Thermopower in Underpotential Deposition-Based Molecular Junctions

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule–electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.

All X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific Nexsa photoelectron spectrometer with a monochromated Al Kα Xray source (1486.6 eV).
Atomic Force Microscopy (AFM) measurements were performed using a Bruker Multimode 8 instrument to assess the surface roughness of Au TS both before and after the UPD process.SNL-10 AFM tips from Bruker, characterized by a resonant frequency of 65 kHz and a spring constant of 0.350 N/m, were utilized for the roughness measurements.During the scanning process, the set point was fixed at 1 V, and the scan rate was set to 1.32 Hz.

Typical Experimental Procedure of Underpotential Deposition (UPD)
The UPD was conducted following the previously reported procedures. 1,2 n a typical experiment, a three-electrode system was utilized, consisting of a freshly prepared template-stripped gold (Au TS ), copper wire, and platinum wire as the working, reference, and counter electrodes, respectively.2][3] Prior to use, the copper wire was polished with sandpaper and briefly immersed in diluted nitric acid to clean its surface.The potential was held at a value just positive of the bulk deposition peak (approximately 0.02 V vs. Cu +2/0 ) over 60 s.The obtained bimetallic electrode was then removed from solution under potential control, rinsed with copious ethanol and dried by N2 flow.

Electrochemical Characterization of UPD Layer
The formation of UPD layer was confirmed by electrochemical analysis, following the previously reported procedures. 1,3,4 T characterize the Cu UPD region, the same conditions as mentioned above were utilized.Cyclic voltammetry (CV) tests were conducted in the potential range of -0.07 to 0.5 V (vs.Cu +2/0 ) at a scan rate of 0.05 V/s.
For verifying the exact UPD window, potentiostatic conditions were employed with various applied potentials (-0.02, -0.01, 0, +0.01 and +0.02 V) for a duration of 60 seconds at room temperature.Subsequently, Cu stripping was carried out using a linear sweeping voltammetry (LSV) method in the same solution, where the potential was swept from the deposition potential to 0.48 V at a scan rate of 0.01 V/s. 4

SAMs Preparation
We prepared SAMs following the procedure reported previously. 2,5After the UPD process, the UPD-modified substrates were immediately transferred into solutions containing acid or thiol derivatives (1 mM; ethanol and n-hexadecane for alkanethiols and alkanoic acids, respectively).All samples were incubated in the solution under N2 atmosphere for at least 12 h.The resulting SAMs of alkanethiols and alkanoic acids were rinsed with ethanol and hexane, respectively.All samples were dried with gentle blowing of N2 prior to characterization.

Eutectic Gallium-Indium (EGaIn) System
The EGaIn based method for molecular thermopower measurement allows easy and convenient collection of large amounts of data for SAMs. 6The soft liquid metal, covered by a spontaneously formed and conductive Ga2O3 layer with a nominal thickness of approximately 1 nm, represents a good choice for the top electrode in molecular junctions. 7The reversibility, non-invasiveness, and well-defined interface of EGaIn-based junction technique enable the creation of a high yield of thermoelectric junctions. 8The process for forming the junction and measuring thermopower followed previously reported procedures. 8In brief, we applied five different temperature gradients (ΔT = 4, 8, 12, 15, 20 K) across the SAM and recorded the output voltage at each temperature.We gathered 1350 -3075 ΔV data from 18 -41 separate junctions in two different samples for each ΔT.

DFT methods
The optimized geometry and ground state Hamiltonian and overlap matrix elements of each structure were self-consistently obtained using the SIESTA implementation of density functional theory (DFT). 9SIESTA employs norm-conserving pseudo-potentials to account for the core electrons and linear combinations of atomic orbitals to construct the valence states.The local density approximation (GGA) of the exchange and correlation functional is used with PBE parameterization, a double-ζ polarized (DZP) basis set, a real-space grid defined with an equivalent energy cut-off of 250 Ry.The geometry optimization for each structure is performed to the forces smaller than 10 meV/Å.The mean-field Hamiltonian obtained from the converged DFT calculation was combined with the GOLLUM implementation of the non-equilibrium Green's function method to calculate the phase-coherent, elastic scattering properties of the each system consist of left gold (source) and right gold (drain) leads and the scattering region. 10,11  transmission coefficient T(E) for electrons of energy E (passing from the source to the drain) is calculated via the relation: and  = ( (0%0 ! )/3 " 4 + 1) %& is the Fermi-Dirac probability distribution function, T is the temperature, EF is the Fermi energy,  ' = 2 5 /ℎ is the conductance quantum, e is electron charge and h is the Planck's constant.

