Growth of Free-standing Diamond Films on Graphite Substrates with Ti Interlayers

Free-standing diamond films, deposited using DC Arc Plasma Jet CVD method onto graphite substrates with titanium interlayers, have been investigated. The Ti interlayers were deposited by arc ion plating equipments. The thickness, morphology and composite phase of Ti interlayers were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The titanium carbide (TiC) was detected in both sides of the interlayers, which played an important role with respect to reasonable adhesion with film and diamond nucleation. The semi-translucent diamond films were characterized by SEM and Raman spectrum. The sharp diamond peak with low intensity of amorphous carbon shows that diamond films have very high quality. The overall results suggest that plating Ti interlayer on graphite substrate is an effective way to obtain optical grade free-standing diamond films.

Growth of Free-Standing Diamond Films on Stainless Steel

Free-standing diamond films have been successfully deposited on stainless steel substrates using microwave plasma-assisted chemical vapor deposition. Although iron, which is the main element of stainless steel, is known to inhibit the nucleation of diamond and enhance the formation of graphite, we were able to grow relatively thick films （-1.2 mm）. The films were easily detachable from the substrates. The poor adhesion made it possible to obtain free-standing diamond films without chemical etching. Raman spectroscopy showed the 1332 cm^-1 characteristic Raman peak of diamond and the 1580 cm^-1, 1350 cm^-1 bands of graphite on the growth surface and backside of the films, respectively. By energy dispersive X-ray spectroscopy it was only possible to detect iron on the back of the films, but not on the surface. The role of iron in the film growth is discussed.

Preparation of free-standing diamond films for high frequency SAW devices

Free-standing diamond films were prepared by hot filament chemical vapor deposition (HFCVD) method under different conditions. Inter-digital transducers (IDTs) were formed on the nucleation sides of free-standing diamond films by photolithography technique. Then piezoelectric ZnO films were deposited by radio-frequency(RF) reactive magnetron sputtering to obtain the ZnO/diamond film structures. Surface morphologies of the nucleation sides and the IDTs were characterized by means of scanning electron microscopy (SEM), atomic force microscope (AFM) and optical microscopy. The results indicate that the surfaces of nucleation sides are very smooth and the IDTs are of high quality without discontinuity and short circuit phenomenon. Raman spectra show the sharp diamond feature peak at about 1 334 cm?1 and the small amount of non-diamond carbon in the nucleation side. X-ray diffraction (XRD) patterns of the structure of ZnO/diamond films show a strong diffraction peak of ZnO (002), which indicates that as-sputtered ZnO films are highly c-axis oriented.

Preparation of ZnO Thin Films on Free-Standing Diamond Substrates

Highly c-axis-oriented ZnO films were deposited successfully on the nucleation sides of free-standing diamond （FD） films by the direct current （DC） magnetron sputtering method. The effect of the sputtering parameters, such as power, gas pressure and sputtering plasma composition of Ar-to-O2, on the properties of ZnO thin films was investigated in detail. X-ray diffraction （XRD） measurements showed that, at a sputtering power of 200 W, gas pressure of 0.5 Pa and an Ar-to- O2 composition of 1：1, a higher intensity of the （002） diffraction peak and a narrower full width at half maximum （FWHM） were detected which meant high c-axis orientation and high quality of the ZnO films. To improve the quality of the ZnO film, a thin ZnO layer was pre-grown as a homo-buffer layer. XRD measurements showed that this buffer layer had a beneficial effect on the structural and morphological properties of the post-grown ZnO film.

Blue-Green Electroluminescence of Free-Standing Diamond Thin Films

Blue-green electroluminescence has been observed in free-standing diamond films which were deposited by microwave plasma assisted CVD on silicon substrates.The electroluminescence device is driven by a 60 Hz power supply.The threshold voltage was about 112 V peak-to-peak.The electroluminescence spectrum at room temperature,showed a blue-green band with the peak centered at 485nm suggesting band A type emission.Electroluminescence was also observed at 77K.

A review on polishing technology of large area free-standing CVD diamond films

Recently,with the rapid development of chemical vapor deposition(CVD)technology,large area free-standing CVD diamond films have been produced successfully.However,the coarse grain size on the surface and the non-uniform thickness of unprocessed CVD diamond films make it difficult to meet the application requirement.The current study evaluates several existing polishing methods for CVD diamond films,including mechanical polishing,chemical mechanical polishing and tribochemical polishing technology.

