Appl Phys B (2010) 100: 443–446DOI 10.1007/s00340-010-4169-5

R A P I D C O M M U N I C AT I O N

Diode laser pumped Gd2O3:Er3+/Yb3+ phosphoras optical nano-heater

S.K. Singh · K. Kumar · S.B. Rai

Received: 11 March 2010 / Revised version: 2 July 2010 / Published online: 27 July 2010© Springer-Verlag 2010

Abstract An intense green upconversion (UC) emission(λexc = 976 nm) followed by the heating effect in Yb3+/Er3+co-doped Gd2O3 nanoparticles has been detected. A temper-ature rise up to 504 K has been observed (on a noteworthylow laser excitation of 290 mW) using fluorescence inten-sity ratio (FIR) method of the thermalized UC luminescencebands 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+ ion. Thereported controlled optical heating of nanoparticles and itsnano-volume has potential applications in biomedicines andin the creation of holes in soft materials.

1 Introduction

The inorganic luminescent materials (also called phosphors)doped with lanthanide ions have been extensively studiedbecause of their immense potential applications viz. whitelight emitting diodes (WLEDs), medical diagnostics (bio-imaging, bio-tagging), radiation and temperature sensors,fingerprint detection etc. [1–7]. The current interest in thisfield is focused on synthesizing nanophosphor materials us-ing improved techniques and looking for their new applica-tions. Out of various phosphors, Er3+/Yb3+ doped Gd2O3

upconversion (UC) nanophosphor has been found to behighly efficient and has been successfully used in differentapplications [5–7]. The high quantum yield is due to lowphonon energy of the Gd2O3 based host materials.

An Yb3+–Er3+ co-dopant sample has been known for itsefficient pumping at 976 nm by commercial laser diodes into

S.K. Singh · K. Kumar · S.B. Rai (�)Laser and Spectroscopy Laboratory, Department of Physics,Banaras Hindu University, Varanasi 221005, Indiae-mail: sbrai49@yahoo.co.inFax: +91-542-230-7308

the absorption band of Yb3+, which has the highest absorp-tion cross-section at 976 nm amongst the rare-earth ions.The large absorption cross-section of Yb3+ and efficientluminescence of Er3+ make Yb3+–Er3+ co-doped coupleespecially promising for doping of nanoparticles aiming athighly efficient luminescence.

In the present work, Yb3+/Er3+ co-doped Gd2O3 nano-phosphor has been synthesized and UC fluorescence hasbeen recorded by pumping the sample with 976 nm radi-ation. In addition to a strong UC fluorescence a substan-tial heating up to several hundred Kelvin has also been ob-served. The value of which has been calculated using fluo-rescence intensity ratio (FIR) method using the UC bands2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+ ion. Very re-cently, Tikhomirov et al. [8] observed similar heating inglass ceramics. However, the effect is reported in PbF2

(18.5 mol%) and CdF2 (31.5 mol%) based glass ceram-ics, which raises the concern regarding heavy metal toxic-ity in practical applications of the material viz. hypothermaltreatment of cells etc. It is notable that the maximum per-missible daily intake limit for Pb and Cd metal is 10 ppm(parts per million) and 0.3 ppm, respectively for the hu-man body [9]. Gadolinium, on the other hand, is relativelymore biocompatible and already in use for different biomed-ical applications [10]. In this work, we report a relativelymore biocompatible Gd2O3:Yb3+–Er3+ material which canbe used for producing heat in nanoparticles and its surround-ing nano-volume at a low laser power.

2 Experimental

The combustion synthesis technique for the preparation ofGd2O3:Yb3+–Er3+ nanophosphor is described in our earlier

444 S.K. Singh et al.

report [7]. The composition of the compounds used is

97.7Gd2O3 + 0.3Er2O3 + 2.0Yb2O3

X-ray diffraction (XRD) pattern of the sample has beenrecorded using 18 kW rotating anode (Cu) based Regakupowder diffractometer fitted with a graphite monochroma-tor in the diffracted beam. The transmission electron micro-scopic (TEM) image of the sample has been recorded usingTechnai 20 G2, Philips microscope. A near-infrared diodelaser emitting at 976 nm has been used to excite the sam-ple and the luminescence has been recorded using a iHR320Jobin Yvon spectrometer equipped with R928 photon count-ing photo multiplier tube. The laser beam is focused onthe sample with spot size of about 0.4 mm in diameter us-ing collimating optics. This radiation with power of about300 mW results a pump power density at the focal point ofabout 240 W/cm2. For fluorescence measurements at differ-ent temperatures, sample has been prepared in the form ofpellets (12 mm in diameter, 0.5 mm thickness). The thick-ness of the pellet has been limited below 0.5 mm. In addi-tion, during the fluorescence measurement the laser has beenfocused at a corner (instead of the middle) of the pellet andthe thermocouple is placed close to the focal spot. Merelya difference of a few millimeters (∼2) is noted in betweenfocal spot and the tip of the thermocouple.

