Crystal Growth, Exponential Optical Absorption Edge, and Ground

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Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO

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Taro Toyoda, Qing Shen, Knane Hori, Naoki Nakazawa, Keita Kamiyama, and Shuzi Hayase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12675 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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The Journal of Physical Chemistry

Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO2 Taro Toyoda,*,†,‡ Qing Shen,*,†,‡Kanae Hori, † Naoki Nakazawa, † Keita Kamiyama, § and Shuzi Hayase //, ‡ †

Department of Engineering Science, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

§

Bunkoukeiki Co., Ltd, 4-8 Takakura, Hachioji, Tokyo 192-0033, Japan

//

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan

‡

Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

ACS Paragon Plus Environment

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ABSTRACT: It is important to investigate the dependencies of the optical absorption and the ground state energy level on the size of the semiconductor quantum dots (QDs) on fully studied single crystal TiO2 surfaces. The present study focuses on the systems comprising PbS QDs on (001), (110), and (111) surfaces of single crystal rutile-TiO2. By the optical absorption characterization, the average diameter of PbS QDs on a (001) surface is independent of the number of adsorption cycles, although those on (110) and (111) surfaces increase with the number of cycles. The rate of adsorption of PbS QDs on a (001) surface is higher than those grown on (110) and (111) surfaces. The results suggest that the crystal growth is caused by the difference of the surface energy of the substrate. The exponential optical absorption edge suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases as the number of adsorption cycles increases. On the other hand, that on a (111) surface decreases as the number of adsorption cycles increases. The ground state energy level of the PbS QDs is independent of the surface orientation of the single crystal rutile-TiO2, but shows negative polarization with the increase of adsorption cycles. It is owing to the possibility of the increase of color centers (electron capture by S- vacancies) in PbS QDs, corresponding to the increase of structural disorder.


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INTRODUCTION Controlling the dimensions of semiconductor materials at the nanoscale level (quantum dots, QDs) is a promising development methodology for many applications, including solar cells (quantum dot-sensitized solar cells, QDSCs).1-11 The most appealing quality of QDs from both industrial and academic perspectives is their size-dependent electronic properties. QDs have been attracting a great deal of attention owing to their tunable energy gap between the ground and excited states and to their high extinction coefficient. Moreover, the electron injection process benefits from the large built-in dipole moments produced in the QDs that facilitate the separation process of the electron-hole pairs.12,13 Although QDs have such merits, there have been few reports on QDSCs with photovoltaic conversion efficiencies equaling or exceeding those of dye-sensitized solar cells. In general, nanoparticle TiO2 electrodes play a key role in QDSC applications, due to the large surface area onto which a great number of QDs can be adsorbed for light harvesting.14 For QDs adsorbed on TiO2 and other metal oxide nanoparticle electrodes, heterogeneity can be produced by distributions in the defects, the abundance of the grain boundaries, the unknown adsorption sites, and amount of exposed surface of the oxide. Also, heterogeneity can be produced by distributions in the parameters of the QDs, such as their size, shape, and charge, as well as their interactions.15 These complexities cloud a detailed understanding of the essential properties that contribute to the electronic structure of QDs adsorbed on nanoparticle TiO2 electrodes (polycrystalline TiO2 assemblies). Also, there are reports that the morphology of the oxide electrode has an impact on electron transfer owing to changes in the dielectric constant and the different densities of band-edge states.8,16 Therefore, an important first step is to investigate the dependencies of the optical absorption and the ground state energy level on the size of the QDs on characterized single crystal metal oxide surfaces,


