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SCIENCE CHINA Chemistry, Volume 61, Issue 11: 1368-1384(2018) https://doi.org/10.1007/s11426-018-9356-2

Towards single-molecule optoelectronic devices

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  • ReceivedJul 2, 2018
  • AcceptedSep 3, 2018
  • PublishedSep 21, 2018

Abstract

Benefiting from the development of molecular electronics and molecular plasmonics, the interplay of light and electronic transport in molecular junctions has attracted growing interest among researchers in both fields, leading to a new research direction of “single-molecule optoelectronics”. Here, we review the latest developments of photo-modulated charge transport, electroluminescence and Raman spectroscopy from single-molecule junctions, and suggest future directions for single-molecule optoelectronics.


Funded by

the National Key R&D Program of China(2017YFA0204901,2017YFA0204902)

the National Natural Science Foundation of China(21673195,61571242,21503179,21727806,21722305)

and the Young Thousand Talent Project of China.


Acknowledgment

This work was supported by the National Key R&D Program of China (2017YFA0204901, 2017YFA0204902), the National Natural Science Foundation of China (21673195, 61571242, 21503179, 21727806, 21722305), and the Young Thousand Talent Project of China.


Interest statement

The authors declare that they have no conflict of interest.


Contributions statement

These authors contributed equally to this work.


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  • Figure 1

    Photo-activated processes in nanoscale junctions without molecules. (a) Thermal expansion due to light-induced heating reduces the effective tunnel barrier between the source and drain electrodes; (b) optical rectification in the case of nonlinear current-voltage characteristics of a nanoscale junction; (c) a photo-emitted electron acquires sufficient energy from the light field to tunnel to the opposite electrode; (d) photon-assisted tunneling: an electron obtains sufficient energy from the optical field to tunnel into an empty state; (e) photothermoelectric current driven by optical heating that warms the electronic distribution more on one electrode than the other; (f) hot-electron photocurrent generated by the small population of electrons at energies comparable to ħω that tunnel before equilibrating with the bulk of the electron gas. Reproduced with permission from Ref. [27], copyright 2018, SPIE (color online).

  • Figure 2

    Photo-induced changes in molecular structure. (a) Schematic of a graphene-diarylethene-graphene junction highlighting the molecular structures; (b) measured current-voltage (I-V) curves through a diarylethene molecule that reversibly switches between the closed and open forms when exposed to ultraviolet (UV) and visible (Vis) radiation, respectively; (c) measured current versus time under periodic exposure to UV and Vis radiation. Reproduced with permission from Ref. [33], copyright 2016, AAAS (color online).

  • Figure 3

    A squeezable-molecule break junction setup for single-molecule conductance measurements. (a) Schematic of the experimental approach. When the prism is illuminated by a laser, free photons and surface plasmons are coupled and confined within the gap between the two electrodes. (b) Representative conductance traces recorded during the separation process without (black) and with (red) laser illumination. Reproduced with permission from Ref. [31], copyright 2013, American Chemical Society (color online).

  • Figure 4

    Mechanism of enhanced conductance under light illumination. (a) Schematic of the measurement strategy. An NH2–PTCDI–NH2 molecule bridging two electrodes is illuminated with laser light. (b) Under dark conditions, the current is dominated by hole-transport through the HOMO. (c) Conductance histograms under dark (gray) and illuminated (blue) conditions, generated from more than 1000 curves. Representative curves are presented in the left panel. (d) Illumination excites the electron to the LUMO, which becomes partially filled. Consequently, a hole entering the HOMO is attracted to it, causing an effective shift of the HOMO level towards the Fermi level, accompanied by enhanced conductance. Reproduced with permission from Ref. [30], copyright 2018, American Chemical Society (color online).

  • Figure 5

    Three excitation mechanisms of plasmon-induced chemical reactions. (a) Indirect hot-electron transfer mechanism. Hot electrons generated by nonradiative decay of a localized surface plasmon are transferred, forming transient negative ion states of the molecule. (b) Direct intramolecular excitation mechanism. The localized surface plasmon induces direct excitation from the occupied state to the unoccupied state of the adsorbate. (c) Charge transfer mechanism. The electrons are resonantly transferred from the metal electrode to the molecule. (e) Schematic of the experiment for real-space investigation of the plasmon-induced chemical reaction in the nanogap between the silver tip and a metal substrate. Topographic STM images of (CH3S)2 molecules on the silver surface (d) before and (f) after irradiation with p-polarized light. Reproduced with permission from Ref. [40], copyright 2018, AAAS (color online).

