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porous Si nanocomposites
Journal of Luminescence 138 (2013) 157–163
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Wavelength tunable photoluminescence of ZnO/porous Si nanocomposites Yuan Ming Huang n, Qing-lan Ma, Bai-gai Zhai School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China
a r t i c l e i n f o
abstract
Article history: Received 22 April 2012 Received in revised form 6 January 2013 Accepted 6 February 2013 Available online 14 February 2013
ZnO/porous Si (PS) nanocomposite films were prepared with the sol–gel spin-coating method. The morphologies, crystal structures, chemical compositions, surface passivations and photoluminescence (PL) spectra of the ZnO/PS nanocomposite films were investigated. It is found that the PL of the ZnO/PS nanocomposites can be tuned from red to green–blue by increasing the concentration of zinc cations in the sol–gels from 7.5 to 240 mM. Post-thermal annealing of the ZnO/PS nanocomposite films at 600 1C can shift the PL further into violet–blue. The roles of SiO2 thin interfacial layer between the ZnO and the Si nanocrystals in the nanocomposites are discussed. Our work provides an affordable means of tuning the emission color of the ZnO/PS nanocomposites across the visible spectral region. & 2013 Elsevier B.V. All rights reserved.
Keywords: Porous Si Zinc oxide Nanocomposite Photoluminescence
1. Introduction Nanometer-sized zinc oxide (ZnO) is a wide bandgap semiconductor (3.37 eV) with a large exciton binding energy (about 60 meV). These features make ZnO nanocrystals suitable for fabrications of ultraviolet nanolasers [1–3], luminescent phosphor [4], light-emitting diodes [5], ultraviolet light detectors [6] and ultraviolet optical switches [7]. Among these applications, the photoluminescence (PL) of ZnO nanocrystals is fundamental to the optoelectronic devices. In general, highly pure ZnO nanocrystals contain little defects in the crystal lattices; the band edge emission of the ZnO nanocrystals produces strong ultraviolet emission at about 385 nm. On the contrary, abundant intrinsic defects (i.e., vacancies of oxygen and zinc, interstitial oxygen and zinc, and antisite oxygen) are present in low quality ZnO nanocrystals, these defect-involved recombinations in the ZnO nanocrystals often generate blue [2], green and yellow emissions. To make ZnO an ideal luminescent material suitable for lightemitting diodes, tunable PL of ZnO nanocrystals in the visible spectral region is highly required. Theoretically speaking, tunable PL of ZnO nanocrystals in the visible spectral region can be realized by the precise control of the type and the population of the defects in the ZnO nanocrystals [8]. However, it is difficult to do so in practice. Incorporation of the ZnO nanocrystals into a foreign matrix to form nanocomposite materials represents one route towards tuning PL of ZnO nanocrystals. In principle, tunable PL within the entire visible spectral region can be expected from a nanocomposite film if
n
Corresponding author. Tel./fax: þ 86 519 86334730. E-mail address: [email protected] (Y.M. Huang).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.002
the guest material gives off blue emission at the short wavelength end while the host material gives off red emission at the long wavelength end [9,10]. It is well known that porous Si (PS) can give off efficient red PL at room temperature [11]. Both the high surface area and the rich nanopores in PS films make it capable of containing a large amount of ZnO nanocrystals in the porous matrix [12,13]. Therefore, tunable PL from red to violet–blue can be expected for the ZnO/PS nanocomposites if the weight percentages of ZnO nanocrystals in the porous matrix can be finely controlled. By embedding ZnO nanocrystals into the matrix of PS, several groups reported white emissions for ZnO/PS nanocomposites [14–17]. For example, Singh et al. reported white light emission for their ZnO/PS nanocomposite film that had been prepared by the sol–gel spin coating method; Kayahan also recorded the white PL from his ZnO/PS nanocomposite that had been prepared by the rf-sputtering technique [16]. Inspite of these pioneering studies, the tunable PL across the entire visible spectrum has not been reported. In this paper, we reported the wavelength tunable PL from red to violet–blue of the ZnO/PS nanocomposite films by controlling the concentration of Zn cations in the sol–gels and the annealing temperature in postthermal annealing process. The roles of SiO2 thin interfacial layer between the ZnO and the Si nanocrystals are discussed for the ZnO/ PS nanocomposites.
2. Experimental details 2.1. Preparation of PS matrix PS films were prepared by electrochemically anodizing boron doped p-type (1 1 1)-oriented Si wafer in hydrofluoric electrolyte at the constant current density of 3 mA/cm2 for 60 min. The
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resistivity of the Si wafer was 8–13 O cm. The electrolyte had an equal volume mixture of hydrofluoric acid (40 wt%) and ethanol (95 wt%). In the electrolytic solution, Si wafer was utilized as the working electrode while the platinum wires were employed to be the counter electrode. Details of the experimental setup and method were reported elsewhere [17]. 2.2. Preparation of ZnO/PS nanocomposites ZnO/PS nanocomposite films were prepared by the sol–gel spin coating technology. Analytical grade of reagents triethanolamine and zinc acetate dihydrate were provided by a local chemical supplier (Sinopharm Chemical Reagent Co., China). Sol–gels were prepared by dissolving specific amount of zinc acetate dihydrate into the mixture of triethanolamine (0.152 g) and ethanol (95 wt%, 6 ml). When the mass of zinc acetate dihydrate increased from 10, 21, 43, 86, 171 to 342 mg, six kinds of sol–gels were obtained with the molar concentrations of zinc cations in the sol–gels to be 7.5, 15, 30, 60, 120 and 240 mM. The speed of the spin coater was fixed to be about 2000 rpm while the duration of the coating was about 20 s. After one sol–gel had been spin-coated on the top surface of a PS film, subsequent baking in an oven at 245 1C was applied to the sol–gel spin-coated PS film to ensure the growth of ZnO nanocrystals in the porous matrix. Serial numbers 1–6 were assigned to the six ZnO/PS nanocomposite films as the molar concentrations of zinc cations in the sol–gels were in the sequence of 7.5, 15, 30, 60, 120 and 240 mM. 2.3. Characterizations The X-ray diffraction (XRD) characterization of the thin films was carried out on an X-ray diffractometer (D/max 2500 PC, Rigaku, Japan). A copper target was utilized to generate the X-ray (l ¼0.154 nm). The operating voltage was 40 kV and the current was 100 mA. A scanning electron microscope (SEM) was employed to characterize the top and cross-sectional morphologies of the films. Fourier transform infrared (FTIR) spectroscopy was employed to analyze the chemical bonding on the surface of ZnO/PS composites; all FTIR spectra were collected on a spectrometer (Nicolet Prote´ge´ 460). The PL spectra of the films were recorded with a spectrophotometer (Aipha Technologies, China). The 325 nm laser line from a helium–cadmium laser (Kimmon Koha Co. Ltd., Japan) was utilized as the excitation source for the PL measurement. The output power of the ultraviolet laser was about 12 mW.
