Thermomechanical nanocutting of 2D materials using thermal scanning probe lithography

The rapidly evolving field of 2D materials has developed a plethora of emerging micro/nano devices where atomically-thin materials play a fundamental role. In most cases, this implies the need of nanostructuring them, but due to their delicate nature, conventional lithographies using electrons or ions can deteriorate their performance. Thermal scanning probe lithography (t-SPL) is an advanced direct-write method that uses a heated nanotip for 2D and 3D subtractive/additive manufacturing [1]. The creation of patterns by t-SPL is accomplished by consecutive indentation of the sample with the heated nanotip while simultaneously scanning the sample. We extend the application of t-SPL beyond polymers and demonstrate the direct cutting of different 2D materials using a heated scanning nanotip [2]. To do so, a 2D material flake is transferred onto a substrate covered by polyphthalaldehyde (PPA), a dedicated polymer that unzips into smaller volatile monomers at around 150 °C, as shown in Figure 1(a). When the tip indents the 2D material, the applied force, together with the heat, induces the rapid sublimation of the PPA and breaks the chemical bonds of the thin flake without damaging the rest of the material. In this way, we created different test structures in the 2D material, i.e. nanoribbons as shown in Figure 1(b). A resolution down to 20 nm is achieved for monolayered MoTe2; yet many other 2D materials, multilayers, and even heterostructures can be patterned. Raman spectroscopy is used to validate the removal of the 2D materials in the cut areas and electrical characterization shows that clean cuts have been achieved. In the future, this high-resolution nanocutting process can be of benefit to scientists and engineers as a fast prototyping technique to fabricate 2D material nanodevices by a clean, one-step thermo-mechanical technique. References [1] S.T. Howell, A. Grushina, F. Holzner, J. Brugger, Microsyst. Nanoeng.,6 21 (2020). [2] X. Liu, S.T. Howell, A. Conde-Rubio, G. Boero, J. Brugger, Adv. Mater. (2020). doi:10.1002/adma.202001232

Thermomechanical Nanocutting of 2D Materials

Giovanni Boero, Jürgen Brugger, Ana Conde Rubio, Samuel Tobias Howell, Xia Liu

Atomically thin materials, such as graphene and transition metal dichalcogenides, are promising candidates for future applications in micro/nanodevices and systems. For most applications, functional nanostructures have to be patterned by lithography. Developing lithography techniques for 2D materials is essential for system integration and wafer-scale manufacturing. Here, a thermomechanical indentation technique is demonstrated, which allows for the direct cutting of 2D materials using a heated scanning nanotip. Arbitrarily shaped cuts with a resolution of 20 nm are obtained in monolayer 2D materials, i.e., molybdenum ditelluride (MoTe2), molybdenum disulfide (MoS2), and molybdenum diselenide (MoSe2), by thermomechanically cleaving the chemical bonds and by rapid sublimation of the polymer layer underneath the 2D material layer. Several micro/nanoribbon structures are fabricated and electrically characterized to demonstrate the process for device fabrication. The proposed direct nanocutting technique allows for precisely tailoring nanostructures of 2D materials with foreseen applications in the fabrication of electronic and photonic nanodevices.

2020

Application of Microstencil Lithography on Polymer Surfaces for Microfluidic Systems with Integrated Microelectrodes

Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart

Microfluidic systems with incorporated microelectrodes are adopted in a variety of applications in life sciences. However, most of micro-electro-fluidic devices are fabricated based on silicon and silicon related materials micromachining processes, which are still expensive for disposable or semi-disposable sensing devices. Plastics and other polymer materials are advantageous to be used as substrates of microfluidic systems in order to realize a low-cost production process of devices, since these materials can be easily structured by e.g. hot-embossing, injection molding or thermal forming with excellent reproducibility. However, the lithographical process of microelectrode fabrication on plastic substrates is one of the critical issues, because many of these materials cannot withstand organic solvents which are unavoidable during conventional photolithography processes. Microstencil lithography, i.e., local deposition of micrometer scale metal patterns through small shadow masks, is a promising method for metal micropattern definition on plastic substrates that are not feasible for solvent-based photoresist technology. Since microstencil lithography is a resistless, single-step direct vacuum patterning method, it enables us to simplify the device fabrication process as well. Thus, we propose to apply microstencil lithography to fabricate microelectrodes onto plastic substrates as part of a low-cost production process of microfluidic devices. However, microstencil lithography is faced with several possible drawbacks. Therefore, we focused on two specific issues accompanying microstencil lithography, which are expected to affect on micro electro fluidic device fabrication process when considered as low-cost, reproducible alternative to standard lithography on plastic substrates: clogging and blurring. One of the main limitations of microstencil lithography is that the method is associated with gradual aperture clogging. To suppress the clogging phenomenon, coating stencils with self-assembled monolayers (SAM) is highly helpful, by which the useful life time of stencils can be extended. But in this study, we traced the clogging of micro-apertures without SAM coating by an observation of the stencil apertures and deposited microstructures after each evaporation step in order to asses the basic feasibility of microstencil lithography. From the series of observation, it was determined that approximately 50 % of the thickness of the evaporated metals (Ti, Cu) was deposited at the side walls of the stencil apertures, i. e., when 200 nm of Ti was evaporated through the microstencil, the thickness of deposited Ti at the side wall of the aperture was 0.1 um. Similar values were observed when Cu was used as a deposited material. The progress of clogging is clearly visible in an SEM image, in which the corresponding microstencil aperture was observed from the side having faced the substrate during the evaporation. These results suggest that the microstencils can be used repeatedly even without a cleaning treatment when the device specifications allow some micrometer tolerance of the electrode size. The other drawback of stencil lithography is related to the gap presence between the stencil and the substrate. We will also present quantitative values of gap induced clogging and discuss the blurring mechanism of deposited structures based on a simplified geometrical model.

