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The luminescence of PbS nanoparticles embedded in sol–gel silica glass
21 September 2001
Chemical Physics Letters 345 (2001) 429±434
www.elsevier.com/locate/cplett
The luminescence of PbS nanoparticles embedded in sol±gel silica glass Ping Yang a, Chun Feng Song a, Meng Kai L u a,*, Xin Yin a, Guang Jun Zhou b, Dong Xu a, Duo Rong Yuan a a b
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Shandong Supervision and Inspection Institute for Product Quality, Jinan 250100, PR China Received 25 April 2001; in ®nal form 28 June 2001
Abstract PbS nanoparticles embedded in a novel silica glass have been achieved in the present study by sol±gel processing. Their ¯uorescence properties have been evaluated and compared with those of un-doped glass. A novel luminescent phenomenon has been observed from the composite of PbS nanoparticles and the sol±gel silica glass. The photoluminescence (PL) spectrum of the composite consists of two emission peaks (440 and 605 nm). The Pb2 ions in the sol± gel silica glass show sharp emission band. This novel luminescence from the samples is assigned to the composite structure of PbS nanoparticles and porous silica glasses. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Semiconductor nanocrystallites are of great interest because of their unique optical properties and potential applications for optoelectronics [1,2]. During recent years, a great deal of work have been carried out which laid a special emphasis on the nanocomposites of nanoparticles embedded in some dielectric matrices such as glasses and polymer for their advantages of stabilizing dots and being adapted to device manufacturing process [3]. These embedded structures have some advantages and oer attractive options: for examples, the chemical and mechanical stabilities, the decrease of the total capacitance to optical waveguides and photonic devices (transparent
*
Corresponding author. Fax: +86-531-8565403. E-mail address: [email protected] (M.K. L u).
matrix) [4]. Extensive researches ± both theoretical and experimental ± have been done on II±VI semiconductor nanoparticles in inorganic and organic matrix [5±7]. In the matrix, the optical properties of II±VI semiconductor quantum dots (QDs) have been extensively investigated. The photoluminescence (PL) and electroluminescence (EL) of the QDs in some matrix have been reported [8,9]. PbS is of considerable interest as a phosphor for luminescent display devices. PbS is a unique semiconductor material with a rather small band gap (0.41 ev, at 300 K) [18]. This band gap can be easily manipulated by the materials dimensions and reached a few electronvolts when PbS particles in the nanometer regime are formed. Thus, the luminescent property of PbS nanoparticles can be varied easily by host material. PbS-doped glass (it is not sol±gel porous silica glass) recently reported as a saturable absorber for
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 2 6 - 5
430
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
mode locking of a Cr:forsterite laser [11,12] is one of the most noteworthy examples of utilization of optical nonlinearities in semiconductor nanocrystals. Also, optical nonlinearities of glass doped with PbS nanocrystals were studied [13]. In this Letter, we present the study of PL properties of PbS nanoparticles embedded in silica glasses by sol±gel processing for the ®rst time. The QDs of PbS are well-distributed in the glass matrices. Strong two emission bands have been observed from PbS nanoparticles embedded in sol±gel silica glasses. The QDs of PbS have sharp emission band in the porous silica glass. 