银纳米晶对铒镱共掺的TeO2-WO3-La2O3微晶玻璃发光性能的影响

陈淑文 林 健 张文俊

（同济大学材料科学与工程学院，教育部先进土木工程材料重点实验室，上海 201800）

0 Introduction

Tellurite glasses have many advantages such as wide range of transparency,large optical nonlinearity,refractive index and density,high solubility of rareearth （RE）ions,as well as low phonon energy compared to other oxide glasses[1].These characteristics make RE doped tellurite glasses promising candidates for the optical fiber amplifier,laser and new band optical communication[2].Er3+-doped tellurite glass has generated considerableinterest duetotheirfavorable emissions of blue,red,green and near-infrared light[3].Yb3+is known as a resourceful dopant can enhance the luminescence emissions of Er3+due to its large absorption cross-section.What′s more,the resonance energy transfer between Yb3+and Er3+can significantly improve the pumping efficiency of Er3+,and improve the upconversion （UC）properties of glasses[4].However,the luminous intensity of RE doped tellurite glasses is still weaker than some luminescent crystals[5].

Over the last few years,glass-ceramics have attracted great interest because of their prospective applications as promising hosts for RE ions.The formation of micro-crystals in the glass can greatly change the coupled environment around RE ions,and reduce the covalence of crystal field as well as the vibration energy of lattice[6].Gao et al.[7]obtain NaYF4∶Tb3+,Yb3+,Li+crystallites in silicate oxide system.RE ions are gathering around the micro-crystals,which increases the probability of the energy transfer between RE ions,and improves the luminous performance of the glasses.In addition,the simple process of preparation and synthesis makes RE ions doped glassceramics become important UC luminescence materials.Ansari et al.[8]report the synthesis of YbF3/ErF3-codoped lithium tungsten tellurite oxyfluoride glass-ceramicscontain LiYbErF4nanocrystals.An intense visible emission originated from Er3+can be observed due to the cooperative UC processed 980 nm excitation.

The localized surface plasmon resonance （LSPR）can increase the local field on the RE ions which near the metal nanoparticles and transferring energy to the RE ions,When the incident light wavelength or photoluminescence （PL）wavelength of the glass is close to the localized surface plasmon resonance wavelength,due to the local field enhancement（LFE）and energy transfer （ET）from Ag to RE ions,the luminescence intensity of glasses can be enhanced[9].Therefore,inorganic glasses doped with silver nanocrystals（Ag NPs）have received considerable attention[10]due to their unique optical properties[11].Amjad et al.[12]report significant luminescence enhancement of Er3+ions as well as raman intensity of Ag NPs embedded zinctellurite glasses.The radiative transition of Er3+ions and Ag NPs induce the electric dipoles,leading to the enhancement of PL[2-13].Ma et al.[14]successfully introduce Ag NPs into SiO2-Al2O3-CaF2system and obtain the glass-ceramics containing CaF2crystallites with significant enhanced luminous intensity.To our best knowledge,the study oftellurite glass-ceramics containing Ag NPs is rare.

In this paper,we report the optical properties of Er3+/Yb3+co-doped tellurite glass-ceramics containing Ag NPs.The effect of different introducing ways of Ag NPs on the UC luminescence properties of Er3+/Yb3+co-doped tellurite glass-ceramics were systematically investigated.Ourstudy furtherdemonstrates the luminescent intensity of the sample co-doped with AgCl and AgNO3has the better UC luminescence properties than the sample which was single doped with AgCl or AgNO3.Tellurite glass-ceramics doped with RE ions and Ag NPs are promising candidates for the development of lasers and optical amplifiers for PL based devices.

