Qingrui Cao

All Publications

• Anomalously enhanced diffusivity of moiré excitons via manipulating the interplay with correlated electrons NATURE COMMUNICATIONS Yan, L., Ma, L., Meng, Y., Xiao, C., Chen, B., Wu, Q., Cui, J., Cao, Q., Banerjee, R., Taniguchi, T., Watanabe, K., Tongay, S., Hunt, B., Cui, Y., Yao, W., Shi, S. 2025; 16 (1): 10569

  Abstract

  Semiconducting transition metal dichalcogenide (TMDC) moiré superlattices provide an unprecedented platform for manipulating excitons. The in-situ control of moiré excitons could enable novel excitonic devices but remains challenging. Meanwhile, as dipolar composite bosons, interlayer excitons in the type-II aligned TMDC moiré superlattices exhibit strong interactions with fermionic charge carriers. Here, we demonstrate active manipulation of exciton diffusivity by tuning their interplay with correlated carriers in moiré potentials. When electrons form Mott insulators, the interlayer exciton energy is blueshifted due to strong electron-exciton repulsion, leading to the enhancement of diffusivity by as much as two orders of magnitude. In contrast, exciton diffusivity is suppressed at fractional fillings, where carriers form generalized Wigner crystals. In between fractional fillings, electrons populate all moiré traps, resulting in enhanced diffusivity with increasing carrier density, owing to the effectively reduced moiré potential confinement experienced by excitons. Our study inspires further engineering and controlling exotic excitonic states in TMDC moiré superlattices for fascinating quantum phenomena and novel excitonic devices.

  View details for DOI 10.1038/s41467-025-65602-5

  View details for Web of Science ID 001626753100013

  View details for PubMedID 41298445

  View details for PubMedCentralID PMC12657994

• Nanoscale Ferroelectric Programming of van der Waals Heterostructures NANO LETTERS Yang, D., Cao, Q., Akyuz, E., Hayden, J., Nordlander, J., Mercer, I., Yu, M., Ramachandran, R., Irvin, P., Maria, J., Hunt, B. M., Levy, J. 2024

  Abstract

  We demonstrate an approach to creating nanoscale potentials in van der Waals layers integrated with a buried programmable ferroelectric layer. Using ultra-low-voltage electron beam lithography (ULV-EBL), we can program the ferroelectric polarization in Al1-xBxN (AlBN) thin films, generating structures with sizes as small as 35 nm. We demonstrate the ferroelectric field effect with a graphene/vdW stack on AlBN by creating a p-n junction. This resist-free, high-resolution, contactless patterning method offers a new pathway to integrate ferroelectric films with a wide range of two-dimensional layers including transition-metal dichalcogenides (TMD), enabling arbitrary programming and top-down creation of multifunctional devices.

  View details for DOI 10.1021/acs.nanolett.4c03574

  View details for Web of Science ID 001378007100001

  View details for PubMedID 39670523

• Tunneling Spectroscopy of Two-Dimensional Materials Based on Via Contacts NANO LETTERS Cao, Q., Telford, E. J., Benyamini, A., Kennedy, I., Zangiabadi, A., Watanabe, K., Taniguchi, T., Dean, C. R., Hunt, B. M. 2022; 22 (22): 8941-8948

  Abstract

  We introduce a novel planar tunneling architecture for van der Waals heterostructures based on via contacts, namely, metallic contacts embedded into through-holes in hexagonal boron nitride (hBN). We use the via-based tunneling method to study the single-particle density of states of two different two-dimensional (2D) materials, NbSe2 and graphene. In NbSe2 devices, we characterize the barrier strength and interface disorder for barrier thicknesses of 0, 1, and 2 layers of hBN and study the dependence on the tunnel-contact area down to (44 ± 14)2 nm2. For 0-layer hBN devices, we demonstrate a crossover from diffusive to point contacts in the small-contact-area limit. In graphene, we show that reducing the tunnel barrier thickness and area can suppress effects due to phonon-assisted tunneling and defects in the hBN barrier. This via-based architecture overcomes limitations of other planar tunneling designs and produces high-quality, ultraclean tunneling structures from a variety of 2D materials.

  View details for DOI 10.1021/acs.nanolett.2c03081

  View details for Web of Science ID 000885500400001

  View details for PubMedID 36356229

• Electroluminescence of atoms in a graphene nanogap SCIENCE ADVANCES Kim, H., Kim, Y., Wu, T., Cao, Q., Herman, I. P., Hone, J., Guo, J., Shepard, K. L. 2022; 8 (3): eabj1742

  Abstract

  Here, we report light emission from single atoms bridging a graphene nanogap that emit bright visible light based on fluorescence of ionized atoms. Oxygen atoms in the gap shows a peak emission wavelength of 569 nm with a full width at half maximum (FWHM) of 208 nm. The energy states produced by these ionized oxygen atoms bridging carbon atoms in the gap also produce a large negative differential resistance (NDR) in the transport across the gap with the highest peak-to-valley current ratio (PVR = 45) and highest peak current density (~90 kA/cm2) ever reported in a solid-state tunneling device. While tunneling transport has been previously observed in graphene nanogaps, the bridging of ionized oxygen observed here shows a low excess current, leading to the observed PVR. On the basis of the highly reproducible light emission and NDR from these structures, we demonstrate a 65,536-pixel light-emitting nanogap array.

  View details for DOI 10.1126/sciadv.abj1742

  View details for Web of Science ID 000745886100009

  View details for PubMedID 35061537

  View details for PubMedCentralID PMC8782453

• Charged Bosons Made of Fermions in Bilayer Structures with Strong Metallic Screening NANO LETTERS Sun, Z., Beaumariage, J., Wan, Q., Alnatah, H., Hougland, N., Chisholm, J., Cao, Q., Watanabe, K., Taniguchi, T., Hunt, B., Bondarev, I., Snoke, D. 2021; 21 (18): 7669-7675

  View details for DOI 10.1021/acs.nanolett.1c02422

  View details for Web of Science ID 000700883900033

• Direct measurement of ferroelectric polarization in a tunable semimetal NATURE COMMUNICATIONS de la Barrera, S. C., Cao, Q., Gao, Y., Gao, Y., Bheemarasetty, V. S., Yan, J., Mandrus, D. G., Zhu, W., Xiao, D., Hunt, B. M. 2021; 12 (1): 5298

  Abstract

  Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

  View details for DOI 10.1038/s41467-021-25587-3

  View details for Web of Science ID 000694666900039

  View details for PubMedID 34489428

  View details for PubMedCentralID PMC8421369

• Observation of the Interlayer Exciton Gases in WSe₂-p:WSe₂ Heterostructures ACS PHOTONICS Sun, Z., Beaumariage, J., Cao, Q., Watanabe, K., Taniguchi, T., Hunt, B., Snoke, D. 2020; 7 (7): 1622-1627

  View details for DOI 10.1021/acsphotonics.0c00476

  View details for Web of Science ID 000551497000006

• Near ultraviolet light emission in hexagonal boron nitride based van der Waals heterostructures Chae, S., Seo, D., Cao, Q., Hua, X., Shih, E., Taniguchi, T., Watanabe, K., Kwon, J., Lee, G., Dean, C. R., Schiminovich, D., Herman, I. P., Choi, H., Kymissis, I., Kim, Y., Hone, J., IEEE IEEE. 2019

  View details for Web of Science ID 000482226302353