SummaryViscoelasticity is a complex yet important phenomenon that drives material response at different scales of time and space. Burgeoning interest in nanoscale dynamic material mechanics has driven, and been driven by two key techniques: instrumented nanoindentation and atomic force microscopy. This review provides an overview of fundamental principles in nanoindentation, and compares and contrasts these two techniques as they are used for characterization of viscoelastic processes at the nanoscale.

Nanocarbon electronic conductors combined with pseudocapacitive materials, such as conducting polymers, display outstanding electrochemical properties and mechanical flexibility. These characteristics enable the fabrication of flexible electrodes for energy‐storage devices; that is, supercapacitors that are wearable or can be formed into shapes that are easily integrated into vehicle parts. To date, most nanocarbon materials such as nanofibers are randomly dispersed as a network in a flexible matrix. This morphology inhibits ion transport, particularly under the high current density necessary for devices requiring high power density. Novel flexible densified horizontally aligned carbon nanotube arrays (HACNTs) with controlled nanomorphology for improved ion transport are introduced and combined with conformally coated poly(3‐methylthiophene) (P3MT) conducting polymer to impart pseudocapacitance. The resulting P3MT/HACNT nanocomposite electrodes exhibit high areal capacitance of 3.1 F cm−2 at 5 mA cm−2, with areal capacitance remaining at 1.8 F cm−2 even at a current density of 200 mA cm−2. The asymmetric supercapacitor cell also delivers more than 1–2 orders of magnitude improvement in both areal energy and power density over state‐of‐the‐art cells. Furthermore, little change in cell performance is observed under high strain, demonstrating the mechanical and electrochemical stability of the electrodes.

Room temperature resistive volatile organic compound (VOC) sensing materials fabricated with vertically aligned-carbon nanotubes (VA-CNT) demonstrated 10-fold improved sensitivity upon application of a thin conformal layer of the conducting polymer coating ((poly­(3,4-ethylenedioxythiophene) (PEDOT)). The PEDOT was directly synthesized on the VA-CNTs via oxidative chemical vapor deposition (oCVD). Conformal PEDOT coatings with thickness of 8 and 17 nm were easily achievable by oCVD. The hybrid VA-CNT/oCVD PEDOT sensing materials exhibited excellent response to low concentrations of analyte gases of different polarity. The projected detection limit for n-pentane is as low as ∼50 ppm. A second polymer layer, nonconducting polystyrene (PS, ∼6 nm), was further conformally coated on the VA-CNT/PEDOT via initiative chemical vapor deposition (iCVD) to enhance the gas selectivity. The iCVD PS enhanced the selectivity of n-pentane over methanol by 2.7-fold and toluene by 4.4-fold. Several unique advantages of these sensing materials include the following: (1) detection of nonpolar hydrocarbon molecule n-pentane at room temperature; (2) high signal quality (signal-to-noise ratio typically ∼30 dB); (3) solvent-free facile fabrication method that preserves the accessible high-surface-area morphology of the VA-CNTs; (4) good reversibility and short response time (∼400 s). Our results indicate that both the polarity of the analyte molecule and the carrier transport regime of the PEDOT layer are important in sensing behavior. Furthermore, this versatile selective layer design is potentially useful for selectivity enhancement for other important target analytes.