3. Fabrication Method
3.6 Calibration of the Nanogap Distance
The calibration of the nanogap distance of MCBJ can be determined by two methods;
considering the mechanics of MCBJ system and applying the tunneling conductance.
By the mechanics of MCBJ system
The sub-angstrom (< 10-10 m) precision control of the space between two metallic electrodes was accomplished by the lithographically defined nanoscale devices, a delicately processed differential screw (M3.5/M2.5), and a DC motor (Faulhaber, 22/2, reduction ratio 1734:1) under control of a motion controller (Faulhaber MCDC 2805) (see Figs.3.4 and 3.5). The resolution of the mechanics (Δs) is determined by the reduction ratio (Rmotor), the difference of differential screw pitch (α=0.1 mm), and the attenuation factor (r) as expressed with following formulas.
Based on our geometry, the thickness of the substrate t = ~0.27 mm, the length of the free standing bridge u = ~2 m, the space between the counter supports to the bending L = 16 mm, and ζ is the correction factor which has a value varying from 2 to 4, we obtain the resolution of the mechanics Δs ~ 1∙10-2 Å per step [56, 71].
By the tunneling conductance
The junction distance can be calibrated from the result of the conductance versus motor positions by the fitting of experimental tunneling curves. We can calibrate the distance calibration factor (R) from the following formulas.
41
2 *
exp 2
G s m (3.2)
* *
2 2
ln G s 2 m const . M R 2 m const .
(3.3)*
ln
2 2 R G
dM m
(3.4)For Au, we used the work function Φ = 5.1 eV, and effective electron mass m* = 1.12 me [56]. For Pt: Φ = 5.6 eV, and m* = 2.06 me were used [95, 96]. Herein we can denote the movement of junctions as a multiple of the movement step of motor (M) and the distance calibration factor (R) as Eq. 3.3. The slope of a logarithmic conductance versus motor movement can be obtained by the linear-fitting procedure as shown in Fig. 3.5(b). Finally, the distance calibration factor (R) is deduced from Eq. 3.4 and demonstrated for many samples as shown in the inset of Fig. 3.5(b). However, there could be a large deviation of the R of about 30 % due to the variation of the effective mass and work function for nanoscale tips [56, 71].
Figure 3.5. Calibration of molecular junction distance. (a) Schematic diagram of the MCBJ system (This figure was obtained from the database of Scheer group). (b) The conductance of Au samples as function of the motor position when the junction is separated, with linear-fits (red solid lines). Inset is the distance calibration factor of several devices.
42
43
Chapter 4
Measurement Setups
In this chapter, the low temperature, and the electronical measurement methods are presented. Measurements at low temperatures provide several merits, for example, clean contacts due to the high cryogenic vacuum conditions, higher stability of metal electrodes, and lower thermal noise, thus they enable IETS measurements. At room temperature, the metal atoms are very mobile showing large conductance fluctuations, whereas at low temperatures, the metal atoms are less mobile and the nanogap width is more stable. The thermal noise also drops by more than one order, and this provides a chance to measure less conducting molecules, the current level of which would be below the thermal noise of room temperature. In addition, the IETS measurement is possible. The vibronic excitation is smeared out at high temperatures as discussed in Chapter 2. Therefore, this thesis concentrates on the low temperature measurements.
4.1 Low Temperature Transport Measurement Setup
Charge transport measurements through a single molecule were carried out in a custom-designed cryogenic vacuum insert equipped with a mechanically controlled break-junction (MCBJ) system. The MCBJ mechanics is presented in Figs. 3.4 of Chapter 3. The devices are mounted into the break-mechanism inside an inner vacuum chamber which is evacuated and purged with He gas before being immersed into a liquid He dewar. Prior to mounting the samples, the electrical connection is grounded to avoid any electrical shock to the samples. The breaking mechanics is controlled by a DC motor with a gear box (a reduction ratio = 1:1734) connected with a vacuum feed-through. In order to reduce the noise signals, low temperature coaxial cables linking between the sample leads and the vacuum feed-through connectors and SMA connectors for room temperature connections were used.
