CVD Protocol for Carbon Nanotubes

Research will be limited to:
- Substrate: Silicon
- Material: Carbon Nanotubes
- Method: Chemical Vapor Deposition (CVD)
- Reactants: Ethylene, Hydrogen

Strategy: The objective is to identify specific CVD protocols for growing carbon nanotubes (CNTs) on silicon substrates using ethylene (C2H4) and hydrogen (H2). I will focus on extracting precise parameters for catalyst deposition (typically Fe, Co, or Ni), gas flow ratios (C2H4:H2), optimal temperature ranges (usually 650-900°C), and growth durations to achieve specific CNT morphologies (SWCNTs vs MWCNTs). I will also look for the role of the buffer layer (e.g., Al2O3 or SiO2) which is critical for preventing silicide formation on silicon.
Queries:
- CVD carbon nanotubes silicon substrate ethylene hydrogen flow rate temperature
- ethylene hydrogen CVD CNT growth protocol catalyst deposition silicon
- C2H4 H2 CVD carbon nanotube growth kinetics silicon substrate
- Fe Al2O3 catalyst ethylene hydrogen CVD carbon nanotubes silicon
- optimization of ethylene hydrogen ratio for CNT growth on silicon CVD

Executing 5 concurrent searches:
- CVD carbon nanotubes silicon substrate ethylene hydrogen flow rate temperature
- ethylene hydrogen CVD CNT growth protocol catalyst deposition silicon
- C2H4 H2 CVD carbon nanotube growth kinetics silicon substrate
- Fe Al2O3 catalyst ethylene hydrogen CVD carbon nanotubes silicon
- optimization of ethylene hydrogen ratio for CNT growth on silicon CVD

Found 10 papers. Key excerpts:

Role of Hydrogen in Ethylene-Based Synthesis of Single-Walled Carbon Nanotubes. — Alisa R Bogdanova et al., 2023

Though carbon nanotubes do not have a clear date of discovery [ 1 ], during the last 30 years, researchers have invested enormous efforts to pursue novel physical effects or next-generation devices based on nanotubes. Being a family of materials with a wide range of properties, carbon nanotubes, as a rule, are distinguished by the number of layers [ 2 ]. Multi-walled carbon nanotubes usually show a lower price, higher defectiveness, and the ability to sacrifice the outer layer for functionalization. Multi-walled carbon nanotubes show promising results in composites, energetics, and functional materials, while single-walled carbon nanotubes (SWCNTs) provide higher prospects in electronics, optics, and medicine [ 3 , 4 , 5 , 6 , 7 ]. However, sometimes SWCNTs also contribute to the polymer composites [ 8 , 9 ] owing to the significant drop in price induced by the OCSiAl company [ 10 , 11 ].
SWCNTs proved to be one of the most promising materials for various applications due to their unique ensemble of optical, electronic, and mechanical properties [ 12 , 13 , 14 ]. A combination of high optical transparency, intrinsic conductivity, and flexibility makes SWCNT-based thin films an ideal candidate for transparent electrodes [ 15 , 16 ]. Since SWCNT properties strongly depend on their structure [ 17 ], precise control over their characteristics is the key barrier to their successful industrial applications. Among different techniques for carbon nanotube growth, the aerosol chemical vapor deposition (CVD) method is one of the most suitable for advanced control over individual nanotube characteristics, such as diameter, chirality, and length [ 18 ]. However, the entangled relationship between the growth conditions/reactor parameters and SWCNT characteristics inhibits the creation of a universal model for the process to reach the tailored properties. This results in the active development of phenomenological methods (e.g., machine learning) or extensive data analysis [ 19 , 20 , 21 ].
Another opportunity is to boost the nanotube growth indirectly using, for example, so-called promotors: species added in small portions to enhance a specific parameter. Such promoters of the nanotube growth as water, potassium, sulfur, and carbon dioxide proved themselves as additives, enhancing the yield of synthesis, etching amorphous carbon from the surface of the catalyst, and increasing nanotube diameters [ 22 , 23 ,

24 , 25 , 26 ]. Usually formed during growth or introduced as a carrier gas, hydrogen is another indirect booster whose role is not fully understood yet. Some researchers report the phenomenological effect on the SWCNT growth and yield enhancement accompanied by defectiveness increase [ 27 ]. On the contrary, others claim that labile hydrogen is unfavorable for growth due to excessive etching [ 28 ].
Moreover, some observe several regimes of the hydrogen effect on the synthesis [ 29 , 30 , 31 , 32 , 33 , 34 ]. Rao et al. [ 30 ] showed a low hydrogen concentration to increase the yield of SWCNTs from methane as deactivation induced by excessive carbon species on the catalyst surface was inhibited, and more particles remained active. Nevertheless, when the hydrogen to methane ratio exceeded unity at 1000 °C, carbon feedstock to the catalyst suffered from inhibited methane decomposition, reducing the yield of SWCNTs. At the same time, Li et al. [ 32 ] observed 15–35% of hydrogen (C 2 H 4 as a carbon source at 750 °C) to improve the SWCNT quality for substrate CVD growth of vertically aligned carbon nanotubes. Ma et al. comprehensively studied the formation of multi-walled carbon nanotubes on quartz and alumina revealing not only the complex role of hydrogen (promotion at low amounts and carbon gasification to methane at high) but also the substrate effect [ 33 ]. Zhang et al. also highlighted the complex relationship between the carbon source and hydrogen [ 34 ]. Moreover, the authors showed that the excess hydrogen [ 35 ] changed the crystal phase of the catalyst from Fe 3 C into body-centered cubic Fe, leading to the formation of more sp 3 structure defects by the saturated carbon atoms. Eventually, under suitable hydrogen concentration (15%), it etched the catalyst to form evenly distributed iron particles, maximizing catalytic efficiency and reducing the formation of amorphous carbon. In addition, hydrogen was long known to inhibit pyrolytic carbon formation [ 36 ], especially in the case of methane pyrolysis [ 36 , 37 , 38 , 39 ]. Hydrogen inhibits active radicals and terminates the chain, thereby stopping pyrolysis and resulting in smaller carbon crystallites [ 40 ]. Thus, studies revealed the following hydrogen effect on carbon nanotube synthesis: hydrogen can help to prolong the catalyst lifetime by etching the impurities on its surface [ 29 , 30 , 31 , 32 ] and shifts the crystal phase of

