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Journal of Materials Research/JMRAdditional services for Journal of Materials Research:Email alerts: Click hereSubscriptions: Click hereCommercial reprints: Click hereTerms of use : Click hereGreatly enhanced broadband near­infrared emission due to energy transfer from Cr3+ to Ni2+ in transparent magnesium aluminosilicate glass ceramicsJin Luo, Shifeng Zhou, Botao Wu, Hucheng Yang, Song Ye, Bin Zhu and Jianrong QiuJournal of Materials Research / Volume 24 / Issue 02 / 2009, pp 310 ­ 315DOI: 10.1557/JMR.2009.0068Link to this article: /abstract_S088429140003140XHow to cite this article:Jin Luo, Shifeng Zhou, Botao Wu, Hucheng Yang, Song Ye, Bin Zhu and Jianrong Qiu (2009). Greatly enhanced broadband near­infrared emission due to energy transfer from Cr3+ to Ni2+ in transparent magnesium aluminosilicate glass ceramics. Journal of Materials Research,24, pp 310­315 doi:10.1557/JMR.2009.0068Request Permissions : Click hereDownloaded from /JMR, IP address: 58.155.210.6 on 11 Sep 2012ARTICLESGreatly enhanced broadband near-infrared emission due toenergy transfer from Cr3+to Ni2+in transparent magnesiumaluminosilicate glass ceramicsJin Luo and Shifeng ZhouState Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou310027,People’s Republic of ChinaBotao WuState Key Laboratory of High Field Laser Physics,Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences,Shanghai201800,People’s Republic of ChinaHucheng Yang,Song Ye,and Bin ZhuState Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou310027,People’s Republic of ChinaJianrong Qiu a)State Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou310027,People’s Republic ofChina;and State Key Laboratory of High Field Laser Physics,Shanghai Institute of Optics and FineMechanics,Chinese Academy of Sciences,Shanghai201800,People’s Republic of China(Received16March2008;accepted11July2008)Cr3+/Ni2+co-doped optically transparent magnesium aluminosilicate glass-ceramicscontaining MgAl2O4nanocrystals have been prepared by heat-treatment.Greatlyenhanced broadband near-infrared emission centered at1216nm in Cr3+/Ni2+co-dopedglass ceramics is observed when compared with the Ni2+single-doped glass ceramicsunder532nm excitation.The observed enhancement of infrared emission is attributed tothe energy transfer from Cr3+to Ni2+ions in the nanocrystalline phase,which leads to the emission due to3T2(3F)!3A2(3F)transition of octahedral Ni2+ions.I.INTRODUCTIONIn recent years,transparent glass ceramics,which can be prepared by precipitating nanocrystalline phase from a glass matrix through suitable heat treatment,have be-come very attractive materials for the photonic applica-tions.1The glass-ceramics are easily fabricated like glass and have crystal’s advantage of high optical activity. With the rapid development of optical communication, the limited bandwidth of traditional rare-earth-ion-doped fiber amplifiers will not meet the need of large-capacity information transmission because of the intrinsic electron transition characteristics of rare-earth ions.In comp-arison,transition metal ions may potentially show broadband luminescence,and it has been reported that Ni2+-doped MgO single crystal shows broadband near-infrared emission.Unfortunately,difficult fabrication restricts its practical applications.2Glass can be easily fabricated into the form of fiber.However,Ni2+ions show weak or even no luminescence in glasses due to the strong nonradiative relaxation.Recently,research was carried out to prepare Ni2+-doped transparent glass ceramics.In glass ceramics,Ni2+ions show broadband near-infrared luminescence with long fluorescent life-time when incorporated into the crystal phase.Therefore, they have potential applications as gain materials for broadband optical amplifiers and tunable lasers.3–6Cr3+-ion-doped materials have broad and strong absorp-tion in the visible region,and their absorption spectra overlap with the emission spectra of a Xe flashlamp and solar light,7–13so they are excellent gain matrixes as lamp-pumped lasers and luminescent solar concentrators.More-over,visible and near-infrared emissions of Cr3+overlap well with the absorption of rare-earth ions.They have been widely used as sensitizers