Transversally and Axially Tunable Carbon Nanotube Resonators In Situ Fabricated and Studied Inside a Scanning Electron Microscope

Abstract

We report a new design of carbon nanotube (CNT) resonator, whose resonance frequency can be tuned not only transversally by a gate voltage, but also by the axial strain applied through directly pulling the CNT. The resonators are fabricated from individual suspended single-walled CNT (SWCNT) in situ inside a scanning electron microscope. The resonance frequency of a SWCNT resonator can be tuned by more than 20 times with an increase of quality factor when the axial strain of the SWCNT is only increased from nearly zero to 2% at room temperature. The transversal gate-tuning ability is found to be weaker than the axial-tuning ability and decrease with increasing the axial strain. The gate voltage can hardly tune the resonance frequency when the initial axial strain is larger than 0.35% and the CNT acts like a tied string. The relationship among resonance frequency, gate voltage, and initial axial strain of the CNT obtained presently will allow for the designs of CNT resonators with high frequency and large tuning range. The present resonator also shows ultrahigh sensitivity in displacement and force detection, with a resolution being better than 2.4 pm and 0.55 pN, respectively.

Fabrication and Characterization of MWCNT-Based Bridge Devices

“…Therefore, a SWCNT with large diameter are better suited for high frequency, low power nanoelectromechanical devices. However, the reported SWCNTs are typically less than 5 nm in outer diameter [1,16,[19][20][21][22][23] limiting their functionality. Therefore, MWCNTs with large inner diameters [51,52] are suited for the desired properties.…”

“…For static applications, such as switches, the device actuation voltages can be reduced by fabricating devices with lower actuation gap. This could be achieved via in-situ fabrication inside a SEM equipped with nano-manipulators [23], which also reduces the randomness presence in the CNT-IPA dispersion and drop cast technique. One should note that lumpedparameter modeling suffers lower accuracy and hence should not be used for final designs; but rather for initial designs and judgments.…”

“…One of the key challenges/requirements for electrically detecting the resonance frequencies of these devices is to reduce the background parasitic currents (due to the common conductive Si as back gate-electrode providing conduction paths to the applied a.c. voltages) such that there is detectable output-to-noise ratio (during the resonating and nonresonating state). Although d.c. or a.c. frequency mixing technique [19,22,23] have been used to reduce the background parasitic currents, they are limited to singlewalled semiconducting CNTs with transconducting property. Hence, the mixing technique is not suitable for the devices reported here due to the non-transconductive nature of CNTs (as seen in Fig.…”

“…These tubular structures can operate at resonance frequencies as high as 39 GHz [19] with quality factors up to 5 million [20], which makes them suitable for sensitive applications, such as mass spectrometer with a resolution of 10 -25 kg/Hz -1/2 [1]. To present, most of the efforts have been focused on SWCNTs-based devices [1,15,16,[20][21][22][23][24][25]. However, MWCNTs bear several advantages including higher resonance frequencies and stiffness, higher chemical inertness, and simpler and less expensive synthesis techniques [26].…”

“…Furthermore, when the CVD approach is used, it is rather challenging to correlate microstructural, mechanical, and electrical analyses for a single interconnect nanotube in bridge-like configuration. All these drawbacks can be eliminated with discrete CNT manipulation and fabrication inside a scanning electron microscope (SEM) [23] and the use of larger, well-structured MWCNTs. Because of their high stiffness, these structures can boost the operating resonance range of bridge-like devices from tens of MHz to GHz.…”