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The study reveals that a two-dimensional (2D) material as substrate for heterogeneous integration acts as a compliant substrate. 

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov–de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth’s Rashba surface states and topological boundary modes1,2,3. Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure. 

Abstract The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe₅samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe₅and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe₅as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures. 

Robust atomic-to-meso-scale chirality is now observed in the one-dimensional form of tellurium. This enables a large and counter-intuitive circular-polarization dependent second harmonic generation response above 0.2 which is not present in two-dimensional tellurium. Orientation variations in 1D tellurium nanowires obtained by four-dimensional scanning transmission electron microscopy (4D-STEM) and their correlation with unconventional non-linear optical properties by second harmonic generation circular dichroism (SHG-CD) uncovers an unexpected circular-polarization dependent SHG response from 1D nanowire bundles – an order-of-magnitude higher than in single-crystal two-dimensional tellurium structures – suggesting the atomic- and meso-scale crystalline structure of the 1D material possesses an inherent chirality not present in its 2D form; and which is strong enough to manifest even in the aggregate non-linear optical (NLO) properties of aggregates. 

A device for measuring a plurality of material properties is designed to include accurate sensors configured to consecutively obtain thermal conductivity, electrical conductivity, and Seebeck coefficient of a single sample while maintaining a vacuum or inert gas environment. Four major design factors are identified as sample-heat spreader mismatch, radiation losses, parasitic losses, and sample surface temperature variance. The design is analyzed using finite element methods for high temperature ranges up to 1000°C as well as ultra-high temperatures up to 2500°C. A temperature uncertainty of 0.46% was estimated for a sample with cold and hot sides at 905.1 and 908.5°C, respectively. The uncertainty at 1000°C was calculated to be 0.7% for a ?T of 5°C between the hot and cold sides. The thermal conductivity uncertainty was calculated to be -8.6% at ~900°C for a case with radiative gains, and +8.2% at ~1000°C for a case with radiative losses, indicating the sensitivity of the measurement to the temperature of the thermal guard in relation to the heat spreader and sample temperature. Lower limits of -17 and -13% error in thermal conductivity measurements were estimated at the ultra-high temperature of ~2500°C for a single-stage and double-stage radiation shield, respectively. It is noted that this design is not limited to electro-thermal characterization and will enable measurement of ionic conductivity and surface temperatures of energy materials under realistic operating conditions in extreme temperature environments.