Nanoscale Piezoelectric Energy Harvesting

by Stephen J. Kilpatrick and Ashley D. Mason

ARL-TR-5229 June 2010

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Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TR-5229 June 2010

Nanoscale Piezoelectric Energy Harvesting

Stephen J. Kilpatrick

Sensors and Electron Devices Directorate, ARL

and

Ashley D. Mason

Oregon State University

Corvallis, OR 97331-4501

Approved for public release; distribution unlimited. 
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Nanoscale Piezoelectric Energy Harvesting

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Stephen J. Kilpatrick and Ashley D. Mason

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U.S. Army Research Laboratory

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14. ABSTRACT

The Sensors and Electron Devices Directorate (SEDD) has initiated an investigation studying nanoscale harvesting elements to 

expand the scope of piezoelectric research. The piezoelectric characteristics of zinc oxide (ZnO) nanowires are explored for 

potential use as a nanogenerator. A mechanical deformation imposed on a nanowire translates into a piezoelectric potential 

generated across the material. The proposed approach involves transferring grown ZnO nanowires from growth substrates to a 

flexible platform for mechanically deforming the wires. Due to the extremely small dimensions of the ZnO nanowires, if the 

mechanical deformation does not generate sufficient voltage to meet application requirements, the integration of a storage 

element may become necessary. We optimized the device fabrication and characterization methods to evaluate a single ZnO 

nanowire. The results gleaned from the initial experiments should aid the search for a viable power generation method for 

nanoscale devices, such as Soldier-borne and vehicle-mounted sensors.

15. SUBJECT TERMS

ZnO, zinc oxide, nanowire, piezoelectric, energy harvesting, hydrothermal

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19a. NAME OF RESPONSIBLE PERSON

Stephen J. Kilpatrick

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(301) 394-0071

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iii

Contents

List of Figures iv

List of Tables iv

1. Introduction/Background 1

2. Experiment 1

2.1 Nanowire Growth .....................................................................................................................1

2.1.1 Vapor-solid Growth......................................................................................................2

2.1.2 Hydrothermal Growth ..................................................................................................2

2.2 Overview ..................................................................................................................................3

2.3 Initial Tests...............................................................................................................................3

3. Results and Discussion 6

4. Summary and Conclusions 7

5. References 8

Distribution 9
iv

List of Figures

Figure 1. Experiment setup diagrams for vapor-solid and hydrothermal NW growth. ..................2

Figure 2. Images of nanowires grown using vapor-solid (left) and hydrothermal (right) 

methods......................................................................................................................................3

Figure 3. Cross-sectional views of (a) growth substrate showing the silicon sample (grey), 

ZnO seed (blue), and nanowire growth; and (b) the transferred wire on substrate. ..................4

Figure 4. SEM images showing transfer of NWs from the growth substrate to the secondary 

substrate and isolation of a single ZnO NW. .............................................................................4

Figure 5. The center square is adhesive side up, which is held in place on the Si wafer by the 

face-down Kapton tape. .............................................................................................................5

Figure 6. The etch on Si showing that when the Au is completely removed, the NWs are 

removed with the Au..................................................................................................................5

Figure 7. SiO2 on SS showing the flakes that resulted from using the Unaxis tool........................6

Figure 8. The influence of large-sized Au contacts on the ZnO NW transfer................................6

List of Tables

Table 1. A summary of the various substrate candidates and their results......................................5
1

1. Introduction/Background

Zinc oxide (ZnO) has previously attracted a lot of attention because of its large bandgap, good 

transparency, high electron mobility, biosafety, and other desirable characteristics. ZnO has 

since found potential applications in optoelectronics, thin-film transistors, sensors, and energy 

harvesting applications (1). The ability to repeatedly create nanostructured ZnO also makes it an 

attractive candidate for future electronic nanodevices. The energy harvesting applications of 

ZnO nanowires are of particular interest to the U.S. Army as a potential means to reduce the 

quantity of batteries that need to be carried by the American Soldier. Previous work by Wang et 

al. from the Georgia Institute of Technology demonstrated that small amounts of energy can be

harvested from bending ZnO nanowires (2). ZnO is piezoelectric, an inherent material property 

in which a mechanical stress results in the generation of an electric potential across the material. 