1 Experimental

1.1 Material preparation

Tellurite glasses with compositions 69TeO2-23WO3-8La2O3（TWL）containing fixed concentration of Er3+（0.5% （n/n）,Yb3+（1.0% （n/n）,AgCl（0～2.0% （w/w））and AgNO3（0～3.0% （w/w）were prepared by conventional melt-quenching method,melting anhydrous mixtures of TeO2（99.99%）,La2O3（99.99%）,Er2O3（99.99%）,Yb2O3（99.99%）,WO3（99.99%）,AgCl （99.9%）and AgNO3（99.9%）.A gold crucible containing the glass constituents was placed in an electric furnace at（790±10）℃ for 15 min and the melt was poured onto a preheated stainless steel plates.Subsequently,the samples were annealed at 380℃for 2 h to remove the thermal and mechanical strains completely.The samples were then cooled down to the room temperature.69TeO2-23WO3-8La2O3-0.5%Er2O3-1.0%Yb2O3（TWL-ErYb base glass）and 69TeO2-23WO3-8La2O3-0.5%Er2O3-1.0%Yb2O3-1.0%AgCl （TWL-ErYb-1AgCl）glasses were heat treated at different heat treatment conditions and other samples were heat treated at appropriate heat treatment condition（at 390 ℃ for 15 min）to form glass ceramics.The samples heat treated at different temperatures （T）and times （t）are denoted as T-t.Finally,all the samples were cut and polished for the structural and optical measurements.

1.2 Characterizations

Differential thermal analysis （DTA）measurement（NETZSCH STA 449C）was carried out by heating about 0.02 g of glass powder in alumina crucible at the heating rate of 10 K·min-1from 200 to 800℃.The used atmosphere in DTA was N2,and the gas flow was set as 40 mL·min-1.The DTA results of samples were referenced to that of alumina powder.Phase identification of the samples was performed by X-ray diffraction （XRD）analysis with Cu Kα （λ=0.154 06 nm,35 kV,30 mA）radiation （D8 Advance,Bruker Inc.,Germany）at room temperature in the 2θ range of 10°～70°with a step size of 0.02°and a step scanning time of 1 s.The UV-NIR absorption spectra of glasses were recorded by a UV-4100 UV/VIS/NIR spectrophotometer in the range of 400～1 100 nm.The fluorescence spectrometer （Model Omni-λ300 Zolix）,together with aphotomultiplierdetector （PMTH-SI-CR131）were used to measure the luminescence spectra under 980 nm diode laser within the range of 500～700 nm.Laser power is 194 mW and focusing methodology is lens focusing.Standard sample was used to compare the intensity between different spectra.Luminescence decay times in the microsecond time scale was measured on an Edinburgh FLSP920 spectrophotometer.All spectroscopic measurements were performed at room temperature.The characterizations of nanocrystal or microcrystal in glasses were carried out by a Transmission electron microscopy （TEM）（JEM-2100）with an accelerating voltage of 200 kV.The samples were grinded into fine powders in an agate mortar.Subsequently,the powders were dissolved in ethanol and then dispersed by supersonic before the solution was dropped on the copper grid.

2 Results and discussion

2.1 Effect of micro-crystallization on luminescence of the TWL-ErYb glasses

Comparing to the base glass,appropriate microcrystallization can improve the strength and thermal stability of glass.For example,oxyfluoride glassceramics doped with RE ions show higher chemical and mechanical stability and lower phonon energy than fluoride glasses[7].

The transparent TWL-ErYb glass-ceramics were prepared by appropriate heat treating.The absorption spectra of TWL-ErYb glasses before and after heat treatment at 390℃for 15 min are presented in Fig.1.After heat treatment,the absorption edge shows no obvious shift.The spectra exhibit a number of distinct absorption bands around 974,800,654,544,522 and 499 nm,which can be well assigned as the electronic transitions of Er3+from its ground4I15/2state to the4I11/2,4I9/2,4F9/2,4S3/2,2H11/2and4F7/2excited states,the absorption bands around 974 nm also include the electronic transitions of Yb3+from its ground2F7/2state to the2F5/2besides[15].This result clearly shows that Er3+ions exists in the glass.In order to further check whether microcrystals were formed,the samples were characterized by TEM,instead of the XRD which is found to be hard to detective small amount of nanometer size micro-crystals in the glass.

