Carbon Nanostructures.pdf

MTN 530: Nanomaterials & Applications Carbon Nanostructures

Indranil Lahiri

Nanocarbon • • • • • • • • •

Fullerene Tubes Sheets Cones Carbon black Horns Rods Foams Nanodiamonds

Bonding

Graphite – sp2

Diamond – sp3

Formation of C Nanostructures 2D graphene sheet

bucky ball

CNT

3D graphite

Why do Carbon Nanotubes form? Carbon

Graphite (Ambient conditions) sp2 hybridization: planar Diamond (High temperature and pressure) sp3 hybridization: cubic

Nanotube/Fullerene (certain growth conditions) sp2 + sp3 character: cylindrical Finite size of graphene layer has dangling bonds. These dangling bonds correspond to high energy states. Eliminates dangling bonds Nanotube formation + Total Energy Increases Strain Energy decreases

Nanocarbon

Shenderova et al. Nanotechnology 12 (2001) 191.

Fullerene ”The most symmetrical large molecule” • Discovered in 1985 - Nobel prize Chemistry 1996, Curl, Kroto, and Smalley

• C60, also 70, 76 and 84. - 32 facets (12 pentagons and 20 hexagons) - prototype

Epcot center, Paris

~1 nm

Architect: R. Buckminster Fuller

Fullerene • Symmetric shape → lubricant

• Large surface area → catalyst

• High temperature (~500oC) • High pressure • Hollow → caging particles

• Ferromagnet? - polymerized C60 - up to 220oC

Fullerene • Chemically stable as graphite - most reactive at pentagons

• Crystal by weak van der Waals force • Superconductivity - K3C60: 19.2 K - RbCs2C60: 33 K

Kittel, Introduction to Solid State Physics, 7the ed. 1996.

Synthesis (discovery) of C60

The experimental set-up used to discover C60. The graphite disk is evaporated with a Nd:YAG laser and the evaporated carbon plasma is cooled by a stream of helium coming from a pulsed valve. The clusters of carbon are produced in the integration cup and are expanded into vacuum. The ions are detected by time of flight mass spectrometry

Synthesis and purification of fullerenes

Schematic illustration of the processes involved in the synthesis and purification of fullerenes. Graphite rods are evaporated in an arc, under He atmosphere. The soot collected is extracted with toluene and subjected to chromatography.

Carbon Nanotubes

Carbon Nanotube • Discovered 1991, Iijima

Roll-up vector:

  Ch = n a1 + m a2

Carbon Nanotube Electrical conductance depending on helicity

  Ch = n a1 + m a2

If

2n + m = i , then metallic 3

else semiconductor

• Current capacity 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: typical few μm

length  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 singlewalled carbon nanotubes

•The thinnest carbon nanotube is armchair (2,2) CNT with a diameter of 3 Å •The thinnest free standing single-walled carbon nanotube is about 4.3 Å in diameter. Researchers suggested that it can be either (5,1) or (4,2) SWCNT, but exact type of carbon nanotube remains questionable.

Carbon Nanotubes: Mechanical Properties Carbon nanotubes are the strongest ever known material. • Young Modulus (stiffness): Carbon nanotubes Carbon fibers High strength steel

1250 GPa 425 GPa (max.) 200 GPa

• Tensile strength (breaking strength) Carbon nanotubes

11- 63 GPa

Carbon fibers

3.5 - 6 GPa

High strength steel

~ 2 GPa

• Elongation to failure : ~ 20-30 %

• Density: Carbon nanotube (SW) 1.33 – 1.40 gram / cm3 Aluminium 2.7 gram / cm3

Carbon Nanotubes: Mechanical Properties ▪

Carbon nanotubes are very flexible

Nanoscience Research Group University of North Carolina (USA) http://www.physics.unc.edu/~rsuper/research/

Synthesis of Carbon Nanotube 1 Laser Ablation – Experimental Devices - graphite pellet containing the catalyst put in an inert gas filled quartz tube; -oven maintained at a temperature of 1,200 ◦C; -energy of the laser beam focused on the pellet; -vaporize and sublime the graphite Sketch of an early laser vaporization apparatus 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.

The synthesis yield is controlled by three parameters: the cooling rate of the medium where the active, secondary catalyst particles are formed, the residence time, and the temperature (in the 1,000– 2,100K range) at which SWNTs nucleate and grow. Fig. 3.10 Sketch of a synthesis reactor with a continuous CO2 laser device

3 Electric-Arc Method – Experimental Devices After the triggering of the arc between two electrodes, a plasma is formed consisting of the mixture of carbon vapor, the rare inert gas (helium or argon), and the vapors of catalysts. The vaporization is the consequence of the energy transfer from the arc to the anode made of graphite doped with catalysts.