3. Results and discussion 3.1. Morphology and structure The morphology of the PS films and the ZnO/PS nanocomposite films have been examined with SEM. Fig. 1(a) and (b) shows the top and cross sectional SEM micrographs of the PS film. It is obvious that the top surface of the PS film is rough with a lot of nanopores. The average size of the nanopores on the top surface of the film is less than 2 nm in diameter. Unlike the columnar microstructures in our previous work [12,13,18], Fig. 1(a) and (b) demonstrates that our PS film is characteristic of spongy nanostructures. The nanopores in the spongy nanostructures make the porous matrix of PS film suitable for the filtration of water from the prepared sol–gels. By the capillary action, the zinc cations and hydroxide anions containing water can be easily sucked into the nanopores from the sol–gels. Once the zinc cations and hydroxide anions in the aqueous solution are sucked into the spongy nanostructures of PS, the heat treatment at 245 1C can readily turn them into ZnO nanocrystals by the reactions Zn2 þ þ 2OH -ZnðOHÞ2 ,
ð1Þ
ZnðOHÞ2 -ZnO þ H2 O:
ð2Þ
Fig. 1(c) and (d) depicts the top and cross sectional the SEM micrographs of a ZnO/PS nanocomposite film. The originally rough surface of the porous matrix is smoothed by the sol–gel spin-coating, suggesting that the nanopores in the spongy nanostructures of PS are filled with the guest materials. As shown in Fig. 1(d), the thickness of the ZnO/PS nanocomposite film is about 1 mm, and the thickness of the ZnO layer on the top of the ZnO/PS nanocomposite film is about only 50 nm. The infiltration of zinc oxide is evident. Fig. 2 gives the powder XRD pattern of a typical ZnO/PS nanocomposite film. The peaks at 29.11 and 48.21 in Fig. 2 can be assigned to the reflections from the 1 1 1 and 2 2 0 planes of Si nanocrystals in the nanocomposite (JCPDS no. 27-1402). According to the standard data for single crystalline Si, the diffraction peaks of the 1 1 1 and 2 2 0 planes are located at 2y ¼28.4421 and 47.3021, respectively. With respect to the peaks of single crystalline Si, the 2y angles of the 1 1 1 and 2 2 0 diffraction peaks of PS film are shifted 0.661 and 0.901 toward the larger angle direction. The observed shift in the diffraction peaks can be attributed to the porosity-introduced distortion in the lattice parameter of Si nanocrystallites in the film. The peaks at 31.7, 34.4, 36.2, 47.5,
Fig. 1. Top (a) and cross sectional (b) SEM micrographs of a PS film. Top (c) and cross sectional (d) SEM micrographs of a ZnO/PS nanocomposite film.
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contribute to the width of the diffraction peak. On the basis of the XRD data in Fig. 2, it is safe for us to conclude that the mean grain size of the ZnO nanocrystals in the porous matrix is in the order of nanometer. 3.2. Chemical composition and surface passivation
Fig. 2. Powder X-ray diffraction pattern of a typical ZnO/PS nanocomposite film.
56.6, 62.9, and 65.4 in Fig. 2 are assigned to the reflections from the 1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0, 1 0 3, and 2 0 0 planes of the hexagonal ZnO with wurtzite structure (JCPDS No.36–1451) respectively. According to the standard data for hexagonal ZnO, diffraction peaks of the 1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0, 1 0 3, and 2 0 0 planes of the hexagonal ZnO with wurtzite structure are located at 2y ¼31.7691, 34.4211, 36.2521, 47.5381, 56.6021, 62.8621 and 66.3781, respectively. Comparison of these data reveals that our recorded data are consistent to the standard ones. Specifically, we have noticed that the diffraction angle of the ZnO (0 0 2) peak in Fig. 2 agrees well with the reported data in the literature for ZnO/PS nanocomposites [16,19–21]. The co-existence of the diffraction peaks of Si and ZnO nanocrystals in Fig. 2 reveals that ZnO nanocrystals have been incorporated in the porous matrix of the host material. The peaks at 39.81 and 43.11 in Fig. 2 can be attributed to the reflections from the 3 1 2 and 2 2 0 planes of lutecite (SiO2) nanocrystals (JCPDF no. 46-1441). By the standard data, the 3 1 2 and 2 2 0 diffraction peaks of lutecite nanocrystals are located at 39.6721 and 42.6111, respectively. The appearance of the SiO2 nanocrystals suggests that the sol–gel spin coating has introduced further oxidation on Si nanocrystals in the nanocomposites. Reaction of Si with bases is proposed to be the reason for the formation of the lutecite. In highly basic solutions, Si atoms are attacked by bases such as aqueous sodium hydroxide to give silicates, which are highly complex species containing the anion [SiO4]4 , by the reaction SiðsÞ þ 4OH ðaqÞ-½SiO4 4 ðaqÞ þ 2H2 ðgÞ:
Energy-dispersive X-ray spectroscopy (EDX) is an analytical technique used for the elemental analysis of a specimen. Fig. 3 shows the EDX spectrum of the top surface of a typical ZnO/PS nanocomposite film. The characteristic emission lines of elements are labeled in Fig. 3 at 0.277 (C Ka1), 0.525 (O Ka1), 1.01 (Zn L), 1.739 (Si K), 2.123 (Au Ma1), 8.63 (Zn Ka), 9.57 (Zn Kb). These data indicate that the surface layer of the ZnO/PS nanocomposite film is rich of carbon, oxygen, platinum, Si and zinc atoms. The presence of gold in the specimens was due to the gold sputtering onto the surfaces of our samples for the SEM and EDX characterizations. The inset at the lower left corner of Fig. 3 represents the depth profiles of zinc (black curve), oxygen (red curve) and Si (white curve) in the ZnO/PS nanocomposite film. The depth was defined to be the distance of a point in the film away from the air/ PS interface. The depth profile analysis shows that the weight percentage of Zn atoms in the nanocomposite film gradually decreases from its maximum value at the air/PS interface to zero at the PS/Si interface. Our EDX results have demonstrated that the spongy PS matrix had been filled with the ZnO nanocrystals. The inset at the upper right corner of Fig. 3 shows the weight percentages of carbon, oxygen, Si and zinc elements on the top surfaces of the nanocomposite films 1–6. From 1 to 6, zinc increases from 6.8 to 21.4 wt%, Si decreases from 63.0 to 43.1 wt%, carbon fluctuates between 13.7 and 15.5 wt%, oxygen varies in the range of 15.9–26.9 wt%. In combination with the data shown in Figs. 1 and 2, the data in Fig. 3 demonstrate that more and more ZnO nanocrystals have been incorporated into the nanopores of PS as the concentrations of zinc cations in the sol– gels increase from 7.5 to 240 mM. FTIR spectra of a PS film and a ZnO/PS nanocomposite film are given in Fig. 4. The bond vibrations in Fig. 4 at about 880, 1040, 2100, and 2260 cm 1 can be attributed to the Si–H bond (scissors mode), the Si–O–Si bond (stretching mode), the Si–H bond (stretching mode), the O–Si–H (stretching mode), respectively [16,22]. With respect to the data in Fig. 4(a), the data in Fig. 4(b) show that there is an increase in the intensity of the Si–O–Si stretching band at 1040 cm 1 and a decrease of the Si–H
ð3Þ
Thermal annealing of the nanocomposites at 245 1C helps to concentrate the silicate solution, and precipitation from silica saturated solutions at high pH gives the lutecite nanocrystals on the surface of Si skeletons in the PS film. The average grain size of the ZnO nanocrystals in the nanocomposites can be estimated by the Scherrer formula using the full width at half-maximum (FWHM) value of the XRD diffraction peak (0 0 2) at 34.41: D¼
0:9l bcosy
ð4Þ
where D, l, y and b are the mean grain size, the X-ray wavelength, the Bragg diffraction angle and the FWHM, respectively. The calculated values of the mean grain size for the ZnO nanocrystals embedded in the porous matrix were about 10 nm. It is important to note that the Scherrer formula provides only a rough estimation on the mean size value of nanocrystals because both the inhomogeneous strain in the film and the instrumental effects can
Fig. 3. X-ray energy dispersive spectroscopy of ZnO/PS nanocomposite films 1–6. Inset at the upper right corner: the weight percentages of carbon, oxygen, Si and zinc elements on the top surfaces of the ZnO/PS nanocomposite films. Inset at the lower left corner: depth profiles of zinc (black curve), oxygen (red curve) and Si (white curve) in the ZnO/PS nanocomposite film.
