Da-Hua Wei

Synthesis of highly transparent ultrananocrystalline diamond films from a low- pressure, low-temperature focused microwave plasma jet

This paper describes a new low-temperature process underlying the synthesis of highly transparent ultrananocrystalline diamond [UNCD] films by low-pressure and unheated microwave plasma jet-enhanced chemical vapor deposition with Ar-1%CH4-10%H2 gas chemistry. The unique low-pressure/low-temperature [LPLT] plasma jet-enhanced growth even with added H2 and unheated substrates yields UNCD films similar to those prepared by plasma-enhanced growth without addition of H2 and heating procedure. This is due to the focused plasma jet which effectively compensated for the sluggish kinetics associated with LPLT growth. The effects of pressure on UNCD film synthesis from the microwave plasma jet were systematically investigated. The results indicated that the substrate temperature, grain size, surface roughness, and sp3 carbon content in the films decreased with decreasing pressure. The reason is due to the great reduction of Hα emission to lower the etching of sp2 carbon phase, resulting from the increase of mean free path with decreasing pressure. We have demonstrated that the transition from nanocrystalline (80 nm) to ultrananocrystalline (3 to 5 nm) diamond films grown via microwave Ar-1%CH4-10%H2 plasma jets could be controlled by changing the pressure from 100 to 30 Torr. The 250-nm-thick UNCD film was synthesized on glass substrates (glass transition temperature [Tg] 557°C) using the unique LPLT (30 Torr/460°C) microwave plasma jet, which produced UNCD films with a high sp3 carbon content (95.65%) and offered high optical transmittance (approximately 86% at 700 nm).

Controlling microstructure and magnetization process of FePd (001) films by staged thermal modification

Staged thermal modifications on the microstructure and magnetic characterizations of epitaxial FePd films have been investigated, while the optimum composition for FePd (001) films with the highest ordering degree was Fe52Pd48. For the FePd films directly grown at the low temperature of 400 °C, the isolated islandlike morphology was observed and displayed a perpendicular magnetic anisotropy with the coercivity value of 8000 Oe. On the other hand, the FePd films grown at 100 °C and then postannealed at 400 °C showed continuous surface and with a lower remanence corresponded to the alternate up and down orientations of the magnetization due to stripe domains formation. The significant distinction in magnetic exhibition of the FePd (001) films was due to the marked change in magnetic domain structures and surface morphology caused by varied interfacial energy during staged thermal modifications.

Magnetic assembles of FePt (001) nanoparticles with SiO2 addition

Isolated FePt (001) nanoparticles surrounded with amorphous SiO2 have been fabricated by electron beam evaporation onto MgO (001) single-crystal substrates via the introduction of a SiO2 intermediate layer into the FePt film structures. The formation of two-dimensional magnetic assemblies of ordered FePt (001) nanoparticles with an average size of about 6nm was directly obtained with this process at only 400°C due to the interpenetration of SiO2, which has a lower surface energy. Studies of angular dependent coercivity show a tendency of a domain-wall motion to weaken toward rotation of reverse-domain type upon thickness of SiO2 additive layer into the FePt film structures. On the other hand, the exchange coupling between neighboring particles in the FePt nanostructures could be reduced with ultrathin SiO2 addition, which is confirmed from the Kelly–Henkel (δM) plot.

Magnetization Reversal Mechanism and Microstructure Refinement of the FePt (001) Nanogranular Films With SiO

Ordered FePt continuous thin films with an amorphous SiO2 single capping layer have been fabricated on MgO (001) single-crystal substrates by the electron beam deposition technique at 400degC in order to investigate the effects of SiO2 capping layer on microstructure and magnetic reversal process of the FePt (001) thin films. The formation of nanogranular-like FePt films was directly obtained with this process due to the interpenetration of SiO2 which has a lower surface energy than that of pure Fe or Pt. Studies of angular dependent coercivity show a tendency of a domain-wall motion weaken towards rotation of reverse-domain type upon thickness of SiO2 capping layer on the FePt thin films. The intergrain interaction was confirmed from the Kelly-Henkel (deltaM) plot that indicated the strong exchange coupling between neighboring grains in the FePt continuous films without SiO2 capping layer. On the other hand, negative deltaM value was obtained when the FePt films with SiO2 capping layer, indicating the SiO2 capping layer, can lead to the reduction of intergrain exchange coupling thus presence of dipole interaction in the SiO2/FePt nanocomposite thin film structures.

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The surfaces of implantable biomaterials improving biocompatibility and bioinertness are critical for new application of bioimplantable devices. Diamond-like carbon (DLC) film is a promising biomaterial with use for coating bioimplantable devices because of its good biocompatibility, bioinertness, and mechanical properties. In this study, concurrent improvement in biocompatibility and bioinertness of DLC films has been achieved using N-incorporation technique. The N doping degree was found to play an important role in affecting the biocompatibility and bioinertness of N-doped DLC films. The results indicated that the N-doped DLC films deposited at N(2) concentration of 5% could help to create suitable condition of surface/structure/adhesion combination of DLC films in the both affinity of the L929 mouse fibroblasts and electrochemical inertness in the Hanks balanced salt solutions (simulating human body fluids). N doping supports the attachment and proliferation of cells and prevents the permeation of electrolyte solutions, thereby simultaneity improved the biocompatibility and bioinertness of DLC films. This finding is useful for the fabrication and encapsulation of in vivo devices without induced immune response in the human body.

