Isotope and Heterostructure Engineering—Tuning Lattice and Optoelectronic Properties of 2D Materials

Isotope engineering allows researchers to tune the lattice and optoelectronic properties of two-dimensional (2D) materials by altering their isotopic composition, enabling systematic investigation and control of various physical properties and providing deeper insights into phonon dynamics and light-matter interactions. Moreover, heterostructure (HS) engineering of 2D materials combined with isotope engineering provides a platform to distinctly tune optoelectronic properties driven by interlayer interaction.<br/>In this study, MoS₂monolayers with four different isotopic compositions—natural sulfur (S), ³²S, ³⁴S and a 50-50% mixture of ³²S and ³⁴S -were synthesized using Chemical Vapor Deposition (CVD). Additionally, MoS₂ bilayers with ³²S and ³⁴S compositions, displaying both AA and AB stacking configurations, were fabricated, and a bilayer HS with a ³²S monolayer bottom layer and ³⁴S top layer was created using a 2-step CVD process.<br/>Raman spectroscopy on monolayers revealed a redshift in A₁' and E' modes with heavier isotopes, consistent with the effective mass variation. The Raman correlation plots between the A₁' and E' modes indicated irregular strain distribution within monolayers with heavier isotopes. Photoluminescence (PL) analysis showed the exciton-to-trion ratio remained consistent in isotopically pure MoS₂, whereas mixed isotope MoS₂ displayed higher levels of trions and B excitons. Temperature-dependent PL analysis demonstrated that exciton peaks for isotopically pure ³²S and ³⁴S MoS₂ followed Varshini's equation, while the 50-50 mixed case, due to its disorder, presented a complex scenario best described by the Manoogian model.<br/>Tip-enhanced Raman spectroscopy (TERS) measurements confirmed the homogeneous distribution of isotopes in the mixed phase. Larger lattice inhomogeneity due to isotopic disorder in natural and mixed monolayers dramatically influenced phonon dynamics and light–matter interactions. Time-resolved photoluminescence (TRPL) measurements indicated nonradiative decay pathways were suppressed in lighter isotopes, while heavier isotopes induced lattice strain and electrostatic doping, supporting faster radiative decay and trion emission.<br/>Further, we report strong interlayer coupling in isotopic HS of MoS₂ monolayers with different sulfur isotopes. The growth propagation revealed distinct triangular domains at separate nucleation centers on the underlying crystal, resulting in heterogeneity in stacking arrangements. The interlayer coupling was affirmed by low-frequency shear and breathing modes, with variability in the shear-to-breathing intensity ratio across different positions, suggesting potential for tuning interlayer coupling through heterogeneous stacking. Raman spectroscopy depicted the modification of fingerprint Raman spectra of individual isotope-modified MoS₂ MLs, indicating a coupling factor.<br/>PL spectra showed a significant decrease in A exciton intensity and a relative enhancement of B exciton intensity, corroborating the interlayer interaction. The minute variation in B exciton over the HS correlated with the heterogeneous stacking pattern. TRPL measurements revealed an additional slower decay channel due to interlayer recombination of carriers and faster lifetimes in the HS, highlighting the nuanced interplay of interlayer coupling dynamics.<br/>These findings underscore the structural and optoelectronic modifications induced by isotopic labelling and heterostructuring in MoS₂ system. This approach opens up new avenues for optimizing the performance of 2D materials in various applications, including optoelectronics, superconductivity, exotic magnetism, and thermoelectricity. The controlled manipulation of isotopic composition and stacking configurations offers a versatile toolkit for advancing the functionality and integration of 2D materials in next-generation technologies.