2. Experimental 2.1. Sample preparation Sols were prepared by sol±gel processing using tetra-ethylorthosilicate (TEOS, Si OC2 H5 4 ). The Si OC2 H5 4 was ®rst hydrolyzed and stirred for one hour. The water:TEOS:ethanol:HCl (6 N) volume ratio of 1:1:0.5:0.00125 was used. Then, the salt solutions of lead acetate (Pb CH3 COO2 3H2 O) and Na2 S were added in the resultant homogeneous sol and stirred for 30 min. The sol so prepared was kept at room temperature until its complete gelation. Final drying of the gel was obtained after one week at room temperature. The xerogel was then heated in atmosphere at a rate of 6 °C/h up to the temperature of 50 °C, and kept 10 h at this temperature. The doped mole ratios of PbS nanoparticles in the sol±gel silica glasses are 0.090%, 0.134%, 0.224%, 0.269%, respectively. 2.2. Apparatus The absorption spectra of the samples were measured on a spectrophotometer (U-3500). The ¯uorescent spectra of the samples were measured on Hitachi M-850 ¯uorescence spectrophotometer. Ideal spectral data were recorded with the slits set at 10 nm. First, the excitation spectra of the samples were measured. The emission spectra of the samples were then obtained. The Fourier transform infrared (FTIR)
spectra of samples were collected using a 5DX FTIR spectrometer. 3. Results and discussion The transparence of the doped sample decreases with the increasing of the impurity mole ratio of PbS. When the impurity mole ratio of PbS is 0.269%, the doped sample is almost opacity. Fig. 1 shows the FTIR spectra of the doped and undoped samples. The FTIR spectra of PbS nanoparticles embedded in the glass and pure glass sample are similar to the spectrum of pure amorphous SiO2 [14]; strong bands associated with Si± O streching and bending vibrations are apparent at 1082, 797, 465 cm 1 . The absorption in 3444 cm 1 indicates the presence of hydroxy (presence of H2 O). Additional weak absorption in the 1638 and 955 cm 1 indicate the presence of oxethyl. Because the bond of Pb±S is mainly electrovalent bond, the FTIR spectra of doped samples do not show strong bands associated with Pb±S streching and bending vibrations. Fig. 2 shows the absorption spectra of PbS nanoparticles embedded in the glass and pure glass sample. It indicates that the absorption spectra have a red shift with the increasing of the impurity mole ratios of PbS. When the impurity mole ratio of PbS P 0:224%, the absorption spectra show a broad absorption band (a maximum at 550 nm) of PbS nanocrystallites. Fig. 3 shows the excitation and emission spectra of the pure glass and PbS nanoparticles-doped samples. The PL spectrum of PbS nanoparticles embedded in sol±gel silica glass consists of two emission peaks. One is at 440 nm (kex 380 nm) while the other is at 605 nm (kex 475 nm). Also, the excitation spectrum of PbS nanoparticles embedded in sol±gel silica glass consists of two excitation peaks. One is at 380 nm while the other is at 475 nm (kem 605 nm). When excitation wavelength is 380 nm, the emission peak of 605 nm is very weak. The relative ¯uorescence intensity of the doped samples increases remarkably than that of the pure glass sample. The emission peak of the pure glass sample is 440 nm. And the excitation peak of the pure glass sample is 380 nm. Therefore,
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
431
Fig. 1. FTIR spectra of the doped and un-doped samples (mean impurity mole ratio).
Fig. 2. Absorption spectra of the doped and un-doped samples (mean impurity mole ratio).
the second emission (605 nm) and excitation peak (475 nm) of the doped samples are assigned to the QDs of PbS in the sol±gel silica glass. Because excessive impurity in luminescent host materials may lead to the ¯uorescence quenching, the relative ¯uorescence intensity of the doped samples
decreases with the increasing of the impurity mole ratios of PbS when the impurity mole ratios of PbS nanocrystallites in sol±gel silica glass exceed certain values (e.g., 0.269%). The luminescence properties of Pb2 ion (6S2 con®guration) are diverse. Usually, they are de-
432
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
Fig. 3. Excitation and emission spectra of the doped and undoped samples (mean impurity mole ratio). Excitation wavelength: (a) kem 380 nm; (b) kem 380 nm.
