Color-Switchable Upconversion And Energy Migration In Core-Shell Nanocrystals

Photon upconversion of lanthanide-doped nanomaterials has sharp emission peak,high optical stability and good biocompatibility,showing promising prospects in various frontier applications,such as volumetric three-dimensional display,super-resolution imaging,micro-nano lasers,and anti-counterfeiting safety.The abundant 4f electronic configurations of lanthanide ions enable the capability in generating tunable emissions,of which visible to ultraviolet upconversion is readily available under traditional near infrared excitation.The rapid development of synthesizing high-quality core-shell nanocrystals offers more possibility for optical manipulation of lanthanide ions.Unlike traditional luminescent materials with a specific emission color,the recently developed orthogonal excitation-emission nanocrystals exhibit different colors by altering the excitation conditions（e.g.,color changes from green to red）,further broadening their optical property and related frontier applications.However,traditional color-switchable upconversion nanoparticles generally consist of well controlled multilayers due to the detrimental cross relaxation and the overlapped absorption spectrum of different lanthanide ions at specific excitation wavelength.On the other hand,energy migration shows particular functions in tuning optical characteristics,which may be helpful in providing potentials for simplifying the design and synthesis of color-switchable upconversion nanoparticles as well as the mechanistic understanding of upconversion process.Based on the above questions,in this thesis,the energy-migratory Na Yb F₄sublattice was used to manipulate the population of intermediate energy state of Er³⁺ions with resultant red to green color-switchable upconversion in a simple core-shell nanostructure.The absorption characteristic of Yb³⁺ions also provides possibility for conveniently versatile multicolor upconversion by using the 915 and 1530 nm excitation in ordinary constructed core-shell nanostructure,which would help to stimulate new concepts for color-switchable upconversion.Moreover,the energy migration at nanometer length scale was investigated by precise control of interfacial energy transfer in a sensitizing-migration-detecting tri-layer nanostructure.The main achievements of this thesis are listed as follows:（1）A new model for color-switchable upconversion was constructed by designing the energy migration core-shell nanostructure.The Na Yb F₄shell can successfully tune the population of intermediate states of Er³⁺ions in Na Er F₄matrix through the resonant energy transfer from Er³⁺ions to Yb³⁺ions.Therefore a red emission upon 980 nm steady state excitation and a green emission under 808（or 1530）nm steady state excitation was achieved.The color-switchable property is closely dependent on the thickness of Na Yb F₄ layer and Yb³⁺ions’concentration as a result of energy losses in migration layer.More interestingly,the energy interaction between Er³⁺and Yb³⁺exhibits temporal characteristics.By simply reducing the pulse width of 980 nm excitation wavelength,the sample showed gradual color change from red to green.Such a core-shell model for the color-switchable upconversion is not only greatly important for the fundamental research of photon upconversion but also helps in exciting new chances for the emerging applications such as information security.（2）A versatile color-switchable core-shell based nanostructure was established by using the selective excitation wavelengths.Considering the relative wide absorption cross-section of²F_5/2 state of Yb³⁺,the 915 nm laser was adoped,and red-to-green,blue-to-red,and blue-to-green tunable upconversion emission was readily realized under 915 and 1530 nm switchable excitations.Unlike the traditional 980 nm excitation,which can activate both Er³⁺and Yb³⁺simultaneously,our design does not require optimization for the photon blocking layer in the nanostructure,providing a more convenient solution to the color-switchable upconversion.Moreover,this design is independent on the locations of the luminescent lanthanide dopants in the nanostructure,also providing more flexibility in the design of the sample.The red-green-blue upconversion emissions at a single nanoparticle level was further achieved by introducing Nd³⁺to harvest 808 nm excitation laser,showing remarkable potential in 3D display and information anti-counterfeiting.（3）A nanoscale model for probing the energy migration was proposed by constructing an IET-mediated core-shell-shell nanostructure.By selecting suitable dopants in the sensitizing layer,migration layer and detecting layer of the nanostructure,the energy migration among Tb³⁺,Gd³⁺and Yb³⁺ions in nanometer length scale was investigated.It was found that the energy migration is closely dependent on the concentration of the energy migratory ions and migration layer thickness,and Gd³⁺exhibits the best energy migration characteristics.The energy migration in Na Yb F₄sublattice permits energy transport over a long distance,resulting in upconversion emission of Eu³⁺and Tb³⁺ions with long decay times.These nanoparticles show enhanced light intensity after decorated with suitable near-infrared dyes（ICG）to improve absorption of 808 nm laser.The distinct lifetimes of lanthanide ions give a chance for high-level information anti-counterfeiting safety based on time-gating technology.