Modification of emission properties of diamond films due to surface treatment process

The modification on the characteristics of the chemical vapor deposited (CVD) diamond films due to H₂-plasma post-treatment, SiO₂-coating and Cr-coating has been examined. A negatively biased H₂-plasma post-treatment process leads to pronounced modification on the morphology, Raman spectroscopy and electron field emission properties of the diamond films, whereas a positively biased H₂-plasma post-treatment process changes these characteristics insignificantly. The emission current density of the diamond films increased markedly to 162.1 μA cm⁻² due to negatively biased H₂-plasma etching, which is presumably caused by the formation of nano-sized columnar grains resulted from the etching of diamonds. On the other hand, the electron field emission capacity of the diamond films was completely suppressed due to the coating and chemical etching of SiO₂ layer, which is ascribed to the formation of a silicon-oxy-carbon (Si_1-x-yC_xO_2y) layer. Only when the SiO₂ layer is subjected to a negatively biased (−50 V) H₂-plasma etching process, can the electron field emission capacity of the diamond films be fully recovered. In contrast, emission current density (J_e) was increased substantially to 642 μA cm⁻² and the turn-on field (E₀) was lowered from 10.2 to 5.8 V μm⁻¹ due to Cr-coating. Scanning electron microscopic (SEM) and Auger electron spectroscopic (AES) examinations reveal that the main factor improving these behaviors for the Cr-coated diamond films is the formation of diamond-like carbon films on their top surface.     

Fluorescence and UV techniques for studying neck growth and equilibration processes during latex film formation

Steady state fluorescence (SSF) and UV–visible techniques have been used to study neck growth and equilibration processes during the coalescence of hard latex particles. Latex films were prepared separately by annealing pyrene (P_y) labelled and unlabelled poly(methyl methacrylate) (PMMA) particles above their glass transition temperature. During the annealing processes, the optical clarity of the films increased considerably. Direct fluorescence emission of excited pyrene from labelled latex films was monitored as a function of annealing temperature to detect this change. Void closure temperature (T_c) and time (t_c) were determined at the point where the fluorescence emission intensity became maximal. Below this point, the increase in fluorescence intensity (I_op) against temperature was used to determine the activation energy for viscous flow (ΔH≈47kcalmol⁻¹). The decrease in I_op above the void closure temperature was used to determine the backbone activation energy (ΔE≈44kcalmol⁻¹) for the interdiffusing chains. Unlabelled PMMA particles were used to prepare films for UV–vis measurements. The transmitted photon intensity (I_tr) from these films increased as the annealing temperature was increased. This behaviour was also used to determine the backbone activation energy (ΔE≈35kcalmol⁻¹) for the interdiffusing chains. © Society of Chemical Industry.     

Characterization of n-hexadecylalkylamine-intercalated graphite oxides as sorbents

Adsorption properties of graphite oxides hydrophobized by n-hexadecylamine, (C16)_xGO, were investigated using pyrene molecules as a model of nonionic organic contaminants. A large quantity of pyrene (28.5 mg/g) was adsorbed from a water-ethanol mixture (1:2) containing 2 mM of pyrene when (C16)_0.6GO was used. The isotherm of pyrene adsorption was better described by Freundlich equation rather than Langmuir equation, which indicated a single adsorption mechanism was involved. The change in the amount of adsorbed pyrene as a function of amine content in GO was very similar to that which occurs upon introduction of pyrene into (C16)_xGO films from chloroform solution, as determined by X-ray measurements. This suggests that pyrene molecules were adsorbed not only on the outer surface but also within the interlayer space of the intercalation compound. Swelling of the intercalation compound by ethanol can render the interlayers more organophilic and make access to hexadecylamine molecules bonded to the graphite oxide layer easier for pyrene molecules, especially in (C16)_xGOs with lower amine contents.     

