Thermopower of Benzenedithiol and C60 Molecular Junctions with Ni

Aug 20, 2014 - electrodes using a home-built scanning tunneling microscope. The thermopower of C60 was negative for both Ni and Au electrodes, indicat...
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Thermopower of Benzenedithiol and C60 Molecular Junctions with Ni and Au Electrodes See Kei Lee, Tatsuhiko Ohto, Ryo Yamada, and Hirokazu Tada* Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531, Japan S Supporting Information *

ABSTRACT: We have performed thermoelectric measurements of benzenedithiol (BDT) and C60 molecules with Ni and Au electrodes using a home-built scanning tunneling microscope. The thermopower of C60 was negative for both Ni and Au electrodes, indicating the transport of carriers through the lowest unoccupied molecular orbital in both cases, as was expected from the work functions. On the other hand, the Ni−BDT−Ni junctions exhibited a negative thermopower, whereas the Au− BDT−Au junctions exhibited a positive thermopower. Firstprinciple calculations revealed that the negative thermopower of Ni−BDT−Ni junctions is due to the spin-split hybridized states generated by the highest occupied molecular orbital of BDT coupled with s- and d-states of the Ni electrode. KEYWORDS: Thermoelectricity, molecular junctions, thermopower, spin-splitting, density functional calculation

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alignment and coupling to delocalized states in the contacts. In a simplified model, a negative value of the differential τ(E), i.e., a positive S, indicates that EF is closer to the highest occupied MO (HOMO). A positive value of the differential τ(E), i.e., a negative S, indicates that EF is closer to the lowest unoccupied MO (LUMO).5,12 In other words, positive and negative values of S indicate that the charge carriers are dominated by holes and electrons, respectively. This relation has been verified in several experimental works.5−10,14 For example, the charge carriers for benzenedithiol (BDT) and C60 molecules are reported to be holes and electrons, respectively.5−8,14−16 The relation between the work function and S has been examined using C60 molecules with low work function electrodes, and the results showed that S increases when EF is aligned nearer to the LUMO level.15 However, this simple relation might not be valid for ferromagnetic electrodes because the interaction between the spin-polarized electronic states (usually the partially filled dband) of the electrode and the molecule can generate new spinsplit hybridized states around EF.17 It had been reported that, when BDT is absorbed on the Ni surface, the spin-down state shifted above the EF, while the spin-up state is located below the EF.18 This should cause a significant change in the value of S compared to the case of Au−BDT−Au junctions. In the present work, the thermoelectricity of BDT and C60 molecular junctions using Au and Ni electrodes was investigated. We demonstrate that the sign of S is tuned when there is strong spin hybridization between the molecule and the ferromagnetic

nderstanding the nature of single-molecule junctions such as charge transport and interactions of molecule−contact is important in the field of molecular electronics.1−4 Recently, the thermoelectric properties of molecular junctions have been intensively studied5−10 not only because important information about the characteristics of the charge transport process can be obtained but also because single-molecule junctions potentially exhibit high efficiency in thermoelectric energy conversion.7 The thermopower or Seebeck coefficient S is related to the transmission function τ(E) as follows11,12 S=−

π 2kB2T ⎛ ∂ ln(τ(E)) ⎞ ⎟ ⎜ ⎠ ∂E 3e ⎝

E = EF

(1)

where kB is the Boltzmann constant, e is the electron charge, T is the average temperature of the junction, and EF is the Fermi energy. τ(E) is expressed by13 τ(E) = Tr[ΓL(E)G†(E)ΓR (E)G(E)]

(2)

where G(E) is the Green’s function matrix of the junction and ΓL/R(E) is the imaginary part of the self-energy matrix of the left/right electrode. One can approximate τ(E) using the Lorentzian function as follows:11,12 τ (E ) ≅

∑∑ m

σ

4γmσ ,Lγmσ ,R (γ mσ ,L+γmσ ,R )2 + 4(E − Emσ )2

(3)

where γmσ,L/R is the broadening of molecular orbital (MO) m with spin σ by the left/right electrode and Emσ is the energy level associated with the transport. Therefore, S is sensitive to changes in γmσ,L/R and Emσ, since τ(E) is defined by the © 2014 American Chemical Society

Received: June 19, 2014 Revised: August 12, 2014 Published: August 20, 2014 5276

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Figure 1. Schematic diagram of the thermoelectric voltage measurement setup. Note that in the experiment, in order to align the sign of the thermoelectric voltage and Seebeck coefficient, equation S = ΔV/ΔT was used where ΔV = (Vcold − Vhot) and ΔT = (Thot − Tcold).

