Download Cluster Assembled Metal Encapsulated Thin Nanotubes of Silicon

EOTD: Chlorine
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Atomic number: 17
Atomic weight: 35.4527
Oxidation State of +/+/- 1,3,5,7
Electron configuration: [Ne
[Ne]] 3s2p5
MP: 172.17 K
BP: 239.18 K
Discovered in Sweden by Carl William Scheele in 1774
Origin of name: From Greek word “chloros
“chloros”” meaning “pale green”
Chlorine is found largely in seawater. The green chlorine gas is recovered
from a solution of sodium chloride in water by electrolysis.
Chlorine is a respiratory irritant. The gas irritates the mucous membranes and
the liquid burns the skin. It was used as a war gas in 1915.
Cluster Assembled Metal
Encapsulated Thin
Nanotubes of Silicon
By: Abhishek Kumar Singh,
Vijay Kumar, Tina M. Briere,
Briere, and Yoshiyuki
Kawazoe
Presented by:
Faisal Rahman
The purpose of this paper:
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Using ab initio total energy calculations will
demonstrate how a metal encapsulated silicon
cluster SixBey can assemble to form hexagonal
nanotube of silicon.
Terms to know:
Binding energies:
“Represents the difference in mass between the nucleus
and its individual protons and neutrons.”
Doping:
“The number of electrons carries can be increased if the
atom with more electrons than the parent element can be
introduced.”
Dimer:
Dimer:
“A molecule formed by joining together of two identical
molecules.”
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Atoms used in the nanostructure:
Silicon:
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located in group 14 just under carbon
has an atomic weight of 28.09
its atomic radius is 1.32 angstroms
electron configuration: 1s22s22p63s23p2
Beryllium:
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located in group 2
has an atomic weight of 9.01
its atomic radius is 1.12 angstroms
electron configuration: 1s22s2
Finite undoped Si nanostructure:
a) Si24
What do these three structure represent?
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The silicon nanotubes represent stacking of sixsix-membered units
of chair shape.
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Atoms are somewhat tetrahedrally coordinated
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This indicates silicon preference for sp3 bonding
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Highly unlikely to get long symmetric Si wire of small dimension
assembled from such units.
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These Silicon clusters were very unstable.
The new development involved creating a stable cluster:
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Introducing a metal inside the silicon cluster.
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The clusters are formed to grow nanotubes of silicon stabilized
by a Beryllium atom.
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By doing this creates higher stability and higher symmetries
compared to pure silicon.
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This method produced in large and size selected quantities.
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The structure and properties can be controlled by a choice of
metal atoms.
c) Si48
b) Si36
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Why was doping used?
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Doping was used to stabilize the finite nanotubes.
nanotubes.
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The two atoms used were Tungsten and Beryllium.
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Doping with tungsten lead to hexagonal prism structure (Si12W).
Due to the large size of Tungsten lead to an unstable hexagonal
nanotube.
nanotube. The structure became distorted.
• These structure involve two and three Be atom in a Si12 cluster.
Doping with Beryllium proved to be more stable while having the
lowest energy. The Be is the main building block for the
development of symmetric hexagonal nanotubes.
nanotubes.
• The Si12 cluster has a chair structure of six membered rings.
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Si12Be2
Si12Be2
Si12Be3
What they found by using Be?
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Be encapsulation lead to several new forms of silicon with different
different
properties.
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The cluster assembly approach was important in developing novel
nanostructure.
Si24Be2
Si24Be2
Si24Be2
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The hexagonal units in the building block form sp2 bonding and is
stabilized using Be encapsulation in Si nanostructure.
• Packing two Si12Be clusters results from transformation of chair
shape to those of hexagonal shape.
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This is achieved using ab initio calculations of the total energy to
obtain the atomic structures and electronic properties.
• Si24Be2 represents a stable unit that can be repeated to form
nanotubes of desired length. Types of units would be (Si24Be2)2
• Doping provides stability and long range ordering in the nanotube.
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Binding Energies and HOMOHOMO-LUMO Gaps for
the Doped Finite Silicon Nanostructure
Si36Be3
Si36Be5
Si36Be3
• The rings again are stable and is hexagonal in shape but the
the Be atoms are not symmetrically arranged due to their odd numbers.
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• In the second diagram the doped portion is symmetric and the undoped
portion is distorted.
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• The third diagram gives a clear idea of the stabilization due to Be doping.
System
Average BE (eV
(eV))
HOMOHOMO-LUMO
gaps (eV
(eV))
Si12Be (2 or 3)
3.67
1.25
Si24Be (2 or 3)
3.81
1.06
Si36Be (3 or 5)
3.83
0.57
Si48Be (4 or 7)
3.85
0.49
As the cluster get bigger the average BE increase and the HOMOHOMO-LUMO gap
decreased.
