Conclusions - Measurement Setups - Controllable and Switchable Properties of Organic Single Mol

4. Measurement Setups

5.3 Conclusions

Figure 5.9. One IETS is selected among our continuously measured spectra. This IETS spectrum is closer to the result of zero tilt-angle.

5.3 Conclusions

In conclusion, the systematic comparison study, describing the stretching dependence for IETS spectra of alkanedithiol single-molecule junctions with two different metal-electrodes (Au and Pt), clearly shows the formation of different molecular conformations. By tracing the metal-sulfur vibrational modes under stretching, we found that both molecular conformation and contact geometries are strongly influenced by the electrode materials.

With the rather stiff Pt as adjustable nanoelectrodes, instead of the conventional and smooth Au, we are able to excite and to tune particular vibrational modes, e.g. the extremity modes of a molecule. Such combination of methodologies provides clear evidence that the conductance variation, when elongating single-molecule junctions, is caused by the change of both the molecular conformation and the contact geometry and will pave the way to establishing a detailed correlation between the precise atomic arrangement and the conductance properties of molecular junctions.

Chapter 6 Characteristics of Amine-Ended and Thiol-Ended Alkane Single-Molecule Junctions Revealed by Inelastic Electron Tunneling Spectroscopy

This work has been published as Y. Kim, T. Hellmuth, M. Bürkle, F. Pauly, and E. Scheer, ACS Nano 2011, 5, 4104. The chapter is an adapted and extended version of the mentioned manuscript.

As introduced in chapter 2, the molecule metal contact is of utmost importance for the transport properties of single-molecule junctions. In the previous chapter, we addressed the role of the contacting metal. The present study is devoted to the role of the moiety terminating the molecule and thus providing the electrical and mechanical contact between both. Again, we chose the molecular class of alkenes because of their rather simple electronic structure, and the fact that they are well-studied theoretically an experimentally.

However, up to now there was no detailed comparison, correlating the configurational, mechanical, and electronic properties of the alkane core with varying end-groups.

Therefore, this chapter reviews a combined experimental and theoretical analysis of the charge transport through octane single-molecule junctions with amine and thiol end-groups for increasing electrode separation. Amine-ended (NH2) and Thiol-ended (SH) molecules were known to bind in different ways to a metal surface. Thiol-ended molecules bind further strongly because the free electrons on sulfur atoms bind covalently on Au, whereas amine-ended molecules bind more gently on Au because the nitrogen atoms do not provide free electrons. In addition, it has been argued that the interaction of the hydrogen atoms in the amine group with the Au atoms, results in a narrow distribution of molecular conductance values compared to the ended molecules. In this study, for both amine-ended and thiol-ended octanes contacted to gold electrodes, signatures of chain formation by analyzing kinks in conductance traces, the junction length, and inelastic electron tunneling spectroscopy (IETS) are studied. The results show that for amine-ended molecular junctions

no atomic chains are pulled under stretching, whereas the Au electrodes strongly deform for thiol-ended molecular junctions. This advanced approach hence provides unambiguous evidence that the amine anchors bind only weakly to Au. This work was performed in cooperation with F. Pauly (now at Lawrence Berkeley National Laboratory, M. Bürkle and T. Hellmuth (KIT) who performed the theoretical calculations). The theoretical methods applied here are summarized in Appendix II.

6.1 Introduction

As discussed in Chapter 1 and 2, single-molecule contacts are extensively studied using various methods because of their promising functionality as active electronic components and their interesting charge transport characteristics. The role of the specific electrode material and the anchoring groups of the contacted molecules received special attention in recent years, because they turn out to be crucial for charge transport through molecular junctions, especially single-molecule junctions. The choice of the anchoring group can tune the charge transport behavior because the binding energy and preferred binding site of the anchoring group to the metallic electrode determine the contact geometry. The resulting alignment of molecular states with respect to the Fermi energy plays the dominant role for the conductance of molecular junctions [18, 90-92, 118]. Among several different anchoring groups of molecules, the amine (NH₂) end group has been suggested as promising building block due to its selective binding to low-coordinated Au surface atoms, resulting in a narrow distribution of molecular conductance values [18, 90-92, 118, 119]. However, depending on temperature, speed of contact formation, and further external parameters, also wider conductance distributions of the amine-ended molecules have been reported [50]. To thoroughly understand the transport mechanism in the hybrid metal-molecule structures, a more sophisticated measurement is required, and the IETS has been introduced due to its high sensitivity to the molecular configuration and contact geometry as introduced in Chapter 2.

