Brian C. Holloway

Free-standing subnanometer graphite sheets

Free-standing graphite sheets with thickness less than 1nm, “carbon nanosheets,” were synthesized on a variety of substrates by radio-frequency plasma-enhanced chemical vapor deposition without any catalyst or special substrate treatment. The nanosheets consist of one to three graphene layers with a large smooth surface topography, standing roughly vertical to the substrate. Due to the atomic thickness and corrugated nature of nanosheets, low-energy vibrational modes are present in the Raman spectra. The low turn-on field of 4.7 V/μm for electron field emission suggests that the carbon nanosheets could be used as a potential edge emitter.

Experimental comparison of N(1s) X-ray photoelectron spectroscopy binding energies of hard and elastic amorphous carbon nitride films with reference organic compounds

Abstract In this work, hard and elastic amorphous carbon nitride (a-CNx) films were deposited by DC magnetron sputtering on heated Si(001) substrates at 400xa0°C. Nanoindentation results confirmed that the films were highly compliant and had high elastic recovery. X-ray photoelectron spectroscopy (XPS) was used to investigate nitrogen bonding by directly comparing the N(1s) spectra of a-CNx with the N(1s) peak positions of a variety of organic compounds that were characterized in the same XPS system. The N(1s) XPS spectra of hard and elastic a-CNx is resolved into two dominant intensity contributions at 398.5 and 400.6 eV. We show that the N(1s) spectra of a-CNx do not conclusively support a film-structure model with nitrogens bonded to sp3 carbons. We offer an alternate interpretation based on the presented data and previous XPS, nuclear magnetic resonance (NMR), and computational work. Together, the data suggest that hard and elastic a-CNx consists of an sp2 carbon network and that single-atom vacancy defects, as found in a graphite layer, may be present in the material. This implies that the low binding energy N(1s) component at 398.5 eV may be due to pyridine-like nitrogen bonded at the perimeter of a vacancy defect.

High field emission reproducibility and stability of carbon nanosheets and nanosheet-based backgated triode emission devices

The authors have characterized field emission properties of freestanding, 1nm thick graphene layers, called carbon nanosheets (CNSs), which were grown perpendicular to the growth surface using a radio-frequency plasma-enhanced chemical vapor deposition technique. The CNSs are metallic impurity-free and have uniform height distribution (standard deviation of 200h at 1.3mA emission current level. Over this time, no degradation has been observed, the variability of the individual I-V curves is small among 7216 voltage cycles, and the standard deviation at the maximum current was no more than 2.3%. A nanosheet-based backgated triode emission device has been developed to take advantage of the nanosheet field emission performance. Prototype devices have confirmed triode operation and stable electron emission.

Interpretation of x-ray photoelectron spectra of elastic amorphous carbon nitride thin films

We report the synthesis and characterization of amorphous carbon nitride (CNx) thin films using a direct current magnetron reactive sputter system. Nanoindentation of the CNx films and amorphous carbon films deposited under similar conditions shows the CNx films are extremely elastic, that the addition of nitrogen fundamentally changes the mechanical properties of the films, and that traditional methods of calculating the hardness and Young’s modulus may not be valid. X-ray photoelectron spectroscopy (XPS) of the N(1s) and C(1s) core levels show multiple bonding arrangements. In a new interpretation of the XPS data, the two predominant N(1s) spectral features have been identified, based on comparison to reference data in the literature, as those belonging to nitrogen in a four-bond arrangement and nitrogen in a three-bond arrangement, independent of hybridization. The formation of a fourth bond allows nitrogen to substitute for C atoms in a carbon-based graphitic system without the formation of dangling b...

Thermal desorption of hydrogen from carbon nanosheets

Carbon nanosheets are a unique nanostructure that, at their thinnest configuration, approach a single freestanding graphene sheet. Temperature desorption spectroscopy (TDS) has shown that the hydrogen adsorption and incorporation during growth of the nanosheets by radio frequency plasma-enhanced chemical vapor deposition are significant. A numerical peak fitting to the desorption spectra (300-1273 K) via the Polanyi-Wigner equation showed that desorption followed a second order process, presumably by the Langmuir-Hinshelwood mechanism. Six peaks provide the best fit to the TDS spectra. Surface desorption activation energies were determined to be 0.59, 0.63, and 0.65 eV for the external graphite surface layers and 0.85, 1.15, and 1.73 eV for desorption and diffusion from the bulk. In contrast to TDS data from previously studied a-C:H films [Schenk et al. J. Appl. Phys. 77, 2462 (1995)], a greater amount of hydrogen bound as sp(2) hybridized carbon was observed. A previous x-ray diffraction study of these films has shown a significant graphitic character with a crystallite dimension of L(a)=10.7 nm. This result is consistent with experimental results by Raman spectroscopy that show as-grown carbon nanosheets to be crystalline as commercial graphite with a crystallite size of L(a)=11 nm. Following TDS, Raman data indicate that the average crystallite increased in size to L(a)=15 nm.

