Controllable and Switchable Properties of Organic Single Molecules
in Nanoscale Devices
Dissertation for the academic degree of Doctor of Natural Science (Dr. rer. nat.)
at the
Mathematics and Science Section Department of Physics
Presented by
Youngsang Kim
Date of examination: 16 May 2012 Reviewers: Prof. Dr. Elke Scheer
Prof. Dr. Juan Carlos Cuevas
Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-194007
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Contents
1. Introduction · · · 1
1.1 Molecular Electronics · · · 1
1.2 This Thesis · · · · 2
2. Background and Method · · · 5
2.1 Analysis of Charge Transport Characteristics · · · 5
2.1.1 Single-Level Model · · · 7
2.1.2 Transition Voltage Spectroscopy · · · 10
2.1.3 Inelastic Electron Tunneling Spectroscopy · · · 12
2.1.4 Point Contact Spectroscopy · · · 19
2.2 Controlling and Switching of Single-Molecule Conductance · · · · 21
2.2.1 Mechanical Controls · · · 21
2.2.2 Light Irradiations · · · 27
2.3 Organic Molecules · · · 28
2.3.1 Molecular Isomers · · · 30
2.3.2 Anchoring Groups · · · 34
3. Fabrication Method · · · 35
3.1 Fabrication of Gold Nanoscale Devices · · · · 35
3.2 Fabrication of Platinum Nanoscale Devices · · · · 36
3.3 Electron Beam Lithography · · · 37
3.4 Deposition of Molecules · · · 38
3.4 Working Principle of MCBJ · · · 40
3.6 Calibration of the Nanogap Distance · · · 40
4. Measurement Setups · · · 43
4.1 Low Temperature Transport Measurement Setup · · · 43
4.2 IETS Measurements · · · · 45
4.3 Optical Setup · · · 46
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5. Conductance and Vibrational States of Single Molecule Junctions controlled by Mechanical Stretching and Material Variation
· · · 49
5.1 Introduction · · · · 50
5.2 Results and Discussion · · · 50
5.3 Conclusions · · · · 60
6. Characteristics of Amine-ended and Thiol-ended Alkane Single- Molecule Junctions Revealed by Inelastic Electron Tunneling Spectroscopy · · · 61
6.1 Introduction · · · · 62
6.2 Results and Discussion · · · 63
6.3 Conclusions · · · · 77
7. Benzenedithiol: A Broad-Range Single-Channel Molecular Conductor · · · 79
7.1 Introduction · · · · 80
7.2 Results and Discussion · · · 82
7.3 Conclusions · · · · 88
8. Charge Transport Characteristics of Diarylethene Photo- Switching Single-Molecule Devices · · · 91
8.1 Introduction · · · · 92
8.2 Results and Discussion · · · 93
8.3 Conclusions · · · · 107
Appendix I · · · 109
Appendix II · · · 111
Summary · · · 113
List of Publications · · · 117
Acknowledgments · · · 119
Bibliography · · · 121
1
Chapter 1
Introduction
1.1 Molecular Electronics
The field of molecular electronics was born to overcome the limits of silicon-based top- down technology. The number of transistors on a unit area has been doubled every two years by downscaling the pattern size as predicted by G. E. Moore for more than 40 years now [1]. In 2011, the 20 nm scale technology, i.e. circuit containing building blocks with smallest lateral dimensions of 20 nm have been developed. However, more downscaling is limited and will result in several inevitable problems. In order to overcome the size-limit of silicon-based technology in the semiconductor industry, scientists are investigating different types of nano-systems. Molecular electronic device has been become one of the potential candidates for future electronic devices because molecules are small (few nanometers), contain fast transit time (~fs), can be functionalized, and are low costs. [2-8] The field investigating the electronic properties of molecules for the sake of applications in functional devices is called molecular electronics.
From the physical point of view, molecular electronics is more interesting. Because of the small size, the electronic transport through molecules can be mostly explained by quantum mechanics. The individual molecules can be functionalized as switches, diodes, transistors, and memories. The features can be measured by electronical measurements with external stimuli such as mechanical stress, light irradiation, gate field, and magnetic field, etc. As mentioned above, the aim of molecular electronics is to build robust electronic logic circuits using molecules. In order to investigate electronical properties of molecules, basically the fabrication of metal-molecule-metal junctions is necessary. Ensembles of molecules [9-11]
and single molecule junctions [12-14] have been fabricated by diverse methods. Deposition methods have been developed in several manners, self-assembled monolayer, Langmuir- Blodgett, and thermal evaporation, etc. Such fabrication methods will be discussed in Chapters 2-4.
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1.2 This Thesis
In this thesis, the charge transport properties through molecules are investigated to answer several physical questions, how the transport properties vary under external stimuli such as mechanical stress and light irradiation, and why the properties are changed under such influences. In order to address these questions, single-molecule junctions were fabricated using the mechanically controlled break-junctions (MCBJs) technique, because this technique offers the chances to control the junctions mechanically and to irradiate them with light. Alkane and benzene molecules were investigated for the first step because these are known as test-bed molecules. Then we extended our studies to functional molecules like, diarylethene and azobenzene derivatives. The preferential conductance of each molecule was determined by repeated opening and closing of the molecular junctions. Finally, the current-voltage (I-V) characteristics, differential conductance (dI/dV-V), and IETS (d2I/dV2- V) were measured. The single-molecule junctions were mechanically controlled and irradiated the light, resulting in the change of molecular conformations, isomers, electrode configurations, and bonding properties, which influence the conductance and the vibrational excitation. Detailed studies how to measure and how to analyze such behaviors in single- molecule junctions are presented in this thesis.
Chapter 2 gives a short overview over the state of the art of the field of molecular electronics and reviews the measurement and analysis methods in single-molecule junctions.
