20190
J. Phys. Chem. C 2008, 112, 20190–20199
Highly Luminescent Water-Soluble InP/ZnS Nanocrystals Prepared via Reactive Phase Transfer and Photochemical Processing Chunliang Li,† Masanori Ando,† Hiroyuki Enomoto,‡ and Norio Murase*,† Photonics Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, and DiVision of Electronics and Applied Physics, Graduate School of Engineering, Osaka Electro-Communication UniVersity, Neyagawa, Osaka 572-8530, Japan ReceiVed: June 22, 2008; ReVised Manuscript ReceiVed: October 24, 2008
Highly luminescent water-soluble InP/ZnS core-shell nanocrystals were prepared using a newly developed method that incorporates reactive phase transfer and photochemical processing. Poor-emitting InP nanocrystals (NCs, 2-4 nm) prepared solvothermally using toluene were transferred into alkaline aqueous solution containing thiol and zinc ions. When these NCs in aqueous solution were subsequently irradiated by ultraviolet (UV) light, they showed intense size-dependent photoluminescence (PL) from green to red due to the formation of a thick (more than 1 nm) ZnS shell on the NCs. The surface dissolution of the NCs, under conditions in which bulk InP does not dissolve due to its covalent bond nature, was observed at two steps: phase transfer and shell formation. This dissolution competed with the formation of the ZnS layer at the start of UV irradiation. Since the UV irradiation enables creation of a thick shell by optimizing the synthesizing conditions, high PL efficiency (30-68%) was obtained in water with sufficient stability. This was quantitatively explained by quantum mechanical calculations. The PL decay behavior of these water-soluble InP/ZnS NCs did not show obvious size-dependence, unlike HF-treated ones. This is attributed to the well-passivated surface states of the NCs due to their thick ZnS shell. The NCs showed a significantly higher In/P ratio than those previously reported. This indicates that In ions were preferentially located on the surface of the InP core in the NCs. Introduction Semiconductor nanocrystals (NCs) have been attracting much attention due to their unique optical properties originating from the quantum confinement effect.1,2 Among them, II-VI nanocrystals have been studied the most extensively over the past two decades owing to their potential application to light-emitting diodes,3-5 lasers,6,7 and biological labels.8-12 With recent advances in colloidal chemistry, high-quality NCs made of various II-VI semiconductor materials have been synthesized in the liquid phase through either an organometallic route13-16 or an aqueous route.17-19 The photoluminescence (PL) efficiencies of CdSe NCs have been increased more than 60% by growing a shell of another semiconductor with a wider bandgap.15,20,21 The prepared CdTe NCs have had PL efficiencies as high as 65% without postpreparative treatment by optimizing the synthesizing conditions.22 However, they were typically composed of highly toxic elements (Cd2+, Se2-, or Te2-), which cause environmental problems with respect to large-scale commercial applications. III-V semiconductor NCs have a robust covalent bond. Among them, InP NCs have been the most extensively studied because they are not toxic and have a broad PL color range in the visible spectrum, one similar to that of II-VI NCs. The most widely used method for synthesizing colloidal InP NCs uses the dehalosilylation reaction between indium chloride (InCl3) and tris(trimethylsilyl)phosphine (P(Si(CH3)3)3) in the presence of a coordinating solvent such as a mixture of * To whom correspondence should be addressed. E-mail: n-murase@ aist.go.jp. Phone: +81-72-751-8483. Fax: +81-72-751-9637. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Osaka Electro-Communication University.
trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) or dodecylamine (DDA). This reaction requires a high temperature (over 200 °C) and takes several days.23-28 A hot injection technique is useful because it enables the nucleation stage to be separated from the growth stage. This makes it possible to obtain InP NCs of various sizes at different stages in the heating process.21,27-31 High-quality InP NCs have been synthesized using fatty acid as the ligand in noncoordinating solvent.29,30 However, this method entails complex synthesis conditions (a relatively high temperature, a long reaction time, and oxygenfree atmosphere). Intense investigations have thus been conducted to improve the synthesis route by eliminating complex reactions and using less toxic and flammable precursors.32-34 Maenosono et al. synthesized InP NCs using a traditional TOPO method at 300 °C and using a safe and easily available phosphorus source (tris(dimethylamino)phosphine, P(N(CH3)2)3).32 The resulting InP NCs had a diameter of 6.4 nm and a size distribution of ∼50% and showed featureless absorption and broad PL spectra. Solvothermal (“hydrothermal” when the solvent is water) synthesis, another method for synthesizing semiconductor NCs, has a short reaction time at low temperatures. It has been used with a flammable phosphorus source to prepare InP NCs.33,34 The NCs were in the form of secondary particles, and their PL efficiencies were quite low. All the InP NCs mentioned above were synthesized in organic solution and showed quite low PL efficiencies immediately after preparation. The traditional method for improving PL efficiency is to coat the InP core with a shell that has a wider bandgap to passive the core.35-38 A typical material for this shell is ZnS because it has a cubic lattice structure with a lattice constant similar to that of InP. The PL efficiency of InP/ZnS core-shell NCs in organic solutions is 23-40%.37,38 Another way to
10.1021/jp805491b CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Luminescent Water-Soluble InP/ZnS Nanocrystals increase PL efficiency is to treat InP NCs with hydrogen fluoride (HF), which removes the surface traps caused by phosphorus atoms located on the surface of the NCs. This increases the PL efficiency in organic solution by 25-40%.25,39 Chemical analysis has shown that all the InP NCs thus prepared have similar In/P molar ratios of 1.0-1.1.27,35 Water solubility and stability are other important factors in biological applications and sol-gel processing of semiconductor NCs. Although water-soluble II-VI NCs have been prepared directly using an aqueous method,17-19,22 there have been no reports on the synthesis of III-V NCs in water except for the work of Qian et al., who used a hydrothermal method.40 Nevertheless, their NCs were in the form of secondary particles, the same as for those prepared solvothermally. The most common way of obtaining water-soluble NCs is to exchange the surfactant on the surfaces for a hydrophilic one. However, this phase transfer generally causes a significant decrease in PL efficiency.41 Parasad et al. transferred the InP/ZnS core-shell of III-V NCs into aqueous solution by exchanging the surfactants for thioglycolic acid (TGA), but the PL efficiency of the NCs in the water was not reported.42 Bawendi et al. prepared water-soluble InAs/ZnSe core-shell NCs of various sizes with PL efficiencies of 6-9%.43 More recently, Peng et al. reported the preparation of water-soluble InP/ZnS NCs with PL efficiencies of 40%; these NCs were transferred into water without any loss of PL intensity by using hydrophilic thiopropionic acid.38 These methods for obtaining water-soluble III-V NCs are based on exchanging the hydrophilic surfactants on the surface of the core/shell NCs prepared by organometallic methods rather than on the direct formation of a hydrophilic layer on the surface of bare NCs in water. Postpreparative irradiation was recently shown to effectively form a ZnS shell on II-VI cores in aqueous solution,44-47 and it should be applicable to bare III-V NCs as well. In short, the challenge we face is to develop a new approach to synthesizing highly luminescent water-soluble InP NCs in a simple and safe way. We have developed a method for synthesizing highly luminescent InP/ZnS core-shell NCs in aqueous solution that incorporates reactive phase transfer and photochemical processing. InP NCs prepared solvothermally48 are transferred into aqueous solution by reactive exchange of the surface In and P for thioglycolic acid molecules in solution. A thick ZnS shell is then created on the surface through postpreparative ultraviolet (UV) irradiation. This preparation process is monitored using elemental analysis at each