A novel hybrid energy harvesting cell by combining thermoelectric and piezoelectric devices with one dimensional ZnO nanostructures

NCKU

Aim for the Top University Project

2013 Annual Report

Research Project Title（Chinese）：

氧化鋅一維奈米結構製作結合熱電及壓電之新

穎複合電池

Research Project Title（English）：

A novel hybrid energy harvesting cell by combining

thermoelectric and piezoelectric devices with one dimensional ZnO nanostructures

Project Duration：

01/2013~12/2015

Principal Investigator（PI）：

Chuan-Pu Liu

Distinguished Professor

Department/Institute/Center：

Department of Materials Science and Engineering

Engineering

Co-Principal Investigator（CO-PI）：None

Position：

Department/Institute/Center：

College：

Signature of Principal Investigator：

Signature of Chairman of Department/Institute：

I.  Abstract ... 3 

II.  Content of Research Proposal ... 4 

III. Results and Discussion ... 8 

IV.  Contribution ... 25 

V.  Expenses ... 26 

I. Abstract

In2O3(ZnO)n superlattice nanowires containing In-O atomic layers inserted between In/ZnO slabs are successfully synthesized by thermal evaporation and condensation method. From transmission electron microscopy analysis, the thickness of the slabs adjacent to each In-O insertion layers is around 7-8 In/Zn-O atomic layers with M-modulated structures in zigzag shape distributed within to relax the lattice mismatch in In/Zn-O slabs. There are two kinds of superlattice nanowires; i.e, the longitudinal or transverse superlattice structure depending on the insertion layers orientated parallel or perpendicular to the axial direction of the nanowires. Thermal conductivity is measured by Joule-Heating on nanodevices containing single nanowires. The In2O3(ZnO)n superlattice nanowires demonstrate great thermoelectric properties.

In moving forward, we further demonstrate a new structure by coupling homologous In2O3(ZnO)n with In-doped ZnO into heterojunction belts synthesized by alloy-evaporation deposition, offering multiple functions with strong confined optical emissions and high power factors. Energy-filter secondary electron images reveal a five layer contrast in the width direction of the belts corresponding to In2O3(ZnO)n/In:ZnO/ZnO/In:ZnO/In2O3(ZnO)n, as confirmed by transmission electron microscopy analysis. The indium-doped ZnO channels confined in two sides behave like quantum wells through band alignment to be predominant in the main UV emission at 385 nm, as revealed by cathodoluminescence spectroscopic imaging. More intriguingly, this novel heterostructure provides an synergistic effect in enhancing electron conduction through the indium doped ZnO layer, while impeding phonon transportation through the homologous In2O3(ZnO)n layer with numerous interfaces, leading to improved power factor.

Piezoelectric nanogenerators (NGs) made of GaN nanowires are systematically investigated as a function of doping concentration, showing the highest output voltage of 80 mV. Results indicate that free carriers would impose a screening effect, which strongly degrades the output performance of NGs. The work provides a crucial guideline in designing a high-output-power AC NG, with the criterion of a nearly depleted semiconductor as the core material. Moreover, we succeed in integrating vertical ZnO nanowire arrays grown on naked p-GaN substrates by a low temperature growth method serving as a perfect template for realizing the piezo-tronic and piezo-phototronic effect. The length and diameter of single crystalline ZnO nanowire arrays can be readily controlled. Thereby, the current-voltage characteristics of the n-ZnO/p-GaN hetero- structure upon strain exhibit the expected piezo-tronic effect, which can be projected into piezo-phototronic applications in the near future.

The work progresses demonstrated not only have promised on the exciting further movement in each cell, but shed a light on the successful integration into hybrid cells in this coming year for higher impact.

II. Content of Research Proposal

Background and Significance of Research

2-1 Thermoelectric devices on one dimensional ZnO nanostructures

Based on our previous research experiences, we will extend to grow one- dimensional superlattice structures and explore their applications into thermoelectric and piezoelectronic devices. Development of thermoelectric devices has been proposed as one of the most important solutions to solve energy shortage, however, we lack of natural high-efficiency bulk thermoelectric materials due to a conflict demand between high electrical conduction and low thermal conduction. Thereby, for better thermoelectric devices, thermoelectric materials have to be artificially designed. In this regard, materials made into both nanostructures and superlattices have been regarded to be the most efficient methods to increase effective surface or interface area. Thus, we propose to combine superlattice structure into one-dimensional nanowires in this three year proposal to aim for high thermoelectric efficiency with thorough analysis of the dependence of thermoelectric properties on composition and structure of constituent materials. Finally, we should be able to demonstrate the best energy management strategy by integrating thermoelectric devices with other energy-harvest nanodevices, here nanogenerators, as the most high-performance hybrid energy nano-devices to harvest different forms of energies simultaneously. The successful hybrid nano-device will be the high impact one.