the catalyst if in excess [ 32 ]. However, many questions about the hydrogen influence remain with no answers. Here, we wish to contribute to the understanding of the hydrogen role during the aerosol chemical vapor deposition (CVD)—specific case of the floating catalyst process under extreme dilution, as the method provides state-of-the-art carbon nanotubes for optoelectronics [ 15 ].
In this work, to reduce the complexity of the effect, we employed aerosol CVD with a single carbon source—ethylene, thereby removing the effect of a substrate or multiple hydrocarbons. We examine the effect of hydrogen on the growth of SWCNTs produced employing the aerosol CVD method, using ethylene as a carbon source and ferrocene as a catalyst precursor. Using a comprehensive set of methods (aerosol spectrometry, SEM, TEM, UV-vis-NIR spectroscopy, Raman spectroscopy, and four-point probe measurements), we thoroughly investigate the role of hydrogen on carbon nanotube growth, assessing the yield, diameter distribution, length distribution, aerosol concentration, quality, and sheet resistance. We observe three distinct SWCNT growth regimes depending on the hydrogen concentration. These findings help to understand the ability of hydrogen to tune the characteristics and purity of the produced SWCNTs.
Single-walled carbon nanotubes were produced using the aerosol chemical vapor deposition method (CVD)—the specific case of floating catalyst CVD with an extreme dilution of catalyst particles. The aerosol CVD synthesis reactor ( Figure 1 ) [ 41 ] consisted of a tubular glass with an inner diameter of 51 mm inserted inside the three-zone furnace with a length of 1300 mm (isothermal hot zone ~550 mm). Ferrocene vapor (the precursor of Fe-based catalyst, Fe(C 5 H 5 ) 2 , 98%, Sigma Aldrich) was transferred from a ferrocene cartridge by nitrogen gas flow (99.999%) to the reactor. The partial vapor pressure of ferrocene was 0.28 Pa during all the experiments. The catalyst-containing flow was fed through the injector near the hot zone of the reactor and mixed with ethylene (carbon source 0–0.03 lpm, 99.9%), CO 2 (0–0.05 lpm, 99.995%), and H 2 (0–0.6 lpm, 99.999%), reaching

Optimization of Mass Flow in the Synthesis of Ferromagnetic Carbon Nanotubes in Chemical Vapor Deposition System. — Grzegorz Raniszewski et al., 2021

Carbon nanotubes are an object of interest for many researchers in many fields such as sensing applications [ 1 , 2 , 3 ], automotive [ 4 ], computer science [ 5 , 6 ], medical diagnosis and therapy [ 7 , 8 , 9 , 10 ], chemical industry [ 11 ], and material engineering [ 12 , 13 , 14 ]. Carbon nanotubes have unique properties such as high thermal and electrical conductivity [ 15 ], an aspect ratio which exceeds 1:1000, high flexibility, very high tensile strength [ 16 ], high thermal stability [ 17 ], and chemical resistance [ 18 ]. Due to its properties, carbon nanotubes can be applied in many industries such as in electronics, e.g., field-effect transistors (FETs) [ 19 , 20 ], as an electronic material for next-generation electronic devices [ 21 , 22 , 23 , 24 ], even in CNT computers [ 25 , 26 ].
Carbon nanotubes were applied in a textile industry [ 27 ], as elements of supercapacitors [ 28 ], as field emitters [ 29 , 30 ], and as an element of composite polymers [ 31 ]. Moreover, carbon nanotubes have a high potential as drug carriers [ 32 , 33 , 34 ]. In the literature, we can find attempts of application in thermal tumor cells ablation [ 35 , 36 ]. Any application of carbon nanotubes requires a high-yield, repetitive production process. The final product has to be characterized by specified, desired properties.
The structure of carbon nanotubes are divided into two groups—multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). The most common method of carbon nanotubes synthesis is the arc discharge method, laser ablation, and chemical vapor deposition (CVD).
Carbon nanotubes filled with a ferromagnetic material may lead to an increase in the capacity of magnetic read-write devices. The nanoparticle size can represent a single domain and the carbon nanotube walls are nonmagnetic separating regions between the nanoparticles [ 37 ].
One of the most common areas of ferromagnetic carbon nanotubes application is that individual nanotubes can be used as cantilever tips in magnetic force microscopy (MFM). These nanotubes can be produced by the selective growth of aligned Fe-filled multi-walled nanotubes [ 38 ]. Ferromagnetic-filled carbon

nanotubes are very interesting for applications in biomedicine for imaging, diagnosis, and therapy [ 39 ].
In this work, a catalytic chemical vapor deposition (CCVD), as an efficient and low-cost method for mass production, is described.
The catalytic chemical vapor deposition is one of the methods of carbon nanotubes synthesis. In [ 40 ], a swirled floating catalyst technique with a xylene/ferrocene as the catalyst was described. There are also attempts to obtain single-walled nanotubes but the final product was not aligned [ 41 , 42 ]. In thermal ablation, multi-walled carbon nanotubes are more convenient for functionalization, targeting, and filling. Single-walled carbon nanotubes, due to their less damaged structure, are difficult for further modification. In the research, a catalyst is injected into the system as a liquid solution. In the literature, the process can be seen as a liquid source chemical vapor deposition (LSCVD). The synthesis of ferromagnetic CNTs employs a limited group of catalysts—mostly metalorganic compounds with a general formula Me–(C 5 H 5 ) 2 , where Me means metal from the transition metals group. Ferrocene Fe(C 5 H 5 ) 2 (Sigma-Aldrich) was used as a solution in xylene (Chempur) in the analytical reagent (AR) grade. The role of xylene is not only to work as a solvent but also to be an additional source of carbon elements. The main criterium for the method selection was the possibility to control the process by an easy change of parameters such as the catalyst dosing rate, temperatures in the zone of catalyst vaporization, temperatures in the zone of carbon material deposition, flow rate of gases, and time of synthesis. The proper adjustment of parameters results in the perpendicular formation of aligned carbon nanotubes. In cases with catalysts in solid states, the amount of sublimated material is limited, thus these processes are periodical. If the catalyst is injected in a continuous way, it results in the continuous formation of ferromagnetic CNTs. This process enables obtaining a relatively large amount of material during the process.
Although mathematical models for iron particles formation and the kinetic model of size control of the generated nanoclusters can be found [ 43 , 44 , 45 ], we focused on the synthesis of MWCNTs with the maximum iron

