Switching charge states in quasi-2D molecular conductors

Abstract

2D molecular entities build next-generation electronic devices, where abundant elements of organic molecules are attractive due to the modern synthetic and stimuli control through chemical, conformational, and electronic modifications in electronics. Despite its promising potential, the insufficient control over charge states and electronic stabilities must be overcome in molecular electronic devices. Here, we show the reversible switching of modulated charge states in an exfoliatable 2D-layered molecular conductor based on bis(ethylenedithio)tetrathiafulvalene molecular dimers. The multiple stimuli application of cooling rate, current, voltage, and laser irradiation in a concurrent manner facilitates the controllable manipulation of charge crystal, glass, liquid, and metal phases. The four orders of magnitude switching of electric resistance are triggered by stimuli-responsive charge distribution among molecular dimers. The tunable charge transport in 2D molecular conductors reveals the kinetic process of charge configurations under stimuli, promising to add electric functions in molecular circuitry.

Keywords: 2D; charge states; molecular conductors; phase control; stimuli-responsive.

Fig. 1.

Crystal structure, charge pattern and stimuli effects on 2D κ-Cl_0.89Br_0.11 crystals for low-temperature metallicity. (A) XRD pattern of κ-Cl_0.89Br_0.11, typical for ET-based molecular crystals, shows a series of (0k0) diffraction peaks (k is even), which is the preferred crystallographic orientation along b-axis. (040) peak is too weak to observe. The inset is an optical image of a 2D κ-Cl_0.89Br_0.11 crystal. (B) Crystal structure of κ-Cl_1-xBr_x system is consisting of conducting ET layer and nonmagnetic insulating Cu[N(CN)₂](Cl_0.89Br_0.11) layer that are alternatively stacking along b-axis. (C) Charge redistribution within ET dimers transforms charge states among charge liquid, charge glass, and charge crystal. (D) Low-temperature metallicity is realized by applying external stimuli (here is 100 uA high current) and a rapid sweeping rate.

Fig. 2.

The induced low-temperature metallicity and phase diagram in κ-Cl_0.89Br_0.11 crystals. (A) A series of electric current (0.1, 1, 10, and 100 uA) are applied to measure the temperature dependent resistivity by cooling that decreases as current increased. The resistivity is almost constant when current is 100 uA. (B) The metallic behavior in low-temperature resistivity is induced between 120 and 270 K by heating up when current is 100 uA. The inset shows the gold-patterned κ-Cl_0.89Br_0.11 crystal with an electric gap of 32.5 um, a width of 128.8 um, and thickness is 64 um. (C) Temperature-current phase diagram of κ-Cl_0.89Br_0.11 crystals shows conducting states upon a cooling rate of 3 K/min. The blue dots present the transition temperatures from charge liquid to charge glass at different electric current 0.1, 1, and 10 uA. The red dots present the temperature boundary of metallic state at 100 uA. The gray, green, and red colors represent the different phases of charge liquid, charge glass, and metal, respectively.

Fig. 3.

The promoted insulator transition in κ-Cl_0.89Br_0.11 crystals. (A) At a low electric current, a low-temperature semiconducting behavior is apparent at a cooling rate of 3 K/min. The spontaneous charge crystallization at low temperature turns κ-Cl_0.89Br_0.11 into an insulating state. The insulator transition temperature is increased by a slower sweeping rate of 1 K/min. (B) The insulator transition can happen at near 300 K on different samples and is enhanced to higher temperature of 318 K by a sweeping rate of 0.5 K/min. The increased electric current of 5 uA and 10 uA allow the insulator transition to occur at 325 K and 328 K, respectively. (C) The current effect on transition temperature at the cooling rate of 0.5 K/min. The dash line is used for eyes guide. (D) Cooling rate versus temperature phase diagram shows charge crystallization.

Fig. 4.

Stimuli effects of voltage and pulsed laser irradiation on κ-Cl_0.89Br_0.11 crystals. (A) A voltage sequence of 6, 3, and 1.5 V reduces the resistance by two magnitude orders compared to the initial value under 1 V. (B) The low resistance can maintain for a long period of the whole measured time. (C) Pulsed Laser irradiation on/off effect on resistance reveals the reduction of about four magnitude orders. The constant electric current 1 uA was applied. (D) Schematic of charge state switching with external stimuli show the resistance change.