Electric-field induced nanostructuring

This chapter deals with the behavior of block copolymer nanostructures subjected to electric fields. We will consider the thin film behavior, elucidate the parameters governing the electric-field-induced alignment, and finally turn to bulk structures and show novel effects of high field strengths on block copolymer... 

Besides conventional nanotubes, other nanostructured anodic Ti02 was also reported. Ti02 nanowires (Fig. 5) were developed under specific anodization condition [38], Like bamboo splitting, the nanowires originated from the vertical splitting of nanotubes, which was caused by the electric field induced longitudinal flow of ions. A small addition of water is essential for the formation of nanowires and the amount of water required decreases with increasing applied potential. 

Zavaliche, F., Zheng, H., Mohaddes-Ardabili, L., Yang, S.Y, Zhan, Q., Shafer, P., Reilly, E., Chopdekar, R., Jia, Y, Wright, P., Schlom, D.G., Suzuki, Y, and Ramesh, R. (2005) Electric field-induced magnetization switching in epitaxial columnar nanostructures. Nano Lett.,... 

Many techniques have been developed to measure the Young s modulus and the stress of the mesoscopic systems [12, 13]. Besides the traditional Vickers microhardness test, techniques mostly used for nanostructures are tensile test using an atomic force microscope (AFM) cantilever, a nanotensile tester, a transmission electron microscopy (TEM)-based tensile tester, an AFM nanoindenter, an AFM three-point bending tester, an AFM wire free-end displacement tester, an AFM elastic-plastic indentation tester, and a nanoindentation tester. Surface acoustic waves (SAWs), ultrasonic waves, atomic force acoustic microscopy (AFAM), and electric field-induced oscillations in AFM and in TEM are also used. Comparatively, the methods of SAWs, ultrasonic waves, field-induced oscillations, and an AFAM could minimize the artifacts because of their nondestructive nature though these techniques collect statistic information from responses of all the chemical bonds involved [14]. 

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

Dielectrophoretic forces, though, can be induced by means other than an applied electric signal through electrodes. Optical tools can be implemented to modify an applied electric field, making these methods more susceptible for dynamic as opposed to static manipulation of electric fields with surface electrodes. Dielectrophoresis applications are not limited to particulate manipulation either. With properly configured surface-electrode geometry, it is possible to induce fluid motion and create nanoliter-sized droplets. Additionally, dielectrophoretic forces can be utilized to manipulate particles to buUd micro- and nanostructures such as wires. 

Surface-enhanced Raman spectroscopy (SERS) is based on the amplification of the Raman effect - the inelastic scattering of light at molecules - at nanostructured metal surfaces. If light is scattered at molecules, a dipole moment H is induced to the molecule by an incident electromagnetic field E as described in Eq. 1. Here, oc represents the polarizability of the molecule - a measure to what extent the electric field E is able to disturb the electron density of a molecule out of its equilibrium configuration ... 

The electric field has three roles in tip-based oxidation. First, it induces the formation of the water bridge. Second, it generates the oxyanions needed for the oxidation by decomposing water molecules. Third, it drives the ox) nions to the sample interface and facilitates the oxidation process. The technique generates ultrasmall silicon oxide nanostructures with a lateral size of about 5 nm and a height in the 1 to 5 nm range. 

The utilization of external fields has recently emerged as a beneficial method to direct the assembly of NPs in polymer-like nanostructures. Electric, magnetic, and shear fields have been used to induce highly anisotropic interactions between NPs. The advantage of field-induced assembly is the reversibility of interpartide interactions, which enables rapid assembly-disassembly transitions when the field is turned on... 

Raman enhancement by SERS is mainly attributed to an electromagnetic (EM) field enhancement via localized optical fields of the metallic nanostmctures that are related to plasmon resonance excitation. The increase of the cross-section with contact between the metal nanostructure and a molecule induces an additional enhancement. Without enhancement by the electrical resonance between incident light and molecules, the total Stokes-Raman signal P (vs) is proportional to the number of molecules in the scattering volume N, the Raman cross-section without surface enhancement and the excitation laser intensity... 

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