Transcript slides from group 2

Development of One-Dimensional
Band Structure in Artificial Gold
Chains
J. R. Edwards
Pierre Emelie
Mike Logue
Zhuang Wu
1-D Band Structure in Gold
Nano-Chains




Background
Theoretical
Experimental
Next Step
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

Metal clusters exhibit absorption
behavior
Small metal aggregates exhibit
catalytic behavior
Problems in Background Work

Preparation and analysis of well-defined nanostructures
is difficult
Continuum of Sizes
 Different Geometries

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
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
Au/Ti02 catalysts prepared by
deposition-precipitation


Uniform clusters are
difficult to prepare
Both size and
geometry varies
How is this work different from
background work


Nano-chain structure is near atom transition
Nano-chains are well-defined and readilymodified geometries that provide useful analysis
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.
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-tobulk transition range.
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.
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

Scanning tunneling
spectroscopy
(STS)
STS & STM
STS & 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 atoms
The atoms are added to the chain one after an other
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
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 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 xpoint is proportional to | Ψ(x)|2
Measuring at different positions
The derived
coefficients are
c1=0.31,
C2=0.29,
2
•P(x)=ΣCnSin (nπx/L)
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.

This can be simulated very well by 1-D infinity
quantum well : Ψ(x)=ΣCnSin(nπx/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.
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
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.
What have we learned?

Scanning Tunneling Microscopy:
Preparation of well-defined
nanosized structures

Scanning Tunneling Spectroscopy:
- dispersion relation
- effective mass
- densitydIof states
  s ( EF  Vs )
dV
Challenges



Can we improve this STM/STS
approach?
What are the alternative techniques?
What is the next step?
Preparation of nanosized
structures

M. F. Crommie et al., Science,
262 218 (1993)

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. T. Cuberes et al., Appl. Phys.
Lett., 69 3016 (1996)
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
Preparation of nanosized
structures
No pulse
4V/5ms

3.8V/5ms
0.4 µm Au thin films
with Au tips
2.8V/5ms
Halo creations:



P. A. Campbell et al., Nanotechnology, 13 69 (2002)
diameters around 210 Ǻ
walls extend to 70 Ǻ laterally
manufacturing time 106
faster
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
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