or Phosphorus

Article pubs.acs.org/JPCC

Atom Probe Tomography Analysis of Boron and/or Phosphorus Distribution in Doped Silicon Nanocrystals Keita Nomoto,† Hiroshi Sugimoto,‡ Andrew Breen,§ Anna V. Ceguerra,§ Takashi Kanno,‡ Simon P. Ringer,∥ Ivan Perez Wurfl,† Gavin Conibeer,*,† and Minoru Fujii*,‡ †

School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia ‡ Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe, 657-8501, Japan § Australian Centre for Microscopy and Microanalysis, and School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia ∥ Australian Institute for Nanoscale Science and Technology, and School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: Silicon nanocrystals (Si NCs) are intensively studied for optoelectronic and biological applications due to having highly attractive features such as band engineering. Although doping is often used to control the optical and electrical properties, the related structural properties of solely doped and codoped Si NCs are not well-understood. In this study, we report the boron (B) and/or phosphorus (P) distribution in Si NCs embedded in borosilicate glass (BSG), phosphosilicate glass (PSG), and borophosphosilicate glass (BPSG) using atom probe tomography (APT). We compared solely and codoped Si NCs grown at different temperatures so that we may compare the effects of codoping and temperature on the B and/or P distribution. Proximity histograms and cluster analyses reveal that there exist boron-rich layers surrounding Si NCs and also B−P clusters within the Si NCs. Raman spectra also show a structural change between codoped Si NCs in solids and free-standing codoped Si NCs. These results lead us to understand that codoped Si NCs disperse in polar solvents.

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INTRODUCTION Semiconductor nanocrystals (NCs) have attracted great interest for potential optoelectronic and biological applications such as thin film transistors,1 light emitting diodes,2 solar cells,3 and bioimaging tools for live cells,4,5 by employing the quantum confinement effect.6 Colloidal silicon (Si) NCs is one candidate that has major advantages because Si (1) is abundant in nature, which can minimize environmental damage to the earth, (2) is favorable for low-cost devices, and (3) is suitable for biomedical applications due to its nontoxicity as a material. Fabrication methods have been developed in the past few years to control Si NCs size, photoluminescence (PL) efficiency, and high dispersibility in solution.7−13 However, understanding the surface science of Si NCs is required to further control the optoelectronic properties and/or solution dispersibility. For example, Si NCs surfaces have dangling bonds induced by strain between Si NCs and the matrix, thus degrading the optoelectronic properties,14 so that hydrogen passivation is used to inactivate the surface defect of Si NCs.15 The function of dopants is another consideration for the fabrication of Si © 2016 American Chemical Society

NCs and their enhanced properties. Dopants such as boron (B) and phosphorus (P) play a key role not only for a fabrication of p- and n-type NCs but also for the surface treatment of Si NCs, which result in the enhancement of PL intensity and efficiency.16−18 To fabricate colloidal NCs, organic and inorganic ligands are used to functionalize the surface of NCs which causes monodispersity in polar solvents.13,19 However, there are disadvantages to this method. One disadvantage is that the long-chain organic ligands (typically 0.4−1.0 nm) degrade carrier mobilities and conductivity, since the width of tunneling barrier between NCs increases. 20,21 Another disadvantage is that an exchange of organic ligands with inorganic ions is not suitable for Si NCs because Si NCs form covalent bonds with organic ligands on Si NCs surfaces,22,23 despite having been successfully demonstrated as having high dispersity in solution for colloidal NCs.19 Received: June 20, 2016 Revised: July 25, 2016 Published: July 27, 2016 17845

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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Figure 1. Three-dimensional reconstructions of solely and codoped Si NCs in (a) BSG (b) PSG, and (c) BPSG annealed at 1150 °C. Sliced images (box size: 24 nm × 24 nm × 7 nm) are shown below each tip (top view).

