Two Different Length-Dependent Regimes in Thermoelectric Large

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Two Different Length-Dependence Regimes in Thermoelectric Large-Area Junctions of n-Alkanethiolates Sohyun Park, Nayoung Cho, and Hyo Jae Yoon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02461 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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Chemistry of Materials

Two Different Length-Dependence Regimes in Thermoelectric Large-Area Junctions of n-Alkanethiolates Sohyun Park, Nayoung Cho, and Hyo Jae Yoon*

Department of Chemistry, Korea University, Seoul, 02841, Korea *Corresponding author, email: [email protected]

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ABSTRACT Molecular thermoelectrics is relatively unexplored compared with its analogous research field, molecular electronics. This is surprising considering that the two research fields share an identical energy landscape across the molecular junctions and similar quantum–chemical mechanisms. This paper describes the length dependence of thermopower in self-assembled monolayers (SAMs) comprising structurally simple wide-bandgap molecules, nalkanethiolates (SCn; n = 2, 4, 6, 8, 10, 12, 14, 16, 18) chemisorbed on gold. Thermovoltage measurements at zero bias have enabled the determination of the Seebeck coefficient of nalkanethiolates for the first time. A plot of the Seebeck coefficient versus the length of the nalkane chain reveals the presence of two different length-dependence regimes. The rate of decrease of the Seebeck coefficient as the molecular length increases changes at SC10 from −0.54 to −0.10 V(K·nC)−1. The theoretically proposed presence of metal-induced gap states (MIGS) in the short but not in the long n-alkanethiolates accounts for the two observed length-dependence regimes. Owing to the length dependence of the transmission function coefficient of MIGS in the short n-alkanethiolates, the Seebeck coefficient decreases linearly as the length increases. The nearly zero rate of decrease in the long n-alkanethiolates mirrors the insignificant MIGS in the long n-alkanethiolates.


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Introduction Studies of molecular thermoelectrics advance the understanding of charge transport behavior in molecular-scale electronic devices.1-5 Simultaneously, molecular thermoelectrics promises the conversion of heat energy into electricity in tiny molecular-scale devices.6 However, the field of molecular thermoelectrics is relatively unexplored compared with its analogous research field, molecular electronics, although the two research fields share an identical energy landscape across the electrode–molecule–electrode junctions and similar quantum– chemical mechanisms in charge transport. We herein show the length dependence of selfassembled monolayers (SAMs)7 comprising structurally simple wide-bandgap molecules that have been widely studied in molecular electronics, n-alkanethiolates (SCn; n = 2, 4, 6, 8, 10, 12, 14, 16, 18; Figure 1a) chemisorbed on gold. Thermovoltage measurements at zero bias using liquid metal (eutectic gallium–indium, denoted as EGaIn; Figure 1b)8 enable the determination of the Seebeck coefficient of n-alkanethiolates for the first time. Notably, the plot of the Seebeck coefficient versus the length of the n-alkane chain reveals the presence of two different length-dependence regimes. Limited studies1, 2, 8-13 have shown the length dependence of the Seebeck coefficient in molecular junctions; these all showed single lengthdependence regimes (the previous studies are summarized in Table S1 in Supporting Information). The rate of decrease of the Seebeck coefficient as the molecular length increases changes at SC10 from −0.54 to −0.10 V(K·nC)−1. This change in the rate of decrease is attributed to the theoretically proposed presence and absence of significant metalinduced gap states (MIGS)14-16 in short and long n-alkanethiolates, respectively. Owing to the length dependence of the transmission function coefficient of the MIGS, the Seebeck coefficient decreases linearly as the molecular length increases in short n-alkanethiolates. The small rate of decrease in the long n-alkanethiolates mirrors the insignificant MIGS.



Figure 1. (a) n-Alkanethiolates used in this study (denoted as SCn where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). (b) Schematic showing the structure of large-area thermoelectric junction we used. EGaIn/Ga2O3 is an eutectic Ga-In alloy covered with a native oxide of Ga2O3, and AuTS is the template-stripped gold substrate.


