Building electronic circuits from molecules is an inspiring idea1-4. The system of metal-molecule-metal tunnel junctions has drawn much attention from theoretical, experimental, and technological studies. Much attention has been devoted to investigate the various transport properties that might be applicable in developing new forms of electronic and energy-conversion devices, such as electron transfer5,6, shot noise7, heat transport8,9, negative differential resistance10, and gate controlled effects11. It is well-known that these electron transport and thermoelectric characteristics are influenced by the intrinsic properties of the molecules, including their lengths, conformations, and the density of states. In the followings, I will briefly introduce several recent investigations on this subject.
1-1 Theoretical and Experimental Researches
M. Di Ventra and N. D. Lang have reported the first-principles calculations of the current-voltage characteristics in a 1,4-benzenedithiolates molecular junction11,15. They find that the shape of the I-V curve is largely determined by the electronic structure of the molecule, while the presence of single atoms at the molecule-electrode interface play a key role in determining the absolute value of the current (Fig.1). The results show that such simulations would be useful for the design of future microelectronic devices for which the Boltzmann-equation approach is no longer applicable.
Moreover, Chao-Cheng Kaun, Brian Larade, and Hong Guo calculated charge transport properties of molecular wires from first principles16. The wires are made of oligophenylene molecules of three different lengths, in contact with atomic scale Au electrodes. (Fig. 2)Most surprising is the quantitative consistency between their theory and the experimental data on the exponential increase of resistance for longer wires.
Fig. 1.The results are that the groups of M. Di Ventra and N. D. Lang varied the source-drain bias and the gate voltage to obtain the I-V characteristics.[Phys. Rev. Lett. 84, 5 (2000)]
Fig. 2. I-V characteristics for planar molecules of I–III.[ Phys. Rev. B 67, 121411(2003)]
J. Taylor and M. Brandbyge have considered the 1,4-benzenedithiolates system.
They presented state of the art calculations of the electron transport through 1,4-benzenedithiolates coupled to Au(1,1,1) surfaces using the code TRANSIESTA. The method is based on density functional theory (DFT) and determines the self-consistent electronic structure of a nanostructure coupled to 3-dimensional electrodes with different electrochemical potentials, using a full atomistic description of both the electrodes and the nanostructure17. Their result is as shown in Fig.3.
Fig. 3The molecular junction of 1,4-benzenedithiolates system as the function of bias(steps of 0.1 V). [C.
M. Science 27, 151(2003)]
Simultaneously, J. Taylor and M. Brandbyge also investigated the transport properties in the mono layers of Tour wires functionalized with different side groups18. (Fig. 4) They found that functionalization of TW’s has a stronger effect on the energetics of the monolayers than on the orbitals responsible for current transport, and a better understanding of the intermolecular interactions in such monolayers could hopefully be exploited in order to design molecular electronic devices with specific properties.
Fig. 4.(Color online) Geometry of monolayers A–C connected with two Au (111) surfaces. Color codes:
C (dark gray or green), H (white), O (black or red), N (black or blue), S (light gray or yellow), and Au (light gray or gold). And I-Vb characteristics for monolayers A, B, and C. [Phys. Rev. B 68,
121101(2003)]
Lately, Mowbray, Jones, and Thygesen also have applied density functional theory (DFT) to analyze the influence of five classes of functional groups, as exemplified by NO2, OCH3, CH3, CCl3, and I, on the transport properties of a 1,4-benzenedithiolate
(BDT) and 1,4-benzenediamine (BDA) molecular junction sandwiched between gold electrodes19. They have found that functional substitutions have a weak influence on a molecule’s conductance (see Table I), and the reason for the weak influence is that charge neutrality pins the HOMO/LUMO molecular levels, making it difficult to shift them relative to EF.
Table I. Conductance G of BDT and BDA species between a gold (111) surface and tip.
[J. Chem. Phys. 128, 111103 (2008)]
In contrast, Many scientists also began measuring the transport properties on experiments. Lortscher, Weber, and Riel presented a statistical approach that combines comprehensive current-voltage data acquisition during the controlled manipulation of a molecular junction with subsequent statistical analysis20. (Fig. 5)
Fig. 5. Schematics of the junction with corresponding I-V curves (experimental data): I-V and GDiff -V characteristics of BDT measured at (a) 250 K and (b) 50 K. The gray area in (a) shows the envelope of all curves measured. [Phys. Rev. Lett. 98, 176807 (2007)]
The groups of Venkataraman, they observed the electronics and chemistry by varying single-molecule Junction conductance using chemical substituents. They still measure the low bias conductance of a series of substituted benzene diamine molecules
while breaking a gold point contact in a solution of the molecules21. Table II is their result. They have observed that the nature of molecular junction can be changed by different functional substitutions in the bridging molecule.
Table II. List of the Molecules Studied Showing the Number and Type of Substituents, Measured Conductance Histogram Peak Position, Calculated IP, and Calculated Relative Conductance a[Nano Lett. 7, 502(2007)].
In this study, we have investigated the effects of function substitutions in the 1,4-benzenedithiolates molecular junctions on the I-V characteristics and the Seebeck coefficients with external biases and gate voltages12,13,14.
Thermoelectric effects were observed long time ago. The macroscopic and microscopic models have been well developed and have been able to successfully explain the thermoelectric properties in the bulk materials. In the past few decades, thermoelectricity has gained renewal interests due to the progress in growing micro and nano structures, such as quantum well, super lattice, and quantum dot. Small structure can significantly alter the features of density of states by changing the dimensionality.
Thus, it leads to novel thermoelectric properties beyond the bulk materials. The efficiency of energy conversion could be enhanced due to the enhancement of the Seebeck coefficient by small structures in materials.
