DOI 10.1007/s10832-006-9930-0
Highly luminescent CdSe nanoparticles embedded
in silica thin films
Chung-Hsin Lu· Baibaswata Bhattacharjee ·
Chia-Hao Hsu· Shih-Yen Chen · Ruoh-Chyu Ruaan · Walter H. Chang
C
Springer Science+ Business Media, LLC 2006
Abstract Thin films of luminescent CdSe nanoparticles with and without silica capping were prepared using sol-gel method. The blue shift observed in the optical absorption spectra suggested quantum confinement effect in the pre-pared films. The films showed photoluminescence emission in the range of 510–590 nm depending upon the particle size of CdSe particles. The emission intensity increased when CdSe particles were embedded in silica matrix. The emission intensity was found to decrease with aging for the films containing CdSe particles without silica capping when they were exposed in relatively humid air (relative humidity 80%). The films containing CdSe nanoparticles embedded in silica matrix showed more stable behavior. The emission intensity practically remained constant with aging in humid atmosphere.
Keywords Nanoparticles· Luminescence · Thin film
1 Introduction
Semiconductor nanoparticles have attracted great interest in both theoretical and applied research areas [1–4] due to
C.-H. Lu () . B. Bhattacharjee . C.-H. Hsu . S.-Y. Chen Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.
e-mail: [email protected]
Permanent address: B. Bhattacharjee, Department of Physics,
Ramananda College, Bishnupur, Bankura, W.B. 722 122 India
R.-C. Ruaan
Department of Chemical and Materials Engineering, National Central University, Chung Li, Taiwan, R.O.C.
W. H. Chang
Department of Biomedical Engineering, Chung Yuan Christian University, Chung Li, Taiwan, R.O.C.
their size- and shape-dependent physical properties. CdSe, an important member of luminescent II–VI family having bright luminescence in the visible range of optical spec-tra, has shown potential to be used in nanocrystalline form in biological field [5, 6], displays [7, 8], diodes and lasers [1], solar cells [9–12] and gas sensors [13–15]. A spe-cial class of materials having improved physical properties can be obtained by incorporating semiconductor nanopar-ticles in a non-conducting (dielectric) matrix [16, 17]. The shell/matrix determines the charge, functionality, and reac-tivity of the nanoparticle surface and enhance the stability and dispersibility of the core. Encasing the nanoparticles in a shell of different compositions may also protect the core from extraneous chemical and physical changes. Protection can be achieved against agglomeration of the particles caused by strong van der Waals attractive forces and also against nanoparticle degradation as a result of chemical etching by this capping procedure. Collective properties of nanoparti-cle assemblies are influenced to a large extent by the sep-aration between the particles. Coating the particles with a uniform shell of inert material could control the distance be-tween the particles, which in turn can control the optical, luminescence and electrical properties of this class of ma-terials. Modification in physical properties of the semicon-ductor nanoparticle due to presence of surrounding dielectric matrix has been studied by different groups [18–24]. Influ-ence of dielectric matrix on third order optical nonlinearity for CdSe nanocrystals embedded in RF-sputtered glass thin films was investigated by Nasu et al. [18]. Mane et al. [19] reported improved properties of dense TiO2/CdSe coupled
thin films synthesized by chemical route at low temperature (<100◦C). Manolis et al. [20] performed photoreflectance study on multilayered structures of nanocrystalline CdSe in insulator matrix like SiOxand GeS2. Correa-Duarte et al. [21]
photo-degradation; however the effect of silica coating on the luminescence property was not reported. Lifshitz et al. [22] studied optical properties of CdSe nanoparticle films pre-pared by chemical deposition and sol-gel methods. Highly luminescent semiconductor nanoparticles were successfully incorporated in ZrO2-SiO2sol-gel glass film [23]. The
chem-istry occurring at the CdSe/Si, CdSe/SiO, and CdSe/SiO2
interfaces has been investigated [24].
In spite of the existence of such large amount of work in this field, very few reports are available that demonstrate the systematic control of physical properties of matrix em-bedded nanoparticles in thin film form employing sol-gel technique [16]. This study reports sol-gel synthesis of very stable, highly luminescent composite thin films containing CdSe nanoparticles embedded in SiO2 matrix having
con-trollable optical properties. The films showed significant im-provement in material properties compared to the thin films of bare CdSe nanoparticles.