Introduction of the thermoelectric properties

To assess the efficiency of a thermoelectric device, a thermoelectric figure of merit is defined as ZT = S2Tσ / κ, where S is the Seebeck coefficient; T being the absolute temperature; σ being the electrical conductivity and κ is the thermal conductivity. To enhance the ZT value of thermoelectric materials, there are generally two approaches:

1. Improve the power factor (power factor = S2σ); which can be affected by four factors: (a) scattering parameter, (b) density of states, (c) carrier mobility, (d) Fermi energy level. The power factor can be optimized by changing doping concentration to adjust Fermi energy level.

2. Reduce thermal conductivity κ = κL + κe; where κe and κL are the thermal conductivity of

electrons and lattices, respectively. Most thermal conductance of thermoelectric materials is governed by lattice vibrations (phonons), thus reducing κL becomes important to improve ZT.

Since κL = 1/3 CvVl, where Cv is specific heat; V being velocity of sound in the material, and l is

Thermoelectric properties of one dimensional nanostructure

In addition to reducing the size of the materials, creating more internal interfaces can also be implemented to decrease thermal conductivity. These coherent interfaces will scatter phonons more than electrons. Thus, thermal conductivity can be reduced, while electrical conductivity is kept. Thereby, superlattice structure is one efficient way to enhance the thermoelectric properties. Yang et al have reported ZnO superlattice nanowires [1]. They deposited In and Ga nanoparticles on the sidewalls of the ZnO nanowire arrays by thermal evaporation, followed by annealing under high temperature oxygen atmosphere. In and Ga atoms at high temperatures diffuse into the nanowires through Zn vacancies to form superlattices, and the superlattice structure is composed of InO2-octahedral planes and Wurtzite MZnnO(n+1) (M = In, Ga) tablet. Thermal conductivity is significantly decreased, while Seebeck coefficient is increased due to low energy electron being filtered by superlattices. A small amount of In and Ga atoms can also be doped into the ZnO nanowires to enhance electrical conductivity. Therefore, while the conductivity and Seebeck coefficient of the nanowires are increased, thermal conductivity is decreased. Three thermoelectric coefficients are enhanced at the same time, resulting in thermoelectric figure of merit rise by 2.5 orders of magnitude (from 1.7x10-4 for ZnO nanowires to 0.055 for the IGZO nanowires at 300 K). It not only shows excellent thermoelectric properties of the 1D superlattice structure but also proves that the structure is one of the important directions in the future thermoelectric field.

Methods for measuring thermoelectric properties

be local heating, and heat will be transported from the middle of the nanowire to both ends. Because the thermal conductivity of Au is much higher than GaAs, the heat will be transported rapidly from both ends of the nanowire to Au. Therefore the temperature in the center will be higher than in both ends, resulting in a temperature gradient. Thermal conductivity can be obtained by the relationship between Raman shift of TO peak and temperature.

2-2 Nanogenerator

As the fossil fuel consumes in an incredible rate, the renewable energy becomes an urgent and important issue now in our world. In order to solve the energy crisis problem, utilizing natural energy such as motion, vibration and liquid flow and convert them to electricity become an important breakthrough in achieving self-powered nanosystems. With these self-powered nanosystems, the energy supplying systems can be further miniaturized. Since 2005, the piezoelectric effect and nanogenerator in Wurtzite nanostructures have been investigated. [5] The unique well-aligned property of ZnO nanowires benefits to the energy harvest from mechanical energy to electricity in nanoscale. Simultaneously controlling the applying forces on the ZnO nanowires by atomic force microscopy (AFM) and Schottky barrier height at metal-semiconductor interface, the mechanical energy can be easily transformed to electricity and stored in the nanowires temporarily. Followed by connecting the nanowires to the desired devices, the electricity can be further released. The fundamental of this nanogenerator system is based on the piezoelectric effect, which generates the electricity itself by deforming the crystal structures under external applied forces. There are lots of motion types in our life, for example, the pressure underneath our shoes when we walk, the muscle stretches and the heart beats, and so on. All of these can be harvested and generate electrical energy. Nowadays, the research of nanogenerators is mainly based on semiconductor compounds that are formed in Wurtzite structure and the most common materials are ZnO[6], ZnS[7], CdS[8], GaN[9] and InN[10]. The basic principle of a nanogenerator is the coupling of semiconductor property and piezoelectric effect. Prof. Zhong Lin Wang is the pioneer in this field. They investigated well-aligned ZnO nanowires by AFM equipped with a Pt-coated Si tip under contact mode. The diameter and length of ZnO nanowires are 20-50um and 1-2um, respectively, and the tip force was kept at 5nF. ZnO nanowires bent and recovered during AFM tip scanning and almost half of them induced 3-12mV output voltage. However, this phenomenon was not observed in Si, WO3 nanowires and carbon nanotubes.

compressive side, the negative piezoelectric potential acts as a forward bias state to the Schottky diode. Thus, the electrons would flow outside and induce a negative output potential, and this also illustrates that the delay of output voltage and the basic principle of a nanogenerator.

