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Molecular Wires Bridging Gaps Between Gold Surfaces and Its Influence on SERS Intensities Klester S Souza, Diego P. dos Santos, Gustavo Fernandes Souza Andrade, Marcelo Barbalho Pereira, Erico Teixeira-Neto, and Marcia L.A. Temperini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04498 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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The Journal of Physical Chemistry

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Molecular Wires Bridging Gaps Between Gold

2

Surfaces and Its Influence on SERS Intensities

3

Klester S. Souza,1,† Diego P. dos Santos,2 Gustavo F. S. Andrade,3 Marcelo B. Pereira,4

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Erico Teixeira-Neto,5 and Marcia L. A. Temperini*,1

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1 –Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, São Paulo, Brazil.

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2 – Departamento de Físico-Química, Instituto de Química, Universidade Estadual de Campinas, Campinas, Brazil

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3 – Laboratório de Nanoestruturas Plasmônicas, Núcleo de Espectroscopia e Estrutura Molecular (NEEM), Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, Brazil

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4 – Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil

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5 – Brazilian Nanotechnology National Laboratory, Brazilian Center for Research in Energy and Materials, Campinas, Brazil

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ABSTRACT: Molecular wires of the Oligophenyleneimine (OPI) families where used

16

as bridging gaps between gold flat surface and gold nanorods, forming molecular junction

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systems as (AuFlat|OPI|AuNR). Systems with different gap sizes were synthesized from

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2.2 nm for the OPI-1p to 9.9 nm for the OPI-13p (where n is equal to the number of

19

phenylene group) and the intensities of the SERS bands at 1078 cm-1 v(CS) and 1168 cm1

20

β(CH) were obtained for each gap length. Our results showed an unusual behaviour for

21

the bands 1078 cm1 v(CS) and 1168 cm-1 β(CH) as function of OPI (gap) size. To address

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these results electromagnetic field simulations by Discrete Dipole Approximation (DDA)

23

method for the systems (AuFlat|Gap|AuNR) were performed. Nevertheless the high

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SERS intensities observed for (AuFlat|OPI|AuNR) with large gap sizes for excitation at

25

785 nm indicated that there is a strong dependence on the electronic properties of the 1

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molecular wire, which supersedes the electromagnetic contribution of the plasmonic

2

coupling. The experimental and simulated results indicated that both electromagnetic

3

(dipole-image interaction and surface plasmon resonance) and molecular properties are

4

contributing to the SERS intensities behaviour. Additionally, it has been noticed that the

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length of the molecular wire that resulted in a decrease in SERS intensity is coincident

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with the reported length in which the transition from tunneling to hoping conduction

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occurs for OPI molecular wires.

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INTRODUCTION

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The first observation of Raman bands of pyridine adsorbed on silver electrode was

10

possible after roughening the electrode surface by oxidation reduction cycles.1 Three

11

years after this first experiment, van Duyne and Creighton groups demonstrated

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independently that this observation was due to the enhancement of Raman bands of

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adsorbed species near metal nanoparticles.1–3 This effect was termed surface-enhanced

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Raman scattering (SERS) and achieved an extensive range of studies and applications

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since its discovery.4–10 It has been generally accepted that two different mechanisms are

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involved in the SERS effect, electromagnetic enhancement (EM)11,12 and chemical

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enhancement (CE).13,14

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The EM enhancement is responsible for most of the SERS intensity12 and is due to a

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concentration of electromagnetic field in the vicinity of a metal surface when illuminated

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by light with wavelength that can excite localized surfaces plasmon modes on

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nanostructured metals. The local field enhancement promoted by surface plasmon

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resonances is specially significant in regions between highly interacting nanostructures,

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named hot-spots (nanoscale gaps or nanoholes) and the SERS signal from molecules in

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such regions dominates the overall detected intensity.15 The CE contribution originates 2


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from the complex interaction between the adsorbed molecule, the substrate and the

2

incident wavelength. The chemisorption interaction increases the polarizability of the

3

molecule, which can be further increased by resonant charge transfer (CT) between the

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adsorbate and substrate induced by the incident radiation.16–19

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SERS intensity is greatly dependent on the distance of the molecule from the metal

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surface. This SERS intensity behavior is due to a steep decrease of the electromagnetic

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field away from the metal nanoparticles surface. For molecules between two

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nanoparticles (sandwiched systems), the observed SERS intensity strongly depends on

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the gap distance and on the orientation of the adsorbate within the gap.20,21

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It has been proposed for sandwiched systems comprised of nanoparticles and flat

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surfaces that the SERS intensities are originated mainly from electromagnetic coupling

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between the dipole moment on the nanoparticle (due to surface plasmon excitation) and

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the induced dipole on the surface of the underlying continuous metal film. 22–25 In the last

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years, considerable effort has been made to describe the observed effects on the optical

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response (plasmon resonance wavelength shifts and scattering profiles) of nanoparticles

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on similar sandwiched situations.26,27 The majority of the results presented in the literature

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for nanoparticle–flat surface optical coupling are for gold nanosphere26-28 and gold

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nanorods.29–33 For the latter the coupling becomes more complex due to the anisotropic

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nature of the Au nanoparticles.

