Temperature-Dependent Electron Transport in Single

Article pubs.acs.org/JPCA

Temperature-Dependent Electron Transport in Single Terphenyldithiol Molecules T. Grellmann,†,‡ D. Mayer,† A. Offenhaü sser,† and R. Wördenweber*,† †

Peter Grünberg Institute (PGI) and JARA-FIT Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany ‡ Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain ABSTRACT: Analyzing the electronic properties of individual terphenyldithiol (TPT) molecules in a temperature range of 30−300 K using cryogenic mechanically controllable break junctions, we observe an unexpected change of the transport mechanism with temperature for this linear and symmetric aromatic molecule. Whereas the work function (∼3.8 eV) and molecular energy level (∼0.8 to ∼1 eV depending on the analysis of the data) of TPT are temperature-independent, elastic tunneling dominates charge transport at low temperatures, whereby an inelastic transport (via hopping) sets in at about 100 K. The molecular level of TPT lies around 1 eV and is temperature-independent. This unusual temperature dependence agrees with recent predictions for other short molecules using density-functional-based transport studies as well as experimental observations obtained for similar relatively short rodlike molecules.

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INTRODUCTION One of the major goals of research into single organic molecules is their possible use as a central element in electronic circuits and devices, known as molecular electronics.1 In order to utilize the electronic properties of molecules, the basic electron transfer through the molecule has to be understood. Well-established methods for creating a metal−molecule−metal contact are, for instance, given by scanning tunneling microscope (STM) experiments or alternatively by mechanically controllable break junctions.2 Most of these experiments are performed at room temperature. However, an important parameter that can affect charge transport in molecules is the temperature.3−5 On one hand, cryogenic temperatures allow measurements to be made under more stable conditions, whereas on the other hand, a temperature-dependent characterization of single molecules helps us to understand the mechanisms that dominate the charge-transport properties of single molecules. Current− voltage characteristics reveal initial information about the conduction mechanism, and their temperature dependence allows us to analyze them in more detail. In this paper, we present studies of charge transport through p-terphenyl-4,4′-dithiol (TPT) performed with a mechanically controllable break-junction setup that can be cooled to liquid He temperature. TPT represents a relatively short molecule with a high degree of conjugation and without bulky side groups that would inhibit rotation along the molecule axis. The molecules are deposited onto a thin, free-standing Au nanocontact where they form a contact between the terminal thiol groups and the Au contacts. By repeated bending of the bridge we generate conductance−position curves (CPC) as well as current−voltage © XXXX American Chemical Society

characteristics (CVC) of the Au−TPT−Au system at temperatures ranging from 30 K to room temperature (300 K). CPCs and CVCs are used to analyze the electronic properties of the TPT single molecule, especially the temperature-dependent electronic-transport mechanism. The length dependence of the charge transport in singlemolecule contacts has been studied for various molecule types. For short molecules, temperature-independent coherent tunneling is dominating. With increasing molecule length, a change of the transport mechanism is reported from elastic tunneling processes to inelastic temperature-dependent hopping processes. Different models for the description of the temperaturedependent conduction have been proposed such as super exchange, steady-state hopping, and intermediate conduction, which result in different predictions for the dependence of the metal−bridge−metal current on molecule length, activation energy for hopping, and electronic coupling between the hopping units.6−8 Typically, the turnover from elastic to inelastic transport occurs at molecule lengths of several nanometers.3 However, the transition can be observed also for shorter molecules if the temperature of the molecules and contacts is reduced. Selzer et al.9 assigned the temperature-dependent transition from elastic to inelastic tunneling to a temperature-induced coplanar alignment of phenyl rings which facilitates an intramolecular hopping process. The activation energy needed to facilitate the Received: January 31, 2017 Revised: March 28, 2017 Published: April 4, 2017 A