Thermal Stress in Free-standing Diamond Films with Cr Interlayer Destroyed

Thermal stress in large area free-standing diamond films was remarkable during the post-deposition cooling of direct current （DC） arc plasma jet chemical vapor deposition （CVD） process.In this research,the stress release caused by delamination of Cr interlayer was of great importance to ensure the integrity of free-standing diamond film.The effects of Cr interlayer on Mo substrate,namely composite substrate,on thermal stress were investigated.Thermo-mechanical coupling analysis of the thermal stress was applied by finite element analysis （FEA） using ANSYS code.It was found that the interlayer could be destroyed first by the large thermal stress,and then the stress could be released and the probability of diamond film crack initiation would be reduced.The stress concentration at the bent edge of diamond film was also discussed.In addition,diamond films deposited on Mo substrates with and without Cr interlayer were prepared by DC arc plasma jet CVD system and experimental measurements were used to characterize these films.It was found that composite substrate could be an effective method of growing free-standing crack-free diamond films by DC arc plasma jet CVD system when there is no special requirement to the film strength.

Edge effect during microwave plasma chemical vapor deposition diamond-film:Multiphysics simulation and experimental verification

This study focused on the investigation of the edge effect of diamond films deposited by microwave plasma chemical vapor de-position.Substrate bulge height△h is a factor that affects the edge effect,and it was used to simulate plasma and guide the diamond-film deposition experiments.Finite-element software COMSOL Multiphysics was used to construct a multiphysics(electromagnetic,plasma,and fluid heat transfer fields)coupling model based on electron collision reaction.Raman spectroscopy and scanning electron microscopy were performed to characterize the experimental growth and validate the model.The simulation results reflected the experimental trends observed.Plasma discharge at the edge of the substrate accelerated due to the increase in△h(△h=0-3 mm),and the values of electron density(n_(c)),molar concentration of H(C_(H)),and molar concentration of CH_(3)(C_(CH_(3)))doubled at the edge(for the special concave sample with△h=−1 mm,the active chemical groups exhibited a decreased molar concentration at the edge of the substrate).At=0-3 mm,a high diamond growth rate and a large diamond grain size were observed at the edge of the substrate,and their values increased with.The uniformity of film thickness decreased with.The Raman spectra of all samples revealed the first-order characteristic peak of dia-mond near 1332 cm^(−1).When△h=−1 mm,tensile stress occurred in all regions of the film.When△h=1-3 mm,all areas in the film ex-hibited compressive stress.

Synthesis of free-standing Ga_2O_3 films for flexible devices by water etching of Sr_3Al_2O_6 sacrificial layers

Flexible electronic devices have attracted much attention due to their practical and commercial value. Integration of thin films with soft substrate is an effective way to fabricate flexible electronic devices. Ga_2O_3 thin films deposited directly on soft substrates would be amorphous mostly. However, the thickness of the thin film obtained by mechanical exfoliation method is difficult to control and the edge of the film is fragile and easy to be damaged. In this work, we fabricated free-standing Ga_2O_3 thin films using the water-soluble perovskite Sr_3Al_2O_6 as a sacrificial buffer layer. The obtained Ga_2O_3 thin films were polycrystalline. The thickness and dimension of the films were controllable. A flexible Ga_2O_3solar-blind UV photodetector was fabricated by transferring the free-standing Ga_2O_3 film on a flexible polyethylene terephthalate substrate. The results displayed that the photoelectric performances of the flexible Ga_2O_3 photodetector were not sensitive to bending of the device. The free-standing Ga_2O_3 thin films synthesized through the method described here can be transferred to any substrates or integrated with other thin films to fabricate electronic devices.

STUDIES ON THE ELECTRICAL CONDUCTIVITY OF FREE-STANDING FILMS OF POLYANILINE

An insulate to metal transition was investigated based on the measurements of the dependence of the conductivity, activation energy on the protonation state of polyaniline (PANI). An isotropy in conductivity for stretched salt form of PANI was observed.For salt film of PANI, the Ⅰ-Ⅴ curve obeys Ohm's law, which shows a typical metal behavior, however, for base film or film with low protonation state, it can be explained by Space Charge Limited Current (SCLC). It is also found that the Ⅰ-Ⅴ curve of base film of PANI is independent of the work function of electrodes and the polymerization temperature.