electrodes. It must be noted that most of the changes in τ(E) between two electrodes will be attributed to spin hybridization because the work function of Ni (≈ 5.01 eV) is very close to that of Au (≈ 5.1 eV). The thermoelectric voltage (TEV) of the molecular junction was measured using the experimental method reported by Reddy and co-workers.5 The setup is schematically shown in Figure 1. The electrical conductance and thermopower measurements were performed at room temperature in an Ar atmosphere using a home-built scanning tunneling microscope (STM). A difference in temperature between the tip and the substrate, ΔT, was generated by controlling the temperature of the substrate with a Peltier device. The STM tip was mounted on a Cu block and kept at approximately room temperature. The temperature of the Cu blocks holding the tip and substrate was monitored by Si diode temperature sensors. The STM tip was prepared by cutting a metal wire with a diameter of 0.25 mm. Au substrates and Ni substrates were prepared by thermal evaporation of Au and electron beam evaporation of Ni onto mica sheets, respectively. The Ni substrates were stored in a glovebox without exposing to air. A self-assembled monolayer of BDT was prepared by immersing the substrates in a 1 mM mesitylene solution of BDT for 30 min immediately after taking the substrate out of the glovebox. In case of the C60 molecule, freshly prepared Ni substrate was immediately transferred to the vacuum chamber through the glovebox to deposit the molecule. The STM tip was brought close to the substrate with a bias voltage of 50 mV until it reached a certain threshold current value larger than the conductance of the molecular junctions.5 Subsequently, the bias voltage source and current amplifier were disconnected, and the voltage amplifier was connected instead to measure the TEV induced by ΔT (Figure 1). After the voltage measurement, the electrical conductance of the junction was measured again to confirm the stability of the junction. If the formation of the junction was reconfirmed, the TEV was measured again. These steps were usually repeated three times. The voltage of the contact was determined from the voltage histogram created from the data obtained. Details of the instruments and measurement procedures are explained in the Supporting Information.

The relation between the Seebeck coefficient of the junction, Sjunction, and the measured TEV, ΔV, is ΔV = (Sjunction − SCu)ΔT

(4)

where SCu is the Seebeck coefficient of bulk Cu, which is ∼1.85 μV/K at 300 K, as reported by Yee et al.15 Figure 2a shows the voltage histogram for BDT with Ni electrodes. The inset of Figure 2a shows the conductance histogram of the BDT molecule. A conductance peak was observed at 0.01 G0, which is consistent with previously reported values.19 Here, G0 is defined as the fundamental quantum of conductance (G0 = 77.4 μS). The threshold conductance to measure the thermoelectric voltage (TEV) was 0.1 G0. Multiple voltage peaks were observed in the voltage histograms obtained for Ni electrodes although single peaks were observed for Au. The multiple peaks can be attributed to different atomic and magnetization configurations of the junctions as discussed in the theoretical analysis and the Supporting Information (SI). Figure 2b shows the peak values of the TEV in the voltage histograms as a function of ΔT. The TEV observed in the Au−BDT−Au junction was also plotted as a reference. The value of SAu−BDT−Au was +7.4 ± 0.5 μV/K, which coincided with previously reported values for Au−BDT− Au junctions.5−8,14 SNi−BDT−Ni was calculated to be in the range of −12.1 ± 1.3 to −13.8 ± 1.8 μV/K. Figure 3a shows the voltage histogram for C60 with Ni electrodes. The inset of Figure 3a shows the conductance histogram of the C60 molecule. A conductance peak was observed at 0.2 G0 only when C60 molecules were deposited on the substrate. Thus, we attributed the conductance at 0.2 G0 to the Ni−C60−Ni junctions. The threshold conductance to measure the TEV was 0.3 G0. Multiple voltage peaks were observed in the voltage histograms, similarly to the result for Ni−BDT−Ni junctions. Figure 3b shows the peak values of the TEV in the voltage histograms as a function of ΔT. The value of SNi−C60−Ni was calculated to be in the range of −12.5 ± 1.2 to −14.1 ± 1.0 μV/K, whereas SAu−C60−Au was −16.1 ± 0.5 μV/K. The latter coincided with previously reported values for Au− C60−Au.15,16 5277

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Figure 2. (a) Voltage histograms for Ni−BDT−Ni junctions at set point of 0.1 G0. (b) Peak values of the thermoelectric voltage in the histograms as a function of ΔT for Ni−BDT−Ni (filled square) and Au−BDT−Au (filled triangle) junctions. The inset in (a) shows the conductance histogram of the BDT molecule. The values and erors of slopes (Sjunction − SCu) were calculated by linear fitting to all of the data points shown in the plot.

