Synthesis and electronic properties of ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_{n}{\mathrm{O}}_{3n+1}$ Ruddlesden-Popper nickelate thin films

Synthesis and electronic properties of ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ Ruddlesden-Popper nickelate thin films

Grace A. Pan, Qi Song, Dan Ferenc Segedin, Myung-Chul Jung, Hesham El-Sherif, Erin E. Fleck, Berit H. Goodge, Spencer Doyle, Denisse Córdova Carrizales, Alpha T. N'Diaye, Padraic Shafer, Hanjong Paik, Lena F. Kourkoutis, Ismail El Baggari, Antia S. Botana, Charles M. Brooks, and Julia A. Mundy

Phys. Rev. Materials 6, 055003 – Published 16 May 2022

Abstract

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite ${NdNiO}_{3}$ , we can reproducibly synthesize the $n = 1 - 5$ member compounds. X-ray absorption spectroscopy at the O $K$ and Ni $L$ edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from $3 d^{8}$ to $3 d^{7}$ as we move across the series from $n = 1$ to $\infty$ . The $n = 3 - 5$ compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward ${NdNiO}_{3}$ $(n = \infty)$ .

Received 31 October 2021
Accepted 11 April 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.055003

Physics Subject Headings (PhySH)

Charge-transfer insulators

Molecular beam epitaxy Resistivity measurements X-ray absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Grace A. Pan ¹, Qi Song¹, Dan Ferenc Segedin¹, Myung-Chul Jung², Hesham El-Sherif³, Erin E. Fleck⁴, Berit H. Goodge ^4,5, Spencer Doyle¹, Denisse Córdova Carrizales¹, Alpha T. N'Diaye⁶, Padraic Shafer⁶, Hanjong Paik ⁷, Lena F. Kourkoutis ^4,5, Ismail El Baggari³, Antia S. Botana ², Charles M. Brooks¹, and Julia A. Mundy ^1,*

¹Department of Physics, Harvard University, Cambridge, Massachusetts, USA
²Department of Physics, Arizona State University (ASU), Tempe, Arizona, USA
³The Rowland Institute, Harvard University, Cambridge, Massachusetts, USA
⁴School of Applied and Engineering Physics, Cornell University, Ithaca, New York, USA
⁵Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York, USA
⁶Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA
⁷Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM), Cornell University, Ithaca, New York, USA

^*mundy@fas.harvard.edu

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Vol. 6, Iss. 5 — May 2022

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Figure 1
Structural description and characterization of the Ruddlesden-Popper ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ compounds. (a) Crystal structures of the ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ . Nd, Ni, and O in turquoise, red, and yellow, respectively. The $n = 4$ and $n = 5$ are drawn without octahedral rotations as there is no existing crystallographic data. (b) X-ray diffraction (XRD) spectra and (c) electrical resistivity characterization of two separate parent perovskite ${NdNiO}_{3}$ calibrations that were used to synthesize the two $n = 5$ compounds in (d). Although both ${NdNiO}_{3}$ films are phase pure and show sub-nanometer surface roughness, using the calibration from an off-stoichiometric calibration forms a higher-order Ruddlesden-Popper of reduced crystallinity. (e) XRD spectra of all the ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ ( $n = 1 - 5$ ) synthesized with optimized ${NdNiO}_{3}$ calibrations. Asterisks denote substrate peaks.
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Figure 2
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ruddlesden-Popper ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ compounds. (a) Large field of view of the ${Nd}_{6} {Ni}_{5} O_{16}$ ( $n = 5$ ) compound with an overlaid strain map to highlight the Ruddlesden-Popper layers. Scale bar, 2 nm. (b) Bar graph showing the number of occurrences of each of the Ruddlesden-Popper layers over the same field of view, plotted on a log scale. (c)–(e) High-resolution images of the (c) $n = 3$ , (d) $n = 4$ , and (e) $n = 5$ compounds, illustrating the precise placement of the Nd-O rock salt double layers every $n$ perovskite layers. Neodymium and nickel atoms are represented by turquoise and red, respectively. Scale bars, 2 nm.
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Figure 3
X-ray absorption spectra (XAS) of the Ruddlesden-Popper ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ compounds. (a) Oxygen $K$ -edge prepeak features. Dashed lines represent the spectral weights of transitions to the nominal $3 d^{7}$ (left) and $3 d^{8}$ (right) states. (b) Spectra across the Ni $L_{2}$ edge. Data are in solid curves. Fits to the data with nonnegative linear least squares fittings using the $n = 1$ and $\infty$ spectra assigned as references are shown as dashed light gray curves. Estimated valences from the fits are shown on the side. (c) X-ray linear dichroic (XLD) signals (bottom) at the Ni $L_{2}$ edge, represented as $E | | c - E | | a, b$ . An example of the polarization-dependent XAS signals used to determine the XLD from the $n = 1$ compound is shown at the top.
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Figure 4
Electrical characterization of the ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ thin films. (a) Temperature-dependent resistivity of the ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ for $n = 2 - 5, \infty$ . Arrows indicate the direction of temperature sweeps. The $n = 1$ compound is insulating as in the bulk. Inset is a zoom-in on the $n = 3 - 5$ compounds. (b) Hall coefficients ( $R_{H}$ ) for the $n = 2 - 5, \infty$ compounds. All the Ruddlesden-Popper compounds possess a sudden increase in $R_{H}$ when cooled through the metal-to-insulator transitions. The $R_{H}$ behavior for ${NdNiO}_{3}$ ( $n = \infty$ ) exhibits a sign change from positive to negative when cooled into the insulating state. Inset is a zoom-in on the $n = 3 - 5$ compounds.
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Physical Review Materials

Synthesis and electronic properties of Ndn+1NinO3n+1 Ruddlesden-Popper nickelate thin films

Grace A. Pan, Qi Song, Dan Ferenc Segedin, Myung-Chul Jung, Hesham El-Sherif, Erin E. Fleck, Berit H. Goodge, Spencer Doyle, Denisse Córdova Carrizales, Alpha T. N'Diaye, Padraic Shafer, Hanjong Paik, Lena F. Kourkoutis, Ismail El Baggari, Antia S. Botana, Charles M. Brooks, and Julia A. Mundy

Phys. Rev. Materials 6, 055003 – Published 16 May 2022

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Synthesis and electronic properties of ${Nd}_{n + 1} {Ni}_{n} O_{3 n + 1}$ Ruddlesden-Popper nickelate thin films