Fluorination effects on the electronic transport properties of dithiophene-tetrathiafulvalene (DT-TTF) molecular junctions

Solid State Communications 157 (2013) 62–67

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Fluorination effects on the electronic transport properties of dithiophene-tetrathiafulvalene (DT-TTF) molecular junctions Ming-Jun Li a, Meng-Qiu Long a,b,n, Ke-Qiu Chen c, Hui Xu a,n a

School of Physics and Electronics, Central South University, Changsha 410083, People’s Republic of China Institute of Super Microstructure and Ultrafast Process, Central South University, Changsha 410083, People’s Republic of China c Department of Applied Physics, Hunan University, Changsha 410082, People’s Republic of China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 July 2012 Received in revised form 25 September 2012 Accepted 3 December 2012 by J. A. Brum Available online 7 December 2012

The fluorination of dithiophene-tetrathiafulvalent (DT-TTF) was investigated by using the density functional theory combined with nonequilibrium Green’s function method. It is demonstrated that fluorination can modify the electronic transport properties of DT-TTF. Negative differential resistance can be observed within a certain bias voltage range in 4FDT-TTF. & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Dithiophene-tetrathiafulvalene C. Fluorination effect D. Transport property D. NDR behavior

1. Introduction The electron transport properties of organic molecular devices have been subject to intensive research because of many interesting physical properties found in these devices [1–6], such as negative differential resistance (NDR) [7–10], field-effect characteristics [11–13], spintronics characteristics [14,15], rectification characteristics [16–18], and so on. Organic based devices are showing great potential in electronic and optoelectronic applications for their cost-effective, large-area coverage, lightweight, structural flexibility, and feature lower power consumption than their inorganic counterparts [19]. Tetrathiafulvallene (TTF) derivatives such as dithiophene-tetrathiafulvalent (DT-TTF) are ideally suited as components because they are reversible and stable electron donors. DT-TTF is an promising candidate molecule, because it is rigid, symmetric and completely conjugated. It is an active material of p-type semiconductor in organic field-effect transistors (OFETs) [20]. Nowadays, TTF is becoming a versatile molecule for organic electronics [21], and many experimental and theoretical groups have investigated charge transport about TTF and its derivatives [22–26]. Mas-Torrent et al. reported that OFET mobility is up to 3.6 cm2/(Vs) n Corresponding authors at: School of Physics and Electronics, Central South University, Changsha 410083, People’s Republic of China. E-mail addresses: [email protected] (M.-Q. Long), [email protected] (H. Xu).

0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.12.001

for DT-TTF [27]. Subramanian and his coworkers studied the molecular switches based on TTF derivatives [28]. Shuai et al. studied the NDR through intermolecular interaction of BEDT-TTF and TCNQ [29]. Although much progress has been achieved on the research of electronic transport properties for TTF and its derivatives, there are still some debates on the theoretical understanding of the carrier scattering mechanism [30]. Especially, how to control the electronic transport properties of molecular devices based on TTF and its derivatives by physical or chemical modulation is not well understood [31]. By chemically modifying their molecular structure and functionality the solid-state structure is feasible to synthesize tailored materials for specific use. Functional molecular devices such as molecular diodes can be realized by introducing different functional groups, which may modify the delocalization of the molecular orbitals and affect the electron transfer from molecule to electrode. Fluorination is extensively used strategy to tune the structural and electronic properties of organic semiconductors [32–34] which induce a route to stability in organics by lowering both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels in the molecule. Fluorine (F) atom is normally introduced as a chemical functionalization dopant in nanostructures. The experimental process of fluorination is easy to control. Geramita and coworker reported that the oligothiophene-fluorinated heterofluorene systems exhibit interesting donor–acceptor properties [35]. Fluorination effects of the electronic transport mechanism are important from fundamental and practical points of view. These may offer key advantages in

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Fig. 1. (Color online) Schematic description of the organic molecular devices and the structure of the two-probe system. M1 and M2 correspond to DT-TTF and 4FDT-TTF molecular devices, respectively.

realizing various electronic applications via fluorination. Different from DT-TTF, the fluorination of DT-TTF will exhibit some novel physical and chemical properties. In this letter, fluorination effects on the electronic transport in DT-TTF are explored based on the first-principle calculation and theoretical analysis. Obviously, full fluoride is easier to control than partial fluoride. Therefore, we choose full fluoride of the DT-TTF molecular junction simulation, in which four hydrogen atoms are substituted with fluorine atom (4FDT-TTF).