Due to the infancy of ZnO nanowires as an energy harvesting system, many challenges must be 

overcome and many questions still need to be answered.

The charge-distribution created in a bent wire is a complicated model dependent on the strain 

gradients (3). Depending on the type of bending and the device being created, different models 

have been proposed. When a single nanowire, or an array of nanowires, is bent with an atomic 

force microscopy (AFM) tip, previous research suggested that the charge-distribution runs along 

the diameter of the nanowire (analogous to piezoelectric films) (2, 4). However, when flexing a 

single microwire, the same research suggested that the charge-distribution in that case runs along 

the length rather than the diameter (5).

Alternative models have been presented that assess the systems used in previous work, theorize 

ideal placement of electrodes, and propose new device designs and measurement systems (6, 7). 

The work on devices inspired by the piezoelectric effect exhibited by ZnO is still dominated by 

one primary research group, and not many instances of independent confirmation of successful 

energy harvesting systems have been documented (8). Additional studies are required to 

determine the viability of such devices and evaluating their success in potential markets is still 

necessary.

2. Experiment

2.1 Nanowire Growth

ZnO nanowires were grown on silicon (Si) substrates with ZnO seed layers using two different 

methods: (1) vapor-solid growth using ZnO and graphite precursors and (2) hydrothermal 

growth using zinc nitrate (Zn(NO3)2) and hexamethylene tetramine (HMT).
2

2.1.1 Vapor-solid Growth

Vapor-solid nanowires (NWs) were grown using a tube furnace with ZnO and graphite 

precursors. The ratio of the precursors was 1:1. The ZnO and graphite were ground and mixed 

together before being placed into the furnace. The temperature of the furnace at the location of 

the precursor mixture was ~915 °C. The substrate was placed downstream, where the 

temperature was ~760 °C. The pressure in the tube was then reduced to 1 torr. Nitrogen was 

flowed through the tube at a rate of 150 sccm, and 1 to 3 sccm of oxygen was injected near the 

substrate to aid in NW nucleation. The growth time was 30 min, after which the furnace was 

turned off and the tube was allowed to cool to room temperature. 

2.1.2 Hydrothermal Growth

Hydrothermal ZnO NWs were grown in solution using Zn(NO3)2 and HMT at 80 °C. The 

solution was heated using a hot plate and mixed continuously using a magnetic stir bar at 60 rpm. 

The growth substrate was taped with two pieces of Kapton tape to the backside of a Petri dish,

which was then floated on top of the solution. During growth, the solution was covered with 

parafilm to minimize evaporation. Although the hydrothermal method is performed at low 

temperature, facilitating the use of various substrates, growth can take up to 24 h.

The purpose of using two different growth methods was to examine the differences in resulting 

morphologies and the respective effects on the piezopotential generated by one NW. The 

experimental setups are shown in figure 1 with the associated results shown in figure 2. Further 

investigation of precursors used for the solution growth method is ongoing to obtain longer 

wires. Development of this growth process will aid in other areas where lower processing 

temperatures are necessary.

Figure 1. Experiment setup diagrams for vapor-solid and hydrothermal NW growth.
3

Figure 2. Images of nanowires grown using vapor-solid (left) and hydrothermal (right) methods.

Note: The difference is shown in scale bars.

2.2 Overview

A general list of tasks was created to outline the steps necessary to design and implement a 

single-nanowire device. An appropriate, non-charging substrate is necessary for identifying the 

NW’s position in a scanning electron microscope (SEM) before e-beam lithography. Due to 

poor visibility of the NWs through the photoresist, the pattern was made using reference 

coordinates taken from an image of the NW before the photoresist was deposited. 

