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. His primary research tool was variable-temperature scanning tunneling microscopy.

Plummer served on many national and international committees both to review existing scientific programs and to identify future directions for science and technology. Recent examples include: Chair of DOE-sponsored Workshop on "Soft X-Ray Science in the Next Millennium: The Future of Photon-In/Photon-Out Experiments, Pikeville, Tennessee, March 15–18, 2000, and Chair of DOE-BESAC (Basic Energy Sciences Advisory Committee) subpanel for the evaluation of the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory and the Los Alamos Neutron Science Center (LANSCE) Manuel Lujan Jr. Neutron Scattering Center. He also was a member of the DOE-Basic Energy Sciences Advisory Committee, 2001–2004.

He was the author of more than 400 refereed papers and included in the list of the 1,000 Most Cited Physicists, a list compiled by the Institute for Scientific Information which is based on papers published between 1981 and 1997. But what Plummer was proudest of in his long and distinguished career was the mentoring of promising young scientists. 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. He died in Baton Rouge on July 23, 2020.