Chemical Physics Letters 536 (2012) 87–91

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Chemical Physics Letters

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Optical properties of SnO2 quantum dots synthesized by laser ablation in liquid

Manish Kumar Singh ⇑, Mohan Chandra Mathpal, Arvind AgarwalDepartment of Physics, Motilal Nehru National Institute of Technology, Allahabad 211004, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 January 2012In final form 23 March 2012Available online 30 March 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.03.084

⇑ Corresponding author. Fax: +91 532 2545341.E-mail address: maksingh4@gmail.com (M.K. Sing

SnO2 quantum dots (QDs) were controllably synthesized by laser ablation in liquid (LAL). The HRTEMimage shows that the diameters of the SnO2 nanoparticles fall into a small range of 1–5 nm, with themajority being less than the exciton Bohr radius of SnO2 (�2.7 nm). The Selected Area Electron Diffraction(SAED) pattern of SnO2 QDs shows tetragonal crystalline structure. The photoluminescence (PL) spectrumof such SnO2 QDs exhibits blue and green emission peaks at 445 nm and 540 nm respectively. These QDshave potential future applications in optoelectronics, biosensor and other modern technologies.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Research on semiconductor quantum dots systems has been ofgreat interest for the last two decades. This is mainly due to novelphysical and chemical properties displayed by these nanometer-scale structures. Quantum dots (QDs) are semiconducting inor-ganic nanocrystallites with critical dimensions smaller than orcomparable to the exciton Bohr radius of the material, in whichcarriers or excitons are confined in all three dimensions to a nano-meter sized region [1,2]. Consequently, discrete electronic states,unusual structural transformations, unique optical properties anda blue shift of the band edge transition energy are induced inQDs [3–6]. Due to the high surface-to-volume ratio, the surfaceatoms play a large role in the properties of QDs, which usually havefewer adjacent coordinate atoms and can be treated as defects ascompared with the bulk atoms. These defects induce additionalelectronic states in the band gap, which can mix with the intrinsicstates to a substantial extent and which may also influence thespacing of the energy levels and the optical properties of QDs.

Recently, tin oxide (SnO2) has attracted great attention becauseof its properties in broad fields such as transparent conductingcoating of glasses, gas sensors, solar cells, and heat mirrors [7–9].SnO2 is an n type semiconductor crystal with direct band gap(3.6 eV at 300 K) having high excitonic binding energy (130 eV).

SnO2 with various nano and microstructures, such as nanopar-ticles, nanorods, nanobelts, hollow microspheres, nanoflowers,mesoporous structures [10–15], nanowhiskers [16] and nanowires[17] have been prepared by different methods. Up till now variousmethods such as molten-salt synthesis [18], hydrothermal method[19,20], carbothermal reduction [21], chemical precipitation [22],microwave technique [23,24], sol–gel [25], sonochemical [26],etc., have been developed to fabricate tin oxide nanostructures.

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h).

In liquid environment the laser ablation of solids is a simple andversatile method to synthesize nanoparticles [27]. The correspond-ing studies are mainly focused on two subjects: (i) preparation ofnanoparticles via laser ablation in liquid medium and (ii) laser-induced modification of the size and shape of the nanoparticles.In this paper the optical properties of these nanoparticles havebeen discussed.

2. Experimental

Tin pellet of 99.9% purity was placed at the bottom of a glassvessel containing 20 ml of double deionized water. The pelletwas ablated for one hour by focused beam of 1064 nm of pulsedNd: YAG laser operating at 35 mJ/pulse energy with 10 Hz repeti-tion rate and 10 ns pulse width. The solution of colloidal nanopar-ticles was collected for characterization.

