Carbon Nanostructures PDF
Carbon Nanostructures PDF
Carbon Nanostructures PDF
Carbon Nanostructures
Indranil Lahiri
Nanocarbon
• Fullerene
• Tubes
• Sheets
• Cones
• Carbon black
• Horns
• Rods
• Foams
• Nanodiamonds
Bonding
~1 nm
Roll-up vector:
Ch = n a1 + m a2
Carbon Nanotube
Electrical conductance depending on helicity
2n + m
Ch = n a1 + m a2 If = i , then metallic
3
• Current capacity else semiconductor
Carbon nanotube 1 GAmps / cm2
Copper wire 1 MAmps / cm2
• Heat transmission
Comparable to pure diamond (3320 W / m.K)
• Temperature stability
Carbon nanotube 750 oC (in air)
Metal wires in microchips 600 – 1000 oC
• Caging
May change electrical properties
→ sensor
Carbon Nanotube
High aspect ratio:
Length: length
typical few μm
1000
diameter
→ quasi 1D solid
Diameter:
as low as 1 nm
Extreme carbon nanotubes
•The longest carbon nanotubes (18.5 cm long) was reported in 2009. These
nanotubes were grown on Si substrates using an improved chemical vapor
deposition (CVD) method and represent electrically uniform arrays of single-
walled carbon nanotubes
The carbon species are there after deposited as soot in different regions:
water-cooled copper collector, quartz tube walls.
2 Synthesis with CO2 laser
Vaporization of a target at a
fixed temperature by a
continuous CO2 laser beam (λ =
10.6μm). The power can be varied
from 100Wto 1,600 W.
Sketch of a solar
energy reactor in use
in Odeilho (France).
(a) Gathering of sun rays,
focused in F;
(b) Example of Pyrex®
chamber placed in (a)
so that the graphite
crucible is at the
point F.
The high temperature of about 4,000K permits both the carbon and the catalysts
to vaporize. The vapors are then dragged by the neutral gas and condense onto
the cold walls of the thermal screen.
CNT Growth by CVD
CVD System
g s
− g tot
al = s
T g − gl
liquid
liquid and solid
liquidus al + a s = 1
solidus
mixed crystal
gl gtot gs
A B
25
Vapour Liquid Solid Method
Eutectic:
-coexistence of 3 phases
- lowest temperature where system is still totally liquid
-minimum of liquidus curve
- solid in solid + liquid phase consists of only one material
liquidus
T
liquid
Mixed crystal
A
solidus B
26
Vapour Liquid Solid Method
• Mix of semiconductor and metal at eutectic
• Melting point of Semiconductor with metal lower - growth of one pure
material
T
→ metal as catalyst l
A+l B+ l
Mixed crystal
Growth procedure: A B
T
liquid
E.g. Au - GaAs pseudobinary phase
Au + GaAs+ liquid
diagram
liquid
Au + GaAs
Au GaAs 28
Vapour Liquid Solid Method
➢In general, the nanowires grown by VLS
- cylindrical morphology
- without facets on the side surface
- uniform diameter
29
VLS – Diameter of Nanostructures
30
VLS – Diameter of Nanostructures
Critical Diameter- liquid catalyst clusters are stable in equilibrium
Problem:
in fluid at according to temperature → critical diameter
Typical value - d = 0.2 mm
Goal:
finding methods to get smaller metal clusters to start NW growth
31
Chemical Vapour Deposition
Fine catalyst particles can be formed by
- Thin film deposition – which cracks to small island upon
heating
- Colloidal suspension of metallic nanocrystals – dried on
substrate
- Evaporated growth species or precursor gas introduced to
reaction chamber
- Supersaturation in Catalyst particle
- Precipitation as nanowire or naontube
32
GaN nanowires grown in CVD reactor
CVD: Principle and Mechanism
1. CxHy decomposes on the top surface of the metal, C diffuses down through the
metal, and CNT precipitates out across the metal bottom, pushing the whole
metal particle off the substrate (step (i)).