step, and the irradiation conditions are optimized to obtain the highest PL efficiency in water. Quantum mechanical calculations indicated that a thick shell of ZnS is required to confine the electrons inside the particles. The prepared InP/ZnS NCs showed band-edge PL tunable from green to red. Lifetime measurement and chemical analysis demonstrated that the NCs differed from those previously reported. They were stable for months under ambient conditions and could be incorporated into a glass matrix using a sol-gel method. Experimental Methods Materials. All chemicals were of analytical grade or the highest purity available. InCl3 (99.999%), DDA (98%), acetonitrile (99.9%), zinc perchlorate hydrate (Zn(ClO4)2 · 6H2O), and 3-aminopropyltrimethoxysilane (APS, 97%) were purchased from Aldrich. Thioglycolic acid (TGA, 90%), P(N(CH3)2)3 (98%), toluene, butanol, and methanol were obtained from Wako Chemicals. The solvents used were of dehydrated grade. A solution containing Zn2+ ions and TGA (“Solution ZT”) was
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20191 prepared by dissolving 0.49 g of Zn(ClO4)2 · 6H2O and 0.29 g of TGA in 10 mL of water; the pH of the solution was adjusted to 11.0 by using 1 M NaOH. The concentration of TGA was changed depending on the situation. Ultrapure deionized water (18.2 MΩ/cm) was obtained from a Milli-Q synthesis system (Millipore). Sample Preparation. Synthesis of InP NCs in Organic Solution (Step 1). InP NCs were synthesized using a solvothermal method through pyrolysis reaction between InCl3 and P(N(CH3)2)3 in the presence of surfactant (DDA) in toluene, as described in a previous report.48 Briefly, InCl3 (0.4 g), P(N(CH3)2)3 (0.45 g), and DDA (5 g) were well mixed with 5 mL of toluene in a glovebox under an argon atmosphere. The mixture was hermetically sealed in a 50 mL Teflon-lined autoclave. The autoclave was then taken out of the glovebox and heated to 180 °C for 24 h. After the autoclave had cooled down to room temperature, the byproducts were precipitated and discarded. The solution of InP NCs was subjected to sizeselective precipitation27,39 using methanol when required. The isolated monodisperse InP NCs fractions were redispersed in hexane. The prepared InP NCs were photoetched with HF to confirm that they had the same properties as those previously reported.39 Phase Transfer of InP NCs to Aqueous Solution (Step 2). The phase transfer was carried out under atmospheric conditions. Typically, an aliquot of InP NCs containing ∼1 × 10-6 mol of particles was dispersed in 2 mL of a hexane and butanol mixture, with a volume ratio of hexane to butanol of 2:1. This solution was then mixed with 2 mL of Solution ZT. After continuous stirring of the mixture at 50 °C for 1 h, the InP NCs were transferred to the aqueous layer. This made the organic layer completely transparent. The InP NCs in the aqueous layer were then precipitated and redispersed in 2 mL of various concentrations of Solution ZT for continued creation of a ZnS shell through postpreparative irradiation, as explained below. The optimum conditions were identified by also using organic solutions such as toluene and chloroform instead of butanol for comparison. The temperature of the solution during phase transfer was varied from room temperature to 76 °C, while the reaction time was changed from 10 min to 2 h. Formation of ZnS Shell on Surface of InP NCs in Aqueous Solution (Step 3). The ZnS shell was coated on the surface of the InP NCs in aqueous solution using a postpreparative irradiation similar to the method previously reported for ZnSe/ ZnS NCs.44-47 The aqueous solution was continuously stirred while it was irradiated with UV light (λ ) 365 nm) from a 250-W UV lamp (SP-7, USHIO) for ∼1 h. The absorption and PL spectra of the NCs were frequently measured during the irradiation. After irradiation, the NCs were precipitated from the reaction mixture by adding 2 mL of acetonitrile and were redispersed in water. The effect of UV irradiation was investigated by changing the light intensity from ∼1 to ∼4 W/cm2. Incorporation of InP/ZnS NCs into Glass Matrix. The prepared InP/ZnS NCs were incorporated into a glass matrix using the method generally used for thiol-capped II-VI NCs.45,46,49 A 10 mL mixture of APS and methanol with a molar ratio of 1:50 was placed in a Teflon Petri dish (50 mm φ). Then, 1 mL of deionized water was added to the solution, and the solution was stirred for 1 h to promote hydrolysis. After the solution had been kept under atmospheric conditions for ∼12 h, an aqueous solution of 0.75 mL of InP/ZnS NCs and 0.25 mL of Solution ZT was added and mixed well. Transparent glass was obtained after the solution had been left in the dark for two days.
