Codoping n- and p-Type Impurities in Colloidal Silicon Nanocrystals

The engineering of the all-inorganic colloidal Si-NC and its optical data reported ... In Si-NCs, the highest luminescence energy can exceed 2.0 eV wh...
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Codoping n- and p‑Type Impurities in Colloidal Silicon Nanocrystals: Controlling Luminescence Energy from below Bulk Band Gap to Visible Range Hiroshi Sugimoto,† Minoru Fujii,*,† Kenji Imakita,† Shinji Hayashi,† and Kensuke Akamatsu‡ †

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan ‡ Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 minatojimaminami, Chuo-ku, Kobe 650-0047, Japan S Supporting Information *

ABSTRACT: We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85− 1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the allinorganic colloidal Si-NC and its optical data reported here are important steps for Sibased optoelectronic and biological applications.

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INTRODUCTION

In Si-NCs, the highest luminescence energy can exceed 2.0 eV when the size is very small, typically when the size is smaller than 2 nm in diameter. On the other hand, the lowest luminescence (band gap) energy is limited to around 1.2 eV at room temperature because the quantum size effects can control the band gap only above the bulk Si band gap (1.12 eV). The size of Si-NCs luminescencing around 1.12 eV is usually much larger than the exciton Bohr radius of bulk Si crystal (∼4.5 nm). The size range is a so-called weak confinement regime, and only the center-of-mass motion of excitons is confined. Si-NCs thus strongly preserve the indirect band gap character of bulk Si crystal, and as a result, the radiative recombination rate is very small and the luminescence efficiency is usually not very high. This sometimes prevents applications of Si-NCs in the near-IR (NIR) range. If the luminescence (band gap) energy of colloidal Si-NCs can be extended around or hopefully below the bulk Si band gap without sacrificing the radiative recombination rate and the luminescence efficiency, their application fields can further be extended. A successful approach to extend the controllable range of the luminescence energy of Si-NCs below the bulk Si band gap is simultaneous doping of n- and p-type impurities in Si-NCs and introducing donor and acceptor states in the band gap. Ossicini et al. demonstrated by first principles calculations that the formation energy of phosphorus (P) and boron (B) codoped Si-NCs is very low, and they have smaller energy gap than

Colloidal semiconductor nanocrystals (NCs) have been attracting great research attention for more than a couple of decades because of their potential optoelectronic and biomedical applications.1−6 Very high quality II−VI and IV− VI semiconductor NCs are already commercially available, and the research stage is shifted to the developments of optoelectronic devices1−4 and in-vivo biomedical applications.5,6 However, the high toxicity of Cd and Pb in these NCs is an indelible problem for the in-vivo biomedical applications as well as for the applications in consumer optoelectronic products. The most promising environmentally benign alternative of II−VI and IV−VI semiconductor NCs is Si-NCs.7−16 Although the quality, i.e., size distribution, crystallinity, luminescence quantum efficiency, stability in solution, etc., of colloidal Si-NCs was lower than that of II− VI and IV−VI semiconductor NCs, it has been improving rapidly. Mangolini et al.9 reported colloidal Si-NCs with the photoluminescence (PL) quantum yields (QYs) of ∼60%. Synthesis of colloidal Si-NCs with very small size distribution was achieved by Mastronardi et al. (average diameter (d): 2.2 nm; polydispersity (PDI): 1.04)10 and Miller et al. (d: 3.79 nm; PDI: 1.005)14 using the density gradient ultracentrifugation (DGU) method. Hessel et al.15 succeeded to control the size of colloidal Si-NCs in a very wide range (2.7−11.8 nm in diameter) and obtained PL tunable in a wide energy range. Because of these rapid improvements of the quality, Si-NCs become a very promising alternative of II−VI and IV−VI semiconductor NCs. © XXXX American Chemical Society

Received: March 20, 2013 Revised: May 10, 2013

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Figure 1. (a) Photographs of Si-NCs dispersed in methanol. Annealing temperature (Ta) and optical transmittance at 1150 nm are shown above and below the photographs, respectively. (b) Photograph of the sample with Ta = 950 °C under UV irradiation.

