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Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent

The reported photocurrent density (JSC) of PbS quantum dot (QD)-sensitized solar cell was less than 19 mA/cm2 despite the capability to generate 38 mA/cm2, which results from inefficient electron injection and fast charge recombination. Here, we report on a PbS:Hg QD-sensitized solar cell with an unprecedentedly high JSC of 30 mA/cm2. By Hg2+ doping into PbS, JSC is almost doubled with improved stability. Femtosecond transient study confirms that the improved JSC is due to enhanced electron injection and suppressed charge recombination. EXAFS reveals that Pb-S bond is reinforced and structural disorder is reduced by interstitially incorporated Hg2+, which is responsible for the enhanced electron injection, suppressed recombination and stability. Thanks to the extremely high JSC, power conversion efficiency of 5.6% is demonstrated at one sun illumination.

Chemical bonding nature surrounding Pb and Hg is investigated by extended X-ray absorption fine structure (EXAFS). Fig. 1a shows deposition process of PbS:Hg QD on nanostructured TiO 2 surface using successive ionic layer adsorption and reaction (SILAR) method. Pb 21 and Hg 21 ions are adsorbed on TiO 2 surface during the first dipping, which is followed by chemical reaction with S 22 ion in second bath to form PbS:Hg QD on TiO 2 surface. Concentration of HgCl 2 is varied from 0 mmol to 8 mmol to evaluate the effect of Hg 21  concentration. It is clearly identified that semi-sphered QDs are sparsely formed on the surface of TiO 2 nanoparticles regardless of HgCl 2 concentration. Pb L3 and Hg L3 edges X-ray absorption near edge spectroscopy (XANES) spectra of bulk PbS and HgS with PbS:Hg QDs deposited on TiO 2 nanoparticles are compared in Supplementary Fig. S1. To identify undesired impurity species, PbO, PbO 2 , Pb(NO 3 ) 2 and HgCl 2 are also measured as references. All electrodes showed almost identical shape with bulk PbS and HgS, which means that lead and mercury ions in samples exist in the form of Pb 21 and Hg 21 bonded to S 22 16,17 . Since it was reported that the peak area in X-ray photoelectron spectroscopic (XPS) data, representing the number of electrons, is assumed to be proportional to the number of atoms in a given state 18 , the doping concentration is estimated from the area of Pb 4f and Hg 4f peaks ( Supplementary  Fig. S2, Table S1). As expected, the ratio of Hg 21 to (Pb 21 1 Hg 21 ) increases as the HgCl 2 concentration increases. Doping concentration of Hg 21 in PbS:Hg is estimated to be 0, 5.4, 6.3, 14.6 and 16.7 at% for 0, 2, 4, 6 and 8 mmol of HgCl 2 , respectively. Relatively high amount of Hg content of 16.7 at% for 8 mmol compared to the nominal composition (7.4 at%) is due to the stronger oxidizing characteristic of Hg 21 than Pb 21 19 .

Results
Optical band gap of the QDs is estimated from reflectance spectra using Kubelka-Munk equation 20 and Tauc plot 21 (Fig. 1c). The optical band gap of PbS:Hg QD increases from 0.94 eV to 1.11 eV as the doping concentration increases from 0 at% to 16.7 at%. UVvis absorption spectra of PbS:Hg QDs look red-shifted compared to the pristine PbS, especially in visible region as can be seen in Supplementary Fig. S3a. However, the threshold for the PbS doped with Hg (equal or greater than 4 mmol) in the NIR region in inset of Fig. S3a is moved to shorter wavelength than the pristine PbS, which is consistent with the increase in band gap. From the detailed investigation on HgCl 2 concentration change from 0 mmol to 2 mmol and from 4 mmol to 6 mmol, optical band gap is almost unchanged in spite of the increased Hg 21 in PbS:Hg, which is related to quantum size effect (ca. 2.4 nm for 0 mmol, ca. 4.0 nm for 2 mmol and 4 mmol, and ca. 5.7 nm for 6 mmol and 8 mmol) as can be seen in Supplementary Fig. S4. As mentioned before, increase in Hg doping concentration is expected to enlarge the optical band gap, which, however, will be compensated with increase in QD size. For this reason, little change in band gap is observed between 0 mmol and 2 mmol or 4 mmol and 6 mmol. Valence band maximum (VBM) is determined by ultraviolet photoelectron spectroscopy (UPS) 22 . Secondary cut-off is fitted to energy of He I light source (21.2 eV), where extrapolation of low energy region corresponds to potential energy of VBM from the vacuum level ( Fig. 1d) 22 . It is noted that VBM is hardly changed by Hg concentration, which indicates that VBM is mainly dominated by S ion. The position of conduction band minimum (CBM) is calculated based on VBM and optical band gap energy. Band edge alignment is demonstrated in Fig. 1e, where CBMs of the QDs move upward as the concentration of HgCl 2 increases, leading to gradual increase in optical band gap. PbS doped with Hg is measured to be p-type semiconductor (Supplementary Table S2), where thin film of PbS:Hg deposited on glass substrate is used for the Hall measurement instead of using QD-adsorbed TiO 2 film because deposited QD on TiO 2 is discontinuous.
