More Like this

1. Forming gas annealing-induced reversible 2D-to-3D bonding transition in 3R-MoS2

   https://doi.org/10.1063/5.0279605  

   Schreier, Caleb; Hasan, Nazmul; Shao, Chen; Wu, Stephen M (September 2025, Applied Physics Letters)

   We report a method of engineering a reversible change in interlayer bonding between layers of exfoliated thin films of MoS2 by means of hydrogen intercalation through forming gas annealing. Interlayer bonding strength is probed through the behavior of MoS2 under process-induced strain engineering, where two-dimensional (2D) flakes are encapsulated with a deposited stressed thin film layer to transfer strain into the underlying 2D materials. It is shown that after forming gas annealing, the depth of the strain transferred into multilayer MoS2 is enhanced as determined through layer-thickness-dependent Raman spectroscopic mapping. This change represents a transition from a 2D van der Waals-bonded material in the as-exfoliated samples to a more three-dimensional (3D)-bonded system in the annealed samples. We demonstrate the reversibility of this effect by means of vacuum annealing of previously forming gas annealed samples. The process of forming gas annealing itself also imparts strain into MoS2 due to a combination of 2D-to-3D bonding transition with differential thermal mismatch between the MoS2 and the substrate. These strains are shown to be retained after the vacuum annealing process, despite the transition back to 2D bonding. Since forming gas annealing is a common technical process in engineering 2D electronic devices, these results represent an important consideration in understanding non-intentionally applied strains due to changes in the mechanical properties of 2D materials. 

   more » « less
2. Rigorous prediction of Raman intensity from multi-layer films

   https://doi.org/10.1364/OE.403705  

   Velson, Nathan_Van; Zobeiri, Hamidreza; Wang, Xinwei (November 2020, Optics Express)

   In the Raman probing of multilayer thin film materials, the intensity of the measured Raman scattered light will be impacted by the thickness of the thin film layers. The Raman signal intensity will vary non-monotonically with thickness due to interference from the multiple reflections of both the incident laser light and the Raman scattered light of thin film interfaces. Here, a method for calculating the Raman signal intensity from a multilayer thin film system based on the transfer matrix method with a rigorous treatment of the Raman signal generation (discontinuity) is presented. This calculation methodology is valid for any thin film stack with an arbitrary number of layers with arbitrary thicknesses. This approach is applied to several thin film material systems, including silicon-on-sapphire thin films, graphene on Si with a SiO₂capping layer, and multilayer MoS₂with the presence of a gap between layers and substrate. Different applications where this method can be used in the Raman probing of thin film material properties are discussed. 

   more » « less
3. Moiré engineering in 2D heterostructures with process-induced strain

   https://doi.org/10.1063/5.0142406  

   Peña, Tara; Dey, Aditya; Chowdhury, Shoieb A.; Azizimanesh, Ahmad; Hou, Wenhui; Sewaket, Arfan; Watson, Carla; Askari, Hesam; Wu, Stephen M. (April 2023, Applied Physics Letters)

   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 C 3 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. 

   more » « less
4. Tuning of the electronic and vibrational properties of epitaxial MoS ₂ through He-ion beam modification

   https://doi.org/10.1088/1361-6528/aca3af  

   Parida, Shayani; Wang, Yongqiang; Zhao, Huan; Htoon, Han; Kucinski, Theresa Marie; Chubarov, Mikhail; Choudhury, Tanushree; Redwing, Joan Marie; Dongare, Avinash; Pettes, Michael Thompson (December 2022, Nanotechnology)

   Abstract Atomically thin transition metal dichalcogenides (TMDs), like MoS 2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4 He + ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS 2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4 He + ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS 2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS 2 and other 2D materials. 

   more » « less
5. Toward understanding the phase-selective growth mechanism of films and geometrically-shaped flakes of 2D MoTe ₂

   https://doi.org/10.1039/d1ra07787b  

   Limbu, Tej B.; Adhikari, Bikram; Song, Seung Keun; Chitara, Basant; Tang, Yongan; Parsons, Gregory N.; Yan, Fei (November 2021, RSC Advances)

   Two-dimensional (2D) molybdenum ditelluride (MoTe 2 ) is an interesting material for fundamental study and applications, due to its ability to exist in different polymorphs of 2H, 1T, and 1T′, their phase change behavior, and unique electronic properties. Although much progress has been made in the growth of high-quality flakes and films of 2H and 1T′-MoTe 2 phases, phase-selective growth of all three phases remains a huge challenge, due to the lack of enough information on their growth mechanism. Herein, we present a novel approach to growing films and geometrical-shaped few-layer flakes of 2D 2H-, 1T-, and 1T′-MoTe 2 by atmospheric-pressure chemical vapor deposition (APCVD) and present a thorough understanding of the phase-selective growth mechanism by employing the concept of thermodynamics and chemical kinetics involved in the growth processes. Our approach involves optimization of growth parameters and understanding using thermodynamical software, HSC Chemistry. A lattice strain-mediated mechanism has been proposed to explain the phase selective growth of 2D MoTe 2 , and different chemical kinetics-guided strategies have been developed to grow MoTe 2 flakes and films. 

   more » « less