Molecular Tunnel Junction-Controlled High-Order Charge Transfer

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Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances Ximin Cui, Feng Qin, Yunhe Lai, Hao Wang, Lei Shao, Huanjun Chen, Jianfang Wang, and Hai-qing Lin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07066 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances Ximin Cui,† Feng Qin,§ Yunhe Lai,† Hao Wang,|| Lei Shao,*,† Huanjun Chen,|| Jianfang Wang,*,† and Hai-qing Lin*,‡ †Department

of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR,

China ‡Beijing §Key

Computational Science Research Center, Beijing 100193, China

Laboratory of Science and Technology of Complex Electromagnetic Environment,

China Academy of Engineering Physics, Mianyang 621999, China ||State

Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province

Key Laboratory of Display Material and Technology, Sun Yat-sen University, Guangzhou 510275, China

KEYWORDS: molecular tunnel junctions, heterodimers, charge transfer plasmon, Fano resonance, gold nanoplates, gold nanospheres

ACS Paragon Plus Environment

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ABSTRACT

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications like plasmon mode excitation, hot carrier generation, and construction of ultra-compact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances, and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate–Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.


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Noble metal nanoparticles provide a powerful and promising means for the confinement of light at nanoscale because of their localized surface plasmon resonances.1,2 If two plasmonic resonators are separated by a small gap, their plasmon resonance modes couple together, producing hybridized plasmon modes.3,4 The plasmon coupling allows for the control of the optical responses of nanostructures and the generation of huge localized near-field enhancements.5,6 These interesting features can lead to improvements in a number of technological applications, such as surface-enhanced spectroscopies,7–9 chemical and biological sensing,10,11 nanomanipulation,12 nonlinear optics,13,14 and optoelectronics.15,16 When the air or vacuum gap between two metal nanoparticles is smaller than ~1 nm, the classical description of the plasmon coupling based on Maxwell’s equations breaks down.17–19 Quantum effects become significant because of non-local charge screening and electron spillout, whose finite spatial profiles will thoroughly alter the plasmonic response.20–22 At a gap distance smaller than ~0.3 nm, the quantum tunneling effect alters the plasmon coupling behavior substantially. A intriguing plasmon mode, charge transfer plasmon (CTP), emerges as a characteristic signature of the interparticle electron tunneling across the junction.23–25 Both state-of-the-art lithography techniques and molecular interaction-based assembly approaches have difficulties in the reliable and repeatable fabrication of the ultra-small gaps.26,27 Recent studies have shown that optically conductive molecular junctions and metallic bridges can facilitate the back-and-forth transfer of electrons across the gap between two metal nanoparticles or between one metal nanoparticle and a supporting metal film, enabling the observation of CTPs.28–31 The presence of conductive molecules lowers the


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tunneling barrier height and therefore results in the transition of the bonding dipolar dimer plasmon mode and the emergence of the CTP mode. A tunneling dipolar CTP mode appears in the infrared region with a low resonance intensity.28 The tunneling CTP mode is strongly dependent on the conductance of the molecules in the plasmonic gap. One can vary the junction conductance by employing different molecules, since their electron tunneling properties, including the tunneling barrier width and height, are strongly dependent on the molecular type, structure and functional groups. One therefore can alter the behavior of the tunneling CTP mode by employing different molecular junctions to assemble metal nanoparticles. The resonance peak blueshifts and the electric near-field enhancement is quenched when the electron tunneling becomes pronounced with the increase in the junction conductance.29,32 Molecular tunnel junctions, as a result, provide solutions to the challenges of fabricating quantum plasmonic devices and controlling plasmons by molecular electronic means,28,33 providing a promising route towards molecular plasmonic devices and ultrafast on-chip integrated plasmonic electronic circuits.34 Almost all previous experimental studies on molecular plasmonic junctions have relied on the dipolar CTP plasmon mode of metal nanoparticle dimers.28,35 In these studies, molecular junction-bridged nanostructure dimers are supported on low-dielectric-constant substrates and the tunneling dipolar CTP mode is observed at near-infrared frequencies. Higher-order CTP modes have been predicted,36 but they have remained less explored compared with the dipolar CTP mode in molecular-tunnel-junction-bridged structures. Higher-order plasmon modes usually possess smaller radiative damping and therefore