2. Model and method The geometries of the organic molecular junctions are illustrated in Fig. 1: the molecular junctions are positioned in the FCC site of the Au surface. Our two-probe system is divided into three regions: the left electrode, the scattering region and the right electrode. The scattering region includes molecule and two layers of metal atoms. The molecule is coupled with two semi-infinite Au(111)-(4  4) electrodes. The sulfur atom is located at the hollow site of the gold triangle. The Au–S distance is 2.1 A˚ which corresponds to the lowest energy of the expanded molecules. Noting that the self-interaction correction is very important for molecules with S as anchor group [36–38]. In this letter, the geometries of anchor group S connected to the Au electrodes are obtained under the same conditions for all systems. We mainly focused on the fluorination effects on the electronic transport properties of DT-TTF molecular junctions. We believe that our calculated results based on standard density-functional schemes are reliable qualitatively. The structures were optimized and the quantum transport calculations were carried out by the Atomistix ToolKit (ATK) package [39,40], which is based on the fully selfconsistent non-equilibrium Green’s functions formalism (NEGR) and density functional theory (DFT). The local-density approximation (LDA) and norm-conserving pseudopotentials are applied. Single zeta plus polarization basis set for Au atoms and doublezeta plus polarization basis set for other atoms are adopted, and a mesh cutoff of 150 Ry and a Monkhorst-Pack mesh of 1  1  100 are employed. The basis set is adopted for elements of systems and the convergence criterion for the total energy is 10  5 Ry to achieve a balance between calculation efficiency and accuracy.

The electron temperature is set to be 300 K. All the structures are relaxed until the maximum atomic force is smaller than ˚ 0.05 eV/A. In the NEGF theory, the transmission function T(E,V) of the system is the sum of transmission probabilities of all channels available at energy E under the external bias voltage V T ðE,V Þ ¼ Tr½GL ðE,V ÞGR ðE,V ÞGR ðE,V ÞGA ðE,V Þ

ð1Þ

where GR/A are the retarded and advanced Green’s functions, and coupling functions GL/R are the imaginary parts of the left and right self-energies, respectively. The self energy depends on the surface Green’s functions of the electrode regions and comes from the nearest-neighbor interaction between the extended molecule region and the electrodes. For the system at equilibrium, the conductance G is evaluated by the transmission function T(E) at the Fermi level (FL) EF of the system G ¼ G0 T ðEF Þ

ð2Þ 2

where G0 ¼ 2e /h is the quantum unit of conductance, h is Planck’s constant and e is the electron charge. The current–voltage characteristics through a molecular junc¨ tion is calculated on the Landauer–Butiker formula Z     2e ½f EmL f EmR T ðE,V ÞdE IðV Þ ¼ ð3Þ h where f(E mL/R) and mL/R are the Fermi function and the electrochemical potential of the left/right electrode, respectively. With the applied bias voltage V, mL ¼ m(0) eV/2 and mR ¼ m(0)þeV/2. Furthermore, mL/R(0)¼ EF is the Fermi level (FL). Considering the fact that the FL is set to be zero, the region of the energy integral window [mL ðVÞ, mR(V)] is actually [V/2, V/2].