In this chapter, the charge transport through single 1,8-octanedithiol (ODT, HS-[CH₂]₈-SH) and 1,8-octanediamine (ODA, H₂N-[CH₂]₈-NH₂) molecules contacted to Au electrodes using mechanically controllable break-junctions (MCBJ) is measured at 4.2 K. A statistical analysis of the conduction properties as a function of junction length (“opening conductance”) is performed, supplemented by theory. Particularly potential signals of chain formation upon junction elongation are focused. Peaks in the IETS spectra are assigned by combined ab-initio electronic structure and transport calculations, taking into account the inelastic effects due to the electron-vibration coupling.

6.2 Results and Discussion

Linear conductance

The ODT and ODA molecules are contacted to the Au electrodes by the MCBJ technique as illustrated in Fig. 6.1(a). To investigate the charge transport through the single-molecule junctions, the samples are mounted in the custom-made cryostat equipped with the MCBJ system, and then all the transport measurements are performed at 4.2 K (see also Chapters 3 and 4 for detailed experimental setups and sample fabrications). In order to characterize the conductance of the metal-molecule junctions, the conductance-distance traces are recorded as displayed in Fig. 6.1(b) while repeatedly opening and closing them.

The conductance histograms measured with molecules (ODT and ODA) and without molecules are presented in Fig. 6.1(c). The appearance of multiple conductance peaks as shown in the histogram of Fig. 6.1(c) was observed in different experimental setups (e.g.

with scanning tunneling microscopes or break junctions) before [39, 50, 73, 118, 120]. This observation can be attributed to the formation of contact geometries with different configurations of the molecule, of the electrodes, or at the molecule-electrode interface [39, 73, 99], and to junctions containing multiple molecules which contribute to the current [73].

In the present study it is restricted to the investigation of junctions with the lowest stable conductance as indicated by the position of the lowest-conductance peak. They are supposed to correspond to single-molecule junctions. The minimum conductance values are shown by arrows in Fig. 6.1(c). By fitting a Lorentzian function to the lowest peak in the histogram, (7.0 ± 1.2)·10^-5G₀ for ODT and (3.0 ± 0.5)·10^-5G₀ for ODA are obtained, where the error is determined by the half width at half maximum (HWHM) and G0 = 2e²/h is the quantum of conductance.

For a better understanding, simulations of the stretching of ODT and ODA single-molecule junctions using a DFT-based scheme are performed (see Figs. 8.2 and 8.3, Appendix II, and Ref. [121] for the theoretical methods). Besides the equilibrium structures at the different elongation stages, the conductance is determined and found it to range between 1-3·10^-4G0

for ODT contacts and between 4-6·10^-5G0 for ODA as shown in Fig. 6.1(d). Compared to the experiment, the conductance of ODA is slightly underestimated in the calculations, while its value is too large by a factor of around 4 for ODT. This may be related to uncertainties in the description of the alignment of the molecular states and the metal Fermi energy, as well as the band gap problem of DFT [49, 122-125]. However, the deviations appear acceptable considering the lack of knowledge with respect to the precise contact geometry. The reduced conductance, by a factor of around 5, for similar junction geometries of ODA as compared to ODT is in qualitative agreement with the experimental observations and previous reports in the literature [50, 92]. This finds that the transport for both ODT and ODA is dominated by electronic states with a high weight on the anchoring groups of the molecules and is slightly more off-resonant for ODA than ODT (see Fig. 8.2).

Figure 6.1. (a) Illustrations of ODT and ODA single-molecule junctions and scanning electron microscopy image of a MCBJ sample. (b) The conductance traces of ODT (black) and ODA (red) molecular junctions are obtained by applying 70 mV of dc voltage and are shown for the opening process. The arrow marks a representative kink or dip at which the slope of the conductance plateau upon stretching changes sign. (c) Conductance histograms of ODT (black) and ODA (red) molecular junctions. Both histograms are collected by repeating the opening and closing process 300 times and are displayed for a bin size of 3·10

-6G₀. The dashed line presents the background conductance measured in vacuum (no molecules). The arrows indicate the lowest-conductance peaks of the histograms. Junctions with these conductance values are used for the IETS measurements. (d) Calculated conductance of ODT and ODA junctions as a function of the electrode displacement. The kink in the ODT trace around 2.8 Å indicates the formation of an Au chain. The simulated atomic arrangements, shortly before and after the chain forms, are presented in the inset.

As displayed in Fig. 6.2, the strong deformations of the Au electrodes are observed upon stretching for ODT junctions [126]. In particular, atomic Au chains are being pulled [126, 127]. The transmission is rather well described by a Lorentz-like shape for the weakly deformed Au electrodes of Fig. 6.2. The resonance peak of this Lorentz curve, marked “*”, is located 1.1 to 1.4 eV below the Fermi energy. As it is visible from the sample molecular orbital (MO) in the figure, it arises mainly from p-like sulfur states on the molecule. Since they are oriented along the transport direction, they are of  character. The small and narrow resonance marked “+”, located around 0.5 eV below EF, is also essentially due to sulfur p states, but they are oriented perpendicular to the transport direction and are hence of  type.