Minor Discussions
Characterization of UPD.We conducted separate control experiments to ensure the desired operation of UPD and the formation of monoatomic Cu adlayer.First, we obtained a CV curve in the electrochemical condition without the Cu 2+ ions (Figure S1a in the Supporting Information); no peaks were observed, confirming that the peaks in the Figure 2a originated from the reduction and oxidation of copper ions. 3Second, we conducted electrochemical experiments at different voltages to determine the suitable threshold voltage for the UPD and bulk deposition. 4The Au TS substates were potentiostatted at several voltages (-0.02, -0.01, 0, +0.01 and +0.02 V) for 60 seconds, and the linear sweeping voltages (LSV) method was followed to remove all copper deposited on the gold surface.In the UPD region, the stripping charge should be independent of the parking time at the deposition potential. 4The LSV curves indicated the bulk deposition occurred when the potential was more negative than 0 V (Figure S1b), and our UPD condition of +0.02 V was relevant to the creation of the monoatomic adlayer avoiding the bulk deposition.This deposition potential was further supported by the constant stripping current with different deposition time (Figure S1c). 4 Surface Topography of Cu Adlayer.We determined whether the surface roughness varied significantly upon the Cu UPD using atomic force microscopy (AFM).
The use of template-stripped metal substrate is justified by the creation of ultraflat surface, which helps avoid creation of significant defects caused by a rough surface. 12ere was no significant difference in the surface roughness between Au TS ME (rms = 0.29 ± 0.07 nm) and Cu/Au TS BE (rms = 0.28 ± 0.12 nm) (Figure S2).This finding indicates homogeneous large-area deposition of monoatomic Cu adlayer rather than bulk or cluster depositions.
Binding Mode of Carboxylic Acid on Cu Adlayer.On bulk copper, a carboxylate group binds asymmetrically-only one oxygen binds to the surface-while the Cu monolayer formed by the UPD prefers the symmetric binding. 5,13 his distinct binding structure of the carboxylate anchor group is attributed to the difference in the extent of surface oxide between the substrates. 14The monoatomic Cu adlayer formed on gold via UPD displays significantly greater resistance against oxidation than the bulk Cu because oxidation on the bimetallic electrode is limited to a maximum of one layer of copper. 15 calculated the DFT T(E)s for the alkanoic acid junctions with two different contacting modalities (Figure S16 and S19.The resonance due to the GWO states in case 1 is far from the Fermi energy and does not significantly contribute to S (Figure S20).In case 2, we found that the GWO state moves toward EF (Figure S18).Also, the weaker interaction between C(=O)O-and the Cu layer resulted in a sharp slope of T(E) close to the GWO states (Figure 4d) leading to a higher  ̅ .Packing Quality of SAMs on UPD surface.It has been reported that the packing quality of alkanethiol and alkanoic acid SAMs on the Cu UPD surface is not significantly different from those of the analogous SAMs on pure gold or silver. 1,5,14 Wlso observed the similar results.Static contact angles of decanoic acid SAMs on the Cu UPD (102 ± 4°) and Ag TS (110 ± 1°) were indistinguishable; there was also no significant difference in water contact angle for 1-decanethiolate SAMs on the Cu UPD (111±2°) and Au TS (115±4°).
Higher Simulated S Values Than Experimental Ones.We note that our simulations predicted a higher value of S than was observed in the experiments, even though the overall trend of the data was similar.This is likely due to the fact that the simulations assumed that the molecular junctions were perfectly stretched and clean, while in reality there are defects in the SAM and some molecules may not be fully stretched.These factors can all affect the shape of the transmission function and broaden the resonance near EF, which can effectively lower the absolute value of S. 16 Conductance.We further estimated conductance of our junctions following previously reported procedure. 17The log|G/A| values at 0 V for octanethiolate SAMs on the BE (0.48 S/cm 2 ) and the gold ME (0.71 S/cm 2 ) were indistinguishable from each other (Figure S24 and Table S7).Here, G is the conductance of a junction and A is the geometrical contact area (cm 2 ).In contrast, the log|G/A| values at 0 V for octanoic acid SAM increased from -0.38 to 0.85 S/cm 2 , when the silver ME was replaced with the BE (Figure S24 and Table S7).These trends concur well with the results of DFT calculations (Figure S22-23).
Power Factor (PF).We followed the previously reported procedure to calculate the PF of the O2CC7 SAM on both BE and ME. 18Generally, PF is determined by PF = σ × S 2 , where σ (μS cm −1 ) represents electrical conductivity, and S (μV K −1 ) represents the Seebeck coefficient.In this case, we obtained the σ values for BE/O2CC7 and ME/O2CC7 as 6.2 × 10 -1 and 3.5 × 10 -2 μS cm −1 .For the distance between two electrodes (d), we used the thickness value of HO2CC7 SAM on pure Ag (0.83 nm, reported by Tao 13 ) and assumed that the thickness of the SAM on both ME and BE is similar for the sake of simplicity. 18Consequently, PF values for BE/O2CC7 and ME/O2CC7 were revealed to be ~1.1 × 10 -8 and ~3.4 × 10 -11 μW m -1 K -2 , respectively.