filling. The aim of this work is to determine a suitable temperature of ferrocene solution vaporization, the rate of catalyst dosing, flow rate of gases, and temperatures in the deposition zone. As a criterium of success, the yield and purity of MWCNTs were established. Carbon nanotubes should form an aligned, perpendicular to the surface forest-like structure and be homogeneous over the entire surface of the substrate. The essential aspect of mass flow selection and gas composition is the final product with a possible minimum amount of impurities such as amorphous carbon, graphite, etc.
The catalytic chemical vapor deposition to obtain ferromagnetic multi-walled carbon nanotubes was used. The system was composed of: The source of gases; Flowmeters with regulators; Injection pump; Tube furnace; Gas exhaust system.
The source of gases;
Flowmeters with regulators;
Injection pump;
Tube furnace;
Gas exhaust system.
The Linde gas cylinders were applied and argon and helium with a purity of 99.9% were used. Hydrogen was produced in the electrolysis process in a Linde HiQ H2 FID hydrogen carrier gas generator. Dosing valves and flowmeters were used to regulate the mass flow of gases. Injection pump Medima S2 was employed to dose the catalyst solution. The tube furnace was divided into three zones. Each zone could be regulated independently in the temperature range up to 1050 °C. The first zone was responsible for the preheating of gases and catalyst vaporization. In the second zone, neutral gases and iron particles were heated to the adjusted temperature. The last zone was the deposition zone where carbon nanotubes were formed and deposited on the substrate. As a substrate, a silicon wafer was used. As an exhaust gas system, a barbotage column and ventilation pump were used. Figure 1 shows the scheme of the process.
The process started with the silicon stripe placement inside the quartz tube in the third zone. The quartz tube with an inner diameter of 30 mm and length of 1200 mm was placed inside the tube furnace, connected to the injection pump on one side, and to the exhaust gas purification system on the other side. Then, the quartz tube was tightly sealed. Air was removed from the reactor by a 10 min ventilation by argon and helium to remove the air present in the quartz tube and prevent the oxidation of synthesis products

Influence of carrier gas flow rate on carbon nanotubes growth by TCVD with Cu catalyst — S.A. Khorrami et al., 2016

Carbon nanostructures have attracted much attention because of unique electrical, optical and mechanical properties ( Ajayan, 1999 ). The application of CNTs is their use in nanoscale electronic such as interconnects, nanosensors and field effect transistors by their specific electronic structures, unique one-dimensional nanostructures and superior transport properties ( Graham et al., 2005 , Kuttle et al., 1998 , Ngo et al., 2004 ). Several major methods have been developed to grow CNTs such as chemical vapor deposition (CVD) ( Fu et al., 2012 , Lisi et al., 2011 , Handuja et al., 2010 , Chen et al., 2009 ), laser ablation ( Thess et al., 1996 ), arc discharge ( Journet et al., 1997 ) and microwave heating ( Teo et al., 2004 ). Also, there are several techniques of CVD such as thermal CVD ( Fu et al., 2012 , Wu et al., 2012 ), plasma-assisted thermal CVD ( Lee et al., 2011 ), Surface wave microwave plasma CVD ( Rusop et al., 2005 , Ghimire et al., 2008 ), hot filament CVD ( Sampaio et al., 2010 , Salgueiredo et al., 2011 ), thermogravimetric CVD ( Kouravelou et al., 2007 ), catalytic chemical vapor deposition ( Zhu et al., 2012 ), radiofrequency CVD ( Mannan et al., 2009 ) and laser CVD ( Guo et al., 2012 ). Among these methods CVD has attracted much attention owing to its advantages including selective growth of CNTs, high yield, high purity and lower cost. In this method, the characteristics of CNTs such as morphology, crystallization and surface density are significantly affected by the synthesis parameters ranging from growth temperature ( Akbarzadeh Pasha et al., 2010 , Zhu et al., 2012 , Sengupta and Jacob, 2010 ), carrier gas flow ( Valles et al., 2009 , Liu et al., 2009 ), hydrocarbon source ( Panda, 2009 , Maity et al., 2008 , Botello-Méndez et al., 2008 , Mordkovich and Karaeva, 2010 , Yu et al., 2006 ), type of wafer and substrate ( Wu et al., 2012 , Sahoo and Daramalla, 2012 , Hong et al., 2012 ) to the catalyst characteristics including its morphology, the technique of catalyst preparation and composition.

Morphology of catalyst used for the CVD technique is one of important factors for CNTs’ growth. Several catalysts such as Fe, Co, Ni and combination of those metals and catalyst support materials ( Ding et al., 2004 , Inoue et al., 2005 ). Some researchers reported the synthesis of single-walled carbon nanotubes (SWNTs) from catalyst film with gradient thickness deposited by a combination masked deposition method ( Lu et al., 2012 , Rinaldi et al., 2011 ). An alcohol catalytic CVD method is expected to be a potential technique for the fabrication of electronic devices ( Yuan et al., 2008 ). For this purpose, several experimental conditions are studied ( Peng et al., 2011 ). Therefore, these parameters such as the carrier gas flow rates, type of wafer, catalyst and hydrocarbon source affect CNTs’ growth.
Relation between synthesis and carrier gas flow rate of CNTs has not been fully studied. In this study, Si wafer P-type was used as substrates and was coated with Cu nanolayer by sputtering technique because Cu catalyst is available and inexpensive with high purity. CNTs have been grown by TCVD. Finally, the influence of carrier gas flow rates on carbon nanostructures, morphology and graphitization is reported.
Substrate preparation: the Si wafer P-type (1 0 0) was used as substrates with the resistivity ranging from 2 to 8 Ω cm. The substrates were cleaned by ultrasonic vibration with acetone, ethanol and deionized water to remove all contaminants, separately and respectively. A Cu thin film was deposited on a mirror-polished silicon wafer as a catalyst by a direct current sputtering technique. Thickness of the Cu nanolayer was 15 nm. This instrument consisted of a cylindrical glass tube and two Cu plates used as a cathode (with 30 mm diameter and 200 mm length) and anode (with 100 mm diameter and 200 mm length) into the chamber. Argon gas was selected as a sputtering gas and the operation pressure was 0.03 torr. The applied voltage between the anode and cathode was about 900 V which produced a current of 120 mA. A silicon wafer used as a substrate was fixed on the anode. The surface morphologies of the Cu thin films were observed using the AFM (AFM Park Scientific Instruments Auto Probe CP). Thick