for rare-earth ion-doped(e.g., Nd3+and Tm3+)materials.13–20It has been demonstrated that the pumping and lasing efficiency were effectively improved by means of co-doping of Cr3+for rare-earth-ion-doped crystals.15–17However,there are few re-ports about enhanced luminescence phenomenon in Cr3+/ Ni2+co-doped materials,especially in glass-ceramics. Magnesium aluminosilicate glass-ceramics have been extensively studied due to their good mechanical properties and chemical durability.The composition of the glass matrix,addition of nucleating agents,and heat-treatment conditions determine crystalline phases and microstructures of glass-ceramics.21,22Magnesiuma)Address all correspondence to this author.e-mail:qjr@DOI:10.1557/JMR.2009.0068J.Mater.Res.,Vol.24,No.2,Feb2009©2009Materials Research Society 310aluminosilicate glass-ceramics containing MgAl2O4 nanocrystals have also been studied as a possible laser medium for tunable lasers.23In this research,we prepared Cr3+/Ni2+co-doped transparent magnesium aluminosilicate glass ceramics. The luminescent properties of the glass-ceramics were investigated,and the results demonstrated that co-doping of Cr3+enhanced the infrared luminescence of Ni2+ obviously.It is expected that the co-doping of Cr3+may be an effective way to improve the luminescence perfor-mance of Ni2+-doped materials.II.EXPERIMENTALGlass-ceramics were prepared by the heat-treatment of glassy materials with a composition of48.7SiO2–19.4Al2O3–19.5MgO–3.8ZrO2–8.6TiO2–0.1Cr2O3–0.1 NiO(mol%;MZTAS).For comparison,single NiO as well as Cr2O3-doped glasses and glass-ceramics with the same concentration were also prepared.A30g stoichio-metric mixture of SiO2,Al2O3,MgO,ZrO2,TiO2,Cr2O3, and NiO was mixed thoroughly in an agate and was melted in an alumina crucible at1560 C for1h.The melt was cast into a stainless steel slab rapidly and then annealed at650 C for4h.The glass was heat treated inair to obtain glass-ceramics,and all the samples were polished to be about2.5mm thick.Thermal properties were measured by differential thermal analysis(DTA)at a heating rate of10 C/min under N2atmosphere.X-ray diffraction(XRD)measure-ments were carried out using Cu/K a1(l=1.54056A˚)as the radiation to identify crystallite phases.The micro-structures of the samples were observed using a JEM-2010high-resolution transmission electron microscope (HRTEM).Optical absorption spectra were recorded by a double-beam spectrophotometer(JASCO FP-6500, Tokyo,Japan).The infrared emission spectra were measured using a ZOLIX SBP300spectrophotometer with InGaAs detector under the excitation of532-nm laser.Fluorescence lifetime decays were measured in an Edinburgh Instrument Ltd.spectrometer(Beijing,China) (model FL920,Edinburgh,UK).The data were obtained using the single photon counting technique equipped with a Ge detector with liquid nitrogen cooling.The sample excitation source was a m F900pulsed Xenon microsecond flashlamp(repetition rates10Hz).III.RESULTS AND DISCUSSIONA.Sample preparationThe inset of Fig.1(a)shows the DTA result of the as-made glass.Glass transition(T g)and crystallization(T x) occur around780and932 C,respectively.Transparent glass-ceramics were obtained by controlling the crystal-lization process of as-made glasses through a two-stage thermal treatment with the nucleation process held at 790 C and the crystal growth held at950 C.Figure1 (a)presents the XRD patterns of the MZTAS glass and MZTAS glass-ceramic(GC).No apparent diffraction patterns can be observed for the glass sample,but a halo characteristic of amorphous phase can be seen around 25 .After heat treatment,several sharp diffraction peaks attributed to the crystal phase of MgAl2O4and ZrTiO4 can be observed.The strongest diffraction peaks around 2y=30.6 and36.8 corresponding to the respective diffraction peaks of ZrTiO4and MgAl2O4were selected to calculate the size of precipitations using the Scher-rer’s equation.The sizes of ZrTiO4and MgAl2O4crys-talline phases are about6.6and4.4nm,respectively. The crystallinity estimated by the ratio of integrated area of peaks to the total area of the XRD pattern is about 52.8%.The photographs of single Cr3+-doped,Ni2+-doped,and co-doped glasses and GCs are shown in Fig.1(b).The color of glasses changed obviously by heat treatment.For single Ni2+-doped and Cr3+-doped glasses,the colors change from yellow to gray and from green to yellowish brown,respectively.Differently,for Cr3+/Ni2+co-doped glass,it varies from deep green to gray yellow.It is noted that all of the samples are trans-parent after heat-treatment.Figures1(c)and1(d)present the HRTEM images of the MZTAS GC,and the HRTEM images reveal that the nanocrystals distribute homogeneously among the glass matrix and the size of nanocrystals is between5and10nm,which agrees well with the estimation based on XRDpatterns.FIG.1.