Synthetic control over the structure and symmetry of carbon nanotubes: Towards biomedical applications
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Synthetic control over the structure and symmetry of carbon nanotubes: Towards biomedical applications Michael S. Lowry, Alfredo Rayms-Keller, Karen J. Long, Francisco Santiago, Victor H. Gehman, Jr., and Kevin A. Boulais Naval Surface Warfare Center – Dahlgren Division Dahlgren, VA 22448
ABSTRACT Carbon nanotubes (CNTs) are appealing materials for biomedical applications due to their unique chemical, electrical and mechanical properties. The emphasis of the present work is on controlling the structure and symmetry of carbon nanotubes by imposing an applied stress at the CNT growth site. CNTs were grown under these conditions using standard chemical vapor deposition (CVD) techniques and were subsequently characterized with a scanning electron microscope; the methodology and implications of this approach are discussed herein.
INTRODUCTION Carbon nanotubes (CNTs) are appealing materials for biomedical applications due to their unique chemical, electrical and mechanical properties.[1] CNTs can be conceptualized as cylindrical materials composed of rolled-up sheets of atomically-thin graphite layers (i.e., graphene). In most instances, the properties and behavior of a CNT can be described according to the symmetry (i.e., chiral vector) and number of concentric layers in its structure.[2] Despite decades of progress involving CNTs, strategic control over their structure and symmetry remains elusive but would enable the development of tunable materials for sensing, delivery and detection applications and could have a profound influence on the advent of nanotechnology in biomedical research. The emphasis of the present work is on controlling the structure and symmetry of carbon nanotubes by imposing an applied stress at the CNT growth site. The influence of applied mechanical stress was first demonstrated by the Slocum group at MIT in 2006[3] and is being considered here as means of gaining strategic synthetic control over individual CNT molecules for the first time. The aims of the experimental work described herein are to assess the effect of an applied stress during the growth process based on post-growth analysis (e.g., appearance, relative proportion of compressed segments) and to observe the relationship between bulk morphology and local molecular shape using an electron microscope.
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EXPERIMENT Vertically aligned conventional carbon nanotubes (CNTs) and stress-grown carbon nanotubes (SG-CNTs) were grown via chemical vapor deposition (CVD) at 750 oC by passing ethylene (reagent gas) over an alumina-supported iron catalyst deposited on the oxidized surface of a silicon substrate (Fe/Al2O3/SiO2/Si). The growth cycle was performed at 10 min and 60 min intervals (i.e., duration of exposure of catalyst to reagent gas). A comparison of the conditions utilized for growth is depicted in figure 1.
Figure 1: Comparison of conventional and applied stress methods for growing CNTs. In the “Applied Stress Method”, mechanical stress (60,000 N/m2)[3] was applied at the site of growth using a tungsten weight. The as-grown CNTs
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