Thesis defence S.J. Cartamil Bueno: freestanding 2D-materials

18 December 2017 15:00 - Location: Aula, TU Delft - By: webredactie

Freestanding 2D materials and their applications. Promotor 1: Prof.dr. P.G. Steeneken (TNW); Promotor 2: Prof.dr.ir. H.S.J. van der Zant (TNW).

This thesis synthesizes the results of my work, in cooperation with other local and international researchers, on the understanding of mechanical devices made out of freestanding graphene and other 2D materials, and some of the potential applications of these nano-membranes.

Its structure follows a dual story with two interspersed themes. On one hand, it is a chronological guide of techniques that were tested or developed, and at the same time it is an exploration of different materials for particular applications. Therefore, the thesis is divided in application parts comprising chapters discussing techniques and findings related to the given material.

After a historical introduction on materials, Chapter 1 uses the mineral called graphite to guide us through the annals of scientific progress to arrive to the definition of nanotechnology and the understanding of what layered materials are. From there, a leap to present times transports us to the current understanding of graphene and other 2D materials, followed by their use in micro-/nano-electromechanical systems (M/NEMS).

Application 1: TaSe2 for write-once-read-many (WORM) memory devices contains Chapter 2. In this chapter, we introduce the laser interferometry setup to study the dependence of the mechanical resonance frequency on the thickness of tantalum diselenide (TaSe2) drumheads. We observe a continuous transition from plate to membrane behavior when reducing the thickness from 100 nm to 6 nm, and we prove the large effect of stress on the resonance frequency of 2D materials by laser-oxidation of the thin membranes. This selective laser-oxidation is comparable to the laser-recording of CD-R and DVD-R optical disks, and therefore each of these TaSe2 NEMS could be a bit of a write-once-read-many (WORM) memory unit.

Application 2: CVD graphene and h-BN for optomechanic sensing spins around the use of NEMS membranes made out of two prominent materials made by chemical vapor deposition: double-layer graphene and single-layer hexagonal boron nitride.

In Chapter 3 we perform a small statistical analysis on the fundamental resonance frequency of CVD DLG drumheads to find its theoretical dependence with the device diameter by means of laser interferometry. Variations of the mechanical resonance are attributed to differences in tension of the membranes, which are compared to values obtained by nanoindentation with an atomic force microscope. The cleanliness of the drums is discussed together with the pressure-induced destruction of the larger devices. Furthermore, the invasive nature of the laser technique due to the heat impact on the measurements is exposed by providing evidence for softening or tensioning of the membranes and their photothermal self-oscillation at higher laser powers. This allow us to present a new theory that explains the heating effects from the laser on the resonance frequency and quality factor of these membranes by a model based on thermally-induced variation of the sidewall length.

Chapter 4 explores the use of single-layer h-BN. We fabricate drums of 5 μm in diameter, extract their mechanical properties with the laser and atomic force microscope techniques, and study their behavior when cooling them to cryogenic temperatures (3 K). The process is repeated after a series of cleaning steps to demonstrate a route for reducing the contamination in this type of fragile structures while exploiting the inorganic nature of this material. We also propose a model that describes an adhesion-mediated influence of temperature on the mechanics of certain 2D materials.

Application 3: CVD graphene for pressure sensing describes the colorimetry technique and the uses we have given to the color-changing of double-layer graphene drumheads.

In Chapter 5, we analyze several samples with single- and double-layer graphene drumheads of different diameters and cavity depths in a very large scale fashion. With the colorimetry technique, we obtained the statistical probabilities of a given device geometry to survive a pressure difference of 1 atm. We found that the device yield follows a trend ruled by a scaling parameter, which indicates that the main failure mode is due to contact between membrane and cavity bottom, and that the survival probability improves considerably when using two instead of one graphene layers. Moreover, we extracted the adhesion energy of the double-layer graphene by studying membranes stuck to the bottom of their cavities.

Chapter 6 explains how to use the technique to extract the pressure-induced deflection of the membranes as a function of time, and shows how this can be used to measure their gas permeability. The observation of Newton's rings enabled their exploitation for exploring the large deflection regime of these membranes where the device stiffness is dominated by the Young's modulus. A study of the pressure decay in the small deflection regime allowed us to measure the permeance of a membrane, and we found that gas molecules escape from these devices faster than when they have to enter sealed cavities under vacuum.

Killer application: CVD graphene for interferometry modulation displays (IMOD) introduces the main innovation in this dissertation: Graphene IMODs. Chapter 7 combines the colorimetry technique with electrostatical actuation to study the dynamical movement of double-layer graphene membranes stroboscopically. The tension of the membranes was extracted non-invasively while vibrating them at frequencies up to 2 kHz without observing mechanical delays nor gas damping, hence proving the suitability of this type of mechanical pixels for applications requiring high refresh rates. The demonstration of GIMOD technology in the Mobile World Congress 2017 in Barcelona served as a test to validate the usefulness of such kind of displays, which was well-received by industry.

Future Applications (Chapter 8) concludes the thesis with some promising uses of 2D materials in the coming years. A continuation of the study on large populations of graphene NEMS will give predictions of the actual applications of those devices for pressure and gas sensing with less speculation. Statistics on the permeability of CVD graphene with different layers could reveal the feasibility of hermetic sensors or molecular sieves. Moreover, their optical properties should be improved to make the GIMOD technology competitive. These enhanced optical properties could also be used in other technologies that require weightless and robust materials with high optical reflectivity such as light sails and other space applications.

More information?

For access to theses by the PhD students you can have a look in TU Delft Repository, the digital storage of publications of TU Delft. Theses will be available within a few weeks after the actual thesis defence.