Photovoltaic performances of PbS:Hg QD-sensitized solar cells are evaluated from current density-voltage (J-V) and external quantum efficiency (EQE) measurements (Fig. 2). Schematic structure of the QD-sensitized solar cell is depicted in Fig. 2a. Dependence of photovoltaic performance on the SILAR coating cycle number is first investigated at the given TiO 2 thickness of 6.5 mm ( Supplementary  Fig. S5). Optimum coating cycle for best efficiency is found to be 2 cycles for PbS (0 mmol of HgCl 2 ), 4 cycles for both 2 mmol and 4 mmol HgCl 2 and 6 cycles for both 6 mmol and 8 mmol HgCl 2 ( Supplementary Fig. S5 and S6 and Table S3). In case of 0 mmol HgCl 2 condition (PbS), efficiency reaches maximum at 2 coating cycles ( Supplementary Fig. S6a) although absorption is further increased after 2 cycles ( Supplementary Fig. S7a). This might be related to decreased band gap due to large sized PbS QD formed after several coating cycles 14 (ca. 5.7 nm for 6 cycle in Supplementary Fig.  S4). No further increase in short-circuit current density (J SC ) is observed after 2 coating cycle ( Supplementary Fig. S6b). On the other hand, PbS:Hg (2,8 mmol HgCl 2 ) shows significant increase in J SC as the coating cycle is increased, which is due to the increased absorption ( Supplementary Fig. S6b and Fig. S7b). Fig. 2b shows J-V curves of the PbS:Hg QD-sensitized solar cells depending on HgCl 2 concentration and photovoltaic parameters are summarized in Table S4, where ca. 10 mm-thick TiO 2 film is used since optimal efficiency is obtained with around 10 mm-thick film from the investigation on dependence of TiO 2 film thickness on photovoltaic performance (Supplementary Fig. S8 and Table S5). J SC of QD-sensitized solar cell is substantially improved after Hg 21 doping. By adding 2 mmol of HgCl 2 in Pb 21 precursor solution, conversion efficiency is significantly improved from 2.38% to 3.78%, which is mainly due to remarkable increase in J SC from 13.68 mA/cm 2 to 22.27 mA/cm 2 (63% increment). For the case of Hg 21 doped PbS, as the concentration of HgCl 2 increases from 2 mmol to 8 mmol, no significant change in J SC is observed, whereas V OC is gradually increased. Since the fill factors are degraded as photocurrent increase, shunt and series resistances are calculated from slope of J-V curves at shortcircuit and at open-circuit region, respectively, using K2400 I-V system program based on LABVIEW (PV Measurements Inc) and plotted in Supplementary Fig. S9. Change in shunt resistance and series resistance are investigated as function of TiO 2 film thickness and number of coating cycle. Upon increasing TiO 2 film thickness or number of coating cycle, photocurrent is increased, while both shunt and series resistances are decreased. As can be seen in Tables S3 and  S5, fill factors are decreased with increasing TiO 2 film thickness or number of coating cycle, which is due to the decreased shunt resistance despite the decreased series resistance.
