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Ni-Ta-O multilayers
Solid State Communications 258 (2017) 33–37
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Communication
Temperature independent Seebeck coefficient through quantum confinement modulation in amorphous Nb-O/Ni-Ta-O multilayers
MARK
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Denis Musica, , Oliver Hunolda, Sarah Coultasb, Adam Robertsb a b
Materials Chemistry, RWTH Aachen University, Kopernikusstr. 10, D-52074 Aachen, Germany Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, UK
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Multilayers C. Amorphous D. Electronic transport E. Photoelectron spectroscopy
Employing a correlative experimental and theoretical methodology, we have investigated amorphous monoxide Nb-O/Ni-Ta-O multilayers. It is feasible to obtain a temperature independent Seebeck coefficient up to 500 °C for these metallic-like conductors, attaining −25 μV K−1. While Nb and Ta strongly interact with O, Ni experiences the metallic and monoxide-like bonding. We observe a 3 eV wide region below the Fermi level convoluted through several first nearest neighbor Ni – Ni and second nearest neighbor Nb – Nb interactions resulting in many confined states. It can be proposed that by increasing temperature these modulated quantum states gradually become thermally accessible eradicating the temperature dependence of the Seebeck coefficient.
1. Introduction NbO (space group Pm 3m) is a cubic compound with Nb and O atoms located at the 3c and 3d Wyckoff sites, respectively [1]. This binary solid exhibits empty 1a and 1b Wyckoff sites, whereby this vacancy ordering phenomenon is peculiar [1,2]. It should be mentioned that NbO is structurally akin to a larger family of NaCl structured carbides, nitrides, and oxides [3]. Unfortunately, properties of cubic NbO have scarcely been explored so far. NbO has been reported to be a superconductor in the excess of 1.4 K [4]. It is catalytically active with CH4 [5] as well as CO and CO2 [6] in the gas phase, but it is not known what occurs on its solid surfaces. In addition, NbO possesses the Seebeck coefficient in the range of −10 to −25 µV K−1 at 1000 K [7,8]. By filling the 1b vacant Wyckoff sites with N, its Seebeck coefficient can attain −70 µV K−1 at 800 K [9]. Based on the positive Cauchy pressure of 83 GPa validated through the nanoindentation data, it appears ductile [10], which is important for the thermal fatigue minimization [11,12]. Besides the possibility for thermoelectric applications, where oxides are particularly interesting since they are stable in atmosphere at elevated temperatures, NbO is also suitable for memory [13] and other electronic devices [14]. To modulate the transport properties of thermoelectric devices, a noteworthy proposal by Nolas and Goldsmid has emerged [15]. These authors have argued that solids with the phonon mean free path larger than that of charge carriers exhibit an enhanced thermoelectric efficiency in energy generation, cooling, and sensing applications, which may be realized in amorphous compounds [15]. This was
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Corresponding author. E-mail address: [email protected] (D. Music).
http://dx.doi.org/10.1016/j.ssc.2017.04.016 Received 30 March 2017; Received in revised form 12 April 2017; Accepted 20 April 2017 Available online 21 April 2017 0038-1098/ © 2017 Elsevier Ltd. All rights reserved.
demonstrated for several amorphous systems, such as NbO2 [16,17], ZnO [18], In-Ga-Zn-O [19], ZnSnO3 [20], In2MgO4 [21], Cu-Ge-Te [22], Zr-Ni-Sn [23], Ge-Au [24], Si-Au [24], and TiO2-PbO-V2O5 [25]. Other related properties, such as electrochemical, may also be enhanced by forming amorphous solids instead of their crystalline counterparts, as reported for amorphous NbOx [26]. However, regardless of interest in amorphous NbO2 [16,17] for thermoelectric applications, amorphous NbO remains unexplored. A very common shortcoming of thermoelectric devices is the fact that their performance is peaked in a very narrow operating temperature range. To tackle this concern, functionally graded or segmented devices have been designed [27–31]. In the functionally graded thermoelectrics, the charge carrier concentration, modulated through compositional gradients, is adjusted to match the thermal gradients allowing for a wider operating temperature window [27]. In the segmented thermoelectrics, several thermoelectric phases are joined together to achieve a similar effect [27]. Multilayers and superlattices represent such a segmented architecture [32–35], but most of the efforts have been dedicated towards the thermal conductivity minimization in crystalline configurations. In this work, we devise a correlative experimental and theoretical strategy to obtain amorphous oxide thermoelectrics with a broad operating temperature range. Nb-O/Ni-Ta-O monoxide multilayers are synthesized and the transport properties are measured. The choice of Ni and Ta is justified by predicted Seebeck coefficient enhancement for amorphous NbO2 [36]. Using density functional theory, we show that quantum confinement modulation in a larger energy window
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below the Fermi level leads to temperature independent Seebeck coefficient in these amorphous monoxides.
