Growth of Low-Dimensional Carbon Nanomaterials
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Wright State University Wright State University CORE Scholar CORE Scholar Physics Seminars Physics 2-8-2013 Growth of Low-Dimensional Carbon Nanomaterials Growth of Low-Dimensional Carbon Nanomaterials John J. Boeckl Weijie Lu William Mitchel Howard Smith Larry Grazulis See next page for additional authors Follow this and additional works at: https://corescholar.libraries.wright.edu/physics_seminars Part of the Physics Commons Repository Citation Repository Citation Boeckl , J. J., Lu , W., Mitchel , W., Smith , H., Grazulis , L., Landis , G., Elhamri , S., & Eyink , K. G. (2013). Growth of Low-Dimensional Carbon Nanomaterials. . This Presentation is brought to you for free and open access by the Physics at CORE Scholar. It has been accepted for inclusion in Physics Seminars by an authorized administrator of CORE Scholar. For more information, please contact [email protected].
Transcript of Growth of Low-Dimensional Carbon Nanomaterials
Wright State University Wright State University
CORE Scholar CORE Scholar
Physics Seminars Physics
2-8-2013
Growth of Low-Dimensional Carbon Nanomaterials Growth of Low-Dimensional Carbon Nanomaterials
John J. Boeckl
Weijie Lu
William Mitchel
Howard Smith
Larry Grazulis
See next page for additional authors
Follow this and additional works at: https://corescholar.libraries.wright.edu/physics_seminars
Part of the Physics Commons
Repository Citation Repository Citation Boeckl , J. J., Lu , W., Mitchel , W., Smith , H., Grazulis , L., Landis , G., Elhamri , S., & Eyink , K. G. (2013). Growth of Low-Dimensional Carbon Nanomaterials. .
This Presentation is brought to you for free and open access by the Physics at CORE Scholar. It has been accepted for inclusion in Physics Seminars by an authorized administrator of CORE Scholar. For more information, please contact [email protected].
Authors Authors John J. Boeckl, Weijie Lu, William Mitchel, Howard Smith, Larry Grazulis, Gerry Landis, Said Elhamri, and Kurt G. Eyink
This presentation is available at CORE Scholar: https://corescholar.libraries.wright.edu/physics_seminars/1
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Growth of Low-Dimensional Carbon Nanomaterials
John Boeckl, Weijie Lu, William Mitchel, Howard Smith, Larry Grazulis, Gerry Landis, Said Elhamri, Kurt Eyink
Materials and Manufacturing DirectorateElectronic and Optical Materials Branch Wright Patterson AFB, OH 45433
Presenter
Presentation Notes
Let me first thank the organizers for inviting me to this beautiful venue and giving me this opportunity to talk to you Today I will present some new ideas on the role of Oxygen on the growth of low dimensional carbon nanomaterials with an emphasis on graphene.
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Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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AFRL Organization
AFRL Headquarters Commander and Staff
AFOSR Propulsion (RZ)
Directed Energy (RD)
Information (RI) 711 Human
Performance Wing
Human Effectiveness
(RH) Munitions
(RW) Sensors
(RY) Space Vehicles
(RV) Materials &
Manufacturing (RX) Air Vehicles
(RB)
Presenter
Presentation Notes
AFOSR: All AF 6.1 Basic Research Funding International Collaborations: EOARD & AOARD Grants & STTRs Mostly Unrestricted Tech Directorates: In-house Research & Funding Agent 6.2 – Applied Research, Exploratory Development 6.3 – Advanced Development 7.8 – Manufacturing Technology Contracts & SBIRs May include export controls, ITAR, security requirements
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Major AFRL Facilities WRIGHT-PATTERSON PROPULSION (RZ) AIR VEHICLES (RB) SENSORS (RY) MATERIALS AND MANUFACTURING (RX) COLLABORATIVE C2 (RI) 711th HUMAN PERFORMANCE WING (HPW)
– HUMAN EFFECTIVENESS (RH)
AFRL HQ
ROME INFORMATION (RI) SURVEILLANCE (RY)
HANSCOM BATTLE SPACE ENVIRONMENTS (RV) ELECTROMAGNETICS (RY)
BALLSTON AIR FORCE OFFICE OF SCIENTIFIC RESEARCH (AFOSR)
TYNDALL AIR BASE TECHNOLOGY (RX)
EGLIN MUNITIONS (RW)
BROOKS 711th HPW - BIOEFFECTS (RH) - AEROSPACE PHYSIOLOGY (RH) - USAFSAM
KIRTLAND DIRECTED ENERGY (RD) SPACE VEHICLES (RV)
MESA WARFIGHTER TRAINING RESEARCH (RH)
EDWARDS ROCKET PROPULSION (RZ)
Plus 40 Sites World-Wide