precipitates and others disperse. As will be shown later, the ratio of Si-NCs forming precipitates depends strongly on Ta. Finally, precipitates were removed by centrifugation (4000 rpm, 2 min), and only a supernatant liquid was stored in a vial. After the removal of precipitates, the liquids were very clear, and precipitates were no more formed for more than a year. All the processes were performed in an ordinary laboratory environment. PL Measurements. PL spectra of colloidal solutions were obtained by using a single spectrometer equipped with a liquidN2 cooled InGaAs diode array (OMA-V-SE, Roper Scientific) and a charge coupled device (CCD) (Roper Scientific). The excitation wavelength was 405 nm. The spectral response of the detection system was corrected with the reference spectrum of a standard halogen lamp. PL decay dynamics were measured by using a NIR photomultiplier (R5509-72, Hamamatsu Photonics) and a multichannel scalar (SR430, Stanford Research). All the measurements were carried out at room temperature.

undoped Si-NCs with the same size due to the donor and acceptor states.17−19 In previous studies,20−23 we demonstrated that P and B codoped Si-NCs embedded in borophosphosilicate (BPSG) matrices show luminescence below the bulk Si band gap at room temperature. The lowest luminescence energy reached 0.9 eV.20−23 Although very low energy luminescence could be achieved by codoping, in the previous samples, 20−23 Si-NCs were embedded in solid matrices, and thus their application was limited to some specific fields. The purpose of this work is to develop colloidal Si-NCs, the PL energy of which can be controlled in a very wide range covering the bulk Si band gap. Recently, we demonstrated that codoped Si-NCs can be extracted from solid matrices by simply dissolving the matrices by hydrofluoric (HF) acid solution, and the extracted Si-NCs can be dispersed in polar solvents without surface functionalization.24,25 The codoped colloidal Si-NCs do not form agglomerates and are stable in polar solvents for more than a year without any special cares for storage. A possible mechanism of the very high solution dispersibility without functionalization by organic ligands is that a high B concentration layer is formed on the surface of Si-NCs and the surface is negatively charged. The negative surface potential prevents agglomeration of Si-NCs in polar solvents. In the previous studies, however, we could produce codoped Si-NCs only with the size of around 3−4 nm in diameter, which showed luminescence in a 1.2−1.3 eV range. In this work, we develop a method to control the size of codoped colloidal Si-NCs in a wide range and study the structure and the optical properties. We show that codoped colloidal Si-NCs with the average diameters from 1 to 14 nm can be synthesized and the PL maximum can be controlled from 0.85 to 1.80 eV. The maximum external PL-QY is higher than 12% when the size is about 3 nm. Even when the PL energy is around 1.2 eV, the PL-QY exceeds 1%. By comparing the PL energies and the lifetimes of codoped and undoped SiNCs with similar sizes, we discuss the PL mechanism of codoped colloidal Si-NCs.

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RESULTS AND DISCUSSION Figure 1a shows the photographs of colloidal Si-NCs prepared from Si-rich BPSG films annealed at different temperatures (Ta). The amounts of Si-rich BPSG films before HF etching are fixed to 20 mg for all the samples. The annealing temperatures are shown above the photographs. Despite the same amount of the initial materials, the color of the solutions is significantly different (see Figure S1, transmittance spectra). Figure 2a shows Si concentration (mg/mL) in the solutions obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Si concentration depends strongly on Ta. It is the largest when Ta is 1075 °C. The high Si concentration implies that most of excess Si atoms in BPSG participate in the formation of Si-NCs by annealing, and they are well-dispersed in methanol without the formation of precipitates. In fact, when Ta is 1075 °C, almost nothing was removed by centrifugation. On the other hand, at higher Ta, a fraction of Si-NCs formed agglomerates and some of them were removed by centrifugation, resulting in smaller Si concentration in the solutions. When Ta is very high, even after centrifugation, small amounts of agglomerates remain in the solutions. In Figure 1a, the numbers below the photographs are the optical transmittance at 1150 nm. When Ta is below 1200 °C, the transmittance is almost 100%, indicating that light scattering due to agglomerates is negligibly small. The transmittance starts to decrease when Ta exceeds 1200 °C, and the solutions become slightly cloudy. This is due to the formation of small agglomerates not being removed by centrifugation. The decrease of Si concentration at lower Ta is not due to the formation of precipitates. When Ta is low, the size of Si-NCs grown in BPSG matrices is small and some of them are etched out during HF etching. It should be stressed here that the color of the solutions is not solely determined by the Si concentration. If we compare solutions with similar Si

