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Optical micro-spectroscopy is an invaluable tool for studying and characterizing samples ranging from classical semiconductors to low-dimensional materials and heterostructures. To date, most implementations are based on point-scanning techniques, which are flexible and reliable, but slow. Here, we describe a setup for highly parallel acquisition of hyperspectral reflection and photoluminescence microscope images using a push-broom technique. Spatial as well as spectral distortions are characterized and their digital corrections are presented. We demonstrate close-to diffraction-limited spatial imaging performance and a spectral resolution limited by the spectrograph. The capabilities of the setup are demonstrated by recording a hyperspectral photoluminescence map of a CVD-grown MoSe$_2$-WSe$_2$ lateral heterostructure, from which we extract the luminescence energies, intensities and peak widths across the interface.

The interface between two different semiconductors is crucial in determining the electronic properties at the heterojunction, therefore novel techniques that can probe these regions are of particular interest. Recently it has been shown that heterojunctions of two-dimensional transition metal dichalcogenides have sharp and epitaxial interfaces that can be used to the next generation of flexible and on chip optoelectronic devices. Here, we show that second harmonic generation (SHG) can be used as an optical tool to reveal these atomically sharp interfaces in different lateral heterostructures. We observed an enhancement of the SH intensity at the heterojunctions, and showed that is due to a coherent superposition of the SH emission from each material. This constructive interference pattern reveals a phase difference arising from the distinct second-order susceptibilities of both materials at the interface. Our results demonstrate that SHG microscopy is a sensitive characterization technique to unveil nanometric features in layered materials and their heterostructures.

The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS$_2$--WS$_2$ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days, and informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS$_2$ domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state., Comment: 27 pages, 5 figures

Two-dimensional lateral heterojunctions are basic components for low-power and flexible optoelectronics. In contrast to monolayers, devices based on few-layer lateral heterostructures could offer superior performance due to their lower susceptibility to environmental conditions. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS2-WS2 and MoSe2-WSe2, where the hetero-junctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layer. With respect to their monolayer counterparts, bilayer lateral heterostructures yield nearly one order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ~103 times larger than those extracted from the monolayers, in addition to room-temperature electroluminescence. The superior performance of bilayer heterostructures significantly expands the functionalities of 2D crystals.

Laser-assisted chemical modification is demonstrated on ultra-thin transition-metal dichalcogenides (TMDs), locally replacing selenium by sulfur atoms. 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 has been further confirmed via transmission electron microscopy as well as Raman and Photoluminescence spatial maps. Our study demonstrates the potential of laser-assisted chemical conversion for on-demand synthesis of heterogeneous two-dimensional materials with applications in nanodevices.

Near-field optical microscopy can be used as a viable route to understand the nanoscale material properties below the diffraction limit. On the other hand, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photon, and phonons. Heterostructures based on Mo or W based monolayer TMDs form type-II band alignment, and hence the optically excited carriers can be easily separated for applications pertaining to photonics and electronics. Mapping these spatially confined carriers or photons in a heterostructure with nanoscale resolution as well as their recombination behavior at the interfaces are necessary for the effective use of these materials in future high performance optoelectronics. We performed tip-enhanced photoluminescence (TEPL) imaging to increase the spatial resolution on multi-junction monolayer MoSe2-WSe2 lateral heterostructures grown by chemical vapor deposition (CVD) method. The near-field nano-PL emission map was used to distinguish the presence of distinct crystalline boundaries and the heterogeneities across the interfaces. This method significantly improves the nanoscale resolution of 2D materials, especially for understanding the PL emission properties at the vicinity of hetero-interfaces.

Two-dimensional monolayer transition metal dichalcogenides (TMDs) have unique optical and electronic properties for applications pertaining to field effect transistors, light emitting diodes, photodetectors, and solar cells. Vertical interfacing of WS2 and MoS2 layered materials in combination with other families of 2D materials were previously reported. On the other hand, lateral heterostructures are particularly promising for the spatial confinement of charged carriers, excitons and phonons within an atomically-thin layer. In the lateral geometry, the quality of the interface in terms of the crystallinity and optical properties is of paramount importance. Using plasmonic near-field tip-enhanced technology, we investigated the detailed nanoscale photoluminescence (nano-PL) characteristics of the hetero-interface in a monolayer WS2-MoS2 lateral heterostructure. Focusing the laser excitation spot at the apex of a plasmonic tip improved the PL spatial resolution by an order of magnitude compared to the conventional far-field PL. Nano-PL spatial line profiles were found to be more pronounced and enhanced at the interfaces. By analyzing the spectral signals of the heterojunctions, we obtained a better understanding of these direct band gap layered semiconductors, which may help to design next-generation smart optoelectronic devices.

Two-dimensional (2D) heterojunctions display a remarkable potential for application in high performance, low power electro-optical systems. p-n junctions based on vertically stacked heterostructures have shown very promising performance as tunneling transistors, light emitting devices and photodetectors, and as photovoltaic cells . Although complex vertical heterostructures were fabricated via van der Waals stacking of different 2D materials, atomically sharp multi-junctions in lateral heterostructures is a quite challenging task, but a viable route towards the development of commercial applications. Previously reported approaches to obtain single-junction lateral heterostructures of the type MoX2-WX2 (X= S and/or Se), involve either a single-step or a two-step growth process. However, these methods lack the flexibility to control the lateral width of the TMD domain as well as its composition. Here, we report a simple and yet scalable synthesis approach for the fabrication of lateral multi-junction heterostructures based on the combination of different TMD monolayers [MoX2-WX2 (X2 = S2, Se2 or SSe)]. Atomically sharp lateral junctions are sequentially synthesized from solid precursors by changing only the reactive gas environment in the presence of water vapor. This allows to selectively control the water-induced oxidation, volatilization and hence the relative amount of a specific metal oxide vapor, leading to the selective edge-epitaxial growth of either MoX2 or WX2. Spatially dependent photoluminescence and atomic-resolution images confirm the high crystallinity of the monolayers and the seamless lateral connectivity between the different TMD domains. These findings could be extended to other families of 2D materials, and creates the foundation towards the development of complex and atomically thin in-plane super-lattices, devices and integrated circuits.