2 Experimental
To prepare the thin films with CdSe nanoparticles embedded in silica matrix, the following method was adopted. A silica sol served as the precursor for the host films was first prepared by dissolving tetraethyl orthosilicate, Si(OC2H5)4 (TEOS,
Merck) in 2-propanol, (CH3)2CHOH (Merck), followed by
drying over activated molecular sieve zeolite 4A and adding distilled water. Hydrochloric acid (0.1N, Merck) was used as a catalyst. A solution of Cd(NO3)2· 4H2O (Merck) mixed
with selenourea, NH2CSeNH2 (Merck) was prepared in
2-propanol and distilled water to be the source for cadmium and selenium, respectively. The as-prepared solution was slowly added into the silica sol under vigorous stirring and the stirring was continued for 1–2 h after the completion of mixing to obtain the final sol ready for the fabrication of films. The equivalent molar ratio of silica to cadmium se-lenide was 70:30. To prepare CdSe thin film without silica matrix for comparison, the silica sol was not added to the solution containing cadmium and selenium precursors.
Using the above sols, spin coated thin films were fabricated on properly cleaned quartz glass substrates (25× 25 mm). All the as-deposited films were colorless. The as-deposited films were annealed in vacuum at different tem-peratures (from 473 to 673 K, with 50 K intervals) for a fixed time of 30 min to study the nucleation and growth of CdSe nanoparticles in SiO2matrix. CdSe films without silica
cap-ping were annealed under same conditions for comparison with the silica capped particles. The films turned pale orange when annealed above 450 K, and the color was found to be getting darker with increasing annealing temperature. Trans-mission electron microscopy (TEM) was performed using a Hitachi H-7100 microscope. Films scratched from the quartz
substrate were carefully placed on the carbon coated Cu grid for TEM study. Optical absorption spectra were recorded us-ing a spectrophotometer (Hitachi-U3410) at the room tem-perature with a resolution ofλ ∼ 0.07 nm along with a pho-tometric accuracy of± 0.3%. Photoluminescence (PL) mea-surement was carried out using a Hitachi F-4500 fluorescence spectrophotometer. Fourier transformed infrared (FTIR) ab-sorption spectra were recorded by using an IR spectrome-ter (Nicolet, Magna-IR). X-ray photoelectron spectroscopic (XPS) measurements were performed on commercial VG Microtech (MT-500) machine using Mg Kαradiation.
3 Result and discussions 3.1 Microstructural study
Figures 1(a) and (b) show respectively the TEM and the cor-responding electron diffraction patterns of CdSe and CdSe-SiO2 films, annealed at 673 K for 30 min. Well dispersed
nanoparticles with an average particle size of 7.5 nm for
Fig. 1 Transmission electron micrographs (TEM) and correspond-ing Electron Diffraction patterns of the films annealed at 673 K for 30 min: (a) thin films containing CdSe nanoparticles embedded in sil-ica matrix and (b) thin films containing CdSe nanoparticles without silica matrix. Circles highlight the particle agglomeration (b), which did not occur in the case of silica (a) capping under same experimental conditions
Table 1 Comparison between particle sizes obtained from TEM study and blue shift of optical band gap for CdSe and CdSe-SiO2films when annealed at different temperatures for 30 min.
CdSe films CdSe-SiO2films
Annealing Particle size (nm) Particle size (nm) Particle size (nm) Particle size (nm) temperature (K) from TEM from blue shift from TEM from blue shift
473 4.0 3.86 3.5 3.29
523 5.1 5.24 4.0 3.98
573 6.0 5.93 4.5 4.47
623 6.5 6.47 5.1 4.95
673 7.5 7.39 5.5 5.42
CdSe and 5.5 nm for CdSe-SiO2films were found under the
above mentioned annealing condition. Diffraction patterns showed central halos with concentric ring patterns. Ring pat-terns showed reflections from (111), (220) and (311) planes, indicating the formation of cubic phase for CdSe in both set of films. Average partcle size changed from 4 to 7.5 nm for the set of CdSe films and 3.5 to 5.5 nm for the set of CdSe-SiO2 films when they were annealed from 473 to
673 K in a step of 50 K (Table 1). It was clear from the micrographs that the films containing CdSe nanoparticles without silica capping tend to lose their nanocrystalline na-ture with increasing annealing temperana-ture at a faster rate when compared to the CdSe-SiO2 films (Fig. 1(b)). In the
films containing CdSe nanoparticles embedded in silica ma-trix, the CdSe particles were completely capped inside the silica matrix (Fig. 1(a)). The distance among the nanoparti-cles did not favor the possible coalescence with one another through silica barrier. Thus silica capping made the parti-cles more stable against agglomeration when the annealing temperature was increased.