Sketch of an electric arc reactor. It consists of a cylinder of about 30 cm in diameter and about 1m in height.

Electric-Arc Method – Experimental Devices

• In view of the numerous results obtained with this electric-arc technique, it appears clearly that both the nanotube morphology and the nanotube production efficiency strongly depend on – the experimental conditions and, in particular, – the nature of the catalysts.

4 Solar energy reactor 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

Precursor Gas

Substrate

C2H2, CH4, H2

✓ Easy process control ✓ Most popular process for CNT growth 24

Vapour Liquid Solid Method Basics about phase diagrams Alloys have phase diagrams

Lever rule: s tot g − g al = s g − gl

T liquid

liquidus

liquid and solid

al + a s = 1

solidus mixed crystal

A

gl gtot

gs

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

Eutectic

A + liquid

B+ liquid Mixed crystal

A solidus

B

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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: reactant vapour

metal

Liquid catalytic nanocluster

reactant vapour

metal +Sc

supersaturating

B

A reactant vapour

metal +Sc

Nanowire nucleation

reactant vapour

metal +Sc

Sc

Nanowire growth 27

Vapour Liquid Solid method Synthesis of multicomponent semiconductor, like binary III-V materials (GaAs, GaP, InAs, InP) ternary III-V materials (GaAs/P, InAs/P) binary II-VI materials ( ZnS, ZnSe, CdS, CdSe) binary Si Ge alloys

Pseudobinary phase diagram T liquid

E.g. Au - GaAs pseudobinary phase diagram

GaAs+ liquid

Au + liquid Au + GaAs

Au

GaAs

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Vapour Liquid Solid Method ➢In general, the nanowires grown by VLS - cylindrical morphology - without facets on the side surface

- uniform diameter

➢Growth rate is much faster (60 times for Si nanowire using a liquid Pt-Si alloy than directly on the silicon substrate at 900C) - liquid acts as a sink for the growth species

- act as a catalyst for the heterogeneous nucleation

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VLS – Diameter of Nanostructures

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VLS – Diameter of Nanostructures Critical Diameter- liquid catalyst clusters are stable in equilibrium

4 dc =  C   RTln  C 

= surface free energy = molar Volume R = gas constant T = absolute temperature C = concentration of semiconductor component in liquid alloy C

= equilibrium concentration

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

GaN nanowires grown in CVD reactor

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CVD: Principle and Mechanism

1.

Generation of active gaseous reactant species

CVD: Principle and Mechanism

2. Transport of the gaseous species into reaction chamber

CVD: Principle and Mechanism

3. Gaseous reactants undergo gas phase reactions forming intermediate species

CVD: Principle and Mechanism

4. Absorption of gaseous reactants onto heated substrate, heterogeneous reaction at gas–solid interface, produces deposit and by-product species

CVD: Principle and Mechanism

5. Absorption of gaseous reactants onto heated substrate, heterogeneous reaction at gas–solid interface, produces deposit and by-product species

CVD: Principle and Mechanism

6. Gaseous by-products removed from boundary layer through diffusion or convection

CVD: Principle and Mechanism

7. Unreacted gaseous precursors and by-products taken away from chamber

CNT Growth by CVD 1. Hydrocarbon vapor comes in contact with the “hot” metal nanoparticles, 2. Hydrocarbon decomposes into carbon and hydrogen species, 3. Hydrogen flies away and carbon gets dissolved into the metal, 4. After reaching the carbon-solubility limit in the metal at that temperature, asdissolved carbon precipitates out and crystallizes in the form of a cylindrical network having no dangling bonds and hence energetically stable. ❑ Hydrocarbon decomposition (being an exothermic process) releases some heat to the metal’s exposed zone, ❑ carbon crystallization (being an endothermic process) absorbs some heat from the

metal’s precipitation zone. ❑ This precise thermal gradient inside the metal particle keeps the process on.

CNT Growth by CVD ❑ Two basic growth mechanisms proposed – depending on the catalyst-substrate interaction ❑ Weak catalyst-substrate interaction – “Tip Growth” model ❑ Strong catalyst-substrate interaction – “Base or Root Growth” model

CNT Growth by CVD: Tip Growth

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. CxHy decomposition and C diffusion – as before, but CNT precipitation fails to push the metal particle up; so the precipitation is compelled to emerge out from the metal’s apex (farthest from the substrate, having minimum interaction with the substrate). First, carbon crystallizes out as a hemispherical dome (the most favorable closed-carbon network on a spherical nanoparticle) which then extends up in the form of seamless graphitic cylinder. 2. Subsequent hydrocarbon deposition takes place on the lower peripheral surface of the metal, and as-dissolved C diffuses upward. Thus CNT grows up with the catalyst particle rooted on its base.