Capping Layer

The effects of underlayer on the microstructure and magnetic characterizations of epitaxial FePt films were studied. Enhanced out-of-plane coercivity and ordering were obtained in the Pt-underlayered FePt samples, which may be due to the proper lattice strain from the Pt underlayer. The larger lattice parameter of Pt may enlarge the a axis and reduce the c axis of the L10 FePt phase and result in low-temperature ordering. However, such an effect does not appear in Fe-underlayered samples because of the smaller lattice parameter and the considerable interdiffusion. The angular dependence coercivity revealed that the dominant reversal mechanism of the FePt films changed from domain-wall motion to rotation, which is caused by the formation of L10 island structures induced by the Pt underlayer. On the other hand, the Fe-underlayered FePt films showed a broad peak roughly at 45°, implying the tilt of the easy axis about 45° away from the film normal.

Concurrent improvement in biocompatibility and bioinertness of diamond-like carbon films with nitrogen doping

Nanocrystalline diamond (NCD) films are promising materials for wide-spread applications due to their outstanding characteristics of chemical, physical, and highly smooth surface. Our present work aimed at the fabrication of high performance diamond-based UV detector. NCD films were prepared by microwave plasma enhanced chemical vapor deposition process, and then Au interdigital electrodes were deposited onto the surface of the as-grown NCD film by sputtering technique. Annealing procedures were conducted at various temperatures to obtain Ohmic contact of NCD/Au structure. The surface morphology, microstructure, and wettablity of the NCD films were analyzed by scanning electron microscopy, atomic forced microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and water contact angle measurement, respectively. The electrical property and photoconductivity of the fabricated devices were tested for UV detection application. It was found that the NCD films possessed high sp3 fraction of 68.6%, low surface roughness of 9.6 nm, and good hydrophobicity, as deposited under working pressure of 40 Torr. Also, the NCD/Au structure annealed at 500°C exhibited a good Ohmic contact characteristic, high detection efficiency, and fast response to UV irradiation in air ambient. The proposed study indeed demonstrates prospective applications of NCD films in UV detector, photocatalyst, solar cell, and so on.

Effects of Pt and Fe underlayers on the microstructure and magnetization reversal of epitaxial FePt films for high areal density magnetic recording

This paper describes a fabrication and characterization of ultraviolet (UV) photodetectors based on Ohmic contacts using Pt electrode onto the epitaxial ZnO (0002) thin film. Plasma enhanced chemical vapor deposition (PECVD) system was employed to deposit ZnO (0002) thin films onto silicon substrates, and radio-frequency (RF) magnetron sputtering was used to deposit Pt top electrode onto the ZnO thin films. The as-deposited Pt/ZnO nanobilayer samples were then annealed at in two different ambients (argon and nitrogen) to obtain optimal Ohmic contacts. The crystal structure, surface morphology, optical properties, and wettability of ZnO thin films were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), photoluminescence (PL), UV-Vis-NIR spectrophotometer, and contact angle meter, respectively. Moreover, the photoconductivity of the Pt/ZnO nanobilayers was also investigated for UV photodetector application. The above results showed that the optimum ZnO sample was synthesized with gas flow rate ratio of 1 : 3 diethylzinc [DEZn, Zn(C2H5)2] to carbon dioxide (CO2) and then combined with Pt electrode annealed at in argon ambient, exhibiting good crystallinity as well as UV photo responsibility.

Development of High-Performance UV Detector Using Nanocrystalline Diamond Thin Film

In this present work, diamond-like carbon (DLC) films were coated onto polycarbonate (PC) substrates as a protective layer at room temperature by radio frequency (rf) magnetron sputtering technique. The ID/IG ratio, roughness (Ra), and contact angle of amorphous DLC films could be controlled by regulating deposition power. The DLC films deposited at 150 W with a good hardness of as high as 13.74 GPa were realized with a surface roughness and contact angle of 0.63 nm (Ra) and 103°, respectively. Ultraviolet/visible (UV/vis) spectrophotometry of single-layer DLC films showed a high transmissive ability (>80%) in the visible wavelengths. No cracks occurred on the surface of DLC films after the flexibility test 500 times at a frequency of 8.6 min-1. This confirms the excellent adhesion of DLC films on PC plastic substrates with potential applications in flexible optoelectronic devices.

Postannealing Effect at Various Gas Ambients on Ohmic Contacts of Pt/ZnO Nanobilayers toward Ultraviolet Photodetectors

The controllable photoluminescence and wettability of NiFe/ZnO heterostructure bilayer films have been demonstrated by applying an ultrathin NiFe capping layer onto ZnO films by radio-frequency magnetron sputtering at room temperature without introducing any oxygen gas during the deposition process. High quality crystalline ZnO(002) textured films were fabricated at first and displayed a remarkable near-band-edge emission peak located at around 370 nm with a bandgap of 3.35 eV confirmed by room temperature photoluminescence spectra. Once the ZnO films were capped with a single NiFe layer, ranging from 5 to 20 nm in thickness, the intensity of their near-band-edge emission peak decreased and the emission band shifted to 414 nm. On the other hand, the contact angle of the uncapped ZnO film increased from 88° to 101° with the addition of a 10 nm thick NiFe capping layer. This means that the ultrathin NiFe layer acted as a surfactant layer. The surface wettability could be switched from hydrophilic to hydrophobic due to the varied surface free energy caused by the controllable grain morphology of the NiFe/ZnO heterostructures. This work demonstrates that a direct NiFe capping layer can effectively control the optical, surface and magnetic characteristics in NiFe/ZnO heterostructures depending on the bimetallic NiFe thickness and provide valuable multifunctional behaviors for potential novel magnetoelectric applications.