  • Figure 6

    (a) Structure of the rod-like molecule in Ref. [50]. (b–e) Fabrication of the NT-M-NT device. (b) A freestanding metallic nanotube (black bar) was fabricated on a palladium electrode (grey) by dielectrophoresis; (c) the nanotube was disintegrated by electrical burning, forming a gap; (d) the NT-M-NT junction was constructed by dielectrophoretic deposition of molecules from solution into the gap; (e) the molecule emitted light under a voltage bias of V. (f) The electroluminescence spectrum of the NT-M-NT device (open red circles) closely resembles the fluorescence spectrum of molecules on highly oriented pyrolytic graphite (black line). Reproduced with permission from Ref. [50], copyright 2010, Nature Publishing Group (color online).

  • Figure 7

    (a) On-chip electrically driven plasmon source in Ref. [54]; (b) Au-SAM-EGaIn based on SCn and O-PE-Fc junctions; (c) bias-dependent electroluminescence spectra of SC12 SAM on 50 nm-thick Au substrate; (d) negative-positive voltage switching of S-OPE-Fc SAM. Reproduced with permission from Ref. [54], copyright 2016, Nature Publishing Group (color online).

  • Figure 8

    The first experiment of combined MCBJ and SERS. (a) Schematic of the MCBJ-SERS setup; (b) gap-width dependence of the SERS intensity at BDT molecular junctions. Reproduced with permission from Ref. [64], copyright 2006, American Chemical Society (color online).

  • Figure 9

    (a) Au constriction with a nanogap fabricated by electromigration; (b) SERS mapping of pMA at the vibrational mode of 1590 cm–1; (c) time differential conductance and SERS spectra of a single or few pMA junctions. Reproduced with permission from Ref. [58], copyright 2008, American Chemical Society (color online).

  • Figure 10

    (a) Work principle of “fishing mode” TERS; (b) bias-dependent TERS spectra of the v8a Raman band of 4,4′-bipyridine; (c) mechanism of applied-bias effect on the peak of the v8a mode split. Reproduced with permission from Ref. [69], copyright 2011, Nature Publishing Group (color online).

  • Figure 11

    (a) Experimental setup and differential conductances of “edge-on” junctions; (b) temperature (K) of the 1585 cm–1 (black) and 1083 cm–1 (red) modes versus the applied bias; (c) Teff (V) of the 1585 cm–1 (black), 1280 cm–1 (blue), and 1083 cm–1 (red) modes versus the applied bias. Reproduced with permission from Ref. [79], copyright 2008, Nature Publishing Group (color online).

  • Figure 12

    (a) Effective vibrational temperatures of two OPV3 modes versus bias voltage: 1317 cm–1 (red) and 1625 cm–1 (blue); (b) effective temperature of electronic heating (blue) and dissipated electrical power (red) versus bias voltage. Reproduced with permission from Ref. [78], copyright 2011, Nature Publishing Group (color online).

  • Figure 13

    (a) Temporal changes in conductance and SERS spectra during self-breaking; (b) the three types of SERS spectra during the breaking process; (c) temporal changes in the conductance and Raman shifts around the b1 mode. Reproduced with permission from Ref. [84], copyright 2012, American Chemical Society (color online).

  • Figure 14

    (a) Bias-dependent Raman spectra of a representative C60 device. Reproduced with permission from Ref. [92], copyright 2014, National Academy of Sciences. (b) Bias-dependent Raman spectra of a PCBM device. Reproduced with permission from Ref. [93], copyright 2016, American Chemical Society (color online).

  • Figure 15

    (a) I-V histogram overlaying the I-V curves of 203 single-molecule BDT junctions; (b) statistical distribution of the coupling energy Γ; (c) SERS spectra of single-molecule BDT junctions at different coupling energies where Γ increasing from bottom to top; (d) the plot is SERS intensity versus coupling energy, averaged from 96 ν1- and ν8-active samples. Reproduced with permission from Ref. [100], copyright 2016, American Chemical Society (color online).

  • Figure 16

    (a) Diagram of NPoM geometry; (b) structures of six different molecular tunneling junctions (MTJs); (c) time-dependent SERS spectra of MTJs IVI; (d) vibrational peak positions of several NPoMs compared to the three redox states of MV. Reproduced with permission from Ref. [108], copyright 2017, Nature Publishing Group (color online).

  • Figure 17

    (a) Schematic of MCBJ-SERS setup; (b) hypothesized configurations of the high- and low-conductance states; (c) SERS spectra collected during breaking of the junction, and the ordinary spectra of BDT powder; (d) conductance histograms of BDT before (blue) and after (red) TCEP addition. Reproduced with permission from Ref. [109], copyright 2018, Royal Society of Chemistry (color online).

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