Energy Transfer in Silicon Nanocrystal Solids Made from All-Inorganic

Letter pubs.acs.org/JPCL

Energy Transfer in Silicon Nanocrystal Solids Made from AllInorganic Colloidal Silicon Nanocrystals Kenta Furuta, Minoru Fujii,* Hiroshi Sugimoto, and Kenji Imakita Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: Energy transfer between silicon (Si) nanocrystals (NCs) in Si-NC solids was demonstrated by photoluminescence (PL) spectroscopy. Clear differences of PL spectra and the decay rates between solutions and solids of Si-NCs were observed. The change in the PL properties caused by the formation of solids could be explained by the energy transfer from small to large NCs in the size distribution. In order to obtain further evidence of NC-to-NC energy transfer, the size distribution was intentionally modified by mixing solutions of NCs with different size distributions. NC solids made from the mixed solutions exhibited significantly different PL spectral shape and decay rates from those made from unmixed solutions, providing clear evidence of NC-to-NC energy transfer in Si-NC solids.

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and properly controlling it are very important to design semiconductor NC-based optoelectronic devices. In direct band gap semiconductor NCs such as CdS, CdSe, CdTe, PbS, PbSe, etc., strong modification of photoluminescence (PL) spectral shape27−33 and shortening of the lifetime34−36 by the formation of dense NC solids have been reported, and these phenomena are explained by FRET between NCs. On the other hand, direct evidence of energy transfer has scarcely been reported in Si-NCs, although there have been many reports that suggest the possibility of energy transfer indirectly from the analysis of the PL decay dynamics.18,37−41 The rate of FRET process is proportional to the oscillator strengths of energy donors and acceptors, the degree of the spectral overlap, and the sixth root of the distance. Owing to the small oscillator strength of Si-NCs due to the indirect nature of the energy band structure, the critical NC−NC distance for FRET is expected to be much smaller than that of direct band gap semiconductor NCs. Therefore, organic ligands on the surface of Si-NCs or even thin native oxides can obscure FRET between NCs effectively. In order to make energy migration between Si-NC by FRET active, very dense Si-NC solids with extremely short NC-to-NC distances are required. Another crucial requirement for the observation of FRET is the formation of a solution in which Si-NCs are perfectly dispersed without any agglomeration. If NCs form agglomerates in solution, FRET occurs even in solution, and no differences are

ilicon (Si) nanocrystals (NCs), i.e., nanometer-size crystallites of Si, have been a subject of intensive research for several decades as materials for future electronic devices and biomedical applications,1,2 because of their high environmental friendliness and the high compatibility with semiconductor manufacturing processes. For electronic device applications, e.g., light-emitting diodes, 3,4 solar cells,5−7 field effect transistors,8 etc., different kinds of Si-NC-based structures have been developed. One of the most widely studied structures is thin films of dielectric9,10 and wide band gap semiconductors such as SiC,11,12 in which Si-NCs are randomly dispersed. Multilayered structures consisting of Si-NC layers and spacer layers are also developed to achieve higher Si-NC density and better size control due to restriction of NC growth within gaps between spacers.13−18 Another type of Si-NC-based structures extensively studied is Si-NC solids, in which Si-NCs are densely packed and three-dimensional lattices of Si-NCs are formed.4,6−8,19 In general, optical properties of NC-based solid structures are different from those of individual NCs due to different dielectric environment of NCs and due to interactions between NCs. Modification of dielectric environment of NCs results in the enhancement of light absorption cross sections20 and emission rates.21,22 If NCs are very closely packed and the wave functions are overlapped, formation of mini-bands is expected.23,24 Even if the NC-to-NC distance is too large for the direct wavevfunction overlap, Förster resonant energy transfer (FRET) could be possible between NCs in close proximity. The Förster process promotes exciton migration between NCs and modifies the light emission properties.25−27 Therefore, understanding Förster energy transfer between NCs © XXXX American Chemical Society

Received: May 22, 2015 Accepted: June 25, 2015

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dynamics are measured by using a near-infrared (NIR) photomultiplier (R5509−72, Hamamatsu Photonics) and a multichannel scalar (SR430, Stanford Research). The excitation source is modulated 405 nm light. All the measurements are performed at room temperature. Figure 1a,b shows photographs of methanol solutions of SiNCs grown at 1050 and 1200 °C, respectively. Hereafter, we

