Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm⁻¹, an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm⁻¹, the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm⁻¹ is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.     

Structural and optical characterizations of nanostructured diamond films grown on Si(0 0 1) substrates

Nanostructured diamond films (NDFs) were grown on fine-polished Si(0 0 1) substrates by radio-frequency plasma-enhanced hot-filament chemical vapor deposition at low gas pressure of approximately 2 Torr. High resolution field-emission scanning electron microscopy (FESEM), X-ray diffraction and Raman scattering spectroscopy were employed to characterize the NDFs. FESEM measurement indicated that diamond nuclei density exceeded 10¹⁰ cm⁻² after growth for 20 min, which should render potential applications, such as in the fabrication of diamond-related field-emission devices. Photoluminescence excitation characterizations revealed the sharp optical absorption band edge of NDFs, and the band gap is 5.34 eV.     

Novel polyimide-based electrospun carbon nanofibers prepared using ion-beam irradiation

This paper describes a novel study focused on preparing carbon nanofibers, with a narrow fiber diameter distribution, from a fluorinated polyimide using both electrospinning and ion-beam irradiation. We specifically focused on the effects of ion species and ion fluences on the electrical conductivity of the nanofiber. The nanofibers were successfully prepared in the diameter range from 340 nm to 1500 nm by varying the concentration of polyimide solution using electrospinning. The Raman spectrum of the ion-irradiated nanofiber included the two well-known D (1360 cm^?1) and G (1580 cm^?1) peaks, indicating that the nanofiber surface changed to a carbon-enriched material. The carbon nanofibers underwent a more ordered graphitic carbon structure with an increase in the ion fluence and the electrical conductivity of the nanofiber irradiated at 1 × 10¹⁶ ions/cm² of Ar⁺ was 0.18 S/cm. In addition, the electrical conductivities of the ion-irradiated nanofibers increased in the order, He⁺ < Ne⁺ < Ar⁺, which indicated that the amount of nuclear energy in the ion species had the most influence on the electrical conductivity. However, the higher electrical conductivity of the carbon nanofibers is required to realize their industrial applications. This paper is the first to address the properties of the electrical conductivity of the carbon nanfibers prepared by electrospinning and ion irradiation as a new approach.     

Local field emission from individual vertical carbon nanofibers grown on tungsten filament

Individual high-aspect-ratio carbon nanofibers (CNFs) were grown on tungsten filament substrates by plasma-enhanced hot filament chemical vapor deposition. They are ∼100 nm in diameter and 6-30 μm in length with a density less than 10⁶/cm². The field emission property of single as-grown carbon nanofibers was measured in a scanning electron microscope equipped with a moveable nanoscale probe tip. The measurement results showed that the threshold field of single carbon nanofibers with different lengths was in the range of 4-5 V/μm with a corresponding emission current density of 20 μA/cm², but an evident difference in the enhancement of emitted current between nanofibers of different lengths could be found when the applied field was increased continuously. This indicates that the field emission property of single carbon nanofibers depends mainly upon their length, which is essentially attributed to the change of field enhancement factor of single carbon nanofibers. In addition, field emission of the different positions on the wall of a single carbon nanofiber was studied.     

Free-standing diamond films grown on cobalt substrates

Diamond films were grown directly on cobalt substrates, using microwave plasma-assisted chemical vapour deposition. Although cobalt is known to inhibit the nucleation of diamond and enhancing the formation of graphite, we were able to grow relatively thick films (∼190 μm). The films were easily detached from the substrates. The poor adhesion allows the possibility of obtaining free-standing diamond films without chemical etching. Micro-Raman spectroscopy showed the 1332 cm⁻¹ characteristic Raman peak of diamond and the 1580 cm⁻¹, 1360 cm⁻¹ bands of graphite, on the growth surface and backside of the films, respectively. Through scanning electron microscopy and X-ray diffraction we were able to monitor film thickness and morphology with growth evolution. The results showed the (111) preferential growth morphology for the film with higher growth rate. By energy dispersive X-ray spectroscopy it was only possible to detect cobalt in the back of the films, but not in the surface. The role of cobalt in the film growth is discussed.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

The characterization and application of p-type semiconducting mesoporous carbon nanofibers

