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Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation.

Peng HY, Li YF, Lin WN, Wang YZ, Gao XY, Wu T - Sci Rep (2012)

Bottom Line: Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

View Article: PubMed Central - PubMed

Affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.

ABSTRACT
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

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Calculated band structures of stoichiometric and Ni-deficient NiO.(a) Calculated density of states (DOS) of NiO. A supercell of Ni32O32 was used. In additional to the total DOS (black lines), partial contributions from O 2p and Ni 3d are also shown. Inset is the enlarged view of the region near the Fermi level. The band gap is determined by the ligand-to-metal charge-transfer energy. (b) Calculated DOS of Ni-deficient NiO as simulated by Ni31O32. Inset is the enlarged view of the DOS near the Fermi level. As a result of the Ni deficiency, the oxygen-derived band expands across the Fermi level and splits due to the Coulomb repulsion between electrons. In this case, the band gap is determined by the negative-charge-transfer gap.
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f4: Calculated band structures of stoichiometric and Ni-deficient NiO.(a) Calculated density of states (DOS) of NiO. A supercell of Ni32O32 was used. In additional to the total DOS (black lines), partial contributions from O 2p and Ni 3d are also shown. Inset is the enlarged view of the region near the Fermi level. The band gap is determined by the ligand-to-metal charge-transfer energy. (b) Calculated DOS of Ni-deficient NiO as simulated by Ni31O32. Inset is the enlarged view of the DOS near the Fermi level. As a result of the Ni deficiency, the oxygen-derived band expands across the Fermi level and splits due to the Coulomb repulsion between electrons. In this case, the band gap is determined by the negative-charge-transfer gap.

Mentions: To elucidate the relationship between the nickel deficiency and the electronic band structure, we performed first-principles calculations on NiO with different chemical stoichiometry. A supercell of Ni32O32 (2×2×2 unit cells) was used. Figure 4a and 4b show the density of states (DOS) for ideal Ni32O32 and Ni-deficient Ni31O32, respectively. As expected, the electronic band structure of Ni32O32 represents a typical charge-transfer insulator, where the valence band showing primarily the oxygen 2p character and the conduction band mainly the nickel 3d character4142. In contrast, for the supercell of Ni31O32 containing one nickel vacancy, the charge transfer energy Δ becomes smaller and turns even negative. The Fermi level enters the oxygen 2p band and the Coulomb repulsion causes the band splitting, in a way analogous to some formally Ni3+ oxides31. In this sense, NiO with nickel vacancies is a negative-charge-transfer insulator, in which both of the conduction band maximum and the valence band minimum are primarily of the oxygen 2p band character. Although other defects besides Ni vacancy and their complexes may exist in NiO, the calculation here captures the basic physics regarding the band structure of Ni-deficient NiO, and more sophisticated models can be developed in the future to shed light on the various physical states during RS operations.


Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation.

Peng HY, Li YF, Lin WN, Wang YZ, Gao XY, Wu T - Sci Rep (2012)

Calculated band structures of stoichiometric and Ni-deficient NiO.(a) Calculated density of states (DOS) of NiO. A supercell of Ni32O32 was used. In additional to the total DOS (black lines), partial contributions from O 2p and Ni 3d are also shown. Inset is the enlarged view of the region near the Fermi level. The band gap is determined by the ligand-to-metal charge-transfer energy. (b) Calculated DOS of Ni-deficient NiO as simulated by Ni31O32. Inset is the enlarged view of the DOS near the Fermi level. As a result of the Ni deficiency, the oxygen-derived band expands across the Fermi level and splits due to the Coulomb repulsion between electrons. In this case, the band gap is determined by the negative-charge-transfer gap.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC3369197&req=5

f4: Calculated band structures of stoichiometric and Ni-deficient NiO.(a) Calculated density of states (DOS) of NiO. A supercell of Ni32O32 was used. In additional to the total DOS (black lines), partial contributions from O 2p and Ni 3d are also shown. Inset is the enlarged view of the region near the Fermi level. The band gap is determined by the ligand-to-metal charge-transfer energy. (b) Calculated DOS of Ni-deficient NiO as simulated by Ni31O32. Inset is the enlarged view of the DOS near the Fermi level. As a result of the Ni deficiency, the oxygen-derived band expands across the Fermi level and splits due to the Coulomb repulsion between electrons. In this case, the band gap is determined by the negative-charge-transfer gap.
Mentions: To elucidate the relationship between the nickel deficiency and the electronic band structure, we performed first-principles calculations on NiO with different chemical stoichiometry. A supercell of Ni32O32 (2×2×2 unit cells) was used. Figure 4a and 4b show the density of states (DOS) for ideal Ni32O32 and Ni-deficient Ni31O32, respectively. As expected, the electronic band structure of Ni32O32 represents a typical charge-transfer insulator, where the valence band showing primarily the oxygen 2p character and the conduction band mainly the nickel 3d character4142. In contrast, for the supercell of Ni31O32 containing one nickel vacancy, the charge transfer energy Δ becomes smaller and turns even negative. The Fermi level enters the oxygen 2p band and the Coulomb repulsion causes the band splitting, in a way analogous to some formally Ni3+ oxides31. In this sense, NiO with nickel vacancies is a negative-charge-transfer insulator, in which both of the conduction band maximum and the valence band minimum are primarily of the oxygen 2p band character. Although other defects besides Ni vacancy and their complexes may exist in NiO, the calculation here captures the basic physics regarding the band structure of Ni-deficient NiO, and more sophisticated models can be developed in the future to shed light on the various physical states during RS operations.

Bottom Line: Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

View Article: PubMed Central - PubMed

Affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.

ABSTRACT
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

Show MeSH