We briefly introduce the theoretical concepts of electron transport through single-molecule devices. These include the single-level model mentioned before as well as charge tunneling models, and the widely used analysis method called transition voltage spectroscopy. We describe how information about the vibrational degrees of freedom of a molecular junction can be revealed from inelastic electron tunneling spectroscopy and point contact spectroscopy. From the experimental side, the molecular energetic states of a single- molecule junction can be controlled by means of mechanical strain, electrostatic field, and light to reveal the intrinsic properties such as molecular conformations, isomers, electrode configurations, and bonding properties, etc.
For the experiments, mainly the MCBJ technique was used at low temperatures. Low temperatures provide the possibility to have more stable molecular junctions and to conduct more diverse measurements. In Chapter 3, the nano-fabrication process of MCBJ is introduced in detail. Then, the electronic measurement methods, the mechanical control technique, and the optical setup are presented in Chapter 4.
The question of how the conductance through single-molecule junctions is affected by the molecular conformation and the contact geometry/contacting metal is addressed in Chapter 5. It presents inelastic electron tunneling spectroscopy (IETS) measurements carried out on hexanedithiol molecules incorporated into a MCBJ of Au and Pt electrodes at low temperatures. The alkane molecules may change the conformation into the so-called trans and gauche conformations resulting in distinct differences of the conductance and the
3 possibility to excite particular vibrational modes. Furthermore, the thiol end-group may bind differently on different electrode materials. The changes of molecular conformation, contact geometry, and metal-molecule bonding are revealed by IETS measurements. This study finds that the conductance through single molecules crucially depends on the contact geometry and molecular conformation.
How the different anchoring groups of molecules behave under stretching of the molecular junctions are investigated in Chapter 6. Here a combined experimental and theoretical analysis of the charge transport through octanedithiol and octanediamine single-molecule junction, which is performed to understand the influence of the molecular end-groups for increasing electrode separation, is presented. 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.
Chapter 7 verifies recent theoretical predictions how the inelastic charge transport correlates with the elastic one provided that the current is carried by a single molecular orbital. For this study, we chose benzenedithiol single molecule junctions, because the benzenedithiol molecules are known to show large conductance distribution ranging from small up to high conductance. It is able to tune the molecular conformation and thus the transport properties by displacing the nanogap electrodes. In order to identify the detailed charge transport properties, in the different conductance regime, the current-voltage characteristics and IETS were measured and analyzed using the single-level model, which deduces the dominant molecular level and the level broadening. Details of the single-level model are introduced in Chapter 2. These observations show unambiguously that the conductance of benzenedithiol is carried by a single transport channel provided by the same molecular level, which is coupled to the metallic electrodes throughout the whole conductance range (10-3~ 0.5 G0, where G0 = 2e2/h).
Chapter 8 addresses how the electrons tunnel through the photo-switching molecules. The diarylethene molecules are most promising photochromic molecules. Under UV or visible- light irradiation, the molecules are isomerized changing the strength of -conjugation resulting in the switching of conductance. The charge transport through the open and closed isomers of photochromic diarylethene single molecular junctions is investigated. The single- molecule conductance and the I-V curves are measured in a MCBJ system at low temperatures. By analyzing I-Vs using the single-level model, the shift of the molecular orbital upon different isomers and the change of coupling upon the geometry of side-chains are investigated because they are crucial to understand the charge transport mechanism of photochromic molecular junctions.
4
5
Chapter 2
Background and Methods
This chapter reviews the analysis and measurement methods in use for studying charge transport through single molecule junctions. We take here an experimentalist’s point of view.
We present the historically important experiments and techniques and use these for introducing the theoretical concepts of charge transport through molecules in terms of tools for analyzing these experiments. In order to reveal how the intrinsic properties of molecular junctions influence the charge transport, external stimuli are necessary. The external stimuli (i.e., mechanical strain, electrostatic field, and light) are chosen such that they assist to demonstrate the impact of molecular isomers, orientations, contact geometries, and bonding properties, etc. Since several theoretical models for the charge transport exist, the analysis method of electronically measured properties is important. There is still plenty of room to develop additional methods for evaluating molecular characteristics and for refining the existing models. We will restrict ourselves to present here the most common models, including the single-level model, conventional tunneling models (Fowler-Nordheim Tunneling, Simmons model), transition voltage spectroscopy, inelastic electron tunneling spectroscopy, and point contact spectroscopy. These are the basic models in use to reveal the role of molecular energy levels, metal-molecule coupling, and electron-phonon interaction. We discuss how such diverse measurement techniques and analysis methods are applied presently, and what one can learn more from these analyses.
2.1 Analysis of Charge Transport Characteristics
The charge transport through single-molecule junctions may be affected by manifold phenomena. These include isomerizations, orientational changes, or changes due to external stimuli. These phenomena will be explained in Sections 2.3. Consequently, the charge transport characteristics may reveal manifold behaviors. In order to detect and investigate the mentioned physical phenomena, we need tools to analyze and measure the properties. A mandatory prerequisite is thus to have models describing the archetypical transport behavior of metal-molecule-metal junctions. Therefore, the single-level model, transition voltage spectroscopy, inelastic electron tunneling spectroscopy, and point contact spectroscopy are
6
presented and discussed in this section. We restrict our considerations here to junctions formed by organic molecules in contact with metal electrodes. These molecules have a discrete energy spectrum given by the molecular orbitals and revealing a large HOMO- LUMO gap (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) that may amount to a few electron volt. Rigorously, the models for analyzing the I-V characteristics are much approximated. The metal electrodes have continuous density of states and the electronic eigenfunctions are plane waves. A molecule has discrete energy levels and its eigenfunctions are molecular orbitals. When brought together, one has to calculate the electronic wave functions of the combined systems. This is a complex task that in general cannot be solved analytically. This is why many approximative models have been put forward, including the single-level model and the various tunneling model.