2. As long as the metal’s top is open for fresh hydrocarbon decomposition
(concentration gradient exists in the metal allowing carbon diffusion), CNT
continues to grow longer and longer (step (ii)).
3. Once the metal is fully covered with excess carbon, its catalytic activity ceases
and the CNT growth is stopped (step (iii)).
CNT Growth by CVD: Root Growth
1. Nature of hydrocarbon,
2. Type of catalyst, catalyst size
3. Temperature,
4. Pressure,
5. Gas-flow rate,
6. Deposition time,
7. Reactor geometry
Mukul Kumar and Yoshinori Ando, Chemical Vapor Deposition of Carbon Nanotubes:
A Review on Growth Mechanism and Mass Production, J. Nanosci. Nanotechnol.
10, 3739–3758, 2010
Graphene
Introduction to graphene
- Electronic properties
- Thermal properties
- Mechanical properties
- Optical properties
- Relativistic charge carriers
- Anomalous quantum Hall effect
Electronic properties
- High electron mobility (at room temperature ~ 200.000 cm2/(V·s),, ex. Si at RT~ 1400 cm2/(V·s),
carbon nanotube: ~ 100.000 cm2/(V·s), organic semiconductors (polymer, oligomer): <10 cm2/(V·s)
Where υd is the drift velocity in m/s (SI units)
E is the applied electric field in V/m (SI)
µ is the mobility in m2/(V·s), in SI units.
- Resistivity of the graphene sheet ~10−6 Ω·cm, less than the resistivity of silver (Ag), the lowest
resistivity substance known at room temperature (electrical resistivity is also as the inverse of the
conductivity σ (sigma), of the material, or
Material Electrical Conductivity (S·m-1) Notes
Graphene ~ 108
Silver 63.0 × 106 Best electrical conductor of any known metal
EF
Mechanical properties
- Graphene is considered as the strongest material ever measured, almost 200 times
stronger than structural steel
Properties of graphene
Optical properties
- Monolayer graphene absorbs πα ≈ 2.3% of white light (97.7 % transmittance),
where α is the fine-structure constant.
Ref: Carbon, 4 8, 2 1 2 7 –2 1 5 0 ( 2 0 1 0 )
Characterization methods
Raman
Spectroscopy
Transmission electron
Microscopy (TEM)
TEM images show the nucleation of (c) one, (d) three, or (e) four
layers during the growth process
Characterization methods
X-ray diffraction
(XRD)
XRD patterns of 400 um diameter graphite flakes oxidized for various lengths of time.
The Big problem with graphene: an imagined conversation:
B. !@#$%%
62
Preparation methods
Top-down approach
(From graphite)
Materials: HOPG
Preparation:
1. prepared 5 mm-deep mesas on top of the platelets (mesas were squares of various
sizes from 20 mm to 2 mm).
2. Pressed the structured surface on a 1-mm-thick layer of a fresh wet photoresist
spun over a glass substrate. After baking, the mesas became attached to the
photoresist layer, which allowed us to cleave them off the rest of the HOPG
sample. Then, using scotch tape to repeatedly peel flakes of graphite off the mesas. 67
3. Thin flakes left in the photoresist were released in acetone.
4. Dipping a Si wafer in the solution and then washed in plenty of water and
propanol, some flakes became captured on the wafer’s surface (chose thick SiO2
with t =300 nm).
5. Used ultrasound cleaning in propanol to remove mostly thick flakes. Thin flakes
(d < 10 nm) were found to attach strongly to SiO2, presumably due to van der
Waals and/or capillary forces.
(Left) Optical photograph in white light of a large Hall bar made from multilayer
graphene (d »5nm). The central wire is 50mm long.
(Right) A short (200 nm) wire made from few-layer graphene.
Advantages : Production of single layer graphene is feasible
68
Drawbacks : Limited quantity
Graphite oxide ( Most common and high yield method)
Oxidation (Hummers’method)
Graphite Oxide
H2SO4/ KMnO4
H2SO4/KClO3
Graphite Or H2SO4/HNO3
……………….