Void closure by viscous flow during coalescence of hard latex particles

Steady state fluorescence (SSF) technique was used to study the void closure process during coalescence of hard latex particles. Latex films were prepared by annealing pyrene (P_y)-labelled poly(methyl methacrylate) particles above the glass transition temperature. During the annealing processes, the optical clarity of the film increased considerably. Direct fluorescence emission from excited pyrene was monitored as a function of annealing temperature to detect these changes. Scanning electron microscopy in conjunction with Monte Carlo simulations of photon diffusion in latex film were used to interpret the fluorescence results. Void Closure temperature (T_c) and time (t_c) were measured at the point at which the fluorescence emission intensity becomes maximum. This was associated with the longest optical path of a photon in latex film. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 651–659, 1997     

Charge density threshold for LbL self-assembly and small molecule diffusion in polyelectrolyte multilayer films

Polyelectrolyte multilayer films were prepared via the layer-by-layer self-assembly using poly(diallydimethylammonium chloride) (PDADMAC) and pyrene labeled polyanions of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and N,N-dimethylacrylamide copolymers with different charge densities (AMPS mole fraction) F_AMPS=0.20-0.999. Multilayer growth with alternating deposition from salt-free polyelectrolyte solutions was monitored by fluorescence intensity and film thickness, showing a charge density threshold between 0.574 and 0.711 for our polyanions, below which the multilayer cannot be formed due to desorption in the following depositions. For the fully charged polyanion, thickness of the multilayer film increased with increasing NaCl concentration in the deposition solution; while for other polyanions with lower F_AMPS, little growth in multilayer films was found when NaCl concentration was 0.02 mol/L or higher. The quenching rate of nitromethane to the pyrene label in the multilayer film was adopted to detect the chain density and permeability of these films. Decreasing the charge density, increasing the salt concentration and reducing the layer number will accelerate the quenching rate. The former two is due to the formation of a looser structure in the multilayer films at low charge density of the polyelectrolytes or high salt concentration in the deposition solutions, while the last one is mainly due to the reduction of diffusion distance.     

Quantitative analysis of the cross-linked structure of microgels using fluorescent probes

Properties of polymeric microgels are influenced by the internal polymer cross-linked structure, but tools to quantitatively analyze this internal structure are limited. With the finding that polymer networks alter the diffusivity and subsequent excimer formation of pyrene, this study used the ratio between pyrene excimer and monomer emission to determine the number of cross-links (N) and average pore size (ξ) in poly(ethylene glycol) diacrylate (PEGDA) microgels. A calibration curve to relate pyrene emission to N and ξ in PEGDA hydrogels was prepared and used to calculate N and ξ in PEGDA microgels. The pyrene emission indicated that PEGDA microgels had a higher cross-linking density and a smaller average pore size when compared with bulk cross-linked hydrogels of the same PEGDA concentration. The analytical method demonstrated in this study may be useful for fine-tuning polymeric microgel properties for a broad array of applications.     

Films formed from polystyrene latex/clay composites: A fluorescence study

This study reports a steady-state fluorescence (SSF) technique for studying film formation from surfactant-free polystyrene (PS) latex and Na-montmorillonite (SNaM) composites. The composite films were prepared from pyrene (P)-labeled PS particles and SNaM clay at room temperature and annealed at elevated temperatures in 10-min intervals above glass transition temperature (t₃) of polystyrene. During the annealing processes, the transparency of the film improved considerably. Scattered light (I_s) and fluorescence intensity (I_p) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of composite films was monitored by using photon transmission intensity, I_tr. Scanning electron microscopy (SEM) was used to detect the variation in physical structure of annealed composite films. Minimum film formation temperature, T_q, and healing temperatures, T_h, were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that both activation energies increased as the percent of SNaM was increased in composite films.     

