Probing the Structure of NaYF4 Nanocrystals using Synchrotron-Based Energy-Dependent X-ray Photoelectron Spectroscopy
Date
2014-09
Authors
Pichaandi, Jothirmayanantham
Das, Gautom Kumar
Johnson, Noah J. J.
Regier, Tom
van Veggel, Frank C.J.M.
Journal Title
Journal ISSN
Volume Title
Publisher
Journal of Physical Chemistry C
Abstract
Understanding the structure and chemical speciation of the synthesized lanthanide-doped NaYF4 nanocrystals is of paramount importance to improve and optimize their physical and chemical properties. Hence in this work we employ synchrotron-based high-resolution X-ray photoelectron spectroscopy (XPS) measurements to analyze lanthanide-doped and undoped NaYF4 nanocrystals. These measurements revealed that there are two doublets for the yttrium ions in the nanocrystal instead of the single doublet in case all Y3+ ions have the same chemical environment, generally observed around a binding energy of 160 eV (3d photoelectrons of Y3+). This second doublet (binding energy ∼ 157.5 eV) was convoluted with the first doublet (binding energy ∼ 160 eV), and the intensity of this doublet increased with a decrease in excitation X-ray energy. The second doublet was confirmed to belong to the yttrium ions as doped and undoped NaYF4 nanocrystals exhibit this second peak. The peaks were deconvoluted showing that the second peak is also a doublet with the ratio of the peaks being 2:3. This is exactly the same as what we have observed for the first doublet of the 3d photoelectrons of yttrium ions. In addition, we observe an increase in intensity of the second doublet in comparison to the original 3d doublet of the yttrium ions as the excitation energy is decreased. This suggests that the second doublet is from surface yttrium ions in the NaYF4 nanocrystal. To confirm our hypothesis, a shell of NaYbF4 or NaTmF4 was grown over the NaYF4 nanocrystal and the second doublet for the yttrium ions was not observed. This is an additional confirmation that the second doublet is indeed from the surface yttrium ions. This implies that the yttriums on the surface of the nanocrystals have a (slightly) different chemical speciation than their counterparts inside the nanocrystals. We attribute the new chemical speciation of surface yttrium ions to the different chemical environment they encounter than their counterparts inside the nanocrystal.
Description
Keywords
Nanocrystals, Lanthanides, Chemical speciation, Upconversion, Surface ions, X-ray Photo-electron Spectroscopy (XPS)
Citation
Pichaandi, J., Das, G.K., Johnson, N.J.J., Regier, T., & van Veggel, F.C.J.M. (2014). Probing the Structure of NaYF4 Nanocrystals using Synchrotron-Based Energy- Dependent X-ray Photoelectron Spectroscopy. Journal of Physical Chemistry C, 118(37), 21639-21646.