Electron Transport Across Plasmonic Molecular Nanogaps

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Electron Transport in Plasmonic Molecular Nanogaps Interrogated with Surface-Enhanced Raman Scattering Li Lin, Qiang Zhang, Xiyao Li, Meng Qiu, Xin Jiang, Wei Jin, Hongchen Gu, Dangyuan Lei, and Jian Ye ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08224 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Electron Transport in Plasmonic Molecular Nanogaps Interrogated with Surface-Enhanced Raman Scattering Li Lin,†,∆ Qiang Zhang,$, ‡,∆ Xiyao Li,† Meng Qiu,¢ Xin Jiang,† Wei Jin,¢ Hongchen Gu,*,† Dang Yuan Lei,*,‡,§ and Jian Ye*,†,#,π

†

State Key Laboratory of Oncogenes and Related Genes, School of Biomedical Engineering, Shanghai

Jiao Tong University, Shanghai 200030, P. R. China

$

School of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of

Technology, Shenzhen 518055, P. R. China ‡

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong 999077, P. R.

China

¢

Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, P.

R. China §

Shenzhen Research Institute, The Hong Kong Polytechnic University, Shenzhen 518057, P. R. China

#

Shanghai Key Laboratory of Gynecologic Oncology, Ren Ji Hospital, School of Medicine, Shanghai

Jiao Tong University, Shanghai 200030, P. R. China

π

Shanghai Med-X Engineering Research Center, School of Biomedical Engineering, Shanghai Jiao

Tong University, Shanghai 200030, P. R. China

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*

To whom correspondence should be addressed. E-mail: [email protected] (H.G.); [email protected]

(D.L.); [email protected] (J.Y.) ∆

These authors contributed equally to this work.

Abstract

Charge transport plays an important role in defining both the far-field and near-field optical response of a plasmonic nanostructure with an ultra-small built-in nanogap. As the gap size of a gold core-shell nanomatryoshka approaches the sub-nanometer length scale, charge transport may occur and strongly alter the near-field enhancement within the molecule-filled nanogap. In this work, we utilize ultrasensitive surface-enhanced Raman spectroscopy (SERS) to investigate the plasmonic near-field variation induced by the molecular junction conductance assisted electron transport in gold nanomatryoshkas, termed gap-enhanced Raman tags (GERTs). The GERTs, with interior gaps from 0.7 to 2 nm, are prepared with a wet chemistry method. Our experimental and theoretical studies suggest that the electron transport through the molecular junction influences both the far-field and near-field optical properties of the GERTs. In the far-field extinction response, the low-energy gap mode predicted by a classical electromagnetic model (CEM) is strongly quenched and hence unobservable in the experiment, which can be well explained by a quantum-corrected model (QCM). In the near-field SERS response, the optimal gap size for maximum Raman enhancement at the excitation wavelength of 785 nm (633 nm) is about 1.35 nm (1.8 nm). Similarly, these near-field results do not tally with the CEM calculations but agree well with the QCM results where the molecular junction conductance in the nanogap is fully considered. Our study may improve understanding of charge transport phenomena in ACS Paragon Plus Environment

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ultra-small plasmonic molecular nanogaps and promote the further development of molecular electronics based plasmonics nanodevices.

Keywords: charge transfer, electron transport, gap-enhanced Raman tags, molecule junction conductance, quantum plasmonics,

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Plasmonic nanostructures have attracted great interest in various disciplines such as nanoscience, optics and materials science due to their fascinating optical properties.1-4 The surface plasmon resonance (SPR) effect responsible for these properties originates from the excitation of collective oscillation of quasifree electrons by external electromagnetic radiation. Due to near-field intensity enhancement and spatial confinement, the SPR effect greatly boosts the efficiencies of light-matter interactions and thus exhibits great potential in the fields such as nanomaterials science,5, 6 analyte detection,1, 7-10 solar cells,11-14 and phase changing processes.15-17 In particular, plasmonic capacitive coupling in ultra-small nanogaps between closely packed metallic nanostructures leads to further enhancement and confinement of their near-fields, which has been well explained by classical electromagnetic theories.18-20 As the gap size of two coupled plasmonic nanostructures approaches an atomic scale,21-24 charge transfer across the gap occurs and consequently influences both the far-field and near-field optical responses of the system.25-27 While the charge-transfer-altered far-field plasmonic properties have been intensively studied in previous reports,21, 28, 29 its effect in the plasmonic near-field response has been seldom investigated.