ness of thin films was measured using the Rutherford Back Scattering (RBS) technique (using He + ion beam of 10 μm in diameter with 2.0 MeV energy). AFM micrographs of Cu nanolayer are shown in Fig. 1 . The morphology and size distribution of CNTs were characterized by transmission electron microscopy (TEM, Philips 200 FEG instruments).
Growth of the carbon nanotubes: a thermal chemical vapor deposition reactor was used for the synthesis of CNTs by catalytic decomposition of ethanol on Si (1 0 0) substrates. TCVD system consists of a furnace, a quartz tube with 60 cm in length and 4 cm inner diameter, a thermocouple and a temperature controller that is shown in Fig. 2 .
The prepared substrate was placed inside of a quartz tube and the CVD chamber was evacuated to 15 mtorr. Then the temperature was increased to 900 °C while being exposed to an argon–hydrogen mixture gas (10% H 2 balanced Ar) with 100, 150 and 250 sccm (denotes cubic centimeter per minute at STP) flow rates. This mixture gas was bubbled into the alcohol chamber. Then, mixture gas with the alcohol vapor was introduced inside of the quartz tube. In a desired reaction temperature (850 °C), controlled high purity anhydrous ethanol (99.95%, Merck) was supplied as a carbon source for the growth of CNTs resulting. To avoid the catalyst oxidation, argon gas with 200 sccm was fed into the tube and then the Ar gas valve was closed. The reaction times for the growth of carbon nanostructures were 30 min. After the finishing the growth process, mixture of H 2 /Ar with 150:150 (sccm) flow rates was fed into the tube until tube temperature was decreased to the room temperature. On the other hand, other parameters of growing carbon nanostructures under these conditions have not been changed. Scanning electron microscopy and Raman spectroscopy were used for the characterization of the CNTs’ morphology and structure.
Si wafer P-type with Cu nanolayer was deposited by sputtering technique. The ethanol concentration in the reaction mixture with different gas flow rates of argon–hydrogen mixture gas varied from 100 to 250 sccm. Whenever a mixture of gases is in contact with the catalyst surface, equilibrium is established

A high-sensitivity hydrogen gas sensor based on carbon nanotubes fabricated on SiO2 substrate — Ahmad M. Al-Diabat et al., 2021

1. IntroductionGreat efforts have been devoted to the development of novel nanostructured materials with specific properties for gas sensors with high performances such as selectivity and sensitivity. Since their discovery in 1991, carbon nanotubes (CNTs) have attracted scientific interest due to their remarkable mechanical, chemical, and electronic properties in addition to their semiconductive/metallic character depending on their diameter [1Qi P, Vermesh O, Grecu M, et al. Toward large arrays of multiplex functionalized carbon nanotube sensors for highly sensitive and selective molecular detection. Nano Lett. 2003;3(3):347–351. [Crossref], [Web of Science ®], [Google Scholar],2Nguyen H-Q, Huh J-S. Behavior of single-walled carbon nanotube-based gas sensors at various temperatures of treatment and operation. Sens Actuators B. 2006;117(2):426–430. [Crossref], [Web of Science ®], [Google Scholar]]. These unique characteristics make them a promising material for various applications including nanoelectronics, field emission devices, and multifunctional composite materials [3De Volder MF, Tawfick SH, Baughman RH, et al. Carbon nanotubes: present and future commercial applications. Science. 2013;339(6119):535–539. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]].The application of CNTs for gas sensing has been widely investigated because of their large surface area due to their nanoscale regime and hollow geometry [4Jaggi N, Dhall S. Hydrogen gas sensing properties of multiwalled carbon nanotubes network partially coated with SnO2 nanoparticles at room temperature. Analysis. 2014;748:9999801. [Google Scholar],5Kong J, Chapline MG, Dai H. Functionalized carbon nanotubes for molecular hydrogen sensors. Adv Mater. 2001;13(18):1384–1386. [Crossref], [Web of Science ®], [Google Scholar]]. Some previous studies confirmed that CNTs-based gas sensors could be fabricated by electrophoresis [6Lee J-H, Kim J, Seo HW, et al. Bias modulated highly sensitive NO2 gas detection using carbon nanotubes

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Effect of growth temperature on the CVD grown Fe filled multi-walled carbon nanotubes using a modified photoresist — Joydip Sengupta et al., 2020

a Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India b Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Keywords: A. Nanostructures B. Vapor deposition C. Electron microscopy C. Raman spectroscopy C. X-ray diffraction
deposition using a simple mixture of iron(III) acetylacetonate (Fe(acac)3) with a conventional photoresist and the effect of growth temperature (550–950 ⁰C) on Fe filled
nanotubes has been studied. Scanning electron microscopy results show that, as the growth temperature increases from 550 to 950 ⁰C, the average diameter of the
nanotubes increases while their number density decreases. High resolution transmission electron microscopy along with energy dispersive X-ray investigation shows that the nanotubes have a multi-walled structure with partial Fe filling for all growth temperatures. The graphitic nature of the nanotubes was observed via X-ray diffraction pattern. Raman analysis demonstrates that the degree of graphitization of the carbon nanotubes depends upon the growth temperature.
1. Introduction
In today’s world of nanotechnology, carbon nanotubes (CNTs) have become a high priority material because of their unique properties [1] with enormous potential in many technological applications. Among the different synthesis methods of CNTs [2– 6], chemical vapor deposition (CVD) is the most preferable method owing to its advantage of producing a large amount of CNTs directly on a desired substrate with high purity. Before the synthesis of CNTs by CVD, high temperature hydrogen treatment of the catalyst is required in order to produce contamination-free catalytic particles and for the removal of oxides that may exist over the catalyst surface [7]. In recent years, magnetic material filled CNTs have become an area of interest for the researchers as they have extended the potential applications to magnetic force microscopy [8], high density magnetic recording media [9] and biology [10]. Though different methods of filled CNT synthesis including capillary incursion [11], chemical method [12], arc-discharge [13] and CVD [14–17] have already been reported, yet the processes are costly as they usually require a two-stage process along with rigorous control of the reaction parameters. Therefore, these processes

are not suitable for the economic large scale production of filled CNTs on the desired substrates. Furthermore, despite recent advancements in CNT synthesis, studies on the growth temperature dependence of the metal filled CNTs are still relatively rare in the literature.
Thus, from the technological point of view, it is absolutely necessary to devise an economic and scalable single stage synthesis method for the magnetic material filled CNT and also to study their growth temperature dependence.
In this article, we report a novel method for the large scale synthesis of Fe filled CNTs employing atmospheric pressure chemical vapor deposition (APCVD) of propane on Si using a modified photoresist (Mod-PR) with a metalorganic molecular
precursor, Fe(acac)3. The effect of growth temperature in the range of 550–950 ⁰C on the Fe filled CNTs was also investigated. Scanning electron microscopy (SEM), X-ray diffraction (XRD), high resolu-tion transmission electron microscopy (HRTEM), energy dispersive X-ray (EDX) and Raman spectroscopy were used to characterize the morphology, phase, internal structure and quality of the resultant products.
2. Experimental procedure
To obtain Mod-PR solution of concentrations 0.2 M, 706.4
mg of Fe(acac)3 was mixed with 10 ml of HPR 504 (Fuji Film). HPR 504 is a positive photoresist which uses ethyl lactate as the solvent and has a viscosity of 40 cps. The solution was then stirred and sonicated for 30 min to achieve a good dispersion of the metallorganic molecular precursor. After that the solution was spin-coated with a rotation speed of 4000 rpm for 20 s on the Si(1 1 1) substrate to get a thin layer of the Mod-PR. The thin Mod-PR film was then annealed in air for 10 min at 200 ⁰C to improve the adhesion to the substrate.
Fig. 1. (a) XRD spectrum and (b) SEM image of the precursor.
Annealed substrates were loaded into a quartz tube furnace, pumped down to10 2 Torr and backfilled with flowing argon
to atmospheric pressure. The samples were then heated in argon up to the growth temperature following which the argon was replaced with hydrogen. Subsequently, the

