Phase Transition By In-situ TEM And Humidity Sensing Property Of NaGdF₄

Rare earth doped up-conversion nanomaterials have the advantage of high optical stability,long luminous life and so on.As structure influences fluorescence property deeply,so correlative research on structure becomes more significant.NaGdF₄ is one of the best up-conversion nanomaterials which it exhibits cubic phase and hexagonal phase.Hexagonal NaGdF₄ have high luminous efficiency,but growth of hexagonal phase was induced by heating cubic phase.Unfortunately,phase transition process has not been identified clearly.Our strategy is based on using in-situ transmission electron microscope to provide energy and up to carrying the point of phase transition.In addition,the humidity sensing property was also studied.The main contents of are as follows:1)Investigations of structural evolution,phase transition of NaGdF₄ nanoparticles induced by in-situ TEM electron beam irradiation.Initially,the vacancies can be found,which there are several nanometers in size under the E-beam irradiation.With the depending irradiation time increases,the structure change from cubic?hexagonal?NaGdF₄ to GdF₃ or Gd₂O₃ on controlling current density of the TEM electron beam.In-situ epitaxial growth behavior of GdF₃?020?was observed at NaGdF₄?111?interface and the two lattice fringes are parallel when the current density of the TEM electron beam was controlled as 40 p A·cm^-1.2)Structural evolution,crystallographic conversion and in-situ phase transition from cubic?hexagonal?NaGdF₄ to Gd₂O₃ in a single nanocrystal induced by in-situ heating through a TEM experimentally.Thanks to continuous development in microscope technology,especially in-situ microscopy with special specimen chambers,it is no longer a dream to pursue the formation mechanism of nanocrystal growth on the nanometer or atomic scale.Firstly,the nanoparticle occured porosity defects at around 150?and became more bigger with the temperature increases.Our experimental results show the contents of Gd and Na atoms decreased as the temperature increases,At last,atomic refactoring of nanoparticles and transform into Gd₂O₃ when the temperature rises to around 300?.3)Humidity sensitivity property of the cubic?hexagonal?NaGdF₄.With the development of society,humidity sensor has been applied in a wide range.So a humidity sensor was fabricated using NaGdF₄.Results show in the humidity sensitive performance of cubic NaGdF₄,the response time of 24 seconds,recovery time of 36 seconds.At 97%relative humidity,the sensitivity was 9.22×10⁵,showing a rapid response recovery performance and particularly high relative humidity.And parameter of hexagonal NaGdF₄ show 16 seconds,36seconds and 258.3,respectively.With excellent long time stability.