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Effect of Zinc Incorporation on the Performance of Red Light Emitting InP Core Nanocrystals Lifei Xi,†,# Deok-Yong Cho,‡ Astrid Besmehn,§ Martial Duchamp,∥ Detlev Grützmacher,† Yeng Ming Lam,*,⊥ and Beata E. Kardynał*,† †

Semiconductor Nanoelectronics (PGI-9), Forschungszentrum Jülich, JARA-FIT, 52425 Jülich, Germany IPIT & Department of Physics, Chonbuk National University, Jeonju 54896, Republic of Korea § Central Institute for Engineering, Electronics and Analytics (ZEA-3), Forschungszentrum Jülich, 52425 Jülich, Germany ∥ Ernst Ruska Centre, PGI-5, Forschungszentrum Jülich, JARA-FIT, 52425 Jülich, Germany ⊥ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore ‡

S Supporting Information *

ABSTRACT: This report presents a systematic study on the effect of zinc (Zn) carboxylate precursor on the structural and optical properties of red light emitting InP nanocrystals (NCs). NC cores were assessed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), energydispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM). When moderate Zn:In ratios in the reaction pot were used, the incorporation of Zn in InP was insufficient to change the crystal structure or band gap of the NCs, but photoluminescence quantum yield (PLQY) increased dramatically compared with pure InP NCs. Zn was found to incorporate mostly in the phosphate layer on the NCs. PL, PLQY, and time-resolved PL (TRPL) show that Zn carboxylates added to the precursors during NC cores facilitate the synthesis of high-quality InP NCs by suppressing nonradiative and sub-band-gap recombination, and the effect is visible also after a ZnS shell is grown on the cores.

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INTRODUCTION Colloidal III−V nanocrystals (NCs) have been a subject of research for both fundamental research and practical applications. Indium phosphide (InP) NCs have attracted substantial interest because of their size-tunable photoluminescence (PL) emission wavelengths ranging from blue to the near-infrared (bulk band gap: 1.35 eV) and low intrinsic toxicity compared with II−VI compounds.1−6 As-prepared InP NCs are not very stable and suffer from nonradiative recombination at surface defects and surface traps.7−9 A few studies found that adding zinc (Zn) carboxylates during the InP core synthesis strongly improves their optical properties.2,7,10−13 For example, Xu et al.2 found that an appropriate amount of Zn undecylenate resulted in fewer surface dangling bonds and enhanced the PL quantum yield (PLQY). Li et al.3 synthesized InP/ZnS NCs with a one-pot method and obtained green-yellow (490−590 nm) emission NCs. They found that the highest PLQY of InP/ZnS NCs is 68%, achieved for green emission (532 nm). A shorter or extended reaction time leads to lower PLQY and a worse full width at half-maximum (fwhm). Ryu et al. synthesized InP/ZnS NCs with addition of Zn acetate and obtained green-, yellow-, and red-emitting NCs.8 The PLQYs of green, yellow, and red emission NCs are 38%, 18%, and 7%, respectively. Similarly, Kim et al. found that © XXXX American Chemical Society

with Zn acetate added during the InP core synthesis, the PLQY increased because the Zn2+ ions attached to the surface and partially removed the surface traps.12 Yang et al.7 found that the absorption spectra and PL emission wavelength can be continuously tuned from blue to the near-infrared (NIR) region by increasing the ratio of InP precursor to Zn stearate. They found a maximum QY of 60.2% at around 515 nm, which showed green emission when the ratio of InP to ZnS is equal to 1.2:1. A ratio higher or lower than this resulted in a lower PLQY and larger fwhm. Similarly, Thuy et al.10 also found that the emission is tunable from 485 to 586 nm by varying the InP precursor to Zn stearate ratio and explained it as resulting from the formation of InZnP alloy. Lim et al. added Zn acetate during their synthesis of InP cores for green-emitting NCs for LED application.11 Song et al. found that adding Zn chloride during the InP core synthesis resulted in the narrowing of the size distribution due to the reduced critical nuclei size.13 They reported that after an additional shelling process, the PLQY of red-emitting (612 nm) NCs is as high as 64%. They found that the PLQY of InP/ZnS NCs slightly drops from green- to redemitting NCs and the fwhm tended to increase from green to Received: March 25, 2016

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DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Typical PL spectra of InP cores synthesized with (red line) and without Zn (black line) and (b) PL peak positions of a set of NCs synthesized with different Zn:In ratios. The wavelength of emission for InP (Zn:In = 0) is shown as a blue point.

