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Thermal rectifiers are essential in optimizing heat dissipation in solid-state devices to enhance energy efficiency, reliability, and overall performance. In this study, we experimentally investigate the thermal rectification phenomenon in suspended asymmetric graphene ribbons (GRs). The asymmetry within the graphene is introduced by incorporating periodic parallel nanoribbons on one side of the GR while maintaining the other side in a pristine form. Our findings reveal a substantial thermal rectification effect in these asymmetric graphene devices, reaching up to 45% at room temperature and increasing further at lower environmental temperatures. This effect is attributed to a significant thermal conductivity contrast between pristine graphene and nanoribbon graphene within the asymmetric structure. We observe that the incorporation of nanoribbons leads to a notable reduction in thermal conductivity, primarily due to phonon scattering and bottleneck effects near the nanoribbon edges. These findings suggest that graphene structures exhibiting asymmetry, facilitated by parallel nanoribbons, hold promise for effective heat management at the nanoscale level and the development of practical phononic devices. [ABSTRACT FROM AUTHOR]

Graphene aligned with hexagonal boron nitride (hBN) undergoes significant structural reconstruction due to the formation of a moiré superlattice. Here, we look at the effect of such structural reconstruction on the Raman spectroscopy of bilayer graphene for both singly aligned and doubly aligned heterostructures. The G peak is found to be particularly sensitive to the hBN alignment as it broadens in doubly aligned bilayer graphene compared to that of singly aligned bilayer graphene. This broadening is attributed to the variation in the phonon frequency as a result of the lattice reconstruction of the bilayer graphene responding to the periodic potential exerted by hBNs on either side of the bilayer graphene. In addition, the position of the G-peak and 2D peak follows a slope of 2.2, which implies the formation of strain in the bilayer graphene, validating the argument of lattice reconstruction. [ABSTRACT FROM AUTHOR]

As a two-dimensional material, graphene has outstanding properties, with high electron mobility and mechanical resistance. Therefore, it has good prospects to be used in nanoelectromechanical resonators for sensitive mass detection. Previously, a graphene resonator with graphene/metal contact able to zeptogram level sensing and a with quality factor of 42 at RT was presented. However, over-etching under the metal contacts degrades the quality factor, and resonators with metal/graphene/metal contacts were fabricated to overcome this issue. A quality factor of 350 at RT was obtained for a device with width = 0.8 μ m and length = 1 μ m. Tunning of the resonance frequency from 9.6 to 13.5 MHz with dc gate bias is demonstrated. Temperature reduction from 300 K to 30 K increased the resonance frequency by about 4–6 MHz. The temperature influence on nonlinearity is also discussed, with a resonator demonstrating softening and hardening nonlinearities at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Graphene is a promising material for sensing applications because of its large specific surface area and low noise. In many applications, graphene will inevitably be in contact with oxygen since it is the second most abundant gas in the atmosphere. Therefore, it is of interest to understand how this gas affects the sensor properties. In this work, the effect of oxygen on the low-frequency noise of suspended graphene is demonstrated. Devices with suspended graphene nanoribbons with a width (W) and length (L) of 200 nm were fabricated. The resistance as a function of time was measured in a vacuum and pure oxygen atmosphere through an ac lock-in method. After signal processing with wavelet denoising and analysis, it is demonstrated that oxygen causes random telegraph signal (RTS) in the millisecond scale, with an average dwell time of 2.9 milliseconds in the high-resistance state and 2 milliseconds in the low-resistance state. It is also shown that this RTS occurs only at some periods, which indicates that, upon adsorption, the molecules take some time until they find the most energetically favorable adsorption state. Also, a slow-down in the RTS time constants is observed, which infers that less active sites are available as time goes on because of oxygen adsorption. Therefore, it is important to consider these effects to guarantee high sensitivity and high durability of gas sensors based on graphene that will be exposed to oxygen during their lifetime. [ABSTRACT FROM AUTHOR]

Graphene is widely used in various real-life applications due to its high sensitivity to the change in the carrier concentration. Here, we demonstrate that graphene can be used for implementing a reliable lightning detection network as it shows excellent sensitivity to the electric field of both positive and negative polarities, with a wide range of magnitude. The lowest electric field detected by our graphene sensor is 67 V/m, which is much smaller than the detection limit of previously reported graphene sensors and comparable to that of field mill and MEMS-based sensors. We also present the results of outdoor experiments where the response of the graphene sensor to the atmospheric electric field on a lightning day was tested and found to be in good agreement with the existing field mill sensor. [ABSTRACT FROM AUTHOR]

Controlling the heat transport and thermal conductivity through a material is of prime importance for thermoelectric applications. Phononic crystals, which are a nanostructured array of specially designed pores, can suppress heat transportation owing to the phonon wave interference, resulting in bandgap formation in their band structure. To control heat phonon propagation in thermoelectric devices, phononic crystals with a bandgap in the THz regime are desirable. In this study, we carried out simulation on snowflake shaped phononic crystal and obtained several phononic bandgaps in the THz regime, with the highest being at ≈2 THz. The phononic bandgap position and the width of the bandgap were found to be tunable by varying the neck-length of the snowflake structure. A unique bandgap map computed by varying the neck-length continuously provides enormous amounts of information as to the size and position of the phononic bandgap for various pore dimensions. We have also carried out transmission spectrum analysis and found good agreement with the band structure calculations. The pressure map visualized at various frequencies validates the effectiveness of snowflake shaped nano-pores in suppressing the phonons partially or completely, depending on the transmission probabilities. [ABSTRACT FROM AUTHOR]