Mechanical Controls

Mechanical control of molecular junctions is a very strong method. The tuning of electrode distance provides an opportunity to change the molecular conformation and electrode configuration. Under mechanical stress, molecules may vary their conformation and orientation as trans/gauche, twist-angle between bipyridine, tilt angle respect to metal surfaces, and also the metal atoms of electrodes can form triangle or chain configuration.

Such geometrical change influences the molecular orbital as well as the conductance properties of the metal-molecule-metal systems [14, 27, 39, 63, 68].

Figure 2.13. (a) Conductance trace under stretching of a Au nanowire measured at 4.2 K in vacuum condition. A clear 1 G0 plateau is observed, indicating a Au-Au single atom contact.

The long plateau indicates the formation of atom chains. (b) Histogram of opening and closing curves. The prominent 1 G0 peak indicates the single atom contact of Au. Below 1 G0, no peak is observed, signaling the vacuum tunneling.

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Figure 2.14. (a) Left, a schematic of the MCBJ with a the bending beam, b the counter supports, c the notched gold wire, d the glue contacts, e the piezo element, and f the glass tube containing the solution. Right, a schematic of a benzene-1,4-dithiolate (BDT) SAM between proximal gold electrodes formed in an MCBJ. (Reproduced from Ref. [12]) (b) A lithographically defined suspended MCBJ sample on polyimide layer with a schematic of Au-benzenedithiol-Au junction. (Reproduced from the author’s previous work, Ref. [32])

Mechanically Controllable Break-Junctions (MCBJ)

MCBJs have high resolution control of nanogap (< 10^-10 m) and high stability because of the principle of operation of MCBJs, which were first introduced by Moreland (1985) [69] and Muller (1992) [70]. The lithographically defined free-standing nanowire is designed on a flexible substrate (i.e. bronze or silicon) with an insulating layer such as polyimide or silicon oxide [12, 56, 60, 71]. A three-point bending mechanism consisting of a step motor with a differential screw or a piezo actuator accomplishes the movement of nanogap in picometer (pm) resolution. When the substrate is bent, the suspended metallic electrodes are stretched, and then finally the single atom contact forms showing 1 G₀ (G₀ = 2e²/h) (see Fig. 2.13).

Further stretching results in a tunneling gap with sudden drop of conductance. If the molecules are deposited onto the surface of metallic electrode before breaking the metal-metal contact, one can trap a single molecule when the tunneling gap forms (see Fig. 2.14).

The breaking of junction in vacuum forms pure metal surfaces of electrodes. In the tunneling regime (below 1 G₀), the molecular conductance plateaus appear as shown in Fig.

2.15(a), while opening the junctions. The histogram of conductance can be produced by repeating of opening and closing the junctions, and then the preferential conductance of a certain molecule can be deduced (see Fig. 2.15(b)). MCBJ system is able to use with the gate voltage, magnetic field, light, and microwave irradiation at low temperature owing to the device-like structure of measurement samples. The advantages of low temperature measurements for the MCBJ system are the following; the atomic-scale electrodes become more stable with less fluctuation (Au atoms are very mobile at room temperature), and it is possible to apply high magnetic field, and to measure vibronic properties. The disadvantage of MCBJ is that the speed of mechanical motion is usually very slow (< 1nm/sec). Detail results using MCBJ system are presented in Chapters 5-8. The sample fabrication recipe and measurement setup are presented in Chapters 3 and 4, respectively.

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Figure 2.15. (a) Examples of conductance traces of benzenedithiol (BDT) molecular junctions are shown for the opening process. The shaded area is metallic contact region. (b) Conductance histograms of BDT molecular junctions. The histogram is collected by repeating the opening and closing process 300 times. The arrows indicate the prominent conductance peaks of the histograms. (Reproduced from the author’s previous work, Ref.