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The SAMs of n-alkanethiolates have been widely studied as two-dimensional nanomaterials in molecular electronics.17-30 In n-alkanethiolates, the length of the n-alkane backbone is a precisely controllable chemical variable, while the energy level of frontier molecular orbitals localized on the sulfur atom is not significantly affected by changing the molecular length.13, 29, 31 The n-alkanethiolates have wide energy gaps between the highest occupied molecular orbital, HOMO, and lowest unoccupied molecular orbital, LUMO. For example, it was previously reported that the HOMO-LUMO energy gaps are ~8 – ~10 eV.14, 15, 27

Due to the wide HOMO-LUMO gaps in n-alkanethiolates, off-resonant tunneling usually

dominates charge transport across the corresponding molecular junctions.21, 32, 33 Although the length dependence of electrical conduction across n-alkanethiolates has been widely described by a rectangular energy barrier model,17, 26, 34 this length dependence remains to be rationalized with a Lorentzian-shaped transmission function particularly for thermoelectric molecular junctions.13

Experimental Section Molecules and Materials. All reagents were used as supplied unless otherwise specified. All organic solvents were purchased from Sigma-Aldrich and Daejung. nAlkanenethiols (HSCn; n=2, 4, 6, 8, 10, 12, 14, 16, 18; purity 95 – 99 %) were purchased from Sigma-Aldrich and TCI (purity 97 %). High purity eutectic gallium-indium (EGaIn; 99.99%) was obtained from Sigma-Aldrich and used as supplied. All thiol derivatives were stored under N2 atmosphere and < 4 °C. Gold thin films (300 nm) were deposited onto silicon wafer (100 mm in diameter; 1–10 ohm-cm, 525 ± 50 microns thick) by e-beam evaporator (ULVAC). Optical adhesive was purchased from Norland (NOA81) and used as supplied. Preparation and Characterization of SAMs. We prepared SAMs following the procedure reported previously.20 Ethanol (anhydrous, 99.9%) solution (3 mM) containing



HSCn was added to a vial. The solution was sealed and degassed by bubbling N2 through the solution for ca. 10 min. A freshly prepared AuTS chip was rinsed with pure ethanol and placed to the solution with the exposed metal face up. The vial was then filled with N2. After 3 h incubation at room temperature, the SAM-bound AuTS chip was removed from the solution and rinsed by repeatedly dipping the chip into pure ethanol (3 × 1 mL). The solvent on the SAM was then evaporated in air for a few seconds. Junction Preparation and Measurements, and Thermopower Analysis. A conical tip of EGaIn for use as a top contact was formed and junction formation and measurements were done following the method reported in the literature.8 All junction formation and measurements in this work were carried out in ambient conditions. We created three different temperature gradients (ΔT = 4, 8 and 12 K) in junction. These moderate temperature gradients help to avoid loss of the desired temperature differentials across junctions due to the thermal conduction to the top-electrode via air. Furthermore, high temperature may lead to decomposition of SAMs particularly through the degradation of sulfur atoms in the anchoring group. We measured 17 – 40 traces per junction; at each temperature, we measured 5 – 12 junctions per sample, and at least three samples. A freshly prepared EGaIn conical tip was used for measuring 3 – 5 junctions, and thereafter a new tip was generated. The yield of working junction was calculated by the ratio of non-shorting junctions to all measured junctions. Shorting junctions were defined as the junction showing the value of SEGaIn (3.4 μV/K), which was measured from the shorting junction of EGaIn conical tip with bare AuTS.8 To analyze thermopower of junctions, we followed the previous method reported by Segalman and Majumdar research groups.2, 8, 35 Using the equations in Figure S1 in the Supporting Information, we were able to estimate thermopower of SAMs. For the Seebeck coefficients of EGaIn and Ga2O3, we used the previously measured values.6 The working junctions show thermoelectric voltages in the range from -100 μV to -102 μV.