Ni
2. Experimental methods
Nb
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Content (at.%)
Nb-O/Ni-Ta-O multilayers were synthesized in a vacuum system using reactive DC magnetron sputtering. Si(100) and Al2O3(0001) oneside polished, single crystalline substrates were employed in these growth experiments without intentional heating. Nb (purity 99.95%, power density 7.6 W cm−2) and Ta (purity 99.95%, power density 2.5 W cm−2) targets were placed 10 cm from the substrate inclined at 45°. Ni sheets were placed onto the Ta target with the effective area of 46% of the whole Ta surface. By rotating the substrates, a formation of homogeneous layers was achieved. The base pressure was 2×10−6 Pa and the samples were formed under an Ar/O2 atmosphere with the working pressure of 0.5 Pa, where the O2 partial pressure was 0.02 Pa (purity 99.9995%). Variations in the O2 partial pressures were also explored, but no intriguing transport properties were obtained so that any discussions are omitted herein. A computer controlled shutter system was applied to allow for the multilayer growth by subsequent closing and opening the shutters, which were positioned in front of the sputter sources. Nb-O/Ni-Ta-O multilayers were analyzed by x-ray photoelectron spectroscopy (XPS) in an AXIS SUPRA instrument (Kratos Analytical) equipped with a monochromatic Kα Al x-ray source (energy 1486.6 eV) and a 180° hemispherical electron energy analyzer (pass energy 20 eV). The energy calibration of the hemispherical analyzer was performed with Au 4f7/5 (83.98 eV), Ag 3d5/2 (368.26 eV), and Cu 2p3/2 (932.67 eV). Depth profiling was made with 4 keV Ar+ ions. Shirley fitting of backgrounds [37] were used in the XPS analysis. A spherical mirror analyzer was employed for parallel imaging. To support the XPS analysis and provide a more detailed evaluation of the interfaces, atom probe tomography (APT) was carried out in a LEAP 4000X HR instrument (Cameca). This system was equipped with an ultra-violet laser (wavelength 355 nm) having a pulse energy of 30 pJ and a repetition frequency of 250 kHz. The tip temperature was held at 60 K during the APT acquisition. The APT tips were prepared with a dualbeam focused ion beam (FEI Helios Nanolab 660) with a Ga+ source and a scanning electron microscope. More details on the sample preparation can be found elsewhere [38]. IVAS 3.6.10a software (Cameca) was employed for the APT data evaluation. The structural analysis of these Nb-O/Ni-Ta-O multilayers was completed by x-ray diffraction in a Bruker AXS D8 Discover General Area Detection Diffraction System (GADDS). Cu Kα radiation, with a current setting of 40 mA and voltage of 40 kV, was employed in the GADDS instrument with a 0.5 mm pinhole collimator and fixed incidence angle of 15°. The Seebeck coefficient and DC electrical resistivity of Nb-O/NiTa-O multilayers grown on Al2O3(0001) were simultaneously measured in a LSR-3/1100 system (Linseis) under He atmosphere at ambient pressure. These transport measurements were carried out in the temperature range from 50 °C to 800 °C with the temperature increment of 10 °C, whereby three data points were acquired at each temperature to enable reliable statistics. The probe current was 10 mA. A complementary heater was used to achieve a 50 °C temperature gradient between the top and bottom electrodes made of Pt. The calibration of the LSR-3/1100 probe was accomplished with a constantan cylinder-shaped sample (Cu0.55Ni0.45).
Nb 3d Ta 4f Ni 2p O 1s Si 2p
1 mm Si 60
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0 0
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Depth (nm) Fig. 1. X-ray photoelectron depth profile of a Nb-O/Ni-Ta-O multilayer thin film sample grown on Si. The inset contains a parallel image of the Ar+ ion etched crater.
approach [43] and the integration in the Brillouin zone was carried out on converged Monkhorst-Pack 4×4×4 k-point mesh [44]. Spin polarization effects were not included as Fe-alloyed amorphous NbO2 showed no significant changes in the electronic structure and hence the transport properties between nonmagnetic and ferromagnetic states [36]. First, the amorphous (random) structure was obtained within the liquid-quench algorithm [16]. This approach was experimentally validated for a similar composition [45]. Second, metal atoms were displaced to form the multilayer configuration. Full structural optimization was carried out with the convergence criterion for the total energy of 0.01 meV and a 500 eV cut-off was used. The electronic structure was explored by analyzing the total and partial density of states.
4. Results and discussion We start the discussion by evaluating the composition of x-ray amorphous multilayers. Fig. 1 contains the depth profile data acquired by XPS. It is evident that 8 Nb-O and 8 Ni-Ta-O single layers are present in the multilayer sample, whereby interlayer mixing appears negligible as the abrupt changes are visible in the quantified data and parallel image provided in the inset of Fig. 1. The surface composition based on XPS is Ni31.5Ta10.2O58.3, reaching Ni36.9Ta14.2O48.9 at the depth of 15 nm below the multilayer surface. High surface sensitivity of XPS is likely to cause an overestimated O content on the pristine surface due to atmosphere contaminations. It should be stated that preferential sputtering during the depth profiling analysis may give rise to minor deviations. The Ni-Ta-O layer exhibits the average composition throughout the sample of Ni36.3Ta16.4O47.3, while the average NbO composition is Nb55.1O44.9. Furthermore, based on the XPS analysis, there are no detectable impurities. In a previous work [9], we have attained a consistency between the composition of Nb containing oxides measured by XPS and elastic recoil detection analysis, which implies that the XPS data in this work are likely sound. After analyzing the composition, we move on with the short-range order evaluation. Fig. 2 shows the high resolution XPS spectra for all metals in the multilayers. Nb in the Nb55.1O44.9 layer possesses Nb (II) and Nb (IV) states, with the literature consistent binding energies [36,46]. Ni in the Ni36.3Ta16.4O47.3 layer exhibits metallic bonding, which is consistent with literature [47], and an unidentified state at a slightly higher binding energy, which is interpreted here as oxidation I-
