Huanhuan Xie

Superlubricity in centimetres-long double-walled carbon nanotubes under ambient conditions

Friction and wear are two main causes of mechanical energy dissipation and component failure, especially in micro/nanomechanical systems with large surface-to-volume ratios. In the past decade there has been an increasing level of research interest regarding superlubricity, a phenomenon, also called structural superlubricity, in which friction almost vanishes between two incommensurate solid surfaces. However, all experimental structural superlubricity has been obtained on the microscale or nanoscale, and predominantly under high vacuum. Here, we show that superlubricity can be realized in centimetres-long double-walled carbon nanotubes (DWCNTs) under ambient conditions. Centimetres-long inner shells can be pulled out continuously from such nanotubes, with an intershell friction lower than 1 nN that is independent of nanotube length. The shear strength of the DWCNTs is only several pascals, four orders of magnitude lower than the lowest reported value in CNTs and graphite. The perfect structure of the ultralong DWCNTs used in our experiments is essential for macroscale superlubricity.

Growth of Half-Meter Long Carbon Nanotubes Based on Schulz–Flory Distribution

The Schulz-Flory distribution is a mathematical function that describes the relative ratios of polymers of different length after a polymerization process, based on their relative probabilities of occurrence. Carbon nanotubes (CNTs) are big carbon molecules which have a very high length-to-diameter ratio, somewhat similar to polymer molecules. Large amounts of ultralong CNTs have not been obtained although they are highly desired. Here, we report that the Schulz-Flory distribution can be applied to describe the relative ratios of CNTs of different lengths produced with a floating chemical vapor deposition process, based on catalyst activity/deactivation probability. With the optimized processing parameters, we successfully synthesized 550-mm-long CNTs, for which the catalyst deactivation probability of a single growth step was ultralow. Our finding bridges the Schulz-Flory distribution and the synthesis of one-dimensional nanomaterials for the first time, and sheds new light on the rational design of process toward controlled production of nanotubes/nanowires.

State of the Art of Single‐Walled Carbon Nanotube Synthesis on Surfaces

Single-walled carbon nanotubes (SWNTs) directly synthesized on surfaces are promising building blocks for nanoelectronics. The structures and the arrangement of the SWNTs on surfaces determine the quality and density of the fabricated nanoelectronics, implying the importance of structure controlled growth of SWNTs on surfaces. This review summarizes the recent research status in controlling the orientation, length, density, diameter, metallicity, and chirality of SWNTs directly synthesized on surfaces by chemical vapor deposition, together with a session presenting the characterization method of the chirality of SWNTs. Finally, the remaining major challenges are discussed and future research directions are proposed.

Optical visualization of individual ultralong carbon nanotubes by chemical vapour deposition of titanium dioxide nanoparticles

Direct visualization and manipulation of individual carbon nanotubes in ambient conditions is of great significance for their characterizations and applications. However, the observation of individual carbon nanotubes usually requires electron microscopes under high vacuum. Optical microscopes are much more convenient to be used, yet their resolution is low. Here we realize the visualization and manipulation of individual ultralong carbon nanotubes under optical microscopes by deposition of TiO2 nanoparticles on them. The strong scattering of TiO2 nanoparticles to visible light renders them visible by optical microscopes. Micro-Raman-spectroscopy measurement of individual carbon nanotubes is greatly facilitated by their optical visualization. With the assistance of TiO2 nanoparticles, individual carbon nanotubes can be easily manipulated under an optical microscope at macroscopic scale and in ambient conditions. Based on our approach, various manipulation of ultralong carbon nanotubes, including cutting, transfer, fabrication of structures/devices and pulling out inner shells of multiwalled carbon nanotubes, are demonstrated.

Interwall Friction and Sliding Behavior of Centimeters Long Double-Walled Carbon Nanotubes

Here, we studied the interwall friction and sliding behaviors of double-walled carbon nanotubes (DWCNTs). The interwall friction shows a linear dependence on the pullout velocity of the inner wall. The axial curvature in DWCNTs causes the significant increase of the interwall friction. The axial curvature also affects the sliding behavior of the inner wall. Compared with the axial curvature, the opening ends of DWCNTs play tiny roles in their interwall friction.