Inside the inner vacuum chamber, the sample holder part is shielded with a copper can to prevent the influence of electromagnetic fields.[39, 94]
When recoding the opening and closing curves the conductance was measured by a sub-femtoamp source-meter (Keithley 6430) operating with an automatic variable gain
pre-44
amplifier. After cooling down the cryostat, the first opening of the junction using MCBJ mechanics is started while monitoring the conductance. Usually the first breaking occurs after 13 turns of the step motor, but this depends on the exact dimensions of the samples.
The maximum was 28 turns for this setup until the differential screw reached to a limit (see Fig. 3.4(b) of Chapter 3). During cooling down the samples, the copper wire on the contact pads of samples may accidentally disconnect resulting in an open electrical circuit. In order to avoid this, it is better to wait longer time to allow the epoxy glue and the silver paint to harden thoroughly. In addition, if the epoxy glue is spread widely on the sample, this may result in the detachment of wires from the sample while bending the samples. The polyimide layer should be protected against any stretching, because breaks in the polyimide may result in the electrical connection between bronze substrate and the electrodes. If the samples are mounted and work successfully, the molecular junctions are very stable at low temperatures as shown in Fig. 4.1.
In order to measure the I-V curves, a programmable dc source (Yokogawa 7651) and a low-noise current amplifier (Ithaco 1211) followed by a digital multimeter (Agilent 34401A) are used. Every ground of the system were carefully designed to avoid ground-loops and electrical noise. All data were collected by a Labview software through GPIB cables. This set-up for the low temperature experiment is presented in Fig. 4.2.
Figure 4.1. The time dependent current is measured through a molecular junction at 230 K in dark with stopping the rotating screw (i.e. a fixed contact distance). The junction is stable during an hour at 230 K. The current is more stable at 4.2 K. However at room temperature, the current fluctuates by more than one order of magnitude.
45
4.2 IETS Measurements
The differential conductance (dI/dV) and the 2nd differential conductance (d2I/dV2) spectra were simultaneously measured using twin lock-in amplifiers (Stanford Research Systems 830) followed by two digital multimeters (DMM) and a low noise current amplifier as shown in Fig. 4.2.[26, 30] Here, the signal is obtained via GPIB of DMM, not of lock-in amps (LIA), because the signals from the GPIB of LIA are not computed by considering the other factors as sensitivity and gain. Therefore, the front output of LIA is used and recorded by DMM in order to obtain better resolution and more precise signals. A dc bias added by an ac modulation of 6 mV (root-mean-square) at a frequency of 317 Hz was applied to the sample. The ac voltage is divided (factor of four) by a custom-made voltage divider operated by Op-amps. Herein the modulated frequency is decided by depending on RC characteristics of the setup. The wiring and electronics used for these experiments provided cutoff frequency of ~ 800Hz. The cutoff frequency is easily checked monitoring the ac amplitude while sweeping the ac frequency. From the approximate cutoff frequency, the amplitude of the ac signal is reduced. The ac phase is modulated manually instead of using an automatic function in order to obtain a more precise value, and then it is inverted to -90 degrees for second harmonic measurement. For the IETS measurement, a dc bias was swept very slowly (~1 mV/sec). The validity of the obtained spectra was checked by the agreement between the repeated measurements, and the spectra (d2I/dV2) were also checked to be consistent with the tendencies found by calculating the derivative of the first-harmonic signal (dI/dV) numerically. Detailed properties and background of IETS are discussed in Chapter 2. In this thesis, the two-point measurement method applying a voltage bias and using a current amplifier is performed because this is an easy and proper method for measuring high resistive samples. This method works for both the current measurement and the IETS measurement. The usual two-point measurements with a reference resistor in series for providing a current bias and measuring the voltage across the sample is the proper method for lower resistance of samples.
46
Figure 4.2. Schematic diagram of the electronic set-up used for the low-temperature transport measurements.
4.3 Optical Setup
For the study of the photochromic molecules presented in Chapter 8, a multi-mode optical fiber is installed in the cryostat via a fiber vacuum feed-through (see Fig. 4.3). The installed fiber is shown in Fig. 4.4(a). Because the optical absorption spectra of the molecules differ from each other and depend on the isomeric state of the species, a palette of different wavelengths has to be used. Therefore, a programmable high power (maximum 40 mW/cm2) LED source containing 365, 442, 550 and 630 nm is connected. The light spot on the sample has a diameter of about 1 mm. The LED spectroscopy is shown in Fig. 4.4(b).