The microstructures of mesoporous carbon nanofibers were characterized by scanning electron microscopy, transmission electron microscopy, nano-Raman, nitrogen adsorption-desorption and optical transmission. They possessed a high specific surface area 840 m² g⁻¹ and a 1.07 eV band gap. All mesoporous carbon nanofiber network can act as the channel material in p-type field-effect transistor devices with field-effect mobilities over 10 cm²/V s. Furthermore, mesoporous carbon nanofiber network exhibits better sensitivity and faster response to NO₂ gas than that of carbon nanotubes, which makes it a promising candidate as poisonous gas sensing nanodevices.     

Evolution of TiOx-SiOx nano-composite during annealing of ultrathin titanium oxide films on Si substrate

This paper discusses the formation of the TiO_x-SiO_x nano-composite phase during annealing of ultrathin titanium oxide films (~27 nm). The amorphous titanium oxide films are deposited on silicon substrates by sputtering. These films are important for high-k dielectrics and sensing applications. Annealing of these films at 750 °C in the O₂ environment (for 15–60 min) resulted in the polycrystalline rutile phase. The films exhibit Raman peaks at 150 cm⁻¹ (B_1g), 435 cm⁻¹ (E_g), and 615 cm⁻¹ (A_1g) confirming the rutile phase. The signature TO (1078 cm⁻¹) and LO (1259 cm⁻¹) infrared active vibrational modes of Si–O–Si bond confirms the presence of silicon-oxide. The X-ray photoelectron spectra of the TiO_x films show multiple peaks corresponding to Ti metal (453.8 eV); Ti⁴⁺ state (458.3 eV (Ti 2p_3/2) and 464 eV (Ti 2p_1/2)); and Ti³⁺ state (456.4 eV (Ti 2p_3/2) and 460.8 eV (Ti 2p_1/2)). The O1s XPS spectra peaks at 530–533 eV can be attributed to Ti–O and Si–O bonds of the TiO_x-SiO_x nano-composite phase in the annealed films. The depth profiling XPS study shows that the top surface of the annealed film is mainly TiO_x and the amount of SiO_x increases with the depth.     

Optical properties of impact diamonds from the Popigai astrobleme

Impact diamonds from Popigai astrobleme were found to consist of different carbon phases: cubic and hexagonal diamond with sp³ bonding according to X-ray structural analysis as well as amorphous, crystalline and disordered graphite with sp²-bonding (Raman scattering). The sizes of graphite domains vary from 10 to 100 nm. Fundamental absorption edge for Popigai impact diamonds is shifted ~ 0.5 eV to lower energies in comparison with kimberlite diamonds (5.47 eV) as a result of the lonsdaleite input, in good agreement with ab initio calculations (E_g = 5.34 and 4.55 eV for 3C cubic and 2H hexagonal diamonds, respectively). Yellowish color of impact diamonds is due to Rayleigh light scattering on structural defects whereas graphite is responsible for gray to black coloring. In the mid-IR region there is a multi-phonon absorption of 3C diamond in the 1800 to 2800 cm^− 1 range and some new bands at 969, 1102, 1225, and 1330 cm^− 1 in the one-phonon region. Micro-Raman study shows inclusions of side noncarbon minerals (quartz, magnetite, and hematite) some of which contain Cr^3 + impurity. The vibration modes of cubic diamond and lonsdaleite exhibited in the Raman spectra were elucidated by the first-principles studies. Popigai impact diamonds demonstrate a broad-band luminescence in 2.1, 2.38, and 2.84 eV components similar to that for nanocrystal polycrystalline 3C diamond. All emissions are excited at band-to-band transitions whereas the last two are observed also at excitation into 2.4 and 3.0 bands supposedly as a result of intracenter processes within the H3(NVN) and NV⁰ centers.     

