Quantum Dot
Bandgap Tuning by Using a Lattice Distortion Induced by Two Symmetries That Coexist in a Quantum Dot Eui-Hyun Kong, Yong-June Chang, Hyun-Jin Park, and Hyun M. Jang*
Among the interests in the application of quantum dots (QDs), the bandgap tuning is of key importance in controlling their material properties. The bandgap of a QD can be adjusted by adopting a variety of different physicochemical methods. Herein, a novel way of the bandgap tuning is developed in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S to cubic Ag and by subsequently inducing a lattice-distorted region of ≈1-nm-scale in a QD. The two distinct crystalline phases of Ag2S and Ag coexisting with the lattice-distorted region are experimentally demonstrated by visually showing this remarkable coexistence in a QD. A new approach is presented to the bandgap tuning (2.51 to 1.64 eV) and enhancing optical properties by suitably tailoring the degree of the lattice-distorted region in a QD. This conceptual method could pave a new way to utilizing quantum effects in various QD applications.
1. Introduction Semiconductor quantum dots (QDs) have become a matter of primary concern in recent research because of their enormous potential for applications in photonic and biological fields which necessitate QDs with excellent optoelectronic properties. The bandgap tuning of QDs has been developed by adopting a variety of different methods. These methods comprise use of i) alloyed QDs (CdZnS,[1–3] CdZnSe,[4,5] CdZnTe[6]), ii) core-shell QDs with different band structures[7,8] and with heteroepitaxial strain [a soft CdTe core and compressive shells (ZnS, ZnSe, ZnTe, CdS, or CdSe)],[9] iii) size-controlled QDs using reverse micelles (CdS,[10,11] CdSe[12]), solvothermal reaction (CdSe),[13] and ultrasonic
E.-H. Kong, Y.-J. Chang, Prof. H. M. Jang Department of Materials Science and Engineering, and Division of Advanced Materials Science Pohang University of Science and Technology (POSTECH) Pohang, 790–784, Republic of Korea E-mail: [email protected] H.-J. Park National Center for Nanomaterials Technology (NCNT) Pohang University of Science and Technology (POSTECH) Pohang, 790–784, Republic of Korea DOI: 10.1002/smll.201303040 small 2013, DOI: 10.1002/smll.201303040
irradiation (ZnO),[14] and iv) a porous three-dimensional fractal network of quantum dots for controlling the quantumconfinement effect (CdSe).[15] Herein, we develop a novel way of tuning the bandgap in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S to cubic Ag and by subsequently inducing an about 1-nmscale lattice-distorted region between these two distinct crystalline phases in a given QD. Small nanoparticles (105 dm3 mol−1 cm−1) can be estimated by using the Tauc model[32] using the absorption spectra, instead of by using PL emission spectra. The Tauc model is represented by "hv ∝ 6(hv − E g )n , where hv is the photon energy, Eg is the optical bandgap energy, and k is the photon-energydependent constant. The power-law exponent, n, depends on the transition type, where n = 1/2 for a direct bandgap transition and n = 2 for an indirect bandgap transition. The absorption coefficient (α) is determined by the transmittance. This Tauc model, which had been originally proposed for singlecomponent semiconductors,[32] has successfully been applied to multicomponent semiconducting systems.[13,33–35] Since Ag2S is a well-known direct bandgap semiconductor;[36] its optical bandgap can thus be determined by a plot of (αhv)2 vs. photon energy, as shown in Figure 7d. The optical bandgap of pure TiO2 is 3.21 eV, as estimated by the Tauc model (see the black arrow in Figure 7d). This value agrees well with the direct interband transition of anatase TiO2 at 3.2 eV.[37] The bandgap of the Ag2S QDs adsorbed on TiO2 decreases as the calcination time varies from 1 to 3 min. Thus, a wide range of the optical bandgap was obtained (2.51 to 1.64 eV; Figure 7d). In particular, the film annealed for 3 min shows a direct transition at a significantly reduced energy of 1.64 eV (red solid line in Figure 7d), which is closely related to the lattice distortion caused by the coexistence of Ag2S and Ag phases in a given QD (Figures 2d and 3c). The distorted lattice consists of various AgxSy stoichiometries with a variety of different energy levels such as alloyed QDs[1–6] (based on STEM-EDS data in Figure 3c). This hypothesis supports the observed interband transition at a significantly reduced energy of 1.64 eV (Figure 7d). Metallic Ag nanocrystallites on a semiconductor such as TiO2 can also reduce the optical bandgap and enhance the absorption coefficient of the semiconductor owing to the local-field enhancement by the surface plasmon resonance of silver nanoparticles.[38] According to our experimental results, however, there is little variation in the optical properties between the Ag2S QDs (deposited at 25 °C by the SILAR process) and the sequentially deposited Ag+Ag2S mixed QDs on TiO2 particles (Figure S2). Here, the Ag QDs in the mixed Ag+Ag2S QDs were prepared by the oxidation of the predeposited Ag2S QDs at 510 °C for more than 4 min, whereas Ag2S QDs in the mixed QDs were subsequently deposited by the SILAR process after the preparation of the Ag QDs on TiO2 particles. This small variation clearly indicates that the enhanced absorbance with the reduced bandgap in a QD with a lattice-distorted region of ca. 1-nm-scale is not directly related to the metallic Ag phase formed by high-temperature oxidation. In addition, transmittance and reflectance of anatase TiO2 films have no relation to the calcination temperature (Figure S3).
