A AuTMA planar device was fabricated by drop casting a 500-nm NP film on a silica substrate supporting a wet-transferred CNT/graphene electrode and an e-beam–evaporated Au electrode. Both electrodes were 5 mm long, 50 μm wide, and 60 nm high and separated from each other by a ~50-μm gap. The KFM measurements were performed on a Bruker MultiMode 8 apparatus in the tapping mode. The electrodes were wired out and connected with a Keithley 2636B source meter. In the absence of bias, a selected area of ~95 μm by 20 μm was scanned in high resolution (for ~4 min). The bias was then applied on the CNT/graphene electrode (+1.0 V), and the surface potential measurement was started when the current became stable (~2 min). The same procedure was conducted when the bias was applied on the Au electrode (−1.0 V). However, before applying bias on the Au electrode, the device was left for ~10 min with no bias applied, which was used to equilibrate the counterion gradient.

The device architecture used for EDS measurement was the same as for KFM. The device was first placed on a SEM sample stage. This stage was locked on the SEM holder in the sample exchange chamber. Constant bias was subsequently applied either on the CNT/graphene electrode or the Au electrode. After polarization, the leads were immediately peeled off and the chamber was pumped. Measurements were then performed on a Hitachi SU8220 SEM using a Horiba EMAX x-ray detector.