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In molecular wires the molecular species of sandwiched systems should be conjugated

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molecules in which charge transfer is very efficient even over long distances.34,35 The

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most important property of such molecules is the electrical transport processes when

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sandwiched between two electrodes, so that they are referred to as molecular junctions.

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Several strategies for building molecular junctions and characterize their electrical

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properties have been reported.36–39 The most highly regarded way to characterize the 3



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conduction regime in such molecules is to connect them between electrodes so that their

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current-voltage (I-V) characteristics can be recorded directly. This approach has been

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used in the literature for studying the electrical transport characteristics of molecular

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wires up to 10 nm.39–42 From those studies, it has been found that for shorter molecular

5

wires the electronic conduction occurs predominantly based on incoherent tunneling, but

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as the molecular wires become longer (> 4 nm), a transition occurs to a hoping

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conduction. The length dependence on the electrical resistance of the wires is a direct

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consequence of the charge transport mechanism.

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theoretically and experimentally 29,30 that for short molecular wires the resistance scales

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exponentially with length (tunneling regime), while for longer molecular wires the

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dependence is linear (hopping transport). The decrease in the conjugation length as the

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wires become longer results in the transition from the tunneling to the hopping charge

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transport; determining the transport mechanism transition would be highly important in

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understanding the performance of molecular wires as a function of wire length.

27,28

It has been pointed out both

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Amongst the molecular wires, seldom studies on oligophenyleneimines (OPI) of

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different lengths have been reported39,41,43,44 The precursor of OPI, 4-aminobenzenethiol

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(4ABT), has been subject of a long discussion in the literature regarding the nature of

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SERS bands at 1140, 1390, 1430 and 1570 cm-1 observed only in the SERS spectra of the

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substance.5,16,45,46 Osawa et al. and Kim’s group have made their point that the observation

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of the new bands is actually assignable to the enhancement by charge-transfer (CT) SERS

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mechanism of very weak bands, assigned to modes with b2 symmetry of the 4ABT

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adsorbate.46–48 On the other hand, Tian’s group have proposed that a laser oxidative

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coupling of 4ABT could result in the formation of 4,4’-dimercaptoazobenzene

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(DMAB).16,49 To the best of our knowledge, no SERS study has been performed on OPI

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of higher order. Studies related to the SERS effect and relative intensity behavior could 4


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be a very powerful way to monitor the changes in molecular structure of the OPI wires,

2

both from the available chemical moieties characteristic bands, as from the SERS

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enhancement changes with the increase in wire length.

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In this work, we present studies for OPI as bridges in the gap between gold flat surface

5

and gold nanorods (AuFlat|OPI|AuNR). Systems with different gap sizes were

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synthesized from 2.2 nm for the OPI-1p to 9.9 nm for the OPI13p (where n is equal to the

7

number of phenylene groups), which had the molecular wire lengthening followed by

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ellipsometry measurements. The intensities of the SERS bands in 1078 cm-1 v(CS) and

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1168 cm-1 β(CH) were monitored for each gap length. To support the experimental SERS

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results, the electromagnetic field in sandwiched systems (AuFlat|Gap|AuNR) with

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different gap sizes was simulated by the Discrete Dipole Approximation (DDA) method

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to understand the dependence of the local electric field enhancement and the degree of

13

coupling between the gold nanorods and the gold flat surface on the expected SERS

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intensities of OPI-np. Finally, DFT calculations of the OPI-np connected to Au clusters

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were performed to support SERS frequency assignment and also to verify the changes in

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polarizability of the molecular wire with increasing length.

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METHODS

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Chemicals

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4-aminobenzenethiol (4-ABT), Terephthal-dicarboxaldehyde (TDA, 99%), 1,4-

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diamino-benzene (DAB, 98%), Ethanol (99%), Hexadecyl-trimethylammonium bromide

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98%, L-ascorbic acid, and Sodium Nitrate were purchased from Sigma-Aldrich, Sodium

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borohydride (99%) were purchased from Fluka. Deionized water (18.1 MΩ cm) was used

24

in all experiments.