carboxylates and find that all of them lead to higher quality NCs. We establish an optimum molar ratio of Zn:In precursors for Zn undecylenate. In order to differentiate between bulk and surface effects of Zn, the NCs are assessed before and after ZnS shell growth using two different shell growth methods: SMP and SILAR. PL spectroscopy including evaluation of PLQY is used as a measure of NCs’ optical quality, while XPS, X-ray absorption spectroscopy (XAS), energy-dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM) are used to get insight into their electronic and structural properties.

red NCs due to the broader size distribution. Although all these studies demonstrated the importance of the Zn presence in the synthesis of high-quality light-emitting InP NCs, it is clear that there is no consensus on why this is the case, and the studies were mostly performed for the synthesis suitable for smaller NCs, emitting in the spectral range below 600 nm. An understanding of the effect of Zn incorporation is particularly important for the synthesis of larger, red light emitting NCs, because of their more complex synthesis (multiple phosphorus injection leading to a worse size distribution) and the need for further improvement in their quality for full-color conversion for white light emitting diodes.14 Analysis of the effect of Zn on InP cores is often performed on the core−shell NCs. Capping of the InP NCs with a larger band gap semiconductor, for instance ZnS (bulk band gap: 3.61 eV), producing a core−shell structure, is commonly used to improve the PLQY and photostability of InP NCs.2,3,5,7,10,12 It has been suggested that Zn during the core formation helps with increasing the thickness of the shell, but structural characterization has not been used to evaluate the shell thickness.4 Evaluation of the performance of the NCs based on optical measurements of core/shell NCs makes it difficult to differentiate between the quality of the InP core and InP surface effects. It is further complicated because various methods can be used to grow a ZnS shell on semiconductor NCs. These include the derived successive ionic layer adsorption/reaction (SILAR)7,11,12 one-pot heating procedure3 and thermal decomposition of the ZnS precursor mixture15 and single-molecule precursor (SMP).2,10,16,17 For example, Virieux et al.4 studied the composition and the surface chemistry of InP/ZnS core/shell NCs using nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) and found that the surface layer of the InP NCs is oxidized by decarboxylative coupling reactions. Using the SMP method, on the other hand, the oxides and even the InP cores are partially etched prior to the shell growth, leading to different InP/ZnS structure.17 The InP/ZnS NCs prepared with this method have high PLQY across the wavelength range from 485 to 675 nm. In this report, we study the effect of Zn incorporation on the performance of red-emitting InP core NCs. We analyze the optical and structural properties of InP cores grown with and without Zn precursors added during the synthesis. Such NCs are particularly needed for color conversion applications, while their optical performance is worse than that of those emitting in the green spectral range. In order to achieve large enough NCs for emission above 600 nm, a multiple P injection method was used with Zn present from the beginning. We study various Zn

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RESULTS AND DISCUSSION Examples of the PL spectra of InP NCs synthesized under identical conditions with (referred to as In(Zn)P) and without Zn undecylenate (referred to as InP) added to the reactants are shown in Figure 1a. The Zn to indium molar ratio of 0.5:1 was used in the synthesis of In(Zn)P NCs. The PL spectrum of the In(Zn)P NCs shows a clear band-to-band emission, centered at 640 nm. The fwhm of the PL spectrum of In(Zn)P NCs is 71 nm. The band-to-band emission is accompanied by an emission tail extending to 1000 nm. This tail is caused by the PL involving defect states (evidence of which is further shown in the PL excitation spectrum shown in Figure S1 in the Supporting Information). In contrast, the band-to-band emission at 630 nm from the InP NCs is very weak compared with the emission from defects, which forms a peak at around 900 nm. The band-to-band emission is slightly blue-shifted compared with In(Zn)P as a result of unavoidable variation in synthesis conditions. Note that PL of InP NCs was measured at twice as high laser excitation power as that of In(Zn)P in order to saturate the defects and reveal band-to-band emission. At lower excitation power, the band-to-band emission was absent. It was also found that the InP core synthesized with Zn undecylenate resulted in the increase of PLQY of the InP cores from less than 0.01% to 6.50%. The quoted number for PLQY of InP cores is a maximum measured at the excitation energy density of 0.4 kJ/cm2, and it decreased abruptly when the excitation power was reduced. It is thus clear that the presence of Zn in the reaction pot during In(Zn)P core synthesis has dramatically reduced nonradiative recombination and recombination through defect states and thus improved optical quality of the resulting material, in agreement with previous reports on NCs emitting in the green spectral range.2,7,10−13 Similar PL improvement was observed when In(Zn)P NCs were synthesized with other Zn carboxylates at the same molar ratios (see Figure S2). It was found that the fwhm of In(Zn)P B

DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. XPS spectra of (a) P 2p and (b) O 1s of InP and In(Zn)P NCs. (c) XPS spectrum of Zn 2p in In(Zn)P NCs. Experimental data, blue line. Dotted red line shows fitted Voigt function centered at 1022.4 eV.

either a change of the P valence number or the lower chemical potential of the material. The presence of InPOx (P3+−5+) in the XPS signal is clear from a P 2p peak at 133.3 eV. In order to compare the relative strength of the P 2p peak from InPOx and from InP, the latter is normalized to one. Upon such normalization, the InPOx (P3+−5+) P 2p peak is much stronger for In(Zn)P NCs than for InP NCs, leading to a conclusion that the amount of phosphates relative to InP is much higher in In(Zn)P NCs. This is likely since adding Zn carboxylates to the reaction pot increases the oxygen content. Figure 2b shows the XPS signal of oxygen. It can be seen that the O 1s line of In(Zn)P is shifted to higher binding energies compared to InP NCs. In addition, a shoulder is observed between 534 and 536 eV, which is absent in the spectrum of InP NCs. The main peak located at around 531 eV for both types of NCs can be fitted with a combination of In2O3, InOx, In(OH)3·nH2O, InPOx, and oxygen from carboxylate groups.4 The high-energy shoulder in In(Zn)P NCs agrees with a previously reported signal for (P2O5)x(ZnO)1−x.18 This is supported by the position of the Zn 2p3/2 peak at 1022.4 eV (see Figure 2c) and is consistent with the previous measurements of ZnO and other compounds of Zn with −O and −OH groups.4,18 The extra oxygen present during In(Zn)P synthesis seems not to incorporate into the InP core, as it would lead to a very poor optical quality of the In(Zn)P cores.19,20 The spectrum of the Zn 2p has been fitted with a Voigt function (after Shirley background subtraction) centered at 1022.4 eV (see Figure 2c). This peak position is consistent with ZnO and Zn(OH)2. There is no evidence for peaks at 1020.6 eV (of Zn line in Zn3P2)21 or at 1020.9 eV (Zn in ZnP2), so the fraction of Zn in InP must be much lower than the average Zn:In ratio in the nanocrystal, evaluated from XPS to be 32%.17 A Zn atom substituting for In in a defect-free InP lattice binds to two P atoms with a dangling bond of the third P atom. It is

synthesized with Zn undecylenate is the lowest among the tested Zn precursors. The possible reason is that Zn undecylenate, with a moderate carboxylate chain length, helps to balance the reactivity of zinc and indium. The evolution of the wavelength of emission with the Zn:In ratio is shown in Figure 1b. It does not change significantly for all but the highest tested Zn:In ratio of 7.5:1, which means that the composition, band gap, and size of the NCs are not affected significantly by the incorporation of Zn. The highest Zn:In molar ratio was found to disturb the nucleation and growth, as evidenced by a much slower color change of the reaction solution (from colorless to dark orange-red). The observed change of the wavelength of emission for these NCs may thus mean that InZnP alloy was formed or that slow nucleation followed by the same growth time resulted in smaller NCs. EDX experiments in a scanning transmission electron microscope (STEM) performed on individual In(Zn)P core NCs have been used to confirm that Zn and In coexist in the core NCs rather than as separate entities. An example of EDX spectra of In, Zn, and P taken on an individual NC, synthesized with a Zn:In ratio of 0.5:1, are shown in Figure S3. The average Zn:In ratio of 11.5 ± 5.5% was estimated from the EDX data as explained in the SI. Unfortunately, due to the damaging effect of the electron beam, the experimental error could not have been reduced with longer acquisition time and thus larger signal. Note that during TEM sample preparation ligands whose presence was confirmed with FTIR (see Figure S4) were removed. In order to get more insight into the incorporation of Zn in the NCs, we used XPS and XAS. Figure 2a compares phosphorus XPS spectra of In(Zn)P with a Zn:In ratio of 0.5:1 and InP NCs. The P 2p peak at 128.9 eV corresponds to P in InP (P3−),18 but its position is slightly shifted to lower energy in In(Zn)P compared with InP. The shift can be due to C

DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) P K-edge and (b) In L3-edge XAS spectra of an InP core synthesized with Zn and a commercial InP crystal for comparison.