Fig.1 Absorption spectra of TWL-ErYb glasses before and after heat treatment（390 ℃-15 min）

Fig.2 shows representative HR-TEM images of TWL-ErYb base glass after heat treatment at 390℃for 15 min.There were many micro-crystals precipitated in the glass,and the size of micro-crystals varies from 40 to 150 nm.The measurement results of crystalline inter-planar space are 0.357 14 and 0.329 8 nm,corresponding to the （311）plane of Er2WO6（PDF No.38-0102）andLa2（WO4）3（PDF No.19-0669）.For this reason,it can be inferred that Er3+ions have been incorporated into micro-crystals,which hasbeen demonstrated to improve the efficiency of the UC luminescence to a certain extent[16].

Fig.2 HR-TEM images of TWL-ErYb base glass with heat treatment（390 ℃-15 min）and （b,c）are the enlarged view of（a）

Fig.3 shows the UC emission spectra of TWLErYb base glasseswith differentheattreatment temperatures under the excitation of 980 nm.Three UC emission bands that located at 538,557 and 674 nm can be clearly observed,which can be assigned to2H11/2→4I15/2,4S3/2→4I15/2and4F9/2→4I15/2transitions of Er3+ions,respectively[17].With the increase of heat treatment temperature,luminous intensity of each band gradually enhanced.As shown in Fig.1,after heat treatment,the transmittance of glass declined,to the contrary,the intensity of RE absorption peaks accordingly increased and the non-radiative-relaxation influences which caused by RE ions co-doping can be eliminated due to the lower phonon energy of glass ceramics[18].

Fig.3 Up-conversion emission spectra of TWL-ErYb base glasses for different heat treatment temperatures with the same treatment time of 15 min

What′s more,comparing to the red emission （674 nm）,the enhancement of the green emission （557 nm）is more obvious （Fig.3）.However,the enhancement ofluminescencecaused bymicro-crystalsisnot significant because of the incomplete structure of micro-crystals. Even when the heat treatment temperature was raised to 410℃,due to the overgrowth of micro-crystals,the luminous intensity of the glass was decreased.In order to investigate the luminescence mechanism of RE ions in glasses,the power dependence of UC emission intensity for TWLErYb base glasses without and with heat treatment at 390℃for 15 min were studied,as shown in the inset of Fig.3.The slope （n）for 557 nm wavelength of the sample without heat treatment was 1.84,and the sample with heat treatment at 390℃for 15 min was 1.71.The result confirms that the green emission （557 nm）originates from the two-photon process absorption of Er3+ions,and micro-crystallization has little effect on the UC mechanism of Er3+and Yb3+ions[19].

2.2 Effect of AgCl on luminescence of the TWLErYb glasses

Introducing Ag NPs into RE ions doped glasses can effectively alter the free space spectral properties of RE ions and enhance the yield of their weak optical transitions[20-21].It is common to introduce AgCl as source of silver in glasses.The introduction of AgCl can reduce the glass transition temperature （Tg）of glass and open the network structure of the glass well,leading to the precipitation of Ag NPs easily despite of the small solubility of AgCl in glass[22].

In order to study the effect of Ag NPs on the micro-crystallization process,XRD patterns of TWLErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions （Fig.4（a～c）and DTA curves of TWL-ErYb and TWL-ErYb-1AgCl glasses （Fig.4（d））at the heating rate of 10 K·min-1from 200 to 800 ℃were measured.Tgand the first crystallization temperature （Tc1）,the second crystallization temperature （Tc2）and the third crystallization temperature （Tc3）are pointed by the arrow in the Fig.4（d）.When the glasses were been heat treated at 420℃for 24 h,TWL-ErYb base glass was still transparent and TWL-ErYb-1AgCl glass became purple.There is no obvious crystallization peak on the XRD patterns,which may be caused by the small content of microcrystal and Ag NPs.When the glasses were been heat treated at 500℃for 24 h and 640℃for 24 h,the glasses were all opaque.The crystalline peaks oflanthanum tellurium oxides compounds were observed in Fig.4（b,c）.In addition,crystallization peaks of TWL-ErYb-1AgCl glasses are more obvious than TWL-ErYb base glass.Therefore,the Ag NPs are expected to be the nucleation agent and promote the precipitation of microcrystals.In general,this kind of glass is very stable and Ag NPs will not destroy the structure of the glass at a moderate heat treatment temperature.