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We now examine the size effect of QD on the bandgap variation. For this, we present two different types of experimental observations. Firstly, as shown in Figures 2b and 2d, and Figure S4a, QDs heat-treated at 510 °C for 3 min are well dispersed without showing appreciable growth in QD size. Thus, the observed bandgap reduction of QDs calcined at 510 °C (Figure 7d) is not directly related to the size growth of QDs. Secondly, to clearly isolate the effect of QD size on the optical properties, we artificially grew Ag2S QDs by using thermal treatment at 300 °C for 15 min. As shown in Figure S4b, this prolonged heat-treatment significantly increases the size of Ag2S QD to about 7 nm in diameter. However, we have found that this remarkable size growth leads to only a small increase in the absorbance over the whole spectral range (as compared with the Ag2S QDs deposited at 25 °C) and a slight reduction in Eg, as shown in Figure S5a and S5b, respectively. The lattice distortion in a QD calcined at 510 °C for 3 min significantly affects both the absorbance and the bandgap (Figure 7d and Figure S5). All the above analysis and experimental results clearly indicate that the enhanced absorbance of a photoanode heattreated for 3 min at 510 °C can be attributed to the lattice distortion that results from two distinct symmetries which coexist in a very small QD of about 4 nm. As discussed previously, the coexistence of two distinct symmetries in a given QD is induced by quenching the transformation from monoclinic Ag2S into cubic Ag. In this way, we are able to develop a novel way of bandgap tuning and enhancing optical properties by suitably inducing the lattice-distorted region of around 1 nm between the Ag2S and Ag phases in a QD. 2.3. Photovoltaic Performance of Ag2S Quantum-DotSensitized Solar Cells The bandgap-tuning method developed herein was applied to quantum-dot-sensitized solar cells (QDSCs) as an example of an application. The photovoltaic experiments were conducted to allow the performance of QDSCs to be evaluated. For this purpose, nanocrystalline TiO2 films were prepared according to the cell-fabrication procedures presented in the Experimental Section. We applied the idea of the nanometer-scale lattice distortion in a QD to QDSCs with optimized SILAR cycles (Figure S6 and Table S1). The photocurrent densityvoltage (J–V) characteristics of the devices are depicted in Figure 8a. In addition, the detailed photovoltaic parameters, i.e., open-circuit voltage (Voc), fill factor (FF), short-circuit photocurrent density (Jsc), and photovoltaic conversion efficiency (η) of the devices, are summarized in Table S2. There is a noticeable enhancement in Jsc when the lattice-distorted QDs are used, over the parent QDs, which can be attributed to the improved absorbance values of our lattice-distorted QDs. As shown in Figure 8b, the quantum efficiency (IPCE) is also higher in the QDSCs with distorted lattices over the whole spectral range, than that of the reference cell. The increased Jsc value contributes to ca. 50 % increase in the powerconversion efficiency (Table S2). However, reported efficiencies (0.148 %, 0.98 %) of the Ag2S QDSCs[19,20] were much
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2013, DOI: 10.1002/smll.201303040
Bandgap Tuning by Using a Lattice Distortion Induced by Two Symmetries That Coexist in a Quantum Dot
Figure 8. Photovoltaic responses of the Ag2S-QDSCs with Ag2S/TiO2 photoanodes heat-treated for various times at 510 °C. a) J–V characteristics and b) IPCE curves.
