Multi-component self-assembled molecular-electronic films: towards new high-performance thermoelectric systems

The thermoelectric properties of parallel arrays of organic molecules on a surface offer the potential for large-area, flexible, solution processed, energy harvesting thin-films, whose room-temperature transport properties are controlled by quantum interference (QI). Recently, it has been demonstrated that constructive QI (CQI) can be translated from single molecules to self-assembled monolayers (SAMs), boosting both electrical conductivities and Seebeck coefficients. However, these CQI-enhanced systems are limited by rigid coupling of the component molecules to metallic electrodes, preventing the introduction of additional layers which would be advantageous for their further development. These rigid couplings also limit our ability to suppress the transport of phonons through these systems, which could act to boost their thermoelectric output, without comprising on their impressive electronic features. Here, through a combined experimental and theoretical study, we show that cross-plane thermoelectricity in SAMs can be enhanced by incorporating extra molecular layers. We utilize a bottom-up approach to assemble multi-component thin-films that combine a rigid, highly conductive ‘sticky’-linker, formed from alkynyl-functionalised anthracenes, and a ‘slippery’-linker consisting of a functionalized metalloporphyrin. Starting from an anthracene-based SAM, we demonstrate that subsequent addition of either a porphyrin layer or a graphene layer increases the Seebeck coefficient, and addition of both porphyrin and graphene leads to a further boost in their Seebeck coefficients. This demonstration of Seebeck-enhanced multi-component SAMs is the first of its kind and presents a new strategy towards the design of thin-film thermoelectric materials.


Materials and Methods
The synthesis of molecules (1-3) has previously been reported. 1,2 All reactions were performed with the use of standard air-sensitive chemistry and Schlenk line techniques, under an atmosphere of nitrogen. No special precautions were taken to exclude air during any work-ups. 1,5-Dibromoanthracene was synthesised through the use of an adapted literature procedure. 3 All other reagents are commercially available and were used as received from suppliers, without further purification. Solvents used in reactions were collected from towers sparged with nitrogen and dried with 3 Å molecular sieves, apart from DIPA, which was distilled onto activated 3 Å molecular sieves under nitrogen.

Instrumentation
1 H-and 13 C{ 1 H}-NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer and referenced to the residual solvent peaks of either CDCl3 at 7.26 and 77.2 ppm or CDCl2 at 5.32 or 54.0 ppm, respectively. 1 H-NMR spectra were fully assigned using 2D correlation spectroscopy. Coupling constants are measured in Hz. Mass spectrometry analyses were conducted by Dr. Lisa Haigh of the Mass Spectrometry Service, Imperial College London. 4 1,5-Dibromoanthracene (0.20 g, 0.60 mmol), 4-ethynylpyridine hydrochloride (0.33 g, 2.38 mmol) and CuI (0.01 g, 0.06 mmol) were dissolved in toluene (100 mL) and DIPA (20 mL). The solution was degassed for 15 minutes and Pd(P -t Bu3)2 was added. The solution was then stirred at 50 o C for >16 hours. The solvent was removed in vacuo to give a black solid that was purified by flash chromatography on an alumina V column, eluting with 1:1 hexane:DCM to give the final product as a yellow solid (0.17 g, 0.45 mmol, 75%).

Coordination of Anthracenes (1-3) to Zinc-Tetraphenylporphyrin (Zn-TPP)
General Procedure: To evaluate the coordinative behaviour of the synthesised anthracene derivatives, we evaluated NMR data produced from both 1:1 and 2:1 ratios of anthracene:porphyrin. For each experiment 10 mg of anthracene was dissolved in 20 mL of DCM. Either 1 or 0.5 equivalents of Zn-TPP were added, to form 6 different samples, the solutions were then stirred for 1 hour before the solvent was removed in vacuo to give a purple solid that was analysed. Characterisation data of molecules 1-3 is included for the sake of comparison. (Proposed molecular structures included within this section are provided as a guide for the reader rather than a definitive molecular identity).

DFT and Transport Calculations
In this section, geometries, electronic structures and transport properties of all junctions are presented. Since the main aim is to examine the change in transport properties when porphyrin and/or graphene are inserted, and since Au and Pt have similar work functions, the top contact will be chosen to be Au.