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Accomplishing the RX Mission
• Revenue • People • Facilities
& Equipment
LOCATIONS: • Wright-Patterson AFB • Tyndall AFB • Program Offices in GA, OK,
UT • Collocates at TDs, SPOs,
Centers
Presenter
Presentation Notes
The Air Force Research Laboratory's Materials and Manufacturing Directorate develops materials, processes, and advanced manufacturing technologies for aircraft, spacecraft, missiles, rockets, and ground-based systems and their structural, electronic and optical components. Air Force product centers, logistic centers, and operating commands rely on the directorate’s expertise in materials, nondestructive inspection, systems support, and advanced manufacturing methods to solve system, expeditionary deployment, and operational challenges.�
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RX Demographics PERSONNEL Civilian
Military
Military
705
84
114 Contractors also have PhDs
DEGREES & SPECIALTY AREAS 388 Civilian/Military Scientist & Engineers (S&E)
Materials Engineers
Chemists/Chemical Engineers
Research Physicists
Aero Engineers
Safety/Environmental Engineers
Civil/Industrial Engineers
Biologists/Microbiologists
Mechanical Engineers
Electrical Engineers
Contractors (S&E, Technician, Ops Support) On-Site 344 (MY Equivalent)
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Functional Materials Division
Photonic Materials Soft Matter Materials Nanoelectronic Materials
PRIMARY R&D FOCUS
PRIMARY APPLICATION OUTLETS
Survivability ISR platforms Unconventional sensing Integrated power/energyDirected Energy
CHALLENGES & OPPORTUNITIES Laser/HPM hardening material solutions to ensure access to denied environments Improved material systems for continuous, robust, and non-conventional ISR appl.
-EO/IR, RF, and human system Facilitate improved human system and warfighter / machine interface Energy dense, compact / lightweight, autonomous power / energy AF-unique system applications Materials and components for near-zero collateral damage tactical strike and airborne self-defense Functional materials & processing motifs as integrated variables in component design
-Non-traditional processing techniques enabling rapid, low-cost, custom devices
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Nanoscale Transport Group
Develop and transition nanoscale engineered transport of electrons & phonons for C4ISR and EW components Impact: Reduced energy consumption, improved capability, reliability and sustainability of C4ISR and EW systems
Material design for enhanced electron and phonon transport
2D-layers, nanowires, nanotubes, nanoparticles, etc.
Thin film processes with ICMSE optimization tools
source drain
gate Predictive modeling of material performance
Evaluation in device prototypes
Game changing materials and processes for above TRL3 maturation in C4ISR and EW
Requirements System Eng. Assessments
Presenter
Presentation Notes
C4ISR - Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance
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Carbon Materials Research Team
AFRL/RXPS: John Boeckl OxyGon/Electron Microscopy Bill Mitchel Transport Measurements Kurt Eyink MBE, SE Shanee Pacley Raman Dave Tomich X-ray Jeongho Park (Visiting Scientist) MBE, XPS Shin Mou (NRC) Transport/Device
AFRL/RXBT Ajit Roy Modeling Adam Waite Thermal Measurements
Contractors: University of Dayton Research Institute Larry Grazulis AFM, STM Said Elhamri Transport Measurements Gerry Landis Sample Preparation Howard Smith SIMS, XPS Wright State University John Hoelscher Raman UTC Weijie Lu Raman, XPS
Presenter
Presentation Notes
Spectroscopic ellipsometry
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Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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Low–D Carbon Basics
Scientific American298(4), 68-75 (2008)
Presenter
Presentation Notes
In this talk I will focus on carbon based nanomaterials. All low dimensional carbon nanostructures can be seen as an evolution of the 2 dimensional graphene sheet that consist of C-atoms arranged in a hexagonal lattice. If we can control this carbon evolution, we attempt to take advantage of unique properties of these various materials. I will initially focus on the latest carbon material to enter this arena, graphene.