The absorption spectra ofTWL-ErYb-1AgCl glasses with different heat treatment conditions are shown in Fig.5.After introducing Ag NPs into the glass,the absorption peak positions didn′t change.However,with the growth of Ag NPs caused by the increase of heat treatment temperature,the transmittance of TWL-ErYb glass with Ag NPs decreased.When the heat treatment temperature was raised to 460℃,in addition to the intrinsic absorption peaks of RE ions,the LSPR peak of Ag NPs was detected at the range of 500～650 nm[23].The emergence of the LSPR peak means that the content of Ag NPs increased significantly in the glass.

Fig.6（a）presents the HR-TEM image of TWLErYb-1AgCl glass with heat treatment at 390℃for 15 min.The shapes of Ag NPs are mainly spherical and ellipsoidal.Moreover,the size of Ag NPs is about 4～6 nm,while the size of micro-crystals is about 6～8 nm.The lattice fringes can be clearly observed in an enlarge image Fig.6（b）.The interplanar spacing of NPs is 0.238 1 nm,corresponding to the （111）plane of silver crystal （PDF No.65-8424）.These results prove the precipitation of Ag NPs in TWL-ErYb-1AgCl glass.It is important to note that,in the Fig.6（c）,we can find the precipitation of La2WO6micro-crystals around the Ag NPs.However,the number of Ag NPs is overall dominant.

Fig.4 XRD patterns of TWL-ErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions of（a）420 ℃-24 h,（b）500 ℃-24 h and （c）640 ℃-24 h;（d）DTA curve of TWL-ErYb and TWL-ErYb-1AgCl glasses

Fig.5 Absorption spectra of TWL-ErYb-1AgCl with different heat treatment

Fig.6 TEM images of the sample TWL-ErYb-1AgCl heat treated at 390℃for 15 min

AgNPsareformedfrom AgClorAgNO3throughout the melting procedure and grown during the annealing.The reduction of the Ag NPs can be discussed by the reduction potentials （E0）of redox system elements,as[24]:

Following reduction processes are likely to ensue:

where ΔE0is the total potential of reduction process.The equation （5～7）are all feasible reactions （with ΔE0＞0）.Therefore,these reactions guarantee the presence of Ag NPs in the system in addition to the absorption spectra results and TEM images.

The luminescence spectra of TWL-ErYb-1AgCl glasses with different heat treatment conditions were studied,the results are shown in Fig.7.After heat treatment,all of the samples obtained the stronger emissions.The sample with heat treatment at 390℃for 15 min acquired the best luminescence property.In this heat treatment condition,Ag NPs precipitated a lot and the average size of the micro-crystals was small.A lot of precipitations of Ag NPs,leading to short distance between Ag NPs and Er3+ions,made the energy transfer from Ag NPs to Er3+ions become a possible explanation for the enhanced luminescence[25].The non-resonance excitation light excites the d-band electron to unoccupied sp-conduction band[1].Subsequently,electron and hole recombine and moves to Fermi level through a phonon-electron interaction.Therefore,luminescence is mainly in visible region[26].The local electric field change cause the enhancement of photoluminescence and the effective electric field）can be written as[17]:

Fig.7 Up-conversion emission spectra of TWL-ErYb-1AgCl glasses at different heat treatment conditions

here ε0is the dielectric constant in the presence of an external electromagnetic field of amplitude,q is the nanocrystal specific volume,ωpis the plasma frequency,i is the ground level,and γ is the damping of the resonance.The damping γ is the contribution of the conduction electrons and it can be described as[2]:

The first term 1/τ0is related to the bulk electron scattering process in the nanoparticle.The interaction between quasi-electron-free and the surface of a sphere causes the second term,where VFis the Fermi velocity,D is the mean core particle size,and gsis the surface factor[26].The shape of the NPs is related to surface plasmon resonance （SPR）and the appropriate size of the NPs can enhance local field[27].Therefore,the glass with more Ag NPs,fewer micro-crystals,as well as high transmittance can obtain the enhancement of the luminescence property[28].The inset of Fig.7 showsthedependenceoftheUC luminescence intensity on the 980 nm pump laser power for 557 nm.The slope （n）for 557 nm wavelength of the sample without heat treatment was found to be 1.87,and the sample with heat treatment at 390℃for 15 min was found to be 2.05.This experimental result confirms that the green emission （557 nm）is due to the twophoton process absorption of Er3+ions.It is worth noting that the slope （n）of the glass doped with Ag NPs is larger than the tellurite base glass （Fig.4）,which implies that Ag NPs can promote the two-photon absorption of Er3+ions in the process of the energy transfer and promote particles jump to a high level[29].

Under the appropriate heat treatment condition at 390℃for 15 min,the up-conversion emission spectra of the glasses with different contents of AgCl are shown in Fig.8.The co-doped AgCl samples are denoted as TWL-ErYb-xAgCl（x=0～2% （w/w）.The reduction and growth ofAg nanocrystalsgenerate an efficient localized electric field around the Ag NPs.The local electric field can increase the rate of excitations of Er3+ions in vicinity of Ag NPs.As a result,the rates of transitions from emitting levels are enhanced[29].According to our testing results,the glass sample containing 0.75% （w/w）AgCl shows the maximum intensity enhancement.Furthermore,we find that the glass will be opaque after introducing 2%（w/w）AgCl into the glass.It means that doping content of AgCl is limited.

Fig.8 Up-conversion emission spectra of TWL-ErYbxAgCl with heat treatment temperature （390 ℃-15 min）

2.3 Effect of co-doped AgCl and AgNO3on luminescence of the TWL-ErYb glassceramics

Ag NPs can be produced by the introduction of AgCl,however,itwillcause the overgrowth of microcrystals at the same time.In order to find a better way to increase the precipitation of Ag NPs,the effect of different contents of AgNO3on luminescence of TWL-ErYb glass-ceramics has been studied,and the luminescence spectra are shown in Fig.9.The single doped AgNO3samples are denoted as TWLErYb-yAgNO3（y=0～3%, （w/w）.Under the heat treatment condition at 390℃for 15 min,the glass sample containing 2.5% （w/w）AgNO3shows the maximum intensity enhancement.Under the condition of large content,the luminescent property of the glass single doped AgNO3is better than that with single doped AgCl.However,the UC emission intensity enhancement of the glasses doped with a small content of AgNO3is not obvious.Besides,it was difficult to find Ag NPs in the TEM images of TWL-ErYb-2.5AgNO3.It is not easy for AgNO3to generate Ag NPs in the glasses.Therefore,we consider co-doping AgCl and AgNO3to inhibitthe precipitation of AgCl crystals and introduce more Ag+ions into the glasses.

Fig.9 Up-conversion emission spectra of TWL-ErYbyAgNO3with heat treatment temperature（390℃-15 min）

We studied the effect of the proportion of AgCl and AgNO3on luminescent properties,the results are shown in Fig.10.The single doped AgCl and AgNO3samples are denoted as TWL-ErYb-xAgCl-yAgNO3（x=0～1%,y=0～1%,（w/w）and the proportion of AgCl and AgNO3is x∶y.The luminescent intensity of the glass co-doped with AgCl and AgNO3was stronger than the glass which was single doped with AgCl or AgNO3.The sample with x ∶y=0.5 ∶0.5 displayed the highest luminescence intensity.The glass with x∶y=0.7∶0.3 show the better luminescence intensity than the glass with x∶y=0.3∶0.7,which proves that AgCl can have greater effect on fluorescence enhancement of glasses when the content of silver source is limited.However,when x∶y=1∶1,the glass didn′t obtain further enhanced luminescence intensity.The luminescence microsecond time resolution were performed on the luminescence of the Er3+ions in TWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO3glass.The samples were excited at 823 nm,and the decay curves were detected at 557 nm,as described in Fig.11.Both the luminescence decays are well fitted to single exponential decay function,the calculated lifetime of the Er3+ions in TWL-ErYb base glass （τEr）and TWL-ErYb-0.5AgCl-0.5AgNO3glass （τErAg）is 81.001 and 95.081 μs,respectively.The energy transfer between Ag NPs and Er3+ions and the effect of LSPR enhanced emission are might responsible for the longer lifetime of the glass with Ag NPs[30].Zhang et al[31].also find a longer lifetime of Er3+ions （4I13/2）in TeO2-WO3-La2O3-AgNO3glass than TeO2-WO3-La2O3glass.