65 Their bad coupling to the C-C  bonds explains why they do not yield an important contribution to the conductance. Overall, the transport is hole-like. The structure of the transmission curves becomes more complicated, when atomic chains have formed, as it is visible in Fig. 6.2(b). In this case also states above the Fermi energy yield a contribution to the conductance. For the ODA junctions shown in Fig. 6.3, we do not find the formation of Au atomic chains. Indeed the Au electrodes remain intact in their initial geometries during the stretching process [126]. This is a result of the weaker binding energy of the Au-N bond as compared to the Au-S and the Au-Au bonds. Similar to ODT, we find the charge transport to be hole-like for the ODA junctions, where the resonance dominating the conductance is positioned ~2 eV below EF and arise mainly from hybridized molecular electronic states located on the amines.

Figure 6.2. Junction geometries for ODT at different electrode displacements and evolution of the energy-dependent transmission. The displacement distance, measured with respect to the initial geometry, is listed above the respective junction structure. At the bottom, fixed and relaxed junction regions during the stretching process are indicated. (a) Transmission for small deformations of the Au electrodes and (b) when an atomic Au chain has formed. In each panel, the Fermi energy is represented by a vertical dashed line. Shown in (a) are isosurface plots of MOs, which contribute to the resonances “*” (lower MO) and “+” (upper MO). The latter resonance is not very pronounced and rather narrow so that “*” dominates the conductance. It is offset from the Fermi energy by around 1.1 to 1.4 eV.

Figure 6.3. Junction geometries for ODA at different electrode displacements and evolution of the energy-dependent transmission. The displacement distance, measured with respect to the initial geometry, is listed above the respective junction structure. At the bottom, fixed and relaxed junction regions during the stretching process are indicated. In the plot showing the transmission, the nature of a sample MO contributing to the dominant resonance in the transmission is shown. Its offset from the Fermi energy amounts to around 2 eV. The Fermi energy is represented by a vertical dashed line.

Inelastic electron tunneling spectroscopy

To date, several IETS studies have been performed using thiol-ended molecules. However, IETS measurements on amine-ended molecules have not been reported so far in spite of their promising transport properties. The vibrational modes of the molecules are detected by their effect on the current through the molecular tunnel barrier. In this process the excitation of a molecular vibration by the charge carriers gives rise to a peak in the d²I/dV² in the typical off-resonant situation (see Chapter 2 for more detailed discussions).

In Fig. 6.4 an antisymmetric I-V and IETS of both Au-ODT-Au and Au-ODA-Au junctions, which indicates that the molecules are symmetrically coupled to both electrodes, are measured [26, 30, 42]. Furthermore, a temperature-independent I-V for studies between 4.2 and 42 K is found, signaling tunneling transport. For the IETS measurement, the second derivative of the I-V curves (d²I/dV²) with the differential conductance (dI/dV) is normalized in order to compensate for the conductance change due to distance changes of the electrodes.

Hence, the IETS amplitude is defined as (d²I/dV²)/(dI/dV) [30, 98]. Both derivatives are measured as the first and second harmonic signals, using lock-in techniques. A dc bias to the samples added to an ac modulation of 6 mV (root-mean-square) at a frequency of 317 Hz is applied (see Chapter 4 for detailed measurement method). Once a stable junction has been established, it was able to sweep the bias voltage several times without significant change of

67 the IETS. This allows us to perform repeated IETS measurements of the same junction.

With changing electrode distance the IETS spectra vary, i.e. peaks vary in height and positions. The positions may shift by some 10 mV due to the changes in the junction geometry (see discussion below).

Figure 6.4. (a) The I-V characteristics of the single-molecule junctions for both ODT and ODA exhibit antisymmetric super-linearity, indicating that the molecules are symmetrically coupled to both electrodes. As shown in the inset, the I-V curves for an Au-ODA-Au junction were also evaluated between 4.2 K and 42 K. From the absence of temperature dependence it is concluded that tunneling is the conduction mechanism for the single-molecule junctions. (b) A IETS spectrum (black) of an ODA single-single-molecule junction is shown together with a curve antisymmetrized (red) with respect to the bias polarity, obtained by the simple formula y=(f(x)-f(-x))/2. The similarity of both traces visualizes the highly antisymmetric shape in both peak positions and peak intensities, implying again that both molecular end groups are symmetrically bonded to the electrodes [26, 30].

Figures 6.5(a) and 6.5(b) show measured IETS spectra of Au-ODT-Au and Au-ODA-Au junctions, respectively. In the low-energy regime of the IETS spectra, i.e. below 200 mV (which is regarded as the “finger-print regime” of a molecular junction because the fundamental vibrational modes typically fall in this energy range), prominent molecular vibrational peaks for both ODT and ODA molecular junctions are observed. Due to the many modes below 25 mV these energies are excluded from the discussion in this subsection, but this will come back to them when Fig. 6.7 is considered.