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Synthesis of gold nanorods

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Gold nanorods (AuNR) were prepared by “seed mediated growth method”.50 Briefly,

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seed nanoparticles (NP) were prepared by mixing 5.0 mL of CTAB solution (0.20 M)

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with 5.0 mL of HAuCl4 (0.50x10-3 M). To the stirring solution, 0.60 mL of icecold

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N-aBH4 (0.010 M) was added, which resulted in the formation of a brownish yellow

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solution. Vigorous stirring of the seed solution continued for 2 min and then the solution

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was kept at 25 °C for 5 min prior to use. For the growth of the NPs, 5.0 mL of CTAB

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solution (0.20 M) was added to 0.20 mL of AgNO3 (4.0×10-3 M). To this solution, 5.0 mL

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of HAuCl4 (1.0x10-3 M) was added, and after gentle mixing of the solution 70 µL of

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ascorbic acid (0.08 M) was added. The final step was the addition of 12 µL of the seed

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solution to the growth solution at 30 °C for 15 min. The colloidal solution was purified

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by three centrifugation cycles (10,000 rpm for 15 min) in water to remove excess of

13

CTAB in the solution.

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Synthesis of OPI wire on gold surfaces: a) On gold flat substrate (AuFlat|OPI wires)

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Scheme 1 shows a molecular structure of the Oligophenyleneimine (OPI) and the

17

synthetic route of this class of compounds on the Au surface. The AuFlat surfaces were

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prepared by the gold evaporation on mica surfaces at 300 °C, the gold thickness were

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180-200 nm. The OPI wires were synthesized according to reported methods.40,51 The

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growth of the OPI wire starts by immersing the gold flat substrate in 1.0 mM of

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4-aminobenzenethiol (4-ABT) in absolute ethanol for 24 h, this procedure forms OPI1p.

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The OPI wires were then grown by step-wise imination, with alternating addition of

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terephthal-dicarboxaldehyde (-TDA) followed by 1,4-diamino-benzene (DAB), as shown

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in Scheme 1, forming OPI-np wires, where ‘np’ stands for the number of phenylene

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groups. For both TDA and DAB, 20 mM solution in absolute ethanol has been used and 6


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the sample was immersed for 12 h in each solution of the reactants. After each growth

2

step the substrate were thoroughly rinsed with absolute ethanol and then dried up in a

3

stream of N2(g).

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Scheme 1. Synthetic route and molecular structure of Oligophenyleneimine (OPI) monolayers on gold surface. b) Formation of the AuFlat|OPI|AuNR substrate

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To synthesize the sandwiched substrate the AuNRs were deposited on the AuFlat|OPI

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by drop casting 10 µL of a diluted AuNRs colloidal solution followed by rinsing with

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absolute ethanol and drying in a stream of N2(g). All SERS analysis were performed

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within the central area of the AuNRs deposition ring after droplet has dried out, so it was

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possible to make sure that all spectra were from region with an AuNRs sub-monolayer.

13

This system is referenced as AuFlat|OPI|AuNR.

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The AuFlat|OPI-np|AuNRs were characterized by SEM images, presented in Figure S1

2

(see Supporting Information file); the SEM images indicate the AuNRs coverage were

3

well below monolayer coverage on the AuFlat|OPI surface

4

Synthesis of the OPI-2p ex-situ (off gold surface)

5

The OPI-2p was synthesized according to reported methods.51 4ABT (0.75 g, 6 mmol)

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and excess of TDA (1.6 g, 12 mmol) were reacted in ethanol (50 mL) for 3 h, at ambient

7

temperature with continuous stirring. The solvent was removed under vacuum and the

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pale yellow product was recrystallized from ethanol and washed with the same solvent:

9

yield, 85%; mp 2-15 °C. Anal. Found: C, 68.5; H, 4.58; N, 5.94. Calculated for

10

C14H11NOS: C, 69.69; H, 4.60; N, 5.81.

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Characterization

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The gold nanorods were characterized by UV-VIS-NIR and electron microscopy. The

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extinction spectra of colloidal aqueous suspension of AuNR were carried out using a

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Shimadzu UVPC-3101. Scaning Electron Microscopy – Field Emission Gun (FEGSEM)

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images were obtained using a JEOL JSM-7401F field emission electron microscope

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operating at 5 kV. The transmission electron microscopy (TEM) images were obtained

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with a JEOL JEM 2100 microscope operated at 200 kV.

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FT-Raman spectra of commercial and synthesized compounds were recorded in a FT-

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Raman Bruker RFS 100 spectrometer with a liquid nitrogen cooled Germanium detector,

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and 1064 nm excitation radiation (Nd:YAG laser, Coherent Compass 1064-500N). The

21

spectra of the solids were obtained at 150 mW and accumulation of 512 scans. The SERS

22

spectra were obtained in a Renishaw InVia Reflex coupled to a Leica DM2500M

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microscope, with the He–Ne excitation laser line at 632.8 nm and diode laser with

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emission at 785 nm; laser power remained at ≈1 mW during SERS measurements. The

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SERS spectra were measured using a 100× objective (NA = 0.9) with an exposure time 8


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of 10 s. All intensities discussed for Raman and SERS experiments have been measured

2

as height of the bands and for SERS analysis 1000 SERS spectra were obtained for each

3

system.

