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On-chip multi-degree-of-freedom control of two-dimensional materials

Nature volume 632, pages 1038–1044 (2024)Cite this article

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Abstract

Two-dimensional materials (2DM) and their heterostructures offer tunable electrical and optical properties, primarily modifiable through electrostatic gating and twisting. Although electrostatic gating is a well-established method for manipulating 2DM, achieving real-time control over interfacial properties remains challenging in exploring 2DM physics and advanced quantum device technology1,2,3,4,5,6. Current methods, often reliant on scanning microscopes, are limited in their scope of application, lacking the accessibility and scalability of electrostatic gating at the device level. Here we introduce an on-chip platform for 2DM with in situ adjustable interfacial properties, using a microelectromechanical system (MEMS). This platform comprises compact and cost-effective devices with the ability of precise voltage-controlled manipulation of 2DM, including approaching, twisting and pressurizing actions. We demonstrate this technology by creating synthetic topological singularities, such as merons, in the nonlinear optical susceptibility of twisted hexagonal boron nitride (h-BN)7,8,9,10. A key application of this technology is the development of integrated light sources with real-time and wide-range tunable polarization. Furthermore, we predict a quantum analogue that can generate entangled photon pairs with adjustable entanglement properties. Our work extends the abilities of existing technologies in manipulating low-dimensional quantum materials and paves the way for new hybrid two- and three-dimensional devices, with promising implications in condensed-matter physics, quantum optics and related fields.

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Fig. 1: MEGA2D, an on-chip MEMS platform for twisting 2D materials.
Fig. 2: Nonlinear optical probing and Raman spectroscopy of twisted h-BN tuned with MEGA2D.
Fig. 3: Experimental realization of synthetic merons (half-skyrmions) in the nonlinear susceptibility of twisted h-BN.
Fig. 4: Tunable classical and quantum light source with MEGA2D.

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Data availability

All relevant data presented in this paper can be found in the data repository at https://doi.org/10.7910/DVN/UTIA0K.

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Acknowledgements

We thank M. Zhang, B. Lou and G. Zhong for their helpful discussions. E.M. acknowledges support from the National Science Foundation (NSF) under contract ECCS-2234513 and the DARPA under contract URFAO: GR510802. A.Y. acknowledges support from the Army Research Office (grant no. W911NF-21-2-0147) and the Gordon and Betty Moore Foundation (grant no. GBMF 9468). S.F. acknowledges the support of a MURI grant from the US Air Force Office of Scientific Research (grant no. FA9550-21-1-0312). The sample fabrication was performed at the Harvard University Center for Nanoscale Systems, which is a member of the National Nanotechnology Coordinated Infrastructure Network and is supported by the NSF (award no. 1541959). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan (grant no. JPMXP0112101001), JSPS KAKENHI (grant no. JP20H00354) and the CREST (JPMJCR15F3), JST. P.J.-H. acknowledges support from the NSF (DMR-1809802), the STC Center for Integrated Quantum Materials (NSF grant no. DMR-1231319), the EPiQS Initiative of the Gordon and Betty Moore Foundation (grant GBMF9463), the Fundación Ramon Areces and the CIFAR Quantum Materials programme.

Author information

Authors and Affiliations

  1. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

    Haoning Tang, Yiting Wang, Xueqi Ni & Eric Mazur

  2. National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe & Takashi Taniguchi

  3. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

    Pablo Jarillo-Herrero

  4. Department of Applied Physics and Ginzton Laboratory, Stanford University, Stanford, CA, USA

    Shanhui Fan

  5. Department of Physics, Faculty of Art and Sciences, Harvard University, Cambridge, MA, USA

    Amir Yacoby & Yuan Cao

  6. Society of Fellows, Harvard University, Cambridge, MA, USA

    Yuan Cao

  7. Department of Electrical Engineering and Computer Science, University of California, Berkeley, Berkeley, CA, USA

    Yuan Cao

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Contributions

Y.C. and A.Y. conceived the experimental ideas. Y.C. designed the MEGA2D platform. H.T., Y.W. and Y.C. performed the sample fabrication. H.T., E.M. and Y.C. performed the optical measurements. K.W. and T.T. provided the h-BN crystals. X.N., S.F. and Y.C. developed the theory. S.F. and P.J.-H. provided insightful discussions. H.T. and Y.C. wrote the paper with input from all authors.

