Direct observation of delithiation as the origin of analog memristance in LixNbO2

The discovery of analog Li x NbO 2 memristors revealed a promising new memristive mechanism wherein the diﬀusion of Li + rather than O 2 − ions enables precise control of the resistive states. However, directly correlating lithium concentration with changes to the electronic structure in active layers remains a challenge and is required to truly understand the underlying physics. Chemically delithiated single crystals of LiNbO 2 present a model system for correlating lithium variation with spectroscopic signatures from operando soft x-ray spectroscopy studies of device active layers. Using electronic structure modeling of the x-ray spectroscopy of Li x NbO 2 single crystals, we demonstrate that the intrinsic memristive behavior in Li x NbO 2 active layers results from ﬁeld-induced degenerate p -type doping. We show that electrical operation of Li x NbO 2 -based memristors is viable even at marginal Li deﬁciency and that the analog memristive switching occurs well before the system is fully metallic. This study serves as a benchmark for material synthesis and characteri-zation of future Li x NbO 2 -based memristor devices, and suggests that valence change switching is a scalable alternative that circumvents the electroforming typically required for ﬁlamentary-based memristors.

The discovery of analog Li x NbO 2 memristors revealed a promising new memristive mechanism wherein the diffusion of Li + rather than O 2− ions enables precise control of the resistive states. However, directly correlating lithium concentration with changes to the electronic structure in active layers remains a challenge and is required to truly understand the underlying physics. Chemically delithiated single crystals of LiNbO 2 present a model system for correlating lithium variation with spectroscopic signatures from operando soft xray spectroscopy studies of device active layers. Using electronic structure modeling of the x-ray spectroscopy of Li x NbO 2 single crystals, we demonstrate that the intrinsic memristive behavior in Li x NbO 2 active layers results from field-induced degenerate p-type doping. We show that electrical operation of Li x NbO 2 -based memristors is viable even at marginal Li deficiency and that the analog memristive switching occurs well before the system is fully metallic. This study serves as a benchmark for material synthesis and characterization of future Li x NbO 2 -based memristor devices, and suggests that valence change switching is a scalable alternative that circumvents the electroforming typically required for filamentary-based memristors.
Functional oxide memristors have the potential to revolutionize neuromorphic computing, which aims to mimic the operation of biological brains using artificial circuits. 1,2 While neuromorphic systems can be implemented with traditional CMOS circuitry, this requires a large number of conventional transistors, resulting in significant power consumption and scalability issues. 3-5 In contrast, truly biomimetic circuits would lead to scalable, low-power processors capable of hardware-level autonomous learning, power-efficient image recognition, and other exciting possibilities. 6,7 Neuristor circuits based on functional oxide memristors have the potential to enable these truly biomimetic circuits. However, the resistive switching of these memristors is typically attributed to a complex combination of processes (e.g., redox reactions, ionic transport, phase changes, etc.), 8 which are dependent on device architecture and are not yet fully understood. 9 Lithium niobite (Li x NbO 2 ), which has been previously studied for its interesting properties such as superconductivity, 10 has recently shown great potential for memristive applications. 11,12 Whereas traditional filamentary devices require the migration of O 2− ions to access discrete resistive states, the resistive states of Li x NbO 2 are analog in nature and are thought to be modulated by the diffusion of Li + ions within the active layer. 13 It has been proposed that the electronic properties of Li x NbO 2 are highly dependent on the Li + content, and that a voltage induced Li-gradient could enable precise control of the resistive state. 14 This voltage-induced Li gradient was previously investigated across an annular LiNbO 2 thin film device using spatially resolved in-situ O K-edge XAS, 13 revealing changes in the XAS lineshape across the active layer interpreted as lithium concentration variations. Recent advances in x-ray absorption simulations of the O K-edge 15 have now presented a opportunity to confirm the predicted evolution of Li x NbO 2 within these devices and accurately describe their operational mechanism.  In this paper, we benchmark the intrinsic material properties responsible for the analog memristive behavior of Li x NbO 2 . High-quality single crystals were grown using liquid phase electro-epitaxy and then chemically delithiated for direct comparison to atomistic modeling. High quality Li x NbO 2 single crystals with well-defined lithium concentrations were preferred over active layer thin films in order to disentangle the effects of Li content variation from other phenomenon. Soft and hard x-ray spectroscopy techniques, as well as first-principles x-ray spectroscopic simulations, were used to monitor the niobium coordination and electronic structure evolution of Li x NbO 2 . From O K-edge XAS, we were able to confirm the depopulation of the Nb 4d z 2 orbital upon Li + ion extraction, considered responsible for the memristive behavior. From the excellent agreement between theory and experiment, we are able to explain the electronic structure evolution of Li x NbO 2 up to x = 0.5, i.e., degenerate p-type semiconductor regime (above which