The in-situ optical switching measurement under light irradiation was examined using Au-octanedithiol-Au molecular junctions for the first step in order to know the stability and to examine other effects by lights. As presented in Fig. 4.5, there is no strong effect of visible light, whereas there is a significant change under UV light. The UV light may heat the junctions and degrade the molecules. Increase of conductance under UV light was not observed in non-photoactive molecules. In photochromic molecular junctions (e.g. Au-4Py-Au), the reversible conductance switching was observed for a single period under UV and visible-light irradiation as shown in Fig. 4.6. Under UV light, the conductance increased about two times, and the conductance decreased back with visible light. However, this effect was not repeatedly observed, and cannot be discussed completely in the framework of this thesis.
47
Figure 4.3. Schematic diagram of the electronic and optic set-up used for the in-situ optical switching measurements. Custom assembled multi-mode optical fiber is installed inside of cryostat through vacuum feed-through (VFT).
Figure 4.4. (a) A MCBJ system and optical fiber installation in a custom designed cryostat.
White box indicates the position where the single molecule sample is mounted. The junctions head for downward and the light is irradiated from the bottom. (b) The light spectra of source (LED) measured with minimum power of light. These four light sources are selectively exposed depending on the absorption properties of switching molecules.
48
Figure 4.5. Au-Octanedithiol-Au junctions were exposed under (a) 630 nm and (b) 365 nm wavelength at 4.2 K while measuring conductance at a fixed contact distance. D and O indicate the dark state without light and the light on of each light, respectively. The light power at the end of optical fiber was 0.05 mW/cm2. Under visible light irradiation (630 nm), the conductance does not change significantly, but the noise enhances. Under UV light irradiation (365 nm), the conductance decreases due to perhaps the change of configurations on metallic atoms.
Figure 4.6. The in-situ conductance switching of a Au-4Py-Au molecular junction at 230 K.
The conductance increased and decreased under UV and visible light, respectively. This effect was not observed repeatedly because the junctions were degraded under UV light while decreasing the conductance. This switching effect was not observed below 200 K. We assume that there is a temperature gating effect for the switching mechanism. In order to clarify this, the reversible switching measurements as a function of temperature is required.
49
Chapter 5
Conductance and Vibrational States of Single-Molecule Junctions controlled by Mechanical
Stretching and Material Variation
This work has been published as Y. Kim, H. Song, F. Strigl, H.-F. Pernau, T. Lee, and E.
Scheer, Phys. Rev. Lett. 2011, 106, 196804. We reproduce here a slightly adapted manuscript.
The aim of this set of experiments is to reveal the role of the contacting electrode metal and the molecular conformation onto the transport properties of single molecule junctions. For that purpose, we chose the well-studied model molecule hexanedithiol (HDT), because it is known to provide a rather well-defined conductance with robust chemical bonds to gold and a prominent conformational change known from earlier experiments and theoretical studies.
The contact configuration is changed by stretching the MCBJ electrodes. The influence of the metal is studied by comparing junction of the same molecule, but one time contacted with gold electrodes, one time with platinum electrodes. This chapter presents inelastic electron tunneling spectroscopy (IETS) measurements carried out on HDT single molecule contacts at low temperature. Under stretching the alkanedithiol molecular junctions, both the molecular conformation and the contact geometry can be changed, and these effects were expected to influence the linear conductance by more than one order of magnitude. As explained in Chapter 2, the changes of the molecular conformation, the contact geometry and the molecule bonding influence the molecular vibrational modes and the metal-phonon modes are thus reflected in the IETS signals. By combining IETS with mechanical control and electrode material variation, we show that the metal electrodes influence the transport properties in a double manner, by virtue of their electrical and their mechanical properties. The electrical properties determine the bonding strength and the linear conductance, the mechanical properties are important for the molecular conformations and thus indirectly also for the transport properties. The mechanical strain of different electrode materials can be imposed onto the molecule, opening a new route for controlling the charge transport through individual molecules.