UV–Visible absorption spectrum of synthesized colloidal solu-tion was recorded using Perkin Elmer, Lambda-35 double beamspectrometer. For HRTEM analysis, a drop of colloidal solution ofnanoparticles was placed on a carbon coated copper grid followedby drying. Field emission transmission electron microscope (FE-TEM) measurements of the samples were taken on a JEOL JEM2100 F instrument with a 200 kV accelerating voltage. FourierTransform Infrared (FTIR) spectra of SnO2 nanoparticles were re-corded by Perkin Elmer RX-IR spectrometer. The photolumines-cence (PL) and photoluminescence excitation (PLE) wereperformed using a Perkin Elmer LS-55 spectrophotometer.

3. Results and discussion

3.1. UV–Visible absorption spectrum

Figure 1a shows UV–Visible absorption spectrum of colloidalsolution obtained by pulsed laser ablation of tin in double

Figure 1. (a) UV–Visible absorption spectrum of SnO2 quantum dots and (b) plot of (ahm)2 versus hm for SnO2 quantum dots synthesized by pulsed laser ablation method.

88 M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91

deionized water. SnO2 exciton broad band starts with a hump at300 nm because of near band edge absorption of electron in con-duction band and exciton broad band further increases up to200 nm. Stronger exciton effect is an important character of quan-tum confinement in nano-semiconductors as carriers are confinedin a very small region which makes the electron and hole to moveonly in a potential well. It can enhance the coupling interactionwith each other and the exciton becomes stronger, probability ofbinding increases, more exciton absorption peak may appear whenthe particle size decreases. Therefore the exciton absorption is ob-served and it shows blue shift near band edge absorption relativeto the bulk exciton absorption (345 nm or 3.6 eV) [19]. This blueshift represents the quantum confinement property of nanoparti-cles. In the quantum confinement range, the band gap of the parti-cle increases as the particle size decreases resulting in the shift ofabsorption edge to lower wavelength [28].

When the dimensions of nanocrystalline particles approach theexciton Bohr radius (aB), a blueshift in energy is observed due tothe quantum confinement phenomenon. The effective mass modelis commonly used to study the size dependence of optical proper-ties of QD systems. In this approach the exciton is treated analo-gously to a hydrogen atom, but is limited by spatial confinement.Therefore the energy of the system is obtained by solving the prop-er Schrodinger equation. In this manner two regimes are defineddue to the coupling of motion of the electron and the hole in theexciton: weak and strong confinements. In the former the particlesize is larger than aB, and the electron and the hole are treated as acorrelated pair. The blue shift of the band gap energy is describedby the following equation:

Eeffg ¼ Eg þ �h2p2=2lR2 � 1:8e2=eR ð1Þ

where R is the particle radius, �h the plank constant, l is the effectivereduced mass, e is the charge of the electron, Eg is the bulk band gapenergy (3.60 eV), static dielectric constant e of approximately 14and Eeff

g is the effective band gap energy. As the effective mass ofthe electrons is much smaller than that of the holes (m�e = 0.27me),since m�e � m�h (m�e and m�h are the electron and hole effectivemasses respectively). The charge carrier confinement mainly affectsthe energy level of the electrons [29,30].

The size dependence of the band gap energies of the quantum-confined SnO2 particles agrees very well with the confinement re-gime. The band gap energy Eg for SnO2 nanoparticles can also bedetermined by extrapolation to the zero absorption coefficients,which are calculated using the following equation:

ðahmÞ1=n ¼ Bðhm� EgÞ ð2Þ

where B is a constant, a is absorption coefficient, Eg is the band gap,hm is the photon energy and n is a value that depends on the natureof the transition. In this case, n is equal to 1/2 for the direct allowedtransition. The band gap can be estimated from a plot of (ahm)2 ver-sus photon energy. The estimated band gap energy is 4.37 eV forsynthesized sample Figure 1b. The increase of the band gap ofSnO2 is due to the smaller size of the particles. By applying Eq. (1)the radius (R) of SnO2 QDs found to be 1.55 nm. This study showsthat the quantum confinement effect of SnO2 is only at particle sizebelow the Bohr radius (2.7 nm).

