In an article recently published in the journal Nature Materials, researchers proposed a synthesis approach where thin bismuth crystals with excellent electronic transport and quantum oscillations were grown in an atomically flat van der Waals (vdW) material-defined nanoscale mold.
Study: Synthesis of Thin Bismuth Crystals with Improved Quantum Oscillations. Image Credit: Declan Hillman/Shutterstock.com
Background
Confining materials to two-dimensional (2D) forms alters the electron behavior and enables new device creation. However, producing thin, uniform crystals from most materials is challenging. Although the study of ultrathin vdW materials isolated using mechanical exfoliation has significantly advanced the understanding of 2D electronic physics, this approach cannot be applied to other materials.
In some instances, the 2D growth of non-vdW materials has been achieved using deposition techniques such as molecular beam epitaxy. However, this method often leads to undesirable substrate interactions or uneven surfaces. An alternative method for 2D synthesis involves growing crystals within the confined spaces between a vdW material's layers.
In this confined growth approach, a mold primarily defines the crystal's geometry. However, any surface roughness on the mold can imprint onto the crystal, potentially degrading its electronic properties. Recent studies have shown that vdW materials can form ultra-thin, nanoscale-thick, atomically smooth channels. These channels, which are challenging to achieve with other techniques, minimize the risk of imperfections.
The Proposed Approach
In this study, researchers demonstrated the ultra-flat growth of bismuth confined between layers of hexagonal boron nitride (hBN). Bismuth is critical in the development of quantum electronic physics due to its small effective mass, capability to grow ultra-pure bulk crystals, and low carrier density.
Bismuth boundaries have recently garnered significant attention for their spin-momentum locking in one-dimensional (1D) helical edge modes and 2D Rashba surface states. These boundary modes have been explored using ARPES and STM, revealing various phenomena.
However, transport studies have been limited due to disorder within thin crystals, where confinement disrupts bulk conduction. The vdW mold technique used in this study enables the production of thin bismuth crystals, facilitating intrinsic transport studies of their surface states.
The crystal growth process within a vdW mold began with standard vdW transfer techniques to encapsulate a micron-sized bismuth flake within thin hBN layers. The bismuth-hBN stack was then compressed between two substrates and underwent sequential heating and cooling to melt and resolidify the bismuth.
During melting, the liquid bismuth spread rapidly between the hBN layers, and the applied pressure reduced its thickness. This compressed form of bismuth was retained upon releasing the pressure and cooling into a solid phase, resulting in a thin bismuth crystal embedded within hBN.
The melt-growth process reduced the flake thickness from 250-500 nm to an ultra-thin 5-30 nm. Hall-bar-shaped devices were fabricated from these crystals and measured in a variable temperature cryostat to electrically characterize the vdW-molded bismuth.
Study Findings
Ultra-flat bismuth crystals with less than 10 nm thickness were grown successfully by compressing and heating bismuth in an hBN mold. All three measurements, including transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and Raman spectroscopy, were consistent with the bismuth's typical rhombohedral structure.
Metallic behavior was observed at low temperatures with a positive slope of resistance versus temperature in all vdW-molded bismuth devices. Specifically, the slope decreased with increasing temperature, unlike the bulk bismuth's linear-T dependence, which indicated the confinement effects in the vdW-molded bismuth.
The vdW-molded devices showed robust metallic dependencies, which suggested that surface-derived states could dominate room-temperature conduction. This also led to larger residual resistance ratios (RRR) values, such as 5.4 and 12 for the flat 13 nm and 8 nm devices, respectively.
The vdW-molded bismuth demonstrated exceptional electronic transport, enabling the observation of Shubnikov–de Haas quantum oscillations originating from the (111) surface state Landau levels. Specifically, the vdW-molded bismuth displayed significantly improved transport properties compared to molecular beam epitaxy-grown thin films.
In the vdW-molded devices, quantum oscillations in the magnetoresistance emerged at high fields. The oscillations were extensive, occurring in 11 devices covering 8 to 107 nm thicknesses with 3 to 4 T onset fields. Additionally, the dominant oscillations were 13-20x larger compared to bulk bismuth and consistent with ARPES and STM studies.
By measuring the gate-dependent magnetoresistance, researchers observed multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. The quantum oscillations strongly dispersed with back-gate voltage, leading to clear Landau fan features.
Overall, the findings of this study demonstrated the feasibility of using the proposed vdW mold growth technique to synthesize flat and ultra-thin bismuth crystals within a nanoscale vdW mold. Beyond bismuth, this approach offers an affordable way to synthesize other ultra-thin crystals and integrate them directly into a vdW heterostructure.
Journal Reference
Chen, L. et al (2024). Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials. Nature Materials, 1-6. https://doi.org/10.1038/s41563-024-01894-0, https://www.nature.com/articles/s41563-024-01894-0
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.