Ching-Hwa Kiang

SIZE EFFECTS IN CARBON NANOTUBES

The intershell spacing of multi-walled carbon nanotubes was determined by analyzing the high resolution transmission electron microscopy images of these nanotubes. For the nanotubes that were studied, the intershell spacing ˆ d002 is found to range from 0.34 to 0.39 nm, increasing with decreasing tube diameter. A model based on the results from real space image analysis is used to explain the variation in intershell spacings obtained from reciprocal space periodicity analysis. The increase in intershell spacing with decreased nanotube diameter is attributed to the high curvature, resulting in an increased repulsive force, associated with the decreased diameter of the nanotube shells. [S0031-9007(98)06983-X]

Carbon nanotubes with single-layer walls

Macroscopic quantities of single-layer carbon nanotubes have recently been synthesized by co-condensing atomic carbon and iron group or lanthanide metal vapors in an inert gas atmosphere. The nanotubes consist solely of carbon, sp 2 -bonded as in graphene strips rolled to form closed cylinders. The structure of the nanotubes has been studied using high-resolution transmission electron microscopy. Iron group catalysts, such as Co, Fe, and Ni, produce single-layer nanotubes with diameters typically between 1 and 2 nm and lengths on the order of micrometers. Groups of shorter nanotubes with similar diameters can grow radially from the surfaces of lanthanide carbide nanoparticles that condense from the gas phase. If the elements S, Bi, or Pb (which by themselves do not catalyze nanotube production) are used together with Co, the yield of nanotubes is greatly increased and tubules with diameters as large as 6 nm are produced. Single-layer nanotubes are anticipated to have novel mechanical and electrical properties, including very high tensile strength and one-dimensional conductivity. Theoretical calculations indicate that the properties of single-layer tubes will depend sensitively on their detailed structure. Other novel structures, including metallic crystallites encapsulated in graphitic polyhedra, are produced under the conditions that lead to nanotube growth.

Stacking nature of graphene layers in carbon nanotubes and nanofibres

Abstract The structure of multilayer carbon nanotubes is studied using digital image analysis to interpret high resolution TEM lattice images containing 002 and 100 fringes, in comparision with very thin vapour-grown carbon fibres with nanometer-sized diameter (nanofibres). The results show that the stacking of graphene shells in a multilayer nanotube glide with respect to one another, which is in contrast to the stacking fidelity of three-dimensional graphite. The diffraction patterns derived from the fast Fourier transform of the lattice images yield angles of 0 °–17 ° for the 100 lattice planes relative to the ideal 100 direction. The median inter-shell spacing d 002 between carbon nanotubes is also characterized, by using the 100 spacing as an internal standard, and d 002 range from 3.4 to 3.6A. The inter-shell spacing d 002 decreases with increasing carbon nanotube diameter, which could be due to a size effect. The so-called vapour-grown carbon fibres (VGCFs), obtained by pyrolytic decomposition of hydrocarbon, are grown through spontaneous deposition of carbon layers on primarily formed nanotubes. In order to clarify the size effect for layer stacking, very thin VGCFs with diameter ~500A, named nanofibers as well as submicron diameter VGCFs, are compared with carbon nanotubes, and are discussed in relation with the curvature of graphene layers. Some gently pulverized VGCFs with nm as well as μm size diameters possess an exposed carbon nanotube at the core of the broken portion, suggesting a difference in the stacking structure of the graphene layers between the central core nanotube and the outer pyrolytic sections.

Experimental free energy surface reconstruction from single-molecule force spectroscopy using Jarzynski's equality.

We used the atomic force microscope to manipulate and unfold individual molecules of the titin I27 domain and reconstructed its free energy surface using Jarzynskis equality. The free energy surface for both stretching and unfolding was reconstructed using an exact formula that relates the nonequilibrium work fluctuations to the molecular free energy. In addition, the unfolding free energy barrier, i.e., the activation energy, was directly obtained from experimental data for the first time. This Letter demonstrates that Jarzynskis equality can be used to analyze nonequilibrium single-molecule experiments, and to obtain the free energy surfaces for molecular systems, including interactions for which only nonequilibrium work can be measured.