Film formation from nano‐sized polystrene latex covered with various TiO2 layers

Steady‐state fluorescence technique was used for studying film formation from TiO₂ covered nano‐sized polystyrene latex particles. The composite films were prepared from pyrene (P)‐labeled PS particles by covering them with various layers of TiO₂ at room temperature. These films then annealed at elevated temperatures in 10 min time interval above glass transition (T_g) temperature of polystyrene. Five different composite films were studied in various TiO₂ layer contents. Fluorescence emission intensity, I_P from P was measured after each annealing step to monitor the stages of film formation. Films present significant increase in I_P above the certain onset temperature called minimum film forming temperature, T₀. However, at higher annealing temperatures, I_P showed a decrease. Increase and decrease in I_P were modeled by void closure and interdiffusion processes and related activation energies were determined, respectively. Dissolution of annealed PS film, with high TiO₂ content presented a nice, ordered nano‐sized ceramic structure, which may predict the construction of nano‐layer photonic crystals. POLYM. COMPOS., 27:651–659, 2006. © 2006 Society of Plastics Engineers     

Fluorescence technique to study thickness effect on dissolution of latex films

In situ steady‐state fluorescence (SSF) measurements were used for studying dissolution of Latex films in real time. Latex films with various thicknesses are formed from pyrene (P) labeled poly(methyl methacrylate) (PMMA) latex particles, sterically stabilized by polyisobutylene (PIB). Annealing of latex films were performed above T_g at 220°C temperature for 60 min. UV‐Visible (UVV) spectrometer was used to measure the transparency of latex films. It was observed that thicker films formed more opaque films than thinner films. Heptane (20%), chloroform (80%) mixture was used as a dissolution agent. Diffusion of pyrene labeled PMMA chains was monitored in real time by the change of pyrene fluorescence intensity, I_P in the polymer‐solvent mixture. Diffusion coefficients, D, and relaxation constants, k₀, of PMMA chains were measured and found to be strongly dependent on the latex films thicknesses. It is observed that thicker and opaque films dissolved much faster than the thinner and transparent films. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1087–1095, 2000     

Functional polymers for colloidal applications. IV. Aggregate formation of lipophile-grafted water-soluble copolymers in aqueous solutions

A water-soluble styrene–maleic anydride copolymer (SMA) is derivatized with different lipophilic groups, butylamine and dodecylamine, with different degrees of substitution (5, 15, and 30%). These lipophile-grafted SMAs are water-soluble, and their solutions are transparent. A plot of I₁/I₃ as a function of polymer concentrations indicates the extent of aggregate formation. Surface tension methods also verify the existence of aggregates. It is found that the aggregates begin to form at concentrations below that of the polymer transfer to the air–water interface. The plots of I₁/I₃ as a function of polymer concentrations for SMAs of different molecular weights derivatized with different lipophile with varying degrees of substitution show that the polymers with a higher degree of substitution and lower molecular weights aggregate at lower concentrations. Polymers substituted by butylamine form aggregates at a very high concentration, independent of the degree of sub-stitution. These phenomena are interpreted in terms of hydrophobic interactions as in micelles formed from surfactants. The higher degree of dodecyl-substituted SMA solubilizes pyrene at higher concentrations, and these pyrene solubilized solutions show pyrene excimer emission spectra. These emission spectra are used to characterize the relative size and hydrophobicity of aggregates. © 1993 John Wiley & Sons, Inc.     

Self-association of hyperbranched poly(sulfone-amine) in water: studies with pyrene-fluorescence probe and fluorescence label

Both pyrene-fluorescence probe and fluorescence label techniques are used to investigate the association behaviors of hyperbranched poly(sulfone-amine) (HPSA) in aqueous solution. In the presence of HPSA, excimer emission peak evidently appeared, while no excimer peak was observed in the emission spectra in the absence of HPSA. The excitation spectrum monitored at excimer emission red shifts by about 38-40 nm compared to that monitored at monomer emission, which shows that the excimer is formed by preassociated pyrene chromophores. In the same concentration of pyrene, monomer emission of pyrene decreases but excimer emission increases with increasing the concentration of HPSA; the ratio of excimer-to-monomer emission intensity (I_E/I_M) gradually increases, reaches a critical point at 5-7 g/l, and sharply increases with the concentration. Pyrene-labeled hyperbranched poly(sulfone-amine) (Py-HPSA) was synthesized from 4-(1-Pyrene)butyroyl chloride and HPSA. The monomer emission and excimer emission of Py-HPSA show the concentration-quenching effect, while I_E/I_M increases monotonously, approaches a critical point, and then suddenly increases with increasing the concentration of Py-HPSA. Influences of acidity and solvents on the fluorescence emission were studied. In high concentrations of hyperbranched polymer, pH and DMSO significantly influence the emission of pyrene, and excimer peak disappears at 72% of DMSO fraction.     

