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1. Strain engineering photonic properties in monolayer semiconductors through mechanically-reconfigurable wrinkling

   https://doi.org/10.1117/12.2567539  

   Hossein, Mohammad A.; Zhang, Yue; van der Zande, Arend (August 2020, Proc. of SPIE Physical Chemistry of Semiconductor Materials and Interfaces IX)

   Congreve, Daniel; Nielsen, Christian; Musser, Andrew J. (Ed.)

   Inhomogeneous and three-dimensional strain engineering in two dimensional materials opens up new avenues to straintronic devices for control strain sensitive photonic properties. Here we present a method to tune strain by wrinkling monolayer WSe2 attached to a 15 nm thick ALD support layer and compressing the heterostructure on a soft substrate. The ALD film stiffens the 2D material, enabling optically resolvable micron scale wrinkling rather than nanometer scale crumpling and folding. Using photoluminescence spectroscopy, we show the wrinkling introduces periodic modulation of the bandgap by 47 meV, corresponding with strain modulation from +0.67% tensile strain at the wrinkle crest to -0.31% compressive strain at the trough. Moreover, we show that cycling the substrate strain mechanically reconfigures the magnitude and direction of wrinkling and resulting band tuning. These results pave the way towards stretchable multifuctional devices based on strained 2D materials. 

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2. Asymmetric 3D Elastic–Plastic Strain‐Modulated Electron Energy Structure in Monolayer Graphene by Laser Shocking

   https://doi.org/10.1002/adma.201900597  

   Motlag, Maithilee; Kumar, Prashant; Hu, Kevin_Y; Jin, Shengyu; Li, Ji; Shao, Jiayi; Yi, Xuan; Lin, Yen‐Hsiang; Walrath, Jenna_C; Tong, Lei; et al (March 2019, Advanced Materials)

   Abstract Graphene has a great potential to replace silicon in prospective semiconductor industries due to its outstanding electronic and transport properties; nonetheless, its lack of energy bandgap is a substantial limitation for practical applications. To date, straining graphene to break its lattice symmetry is perhaps the most efficient approach toward realizing bandgap tunability in graphene. However, due to the weak lattice deformation induced by uniaxial or in‐plane shear strain, most strained graphene studies have yielded bandgaps <1 eV. In this work, a modulated inhomogeneous local asymmetric elastic–plastic straining is reported that utilizes GPa‐level laser shocking at a high strain rate (dε/dt) ≈ 10⁶–10⁷s⁻¹, with excellent formability, inducing tunable bandgaps in graphene of up to 2.1 eV, as determined by scanning tunneling spectroscopy. High‐resolution imaging and Raman spectroscopy reveal strain‐induced modifications to the atomic and electronic structure in graphene and first‐principles simulations predict the measured bandgap openings. Laser shock modulation of semimetallic graphene to a semiconducting material with controllable bandgap has the potential to benefit the electronic and optoelectronic industries. 

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3. Strain‐Engineered Anisotropic Optical and Electrical Properties in 2D Chiral‐Chain Tellurium

   https://doi.org/10.1002/adma.202002342  

   Wang, Yixiu; Yao, Shukai; Liao, Peilin; Jin, Shengyu; Wang, Qingxiao; Kim, Moon_J; Cheng, Gary_J; Wu, Wenzhuo (June 2020, Advanced Materials)

   Abstract Atomically thin materials, leveraging their low‐dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin‐film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain‐engineered anisotropic optical and electrical properties in solution‐grown, sub‐millimeter‐size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral‐chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain‐engineered functionalities. 

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4. High-pressure characterization of Ag3AuTe2: Implications for strain-induced band tuning

   https://doi.org/10.1063/5.0223472  

   Won, Juyeon; Zhang, Rong; Peng, Cheng; Kumar, Ravhi; Gebre, Mebatsion S; Popov, Dmitry; Hemley, Russell J; Bradlyn, Barry; Devereaux, Thomas P; Shoemaker, Daniel P (November 2024, Applied Physics Letters)

   Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag3AuTe2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag3AuTe2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the semiconducting behavior of Ag3AuTe2 with high resistivity and an activation energy Ea of 0.2 eV. The optical bandgap determined by diffuse reflectance measurements is about three times wider than the experimental Ea. Despite the difference, both experimental gaps fall within the range of predicted bandgaps by our first-principles density functional theory (DFT) calculations employing the Perdew–Burke–Ernzerhof and modified Becke–Johnson methods. Furthermore, our DFT simulations predict a progressive narrowing of the bandgap under compressive strain, with a full closure expected at a strain of −4% relative to the lattice parameter. To evaluate the feasibility of gap tunability at such substantial strain, the high-pressure behavior of Ag3AuTe2 was investigated by in situ high-pressure x-ray diffraction up to 47 GPa. Mechanical compression beyond 4% resulted in a pressure-induced structural transformation, indicating the possibility of substantial gap modulation under extreme compression conditions. 

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5. Strain-engineered high-responsivity MoTe2 photodetector for silicon photonic integrated circuits

   https://doi.org/10.1038/s41566-020-0647-4  

   Maiti, R.; Patil, C.; Saadi, M. A.; Xie, T.; Azadani, J. G.; Uluutku, B.; Amin, R.; Briggs, A. F.; Miscuglio, M.; Van Thourhout, D.; et al (June 2020, Nature Photonics)

   In integrated photonics, specific wavelengths such as 1,550 nm are preferred due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, two-dimensional materials bear scientifically and technologically relevant properties such as electrostatic tunability and strong light–matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with two-dimensional transition metal dichalcogenide materials due to their large optical bandgaps. Here we demonstrate a MoTe2-based photodetector featuring a strong photoresponse (responsivity 0.5 A W–1) operating at 1,550 nm in silicon photonics enabled by strain engineering the two-dimensional material. Non-planarized waveguide structures show a bandgap modulation of 0.2 eV, resulting in a large photoresponse in an otherwise photoinactive medium when unstrained. Unlike graphene-based photodetectors that rely on a gapless band structure, this photodetector shows an approximately 100-fold reduction in dark current, enabling an efficient noise-equivalent power of 90 pW Hz–0.5. Such a strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems. 

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