🔬 materials science

Multiwavelength Raman investigation of mono- and few-layer MoS2 grown by Pulsed Laser Deposition on SiO2

This paper demonstrates the room-temperature growth of mono- and few-layer MoS $_2$ on SiO $_2$ substrates via pulsed laser deposition and utilizes multiwavelength Raman spectroscopy to characterize film thickness and defect density while providing experimental evidence of symmetry-dependent exciton-phonon coupling in the resulting material.

Original authors: Alice Cartoceti (Department of Energy, Politecnico di Milano, Milano, Italy), Paolo D'Agosta (Department of Energy, Politecnico di Milano, Milano, Italy), Francesco Tumino (Department of Energy, Polit

Published 2026-07-16

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Alice Cartoceti (Department of Energy, Politecnico di Milano, Milano, Italy), Paolo D'Agosta (Department of Energy, Politecnico di Milano, Milano, Italy), Francesco Tumino (Department of Energy, Politecnico di Milano, Milano, Italy), Valeria Russo (Department of Energy, Politecnico di Milano, Milano, Italy), Carlo S. Casari (Department of Energy, Politecnico di Milano, Milano, Italy), Andrea Li Bassi (Department of Energy, Politecnico di Milano, Milano, Italy)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where the tiny circuits inside your phone and computer are built not from silicon, but from materials so thin they are essentially two-dimensional sheets of atoms. This is the frontier of "2D materials," a hot topic in physics and chemistry where scientists are hunting for the next generation of super-efficient electronics. One of the most promising stars in this field is Molybdenum Disulfide (MoS2). Think of MoS2 like a microscopic sandwich: a layer of molybdenum atoms stuck between two layers of sulfur atoms. When you stack many of these sandwiches, they act like a standard semiconductor. But if you peel them down to just a single layer, they transform into something magical, becoming incredibly efficient at absorbing light and conducting electricity. The challenge? Making these perfect, single-layer sheets over large areas without breaking them or introducing "dirt" (defects) that ruins their superpowers. Scientists have been trying to figure out the best way to grow these sheets on the glass-like surfaces used in electronics, and they need a way to count exactly how many layers they have made and check if the material is healthy.

In this study, a team of researchers from Politecnico di Milano in Italy decided to try a new recipe for growing these MoS2 sandwiches on a standard silicon dioxide (SiO2) surface, which is the kind of glassy material found on computer chips. Instead of using high heat, which can sometimes damage the delicate layers, they used a technique called Pulsed Laser Deposition (PLD). You can imagine this like a high-tech paintball gun: a powerful laser shoots at a solid block of MoS2, blasting tiny bits of it off the surface. These bits fly through a vacuum and land on the cool SiO2 substrate below. By simply counting how many times they pull the trigger (the number of laser pulses), they could control exactly how thick the new film would be, growing everything from a single layer up to several layers.

The researchers then used a special tool called Raman spectroscopy to "listen" to the vibrations of the atoms in their new films. It's like tapping a drum to hear its pitch; different thicknesses of MoS2 vibrate at slightly different frequencies. They found that by adjusting the number of laser pulses, they could successfully grow a perfect single layer of MoS2 on the SiO2 surface. However, they also discovered that their laser-grown films were a bit "messier" than films made by peeling them off a larger crystal (a method called mechanical exfoliation). The laser-grown sheets had more defects, or missing atoms, which changed how they interacted with light.

The most exciting discovery came when they shined different colors of laser light on the samples. They found that the way the atoms vibrated depended on which color of light they used, a phenomenon called "exciton-phonon coupling." In simple terms, the light creates excited particles (excitons) that talk to the vibrating atoms (phonons). The researchers observed that the "out-of-plane" vibrations (atoms moving up and down like a jump rope) and the "in-plane" vibrations (atoms moving side-to-side) reacted differently to the light, depending on the specific type of exciton they were talking to. Crucially, they found that the extra defects in their laser-grown films changed this conversation, suppressing the response of certain vibrations more than others. This suggests that while their room-temperature laser method is a viable way to grow these materials on electronic chips, the quality of the growth (how few defects it has) plays a huge role in how the material behaves optically. The study confirms that this laser approach works for making tunable 2D MoS2, but it also highlights that getting the material perfectly clean is the next big hurdle to overcome for high-performance devices.

Technical Summary: Multiwavelength Raman Investigation of Mono- and Few-Layer MoS2 Grown by Pulsed Laser Deposition on SiO2

Problem Statement
Molybdenum disulfide (MoS2) is a promising transition metal dichalcogenide (TMD) for low-power electronics and optoelectronics due to its tunable bandgap and strong optical absorption. However, the scalable fabrication of large-area, uniform, high-crystalline-quality MoS2 films remains a significant challenge. Top-down methods like mechanical exfoliation lack scalability and thickness control, while bottom-up approaches like Chemical Vapor Deposition (CVD) often require high growth temperatures (1020–1270 K) and suffer from reproducibility issues. Conventional Pulsed Laser Deposition (PLD) at high temperatures can induce chalcogen deficiency, vacancy formation, and oxidation. While previous work by the authors demonstrated room-temperature PLD followed by annealing for MoS2 growth on noble metals and few-layer films on SiO2, the production of high-quality MoS2 monolayers on electronics-compatible SiO2 substrates via room-temperature PLD had not yet been reported.