Optimised DFT Structures of Isolated Molecular-scale Structures
Using the density functional code SIESTA the optimum geometries of the isolated molecules were obtained by relaxing the molecules until all forces on the atoms were less than 0.01 eV / Å. 4,5 A double-zeta plus polarization orbital basis set, norm-conserving pseudopotentials, an energy cut-off of 250 Rydbergs defining the real space grid were used and the local density approximation (LDA) was chosen as the exchange correlation functional. We also computed results using GGA and found that the resulting transmission functions were comparable with those obtained using LDA. [6][7][8] The basic building blocks I-IV of this study are shown in Fig. S40:

Frontier orbitals
The plots below show isosurfaces of the HOMO, LUMO, HOMO-1 and LUMO+1 of isolated molecules 3 and 4.

Binding energies
To calculate the optimum binding distance between any two components, we used DFT and the counterpoise method, which removes basis set superposition errors (BSSE). The binding distance d is defined as the distance between compound 1 and compound 2. Here, compound 1 is defined as entity A and compound 2 as entity B. The ground state energy of the total system is calculated using SIESTA and is denoted . The energy of each entity is then calculated in a fixed basis, which is achieved using ghost atoms in SIESTA. Hence, the energy of the individual 1 in the presence of the fixed basis is defined as and for the gold as . The binding energy is then calculated using the following equation:

Binding energy of two components
In this section, four binding energies are calculated. B1 to find the optimum distance between the anthracene and graphene sheet, B2 to find the optimum distance between the anthracene and porphyrin (Zn-TPP, Zn-N), B3 between the anthracene porphyrin (Zn-TPP) molecule, and graphene sheet. B4 between the anthracene and porphyrin (Zn-TPP, Zn-SMe), Figure S44 illustrates the four structures.

Binding energy of molecules on Au
After obtaining the optimum structures of the molecules of study, shown in Fig. S45, the next step was to find the optimum distance between the Au electrode and the molecules. B4 was added to this study to investigate whether porphyrin (Zn-TPP) binds to anthracene with SMe anchor. Comparing B2 (anthracene Py) binding energy curve against B4 (anthracene SMe) curves, shows that anthracene Py binds 10 times more strongly than anthracene SMe (0.5 and 0.05 eV), see B4 and B2 of Fig. S45.

The tilt angle (θ)
In this section, we determine the tilt angle of each compound on a gold substrate, which corresponds to the experimentally measured most-probable break-off distance. In previous work we demonstrated how the tilt angle varies between single molecules and SAMs. [9][10][11][12][13] Table  S2 shows each compound for a range of tilt angles. Break-off distance values suggest that the eight compounds tilt with angle θ ranging from 45 o to 50 o . Note, that experimentally, the Au/3 and Au/3/P, and Au/4/G and Au/4/P/G were contacted by Pt probes, in all cases the film thickness and roughness is identical whether contacted by Pt probe or graphene-coated Pt probe.  Figure S48: Optimised structures of the eight junctions, demonstrating the tilt-angle of the anthracene (side-view).

Transport calculations
The transmission coefficient curves T(E), obtained using the Gollum transport code, were calculated for the eight junctions based on the tilt angles in Table S2 (different curves of the same colour correspond to different title angles and the yellow line is the average), as shown in the right panels of Figs. S49-52. Although the LUMO resonance is predicted to be pinned near the Fermi Level of the electrodes for these eight systems, previous comparisons between theory and experiment suggest that better agreement is obtained when the Fermi level is closer to the mid gap (see black-dashed lines in the right panels of Figs. S49-52).  Table S2. Different curves of the same colour correspond to different tilt angles. The yellow line is the average of four curves.  Table S2. Different curves of the same colour correspond to different tilt angles. The yellow line is the average of four curves.   Table S2 Different curves of the same colour correspond to different title angles and the yellow line is the average of four curves.