the Li x NbO 2 is metallic). From reexamining previous in-situ O K-edge XAS measurements of Li x NbO 2 , 13 we conclude that the linear memristive switching in the devices occurs within the degenerate p-doping regime rather than being associated with a full insulator-to-metal transition. LiNbO 2 single crystals were grown using a liquid phase electro-epitaxy (LPEE) method. The LPEE growth method makes use of Nb 2 O 5 (99.9%) and LiBO 2 (99.9%). The LiNbO 2 crystals were grown over a 24 hour period and nucleated on a niobium rod at 1.1 V with a 10:1 LiBO 2 to Nb 2 O 5 ratio. Some LiNbO 2 crystals were delithiated in a 37% HCl aqueous bath at room temperature for 24 hours followed by a rinse with DI water, and then dried with nitrogen. Other crystals were set aside to act as pristine references, while some large crystals were reserved for device fabrication. To create the memristor devices, 100 nm of Ti and 500 nm of Au were deposited on the large crystals using evaporation and then patterned into device contacts using a lift-off process. These volatile ring dot devices were then tested with IV sweeps. Initial scans were performed to find minimum programming voltage starting at 0.1 V and increasing in steps of 0.1 V, followed by a collection of data at minimum programming voltage. 16 Some crystals were ball milled or ground manually using a mortar and pestle for powder measurements. X-ray diffraction (XRD) was performed on powderized LiNbO 2 using a Bruker D8 Advance diffractometer with Bragg-Brentano geometry and a Cu K α source at Georgia Institute of Technology. Phase identification was performed using Bruker DIFFRAC.EVA software coupled with the PDF-2016 database. X-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) measurements were also performed on powderized LiNbO 2 to determine the effective Nb oxidation state in the bulk, as well as local electronic/atomic structure. The XANES and EXAFS were performed at beamline 20-BM of the Advanced Photon Source at Argonne National Laboratory in Lemont, IL (see section I A. of the supplementary material). Soft and hard x-ray photoelectron spectroscopy (XPS and HAXPES), which have an effective probing depth of ∼ 4 nm (surface) and ∼ 15 nm (sub-surface), respectively, were performed on LiNbO 2 crystals to determine the Li content, Nb oxidation state, and the valence band electronic structure. In addition, x-ray absorption spectroscopy (XAS) measurements of the oxygen K-edge were performed in total electron yield (TEY) mode, which can probe up to 5 nm deep. 17  .The x-ray formalism of the core-hole approach for simulating the O k-edge in pristine and delithiated cases was performed within the ShirleyXAS + MBX-ASPY environment. 15,[19][20][21] The DFT information of the x-ray final state was obtained using full core-hole (FCH) approach in which an electron is removed from the inner shell of a designated excited atom within a supercell. Therefore, the interaction between the core hole and the electron was not explicitly included via many-body approaches, such as the Feynman diagram technique, but was instead accounted for using a modified oxygen pseudo-potential with one electron removed from the 1s orbital for the O K-edge. The excited electron was then added to the occupied electronic structure. Next, the modified electronic system was relaxed to its ground state using DFT. Finally, the initial-state and final-state DFT orbitals and energies were provided as input for the MBXASPY software codes to produce the determinant spectra. We utilized a 1×1×1 supercell structure for the rutile and BCT NbO 2 calculations, as well as the Li 0.5 NbO 2 . The fully lithiated LiNbO 2 was calculated using a 3 × 3 × 1 supercell. All supercells were chosen such that their dimensions were large enough to avoid effects due to neighboring periodic images A representative pinched hysteresis IV curve taken on a high-quality, single crystal LiNbO 2 ring dot memristor is shown in Figure 1(a), consistent with previous reports of analog memristive behavior for LiNbO 2 . 16,22 Since the crystals show similar IV curve responses to LiNbO 2 devices grown via both molecular beam epitaxy 11,13 and sputter deposition 23 , we consider the underlying mechanism to be inherent to the material rather than the device processing. As such, chemically delithiated LiNbO 2 single crystals should display the same electronic structure modifications as those electrochemically induced in the thin film devices. As shown in Figure 1(b), the chemical potential (µ) is expected to lower into the Nb 4d z 2 -derived valance band as lithium is extracted, thereby favoring hole formation (ptype). Although the Nb-O bond length and O K-edge XAS lineshape are reportedly sensitive to d z 2 occupancy, 13,14,24 direct measurement of Li concentration combined with accurate simulations of the O K-edge XAS are required to fully verify the electronic structure evolution of Li x NbO 2 and understand the memristive mechanism.
To evaluate the quality of our pristine LiNbO 2 crystals, powder x-ray diffraction (XRD) was performed. Figure 2(a) shows the hexagonal P 6 3 /mmc crystal structure (inset) and XRD pattern of our LiNbO 2 powder with the reflections indexed with PDF 029-0815. The XRD pattern predominantly shows the expected structure with some negligible contribution from LiNbO 3 impurities, which were likely introduced during the powder preparation process. XPS studies confirmed that these impurities were mainly limited to the crystal surface and could be reduced by exposing fresh crystal surfaces using a razor blade (see section II A. of the supplementary material). Henceforth, all results shown were taken on carefully cleaved samples and/or measured using bulk-sensitive techniques to avoid spectral contamination from over-oxidized surface species.