4

The OPI-np films were also characterized with Spectroscopic Ellipsometer SOPRA

5

GES-5E. All measurements were made in the wavelength range from 350 nm to 800 nm

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(angle of incidence of 68°), using a microspot accessory (numerical aperture of 3º) that

7

focuses the light beam in a small region of the film surface (approximately 365 µm x

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270 µm).

9

Discrete Dipole Approximation (DDA) simulations

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The plasmonic properties of the AuNR on the surface of a flat Au substrate were

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simulated according to the DDA method, using the software developed by Draine and

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Flatau.52,53 In all simulations, the optical properties of Au were described in terms of the

13

experimental values for the dielectric function made available by Johnson and Christy.54

14

The Au flat substrate was approximated to a cylindrical slab of 250 nm diameter and

15

30 nm thickness, following the criteria proposed by Malinsky et al to describe the effect

16

of a (dielectric) substrate on the plasmonic properties of the AuNR simulated by the DDA

17

method. Since the substrate is metallic and is represented by a finite nanometer sized Au

18

cylinder, it is reasonable that such structure presents an optical response in the simulated

19

spectra. However, this contribution is constant and it had not negatively influenced the

20

simulated spectral changes due to the AuNRs.

21

Figure 1 shows a representation of the DDA simulation setup in terms of the array of

22

point dipoles. In all simulations, the distance between such dipoles was taken to be 1 nm.

9



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Figure 1. Array of point dipoles used in DDA simulations. The incident field propagation

3

direction is perpendicular to the cylinder surface and the electric field vector is parallel to

4

the AuNR long axis.

5

DFT calculations

6

To support Raman e SERS characterization of molecular wires OPI-1p and OPI-2p DFT

7

calculations have been performed. DFT optimized structures for the OPI-np were also

8

used to calculate the volumetric polarizability of the OPI wires in order to verify the

9

contribution of the number of phenyleneimine units in the Raman intensity of the wires.

10

DFT calculations have been performed using Density Functional Theory as

11

implemented in the Gaussian09 suite of programs, using the hybrid functional

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B3LYP.55,56 The basis set for OPI-1p, OPI-2p and OPI-3p was the split valence triple-ζ

13

6-311G(d) for C, H, N, S, O, and the pseudo-potential core double-ζ LANL2DZ basis set

14

for Au. All optimizations have performed with no geometry constraint. To simulate the

15

influence of the Au surfaces on the Raman spectra of OPI, the molecules have been set

16

to interact with an Au10 cluster, which had the geometry optimized before adding to the

17

optimization of the complex with OPIs. The obtained Raman activity spectra were

18

corrected for finite temperature and scattering dependence on frequency before plotting.57

19

In the calculation of the volumetric polarizability, the OPI-np (n=1-13) were optimized

10


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using the double-ζ 3-21G(d) for all OPI atoms, and the calculated molecular volume was

2

used to calculate the property.58

3 4

RESULTS AND DISCUSSION

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AuNR were characterized by electron microscopy and UV-Vis spectroscopy and

6

the results of such characterization are presented as supporting information in the SI file

7

(See, for instance, Figures S1 and S2 and the related discussion). The AuNR synthesis

8

yielded low dispersion of nanorods with aspect ratio 3 (longitudinal and transversal

9

dimensions of 45±5 nm and 15±2 nm, respectively).

10

Characterization of the OPI-1p and OPI-2p

11

The FT-Raman spectra of the OPI-1p and OPI-2p are presented in the Figure 2A,

12 13

Figure 2. (A) Raman of the OPI-1p and 2p in solid state (λ0= 1064 nm), (B) DFT of the

14

OPI-1p and 2p in isolated form.

15

It is possible to observe that when there is an increase in the OPI wire units there is a

16

decrease in the relative intensity of the band at 1078 cm-1 assigned to v(CS) (Table S1)

17

and an increase in the relative intensity of the aromatic ring bands. In the 1600 cm-1 region

18

together with the aromatic v(C=C) stretching there are bands due to iminic v(CN) and

19

aldehydic v(CO) characteristic chemical group wavenumbers.

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Figure 2B shows the DFT Raman spectra of OPI-1p and OPi-2p. The comparison

2

between experimental and DFT calculated spectra show a qualitative agreement in the

3

observed spectral changes discussed above.

4

To confirm that, the OPI-2p synthesized on gold surface refer to the expected molecule,

5

OPI-2p was also synthesized ex situ (off gold surface) and its SERS spectrum was

6

obtained. Comparing this spectrum to that of OPI-2p synthesized on gold surface we can

7

see that they have a similar pattern (Figure S3), indicating that the synthesis of OPI-2p

8

on gold was successfully performed. In Figure S4 the SERS spectra of OPI-1p to OPI-3p

9

in AuNR colloidal solutions and their calculated spectra are presented in order to confirm

10

molecular wire growth and the frequencies assignment (Table S1). These results strongly

11

indicate that the synthesis is generating progressively longer molecular wires of different

12

sizes. The above discussion suggests the possibility of using such systems to generate

13

controlled gap distances between Au surfaces, such as AuFlat|OPI|AuNB.

