Development of One-Dimensional Band Structure in Artificial Gold Chains

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Development of One-Dimensional Band Structure in
Artificial Gold Chains

• J. R. Edwards
• Pierre Emelie
• Mike Logue 
• Zhuang Wu

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1-D Band Structure in Gold Nano-Chains

• Background
• Theoretical
• Experimental
• Next Step

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Background Work

• Atoms exhibit different properties than
  bulk---with a continuum of properties in-between
• Metal clusters exhibit absorption behavior
• Small metal aggregates exhibit catalytic behavior

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Metal clusters exhibit absorption behavior
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Small metal aggregates exhibit catalytic behavior
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Problems in Background Work

• Preparation and analysis of well-defined
  nano-structuresis difficult
• Continuum of Sizes
• Different Geometries

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Geometric structures and Size Continuum

• Geometric structures near the bulk transition
• 2-D states on surface
• 1-D states in step edges
• Geometric structures near the atom transition
• Nano-clusters

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

• The Cu atoms form an island with local hexagonal
  order
• Single Cu atoms are trapped in front of a
  descending step edge

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Au/Ti02 catalysts prepared by deposition-precipita
tion

• Uniform clusters are difficult to prepare
• Both size and geometry varies

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How is this work different from background work

• Nano-chain structure is near atom transition
• Nano-chains are well-defined and readily-modified
  geometries that provide useful analysis

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What is being studied

• Interrelation between geometric structure,
  elemental composition and electronic properties
  in metallic nanostructures.
• The behavior of matter in the atom-to-bulk
  transition range for well defined 1-D structures.
• How values for the effective mass and density of
  states of the 1-D, 20-atom length, gold chain
  compare to known results from other experiments.

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Why do this

• To be able to understand the electronic
  properties of metallic nanostructures on the size
  of a few atoms as related to geometric structure
  and elemental makeup.
• Demonstrate a strategy for studying this
  relationship.
• Use the knowledge gained to be able to control
  the intrinsic properties of metallic
  nanostructures whose size is in the
  atomic-to-bulk transition range.

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Why gold chains on NiAl(110)

• The NiAl(110) structure is made up of alternating
  rows of Ni troughs and protruding Al rows.
• This structure acts as a natural template for
  building the 1-D gold chain.
• The distance between adjacent Ni bridge sites
  (2.89 Å) matches almost exactly the nearest
  neighbor distance (2.88 Å) in bulk Au.

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How are the electronic properties measured

• STS-Scanning Tunneling Spectroscopy
• Uses the STM (Scanning Tunneling Microscope) to
  take very precise and accurate measurements of
  the electronic properties
• Why use STS?
• Because of its sensitivity to vibrational,
  optical, and magnetic properties.
• Because it can move atoms around as well as image
  atomic scale surfaces

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Scanning tunneling spectroscopy(STS)
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STS STM
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STS DOS
?s is the density of the electronic state of the
sample surface ?t is the density of the
electronic state of the tip
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How to move the atoms
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The atoms are added to the chain one after an
other
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Conductivity changes during 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
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1-D quantum well
The energy levels are discrete For infinity 1-D
quantum well, the wave function of the electron
at certain energy level En is fnsin(npx/r0) The
wave function of the electron is the
superposition of a series of fn
?(x)SAnSin(npx/r0) The probability to find a
electron at x-point is proportional to ?(x)2
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Measuring at different positions
The derived coefficients are c10.31, C20.29,
c30.26, c40.11 for 0.78 V C50.26, C60.50,
c70.24 for1.51 V and c60.13, C70.29,
c80.39, c90.19 for 2.01 V.

• P(x)SCnSin2(npx/L)

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• This can be simulated very well by 1-D infinity
  quantum well ?(x)SCnSin(npx/L)
• To account for a finite barrier height, the
  absolute length of the well (L) is treated as an
  adjustable parameter. For Au20, L varies from 59
  to 62 Å with increasing energy.
• The measured dI/dV signal is high, when the
  sample bias matches one of the energy levels En.

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Conductivity changes during 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
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Density of States (DOS) comparison

• The picture to the left compares the relative DOS
  for a Au20 chain to that of a 60 A long quantum
  well and a 1D free-electron gas. Quantum well
  states are marked with bars along the left axis.
• This data corresponds well with the predictions
  of an E-1/2 dependence and variations from the
  perfect 1-D behavior is attributed to the finite
  length of the chain and the limited number of
  states on the parabolic band.

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What have we learned?

• Scanning Tunneling Microscopy Preparation of
  well-defined nanosized structures
• Scanning Tunneling Spectroscopy - dispersion
  relation - effective
  mass - density of states

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Challenges

• Can we improve this STM/STS approach?
• What are the alternative techniques?
• What is the next step?

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Preparation of nanosized structures

• STM is a very useful tool to manipulate single
  atoms at low temperature
• It has also been used to manipulate single
  molecules at room temperature
• Problem time required to obtain these
  structures by STM

M. F. Crommie et al., Science, 262 218 (1993)
M. T. Cuberes et al., Appl. Phys. Lett., 69 3016
(1996)
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Preparation of nanosized structures

• Novel approach use tip geometries combined with
  millisecond voltage pulses

• Facet terminated STM tips are employed
• Polarity of the field is arranged to have the tip
  positive
• Both electrodes are of the same material

• Field enhanced evaporation

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Preparation of nanosized structures
No pulse
4V/5ms

• 0.4 µm Au thin films with Au tips

3.8V/5ms
2.8V/5ms

• Halo creations
• diameters around 210 ?
• walls extend to 70 ? laterally
• manufacturing time 106 faster

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

• UPS is one of the many complementary/alternative
  techniques to STS to study the electronic
  properties of nanosized structures
• Based on the absorption of a photon by an
  electron in the valence band
• Applications - electronic structure of
  solids - adsorption of molecules on metals

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Next Step

• 2D and 3D systems can be analyzed by this STM/STS
  approach
• Electronic and optical properties have to be
  characterized to potentially develop nanosized
  devices with novel applications
• Preparation and manufacturing of these nanosized
  structures will be a challenge