I. REALIDAD SOCIO-CULTURAL
1.3 Consecuencias del abuso sexual infantil
1.3.2. Legales
6.1 PLD growth of cr-ZnO/am-ZnO core/shell nanorods on ZnO-seeded Si substrates: Self-organised growth and 3.331 eV luminescence
In section 4.1 of this thesis, we have reported for the first time the self-organised crystalline (cr)-ZnO/amorphous (am)-ZnO core/shell nanorods by pulsed laser deposition (PLD) on ZnO-seeded Si (100) substrates. These core/shell nanorods were grown without using a metal catalyst seed and without the need for a separate growth stage for the shell region. The structural, morphological and luminescent properties of the ZnO core/shell nanorod samples were established and show that the core/shell nanorods are highly textured with their c-axis oriented normal to the substrate surface, but without epitaxial in-plane ordering. The core/shell nanorods have a closely packed morphology and they also have conical terminations with rounded/blunt tips. A ZnO emission band at 3.331 eV is seen and its origin linked to the defects observed at the crystalline/amorphous interface of the core/shell
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structure, specifically that the 3.331 eV emission arises from a single electron-hole pair recombination involving deeply bound constituents likely associated with the structural defects at the core/shell boundary interface region. This emission feature appears to have a different origin compared to the emissions at this energy reported by other workers and thus to be a new contribution to the body of knowledge concerned with ZnO nanostructures and their PL properties.
In section 4.2, we have grown crystalline (cr)-ZnO/amorphous (am)-ZnO core/shell nanorods in interconnected architectures. These interconnected cr-ZnO/am-ZnO core/shell nanorods were grown by catalyst free-PLD on ZnO-seeded Si (100) substrates. These deposits were characterised using x-ray diffraction, electron microscopies, photoluminescence and Raman spectroscopy, and four point probe/Hall effect instruments. The interconnected core/shell nanorods have a similar morphology to the previously discussed cr-ZnO/am-ZnO core/shell nanorods with a high degree of c-axis orientation. These nanorods also exhibit the characteristic emission at 3.331 eV. This study strongly supports our previous assignment concerning this defect related emission. No substantial differences in optical properties are seen following annealing at 500 °C. In terms of the electrical properties, the results reveal that the nanorods show good ohmic behaviour.
This work contains important new results in the field of ZnO nanorod growth and optical properties. Detailed characterisations of the ZnO nanorod samples were carried out and their analyses provide a deep physical insight into the nature of the new data reported. The most important findings are: (a) the self-organised growth of highly c-axis oriented cr-ZnO/am-ZnO core/shell nanorods without the need for (i) a separate shell growth and (ii) the use of a metal catalyst; (b) the formation of a crystalline ZnO core and an amorphous ZnO shell achieved as part of a unique two-staged sequence of growths at different temperatures and a single ambient oxygen pressure and (c) importantly, the identification of a emission band at 3.331 eV in the low temperature photoluminescence spectrum of the cr-ZnO/am-ZnO core/shell nanorods and its relationship with the defect structure observed at the irregular interface of the core-shell region.
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We confirmed the origin of this emission from the ‘interconnected cr-ZnO/am-ZnO core/shell nanorods’ work (section 4.2), where this interconnected core/shell nanorod sample also exhibits this emission band with the same characteristic broad and asymmetric profile. Furthermore, the complete absence of this emission for either PLD-grown ZnO seed layer or VPT-grown ZnO nanorod samples (see section 6.2 below) further supports and strengthens our previous assignment on the origin of this emission band. Our extensive investigations on this defect-related ZnO emission contributes to an important increase in understanding of the different optically-active defects which contribute to the near-UV band edge photoluminescence in ZnO nanostructures, and the relationship of these defects to the nanostructure morphology is key to the choice of the optimum deposition methods and conditions for a particular application.
We believe that these features and properties of the cr-ZnO/am-ZnO core/shell nanorods would be advantageous in a number of state-of-the art applications based on the core/shell architecture. Specifically, the unique architecture and properties of the core/shell cr-ZnO/am-ZnO nanorods produced in this work should prove useful in applications where the functionality arises from the presence of an amorphous shell on a ZnO crystalline nanorod core. Examples of such applications would be in ZnO supercapacitor electrodes for energy storage, the passivation of ZnO photoanodes in dye-sensitized solar cells, or the control of the emission properties of ZnO nanolasers.