14

Molecular wires bridging gaps between gold surfaces

15

To study the influence of the gap distance in the SERS spectra of AuFlat|OPI|AuNR

16

molecular junction system, OPI wires with different lengths were synthesized. The gap

17

distance varies from 2.2 nm (OPI-1p) to 9.9 nm (OPI-13p) calculated using the theoretical

18

values of the length of the molecular wires and the length of the CTAB monolayer under

19

the AuNR.29,59

20

Ellipsometry measurements were performed to confirm the growth of the films and it

21

was found strong evidence of the molecular wire growth with the click-reaction steps.

22

The results and discussion on the evidence for wire growth are presented in the SI file,

23

Figure S5 and text associated to it.

24 25

Figure 3 shows a diagram of the systems and the average SERS spectra for some of the synthesized molecular wires. 12


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Figure 3. Schematic diagram and SERS spectra (λ0=785 nm) of the molecular junction

3

systems (AuFlat|OPI-np|AuNR). The vertical bar indicates the SERS intensity and is valid

4

for all the spectra.

5

As shown in Figure 3, there is an increase in the relative intensity of the band at ca.

6

1168 cm-1, assigned to β(CH), relative to the band at ca. 1078 cm-1, assigned to v(CS)

7

(both wavenumber values refer to OPI-2p, to keep discussion concise), with the increase

8

of the molecular length, until OPI-5p (4.7 nm). For systems with gap lengths larger than

9

6.0 nm (OPI-7p), it has been observed a decrease in the overall intensity and a broadening

10

of the bands in the SERS spectra.

11

To better understand the variation in the relative intensities of the 1078 cm-1 [v(CS)]

12

and 1168 cm-1 [β(CH)] bands with molecular length, the molecular wire systems with

13

increasing lengths were studied using SERS at λ0= 632.8 nm and 785 nm. Figure 4 shows

14

the SERS intensities of the bands at 1078 cm-1 and 1168 cm-1 as a function of the gap

15

distance. 13



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Figure 4. SERS intensity of the bands 1078 cm-1 and 1168 cm-1 for the systems

3

AuFlat|OPInp|AuNR- measured using A) λ0= 632.8 nm and B) λ0= 785 nm and the gap

4

sizes.

5

The SERS intensities in Figure 4A and B may be separated in two groups, the first

6

group containing the wires from OPI-1p to OPI-5p (gap size from 2.2 nm to 4.7 nm) and

7

the second one going from OPI-7p to 13p (gap size 5.9 nm to 9.9 nm). In the first group,

8

it can be observed a small increase in the intensity of the band 1078 cm-1 at 632.8 nm

9

excitation radiation (Figure 4A) whereas the intensity of this band is almost constant at

10

785 nm. For both exciting radiations, the intensity of the band at 1168 cm-1 increases for

11

this first group until OPI-3p, and remains constant from OPI-3p to OPI-5p. For molecular

12

wires longer than OPI-5p, which constitute the second group of intensities, it is possible

13

to observe a decrease in SERS intensities for both vibrational bands and both exciting

14

radiation. Another behaviour observed in Figure 4 is the overall lower SERS intensities

15

values for 785 nm compared to the intensities for excitation at 632.8 nm.

16

SERS Intensity behavior for OPI-np

17

a) DDA simulations

18

Seeking for a deeper understanding of the observed experimental results (intensity

19

profile as function of wire length and overall larger intensity for 632.8 nm in respect to

20

785 nm excitation), DDA simulations of AuNR on Au substrate and air as dielectric 14


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environment were performed for gap lengths of 2 nm to 10 nm. Since the wire length

2

leads to different gap sizes separating the nanorod and the flat surface, which in turn

3

results in different electromagnetic interactions, a tentative explanation of the

4

experimental results could be performed in terms of the electromagnetic mechanism of

5

SERS, which can be exploited by DDA simulations. In this section we present a detailed

6

description of the optical properties of the AuNR in each system. Figure 5 presents the

7

results for the absorption and scattering spectra.

8 9 10

Figure 5. Absorption (A) and scattering (B) DDA simulated spectra for different gap distances between AuNR and the Au substrate surface.

11

The DDA absorption spectra in Figure 5A show that the AuNR longitudinal surface

12

plasmon resonance presents a redshift as the distance to the flat surface is decreased,

13

which is in accordance to the literature.21,23–25,35,60–62 The redshift reflects the progressive

14

increase in the effective dielectric function of the medium surrounding the AuNR;

15

additionally, the inset presents a fit to the simulated data, that allows predicting the shift

16

of the resonance to 830 nm for a 2 nm gap. There is a contribution at ca. 800 nm, which

17

corresponds to the Au cylinder slab that has been employed as part of the simulation

15



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Page 16 of 29 1

model (see section 2.6 and Figure 1), and must not be considered in the following

2

discussion.

