Fig. 1. (Color online) Ferroelectric polarization switching by PFM for 50 nm-thick P(VDF-TrFE) film. (a) Schematic of the operation for P(VDF-TrFE) polarization switching with PFM probe appling a positive or negative voltage. (b) The PFM amplitude (red) and phase (black) hysteresis loops during the switching process, scale bar is 10 μm. (c) Amplitude signal and (d) phase signal of the P(VDF-TrFE) film after writing half-to-half rectangle patterns with reversed DC bias, scale bar is 10 μm.

Fig. 2. (Color online) Structure illustration of the gate-free MoS₂ photodetector. (a) A 3D schematic of the device structure. (b) The optical image of the device, which includes a 3.5 nm MoS₂ and Cr/Au source and drain electrodes covered by 50 nm-thick P(VDF-TrFE) film, scale bar is 10 μm. (c) The PFM phase image of the device (red square part in (b)), two polarization states of P_down and P_up represent the opposite direction of the electric dipole moment in different parts (orange and black), scale bar is 3 μm. (d) Phase profile extracted along the white dashed line in (c), which demonstrated that the MoS₂ in the channel has a 180° phase difference with the other part.

Fig. 3. (Color online) Electrical characteristics of the device with three different polarization states of P(VDF-TrFE). (a) The I_sd–V_sd characteristics of the device with three different polarization states of P(VDF-TrFE). These three polarization states are fresh (without polarization), P_up (upward polarization) and P_down state (downward polarization). (b) Sectional view of the device for illustrating the distribution of electron and hole under downward and (c) upward polarization state. (d–f) The energy band diagrams of the device with three polarization states at V_sd = 0 V. Φ_b is the Schottky barrier height, E_g is the energy gap of MoS₂, δ is the barrier height from conduction band to Fermi level.

Fig. 4. (Color online) Optoelectronic properties of the device. (a) I_sd–V_sd curves of the device under dark and different power of incident light (520 nm). (b) photocurrent switching characteristic of the device with 520 nm laser illumination. The light power is 6.56 μW and V_sd = 1 V. (c) The photocurrent map of the device recorded at P_up state and V_sd = 1 V, the incident light power is 0.28 μW, scale bar = 3 μm. (d, e) The photocurrent rise and fall time extracted from a photocurrent switching cycle. (f) The photoresponse characteristic of the device under incident light with the different wavelengths but the same power.

Table 1. Photoresponse speed comparation between our device and other MoS₂-based photodetectors.