One-dimensional nanomaterials including nanotubes are building blocks for constructing various co... more One-dimensional nanomaterials including nanotubes are building blocks for constructing various complex nanodevices. Boron nitride (BN) nanotubes with structure similar to carbon nanotube are known to be among the strongest insulators in the world. In this work, deformation of an individual BN nanotube is performed inside a high-resolution transmission electron microscope (TEM) using a piezo-driven atomic force microscope (AFM)-TEM holder. The mechanical properties of individual BN nanotubes are obtained from the experimentally recorded force-displacement curves.
Bulletin of the American Physical Society, Mar 13, 2006
CNTs) are known to be excellent electron field emitters. However, the fundamental factors that co... more CNTs) are known to be excellent electron field emitters. However, the fundamental factors that contribute to the emission stability have not been well studied. Here, we found that stability of emission current from CNTs is related to their graphitic orders. We have tested various types of CNTs grown by thermal chemical vapor deposition (CVD) and plasma enhanced CVD (PECVD). Our samples were grown in a circular area of 0.385cm 2 on low resistance Si substrates. Field emission measurements were conducted in a planar diode configuration, with a pair of electrodes separated with a gap of 1000 ± 10 µm. The vacuum level during the measurement is ∼2.0 X 10 -7 mbar. We found that the emission currents from PECVD grown CNTs degraded by as much as 70% within a period of 20 hours. In contrast, random CNTs grown by thermal CVD exhibit stable emission current for at least 20 hours. These CNTs also have relatively lower threshold electric field of field emission. Since all samples are tested in a same condition, the detected results are thus related to the structural order of the CNTs. Transmission electron microscopy and Raman spectroscopy confirmed that field emission stability is depends on the graphitic structures of these CNTs.
Bulletin of the American Physical Society, Mar 12, 2008
Low temperature growth of boron nitride nanotubes 1 CHEE HUEI LEE, MING XIE, JIESHENG WANG, YOKE ... more Low temperature growth of boron nitride nanotubes 1 CHEE HUEI LEE, MING XIE, JIESHENG WANG, YOKE KHIN YAP -Boron nitride nanotubes (BNNTs) are promising nanostuctures that will complement the applications of carbon nanotubes in various emerging areas. However, the synthesis of BNNTs is still challenging and required high growth temperatures (1500˚C to 3000˚C). Here we will discuss about two approaches for low temperature growth of BN-NTs. First, we have reported on the growth of pure BNNTs at 600˚C by a plasma-enhanced pulsed-laser deposition (PE-PLD) technique [1]. These BNNTs were grown vertically-aligned on substrates. Latest result on the effect of catalyst, growth temperatures, ambient gas pressures, substrate bias voltages and the growth mechanism will be discussed in the meeting. Secondly, effective growth of BNNTs is recently achieved by conventional thermal chemical vapor deposition (CVD). Our new CVD approach leads to effective growth of long and clean BNNTs at 1200 ˚C. SEM, TEM, EELS, Raman, FTIR, and UV absorption data indicate that these BN-NTs are having high structural ordered and a energy band gap > 5.6 eV. [1]. J. Wang et. al, Nano Lett. 5, 2528 (2005). 1 Y.K.Yap acknowledges support from the National Science Foundation CAREER awards (Award No. 0447555, Division of Materials Research).
Bulletin of the American Physical Society, Mar 13, 2006
nitride nanotubes (BNNTs) are well recognized as the candidate that will complement the uses of c... more nitride nanotubes (BNNTs) are well recognized as the candidate that will complement the uses of carbon nanotubes (CNTs) in nanotechnology. However, high growth temperatures (>1100 o C), low production yield, and impurities have prevented effective synthesis and applications of boron nitride nanotubes (BNNTs) in the past ten years. For the first time, we have succeeded on the growth of pure BNNTs on substrates [1, 2]. This has been realized based on our experiences of growing CNTs and boron nitride (BN) phases (cubic phase BN, hexagonal phase BN). According to our hypothetical model, energetic growth species play an important role on controlling the phases of BN solids. We have experimentally verified that BNNTs can be grown by energetic growth species by a plasma-enhanced pulsed laser deposition (PEPLD) technique. These BNNTs can be grown vertically aligned into arrays of regular patterns at 600 o C, and can be used for applications without purification. The growth mechanism of thee BNNTs will be discussed.
Ammonia ͑NH 3 ͒ gas was thought to be essential for the growth of vertically aligned multiwalled ... more Ammonia ͑NH 3 ͒ gas was thought to be essential for the growth of vertically aligned multiwalled carbon nanotubes ͑VA-MWCNTs͒ and led to the formation of bamboo-like structures. Here, we show that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixed gases with or without NH 3 gas. The growth of these VA-MWCNTs is guided by a growth model that combined the dissociative adsorption of acetylene molecules ͑C 2 H 2 ͒ and the successive vapor-liquid-solid growth mechanism. Results indicate that the key factor for growing these VA-MWCNTs is a balance between the decomposition rate of the C 2 H 2 molecules on the iron catalyst and the subsequent diffusion and segregation rates of carbon.
Real-time deformation of individual multiwalled boron nitride nanotubes ͑BNNTs͒ was investigated ... more Real-time deformation of individual multiwalled boron nitride nanotubes ͑BNNTs͒ was investigated using an atomic force microscopy ͑AFM͒ stage installed inside the chamber of a transmission electron microscopy ͑TEM͒ system. These in situ AFM-TEM experiments were conducted in following two deformation regimes: a small-angle ͑ϳ65°͒ and a large-angle ͑ϳ120°͒ cyclic bending process. BNNTs survived from the low-angle test and their modulus was determined as ϳ0.5 TPa. Fracture failure of individual BNNTs was discovered in the large-angle cyclic bending. The brittle failure mechanism was initiated from the outermost walls and propagated toward the tubular axis with discrete drops of applied forces.
