Electrostatically Driven Nanoballoon Actuator

Collapse of carbon nanotubes due to local high-pressure from van der Waals encapsulation

Van der Waals (vdW) assembly of low-dimensional materials has proven the capability of creating structures with on-demand properties. It is predicted that the vdW encapsulation can induce a local high-pressure of a few GPa, which will strongly modify the structure and property of trapped materials. Here, we report on the structural collapse of carbon nanotubes (CNTs) induced by the vdW encapsulation. By simply covering CNTs with a hexagonal boron nitride flake, most of the CNTs (≈77%) convert from a tubular structure to a collapsed flat structure. Regardless of their original diameters, all the collapsed CNTs exhibit a uniform height of ≈0.7 nm, which is roughly the thickness of bilayer graphene. Such structural collapse is further confirmed by Raman spectroscopy, which shows a prominent broadening and blue shift in the Raman G-peak. The vdW encapsulation-induced collapse of CNTs is fully captured by molecular dynamics simulations of the local vdW pressure. Further near-field optical characterization reveals a metal-semiconductor transition in accompany with the CNT structural collapse. Our study provides not only a convenient approach to generate local high-pressure for fundamental research, but also a collapsed-CNT semiconductor for nanoelectronic applications.

Bidirectional regulation of configuration of the carbon nanotube containing a water droplet

Carbon nanotube complexes are known for their miraculous mechanical and electronic properties that are crucial for nano-electromechanical systems (MEMS). In this study, through molecular dynamics simulations we found for the first time that the electric field and temperature can be used to co-regulate a reversible change of cross-sectional configuration of single-wall carbon nanotubes (SWCNTs). We showed that the electric field can help induce the collapse of an SWCNT when it contains a water droplet, while the increase of temperature can quickly recover its configuration. This controllable bistability of SWCNTs is promising for the design of nanodevices such as electromechanical switches in NEMS.

Advance in Close-Edged Graphene Nanoribbon: Property Investigation and Structure Fabrication

The absence of dangling bonds in close-edged graphene nanoribbons (CEGNRs) confers upon them a series of fascinating properties, especially when compared with cylindrical carbon nanotubes and open-edged GNRs. Here, the configuration of CEGNRs is described, followed by the structure-related properties, including mechanical, thermal, electrical, optical, and magnetic properties. Based on the unique structures and extraordinary properties, their potential applications in a variety of fields, such as field-effect transistors, energy suppliers, nanoactuators, and fibers, are discussed. Remarkably, the strategies applied for generating CEGNRs, mainly from the collapse of carbon nanotubes and graphene tubes, are depicted in detail. Finally, the prospects in the research area of CEGNRs are proposed.

An intrinsic energy conversion mechanism via telescopic extension and retraction of concentric carbon nanotubes

The conversion of other forms of energy into mechanical work through the geometrical extension and retraction of nanomaterials has a wide variety of potential applications, including for mimicking biomotors. Here, using molecular dynamics simulations, we demonstrate that there exists an intrinsic energy conversion mechanism between thermal energy and mechanical work in the telescopic motions of double-walled carbon nanotubes (DWCNTs). A DWCNT can inherently convert heat into mechanical work in its telescopic extension process, while convert mechanical energy into heat in its telescopic retraction process. These two processes are nearly thermodynamically reversible. The underlying mechanism for this energy conversion is that the configurational entropy changes with the telescopic overlapping length of concentric individual tubes. We also find that the entropy effect enlarges with the decreasing intertube space of DWCNTs. As a result, the spontaneous telescopic motion of a condensed DWCNT can be switched to extrusion by increasing the system temperature above a critical value. These findings are important for fundamentally understanding the mechanical behavior of concentric nanotubes, and may have general implications in the application of DWCNTs as linear motors in nanodevices.

Closed-edged bilayer phosphorene nanoribbons producing from collapsing armchair phosphorene nanotubes

A new phosphorous allotrope, closed-edged bilayer phosphorene nanoribbon, is proposed via radially deforming armchair phosphorene nanotubes. Using molecular dynamics simulations, the transformation pathway from round PNTs falls into two types of collapsed structures: arc-like and sigmoidal bilayer nanoribbons, dependent on the number of phosphorene unit cells. The fabricated nanoribbions are energetically more stable than their parent nanotubes. It is also found via ab initio calculations that the band structure along tube axis substantially changes with the structural transformation. The direct-to-indirect transition of band gap is highlighted when collapsing into the arc-like nanoribbons but not the sigmoidal ones. Furthermore, the band gaps of these two types of nanoribbons show significant size-dependence of the nanoribbon width, indicative of wider tunability of their electrical properties.

Electrochemically controlled winding and unwinding of substrate-supported carbon nanoscrolls

Carbon nanoscrolls (CNSs) formed spontaneously on the basal plane of highly ordered pyrolytic graphite (HOPG) show winding and unwinding movements when potential steps from 0 V to -0.5 V, -0.6 V and -0.9 V are applied on HOPG immersed in an aqueous electrolyte solution (0.1 M H₂SO₄). Reversible changes in CNS radial dimensions exceeding 10 nm in the axial direction and 50 nm in the lateral direction are ascribed to variations in the surface tension and electric double-layer structure under applied potentials. Radial motion is observed exclusively on scrolled tube-shaped nanostructures, while other parts of the HOPG surface including planar areas, simple bended and lifted step edges, and kinks remain intact. The mechanism explaining the observed phenomenon is proposed and its significance for prospective applications in electrochemically controlled nanomechanical actuators is outlined.

Reversible and Continuously Tunable Control of Charge of Close Surfaces

Surfaces of almost all types of materials are often charged easily by contact electrification or deposition of ions; hence, surface charge is ubiquitous and has a vast range of influences in our lives and in industry. Since the 19th century, scientists have been measuring the charge of multiple materials collectively. The common expectation is that the total charge of multiple materials is equal to the sum of the charges of the individual materials. This study describes a previously unreported phenomenon in which the total charge of two insulating surfaces decreases when the surfaces are brought close to each other. The charge varies continuously and reversibly depending on the distance of separation between the surfaces. Experimental results derived from analyzing the movement of charge suggest that the changes are due to a rapid exchange of charge between the surfaces and their surrounding air. This change can be used to control the surface charge of the materials flexibly and reversibly.