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Growth of Large Crystals of Janus Phase RhSeCl Using Self-Selecting Vapour Growth

Anastasiia Lukovkina, Maria A. Herz, Xiaohanwen Lin, Volodymyr Multian, Alberto Morpurgo, Enrico Giannini, Fabian O. von Rohr

TL;DR

This work addresses the challenge of growing large, high-purity RhSeCl Janus crystals by systematically comparing chemical vapour transport (CVT) with self-selecting vapour growth (SSVG) using two precursor chemistries (RhCl3 and SeCl4). A two-step SSVG approach, especially with SeCl4, enables reproducible crystals up to 6 mm lateral size and yields phase-pure RhSeCl, while CVT remains limited in crystal size. Structural validation via PXRD and SCXRD confirms the hexagonal P6_3mc Janus structure with minimal disorder, and exfoliation demonstrates flakes down to few-layer and monolayer, with SeCl4-derived crystals showing no intergrowth impurities. The optimized synthesis provides a reliable platform for fundamental studies and device applications of 2D Janus materials, highlighting the importance of reagent choice, growth temperature, and sealed vapour chemistry.

Abstract

In recent years, interest in 2D Janus materials has grown exponentially, particularly with regard to their applications in spintronics and optoelectronic devices. The defining feature of Janus materials is the ordered arrangement of different layer terminations - creating chemically distinct surfaces and an inherent out-of-plane polarity. Among the few known Janus materials, RhSeCl is particularly intriguing as a rare example of an intrinsic Janus compound. Owing to its exceptional chemical stability, RhSeCl offers a promising platform for exploring the physics related to the Janus-structure. However, synthesising large, high-quality crystals of this compound remains a significant challenge. Here, we report a novel synthetic pathway for growing crystals up to 6 mm in lateral size via a two-step self-selecting vapour growth reaction. We further present a comprehensive comparison of newly developed synthesis routes with all previously reported methods for RhSeCl. During these investigations, we identified a previously unreported impurity that forms in specific growth pathways and demonstrate how it can be avoided to obtain phase-pure few- and monolayer flakes. We showcase the reproducibility of the process to obtain high-quality, large single-crystals and flakes.

Growth of Large Crystals of Janus Phase RhSeCl Using Self-Selecting Vapour Growth

TL;DR

This work addresses the challenge of growing large, high-purity RhSeCl Janus crystals by systematically comparing chemical vapour transport (CVT) with self-selecting vapour growth (SSVG) using two precursor chemistries (RhCl3 and SeCl4). A two-step SSVG approach, especially with SeCl4, enables reproducible crystals up to 6 mm lateral size and yields phase-pure RhSeCl, while CVT remains limited in crystal size. Structural validation via PXRD and SCXRD confirms the hexagonal P6_3mc Janus structure with minimal disorder, and exfoliation demonstrates flakes down to few-layer and monolayer, with SeCl4-derived crystals showing no intergrowth impurities. The optimized synthesis provides a reliable platform for fundamental studies and device applications of 2D Janus materials, highlighting the importance of reagent choice, growth temperature, and sealed vapour chemistry.

Abstract

In recent years, interest in 2D Janus materials has grown exponentially, particularly with regard to their applications in spintronics and optoelectronic devices. The defining feature of Janus materials is the ordered arrangement of different layer terminations - creating chemically distinct surfaces and an inherent out-of-plane polarity. Among the few known Janus materials, RhSeCl is particularly intriguing as a rare example of an intrinsic Janus compound. Owing to its exceptional chemical stability, RhSeCl offers a promising platform for exploring the physics related to the Janus-structure. However, synthesising large, high-quality crystals of this compound remains a significant challenge. Here, we report a novel synthetic pathway for growing crystals up to 6 mm in lateral size via a two-step self-selecting vapour growth reaction. We further present a comprehensive comparison of newly developed synthesis routes with all previously reported methods for RhSeCl. During these investigations, we identified a previously unreported impurity that forms in specific growth pathways and demonstrate how it can be avoided to obtain phase-pure few- and monolayer flakes. We showcase the reproducibility of the process to obtain high-quality, large single-crystals and flakes.
Paper Structure (18 sections, 4 equations, 3 figures)

This paper contains 18 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Experimental Methods
  3. Synthesis
  4. Chemical Vapour Transport Method
  5. Self-Selecting Vapour Growth Method
  6. Scanning Electron Microscopy and Elemental Analysis
  7. Powder X-ray Diffraction
  8. Single Crystal X-ray Diffraction
  9. Exfoliation and Raman spectroscopy
  10. Results and Discussion
  11. Chemical Vapour Transport Crystal Growth
  12. Self-Selecting Vapour Growth
  13. Crystal Growth Optimization
  14. Self-Selecting Vapour Growth for Crystals with Needle-like Morphology
  15. Comparison of the Crystal Growth Methods
  16. ...and 3 more sections

Figures (3)

  • Figure 1: Synthesis map for RhSeCl crystal growth. Investigated methods are shown in the upper panel. Temperature profiles for each technique are presented in the middle. Pictures of the RhSeCl crystals taken with an optical microscope on millimetre paper at the bottom correspond to the synthesis method above. The turquoise and orange backgrounds represent different synthesis paths: via Rh, Se, RhCl3 or Rh, Se, SeCl4 reagent combinations, respectively. For the SSVG in a tubular furnace, the shown crystals are obtained by two-step SSVG process starting from RhCl3 and SeCl4, with the first step being SSVG in a ‘flipped’ box furnace. The green background and the dashed frame highlight the best synthesis methods and conditions, which allow the growth of the largest RhSeCl crystals.
  • Figure 2: Grown crystals of RhSeCl and crystal structure. (a) Comparison of the RhSeCl crystals grown by chemical vapour transport (CVT) and self-selecting vapour growth (SSVG). The crystals grown with RhCl3 are highlighted via a turquoise background, while the crystals grown with SeCl4 are highlighted via an orange background. (b) PXRD patterns of washed crystals from the different types of reagent combinations and synthesis types. The crystals grown from RhCl3 are in turquoise, the crystals grown with SeCl4 are in orange, and the RhSeCl reference (ICSD 206914) is in black. (c) The crystal structure from a refinement of a RhSeCl crystal grown via the two-step SSVG synthesis with RhCl3 as a reagent. The structure here is viewed along the crystallographic b-axis to highlight the ordered Janus phase with the Cl atoms forming one layer on one side of the central Rh and the Se atoms forming a layer on the other side.
  • Figure 3: Elemental analysis and Raman spectra of bulk and exfoliated RhSeCl crystals grown via two-step SSVG starting from two different reagent combinations. The upper panels show results for RhSeCl crystals grown from RhCl3, the bottom ones from SeCl4. (a,b) Micrograph and elemental analysis of exfoliated RhSeCl crystals. The dominant phase observed is RhCl3 for crystals grown from RhCl3, while for crystals grown from SeCl4 it is RhSeCl. (c,d) Comparison of Raman spectra of bulk and exfoliated RhSeCl crystals. (e,f) Polarised optical microscope pictures of RhSeCl flakes on which Raman measurements were performed.