Yanli Qu

A programmable AFM-based nanomanipulation method using vibration-mode operation

A novel method is developed to effectively manipulating nano-entities to predefined positions and orientations autonomously. In this method, the nanomanipulation process is programmed by planning and executing the AFM tips movement, and the amplitude of probe-tips vibration is measured in real-time to detect the boundary of the nano-entities under manipulation - which will change as a function of the distance between the probe-tip and a nano-entity due to their inter-molecular force interactions. After the start- position and destination are defined by the user interface software, the nanomanipulation process is operated automatically. The result from each manipulation step is detected and used as the information for feedback control on the subsequent operating step. Using this method, CNTs can be manipulated to any position and orientation. Experimental results show that this manipulation method is effective and more efficient (reduce operation time by >50%) than nanomanipulation processes with a human operator in the feedback loop.

Carbon nanotube-sensor-integrated microfluidic platform for real-time chemical concentration detection

This paper presents the development of a chemical sensor employing electronic‐grade carbon nanotubes (EG‐CNTs) as the active sensing element for sodium hypochlorite detection. The sensor, integrated in a PDMS‐glass microfluidic chamber, was fabricated by bulk aligning of EG‐CNTs between gold microelectrode pairs using dielectrophoretic technique. Upon exposure to sodium hypochlorite solution, the characteristics of the carbon nanotube chemical sensor were investigated at room temperature under constant current mode. The sensor exhibited responsivity, which fits a linear logarithmic dependence on concentration in the range of 1/32 to 8 ppm, a detection limit lower than 5 ppb, while saturating at 16 ppm. The typical response time of the sensor at room temperature is on the order of minutes and the recovery time is a few hours. In particular, the sensor showed an obvious sensitivity to the volume of detected solution. It was found that the activation power of the sensor was extremely low, i.e. in the range of nanowatts. These results indicate great potential of EG‐CNT for advanced nanosensors with superior sensitivity, ultra‐low power consumption, and less fabrication complexity.

Ultra-Low-Powered Aqueous Shear Stress Sensors Based on Bulk EG-CNTs Integrated in Microfluidic Systems

Novel aqueous shear stress sensors based on bulk carbon nanotubes (CNTs) were developed by utilizing microelectrical mechanical system (MEMS) compatible fabrication technology. The sensors were fabricated on glass substrates by batch assembling electronics-grade CNTs (EG-CNTs) as sensing elements between microelectrode pairs using dielectrophoretic technique. Then, the CNT sensors were permanently integrated in glass-polydimethylsiloxane (PDMS) microfluidic channels by using standard glass-PDMS bonding process. Upon exposure to deionized (DI) water flow in the microchannel, the characteristics of the CNT sensors were investigated at room temperature under constant current (CC) mode. The specific electrical responses of the CNT sensors at different currents have been measured. It was found that the electrical resistance of the CNT sensors increased noticeably in response to the introduction of fluid shear stress when low activation current (Lt1 mA) was used, and unexpectedly decreased when the current exceeded 5 mA. We have shown that the sensor could be activated using input currents as low as 100 muA to measure the flow shear stress. The experimental results showed that the output resistance change could be plotted as a linear function of the shear stress to the one-third power. This result proved that the EG-CNT sensors can be operated as conventional thermal flow sensors but only require ultra-low activation power ( ~ 1 muW), which is ~ 1000 times lower than the conventional MEMS thermal flow sensors.

Limitations of Au Particle Nanoassembly Using Dielectrophoretic Force—A Parametric Experimental and Theoretical Study

When a gold colloidal suspension is subjected to ac electric field, ldquogold pearl chainsrdquo will form due to the dielectrophoretic (DEP) force. Our latest experiments show that the formation rate of gold pearl chains, which tends to zero at high and low frequency limits and has a maximum at a narrow mid range of frequency, is dependent on the applied field frequency. This letter analyzes the frequency-dependent DEP manipulation of gold colloid suspensions using the protoplast model. Simulated results show that the relationship curve between the frequency of applied field and the velocity of gold colloids motion due to DEP agrees with our experimental observations. In addition, the orders of magnitude of the velocity due to various effects in our experimental system, such as DEP force, Brownian motion, gravity, and fluid flows induced by electric field, were also estimated. The result implies that the DEP-based manipulation of less than 2 nm gold colloids is extremely difficult to be controlled.

An equivalent electrical model for numerical analyses of ODEP manipulation

Opticaldielectrophoresis(ODEP)has been explored experimentally with success in manipulating microscale objects in the last 5 years. However, not much theoretical analyses have been performed to understand its operating principles in depth and also determine its limitations as a tool to manipulate micro- and nano-scale objects. In this paper, we present our work on establishing an equivalent electrical model to analyze the important physical interactions when optically induced dielectrophoretic force is used to manipulate micron-sized polystyrene beads. Simulation results show that the ODEP manipulation of microbeads is frequency-dependent and that the electrothermal effect is negligible. Furthermore, the relationship between the frequency of the applied voltage and the maximum manipulation velocity of the microbeads obtained from simulation is consistent with our experimental measurements. In addition, simulation results also show that the minimum radius of a bead that can be manipulated exponentially decreases with respect to the size of the illuminated spot. For instance, when the illuminated spot size is 1µm, ODEP can theoretically manipulate 100nm beads - indicating that ODEP can be extended to manipulate nano-scale objects if the illuminated spot size can be significantly reduced.