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Results and Discussion As the thermopower of molecular junctions permits access to thermovoltage at zero bias across the junctions,1, 3, 5 we anticipated that thermoelectric characterization of nalkanethiolate junctions could offer an unprecedented opportunity to experimentally probe interfacial energy states between the HOMO-LUMO gap. Thermoelectric characterization of molecular junctions remains relatively unexplored compared to analogous tunneling characterization.1 Indeed, limited junction test beds such as scanning probe microscopy-based single2, 36, 37 and small-area (containing 101–102 molecules)9 junctions have been reported. Herein, we used a recently developed large-area junction technique based on a EGaIn microelectrode (Figure 1b).8 The EGaIn top-contact allows reversible, non-invasive, and geometrically defined thermoelectric top-contacts on delicate SAM surfaces under ambient conditions. Hence, the EGaIn technique allows collection of statistically sufficient thermoelectric data in high yields. There are differences between odd- and even-numbered nalkanethiolates in terms of the tilt angle with respect to the surface normal, the orientation of terminal group, and surface dipole.7, 38, 39 These can affect charge transport phenomena across SAMs, often leading to odd-even effects. In this work, we used even-numbered nalkanethiolates from SC2 to SC18 to form SAMs (Figure 1a) to eliminate the complexity arising from a possible (but yet undefined) odd-even effect of thermopower. For a bottomelectrode, we used ultraflat AuTS to minimize the degree of structural defects resulting from the roughness of the substrate.40 Following previously reported procedures,8 we obtained statistically significant thermoelectric voltage (ΔV, V) data at various temperature differentials (ΔT = 4, 8, 12 K) on many separate AuTS/SCn//Ga2O3/EGaIn junctions (where “/” and “//” indicate covalent and van der Waals contacts, respectively; see Figure 1b). The number of data points ranged from 1368–3040, obtained from 18–40 separate junctions in three different samples. Table 1



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Table 1. Summary of length dependence in thermoelectric measurements over AuTS/SCn//Ga2O3/EGaIn junctions. n in SCn

n=2

n=4

n=6

n=8

n = 10

n = 12

n = 14

n = 16

n = 18

SSAM (V/K)a

∆T (K)

# of samples

# of tips

# of junctions

counts

ΔVmean ± σΔV

4

3

6

18

1368

−40 ± 9

8

3

6

20

1520

−68 ± 10

12

3

7

23

1748

−87 ± 21

56

4

3

6

25

1900

−28 ± 5

78

8

3

6

22

1672

−53 ± 11

12

3

6

20

1520

−68 ± 17

61

4

3

6

23

2204

−28 ± 9

85

8

3

6

20

1520

−42 ± 9

12

3

6

20

1520

−63 ± 15

77

4

3

7

31

1356

−27 ± 6

86

8

3

7

36

2736

−36 ± 12

12

3

8

36

2736

−48 ± 11

84

4

4

7

30

2280

−26 ± 6

90

8

3

6

31

2356

−36 ± 12

12

4

6

26

1976

−44 ± 13

87

4

3

6

30

2356

−23 ± 7

91

8

3

6

26

1976

−31 ± 7

12

3

6

26

1976

−39 ± 6

89

4

3

6

29

2204

−24 ± 8

93

8

3

7

31

2356

−33 ± 10

12

3

6

30

1976

−39 ± 10

87

4

3

7

27

2052

−26 ± 7

93

8

3

9

40

3040

−34 ± 9

12

3

7

26

1976

−35 ± 12

91

4

3

8

37

2812

−21 ± 10

97

8

3

8

33

2508

−27 ± 13

12

3

8

35

2660

−33 ± 23

a

yield (%) 72

7.2 ± 0.6

6.4 ± 0.7

5.1 ± 0.6

3.5 ± 0.2

3.3 ± 0.1

3.0 ± 0.1

2.9 ± 0.2

2.3 ± 0.4

2.5 ± 0.0

68

65

81

84

89

89

91

91

95 93

SSAM was estimated by a Gaussian mean-based statistical method (see the main text for details). See Figure S5 and Table S2 in the Supporting Information for all data-based linear square (LS) and linear absolute deviation (LAD) fittings and the corresponding SSAM values, respectively.