The single-level model as well as the tunneling models use the framework of the Landauer formalism, i.e. the current is given by an integral over an energy and voltage dependent transmission function. The transmission function itself can be derived considering specific model systems like, e.g. the single-level model. In this model, this will be explained in detail in Section 2.1.1, it is assumed that one channel dominates the elastic transport. The term channel denotes a delocalized electronic state of the coupled system consisting of the molecule and the electrodes. Through the hybridization of the molecular orbital with the metal electrodes, the discrete energy levels become broadened. In fact, there are several channels in the metal-molecule-metal system. However, the contribution of most channels may be negligible. Furthermore, for the molecular junctions (i.e. organic molecules with anchoring groups), we assume that the coupling is much larger than the charging energy and the energy level (HOMO or LUMO) is further away from the Fermi energy, resulting in the off-resonant coherent transport. The coupling and the energy level are changed depending on the geometry of molecular backbones and the anchoring groups providing the binding to the electrodes. The entire current through molecular junctions is a combination of elastic and inelastic contribution. Unfortunately, this model does not include inelastic transport processes. However, the inelastic processes contribute only a few percent to the total current flow. A model accounting for inelastic processes is discussed in Section 2.1.3.
The “conventional” tunneling model, describing the tunneling of electrons through a potential barrier with the WKB approximation was popularized by Simmons[15]. In this model, the molecules are assumed as insulators having finite tunneling length and without discrete energy levels. Therefore, in this model the specific properties of the system are the work function of the electrodes and the length of the molecule equivalent to the width of tunneling barrier. Accordingly, the conductance depends exponentially on the length of the molecule. When applying a voltage to the junction the rectangular barrier is inclined, resulting finally in a triangular barrier for biases larger than the work function. In the framework of electron emission from metals, this corresponds to the regime of field emission. This latter regime is called Fowler-Nordheim (F-N) tunneling giving rise to a
7 characteristic voltage dependence of the current. This in mind, the so-called F-N plot was suggested to examine the tunneling mechanism. By re-plotting a I-V curve as ln(I/V2) vs. 1/V, one can observe an inflection on the curve (see Fig.2.3). This inflection is the point where the alteration between direct tunneling (inclined rectangular barrier) and F-N tunneling (triangular barrier) takes place. This model is widely used, although no microscopic properties of the molecule, e.g. the electronic spectrum can be deduced from it.
Figure 2.1. (a) Schematic diagram for single-level model with molecular level E0 and level broadening . (b) Energy dependent transmission spectrum reveals Lorentzian nature as described by Eq. 2.3., and is plotted for arbitrarily chosen parameters. Zero energy indicates the Fermi energy.
2.1.1 Single-Level Model
Recently, in order to understand the transport mechanisms in molecular devices, the single- level model was introduced. It assumes that despite the fact that a molecule provides a multitude of electronic states; the current is carried by one single molecular orbital coupled to the Fermi seas of the electrodes. In the generic situation, this level is either the HOMO or the LUMO, whatever is closest to the Fermi energy and well-enough coupled to them. Using this model for analyzing transport measurements through single-molecule devices, this single-level model gives thus information about the electrode-molecule coupling and the energetic position of the current-dominating molecular orbital relative to the metal Fermi level. In order to apply this model, the charge transport should be furthermore phase coherent. Here we consider mainly HOMO and LUMO levels, which are assumed to determine the charge transport properties. The molecular orbital in gas phase are occupied up to the HOMO below the Fermi energy. When the molecule bridges two electrodes, for most molecule-metal combinations, the levels align such that the Fermi energy of electrodes
8
lies within the HOMO-LUMO gap of the molecule, because of the interaction between the molecule and the metal electrodes. Then the molecular orbital hybridizes with the electronic states of the electrodes. This process makes the molecular level broaden depending on the strength of the metal-molecule coupling (). In Fig. 2.1 (a), the molecular energy level is positioned below the Fermi level, and this level (E0) is the dominant level for the charge transport. The E0 is broadened by coupling ( =L+R, where L andR are the coupling of left and right electrodes, respectively).
Using the Landauer transport picture the current can be expressed as an integral over the transmission function with the Fermi function f(E):
( ) 2 e ( , ) / 2 / 2 (2.1)
I V T E V f E eV f E eV dE h
( ) 1 (2.2)
1 exp
B
f E E
k T
Within this model the transmission function T(E,V) adopts a Lorentzian shape described by the Breit-Wigner formula (Eq. 2.3):
0
2
2( , ) 4Γ Γ (2.3)
( ) Γ Γ
L R
L R
T E V
E E V
0 0
Γ Γ
( ) (2.4)
Γ Γ 2
L R
L R
E V E eV
where f(E) is the Fermi function with the lead chemical potential (μ) and the thermal energy (kBT). The transmission function (T(E,V)) depends on the energy of the molecular level (E0) and the coupling constant to left (L) and right (R) electrodes. Eq. 2.4 describes that the energy of the current-carrying orbital shifts when a voltage bias is applied and the coupling to the electrodes is not of equal strength. In the case of symmetric coupling (L = R), E0
does not depend on the bias voltage, and I-V characteristics become symmetric. Therefore, the molecular conductance is determined by the position of the energy level and the strength of coupling as shown in Fig. 2.1 (b). The degree of coupling asymmetry is defined by =
R / L or L / R (with the bigger of both values in the denominator. = 1 indicates the symmetric coupling.). Here the conductance indicates the transmission at the Fermi level
9 (eV = 0). An example for symmetric and asymmetric coupling is shown in Fig. 2.2. Since in Eq. 2.1, the Fermi function appears that the current is in principle temperature dependent.
This influence is negligible when the resonant level lies away from Fermi level (i.e. off- resonant tunneling). However, if the charge transport takes place on resonance, the current depends significantly on the temperature. From Eq. 2.1, the analytical equation Eq. 2.5 is derived for symmetric coupling (L = R) and Eq. 2.6 for asymmetric coupling (L ≠ R).