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Potential Graphene/CNT Applications Motivation
Material P roperties
Ultra-high carrier mobility- µ ∼ 104 cm2/V·s High saturation velocity- v = 108 cm/s
High current carrying capacity – 1000x that of Cu Excellent thermal conductivity - ~5000 W/mK Extreme strength - Young’s modulus ∼ 1.0 TPa
Flexible – Elastic modulus ~ 0.25 TPa Ultra-thin geometry – Single atomic layer
Application areas • Digital electronics • RF Electronics • Flexible electronics • THz/IR sources and detectors • Spintronics at room temperature • Thermal management for electronics • Chemical sensors
Presenter
Presentation Notes
Many applications have been proposed in the literature to take advantage of the unique properties of graphene listed in the box in the upper right. Our current research on graphene is primarily focused on controlling the material quality to achieve these properties, but we our ultimately motivated to do this by the potential applicationsSo we are teaming with cross-division and cross directorate teams to investigate many of these application areas and characterize the properties. Some specifics of these include RF electronics with the Sensors Directorate and Thermal management with RXB. More general Air Force benefits include Size weight and power improvements and radar and sensing benefits for C4ISR.� higher power density, greater operating voltage, and higher operating temperature to enable superior performance and wider bandwidth capabilities.
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Graphene Fabrication
1. Exfoliation from HOPG on SiO2/Si wafers 2. CVD growth on substrates 3. Surface segregation on metal substrates 4. Epitaxial growth on SiC by Si sublimation * Our Research Focus
Epitaxial Growth 1300°C - 1700°C
6√3 X 6√3 Surface reconstruction Graphene is aligned with SiC crystal structure
Presenter
Presentation Notes
Several techniques have been used to fabricate graphene samples. Exfoliation of graphene from highly order pyrolytic graphite onto oxidized Si wafers is very common and produces graphene platelets up to a hundred micrometers across. Chemical vapor deposition of carbon on metal substrates has also been reported, as well as controlled surface segregation of carbon from saturated metal substrates. However, the focus of this project is epitaxial growth of graphene on SiC by sublimation of Si. This takes place at temperatures between 1300 and 1600°C for a few minutes. The SiC surface needs to have the correct surface reconstruction as will be discussed later. This process produces graphene domains align with the substrate crystal orientation.
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Exfoliation – “The Tape Method”
Science22, Vol.306. pp.666-669 •Method is ideal for physics •Non-scalable & Cost prohibitive •Graphene discovered via exfoliation from graphite •Simplest method for graphene research •Generally creates small sheets (<100um2) Bulk GraphiteGraphene Synthesis: Exfoliation
2010 Nobel Physics
Laureates
• Discovered via exfoliation from graphite (circa 2004 – University of Manchester, UK) • Simple method for research • Generally creates small sheets on the order of hundreds of square microns
100 μm flake ~ $1000
PRL 97, 187401 (2006)
Presenter
Presentation Notes
Keys to this method 1. Discovered existence Optically see one layer Simple method Thickness confoirmable by RAMAN spectroscopy
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Graphene impact on nano-Science
• Graphene a new era of “Conductor Electronics” • Band-gap created by interfaces, leading to possible
new techniques toward bandgap engineering (substrate effects, nano-nano effects etc.)
• From 3D to 2D, new sciences and new knowledge are being created. 2D is used as a new approach in materials research even beyond carbon materials.
• Graphene has “multi-scale” structural integration from atomic scale to microns and beyond, which is an ideal nano-block materials for new 3D nano structures.
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Challenges to Nanostructure Development
• The bridge between atomic design (chemistry synthesis, <1nm) and crystal growth (up to Inches) is not established
• The general methods and principles for growth from atomic structures (chemistry) to nanostructures (1 -100 nm) is not available. (only nanotechnology)
• Controllable assembly is not achieved, “materials by design” and “composites by design” have not been established
– Examples: chemical synthesis of graphene, controlled CNTs growth, high G dendrimer polymers, etc. are not yet fully successful
Chemical synthesis (< 1 nm)
? nano-structures (1-100 nm)
? Crystal growth (> microns)
Presenter
Presentation Notes
One of the general challenges to achieving the developments for nanomaterials shown on the previous slide lis bridging these materials across multiple structural scales . I liken this time to early transistor development in the 50’s where niche applications were being developed. But not until integrated circuit fabrication took place did it become a revolutionary science. Now that technology is approaching its effective limit as the scales continue to reduce. What is the next paradigm shift that will take nanomaterials into the future. What we have now is nanotechnology what we need is nanoscience. Methods are being investigated, but too date none have been fully suuccessful Nanostructures can be predictably produced, Taking nanostructures across the bridge to macro scales where they can be easily handled and manipulated.
