Session Y27: Nanowire Sensors and Oscillators

Effect of Chamber Pressure on the Vibrational Properties of Micro- and Nano-Cantilevers

We have studied the nonlinear dynamics of micro- and nano-cantilevers under varying ambient conditions using the Harmonic Detection of Resonance technique (HDR)$^{1}$. In our studies, a cantilever is either microstructure shaped like a diving board, or a cantilevered MWNT. In this work, we report the dependence of the amplitude and Q of a silicon microcantilever (300 $\mu$m long, 35 $\mu$m wide, 2 $\mu$m thick) on ambient pressure. An environment of air at a pressure of 10$^{-3}$ Torr gives a high quality factor of $\sim$7000. The response of higher harmonics of the ac voltage that drives the cantilever is also observed with varying chamber pressure. An investigation of the influence of ac and dc voltages on sensitivity shows very good agreement with a model calculation. The shift in the resonant frequency of cantilevers under different environments such as helium, air and argon at different pressures will be discussed. \textbf{References:} 1. J. Gaillard, M. J. Skove, R. Ciocan, and A. M. Rao, Rev. Sci. Instrum. 77, 073907 (2006). \textbf{Contact Info: arao@clemson.edu}   

Chemical and bio- sensor chips based on nanowires and carbon nanotube

Significant effort has been devoted to the study of sensors using one-dimensional structured nanomaterials. Integration with other functionalities or combinational use of such sensors is a promising direction with which highly sophisticated functionalities can be realized. In this talk, we will present the use of the integrated/combined nano-chemical/bio sensors for more sensitive, precise, and selective sensing. We integrated micromachined heater into the chemical sensor based on In2O3 nanowire (NW), and the sensing experiments at the elevated temperatures demonstrated the detection of ethanol, which is undetectable at room temperature. Furthermore, combinational use of NW and carbon nanotube (CNT) chemical sensors combined with the micromachined heater were demonstrated toward the construction of electronic nose system. In addition, manufacturable multiplexed biosensor chips based on In2O3 NW, CNT, and aligned CNT were successfully fabricated with a highly generic strategy that can be extended to other one-dimensional materials. These sensors chips can be used as the platform for multiplexed sensing combined with selective functionalization.   

Detection of adsorbed monolayers on suspended singlewalled carbon nanotubes.

Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two.~ This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed.~ The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance.~ We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen.~ The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time.~ The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important.~ We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.   

Modeling of the Nonlinear Response of a Microcantilever: Understanding Higher Order Harmonics and Resonances

The most promising and readily scalable detection scheme for micro and nanocantilevers is electrostatic excitation and capacitive detection. This method has proven difficult to implement because of a large parasitic capacitance which masks the dynamic signal from the cantilever. Fortunately, the cantilever response exhibits several higher order harmonics and resonant peaks that can be exploited to avoid the parasitic capacitance and dramatically improve the signal to noise ratio. In this report a theoretical model of the cantilever response is presented that explains these higher order harmonics and resonances by considering nonlinear effects. Also, an experiment is presented in which the response of a microcantilever is measured simultaneously using laser reflectometry and capacitive techniques in order to separate the effects of current modulation and mechanical motion.   

Sequence Dependent Single Stranded DNA-Single Walled Carbon Nanotube Interactions and Their Applications in Detection of Gaseous Analytes

Recently there has been great interest in sensing strategies based on the use of non-covalent means to tune the chemical affinity of single walled carbon nanotube field effect transistors (SWNT FET's). The combination of single-stranded DNA (ssDNA) and SWNT FET's is particularly intriguing because of their chemical compatibility and diverse chemical recognition properties. We have demonstrated the utility of such devices for vapor sensing$^{1}$, and report here on results involving more than a dozen different ssDNA sequences. ssDNA/SWNT based sensors are sensitive to ppms of said gases, with response and recovery times on the scale of seconds. In tests involving a gas panel that includes explosives, neuro-toxins, and disease defining compounds, sensor response is specific to particular sequences of ssDNA. Given the extremely large number of different ssDNA sequences available, this observation opens up possibility of creating a large number of sensors with widely varying response characteristics, as required for an ``electronic nose'' system for the detection and classification of vapor mixtures. $^{1}$ C. Staii, M. Chen, A. Gelperin, and A.T. Johnson, Nano Lett. 2005, 5, 1774-1778 This work supported by the JSTO DTRA and the Army Research Office Grant {\#} W911NF-06-1-0462   

