Anomalous Isotope-Induced Red Shift in Optical Bandgap of Monolayer MoS2

Isotope engineering, a technique involving the substitution of naturally abundant elements with their isotopically pure counterparts, has emerged as a powerful tool in materials science. This approach is instrumental in modulating various material properties, including thermal conductivity, electronic energy gaps, and quantum characteristics. Traditional semiconductors such as diamond, silicon, germanium, and GaAs have been extensively studied, revealing that isotopic substitution can significantly enhance thermal management and alter electronic and optical properties through changes in electron-phonon coupling. In recent years, attention has turned to two-dimensional (2D) materials, especially transition metal dichalcogenides (TMDs) like MoS₂. These materials exhibit unique optical properties, including strong photoluminescence and large exciton binding energies, which make them ideal candidates for exploring isotope effects on optoelectronic properties. However, understanding how isotopic mass affects the optoelectronic properties in 2D semiconductors remains a significant challenge. This is primarily due to measurement uncertainties arising from sample heterogeneities such as defects, impurities, and interactions with substrates, which can obscure the intrinsic effects of isotopic substitution. To address these challenges, a team of researchers from the Oak Ridge National Laboratory, including Dr. Yiling Yu, Dr. Volodymyr Turkowski, and others, led by Dr. David Geohegan and Dr. Kai Xiao, conducted a comprehensive study published in Science Advances. Their goal was to investigate the effects of isotopic mass on the optical properties of 2D TMD semiconductors, specifically monolayer MoS₂. By developing a two-step chemical vapor deposition (CVD) method to create isotopically pure 100MoS₂-92MoS₂ lateral structures, the researchers aimed to minimize sample heterogeneities and provide a clear understanding of the intrinsic isotope effects on the optoelectronic properties. The primary motivation behind this study was to resolve the ambiguities in previous research regarding how isotopes influence the electronic and excitonic properties of 2D materials. Conventional semiconductors have shown an increase in electronic energy gaps with increasing isotopic mass, primarily due to electron-phonon coupling. However, the behavior in 2D TMDs like MoS2, which have strong exciton-phonon interactions due to their reduced dimensionality, was unclear and required further investigation.

The researchers synthesized isotopically pure 100MoS₂-92MoS₂ lateral structures using a two-step chemical vapor deposition (CVD) method. Initially, monolayer 100MoS₂ crystals were grown on sapphire substrates using a 100MoO3 precursor. After cooling, the substrates were reintroduced into the CVD reactor with a 92MoO₃ precursor to grow the 92MoS₂ regions. This method ensured uniform monolayer crystals with distinct isotopic regions, effectively mitigating the effects of sample heterogeneities. The successful synthesis was confirmed through Raman spectroscopy, which showed distinct vibrational modes for 100MoS₂  and 92MoS₂, indicating the presence of uniform isotopic compositions in each region. Raman spectroscopy was a critical tool in identifying the different isotopic regions of MoS2 based on their vibrational modes. The E′ mode, which involves Mo atoms, exhibited shifts corresponding to the reduced mass of the isotopes. For instance, the E′ mode for 92MoS2 appeared at 387.7 cm−1, while for 100MoS₂, it was at 381.0 cm−1. These findings confirmed the successful creation of isotopically pure regions within the monolayer structures. Additionally, the researchers used photoluminescence (PL) spectroscopy over a temperature range of 4 to 300 K to examine the optical bandgap shifts. The PL measurements revealed a significant red shift of approximately 13 meV in the 100MoS₂ regions compared to the 92MoS₂ regions, contrary to the blue shift typically observed in conventional semiconductors with increasing isotope mass. To verify the spatial distribution and uniformity of Mo isotopes within the lateral structures, the researchers employed time-of-flight secondary ion mass spectrometry (ToF-SIMS). The ToF-SIMS images confirmed distinct regions of 100Mo and 92Mo within the monolayer structure, with minimal cross-contamination. This spatial uniformity was crucial in ensuring that the observed optical properties were intrinsic to the isotopic compositions rather than due to sample impurities or defects. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was used to characterize the atomic structures of the isotopic junctions. Unlike conventional lateral heterostructures that often exhibit dislocation defects, the 100MoS₂-92MoS₂ junctions were found to be dislocation-free. This absence of dislocations suggests minimal scattering during charge transport, an advantageous feature for potential electronic applications. The pristine quality of the junctions further supported the reliability of the optical measurements, ensuring that the red shift observed in the PL spectra was due to intrinsic isotope effects. The researchers used density functional perturbation theory (DFPT) and time-dependent density functional theory (TDDFT) to understand the underlying mechanisms behind the observed optical bandgap shifts. DFPT calculations indicated that the electronic bandgap renormalization due to electron-phonon coupling followed the conventional trend, with lighter isotopes showing larger bandgap renormalizations. However, TDDFT calculations revealed that the exciton binding energy renormalization, influenced by strong exciton-phonon coupling, was significantly larger for heavier isotopes. This led to an overall red shift in the optical bandgap for 100MoS2 compared to 92MoS2, counter to the trend seen in bulk semiconductors.

