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Tuning the Electrical and Thermoelectric Properties of Phthalocyanine and Metallo-Phthalocyanine Molecular Junction

In this work, a comparative study is presented that analyses the electrical and thermoelectrical properties of free base-Phthalocyanine (Bare-Pc) and Metallo-Phthalocyanine (MPc) molecule sandwiched between gold electrodes. The study investigates the transmission and conductance characteristics of Bare-Pc without central metal ion and with different transition metal ions at the centre of (Pc) such as (Mn, Co, Ni, and Zn) in two different molecular configurations, cis and trans. The results reveal that the conductance changed by varying the transition metal-centre in the order of Co > Ni > Zn in trans, and closely similar pattern Co > Ni > Mn in cis with higher values in trans configuration. This reflects the correlation between the spin-dependent transport properties with the transition metal-centre type and the molecular structure. Further, thermoelectrical properties such as thermal conductance (k), Seebeck coefficient (S) and figure of merit (ZT) are investigated. It was found that (k), (S) and (ZT) have higher values in case of trans configuration for Co-Pc, Ni-Pc and Zn-Pc structures. Thus, by varying the transition metal-centre type and the molecular configuration of (Pc) molecule, transport properties can be tuned to produce a good value of the transmission and electrical conductance, thermal conductance, Seebeck coefficient, and figure of merit of the (Pc) molecular junction for potential high conductance and efficient thermoelectric applications.


Introduction:
Nowadays, more than 10% of the world's electricity that is used by computers and the internet is converted to heat. This wasted heat could be used to generate electricity economically by identifying new materials with a high thermoelectric efficiency [1]. The possibility of generating a voltage difference from a heat gradient, Seebeck effect, has been already introduced since the early 19 th century [2]. This effect is devoted to enhancing electric current generation by investing the wasted heat that exists in some physical structures like automobile exhausts or accompanies some mechanisms such as 2 most industrial manufacturing processes. Recently, much attention has been given to enhance the efficiency of this effect by studying the parameters that control the thermoelectrical properties, especially of molecular materials and devices [3][4][5][6][7]. Thermoelectric figure of merit (ZT) is mainly used to determine the efficiency of thermoelectrical materials. It is given by (ZT=S 2 GT/κ), where S is the Seebeck coefficient, G is the electrical conductance, T is the temperate and k is the electronic thermal conductivity.
Although the commonly used thermoelectric materials are inorganic, however, there is an increased interest in using organic thermoelectric materials to avoid many downsides of using inorganic materials such as toxicity, availability, and high cost. Besides, the thermoelectric properties of inorganic materials can be tuned by considering nano-structuring characteristics [8,9], thus, there is particular interest in exploiting the room-temperature characteristics of single-molecules attached to nanogap electrodes, which represents a nano-device. Up to now, different experimental and theoretical studies have been carried out to develop and control electronic molecules [4,[10][11][12]. They have been tuned for future nano electronics circuits applications such as molecular wires [13], electrodes [14,15], wiring circuit [16], magnets [17], quantum dots [18,19] switches [20,21], and biology systems [22,23].
Among several potential organic molecular candidates for nano scale application, phthalocyanine (Pc) has attracted huge attention for many years due to its distinguished high chemical and thermal stabilities and ideal symmetric geometrical structure [24][25][26]. Its electronic structure, magnetic, chemical, as well as transport characteristics can be tuned by inserting host central metallic ion. Such materials are known as metallo-phthalocyanines (MPc) that have been considered in many applications from organic thin film transistors [27], chemical sensors [28] to optoelectronic and photovoltaic devices [29][30][31][32], and have been investigated with techniques such as scanning tunneling spectroscopy (STS) [33] photoemission spectroscopy [34], gas phase electron diffraction [35], nuclear magnetic resonance [36], etc. However, much of these studies were based on experiments approach, and thus more deep theoretical analysis needed to be addressed in this field.
In what follows, the aim in this work is to explore the potential of the metallo-phthalocyanine (MPc) molecular junctions shown in Fig. 1 for high-efficiency thermoelectricity. We will demonstrate the use of density functional theory DFT to calculate the transmission coefficient T(E), which describes electrons of energy E and spin σ=[↑, ↓] passing from one electrode to the other through the metallo-phthalocyanine (MPc) single-molecule, and then discuss the electrical conductance G obtained using the Landauer formula (see method), Here, we will present a comparative theoretical study of the transmission and conductance characteristics as well as Seebeck coefficient and figure of merit behaviour of phthalocyanine (Pc) and metallo-phthalocyanine (MPc) single-molecule as a promising material for thermoelectrical applications. As shown in Fig. 1 the single (Pc) molecule is sandwiched between gold electrodes in two different configurations cis and trans, and it used first in the form of free base (Bare-Pc) and then variety of different metals such as (M= Mn, Co, Ni, and Zn) are embedded in the centre of (Pc) molecule to present the comparison.

Methods:
First, the cis and trans molecular geometry that shown in Fig. 1 was optimized before building the junction model. Then, the geometry of each structure that consist of two gold electrodes and a single Bare-phthalocyanine (Bare-Pc) and metallo-phthalocyanine (MPc) molecule were optimized using the SIESTA code [37] implementation of density functional theory (DFT), with a double-ζ polarized basis set (DZP) and generalized gradient functional approximation (GGA-PBE) for the exchange and correlation functionals. We then merge the DFT Hamiltonian with our quantum transport code, GOLLUM [38] to calculate the transmission coefficient T el (E) that refers to the probability that electrons with energy E and spin σ= [↑, ↓] can pass from the left to the right gold electrode through the single molecule. After that, the conductance was calculated using the calculated transmission coefficient T el (E), and the is the quantum of conductance. Further, the Seebeck coefficient which characterizes the ability of a thermoelectric material to convert heat to electricity is obtained, which is defined as S = −ΔV/ΔT, where ΔV is the voltage difference generated between the two ends of the junction when temperature difference ΔT is established between them. This leads to finally evaluate the figure of merit values given by (ZT=S 2 GT/ κ), where T is the temperature and κ is the thermal conductivity in which only the electronic part is considered while the phonon contribution is neglected as considered by most studied because there are no direct measuring methods.