Detailed studies using this model are presented in Chapters 7 and 8. The reliability of the fitting procedure is discussed in Chapter 8. Due to this crude assumption – only one single model, voltage independent coupling strength, shift of the energy level for asymmetric coupling entirely described by the asymmetry of the coupling strength, fitting the experimental I-Vs with this model does not provide perfectly correct values for E0 and .
However, we can deduce tendencies of the dependence of E0 and , e.g. the stretching distance, gate voltage, or other external parameters.
0 0
/ 2 / 2
( ) 2e arctan eV arctan eV
I V h
(2.5)
0 0
2 2
( ) ( )
2 4
( ) arctan arctan
( ) ( )
R L R L L R
L R
L R L R L R
eV eV
I V e h
(2.6)
Figure 2.2. The I-V (square) and fitting (solid line) curves of a Au-BDT-Au junction for (a) symmetric coupling and (b) asymmetric coupling.
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2.1.2 Transition Voltage Spectroscopy
From the current-voltage characteristics of metal-molecule-metal systems, the transition voltage (Vtrans) between the direct tunneling to the Fowler-Nordheim (F-N) tunneling is deduced, indicating the position of molecular level. Historically, the first transport experiments through single-molecule devices were analyzed in terms of tunneling transport, assuming that the molecule represents a tunnel barrier to the electrons with a barrier height (between the Fermi energy and the vacuum level) and a width d corresponding to the length of the molecule (see Fig. 2.3 (a)). The direct tunneling happens at low bias regime when the applied voltage is smaller than the barrier height, whereas the F-N tunneling happens at high bias regime when the applied voltage is larger than the barrier height and the barrier inclines.
While increasing the applied voltage through a system, the current increases drastically and then saturates above the transition voltage point. In a F-N plot, i.e. ln(I/V2) as a function of 1/V from I-V characteristics, the inflection behavior indicates the Vtrans, as shown as a vertical dash-line in Fig. 2.3(a).[16] The Vtrans roughly corresponds to the barrier height, the energy gap from the Fermi level of the electrode to the nearest molecular level (see Fig.
2.3(a)). Additionally, it was reported that the inflection of Vtrans is not necessary to understand the tunnel barrier model, i.e. Simmons model as shown in Fig. 2.3(a). Recently, it was pointed out that the experimental data and the transport mechanism through molecular junctions are inappropriately described with the simple tunnel barrier model but much better with the coherent Landauer approach with a single transport level (see Fig.
2.3(b)).[17, 18] By examining the F-N curve and transmission function, the origin of transition between the two regimes takes place when the frontier molecular level approaches to the edge of the bias window.[19, 20] The length dependence of TVS measurements is suggested for comparing with saturated alkane molecules to distinguish true molecular junctions and a vacuum tunnel junction. As expected, Vtrans of longer molecules with more than 8 carbon atoms is independent as a function of molecular length as shown in Fig.
2.4.[21] However, for the case of conjugated molecules as shown in Fig, 2.5, Vtrans decreases with molecular length for phenylenedithiols, because the HOMO-LUMO gap of - conjugated molecules is known to decrease with an increase in conjugated length.[22, 23]
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Figure 2.3. (a) Solid circle represent the average of 100 I-V curves for a Au-anthracenethiol- Au junction measured by CP-AFM. The dashed line corresponds to the voltage at which the tunneling barrier transitions from trapezoidal to triangular. Also shown are representations of the barrier shape at various values of applied bias. The inset shows current-voltage data on standard axes. (Reproduced from Ref. [16]) (b) Schematic of the theoretical model of the inflection of F-N curve. (Reproduced from Ref. [19])
Figure 2.4. (a) ln(I/V2) versus 1/V curves for five different length alkanedithiols, where the vertical dashed line demoted the transition voltage (Vtrans). The inset shows corresponding I- V curves. All data were obtained at 4.2 K. (b) Vtrans as a function of molecule length for a series of alkanedithiols from DC8 to DC12. The solid line represents the mean value of Vtrans
for five different length alkanedithiols, and two dashed lines show the standard deviation for averaging. Error bars on each data point also denote the standard deviation across individual measurements for different devices. Chemical structures for each molecule are displayed in the inset. (Reproduced from the author’s previous work, Ref. [21])
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Figure 2.5. (a) ln(I/V2) vs. 1/V plots for BDT, DBDT, and TBDT, where the arrows denote transition voltage (Vtrans). The inset shows corresponding I-V curves. All data were obtained at 4.2 K. (b) Plot of Vtrans as a function of the number of phenyl rings for phenylenedithiols.
The solid line is a linear fit to the three data points. Error bars on each data point also represent the standard deviation across individual measurements for different devices.