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where the electronic structure has been well studied.15,17 Nevertheless, a detailed study of the influence of electrodes with different crystal orientations has seldom been studied, except in a few reports.18-23 The present study focuses on the QD size-dependencies of the optical absorption and the ground state energy level in systems comprising PbS QDs on single crystal rutile-TiO2 (R-TiO2). We chose single crystal R-TiO2, which is not only ideal for studying the crystal growth of the QDs but is also useful for studying the interactions between the QDs and TiO2. The rutile phase is the most stable and has been the subject of most fundamental studies because of the ready access to large bulk single crystals and relatively easy surface preparation procedures.17 Knowledge of the QD size-dependencies of the optical absorption and ground state energy level is important to clarify the nature of the interaction between the QDs and R-TiO2. We chose PbS QDs as the sensitizer as these have been extensively studied for many years. PbS QDs are known for their high absorbance in the visible and near infrared regions. PbS has a relatively large Bohr radius (~ 18 nm), and the QDs have a large ground state absorption cross section, and long excitonic lifetime (~ 200 – 800 ns). The bandgap can be tuned between 0.3 – 2.0 eV by controlling the size of the QDs. For optical absorption measurements, we applied photoacoustic (PA) spectroscopy, which is based on photothermal phenomena, to characterize the optical absorption, especially in the sub bandgap region.11,24,25 The PA technique detects the acoustic energy produced by heat generated through non-radiative processes in materials.11 The PA signal is less sensitive to light-scattering effects than the signals in conventional transmission spectroscopy and it shows high sensitivity. In order to study the ground state energy level, we applied photoelectron yield (PY) spectroscopy.26-28 PY spectroscopy is usually used to determine the ionization energy of bulk


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semiconductors and metals. However, it has not been applied to examine the ionization energy of QDs as a function of size. In contrast to electrochemical measurements, ionization energy measurements based on PY spectroscopy are expected to give direct information on the ground state energy levels of QDs and a reliable estimation of their excited state energy levels.28 While UV-VIS optical absorption spectra specify the energies of bands relative to each other and give information only at the band edge, they do not determine the absolute energy levels. An understanding of the absolute energy levels is necessary to get a complete picture of the electronic structure, including the quantum confinement effect in the system. PY spectroscopy is advantageous in determining the absolute ground state energy level of PbS QDs on R-TiO2, in contrast to UV-VIS spectra.26,27 EXPERIMENTAL SECTION Materials and Chemicals. The characteristics of single crystal R-TiO2 have already been reported.20 Single-crystal R-TiO2 wafers, 5 mm x 7 mm in area and 0.5 mm thick, with (001)-, (110)-, and (111)-cuts were obtained from Furuuchi Chemical Co., Ltd., Japan. The surface roughness of the (001), (110), and (111) surfaces were 0.322 nm, 0.356 nm, and 0.394 nm, respectively. The flat surfaces were treated by washing them in acetone for 30 min, immersing them in distilled water for 30 min, and exposing them to ozone for 10 min. PbS QDs were adsorbed by a successive ionic layer adsorption and reaction (SILAR) method.29,30 The wafers were immersed in a (CH3COO)2Pb solution (0.02 M in methanol) for 1 min and then rinsed with solvent. Next they were immersed in Na2S solution (0.02 M in pure water and methanol, mixed solvent) for 1 min and then rinsed with solvent again. This immersion cycle was repeated 5, 7, and 10 times to change the QD size. After adsorption of the PbS QDs, all the wafers were coated with zinc sulfide (ZnS) using a SILAR method. The wafers were immersed in (CH3COO)2Zn


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solution (0.1 M in methanol) for 1 min and rinsed with solvent, then they were immersed in Na2S solution (0.1 M in pure water and methanol mixed solvent) for 1 min and rinsed with solvent again. This immersion cycle was repeated 6 times.29,30 PA Spectroscopy Characterization. First, we applied a UV-vis spectrometer (JASCO V-670 Spectrophotometer) to characterize the absorbance (ABS) of the PbS QDs on R-TiO2 at a photon energy of 2.8 eV which is higher than the energy gap between the ground state and excited state of PbS QDs. Second, their optical absorption was investigated using a single beam PA spectrometer.11 The PA cell consists of an aluminum cylinder with a small channel at the periphery into which a microphone is inserted. The sealed PA cell is used to detect pressure changes due to temperature changes in the sample by the absorption of light. Monochromatic light from a 300 W xenon short arc lamp modulated at 33 Hz was focused onto the sample surface in the PA cell. The PA signal was detected by first passing the output from the sensitive microphone through a preamplifier and then a lock-in amplifier. The spectra were taken at room temperature in the wavelength range of 400 – 800 nm. The PA signal intensity below the fundamental absorption edge is proportional to the optical absorption coefficient owing to the relationship between the optical absorption length and the thermal diffusion length.31 The spectra were calibrated using the PA signals from a carbon black sheet.11,31 PY Spectroscopy Characterization. PY measurements were performed by means of an ionization energy measurement system (BIP-KV201, Bunkoukeiki, Co., Ltd., Japan).11 The ionization energy (I) is defined as the minimum energy for removing an electron from the system. PY spectroscopy is a method to determine the value of I, where monochromatic light of tunable photon energy is irradiated onto the sample. A 30 W deuterium lamp was used for the light source and the PY spectra were collected.11,21 The number of photoelectrons was obtained using