3

In Figure 5A it is also shown the incident laser wavelength (0 = 632.8 nm) for the

4

SERS experiments and a representation for the scattered light wavelength at the Stokes

5

side of the SERS spectrum (Stokes) by vertical dashed lines. The value 680 nm represents

6

the wavelength of a SERS band at ~1100 cm-1, which is in between the two frequencies

7

of the analyzed bands in the experimental results Figure 4. The simulated results show

8

that surface plasmon resonances are redshifted with respect to the incident laser

9

wavelength in the SERS experiments, i.e. the resonances are at the Stokes side of the

10

spectra. For a SERS band close to 1100 cm-1 (680 nm), such scattered radiation is in

11

resonance with the surface plasmon for a gap size of 6 nm. Once SERS intensities depend

12

on the incident and scattered light wavelength with respect to the surface plasmon

13

resonance wavelength, the above result indicate that SERS intensities do not necessarily

14

decrease with the increase of molecular wire size. We will address this point later.

15

The simulated scattering spectra of Figure 5B show a damping in the scattering

16

efficiency at the AuNR surface plasmon resonances, which indicates an interference

17

interaction between the AuNR and the Au substrate. This is also in accordance to previous

18

reports,33 and it is due to a dipole-image interaction for the longitudinal dipolar plasmon

19

mode (parallel to the substrate surface) and the induced image dipole in the substrate.29

20

In such interaction, the real and induced dipoles are out of phase, which leads to an

21

electric field coupling between the opposing charge densities (in the AuNR and in the Au

22

substrate). This coupling leads to a concentration of electric field between the AuNR and

23

the Au flat surface (see Figure 6), which directly contributes to the observed SERS

24

intensities. The strength of electromagnetic coupling between such dipoles is larger for

25

smaller gap distances. Therefore, we could expect two contributions to the total SERS 16


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signal: (I) the resonance condition (between surface plasmon and incident and scattered

2

radiations) and (II) the strength of electromagnetic coupling between the real and induced

3

dipoles. To account for such effects, DDA simulations for the SERS enhancement factor

4

(EF) were carried out, according to Eq. 1:63

5

EF = [

𝐸loc (632.8nm) 2 𝐸loc (680nm) 2 ] [ (680nm) ] 𝐸0 (632.8nm) 𝐸0

Eq. 1

6

where Eloc and E0 represent the amplitude of the local electric field in the presence and

7

absence of Au structures (both AuNR and Au substrate), respectively. The two terms of

8

Eq. 1 represent the incident and (Stokes) scattered electric field enhancements,

9

respectively. Figure 6 presents the results for EF simulations for different gap distances.

10 17



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Page 18 of 29 1

Figure 6. (A-C) Local EF maps between a gold nanorod and gold flat for gap distance

2

2 nm, 6 nm and 10 nm. D) EF profiles along the dashed lines of (A-C), i.e., close to the

3

Au cylinder slab surface that represents the flat Au substrate; the dashed line represents

4

an approximate dimension of the AuNR. E) Average EF on the Au slab surface (red) for

5

different gap distances, in comparison to the usual E4 approximation. The EF maps were

6

all simulated for electric field excitation parallel to rod long axis.63,64

7

Figure 6 A-C show maps of the simulated EF for gap sizes of 2, 6 and 10 nm, which

8

correspond to the approximate gap sizes created by OPI-1p, OPI-7p and OPI-13p,

9

respectively, in the experimental conditions. The dashed horizontal white lines in the EF

10

maps in Figure 6A-C represent the region on the surface of the Au substrate (Au slab)

11

where further analysis was performed. For a 2 nm- gap (Figure 6A), the highest EF values

12

are concentrated in the gap region and the maximum EF values are observed very close

13

to the AuNR ends. This can be better visualized in Figure 6D (black line), where the EF

14

values on the Au cylinder slab surface are presented. It is possible to observe a sudden

15

drop in the EF values extending away from the AuNR ends as well as approaching the

16

AuNR middle point region. This result is consistent with the picture of local field

17

enhancement generated by dipole-image interaction (electric field vectors in Figure 6C,

18

represented as white arrows).

19

The EF profile close to the Au slab surface (Figure 6D) indicates that the maximum

20

enhancement is observed for a gap distance of 6 nm and not for a 2 nm gap as one would

21

expect.64–68 In the case of the SERS experiments reported in this study, an important

22

figure-of-merit is the average SERS enhancement. Figure 6E shows the average EF values

23

on the surface of the Au slab (red points), here called . Note that the synthesis of

24

the molecular wires has occurred in such a way that the CS group is always on the flat Au

25

substrate surface. Therefore, the intensities for the band attributed to v(CS) at 1078 cm-1 18


Page 18 of 29

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Page 19 of 29 1

can be simulated in a first approximation, by such values. For comparison, an

2

usual approximation to calculate EF, the E4 approximation, is also presented (black).

