Correlation of growth, structure, optical and electronic properties of novel Nb3O7(OH) and Nb2O5 nanostructures
Final Report Abstract
TiO2 and Nb3O7(OH) nanostructures can be applied in photocatalytic and energy storage systems due to their large surface-to-volume ratio, chemical stability and good charge transport properties. In the present project we discovered that both can also be used as memristive switching material. The reason for this is related to the unique defect configuration possessing a large number of oxygen vacancies, which can be tailored by tuning the growth condition. A thermally varied hydrothermal growth of rutile TiO2 nanowire arrays on fluorine doped tin oxide led to samples with different nanowire diameters and densities. At lower temperatures thinner nanowires were grown which had a larger number of defects, e.g. oxygen vacancies, compared to thicker wires as shown by transmission electron microscopy. The different nanowire arrays were used to study the difference in metastable switching behavior via voltage pulse-probe measurements. Caused by the different defect density and localization in the nanowires they differ in their diode behavior, leading to a synthesisdependent behavior over time. For thicker nanowires the overall conductivity per voltage cycle increased more dominantly. The decay of intermediate and low resistance states studied in temporal behavior analyses also showed a longer lifetime with increasing growth temperature. The observed behavior of the nanowire arrays can be correlated with the findings from transmission electron microscopy analyses showing a gradient in stoichiometry along the length of the nanorods, leading to different independent switching mechanisms for positive and negative applied voltages. The effect of higher temperature in the synthesis leading to less grain boundaries and therefore slower oxygen vacancy migration as well as a smaller defect density resulted in longer retention times and a reduced saturation effect in TiO2 nanowires. A general possibility to use the arrays as artificial synapse to emulate important biological functions could be proven. The studies on the unusual metastable switching behavior of TiO2 allowed us to construct and develop a new setup, where current-voltage (I-V) curves of nanowire arrays as well as individual nanowires could be measured. With the studies on TiO2 we also gained the required experience to analyze novel Nb3O7(OH) nanowires at a later stage of the project. Their growth and resulting morphologies were studied in detail as a function of various precursors such as Nb, NbO2, NbCl5 and NbCl4 · 2THF. The latter two showed not only the formation of highly crystalline individual nanowires but also the formation of cubic µm-sized mesocrystals, where the nanowires form interlinked networks. The growth starts from the outside of the initial precursor. At earlier stages of the growth the interior of the cubes consists of a nanocrystalline material with well aligned pores, which grow together with increasing synthesis times. The Nb3O7(OH) nanowires itself are highly facetted with the long facets being parallel to the (001) and (100) planes of the orthorhombic crystal structure. The nanowires contain planar defects, which were identified as stacking defects that form to compensate a non-stoichiometric composition. Accordingly, a high number of oxygen vacancies exist within the Nb3O7(OH) nanowires. They can be transformed via heating treatments to orthorhombic T-Nb2O5 nanostructures retaining their overall morphology as shown in earlier studies. We investigated individual Nb3O7(OH) nanowires and demonstrated that they possess a complementary resistive switching behavior. Analysis of the data revealed that the complementary resistive switching behavior is due to the migration of oxygen vacancy to the interface between the Au electrode and Nb3O7(OH). The amount of oxygen vacancies was reduced by an oxygen plasma treatment which led to a severely lower conductivity of the device. The current–voltage measurements were also done as a function of time, which revealed that the destructive readout process of the resistance states depend on the voltage amplitude and polarity.
Publications
- Controlling the density of hydrothermally grown rutile TiO2 nanorods on anatase TiO2 films, Surfaces and Interfaces (2019), 15, 141-147
J. Kalb, A. Folger, E. Zimmermann, M. Gerigk, B. Trepka, C. Scheu, S. Polarz, L. Schmidt-Mende
(See online at https://doi.org/10.1016/j.surfin.2019.02.010) - Atomic Resolution Observation of the Oxidation of Niobium Nanowires: Implications for Renewable Energy Applications, ACS Applied Nano Materials 3 (2020), 3, 9285−9292
S. B. Betzler, A. L. Koh, B. V. Lotsch, R. Sinclair, C. Scheu
(See online at https://doi.org/10.1021/acsanm.0c01952) - Hydrothermally Grown TiO2 Nanorod Array Memristors with Volatile States, ACS Applied Materials & Interfaces 12 (20), (2020) 23363-23369
C. Ebenhoch, J. Kalb, J. Lim, T. Seewald, C. Scheu, L. Schmidt-Mende
(See online at https://doi.org/10.1021/acsami.0c05164) - Complementary Switching in Single Nb3O7(OH) Nanowires, APL Mater. 9, (2021) 071105
C. Ebenhoch, T. Gänsler, S. Schupp, C. Scheu, L. Schmidt-Mende
(See online at https://doi.org/10.1063/5.0052589) - TiO2 Nanowire Array Memristive Devices Emulating Functionalities of Biological Synapses, Advanced Electronic Materials 7 (2), (2021) 2000950
C. Ebenhoch, L. Schmidt‐Mende
(See online at https://doi.org/10.1002/aelm.202000950)
