One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon Nanoparticles Bhaskar Das,†,‡ Syed Minhaz Hossain,‡ Ashit Kumar Pramanick,§ Arjun Dey,∥ and Mallar Ray*,† Dr. M. N. Dastur School of Materials Science and Engineering and ‡Department of Physics, Indian Institute of Engineering Science and Technology, Shibpur, P.O. Botanic Garden, Howrah 711103, India § Materials Science Division, National Metallurgical Laboratory, Jamshedpur 831007, India ∥ Thermal Systems Group, U. R. Rao Satellite Centre (Formerly Known as ISRO Satellite Centre), Indian Space Research Organisation, Bengaluru 560017, India ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/05/19. For personal use only.

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S Supporting Information *

ABSTRACT: Preparation of highly luminescent glasses involves expensive and complicated processes and usually requires high temperature. In this work, we show that luminescent silicon (Si) nanoparticle (NP)- embedded silicate gel glasses can be developed under near-ambient conditions by a remarkably simple, one-pot strategy, without using any sophisticated instrumentation or technique. Simultaneous hydrolysis and reduction of (3-aminopropyl)triethoxysilane leads to the formation of colloidal Si nanocrystals that can be transformed to a glassy phase upon slow evaporation followed by freezing. Structural investigations reveal the formation of a sodium silicate gel glass framework having discernible shear bands, along with embedded Si NPs. High photoluminescence quantum yield (ca. 35−40%), low glass-transition temperature (Tg ≈ 66−73 °C), strain-tolerant mechanical stability, and inexpensive preparation make the glass attractive for applications as display materials and photonic converters. KEYWORDS: silicon, nanoparticles, gel glass, luminescence, photon conversion

1. INTRODUCTION Luminescent glasses are attractive candidates for the development of next-generation solid-state display devices and photonic converters of solar cells. Among different types of glasses, sodium-based silicate glasses are inexpensive, easy to manufacture, and widely used for a variety of applications. A standard silicate glass is nonluminescent and cannot be directly used for displays or photonic converters.1 Incorporation of luminescent structures, usually NPs with high emission yields, is an option that has been recently explored for making these glasses photoactive.2,3 A variety of approaches have been used to incorporate luminescent NPs in a glassy network, such as melt quenching,4 sputtering,5 chemical vapor deposition,6 ion implantation,7 and laser-based processes.8 All of these methods rely on deposition or incorporation of NPs in a glassy matrix and consequently require elevated temperatures and/or special treatments like laser ablation.4−8 Majority of them involve ex situ preparation of NPs, followed by incorporation in a host matrix, resulting in NP-embedded films or typical spin on glass systems.5−7,9 In situ nucleation and growth of NP-embedded glass, usually nanocrystalline Si in SiO2 matrix, have also been well studied. Such processes require a very high temperature (typically ∼1100 °C) and preferably an inert atmosphere.10,11 To the best of our knowledge, there is no report on the © XXXX American Chemical Society

preparation of luminescent Si NP-embedded glass under nearambient conditions. During the last couple of years, attempts to induce luminescence in different glasses have been made by doping them with a variety of rare earths like Tb3+, Eu3+, and Dy3+ or with direct band gap semiconductor quantum dots like CdS.12,13 Recently, Gu et al.8 have reported modification of the photonic properties of chalcogenide glass by embedding Ge NPs using pulsed laser ablation in liquids. However, among the different light-emitting NPs, luminescent Si NPs are one of the most well-researched nanosystems.14,15 This is because Si is still the building block of nearly all microelectronic devices, and consequently the promise of Si NPs as a potential light source in photonic communication is unmatched by any other luminescent nanosystem. Additionally, Si NPs provide a benign alternative compared to the other binary or ternary semiconductor quantum dots. Despite these advantages, light emission from Si NPs is still somewhat controversial and widespread application of luminescent Si NPs has remained largely elusive.14−16 Another key concern about luminescent Si Received: October 12, 2018 Accepted: December 18, 2018 Published: December 18, 2018 A

