AVS2011 Session EN+NS-TuA: Nanostructured Materials for Thermophotovoltaics, Thermoelectrics & Plasmonics

Patterned metallic films allow the generation and manipulation of special electromagnetic waves known as surface plasmons that propagate along a metal interface. Because these waves allow the concentration of light in nanometer-scale volumes, they have implications for fundamental phenomena as well as applications such as imaging, sensing, and solar cells. Thus, the field of plasmonics has arisen to study and utilize surface plasmons. While light is typically used to create these waves, in some cases it would be more convenient to have a non-optical source for surface plasmons. One route is thermal excitation (i.e., heat). Here, we will explore hot plasmonic structures for obtaining new optical behavior. For example, we will examine metallic films patterned with a series of circular concentric grooves (a bull's eye pattern). We show that, when heated, these films can emit light that is amazingly narrow, both in terms of its spectrum and its angular divergence. Thus, a simple metallic foil can generate a highly directional beam of monochromatic light by a thermal process. This effect has implications for creating efficient thermophotovoltaic devices, which convert heat into electricity. During these studies, we also developed a simple, high-throughput method for obtaining ultrasmooth patterned metal films. Previously, roughness in such films has hindered the field of plasmonics. Therefore, our approach solves a critical problem and allows many high-quality plasmonic structures to be available for the first time.

Colloidal nanocrystals can combine the advantages of crystalline inorganic semiconductors with the size-tunable electronic structure and inexpensive solution-based device fabrication. Single- and multicomponent nanocrystal assemblies, also known as superlattices, provide a powerful general platform for designing two- and three-dimensional solids with tailored electronic, magnetic, and optical properties. Unlike atomic and molecular crystals where atoms, lattice geometry, and interatomic distances are fixed entities, the nanocrystal arrays represent ensembles of “designer atoms” with potential for tuning their electronic structure and transport properties. Generally speaking, nanocrystal assemblies can be considered as a novel type of condensed matter, whose behavior depends both on the properties of the individual building blocks and on the interparticle exchange interactions.

The ability to assemble precisely engineered nanoscale building blocks into complex structures is opening the door to materials where components and functionalities can be added, tuned or combined in a predictable manner. I will show how self-assembly of nanocrystals can lead to a palette of unprecedented phases including superlattices isostructural with the Archimedean tilings and dodecagonal quasicrystals.

Thermoelectric materials for energy generation have several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturizability, and CO2 free conversion of heat to electricity. Excellent thermoelectric efficiency requires a combination of high thermopower (S, V/K), high electrical conductivity (σ, S/cm), and low thermal conductivity (κ, W/mK). To date the best materials available have been inorganic compounds with relatively low earth abundance and highly complex, vacuum processing routes (and hence greater expense), such as Bi₂Te_3. Molecular materials and hybrid organic-inorganics bring the promise of inexpensive, solution processible, mechanically durable devices. While highly conductive polymers are now common place, they generally demonstrate low thermopower. Our work on molecular scale junctions suggests that nanostructuring of organics allows them to act as thermionic filters between inorganic junctions which can lead to enhanced thermoelectric properties. We have taken inspiration from this fundamental understanding to design material systems in which we combine a high electrical conductivity, low thermal conductivity polymer with a nanoparticle that contributes high thermopower. Additionally, the work functions of the two materials are well-aligned which introduces the possibility of thermionic filtering at the interface and an additional boost to the power factor. The combination of these effects results in a new hybrid, solution processible material with a thermoelectric figure of merit approaching those of Bi₂Te₃. In this talk, I will discuss both the use of thermoelectric measurements to gain insight to molecular junctions and how this insight translates to design principles for polymer and hybrid thermoelectrics.

Current energy production mechanisms for electrical power and transportation are plagued by inefficiencies which results in most of the energy source being lost as heat. In most cases that heat is found in the form of low-grade heat with temperatures below approximately 200 degrees C. Unfortunately, typical methods such as the Rankine cycle for converting heat to electricity suffer from poor efficiency for low-grade heat. Direct thermoelectric conversion is currently struggling to match the efficiency of the Rankine cycle at high temperatures but offers advantages in terms of reduced maintenance and form-factor which enable energy scavenging in places such as the exhaust line of a vehicle where larger systems could not be implemented.

Dominated by recent progress in nanostructured materials, the unitless thermoelectric figure of merit ZT has been increased to well beyond 1 such that efficiencies are reaching a range which makes them cost effective. Typically thermoelectric materials include elements such as lead or tellurium which are toxic and rare therefore alternative materials are being sought. Recent progress in silicon nanowire thermoelectric has shown a reduction in thermal conductivity, and therefore an increase in ZT of two orders of magnitude, making them viable candidates in the thermoelectric marketplace. Decreased cost and toxicity of silicon as compared to conventional thermoelectric materials make it an attractive candidate but to date nearly all studies on thermoelectricity of nanowires have focused on nanowires in isolation. Our platform based on interconnected 3-dimensional nanowire networks grown directly on metallic substrates provides large area thermoelectric modules capable of scavenging low-grade heat for low cost. The materials properties comprising ZT: thermal conductivity, electrical conductivity and Seebeck coefficient will be discussed for undoped, p-type and n-type silicon nanowire networks with particular emphasis on electrical conductivity and Seebeck coefficient within the temperature range of low-grade heat.

Introduction

Thermionic energy conversion is a candidate technology for the efficient conversion of thermal energy directly to electrical energy. In a thermionic converter, thermally excited electrons are emitted from the surface of a heated cathode into a vacuum gap. These electrons are then collected by a cooler anode and driven through an external load back to the cathode. The thermionic emission current density of a heated cathode can be described by the Richardson Equation (Eq. 1).

J = AT²e^-Φ⁄kT (1)

where: J: Thermionic emission current density (A/cm²); A: Richardson constant (A/cm² T²); T: Temperature (K); Φ: Material’s work function (eV); and k: Boltzmann constant (eV/T)

It can be seen from Eq. 1 that a material with a lower work function, Φ, can achieve higher current densities at lower temperatures than a material with a higher work function, which implies lower work function values translate into better energy converters. Prior attempts to construct efficient thermionic converters were limited by available materials with work function values ranging from 3.5eV to 5eV requiring extreme cathode temperatures in order to achieve useable output power values. In this study, the thermionic emission properties of nitrogen-incorporated diamond films are examined as a potential electrode material to enhance the efficiency of such a device.

Experimental

Polycrystalline diamond films were synthesized on molybdenum substrate via Microwave Plasma-Enhanced Chemical Vapor Deposition (MPCVD). The source gasses were H₂, CH₄, and N₂, microwave power was 1.5KW, and the nominal pressure was 50Torr. Scanning electron micrographs of the samples demonstrated uniform film coverage and a thickness of ~100µm.

Thermionic emission characterization was performed in a vacuum environment with a base pressure of 1 x 10^-7 Torr. The diamond films were resistively heated and the temperature was constantly observed by a dual color pyrometer. Electron emission current was collected with an electrically isolated anode biased at a constant voltage of 100V positioned 0.5cm above the heated cathode.

Results and Discussion