Chemical structures for each molecule are also shown in the inset. (Reproduced from the author’s previous work, Ref. [22])
2.1.3 Inelastic Electron Tunneling Spectroscopy
Since the inelastic electron tunneling spectroscopy (IETS) was introduced in 1966 by Jaklevic and Lambe [24], this method became a crucial tool for investigating a metal- molecule-metal junction, which is able to detect the vibrational characteristics of molecules buried in the sandwiched interface. IETS is also useful to investigate the molecular conformation, contact geometry, chemical bonding, and the interface states in metal-oxide- semiconductor (MOS) systems including high-k dielectrics.[7, 25-31] In molecular junctions, when molecular conformation, orientation, and contact geometry varies, vibrational states or metal phonons are reflected in IETS signals. Herein let us assume that the transmission at the Fermi level of a molecule is below 0.5 G0. The reason is introduced below. Principle of IETS is presented in Fig 2.6. When bias voltage is applied to the metal- molecule-metal junction, electrons tunnel elastically from an occupied state of the left electrode to an empty state of the right electrode conserving the energy (process ‘a’) as shown in Fig. 2.6(a). However, when the applied bias (eV) exceeds the excitation energy (ћ) of a vibrational mode, electrons can tunnel into another empty state inelastically losing a quantum of energy (ћ) by emitting a vibrational mode (process ‘b’). This opening of another channel for the electrons increases the total tunneling current, showing a kink at V =
± ћ/e in Fig. 2.6(b). This kink represented as a step in the differential conductance (dI/dV) plot and as a peak in the d2I/dV2 plot (in the range of positive bias).[5, 25] In Fig. 2.11, representative results of I-V, dI/dV-V, and (d2I/dV2)/(dI/dV)-V are measured in an identical
13 Au-octanediamine-Au junction. (d2I/dV2)/(dI/dV) is called normalized IETS because IETS is divided by the conductance to compensate the change of conductance.[13, 27, 32]
In the low conductance regime, as explained above, the possibility of inelastic charge transport results in an enhanced probability of forward scattering [33-37]. However, in the range of higher transmission, the electron backscattering increases due to a momentum transfer to the excited mode, leading to a negative contribution (or reduction) of the transmission probability (this is related with the point contact spectroscopy (PCS) of Section 2.1.4) [33-37]. Hence, in the tunneling regime (low conductance) the inelastic excitations appear as peaks in the second derivative of the I-V characteristics, corresponding to enhanced differential conductance above this threshold voltage (see Fig. 2.9 and 2.10) [13, 27, 33-41]. In contrast to that, the excitation of vibrational modes gives rise to dips in the IETS, i.e. a reduced differential conductance, when the transmission (T) exceeds the so- called crossover transmission (Tcrossover), which is given by half the value of the maximum transmission (Tmax) of a junction, where T is the transmission in the linear conductance regime (see also Section 2.1.4) [33-38]. Therefore, in the symmetric coupling case, the crossover transmission that separates peaks or dips in the positive bias regime of IETS or PCS becomes 0.5.
Figure 2.6. (a) A schematic of a metal-molecule-metal junction and energy band diagram with a vibrational mode of frequency localized inside: ‘a’ is the elastic tunneling process;
‘b’ is the inelastic tunneling process. (b) Corresponding I-V, dI/dV, and d2I/dV2 characteristics. (Reproduced from Ref.[25] )
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The IETS can be described by the single-level model as Eq. 2.7 and 2.8. The intensity of normalized IETS, (d2I/dV2)/(dI/dV), is proportional to the transmission (T) for the case of symmetric coupling [35, 37]. This formula demonstrates the change of the IETS intensity (into peak and dip) respect to T = 0.5. The related study is presented in Chapter 7. (see also
‘http://www.magnuspaulsson.se/FGRpaper/’ produced by M. Paulsson for gaining an intuitive understanding of the single-level model and IETS)
2 2
2
2 1 2
el in
G G G e T T
h t
(2.7)
/ ~ 1 2
G T T
(2.8)where is the electron-phonon coupling, t is the hopping term.
Vibrational Mode Assignments in IETS Spectra
The assignment of each molecular vibrational mode is very important and a difficult step of the analysis. Some information can be won by comparing the IETS with Raman or Infra-red (IR) spectroscopy. However, particular vibrational modes can be silent in Raman or IR investigations because of their rigorous selection rules. In principle, the IETS measurement can detect all modes, although the amplitudes of some modes may be small or vanishing electron-phonon coupling constant. This effect is summarized under the term “propensity rules”.[27, 42] The electron-phonon coupling constants depend among others on the exact conformation. Therefore, the assignment of IETS is suggested to compare with the theoretical or other experimental IETS studies. The comparison with theoretical calculation is also not easy because each situation of metal-molecule-metal (e.g. the contact configuration or molecular conformation is different) is different resulting in both the swift of vibrational energies and the change of intensity. Although same molecules are measured, if the junction situation is different, the shape of IETS can be different. However, the position of modes does not swift significantly and drops in a certain range of the energy window. If the structure of molecules is complex with diverse components, several modes appear together at a same vibrational energy, and this is very challenging work even for theoretical calculations. The representative work is shown in Chapter 6 for simple alkanediamine and alkanedithiol molecules.
15 Evaluation of IETS Spectra
In order to examine the validity of IETS, the broadening and the symmetry of IETS are checked. The IETS spectra are broadened by intrinsic linewidth (WI), thermal broadening, and ac modulation broadening as presented in Fig. 2.7.[13, 30, 43] Certain vibrational peaks are fitted by a Gaussian function to determine the full width as half maximum (FWHM) of the experimental curve obtained in different temperature and ac modulation. The FWHM values (Wexp) plotted as a function of temperatures and ac modulation are fitted by using Wexp = [(5.4kBT)2 + (1.7Vac)2 + (WI)2]1/2 in order to know the intrinsic linewidth, where thermal broadening (kBT), the modulation broadening (Vac), and the intrinsic line width (WI) (see Fig. 2.7).[24, 30, 44, 45] The symmetry of IETS should be checked as well to verify the IETS as shown in Fig. 2.8. The vibrational modes of peaks or dips appear at places of positive and negative bias. 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.
Figure 2.7. Linewidth broadening study performed for an Au-ODA-Au molecular junction.
(a) Full width at half maximum (FWHM), black dots with error bars, of one particular mode labeled the (C-C) mode of the ODA junction as a function of the ac modulation voltage at 4.2 K. The intrinsic linewidth (WI) is determined to be 4.87 ± 0.77 meV by fitting (red solid line) the nonlinear least squares using Wexp = [(5.4kBT)2 + (1.7Vac)2 + (WI)2]1/2 to the modulation broadening data.[30, 44, 45] The measured peak width is determined by taking into account the known thermal broadening, the modulation broadening (Vac), and WI. The inset indicates the (C-C) mode deduced from the IET spectra for increasing ac modulation voltage from 4.5 mV to 12 mV in 1.5 mV steps. (b) FWHM, red dots with error bars, of the
(C-C) mode of the ODA junction as a function of temperature at a fixed ac modulation voltage of 6 mV. The theoretical values (black squares), obtained by [(5.4kBT)2 + (1.7Vac)2 + (WI)2]1/2, are in good agreement with the experimental FWHM values.