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Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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•Commercial grade SiC wafer • 6H and 4H-SiC, Si-face (0001)
•Standard chemical clean •Anneal in various chambers
• Temperature: RT - 2100°C (TC,Pyrometer) • Pressure: Ar-ATM to 10-10 Torr
Graphene Preparation
2 nm
Presenter
Presentation Notes
For anyone not familiar with the procedure the basic steps in the epitaxial graphene growth include a commercially available SiC wafer (semiconductor grade 4” wafers are easily obtained). I will focus here on only the of Si-face SiC .The wafer receives a surface treatment (in our vacuum furnace growth a standard chemical clean). Then the sample is loaded into the chamber heated to the desired decompostion temperature, held there for a growth increment, and then cooled back to room temp. This all looks like a very simple growth process. But depending on certain variables we found the resulting material varied greatly. Pieces from the samples were cleaned with a standard RCA clean and then put in a resistively heated Oxy-Gon furnace and ramped to a temperature of 1700 C and held at this temperature for times ranging from 30 minutes to 12 hours. RCA was developed by Werner Kern 1965 while working at RCA Furnace has graphite resistance elements but another furnace on base with Tungsten heating elements was also effective for forming tubes.
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Growth of Graphene/SiC
Growth in UHV Growth in Ar ATM Growth Conditions @ 1200-1500˚C in Vacuum @1600-1700˚C in Ar ATM
1-30 min duration
2 nm
2D
SiC bilayer: 0.5nm
Presenter
Presentation Notes
On SiC there are two similar but separate growth regimes, one in a vacuum environment be it UHV or HV and the other in atmospheric pressure Argon. Here I show two AFM images to give example of the resulting surface morphology. From these growth modes. Note the presence of wrinkles in the vacuum mode of growth. Typical growth conditions are 1200-1500In UHV 1600-17000 in atmospheric pressure with runtimes of up to 30 minutes. In the bottom left AFM scan I identify the step height on the SiC suface, of ~ .5nm that correspons to the surface steps on a nominally on-axis Si-face SiC sample.. The graphene uniformely covers these steps and formsa a continuous film over the SiC sample. Other charcaterization tools are highlighted here including cross sectional TEM which yields conclusive evidence for number of layers, RAMAN spectroscopy and we perform transport measurements as well.
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Current Growth Understanding Mechanism based on surface morphology
Hupalo, et al Phy. Rev. B 80, 041401R (2009) Bolen et al Phys. Rev.B 80, 115433 (2009)
Mechanism based on conversion of SiC lattice
Kusunoki et al, Phys. Rev.B 81, 161410R 2010, and Chem. Phys. Lett. 468 (2009) 52–56
The formation of one graphene layer requires about three SiC bilayers, and a transitional interface is formed when one or two SiC bilayers are decomposed. All assuming SiC → Si (g) + C (s)
Presenter
Presentation Notes
The current models available for growth on SiC consist of the surface morphology model on the top of this slide propised by researchers at both Georgia tech and Purdue University. In this model, growth initiates at step edges , the lines in theses AFM images, The growth proceeds onto the step in a finger-like fashion. The second model proposed by Kusunoki from the Fine Ceramics Center in Japan, suggests that three SiC layers contribute enough C atoms to produce one graphene layer before complete layer formation an interface transition layer Each of these models assume the chemical reaction that takes place is SiC decomposing as Si leaves the surface
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Quality vs Growth Conditions
• The quality of graphene grown in Ar is better than in vacuum
• The quality of graphene grown in high vacuum 10-4-5 torr is better than in ultra-high vacuum (<10-8 torr)
High purity Ar contains 0.1-1 ppm oxygen, about
10 - 3-4 torr oxygen in 1 atm Ar
The available models cannot explain why
oxygen helps the growth of graphene?
Presenter
Presentation Notes
We observe a trend in the quality of the graphene in both the morphology and electronic quality that indicates ipmrovments as the vacuum becomes poorer, In addition the quality is further improved within the Argon atmospheric pressure growth regime. I will show that this can be an effect of increased oxygen present in the chamber leading to higher quality material. Observe that the residual oxygen pressure increases as the quality of the graphene improves and can be even higher when Argon is flowing. However there is no current model available that can explain oxygen ‘s role in this .