Catalytic effects of heavy metals on the growth of carbon nanotubes and nanoparticles

The effects of bismuth, lead, and tungsten on the cobalt-catalyzed synthesis of single-layer carbon nanotubes and nanoparticles have been studied. When co-vaporized with cobalt in an electric arc, bismuth and lead increase the yield of single-layer nanotubes and broaden the range of tube diameters, compared to the case where only cobalt is used. Tungsten is found to reduce the nanotube yield and does not change the tube diameter distribution. Both tungsten and bismuth increase the graphitization of the carbon encapsulating the cobalt particles found in the soot, while lead does not. None of these three heavy metals catalyzes the formation of single-layer nanotubes without a transition metal present.

Phase transition of DNA-linked gold nanoparticles

Melting and hybridization of DNA-capped gold nanoparticle networks are investigated with optical absorption spectroscopy. Single-stranded, 12-base DNA-capped gold nanoparticles are linked with complementary, single-stranded, 24-base linker DNA to form particle networks. Compared to free DNA, a sharp melting transition is seen in these networked DNA-nanoparticle systems. The sharpness is explained by percolation transition phenomena.

Defects Can Increase the Melting Temperature of DNA-Nanoparticle Assemblies

DNA-gold nanoparticle assemblies have shown promise as an alternative technology to DNA microarrays for DNA detection and RNA profiling. Understanding the effect of DNA sequences on the melting temperature of the system is central to developing reliable detection technology. We studied the effects of DNA base-pairing defects, such as mismatches and deletions, on the melting temperature of DNA-nanoparticle assemblies. We found that, contrary to the general assumption that defects lower the melting temperature of DNA, some defects increase the melting temperature of DNA-linked nanoparticle assemblies. The effects of mismatches and deletions were found to depend on the specific base pair, the sequence, and the location of the defects. Our results demonstrate that the surface-bound DNA exhibit hybridization behavior different from that of free DNA. Such findings indicate that a detailed understanding of DNA-nanoparticle assembly phase behavior is required for quantitative interpretation of DNA-nanoparticle aggregation.

Quantifying DNA melting transitions using single-molecule force spectroscopy

We stretched a DNA molecule using atomic force microscope and quantified the mechanical properties associated with B and S forms of double-stranded DNA (dsDNA), molten DNA, and single-stranded DNA (ssDNA). We also fit overdamped diffusion models to the AFM time series and used these models to extract additional kinetic information about the system. Our analysis provides additional evidence supporting the view that S-DNA is a stable intermediate encountered during dsDNA melting by mechanical force. In addition, we demonstrated that the estimated diffusion models can detect dynamical signatures of conformational degrees of freedom not directly observed in experiments.

Melting transition of directly linked gold nanoparticle DNA assembly

DNA melting and hybridization is a fundamental biological process as well as a crucial step in many modern biotechnology applications. DNA confined on surfaces exhibits a behavior different from that in free solutions. The system of DNA-capped gold nanoparticles exhibits unique phase transitions and represents a new class of complex fluids. Depending on the sequence of the DNA, particles can be linked to each other through direct complementary DNA sequences or via a ‘linker’ DNA, whose sequence is complementary to the sequence attached to the gold nanoparticles. We observed different melting transitions for these two distinct systems.

Carbon rings and cages in the growth of single-walled carbon nanotubes

We present a growth model for single-walled carbon nanotubes (SWNTs) based on experimental observation and energetic considerations. Carbon rings, observed experimentally and suggested to be fullerene precursors, are the templates for SWNTs nucleation. SWNTs grow by addition of C2 radicals to catalytically induced reactive sites. This model is consistent with known experimental results and explains the characteristic diameter distribution of SWNTs as resulting from the gas-phase distribution of carbon ring isomers.