ICMCTF2016 Session C4-1: Thin Films for Energy Related Applications

SnO₂/multi-layer graphene nanocomposites was synthesized via a electroless plating route. Aqueous suspension containing Sn(BF₄)₂ with multi-layer graphene was reacted at 60 °C in an acidic environment for 1 hour, and Na₂S₂O₄ was used to reduce tin ion from Sn(BF₄)_2, the final nanocomposite will be collected by using suction filtration. The morphology and structure of as-synthesized nanocomposites were analyzed by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and electrochemical characterization. After electroless plating, the presence of SnO₂ particle attaching on the multi-layer graphene is confirmed by TEM and its particle size is around 15 to 35 nm. Tin oxide (SnO₂) can be used to modify multi-layer graphene via electroless plating process decorating with oxygen-containing functional groups, hence the electrical conductivity of multi-layer graphene can be obviously improved. The preliminary results of electrochemical characterization indicated multi-layer graphene can offer 340 mAhg^-1 in first cycle and 273 mAhg^-1 after 50 cycles, SnO₂/multi-layer graphene nanocomposites can offer 468 mAhg^-1 in first cycle and 302 mAhg^-1 after 50 cycles. It was found that due to the electroless plating, which enhanced the electrochemical performance in SnO₂ and multi-layer graphene. Hence SnO₂/multi-layer graphene nanocomposites is a possible strategy for achieving enhanced capacity, rate capability and cyclability performance.

The cubic phase of Bi₂O₃ (δ-Bi₂O₃) is one of the materials with the highest ionic conductivity, property required for the development of solid oxide fuel cells. The d-phase has a defect fluorite-type structure that contains a large amount of vacant sites in the oxide ion sublattice. However, this phase is only stable between 725-850 ^oC, i.e. a reduced range for technological applications. It has been shown that the δ-phase can be stabilized at room temperature when the material is grown as a thin film, either as a consequence of low-dimensional confinement or due to the non-equilibrium growth conditions. Nevertheless, the δ-phase is lost when the films are heated above 250-300^oC, leading to tetragonal or monoclinic phases, which present lower ionic conductivities. In this work, our aim was to study the effect of doping to increase the temperature range in which the defect-fluorite structure can be maintained without drastic changes in the dc and ac-conductivities. For such purpose, δ-Bi₂O₃ thin films were deposited using reactive magnetron sputtering at conditions were the phase can be obtained; Ar:O₂ (80:20) atmosphere, 100 W power applied to an α-Bi₂O₃ target and 125 ºC substrate temperature at 10 min. The thin films were deposited on glass substrates and the substrate-target distance was fixed at 5 cm, the base pressure was below 5.3 x10^-4 Pa and the deposition pressure was fixed at 2.7 Pa. Cooper (Cu), aluminum (Al), tantalum (Ta), tungsten (W) and zinc (Zn) were used for the doping, attaching small pieces of each element to the Bi₂O₃ target. The structure was mainly studied using X-ray Diffraction in the films as-deposited or during annealing experiments, but also using Raman spectroscopy. The results indicated that the cubic-fluorite structure could be stabilized using either Zn or Cu, the films were amorphous as-deposited and after annealing the tetragonal-phase was obtained. Scanning electron microscopy showed a clear difference on the morphology of the doped-films compared to the pure Bi₂O₃. The ac and dc surface conductivity of the films (Ta, W and Al-doped) were evaluated as a function of the temperature to obtain the activation energies and then evaluate the effect of the dopants on the conductivity properties of the films.

Chromium nitride (CrN) - a well-known transition metal nitride used for medical implants, decorative coatings, and high temperature wear-resistant coatings [1] - is also of interest as a thermoelectric material since it possesses a high Seebeck coefficient [2-3]. Alloying with vanadium to modify the electrical conductivity by forming the solid solution (Cr,V)N may benefit the latter application [4].

