Carbon Nanowall Films Research Articles

Carbon nanowall-based gas sensors for carbon dioxide gas detection

Carbon nanowalls (CNWs) have attracted significant attention for gas sensing applications due to their exceptional material properties such as large specific surface area, electric conductivity, nano- and/or micro-porous structure, and high charge carrier mobility. In this work, CNW films were synthesized and used to fabricate gas sensors for carbon dioxide (CO2) gas sensing. The CNW films were synthesized using an inductively-coupled plasma (ICP) plasma-enhanced chemical vapor deposition (PECVD) method and their structural and morphological properties were characterized using Raman spectroscopy and electron microscopy. The obtained CNW films were used to fabricate gas sensors employing interdigitated gold (Au) microelectrodes. The gas sensors were fabricated using both direct synthesis of CNW films on interdigitated Au microelectrodes on quartz and also transferring presynthesized CNW films onto interdigitated Au microelectrodes on glass. The CO2 gas-sensing properties of fabricated devices were investigated for different concentrations of CO2 gas and temperature-ranges. The sensitivities of fabricated devices were found to have a linear dependence on the concentration of CO2 gas and increase with temperature. It was revealed that devices, in which CNW films have a maze-like structure, perform better compared to the ones that have a petal-like structure. A sensitivity value of 1.18% was obtained at 500 ppm CO2 concentration and 100 °C device temperature. The CNW-based gas sensors have the potential for the development of easy-to-manufacture and efficient gas sensors for toxic gas monitoring.

Synthesis of carbon nanowalls by oxy-acetylene torch method

This work presents a relatively new method for the synthesis of carbon nanowalls (CNWs) based on oxy-acetylene torch as a function of deposition time. The morphological and structural properties of the obtained CNW films were studied by scanning electron microscopy and Raman spectroscopy. Changes in the morphology and structural properties of the CNW films depending on the synthesis time were revealed. Shorter growth times lead to the formation of thinner CNW films with a dense labyrinth-like structure, while longer growth times lead to thicker CNW films with a petal-like structure. In addition, this study opens up the possibility of synthesizing CNWs on a production scale, since the proposed method is relatively environmentally friendly and efficient from an economical point of view.

Wavelength‐Dependent Photocurrent Generation Efficiency in the Carbon Nanowall Films

Herein, the efficiency of generation of unipolar photocurrent pulses under action of obliquely incident nanosecond laser pulses is studied in carbon nanowall (CNW) films on silicon substrate depending on the direction of the wave vector and polarization of the laser radiation. The films consist of flake‐like graphite crystallites of nanometer thickness. Each of the crystallites comprises stacked graphene atomic sheets oriented mostly perpendicular to the substrate surface with random orientation in other directions. The angular and polarization dependencies of the longitudinal and transverse photocurrents at wavelengths of 266, 354.7, 532, and 1064 nm are measured. The longitudinal and transverse photocurrents are odd functions of the incidence angle, and, for a given angle of incidence, they are even and odd functions of the polarization azimuth, respectively. It is noteworthy that with the decrease in exciting radiation wavelength, the conversion coefficients of laser pulse power into longitudinal and transverse photocurrents increase and decrease, respectively. At wavelength of 266 nm, the transverse photocurrent changes its polarity, and the longitudinal photocurrent generated by s‐polarized radiation exceeds the p‐polarized radiation photocurrent. The obtained results are explained by morphology peculiarities of the CNW films and the surface photogalvanic effect photocurrent generation.

Effect of Electron and Proton Irradiation on Structural and Electronic Properties of Carbon Nanowalls.

In this work, a complex experimental study of the effect of electron and proton ionizing radiation on the properties of carbon nanowalls (CNWs) is carried out using various state-of-the-art materials characterization techniques. CNW layers on quartz substrates were exposed to 5 MeV electron and 1.8 MeV proton irradiation with accumulated fluences of 7 × 1013 e/cm2 and 1012 p/cm2, respectively. It is found that depending on the type of irradiation (electron or proton), the morphology and structural properties of CNWs change; in particular, the wall density decreases, and the sp2 hybridization component increases. The morphological and structural changes in turn lead to changes in the electronic, optical, and electrical characteristics of the material, in particular, change in the work function, improvement in optical transmission, an increase in the surface resistance, and a decrease in the specific conductivity of the CNW films. Lastly, this study highlights the potential of CNWs as nanostructured functional materials for novel high-performance radiation-resistant electronic and optoelectronic devices.