The emergence of two-dimensional (2D) materials has led to tremendous interest in the study of gr... more The emergence of two-dimensional (2D) materials has led to tremendous interest in the study of graphene and a series of mono-and few-layered transition metal dichalcogenides (TMDCs). Among these TMDCs, the study of molybdenum disulfide (MoS2) has gained increasing attention due to its promising optical, electronic, and optoelectronic properties. Of particular interest is the indirect to direct band-gap transition from bulk and few-layered structures to mono-layered MoS2, respectively. In this review, the study of these properties is summarized. The use of Raman and Photoluminescence (PL) spectroscopy of MoS2 has become a reliable technique for differentiating the number of molecular layers in 2D MoS2.
The inherent problem of a zero-band gap in graphene has provided motivation to search for the nex... more The inherent problem of a zero-band gap in graphene has provided motivation to search for the next-generation electronic materials including transition metal dichalcogenides, such as MoS 2 . In this study, a triangular MoS 2 quantum dot (QD) is investigated to see the effects of passivation, additional layer, and the h-BN substrate on its geometry, energetics, and electronic properties. The results of density functional theory calculations show that the monolayer QD is metallic in nature, mainly due to the coordinatively unsaturated Mo atoms at the edges. This is reaffirmed by the passivation of the S edge atoms, which does not significantly modify its metallic character. Analysis of the chemical topology finds that the Mo-S bonds associated with the edge atoms are predominantly covalent despite the presence of metallic states. A bilayer QD is more stable than its monolayer counterpart, mainly due to stabilization of the dangling bonds of the edge atoms. The degree of the metallic character is also considerably reduced as demonstrated by the I-V characteristics of a bilayer QD. The binding strength of a monolayer QD to the h-BN substrate is predicted to be weak. The substrate-induced modifications in the electronic structure of the quantum dot are therefore not discernible. We find that the metallic character of the QD deposited on the insulating substrate can therefore be exploited to extend the functionality of MoS 2 -based nanostructures in catalysis and electronics applications at the nanoscale level.
Supercooled water is found to have a significantly enhanced freezing temperature during transient... more Supercooled water is found to have a significantly enhanced freezing temperature during transient electrowetting with electric fields of order 1 V/lm. High speed imaging reveals that the nucleation occurs randomly at the three-phase contact line (droplet perimeter) and can occur at multiple points during one freezing event. Possible nucleation mechanisms are explored by testing various substrate geometries and materials. Results demonstrate that electric field alone has no detectable effect on ice nucleation, but the moving boundary of the droplet on the substrate due to electrowetting is associated with the triggering of nucleation at a much higher temperature. V
Clusters of diamond-phase carbon, known as nanodiamonds, exhibit novel mechanical, optical and bi... more Clusters of diamond-phase carbon, known as nanodiamonds, exhibit novel mechanical, optical and biological properties that have elicited interest for a wide range of technological applications. Although diamond is predicted to be more stable than graphite at the nanoscale, extreme environments are typically used to produce nanodiamonds. Here we show that nanodiamonds can be stably formed in the gas phase at atmospheric pressure and neutral gas temperatures o100 °C by dissociation of ethanol vapour in a novel microplasma process. Addition of hydrogen gas to the process allows in flight purification by selective etching of the non-diamond carbon and stabilization of the nanodiamonds. The nanodiamond particles are predominantly between 2 and 5 nm in diameter, and exhibit cubic diamond, n-diamond and lonsdaleite crystal structures, similar to nanodiamonds recovered from meteoritic residues. These results may help explain the origin of nanodiamonds in the cosmos, and offer a simple and inexpensive route for the production of high-purity nanodiamonds.
One-dimensional nanomaterials including nanotubes are building blocks for constructing various co... more One-dimensional nanomaterials including nanotubes are building blocks for constructing various complex nanodevices. Boron nitride (BN) nanotubes with structure similar to carbon nanotube are known to be among the strongest insulators in the world. In this work, deformation of an individual BN nanotube is performed inside a high-resolution transmission electron microscope (TEM) using a piezo-driven atomic force microscope (AFM)-TEM holder. The mechanical properties of individual BN nanotubes are obtained from the experimentally recorded force-displacement curves.
Tellurium can form nanowires of helical atomic chains. Given their unique onedimensional van der ... more Tellurium can form nanowires of helical atomic chains. Given their unique onedimensional van der Waals structure, these nanowires are expected to show remarkably different physical and electronic properties than bulk tellurium. Here we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With the approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain and a carbon nanotube is weak, and that the inter-chain interaction becomes stronger as the number of chains increases. Compared with bare tellurium nanowires on SiO2, nanowires encapsulated in boron nitride nanotubes exhibit a dramatically enhanced current-carrying capacity, with a current density of 1.5×10 8 A cm -2 , which exceeds that of most semiconducting nanowires. We also use our tellurium nanowires encapsulated in boron nitride nanotubes to create field-effect transistors that have a diameter of only 2 nm.
We report here, an investigation on electrical and structural-microstructural properties of an in... more We report here, an investigation on electrical and structural-microstructural properties of an individual ZnO nanobelt via in situ transmission electron microscopy using an atomic force microscopy ͑AFM͒ system. The I-V characteristics of the ZnO nanobelt, just in contact with the AFM tip indicates the insulating behavior, however, it behaves like a semiconductor under applied stress. Analysis of the high resolution lattice images and the corresponding electron diffraction patterns shows that each ZnO nanobelt is a single crystalline, having wurtzite hexagonal structure ͑a = 0.324 nm, c = 0.520 66 nm͒ with a general growth direction of ͓1010͔.
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Papers by Yoke Khin Yap