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Fig. 4. FTIR spectra of (a) a PS film and (b) a ZnO/PS nanocomposite film.
stretching mode at 2100 cm 1. The FTIR results suggest that some of the unstable Si–H bonds on the PS film are replaced with Si–O bonds after sol–gel spin coating. Thus our data in Fig. 4 confirm the enhanced oxidation on the Si nanocrystals in the nanocomposites. The sol–gel spin coating induced further oxidation on the Si nanocrystals can modify the PL of the nanocomposites films: (i) a blueshift in the PL of PS is expected due to the quantum confinement effect [11,22]; (ii) the oxide layer on the Si nanocrystals can give off blue emissions due to the intrinsic defects and Si–O related species in the oxide layer [23]; and (iii) the tunneling effect of photogenerated carriers across the SiO2 interfacial layer will be significant if the barrier is narrowed in thickness. 3.3. Tunable PL Fig. 5 depicts the PL spectra of the ZnO/PS nanocomposite films 1–6. For the reason of comparison, the PL spectrum of a PS film is shown in Fig. 5 as the first curve; while the PL spectrum of the ZnO nanocrystals is placed in Fig. 5 as the last curve. The first curve in Fig. 5 shows that the Si nanocrystal exhibits strong red PL with its peak located at about 620 nm while the last curve in Fig. 5 exhibits that the ZnO nanocrystal gives off efficient green– blue PL spectrum with its peak located at about 475 nm. As shown by the PL curves 1–6 in Fig. 5, the green–blue luminescent band gains more in its intensity while the red luminescent band loses more in its intensity as more and more ZnO nanocrystals are incorporated into the porous matrix of PS film. For example, the PL spectrum of nanocomposite film 3 in Fig. 5 consists of two luminescent bands: the peak of orange luminescent band is located at about 600 nm while the peak of the green luminescent band is located at about 500 nm. The results in Fig. 5 show that the PL of ZnO/PS nanocomposite films is tunable from red to green–blue as more and more ZnO nanocrystals are incorporated into the porous matrix. In the course of the progressive blueshift from red to green–blue, white light emission can be expected. A quick check on the PL spectra in Fig. 5 suggests composite 4 is likely to give off white light emission because its PL covers the entire visible spectrum. White emissions are important for solidstate lighting industry to fabricate white light emitting diodes, and several groups reported white PL emissions for their ZnO/PS nanocomposite films [15,16,24,25], but the PL mechanisms are still in debate. For example, Kayahan attributed his white PL to the simultaneous emission of blue–green and green–yellow emissions from the ZnO and the red emission from the PS film [16], while Singh et al. [15] and Liu et al. [26] proposed that the
Fig. 5. PL spectra of the ZnO/PS nanocomposite films 1–6. For comparison, the PL spectrum of PS is listed as the first curve while the PL of ZnO nanocrystals is listed as the last one. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
electron tunneling between the interface of ZnO and PS through the siloxene structure were responsible for the white light emissions. The SiO2 interface between ZnO and Si nanocrystals plays an important role in the carrier recombinations of the composite system. In the absence of the SiO2 interface, one ZnO nanocrystal contacts directly with a Si nanocrystal. Assuming the continuity of vacuum levels and neglecting the effects of dipole and interfacial state, the heterojunction band structure of the ZnO/PS can be constructed. Values of electron affinity of w(ZnO)¼4.5 eV and w(Si)¼4.05 eV, the band gaps of Eg(ZnO)¼3.4 eV and Eg(PS)¼ 2.7 eV were taken. The band structure of such a heterojunction is shown in Fig. 6(a). As shown in Fig. 6(a), the barrier for electron is equal to DEC ¼ w(ZnO) w(Si)¼ 0.45 eV and that for hole (DEV) is equal to 1.15 eV. Both the conduction band and the valance band have band offsets, which are originated from the difference in the electron affinities and the band gaps of the two materials. It is noticed that the valance band offset DEV is much larger than the conduction band offset DEC. Because the value of DEC is positive, electrons in the conduction band of Si nanocrystals can easily flow into the conduction band of ZnO nanocrystals. Similarly, holes in the valence band of ZnO nanocrystals can easily flow into the valence band of Si nanocrystals. Therefore, in the absence of the SiO2 interfacial layer, photogenerated carriers in the a nanocomposite film can be recombined either in the ZnO nanocrystals or in the Si nanocrystals, but the PL of the nanocomposite will deviate away from the arithmetic summation of the host’s PL spectrum and the guest’s PL spectrum due to the ‘‘cross-talking’’ between them. Fig. 6(b) illustrates the flat band diagram of the ZnO/PS composite when the SiO2 interface layer is present. In such case, carriers can readily tunnel across the barrier if the width of the interface layer is very thin. In addition to the recombinations in
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The three replicas, which were labeled as T1, T2 and T3, were thermally annealed for 20 min in air at 200, 400 and 600 1C, respectively. The PL spectra of thermally annealed ZnO/PS nanocomposite films T1–T3 are given in Fig. 7. It is clear that the violet–blue emission band at about 400 nm gradually gains its intensity as the annealing temperature is raised. To obtain a better understanding on the underlying mechanisms, the PL spectra in Fig. 7 can be resolved into three Gaussian components by the following equation: " # 3 X ðllci Þ2 IðlÞ ¼ Ii exp , ð5Þ 2 2bi i¼1
Fig. 6. Energy band diagrams for ZnO/PS nanocomposite (a) in the absence of thin SiO2 interfacial layer and (b) in the presence of thin SiO2 interfacial layer.
the ZnO and Si nanocrystals, photogenerated carriers in the ZnO/ PS nanocomposite film can also recombine in the SiO2 interfrace layers. Within the SiO2 interface layer, there are a large number of interface traps and defect levels from the broken bonds and impurities of the Si nanocrystals. The carrier recombination through these interface traps and defect levels in the SiO2 interface layer can produce extra luminescent bands [11,21,23]. In our previous work, we demonstrated the defect-related blue emissions from the oxidized PS films [23]. In an extreme case, the interface layer is sufficiently thick so that the tunneling effect can be ignored. The PL spectrum of a nanocomposite in this extreme case will be the arithmetic summation of the PL spectra of Si nanocrystals, ZnO nanocrystals and the interface layer. No matter if the SiO2 interface layer is present or not, the emission of the ZnO/PS nanocomposites can be tuned by controlling the weight percentage the ZnO nanocrystals in the nanocomposites although the detailed mechanisms are different. 3.4. Thermal annealing effects In the semiconductor industry, semiconductors are annealed for diffusion of dopants, oxidation and annihilation of many kinds of point defects with the result of drastic changes in the electrical and optical properties of the semiconducting materials. Because the intrinsic defects in ZnO nanocrystals are responsible for the recorded green–blue and yellow–green luminescent bands [2], the PL properties of ZnO/PS nanocomposite can be modified by thermal annealing process [21]. In order to tune the PL of the ZnO/ PS nanocomposite further into the violet–blue spectral region, we prepared three replicas of ZnO/PS nanocomposite film 5.