Physical properties of carbon nanowalls synthesized by the ICP-PECVD method vs. the growth time

Investigation of the physical properties of carbon nanowall (CNW) films is carried out in correlation with the growth time. The structural, electronic, optical and electrical properties of CNW films are investigated using electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, UV–Vis spectroscopy, Hall Effect measurement system, Four Point Probing system, and thermoelectric measurements. Shorter growth time results in thinner CNW films with a densely spaced labyrinth structure, while a longer growth time results in thicker CNW films with a petal structure. These changes in morphology further lead to changes in the structural, optical, and electrical properties of the CNW.

Functionalized Carbon Nanowalls as Pro-Angiogenic Scaffolds for Endothelial Cell Activation.

Substrates that can be utilized to assist in expression of the inherent characteristics of cells under in vitro conditions are necessary to mimic the in vivo scenario to the maximum extent possible. We demonstrate the application of functionalized carbon nanowalls (CNW) in facilitating human endothelial cell attachment and proliferation. The CNW films were grown on silicon substrates by surface-wave microwave plasma-enhanced chemical vapor deposition (SWMWPECVD) technique. These CNW were then functionalized using nitrogen (N2) gas plasma to form functionalized N2 doped CNW (CNW-N2). Characterization of CNWs revealed a uniform petal-like morphology with the individual CNW width measured to be 1-5 nm and the film thickness in the range of 10-12 μm. The N2 functionalized CNWs proved to be highly efficient in providing cell-anchorage to the endothelial cells. Profiles of major cytoskeletal proteins revealed a higher degree of expression in the functionalized CNWs depicting the maintenance of structural integrity of cells. Interestingly, the CNW-N2 substrate was found to promote pro-angiogenic factors in the cells, which is observed here for the first time, that could pave way for the utilization of this substrate for detailed studies of angiogenesis processes in vitro and further biomedical applications.

Synthesis of carbon nanowalls from a single-source metal-organic precursor.

In this work, the deposition of carbon nanowalls (CNWs) by inductively coupled plasma enhanced chemical vapor deposition (ICP-PECVD) is investigated. The CNWs are electrically conducting and show a large specific surface area, which is a key characteristic to make them interesting for sensors, catalytic applications or energy-storage systems. It was recently discovered that CNW films can be deposited by the use of the single-source metal-organic precursor aluminium acetylacetonate. This precursor is relatively unknown in combination with the ICP-PECVD deposition method in literature and, thus, based on our previous publication is further investigated in this work to better understand the influence of the various deposition parameters on the growth. Silicon, stainless steel, nickel and copper are used as substrate materials. The CNWs deposited are characterized by scanning electron microscopy (SEM), Raman spectroscopy and Auger electron spectroscopy (AES). The combination of bias voltage, the temperature of the substrate and the substrate material had a strong influence on the morphology of the graphitic carbon nanowall structures. With regard to these results, a first growth model for the deposition of CNWs by ICP-PECVD and aluminium acetylacetonate is proposed. This model explains the formation of four different morphologies (nanorods as well as thorny, straight and curled CNWs) by taking the surface diffusion into account. The surface diffusion depends on the particle energies and the substrate material and thus explains the influence of these parameters.

Characteristic Study of Boron Doped Carbon Nanowalls Films Deposited by Microwave Plasma Enhanced Chemical Vapor Deposition.

In this research, catalyst-free vertically aligned boron doped carbon nanowalls films were fabricated on silicon (100) substrates by MPECVD using feeding gases CH4, H2 and B2H6 (diluted with H2 to 5% vol) as precursors. The substrates were pre-seeded with nanodiamond colloid. The fabricated CNWs films were characterized by Scanning Electron Microscopy (SEM) and Raman Spectroscopy. The data obtained from SEM confirms that the CNWs films have different density and wall thickness. From Raman spectrum, a G peak around 1588 cm(-1) and a D band peak at 1362 cm(-1) were observed, which indicates a successful fabrication of CNWs films. The EDX spectrum of boron doped CNWs film shows the existence of boron and carbon. Furthermore, field emission properties of boron doped carbon nanowalls films were measured and field enhancement factor was calculated using Fowler-Nordheim plot. The result indicates that boron doped CNWs films could be potential electron emitting materials.