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narrower linewidths than the dipolar plasmon mode.37 These attractive features are beneficial to the development of ultrasensitive sensing,38 plasmonic nanoantennas,39 superlens imaging,40 and low-loss waveguides.41 However, it has been challenging to produce molecular tunnel junction-bridged nanostructures that can support strong higher-order CTP modes, because nanoparticle assemblies with complex geometries are commonly required. Herein we report on our observation of a strong, well-controlled, higher-order CTP mode in molecular junction-bridged Au nanoplate (NPL)–Au nanosphere (NS) heterodimers on Si substrates, which are geometrically simple and easy to prepare. The higher-order CTP mode, together with the spectral features of Fano resonance, of the heterodimers can be tailored by varying the molecular junction in the plasmonic gap. The use of high-dielectricconstant Si substrates allows for the excitation of strong higher-order plasmon modes in the Au NPLs.42 The small gap between the Au NPL and Au NS is created with a self-assembled monolayer of molecules. When nonconductive molecules in the gap are replaced with chemically equivalent, conductive ones differing by only one atom, a strong blueshift of the scattering spectral peak is produced. The blueshift is ascribed to the emergence of a higherorder CTP mode due to the electron tunneling across the molecular junction. This tunneling effect also largely modifies the Fano resonance feature in the scattering spectra of the heterodimers. We additionally performed electromagnetic simulations to understand the observed dependences of the higher-order CTP and associated Fano resonance on the molecular junction conductance. The molecular conductance at optical frequencies was therefore estimated by matching the simulation-obtained results with the experiments.


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Furthermore, the electron tunneling across the molecular junctions was confirmed by surface-enhanced Raman scattering (SERS) measurements. When the molecules in the plasmonic gap change from insulating to conductive, the reduced SERS intensities clearly indicate the decrease in the local electric field enhancement induced by the electron tunneling.

RESULTS AND DISCUSSION We prepared the Au NPL and NS samples by a seed-mediated growth method, using cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) as the stabilizing and structure-directing agents.43,44 The NS sample was measured to have an average diameter of 60 ± 2 nm by transmission electron microscopy (TEM) imaging (Figure 1a). The average lateral size of the Au NPL sample, which is defined as the distance between two opposite parallel edges of a hexagonal plate, was measured by scanning electron microscopy (SEM) imaging to be 144 ± 4 nm (Figure 1b). The Au NPLs possess atomically flat surfaces with an average thickness of 50.1 ± 1.7 nm, which was determined from the height profiles acquired with an atomic force microscope (AFM) (Figure S1, Supporting Information). The Au NS sample in aqueous solutions has a dipolar plasmon resonance wavelength of 533 nm, as revealed by optical extinction characterization (Figure 1c). The Au NPL sample exhibits three plasmon resonance peaks at 769 nm, 609 nm and 524 nm (very weak) in the extinction spectrum (Figure 1c), which are ascribed to the in-plane dipolar, inplane quadrupolar and out-of-plane octupolar modes, respectively.43 In order to excite the


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out-of-plane octupolar mode of the Au NPLs efficiently, we deposited them on highdielectric-constant Si substrates. After removing as much residual surfactant molecules as possible from the surface of the Au nanocrystals, we successfully assembled the Au NSs onto the Si-supported Au NPLs to form heterodimers, with the two components separated by a monolayer of selected molecules (Figure 1d). In brief, the CTAB-capped Au NPLs were first deposited on a Si substrate, which was then immersed in ethanol to remove the CTAB bilayer off the surfaces of the Au NPLs.45 After the ethanol treatment, the vibration peaks of CTAB can be hardly detected in the Fourier-transform infrared (FTIR) transmission spectrum (Figure S2, Supporting Information). The Au NPLs were thereafter functionalized with a self-assembled monolayer of thiol molecules and the heterodimers were prepared by depositing the Au NSs on the functionalized Au NPLs mainly through Au–Au attractive van der Waals interaction. The observation of the S–H stretching peak at 2580 cm–1 in the Raman scattering spectra (Figure S3, Supporting Information) of the NPLs right after the dithiol functionalization suggests that the dithiol molecules are bonded to the surface of the Au NPLs through one thiol group and the other thiol group is free. The free dangling thiol group can subsequently assist in the assembly of the Au NSs through the formation of the second Au–S bond. As a result, the formation yields of the Au NPL–NS heterodimers with the dithiol molecules are slightly higher than those of the heterodimers prepared from the Au NPLs functionalized with the monothiol molecules. For the thiol molecules used in our study, the number yields of the Au NPL–NS heterodimers can be as high as 70%. More details about the sample preparation procedure are provided in the Methods section. The far-


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field scattering patterns of the Au NPLs appeared as green doughnut spots (Figure 1e and Figure S4a, Supporting Information). The green color arises from the out-of-plane octupolar plasmon mode of the NPLs situated near 535 nm.42 The doughnut-shaped spots are the farfield scattering patterns that are induced through the near-field interaction between the individual NPLs and the Si substrate.46 The scattering patterns turned orange after the Au NSs were deposited onto the Au NPLs (Figure 1f and Figure S4b, Supporting Information). Besides the color change, the scattering patterns of the Au NPL–NS heterodimers were observed to exhibit a much clearer doughnut-shaped appearance in both of the geometry and intensity profile than the individual NPLs (Figure S4, Supporting Information). We can therefore optically identify the Au NPL–NS heterodimers properly, preventing electron beam exposure-caused damage on the junction molecules during SEM imaging. As shown below, the Au NPL–NS heterodimers on Si substrates have the greatest advantage of providing strongly enhanced interaction between the gap molecules and the plasmons because of the efficient excitation of the higher-order modes in the Au NPL.


