Supporting Information

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1 Supporting Information Molecular Orbital Gating Surface-Enhanced Raman Scattering Chenyang Guo, 1, Xing Chen, 2, Song-Yuan Ding, 3, Dirk Mayer, 4 Qingling Wang, 1 Zhikai Zhao, 1,5 Lifa Ni, 1,6 Haitao Liu, 1 Takhee Lee, 5,* Bingqian Xu, 6,* & Dong Xiang 1,* 1 Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology, Key Laboratory of Optical Information Science and Technology, Institute of Modern Optics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin , China 2 Department of Chemistry, The Pennsylvania State University, Pennsylvania 16802, USA 3 State Key Laboratory of Physical Chemistry of Solid Surfaces (PCOSS), Collaborative Innovation Centre of Chemistry for Energy Materials (ichem), and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen , China. 4 Peter-Grünberg-Institute PGI-8, Bioelectronic Research Center Jülich GmbH and JARA Fundamentals of Future Information Technology, Jülich 52425, Germany. 5 Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea. 6 College of Engineering, University of Georgia, Athens, Georgia 30602, USA

2 This PDF file includes: Figure S1. Working principle of the MCBJ device Figure S2. Precisely gap control of the MCBJ device Figure S3. Measurement system for gating control SERS Figure S4. The laser polarization effect to Raman scattering spectroscopy Figure S5. Electric field distribution upon laser illumination Figure S6. Fabrication process of side-gated MCBJ chips Figure S7. Source-drain current (ISD) versus gate voltage Figure S8. The calculation of field distribution upon gating voltage Figure S9. The selected normal modes under gating voltage 1a X = 0 1b X S L Z = 0 1d Z Figure S1. Working principle of the MCBJ device. (a) A suspended metal bridge with a constriction was micro-fabricated on the substrate. The substrate was fixed on a three-point bending apparatus. (b) When the push rod exerts a bending force on the substrate, the movement (ΔZ) causes an elongation of the constriction until it breaks, resulting in the formation of two separated nanoscale electrodes. The gap size (ΔX) between the two electrodes can be precisely controlled by bending or relaxing the substrate. In our device, the attenuation r = ΔX / ΔZ

3 The gap size can be precisely controlled at sub-nanometer accuracy is due to fact that an extremely low value of attenuation factor can be achieved by the MCBJ setup. The attenuation factor is defined as, r = ΔX/ΔZ, where, ΔX is the gap size change between the two nanoelectrodes and ΔZ is the vertical displacement of push rod referenced to the substrate, see Figure S2.The attenuation factor can be estimated as r = ΔX/ΔZ = 6ut/L^2, where the t is the thickness of substrate, u is the length of suspended metal bridge (Science, 1997, 278, 252 ). In our devices, the attenuation factor is calculated to be r = ΔX/ΔZ = 6ut/L^2 6 1µm 0.2mm / 3cm^ The value of attenuation factor (10E-6) indicates that, in principle, we can adjust the gap size with picometer accuracy if we can control the movement of push rod with micrometer accuracy. Actually, the movement of push rod is controlled by a piezoelectric actuator in our devices, thus it is feasible to control the movement of push rod at nanometer accuracy and the movement of push rod can be precisely controlled at picometer accuracy. L t 1u X X Z Figure S2. Precise control of the gap size with MCBJ devices. (a) The parameters of a MCBJ devices for the attenuation factor calculation. (b) SEM image of a MCBJ sample (chip). 3

4 (a) To Raman spectrometer (b) Bending substrate Push rod Counter support Bending substrate Push rod Figure S3. The measurement system. (a) Schematic diagram of the experimental system combined mechanically controllable break junction (MCBJ) with surface enhanced Raman spectroscopy. (b) Experimental system combined with MCBJ and SERS. Laser Polarization Intensity (Arb.Unit) Polarization Wavenumber/cm -1 Figure S4. The laser polarization effect to Raman scattering spectroscopy in 1,4-benzendithiol molecule junction. The gold electrodes pair has its axis (a) parallel and (b) perpendicular to the incident laser polarization. Laser wavelength: nm. The Raman intensity was greatly enhanced when the laser polarization parallel to the electrodes axis. 4