(a)XRD patterns of as-made MZTAS glass and MZTAS GC, the inset shows the DTA results of the as-made MZTAS glass;(b) photographs of single Cr3+-doped,Ni2+-doped,and co-doped glasses and GCs;(c,d)HRTEM images of MZTAS GC.B.Optical absorption propertiesOptical absorption spectra of single Cr 3+,Ni 2+-doped and co-doped glasses and GCs are shown in Fig.2.It is obvious that the absorption positions for the Cr 3+-doped glass and GC in Fig.2(a)are different from each other.Cr 3+occupies more preferentially the octahedral site than the tetrahedral site in glasses and crystals because three 3d electrons of Cr 3+can take a lower t ag level in the octahedral site.7Two broad bands are observed in the spectrum of Cr 3+-doped glass sample:the low-energy one (at about 650nm)can be assigned to the 4A 2g (F)!4T 2g (F)spin-allowed transition,and the high-energy one (at about 443nm)is related to the 4A 2g (F)!4T 1g (F)transition of octahedral Cr 3+.8For Cr 3+-doped GC,two broad bands are also observed in the spectrum,and they can be assigned to the same origin as Cr 3+-doped glass.However,it is necessary to point out that these absorption bands show a slight shift toward high energy in the Cr 3+-doped GC.The blue shift of the absorption can be understood if the octahedral sites in the crystal lattice are substituted by Cr 3+ions.After incorporation into the crystalline phase,the crystal field strength may be expected to become large,and the change can be viewed from the Dq parameter.The Dq value was calculated.7The Dq value of Cr 3+ions changes from 1538cm À1for Cr 3+-doped glass to 1754cm À1for Cr 3+-doped GC.The magnitude of Dq is a measure of the interaction of the 3d -electrons with the rest of the lattice,and the main contribution to the inter-action arises from the nearest neighbors.5Thus,the weaker crystal field in the glass around Cr 3+can be considered to be caused by the more open structure.An increase of crystal field strength leads to an increase of the energy of the levels 4T 2g (F)and 4T 1g (F)and a blue shift of the absorption bands due to the transitions from 4A 2g (F)ground state to 4T 2g (F)and 4T 1g (F)excited states.9The absorption spectrum of single Ni 2+-doped glass resembles the absorption of Ni 2+in tetrahedral 4-fold and trigonal bypyramid coordinated sites in sili-cate glasses.In contrast,the absorption spectrum of the Ni 2+-doped GC is similar to the absorption spectra of Ni 2+ions in the octahedral sites in the other materials.3,5,6The broad absorption bands at around 630and 1020nm in Ni 2+-doped GC can be attributed to spin-allowed tran-sitions from 3A 2g (3F)ground state to the 3T 1g (3F)and 3T 2g (3F)excited state of Ni 2+in octahedral sites,respec-tively.3–6It is supposed that Ni 2+substituted the octahe-dral positions in the crystalline phase.The absorption spectrum of Cr 3+/Ni 2+co-doped glass and GC in Fig.2(c)superposes the corresponding absorption spectra of the Cr 3+and Ni 2+single-doped glasses and GCs,respec-tively.Therefore,it is reasonable to assume that at least some Cr 3+and Ni 2+are incorporated into the crystalline phase in Cr 3+/Ni 2+co-doped GC.20It is known the MgAl 2O 4crystals are of the spinel structure withspace group O h 7(Fd 3m ),and Al3+ions (r Al3+=0.535A ˚)occupy octahedral sites banded to six oxygens at thesame distance,1.928A˚,with D 3d point symmetry.10,24In addition,ZrTiO 4has the a –PbO 2structure with space group D 142h (Pbcn ),and it is composed of hexagonally close-packed oxygen layers and small-sized cations(r Ti4+=0.605A˚and r Zr4+=0.72A ˚)that randomly occupy 50%of the available distorted octahedral sites.25Each metal ion of ZrTiO 4is bonded to six