Steven Tran

All Publications

• 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