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Figure 2.8. A IETS spectrum (black solid curve) of an benzenedithiol (BDT) single- molecule junction is shown together with a curve antisymmetrized (red dash-dot line) with respect to the bias polarity, obtained by the simple formula y=(f(x)-f(-x))/2.
Figure 2.9. Second derivative of the current versus bias voltage for three characteristic situations: (a) contact, (b) crossover, and (c) tunneling. In each situation, we consider different active vibrational regions: the two apex atoms only (thick solid line), the ten pyramid atoms (thick dashed curve), and both pyramids and first-layer atoms (thin dotted curve). The signal broadening is due to temperature (4.2 K). (from Ref.[37])
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Figure 2.10. (a) Left schematic is for dominating forward scattering when the conductance is lower than 0.5 G0. Right schematic is for dominating electron back-scattering when the conductance is higher than 0.5 G0. (b) Phase diagram for a one-level model (inset) illustrating the sign of the conductance change at the onset of phonon emission. At a given asymmetry factor the elastic transmission has an upper bound max (black line), and the inelastic conductance change undergoes a sign change at crossover = max / 2 (dashed line).
(From Ref. [34] )
18
Figure 2.11. Transport properties of Au-octanediamine (ODA)-Au junction (inset of (a)) are obtained using standard lock-in technique at 4.2 K in a MCBJ system. (a) I-V, (b) differential conductance (dI/dV), and (c) normalized IETS (d2I/dV2)/(dI/dV) are presented and measured in an identical sample. Each vibrational mode is presented in Chapter 6.
19
2.1.4 Point Contact Spectroscopy
The point contact spectroscopy (PCS) is a similar measurement method as the IETS.
Historically, PCS has been developed for metallic contacts with rather high conductance in the order of 1000 G0. In the framework of molecular electronics, IETS applied to highly conductive junctions with G > 0.5 G0 is called PCS.[46] In the range of higher transmission, the contribution of inelastic excitations reduces the transmission probability due to a higher probability of electrons being back scattered through the molecular junction due to the momentum transfer to the excited mode (see Fig. 2.10). As a result, the excitation of vibrational modes gives rise to dips in the second harmonics (d2I/dV2) and to a reduced differential conductance (in the range of positive bias) as shown in Figs. 2.9(a) and 2.12.
Therefore, it was expected to observe the change of sign of second harmonics from peaks (IETS) to dips (PCS) in a system whose conductance is variable in a wide range.[34, 37]
For example, 1,4-benzenedithiol (BDT), as a conjugated molecule with one aromatic ring, is reported to show a variable conductance ranging from ~ 10-4 G0 to ~ 0.5 G0 , with the quantum of conductance G0 = 2e2/h.[47-51] Such large variation occurs because the BDT molecules may adopt several configurations in the junction including tilting of the ring plane with respect to the electrodes or bonding to different sites on the metal atoms (i.e., top or hollow) when the molecular junction is stretched or compressed [47-52]. This makes BDT as a very promising candidate for studying fundamental aspects of quantum transport – provided that the configuration can be controlled – and also for manifold applications in molecular electronics devices. Details of this study are presented in Chapter 7.
Agraït, et al. [53] reported that the mechanical stretching of the atomic chain results in bond softening, which is reflected in a red-shift of the phonon mode and an enhancement of the electron-phonon interaction. Typical data reproduced from their publication Ref. [117] is shown in Fig. 2.12. In the one-dimensional atomic metal chain, the intensity of the longitudinal phonon mode indicating the emission probability increases due to an enhancement of the electron-phonon interaction, and the shift to low energy of longitudinal phonon mode can be understood by the decrease of the elastic constant of the atomic chain.
When the atomic chain is short (just 1~2 atoms), the excitation of the longitudinal phonon mode is very weak, and we cannot detect the enhancement of inelastic signals while elongating the chain. In the long atomic chain (> 6 atoms), the inelastic signal can strongly increase, and its energy can shift to lower values. Consequently, the increase in intensity and the red-shift of longitudinal phonon modes, while the junction is stretched, are clear indications that the metal atoms form a long chain in one dimension.
20
Figure 2.12. (a) Short and long atomic wire, ~ 4 Å long and 22 Å long, respectively. At the point of rupture, the atomic wire collapses and the conductance, which is negligible in the scale of this figure, corresponds to the tunneling regime. To reestablish contact, the electrodes must re-approach by a distance of the order of the chain length. Panels (b), (c), and (d) show the differential conductance and its derivative at point S, M, and L, respectively, marked by the arrows. The various curves in (b), (c), and (d) were acquired at intervals of 0.3, 0.3 and 0.5 Å, respectively. Note that the vertical scales are identical in these panels. (From Ref. [53])
21
2.2 Controlling and Switching of Single-Molecule Conductance
In this section, we summarize the most common methods for fabricating electrodes suitable for the formation of single-molecule junctions and describe which stimuli are used for tuning the electronic properties of single-molecule junctions. Recently controlling or switching of molecular energetic states became a main issue, because this may be used to vary the molecular conductance by under external excitations.[5, 7, 13, 14, 54-59]
Molecular orbitals (MO) can be tuned by mechanical controls, and light irradiations, etc.
This change of MO influences the molecular conductance. The detailed mechanism and method of each technique are presented here. For the mechanical control of single- molecules, mechanically controlled break-junctions (MCBJ) [12, 32, 46, 60-62] and modified STM [14, 40, 63] are widely used. For the light irradiation, photochromic molecules [10, 64-67] are adopted to use the intrinsic properties of molecules.
2.2.1 Mechanical Control
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.