Structural characterization of carbon nanosheets via x-ray scattering

The structure of carbon nanosheets deposited by radio frequency plasma-enhanced chemical-vapor deposition at different substrate temperatures is investigated via x-ray scattering. Carbon nanosheets consist of vertically aligned graphene-layer stacks, one to nine layers thick, which can attain micron-scale lengths. Histograms of both molecule length and thickness are generated by fitting the experimental data with a linear combination of x-ray scattering intensities, which are calculated for rhombus-shaped molecules of different dimensions. These histograms show that the average uncorrugated length within a nanosheet decreases from 107A at a 670°C deposition temperature to 50A at 950°C. The distribution of nanosheet thickness remains qualitatively similar at each deposition temperature, but decreases from an average of eight graphene layers at 670°C to about six layers at 950°C. With increasing temperature large nestlike structures are observed, but are found to consist of the same nanosheet constituents i...

Analysis of diamond-like carbon and Ti/MoS2 coatings on Ti–6Al–4V substrates for applicability to turbine engine applications

Abstract Ti–6Al–4V substrates have been coated by diamond-like carbon (DLC) films, with no surface pretreatment, and have been coated by Ti/MoS 2 films, with a simple surface pre-cleaning. The DLC films were deposited by planar coil r.f. inductively-coupled plasma-enhanced chemical vapor deposition (r.f. ICPECVD); the Ti/MoS 2 films were deposited by magnetron sputtering. Both the DLC and Ti/MoS 2 films were characterized by pull tests, hardness tests, scanning electron microscopy (SEM), and wear tests (pin-on-disk and block-on-ring) to compare their adhesion, hardness, surface topology, and wear properties to plasma-sprayed Cu–Ni–In coating currently used for turbine engine applications. The DLC films were easily characterized by their optical properties because they were highly transparent. We used variable-angle spectroscopic ellipsometry (VASE) to characterize thickness and to unequivocally extract real and complex index of refraction, providing a rapid assessment of film quality. Thicker coatings yielded the largest hardness values. The DLC coatings did not require abrasive pretreatment or the formation of bond-layers to ensure good adhesion to the substrate. Simple surface pre-cleaning was also adequate to form well-adhered Ti/MoS 2 on Ti–6Al–4V. The results show that the DLC and Ti/MoS 2 coatings are both much better fretting- and wear-resistant coatings than plasma-sprayed Cu–Ni–In. Both show excellent adhesion to the substrates, less surface roughness, harder surfaces, and more wear resistance than the Cu–Ni–In films.

Role of delocalized nitrogen in determining the local atomic arrangement and mechanical properties of amorphous carbon nitride thin films

We report the results of a comprehensive study of the mechanical properties and chemical structure of carbon nitride thin films deposited by magnetron sputtering. Nanoindentation measurements show that the films are very elastic. Using a modified Oliver–Pharr method, to account for the elasticity of the films, the reduced elastic modulus was calculated to fall in the range of 35–55 GPa for all films measured. No quantifiable plastic deformation was measured in the tested films, therefore the hardness calculated (6 GPa) can only be assumed to be a lower bound. Fourier transform infrared spectroscopy of the carbon nitride films indicates the presence of large amounts of single and double bonds, and very small amounts of triple bonding, between carbon and nitrogen. Near edge x-ray absorption fine structure data suggests that there are three predominant types of bonds between carbon and nitrogen. This is consistent with the x-ray photoelectron spectroscopy data which shows N(1s) and C(1s) core spectra with mu...

Buried-Line Back-Gated Triode Field Emission Devices

Reported in this paper is recent work on a new type of back-gated triode device. Fabrication of the device and modeled performance predictions are discussed

Field emission observation of carbon nanosheet thin film by photoelectron emission microscopy (PEEM)

In this study, the field emission characterization of carbon nanosheet thin film was conducted using a diode configuration with an anode-cathode distance of 254 mum. Photoelectron emission microscopy (PEEM) was used to investigate the field emission uniformity over the surfaces of carbon nanosheet thin films.