Recording,Decoding And Exploring The Applications Of Nanopatterns In Ferroelectric Polymer Thin Films

Ferroelectric materials are characterized by a spontaneous polarization that can be reversed by the application of an external electric field in a certain temperature range.This behavior has been well understood and exploited to fabricate nonvolatile memory devices.The coupling of polarization with mechanical deformation leads to piezoelectric properties with applications in sensors and actuators.In addition,the spontaneous polarization can build an internal electric field and induces macroscopic charges on the surface of ferroelectrics.Therefore,it is demonstrated recently the ferroelectrics has the potential in manipulating superconducting,semiconducting and conducting properties of materials.Among the various organic,inorganic and hybrid ferroelectric materials,poly(vinylidene fluoride)(PVDF)and its copolymers such as poly(vinylidene fluoride-trifluoroethylene)(P(VDF-Tr FE)),have received considerable attentions due to their excellent ferroelectric,piezoelectric and pyroelectric performance combined with unique properties of structural flexibility,low cost,good chemical resistance and easier processability.However,it is still a challenge to precisely manipulate the domain switching of P(VDF-Tr FE)at the nanometer scale and to extend the applications beyond the realm of electronic devices.In this thesis,we have developed a new strategy for patterning and manipulating the ferroelectric domains at the nanometer scale in a non-contact mode by utilizing the electron beam generated in electron beam lithography equipment.The amount and sign of charges generated by electron beam are thus recorded in the nanopatterns of P(VDF-Tr FE)thin films.Going beyond the realm of electronic devices,we found that the recorded patterns in P(VDFTr FE)thin films can be decoded by exploring the electrohydrodynamic instability of polymer thin films.In addition,we demonstrated that the nanopatterns of P(VDF-Tr FE)thin films can be used to direct the self-assembly of block copolymer thin films with a high interaction parameter ?,which is extremely difficult to achieve in other ways.Therefore,the main results of the thesis are divided to three parts.In the first part,we demonstrate the patterning and manipulating the ferroelectric domains at the nanometer scale in a non-contact mode by utilizing the electron beam.Local accumulation of charges generates a built-in electric field which can remotely induce ferroelectric domain switching and patterning in P(VDF-Tr FE)thin films at the nanometer scale within microseconds.As the amount and sign of charges depend of writing dose,both upward and downward polarizations can be obtained by simply changing the dose or the dwell time on the electron beam.Our method allows us to manipulate domains in ferroelectric thin films at the nanometer scale without potentially-damaging physical contact with an electrode or tip,and is therefore ideal for the fabrication of patterned ferroelectric devices.In addition,the measurements of piezoresponse support the notion that the domain switching in P(VDF-Tr FE)proceeds by 60 o axial rotation steps of the chains and the feasibility of multi-bit data storage in ferroelectric memories.In order to decode the patterns in P(VDF-Tr FE)thin films,a thin layer of polymers such as poly(2-vinylpyridine),polystyrene,and polymethylmethacrylate is coated onto the surface of patterned P(VDF-Tr FE)thin films.By triggering the instability of polymer thin films via dewetting,a large area of periodic polymer nanostructure arrays can be constructed.Therefore,the two-dimensional patterns in P(VDF-Tr FE)thin films are transferred to threedimensional patterns composed of another polymer.This is achieved by exploring the electrohydrodynamic instability(EHD)and dewetting behavior of polymer thin films.The results indicate that a lateral gradient electrostatic stress on the patterning surface is generated due to a potential difference between the unpolarized and polarized areas,making the surface of polymer film more unstable and driving molecular chain flowing from the unpolarized to the polarized region during annealing.In addition,the presence of local electric field reduces the surface tension between P(VDF-Tr FE)and polymer compared to the unpolarized region,making the polymer more prone to wet in the polarized region.We have further extended the applications of ferroelectric P(VDF-Tr FE)thin films to direct the self-assembly of block copolymer.The self-assembly of block copolymer is a promising complementary approach to circumvent the feature size limits of conventional photolithography.However,the self-assembly of block copolymers requires the control of interfacial properties on both interfaces of a block copolymer film to induce the formation of domains with a perpendicular orientation and long-range ordered parallel orientation.Orienting such materials with a high interaction parameter(?),however,is extremely challenging due to their disparate polarities.Here,we demonstrate that P(VDF-Tr FE)thin films with nanopatterns provide an effective,simple method that can solve both of these problems at the same time.The local electric field generated by remanent polarization induced the formation of perpendicularly oriented microdomain of BCP film.The accurate orientation control of BCP film can be realized by EBL to fabricate nanoscale arbitrary lateral patterns down to two or even single column BCP cylinder pattern.Finally,the periodic ordered or hierarchical structure of polymer materials at different scales can be realized.