Development of One-Development of One-Dimensional Band Dimensional Band

Structure in Artificial Structure in Artificial Gold ChainsGold Chains

J. R. EdwardsJ. R. EdwardsPierre EmeliePierre EmelieMike Logue  Mike Logue  Zhuang Wu Zhuang Wu

1-D Band Structure in 1-D Band Structure in Gold Nano-ChainsGold Nano-Chains

BackgroundBackground TheoreticalTheoretical ExperimentalExperimental Next StepNext Step

Background WorkBackground Work

Atoms exhibit different properties Atoms exhibit different properties than bulk---with a continuum of than bulk---with a continuum of properties in-betweenproperties in-between Metal clusters exhibit absorption Metal clusters exhibit absorption

behaviorbehavior Small metal aggregates exhibit catalytic Small metal aggregates exhibit catalytic

behaviorbehavior

Metal clusters exhibit Metal clusters exhibit absorption behaviorabsorption behavior

Small metal aggregates Small metal aggregates exhibit catalytic behaviorexhibit catalytic behavior

Problems in Background WorkProblems in Background Work

Preparation and analysis of well-Preparation and analysis of well-defined nano-structuresdefined nano-structuresis difficultis difficult Continuum of SizesContinuum of Sizes Different GeometriesDifferent Geometries

Geometric structures Geometric structures and Size Continuumand Size Continuum

Geometric structures near the bulk Geometric structures near the bulk transitiontransition 2-D states on surface2-D states on surface 1-D states in step edges1-D states in step edges

Geometric structures near the atom Geometric structures near the atom transitiontransition Nano-clustersNano-clusters

Single Cu atoms evaporated Single Cu atoms evaporated at 15 K on Cu(111).at 15 K on Cu(111).

The Cu atoms form an The Cu atoms form an island with local island with local hexagonal orderhexagonal order

Single Cu atoms are Single Cu atoms are trapped in front of a trapped in front of a descending step edgedescending step edge

Au/Ti02 catalysts prepared Au/Ti02 catalysts prepared by deposition-precipitationby deposition-precipitation

Uniform Uniform clusters are clusters are difficult to difficult to prepareprepare

Both size and Both size and geometry geometry variesvaries

How is this work How is this work different from different from

background workbackground work Nano-chain structure is near atom Nano-chain structure is near atom

transitiontransition Nano-chains are well-defined and Nano-chains are well-defined and

readily-modified geometries that readily-modified geometries that provide useful analysisprovide useful analysis

What is being studiedWhat is being studied

Interrelation between geometric Interrelation between geometric structure, elemental composition and structure, elemental composition and electronic properties in metallic electronic properties in metallic nanostructures.nanostructures.

The behavior of matter in the atom-The behavior of matter in the atom-to-bulk transition range for well to-bulk transition range for well defined 1-D structures.defined 1-D structures.

How values for the effective mass and How values for the effective mass and density of states of the 1-D, 20-atom density of states of the 1-D, 20-atom length, gold chain compare to known length, gold chain compare to known results from other experiments.results from other experiments.

Why do thisWhy do this

To be able to understand the To be able to understand the electronic properties of metallic electronic properties of metallic nanostructures on the size of a few nanostructures on the size of a few atoms as related to geometric atoms as related to geometric structure and elemental makeup.structure and elemental makeup.

Demonstrate a strategy for studying Demonstrate a strategy for studying this relationship.this relationship.

Use the knowledge gained to be able Use the knowledge gained to be able to control the intrinsic properties of to control the intrinsic properties of metallic nanostructures whose size is metallic nanostructures whose size is in the atomic-to-bulk transition range.in the atomic-to-bulk transition range.

Why gold chains on Why gold chains on NiAl(110) NiAl(110)

The NiAl(110) structure is made up of The NiAl(110) structure is made up of alternating rows of Ni troughs and alternating rows of Ni troughs and protruding Al rows.protruding Al rows.

This structure acts as a natural This structure acts as a natural template for building the 1-D gold template for building the 1-D gold chain.chain.

The distance between adjacent Ni The distance between adjacent Ni bridge sites (2.89 Å) matches almost bridge sites (2.89 Å) matches almost exactly the nearest neighbor distance exactly the nearest neighbor distance (2.88 Å) in bulk Au.(2.88 Å) in bulk Au.

How are the electronic How are the electronic properties measuredproperties measured

STS-Scanning Tunneling SpectroscopySTS-Scanning Tunneling Spectroscopy Uses the STM (Scanning Tunneling Uses the STM (Scanning Tunneling

Microscope) to take very precise and Microscope) to take very precise and accurate measurements of the electronic accurate measurements of the electronic propertiesproperties

Why use STS?Why use STS? Because of its sensitivity to vibrational, Because of its sensitivity to vibrational,

optical, and magnetic properties.optical, and magnetic properties. Because it can move atoms around as well Because it can move atoms around as well

as image atomic scale surfacesas image atomic scale surfaces

Scanning tunneling Scanning tunneling spectroscopyspectroscopy

(STS)(STS)

STS & STMSTS & STM

STS & DOSSTS & DOS

ρs is the density of the electronic state of the sample surface

ρt is the density of the electronic state of the tip

How to move the atomsHow to move the atoms

The atoms are added to the chain one after an The atoms are added to the chain one after an otherother

Conductivity changes Conductivity changes during this processduring this process

Measurements are taken in the center of the chains

Peak splits due to strong coupling between atoms, and there is a downshift of the peaks

Due to the overlap between neighboring peaks, conductivities become indistinguishable for chains with 4 or more atoms

1-D quantum well1-D quantum wellThe energy levels are discrete

For infinity 1-D quantum well, the wave function of the electron at certain energy level En is φn=sin(nπx/r0)

The wave function of the electron is the superposition of a series of φn

Ψ(x)=ΣAnSin(nπx/r0)

The probability to find a electron at x-point is proportional to | Ψ(x)|2

Measuring at different Measuring at different positionspositions

The derivedcoefficients are c1=0.31,C2=0.29, c3=0.26, c4=0.11 for 0.78 V;

C5=0.26,C6=0.50, c7=0.24 for1.51 V;

and c6=0.13,C7=0.29, c8=0.39, c9=0.19 for 2.01 V.