The binding energies for the unit cells containing 2424-48 Si atoms decreased
very slightly with increasing number of Be atoms.
The results indicates that Be doping results in clusters that are
are stable, straight
and symmetric. This would help researcher develop nanotubes as nanowires
for device applications.
Binding Energies for the Doped Infinite Nanotubes
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Si48Be4
Si48Be4
Si48Be7
• In diagram one, packing leads to symmetric nanotube with a
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reflection passing through its center. This is composed of Si24Be2 structure.
• In the second diagram the Be atom is placed between successive
hexagons and the doped portion is symmetric while the remaining nanotube becomes
distorted.
• In the last diagram show further doping between hexagons leads to slight distortion.
This is due to variations in bond lengths of Si-Be as Be atom is not symmetrically
distributed.
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The structure was optimized with
respect to the cell size along the
nanotube axis allowing the atoms
to freely relax.
The Be atom does not lie at the
center of the hexagonal ring like
the finite structure but slightly
towards one ring.
This leads to a higher binding
energy as compared to the finite
nanotubes.
nanotubes.
The binding energies are the same
for all the structure indicating a
weak interaction between Be
atoms.
System
BE (eV
/atom)
(eV/atom)
Si12Be
3.92
Si24Be2
3.92
Si24Be3
3.93
Si24Be4
3.90
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Infinite Si Nanotubes with Different Doping Concentrations
The Fermi Level
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(I)
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(II)
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(III)
In diagram (I) the Be atom is centered.
Diagram (I) and (III) are degenerate with the ground state and has
has the same
structure and BE as Si12Be.
The placement of the Be atom in diagram (II) has the lowest energy
energy which is
the same as the finite structure in Si24Be2 and Si48Be4.
This tells us that Si24Be2 is the building block for the infinite nanostructure.
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These figures
represent the infinite
Si24Be4 nanotube:
nanotube:
What is the main role of the Be atom?
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There is a small charge transfer to Be because of its deeper
potential. This leads to occupation of the p state.
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Bonding can be considered sp2 like within the hexagonal rings
and p-p like between the rings.
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There is a depletion of charge from the lobes protruding from
the ring to the space between the Si and Be atoms.
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Be main role is to stabilize the hexagonal ring.
If atom supplies one s electron
then at T=0 the lowest ½ atom are
occupied.
At T=0 the highest occupied
orbital is called the Fermi level.
This lies at the center of the band.
If a band is not full, an electron
near the Fermi level can easily be
placed to an empty level. As a
result the electron is more mobile
and can easily move through the
solid.
Therefore making it a good
electron conductor.
In figures (I) & (II) the
states near the Fermi
energy arise from the Be
and Si energy levels. This
indicates hybridization of
the p state.
„ The energy levels can be
changed by varying the
doped atom.
„ The partial density of
states are represented by
s, p and d.
s – red
p – green
d - blue
(I) Si
Fermi energy
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(II) Be
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Charge Density Analysis:
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In order to calculate the charge
accumulation and depletion
involves subtracting the charge
density of isolated Si24 and Be4 at
their position from the charge
density of the infinite Si24Be4
nanotube.
nanotube.
This analysis shows that a strong
depletion takes place at the Be
atom. This imply that there is no
dimerization.
dimerization.
The maximum accumulation of
charge takes place between Be atom
and the nearest Si ring. Very slight
charge transfer takes place between
the Be atom and the other Si ring.
This provides further evidence that
the Be atom stabilizes the Si
hexagonal ring.
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In order to improve the transport behavior of the wire, proper
doping that would affect the distribution around the Fermi
energy should be modified.
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Therefore this would lead to interesting transport property along
along
the center surface of the nanotube.
nanotube.
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Metal encapsulation of silicon offer new ways of combining
properties of metal atomic wires.
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Tubular structure involving Si including tubular superlattices give
future hope for future nanodevice technology.
Charge Accumulation
Charge Depletion
Conclusion:
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Determined the stability of finite and infinite Si nanotubes doped
with Be.
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When silicon is used, metal doping is very important in controlling
controlling
the structure.
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Bonding with only Si cannot form thin nonowires/nanotubes but
doping with Be forms stability of the nanotube structure.
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The best structure are normally assembled from Si12Be units.
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The doped infinite Si nanotubes are the most stable, symmetric,
and metallic.
References
1)
Singh K. Abhishek;
Abhishek; Cluster Assembled Metal Encapsulated Thin
Nanotube of Silicon;
Silicon; American Chemical Society; 2002; Vol. 0 No. 0;
published on web.
2)
Shriver D. and Atkins P.; Inorganic Chemistry 3rd edition; Freeman and
Company; New York; 1999.
3)
Bruice Y. Paula; Organic Chemistry 2nd edition; Prentice Hall;
New Jersey; 1998.
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