molar ratio of Na/Si/Sn=5.75:2:1.

Keywords

  • A2. Single crystal growth; 
  • A2. Growth from solutions; 
  • A2. Tin flux growth; 
  • B1. Inorganic compounds; 
  • B1. Silicon compounds; 
  • B1. Clathrate compound
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Tuesday, June 21, 2016

Growth of single crystals of BaFe12O19 by solid state crystal growth

Highlights

•
Single crystals of BaFe12O19 are grown by solid state crystal growth.
•
A single crystal up to ∼130 μm thick (c-axis direction) grows on the seed crystal.
•
The single crystal and surrounding ceramic matrix have similar composition.
•
Micro-Raman scattering shows the single crystal has the BaFe12O19 structure.

Abstract

Single crystals of BaFe12O19 are grown for the first time by solid state crystal growth. Seed crystals of BaFe12O19 are buried in BaFe12O19+1 wt% BaCO3 powder, which are then pressed into pellets containing the seed crystals. During sintering, single crystals of BaFe12O19 up to ∼130 μm thick in the c-axis direction grow on the seed crystals by consuming grains from the surrounding polycrystalline matrix. Scanning electron microscopy-energy dispersive spectroscopy analysis shows that the single crystal and the surrounding polycrystalline matrix have the same chemical composition. Micro-Raman scattering shows the single crystal to have the BaFe12O19 structure. The optimum growth temperature is found to be 1200 °C. The single crystal growth behavior is explained using the mixed control theory of grain growth.

Keywords

  • BaFe12O19; 
  • Single crystal; 
  • Scanning electron microscopy; 
  • Raman scattering; 
  • Grain growth
If you need more information about single crystal, please visit our website:http://www.qualtiymaterial.net, send us email at sales@qualitymaterial.net. 

      • Soure:sciencedirect
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Wednesday, May 18, 2016

Growth of β-Ga2O3 single crystals using vertical Bridgman method in ambient air

Highlights

•
Single-crystal growth of β-Ga2O3 was studied by the vertical Bridgman (VB) method.
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β-Ga2O3 crystals were grown in platinum–rhodium alloy crucibles in ambient air.
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Single crystals with (100) faceted plane were grown without seeding.
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Growth direction of the crystals grown was perpendicular to the (100) faceted plane.

Abstract

A new approach to β-Ga2O3 single crystal growth was studied, using the vertical Bridgman (VB) method in ambient air, while measuring the β-Ga2O3 melting temperature and investigating the effects of crucible composition and shape. β-Ga2O3 single crystals 25 mm in diameter were grown in platinum–rhodium alloy crucibles in ambient air, with no adhesion of the crystals to the crucible wall. Single crystal growth without a crystal seed was realized by (100) faceted growth with a growth direction perpendicular to the (100) faceted plane.

Keywords

  • A2. Bridgman technique; 
  • A2. Growth from melt; 
  • A2. Single crystal growth; 
  • B1. Oxides; 
  • B2. Semiconducting gallium compounds
      • If you need more information about single crystal, please visit our website:http://www.qualtiymaterial.net, send us email at sales@qualitymaterial.net. 

        Soure:sciencedirect


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