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summarizes the thermoelectric measurement data, and Figure 2 shows histograms of the thermovoltage. For statistical analysis, we excluded the thermovoltage data for short junctions, assuming that it did not contain meaningful information derived from molecular features. Five features were observed in the thermovoltage data. i) All the ΔV histograms exhibited approximately normal distributions, further confirmed by the corresponding heatmaps in Figure 3a. ii) The absolute value of V increased with T regardless of the molecular length, which is further confirmed by the plot of Vmean (the mean of V extracted from the histogram) in Figure 3b. iii) Overall, the V dispersion increased with T for each molecule, confirmed by the plot of V (standard deviation of V estimated from the Gaussian fitting curve in the histogram; see Figure S3 in the Supporting Information for detailed procedure for estimating the standard deviation) in Figure 3b. iv) Overall, the V dispersion for the intermediate lengths (SC8–SC14) was smaller than that for the short (SC2– SC6) and long (SC16–SC18) lengths, and this trend became significant as T increased. This finding is confirmed in Figure 3c.) The yield of the working junction increased as the nalkane chain length increased (Figure S4 in the Supporting Information). Statistical Analysis. Previous studies demonstrated that the dispersion of ΔV histograms for molecular junctions largely depends on the chemical structure of the molecule, the supramolecular structure of the monolayer, and the temperature.2, 41 Tails and outliers in the ΔV distributions become significant and the data dispersion increases with ΔT, the complexity of the internal structure (the degree of freedom), and/or the lateral intermolecular interactions. As shown in Table S3 in the Supporting Information, the degree of skewness of Gaussian fitting curves in ΔV histograms was estimated following the previously reported procedures.42 The estimated values of skewness were positive in sign, indicating the positive skewness (i.e., the tail of ΔV dispersion biased towards negative values in the histogram).



Figure 2. Histograms of thermoelectric voltage (ΔV, V) measured on AuTS/ SCn//Ga2O3/EGaIn junctions at various temperature differentials (ΔT = 4, 8, 12 K).


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Figure 3. (a) Heatmaps of thermoelectric voltage (ΔV, μV) measured at various temperature differentials (ΔT = 4, 8, 12 K). (b) Plots of mean values of ΔV (ΔVmean) at various ΔT and the corresponding standard deviation of ΔV (σΔV) against the length of n-alkanethiolates (n in SCn). (c) Trends of σΔV versus the molecular length at different ΔT.



It was also found that the averaged value of skewness (0.68) for the low ΔT (4 K) was smaller than those (1.43 and 1.34) for the high ΔT (8 and 12 K). This finding indicates more positive skewness of ΔV distribution for higher ΔT. We could rationalize the ΔV dispersion in our junction measurements with the chemical factors. The monolayers of the short nalkanethiolates are loosely packed owing to weak lateral vdW interactions, resulting in liquid-like monolayers with many gauche defects.39, 43 Thus, the large V dispersion in the short n-alkanethiolates could be attributed to the disordered monolayer structure. Currently, we do not know the exact reason for the increase in σΔV for the long n-alkanethiolates. The monolayers of long n-alkanethiolates are well packed and relatively ordered relative to those of the shorter ones.38 Thus, the V dispersion is probably due to the increased internal vibration on adding methylene groups41 or the poor electrical conductance of the long nalkanethiolates. The Seebeck coefficient (S, S = ΔV/ΔT, V/K) of the junction is estimated by measuring the linear regression slope in the plot of ΔV versus ΔT. Therefore, the accuracy of S depends on the magnitude of ΔV and its distribution. This implies that S can vary according to the statistical analytical method used to determine the linear regression slope. Previous studies on molecular thermoelectrics use Gaussian mean-based fitting to estimate ΔVmean and σΔV, and linear square (LS) fitting over the scattered plot of ΔVmean ± V data points is conducted to estimate S. This Gaussian mean-based LS fitting is a reasonable and convenient statistical method to estimate S values from thermovoltage data. Here, we compared S values obtained by three different statistical methods: conventional Gaussian mean-based LS fitting, all data-based LS fitting, and all data-based least absolute deviation (LAD) fitting (Figure 4a). The LS and LAD methods give different weights to the dispersion of ΔV histograms.44 Detailed procedures for all data-based LS and LAD fittings are summarized in the Supporting Information (Figure S5 and Table S2). The all data-based fitting methods consider more the


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Figure 4. (a) Comparison of statistical methods for determining the Seebeck coefficient (S, V/K) from a series of ΔV histograms measured at different ΔT in an exemplary SAM (SC2). (b) Plots of the Seebeck coefficient (S, V/K) against the molecular length (n in SCn), obtained by three different statistical methods. (c) Comparison of our length dependence with those demonstrated in other thermoelectric junctions having similar molecular backbones or anchoring groups. The units of βS for alkane chain and oligophenylene molecules are V(K·nC)−1 and V(K·nPh)−1, respectively.



