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 the mean free path, which can be altered by impurities and grain boundaries. Therefore, in order to enhance the ZT value by reducing thermal conductivity, we generally reduce the mean free path of phonons by increasing phonon scattering probability, by which only thermal conductivity will be reduced without affecting electrical conductivity. Low dimensional nanomaterials have the characteristic size smaller than phonon mean free path, enforcing phonon mean free path limited by the size. The scattering probability of phonons will be substantially increased, resulting in the reduction of phonon contribution to thermal conductivity1.
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
Measuring thermoelectric properties of 1D structures is extremely hard and non-standard. There are two methods developed depending on whether nanowires are in contact with the substrate. In the first approach, the nanowire is placed on a silicon substrate with a SiO2 layer, and the electrodes and the heater are made by electron beam lithography on the substrate. Electrical current into the heater converts to Joule heat, by P = IV, to produce a temperature gradient on the substrate. By establishing relationship between resistance and temperature of a material, the temperature gradient, ΔT, can be extracted by measuring resistance at the both ends of the nanowire. At the same time, the voltage difference ΔVP between the both ends of the nanowire is also taken, so that Seebeck coefficient can be learned by S = -ΔVP/ΔT, and also temperature-dependent thermoelectric power (TEP) at different temperatures. We can apply electric field effect to make carriers injection into nanowires on a highly doped silicon substrate.
This method will enhance the conductivity of the nanowire. This device also can measure the variance of resistance at different temperatures.[2] In the second method, the nanowire is suspended from the substrate, and only both ends of the nanowire contact with the electrodes.
With this method, thermal conductivity of a substrate will not be counted, and thermal conductivity can be measured by 3ω method.[3] In addition, with regard to the measurement of thermal conductivity, Abstreiter et al. employed micro-Raman to measure thermal conductivity of a single nanowire[4]. The authors focused a micro-Raman laser onto the middle of a suspended GaAs nanowire, which contacts with Au films at both ends. At the laser focal point, there would
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.
Another key discovery is that the output voltage is induced only when ZnO nanowires are under compressive strain. As a ZnO nanowire is bent by an AFM tip, the ZnO nanowire bends and exhibits tensile strain on one side and compressive strain on the other side. Owing to the ions’
displacement, the ZnO nanowire induces a positive piezoelectric potential under tensile strain, and a negative one under compressive strain. When the Pt-coated Si tip contacts with the tensile side of the ZnO nanowire, the positive piezoelectric potential applies on the Schottky diode formed by Pt and ZnO. Under this situation, the Schottky diode is at a reversed bias state, and no current goes through the diode. On the other hand, when the tip scans over and contacts with the
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.
Since the synthesis of ZnO nanowires is well developed, ZnO nanowires become the major material for fabricating nanogenerators. Compared to ZnO nanowires, CdS, ZnS, GaN and InN nanowires are just at its infancy on nanogenerator research and only single nanowire based properties were investigated. Under conductive AFM measurements, single CdS nanowire exhibited 3mV output voltage [8], 2mV for ZnS [7], 20mV for GaN [9] and 110-120mV for InN.
Some work manifested that InN exhibited over 1V output voltage. [10]