shell synthesis were employed in this work: SILAR and SMP (both described in detail in the Supporting Information). In the SILAR method, it has been reported that Zn and sulfur ions react on the phosphate layers while the size of the unoxidized InP core slightly decreases because of an additional oxidation process by decarboxylative coupling reactions during ZnS shell coating.4 This results in forming an even thicker InPxOy middle layer between the InP core and ZnS shell. In the synthesis of the shell using the SMP method, the phosphate layer and even InP are partially etched, as is clear from reduction of the NC size after shelling (2.67 ± 0.46 nm) compared with the In(Zn) P core size (3.15 ± 0.51 nm).19 Using data from HRTEM, an example of which is shown in Figure 4 supported by electron energy loss spectroscopy

known, however, that Zn in InP exhibits complex behavior. It is often found in interstitial positions and forms complexes with double P vacancies, which may be readily available at the core surface. In such cases, it is not clear what the XPS signal would look like, but Zn would also not function as a dopant or an element of an alloy.22 The ratio of Zn:In determined from XPS is higher than the value determined from STEM EDX, and the difference could result from the use of oxygen plasma in the preparation of samples for the latter or could simply be a statistical variation between the nanocrystals. The NC cores were further characterized by XAS, which provides complementary information to XPS. Figure 3 shows the P K-edge (a) and In L3-edge (b) XAS spectra of the In(Zn) P cores and a commercial InP monocrystal. In the case of the InP monocrystal (see Figure 3a), a white line (strongest feature) observed at 2144 eV is characteristic of dominant P3− ionic species, while many additional features are present due to high structural order (X-ray absorption fine structures; XAFS). In contrast, in the case of the In(Zn)P NCs, only three white lines located at 2144, 2151, and 2153 eV are observed with no XAFS oscillations, a result of weak structural order in the NCs. The three white lines can be attributed to the P3−, P3+, and P5+ ionic species, respectively, thus confirming significant oxidation of P in the specimen. Figure 3b shows the influence of the P oxidation on the In ions. A huge difference in the overall line shape observed for L3-edge spectra of In(Zn)P NCs and the InP monocrystal indicates that In in In(Zn)P NCs has, in addition to In−P bonding, a different local electronic structure, such as that in the InPOx phase at the surface of the NCs. According to XPS and XAS the NCs contain a layer of phosphate and other oxides on the surface that is much thicker in the case of In(Zn)P than InP. Zn incorporation in the InP core was smaller than the detection limit of the techniques employed here. Only Zn bound to oxygen or −OH groups was observed in XPS. In addition, HRTEM of the NCs showed that the lattice spacing in the In(Zn)P NCs is very similar to that of pure InP (crystal) and InP NCs.17 Within experimental error, there is no difference in the band-to-band PL wavelength for moderate ratios of Zn:In precursors as seen in Figure 1b, as also reported before.2,7 Apart from the presence of Zn in InP, one needs to consider the effect of change in the thickness of the phosphate layer on the quality of the NCs. It has been previously proposed that it acts as a middle shell that is responsible for the high quality of InP/ZnS NCs by allowing the lattice-mismatched ZnS shell to grow on InP cores while not having any detrimental effect on luminescence.4 In order to test this possibility, two methods of

Figure 4. HRTEM of In(Zn)P/ZnS core−shell NCs synthesized with the SILAR method. Inset: Enlargement of one of the NCs.

(EELS) (see Figure S5), we calculated the shell thickness from the SILAR method to be about one monolayer (see Table S1), while the ZnS shell formed using SMP has been previously found to be around two monolayers.17 Considering large scatter in the data (from the statistically small sample size), we must conclude that etching away the phosphate (InPOx) layer did not change the average coverage of the NCs with ZnS. It is, however, possible that it is not the thickness of the shell but the completeness of the core coverage (which cannot be easily verified) that determines the quality of the passivation. After adding the shell, both types of NCs have a dominant band-to-band photoluminescence at 640 nm and a weak tail of D

DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) PLQY and (b) PL peak emission wavelength of In(Zn)P/ZnS NCs synthesized using the SMP method as a function of Zn:In molar ratio during synthesis of the core.