The chosen substrate was patterned with a large-scale mask to reduce the amount of time 

necessary to finish the e-beam write to the NW. The NWs were to be transferred between two 

separate lithography steps. A system for bending and measuring the piezoelectric potential was 

designed for use after contacting the NW. Preliminary tests were performed using existing 

equipment: a probe station and a Keithley 4200 Semiconductor Characterization System. As 

needed, alterations were made to reduce ambient noise or interference. If a higher frequency 

actuation is needed, the voice coil and associated controller (at Oregon State University) can be 

modified to bend the NW.

2.3 Initial Tests

After NW growth, a test of the mechanical transfer process was performed to move the vertically

aligned NWs to a new substrate where the NWs were laid horizontally on the substrate 

(figure 3). Images were taken using a SEM to see if pressing the two substrates together and 

lightly rubbing them was enough to transfer wires from one substrate to another. Figure 4 shows 

that NWs could be successfully transferred to a silicon substrate with native oxide, and that a 

single nanowire could be isolated.
4

Figure 3. Cross-sectional views of (a) growth substrate showing the 

silicon sample (grey), ZnO seed (blue), and nanowire growth; and 

(b) the transferred wire on substrate.

Figure 4. SEM images showing transfer of NWs from the growth substrate to 

the secondary substrate and isolation of a single ZnO NW.

When choosing a substrate, there are a few considerations which limit the viable options. 

Potential transfer substrates should be flexible enough to bend without damaging the substrate or 

the measurement apparatus, but robust enough to withstand the device fabrication processing. 

Preliminary bending tests were performed using a probe station. Alternatively, a carrier wafer 

could be used as long as the flexible substrate could be removed and handled without damaging 

the device. As mentioned previously, the contacts to the wire are written using e-beam 

lithography. The substrate must not undergo excessive charging in the environmental SEM

(ESEM), so that the coordinates taken from images prior to photoresist (PR) placement are 

reliable.

Various substrates were examined as candidates. Table 1 summarizes the experiments and 

results obtained when trying to complete processing up through the first metallization and NW 

transfer. Figure 5 shows the Kapton tape double-layer on an Si carrier. Figure 6 illustrates the 

problems with the Kapton tape and gold (Au). Figure 7 shows the SiO2 flaking off of the 

stainless steel (SS) substrate. 
5

Table 1. A summary of the various substrate candidates and their results.

Substrate Description Summary of Results

Kapton tape on SS There were adhesion problems with platinum on Kapton tape. During 

lithography, the Kapton started to peel off of the SS substrate. A metal etch was 

unsuccessful because of adhesion problems. We tried to sonicate the excess 

metal that was left and all metal was removed.

Kapton tape double-layer on 

Si carrier 

(see figure 5)

This sample was created as a double-layer Kapton tape sample that could be 

removed from the Si wafer once processing was complete. Problems with the 

adhesion of the platinum on Kapton ruled out this substrate option as well.

Aluminum oxide (Al2O3) on 

aluminum

The thermal Al2O3 layer could not be grown thick enough on aluminum in the 

Blue M oven.

Kapton The Kapton has to be coated to reduce charging in the ESEM. We tried an Au 

coating because oxygen plasma needed for removing carbon could change the 

electrical properties of the wires. We tested the ability for Kapton to withstand 

potassium iodide (Au etch). The Kapton survived the necessary etch times. We 

tried coating and etching of ZnO wires on a Si substrate in parallel with trying to 

determine how much Au was needed to reduce charging. The Kapton could not 

be coated with Au well enough to reduce charging in the ESEM in high vacuum 

mode, which was needed for e-beam writing. Also, on the Si samples, when the 

Au was removed, the NWs were removed as well (see figure 6). 

Silicon dioxide (SiO2) on SS First, the Unaxis VLR 700 deposition tool was used to deposit SiO2 on the SS. 

The SiO2 flaked off of the substrate (see figure 7). After some discussion, we

discovered that the recipe on the Plasma Therm 790 was better developed. We 

completed a successful deposition. We successfully imaged the NWs as well.

