Low-Temperature Thermal Transport Characteristics in Epitaxial Bilayer Graphene Microbridges.

In this paper, we present a study of the thermal transport of epitaxial bilayer graphene microbridges. The thermal conductance of three graphene microbridges with different lengths was measured at different temperatures using Johnson noise thermometry. We find that with the decrease of the temperature, the thermal transport in the graphene microbridges switches from electron-phonon coupling to electron diffusion, and the switching temperature is dependent on the length of the microbridge, which is in good agreement with the simulation based on a distributed hot-spot model. Moreover, the electron-phonon thermal conductance has a temperature power law of T3 as predicted for pristine graphene and the electron-phonon coupling coefficient σep is found to be approximately 0.18 W/(m2 K4), corresponding to a deformation potential D of 55 eV. In addition, the electron diffusion in the graphene microbridges adheres to the Wiedemann-Franz law, requiring no corrections to the Lorentz number.

4-phenylbutyrate exerts stage-specific effects on cardiac differentiation via HDAC inhibition.

4-phenylbutyrate (4-PBA), a terminal aromatic substituted fatty acid, is used widely to specifically attenuate endoplasmic reticulum (ER) stress and inhibit histone deacetylases (HDACs). In this study, we investigated the effect of 4-PBA on cardiac differentiation of mouse embryonic stem (ES) cells. Herein, we found that 4-PBA regulated cardiac differentiation in a stage-specific manner just like trichostatin A (TSA), a well-known HDAC inhibitor. 4-PBA and TSA favored the early-stage differentiation, but inhibited the late-stage cardiac differentiation via acetylation. Mechanistic studies suggested that HDACs exhibited a temporal expression profiling during cardiomyogenesis. Hdac1 expression underwent a decrease at the early stage, while was upregulated at the late stage of cardiac induction. During the early stage of cardiac differentiation, acetylation favored the induction of Isl1 and Nkx2.5, two transcription factors of cardiac progenitors. During the late stage, histone acetylation induced by 4-PBA or TSA interrupted the gene silence of Oct4, a key determinant of self-renewal and pluripotency. Thereby, 4-PBA and TSA at the late stage hindered the exit from pluripotency, and attenuated the expression of cardiac-specific contractile proteins. Overexpression of HDAC1 and p300 exerted different effects at the distinct stages of cardiac induction. Collectively, our study shows that timely manipulation of HDACs exhibits distinct effects on cardiac differentiation. And the context-dependent effects of HDAC inhibitors depend on cell differentiation states marked by the temporal expression of pluripotency-associated genes.

Ultra-extraordinary optical transmission induced by cascade coupling of surface plasmon polaritons in composite graphene-dielectric stack.

Surface plasmon polaritons have been extensively studied owing to the promising characteristics of near fields. In this paper, the cascade coupling of graphene surface plasmon polaritons (GSPPs) originating from cascading excitation and multiple coupling within a composite graphene-dielectric stack is presented. GSPPs confined to graphene layers are distributed in the entire stack as waveguide modes. Owing to the near-field enhancement effect and large lifetime of the GSPPs, the terahertz wave-graphene interaction is significantly enhanced, which induces an ultra-extraordinary optical transmission (UEOT) together with the reported negative dynamic conductivity of graphene. Furthermore, owing to cascade coupling, the UEOT exhibits considerable transmission enhancement, up to three orders of magnitude, and frequency and angle selections. Based on the key characteristics of cascade coupling, the mode density and coupling intensity of GSPPs, the dependences of the number of graphene layers in the stack, the thickness of dielectric buffers, and the effective Fermi levels of the graphene on the UEOT are also analyzed. The proposed mechanism can pave the way for using layered plasmonic materials in electric devices, such as amplifiers, sensors, detectors, and modulators.

Photo-induced enhanced negative absorption in the graphene-dielectric hybrid meta-structure.

Recently, the negative absorption in graphene-based metamaterials became a very attractive direction of THz electronic devices. Here we propose a graphene-dielectric hybrid meta-structure to realize photo-induced enhanced negative absorption in the THz regime, which results from strong graphene-light interaction. The negative absorption is derived from the degradation of the conductivity of graphene under optical pump. Meanwhile, the graphene-dielectric hybrid meta-structure introduces dispersion relation and resonance mode, which can couple with the incident wave to construct a strong resonance. In this case, both the dispersion of the propagating waves and resonance are contributed to the graphene-light interaction and enhance the negative absorption, in which the resonance coupling determines the distribution of negative absorption, and the maximum is dominated by dispersion. More importantly, compared with the previous work, the negative absorption is increased by nearly 100 times by adopting this meta-structure.

Growth of graphene with large single-crystal domains by Ni foam-assisted structure and its high-gain field-effect transistors.

High-quality graphene materials and high-performance graphene transistors have attracted much attention in recent years. To obtain high-performance graphene transistors, large single-crystal graphene is needed. The synthesis of large-domain-sized single-crystal graphene requires low nucleation density; this can lead to a lower growth rate. In this study, a Ni-foam assisted structure was developed to control the nucleation density and growth rate of graphene by tuning the flow dynamics. Lower nucleation density and high growth rate (∼50 µm min-1) were achieved with a 4 mm-gap Ni foam. With the graphene transistor fabrication process, a pre-deposited Au film as the protective layer was used during the graphene transfer. Graphene transistors showed good current saturation with drain differential conductance as low as 0.04 S mm-1 in the strong saturation region. For the devices with gate length of 2 µm, the intrinsic cut-off frequency f T and maximum oscillation frequency f max were 8.4 and 16.3 GHz, respectively, with f max/f T = 1.9 and power gain of up to 6.4 dB at 1 GHz. The electron velocity saturation induced by the surface optical phonons of SiO2 substrates was analyzed. Electron velocity saturation and ultra-thin Al2O3 gate dielectrics were thought to be the reasons for the good current saturation and high power gain of the graphene transistors.

The status of MAPK cascades contributes to the induction and activation of Gata4 and Nkx2.5 during the stepwise process of cardiac differentiation.

Cardiac differentiation in vitro is a complex, stepwise process that is rigidly governed by a subset of transcription factors and signaling cascades. In this study, we investigated the cooperation of cardiac-specific transcription factors Gata4 and Nkx2.5, as well as mitogen-activated protein kinase (MAPK) cascades. P19 embryonic carcinoma cells were induced into spontaneously beating cardiomyocytes utilizing a two-step protocol that comprised an early stage and a late stage of differentiation. During early-stage differentiation in suspension culture, P19 cells aggregated to form embryoid bodies (EBs), and the Gata4 and Nkx2.5 genes were induced. However, Gata4 expressed at the early stage of differentiation was incapable of activating downstream gene expression, as it was localized in the cytoplasm and prone to degradation. After EBs were plated for late-stage differentiation in adherent culture, the MAPK cascades were highly activated and contributed to the activation of Gata4 and Nkx2.5. Specifically, we revealed that p38 signaling participated in regulating the localization and stabilization of Gata4 and Nkx2.5. Additionally, the JNK cascade regulated late-stage cardiac differentiation; JNK kinase reduced Gata4 stabilization and conversely alleviated Nkx2.5 degradation by direct interaction and phosphorylation of Nkx2.5. Finally, we found that the C-terminal domain of Nkx2.5 was required for its stabilization under conditions of oxidative stress and JNK activation. Overall, our results indicated that the induction and activation of Gata4 and Nkx2.5 during early- and late-stage cardiac differentiation was closely associated with the function of the MAPK signaling cascades.