Raman spectral mapping of self-aligned carbon nanowalls

Carbon nanowalls (CNWs) are a nano-carbon material constructed with a few layers of graphene and thus may exhibit similar properties to graphene. We developed a self-alignment process for the creation of carbon nanowalls using graphoepitaxy, which can be used for the structural fabrication of field-effect transistors. We used Raman spectral mapping to study the mechanism of the self-alignment process. The dependence of growth on temperature and the use of Raman spectra enabled the observation of geometrical catalyst effects and the induction of defects for self-aligned regions. The same effects were observed when analyzing the dependence of deposition on time. For low-temperature growth, which corresponds to the initial growth, the growth of CNW flakes can be observed only on the edge of processed patterns. On the other hand, the self-alignment process should change the shape of the flakes, thereby affecting grain structures and inducing carrier scattering. The use of graphoepitaxy can therefore help in the initial growth, and then mainly induces defects in CNW films. Finally, these defects are cured with the overgrowth.

Optical and IR absorption of multilayer carbon nanowalls

Composition, optical and IR absorption spectroscopy, and complex characterization of multilayer carbon nanowall (CNW) coatings are studied. CNW coatings are made using multistep chemical vapor deposition (CVD) growth by DC discharge plasma enhancement method. Each of the growth steps leads to the formation of structurally and optically similar CNW layers with the average thickness of 1μm. Optical properties of CNW films reveal no polarization dependence. The absorption of a single CNW layer with the average thickness of 1μm has record values of 96–99% in the wavelength range from 0.4 to 10μm, while the mirror reflection from the CNW films decreases steadily with the films thickness. We also demonstrate that the designed CNW films are stable with respect to water and alcohol environment.

Carbon nanowalls: the next step for physical manifestation of the black body coating

The optical properties of carbon nanowall (CNW) films in the visible range have been studied and reported for the first time. Depending on the film structure, ultra-low total reflectance up to 0.13% can be reached, which makes the CNW films a promising candidate for the black body-like coating, and thus for a wide range of applications as a light absorber. We have estimated important trends in the optical property variation from sample to sample, and identified the presence of edge states and domain boundaries in carbon nanowalls as well as the film mass density variation as the key factors. Also we demonstrated that at much lower film thickness and density than for a carbon nanotube forest the CNWs yield one order higher specific light absorption.

Electrochemical anodic oxidation of nitrogen doped carbon nanowall films: X-ray photoelectron and Micro-Raman spectroscopy study

Unintentional nitrogen doped carbon nanowall (CNW) films were oxidized through anodic polarization in different applied potential windows, in a mild neutral K2SO4 electrolyte solution. Applied potentials in the range of [0–1], [0–1.5] and [0–2]V vs. SCE were explored. The films were characterized with X-ray photoelectron (XPS) and Micro-Raman spectroscopy, in order to investigate the surface chemistry and structural changes after treatment, respectively. The XPS analysis revealed that this electrochemical treatment leads to an increase of oxygen functional groups, and influences the nitrogen proportion and bonding configuration (such as pyridinic/pyridonic nitrogen) on the film surface at room temperature. In particular, an obvious enhancement of pyrrolic/pyridonic nitrogen doping of CNWs via electrochemical cycling in the range of [0–1.5] and [0–2] V vs. SCE was achieved. Such enhancement happened, because of the oxidation of nitrogen atoms in pyridine as a result of OH ions injection upon electrochemical cycling. Micro-Raman analysis indicates structural quality degradation with increasing the applied potential window. Moreover, the electrochemical capacitance of CNW films was increased after treatment in the range of [0–1] and [0–1.5] and decreased in the range of [0–2]V vs. SCE. The results show that harsh oxidation happened in the range [0–2]V.

Electrical properties of carbon nanowall films

The dark conductivity of carbon nanowall (CNW) films deposited by hot-wire CVD was investigated. The conductivity of CNW films increased with increasing the film thickness. The CNW films have wall structure stood on substrate and the wall structure grows after initial layer was formed. The conductivity of the wall structure was estimated from above result. The conductivity of the wall structure increased with increasing temperature. Based on these results, the electrical properties of CNW films were discussed.

