ABSTRACT
Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS2 modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to â¼0.62 V/nm through the MoS2 layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.
ABSTRACT
Robust room-temperature interfacial ferroelectricity has been formed in the 2D limit by simply twisting two atomic layers of non-ferroelectric hexagonal boron nitride (hBN). A thorough understanding of this newly discovered ferroelectric system is required. Here, twisted hBN is used as a tunneling junction and it is studied at the nanometer scale using conductive atomic force microscopy. Three properties unique to this system are discovered. First, the polarization dependence of the tunneling resistance contrasts with the conventional theory. Second, the ferroelectric domains can be controlled using mechanical stress, highlighting the original meaning of the emergent "slidetronics". Third, ferroelectric hysteresis is highly spatially dependent. The hysteresis is symmetric at the domain walls. A few nanometers away, the hysteresis shifts completely to the positive or negative side, depending on the original polarization. These findings reveal the unconventional ferroelectricity in this 2D system.
ABSTRACT
PbS colloidal quantum dots (CQDs) are emerging as promising candidates for next-generation, low-cost, and high-performance infrared photodetectors. Recently, photomultiplication has been explored to improve the detectivity of CQD infrared photodetectors by doping charge-trapping material into a matrix. However, this relies on remote doping that could influence carrier transfer giving rise to limited photomultiplication. Herein, a charge-self-trapped ZnO layer is prepared by a surface reaction between acid and ZnO. Photogenerated electrons trapped by oxygen vacancy defects at the ZnO surface generate a strong interfacial electrical field and induce large photomultiplication at extremely low bias. A PbS CQD infrared photodiode based on this structure shows a response (R) of 77.0 A·W-1 and specific detectivity of 1.5 × 1011 Jones at 1550 nm under a -0.3 V bias. This self-trapped ZnO layer can be applied to other photodetectors such as perovskite-based devices.
ABSTRACT
Heterostructure devices based on two-dimensional materials have been under intensive study due to their intriguing electrical and optical properties. One key factor in understanding these devices is their nanometer-scale band profiles, which is challenging to obtain in devices. Here, we use a technique named contact-mode scanning tunneling spectroscopy to directly visualize the band profiles of MoS2/WSe2 heterostructure devices at different gate voltages with nanometer resolution. The long-held view of a conventional p-n junction in the MoS2/WSe2 heterostructure is reexamined. Due to strong inter- and intralayer charge transfer, the MoS2 layer in contact with WSe2 is found to convert from n-type to p-type, and a series of gate-tunable p-n and p-p+ junctions are developed in the devices. Highly conductive edges are also discovered which could strongly affect the device properties.
