Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

Saxena, Shailendra K.; Smith, Scott R.; Supur, Mustafa; McCreery, Richard L.

doi:10.1002/adom.201901053

Cited by 20 publications

(37 citation statements)

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“…Many studies has been reported quite recently using diazonium electroreduction to graft conductive oligomers on surfaces [38,[43][44][45][46][47]. Ultrathin layers can be generated since the growth is limited by the conductivity of the organic layer in the potential range required for the electroreduction of the diazonium salt.…”

Section: Introductionmentioning

confidence: 99%

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

et al. 2020

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The control of the interface and the adhesion process are key issues for the development of new application based on electrochromic materials. In this work the functionalization of an electrode’s surface through electroreduction of diazonium generated in situ from 4-(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenylamine (SNS-An) has been proposed. The synthesis of the aniline derivative SNS-An was performed and the electrografting was investigated by cyclic voltammetry on various electrodes. Then the organic thin film was fully characterized by several techniques and XPS analysis confirms the presence of an organic film based on the chemical composition of the starting monomer and allows an estimation of its thickness confirmed by AFM scratching measurements. Depending on the number of electrodeposition cycles, the thickness varies from 2 nm to 10 nm, which corresponds to a few grafted oligomers. In addition, the grafted film showed a good electrochemical stability depending on the scan rates up to 400 V/s and the electrochemical response of the modified electrode towards several redox probes showed that the attached layer acts as a conductive switch. Therefore, the electrode behaves as a barrier to electron transfer when the standard redox potential of the probe is below the layer switching potential, whereas the layer can be considered as transparent towards the electron transfer for redox probes with a redox potential above it.

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Section: Introductionmentioning

confidence: 99%

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

et al. 2020

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“…The first type of MJ considered has the structure shown in Figure A, consisting of 4–100 nm oligomers of NAB between conducting carbon contacts, designated as Q/Cr 4 /Au 30 /eC 10 /NAB d /eC 10 /Au 20 , where subscripts indicate layer thickessess in nanometers, eC is electron beam deposited carbon derived from graphite rods as described in detail previously, , and Q is a quartz substrate to avoid silicon due to its photoactivity. A second type of MJ is a bilayer structure also previously reported to be photoactive, containing anthraquinone and tetraphenyl porphyrin oligomeric layers deposited from diazonium reagents. , The fabrication of junctions is detailed in Supporting Information section 1.…”

Section: Resultsmentioning

confidence: 99%

“…Several examples of the interactions of light with MJs with d = 1–10 nm have been reported, − as have several treatments of the relevant theory. − Recent research on all-carbon molecular junctions reveals activationless, photoassisted transport across 4–60 nm of an aromatic oligomer illuminated by UV–vis light. , Figure C illustrates three modes of photon/MJ interactions which both absorb and emit light during electron transport across the molecular layer. Light absorption in the conducting contacts can excite electrons sufficiently to enter empty molecular orbitals (mode #1 Figure C), resulting in current flow at zero bias.…”

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confidence: 99%

“…IPE occurs in MJs with no AlOx present and provides a measure of the offsets between contact Fermi levels and molecular orbitals. , Photon absorption occurring in the molecular layer generates holes and electrons which can then dissociate to generate photocurrent (PC) at zero bias (mode #2), or an increase in photoconductance under bias. ,, Reported examples show that the PC spectrum is similar to the absorption spectrum of the molecule inside the junction, and that PC transport is nearly activationless below T = 150 K. , Light emission by MJs has been reported for two distinct mechanisms, with the first involving hot electrons in the positive electrode coupling to plasmons and generating light (mode #3). ,,− Hot electrons resulting from bias-induced tunneling across thin inorganic films then undergoing photoemission have been reported, , and photoemission accompanying scanning tunneling microscope (STM) experiments has also been reported. − A second mechanism for emission from few-nanometer thick MJs involves electron and hole injection at opposite electrodes followed by recombination to produce light, , analogous to light emitting diodes but with fewer and much thinner molecular layers and likely distinct transport modes. Finally, optical absorption by molecular bilayers with total thickness of ∼10 nm has been reported, with the PC tracking the molecular absorption spectrum and the polarity depending on relative orbital energies. , As will be discussed below, optical generation of carriers within the molecular layer (mode #2 in Figure C) is the most suitable mechanism for practical photodetection, and is the basis of all experiments illustrated in the remaining figures.…”

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Evaluation of Carbon Based Molecular Junctions as Practical Photosensors

et al. 2020

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Molecular junctions with partially transparent top contacts permit monitoring photocurrents as probes of transport mechanism and potentially could act as photosensors with characteristics determined by the molecular layer inside the device. Previously reported molecular junctions containing nitroazobenzene (NAB) oligomers and oligomers of two different aromatic molecules in bilayers were evaluated for sensitivity, dark signal, responsivity, and limits of detection, in order to determine the device parameters which have the largest effects on photodetection performance. The long-range transport of photogenerated charge carriers permits the use of molecular layers thick enough to absorb a large fraction of the light incident on the layer. Thick layers also reduce the dark current and its associated noise, thus improving the limit of detection to a few nanowatts on a detector area of 0.00125 cm2. Since the photocurrents have much lower activation energy than dark currents do, lowering the detector temperature significantly improves the limit of detection, although the present experiments were limited by environmental and instrumentation noise rather than detector noise. The highest specific detectivity (D*) for the current molecular devices was 3 × 107 cm s1/2 /W (∼109, if only shot noise is considered) at 407 nm in a carbon/NAB/carbon junction with a molecular layer thickness of 28 nm. Although this is in the low end of the 106–1012 range for commonly used photodetectors, improvements in device design based on the current results should increase D* by 3–4 orders of magnitude, while preserving the wavelength selectivity and tunability associated with molecular absorbers. In addition, operation outside the 300–1000 nm range of silicon detectors and very low dark currents may be possible with molecular junctions.

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“…Nanoelectronics is based on switching devices, and molecular electronics have tried to use various active molecules to develop resistive switches. Some approaches are based on redox-active molecules − while others are based on photoactive systems, − with few studies combining the two properties. − In this context, photochromic molecules, i.e. molecules with two different forms (open/nonconjugated and closed/conjugated), which can be interconverted by light irradiation, have been widely used and incorporated into single-molecule and large-area Molecular Junctions (MJs). , Most studies are devoted to monolayers or single-molecule-based MJs where the dominant transport mechanism is direct tunneling.…”

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Highly Efficient Photoswitch in Diarylethene-Based Molecular Junctions

et al. 2020

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Thin layers of diarylethene oligomers (oligo(DAE)) were deposited by electrochemical reduction of a diazonium salt on glassy carbon and gold electrodes. The layers were fully characterized using electrochemistry, XPS, and AFM, and switching between open and closed forms using light was evidenced. Solid-state molecular junctions (MJs), in which a C-AFM tip is used as the top contact, were fabricated with total layer thicknesses fixed at 2–3 nm and 8–9 nm, i.e. below and above the direct tunneling limit. DAE was then photoswitched between its open and closed forms. Oligo(DAE) MJs using the open form of DAE are highly resistive while those with DAE in the closed form are more conductive. ON/OFF ratios of 2–3 and 200–400 were obtained for 3-nm- and 9-nm-thick DAE MJs, respectively.

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Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

Cited by 20 publications

References 64 publications

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

Evaluation of Carbon Based Molecular Junctions as Practical Photosensors

Highly Efficient Photoswitch in Diarylethene-Based Molecular Junctions

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