Bulk-sensitive extended x-ray absorption fine structure (EXAFS) of lithium niobite ground pellets was performed to confirm the local atomic structure, shown in Figure 2(b). The data was fit using the LiNbO 2 crystal structure from the materials project database (ICSD-451) 25 (see section II B. of the supplementary materials). The first peak around 1.654Åcorresponds to the Nb-O interaction in the first coordination shell, while the second peak around 2.596Åis due to the Nb-Nb interaction in the second coordination shell. Excellent agreement between the experimental data and theory is achieved up to ∼5Å. Taken together, the XRD and EXAFS results confirm the overall phase purity of our LiNbO 2 crystals.
The effect of chemical delthiation on the electronic structure of LiNbO 2 was studied via XPS and XAS, as shown in Figure 3. The lower photon energy (hν = 700 eV) used for the XPS results in a higher Li 1s photo-ionization cross section than is possible with a traditional laboratory-based XPS system, thus improving our sensitivity to variations in Li content associated with delithiation (see section II C. of the supplementary materials). After the crystal was soaked for 24 hours in an HCl bath, a drop in the Li 1s peak intensity is observed in Figure 3(a), associated with a reduction in bulk Li content. This delithiation is accompanied by a compensating change to a higher Nb oxidation state observed in both the Nb 4s and 3d core regions, as well as a reduced Nb-O bond length in EXAFS fitting and a shifted Nb K-edge XANES spectra (see section II B. of the supplementary materials). This delithiation also results in the depopulation of Nb states near the Fermi level, as shown in Figure 3(b). Importantly, the lack of a clear Fermi edge upon delithiation indicates that the material stays within p-type semiconductor regime, i.e., not fully metallic. Figure 3(c) shows experimental O K-edge spectra taken using TEY mode XAS and simulations of the O K-edge calculated using the x-ray formalism of the core-hole approach. 19 One feature that clearly changes with lithium content is a pre-edge feature located around 1 -2 eV, which is found to increase in intensity with delithiation. While simulations show a complete lack of this pre-edge feature at 100% lithiation, our pristine experimental spectra still displays some weight at this energy, indicating the pristine single crystals may not possess a 100% lithium concentration. It is important to note that our O K-edge lineshapes are in agreement with those previously reported by Greenlee et al. from spatially resolved in-situ O K-edge XAS of LiNbO 2 devices. This suggests that perfectly stoichiometric LiNbO 2 is not necessary to achieve a memristive response. 13 Additionally, the trends Greenlee et al. observed across their device from the positive to negative electrode matches our observed changes with delithiation, confirming that changing Li content produces the observed analog memristive response.
Additional computational methods were used to qualitatively estimate the regime of p-type doping observed in our samples. Figure 4 shows the projected density of states of Li x NbO 2 as a function of Li content. Starting with the stoichiometric x = 100% case, the semiconducting gap is inside the Nb d manifold of ∼1.5 eV. While the experimental gap is closer to 2 eV 14 , this is consistent with other theoretical predictions 24,33 and the general tendency of LDA methods to under-estimate gap strengths. Looking at the first principles simulations for delithitated cases (x < 100%), we can now clarify the electronic response without assuming a rigid band model. As Li is driven out of the system, there is a clear auto-doping effect where the top of the valence band is heavily p-doped as previously reported, 14 however we also notice that the curvature of the bands is still high at marginal delithitation and only flattens out upon deeper delithiation. Formation of light hole states, namely depopulation of d z 2 observed in operando O K-edge XAS, 13 can give rise to the low field linear memristive response before truly metallic behavior sets in. Figure 4 reveals that the onset of a divergent dielectric function at Li 0.5 NbO 2 (see section II D. of the supplementary material), meaning that metallicity doesn't occur until nearly 50% delithitation. By comparing the measured valence band (Fig. 3(b)) to simulations and to operando XAS studies of active layers, we conclude that the memristive response is due to lithium variation within the p-type semiconductor (marginal delithitation) regime. In light of this, we determine that the memristive response does not require the metallic phase, suggesting that the dominant turn-on effect is the formation of light p-carriers.
The analog memristive behavior observed to Li x NbO 2 single crystals has been investigated with a variety of experimental and theoretical techniques. We show intrinsic memristive behavior in lithium niobite phase pure single crystals, and use chemical delithiation of these crystals to benchmark the resulting electronic structure changes. We show that removal of Li oxidizes the Nb and thus depopulates the top Nb d-states within the valence band, facilitating early onset p-type conduction prior to metallicity. This work clarifies that the field driven Li-ion motion inherent to Li x NbO 2 is a viable analog switching mechanism that does not require any complex interfacial effects or pre-forming/electroforming.