where I stands for the recorded PL intensity. The parameters of the three-component Gausssian decomposition of the PL spectra are listed in Table 1. As shown in Fig. 7(a), the PL spectrum of nanocomposite film T1 (solid circles) can be decomposed into three Gaussian components whose peaks are located at 409.3 (blue dash line), 500.1 (green dash line) and 624.5 nm (red dash line). The arithmetic sum of the three decomposed luminescent bands is represented by the heavy green solid curve. The weak violet–blue PL band at 409.3 nm can be assigned to the nearband-edge emission of ZnO nanocrystals while the strong green band at 500.1 nm can be assigned to the deep-level emissions of ZnO nanocrystals. The deep-level emissions of ZnO nanocrystals are related to various types of defects such as oxygen vacancies (Vo) [27,28], zinc vacancies (VZn) [29], oxygen atoms at the Zn position in the crystal lattice (OZn) [30], and donor–acceptor pairs [31]. Although the detailed relation between the recorded visible emissions and the defects in the ZnO nanocrystals is still under debate, it is clear that the violet–blue band in Fig. 7 gradually gains its intensity as the annealing temperature increases from 200 to 600 1C; in the meanwhile the defect-related green emission loses its intensity. It is understandable because thermal annealing of the nanocomposites can annihilate many kinds of defects (such as interstitial vacancies) in the ZnO nanocrystals. Once the defect introduced deep traps in the bandgap are cleaned, the near band edge recombination will dominate. This is why the violet–blue PL band at about 400 nm in Fig. 7 gets stronger as the annealing temperature increases from 200 to 600 1C. The weak red PL band at 624.5 nm in Fig. 7(a) can be assigned to the emission of Si nanocrystals. As the annealing temperature is raised from 200 to 600 1C, the red PL band gets broad in its shape with its peak wavelength blue shifted from 624.5 to about 550 nm. On one hand, thermal annealing introduces further oxidation on Si nanocrystals. The annealing induced further oxidation on Si nanocrystals reduces the size of Si nanocrystals. According to the quantum confinement effect, blue shift in the PL of Si nanocrystals will be resulted once the size of Si nanocrystals is reduced. On the other hand, thermal annealing generates a lot of broken bonds on the surface of Si nanocrystals, and a large number of interface traps and defect levels will be introduced into the SiO2 interface layer. The carrier recombination through these interface traps and defect levels will broaden the luminescent band. Consequently, the PL band of Si nanocrystals will be blue shifted and broadened after the thermal annealing. Furthermore, the photogenerated carriers in the Si and ZnO nanocrystals will be blocked from tunneling across the interfacial layer when the oxide layer is thick enough. It is clear that the SiO2 interface layer has played important roles in the carrier recombinations of the nanocomposites. 3.5. CIE color coordinates Once the PL spectrum of a nanocomposite is modified, the color of its PL will be changed correspondingly [9]. In order to
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Fig. 7. Three-component Gaussian fitted PL spectra of a ZnO/PS nanocomposite film after thermal annealing in air for 20 min under the temperatures of (a) 200 1C, (b) 400 1C, and (c) 600 1C.
Table 1 Parameters of the three-component Gaussian decomposition of the PL spectra of ZnO/PS nanocomposite films T1, T2 and T3. Sample
Annealing temperature ( 1C)
T1
200
T2
400
T3
600
Value of i
Ii
lci (nm)
bi (nm)
1 2 3 1 2 3 1 2 3
41732 123,897 28,572 1,201,970 322,001 136,467 68,430 50,872 10,251
409.3 500.1 624.5 396.7 469.5 550.0 404.3 472.5 546.9
33.65 63.38 41.17 29.21 62.68 79.60 35.33 54.23 69.96
quantitatively study the evolution of the colors of the ZnO/PS nanocomposites, we should derive the chromaticity coordinates x and y in the CIE 1931 XYZ color space for these luminescent nanomaterials [9,32]. Fig. 8 illustrates the evolution of the colors of the photoluminescent ZnO/PS nanocomposite films in the CIE 1931 XYZ color space. The chromaticity coordinates of the host PS and the guest ZnO nanocrystals are shown in Fig. 8 for comparison. The solid line in Fig. 8 is for eye guidance only. As shown in Fig. 8, the perception of the PL color of the ZnO/PS nanocomposites can be effectively shifted from the red point (0.624, 0.374) to the blue point (0.184, 0.185). The realization of the tunable color for the ZnO/PS nanocomposites is a positive step toward its application in solid state lighting industry. White light emission is of special interest for white light-emitting diodes. In our case, white light emission can be achieved if appropriate amount of ZnO is incorporated into the porous matrix. As shown in Fig. 5, the ZnO/PS nanocomposite film 4 is likely to give off white colored PL because the contributions from its red, green and blue luminescent bands are nearly equal. In Fig. 8, the chromaticity coordinates of film 4 were (0.357, 0.432), which is very close to achromatic point (1/3, 1/3) in the CIE diagram. As shown by the square in Fig. 8, we achieved bright and efficient white
Fig. 8. Evolution of the colors of ZnO/PS nanocomposite films in the CIE 1931 XYZ color space. The solid line is for eye guidance only.
light-emitting ZnO/PS nanocomposites whose chromaticity coordinates were (0.33, 0.33) by fine adjusting the concentration of Zn cations in the sol–gels. Fine color tuning and optimum level of ZnO incorporation in the ZnO/PS nanomaterials are currently in progress to reach better color points in CIE chromaticity diagram. The data in Fig. 8 demonstrate that wavelength tunable PL of ZnO/PS nanocomposite films can be achieved from red to blue by controlling the concentration of zinc cations in the sol–gels and
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the parameters of post-thermal annealing. The high versatility of the ZnO/PS nanocomposites offers a wide range of possibilities for designing tailor-made luminescent materials for optical applications. That is why numerous ZnO-based nanocomposites were developed in the past few years [33–36]. Besides the intrinsic emissions of the guest and the host themselves, the key features of these nanocomposites lie at the interfacial interactions between the host and the guest materials. The limitation of our approach in this work is the fine control of the required amount of ZnO in the ZnO/PS nanomaterials.
4. Conclusions A series of ZnO/PS nanocomposite films have been prepared by sol–gel spin-coating technology. The morphologies, crystal structures, chemical compositions, surface passivations and PL spectra of the ZnO/PS nanocomposite films have been characterized with SEM, XRD, EDX, FTIR and photospectroscopy, respectively. Our results have demonstrated that the PL of the ZnO/PS nanocomposites can be tuned from red to violet–blue by controlling the concentration of Zn cations in the sol–gels and the annealing temperature in postthermal annealing process. Fine control of the required amount of ZnO in the PS matrix is the key factor to determine the PL properties of the nanocomposite. In the meanwhile the SiO2 interfacial layer between the ZnO and Si nanocrystals plays an important role in the recombination processes of the nanocomposites. In addition to the advantages of low cost, high processibility and reasonable reproducibility, the ZnO/PS nanocomposites with tunable PL properties are of considerable interest for the fabrication of nano-electronic and nano-optical devices if we consider its compatibility with the mature Si technologies.
