Anchoring Groups - Organic Molecules

2. Background and Method

2.3 Organic Molecules

2.3.2 Anchoring Groups

One important parameter to determine the molecular conductance bridging metal electrodes is the strength of molecular coupling to the electrodes. Depending on anchoring groups (or end groups) such as sulfur (S-), amine (NH2-), dicarboxylic-acid (COOH-), cyano (CN-), and nitro (N-), the coupling can be tuned. The molecular level can be influenced when the coupling changes because of the capacitive coupling. Practically, in the order of S > Au-NH2 > Au-COOH, the bonding strengths are known as shown in Fig. 2.27.[41, 90]

Moreover the anchoring groups have binding preference on the metal surface as adatom, bridge, and hollow sites.[73, 91] Depending on the position of binding sites, the molecular conductance also varies. Recently NH2 molecules were suggested as a conductor for a well-defined conductance, since it has a narrower distribution of conductance than other anchoring groups as e.g. S.[91, 92] In addition, depending anchoring groups, the S and NH₂ have an electron-donating character reflecting the HOMO is a dominant level, whereas N and CN have an electron-withdrawing character indicating that the LUMO becomes closer to the Fermi level. [5, 13, 18, 93] Summarizing, the anchoring group plays an important role for the charge transport mechanism, although the exact role depends on the molecule as a whole system; the anchoring group is strongly related with the strength of coupling and the molecular level.

Figure 2.27. Logarithmic plots of single-molecule conductance vs. molecular length for dithiol- (orange), diamine- (blue), and dicarboxylic-acid-terminated (purple) alkanes: (a) high conductance and (b) low conductance. Each conductance value is determined by fitting the first peak of the conductance histogram of each molecule with a Gaussian function, and the corresponding error bar is the full width at half-maximum (FWHM) conductance. (from Ref.[90])

Chapter 3 Fabrication Method

In this thesis, we use the technique of mechanically controlled break-junctions (MCBJs).

Sophisticated nano-fabrication is a crucial step for obtaining well-working samples. The MCBJ method provides nanogaps with width ranging from zero to a few nanometers, adjustable with sub-picometer precision that enables the formation of highly stable metal-molecule contacts that last for several hours when arranged at low temperatures. The high stability is necessary for systematically studying the influence of external stimuli as temperature, change of conformation, and contact geometry. In this chapter, we describe the fabrication of the MCBJ electrodes, the deposition of molecules, the working principle of MCBJ, and the calibration method of nanogap distance.

3.1 Fabrication of Gold Nanoscale Devices

The softly polished bronze wafer (60 mm in diameter and 270 μm in thickness) is spin-coated with a layer of polyimide (~2 μm in thickness), which serves as an electrical insulator and a sacrificial layer in the subsequent etching process. In order to polish the bronze wafer, two different grain sizes of sand papers are used, and then the wafer is polished again using a polishing past. The spin-coated polyimide is baked at 430 °C for 100 min in vacuum. On top of these prepared wafers, a double layer of electron-beam resists (ER), MMA-MAA / PMMA, is deposited by spin-coating. Prior to performing the electron beam lithography (EBL) process, the wafer is cut into proper dimension (4x19 mm²). After developing, gold of about 80 nm thick is deposited using electron beam evaporation at a pressure of about 10^-8 mbar. Finally, in order to form a free-standing bridge, the samples are installed into a vacuum chamber of a reactive ion etcher (RIE). Oxygen (O₂) removes about 700 nm of the polyimide layer in a microwave plasma of 50 W in Oxygen flow of 50 ccm for 30 min. The fabrication procedure is presented in Fig. 3.1, and the detailed lithography method is explained in Fig. 3.2. The optical microscope and the scanning electron microscopy (SEM) image of the completely fabricated device are shown in Fig. 3.3 Detailed recipes are listed in the Appendix I at the end of this thesis.[39, 41, 94]

Figure 3.1. The procedure of sample fabrication. (a) MMA-MAA (pink) / PMMA (red) layer is spin-coated on the bronze (yellow) / polyimide (green) substrate. (b) The electron beam lithography is conducted. More detailed method is shown in Fig. 3.2. (c) The development is performed in MIBK:IPA=1:1 solution. (d) The Au layer is deposited using the electron beam evaporator in an UHV chamber. (e) The sample is immersed in Acetone over night for the lift-off of MMA-MAA / PMMA layer. (f) The dry etching about 700 nm depths of polyimide layer is carried out in a RIE chamber to form a free-standing bridge.

(This figure was obtained from the database of Scheer group)

3.2 Fabrication of Platinum Nanoscale Devices

Fabrication of platinum (Pt) electrodes by using the same lift-off fabrication process as the gold electrodes for the MCBJ is difficult because of the high evaporation temperature of Pt.

The ER is destroyed by heating above its glass temperature and by the different thermal expansions of polyimide. To overcome this problem, a pure Pt layer (~80 nm) is formed by sputtering on a polyimide sacrificial layer prior to the EBL process. The double layer (MMA-AAM / PMMA) of ERs is spin-coated on this Pt surface in order to perform the EBL process. After the developing process, an aluminum (Al) layer about 100 nm forming a nanoscale structure of electrodes is deposited by thermal evaporation. The resin mask covered with Al is then dissolved in acetone. For the last step, the sample is put into the RIE chamber. The Al structure on the Pt layer and the Pt layer which is not covered by the Al are etched off anisotropically by sulfur hexafluoride (SF6) at 150 W while monitoring the interferometer until the Al layer is removed, and then the polyimide layer is etched away isotropically forming a free-standing Pt bridge by O₂. This method was developed by F.