22
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.
24
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.
26
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 VG. (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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2.2.2 Light Irradiation
Of particular interest for building functional molecular devices are such molecules that exists in two distinct states with clearly distinct electronic properties and that can be switched between these two states by optical means. The goal is to develop junctions the conductance of which can be switched between the “on” (high conductance) and “off” (low conductance) states. In Fig. 2.20, ensembles of molecules were assembled in large area molecular junctions [77] and nanoparticle array junctions [10]. Reversible conductance switching was observed under irradiation of UV and visible light at room temperature. Upon absorbing a certain wavelength, photochromic molecules isomerize showing different chemical and physical properties such as color, hydrophobicity, redox chemistry, refractive index, dielectric constant, and energy level.[78] However, in this thesis, the charge transport properties, which change upon variation of molecular orbitals, are investigated in single molecule junctions at low temperatures. Famous photochromic molecules are diarylethene, azobenzene, stilbene, and spiropyran (shown in Fig. 2.24). Light irradiation on a single molecule junction is a challenging task. Although focusing the light on a single molecule junction is achieved, there are still many problems, e.g. deformation of metal electrodes due to heating, other excitations, degradation of molecules, or screening of photons by metal atoms, etc. Moreover, the photochromic molecules may not switch because of improper electronic coupling between electrodes and switching core may hinder the switching process.
In addition, the length difference between two isomers also should be considered for metal- molecule-metal junctions. Detailed studies of photochromic molecules are presented in Chapter 8 and also Section 2.3 of this Chapter.
Under light irradiation, another interesting effect can occur on single-molecular junctions.
Surface plasmons are enhanced in nanoscale junctions, and Raman spectroscopy of single molecules can be detected due to this enhancement. This phenomenon is called surface enhanced Raman spectroscopy (SERS) [79-82]. Although, in this thesis, we are not studying SERS, it gives us valuable information for the study of the transport properties of photochromic molecules under light irradiation, because a similar field-enhancement effect at the tips of the electrodes can be expected.
Nanostructures act as an optical antenna, meaning that they provide signal amplification.
The plasmon enhancement (excitation field enhancement) is crucial for the Raman scattering. Surface plasmons are quasi-particles resulting from the vibration of photons and electrons at metal-dielectric interfaces. When the local electromagnetic modes are excited by light irradiation, the surface plasmons propagate along the interface, and huge electromagnetic field is localized at the edge of nanostructures. With this plasmon enhancement, Raman scattering greatly enhances because it is proportional to the product of both electric field components at the frequency of incident light and the component at the scattered frequency.[83] This technique allows one to study the optically excited vibrational signals of single molecules as shown in Fig. 2.19.
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Figure 2.19. (a) SEM false-color image of a typical nanogap. (b) Schematic of electrical and optical measurements. (c) Waterfall plot showing the Raman response of an OPV3 junction (in CCD counts) as a function of dc bias V (y-axis) and Raman shift (x-axis). The anti- Stokes spectrum (left) shows a string dependence on V, whereas the Stokes spectrum (right) is relatively constant as a function of V. The strong peak at 520 cm-1 is from the silicon substrate. Note, the false-color scales for Stokes and anti-Stokes signals are different. (d) Current (top) and differential conductance (bottom) measured simultaneously as a function of V. (Reproduced from Ref. [84])
2.3 Organic Molecules
The charge transport through organic molecules bridging metallic electrodes is usually off- resonant tunneling because these molecules possess a large HOMO-LUMO gap. In this study, the simple testbed molecules as alkane and benzene, and the functional molecules react under light as diarylethene and azobenzene derivatives were investigated. Alkane molecules are saturated with carbon chains, and are usually low conducting. The conformation of carbon chains can be changed to trans or cis isomer, and this results in high and low conductance, respectively (see Fig. 2.21). Benzene molecules are conjugated and high conducting. The benzene is rather stiff and does not have isomers. However, the benzene molecules are very small and result in the change of orientation respect to the current path through electrodes (see Fig. 2.22). Diarylethene and azobenzene molecules switches to “on” (high conducting) or “off” (low conducting) state under light irradiation
29 (see Fig. 2.24). When these molecules between metallic electrodes change the conformation, orientation, and forms, the molecular orbitals of each molecule vary. The dominant current carrying molecular orbital is usually determined by the end-group of molecules because the end-groups have electron donating or withdrawing character. Usually it is HOMO transport, except molecules that contain nitro-, cyano-, or pyridine end-groups.[13, 18, 85] In addition the dominating molecular orbital varies when the molecular conformation and contact geometry change. Such variation of molecular orbital determines the molecular conductance.
In this section, the influences of molecular conformation and end-groups on the molecular conductance are discussed. More detailed investigations are presented in Chapter 5-8.
Figure 2.20. (a) Schematic cross section of the device layout of a large-area molecular junction in with the diarylethene is sandwiched between Au and PEDOT:PSS/Au. (b) In situ optical switching of a monolayer of diarylethenes at 0.5 V bias results in a direct current modulation of the molecular junctions. Comparison of the current densities of the as- assembled devices based on the open (red) and closed (green) isomer with the in situ irradiated molecular junctions. (Reproduced from Ref. [77]) (c) Switchable diarylethene molecules: top, closed (on); bottom, open (off) state. Switching from on to off is possible by illumination with visible light. The reverse is achieved by UV illumination. Schematic setup to measure light-induced conductance switching is presented. In addition, optical spectroscopy is possible using a low-intensity white-light source. (d) Repeated conductance switching. The conductance G is plotted vs illumination time t. In the dark, the sample conductance is constant. At t = 0, illumination with visible light is commenced, leading to an immediate conductance decrease. Upon UV irradiation, G increased again. (Reproduced from Ref. [10])
30
2.3.1 Molecular Isomers
Charge transport through alkane and benzene molecules is extensively investigated because they have a simple structure and are considered a basis unit for molecular circuits.[9, 12, 13, 57] Alkanedithiol (ADT), as a saturated molecule, is one of the most appropriate candidates to study these properties and can adopt the usual trans conformation as well as a gauche (or defect) conformation (see Fig. 2.22), which is predicted to give rise to alterations of the charge tunneling.[27, 39, 40, 73] As illustrated in Fig. 2.21, the alkane molecule shows trans and gauche conformations. In each conformation, the potential energy is different and influences the molecular conductance by a factor of ~ 10.[27] The 1,4-benzenedithiol (BDT) molecule, as a conjugated molecule with one aromatic ring, varies orientation between electrodes, including tilting of the ring plane with respect to the electrodes varying conductance ranging from ~ 0.5 G0 to ~ 10-4 G0, when the molecular junction is stretched or compressed.[32, 47-50] In Fig. 2.22, the transmission as a function of tilt angle is well presented theoretically. These properties make both ADT and BDT very promising candidates for studying fundamental aspects of quantum transport– provided that the configuration can be controlled – and also for manifold applications in molecular electronics devices.