EQE spectra are demonstrated in inset of Fig. 2b. Maximum EQE of PbS:Hg QD sensitized solar cell reaches 74.6% at 470 nm, which is improved by 78% compared to the maximum EQE of 41.8% at 400 nm for PbS QD sensitized one. EQE is substantially enhanced from visible to near IR region, where 123% at 500 nm (from 33.1% to 74%), 472% at 700 nm (from 9.41% to 53.8%) and 440% at 1000 nm (from 1.89% to 10.2%) are improved. Stability of the devices is evaluated by measuring J SC with respect to time after electrolyte injection (Fig. 2c). J SC of PbS QD sensitized solar cell sharply decreases immediately after electrolyte injection, where almost 80% of J SC degrades during the initial 180 seconds. Instability of PbS QD in polysulfide electrolyte is also observed in previous research 12 . However, degradation of J SC surprisingly is overcome as the HgCl 2 concentration increases. Only 8% degradation of J SC is observed for the 6 mmol sample. This is considered to be attributed to change in chemical bonding nature of Pb-S owing to Hg 21 doping, which will be discussed in detail based on EXAFS analysis.
Femtosecond transient absorption (TA) spectra are measured to analyze the carrier dynamics in PbS:Hg QD-TiO 2 system along with pristine PbS QD-TiO 2 system. Fig. 2d shows femtosecond TA spectra in the near IR (NIR) region from 840 nm to 1200 nm, where the source of absorption is related to electrons in conduction band of TiO 2 23,24 . When considering the wavelength of pump light (490 nm) which is only absorbed by the QD, NIR absorption by electrons in TiO 2 conduction band can be ascribed to the injected photoelectrons from PbS and PbS:Hg QD to TiO 2 . It can be noticed that DO.D. increases as the HgCl 2 concentration increases and is highest for HgCl 2 concentration of 6 mmol. Since DO.D. is related to the injected electrons , the higher DO.D. is indicative of more populated electrons in TiO 2 conduction band, which is responsible not only for higher J SC but also for higher V OC after doping of Hg 21   expected that increase in DO.D. leads to upward shift of Fermi level, resulting in increase in V OC 25 . The observed DO.D. is well correlated with change in V OC measured in Table S4, where V OC increases from 0.387 V to 0.426 V as the HgCl 2 increases from 0 mmol to 6 mmol and slightly decreases to 0.424 with 8 mmol. Decreased DO.D. for 8 mmol HgCl 2 condition might be related to blue shift of absorption caused by the increased band gap. TA time profile in this region exhibits triply exponential decays with an ultrafast rising component as a result of numerical convolutions with cross-correlation function (FWHM 5 200 fs). In order to compare the carrier dynamics, DO.D. is normalized as a function of time (Fig. 2e). The decay dynamics slow down monotonically with an increased concentration of HgCl 2 from 0 mmol to 8 mmol. In Fig. 2f, DO.D. at arbitrary time delays of 2 ps, 20 ps, and 300 ps are elucidated with different HgCl 2 concentration. The DO.D. at arbitrary time increases with increased HgCl 2 concentration, which have the same trend with the shorter time delay (21 ps,13 ps). Numerically fitted parameters for decay time profile are listed in Table S6 where triply exponential model (eq 1) is used, and average decay time constants (t d ) are estimated by eq 2 26 , where A i and t i are triply exponential decay components constant. Average decay time constant increases from 151 ps to 186 ps as the HgCl 2 concentration increases from 0 mmol to 8 mmol (Fig. 2g). Absorption decay dynamics of electrons in conduction band of TiO 2 result from recombination of injected electrons with holes in QDs in absence of electrolyte. Therefore it is obvious that charge recombination in TiO 2 is substantially suppressed as the HgCl 2 concentration increases, which is responsible for the increased V OC . Also, considering that charge collecting efficiency is inversely proportional to recombination kinetics, suppressed charge recombination may lead to improvement of charge collecting efficiency (F c ) 27 . Ultrafast rising of DO.D. signal in Fig. 2e is originated from injection of photoexcited electrons from QD to TiO 2 23,24 . Therefore, it is possible to estimate the electron injection kinetics by measuring the time for DO.D. signal to reach its maximum. Fitted rise time constants are listed in Table S6 and Fig. 2g. Rise time constant decreases from 340 fs to 270 fs as the HgCl 2 concentration increases from 0 mmol to 8 mmol, which implies an improved electron injection efficiency (F inj ) 28 . It may not be ruled out that upward shift of PbS QD conduction band upon Hg 21 doping provides a driving force for faster electron injection in PbS:Hg QD than in PbS QD 29 . However, since the band shift of CBM of QDs seems favoring electron injection toward TiO 2 , one hypothesis for the improved functional properties of the cell and the mechanism for the inhibition of charge recombination seems to be related to structural reinforcement by Hg doping.