lower than those of QDSCs made by using other sensitizers such as CdS (1.9 %),[39] CdSe (2.5 %, 5.21 %),[13,40] CdS/CdSe (5.32 %, 4.2 %, 4.6 %),[8,41,42] etc. because proper electrolytes and counter electrodes were unexploited in this work. In view of this, noticeably enhanced optical properties of the present lattice-distorted Ag2S QD will play an important part for the development of highly efficient QDSCs when these problems are overcome.
3. Conclusion We have demonstrated a novel way of tuning the bandgap in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S into cubic Ag. On a few-nanometers scale, the coexistence of two distinct phases, monoclinic Ag2S and cubic Ag, with a lattice-distorted region between these crystalline phases significantly improved optical properties (e.g., absorbance values) in the visiblewavelength region. This lattice-distortion mechanism was demonstrated by analysis of HRTEM-EDS and EELS data. A wide range of the optical bandgap (2.51 to 1.64 eV) was obtained by suitably tailoring the lattice-distorted region that coexists with the monoclinic Ag2S and cubic Ag phases in a QD. We believe that the present methodology can be extended to various QD systems, which potentially paves the way to unexplored quantum effects in QD-based applications.
dipping into one solution, electrodes were rinsed with ethanol and methanol to remove the excess of each precursor. The optimal number of SILAR cycles was 2. The counter electrode was prepared by dripping a Pt solution (Solaronix, Platisol T) on the FTO glass substrate, which was followed by thermal treatment at 500 °C for 30 min. For the cell assembly, a hot-melt 60-μm-thick Surlyn (Solaronix, Meltonix 1170–60) was used as a spacer between the working electrode and the Pt electrode. The polysulfide electrolyte was composed of 2.5 M S, 1 M Na2S, and 0.1 M KCl in the solvent with the methanol:water ratio of 9:1 (by volume). Characterizations: Crystallography of the Ag2S QDs was studied by employing X-ray diffraction (Rigaku RINT 2000 with Cu Kα ray). For the Ag2S-sensitized TiO2 samples, high-resolution TEM images with EELS and EDS results were obtained by a TEM (JEOL JEM2200FS) located at NCNT (National Center for Nanomaterials and Technology in Pohang). UV-Vis reflectance spectra were recorded with a Perkin Elmer UV-Vis spectrometer (Lambda 750S). The photovoltaic performance was measured under the illumination of a solar simulator (Newport, Oriel class A, 92251A) at one sun (AM 1.5, 100 mW cm−2). The incident-photon-to-current conversion efficiency (IPCE) values were recorded as a function of the wavelength from 350 to 1400 nm (PV Measurements, Inc). A 75 W Xenon lamp was used as a light source with a monochromator. Calibration was performed with a NIST-calibrated photodiode G425 as a reference (at a chopping frequency of 10 Hz). The monochromatic power density was calibrated by using a reference Si photodiode as a standard from the NIST.
4. Experimental Section Synthesis and Fabrication: TiO2 pastes were screen-printed on FTO glass. A conventional box furnace was used for the thermal treatment of the films. The TiO2 films were gradually heated under air at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min.[43] The thickness of the TiO2 film was ca. 7 μm as estimated by using a surface profiler (Tencor, Alpha-Step 500). The SILAR technique,[29] a modified version of chemical bath deposition, was employed to assemble Ag2S QDs on the TiO2 anode. For the deposition of Ag2S QDs from the precursor solutions, two separate solutions were prepared: 0.1 M AgNO3 in ethanol and 0.1 M Na2S in methanol. The working electrodes were immersed into the Ag+ solution and the S2− solution successively for 3 min each. After small 2013, DOI: 10.1002/smll.201303040
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the Basic Science Research Program (Grant No. 2012R1A1A2041628) and the World Class University program (Grant R31–2008–000–10059–0) through the
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National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology.