50
5.1 Introduction
Extensive studies on charge transport through single molecules have been performed for the implementation of molecular-scale devices, as well as with the objective of understanding how the molecules are able to carry charges in such devices as discussed in Chapter 2 [12, 13, 26, 30, 97, 98]. The conductance of the same single molecule contacted with a mechanically controllable break-junction (MCBJ) or with a scanning tunneling microscope (STM) is reported to have various values because the contact geometry and the molecular conformation may vary as shown in Chapter 2. To understand precisely such dependences, more sophisticated experimental studies are required, in spite of the complexity of those measurements. In such single-molecule devices, both the contact geometry and the material of electrodes (e.g., gold (Au) or platinum (Pt)) can significantly influence the charge transport through the single molecule, although it is specifically anchored by functional end groups (e.g., thiol (-SH)) [95, 99-101]. Alkanedithiol is one of the most appropriate candidates to study these properties, owing to its simple and flexible structure with the -bonding (see also Fig. 2.20 in Chapter 2). Imbedded into a junction it can adopt the usual trans conformation as well as a defect conformation. A well-known defect is given by the so-called gauche conformation, which is predicted to give rise to alterations of the charge tunneling [27, 40, 73, 102]. To understand such manifold behavior of a single-molecule junction, inelastic electron tunneling spectroscopy (IETS) has been introduced as a powerful tool, which is very sensitive and applicable for detecting vibrational excitation in solid-state molecular devices as introduced in Chapter 2 [25, 26, 30, 34, 98]. Here IETS measurements for 1,6-hexanedithiol [SH-(CH2)6-SH, denoted as HDT] molecules when stretching both Au and Pt MCBJs used as adjustable electrodes at low-temperature are discussed. The signature of the different molecular conformations and contact geometry are demonstrated by the appearance of particular IETS signals as well as by changes in the conductance.
5.2 Results and Discussion
The HDT molecules are connected to the electrodes formed by the MCBJ technique as illustrated in Fig. 5.1(a). The details of the device fabrication are described in Chapter 3. In order to determine the preferred conductance values of the molecular junctions, the junctions are repeatedly opened and closed, and then the conductance histograms as shown in Fig. 5.1(b) is recorded. The inset of Fig. 5.1(b) presents typical conductance traces acquired during opening processes. The molecular junctions show several plateaus of conductance as well as conductance variations within a plateau. In both histograms two clear conductance maxima are observed and denoted as ‘highest conductance’ (HC) and
‘lowest conductance’ (LC), respectively, as indicated by the arrows. For the Au-HDT-Au, two additional intermediate conductance peaks are observed. These peaks arise from multiple possible contact geometries of Au junctions are assumed. The appearance of
51 several preferred conductance values of an individual alkanedithiol molecule between Au electrodes had been observed and described theoretically before [73]. Here it is concentrated with our IETS measurements on the HC and LC regimes to analyze distinct changes and differences between the two electrode materials. The conductance values of these maxima are approximately twofold higher in the Pt-HDT-Pt junctions than for the Au-HDT-Au junctions, in agreement with previous studies [95, 99]. Since a narrow 5d band of Pt is located at the Fermi level (EF), the local density of states (LDOS) of the d band for Pt at EF is higher by one order of magnitude than that of Au which exhibits stronger s-orbital contribution, resulting in an enhancement of the conductance [95, 99-101, 103]. The current-voltage (I-V) characteristics (see Fig. 5.2) can be well described by the single-level model (see Chapter 2 and Eq. 2.1.) for symmetric metal-molecule coupling for both junction types, Au-HDT-Au and Pt-HDT-Pt. No temperature dependence of the I-V characteristics is observed, concluding that tunneling is the conduction mechanism for these single-molecule junctions.
