ENGINEERING ATOMIC AND MOLECULAR NANOSTRUCTURES AT SURFACESJohannes V.Barth, Giovanni Constantini & Klaus Kern

Presented by:Kiran ManjunathAravind Poolavalasu Ramasamy

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TABLE OF CONTENTS•Motivation•Fabrication methods at Nanoscale•Understanding Surface Structuring• Semiconductor artificial atoms• Supramolecular Engineering• Outlook•Bibliography

MOTIVATION

• The need for a promising alternative route to produce structures of nanometer dimensions on atomically well defined surfaces. (limit)

• Autonomous ordering and assembly of atoms and molecules- combines ease of fabrication with control over shape, composition and mesoscale organìsation.

• Applications --Controlled self assembly and growth processes produce a wide range of metallic semiconducting and molecular materials.

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TABLE OF CONTENTS

•Motivation•Fabrication methods at Nanoscale•Understanding Surface Structuring• Semiconductor artificial atoms• Supramolecular Engineering• Outlook•Bibliography

FABRICATION METHODS AT NANOSCALE

TOP DOWN METHOD• Use of lithography,

writing and short wavelength optical sources to define structures

• impose structures on substrate

BOTTOM UP METHOD

• Use of self processes for ordering of supramolecular and solid state architectures from the

atomic to the mesoscopic scale.

• Guide the assembly of atoms and molecular constituents

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©2005 Nature publishing group

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TABLE OF CONTENTS

•Motivation•Fabrication methods at Nanoscale•Understanding Surface Structuring• Semiconductor artificial atoms• Supramolecular Engineering• Outlook•Bibliography

UNDERSTANDING SURFACE STRUCTURING

SURFACE DIFFUSION

• Transport of adsorbed species on a flat terrace at substrate atomic lattice.

• Surface diffusion is

thermally activated

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GROWTH KINETICS

• Diffusivity D = Mean square distance travelled by and adsorbate per unit time.

F=Deposition rate

•D/F : the ratio of diffusion to deposition rate is the key parameter in growth kinetics

Large (D/F) Deposition is slower than diffusion,

growth occurs close to equilibrium conditions.

Small (D/F) Deposition is faster than diffusion,

growth determined by individual processes leading to

metastable structures

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MAGNETISM AT THE PHYSICAL LIMIT

• Advancements in CMOS technology has increased the information processing speeds.

• As the scaling down continues, decrease in ferromagnetic domain size is accompanied by decrease in magnetic anisotropic energy K.

• For very small domains, thermal fluctuations flip the magnetisation direction. This renders the domain superparamagnetic with all stable magnetic order lost.

Semiconductor artificial atoms• Semiconductor nanocrystals are called

artificial atoms, ., Quantum Dots

• Quantum dots are solid state nanostructures

• Energy spectrum can be controlled by tuning composition, size, lattice strain and morphology

• Why?

• How?

•Deals with semiconductor quantum dots supported on surfaces or embedded in solids

•Efficient fabrication method – self organized growth at surface – wide separated energy levels - no interface defects

•By using Stranski-Krastanow growth mode they have demonstrated the growth of :

Ge on Si and InAs on GaAs

•Found that well defined island shapes occur during growth and evolution, independent of specific material system considered

•Quantum dots are laterally organized using self-ordering processes - mediated by elastic interactions, or using patterned substrates

•In Stranski–Krastanow growth mode, highly regular structures are obtained by using patterned substrates

•As they have extremely small dimension and low power dissipation – driving the desire to develop

•Notable applications are Lasers, information technology, and in quantum computation application

Supramolecular Engineering•Supramolecule - well defined complex of molecules

held together by noncovalent bonds

•Chemical Synthesis provide access to various building blocks through weak and non-covalent interactions

•With Limitation, only used as tool for steering interactions with appropriate surface patterns

•Planar molecules tend to bond to substrate easily and provides mobility on surface

•Surface pattern can be controlled by tecton design

•Network stability is essential feature if nanoporous surface patterns are used as templates for successive layers

•Metal-Ligand interactions are stronger than Hbonds

•Metal incorporation increases nano-architecture functional properties

•Deposition sequence and temperature have to be carefully controlled to avoid spurious effects and to achieve regular nanosystems

•Resultant structure depends on the relative concentrations of metal atoms and ligand molecules

• The interaction of benzenepolycarboxylic acid species and iron atom on a copper as follows

• Mononuclear complexes are obtained on depositing about 0.3 Fe atoms per linear terephthalate linker molecule

• Individual Fe centers are thus arranged in a perfect square lattice with a 15 Å periodicity

• Is the deposited metal is about 1 Fe atom per linker molecule results in polymeric ladder structures

•Constitutes a regular array of open nanocavities with an effective opening of (3 x10) Å

•If the metal concentration is about two Fe atoms per linker molecule, yields fully interconnected 2D metal–organic networks with complete two-dimensional reticulation

•Linker structures should be selected in conjunction with appropriate metals gives effective control over pores

•Side groups present in the linker determines the lining of pores walls

Outlook•Self-organized growth and self assembly at

surfaces can serve as efficient tool for creating low dimensional nanostructures

•Gives exquisite control over size and organization on the atomic and mesoscopic length scales

•Incorporation of nanostructures into more complex organized architectures can be achieved by combining bottom-up and top-down technique

Applications•Nano-Scale Machinery

•Drug delivery applications

•Small Circuits for Ultra fast switching, in solar power generation

•Single electron Transistors

•Electroluminescent displays