A research team has developed a symmetry-engineered two-dimensional heterostructure that can distinguish how photocurrent is generated along different crystal directions. By stacking black arsenic (b-As) with monolayer 2H transition-metal dichalcogenides(2H-TMDs), the researchers created b-As/2H-TMDs van der Waals heterostructures in which the armchair (AC) and zigzag (ZZ) directions produce photocurrent through different mechanisms.
The key lies in interfacial symmetry breaking. b-As has twofold rotational symmetry, while monolayer 2H-TMDs such as MoS 2 , WSe 2 and SnS 2 have threefold rotational symmetry. When the two materials are aligned along the AC direction, this symmetry mismatch creates a non-centrosymmetric interface with directional in-plane polarization.
This built-in polarization drives a spontaneous bulk photovoltaic current along the AC direction, allowing the device to generate photocurrent at zero bias. In contrast, the ZZ direction does not show the same bulk photovoltaic behavior. Instead, its photocurrent is much weaker and is mainly associated with contact-related effects, such as Schottky-barrier-driven or photothermal responses near the electrodes.
The devices also showed strong broadband and polarization-sensitive performance. Optimized b-As/WSe 2 heterostructures responded from 405 nm to 1064 nm and reached a responsivity of 946 mA/W, a detectivity of 9.28 × 10 9 Jones and a BPV coefficient of 0.95 V −1 under 638 nm illumination. Their BPV dichroic ratio reached 10.82 at 638 nm, showing that the photocurrent is highly sensitive to the polarization direction of incoming light.
By linking crystal direction to photocurrent mechanism, this work provides a design principle for ultrathin, self-powered and polarization-sensitive photodetectors. Such devices may be useful for integrated photonics, polarization imaging and optical sensing, where understanding and controlling the origin of photocurrent is essential.
Science Bulletin
Experimental study