Molecular Electronics

Our present semiconductor based solid-state microelectronics follows one of the most famous axioms in technology: Moore's law. It states that the number of transistors that can be fabricated on a silicon integrated circuit - and therefore the computing speed of such a circuit - doubling every 1.5 to 2 years. After following this remarkable curve for four decades, solid-state microelectronics has advanced to the point at which engineers can now put on a few square centimetres of silicon some 100 million transistors, with key features measuring 130-180 nm. However, a number of serious problems in the fabrication of efficient smaller silicon transistors and their interconnections, such as the need to dissipate the heat from so many closed packed devices, the effect of stray signals, and the difficulty to create the devices in the first place rise at the horizon of the multibillion-dollar semiconductor industry. Because of these difficulties the exponential increase in transistor densities and processing rates of integrated circuits is being sustained only by a similar exponential rise in the financial outlays. Eventually the drive to downscale will run headlong into these extreme facility costs, and the market will reach equilibrium. Many experts in the field expect that this saturation will take place around or even before 2015. Therefore it is appropriate to look for alternative approaches to circumvent these severe problems. A very promising candidate is molecular electronics, a field emerging around the premise that it is possible to build individual molecules that can perform functions identical or analogous to those of the switches, transistors, conductors, diodes and other key components. By pushing devices to such a small length scale novel quantum phenomena - such as single-electron charging effects, or energy-level or conductance quantization - might readily be exploited near room temperature (at present, in current devices such effects are only observed at millikelvin temperature range). The idea that a single molecule, could be embedded between electrodes and perform the basic functions of digital electronics, such as rectification, amplification and storage, was already put forward in the mid-1970s by Ratner and Aviram from IBM]. But their suggestion remained a dream until the advent of the scanning tunnelling microscope in the 1980s, which gave researchers the tools to probe individual molecules and move them around at will. Recent studies showed that individual molecules could conduct electricity like wires or semiconductors, the building blocks of modern microprocessors.

Introduction

Already in the 1970's Aviram and Ratner came up with the idea to use organic molecules for building electrical devices such as transistors, memories and diodes. The term molecular electronics was introduced. However, some time elapsed until experimental techniques reached such a refined level that single molecule measurements were possible. The last decades a dramatic miniaturisation of electronic devices has taken place, now approaching nanometer dimensions. Soon the manufacturing and economical limits for this down-sizing will be reached, which raises the needs for a new approach to electronic circuitry. At these length dimensions, quantum effects come into play. Due to their nanometer scale dimensions and the fact that they can be designed on an atomic level to exhibit specific properties molecules present an interesting option for nanoelectronics, and the last decennia the study of electronic properties of molecules has drawn much attention.
In this project we study electrical properties and electrical transport phenomena of different kinds of molecules on both metal and semiconductor surfaces, by using scanning tunnelling microscopy and spectroscopy.

Experimental set-up

For our measurements we use a low temperature scanning tunneling microscope, LT-STM. With this STM measurements can be performed in a temperature range from room temperature down to 4.7 K. Low temperatures are advantageous in order to reduce the influence of thermal energy, kT, which is of importance e.g. when recording molecular vibrational spectra.
The experimental set-up consists of a main vacuum chamber where the STM is placed and a combined load-lock and preparation chamber. In the main chamber there is an ultra-high vacuum, of less than 3·10^-11 Torr. In the preparation chamber a pressure of about 2·10^-10 Torr can be reached. The preparation chamber has facilities for sample annealing and deposition of molecules onto samples. Via the preparation chamber samples can be inserted into the system without breaking the UHV of the main chamber.

4K STM

Research areas

This far, our research has focused mainly on self-assembled monolayers on gold substrates. Alkanethiol molecules self-assemble into well-ordered, dense monolayers on Au(111), where the molecules arrange in parallel rows along the three main directions of the substrate. In STM images 'holes' can be seen, these are vacancy islands of the Au substrate, one atom deep and filled with molecules ordered in the same way as on the terraces.