3 Results and discussions

XRD pattern of the sample shows two crystalline phases,monoclinic and cubic, with cubic phase as the major one.Mean crystallite size of 50 ± 4.0 nm has been calculated forthe cubic phase using Debye–Scherrer formula. The trans-mission electron microscopic (TEM) image shows the pres-ence of the agglomerated crystallites of polygonal shape.A detailed structural characterization of the material is givenin our previous report [11].

This sample, irradiated with 976 nm laser excitation,emits a very strong anti-Stokes radiation with peak wave-lengths at 366, 380, 523, 546 and 667 nm correspond-ing to 4G9/2 → 4I15/2, 4G11/2 → 4I15/2, 2H11/2 → 4I15/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ion,respectively. Yb3+ ions act as the sensitizer for Er3+ ions.Figure 1 presents the laser input power dependent emissionof the sample for the two green bands 2H11/2 → 4I15/2 and4S3/2 → 4I15/2. It is evident from the spectra that thoughemission intensity of the two green bands increases with theincrease in input laser power however, the increase is notidentical and so a variation in the ratio in intensity of the twobands is observed. The increase in the intensity of the bandcorresponding to 2H11/2 → 4I15/2 is higher compared to4S3/2 → 4I15/2 and so the ratio (2H11/2 → 4I15/2)/(

4S3/2 →4I15/2) increases with increase in input laser power. This

Fig. 1 Effect of pump laser power on the emission intensity of thegreen bands (4S3/2 → 4I15/2 and 2H11/2 → 4I15/2) of the Er3+ ion(λexc = 976 nm, room temperature) in Er3+/Yb3+ co-doped Gd2O3phosphor

variation in the ratio indicates the increase in the sampletemperature (according to the basic theory of fluorescenceintensity ratio (FIR method)) and leads to the concept of anano-volume optical heater.

The intensity of the two bands (2H11/2 → 4I15/2 and4S3/2 → 4I15/2) not only in the present case but in otherhosts also has been shown to depend on the temperature ofthe sample [12–15]. The 2H11/2 and 4S3/2 levels of Er3+ions are separated in energy by only ∼700 cm−1. Hencetheir population follows Boltzmann distribution and rela-tive intensities of the lines emitted from them are tempera-ture dependent and environment sensitive. The temperaturerise can be detected by the temperature-dependent fluores-cence from two thermally coupled energy levels using FIRmethod.

The FIR method has been discussed in detail in severalpapers [16, 17]. In brief, if I10 and I20 are the fluorescenceintensities of the bands emitted from the two close lying lev-els (1 and 2) to a common level (0) then the intensity ratio(R) of the two transitions is

R = I20

I10= N2ω20A20

N1ω10A10= ω20A20g2

ω10A10g1exp

(−�E21

kT

)

= B exp

(−�E21

kT

), (1)

where

B = ω20A20g2

ω10A10g1,

where Aij and ωij are the spontaneous emission rate and theangular frequency of emission, respectively. Equation (1),

Diode laser pumped Gd2O3:Er3+/Yb3+ phosphor as optical nano-heater 445

in fact is the physical basis of the FIR method. It resultsan equation which is independent of the source intensity, asrequired for the effective temperature-monitoring scheme,and which is also a nonlinear function of temperature.

Equation (1) further transforms to a simple equation:

ln

(I20

I10

)= B − C

T, (2)

where C and B are the constants. It is seen from (2) that theratio R is related to the temperature T of the emitting sam-ple. However, the exact value of constants B and C shouldbe experimentally found for the sample before (2) could beapplied to evaluate the sample temperature precisely.