3. Results and discussion Fig. 2 shows the transmission coefficient T(E,V) at zero bias V¼0 V for M1 and M2 given in Fig. 1. In the process of calculation energy is normalized such that the Fermi level is set to be the origin of energy. The T(E)in the Fermi level is refer to the value of equilibrium conductance. We can find the value is 0.0214 in

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Fig. 2(a) and 0.0211 in Fig. 2(b), corresponding to the equilibrium conductance is 0.0214G0 and 0.0211G0 for M1 and M2.There are two large transmission peaks corresponding to HOMO and LUMO for each system. This figure also presents the frontier molecular orbitals (HOMO and LUMO) for M1 and M2. It is clearly seen that

all of the frontier orbitals are delocalized and contributed to electronic transport. Comparison of the transmission coefficient peaks shows that a strong migration to lower energy for M2. The results reveal that introduction of fluorine to DT-TTF reduces both HOMO and LUMO energy levels. Besides, a larger HOMO–LUMO

Fig. 2. (Color online) Transmission spectra T(E,Vb) and molecularly projected self-consistent Hamiltonian (MPSH) of HOMOs and LUMOs under zero bias voltage for M1 and M2, respectively. The Fermi level is set to be the origin of energy.

Fig. 3. (Color online) Mulliken populations at zero bias voltage for M1 and M2.

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gap (HLG) exists for each junction. The gap in the transmission spectra can be attributed to the intrinsic semiconductor nature of TTF molecule. It can be found that the broadening of transmission peaks in Fig. 2(a) is wider than that in Fig. 2(b). The transmission peak corresponds to the resonant transmission through the molecular states, the broadening due to the strong molecule– electrode coupling [41]. It means that the fluorination decreases the coupling of TTF and gold electrode and may affect the electronic transport of molecular junction. To understand the fluorination effect, based on the Mulliken population analysis, Fig. 3 shows the net atomic charge for M1 and M2. For the symmetric systems, only the atoms on the left moiety have been presented. Compared with M1, we can find fluorination can result in the charge transfer. In the system of M1, it can be found that C2 and C4 receive about 0.20 electron from neighboring H and S. Instead, in the system of M2, the corresponding C2 and C4 donate about 0.05 electrons to neighboring F. This charge transfer results from different electron affinities of H and F. Therefore, the electronic transport properties of TTF molecular devices should be affected. Using the first-principle nonequilibrium Green’s function method, we present the I–V characteristic curves for M1 and M2 in Fig. 4(a). The convergence precisions of 10–4, 10  5 and 10  6 have been selected to calculate the I–V curves of M1, and almost no difference have be observed. No significant difference on the currents was found for the three precisions. Therefore, 10  5 was applied to calculate currents for the appropriate precision in this work. The currents through molecular devices are initially very low and increase slowly in the lower bias range of [ 1.0 V, 1.0 V]. In the inset Fig. 4(a’), we present the local expansion for lower bias range. We can clearly see that the current of M1 is about ten percent higher than that of M2 under the same bias (e.g. the current is 0.0195 uA for M1 and 0.0175 uA for M2 under the bias of 0.5 V), which corresponding to a wider transmission peak broadening for M1 than M2 in Fig. 2. When 9V941.0 V, the currents rapidly increase with bias voltage. The current of M1 increases continuously with bias, while that of M2 shows nonlinear behavior. When 9V9 ranges from 1.4 V to 1.7 V, we can clearly see that the current of M1 decreases rapidly with the augment of bias and the NDR behavior can be observed. In Fig. 4(b), we also plot the corresponding conductance (dI/dV)