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SCHEME 1: (a) Illustration of Synthesis of Highly Luminescent Core-Shell InP NCs in Aqueous Solutiona. (b) Schematic Drawing of InP NCs at Different Synthesis Stages
a Step 1: solvothermal preparation of InP NCs in organic solution. Step 2: phase transfer of InP NCs from organic to aqueous solution. Step 3: creation of ZnS shell on surface of InP NCs by postpreparative UV irradiation.
Characterization. X-ray powder diffraction (XRD) patterns were obtained with a Rigaku Geigerflex diffractometer with Cu KR radiation (λ ) 1.5418 Å). High-resolution transmission electron microscopy (TEM) was performed using an FEI Tecnai G2 electron microscope operated at an accelerating voltage of 200 kV. The specimens were prepared by putting one drop of the NC solution onto a surface of high-resolution carbonsupported copper grids (pitch of 100 µm). Elemental analyses (In, P, Zn, and S) of the NCs at each step (Steps 1 to 3) were carried out using an electron probe microanalyzer (EPMA, EPMA-1610, Shimadzu). The elements (In, P) in some of the samples were also determined by inductively coupled plasma spectrometry (ICP, SPS-4000, SII). UV-visible absorption spectra of the diluted NC solution were measured with a spectrophotometer (U-4000, Hitachi), and fluorescent measurements were performed with a spectrofluorimeter (F-4500, Hitachi) using an excitation wavelength of 400 nm. The PL efficiencies of the colloidal solution and glass plate were estimated by a recently developed method50 using standard solutions of quinine in an aqueous 0.05 M H2SO4 solution (η ) 54.6%51). PL decay curves for three sizes of NCs were obtained at room temperature using a time-correlated singlephoton-counting spectrofluometer system (FluoroCube-3000U, Horiba). The excitation wavelength was set to 374 nm. The lifetime components were determined by applying threecomponent exponential functions to the decay data. Results and Discussion A. Synthesis. The procedure for synthesizing InP/ZnS core-shell NCs can be divided into three steps, as shown in Scheme 1a. Schematic drawings of the NCs at the three steps are shown in Scheme 1b. SolWothermal Synthesis in Organic Solution (Step 1). The InP NCs, which were prepared solvothermally, had a broad size distribution because the nucleation and growth occurred simultaneously. However, after the size-selective precipitation, the fractions of InP NCs had a size distribution as narrow as 10%, as revealed by TEM (described below). Excitonic peaks were observed in the absorption spectra (Figure 1a). The particle sizes were estimated using the reported relationship between the absorption excitonic peak wavelengths and particle sizes (Figure S1, Supporting Information). The estimated sizes (2.0-4.0 nm) were confirmed by TEM observation. The concentration of NCs was estimated from the absorption spectra using the reported molar extinction coefficient.39,52 As explained later in detail, the
Figure 1. (a) Absorption spectra of solvothermally synthesized NCs after 12 rounds of size-selective precipitation. (b) Absorption and PL spectra of HF photo etched NCs.
timing of the size-selective precipitation in Scheme 1 affects the PL properties after coating of the ZnS shell most probably through the oxidization of the surface during processing. Therefore, hereafter, we mention the timing of the size-selective precipitation in our description of PL properties. When these size-selected InP NCs in butanol were treated with HF,39 the absorption and PL spectra changed, as shown in Figure 1b. The PL efficiencies were between 27 and 58% depending on the particle size. The details of this are described elsewhere.48 These results indicate that high-quality NCs can be prepared solvothermally. Phase Transfer into Aqueous Solution (Step 2). We carefully investigated the conditions required to effectively transfer the InP NCs into aqueous solution. We tested the use of several kinds of organic solvent, of several concentrations of Solution ZT in water, of various solution temperatures, and of various reaction times. After redispersion of the NCs in one of the organic solvents (toluene, hexane, butanol, and chloroform), it was mixed with a water phase (Solution ZT) by stirring. The butanol was the most effective for transferring the NCs into the water phase (Figure S2, Supporting Information). Since butanol is miscible with water, the effective surface area between butanol and water is significantly larger. This should account for the efficient transfer of the NCs in butanol. However, InP NCs after precipitation from toluene were not readily soluble in butanol but were well redissolved in hexane. Therefore, we used a mixture of hexane and butanol (67/33 v/v) instead of pure butanol. When the NCs were transferred into the water phase, the color of the organic phase quickly faded out accompanied by the coloration of the water phase. Increasing the temperature
Luminescent Water-Soluble InP/ZnS Nanocrystals
Figure 2. Absorption and PL spectra of NCs in organic solution (Step 1), after phase transformation (Step 2), and after UV irradiation (Step 3).