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Oxygen Impact on Growth
1450ºC in UHV chamber for 30min (oxygen was not detected)
O2<10-14 torr
1450°C in UHV chamber for 30 min (with oxygen leak valve)
O2 @ 8 x10-11 torr
Presenter
Presentation Notes
As some direct evidence of the role of oxygen on graphene growth we performed a study in ourUHV growth chamber in which an initial run was made in UHV conditions at 1450 C for 30 minutes. We then repeated the growth with an oxygen leak valve at 10-8 Torr. Results show the oxygen improved the growth yielding a smoother surface with less pitting indicating more uniform growth with less site specific initiation o the growth. In UHV SIC – Si +C dominates whereas under lower vacuum chemical reaction dominates.
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Oxygen Effect on SiC surface
Annealed at 1400°C, at 10-8 torr (UHV) for 30min
Preheated at 700°C in 0xygen at 10-3 Torr for 5 min, cooled and then heated to 1400 C for 30 mins at 10-8torr (UHV)
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Surface Chemistry w/Oxygen
SiC (s) + ½ O2 (g) → SiO (g) + C (s) Eq. 1 ½ SiC (s) + ½ CO (g) → ½ SiO (g) + C (s) Eq. 2 SiC (s) + H2O (g) → SiO (g) + C (s) + H2 (g) Eq. 3 SiC (s) + CO (g) → SiO (g) + CO (g) + C (s) 2 Eq. 4 SiC (s) → Si (g) + C (s) Eq. 5
• Preferential sublimation of Si from SiC surface and subsequent conversion of free carbon to graphene or CNTs
• Structure of the interface? Nature of chemical bonding? • Transition from SiC to graphene • Mechanism of the growth process?
Presenter
Presentation Notes
Here we establish the basis of an alternate model based on these five equations to demonstrate the surface chemistry on the SiC surface during growth conditions. The basis is again the preferential sublimation of Si from the SiC and conversion of free carbon into low dimensional carbon but with the important addition of oxygen containing reactions. Now we ask ourselves what model can we propose taking into account the structure of the growth interface the nature of the chemical bonding there and the basic mechanism for the growth process.
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Mechanism Based on Interface Chemical Reactions
Graphene growth process on SiC via a defective surface layer
• An interface transition layer a few nm thick is formed above 1100˚C
• The structures of this interface layer is unknown
• The interface layer decomposes at a lower temperature than SiC
“Surface compositional analysis of graphene/SiC (0001)”, R. L. Barbosa, et.al., submitted to ICSCRM 2011
SiC (s) + O2 (g) → SiCxOy (s) → SiO (g) + C (s)
Presenter
Presentation Notes
SO we propose a growth model with defective surface layer that forma around 1100C . The structure of this surface layer is unknown. But though to contain oxygen. What evidence do we have to support such a model?
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Partially Disordered Interface
1
1100˚C To form an interface layer
2
The layer decomposes at 1400˚C
2 nm
3
1600˚C
Atomic Growth Model 1. A carbon rich layer on the SiC surface is formed at high temperature 2. The interaction of the residual oxygen with carbon forms thermally
stable oxygen-carbon embedded structures. 3. Depending on environment such structures lead to curvature of
hexagonal structures resulting in the formation of CNT nanocaps or planar graphene
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Interface transition layer before/after graphene formation
Presenter
Presentation Notes
As evidence of the role of Oxygen in our process here I show some cross sectional transmission electron micrographs. In each there was a Pt protection layer applied to the surface as an initial stage of the sample preparation to ensure surface integrity. In our growth model, as the temperature is ramped up, an initial carbon rich surface forms as Si is decomposed from the SiC which reacts with residual oxygen on the background and forms stable O-C compunds. These compounds then act as catalysts to carbon as it continues to be provided by the substrate decomposition. Depending on the growth environment these can form vertical CNTs or planar graphene. At higher temperatures the O-C compounds destabilize and the interface region disappears.
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X-Ray Photoelectron Spectroscopy
SiC
C-C sp2
C-C sp3
C-O
(a) (c)
C-1s
C-O
C=O *O-C
O-1s
SiC
SiOxCy
(b)
Si-2p
Sample Composition • SiC • Graphene ( C-C sp2) • SiOxCy
• C 1s sp3
• Si 2p SiOxCy • O 1s C-O
•*O-C (Bulk component)
Presenter
Presentation Notes
Deconvoluted C 1s (a), Si 2p (b) and O 1s (c) spectra of the graphene/SiC sample. Here we look at deconvoluted XPS peaks for the C-1s Si-2p and O-1s for bond species in the sample. If we look at the SiC first. Then graphene which would yieldsp2 bonded carbon then add some Si oxy carbide group fo rthe interface transition region
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Role of Oxygen in SiC Decomposition
Oxygen in the range of 2 to 20 torr accelerates the graphitization process. Ar and N2 have no effect on graphitization. Oxygen atoms may remove the amorphous carbon atoms by chemical reaction. “Effect of gas phase on graphitization of carbon”, T. Noda, and M. Inagaki, Carbon, 2, 127 (1964).