H2O
Ultrasonication (exfoliation
Graphite oxide
Graphite/Graphene oxide
Hydroxyl group
Epoxy group
Lerf-Klinowski model of graphene oxide
Steps:
(1) hydrogen etching to produce atomically
flat surfaces;
(2) vacuum graphitization to produce an
ultrathin epitaxial graphite layer;
(3) application of metal contacts (Pd, Au),
(4) electron-beam patterning and
development;
(5) oxygen plasma etch to define graphite
structures;
(6) wire bonding.
When SiC substrates are annealed at high temp., Si atoms selectively desorb from the surface
and the C atoms left behind naturally form FLG (few-layer graphene)
Graphene nanoribbons
(from carbon nanotube)
Mechanisms:
1.Adoption of Cu:(a) low solubility of carbon in Cu,
(b) surface diffusion of carbon atoms on Cu;
2.Absorption and de-absorption of hydrocarbon
molecules on Cu;
3.Decomposition of hydrocarbon to form carbon atoms;
4.Aggregation of carbon atoms on Cu surface to form
graphene nucleation centers;
5.Diffusion and attachment of carbon atoms to
nucleation centers to form graphene film 80
Chemical vapor deposition
Effect of Hydrocarbon precursor and substrate
1.Methane is a relatively stable hydrocarbon compound due to strong C-H bond,
as a result, decomposition occurs at elevated temperature (>1200C);
2.Other hydrocarbon compounds such as ethane and acetylene are not suggested
due to rapid decomposition at high temperature;
3.Other transition metal such as Fe, Co, and Ni are not preferred for mono or
bilayer graphene growth due to their higher-than-desirable capacity to
decompose hydrocarbons.
4.The low decomposition rate of methane on Cu allows the possibility of controlling
the number of graphene layers.
Decomposition of hydrocarbon
1.Cu foil: Cu foil is usually not single crystal possessing grain boundaries and steps;
2.The sites have much higher chemical activation energy than those of the flat
regions of Cu; as a result, hydrocarbons prefer to decompose on the sites to form
nucleation centers;
3.Cu foil with smooth surface is preferred: pre-polish and in-situ polish.
81
(a) Schematic of graphene growth process at low-pressure CVD condition:
(1) Nucleation starts at the steps and grain boundaries,
(2) Growth process after nucleation.
(b) Graphene domains on the Cu surface. Arrow indicates the direction of polish lines.
(c) Image of a single tetragonal graphene domain. 82
(d) OM image of a grown graphene sheet verifying the growth mechanism outlined in (a).
Processing parameters-substrate pretreatment
1.Treated by dilute acid
2.Ultrasonic Cu foil in acetone
3.Annealing at lower pressure (due to sublimation of Cu at low pressure)
1. After low pressure annealing, the Cu surface becomes smoother and has low-index
planes (such as (1 0 0) plane). due to the restructuring of the Cu atoms enabled by
increased diffusivity at high temperatures during the low pressure annealing.
2. Low partial pressure of hydrocarbon decrease the size of nucleation centers.
84
Processing parameters-purity of the Cu substrate
1.The impurity in the catalyst greatly enhances the catalytic capability of the catalyst.
Could the purity of the Cu foil affect the number of layers?*
2.Cu foil with lesser purity: 2638–2641 cm-1, I2D/IG: over 3: monolayer
Cu foil with higher purity: 2641–2646 cm-1, I2D/IG:1.8-2.4:bilayer.**
85
Graphene Growth by CVD
1. Graphitic structure starts forming at temperature over 2500C – difficult to
control process, energetically unfavorable – needs catalyst
2. Catalysts lower temperature significantly – but can introduce CNT growth
Graphene Growth by CVD – on Ni and Cu
Cu can evaporate more than Ni (due to lower M.P.) – frequent chamber cleaning
required
Congqin Miao, Churan Zheng, Owen Liang and Ya-Hong Xie,
Chemical Vapor Deposition of Graphene, www.intechopen.com
Grain Boundaries
• Grain Boundaries give rise to surface roughness.
• On precipitation, carbon prefers to stay in the areas with
higher surface energy, such as grain boundaries, surface
trenches and so on.