Reactive Ion Etching of Carbon Nanowalls

Two-dimensionally standing graphene sheets, i.e., carbon nanowalls (CNWs), were synthesized on a Si substrate employing a capacitively coupled fluorocarbon plasma-enhanced chemical vapor deposition system together with H radical injection. To apply CNWs in electronic devices and/or membrane filters, we have demonstrated the reactive ion etching (RIE) of CNWs. RIE employing H2/N2 gases showed that the CNW films were anisotropically etched at a relatively high rate of more than 250 nm/min. However, the 10-nm-thick interface layer between a CNW film and the Si substrate remained and the interface layer was not completely etched. In contrast, RIE employing Ar/H2 gases enabled us to completely remove the interface layer. Ar/H2 RIE was also carried out from the bottom surface of CNW films after exfoliating them from the Si substrate. As a result, a free-standing CNW film of 550 nm thickness without an interface layer as a membrane filter was successfully formed.

Carbon nanowall growth on carbon paper by hot filament chemical vapour deposition and its microstructure

Carbon nanowall films were synthesized by plasma enhanced hot filament chemical vapour deposition on carbon paper, a three dimensionally structured material. The micro-structured nature of the carbon paper, which is composed of an irregular and open mesh of carbon fibres, allowed one to determine the microstructure of the carbon nanowalls both at the tip and at the fibre–nanowall base interface. The number of graphenes which pile up to form the structure of a single nanowall ranges from 1 to 2 at the tip up to several 10s at the base, making this material suitable to study and eventually exploit the electronic properties of graphenes on a macroscopic scale. Such material is promising for electrochemical applications.

Monolithic self‐sustaining nanographene sheet grown using plasma‐enhanced chemical vapor deposition

AbstractWe have fabricated carbon nanowalls (CNWs) composed of monolithic self‐sustaining nanographene sheets standing vertically on a Si substrate, using plasma‐enhanced chemical vapor deposition with a C2F6/H2 mixture. The crystallinity, evaluated by Raman spectroscopy and synchrotron X‐ray surface diffraction, and the electrical properties of the CNWs were improved by introducing O2 gas into the source gas mixture during the CNW growth process. The temperature dependence of the resistivity of the CNW films exhibited semiconductor behavior.

Effect of hydrogen dilution in preparation of carbon nanowall by hot-wire CVD

Carbon nanowall films prepared on the stainless steel substrates by hot-wire chemical vapor deposition using CH 4 with different hydrogen dilution ratios and structure variation in the CNWs against hydrogen dilution have been studied. In the scanning electron microscope images, the wall height and width in the samples prepared with the hydrogen dilution ratio, H 2/(CH 4 + H 2), between 10% and 25% were larger than that prepared without hydrogen dilution. In the Raman spectra for the samples prepared with the H 2/(CH 4 + H 2) below 25%, the intensity ratio of the G peak to the D peak, I G/ I D, increased with increasing the H 2/(CH 4 + H 2). In the samples prepared with the H 2/(CH 4 + H 2) over 25%, the wall size and the I G/ I D decreased.

DC plasma enhanced growth of oriented carbon nanowall films by HFCVD

Two dimensional graphitic carbon structures, commonly referred to as carbon nanowalls (CNW), are raising increasing interest in the scientific community. Their surface area is theoretically twice that of the closed boundary structures, such as carbon nanotubes, making them extremely attractive for chemical and biosensor applications. In this work CNW, with maximum longitudinal dimension ranging from 10 to 200 nm and wall thickness lower than 5 nm, have been grown in a HFCVD reactor on Si substrates. The growth precursors consisted of methane diluted into He noble gas. The effect of a DC plasma on the growth rate and film morphology was explored. The experimental setup consisted of a two grid system which allowed to vary independently the plasma voltage and current density on the substrate surface. An increase of growth rate was observed as the film thickness increased from a few nanometers to about 200 nm when the substrate current density was increased from 0 to 3.5 mA/cm 2 for 30 min.

Preparation and electron field emission of carbon nanowall by Cat-CVD

Carbon nanowall (CNW) films have been prepared by catalytic chemical vapor deposition using only CH 4 gas. In the SEM images of the samples prepared at a substrate heating temperature, T s, of 500 °C, the wall structures on the substrates were observed. In those prepared at T s of 400 °C, however, the wall structure was not observed. The electron field emission from the CNW films was observed at small electric field. The wall width and height increased when the Raman intensity ratio, I D/ I G, decreased. Then, the threshold electric field decreased. The smallest threshold electric field was 2.6 V/μm in this work.