5 We further calculate the electric field distribution upon laser illumination in the electrode junction of different morphology employing discrete interaction model (DIM). The DIM is an atomistic electrodynamics model. Each atom is represented by a spherical Gaussian charge distribution with an atomic isotropic polarizability which is obtained from a Clausius-Mossotti relation, α = R, where R is an atom radius, which is Å for Au atom, ϵ and ϵ are the dielectric constants of metal and environment, respectively. In this work, ϵ is the frequency-dependent complex taken from Johnson and Cristy 1. The total polarizability of the nanoparticle is derived from the total energy minimization with respect to the induced atomic dipoles, which leads to a set of linear response equations. μ, = α, E + T (), μ,, where T () is the second order interaction tensor for the dipole-dipole interaction between I and J, E is the external field, and μ is the induced atomic dipole I. Adapting the damping scheme in renormalized interaction tensors leads to the screening of the interaction at short distances, which was set to 1.59 Å in our calculations. The near field at the real space (r) is given by, E = T (), (r r )μ,, electric field is polarized along the dimer symmetric axis.. In this work, the near field intensity is defined as I = E / E. The The near field is drastically confined and enhanced in the junction as the gap is 1 nm. With the increasing of gap distance, the fields become weaker and localized on the electrodes, especially on the corner sites. It shows that the field distribution in two types of electrodes is extremely sensitive to the gap distance, where the gap plasmon is similar to a combination of plasmon in the individual electrode with the gap distance of 3 nm. However, when the gap distance reaches 5 nm, we can identify the plasmon localized on the well-separated electrodes. The features of near field distribution can reflect the SERS enhancement factor, which is roughly estimated as E. 5

6 a b c E 2 / E d 3 nm 3 nm 3 nm 3 nm 3 nm 3 nm Figure S5. The simulation of electric field based on atomistic electrodynamics model. (a) Atomistic representation of computational models for electrodes in different morphology. The truncated icosahedral dimer represents the pair electrodes in the cone shape (left) and cube dimer represents the pair electrodes in plane shape (right). The gap distance is defined by the shortest length between two nanoparticles, which increases from 1 to 6 nm to explore the varying field distributions. Near field intensity in the vicinity of plane-to-plane (d, e, f) and cone-to-cone Au electrodes (g, h, i) with the gap distances of 1 nm (d, g), 3 nm (e, h), and 5 nm (f, i). 6

7 (a) (d) Au PMMA Si N 3 4 (b) Si N 3 4 (e) Au Si N 3 4 Substrate (c)1 PMMA Si3N4 (f) Au Substrate Figure S6. Fabrication process of side-gated MCBJ chips. (a) Top view SEM image of the fabricated chip. Scale bar: 100 nm. (b-f) Side view of the chip fabrication process along the cross section in dashed line marked in (a). (b) Deposition isolating layer (Si3N4) on the substrate. (c) Electron beam lithography process. (d) Deposition of Au layer as electrodes. (e) Lift-off process to get rid of undesired gold layer. (f) Reactive ion etching process to obtain suspended gold bridge and side-gate electrode. The insulation layer Si3N4 was etched by gas mixture 15 CF4 / 30 O2 at the condition of power 75 W, DC bias 319 V, gas pressure 150 mtor. 7

8 a I SD (na) V G V SD b 90 V SD I SD (na) V G Gate voltage (V) Figure S7. Source-drain current (ISD) versus gate voltage (VG) for the Au-BDT-Au molecular junction and molecule absent junction. (a) The current was dramatically modulated by the gate voltage in the molecule bridged junction. Insert: schematic of three-terminal molecule junctions. (b) No gate-controlled current was observed in the molecule-absent junction. The inset schematics illustrate the three-terminal junction. (a) (b) V/m V Figure S8. The simulation of the (a) electric field and (b) potential distribution with the source-drain voltage of 12 mv. The distance between source and drain electrodes is 1.2 nm, and the perpendicular distance between the source-drain axis and gate electrodes is 5 nm. The applied gate voltage is 10 V. 8

9 The frequency-dependent polarizability under external electric field was simulated using the Adiabatic Local Density Approximation (ALDA) implemented in AO Response module of ADF. 2 The polarizability derivatives were obtained from the numerical three-point differentiation with respect to the normal mode displacement. The Raman cross section is calculated by dσ dω = π ε (ν h p ν ) 8π cν 45(1 exp( hcν /k T)) where ν and ν are frequency of the incident light which is 632 nm in our calculations and the k th normal mode, respectively. ε is dielectric constant in vacuum, k is Boltzmann constant. p is Raman scattering factor, which is given by p = 45( α Q )+7( γ Q ) Where α and γ are the isotropic and anisotropic polarizabilities. As the molecule axis is aligned with z- axis of coordinate system, the zz component of polarizability is mainly responsible for TERS. Accordingly, only zz polarizabilities are taken into account in TERS simulations. 9

10 (a) (b) cm 1095 cm -1 (c)1 (d) cm 1542 cm -1 Figure S9. The selected normal modes of benzene-1,4-dithiol responsible for the enhanced Raman signals under electric field effects. The arrow in pink indicate the direction of electric field (Y axis), and the arrow in blue indicate the direction of Z axis. REFERENCES 1. Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, Jensen, L.; Zhao, L. L.; Autschbach, J.; Schatz, G. C. Theory and Method for Calculating Resonance Raman Scattering from Resonance Polarizability Derivatives. J.Chem. Phys. 2005, 123,