3.2. Transmission electron microscopy

The morphology, particle size and structure of the synthesizedsample were characterized by TEM and HRTEM. The TEM imagesare shown in Figure 2a and b which reveals that the product iscomposed of homogeneous ultrafine self assembled nanoparticleswith a diameter below 5 nm. The SAED ring pattern in Figure 2cfor SnO2 QDs shows that the particles are well crystallized [31].The diameter of the diffraction ring in SAED pattern is proportionalto (h2 + k2 + l2)2 where (hkl) are the Miller indices of the planes cor-responding to the ring, counting from the center. First, second andthird ring correspond to (111), (200) and (220) planes respec-tively. Figure 2d is the HRTEM image of the SnO2 QDs, whichclearly reveals the lattice fringes with a d-spacing of about0.176 nm, corresponding to (211) planes of SnO2 (JCPDS No. 41-1445). Highly ordered lattice fringes, even in the surface region,are evidence of the crystallinity of the particles.

From the data obtained by TEM images, the particle size distri-bution graph shown in Figure 2e, and the mean size of the particlescan be determined. It can be seen that the diameter (2R) of thequantum dot is from 1 nm to 5 nm, and the median diameter (takenas average particle diameter) is about 2.5 nm. The radius (R) of QDscalculated from absorption data is 1.55 nm. Both the results showthat the size of QDs is less than the Bohr radius (R = 2.7 nm).Therefore, the SnO2 nanoparticles are called SnO2 quantum dots(QDs).

3.3. FTIR spectra

The FTIR spectrum is shown in Figure 3 for SnO2 nanoparticlessynthesized by LAL in water. The spectrum was recorded of a filmcoated on a glass substrate by suspending the solution drop by

Figure 2. TEM images of the SnO2 quantum dots: (a and b) TEM images at 20 nm and 5 nm scale respectively (c) HRTEM image (d) SAED pattern (e) particle size histogramcurve of SnO2 QDs.

M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91 89

drop at 80 �C for evaporating the water. FTIR is recorded in therange of 1000–400 cm�1 for confirmation of SnO2. A band appearsin the range of 400–700 cm�1 is assigned to Sn–O antisymmetricvibration [32,33].

3.4. Photoluminescence spectrum

The photoluminescence (PL) emission spectrum for SnO2 quan-tum dots synthesized by PLA in water at an excitation wavelengthof 271 nm is shown in Figure 4. The peak shows three individualpeaks at 409 nm, 447 nm and 540 nm respectively indicating theblue and green emissions. These three peaks are smaller thanthe band gap of SnO2 which is 4.37 eV therefore the electron is

not directly recombining to a hole in the valance band from con-duction band [34].

Generally, defects such as oxygen vacancies are known to be themost common defects in oxides and usually act as radiative centersin luminescence processes. Among the oxygen vacancies in theoxide, only the single ionized oxygen vacancy (V 0o) state observedby EPR is paramagnetic, and it is expected that most oxygen vacan-cies will be in their paramagnetic V 0o state under flat-band condi-tions [34]. V 0o is assumed to be the recombination center for theluminescence emission, which has an effective monovalent posi-tive charge with respect to the regular O2� site. After such a recom-bination the effectively neutral center V0

o (neutral charge vacancyfor oxygen) will be formed, whose energy is very close to theconduction band edge due to the correlation energy of the two

Figure 3. FTIR spectrum of the SnO2 quantum dots synthesized by pulsed laserablation.

Figure 4. Photolumniscence spectrum of SnO2 quantum dots synthesized by pulsedlaser ablation.