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EXPERIMENTAL PROCEDURE Fabrication of Colloidal Si-NCs. P and B codoped colloidal Si-NCs were synthesized by a previously reported procedure.24,25 Si-rich BPSG films were deposited on thin stainless steel plates by cosputtering Si, SiO2, B2O3, and P2O5 in an rf-sputtering apparatus.20−25 The films were peeled from the plates and crushed to powder in a mortar. The powder was then annealed at different temperatures (850−1300 °C) in a N2 gas atmosphere for 30 min to grow Si-NCs with different sizes in BPSG matrices. During the growth of Si-NCs, P and B atoms are incorporated into Si-NCs from BPSG matrices. Impurity codoped Si-NCs were isolated from BPSG matrices by etching in HF solutions (46 wt %) for 1 h. Isolated Si-NCs were then transferred to methanol. In methanol, a part of Si-NCs form B

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Figure 3. TEM images of codoped Si-NCs prepared with the annealing temperatures of (a) 1050, (b) 1100, (c) 1200, and (d) 1300 °C. Electron diffraction patterns are shown in the insets. The bars in the insets represent diamond structure Si crystal.

diamond structure. Therefore, the term “nanocrystal” may not be suitable for these particles, and the term “nanocluster” is more appropriate. However, in the following, we will not distinguish “nanocrystal” and “nanocluster” and refer to both of them as NC. Since accurate measurements of the sizes by TEM and DLS were not possible for Si-NCs prepared with Ta < 1000 °C, we roughly estimate the sizes by the following procedure (see the Supporting Information for details). We first obtain diffusion coefficients of Si in BPSG at different T a from the experimentally obtained sizes under the assumption that the radius of as-deposited Si clusters in BPSG is 0.5 nm.26−29 From the Ta dependence of the diffusion coefficients, the activation energy (Q) can be estimated. The estimated value of Q is 218 kJ/mol. By using the value, the size vs Ta relation is calculated. The results are shown as a solid curve in Figure 2b. The calculated result reproduces experimentally obtained data well, and thus we can roughly estimate the size of Si-NCs prepared at low annealing temperatures by extrapolating the curve. Figure 2c shows the ζ-potentials of the solution. The ζpotentials are negative for all the samples and in the range of 27−42 mV. This relatively large ζ-potential implies that Si-NCs are stabilized in methanol by electrostatic repulsion. The ζpotential value seems to depend slightly on Ta, but the reason is not clear. Figure 4a,b shows P and B concentration in a Si-NC measured by ICP-AES. The P and B concentration is very high. Especially, the B concentration is much higher than the solid solubility of B in bulk Si crystal.30 Our previous work24,25 revealed that not all P and B in Si-NCs are doped in the substitutional sites of Si-NCs, and many of them are accumulated on or near the surface of Si-NCs. Furthermore, we showed that the formation of B-rich surface layers is the origin of the high solution dispersibility of codoped SiNCs.24,25,31 In Figure 4a,b, the P and B concentration is almost independent of Ta, although the data are scattered. This

Figure 2. (a) Si concentration, (b) average diameters of Si-NCs, and (c) ζ-potentials of colloids in Figure 1.

concentration, e.g., Ta = 1050 and 1200 °C, the color of a lower-temperature-annealed one is much lighter than that of a higher-temperature-annealed one. This indicates that the energy state structure is very much different between Si-NCs annealed at different temperatures. Note that, although the color of the solution for the samples annealed below 950 °C is very light, they show bright PL under UV irradiation as shown in Figure 1b. Figure 3a−d shows transmission electron microscope (TEM) (JEOL JEM-200CX) images and electron diffraction patterns of Si-NCs dispersed in methanol. For the TEM observations, the solutions were dropped on carbon-coated copper grids. The diffraction rings agree with that of the diamond structure Si crystal (JCPDS No. 27-1402). In the images, we can clearly see the increase of the size with increasing Ta. When Ta is below 1200 °C, Si-NCs form two-dimensional arrays on carbon thin films, which evidence that Si-NCs are well-dispersed in solution without the formation of three-dimensional agglomerates. On the other hand, when Ta = 1300 °C, the formation of threedimensional agglomerates is observed, although the sizes of the agglomerates are relatively small (100−200 nm). This is consistent with the slight decrease of NIR transmittance at high Ta. Figure 2b shows the average diameters of Si-NCs obtained from TEM images (■) and by dynamic light scattering (DLS) measurements (▲) (Malvern: Zetasizer Nano ZS). The agreement between the two methods is very good. We can see that the average size is controlled in a very wide range (2.2−14.1 nm) by Ta. When Ta is below 950 °C, particles were not clearly seen by TEM and the electron diffraction patterns were halo. This suggests that the structure of Si nanoparticles prepared with Ta ≤ 950 °C is strongly distorted from the C