Seebeck coefficient
After computing the electronic transmission coefficient for the eight molecules, we now compute their Seebeck coefficients . To this end, it is useful to introduce the non-normalised probability distribution ( ) defined by; Where ( ) is the Fermi function and ( ) are the transmission coefficients and whose moments are denoted as follows; Where is the Fermi energy. The Seebeck coefficient, and electrical conductance are then given by; Where is the electronic charge. Supplementary Figures S53-56 show the Seebeck coefficient evaluated at room temperature for different energy range − .  Table S2 Different curves of the same colour correspond to different title angles and the yellow line is the average of four curves.   Table S2 Different curves of the same colour correspond to different title angles and the yellow line is the average of four curves.  Table S2 Different curves of the same colour correspond to different title angles and the yellow line is the average of the four curves.

Au preparation
Template stripped gold (Au TS ) was prepared by a modified method of Whitesides and Pinkhassik. 14,15 A Si wafer (5 mm x 5 mm) was cleaned using an ultrasonic bath with acetone, methanol and isopropanol (IPA), then cleaned with oxygen plasma for 5 minutes. The cleaned wafer was glued onto the top surface of a thermal evaporated gold chip (100 nm thickness) with Epotek 353 nd epoxy adhesive. The adhesive was cured for 40 minutes at 150 o C, then cooled down to room temperature. The Si contact with gold without epoxy and adhesive was carefully removed using a sharp blade and leaving an atomically-flat Au surface.
The prepared gold was scanned by AFM for 3-5 random spots for quality tests. For all cases, only the substrates with roughness below 0.2 nm were used for SAMs growth.

SAMs growth
Growth of anthracene-based SAMs: 1mM solution of molecule (1-4) was dissolved in toluene (>99.5%, Sigma Aldrich), with 10 minute nitrogen bubbling for oxygenation. The prepared Au TS was immersed into the solution, and incubated for 24 hours under nitrogen atmosphere, and the molecules will spontaneously form an organic thin film on Au surface due to the specialized anchor and the intermolecular interaction. The SAMs modified Au was rinsed by toluene and IPA several times to wash-off the physisorped molecules. The sample after rinsing was blown with nitrogen for drying and incubated in vacuum oven (10 -2 mbar) overnight at 35 o C for solvent evaporation.

Au/3/P/G/Au Au/4/P/G/Au
Zn-TPP deposition: The SAMs modified Au substrates were immersed in 100 µM Zn-TPP solutions dissolved in toluene for Zn-TPP deposition. Among different deposition time, we found that 20 minutes of deposition gives a Zn-TPP layer with thickness relatively comparable with a single Zn-TPP lying on a flat surface.
The SAMs growth procedure on QCM substrate was the same as the growth on Au TS . The resonance frequency of the substrate before and after SAMs growth was recorded by the QCM instrument (OpenQCM), and the shift in frequency, ∆ , implies the amount of molecule adsorbed on the substrate surface, through use of the Sauerbrey equation (S7). 16 Where n the amount of molecule adsorbed on Au surface, A the electrode area, NA the Avogadro's number, Mw the molecular weight, µ the shear modulus of quartz, ρ the density of quartz, f0 the initial frequency. .

SAM Topography
AFM characterization: peak force AFM (PF-AFM, Brucker, 500 pN force at 2 kHz) was used to characterize the surface of the Au substrate, before and after SAMs growth. Our previous publication demonstrated that these anthracene molecules forms well-ordered SAMs. The nano-scratching was performed in contact mode at high set force (F = 15 -40 nN) using a soft probe (Multi-75-G, k = 3 N/m) to 'sweep away' the molecular film from a defined area. The topography of each sample, after scratching, was again characterized in peak force mode, where the scratched window is easily observed. Nano-scratching was also conducted on a bare gold sample under the same conditions to ensure no gold is scratched away in the force range that we used. The height difference between the 'scratched' part and 'un-scratched' part indicates the thickness of SAMs. With a combination of QCM, AFM imaging and Nano scratching, we conclude that under the same deposition conditions (100 µM for 20 min), the amount of Zn-TPP deposited on each SAM was as follows; SAM1 < SAM3 ≈ SAM4 < SAM2. For SAM2, topological image's suggested that there were quite a number of large clusters on the sample surface instead of continuous film. However, SAM3 and SAM4 formed well defined Zn-TPP layer with reasonable thickness, which was useful for further characterization.