DOI: 10.1021/acs.jpca.7b00977 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

atomically sharp Au electrodes. If a molecule is trapped between these electrodes, the electronic properties of the resulting metalmolecule-metal contact can be analyzed. The z-position (and thus the elongation of the bridge) is established via a piezo actuator (Attocube, ANPz101/RES) with an accuracy of 50 nm and a maximum hub of 5 mm. Together with the attenuation factor (ratio between the lateral expansion of the Au bridge Δu and the vertical motion of the piezo stack Δz) of our setup of af = Δu/Δz ≈ (1−6) × 10−6, this translates into an accuracy and maximum expansion in the lateral direction of the Au nanobridge of (5−30) × 10−5 nm and (5−30) nm. The complete setup is mounted on a cryogenic sample holder that is placed in a He-flow cryostat allowing a He atmosphere and a temperature variation between 4 K and room temperature. An important issue is the immobilization of the molecules onto the nanoelectrodes. In this paper, we concentrate on p-terphenyl4,4″-dithiol (TPT). TPT possesses a conjugated π-system with strongly delocalized electrons. It consists of three connected benzol rings in a row with two thiol groups (SH) at the ends (see sketch in Figure 2). The thiol groups provide a relatively strong bond to the Au electrodes. The TPT with a nominal purity of 96% (Sigma-Aldrich) is controlled via elementary analysis and gas chromatography. Different solvents and concentrations were tested. For the MCBJ experiments, 0.2 mM solutions of TPT in pure ethanol produced the best results in terms of the single molecule junction yield. Small amounts (typically 20 μL) of the solution are deposited onto the break junction immediately before placing the sample holder in the cryostat, which is then evacuated and refilled with He gas at a pressure of ∼10 mbar. As a result, the ethanol evaporates and leaves the TPT, which bonds to the Au via its thiol groups.

hopping process is the energy that is required to bring the rings into the coplanar conformation. So far, such temperature dependencies were mainly observed for molecules with bulky side groups at the phenyl rings which lead to an increase of the activation energy. In this work, we demonstrate a temperaturedependent transition from elastic to inelastic transport for p-terphenyl-4,4′-dithiol, a short molecule composed of three phenyl rings with only hydrogen atoms attached the carbon atoms.

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EXPERIMENTAL TECHNIQUE A cryogenic mechanically controllable break junction (MCBJ) setup was developed to analyze charge transport in single molecules. Basically, the setup consists of an Au nanobridge that is suspended above a flexible substrate and a flexible steel-tape 0.1 mm thickness coated with a 10 μm polyimide film (see Figure 1a). The Au nanobridges are prepared via a lift-off

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EXPERIMENTAL RESULTS AND DISCUSSION Figure 2a shows a typical example of a conductance-versusposition characteristic (CPC) of TPT measured at low temperature. During this procedure, the conductance G = I/V is continuously measured as a function of the z-position applying a constant bias voltage (here 50 mV) across the electrodes and a continuous increase of the z-position (here at a velocity of 3 μm/s, which is equivalent to a lateral expansion of ∼0.01 nm/s). With increasing z-position the following steps are visible: (i) At the beginning (small z-values), the conductance decreases gradually due to the reduction of the cross section of the Au nanobridge with increasing expansion, whereas during and after the breaking process the conductance decreases in a more stepwise manner. (ii) Immediately before complete rupture of the junction, a single Au atom bridges the electrodes, which results in a conductance close to the quantum of conductance Go = 2e2/h ≈ 77 μS.10 A further stretching of the bridge does not change the conductance value. Due to the “ductile” behavior of gold, a chain of Au atoms is formed, and therefore, a clearly visible plateau at ∼Go appears in the CPC. (iii) When the one-atom contact finally breaks, the conductance drops abruptly due to the mechanical relaxation of the Au chain and atomic rearrangements at the electrode apexes11 (iv) If a molecule is trapped between the electrodes, a second conductance plateau can be observed at even larger electrode displacement. This plateau characterizes the metal-molecule-metal contact, i.e., the conductivity at a given bias voltage. For TPT at 36 K and 50 mV, we obtain

Figure 1. (a) Image of the mounted sample and sketch of the working principle, the distance L between the counter supports, and the thickness t of the sample determines the attenuation factor of Δx/Δz ≈ (1−6) × 10−6 for this setup. (b) Sample design consisting of four identical bridges and resistive shunts for the protection of the nanobridges during preparation and mounting. (c) Microscopic image of a free-standing Au nanobridge on polyimide-coated steel (image in pseudocolor).