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20193
Figure 3. PL spectra of InP/ZnS NCs with same PL peak wavelength for size-selective precipitation carried in three steps.
TABLE 1: PL Properties of InP/ZnS NCs Size-Selectively Precipitated at Different Steps
of the solution increased the speed of the transfer. The optimum concentration of InP NCs in the organic solution was related to the concentration of Solution ZT in the water. A high concentration (>0.32 mol/L) of TGA in Solution ZT blue-shifted the absorption spectra of InP NCs in aqueous solution along with a decreased absorbance. These blue shifts were due to the size decrease in the InP NCs caused by the high concentration of TGA dissolved the NCs during the transfer. Extending the reaction time increased the number of NCs transferred. However, no significant increase in the PL efficiency of the irradiated InP NCs in the next step was obtained when the transfer was carried out for more than 1 h. Creating a ZnS Shell by PostpreparatiWe Irradiation (Step 3). UV irradiation creates a thickness-controlled ZnS shell on the surface of II-VI NCs.45-47 The power of the UV light had to be increased to 4 W/cm2 (from the recently used value of 1 W/cm2 for the preparation of blue-emitting ZnSe1-xTex/ZnS NCs46) in order to create a ZnS shell quickly. The absorption and PL spectral evolution of InP NCs from Steps 1 to 3 are shown in Figure 2. Size-selective precipitation was carried out in Step 1. The obtained NCs with a size of ∼3.1 nm (Figure 1a, no. 6) were used. At Step 1, the NCs showed very weak PL on the lower energy side with the first excitonic absorption peak at 570 nm. After the NCs were transferred into aqueous solution (Step 2), the first excitonic peak was shifted to a shorter wavelength by approximately 30 nm (Figure 2, Step 2). This is ascribed to a decrease in particle size. From the elemental analysis described later, roughly one monolayer of InP dissolved from the surface during this transfer. The thickness of a monolayer was 3.4 Å, which was defined by the distance between consecutive planes along the 〈111〉 axis in bulk InP. The absorbance of InP NCs at the first excitonic peak was decreased by 30%. Considering that the molar extinction coefficient decreased along with the particle size,39,52 the concentration of InP NCs in the aqueous solution remained almost the same as that in the organic solution. The PL efficiency of InP NCs was increased to ∼2% when they were transferred into aqueous solution. This increment was caused by the creation of a thin ZnS shell on the surface of the NCs, as clarified by XRD and elemental analysis (discussed below). The phase transfer process here differs a great deal from that for obtaining water-soluble III-V NCs by simple exchange of the surfactants for hydrophilic ones. The surface of the InP NCs was dissolved, and a hydrophilic layer formed on the surface simultaneously in the present case. The first excitonic peak of absorption spectra discernible in Step 2 changed into a shoulder after UV irradiation (Figure 2, Step 3). The PL intensities dramatically increased with the
sample no.