detector, and a back projection algorithm enables the atoms to be reconstructed in three dimensions (3D). The time-of-flight measurements enable the mass-to-charge ratio of the ions to be identified.32 Khelifi et al. had used the results of APT to show the incorporation of As and P atoms into Si NCs.34 Gnaser et al. demonstrated P distribution around Si NCs in an oxynitride matrix and showed the enrichment of P at the interfaces.35 To our knowledge, there are yet to be any published studies that characterizes the structure of B and P codoped Si NCs by APT. In this study, we studied several types of Si NCs using APT and Raman spectroscopy. First, we analyze the structure of solely and codoped Si NCs using APT. The distribution of B and/or P around Si NCs is revealed by proximity histogram (proxigram) analysis,36 and the existence of B−P clusters is confirmed by cluster analysis.37 By comparing the behavior of B and P between solely doped Si NCs in borosilicate glass (BSG), phosphosilicate glass (PSG), and codoped Si NCs in borophosphosilicate glass (BPSG) grown at different annealing temperatures, we show how the B and P distribution and their incorporation into the Si NCs is affected by codoping and annealing temperature. Finally, we study the structure between codoped Si NCs in solids and free-standing codoped Si NCs using Raman spectroscopy.

Previously, we had developed a new and cost-effective method to fabricate Si NCs using a B and P codoping technique.24−30 This method is a simple procedure using sputtering, annealing, and HF etching. The Si NCs are stable in polar solvents without any surface treatments by ligands. Moreover, our Si NCs have a wide range of luminescence energies typically from 0.9−1.8 eV, including near-infrared range that is below the band gap of bulk Si (1.12 eV). This phenomenon can be explained by the formation of the donorto-acceptor transitions by B and P codoping,24,31 giving another reason why codoping is important. It is hypothesized that, for the codoped Si NCs samples, the B-rich region at the interface is paired with P and, if the outer B has negative potential, this enables the Si NCs surfaces to become hydrophilic.26−30 To verify this model, a direct insight into the distribution of B and P atoms in Si NCs is essential. Atom probe tomography (APT) enables the visualization of the structure of Si NCs as well as dopant distribution with subnanometer resolution.32,33 For semiconductor materials, a high voltage assisted with pulsed laser is applied to a sharp needle-shaped tip (typically less than 50 nm in diameter at apex) to achieve an electric field high enough to evaporate atoms from the surface of the tip (∼33 V nm−1 is required to field-evaporate pure Si at temperatures near absolute zero). The field-evaporated atoms are projected onto a position-sensitive 17846

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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EXPERIMENTAL PROCEDURE Samples were either doped solely with B or P, or codoped with both B and P. The solely and codoped Si NCs were fabricated by a cosputtering method as described in our previous papers.24−26 For codoped Si NCs in BPSG, Si, SiO2, B2O3, and P2O5 were simultaneously sputtered on a quartz substrate in Ar atmosphere. The composition of SiOx (including doped samples) was in the range between x = 1.45 and 1.55 for all samples,26 and the film thickness was ∼2.5 μm. The Si-rich BPSG films were annealed at 1050, 1150, and 1250 °C in N2 for 30 min. This enables the formation of Si NCs in BPSG matrixes.24 Similarly, for solely doped Si NCs in BSG or PSG, either B2O3 or P2O5 was used to dope the matrix and annealed at 1150 °C in the same manner. From previous work, the B and P concentration was estimated at 0.75 and 1.9 atom %, respectively.25,26 For APT specimen preparation, sharp needle-shaped tips in each sample were prepared using a Zeiss Auriga focused ion beam scanning electron microscope (FIB-SEM). Approximately 300 nm layer of Pt was deposited onto the surface of the samples to protect the film from Ga ions damage during focused ion beam milling. The film, including the substrate, was vertically attached onto the apex of a Mo support grid using the standard lift-out procedure.38 The film is oriented such that it is parallel to the long axis of the tip. By this method, we were able to collect atomic-scale data from the tip up to a high voltage without sample fracture as observed by Heck et al.39 The APT measurements were performed using a LEAP 4000XSi (CAMECA) with a pulsed UV laser (λ = 355 nm) and a detector efficiency of 0.57. The chamber pressure was within the range of 10−12−10−11 Torr. The laser pulse energy was set to 100 pJ at a pulse rate of 250 kHz. The specimen holder was cooled to ∼40 K. The APT data was then reconstructed, and the proxigram36 and cluster analysis37 were performed using the commercially available IVAS software (version 3.6.6). Raman spectra were taken at room temperature using a confocal microscope (50× objective lens, NA = 0.8) equipped with a single monochromator and a charge-coupled device. A 514.5 nm wavelength Ar+ laser was used as the excitation source.

