Study On The Preparation And Optical Properties Of NaP₁₅

Two-dimensional（2D）materials with remarkable physical properties have drawn much attention.In particular,many interests are attracted to anisotropic 2D materials due to their anisotropic electrical and optical properties,which give the possibility of designing new optoelectronic and electronic devices.However,most 2D materials are isotropic.It is necessary to discover new anisotropic 2D materials and study their properties.NaP₁₅ is a phosphorus rich compound,which consists of Na atoms and pentagonal P tubes.Two rows of P tubes connected by Na atoms are antiparallel in a single layer.And layers are connected through van der Waals force.Therefore,NaP₁₅ is a kind of layered material that can be exfoliated into two dimensional material and has anisotropy due to the structure of its P tube.Until now,the studies of NaP₁₅ include only basic Raman spectroscopy,thermodynamic properties and photoluminescence.And the studies are about bulk properties,ignoring the layered structure nature,let alone the anisotropic properties.In this paper,NaP₁₅ was prepared and exfoliated.Then it was characterized and its optical properties were studied.The best process parameters of preparing bulk NaP₁₅ via chemical vapor phase transfer（CVT）were explored through a series of experiments.Subsequently,two-dimensional NaP₁₅ was obtained by mechanical and liquid exfoliation.The NaP₁₅ was characterized by X-ray diffraction（XRD）,atomic force microscope（AFM）,transmission electron microscope（TEM）,scanning electron microscope（SEM）and Raman spectrometer.The research results are as follows:NaP₁₅with good crystallinity was successfully prepared,and the length of the sample is along the direction of[110];the thickness of NaP₁₅ by liquid exfoliation is mostly below 40nm and the ratio of width to thickness is 5-10 times and most of which are linear.Mechanical exfoliation NaP₁₅ were studied by Raman spectroscopy and photoluminescence.Angle-resolved polarized Raman spectra shows that NaP₁₅ has anisotropic Raman features:the Raman peak intensity reached its maximum value when the incident laser polarization direction was along the direction of[110],and the intensity reached its minimum value even almost disappeared when the direction of the[110]was perpendicular to the direction of the laser.The intensity of the Raman peak is sensitive to the laser polarization angleθand is fitted well by cos²θ.The Raman peaks have a red shift as the temperature increasing.The peak positions have a linear relationship with the temperature,and the temperature coefficients of each peak were calculated.The maximum temperature coefficient is-0.0289 cm^-1/K,which proves that the phonon frequencies in NaP15 are sensitive to the temperature modulation.Angle-resolved polarized photoluminescence spectra shows that NaP₁₅ has anisotropic photoluminescence properties.When the direction of incident laser and the collection are all along the[110]direction,the peak intensity reached the largest.The emission of photoluminescence originates from exciton and exciton binding energy is as high as70.5 meV.