over a certain value, the growth of the NCs was affected and the optical properties of InP NCs deteriorated. The highest quantum yield was measured for In(Zn)P/ZnS NCs with the shell synthesized using a method in which the phosphate layer and even the surface of In(Zn)P are etched away, showing that direct ZnS shelling provides more effective passivation than ZnS on the phosphate layer formed on InP core NCs. We found that synthesis with Zn undecylenate with a Zn:In molar ratio on the order of 0.5:1 gave the highest PLQY of InP/ZnS core−shell NCs. No evidence that the surface phosphate layer, identified with XAS and XPS, helps with ZnS shell formation was found. Since, in addition there was a difference in PLQY between In(Zn)P/ZnS NCs with cores synthesized with different In:Zn ratios, Zn during the core reaction simply improves the core quality. It may also result in a better protection of the cores with a thicker phosphate layer during their transfer to the shell reaction pot.

counts from defect states extending to longer wavelengths (see Figure S6). The defect PL of In(Zn)P NCs is significantly smaller than that of core NCs, and time-resolved PL (TRPL) shows almost monoexponential decay (see Figure S7). PL spectra of the cores capped with the SMP method are slightly broaderas a result of the core etchingbut have slightly smaller defect emission and a higher PLQY when compared to that of SILAR. Figure 5 shows the normalized to maximum PLQY and PL peak emission wavelength of In(Zn)P/ZnS NCs with a shell synthesized with the SMP method as a function of Zn:In molar ratio (0.1, 0.25, 0.5, 0.63, 1.0, and 7.5) during core synthesis. Cores whose PL was shown in Figure 1b have been used. Note that we choose to compare the PLQY of core/shell NCs, as that of InP cores only is not easily comparable, as discussed earlier. Figure 5a shows that the highest PLQY from core−shell NCs is obtained for a Zn:In molar ratio of 0.5:1. Deviation from this ratio results in steady reduction of PLQY. Figure 5b shows the wavelength of PL spectra of In(Zn)P/ZnS NCs. The wavelength of PL is shorter than that of core NCs (Figure 1a) as a result of a H2S gas etching effect during shell formation,17 but it is independent of the Zn:In ratio up to the value of 1:1. The PLQY for the highest Zn:In ratio is significantly smaller than the optimum, and the wavelength of emission is shorter than for InP. This means that deterioration of growth rate rather than In(Zn)P alloy formation is a more likely origin of the shorter emission wavelength, and so in the multiply P injection method there is an optimum Zn:In ratio.

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EXPERIMENTAL SECTION

Chemicals. Indium acetate (99.99%), stearic acid (99%), myristic acid (99%), Zn undecylenate (98%), Zn stearate (10−12% Zn base), tris(trimethylsilyl)phosphine (PTMS, 98%), 1-dodecanethiol (DDT, 98%), oleylamine (OLA, tech. 70%), chloroform (99.99%), and 1octadecene (ODE, tech. 90%) were purchased from Sigma-Aldrich. Octanoic acid (>98%) was purchased from Alfa Aesar. Zn dibenzyldithiocarbamate (ZDBz, >97%) was purchased from TCI Europe. Zn octoate or myristate was prepared as follows: Zn acetate (0.1 mmol) was added to a mixture of octanoic acid or myristic acid (0.5 mmol) and 20 mL of ODE, heated to 120 °C, and kept under vacuum for 1 h. After that, the mixture was repeatedly washed with acetone to remove unreacted acid and ODE. Synthesis of In(Zn)P Core Stock Solutions. In a typical synthesis, indium acetate (1 mmol), Zn carboxylate (0.5 mmol of Zn octoate, undecylenate, myristate, or stearate), stearic acid (3.5 mmol), and ODE (20 mL) were mixed in a 25 mL flask. The solution was heated to 100 °C and kept under vacuum for 1 h. The solution was then purged with argon and heated to 300 °C. PTMS (1 mmol) in ODE (5 mL) was prepared inside a glovebox and rapidly injected into the flask. The reaction was carried out at 280 °C for 9 min. After that, the solution was quickly cooled to room temperature. Without any washing, the crude solution was stored in a refrigerator and used for the shell growth. Different molar ratios of Zn carboxylate to indium (0, 0.1, 0.25, 0.5, 0.63, 1, and 7.5) were studied by adjusting the initial amount of Zn carboxylate. For characterization, the InP NCs are repeatedly washed with a chloroform/acetone mixture (1:9) and centrifuged (13 500 rpm) for 15 min. NCs can be easily redispersed in chloroform, toluene, or other nonpolar solvents. Synthesis of InP/ZnS NCs Using the SMP Method. A 3.2 mL amount of the above crude solution that contains 7.0 × 10−7 mol of InP NCs with a diameter of 3.15 nm, calculated as described elsewhere,1 10 mL of ODE, and 2.5 mL of OLA were mixed and

