Flexible, Transparent, and Noncytotoxic Graphene Electric Field Stimulator for Effective Cerebral Blood Volume Enhancement

Enhancing cerebral blood volume (CBV) of a targeted area without causing side effects is a primary strategy for treating cerebral hypoperfusion. Here, we report a new nonpharmaceutical and nonvascular surgical method to increase CBV. A flexible, transparent, and skin-like biocompatible graphene electrical field stimulator was placed directly onto the cortical brain, and a noncontact electric field was applied at a specific local blood vessel. Effective CBV increases in the blood vessels of mouse brains were directly observed from <i>in vivo</i> optical recordings of intrinsic signal imaging. The CBV was significantly increased in arteries of the stimulated area, but neither tissue damage nor unnecessary neuronal activation was observed. No transient hypoxia was observed. This technique provides a new method to treat cerebral blood circulation deficiencies at local vessels and can be applied to brain regeneration and rehabilitation

Photochemical Reaction in Monolayer MoS₂ <i>via</i> Correlated Photoluminescence, Raman Spectroscopy, and Atomic Force Microscopy

Photoluminescence (PL) from monolayer MoS₂ has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damage and associated oxygen adsorption on the sample in ambient conditions. Three distinct behaviors were observed according to the laser irradiation time: (i) slow photo-oxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity; (ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly; and (iii) photoquenching, with complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage; however, the structural disorder is invoked during the fast photo-oxidation stage, and severe structural degradation is generated during the photoquenching stage. The effect of oxidation is further verified by repeating experiments in vacuum, where the PL intensity is simply degraded with laser irradiation in a vacuum due to a simple structural degradation without involving oxygen functional groups. The charge scattering by oxidation is further explained by the emergence/disappearance of neutral excitons and multiexcitons during each stage

Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor

Although aqueous asymmetric supercapacitors are promising technologies because of their high-energy density and enhanced safety, their voltage window is still limited by the narrow stability window of water. Redox reactions at suitable electrodes near the water splitting potential can increase the working potential. Here, we demonstrate a kinetic approach for expanding the voltage window of aqueous asymmetric supercapacitors using <i>in situ</i> activated Mn₃O₄ and VO₂ electrodes. The underlying mechanism indicates a specific potential of ∼1 V <i>vs</i> Ag/AgCl for the oxidation of Mn⁴⁺-to-Mn⁷⁺ at the positive electrode and ∼ –0.8 V <i>vs</i> Ag/AgCl for the reduction of V³⁺-to-V²⁺ at the negative electrode, which limits oxygen and hydrogen evolution reactions, respectively. The as-fabricated aqueous asymmetric supercapacitor exhibited a working voltage of 2.2 V with a high-energy density of 42.7 Wh/kg and a power density of ∼1.1 kW/kg. This mechanism improves the voltage window and energy and power densities

Hyperdislocations in van der Waals Layered Materials

Dislocations are one-dimensional line defects in three-dimensional crystals or periodic structures. It is common that the dislocation networks made of interactive dislocations be generated during plastic deformation. In van der Waals layered materials, the highly anisotropic nature facilitates the formation of such dislocation networks, which is critical for the friction or exfoliation behavior for these materials. By transmission electron microscopy analysis, we found the topological defects in such dislocation networks can be perfectly rationalized in the framework of traditional dislocation theory, which we applied the name “hyperdislocations”. Due to the strong pinning effect of hyperdislocations, the state of exfoliation can be easily triggered by 1° twisting between two layers, which also explains the origin of disregistry and frictionlessness for all of the superlubricants that are widely used for friction reduction and wear protection

Unsaturated Drift Velocity of Monolayer Graphene

We observe that carriers in graphene can be accelerated to the Fermi velocity without heating the lattice. At large Fermi energy |<i>E</i>_F| > 110 meV, electrons excited by a high-power terahertz pulse <i>E</i>_THz relax by emitting optical phonons, resulting in heating of the graphene lattice and optical-phonon generation. This is owing to enhanced electron–phonon scattering at large Fermi energy, at which the large phase space is available for hot electrons. The emitted optical phonons cause carrier scattering, reducing the drift velocity or carrier mobility. However, for |<i>E</i>_F| ≤ 110 meV, electron–phonon scattering rate is suppressed owing to the diminishing density of states near the Dirac point. Therefore, <i>E</i>_THz continues to accelerate carriers without them losing energy to optical phonons, allowing the carriers to travel at the Fermi velocity. The exotic carrier dynamics does not result from the massless nature, but the electron–optical-phonon scattering rate depends on Fermi level in the graphene. Our observations provide insight into the application of graphene for high-speed electronics without degrading carrier mobility

