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1. Giant Thermomechanical Bandgap Modulation in Quasi‐2D Tellurium

   https://doi.org/10.1002/adfm.202407812  

   Hussain, Naveed; Ahmed, Shehzad; Tepe, Hüseyin U; Ullah, Kaleem; Shehzad, Khurram; Wu, Hui; Shcherbakov, Maxim R (November 2024, Advanced Functional Materials)

   Lattice deformation via substrate‐driven mechanical straining of 2D materials can profoundly modulate their bandgap by altering the electronic band structure. However, such bandgap modulation is typically short‐lived and weak due to substrate slippage, which restores lattice symmetry and limits strain transfer. Here, it is shown that a non‐volatile thermomechanical strain induced during hot‐press synthesis results in giant modulation of the inherent bandgap in quasi‐2D tellurium nanoflakes (TeNFs). By leveraging the thermal expansion coefficient (TEC) mismatch and maintaining a pressure‐enforced non‐slip condition between TeNFs and the substrate, a non‐volatile and anisotropic compressive strain is attained with ε = −4.01% along zigzag lattice orientation and average biaxial strain of −3.46%. This results in a massive permanent bandgap modulation of 2.3 eV at a rate S (ΔEg) of up to 815 meV/% (TeNF/ITO), exceeding the highest reported values by 200%. Furthermore, TeNFs display long‐term strain retention and exhibit robust band‐to‐band blue photoemission featuring an intrinsic quantum efficiency of 80%. The results show that non‐volatile thermomechanical straining is an efficient substrate‐based bandgap modulation technique scalable to other 2D semiconductors and van der Waals materials for on‐demand nano‐optoelectronic properties. 

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2. Strain tuning of the emission axis of quantum emitters in an atomically thin semiconductor

   https://doi.org/10.1364/OPTICA.377886  

   Chakraborty, Chitraleema; Mukherjee, Arunabh; Moon, Hyowon; Konthasinghe, Kumarasiri; Qiu, Liangyu; Hou, Wenhui; Peña, Tara; Watson, Carla; Wu, Stephen_M; Englund, Dirk; et al (May 2020, Optica)

   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 $\sim <\#comment/>{40}^{\circ <\#comment/>}$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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3. Atomic Layer Deposition for Membranes, Metamaterials, and Mechanisms

   https://doi.org/10.1002/adma.201901944  

   Dorsey, Kyle J.; Pearson, Tanner G.; Esposito, Edward; Russell, Sierra; Bircan, Baris; Han, Yimo; Miskin, Marc Z.; Muller, David A.; Cohen, Itai; McEuen, Paul L. (May 2019, Advanced Materials)

   Abstract Bending and folding techniques such as origami and kirigami enable the scale‐invariant design of 3D structures, metamaterials, and robots from 2D starting materials. These design principles are especially valuable for small systems because most micro‐ and nanofabrication involves lithographic patterning of planar materials. Ultrathin films of inorganic materials serve as an ideal substrate for the fabrication of flexible microsystems because they possess high intrinsic strength, are not susceptible to plasticity, and are easily integrated into microfabrication processes. Here, atomic layer deposition (ALD) is employed to synthesize films down to 2 nm thickness to create membranes, metamaterials, and machines with micrometer‐scale dimensions. Two materials are studied as model systems: ultrathin SiO₂and Pt. In this thickness limit, ALD films of these materials behave elastically and can be fabricated with fJ‐scale bending stiffnesses. Further, ALD membranes are utilized to design micrometer‐scale mechanical metamaterials and magnetically actuated 3D devices. These results establish thin ALD films as a scalable basis for micrometer‐scale actuators and robotics. 

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4. Holey Substrate-Directed Strain Patterning in Bilayer MoS₂

   https://doi.org/10.1021/acsnano.1c08348  

   Zhang, Yichao; Choi, Moon-Ki; Haugstad, Greg; Tadmor, Ellad B.; Flannigan, David J. (November 2021, ACS Nano)

   null (Ed.)

   Key properties of two-dimensional (2D) layered materials are highly strain tunable, arising from bond modulation and associated reconfiguration of the energy bands around the Fermi level. Approaches to locally controlling and patterning strain have included both active and passive elastic deformation via sustained loading and templating with nanostructures. Here, by float-capturing ultrathin flakes of single-crystal 2H-MoS₂ on amorphous holey silicon nitride substrates, we find that highly symmetric, high-fidelity strain patterns are formed. The hexagonally arranged holes and surface topography combine to generate highly conformal flake-substrate coverage creating patterns that match optimal centroidal Voronoi tessellation in 2D Euclidean space. Using TEM imaging and diffraction, as well as AFM topographic mapping, we determine that the substrate-driven 3D geometry of the flakes over the holes consists of symmetric, out-of-plane bowl-like deformation of up to 35 nm, with in-plane, isotropic tensile strains of up to 1.8% (measured with both selected-area diffraction and AFM). Atomistic and image simulations accurately predict spontaneous formation of the strain patterns, with van der Waals forces and substrate topography as the input parameters. These results show that predictable patterns and 3D topography can be spontaneously induced in 2D materials captured on bare, holey substrates. The method also enables electron scattering studies of precisely aligned, substrate-free strained regions in transmission mode. 

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5. Interaction of 2D materials with liquids: wettability, electrochemical properties, friction, and emerging directions

   https://doi.org/10.1038/s41427-020-0203-1  

   Snapp, Peter; Kim, Jin Myung; Cho, Chullhee; Leem, Juyoung; Haque, Md Farhadul; Nam, SungWoo (March 2020, NPG Asia Materials)

   Abstract The emergence of two-dimensional (2D) materials as functional surfaces for sensing, electronics, mechanics, and other myriad applications underscores the importance of understanding 2D material–liquid interactions. The thinness and environmental sensitivity of 2D materials induce novel surface forces that drive liquid interactions. This complexity makes fundamental 2D material–liquid interactions variable. In this review, we discuss the (1) wettability, (2) electrical double layer (EDL) structure, and (3) frictional interactions originating from 2D material–liquid interactions. While many 2D materials are inherently hydrophilic, their wettability is perturbed by their substrate and contaminants, which can shift the contact angle. This modulation of the wetting behavior enables templating, filtration, and actuation. Similarly, the inherent EDL at 2D material–liquid interfaces is easily perturbed. This EDL modulation partially explains the wettability modulation and enables distinctive electrofluidic systems, including supercapacitors, energy harvesters, microfluidic sensors, and nanojunction gating devices. Furthermore, nanoconfinement of liquid molecules at 2D material surfaces arising from a perturbed liquid structure results in distinctive hydrofrictional behavior, influencing the use of 2D materials in microchannels. We expect 2D material–liquid interactions to inform future fields of study, including modulation of the chemical reactivity of 2D materials via tuning 2D material–liquid interactions. Overall, 2D material–liquid interactions are a rich area for research that enables the unique tuning of surface properties, electrical and mechanical interactions, and chemistry. 

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