Si-Ping Han

Thermal Decomposition of Condensed-Phase Nitromethane from Molecular Dynamics from ReaxFF Reactive Dynamics

We studied the thermal decomposition and subsequent reaction of the energetic material nitromethane (CH(3)NO(2)) using molecular dynamics with ReaxFF, a first principles-based reactive force field. We characterize the chemistry of liquid and solid nitromethane at high temperatures (2000-3000 K) and density 1.97 g/cm(3) for times up to 200 ps. At T = 3000 K the first reaction in the decomposition of nitromethane is an intermolecular proton transfer leading to CH(3)NOOH and CH(2)NO(2). For lower temperatures (T = 2500 and 2000 K) the first reaction during decomposition is often an isomerization reaction involving the scission of the C-N bond the formation of a C-O bond to form methyl nitrate (CH(3)ONO). Also at very early times we observe intramolecular proton transfer events. The main product of these reactions is H(2)O which starts forming following those initiation steps. The appearance of H(2)O marks the beginning of the exothermic chemistry. Recent quantum-mechanics-based molecular dynamics simulations on the chemical reactions and time scales for decomposition of a crystalline sample heated to T = 3000 K for a few picoseconds are in excellent agreement with our results, providing an important, direct validation of ReaxFF.

DNA-Linker-Induced Surface Assembly of Ultra Dense Parallel Single Walled Carbon Nanotube Arrays

Ultrathin film preparations of single-walled carbon nanotube (SWNT) allow economical utilization of nanotube properties in electronics applications. Recent advances have enabled production of micrometer scale SWNT transistors and sensors but scaling these devices down to the nanoscale, and improving the coupling of SWNTs to other nanoscale components, may require techniques that can generate a greater degree of nanoscale geometric order than has thus far been achieved. Here, we introduce linker-induced surface assembly, a new technique that uses small structured DNA linkers to assemble solution dispersed nanotubes into parallel arrays on charged surfaces. Parts of our linkers act as spacers to precisely control the internanotube separation distance down to <3 nm and can serve as scaffolds to position components such as proteins between adjacent parallel nanotubes. The resulting arrays can then be stamped onto other substrates. Our results demonstrate a new paradigm for the self-assembly of anisotropic colloidal nanomaterials into ordered structures and provide a potentially simple, low cost, and scalable route for preparation of exquisitely structured parallel SWNT films with applications in high-performance nanoscale switches, sensors, and meta-materials.

Coupling of Raman Radial Breathing Modes in Double-Wall Carbon Nanotubes and Bundles of Nanotubes

Measurements of the radial breathing modes from Raman Spectroscopy have been most useful in characterizing the diameters of single-wall carbon nanotubes (SWNT), where there is a simple monotonic relationship between frequency and diameter. Similar correlations have also been used to predict sizes for double and multiple wall nanotubes and for bundles of SWNT. However this can lead to significant errors because the relationship between frequencies and diameter is much more complicated for DWNT. This is because of couplings between the vibrations of various walls. To provide guidance in such assignments we used the GraFF atomistic force field to predict the in-phase and counter-phase radial breathing modes (RBMs) of double wall carbon nanotubes (DWNTs) over a broad range of inner and outer diameters and chiralities. We then developed an analytical model to describe the RBMs of dispersed DWNTs. This enables the inner and outer shell diameters to be extracted from pairs of RBM peaks. We find that nanotubes bundles show significant dependent peak broadening and shifting compared to dispersed nanotubes. For bundles of SWNT and DWNT, the relationships are much more complicated.

DNA-Directed Self-Assembly of Highly Ordered and Dense Single-Walled Carbon Nanotube Arrays

Single-walled carbon nanotubes (SWNT or CNT) have unique and well-known high-performance material properties that can enable revolutionary increases in the performance of electronic devices and architectures. However, fabrication of large-scale SWNT-based ICs is an enormously challenging, unsolved problem, and self-assembly is likely needed for critical steps. Over the past several years, methods have been introduced to created ordered carbon nanotube structures using DNA guided self-assembly. In this chapter, we briefly review the challenges involved in using DNA to assemble SWNT nanostructures, and then give detailed methods to assemble dense, aligned SWNT arrays. In particular, we discuss the preparation of DNA wrapped single-walled nanotubes (DNA-CNTs) using commercial carbon nanotube products that are suitable for electronics applications. Then, we discuss methods to characterize DNA-CNTs using fluid mode atomic force microscopy (AFM). Finally, we give detailed procedures for assembly of DNA-CNTs into dense parallel arrays via linker induced surface assembly (LISA).