III. Results and Discussion

3-1 Thermoelectric devices

Highlighted results that are World-leading or Taiwan-leading:

1. Two types of homologous In2O3(ZnO)n nanowires with vertical and horizontal

superlattices first synthesized

Figure 1 shows the FE-SEM image of the as-grown products, consisting of wire- and belt-like structures. Of which, the typical nanowires are characterized by 5-15μm in lengths and 50-120 nm in diameter, which are composed of homologous In2O3(ZnO)n superlattice structures. Figure 2(a) shows the low magnification TEM image of such a single nanowire. Like the other nanowires, this nanowire has a gold nanoparticle at its tip acting as a catalyst via the vapor–liquid–solid growth mechanism. Figure 2(b) is the HRTEM image, showing modulation contrast along the longitudinal axis, confirming to a In2O3(ZnO)n superlattice structure. This superlattice structure consists of In-O and In/Zn-O layers stacked alternately along the c axis with the spacing between the In planes and the nearest Zn planes being 0.32nm, which is larger than the Zn (0002) interplanar spacing of 0.27nm. The increase in the d spacing is due to the larger ionic radius of In than that of Zn. In addition, the HRTEM image reveals periodic superlattice structure with identical distance between two In-O layers, being around 7-8 In/Zn-O atomic layers. Therefore, this nanowire can be described by a precisely defined unit cell as expected for the In2O3(ZnO)n compounds in theory. Moreover, this structure is obviously reflected in Figure 2(c) of the SAED pattern, with a series of satellite spots between two main spots of ZnO.[11]

To confirm the microstructure of the In2O3(ZnO)n nanowires, Z-contrast STEM imaging is employed as shown in Fig. 2(f), where the intensity is approximately proportional to the square of the atomic number of the constituent atoms. Thus, using Z-contrast imaging, the location of In (Z= 49) can be unambiguously determined, but oxygen cannot be imaged due to its relatively small atomic number[1]. Fig. 2(e) clearly shows the presence of In-enriched layers (bright lines) oriented perpendicular to the [002] direction. As observed in Fig. 2(f), the In atoms segregate on individual planes and are separated by wurtzite In/Zn-O slabs. Only single layers of In atoms are observed, consistent with the layers being composed of octahedrally-coordinated InO2-, which has been shown to be the most stable configuration for In within the superlattice structure.

Besides the nanowires with a longitudinal superlattice structure, nanowires with transverse superlattice structure have also been observed in the product. As shown in Figure 3(a), layers do not stack along the growth direction [10-10] of the nanowire but along the transverse direction of the ZnO [0001] direction, which is confirmed the corresponding SAED pattern, in Figure 3(b). If we enlarge the Z-contrast image, shown in Figure 3(c), the zig-zag modulation is more apparent than the transverse superlattice structure. However, unlike the longitudinal superlattice structure, the In-O stacking sequence along the c axis is not identical. Therefore, this nanowire cannot be described by a precisely defined unit cell. Lastly, from the Z contrast image shown in Figure 3(d), the spacing between the In planes to the nearest Zn planes is 0.31nm whereas the In/ZnO (0002) interplanar spacing is about 0.27nm.

Figure 1: FE-SEM image of the as-synthesized In2O3(ZnO)n nanowires.

image from (b). (e) STEM Z-contrast image. (f) Enlarged Z-contrast image.

Fig.3: (a) HR-TEM image of a transverse superlattice structure with different stacking sequences. (b) SAED pattern. (c) Enlarged HRTEM image from (a). (d) Z contrast image.

2. Enhanced thermoelectric properties with the In2O3(ZnO)n superlattice nanowires

Various nanodevices are first devised to take measurements for thermal conductivity and Seeback coefficient from individual nanowires. As for nanodevices measuring thermal conductivity, the nanowire is designed to be suspended as shown in Fig. 4 so that heat is ensured to pass only through the nanowire without any excursion through the underlined layer. The heat is self-generated through resistive heating or Joule heating by a constant DC current through the nanowire. Therefore, the heating power of the nanowire can be described as P = I2R. The both gold electrodes are regarded as heat sinks due to inherently excellent thermal conductivity to establish equilibrium temperature with the ambient. Therefore, during Joule heating, heat flows from the highest temperature at the central part of the nanowire toward both ends of the electrodes until a steady state for establishing temperature gradients is reached where the power input through Joule heating is equal to the power output leaving from both electrodes. Meanwhile, to avoid heat dissipation by thermal convection caused by ambient gas molecules in the theoretical derivation, the measurements are carried out under vacuum. Based on this model, thermal conductivity has been derived and given by: [14]

where m = R/T. R is determined from the change of the resistance between initial state and steady state during Joule heating. The parameter of m has to be determined independently by reading the resistance value against ambient temperature when heating the nanowire as a whole on a hot plate. The rest of the parameters including length, L, and area, A, can be measured from the SEM image in Fig. 4. Ultimately, thermal conductivity is calculated by the above formula.

