The Optical Property Of Cellulose Nanocrystal-based Composite Films

Cellulose nanocrystals can be extracted from the nature source,which is a kind of renewable bio-nanomaterials and exhibiting the advantages of high mechanical strength,high degree of crystallinity and biocompatibility.To the surprise of the scientists,cellulose nanocrystals can form chiral nematic liquid crystal materials by evaporation induced self-assembly.Since the year of 2010,the research of cellulose nanocrystal has developed greatly,including?1?different kinds of inorganic and organic chiral nematic materials by using cellulose nanocrystal as template,?2?host-guest method,such as the loading of gold or silver nanoparticles into cellulose nanocrystal films or the mesopores of inorganic or organic chiral nematic liquid crystal materials,to study the physical and chemical properties.?3?the application of cellulose-based chiral photonic crystal films,such as the field of sensing ability,asymmetric catalysis and chiral separation.However,the research of the modulation of chiral nematic cellulose?CNC?liquid crystal to the spontaneous emission of upconversion nanoparticles is rare.Cellulose nanocrystal is able to form left-handed chiral nematic liquid crystal.The characteristic structure can selectively reflect certain wavelength of left-handed circularly polarized light and transmit right-handed circularly polarized light.And the wavelength can be tuned by physical and chemical methods in the range from near violet to near infrared.The smart ability can be applicable in the field of sensing and display,as well as anti-counterfeiting.On this background,the text has explored the modulation of cellulose nanocrystal film as one-dimensional photonic crystal to the spontaneous emission of upconversion nanoparticles and the circularly polarized potential of chiral nematic mesoporous silica using cellulose nanocrystal as template.It's the first time to reveal the intrinsic circularly polarized ability of chiral nematic mesoporous silica film and its property to transform the emission of guest luminophores into right-handed circularly polarized luminescence.We propose and prove that the photonic bandgap,the luminescence intensity of the guest molecules,excitation wavelength,the thickness of chiral nematic mesoporous silica and the loading amount of luminophores have great influence on the circularly polarized luminescence.The intrinsic circularly polarized ability of free-standing chiral nematic mesoporous silica films has promsing application in the field of anti-counterfeit,chiral separation and chiral switch.By host-guest method,we design and build cellulose nanocrystal-based and silica-based nanocomposites,which provides new thought to obtain chirality controllable,wavelength and intensity tunable circularly polarized luminescence materials.This thesis includes five chapters.The first chapter is introduction,mainly representing the unique chemical and physical properties of cellulose nanocrystals,the current research and also the intention of this thesis.The second chapter is around CNC as one-dimensional photonic crystal.By adding upconversion nanoparticles NaYF₄:Yb,Er into cellulose suspension,the free-standing CNC composite films loaded with upconversion nanoparticles are formed through evaporation induced co-assembly.We explore the modulation of the photonic bandgap of CNC to the spontaneous emission of upconversion nanoparticles.The result shows when the luminescence peak is in the photonic bandgap of cellulose nanocrystal,the spontaneous emission is suppressed,and when the emission is at the photonic bandedge,the spontanenous emission is enhanced.The third chapter is exploring the optical property of the chiral nematic mesoporous silica-polymer dot composite film templated from cellulose nanocrystal.Chiral nematic mesoporous silica film can selectively reflect left-handed circularly polarized light based on its photonic bandgap,and transmit right-handed circularly polarized light.Cholesteric silica-PDot composite film is obtained by loading red,green and blue polymer dot?PDot?into mesopores.After excitation,left-handed circularly polarized luminescence is inhibited because of the photonic bandgap?PBG?effect,and then right-handed circularly polarized light is released.Therefore,the composite film is able to transform guest emission to right-handed circularly polarized luminescence.The dissymmetry factor g can reach-0.396.We study the influence of PBG and loading amount of guest luminophores on circularly polarized luminescence preliminarily.Furthermore,we design to obtain bi-color CPL by loading binary luminophores simultanemously.The response of composite film to alcohol is demonstrated as well.The fourth chapter is based on the previous work of the circularly polarized potential of chiral nematic mesoporous silica.We use the cellulose nanocrystal as the template and TMOS as the silicon source.Through hydrolysis and condensation,the structure of chiral nematic cellulose nanocrystal is templated.The cellulose nanocrystal is removed by calcination.Then the inorganic chiral nematic mesoporous silica nanomaterial is obtained.We avoid the problem that guest luminophores disperse inhomogeneously in host silica film.Positive charged luminescent materials such as carbon dots,light-emitting polymer and dye Rhodamine B are the best choice.In this work we take Rhodamine B as an example to explore the influence of photonic bandgap effect,luminescence intensity,the thickness of silica film and the loading amount of guest luminophores on CPL intensity and the dissymmetry factor g systematically.We also exhibit the universality of this method and obtained optical actively right-handed circularly polarized luminescence material with excellent g value reaching-0.38.By the method of widening photonic bandgap and loading three kinds of luminophores,we realize to obtain multicolor CPL from one piece of film the first time.Otherwise the CPL On-Off switchablity of composite film is demonstrated.The fifth chapter is conclusion and prospect.