compared to solely doped samples. This B and P enrichment is more prominent when the annealing temperature is increased from 1050 to 1250 °C (Supporting Information, Figure S2, for 3D reconstructions of codoped Si NCs in BPSG annealed at 1050 and 1250 °C). Assuming the Si NCs morphologies are close to spherical, the mean diameter is derived from the volume of the Si NCs, whose boundaries were defined above, and summarized in Figure 2. (See Supporting Information in Figure S3 for the

Figure 2. Mean diameter of the measured Si NCs between solely and codoped Si NCs as a function of annealing temperature. The error bars represent the standard deviation.

histogram of the size distribution for all samples.) Table S1 in Supporting Information lists the number density of Si NCs. For codoped samples, the trend of the decrease in the number density can be correlated with increasing the annealing temperature. This indicates that neighboring Si NCs may become connected, resulting in the increase in the size of the individual Si NCs. In fact, at the annealing temperature of 1150 °C, the Si NCs size of codoped samples is larger than those of solely doped ones. Although the effect of dopants on the formation and evolution of Si NCs is not well-understood, it can be observed that doped Si NCs are often larger than undoped NCs despite using the same annealing procedure. This difference in Si NCs size is thought to be due to dopants causing a “softening” the oxide matrix and accelerating the diffusion of Si and/or O.24 When B and P atoms are codoped in the sample, this softening of the matrix may be enhanced. As the annealing temperature increases at a given annealing time for codoped samples, the Si NCs size increases, which is simply explained by an increase in the diffusion length. Note that the morphology of the APT reconstructions, and therefore the NCs contained within, are influenced by user-defined reconstruction parameters, including but not limited to the image compression factor (ICF) and field factor (kf), so absolute size measurements must be treated with caution.32 If sufficient crystallographic information is present within the reconstruction, a calibration procedure may be used, but this was not possible in the collected data sets, so the default ICF (1.65) and kf (3.3) were used. However, the standard reconstruction algorithm, which was used here, is volume conserving, so the calculated diameter should be fairly constant regardless of the reconstruction parameters used. It is also worth mentioning

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RESULTS AND DISCUSSION Figure 1 provides 3D reconstructions of solely and codoped Si NCs in solids annealed at 1150 °C. Sliced images of 7 nm thickness in the z-direction are also shown below each tip. The Si NCs interfaces can be qualitatively visualized using isoconcentration surfaces that create bounding volumes that contain at least 57 atom % Si. To define the Si NC boundary, an iterative process was used to determine the concentration for the isoconcentration surface such that the Si and O proxigram profiles intersected at 0 nm. The isoconcentration surfaces were defined with a voxel size of 1 nm and a delocalization value of x = 3 nm, y = 3 nm, z = 3.5 nm. The mass spectra used in the reconstruction are shown in Supporting Information (Figure S1).The reconstructions indicate that the Si NCs were well-separated from each other in 3D, despite the Si-rich oxide film being deposited by cosputtering of Si and SiO2 targets, which is different from creating a multilayer structure of Si-rich oxide and SiO2 films that is known to create well-distributed Si NCs.40,41 In the sliced images, the codoped sample has more B and P enrichment at the position where the local Si density is high 17847

