The aim of this study is to discuss the new technology in the field of Nano-Tweezers. These tweezers integrate the electrostatic and thermal actuation. In this new technology, the Nano-tweezers will have the ability work in the field of biotechnology for instance in the manipulation of DNA molecules or force sensing. The project will involve the design and the fabrication part. The new design will feature two tungsten tips that are chemically etched and attached to a carbon fiber-reinforced polymers. It will have electrostatic and thermal actuators. The fabrication part will combine the tungsten etching and the ion deep reactive etching on an insulator made of the silicon wafer to form Nanotips that are sharp and microstructures that are of high aspect ratio respectively.
Keywords: Nano-tweezers, Nanotubes, Silicon-on-Insulator, electrical and thermal actuators.
In the field of nanotechnology and nanomaterial, reliable and accurate tools for the manipulation of the Nanoscale are sought after to complement new breakthroughs in this fields (Castilla et al, 2011). In order to manipulate a single molecule or some other polymer, many techniques have consequently been developed. The most commonly used tools are the optical tweezers and the Nanotweezers or traps. In this technology, a bio-molecule is anchored between a force transducer and movable surface. The magnitude is so little in terms of piconewtons and the sensing is conducted with the help a trapped micron- sized bead or a cantilever whose displacement used as a measure of force. MEMS or microelectromechanical system have a pair of tips that are opposing with the ability to accurately adjust through a high-resolution differential sensor that is capacitive (IEEE, 2004). It will have an electrostatic mode to show a resolution of 5nm for a displacement range of 3um. The resonant frequency of 2 kHz and a 40 quality factor in the air and in a 550 in a vacuum.
The shape recovery effect of the spring in the electrostatic and thermal actuators is exploited as a mechanism to control the bending and relaxation modes of the Nano-tweezers. The activation is done by driving a potential difference of less than a single volt across the coils. Assembling involved putting the individual Au Nanowires averaging to 5-10 um in length and 200nm in diameter on a silicon substrates using the tips of a tungsten. Drawing initials out of the many shapes of the nanowires such as loops, curls, Zigzags and crosses will make this new nano-tweeZers have applications in the manufacturing of nanostructures that are complex or in modifying minute surface materials in the field of biotechnology among others (Kharisov et al, 2012).
The Nanotweezers described here was pneumatically actuated and was designed to be operated in a range of environments including liquid and air at a range of scales (Zhou et al, 2004). It comprises of two sharp tungsten tips that are chemically attached and act as electrodes for attracting the molecules. One of the electrodes was fixed while the other was electrostatically and thermally actuated. The gap between the tips (x-direction) was adjusted due to a transverse capacitor that is differential that will measure the relative displacement of the tungsten tips that are moving and measures the relative displacement of the moving tips. It introduced a limit of the voltage of pull-in dependent on the current position so that it allows a residual spacing between the tips of the gripper. The electrostatic and thermal actuator is an essential flexible membrane that applies force to the tweezers pad when the air inlet is pressured (Chang et al, 2009). The sensor consisted of the central plates that are relatively more fixed to the external plates thus forming two capacities. The structure used was obtained by the deployment of two polysilicon plates signed to integrate the electrostatic actuator and the tungsten tips of the Nano tweezers. The electrostatic tips use the coulomb forces present between them subject to a difference in potential to maximize the capacity. The thermal tips use a difference in temperature between the two tips and therefore a dissymmetry of dilation between the two tips that have tungsten as its unique material. Part of the material that makes the tips is heated up to obtain a lateral bending. By joules effect, the heating is obtained through the conductive material which in this case is the shrinking of the local part. This part imposes the electrical resistance thus the dissipated power and a higher temperature. Combining this two actuation to make an electrostatic and thermal Nano tweezers is very effective and efficient (Hawkes, 2012). They will manipulate molecules in the most efficient way possible.
The fabrication process of the sharp tungsten and silicon Nanotweezers was based on the Silicon-On-insulator. A thin silicon layer was first deposited by the Low-pressure Chemical Vapor Deposition and patterned to form rectangular in the right direction. Deep Reactive Ion etching was used to etch the over silicon layer. Then, an oxidation process that is specifically wet was used to cover the structured silicon with SiO2. A tungsten thin nanowire was installed then a KOH anisotropic etching of silicon is performed after removing the Silicon layer to obtain the facets which form the sharp tips. HF removed the buried oxide and the handled silicon was structured by the Deep Reactive Ion etching. The carbon fiber was attached to the design that is field emissive-type. In order to obtain a symmetrical movement of the tips, an electrostatic force was applied.
Fig 1: images of motion of the Nano tweezers for AFM: (a) opened and (b) closed.
For fabrication of a pair of Nanotweezers, carbon fiber was used to make the reinforced polymers (Liu, 2008). This is due to its properties in electrical and thermal conduction in microscale levels. One side of the tungsten tip was manipulated to be in contact with the carbon fiber that is targeted in the polymer and then a carbon film that is amorphous was deposited on the contact portion. The target carbon fiber was finally pulled off. The same process is performed in the attachment of the other carbon fiber polymer. The positions of the two carbon fiber polymers are adjusted using the stages with the tungsten needle to make the arms parallel and symmetrical in space. Also, it is fixed by deposition of the carbon films at the arms base. The entire carbon fiber polymers area was coated with a very thin carbon film ( 5 nm) to be insulated on the outside as the carbon films are insulators by nature. This film will prevent large current and heat flow when the two nanotubes-arms pick-up or close a conductive particle to make an electrostatic or thermal contact (Luo et al, 2005).
This study has demonstrated a recent technology in the field of nano-tweeZers. The electrostatic and thermally actuated nano-tweezers have the ability to manipulate very tiny molecules in the field of biotechnology and then provide feedback. The two carbon fiber polymers that form the arms are fabricated using low-cost and the most efficient manufacturing techniques with a range of dimensions. This allowed the design to be scaled to fit the chosen application. This structure will be appropriate and suitable for grasping micro objects of smaller sizes. The Nano-tweezers could also be used as an STM two-tip or conduct an AFM probe (Sattler, 2009). The probe can measure the single electron Greens function between the local two tunneling junctions and thus provide detailed information about the electronic and thermal properties of the material (Sattler, 2011). Future work will focus on designing a Nanotweezers that manipulate even smaller and tiny cells thus opening up exciting opportunities for modification and manipulation of biological systems such as structures of a cell.
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