(a) 

(b) 

(c) 

Fig. 5 shows an optical microscopy image of a real device measuring Seeback coefficient by using electron beam lithography process. First, the temperature coefficient of resistance, α, of the Ti/Au electrode on the nanowire is measured by four-point probe measurements. Then, a dc current is applied to one of the two micro-heaters to generate heat through Joule heating, establishing a temperature gradient along the nanowire. The temperature of each Ti/Au electrode on both terminal ends can be determined by measuring resistance of each electrode with the pre-determined  value to calculate the temperature difference across the nanowire. The voltage difference across the nanowire is read out directly. Consequently, Seeback coefficient can be determined by sweeping the applied dc current for different heating power. Table 1 compares the measured results between the In2O3(ZnO)n with pristine ZnO nanowires. Compared to ZnO, both thermal conductivity and seekbeck coefficient of the In2O3(ZnO)n nanowire are enhanced by introducing superlattice, at the expense of electrical conductivity. Therefore, ZT value of the In2O3(ZnO)n nanowire is only two times larger than ZnO. We, therefore, come out with another novel microstructure as further improvement in the next section.

Fig. 4: SEM image of a suspended In2O3(ZnO)n nanowire device allowing to measure the change of resistance after Joule heating by applying a constant current to the nanowire

Table1: Thermoelectric properties of ZnO and In2O3(ZnO)n nanowires

3. A novel heterojunction belts by coupling homologous In2O3(ZnO)n with In-doped ZnO for further enhanced thermoelectric properties

The coupled structure of homologous In2O3(ZnO)n with In-doped ZnO into heterojunction belts is successfully fabricated by alloy-evaporation deposition. This work demonstrates a new epitaxial layered structure by combining doping in one layer and homologous structures in another.

Figure 6(a) is a low-magnification SEM image, showing belts of lower density formed accompanied by higher density nanodisks (NDs) on a silicon substrate, where the red dotted circles mark the nucleation sites of the belts. The belts exhibit a tapered morphology, shrinking from bottom to top, with typical lengths and widths falling in the range of 10-20 µm and 1-2 µm, respectively. Figures 6(b) and 6(c) show higher magnification SEM images of the NDs and belts, respectively. The NDs are partially hexagons and partially half-hexagons, with a diagonal size of around 0.8~1 m randomly oriented on the substrate. Careful examination of some vertically oriented NDs reveals that their thicknesses are in the range of 60-100 nm, with most having two parallel side edges with a cone on top. Interestingly, the belts with thicknesses in the range of 250~350 nm also show a sword-like shape, with two parallel side edges and a tip of around 60~70° on top. The similar features of these structures, except for the aspect ratio and size, indicate that the belts grow longer, extending from nanodisks as embryos.

contrast implies that the belt is characterized by a flat surface morphology. In contrast, Figure 6(e) exhibits a distinct sandwich-like contrast for the belt, indicative of being composed of a heterojunction layered structure. As revealed in the projected intensity line profile in the inset, two dark bands are sandwiched by the bright central area and two bright bands at either side, forming a five-layer structure, where the two brightest lines at the edges due to edge effect are excluded. The symmetric five-layer structure can also be seen in a higher magnification SEM image in Figure 6(f), which clearly shows a heterojunction structure rendered by non-uniform In doping. In order to confirm this conjecture, Figures 6(g)-(i) show EDS elemental maps of indium, zinc, and oxygen, respectively, with reference to the SEM image in Figure 6(f). Intriguingly, these three elements demonstrate drastically different spatial distributions. Whereas oxygen is uniformly distributed over the entire structure, In is richer at the bottom and two sides covering from the area corresponding to the side bright bands, and perhaps extending to the two dark bands in Figure 6(f). Meanwhile, zinc seems to decay slightly and symmetrically toward the two sides, and probably also in the two bright bands. These results reveal the complex dependence of indium doping on the five heterojunction layers through the influence of the electronic band structure, responsible for the strong SE contrast.