CNT Growth by CVD: Parameters 1. Nature of hydrocarbon, 2. Type of catalyst, catalyst size 3. Temperature,

4. Pressure, 5. Gas-flow rate, 6. Deposition time,

7. Reactor geometry

Formation of SWCNT or MWCNT – dictated by CVD parameters

CNT Growth by CVD: Precursors 1. Methane, 2. Ethylene

3. Acetylene 4. Benzene 5. Carbon Monoxide

Linear hydrocarbons such as methane, ethylene, acetylene, thermally decompose into atomic carbons or linear dimers/trimers of carbon, and generally produce straight hollow CNTs. Cyclic hydrocarbons such as benzene, xylene, cyclohexane, fullerene, produce relatively curved/hunched CNTs with the tube walls often bridged inside

Formation of SWCNT or MWCNT – low-temperature CVD (600–900 C) yields MWCNTs, whereas high-temperature (900–1200 C) reaction favors SWCNT growth Formation of SWCNT or MWCNT – SWCNTs grow from selected hydrocarbons (viz. carbon monoxide, methane, etc.) which have a reasonable stability in the temperature range of 900–1200 C). Commonly efficient precursors of MWCNTs (viz. acetylene, benzene, etc.) are unstable at higher temperature and lead to the deposition of large amounts of carbonaceous compounds other than nanotubes

CNT Growth by CVD: Catalysts 1. Fe, 2. Co

3. Ni

(i) High solubility of carbon in these metals at high temperatures; (ii) high carbon diffusion rate in these metals. (iii) High melting point and low equilibrium-vapor pressure of these metals offer a wide temperature window of CVD for a wide range of carbon precursors. (iv) Fe, Co, and Ni have stronger adhesion with the growing CNTs (than other transition metals do)

Solid organometallocenes (ferrocene, cobaltocene, nickelocene) – widely used as CNT catalyst, as they liberate metal nanoparticles in-situ which catalyze the hydrocarbon decomposition more efficiently Formation of SWCNT/MWCNT – catalyst-particle size dictates the tube diameter

CNT Growth by CVD: Substrates 1. Graphite, 2. Quartz

3. Si 4. SiC 5. Silica

Catalyst–substrate reaction (chemical bond formation) – affects catalytic behavior of the metal. The substrate material, its surface morphology and textural properties greatly affect the yield and quality of the resulting CNTs.

6. Alumina 7. Alumino-silicate 8. CaCO3

9. MgO SWCNT growth on a Fe catalyst, as observed in ab initio simulations. (i) Diffusion of single C atoms (red spheres) on the surface of the catalyst. (ii) Formation of an sp2 graphene sheet floating on the catalyst surface with edge atoms covalently bonded to the metal. (iii) Root incorporation of diffusing single C atoms (or dimers).

CNT Growth by CVD: Challenges 1. Growing fixed diameter of CNTs, 2. Chirality control

3. Controlling number of walls in MWCNTs 4. Even a single CNT has different diameter and chirality!! 5. Lowering CVD temperature

6. Understanding growth mechanism 7. Improving crystallinity of CVD grown CNTs

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

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice The name ‘graphene’ comes from graphite + -ene = graphene

Molecular structure of graphene

High resolution transmission electron microscope images (TEM) of graphene

Introduction Properties of 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

Copper

59.6 × 106

Commonly used in electrical wire applications due to very good conductivity and price compared to silver.

Annealed Copper

58.0 × 106

Referred to as 100% IACS or International Annealed Copper Standard. The unit for expressing the conductivity of nonmagnetic materials by testing using the eddy-current method. Generally used for temper and alloy verification of aluminium.

Gold

45.2 × 106

Gold is commonly used in electrical contacts because it does not easily corrode.

Aluminium

37.8 × 106

Sea water

4.8

Commonly used for high voltage electricity distribution cables[citation needed] Corresponds to an average salinity of 35 g/kg at 20 °C.[1]

Drinking water

0.0005 to 0.05

This value range is typical of high quality drinking water and not an indicator of water quality

Deionized water

5.5 × 10-6

Conductivity is lowest with monoatomic gases present; changes to 1.2 × 10-4 upon complete de-gassing, or to 7.5 × 10-5 upon equilibration to the atmosphere due to dissolved CO2 [2]

Jet A-1 Kerosene n-hexane Air

50 to 450 × 10-12 100 × 10-12 0.3 to 0.8 × 10-14

[3]