3.2 Fourier transformed infra red (FTIR) absorption study FTIR has been employed to examine the chemical purity of CdSe-SiO2 films annealed from 473 to 673 K. All the
spectra (Fig. 2(a)–(e)) were found to be dominated by three
400 600 800 1000 1200 1400 1600 463 806 (d) (e) Absorbance (arb.u.) (c) (b) 1043 1075 Wave numbers (cm-1) (a) 451 799
(position for free amorphous silica) 1058
Fig. 2 Fourier transformed infra red (FTIR) absorption spectra of thin films containing CdSe nanoparticles embedded in SiO2matrix annealed for 30 min at (a) 473 K, (b) 523 K, (c) 573 K, (d) 623 K and (e) 673 K
absorption bands centered at frequencies about 450, 800 and 1050 cm−1. The absorption peaks near 450, 800, and 1050 cm−1were associated with the Si–O stretching modes [25]. The peak occurred at about 1050 cm–1was due to the
broad asymmetrical stretch of Si–O–Si and that at about 800 cm–1was due to the symmetrical Si–O–Si stretch [26]. The peak at 1050 cm–1is resulted from the oxygen atom
stretch-ing parallel to Si–O–Si and is also called the transverse opti-cal (TO) mode [26]. The peak of the asymmetric stretching mode lied at a frequency lower than that of amorphous SiO2
(∼1075 cm−1) [27] in all the films. Shifting of the band to the lower frequencies could be attributed to the partial bond for-mation with the surface Se atoms which eventually passivate the surface of CdSe nanoparticles. The absorption band re-lated to Si O Si asymmetric stretching mode was found to
become more intense with a narrower bandwidth for the films annealed at higher temperatures. The band was also found to be shifted gradually towards higher frequencies with in-creasing annealing temperature. Broadening of the band cor-responding to the asymmetric stretching mode can be related to a statistical distribution of different bonding arrangements at each silicon atom site, and so to a structural inhomogeneity of the film [28]. On the other hand, the shift toward lower fre-quencies of the peak intensity can be ascribed to greater film porosity or lower packing density [28–30]. Thus the above observation clearly demonstrates the gradual densification of silica matrix with increasing annealing temperature.
The spectra showed no Se-O absorption peaks around 890 cm−1as seen in bulk CdSe that had been oxidized [31]. No absorption peaks from bulk SeO2or SeO3 group were
1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Absorption (arb.u.) hν(e V )
(a)
(b)
(c)
450 500 550 600 650 700 2.0 2.1 2.2 2.3 2.4 2.5 450 500 550 600 650 700 2.0 2.1 2.2 2.3 2.4 2.5 (a) (b) Eg (e V ) Annealing temperature (K) Fig. 3 Optical absorptionspectra of thin films containing CdSe nanoparticles embedded in SiO2matrix annealed for 30 min at (a) 473 K, (b) 573 K and (c) 673 K. Inset shows variation of optical band gap (Eg) with
annealing temperatures for (a) thin films containing CdSe nanoparticles embedded in silica matrix and (b) thin films containing CdSe nanoparticles without silica matrix
observed in the spectra within the range of 680– 695 cm−1[32]. These results strongly suggested that CdSe nanoparticles were completely passivated by the silica host and surface oxidation was prevented successfully in all of the capped films. No trace of residual precursors or any other impurities were found in the CdSe-SiO2films when annealed
at and above 473 K. 3.3 Optical study
Optical absorption spectra were recorded for the films an-nealed at different temperatures (T). Some representative spectra for CdSe-SiO2 films are shown in Fig. 3. The
ap-pearance of absorption peaks at higher energies as com-pared to the bulk material was indicative of the formation of CdSe nanoparticles. Absorption edges were red-shifted
with increasing annealing temperature, indicating growth of the particles at higher annealing temperatures. It could be no-ticed that the line shape of the absorption spectrum with de-creasing photon energy (hν) at the band edge was sharper for the films baked at higher temperatures, suggesting stronger band-tailing effect in the films containing smaller particles. The optical band gaps (Eg) were calculated using the con-ventional method by extrapolating the straight line portion of the (αhν)2vs. hν curve to α = 0 (figure not shown), where α is the optical absorption coefficient derived from the ab-sorption data. The band gaps for different films varied within the range of 2.46–2.12 eV, which were always greater than the bulk value of 1.75 eV at 293 K [33]. The increase in the fundamental band gap of the nanostructured material could be attributed to the quantum size effect [34]. The band gaps in different films varied with the annealing temperature as