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electrons at room temperature, all V0o centers are thermally disso-

ciated into V 0o centers and conduction band electrons. Therefore,photons with an energy of 2.7 eV (445 nm) will not be emittedthrough a transition of an electron from the conduction band toa V 0o level, as such a transition effectively occurs between the con-duction band edge and the V0

o level. However, recombination of aconduction band electron with a V 00o center (an oxygen vacancycontaining no electrons, having an effective divalent positivecharge with respect to the normal O2� site) can yield such a blueemission band. Such V 00o centers can be formed when a hole istrapped at a V 0o center. Thus, a model for this blue emission ofthe photoexcited SnO2 nanoparticles can be proposed. The excita-tion of the SnO2 nanoparticle starts with the creation of an elec-tron–hole pair. The electron is promoted from the valence bandto the conduction band, leaving a hole in the valence band. The ac-tive hole formed can be trapped at the V 0o center directly to formthe V 00o center or at the surface of the particle. Then the surface-trapped hole may transfer back into the particle to recombine withan electron in a deep trap (V 0o) to form the V 00o center. Thereafter,recombination of a V 00o center with a conduction band electron givesrise to the blue emission. The blue emission band has been gener-ated due to the recombination of the deep trapped charges andphotogenerated electrons from the conduction band [35]. The

green emission band (540 nm) can be atrributed to radiativerecombination from defect such as oxygen vacancies [36]. The de-fects may arise because of non-equilibrium process in LAL [37].Zeng et al. have shown the presence of defects formed in highlynon-equilibrium conditions had a significant impact on the lumi-nescence of ZnO. A detailed study about blue emissions, greenemissions and quantum confinement effect for the increase inthe band gap of ZnO and other semiconducting nanoparticles dueto smaller particle size is reported by Zeng et al. [38,39].

4. Conclusions

SnO2 quantum dots were synthesized by LAL. The estimatedband gap energy is 4.37 eV for SnO2 QDs. The increase of the bandgap of SnO2 is due to the smaller size of the particles (quantumcomfinement effect). The diameters of the SnO2 nanoparticles fallinto a small range of 1–5 nm, with the majority of them are lessthan the exciton Bohr radius of SnO2 (2.7 nm). Therefore, theSnO2 nanoparticles are called SnO2 (QDs). The SAED ring patternshows that the SnO2 QDs are well crystallized. The photolumines-cence (PL) emission spectrum shows the blue and green emission.Blue and green emission bands occur due to the recombination ofthe deep trapped charges and photogenerated electrons from theconduction band, and also due to the defect such as oxygenvacancies.

Acknowledgments

Authors are thankful to Dr. Gajender Saini, AIRF-JNU Delhi forTEM facilities and Prof. Ram Gopal, University of Allahabad forextending the facilities available with them.

References

[1] A.P. Alivisatos, Science 271 (1996) 933.[2] A. Henglein, Chem. Rev. 89 (1989) 1861.[3] D. Granados, J.M. Garcıa, Nanotechnology 16 (2005) S282.[4] X. Peng, Adv. Mater. 15 (2003) 459.[5] L. Qu, W.W. Yu, X. Peng, Nano Lett. 4 (2004) 465.[6] C. Burda, S. Link, M. Mohamed, M. El-Sayed, J. Phys. Chem. B 105 (2001) 12286.[7] A. Diéguez, A. Romano-Rodríguez, J.R. Morante, U. Weimar, M. Schweizer-

Berberich, W. Göpel, Sens. Actuat. B: Chem. 31 (1996) 1.[8] N. Kudo, Y. Shimazaki, H. Ohkita, M. Ohoka, S. Ito, Sol. Cell 91 (2007) 1243.[9] C. Choudhury, H.K. Sehgal, Energy Convers. Manage. 29 (1989) 265.

