The primary mechanism of operation of almost all transistors today relies on electric-field effect in a semiconducting channel to tune its conductivity from the conducting 'on'-state to a nonconducting 'off'-state. As transistors continue to scale down to increase computational performance, physical limitations from nanoscale field-effect operation begin to cause undesirable current leakage that is detrimental to the continued advancement of computing 1,2 . Using a fundamentally different mechanism of operation, we show that through nanoscale strain engineering with thin films and ferroelectrics (FEs) the transition metal dichalcogenide (TMDC) MoTe2 can be reversibly switched with electric-field induced strain between the 1T'-MoTe2 (semimetallic) phase to a semiconducting MoTe2 phase in a field effect transistor geometry. This alternative mechanism for transistor switching sidesteps all the static and dynamic power consumption problems in conventional field-effect transistors (FETs) 3,4 . Using strain, we achieve large non-volatile changes in channel conductivity (Gon/Goff~10 7 vs. Gon/Goff~0.04 in the control device) at room temperature. Ferroelectric devices offer the potential to reach sub-ns nonvolatile strain switching at the attojoule/bit level 5-7 , having immediate applications in ultra-fast low-power non-volatile logic and memory 8 while also transforming the landscape of computational architectures since conventional power, speed, and volatility considerations for microelectronics may no longer exist.We design our device using single crystal oxide substrates of relaxor ferroelectric Pb(Mg1/3Nb2/3)0.71Ti0.29O3 (PMN-PT) as the gate dielectric (0.25-0.3 mm thickness). On top of this ferroelectric substrate we exfoliate 1T'-MoTe2 (13-70 nm) from a single crystal source, and pattern devices using Ni contact pads (Figure 1a,b). Exfoliation was performed in a humidity-controlled environment for increased adhesion. Depending on the contact material, stress from the deposited thin film strains the MoTe2 channel at the contact pads, analogous to the uniaxial strain techniques from strained silicon technology, widely adopted in industrial CMOS processes 9 . We find that contact metal stress and TMDC to substrate adhesion are both critically important for obtaining a functional device. During fabrication we are careful not to increase the temperature of the ferroelectric above 80° C, well below the Curie temperature of the ferroelectric at 135° C. Upon reaching the Curie temperature, the Device fabrication performed by W.H., A.S., T.P., and A.A.; Device characterization performed by W.H., A.A., and S.M.W.; Conductive atomic force microscopy performed by W.H., A.S., and S.M.W.; Strain gauge calibration performed by W.H. and S.M.W.; Topographic atomic force microscopy and optical contrast calibration performed by T.P.; Thin film stress measurements performed by C.W., A.A., W.H., and S.M.W.; Piezoresponse force microscopy performed by C.W.; Raman spectroscopy performed by A.A. and S.M.W.; Finite element analysis simul...

We introduce a controllable approach to selectively strain (uniaxially or biaxially) MoS2 by depositing e-beam evaporated thin film stressors with a lithographically patterned stripe geometry. This type of strain engineering has been highly successful in commercial silicon-based CMOS processes to enhance carrier mobility by applying uniaxial strain in MOSFET channels. We attempt to outline the basis for using the same techniques with 2D van der Waals materials with weak out-of-plane bonding. The stressor in this work is chosen to be optically transparent to examine the strain distribution within MoS2 using Raman spectroscopic mapping. MoS2 flakes with partial tensile stressor coverage show large tensile strains close to free edges and compressive strain at the center of the stressor strip. Both in-plane and out-of-plane strains are observed. By varying strip width and MoS2 flake thickness, the geometric distribution of both tensile and compressive strained regions can be controlled. The directionality of strain induced by the stressor strip is also explored through polarized Raman spectroscopy where MoS2 shows 0.85% uniaxial strains occurring at strip edges for 25 N/m film force and biaxial strains occurring at strip centers using the same stressor. Using these combined techniques, we show that strain in 2D materials can be uniquely engineered by design to selectively exhibit tension/compression, uniaxiality/biaxiality, and directionality relative to crystal axes through simple lithographic patterning of stressed thin films. This opens the opportunity to create strain patterned devices with a wide variety of strain-tunable 2D materials properties (electronic, optical, superconducting, etc.), now controllable by micro/nanolithographic design.

Strain engineering is a natural route to control the electronic and optical properties of two-dimensional (2D) materials. Recently, 2D semiconductors have also been demonstrated as an intriguing host of strain-induced quantum-confined emitters with unique valley properties inherited from the host semiconductor. Here, we study the continuous and reversible tuning of the light emitted by such localized emitters in a monolayer tungsten diselenide embedded in a van der Waals heterostructure. Biaxial strain is applied on the emitters via strain transfer from a lead magnesium niobate–lead titanate (PMN-PT) piezoelectric substrate. Efficient modulation of the emission energy of several localized emitters up to 10 meV has been demonstrated on application of a voltage on the piezoelectric substrate. Further, we also find that the emission axis rotates by ∼ 40 ∘ as the magnitude of the biaxial strain is varied on these emitters. These results elevate the prospect of using all electrically controlled devices where the property of the localized emitters in a 2D host can be engineered with elastic fields for an integrated opto-electronics and nano-photonics platform.

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We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.