DOI: 10.1021/acsami.8b17604 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Structural characteristics of the Si NP-embedded silicate gel glass: (a) FESEM image showing Si NPs dispersed in a matrix interspersed with shear bands; (b) bright-field transmission electron microscopy (TEM) image, where the darker spots represent the Si NPs along with the corresponding selected area electron diffraction pattern (SAEDP) (top inset) and the histogram representing the size distribution (bottom inset); (c) high-magnification high-resolution (HR)TEM image capturing a single large NP with distinct crystalline fringes; (d) X-ray diffraction (XRD) profile of the sample revealing dominant amorphous character; and (e) energy-dispersive X-ray (EDX) spectrum of the sample. which we refer to as “silicate gel glass” to distinguish it from the conventional silicate glasses formed by high-temperature vitrification.18 All of the chemicals used were of analytical grade procured from Sigma-Aldrich. 2.2. Characterization. The structural features of the NPembedded gel glass were characterized by a Zeiss Sigma field emission scanning electron microscope, a 200 kV JEOL JEM 2200FS high-resolution transmission electron microscope, and a Bruker D8 advanced X-ray diffractometer using Cu Kα1 (λ = 1.54 Å) radiation. X-ray photoelectron spectroscopy (XPS) was performed using by an Omicron Multiprobe spectrometer (Omicron NanoTechnology GmbH, U.K.) fitted with an EA125 (Omicron) hemispherical analyzer. Monochromatic Al Kα source operated at 150 W was used, and the pass energy of the analyzer was kept at 40 eV. A lowenergy electron gun (SL1000, Omicron) with a large spot size was used for sample neutralization. The voltage of the electron gun was fixed at −3 V. Thermogravimetric analysis (TGA) and differential thermal analysis (TG−DTA) of the NP-embedded glass were performed with a TGA/SDTA851e (Mettler Toledo AG), and differential scanning calorimetry (DSC) data were recorded by a Netzsch DSC-200 PC Phoenix, Germany. Load control nanoindentation (CSM Instruments, Anton Paar Tritec) technique was utilized to measure nanohardness and Young’s modulus of the prepared gel glass sample. In-built scanning probe microscopy facility was also used to characterize the topology of the glass surface and the indentation footmark. The experiments were conducted at four different loads: 2, 3, 5, and 10 mN, with a Berkovich diamond indenter. The well-established Oliver−Pharr model19 was utilized to evaluate nanohardness and Young’s modulus of the prepared NPembedded gel glass. UV−visible spectroscopy was performed using a JASCO V-750 UV−vis spectrophotometer, while the steady-state PL property of different samples was investigated using a Horiba Jobin Yvon Fluorolog-3 (Nanolog) spectrofluorometer (model FL3-11) fitted with a 450 W xenon lamp source, photomultiplier tube detector, and single grating monochromator.

NPs is their stabilitycolloidal Si NPs or the conventional porous Si NPs exhibit a time-dependent blue shift of the photoluminescence (PL) spectra and degrade with time due to spontaneous ambient oxidation.17 One of the strategies to protect the NPs from atmospheric degradation is to encapsulate or embed them in some protective material.15 A transparent glassy matrix in this regard is much desirable since it can directly be used for various applications. Here, we demonstrate that simultaneous reduction and hydrolysis of (3-aminopropyl)triethoxysilane (APTES) followed by ambient-temperature evaporation and freezing results in the formation of Si NP-embedded gel glass that exhibits intense and stable luminescence under UV excitation. The preparation method is surprisingly simple, independent of any costly equipment/process, easily scalable, and hence attractive from a commercial standpoint.

2. EXPERIMENTAL SECTION 2.1. Preparation of Si NP-Embedded Glass. The first step in the preparation of Si NP-embedded gel glass involves reduction of an aminosilane, APTES (99%), by sodium ascorbate. Sodium ascorbate stock solution (0.1 M) was prepared by mixing a requisite amount of NaOH with ascorbic acid. Deionized water (∼8 mL, Millipore, 18.2 MΩ cm−1) was added to 2 mL of APTES under continuous magnetic stirring. An aliquot of 2.5 mL (0.1 M) of sodium ascorbate was then added to the aqueous solution of APTES, and the solution was stirred magnetically for another ca. 20−25 min, which resulted in the formation of light red Si NP colloids. Starting from the mixing of reagents, it takes barely 30 min to complete the entire process of synthesis of colloidal Si NPs. A part of this colloid was kept aside for characterization, while another part was very slowly heated at ca. 50− 60 °C in a water bath under ambient conditions for around 5−6 h to facilitate slow evaporation of water. Subsequently, the transparent solution was transferred to the freezer of a household refrigerator and allowed to freeze for 2−3 days at ∼4 °C. This leads to the formation of a Si NP-embedded luminescent, solid-state, glasslike structure, B


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Figure 2. (a) Wide-range XPS image of the gel glass sample and high-resolution spectra of (b) C 1s, (c) N 1s, (d) O 1s, (e) Si 2p, and (f) Si 2s.