Surface-enhanced Raman scattering (SERS), in which vibrational transitions in molecules are probed using enhanced inelastic Raman scattering, is a powerful technique to explore the near-field properties of plasmonic nanostructures.30-33 Since the cross-section of Raman scattering scales approximately as the fourth power of the near-field strength enhancement, SERS has become a more sensitive tool than far-field spectroscopy (e.g., dark-field scattering spectroscopy) for investigating electron transport and spatial nonlocality induced near-field variation.24, 34 However, it is still challenging to fabricate metallic nanostructures with precisely controlled gap size down to the sub-nanometer scale with Raman molecules simultaneously trapped inside the gaps. ACS Paragon Plus Environment

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Recently, plasmonic gap-enhanced Raman tags (GERTs), which are composed of a metallic coreshell nanoparticle with an internal nanogap, have found great potential in biosensing and bioimaging applications.35-49 Compared to the nanogaps randomly formed in nanoparticle aggregates, GERTs provide uniform and stable SERS signals by loading Raman molecules in the nanogap “hot spots”,50, 51 and potentially have minimal photothermal damage to biological tissues.52 Such interior nanogaps are typically filled by a dielectric layer, such as silica,19, 46 DNA,39, 40 polymer,38 and dithiol molecules,36, 37 generally with a thickness down to a few angstroms. This makes GERTs a model system (in addition to metallic nanoparticle dimers widely used in the literature) well suited for investigation of the quantum mechanical effects in plasmonic nanostructures. Calculations using time-dependent density functional theory (TDDFT) have predicted the emergence of pronounced quantum electron tunneling across a 0.5 nm nanogap formed in a gold nanoparticle dimer, which significantly changes the far-field absorption cross-section of the dimer and the near-field enhancement in the nanogap.23 However, such nanogap in the dimer is quite different from the gap between the core and shell of a GERT. The nanogap in a metallic nanosphere dimer is essentially zero-dimensional (0D) with only one small junction between the two nanospheres (Figure S1a). The gap in a metallic nanocube dimer is two-dimensional (2D) (Figure S1b), with a planar interface between the two nanocubes. In these two systems, charge transfer may occur when the incident light is polarized along the dimer axis. In contrast, the nanogap in GERTs forms a three-dimensional (3D) hot spot, which has a spherical interface and quite large interaction area (Figure S1c). Because of the spherical symmetry of the GERT structures, the gap mode excitation and the charge transfer are expected to be independent on the polarization of incident light. Consequently, the 3D nanogap in GERTs may be expected to produce quite different far-field and near-field optical


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response compared to the 0D and 2D hot spots. Some theoretical studies have discussed the quantum plasmonic properties of 3D vacuum nanogaps,69 which still call for experimental verification. We have previously synthesized gold (Au) GERTs with 1,4-benzenedithiol (1,4-BDT) molecules embedded in their interior nanogaps, and studied electron transport through the molecular junction of the GERTs by monitoring the gap mode in the far-field extinction response.37 However, using far-field spectroscopies to probe quantum plasmonic effects such as electron transport may suffer from low signal-to-noise ratios and spectral broadening of the gap mode due to a relatively wide distribution of nanoparticle size. Therefore, investigating the electron transport induced plasmonic near-field variation with SERS spectroscopy may be a more convincing approach, complementary to far-field spectroscopy. In addition, the GERTs we studied previously had a fixed gap size of ~0.7 nm, which makes it impossible to investigate the gap size dependence of electron transport through the embedded conductive molecules. Recently, we have demonstrated successful synthesis of GERTs with a variety of gap sizes, taking advantage of the formation of multi-layers of molecules in the nanogaps.53 Given a long-enough incubation time, dithiol molecules can form multilayers on the surface of gold through disulfide bonds,53-55 which allows tuning of the thickness of the assembled molecular layers and consequently control over the size of the interior gaps after shell growth. This enables us to study the molecule-assisted electron transport through plasmonic molecular nanogaps with tunable gap size. In this work, we study the far- and near-field optical properties of Au GERTs with built-in gaps ranging from 0.7 to 2 nm, which are precisely tuned by controlling the immobilization of Raman molecules 1,4-BDT on the surface of Au cores. Measurements of extinction and SERS spectra jointly demonstrate the occurrence of electron transport in the GERTs with ultra-small interior gaps, which is consistent with the calculations considering the molecular junction conductance in a quantum-corrected ACS Paragon Plus Environment