3. Theoretical methods A Nb50O50/Ni35Ta15O50 multilayer configuration (120 atoms) was considered within the framework of Vienna ab initio simulation package (VASP) and projector augmented wave potentials [39–41]. The potential parametrization was carried out within the generalizedgradient approximation, as described by Perdew, Burke, and Ernzerhof [42]. The total energy of this system was treated within the Blöchl 34
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II
IV
II IV
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II 203.2 eV IV 205.2 eV
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analysis. The oxygen to metal ratio is approx. 1.07, except of the drop in the Ni-Ta-O layer. This is consistent with Ni metallic states found in the XPS analysis, pointing that this is not just an XPS artefact. It seems that Ni is less oxidized than expected. Furthermore, there is no evidence of intermixing. This is consistent with the XPS data as well. Hence, based on the XPS and APT analyzes, Nb is of monoxide type, while Ni partly exhibits metallic states. This may be of relevance for the transport properties. The measured Seebeck coefficient and electrical resistivity data for the x-ray amorphous Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample are provided in Fig. 4. This specimen is an n-type thermoelectric exhibiting a relatively low electrical resistivity in the range of 20 μΩ m. In general, this low resistivity is unusual for oxides, such as Ta2O5 being a common dielectric [50], since it is close to ordinary metals and well-known conductive oxides, such as RuO2 [51,52] and In-Sn-O [53]. As this value is also lower than the resistivity of cubic NbO [9], this could be induced by metallic Ni reveled in the XPS (Fig. 2) and APT (Fig. 3) analyzes, but it needs to be further explored and we provide additional argumentations below. Considering the Seebeck coefficient data in Fig. 4, it is constant in the vast temperature range, following a similar behavior of the electrical resistivity. We distinguish two plateau regions: (i) from room temperature to 300 °C with the Seebeck coefficient of about −8 μV K−1 and (ii) from 300 °C to 500 °C, where the Seebeck coefficient yields approx. −25 μV K−1. This is similar to or a slight improvement over cubic NbO at elevated temperatures [7,9], but it is striking that the Seebeck coefficient is constant over a wide temperature range, which resembles the behavior of functionally graded or segmented crystalline bulk thermoelectrics [27–31]. However, it is possible to synthesize amorphous multilayers on any kind of substrates, especially temperature sensitive substrates, such as polymers. This unusual phenomenon is discussed below based on the quantum mechanical data. Adopting a nominal composition of Nb50O50/Ni35Ta15O50, we explore the atomic and electronic structure of these multiplayers using density functional theory. Fig. 5 shows the structure of such a multilayer configuration. Nb is bonded to O having the bond length of 2.07 Å, which is comparable to the corresponding bond length in cubic NbO of 2.10 Å [1] and consistent with amorphous Nb-Ru-O [54] and our XPS observations above. Unlike Nb, Ni exhibits two distinctive classes of the short-range ordering. There are Ni – O bonds with the length of 1.87 Å, which is shorter than the corresponding bond length in crystalline NiO of 2.08 Å [55]. This indicates that some rearrange-
50 Nb
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10 0 4
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Fig. 4. Measured Seebeck coefficient and electrical resistivity as a function of temperature for the x-ray amorphous Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample. Two transport plateaus are indicated. At about 700 °C the sample delaminates, most likely due to crystallization, so that the data in the excess of this temperature range are not considered.
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0
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II as the binding energy is ranged between metallic Ni and NiO. This may be due to the fact that Ni binary oxides are the least stable among possible binary phases in this sample [48] so that a pristine Ni containing oxide reduces upon ion irradiation and only the decomposition products are observed via XPS. Furthermore, some authors argue about the existence of tervalent Ni oxides, i.e. lower Ni oxidation states than in NiO [49]. On the other hand, Ta in the Ni36.3Ta16.4O47.3 layer exhibits the following states: Ta (I), Ta (II), and Ta (V), with the binding energies consistent with literature [46,47]. Some reduction and creation of previously non-existing bonds may occur during ion irradiation, but it seems that the monoxide short-range ordering is dominating in this sample. To examine if there are considerable reduction and preferential resputtering artefacts in the XPS analysis, we carry out an APT evaluation of a single interface. Fig. 3 shows an example of such an
Content (at.%)
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II
I
Fig. 2. High resolution x-ray photoelectron spectra for Nb 3d, Ni 2p, and Ta 4f states in the Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample at the depth of approx. 400 nm from the pristine surface. The deconvoluted binding energy values (Nb 3d5/2, Ni 2p3/2, and Ta 4f7/2) are provided for each element. The raw data are provided as circles and the solid lines correspond to fitted states and Shirley backgrounds.
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crystallization
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Distance (nm) Fig. 3. Depth profile of the Nb-O/Ni-Ta-O single interface obtained by atom probe tomography. The inset shows a scanning electron micrograph of an atom probe tip during preparation in which the individual layers can easily be discerned.
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Density of states (states / eV cell)
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Fig. 5. Structure of amorphous Nb50O50/Ni35Ta15O50, as obtained from the quantum mechanical calculations. The 2×2×2 superstructure is plotted for visualization purposes only.