samples were annealed in hydrogen atmosphere for 10 min. Finally, the hydrogen was turned off; thereafter propane was introduced into the gas stream at a flow rate of 200 sccm for 1 h for CNT synthesis. The synthesis of Fe filled CNTs was performed at 550, 650, 850 and 950 ⁰C.
SEM (ZEISS SUPRA 40) and HRTEM (JEOL JEM 2100) equipped with EDX (OXFORD Instruments) were employed for examination of the morphology and microstructure of the products. Samples were also characterized by a Philips X-ray diffractometer (PW1729) with a Cu source and a u–2u geometry to analyze the crystallinity and phases of the products. Raman measurements were carried out with a RENISHAW RM1000B LRM at room temperature in the backscattering geometry using a 514.5 nm air-cooled Ar+ laser as an excitation source for compositional analysis.
Fig. 3. (a) The XRD spectrum of the MWCNTs prepared from 0.2 M Mod-PR solution after CVD growth at 850 ⁰C and (b) SEM micrograph of the catalytic nanoparticles prepared from 0.2 M Mod-PR solution after annealing at 850 ⁰C.
Fig. 4. HRTEM images of carbon nanotubes grown at different temperatures: (a) 550 ⁰C, (b) 650 ⁰C, (c) 850 ⁰C, (d) 950 ⁰C, (e) lattice image from a CNT grown at 650 ⁰C and (f) lattice image from a CNT grown at 850 ⁰C.
.
3. Results and discussion
XRD measurements were carried out to investigate the structure of the precursor and the resulting u–2u scan is shown in Fig. 1a. All its diffraction peaks can be assigned to the orthorhombic structure of Fe(acac)3 which is in agreement with the JCPDS card no. 30-1763. SEM image (Fig. 1b) shows that the precursor comprises thin platelets with a wide range of crystallite sizes.
The analysis of the morphology and number density of the as-grown CNTs was performed using SEM. Fig. 2

CVD-Grown Carbon Nanotube Branches on Black Silicon Stems for Ultrahigh Absorbance in Wide Wavelength Range. — Thanh Luan Phan et al., 2020

Introduction The intensive research on black silicon (bSi) 1 , 2 over the past decade has inspired a promising approach for increasing the efficiency and reducing the manufacturing costs of many applications, including photovoltaics 3 – 8 , photodetectors 9 , 10 , and water splitting via photoelectro-chemical catalysis 11 – 13 . Recently, considerable effort has been directed toward enhancing the light absorbance by using Si nanowires (NWs) 14 , 15 , Si nanocones (NCs) 16 , 17 , and porous Si 18 , 19 , as Si has a small band gap (E g = 1.1 eV) that allows for light absorption in the solar spectrum 20 . Many different methods for fabricating bSi have been introduced, such as laser texturization 21 , 22 , reactive-ion etching 23 – 25 , and metal-assisted wet etching 26 , 27 . However, the performance of bSi nanostructures obtained via such methods is limited by two main factors: the absorbance efficiency and the absorbance over a wide wavelength range. To overcome the first limitation, many studies have been conducted to improve the absorbance efficiency 21 – 27 , which is increased the fabrication cost and complex facility requirements. The second limitation leads to a low absorbance efficiency over a wide spectral range. Recently, the surface structure has been modified to enhance the efficient antireflection by coating an oxide layer via atomic layer deposition (ALD) method 23 , 28 , thin film deposition 29 – 31 or metal nanoparticles deposition 32 . However, ALD or deposition technique may limit the applications of the Si device because the outer thin-film layer can cover whole area of Si; while, the metal nanoparticles deposition has a high production cost. On the other hand, carbon materials such as single-wall carbon nanotube (SWCNT) 33 , multi-wall carbon nanotube (MWCNT) 34 , 35 or graphene 36 as known as excellent absorption material for wide range spectral of wavelength. Here, carbon materials are not only absorbing the light, but also enhance the light trapping in vertical array structure, which attributed to achieve high optical absorption 33 – 36 . However, silicon based application such as solar cell or photo-electrochemical water splitting, where the practical implementations required several important aspects such are band edge energy for light absorption and charge transport, or thermodynamic at semiconductor/liquid interface 12

. Thus, wide range of wavelength operation in silicon based practical application still remains the significant technical challenges to overcome. Herein, we report a black silicon-carbon nanotube (bSi-CNT) hybrid structure for ultrahigh absorbance at wide spectral range of wavelength (300–1200 nm). CNTs are densely grown on entire bSi side walls by chemical vapor deposition (CVD) through uniformly coating Fe catalyst on bSi. The bSi-CNT hybrid structure not only increases the surface roughness for enhancing the light suppression and trapping, but also allows the absorption of light in a wide wavelength range over the Si band gap (>1000 nm owing to 1.1 eV) due to the small band gap of CNT (0.6 eV) 37 . Therefore, the absorbance of the bSi-CNT hybrid sample was exhibited average absorbance values of 96.3% in the wavelength range of 300–1200 nm. In particular, at short wavelength below Si band gap (<1000 nm), the absorbance of bSi-CNT shows average of 98.1%, while bSi shows 89.4%, which is because of high surface roughness of bSi-CNT that enhancing the light suppression and trapping. Meanwhile, at long wavelength over Si band gap (>1000 nm), the absorbance of bSi-CNT was maintained to 96.3% because of the absorption in CNT (0.6 eV), while absorbance of bSi abruptly reduces with increase wavelength. Importantly, the absorbance of bSi-CNT was showed 93.5% at 1200 nm of wavelength, which is about 30~90% higher than previously reported bSi. Furthermore, we demonstrated the impact of the CNTs by adjusting the density of the CNTs-grown on the side of the bSi stems, where the absorbance of bSi-CNT hybrid sample was increased along to the increment of the CNT density. We proposed a simple method to integrate of CNTs and bSi, which can dramatically enhances the absorbance without using any antireflection coating layer. The results can be employed for realizing high-efficiency photodiodes, solar cells, and photocatalytic water splitting in future application devices. Results and Discussion Figure  1a shows a schematic of the fabrication process for the b

