Study On The Ferroelectricity And Optoelectronic Properties Of Two-Dimensional Layered Van Der Waals Materials

Ferroelectrics are materials with spontaneous polarization and the polarization direction can be reversed under an external electric field.Its application fields are very wide,such as ferroelectric tunnel junctions,ferroelectric field effect transistors and ferroelectric capacitors and other modern electronic devices.However,due to the lattice mismatch and critical size effect of traditional bulk three-dimensional ferroelectrics,the ferroelectricity in the limit thickness is still rare,which brings challenges to the study of low-dimensional ferroelectricity and the preparation of ultra-thin ferroelectric devices.In recent years,the emergence of layered van der Waals two-dimensional materials has brought the possibility of exploring this field.Benefiting from the naturally layered structure and saturated interface chemical environment of the two-dimensional material,a single layer can exist stably and has a weak interaction with the substrate.The research on the ferroelectric properties in the two-dimensional materials could break through the bottleneck of conventional ferroelectrics,and then develop ultra-thin all two-dimensional ferroelectric devices for high density data storage.Besides,the two-dimensional ferroelectrics have natural inversion center symmetry breaking characteristics and atomic layer thickness,which provides an ideal platform for developing photovoltaic devices whose power conversion efficiency is not limited by the Shockley-Queisser limit.At present,a variety of two-dimensional ferroelectric materials have been experimentally confirmed,among them,the two-dimensional ferroelectric ?-In2Se3 and CuInP2S6 are representatives.For these reason,our thesis is based on the two-dimensional ferroelectric system,we carry out the research of two-dimensional ferroelectricity and heterostructure properties modulation,explore the two-dimensional layered van der Waals ferroelectric materials in the potential applications of non-volatile memory,the novel type of external electric field control and bulk photovoltaic effect.In chapter 1,we first summarize and review the history and progress of two-dimensional materials in piezoelectricity and ferroelectricity,as well as the key breakthroughs in the corresponding device applications.Among them,the ferroelectricity of two-dimensional materials is emphatically introduced.Finally,we introduce the generation mechanism and research progress of the photovoltaic characteristics in polar materials.In chapter 2,we introduce the commonly used layered material preparation and characterization methods in the experiment,including the chemical vapor deposition,mechanical exfoliation method,the optimized plasma-assisted peeling method and ionic liquid intercalation method and all-dry heterostructure stacking process.Raman spectrum is commonly used for sample characterization,we also introduce the mechanism of characteristic peaks in graphene,and the dependence of Raman spectrum on material thickness and carrier concentration.In Chapter 3,we carry out the verification of out-of-plane ferroelectricity in ?-In2Se3 through PFM and ferroelectric field effect transistors.Through the observation of spontaneous ferroelectric domains and the construction of artificial ferroelectric domains,we verify the out-of-plane ferroelectricity in the few layers of ?-In2Se3,and construct an all two-dimensional materials based non-volatile memory.The polarization density of two-dimensional ?-In2Se3 is estimated based on the dual-capacitance model,and the two stable polarization states in the ferroelectric are used to demonstrate the non-volatile memory characteristics.In Chapter 4,we explore the unique in-plane and out-of-plane ferroelectric coupling effect in ?-In2Se3 predicted by the theoretical prediction,and demonstrate experimentally the in-plane electric field control of the out-of-plane field effect for the first time.Starting from the crystal structure of ?-In2Se3,we analyze the mechanism of the in-plane and out-of-plane ferroelectric polarization coupling.By a piezoelectric force microscope,we observe the locking effect of the in-plane and out-of-plane ferroelectric domains in the system and verified the in-plane and out-of-plane ferroelectric polarization coupling characteristics of ?-In2Se3 by the external electric field.Based on the unique in-plane and out-of-plane coupling ferroelectricity of ?-In2Se3,we construct an orthogonally controlled ferroelectric gate transistor with ?-In2Se3 as the planar gate,which effectively modulates the channel conduction of the transistor through the in-plane electric field.In Chapter 5,we study the bulk photovoltaic effect in two-dimensional ferroelectric materials and propose that the material dimension has a decisive influence on the photovoltaic effect.We adopt the two-dimensional ferroelectric CuInP2S6 as the optical absorption layer and graphene as the transparent electrode to construct a two-dimensional bulk photovoltaic device with a vertical capacitor structure.We observe that under the excitation of photon energy larger than the band gap of CuInP2S6,the device has a spontaneous non-zero bulk photovoltaic current,the photovoltaic current and voltage polarity are significantly modulated by the ferroelectric polarization,and shows obvious hysteresis loop with the external electric field.Compared with the results obtained in traditional bulk materials,the photocurrent density based on the bulk photovoltaic effect of the two-dimensional ferroelectric CuInP2S6 is improved by two orders of magnitude.In Chapter 6,we summarize and prospect the work of this thesis.