Two dimensional materials are a promising candidate for future electronic, (bio)sensing, photonic and even energy applications. The list of available 2D materials is very long and include transition metal dichalcogenides (e.g. MoS2, MoSe2, WS2...), graphene, phosphorene, h-BN... The range of interesting physical properties of these 2D materials is extended and includes high thermal conductivity, flexibility, tunable light absorption, superconductivity, high mobility... All these properties are combined with the ultimate thickness control that can be achieved with these 2D materials. As a result, van der Waals (hetero)structures are very exciting for future electronic applications.
It is clear that a low 2D material defect density is required for the development of electronic devices. The best synthetic 2D material quality is currently obtained during high temperatures growth processes (>>700 oC). This high growth temperatures makes it practically impossible to directly deposit the 2D material at the desired location in a device. As a result, the transfer of these 2D materials seems unavoidable, while the knowledge to perform such a transfer process is currently very poor. A typical transfer process consists of the delamination of the 2D material from its growth substrate, followed by the lamination on a desired target substrate. As these 2D materials are only van der Waals bonded, it is immediately clear that such a pick and place process is already very challenging on its own. To complicate the transfer process further, the ultimate thickness scaling of these 2D materials makes them strongly influenced by their surroundings. Some of the materials are not only sensitive to doping by surrounding molecules, but are even very vulnerable to oxidation. This makes control over both top and bottom interface extremely important during and after transfer. Furthermore, the flexible nature of these materials make stress/strain control very challenging. On the contrary, one could use the sensitivity of these materials to dope them in a controlled way or even change its properties by varying the strain of the layer. In order to obtain this level of control, one has to gain much more fundamental insights in the behavior of these 2D materials when handling and laminating them on different materials.
It is very clear that different challenges lie ahead to achieve a successful transfer. For example, one needs to understand how adhesion forces can be tuned to achieve a successful 2D material delamination from the growth substrate, but also a successful lamination on a target substrate. Since everything occurs at interfaces, experimental research is challenging and state-of-the-art characterization techniques will have to be used to achieve the necessary level of understanding. Heterogeneous 2D material stacking might be a possible way forward to control interfaces, but also the use of self-assembled monolayers or even other out-of-the-box routes can lead to the desired level of transfer control. Electrical device characterization will be the final proof of the achieved level of interface and strain control.
Type of work: 10% Literature study, 60% experimental work, 30% data analysis
Supervisor: Stefan De Gendt
Daily advisors: Steven Brems, Alain Phommahaxay
The reference code for this PhD position is STS1712-43. Mention this reference code on your application form.