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The Journal of Physical Chemistry C that the reconstructed data includes an artifact called local magnification.42 In the case of the Si and SiO2 interface, it has been observed in this study that Si atoms have a higher probability of being evaporated from the surface of the specimen compared to SiO2, due to the lower evaporation field of Si. This causes atoms at the interface to be projected inward, and the reconstruction has an increase in the measured local density.43 Considering HF etching is used to fabricate free-standing Si NCs, which normally removes the outer surface of Si NCs, the actual Si NCs size in solids should be larger than that obtained in this work. However, the trend of Si NCs size dependence on annealing temperatures is in good agreement with the results from observations of the free-standing Si NCs.29 To investigate the B and P distribution inside and outside of the Si NCs, a proxigram analysis on solely and codoped Si NCs was performed as shown in Figure 3. The profiles were calculated for a number of Si NCs as shown in Figure 1. Note that the profiles were almost the same even when the proxigram analysis was performed for a variety of Si NCs size ranges (Figure S4 in Supporting Information). The distance at 0 nm represents the interface between the Si NC and the matrix. The positive distances represent the inside of the Si NC, while the negative distances represent the matrix. The large error bar at the center of the Si NCs is caused by the reduced atom counts toward the center of the Si NCs and farther from the interface. The incorporation of B and P into the NCs is clearly detected for all samples and annealing conditions; however, significant differences in the dopant distribution profiles can be observed. For the sample that was solely doped with P and annealed at 1150 °C, the P concentration increases rapidly from approximately 1 nm outside the NCs interfaces and into the Si NCs core. However, in the sample that was solely doped with B and annealed under the same conditions, the B concentration slightly decreases after the Si NCs interfaces toward the core. For the codoped samples, the P concentration still increases rapidly from approximately 1 nm outside the interface at the lowest annealing temperature of 1050 °C, becoming more pronounced at the next annealing condition of 1150 °C, and eventually starts to concentrate approximately 1.5 nm inside the interface at the highest annealing condition of 1250 °C. The change in B concentration is weaker and appears to penetrate less into the Si NCs core, accumulating at the interface region, an effect which becomes more pronounced at higher annealing temperatures. Similar B and P distribution behaviors have been reported on doped Si nanowires (Si NWs) in SiO2, showing B is more likely to be located in the surface oxide layer, while P prefers to remain inside the Si region around the interface of Si NWs.44 The B enrichment of codoped samples supports the model that the B-rich layers surround the Si NCs and these act as a resistance to HF etching in the process of making free-standing Si NCs.30 Figure 4 compares the extracted B and P concentration values between the peak (near interface) and the matrix (at −2 nm) obtained by the proxigram analysis in Figure 3. Interestingly, it is found that the peak B and P concentrations increase while the concentrations in the matrix decrease not only by annealing temperature but also by codoping. Codoping, when compared to solely doped samples, therefore appears to be an effective means of promoting segregation of the B and P atoms near the interface and within the NC regions. Segregation is also enhanced by higher annealing temperatures. Ossicini and co-workers have simulated that P atoms are stable

Figure 3. Proxigram analysis on solely and codoped Si NCs in (a) BSG, (b) PSG, and (c−e) BPSG annealed at 1050, 1150, and 1250 °C, respectively. The model images of the B and/or P distribution are shown at the top of each figure. B and P are represented in green and red, respectively. The error bars represent a statistical error of 1σ.

at the Si NC core in the case of solely doping, whereas in the case of codoping with additional B atoms, P and B prefer to be located together in more energetically stable pairs near the surface of the Si NC.31,45,46 Fukata et al. have also shown that B can be located at the Si side of an interface between the Si NW and the oxide matrix, by pairing with P atoms in codoped Si NWs.47 A cluster analysis was performed for codoped samples to find B and P paired clusters (B−P clusters) in Si NCs as shown in Figure 5. The maximum distance to detect clusters was defined to be 0.25 nm, with the error bars defined by changing dmax to ±0.02 nm of this value. This is close to B−P bond length (0.232 nm).45 The percentage of B−P clusters, B−B clusters, P−P clusters, isolated B, and isolated P in the Si NCs was calculated by counting B and P atoms in clusters if they were 17848

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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the annealing temperature increases. While there is no remarkable change for the B−B clusters with annealing temperature, the P−P clusters shows the same trend as the B−P clusters. The isolated B and P atoms decreases with increasing annealing temperature, implying that more B and P atoms are consumed to form B−P and P−P clusters as annealing temperature increases. This finding for B−P clusters provides an important clue for the model that the B−P pairs form the donor and acceptor state. This induces low PL energy below the bulk Si and also acts as a negatively charged surface on Si NCs, which enables the Si NCs to disperse in polar solvents.28,29 As the Si NCs size is widely distributed (Supporting Information, Figure S3), studying the B and P concentration depending on the Si NCs size is of interest to further understand the behavior of B and P atoms in codoped Si NCs. Individual Si NCs with solely and codoped Si NCs annealed at 1150 °C are shown in Figure 6. The representative Si NCs were

Figure 4. Extracted B and P concentrations from the proxigram analysis in Figure 3. The peak concentration is extracted near the interfaces, and the concentration in matrix is extracted the value at −2 nm. The error bars represent a statistical error of 1σ.