Electronic properties Graphene’s band structure yields unusual properties

EF The velocity of an electron at the Fermi level (vF) Is inversely related to meff Effective mass (m*) ~ [dE2/dk2]-1

Most semiconductors, 0.1 m0 < m* < 1 me Graphene, m* < 0.01 m0 (depending on number of carriers) Therefore, expect VERY high mobility in graphene Both holes and electrons can be carriers

Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109 53

Thermal properties Material

Thermal conductivity W/(m·K)

Silica Aerogel

0.004 - 0.04

Air

0.025

Wood

0.04 - 0.4

Hollow Fill Fibre Insulation Polartherm

0.042

Alcohols and oils

0.1 - 0.21

Polypropylene

0.25 [6]

Mineral oil

0.138

Rubber

0.16

LPG

0.23 - 0.26

Cement, Portland

0.29

Epoxy (silica-filled)

0.30

Epoxy (unfilled)

0.59

Water (liquid)

0.6

Thermal grease

0.7 - 3

Thermal epoxy

1-7

Glass

1.1

Soil

1.5

Concrete, stone

1.7

Ice

2

Sandstone

2.4

Stainless steel

12.11 ~ 45.0

Lead

35.3

Aluminium

237 (pure) 120—180 (alloys)

Gold

318

Copper

401

Silver

429

Diamond

900 - 2320

Graphene

(4840±440) - (5300±480)

Properties of graphene Mechanical properties

- High Young’s modulus (~1,100 Gpa) High fracture strength (125 Gpa) A representation of a diamond tip with a two nanometer radius indenting into a single atomic sheet of graphene (Science, 321 (5887): 385)

- 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.

F. Bonaccorso et al. Nat. Photon. 4, 611 (2010)

Preparation and characterization graphene Preparation methods Top-down approach (From graphite) - Micromechanical exfoliation of graphite (Scotch tape or peel-off method) - Creation of colloidal suspensions from graphite oxide or graphite intercalation compounds (GICs)

Ref: Carbon, 4 8, 2 1 2 7 –2 1 5 0 ( 2 0 1 0 )

Bottom up approach (from carbon precursors) - By chemical vapour deposition (CVD) of hydrocarbon - By epitaxial growth on electrically insulating surfaces such as SiC

Characterization methods

Scanning Probe Microscopy (SPM):

Raman Spectroscopy

Transmission electron Microscopy (TEM)

X-ray diffraction (XRD)

- Atomic force microscopes (AFMs) - Scanning tunneling microscopy (STM)

Atomic force microscopy images of a graphite oxide film deposited by Langmuir-Blodgett assembly

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: A. OK: Graphene is great, lots of interesting properties for devices! B. How do you make a device?

A. You need a sheet of graphene! B. OK, how do you get a sheet of graphene? A. HOPG, scotch tape, and tweezers! B. !@#$%%

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Preparation methods Top-down approach (From graphite) Direct exfoliation of graphite

Graphite intercalation compound

Graphite oxide method

Direct exfoliation of graphite

Dispersions of microcrystalline synthetic graphite have a concentration of 0.03 mg mL-1. Dispersions of expanded graphite and HOPG are less concentrated (0.02 mg mL-1).

Direct exfoliation of graphite

Graphene sheets ionic-liquid-modified by electrochemistry using graphite electrodes. Liu, N. et al. One-step ionic-liquid-assisted electrochemical synthesis of ionicliquidfunctionalized graphene sheets directly from graphite. Adv. Funct. Mater. 18, 1518–1525 (2008).

Graphite intercalation compound

J. Mater. Chem. 2005, 15, 974.

Micromechanical cleavage SiO2 Si 3 nm

3 nm

2.5 nm 0.8 nm SiO2 1.2 nm

Fig. 1. Graphene films. (A) Photograph (in normal white light) of a large multilayer graphene flake on top of an oxidized Si wafer. (B) AFM image of 2 mm by 2 mm area of this flake near its edge. Colors: dark brown, SiO2 surface; orange, 3 nm height above the SiO2 surface. (C) AFM image of single-layer graphene.

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.

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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 Drawbacks : Limited quantity

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Graphite oxide ( Most common and high yield method) Oxidation (Hummers’method)

Graphite Oxide Graphite

H2SO4/ KMnO4 H2SO4/KClO3 Or H2SO4/HNO3 ……………….

H2O

Ultrasonication (exfoliation

Graphene Oxide monolayer or few layers

Fuctionalization (for better dispersion)

Chemical reduction to restore graphitic structures

Making composite with polymers

Graphite oxide method

Tung, V. C., Allen, M. J., Yang, Y. & Kaner, R. B. High-throughput solution processing of large-scale graphene. Nature Nanotech. 4, 25–29 (2008).

