Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

Fereiro, Jerry A.; Kondratenko, Mykola; Bergren, Adam Johan; McCreery, Richard L.

doi:10.1021/ja511592s

Cited by 31 publications

(49 citation statements)

References 67 publications

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“…The PCs observed in thin molecular junctions (thickness, d < 4 nm) with copper as a partially transparent top contact are consistent with the IPE mechanism, provided the photon energy is in a region where light absorption by direct HOMO−LUMO (H−L) transitions in the molecule are insignificant. 34,35 The energy barriers observed with IPE on carbon/molecule/Cu MJs are similar to those measured by Ultraviolet Photoelectron Spectroscopy, and are consistent with transport measurements of similar MJs. 34−36 However, charge transport in carbon/molecule/Cu MJs with d < 5 nm is weakly dependent on variations in molecular structure for aromatic molecules due to the strong electronic coupling between the molecular layer and the contacts.…”

Section: ■ Introductionsupporting

confidence: 82%

“…Regarding the origin of the PC, we can rule out the IPE mechanism observed for thinner MJs (d < 5 nm) in which the molecular absorption is weak. 34,35 The IPE mechanism involves optical excitation in the top contact and further transport to the bottom contact through the barrier determined by either HOMO or LUMO offset from the E F . IPE provided a direct probe of the hole or electron barrier for direct tunneling when d < 5 nm, but was complicated when the molecule as well as the contact absorbed incident light.…”

Section: ■ Discussionmentioning

confidence: 99%

“…The PC spectra shapes and maxima for MJs with d > 5 nm differ significantly from those for IPE spectra reported previously (e.g., for AQ). 34 The energy required for IPE is inherently smaller than the H−L gap, and the absence of PC for photon energies below the absorption onset of the molecular layer indicates that carriers excited in the contacts cannot traverse the thicker layers. Furthermore, the PC polarity and magnitude for the current MJs are the same when either the top or bottom electrode is illuminated ( Figure 5), which is not consistent with the IPE mechanism.…”

Section: ■ Discussionmentioning

confidence: 99%

“…Photocurrents generated by internal photoemission (IPE) have been used to probe energy barriers in classical inorganic tunnel junctions, 27−32 semiconductor/molecule devices, 33 and more recently molecular electronics. 34,35 The minimum photon energy required to generate a photocurrent (PC) is directly related to internal energy barriers, often the offsets between the contact Fermi levels and the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO), depending on the molecule involved. The PCs observed in thin molecular junctions (thickness, d < 4 nm) with copper as a partially transparent top contact are consistent with the IPE mechanism, provided the photon energy is in a region where light absorption by direct HOMO−LUMO (H−L) transitions in the molecule are insignificant.…”

Section: ■ Introductionmentioning

confidence: 99%

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Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions

2018

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Photocurrents generated by illumination of carbon-based molecular junctions were investigated as diagnostics of how molecular structure and orbital energies control electronic behavior. Oligomers of eight aromatic molecules covalently bonded to an electron-beam deposited carbon surface were formed by electrochemical reduction of diazonium reagents, with layer thicknesses in the range of 5-12 nm. Illumination through either the top or bottom partially transparent electrodes produced both an open circuit potential (OCP) and a photocurrent (PC), and the polarity and spectrum of the photocurrent depended directly on the relative positions of the frontier orbitals and the electrode Fermi level (E). Electron donors with relatively high HOMO energies yielded positive OCP and PC, and electron acceptors with LUMO energies closer to E than the HOMO energy produced negative OCP and PC. In all cases, the PC spectrum and the absorption spectrum of the oligomer in the molecular junction had very similar shapes and wavelength maxima. Asymmetry of electronic coupling at the top and bottom electrodes due to differences in bonding and contact area cause an internal potential gradient which controls PC and OCP polarities. The results provide a direct indication of which orbital energies are closest to E and also indicate that transport in molecular junctions thicker than 5 nm is controlled by the difference in energy of the HOMO and LUMO orbitals.

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Section: ■ Introductionsupporting

confidence: 82%

Section: ■ Discussionmentioning

confidence: 99%

Section: ■ Discussionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

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Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions

2018

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“…This suggests that transport in BTB involves holes and agrees with previous reports based on transport 15 and photocurrents. 88,91 Moreover, when using BTB layers where covalent bond formation at both bottom and top electrodes is indicated experimentally, we propose that the difference in barriers for hole injection, associated with strong coupling at both electrode, is the origin of the large rectification (RR > 1000) behavior observed for BTB based devices. Figure 5 shows a plausible scheme explaining the rectification of BTB devices.…”

Section: Journal Of the American Chemical Societymentioning

confidence: 81%

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature

et al. 2017

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Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

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Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

Saxena

Smith

Supur

et al. 2019

Advanced Optical Materials

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Eleven bilayer molecular junctions (MJs) consisting of two different 5–7 nm thick molecular layers between conducting contacts are investigated to determine how orbital energies and optical absorbance spectra of the oligomers affect the photocurrent (PC) response, the direction of photoinduced charge transport, and maximum response wavelength. Photometric sensitivity of 2 mA W−1 and a detection limit of 11 pW are demonstrated for MJs, yielding an internal quantum efficiency of 0.14 electrons per absorbed photon. For unbiased MJs, the PC tracks the absorption spectrum of the molecular layer, and is stable for >5 h of illumination. The organic/organic (O/O) interface between the molecular layers within bilayer MJs is the primary determinant of PC polarity, and the bilayer MJ mechanism is conceptually similar to that of a single O/O heterojunction studied in bilayers of much greater thickness. The charge transport direction of the 11 MJs is completely consistent with hole‐dominated transport of photogenerated carriers. For MJs illuminated while an external bias is applied, the PC greatly exceeds the dark current by factors of 102 to 105, depending on bias, bilayer structure, and wavelength. The bilayer MJs are amenable to flexible substrates, and may have applications as sensitive, wavelength‐specific photodetectors.

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Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

Cited by 31 publications

References 67 publications

Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions

Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature

Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

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