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model (QCM). Our GERTs provide a platform for investigating charge transport induced by the molecular junction in sub-nanometer scale plasmonic molecular gaps and could potentially facilitate development of quantum plasmonic devices. Our findings may provoke re-consideration of SERS probe design and fabrication methods by optimizing various structural parameters such as inter-particle distances. RESULTS AND DISCUSSION

A GERT consists of a spherical Au core and a concentric Au shell with a built-in nanogap, which is occupied by a self-assembled 1,4-BDT molecular layer to form a nanoscale molecular junction.56 We fabricated GERTs and established a standard protocol to tune their interior gap size. Uniform Au cores with an average diameter of 18 ± 2 nm were prepared and then 1 mM 1,4-BDT was added into the Au core solution to dilute the 1,4-BDT concentration to be 50 µM. The amount of 1,4-BDT added was in great excess of that needed to form monolayers on the Au cores. Therefore, either monolayer or multilayer 1,4-BDT could be formed on the Au cores, depending on the incubation time in the experiment (Figure 1a). The refractive index change of the surrounding environment induced by the molecular decoration causes a redshift in the localized surface plasmon resonance (LSPR) energy of the Au cores.57 Thus, we are able to perform a real-time monitoring of 1,4-BDT immobilization on the Au cores by measuring their LSPR spectra.53 Here, the LSPR peak shift of the mixture of Au cores and 1,4BDT molecules was recorded with an UV-Vis spectrometer. As shown in Figure 1b and 1c, the extinction peak of the Au cores exhibits a continuous redshift during 96 h incubation with 1,4-BDT. From the data recorded in the initial 4 h (inset of Figure 1c), we observe that the LSPR shift can be divided into two stages. In the first stage, the redshift immediately appears as soon as 1,4-BDT is added ACS Paragon Plus Environment

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into the Au core solution and reaches a ~3 nm redshift within 30 min; in the second stage, the redshift rate dramatically decreases and gradually reaches an amount of over 9 nm in the next 96 h (Figure 1c). These two stages are governed by different mechanisms. The former is due to a rapid chemical interaction between 1,4-BDT and the Au surface, i.e. formation of Au-S bonds, which creates selfassembled 1,4-BDT monolayers; the latter is supposed to be induced by the formation of disulfide bonds between 1,4-BDT molecules, which drives the growth of molecular multilayers. In contrast, we also used another Raman reporter molecule 4-methylbezenethiol (4-MBT), which has a very similar molecular structure to 1,4-BDT except it lacks a second thiol group and therefore cannot form multilayered structures. During the 52-h incubation of the Au cores and 4-MBT, we observe no subsequent redshift after the initial adsorption of 4-MBT on Au (Figure S2). This result further confirms that the continuous immobilization of 1,4-BDT molecules is driven by the formation of disulfide bonds.

Figure 1. (a) Schematic molecular packing structure on the Au core surface with different packing density and number of molecular layers. (b) Extinction spectra and (c) LSPR shift of bare Au cores during 96 h incubation with 1,4-BDT molecules. The inset in (b) is a TEM image of a Au core used in our experiment. Scale bar is 20 nm. The inset in (c) enlarges the region enclosed by the dashed rectangle. (d) Schematic illustration for the release of 1,4-BDT molecules from CTAC micelles. After 96 h incubation, the LSPR redshift gradually slows down and saturates. This indicates the adsorption of 1,4-BDT molecules reaching equilibrium. Taking into consideration the fact that thiol groups are easily oxidized, this LSPR redshift lasting for 96 h is not a common phenomenon.54, 58 We ACS Paragon Plus Environment