Figure 5.1. (a) Schematic illustration of MCBJ system (bottom) and a scanning electron microscope image of Au break-junction electrodes with a conceptual image of the Au-HDT-Au junction. (b) Recorded histograms of Au-HDT-Au (black) and Pt (red) junctions, repeated 2000 and 300 times, respectively. Inset shows some representative conductance traces. (c) IETS (black) of HDT single-molecule connected with Au and Pt. For negative polarity the sign of d2I/dV2 has been inverted for better illustrating the symmetry. The red curves are symmetrized with respect to the bias polarity obtained by the simple formula ( y( ( )f x f(x)) / 2 ) which applies for the symmetrization of point-symmetric functions.
52
Figure 5.2. Current-voltage characteristics of single-molecule junctions. (a) The temperature dependence of current-voltage (I-V) curves in an Au-HDT-Au junction, when the junction is stretched about 8 Å. (b) The I-V curves of Au-HDT-Au and Pt-HDT-Pt are recorded after stretching ~10 Å at 4.2 K. Each I-V curve is fitted (solid lines) by the single-level model.
The parameters used are: Γ = 4.5 meV, E0 = 0.366 eV for Au-HDT-Au of (a), Γ = 3.2 meV, E0 = 0.364 eV for Au-HDT-Au of (b), Γ = 7 meV, E0 = 0.364 eV for Pt-HDT-Pt of (b).
Representative IETS spectra are shown in Fig. 5.1(c). The spectra seem highly symmetric, implying that the IETS signals originate from the excitation of molecular vibrations [26, 30, 98]. The detailed IETS measurement method is explained in Chapter 4. The IETS spectra defined as (d2I/dV2)/(dI/dV) are presented while separating the junction from the HC (Fig.
5.3(a)) to LC (Fig. 5.3(b)). The distance scale is set to zero at the beginning of the HC plateau. After stretching the junction for about 4.5 Å, the conductance jumps to the LC regime. The junction is continuously stretched to a total elongation of 14 Å. The distance values are calibrated by the linear-fitting of experimental tunneling curves as presented in Chapter 4. By comparison with previously studied IETS measurements and theoretical calculations, the vibrational peaks in the spectra are assigned: Z: longitudinal metal phonon, I: gold-sulfur stretching ((Au-S)), II: sulfur stretching ((C-S)), III: carbon-hydrogen rocking (r(CH2)), IV: carbon-carbon stretching ((C-C)), V: carbon-hydrogen wagging (w(CH2)), VI: carbon-hydrogen scissoring (s(CH2)) modes, (see also Table 5.1).
The shapes of IETS spectra in HC of Fig. 5.3(a) and LC of Fig. 5.3(b) vary, because the IETS depends on the molecular conformation, the atomic arrangement, and the metal-molecule coupling. Moreover these variables can influence the molecular conductance as well [42, 73, 104]. Firstly, the potential changes in molecular conformation for the Au-HDT-Au junctions, e.g., the gauche and trans molecular conformation are investigated.
53
Table 5.1. Summary of vibrational mode assignments in IETS spectra for alkanedithiol molecules. Each peak position of IETS spectra is determined by previously studied IETS experiments and calculations. IP and OP indicate in-plain and out-of-plain, respectively.
Modes Description Peak Position
References
mV cm-1
I: (Au-S) Au-S stretching 34-38 274-306 [30, 40, 44, 105-109]
II: (C-S) C-S stretching 75-91 605-734 [27, 29, 30, 40, 44, 106-109]
III: r(CH2) CH2 rocking IP 91-124 734-1000 [26, 27, 29, 30, 40, 44, 105, 107-109]
IV: (C-C) C-C stretching 138-147 1113-1186 [26, 27, 29, 30, 40, 44, 106-109]
V: w(CH2) CH2 wagging OP 159-167 1282-1347 [26, 27, 29, 30, 40, 44, 105, 107-109]
VI: s(CH2) CH2 scissoring IP 175-196 1411-1581 [29, 30, 40, 44, 105, 107-109]
The trans conformation is the usual one, in which the H atoms attached to neighboring C atoms are positioned opposite to each other. The carbon chain has a zig-zag shape all over the molecule. If one gauche defect is present, the H atoms attached to the two neighboring C
The trans conformation is the usual one, in which the H atoms attached to neighboring C atoms are positioned opposite to each other. The carbon chain has a zig-zag shape all over the molecule. If one gauche defect is present, the H atoms attached to the two neighboring C