Hang
Chen‡
a,
Sara
Sangtarash‡
*bc,
Guopeng
Li‡
a,
Markus
Gantenbein
d,
Wenqiang
Cao
a,
Afaf
Alqorashi
b,
Junyang
Liu
a,
Chunquan
Zhang
e,
Yulong
Zhang
e,
Lijue
Chen
a,
Yaorong
Chen
a,
Gunnar
Olsen
d,
Hatef
Sadeghi
c,
Martin R.
Bryce
*d,
Colin J.
Lambert
*b and
Wenjing
Hong§
*a
aState Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, iChEM, Xiamen University, 361005, Xiamen, China. E-mail: whong@xmu.edu.cn
bDepartment of Physics, Lancaster University, LA1 4YB, Lancaster, UK. E-mail: c.lambert@lancaster.ac.uk; s.sangtarash@lancaster.ac.uk
cSchool of Engineering, University of Warwick, Coventry CV4 7AL, UK
dDepartment of Chemistry, Durham University, DH1 3LE, Durham, UK. E-mail: m.r.bryce@durham.ac.uk
ePen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, 361005, China
First published on 13th July 2020
Seebeck coefficient measurements provide unique insights into the electronic structure of single-molecule junctions, which underpins their charge and heat transport properties. Since the Seebeck coefficient depends on the slope of the transmission function at the Fermi energy (EF), the sign of the thermoelectric voltage will be determined by the location of the molecular orbital levels relative to EF. Here we investigate thermoelectricity in molecular junctions formed from a series of oligophenylene-ethynylene (OPE) derivatives with biphenylene, naphthalene and anthracene cores and pyridyl or methylthio end-groups. Single-molecule conductance and thermoelectric voltage data were obtained using a home-built scanning tunneling microscope break junction technique. The results show that all the OPE derivatives studied here are dominated by the lowest unoccupied molecular orbital level. The Seebeck coefficients for these molecules follow the same trend as the energy derivatives of their corresponding transmission spectra around the Fermi level. The molecule terminated with pyridyl units has the largest Seebeck coefficient corresponding to the highest slope of the transmission function at EF. Density-functional-theory-based quantum transport calculations support the experimental results.
(1) |
Since the thermoelectric properties of a molecular junction were investigated through the STM break-junction (STM-BJ),18 scanning thermal microscopy (SThM),19 mechanically controlled break junction (MCBJ)20 and electromigration21 techniques, those advances have transformed studies of thermal and thermoelectric properties at the single-molecule scale and even enabled fabrication of single-molecule energy conversion devices, thermal rectifiers or transistors.19,22–27 The thermoelectric properties of molecules can be tuned by molecular length,28–30 electrode coupling,31–33 quantum interference effects,34–36 intermolecular interactions,37 substituent effects,38 variations of the end group,24 and also by using pi-stacked systems to increase the heat current, controlling the molecular energy levels relative to the Fermi energy of the electrodes.12 Further investigation of the thermoelectric properties of a family of molecules with varying electronic structures will provide the experimental and theoretical insights into the correlation between the transmission function and the Seebeck coefficient based on eqn (1).
Here we investigate the Seebeck coefficient of four oligo(phenylene-ethynylene) (OPE) derivatives shown in Fig. 1a using a home-built scanning tunnelling microscope break junction technique (STM-BJ).18 All the molecules have 1,4-difunctionalized pyridyl39 or methylthio28 terminals at both ends and the OPE cores in their backbone.40 The choice of these four molecules is motivated by their expected different energy derivatives in electron-transmission characteristics. Seebeck coefficient measurements show that the transport through all the four molecules is dominated by their LUMO channels. The Seebeck coefficients for these molecules follow the same trend as the energy derivatives of their corresponding transmission spectra around the Fermi level and are supported by our Seebeck coefficient calculations.