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ΔV dispersion than the Gaussian mean-based fitting method, and the LS method gives more weight to the dispersion than the LAD method. If the outliers and tails contain meaningful information on the (supra)molecular features, all data rather than the mean value of ΔV should be considered. In this work, we focused on S values obtained by the Gaussian meanbased fitting method, because this was used in other studies and allowed us to directly compare the literature results with our result (see the Supporting Information for the S values estimated using all the data-based LS and LAD methods). Length Dependence. The data fitting yielded the S value of the junction (Sjunction). We obtained the S value of the SAM (SSAM) from Sjunction by considering the junctions as thermopower circuits based on a previously reported method.2, 8, 35 Such a circuit analysis could be performed with the experimentally measured thermopowers of the surface Ga2O3 and EGaIn: the value of Stop electrode (that of the conical tip) is 6.8 ± 0.2 μV/K; those of SEGaIn and SGa2O3 are 3.4 ± 0.1 and 3.4 ± 0.2 μV/K, respectively.8 Using Eq. 1, we were finally able to estimate S for the SAM: ΔV = − (SSAM − SW tip) × ΔT

(1)

Figure 4b shows the linear regression in the plot of S versus the molecular length (n in SCn). The trend in the scattered plot of S showed that S decreased as the molecular length increased regardless of the type of statistical method used. The S values for individual SCn SAMs are summarized in Table 1. All the S values were positive, indicating that the high-lying occupied molecular orbitals dominate thermovoltage generation across the junctions.1 The length dependence of the thermopower in molecular junctions could be explained using a semi-empirical parametric equation (Eq. 2), SSAM = SC + n∙βS

(2)

established by Quek et al.45 This suggests that SSAM depends linearly on the width of an energy barrier (n in SCn) with a slope of βS (the rate of thermopower change with the number


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of carbons or phenylene units; V(K·nC)−1 or V(K·nPh)−1).45 The SC (V/K) is the thermopower of a hypothetical non-shorting junction where n = 0 and is associated with the molecule-electrode interfacial characteristics (e.g., the work function of the electrode, the strength of electronic coupling between the molecule and electrode, and the effective contact area). The SC may vary according to the type of junction platform and the anchoring group, whereas βS depends on the shape of the energy barrier of the most insulating component in the junction, in principle. MIGS in n-Alkanethiolate SAMs. Interestingly, a transition in the linear regression slope occurred at SC10 (Figure 4b). S decreased linearly from SC2 to SC10, followed by a near-plateau region, where it did not respond significantly to length variation. The values of

S for the short (SC2–SC10) and long (SC12–SC18) length regions were −0.54 ± 0.06 and −0.10 ± 0.05 V(K·nC)−1, respectively (the Gaussian mean-based LS in Figure 4b). Note that the low Seebeck coefficients for the long SCn molecules remained positive. The presence of two different length-dependence regimes in thermopower is surprising given that previous molecular thermoelectric studies1, 2, 8-13 show single S values for the length dependence. While most of these studies focused on oligophenylene derivatives as emphasized in Table S1 in the Supporting Information, to our knowledge, only one study13 examined the length dependence of n-alkane chains and was related to our work. The significant linear regression we observed for the short n-alkanethiolates cannot be explained by the mechanism commonly used for oligophenylene derivatives. For oligophenylene-based thermoelectric junctions, the decrease in the HOMO-LUMO gap originates from the increase in conjugation length. This accounts for the decrease in the energy offset (E = EF − ) between the Fermi level of the electrode (EF) and the transport orbital (), which causes S to increase with the number of phenylene units (i.e., positive S). This scenario cannot be applied to explain the negative S