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Figure 1. Assembly of the Au NPL–NS heterodimers on Si substrates. (a) TEM image of the Au NS sample. (b) SEM image of the Au NPL sample. (c) Normalized extinction spectra of the Au NS and NPL samples dispersed in aqueous solutions. (d) Schematic illustration of the assembly strategy for the Au NPL–NS heterodimers. The Au NPLs and NSs were bridged by a self-assembled monolayer of molecules on a Si substrate. (e, f) Dark-field scattering images of the individual Au NPLs and the Au NPL–NS heterodimers on Si substrates, respectively. The images were recorded with a digital color camera.

Correlated SEM imaging was performed after the dark-field scattering imaging and spectral measurements of the individual heterodimers, confirming the successful preparation


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of the Au NPL–NS heterodimers (Figure 2a). We employed four different types of thiol molecules to form the molecular junctions between the Au NSs and the flat NPLs, including biphenyl-4,4’-dithiol (BPDT), biphenyl-4-thiol (BPT), 1,4-benzenedithiol (BDT), and 4mercaptophenol (MP) (Figure 2b). BPT and BPDT molecules have a similar length of ~1.3 nm while the molecular lengths of BDT and MP are both ~0.7 nm. For vacuum/air gaps, a gap distance of either 1.3 nm or 0.7 nm is still too large for the observation of any tunneling effect on the plasmonic response.26 However, both BPDT and BDT have two thiol groups that form covalent bonds to gold, providing conductive links through the π-orbitals of the phenyl rings to overlap with the electron waves in Au.47 When the gap is connected by BPDT or BDT, electrons can transfer across the molecular junction. BPT and MP lack the second thiol group and therefore fail to create conductive links. The tunneling of electrons is much more difficult. The tunneling behaviors of the molecules can greatly alter the plasmonic response of the heterodimers. We observed distinct difference in the experimental dark-field scattering spectra of the Au NPL–NS heterodimers with BPT and BPDT in their gap (Figure 2c). The scattering spectra of the insulating BPT-gapped heterodimers exhibit a clear asymmetric dip in the 600–700 nm wavelength range, suggesting the occurrence of Fano resonance supported on the Au NPL–NS heterodimers. When BPT molecules were replaced with conductive BPDT molecules in the plasmonic gap, the electromagnetic interaction between the Au NS and NPL dramatically decreases because of the formation of a conductive bridge in the molecular junction. A strong blueshift of the scattering spectral peak was observed and the depth of the asymmetric Fano dip was greatly reduced. Since


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BPDT and BPT molecules have similar lengths (~1.3 nm) and refractive indexes, the observed blueshift can be excluded from the variations in the gap distance or refractive index.29 We therefore referred the significant spectral change to a signature of the direct charge transfer between the plasmonic nanoparticles, according to the prediction by the quantum-corrected model.19 For the heterodimers with BDT or MP molecules in the gap, the replacement of insulating MP with conductive BDT produced a smaller, but still distinct blueshift of the scattering spectral peak and a limited reduction in the depth of the Fano dip (Figure 2d). The result is consistent with the measurement of the heterodimers with BPDT or BPT in the gap. We further performed statistical analysis on the measured scattering spectra of many heterodimers when the different molecular junctions were employed. More than 50 heterodimers for each type of the thiol molecules were measured to analyze the scattering spectra. The major peaks in the scattering spectra of the heterodimers are located at 643 ± 10 nm, 707 ± 13 nm, 676 ± 12 nm and 712 ± 16 nm, when the plasmonic gap is connected by BPDT, BPT, BDT and MP, respectively (Figure 2e and f). The distributions of the major scattering peak position show a distinct blueshift as the molecular junction turns from insulting to conductive. Containing one more phenyl unit than BDT, BPDT has a higher junction conductance at optical frequencies.48 The average peak wavelength difference between the heterodimers connected by conductive BPDT and BDT molecules is up to 33 nm, which is much larger than that between the heterodimers connected by insulating BPT and MP molecules (5 nm). Given that the changes in the gap size are nearly identical, such a


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large difference in the peak shift indicates that the conductance, instead of the gap size, of the molecular junction plays a dominant role on the modulation of the plasmon coupling.