[32])

Modified STM Break-Junctions (STM-BJ)

Since Tao and coworkers developed the STM-BJ technique, the statistical behavior of single molecular junctions is intensively investigated [14, 72]. Basic principle of this method is similar as MCBJ method. The mechanic of this technique is as follows. The STM tip approaches and presses the metal surface. While lifting the tip from the surface, the metal electrodes are elongating forming atom chains. At this moment, the molecules assembled on the metal surface are bridged on the electrodes, and then finally a single molecule binds at the end of metal atoms on both sides showing a single molecular conductance as shown in Fig. 2.16. The fast repetition (~40 nm/sec) of breaking and forming the atomic contacts builds robust statistical analysis of conductance [14, 72, 73]. In the study of Fig. 2.16, each conductance histogram was constructed from more than 10,000 individual opening and closing curves. There are disadvantages; this method is used with molecular solution at room temperature and therefore difficult for low temperature experiments. It is more difficult to measure current-voltage characteristics, and to apply a gate-voltage or magnetic field.

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Figure 2.16. (a) Conductance histogram of 1 and 2 molecules on a log-log scale, along with a control histogram of Au measured without molecules in the junction. (b) Illustration of a gap between a gold point-contact and a gold surface that can be bridged with either molecule 1 or 2 from the surrounding solution. (Reproduced from Ref. [14])

Electromigrated Nanogap Junctions

Although strictly speaking this method does not provide controlled mechanically, this is presented because the method is a promising candidate to build a nanogap junction on a silicon substrate. During the last decade, the electromigration technique was investigated extensively because it is very useful to form a nanometer (< 2 nm) scale gap for studying molecular electronics and atomic contacts on a silicon substrate.[74-76] The nanogap junctions are produced by applying a large current to metallic wires. The high current drives the momentum transfer of electrons to the metal atoms. This process makes the metal atoms migrate in the direction of current flow or in the opposite direction, and then some voids form showing a nanoscale gap. An example is presented in Fig. 2.17. To obtain reproducible and well-defined nanogaps, these three things are suggested. First, the series resistance of the electrodes between source and drain should be minimized to reduce the temperature of leads during the electromigration process. Second, the cycling process of electromigration improves the yields of nanogap formation because it limits the power dissipation. Third, Joule heating is required to migrate the metal atoms because the electromigration process starts when the temperature of lead reaches ~ 400 K.[74-76] The yield (i.e. probability of success to obtain well-defined nanogaps) is very low compared to other methods, and the gap distance is not very controllable. However, because the electrodes are not suspended, the contact electrodes are stiffer and stable. If we use gate fields, the performance is more robust, and the gating efficiency is higher, compared to MCBJ samples.

25 Using this electromigration technique, a solid-state molecular transistor was recently demonstrated by the author and coworkers. In this device, the charge transport is controlled by modulating the energy of molecular orbitals.[13] As a local gate electrode, an oxidized aluminum (Al₂O₃) layer is defined below the nanogap junctions. The gate-dependent I-V curves and inelastic electron tunneling spectroscopy (IETS) of octanedithiol (ODT) and benzenedithiol (BDT) molecular junctions prove that the charge carriers in fact pass through the molecules, and the dominant molecular orbitals (i.e. HOMO) of BDT (i.e. near resonance) are resonantly enhanced by the gate modulation (see Fig. 2.18). The details of IETS were discussed in the section 2.1.3.

Figure 2.17. (a) Scanning microscope image of an electromigrated nanogap junction. (b) The feed-back controlled current-voltage curves (black dots) during the electromigration process in ambient condition. The process starts from high conductance to low conductance as the direction of arrow. The red solid lines (slopes) indicate the 20 and 10 G0 of leads. The initial resistance between the source and drain is 127 Ω. (c) Resistance-voltage curve shows that the resistance increases while the voltage is swept repeatedly. (d) Conductance steps at few atoms contact regimes are clearly observed during the feed-back looped electromigration process.

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Figure 2.18. (a) SEM image of fabricated gated molecular devices. (B) Representative I-V curves measured at 4.2 K for different values of gate-voltages V_G. (c) IETS spectra of a Au-ODT-Au junction measured at 4.2 K for different values of effective gate voltages with vibration modes assigned. (d) IETS spectra of a Au-BDT-Au junction for different values of effective gate voltages. (Reproduced from the author’s previous work, Ref. [13])

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