scribed by the 1 S0 ±3 P0;1 transitions which originate from the 6S2 ±6S6P intercon®gurational transition [15]. Besides, it is known that a complication of dierent origin arises if the excited states of the
host lattice or the surroundings of the Pb2 ion are at about the same energy as the levels of the 6s6p con®guration of the Pb2 ion [16,17]. Therefore, the luminescence properties of Pb2 ion can be aect by host materials. Pb2 ions have dierent luminescent spectra in dierent matrix materials. Our experimental result is signi®cantly dierent from that of reported by Chen et al. [18]. In their work, PbS nanoclusters have a broad emission band (a maximum at 804 nm). In SrTiO3 matrix, the broad PL band of Pb2 is at 600±700 nm [15]. For CaS:Pb EL material, a blue phosphor has been observed [20]. Also, this test result is dierent from that of our previous work [10]. Pb2 ions have broad emission band in ZnS nanocrystallites. The ¯uorescence intensity of Pb2 -doped ZnS nanocrystallites (the relative ¯uorescence intensity is about 2) [10] is lower than that of the sol±gel silica glass doped with PbS nanoparticles (the relative ¯uorescence intensity of PbS nanocrystallites-doped sol±gel silica glass is about 24). Usually, two emissions are observed from luminescent materials-excitonic and trapped luminescence [21]. The excitonic emission is sharp and is located near the absorption edge of the materials while the trapped emission broad and is stokesshifted. The trapped luminescence arising from the surface states is observed in the doped samples. For the broad emission (trapped emission) of Pb2 , non-recombination of luminescent process increases than that of sharp emission peak (excitonic emission) of Pb2 . Therefore the ¯uorescence intensity of PbS nanoparticles embedded in the sol±gel silica glass dramatically increases than that of PbS nanoparticles in ZnS nanocrystallites. The luminescence of the sol±gel silica glass is well-known and has been ascribed to a defect luminescence (such as, Si dangling and oxygen vacancy, . . .) [14]. By introducing PbS nanoparticles, one can expect luminescence from a dierent origin. The structure defects of the sol±gel silica glass can be aected by the neighboring PbS nanoparticle, or the PbS nanoparticle can be active centers. By comparing the luminescence of the pure sol± gel silica glass with the PbS embedded in the sol± gel silica glass, an emission band consisting of two contributions is observed. The shorter wavelength contribution has a maximum at 440 nm, and is
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
ascribed to the defect luminescence in the sol±gel silica glass. The second contribution is ascribed to centers involving PbS nanoparticles (a maximum at 605 nm). Since the luminescence of lead centers in the PbS nanocrystallites embedded in the sol±gel silica glass involves a transition from the conduction band to the ground state level of divalent lead, situated near the valence band, no concentration dependence on the spectral position can be observed. The sol±gel SiO2 glass is a porous phosphor material. Due to the inclusion of many defects, it can exhibit a novel luminescent behavior (common SiO2 glass does not show PL) [14]. The ¯uorescence eciency of the sol±gel glass increases with the increasing of the defects in silica glass network. As for PbS particles in nanometer size, we cannot ignore the existence of the electron- or hole-trapped surface levels on PbS. Because most of the ions of the nanoparticles are located on the surface and they are non-saturated in coordination, electrons or holes may be excited easily and escape from the ions. Then they are trapped on the surface states located in the forbidden gap. The PbS nanoparticles are embedded in the networks of the silica glass in sol±gel processing. First, the nanoparticles can lead to form more Si dangling and oxygen vacancy in the network of the silica glass. More electrons or holes excited easily and radiative recombinations are increased. Thus, the relative ¯uorescence intensity of the doped samples increases remarkably. Second, the luminescent processing of ¯uorescent materials involves radiative combination and non-radiative combination. The PbS QDs in the silica glass are isolated and stabilized. And they occupy narrow distribution of size. The isolative distribution of PbS nanoparticles in the matrix leads to decrease non-radiative combinations in luminescence processing. Therefore, the ¯uorescence intensity of the emission band of PbS increases remarkably. The relative ¯uorescence intensity of PbS nanoparticles embedded in the sol±gel silica glass is about 12 times of that of Pb-doped ZnS nanoparticles [10]. The band gap of semiconductor nanocrystallites Eg0 as a function of particles radius is given by [19]: Eg0 Eg2 2 h2 Eg p=r2 =m 1=2 , Eg : the band gap of the bulk semiconductor, h: Planck's constant, m :
433
the eective mass of electrons, taking m 0:085 me , me : being the free electron mass. The narrow distribution of size of PbS nanoparticles leads to increase the luminescent monochromaticity of PbS nanoparticles. Therefore, the sharp emission band of divalent lead has been observed from the PbS nanoparticles embedded in the sol±gel silica glass.\
4. Conclusion By introducing PbS nanoparticles into the sol± gel silica glass, two strong luminescent bands (kem1 440; kem2 605 nm) have been observed. Their novel emission phenomenon is due to the interaction between the PbS nanoparticle and the porous phosphor glass. The sharp emission band of divalent lead has been observed from the PbS nanoparticles embedded in the sol±gel silica glass. The novel emission phenomenon is very important for the applications and research of nanometer scale luminescence materials and the sol±gel silica glass.
Acknowledgements The authors thank the Natural Science Foundation of China for support (Grant No. 69890230).