Acknowledgments This work was financially supported by Changzhou University under the Grant no. ZMF1002132. References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Wavelength tunable photoluminescence of ZnO/porous Si nanocomposites Yuan Ming Huang n, Qing-lan Ma, Bai-gai Zhai School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China
a r t i c l e i n f o
abstract
Article history: Received 22 April 2012 Received in revised form 6 January 2013 Accepted 6 February 2013 Available online 14 February 2013
ZnO/porous Si (PS) nanocomposite films were prepared with the sol–gel spin-coating method. The morphologies, crystal structures, chemical compositions, surface passivations and photoluminescence (PL) spectra of the ZnO/PS nanocomposite films were investigated. It is found that the PL of the ZnO/PS nanocomposites can be tuned from red to green–blue by increasing the concentration of zinc cations in the sol–gels from 7.5 to 240 mM. Post-thermal annealing of the ZnO/PS nanocomposite films at 600 1C can shift the PL further into violet–blue. The roles of SiO2 thin interfacial layer between the ZnO and the Si nanocrystals in the nanocomposites are discussed. Our work provides an affordable means of tuning the emission color of the ZnO/PS nanocomposites across the visible spectral region. & 2013 Elsevier B.V. All rights reserved.
Keywords: Porous Si Zinc oxide Nanocomposite Photoluminescence
1. Introduction Nanometer-sized zinc oxide (ZnO) is a wide bandgap semiconductor (3.37 eV) with a large exciton binding energy (about 60 meV). These features make ZnO nanocrystals suitable for fabrications of ultraviolet nanolasers [1–3], luminescent phosphor [4], light-emitting diodes [5], ultraviolet light detectors [6] and ultraviolet optical switches [7]. Among these applications, the photoluminescence (PL) of ZnO nanocrystals is fundamental to the optoelectronic devices. In general, highly pure ZnO nanocrystals contain little defects in the crystal lattices; the band edge emission of the ZnO nanocrystals produces strong ultraviolet emission at about 385 nm. On the contrary, abundant intrinsic defects (i.e., vacancies of oxygen and zinc, interstitial oxygen and zinc, and antisite oxygen) are present in low quality ZnO nanocrystals, these defect-involved recombinations in the ZnO nanocrystals often generate blue [2], green and yellow emissions. To make ZnO an ideal luminescent material suitable for lightemitting diodes, tunable PL of ZnO nanocrystals in the visible spectral region is highly required. Theoretically speaking, tunable PL of ZnO nanocrystals in the visible spectral region can be realized by the precise control of the type and the population of the defects in the ZnO nanocrystals [8]. However, it is difficult to do so in practice. Incorporation of the ZnO nanocrystals into a foreign matrix to form nanocomposite materials represents one route towards tuning PL of ZnO nanocrystals. In principle, tunable PL within the entire visible spectral region can be expected from a nanocomposite film if
n
Corresponding author. Tel./fax: þ 86 519 86334730. E-mail address: [email protected] (Y.M. Huang).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.002
the guest material gives off blue emission at the short wavelength end while the host material gives off red emission at the long wavelength end [9,10]. It is well known that porous Si (PS) can give off efficient red PL at room temperature [11]. Both the high surface area and the rich nanopores in PS films make it capable of containing a large amount of ZnO nanocrystals in the porous matrix [12,13]. Therefore, tunable PL from red to violet–blue can be expected for the ZnO/PS nanocomposites if the weight percentages of ZnO nanocrystals in the porous matrix can be finely controlled. By embedding ZnO nanocrystals into the matrix of PS, several groups reported white emissions for ZnO/PS nanocomposites [14–17]. For example, Singh et al. reported white light emission for their ZnO/PS nanocomposite film that had been prepared by the sol–gel spin coating method; Kayahan also recorded the white PL from his ZnO/PS nanocomposite that had been prepared by the rf-sputtering technique [16]. Inspite of these pioneering studies, the tunable PL across the entire visible spectrum has not been reported. In this paper, we reported the wavelength tunable PL from red to violet–blue of the ZnO/PS nanocomposite films by controlling the concentration of Zn cations in the sol–gels and the annealing temperature in postthermal annealing process. The roles of SiO2 thin interfacial layer between the ZnO and the Si nanocrystals are discussed for the ZnO/ PS nanocomposites.
2. Experimental details 2.1. Preparation of PS matrix PS films were prepared by electrochemically anodizing boron doped p-type (1 1 1)-oriented Si wafer in hydrofluoric electrolyte at the constant current density of 3 mA/cm2 for 60 min. The
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resistivity of the Si wafer was 8–13 O cm. The electrolyte had an equal volume mixture of hydrofluoric acid (40 wt%) and ethanol (95 wt%). In the electrolytic solution, Si wafer was utilized as the working electrode while the platinum wires were employed to be the counter electrode. Details of the experimental setup and method were reported elsewhere [17]. 2.2. Preparation of ZnO/PS nanocomposites ZnO/PS nanocomposite films were prepared by the sol–gel spin coating technology. Analytical grade of reagents triethanolamine and zinc acetate dihydrate were provided by a local chemical supplier (Sinopharm Chemical Reagent Co., China). Sol–gels were prepared by dissolving specific amount of zinc acetate dihydrate into the mixture of triethanolamine (0.152 g) and ethanol (95 wt%, 6 ml). When the mass of zinc acetate dihydrate increased from 10, 21, 43, 86, 171 to 342 mg, six kinds of sol–gels were obtained with the molar concentrations of zinc cations in the sol–gels to be 7.5, 15, 30, 60, 120 and 240 mM. The speed of the spin coater was fixed to be about 2000 rpm while the duration of the coating was about 20 s. After one sol–gel had been spin-coated on the top surface of a PS film, subsequent baking in an oven at 245 1C was applied to the sol–gel spin-coated PS film to ensure the growth of ZnO nanocrystals in the porous matrix. Serial numbers 1–6 were assigned to the six ZnO/PS nanocomposite films as the molar concentrations of zinc cations in the sol–gels were in the sequence of 7.5, 15, 30, 60, 120 and 240 mM. 2.3. Characterizations The X-ray diffraction (XRD) characterization of the thin films was carried out on an X-ray diffractometer (D/max 2500 PC, Rigaku, Japan). A copper target was utilized to generate the X-ray (l ¼0.154 nm). The operating voltage was 40 kV and the current was 100 mA. A scanning electron microscope (SEM) was employed to characterize the top and cross-sectional morphologies of the films. Fourier transform infrared (FTIR) spectroscopy was employed to analyze the chemical bonding on the surface of ZnO/PS composites; all FTIR spectra were collected on a spectrometer (Nicolet Prote´ge´ 460). The PL spectra of the films were recorded with a spectrophotometer (Aipha Technologies, China). The 325 nm laser line from a helium–cadmium laser (Kimmon Koha Co. Ltd., Japan) was utilized as the excitation source for the PL measurement. The output power of the ultraviolet laser was about 12 mW.