technique using electron beam lithography and Au evaporation. The optimum dimension for the nanobridge was found to be 30 nm for the width and thickness at the constriction of the contact. In order to obtain a free-standing nanocontact, an additional etching procedure (reactive ion etching at 100 W and a pressure of 20mbar O2/CH3 for 8 min) is applied, which removes the isolating polyimide layer below the Au (Figure 1c). For redundancy reasons, four identical structures are patterned simultaneously. Furthermore, a shunt is added for the protection of the bridge during mounting (see Figure 1b). For mounting, the substrate with the Au bridge is clamped in a three-point bending configuration (see Figure 1a), then the two electrodes are contacted and, finally, the electronic shunt is removed. After the molecules have been deposited, the suspended Au nanobridge is stretched by driving a piezo stack in the z-direction against the central position of the MCBJ sample until the Au nanobridge breaks, forming a pair of B


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Figure 2. (a) Typical example of a CPC of TPT recorded at 36 K and a piezo velocity of 3 μm/s plotted as a function of the piezo-stack position z (bottom axis) and lateral expansion of the nanocontact Δu (top axis12) and (b) temperature dependence of the Au−TPT−Au conductance obtained from a series of CPCs taken at different temperatures. All CPCs are recorded at 50 mV; the inset in (a) shows a sketch of the structure of TPT. The bars in (b) represent the standard deviation of the normal distribution fit of the conduction peak at Gm in the histogram obtained from several hundred CPCs recorded at each temperature.

Figure 3. Trace histograms obtained from 80 CPCs for (a) reference measurement on a bare Au−Au contact without a molecule and (b, c) Au−TPT−Au at different temperatures. The dashed lines in the plots indicate the dominant z-dependencies of the conductance in the semilogarithmic plots. Its slope m = d(G/G0)/dz can be translated into a decay constant δ = m/af (af is the attenuation factor) that describes the change of the conductance upon elongation Δz of the contact. Due to the small difference in readings, the molecular conductance Gm yields only a small change in color which is visible around (2−9) × 10−4Go and at (1−10) × 10−5Go in (b) and (c), respectively.

a conductivity of Gm ≈ 2.5 × 10−4Go. Since similarly to the first plateau, gold atoms are “pulled out” of the bridge, leading to a constant conductance with increasing z-position. The strength of the chemical bond between the molecule end groups (here thiol) and the Au, as well as the length of the molecule, affect the length of the plateau. In the case of terphenyldithiol, a relatively strong sulfur− gold bond leads to a quite stable (i.e., extended) plateau. (v) Finally, at large z-positions one of the electrode-molecule contacts breaks, resulting in a sudden drop of conductance caused by mechanical relaxation and atomic rearrangements. The conduction in this regime can be described by a vacuum-tunneling process. Figure 2b shows the resulting temperature dependence of the plateau Gm. It characterizes the conductivity of the complete Au− TPT−Au contact. The data show an increase of the conductance with increasing temperature. Below we will show that this increase is most likely caused by the increase of the conductivity of the Au−TPT contacts. Generally, the CPCs represent the basic MCBJ experiment. A better analysis of the electronic properties of the system can be obtained by histograms (i.e., a statistical evaluation of CPCs) or current−voltage characteristics (CVC) measured at constant

z-position in the regime of the plateau at Gm. Both types of analysis are discussed in detail in the following text. Trace Histograms. Whereas in a classical histogram (see, for instance, inset of Figure 5) only the conductance values measured in a CPC are used, in this type of histogram all information on CPCs is included, i.e., conductance values and z-position. A large number of CPCs obtained by repeatedly opening and closing the contact are merged in a conductance− position histogram (only the recording for opening the contact is taken) in the form of a contour plot. Figure 3 shows a comparison of trace histograms obtained for measurements without molecules and with TPT measured at different temperatures. The reference measurement without TPT shows an interesting result. After the rupture of the Au nanocontact at Go, the reference measurement without molecules (Figure 3a) displays the expected quantum mechanical tunneling behavior I ∝ V exp( −d 2me(Φ − E)/ℏ2 ) = V exp( −δd)