PL wavelength λ/nm
PL efficiency η (%)
fwhm/nm
1 2 3 4 5
Size-Selected at Step 1 554 34 566 34 590 48 607 45 621 40
84.4 83.4 83.4 78.6 80.6
1 2 3 4 5 6
Size-Selected at Step 2 540 30 584 49 599 49 619 52 635 46 651 46
80.2 76.4 73.4 69.8 66.0 69.4
1 2 3 4 5
Size-Selected at Step 3 604 59 616 66 625 65 636 68 648 56
93.4 92.4 91.8 88.4 81.2
irradiation, which was accompanied by the widening of the PL spectra. Small red shifts of both the excitonic absorption and PL peak were observed. However, they were much smaller than previously reported for ZnSe1-xTex/ZnS NCs.46 This is attributed to the larger band offset between the InP core and ZnS shell, which makes the carrier wave functions more confined in the core, as shown quantitatively by calculation (described below). Condition Optimization for Obtaining High PL Efficiency in Wide WaWelength Range. Even though size-selective precipitation of the as-prepared NCs was required to obtain a narrow PL width, as described above, the surface of the NCs was easily oxidized when these NCs were size-selected under atmospheric conditions.27 After the postpreparative irradiation, defect emissions probably caused by this oxidization were clearly observed at the longer wavelengths of the PL spectra. To reduce this phenomenon, we did the size-selective precipitation at Step 2 or 3 instead of at Step 1. Figure 3 shows the PL spectra of irradiated NCs with the same PL peak wavelength for size-selective precipitation carried out in the three steps. Compared with the PL spectra of the sample size-selected at Step 1, the defect emissions at longer wavelengths for the sample size-selected at Step 2 were restrained. The same results were observed for the case size-selected at Step 3. Table 1 shows the PL properties of irradiated InP NCs sizeselectively precipitated at the three different steps. The fwhm of those size-selected at Step 3 were wider than those sizeselected at Step 2. Moreover, the PL wavelengths were only
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Figure 4. (a) Absorption and PL spectra of InP/ZnS NCs in the aqueous solution. The NCs were size-selectively precipitated at Step 2 and irradiated by UV light at Step 3. Fluorescence images of the prepared NCs placed under the room light (b) and excited by UV lamp at 365 nm (c).
TABLE 2: PL Properties of InP/ZnS NCs after Irradiation in Solutions with Different Ratios of TGA and Zn Ions
Figure 5. Absorption and PL spectra of InP NCs after UV irradiation in solutions with different ratios of TGA and Zn ions.
tunable from orange to red, a much narrower range than that for those size-selected at Step 2. TEM observation (described below) showed that a thick shell (1.5 nm) had formed on the surface of the InP cores after irradiation at Step 3. The final size distribution is the summation for the distribution of the original core size and the thickness of the shell. This made the core-size range selected after Step 3 wider, resulting in a wider PL spectrum with a narrower wavelength tuning range. The highest PL efficiency (68%) was obtained when the size selection was done at Step 3 because the surface was protected by the thick ZnS shell (Table. 1). Therefore, the size selection should be done at Step 3 to obtain high PL efficiency, and it should be done at Step 2 to obtain a wider wavelength tuning range with a narrower PL spectrum. Figure 4a shows a series of absorption and PL spectra of NCs size-selectively precipitated at Step 2 and UV irradiated at Step 3. Photographs of prepared NCs under room and UV light are shown in Figure 4, b and c. After postpreparative irradiation, each absorption spectrum in the series became rather smooth and featureless. The PL peak wavelengths were from green to red with PL efficiencies between 30 and 52% (Table 1). The fwhm ranged from 66 to 80 nm, which is similar to that reported for high-quality InP NCs in organic solution.31 Even when the size selection was done at Step 3, there were still defect emissions. Therefore, we further optimized the postpreparative irradiation conditions. Figure 5 shows the absorption and PL spectra of NCs when the molar ratios of TGA and Zn2+ (TGA/Zn2+ ratio) in Solution ZT used for the irradiation were varied. The concentration of Zn2+ remained
TGA/Zn2+ ratio
PL peak/nm
PL efficiency (%)
fwhm/nm
1.5 1.9 2.4 4.0
590 589 604 596
49 55 50 46
81.2 83.6 88.8 97.0