UPD Does Not Impact the Tunneling Attenuation Coefficient (β). It has
been reported that there is no significant difference in β for alkanoic acid on copper UPD (0.92 per carbon) and pure silver (0.95 per carbon). 19We have calculated β values of our molecules (Figures S25 and S26) and found that the β values are consistent with the previous report 19 and do not vary significantly according to the presence or absence of Cu UPD layer (red and blue curves in Figure S26).Note that the shape, amplitude, and the position of the transmission resonance due to the GWO change by the alkyl chain length.Altogether, these lead to non-linear decrease of S as a function of the length which is also supported by our first-principle calculations in Figure 4.

- 17 )
: case 1 when only one oxygen atom is connected to Cu layer, and case 2 when both oxygen atoms are connected to Cu adlayer.The GWO state is due to the hybridized orbitals of both -C(=O)O -and Cu in case 2, while it is due to the Cu adlayer only in case 1 as confirmed by our LDOS calculations in Figures S18

Figure S2 .
Figure S1.(a) CV of Au TS in 0.1 M H2SO4 solution with or without 1 mM CuSO4.(b)

Figure S3 .
Figure S3.High-resolution X-ray photoelectron spectra of Cu 2p for Cu/Au TS

Figure S4 .Figure S5 .
Figure S4.High-resolution X-ray photoelectron spectra of S 2p for BE/SC8 SAM and

Figure S8 .
Figure S8.Molecular structures of alkanethiol wires sandwiched between two

Figure S9 .
Figure S9.Molecular structures of alkane-carboxylic wires sandwiched between two

Figure S10 .
Figure S10.The effect of sulfur GWO on transport through alkanes.(a) Molecular

Figure S11 .
Figure S11.Local density of state iso surfaces for Au/SCn//Au junctions calculated

Figure S12 .
Figure S12.Local density of state iso surfaces for BE/SCn//Au junctions calculated

Figure S20 .
Figure S20.The effect of contacting modalities between -COOH and Cu adlayer.

Figure S22 .
Figure S22.Transport through alkanes with -SH between gold electrodes with and

Figure S23 .
Figure S23.Transport through alkanes with -COOH between gold electrodes with and

Figure S25 .Figure S26 .
Figure S25.Room temperature electrical conductance as a function of electrodes Fermi

Table S1 .
Summary of data of thermoelectric junction measurements of HO2CCn-1 on

Table S2 .
Summary of data of thermoelectric junction measurements of HSCn on

Table S4 .
21mmary of reported Seebeck coefficient of HSCn on Au TS .21

Table S7 .
Summary of effects of electrode materials for HO2CC7 and HSC8 on current density and conductance.Cu/Au TS ; b Ag TS ; c Au TS