Flexible free-standing graphene-like film electrode for supercapacitors by electrophoretic deposition and electrochemical reduction

Electrophoretic deposition in conjunction with electrochemical reduction was used to make flexible free-standing graphene-like films. Firstly, graphene oxide （GO） film was deposited on graphite substrate by electrophoretic deposition method, and then reduced by subsequent electrochemical reduction of GO to obtain reduced GO （ERGO） film with high electrochemical performance. The morphology, structure and electrochemical performance of the prepared graphene-like film were confirmed by SEM, XRD and FT-IR. These unique materials were found to provide high specific capacitance and good cycling stability. The high specific capacitance of 254 F/g was obtained from cyclic voltammetry measurement at a scan rate of 10 mV/s. When the current density increased to 83.3 A/g, the specific capacitance values still remained 132 F/g. Meanwhile, the high powder density of 39.1 kW/kg was measured at energy density of 11.8 W-h/kg in 1 mol/L H2SO4 solution. Furthermore, at a constant scan rate of 50 mV/s, 97.02% of its capacitance was retained for 1000 cycles. These promising results were attributed to the unique assembly structure of graphene film and low contact resistance, which indicated their potential application to electrochemical capacitors.

Evaluation on residual stresses of silicon-doped CVD diamond films using X-ray diffraction and Raman spectroscopy

The effect of silicon doping on the residual stress of CVD diamond films is examined using both X-ray diffraction (XRD) analysis and Raman spectroscopy measurements. The examined Si-doped diamond films are deposited on WC-Co substrates in a home-made bias-enhanced HFCVD apparatus. Ethyl silicate (Si(OC2H5)4) is dissolved in acetone to obtain various Si/C mole ratio ranging from 0.1% to 1.4% in the reaction gas. Characterizations with SEM and XRD indicate increasing silicon concentration may result in grain size decreasing and diamond [110] texture becoming dominant. The residual stress values of as-deposited Si-doped diamond films are evaluated by both sin2ψ method, which measures the (220) diamond Bragg diffraction peaks using XRD, with ψ-values ranging from 0° to 45°, and Raman spectroscopy, which detects the diamond Raman peak shift from the natural diamond line at 1332 cm-1. The residual stress evolution on the silicon doping level estimated from the above two methods presents rather good agreements, exhibiting that all deposited Si-doped diamond films present compressive stress and the sample with Si/C mole ratio of 0.1% possesses the largest residual stress of ~1.75 GPa (Raman) or ~2.3 GPa (XRD). As the silicon doping level is up further, the residual stress reduces to a relative stable value around 1.3 GPa.

Effect of working pressure and temperature on ZnO film deposited on free-standing diamond substrates

The structure characteristic and electric performance of ZnO film deposited on nucleation side of free-standing diamond substrates under different heating temperatures (Th) of substrate and working pressures (p) were studied. The structure of the ZnO films tested by X-ray diffraction shows that ZnO film of high c-axis orientation is deposited on the nucleation side of free-standing diamond substrate which is extremely smooth when Th=250 ℃ and p=0.4 Pa. After annealing at 480 ℃ in N2 atmosphere, the SEM and the AFM analyses demonstrate that the c-axis orientation of ZnO film is obviously enhanced. The resistivity of ZnO films also increases up to 8×105 ■·cm which is observed by I?V test.

Effect of deposition parameters on micro-and nano-crystalline diamond films growth on WC-Co substrates by HFCVD

The characteristics of hot filament chemical vapor deposition（HFCVD） diamond films are significantly influenced by the deposition parameters, such as the substrate temperature, total pressure and carbon concentration. Orthogonal experiments were introduced to study the comprehensive effects of such three parameters on diamond films deposited on WC-Co substrates. Field emission scanning electron microscopy, atomic force microscopy and Raman spectrum were employed to analyze the morphology, growth rate and composition of as-deposited diamond films. The morphology varies from pyramidal to cluster features with temperature decreasing. It is found that the low total pressure is suitable for nano-crystalline diamond films growth. Moreover, the substrate temperature and total pressure have combined influence on the growth rate of the diamond films.

Enhancement of nucleation of diamond films deposited on copper substrate by nickel modification layer

A Ni layer with a thickness of about 100 nm was sputtered on Cu substrates,followed by an ultrasonic seeding with nanodiamond suspension.High-quality diamond film with its crystalline grains close to thermal equilibrium shape was deposited on Cu substrates by hot-filament chemical vapor deposition（HF-CVD）,and the sp2 carbon content was less than 5.56%.The nucleation and growth of diamond film were investigated by micro-Raman spectroscopy,scanning electron microscopy,and X-ray diffraction.The results show that the nucleation density of diamond on the Ni-modified Cu substrates is 10 times higher than that on blank Cu substrates.The enhancement mechanism of the nucleation kinetics by Ni modification layer results from two effects：namely,the nanometer rough Ni-modified surface shows an improved absorption of nanodiamond particles that act as starting points for the diamond nucleation during HF-CVD process;the strong catalytic effect of the Ni-modified surface causes the formation of graphite layer that acts as an intermediate to facilitate diamond nucleation quickly.