The Key Issue to produce carbon on SiC The reaction must be controlled in active oxidation, not forming SiO2
Y. Song et al, Appl. Phys. Lett., 81, 3061 (2002).
Region 1: Active oxidation, Si(g)/SiO(g)
SiC(s) + ½ O2(g) = SiO(g) + C(s) SiC(s) = Si(g) + C(s)
Region 2: Active oxidation, SiO(g)
SiC(s) + O2(g) = SiO(g) + CO(g) Region 3: Passive oxidation, high O2, SiO2 (s)
SiC(s) + 3/2 O2(g) = SiO2(s) + CO(g)
Presenter
Presentation Notes
SO what is the role of oxygen in the graphitization process> First we note that in graphitization science it is long been establiished that oxygen and not other common gases acts as potential graphitization catalyst as shown in the reference from 1964. Here we drawon the earlier results, from Song and Smith , in which they explained several regions in the oxidation process of SiC and the role of Oxygen content in that process. In region 1 ……… in region 2, …….. And in region 3, ………… We proposed that a similar effect is responsible in the production of carbon on the SiC surface. What we found through various growth experiments, in different chambers, under different pressures and therefore different residual oxygen pressure the fration of carbon required us to bein the active oxidation regions1 or 2. Oxygen as a gas phase graphitization catalyst Oxygen in the range of 2 to 20 torr accelerates the graphitization process. Ar and N2 have no effect on graphitization. Oxygen atoms may remove the amorphous carbon atoms by chemical reaction. “Effect of gas phase on graphitization of carbon”, T. Noda, and M. Inagaki, Carbon, 2, 127 (1964).
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Growth by Interface Decomposition
~1100˚C To form an interface layer 2D peak
“Graphene growth on oxidized SiC surface”, W. Lu, J. J. Boeckl, W. C. Mitchel, T. R. Crenshaw, W. E. Collins, R. P. H. Chang, L. C. Feldman, J. Electronic Mat. 38 (6), 731 (2009)
“Transition layers in SiO2/SiC interface”, T. Zheleva et al., APL, 93, 022108 (2008).
Presenter
Presentation Notes
As further evidence of this process, the top RAMAN spectra shows the graphene 2D peak, centered around 2700 inverse cm, and shifting to higher wavenumber for greater number of graphene layers. In somewhat parallel work on the on the oxidation process of the SiC surface Zheleva and co-workers also show an interface region.
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Thermally stable O-C compounds
Goal to understand the thermal stable O-C compounds by TEM, TPD, and modeling
Thermal desorption spectra of CO (a) and CO2 (b) after oxygen implantation for various carbon materials. (EK98: pure graphite, and USB15: 15%B in C),
.
Temperature region for CNT and graphene growth
A. Refke, , et al. J. Nuclear Materials, 212-215, 1255 (1994)
Presenter
Presentation Notes
TPD = temperature programed desorption
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Graphene Growth Conclusions
• Graphene is formed above ~1400°C • Oxygen may play a role in the initial growth
of graphene on SiC • XPS shows oxy-carbides, sp3 and sp2 carbon
structures • Proposed growth model with oxygen SiC (s) + O2 (g) → SiCxOy (s) → SiO (g) + C (s)
• The controlling steps: SiC SiO C SiO + C C sp3 C sp2
x y
Presenter
Presentation Notes
The controlling steps in the growth process are SiC converting to stable Oxy-carbide compounds which then decmpose into volatile components removing the Si leaving behind sp3 bonded carbon that then reverts to the more favorable sp2 carbon.
