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Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach
Author(s) -
Li Zhihai,
Smeu Manuel,
Afsari Sepideh,
Xing Yangjun,
Ratner Mark A.,
Borguet Eric
Publication year - 2014
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201308398
Subject(s) - homo/lumo , density functional theory , molecule , scanning tunneling microscope , conjugated system , work function , quantum tunnelling , chemical physics , band gap , materials science , metal , chemistry , photochemistry , nanotechnology , optoelectronics , computational chemistry , organic chemistry , polymer
Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single‐molecule‐based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal–molecule–metal (m‐M‐m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH‐induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO–LUMO gap. The dye‐mediated m‐M‐m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non‐equilibrium Green’s function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO–LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.