(1)

with me the electron mass, d the separation of the electrodes, and Φ the working function that describes the potential barrier. The resulting logarithmic decrease δ ≈ C

2me Φ/ℏ2 of the


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Histograms. Standard histograms are recorded in order to analyze the temperature dependence of the molecular conductance level Gm. The inset of Figure 5 shows a typical example

conductivity with increasing electrode separation d is indicated by the dashed line in Figure 3a. Inserting the geometrical attenuation factor, here (5.6 ± 0.2) × 10−6 and the slope m ≈ −(57 ± 8) mm−1 obtained from the histogram, we can evaluate the working function for the bare junction of ΦAu−Au ≈ 3.8 eV, which is smaller than the literature value ΦAu = 4.8 eV for the working function of pure Au.13 Due to the additional plateaus which are characteristics of the Au−molecule−Au contact, first of all, the trace histograms of the measurements with TPT are generally broader than those obtained for the reference measurement. For example, Figure 3b shows an accumulation of plateaus around ∼8 × 10−4Go. Especially in this conductance regime and below, the contour plot is broader compared to the plot shown for the reference measurement. Nevertheless, a logarithmic dependence of the conductivity on the z-position can also be seen for this experiment (see dashed line in Figure 3b). The slope in this case is flatter compared to the reference measurement in Figure 3a. It translates to a decay constant of δ ≈ 0.6 Å−1. The resulting working function in this case would be ΦAu−TPT−Au ≈ 1.08 eV. Later (see Figure 7), we show that this is the energy level of the molecular conducting band (homo or lumo) compared to the energy level of Au. In contrast, the histograms recorded at low temperatures (see, for instance, Figure 3c) show a branching of the conductivity dependence. These measurements were made after a few hours of cooling and breaking events, which were performed during cooling. Therefore, the probability of trapping a molecule between the electrodes is reduced and more breaking events without molecules are recorded. As a consequence, two different logarithmic dependences of the conductivity on the z-position are visible. Both of them are marked in Figure 3c. The steeper one describes measurements without molecules (i.e., direct tunneling between Au electrodes), where the slope is comparable to that shown in the reference measurement (Figure 3a). The flatter one, however, refers to transport processes via molecules with a slope that is comparable to that shown in Figure 3b. Figure 4 summarizes the data measured for the decay constant and resulting work function with and without TPT. In both cases there seems to be no temperature dependence of the work functions, on average they are ∼1.1 eV and ∼3.8 eV for the experiments with and without molecules, respectively.

Figure 5. Arrhenius plot of the molecular conductance Gm obtained from standard histogams for TPT recorded at different temperatures and a bias voltage of 150 mV. The dashed lines indicate the two different temperature dependencies, which could be described by elastic tunneling and inelastic hopping, respectively. The inset shows a typical example of a histogram taken at room temperature.

of a histogram measured at room temperature. A fit of the data (Gaussian fit) provides an average value for Gm and the statistical accuracy of the experiment. The resulting temperature dependence of these measurements is shown in the form of an Arrhenius plot in Figure 5. Two different temperature regimes are visible: (i) For low temperatures (T < 100 K), the conductivity seems to be temperature-independent. This behavior is characteristic of direct (elastic) tunneling. It can be described by the Simmons model, which for small voltages predicts a conductivity14 C=

Jk ΦE

with Jk = e

d 2

exp( −bd ΦE )

(2)

2

2m* /4π αℏ, b = α 8m* /ℏ, m* the effec-

tive electron mass, α a constant which is 1 for free electrons, and ΦB the potential barrier defined by the difference in the conducting energy levels of the molecule and Au. Since d is approximately equivalent to the size of the molecule and ΦB is temperature-independent as demonstrated above, the conductance is also expected to be temperatureindependent, which is in good agreement with the experimental observation. (ii) However, at higher temperatures (T > 100 K), a change in the conductance mechanism is observed. The conductance increases with increasing temperature. This increase can be explained by a hopping mechanism.15,16 The conductivity of this mechanism is given by17

⎡ E ⎤ C ∝ d −1 exp⎢ A ⎥ ⎣ kBT ⎦

(3)

with an activation energy EA and Boltzmann constant KB. The resulting temperature dependence ln(C) ∝ T−1 agrees with our measurements for T > 100 K.

