Nitrogen incorporation into tetrahedral hydrogenated amorphous carbon

Structural changes induced by the incorporation of nitrogen into ta-C : H films have been studied by Electron Energy Loss Spectroscopy, X-Ray Photoelectron Spectroscopy, Fourier Transformed Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy. ta-C:H films have been synthesised using a low pressure Electron Cyclotron Wave Resonance (ECWR) source which provides a plasma beam with a high degree of ionisation and dissociation. Nitrogen was incorporated by adding N2 to the C2H2 plasma used for the deposition of ta-C : H films. The N/C atomic ratio in the films rises rapidly until the N2/C2H2 gas ratio reaches three, and then increases more gradually, while the deposition rate decreases steeply. Chemical sputtering of the forming films and the formation of molecular nitrogen within the films limit the maximum nitrogen content to about N/C = 0.6. For low nitrogen content the films retain their diamond-like properties, however as N/C atomic ratio increases, a polymeric-like material is formed, with >C=N- structures and terminating C=N and NH groups that decrease the connectivity of the network.

Bonding regimes of nitrogen in amorphous carbon

Nitrogen can have numerous effects on diamond-like carbon: it can dope, it can form the hypothetical superhard compound C3N4, or it can create fullerene-like bonding structures. We studied amorphous carbon nitrogen films deposited by a filtered cathodic vacuum arc as a function of nitrogen content, ion energy and deposition temperature. The incorporation of nitrogen from 10-2 to 10 at% was measured by secondary ion mass spectrometry and elastic recoil detection analysis and was found to vary slightly sublinearly with N2 partial pressure during deposition. In the doping regime from 0 to about 0.4% N, the conductivity changes while the sp3 content and optical gap remain constant. From 0.4 to approximately 10% N, existing sp2 sites condense into clusters and reduce the band gap. Nitrogen contents over 10% change the bonding from mainly sp3 to mainly sp2. Ion energies between 20 and 250 eV do not greatly modify this behaviour. Deposition at higher temperatures causes a sudden loss of sp3 bonding above about 150 °C. Raman spectroscopy and optical gap data show that existing sp2 sites begin to cluster below this temperature, and the clustering continues above this temperature. This transition is found to vary only weakly with nitrogen addition, for N contents below 10%.

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesized using a high plasma density electron cyclotron wave resonance (ECWR) technique using N2 and C2H2 as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N2/C2H2 gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp3-bonded carbon fraction decrease from 1.7 to 1.1 eV and approximately 65 to 40%, respectively. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (109 Ω cm) and moderate sp3-bonded carbon fraction (40%), they should be classed as polymeric in nature.

Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films

A balanced planar r.f. powered magnetron sputter source has been used to deposit carbon nitride films from a graphite target under various conditions. Sample temperature, bias voltage and nitrogen content in the gas mixture were varied. The effects of oxygen, methane and ammonia on the film growth were also studied. Special attention was paid to the effects of the deposition parameters on the structure of the films, in particular the hybridisation of the carbon and nitrogen bonding. The chemical bonding of the carbon and nitrogen atoms was studied by electron energy loss spectroscopy (EELS). The chemical composition was evaluated by Rutherford back-scattering. The intensity of transitions to π antibonding orbitals, as revealed by EELS, was found to increase with the nitrogen content in the films. Ion bombardment of the films during growth and the addition of oxygen or hydrogen-rich gases further increased the proportion of π bonds of both the carbon and nitrogen atoms. It is suggested that the increase in the transitions to μ antibond orbitals is to be explained by increased sp2 or possibly sp hybridisation of the carbon and nitrogen. Also, the effect of annealing on the bonding of nitrogen rich films after deposition was tested. The changes caused by nitrogen and deposition conditions are consistent with previous reports on the formation of paracyanogen structures.

Field emission from tetrahedrally bonded amorphous carbon

The field emission behaviour of a series of Tetrahedrally Bonded Amorphous Carbon (ta-C) films has been measured. The films were produced using a Filtered Cathodic Vacuum Arc System. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have a threshold field of order 10-15 V/μm and optimally nitrogen-doped films exhibit fields as low as 5 V/μm. The emission as a function of back contact and front surface condition has also been considered and shows that the back contact has only a minor effect on emission efficiency. However, after etching in either an oxygen or hydrogen plasma, the films show a marked reduction in threshold field, down to as low as 2-3 V/μm, and a marked improvement in emission site density.