a

65

curves for M1 and M2. It can be found that the conductance increases smoothly under lower bias voltage (about 9V9o1.0 V). Beyond 71.0V, the conductance increases rapidly, which means that the corresponding transmission channels are opened and make contributions to the electronic transport. However, when 9V9 is larger than 1.4 V, the conductance oscillates, especially, the negative values of conductance can be found in bias range of 1.4o9V9o1.7 V for M2. Conductance oscillation in such molecule devices originates from two important aspects [29,42,43]: (i) the effect of the electronic structure and (ii) the effect of molecule– electrode coupling. In order to understand this conductance phenomenon, in Fig. 4(c), we plot the change in the charge transfer from the electrodes to the molecule as a function of bias. Transfer charge is closely related to the coupling between the electrodes and molecule, which affects the molecular conductance [7,44–46]. With the increase of the bias, it is clearly seen that the transferred charge is increased at low biases. However, when the bias is higher than 1.4 V, the transferred charge is reduced in both systems, which matches well with conductance curve for the explored bias region. We can find the interesting NDR behavior in bias range of 1.4o9V9o1.7 V for M2 in Fig. 4(a). The NDR character is mainly attributed to the particular electronic structure. The resonant tunneling originated from the shift of the molecular energy level by external electric field [47], which is determined by the charge transfer of introducing F. To further understand the transport characteristics of the molecular junction, in Fig. 5(a) and (b), we give the total transmission coefficient as a function of the bias and electron energy. From Fig. 5(a) and (b), we find that the total transmission coefficients are symmetric under positive and negative biases for M1 and M2. At the lower bias range, there are smaller transmission coefficients in bias windows for both M1 and M2 systems. With an increase in the bias, the transmission enters into bias windows which result in the increasing of currents. However, we can find the total magnitude of transmission coefficient entering into the bias window becomes smaller in the range from 9V9¼ 71.4 V to 9V9¼ 71.7 V in Fig. 5(b), so the NDR behavior appears in M2 system, as shown in Fig. 4(a). In Fig. 5(c) and (d), we give the transmission coefficient of M2 under biases V¼1.4 V and 1.7 V, respectively. As we known, the current is determined by T(E,V) in the bias window and is further only determined by the integral area (namely, the shaded area in the bias

b

c

Fig. 4. (Color online) (a) Calculated currents as function of bias (I–V curves) for M1 and M2. The inset (a’) shows the local expansion for lower bias range. (b) Differential conductance (dI/dV) against bias voltage. (c) Changes in transferred charges from the electrode to the molecule at different bias.

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Fig. 5. (Color online) (a) and (b) show the total transmission as a function of the bias voltage and electron energy for M1 and M2, and the white lines stand for bias windows. (c) and (d) Describes the transmission coefficient and the corresponding HOMOs and LUMOs for the system M2 under biases V ¼ 1.4 V and 1.7 V, respectively. Region between the solid lines is the bias window, the dotted lines correspond to frontier molecular orbitals, and the shaded area denotes the integral area in the bias window.

Fig. 6. (Color online) Contour plots of the electrostatic difference potential at 1.4 V and 1.7 V for M2. Blue (red) represents low (high) effective potential.

window). When the bias is 1.4 V, there is a strong and broad transmission peak originating from LUMO that lies in bias window, which leads to a large current. When the bias is 1.7 V, the bias window is increased with the bias voltage, but the total integral area gets smaller. As a result, the current decreases and the NDR appears.

From the frontier molecular orbitals, we can find LUMO is fully delocalized under bias of 1.4 V, but it is suppressed at a higher bias value (1.7 V). So electrons are much easier to transport through molecular states by LUMO under the bias of 1.4 V rather than 1.7 V. Therefore, the NDR behavior appears in this bias range.

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We also calculated the real space effective potential for M2 under bias V ¼1.4 V and 1.7 V, as shown in Fig. 6, which shows how the applied bias is distributed across the junction [48,49]. From the change in the potential, in Fig. 6(a), we can find that the potential drops through the junction, and there is almost no voltage blockage across molecular junction under bias of 1.4 V. However, when the bias is 1.7 V, we can find strong voltage blockage is localized at left contact of anchoring atom S and Au electrode, which indicates that the resistance of M2 mainly takes place in the left contact. As we know, voltage blockage under external bias means transport channels should be suppressed.

4. Conclusions Using the density functional theory combined with the nonequilibrium Green’s function approach, we calculated the electron transport properties of DT-TTF and 4FDT-TTF. We demonstrated that F-substitution can modify the electron transport properties of DT-TTF and the NDR behavior can be observed within a certain bias voltage range for 4FDT-TTF. We demonstrated that F-substitution can modify the electron transport properties of DT-TTF and the NDR behavior can be observed within a certain bias voltage range for 4FDT-TTF. It is suggested that the NDR behavior originates from the suppression of the frontier molecular orbitals LUMO of 4FDT-TTF with the bias increasing.

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Acknowledgments This work was supported by the Natural Science Foundation of China (No. 21103232), the Fundamental Research Funds for the Central South University (CSU), and High Performance Computing Center of CSU.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ssc.2012.12. 001.

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