STM image 1 STM image 2

Self-assembled monolayers of alkanethiol molecules form an interesting field of study due to these advantageous properties, which make them useful in applications. Conjugated organic molecules can insert at defects in the monolayer, where they insert either one by one or in groups of a few molecules. In this way, electrically conducting molecules can be isolated from each other as the monolayer acts as an insulating matrix, hence making single molecule measurements possible. With the STM, electrical characterization of the single molecule can be done. Furthermore, the alkanethiol monolayer can function as an isolating layer, i.e., between a metal particle and the metal substrate.

Inelastic Electron Tunneling Spectroscopy

Inelastic Electron Tunnelling Spectroscopy

Inelastic electron tunnelling spectroscopy is based on inelastic scattering of the tunnelling electrons, where the tunnelling electrons loose part of their energy to vibrational excitation of molecules placed between the electrodes. In this technique, a small ac modulation voltage is added to the dc voltage, and during scanning of the dc voltage the second derivative, d²I/dV², of the resulting current is detected. Excitation of a vibrational mode will show up as a peak in the d²I/dV² vs V spectrum, and in this way the vibrational modes of molecules can be probed and identification of molecular species is possible. This can be done with several different types of experimental set-ups, among them STM. The advantage of STM-IETS is the possibility of recording vibrational spectra for a single molecule as well as topographical imaging and vibrational mapping. In vibrational mapping different isotopes of a molecule can be distinguished from each other, which is not possible in topographic imaging. During vibrational mapping the surface is scanned while applying a voltage modulation with the same frequency as the vibrational frequency of the molecular excitation to the bias voltage and detecting the second derivative of the tunneling current. Since the frequencies of the vibrational modes vary between different isotopes, only the isotope with the corresponding vibration will be seen in the vibrational map.
In an STM-IETS measurement of a self-assembled monolayer of decanethiol at 77 K we could distinguish two peaks, that could be assigned to the Au-S or S-C stretch mode and the C-C stretch mode or a CH₂ wag or twist mode, respectively. An important criterion for the assignment of peaks in an IET spectrum is the mirror symmetry with respect to zero voltage, which our spectrum fulfills, within the experimental accuracy.

STS spectrum

Coulomb blockade

By depositing small metal particles on an insulating SAM on a metal substrate the tunnelling of single electrons through a metal-molecule-metal junction can be studied. This system can be modelled by a simple electrical circuit consisting of two resistors and two capacitors. By comparing experimental results with simulations the values of R₁, R₂, C₁ and C₂ can be determined.

Alkanethiol SAM

From I/V measurements on Pd clusters on a decanethiol monolayer on Au(111) at 77 K Coulomb blockade could be observed. The tunnelling of an electron to the metal particle is blocked until the applied voltage reaches a value equal or higher than the charging energy of the particle. Due to this, the current will increase step-wise, and the I/V curve will get the shape of a staircase, the so-called Coulomb staircase. On top of the bare monolayer the I/V curve looks more like a straight line.

Coulomb blockade I-V

By performing model calculations and fitting the theoretical curve to the experimental one, the resistance and capacitance values can be determined. From the derivative of the I(V) data, dI/dV, the fractional charge on the cluster can be determined. As seen in the curves, the experiment and the theory are in very good agreement.

Coulomb blockade I-V, #2

Organic molecules on semi-conductor surfaces

Since today's microelectronics industry is based on silicon, incorporating functional organic molecules into these structures provide a much more straightforward way to applications than do molecules on metal surfaces. Therefore, we plan to study the electrical properties of different types of organic molecules adsorbed on silicon surfaces.

Research plans

The plan is to follow both paths outlined above; the inserted conjugated molecules in insulating monolayers on gold substrates, and single organic molecules on silicon surfaces. The ultimate way to study electrical transport and inelastic tunneling spectroscopy on individual molecules is by making a chemical bond between the adsorbed molecule and the STM tip, so that the molecule is trapped between the substrate and the STM tip. In the first instance we would like to study the electrical transport through different kinds of molecules and their vibrational properties, and the influence of different functional groups on the electrical properties. The long-term goal is to be able to controllably switch a molecule between two states of different conductance, i.e., between ON and OFF states.

Individual molecule I-V

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