3. Results and discussion 3.1. Morphology and structure The morphology of the PS films and the ZnO/PS nanocomposite films have been examined with SEM. Fig. 1(a) and (b) shows the top and cross sectional SEM micrographs of the PS film. It is obvious that the top surface of the PS film is rough with a lot of nanopores. The average size of the nanopores on the top surface of the film is less than 2 nm in diameter. Unlike the columnar microstructures in our previous work [12,13,18], Fig. 1(a) and (b) demonstrates that our PS film is characteristic of spongy nanostructures. The nanopores in the spongy nanostructures make the porous matrix of PS film suitable for the filtration of water from the prepared sol–gels. By the capillary action, the zinc cations and hydroxide anions containing water can be easily sucked into the nanopores from the sol–gels. Once the zinc cations and hydroxide anions in the aqueous solution are sucked into the spongy nanostructures of PS, the heat treatment at 245 1C can readily turn them into ZnO nanocrystals by the reactions Zn2 þ þ 2OH -ZnðOHÞ2 ,
ð1Þ
ZnðOHÞ2 -ZnO þ H2 O:
ð2Þ
Fig. 1(c) and (d) depicts the top and cross sectional the SEM micrographs of a ZnO/PS nanocomposite film. The originally rough surface of the porous matrix is smoothed by the sol–gel spin-coating, suggesting that the nanopores in the spongy nanostructures of PS are filled with the guest materials. As shown in Fig. 1(d), the thickness of the ZnO/PS nanocomposite film is about 1 mm, and the thickness of the ZnO layer on the top of the ZnO/PS nanocomposite film is about only 50 nm. The infiltration of zinc oxide is evident. Fig. 2 gives the powder XRD pattern of a typical ZnO/PS nanocomposite film. The peaks at 29.11 and 48.21 in Fig. 2 can be assigned to the reflections from the 1 1 1 and 2 2 0 planes of Si nanocrystals in the nanocomposite (JCPDS no. 27-1402). According to the standard data for single crystalline Si, the diffraction peaks of the 1 1 1 and 2 2 0 planes are located at 2y ¼28.4421 and 47.3021, respectively. With respect to the peaks of single crystalline Si, the 2y angles of the 1 1 1 and 2 2 0 diffraction peaks of PS film are shifted 0.661 and 0.901 toward the larger angle direction. The observed shift in the diffraction peaks can be attributed to the porosity-introduced distortion in the lattice parameter of Si nanocrystallites in the film. The peaks at 31.7, 34.4, 36.2, 47.5,
Fig. 1. Top (a) and cross sectional (b) SEM micrographs of a PS film. Top (c) and cross sectional (d) SEM micrographs of a ZnO/PS nanocomposite film.
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contribute to the width of the diffraction peak. On the basis of the XRD data in Fig. 2, it is safe for us to conclude that the mean grain size of the ZnO nanocrystals in the porous matrix is in the order of nanometer. 3.2. Chemical composition and surface passivation
Fig. 2. Powder X-ray diffraction pattern of a typical ZnO/PS nanocomposite film.
56.6, 62.9, and 65.4 in Fig. 2 are assigned to the reflections from the 1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0, 1 0 3, and 2 0 0 planes of the hexagonal ZnO with wurtzite structure (JCPDS No.36–1451) respectively. According to the standard data for hexagonal ZnO, diffraction peaks of the 1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0, 1 0 3, and 2 0 0 planes of the hexagonal ZnO with wurtzite structure are located at 2y ¼31.7691, 34.4211, 36.2521, 47.5381, 56.6021, 62.8621 and 66.3781, respectively. Comparison of these data reveals that our recorded data are consistent to the standard ones. Specifically, we have noticed that the diffraction angle of the ZnO (0 0 2) peak in Fig. 2 agrees well with the reported data in the literature for ZnO/PS nanocomposites [16,19–21]. The co-existence of the diffraction peaks of Si and ZnO nanocrystals in Fig. 2 reveals that ZnO nanocrystals have been incorporated in the porous matrix of the host material. The peaks at 39.81 and 43.11 in Fig. 2 can be attributed to the reflections from the 3 1 2 and 2 2 0 planes of lutecite (SiO2) nanocrystals (JCPDF no. 46-1441). By the standard data, the 3 1 2 and 2 2 0 diffraction peaks of lutecite nanocrystals are located at 39.6721 and 42.6111, respectively. The appearance of the SiO2 nanocrystals suggests that the sol–gel spin coating has introduced further oxidation on Si nanocrystals in the nanocomposites. Reaction of Si with bases is proposed to be the reason for the formation of the lutecite. In highly basic solutions, Si atoms are attacked by bases such as aqueous sodium hydroxide to give silicates, which are highly complex species containing the anion [SiO4]4 , by the reaction SiðsÞ þ 4OH ðaqÞ-½SiO4 4 ðaqÞ þ 2H2 ðgÞ:
Energy-dispersive X-ray spectroscopy (EDX) is an analytical technique used for the elemental analysis of a specimen. Fig. 3 shows the EDX spectrum of the top surface of a typical ZnO/PS nanocomposite film. The characteristic emission lines of elements are labeled in Fig. 3 at 0.277 (C Ka1), 0.525 (O Ka1), 1.01 (Zn L), 1.739 (Si K), 2.123 (Au Ma1), 8.63 (Zn Ka), 9.57 (Zn Kb). These data indicate that the surface layer of the ZnO/PS nanocomposite film is rich of carbon, oxygen, platinum, Si and zinc atoms. The presence of gold in the specimens was due to the gold sputtering onto the surfaces of our samples for the SEM and EDX characterizations. The inset at the lower left corner of Fig. 3 represents the depth profiles of zinc (black curve), oxygen (red curve) and Si (white curve) in the ZnO/PS nanocomposite film. The depth was defined to be the distance of a point in the film away from the air/ PS interface. The depth profile analysis shows that the weight percentage of Zn atoms in the nanocomposite film gradually decreases from its maximum value at the air/PS interface to zero at the PS/Si interface. Our EDX results have demonstrated that the spongy PS matrix had been filled with the ZnO nanocrystals. The inset at the upper right corner of Fig. 3 shows the weight percentages of carbon, oxygen, Si and zinc elements on the top surfaces of the nanocomposite films 1–6. From 1 to 6, zinc increases from 6.8 to 21.4 wt%, Si decreases from 63.0 to 43.1 wt%, carbon fluctuates between 13.7 and 15.5 wt%, oxygen varies in the range of 15.9–26.9 wt%. In combination with the data shown in Figs. 1 and 2, the data in Fig. 3 demonstrate that more and more ZnO nanocrystals have been incorporated into the nanopores of PS as the concentrations of zinc cations in the sol– gels increase from 7.5 to 240 mM. FTIR spectra of a PS film and a ZnO/PS nanocomposite film are given in Fig. 4. The bond vibrations in Fig. 4 at about 880, 1040, 2100, and 2260 cm 1 can be attributed to the Si–H bond (scissors mode), the Si–O–Si bond (stretching mode), the Si–H bond (stretching mode), the O–Si–H (stretching mode), respectively [16,22]. With respect to the data in Fig. 4(a), the data in Fig. 4(b) show that there is an increase in the intensity of the Si–O–Si stretching band at 1040 cm 1 and a decrease of the Si–H
ð3Þ
Thermal annealing of the nanocomposites at 245 1C helps to concentrate the silicate solution, and precipitation from silica saturated solutions at high pH gives the lutecite nanocrystals on the surface of Si skeletons in the PS film. The average grain size of the ZnO nanocrystals in the nanocomposites can be estimated by the Scherrer formula using the full width at half-maximum (FWHM) value of the XRD diffraction peak (0 0 2) at 34.41: D¼
0:9l bcosy
ð4Þ
where D, l, y and b are the mean grain size, the X-ray wavelength, the Bragg diffraction angle and the FWHM, respectively. The calculated values of the mean grain size for the ZnO nanocrystals embedded in the porous matrix were about 10 nm. It is important to note that the Scherrer formula provides only a rough estimation on the mean size value of nanocrystals because both the inhomogeneous strain in the film and the instrumental effects can
Fig. 3. X-ray energy dispersive spectroscopy of ZnO/PS nanocomposite films 1–6. Inset at the upper right corner: the weight percentages of carbon, oxygen, Si and zinc elements on the top surfaces of the ZnO/PS nanocomposite films. Inset at the lower left corner: depth profiles of zinc (black curve), oxygen (red curve) and Si (white curve) in the ZnO/PS nanocomposite film.