2006

Functional polymer brushes via surface-initiated controlled radical polymerizations

Stefano Tugulu

The term polymer brush refers to a well defined arrangement of polymer chains, which are tethered with one end to an interface and usually the surface of a solid substrate. Due to steric repulsion the polymer chains furthermore adopt a defined, stretched chain conformation, which significantly differs from the random walk conformation of free polymer chains in solution or in conventionally solution casted polymer coatings. From a structural point of view polymer brushes can therefore be regarded as the most defined type of polymer coating that can currently be realized. Especially the combination of surface-initiated polymerizations (SIP) with modern, controlled polymerization methods thereby allows tailoring the structure of the polymer brushes almost on the molecular level. Specifically precise control over the thickness, composition, chain architecture, grafting density and via the use of lithographic techniques also over the topography of the brushes can be achieved. This high degree of control over the structural and implicated (physico-)chemical properties of the brush allows at the same time tailoring and fine tuning the interfacial properties of the brush. This thesis describes the systematic exploitation of well-defined polymer brushes as functional coatings with fine-tuned physicochemical properties to direct and control the interaction with and the response of specific biological, bioanalytical and even reactive chemical environments. In detail the use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the fabrication of functional polymer brushes will be presented. These brushes were used as highly defined coatings in various applications ranging from biomedical and bioanalytical applications to biomimetic materials fabrication (Figure 1). Figure 1 Numerous synthetic approaches for the preparation of polymer brushes have been described in the literature. Chapter 2 will therefore present a summary of the most important types of surface-initiated polymerizations, focusing especially on controlled SIP methods and SI-ATRP. Chapter 3 describes a chemoselective and specific immobilization strategy to decorate intrinsically bioinert polymer brushes with proteins of interest in a defined orientation and density. Specifically the resulting substrates represent attractive candidates for the realization of protein microarrays. In view of this application protein-small molecule and protein-protein interactions as well as posttranslational modifications were analyzed within a feasibility study. Due to their well-defined structure polymer brushes represent also attractive candidates for the realization of molecularly defined cell adhesion substrates for tissue engineering or as highly defined biophysical model system to analyze cell adhesion and mechanics. Chapter 4 describes the decoration of intrinsically bioinert polymer brushes with small peptide ligands with defined density to induce integrin specific cell adhesion on the brushes. Peptide functionalized polymer brushes could be readily endothelialized and adhering cell layers have shown to preserve their homeostatic ability to respond to an applied mechanical stimulus in a physiological manner. In Chapter 5 the limitations of high density polymer brushes in biomedical applications were assessed by analyzing their stability under in vitro cell culture conditions. Intrinsically bioinert high density polymer brushes were found to degraft from glass or silicon substrates upon swelling in good solvents, which lead to deterioration of the bioinert properties of the substrate. The degrafting of the brushes might be connected to swelling induced mechanical activation of chemical bonds, which link the brushes to the substrate. Chapter 6 describes the transfer of the developed SIP approach onto flexible PDMS substrates by exploiting the formation of interpenetrating networks of ATRP-initiator siloxane and PDMS. The modification of the PDMS substrates with hydrophilic brushes effectively reduces the unspecific protein adsorption on the substrates. This approach might therefore represent an attractive and facile route for the surface modification of miniaturized PDMS devices for biomedical or bioanalytical applications. In Chapter 7 the systematic development of a reliable and robust protocol for the aqueous SI-ATRP of sodium methacrylate is presented, which gives access to poly(methacrylic acid) (PMAA) brushes with defined molecular weight and grafting density. The resulting brushes were assessed for their ability to direct the mineralization of calcium carbonate. Chapter 8 describes the use of photolithographically patterned poly(methacrylic acid) brushes as biomimetic, acidic macromolecular matrix to fabricate microstructured calcite thin films that are an exact 3D replica of the PMAA brush. The presented strategy relies on three key elements: (i) the use of photolithographic techniques to prepare microstructured PMAA brushes; (ii) the ability of PMAA brushes to stabilize amorphous calcium carbonate (ACC) and (iii) the possibility to convert the metastable ACC phase into a polycrystalline calcite film via a thermal treatment.