Graphite oxide method More intercalation for better exfoliation to monolayers

Graphite oxide

Graphite/Graphene oxide Hydroxyl group

Epoxy group Lerf-Klinowski model of graphene oxide

GO : interlayer spacing between 6 and 10 A ° depending on the oxygen content

GO : C:O ratio between 2.1-2.9

r-GO : C:O ratio around 11-12

Bottom up approach (from carbon precursors)

Epitaxial growth Production of EG on diced (3 mm by 4 mm) commercial SiC wafers. 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.

Advantage: Patterned graphene structure 75

Drawback: Ultra-high vacuum, high cost

Epitaxial growth - SiC decomposition

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)

NATURE, Vol , 458, 16 , April (2009)

Potential applications of graphene - Single molecule gas detection - Graphene transistors - Integrated circuits - Transparent conducting electrodes for the replacement of ITO - Ultracapacitors - Graphene biodevices - Reinforcement for polymer nanocomposites: Electrical, thermally conductive nanocomposites, antistatic coating, transparent conductive composites..ect

Graphene Growth by CVD

Chemical vapor deposition Chamber:12-in chamber quartz reactor Heater: graphite electrodes placed at the top and bottom with a separating distance of 10 cm IR detector: temp. measurement Substrate: A cupper foil was placed on the bottom heater Annealing: by H2 and Ar (increase Cu grains) Deposition: using CH4 and H2 as precursor Cooling: rapidly cooling under Ar 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 80 nucleation centers to form graphene film

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.

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(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. 82 (c) Image of a single tetragonal graphene domain. (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)

Fig. (a)-(c): Treated in relatively high pressure Fig. (d)-(f): Treated in lower pressure Fig. (g):OM of Fig.(a); (i):OM of Fig.(d),

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Processing parameters-hydrocarbon concentration

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 2500C – 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 is more sensitive to oxidation than Ni 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

1. Solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400 C. 2. Benzene as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300 C.

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 SiC sublimation

Metal catalysis CVD Ni: non uniform multi➔ Cu: uniform single ➔ Cu: layer by layer growth

Current Status

Solid Carbon : Low temp.

Nat.mat.2009.203. Ar1atm,1450~1650°C Terrace size increase. Nat.2010.549.

Pros& Cons

High temperature growth :1200~1500°C Non-uniform growth in Step edge and terrace. High cost SiC wafer : SiC growth on Si No transfer required

ACS nano,2011

Low temperature growth :below 1000°C Unform growth : Capet like (Large area) Si CMOS compatible process.

“Transfer required” 95

Motivation: Direct Growth on Dielectric Substrates: Toward Industrially Practical, Scalable Graphene—Based Devices Graphene Growth: Conventional Approaches CVD graphene monolayer

transfer SiO2

Metal or HOPG

SiC(0001)

Focus: Direct CVD, PVD or MBE On Dielectrics

Si

Si evaporation > 1500 K SiC(0001)

Charge-based devices

Result: graphene monolayer, interfacial inhomogeneities Result: graphene monolayer or multilayer on SiC(0001)

FET: Band gap n

Spintronics

graphene

MgO(111) Si(100) graphene

Coherent-Spin FET:

Top Gate

Co3O4(111) Multi-functional, nonvolatile devices 96

Direct Growth of Graphene on Dielectric Substrates: Summary

Substrate

Growth Temperature

Method

MgO(111)

~ 1000 K

L. Kong, et al. J. Phys. Chem. C. 114 (2010) 21618

Co3O4(111)

1000 K

Mica

~1000 K

Al2O3(0001)

1800 K

CVD, PVD Interfacial reaction, band gap MBE Incommensurate interface, Ferromagnetism1 MBE Oxidation at C(111) edge sites? CVD High temp. required for fewdefect films

BN(0001)

1000 K

CVD

C. Bjelkevig, et al., J. Phys.: Cond. Matt. 22 (2010) 302002

1

Remarks

Monolayer BN by ALD, strong BN→ graph charge transfer

References

M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 G. Lippert, et al. Phys. Stat. Sol. B. 248 (2011) 2619 M. Fanton,et al., Conf. Abstract (Graphene 2011, Bilbao, Spain)

Unpublished result

97

Generalization, Directly Grown Graphene and Charge Transfer: Oxides (p-type) vs. Metals (n-type) e-

graphene

Transition metals (Ru, Ni, Cu, Ir…)

e-

graphene Oxides, SiC

EF n-type; metal to graphene charge transfer

p-type; graphene to substrate charge transfer

EF 98