Optical properties of ultrananocrystalline diamond/amorphous carbon composite films prepared by pulsed laser deposition

Ultrananocrystalline diamond (UNCD)/amorphous carbon (a-C) composite thin films were grown in ambient hydrogen by pulsed laser deposition using a graphite target, and their optical properties were determined by optical absorption spectroscopy and Raman scattering spectroscopy. Three optical bandgaps exist. Two bandgaps are indirect and their values were estimated to be 1.0 eV and 5.4 eV; these bandgaps correspond to the a-C surrounding the UNCDs and the UNCDs respectively. The third bandgap is direct and has a value of 2.2 eV, which significantly contributes to a large absorption coefficient, (10⁶ cm^− 1 at 3.0 eV). Possible origins of the third bandgaps are the grain boundaries (GBs) between the UNCDs and the a-C since they are specific to the UNCD/a-C composite films. The infrared absorption spectrum and the Raman scattering spectrum revealed the incorporation of hydrogen in the GBs. The hydrogen incorporated in the GBs might also have some influence on the appearance of the direct bandgap and its value.     

Surface-Enhanced Vibrational Spectroscopy: SERS and SEIRA

With respect to the origin of single-molecule sensitivity in surface-enhanced Raman scattering, elastic scattering and emission spectra were investigated for Ag particles adsorbed with dye. The scattering peak observed at 600–650 nm was extinguished during the inactivation process of an enormous SERS signal, whereas localized surface plasmon (LSP) peaks located at 520 nm and 730 nm did not change significantly. The scattering peak at 600–650 nm arises from increased electromagnetic coupling between the LSP of adjacent Ag particles through dye molecules. In addition, distinct emission peaks were observed at 550–600 nm and 600–750 nm for hot Ag particles with adsorbates. These bands were attributed to emissive relaxation of metal electrons and fluorescence of molecules, respectively. Furthermore, the shorter wavelength peak showed invariant Stokes shift irrespective of excitation wavelengths, most probably arising from inelastic scattering of excited electrons by adsorbed molecules. The adsorbed state of CO and related species on the Pt film electrode was investigated using attenuated total reflection—surface-enhanced infrared absorption spectroscopy. Intermediate species were found on the bare Pt surface in 1 mM CH₃OH + HClO₄ solutions at +0.2 V ≤ E ≤ +0.6 V that give absorption peaks at 1405 cm⁻¹ and 1300 cm⁻¹. These bands can be attributed to carbonate species or -COH. Water molecules located at the hydrophobic interfaces between CO and electrolyte solutions were evidenced by a quite high OH stretch absorption at 3664–3646 cm⁻¹, as well as a lower broad peak at ca. 3480 cm⁻¹.     

Inelastic neutron scattering,Raman and DFT investigations of the adsorption of phenanthrenequinone on onion-like carbon

The orientation and bonding of adsorbed 9,10-phenanthrenequinone (PQ) on onion-like carbon (OLC) were determined by combining spectroscopy and density functional theory (DFT) calculations. Electrochemical measurements demonstrated relatively strong bonding of PQ to the OLC as indicated by persistent and reversible features in the cyclic voltammetry. Spectra of bulk solid and adsorbed PQ were obtained by inelastic neutron scattering (INS) and Raman spectroscopy, and the bands were compared with vibrational energies calculated from DFT. At energy losses (frequencies) above 400 cm⁻¹, no band shifts in INS or Raman spectra were observed between bulk solid and adsorbed PQ. However, adsorption of PQ resulted in shifts in the lowest frequency modes (<400 cm⁻¹), compared to crystalline PQ, which could only be identified by INS. DFT calculations also provided adsorption energies and from these and comparison of computed and experimental spectra it is determined that the molecule adsorbs parallel to the onion-like carbon surface through π–π stacking interaction.     

Epitaxial growth of β-SiC on Si (100) by low energy ion beam deposition

Epitaxial β-silicon carbide (SiC) films on mirror-polished (100) Si substrates were deposited using low energy (150 eV) ion beam modification at different ion doses (1×10¹⁷/cm², 1×10¹⁸/cm², 1×10¹⁹/cm²). The Si substrates were exposed to a broad ion beam of hydrocarbon, argon, and hydrogen ions, generated in a Kaufman ion source. These ions reacted with the Si substrates and formed the epitaxial β-SiC films. Amorphous carbon films were generated on top of β-SiC films. Some voids were found with transmission electron microscopy (TEM) at the interface between the Si substrate and the β-SiC film. By studying the ion dose dependence of the quality and thickness of the SiC and amorphous carbon films, we suggest that the formation of the epitaxial β-SiC was driven by thermal diffusion of Si atoms in SiC and preferential etching of non-epitaxial SiC crystals by H ions.     