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Fig. 4. FTIR spectra of (a) a PS film and (b) a ZnO/PS nanocomposite film.
stretching mode at 2100 cm 1. The FTIR results suggest that some of the unstable Si–H bonds on the PS film are replaced with Si–O bonds after sol–gel spin coating. Thus our data in Fig. 4 confirm the enhanced oxidation on the Si nanocrystals in the nanocomposites. The sol–gel spin coating induced further oxidation on the Si nanocrystals can modify the PL of the nanocomposites films: (i) a blueshift in the PL of PS is expected due to the quantum confinement effect [11,22]; (ii) the oxide layer on the Si nanocrystals can give off blue emissions due to the intrinsic defects and Si–O related species in the oxide layer [23]; and (iii) the tunneling effect of photogenerated carriers across the SiO2 interfacial layer will be significant if the barrier is narrowed in thickness. 3.3. Tunable PL Fig. 5 depicts the PL spectra of the ZnO/PS nanocomposite films 1–6. For the reason of comparison, the PL spectrum of a PS film is shown in Fig. 5 as the first curve; while the PL spectrum of the ZnO nanocrystals is placed in Fig. 5 as the last curve. The first curve in Fig. 5 shows that the Si nanocrystal exhibits strong red PL with its peak located at about 620 nm while the last curve in Fig. 5 exhibits that the ZnO nanocrystal gives off efficient green– blue PL spectrum with its peak located at about 475 nm. As shown by the PL curves 1–6 in Fig. 5, the green–blue luminescent band gains more in its intensity while the red luminescent band loses more in its intensity as more and more ZnO nanocrystals are incorporated into the porous matrix of PS film. For example, the PL spectrum of nanocomposite film 3 in Fig. 5 consists of two luminescent bands: the peak of orange luminescent band is located at about 600 nm while the peak of the green luminescent band is located at about 500 nm. The results in Fig. 5 show that the PL of ZnO/PS nanocomposite films is tunable from red to green–blue as more and more ZnO nanocrystals are incorporated into the porous matrix. In the course of the progressive blueshift from red to green–blue, white light emission can be expected. A quick check on the PL spectra in Fig. 5 suggests composite 4 is likely to give off white light emission because its PL covers the entire visible spectrum. White emissions are important for solidstate lighting industry to fabricate white light emitting diodes, and several groups reported white PL emissions for their ZnO/PS nanocomposite films [15,16,24,25], but the PL mechanisms are still in debate. For example, Kayahan attributed his white PL to the simultaneous emission of blue–green and green–yellow emissions from the ZnO and the red emission from the PS film [16], while Singh et al. [15] and Liu et al. [26] proposed that the
Fig. 5. PL spectra of the ZnO/PS nanocomposite films 1–6. For comparison, the PL spectrum of PS is listed as the first curve while the PL of ZnO nanocrystals is listed as the last one. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
electron tunneling between the interface of ZnO and PS through the siloxene structure were responsible for the white light emissions. The SiO2 interface between ZnO and Si nanocrystals plays an important role in the carrier recombinations of the composite system. In the absence of the SiO2 interface, one ZnO nanocrystal contacts directly with a Si nanocrystal. Assuming the continuity of vacuum levels and neglecting the effects of dipole and interfacial state, the heterojunction band structure of the ZnO/PS can be constructed. Values of electron affinity of w(ZnO)¼4.5 eV and w(Si)¼4.05 eV, the band gaps of Eg(ZnO)¼3.4 eV and Eg(PS)¼ 2.7 eV were taken. The band structure of such a heterojunction is shown in Fig. 6(a). As shown in Fig. 6(a), the barrier for electron is equal to DEC ¼ w(ZnO) w(Si)¼ 0.45 eV and that for hole (DEV) is equal to 1.15 eV. Both the conduction band and the valance band have band offsets, which are originated from the difference in the electron affinities and the band gaps of the two materials. It is noticed that the valance band offset DEV is much larger than the conduction band offset DEC. Because the value of DEC is positive, electrons in the conduction band of Si nanocrystals can easily flow into the conduction band of ZnO nanocrystals. Similarly, holes in the valence band of ZnO nanocrystals can easily flow into the valence band of Si nanocrystals. Therefore, in the absence of the SiO2 interfacial layer, photogenerated carriers in the a nanocomposite film can be recombined either in the ZnO nanocrystals or in the Si nanocrystals, but the PL of the nanocomposite will deviate away from the arithmetic summation of the host’s PL spectrum and the guest’s PL spectrum due to the ‘‘cross-talking’’ between them. Fig. 6(b) illustrates the flat band diagram of the ZnO/PS composite when the SiO2 interface layer is present. In such case, carriers can readily tunnel across the barrier if the width of the interface layer is very thin. In addition to the recombinations in
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The three replicas, which were labeled as T1, T2 and T3, were thermally annealed for 20 min in air at 200, 400 and 600 1C, respectively. The PL spectra of thermally annealed ZnO/PS nanocomposite films T1–T3 are given in Fig. 7. It is clear that the violet–blue emission band at about 400 nm gradually gains its intensity as the annealing temperature is raised. To obtain a better understanding on the underlying mechanisms, the PL spectra in Fig. 7 can be resolved into three Gaussian components by the following equation: " # 3 X ðllci Þ2 IðlÞ ¼ Ii exp , ð5Þ 2 2bi i¼1
Fig. 6. Energy band diagrams for ZnO/PS nanocomposite (a) in the absence of thin SiO2 interfacial layer and (b) in the presence of thin SiO2 interfacial layer.
the ZnO and Si nanocrystals, photogenerated carriers in the ZnO/ PS nanocomposite film can also recombine in the SiO2 interfrace layers. Within the SiO2 interface layer, there are a large number of interface traps and defect levels from the broken bonds and impurities of the Si nanocrystals. The carrier recombination through these interface traps and defect levels in the SiO2 interface layer can produce extra luminescent bands [11,21,23]. In our previous work, we demonstrated the defect-related blue emissions from the oxidized PS films [23]. In an extreme case, the interface layer is sufficiently thick so that the tunneling effect can be ignored. The PL spectrum of a nanocomposite in this extreme case will be the arithmetic summation of the PL spectra of Si nanocrystals, ZnO nanocrystals and the interface layer. No matter if the SiO2 interface layer is present or not, the emission of the ZnO/PS nanocomposites can be tuned by controlling the weight percentage the ZnO nanocrystals in the nanocomposites although the detailed mechanisms are different. 3.4. Thermal annealing effects In the semiconductor industry, semiconductors are annealed for diffusion of dopants, oxidation and annihilation of many kinds of point defects with the result of drastic changes in the electrical and optical properties of the semiconducting materials. Because the intrinsic defects in ZnO nanocrystals are responsible for the recorded green–blue and yellow–green luminescent bands [2], the PL properties of ZnO/PS nanocomposite can be modified by thermal annealing process [21]. In order to tune the PL of the ZnO/ PS nanocomposite further into the violet–blue spectral region, we prepared three replicas of ZnO/PS nanocomposite film 5.
