Simple Physical Vapor Deposition Method Research Articles

Oxygen Reduction Reaction Activity of Nanocolumnar Pt Thin Film Electrocatalyst Deposited on Carbon Support By High Pressure Sputtering

Proton-exchange membrane fuel cell (PEMFC) is one of the most important sources of clean energy especially for automotive applications, which currently utilizes platinum nanoparticles dispersed on carbon (Pt/C) as a catalyst. However, the catalyst activity and durability need to be improved and the cost of the fuel cell need to be reduced for successful commercialization. Extensive research has been done to improve the catalyst activity and durability, reduce the amount of platinum used and reduce its manufacturing cost. Continuous thin film layer approach is a promising candidate for non-conventional catalysts to address these challenges. For this purpose, nanocolumnar Pt thin film (Pt-TF) layers supported on carbon was fabricated by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction (ORR) electrocatalysts for PEMFCs. HIPS is a simple physical vapor deposition method that is scalable and easily applicable to industrial sputter deposition systems, in which atoms come to the substrate surface with oblique angles and form columnar structures. Different Pt-TF/C weight ratios ranging from 5% to 20% and Pt:Ni (1:3) TF/C were studied. Weight loading was controlled by changing the sputter deposition time. X-ray diffraction analysis revealed the existence of Pt and formation of the Pt:Ni alloy on carbon support. Electrochemical characterization of the carbon-supported Pt-TF samples was conducted by cyclic voltammetry and rotating disk electrode measurements in an aqueous perchloric acid electrolyte. The electrochemically active surface area, mass activity and specific activity of the Pt-TF/C samples were found to be increasing as the Pt-TF/C ratio was increased.

Oxygen Reduction Reaction Activity of Nanocolumnar Pt:Ni Alloy Thin Films By High Pressure Sputtering

Self-supported nanocolumnar Pt-Ni alloy thin films (TFs) with different Pt:Ni compositions and Pt mass loadings were fabricated by high pressure sputtering (HIPS) on a microporous layer (MPL)-like surface composed of carbon particles in order to mimic the catalyst-coated gas diffusion layer (gas diffusion electrode) in a membrane electrode assembly and investigated as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. HIPS is a simple physical vapor deposition method that is scalable and easily applicable to industrial sputter deposition systems. At high working gas pressures, columnar and less-dense structures are formed because of angular distribution of sputtered atoms that leads to a shadowing effect. Cauliflower-like microstructures were observed from scanning electron microscopy imaging. X-ray diffraction and quartz crystal microbalance analysis revealed that by simply changing the relative deposition power between Pt and Ni source, different Pt:Ni compositions can be achieved. Benchtop cyclic voltammetry and rotating disk electrode measurements were performed for electrochemical characterization of the Pt:Ni-TF/MPL-like-layer/glassy-carbon samples in an aqueous perchloric acid electrolyte. The electrochemically active surface area (ECSA) ranged between 22-42 m2/g for varying Pt:Ni compositions. Lower Pt mass loadings showed a higher ECSA likely due to smaller nanocauliflower diameters, while the ORR activity of all compositions increased as the Pt mass loading is increased. The catalytic performance of the Pt:Ni-TFs increased in the order of 3:1 < 1:1 < 1:3 with the 1:3 films exhibiting a specific activity of 1781 µA/cm2 and mass activity of 0.66 A/mg, indicating efficient catalyst utilization. The Pt:Ni-TFs were found to exhibit higher ORR activity than traditional high surface area carbon supported Pt nanoparticles, elemental Pt nanorods, and Pt-Ni nanorods.

Catalytic growth of one-dimensional single-crystalline ZnO nanostructures on glass substrate by vapor transport

Novel one-dimensional single-crystalline ZnO nanorod and nanoneedle arrays on a Cu catalyst layer-coated glass substrate were investigated via a simple physical vapor-deposition method by thermal evaporation of Zn powder in the presence of O2 gas. The ZnO nanorods and nanoneedles were synthesized along the c-axis growth direction of the hexagonal crystal structure. The diameter and growth rate of the high-quality and well-oriented one-dimensional ZnO nanostructures is achieved as a function of varying growth temperature and growth time. This study serves as the basis for further research on the growth of 1D ZnO nanostructures on a cost-effective glass substrate and at low temperature.

Mid-infrared photodetectors based on InSb micro/nanostructures grown on low-cost mica substrates

Thin films consisted of InSb micro/nanostructures are grown on mica substrates by a simple physical vapor deposition method. The deposition temperature plays a major role in the surface morphology and material quality of the InSb films synthesized on mica substrates as evidenced by scanning electron microscopy and X-ray diffraction measurements. Mid-infrared photodetectors were also fabricated from the InSb films and superior device performance was obtained from the InSb films grown at elevated temperature.

Physical vapor deposition synthesis of two-dimensional orthorhombic SnS flakes with strong angle/temperature-dependent Raman responses.

