Electron-phonon interactions - Nanotechnology - Lecture Slides, Slides of Nanotechnology

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Alfredo D. Bobadilla

Nanotechnology course – Prof Jorge M. Seminario

Spring 2010

Texas A&M University

Electron-phonon interactions in a SWCNT-DNA hybrid nanostructure

Table of Contents

 INTRODUCTION

 MOTIVATION

 OBJECTIVE

 BASIC CONCEPTS

 REVIEW OF PREVIOUS WORKS

 RESEARCH PROJECT PROPOSAL

 CONCLUSIONS

 REFERENCES

MOTIVATION

 Optical spectroscopy methods are not adequate for characterization of nanoelectronic sensors and devices, they were developed for studying bulk material properties.

 Novel methods are required to characterize a single molecule or few molecules nanosystem.

 DNA and CNT own unique physical and chemical properties which make them ideal elements for a broad range of engineering applications.

OBJECTIVE

 Detecting vibrational modes in a suspended CNT-DNA hybrid nanostructure, necessary requirement to ensure the nanodevice was properly assembled.

SWCNT diameter ~1.5nm, DNA diameter ~1.8nm

Si wafer

Au electrodes

Basic concepts: Quantum tunnelling

 Tunneling is the quantum mechanical process by which a particle can penetrate a classically forbidden region of space (for example, passing from two separate points A and B without passing through intermediate points). The phenomenon is so named because the particle, in traveling from A to B , creates a sort of "tunnel" for itself, bypassing the usual route.

http://scienceworld.wolfram.com/physics/Tunneling.html

Basic concepts: Quantum tunnelling

Schematic representation of quantum tunnelling through a barrier. The energy of the tunneled particle is the same, only the quantum amplitude (and hence the probability of the process) is decreased.

The scale on which these "tunnelling-like phenomena" occur depends on the wavelength of the traveling wave. For electrons, the thickness of "the tunnelling barrier" is typically a few nanometres.

http://en.wikipedia.org/wiki/Quantum_tunnelling

Basic concepts: Inelastic Electron

Tunneling Spectroscopy (IETS)

 IETS is an experimental tool for obtaining the vibrational spectra by detecting changes in a tunneling current due to inelastic scattering process.

 In a tunneling current electrons experience elastic scattering, but if the applied bias voltage is high enough some electrons will experience inelastic scattering, exciting vibrations in the molecule and observing a small change in the current.

 The small change in the current is clearly noticeable in the second derivative.

http://en.wikipedia.org/wiki/Inelastic_electron_tunneling_spectroscopy

Basic concepts: Inelastic Electron Tunneling Spectroscopy (IETS)  Experimentally, the first and second derivative of the I –V curve is obtained by a lock-in amplifier technique.  The voltage applied across the tunnel junction is an AC small signal Vω superimposed onto a DC bias signal VG. The current through the junction can be written as a Taylor expansion.

C. Petita and G. Salace, Rev. Sci. Instrum. (2003)

Review of previous works: STM-based molecular junctions

 An STM is used to form a molecular junction aiming to measure the conductivity of an N-alkanedithiol molecule.

 The gold-Nitrogen affinity make it possible the formation of a covalent bond between N- alkanedithiol molecules and each metal gold electrode.

 Alkanedithiol molecules are in solution on a gold substrate.

http://www.zurich.ibm.com/~bmi/REVFIG4.JPG

Review of previous works:

STM-based molecular junctions

 Initially the STM gold tip is approached until making contact with the gold substrate, and when slowly retracted it is formed a gold wire of a few atoms.  When the gold nanowire is present it’s detected quantized values of conductance which decrease (as the tip is moved out) until a minimum value before the gold nanowire breaks.

B Xu and NJ Tao, Science (2003)

Review of previous works: STM-based molecular junctions

 After the molecular junction is formed, it continues to be separated by pulling out the STM tip until another molecule configuration occurs and the first and second derivative is again recorded.  The phonon spectrum for the molecular junction changes with changes in contact geometry or molecular configuration due to stretching conditions.

A STM allows conductance measurements and a Lock-in amplifier allows measuring first an second derivative of the I-V curve at each molecular configuration. Hihath et al, Nano Lett. (2008)

Review of previous works:

STM-based molecular junctions

 Figure (a) is the histogram of conductance, it shows how many times was found each measured value of conductance.  The big peak correspond to the gold wire and the small one corresponds to the N-alkanedithiol molecule.  Figure (b) shows how measured conductance values change as the molecule is stretched.

Hihath et al, Nano Lett. (2008)

Review of previous works:

microfabricated molecular junctions

 Single-molecule conductance and IET spectra are measured on benzenedithiolate connected to gold electrodes to identify the number and type of organic molecules within metal–molecule– metal junctions.

M. Taniguchi, IOP nanotechnology (2009)

Review of previous works:

microfabricated molecular junctions

 An electromechanical system allow a precise control on the molecule stretching.  The piezo-driven pushing element cause a bending of the substrate on which it is supported the molecular junction.

M. Taniguchi, IOP nanotechnology (2009)