The effect of Hg 21 doping on the local structure of Pb 21 ions in the PbS:Hg QD adsorbed on TiO 2 surface is investigated with EXAFS analysis at Pb and Hg L 3 -edge. As can be seen in Fig. 3a, regardless of Hg doping, all the present PbS:Hg QDs adsorbed on TiO 2 show typical EXAFS oscillation of the reference PbS, confirming the formation of rocksalt-structured PbS phase. A closer inspection on the experimental spectra reveals that the wavelength of EXAFS oscillation becomes greater with the increase of Hg content, strongly suggesting the shortening of (Pb-S) bonds upon Hg doping 30 . The decrease of (Pb-S) bond distance upon the Hg doping is obviously verified by the quantitative EXAFS fitting analysis showing the gradual decrease of (Pb-S) distance with the increase of HgCl 2 concentration (Fig. 3c, Fig. 3e and Table 1). This result provides strong evidence for the reinforcement of the (Pb-S) bonds by the doping of Hg 21 ions. In addition to the systematic variation of bond distance, there is an overall decreasing trend of Debye-Waller factor (s 2 ) with the increase of HgCl 2 concentration, indicating the depression of structural disorder around Pb ions (Table 1) 31 . Such a decrease of structural disorder is also attributable to the reinforcement of (Pb-S) bonds upon the Hg doping. The strengthening of chemical bonds between Pb and S leads not only to the increase of the optical band gap of PbS QD but also to the upward shift of the conduction band of PbS with antibonding character. For local structure of mercury ion, detailed analysis on the Hg L 3 -edge EXAFS spectra (Fig. 3b, 3d and  3f) is performed as follows. Since the value of coordination number obtained from the EXAFS fitting analysis corresponds to the product of coordination number (CN) and amplitude reduction factor (S 0 2 ), it is necessary to get the knowledge of amplitude reduction factor for a Hg-S pair, prior to determining the exact coordination number of Hg ion. In the present analysis, the S 0 2 value obtained from the pure HgS reference is used to calculate the exact CN of mercury in the present PbS:Hg QDs. As listed in Table 1, this fitting analysis yields the CN of ,3.7-4 for mercury ions in the PbS:Hg QDs, strongly suggesting the stabilization of Hg ions in the interstitial tetrahedral sites. The tetrahedral symmetry of Hg ions in the PbS:Hg QDs is further confirmed by the comparison of their bond distance with that of pure (bulk) HgS material. In comparison with the reference HgS where the Hg ions are stabilized in the tetrahedral sites with the (Hg-S) bond distance of 2.49 Å , the present PbS:Hg QDs show a shorter bond distance of ,2.36-2.38 Å . Since the decrease of CN shortens the bond distance, the shorter (Hg-S) bond distance of the PbS:Hg QDs than that of the pure HgS underscores that the CN of Hg ion is smaller than or similar to four (i.e. the CN of Hg in the reference HgS). This result provides strong evidence for the stabilization of mercury ions in interstitial tetrahedral sites. If the Hg ions exist in octahedral symmetry (Pb site) with CN 5 6, the PbS:Hg QDs should show a longer (Hg-S) bond distance than that of the reference HgS (2.49 Å ) with CN 5 4. The tetrahedrally coordinated interstitial Hg 21 ions contribute to contraction of lattice and consequently causing a reinforcement of Pb-S bond (Fig. 3g). Theoretically calculated (Hg-S) bond distance for 6 mmol HgCl 2 case is 2.386 Å , which is well consistent with the observed bond distance of 2.369 Å . Both the enhancement of (Pb-S) bond strength and the improvement of structural ordering around Pb 21 ions strongly suggest the enhanced structural stability of PbS QD via the Hg doping. In addition, the enhanced structural ordering is likely to lead to reduction of deep trap density 32,33 , which contributes to suppression of charge recombination. Thus, the structural reinforcement by Hg addition is playing major role in improving the functional properties of the cell, and is the main mechanism for the inhibition of charge recombination compared to the role of band shift.
Best performance of PbS:Hg QD sensitized solar cell in this study is obtained by combination of 6 mmol HgCl 2 condition, scattering layer and CsOH additive in electrolyte. Addition of 0.3 M CsOH into the electrolyte improves fill factor of the device by 13% (Supplementary Fig. S10 and Table S7), which is probably due to decrease in interfacial resistance at electrolyte/Cu 2 S counter electrode 34 . Light scattering layer increases further EQE at longer wavelength (EQE of 56% at 800 nm and 32% at 1000 nm). As a result, J SC reaches 29.98 mA/cm 2 , leading to a PCE as high as 5.58% at AM 1.5G one sun illumination (Fig. 4).