440 460 480 500 520 540 560 580 600 620 640 660 680 700 λ (nm) 518 nm 590 nm 473 K (a) PL In te n s ity (a rb .u .) 566 nm 573 K 673 K (b) (c) 591 nm 440 480 520 560 440 480 520 560 (a) (b) PL i n te n s it y ( a rb .u .) λ(nm) Fig. 4 Photoluminescence (PL)
emission spectra of thin films containing CdSe nanoparticles without SiO2matrix annealed for 30 min at (a) 473 K, (b) 573 K and (c) 673 K. Inset shows photoluminescence (PL) emission spectra of (a) thin film containing CdSe nanoparticles embedded in silica matrix and (b) thin film containing CdSe nanoparticles without SiO2 matrix, both annealed at 473 K for 30 min
shown in Fig. 3 (inset). This figure depicted that the band gap progressively decreased with increasing annealing tempera-ture for both CdSe and CdSe-SiO2films. This observation is
obviously an indication of the increase in particle size with increasing annealing temperature. The average particle sizes of the CdSe nanoparticles in the films with and without silica matrix were determined using the value of blue shift in the optical band gaps. The values obtained via this method match well with that form TEM as the results are shown in Table 1. 3.4 Photoluminescence (PL) study
Figure 4 shows photoluminescence (PL) emission spectra for three CdSe films recorded at room temperature using
an excitation wavelength of 445 nm. The line shapes of PL peaks were smooth, symmetric and sharp in all cases. The Stoke-shifts of the emission were small (∼20–30 nm). No shift in peak position was observed with changing excitation energy, indicating band edge emissions. For the sample syn-thesized at 473 K, a less intense broad peak was observed at 590 nm in addition to the intense band-edge luminescence at 518 nm (Fig. 4(a)). Origin of this peak at lower energy could be attributed to the luminescence coming from the sur-face states. As this film was annealed at lower temperature, smaller particle size culminated higher surface to volume ra-tio of the nanoparticles present in this film giving rise of this Stoke shifted broad luminescence band. The films synthe-sized at elevated temperatures did not show luminescence
0 1 2 3 4 5 6 0.5 0.6 0.7 0.8 0.9 1.0 N o rmaliz e d PL in tens it y
Aging time (Weeks)
(a) (b) (c) 52 54 56 58 60 62 (b) (c) (d) Int e ns ity (arb. u. ) (e)
Binding Energy (eV) (a) (f) CdSe SeO2 1 52 54 56 58 60 62 (f) CdSe 2 No SeO2 peak
Binding Energy (eV) (a) (b) (c) Intensity (arb.u.) (d) (e) Fig. 5 Change in normalized
intensity of emission peak with aging time (degradation curves) for CdSe films synthesized at different annealing
temperatures: (a) 473 K, (b) 573 K and (d) 673 K. Inset 1 shows the X-ray Photoelectron spectra of Se 3d core level of a representative film (annealed at 573 K) containing bare CdSe nanoparticles after different period of aging: (a) freshly prepared, (b) 2 days, (c) 4 days, (d) 6 days, (e) 8 days and (f) 10 days. Inset 2 shows X-ray Photoelectron Spectra of Se 3d core level of a representative film (annealed at 573 K for 30 min) containing CdSe nanoparticles embedded in SiO2 matrix after different period of aging: (a) freshly prepared, (b) 2 days, (c) 4 days, (d) 6 days, (e) 8 days and (f) 10 days
related to surface states, indicating particle growth and rel-atively lower surface-to-volume ratio in these films. The PL peaks were red-shifted with increasing annealing tempera-ture as shown in the figure, revealing particle growth in the films. The full width at half maxima (FWHM) of the emis-sion peaks were found to increase with increasing annealing temperature. Broadening trend of PL line shape could be re-lated to the broadening of the particle size distribution with raising reaction temperature.
CdSe-SiO2 films showed substantially higher
lumines-cence compared to CdSe films prepared under the same ex-perimental conditions (Fig. 4, inset). The increase in the luminescence intensity could be attributed to the surface passivation (supported by FTIR study) and subsequent re-duction in the non-radiative recombination in the thin films containing CdSe nanoparticles embedded in silica matrices. The emission from CdSe-SiO2films also experienced
red-shift with increasing annealing temperature, but at a slower rate compared to CdSe films.
