Investigation of the Electromechanical Properties of Graphene

This thesis studies the optical and electro-mechanical properties of novel atomically thin crystals. It deals with the carbon based material, graphene, and with layered transition metal dichalcogenides with the common formula MX2. The first part describes a new optical model for detecting ultra-thin, two dimensional nanocrystals under optical microscope. A simple optical model is used to calculate the contrast of nanolayers on SiO2. The model is extended for imaging using the green channel of a video camera. Atomic force microscopy combined with optical imaging confirms that single layers of graphene, MoS2, WSe2 and NbSe2 are detectable on 90 nm and 270 nm SiO2. The optical contrast of multi-layer nanocrystals is also reported and allows easy differentiation between single, double and triple layers. Optical imaging is proposed as a rapid, non invasive and low cost method for the detection of ultra-thin nanocrystals. The second part is devoted to studying the electromechanical properties of mono-layer and bilayer graphene. The interplay between the mechanical and the electrical properties of graphene nanoribbons is probed by nanoindentation techniques based on AFM. Deflection of monolayer graphene nanoribbons with widths ranging between 60 nm and 300 nm results in a linear increase in their electrical resistance. The sensitivity of the response increases for nar- rower ribbons and variations of the resistance up to 15% are measured. The electromechanical response of bilayer graphene shows a superposition of a linear response with oscillations of the electrical resistance. We present possible scenarios to explain the origin of these oscillations.

Electromechanical oscillations in bilayer graphene

Gabriel Albert Autes, Mohamed Malik Benameur, Fernando Gargiulo, Andras Kis, Sajedeh Manzeli, Mahmut Tosun, Oleg Yazyev

Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.

2015

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

Nowadays, the interest in 2D materials has gone far beyond graphene. Specially, monolayers of transition metal dichalcogenides (TMDs) offer a broad spectrum of electronic and optical properties, and show the potential to revolutionize the electronics industry. The promising electronic properties of 2D semiconductors combined with their mechanical strength and flexibility, makes them ideal candidates for nanoelectromechanical systems (NEMS). This thesis focuses on realizing NEMS based on graphene and MoS2, which is one of the most appealing TMDs. First, we present the realization of graphene NEMS by fabricating single and bilayer graphene transistors featuring a doubly clamped suspended channel. The electromechanical response of monolayer graphene nanoribbons show a strain-induced increase in their electrical resistance, making it possible to estimate an upper limit for the piezoresistive gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of the two graphene layers with respect to each other. Our results report on the rare observation of room temperature electronic interference in bilayer graphene. Next, we investigate the static electromechanical response of atomically thin MoS2. MoS2 exhibits high youngâs modulus and fracture strength. Furthermore, the bandgap of MoS2 is highly strain-tunable. This coupling between electrical and mechanical properties makes MoS2 a promising material for NEMS. Here we incorporate monolayer, bilayer and trilayer MoS2 in a suspended membrane configuration with the electrodes acting as mechanical clamps. Strain-induced bandgap tuning is detected via electrical conductivity measurements and the emergence of the piezoresistive effect in MoS2 is demonstrated. We observe a reversible bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element method (FEM) simulations are used to obtain the spatially varying bandgap profile on the membrane and to quantify the rate of bandgap change. The piezoresistive gauge factor is calculated for single layer, bilayer and trilayer MoS2. Our results reveal that monolayer and bilayer MoS2 show a piezoresistive effect which is comparable to the state-of-the-art silicon strain sensors and two orders of magnitude higher than in graphene. Finally, we present the investigation of MoS2 NEMS resonators working in the VHF range and featuring piezoresistive transduction. The atomic thickness of monolayer MoS2 places it as a promising candidate for miniaturization of electromechanical devices to the limits of vertical scaling. While the small mass of MoS2 leads to an increased resonant frequency and a higher mass sensitivity, the presence of piezoresistivity offers a transduction mechanism in addition to the traditional capacitive transduction. Operating in the tension-dominated regime, monolayer MoS2 NEMS resonators not only allow tunability of the resonant frequency using an external voltage, but also show the strain-induced enhancement of their dynamic range. Furthermore, the resonators are driven into the nonlinear regime allowing the study of nonlinear effects. This work sheds light on the potential of TMD based NEMS as ultra-low power switches, sensors and resonators for applications in RF range.

EPFL2018

MAGNETIC NANOCRYSTAL MODIFIED EPOXY PHOTORESIST FOR MICROFABRICATION OF AFM PROBES

Jürgen Brugger, Cristina Martin Olmos

Nanocomposites based on an organic polymer and inorganic nanocrystals (NCs) represent a class of high impact functional materials able to convey the unique size and shape dependent properties of nano-objects to highly processable resists.[1] In this work, a novel magnetic nanocomposite based on a negative tone epoxy photoresist and magnetic colloidal Fe2O3 NCs has been manufactured for fabricating AFM probes. Epoxy-type photoresist grant superior lithographic performances when patterned by standard near- ultraviolet (UV) optical lithography, providing structures with high aspect-ratio and nearly vertical sidewalls. Such resists are at present employed in optical and micromechanical applications for the fabrication of optical waveguides, microfluidic systems and scanning probes.[2] However, these materials lack of any inherent functionality, e.g. electrical conductivity, luminescence, magnetism, piezoresistivity and dielectricity. Hence, the incorporation of NCs can confer them new properties maintaining the patterning resolutions required for the manufacturing of highly miniaturized devices. These added properties can be interesting for the fabrication of novel micro/nanoelectromechanical systems (MEMS/NEMS). The current challenge consists on incorporating NCs in the photoresist host matrix to add functionality preserving at the same time the polymer photostructurability. The NC addition has relied on the efficient dispersion of pre-synthesized functional nanosized fillers into the pre-made epoxy photoresist, by using a common solvent as recently reported by the authors in the incorporation of highly luminescent CdSe@ZnS NCs in the same epoxy photoresist.[3] Here, the magnetic properties, UV-photostructurability and mechanical characteristics of the nanocomposite have been investigated. The results show that, after the incorporation, the Fe2O3 NCs preserve their magnetism which is conveyed to the photoresist and retained after the UV-lithography process (Figure 1).