expected to be observed in PL spectra of NC solutions and solids. Recently, we have succeed in producing Si-NCs that can be dispersed in polar solvents without any surface functionalization processes, i.e., without organic capping.42,43 The all-inorganic Si-NCs are very stable in alcohol, and no agglomerates and precipitates are observed for more than several years. In the SiNCs, B and P are very heavily doped and high B and P concentration shells are formed on the surface.44 The shell induces negative potential on the surface and prevents agglomeration of NCs in polar solvents by the electrostatic repulsion. Because of the agglomeration-free perfect dispersion of NCs in solution, very uniform Si-NC solids can be produced by spin-coating.45 In the NC solids, the distance between adjacent NCs is expected to be much smaller than those made from conventional organic-functionalized Si-NCs, and thus a larger energy transfer rate is expected. The purpose of this work is to provide direct evidence of energy transfer between Si-NCs in Si-NC solids made from B and P codoped colloidal Si-NCs. Thanks to the perfect dispersion of codoped Si-NCs in solution, energy transfer between NCs is not expected in solution. Therefore, the PL spectrum of colloidal Si-NCs is a convolution of the size distribution and the size-dependent PL quantum efficiency of individual NCs. On the other hand, in densely packed Si-NC solids, energy transfer from larger band gap NCs, i.e., smaller NCs, to smaller band gap NCs, i.e., larger NCs, in size distributions is expected. The energy transfer results in the enhancement of PL from larger NCs and quenching of that from smaller NCs. This appears as a low-energy shift of the PL from NC solids compared to that of NC solutions. Furthermore, quenching of PL from smaller NCs, i.e., highenergy tail of the spectrum, due to nonradiative FRET, is accompanied by the shortening of the lifetime. Therefore, observation of these phenomena is a direct proof of NC-to-NC energy transfer in Si-NC solids. In this work, we first compare PL properties of solutions and solids of B and P codoped Si-NCs for two series of samples with different size distributions of NCs and demonstrate the evidence of FRET between NCs in NC solids. We then intentionally modify the size distributions of NCs by mixing samples with different sizes of NCs to obtain further evidence of FRET. We also discuss the effect of dielectric environments in the analyses of PL properties of Si-NC solutions and solids. B and P codoped Si-NCs are prepared by the procedure described in refs 42 and 46. First, Si-rich borophosphosilicate glass (BPSG) films are prepared by a radio frequancy (rf) sputtering method. In order to grow B and P codoped Si-NCs in BPSG films, sputter-deposited films are annealed in N2 gas atmosphere at 1050 and 1200 °C for 30 min.47,48 Codoped SiNCs were then extracted from matrices by HF (46 wt %) etching and finally dispersed in methanol. The results of detailed analyses of codoped colloidal Si-NCs by infrared absorption, Raman, X-ray photoelectron and inductively coupled plasma atomic emission spectroscopy are shown in our previous papers.42−44 Si-NC solids are prepared by dropcasting the solution onto fused silica plates (7.5 × 7.5 mm2) and dried in air at room temperature. Size distributions of NCs are obtained by transmission electron microscope (TEM) observations (JEM-2100F, JEOL) operated at 200 keV. PL spectra are measured by spectrophotometer (Fluorolog-3, Horiba Jovin Yvon). The excitation source is monochromatized 325 nm light from a Xe lamp (3.2 mW/cm2). PL decay

Figure 1. (a,b) Photographs of methanol solutions of Si-NCs grown at 1050 °C (C1050) and 1200 °C (C1200). (c,d) SEM images of S1050 and S1200. (e,f) TEM images of Si-NCs grown at 1050 and 1200 °C, and size distributions obtained from the TEM images.

designate these colloidal solutions as C1050 and C1200, respectively. The Si concentration in the solutions is 0.88 and 0.91 mg/mL for C1050 and C1200, respectively. The solutions are very clear because of perfect dispersion of NCs. No agglomerates are observed even after a few years since production. The difference of the color between Figure 1a,b is mainly due to the difference in the size of NCs.49 Figure 1c,d shows scanning electron microscope (SEM) images of NC solids prepared from C1050 and C1200, respectively. Dense films with very smooth surfaces are formed. The thicknesses are about 350 nm. Hereafter, we designate these NC solids as S1050 and S1200, respectively. Figure 1e,f shows TEM images taken for samples produced by drop-casting C1050 and C1200, respectively, after dilution on carbon-coated TEM meshes. The size distributions obtained by TEM images in wider regions are shown below the images. The average diameter and the standard deviation are 2.8 and 1.1 nm, respectively, in Figure 1e, and 7.3 and 1.5 nm, respectively, in Figure 1f. 2762

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The Journal of Physical Chemistry Letters Figure 2 shows normalized PL spectra of solutions (solid curves) and solids (dashed curves) of Si-NCs grown at 1050

Figure 2. Normalized PL spectra of solutions (solid curves) and solids (dashed curves) of Si-NCs grown at 1050 and 1200 °C.

