Solution-processed 2D Ultrathin Molecular Semiconductors And Ferroelectric Films: Material Preparation,Property Exploration,and Transistor Applications

As an emerging electronic material,2D ultrathin functional organic crystals(2DOCs)have attracted particular attention since their discovery.The material structure of 2DOC features a monolayer or few layers of molecules that are assembled via weak van der Waals bonding along two dimensions.2DOCs have also demonstrated unique characteristics of thickness uniformity over a large scale,perfect lateral continuity with an atomically flat surface,and long-range molecular ordering.Thus,2DOCs are extremely fascinating for the direct exploration of the charge accumulation and transport behaviors regarding with the properties of the semiconductor/insulator interface,disorder effects,and polaronic relaxation;and 2DOCs also possess unique optoelectronic properties that cannot be achieved in conventional bulk molecular crystals.Additionally,ultrathin ferroelectric crystalline films can demonstrate outstanding film properties at a quasi 2D limit,thus providing a promising avenue for the scalability and stability of 2DOC-based electronic devices.Besides,utilizing their ultrathin structural features,superior interface qualities,combined with their intrinsic properties,such as lightweight construction,material versatility,and chemical/environment stability,explicitly enable these 2D crystals fascinating prospects for realistic applications in advanced electronic technologies.The research of this thesis can be summarized as follows:(1)First of all,we propose a new strategy,named“floating-coffee-ring-driven assembly”,for the fabrication of 2D molecular semiconducting films on the Si O₂/Si substrates.The highly uniform and layer-controllable 2D C₈-BTBT films are particularly composed of single-crystalline phase.The charge carrier mobility based on bilayer C₈-BTBT films can reach up to 13.0 cm²/Vs,which is comparable to that of polycrystalline silicon.Thus,our solution-grown 2D single crystalline molecular semiconductors are greatly favourable for low-cost,high-performance organic electronic devices and circuits.Additionally,we also studied the interfacial charge-transport physics,finding that the Coulomb interactions play a vital role on the interfacial carrier transport at a relatively high carrier density level.Besides,we also constructed an optically-modulated organic Schottky-barrier planar diode for the realization of outstanding optoelectronic synaptic behaviours.(2)Next,we used an antisolvent-assisted crystallization to prepare the ultrathin ferroelectric thin films.The ferroelectric crystalline films are extremely smooth,neat and uniform.Then,we exploited atomic force microscopy(AFM),Raman spectroscopy,GIXRD,highly-sensitive piezoresponse force microscopy(PFM),and conducting AFM for the examination of film properties,such as film morphology,crystalline conformation,lattice orientation,dipole alignment,piezoelectric activity,and polarization relaxation.Especially,on applying a 40?annealing treatment,the optimum film properties are achieved owing to a high degree of polymer chains oriented parallel to the substrate plane.Our results can thus reveal a promising avenue for nano-electro-mechanical and nano-ferroelectric electronic applications using ultrathin P(VDF–Tr FE)films.(3)In modern society,the information storage technology is undergoing a rapid development with a great increase of operation speed and integration density of the functional devices.Recently,organic transistor-type nonvolatile ferroelectric memories have attracted an increased attention because of their inherent merits of simplicity in low-temperature solution processing,high speed programming/reading behaviors,non-destructive readout,and long-term device lifetime.In particular,obtaining high-performance,low power consumption,and highly reliable ferroelectric memories toward smart and portable systems is believed to be a major roadmap for advanced electronic technologies.As an emerging electronic material,2D molecular semiconducting crystals(2DMCs)can serve as an ideal platform to directly probe the molecular structure dependent charge transport behaviors and contact characteristics.Besides,ultrathin ferroelectric crystalline polymers with considerable film properties are preferably favorable for superior interface qualities and low-voltage operation.In this project,our objective is to develop ultra-low power transistor-type memories based on 2DMCs and ultrathin ferroelectric polymeric films.Under a field-effect transistor device structure,we will study the microscopic behaviors of charge carriers modulated by ferroelectric polarization.Furthermore,the investigation and optimization of device characteristics(such as charge carrier mobility,contact resistance,and power consumption,etc.)will be realized by process and interface engineering.And then,the relationship between the ferroelectric negative capacitance effect and device operation features will also be explored.Besides,other low-power ferroelectric field-effect devices will be developed based on our research basis.Our results will be greatly beneficial to satisfy the fundamental studies and sustain the revolutionary advancement of organic electronics,ferroelectric materials science,and nanoelectronic devices.