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ments occur compared to the ideal bulk, which is not surprising for an amorphous state. For instance, in amorphous TiO2 bond shortening and coordination loss occurs [56]. This coordination loss in Ni-O shortrange environment is consistent with the XPS binding energy, as we measure the oxidation state between metallic Ni and Ni in NiO, termed Ni (I-II) above. This is also in agreement with the APT data as there is some oxygen depletion in the Ni-Ta-O layers. More strikingly, there is also a large population of Ni – Ni bonds with the bond length of 2.51 Å, which is identical to the bond length in bulk Ni [57]. We also detect these metallic bonds via XPS. Finally, Ta forms bonds with O with the length of 1.96 Å, which is consistent with the bond length in amorphous Ta2O5 [58] and our XPS observations above. As NiO is the least stable binary oxide in this system [48], we speculate that O preferably interacts with Ta within the Ni-Ta-O layer rather than with Ni. It appears that a larger population of metallic Ni bonds found during the XPS analysis (see Fig. 2) is not only a high energy ion irradiation artefact. The APT data (see Fig. 3) also confirm this argumentation. After analyzing the atomic structure obtained by density functional theory, we continue with the electronic structure evaluation. Fig. 6 contains the total and partial density of states for Nb50O50/ Ni35Ta15O50. There are finite states at the Fermi level, implying that this system is a good electrical conductor, which is consistent with the electrical resistivity data (see Fig. 4). Based on the partial densities, we observe the following hybridizations: Nb 4d – O 2p in the range of −7.8 to −3.2 eV and (ii) Ta 5d – O 2p in the similar energy window. It should be mentioned that the Ta states are not largely populated in close proximity of the Fermi level and hence of less relevance for the electronic transport as in amorphous Ta-alloyed NbO2 [36], but here Ta warrants the depletion of the Ni – O nearest neighbors. The hybridization of Ni 3d and O 2p states is thus considerably weaker, but there is a strong Ni 3d – Ni 3d interaction from the Fermi level to approx. −3 eV. These observations are consistent with the atomic structure (Fig. 5) and XPS (Fig. 2) as well as APT (Fig. 3) assessments. Enhanced thermoelectric phases typically exhibit a pronounced and rather narrow peak in the electronic structure in the close vicinity of the Fermi level, which is known as quantum confinement [51,59,60]. In these Nb50O50/Ni35Ta15O50 multilayers, there is no a very well defined peak, it is rather a broader region from the Fermi level to approx. −3 eV emerging in the density of states, which is dominated by first nearest neighbor Ni 3d – Ni 3d interactions, exhibiting peaks at −0.1, −0.9,
Fig. 6. Calculated total and partial density of states for Nb50O50/Ni35Ta15O50. The Fermi level is set to 0. The vertical dashed lines indicate the width of the Fermi-Dirac distribution at room temperature and 900 K indicating the thermally accessible states during the electronic transport.
−1.5, −2.2, and −2.8 eV, as well as second nearest neighbor Nb 4d – Nb 4d interactions with the peaks at −0.2, −0.9, −1.5, and −2.2 eV. As three of the Ni and Nb related peaks actually overlap, it is apparent that there is some electronic interaction across the interface in Nb50O50/ Ni35Ta15O50. A single orbital overlap in the vicinity of the Fermi level leads to the desired quantum confinement effect, but here we come across an unusual electronic structure pattern likely induced by the layered structure. This misalignment of the confined states is not the paramount electronic feature to facilitate the Seebeck coefficient enhancement, but it may have an effect on the temperature behavior. To rationalize the temperature response of the Seebeck coefficient (see Fig. 4), we plot the range of the Fermi-Dirac distribution at room temperature and 900 K in Fig. 6. This quantum distribution indicates the accessible states for the electronic transport at a given temperature according to the Boltzmann transport theory [9,61]. At room temperature, we speculate that a single Ni 3d – Ni 3d confined state and one Nb 4d – Nb 4d state give rise to the Seebeck coefficient. As the temperature increases, different electronic confined states, evidenced by the peaks in the density of states, are activated and thus become thermally accessible. We suggest that this modulation of the confined states is the physical origin for the two plateaus identified in the Seebeck coefficient data (see Fig. 4). Superlattice may be perceived as a multilayered sample with a long range structural coherence [62]. The critical monolayer thickness in superlattices is usually in the order of a unit cell [62]. Based on the pioneering theoretical work of Hicks and Dresselhaus, 2D confinement found in superlattices, i.e. a realization of a quantum well, can give rise to an enhanced thermoelectric figure of merit [63]. One example of such a structure is SrTiO3/SrTi0.8Nb0.2O3 superlattice [32,64]. This has also been demonstrated for Pb-Te/Pb-Eu-Te [65] and amorphous InZn-O/In-Ga-Zn-O [66] quantum well realizations. In the case of amorphous monoxide Nb-O/Ni-Ta-O multilayers explored in this work, we do not claim any superlattice (quantum well) effect as the monolayers are likely beyond the critical monolayer thickness (see Fig. 1). Furthermore, due to limited computational resources we
