Piezoelectric ZnO Nanostructure for Energy Harvesting

By Yamin Leprince-Wang

Over the past decade, ZnO as an important II-VI semiconductor has attracted much attention within the scientific community over the world owing to its numerous unique and prosperous properties. This material, considered as a “future material”, especially in nanostructural format, has aroused many interesting research works due to its large range of applications in electronics, photonics, acoustics, energy and sensing. The bio-compatibility, piezoelectricity & low cost fabrication make ZnO nanostructure a very promising material for energy harvesting.

• Language: English
• Publisher: Wiley
• Release date: Mar 24, 2015
• ISBN: 9781119007449

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Contents

Cover

Contents

Title Page

Copyright Page

Preface

Acknowledgements

Introduction

1 Properties of ZnO

1.1. Crystal structure of ZnO

1.2. Electrical properties of ZnO and Schottky junction ZnO/Au

1.3. Optical properties of ZnO

1.4. Piezoelectricity of ZnO

2 ZnO Nanostructure Synthesis

2.1. Electrochemical deposition for ZnO nanostructure

2.2. Hydrothermal method for ZnO nanowire array grow

2.3. Comparative discussion on ZnO nanowire arrays obtained via electrodeposition and hydrothermal method

2.4. Influence of main parameters of hydrothermal method on ZnO nanowire growth morphology

2.5. Electrospinning method for ZnO micro/nanofiber synthesis

3 Modeling and Simulation of ZnO-Nanowire-Based Energy Harvesting

3.1. Nanowire in bending mode

3.2. Nanowire in compression mode

3.3. Nanowire arrays in static and vibrational responses

4 ZnO-Nanowire-Based Nanogenerators: Principle, Characterization and Device Fabrication

4.1. Working principle of nanogenerators

4.2. ZnO-nanowire-based energy harvesting device fabrication

4.3. ZnO-nanowire-based energy harvesting device characterization

4.4. ZnO-nanostructure-based hybrid nanogenerators

Conclusion

Bibliography

Index

List of Tables

1 Properties of ZnO

Table 1.1. Work function of some metals and their contact type with ZnO

Table 1.2. Different Schottky barrier height values of the Au/ZnO junction obtained from different synthesis and measurement methods

4 ZnO-Nanowire-Based Nanogenerators: Principle, Characterization and Device Fabrication

Table 4.1. Surrounding harvestable mechanical energy.

List of Illustrations

Introduction

Figure I.1. Electromechanical conversion by piezoelectric effect

Figure I.2. Most commonly ZnO nanostructures a) individual nanowires, and b) plan view and c) cross-section view of nanowire arrays

1 Properties of ZnO

Figure 1.1. Three crystalline structures of ZnO a) wurtzite (hexagonal symmetry), b) blende (cubic symmetry) and c) rocksalt (cubic symmetry)

Figure 1.2. Electronic levels in a metal a) and an n-type semiconductor b). c) Energy diagram of the Schottky junction under forward bias without c) and with d) the interface layer

Figure 1.3. Sample preparation for I–V measurements on electrodeposited ZnO thin film with top and bottom Ohmic contacts

Figure 1.4. a) and b) experimental setup for I–V measurements used both for ZnO thin film and the nanowires. c) a typical I–V characteristic obtained between one of the microelectrodes and the bottom electrode

Figure 1.5. a) Schematic description of electrical contact made on ZnO nanowire arrays a PMMA layer was spin-coated before Al top electrodes deposition. b) Tilted and c) top-view SEM images of the ZnO nanowire arrays after PMMA layer deposition. d) SEM image of the top aluminum microelectrodes (80 × 80 μm²) deposited on the ZnO nanowire arrays

Figure 1.6. Representative I–V characteristics under Al/ZnO/Au configuration from the ZnO thin film a) and nanowire arrays b), respectively, demonstrating typical Schottky contact behavior

Figure 1.7. I–V curves of ZnO thin films a) and ZnO nanowires b). Experimental data (bold line) compared to the fits without series resistance (solid line) and with series resistance (dashed line) according to Equation [1.1]

Figure 1.8. Equivalent electrical circuits of the ZnO nanowire arrays

Figure 1.9. Dispersion of the regular refractive index n0 and the extraordinary refraction index ne of the crystal ZnO

Figure 1.10. PL spectrum of a hydrothermal-grown ZnO nanowire array a) and visible emission fitted by three Gaussians centered at ~525 (G1), ~580 (G2) and ~660 nm (G3), respectively b)

Figure 1.11. PL spectra of a hydrothermal-grown ZnO nanowire sample before and after an annealing treatment at 400°C during 30 min in the air

Figure 1.12. Piezoelectric effect in ZnO unit cell (P – dipole vector)

Figure 1.13. Illustration of two operating modes for the piezoelectric material a) 33 mode and b) 31 mode

2 ZnO Nanostructure Synthesis

Figure 2.1. Experimental set-up of a three electrodes electrochemical cell

Figure 2.2. Schematic illustration for the template method using polycarbonate membrane for the ZnO nanowire growth

Figure 2.3. Extraction of the ZnO nanowires from template matrix after electrodeposition using chloroform drops

Figure 2.4. TEM images showing a general morphology a) & b) of the electrodeposited ZnO nanowires via template method. Both TEM observation c) and HRTEM image d) showing an individual monocrystalline ZnO nanowire with near [100] growth direction

Figure 2.5. SEM image of a nanostructured PMMA template

Figure 2.6. SEM images of a ZnO nanopillar array on a gilded silicon substrate a) top view, b) side view with broad nanopillars and c) side view of candle-shaped nanopillars

Figure 2.7. Planar & cross-sectional view SEM images of an electrodeposited seed layer before ZnO nanowire arrays growth

Figure 2.8. Planar view SEM image of the ZnO nanowire array obtained by electrodeposition (sample A)

Figure 2.9. Planar view SEM images a) ITO substrate, b) a homogeneous seed layer composed of small grains (size ~ 20 nm).

Figure 2.10. Planar view SEM image of the ZnO nanowire array obtained by hydrothermal method (sample B)

Figure 2.11. HRTEM images show an excellent monocrystalline microstructure of the ZnO nanowires obtained by electrodeposition a) and by hydrothermal method b), respectively

Figure 2.12. PL spectra of the ZnO nanowire arrays grown by the hydrothermal method and electrodeposition

Figure 2.13. Schematic sketches of two possible routes for ZnO nanowire array synthesis

Figure 2.14. SEM images of ZnO nanowires synthesized via way 1 a) and way 2 b)

Figure 2.15. SEM images of ZnO nanowires synthesized at pH of 6.4 a), 6.6 b), 6.8 c), 7.0 d), 7.2 e), and 7.4 f) respectively. Scale bar 500 nm

Figure 2.16. SEM images of ZnO nanowires grown at a) 60°C (top view), b) 70°C, c) 80°C and d) 90°C respectively. Scale bar 300 nm

Figure 2.17. SEM images of ZnO nanowires synthesized for growth time of 30 min a), 1 h b), 2 h c), 3 h d), 5 h e) and 6 h f), respectively. Scale bar 1 μm

Figure 2.18. Average lengths of ZnO nanowires as a function of the growth time

Figure 2.19. Coordination polyhedron in crystalline structure of the ZnO hexagonal wurtzite structure

Figure 2.20. Electrospinning experimental setup a) and schematic illustration of the principle of the electrospinning process b)

Figure 2.21. a) Schematic illustration of the setup to obtain the aligned nanofibers between two parallel conductive Si electrodes with a controlled gap. b) SEM image shows aligned nanofibers between electrodes [from YAN 09]

Figure 2.22. SEM images of a) PVA/zinc acetate composite fibers obtained with 8 kV high voltage during 10 min; b) fibers after calcination at 700°C during 2 h for PVP evaporation and c) ZnO nanorods/nanowires on ZnO nanofiber (hydrothermal method, 2 h at 95°C)

3 Modeling and Simulation of ZnO-Nanowire-Based Energy Harvesting

Figure 3.1. a) Individual nanowire geometry D = 50 nm and L = 800 nm and meshing structure in COMSOL multiphysics b)

Figure 3.2. a) A force of 70 nN is applied at the top of nanowire. b) The displacement in the bent nanowire

Figure 3.3. Electric potential generation in the ZnO nanowire undergoing flexion. a) Longitudinal section electric potential profile. b) and c) The electric potential value plotting along the nanowire. d) Cross-section electric potential profile. e) The electric potential value plotting across the nanowire (L = 800 nm, D = 50 nm)

Figure 3.4. Variation of the generated electric potential according to the length of the nanowire ZnO (F = 70 nN, D = 50 nm)

Figure 3.5. Variation of the electric potential as a function of the nanowire radius (F = 70 nN, L = 800 nm)

Figure 3.6. Variation of the ele ctric potential as a function of the aspect ratio (length/diameter) with the fixed displacement Y = 100 nm

Figure 3.7. Nanowire in axial compression a) and its axial deformation b). c) The electric potential variation along z-axis and the electric potential longitudinal profile of ZnO nanowire (L = 800 nm and D = 50 nm) undergoing an axial compression

Figure 3.8. Variation of the electric potential as a function of the nanowire length (F = 70 nN and D = 50 nm)

Figure 3.9. Variation of the electric potential as a function of the nanowire diameter (F = 70 nN and L = 800 nm)

Figure 3.10. Variation of the electric potential as a function of the aspect ratio (length/diameter) with a fixed axial displacement of 0.16 nm

Figure 3.11. Modeling of a ZnO nanowire network undergone a compressive force

Figure 3.12. ZnO nanowire deformation of the network under a static force of 6 µN a) and the generated electric potential b) – without top and bottom electrodes

Figure 3.13. a) Schematic of nanogenerator based on ZnO nanowires under periodic pressure. b) Alternating electric potential generated in the ZnO nanowires for different pressure values

4 ZnO-Nanowire-Based Nanogenerators: Principle, Characterization and Device Fabrication

Figure 4.1. a) Experimental setup and procedures for generating electricity by deforming a ZnO piezoelectric nanowire with a conductive AFM tip in contact mode. b) An overlap plot of the AFM topological image (dashed line) and the corresponding generated voltage (solid line) for a single scan of the tip across a ZnO nanowire – a delay in the electricity generation is apparent (from [WAN 06])

Figure 4.2. a) Schematic definition of a nanowire and the coordination system used in the simulation L = 800 nm and T = 50 nm (with the same aspect ratio than that of the nanowires in Chapter 3). b) Longitudinal strain εz distribution in the nanowire bent by an AFM tip. c) and d) The corresponding electric field Ez and potential distribution resulted by the piezoelectric effect in the bent ZnO nanowire

Figure 4.3. Working mechanism for the power generation process of a piezoelectric ZnO nanowire as a result of coupled piezoelectric and semiconducting properties in conjunction with the Schottky barrier at the AFM metal tip–semiconductor ZnO nanowire interface. The contact of Pt/ZnO shows a Schottky rectifying behavior. The inset shows a typical current–voltage (I–V) characteristic for a metal/n-type semiconductor Schottky contact (from [WAN 06])

Figure 4.4. Band diagram for understanding the charge outputting and flowing processes in the nanogenerator (from [WAN 11])

Figure 4.5. Main steps for fabrication of the piezoelectric nanogenerator based on ZnO nanowire arrays

Figure 4.6. Schematic illustrations of four types of the top electrode for nanogenerator device based on the ZnO nanowire array

Figure 4.7. Design a), working principle b) and fabrication process c) of lateral nanowire array nanogenerator (from [XU