Flexible, Low Defect Density and Single-Layer MoS₂ Piezoelectric Sensor for Slippage Detection in Soft Gripper

Transition metal dichalcogenides (TMDs) have garnered considerable interest due to their exceptional flexibility and superior piezoelectric properties compared to conventional piezoelectric ceramics and polymers. A key challenge in realizing their practical potential lies in the fabrication of continuous, scalable, and thickness-controlled TMD films, as their piezoelectric properties are layer-dependent. Herein, we develop a multi-holding atomic layer deposition (MH-ALD) strategy to enable the homogeneous deposition of MoS₂. This approach facilitates rapid nucleation during the initial ALD cycles while mitigating the steric hindrance effect due to the sufficient feed time of precursor and reactant. Moreover, the repeated pulse-hold-purge sequence of precursor in each ALD cycle effectively eliminates physisorbed by-products, which clears the surface and promotes further lateral growth. This process demonstrates greater efficiency than the holding ALD (single pulse-hold-purge sequence of precursor per cycle, referred to as the H-ALD process), as it achieves superior coverage of single-layer MoS₂. Specifically, the MH-ALD process results in a coverage rate of 99.95%, compared to 92.51% observed with the H-ALD process. Based on these mechanisms, a single-layer centimeter-scale MoS₂ film with low defect density and remarkable piezoelectric performance (d₁₁ = 4.3 pm/V) is achieved. The evolution of growth modes in the advanced ALD strategy is carefully studied on the films deposited under varying temperatures and repetition times of pulse-hold of precursor. To showcase the piezoelectric performance, both bendable and stretchable piezoelectric devices are integrated onto PET and TPU substrates respectively, demonstrating outstanding linear sensitivity (approximately 141.38 pA per 1% lateral strain for bendable one, the highest among 2D binary materials). The fabricated bendable devices exhibit superior response to uniaxial lateral strain (1-1 mode), showing an output voltage of around 1.74 V when subjected to a lateral strain of 0.88% and a bending velocity of 6 cm/s. Furthermore, the integrated bendable device is embedded into a 3D-printed pneumatic soft gripper system for detecting slippage. The sensor generates real-time signals as soon as slippage occurs, with the signal amplitude proportional to the object's weight—635.5 pA for a tomato and 92.11 pA for a grape. This feedback system enables closed-loop control, allowing the gripper to dynamically adjust its holding force to prevent further slippage.