Abstract

We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF3 and BaLaF5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na+ for La3+ ions (97% of total Na+ codopant) in the crystal system, and interstitial Na+ (3% of detected Na+) and La3+ (3% of detected La3+) present in (Na)LaF3 and only direct substitution of Na+ for Ba2+ in Ba(Na)LaF5. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08–0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E.3+) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5–100 W/cm2 range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm3+ 3F3 and 3H4 states was observed and noted as a key initial condition for effective population of the 1D2 and 1I6 UV emitting states, via Tm-Tm cross-relaxation.

Highlights

  • The design of upconverting nanocrystals (UCNCs) with specific, tunable electronic spectroscopic profiles [1,2,3,4], based on multiple dopants and mixed host crystal structures [5,6], has shown great promise for applications that range from biomedical detection [7,8,9] to triggered photoresponsive reactions [10,11]

  • Sodium codoped LaF3 and BaLaF5 ( denoted as (Na)LaF3 and Ba(Na)LaF5) UCNCs were generated via a solvothermal synthesis, similar to that reported by Wang et al [34]

  • Though binding energy shifts were undetected in the X-ray photoelectron spectroscopic (XPS) spectrum of the Yb 4d region, a noteworthy increase in the NIR 3H4 → 3H6 (803 nm) Tm3+ transition intensity in (Na)LaF3 core@shell material is observed relative to (Na)LaF3 UCNC cores (Figure 2a), consistent with highly efficient Yb-to-Tm energy transfer upconversion (ETU) processes and dominant NIR emission previously reported for Tm3+ doped, NaYbF4 [7]

Read more

Summary

Introduction

The design of upconverting nanocrystals (UCNCs) with specific, tunable electronic spectroscopic profiles [1,2,3,4], based on multiple dopants and mixed host crystal structures [5,6], has shown great promise for applications that range from biomedical detection [7,8,9] to triggered photoresponsive reactions [10,11]. Diminished symmetry proximal to emitting R.E. ions permits otherwise disallowed ETU transitions to become more favorable These studies have reported between two- and 50-fold enhancements of luminescence intensity in two-photon upconversion processes extending from the NIR to visible spectral regime [19,20,21,22], but do not address the potential for enhancement of higher energy, UV emitting transitions or their possible upconversion mechanisms. Similar power dependence studies of upconverted emission intensity conducted on the core@shell materials vs their unshelled counterparts reveal shifts in the upconversion mechanism at low laser densities (

UCNC Materials and Emission Profiles
Crystal Structure and Emissive Ion Environment
Materials
Na-Doped LaF3 Synthesis
Na-Doped BaLaF5 Synthesis
Characterization
Conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call