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an ammeter to measure the current needed to compensate for the photoexcited holes generated in the sample. A negative voltage of -50 V was applied to the base plate to prevent carrier recombination of photoelectrons emitted from the sample surface with photogenerated holes in the sample. In the PY measurements, the photoemission yield (Y) was measured as a function of photon energy (hν), and the value of I was determined from the onset of the PY spectrum. The PY spectrum around the photoelectric threshold I can be expressed by the following equation Y = K (hν – I )n

(1)

where K is a constant and n is a parameter that mainly depends on the shape of the density of electronic states at the upper edge of the valence band and the probability of the transmission of electrons across the surface.26 In this study, we employed a cubic function (n = 3) based on a theoretical analysis.26,32 The value of I was determined by extrapolating the linear part of Y1/3 to the baseline.11 An energy scan of the incident photons was performed with UV light in the wavelength range of 130 – 310 nm (4 ~ 9.5 eV). The UV light was focused on the sample over an area of 1 × 3 mm2. All the measurements were performed in a vacuum chamber (~ 4 × 10-3 Pa) at room temperature. RESULTS AND DISCUSSION ABS and PA characterizations of PbS QDs on Single Crystal R-TiO2. Figure 1(a), 1(b), and 1(c) show the SILAR cycle dependence of the ABS of PbS QDs adsorbed on (001), (110), and (111) surfaces of single crystal R-TiO2, respectively, at a photon energy of 2.8 eV. ABS is product of the optical absorption coefficient (α) and the thickness (L) of the material (ABS = αLlog10 e). In reference 33, the optical absorption coefficient higher than the energy gap is size-independent, although size-dependent optical absorption coefficient is dominated only at the


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energy gap region.33 So, we assume that the ABS at a photon energy of 2.8 eV is proportional to the adsorption of PbS QDs. The adsorption is nearly linearly proportional to the number of SILAR cycles. Figure 2 shows the ABS of PbS QDs at 5 cycles on (001), (110), and (111) surfaces of single crystal R-TiO2. The ABS of PbS QDs on (001) R-TiO2 is higher than those on (110) and (111) surfaces, suggesting the possibility that the seeds of PbS QDs on (001) grow rapidly in the first step. The ABS rate with SILAR cycle is different for each crystal orientation. Figure 3 shows the ABS rate of PbS QDs on different crystal orientations of single crystal R-TiO2. The ABS rate of in QDs on the (001) surface (0.056/cycle) is higher than those grown on the (110) surface (0.051/cycle) and the (111) surface (0.049/cycle) within the limits of experimental accuracy ((001) ˃ (110) ˃ (111)). Hence, the adsorption of PbS QDs on R-TiO2 is related to the orientation of the R-TiO2 which corresponds to different surface energy. Characterization of the crystal orientation of the PbS QDs by conventional XRD measurements was impossible owing to their small number. High resolution XRD observations are important since the ground state energy level of the QDs depends on the crystal orientation.34 Figure 4(a), 4(b), and 4(c) show the PA spectra for PbS QDs adsorbed on (001), (110), and (111) surfaces of single crystal R-TiO2 (number of SILAR cycles = 7), respectively. Similar PA spectra can be obtained for PbS QDs with SILAR cycles of 5 and 10 on single crystal R-TiO2 (see Figure S1, Supporting Information). The energy gap between the ground state and excited states (first excitation energy) was evaluated from the knee point (↓, confinement energy value is E1). Also, PA spectra allowed us to identify two excitation states (↑, confinement energy values are E2 and E3, respectively). Figure 5(a), 5(b), and 5(c) show the SILAR cycle dependence of the excitation energies (E1, E2, and E3) of PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2, respectively. This shows that E1, E2, and E3 on a (001) surface is independent of