Fig.10 Up-conversion emission spectra of TWL-ErYbxAgCl-yAgNO3glasses with heat treatment temperature （390 ℃-15 min）

Fig.11 Luminescence decay curves of the Er3+ions in TWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO3glass

LSPR and plasma coupling effectbetween particles make the effective enhancement of local electric field near the nanoparticles,leading to the increase of the radiative transition probability of each energy level of Er3+ions,which eventually makes luminescence emission enhanced.Comparing to LSPR and plasma coupling effect,the energy transfer between Ag NPs and Er3+ions is however the secon-dary factor lead to the enhancement of luminescence[30].Besides,micro-crystallization can decrease the phonon energy of glasses,and boost the energy level transition probability of RE ions[32].

Fig.12 TEM images of the sample TWL-ErYb-0.5AgCl-0.5AgNO3annealed at 380 ℃ for 2 h:（a）is the enlarged view of（c）and （b）is the enlarged view of（a）

TWL-ErYb-0.5AgCl-0.5AgNO3glasswith heat treatment at 390℃for 15 min shows the maximum enhancement and the HR-TEM images of the glass are shown in Fig.12.A large number of Ag NPs in uniform distribution were observed.The shapes of Ag NPs are mainly spherical and ellipsoidal.It can be clearly seen the regular arrangement of silver atoms,and the size is about 4～6 nm.However,we didn′t find micro-crystals in this TEM image.It is probably that the amount of Cl-ions of TWL-ErYb-0.5AgCl-0.5AgNO3glass is less than TWL-ErYb-1AgCl glass.The effect of AgCl on the precipitation of microcrystals becomes weak,and the glass becomes more stable.Therefore,the precipitation of micro-crystals wasnotobviousinTWL-ErYb-0.5AgCl-0.5AgNO3glass.Besides,TWL-ErYb-0.5AgCl-0.5AgNO3glass can provide the same amount of Ag+ions comparing to the TWL-ErYb-1AgCl glass.According to the above results,we conclude the follow results: （1）The introduction of AgCl can be helpful to produce Ag NPs,but it will cause the overgrowth of microcrystals;（2）It is not easy for AgNO3to produce Ag NPs,but introducing AgNO3can increase the content of Ag+ions.Therefore,co-doping AgCl and AgNO3can combine the characteristics of AgCl and AgNO3.Comparing to single doped AgCl or AgNO3,co-doping AgCl and AgNO3can bring more Ag NPs into the glass,and keep the glass transparent.Based on the above reasons,the glass with appropriate co-doping proportion of AgCl and AgNO3show an enhancement ofup-conversion emission intensity due to the formation of a lot of Ag NPs and a small amount of micro-crystals.

3 Conclusions

The role of micro-crystals and Ag NPs on the thermal,structural and spectroscopic properties of TWL-ErYb glasses have been studied in this paper.Micro-crystallization can improve the efficiency of luminescence emission.In the meantime,Ag NPs can also increase the strength of luminescence emission.The luminous efficiency of glasses can be further increased by introducing micro-crystals and Ag NPs at the same time.However,the excessive growth of micro-crystals will decrease the strength of luminescence emission.We find that co-doping AgCl and AgNO3can increase the precipitation of Ag NPs and reducetheovergrowth ofmicro-crystals.Besides,appropriate heattreatmenttemperature can also promote the precipitation of Ag NPs,and help to control the precipitation of micro-crystals.TWL-ErYb glasses with a lot of Ag NPs and a small amount of micro-crystals can further improve the up-conversion luminescence intensity due to the enhanced LSPR effect and a low phonon energy environment.

Acknowledgements:The research is supported by theShanghaiScienceTechnologyCommittee （GrantNo.12nm0504700）.

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