9 research outputs found
Too Hot for Photon-Assisted Transport: Hot-Electrons Dominate Conductance Enhancement in Illuminated Single-Molecule Junctions
We investigate light-induced
conductance enhancement in single-molecule
junctions via photon-assisted transport and hot-electron transport.
Using 4,4â˛-bipyridine bound to Au electrodes as a prototypical
single-molecule junction, we report a 20â40% enhancement in
conductance under illumination with 980 nm wavelength radiation. We
probe the effects of subtle changes in the transmission function on
light-enhanced current and show that discrete variations in the binding
geometry result in a 10% change in enhancement. Importantly, we prove
theoretically that the steady-state behavior of photon-assisted transport
and hot-electron transport is identical but that hot-electron transport
is the dominant mechanism for optically induced conductance enhancement
in single-molecule junctions when the wavelength used is absorbed
by the electrodes and the hot-electron relaxation time is long. We
confirm this experimentally by performing polarization-dependent conductance
measurements of illuminated 4,4â˛-bipyridine junctions. Finally,
we perform lock-in type measurements of optical current and conclude
that currents due to laser-induced thermal expansion mask optical
currents. This work provides a robust experimental framework for studying
mechanisms of light-enhanced transport in single-molecule junctions
and offers tools for tuning the performance of organic optoelectronic
devices by analyzing detailed transport properties of the molecules
involved
TiO<sub>2</sub>(110) Charge Donation to an Extended ĎâConjugated Molecule
The surface reduction of rutile TiO<sub>2</sub>(110) generates
a state in the band gap whose excess electrons are spread among multiple
sites, making the surface conductive and reactive. The charge extraction,
hence the surface catalytic properties, depends critically on the
spatial extent of the charge redistribution, which has been hitherto
probed by small molecules that recombine at oxygen vacancy (O<sub>vac</sub>) sites. We demonstrate by valence band resonant photoemission
(RESPES) a very general charge extraction mechanism from a reduced
TiO<sub>2</sub>(110) surface to an extended electron-acceptor organic
molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped
at O<sub>vac</sub> sites and forms a closely packed, planar layer
on TiO<sub>2</sub>(110). In this configuration, the perylene core
spills out the substrate excess electrons, filling the lowest unoccupied
molecular orbital (LUMO). The charge transfer from the reduced surface
to an extended Ď-conjugated system demonstrates the universality
of the injection/extraction mechanism, opening new perspectives for
the coupling of reducible oxides to organic semiconductors and supported
catalysts
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ď-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
Understanding Energy-Level Alignment in DonorâAcceptor/Metal Interfaces from Core-Level Shifts
The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donorâacceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donorâacceptor stoichiometry in the contact blend