Paper 2D+EM+IS+MC+NS+SP+SS-WeA7
Defects Compensation and Refining Optical Luminescence in Organic/Transition Metal Dichalcogenide Heterostructure

Wednesday, October 21, 2015, 4:20 pm, Room 212C

Since layered transition-metal dichalcogenides(TMD) have demonstrated novel electronic and optoelectronic property, intense research has focused synthesis and integration into future electronic devices. Unlike graphene, TMD materials have band gaps, and these band structures can be tuned by thickness. However, in many cases, unintentional defects can be observed on TMD giving rise to the degradation of performance in the devices. Even for mechanical exfoliated TMD, there is a high density of defects, such as vacancies. For successful integration of TMD into devices, proper passivation of defects on TMD requires high stability in ambient conditions. In this study, a TiOPc monolayer was employed for passivation of defects to improve electrical and optical properties in TMD devices. Multilayer MoS₂ flakes were cleaved in ambient condition and transferred into the UHV chamber; afterwards. TiOPc monolayers were deposited on the MoS₂ surfaces by organic molecular beam epitaxy. After deposition, TiOPc forms a monolayer with only few defects, and the TiOPc monolayer structure has square lattice in a 1.5x1.5 nm grid. This crystal structure indicates that each TiOPc in the monolayer is directed outward to vacuum. The deposited TiOPc layer has very high thermal stability on MoS₂; the TiOPc layer on MoS₂ requires annealing above of 673K for desorption. This high thermal stability indicates there are strong interaction between TiOPc and MoS₂ surface. STS shows the band gap of the monolayer is 1.8 eV, while bulk MoS₂ has a 1.3eV band gap. Moreover, the Fermi level of TiOPc/bulk MoS₂ is shifted to the valence band, consistent with a P type shift. However, bulk MoS₂ surface, where less than monolayer of TiOPc was deposited, has Fermi level shifted towards the conduction band, consistent with N type doping. In the single layer MoS₂ deposited TiOPc monolayer, threshold bias is shifted from -30 V to near O V, indicating P-doping of MoS₂. It can be hypothesized that the work function transition of MoS2 is changed as a function of thickness. Before deposition of the TiOPc monolayer, the defects peak corresponded to S vacancy is displayed at 1.7 eV in photoluminescence. Conversely, the deposition of TiOPc monolayer almost completely suppresses S vacancy peak located 1.7 eV. Moreover, in the single layer MoS₂ FET, the on/off ratio is enhanced more than 2 orders magnitude. The similar charge transfer behavior also can be observed in TiOPc/WSe₂; on the bilayer WSe₂/HOPG, the TiOPc monolayer deposited on the first layer of WSe₂ shows the a conduction band shifted Fermi level, while a TiOPc monolayer deposited on the second layer of WSe₂ shows a valence band shifted Fermi level.