Anisotropic layered semiconductors have attracted significant interest due to the huge possibility of bringing new functionalities to thermoelectric, electronic and optoelectronic devices. Currently, most reports on anisotropy have concentrated on black phosphorus and ReS2, less effort has been contributed to other layered materials. In this work, two-dimensional (2D) orthorhombic SnS flakes on a large scale have been successfully synthesized via a simple physical vapor deposition method. Angle-dependent Raman spectroscopy indicated that the orthorhombic SnS flakes possess a strong anisotropic Raman response. Under a parallel-polarization configuration, the peak intensity of Ag (190.7 cm(-1)) Raman mode reaches the maximum when incident light polarization is parallel to the armchair direction of the 2D SnS flakes, which strongly suggests that the Ag (190.7 cm(-1)) mode can be used to determine the crystallographic orientation of the 2D SnS. In addition, temperature-dependent Raman characterization confirmed that the 2D SnS flakes have a higher sensitivity to temperature than graphene, MoS2 and black phosphorus. These results are useful for the future studies of the optical and thermal properties of 2D orthorhombic SnS.

A high performance HfSiON/TaN NMOSFET fabricated using a gate-last process

A gate-last process for fabricating HfSiON/TaN n-channel metal-oxide-semiconductor-field-effect transistors (NMOSFETs) is presented. In the process, a HfSiON gate dielectric with an equivalent oxide thickness of 10 Å was prepared by a simple physical vapor deposition method. Poly-Si was deposited on the HfSiON gate dielectric as a dummy gate. After the source/drain formation, the poly-Si dummy gate was removed by tetramethylammonium hydroxide (TMAH) wet-etching and replaced by a TaN metal gate. Because the metal gate was formed after the ion-implant doping activation process, the effects of the high temperature process on the metal gate were avoided. The fabricated device exhibits good electrical characteristics, including good driving ability and excellent sub-threshold characteristics. The device's gate length is 73 nm, the driving current is 117 μA/μm under power supply voltages of VGS = VDS = 1.5 V and the off-state current is only 4.4 nA/μm. The lower effective work function of TaN on HfSiON gives the device a suitable threshold voltage (∼ 0.24 V) for high performance NMOSFETs. The device's excellent performance indicates that this novel gate-last process is practical for fabricating high performance MOSFETs.

Sputtered magnesium diboride thin films: Growth conditions and surface morphology

Magnesium diboride (MgB 2) thin films were deposited on C-plane sapphire substrates by sputtering pure B and Mg targets at different substrate temperatures, and were followed by in situ annealing. A systematic study about the effects of the various growth and annealing parameters on the physical properties of MgB 2 thin films showed that the substrate temperature is the most critical factor that determines the superconducting transition temperature ( T c ), while annealing plays a minor role. There was no superconducting transition in the thin films grown at room temperature without post-annealing. The highest T c of the samples grown at room temperature after the optimized annealing was 22 K. As the temperature of the substrate ( T s ) increased, T c rose. However, the maximum T s was limited due to the low magnesium sticking coefficient and thus the T c value was limited as well. The highest T c , 29 K, was obtained for the sample deposited at 180 °C, annealed at 620 °C, and was subsequently annealed a second time at 800 °C. Three-dimensional (3D) AFM images clearly demonstrated that the thin films with no transition, or very low T c , did not have the well-developed MgB 2 grains while the films with higher T c displayed the well-developed grains and smooth surface. Although the T c of sputtered MgB 2 films in the current work is lower than that for the bulk and ex situ annealed thin films, this work presents an important step towards the fabrication of MgB 2 heterostructures using rather simple physical vapor deposition method such as sputtering.

Synthesis and electrical properties of TiSi2 nanocables

Uniform TiSi2 nanocables were synthesized on large substrates using a simple physical vapor deposition method. X-ray diffraction, scanning electron microscopy, and transmission electronic microscopy were employed to characterize the samples. It reveals that the as-grown TiSi2 nanocables are of high quality single crystal inside with thin amorphous SiO2 sheathing layer. Electrical properties of these core-shell structure nanowires are also presented. Insulativity of the outer layer and stable metallic characters of the inside single crystal TiSi2 were observed. Moreover, carrying capacity of current density is up to 1011A∕m2. The nanoscale structure and excellent electrical performance make the TiSi2 nanocables good candidate for electrical interconnection in potential nanodevices.

Structure and Photoluminescence Properties of Aligned ZnO Nanobolt Arrays

Well-aligned ZnO nanorod and nanobolt arrays were synthesized on p-type Si(100) substrates by a simple physical vapor deposition method. During the growth process when the carrier gas was switched from Ar to air, the nanorods changed into nanobolts completely. The mechanism for this transformation process is discussed. The room-temperature photoluminescence spectrum of nanobolts showed a strong ultraviolet emission at 3.26 eV coupled with a weak deep level energy emission at 2.48 eV. The origin of the ultraviolet emission of nanobolts was investigated by temperature-dependent photoluminescence spectra.

