This Thesis - Controllable and Switchable Properties of Organic Single Molecules in Nanoscale D

1. Introduction

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.

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

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, HOMO-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

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 andR 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):

   

Within this model the transmission function T(E,V) adopts a Lorentzian shape described by the Breit-Wigner formula (Eq. 2.3):

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

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.

2.1.2 Transition Voltage Spectroscopy

From the current-voltage characteristics of metal-molecule-metal systems, the transition voltage (V_trans) 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 V_transis 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, V_trans 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]

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) I-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])

Figure 2.5. (a) ln(I/V²) vs. 1/V plots for BDT, DBDT, and TBDT, where the arrows denote transition voltage (V_trans). The inset shows corresponding I-V curves. All data were obtained at 4.2 K. (b) Plot of V_trans 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

Im Dokument Controllable and Switchable Properties of Organic Single Molecules in Nanoscale Devices (Seite 6-0)