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Figure 5. (a) Normalized PL spectra of codoped Si-NCs in BPSG annealed at different temperatures. A dashed vertical line represents the bulk Si band gap energy (1.12 eV). (b) PL peak energy and intensity as a function of Ta. Figure 4. (a) P and (b) B concentration in a Si-NC as a function of the annealing temperature. (c) Numbers of P and B atoms per a NC as a function of NC diameter. In the size range larger than 2 nm in diameter (solid symbols), the size is estimated by TEM and/or DLS, while that smaller than 2 nm (open symbols), the size is estimated by the procedure explained in the main text.

means that the number of impurity per a NC depends strongly on Ta. The average numbers of P and B atoms per a NC are plotted in Figure 4c as a function of the diameter. It changes 3 orders of magnitude depending on the diameter. Figure 5a shows normalized PL spectra of codoped Si-NC samples before etching, i.e., Si-NCs in BPSG, annealed at different temperatures (850−1250 °C). The spectra depend strongly on Ta. The PL peak energies and intensities are plotted in Figure 5b as a function of Ta. We can see the shift of the PL peak in a very wide range by changing Ta. The most remarkable feature in Figure 5 is that the PL peak energy is below the bulk Si band gap (1.12 eV) when Ta is higher than 1100 °C. The very low energy PL cannot be observed in intrinsic Si-NCs at room temperature, and the observation of the below bulk-band gap PL is one of the evidence that the PL arises from the transitions between donor and acceptor states.20−25 In Figure 5b, the PL intensity depends on Ta. Especially, the PL intensity decreases rapidly at high Ta. We will discuss the mechanism later. Figures 6a and 6b show PL spectra of colloidal Si-NCs prepared from Si-rich BPSG films annealed at different Ta measured 1 day and 1 month after etching, respectively. Small bumps in the spectra in the NIR region are due to reabsorption of emitted light by methanol (Figure S3a). Similar to codoped Si-NCs in BPSG, codoped colloidal Si-NCs show the PL in a

Figure 6. Normalized PL spectra of codoped colloidal Si-NCs annealed at different temperatures: (a) 1 day and (b) 1 month after preparation; (c) Si-NC solids prepared by drop-casting colloidal SiNCs.

very wide energy range. Figure 6c shows PL spectra of Si-NC solids prepared by drop-casting the colloidal solutions onto D

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quartz substrates in air. The spectra are very similar to those of the solution 1 month after etching. The PL peak energies of codoped Si-NCs in BPSG, colloidal Si-NCs (1 day and 1 month after etching) and Si-NC solids are summarized in Figure 7a. The peak energies of colloidal Si-NCs

Figure 8. PL lifetimes of codoped colloidal Si-NCs prepared with different annealing temperatures (900−1250 °C) as a function of detection energy. For all the samples, the lifetime is measured at several different energies. The lifetimes of undoped Si-NCs in SiO2 (□) and in solution (○) are also shown. The curve is guide to the eyes.

on Ta. In particular, when Ta is 925 and 950 °C, the lifetimes are much shorter than others, suggesting that NCs in these samples are qualitatively different from others. Up to now, we study the structure and PL properties of codoped colloidal Si-NCs as a function of Ta. We are now ready to discuss the size dependence. Figure 9a shows the PL peak energy of codoped samples as a function of the NC diameter. For comparison, the data of undoped Si-NCs in the literature are also shown.15,16,35−37 The PL peak of undoped samples shifts from the bulk Si band gap to the visible range with decreasing the size, although the data are scattered. The large scattering of the data is mainly due to different surface terminations. In general, the PL energy of small Si-NCs decreases by oxygen termination by the formation of defect states in the band gap.38 In fact, in Figure 9a, the PL energy of undoped Si-NCs in silica matrices (□) is lower than those terminated by hydrogen or organic molecules (△, ◊, and ▽). The PL peak of codoped Si-NCs exhibits qualitatively the same size dependence as that of undoped Si-NCs. However, the PL peak energy is much lower. Especially, the PL peak energy of codoped Si-NCs in BPSG matrices is always 300−400 meV lower than that of undoped Si-NCs. In codoped colloidal SiNCs, the difference of the PL energy from that of undoped SiNCs is smaller than that of codoped Si-NCs in BPSG matrices. However, when we compare the data of codoped colloidal SiNCs with those of undoped Si-NCs not heavily oxidized (△, ◊, and ▽), the difference is clear. In the whole energy range, the PL energy of codoped colloidal Si-NCs is smaller. This is one of the evidence that donor and acceptor states are involved in the optical transitions even when the size of Si-NCs is very small. In some specific sizes of Si-NCs, the energy differences between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are calculated by first principles calculations when a pair of P and B is doped.18,19 For example, the HOMO−LUMO gap of a P and B codoped Si-NC 1.8 nm in diameter is calculated to be in the range between 1.55 and 2.03 eV depending on the P and B distance. In the present work, the PL peak energy of codoped Si-NCs 1.8 nm in diameter is around 1.7 eV, in good agreement with the theoretical prediction.