Si-CNT samples. Corresponding scanning electron microscopy (SEM) images are shown in Fig.  1b–d . First, an n -type Si (100) substrate was immersed in a 5 M hydrofluoric acid (HF) / 0.02 M AgNO 3 solution for 1 h at 50 °C. Then, the sample was washed with deionized (DI) water and dried with N 2 gas 26 . The SEM image results (side and top views) are shown in Fig.  1b , where the bSi arrays are well aligned. As shown in Fig.  1c and Fig.  S1 , Supplementary Information, the Fe nanoparticle was uniformly deposited on the side of the bSi via electro-deposition method. The deposition was performed at room temperature in a solution containing of 5 g of FeCl 3 and 10 g of NH 4 Cl in 100 mL of DI water for 1 min under an applied voltage (2 V) and current (0.01 A). The distance between the cathode (Pt electrode) and the anode (bSi) was 3 cm. The sample was rinsed in DI water for 5 min and then dried with N 2 gas. Here, the top part of bSi were shrink due to the precursor solution under deposition process. Next, a CNT random network was synthesized on a bSi substrate via CVD method with methane (CH 4 ) as the carbon source. The bSi substrate with a Fe catalyst precursor was then placed in a horizontal 1-inch quartz tube furnace with the catalyst end facing the gas flow. The catalyst precursor was reduced in a flowing Ar/H 2 gas mixture (200 sccm/100 sccm) at 1000 °C for 20 min, and then 10 sccm methane with 20 sccm H 2 gas was introduced into the furnace for the growth of the CNT random network. At the end of the growth process, Ar gas (500 sccm) was applied during cooling to room temperature 38 . As shown in Fig.  1d , the CNT random network was successfully grown on the side of the bSi sample, which was confirmed by energy-dispersive X-ray spectroscopy (EDS) measurement

Direct Application of Carbon Nanotubes (CNTs) Grown by Chemical Vapor Deposition (CVD) for Integrated Circuits (ICs) Interconnection: Challenges and Developments. — Zhenbang Chu et al., 2023

For a long time, copper material has dominated the field of interconnects in ICs due to its excellent electrical performance, good ductility, relatively low cost, and mature process [ 1 ]. However, as manufacturing technology continues to advance and IC dimensions continue to shrink, the size of interconnections also needs to be reduced accordingly, and copper-based interconnects are increasingly exposed to drawbacks. For example, the increase in resistance and capacitance [ 2 ], thermal effects [ 3 ], and electromigration (EM) [ 4 ]. The most direct and effective way to solve the above problems is to find a new conductive material to replace copper. Among various candidate materials, metallic carbon nanotubes (CNTs) are suitable as the new generation of interconnect materials for IC due to their quasi-one-dimensional properties and excellent electrical properties. Further, due to the stable structure of CNTs, problems in copper interconnects will be greatly improved, which increases the possibility for their substitution. Liao et al. [ 5 , 6 , 7 ] demonstrated the application of CNTs as interconnection materials in 3D IC packaging, and successfully achieved the connection between two different silicon test vehicles through CVD growth of CNTs. These studies [ 5 , 6 , 7 ] have demonstrated the enormous potential of CNTs as a new generation of interconnection materials. Currently, there are many methods for producing CNT, including arc discharge [ 8 ], laser ablation [ 9 ], and CVD [ 10 , 11 , 12 , 13 ]. Among them, the CVD method with advantages such as a relatively simple process, low production cost, and mild reaction conditions, has attracted extensive attention and research due to its potential compatibility with IC manufacturing processes [ 10 , 11 , 12 , 13 ].
Among various interconnection methods, through silicon via (TSV) technology is a highly promising approach that enables vertical interconnects and serves as a core technology for 3D packaging [ 14 ]. TSV technology involves drilling holes in the silicon layer and filling them with conductive materials to realize vertical interconnects within the chip. As shown in Figure 1 , the CVD method can be combined with TSV technology by placing a metal catalyst at the bottom of the via, allowing the growth of CNTs through the CVD process to fill the via. Simultaneously, the metal catalyst acts as an electrode, connecting to the CNTs that grow on top, thereby achieving IC

interconnects. Through this approach, the direct application of CVD growth CNTs in IC interconnects can be realized. Based on this, three main challenges need to be addressed: firstly, controlling the temperature of the CVD growth process within a low range, preferably below 500 °C, to be compatible with IC manufacturing processes; secondly, achieving the enrichment of metallic CNTs; and thirdly, overcoming the contact resistance between the metal catalysts as electrodes and the CNTs grown on it. The work of Liao et al. [ 5 ] demonstrates the enormous potential of this method for application. They used ferrocene as a catalyst to grow CNTs in pores (TSVs) with high aspect ratios through TCVD to achieve multi-layer 3D IC interconnection. The subsequent test results indicate that electrical connection has been successfully achieved in the 3D IC structure, and the contact resistance between the metal and CNTs is as low as approximately 10 Ω. Further, the CVD process can be carried out at low temperatures, and the grown CNTs have high thermal conductivity.
This paper summarizes representative research progress on the aforementioned issues and provides related discussions, aiming to provide insights for future research endeavors.
In 1991, Iijima et al. [ 15 ] discovered CNTs during the process of producing carbon fibers by arc discharge, which drew widespread attention. Researchers have developed various methods for preparing CNTs in the past few decades, including arc discharge, laser ablation, and chemical vapor deposition (CVD). Among these methods, CVD has become a mainstream method for producing CNTs due to its ease of operation, relatively mild reaction conditions, and relatively low cost. At the same time, the CVD method is also the most promising for achieving compatibility and integration with semiconductor manufacturing processes. In the IC manufacturing process, the temperature is usually lower than 500 °C. To be compatible with it, the temperature of CVD growth CNTs should also be controlled at a lower level. Extensive and continuous research has been conducted in this field [ 10 , 11 , 12 , 13 ]. The process of CVD growing CNTs can be divided into two mechanisms. One is the vapor–liquid–solid (VLS) growth mechanism, as shown in Figure 2 a, where “vapor” refers to gaseous carbon source molecules, which decompose and produce carbon atoms at high temperatures

