ZnO Nanostructured Diodes: The Influence of Synthesis Conditions and p-type Material on Device Performance
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ZnO Nanostructured Diodes: The Influence of Synthesis Conditions and p-type Material on Device Performance S.M. Hatcha and S. Dunna a
School and Engineering and Materials, Queen Mary University of London, E1 4NS, UK
Email: [email protected] ABSTRACT We produce four distinct ZnO nanorod diode structures that are based on ZnO nanorods produced at pH 6 and pH 11 and have the p-type material PEDOT:PSS (hybrid device) or CuSCN (all inorganic device). After testing the performance of the diodes we show a rectification of 1050 at ±1V in the dark for the inorganic device. The device also exhibits good UV photodetection showing a rapid ca 0.1ms turn on and off to a source of illumination. The hybrid devices performed as previously reported with a rectification of 25 at ±1V in both dark and under illumination. We ascribe the performance of the devices to the differences in morphology in the ZnO brought about by the processing conditions and the way in which the ptype layer coats the nanostructure. INTRODUCTION ZnO NRs are finding increasing use in a variety of applications 1,2,3,4,5,6,7. A range of synthesis techniques have allowed an influence over the optical, electrical and morphological properties of this direct wide band-gap (3.4eV) material. One of the most environmentallyfriendly and suitable methods for mass production is that of low-temperature aqueous solution growth, or hydrothermal growth when pressures are involved8. Numerous studies have since discussed ZnO sensitivity to synthesis conditions9,10. Photoconductive properties of ZnO have been linked to the adsorbed oxygen ions that trap electrons at the surface11,12. In the case of nanorods and other high-surface area ZnO structures this can lead to a significant reduction of conductivity. Upon super-bandgap illumination, photo-induced holes migrate to trap states located on the nanorod surface neutralising the oxygen bound electrons and cause the oxygen molecules to desorb from the nanorod surface. This permits the photo-excited electrons to remain mobile and results in increased conductivity of the nanorod. By introducing further trap states on the nanorod surface, it is possible to enhance the surface chemistry of the nanorod and therefore the electrical response to gaseous molecules. Hexamethylenetetramine (HMT) is reported to have dual functionality when incorporated in the aqueous synthesis of ZnO nanorods13. Primarily, HMT decomposes into formaldehyde and ammonia. Ammonia supplies hydroxyl ions, which react with free Zn2+ cations to form ZnO14. Secondly, HMT acts as a capping agent that preferentially attaches to non-polar ZnO facets. This promotes growth in the (0001) c-axis direction by restricting the availability of growth sites on the non-polar facets. Strom et al15 found the rate at which HMT decomposes into its constituents decreases with increasing pH at 35˚C. Above pH 7.4 no HMT degradation was detected after
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6hours. Therefore, at pH>7.4 temperature plays an important role in the rate of OH- ion production. Growth solutions comprised sole
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