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cannot explore the same monolayer thickness as obtained experimentally. Nevertheless, there are clearly peaks in the vicinity of the Fermi level found in each monolayer (see Fig. 6), which we interpret as quantum confinement in individual layers, and we observe some smaller interactions between the monolayers as interfacial atoms give rise to an electronic structure modulation. We suggest that precisely due to lack of long range coherency, different states become active at different temperatures eradicating the temperature dependence of the Seebeck coefficient. 5. Conclusions Amorphous metallic Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayers exhibit two temperature independent Seebeck coefficient plateaus, from room temperature to 300 °C at −8 μV K−1 and from 300 °C to 500 °C at −25 μV K−1. While Nb and O are primarily of monoxide bonding type induced by Nb 4d – O 2p hybridization, which is essentially mimicked in the Ta – O bonding as well, Ni possesses a dual short-range nature, i.e. metallic states and O depleted Ni – O bonds are present. Instead of a narrow quantum confinement state in the vicinity of the Fermi level, which would in turn yield an enhanced Seebeck coefficient, we identify a 3 eV wide region below the Fermi level convoluted through several first nearest neighbor Ni 3d – Ni 3d and second nearest neighbor Nb 4d – Nb 4d interactions resulting in various confined states at different energies. This unique modulation of the electronic structure is at least in part interface induced as long-range interactions between Nb-O and Ni-Ta-O layers are present. We suggest that upon a temperature increase, these states gradually become thermally accessible giving rise to a temperature independent Seebeck coefficient, as observed experimentally. Hence, a large operating temperature window is obtainable and temperature sensitive substrates, such as polymers, can be used. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the project MU 2866/2. Simulations were performed with computing resources granted by JARA-HPC from RWTH Aachen University under project JARA0131. We also acknowledge P. Keuter, RWTH Aachen University, and K. Oswald, Kratos Analytical, for discussions. References [1] A.L. Bowman, T.C. Wallace, J.L. Yarnell, R.G. Wenzel, Acta Cryst. 21 (1966) 843. [2] A. Miura, T. Takei, N. Kumada, S. Wada, E. Magome, C. Moriyoshi, Y. Kuroiwa, Inorg. Chem. 52 (2013) 9699. [3] S.P. Denker, J. Less-Common Met. 14 (1968) 1. [4] J.K. Hulm, C.K. Jones, R.A. Hein, J.W. Gibson, J. Low Temp. Phys. 7 (1972) 291. [5] G. Wang, S. Lai, M. Chen, M. Zhou, J. Phys. Chem. A 109 (2005) 9514. [6] M.R. Sievers, P.B. Armentrout, Int. J. Mass Spectrom. 179/180 (1998) 103. [7] J.A. Roberson, R.A. Rapp, J. Phys. Chem. Solids 30 (1969) 1119. [8] M. Backhaus-Ricoult, J. Rustad, L. Moore, C. Smith, J. Brown, Appl. Phys. A 116 (2014) 433. [9] D. Music, R.W. Geyer, P. Bliem, M. Hans, D. Primetzhofer, J. Phys.: Condens. Matter 27 (2015) 115501. [10] D. Music, P. Bliem, M. Hans, Solid State Commun. 212 (2015) 5. [11] E.D. Case, J. Electron. Mater. 41 (2012) 1811. [12] D. Music, R.W. Geyer, P. Keuter, Appl. Phys. Lett. 109 (2016) 223903. [13] H.-D. Kim, M.J. Yun, T.G. Kim, Appl. Phys. Lett. 105 (2014) 213510. [14] C. Nico, M.R.N. Soares, J. Rodrigues, M. Matos, R. Monteiro, M.P.F. Graça, M.A. Valente, F.M. Costa, T. Monteiro, J. Phys. Chem. C 115 (2011) 4879. [15] G.S. Nolas, H.J. Goldsmid, Physica Status Solidi A 194 (2002) 271. [16] D. Music, Y.-T. Chen, P. Bliem, R.W. Geyer, J. Phys. D: Appl. Phys. 48 (2015) 275301.
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Temperature independent Seebeck coefficient through quantum confinement modulation in amorphous Nb-O/Ni-Ta-O multilayers
MARK
⁎
Denis Musica, , Oliver Hunolda, Sarah Coultasb, Adam Robertsb a b
Materials Chemistry, RWTH Aachen University, Kopernikusstr. 10, D-52074 Aachen, Germany Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, UK
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Multilayers C. Amorphous D. Electronic transport E. Photoelectron spectroscopy
Employing a correlative experimental and theoretical methodology, we have investigated amorphous monoxide Nb-O/Ni-Ta-O multilayers. It is feasible to obtain a temperature independent Seebeck coefficient up to 500 °C for these metallic-like conductors, attaining −25 μV K−1. While Nb and Ta strongly interact with O, Ni experiences the metallic and monoxide-like bonding. We observe a 3 eV wide region below the Fermi level convoluted through several first nearest neighbor Ni – Ni and second nearest neighbor Nb – Nb interactions resulting in many confined states. It can be proposed that by increasing temperature these modulated quantum states gradually become thermally accessible eradicating the temperature dependence of the Seebeck coefficient.
1. Introduction NbO (space group Pm 3m) is a cubic compound with Nb and O atoms located at the 3c and 3d Wyckoff sites, respectively [1]. This binary solid exhibits empty 1a and 1b Wyckoff sites, whereby this vacancy ordering phenomenon is peculiar [1,2]. It should be mentioned that NbO is structurally akin to a larger family of NaCl structured carbides, nitrides, and oxides [3]. Unfortunately, properties of cubic NbO have scarcely been explored so far. NbO has been reported to be a superconductor in the excess of 1.4 K [4]. It is catalytically active with CH4 [5] as well as CO and CO2 [6] in the gas phase, but it is not known what occurs on its solid surfaces. In addition, NbO possesses the Seebeck coefficient in the range of −10 to −25 µV K−1 at 1000 K [7,8]. By filling the 1b vacant Wyckoff sites with N, its Seebeck coefficient can attain −70 µV K−1 at 800 K [9]. Based on the positive Cauchy pressure of 83 GPa validated through the nanoindentation data, it appears ductile [10], which is important for the thermal fatigue minimization [11,12]. Besides the possibility for thermoelectric applications, where oxides are particularly interesting since they are stable in atmosphere at elevated temperatures, NbO is also suitable for memory [13] and other electronic devices [14]. To modulate the transport properties of thermoelectric devices, a noteworthy proposal by Nolas and Goldsmid has emerged [15]. These authors have argued that solids with the phonon mean free path larger than that of charge carriers exhibit an enhanced thermoelectric efficiency in energy generation, cooling, and sensing applications, which may be realized in amorphous compounds [15]. This was
⁎
Corresponding author. E-mail address: [email protected] (D. Music).
http://dx.doi.org/10.1016/j.ssc.2017.04.016 Received 30 March 2017; Received in revised form 12 April 2017; Accepted 20 April 2017 Available online 21 April 2017 0038-1098/ © 2017 Elsevier Ltd. All rights reserved.
