Anisotropic Ti x Sn1- x O2 nanostructures prepared by magnetron sputter deposition
Regular arrays of Ti x Sn1- x O2 nanoflakes were fabricated through glancing angle sputter deposition onto self-assembled close-packed arrays of 200-nm-diameter polystyrene spheres. The morphology of nanostructures could be controlled by simply adjusting the sputtering power of the Ti target. The reflectance measurements showed that the melon seed-shaped nanoflakes exhibited optimal properties of antireflection in the entire visible and ultraviolet region. In addition, we determined their anisotropic reflectance in the direction parallel to the surface of nanoflakes and perpendicular to it, arising from the anisotropic morphology.
KeywordsSnO2 Polystyrene Sphere Incident Flux Typical Scanning Electron Microscopy Micrograph Physical Vapor Deposition Process
glancing angle deposition
scanning electron microscopy
Much research has recently been focused on the design of nanoscale semiconductor oxide materials with controlled morphology and novel morphology-dependent physical properties. As an n-type semiconductor oxide with a wide band gap (Eg = 3.6 eV at 300 K), SnO2 is well known for its potential applications in gas sensing , photoconductors , photocatalysis , and dye-sensitized solar cells . Similar to other highly transparent semiconductors, SnO2 is also expected to be a competitive candidate for optical devices . To optimize the optical properties, tuning the composition and tailoring the morphology are very important.
Numerous techniques have been developed to synthesize SnO2 films. The advantage of magnetron sputter deposition lies in the ease and flexibility of doping, which is an effective method to adjust the composition, and to ensure good homogeneity and repeatability. Ti is an appealing alternative for doping, compared to other elements quantities of which are restricted due to the emergence of the second phase. Owing to the isostructure (rutile type) and slightly different lattice parameters of TiO2 and SnO2, the metal cations can replace each other in a wide concentration region .
On the other hand, the morphology of films can be modulated by the glancing angle deposition (GLAD) technique. The GLAD technique, which exploits the shadowing effects, is applied to fabricating a variety of nanostructures, such as columns, helixes, and springs [7, 8, 9, 10, 11]. With the combination of GLAD and magnetron sputter deposition techniques, arrays of uniquely shaped nanostructures built from a wide range of material systems can be created. However, the preparation of oxides by glancing angle sputter deposition is rarely reported, as the sputtering of oxide targets is hard to maintain in the pressure technically required for GLAD. In this study, periodic arrays of Ti x Sn1- x O2 nanostructures were grown on patterned Si substrates using GLAD with the simultaneous deposition of DC and RF sputtering sources. The RF sputtering of the SnO2 target at low pressures resulted owing to the DC sputtering of the Ti target. The morphology of nanostructures can be modulated by the regulation of the sputtering power.
Results and discussion
The average length-width ratios of the samples and corresponding parameters
Discharge current (A)
The effect of the deposition parameter to the morphology of nanostructures was studied. As GLAD is a physical vapor deposition process in which the incident flux impinges the substrate from an oblique angle, causing atomic shadowing and resulting in highly porous nanostructures , the morphology of nanostructures is closely related to the direction and velocity of the incident flux. With the incident flux from lateral SnO2 target, the direction of the growth front was changed from perpendicular to lateral, leading to an increase of the growth rate in the direction parallel to the SnO2 flux and a decrease in the perpendicular direction . As a result, the width of the nanostructures became broadened parallel to the SnO2 flux, and suppressed perpendicular to it, as shown in Figure 3a. The arrangement of the nanoflakes corresponds with the direction of SnO2 flux illustrated in Figure 1a, confirming that the nanostructures were deformed by the incident flux that caused anisotropic lateral growth. With the increased sputtering rate of the Ti target, the effect of lateral SnO2 flux was gradually overweighed by Ti flux, resulting in the increase of the length-width ratio. Thus, it can be concluded that the sputtering rates of Ti target and SnO2 target have a great influence on the morphology of nanostructures.
We have reported a general and an effective process for producing semiconductor oxide materials with controllable morphology and properties by glancing angle sputter deposition. Periodic arrays of Ti x Sn1- x O2 nanostructures were prepared on patterned Si substrates through the co-sputtering of Ti and SnO2. The shape of Ti x Sn1- x O2 nanostructures, characterized by the length-width ratio which linearly depended on the sputtering rate of Ti target, could be controlled by adjusting the sputtering power of Ti target. Optical properties were studied, and the results confirmed that the melon seed-shaped nanoflakes, which were approximately equivalent to gradient-duty cycle subwavelength nanogratings, possessed very low reflectance. Furthermore, their reflectances were anisotropic, which can be ascribed to the anisotropism of the morphology. Consequently, this study provides the opportunity to design optical devices by creating nanostructures with tailored morphology and unique performance.
The authors are grateful for the financial support provided by the National Natural Science Foundation of China (51072094 and 61076003), and the National Basic Research Program of China (973 program, 2010CB731600).
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