[10] Q.Y. Pan, J.Q. Xu, X.W. Dong, J.P. Zhang, Sens. Actuat. B: Chem. 66 (2000) 237.[11] A.A. Firooz, A.R. Mahjoub, A.A. Khodadadi, Mater. Lett. 62 (2008) 1789.[12] P. Andrei, L.L. Fields, J.P. Zheng, Y. Cheng, Actuat. B: Chem. 128 (2007) 226.[13] Q.R. Zhao, Y. Gao, X. Bai, C.Z. Wu, Y. Xie, Eur. J. Inorg. Chem. 8 (2006) 1643.[14] A.C. Chen, X.S. Peng, K. Koczkur, B. Miller, Chem. Commun. 17 (2004) 1964.[15] Y.D. Wang, C.L. Ma, X.D. Sun, H. Li, Micropor. Mesopor. Mater. 49 (2001) 171.[16] S.H. Luo et al., Nanotechnology 15 (2004) 1424.[17] S. Mathur, S. Barth, H. Shen, J.C. Pyun, U. Werner, Small 1 (2005) 713.[18] D. Wang, X.F. Chu, M.L. Gong, Sens. Actuat. B: Chem. 117 (2006) 183.[19] B. Liu, H.C. Zeng, J. Phys. Chem. B 108 (2004) 5867.[20] H. Huang, O.K. Tan, Y.C. Lee, M.S. Tse, J. Guo, T. White, Nanotechnology 17

(2006) 3668.[21] S. Thanasanvorakun, P. Mangkorntong, S. Choopun, N. Mangkorntong, Ceram.

Int. 34 (2008) 1127.[22] Y. Dabin, W. Debao, Y. Weichao, Q. Yitai, Mater. Lett. 58 (2006) 84.[23] T. Krishnakumar, R. Jayaprakash, N. Pinna, V.N. Singh, B.R. Mehta, A.R. Phani,

Mater. Lett. 63 (2009) 242.[24] T. Krishnakuma, R. Jayaprakash, M. Parthibavarman, A.R. Phani, V.N. Singh, B.R.

Mehta, Mater. Lett. 63 (2009) 896.[25] L. Korosi, S. Papp, V. Meynen, P. Cool, E.F. Vansant, I. Dekany, Colloid Surf. A:

Physicochem. Eng. Aspects 268 (2005) 147.[26] H.X. Luo, Z. Ying-Ji, W. Shi-Wei, Mater. Chem. Phys. 88 (2004) 421.[27] M.K. Singh, A. Agarwal, R. Gopal, R.K. Swarnkar, R.K. Kotnala, J. Mater. Chem. 21

(2011) 11074.[28] M.A. Gondal, Q.A. Drmosh, T.A. Saleh, Appl. Surf. Sci. 256 (2010) 7067.[29] L. Brus, J. Phys. Chem. 90 (1986) 2555.[30] E.J.H. Lee, C. Ribeiro, T.R. Giraldi, E. Longo, E.R. Leite, J.A. Varela, Appl. Phys.

Lett. 84 (2004) 1745.[31] J. Ba, J. Polleux, M. Antonietti, M. Niederberger, Adv. Mater. 17 (2005) 2509.[32] S. Emiroglu, N. Barson, V. Weimar, V. Hoffman, Thin Solid Films 391 (2001)

176.

M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91 91

[33] P. Shah, A.V. Ramaswamy, K. Lazar, V. Ramaswamy, Appl. Catal. A: Gen. 273(2004) 239.

[34] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J.Appl. Phys. 79 (1996) 7983.

[35] F. Gu, S.F. Wang, M.K. Lu1, G.J. Zhou, D. Xu, D.R. Yuan, J. Phys. Chem. B 108(2004) 8119.

[36] X. An, G. Mang, Q. Wei, X. Zhang, Y. Hao, L. Zhang, Adv. Mater. 17 (2007) 1781.[37] H. Zeng, X.W. Du, S.C. Singh, S.A. Kulinich, S. Yang, J. He, W. Cai, Adv. Funct.

Mater. (2012), http://dx.doi.org/10.1002/adfm.201102295.[38] H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, W. Cai, Adv. Funct. Mater. 20 (2010) 561.[39] H. Zeng, W. Cai, P. Liu, X. Xu, H. Zhou, C. Klingshirn, H. Kalt, ACS Nano 2 (2008)

1661.