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Observations Explained by Model Literature evidence for large variation of growth temperature (~ 300°C)
Reference H-etch T(√3 x √3) (°C) T(6√3 x 6√3) (°C) Graphene growth (°C)
Luxmi et al. [54] Yes NR 1150 >1285 Starke and Riedl [15] Yes 950 1100 1200 Jernigan et al. [55] Yes NR 1400 1500 Van Bommel et al.[56] No NR 1000 NR Rollings et al.[20] No 1100 1250 NR Riedl et al.[16] Yes 1000 1250 1350 Huang et al.[19] No NR 1100 1200 Charrier et al.[15] No NR <1080 1200 Seyller et al[21] No 1050 1150 1400 Aoki et al [57] No 1100 1250 1550 Berger et al.[58] Yes 1100 1250 1400 Park et al.[59] No 1050 1200 >1200 Chen et al. [17] No NR 1100 >1100 Johansson et al.[60] No 900 1000 NR Hannon et al.[61] No 1050 1060 1160 Forbeaux et al. [13] No 1050 1150 1350
“A critical review of growth of low dimensional carbon nanostructures on SiC (0001): Impact of growth environment”, W. Lu, J. Boeckl, and W. C. Mitchel, J. of Physics D: Applied Physics, 43, 374004 (2010)
Presenter
Presentation Notes
Variations in growth conditions —explained by variations in oxygen content in different growth chambers
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WSU Seminar – Feb. 2013
Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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Low-D Carbon Nanostructures on SiC
Growth of graphene/SiC • Growth at 1200-1500˚C in UHV, or at 1600-1800˚C in Ar for 1-30 min
Growth of SWNTs/SiC structure • Growth in high vacuum at 10-6 to -7 torr ~1400˚C
Growth of MWNTs/SiC structure • Growth in high vacuum at 10-3 to -5 torr at 1400-1700˚C up to 2 um long
Standard growth conditions and results Annealing from 1400-1700°C
Vacuum from 10-2 to 10-10 torr Products: SWNTs, MWNTs and graphene.
No amorphous carbon and no Si in the products Metal-free and well aligned structures
An ideal space for studying mechanism/ kinetics of Low -D materials
Presenter
Presentation Notes
To setup this 3D structure let me talk a bit about growth of other low D carbon materials. I’ve already gone into some great deal about grpahene. Which I showed we can grow under two conditions. This growth actually evolved out of our growth of vertical carbon structures on the SiC surface during high temperature anneals in a vacuum furnace.
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Growth Parameter: Pressure
• UHV- (10-8 to 10-10 Torr) Graphene • LEEM, LEED, XPEEM, MBE studies • Graphene formation
• HV- (10-5 to 10-7 Torr) Low CNT growth rate • Turbo pump on
• LV- (10-2 to 10-4 Torr) High CNT growth rate
• LLV- (<10-2 Torr) No growth, burning?
• ATM – (Argon background) graphene
Vacu
um p
ress
ure
UHV
ATM
Presenter
Presentation Notes
Listed on the chart are the critical part of our model for CNT catalization. The low oxygen pressure prevents the formation of solid SiO2 films on the surface and encourages the graphitization of the free carbon, which can then grow into CNT’s.
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Oxygen as a Graphitization Catalyst
• Oxygen as a gas phase graphitization catalyst Oxygen in the range of 2 to 20 torr accelerates the graphitization process. Ar and N2 have no effect on graphitization. Oxygen atoms may remove the amorphous carbon atoms by chemical reaction. “Effect of gas phase on graphitization of carbon”, T. Noda, and M. Inagaki, Carbon, 2, 127 (1964).
• Metal catalytic CNT synthesis involving oxygen 1. Hata, Iijima, et al. reported that adding water vapor in the feeding gases
significantly increases the efficiency of SWNT growth. Science, 306, 1362 (2004). 2. Rümmeli et al. have suggested a mechanism of oxygen assisted SWNTs metal
catalytic growth, i.e., nucleation via oxygen etching carbon shells. Nano-Lett. 5, 1209 (2005).
3. Dai et al suggested that oxygen controls the C/H ratio and provides a C-rich condition in favor of CNT formation, PNAS, 102, 16141 (2005).
• Oxides as catalysts in CNTs synthesis Liu et al., Importance of Oxygen in the Metal-Free Catalytic Growth of Single-Walled Carbon Nanotubes from SiOx by a Vapor-Solid-Solid Mechanism”, JACS, 2011
Presenter
Presentation Notes
I’ve already noted the top reference here pointing at oxygen as a graphitization catalyst while talking about our graphene growth. The center references are form other research groups around the world showing additional evidence for the role of oxygen in the growth of CNTs. And finally the last reference shows a group in china that are able to grow CNTs directly from oxides with no metal catalyst.
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Metal-free SWNTs on SiC XTEM RAMAN
RBM
• Si-face SiC, at 1400°C, 10-6-7 Torr • Radial breathing mode centered at 191 cm-1
• Diameter of SWNT is 1.3 nm Other diameters demonstrated by: M. Kusunoki et al. Appl. Phys. Lett. 71, 2620 (1997) M. Kusunoki et al. Jpn. J. Appl. Phys. (part 2) 37, L605 (1998)
Presenter
Presentation Notes
Growth rate 3x on the C-Face compared to the Si Face.