Figure 2. Spectral behaviors of the molecular junction-bridged heterodimers. (a) SEM image of an individual Au NPL–NS heterodimer. (b) Schematic illustrations of the plasmonic junctions formed with the different molecules, whose molecular structures are also shown. The possible tilt of the thiol molecules in the gap is not considered. (c, d) Normalized singleparticle dark-field scattering spectra of the Au NPL–NS heterodimers with BPDT (yellow), BPT (blue), BDT (red) and MP (green) in their gaps. (e, f) Distributions of the major peak wavelengths in the scattering spectra of the heterodimers with BPDT (yellow), BPT (blue), BDT (red) and MP (green) in the gap.


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The finite-difference time-domain (FDTD) method and an ab initio theory for Fano resonance were performed to investigate the effect of the charge transfer in the molecular junctions on the plasmon coupling of the Au NPL–NS heterodimers and unravel the nature of the emergent plasmon modes. We reproduced the experimental conditions for the Au NPL–NS heterodimers supported on Si substrates in the simulations. The refractive indexes of BPT and MP were set to be 1.62 and 1.64, respectively. The molecular junctions in the BPDT/BDT-bridged heterodimers were modelled as a fictitious conductive material to mimic the charge transfer according to the quantum-corrected model.19 The dielectric function ε(ω) of the conductive BPDT/BDT bridge was simplified as ε(ω) = n02[1+i4πσ(ω)/ω], with n0 being the refractive index of BPT/MP.29 We assumed the conductive molecular bridge to be a cylindrical conductor with a radius r, height d and conductance G. The possible tilt of the thiol molecules in the gap was not considered. As a result, the conductivity σ(ω) can be obtained from the junction conductance G according to G = σ(ω)πr2/d. G is also affected by the number of the molecules connecting the plasmonic nanoparticles, N, through G = NGM, with GM being the molecular conductance.32 In the FDTD simulations, the excitation electromagnetic wave was propagating with its polarization parallel (in-plane excitation) or perpendicular (out-of-plane excitation) to the Si substrate (Figure S5a, Supporting Information). The scattering spectra of the heterodimers excited under the different polarization states can be calculated accordingly. For example, the simulation result of the BPT-gapped heterodimers shows that the scattering spectral peak under the in-plane excitation is redshifted to the infrared region because of the substrate effect (Figure S5b,


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Supporting Information). The out-of-plane component dominates the scattering in the wavelength range of 500–900 nm, which can be recorded by our optical detection system. Hence, the plasmonic response under the in-plane excitation is omitted in the discussion below. The simulation-obtained scattering spectra of the heterodimers with the four different types of molecules in the gap (Figure 3a) agree quite well with those measured experimentally (Figure 3b) in both of the resonance peak positions and relative scattering intensities. The total conductance values of 10G0 and 1G0, where G0 = 2e/h is the conductance quantum, were employed in the simulations for BPDT and BDT, respectively. The radius of the simulated cylindrical conductor was assumed to be 2 nm.29 Based on the typical area (0.22 nm2) per molecule in the self-assembled monolayer,49 the number of the molecules in the gap can be estimated to be ~57. The conductance per molecule GM is found to be 0.18G0 and 0.02G0 for BPDT and BDT, respectively. The optical-frequency conductance values of the dithiol molecules have been reported in previous works to be on the order of 10–1–10–2

G0 at charge transfer plasmon resonances.28,29,50,51 Our results are on the same order of magnitude as those in previous works.


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Figure 3. Simulated, measured and fitted scattering spectra. (a) Simulated scattering spectra of the Au NPL–NS heterodimers with BPDT, BPT, BDT and MP in the gap, respectively. (b) Representative measured scattering spectra. (c) Fitted curves according to the two-oscillator model based on the ab initio theory.

To quantify the change in the Fano resonance caused by the charge transfer across the molecular junctions, the representative experimental scattering spectra (Figure 3b) were fitted with a two-oscillator model based on the ab initio theory according to52–54

𝑅(𝜔) = 𝑅s(𝜔)𝑅a(𝜔) =

𝑎2

(

2 𝜔2 ― 𝜔2 s 2𝛾s𝜔s

)

∙ +1

(

𝜔2 ― 𝜔2 a 2𝛾a𝜔a

(

)

+𝑞

2 𝜔2 ― 𝜔2 a 2𝛾a𝜔a

)

2

+𝑏

(1)

