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Ultrathin metallic layers, such as atomic monolayer palladium (Pd), offer tunable redox behavior well suited for switchable components in nanoscale electronics. Here, we present an in situ plasmonic-electrochemical platform that probes redox processes within subnanometer gaps using a nanoparticle-on-mirror (NPoM) geometry. Atomically thin Pd layers are grown by underpotential deposition, and their redox is monitored via surface enhanced Raman spectroscopy, revealing reversible transformations between metallic and dielectric Pd redox states. This confined plasmonic cavity enhances sensitivity to vibrational and electronic changes, allowing nondestructive optical readout of redox switching behaviors. By reducing the active switching volume to the monolayer limit while maintaining precise geometric control, this work establishes a powerful platform for probing redox dynamics at the atomic scale and points toward highly energy-efficient nanoswitches based on redox-active ultrathin materials.<\/jats:p>","DOI":"10.1021\/acs.nanolett.6c02438","type":"journal-article","created":{"date-parts":[[2026,9,21]],"date-time":"2026-09-21T20:03:47Z","timestamp":1790021027000},"page":"13272-13278","source":"Crossref","is-referenced-by-count":0,"title":["Optically Tracked\nReversible Single-Atomic-Monolayer\nRedox in Plasmonic Nanocavities"],"prefix":"10.1021","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4088-929X","authenticated-orcid":true,"given":"Eric S. 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Commun."},{"key":"2026100710092601900_cit20","doi-asserted-by":"publisher","first-page":"2879","DOI":"10.1021\/acssensors.3c00967","article-title":"Controlling\nAtomic-Scale\nRestructuring and Cleaning of Gold Nanogap Multilayers for Surface-Enhanced\nRaman Scattering Sensing","volume":"8","author":"Grys","year":"2023","journal-title":"ACS Sens."},{"key":"2026100710092601900_cit21","doi-asserted-by":"publisher","first-page":"8689","DOI":"10.1021\/acsnano.0c03050","article-title":"Citrate Coordination\nand Bridging of Gold Nanoparticles: The Role of Gold Adatoms in AuNP\nAging","volume":"14","author":"Grys","year":"2020","journal-title":"ACS Nano"},{"key":"2026100710092601900_cit22","doi-asserted-by":"publisher","first-page":"6327","DOI":"10.1039\/C6NR08763A","article-title":"An In situ TEM study\nof the surface oxidation of palladium nanocrystals assisted by electron\nirradiation","volume":"9","author":"Zhang","year":"2017","journal-title":"Nanoscale"},{"key":"2026100710092601900_cit23","doi-asserted-by":"publisher","first-page":"2022","DOI":"10.1038\/s41467-024-46097-y","article-title":"In situ\nelectrochemical regeneration of nanogap hotspots for continuously\nreusable ultrathin SERS sensors","volume":"15","author":"Sibug-Torres","year":"2024","journal-title":"Nat. Commun."},{"key":"2026100710092601900_cit24","doi-asserted-by":"publisher","first-page":"16576","DOI":"10.1021\/acsomega.8b02677","article-title":"In Situ Creation\nof Surface-Enhanced Raman Scattering Active Au\u2013AuO x Nanostructures through Electrochemical Process for\nPigment Detection","volume":"3","author":"Chen","year":"2018","journal-title":"ACS Omega"},{"key":"2026100710092601900_cit25","doi-asserted-by":"publisher","first-page":"294","DOI":"10.1038\/s41557-025-01989-4","article-title":"Transient Au\u2013Cl\nadlayers modulate\nthe surface chemistry of gold nanoparticles during redox reactions","volume":"18","author":"Sibug-Torres","year":"2026","journal-title":"Nat. Chem."},{"key":"2026100710092601900_cit26","doi-asserted-by":"publisher","first-page":"3197","DOI":"10.1088\/0022-3727\/30\/23\/003","article-title":"Chemical and\nmorphological characterization of Pd aggregates\ngrown by plasma sputter deposition","volume":"30","author":"Thomann","year":"1997","journal-title":"J. Phys.\nAppl. Phys."},{"key":"2026100710092601900_cit27","doi-asserted-by":"publisher","first-page":"L447","DOI":"10.1016\/S0039-6028(03)00640-X","article-title":"Instability\nof gold oxide Au2O3","volume":"537","author":"Tsai","year":"2003","journal-title":"Surf. 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Engl."},{"key":"2026100710092601900_cit30","doi-asserted-by":"publisher","first-page":"3878","DOI":"10.1021\/nn200250v","article-title":"Precise subnanometer\nplasmonic junctions for SERS within gold nanoparticle assemblies using\ncucurbit[n]uril \u201cglue\u201d","volume":"5","author":"Taylor","year":"2011","journal-title":"ACS Nano"},{"key":"2026100710092601900_cit31","doi-asserted-by":"publisher","first-page":"281","DOI":"10.1038\/s41377-022-00943-0","article-title":"Giant mid-IR resonant\ncoupling to molecular vibrations in sub-nm gaps of plasmonic multilayer\nmetafilms","volume":"11","author":"Arul","year":"2022","journal-title":"Light Sci. 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C Solid State Phys."},{"key":"2026100710092601900_cit35","doi-asserted-by":"publisher","first-page":"4348","DOI":"10.1038\/ncomms5348","article-title":"Quantifying thiol\u2013gold interactions\ntowards the efficient strength control","volume":"5","author":"Xue","year":"2014","journal-title":"Nat.\nCommun."}],"container-title":["Nano Letters"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/pubs.acs.org\/doi\/pdf\/10.1021\/acs.nanolett.6c02438","content-type":"application\/pdf","content-version":"vor","intended-application":"unspecified"},{"URL":"https:\/\/pubs.acs.org\/nalefd\/article-pdf\/26\/39\/13272\/68587454\/acs.nanolett.6c02438.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"syndication"},{"URL":"https:\/\/pubs.acs.org\/nalefd\/article-pdf\/26\/39\/13272\/68587454\/acs.nanolett.6c02438.pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,10,7]],"date-time":"2026-10-07T14:18:50Z","timestamp":1791382730000},"score":1,"resource":{"primary":{"URL":"https:\/\/pubs.acs.org\/nalefd\/article\/26\/39\/13272\/5434706\/Optically-Tracked-Reversible-Single-Atomic"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,21]]},"references-count":35,"journal-issue":{"issue":"39","published-online":{"date-parts":[[2026,9,21]]},"published-print":{"date-parts":[[2026,10,7]]}},"URL":"https:\/\/doi.org\/10.1021\/acs.nanolett.6c02438","relation":{},"ISSN":["1530-6984","1530-6992"],"issn-type":[{"value":"1530-6984","type":"print"},{"value":"1530-6992","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,21]]}}}