Strain-Induced Work Function Shifts in 2D Materials

Two-dimensional (2D) materials are increasingly recognized as promising candidates for beyond-silicon electronics owing to their favorable size scaling of electronic performance. The heterogeneous integration of 2D materials with CMOS technologies often introduces the process-induced strain arising from mechanical mismatches, necessitating a comprehensive understanding of how strain impacts their electrical characteristics. Numerous studies have explored the effects of strain on the band gap alterations in 2D materials, such as WSe₂ and MoS₂, and consequent changes in the electrical performance of devices. For instance, strain tunes the optical band gap of WSe₂ at a rate of -54 meV/%,^[1] and enhances the field effect mobility of MoS₂ transistors, 1.3 times/%.^[2] However, one of the lesser understood yet crucial parameters that strain affects is the work function, which is crucial for understanding metal-semiconductor Schottky barriers, contact resistance and threshold voltage in transistors. Understanding the interplay between strain and work function in 2D materials is pivotal determining the ultimate performance of strain engineered transistors, and the behavior of stretchable electronics from 2D heterostructures.^[3]<br/>Here, we analyzed the strain induced tuning of the work function in wrinkled monolayer WSe₂. We fabricated 2D materials with periodically modulated strain by transferring WSe₂ with a 15 nm thick ALD grown HfO₂ support layer onto 2.5% pre-stretched elastomers. After releasing the pre-stretch, the 2D material-ALD heterostructure self-assembles into quasi-periodic micrometer wrinkles, which show periodic modulations in the strain of up to 0.67%.^[4] We then used Kelvin Probe Force Microscopy (KPFM) to simultaneously measure the local topography and change in potential energy across the wrinkles. We then correlated the results to the optical properties through hyperspectral mapping of both Photoluminescence and Raman modes. There are two advantages of this approach. First, all strain conditions can be measured in a single acquisition, avoiding challenges of sample-to-sample variation or changes in tip measurements between samples. Second, we can directly correlate the topography and resulting strain state to the local work function.<br/>From the topography, we found that the maximum strain occurred on the wrinkle peak of monolayer WSe₂, at a value of 0.39%, and the minimum strain occurred on the wrinkle valley at a value of -0.43%. Between the wrinkle peak and valley, the work function shifted at a rate of -79 ± 26 meV/%, while the band gap shifted at a rate of -58.6 ± 3.4 meV/%. The tuning of the work function is new, but bandgap shift is consistent with what is observed in uniaxial tension of WSe₂, where shifts of -54 meV/% have been observed.^[1] These results suggest that tensile (compressive) strain decreases (increases) both the band gap and work function of WSe₂. Further, our approach is materials agnostic and can be applied to measure the strain relationships in many 2D materials.<br/>This work elucidates the relationship between strain, work function, and optical band gap in 2D materials, which is important across device applications in the emerging field of 2D straintronics.<br/><br/><br/>References:<br/>1. Schmidt, Robert, et al. "Reversible uniaxial strain tuning in atomically thin WSe₂." 2D Materials 3.2 (2016): 021011.<br/>2. Zhang, Yue, et al. "Enhancing Carrier Mobility in Monolayer MoS2 Transistors with Process-Induced Strain." ACS nano 18.19 (2024): 12377-12385.<br/>3. Kim, Hyunchul, He Lin Zhao, and Arend M. van der Zande. "Stretchable Thin-Film Transistors Based on Wrinkled Graphene and MoS2 Heterostructures." Nano letters 24.4 (2024): 1454-1461.<br/>4. Hossain, M. Abir, Yue Zhang, and Arend M. Van Der Zande. "Strain engineering photonic properties in monolayer semiconductors through mechanically-reconfigurable wrinkling." Physical Chemistry of Semiconductor Materials and Interfaces XIX. Vol. 11464. SPIE, 2020.