In this study, we synthesized CrN and (Cr,V)N thin films by reactive dc magnetron sputtering. CrN is known to be a semiconductor with an approximate band gap of 71 meV and a resistivity of approximately 20 mΩ·cm at room temperature [5]. It crystallizes into the cubic rocksalt structure with a lattice parameter of 4.140 Å [5]. Likewise, vanadium nitride (VN) also crystallizes in the cubic rocksalt structure with a lattice parameter of 4.126 Å [6], which facilitates an easy substitution of chromium with vanadium in the crystal structure. The thin films were deposited onto 0001 sapphire (Al₂O₃) substrates at 600 °C under various N₂/Ar gas flow ratios, keeping the total gas pressure at 5 mTorr (0.67 Pa). Characterization by θ-2θ X-ray diffraction (XRD) and pole figure measurements confirmed CrN (111) epitaxial growth. The films with an N/Cr = 0.91 atomic ratio measured by X-ray photoelectron spectroscopy (XPS) were found to yield an electrical resistivity of ρ = 6 mΩ·cm as measured by a 4-point-probe station when the Cr target power was set to 50 W. Samples with N/Cr ratios below 0.91 also include the Cr₂N phase, causing a semiconductor to metallic phase transformation in the films (Cr target power: 150 W, N/Cr = 0.61, ρ = 48 μΩ·cm). Pole figure measurements obtained from samples alloyed with 10-20 at. % vanadium also show cubic rocksalt epitaxial growth with a resistivity in the range of 90-180 μΩ·cm, demonstrating vanadium as a suitable alloying element to enhance the electrical conductivity.

References

[1] Hones et al., J. Phys. D: Appl. Phys. 36 (2003) 1023–1029

[2] Quintela et al., Adv. Mater. 27 (2015) 3032–3037

[3] O. Jankovsky´ et al., Journal of the European Ceramic Society 34 (2014) 4131–4136

[4] Quintela et al., Appl. Phys. Lett., 94 (2009) 152103 and PHYSICAL REVIEW B 82 (2010) 245201

[5] Constantin et al., Appl. Phys. Lett., 85 (2004) 3671-6373

[6] Hugh O. Pierson, Handbook of Refractory Carbides and Nitrides, Noyes Publications (1996), p 201

Transition metal nitrides have recently attracted attention for possible thermoelectric application, [1,2,3] since they show excellent mechanical properties and a wide range of electrical properties. ScN is one of the explored transition metal nitrides and it was discovered to have a relatively high Seebeck coefficient and low electrical resistivity. However, because of its high thermal conductivity, the thermoelectric figure of merit (ZT) of ScN is low. [1,2] Recent theoretical calculations [4] showed a possibility to increase Seebeck coefficient of ScN by varying N stoichiometry and introducing doping. In this work, ScN thin films were deposited with reactive magnetron sputtering on MgO, fused silica, and sapphire substrates and by varying both the N concentration (0-25 % of total gas flow) and other growth parameters such as deposition temperature (500-800 ⁰C); the structure, composition, crystalline quality, and thermoelectric properties were explored. A change of the optical band gap from a yellow to a light green color of ScN was observed when varying the N concentration. This also results in different cell parameters for ScN. Another strategy for improving the thermoelectric properties can be alloying ScN with heavy transition elements in order to reduce thermal conductivity by introducing phonon scattering. A previous study by Kerdsongpanya et al suggested that ScN can be alloyed by several heavier elements such as niobium, tantalum and lutetium.[5] By using the same growing and analyzing techniques as mentioned before, the effect on the thermoelectric properties have been explored.

[1] Kerdsongpanya, S. et al, Appl. Phys. Lett. 99 232113 (2011)

[2] Burmistrova, P. V. et al, J. Appl. Phys 113 153704 (2013)

[3] Quintela, C. X. et al, Adv. Mater. 27 3032 (2015)

[4] Kerdsongpanya, S et al, Phys. Rev. B 86 195140 (2012)

[5] Kerdsongpanya, S. et al, J. Appl. Phys. 114 073512 (2013)

Thermoelectric devices interconvert thermal and electrical energy and provide the ability to capture waste heat and to control temperature without moving parts. Organic semiconductors are a promising class of thermoelectric materials because they can be processed from solution simply using printing techniques and have unique transport properties. There have been significant advances in understanding charge transport in organic semiconductors, but there are few design rules to guide the development of new thermoelectric materials. We will present recent work in our lab on the thermoelectric properties of semiconducting polymers and small molecules with p-type and n-type conductivity. Our studies of these materials find an important link between processing conditions, thermopower and electrical conductivity. For example, we find that the electrical conductivity can vary by orders of magnitude while the thermopower is relatively unaffected. We will discuss the application of common transport models for homogeneous and heterogeneous thermoelectrics to understand these results and provide guidance for the limits of performance.