17 research outputs found
Czochralski technique growth of pure and rare-earth-doped SrWO 4 crystals
Abstract Pure and rare-earth (Nd 3+ , Tm 3+ and Er 3+ )-doped strontium tungstates of good optical quality with sizes of about f20 mm Ă 50 mm were grown successfully by the Czochralski technique. The (0 0 1) orientation was found to be the favorable direction for crystal growth. X-ray powder diffraction (XRD), differential thermal analysis (DTA) and differential scanning calorimeter (DSC) of pure strontium tungstate were measured. The concentrations of Nd 3+ , Tm 3+ and Er 3+ were measured and their segregation coefficients were also calculated. The absorption and emission spectra of rare-earth-doped crystals as a function of the s and p polarizations were presented and discussed. Favorable values of the absorption cross section centered at about 800 nm suggest that Nd 3+ -and Tm 3+ -doped strontium tungstates are promising candidates for laser diode (LD) pumping.
Two-step flux synthesis of ultrapure transition metal dichalcogenides
Here, we describe synthesis of TMD crystals using a two-step flux growth
method that eliminates a major potential source of contamination. Detailed
characterization of TMDs grown by this two-step method reveals charged and
isovalent defects with densities an order of magnitude lower than in TMDs grown
by a single-step flux technique. Initial temperature-dependent electrical
transport measurements of monolayer WSe2 yield room-temperature hole mobility
above 840 cm2/Vs and low-temperature disorder-limited mobility above 44,000
cm2/Vs. Electrical transport measurements of graphene-WSe2 heterostructures
fabricated from the two-step flux grown WSe2 also show superior performance:
higher graphene mobility, lower charged impurity density, and well-resolved
integer quantum Hall states
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Catalytic and Electronic Activity of Transition Metal Dichalcogenides Heterostructures
The synthesis of transition metal dichalcogenides (TMDs) are crucial to realization of their real-world applications in electronic, optoelectronic and chemical devices. However, the fabrication yield in terms of material quality, crystal size, defect density are poorly controlled. In this work, by employing the up-to-date stack-and-transfer and nano fabrication techniques, synthetic TMDs that obtained from different growth methods with various crystal qualities were studied.
In most of the cases, better crystals with lower defect densities and larger crystal domain sizes are preferred. Self-flux method was developed to obtain better quality crystals comparing to the traditional chemical vapor transport, as characterized by lower defect densities. BN encapsulating graphene device platform was utilized and TMDs monolayers with different defect densities was inserted in between the BN/graphene interface, where intrinsic defects from the TMDs disturbed the electronic environment of graphene. With the better TMD crystal insertion, we obtain much better electrical performed device in terms of hysteresis, FWHM of Dirac peak and electron mobility. This device also showed advantage in quantum transport measurements
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On the other hand, the presence of defects are not always undesired, especially when it comes to serve as electrocatalysts, in which most of the reactions take place at vacancy sites. However, similar to most of the MoS2 electronic devices, forming barrier-free metal semiconductor contact is the major challenge. We develop a platform that contact resistance could be monitored simultaneously with electrochemical activity. In this platform, the total device resistance is significantly reduced before electrochemical reaction happens while the intrinsic catalytic activity of the MoSâ can be extracted. With this platform, we found the intrinsic catalytic activity of MoSâ strongly correlated to H-coverage on its surface. By adding molecular mediator into electrolytes, H-coverage and the resulting HER activity was enhanced via âCatch and Releaseâ mechanism. Molecular simulation was performed to support our experimental results
Effects of reverse engineering pedagogy on studentsâ learning performance in STEM education: The bridge-design project as an example
In K-12 STEM education, engineering design is emphasized, as demonstrated by the bridge-design project. Due to the iterative nature of engineering design, engineering practice is frequently complicated and requires pedagogical guidance. As an emerging pedagogy in STEM education, REP (Reverse Engineering Pedagogy) is showing, but not enough, some benefits in several cases. This paper aims to explore the effects of REP in a bridge-design course. A comparison experiment, REP versus PBL (Project-Based Learning), was conducted by randomly forming two groups of fourth-grade students from a primary school in China. Results indicated that REP was more advantageous than PBL in terms of decreasing students' cognitive load, boosting their scientific knowledge level and engineering design skills. However, REP and PBL have the same effect on the studentsâ learning attitude and engagement. The key findings, possible reasons, and suggestions for practice are also discussed
