Atomically thin layers are known as two-dimensional (2D) materials and have attracted a growing attention due to their great potential as building blocks for a future generation of low-power and flexible 2D optoelectronic devices. Similar to the well-established 3D electronics, the development of functional 2D devices will depend on our ability to fabricate heterostructures and junctions where the optical and electronic properties of different compounds are brought together to create new functionalities. Vertical heterostructures can be produced by selective van der Waals stacking of different monolayers with distinct chemical composition. However, in-plane lateral heterostructures, where different materials are combined within a single 2D layer, have proven to be more challenging. During the formation of the hetero-junction, it is important to minimize the incorporation of undesired impurities and the formation of crystal defects at the junction that will impact the functionality of the 2D device. When fabricating periodic structures, it is equally important to develop the ability of controlling the domain size of each material.
In this talk, we will review different techniques that have been used to create 2D lateral heterostructures of transition metal dichalcogenide compounds. Emphasis will be made in two synthesis approaches developed in our group. The first, a one-pot modified CVD, utilizes a single heterogeneous solid source, for the continuous fabrication of lateral multi-junction heterostructures of TMD monolayers. In this method, the heterojunctions are sequentially created by only changing the composition of the reactive gas environment in the presence of water vapor. This allows to selectively control the water-induced oxidation and volatilization of each transition metal precursors, as well as its nucleation on the substrate, leading to sequential edge-epitaxy of distinct TMDs. This simple method have proven to be effective for continuous growth of TMD-based multi-junction lateral heterostructures, including selenides, sulfides and ternary alloys. Basic devices with field effect transistor configuration were fabricated to study the electrical behavior of these heterojunctions, their diode-like response, photo-response as a function of laser power as well as photovoltaic behavior of the heterojunctions will be discussed.
The second approach consists in a laser-assisted chemical modification of ultra-thin TMDs, locally replacing selenium by sulfur atoms that allows the local tuning of the physical properties. The photo-conversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real-time Raman and photoluminescence spectroscopies. The spatially localized photo-conversion resulted in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This was confirmed via transmission electron microscopy as well as Raman and Photoluminescence spatial maps. Additionally, we also applied this method for post-growth local electronic doping, where small amounts of chalcogen atoms are replaced by nitrogen increasing the hole concentration and hence the p-type doping.