demonstrated for several amorphous systems, such as NbO2 [16,17], ZnO [18], In-Ga-Zn-O [19], ZnSnO3 [20], In2MgO4 [21], Cu-Ge-Te [22], Zr-Ni-Sn [23], Ge-Au [24], Si-Au [24], and TiO2-PbO-V2O5 [25]. Other related properties, such as electrochemical, may also be enhanced by forming amorphous solids instead of their crystalline counterparts, as reported for amorphous NbOx [26]. However, regardless of interest in amorphous NbO2 [16,17] for thermoelectric applications, amorphous NbO remains unexplored. A very common shortcoming of thermoelectric devices is the fact that their performance is peaked in a very narrow operating temperature range. To tackle this concern, functionally graded or segmented devices have been designed [27–31]. In the functionally graded thermoelectrics, the charge carrier concentration, modulated through compositional gradients, is adjusted to match the thermal gradients allowing for a wider operating temperature window [27]. In the segmented thermoelectrics, several thermoelectric phases are joined together to achieve a similar effect [27]. Multilayers and superlattices represent such a segmented architecture [32–35], but most of the efforts have been dedicated towards the thermal conductivity minimization in crystalline configurations. In this work, we devise a correlative experimental and theoretical strategy to obtain amorphous oxide thermoelectrics with a broad operating temperature range. Nb-O/Ni-Ta-O monoxide multilayers are synthesized and the transport properties are measured. The choice of Ni and Ta is justified by predicted Seebeck coefficient enhancement for amorphous NbO2 [36]. Using density functional theory, we show that quantum confinement modulation in a larger energy window
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below the Fermi level leads to temperature independent Seebeck coefficient in these amorphous monoxides.
Ni
2. Experimental methods
Nb
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Content (at.%)
Nb-O/Ni-Ta-O multilayers were synthesized in a vacuum system using reactive DC magnetron sputtering. Si(100) and Al2O3(0001) oneside polished, single crystalline substrates were employed in these growth experiments without intentional heating. Nb (purity 99.95%, power density 7.6 W cm−2) and Ta (purity 99.95%, power density 2.5 W cm−2) targets were placed 10 cm from the substrate inclined at 45°. Ni sheets were placed onto the Ta target with the effective area of 46% of the whole Ta surface. By rotating the substrates, a formation of homogeneous layers was achieved. The base pressure was 2×10−6 Pa and the samples were formed under an Ar/O2 atmosphere with the working pressure of 0.5 Pa, where the O2 partial pressure was 0.02 Pa (purity 99.9995%). Variations in the O2 partial pressures were also explored, but no intriguing transport properties were obtained so that any discussions are omitted herein. A computer controlled shutter system was applied to allow for the multilayer growth by subsequent closing and opening the shutters, which were positioned in front of the sputter sources. Nb-O/Ni-Ta-O multilayers were analyzed by x-ray photoelectron spectroscopy (XPS) in an AXIS SUPRA instrument (Kratos Analytical) equipped with a monochromatic Kα Al x-ray source (energy 1486.6 eV) and a 180° hemispherical electron energy analyzer (pass energy 20 eV). The energy calibration of the hemispherical analyzer was performed with Au 4f7/5 (83.98 eV), Ag 3d5/2 (368.26 eV), and Cu 2p3/2 (932.67 eV). Depth profiling was made with 4 keV Ar+ ions. Shirley fitting of backgrounds [37] were used in the XPS analysis. A spherical mirror analyzer was employed for parallel imaging. To support the XPS analysis and provide a more detailed evaluation of the interfaces, atom probe tomography (APT) was carried out in a LEAP 4000X HR instrument (Cameca). This system was equipped with an ultra-violet laser (wavelength 355 nm) having a pulse energy of 30 pJ and a repetition frequency of 250 kHz. The tip temperature was held at 60 K during the APT acquisition. The APT tips were prepared with a dualbeam focused ion beam (FEI Helios Nanolab 660) with a Ga+ source and a scanning electron microscope. More details on the sample preparation can be found elsewhere [38]. IVAS 3.6.10a software (Cameca) was employed for the APT data evaluation. The structural analysis of these Nb-O/Ni-Ta-O multilayers was completed by x-ray diffraction in a Bruker AXS D8 Discover General Area Detection Diffraction System (GADDS). Cu Kα radiation, with a current setting of 40 mA and voltage of 40 kV, was employed in the GADDS instrument with a 0.5 mm pinhole collimator and fixed incidence angle of 15°. The Seebeck coefficient and DC electrical resistivity of Nb-O/NiTa-O multilayers grown on Al2O3(0001) were simultaneously measured in a LSR-3/1100 system (Linseis) under He atmosphere at ambient pressure. These transport measurements were carried out in the temperature range from 50 °C to 800 °C with the temperature increment of 10 °C, whereby three data points were acquired at each temperature to enable reliable statistics. The probe current was 10 mA. A complementary heater was used to achieve a 50 °C temperature gradient between the top and bottom electrodes made of Pt. The calibration of the LSR-3/1100 probe was accomplished with a constantan cylinder-shaped sample (Cu0.55Ni0.45).
Nb 3d Ta 4f Ni 2p O 1s Si 2p
1 mm Si 60
40
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0 0
100
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Depth (nm) Fig. 1. X-ray photoelectron depth profile of a Nb-O/Ni-Ta-O multilayer thin film sample grown on Si. The inset contains a parallel image of the Ar+ ion etched crater.
approach [43] and the integration in the Brillouin zone was carried out on converged Monkhorst-Pack 4×4×4 k-point mesh [44]. Spin polarization effects were not included as Fe-alloyed amorphous NbO2 showed no significant changes in the electronic structure and hence the transport properties between nonmagnetic and ferromagnetic states [36]. First, the amorphous (random) structure was obtained within the liquid-quench algorithm [16]. This approach was experimentally validated for a similar composition [45]. Second, metal atoms were displaced to form the multilayer configuration. Full structural optimization was carried out with the convergence criterion for the total energy of 0.01 meV and a 500 eV cut-off was used. The electronic structure was explored by analyzing the total and partial density of states.