Rheological Properties of Modified Coal Tar Pitches
Different modifiers composed of polyethylene glycol, paraformaldehyde, polystyrene, and polyphosphoric acid at different ratios (designated CD-0, CD-1, CD-2. CD-3, and CD-4) were added to coal tar pitch. The resulting modified pitches were prepared and designated control D-0 and D-1, D-2, D-3, and D-4, correspondingly. The objective of this study was to evaluate the properties of the control and modified coal tar pitches by the dynamic shear rheometer (DSR) test and microscopic imaging technology. The complex shear modulus G* role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3eGâG* and phase angle δ role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3eδδ of the modified coal tar pitches decreased with increased temperature, which indicated a decline in the gradual capacity for resistance to permanent deformation at high temperatures. There was good correlation between the Christensen-Andersen-Marasteanu (CAM) model and the complex modulus master curve of the modified coal tar pitches. The test results showed that D-4 had a higher complex viscosity Ρ* role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3eΡâΡ* than that of the control D-0 and the other modified coal tar pitches (D-1, D-2, and D-3), along with a higher glassy complex modulus Gg* role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3eGâgGg*, cross frequency fc role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3efcfc, rheological parameter r role= presentation style= box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3err, and better resistance to rheological deformation. Control D-4 also had a strong capability to resist high-temperature permanent deformation. In addition, the homogeneity of D-4 was better than that of the control D-0 and the other modified pitches, as observed in microscopic images. Therefore, it could be concluded that D-4 had better properties compared with the others
Aging of Transition Metal Dichalcogenide Monolayers
Two-dimensional
sheets of transition metal dichalcogenides are an emerging class of
atomically thin semiconductors that are considered to be âair-stableâ,
similar to graphene. Here we report that, contrary to current understanding,
chemical vapor deposited transition metal dichalcogenide monolayers
exhibit poor long-term stability in air. After room-temperature exposure
to the environment for several months, monolayers of molybdenum disulfide
and tungsten disulfide undergo dramatic aging effects including extensive
cracking, changes in morphology, and severe quenching of the direct
gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy
reveal that this effect is related to gradual oxidation along the
grain boundaries and the adsorption of organic contaminants. These
results highlight important challenges associated with the utilization
of transition metal dichalcogenide monolayers in electronic and optoelectronic
devices. We also demonstrate a potential solution to this problem,
featuring encapsulation of the monolayer sheet by a 10â20 nm
thick optically transparent polymer (parylene C). This strategy is
shown to successfully prevent the degradation of the monolayer material
under accelerated aging (<i>i</i>.<i>e</i>., high-temperature,
oxygen-rich) conditions
Unzipping hBN with ultrashort mid-infrared pulses
Manipulating the nanostructure of materials is critical for numerous
applications in electronics, magnetics, and photonics. However, conventional
methods such as lithography and laser-writing require cleanroom facilities or
leave residue. Here, we describe a new approach to create atomically sharp line
defects in hexagonal boron nitride (hBN) at room temperature by direct optical
phonon excitation in the mid-infrared (mid-IR). We term this phenomenon
"unzipping" to describe the rapid formation and growth of a <30-nm-wide crack
from a point within the laser-driven region. The formation of these features is
attributed to large atomic displacements and high local bond strain from
driving the crystal at a natural resonance. This process is distinguished by
(i) occurring only under resonant phonon excitation, (ii) producing highly
sub-wavelength features, and (iii) sensitivity to crystal orientation and pump
laser polarization. Its cleanliness, directionality, and sharpness enable
applications in in-situ flake cleaving and phonon-wave-coupling via free space
optical excitation.Comment: 11 pages, 4 figure