Alain Jungen

Raman imaging of doping domains in graphene on SiO2

We present spatially resolved Raman images of the G and 2D lines of single-layer graphene flakes. The spatial fluctuations of G and 2D lines are correlated and are thus shown to be affiliated with local doping domains. We investigate the position of the 2D line—the most significant Raman peak to identify single-layer graphene—as a function of charging up to ∣n∣≈4×1012cm−2. Contrary to the G line which exhibits a strong and symmetric stiffening with respect to electron and hole doping, the 2D line shows a weak and slightly asymmetric stiffening for low doping. Additionally, the linewidth of the 2D line is, in contrast to the G line, doping independent making this quantity a reliable measure for identifying single-layer graphene.

Fabrication of discrete nanoscaled force sensors based on single-walled carbon nanotubes

We present a fabrication technique for discrete, released carbon-nanotube-based nanomechanical force sensors. The fabrication technique uses prepatterned coordinate markers to align the device design to predeposited single-walled carbon nanotubes (SWNTs): Atomic force microscope (AFM) images are recorded to determine spatial orientation and location of each discrete nanotube to be integrated in a nanoscaled force sensor. Electron beam lithography is subsequently used to pattern the metallic electrodes for the nanoscale structures. Diluted hydrofluoric acid etching followed by critical point drying completes the nanosized device fabrication. We use discrete, highly purified, and chemically stable carbon nanotubes as active elements. We show AFM and scanning electron microscope images of the successfully realized SWNTs embedded nanoelectromechanical systems (NEMS). Finally, we present electromechanical measurements of the suspended SWNT NEMS structures

Synthesis of individual single-walled carbon nanotube bridges controlled by support micromachining

Single-walled carbon nanotubes (SWNTs) were directly grown onto poly-crystalline silicon grids by catalytic thermal chemical vapour deposition. We demonstrate that simple micromachining of the catalyst-covered support can influence the number, location and alignment of suspended SWNTs. Sharp apexes formed by over-etching circular microstructures enable the scalable, cost-efficient formation of mostly individual, straight SWNT bridges, as verified by Raman scattering and electron diffraction.

Electrothermal effects at the microscale and their consequences on system design

Electrothermal devices exhibit increasing secondary effects upon downscaling. These effects alter their temperature profiles and thermal energy distributions. Based on scaling laws, we show that the Thomson effect is increased over Joule heating as the structures are sized down. Thermal models are upgraded to take into account this secondary effect, and finite element simulations together with experimental runs validate them.

Amorphous carbon contamination monitoring and process optimization for single-walled carbon nanotube integration

We detail the monitoring of amorphous carbon deposition during thermal chemical vapour deposition of carbon nanotubes and propose a contamination-less process to integrate high-quality single-walled carbon nanotubes into micro-electromechanical systems. The amorphous content is evaluated by confocal micro-Raman spectroscopy and by scanning/transmission electron microscopy. We show how properly chosen process parameters can lead to successful integration of single-walled nanotubes, enabling nano-electromechanical system synthesis.

Thermography on a suspended microbridge using confocal Raman scattering

Confocal Raman scattering to perform submicron spatial resolution thermography on suspended microbridges was carried out by tracking the frequency shift of the Raman-active first-order optical phonon mode of polycrystalline silicon. The measurements reveal that microsystem design allows the definition of structures with very high thermal gradients of about 50K∕μm. The Thomson effect is pronounced in such structures and was actually sensed at different temperatures. Fluorescence measurements at elevated temperatures confirm the observation of this secondary effect.

Fabrication of discrete carbon nanotube based nano-scaled force sensors

We present a novel fabrication technique for discrete and released carbon nanotube based nano-mechanical force sensors. The simple fabrication technique involves pre-patterned coordinate markers used to align the device design to pre-deposited single walled carbon nanotubes (SWCNT): atomic force microscope (AFM) images are recorded to determine spatial orientation of each discrete nanotube to be integrated in a nano-scaled force sensor. Electron beam (e-beam) lithography is subsequently used to pattern the metallic electrodes for the nanoscale structures. Diluted HF etching followed by critical point drying finalizes the nano-sized device fabrication. We use discrete, highly purified and chemically stable CNT as active elements to reliably withstand HF release, rather than CNT grown by chemical vapor deposition (CVD). We show AFM and FESEM images to prove the successful realization and release of SWCNT embedded in true NEMS.

Piezoresistance of Single-Walled Carbon Nanotubes

We report on nano electromechanical transducers based on single-walled -carbon nanotubes (SWNTs) in order to investigate the piezoresistance of individual SWNTs, which are at present one of the most studied nano structures, showing promising physical properties. We show a theoretical and experimental analysis of electromechanical properties of SWNTs, highlighting their extraordinary potential and diversity. It is shown theoretically and experimentally that ballistically conducting SWNTs show nonlinear piezoresistive gauge factors of up to e.g. ap 1500 (applied strain epsivmax ap 1%), which clearly exceeds state-of-the-art silicon based strain gauges (values of ap 200).

A MEMS actuator for integrated carbon nanotube strain sensing

We report on the performance of a multi-purpose strain sensor for embedded carbon nanotubes (CNT). The sensor is based on a microactuator operable in various environments including in situ scanning electron microscopy (SEM) and Raman microscopy from the same tube. The fabrication of the MEMS was outsourced to a foundry process first and then post-processed to integrate the nanotubes and to complete the sensor synthesis. The electrothermal actuator, applying tensile loads to the integrated nanotubes, is designed to decouple thermally and electrically from the device under test. The displacement resolution was investigated under SEM. We performed electrical and mechanical characterizations of the actuator as well as reliability tests. The fabrication of the system, together with the previously developed CNT integration process, is batch-fabrication compatible

Nanoscale Straining of Individual Carbon Nanotubes by Micromachined Transducers

We report on the (opto-)mechanical properties of integrated single-walled carbon nanotubes. Individual nanotubes were integrated into micro-actuators by a simple location-controlled integration process. These structures allowed for reproducible strain-dependent Raman analysis of individual nanotubes under various levels of tensile strain. The nanotubes were suspended in air, reducing the support interaction and enabling characterization based on transmission beams.