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Received: September 19, 2013 Published online:
small 2013, DOI: 10.1002/smll.201303040
Bandgap Tuning by Using a Lattice Distortion Induced by Two Symmetries That Coexist in a Quantum Dot Eui-Hyun Kong, Yong-June Chang, Hyun-Jin Park, and Hyun M. Jang*
Among the interests in the application of quantum dots (QDs), the bandgap tuning is of key importance in controlling their material properties. The bandgap of a QD can be adjusted by adopting a variety of different physicochemical methods. Herein, a novel way of the bandgap tuning is developed in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S to cubic Ag and by subsequently inducing a lattice-distorted region of ≈1-nm-scale in a QD. The two distinct crystalline phases of Ag2S and Ag coexisting with the lattice-distorted region are experimentally demonstrated by visually showing this remarkable coexistence in a QD. A new approach is presented to the bandgap tuning (2.51 to 1.64 eV) and enhancing optical properties by suitably tailoring the degree of the lattice-distorted region in a QD. This conceptual method could pave a new way to utilizing quantum effects in various QD applications.
1. Introduction Semiconductor quantum dots (QDs) have become a matter of primary concern in recent research because of their enormous potential for applications in photonic and biological fields which necessitate QDs with excellent optoelectronic properties. The bandgap tuning of QDs has been developed by adopting a variety of different methods. These methods comprise use of i) alloyed QDs (CdZnS,[1–3] CdZnSe,[4,5] CdZnTe[6]), ii) core-shell QDs with different band structures[7,8] and with heteroepitaxial strain [a soft CdTe core and compressive shells (ZnS, ZnSe, ZnTe, CdS, or CdSe)],[9] iii) size-controlled QDs using reverse micelles (CdS,[10,11] CdSe[12]), solvothermal reaction (CdSe),[13] and ultrasonic
E.-H. Kong, Y.-J. Chang, Prof. H. M. Jang Department of Materials Science and Engineering, and Division of Advanced Materials Science Pohang University of Science and Technology (POSTECH) Pohang, 790–784, Republic of Korea E-mail: [email protected] H.-J. Park National Center for Nanomaterials Technology (NCNT) Pohang University of Science and Technology (POSTECH) Pohang, 790–784, Republic of Korea DOI: 10.1002/smll.201303040 small 2013, DOI: 10.1002/smll.201303040
irradiation (ZnO),[14] and iv) a porous three-dimensional fractal network of quantum dots for controlling the quantumconfinement effect (CdSe).[15] Herein, we develop a novel way of tuning the bandgap in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S to cubic Ag and by subsequently inducing an about 1-nmscale lattice-distorted region between these two distinct crystalline phases in a given QD. Small nanoparticles (105 dm3 mol−1 cm−1) can be estimated by using the Tauc model[32] using the absorption spectra, instead of by using PL emission spectra. The Tauc model is represented by "hv ∝ 6(hv − E g )n , where hv is the photon energy, Eg is the optical bandgap energy, and k is the photon-energydependent constant. The power-law exponent, n, depends on the transition type, where n = 1/2 for a direct bandgap transition and n = 2 for an indirect bandgap transition. The absorption coefficient (α) is determined by the transmittance. This Tauc model, which had been originally proposed for singlecomponent semiconductors,[32] has successfully been applied to multicomponent semiconducting systems.[13,33–35] Since Ag2S is a well-known direct bandgap semiconductor;[36] its optical bandgap can thus be determined by a plot of (αhv)2 vs. photon energy, as shown in Figure 7d. The optical bandgap of pure TiO2 is 3.21 eV, as estimated by the Tauc model (see the black arrow in Figure 7d). This value agrees well with the direct interband transition of anatase TiO2 at 3.2 eV.[37] The bandgap of the Ag2S QDs adsorbed on TiO2 decreases as the calcination time varies from 1 to 3 min. Thus, a wide range of the optical bandgap was obtained (2.51 to 1.64 eV; Figure 7d). In particular, the film annealed for 3 min shows a direct transition at a significantly reduced energy of 1.64 eV (red solid line in Figure 7d), which is closely related to the lattice distortion caused by the coexistence of Ag2S and Ag phases in a given QD (Figures 2d and 3c). The distorted lattice consists of various AgxSy stoichiometries with a variety of different energy levels such as alloyed QDs[1–6] (based on STEM-EDS data in Figure 3c). This hypothesis supports the observed interband transition at a significantly reduced energy of 1.64 eV (Figure 7d). Metallic Ag nanocrystallites on a semiconductor such as TiO2 can also reduce the optical bandgap and enhance the absorption coefficient of the semiconductor owing to the local-field enhancement by the surface plasmon resonance of silver nanoparticles.[38] According to our experimental results, however, there is little variation in the optical properties between the Ag2S QDs (deposited at 25 °C by the SILAR process) and the sequentially deposited Ag+Ag2S mixed QDs on TiO2 particles (Figure S2). Here, the Ag QDs in the mixed Ag+Ag2S QDs were prepared by the oxidation of the predeposited Ag2S QDs at 510 °C for more than 4 min, whereas Ag2S QDs in the mixed QDs were subsequently deposited by the SILAR process after the preparation of the Ag QDs on TiO2 particles. This small variation clearly indicates that the enhanced absorbance with the reduced bandgap in a QD with a lattice-distorted region of ca. 1-nm-scale is not directly related to the metallic Ag phase formed by high-temperature oxidation. In addition, transmittance and reflectance of anatase TiO2 films have no relation to the calcination temperature (Figure S3).
