Fig. 2

Measured open-circuit photovoltage (OCPV) and analysis for three different materials. a–c Scanned OCPV maps for nanogap devices made from SC Au, lithographically patterned PC Au, and PC Au with a 1 nm Cr adhesion layer as plasmon damping medium, respectively. The nanowire and bowtie shape fanout is outlined, with the maximum OCPV value indicated in the legend. The unit is \({\rm{\mu }}\)V/mW. The black scale bar in (a–c) is 500 nm. d OCPV linecuts on logarithmic scale for three materials taken along the maximum OCPV signal in the OCPV maps shown in (a-c). e Scatter plot showing the sign and magnitude of the OCPV across all the devices for different materials. Here, the positive sign indicates the hot electron tunnels towards the ground. f Polarization-dependent OCPV signal before and after the formation of the electromigrated nanogap. g Statistical analysis of OCPV signal showing scatter plots on a logarithmic scale with varying zero-bias conductance of three materials. The confidence intervals of hot electron tunneling currents are indicated by the shaded areas for different materials. The average values are labeled in the legends