. “Liquid” refers to the catalyst exhibiting a liquid or quasi-liquid state at high temperatures. When the concentration of carbon atoms dissolved into the catalyst reaches saturation, they will precipitate and grow into CNT, and “solid” refers to the solid CNTs. The other is the vapor-solid-solid (VSS) growth mechanism, as shown in Figure 2 b, which differs from the VLS growth mechanism in that the catalyst remains in a solid state during the CVD process. Carbon atoms do not dissolve into the interior of the catalyst but diffuse and grow into CNTs on the surface of the catalyst. Compared to the VLS mechanism, the reaction temperature of the VSS mechanism is usually lower, which is more conducive to compatibility with IC manufacturing processes. Moreover, since the catalyst remains in a solid state, its lattice structure does not change, which provides conditions for designing the lattice structure of the catalyst to achieve the purpose of controlling the chirality of CNTs. In summary, compared to the VLS growth mechanism, the VSS growth mechanism is more suitable for the needs of IC manufacturing processes. Therefore, the works described in this paper are based on the VSS growth mechanism.
During the process of CVD growth of CNTs, there are many factors that affect the reaction temperature. For example, the selections of carbon source gas and catalyst, carrier gas flow rate and flow rate, and H 2 flow rate and flow rate. Among them, carbon source materials and catalysts are the two most important factors, at the same time, they are also the two most easily controlled factors, considering compatibility issues, they will not have a significant impact on the IC manufacturing process. This chapter will discuss from the perspective of carbon source materials and catalysts.
Common carbon source materials include ethanol (C 2 H 5 OH), methane (CH 4 ), and acetylene (C 2 H 2 ). The growth temperatures corresponding to several commonly used carbon sources in experiments are shown in Table 1 .
As can be seen, among the various commonly used carbon source materials, acetylene (C 2 H 2 ) can control the CVD growth temperature of CNTs within the temperature range of 400 °C to 600 °C, which is more in line with the requirements of the IC manufacturing process. Magrez et al. [ 18 ] first applied the oxidative dehydrogenation reaction of acetylene (C 2 H 2 ) to the

Critical Oxide Thickness for Efficient Single-walled Carbon Nanotube Growth on Silicon Using Thin SiO2 Diffusion Barriers — J. M. Simmons et al., 2018

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The ability to integrate carbon nanotubes, especially single-walled carbon nanotubes, seamlessly
onto silicon would expand the range of applications considerably. Though direct integration using
chemical vapor deposition is the simplest method, the growth of single-walled carbon nanotubes
on bare silicon and on ultra-thin oxides is greatly inhibited due to the formation of a non-catalytic
silicide. Using x-ray photoelectron spectroscopy, we show that silicide formation occurs on
ultra-thin oxides due to thermally activated metal diffusion through the oxide. Silicides affect
the growth of single-walled nanotubes more than multi-walled nanotubes due to the increased
kinetics at the higher single-walled nanotube growth temperature. We demonstrate that nickel
and iron catalysts, when deposited on clean silicon or ultra-thin silicon dioxide layers, begin to form silicides at relatively low temperatures, and that by 900 oC, all of the catalyst has been
incorporated into the silicide, rendering it inactive for subsequent single-walled nanotube growth.
We further show that a 4 nm silicon dioxide layer is the minimum diffusion barrier thickness which
allows for efficient single-walled nanotube growth.
Keywords: Carbon nanotubes, Catalysis, Chemical vapor deposition, Photoelectron spectroscopy
Corresponding author:
M. A. Eriksson
Department of Physics, University of Wisconsin - Madison
1150 University Ave.
Madison, WI 53706
e-mail: maeriksson@wisc.edu
I. INTRODUCTION
Since their discovery, carbon nanotubes have shown great promise for a wide variety of applications which utilize their unique electronic and mechanical properties.[1] For applications in which individual nanotubes act as the working element, such as nanotube field effect transistors (FETs)[2,3] or chemical sensors,[4,5,6] it is important to control the location and orientation of the nanotubes. Nanotubes can be prefabricated and then assembled into the desired geometry, or they can be fabricated in place using chemical vapor deposition (C

VD).[7,8] CVD is preferred since the growth can be widely tuned, both in yield and structure (single- vs. multi-walled), by modifying the experimental conditions. While there are a vast number of CVD recipes available in the literature for both single- and multiwalled nanotube growth, most studies of the growth process have focused on the role of the carbon precursor (e.g. CO, CH4, C2H2) and temperature as control parameters with less attention placed on the choice of catalyst and substrate.[9] Optimization of the catalytic process requires an understanding of the catalyst chemistry throughout the growth process, including the initial chemical state of the catalyst before the introduction of the feedstock, as well as the catalytic decomposition of the feedstock in or on the catalyst particle.[10,11,12,13,14,15,16] Only recently have studies focused on the catalyst-feedstock and catalyst-substrate interaction.[14,15,16,17,18,19,20]
It is well established that the yield of single-walled nanotube growth on silicon substrates is dramatically reduced compared with growth on thick silicon dioxide layers, due to poisoning of the catalyst by the formation of a silicide.[12,21,22,23] Intriguingly, the growth of multi-walled nanotubes on silicon substrates is regularly reported and seems less susceptible to catalyst poisoning,[24,25,26,27] yet the cause of this difference has not been addressed. For device applications it is desirable to use thin oxides to increase the gate capacitance and gate efficiency, and thus it is important to understand how to minimize oxide thickness while still preventing catalyst poisoning. Though thick silicon dioxide layers have been used as diffusion barriers during nanotube growth, there has been no investigations to date that determine the the effectiveness of ultra-thin oxides.
Here we demonstrate explicitly that catalyst diffusion through the ultra-thin silicon dioxide layer controls the formation of the non-catalytic silicide. Using x-ray photoelectron spectroscopy (XPS), we study the interfacial reactions between the substrate and iron-
or nickel-based nanotube catalysts during the initial temperature ramp portion of a CVD growth cycle. We show that ultra-thin oxide layers (4 nm or greater) are sufficient to inhibit the silicide formation and permit high yield growth of

single-walled carbon nanotubes. On thinner oxides or clean silicon, silicide formation begins by 600 oC (Ni) or 800 oC (Fe) and the catalyst is entirely consumed in the silicide at the growth temperature of single-walled nanotubes. The silicide formation temperatures account for the difference in single- versus multi-walled nanotube growth because multi-walled nanotubes are grown at lower temperatures where some of the catalyst remains unreacted and active for catalysis. Interestingly, the silicides form while the silicon dioxide is still present on the substrate, indicating that diffusion of metal or silicon through the oxide is occurring. We show that it is metal diffusion through the oxide, forming a metal silicide underneath the oxide, which dominates the interfacial reactions between the nanotube catalyst and the silicon substrate.
II. RESULTS AND DISCUSSION
To understand the catalyst-substrate interfacial reactions, we first analyze XPS spectra of iron nitrate catalyst that has been deposited on a thick (100 nm) thermal oxide. On thick oxides, silicides will not form and single-walled nanotube growth occurs with high yield (Figure 1a). Before annealing, the iron core level (Figure 1b) exhibits broad, asymmetric peaks at 710.8 eV and 724.5 eV, corresponding to the Fe 2p3/2 and 2p1/2 levels of iron oxide in a mixed Fe2+/Fe3+ oxidation state.[28,29] The extra intensity on the high binding energy side of the core levels is due to unresolved satellites that are characteristic of oxidized iron.[29,30] As the catalyst is heated on the thick oxide, there are no major changes in the chemistry of the iron oxide. There is a small loss in iron intensity, due to either agglomeration of the catalyst on the surface (thereby reducing the measured intensity due to the finite electron escape depth) or desorption of the iron into the vacuum while annealing.[13] There is also a shift in the center of gravity of the Fe 2p3/2 core level as the Fe 2+ oxide (FeO) transforms into the more stable Fe3+ oxide (Fe2O3) [31] and the satellite peaks at 715 eV and 730 eV become better defined. Even at 1000 oC, the

