Ultrasensitive Quartz Crystal Microbalance Integrated with Carbon Nanotubes
Open Access
- Author:
- Goyal, Abhijat
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 26, 2006
- Committee Members:
- Srinivas A Tadigadapa, Committee Chair/Co-Chair
Karen Eklund, Committee Member
Joan Marie Redwing, Committee Member
Charles Lee Croskey, Committee Member
David Lawrence Allara, Committee Member - Keywords:
- Gas Adsorption
Quality Factor
Carbon Nanotubes
Automatic Gain Control Circuit
Sensor
Etching
Quartz Crystal Microbalance
Y-cut quartz
infra red sensor
self assembled monolayers. - Abstract:
- In this thesis, an ultrasensitive Quartz Crystal Microbalance (QCM) which can be configured as a versatile (bio)chemical sensor is presented. The high sensitivity of the QCM was achieved via miniaturization of the QCM using micromachining techniques. The absolute mass sensitivity of sensor was increased by decreasing the thickness and the area of the electrodes of the resonators. Through optimal design, fabrication, and miniaturization the mass sensitivity of the sensors was increased by more than four orders of magnitude to less than 1 pg/Hz; as compared to 17 ng/Hz for commercially available 5 MHz bulk resonators. Miniaturization of the resonators enables their fabrication in an array format with each pixel of the array being individually addressed. This enables true spatial and temporal mass sensing capabilities. The fabricated resonators were tested for operation in air and water and high quality factors of 7500 and ~2000 were obtained respectively. A dielectric etch process was developed to achieve the miniaturization of the sensors. The optimization of the dielectric etch process was achieved using statistical techniques such as Design of Experiment (DOE). An etch rate of 0.5 ìm/min at rms surface roughness of less than 2 nm was achieved after the optimization process. The process parameters, namely the ICP power, the substrate power, the flow rate of gases, the operating pressure of the etch tool, distance of substrate holder from the source, and the temperature of substrate holder, were quantitatively related to the etch rate and rms surface roughness using least square fit to the etch data. The QCMs were integrated with carbon nanotubes using a simple spray-on technique. It was found that the addition of carbon nanotubes onto the electroded surface of the resonator increased its Q-factor by as much as 100%. It was proposed that the carbon nanotubes due to their high stiffness suppress the out-of-plane flexural vibrations in the QCMs thereby suppressing an energy loss channel and a consequent increase in the Q-factor. Measurement of out-of-plane vibrations of the quartz crystals using a laser based optical vibrometer revealed that the out-of-plane vibrations of QCM increase from 13 pm to 26 pm when carbon nanotubes are removed from the surface of the resonator – directly confirming the suppression of the out-of-plane motion on the resonator surface by carbon nanotubes. Additionally, the QCMs were used to study the gas adsorption and desorption behavior of nominally “open-ended” isolated and nominally “close-ended” bundled SWNTs. Using the ultrasensitive QCM, we were able to probe gas storage properties of carbon nanotubes. It was found that carbon nanotubes can adsorb large amount of gas molecules not only in the cylindrical pore that they enclose, but also on their external surface. Four different gases were tested, namely Helium, Nitrogen, Argon, and SF6. It was found that the change in resonance frequency and quality factor for the “fill in” and “evacuation” of gases from carbon nanotubes exhibited a characteristic ~ relationship, where MW is the atomic/molecular weight of gas species adsorbed. Such a behavior was consistently observed both for change in resonance frequency and Q-factor during the events of “fill in” and “evacuation” in the case of bare quartz with gold electrode, gold electrode covered with isolated “open-ended” SWNTs, and gold electrode with bundled “close-ended” SWNTs. In the case of bare quartz with gold electrode, the observed change in resonance frequency and Q-factor and their characteristic ~ relationship can be explained on the basis of the viscous dissipation arising in gas ambient through the Gordon-Kanazawa equation. In the case of QCM with carbon nanotubes, the change in Q-factor and its characteristic ~ relationship could be explained on the basis of enhanced viscous dissipation arising due to surface roughness or modified Gordon Kanazawa equation. However, for the case of frequency change in the presence of carbon nanotubes, the characteristic ~ relationship and the observed change in magnitude of resonance frequency was explained in terms of physical adsorption of gas molecules near the inside and outside walls of carbon nanotubes, in regions where the potential due to the carbon atoms for gas atoms and molecules is attractive.