Abstract
Atomic layers of hexagonal boron nitride (h-BN) crystal are excellent candidates for structural materials as enabling ultrathin, two-dimensional (2D) nanoelectromechanical systems (NEMS) due to the outstanding mechanical properties and very wide bandgap (5.9 eV) of h-BN. In this work, we report the experimental demonstration of h-BN 2D nanomechanical resonators vibrating at high and very high frequencies (from ~5 to ~70 MHz), and investigations of the elastic properties of h-BN by measuring the multimode resonant behavior of these devices. First, we demonstrate a dry-transferred doubly clamped h-BN membrane with ~6.7 nm thickness, the thinnest h-BN resonator known to date. In addition, we fabricate circular drumhead h-BN resonators with thicknesses ranging from ~9 to 292 nm, from which we measure up to eight resonance modes in the range of ~18 to 35 MHz. Combining measurements and modeling of the rich multimode resonances, we resolve h-BN’s elastic behavior, including the transition from membrane to disk regime, with built-in tension ranging from 0.02 to 2 N m−1. The Young’s modulus of h-BN is determined to be EY≈392 GPa from the measured resonances. The ultrasensitive measurements further reveal subtle structural characteristics and mechanical properties of the suspended h-BN diaphragms, including anisotropic built-in tension and bulging, thus suggesting guidelines on how these effects can be exploited for engineering multimode resonant functions in 2D NEMS transducers.
Highlights
Nanoelectromechanical systems (NEMS) vibrating at their resonance modes and made from atomic layer crystalline materials have attracted increasing research interest owing to their promises for exceptionally high responsivities and sensitivities to external stimuli, enabled by their ultralow weight and ultrahigh surface-area-to-volume ratio[1,2,3]
Following semi-metallic graphene, the early hallmark of two-dimensional (2D) crystals, a variety of 2D materials have been studied as structural materials for 2D NEMS resonators, including superconducting NbSe2 (Ref. 4), semiconducting MoS2 (Refs. 3,5–7), and black phosphorus[8,9], which opens a wide spectrum of emerging applications, such as sensing[10,11] and signal processing with ultralow power and broad tunability[12,13]
The hexagonal boron nitride (h-BN) material has a very wide bandgap (5.9 eV)[14] and excellent chemical and thermal stability beyond that offered by graphene[15,16], making h-BN attractive for wide bandgap 2D NEMS resonators
Summary
Nanoelectromechanical systems (NEMS) vibrating at their resonance modes and made from atomic layer crystalline materials have attracted increasing research interest owing to their promises for exceptionally high responsivities and sensitivities to external stimuli, enabled by their ultralow weight (mass) and ultrahigh surface-area-to-volume ratio[1,2,3]. We first fabrication and detection of ultrathin h-BN 2D NEMS obtain h-BN layers a few nanometers thick from high-quality bulk resonators, which are the smallest ultrawide bandgap crystalline h-BN by exfoliating it onto a polydimethylsiloxane (PDMS) stamp. After exfoliation and careful optical identification, we transfer the h-BN nanosheet, with controlled alignment, to a pre-defined microtrench with the aid of a micromanipulator to achieve a ranging from ~ 6.7 to ~ 292 nm) exfoliated from their layered bulk, suspended structure. This technique enables the fabrication of and utilize them to fabricate suspended h-BN devices that pristine suspended h-BN resonators free from wet chemistry function as new nanomechanical resonators. Monolayer h-BN on PDMS has showed extremely low optical
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