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We now examine the size effect of QD on the bandgap variation. For this, we present two different types of experimental observations. Firstly, as shown in Figures 2b and 2d, and Figure S4a, QDs heat-treated at 510 °C for 3 min are well dispersed without showing appreciable growth in QD size. Thus, the observed bandgap reduction of QDs calcined at 510 °C (Figure 7d) is not directly related to the size growth of QDs. Secondly, to clearly isolate the effect of QD size on the optical properties, we artificially grew Ag2S QDs by using thermal treatment at 300 °C for 15 min. As shown in Figure S4b, this prolonged heat-treatment significantly increases the size of Ag2S QD to about 7 nm in diameter. However, we have found that this remarkable size growth leads to only a small increase in the absorbance over the whole spectral range (as compared with the Ag2S QDs deposited at 25 °C) and a slight reduction in Eg, as shown in Figure S5a and S5b, respectively. The lattice distortion in a QD calcined at 510 °C for 3 min significantly affects both the absorbance and the bandgap (Figure 7d and Figure S5). All the above analysis and experimental results clearly indicate that the enhanced absorbance of a photoanode heattreated for 3 min at 510 °C can be attributed to the lattice distortion that results from two distinct symmetries which coexist in a very small QD of about 4 nm. As discussed previously, the coexistence of two distinct symmetries in a given QD is induced by quenching the transformation from monoclinic Ag2S into cubic Ag. In this way, we are able to develop a novel way of bandgap tuning and enhancing optical properties by suitably inducing the lattice-distorted region of around 1 nm between the Ag2S and Ag phases in a QD. 2.3. Photovoltaic Performance of Ag2S Quantum-DotSensitized Solar Cells The bandgap-tuning method developed herein was applied to quantum-dot-sensitized solar cells (QDSCs) as an example of an application. The photovoltaic experiments were conducted to allow the performance of QDSCs to be evaluated. For this purpose, nanocrystalline TiO2 films were prepared according to the cell-fabrication procedures presented in the Experimental Section. We applied the idea of the nanometer-scale lattice distortion in a QD to QDSCs with optimized SILAR cycles (Figure S6 and Table S1). The photocurrent densityvoltage (J–V) characteristics of the devices are depicted in Figure 8a. In addition, the detailed photovoltaic parameters, i.e., open-circuit voltage (Voc), fill factor (FF), short-circuit photocurrent density (Jsc), and photovoltaic conversion efficiency (η) of the devices, are summarized in Table S2. There is a noticeable enhancement in Jsc when the lattice-distorted QDs are used, over the parent QDs, which can be attributed to the improved absorbance values of our lattice-distorted QDs. As shown in Figure 8b, the quantum efficiency (IPCE) is also higher in the QDSCs with distorted lattices over the whole spectral range, than that of the reference cell. The increased Jsc value contributes to ca. 50 % increase in the powerconversion efficiency (Table S2). However, reported efficiencies (0.148 %, 0.98 %) of the Ag2S QDSCs[19,20] were much
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2013, DOI: 10.1002/smll.201303040
Bandgap Tuning by Using a Lattice Distortion Induced by Two Symmetries That Coexist in a Quantum Dot
Figure 8. Photovoltaic responses of the Ag2S-QDSCs with Ag2S/TiO2 photoanodes heat-treated for various times at 510 °C. a) J–V characteristics and b) IPCE curves.