Figure 4. Decay constant δ (left-hand scale) and resulting work function (right-hand scale) for different temperatures with (solid symbols) and without (open symbols) molecules. D


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Figure 6. IVC of TPT at 150 K in a linear form (a) and a Fowler−Nordheim plot (b). The RTM fit in (a) yields molecular levels of εR = (766 ± 3) meV and εL = (834 ± 3) meV and coupling constants ΓR = (91 ± 1) μeV and ΓL = (82.2 ± 0.8) μeV for the positive and negative voltage regimes, respectively, whereas the Fowler−Nordheim plot in (b) indicates εR = 1.05 V and εL = 1.17 V.

For some molecules with very small decay constant δ (e.g., (0.040 + 0.006) Å−1 for oligo-porphyrins) a temperature dependent but still phase-coherent tunneling was observed.4 However, the relatively high decay constant of δ ≈ 0.6 Å−1 observed in our case excludes this transport regime for TPT. Current−Voltage Curves. Current−voltage characteristics (IVC) can provide a detailed characteristic of the Au−molecule− Au contact. However, modifications of the Au−molecule contacts and the limited voltage range of the individual runs restrict the interpretation of these data. This is the reason for the use of histograms for this type of experiments. Nevertheless, IVCs are recorded for TPT at different temperatures and at the z-position at which the molecular conduction Gm is obtained (see for instance Figure 2). At each temperature, first the position for adequate conductance is established via a CPC followed by a scan of the voltage dependence of the current. It was found that TPT can be measured up to rather large voltages of ±1.5 V in a relatively stable and reproducible manner. However, quite significant modifications of the Au-molecule contacts are visible from run to run. Even asymmetric ICVs are usually observed, which might be caused by asymmetric bonding of the molecule in the break junction. Figure 6 shows a typical measurement of a (slightly asymmetric) IVC of TPT recorded at 150 K, displayed in the linear form and in a Fowler−Nordheim plot. The linear plot shows the characteristic S-shape that can be interpreted using, for instance, the Simmons model or the resonant tunnel model (RTM). Similar to the Simmons model, the RTM separates the metal−molecular−metal junction into two metal−molecular contacts described by two scattering factors ΓL and ΓR and a molecule with one orbital with energy ε (HOMO or LUMO) that contributes to the conductance. The resulting current is given by an integral over the energy of the electrons, I(V) = (2e/h)∫ dET(E,V)[f L(E) − f R(E)], where T is the transmission probability. Within the wide-band approximation, the transmission probability given by Tres(E) = (4ΓLΓR)/((E − ε0)2 + Γ2) with Γ = ΓL + ΓR and ΓL and ΓR is independent of voltage or energy.14 This approximation is reasonable if the density of states is relatively flat at the Fermi energy, which is true for noble metals such as Au. As a result, the RTM predicts an IVC in the symmetric case ΓL = ΓR according to