Figure 7. (a) PL peak energies and (b) quantum yields of codoped SiNC samples as a function of annealing temperature. The data of colloids 1 day and 1 month after preparation and that of Si-NC solids are shown in (a).

are estimated after correcting the spectral shape by taking into account the reabsorption by methanol (Figure S3b,c). The PL peak always shifts to higher energy by HF etching. This is considered to be due to the change of the surface termination of Si-NCs by etching.31 The shift is larger at lower Ta, especially when Ta is below 1050 °C. By keeping Si-NCs in methanol for 1 month, the PL peak slightly shifts to higher energy, especially when Ta is high. It should be stressed here that, after 1 month from etching, no further change of the spectra was observed. The PL peak energies of Si-NC solids almost perfectly coincide with those of colloidal Si-NCs 1 month after etching. Figure 7b shows the PL-QYs of colloidal solutions 1 month after etching as a function of Ta. The PL-QY was determined by a comparative method (Supporting Information).32−34 The maximum QY of about 12.8% is obtained for the sample with Ta = 1050 °C. The QY decreases significantly to higher Ta and slightly to lower Ta. Figure 8 shows the PL lifetimes, i.e., the time where the emission intensity drops to 1/e of the initial value, of codoped colloidal Si-NCs as a function of the detection energy. The PL decay curves are shown in the Supporting Information (Figure S4). For the comparison, the PL lifetimes of undoped Si-NCs in SiO2 and in solution are also shown. Since the PL of undoped Si-NCs arises from quantum confined excitons, the data exist only above 1.12 eV. The PL energy dependence of the lifetime of codoped colloidal Si-NCs is much different from that of undoped Si-NCs in some aspects; the PL lifetime at the same energy is much shorter, and the energy dependence of the lifetime is much smaller. In codoped colloidal Si-NCs, the lifetime depends slightly on Ta at relatively high Ta. On the other hand, when Ta is lower than 1000 °C, it depends strongly E

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number of Si-NCs that do not contribute to PL (dark Si-NCs) increases. Iori et al.39 demonstrated that, when the size of SiNCs is very small, the formation energy of Si-NCs in which exactly the same number of P and B atoms are doped is much smaller than those in which unequal number of P and B atoms are doped. This implies that perfectly compensated Si-NCs are preferentially grown by annealing Si-rich BPSG. On the other hand, when the size increases and the number of impurities per a NC increases, the difference of the formation energy between perfectly and imperfectly compensated Si-NCs is considered to be small. As a result, Si-NCs in which different number of nand p-type impurities increase. If excess carriers exist in Si-NCs, Auger recombination of photoexcited carriers becomes possible. The Auger time is estimated to be 0.1−100 ns40 and is more than 2 orders of magnitude shorter than the radiative recombination time. The imperfectly compensated SiNCs thus become almost completely dark. The increase of the number of imperfectly compensated dark Si-NCs results in the decrease of the PL-QY when the size increases. It should be stressed here that the PL-QY is not low if we compare the value with those of other materials having luminescence in the NIR range. For example, the QY of codoped Si-NCs with 6.2 nm in diameter, which exhibit PL around 1150 nm, is 0.14%. This value is larger than that of a NIR luminescent dye, e.g., IR-26 (QY: ∼0.05%), exhibiting PL in the same energy range (∼1130 nm).41 Furthermore, the PL is more stable than NIR luminescent dyes. When the diameter is smaller than 3 nm, the PL lifetime and the PL-QY decreases as can be seen in Figures 9b and 9c, respectively. Since the shortening of the lifetime is accompanied by the decrease of the PL-QY, it is considered to be caused by the formation of nonradiative recombination processes due to disorders in or on the surface of Si-NCs. The shortening of the lifetime and the decrease of the PL-QY are significant when the size is smaller than 2 nm. This is probably due to the qualitative change of the structure from nanocrystals to nanoclusters and further introduction of nonradiative recombination routes.

