Enhanced thermoelectric performance of rough silicon nanowires

Abstract

Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Mechanical Properties of Vapor−Liquid−Solid Synthesized Silicon Nanowires

“…[7][8][9] As such, Si NWs have been used in a broad range of applications including nanoelectronics, 10-12 nanosensors, 13 nanoresonators, 14 light-emitting diodes, 15 and thermoelectric energy scavengers. 7,8 The operation and reliability of these nanodevices depend on the mechanical properties of Si NWs, which are expected to be different from their bulk counterparts due to their increasing surface-to-volume ratio.Existing techniques for measuring the mechanics of individual NWs include observing the vibration (or resonance) of cantilevered NWs inside a transmission or scanning electron microscope (TEM/SEM), [16][17][18] measuring the lateral bending of suspended NWs with an atomic force microscope (AFM), 3,19-21 measuring uniaxial tension of suspended NWs in SEM or TEM, 2,22-26 and nanoindentation of NWs on a substate. 27 Available experimental results on Si NWs exhibit significant scatter including the following: (1) some reported a decrease in Young's modulus with decreasing size, 2,9,24,28 while others showed an opposite trend; 20,21 (2) the reported strength values of vapor-liquid-solid (VLS) grown Si NWs ranged from 500 MPa to 12 GPa; 3,28 (3) Han et al 2 observed pronounced plastic deformation of Si NWs by in situ TEM tensile tests at room temperature, while Gordon et al 21 reported linear elastic behavior followed by brittle fracture using AFM bending tests.…”

Nanoscale Electromechanics To Measure Thermal Conductivity, Expansion, and Interfacial Losses

“…show near linear temperature dependence of thermal conductivity at cryogenic temperatures. [20][21][22] Equation 1 is solved to obtain the temperature profile along the nanowire for an applied V sd assuming the contacts are in thermal equilibrium with the bath temperature (T (x) = T b for x = 0, L). The temperature profile along the nanowire can be analytically written as:…”

Heteroepitaxial Growth of Gold Nanostructures on Silicon by Galvanic Displacement

“…11,17,32,57 In a completely unrelated application, galvanic displacement of silver nanoparticles on silicon can be used to etch silicon nanowire arrays in a wet chemical fashion. 58 Although gold-on-silicon nanostructures, prepared by galvanic displacement, have been used in the fabrication of many device architectures, the AuϪSi interface is not well understood, and is the source of much interest. A detailed understanding of the nature and the structure of the goldϪsilicon interface as prepared by galvanic displacement, and the subsequent growth mode of the gold nanostructures merits detailed consideration not only from a technological perspective, but also to elucidate fundamentals in interfacial nanoscience.…”