Thus, the experimental data demonstrate that for low temperatures the conductivity does not change with temperature, indicating direct tunneling is the dominant conduction mechanism in this temperature regime. At approximately 100 K a change in the behavior sets in, and the current increases rapidly with increasing temperature.15,16 The dominant electronic transport mechanism in the system seems to change from elastic tunneling to an inelastic hopping mechanism for T = 100 K. The latter seems to be unusual, since a hopping mechanism is usually reported for long molecules at room temperature. Temperature-dependent hopping transport in molecules is commonly observed for charge transport over long distances.3 Therefore, hopping transport has usually been reported for molecules that are too long to allow a coherent tunneling process at room temperature. However, recently it has been found that relatively short rodlike molecules, which are composed of aromatic phenyl rings, can also display temperature-dependent charge transport in a temperature range between 0 K and room temperature.9,18,19 Coherent transport was observed at low temperature and changes into a thermally activated sequential hopping mechanism at temperatures higher than 100 K. This is very similar to the observations made in this work. It is assumed that the charge carriers hop between two adjacent rings when they are in coplanar conformation. The transport transition at 100 K has been assigned to the onset of torsional fluctuations of the phenyl rings as the temperature increases.19,20 Moreover, the change in the transport mechanism requires isolated individual molecules and can be observed in single molecule experiments, but not usually for molecules assembled in films.18 Also density-functional-based transport studies recently predicted temperature-dependent conductivity for biphenylderived molecules with bulky side groups but not for hydrogen groups.21 The results obtained in this work indicate that even without steric side groups a transition of the transport mechanism can occur at low temperatures, which agrees with the prediction that both metal−lead-induced and configurationdependent contributions can affect the temperature-dependent transport of molecules without bulky side groups. Furthermore, Ballmann et al.22 reported recently that vibrationally induced decoherence can lead to an increase of the current with temperature when the charge carriers are transported via quasidegenerated electronic states. These states originate from the strong coupling of the thiol groups to the gold electrodes. E


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The Journal of Physical Chemistry A I (V ) =

⎛ eV/2 − ε0 ⎞ G0 4ΓLΓR ⎡ ⎟ ⎢arctan⎜ ⎝ ⎠ ⎣ e Γ Γ ⎛ eV/2 + ε0 ⎞⎤ ⎟⎥ + arctan⎜ ⎝ ⎠⎦ Γ

those obtained from the Fowler−Nordheim plots (εFN = (1.0 ± 0.3) eV). Interestingly, the values obtained for the work function of the Au−TPT−Au system, which was derived from the trace histograms (see Figures 3 and 4), behave very similarly. The work function for Au−TPT−Au is also temperature-independent and has a similar value ϕAu−TPT−Au = (1.08 ± 0.7) eV. It seems that ϕAu−TPT−Au is not the work function of the system Au−TPT−Au but the energy necessary to move electrons from the energy level of Au to that of TPT, i.e., ϕAu−TPT−Au = εTPT. This situation is sketched in the inset of Figure 7.

(4)

Although this equation is only strictly valid for symmetric IVCs, small deviations like that shown in Figure 6a can be approximated using this expression and separate fit parameters for the positive and negative voltage regime. This is demonstrated in Figure 6a where the RTM fit agrees nicely with the data and the resulting fit parameters (molecular energy level ε and coupling constant Γ) can be obtained for both parts of the fit. Both sides yield quite similar values for the molecular orbital (0.77 and 0.83 eV) and the coupling constant (Γ = (9.1 ± 0.1) × 10−5 eV and Γ = (8.2 ± 0.8) × 10−5 eV). This indicates that there is only a small anisotropy, most likely in the Au−molecule bond on both sides. Alternatively, the molecular energy level can be obtained using the Fowler−Nordheim plot by plotting ln(I/V2) versus V−1 (Figure 6b). The Fowler−Nordheim plot shows a clear minimum at V = −1.05 V and +1.17 V, which can be associated with the molecular orbital levels of the left and right contact, respectively. The values are slightly greater (40%) compared to the values obtained via the RTM fit in Figure 6a. However, they show the same tendency; i.e., the molecular orbital level of the left contact is about 8% smaller than that of the right contact. Finally, we compare the temperature dependence of the molecular energy level and work function of TPT. Figure 7 indicates that there is no temperature dependence of the molecular energy level irrespective of the way in which the energy level is obtained. RTM fits and Fowler−Nordheim plots obtained from IVCs provide similar values that are temperature-independent over the whole temperature regime from 30 K to room temperature. The values obtained for the RTM fits are slightly smaller (εRTM = (0.8 ± 0.2) eV) compared to