Figure 7(a) shows a low-magnification TEM bright-field image of a single belt with the width and length of about 800 nm and 1 m, respectively. By comparing this with the SEM results, we suggest that the “region Ⅰ” feature with a larger width may correspond to the bottom part of the heterojunction belt, while the sharp tip feature of “region Ⅲ” may correspond to the top of the belt. The corresponding SAED pattern on the [0001] projection in Figure 7(a) shows that the entire belt is a single-crystalline wurtzite structure growing along the [10-10] direction without any extra diffraction spots, indicative of epitaxial interfaces across the heterojunctions. However, the average indium concentrations with respect to ZnO acquired from EDS reveal large variations in spatial distribution, at about 10.3, 2.3, and 2.5 at. % for the bottom (Ⅰ), middle (Ⅱ) and top (Ⅲ) regions of the belt, respectively. Notably, the nucleation region corresponds to the highest indium concentration.

the EDS characterization. In region ii, the average indium concentration is around 1.7 at%, which is associated with the brighter contrast in the HAADF image. The DP in the inset also corresponds to the single crystalline wurtzite structure, with the diffraction spots shifted to a slightly lower angle compared with that of region i, suggesting that this region is single crystalline ZnO homogenously doped with In. Upon careful examination of the HAADF image, region iii is found to exhibit an amazingly complex structure composed of many indium-enriched layers, as evidenced by bright lines oriented parallel to the <11-20> direction, with a few lines extending slightly into region ii, causing weak superlattice spots and prominent streaks in the SEAD pattern. The average In concentration in this region is even higher than region ii of around 2.4 at%, as measured by EDS, which in combination with the superlattice results suggest that this region has a modulated homologous In2O3(ZnO)n structure. Notably, a slight increase in indium incorporation switches the homogeneous In doping in region ii to the superlattice structure in region iii.

Similarly, the In2O3(ZnO)n superlattice structure generally consists of atomic layers of InO2- octahedra separated by slabs of wurtzite InZnnO(n+1)+ of varying thicknesses [1, 16], exactly as observed in the high-magnification HADDF image of region iii in Figure 7(d). The formation of a superlattice structure will induce a large strain between the ZnO slabs and InO2 insertion layers. The ends of the partial inclusions through incomplete indium lattice diffusion are thus usually associated with edge dislocations, as shown in the white dashed circle in Figure 7(d). The accumulative strain is relaxed via two routes. First, indium atoms distribute along the hexagonal c axis in zigzag modulated structures with the inclined angle of 53°, thus reducing total strain, as shown in Figure 7(e), preferably for thicker ZnO slabs, consistent with theoretical predictions [17]. In addition, the octahedral indium inclusion layers act as inversion domain boundaries (IDB) to relax the strain, as shown in Figure 7(f) in a high-resolution HAADF image, where the stacking sequence of the close-packed metal layers switches from ABAB to CACA passing the IDB. Figure 7(g) shows a projected line profile across an IDB, where the d- spacing in the InZnnO(n+1)+ layer remains almost the same as in pure ZnO (0002) planes (0.26 nm), but with a 19 % increase in the d-spacing (0.31 nm) on the either side of the In-O insertion layer.

VS process is also operating[19], and that the decrease in width may be as a result of the varied growth rate in the [11-20] direction due to the gradual decrease in indium concentration. (2) Surface diffusion of indium: The indium concentration decreases from the bottom to top, implying that indium diffuses upward. Moreover, the large amount of indium available at the bottom establishes a large concentration gradient as the driving force for diffusion. Under this condition, surface diffusion is much more favored than slow lattice diffusion. (3) and (4) Redistribution of indium to form a heterojunction structure: Subsequently, indium diffuses inward to result in a three-zone sandwich-like belt structure, where each layer structure is determined by the average indium concentration and strain involved. Since the average indium concentration may exceed the solid solubility limit in ZnO in the outermost region (region iii), an In2O3(ZnO)m homologous structure is generated to compromise between indium concentration and strain. Region ii certainly results from residual diffusion of indium into ZnO lattices, a process that is limited by indium supply and temperature. The equilibrium structure is finally established through competition between belt growth and indium diffusion inward from the surfaces, causing region i top shrink more than regions ii and iii with regard to the width towards the top of the belt.

Figure 7. (a) TEM bright-filed image of a In2O3(ZnO)n/In:ZnO/ZnO belt with indium concentrations in distinct regions (I, II, and III) obtained from EDS. The inset is the corresponding SAED pattern recorded from the entire belt structure. (b) TEM cross-sectional bright-field (up) and HAADF (down) images of an belt. (c) Medium-magnification HAADF image recorded from the rectangular area in (b) with SAED patterns and In EDS data obtained from regions i, ii, and iii. High-magnification HAADF images obtained from region iii in (c) showing (d) alternate stacking of In-O layers and In/Zn-O slabs along the c-axis, and (e) zigzag modulated structures in the In/Zn-O slabs. (f) Atomic-resolution HAADF image showing columns of Zn and In cations. (g) Intensity line profile and d-spacing across an IDB. (h) Schematic diagram of the proposed growth mechanisms of the belts.

