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Optical contrast-based determination of number of layers for two-dimensional van der Waals magnet Fe$_3$GeTe$_2$

Neesha Yadav, Sandeep, Pintu Das

TL;DR

FGT is studied as a 2D van der Waals magnet where layer-dependent properties are crucial and AFM-based thickness determination is impractical inside glove boxes. The authors introduce a fast, non-destructive optical-contrast method that correlates image contrast with thickness, calibrated against AFM measurements, and validate it with Raman spectroscopy. They find a linear relationship between optical contrast and layer number up to $25$ layers, while Raman peak positions remain thickness-insensitive though intensities of $A_{1g}$-type modes decline with fewer layers; two intrinsic modes at $129$ cm$^{-1}$ and $190$ cm$^{-1}$ are identified. This approach enables in-situ, glove-box-compatible layer counting, preserving sample quality for magnetic and transport studies of pristine FGT.

Abstract

Recent advances in revealing intrinsic magnetism in two-dimensional (2D) materials have highlighted their potential for future spintronic applications, driven by their novel physical properties, promising for future spintronic devices. In order to explore layer dependent magnetic behavior, in general, mechanically exfoliated flakes from high-quality single crystals are used. It is crucial to determine the number of layers of these materials accurately. In the absence of an efficient and quick method, researchers often rely on atomic force microscopy (AFM) imaging to identify their number of layers. In this work, we report an optical contrast study as a quick and cost-effective technique to determine the number of layers of Fe$_3$GeTe$_2$ (FGT). Here, we observed a linear relationship between the optical contrast (derived from optical microscopic images) observed for mechanically exfoliated FGT nano-flakes and their thickness, as measured by the AFM imaging method. This technique requires no additional equipment; it relies solely on a conventional optical microscope. Additionally, our results reveal a thickness-dependent evolution of the intensity; in contrast, the Raman frequency demonstrates no significant dependence on layer thickness. Also, our studies reveal two additional Raman modes of FGT, at the frequency of 129\,cm$^-1$ \& 190\,cm$^-1$. Both modes show the intensity dependence on the thickness of FGT, same as out-of-plane (A$_{1g}$) Raman modes.

Optical contrast-based determination of number of layers for two-dimensional van der Waals magnet Fe$_3$GeTe$_2$

TL;DR

FGT is studied as a 2D van der Waals magnet where layer-dependent properties are crucial and AFM-based thickness determination is impractical inside glove boxes. The authors introduce a fast, non-destructive optical-contrast method that correlates image contrast with thickness, calibrated against AFM measurements, and validate it with Raman spectroscopy. They find a linear relationship between optical contrast and layer number up to layers, while Raman peak positions remain thickness-insensitive though intensities of -type modes decline with fewer layers; two intrinsic modes at cm and cm are identified. This approach enables in-situ, glove-box-compatible layer counting, preserving sample quality for magnetic and transport studies of pristine FGT.

Abstract

Recent advances in revealing intrinsic magnetism in two-dimensional (2D) materials have highlighted their potential for future spintronic applications, driven by their novel physical properties, promising for future spintronic devices. In order to explore layer dependent magnetic behavior, in general, mechanically exfoliated flakes from high-quality single crystals are used. It is crucial to determine the number of layers of these materials accurately. In the absence of an efficient and quick method, researchers often rely on atomic force microscopy (AFM) imaging to identify their number of layers. In this work, we report an optical contrast study as a quick and cost-effective technique to determine the number of layers of FeGeTe (FGT). Here, we observed a linear relationship between the optical contrast (derived from optical microscopic images) observed for mechanically exfoliated FGT nano-flakes and their thickness, as measured by the AFM imaging method. This technique requires no additional equipment; it relies solely on a conventional optical microscope. Additionally, our results reveal a thickness-dependent evolution of the intensity; in contrast, the Raman frequency demonstrates no significant dependence on layer thickness. Also, our studies reveal two additional Raman modes of FGT, at the frequency of 129\,cm \& 190\,cm. Both modes show the intensity dependence on the thickness of FGT, same as out-of-plane (A) Raman modes.

Paper Structure

This paper contains 7 sections, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methods
  3. Fabrication of 2D nanoflakes
  4. Characterization of the number of layers of fabricated 2D nanoflakes
  5. Results & Discussion
  6. Conclusion
  7. Acknowledgments

Figures (4)

  • Figure 1: (a) - (f) AFM images of mechanically exfoliated nanoflakes with the thickness analysis. (g) - (l) The corresponding optical microscopic images of the flakes.
  • Figure 2: Optical contrast and Raman spectroscopy analysis of the FGT samples. (a) Thickness-dependent optical contrast of FGT exfoliated on Si/SiO$_2$ substrate. (b) The Raman spectra of several FGT samples with varying layer numbers from 2 to 25 monolayers. (c) Raman spectra of FGT, encapsulated with hBN and PMMA.
  • Figure 3: Raman spectroscopy analysis of various FGT samples of different thicknesses. (a) The peak positions of all three peaks, $E_{2g}^1$, $A_{1g}^1$ & $A_{1g}^*$ with the number of layers of FGT and optical contrast. The Raman intensity of $A_{1g}^1$ mode & $A_{1g}^*$ mode of FGT with the number of layers (b) and optical contrast (c).
  • Figure 4: (a) & (c) The Raman spectra of the FGT and encapsulated FGT nanoflakes at higher frequencies, respectively. (b) & (d) The zoomed spectra between in the range of 150-325 cm$^{-1}$ as shown in (a) & (c).