Tetrahedrally bonded amorphous carbon for field-emission displays

Field emission from a series of tetrahedrally bonded amorphous-carbon (ta-C) films, deposited in a filtered cathodic vacuum arc, has been measured. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have threshold fields of the order 10-15 V/μm and optimally nitrogen doped films exhibited fields as low as 5 V/μm. In order to gain further understanding of the mechanism of field emission, the films were also subjected to H2, Ar, and O2 plasma treatments and were also deposited onto substrates of different work function. The threshold field, emission current, emission site densities were all significantly improved by the plasma treatment, but little dependence of these properties on work function of the substrate was observed. This suggests that the main barrier to emission in these films is at the front surface.

Optical gap in carbon nitride films

In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N2 plasma source, N was introduced into sp3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the optical gap initially decreases as the N content and the sp2 fraction rises, but above a certain N quantity there is a level-off of the value, and the gap then remains constant despite further increases in the fraction and clustering of the sp2 phase. However, for a-CN films the optical gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band gap is associated to an increase in the sp2 fraction or its clustering. Here we discuss the conditions that lead to high optical gap in sp2-bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra. © 2003 Elsevier Science B.V. All rights reserved.

Preliminary model development for predicting strength and stiffness of cement-stabilized soils using artificial neural networks

This paper presents ongoing work on data collection and collation from a large number of laboratory cement-stabilization projects worldwide. The aim is to employ Artificial Neural Networks (ANN) to establish relationships between variables, which define the properties of cement-stabilized soils, and the two parameters determined by the Unconfined Compression Test, the Unconfined Compressive Strength (UCS), and stiffness, using E50 calculated from UCS results. Bayesian predictive neural network models are developed to predict the UCS values of cement-stabilized inorganic clays/silts, as well as sands as a function of selected soil mix variables, such as grain size distribution, water content, cement content and curing time. A model which can predict the stiffness values of cement-stabilized clays/silts is also developed and compared to the UCS model. The UCS model results emulate known trends better and provide more accurate estimates than the results from the E50 stiffness model. © 2013 American Society of Civil Engineers.

Development of predictive models for cement stabilized soils

Factors that affect the engineering properties of cement stabilized soils such as strength are discussed in this paper using data on these factors. The selected factors studied in this paper are initial soil water content, grain size distribution, organic matter content, binder dosage, age and curing temperature, which has been collated from a number of international deep mixing projects. Some resulting correlations from this data are discussed and presented. The concept of Artificial Neural Networks and its applicability in developing predictive models for deep mixed soils is presented and discussed using a subset of the collated data. The results from the neural network model were found to emulate the known trends and reasonable estimates of strength as a function of the selected variables were obtained. © 2012 American Society of Civil Engineers.

Properties of two model soils stabilized with different blends and contents of GGBS, MgO, lime, and PC

This paper addresses the use of ground granulated blast furnace slag (GGBS) and reactive magnesia (MgO) blends for soil stabilization, comparing them with GGBS-lime blends and Portland cement (PC) for enhanced technical performance. A range of tests were conducted to investigate the properties of stabilized soils, including unconfined compressive strength (UCS), permeability, and microstructural analyses by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influence of GGBS:MgO ratio, binder content, soil type, and curing period were addressed. The UCS results revealed that GGBS-MgO was more efficient than GGBS-lime as a binder for soil stabilization, with an optimum MgO content in the range of 5-20% of the blends content, varying with binder content and curing age. The 28-day UCS values of the optimum GGBS-MgO mixes were up to almost four times higher than that of corresponding PC mixes. The microstructural analyses showed the hydrotalcite was produced during the GGBS hydration activated by MgO, although the main hydration products of the GGBS-MgO stabilized soils were similar to those of PC. © 2014 American Society of Civil Engineers.

Strength correlations of cement-mixed soils using data mapping

Development of comparisons and correlations between the unconfined compressive strength (UCS) and the undrained triaxial compressive strength, qu, is essential for generalising performance and optimising the design of cement-stabilised soils. This paper introduces current work in collecting and collating data from a number of research projects involving both laboratory strength tests performed on identical cement-stabilised soil samples. The research project on cement-stabilised Singapore marine clays at the National University of Singapore has been used as an example to explain the work on comparing and correlating results from both tests by normalising data and constructing contour plots. The effect of variables on strength comparison and correlations was evaluated. The variation in strength correlations was found to be dependent on a number of factors including: soil properties, cement content, curing time and stress, total water/cement ratio, confining stress and strain rate. The results showed that at ~ 100 kPa confining stress, UCS and qu, had similar magnitudes. Correlations between strengths and other design variables are discussed and presented.