Effects of deposition parameters on HFCVD diamond films growth on inner hole surfaces of WC-Co substrates

Deposition parameters that have great influences on hot filament chemical vapor deposition (HFCVD) diamond films growth on inner hole surfaces of WC?Co substrates mainly include the substrate temperature (t), carbon content (φ), total pressure (p) and total mass flow (F). Taguchi method was used for the experimental design in order to study the combined effects of the four parameters on the properties of as-deposited diamond films. A new figure-of-merit (FOM) was defined to assess their comprehensive performance. It is clarified thatt,φandp all have significant and complicated effects on the performance of the diamond film and the FOM, which also present some differences as compared with the previous studies on CVD diamond films growth on plane or external surfaces. Aiming to deposit HFCVD diamond films with the best comprehensive performance, the key deposition parameters were finally optimized as:t=830 °C,φ=4.5%,p=4000 Pa,F=800 mL/min.

DIAMOND FILMS DEPOSITED AT LOW TEMPERATURES MICROWAVE PLASMA-ASSISTED CVD METHOD

Low-temperature deposition of diamond thin films in the range of 280￣445℃ has been successfully carried out by microwave plasma-assisted CVD method.At lower deposition temperatures (280￣445℃),the large increase in the nucleation density and great improvement in the average surfae roughness of the diamond were observed. Results of low temperature deposition and characterization of diamond thin films obtained are presented.

Preparation, characterization and electrochemical properties of boron-doped diamond films on Nb substrates

A series of boron-doped polycrystalline diamond films were prepared by hot filament （HF） chemical vapor deposition on Nb substrates. The effects of B/C ratio of reaction gas on film morphology, growth rate, chemical bonding states, phase composition and electrochemical properties of each deposited sample were studied by scanning electron microscopy, Raman spectra, X-ray diffraction, microhardness indentation, and electrochemical analysis. Results show that the average grain size of diamond and the growth rate decrease with increasing the B/C ratio. The diamond films exhibit excellent adhesion under Vickers microhardness testing （9.8 N load）. The sample with 2% B/C ratio has a wider potential window and a lower background current as well as a faster redox reaction rate in H2SO4 solution and KFe（CN）6 redox system compared with other doping level electrodes.

EFFECT OF GRID BIAS ON DEPOSITION OF NANOCRYSTALLINE DIAMOND FILMS

A positive grid bias and a negative substrate bias voltages are applied to the self-made hot filament chemical vapor deposited （HFCVD） system. The high quality nanocrystalline diamond （NCD） film is successfully deposited by double bias voltage nucleation and grid bias voltage growth. The Micro-Raman XRD SEM and AFM are used to investigate the diamond grain size, microstructure, surface morphology, and nucleation density. Results show that the obtained NCD has grain size of about 20 nm. The effect of grid bias voltage on the nucleation and the diamond growth is studied. Experimental results and theoretical analysis show that the positive grid bias increases the plasma density near the hot filaments, enhances the diamond nucleation, keeps the nanometer size of the diamond grains, and improves the quality of diamond film.

Mechanically flexible reduced graphene oxide/carbon composite films for high-performance quasi-solid-state lithium-ion capacitors

Practical applications of diverse flexible wearable electronics require electrochemical energy storage(EES)devices with multiple configurations.Moreover,to fabricate flexible EES devices with high energy density and stability,organic integration from electrode design to device assembly is required.To address these challenges,a free-standing reduced graphene oxide(rGO)/carbon film with a unique sandwich structure has been designed via the assistance of vacuum-assistant filtration for lithium-ion capacitors(LICs).The graphene acts as not only a binder to construct a three-dimensional conductive network but also an active material to provide additional capacitive lithium storage sites,thus enabling fast ion/electron transport and improving the capacity.The designed rGO/hard carbon(rGO/HC)and rGO/activated carbon(rGO/AC)free-standing films exhibit enhanced specific capacities(513.7 mA h g^(-1)for rGO/HC and 102.8 mA h g^(-1)for rGO/AC)and excellent stability.Moreover,the integrated flexible quasi-solid-state rGO/AC//rGO/HC LIC devices possess a maximum energy density of 138.3 Wh kg^(-1),a high power density of 11 kW kg^(-1),and improved cycling performance(84.4%capacitance maintained after 10,000 cycles),superior to the AC//HC LIC(43.5%retention).Such a strategy enlightens the development of portable flexible LICs.