Figure 6. Individual Si NCs in (a) BSG, (b) PSG, and (c) BPSG annealed at 1150 °C. The diameter of each sample is (a) 3.0, (b) 3.0, and (c) 4.0 nm, respectively. Figure 5. Cluster analysis to find B−P clusters, B clusters, P clusters, isolated B, and isolated P in Si NCs. The error bars represent the standard deviation.

selected within ±0.3 nm of the average diameter of each sample. Other individual Si NCs with different diameters (down to 2 nm) and annealing temperatures are also shown (Supporting Information, Figure S5). It was confirmed that the B and P atoms were successfully incorporated into the Si NCs even for small Si NCs in a range of 2−4 nm in diameter. Table 1 summarizes the probability of finding B and/or P atoms in Si NCs, as well as the different size ranges of Si NCs (less than 4, 3, and 2 nm in diameter). In this case, if either B or P was detected in a particular Si NC, that Si NC was counted as “solely B-doped” or “solely P-doped”, while if both B and P were detected, that Si NC was counted as “B/P codoped”. If neither B nor P was detected, that Si NC was counted as “undoped”. Similar to the results mentioned above, the ratio of codoped Si NCs increases by increasing annealing temperature.

clustering, or by counting isolated B or P atoms if they were randomly distributed, and then dividing the counted number by the total number of B and P atoms in all Si NCs. Although these atoms should be located at a substitutional site or interstitial site, another technique such as electron paramagnetic resonance (EPR)48 would be required to study the doping efficiency and is outside the scope of this paper. The cluster analysis found that approximately 8−14% of B and P atoms were consumed to form B−P clusters in Si NCs. The graph shows that the number of B−P clusters increases as the annealing temperature increases; this is thought to be due to the formation energy for B−P clusters in Si NCs reducing as 17849

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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The Journal of Physical Chemistry C Table 1. Probability of Finding B and/or P Atoms in Si NCsa size range of Si NCs sample name

dopant in Si NCs

all NCs (%)

d ≤ 4 nm (%)

d ≤ 3 nm (%)

d ≤ 2 nm (%)

BSG 1150 °C

solely B-doped undoped solely P-doped undoped solely B-doped solely P-doped undoped B/P codoped solely B-doped solely P-doped undoped B/P codoped solely B-doped solely P-doped undoped B/P codoped

95.0 5.0 98.4 1.6 1.0 4.9 0.3 93.8 0.0 2.2 0 97.8 0 0 0 100

93.7 6.3 98.3 1.7 1.1 5.3 0.4 93.2 0.0 4.7 0 95.3 0 0 0 100

89.8 10.2 97.4 2.6 1.5 7.7 0.5 90.2 0.0 9.8 0 90.2 0 0 0 100

76.6 23.4 92.3 7.7 4.1 20.5 1.4 74.0 0.0 17.6 0 82.4 N/Ab N/Ab N/Ab N/Ab

PSG 1150 °C BPSG 1050 °C

BPSG 1150 °C

BPSG 1250 °C

a

The different Si NCs size ranges are provided to be less than 4, 3, and 2 nm. bThere were no Si NCs in this range.

In Figure 7, we show the B and P concentration of solely and codoped samples as a function of the Si NCs diameter. The B

Table 2. Average B and P Concentration in Si NCs, in the Matrix, and the Whole Tip (Nominal Concentration) of Solely and Codoped Samples av concn dopant

sample

B

BSG BPSG

P

PSG BPSG

annealing temp (°C)

NCs (%)

matrix (%)

whole tip (%)