The vibrational modes are characterized both by performing DFT calculations of the eigenmodes of the molecular junctions and by computing IETS spectra by means of a lowest order expansion in the electron-vibration coupling (see Appendix II). In this way, peak positions, peak heights, and the evolution of vibrational energies are determined during the stretching of the molecular contacts.

Following the experiments, the theoretical IETS spectra are intentionally broadened by an ac-voltage of 6 mV (root-mean-square) [128, 129]. We show them as the dashed lines beside the experimental spectra in Fig. 4.5. The main character of those modes, which are responsible for the peaks in the calculated spectra, is indicated. Since the theoretical IETS spectra have been calculated close to the starting geometries (see the geometries with an elongation of 0.00 Å in Figs. 6.2 and 6.3), we compare them to experimental spectra taken from the middle part of stretching experiments, because in this situation a straight, but not strongly elongated molecular junction can be assumed. Those modes identified by the comparison between theory and experiment are summarized in Table 6.1.

According to the analysis of the theoretical results, the peak at around 35 mV in Fig. 6.5(a) results from the S) stretching mode and those at 29 mV in Fig. 6.5(b) from the (Au-N) stretching mode. They signal that both amine and thiol end groups are robustly bonded to the Au atomic electrodes. Further main peaks in the experimental IETS at around 53, 78, 125, and 164 mV for the Au-ODT-Au junction are attributed to _w(CH₂) wagging, (C-S) stretching, (C-C) stretching and _t(CH₂) twisting, and _w(CH₂) wagging, respectively, in accordance with previous studies [13, 26, 27, 29, 30, 40, 44, 106-109]. For the Au-ODA-Au junction, we assign the main peaks at 63, 110, 124, 139, 168, 180, and 200 mV to combined

(C-C-C) and (C-C-N) bending, combined (C-N) stretching,_w(NH₂) wagging, (C-C) stretching, w(NH₂) wagging, w(CH₂) wagging s(CH₂) scissoring, and s(NH₂) scissoring, respectively.

As expected, the higher energy modes above 100 meV, affecting mainly the C and H atoms of the molecule appear at similar energies for ODA and ODT, however with different amplitudes. Modes including motions of the NH2 groups appear in a broad energy range. It is observed that some modes appearing in the experimental spectra are absent in the theoretical ones and vice versa. This discrepancy is due to the fact that only single spectra are compared, likely corresponding to different contact configurations. Choosing the peak at 104 mV in the spectrum of ODT as an example, it has been show [108] that the _r(CH₂) rocking modes, to which we ascribe the peak, may be excited only in certain configurations which lift particular symmetries. Other prominent features, not explained by our calculations, could be due to combined _w(CH₂) wagging and _t(CH₂) twisting modes at 147 meV for ODT and _r(CH₂) rocking modes at 90 meV for ODA.

In summary, the detailed comparison of theoretical and experimental IETS spectra reveals the complex interplay of molecular conformation and inelastic transport. Changing the anchoring group has an important influence on the vibrational spectrum and the possibility to excite modes electrically. In the following we shall concentrate on metal-anchor group vibrations, and also the low-energy Au vibrational modes are discussed in this context below.

Figure 6.5. Comparison of experimental (solid lines) and theoretical (dashed lines) IETS spectra of (a) ODT and (b) ODA single-molecule junctions. Experimental results are obtained from the lock-in second harmonic signals at 4.2 K. The vertical lines show the peak positions in the theoretically obtained spectra, and modes of the same character are summarized by horizontal lines. The character of the modes is specified by the symbols explained in the text. The separation by a comma means that the peak is due to several modes, while “+” is used to indicate the mixed character of a mode.

Table 6.1. Summary of the vibrational mode assignment in the IETS spectra for ODT and ODA molecular junctions, shown in Fig. 6.3 Peak positions in the spectra are identified by our IETS calculations and previous IETS experiments and calculations for ODT.[13, 26, 27, 29, 30, 40, 44, 106-109].

(a) When we separate modes by a comma, there are several ones contributing to the same peak. When we use “+” or “with”, a single mode has a mixed character.

The gold-molecule interface

According to our theoretical investigations, the binding energy of the Au-N bond (0.92 eV) is significantly weaker than those of the Au-S bond (1.76 eV) for the binding in top position [91, 126]. Indeed, in our simulations we find that this difference results in the pulling of Au chains for ODT [27, 127], while they are absent for ODA (see Figs. 6.1(d) and 6.6).

Im Dokument Controllable and Switchable Properties of Organic Single Molecules in Nanoscale Devices (Seite 64-121)