Discussion
We presented a mercury doped PbS QD-sensitized solar cell with extremely high J SC of 30 mA/cm 2 . Such a high J SC was realized by reinforcing Pb-S bond via incorporation of Hg 21 ion into the interstitial site of PbS lattice. The improved ordering and covalency surrounding Pb ion led to faster electron injection and suppressed charge recombination. It is therefore concluded that structural www.nature.com/scientificreports SCIENTIFIC REPORTS | 3 : 1050 | DOI: 10.1038/srep01050 reinforcement by Hg addition plays a major role in improving the functional properties of the cell, and is the main mechanism for the inhibition of charge recombination. Based on our findings, optoelectronic and photovoltaic properties of QD materials are expected to be significantly improved by careful investigation on chemical bonding nature. Furthermore, such a preponderant enhancement in QD photovoltaic materials is believed to provide new insight into super high efficiency solar cell technology such as third generation MEG photovoltaics.

Methods
Sensitization of PbS:Hg QD on mesoporous TiO 2 layer. TiO 2 nanoparticles and films were prepared according to the method described elsewhere 35 . Fluorine-doped tin oxide (FTO) glass (Pilkington, TEC-8,8 V/sq ) was cleaned with ethanol using ultrasonic cleaner for 15 min and subsequently UV/Ozone cleaning was conducted for 15 min to remove organic contaminants. A dense blocking layer was prepared on the cleaned FTO glass using 0.1 M Ti(IV) bis (ethyl acetoacetato) diisopropoxide (Aldrich, 98%) in 1-butanol (Aldrich, 99.8%) solution. TiO 2 nanoparticles were hydrothermally synthesized using titanium isopropoxide (Aldrich, 97%) in autoclave at 230uC. TiO 2 paste was prepared by mixing the synthesized nanocrystalline TiO 2 , terpineol (Aldrich, 99.5%), ethyl cellulose (Aldrich, 46 cp) and lauric acid (Fluka, The paste was further treated using three-roll-mill for 40 min to disperse nanoparticles sufficiently. The TiO 2 paste was coated on the blocking layer coated FTO substrate using doctor-blade method and annealed at 550uC for 1 h in air. Thickness of the annealed mesoporous TiO 2 layers was measured by an alpha-step IQ surface profiler (KLA Tencor). Successive ionic layer adsorption and reaction (SILAR) method was used to sensitize the TiO 2 nanoparticles with PbS:Hg QDs as described in Fig. 1a. Briefly, mesoporous TiO 2 coated electrode was first dipped in aqueous solution of 0.1 M Pb(NO 3 ) 2 (Aldrich, 99.99%) for 1 min, followed by dipped in 0.1 M Na 2 S (Aldrich) for 1 min. Between each dipping, the electrode was thoroughly washed with DI (deionized) water. For PbS:Hg, 2,8 mmol of HgCl 2 (Sigma-Aldrich, 99.5%) was added to cationic precursor solution. These processes were defined as one cycle and several coating cycle was repeated to obtain the optimum photovoltaic performance in each condition (2 cycles for 0 mmol, 4 cycles for 2 mmol and 4 mmol and 6 cycles for 6 mmol and 8 mmol) ( Supplementary Fig. S5, Fig. S6 and Table S3). To enhance the photovoltaic performance, ZnS passivation layer was deposited after PbS:Hg deposition on TiO 2 by using aqueous solution of 0.1 M Zn(CH 3 COO) 2 (Aldrich, 99.99%) and 0.1 M Na 2 S 36 .
Solar cell fabrication. Cu 2 S formed on a brass sheet was used for counter electrode. Brass foil (Alfa aesar, 0.25 mm thick) was etched in hydrochloric acid (SAMCHUM, 35.0,37.0%) at 80uC for 15 min and subsequently washed with DI water and dried using air gun. Polysulfide solution composed of 1 M Na 2 S (Aldrich) and 1 M S (Sigma-Aldrich, 99.5%) in DI water was dropped on unmasked area of etched brass and black colored Cu 2 S immediately formed on the brass foil. The PbS:Hg QDsensitized working electrode and the Cu 2 S-brass counter electrode were assembled using double-side silicon-binder-coated polyimide tape (ca. 80 mm) as a spacer and sealant. Polysulfide redox electrolyte composed of 1 M Na 2 S (Aldrich) and 1 M S (Sigma-Aldrich, 99.5%) in DI water was injected through the 0.75 mm-diameter hole at working electrode and the holes were sealed using polyimide tape. For the best performance, 0.3 M of CsOH (Aldrich, 99.9%, 50 wt% solution in water) was added to electrolyte.