4. Results and discussion We start the discussion by evaluating the composition of x-ray amorphous multilayers. Fig. 1 contains the depth profile data acquired by XPS. It is evident that 8 Nb-O and 8 Ni-Ta-O single layers are present in the multilayer sample, whereby interlayer mixing appears negligible as the abrupt changes are visible in the quantified data and parallel image provided in the inset of Fig. 1. The surface composition based on XPS is Ni31.5Ta10.2O58.3, reaching Ni36.9Ta14.2O48.9 at the depth of 15 nm below the multilayer surface. High surface sensitivity of XPS is likely to cause an overestimated O content on the pristine surface due to atmosphere contaminations. It should be stated that preferential sputtering during the depth profiling analysis may give rise to minor deviations. The Ni-Ta-O layer exhibits the average composition throughout the sample of Ni36.3Ta16.4O47.3, while the average NbO composition is Nb55.1O44.9. Furthermore, based on the XPS analysis, there are no detectable impurities. In a previous work [9], we have attained a consistency between the composition of Nb containing oxides measured by XPS and elastic recoil detection analysis, which implies that the XPS data in this work are likely sound. After analyzing the composition, we move on with the short-range order evaluation. Fig. 2 shows the high resolution XPS spectra for all metals in the multilayers. Nb in the Nb55.1O44.9 layer possesses Nb (II) and Nb (IV) states, with the literature consistent binding energies [36,46]. Ni in the Ni36.3Ta16.4O47.3 layer exhibits metallic bonding, which is consistent with literature [47], and an unidentified state at a slightly higher binding energy, which is interpreted here as oxidation I-
3. Theoretical methods A Nb50O50/Ni35Ta15O50 multilayer configuration (120 atoms) was considered within the framework of Vienna ab initio simulation package (VASP) and projector augmented wave potentials [39–41]. The potential parametrization was carried out within the generalizedgradient approximation, as described by Perdew, Burke, and Ernzerhof [42]. The total energy of this system was treated within the Blöchl 34
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II
IV
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II 203.2 eV IV 205.2 eV
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analysis. The oxygen to metal ratio is approx. 1.07, except of the drop in the Ni-Ta-O layer. This is consistent with Ni metallic states found in the XPS analysis, pointing that this is not just an XPS artefact. It seems that Ni is less oxidized than expected. Furthermore, there is no evidence of intermixing. This is consistent with the XPS data as well. Hence, based on the XPS and APT analyzes, Nb is of monoxide type, while Ni partly exhibits metallic states. This may be of relevance for the transport properties. The measured Seebeck coefficient and electrical resistivity data for the x-ray amorphous Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample are provided in Fig. 4. This specimen is an n-type thermoelectric exhibiting a relatively low electrical resistivity in the range of 20 μΩ m. In general, this low resistivity is unusual for oxides, such as Ta2O5 being a common dielectric [50], since it is close to ordinary metals and well-known conductive oxides, such as RuO2 [51,52] and In-Sn-O [53]. As this value is also lower than the resistivity of cubic NbO [9], this could be induced by metallic Ni reveled in the XPS (Fig. 2) and APT (Fig. 3) analyzes, but it needs to be further explored and we provide additional argumentations below. Considering the Seebeck coefficient data in Fig. 4, it is constant in the vast temperature range, following a similar behavior of the electrical resistivity. We distinguish two plateau regions: (i) from room temperature to 300 °C with the Seebeck coefficient of about −8 μV K−1 and (ii) from 300 °C to 500 °C, where the Seebeck coefficient yields approx. −25 μV K−1. This is similar to or a slight improvement over cubic NbO at elevated temperatures [7,9], but it is striking that the Seebeck coefficient is constant over a wide temperature range, which resembles the behavior of functionally graded or segmented crystalline bulk thermoelectrics [27–31]. However, it is possible to synthesize amorphous multilayers on any kind of substrates, especially temperature sensitive substrates, such as polymers. This unusual phenomenon is discussed below based on the quantum mechanical data. Adopting a nominal composition of Nb50O50/Ni35Ta15O50, we explore the atomic and electronic structure of these multiplayers using density functional theory. Fig. 5 shows the structure of such a multilayer configuration. Nb is bonded to O having the bond length of 2.07 Å, which is comparable to the corresponding bond length in cubic NbO of 2.10 Å [1] and consistent with amorphous Nb-Ru-O [54] and our XPS observations above. Unlike Nb, Ni exhibits two distinctive classes of the short-range ordering. There are Ni – O bonds with the length of 1.87 Å, which is shorter than the corresponding bond length in crystalline NiO of 2.08 Å [55]. This indicates that some rearrange-
50 Nb
Ni
O
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10 0 4
6
8
0 100 200 300 400 500 600 700 800
Fig. 4. Measured Seebeck coefficient and electrical resistivity as a function of temperature for the x-ray amorphous Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample. Two transport plateaus are indicated. At about 700 °C the sample delaminates, most likely due to crystallization, so that the data in the excess of this temperature range are not considered.
60
40
0
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II as the binding energy is ranged between metallic Ni and NiO. This may be due to the fact that Ni binary oxides are the least stable among possible binary phases in this sample [48] so that a pristine Ni containing oxide reduces upon ion irradiation and only the decomposition products are observed via XPS. Furthermore, some authors argue about the existence of tervalent Ni oxides, i.e. lower Ni oxidation states than in NiO [49]. On the other hand, Ta in the Ni36.3Ta16.4O47.3 layer exhibits the following states: Ta (I), Ta (II), and Ta (V), with the binding energies consistent with literature [46,47]. Some reduction and creation of previously non-existing bonds may occur during ion irradiation, but it seems that the monoxide short-range ordering is dominating in this sample. To examine if there are considerable reduction and preferential resputtering artefacts in the XPS analysis, we carry out an APT evaluation of a single interface. Fig. 3 shows an example of such an
Content (at.%)
-200
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II
I
Fig. 2. High resolution x-ray photoelectron spectra for Nb 3d, Ni 2p, and Ta 4f states in the Nb55.1O44.9/Ni36.3Ta16.4O47.3 multilayer sample at the depth of approx. 400 nm from the pristine surface. The deconvoluted binding energy values (Nb 3d5/2, Ni 2p3/2, and Ta 4f7/2) are provided for each element. The raw data are provided as circles and the solid lines correspond to fitted states and Shirley backgrounds.
