Dielectric screening in atomically thin boron nitride nanosheets

Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas-Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.

Change in dielectric properties in the microwave frequency region of polypyrrole-coated textiles during aging

Complex permittivity of conducting polypyrrole (PPy)-coated Nylon-Lycra textiles ismeasured using a free space transmission measurement technique over the frequency range of1–18 GHz. The aging of microwave dielectric properties and reflection, transmission and absorptionfor a period of 18 months is demonstrated. PPy-coated fabrics are shown to be lossy over thefull frequency range. The levels of absorption are shown to be higher than reflection in the testedsamples. This is attributed to the relatively high resistivity of the PPy-coated fabrics. Both the dopantconcentration and polymerisation time affect the total shielding effectiveness and microwave agingbehaviour. Distinguishing either of these two factors as being exclusively the dominant mechanismof shielding effectiveness is shown to be difficult. It is observed that the PPy-coated Nylon-Lycrasamples with a p-toluene sulfonic acid (pTSA) concentration of 0.015 M and polymerisation times of60 min and 180 min have 37% and 26% decrease in total transmission loss, respectively, upon agingfor 72 weeks at room temperature (20 C, 65% Relative humidity (RH)). The concentration of thedopant also influences the microwave aging behaviour of the PPy-coated fabrics. The samples with ahigher dopant concentration of 0.027 mol/L pTSA are shown to have a transmission loss of 32.6% and16.5% for short and long polymerisation times, respectively, when aged for 72 weeks. The microwaveproperties exhibit better stability with high dopant concentration and/or longer polymerization times.High pTSA dopant concentrations and/or longer polymerisation times result in high microwaveinsertion loss and are more effective in reducing the transmission and also increasing the longevity ofthe electrical properties.