Research ArticleMATERIALS SCIENCE

Memristive phase switching in two-dimensional 1T-TaS2 crystals

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Science Advances  02 Oct 2015:
Vol. 1, no. 9, e1500606
DOI: 10.1126/sciadv.1500606

Figures

  • Fig. 1 1T-TaS2 single crystals with reduced thickness.

    (A) Crystal structure of the layered 1T-TaS2, where the planes of tantalum (Ta) atoms are surrounded by sulfur (S) atoms in an octahedral arrangement. The top view of the crystal structure shows a David-star cluster, where 12 Ta atoms within the layer move toward a 13th central Ta atom. Also shown is the optical microscope image of a typical nano-thick crystal device. (B) Schematic pictures of a Ta atom network in the CCDW (left), hexagonal NCCDW (middle), and ICCDW (right) phases. The dark blue circles represent the Ta atoms displaced from their undistorted lattice coordinates, forming the David-star clusters. (C) Temperature (T) dependence of the resistivity (ρ) for bulk and nano-thick crystals of 1T-TaS2. The solid and broken lines represent the ρ in the cooling and warming cycle, respectively. The notation sc-NCCDW represents supercooled NCCDW. (D) Temperature-thickness phase diagram of 1T-TaS2 nano-thick crystals upon cooling at 1 K/min.

  • Fig. 2 Cooling rate–dependent behavior of 1T-TaS2 nano-thick crystals.

    (A and B) Temperature (T) dependence of the resistivity (ρ) for different temperature-sweeping rates in crystals with thicknesses of 51 nm (A) and 15 nm (B). The inset in (B) is the magnified view of the ρ-T curves at low temperatures. (C) Temperature-thickness phase diagram of 1T-TaS2 nano-thick crystals for different temperature-sweeping rates. (D) Critical cooling rate (Rc) versus thickness phase diagram.

  • Fig. 3 Voltage-driven phase switching in a 1T-TaS2 nano-thick crystal.

    (A and B) Current-voltage (I-V) characteristics (A) and simultaneously measured Rs versus voltage curves (B) of a 24-nm-thick crystal measured at 95 K (blue lines) and 165 K (red lines). The sweeping rate was 10 mV/s at 95 K and 50 mV/s at 165 K, respectively. (C) Temperature dependence of Rs before and after the applications of in-plane voltage. The green curve was taken before the application of voltage. After applying voltage at 95 K, the crystal was cooled down to 2 K and warmed up to 165 K, which is shown as a blue curve. Subsequently, we applied voltage on the crystal at 165 K and cooled and heated the crystal, which is shown as a red curve. T* is the temperature above which the crystal recovers to its normal NCCDW phase.

  • Fig. 4 Memristive characteristics in a 1T-TaS2 nano-thick crystal.

    (A) Rs versus voltage (V) curves of another 24-nm-thick crystal measured at 90 K with a scanning rate of 10 mV/s. (B) Variation of Rs near V = 0 V at 90 K as a function of the number of cycles. The applications of voltage realized the switching from a supercooled NCCDW (sc-NCCDW) state through another supercooled NCCDW (sc-NC'CDW) state to the CCDW state. Insets are the possible microscopic structures of each state. The dark blue hexagons refer to CCDW domains composed of David-star clusters, and the light blue areas represent the discommensurations. (C) The Rs-T relations before and after the applications of voltage. The green curve was taken before applying voltage. First, the crystal was switched from the supercooled NCCDW state to the CCDW state by applying voltage at 90 K [shown in (A)] and heated to 150 K. The blue curve is the result of cooling and warming measurements after the application of voltage on the crystal in the CCDW phase at 150 K. The red curve is the result of applying voltage on the crystal in the CCDW phase at 165 K. The orange curve was measured after the voltage application on the crystal in the NCCDW phase at 165 K. (D) Variation of Rs near V = 0 V at 90 K as a function of T*, above which the system recovers to its normal NCCDW state. Insets are the possible patterns of each metastable state. The notation mNCCDW represents metallic NCCDW. A metallic NCCDW state with a higher T* (represented by the notation mNC’CDW) may contain a larger area of discommensurations. (E) Schematic energy diagram of a 1T-TaS2 nano-thick memristive system, showing a multiminimum potential. Rainbow- and gray-colored arrows indicate the application of voltage at low and high temperatures, respectively.