Figure 5. The center square is adhesive side up,

which is held in place on the Si wafer 

by the face-down Kapton tape.

Figure 6. The etch on Si showing that when the Au is completely removed, the 

NWs are removed with the Au.
6

Figure 7. SiO2 on SS showing the flakes that resulted from using the Unaxis tool.

3. Results and Discussion

A non-charging substrate was discovered that could be integrated into a NW bending system 

through systematic trials. NW transfer to the flexible substrate became more difficult with the 

integration of the large-size contacts. Figure 8 shows the resulting transfers, where wires would 

gather around the Au contacts or not transfer as desired. For future devices, only one set of 

large-size contacts (two per sample previously) will be patterned to leave space available for 

direct transfer of wires to the SiO2/SS sample. 

Figure 8. The influence of large-sized Au contacts on the ZnO NW transfer.

Initial measurements using the probe station and Keithley 4200 showed that ambient noise was 

greater than the piezoelectric measurements from actuation. Preliminary testing was performed 

on samples that had NWs connecting the large-scale contacts rather than an e-beam patterned 

connection to a single wire. The mechanical actuation of the sample was performed by mounting 

a sample to a larger stage and applying pressure with a probe tip on one side of the substrate.
7

4. Summary and Conclusions

The results gleaned from experiments over a two-month period showed a necessity for minor 

revisions to the previously used processing technique. We will continue this work at Oregon 

State University with an investigation of modified hydrothermal growth methods. Using a 

microwave as a heating mechanism to decrease growth time has been suggested by Unalan et al.

(9). We will design experiments around determining the best growth parameters for obtaining 

NWs that are long enough (~1 μm) to pattern with e-beam lithography. 

To facilitate easier alignment of a single NW, we will perform electrophoresis studies. 

Additionally, the project could be extended to actuate multiple, horizontally aligned NWs at once 

to potentially increase the usable piezoelectric potential. Measurement and actuation systems at 

Oregon State University will be developed in parallel. 

Continuing partnerships with ON Semiconductor and Hewlett-Packard will help ease the load of 

SEM analysis, which needs to be performed to examine the growth, transfer, and alignment of 

the ZnO NWs. A research grade microwave, with integrated temperature control, is available for

demo from CEM Corporation. Plans to compare the piezoelectric output from ZnO NWs grown 

using different methods are in progress as well.
8

5. References 

1. Wang, Z. L. Materials Science and Engineering R 2009, 64, 33–71.

2. Wang, Z. L. Advanced Functional Materials 2008, 18, 3553–3567.

3. Gao, Z.; Zhou, J.; Gu, Y.; Fei, P.; Hao, Y.; Bao, G.; Wang, Z. L. Journal of Applied Physics

2009, 105, 113707-1–113707-6.

4. Wang, Z. L. Advanced Materials 2007, 19, 889–892.

5. Zhou, J.; Fei, P.; Gu, Y.; Mai, W.; Gao, Y.; Yang, R.; Bao, G.; Wang, Z. L. Nano Letters

2008, 8, 3973–3977.

6. Alexe, M.; Senz, S.; Schubert, M. A.; Hesse, D.; Gösele, U. Advanced Materials 2008, 20, 

4021–4026.

7. Schubert, M. A.; Senz, S.; Alexe, M.; Hesse, D.; Gösele, U. Applied Physics Letters 2008, 

92, 122904-1–122904-3.

8. Choi, M.-Y.; Choi, D.; Jin, M.-J.; Kim, I.; Kim, S.-H.; Choi, J.-Y.; Lee, S. Y.; Kim, J. M.;

Kim, S.-W. Advanced Materials 2009, 21, 2185–2189. 

9. Unalan, H. E.; Hirala, P.; Rupesinghe, S.; Dala, S.; Milne, W. I.; Amaratuga, G.A.J.

Nanotechnology 2008, 19, 255608-1–255608-5.
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