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OnderzoekZernike (ZIAM)FYSNDResearch

Single molecule transport

Single molecule transport studied in Mechanically Controllable Break Junctions

Aim of the project

In the near future we want to study the electronic transport properties of (single) molecules using the mechanically controllable break junction (MCB-junction) technique to realize metal-molecule(s)-metal junctions. With these MCB-junctions, it is possible to realize atomic-size metallic (quantum) point contacts (QPCs). The short term focus is still the nanofabrication and optimization of these MCB-junctions. A possible future application of these junctions is to obtain light-controlled molecular transistors, the so-called Photochromic Molecular Switch (PMS).

Conventional microstransistor and the photochromic molecular switch. Image taken from M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin and J.M. Tour, Science 278, 1997.
Conventional microstransistor and the photochromic molecular switch. Image taken from M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin and J.M. Tour, Science 278, 1997.

Introduction

In above figure the analogy between a conventional transistor and a photochromic molecular switch is given. The idea is to be able to optically switch a special type of molecule(s), immersed between the two Au electrodes of the MCB-junction, from a conducting to a nonconducting state.

MCB-junction technique

Two metallic Au tips facing one another are fabricated in situ by breaking a thin wire clamped to an elastic substrate. The MCB-junction technique is a very good technique to perform tunneling-resistance and conductance-quantization measurements in clean stable point contacts. Measurements can take place at room temperature due to the large signal associated with the quantum resistance (G0=2e2/h ~13 kW ). With the MCB-junction technique one is able to adjust the number of atomic contacts in the constriction mechanically (!) by bending the substrate of the sample. Because of the smaller dimensions of the MCBJ over the STM it has a better interelectrode stability (values of 1 pm/min have been observed). An image of a conventional break junction is shown below in the figure below.

Conventional break junction technique. A tree-point bending system bends the bending beam which results in an elongation of the metallic strip on top of this substrate. Image taken from C.J. Muller, J.M. van Ruitenbeek and L.J. de Jongh, Physica C, 191, 1992.
Conventional break junction technique. A tree-point bending system bends the bending beam which results in an elongation of the metallic strip on top of this substrate. Image taken from C.J. Muller, J.M. van Ruitenbeek and L.J. de Jongh, Physica C, 191, 1992.

Theory

The theory describing the conductance inquantum point contacts was developed by Landauer and Büttiker around 1957 and is called the Landauer-Büttiker formalism. The current flowing through such a very narrow constriction is found to be quantized and given by the total number of conduction channels which are quantum mechanically permitted in the confined area of the junction.

The next formula describes the 2-terminal conductance of the constriction when a voltage difference V12 is applied to the two electron reservoirs connected to the junction

2-terminal conductance of the constriction
2-terminal conductance of the constriction
In the next figure a typical conductance trace of an operating MCB-junction in Au is shown. The junction is already broken, but the interelectrode distance is continuously increased in time and the conductance is measured continuously. It shows conductance plateaus near integer multiples of the conductance quantum. After the contact is broken, we enter the so called tunnel regime where the current decreases exponentially with the interelectrode distance.
Conductance trace of a broken break junction. The last plateau in the contact regime is near 2e2/h. Data taken from C.Zhou, C.J. Muller, M.R. Deshpande, J.W. Sleight and M.A. Reed, Appl.Phys.Lett. 67, 1995.
Conductance trace of a broken break junction. The last plateau in the contact regime is near 2e2/h. Data taken from C.Zhou, C.J. Muller, M.R. Deshpande, J.W. Sleight and M.A. Reed, Appl.Phys.Lett. 67, 1995.

Nanofabrication

In the group we have been working on different processing techniques to fabricate the MCB-junctions. One technique is based on a silicon substrate, the other is based on a phosphor-bronze bending beam. Each of these techniques requires it own specific fabrication procedures.

Silicon technique

The SEM photo (figure below) shown is the point contact region in a MCB-junction based on a silicon substrate. The MCB-junctions are fabricated using standard optical and e-beam lithography.

First MCB-junction using a Si substrate. The contact is about 100 nm wide and 200 nm long.
First MCB-junction using a Si substrate. The contact is about 100 nm wide and 200 nm long.

Phosphor-bronze technique

We soon discovered that the nanofabrication of the MCB-junctions using phosphor-bronze as a substrate is more promising than the silicon based one due to its intrinsic material properties. We use polished phosphor-bronze plates on which we deposit an insulating layer of polyimide. This is a very elastic and low-dielectric material. On top of this layer we evaporate our structures and end the fabrication with a dry etch step to carve the polyimide to free the junctions. The latest MCB-junctions made are shown below.

Break junction set-up after Ti/Au evaporation and a lift-off step. The arrows indicate a wire width of 1 mm.
Break junction set-up after Ti/Au evaporation and a lift-off step. The arrows indicate a wire width of 1 mm.

Measurement results

The I-V characteristics of a phosphor-bronze based MCB-junction was measured upon breaking the constriction. Using a lock-in amplifier set-up, an a.c. current of 1 or 10 m A is sent through the MCB-junction and the voltage response is continuously measured in a four-terminal set-up, which gives us direct information about the conductance. It is known however, an one-atom point contact can sustain currents about 100 m A, corresponding to the very large current density of 1011 Acm-2. De data clearly shows some steps in the conductance.

Conductance trace of a MCB-junction upon bending the sample. Some equidistant conductance plateaus are clearly visible. The data was obtained at room temperature. The noise in the conductance increases when a higher current is sent through the contact.
Conductance trace of a MCB-junction upon bending the sample. Some equidistant conductance plateaus are clearly visible. The data was obtained at room temperature. The noise in the conductance increases when a higher current is sent through the contact.

Current work

The fabrication process of the MCB-junctions can still be simplified and optimized. We are currently working on a way to pattern the sample in only one e-beam step and also try to understand how to optimize the adhesion of the polymer-metal interface. The first results show that we are on the good way.

Last modified:09 July 2015 10.59 a.m.