-2-Black phosphorus (P) is a layered material in which individual atomic layers of P are stacked together by weak van der Waals forces (similar to bulk graphite) 1 . Inside a single layer, each P atom is covalently bonded with three adjacent P atoms to form a corrugated plane of honeycomb structure (Fig. 1a, note top view of each crystal plane is in honeycomb structure). The three bonds take up all three valence electrons of P (different than graphene and graphite). This makes monolayer black P ('phosphorene') a semiconductor with a direct bandgap of ~2eV. The bandgap is reduced in few-layer phosphorene, and becomes ~0.3eV for bulk black P. 2,3,4,5,6,7,8 The bandgap and its dependence on thickness has brought mono-and few-layer phosphorene to the family of 2D crystals, especially for enabling field-effect transistors (FETs) 9,10,11,12 and optoelectronic devices with potential applications in the infrared regime, 13,14 with prototypes recently demonstrated.In parallel to its potential for making novel electronic and optoelectronic devices, black P possesses attractive mechanical properties that are unavailable in other peer materials: it has very large strain limit (30%), and is much more stretchable (Young's modulus of E Y =44GPa for single layer) than other layered materials (e.g., E Y =1TPa for graphene), especially in the armchair direction (x axis in Fig. 1a). 15Such superior mechanical flexibility, 15,16,17 together with the exotic negative Poisson's ratio 18 arising from its corrugated atomic planes, offers unique opportunities for effectively inducing and controlling sizable strains, and thus the electronic, optoelectronic, and thermoelectric properties in this nanomaterial. 19,20,21,22,23 For example, with a 2D tension (in N/m, as in surface tension) of 0. and frequency-shift-based resonant infrared sensors. 28To date, however, the exploration and implementation of black P mechanical devices have not yet been reported; such efforts have been plagued by the relative chemical activeness of black P:8 , 9 it can be readily oxidized in air, and the multiple processing steps (many involving wet chemistry) required in fabricating mechanical devices from layered 2D materials (lithography, metallization, etching and suspension, etc.) make it particularly challenging to preserve the quality of black P crystal throughout the process. Here, we make the initial experimental study on exploring and exploiting the mechanical properties of black P to realize the first robust black P crystalline nanomechanical devices, by employing a set of specially-engineered processing and measurement techniques. We fabricate suspended black P NEMS resonators with electrical contacts using a facile dry transfer technique, minimizing sample exposure to the ambient and completely avoiding chemical processes. We characterize the material properties in vacuum, and implement nanomechanical measurements on the black P NEMS resonators using a number of experimental schemes, including Brownian motion-induced thermomechanical resonance, elec...

Black phosphorus (P) has emerged asWang Z.H., Jia H., Zheng X.-Q., Yang R., Ye G.J., Chen X.H., Feng P.X.-L., Nano Letters 16, 5394-5400 (2016) [Accepted Version] DOI: 10.1021/acs.nanolett.6b01598, Online Publication: August 9, 2016-2-Efficiently exploiting anisotropic properties in crystals plays important roles in many areas in science and technology, ranging from timing and signal processing using a rich variety of crystalline cut orientations in quartz to the modulation and conversion of light using anisotropic crystals. In state-of-the-art miniature devices and integrated systems, crystalline anisotropy enables a number of important dynamic characteristics in microelectromechanical systems (MEMS) such as gyroscopes, rotation rate sensors, and accelerometers. 1,2,3,4 Single crystal silicon (Si), the hallmark of semiconductors and the most commonly used crystal in MEMS, possesses clear mechanical anisotropy that has been extensively characterized and utilized (e.g., Young's moduli in the <110> and <100> directions are E Y<110> = 169 GPa and E Y<100> = 130 GPa). 5 , 6 As devices continue to be scaled down to nanoscale, anisotropy in mechanical properties may not always be readily preserved at device level due to lattice defects or surface effects, 7 , 8 , 9 and thus remain largely unexplored in emerging nanoelectromechanical systems (NEMS) built upon conventional crystals.The recent advent of atomic-layer crystals offer exciting opportunities for building twodimensional (2D) NEMS using single-crystal layered materials, in which many desired material properties are preserved, or even intensified, as the crystal thickness approaches genuinely atomic scale. One unique crystal is black phosphorus (P), not only a single-element directbandgap semiconductor with bandgap depending on the number of atomic layers (covering a wide range from visible light to IR) and with high carrier mobility, but also hitherto the bestknown atomic-layer crystal with strong in-plane anisotropy. The intrinsically anisotropic lattice structure (Figure 1a) of black P dictates a number of anisotropic material properties. In particular, it is theoretically predicted to exhibit in-plane mechanical anisotropy ( Figure 1a) 10,11,12,13 much stronger than that of Si, which shall lead to previously inaccessible dynamic responses in resonant NEMS 14 and new opportunities for studying carrier-lattice interaction in atomic layers. 15,16,17,18,19 To date, while extensive and rapidly growing efforts have been devoted to studying optical and electrical properties of black P and anisotropic effects in such devices, experiments on black P mechanical devices and mechanical anisotropic effects therein have been lacking. It is therefore of both fundamental and technological importance to systematically investigate the mechanical anisotropy in black P crystal.In bulk materials, such as crystalline Si, mechanical anisotropy is often characterized by measuring the sound velocity in different directions, 20,21,22,23 and fitting data to the Christoff...

We report the fabrication of large-scale arrays of suspended molybdenum disulfide (MoS2) atomic layers, as two-dimensional (2D) MoS2 nanomechanical resonators. We employ a water-assisted lift-off process to release chemical vapor deposited (CVD) MoS2 atomic layers from a donor substrate, followed by an all-dry transfer onto microtrench arrays. The resultant large arrays of suspended single- and few-layer MoS2 drumhead resonators (0.5-2 μm in diameter) offer fundamental resonances (f0) in the very high frequency (VHF) band (up to ∼120 MHz) and excellent figures-of-merit up to f0 × Q ≈ 3 × 10(10) Hz. A stretched circular diaphragm model allows us to estimate low pre-tension levels of typically ∼15 mN m(-1) in these devices. Compared to previous approaches, our transfer process features high yield and uniformity with minimal liquid and chemical exposure (only involving DI water), resulting in high-quality MoS2 crystals and exceptional device performance and homogeneity; and our process is readily applicable to other 2D materials.

The quest for realizing and manipulating ever smaller man-made movable structures and dynamical machines has spurred tremendous endeavors, led to important discoveries, and inspired researchers to venture to new grounds. Scientific feats and technological milestones of miniaturization of mechanical structures have been widely accomplished by advances in machining and sculpturing ever shrinking features out of bulk materials such as silicon. With the flourishing multidisciplinary field of low-dimensional nanomaterials, including onedimensional (1D) nanowires/nanotubes, and two-dimensional (2D) atomic layers such as graphene/phosphorene, growing interests and sustained efforts have been devoted to creating mechanical devices toward the ultimate limit of miniaturization-genuinely down to the molecular or even atomic scale. These ultrasmall movable structures, particularly nanomechanical resonators that exploit the vibratory motion in these 1D and 2D nano-toatomic-scale structures, offer exceptional device-level attributes, such as ultralow mass, ultrawide frequency tuning range, broad dynamic range, and ultralow power consumption, thus holding strong promises for both fundamental studies and engineering applications. In this Review, we offer a comprehensive overview and summary of this vibrant field, present the state-of-the-art devices and evaluate their specifications and performance, outline important achievements, and postulate future directions for studying these miniscule yet intriguing molecular-scale machines.