Carbon nanotube light sensors with linear dynamic range of over 120 dB

We show that a carbon nanotube (CNT) diode fabricated by asymmetric contacts shows a linear photocurrent in response to illumination for over six decades or dynamic range of 120 dB; in particular, it shows no sign of degradation under illumination intensity of up to 100 kW/cm2. This CNT diode also exhibits a continued response for incident wavelength from 1165 nm to 2100 nm, promising potentials applications in robust and wide bandwidth light sensing.

Acoustic-assisted assembly of an individual monochromatic ultralong carbon nanotube for high on-current transistors

Consistent-chirality carbon nanotube tangles with high on-currents in transistors. Great effort has been applied to scientific research on the controllable synthesis of carbon nanotubes (CNTs) with high semiconducting selectivity or high areal density toward the macroscale applications of high-performance carbon-based electronics. However, the key issue of compatibility between these two requirements for CNTs remains a challenge, blocking the expected performance boost of CNT devices. We report an in situ acoustic-assisted assembly of high-density monochromatic CNT tangles (m-CNT-Ts), consisting of one self-entangled CNT with a length of up to 100 mm and consistent chirality. On the basis of a minimum consumed energy model with a Strouhal number of approximately 0.3, the scale could be controlled within the range of 1 × 104 to 3 × 104 μm2 or even a larger range. Transistors fabricated with one m-CNT-T showed an on/off ratio of 103 to 106 with 4-mA on-state current, which is also the highest on-state current recorded so far for single CNT–based transistors. This acoustic-assisted assembly of chiral-consistent m-CNT-Ts will provide new opportunities for the fabrication of high-performance electronics based on perfect CNTs with high purity and high density.

Carbon nanotube bundles with tensile strength over 80 GPa

Carbon nanotubes (CNTs) are one of the strongest known materials. When assembled into fibres, however, their strength becomes impaired by defects, impurities, random orientations and discontinuous lengths. Fabricating CNT fibres with strength reaching that of a single CNT has been an enduring challenge. Here, we demonstrate the fabrication of CNT bundles (CNTBs) that are centimetres long with tensile strength over 80 GPa using ultralong defect-free CNTs. The tensile strength of CNTBs is controlled by the Daniels effect owing to the non-uniformity of the initial strains in the components. We propose a synchronous tightening and relaxing strategy to release these non-uniform initial strains. The fabricated CNTBs, consisting of a large number of components with parallel alignment, defect-free structures, continuous lengths and uniform initial strains, exhibit a tensile strength of 80 GPa (corresponding to an engineering tensile strength of 43 GPa), which is far higher than that of any other strong fibre.Ultralong defect-free and well-aligned carbon nanotube bundles exhibit the highest tensile strength among known strong fibres.

Nanoscale color sensors made on semiconducting multi-wall carbon nanotubes

Sub-micron color sensors are developed, using carbon nanotubes (CNTs). The color sensor consists of an array of two photodiodes with different spectral responses, fabricated using controlled electric peeling-off and doping-free techniques on a single semiconducting double-wall CNT. The CNT photodiodes exhibit intrinsic broad spectral responses from 640 to 2,100 nm, large linear dynamic ranges of over 60 dB, and sub-micron pixel size. This method explores the unique properties of multi-wall CNTs, and may be readily used for large-scale fabrication of high performance color sensor arrays, when arrays of parallel multi-wall CNTs become available.

Controlled synthesis and property of horizontally aligned carbon nanotubes

Horizontally aligned carbon nanotubes (HACNTs) refers to the horizontally aligned CNT arrays grown and parallel on flat substrates, with relatively large intertube distances. Due to their perfect structures, excellent properties and lengths up to millimeters, even centimeters, HACNTs show great potential as building blocks for transparent displays, nano electronics, quantum lines, field emission transistors, superstrong tethers, and aeronautics and aerospace materials, etc. Great development was obtained on the study of ultralong CNTs during the past decade. In this review, we summarize the recent progress on the growth mechanism, structure-controlled synthesis, optical visualization, controllable manipulation, mechanical, electrical and thermal properties, as well as the superlubricity between the shells of HACNTs. First we review the growth mechanism of ultralong CNTs, pointing out the key for obtaining CNTs with perfect structures, extraordinary properties and macroscale lengths. We then discuss the controlled synthesis of ultralong CNTs with various chirality, diameters, lengths, and morphologies, etc. After that, we discuss the optical visualization, controllable manipulation, structure and property characterization, and superlubricity of HACNTs. Finally, an outlook is presented to shed light on the future study of ultralong CNTs.