Fabrication and characterization of individual nanotube based nanoelectrodes for chemical and biological sensing

We present the fabrication and characterization of individual nanotube based high aspect ratio nanoelectrodes for chemical and biological sensing. The nanoelectrodes are fabricated by coating nanotubes with metal and, subsequently, with thin insulating layers, and cutting the end of the nanotubes. This process yields ring-shaped nanoelectrodes with total structural diameter of $\sim $ 100 nm, including insulating layers, and length up to $\sim $ 30 $\mu $m. The nanoelectrodes are characterized by cyclic voltammetry (CV), and the structure of the nanoelectrodes is examined in transmission electron microscope (TEM). The nanoelectrodes show steady-state voltammetric current responses and good insulation of the side wall of nanoelectrodes. These high aspect ratio nanoelectrodes will open up a new opportunity for electrochemical sensing in microscale environments, e.g. probing local intracellular environments without damaging cells, with high temporal and spatial resolution.   

Transmission line impedance of carbon nanotube thin films for chemical sensing applications.

We measure the resistance and frequency-dependent (50 Hz - 20 KHz) gate capacitance of carbon nanotube (CNT) thin films as a function of DC gate bias under ambient conditions, in ultra-high vacuum, and under low-pressure (10$^{-6}$ torr) gaseous environments of water, acetone, and argon. We have analyzed our results by modeling the CNT film as an RC transmission line. We show that changes in the measured capacitance as a function of gate bias and analyte pressure can be explained by the changes in the CNT film resistivity alone; the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption to within the resolution of our measurement. We also show that the resistance of the CNT film is enormously sensitive to exposure to low pressures ($<$ 10$^{-6}$ Torr) of analytes. This research was supported by the Laboratory for Physical Sciences and the U.S. Army Research Laboratory MICRA Program. MI received support from the Director of Central Intelligence Postdoctoral Fellowship program.   

Harmonic Detection of $\omega _{0}$/n Superharmonics in Microcantilevers via Electrostatic Actuation/ Detection

In nonlinear dynamics, mechanical motion can be made up of a complicated mixture of vibrations. In resonating structures, nonlinearities are ubiquitous and more often than not are undesirable. On the other hand, nonlinear dynamics and chaos in electrostatic microelectro-mechanical systems (MEMS) has been shown to be useful for various applications, including secure communications, MEMS filters, and scanning force microscopy. Exploiting these dynamics opens the door for nanoelectro-mechanical systems (NEMS) by providing signals with higher quality factors and better signal-to-background ratios. In cantilever-based MEMS, the nonlinear dynamics usually stem from harmonically forced excitation in which only the second superharmonic has been theorized or detected. Here we measure the nonlinearly modulated charge on a silicon microcantilever up to the seventh superharmonic of the fundamental resonant mode via electrostatic actuation/detection. In agreement with experiment, simulated results reveal that the time dependence of the modulated charge due to the cantilever's motion carries a set of harmonics for each superharmonic of order $\omega $0/n. We propose that using a system of harmonics and modes of the cantilever increases applications for cantilevered and doubly-clamped microbeams, and that exploiting the nonlinearities in the modulated charge provides a valuable tool for the study of the dynamics in electrostatic transduction.   

Electromechanical Response of Single-wall Carbon Nanotubes to Torsional Strain in a Self-Contained Device

The response of single-wall carbon nanotube transport properties to applied shear strain has been measured. The strain is applied in a self-contained nanoelectromechanical device. We find that the measured resistance of an individual nanotube can increase or decrease depending on initial band structure, and that this change is approximately proportional to the applied strain.   

Kinetics of Desorption of Oxygen

The kinetics of desorption of oxygen was studied by measuring in-situ thermopower of the Single Walled Carbon Nanotube samples subjected to (a) post synthesis acid treatment by refluxing (b) high temperature annealing at 10$^{-7}$ Torr (c) plasma (Ar, O$_{2}$, H$_{2})$ treatment using an inductively coupled plasma reactor. Raman Spectroscopy and X-Ray Photoluminescence Spectroscopy were used to identify wall defects and other disorders created due to each treatment on the nanotubes. Also we have estimated binding energy of oxygen with the carbon in all cases.   