+1


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where the symmetric Lorentzian function Rs(ω) represents the radiative contribution of the superradiant mode and the asymmetric envelope Ra(ω) describes the destructive or constructive interference due to the coherent interaction between the superradiant and subradiant modes. The frequencies ωs and ωa determine the resonance positions, and the damping coefficients γs and γa characterize the peak widths of the superradiant and subradiant modes. a is the maximal amplitude of the superradiant mode while q and b denote the shape parameters of the Fano line. q is the asymmetry parameter and b represents the modulation damping parameter stemming from the intrinsic loss. The fitting curves are shown in Figure 3c, and the fitting parameters are given in Table S1 (Supporting Information). First, we compare the fitting results of the heterodimers gapped with the insulating molecules of different lengths. The asymmetry parameter q increases from 0.53 to 0.63 if the molecules in the gap are changed from longer BPT to shorter MP, indicating that a stronger Fano interference is induced at a shorter gap distance. Second, as the insulating molecules are replaced by the conductive ones, the parameter q is reduced. q changes from 0.53 to 0.05 if BPT is replaced with BPDT in the plasmonic gap, and from 0.63 to 0.58 if MP is replaced with BDT. We then tried to understand the rich spectral features of the heterodimers. For the Au NPL–NS heterodimers with BPT molecules in the gap, the coupling between the Au NS and NPL results in a clear Fano resonance with two scattering peaks and one asymmetric dip (Figure 3a). We denote them as Peak I, Dip II and Peak III. The charge distributions at the peak and dip wavelengths were calculated from the FDTD-simulated results (Figure 4a).


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According to the calculation, we assign Peak I to a bonding dipole (NS)–quadrupole (NPL) mode and Peak III to a bonding quadrupole–octupole mode. At Dip II, the bonding dipole– octupole mode dominates. Our previous work showed that a quadrupole mode (dark mode) of individual NPLs deposited on Si substrates can be excited, and therefore interact with an octupole mode (bright mode) destructively to induce a weak Fano resonance.42 For the Au NPL–NS heterodimers on a Si substrate, the destructive interference between the dipole– octupole mode (superradiant mode) and the dipole–quadrupole mode (subradiant mode) gives rise to a strong Fano resonance. The presence of the Au NS reduces the energy mismatch between the superradiant mode and the subradiant mode by hybridizing its dipole mode with the NPL octupole (superradiant) and quadrupole (subradiant) modes. If we replace BPT with conductive BPDT in the gap, the bonding higher-order dimer plasmon mode (Peak I) surprisingly split into two peaks (1 and β) with a dip (α) in between while Peak III and Dip II blueshift to Peak 3 and Dip 2 (Figure 3a). Peak 1, 3 and Dip 2 maintain the dipole–quadrupole, quadrupole–octupole and dipole–octupole interactions (Figure 4b), which have nearly the same charge distributions as Peak I, III and Dip II, respectively. At the emergent Peak β, the heterodimer exhibits a dipole–quadrupole interaction nature like that at Peak I, but the charge distribution becomes completely different (Figure 4c). By carefully examining the details in the calculated charge distributions, we ascertain that this dipole– quadrupole mode at Peak β is a higher-order CTP mode induced by the conductive molecular bridge. Because of the back-and-forth transitions of charges between the NS and the NPL, the oscillating charges are relocated at the interface of the nanoparticles, resulting in the


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transition from Peak I to Peak β concurrently. However, because of the geometrical structure of the heterodimer, Peak I cannot fully transform into Peak β, with a low-intensity Peak 1 appearing at a similar wavelength with Peak I. When the Au NS touch the Au NPL directly, the connecting heterodimer presents a similar spectral feature to the Au NPL–NS heterodimer separated by BPDT (Figure S6, Supporting Information). As expected, a peak (β’), which shares the same charge distribution with Peak β of the molecule-bridged heterodimer, appears together with a low-energy and low-intensity peak (1’). This highenergy peak (β and β’) can therefore be identified as a higher-order CTP peak.

Figure 4. Simulated charge distribution contours. (a) Contours at the peak and dip wavelengths for the heterodimers with BPT in the gap. (b, c) With BPDT in the gap. The inset in (c) shows the schematic of the back-and-forth charge transfer across the molecular junction formed by BPDT. The red and blue colors represent the positive and negative charges, respectively.


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We also investigated how the higher-mode CTP varies with the molecular conductance by simulation (Figure S7a, Supporting Information). The BPDT-bridged heterodimers were considered because they exhibit a clearer higher-order CTP mode (Figure 3a and b). When the conductance of the molecular junction is increased gradually from 0 to 0.05 G0, the intensity of the bonding higher-order dimer plasmon peak (Peak I) and the depth of the Fano dip are first greatly reduced without the appearance of the higher-order CTP peak. As the junction conductance is further increased from 0.05 G0 to 1 G0, the bonding dipole– quadrupole mode slightly blueshifts and decreases in intensity. At the same time, the higherorder CTP mode emerges at the higher-energy side of the bonding dimer plasmon mode. Because of the nature of the higher-order plasmon mode, the higher-order CTP does not exhibit a significant blueshift but shows a prominent increase in intensity as the molecular conductance is increased (Figure S7a, Supporting Information). To verify the above simulated results, we put the Au NPL–NS heterodimers formed with BPDT under the exposure of the electron beam (20 kV, 30 s) in an SEM chamber to break the chemical bonds of the conductive molecules in the gap. A recent study has shown that secondary electrons scattered off Au nanoparticles during SEM and X-ray photoelectron spectroscopy characterization can thermally drive chemical reactions on the adsorbed molecules and induce the cross-linking of phenyl rings.55 We did not observe any considerable changes on the heterodimer geometrical structure after the electron beam exposure (Figure S7b, Supporting Information). However, the scattering spectra of the same heterodimer before and after the exposure are very different (Figure S7c, Supporting Information). After the first