Separation of mixed SWNTs and MWNTs by centrifugal force - an experimental study

For eventual commercial applications of carbon nanotubes (CNTs) in nanoscale devices, it will be very important to realize effective separation of mixed single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). We have developed a new method to separate a mixture of SWNTs and MWNTs by centrifugal force in an aqueous solution of glycerol. The method proposed in this paper takes advantage in the difference of the density and diameter of the SWNTs and MWNTs, which effectively produce a difference of sedimentation velocity between them along the direction of spinning radius when centrifugal force is exerted. The inert gradient agent acts to both stabilize the fluid environment of the centrifuge tube and facilitate sharp resolution of zones of the centrifuge fluid in the tubes after the centrifugation. Experiments show that SWNTs and MWNTs subside at different speeds during the procedure of centrifugation and suspend at different levels in the centrifugal tube, due to difference in their buoyant densities, at the end of centrifugation. Our results indicate that the centrifugal technique can provide a rapid and repeatable separation of mixtures of SWNTs and MWNTs.

Purification of SWNTs using high-speed centrifugation

For eventual commercial applications of single- walled carbon nanotubes (SWNTs) in nanoscale devices, it will be very important to realize effective purification of SWNTs. We have developed a simple method to purify as-synthesized SWNTs by centrifugal force in an aqueous solution of sodium dodecyl sulfate (SDS). The method proposed in this paper can effectively remove impurities such as metal catalyst particles, graphitic nanoparticles and amorphous carbon. Our method takes advantage of the fact that more massive particles sediment faster than less massive particles. Through adjusting relative centrifugal force, we can respectively remove metal catalyst particles and carbon nanoparticles from SWNTs by centrifugation. SEM results indicate that the concentration of the SWNT solution decreased when the number of times of centrifugation is increased. Raman spectra suggest that the purity of the SWNT solution increased when the number of times of centrifugation is increased. These experimental results indicate that the centrifugal technique can provide a rapid and repeatable purification of SWNTs.

Vibration-mode based real-time nanoimaging and nanomanipulation

A novel method based on vibration-mode of the atomic force microscope (AFM) for nanoimaging and nanomanipulation is introduced in this paper. With this approach, the amplitude of OMSPV (opto-electronic measurement signal of probe vibration) can be used as a feedback signal to detect and control the operation state under vibration-mode. By controlling the amplitude of AFM probe, the tip-sample interaction force can be sufficiently adjusted. Therefore, vibration-mode of AFM system can be used to manipulate nano-entities just as in using the standard contact- mode manipulation strategy. Using the novel method, nanoimaging and nanomanipulation could both be performed in vibration-mode, and the damages of the sample and probe tip can be reduced significantly. Furthermore, by detecting the amplitude of OMSPV change during operation, the precise position information between the probe tip and nano-entities (e.g., CNTs) can be determined automatically as well. Experiments show that the amplitude of OMSPV will be in USPV (unstable state of probe vibration) when the tip is at the edge of CNTs due to the real-time adjustment of parameters of the AFM control system. Such USPV can be used to determine whether the manipulation succeeds in real-time. The correlative analysis and signal processing method is also presented in the paper.

Nanoscale welding by AFM tip induced electric field

The most difficult challenges in fabricating SWCNT-based nanosystems or nanodevices have proven to be the assembly and anchoring of SWCNTs to form a stable physical and electric contact between SWCNTs and the electrodes. For example, for SWCNT-based nanosensors or field effect transistors (FETs), the need to fix the SWCNT between electrodes is extremely important, i.e., it affects the electronic transport properties at the connection point. Currently, researchers usually focus on the assembly between SWCNTs and the electrodes by using dielectrophoresis (DEP), direct growth, or atomic force microscopy (AFM). In this paper, we present a new method to realize nanoscale welding by using an AFM tip coated with conductive materials. This method is very useful in welding the SWCNTs on the micro electrodes after the manipulation of the SWCNTs in between the microelectrodes by AFM-based or DEP-based manipulation. In our experiments, we first assembled individual SWCNTs or bundles of SWCNTs between two electrodes using DEP force. Then, SWCNTs are welded on the surface of the electrodes when a bias impulse voltage is exerted between the AFM tip and sample, which produces an electric field. The experimental results have demonstrated that SWCNTs can effectively be welded on the surface of the electrodes.

Theoretical analysis based on particle electro-mechanics for Au Pearl Chain Formation

This paper analyzes the fundamental mechanisms in driving Au pearl chain formation (PCF) based on dielectrophoresis (DEP) force. From experimental results, the PCF process strongly depends on the voltage and the frequency applied on electrodes, but weakly on the sizes of particles, which appears to be contrary to theoretical expectations. To explain the above phenomenon, we estimated the DEP force and the Brownian motion imposed on the Au nanoparticles, and then investigated the AC electro-osmosis force and the electro-thermal force which may possibly affect the PCF rate. Numerical modeling to compare the forces is presented. By matching experimental and numerical results, we validate the scaling laws of the DEP force and electro-mechanics in the PCF of Au nano-particles.