4. Even higher enhanced optical and thermoelectric properties of the novel heterojunction structure

This novel heterostructure provides an ideal pathway to enhance electron conduction through the indium doped ZnO layer, and the homologous In2O3(ZnO)n layer contains numerous interfaces to impede phonon transportation. This structure can offer an efficient way of separating the transportation of electrons and phonons for high thermoelectric power factors in semiconductor devices.

Although the band offset should also exist for the interface between regions ii and region iii, the presence of both a large amount of defects and interfaces in region iii of the homologous structure might stop electrons from diving away and promote recombination through defect levels, as in route  depicted in Figure 8(e). Nevertheless, CL emissions are successfully confined in the two symmetric In:ZnO regions as quantum wells in this interesting heterojunction belt structure, which may have potential applications in optoelectronic devices.

Moreover, this unique belt is not only useful for optoelectronic applications, but is also an ideal structure for exploring thermoelectric effects, since the many interfaces introduced in region iii can impede phonon transportation with an electron conduction channel decoupled in region ii. To further explore the thermoelectric properties of the belt, in addition to the field-effect device shown in Figure 9(a), another device incorporating nano-heaters was also fabricated on a SiO2/Si chip by e-beam lithography, as shown in Figure 9(b). The electrical conductivity and carrier concentration of the belt determined from Figure 9(a) are 4.48 × 104 Ω-1m-1 and 8.8 × 1019 cm-3, respectively, while the Seebeck coefficient is determined to be -63.32 μV/K from Figure 9(b), and the power factor can thus be calculated as 2.07 × 10−4 Wm-1K-2 at room temperature. Comparing these results with those of previous studies [1], the power factor of a single ZnO / In:ZnO / In2O3(ZnO)n heterojunction belt is larger by a factor of 20 than that of undoped ZnO nanowires (about 1.03 × 10−5 Wm-1K-2), and comparable to the best reported results for ZnO-based nanomaterials, including Sb-doped ZnO microbelts (3.2 × 10−4 Wm-1K-2)[28] and IGZO homologous nanowires (5~6 × 10−4 Wm-1K-2)[1]. The high power factor of the belt is attributed to the higher carrier concentration due to indium doping, as compared with that of about 3 × 1017 cm-3 for undoped ZnO nanowires. The unique sandwich-like belt with a high power factor produced in this work may make possible more efficient thermoelectric device applications, because the good separation of the high-conductivity In:ZnO channels enables better carrier conduction and the layer-by-layer In2O3(ZnO)n structures can impede the propagation of phonons due to multiple scattering, as schematically shown in Figure 9(c) for a ZnO / In:ZnO / In2O3(ZnO)n belt. The homologous belt also acts as a double core-shell phonon transfer system to reduce thermal conductivity via the depression and localization of long wavelength phonons at the interfaces of the insertion layers in In2O3(ZnO)n regions, and the interfaces between ZnO, In: ZnO, and In2O3(ZnO)n regions [29]. Although the size of the belts is not yet optimized for use with thermoelectric devices, the ideal structure to separate electron conduction from phonon transportation can be easily achieved by one-step synthesis. The thermoelectric efficiency is expected to be further enhanced by reducing the widths of the In: ZnO and In2O3(ZnO)n regions in future work. Overall, this work demonstrates a concept to fabricate an efficient thermoelectric structure by generating indium doped ZnO channels for efficient carrier transportation and numerous interfaces to resist phonon propagation.

Figure 8. (a) Room-temperature CL spectrum, (b) SEM image, and (c) monochromatic CL image at 385 nm of a typical belt. (d) Line-scan profiles of indium concentration, energy filter SE image, and CL emission at 385nm acquired from the rectangular area in the inset. (e) Schematic diagram of the proposed band structure and radiative recombination routes of the belt.

Figure 9. SEM micrographs of an belt-based (a) field effect transistor, and (b) thermoelectric device prepared by E-beam lithography. (c) Schematic diagram of the proposed mechanisms for electron and phonon transportation in an belt.

1. AC Nanogenerator based on GaN nanowires with 80 mV output voltage

A single vertical integrated nanogenerator (VING) device based on GaN nanowires is successfully fabricated with output voltage of up to 80 mV. The role of carrier screening effect was first exploited in AC typed VING.