oxygens at anaverage distance of 2.04A˚:four at 1.94A ˚and two more at 2.23A˚.26According to Grimes,27large impurity ions are generally held at the high-symmetry site,while smaller ions are able to move closer to the positions of minimum energy.Moreover,the valence state of Cr 3+and Ni 2+are so different from that of Zr 4+and Ti 4+,and nanocrystals will be not stable when Zr 4+and Ti 4+are substituted for Cr 3+and Ni 2+.Therefore,it seems to be reasonable toconsider that the doped Cr 3+(r Cr3+=0.69A˚)24and Ni 2+(r Ni2+=0.69A˚)5ions are both incorporated into MgAl 2O 4spinel nanocrystals.4,5,10,24C.Fluorescence propertiesThe near-infrared emission spectra of single Cr 3+-doped,Ni 2+-doped,and co-doped glasses and GCs are shown in Fig.3.The weak emission of single Ni 2+-doped GC peaked around 1200nm under the excitation with 532nm could be attributed to the transition from the 3T 2g (3F)excited state to the 3A 2g (3F)ground state of Ni 2+ions in octahedral sites.3–6,20An intensely broad emission band centered at 1216nm in the Cr 3+/Ni 2+co-doped GC was also due to the 3T 2g (3F)!3A 2g (3F)FIG.2.Optical absorption spectra of (a)Cr 3+-doped,(b)Ni 2+-doped,and (c)Cr 3+/Ni 2+-doped glasses (dotted line)and GCs (solid line).transition.In contrast,the single Cr 3+-doped GC and Cr 3+/Ni 2+co-doped glass do not show any emission at this band.The near-infrared emission in single Cr 3+-doped and Cr 3+/Ni 2+co-doped glass are centered at about 950nm,and they can be assigned to the 4T 2g (F)!4A 2g (F)transition of Cr 3+in octahedral sites.8The absorption peaks around 880–1300nm of Ni 2+in glass result in the decrease of near-infrared emission intensity of Cr 3+in Cr 3+/Ni 2+co-doped glass compared with that of Cr 3+in Cr 3+-doped glass.This emission band shifts to higher energies (at about 914nm)in Cr 3+-doped and Cr 3+/Ni 2+co-doped GCs,and the emission intensity of these samples is greatly increased compared with those of Cr 3+-doped and Cr 3+/Ni 2+co-doped glasses,which further proves that the Cr 3+ions are incorporated into high-field sites.In other words,the Cr 3+ions substitute the octahe-dral sites in the crystalline phase,which results in the increase of the crystal field and decrease of nonradiative relaxation.7The emission shape originating from the 4T 2g (F)!4A 2g (F)transition of Cr 3+ions is not affected due to Ni 2+co-doping in Cr 3+/Ni 2+co-doped GC,while a decrease of the emission intensity of Cr 3+ions can also be observed in Cr 3+/Ni 2+co-doped GC compared with Cr 3+single-doped GC.These phenomenon could be attributed to the Cr 3+!Ni 2+energy transfer,which can also explain the great enhancement of near-infrared emission of Ni 2+ions centered at 1216nm in Cr 3+/Ni 2+GC.20It is observed that the excitation wave length of532nm locates in the front-end of 3T 1(3F)absorption band of Ni 2+in GC [Fig.2(b)],and only weak infrared emission occurs under the excitation of 532nm for sin-gle Ni 2+-doped GC.The broad-band emission from the 4T 2g (F)excited state to 4A 2g (F)ground state of the Cr 3+ions overlaps the strong absorption of Ni 2+in the 880–1300nm region due to spin-allowed transitions from 3A 2g (3F)ground state to the 3T 2g (3F)excited state of Ni 2+.Thus,after the Cr 3+ions are excited,energy trans-fer occurs between Cr 3+and Ni 2+,and the energy transfer from Cr 3+to Ni 2+ions associated with the vibro-nically assisted 4T 2g (F)!4A 2g (F)transition of Cr 3+ions can be confirmed by the fluorescence decay curves at 914nm of Cr 3+in Cr 3+-doped and Cr 3+/Ni 2+co-doped GC,which are presented in the inset of Fig.3.The fluorescence decay curves measured at 914nm exhibit non-exponential decay.For practical purposes we use the “average lifetime”t 0defined by equation t 0=R tI (t )d t /R I (t )d t .13A decrease from the Cr 3+average lifetime of 51m s for Cr 3+-doped GC to 37m s for Cr 3+/Ni 2+co-doped GC was observedThe fluorescence spectra of Cr 3+single-doped,Cr 3+/Ni 2+co-doped glasses,and GCs in visible region are shown in Fig.4.No emission was observed in this region for single Cr 3+-doped and Cr 3+/Ni 2+co-doped glasses.After heat treatment,an emission can be seen for single Cr 3+-doped and Cr 3+/Ni 2+co-doped GCs.The intense fluorescence peak centered at 686nm is attributed to the