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crystallization
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Distance (nm) Fig. 3. Depth profile of the Nb-O/Ni-Ta-O single interface obtained by atom probe tomography. The inset shows a scanning electron micrograph of an atom probe tip during preparation in which the individual layers can easily be discerned.
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Density of states (states / eV cell)
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Fig. 5. Structure of amorphous Nb50O50/Ni35Ta15O50, as obtained from the quantum mechanical calculations. The 2×2×2 superstructure is plotted for visualization purposes only.
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5
Energy (eV)
ments occur compared to the ideal bulk, which is not surprising for an amorphous state. For instance, in amorphous TiO2 bond shortening and coordination loss occurs [56]. This coordination loss in Ni-O shortrange environment is consistent with the XPS binding energy, as we measure the oxidation state between metallic Ni and Ni in NiO, termed Ni (I-II) above. This is also in agreement with the APT data as there is some oxygen depletion in the Ni-Ta-O layers. More strikingly, there is also a large population of Ni – Ni bonds with the bond length of 2.51 Å, which is identical to the bond length in bulk Ni [57]. We also detect these metallic bonds via XPS. Finally, Ta forms bonds with O with the length of 1.96 Å, which is consistent with the bond length in amorphous Ta2O5 [58] and our XPS observations above. As NiO is the least stable binary oxide in this system [48], we speculate that O preferably interacts with Ta within the Ni-Ta-O layer rather than with Ni. It appears that a larger population of metallic Ni bonds found during the XPS analysis (see Fig. 2) is not only a high energy ion irradiation artefact. The APT data (see Fig. 3) also confirm this argumentation. After analyzing the atomic structure obtained by density functional theory, we continue with the electronic structure evaluation. Fig. 6 contains the total and partial density of states for Nb50O50/ Ni35Ta15O50. There are finite states at the Fermi level, implying that this system is a good electrical conductor, which is consistent with the electrical resistivity data (see Fig. 4). Based on the partial densities, we observe the following hybridizations: Nb 4d – O 2p in the range of −7.8 to −3.2 eV and (ii) Ta 5d – O 2p in the similar energy window. It should be mentioned that the Ta states are not largely populated in close proximity of the Fermi level and hence of less relevance for the electronic transport as in amorphous Ta-alloyed NbO2 [36], but here Ta warrants the depletion of the Ni – O nearest neighbors. The hybridization of Ni 3d and O 2p states is thus considerably weaker, but there is a strong Ni 3d – Ni 3d interaction from the Fermi level to approx. −3 eV. These observations are consistent with the atomic structure (Fig. 5) and XPS (Fig. 2) as well as APT (Fig. 3) assessments. Enhanced thermoelectric phases typically exhibit a pronounced and rather narrow peak in the electronic structure in the close vicinity of the Fermi level, which is known as quantum confinement [51,59,60]. In these Nb50O50/Ni35Ta15O50 multilayers, there is no a very well defined peak, it is rather a broader region from the Fermi level to approx. −3 eV emerging in the density of states, which is dominated by first nearest neighbor Ni 3d – Ni 3d interactions, exhibiting peaks at −0.1, −0.9,
Fig. 6. Calculated total and partial density of states for Nb50O50/Ni35Ta15O50. The Fermi level is set to 0. The vertical dashed lines indicate the width of the Fermi-Dirac distribution at room temperature and 900 K indicating the thermally accessible states during the electronic transport.
−1.5, −2.2, and −2.8 eV, as well as second nearest neighbor Nb 4d – Nb 4d interactions with the peaks at −0.2, −0.9, −1.5, and −2.2 eV. As three of the Ni and Nb related peaks actually overlap, it is apparent that there is some electronic interaction across the interface in Nb50O50/ Ni35Ta15O50. A single orbital overlap in the vicinity of the Fermi level leads to the desired quantum confinement effect, but here we come across an unusual electronic structure pattern likely induced by the layered structure. This misalignment of the confined states is not the paramount electronic feature to facilitate the Seebeck coefficient enhancement, but it may have an effect on the temperature behavior. To rationalize the temperature response of the Seebeck coefficient (see Fig. 4), we plot the range of the Fermi-Dirac distribution at room temperature and 900 K in Fig. 6. This quantum distribution indicates the accessible states for the electronic transport at a given temperature according to the Boltzmann transport theory [9,61]. At room temperature, we speculate that a single Ni 3d – Ni 3d confined state and one Nb 4d – Nb 4d state give rise to the Seebeck coefficient. As the temperature increases, different electronic confined states, evidenced by the peaks in the density of states, are activated and thus become thermally accessible. We suggest that this modulation of the confined states is the physical origin for the two plateaus identified in the Seebeck coefficient data (see Fig. 4). Superlattice may be perceived as a multilayered sample with a long range structural coherence [62]. The critical monolayer thickness in superlattices is usually in the order of a unit cell [62]. Based on the pioneering theoretical work of Hicks and Dresselhaus, 2D confinement found in superlattices, i.e. a realization of a quantum well, can give rise to an enhanced thermoelectric figure of merit [63]. One example of such a structure is SrTiO3/SrTi0.8Nb0.2O3 superlattice [32,64]. This has also been demonstrated for Pb-Te/Pb-Eu-Te [65] and amorphous InZn-O/In-Ga-Zn-O [66] quantum well realizations. In the case of amorphous monoxide Nb-O/Ni-Ta-O multilayers explored in this work, we do not claim any superlattice (quantum well) effect as the monolayers are likely beyond the critical monolayer thickness (see Fig. 1). Furthermore, due to limited computational resources we
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