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the number of SILAR cycles, although those on (110) and (111) surfaces decreases. Here, normalized confinement energy is defined as (En – Eg)/(E1 – Eg)35,36 (n = 2, 3; Eg = 0.41 eV, band gap of bulk PbS). Those values are independent of SILAR cycle and surface orientation of single crystal R-TiO2 ((E2 – Eg)/(E1 – Eg) ~1.2 and (E3 – Eg)/(E1 – Eg) ~1.4). In the case of PbS colloidal nanocrystal, the former and the latter values are ~1.4 and ~1.7, respectively, higher than those on single crystal R-TiO2.35,36 The smaller value of normalized confinement energy of PbS QDs on single crystal R-TiO2 than PbS colloidal nanocrystals reflects the difference of the interaction between PbS QDs and the background including electron transfer. In general, the value of E1 in the logarithmic PA spectra agree with the reported values of the bandgaps.37 The average diameter, R, can be estimated from the relationship between E1 and R using the following equation:33

E1 = 0.41 +

ଵ

଴.଴ଶହଶோమ ା଴.ଶ଼ଷோ

(2)

This equation allows us to determine R directly from E1, avoiding lengthy TEM analysis of each sample.33 Also, equation (2) agrees well with the calculated data for less than 3 nm diameter.38 Reference 33 showed a relative size dispersion σd = 10% for the PbS QD particle suspension by a typical transmission electron microscopy (TEM).33 There is a possibility of size distribution in our SILAR synthesized PbS QDs. However, our PA measurements suggest that there is not so much size distribution because the position E1 (knee point in PA spectra) is independent of the modulation frequency of irradiated light in PA measurement. If there is much amount of size distribution, the value of E1 depends on the modulation frequency. Figure 6(a), 6(b), and 6(c) show the SILAR cycle dependence of R of the PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2, respectively. This shows that R on a (001) surface is independent of the


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number of SILAR cycles (~ 1.8 nm), although those on (110) and (111) surfaces increase from 1.7 to 2.1 nm. Normal growth plus the suppression (negative growth or dissolution) of QDs can be observed. Both of them depend on the morphology of TiO2, indicating the importance of the surface energy.39,40 One of the possibility of the constant R of PbS QDs is that the surface energy of (001) surface is different from those of (110) and (111) surfaces. Different surface energy of (001) surface contributed to the different contribution of the normal growth and the suppression compared to other surface orientations. Therefore, the saturation of R of PbS QDs on (001) surface at equilibrium (critical size) might be faster than other surface orientations due to higher suppression ratio.39,40 The increasing rate of R on (111) is highest ((111) ˃ (110) ˃ (001)), suggesting a difference in the Pb-O coupling at the interface between the PbS QDs and the R-TiO2 surface. Figure 7 shows a schematic illustration of the crystal growth of PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2 after 5 and 10 SILAR cycles. In semiconductors, the optical absorption below the fundamental absorption region increases exponentially (Urbach tail).41 The origin of the Urbach tail has been investigated extensively. Examination of the Urbach tail affords fundamental information about the band structure, disorder, defects, impurities, and electron-phonon interactions.31,42-48 The PA signal (P) in the sub bandgap region is given by the following equation:47

P = P0 exp

ቂ

ఙ(௛ఔି௛ఔబ ) ௞ಳ ்

ቃ

(3)

where h is Planck’s constant (hν, incident photon energy), kB is the Boltzmann constant, T is absolute temperature, and P0 and ν0 are fitting parameters. σ is a characteristic of the logarithmic slope (Urbach tail) and is called the steepness parameter. Figure 8(a), 8(b), and 8(c) show the


