NIST selected his 1969 paper "Resonance Tunneling of Field-Emitted Electrons Through Adsorbates on Metal Surfaces", co-authored with J. W. Gadzuk and R. D. Young, for inclusion in the agency's centennial collection of its top 100 articles of the 20th century.

In 1973, Plummer accepted a position in the Physics Department at the University of Pennsylvania[1][5] where his work mainly focused on angle-resolved photoemission, momentum-resolved inelastic electron scattering and nonlinear optical response from surfaces.

[1] His research interests shifted to the study on an atomic scale of phase transitions in reduced dimensionality and surfaces of highly correlated electron systems such as transition-metal oxides.

[1] Plummer served on many national and international committees both to review existing scientific programs and to identify future directions for science and technology.

This included advising or co-advising Ph.D. theses of 40 graduate students, hosting ~25 postdoctoral fellows, and assisting many young scientists in advancing their careers.