Effect of non-diamond carbon content on the spatial distributions of emission sites of the diamond films

The influence of non-diamond carbon in diamond films on the spatial distributions of emission sites was investigated, and consequently the field emission mechanism was discussed. It was confirmed that when increasing the positive substrate bias or the CH₄/H₂ concentration ratio, the non-diamond carbon content in the resulting films was markedly increased and the surface morphologies of the films lost their unique facet shape. An increase of non-diamond carbon content improved significantly the field emission properties and caused a dramatic increase of emission sites. It was observed from the images of the films containing a small amount of non-diamond carbon that electron emission occurs predominantly at the sample edges and the reason for this phenomenon could be attributed to the surface electron emission. With increasing non-diamond carbon, the emission occurs at the center as well as at the edge of the sample. From this observation, it is proposed that the emission from central region was caused by a preferential transport through the conducting pathways such as grain boundaries.     

In situ fluorescence experiments for real-time monitoring of annealed high-T latex film dissolution

A new technique, based on steady-state fluorescence measurements, is introduced for studying dissolution of polymer films. These films are formed from naphthalene and pyrene labeled poly(methyl methacrylate) (PMMA) latex particles, sterically stabilized by polyisobutylene. Diffusion of solvent (chloroform) into the annealed latex film was followed by desorption of polymer chains. Annealing was performed above T_g at various temperatures for 30-min time intervals. Desorption of pyrene labeled PMMA chains was monitored in real time by the pyrene fluorescence intensity change. Desorption coefficients were found to be between 1 and 4 × 10⁻¹⁰ cm²/s and two different dissolution mechanisms were detected. © 1996 John Wiley & Sons, Inc.     

Percolation cluster on partially dissolving polymer film

Steady state fluorescence (SSF) technique was used for studying dissolution of UV-induced polymer films. These films are formed from poly(methyl methacrylate) (PMMA) chains labeled with pyrene (P) which was introduced as a fluorescence probe to monitor the dissolution processes. PMMA films in various ethylene glycol dimethacrylate (EGDM) content, cured by UV radiation, were dissolved in chlorofrom-heptane (20-80%) mixtures. Dissolution of the films were monitored in real-time by the P fluorescence intensity change in the solvent reservoir. It is observed that P intensity decreased dramatically above a critical EGDM content, which was atributed to the formation of a percolation cluster. The measured percolation threshold, (p_c=0.25) was found to be in accord with the bond percolation model. Desorption coefficients, D_d were measured for films prepared with various EGDM content and it was observed that D_d values are much lower above p_c than below p_c.     

Solubility and diffusion in latex films formed from high glass transition temperature particles

In situ steady-state fluorescence measurements were used to study the dissolution of polymer films. These films were formed from pyrene labeled poly(methyl methacrylate) (PMMA) latex particles that were sterically stabilized by polyisobutylene. Annealing was performed above the glass transition temperature at 180°C at 1-h time intervals for film formation. Desorption of pyrene labeled PMMA chains was monitored in real time by the change of pyrene fluorescence intensity. Dissolution experiments were performed in various solvents with different solubility parameters, δ, at room temperature. Diffusion coefficients, D, in various solvents were measured and found to be around 10⁻¹⁰ cm²/s. A strong relation between D and δ was observed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1493–1502, 1998     

The effect of solid content on latex coalescence and film formation: Steady-state energy transfer study with fluorescence labeled polymers

The steady-state fluorescence technique was used to examine the healing and interdiffusion of polymer molecules as a function of solid content during annealing of latex films above the glass transition (T_g). Films were prepared from a mixture of naphthalene (N)- and pyrene (P)-labeled poly(methy methacrylate) (PMMA) latex particles. Above T_g, interdiffusion of polymer chains was observed by detecting the steady-state energy transfer from excited naphthalene to pyrene molecules. Various latex films with different latex content were used to measure the critical occupation percent for the reliable steady-state fluorescence measurements. Diffusion activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains. © 1996 John Wiley & Sons, Inc.     