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dependence of the value of σ on the number of SILAR cycles for PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2, respectively. We assume firstly that the value of σ is a reflection of the structural disorder in the PbS QDs. Smaller σ corresponds to a broader exponential tail and hence to larger structural disorder.22 The values of σ for PbS QDs on (001) and (110) decrease with increasing number of SILAR cycles, indicating an increase in structural disorder. Although the value of σ for PbS QDs on the (111) surface is lower than those on (001) and (110) surfaces, that for PbS on the (111) surface increases with increasing number of SILAR cycles, indicating a relatively smaller decrease in structural disorder. Another possibility for the SILAR cycle dependence of σ is the effect of electron-phonon interactions.47 Our results suggest that the electron-phonon interaction for PbS QDs is larger for a greater number of SILAR cycles (corresponds to larger R) for the (001) and (110) surfaces. On the other hand, the electron-phonon interaction for PbS QDs on the (111) surface decreases with increasing number of SILAR cycles. In the future, the temperature dependence of σ is needed in order to clarify the nature of the structural disorder and/or electron-phonon interactions of PbS QDs on different substrate orientations.47 PY Characterization of PbS QDs on Single Crystal R-TiO2. We utilized the PY spectroscopy method to determine the size-dependent ground state energy levels of PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2. Figure 9(a), 9(b), and 9(c) show the PY spectra for PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2 (after 7 SILAR cycles), respectively. Similar PY spectra can be obtained for PbS QDs with SILAR cycles of 5 and 10 on single crystal R-TiO2 (see Figure S2, Supporting Information). The error bars in Figure 9 are no bigger than the size of the symbols. Figure 10(a), 10(b), and 10(c) show the alignment of the energy levels of PbS QDs on (001), (110), and (111) surfaces of single crystal


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R-TiO2, respectively, together with the valence band maximum (VBM) positions of (001), (110), and (111) R-TiO2.20 The positions of the ground state energy levels of PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2 are the same for each SILAR cycle within the limits of experimental accuracy (5 cycles, ~ −5.0 eV; −7 cycles, ~ −6.2 eV; 10 cycles, ~ −6.4 eV). Figure 11(a), 11(b), and 11(c) show the SILAR cycle-dependent ground state and excited state energy levels of PbS QDs on (001), (110), and (111) surfaces of single crystal R-TiO2, respectively. Here, the excited state energy levels of PbS QDs are evaluated from the PA characterization of E1. This shows that the ground state and excited energy levels of PbS on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2 shift downward with increasing number of SILAR cycles and agree with each other. It indicates that they are independent of the surface orientation of the substrate. The ground state energy levels for 5 cycles agree with each other, and is around −5.0 eV which is higher than the values after 7 and 10 cycles. The value of the ground state energy levels for 5 cycles are in good agreement with the reported value (−5.32 eV, R ~ 2.7 nm).49 These results are different from those obtained for CdSe QDs on R-TiO2.23 In the case of CdSe QDs on R-TiO2, the ground state energy level on the (001) surface shifts upward slightly, while that on the (110) surface shifts downward slightly with increasing QD size. That on the (111) surface is independent of the QD size, indicating that the surface orientation has an effect on the adsorption of the QDs.23 A downward shift in the ground state energy levels of PbS QDs corresponds to more negative polarization with increasing number of SILAR cycles (increase of R). One possibility is as follows. Since PbS is semi-covalent material (iconicity: ~80%),50 we assume that more amount of color centers than in CdSe QDs are produced with the increase of SILAR cycle. Color center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electrons. Thus, the


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negative polarization is electron capture by S- defects or disordered states. This corresponds to the decrease of σ of PbS QDs for (001) and (110) surface orientations (Figure 5 (a) and (b)) with the increasing number of SILAR cycles. On the other hand, σ for (111) increases with the increasing number of SILAR cycles. Although this signifies a decrease in defects and disordered states for PbS QDs on the (111) surface, there is a possibility that the electron capture cross section might be higher than that on (001) and (110) surfaces due to the different Coulomb interaction of the R-TiO2 (111) surface. CONCLUSIONS

We have shown the dependencies of the optical absorption and the ground state energy level on the number of SILAR cycles used to adsorb PbS QDs on the (001), (110), and (111) surfaces of single crystal R-TiO2. By the optical absorption characterization, the average diameter of PbS QDs on the (001) surface is independent of the number of adsorption cycles (~ 1.8 nm), while those on (110) and (111) surfaces increase from 1.7 to 2.1 nm. The rate of adsorption in PbS QDs on the (110) surface is higher than those grown on (001) and (111) surfaces ((110) ˃ (111) ˃ (001)). Hence, the average diameter and the rate of adsorption of PbS QDs on R-TiO2 (crystal growth) are related to the orientation of the R-TiO2 surface. The results suggest that the crystal growth may be caused by the difference of the surface energy of the substrate. The exponential optical absorption edge (Urbach tail) suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases with increasing number of adsorption cycles. On the other hand, that on the (111) surface decreases with the increasing number of adsorption cycle although the disorder of PbS on (111) surface is originally higher than those on (001) and (110) surfaces. The ground state energy level for the PbS QDs is independent of the surface orientation, but shifts


