Topological and optoelectronic properties of stacked two-dimensional materials
Jort Verbakel is a PhD student in the Department Physics of Interfaces and Nanomaterials. (Co)Promotors are prof.dr.ir. H.J.W. Zandvliet and dr.ir. P. Bampoulis of the Faculty of Science & Technology.
Two-dimensional (2D) materials are layered materials that can be isolated to single layer forms. This confines the charge carriers in these materials, giving rise to many unusual electronic properties. However, by stacking these materials on top of each other, we alter these properties substantially. Not only can stacks combining different 2D materials be made, intentionally rotationally misaligning (twisting) the layers with respect to each other gives rise to new emergent properties. We can exploit these properties to create unconventional electronics that consume less energy, or to, paradoxically, harvest energy.
This thesis is focused on two aspects of 2D materials that can change drastically upon stacking: the topological properties and optoelectronic properties. Firstly, the topological properties can be changed due to stacking or twisting, breaking crucial symmetries of the crystal. In this work, we consider bilayer graphene, which upon twisting can undergo a topological phase transition inside the moiré superlattice, which we study using scanning tunneling microscopy. On the other hand, the optoelectronic properties of transition metal dichalcogenides (TMDCs) can also be drastically changed by stacking them. In this thesis, we show that we can measure excitons in stacked TMDCs with ultra-high resolution through photocurrent atomic force microscopy.
To create our stacked 2D materials, we follow a specific experimental procedure, described in detail in Chapter 2. We start the exfoliation process of 2D materials, then we elaborate on how to the stack these materials. We explain the procedure of picking up flakes, how to stack them into (twisted) heterostructures, and how to transfer them to a substrate of choice. We explain how to flip the stacks, such that the layers of interest are facing upwards for SPM characterization. Finally, we explain the microsoldering method to electrically connect the samples to the SPM system.
The measurement techniques used to study the topological and optoelectronic properties of the samples of stacked 2D materials are described in Chapter 3. First we introduce the concepts of atomic force microscopy (AFM) and scanning tunneling microscopy (STM). We then explain the concepts and set-ups for a back-gated STM system and the photoconductive AFM system.
In Chapter 4, we study a topological valley Hall network that emerges in bilayer graphene with a small twist angle. The twist creates domains of different stacking orders, which form a triagonal lattice. When an electric field is applied perpendicular to the graphene, the graphene develops a band gap. Furthermore, the symmetry between the AB and BA stacking domains is broken, creating a topological boundary. At this boundary, electrons from different valleys counterpropagate, and they do so dissipationlessly, as long as no intervalley scattering occurs.
We firstly explore the emergence of the network in a naturally occuring twisted bilayer in highly oriented pyrolithic graphite. Using Fourier transform STM, we prove that the topological transport inside the network is indeed valley protected.
In this chapter, we also study the network in a stacked sample of twisted bilayer graphene on hexagonal boron nitride. Using an electrostatic back gate, we perform scanning tunneling spectroscopy to study the density of states of different regions of the moiré unit cell in the twisted bilayer at different electric field strengths. We show that at sufficient electric field strengths, the bilayer graphene becomes gapped. Additionally, we show that at the domain walls, the carriers in the twisted bilayer are more massive than in the other regions of the moiré pattern.
In Chapter 5, we study the nanoscale optoelectronic properties of transition metal dichalcogenides (TMDCs). Using a photoconductive AFM, we measure the photocurrent at sub-nanometer resolution. We show that exciton peaks can be clearly resolved, even at room temperature. We then study the excitonic properties of a stacked TMDC sample, were we show that excitons from multiple layers can be measured in the photocurrent. We then dive deeper into the effects of atomic defects and flake edges on the excitonic structure of molybdenum diselenide on hBN, were we show that the excitonic response is significantly altered as compared to the pristine material.
