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

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


•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

Tuesday, September 6, 2016

Growth of single crystals of BaFe12O19 by solid state crystal growth


•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

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


•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