Contact

Additional Info

  • Mail Code: 4060

All Publications

  • Deterministic fabrication of graphene hexagonal boron nitride moire superlattices. Proceedings of the National Academy of Sciences of the United States of America Kamat, R. V., Sharpe, A. L., Pendharkar, M., Hu, J., Tran, S. J., Zaborski, G. J., Hocking, M., Finney, J., Watanabe, K., Taniguchi, T., Kastner, M. A., Mannix, A. J., Heinz, T., Goldhaber-Gordon, D. 2024; 121 (40): e2410993121

    Abstract

    The electronic properties of moire heterostructures depend sensitively on the relative orientation between layers of the stack. For example, near-magic-angle twisted bilayer graphene (TBG) commonly shows superconductivity, yet a TBG sample with one of the graphene layers rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided experimental observation of orbital ferromagnetism. To create samples with aligned graphene/hBN, researchers often align edges of exfoliated flakes that appear straight in optical micrographs. However, graphene or hBN can cleave along either zig-zag or armchair lattice directions, introducing a [Formula: see text] ambiguity in the relative orientation of two flakes. By characterizing the crystal lattice orientation of exfoliated flakes prior to stacking using Raman and second-harmonic generation for graphene and hBN, respectively, we unambiguously align monolayer graphene to hBN at a near-[Formula: see text], not [Formula: see text], relative twist angle. We confirm this alignment by torsional force microscopy of the graphene/hBN moire on an open-face stack, and then by cryogenic transport measurements, after full encapsulation with a second, nonaligned hBN layer. This work demonstrates a key step toward systematically exploring the effects of the relative twist angle between dissimilar materials within moire heterostructures.

    View details for DOI 10.1073/pnas.2410993121

    View details for PubMedID 39331413

  • Quantitative determination of twist angle and strain in Van der Waals moiré superlattices APPLIED PHYSICS LETTERS Tran, S. J., Uslu, J., Pendharkar, M., Finney, J., Sharpe, A. L., Hocking, M., Bittner, N. J., Watanabe, K., Taniguchi, T., Kastner, M. A., Mannix, A. J., Goldhaber-Gordon, D. 2024; 125 (11)

    View details for DOI 10.1063/5.0223777

    View details for Web of Science ID 001313187100002

  • Thermal relaxation of strain and twist in ferroelectric hexagonal boron nitride moiré interfaces JOURNAL OF APPLIED PHYSICS Hocking, M., Henzinger, C. E., Tran, S. J., Pendharkar, M., Bittner, N. J., Watanabe, K., Taniguchi, T., Goldhaber-Gordon, D., Mannix, A. J. 2024; 136 (2)

    View details for DOI 10.1063/5.0210112

    View details for Web of Science ID 001272420800003

  • Torsional force microscopy of van der Waals moirés and atomic lattices. Proceedings of the National Academy of Sciences of the United States of America Pendharkar, M., Tran, S. J., Zaborski, G., Finney, J., Sharpe, A. L., Kamat, R. V., Kalantre, S. S., Hocking, M., Bittner, N. J., Watanabe, K., Taniguchi, T., Pittenger, B., Newcomb, C. J., Kastner, M. A., Mannix, A. J., Goldhaber-Gordon, D. 2024; 121 (10): e2314083121

    Abstract

    In a stack of atomically thin van der Waals layers, introducing interlayer twist creates a moiré superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult; hence, determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moiré, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that torsional force microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of van der Waals stacks on multiple length scales: the moirés formed between bi-layers of graphene and between graphene and hexagonal boron nitride (hBN) and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an Atomic Force Microscope (AFM) cantilever is monitored as it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the sample surface. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moiré superlattices and crystallographic orientation of van der Waals flakes to support predictable moiré heterostructure fabrication.

    View details for DOI 10.1073/pnas.2314083121

    View details for PubMedID 38427599

  • Nanoscale Electronic Transparency of Wafer-Scale Hexagonal Boron Nitride. Nano letters Zerger, C. Z., Rodenbach, L. K., Chen, Y., Safvati, B., Brubaker, M. Z., Tran, S., Chen, T., Li, M., Li, L., Goldhaber-Gordon, D., Manoharan, H. C. 2022

    Abstract

    Monolayer hexagonal boron nitride (hBN) has attracted interest as an ultrathin tunnel barrier or environmental protection layer. Recently, wafer-scale hBN growth on Cu(111) was developed for semiconductor chip applications. For basic research and technology, understanding how hBN perturbs underlying electronically active layers is critical. Encouragingly, hBN/Cu(111) has been shown to preserve the Cu(111) surface state (SS), but it was unknown how tunneling into this SS through hBN varies spatially. Here, we demonstrate that the Cu(111) SS under wafer-scale hBN is homogeneous in energy and spectral weight over nanometer length scales and across atomic terraces. In contrast, a new spectral feature─not seen on bare Cu(111)─varies with atomic registry and shares the spatial periodicity of the hBN/Cu(111) moire. This work demonstrates that, for some 2D electron systems, an hBN overlayer can act as a protective yet remarkably transparent window on fragile low-energy electronic structure below.

    View details for DOI 10.1021/acs.nanolett.1c04274

    View details for PubMedID 35536749