References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] L.E. Brus, Appl. Phys. A 53 (1991) 465. [3] J. Zhou, L. Li, Z. Gui, S. Buddhudu, Y. Zhou, Appl. Phys. Lett. 76 (2000) 1540. [4] D.M. Burland, R.D. Miller, C.A. Walsh, Chem. Rev. 94 (1994) 31. [5] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [6] P. Maly, T. Miyoshi, J. Lumin. 90 (2000) 129. [7] T. Hayakawa, S.T. Selvan, M. Nogami, J. Lumin. 87±89 (2000) 532. [8] S.A. Empedocles, M.G. Bawendi, Science 278 (1997) 2114. [9] B.O. Dabbousi, M.G. Bawedi, O. Onitsuka, F. Rubner, Appl. Phys. Lett. 66 (1995) 1316. [10] P. Yang, M.K. L u, D. Xu, D. Yuan, G. Zhou, Chem. Phys. Lett. 336 (2001) 76.
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P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
[11] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. [12] K. Wundke, S. P otting, J. Auxier, A. Sch ulzgen, N. Peyghambarian, N.F. Borrelli, Appl. Phys. Lett. 76 (2000) 10. [13] G. Tamulaitis, V. Gulbinas, G. Kodis, A. Dementjer, L. Valkunas, I. Motchalov, H. Raaben, J. Appl. Phys. 88 (2000) 178. [14] W.H. Green, K.L. Le, J. Grey, T.T. Au, M.J. Sailor, Science 276 (1997) 1826.
[15] H.F. Folkerts, G. Blasse, Chem. Mater. 6 (1994) 969. [16] G. Blasse, J. Solid State Chem. 18 (1988) 79. [17] A. Ragfagni, D. Mugnai, M. Bacci, Adv. Phys. 32 (1983) 823. [18] S.W. Chen, L.A. Truax, J.M. Sommer, Chem. Mater. 12 (2000) 3864. [19] M. Mukherjee, A. Datta, S.K. Pradhan, D. Chakravorty, J. Mater. Sci. Lett. 15 (1996) 654. [20] S.J.Yun,Y.S.Kim,S.H.Park,Appl.Phys.Lett.78(2001)721. [21] W. Chen, Z. Wang, Z. Lin, L. Lin, Appl. Phys. Lett. 70 (1997) 1466.
Chemical Physics Letters 345 (2001) 429±434
www.elsevier.com/locate/cplett
The luminescence of PbS nanoparticles embedded in sol±gel silica glass Ping Yang a, Chun Feng Song a, Meng Kai L u a,*, Xin Yin a, Guang Jun Zhou b, Dong Xu a, Duo Rong Yuan a a b
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Shandong Supervision and Inspection Institute for Product Quality, Jinan 250100, PR China Received 25 April 2001; in ®nal form 28 June 2001
Abstract PbS nanoparticles embedded in a novel silica glass have been achieved in the present study by sol±gel processing. Their ¯uorescence properties have been evaluated and compared with those of un-doped glass. A novel luminescent phenomenon has been observed from the composite of PbS nanoparticles and the sol±gel silica glass. The photoluminescence (PL) spectrum of the composite consists of two emission peaks (440 and 605 nm). The Pb2 ions in the sol± gel silica glass show sharp emission band. This novel luminescence from the samples is assigned to the composite structure of PbS nanoparticles and porous silica glasses. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Semiconductor nanocrystallites are of great interest because of their unique optical properties and potential applications for optoelectronics [1,2]. During recent years, a great deal of work have been carried out which laid a special emphasis on the nanocomposites of nanoparticles embedded in some dielectric matrices such as glasses and polymer for their advantages of stabilizing dots and being adapted to device manufacturing process [3]. These embedded structures have some advantages and oer attractive options: for examples, the chemical and mechanical stabilities, the decrease of the total capacitance to optical waveguides and photonic devices (transparent
*
Corresponding author. Fax: +86-531-8565403. E-mail address: [email protected] (M.K. L u).
