Multi-phonon assisted upconversion emission and …

1 Multi-phonon assisted upconversion emission and powerdependence studies in LaF3:Er3+ Singha, K. Kumarb,c, , Pandeyc, Raid, D. KumaraaDepartment of Ceramic Engineering, IIT-BHU, Varanasi 221 005, IndiabDepartment of Applied Physics, ISM, Dhanbad 826 004, IndiacNanotechnology Application Centre, Allahabad University, Allahabad 211 002, IndiadLaser & Spectroscopy Laboratory, Physics Department, BHU, Varanasi 221 005, Indiahighlights"Observation of UV bands in LaF3:Er3+phosphor on 532-nm excitation."Multiphonon assisted energymigration from2H11/2(4S3/2) level tothe upper4F3/2,4F5/2and4F7/2levels."Reversal in intensity of 411 and488 nm emission bands withexcitation power or the sampletemperature."Continuum emission at increasedexcitation powers due to the largemultiphonon abstractExcitation power dependence of upconversion emission intensity of Er3+ions in LaF3phosphor is per-formed.

2 Phosphor has shown two-photon excited UV/visible UC bands at the 325 nm, 342 nm, 383 nm,403 nm, 411 nm, 440 nm, 453 nm, 470 nm and 488 nm wavelengths on 532 nm excitation the increased excitation powers Multi-phonon assisted energy migration from2H11/2(4S3/2) level tothe upper4F3/2,4F5/2and4F7/2levels has observed which opened up new channels for the emission at440 nm, 453 nm and 488 nm due to the4F3/2?4I15/2,4F5/2?4I15/2and4F7/2?4I1 5/2transitions, respec-tively. Planck blackbody like continuum emission has also observed at increased excitation infoArticle history:Received 9 September 2012 Received in revised form 6 January 2013 Accepted 10 January 2013 Available online 19 January 2013 Keywords:PhosphorRare-earth ionsUpconversion emissionMulti- phonon absorptionabstractLaF3:Er3+phosphor is synthesized through chemical precipitation method and its upconversion (UC) emission studies have carried out using 532-nm excitation.

3 Phosphor has shown two-photon absorptionUV bands at the 325 nm, 342 nm, 383 nm, 403 nm and 411 nm wavelengths. At relatively higher excita-tion powers Multi-phonon assisted energy migration from2H11/2(4S3/2) level to the upper4F3/2,4F5/2and4F7/2levels has observed and this energy migration opened new channel of emission at 440 nm, 453 nmand 488 nm due to the4F3/2?4I15/2,4F5/2?4I15/2and4F7/2?4I1 5/2transitions, respectively. Temperaturedependent UC measurement is also done and observed emission pattern is correlated with the powerdependence studies. upconversion bands at 411 and 488 nm wavelengths have shown reversal in inten-sity as either excitation power or sample temperature is increased and hence these two bands are foundto act as optical switch. Planck blackbody like continuum emission has also observed at higher excitationpowers.

4 2013 Elsevier All rights $ - see front matter 2013 Elsevier All rights Corresponding author. Tel.: +91 Kumar).Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 106 (2013) 236 241 Contents lists available atSciVerse ScienceDirectSpectrochimica Acta Part A: Molecular andBiomolecular Spectroscopyjournal homepage: the recent years upconversion (antiStokes) emission studiesof rare earth (RE) ions doped fine powder phosphors preparedthrough soft chemistry routes have attracted much attention[1 6]. Size confined phosphors have shown modified and sometimesstrange optical properties compared to the same phosphor pre-pared through traditional solid state route[7 11]. RE powderphosphors have shown newer potential applications in opticalswitches, fingerprint imaging, solar cells, random lasing etc.

5 [9,10,12].In many cases RE ions in real nano-phosphors have shown de-creased quantum efficiency than their bulk counterparts becauseof the decrease of RE concentration in nanoparticles and increasein electron phonon interaction at the surface of nanoparticles. Also,the optical emission in RE ions occurs through atomic transitionswhich are very less affected by the quantum size effect. To obtainhigh quantum yield UC phosphor materials, three points must bekept in mind. These are: (1) particle morphology and size; (2) com-position; and (3) surface chemistry (electron phonon coupling).The low electron phonon coupling results high UC emission . In ran-dom nanoparticles/nanopowders this electron phonon couplinghas resulted anomalous optical properties. Nanopowders also exhi-bitmultiplescattering, lowthermalconductivityandradiationtrap-p ing which lead to the observation of anomalous optical properties[13 15].