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Metal-free MWNTs on SiC
50 nm
C-face 1400oC 10-5 torr. 50 nm
C-face 1600oC 10-5 torr. C-face 1700oC 10-5 torr
SEM Plan View
100nm
• Metal-free CNTs • Well-aligned on SiC • Low structural defects
Oxygen is at 10-8 torr when the total pressure is at 10-5 torr
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Carbon on SiC: Summary
• Complex growth process
• Difficult environment for in-situ tools • Use modeling to assist in understanding
• We predict oxygen plays critical role in controlling growth
• Need to know residual gas levels in system • Can be used for preferential structure growth • Evaluating model to study molecular oxygen adsorption
• Graphene • Layer control reasonable • Electronic quality improvements still needed
• Structurally attached to the SiC substrate • CNT’s
• Vertically aligned to SiC substrate • Non-metal catalyst method
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Pillared Carbon Architecture
• Bottom-up approach: 3-D Pillared Architecture • Thermal transport limited by CNT-graphene junction • Variations by changing CNTs density and distance between graphene layers • Growth based on theory of oxygen atomic catalysis for Low-D carbon
“Modeling of Thermal Transport in Pillared Graphene Architectures”, V. Varshney, S. S. Patnaik, A. K. Roy, G. Froudakis, and B. L. Farmer, ACS Nano, 4 (2), 1153, (2010).
Presenter
Presentation Notes
Now switching to a new topic and coming from it at the exact reverse process, In theoretical work by Ajit Roy from RXB the use of three dimensional pillared architectures is shown to be a superior to either solo CNTs or graphene since they each exhibit significant anisotropic thermal conduction behaviors. Their results show that in the Pillared structures, the thermal transport is governed by the minimum interpillar distance and the CNT−pillar length. This is primarily attributed to scattering of phonons occurring at the CNT−graphene junctions in these nanostructures. They foresee that such architecture could potentially be used as a template for designing future structurally stable microscale systems with tailorable in-plane and out-of-plane thermal transport. Furthermore as a composite material the requirement of directly coupling to the carbon material has been shown to be important. The need for a functionalized connection to the carbon structure will be essential for polymer based applications.
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Pillared Carbon Structures on SiC
• Pillared graphene-CNTs on SiC can be produced
• Great for modeling • Excellent structures
for investigating thermal nano-science
Goal To control the growth of low-
dimensional carbon nanostructures enabling
application specific materials to be produced.
Demonstration
CNTs
100 nm SiC
Carbon nanostructures
Vertical and horizontal
2 nm 5nm
Presenter
Presentation Notes
From this theoretical concept, we have demonstrated the ability to form these carbon nanostructures using SiC decomposition Here we show a proof of concept that we can form both these vertical and horizontal structures on the same sample. Our approach is a bottom up growh that will enable complex heterostructures to be formed during the growth procedure. By varying growth parameters such as temperature, pressure, and ambient environment we can produce three dimensional structures that coexist in any number of geometries. In addition, by use of lithography we can also control the growth of CNts vs. Graphene Our goal is to be able to control the growth to allow application specific material to be formed.
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Preliminary Conclusions
Approach • Evaluate the atomic structures
by TEM, SEM, Raman, and IR • Understand the structures of
CNTs-graphene junctions and correlate with phonon properties
Result • The first group to grow Pillared graphene
structures at the atomic scale • A new high performance thermal material
with promising applications in high power electronics
70 nm
350 µm
100 µm
Heated from bottom
IR microscopy (x12) 120°C on the substrate
(unpublished data)
Pillars of CNT/graphene
stacks
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Outline
• AFRL Overview • Low Dimensional carbon preview • Atomic scale growth of Low-D carbon/SiC • Growth of 3-D carbon nanostructures for
potential thermal applications • Summary
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Conclusions
• Focus on chemistry and atomic catalysis for growth from atomic to nano scale and beyond
• A preliminary atomic oxygen catalytic growth mechanism for low-D carbon nano-structure growth
• Experimental data has shown that the proposed mechanism could be applied to growth of pillared graphene architectures
Chemical synthesis (< 1 nm)
Atomic growth mechanism for Low-D carbon nano-
structures (1-100 nm)
Structure vs
Property Crystal growth
(> microns)