Fig. 1 (a) The structures of OPE derivatives 1–4 studied in this work. (b) Schematic of the experimental setup. |
To investigate the Seebeck coefficient of single-molecule devices, we incorporated a home-built temperature control unit into the STM-BJ setup.23,34 The scheme of the home-built STM-BJ set-up is shown in Fig. 1b, and the set-up incorporated a conductance mode (conductance mode: only current amplifier switched on) and a thermoelectricity mode (thermoelectricity mode: only voltage amplifier switched on). To create a stable temperature difference between the tip and the substrate, the substrate was heated via a Peltier device, and the temperature difference was modulated by a thermocouple mounted under the substrate with feedback control. Meanwhile, the tip was kept at room temperature.23,43 A series of temperature differences (ΔT = ΔTsubstrate − ΔTtip ≈ 0, 5, 10 to 15 K) between the tip and the substrate were set respectively in the air. During the thermoelectric measurements, the tip, controlled by a piezoelectric actuator, was withdrawn from the substrate until a stable molecular junction was formed. Once the plateau was created, the circuit was switched from conductance mode to thermoelectricity mode through a relay switch. The thermoelectric voltage (ΔVth = ΔVsubstrate − ΔVtip) across a single-molecule junction was directly recorded using a voltage amplifier for a period of 100 ms, which was induced by the corresponding temperature difference. Then, the molecular junction broke down, and the set-up switched back to conductance mode. Histograms of the thermoelectric voltage were used to estimate the variation of the Seebeck coefficient and then normalized to make sure each temperature difference contains the same number of counts. Gaussian fitting gave the most probable thermoelectric voltage ΔVth. The heated substrate was connected to the voltage amplifier using copper wires, which contribute an additional thermoelectric voltage to the system. Therefore, the Seebeck coefficient of the molecular junction function is given by34
(2) |
Compounds | Seebeck coefficienta (μV K−1) | Measured conductance/log(G/G0)b-STM-BJ | Measured conductance/log(G/G0)b-MCBJ40 |
---|---|---|---|
a The error bars are based on the standard deviation in the linear fitting of the most probable thermoelectric voltage as a function of ΔT. b Most probable conductance values and the error bars are based on the standard deviation in the Gaussian fitting of the 1D conductance histograms. | |||
1 | −9.71 ± 1.06 | −4.73 ± 0.22 | −4.6 ± 0.41 |
2 | −6.88 ± 0.67 | −4.52 ± 0.23 | −4.4 ± 0.52 |
3 | −1.62 ± 0.21 | −4.32 ± 0.18 | −4.2 ± 0.46 |
4 | −4.13 ± 1.52 | −4.30 ± 0.33 | −4.1 ± 0.49 |
Typical histograms of the thermoelectric voltage for molecule 1 are shown in Fig. 3a. Since the fluctuation of molecular junction configurations is inevitable during the measurement process in ambient environment, the fluctuation of the molecular configuration and the electrode contact coupling is likely to lead to the fluctuation of the Seebeck coefficient.46,47 Therefore, the thermoelectric voltage from different molecular junctions might exhibit different distributions. Thus the thermoelectric voltage values of more than 1000 junctions were recorded and summarized into histograms during the thermovoltage measurement. The histogram peaks, which are based on the Gaussian fitting, representing the most probable measured Vpeak were chosen and plotted as a function of ΔT, and the Seebeck coefficient was obtained from the thermoelectric slope as shown in Fig. 3b. These values are plotted in Fig. 3c and summarized in Table 1. The Seebeck coefficients for molecules 1–4 are −9.71 ± 1.06 μV K−1, −6.88 ± 0.67 μV K−1, −1.62 ± 0.21 μV K−1 and −4.13 ± 1.52 μV K−1, respectively (see ESI, Fig. S5† for more details). It can be clearly seen that the sign of the Seebeck coefficients for all four OPE derivatives is negative, which means that EF is closer to the lowest unoccupied molecular orbital (LUMO) level and their transport properties are electron-dominated. The Seebeck coefficient value for molecule 1 was found to be the largest, followed by molecules 2 > 4 > 3. Moreover, the methylthio-terminal was found to impart HOMO-dominated transport28 or LUMO-dominated transport48 in the previous studies, whereas in these molecular systems, molecules 1–4 all show LUMO-dominated transport (negative Seebeck coefficient). The only difference between 1 and 2 is the anchor group, but the magnitude of the Seebeck coefficient of molecule 1 is much higher (more negative) than molecule 2. Considering the above-mentioned factor, we suppose such a difference in the thermoelectric performance of the two molecules is caused by destructive quantum interference in molecule 1.
