Two-dimensional layered semiconductors, for example, the monolayer transition metal dichalcogenides, serve as excellent platforms to study fundamental properties of exciton complexes, which are electron-hole complexes bound through Coulomb interaction [1,2]. In particular, these layers exhibit strong luminescence in spite of being ultra-thin (~0.7 nm) due to high radiative decay rate of the excitons  – making them promising candidates for optoelectronic applications. One can make atomically sharp vertical van der Waals heterojunctions of these layers without worrying about the lattice mismatch between two successive layers – a unique property which is difficult to achieve with conventional semiconductors. Using such heterojunction, we can make vertical devices with atomic length scales, which open up a new regime of atomic devices. In one such example, we demonstrated a 15-fold enhancement in the luminescence intensity from a monolayer through non-radiative resonant energy transfer across such vertical heterojunction . In another work, we demonstrated a coherent formation of a trion – a charged exciton, followed by its electrical detection through ultra-fast vertical transport, opening up a different era of devices called trionics . Recently, we discovered a surprisingly strong luminescence from a layered metal, and were able to combine both electrical conductivity and luminescence properties in a variety of electronic and optoelectronic applications . We have shown the usefulness of such vertical heterojunctions in different applications, for example, highly sensitive, fast photodetectors [5-7], and gate controlled electronic switching in charge density wave devices .
G. Gupta and K. Majumdar, “Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides,” Physical Review B, 99, 085412, 2019.
S. Das, G. Gupta, and K. Majumdar, “Layer degree of freedom for excitons in transition metal dichalcogenides,” Physical Review B, 99, 165411, 2019.
M. Dandu, R. Biswas, S. Das, S. Kallatt, S. Chatterjee, M. Mahajan, V. Raghunathan, and K. Majumdar, “Strong Single- and Two-Photon Luminescence Enhancement by Non-Radiative Energy Transfer across Layered Heterostructure,” ACS Nano, 13, 4795, 2019.
S. Kallatt, S. Das, S. Chatterjee, and K. Majumdar, “Inter-layer charge transport controlled by exciton-trion coherent coupling,” npj 2D Materials and Applications, 3, Article number 15, 2019.
M. Mahajan, S. Kallatt, M. Dandu, N. Sharma, S. Gupta, and K. Majumdar, “Light emission from the layered metal 2H-TaSe2 and its potential applications,” Communications Physics, 2, Article number 88, 2019.
K. Murali, N. Abraham, S. Das, S. Kallatt, and K. Majumdar, “Highly Sensitive, Fast Graphene Photodetector with Responsivity >106 A/W Using Floating Quantum Well Gate,” ACS Applied Materials & Interfaces, 2019 (DOI: 10.1021/acsami.9b06835).
K. Murali and K. Majumdar, “Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction,” Invited paper, IEEE Transactions on Electron Devices, Special Issue on 2D Materials for Electronic, Optoelectronic and Sensor Devices, 65, 4141-4148, 2018.
M. Mahajan, K. Murali, N. Kawatra, and K. Majumdar, “Gate-controlled large resistance switching driven by charge-density wave in 1T-TaS2/2H-MoS2 heterojunction,” Physical Review Applied, 11, 024031, 2019.