In the first part, piezoelectric nanogenerator (NG) performance is investigated with basic semiconductor material characteristics, in an attempt to push its physical limit by exploring its deep physical insight toward higher power NG. Except for geometrical design of nanowires in NGs, among the semiconductor characteristics, we choose to study the effect of carrier concentration, which is expected to be crucial for improving NG output performances by simulations. However, few reports have investigated this issue by experiments. Here, n-GaN nanowire (NW) arrays with a series of doping concentrations are assembled into vertical integrated nanogenerators (VING) with PMMA inserted into gaps between nanowires to increase mechanical robustness and prevent electrical shortening. The detailed VING device structure is sketched in Figure 10 (a), and the resulting peak output voltage and current density are about 80 mV and 10 nA/cm2, respectively, demonstrated in Figure 10 (b) and (c). This NG output performance is Taiwan-leading in GaN-based piezoelectric NGs. Next, the excess carriers would screen the piezopotential, and degrade the output performance of a NG, but how strong it would be remains un-clear. Therefore, GaN NW arrays are prepared with doping concentration varying from 7.58×1017 cm-3 to 1.53×1019 cm-3, and the output performances are demonstrated in Figure 11 (a) to (d). By increasing the carrier concentration in the GaN NWs within the above range, the average output voltage drops from ~70 mV down to ~4 mV, and the average output charge density, which is normalized from the current density for a fair comparison, decreases from ~0.464 nC/cm2 to ~0.064 nC/cm2. It is about 17.5-fold fall for output voltage when the carrier concentration increases by about 20 times. Strikingly, a good relationship is apparent between average output voltage drop and carrier concentration increment, indicating that the reduced output voltage is caused by the strong carrier screening effect on the piezopotential via increased electron density inside the GaN NWs. It is, for the first time, observed that the free carriers degrade the piezopotential output drastically by 17.5 times where the output piezopotential is almost diminished.

Subsequently, the maximum output power density is further investigated and can be calculated by the following equation: Pmax = Jsc-max X Voc-max; where Jsc-max and Voc-max refer to the

conclusion is totally different from DC typed NGs, which require an optimum carrier concentration to generate a maximum electrical power. The systematic study of the dependence of the strong screening effect on the output of NGs is world-leading where a 20-fold drop in output voltage is observed, simply by increasing the carrier concentration.

Figure 11: (a) and (c) output voltage and current density of 4 VINGs with different carrier concentrations in n-GaN NW arrays; (b) and (d) extracted average output voltage and maximum charge density with carrier concentration. The inset in (d) is the current density versus carrier concentration and (e) power density of VINGs with respect to different carrier concentrations. 2. Peizoelectric Nano-light-emitting diode (Piezo-Nano-LED) based on vertical ZnO

nanowire arrays on p-GaN substrate by low temperature growth method.

Piezoelectric ZnO nanowire arrays grown on a piezoelectric GaN thin film provides a perfect template for investigating both piezo-tronic and piezo-phototronic effects. The length and diameter of the as-grown ZnO nanowire arrays by a low temperature solution method can be readily controlled. With increasing the molar concentration of Zn precursors in the recipe, ZnO nanowire arrays can lengthen from 1.25 μm to 4.25 μm and widen from 250 nm to 750 nm, as depicted in Figure 12. The microstructure of the heterojunction device is studied by high-resolution TEM, shown in Figure 13. The ZnO nanowires are confirmed to be single crystalline hexagonal structure with free of planar or linear defects. Moreover, the diffraction patterns further indicate the epitxial growth between the ZnO nanowires and the GaN thin film along the [0002] direction, implying the strong substrate guiding effect.

piezoelectric charges interact with the depletion layer close to the heterojunction and the interfacial Schottky barrier between ZnO and metal electrode. The positive piezoelectric charges would lower the barrier height and the negative piezoelectric charges further raise the barrier, which illustrates the opposite current responses of the heterojunction device when subjected to compressive strain. As a consequence, the preliminary results manifest the effect of piezoelectric charges on the electronic device and open a new research field when the piezoelectric effect works with semiconductor and couples with photon.

Figure 12: Dependence of length and diameter of ZnO nanowire arrays grown on p-GaN thin film with molar concentration of Zn precursors. The Insets show the corresponding SEM images with respect to each molar concentration.

(a)

(b)

Figure 13: (a) Low magnification TEM image of ZnO nanowire arrays grown on a p-GaN thin film. (b) High resolution image of a single ZnO nanowire taken from the yellow region in (a). (c) Diffraction pattern of a single ZnO nanowire. (d) Diffraction pattern of the p-GaN thin film.

Figure 14: (a) Logarithm of current versus voltage plot of the heterojunction device on straining. (b) Extracted forward and reversed current at 20 V and -5 V biased voltage as a function of strain.

References

[1] S.C. Andrews, M.A. Fardy, M.C. Moore, S. Aloni, M.J. Zhang, V. Radmilovic, P.D. Yang, Chem Sci 2 (2011) 706.

[2] C.H. Lee, G.C. Yi, Y.M. Zuev, P. Kim, Appl Phys Lett 94 (2009).

[3] M.N. Ou, S.R. Harutyunyan, S.J. Lai, C.D. Chen, T.J. Yang, Y.Y. Chen, Phys Status Solidi B 244 (2007) 4512.

[4] M. Soini, I. Zardo, E. Uccelli, S. Funk, G. Koblmuller, A.F.I. Morral, G. Abstreiter, Appl Phys Lett 97 (2010).