In order to confirm the hypothesis that the destructive quantum interference occurred in molecule 1, we used density functional theory (DFT) to obtain the mean field Hamiltonian from the relaxed ground state geometry of the junctions (see Methods in the ESI†) and calculated the transmission coefficient T(E) of electrons with energy E passing from one electrode to the other. From the calculated T(E) value shown in Fig. 4a, an antiresonance peak is observed between the HOMO and LUMO of molecule 1, which causes a steeper slope around the Fermi level of the electrode. However, the transmission functions of molecule 2 show a typical feature of constructive quantum interference, suggesting that the significant difference between molecules 1 and 2 is due to the quantum interference effect.
Fig. 4 (a) DFT results of the transmission coefficients for compounds 1–4. (b) Seebeck coefficients of the corresponding molecules. The highlighted area shows the Fermi energy at which the calculated transmission and Seebeck coefficient are in qualitative agreement with the experimental findings. The relaxed structures are shown in Fig. S8 in the ESI.† (c) Calculated electrical conductance of molecules 1–4. |
Calculations show that the conductance of 4 ≈ 3 > 2 is in good agreement with the experiment for a wide energy range around E − EF = −0.5 eV. From the transmission functions in Fig. 4a, we can attribute the trend to the reduction of the HOMO–LUMO gap. The LUMO is closer to the Fermi level, suggesting that the transport channels of all three molecules are dominated by the LUMO which is consistent with the negative Seebeck coefficient measured in our experiments. The LUMOs of molecules 2–4 show no obvious changes with the different pendant units on the central phenylene ring. However, the HOMO of the three molecules is sequentially shifted towards the Fermi level, and this phenomenon was also observed in the investigation of a series of thiophene-1,1-dioxide derivatives.28 Thus, the conductance increased and the Seebeck coefficient decreased caused by the LUMO moving closer to the Fermi level.
We used the calculated T(E) value to obtain the Seebeck coefficients shown in Fig. 4b and the corresponding electrical conductance shown in Fig. 4c. We considered that all the Fermi energies of the electrodes in the case of each molecule are in the same range, as indicated by a green band in Fig. 4a. This yields the correct trend for molecules 1, 2 and 4. Molecule 3 is an outlier and has a smaller Seebeck coefficient than expected, which indicates that the Fermi energy in this case is closer to the middle of the HOMO–LUMO gap. However, the predicted values of the Seebeck coefficient were higher than the measured values. In order to correct the level alignment obtained from DFT, a scissor correction was performed using the method described in ref. 49 and 50. This leads to a better agreement between the theory and the experiment as shown in Fig. S8 in the ESI.† We found that the Seebeck coefficient of 1 is about two times higher than that of 2 for a wide energy range. Furthermore, the Seebeck coefficient of 4 is higher than that of 3; both of the above results are in agreement with the experiment. The higher overall values obtained from calculations when compared to the experimental results can be induced by different binding configurations to the electrodes in the experiment which lead to energy level broadening. The values of the Seebeck coefficient are sensitive to the slope of T(E) as shown in eqn (1), and therefore are more sensitive to the broadening of resonances compared to the electrical conductance.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental technique and computational methods for calculating the transmission coefficients. See DOI: 10.1039/d0nr03303k |
‡ These authors contributed equally to this work. |
§ W. H. coordinated the writing of the manuscript with contributions from all authors. All authors have given approval to the final version of the manuscript. |
This journal is © The Royal Society of Chemistry 2020 |