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Energy Dissipation, Clamping and Motional Currents in Suspended Room Temperature Carbon Nanotube Resonators

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Energy Dissipation, Clamping and Motional Currents in Suspended Room Temperature Carbon Nanotube Resonators

Lalit Kumar (Unbekannter Einband, Englisch)

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With continuous downscaling in NEMS based devices such as nanomechan- ical resonators, clamping is argued to be of signi cant importance as it controls two important aspects - mechanical stability and energy dissipa- tion mechanisms. While theoretical work including analytical and atomistic simulations has been reported, the lack of su cient experimental investiga- tions currently limits our understanding of clamping mechanisms, especially at the nanoscale. This thesis reports on the experimental investigation of clamping e ects performed with an individual suspended carbon nanotube based nanomechanical resonator at room temperature. Non-linear dynamics of resonators were exploited for insights into clamping strength and energy dissipation at room temperature. The research was carried out in collabo- ration with Laura V. Jenni and Miroslav Haluska. The resonator devices were based on a bottom gate eld e ect transistor based architecture with a designed channel length of 2 μm and a chan- nel gate distance of 270 nm. A selectively pre-characterized individual carbon nanotube was dry-transfered onto the source-drain palladium elec- trodes resulting in a bottom clamped con guration. Subsequent optional top-metallization of contact electrodes with 20 nm platinum using atomic layer deposition was performed to obtain a top-bottom clamped con gu- ration. The clamping between the nanotube and the metal was presum- ably governed by surface interactions such as van der Waals and frictional forces. The mechanical resonances of suspended carbon nanotubes were character- ized by measuring both conduction modulation current and piezoresistive current. The former was attributed to the gate induced eld e ect while the latter was the result of strain modulation, both being proportional to nanotube motional amplitude. As a result of static displacement, induced by the DC gate bias, mixing of both motional currents at resonance was ob- served. For instance, at a DC gate bias of 1.4 V, 75% of the output current was conduction modulation current. The remaining 25% was piezoresistive current generated from the DC pre-strain of ∼0.0056% in the nanotube. For a 2.1 μm long and 1.8 nm diameter small bandgap semiconducting nanotube, the intermixing of motional currents was used to extract the low strain Gauge factor of ∼1.8. The mechanical resonances were used to investigate clamping stability of both bottom clamped and top-bottom clamped devices. Under high DC gate bias > 1.5 V, non-linear Du ng response, attributed to large nanotube motional amplitude, was observed for all devices. Continuous operation un- der large DC gate bias resulted in an irreversible downshift in the resonance frequency. The Large motional amplitude of the nanotube initiated slipping which led to the decrease in the nanotube tension, or mechanical stress re- laxation, which was observed as a decrease in the resonance frequency. For a bottom clamped nanotube device with a 2.1 μm suspended length and 2.4 nm diameter clamped on either side onto palladium electrodes, the onset of slipping was observed at a motional amplitude of 7.4 nm at a DC gate bias of 1.1 V. An additional nanotube tension of 70 pN was estimated to over- come the clamping forces to initiate slipping, quantifying the weak nature of clamping strength. The resonance frequencies decreased until saturation was attained. The saturated resonance frequencies were two to three times higher than the fundamental beam mode and were attributed to clamping induced residual strains. Residual strains for both clamping con gurations were estimated in the range of 94-130 pN. Weak clamping behavior was also assumed to in uence energy dissipation through clamping losses. An already saturated bottom clamped device was subjected to top metallization resulting in two di erent clamping con gura- tions for the same device. Top-bottom clamped device showed an improve- ment in quality factors up to two times and was attributed to the reduction in the non-linear damping. The amplitude dependent non-linear damping was signi cantly observed in bottom clamped devices. The same bottom clamped device, which observed an onset of slipping behavior at ∼ 7.4 nm, exhibited amplitude dependent non-linear damping at a motional amplitude of ∼ 11 nm (and higher). The non-linearity in energy losses and improved quality factors were attributed to possible external dissipation channels as- sociated with clamping, which previously has only been discussed from a phenomenological and mathematical point of view. The dynamic measurements based on resonance frequency downshift and saturation provides an alternative way for investigation of clamping e ects on both mechanical stability and energy dissipation. The dependency on motional amplitude suggested a signi cant role of surface interactions on the clamped edges with forces in the order of pN. The experimental ndings reported here argues against the assumption of perfect clamping conditions, which are often taken into consideration in various reported experimental and theoretical studies. Such assumptions might explain the discrepancies between the experimental ndings and analytical or simulation based models for nanomechanical resonators. Weniger zeigen

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Produktinformationen

Technische Daten

Erscheinungsdatum

05.08.2019

Sprache

Englisch

EAN

9783866286504

Herausgeber

Hartung-Gorre

Serien- oder Bandtitel

Scientific Reports on Micro and Nanosystems

Sonderedition

Nein

Autor

Lalit Kumar

Seitenanzahl

182

Auflage

Einbandart

Unbekannter Einband

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