for the short n-alkanethiolates because the HOMO energy level does not change on adding or removing methylene groups. The negative S we observed provides experimental evidence for the presence of the MIGS and its length dependence. In conjunction with a scanning tunneling microscopy experiment,14 several theoretical studies based on density functional theory (DFT) and nonequilibrium Green’s function formalism proposed that the new energy states, MIGS, emerge at the gold-thiolate interface resulting from orbital mixing with a small amount of hydrocarbons in the n-alkane backbone on chemisorption of n-alkanethiol (and nalkanedithiol) on a gold substrate.15, 16 That is, the broadening and transmission coefficients of the MIGS depend on coupling of the molecular orbital energy with the energy of the gold substrate (Table S4 in the Supporting Information). Based on the theoretical work, the energy level of the MIGS is closer to the Fermi level (EF) by approximately an order of magnitude than the renormalized frontier molecular orbitals, HOMO and LUMO energy levels.15 MIGS peaks and tails of frontal molecular orbital energy levels can be overlapped in part with each other in n-alkanedithiolates on gold.16 Furthermore, it was proposed that the length dependence of MIGS is valid for short n-alkanethiolates, and that weak coupling between Au-S and n-alkane orbitals in long n-alkanethiolates causes MIGS to disappear. The large negative S in the SC2–SC10 region in Figure 4b reflects the decreasing electronic coupling between S-Au and the hydrocarbon orbitals, indicating that the MIGS dominates the thermopower as theoretically proposed.16 By surveying a wide range of n-alkane lengths, we were able to identify the molecular length at which the MIGS became insignificant and the dominant orbital was likely to switch to the renormalized HOMO. Considering MIGS is usually observed in metal-semiconductor interfaces,46 our results suggest that some portion of short n-alkanethiolates seem to behave like a semiconductor.


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At present, we do not know exactly what factor(s) affect the degree of the orbital mixing between Au-S and n-alkane chain parts as the length of n-alkane chain varies. The following factors may be important: i) surface dipole, ii) hyperconjugation between the Au-S and the adjacent alkane parts, and iii) the degree of structural disorder.38, 46, 47 Furthermore, it is unclear which part of alkane chain (the alkane near the Au-S vs. the terminal alkane group) participates the orbital mixing.14-16 Further studies are needed to unravel these issues. Comparison of Length Dependence for Thermopower and Electrical Conduction in n-Alkanethiolate SAMs. The tunneling current usually decreases as the length increases for the identical length range of n-alkanethiolate monolayers formed on template-stripped metal substrate, exhibiting a single linear regression slope.17, 30, 48 Note that the rough metal substrate (e.g., as-deposited metal substrates) can give the two length-dependence regimes in tunneling junctions.49 Although the detection of the MIGS in the S but not in the tunneling decay constant (G) is plausible given that the tunneling current is dominated by inclusion of a strong transmission coefficient-based energy level (e.g., the HOMO) within an external bias window (e.g., |105|–|106| V, equivalent to 0.1–1.0 V). Note that the thermoelectric characterization is performed in a much smaller voltage range (~|101|–|102| V), and the proximity of the energy to the EF is likely more important than the magnitude of the transmission coefficient. Hence, although weaker by one or two orders of magnitude than the HOMO and LUMO according to the previous theoretical work,15 the MIGS play a critical role in the thermopower performance in the short n-alkanethiolates. This accounts for the discrepancy in the length dependence for the electrical conduction and thermopower across nalkanethiolate SAM-based junctions. Comparisons with Other Molecular Thermoelectric Junctions. The polarity of S for n-alkanethiolates was opposite to those for oligophenylene thiolates (S = 2.1 ± 0.3