6 Ananomalousthermalizationin Er3+ion emission hasbeenreported by Liu et al.[13]in Er3+doped Y2O2S nanocrystals and inEu3+doped nanotubes by Liu et al.[14]. Lawandy et al.[15]have ob-served random lasing action in dye solutions containing micro-par-ticles. They supposed close path scattering process in sampleamplify the emitted radiation. In achieving random lasing thermaleffects should be considered carefully due to the low thermal [16]havestudiedindeepthe luminescence quenching in random Y2O3medium doped withYb3+ions by considering the low thermal conductivity, multiplescattering and enhanced absorption of the medium. Some otherauthors have also studied pump power dependence of emissionspectra and have observed interesting results[17 19]. multi -pho-non assisted UC with the particular excitation wavelengths was ob-servedinsomeglassesandcrystalsdopedwi thNd3+[20,21]andEr3+[22]ions.

7 Park[22]in a recent investigation has observed thermallyenhanced UC at 406 nm arising from2H9/2level of Er3+ion doped nanostructured phosphors above such studies have not yetbeen done intensely and it is expected that nanopowders pro-foundly would show these properties. In our previous work[23]pump power dependence of emission intensity with 976 nm exci-tation was done on La2O3:Er3+/Yb3+nanostructured phosphor. Thissample showed moderate thermal heating upto to the studiedpower and high UC emission was observed. In the next study whenthis phosphor was heat treated at 1200 C a profound heating wasobserved which forced to emit bright black body radiation in placeof discrete Er3+ emission [24]. This effect is supposed to happendue to the decreased thermal conductivity of the samples.

8 Similarobservation is also observed in pure RE materials under infraredexcitation[18]but the authors have explained it Er3+ion doped phosphors show UC emission on excita-tion with a number of wavelengths. Most of the UC studies inEr3+doped materials are reported by the excitation of infrared laserradiation. UC studies by employing 532-nm or other green wave-lengths are rare, since the UC emission goes weaker with decreasein excitation wavelength and poses a problem to record it. Tanner[25]and Gan et al.[26]have reported the UC spectra of Er3+ion influorozirconate glasses and Cs2 NaErCl6crystal using In another work, Guo et al.[27]prepared cubic Gd2O3:-Er3+phosphor through sol gel method and observed ex-cited UC emissions at 408, 478 and 504 nm due to the2P3/2and4G11 of such studies are in materials that are in bulk phases.

9 Inpowdered samples little attention is paid on the measurement ofemission intensity at high excitation powers. There are some re-cent observations available, however[17 19]. In the present work532-nm wavelength excited single Er3+ion doped LaF3nano-crys-talline phosphor is studied upto the high excitation powers to seethe variation in UC emission intensity and whether thermal heat-ing in sample will occur or not. The prepared samples have showngood UC emissions in ultra-violet (UV), blue and green regions. Theincrease in excitation power or in temperature have shown inter-esting consequences like observation of new bands, switching inintensity in some emission bands, and black body (thermal) emis-sion. The Er3+ion was chosen to see whether this ion is capable toproduce heating effect on 532 nm excitation in low phonon fre-quency co-precipitation method is used for the synthesis ofphosphor material.

10 The reagent grade La(NO3)3 9H2O ( ) andEr(NO3)3 6H2O ( ) compounds were weighted in appropriateamount and then dissolved in triply distilled water. After stirringabout half an hour, drop wise HF (46%) was added under stirringto the solution until pH of the solution goes around Additionof HF gives whitish colloidal solution and this solution was boiledat 100 C for half an hour under stirring to complete the cooling down, the solution was filtered and white precipitatewas collected. The precipitate was washed with water severaltimes to remove the unused HF and then dried for 24 h. This as-synthesized sample was heat treated (calcinated) at 800 C for2h. in N2gas environment. Final phosphor composition was setat: + ErF3where numerical values are in mol%.The crystalline structure of the heat treated powder sampleswas verified by the powder X-ray diffraction (XRD) using Molybde-num (Mo) source.