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downward with increasing number of adsorption cycles (negative polarization). This is different from the nature of CdSe QDs on R-TiO2 owing to the possibility of the increase of color centers in PbS QDs (electron capture by S- vacancies).

ASSOCIATED CONTENT Supporting Information Photoacoustic spectra for PbS quantum dots adsorbed on the (001), (110), and (111) surfaces of single crystal R-TiO2 (SILAR cycles, 5 and 10). Photoelectron yield spectra for PbS quantum dots adsorbed on the (001), (110), and (111) surfaces of single crystal R-TiO2 (SILAR cycles, 5 and 10). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. T.); Tel.: +81-42-443-5464 *E-mail: [email protected] (Q. S.); Tel. +81-42-443-5471 ORCID Taro Toyoda: 0000-0002-2067-3689 Qing Shen: 0000-0001-8359-3275 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Part of this work was supported by Core Research for Evolutional Science and Technology (CREST), Japan Science Technology Agency (JST). The work was also supported by JSPS


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Kakenhi Grant Numbers 26390016 and 17K04988. We thank T. Amano and Y. Takeshita (Bunkoukeiki Co., Ltd.) for cooperation with the PY spectroscopy measurements. REFERENCES (1) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Rühle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol. A: Chemistry 2006, 181, 306-313. (2) Diguna, L. J.; Shen, Q.; Sato, A.; Katayama, K.; Sawada, T.; Toyoda, T. Optical Absorption and Ultrafast Carrier Dynamics Characterization of CdSe Quantum Dots Deposited on Different Morphologies of Nanostructured TiO2 Films. Mater. Sci. Eng. C 2007, 27, 1514-1520. (3) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High Efficiency of CdSe Quantum-Dot-Sensitized TiO2 Inverse Opal Solar Cells. Appl. Phys. Lett. 2007, 91, 023116. (4) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007-4015. (5) Mora-Seró, I.; Giménez, S.; Fabregat-Santiago, F.; Gómez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848-1857. (6) Mora-Seró, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046-3052. (7) Emin, S.; Singh, S. P.; Han, L.; Satoh, N.; Islam, A. Colloidal Quantum Dot Solar Cells. Sol. Energy 2011, 85, 1264-1282.


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Figure 1. SILAR cycle dependence of the absorbance of PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2 at a photon energy of 2.0 eV.


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Figure 2. Absorbance of PbS quantum dots at 5 cycles on (001), (110), and (111) surfaces of single crystal R-TiO2.

Figure 3. Absorbance rate of PbS quantum dots on single crystal R-TiO2 with different crystal orientations.


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Figure 4. Photoacoustic spectra for PbS quantum dots adsorbed on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2 (number of SILAR cycles=7).


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Figure 5. SILAR cycle dependence of the excitation energy (E1, E2, and E3) of PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2.


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Figure 6. SILAR cycle dependence of the average diameter of PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2.


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Figure 7. Schematic illustration of the crystal growth of PbS quantum dots on (001), (110), and (111) surfaces of single crystal R-TiO2 after 5 and 10 SILAR cycles.


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Figure 8. Dependence of steepness parameter on SILAR cycles for PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2.


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Figure 9. Photoelectron yield spectra for PbS quantum dots adsorbed on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2 (number of SILAR cycles=7).


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Figure 10. Alignment of energy levels of PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2 together with the positions of the valence band maxima of R-TiO2.


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Figure 11. SILAR cycle dependence of the ground and excited state energy levels of PbS quantum dots on (a) (001), (b) (110), and (c) (111) surfaces of single crystal R-TiO2.


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PbS QDs / R-TiO2

1 cycle

TOC Graphic


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Crystal Growth, Exponential Optical Absorption Edge, and Ground

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