3

For the E4 approximation it can be observed a rapid decrease in the average EF as the

4

gap size is increased. For such approximation, the considered wavelength (632.8 nm) is

5

far from any resonance and, therefore, the contribution is dominated by the decrease in

6

the strength of the dipole-image interaction as the gap increases, predicting a decrease in

7

the SERS intensities for this mode (1078 cm-1) with the length of the molecular wire.

8

Such a behavior is also present in the EF values calculated by Eq. 1 (red). However, due

9

to the resonance condition for the scattered radiation, the profile shows an

10

approximate constant behaviour between 2 nm and 6 nm gap, followed by a decrease for

11

larger gap sizes. This result is in very good agreement with the experimental results for

12

the 1078 cm-1 SERS band in Figure 4A, which would suggest an important

13

electromagnetic contribution to the SERS intensity profile of v(CS) mode.

14

Interestingly the same behaviour in the intensity profile is observed for 785 nm

15

excitation (which is far from resonance for most of the gap sizes). This result cannot be

16

explained by the electromagnetic model, suggesting another important contribution to the

17

SERS enhancement. In fact, the EFs obtained in each configuration is not very large. For

18

comparison the surface average EF for a single AuNR in water (single-particle SERS

19

configuration, where plasmonic couplings are absent) is of the order 106 (see supporting

20

information Figure S6). This shows that the dipole-image coupling does not lead to very

21

large electromagnetic enhancements. Therefore, the above results show a detailed

22

analysis that is suitable to explain the overall larger SERS intensity for 632.8 nm in

23

respect to 785 nm. Although the DDA simulations are not capable to fully explain the

24

experimental results, they are of fundamental importance in this study to rule out the

25

electromagnetic effect to SERS as the main contribution, paving the way to investigate 19



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Page 20 of 29

Page 20 of 29 1

other possibilities to explain the intensity profile as function of wire size for both

2

radiations.

3

b) Relative intensity changes of 1168 and 1078 cm-1 bands

4

The band at 1168 cm-1 presents a higher increase in SERS intensity than the band at

5

1078 cm-1 for every OPI-np SERS spectrum (this may be easily observed if the relative

6

intensity of the two bands are plotted as a function of OPI-np; see Figure S7 in the

7

Supporting Information file) for both 632.8 and 785 nm excitation. The higher increase

8

in intensity for the band 1168 cm-1 (β(CH) mode) observed in Figure 4A cannot be solely

9

described by the plasmonic electromagnetic contributions discussed above. Besides the

10

electromagnetic effect, changes in molecular electronic structure could contribute to the

11

intensity of the band. Three possible explanations for the observed experimental results

12

are:

13

1 - Increase in the number of aromatic rings with growth of the molecular wire. As this

14

band is assigned to the vibrational mode β(CH) from CH bonds of the aromatic ring, the

15

synthesis of larger molecular wires leads to an increase in the intensities of such mode

16

solely by the addition of CH bonds;

17

2 – Increase of molecular polarizability due the increasing amount of electrons with the

18

growth of the molecular wire, which could result in the increase of the Raman intensity

19

of the wires;

20

3 - A CT SERS mechanism could be operating in the OPI-np system, for both AuNR

21

colloidal solution and sandwiched system. The energy of the new electronic state that

22

characterizes the CT effect would be in resonance with the exciting radiation, resulting in

23

the enhancement of vibrational bands of certain symmetry.

24

Figure S8 presents the change of calculated volume polarizability of OPI with the

25

molecular wire length (see Supporting Information file). The values have been 20


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Page 21 of 29 1

normalized for OPI-2p (molecular length = 1.4 nm) in order to highlight the dependence

2

on the length. It is possible to notice a ca. 6 times increase in the volume polarizability

3

with the molecular wire length growing from 0.7 to 8.4 nm. Considering that the Raman

4

effect depends on the change of polarizability of a vibrational mode, it also presents a

5

dependence on the magnitude of the molecular polarizability. Similar conclusion could

6

be drawn on the consideration that the increase in intensity is due mostly to the addition

7

of CH groups to the molecular wire structure: if the addition of CH was the only effect to

8

be considered, the intensity of the 1168 cm-1 band should increase as the molecular wire

9

grows. The third hypothesis, of an operating CT mechanism, could explain the high

10

increase in the relative intensity of the band at 1168 cm-1 with the increase in the wire

11

lengths up to OPI-5p. The CT mechanism was used by Osawa et al.46 and Kim et al.43 to

12

explain the observation of the bands at 1178, 1433 and 1587 cm-1 in the SERS spectrum

13

of the 4-ABT (OPI-1p).