[5] H.A. Nilsson, C. Thelander, L.E. Froberg, J.B. Wagner, L. Samuelson, Appl Phys Lett 89 (2006).

[6] G.A. Zhu, R.S. Yang, S.H. Wang, Z.L. Wang, Nano Lett 10 (2010) 3151.

[7] M.Y. Lu, J.H. Song, M.P. Lu, C.Y. Lee, L.J. Chen, Z.L. Wang, Acs Nano 3 (2009) 357. [8] Y.F. Lin, J. Song, Y. Ding, S.Y. Lu, Z.L. Wang, Appl Phys Lett 92 (2008).

[9] C.T. Huang, J.H. Song, W.F. Lee, Y. Ding, Z.Y. Gao, Y. Hao, L.J. Chen, Z.L. Wang, J Am Chem Soc 132 (2010) 4766.

[10] C.T. Huang, J.H. Song, C.M. Tsai, W.F. Lee, D.H. Lien, Z.Y. Gao, Y. Hao, L.J. Chen, Z.L. Wang, Adv Mater 22 (2010) 4008.

[11] J.S. Jie, G.Z. Wang, X.H. Han, J.G. Hou, J Phys Chem B 108 (2004) 17027. [12] Y.F. Yan, J.L.F. Da Silva, S.H. Wei, M. Al-Jassim, Appl Phys Lett 90 (2007).

[13] X.T. Zhang, H.Q. Lu, H. Gao, X.J. Wang, H.Y. Xu, Q. Li, S.K. Hark, Cryst Growth Des 9 (2009) 364.

[16] N. Kimizuka, M. Isobe, M. Nakamura, Journal of Solid State Chemistry 116 (1995) 170. [17] J.L.F. Da Silva, Y.F. Yan, S.H. Wei, Physical Review Letters 100 (2008) 255501.

[18] Y.S. Gu, J.J. Qi, Y. Zhang, Materials Science Forum 561-565 (2007) 1861.

[19] J. Jie, G. Wang, X. Han, Q. Yu, Y. Liao, G. Li, J.G. Hou, Chemical Physics Letters 387 (2004) 466.

[20] Y.C. Chang, L.J. Chen, J. Phys. Chem. C 111 (2007) 1268. [21] K.J. Kim, Y.R. Park, Appl. Phys. Lett. 78 (2001) 475.

[22] H.J. Fan, B. Fuhrmann, R. Scholz, C. Himcinschi, A. Berger, H. Leipner, A. Dadgar, A. Krost, S. Christiansen, U. Gosele, M. Zacharias, Nanotechnology 17 (2006) S231.

[23] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983.

[24] A. Shih, J. Yater, C. Hor, R. Abrams, Appl. Surf. Sci. 111 (1997) 251. [25] A.A. Suvorova, S. Samarin, Surface Science 601 (2007) 4428.

[26] N.F. Mott, Metal-insulator transitions, 2nd ed., Taylor & Francis, London ; New York, 1990. [27] A.P. Roth, J.B. Webb, D.F. Williams, Physical Review B 25 (1982) 7836.

[28] Y. Yang, K.C. Pradel, Q.S. Jing, J.M. Wu, F. Zhang, Y.S. Zhou, Y. Zhang, Z.L. Wang, ACS Nano 6 (2012) 6984.

[29] M. Hu, K.P. Giapis, J.V. Goicochea, X.L. Zhang, D. Poulikakos, Nano Lett. 11 (2011) 618.  

IV. Contributions

(Please complete the table as below and describe the major contributions.)

Category No. KPI Estimated

Performances

Achieved

Performances Justification

Research 1 SCI papers 2 2

Advanced materials, The Journal of Physical Chemistry C

Research 2 SSCI papers

Research 3 A&HCI papers

Research 4 Top 5% SCI & SSCI papers 1 1 Advanced materials Research 5 Nature papers

Research 6 Science papers

Research 7 TSSCI papers

Research 8 Academic books

Research 9 Academic chapters

Research 10

Non-Chinese papers published in the field of

Humanities and Social (outside SSCI)

Research 11 Domestic & Foreign

Academicians

Recruiting Distinguished Scholars 1

Newly recruited outstanding teacher and researchers (postdoctoral research fellow is not included)

Recruiting Distinguished Scholars 2

V. Expenses

Budget Categories _{Grant Amount Grant Utilized Justification}

Operating Expenses 692200 692200

Personnel Expenses 0 0

Recruitment Expenses 0 0

Consumable & Miscellaneous Expenses 352180 352180 Chemicals , EM Consumable..etc

International Travel Expenses 340020 340020 Visit the Professor Zhong Lin Wang's Lab at

Georgia Institute of Technology

Capital Expenses 400000 400000

Equipment Expenses 400000 400000 shaker, MFC, MFC controller, vacuum pump

Subtotal (NTD) 1092200 1092200