Nickel-Fullerene Nanocomposites as Thermoelectric Materials

Abstract

Nickel films with nanovoids filled with fullerene molecules have been fabricated. The thermoelectric properties of the nanocomposites have been measured from room temperature down to about 30 K. The main idea is that the phonon scattering can be enhanced at the C₆₀/matrix heterointerface. The distribution of atoms within the Ni and Ni-C₆₀ layers has been characterized by Auger depth profiling. The morphology of the grown samples has been checked using cross-sectional scanning electron microscopy (SEM). The Seebeck coefficient and electrical conductivity have been addressed employing an automatic home-built measuring system. It has been found that nanostructuring using Ar⁺ ion treatment increases the thermopower magnitude over the entire temperature range. Incorporating C₆₀ into the resulting voids further increased the thermopower magnitude below ≈200 K. A maximum increase in the Seebeck coefficient has been measured up to four times in different fabricated samples. This effect is attributed to enhanced scattering of charge carriers and phonons at the Ni/C₆₀ boundary.

Keywords: Seebeck coefficient; fullerene; metal; nanocomposite.

Figure 1

Schematics of the as-grown (top image), Ar⁺–treated (middle image) Ni film and Ni-C₆₀ composite layer (bottom image). Dashed line in the bottom image schematically illustrate the side edge of the Ar⁺ ion etching crater used to obtain scanning electron microscopy (SEM) imagesgiven below.

Figure 2

Block diagram of the automated system for the temperature-dependent Seebeck coefficient and electrical conductivity measurements. 1—sample, 2—cryostat, 3—current source, 4—switch card and amplifier, 5—analog-to-digital converter (ADC), 6—temperature controller, 7—computer, 8—temperature sensors.

Figure 3

Schematics of experimental configuration for measuring Seebeck coefficient. 1—sample, 2—heater, 3—thermostat (temperature-controlled heat sink), 4—diode temperature sensors.

Figure 4

Measured temperature dependence of $S_{Cu}-S_{Al}$ (squares). Line is the difference of $S_{Cu}$ and $S_{Al}$ adapted from [25] and [24], respectively, with permissions from IOP publishing, 1958 (© IOP Publishing. Reproduced with permission. All rights reserved) and Taylor & Francis, 1977.

Figure 5

Elemental concentration as a function of depth obtained by Auger Electron Spectroscopy analysis in samples Ni (a) and Ni-C₆₀ (b).

Figure 6

Cross-sectional SEM images of Ni (a) and Ni-C₆₀ (b) samples. Lower images enlarge the middle parts of the upper images.

Figure 7

Measured temperature dependence of $S_{Ni}-S_{Cu}$ (a) and electrical resistivity; (b) in samples Ni (open and closed triangles for two different samples) and Ni-C₆₀ (open and closed circles for two different samples). Squares represent the data obtained in Ar⁺–treated Ni film. Line is a fit of the closed circles to Equation (4).