• These areas have more atomic dangling bonds that could
easily attract precipitated carbon atoms.
• Deposited graphene exhibits non-uniformity with thick
graphene around grain boundaries and other surface defects,
and thin sheet on the other areas.
• Thus important to pre-anneal catalyst substrates in order to
have large grains to reduce total length of grain boundaries, as
well as other minor surface defects.
Congqin Miao, Churan Zheng, Owen Liang and Ya-Hong Xie,University of California, Los Angeles,United States: Chemical Vapor
Deposition of Graphene
Rate Of Cooling(in case of Nickel)
• At elevated temperature, dissociated carbon atoms on the catalyst surface
may dissolve into the bulk due to the finite solubility.
• As these dissolved carbon atoms precipitate back onto Ni surface as
temperature drops and hence unwanted carbon deposition may occur from
bottom.
• Different cooling rate suggests the different thickness of graphene.
• Hence the control of deposition is of more difficulties.
• Figure (next slide) shows the schematic drawing of graphene grown on Ni with
different cooling rate.
▪ Extreme fast cooling leads to little carbon precipitation, because not
sufficient time is allowed for carbon to precipitate.
▪ Medium cooling gives graphene, and
▪ Slow cooling has nothing on the surface in that carbon atoms diffuse
deep into the bulk catalyst.
Congqin Miao, Churan Zheng, Owen Liang and Ya-Hong Xie,University of California, Los Angeles,United States: Chemical Vapor
Deposition of Graphene
Illustration of carbon segregation at metal(Ni) surface
Congqin Miao, Churan Zheng, Owen Liang and Ya-Hong Xie,University of California, Los Angeles,United States: Chemical Vapor Deposition
of Graphene
Copper As Catalyst
• Copper is put into a furnace and heated under low vacuum to
around 1000°C. The heat anneals the copper, increasing its
domain size.
• The hydrogen catalyzes a reaction between methane and the
surface of the metal substrate, causing carbon atoms from the
methane to be deposited onto the surface of the metal through
chemical adsorption (see Figure).
• The furnace is quickly cooled to keep the deposited carbon layer
from aggregating into bulk graphite, which crystallizes into a
contiguous graphene layer on the surface of the metal.
Benjamin Pollard
Department of Physics, Pomona College : Growing Graphene via Chemical Vapor
Deposition
Diagram of CVD growth on copper.
Benjamin Pollard
Department of Physics, Pomona College : Growing Graphene via Chemical Vapor
Deposition
Graphene Growth by CVD – Low Temp
Zhancheng Li, Ping Wu, Chenxi Wang, Xiaodong Fan, Wenhua Zhang, Xiaofang Zhai, Changgan Zeng,*
Zhenyu Li,* Jinlong Yang, and Jianguo Hou, Low-Temperature Growth of Graphene by Chemical Vapor
Deposition Using Solid and Liquid Carbon Sources, ACS Nano, VOL. 5 ’ NO. 4 ’ 3385–3390 ’ 2011
Graphene Growth by CVD – Challenges
1. Controlling number of layers
2. Control of domain size
3. Uniformity
4. Transfer – causing defects
5. Effect of substrate texture
6. Understanding growth mechanism
Graphene growth: Comparison
Current
Status Solid Carbon : Low temp.
Nat.mat.2009.203. Ar1atm,1450~1650°C
Terrace size increase.
95
Motivation: Direct Growth on Dielectric Substrates: Toward Industrially
Practical, Scalable Graphene—Based Devices
Graphene Growth: Conventional Approaches
transfer
CVD graphene monolayer
Result: graphene
SiO2 monolayer, interfacial inhomogeneities
Metal or HOPG Si
Si evaporation Result: graphene
> 1500 K
monolayer or multilayer
SiC(0001) SiC(0001) on SiC(0001)
96
Direct Growth of Graphene on Dielectric Substrates: Summary
97
Generalization, Directly Grown Graphene and Charge Transfer: Oxides
(p-type) vs. Metals (n-type)
e- graphene EF
n-type; metal to
Transition metals graphene charge transfer
(Ru, Ni, Cu, Ir…)
EF
98