1150 1050 1150 1250 1150 1050 1150 1250

1.0 1.2 1.5 1.7 2.9 3.1 4.0 4.4

1.1 1.3 1.2 0.9 1.7 1.9 1.6 1.1

1.1 1.3 1.3 1.0 1.8 2.0 2.0 1.7

temperature, the result of which is consistent with what we already mentioned above. There is a difference in the B and P concentration between codoped Si NCs in solids and free-standing codoped Si NCs. For instance, the B concentration of codoped Si NCs in solids (1.2−1.7 atom %) is much lower than that of free-standing codoped Si NCs (10−15 atom %).29 One possibility might be due to undetected B counts in a multiple event as demonstrated in the literature.49 Assuming only 50% of B atoms are successfully detected, the corrected B concentration in Si NCs is approximately 2.4−3.4 atom %, which is still lower than the measured B concentration in the free-standing codoped Si NCs.29 This discrepancy between codoped Si NCs in solids and free-standing codoped Si NCs is probably due to the structural change in the Si NCs by HF etching. To test this, we measured the Raman spectra of these samples. Figure 8 shows Raman spectra of codoped Si NCs in solids annealed at 1150 °C and free-standing codoped Si NCs after HF etching. The signal from Si NCs can be identified at 520 cm−1 for both samples. In addition, the B-related local vibrational modes in Si crystal can be observed as a broad peak around 650 cm−1 for both samples.30 However, the P-related local vibrational mode in the Si crystal, the broad peak around 441 cm−1,30 is only observed for codoped Si NCs in solids. This indicates that P-rich layers are removed during HF etching, which results in the decrease in

Figure 7. B and P concentration of solely and codoped samples as a function of Si NCs diameter. The error bars represent the standard deviation.

and P concentration may increase as the Si NCs size decreases; however, it is difficult to find out the clear trend depending on the Si NCs size due to the large error bars. Here, the average concentration apparently shows different concentration levels between the samples. Table 2 summarizes the average B and P concentration in Si NCs, in the matrix, and in the whole tip (nominal concentration). Even though the nominal concentration is almost the same, the average B and P concentration in Si NCs increases by codoping or by increasing the annealing 17850

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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The authors declare no competing financial interest.

P concentration and relative increase in B concentration after HF etching in free-standing codoped Si NCs. APT measurements on the free-standing codoped Si NCs are necessary to further understand the structural and concentration change between codoped Si NCs in solids and free-standing codoped Si NCs.

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REFERENCES

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CONCLUSION The B and/or P doped Si NCs have been studied using APT and Raman spectroscopy. APT proxigram analysis showed B enrichment around the interface between Si NCs and the matrix for codoped Si NCs in BPSG and found that codoping is an effective means of promoting segregation and stability of the B and P atoms to the Si NC regions. APT cluster analysis found the presence of B−P clusters in the codoped Si NCs. These findings verify the model that the B−P pairs form donor and acceptor states, inducing low PL energy below the bulk Si, and that these pairs also act as a negatively charged surface on Si NCs that enables the Si NCs to disperse in polar solvents. The difference in the B and P concentrations in the codoped Si NCs in solids and free-standing codoped Si NCs was investigated using Raman spectroscopy which showed the removal of the Prelated species from the Si NCs surface. In addition, the Raman spectra showed no significant change for B-related local vibrational mode showing B-rich layers were not removed by HF etching. The APT and Raman spectroscopy findings in this work give us more confidence about the role of B enrichment and B−P clusters which act as the resistance to HF etching and the dispersibility in polar solvents. This will have implications in the fabrication of free-standing Si NCs in polar solvents using our method. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06197. Additional APT data of solely and codoped Si NCs in solids (PDF)

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ACKNOWLEDGMENTS

This research has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Australian Centre for Microscopy and Microanalysis at the University of Sydney. This research was supported by the Faculty of Engineering and Information Technologies, The University of Sydney, under the Faculty Research Cluster Program. This work is partly supported by 2015 JST Visegrad Group (V4)−Japan Joint Research Project on Advanced Materials, and KAKENHI (16H03828). The authors thank T. Hartley for creating the model images in Figure 3 in this work.

Figure 8. Raman spectra of codoped Si NCs in solids and freestanding codoped Si NCs after HF etching.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +61-2-9385-7858. *E-mail: [email protected]. Phone: +81-78-803-6081. 17851

DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852

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DOI: 10.1021/acs.jpcc.6b06197 J. Phys. Chem. C 2016, 120, 17845−17852