matrix) [4]. Extensive researches ± both theoretical and experimental ± have been done on II±VI semiconductor nanoparticles in inorganic and organic matrix [5±7]. In the matrix, the optical properties of II±VI semiconductor quantum dots (QDs) have been extensively investigated. The photoluminescence (PL) and electroluminescence (EL) of the QDs in some matrix have been reported [8,9]. PbS is of considerable interest as a phosphor for luminescent display devices. PbS is a unique semiconductor material with a rather small band gap (0.41 ev, at 300 K) [18]. This band gap can be easily manipulated by the materials dimensions and reached a few electronvolts when PbS particles in the nanometer regime are formed. Thus, the luminescent property of PbS nanoparticles can be varied easily by host material. PbS-doped glass (it is not sol±gel porous silica glass) recently reported as a saturable absorber for
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 2 6 - 5
430
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
mode locking of a Cr:forsterite laser [11,12] is one of the most noteworthy examples of utilization of optical nonlinearities in semiconductor nanocrystals. Also, optical nonlinearities of glass doped with PbS nanocrystals were studied [13]. In this Letter, we present the study of PL properties of PbS nanoparticles embedded in silica glasses by sol±gel processing for the ®rst time. The QDs of PbS are well-distributed in the glass matrices. Strong two emission bands have been observed from PbS nanoparticles embedded in sol±gel silica glasses. The QDs of PbS have sharp emission band in the porous silica glass. 2. Experimental 2.1. Sample preparation Sols were prepared by sol±gel processing using tetra-ethylorthosilicate (TEOS, Si OC2 H5 4 ). The Si OC2 H5 4 was ®rst hydrolyzed and stirred for one hour. The water:TEOS:ethanol:HCl (6 N) volume ratio of 1:1:0.5:0.00125 was used. Then, the salt solutions of lead acetate (Pb CH3 COO2 3H2 O) and Na2 S were added in the resultant homogeneous sol and stirred for 30 min. The sol so prepared was kept at room temperature until its complete gelation. Final drying of the gel was obtained after one week at room temperature. The xerogel was then heated in atmosphere at a rate of 6 °C/h up to the temperature of 50 °C, and kept 10 h at this temperature. The doped mole ratios of PbS nanoparticles in the sol±gel silica glasses are 0.090%, 0.134%, 0.224%, 0.269%, respectively. 2.2. Apparatus The absorption spectra of the samples were measured on a spectrophotometer (U-3500). The ¯uorescent spectra of the samples were measured on Hitachi M-850 ¯uorescence spectrophotometer. Ideal spectral data were recorded with the slits set at 10 nm. First, the excitation spectra of the samples were measured. The emission spectra of the samples were then obtained. The Fourier transform infrared (FTIR)
spectra of samples were collected using a 5DX FTIR spectrometer. 3. Results and discussion The transparence of the doped sample decreases with the increasing of the impurity mole ratio of PbS. When the impurity mole ratio of PbS is 0.269%, the doped sample is almost opacity. Fig. 1 shows the FTIR spectra of the doped and undoped samples. The FTIR spectra of PbS nanoparticles embedded in the glass and pure glass sample are similar to the spectrum of pure amorphous SiO2 [14]; strong bands associated with Si± O streching and bending vibrations are apparent at 1082, 797, 465 cm 1 . The absorption in 3444 cm 1 indicates the presence of hydroxy (presence of H2 O). Additional weak absorption in the 1638 and 955 cm 1 indicate the presence of oxethyl. Because the bond of Pb±S is mainly electrovalent bond, the FTIR spectra of doped samples do not show strong bands associated with Pb±S streching and bending vibrations. Fig. 2 shows the absorption spectra of PbS nanoparticles embedded in the glass and pure glass sample. It indicates that the absorption spectra have a red shift with the increasing of the impurity mole ratios of PbS. When the impurity mole ratio of PbS P 0:224%, the absorption spectra show a broad absorption band (a maximum at 550 nm) of PbS nanocrystallites. Fig. 3 shows the excitation and emission spectra of the pure glass and PbS nanoparticles-doped samples. The PL spectrum of PbS nanoparticles embedded in sol±gel silica glass consists of two emission peaks. One is at 440 nm (kex 380 nm) while the other is at 605 nm (kex 475 nm). Also, the excitation spectrum of PbS nanoparticles embedded in sol±gel silica glass consists of two excitation peaks. One is at 380 nm while the other is at 475 nm (kem 605 nm). When excitation wavelength is 380 nm, the emission peak of 605 nm is very weak. The relative ¯uorescence intensity of the doped samples increases remarkably than that of the pure glass sample. The emission peak of the pure glass sample is 440 nm. And the excitation peak of the pure glass sample is 380 nm. Therefore,
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
431
Fig. 1. FTIR spectra of the doped and un-doped samples (mean impurity mole ratio).