Materials and photovoltaic characterizations. The active area was measured by a digital microscope camera (DCMe 500) equipped with an image analysis program (Leopard 2009). Current density and voltage were measured using solar simulator (Oriel Sol 3A class AAA) equipped with 450 W Xenon lamp (Newport 6279NS) and a Keithley 2400 source meter. Light intensity was adjusted with the NREL-calibrated Si solar cell having KG-2 filter for approximating one sun light intensity (100 mW/cm 2 ). While measuring current and voltage, the cell was covered with an aperture mask. External quantum efficiency (EQE) was measured by a specially designed EQE system (PV measurement Inc.). A 75 W Xenon lamp (USHIO, Japan) was used as a white light source to generate monochromatic beam. Morphology and size of PbS:Hg QDs adsorbed on the nanocrystalline TiO 2 surface were investigated using a highresolution transmission electron microscope (HR-TEM, Jeol, JEM-2100F) at an acceleration voltage of 200 kV. Average diameter of QDs was determined from TEM micrograph using image J program (version 1.45s). Stoichiometry of QDs was analyzed using X-ray photoelectron spectroscopy (VG microtech, ESCA 2000). X-ray source was Al Ka and x axis was fitted for C 1s peak to have 284.6 eV. Removal of background signal and integration of peak was conducted using VGX900-W system. Ultraviolet photoelectron spectroscopy (UPS) spectra were measured with an AXIS-NOVA using a He I (21.2 eV) gas discharge lamp. Absorbance and reflectance was measured by UV/Vis spectrometer (PerkinElmer, Lambda 1050).
Hall measurement. To prepare samples for Hall measurement, chemical bath deposition of PbS and PbS:Hg thin film was conducted as described elesewhere 37 . 1 M Pb(CH 3 COO) 2 , 1 M thiourea and 0.5 M NaOH in DI water were used for deposition of PbS while 0.85 M Pb(CH 3 COO) 2 and 0.15 M HgCl 2 were used for deposition of PbS:Hg film. Both of the deposition was performed for 10 min at room temperature, which led to 300 nm thick-PbS and 235 nm thick-PbS:Hg on glass substrate. Hall voltage was measured under bias current of 1 mA for PbS film and 3 mA for PbS:Hg film, and applied field was adjusted from 1 to 5 kG using LaKeShore 7500 series Hall Measurement System at room temperature. Resistivity, type of conductivity, sheet carrier density, and mobility of majority carrier were calculated by van der Pauw method.
Femtosecond transient absorption measurement. Pump-probe transient absorption (TA) experiments were carried out using commercial Helios system (Ultrafast Systmes). The pump beam of 490 nm with a pulse energy of 1.6 mJ was obtained from an optical parametric amplifier (Light conversion, TOPAS-C), pumped by a regeneratively amplified Ti:Sapphire laser system (Coherent, Libra) with 1-k Hz repetition rate. The probe pulse was a white light continuum generated by focusing a small portion of the amplifier output though a c-axis sapphire window.
XANES and EXAFS measurements. Local structure of QD was analyzed by XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure). The Pb L 3 -edge XANES/EXAFS data were collected at room temperature in a fluorescent mode at the beam line 10C at the Pohang Accelerator Laboratory (PAL), Korea. The spectra were calibrated by measuring the spectrum of Pb foil simultaneously. Data analysis for the experimental spectra was performed with the standard procedure by using UWXAFS 2.0 program 38 . For the fitting of the reference, the amplitude reduction factor (S 0 2 ) was varied while the CN was fixed to the crystallographic value. All the bond distances (R), Debye-Waller factors (s 2 ), and energy shifts (DE) were allowed to vary. In order to exactly estimate the CNs of lead ions in the present PbS:Hg QDs adsorbed on TiO 2 , S 0 2 was fixed to the value from the reference PbS.  www.nature.com/scientificreports