The influence of growth temperature on ZnO nanowires

ZnO nanowires have been successfully synthesized on Si(100) substrate by a simple physical vapor deposition method. Thin film ZnO layer used as the nucleation site can avoid the contamination from the metal catalysts, and it can also control the growth direction of ZnO nanowires. Well-aligned ZnO nanowire arrays along the normal direction of the substrate can be obtained by controlling different growth temperature, which was demonstrated by XRD and FESEM analysis. A strong ultraviolet emission at room temperature was observed in all ZnO nanostructures. In addition, the growth mechanisms of the ZnO nanowires with different growth temperature is discussed in details.

Correlation between photoluminescence and varied growth pressure of well-aligned ZnO nanorods on fused silica substrate

Well-aligned ZnO nanorods with variable diameters were fabricated on fused silica substrates by the simple physical vapor deposition method with and without pre-deposited NiO particles. The diameter control of ZnO nanorods on the fused silica substrates was achieved by varying the growth pressure. The room-temperature photoluminescence spectra of the ZnO nanorods exhibit strong exciton emission at 3.25eV that reveals good crystal quality. The exciton emission peak shifts toward 3.22eV for the samples made at the larger growth pressure that is attributed to the influence of the electron-acceptor-level transition. The ZnO nanorods were examined by high resolution transmission electron microscopy and X-ray diffraction to show single crystal quality and preferentially c-axis alignment. Furthermore, we used scanning electron microscope to verify not only the size but also the growth mechanisms of ZnO nanorods. The results show a ZnO buffer layer formed between the flat top-facet nanorods and the substrate. In comparison with the samples without pre-deposited NiO particles on the substrate, the pre-deposited NiO particles act as nucleation centers for Zn vapor during the deposition process that improves the quality of the buffer layer.

Optical properties of highly ordered AlN nanowire arrays grown on sapphire substrate

Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip &amp;lt;10nm, with the average length around 3 μm. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift ∼0.27eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires.

Growth temperature controlled shape variety of ZnO nanowires

ZnO nanowires array has been synthesized via a simple physical vapor deposition method at low growth temperature without using any catalyst. By controlling different growth parameters, the nanowires exhibit different shapes: needle-like with sharp tips or rod-like with flat tips. All the nanowires are single crystal with a growth direction along c-axis. The rod-like nanowires are well vertical aligned to the substrate surface with c-axis preferred orientation. The morphology difference of ZnO nanowires is believed originated from different growth mechanisms.

Formation of ZnS nanorods by simple evaporation technique

Semiconductor nanocrystals and nanorods whose properties are largely determined by the quantum confinement effect are currently being intensively studied by materials scientists, physicists and chemists. Zinc sulphide (ZnS), a II–VI group semiconductor material possessing a direct band gap of 3.66 eV, has recently been extensively investigated due to its multifaceted applications. We report the synthesis of ZnS nanorods by a simple physical vapor deposition method and an in-detail surface analysis for device applications. Our interest in this material mainly lies behind its use as an n-window layer for our investigations on different window layers for CdTe- and CIS (Copper Indium diselenide) based solar cells and for photocatalytic production of hydrogen from water using the photocatalysts CdS/ZnS. ZnS films are deposited onto well-cleaned glass substrates at a vacuum of 5×10-5 Torr and various parameters are determined. The distance between the substrate and the source was maintained at 0.15 cm. The deposition time was about 20 min at a constant rate of evaporation and the substrates were maintained at room temperature. Structural analysis reveals the cubic nature of the crystallites, which is confirmed from atomic force microscopy (AFM) analysis. The AFM analysis reveals the formation of nanorods due to coalescence, which is substantiated from sectional analysis. A further analysis reveals the preferential growth of the nanorods and the coalescence limited by the energy in the (002) face. The composition was analyzed using an energy-dispersive X-ray method (EDX) and the film was found to possess excess sulfur. The band gap of the vacuum-deposited ZnS film was found to be 3.6 eV.

Low-Temperature Growth of ZnO Nanowire Array by a Simple Physical Vapor-Deposition Method

Well-aligned single-crystalline wurzite zinc oxide (ZnO) nanowire array was successfully fabricated on an Al2O3 substrate by a simple physical vapor-deposition method at a low temperature of 450 °C. The diameter and growth rate of ZnO nanowires increased as a function of growth temperature. TEM observation showed that the ZnO nanowires were synthesized along the c-axial direction of the hexagonal crystal structure. We demonstrate that ZnO nanowires followed the self-catalyzed growth mechanism on the ZnO nuclei. Besides high-quality ZnO nanowires, sometimes a fascinating hierarchically ordered ZnO structure was also observed.

Raman characterization of epitaxial Cu–In–Se thin films

We have fabricated epitaxial chalcopyrite alloys with varying compositions and thicknesses on GaAs (001) substrates using a simple physical vapor deposition method. Growth of the films was characterized with Raman spectroscopy as well as by other analytical techniques. We identified both CuInSe 2 (001) and CuIn 3Se 5 (001). All of the spectra were dominated by the A 1(Γ 1 (1) [W 1]) non-polar optical mode at 172 cm −1 for CuInSe 2 and 152 cm −1 for the CuIn 3Se 5 phase. In addition, Raman spectra for the thinner layers indicated that these films are under compressive stress due to the lattice mismatch between the films and the substrate.