We have calculated the values of these constants for thepresent sample. The value of C was found to be ≈1015 K(taking �E = 700 cm−1 [18] and the value of Boltzmannconstant k = 0.6950356 cm−1/K). Further, for the deter-mination of value B , we recorded the UC spectrum of thesample on different temperatures at low input laser power(120 mW) with the aid of a chopper. An external chopper hasbeen used to chop the excitation beam at regular interval toavoid the heating of the sample due to laser power itself. Thetemperature-dependent UC emission of the sample is shownin Fig. 2. It is noticed that though the intensity of emissionoverall decreases on increasing the sample temperature, twoUC bands positions show no change. In addition, the ratioof the intensity of the two bands (I523/I546) increases.

Figure 2 demonstrates the variation of fluorescence inten-sity of the two bands at different temperatures and providesopportunity to calculate the value of the constant B . A sub-stitution of value of C, fluorescence intensity ratio (FIR) ofthe two bands and temperature T in (2) enables us to de-termine the average value of B . This value is found to be∼1.8 which is consistent with the range reported in the ear-lier work also [19, 20].

Now after having the values of the two constants B

and C, ratio of the intensity of the two green bands obtainedat different laser powers enable us to calculate the temper-ature gain of the sample using (2). A temperature rise ofabout 470 K was observed in the case of the sample beingirradiated at maximum laser power of 290 mW, using (2).

A plot of the FIR at different temperature (recorded withthe aid of chopper in input beam) versus temperature (T )

has been taken as the standard to calculate the temperaturegain of the sample. In Fig. 3 black asterisks correspond tothe fluorescence intensity ratio (FIR) of green UC emissionsat 523 and 546 nm (i.e. I523/I546) relative to the absolutetemperature recorded at a low input laser power and withthe aid of chopper, while a black line is the linear fit to theblack asterisks. Inset in Fig. 3 shows the values of FIR ofthe two bands at different laser pump powers. These valuesof the FIR (presented as blue squires) can be placed on the

Fig. 2 Effect of temperature (external heating of the material) on theintensity of the green bands (4S3/2 → 4I15/2 and 2H11/2 → 4I15/2) ofEr3+ ion (λexc = 976 nm, 120 mW power) in Er3+/Yb3+ co-dopedGd2O3 phosphor. External chopper has been used to chop the excita-tion beam at regular interval to avoid the heating of the sample due tolaser power itself

Fig. 3 Black asterisks correspond to the fluorescence intensity ratio(FIR) of the green upconversion emissions at 523 and 546 nm (i.e.I523/I546) relative to the absolute temperature. Black line is the linear fitto the black asterisks. Blue squire presents the values of FIR (I523/I546)

at higher input power (∼290 mW) resulting heating of sample

black straight line to find out the temperature gain at partic-ular pump power. The blue squire corresponding to the inputpower of 290 mW at the black straight line shows a temper-ature of ∼504 K, which is consistent to the value obtainedearlier by calculations using (2). Thus, a temperature riseof ∼504 K is obtained in nanoparticles and its surroundingvolumes at a laser power of 290 mW.

446 S.K. Singh et al.

The temperature rise of the sample at such a low laserpower can be understood in terms of the nanocrystalline na-ture of the material. Crystalline nanoparticles can efficientlyrelease heat under optical excitation. The laser electric fieldstrongly drives mobile carriers inside the nanocrystals, andthe energy gained by the carriers turns into heat through non-radiative channels. Heat diffuses away from the nanocrys-tal and leads to an elevated temperature of the surround-ing volume [21]. Thus, a small size of Gd2O3 nanocrys-tal hosting the dopants Er3+ and Yb3+ plays an importantrole in thermal release of absorbed energy into surround-ing medium due to their large surface-to-volume ratio. Inaddition, a quantum confinement of phonons and thereforeenhanced electron-phonon interaction taking place in thesenanoparticles [22], results in extra heating rate of nanoparti-cles.

4 Conclusion

In conclusion, Er3+–Yb3+ co-doped multifunctional Gd2O3

nanoparticles have been synthesized. Laser-induced temper-ature rise up to ∼504 K in the nanoparticles and its nano-volume along with the strong visible UC fluorescence hasbeen observed. This effect of optical heating in nano-volumecould find applications in biomedicine (viz. local hypother-mal treatment of cells) and creation of holes in soft materialssuch as organic matters.

Acknowledgements One of the authors, S.K. Singh thankfully ac-knowledges Council of Scientific and Industrial Research (CSIR), NewDelhi for providing financial assistance as SRF. We are also grateful toDST, New Delhi for financial assistance.

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