Conductance Quantization in Au Nanocontacts

Mariangela Lisanti
Staples High School, Westport, Connecticut
2001 Intel STS First Place Winner

(This is an abridged version of Mariangela’s report.  The original report included Summary, Abstract, Introduction, Materials and Methods, Results, Discussion, Conclusion and Acknowledgements sections.)

Summary

A novel apparatus was developed for measuring conductance quantization in metallic nanowires.  The technique allows for data acquisition at the unprecedented rate of 86 million data points per 24 hours and its accuracy is demonstrated using gold wires, which show clear quantization.  The results indicate a very stable formation of a nanowire and show quantization at higher conductance values, a phenomenon that has never before been reported.

Introduction

The study of nanostructures has become an increasingly important field of investigation now that the fabrication of electronic devices based on single atoms or molecules is within our technological reach.  The optical, mechanical, and electrical properties of nanostructures, though, are not the same as those of larger devices because quantum phenomena are observed at this small scale.  Because a wire is the most basic element of a circuit, studying the properties of metallic nanowires will increase the understanding of electron transport in nanostructures.

If electronic transport through a small wire is truly ballistic, the electrical conductance G through the constriction is given by the Landauer formula

G=G_o

 where Go = 2e2/h is the quantum of conductance (e is electron charge and h is Planck’s constant) and Ti is the transmission probability of the ith channel.

Histograms of the relative frequency of detection for each conductance value have been used to analyze large quantities of conductance quantization data.  Peaks in these histograms become more pronounced as the sample size increases.  The development of an apparatus that is easy to use in ambient conditions and enables the collection of many sample traces is especially important because larger data sets would magnify the peaks and patterns in the histograms.  Such an apparatus would enable the analysis of a great variety of substances, including the transition metals where conductance quantization is difficult to observe. 

Statement of Purpose

I sought to develop an apparatus that would enable the collection of larger amounts of conductance quantization data on a wider range of metals.

Experimental Design

The novel data collection technique developed enables experiments to be carried out at ambient conditions without the use of sophisticated equipment.  Gold tips were soldered onto the ends of two copper wires.  One copper wire was clipped onto the end of a piezoelectric speaker element and rested along the length of the speaker.  The speaker was connected to a function generator.  The piezoelectric speaker element gently pulled the wires apart and at the last stages of breakage, a nanowire formed.  The copper wires were connected to an I-V converter, which connected to an oscilloscope.

Results and Discussion

The cumulative histogram that was obtained shows a clear peak at 1Go and 2Go  and fractional peaks at 2.7Go, 3.5Go, and 5.1Go.  There are two possibilities for the fractional conductance peaks.  First, fractions of channels may have formed.  For instance, two channels plus a fraction of a third may have formed to give a conductance of 2.7Go.  The second possibility is that the disorder of the atoms during the elongation of the junction may have created an additional resistance that decreased the conductance values.

In the cumulative histogram observed, the peaks broaden and appear to wash out as the conductance increases.  Zooming in on this histogram, though, shows an interesting peak structure at higher values of conductance.  Such clear conductance plateaus have never before been observed at these high conductance values.  The additional resistance between jumps at higher conductance values is due to the fact that a junction with many channels contains more atoms and thus has a higher probability of being disordered than a junction with only one channel.  This resistance accounts for the less prominent histogram peaks at higher multiples of Go.

Conclusion

The understanding of electron transport in nanostructures is critical in continuing to miniaturize electronics.  Moore’s Law states that the number of transistors fabricated on a silicon integrated circuit – and therefore the computing power of these circuits – doubles every 18-24 months, but silicon-based microelectronics will cease following this trend around the year 2015.  This will have a tremendous impact on society since the levels of growth we have seen in the computer industry over the past forty years will no longer continue.  The understanding of quantum phenomena is crucial in leading to the next generation of electronics where single atoms or molecules will be used to fabricate electronic devices.  The development of such smaller electronics, which will lead to faster and more efficient computers, will impact all the sciences.