unchanged (0.13 M), while the molar ratio of TGA against Zn2+ was changed from 1.5 to 4.0. There were negligible differences in the absorption and PL spectra when the TGA/Zn2+ ratio was between 1.5 and 1.9. However, the excitonic peak in the absorption spectrum became a shoulder when the ratio exceeded 2.4. The PL spectra shifted to the red in combination with a significant increase in defect emissions. Table 2 shows the PL properties of InP/ZnS NCs after irradiation in the solutions with different ratios of TGA and Zn ions. The highest PL efficiency was obtained when the TGA/ Zn2+ ratio was 1.9. When the TGA concentration was high, an excess amount of S2- ions was produced by irradiation. This resulted in fast formation of the ZnS shell with defect centers contributing to the emissions at ∼700 nm and broadening of the PL width. B. Structural Characterization. X-ray Diffraction. Figure 6 shows the powder XRD patterns of the NCs for the three steps. That for Step 1 exhibited three prominent peaks, reflecting InP NC peaks corresponding to a zinc blende structure. The particle size was estimated to be 3.4 nm by applying the Debye-Scherrer equation to the three peaks. This closely agrees with the size estimated from both the absorption spectra and TEM images (shown below). The XRD peaks for Step 2 were located at almost the same positions as those for Step 1. However, slight shifts to larger angles were detected for the three peaks. They were caused by the formation of a thin layer of ZnS on the surface. The particle size estimated from the Debye-Scherrer equation for this step was ∼2.8 nm. This means the size was slightly reduced by the phase transfer. This coincides with the dissolution of NCs discussed above. For Step 3, the XRD peaks were shifted to larger angles and matched those of pure ZnS. From the elemental analysis discussed later, the molar ratio of InP and ZnS in the irradiated NCs was roughly 1:60. Therefore, the XRD peaks of InP almost disappeared. This clearly indicates that a thick ZnS shell had formed on the surface of the InP NCs.
Luminescent Water-Soluble InP/ZnS Nanocrystals
Figure 6. X-ray diffraction patterns of NCs in organic solution (Step 1), after phase transformation (Step 2), and after postpreparative UV irradiation (Step 3). Bars at the top and bottom represent peak positions of bulk ZnS and InP, respectively. ZnS shell shifted the peaks over wider angles.
Transmission Electron Microscopy. Figure 7 shows TEM images of NCs in organic solution (Step 1, (a)), after phase transfer (Step 2, (b)), and after postpreparative irradiation (Step 3, (c)). The size distributions at each step are depicted in Figures 7d-f. At Step 1, the NCs were spherical and well dispersed. The average particle size obtained from the images was 3.1 ( 0.3 nm. The particles were oriented to the 〈111〉 axis in the plane of images (Figure 7a, insert) with interplanar spacing of 3.4 Å. This corresponds to the value from an XRD database (JCPDS 10-0216). The average particle size at Step 2, as obtained from the images, was 3.0 ( 0.5 nm, a bit smaller than that in organic solution (Step 1). It had increased to 6.0 nm after postpreparative irradiation (Step 3). This means the InP NCs were covered by a 1.5 nm (5 monolayer) ZnS shell. One monolayer here corresponds to a thickness of 3.1 Å, which is the distance between consecutive planes along the 〈111〉 axis in bulk sphalerite ZnS. The ZnS shell created by UV irradiation was much thicker than that prepared by conventional epitaxial growth (1-2 monolayers).37,42 The lattice parameters of the InP and ZnS were 5.869 and 5.406 Å, respectively. Since the difference in lattice parameters is relatively small, a thicker shell could be formed on the InP core without degrading PL efficiency. This situation is the same as that previously reported for ZnSe1-xTex/ZnS NCs.46 The particle size distribution of the irradiated InP NCs at Step 3 was shifted to higher values, as shown in Figure 7f. This resulted in the broadening of the PL spectra, as mentioned above for Figure 2. Elemental Analysis. Elemental analyses of the NCs at different steps were carried out using EPMA. To improve the accuracy, a sample at Step 1 was used as the In and P standard for analysis, where the molar ratio of In and P was determined in advance by ICP analysis. Corrections based on atomic number (Z), absorption (A), and fluorescence (F) (“ZAF correction”) were made to obtain accurate values. The results are shown in Table 3 for the NCs for which the absorption and PL spectra are plotted in Figure 2. The NCs contained a larger amount of In relative to P at all steps. Roughly half the atoms were located on the surface of the particles, which had a size of ∼3.1 nm. The high ratio of In/P in the organic solution indicates that the surface of the InP NCs was preferentially populated by In atoms. When the NCs were transferred into aqueous solution (Step 2), the In/P ratio dropped to 1.3. Both Zn and S elements were also detected with compositions of 5.6 and 13.9 mol %, respectively. This corresponds to a molar ratio of Zn/S of 0.41