Figure 3. (a) Voltage histograms for Ni−C60−Ni junctions at a set point of 0.3 G0. (b) Peak values of the thermoelectric voltage in the histograms as a function of ΔT for Ni−C60−Ni (filled square) and Au−C60−Au (filled triangle) junctions. The inset in (a) shows the conductance histogram of the C60 molecule. The values and errors of slopes (Sjunction − SCu) were calculated by linear fitting to all of the data points shown in the plot.

The negative S observed for the Ni−BDT−Ni junctions indicates that the charge carriers are electrons. However, LUMO-mediated transport is not expected since the work function of Ni is very close to that of Au, where the BDT molecular junction exhibits a positive S, i.e., hole transport. One of the main factors that can affect the electronic properties of a molecular junction is the nature of the bonding between the molecule and electrodes.4 He et al. reported that the calculated density of states for BDT on Ni(111) exhibits a spin-split around EF.18 Unfortunately, the transmission function around EF, which is required to estimate S, was not calculated in their article. They also reported the transmission function of Ni− C60−Ni junctions based on a simplified model in which a C60 molecule is bound to a Ni atom.20 They found that EF is located near the LUMO of C60, and thus, negative thermopower is expected, which qualitatively coincides with the present experimental result for Ni−C60-Ni. To investigate the origin of the changes in S, we calculated the transmission functions of Ni−BDT−Ni and Ni−C60-Ni junctions by first-principles calculations based on more realistic models. The electronic and transport properties of BDT and C60 between Ni(111) were calculated using the SMEAGOL code21−23 based on the SIESTA program.24 SMEAGOL

employs the nonequilibrium Green’s function method combined with density functional theory. The exchangecorrelation functional was treated within the local density approximation. The electrode was modeled as a Ni slab having p(5 × 5) and p(6 × 6) surface periodicities for BDT and C60, respectively. The k-points were sampled by a 2 × 2 × 1 uniform grid. In order to obtain a smooth τ(E), we used a finer k-point mesh of 8 × 8 × 1 for the converged charge density. Double and single-ζ plus polarization basis sets were used for the molecules and Ni, respectively. Each electrode had six Ni monolayers, and the atomic positions were fixed to the experimental values except for the molecules and the two layers beneath them. The S atom of BDT and the center of the hexagonal ring of C60 (the most stable configuration)20,25 were located on the hollow site of the clean Ni (111) surface. We also calculated various configurations of the junctions. All of the results are summarized in the SI. S(T) was calculated using eq 5 at T = 300 K. S( T ) = −

5278

1 ∫ dEτ(E)(E − E F)( −∂f (E , T )/∂E) T ∫ dEτ(E)(−∂f (E , T )/∂E)

(5)

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Figure 5. Schematic illustration of the effect of spin hybridization on τ(E) for a molecular junction. The red (spin up) and blue (spin down) lines show the up and down spin components of the total τ(E), respectively. (a) Paramagnetic electrode and negligible spin hybridization, (b) weak/moderate hybridization, and (c) strong spin hybridization with a ferromagnetic electrode.

state generated from the spin-split hybridization between HOMO of BDT and the d-band of the Ni electrode. Figure 4b shows τ(E) for the Ni(111)−C60−Ni(111) junction. In contrast with the result for Ni(111)−BDT− Ni(111), no significant spin-splitting effect was observed for the Ni(111)−C60−Ni(111) junction around EF. From eq 1, S at EF is calculated to be −11.9 μV/K at 300 K, which coincides with the experimental result. Therefore, we conclude that the negative S observed in Ni−C60−Ni junctions is attributed to spin-degenerated LUMO. On the basis of the theoretical and experimental results, we propose a scheme as shown in Figure 5 in which we consider the case when HOMO contributes to the charge transport. When there is negligible spin hybridization with the ferromagnetic electrodes, a single spin-degenerated transmission peak is expected (Figure 5a). Weak/moderate hybridization with the ferromagnetic electrodes (Figure 5b) broadens the total τ(E). In this case, the absolute value of S increases since the slope of τ(E) at EF is large. This type of negligible or weak splitting has been observed in physisorbed molecules28 and also in our result of Ni−C60−Ni junctions. Figure 5c represents the last scenario, when there is strong spin hybridization. In this case, MO splits into two spin-dependent transmission peaks and is realized in our result of Ni−BDT−Ni junctions. In summary, we have investigated the thermopower of BDT and C60 molecules with Au and Ni electrodes. A negative thermopower was obtained for BDT molecules with Ni electrodes and is attributed to the strong spin hybridization of the HOMO level at EF. Our results suggest the possibility of tuning the thermopower of molecular junctions using the spin degrees of freedom.