Figure 9. (a) PL peak energy of codoped Si-NCs as a function of the diameter. The data of undoped Si-NCs obtained from ref 15 (△), ref 16 (◊), ref 35 (▽), ref 36 (○), and ref 37 (□) are also shown. (b) PL lifetime of codoped Si-NCs as a function of the diameter (▼). The data of undoped Si-NCs in SiO2 is also shown (○). (c) PL quantum yield of codoped colloidal Si-NCs as a function of NC diameter.

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Figure 9b shows the PL lifetimes of codoped and undoped Si-NCs as a function of the diameter. The lifetimes are obtained at the PL maxima. The size dependence of the lifetime is very much different between codoped and undoped Si-NCs. In undoped Si-NCs, the lifetime depends strongly on the size, while it is almost independent of the size in codoped Si-NCs when the size is larger than 3 nm. The insensitiveness of the lifetime on the size can be explained by the following model. In codoped Si-NCs, carriers are localized in the impurity states. The localization further relaxes the momentum conservation rule during the optical transition and enhances the radiative recombination rate compared to that of undoped Si-NCs. As a result, the physical size of NCs, which determine the degree of the relaxation of the momentum conservation rule and the radiative recombination rate in undoped Si-NCs, is not a decisive factor to determine the PL lifetime. In other words, in codoped Si-NCs, carriers are localized not only by potential barriers on the surface but also by potentials induced by impurities. When the size is relatively large, the latter one is a decisive factor to determine the PL properties including the lifetime. On the other hand, when the size is below a certain value, the former one becomes more important. Figure 9c shows the PL-QY as a function of the size of SiNCs. When the diameter is larger than 3 nm, the PL-QY decreases despite the almost constant PL lifetime as shown in Figure 9b. This implies that with increasing the average size the

CONCLUSION We have succeeded in synthesizing colloidal Si-NCs dispersible in polar solvents without functionalization by organic ligands and exhibiting PL in a very wide energy range from 0.85 to 1.80 eV. The key technology to achieve the very wide range controllable PL is the combination of P and B codoping and the formation of donor and acceptor states in the band gap and the size control in a wide range (1−14 nm in diameter). The highest PL-QY of 12.8% is obtained when the size is 3 nm. The PL of the colloids is very stable in laboratory atmosphere and is not at all affected by drop-casting on substrates and drying. The codoped colloidal Si-NCs reported here are promising for applications not only in optoelectronics but also in biology since the PL energies are in the biological window and they consist of low-toxic materials.

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ASSOCIATED CONTENT

S Supporting Information *

Optical transmittance spectra, photoluminescence spectra after correcting reabsorption by methanol, and photoluminescence decay curves of colloidal codoped Si-NCs; procedures to estimate the average diameters of NCs when Ta is low; photoluminescence quantum yields. This material is available free of charge via the Internet at http://pubs.acs.org. F