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exposure, the higher-order CTP peak redshifts and decreases in intensity. The bonding higher-order dimer plasmon peak rises up at the lower-energy side. After the second exposure, the higher-order CTP vanishes. Only the bonding higher-order dimer plasmon peak is present, and it is shifted by 60 nm to the lower-energy side, compared with the higher-order CTP mode. A distinct Fano profile also appears. The resultant scattering spectrum is very similar to that of the heterodimer with the insulating BPT in the gap. The spectral evolution introduced by the electron beam exposure indicates that the higher-order CTP is highly sensitive to the molecular conductance. As a control experiment, we exposed the BPT-gapped heterodimers to the same electron beam. Each exposure step only caused a very small peak shift around a few nanometers in the scattering spectra (Figure S7d, Supporting Information). The observed difference between the BPDT- and BPT-gapped heterodimers further suggests that a conductive pathway is indeed established between the NS and NPL through the dithiol monolayer in the gap. In addition, the higher-order CTP was found to be affected by the diameter of the Au NS while the molecular conductance and the sizes of the Au NPL were fixed. In our experiments, we assembled Au NSs of an average diameter of 70 ± 2 nm with the Au NPLs using BPDT (Figure S8, Supporting Information). From simulation, the higher-order CTP peak was found to gradually redshift as the Au NS diameter is increased from 60 nm to 70 nm (Figure S9, Supporting Information). The larger NS increases the plasmon coupling strength, which in turn leads to a lower energy of the higher-order CTP. As a result, the


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higher-order CTP peak and the bonding higher-order dimer plasmon peak overlap to form a broad peak, which was observed in both of our simulations and measurements. SERS measurements were further performed on the individual Au NPL–NS heterodimers with the different molecules in their gaps to study the influence of the charge transfer across the molecular junction on the plasmonic near-field enhancement. Figure 5a shows the representative Raman spectra measured on the heterodimers with the conductive and insulating molecules in the gap. In all spectra, the main Raman scattering peak around 1590 cm–1 is originated from a coupled vibrations of the phenyl rings.56 This main peak is split into two peaks at 1586 cm–1 and 1598 cm–1 for BPT because of the lack of the second thiol group.29 To better quantify the charge transfer effect on the local electric field enhancement, we integrated the area of the main Raman scattering peak and calculated the average integral intensity of more than 20 heterodimers for each type of the molecules (Figure 5b). As the same-sized Au NSs and NPLs were employed to form the heterodimers, the numbers of the molecules in the plasmonic gaps were assumed to be approximately identical in all cases. The experiments showed that the SERS intensities of the insulating molecules in the plasmonic gap are two to three folds larger than those of their conductive counterparts with almost the same molecular lengths. The Raman signals are subdued for the conductive molecules, indicating the quenching of the plasmonic near-field enhancement. FDTD simulations were also performed to calculate the electric field enhancements at the laser wavelength (633 nm) and the Raman scattering wavelength (704 nm, corresponding to the Stokes line of 1590 cm–1) (Figure S10, Supporting Information). The electric field


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intensity enhancement in the gap is highest for the heterodimers bridged with the nonconductive BPT and MP molecules, which is typical for plasmonic dimers with air or vacuum gaps. In contrast, the electric field intensity enhancement in the gap is clearly suppressed compared to that in the adjacent region for the heterodimers bridged with the conductive BPDT and BDT molecules. The zoomed-in electric field intensity enhancement contours for the BPDT-bridged heterodimer are shown in Figure 5c as an example. These results further confirm the attribution of the quenching of the electric field intensity enhancement in the gap to the electron tunneling enabled by the conductive molecules. To quantitatively examine the reduction in the electric field intensity enhancement, we considered the product of the near-field intensities at the laser wavelength |E(λlaser)/E0|2 and at the Raman wavelength |E(λSERS)/E0|2 and averaged it along the line that is 10-nm long, passes the central point of the gap and is perpendicular to the dimer axis. The resultant value was taken as the SERS enhancement factor (EF) (Figure 5d). A 670-fold decrease in EF is obtained as BPT is replaced with BPDT in the molecular junction. The change from MP to BDT causes a 170-fold reduction in EF. This reduction in EF suggests that the molecular conductance in the plasmonic junction significantly affects the plasmonic near-field performance through the charge transfer effect. The large difference in the conductive molecule-induced reduction in the electric field intensity enhancement between the SERS measurements and the FDTD simulations is believed to result from the large increase in the Raman cross-sections of the conductive molecules. The inelastic scattering of tunneling electrons in a conductive molecular junction can excite molecular vibrations, which