Polymer/carbon nanotube composite film formation: A fluorescence study

In this study, the effect of multi‐walled Carbon nanotube (MWNT) on film formation behavior of Polystrene (PS) latex film was investigated by using steady state fluorescence technique. Films were prepared by mixing of pyrene (P)‐labeled PS latex with different amounts of MWNTs varying in the range between 0 and 20 wt%. After drying, MWNT containing films were separately annealed above glass transition temperature (T_g) of PS ranging from 100 to 270°C for 10 min. In order to monitor film formation behavior of PS/MWNT composites, Scattered light (I_s) and fluorescence intensities (I_P) from P were measured after each annealing step to monitor the stages of film formation. At 0–20 wt% range of MWNT content films, minimum film formation (T_o), void closure (T_v), and healing, (T_h) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that while void closure activation energies increased, backbone activation energies decreased as the percent of MWNT is increased in the composite films. POLYM. COMPOS., 35:817–826, 2014. © 2013 Society of Plastics Engineers     

Effect of surface treatment on the electron field emission property of nano-diamond films

Nano-diamond films having grain size around 20 nm were deposited by bias enhanced growth (BEG) method. Different surface treatments were carried out to increase their field emission properties and their effects are clearly noticed. Surface morphology of different surface treated nano-diamond films was examined. There was no significant change in the curve of Raman spectra of different surface treated samples. Raman spectra were typically of similar nature to nano-diamond film. Field emission results were more interesting. Biased in hydrogen plasma treated nano-diamond film has shown best electron emission behavior and low turn-on-field (E₀). The turn-on-field of bias-treated nano-diamond film was 19.5 V/μm. The decrease of turn-on-field (6 V/μm) of biased treated nano-diamond film from as-grown BEG film was attributed to the formation of thin sp² layer and more defects on the surface of film by hydrogen ion bombardment. Moreover, hydrogen-plasma treated nano-diamond film was also found to be good for electron emission but there was no improvement in electron emission as in the case of air plasma treated nano-diamond films.     

Dissolution of latex films after γ‐ray treatment

Latex films were prepared by annealing pyrene (Py)‐labeled poly(methyl methacrylate) particles at glass‐transition temperature (100°C). These films were then irradiated by γ‐rays from ⁶⁰Co in a gamma cell at room temperature at the same dose rate (rad/h) for 30 min. Before dissolution films were annealed at elevated temperatures for a 30‐min time interval to complete the film formation process. Steady‐state fluorescence (SSF) technique were used to monitor the dissolution of these irradiated latex films. The dissolution of films in chloroform–heptane (80–20%) mixture was monitored in real time by the Py fluorescence intensity change. Relaxation constants k₀ and desorption coefficients D_d of polymer chains were measured. It was observed that both D_d and k₀ values first increased and then decreased by increasing the annealing temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 129–137, 2002     

Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency

This experimental work investigated major gaseous (CO and NO_x) and PAH emissions from a 400 kW_th fluidized-bed combustor with a cone-shaped bed (referred to as ‘conical FBC’) firing rice husk with high, over 99%, combustion efficiency. Experimental tests were carried out at the fuel feed rate of 80 kg/h for different values of excess air (EA). As revealed by the experimental results, EA had substantial effects on the axial CO and NO_x concentration profiles and corresponding emissions from the combustor. The concentration (mg/kg-ash) and specific emission (μg/kW h) of twelve polycyclic aromatic hydrocarbons (PAHs), acenaphthylene, fluorene, phenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene and indeno[1,2,3-cd]pyrene, were quantified in this work for different size fractions of ash emitted from the conical FBC firing rice husk at EA = 20.9%. The total PAHs emission was found to be predominant for the coarsest ash particles, due to the effects of a highly developed internal surface in a particle volume. The highest emission was shown by acenaphthylene, 4.1 μg/kW h, when the total yield of PAHs via fly ash was about 10 μg/kW h.