Strigl. [39]

Figure 3.2. Design of the break-junctions structure used for the EBL process in Elphy Quantum software.

3.3 Electron Beam Lithography

In order to fabricate the sophisticated nanoscale devices, the electron beam lithography (EBL) was conducted. The EBL was performed in two layers, 100 m and 1000 m working field as shown in Fig. 3.2. Technically, the size of the working field is chosen by the magnification of the scanning electron microscope (SEM) used for the lithography. Two working fields have been used because of the finite resolution of the pattern generator and the precision with which the electron beam can be focused. For writing the nanoscale elements, small writing currents in the range of picoamperes (pA) are necessary, while the larger contact pads require higher writing currents in order to save time. The possible current range also depends on the magnification. The italic numbers in the working field area (blue and red box) is the order of the process. For the first step, the nanoscale patterns in the working field of 100 m were written. Next the large patterns in the working field of 1000 m were written in six steps while moving the sample stage because the 1000 m is the maximum working area of the SEM. Approximately, 10 pA and 5 nA of current were exposed on the samples for 100 m and 1000 m working field, respectively. The working distance between the samples and the end of electron-beam column was kept ~ 5.5 mm. The contact pads on the left and right end of sample were written as a grid shape instead of a

large box in order to save writing time. 145 As/cm² dose was preset for all structures, however, for writing the nanoscale patterns in the enlarged panel, different dose factors (1.0, 0.99, and 1.1) were used as shown in the figure by different colors. The dose factors are necessary to account for the “proximity effect” of EBL. This term denotes that due to the finite focus of the electron beam and because of back-scattering electrons from the substrate also neighboring pixels are exposed. Therefore, the optimum dose depends on the size, the geometry, and the neighborhood of the individual elements. The optimum dose factors are determined empirically. The indicated dose value of 145 As/cm² is an example. In reality, this value varies depending on the quality of ER. Therefore, when the ER is spin-coated on a new substrate, the dose test was performed every time to obtain more precise nano-structures. The writing order (italic number) was from left to right direction in series in order to obtain better alignment of structures. Between the 1000 m working fields, vertical rectangular patterns were written to avoid the disconnection between patterns, because sometimes the movement of sample stage is not accurate. The vertical marker at the center of sample was used as a guide for optical fiber alignment. The beam park position, when the beam blanker works, is usually set at the center of the working field in default. This beam parking procedure influences the nano patterns, i.e. patterns at the center of geometry can be exposed more, if the beam blanker works slowly or is not accurate. Therefore, it is better to change the beam park position to another place, e.g. at the corner of the working field.

3.4 Deposition of Molecules

After the etching procedure, the samples are immediately immersed into solution of molecules to assemble the molecule on the surface of the metal (Au or Pt) electrode. This method is called formation of a self-assembled monolayer (SAM), although it cannot be checked with our methods, whether in fact a single and complete monolayers is formed. The dilute solution (~0.1 mM) was prepared, and then the patterned substrates were immersed in the molecular solution for 5 hours. Each sample was then rinsed with a few milliliters of ethanol and gently blown dry in a stream of nitrogen gas to remove noncovalently attached molecules from the metal surface.[39, 41] This method is for chemical adsorption of end-group on metal surface. In this thesis, the SAM method was mainly used. There are other methods to deposit molecules on metal surfaces, for example, the Langmuir-Blodgett (LB) technique, the evaporation of molecules with a volatile solvent, flowing molecular gas, and thermal evaporation in vacuum. LB methods require the amphiphilic molecules with a hydrophilic head and hydrophobic tails. The evaporation with a volatile solvent is proper for light molecules, because the light small molecules can be vaporized with the volatile solvent.

The flowing gaseous molecules are also useful for small molecules as hydrogen, oxygen, nitrogen, etc. The gaseous molecules are directly deposited in gas phase by condensation on the cold metal surface in a cryostat. The thermal evaporation is used for stable and larger molecules as C60 and metal incorporated molecules. However, the molecules should have a high melting point.[5]

Figure 3.3 Image of a Au break-junction. Top: Optical microscope image. The left and right large pads are connected to copper wires by a silver conductive paint (Electrolube) for the electrical measurements. The wires are rigidly bonded again on the edge of substrate by epoxy glue to fix the wires. Bottom: SEM image of a nanoscale area of the break-junction.

Figure 3.4. Illustration of MCBJ mechanics. (a) Simplified MCBJ mechanics consists of a pushing rod and two counter supports (This figure was obtained from the database of Scheer group). (b) The detailed structure of MCBJ mechanics. The counter supports moves upward and downward by rotating both a rotary axis and a differential screw (M3.5/M2.5). The moving counter support bends the sample, and it is bent by a pushing rod relative to the counter supports.

3.5 Working Principle of MCBJ

MCBJ is bent by a three point bending mechanism consisting of a pushing rod and two counter supports as shown in Fig. 3.4. The pushing rod moves relative to the counter supports by controlling a step motor, piezo, or combination both. While bending the flexible substrate, the suspended nanowire is elongated, and with further bending the nanowire is broken. This motion can be reversed with pulling back the pushing rod. In order to repeat this movement, high elasticity of metal substrate is necessary, and usually bronze and spring steel are used. This mechanism provides controllable nanogaps with width ranging from zero to a few nanometers, adjustable with sub-picometer precision.

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 (R_motor), 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.

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 m_e [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.

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.

4.2 IETS Measurements

The differential conductance (dI/dV) and the 2^nd 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

Im Dokument Controllable and Switchable Properties of Organic Single Molecules in Nanoscale Devices (Seite 38-0)