Figure 4. τ(E) for (a) Ni(111)−BDT−Ni(111) and (b) Ni(111− C60−Ni(111) junctions.

Figure 4a shows τ(E) for the Ni(111)−BDT−Ni(111) junction. The red solid curve shows the tota τ(E), in which the spin up (green dashed curve) and spin down (blue dotted curve) components are combined. Three peaks (I, II, and III) are observed in this figure, and EF is located between peaks I and II. Peak I is mainly composed of the spin-up component, and it originates from the hybridization of the HOMO level with the s-band of the Ni substrate. This spin-up component of τ(E) for the Ni(111)−BDT−Ni(111) junction is very similar to that for Au−BDT−Au junctions.13 On the other hand, peak II is composed of the spin-down component, and it originates from the hybridization of the HOMO level with the d-band of the Ni substrate. Peak III is attributed to the LUMO level. Similar spin-splitting of the calculated density of states for the self-assembled monolayer of BDT adsorbed on Ni(111) was reported by He et al.,18 which supports our calculated results. From eq 5, S at EF is calculated to be −14.1 μV/K at 300 K, which coincides with our experimental results. Effects of atomic structure and spin configuration of the electrodes were also investigated as shown in the SI. The sign of S is not altered in the conditions examined whereas the value changes. These results suggest that multiple voltage peaks observed in the present experiment can be attributed to different atomic configurations, which result in different degrees of delocalization of the d-band of Ni, and spin configurations. The inclusion of the self-interaction correction,26,27 which is missing in the standard density functional calculation, also gives negative S. From all of these results, we conclude that the negative S observed in the present experiment is due to the spin-down 5279

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(20) He, H.; Pandey, R.; Karna, S. P. Chem. Phys. Lett. 2007, 439, 110−114. (21) Rocha, A. R.; Garcia-Suarez, V. M.; Bailey, S.; Lambert, C.; Ferrer, J.; Sanvito, S. Phys. Rev. B 2006, 73, 085414. (22) Rungger, I.; Sanvito, S. Phys. Rev. B 2008, 78, 035407. (23) Ohto, T.; Rungger, I.; Yamashita, K.; Nakamura, H.; Sanvito, S. Phys. Rev. B 2013, 87, 205439. (24) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745−2778. (25) Yoshida, K.; Hamada, I.; Sakata, S.; Umedo, A.; Tsukada, M.; Hirakawa, K. Nano Lett. 2013, 13, 481−485. (26) Toher, C.; Sanvito, S. Phys. Rev. B 2006, 77, 1554024. (27) Pemmaraju, C. D.; Archer, T.; Sanchez-Portal, D.; Sanvito, S. Phys. Rev. B 2007, 75, 045101. (28) Schwobel, J.; Fu, T.; Brede, J.; Dilullo, A.; Hoffmann, G.; Klyatskaya, S.; Ruben, M.; Wiesendanger, R. Nat. Commmun. 2012, 3, 1−5.

ASSOCIATED CONTENT

S Supporting Information *

Additional information on experimental methods, results, and computational calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. S.K.L. performed the experiments. T.O. performed the theoretical calculations. S.K.L., R.Y., and H.T. conceived and planned the experiments. H.T. supervised the project. S.K.L. wrote the paper with inputs from T.O., R.Y., and H.T. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant-in-Aid for Young Scientists (A) (Grant No. 20343741) from the Ministry of Education, Culture, Sports, Science and Technology and Grantin-Aid for Scientific Research on Innovative Areas “Molecular Architectonics” (Grant No. 25110012). T.O. would like to thank the Nano Research Institute of Advanced Institute of Science and Technology and the Supercomputer Center, the Institute of Solid State Physics, the University of Tokyo for providing computational resources.

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