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clusters: Formation energies and electronic properties. Appl. Phys. Lett. 2005, 87, 1731200−1731203. (18) Iori, F.; Degoli, E.; Magri, R.; Marri, I.; Cantele, G.; Ninno, D.; Trani, F.; Pulci, O.; Ossicini, S. Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus. Phys. Rev. B 2007, 76, 085302−1−085302−14. (19) Iori, F.; Degoli, E.; Palummo, M.; Ossicini, S. Novel optoelectronic properties of simultaneously n-and p-doped silicon nanostructures. Superlattices Microstruct. 2008, 44, 337−347. (20) Fujii, M.; Toshikiyo, K.; Takase, Y.; Yamaguchi, Y.; Hayashi, S. Below bulk-band gap photoluminescence at room temperature from heavily P-and B-doped Si nanocrystals. J. Appl. Phys. 2003, 94, 1990− 1995. (21) Fujii, M.; Yamaguchi, Y.; Takase, Y.; Ninomiya, K.; Hayashi, S. Control of photoluminescence properties of Si nanocrystals by simultaneously doping n- and p-type impurities. Appl. Phys. Lett. 2004, 85, 1158−1160. (22) Fujii, M.; Yamaguchi, Y.; Takase, Y.; Ninomiya, K.; Hayashi, S. Photoluminescence from impurity codoped and compensated Si nanocrystals. Appl. Phys. Lett. 2005, 87, 2119190−2119193. (23) Fukuda, M.; Fujii, M.; Imakita, K.; Hayashi, S. Roomtemperature below bulk-Si band gap photoluminescence from P and B co-doped and compensated Si nanocrystals with narrow size distributions. J. Lumin. 2011, 131, 1066−1069. (24) Fukuda, M.; Fujii, M.; Sugimoto, H.; Imakita, K.; Hayashi, S. Surfactant-free solution-dispersible Si nanocrystals surface modification by impurity control. Opt. Lett. 2011, 36, 4026−4028. (25) Sugimoto, H.; M.; Fujii, M.; Imakita, K.; Hayashi, S.; Akamatsu, K. All-inorganic near-infrared luminescent colloidal silicon nanocrystals−high dispersibility in polar liquid by phosphorus and boron codoping. J. Phys. Chem. C 2012, 116, 17969−17974. (26) Nesbit, L. A. Annealing characteristics of Si-rich SiO2 films. Appl. Phys. Lett. 1985, 46, 38−40. (27) Hayashi, S.; Yamamoto, K. Optical properties of Si-rich SiO2 films in relation with embedded Si mesoscopic particles. J. Lumin. 1996, 70, 352−363. (28) Kanzawa, Y.; Hayashi, S.; Yamamoto, K. Raman spectroscopy of Si-rich films: possibility of Si cluster formation. J. Phys.: Condens. Matter 1996, 8, 4823−4835. (29) Kumar, V. Nanosilicon; Elsevier: Amsterdam, 2007. (30) Sze, S. M. Physics of Semiconductor Devices, 2nd ed.; Wiley: New York, 1981. (31) Sugimoto, H.; M.; Fujii, M.; Imakita, K.; Hayashi, S.; Akamatsu, K. Phosphorus and boron codoped colloidal silicon nanocrystals with inorganic atomic ligands. J. Phys. Chem. C 2013, 117 (13), 6807−6813. (32) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (33) Yvon, J. A guide to recording Fluorescence Quantum Yields; www.jobinyvon.com. (34) Bindhu, C. V.; Harilal, S. S.; Nampoori, V. P. N.; Vallabhan, C. P. G. Mod. Phys. Lett. 1999, B13, 563−576. (35) Jurbergs, D.; Rogojina, E.; Mangolini, L.; Kortshagen, U. Silicon nanocrystals with ensemble quantum yields exceeding 60%. Appl. Phys. Lett. 2006, 88, 2331160−2331163. (36) Takeoka, S.; Fujii, M.; Hayashi, S. Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime. Phys. Rev. B 2000, 62, 16820−16825. (37) Kanzawa, Y.; Kageyama, T.; Takeoka, S.; Fujii, M.; Hayashi, S.; Yamamoto, K. Size-dependent near-infrared photoluminescence spectra of Si nanocrystals embedded in SiO2 matrices. Solid State Commun. 1997, 102, 533−537. (38) Wolkin, M.; Jorne, J.; Fauchet, P.; Allan, G.; Delerue, C. Electronic states and luminescence in porous silicon quantum dots: the role of oxygen. Phys. Rev. Lett. 1999, 82, 197−200. (39) Iori, F.; Ossicini, S. Effects of simultaneous doping with boron and phosphorous on the structural, electronic and optical properties of silicon nanostructures. Physica E 2009, 41, 939−946. (40) Delerue, C.; Lannoo, M.; Allan, G.; Martin, E.; Mihalcescu, I.; Vial, J. C.; Romestain, R.; Muller, F.; Bsiesy, A. Auger and Coulomb

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Corresponding Author

*E-mail: [email protected] (M.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by KAKENHI (23310077, 24651143). REFERENCES