Although extensive researches have been made on electron transport in the nanoscale junctions, the thermoelectricity in molecular junctions has never been measured until very recent. In 2007, Prof. Majumdar’s22 group at UC, Berkeley has measured the Seebeck coefficients in a single-molecule junction. These experiments open a new era to study the thermoelectric effects at atomic level and demonstrated the capability to fabricate the thermoelectric molecular devices.
Majumdar’s experimental setup is shown schematically in Fig. 6.
Fig. 6. Schematic description of the experimental set up based on an STM break junction. Molecules of BDT, DBDT, or TBDT are trapped between the Au STM tip kept at ambient temperature and a heated Au substrate kept at temperature ∆T above the ambient. When the tip approaches the substrate, a voltage bias is applied and the current is monitored to estimate the conductance. When the conductance reaches a threshold of 0.1 G0, the voltage bias and the current amplifier are disconnected. A voltage amplifier is then used to measure the induced thermoelectric voltage, ∆V, and the tip is gradually pulled away from the substrate.
The results they have obtained are: (i) the Seebeck coefficients is independent of the number of molecules as shown in Fig. 7; (ii) the length dependence of molecular junction on the Seebeck coefficients is shown in Fig. 8; and (iii) the Seebeck coefficients can reveal more detailed information about the electronic structures of the molecule sandwiched between the nanojunctions beyond what the conductance measurements can provide as shown in Fig. 9.
Fig. 7.(A) A plot of the thermoelectric voltage Typical thermoelectric voltage traces for tip-substrate temperature differentials of 0, 10, 20, and 30 K for Au-BDT-Au junctions.
In Fig. 7, blue curve, we observed a constant thermoelectric voltage of about ∆V=
-200 μV, which lasted until all of the molecules trapped in the junction broken away, suggesting that the Seebeck coefficient is independent of the number of molecules.
Fig. 8.Plot of measured junction Seebeck coefficient as a function of molecular length for BDT, DBDT, and TBDT.
Fig. 8 shows a weak linear dependence of the thermopower on the lengths of molecules sandwiched in the junctions. The Seebeck coefficients increase as the lengths of molecules increase. The Seebeck coefficient in Au-BDT-Au obtained from experiments is around +8.7 2.1 μV/K, which depends on the slope of DOSs as shown in Fig. 9(B).
Fig. 9. Relating the measured Seebeck coefficient of Au-BDT-Au junction to the position of Fermi level. (A) theoretical prediction of the transmission function of a Au-BDT-Au junction plotted as a function of the relative position of the Fermi level of the Au electrodes with respect to the HOMO and LUMO levels. (B) The predicted Seebeck coefficient of a Au-BDT-Au junction as a function of the relative position of the Fermi level with respect to the HOMO and LUMO levels. When the measured value of SAu-BDT-Au = +8.7 2.1 μV/K (blue band) is shown, it is clear that the Fermi level is 1.2 eV above the HOMO level. At this energy level, the transmission function is( )E 0.01.
Measurements of the Seebeck coefficient in nanojunctions can provide insight into the electronic structure of the heterojunction, but the results also bear on an as-yet unexplored field of thermoelectric energy conversion based on molecules. The efficiency in thermoelectric device can be optimized if resonant tunneling occurs through an energy level between the left and right Fermi levels. Metal-molecule-metal heterojunctions are ideal in this regard because they (i) the overwhelming joule heating may be absent for the bias smaller than the threshold voltage, where no heating is possible; (ii) Diversified atomic-sized junctions may be achieved by manipulating the species of nano-structured objects and the contact region. Such manipulations may lead to a significant change in the density of states, consequently varying the the Seebeck coefficients of nanojunctions. A full exploration of all the possibilities in such an unknown system may lead to observations of practical thermoelectric devices at atomic level.
1-2 Our Systems
In this work, we model two systems of molecular junctions: the amino-substituted (-NH2) and nitro-substituted (-NO) 1,4-benzenedithiolates sandwich between two gold electrodes.
At first, we optimize a 1,4-benzenedithiolates molecule using the program, Gaussian, with the method of Hartree-Fock theory. We choose the basis set 3-21G. The optimized 1,4-benzenedithiolates molecule are taken out to reconstruct the amino-substituted (-NH2) and nitro-substituted (-NO) 1,4-benzenedithiolates molecule and optimize the structure of -NH2 and -NO 1,4-benzenedithiolates again. These relaxed molecules are put into the designed molecular junctions as shown in Fig. 10.
Fig. 10. The schemes of the three terminal junctions used in the present study. The left panel is the -NH2 substituted 1,4-benzenedithiolates molecular junction and the right panel is the -NO substituted 1,4-benzenedithiolates molecular junction.
The benzene is a stable and symmetrical molecule, and it is also a fundamental module in the chemistry. The interesting thing is that we can make the different chemical substituents, and the molecular feature can be changed easily. Charge distributions determining the electrostatic potential in monosubstituted benzenes are investigated23. So we can use the characteristic to look for some applicable materials.
We investigate the dependence of conductance on the external biases and gate field in the two nanoscale junction. In the two terminal systems, the Fermi level in the right/left electrodes is determined by filling the conduction band with the valence electrons in the bulk metal electrode described by jellium model (Rs=3). The stationary scattering wave functions of the whole system are calculated by solving the Lippmann–Schwinger equation iteratively until self-consistency is obtained. Finally, we calculate the density of stat and wave functions to simulate the I-V quality.
After studying the electronic transport properties, we also propose using the external biases and gate field as means to modulate the Seebeck coefficients in our molecular junctions. The Seebeck coefficients are relevant not only to the magnitude but also to the slope of electronic transmission functions near the Fermi levels in the metal electrodes. Therefore, observing the probability of transmission makes us understand the trend of Seebeck coefficients.