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An accurate as well as highly sensitive label-free chemical sensing platform for the detection of various metal ions was demonstrated. The chemical sensor was derived from the micro-tapered long-period fiber grating (MLPG) by depositing graphene oxide (GO) by chemical-bonding and optical-tweezer effects. The enhancement in refractive index (RI) sensitivity as well as reusability was obtained by evaluating the deposition thickness in the range of approximately 97.7 to 158.9 nm. Based on the analysis of adsorption principles, the enhanced RI sensitivity leads to a limit of detection as low as 3.2 ppb. The highest sensitivities for the cases studied using sodium and manganese ions in a wide concentration range of 1 ppb to 1 × 106 ppb are respectively 2.2 × 10−3 dB per ppb and 3.2 × 10−3 dB per ppb. Mixture samples were also studied to evaluate the properties of sensing the doped ions. This demonstration of GO modified MLPG is bound to find potential applications that require sensing of mixed samples and illustrates significant importance in developing cost-effective, label-free, reusable, and real-time chemical sensors. [ABSTRACT FROM AUTHOR]

The potential of solution-processable reduced graphene oxide (rGO) films as wave impedance matching layers has been examined in a broad terahertz (THz) spectral bandwidth. The THz sheet conductivities of rGO films measured by THz time-domain spectroscopy were observed to be tunable and sensitive to the film thicknesses and reduction degrees, which can be efficiently controlled by our solution-processable fabrication method. Remarkable broadband impedance matching was achieved with a suitable rGO film, as shown by the suppression of the internal reflected THz pulses from the substrate in the spectra. The underlying mechanisms have been revealed both experimentally and theoretically. This work paves the way for developing rGO-based broadband and large-scale anti-reflection layers for THz components. [ABSTRACT FROM AUTHOR]

The possibility of forming quantum wells (QWs) in transition-metal dichalcogenide nanosheet assembled superlattices (SLs) was investigated by using the first principles calculations. The interfacial binding energies and electronic structures of MoS2/MX2 (MX2 = MoSe2, WS2, and WSe2) SLs were calculated. The interfacial binding energies show that all the SLs are stable, and the most stable atomic configuration is that where M atoms are located right above S atoms. By calculating the band offsets in the SLs, it was found that a QW with a depth of 0.17 eV can be formed in the MoS2 layer in MoS2/WSe2 SLs. The calculated band structure shows that this SL has an indirect band gap due to the tensile strained state of the MoS2 layer. The charge transfer between the two layers is very small, which is in favor of the QWs' formation. In particular, the depth of the QW in the SLs can be adjusted by strain engineering, which can be attributed to the different strain dependencies of the two materials' band gaps. These findings will guide the choice of future nanosheet assembled SLs to work on and suggest a new route to facilitate the design of QW based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Terahertz (THz) technology has been a promising tool for sensing, spectroscopy, imaging, and communication. However, only few devices have shown efficient performance for future THz technology. Herein, we propose a device based on tunable magnetoplasmons in gated monolayer graphene for THz wave modulation and isolation. The relative transmission and the Faraday rotation angle of the device have been calculated by combining the Fresnel method with the voltage-dependent Drude model. Our results suggest that a superior modulation depth and giant Faraday rotation due to the cyclotron effect in the classical regime by intraband transitions in graphene offer an effective, uniform, and flexible tunability for THz wave. And the modulating and isolating manipulations by graphene can range from 0 to 2 THz, with electron-hole asymmetry originating from variable scattering rate of magnetoplasmons. Moreover, the thickness effect of the thin substrate is also studied for better performance of the device, taking advantage of the unavoidable Fabry-Perot (F-P) effect. This work demonstrates a pathway for efficient THz modulator and isolator based on the magneto-optical polarization effect in graphene.