Figure 2.21. (a) The trans conformation is the usual one, having lower potential energy, in which the H atoms attached to neighboring C atoms are positioned opposite to each other.
(b) 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 atoms are rotated along the long axis of the molecule such that they enclose an angle of roughly 120 degrees, having higher potential energy. The image shows a gauche defect between the C atoms labeled 1 and 2.
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Figure 2.22. (a) Graphics of the different stages of the evolution of BDT on gold of different junction distances. The initial configuration is that of a flat BDT molecule on the surface. L is the leads separation at each stage. (b) GGA (generalized gradient approximation) and ASIC (approximate self-interaction correction scheme) transmission coefficient at the Fermi level calculated for BDT in the flat initial configuration as a function of the lead-lead separation. (Reproduced from Ref. [49])
Figure 2.23. (a) Structures of a subset of the biphenyl series studied, shown in order of increasing twist angle or decreasing conjugation. (b) Conductance histograms obtained from measurements using molecule 2 (purple; constructed from 15,000 traces and scaled by 1/15), 4 (cyan; constructed from 7,000 traces and scaled by 1/7), 6 (pink; constructed from 11,000 traces and scaled by 1/11) and 8 (blue; constructed from 5,000 traces and scaled by 1/5).
Also shown is the control histogram obtained from measurements without molecules between the contacts (yellow; constructed from 6,000 traces and scaled by 1/6). Arrows point to the peak conductance values obtained from Lorentzian fits (solid black curves). All data were taken at a bias voltage of 25 mV. (c) Position of the peaks for all the molecules studied plotted against cos2θ, where θ is the calculated twist angle for each molecule. Error bars are determined from the standard deviation of the peak locations determined from the fits to histograms of 1,000 traces. (Reproduced from Ref. [14])
32
Finally, we present another interesting example under the influence of the molecular conformation onto the conductance. This study addresses the role of the conjugation of aromatic molecules for the conductance. The investigated molecules contain two aromatic rings that can be tilted with respect to each other and thereby continuously suppressing the conjugation changing the entire set of molecular orbitals. The conductance of a series of molecules with carrying torsion angle has been studied and shown to follow a cos2φ law.
[14, 68, 86] While the torsion angle varies from 0˚ to 90˚ between two phenyl rings, the conductance decreases linearly as shown in Fig. 2.23. For these studies, the molecules are engineered such that the phenyl rings include a certain angle intrinsically. Another experiment was carried out while controlling the torsion angle stressing a bipyridine molecule using a external mechanical stress.[63]
The photochromic switching molecules, as shown in Figs. 2.24 and 2.25, isomerize and change their energy states, refractive index, dielectric constant, and redox potential under external stimuli such as light irradiations. Basically, under UV and visible (Vis) light irradiation, the photochromic molecules switch between two isomers.[10, 58, 87-89] In a conjugated molecule, when electrons in a ground state absorb certain energy, double bonding becomes a single bonding, and the electrons are excited. Then the orbitals rotate and vibrate until they are relaxed in another ground state (see Fig. 2.26). Therefore, the bonding sequence can be changed under light irradiation. This switching behavior is easily examined by UV/Vis spectroscopy as shown in Fig. 2.25.
Figure 2.24. Several types of switching molecules and their switching reaction under UV/Vis light. (a) Sketches of open (left) and closed (right) forms of diarylethene molecules.
(b) Sketches of trans (up) and cis (down) forms of helix molecules. (c) Sketches of trans (up) and cis (down) forms of azobenzene molecules. (d) Sketches of closed (up) and open (down) forms of spiropyran molecules.
33
Figure 2.25. UV/Visible spectroscopy of photochromic molecules. (These molecules are synthesized by D. Sysoiev. Related work will be presented in Chapter 8 and Ref.[65]) (a) Absorption spectra of YnPhT diarylethene molecules under UV irradiation from 0 to 150 sec. (b) Absorption spectra of azobenzene derivative molecules (AzoATM) under UV irradiation from 0 to 1800 sec. The longest-wavelength absorption edge is indicative for the possible optical excitation with lowest energy. A variation in the absorption band thus signals a variation of the HOMO-LUMO gap of the molecules.
Figure 2.26. (illustration by Prof. Ulrich E. Steiner) Potential energy of a diarylethene molecule. S0 and S1 indicate ground state and excited state, respectively. A and B are the ground state of closed and open form. When electrons on B state absorb certain energy by UV light, the electrons excite and jump to S1 state. Then along the reaction coordinate, electrons relax to A state. The reverse reaction is vice versa. Depending on the position of low energy point C along the reaction coordinate, the excited electrons from B state can relax back to B state, again. In the ring opening process, there could be a potential barrier D, which hinders the relaxation of excited electrons. This energy coordinate is different for every type of molecules.