Ultrafast Patterning Vertically Aligned Carbon Nanotube Forest on Al Foil and Si Substrate Using Chemical Vapor Deposition (CVD) — Yan-Rui Li et al., 2019

1. IntroductionKnown for their superior electrical, chemical, and mechanical strength, carbon nanotubes (CNTs) possess a vast array of prospective applications. In particular, their abnormally high current-carrying capacity and excellent field emission properties makes them an integral component [1,2,3] in the field of future electronics. The preparation of patterned CNTs also holds development potential for real-life applications, such as field-emission displays [4,5,6,7], CNT sensor chips [8,9,10], probes [11,12,13,14,15,16], and field-effect transistors [17,18,19]. To incorporate CNTs into nanoelectromechanical systems, effective and controllable patterned synthesis of CNTs on specific substrates is crucial. A variety of methods have contributed to the preparation of CNT patterning and are still in use today, including shadow masking, photolithography, electron beam lithography, and soft lithography [20,21,22,23,24,25,26,27,28].However, all such methods rely on the photolithographic process to produce even the most basic catalyst patterns. Further, such methods rely on the photolithographic process to coat CNTs in unsuitable materials and then synthesize arrays of patterned CNTs. Other methods employ anodic aluminum oxide nano-templates as a growth-limiting template [29] or utilize plasma etching [30,31] for post-synthesis processing. Though not reliant on the photolithographic process, these latter methods are limited to producing arrays that are tubular or fascicular in form and lack the control necessary to create the required patterns. Therefore, there is an urgent need to develop a low-cost CNT patterning process that features high pattern accuracy and quick and simple specimen handling.This study introduces a method that obviates the need for photolithography and its various time-consuming steps: evaporation, etching, and masking. Instead, it draws on certain polymer materials, transforming them into ink and using the resulting composition to form a barrier between the substrate and the catalyst, ferrocene (Merck, Darmstadt, Germany). This method allows for rapid CNT forest patterning and can be carried out using either ink printing or laser stripping. 2. ExperimentsThis experiment employed a high-

temperature furnace to synthesize vertically aligned carbon nanotubes using a three-step chemical vapor deposition (CVD) method. The furnace itself comprised three separate compartments or chambers: the sublimation, transition, and growth chambers; each of which allowed for independent temperature control. All three chambers also contained a heater and a thermocouple, with temperatures set at 250 °C, 400 °C, and 600 °C, respectively. In addition, glass fiber blankets were used for isolation, enabling each chamber to maintain its own distinct temperature. Substrates used in the experiment included commercially available household aluminum foil as well as silicon wafers, both of which were cleaned thoroughly with acetone, isopropanol, and deionized water, in that particular sequence, to remove any impurities from the substrate surface. The cleaned aluminum foil was placed on a quartz plate and then loaded into the growth chamber. Next, a quartz boat carrying ferrocene (Merck, Germany) was placed outside the sublimation chamber. Once the reaction began, it was moved into the chamber to undergo the CNT synthesis reaction. When the ferrocene was heated to over 400 °C it pyrolyzed to produce iron and carbon atoms, which were further used in the production of CNT. Specifically, iron atoms were used as catalysts, while carbon atoms constituted a partial carbon source for CNT forests. During the experiment, as the temperature began to rise, 1000 sccm of Ar and 250 sccm of H2 were funneled through a three-inch quartz tube; and the temperature increase duration was set to 20 min to ensure that all three chambers reached their respective temperatures. Next, the tube pumped through 50 sccm of C2H2, followed by powdered ferrocene, which was pushed through the tube to sublimate, and then moved with the gas flow into the growth chamber for the synthesis reaction, as shown in Figure 1. When the substrate used was aluminum foil, the temperature of the growth chamber was 600 °C; when using silicon wafers, the temperature was 750 °C. No other changes were made. However, the height of the nanotubes were affected by the growth time applied, but this can be adjusted as required. 3. Results and DiscussionsThe method adopted in this study was the floating catalyst method [32]. Moreover, the CNTs synthesized were Fe-filled Multi-wall carbon nan

otubes. When using aluminum foil as a substrate, the CNTs grew to a height of around 50 µm in 15 min; with silicon as a substrate, they grew to a height of around 250 µm in 15 min. Figure 2 presents a photograph obtained through transmission electron microscopy (TEM; JEOL JEM-2100F, Tokyo, Japan at 200 kV), they have an approximate diameter of tens of nm. Through observations made by way of scanning electron microscopy (SEM; JEOL JSM-6390, Tokyo, Japan at 15 kV), we found that the areas of samples coated in ink developed a thin film that directly impacted nanotube morphology, as shown in Figure 3a. Conversely, and as anticipated, CNT forests grew unimpeded in the areas where no ink was applied. We recognized the potential value of this phenomenon and inferred that a specific element that was present in ink caused a thin film to form on the surface of the sample before the ferrocene was sublimated. When the ferrocene reaches the test specimen, the film layer blocks it from coming in contact with the substrate just as the CNT synthesis reaction is set to occur. Moreover, as CNTs display growth selectivity during synthesis, they only grow on certain materials [33]. Therefore, by blocking contact between the catalyst and the substrate surface, the thin film effectively prevents the growth of CNTs.However, the consistency of the film also determines the degree to which growth is stifled. Thus, we can see that some catalysts still managed to drill through the bottom of the film layer and make contact with the substrate, producing a CNT forest which in turn ruptured the surface of the film. This is clearly evident in Figure 3b, while Figure 3c shows how the entire film structure was elevated by the CNT forest growing below it. We therefore believe that gaining a better grasp on the characteristics of this film layer, to the point of being able to effectively control its behavior, will be of substantial benefit to the process of CNT patterning.Our initial step was to obtain ink from numerous commercially available, oil-based markers and smear it across aluminum foil to conduct preliminary synthesis testing. (See Figure S1 in Supplementary Materials for details.) In the end, we found that the vast majority of tested inks produced the desired effect. Commercially available inks comprise various elements, including solvents

Growth of aligned carbon nanotubes on ALD-Al2O3 coated silicon and quartz substrates — Hengzhi Wang et al., 2011

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