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SUMMARY We developed a cryogenic setup for the temperature-dependent analysis of the electronic properties of single molecules. Temperature-dependent measurements of p-terphenyl-4,4″dithiol were performed from 300 K down to 30 K using conductance−position curves for statistical analysis. Additionally, current−voltage curves are recorded and analyzed via different models (Fowler−Nordheim plot and fits using the resonant tunneling model). A detailed analysis of the CPCs using, for instance, trace histograms yields a temperature-independent work function of ∼1.08 eV and a temperature-independent molecular energy level of εRTM = (0.8 ± 0.2) eV and εFN = (1.0 ± 0.3) eV for the RTM and FN plot, respectively. The experimental results agree with literature values obtained, for instance, from measurements of self-assembled monolayers.23 In contrast, the charge transport of TPT seems to be temperature-dependent. With increasing temperature, the conductivity of the Au−TPT−Au contact increases in a characteristic manner indicating a change of the transport mechanism with temperature. At low temperature, a direct tunneling mechanism is observed, which changes to an inelastic (hopping) mechanism at high temperature. This change takes place at approximately 100 K. Although hopping transport is usually only reported for molecules that are too long to allow a coherent tunneling process, such a change of the transport mechanism is not unusual. Recently, similar transitions have been seen at a similar temperature for relatively short rodlike molecules composed of aromatic phenyl rings. They have been assigned to an onset of torsional fluctuations of the phenyl rings as the temperature increases.9,18−20 However, the absence of steric side groups and the resulting small activation energy for torsional conformation changes makes this explanation unlikely for p-terphenyl-4,4″dithiol. It rather confirms predictions that ascribe the temperature-dependent conductivity of short heterocycles to Fermi distributions in the leads24 and vibrational coupling of the molecules to the electrode.21,22 The current study shows that also short symmetrical heterocycles without steric side groups can exhibit a temperaturedependent change of the transport mechanism. Further experimental and theoretical studies are required to reveal details of the effect of temperature on the transport of charge carriers in molecular junctions.

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Figure 7. Temperature dependence of the molecular orbital level of TPT obtained from IVCs via Fowler−Nordheim plot (blue) or RTM fits (red). Triangles pointing to the right and triangles pointing to the left indicate the values obtained in the positive and in the negative voltage regime, respectively. For comparison, the energy level of the work function obtained from trace histograms of the Au−TPT−Au contact (Figure 3 and 4) is added (green line). The inset shows schematically the different energy levels of Au, TPT, and vacuum and the resulting working functions and relative energy level of TPT.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

T. Grellmann: 0000-0003-4072-1657 D. Mayer: 0000-0003-1296-8265 A. Offenhäusser: 0000-0001-6143-2702 F


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(19) Selzer, Y.; Cabassi, M. A.; Mayer, T. S.; Allara, D. L. Temperature Effects on Conduction through a Molecular Junction. Nanotechnology 2004, 15, S483. (20) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Dependence of Single-Molecule Junction Conductance on Molecular Conformation. Nature 2006, 442, 904. (21) Pauly, F.; Viljas, J. K.; Cuevas, J. C.; Schön, G. Density-Functional Study of Tilt-Angle and Temperature-Dependent Conductance in Biphenyl Dithiol Single-Molecule Junctions. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 155312. (22) Ballmann, S.; Haertle, R.; Coto, P. B.; Elbing, M.; Mayor, M.; Bryce, M. R.; Thoss, M.; Weber, H. B. Experimental Evidence for Quantum Interference and Vibrationally Induced Decoherence in Single-Molecule Junctions. Phys. Rev. Lett. 2012, 109, 056801. (23) Hong, S.; Reifenberger, R.; Tian, W.; Datta, S.; Henderson, J.; Kubiak, C. Molecular Conductance Spectroscopy of Conjugated, Phenyl-Based Molecules on Au(111): the Effect of End Groups on Molecular Conduction. Superlattices Microstruct. 2000, 28, 289−303. (24) Poot, M.; Osorio, E.; O’Neill, K.; Thijssen, J. M.; Vanmaekelbergh, D.; van Walree, C. A.; Jenneskens, L. W.; van der Zant, H. S. Temperature dependence of three-terminal molecular junctions with sulfur end-functionalized tercyclohexylidenes. Nano Lett. 2006, 6, 1031−1035.

R. Wördenweber: 0000-0002-1898-0751 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank R. Kutzner, S. Trellenkamp, and E. Hollmann for their valuable support. REFERENCES

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Temperature-Dependent Electron Transport in Single

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