X.W. Liu

WELL-ALIGNED CARBON NITRIDE NANOTUBES SYNTHESIZED IN ANODIC ALUMINA BY ELECTRON CYCLOTRON RESONANCE CHEMICAL VAPOR DEPOSITION

Vertically aligned carbon nitride nanotubes with a uniform diameter of about 250 nm have been synthesized on a porous alumina membrane template (50–80 μm thick) in a microwave excited plasma of C2H2 and N2 using an electron cyclotron resonance chemical vapor deposition system. A negative dc bias voltage was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nanochannels of the alumina template. This allowed the physical, and subsequent chemical, absorption of species on the walls of the nanochannels that resulted in the formation of the carbon nitride nanotubes. The hollow structure and vertically aligned properties of the nanotubes have been clearly verified by field-emission scanning electron microscope images. The absorption band between 1250 and 1750 cm−1 in the Fourier transform infrared spectroscopy spectrum proves that nitrogen atoms have been incorporated into an amorphous network of carbon.

A novel approach to the formation of amorphous carbon nitride film on silicon by ECR-CVD

Amorphous carbon nitride films have been synthesized on silicon by using an ECR-CVD system equipped with a DC bias and a mixture of C2H2, N2 and Ar. Excess argon together with the application of DC bias can increase the ratio of nitrogen to carbon in the film up to 41% as determined by XPS. FTIR spectrum shows an absorption band between 1000 and 1700 cm−1 which proves the incorporation of nitrogen atoms into the amorphous network of carbon. The plasma chemistry of the system was also analyzed by OES to investigate the active chemical species that were involved in the formation of carbon nitride. The result indicated that the addition of excess argon (four times more than nitrogen) can effectively enrich the excited-state CN radicals which subsequently promotes the concentration of nitrogen in the amorphous carbon nitride film. This observation is likely due to the lower ionization energy of argon (15.8 eV), argons larger cross-section area for collision and its massive weight in comparison with the indispensable hydrogen gas as employed in the synthesis of other related materials.

Optical properties of amorphous carbon nitride synthesized on si by ECR-CVD

Amorphous carbon nitride films have been synthesized on silicon using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system combined with a negative dc bias and a mixture of C2H2, N2 and Ar as precursors. The refractive index and extinction coefficient of the amorphous carbon nitride films are characterized by N&K analyzer. The optical band gap energy derived by Taucs equation decays with increasing substrate negative dc bias, ECR-power, flow rate ratio of N2/C2H2, nitrogen to carbon ratio (N/C) determined by X-ray photoelectron spectroscopy (XPS) and with the increase of graphitic content was examined by Raman spectroscopy.

The crystalline properties of carbon nitride nanotubes synthesized by electron cyclotron resonance plasma

Abstract Carbon nitride nanotubes (CN-NT) have been synthesized by an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system with a mixture of C 2 H 2 and N 2 as precursors without using any catalyst. The carbon nitride nanotubes were synthesized in an anodic alumina membrane as template in which a packed array of parallel, straight and uniform channels with a diameter of approximately 50-nm and 30-μm thick exists. Samples were analyzed by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Preliminary results showed that the properties of the deposited carbon nitride nanostructures depend on process parameters, such as deposited temperature, ratio of the precursors and microwave power. The aligned nanostructures have been verified by FESEM and HRTEM. The FTIR spectra reveal that some of the carbon atoms may have possibly been substituted by oxygen atom in the carbon nitride nanotubes. From the XPS results, N is either bonded to two C atoms (sp 2 pyridine-like type) or to three (sp 3 urotropine-like type) in the hexagonal sheets. The Raman spectrum showed that carbon nitride nanotubes have a high degree of graphitization.

Optical and structural properties of the amorphous carbon nitride by ECR-plasma

Abstract Amorphous carbon nitride films have been synthesized on silicon using an electron-cyclotron-resonance chemical vapor deposition (ECR-CVD) system combined with a negative d.c. bias in a mixture of C2H2, N2 and Ar as precursors. The refractive index and extinction coefficient of the amorphous carbon nitride films were determined by the N&K analyzer. The optical band gap was derived according to Tauc’s equation. The optical constants (n, k) and structural properties of the amorphous carbon nitride films were studied as a function of the substrate negative d.c. bias, ECR-power, flow rate ratio (N2/C2H2), nitrogen to carbon ratio (N/C) by X-ray photoelectron spectroscopy (XPS), degree of graphitization (sp2 content) by Raman spectroscopy and the radicals’ density in the ECR-plasma by the optical emission spectrometer (OES). Through the relationship between OES measurements and the Raman studies with optical band gap, the result indicated that a progressive graphitization of the film occurs with increasing N2/C2H2, N/C, ECR-power and substrate negative d.c. bias. The optical constants (n, k) of the a-C:N films prove an unambiguous dependence of the optical band gap on the substrate negative d.c. bias for the ECR-power.

