Free-Standing All-Nanoparticle Thin Fibers: A Novel Nanostructure Bridging Zero- and One-Dimensional Nanoscale Features

2009 WILEY-VCH Verlag Gm Nanoscience and nanotechnology have had great success in revealing nanostructures with unique dimensionalities. To highlight a few, nacre is composed of nanolayers of inorganic crystallites and biopolymers and is therefore 2D in nature and exhibits extraordinary toughness. Derivatives of such a structure can be utilized to suit a variety of engineering applications due to their fine structure. Nanotubes, nanobelts, or nanofibers all possess a 1D structure, where the long dimension promotes applications for advanced functional materials and the short dimension delivers superior electronic, mechanical, or electromechanical properties. Long-lasting impacts of these 1D features are being generated in multiple fields. For example, nanotubes could be used as scanning probes to provide unprecedented resolution to scanning probe microscopy, continuous nanofibers can be efficient intermediate layers in nanocomposites, and functional nanobelts demonstrating an electromechanical coupling effect can be potential candidates to power future nanosystems. While nanoparticles or nanodots qualify as 0D structures and also have peculiar electronic or optoelectronic properties, their role as building blocks for higher-order subjects is one of their most prominent features. Demonstrated examples include packed particle films for high sensitivity detection, fabricated arrays for photonic crystals, and layered pores for energy storage. More interestingly, it is even possible to form laminates of nanolayers with a carefully designed manufacturing and surface chemistry approach, rendering the formation of artificial nacre. Properties of resulting assemblies usually exceed that of individual nanoparticles, where a large surface-to-volume ratio is believed to be one of the reasons for such a collective effect. With so much evidence for the structural flexibility of nanoparticles, stumbling blocks still impede the control of the growth of these 0D elements into a 1D assembly. Major barriers to unidirectional growth are difficulties to attenuate the robust interactions between nanoparticles and their isotropic shape. Patterned templates or surfaces could guide the growth of these building blocks. An anisotropic surface treatment atop nanoparticles could regulate their assembly to a certain degree. Free-standing strings of nanoparticles are rarely observed. A good strategy to devise a high throughput manufacturing of these assemblies with decent uniformity and a large aspect ratio has not been reported. We present our strategy in this communication to make unidirectionally grown and template-free, i.e., free-standing, 1D nanostructures by using uniform nanoparticles. We call this newly discovered structure a particle-fiber, implying the 0D nature of the building blocks. These string-like structures are grouped into forests after fabrication and have a few unique features, such as a high aspect ratio (length vs. diameter 2500), a uniform profile (diameter of 1.5mm), and a relatively strong mechanical strength. Since these 1D assemblies have many gaps between the building blocks, chemical vapor deposition (CVD) can be utilized to further reinforce these novel nanostructures. The principle of our strategy is to take advantage of the unidirectional growth of ice and its easy removal during a subsequent freeze-drying process. The key elements of our methodology are shown schematically in Figure 1a–c. An aqueous suspension of polymeric nanoparticles is filled in a cold finger (Fig. 1a, left), which is designed to have one side particularly sensitive to a temperature change and all other sides insulated from the cold source. Then, we load the suspension and the finger into an alcohol bath exposed over a vapor of liquid nitrogen (Fig. 1a, middle). Since the sensitive side of the cold finger faces downward in the bath, water in the suspension freezes in an upward, unidirectional fashion by forming ice crystals. Since ice forms by expelling all other ingredients previously dispersed, many gaps will form either inside or between the ice nuclei. Since these gaps also grow in a predetermined unidirectional fashion, free-standing forests of 1D string-like features form after all the nanoparticles fall into these gaps (Fig. 1a, right). Scanning electron microscope (SEM) images of fabricated particle-fibers are shown in Figures 1b–f. The main bodies of forests (Fig. 1b) have a thickness of 4.0mm and are fibrous in texture. Figure 1c shows a zoomed-in image of the bundles, where a uniform diameter is evident. Figures 1d and e reveal two