Fig. 2. Absorption spectra of the doped and un-doped samples (mean impurity mole ratio).
the second emission (605 nm) and excitation peak (475 nm) of the doped samples are assigned to the QDs of PbS in the sol±gel silica glass. Because excessive impurity in luminescent host materials may lead to the ¯uorescence quenching, the relative ¯uorescence intensity of the doped samples
decreases with the increasing of the impurity mole ratios of PbS when the impurity mole ratios of PbS nanocrystallites in sol±gel silica glass exceed certain values (e.g., 0.269%). The luminescence properties of Pb2 ion (6S2 con®guration) are diverse. Usually, they are de-
432
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
Fig. 3. Excitation and emission spectra of the doped and undoped samples (mean impurity mole ratio). Excitation wavelength: (a) kem 380 nm; (b) kem 380 nm.
scribed by the 1 S0 ±3 P0;1 transitions which originate from the 6S2 ±6S6P intercon®gurational transition [15]. Besides, it is known that a complication of dierent origin arises if the excited states of the
host lattice or the surroundings of the Pb2 ion are at about the same energy as the levels of the 6s6p con®guration of the Pb2 ion [16,17]. Therefore, the luminescence properties of Pb2 ion can be aect by host materials. Pb2 ions have dierent luminescent spectra in dierent matrix materials. Our experimental result is signi®cantly dierent from that of reported by Chen et al. [18]. In their work, PbS nanoclusters have a broad emission band (a maximum at 804 nm). In SrTiO3 matrix, the broad PL band of Pb2 is at 600±700 nm [15]. For CaS:Pb EL material, a blue phosphor has been observed [20]. Also, this test result is dierent from that of our previous work [10]. Pb2 ions have broad emission band in ZnS nanocrystallites. The ¯uorescence intensity of Pb2 -doped ZnS nanocrystallites (the relative ¯uorescence intensity is about 2) [10] is lower than that of the sol±gel silica glass doped with PbS nanoparticles (the relative ¯uorescence intensity of PbS nanocrystallites-doped sol±gel silica glass is about 24). Usually, two emissions are observed from luminescent materials-excitonic and trapped luminescence [21]. The excitonic emission is sharp and is located near the absorption edge of the materials while the trapped emission broad and is stokesshifted. The trapped luminescence arising from the surface states is observed in the doped samples. For the broad emission (trapped emission) of Pb2 , non-recombination of luminescent process increases than that of sharp emission peak (excitonic emission) of Pb2 . Therefore the ¯uorescence intensity of PbS nanoparticles embedded in the sol±gel silica glass dramatically increases than that of PbS nanoparticles in ZnS nanocrystallites. The luminescence of the sol±gel silica glass is well-known and has been ascribed to a defect luminescence (such as, Si dangling and oxygen vacancy, . . .) [14]. By introducing PbS nanoparticles, one can expect luminescence from a dierent origin. The structure defects of the sol±gel silica glass can be aected by the neighboring PbS nanoparticle, or the PbS nanoparticle can be active centers. By comparing the luminescence of the pure sol± gel silica glass with the PbS embedded in the sol± gel silica glass, an emission band consisting of two contributions is observed. The shorter wavelength contribution has a maximum at 440 nm, and is
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
ascribed to the defect luminescence in the sol±gel silica glass. The second contribution is ascribed to centers involving PbS nanoparticles (a maximum at 605 nm). Since the luminescence of lead centers in the PbS nanocrystallites embedded in the sol±gel silica glass involves a transition from the conduction band to the ground state level of divalent lead, situated near the valence band, no concentration dependence on the spectral position can be observed. The sol±gel SiO2 glass is a porous phosphor material. Due to the inclusion of many defects, it can exhibit a novel luminescent behavior (common SiO2 glass does not show PL) [14]. The ¯uorescence eciency of the sol±gel glass increases with the increasing of the defects