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20195 and means that a thin ZnS layer formed on the surface of the NCs during the transfer. The excessive amount of S is ascribed to S ions being released by the decomposition of TGA in the solution and then reacting with the relatively larger amount of In atoms on the surface of the NCs. The decrease in the In/P ratio is attributed to the dissolution of the surface of the NCs during processing. The size reduction was detected by the absorption peak wavelength, the peak width from XRD, and TEM images. Further elemental determination supported this explanation. Namely, the InP NCs after phase transfer into aqueous solution were precipitated and removed from the solution. The clear supernatant solution thus obtained was analyzed using ICP. The resultant concentrations of In and P in the solution are shown in Table 4. The InP NCs had a diameter of 3.2 nm and a concentration of 2.9 × 10-6 M · particles in the organic solution. The measured concentrations of In and P in the supernatant were 6.6 and 3.1 × 10-4 M, respectively. The amount of dissolved In was twice that of P after phase transfer. This also indicates that the In atoms preferentially populated the surface of the NCs. We assumed that one monolayer (3.4 Å) of InP was dissolved and calculated the amount of In3+ and P3- ions in the supernatant solution. The molar ratios of In/P in the NCs at each step were taken from the EPMA results (Table 3). The density of NCs was assumed to be the same as in the bulk counterpart. As shown in Table 4, the calculated amount of dissolved atoms for the current experimental conditions agreed quite well with the ICP analysis results. As for the smaller NCs (2.6 nm), the calculated amount of dissolved atoms also agreed well with the elemental analysis. We investigated the reaction conditions required for this transfer and subsequent growth of the ZnS layer. The InP NCs in organic solution could be transferred to 1.0 M NaOH aqueous solution. This means that the DDA molecules on the surface of the NCs were at least partially exchanged for OH- during processing. However, the NCs in aqueous solution were agglomerated immediately after this transfer. When the NCs in organic solution were mixed with 1.0 M water solution of TGA, they dissolved quickly, whereas bulk InP does not dissolve when it is immersed in the same TGA solution. The reaction of the NCs proceeded because the binding energies of In-S (289 kJ/ mol) and P-S (444 kJ/mol) were much higher than that of In-P (198 kJ/mol).53 When there were Zn2+ and S2- ions in the solution, a ZnS layer formed on the surface of the InP NCs. This prevented their further dissolution. After postpreparative UV irradiation (Step 3), the compositions of In and P in the NCs were roughly 1 mol % and the In/P molar ratio had increased to 1.9. The compositions of Zn and S in the NCs were significantly increased to ∼60 mol %, with a molar ratio (Zn/S) of 0.93, as shown in Table 3. The dissolution of InP NCs occurred during UV irradiation as well. Figure 8a shows the absorption spectra of NCs at the beginning of irradiation in Step 3 together with the spectra at Steps 1 and 2. The first excitonic peak showed a blue shift when irradiation began. This indicates the dissolution of the NCs. Along with this dissolution, the In/P ratio further increased at Step 3. As the irradiation continued, the first excitonic peak started to shift to red accompanied by the formation of a ZnS layer. Figure 8b shows the time evolution of PL and first excitonic peak wavelengths during UV irradiation. Sulfur ions were produced in the solution by photodecomposition of TGA in an alkaline solution.45,46 After irradiation for 100 s, a ZnS layer started to form on the surface of the InP NCs. The use of low UV power (