zero-phonon line of the 2E !4A 2transition for the Cr 3+in a “strong”octahedral field.7The broad tail in the long wave length in the fluorescence spectra of the Cr 3+-doped GC,as well as the Cr 3+/Ni 2+co-doped GC,is attributed to 4T 2g (F)!4A 2g (F)vibronic transition.The crystal-like 686-nm peak indicates that themajorFIG.3.Near-infrared emission spectra of (a)single Cr 3+-doped GC,(b)Cr 3+/Ni 2+co-doped GC,(c)single Cr 3+-doped glass,(d)Cr 3+/Ni 2+co-doped glass,and (e)single Ni 2+-doped GC under 532nm excita-tion.The inset shows the experimental time decay profiles of the Cr 3+luminescence at 914nm obtained from single Cr 3+-doped and Cr 3+/Ni 2+co-dopedGCs.FIG.4.Fluorescence spectra of single Cr 3+-doped and Cr 3+/Ni 2+co-doped glass and GCs with 532nm excitation;the inset shows the experimental time decay profiles of the Cr 3+luminescence at 686nm obtained from single Cr 3+-doped and Cr 3+/Ni 2+co-doped GCs.fraction of Cr 3+ions in the glass matrix is located in strong crystal field octahedral sites,which are available only within the nanocrystalline phase nucleated in the host glass matrix of the composite material.It should be noted that there is a decrease of emission intensity for Cr 3+/Ni 2+co-doped GC in contrast with that of Cr 3+-doped GC.The decrease of the emission intensity for Cr 3+/Ni 2+co-doped GC in this region is assigned to the energy transfer from Cr 3+to Ni 2+ions.The inset of Fig.4shows the fluorescence decay curves at 686nm of Cr 3+in Cr 3+-doped and Cr 3+/Ni 2+co-doped GCs.A decrease in the Cr 3+average fluorescence lifetime of 1.67ms for Cr 3+-doped GC to 0.77ms for Cr 3+/Ni 2+co-doped GC was observed.The decrease of lifetime of 686nm emissions for Cr 3+/Ni 2+co-doped GC directly indicates the energy transfer between Cr 3+and Ni 2+.The fact that the lifetime of visible emission and in-frared emission of Cr 3+in Cr 3+/Ni 2+co-doped GC decre-ased compared with those in Cr 3+singly doped GC indirectly indicates that the energy transfer from Cr 3+to Ni 2+should be nonradiative because a decrease in the sensitizer’s emission fluorescence lifetime is a distin-guishing feature related to nonradiative energy trans-fer.20Figure 5illuminates the schematic mechanism of the energy transfer from Cr 3+to Ni 2+.Cr 3+ions are excited into 4T 2(F)excited state under the 532nm exci-tation.Nonradiative energy transfer occurs from the 4T 2(F)excited state of Cr 3+to the 3T 2(3F)or 3T 1(3F)excited state of Ni 2+,which stimulates the broadband near-infrared emission of octahedral Ni 2+.However,the exact energy transfer mechanism still needs to be studied further.The nonradiative energy-transfer efficiency Z can be evaluated by calculating the shortening of the 686nm luminescence decay time of Cr 3+in the Cr 3+/Ni 2+co-doped GC with respect to the luminescencedecay time in Cr 3+singly doped one using the following formula:¼1Àt 0Cr Ni =t 0Cr;where the t 0Cr–Ni and t 0Cr are average fluorescence life-time of Cr 3+emission in the Cr 3+/Ni 2+co-doped and Cr 3+singly doped GC,respectively.17,18The calculated Z is about 53%.IV.CONCLUSIONSSingle Cr 3+-doped,Ni 2+-doped,and co-doped MgAl 2O 4GCs were prepared by heat treatment.The broadband infrared emission of Ni 2+around 1216nm was observed in the Cr 3+/Ni 2+co-doped GC under the excitation with a 532-nm laser.The intensity of Ni 2+emission was greatly enhanced due to the energy transfer between Cr 3+and Ni 2+ions in the Cr 3+/Ni 2+co-doped GC.It can be confirmed from the spectral overlap be-tween the Cr 3+emission and absorption of Ni 2+and fluorescence decay curves of the emission of Cr 3+ions.The energy transfer efficiency was evaluated to be 53%.It is suggested that Cr 3+/Ni 2+co-doped MZTAS GC are promising materials application as broadband optical amplifiers and tunable lasers.ACKNOWLEDGMENTSThis work was financially supported by the National Natural Science foundation of China (Grant No.50872123),the National Basic Research Program of China (2006CB806000b),and Program for Changjiang Scholars and Innovative Research Team in University (IRT0651).REFERENCES1.B.N.Samson,L.R.Pinckney,J.Wang,G.H.Beall,and N.F.Borrelli:Nickel-doped nanocrystalline glass-ceramic fiber.Opt.Lett 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