Figure 3. (a−c) PL decay curves of solution and solid of Si-NCs grown at 1200 °C detected at 1.0, 1.3, and 1.6 eV, respectively. (d) Decay rates of solution and solid of Si-NCs grown at 1200 °C. Normalized PL spectra of solution and solid are also shown.

and 1200 °C. In order to excite NCs in solutions uniformly, the solutions are diluted about 10 times from those shown in Figure 1a,b. The PL properties of B and P codoped Si-NCs have been studied in detail previously.49 The PL energy depends strongly on the size and increases with decreasing the size. The PL energy of codoped Si-NCs is always 200 to 300 meV smaller than that of undoped Si-NCs with identical sizes, and the lifetime is shorter than that of undoped ones. From the low-energy shift of the PL by doping, donor and acceptor states are considered to be involved in the optical transition.49 The PL quantum efficiency in solutions is about 10% when the size is about 3 nm (grown at 1050 °C). It decreases gradually to smaller size and rapidly to larger size.49 In Figure 2, C1050 exhibits PL around 1.7 eV, while C1200 exhibits it around 1.2 eV. The size dependence of the PL energy is consistent with the previous work.49 The PL spectra are very broad, with a full width at half-maximum (fwhm) of about 350 nm. The large width is mainly due to inhomogeneous broadening caused by the size distributions. In both series of samples, the spectra shift about 80 meV to lower energy by the formation of solids. In other word, the high-energy tail of the PL, where smaller NCs in the size distribution contribute, is quenched, while the lowenergy tail, where larger NCs in the size distribution contribute, is enhanced. An explanation of the low energy shift is reabsorption of emitted photons in solids because of higher density of NCs in solids than in solutions. However, the Si-NC solids are almost transparent around the emission wavelengths (transmittance: 72.2% at 715 nm (S1050), 82.3% at 995 nm (S1200)), and thus reabsorption is expected to be negligibly small. Reabsorption and FRET can be distinguished from PL decay dynamics. If the PL quenching at the high-energy tail is due to reabsorption of photons emitted by small NCs by large NCs, the PL lifetime is not affected. On the other hand, if the quenching is due to FRET, PL quenching should be accompanied by the shortening of the lifetime. Figure 3a−c compares PL decay curves of C1200 and S1200 detected at 1.0, 1.3, and 1.6 eV, respectively. In all the detection energies, the lifetime is significantly shortened by the formation of solid. The shortening is the most significant at the high energy trail of the PL spectrum (1.6 eV). In Figure 3d, the PL decay rates, i.e., inverse of the lifetimes, are plotted as a function of the detection energy together with the PL spectra. The lifetimes are average lifetimes defined as τave = τβ−1Γ(β−1), where Γ is the Euler gamma function, and τ and β are a decay constant and a stretching parameter, respectively, obtained by fitting a decay curve with a stretched exponential function, I = I0 exp(−t/τ)β.50 The decay rate of the solution

increases with increasing the detection energy. This is consistent with previous work10,49,51 and can be explained by better overlap of electron and hole wave functions in momentum space. The decay rates of the solid are much larger than those of the solution. In particular, the decay rate is significantly different at the high-energy tail of the PL spectrum. This is strong evidence of FRET from smaller to larger NCs in Si-NC solids; excitons in smaller NCs transfer energy to larger NCs by FRET and recombine nonradiatively, resulting in quenching of the high-energy side of the spectrum. Similar results are obtained for Si-NCs grown at 1050 °C (Supporting Information Figure S1). In Figure 3d, the lifetime shortening occurs not only at the high-energy tail of the spectrum, but also at the lowest detection energy. This cannot simply be explained by NC-toNC energy transfer, because NCs exhibiting PL in the lowenergy tail are considered to be the largest NCs in the size distribution and have no route to transfer energy. A possible mechanism of the shortening of the lifetime at the low-energy tail is different dielectric environment between Si-NCs in methanol and in solids. It is well-known that the radiative rate of an emitter is a product of its intrinsic decay probability determined by the electronic wavefuntions of the initial and final states and the local photonic mode density (Purcell factor). In the present experiments, the latter one is different between Si-NCs in solution and that in solids due to the large difference in the dielectric constants. In order to roughly estimate Purcell factors in Si-NC solids, we assume that a SiNC is placed in a dielectric environment of a medium consisting of Si-NCs and air, and that the dielectric function can be determined by the Bruggeman effective medium approximation.52 Purcell factors calculated for different filling factors of Si-NCs are shown in the Supporting Information with details of the calculation procedure.53,54 For comparison, the Purcell factor in methanol (εmethanol = 1.77) is also shown. The Purcell factors in Si-NC solids are larger than those in methanol and increase with increasing the filling factor. Figure 4 shows Purcell factors in Si-NC solids normalized by that in methanol. The enhancement factor of the decay rate by the formation of solid, i.e., PL decay rates of S1200 divided by those of C1200, are also shown. The enhancement factor agrees with the calculated Purcell factor at the smallest energy when the filling factor is about 0.35. This value is about half of the filling factors of typical simple regular lattices and seems to be reasonable 2763