•P(x)=ΣCnSin2(nπx/L)

This can be simulated very well by 1-D infinity This can be simulated very well by 1-D infinity quantum wellquantum well : : ΨΨ(x)=(x)=ΣΣCnSin(nCnSin(nππx/L)x/L)

To account for a finite barrier height, the To account for a finite barrier height, the absolute length of the well (absolute length of the well (LL) is treated as an ) is treated as an adjustable parameter. For Au20, adjustable parameter. For Au20, L L varies from varies from 59 to 62 Å with increasing energy.59 to 62 Å with increasing energy.

The measured The measured dIdI//dV dV signal is high, when the signal is high, when the sample bias matches one of the energy levels sample bias matches one of the energy levels En.En.

Conductivity changes Conductivity changes during this processduring this process

Measurements are taken in the center of the chains

Peak splits due to strong coupling between atoms, and there is a downshift of the peaks

Due to the overlap between neighboring peaks, conductivities become indistinguishable for chains with 4 or more atoms

Density of States (DOS) Density of States (DOS) comparisoncomparison

The picture to the left compares The picture to the left compares the relative DOS for a Authe relative DOS for a Au2020 chain chain to that of a 60 A long quantum to that of a 60 A long quantum well and a 1D free-electron gas. well and a 1D free-electron gas. Quantum well states are marked Quantum well states are marked with bars along the left axis.with bars along the left axis.

This data corresponds well with This data corresponds well with the predictions of an Ethe predictions of an E-1/2-1/2 dependence and variations from dependence and variations from the perfect 1-D behavior is the perfect 1-D behavior is attributed to the finite length of attributed to the finite length of the chain and the limited number the chain and the limited number of states on the parabolic band.of states on the parabolic band.

What have we learned? What have we learned?

Scanning Tunneling Scanning Tunneling Microscopy:Microscopy: Preparation of well- Preparation of well-defined nanosized structuresdefined nanosized structures

Scanning Tunneling Scanning Tunneling Spectroscopy:Spectroscopy: - dispersion - dispersion relationrelation - - effective masseffective mass - - density of statesdensity of states

)( sFs VEdV

dI

Challenges Challenges

Can we improve this Can we improve this STM/STS approach?STM/STS approach?

What are the alternative What are the alternative techniques?techniques?

What is the next step?What is the next step?

Preparation of nanosized Preparation of nanosized structuresstructures

STM is a very useful tool to STM is a very useful tool to manipulate single atoms at low manipulate single atoms at low temperaturetemperature

It has also been used to It has also been used to manipulate single molecules at manipulate single molecules at room temperatureroom temperature

Problem:Problem: time required to obtain time required to obtain these these structures by STM structures by STM

M. T. Cuberes et al., Appl. Phys. Lett., 69 3016 (1996)

M. F. Crommie et al., Science, 262 218 (1993)

Preparation of nanosized Preparation of nanosized structuresstructures

Novel approach:Novel approach: use tip geometries use tip geometries combined with millisecond voltage pulsescombined with millisecond voltage pulses

Field enhanced evaporationField enhanced evaporation

Facet terminated STM tips are employedFacet terminated STM tips are employed Polarity of the field is arranged to have Polarity of the field is arranged to have

the tip positivethe tip positive Both electrodes are of the same materialBoth electrodes are of the same material

Preparation of nanosized Preparation of nanosized structuresstructures

0.4 0.4 µµm Au thin m Au thin films with Au films with Au tipstips

P. A. Campbell et al., Nanotechnology, 13 69 (2002)

No pulse

3.8V/5ms

4V/5ms

2.8V/5ms

Halo creations:Halo creations: diameters around 210 diameters around 210 ǺǺ walls extend to 70 Ǻ laterallywalls extend to 70 Ǻ laterally manufacturing time 10manufacturing time 1066

fasterfaster

Photoelectron Photoelectron Spectroscopy Spectroscopy

(UPS)(UPS) UPS is one of the many UPS is one of the many

complementary/alternative complementary/alternative techniques to STS to study the techniques to STS to study the electronic properties of nanosized electronic properties of nanosized structuresstructures

Based on the absorption of a photon Based on the absorption of a photon by an electron in the valence bandby an electron in the valence band

Applications:Applications: - electronic structure - electronic structure of solidsof solids - adsorption of - adsorption of molecules on metalsmolecules on metals

Next StepNext Step

2D and 3D systems can be analyzed 2D and 3D systems can be analyzed by this STM/STS approachby this STM/STS approach

Electronic and optical properties Electronic and optical properties have to be characterized to have to be characterized to potentially develop nanosized potentially develop nanosized devices with novel applicationsdevices with novel applications

Preparation and manufacturing of Preparation and manufacturing of these nanosized structures will be a these nanosized structures will be a challengechallenge