Electric and thermoelectric characterization
A modified AFM system was used for electric and thermoelectric characterization. The electrical transport properties of the SAMs were characterized by a cAFM system. The cAFM setup is based on a Multi-mode 8 AFM system (Bruker Nano Surfaces). The bottom gold substrate was used as the source, and a Pt/Cr coated probe (Multi75 E, BugetSensors) was used as the drain. The force between probe and molecule was controlled at 2 nN, as this force is strong enough for the probe to penetrate through the water layer on the sample surface but not too strong so as to destroy the molecular thin film. The triangular shape AC bias was added between the source and drain by a voltage generator (Aglient 33500B), the source to drain current was acquired by a current pre-amplifier (SR570, Stanford Research Systems) providing current-to-voltage conversion. The I-V characteristics were obtained by Nanoscope 8 controller simultaneously collecting drive bias and current with subsequent correlation of these values at each time point.
The conductance of the molecule was calculated by the derivative of the IV curve, dI/dV, at near 0 bias region (-10 mV to 10 mV), and the amount of molecules getting contact with the probe was estimated utilising the RKJ model (S8). 17,18 Where r is the contact radius, F is the loading force from probe to sample, R is the radius of the probe, v1 and v2 the Poisson ratio of the material, E1 and E2 are the Young's Modulus for probe and SAMs respectively. The radius of the probe was obtained from SEM image and estimated to be 25 nm. The Young's modulus was obtained from AFM in peakforce mode, which was about 2 GPa for the SAMs. Other parameters were obtained from literature working on similar systems. The graphene was coated onto the conductive probe by LB film.  The Seebeck coefficients of SAMs were obtained by a ThEFM, modified from the cAFM system used for electrical transport measurement. A Peltier stage driven by a voltage generator (Agilent 33500B, with auxiliary amplifier to provide sufficient drive current) and was used for heating up and cooling the sample, therefore providing a temperature difference between sample and probe. The probe used was a commercially available Pt coated probe coated with additional layers of 5 nm Cr and 30 nm Au to enlarge the contact area for voltage stability. The graphene was coated on the top of the probe with same method as described in the cAFM section.
The sample temperature was measured by a Type T thermal couple, and the probe temperature can be assumed to be equal to ambient temperature as is in the case of low thermal conductivity sample's (organic film) as shown elsewhere. 2 The thermal voltage between sample and probe was amplified by a high impedance differential pre-amplifier (SR560, Stanford Research Systems), and recorded by a computer.    XPS was performed on self-assembled monolayers of Au/2 with and without Zn-TPP. Spectra for Au/2 in the N 1s region shows two peaks at 399.8 eV and 401.68 eV, representing bound and unbound pyridyl units ( Fig. S67 (a)). After Zn-TPP deposition, bound and unbound nitrogen peaks were still present at similar binding energies, 400.1 eV and 402.12 eV, however an additional nitrogen appears due to the binding of nitrogen within porphyrin showing three peaks at 398.4 eV, as shown in Fig. S67 b. Similar to Au/3 and Au/4, a peak is expected from the anthracene Zn-TPP interaction and cannot be seen under the binding energy from Au-N interactions. We note that the unbound peak following Zn-TPP deposition has increased which we attribute to toluene solvent removing some of the bound anthracene layer.
As expected from characterisation of the sulphur region two sets of doublets were observed representing bound and unbound thiomethyl. (Peaks were fitted using standard methods with two Guassian-Lorentzian S 2p doublets with a 2:1 area ratio and splitting 1.2 eV). Identical doublets were found at 161.8 eV and 164.6 eV, as shown in Fig. S67 (c-d). In the Au/2 system, we note that there is a mixture of both bound and unbound nitrogen and thiol units suggesting that Au/2 forms a mixed layer. Fig S67 (e) shows spectra recorded in the zinc region to confirm presence of Zn-TPP. A peak was found at 1021.7 eV, similar to the Au/3 and Au/4 systems. Broad scanning indicated the presence of a small amount of iodide species in Au/2 (6.7%) and Au/3 (12.6%) believed to originate from the copper-iodide co-catalyst used in the preparation of these systems, but is believed to play no detrimental role in the SAMs assembly or performance.