The capillary-force-assisted transfer has shown application
potential
for constructing two-dimensional (2D) electronic and optoelectronic
devices for the advantage of free of spin coating the organic compound
and etching the substrate. Currently, the transfer mechanism remains
obscure. The capillary adhesion mechanism and capillary invasion separation
mechanism were proposed independently and rarely discussed in a comprehensive
manner. What is more, the integrity and utilization remain to be improved.
Here, we developed the capillary-force-assisted transfer method with
high utilization and integrity. Uniformity of water transport was
improved by introducing water from the sidewall of the small polydimethylsiloxane
(PDMS) stamp driven by capillary force. The transfer integrity rate
increased, and the location of the complete samples became predictable.
The transfer utilization increased as the limited water transportation
minimized the impact on the surrounding WS2. The monolayer
triangle WS2 crystals from adjacent areas on the sapphire
substrate were transferred one after another. Besides, local mechanical
exfoliation of the continuous WS2 thin films was demonstrated,
implying that the capillary adhesion is strong enough to break the
strong in-plane covalent bond and overcome the van der Waals force
between WS2 and sapphire substrate. Finally, the water
transport model between two surfaces with different hydrophobicity
combinations was derived on the basis of the Young–Laplace
equation. The analysis of water transport between different interfaces
reveals how capillary adhesion and capillary invasion work together
to achieve capillary force transfer. This study highlights the potential
of the capillary-force-assisted transfer as an efficient technique
for fabricating van der Waals structures based on two-dimensional
atomic crystals, especially periodic structures.
The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment. Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm−2 and 12 J cm−2, respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm−2, the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 ms > Δt > 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.
Chemical
vapor deposition (CVD) is a promising method to obtain
monolayer transition metal dichalcogenides (TMDCs) with high quality
and enough size to meet the requirements of practical photoelectric
devices. However, the as-grown monolayers often exhibit a lower PL
performance due to the stress between the as-grown TMDCs flakes and
the substrate. Therefore, finding a facile method to effectively promote
the photoluminescence quantum yield (PL QY) of CVD monolayer TMDCs
with a clean surface is highly desirable for practical applications.
In this work, based on the CVD monolayers MoS2 and MoSe2, the effect of various stress relaxation methods on the TMDCs
PL enhancement is systemically studied. By comparing the different
kinds of volatile solution treatment processes, as well as the traditional
transfer process, it can be found that the volatile solution with
a moderate volatilization rate such as ethanol or IPA is a preferred
option to improve the PL performance of the CVD monolayer TMDCs, which
also surpasses the traditional transfer method by avoiding wrinkles,
defects, and contamination to the samples. PL QY of ethanol-treated
CVD samples could increase by 6 times on average. Significantly, PL
QY of CVD MoSe2 treated by ethanol can reach ∼16%,
which is at the forefront of the previous reports of 2D MoSe2. Our study demonstrated an optimized method to enhance the PL QY
of CVD monolayer TMDCs, which would facilitate TMDCs optoelectronics.
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