A novel form of carbon nitrides: Well-aligned carbon nitride nanotubes and their characterization

Well-aligned carbon nitride nanotubes were prepared with a porous alumina membrane as a template when using electron cyclotron resonance (ECR) plasma in a mixture of C 2 H 2 and N 2 as the precursor with an applied negative bias to the graphite sample holder. The hollow structure and good alignment of the nanotubes were verified by field-emission scanning electron microscopy. Carbon nitride nanotubes were transparent when viewed by transmission electron microscopy, which showed that the nanotubes were hollow with a diameter of about 250 nm and a length of about 50–80 μm. The amorphous nature of the nanotubes was confirmed by the absence of crystalline phases arising from selected-area diffraction patterns. Both Auger electron microscopy and x-ray photoelectron spectroscopy spectra indicated that these nanotubes are composed of nitrogen and carbon. The total N/C ratio is 0.72, which is considerably higher than other forms of carbon nitrides. No free-carbon phase was observed in the amorphous carbon nitride nanotubes. The absorption bands between 1250 and 1750 cm −1 in Fourier transform infrared spectroscopy provided direct evidence for nitrogen atoms, effectively incorporated within the amorphous carbon network. Such growth of well-aligned carbon nitride nanotubes can be controlled by tuning the ECR plasma conditions and the applied negative voltage to the alumina template.

The effect of argon on the electron field emission properties of α-C:N thin films

Abstract Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C2H2 and N2 was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm−1 compared with 9.5 V μm−1 of the film without the addition of Ar.

Effect of bias voltage on the formation of a-C:N nanostructures in ECR plasmas ☆

Amorphous carbon nitride (a-C:N) nanotubes and nanofibers on porous alumina templates were synthesized by an electron cyclotron resonance chemical vapor deposition system in which a variable negative d.c. bias was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nano-channels of the alumina template in microwave excited plasmas of C2H2 or N2. The aligned structures of a-C:N nanotubes or nanofibers were verified by field emission scanning electron microscopy. Transmission electron microscopy micrographs showed that a-C:N nanotubes and nanofibers were the size with a diameter of ∼100–250 nm and a length of ∼50–80 μm. The amorphous nature of the nanostructures was confirmed by the absence of crystalline phases arising from selected area diffraction patterns. X-ray photoelectron spectroscopy spectra indicated that a-C:N nanotubes and nanofibers were composed of nitrogen and carbon, and the N/C ratios could reach as high as 72%. The absorption bands between 1250 and 1750 cm−1 in Fourier transform infrared spectroscopy provided direct evidence for the presence of nitrogen atoms in the amorphous carbon network. The well-aligned a-C:N nanotubes and nanofibers are expected to have potential applications in optical, electronic and optoelectronic devices.

The effect of pressure control on a thermally stable a-C:N thin film with low dielectric constant by electron cyclotron resonance-plasma

Amorphous carbon nitride (a-C:N) thin films have been prepared on silicon as low-dielectric-constant materials using an electron cyclotron resonance plasma with an application of a negative r.f. bias to the silicon substrate in a mixture of C2H2 and N2 as precursors. The dielectric constants (k) of a-C:N thin films can be as low as 2.0 at 1 MHz in this work. The thermal stability of the film has been improved by the incorporation of nitrogen atoms into the amorphous carbon network with addition of Ar into precursors. The basic structure, composition and electronic properties of these films were analyzed by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), field emission scanning electron microscopy (FE-SEM), and dielectric constant measurements. The dielectric constant of a-C:N thin films can be significantly lowered by the formation of the cross-linked structure and tetrahedral CN bonds in the film due to the increase of the N/C ratios, and sp3-bonded carbon contents in the a-C:N thin films with the increase of precursor pressure from 5 to 8 mtorr.

Morphology and characterization of highly nitrogenated, aligned, amorphous carbon nano-rods formed on an alumina template by ECR-CVD

Abstract Highly nitrogenated amorphous carbon (a-C:N) nano-rods on a porous alumina template were synthesized using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system, in which a negative DC bias was applied to the graphite substrate holder to promote the flow of ionic fluxes through the nano-channels of the alumina template in a microwave-excited plasma of C 2 H 2 and N 2 as precursors. The aligned structure of a-C:N nano-rods was verified by field-emission scanning electron microscopy (FE-SEM). Transmission electron microscopy (TEM) micrographs of a-C:N nano-rods showed that the nano-rods are well aligned with a diameter of approximately 100–250 nm and a length of approximately 50–80 μm. The amorphous nature of the nano-rods was confirmed by the absence of crystalline phases arising from selected-area diffraction (SAD) patterns. X-Ray photoelectron spectroscopy (XPS) spectra indicated that these nano-rods were composed of nitrogen and carbon, and the N/C ratios could reach as high as 56%. The absorption bands between 1250 and 1750 cm −1 in Fourier-transform infrared (FTIR) spectra provided direct evidence for the effective incorporation of nitrogen atoms into the amorphous carbon network. Raman spectra showed the same feature, with a G-band at ∼1580 and a D-band at ∼1370 cm −1 in the amorphous carbon film. The well-aligned a-C:N nano-rods are expected to have potential applications in optic, electronic and optoelectronic devices.