High-Performance ZnO Nanowire FETs

Zinc oxide (ZnO) nanowires have attracted tremendous interest due to their remarkable physical properties and versatile applications in electronic devices, such as logic circuit, UV emitter and photodetector, as well as chemical sensor. In the previous research report, ZnO nanowire configured as field effect transistor (FET) shows an electron mobility ranging from 3 to 80 cm$^{2}$/V$\cdot $s without surface treatments. In order to optimize the performance of devices, it is crucial to improve the carrier transconductance and mobility in the nanochannel. In our work, single-crystalline ZnO nanowires were synthesized via a catalyst-assisted chemical vapor deposition method. Temperature dependent photoluminescence measurements demonstrate the evolution of peak intensities resulted from different radiative mechanisms. The sharp peak with its full-width half maximum of 3.6meV at 12 K and the absence of other bound-exciton lines indicate that the as-grown nanowires are of high crystal and optical qualities. Following CMOS compatible procedures to passivate surface defect states and also to reduce chemisorption processes, the nanowire FETs exhibit orders of magnitude improvement in the on/off ratio, sub-threshold swing and field effect mobility. Remarkable mobility exceeding 4000~cm$^{2}$/ V$\cdot $s was estimated.   

N-doped carbon nanotubes and their behavior as ammonia sensors

CNx nanotubes can display a measurable variation in resistance upon exposure to ammonia. We present a microscopic model for the origin of these variations. We studied, using Total Energy DFT calculations, a (5,5) CNT containing pyridine-like N atoms replacing C atoms, and how the NH$_{3}$ molecule binds to these sites. We also investigate how these defects affect the charge transport properties using a Non-Equilibrium Greens Function formalism. We initially studied a defect composed by a vacancy surrounded by 3 pyridine-like rings. The most stable adsorption configuration for the ammonia molecule adsorbed close to this defect is dissociative, with an amino group (NH$_{2})$ fragment bound to one of the nitrogens and a H atom bound to another. This configuration leads to an increase in the conductance and cannot, therefore, explain the increase of resistance that has been experimentally observed. We then investigate a variety of other configurations in order to propose possible causes for the resistance increase. We find that a divacancy surrounded by 4 pyridine-like defects is the most stable N-defect, instead of the previously proposed one. The ammonia also dissociates into NH$_{2}$ and H. Moreover, the calculated change in conductance after the NH$_{3 }$dissociation has the correct trend when compared to the experimental results. We acknowledge FAPESP, CNPq and CENAPAD-SP.   

First principles study of hydrogen adsorption on carbon nanowires.

Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. In the present work we have studied the changes in the electronic structure of a carbon nanowires and (5,5) single-walled carbon nanotubes (SWCN) when a hydrogen atom is adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure, density of states (LDOS), and the local orbital population. We found charge transfer from the nanotube to the linear chain and the hydrogen atom, the electronic character of the chain and nanotube sub-systems in chain@SWCN is the same that in the corresponding isolated systems, chain or SWCN. But the hydrogen adsorption produced changes in the atomic estructure and the electronic properties. This research was supported by PRIORI-UADY under Grant No. FING-05-004 and Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grants No. 43830-F and 49985-J.   

Hydrogen Physisorption Properties of Single-Walled Carbon Nanotubes Studied by Soft-X-Ray Spectroscopy.

Single-walled carbon nanotubes (SWNT) for nano gas sensors becomes a subject of active research. Different mechanisms for interaction of gas and SWNTs were reported. The electrical resistance was reported to change in a semiconductor SWNT when exposed to gases. The mechanism was interpreted to be collisions between gas molecules and tube wall. The major experimental effect is transport response to inert gases. It is said that deformation from collisions can change the electronic properties of SWNTs. So far there is few study to verify the mechanism and thus further investigation is needed. We have performed soft-x-ray absorption experiment to show the in-situ interaction between SWNTs and gas molecules under ambient temperature and pressure. The gas adsorption caused changes in electronic structure of SWNTs can be recovered by evacuation of gas. The collision of gas molecules to SWNTs can be responsible to the pressure-dependent spectral change. The raising gas pressure up to 10 torr induces deformation of SWNTs thus decreases the conductance, and further increase of pressure will recover the deformation due to inner tube collisions.