Ultrafast spectroscopic studies of two dimensional nanomaterials

Abstract

When the world goes flat, we see through it differently. The key to reveal the secrets of low dimensional world lies within one class of newly emerged material – Two dimensional (2D) nanomaterials. During past few years, two dimensional nanomaterials have drawn great attention for its simple physical model in theoretical research and unique properties in practical application. In this thesis, we concentrate on the optical properties related to the ultrafast dynamics of three typical families of 2D nanomaterials: graphene, transition metal dichalcogenides (MoS2, WSe2). A comprehensive investigation on the ultrafast dynamics studies of these 2D nanomaterials is conducted through a combination of several ultrafast spectroscopy techniques, such as time resolved photoluminescence (TRPL), transient absorption spectroscopy (TAS) as well as supplementary material characterizations including Raman Spectroscopy, x-ray photoemission (XPS), scanning electron microscopy (SEM) and etc.

Graphene, as the most classic 2D nanomaterial, possesses unique conical band structure and exceptional optical properties. Presently, high quality large area graphene which is widely used as the flexible transparent electrode, has been successfully prepared through chemical vapor deposition (CVD). Revealing the charge carrier dynamics of this CVD graphene is crucial for its related optoelectronic device. The application of CVD graphene requires the polymer-assisted substrate transfer process, the possible polymer residual macromolecule interaction with the graphene surface address this issue great research significance, the understanding of the properties in this polymer-grafted graphene region is far and few between. There are three processes of the carrier relaxation dynamics in graphene after initial excitation mainly consists of three processes; electron-electron scattering (< 10 fs), electron-optical phonon coupling (500 fs ~ 2 ps), optical phonon and acoustic phonon coupling in sequence. A systematical investigation was performed to clarify the carrier relaxation channel modulation in polymer-grafted region compared to pristine graphene. Meanwhile, the obvious features of Auger recombination and hot phonon bottleneck effect were also observed in the polymer-grafted graphene.

Another important family of 2D nanomaterials is transition metal dichalcogenides (TMD). The monolayer MX2 TMD obtains binary hexagonal lattice and intrinsic optical bandgap ranging from the near infrared to visible. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron–hole separation for light detection and harvesting. Regarding all possible combination of 2D crystals, MoS2 and WeSe2 van der Waals heterosturctures is the only type II p-n junctions, spatial temporal dynamics studies was conducted on this heterostructures, and we observe that hole transfer from the MoS2 layer to WSe2 layer occurs with 100 fs after the optical excitation. Such ultrafast charge transfer in van der Waals can enable advanced 2D devices requiring fast processing.

CH3NH3PbI3 is one of the most extensively studied materials due to its strong optical absorption and long carrier diffusion length. Integrating the strengths of 2D nanomaterial and CH3NH3PbI3 on device designing is one promising field for the future development of photovoltaics. We achieve to tune the band gap offset of MoS2 flakes through mild oxygen plasma treatment to form a type II interface in 2D MoS2/CH3NH3PbI3 hetero-structure. The ultrafast hole transfer process is observed in the heterostructure interface which suggest that vacancy tuned MoS2 a perfect hole transporting layer for the perovskite-based solar cells.

In summary, our spectroscopic studies of these 2D nanomaterials provide valuable information on relaxation dynamics analysis and pave the way for further applications of novel 2D nanomaterials based device for optoelectronic and light harvesting.