Negative Differential Resistance
Negative differential resistance (NDR) is a feature of a current-voltage curve where the current decreases as the magnitude of the applied voltage increases, giving a local current maximum (a peak) in the spectrum. In scanning tunneling spectroscopy of molecular layers, NDR peaks arise from discrete molecular electronic levels and can serve as fingerprints of the molecule in the tunneling junction. For heteropolyacids on graphite, the NDR peak position correlates with the molecule’s reduction potential. Related: Scanning tunneling microscopy, Heteropolyacids.
NDR as a test for molecules on an STM tip (2002)
Song, Kitchin and Barteau used the NDR signature of H3PW12O40 to confirm that the molecule was attached to a Pt/Ir tip. Spectra on bare graphite with the modified tip showed an NDR peak averaging -1.15 ± 0.07 V (62 spectra), nearly identical to the -1.14 ± 0.09 V (24 spectra) found for an H3PW12O40 monolayer on graphite probed with a bare tip. NDR appears whether the molecule sits on the sample or on the tip, so it identifies what is in the junction (Song et al. 2002). Summary: song2002.