Interfacial Thermal Conductance Observed to be Higher in Semiconducting than Metallic Carbon Nanotubes

Thermal transport at carbon nanotube (CNT) interfaces was investigated by characterizing the interfacial thermal conductance between metallic or semiconducting CNTs and three different surfactants. We thereby resolved a difference between metallic and semiconducting CNTs. CNT portions separated by their electronic type were prepared in aqueous suspensions. After slightly heating the CNTs dispersed in the suspension, we obtained cooling curves by monitoring the transient changes in absorption, and from these cooling curves, we extracted the interfacial thermal conductance by modeling the thermal system. We found that the semiconducting CNTs unexpectedly exhibited a higher conductance of 11.5 MW/m²·K than that of metallic CNTs (9 MW/m²·K). Meanwhile, the type of surfactants hardly influenced the heat transport at the interface. The surfactant dependence is understood in terms of the coupling between the low-frequency vibrational modes of the CNTs and the surfactants. Explanations for the electronic-type dependency are considered based on the defect density in CNTs and the packing density of surfactants

Understanding Coulomb Scattering Mechanism in Monolayer MoS₂ Channel in the Presence of <i>h</i>‑BN Buffer Layer

As the thickness becomes thinner, the importance of Coulomb scattering in two-dimensional layered materials increases because of the close proximity between channel and interfacial layer and the reduced screening effects. The Coulomb scattering in the channel is usually obscured mainly by the Schottky barrier at the contact in the noise measurements. Here, we report low-temperature (<i>T</i>) noise measurements to understand the Coulomb scattering mechanism in the MoS₂ channel in the presence of <i>h</i>-BN buffer layer on the silicon dioxide (SiO₂) insulating layer. One essential measure in the noise analysis is the Coulomb scattering parameter (α_SC) which is different for channel materials and electron excess doping concentrations. This was extracted exclusively from a 4-probe method by eliminating the Schottky contact effect. We found that the presence of <i>h</i>-BN on SiO₂ provides the suppression of α_SC twice, the reduction of interfacial traps density by 100 times, and the lowered Schottky barrier noise by 50 times compared to those on SiO₂ at <i>T</i> = 25 K. These improvements enable us to successfully identify the main noise source in the channel, which is the trapping–detrapping process at gate dielectrics rather than the charged impurities localized at the channel, as confirmed by fitting the noise features to the carrier number and correlated mobility fluctuation model. Further, the reduction in contact noise at low temperature in our system is attributed to inhomogeneous distributed Schottky barrier height distribution in the metal–MoS₂ contact region

Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor

Although aqueous asymmetric supercapacitors are promising technologies because of their high-energy density and enhanced safety, their voltage window is still limited by the narrow stability window of water. Redox reactions at suitable electrodes near the water splitting potential can increase the working potential. Here, we demonstrate a kinetic approach for expanding the voltage window of aqueous asymmetric supercapacitors using <i>in situ</i> activated Mn₃O₄ and VO₂ electrodes. The underlying mechanism indicates a specific potential of ∼1 V <i>vs</i> Ag/AgCl for the oxidation of Mn⁴⁺-to-Mn⁷⁺ at the positive electrode and ∼ –0.8 V <i>vs</i> Ag/AgCl for the reduction of V³⁺-to-V²⁺ at the negative electrode, which limits oxygen and hydrogen evolution reactions, respectively. The as-fabricated aqueous asymmetric supercapacitor exhibited a working voltage of 2.2 V with a high-energy density of 42.7 Wh/kg and a power density of ∼1.1 kW/kg. This mechanism improves the voltage window and energy and power densities

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS₂/<i>h</i>‑BN Heterostructure

Layered hexagonal boron nitride (<i>h</i>-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the <i>h</i>-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of <i>h</i>-BN thin film as a substrate for monolayer MoS₂ can induce ∼6.5 × 10¹¹ cm^–2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS₂ on <i>h</i>-BN was found to be ∼5 × 10¹³ cm^–2 at high temperature and was significantly reduced at low temperature. Further, the inserted <i>h</i>-BN enables us to reduce the Coulombic charge scattering in MoS₂/<i>h</i>-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal–insulator transition at a much lower charge density of ∼1.0 × 10¹² cm^–2 (<i>T</i> = 25 K). The reduced effective Schottky barrier height in MoS₂/<i>h</i>-BN is attributed to the decreased effective work function of MoS₂ arisen from <i>h</i>-BN induced <i>n</i>-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO₂

Gate-Controlled Nonlinear Conductivity of Dirac Fermion in Graphene Field-Effect Transistors Measured by Terahertz Time-Domain Spectroscopy

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene