04-03-2013, 12:32 PM
Nanowire Piezoelectric Nanogenerators on Plastic Substrates as Flexible Power Sources for Nanodevices
[attachment=52529]
INTRODUCTION
Research applications in biomedical science and technology
usually require various portable, wearable, easy-to-use, and/or
implantable devices that can interface with biological systems.[
1,2] Organic or hybrid organic–inorganic microelectronics
and nanoelectronics have long been a possibility.[3–7] However,
these devices require a power source, such as
electrochemical cells[8] or piezoelectric,[9] thermoelectric,[10]
and pyroelectric transducers,[11] to generate or store the electrical
energy created through chemical, mechanical, or thermal
processes. Finding a suitable power source has remained
a major challenge for many devices in bioengineering and
medical fields.
ZnO is a typical piezoelectric and pyroelectric inorganic
semiconducting material used for electromechanical and thermoelectrical
energy conversion. Nanostructures of ZnO,[12]
such as nanowires (NWs),[13,14] nanobelts (NBs),[15] nanotubes,[
16–18] nanorings,[19] nanosprings,[20,21] and nanohelices,[22]
have attracted extensive research interest because of their potential
applications as nanoscale sensors and actuators. While
most of the current applications focus on its semiconducting
properties, only a few efforts have utilized the nanometerscale
piezoelectric properties of ZnO. Using ZnO NW arrays
grown on a single-crystal sapphire substrate, we have successfully
converted mechanical energy into electrical energy at
the nanoscale.[23] A conductive atomic force microscopy
(AFM) tip was used in contact mode to deflect the aligned
NWs. The coupling of piezoelectric and semiconducting properties
in ZnO creates a strain field and charge separation
across the NWs as a result of their bending. The rectifying
characteristic of the Schottky barrier formed between the
metal tip and the NW leads to electrical current generation.
This is the principle behind piezoelectric nanogenerators.
Experimental
The plastic substrates used were 50 lm thick Kapton polyimide
films provided by Dupont. Zinc nitrate hydrate (Zn(NO3)2·6H2O)
and hexamethylenetetramine were purchased from Fluka.
Synthesis of Aligned ZnO Nanowires: The aligned ZnO NWs arrays
were grown on a plastic film using a solution-based method [27–31].
Serving as one of the electrodes for a later electrical connection and
also functioning as uniform nucleation sites for NW growth, a thin,
ca. 100 nm, layer of Au was deposited on the plastic substrate by
using thermal evaporation at 0.3-0.5 Ås–1 before the hydrothermal
growth of ZnO NW arrays. The growth of ZnO NWs was conducted
by suspending the Au-coated plastic substrate in a Pyrex glass bottle
filled with an equal molar aqueous solution of Zn(NO3)2·6H2O
(0.01–0.04M) and hexamethylenetetramine (0.01–0.04M) at temperatures
between 60 and 80 °C. The temperature, solution concentration,
reaction time (1–72 h), and substrate surface quality were optimized
for growing NW arrays with controlled dimensions and orientation.
After reaction, the plastic substrates were removed from the solution,
rinsed with deionized water, and dried in air at 60–80 °C overnight.
Morphology and Structure Characterization: The as-grown ZnO
NWs were characterized with a scanning electron microscope
(LEO 1530 and 1550 FEG at 5 and 10 kV) and a transmission electron
microscope (Hitachi HF-2000 at 200 kV).
[attachment=52529]
INTRODUCTION
Research applications in biomedical science and technology
usually require various portable, wearable, easy-to-use, and/or
implantable devices that can interface with biological systems.[
1,2] Organic or hybrid organic–inorganic microelectronics
and nanoelectronics have long been a possibility.[3–7] However,
these devices require a power source, such as
electrochemical cells[8] or piezoelectric,[9] thermoelectric,[10]
and pyroelectric transducers,[11] to generate or store the electrical
energy created through chemical, mechanical, or thermal
processes. Finding a suitable power source has remained
a major challenge for many devices in bioengineering and
medical fields.
ZnO is a typical piezoelectric and pyroelectric inorganic
semiconducting material used for electromechanical and thermoelectrical
energy conversion. Nanostructures of ZnO,[12]
such as nanowires (NWs),[13,14] nanobelts (NBs),[15] nanotubes,[
16–18] nanorings,[19] nanosprings,[20,21] and nanohelices,[22]
have attracted extensive research interest because of their potential
applications as nanoscale sensors and actuators. While
most of the current applications focus on its semiconducting
properties, only a few efforts have utilized the nanometerscale
piezoelectric properties of ZnO. Using ZnO NW arrays
grown on a single-crystal sapphire substrate, we have successfully
converted mechanical energy into electrical energy at
the nanoscale.[23] A conductive atomic force microscopy
(AFM) tip was used in contact mode to deflect the aligned
NWs. The coupling of piezoelectric and semiconducting properties
in ZnO creates a strain field and charge separation
across the NWs as a result of their bending. The rectifying
characteristic of the Schottky barrier formed between the
metal tip and the NW leads to electrical current generation.
This is the principle behind piezoelectric nanogenerators.
Experimental
The plastic substrates used were 50 lm thick Kapton polyimide
films provided by Dupont. Zinc nitrate hydrate (Zn(NO3)2·6H2O)
and hexamethylenetetramine were purchased from Fluka.
Synthesis of Aligned ZnO Nanowires: The aligned ZnO NWs arrays
were grown on a plastic film using a solution-based method [27–31].
Serving as one of the electrodes for a later electrical connection and
also functioning as uniform nucleation sites for NW growth, a thin,
ca. 100 nm, layer of Au was deposited on the plastic substrate by
using thermal evaporation at 0.3-0.5 Ås–1 before the hydrothermal
growth of ZnO NW arrays. The growth of ZnO NWs was conducted
by suspending the Au-coated plastic substrate in a Pyrex glass bottle
filled with an equal molar aqueous solution of Zn(NO3)2·6H2O
(0.01–0.04M) and hexamethylenetetramine (0.01–0.04M) at temperatures
between 60 and 80 °C. The temperature, solution concentration,
reaction time (1–72 h), and substrate surface quality were optimized
for growing NW arrays with controlled dimensions and orientation.
After reaction, the plastic substrates were removed from the solution,
rinsed with deionized water, and dried in air at 60–80 °C overnight.
Morphology and Structure Characterization: The as-grown ZnO
NWs were characterized with a scanning electron microscope
(LEO 1530 and 1550 FEG at 5 and 10 kV) and a transmission electron
microscope (Hitachi HF-2000 at 200 kV).