in silica glass network. As for PbS particles in nanometer size, we cannot ignore the existence of the electron- or hole-trapped surface levels on PbS. Because most of the ions of the nanoparticles are located on the surface and they are non-saturated in coordination, electrons or holes may be excited easily and escape from the ions. Then they are trapped on the surface states located in the forbidden gap. The PbS nanoparticles are embedded in the networks of the silica glass in sol±gel processing. First, the nanoparticles can lead to form more Si dangling and oxygen vacancy in the network of the silica glass. More electrons or holes excited easily and radiative recombinations are increased. Thus, the relative ¯uorescence intensity of the doped samples increases remarkably. Second, the luminescent processing of ¯uorescent materials involves radiative combination and non-radiative combination. The PbS QDs in the silica glass are isolated and stabilized. And they occupy narrow distribution of size. The isolative distribution of PbS nanoparticles in the matrix leads to decrease non-radiative combinations in luminescence processing. Therefore, the ¯uorescence intensity of the emission band of PbS increases remarkably. The relative ¯uorescence intensity of PbS nanoparticles embedded in the sol±gel silica glass is about 12 times of that of Pb-doped ZnS nanoparticles [10]. The band gap of semiconductor nanocrystallites Eg0 as a function of particles radius is given by [19]: Eg0 Eg2 2 h2 Eg p=r2 =m 1=2 , Eg : the band gap of the bulk semiconductor, h: Planck's constant, m :
433
the eective mass of electrons, taking m 0:085 me , me : being the free electron mass. The narrow distribution of size of PbS nanoparticles leads to increase the luminescent monochromaticity of PbS nanoparticles. Therefore, the sharp emission band of divalent lead has been observed from the PbS nanoparticles embedded in the sol±gel silica glass.\
4. Conclusion By introducing PbS nanoparticles into the sol± gel silica glass, two strong luminescent bands (kem1 440; kem2 605 nm) have been observed. Their novel emission phenomenon is due to the interaction between the PbS nanoparticle and the porous phosphor glass. The sharp emission band of divalent lead has been observed from the PbS nanoparticles embedded in the sol±gel silica glass. The novel emission phenomenon is very important for the applications and research of nanometer scale luminescence materials and the sol±gel silica glass.
Acknowledgements The authors thank the Natural Science Foundation of China for support (Grant No. 69890230).
References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] L.E. Brus, Appl. Phys. A 53 (1991) 465. [3] J. Zhou, L. Li, Z. Gui, S. Buddhudu, Y. Zhou, Appl. Phys. Lett. 76 (2000) 1540. [4] D.M. Burland, R.D. Miller, C.A. Walsh, Chem. Rev. 94 (1994) 31. [5] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [6] P. Maly, T. Miyoshi, J. Lumin. 90 (2000) 129. [7] T. Hayakawa, S.T. Selvan, M. Nogami, J. Lumin. 87±89 (2000) 532. [8] S.A. Empedocles, M.G. Bawendi, Science 278 (1997) 2114. [9] B.O. Dabbousi, M.G. Bawedi, O. Onitsuka, F. Rubner, Appl. Phys. Lett. 66 (1995) 1316. [10] P. Yang, M.K. L u, D. Xu, D. Yuan, G. Zhou, Chem. Phys. Lett. 336 (2001) 76.
434
P. Yang et al. / Chemical Physics Letters 345 (2001) 429±434
[11] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. [12] K. Wundke, S. P otting, J. Auxier, A. Sch ulzgen, N. Peyghambarian, N.F. Borrelli, Appl. Phys. Lett. 76 (2000) 10. [13] G. Tamulaitis, V. Gulbinas, G. Kodis, A. Dementjer, L. Valkunas, I. Motchalov, H. Raaben, J. Appl. Phys. 88 (2000) 178. [14] W.H. Green, K.L. Le, J. Grey, T.T. Au, M.J. Sailor, Science 276 (1997) 1826.
[15] H.F. Folkerts, G. Blasse, Chem. Mater. 6 (1994) 969. [16] G. Blasse, J. Solid State Chem. 18 (1988) 79. [17] A. Ragfagni, D. Mugnai, M. Bacci, Adv. Phys. 32 (1983) 823. [18] S.W. Chen, L.A. Truax, J.M. Sommer, Chem. Mater. 12 (2000) 3864. [19] M. Mukherjee, A. Datta, S.K. Pradhan, D. Chakravorty, J. Mater. Sci. Lett. 15 (1996) 654. [20] S.J.Yun,Y.S.Kim,S.H.Park,Appl.Phys.Lett.78(2001)721. [21] W. Chen, Z. Wang, Z. Lin, L. Lin, Appl. Phys. Lett. 70 (1997) 1466.