Nguyen Le Thao Nguyen

NANOBIOCHEMISTRY LAB - ChungAng University

On the Basis of Molecular

Rectification in Tunneling

Junctions

Chemistry Colloquium Seminar

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On the Basis of Molecular Rectification in Tunneling

Junctions

Hyo Jae Yoon

Department of Chemistry, Korea University, Seoul, Korea

E-mail: hyoon@korea.ac.kr

A. Introduction

Molecular electronics is the study and application of molecular building blocks for the

fabrication of electronic components. It is an interdisciplinary area that spans physics, chemistry,

and materials science. A central goal of the field of molecular electronics is to relate the

electrical behavior of molecular junctions to the chemical structure of the molecules they

incorporate.

Molecular rectification is a particularly attractive phenomenon to study structure-property

relationships in molecular electronics, since the tunneling currents across the same molecular

junction are measured, at opposite biases, with the same electrodes, molecule(s), and contacts.

This type of experiment minimizes the complexities arising from measurements of current

densities at one polarity. Although there are many reports of molecular rectifiers, the basis of

the rectification remains, in most instances, unclear.

B. Contents:

Nanostructure-based solar- and fuel cells, both organic and inorganic, hold promise for

efficiently and cheaply converting solar energy into forms convenient for storage and

transportation. A primary reason for the currently low efficiency of these devices is the absence

of a quantitative picture of the fundamental nonequilibrium electronic structure underlying key

processes in solar energy conversion - photon absorption and charge separation, transport, and

collection at organic/metal and organic/inorganic interfaces.

Single molecule junctions are model interfaces between a single organic molecule

connected to macroscopic metallic electrodes, and as such provide an ideal platform for

developing the fundamental understanding needed.Tunneling refers to the quantum

mechanical phenomenon where a particle tunnels through a barrier that it classically could not

surmount. This plays an essential role in several physical phenomena, such as the nuclear

fusion that occurs in main sequence stars like the Sun.

It has important applications to modern devices such as the tunnel diode, quantum

computing, and the scanning tunnelling microscope. The effect was predicted in the early 20th

century and its acceptance, as a general physical phenomenon, came mid-century.

Chemistry Colloquium Seminar

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Tunnelling is often explained using the Heisenberg uncertainty principle and the wave–

particle duality of matter. Pure quantum mechanical concepts are central to the phenomenon, so

quantum tunnelling is one of the novel implications of quantum mechanics.

In electronics, a tunnel junction is a barrier, such as a thin insulating layer or electric

potential, between two electrically conducting materials. Electrons pass through the barrier by

the process of quantum tunneling. Classically, the electron has zero probability of passing

through the barrier. However, according to quantum mechanics, the electron has a non-zero

wave amplitude in the barrier, and hence it has some probability of passing through the barrier.

Tunnel junctions serve a variety of different purposes.

1. Molecular transport junctions

Molecular transport junctions (MTJs), the simplest components of molecular electronics,

are structures in which a molecule is inserted between two electrodes, and subjected to applied

voltage. Monitoring MTJ current as a function of applied voltage can be viewed as a kind of

spectroscopy.This spectroscopy is characterized by several factors.

First, is the identity of the molecule and the geometry that the molecule adopts within the

junction. Second are the parameters of the Hamiltonian that describe the system and determine

the band structure of the electrodes, the electronic structure of the molecule and the electronic

coupling between the electrodes and the molecule. The latter includes electronic correlations

such as the image effect that is often disregarded in theoretical studies. Finally, effects of the

underlying nuclear configuration as well as dynamic coupling between transmitted electrons

and molecular vibrations can strongly affect the electron transmission process.

Previous Junctions Syste: single-molecule junctions in a metal–molecule–metal motif have

contributed significantly to our fundamental understanding of the principles required to realize

molecular-scale electronic components from resistive wires to reversible switches. The success

of these techniques and the growing interest of other disciplines in single-molecule-level

characterization are prompting new approaches to investigate metal–molecule–metal junctions

with multiple probes. Quantum interference and manipulation of electronic and nuclear spins in

single-molecule circuits are heralding new device concepts with no classical analogues.

2. Self-assembled monolayers

Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed

spontaneously on surfaces by adsorption and are organized into more or less large ordered

domains. In some cases molecules that form the monolayer do not interact strongly with the

substrate.

This is the case for instance of the two-dimensional supramolecular networks of Perylene-

tetracarboxylicacid-dianhydride (PTCDA) on gold or of porphyrins on highly oriented

pyrolitic graphite (HOPG). In other cases the molecules possess a functional group that has a

strong affinity to the substrate and anchors the molecule to it. Common head groups

include thiols, silanes, phosphonates.

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SAMs are created by the chemisorption of "head groups" onto a substrate from either

the vapor or liquid phase followed by a slow organization of "tail groups". Initially, at small

molecular density on the surface, adsorbate molecules form either a disordered mass of

molecules or form an ordered two-dimensional "lying down phase", and at higher molecular

coverage, over a period of minutes to hours, begin to form three-dimensional crystalline or

semicrystalline structures on the substrate surface. The "head groups" assemble together on the

substrate, while the tail groups assemble far from the substrate. Areas of close-packed molecules

nucleate and grow until the surface of the substrate is covered in a single monolayer.

Type of Sam: Selecting the type of head group depends on the application of the

SAM. Typically, head groups are connected to a molecular chain in which the terminal end can

be functionalized to vary the wetting and interfacial properties. An appropriate substrate is

chosen to react with the head group. Substrates can be planar surfaces, such as silicon and metals,

or curved surfaces, such as nanoparticles.

Alkanethiols are the most commonly used molecules for SAMs. Alkanethiols are

molecules with an alkyl chain, (C-C)ⁿ chain, as the back bone, a tail group, and a S-H head

group. Other types of interesting molecules include aromatic thiols, of interest in molecular

electronics, in which the alkane chain is replaced by aromatic rings.

C. Conclusion

The professor speech described a new organic molecular rectifier: a junction having the

structure AgTS/S(CH2)11-2,2’-bipyridyl//Ga2O3/EGaIn which shows reproducible

rectification with mean r+. Comparisons among SAM-based junctions incorporating the

Ga2O3/EGaIn top-electrode and a variety of heterocyclic terminal groups indicate that the

metal-free bipyridyl group, not other features of the junction, is responsible for the rectification.

On the other hand, the professor also present about the robust M-SAM, Ga2O3,

Conveniences, versatile, and relatively well-defined junction characteristics. The statistical

analysis rectification. Insensitivity of current to many organic functional groups and the size of

rectifying moiety is a key feature for determining the magnitude of rectification