Figure 1: Graphene–ferroelectric amplitude and phase memory metadevice.
From: Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations
(a) Schematic representation of the graphene–ferroelectric memory metadevice composed of a THz transparent electrode (TTE) with periodical metallic lines (4 μm linewidth and 2 μm spaces between the lines), ferroelectric polymer layer (2.1 μm, represented by green), single-layer graphene, hexagonal MAs and polyimide (1 μm, represented by light red) as substrate. Polarization of the incident THz is perpendicular to the TTE lines. (b) Schematic representation of the principle of nonvolatile doping in inherently p-doped graphene by ferroelectric polarization (P). Positive external voltage induces +PR at the surface of a ferroelectric, resulting in hole depletion in graphene while negative external voltage changes P to −PR, thereby resulting in the accumulation of holes in graphene. (c) Measured (open circles) and simulated (solid lines) THz transmission spectra for external pulsed gating voltage (VG,pulse) lasting for 1 s. The red, blue and yellow lines and circles represent the results of application of VG,pulse(+200 V), VG,pulse(−200 V) and VG,pulse(−120 V), respectively. (d) Hysteresis in the measured transmission amplitude (TA) and the calculated Fermi level of graphene for VG,pulse within a range of +200 and −200 V at a specific frequency of 0.5 THz. 
and 
are the positive and negative coercive voltages, respectively. Arrow refers to the VG,pulse sweep direction. Logic states denoted as 00, 01, 10 and 11 correspond to the multi-level transmission amplitudes for the retention time measurement. Error bars indicate the variation of each measured logic state. (e) Timing diagram of the transmission amplitude (TA) measured at 0.5 THz for various VG,pulse values of −80, −105, −115 and −130 V. Counting from the highest transmission amplitude, data states were designated as 00, 01, 10 and 11. Before each VG,pulse was applied, VG,pulse(+200 V) was applied to reset the ferroelectric polarization to +PR. (f) Transmission amplitude (TA) retention time measured at 0.5 THz for 1 × 105 s and a histogram for each state.