(1) Talapin, D.; Murray, C. PbSe nanocrystal solids for n- and pchannel thin film field-effect transistors. Science 2005, 310, 86−89. (2) Urban, J.; Talapin, D.; Shevchenko, E.; Kagan, C.; Murray, C. Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films. Nat. Mater. 2007, 6, 115−121. (3) Gur, I.; Fromer, N.; Geier, M.; Alivisatos, A. Air-stable allinorganic nanocrystal solar cells processed from solution. Science 2005, 310, 462−465. (4) Mcdonald, S.; Konstantatos, G.; Zhang, S.; Cyr, P.; Klem, E.; Levina, L.; Sargent, E. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat. Mater. 2005, 4, 138−142. (5) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 2005, 307, 538−544. (6) Cai, W. B.; Shin, D. W.; Chen, K.; Gheysens, O.; Cao, Q. Z.; Wang, S. X.; Gambhir, S. S.; Chen, X. Y. Peptide-labeled near-infrared quantum dots for imaging tumor vasculature in living subjects. Nano Lett. 2006, 6 (4), 669−676. (7) Erogbogbo, F.; Yong, K.; Roy, I.; Xu, G.; Prasad, P.; Swihart, M. Biocompatible luminescent silicon quantum dots for imaging of cancer cells. ACS Nano 2008, 2, 873−878. (8) Gupta, A.; Swihart, M.; Wiggers, H. Luminescent colloidal dispersion of silicon quantum dots from microwave plasma synthesis exploring the photoluminescence behavior across the visible spectrum. Adv. Funct. Mater. 2009, 19, 696−703. (9) Mangolini, L.; Kortshagen, U. High-yield plasma synthesis of luminescent silicon nanocrystals. Adv. Mater. 2007, 19, 2513−2519. (10) Mastronardi, M. L.; Hennrich, F.; Henderson, E. J.; Maier-Flaig, F.; Blum, C.; Reichenbach, J.; Lemmer, U.; Kübel, C.; Wang, D.; Kappes, M. M.; Ozin, G. A. Preparation of monodisperse silicon nanocrystals using density gradient ultracentrifugation. J. Am. Chem. Soc. 2011, 133, 11928−11931. (11) Pettigrew, K.; Liu, Q.; Philip, P.; Kauzlarich, S. Solution synthesis of alkyl- and alkyl/alkoxy-capped silicon nanoparticles via oxidation of Mg2Si. Chem. Mater. 2003, 15, 4005−4011. (12) Sato, S.; Swihart, M. Propionic-acid-terminated silicon nanoparticles: synthesis and optical characterization. Chem. Mater. 2006, 18, 4083−4088. (13) Beard, M.; Knutsen, K.; Yu, P.; Luther, J.; Song, Q.; Metzger, W.; Ellingson, R.; Nozik, A. Multiple exciton generation in colloidal silicon nanocrystals. Nano Lett. 2007, 7, 2506−2512. (14) Miller, J. B.; Van Sickle, A. R.; Anthony, R. J.; Kroll, D. M.; Kortshagen, U. R.; Hobbie, E. K. Ensemble brightening and enhanced quantum yield in size purified silicon nanocrystals. ACS Nano 2012, 6, 7389−7396. (15) Mastronardi, M. L.; Maier-Flaig, F.; Faulkner, D.; Henderson, E. J.; Kübel, C.; Lemmer, U.; Ozin, G. A. Size-dependent absolute quantum yields for size-separated colloidally-stable silicon nanocrystals. Nano Lett. 2011, 12, 337−342. (16) Hessel, C.; Reid, D.; Panthani, M.; Rasch, M.; Goodfellow, B.; Wei, J.; Fujii, H.; Akhavan, V.; Korgel, B. Synthesis of ligand-stabilized silicon nanocrystals with size-dependent photoluminescence spanning visible to near-infrared wavelengths. Chem. Mater. 2012, 24, 393−401. (17) Ossicini, S.; Degoli, E.; Iori, F.; Luppi, E.; Magri, R.; Cantele, G.; Trani, F.; Ninno, D. Simultaneously B-and P-doped silicon nanoG

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The Journal of Physical Chemistry C

Article

charging effects in semiconductor nanocrystallites. Phys. Rev. Lett. 1995, 75, 2228−2231. (41) Semonin, O. E.; Johnson, J. C.; Luther, J. M.; Midgett, A. G.; Nozik, A. J.; Beard, M. C. Absolute photoluminescence quantum yields of IR-26 dye, PbS, and PbSe quantum dots. J. Phys. Chem. Lett. 2010, 1, 2445−2450.

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