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increases the Raman cross-sections of the conductive molecules in the junction gap, as discussed in a previous work.29

Figure 5. SERS performances. (a) Representative SERS spectra of the individual Au NPL–NS heterodimers on Si substrates, with BPT, BPDT, MP and BDT in the gap, respectively. Excitation laser wavelength 633 nm, power 0.225 mW, integration time 1 s. (b) Average integral SERS intensities of more than 20 heterodimers. (c) Zoomed-in electric field intensity enhancement contours in the gap region for the BPDT-bridged heterodimer at the laser wavelength of 633 nm (left) and the Raman scattering wavelength of 704 nm (right). (d) Calculated EFs.


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CONCLUSION In this work, we have successfully developed an approach for the large-scale, bottom-up fabrication of Au NPL–NS heterodimers in high yield with different molecules in the gap. The heterodimers with insulating molecules in the gap exhibit a clear Fano resonance with a deep dip because of the excitation of the higher-order plasmon modes of the Au NPLs in the presence of the high-dielectric-constant Si substrate. When the gap is bridged by conductive molecules, the dark-field scattering spectra show a strong blueshift in the major scattering peak as a signature of the direct charge transfer across the molecular junction. Especially, a higher-order CTP mode emerges at the high-energy side of the bonding higher-order dimer plasmon for the heterodimers bridged with BPDT molecules. FDTD simulations reveal the nature of the higher-order CTP and the evolution from the bonding higher-order dimer plasmon to the higher-order CTP. In parallel, the reduced SERS signals of the conductive molecules in the plasmonic gap demonstrate that the charge transfer across the molecular junction quenches the plasmonic near-field enhancement. We believe that our findings provide a facile and effective platform for generating higher-order CTP modes in plasmonic structures connected by molecular junctions, which will help to deepen our understanding of the charge transfer effect in plasmonic structures and provide a rich area of exploration on molecular–plasmonic devices.

METHODS


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Preparation of the Au NPL–NS Heterodimers. The Au NPL and NS samples were synthesized by seed-mediated growth methods, as described in our previous works. The asprepared Au NPLs were centrifuged twice with the particle concentration adjusted to ~1 pM and kept for further use. The NS solution was prepared by centrifugation and redispersion in a water/acetonitrile mixture (1:4 v/v) with the particle concentration adjusted to ~50 pM. Water/acetonitrile mixture solvents have been reported to be able to destroy the CTAB bilayer on the surface of Au nanocrystals. In acetonitrile-rich solvents, the number densities of the CTAB molecules adsorbed on Au nanocrystals are relatively low, which allows for the access of thiol molecules to the Au surface. The thiol groups on BPDT (Sigma-Aldrich, 95%), BPT (Sigma-Aldrich, 97%), BDT (Sigma-Aldrich, 99%) and MP (Sigma-Aldrich, 90%) can form covalent bonds to Au atoms. All of the thiol molecules were dissolved in ethanol to produce 1 mM solutions. Si wafers (Semiconductor Wafer, Taiwan) were cleaned under ultrasonication (Branson, 2510) in ethanol for 30 min and then treated in a plasma cleaner (Harrick Scientific, PDC-32G, 18 W) for 5 min in an air environment pumped down to ~30 Pa. The cleaned Si substrates were then immersed in the Au NPL solution for several minutes for the deposition of the Au NPLs. The number density of the Au NPLs was adjusted to be appropriate for optical measurements at the single-particle level by varying the immersion time. The substrates were thereafter immersed in ethanol to remove the CTAB bilayer on the Au NPL surface for the functionalization with the thiol molecules. After incubation in a thiol molecule solution for 2 h, the substrates were taken out and rinsed thoroughly with ethanol. For the adsorption of the Au NSs on the NPLs, the substrates were further incubated in the


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NS solution for 2 h. The Au NSs in the water/acetonitrile mixture (1:4 v/v) solvent were negatively charged.45 The adsorption of one NS on the NPL would preclude the adsorption of a second NS owing to the electrostatic repulsion. Therefore the heterodimers composed of one NPL and one NS were preferentially formed. The substrates were finally taken out and blown dry with N2. The Au NPL–NS heterodimers were obtained with the NS attached at the top facet of the NPL. Based on the statistics on more than 500 nanostructures characterized by dark-field scattering and SEM, the yield of the heterodimers, which is defined as the number percentage of the formed NPL–NS heterodimers relative to the predeposited NPL monomers, was found to be ~70%. The remaining ~30% are the NPL monomers. Characterization.

