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revolutionary breakthrough in reducing cost and increasingthrough put in the manufacture and of photovoltaic or electronic devices can be achieved through reel-to-reel coating of material from solution onto a large, flexible and lightweight platform. Thus, it important to develop a low temperature, cheap and large-scale solution-based synthesis
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method and patterning of inorganic nanostructures. Recently, patterning and self-assembly growth of nanostructures have received considerable attention.However, many of these methods require expensive equipments, multiple complex steps as well as the use of photoresist and other harmful chemicals to obtain selective growth of nanostructures. On the otherhand, soft-lithography techniques can easily be employed to pattern any nanomaterials and biomaterials on desired sites.
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Figure 1: SEM images of ZnO nanowires grown on various periodic patterns of approximately (a) 5 μm line and 10 μm spacing (b) 1 μm line and 1.5 μm spacing. |
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Here we have looked into the growth of avertically-aligned inorganic nanomaterial, ZnO on a selective area using a solution processable, low temperature, substrate independent and environmentally friendly method.Previously, the photoactive layer of photovoltaic cells was made of randomly nterdispersed electron accepting and hole conducting polymer/ inorganic material, where charge transport is limited by the hopping of electrons along the poorly connected network. To improve electron transport in these photovoltaic cells, arrays of one-dimensional nanostructures infiltrated with inorganic material or conjugated polymers have been designed to provide a direct path to the electrode.
Recently, there have been many reports on solution-based synthesis of ZnO nanostructures using zinc nitrate acetate along with organic amine additives namely oleylamine, hexadecylamine, dioctylamine, dodecylamine, methenamine etc. The commonly employed aminemediated additives, being non-polar chelating agents, would preferentially attach to the non-polar facets thereby exposing the polar planes (c-axis) for s anisotropic growth. However, in view of environmental concerns to eliminate harmful and corrosive organic additives in the synthesis process, as well as developer solvents in the patterning process, we have successfully developed a simple wayto transfer patterns of ZnO nanocrystals onto various substrates and then use a facile solution-based synthesis to grow highly ordered ZnO nanostructures on selective areas as shown in Figure 1. The patterning and growth of highly ordered arrays of crystalline ZnO inorganic nano structures use a simple soft lithography technique
and mild reaction conditions. |
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An optimized concentration of the inking solution is 5 mM since it is observed that the transferred and post-annealed film is uniformly covered with nanocrystals as shown in the AFM image (Figure 2a). The average diameter of the nanocystals is approximately 5 nm as determined from the height profile. Figure 2b shows the two-dimensional XRD Debye diffraction pattern obtained on the nanocrystals film. A high intensity and non-dispersive Debye ring shows only the existence of the ZnO (002) plane which indicates that complete c-axis textured alignment has occurred on the film. This result complements the XRD pattern where a sharp and narrow full-width-half-maximum (FWHM) peak at 2θ = 34.4°, attributed to ZnO (002) crystal plane (lattice constant of 5.206 Å), is observed. When the zinc acetate concentration is kept at 5 mM, vertically-aligned and discrete nanowires of diameter 30-60 nm and length 200 nm are produced
(Figure 2c). It is noted that at a controlled intermediate concentration, the synthesized nanostructures are no longer randomly aligned as is evident in the cross-sectional SEM image of Figure 2d. Finally, it is expected that with good control of the ZnO nanocrystals seeding layers and reaction parameters, the dimension, gap separation and density of the ZnO nanowires can be tailored for various applications.
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Figure
2: (a) AFM topographic images of microcontact-printed ZnO nanocrystals. (b) Two-dimensional XRD Debye pattern showing the c-axis (002) texturing of the wurtzite ZnO structure. (c-d) Top and cross-sectional SEM images of ZnO nanostructures. |
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Dr
Ho Ghim Wei is an assistant professor of Electrical and Computer Engineering. She received her Ph.D. degree from the University of Cambridge in 2007, and M.Sc and B.Sc. degrees from the National University of Singapore. She specializes in the understanding of the physical and chemical processes of nanostructured materials synthesis, properties and nanosystems. The study of the phenomena and manipulation of materials at the nanoscale revolves around the realization of solar cell, chemical sensor and nanowire based devices.
Email: elehgw@nus.edu.sg |
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