The temperature-dependent PL spectra provided further insights into the isotope effects on exciton-phonon coupling. The researchers observed that the exciton-phonon interactions were stronger in 100MoS₂, resulting in a larger linewidth at low temperatures compared to 92MoS₂. However, at higher temperatures, the 92MoS₂ regions exhibited a faster increase in linewidth, surpassing 100MoS₂. This temperature-dependent behavior highlighted the complex interplay between isotopic mass and exciton-phonon coupling in 2D materials, with significant implications for their optoelectronic properties. The combined experimental and theoretical findings underscored the critical role of exciton-phonon coupling in modulating the optoelectronic properties of 2D semiconductors. The observed isotope-induced red shift in the optical bandgap opens new avenues for designing advanced optoelectronic devices through isotope engineering. The ability to tailor the optical properties by manipulating isotopic compositions offers potential applications in developing high-performance transistors, photodetectors, and quantum computing components.

In conclusion, the authors, provided valuable data into how isotopic substitution affects the optoelectronic properties of 2D transition metal dichalcogenides (TMDs), such as MoS₂. By revealing an anomalous red shift in the optical bandgap with increasing isotopic mass, the research challenges conventional understanding based on bulk semiconductors. This phenomenon underscores the importance of exciton-phonon coupling in 2D materials, which differs significantly from the behavior observed in three-dimensional systems. Moreover, the findings highlight the crucial role of exciton-phonon interactions in determining the optoelectronic properties of 2D semiconductors. The study shows that the renormalization of exciton binding energy, driven by strong exciton-phonon coupling, can dominate the optical bandgap shifts in these materials. This deeper understanding can guide future theoretical and experimental studies aimed at exploring and manipulating exciton-phonon dynamics in low-dimensional systems. Additionally, the ability to engineer the optical bandgap through isotopic substitution opens new avenues for designing advanced optoelectronic devices. By tailoring the isotopic composition, it is possible to fine-tune the electronic and optical properties of 2D materials, leading to the development of high-performance transistors, photodetectors, and light-emitting devices with improved efficiency and functionality.  The findings have potential implications for quantum computing and spintronics. The manipulation of isotope-induced spin properties in 2D materials could enhance spin and valley coherence times, which are critical for quantum information processing devices. Isotopically engineered 2D materials may exhibit improved performance in quantum computing applications due to reduced decoherence and enhanced control over quantum states. Isotopic engineering can also impact thermal management in microelectronics. By enhancing thermal conductivity through the suppression of isotopic disorder, isotopically pure 2D materials can better dissipate heat, thereby improving the reliability and longevity of electronic devices. This aspect is particularly important for high-power electronics and densely packed microelectronic circuits. The ability to customize material properties through isotopic substitution enables the development of materials tailored for specific applications. For instance, materials with precisely controlled bandgaps and thermal properties can be designed for use in specialized sensors, photovoltaic cells, and other optoelectronic systems. Finally, this study exemplifies the potential of isotope engineering to unlock new functionalities in low-dimensional materials, driving future advancements in nanotechnology and material science.