CONCLUSIONS

In summary, we have studied the effect of adding a Zn precursor during the synthesis of InP cores designed to emit light in the red wavelength range. EDX/STEM-based studies of individual core clusters confirmed the presence of Zn in the NCs but were not able to discriminate between incorporation in InP or phosphate on the surface. XPS, however, showed that a large fraction of Zn was in the phosphate layer on the NC surface. No direct evidence of Zn substituting for In in InP has been found. Indirect measurements of incorporation of Zn in InP such as through the lattice constant of the NC (as measured with TEM) or the band gap (as measured with PL) did not show any difference between InP and In(Zn)P within experimental error either; yet the PLQY of In(Zn)P increased significantly. We found that synthesis with Zn undecylenate with a Zn:In molar ratio on the order of 0.5:1 improved the PLQY compared to pure InP core NCs by over an order of magnitude. When the Zn:In ratio in the reaction pot increased E

DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX

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stirred for 15 min. After that, 1.52 mmol of ZDBz was added. The mixture was stirred for another 30 min. The flask was evacuated and flushed with argon to remove water and oxygen gas from the solution. The solution was then slowly heated to 170 °C for 45 min. After that, the temperature was kept at this temperature for another 2 h of reaction. After reaction, the solution was allowed to cool to 50 °C. The NCs were precipitated from solution using acetone and centrifuged at 13 500 rpm for 15 min. The resulting NCs could be redispersed in chloroform, toluene, or other nonpolar solvents. Synthesis of InP/ZnS NCs Using the SILAR Method. Zn stock solution: 0.2 mmol of Zn undecylenate was mixed with 2 mL of ODE and heated to 130 °C to obtain a clear solution. DDT/ODE stock solution: 0.2 mmol of DDT was also mixed with 2 mL of ODE. A 3.2 mL amount of the above crude In(Zn)P NC solution and 10 mL of ODE were mixed for 15 min and heated to 170 °C. A 1 mL amount of Zn stock solution was slowly injected. After 3 min, 1 mL of DDT solution was slowly injected. The above injection was repeated once with a 3 min interval. The reaction was stopped after 30 min. The same workup procedure was applied as described earlier. Characterization. Photoluminescence spectra were obtained using a homemade PL setup. The excitation wavelength (405 nm) from a diode laser was focused with an achromatic objective lens with NA = 0.42 to a spot on a sample. PL from the investigated sample was collected and collimated with the same lens and then passed through the dichroic mirror onto an entrance slit of a spectrometer. A spectrometer (Andor 303i) equipped with an Andor iDus Si CCD camera on an output was used. The system was calibrated using a lamp with a known blackbody spectrum, which allows conversion of the counts recorded on the CCD into photons/nm/m2. All the experiments were carried out at room temperature. The QYs of NCs were measured by comparison with rhodamine 6G (Sigma, 83697, QY = 95%) and rhodamine 101 (Sigma, 83694, QY = 95%) in absolute ethanol.2 The optical density (OD) at the excitation wavelength of the QDs and a dye sample was set at the same value. The ODs of the NCs solutions and the reference samples were less than 0.1 to reduce the error from reabsorption. The QY of the NCs sample was obtained by comparing the integrated areas of PL emission of the NCs and that of the reference samples. Fourier transform infrared (FT-IR) spectra were recorded on a Nexus 470 spectrometer (Thermo Nicolet) using the transmission mode. TEM, EELS, and EDX were taken on JEOL 2100F, FEI Titan “Pico”, and ChemiSTEM microscopes operated at an acceleration voltage of 200 kV. XPS was performed under ultrahigh vacuum using a PHI5000 VersaProbe II with monochromatic Al Kα radiation in a large area mode (1.4 mm × 200 μm, 100 W, 20 kV). A 20 eV pass energy was used for the elemental scans. Quantification of the survey scans was carried using MULTIPAK software. Core level spectra were fitted with a mixed Gauss−Lorentz function after subtraction of a Shireley background. XAS was performed at the 16A1 beamline in Taiwan Light Source. Data were collected in fluorescence yield mode with a Lytle detector at room temperature.

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NWs4LIGHT project. D.-Y.C. acknowledges the support from the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (No. 2015R1C1A1A02037514).

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00747.

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REFERENCES

Additional figures, table, and calculation (PDF)

AUTHOR INFORMATION

Corresponding Authors

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Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien and Energie GmbH, 12489 Berlin, Germany. F

DOI: 10.1021/acs.inorgchem.6b00747 Inorg. Chem. XXXX, XXX, XXX−XXX