Results and Discussion
After obtaining the relaxed geometry of each system, molecule sandwiched between two gold electrodes in two different configurations cis and trans as shown in Fig. 1. In order to investigate the electrical and thermoelectric properties of each MPc, and since the d-orbitals of metallic atoms in the centre of phthalocyanine could be partially filled, so for each molecular junction we used the spin density functional theory (DFT) code SIESTA, to obtain the electron and spin transport properties, using the Gollum code. Here, the transmission function has been calculated for a majority and minority spin denoted by spin up (↑) and down (↓). The total function of the transmission is defined as T(E) = (T↑ + T↓). Figure 2 illustrates the spin-up (red line), the spin-down (yellow line), and the total transmission coefficient (blue line) for Bare-Pc and MPc, where M=Mn, Co, Ni, and Zn in two different configurations, cis and trans. From the transmission curves, we can carry out a spin state test. In Fig. 2  The total room temperature electrical conductance for Bare-Pc and MPc in both cis and trans configurations are shown in Fig.3a and Fig.3b, respectively. In both cis and trans configurations, the electrical conductance of Co-Pc, and Ni-Pc show higher values than of other cases. Fig.3c shows the total conductance at the DFT Fermi energy (EF=0 eV) and shows that the pattern of increasing conductance in trans is Co > Ni > Zn, and closely similar pattern in cis Co > Ni > Mn. Fig.3c also shows that the conductance of Bare-Pc and Mn-Pc has close values in both cis and trans cases.  Thermal conductance k as a function of energy is found to be higher in trans than cis configuration in the vicinity of DFT predicted Fermi energy for Bare-Pc, Co-Pc, Ni-Pc, and Zn-Pc as shown in Fig. 4. While k is equal in cis and trans in the case of Mn-Pc.  To test the thermoelectric properties of all structures in both configurations, we have obtained the Seebeck coefficient (S) as described in the Method section and shown in Fig. 5 (a  and b) as a function of Fermi energy E F . It should be mentioned that it is well-known that the Fermi energy E F predicted by DFT could be inaccurate. Thus, Fermi energy E F treat here as a single free parameter. The results demonstrate that both the magnitude and sign of thermopower (S) are sensitive to the metal atoms at the centre of the phthalocyanines, which determine the location of transport resonances relative to the Fermi energy. Fig. 5c shows the comparison between the room-temperature values of (S) for Bare-Pc and MPc evaluated at the DFT Fermi energy (EF=0 eV). It shows that the highest values of the thermopower occurs at the DFT-predicted Fermi energy are obtained in the presence of Ni, Co, and Zn in both cis and trans configurations. The lowest values of the thermopower of all are obtained for Mn, closely followed by Bare-Pc. As a result, the figure of merit (ZT) can be obtained next as explained in the method and as shown in Fig. (6), which shows that (ZT) in trans connection is higher than cis connection in case of Co-Pc, Ni-Pc and Zn-Pc. This is due to the better transmission T(E), electrical conductance (G), thermal conductance (k) and Seebeck coefficient (S) values in trans connection than cis connection in these structures. However, (ZT) values in cis connection are higher in case of Bare-Pc and Mn-Pc due to their slightly higher (S) values in cis connection. This can clearly interduce that (ZT) values are very sensitive to the change of (S) values as well as type of used configuration. High values of (ZT), (greater than 1 as shown in Fig. 6c), are obtained for Co-Pc, Ni-Pc in trans configuration when E F lies in an energy window in the vicinity of the DFT-predicted Fermi energy. Therefore, this suggests that these structures if used in trans configuration are promising candidates for thermoelectrical application.

Conclusion
We have investigated the electrical properties of free base-Phthalocyanine (Bare-Pc) and Metallo-Phthalocyanine (MPc) molecule sandwiched between gold electrodes in two different molecular configurations, cis and trans. SIESTA implementation of density functional theory (DFT) has been used to carry out the analysis. Transmission calculations T(E) of Bare-Pc and MPc, where M=Mn, Co, Ni, and Zn were presented, and it shows that T(E) is higher in trans connection in all cases except for Mn-Pc. Also, the electrical conductance (G) results show that the highest value recorded in trans configuration for Co, Ni, and Zn. It was found that the conductance varies through this family of molecules in the order Co > Ni > Zn in trans, and closely similar pattern Co > Ni > Mn in cis. Therefore, single molecule junctions in the order of Co > Ni > Zn in trans configuration can be considered as a potential system for high-conductance (Pc) single-molecule applications. Further, thermoelectric properties were investigated by calculating the thermal conductance (k), Seebeck coefficient (S) and figure of merit (ZT). It was found that (k) is higher in trans than cis configuration in the vicinity of DFT predicted Fermi energy in all structures except Mn-Pc. Also, it was found that the energies of transport resonances can be tuned through the choice of central metal atom, leading to rather large positive and negative thermopowers (S) range from ~ -60 µV/K for Mn-Pc in cis configuration to ~205 µV/K for Ni in trans configuration. Consequently, enhanced (ZT) values (greater than 1) obtained in trans connection in case of Co-Pc and Ni-Pc which suggested them to be highly recommended and promising candidates for the application of thermoelectrical field.

Conflicts of interest
There are no conflicts to declare.