3.5 Study on the effect of aging on luminescence property The films were kept in air under the exposure of room light during the tenure of the aging experiments. The luminescence peak intensities normalized to the freshly prepared samples were investigated as a function of aging time in both sets of films with and without silica matrix. A loss in luminescence intensity with prolonging aging time was noticed in all of the CdSe films. The luminescence intensity was found to dwindle down to nearly 56% of its initial value for the CdSe film annealed at 473 K for 30 min when kept in humid air (relative humidity 80%) for 6 weeks (Fig. 5(a)). The loss in luminescence intensity for the bare CdSe films could be explained by the instability of CdSe due to photo-oxidation [35–37].
Figure 5 (inset 1) shows the XPS spectra of Se 3d core level of a representative CdSe film (annealed at 573 K for 30 min) recorded at different aging times. The formation of SeO2
peak at energy (∼59 eV) higher than the main Se peak (∼54
400 600 800 1000 1200 1400 1600 Wavenumbers (cm-1) 1 week 2 weeks 3 weeks Absorbance (arb.u.) 4 weeks 5 weeks 6 weeks
Si-O Si-O Si-OH
Si-O
Fig. 6 Fourier transformed infra red (FTIR) absorption spectra of a thin film containing CdSe nanoparticles embedded in SiO2matrix annealed at 573 K 30 min at different periods of aging times
eV) after two days was an indication of the surface oxidation of the nanoparticles when it was exposed to visible light in normal humid atmosphere. It was interesting to note the tem-poral behavior of the SeO2peak with increasing aging time.
After initial rise of the oxide peak, the peak decayed and then rose again. Oscillations continued over the period of several weeks indicating that the oxide leaves the surface as a molec-ular species, leaving Cd and a freshly exposed layer of CdSe behind. During the period of decaying the oxidized peak, the Cd/Se ratio was found to rise, confirming the loss of Se from the nanoparticles. Surface of the nanocrystals of CdSe ex-posed to air and light was thus effectively destroyed by these redox cycles within a few days and resulted in the reduction of PL intensity [35]. It can be noticed from Fig. 5, that the slope of the degradation curves were steeper for the films syn-thesized at lower annealing temperature and the slope grad-ually became gentler for the films prepared with increasing annealing temperature. The films synthesized at lower tem-perature contained smaller nanoparticles with greater sur-face area. This made the particles more sensitive to sursur-face photo-oxidation process, leading to a faster degradation com-pared to the films precom-pared at higher temperature having larger particle sizes. No such evidence of surface oxidation was found (Fig. 5, inset 2) for the films containing CdSe nanoparticles embedded in SiO2matrix due to the effective
silica capping on the CdSe nanoparticles. This resulted in much more stable luminescence behavior of the CdSe-SiO2
films (luminescence intensity practically remains constant throughout the period of aging experiments) compared to that of the films containing CdSe nanoparticles without silica capping.
The overall quality of the CdSe-SiO2 films during
ag-ing was studied usag-ing FTIR data colleted at different stages of aging and the results for the film annealed at 573 K for 30 min are shown in Fig. 6. No significant change was ob-served in the FTIR spectra, depicting stable and effective silica capping of CdSe nanoparticles. After 5 weeks of ag-ing, a broad and weak band appeared around 935 cm−1 in the spectrum. The intensity of this additional band was in-creased slightly when the film was aged for 6 weeks. No further increase in the intensity of this band was found be-yond 6 weeks of aging. The presence of a band peaking at
∼935 cm−1 in the FTIR spectrum has been ascribed to the
vibrational stretching mode of Si OH groups [38]. Appear-ance of this weak band could be attributed to the absorption of small amount of water vapor on the outer surface of the SiO2matrix after the films were being aged for a long period
of time (at least 5 weeks) in the humid environmental condi-tions. However, this process remained confined to the outer surface of the silica matrix and did not attack the nanoparticle surface. Prevention of surface oxidation was revealed from the absence of any Se O band in FTIR spectra and also from
XPS study. As a result, the luminescence intensity remained practically unchanged with aging in the CdSe-SiO2 set of
films.
4 Conclusions
Sol-gel technique was adopted to synthesize thin films con-taining CdSe nanoparticles with and without silica capping. All of the films exhibited quantum-size effects. The increase in particle size was observed with increasing annealing tem-perature. The emission intensity was found to increase when CdSe nanoparticles were encapped in silica matrix. The emission intensity was found to decrease with aging for the films containing CdSe nanoparticles without silica capping. CdSe-SiO2films showed more stable luminescence behavior
and less degradation against aging.
Acknowledgments The authors would like to thank National Science Council, Taiwan, R.O.C. for financially supporting this research under contract No. NSC 93-2214-E-002-019 and NSC 93-2120-M-033 -001. One of the authors (B.B.) would like to thank Ramananda College, Bishnupur, India for providing leave to participate in this research work.
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