where I stands for the recorded PL intensity. The parameters of the three-component Gausssian decomposition of the PL spectra are listed in Table 1. As shown in Fig. 7(a), the PL spectrum of nanocomposite film T1 (solid circles) can be decomposed into three Gaussian components whose peaks are located at 409.3 (blue dash line), 500.1 (green dash line) and 624.5 nm (red dash line). The arithmetic sum of the three decomposed luminescent bands is represented by the heavy green solid curve. The weak violet–blue PL band at 409.3 nm can be assigned to the nearband-edge emission of ZnO nanocrystals while the strong green band at 500.1 nm can be assigned to the deep-level emissions of ZnO nanocrystals. The deep-level emissions of ZnO nanocrystals are related to various types of defects such as oxygen vacancies (Vo) [27,28], zinc vacancies (VZn) [29], oxygen atoms at the Zn position in the crystal lattice (OZn) [30], and donor–acceptor pairs [31]. Although the detailed relation between the recorded visible emissions and the defects in the ZnO nanocrystals is still under debate, it is clear that the violet–blue band in Fig. 7 gradually gains its intensity as the annealing temperature increases from 200 to 600 1C; in the meanwhile the defect-related green emission loses its intensity. It is understandable because thermal annealing of the nanocomposites can annihilate many kinds of defects (such as interstitial vacancies) in the ZnO nanocrystals. Once the defect introduced deep traps in the bandgap are cleaned, the near band edge recombination will dominate. This is why the violet–blue PL band at about 400 nm in Fig. 7 gets stronger as the annealing temperature increases from 200 to 600 1C. The weak red PL band at 624.5 nm in Fig. 7(a) can be assigned to the emission of Si nanocrystals. As the annealing temperature is raised from 200 to 600 1C, the red PL band gets broad in its shape with its peak wavelength blue shifted from 624.5 to about 550 nm. On one hand, thermal annealing introduces further oxidation on Si nanocrystals. The annealing induced further oxidation on Si nanocrystals reduces the size of Si nanocrystals. According to the quantum confinement effect, blue shift in the PL of Si nanocrystals will be resulted once the size of Si nanocrystals is reduced. On the other hand, thermal annealing generates a lot of broken bonds on the surface of Si nanocrystals, and a large number of interface traps and defect levels will be introduced into the SiO2 interface layer. The carrier recombination through these interface traps and defect levels will broaden the luminescent band. Consequently, the PL band of Si nanocrystals will be blue shifted and broadened after the thermal annealing. Furthermore, the photogenerated carriers in the Si and ZnO nanocrystals will be blocked from tunneling across the interfacial layer when the oxide layer is thick enough. It is clear that the SiO2 interface layer has played important roles in the carrier recombinations of the nanocomposites. 3.5. CIE color coordinates Once the PL spectrum of a nanocomposite is modified, the color of its PL will be changed correspondingly [9]. In order to
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Fig. 7. Three-component Gaussian fitted PL spectra of a ZnO/PS nanocomposite film after thermal annealing in air for 20 min under the temperatures of (a) 200 1C, (b) 400 1C, and (c) 600 1C.
Table 1 Parameters of the three-component Gaussian decomposition of the PL spectra of ZnO/PS nanocomposite films T1, T2 and T3. Sample
Annealing temperature ( 1C)
T1
200
T2
400
T3
600
Value of i
Ii
lci (nm)
bi (nm)
1 2 3 1 2 3 1 2 3
41732 123,897 28,572 1,201,970 322,001 136,467 68,430 50,872 10,251
409.3 500.1 624.5 396.7 469.5 550.0 404.3 472.5 546.9
33.65 63.38 41.17 29.21 62.68 79.60 35.33 54.23 69.96
quantitatively study the evolution of the colors of the ZnO/PS nanocomposites, we should derive the chromaticity coordinates x and y in the CIE 1931 XYZ color space for these luminescent nanomaterials [9,32]. Fig. 8 illustrates the evolution of the colors of the photoluminescent ZnO/PS nanocomposite films in the CIE 1931 XYZ color space. The chromaticity coordinates of the host PS and the guest ZnO nanocrystals are shown in Fig. 8 for comparison. The solid line in Fig. 8 is for eye guidance only. As shown in Fig. 8, the perception of the PL color of the ZnO/PS nanocomposites can be effectively shifted from the red point (0.624, 0.374) to the blue point (0.184, 0.185). The realization of the tunable color for the ZnO/PS nanocomposites is a positive step toward its application in solid state lighting industry. White light emission is of special interest for white light-emitting diodes. In our case, white light emission can be achieved if appropriate amount of ZnO is incorporated into the porous matrix. As shown in Fig. 5, the ZnO/PS nanocomposite film 4 is likely to give off white colored PL because the contributions from its red, green and blue luminescent bands are nearly equal. In Fig. 8, the chromaticity coordinates of film 4 were (0.357, 0.432), which is very close to achromatic point (1/3, 1/3) in the CIE diagram. As shown by the square in Fig. 8, we achieved bright and efficient white
Fig. 8. Evolution of the colors of ZnO/PS nanocomposite films in the CIE 1931 XYZ color space. The solid line is for eye guidance only.
light-emitting ZnO/PS nanocomposites whose chromaticity coordinates were (0.33, 0.33) by fine adjusting the concentration of Zn cations in the sol–gels. Fine color tuning and optimum level of ZnO incorporation in the ZnO/PS nanomaterials are currently in progress to reach better color points in CIE chromaticity diagram. The data in Fig. 8 demonstrate that wavelength tunable PL of ZnO/PS nanocomposite films can be achieved from red to blue by controlling the concentration of zinc cations in the sol–gels and
Y.M. Huang et al. / Journal of Luminescence 138 (2013) 157–163
the parameters of post-thermal annealing. The high versatility of the ZnO/PS nanocomposites offers a wide range of possibilities for designing tailor-made luminescent materials for optical applications. That is why numerous ZnO-based nanocomposites were developed in the past few years [33–36]. Besides the intrinsic emissions of the guest and the host themselves, the key features of these nanocomposites lie at the interfacial interactions between the host and the guest materials. The limitation of our approach in this work is the fine control of the required amount of ZnO in the ZnO/PS nanomaterials.
4. Conclusions A series of ZnO/PS nanocomposite films have been prepared by sol–gel spin-coating technology. The morphologies, crystal structures, chemical compositions, surface passivations and PL spectra of the ZnO/PS nanocomposite films have been characterized with SEM, XRD, EDX, FTIR and photospectroscopy, respectively. Our results have demonstrated that the PL of the ZnO/PS nanocomposites can be tuned from red to violet–blue by controlling the concentration of Zn cations in the sol–gels and the annealing temperature in postthermal annealing process. Fine control of the required amount of ZnO in the PS matrix is the key factor to determine the PL properties of the nanocomposite. In the meanwhile the SiO2 interfacial layer between the ZnO and Si nanocrystals plays an important role in the recombination processes of the nanocomposites. In addition to the advantages of low cost, high processibility and reasonable reproducibility, the ZnO/PS nanocomposites with tunable PL properties are of considerable interest for the fabrication of nano-electronic and nano-optical devices if we consider its compatibility with the mature Si technologies.
Acknowledgments This work was financially supported by Changzhou University under the Grant no. ZMF1002132. References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.
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