Investigating the metal to insulator transition in crystalline NbO₂ for neuromorphic computing applications

The metal-insulator transition (MIT) of NbO2 is promising for technological applications where a self-regulated resistivity is needed like in neuromorphic computing. Though the Mott nature (electron correlation) of the MIT of NbO2 arises purely by comparison to VO₂, the actual transition mechanism in NbOx-based memristors remains unclear mainly due to the degree of participation of multi-phases of niobium oxides, likely induced after electroforming. By investigating phase-pure, crystalline NbO2, we avoid the uncertainty created in electro-formed NbOx-based devices. Using surface sensitive techniques like LEED, HAXPES, and LEEM on NbO2(440)/Al2O3(006) thin films, we show that the phase transition does not extend to the film surface. XPS of the crystalline NbO2 reveals a second-order Peierls transition in the bulk, in agreement with DFT results, indicating electron correlation effects do not play a significant role. In addition, the observed temperature dependence of the Nb-Nb dimer distance which controls the NbO2 resistivity suggests that the switching of future phase-pure NbO2 memristors could be controlled by resistive heating in an analog rather than digital fashion.