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thus presume that the existence of CTAC micelles in the mixture solution of Au cores and 1,4-BDT may be a contributing factor during the molecular assembly process (Figure 1d). Initially, a number of 1,4BDT molecules are located in the interior or at the interface of CTAC micelles due to their hydrophobicity;59, 60 as the 1,4-BDT molecules are gradually immobilized on the Au cores, extra 1,4BDT molecules are released from the micelles until the adsorption process reaches equilibrium. The oxidization of 1,4-BDT can be partially minimized by the existence of CTAC micelles and thus the formation of 1,4-BDT multilayers can continue for 96 h. The multilayer structure of 1,4-BDT molecules constructed by disulfide bonds is robust and stable, and is not easily destroyed by subsequent washing treatment. This is further confirmed by the observation that the LSPR of Au cores decorated with 1,4BDT molecules (for 48 h) remains nearly constant for 6 days after washing with H2O or 0.2 M CTAC (Figure S3). Previously, it was reported that 1,4-BDT molecules physically adsorbed on a Au surface could be washed away by high-concentration surfactants (e.g., CTAC).61 Herein, the sample washed with 0.2 M CTAC solution showed only a 0.8 nm blueshift of the LSPR peak, corresponding to 18.0 MΩ-cm) was used for all experiments.

Characterizations. Raman spectra were measured on three different spectrophotometers: Renishaw Invia reflex Raman spectrophotometer; Horiba Scientific LabRAM HR Evolution Raman spectrophotometer; a home built Raman microscope equipped with a spectrometer (SR-500, Andor, Northern Ireland), a back-illuminated deep-depletion CCD detector (Andor, Northern Ireland) and a 785 nm single mode diode laser (20 mW, Innovative Photonic Solutions, America). TEM images were obtained from a JEM-2100F Transmission Electron Microscopy (JEOL, Japan) operated at 200 kV. UVACS Paragon Plus Environment

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Vis spectra were collected on a UV1900 UV-Vis spectrophotometer (Aucybest, China) with a data interval of 0.1 nm.

Synthesis of molecule-decorated Au cores. The CTAC-capped Au cores were synthesized using our reported method.56 The obtained Au cores (1 nM) were washed once to remove excess CTAC and then re-dispersed in water. 1,4-BDT powder was dissolved in ethanol. Then 50 µL of 1 mM 1,4-BDT molecule solution were slowly added to the 1 mL of Au core (1 nM) solution under vigorous ultrasonication. The mixtures were left for incubation.

Synthesis of Au core-shell GERTs. The molecule-modified cores were washed with water three times to remove excess molecules. The Au core-shell nanoparticles were prepared by adding 190 µL of molecule-modified core solution into the mixture of 4 mL CTAC solution (0.1 M), 200 µL of ascorbic acid (0.04 M), and 200 µL of HAuCl4 (4.86 mM) under vigorous ultra-sonication. Finally, the obtained Au GERTs were washed and stored in CTAC solution (0.1 M).

Study of the GERTs Au shell Growth Evolution. The intermediate morphologies of the growing nanostructures were recorded by quenching the reaction using liquid nitrogen using a reported method.78 After Au cores were introduced into the reaction solution during the synthesis of Au core-shell GERTs, a holey carbon coated copper grid was quickly immersed into the mixture at different reaction time, and then immediately put into liquid nitrogen to quench the reaction process by dramatically decreasing the temperature. After an overnight freeze-drying, these copper grids were used for further TEM characterization.


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Nanogap electromagnetic field calculation. We calculated the electromagnetic field distributions of GERTs particles using the commercial full wave simulation software (COMSOL Multiphysics). The radius of core and GERT particle was 9 nm and 25 nm, respectively. The interior nanogap was formed between Au core and shell, and the gap size was adjusted from 0.7 to 2.4 nm. The mesh used in the simulation is fine enough to make sure all the results are converged.

Acknowledgement This work was support by National Natural Science Foundation of China (Nos. 81571763, 81622026, 11474240 and 21511130019), Shanghai Jiao Tong University (Nos. YG2016MS51 and YG2017MS54), the Hong Kong Polytechnic University (No. 4-BCCB) and the Grant from the Shanghai Key Laboratory of Gynecologic Oncology. The author would like to acknowledge Dr. B. Thackray for his kindhearted help in revising the manuscript. Supporting Information: Supporting Information is available free of charge on the ACS Publications website. Schematic of nanogaps formed in metallic junctions, LSPR measurements, TEM images of GERTs during the shell growth, gap size calculations, Raman measurements, and more CEM and QCM simulation results.

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Electron Transport Across Plasmonic Molecular Nanogaps

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