6.2 High optical quality ZnO nanorods on ZnO-seeded Si substrates: 3.331 eV luminescence
In section 4.3 of this thesis, we have also reported the growth of vertically aligned ZnO nanorods with excellent optical quality by catalyst-free vapour phase transport (VPT) on the PLD prepared ZnO-seeded Si (100) substrates. We have mainly compared the near band edge emission of such VPT nanorod deposits to the previously discussed PLD core/shell nanorod deposits (where identical PLD-grown ZnO seed layers were used for both VPT- and PLD-grown nanorods), with a focus on the identification of the origin of the 3.331 eV emission feature. The main difference between the PLD- and VPT-grown nanorod samples is the presence of the
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3.331 eV emission in the former, and its complete absence in the latter (as well as in continuous PLD-grown seed layers) which was discussed in light of the differing surface morphologies and which provides strong support for our previous assignment of the origin of this defect to structural defects at the nanorod interface region.
The most important findings of this work are: (a) the nanorods are well separated and show smooth, facetted surfaces with a high c-axis orientation; (b) the nanorods also have a very high surface coverage density of ~ 18 per μm2, compared to the previous literature; (c) importantly, the nanorods have an excellent optical quality, revealed by their low-temperature PL analyses and (d) finally, this study allows us to confidently assign the 3.331 eV emission to recombination at structural defects at the core/shell boundary region as this emission band was not seen for either the PLD-grown seed layer or VPT-grown nanorod samples.
6.3 Transparent and conductive ZnO and AZO nanocrystalline thin films on flexible Zeonor plastic substrates
Zeonor (a brand of COP) plastics are highly versatile due to exceptional optical and mechanical properties which make them the choice material in many novel applications. In section 5.1 and 5.2 of this thesis, we have investigated for the first time, the use of Zeonor as a flexible substrate for the deposition of high quality ZnO and Al-doped ZnO (AZO: 3 at% Al) thin films. Films were prepared by PLD at room temperature in oxygen ambient pressures between 1 and 300 mTorr. The growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of as-grown films with thicknesses in the range 65 nm - 420 nm were measured. The films obtained are highly reproducible, with high optical transparency (> 90%), and optically very smooth (rms roughness ~ 4-8 nm for ZnO and ~ 1-2 nm for AZO). The films are also highly crystalline (average crystallite size
~ 4-22 nm for ZnO and ~ 3-18 nm for AZO) with strong c-axis orientation, and in-plane residual compressive stress in the ranges 2-7 GPa and 0.5-4 GPa for ZnO and AZO, respectively. Their electrical properties show low resistivities (10-2-10-3 Ω cm for ZnO and 10-3-10-4 Ω cm for AZO), high carrier concentrations (1020-1021 cm-3 for
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ZnO and 1021-1022 cm-3 for AZO) and reasonable Hall mobilities (4-35 cm2/Vs for ZnO and 1-18 cm2/Vs for AZO). All films display a marked hydrophobic behaviour (water contact angle > 90°). Overall, the film properties are found to depend strongly on oxygen growth pressure and mildly on film thickness. The possible applications for these films are suggested. Furthermore, the effect of ageing on the properties of these films was also investigated over a 6-month period. This ageing study shows that the AZO samples have greater stability than the ZnO samples.
The work reported in this thesis shows that the high-quality ZnO and AZO electrodes can be successfully deposited at room temperature on amorphous, flexible Zeonor plastic substrates using PLD. The most important findings of this work are:
(a) for the first time, Zeonor (a flexible, highly transparent (> 90%), low water absorption (< 0.01%) and hydrophobic) was used as a substrate for the deposition of high quality ZnO and AZO nanocrystalline thin films by PLD at room temperature;
(b) we have successfully grown high transmittance, optically smooth, low stress, highly reproducible ZnO and AZO thin films at room temperature, which show hydrophobic surfaces; (c) we have extensively investigated the film properties as a function of thickness and oxygen ambient pressure, and shed light on the aspects of the growth mechanisms and (d) the large variations of film properties with oxygen growth pressure (especially for 40 and 1 mTorr range) will attract significant attention from a wide range of scientists working in many disciplines, especially in flexible TCO-based optoelectronics, as well as the PLD community.
The work broadly discussed in the context of current literature in the field of TCO growth on plastics. This work contains new and important results in the field of flexible TCOs for the flexible optoelectronic applications. In addition, as Zeonor plastics are a widely used material in many healthcare and medical applications, the work could also find applications in the fields such as microfluidics, biosensors and biofuel-cells.
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