Editing Chemical vapor deposition (section)

=== Graphene ===
Many variations of CVD can be utilized to synthesize graphene. Although many advancements have been made, the processes listed below are not commercially viable yet.
* Carbon source

The most popular carbon source that is used to produce graphene is methane gas. One of the less popular choices is petroleum asphalt, notable for being inexpensive but more difficult to work with.<ref name=":0">{{Cite journal|title = Synthesis of three-dimensional graphene from petroleum asphalt by chemical vapor deposition|journal = Materials Letters|date = 2014-05-01|pages = 285–288|volume = 122|doi = 10.1016/j.matlet.2014.02.077|first1 = Zhuchen|last1 = Liu|first2 = Zhiqiang|last2 = Tu|first3 = Yongfeng|last3 = Li|first4 = Fan|last4 = Yang|first5 = Shuang|last5 = Han|first6 = Wang|last6 = Yang|first7 = Liqiang|last7 = Zhang|first8 = Gang|last8 = Wang|first9 = Chunming|last9 = Xu| bibcode=2014MatL..122..285L }}</ref>

Although methane is the most popular carbon source, hydrogen is required during the preparation process to promote carbon deposition on the substrate. If the flow ratio of methane and hydrogen are not appropriate, it will cause undesirable results. During the growth of graphene, the role of methane is to provide a carbon source, the role of hydrogen is to provide H atoms to corrode amorphous C,<ref>{{Cite journal|last1=Park|first1=Hye Jin|last2=Meyer|first2=Jannik|last3=Roth|first3=Siegmar|last4=Skákalová|first4=Viera|date=Spring 2010|title=Growth and properties of few-layer graphene prepared by chemical vapor deposition|journal=Carbon|volume=48|issue=4|pages=1088–1094|doi=10.1016/j.carbon.2009.11.030|issn=0008-6223|arxiv=0910.5841|bibcode=2010Carbo..48.1088P |s2cid=15891662}}</ref> and improve the quality of graphene. But excessive H atoms can also corrode graphene.<ref>{{Cite journal|last1=Wei|first1=Dacheng|last2=Lu|first2=Yunhao|last3=Han|first3=Cheng|last4=Niu|first4=Tianchao|last5=Chen|first5=Wei|last6=Wee|first6=Andrew Thye Shen|date=2013-10-31|title=Critical Crystal Growth of Graphene on Dielectric Substrates at Low Temperature for Electronic Devices|journal=Angewandte Chemie|volume=125|issue=52|pages=14371–14376|doi=10.1002/ange.201306086|pmid=24173776|bibcode=2013AngCh.12514371W|issn=0044-8249}}</ref> As a result, the integrity of the crystal lattice is destroyed, and the quality of graphene is deteriorated.<ref>{{Cite journal|last1=Chen|first1=Jianyi|last2=Guo|first2=Yunlong|last3=Wen|first3=Yugeng|last4=Huang|first4=Liping|last5=Xue|first5=Yunzhou|last6=Geng|first6=Dechao|last7=Wu|first7=Bin|last8=Luo|first8=Birong|last9=Yu|first9=Gui|date=2013-02-14|title=Graphene: Two-Stage Metal-Catalyst-Free Growth of High-Quality Polycrystalline Graphene Films on Silicon Nitride Substrates (Adv. Mater. 7/2013)|journal=Advanced Materials|volume=25|issue=7|pages=992–997|doi=10.1002/adma.201370040|bibcode=2013AdM....25..938C |issn=0935-9648|doi-access=free}}</ref> Therefore, by optimizing the flow rate of methane and hydrogen gases in the growth process, the quality of graphene can be improved.
* Use of catalyst
The use of catalyst is viable in changing the physical process of graphene production. Notable examples include iron nanoparticles, nickel foam, and gallium vapor. These catalysts can either be used in situ during graphene buildup,<ref name=":0" /><ref name=":1">{{Cite journal|title = Novel synthesis route to graphene using iron nanoparticles|journal = Journal of Materials Research|date = 2014|pages = 1522–1527|volume = 29|issue = 14|doi = 10.1557/jmr.2014.165|first1 = Rajen B.|last1 = Patel|first2 = Chi|last2 = Yu|first3 = Tsengming|last3 = Chou|first4 = Zafar|last4 = Iqbal|bibcode = 2014JMatR..29.1522P| s2cid=137786071 }}</ref> or situated at some distance away at the deposition area.<ref name=":2">{{Cite journal|title = Direct synthesis of large area graphene on insulating substrate by gallium vapor-assisted chemical vapor deposition|journal = Applied Physics Letters|date = 2015|pages = 093112|volume = 106|issue = 9|doi = 10.1063/1.4914114|first1 = Katsuhisa|last1 = Murakami|first2 = Shunsuke|last2 = Tanaka|first3 = Ayaka|last3 = Hirukawa|first4 = Takaki|last4 = Hiyama|first5 = Tomoya|last5 = Kuwajima|first6 = Emi|last6 = Kano|first7 = Masaki|last7 = Takeguchi|first8 = Jun-ichi|last8 = Fujita|bibcode = 2015ApPhL.106i3112M}}</ref> Some catalysts require another step to remove them from the sample material.<ref name=":1" />