Encapsulation mechanism of N2 molecules into the central hollow of carbon nitride multiwalled nanofibers

Nitrogen molecules have been encapsulated into the central hollows of vertically aligned carbon nitride (CN) multiwalled nanofibers by dc plasma-enhanced chemical vapor deposition with C₂H₂, NH₃, and N₂ gases on a Ni/TiN/Si(1 0 0) substrate at 650 °C. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectra showed the existence of nitrogen molecules in CN nanofibers. Elemental mapping images with electron energy loss spectroscopy of the CN nanofiber and catalyst metal, and optical emission spectroscopy spectra of the plasma showed the distribution of nitrogen atoms and molecules in the CN nanofiber, catalyst metal, and gaseous precursor, respectively. These studies showed that atomic nitrogen diffused into the catalytic metal particle because of the concentration gradient and then saturated at the bottom of the particle. Saturated nitrogen atom participated in the formation of the CN nanofiber wall but most of nitrogen was trapped in the central hollow of the nanofiber as molecules.     

Optical Properties of GaSb Nanofibers

Amorphous GaSb nanofibers were obtained by ion beam irradiation of bulk GaSb single-crystal wafers, resulting in fibers with diameters of ~20 nm. The Raman spectra and photoluminescence (PL) of the ion irradiation-induced nanofibers before and after annealing were studied. Results show that the Raman intensity of the GaSb LO phonon mode decreased after ion beam irradiation as a result of the formation of the amorphous nanofibers. A new mode is observed at ~155 cm^-1 both from the unannealed and annealed GaSb nanofiber samples related to the A_1g mode of Sb–Sb bond vibration. Room temperature PL measurements of the annealed nanofibers present a wide feature band at ~1.4–1.6 eV. The room temperature PL properties of the irradiated samples presents a large blue shift compared to bulk GaSb. Annealed nanofibers and annealed nanofibers with Au nanodots present two different PL peaks (400 and 540 nm), both of which may originate from Ga or O vacancies in GaO. The enhanced PL and new band characteristics in nanostructured GaSb suggest that the nanostructured fibers may have unique applications in optoelectronic devices.     

Spectral characterization of lysozyme adsorption on dye-affinity beads

Cibacron Blue F3GA-attached magnetic poly(2-hydroxyethyl methacrylate) [mPHEMA] beads were prepared by suspension polymerization of HEMA in the presence of magnetite (Fe₃O₄) nanopowder. Average diameter size of the mPHEMA beads was 150–200 μm. The characteristic functional groups of Cibacron Blue F3GA-attached mPHEMA beads were analyzed by Fourier transform infrared spectrometer (FTIR) and Raman scattering spectrometer. The lysozyme adsorption and desorption characteristics of Cibacron Blue F3GA-attached mPHEMA beads were also investigated using FTIR and Raman spectroscopic techniques. When the Raman spectrum of lysozyme adsorbed mPHEMA is evaluated characteristic Amide-I band appears at 1657 cm⁻¹. The intensity of this band decreases in the spectrum of lysozyme desorbed mPHEMA sample. When the characteristic bands of lysozyme adsorbed and desorbed mPHEMA samples are compared, the band intensities of desorbed sample are lower than those of lysozyme adsorbed sample except for the band appearing at 656 cm⁻¹ (Tyr vC S). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Polymeric semiconducting carbon from fullerene by pulsed laser ablation

The polymeric semiconducting carbon films are grown on silicon and quartz substrates by excimer (XeCl) pulsed laser deposition (PLD) technique using fullerene C₆₀ precursor. The substrate temperature is varied up to 300 °C. The structure and optical properties of the films strongly depend on the substrate temperature. The grain size is increased and uniform polymeric film with improved morphology at higher temperature is observed. The Tauc gap is about 1.35 eV for the film deposited at 100°C and with temperature the gap is decreased upto 1.1 eV for the film deposited at 250 °C and increased to about 1.4 eV for the film deposited at 300 °C. The optical absorption properties are improved with substrate temperature. Raman spectra show the presence of both G peak and D peak and are peaked at about 1590 cm^− 1 and 1360 cm^− 1, respectively for the film deposited at 100 °C. The G peak position remains almost unchanged while D peak has changed only a little with temperature might be due to its better crystalline structure compared to the typical amorphous carbon films and might show interesting in device such as, optoelectronic applications.     