Corresponding authors

Correspondence to Eric Mazur, Amir Yacoby or Yuan Cao.

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Extended data figures and tables

Extended Data Fig. 1 Illustration of the principle of the vertical actuator and rotary three-phase stepper.

(a) Illustration of the vertical actuator at rest position. (b) Vertical actuator driven by a voltage Vz. (c) Illustration of the rotary actuator with a three-phase electrode. (d) Micro-stepping voltage waveform for driving the rotary actuator. The vertical axis is normalised voltage on each phase.

Extended Data Fig. 2 MEMS Fabrication process.

(a) Thermal silicon dioxide is grown on a commercial SOI wafer. (b) Back-side oxide patterning. (c) Tip KOH etching and oxide removal. (d) TSV etching and coating. (e) Back-side and front-side RIE etching and vapour HF release. (f) Bonding to bottom substrate. Blue: Silicon. Gray: Silicon oxide. Orange: Poly-silicon. Yellow: SU-8.

Extended Data Fig. 3 Example of an h-BN flake before and after transferring to MEMS.

(a) the as-exfoliated flake. (b) the transferred and annealed flake.

Extended Data Fig. 4 Detailed measurement setups.

(a) Parallel SHG (χ(2)) measurement setup. The quarter wave plate (QWP) before the objective is optional and only used for SHG CD measurements. (b) Quarter-wave plate polarimetry setup for arbitrary polarisation generation.

Extended Data Fig. 5 Correction of Z/R crosstalk in measured SHG data.

(a) Raw data of SHG CD corresponding to Fig. 2h. (b) Data corrected for a quadratic drift with respect to θ.

Extended Data Fig. 6 Additional SHG CD simulation and experimental data.

(a) Simulated SHG CD corresponding to Fig. 2h. (b-c) Measured SHG CD data at the anti-meron (Fig. 3h,i) and meron (Fig. 3j,k), respectively. The sign of SHG CD at the meron/anti-meron core is used to determine their polarity p, which is +1 in both cases. Combined with the vorticity (v), which is determined from α measurements shown in Fig. 3h–k, their meron (Q = + 1/2) or anti-meron (Q = −1/2) nature could be pinpointed.

Extended Data Fig. 7 Interfacial quality of MEGA2D devices.

(a) AFM line scan of a clean h-BN flake on a Si pyramid. The average roughness is 0.11 nm over about 4 μm. (b) Large-scale AFM line scan of a Si pyramid, showing the parallelism between the Si pyramid (pink) and Si pillar surface (green). While the Si pillar has an increased roughness, the Si pyramid has the intrinsic roughness of a commercial Si wafer. (c-d) The optical image of (c) a normal MEGA2D device and (d) a defective MEGA2D device, taken through the fused silica substrate. The colours are saturated to show the colour variation. Defective devices typically show colour bands that indicate a tilt angle, whereas working devices do not show such bands. (e-f) Simulated colour (saturated) of an air gap (540 nm) between fused silica and silicon, (e) without tilt and (f) with a tilt of 0.01°. Colour variation can be seen on the upper-right corner of the Si pillar (circle). (g-h) Our MEMS actuators can be driven differentially to tilt the Si pillar/pyramid in either (g) y direction or (h) x direction, by an amount that is on the order of 0.008°. (i) Illustration of patterned h-BN for strain measurements. (j) Extracted strain profile along horizontal direction for a 45 nm thick h-BN flake on square pyramid (denoted by red dashed lines). The right panel shows the raw AFM image (amplitude channel). (k) Same measurements for a rounded pyramid and a 30 nm thick h-BN flake.

Supplementary information

Supplementary Discussion

This file includes a detailed derivation of nonlinear pseudospin, transfer-matrix method and SPDC. It includes Supplementary Figs. 1 and 2 for illustration purposes.

Supplementary Video 1

In situ rotation of two h-BN flakes. This video shows the in situ rotation of two pieces of h-BN on MEGA2D. The scale of the square pyramid at the centre is 4 × 4 μm.

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Tang, H., Wang, Y., Ni, X. et al. On-chip multi-degree-of-freedom control of two-dimensional materials. Nature 632, 1038–1044 (2024). https://doi.org/10.1038/s41586-024-07826-x

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