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Figure 4. (Left axis) Calculated Purcell factors when the filling factor of Si-NCs are 0.25, 0.35, and 0.45 normalized by that in methanol. (Right axis) PL decay rates of Si-NC solid normalized by those in methanol.

considering the imperfection of NC lattices due to size distributions and the relatively small thickness of the solid. In Figure 4, the energy dependence of the normalized decay rate is apparently different from that of the normalized Purcell factor. This indicates that the significant increase of the decay rate at the high-energy tail of the spectrum by the formation of Si-NC solid cannot be explained by the model and thus energy transfer from small to large NCs is the most plausible explanation of the observed phenomenon. In Figure 4, at 1.6 eV, the decay rate increases about 3-fold due to the increase of the Purcell factor and about 2-fold due to energy transfer. This suggests that the time scale of energy transfer is comparable to that of PL lifetimes without NC-to-NC energy transfer and it is of the order of microseconds to tens of microseconds. In order to obtain additional evidence of energy transfer, we study the PL properties of the mixture of C1050 and C1200 and NC solids made from them. Since the average size of NCs in C1050 is much smaller than that in C1200, NCs in C1050 mainly play the role of energy donors and those in C1200 are energy acceptors. We mixed the solutions so that the atomic ratio of Si becomes C1050:C1200 = 1:15, which corresponds to the number ratio of NCs of 5:6. Figure 5a shows the PL spectrum of the mixed solution (solid curve). The results of deconvolution of the spectrum with those of C1050 and C1200 are also shown by dashed and dotted curves, respectively. The different contribution of C1050 and C1200 to the total spectrum despite the similar number of NCs is due to different PL quantum yields and absorption cross sections between NCs grown at 1050 and 1200 °C. In the figure, PL decay rates of the mixed solution are also shown (filled triangles). For comparison, the data of C1050 are also plotted by filled squares. The decay rates of the mixed solution are very close to those of C1050 in a wide energy range. This means that the decay rate of NCs in C1050 (energy donor) is almost not affected by addition of C1200 (energy acceptor). The situation is significantly different in the solid prepared from the mixed solution (Figure 5b). The spectral shape is strongly modified from that of the mixed solution, and the contribution of S1200 (dotted curve) to the total spectrum is strongly enhanced. This can be explained by energy transfer from NCs in S1050 to those in S1200 in the mixed solid. In Figure 5b, we also compare the decay rates of the mixed solid with those of S1050. In the high energy range, the decay rates are strongly enhanced by mixing, i.e., by adding energy acceptors (S1200) to S1050. This is clear evidence of FRET in the Si-NC solid.

Figure 5. (a) PL spectrum (solid curve) and decay rates (filled triangles) of a mixed solution. The decay rates of C1050 are also shown (filled squares). Dashed and dotted curves are the results of deconvolution of the spectra with those of C1050 and C1200, respectively. (b) PL spectrum and decay rates of Si-NC solid made from the mixed solution in panel a. The decay rates of S1050 are also shown (filled squares). Dashed and dotted curves are the results of deconvolution of the spectra by S1050 and S1200.

In conclusion, from the comparison of PL spectra and decay rates of solutions and solids of B and P codoped Si-NCs, we have successfully demonstrated the existence of energy transfer between Si-NCs in densely packed Si-NC solids. We also demonstrated that the energy transfer can be controlled by controlling the size distribution. To our knowledge, this is the first direct experimental evidence of the energy transfer between Si-NCs. The present results indicate that luminescence properties of Si-NC solids are modified from those of individual Si-NCs due to energy transfer between NCs, and thus the effect should be taken into account to design optical devices from SiNC solids.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: PL decay curves and decay rates of C1050 and S1050 samples. Procedure for the calculation of Purcell factors in solution and in Si-NC solid. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01067.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI (23310077 and 24651143) and 2014 JSPS Bilateral Joint Research Projects (Japan-Czech Republic). 2764

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DOI: 10.1021/acs.jpclett.5b01067 J. Phys. Chem. Lett. 2015, 6, 2761−2766

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DOI: 10.1021/acs.jpclett.5b01067 J. Phys. Chem. Lett. 2015, 6, 2761−2766