14

c) Abrupt decreasing in SERS intensities for large gaps

15

The experimental SERS intensity of OPI-np in Figure 4 increased up to 4.7 nm,

16

followed by a steep decrease for longer molecular lengths, which indicates that the

17

influence of polarizability would be important but for longer gap the decrease in the

18

would be mandatory.

19

The above results indicate that both electromagnetic and chemical effects are

20

contributing to the observed intensities, but the reason for the same intensity profile with

21

the gaps distance for the two exciting radiation remains an open question. It should,

22

nevertheless, be noticed that the change in the SERS intensity profile with OPI molecular

23

wires length is coincident with changes in the conduction mechanism for this family of

24

substances for both exciting radiations used in the present paper. For shorter OPI wires

25

the conduction has been determined to be predominantly based on incoherent tunneling, 21



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Page 22 of 29

Page 22 of 29 1

but as the OPI wires become longer, a transition occurs at OPI-5p to a hoping

2

conduction.40 Although this is an initial study, which is still being experimentally

3

explored in our group, this may be an important indicative that the SERS technique may

4

be used to monitor the transition in conduction regime of molecular wire systems.

5 6

CONCLUSION

7

We have synthesized a set of molecular junction systems ranging in length from 2.2 nm

8

to 9.9 nm. The OPI wires were grown on gold flat surfaces from SAMs of

9

4aminothiophenol

reacted

stepwise

with

alternating

addition

of

10

terephthaldicarboxaldehyde and/or 1,4-diamino-benzene. To the best of our knowledge

11

this was the first time that molecular wires of the OPI family were characterized by SERS.

12

The SERS results as a function of OPI length indicated a similar trend for both

13

632.8 and 785 nm excitation, with increasing SERS intensity up to OPI-4p, followed by

14

an intensity plateau up to OPI-6p, and finally a sudden decrease in intensity for longer

15

molecular wires. Electrodynamical simulations based on DDA of the local electric field

16

in the sandwich system are in good agreement with SERS intensity profile for 632.8 nm

17

and the observed higher SERS intensity at 632.8 nm compared with that at 785 nm.

18

Nevertheless, they could not explain the SERS intensity profile with the gap distance for

19

785 nm. The experimental and simulated results indicated that both electromagnetic

20

(dipole-image interaction and surface plasmon resonance) properties of the gold

21

sandwiched structures and chemical contributions from the molecular structures of the

22

molecular wires are contributing to the observed intensities.

23

It has been noticed that the length of the molecular wire that resulted in a decrease in

24

SERS intensity is coincident with the reported length in which the transition from

25

tunneling to hoping conduction occurs for OPI molecular wires. Additional evidence is 22


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Page 23 of 29 1

still necessary, but this is an indicative that SERS experiments may be a tool for studying

2

the conduction regime transition in other molecular wire systems.

3 4

ASSOCIATED CONTENT

5

Supporting Information: SEM characterization of the AuFlat|OPI-np|AuNR;

6

Characterization of the AuNR; SERS spectrum of OPI-2p synthesized off gold

7

nanoparticles compared to OPI-2p synthesized on the AuNRs; Synthesis and SERS

8

spectra of OPI-1p,2p and 3p wires on gold nanorods (AuNRs|OPI wires) and frequencies

9

assignment; Growth Characterization of the Molecular Wires by Ellipsometry; DDA

10

SERS enhancement factor (EF) simulation for single nanorod in water; Relative intensity

11

of the band 1168 cm-1 and 1078 cm-1 [Irel(1168/1078)] plotted for the different OPI-np;

12

Volume Polarazability as a function of molecular wire length optimized using DFT

13

calculations. This material is available free of charge via the Internet at

14

http://pubs.acs.org.

15

AUTHOR INFORMATION

16

Corresponding Author

17

Prof. Marcia L.A. Temperini, Departamento de Química Fundamental, Instituto de

18

Química, Universidade de São Paulo, São Paulo, Brazil. Phone:+55 11 3091-3890, E-

19

mail: [email protected]

20

Present Addresses

21

†Klester S. Souza: Departamento de Física, Universidade Federal do Rio Grande do

22

Sul, Porto Alegre, RS. Phone: +55(51) 3308-6514 E-mail: [email protected]

23

Author Contributions 23



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Page 24 of 29

Page 24 of 29 1

The manuscript was written through contributions of all authors. All authors have given

2

approval to the final version of the manuscript.

3

Notes

4

The authors declare no competing financial interest.

5

ACKNOWLEDGMENTS

6

We would like to thank FAPESP (Fundação de Amparo à Pesquisa do Estado de São

7

Paulo) N. 2011/17923-9 and N. 2012/13119-3, for financial support. Alfredo Duarte (CA-

8

IQ.USP) for his help with the TEM system and Gustavo R. T. da Silva for his help with

9

the ellipsometry measurements. MLAT, GFSA and DPS thank CNPq for research

10

fellowships. GFSA thanks FAPEMIG. We also thank CENAPAD-UNICAMP for

11

computational resources.

12

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