Optoelectronics Properties Of Organic/Inorganic Heterostructures

Two-dimensional transition metal dichalcogenides(2D TMD)and the related Van der Waals Heterostructures(vdWHs)have provided an excellent platform to research the light-matter interactions.Introducing organic counterparts could modulate the light-matter interactions in TMD,enabling the intriguing optoelectronics,such as diodes,photodiodes,bipolar transistors,LEDs and solar cells,etc.Currently,the integrations of organic materials and TMDs have been reported,whereas the organic counterparts are usually above 20 nm,which do not fall within the two-dimensional limit.Two-dimensional organic materials usually possess an ultraclean interface and high excitonic states with high binding energies.They could interact with excitonic states of 2D TMD,enabling a number of interesting applications.In addition,the arrangement of organic molecules is hard to be precisely controlled,nevertheless,the aggregation and arrangement of organic materials influence the optoelectronic features of devices significantly.Furthermore,a recent report has shown that 2D high-quality organic crystals could present a highly oriented molecular arrangement,and the organic materials’transport properties are demonstrated to be layer-dependent,allowing us to fabricate the atomically thin O-I heterostructures and explore how the molecular aggregation affects light-matter interactions in the hybrid system.In this study,we firstly fabricate atomically thin organic-inorganic heterostructures and research interactions of light-matter in the hybrid system at the two-dimensional system.The paper demonstrates the growth of 2D organic crystals,the corresponding fabrication and characterization process of atomically thin O-I heterostructures and the study of many-body effects,etc.The concrete content is as follows:Chapter 1:We mainly describe the basis of O-I vdWHs,which includes the classifications,fabrications,optoelectronics properties and related exciton physics of 2D organic materials,TMD and correlated vdWHs respectively.Chapter 2:The experimental methods are introduced here.We present the growth method of organic crystals and show the fabrication of 2D TMD.Also,experimental tools which are used in our study have been presented.Chapter 3:Firstly,the 2D organic crystals are grown on h-BN via physical vapor deposition(PVD)and corresponding optoelectronic properties have been investigated using Photoluminescence(PL)and Raman.2D pentacene film shows layer-dependent optical properties,where the wetting layer(WL)pentacene gievs out the strongest PL intensity.We ascribed this to layer-dependent charge transport properties in pentacene.In addition,we employed the dry-transfer method to fabricate atomically thin O-I heterostructures,consisting of a monolayer MoSe2 and a monolayer/few layers pentacene.PL measurements demonstrate that the three heterostructures have a PL enhancement at the peak position of MoSe2 monolayer and the corresponding PL enhancement factor decreases with the increase of the pentacene thickness.This is due to the layer-dependent charge transport mechanism and quantum yield of 2D pentacene.Furthermore,photoluminescence excitation(PLE)verifies that the heterostructures are Type Ⅰ,which is different from the reported type Ⅱ band alignment.Chapter 4:We explore the high efficiency of the exciton pumping and modulate many-body effects in the hybrid system.When we compared the various heterostructures,the exciton pumping efficiency of 1L MoSe2+ WL pentacene,1L MoSe2 + 1L pentacene,and 1L MoSe2 + 2L pentacene heterostructures are 86.4,20.4,and 3.2 respectively,attributed to the ultrafast energy transfer in O-1 interface and the high quantum yield of pentacene.Additionally,PL spectra of 1L MoSe2 + Bulk pentacene show the lowest efficiency of the exciton pumping as compared to other heterostructures,which reveals that the high exciton pumping effects are only prominent at two dimensional and interface effects play an important role in the O-1 hybrid heterostructures.Moreover,the varying temperature and gate-dependent PL measurements are conducted to investigate how the hybridization with organic materials modulates the trion binding energy of TMD.As compared to monolayer MoSe2,the heterostructures show much higher trion binding energy and this is because of the smaller dielectric constant of pentacene.Among the three kinds of heterostructures,the thicker the pentacene,the higher the trion binding energy,where the trion binding energy of 1L MoSe2 + 2L pentacene heterostructure shows an increase of 4.9 meV as compared to monolayer MoSe2,due to the weakening of shielding effects induced by the thicker pentacene.Our results provide new references and a new way for exploring the fundamental physics phenomenon as well as enabling the new optoelectronic application of the ultrathin O-I heterostructures based devices.Chapter 5:We summarize this work and discuss the challenges to exploit the potential of 2D O-I based vdWHs.Additionally,some research directions are presented as well.