The direct growth of high-quality, large single-crystalline domains of graphene on a dielectric substrate is of vital importance for applications in electronics and optoelectronics. Combining the advantages of both catalytic CVD and the ultra-flat dielectric substrate, gaseous catalyst-assisted CVD<ref>{{cite journal |title=Silane-catalysed fast growth of large single-crystalline graphene on hexagonal boron nitride |journal=Nature Communications  |volume=6  |year=2015|page=6499|doi=10.1038/ncomms7499 |pmid=25757864  |pmc=4382696  |last1=Tang |first1=Shujie |last2=Wang |first2=Haomin|last3=Wang |first3=Huishan |arxiv=1503.02806|bibcode=2015NatCo...6.6499T}}</ref> paves the way for synthesizing high-quality graphene for device applications while avoiding the transfer process.
* Physical conditions
Physical conditions such as surrounding pressure, temperature, carrier gas, and chamber material play a big role in production of graphene.

Most systems use LPCVD with pressures ranging from 1 to 1500 Pa.<ref name=":0" /><ref name=":2" /> However, some still use APCVD.<ref name=":1" /> Low pressures are used more commonly as they help prevent unwanted reactions and produce more uniform thickness of deposition on the substrate.

On the other hand, temperatures used range from 800 to 1050&nbsp;°C.<ref name=":0" /><ref name=":1" /> High temperatures translate to an increase of the rate of reaction. Caution has to be exercised as high temperatures do pose higher danger levels in addition to greater energy costs.
* Carrier gas
Hydrogen gas and inert gases such as argon are flowed into the system.<ref name=":0" /><ref name=":1" /> These gases act as a carrier, enhancing surface reaction and improving reaction rate, thereby increasing deposition of graphene onto the substrate.
* Chamber material
Standard quartz tubing and chambers are used in CVD of graphene.<ref name=":3">{{Cite journal |last1=Zhang |first1=CanKun |last2=Lin |first2=WeiYi |last3=Zhao |first3=ZhiJuan |last4=Zhuang |first4=PingPing |last5=Zhan |first5=LinJie |last6=Zhou |first6=YingHui |last7=Cai |first7=WeiWei |date=2015-09-05 |title=CVD synthesis of nitrogen-doped graphene using urea |journal=Science China Physics, Mechanics & Astronomy |volume=58 |issue=10 |pages=107801 |bibcode=2015SCPMA..58.7801Z |doi=10.1007/s11433-015-5717-0 |s2cid=101408264}}</ref><ref name=":4">{{Cite journal |last1=Kim |first1=Sang-Min |last2=Kim |first2=Jae-Hyun |last3=Kim |first3=Kwang-Seop |last4=Hwangbo |first4=Yun |last5=Yoon |first5=Jong-Hyuk |last6=Lee |first6=Eun-Kyu |last7=Ryu |first7=Jaechul |last8=Lee |first8=Hak-Joo |last9=Cho |first9=Seungmin |year=2014 |title=Synthesis of CVD-graphene on rapidly heated copper foils |journal=Nanoscale |volume=6 |issue=9 |pages=4728–34 |bibcode=2014Nanos...6.4728K |doi=10.1039/c3nr06434d |pmid=24658264 |s2cid=5241809}}</ref> Quartz is chosen because it has a very high melting point and is chemically inert. In other words, quartz does not interfere with any physical or chemical reactions regardless of the conditions.
* Methods of analysis of results
Raman spectroscopy, X-ray spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) are used to examine and characterize the graphene samples.<ref name=":3" /><ref name=":4" />

Raman spectroscopy is used to characterize and identify the graphene particles; X-ray spectroscopy is used to characterize chemical states; TEM is used to provide fine details regarding the internal composition of graphene; SEM is used to examine the surface and topography.

Sometimes, atomic force microscopy (AFM) is used to measure local properties such as friction and magnetism.<ref name=":3" /><ref name=":4" />

Cold wall CVD technique can be used to study the underlying surface science involved in graphene nucleation and growth as it allows unprecedented control of process parameters like gas flow rates, temperature and pressure as demonstrated in a recent study. The study was carried out in a home-built vertical cold wall system utilizing resistive heating by passing direct current through the substrate. It provided conclusive insight into a typical surface-mediated nucleation and growth mechanism involved in two-dimensional materials grown using catalytic CVD under conditions sought out in the semiconductor industry.<ref>{{cite journal|last1=Das|first1=Shantanu|last2=Drucker|first2=Jeff|title=Nucleation and growth of single layer graphene on electrodeposited Cu by cold wall chemical vapor deposition|journal=Nanotechnology|volume=28|issue=10|pages=105601|doi=10.1088/1361-6528/aa593b|pmid=28084218|bibcode=2017Nanot..28j5601D|year=2017|s2cid=13407439 |url=https://zenodo.org/record/895408}}</ref><ref>{{cite journal|last1=Das|first1=Shantanu|last2=Drucker|first2=Jeff|title=Pre-coalescence scaling of graphene island sizes|journal=Journal of Applied Physics|date=28 May 2018|volume=123|issue=20|pages=205306|doi=10.1063/1.5021341|bibcode=2018JAP...123t5306D|s2cid=126154018}}</ref>