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V(K·nC)−1), in which S increased with the molecular length (Figure 4c). This observation was consistent with the previous result involving the corresponding dithiolates;13 the linear regression slopes for n-alkane and oligophenylene dithiolates are −0.72 ± 0.08 V(K·nC)−1 and 2.8 ± 0.8 V(K·nPh)−1, respectively (Figure 4c). The y-intercept in the plot of S against molecular length corresponds to SC in Eq. 2. The value of SC obtained from the short nalkanethiolates was 8.3 ± 0.4 μV/K. This corresponds to the thermopower of the top and bottom interfaces. Most molecular thermoelectric studies have thus far focused on symmetric junctions with two covalent molecule-electrode interfaces (see Table S1 in the Supporting Information). Few studies have discussed the thermopower of the vdW interface.8, 9, 50 Our SC value could be considered as the thermopower of gold-thiolate (SS-Au) plus that of the CH3//Ga2O3 vdW interface (SvdW) for the sake of simplicity. The previously studied n-alkane dithiolate junction of the form, Au/SCnS/Au, exhibited an SC of 8.0  0.4 μV/K.13 Provided that this literature value is the product of two linearly summed SAu-S values, the value for the single SAu-S interface (4.0  0.4 μV/K) is equal to half the measured SC. Finally, SvdW (4.3  0.4 μV/K) could be estimated using the following equation: SvdW = SC − SAu-S

(3)

This indicates that the Seebeck coefficient of the vdW (SvdW in the SCn//Ga2O3) interface was not significantly different from (~92% of) that of the covalent (SAu-S in the Au-S) interface. Considering that the difference in bond strength between vdW and covalent bonds is generally substantial, the insignificant difference between SvdW and SAu-S is surprising because other studies previously demonstrated that the thermal conductance of a SAM-based junction strongly depends on the bonding nature at the molecule-electrode interface.51-54 A similar result to our observation was reported by Tan et al.9 The Seebeck coefficients for noligophenylene dithiolate (6.4  1.8 μV/K measured for the Au/S(Ph)nS/Au junction) and


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monothiolate (5.6  0.5 μV/K measured for the Au/S(Ph)n//Au junction) do not differ significantly (only a few percent differences), whereas the electrical conduction differed considerably (by an order of magnitude).

Conclusion In conclusion, this work has examined the electronic function of newly formed energy states at the organic-inorganic interface of molecular junction using a well-defined system, nalkanethiolates on gold. The investigation on the length dependence of thermopower in the large-area junctions of n-alkanethiolates confirms that the thermopower in the junction decreases linearly as the length of the n-alkane increases, and the rate of decrease becomes nearly zero at n-decanethiolate. These results experimentally verify the presence of two different length-dependence regimes in n-alkanethiolate monolayers. Our work experimentally shows that the weak energy states newly formed at the organic-inorganic interface should be considered for understanding the thermopower of wide-bandgap molecules. Given the straightforward synthetic tailorability at the atomic level in SAMs, we envisage that the new energy states at the SAM-metal interface can be chemically tuned (perhaps with respect to size and position) and harnessed for new applications in organic and molecular thermoelectrics and electronics. Hybridization of organic and inorganic matter commonly leads to the unexpected emergence of new energy states at the organic-inorganic interface. Their electronic functions are difficult to define, primarily because the signal of newly formed interfacial energy states is weak, compared to those of bulk materials, and no straightforward operando analytical technique is available that can detect them. When molecules are adsorbed on a metallic substrate, the resulting organic-metal interface can create such new energy states, yet direct probing of their electronic function remains challenging for many cases. Our work shows that



thermoelectric measurements over electrode–molecule–electrode molecular junctions allow identification of such interfacial energy states.

Associated Content Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Statistical methods for analyzing thermoelectric data, thermopower analysis of junction, histograms of thermoelectric voltage, Gaussian mean-based fitting method, plot of yield of working junctions against the length of n-alkane chain, all data-based linear square (LS) and linear absolute deviation (LAD) fitting data, summary of previous studies, determination of skewness for thermovoltage histograms (PDF)

Author Information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements This research was supported by the National Research Foundation of Korea (NRF2019R1A2C2011003; NRF-2017M3A7B8064518; NRF-2019R1A6A1A11044070). S. Park acknowledges the support of the Korea University Graduate School Junior Fellowship and the Hyundai Motor Chung Mong-Koo Foundation.


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ToC graphic (4.76 cm × 8.47 cm)


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Two Different Length-Dependent Regimes in Thermoelectric Large

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