Phase transitions of electron–hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. To investigate interfacial dynamical transitions of charged quasiparticles, however, remains a grand challenge. By employing ultrafast mid-infrared microspectroscopic probes to detect excitonic internal quantum transitions and two-dimensional atomic device fabrications, we are able to directly monitor the interplay between free carriers and insulating interlayer excitons between two atomic layers. Our observations reveal unexpected ultrafast formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2. The result suggests carriers in the doped graphene are no longer massless, and an effective mass as small as one percent of free electron mass is sufficient to confine carriers within a 2D hetero space with energy 10 times larger than the room-temperature thermal energy. The interlayer excitons arise within 1 ps. Their formation effectively blocks charge recombination and improves charge separation efficiency for more than one order of magnitude.
It has been generally accepted that
iron-group metals (iron, cobalt,
nickel) consistently show the highest catalytic activity for the growth
of carbon nanomaterials, including carbon nanotubes (CNTs) and graphene.
However, it still remains a challenge for them to obtain uniform graphene,
because of their high carbon solubility, which can be attributed to
an uncontrollable precipitation in cooling process. The quality and
uniformity of the graphene grown on low-cost iron-group metals determine
whether graphene can be put into the mass productions or not. Here,
we develop a novel strategy to form an antiperovskite layer using
ambient-pressure chemical vapor deposition (APCVD), which, so far,
is the only known way for iron-group metals to prepare uniform monolayer
graphene with 100% surface coverage. Our strategy utilizes liquid
metal (e.g., gallium) to assist iron-group metals to form an antiperovskite
layer that is chemically stable throughout the high-temperature growth
process and then to seal the passageway of carbon segregation from
the metal bulk during cooling. With the advantage of forming antiperovskite
structure, the uniform monolayer graphene can always be obtained under
the variations of experimental conditions. Our strategy solves the
problem about how to get uniform graphene film on high-solubility
carbon substrate, to utilize the high catalytic activity of low-cost
iron-group metals and to realize low-temperature growth by chemical
vapor deposition.
The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate...
In this paper, a three-dimensional thermo-viscoelastic constitutive equation is used to simulate and analyze the hot stamping process of thermoplastic resin matrix composites. Dynamic mechanical analyzer (DMA) was used to measure the relaxation behavior of resin matrix at different temperatures. Then, the relaxation times and the weight factors of the resin matrix were obtained by regression analysis of the data of the resin matrix relaxation behavior using genetic intelligent algorithm. According to the classical Maxwell model with N Maxwell elements and representative volume element (RVE), the integral constitutive model was established, and the corresponding integral finite element program was incorporated into commercial software ABAQUS with an UMAT subroutine.Thermal expansion was also taken into account in the present model. The constitutive model and its finite element program in this paper were verified by experiments with a self-designed hot stamping die for thermoplastic composites.In addition, the simulation results were compared with the actual failure areas to further verify the applicability of the constitutive model adopted in this paper. The results indicate that the ball edge of the hemispherical part is easy to be damaged.
In this study, hot stamping tests on continuous glass fiber (GF)-reinforced thermoplastic (PP) composites were conducted under different process parameters using a self-designed hemispherical hot stamping die with a heating system. The effects of parameters such as preheating temperature, stamping depth, and stamping speed on the formability of the fabricated parts were analyzed using optical microscopy and scanning electron microscopy (SEM). The test results show that the suitable stamping depth should be less than 15 mm, the stamping speed should be less than 150 mm/min, and the preheating temperature should be about 200 °C. From the edge of the formed parts to their pole area, a thin-thick-thin characteristic in thickness was observed. Under the same preheating temperature, the influence of stamping depth on the thickness variation of the formed parts was more significant than the stamping speed. The primary defects of the formed parts were cracking, wrinkling, delamination, and fiber exposure. Resin poverty often occurred in the defect area of the formed parts and increased with stamping depth and stamping speed.
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