Effects of size and surface on luminescence properties of submicron upconversion NaYF 4 :Yb,Er particles
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Bulk NaYF4:Yb,Er particles (1.4 mm particle size) were synthesized using a hydrothermal method. As-synthesized particles were subsequently ball milled to three average particle sizes, namely, 260 nm, 160 nm, and 100 nm. The x-ray diffraction pattern showed an hcp phase for as-synthesized and ball-milled particles with a predominant (100) peak. Room temperature emission spectra showed no size dependent peak shifts or peak broadening. The intensities of both green and red emissions decreased with increasing milling time. Segregation of Er ions was detected on the surfaces of milled particle that reduced the sensitizer-activator transition probability, resulting in decreased emission intensities. The green-to-red emission ratio was correlated to the surface enrichment of Er, which affected the cross-relaxation of luminescence dynamics. I. INTRODUCTION
Upconversion phosphors find applications in solid state lasers, 3D flat-panel displays, infrared quantum counters, and bio-imaging.1–4 NaYF4:Yb,Er is one of the most efficient near infrared (NIR)-to-visible upconversion materials.5 In this material system, Yb acts as the photosensitizer that absorbs the NIR photons and Er is the activator that emits the visible photons.6 Many methods exist for the synthesis of NaYF4:Yb,Er. For example, solid state reactions at temperatures as high as 1000 C in HF yielded particles with micron size or larger.7 Hydrothermal synthesis occurred at temperatures typically less than 300 C.8,9 NaYF4:Yb,Er nanoparticles were also chemically synthesized in trifluoroacetate salts above 300 C.10 In chemical synthesis, surfactants are commonly used to prevent undesirable agglomeration of particles. However, compared to bulk NaYF4:Yb,Er made by the hydrothermal method, a decrease in emission intensity of NaYF4:Yb,Er (particle size 10 nm) by a factor of 10 was reported.10 The effects of particle size and surface on the luminescent properties of these materials warrant further consideration. The 4f electrons in rare earth atoms (ions) are shielded by outer 5s2 and 5p6 orbitals. Hence, quantum confinement effects on electronic states of these highly localized electrons around the nucleus are not expected.11 However, in rare earth doped nanosize phosphors, luminescence efficiency and dynamics have been attributed a)
Currently at nanoBright Technologies Pte Ltd., Singapore 609964, Republic of Singapore. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0258 2042
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 15 Mar 2015
to size dependent confinement effects on electronphonon interaction.11,12 The phonon density of states (PDOS) in nanoparticles became discrete typically for sizes below 20 nm,13,14 as compared to the continuous PDOS in the bulk counterpart.15 Modified electronphonon coupling due to discrete PDOS and cutoff of low energy phonon modes (where the cutoff phonon energy is inversely proportional to particle size),13,14 may lead to a change in temperatur
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