Methodology
The authors employed a room-temperature PLD approach followed by post-deposition annealing to grow MoS2 on oxidized p-doped Si substrates (285 nm SiO2).

Deposition: A bulk MoS2 target was ablated using a KrF excimer laser (248 nm, 20 ns pulses) at a fluence of 2 J/cm². The SiO2 substrate was maintained at room temperature during deposition. The number of laser pulses (ranging from 20 to 120) served as the sole parameter to control film thickness.
Annealing: Post-deposition, samples were annealed in ultra-high vacuum (UHV) at 720–810 K for 30 minutes to promote crystallization.
Characterization: The study utilized multiwavelength Raman spectroscopy (excitation wavelengths: 457 nm, 532 nm, 660 nm) and Photoluminescence (PL) spectroscopy (514 nm excitation). Scanning Electron Microscopy (SEM) was used for morphological analysis. Reference samples of mechanically exfoliated MoS2 monolayers on SiO2 were prepared and analyzed under identical conditions for comparison.

Key Contributions and Results

Controlled Growth of Monolayer MoS2 on SiO2:
The study successfully demonstrated the production of MoS2 monolayers on SiO2 using room-temperature PLD. By varying the laser pulse count, the authors tuned the film thickness from mono- to multilayer.
- Thickness Determination: The Raman shift difference ( $\Delta\omega$ ) between the in-plane $E_{2g}^1$ and out-of-plane $A_{1g}$ modes was used as a metric for layer count. The 30-pulse sample exhibited a $\Delta\omega$ of $20.3 \pm 0.3$ cm $^{-1}$ , consistent with monolayer values. Samples with 90 and 120 pulses showed progressively higher $\Delta\omega$ values, corresponding to trilayer and quadrilayer configurations, respectively.
- Growth Mechanism: An anomalous increase in $\Delta\omega$ was observed for the 20-pulse sample. SEM analysis revealed that below the monolayer threshold, MoS2 forms isolated islands with exposed SiO2 regions, whereas above the threshold (30 pulses), a continuous monolayer forms. This contrasts with growth on metals, where strong substrate interaction facilitates contiguous monolayer formation even at lower coverage.
Defect Characterization and Density:
The PLD-grown films were found to be nanocrystalline with a high density of defects compared to mechanically exfoliated samples.
- Defect-Activated Modes: Raman spectra of PLD samples showed distinct disorder-activated peaks (e.g., LA(M) at ~227 cm $^{-1}$ ) absent in pristine exfoliated flakes.
- Interdefect Distance: Using the intensity ratio of the LA(M) to $A_{1g}$ modes, the average interdefect distance ( $L_d$ ) was estimated. The monolayer PLD sample exhibited an $L_d$ of $1.48 \pm 0.04$ nm, significantly smaller than the $2.15 \pm 0.05$ nm observed in the 4-layer sample, indicating that defect density increases as the film thickness decreases toward the monolayer limit in this growth regime.
Symmetry-Dependent Exciton-Phonon Coupling:
A central finding of the paper is the experimental evidence of symmetry-dependent exciton-phonon coupling in PLD-grown MoS2, revealed through multiwavelength Raman spectroscopy.
- Resonance Effects: The study observed that the $A_{1g}/E_{2g}^1$ intensity ratio is highly sensitive to the excitation wavelength.
- Exfoliated vs. PLD Behavior: In mechanically exfoliated monolayers, the $A_{1g}/E_{2g}^1$ ratio increases under 660 nm excitation (resonant with A excitons) and decreases under 457 nm excitation (resonant with C excitons). This aligns with theoretical predictions where A excitons (derived from Mo $d_{z^2}$ orbitals) couple strongly to out-of-plane $A_{1g}$ phonons, while C excitons (mixed orbital character) couple to in-plane $E_{2g}^1$ phonons.
- Defect-Induced Modulation: In PLD-grown monolayers, the expected enhancement of the $A_{1g}$ mode under 660 nm excitation was suppressed, resulting in a decreased $A_{1g}/E_{2g}^1$ ratio. The authors attribute this to the higher defect density in PLD films, which quenches A-exciton photoluminescence more strongly than B or C excitons, thereby reducing the resonant enhancement of the $A_{1g}$ mode.

Significance and Claims
The paper claims to establish room-temperature PLD as a viable strategy for growing two-dimensional MoS2 on inert, electronics-compatible SiO2 substrates with fine controllability over layer number down to the monolayer. The work provides new insights into the interplay between excitonic resonances and growth-induced disorder. Specifically, it demonstrates that while PLD-grown MoS2 shares intrinsic vibrational properties with exfoliated samples, the high defect density inherent to the growth process significantly modulates exciton-phonon coupling. This modulation manifests as a suppression of resonant Raman enhancement for specific modes, highlighting the critical role of structural disorder in the optoelectronic response of 2D TMDs. The findings suggest that while PLD offers scalability and substrate versatility, the resulting defectivity must be accounted for when interpreting optical properties and designing devices.

More like this