Extinction

spectra

were

measured

on

a

Lambda

950

ultraviolet/visible/near-infrared spectrophotometer with plastic cuvettes of an optical path length of 1.0 cm. SEM imaging was performed on an FEI Quanta 400 FEG microscope operated at 20 kV. For SEM imaging, the as-prepared Au NPLs (1 mL) were washed twice by centrifugation and redispersed in water (0.1 mL). The resultant NPL dispersion (10 μL) was drop-cast carefully onto a clean Si substrate and kept at room temperature for the evaporation of water. TEM imaging was carried out on an FEI Tecnai Spirit microscope operated at 120 kV. For TEM imaging, the as-prepared Au NSs (1 mL) were processed by centrifugation twice and redispersed into water (0.1 mL). A drop (10 μL) of the processed dispersion of the Au NSs was placed on a carbon-film-covered copper TEM grid and allowed to dry in air. AFM images were acquired in air on a Veeco Metrology system (Model No.


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920-006-101) that was operated at the contact mode using a super-sharp silicon nitride tip (Bruker). The FTIR transmission spectra were acquired on a Thermo Nicolet NEXUS 670 Fourier-transform infrared spectrometer. The spectral resolution was 4 cm–1, and the infrared transmission spectra were collected by averaging 36 scans. The NPL sample was dropped on the surface of an infrared-transparent MgF2 substrate for the infrared transmission measurements. Single-particle dark-field scattering spectra and grey images were recorded on an upright optical microscope (Olympus, BX60) that was integrated with a quartz–tungsten–halogen lamp (100 W), a monochromator (Acton, SpectraPro 2360i), and a monochrome charge-coupled device camera (Princeton Instruments, Pixis 400, cooled to -70 °C). A dark-field objective (100×, numerical aperture 0.9) was employed for both exciting the individual nanostructures with the white light and collecting the scattered light. The scattering spectra were corrected by first subtracting the background spectrum taken from the adjacent region without any nanoparticles and then dividing them with the precalibrated response curve of the entire optical system. The exposure time was set at 60 s. The color images were recorded on another upright optical microscope (Olympus, BX53M), which was integrated with a quartz-tungsten-halogen lamp (100 W) and a digital color camera (Olympus, DP73). After the dark-field scattering spectral and imaging measurements, we used a pattern-matching method to correlate the single-particle dark-field scattering characterization with the geometrical structure of the NPL–NS heterodimers from SEM imaging.57 The Raman spectra of the thiol-gapped NPL–NS heterodimers were collected using a Renishaw inVia Reflex system with a dark-field microscope (Leica). The excitation


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laser (wavelength 633 nm, power 0.225 mW) was focused onto the samples at a diameter of ~1 μm through a 50× objective (numerical aperture 0.8). The integration time was set at 1 s. The Raman spectra of the thiol-functionalized NPLs were recorded on another Raman system (Renishaw, RM1000B) with an argon-ion laser (wavelength 514 nm, power 10 mW) as the excitation source and a 50× objective (numerical aperture 0.75). The integration time was set at 10 s. Finite-Difference Time-Domain Simulations. The FDTD simulations were performed using FDTD Solutions 8.7 developed by Lumerical Solutions. During the simulations, an electromagnetic pulse was launched into a box containing the metal nanostructure. A mesh size of 0.25 nm was used in the gap region of the Au NPL–NS heterodimers, and a mesh size of 0.5 nm was used for the Au nanocrystals and their surrounding space. The sizes of the Au NSs and NPLs were set according to their measured average values. The refractive index of the medium in the top and side regions was set at 1.0 and that in the bottom was set according to the dielectric function of the Si wafer. The refractive index of Si was calculated from the dielectric function fitted from the measured data of Palik. The dielectric function of gold was taken from Johnson and Christy’s data. The EFs were calculated by averaging the product of the electric field intensity enhancements at the laser wavelength |E(λlaser)/E0|2 and at the Raman wavelength |E(λSERS)/E0|2. The averaging was performed along the line that passes the central point of the gap, is perpendicular to the dimer axis, 10-nm long and symmetric with respect to the central point of the gap.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. AFM results, additional SEM images, scattering images and spectra of the heterodimers, additional simulated spectra, electric field intensity enhancement contours and charge distribution contours, extinction spectrum and TEM image of the Au NSs, fitting parameters (PDF) The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID Lei Shao: 0000-0003-2161-5103 Jianfang Wang: 0000-0002-2467-8751

ACKNOWLEDGMENTS


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L.S. acknowledges support from CUHK (Direct Grant, Ref. No. 2017–18, Project Code 4053307). J.F.W. acknowledges support from Hong Kong Research Grants Council (General Research Fund 14320916). H.Q.L. acknowledges support from NSAF (U1530401) and Ministry of Science and Technology of China (2017YFA0303404).

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Molecular Tunnel Junction-Controlled High-Order Charge Transfer

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