Raman spectroscopy measurements of DC-magnetron sputtered carbon nitride (a-C:N) thin films for magnetic hard disk coatings

As a protective coating for hard disks in magnetic storage applications, amorphous carbon nitride (a-C:N) thin films have proved superior to DLC (diamond-like carbon) a-C:H films in terms of durability, wear-resistance and adhesion properties. In this study, we present Raman spectroscopy investigations of a-C:N films which were produced by DC-magnetron sputtering systems. The layers were deposited with a variable nitrogen content, thickness and substrate temperature. Raman measurements were carried out with two different excitation lasers at wavelengths of 488 and 532 nm. The spectra show that besides the typical carbon D- and G-bands, two other characteristic bands are present at approximately 690 and 1090 cm⁻¹. The meaning and identification of these bands is not clear in the literature. In order to obtain more information, the films were also characterized by various analytical techniques, e.g. time-of-flight secondary ion mass spectrometry (ToF-SIMS), Auger electron spectroscopy (AES), ellipsometry, and n+k optical measurements. The Raman G-band position shows a systematic shift with the varying nitrogen content of the films. A comparison of layer thickness and the total area of D-, G- and 1090 cm⁻¹ bands also shows a significant correlation. The results offer Raman spectroscopy as a possible monitoring tool for carbon nitride coatings in the production of magnetic hard disk drives.     

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm⁻¹, observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm⁻¹ up-shifted replicas i.e. the bands at 178 and 190 cm⁻¹. A different behavior of these bands in the anti-Stokes side was observed.     

Raman study of the microstructure changes of phenolic resin during pyrolysis

After curing, phenol‐formaldehyde resins were post‐cured at 160°C, and then carbonized and graphitized from 300°C to 2400°C. The structure of the resulting carbonized and graphitized resins were studied using X‐ray diffraction and Raman spectroscopy. Thermal fragmentation and condensation of the polymer structure occurred above 300°C. The crystal size of the cured phenolic resins increased with an increase in temperature. The crystal size increased from 0.997 nm to 1.085 nm when the heat‐treatment temperature rose from 160°C to 500°C. Above 600°C, the original resin structures disappeared completely. Below 1000°C, the stack size (L_c) increased very slowly. The values increased from 0.992 to 1.192 nm when the heattreatment temperature rose from 600°C to 1000°C. Above 1000°C, the stack size showed an increase with the increase in heat‐treatment temperature. The values increased from 1.192 to 2.366 nm when the temperature rose from 1000°C to 2400°C. The carbonized and graphitized resins were characterized using Raman spectroscopy. The Raman spectrra were recorded between 700 and 2000 cm⁻¹. Below 400°C, there were no carbon structures in the Raman spectra analysis. Above 500°C, G and D bands appeared. Raman spectra confirmed progressive structure ordering as heat‐treatment temperature increased. The frequency of the G band of all carbonized and graphitized samples shifted to 1600 cm⁻¹ from the 1582 cm⁻¹ of graphite. At the same temperature, the D band shifted to 1330 cm⁻¹ from the 1357 cm⁻¹ of the imperfect carbon. In the curve fitting analysis of the Raman spectra, a Gaussian shaped band centered at 1165 cm⁻¹ was included. This band has not been described before in the literature and is attributed to disordered structures, which are formed from the original polymeric structures. These polymeric structures formed unknown disordered structures and remained in the carbonized phenolic resins. Above 1800°C, this band disappeared completely. But, a weak peak is present near 1620 cm⁻¹. This indicated that those disoriented molecules and some disordered carbons were removed as volatiles or repacked into the glassy carbon structures during graphitization. The carbonized and graphitized phenolic resins were found to correspond to low order sp² bonded carbon, but cannot be considered as truly glassy or amorphous carbon materials since they have some degree of order in the basal plane.