Abstract Body: We recently demonstrated a non-volatile, two-dimensional (2D) crystal random access memory (RAM) using a monolayer electrolyte1 consisting of cobalt crown ether phthalocyanine (CoCrPc) and lithium salt. Each CoCrPc molecule includes four crown ethers that each solvate one lithium ion and form an ordered array on 2D crystals including graphene, WSe2 and MoS2. The electrolyte is deposited by drop casting from solvent followed by annealing in argon2. The key mechanism of the nonvolatile memory is the toggling of lithium ions back and forth through the crown ether cavity by an applied electrical field. The positioning of the ions near the semiconductor surface results in electric double layer (EDL) gating which creates a low resistance (ON) state; when ions are moved away from the semiconductor surface, a high resistance (OFF) state is created3. Our published work primarily focused on the combination of 15-crown 5-ether (15C5) CoCrPc and LiClO4 where the salt is deposited by drop-casting in ethanol. However, this salt-solvent combination increased the number of features on the semiconductor surface resembling aggregates of CoCrPc and/or lithium salt. Literature indicates additional combinations of lithium salts (such as LiCl and LiI) and solvents that promote the complexation between 15C5 and Li+. Therefore, in this study we explore additional salt-solvent pairs to decrease the size and density of the aggregates or eliminate them entirely. The quality of the deposition can be characterized by root mean square roughness (Rq), skewness (Rsk), and kurtosis (Rku) using atomic force microscope (AFM) topological scans. In this study, WSe2 flakes were exfoliated on p-type Si (90 nm SiO2) and a monolayer of 15C5 CoCrPc was deposited on the flake surfaces. To isolate the impact of anion identity on the monolayer quality, we kept the deposition solvent constant as ethanol while varying the lithium anions: LiClO4, LiCl, and LiI. To isolate the impact of solvent identity, we kept the salt, LiClO4, constant and varied the solvent from ethanol to acetone and acetonitrile (note that acetone and acetonitrile were chosen for their reported tendency to promote ion-(15C5) crown complexation4.) The monolayer quality is evaluated using AFM in an Argon-filled glovebox with water and oxygen concentration < 0.6 ppm. Among the three salts, LiCl in ethanol gives the most homogenous monolayer with a Rq,LiCl = 0.25 ± 0.01 nm. Specifically, with ethanol as the solvent, LiCl gives a monolayer that is 2.4 and 6.2 x less rough than LiClO4 and LiI, respectively. The skewness of LiCl (Rsk,LiCl = 0.15 ± 0.01) is close to zero indicating that the surface topology is relatively flat, which is also consistent with the small surface roughness of 2.5 angstrom. Among the three solvents, we observed that acetone gives the best LiClO4 deposition with the lowest Rq (0.42 ± 0.05 nm), which is ~1.5 and 3 x lower than ethanol and acetonitrile, respectively. Also, the skewness (Rsk,acetone = 0.72 ± 0.24) and the kurtosis (Rku,acetone = 5.23 ± 0.31) were approximately 1.5 times lower than the other two solvents. These data were taken by averaging over six, 200 x 200 nm locations on each flake. The data indicate LiCl and acetone as the most promising salt-solvent combination; these results will be reported in the talk. This work is supported by the NSF under Grant # NSF-DMR-EPM CAREER: 1847808. (1) Liang, et. al., Nano Lett. 2019, 19 (12), 8911–8919. (2) Lu, et.al., J. Phys. Chem. C 2015, 119 (38), 21992–22000. (3) Xu, et. al., ACS Nano 2017, 11 (6), 5453–5464. (4) Izatt, et. al., Chem. Rev. 1985, 85 (4), 271–339.

Impact of Salt and Solvent Identity on the Deposition of a Cobalt Crown Ether Phthalocyanine Monolayer Electrolyte

P: Synthesis and Processing of 2D Materials

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Impact of Salt and Solvent Identity on the Deposition of a Cobalt Crown Ether Phthalocyanine Monolayer Electrolyte

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