Reorganization energy upon charging a single molecule on an insulator measured by atomic force microscopy

Abstract

Intermolecular single-electron transfer on electrically insulating films is a key process in molecular electronics and an important example of a redox reaction. Electron-transfer rates in molecular systems depend on a few fundamental parameters, such as interadsorbate distance, temperature and, in particular, the Marcus reorganization energy . This crucial parameter is the energy gain that results from the distortion of the equilibrium nuclear geometry in the molecule and its environment on charging. The substrate, especially ionic films , can have an important influence on the reorganization energy. Reorganization energies are measured in electrochemistry as well as with optical and photoemission spectroscopies, but not at the single-molecule limit and nor on insulating surfaces. Atomic force microscopy (AFM), with single-charge sensitivity, atomic-scale spatial resolution and operable on insulating films, overcomes these challenges. Here, we investigate redox reactions of single naphthalocyanine (NPc) molecules on multilayered NaCl films. Employing the atomic force microscope as an ultralow current meter allows us to measure the differential conductance related to transitions between two charge states in both directions. Thereby, the reorganization energy of NPc on NaCl is determined as (0.8 ± 0.2) eV, and density functional theory (DFT) calculations provide the atomistic picture of the nuclear relaxations on charging. Our approach presents a route to perform tunnelling spectroscopy of single adsorbates on insulating substrates and provides insight into single-electron intermolecular transport.

Electrofluorochromism at the single-molecule level

“…The observation of charged atoms and molecules was reported for several double tunneling junctions (9)(10)(11)(12)(13)(14). It is generally associated to a sharp feature in the dI/dV spectra at voltages characteristic of the charging energy.…”

“…Scanning probe microscopies can be used to control the charge state of atoms and molecules (9)(10)(11)(12)(13)(14) and excite their vibrationaly resolved fluorescence (15)(16)(17)(18)(19)(20)(21) with sub-molecular precision. Here we combine these approaches and report on the STM-induced fluorescence of a ZnPc molecule in its neutral and (radical) cationic states.…”

Charge-Induced Structural Changes in a Single Molecule Investigated by Atomic Force Microscopy

“…The resulting energy gain, the Marcus reorganization energy [2], governs the rate of redox reactions, being decisive also for hopping conduction in organic materials [6][7][8]. Scanning probe microscopy has provided insight into the mechanisms governing the electron transfer at the single-molecule level [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. For example, the influence of an additional charge on the tunneling barrier was revealed by means of scanning tunneling microscopy (STM) [13].…”

“…For example, the influence of an additional charge on the tunneling barrier was revealed by means of scanning tunneling microscopy (STM) [13]. Recent experiments based on atomic force microscopy (AFM) for detecting single-electron tunneling events in individual molecules deposited on non conductive substrates allowed determining the reorganization energy [25] and imaging orbital changes upon electron transfer with angstrom resolution [26]. Whereas all these effects are governed by the complex interplay of charge and structure, mostly the electronic aspect of this interplay has been investigated so far.…”

Nanoscale Surface Redox Chemistry Triggered by Plasmon‐Generated Hot Carriers

“…The HeNe laser was set at an incident power of 30 µW in order to avoid the photo‐bleaching and thermal damaging on the surface of nano‐antennas . During TERS measurements, the bias voltage was set to 0.3 V (to prevent bias triggered reaction), and the tunneling current was kept at 0.2 nA to avoid scratching the sheet. All TERS images were obtained after overnight measurements to minimize the thermal drift of the system.…”