lower than those of QDSCs made by using other sensitizers such as CdS (1.9 %),[39] CdSe (2.5 %, 5.21 %),[13,40] CdS/CdSe (5.32 %, 4.2 %, 4.6 %),[8,41,42] etc. because proper electrolytes and counter electrodes were unexploited in this work. In view of this, noticeably enhanced optical properties of the present lattice-distorted Ag2S QD will play an important part for the development of highly efficient QDSCs when these problems are overcome.
3. Conclusion We have demonstrated a novel way of tuning the bandgap in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S into cubic Ag. On a few-nanometers scale, the coexistence of two distinct phases, monoclinic Ag2S and cubic Ag, with a lattice-distorted region between these crystalline phases significantly improved optical properties (e.g., absorbance values) in the visiblewavelength region. This lattice-distortion mechanism was demonstrated by analysis of HRTEM-EDS and EELS data. A wide range of the optical bandgap (2.51 to 1.64 eV) was obtained by suitably tailoring the lattice-distorted region that coexists with the monoclinic Ag2S and cubic Ag phases in a QD. We believe that the present methodology can be extended to various QD systems, which potentially paves the way to unexplored quantum effects in QD-based applications.
dipping into one solution, electrodes were rinsed with ethanol and methanol to remove the excess of each precursor. The optimal number of SILAR cycles was 2. The counter electrode was prepared by dripping a Pt solution (Solaronix, Platisol T) on the FTO glass substrate, which was followed by thermal treatment at 500 °C for 30 min. For the cell assembly, a hot-melt 60-μm-thick Surlyn (Solaronix, Meltonix 1170–60) was used as a spacer between the working electrode and the Pt electrode. The polysulfide electrolyte was composed of 2.5 M S, 1 M Na2S, and 0.1 M KCl in the solvent with the methanol:water ratio of 9:1 (by volume). Characterizations: Crystallography of the Ag2S QDs was studied by employing X-ray diffraction (Rigaku RINT 2000 with Cu Kα ray). For the Ag2S-sensitized TiO2 samples, high-resolution TEM images with EELS and EDS results were obtained by a TEM (JEOL JEM2200FS) located at NCNT (National Center for Nanomaterials and Technology in Pohang). UV-Vis reflectance spectra were recorded with a Perkin Elmer UV-Vis spectrometer (Lambda 750S). The photovoltaic performance was measured under the illumination of a solar simulator (Newport, Oriel class A, 92251A) at one sun (AM 1.5, 100 mW cm−2). The incident-photon-to-current conversion efficiency (IPCE) values were recorded as a function of the wavelength from 350 to 1400 nm (PV Measurements, Inc). A 75 W Xenon lamp was used as a light source with a monochromator. Calibration was performed with a NIST-calibrated photodiode G425 as a reference (at a chopping frequency of 10 Hz). The monochromatic power density was calibrated by using a reference Si photodiode as a standard from the NIST.
4. Experimental Section Synthesis and Fabrication: TiO2 pastes were screen-printed on FTO glass. A conventional box furnace was used for the thermal treatment of the films. The TiO2 films were gradually heated under air at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min.[43] The thickness of the TiO2 film was ca. 7 μm as estimated by using a surface profiler (Tencor, Alpha-Step 500). The SILAR technique,[29] a modified version of chemical bath deposition, was employed to assemble Ag2S QDs on the TiO2 anode. For the deposition of Ag2S QDs from the precursor solutions, two separate solutions were prepared: 0.1 M AgNO3 in ethanol and 0.1 M Na2S in methanol. The working electrodes were immersed into the Ag+ solution and the S2− solution successively for 3 min each. After small 2013, DOI: 10.1002/smll.201303040
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the Basic Science Research Program (Grant No. 2012R1A1A2041628) and the World Class University program (Grant R31–2008–000–10059–0) through the
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National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology.
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Received: September 19, 2013 Published online:
small 2013, DOI: 10.1002/smll.201303040
