High thermopower of mechanically stretched single-molecule junctions

Abstract

Metal-molecule-metal junction is a promising candidate for thermoelectric applications that utilizes quantum confinement effects in the chemically defined zero-dimensional atomic structure to achieve enhanced dimensionless figure of merit ZT. A key issue in this new class of thermoelectric nanomaterials is to clarify the sensitivity of thermoelectricity on the molecular junction configurations. Here we report simultaneous measurements of the thermoelectric voltage and conductance on Au-1,4-benzenedithiol (BDT)-Au junctions mechanically-stretched in-situ at sub-nanoscale. We obtained the average single-molecule conductance and thermopower of 0.01 G0 and 15 μV/K, respectively, suggesting charge transport through the highest occupied molecular orbital. Meanwhile, we found the single-molecule thermoelectric transport properties extremely-sensitive to the BDT bridge configurations, whereby manifesting the importance to design the electrode-molecule contact motifs for optimizing the thermoelectric performance of molecular junctions.

Figure 1. Simultaneous measurements of conductance and thermoelectric voltage of molecular junctions.

(a), A microheater-embedded mechanically-controllable break junction used for forming stable Au-BDT-Au junctions at room temperatures. The free-standing Au junction was broken and reformed through manipulating the substrate bending via piezo-control. The adjacent Pt microheater was heated under the applied voltage V_h to create a temperature gradient at the junction for inducing detectable level of thermovoltage. (b), Schematic description of the measurement scheme. The conductance of molecular junctions was obtained by recording current under dc voltage V_b = 0.2 V. Subsequently, the potential drop at the 100 kΩ sensing resistor ΔV_c was acquired at V_b = 0 V, from which the thermovoltage at the junction ΔV was deduced. The sequential G - ΔV_c measurements were performed in the course of repeated formation and breaking of BDT molecular junctions. c-d, Partial G – t curve (c) and corresponding two-dimensional histogram (d) obtained at V_h = 3.0 V. e-f, The simultaneously recorded ΔV – t trace (e) and the two-dimensional histogram (f).

Figure 2. Single-molecule conductance versus thermoelectric voltage characteristics.

a-c, ΔV – G diagrams of BDT junctions at V_h = 2.0 V (a), 3.0 V (b), and 4.0 V (c). d, ΔV_ave versus G semilog plots at V_h = 2.0 V. Dashed line denote ΔV_ave = 0 V. e, The standard deviation σ in ΔV plotted against G. Dashed lines represent G = nG_BDT for n = 1 to 3, where G_BDT = 0.011 G₀ is the single-molecule conductance of Au-BDT-Au junctions. A sharp rise in σ at G ~ 0.2 G₀ signifies a transition between Au atomic wires and BDT molecular junctions during the break junction experiments whereat the thermoelectric voltage changes its sign from positive to negative (see also Fig. S8).

Figure 3. Single-molecule thermopower.

a, Histogram of ΔV at V_h = 2.0 V. Peaks at the positive and negative regimes correspond to the thermovoltage occurred at Au atom-sized contacts and BDT molecular junctions, respectively. Red solid curve is Gaussian fitting to the thermoelectric voltage distribution that define the peak voltage ΔV_p for BDT single molecule junctions. Green curve is the fit to the thermovoltage distribution for Au contacts. b, Heater voltage dependence of ΔV_p. The negative values reflect the thermoelectric characteristics of BDT junctions. Dashed lines are V_h² fits. Error bars denote the full-width at half-maximum of the V_p distributions. c, Au single-atom contact lifetime τ (blue) and the estimated local contact temperature T_c (red) plotted with respect to V_h. Dashed line is a quadratic fit to T_c. d, Thermopower S of BDT molecular junctions plotted against ΔT_c. Dashed lines are the average of S.

Figure 4. Mechanical response of thermovoltage.

a, Three consecutive G (blue) and ΔV (red) traces at V_h = 3.0 V. Green dashed line indicates ΔV_ave. Whereas the first two junctions possessed ΔV close to the average value, the third junction show a high thermovoltage exceeding ΔV_ave by more than an order of magnitude. b, Three types of stretching dependence of ΔV (red) in BDT junctions demonstrating rapid increase (RI), steady rise (TR), and steady decrease (TD) in the absolute value of the thermoelectric voltage by junction elongation. The graph also displays the simultaneously recorded G (blue). c, BDT junction with optimal thermoelectric properties observed in the present work that possess S_BDT = 120 μV/K and G_BDT = 0.21 G₀ after RI. Red and blue curves denote the mechanical response of G and ΔV, respectively. d, Power factor GS² calculated from data in (c).

Figure 5. Geometrical dependence of thermoelectric voltage in single-molecule junctions.

a, Coupling-insensitive thermopower of BDT tunnelling junctions. A transmission curve calculated for the average properties of S_BDT = 15 μV/K and G_BDT = 0.011 G₀ (blue) using Lorentzian expression of the transmission and the thermopower with E_F = 5.0 eV, where E_gap is the HOMO-Fermi level gap and Γ_L,R is the coupling strength at the left and right electrodes. While the curve tends to become sharper under lower Γ (red), the slope of the transmission curve at the electrode Fermi level E_F (black dotted lines) changes little due to the relatively large E_gap of E_gap = 0.97 eV. b, Transmission curve for BDT junctions having E_gap = 0.10 eV with Γ_L,R = 0.05 eV (blue) as deduced from S_BDT = 120 μV/K with G_BDT = 0.21 G₀ attained by a specific junction displayed in Fig. 4c. A curve for the average junctions is also shown (red). The schematic models denote possible junction motifs responsible for the two different thermoelectric properties having a hollow-hollow geometry and an adatom-coordinated configuration. c, The normalized number of events for the three ΔV – t characteristics.