3. RESULTS AND DISCUSSION The salient structural features of the as-synthesized gel glass are summarized in Figure 1a−e. In the field emission scanning electron microscopy (FESEM) image shown in Figure 1a, abundant tiny bright spots representing the Si NPs are found to be reasonably well distributed in a background matrix. We also see some randomly distributed larger features of arbitrary shapes, which are commonly observed in glassy samples due to surface relief contrast.20 Local agglomerations of the NPs also contribute to the formation of such random features. Interestingly, the FESEM image reveals that the matrix is interspersed with a network of shear bands formed due to localized shear strains that are confined in relatively thin bandlike regions. Shear bands are ubiquitously found in a wide variety of materials, ranging from metallic alloys to soft matter, and have been extensively investigated, particularly for bulk metallic glasses.21,22 Examination of shear bands is important since localization of plastic deformation in these bands is one of the primary causes for decohesion and catastrophic failure in materials.23 For the purpose of the present study, the presence of shear bands could be significant because of a different reasonsuch bands have been reported to induce crystallization in metallic glasses.24,25 Although the exact mechanism of crystallization due to plastic deformation in shear bands is still not clear, it is well accepted that nanocrystallites nucleate around shear bands. It seems that increasing excess free volume and a sharp rise in local temperature during shear banding lead to the formation of NPs under ambient conditions.26 However, a shear banding-induced nucleation

would entail localization of the NPs around the bands, which we do not see in this case, suggesting some other dominant mechanism at play. A detailed discussion on the possible formation mechanism is presented later. The crystalline character of the NPs is confirmed from the HRTEM image, selected area electron diffraction pattern (SAEDP), and high-magnification TEM image of a single NP, shown in Figure 1b,c, respectively. Tiny dark spots representing the Si NPs are found in abundance in the bright-field image shown in Figure 1b. The sizes of these NPs estimated from several such bright-field and dark-field images (Figure S2a−d in the Supporting Information) are found to vary over a very narrow range: 2−7 nm, as shown in the histogram (bottom inset, Figure 1b). The SAEDP (top inset, Figure 1b) corresponding to the bright-field image consists of diffused spots defining ringlike patterns along with a bright halo, which are typical characteristics of randomly oriented nanocrystallites. The rings corresponding to (111), (200), and (311) reflections from Si are identifiable and are marked in the image. From the high-magnification image capturing a single Si NP, shown in Figure 1c, we can see the fringes, which further establish the crystalline nature of the NPs. The (111) exposed plane is marked for easy understanding. Although electron diffraction captures the crystalline feature, the XRD profile of the gel glass shown in Figure 1d exhibits a dominant amorphous nature. The profile consists of broad diffused scattering, confirming the absence of any long-range structural order. The only distinguishable feature is a broad diffraction hump centered at 2θ ∼ 21.8°, due to the amorphous SiO2 C


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Figure 3. Schematic of the mechanism leading to the formation of Si NP-embedded silicate gel glass.

phase.27 The tiny nanocrystals observed in the electron micrographs are masked by the dominant glassy phase and the amorphous oxide surrounding the NPs, which overshadows the crystalline features. The dominant presence of Si along with O, C, and Na is confirmed from the energy-dispersive Xray (EDX, obtained using a Horiba EX-250 attached with a scanning electron microscope) spectrum shown in Figure 1e. For detailed composition and elemental analysis of the prepared sample, XPS measurements were performed. Six major peaks at 100.1, 152.3, 284.8, 398, 530, and 1070 eV, corresponding to Si 2p, Si 2s, C 1s, N 1s, O 1s and Na 1s, are observed in the full-range XPS image shown in Figure 2a.28−30 Each of these peaks are resolved and deconvoluted using multiple Gaussian peaks (R2 between the experimental and fitted data >0.99, for all cases), as shown in Figure 2b−f. The details of the deconvoluted XPS peak positions and their assignments are listed in Table S1 (Supporting Information). From Figure 2b and Table S1, we see that the high-resolution C 1s spectrum deconvolutes to four peaks, which correspond very closely to the peaks of C−Si, C−C, C−N, C−O, and C O groups.29 While the presence of C−C, C−N, C−O, and CO groups is expected, the assignment of the small peak appearing at 284.3 eV to C−Si linkage is not so obvious. In fact, C−Si linkages are not formed in standard silanization reactions of Si surfaces with APTES.31 Yet this peak can be assigned to C−Si since this assignment is perfectly consistent with the high-resolution Si 2p spectrum of the sample that also exhibits a peak due to C−Si linkages. Additionally, a satellite peak appearing at a marginally lower binding energy than the C−C peak is suggestive of a C−Si bond, considering the difference in Pauling’s electronegativity of Si (1.9) and C (2.55).32 The three peaks deconvoluted from the N 1s spectrum (Figure 2c) indicate the anticipated presence of Si− N−Si, C−N, and −NH− groups.33 The high-resolution O 1s spectrum shown in Figure 2d reveals two peaks, which may also be attributed to expected Si−O and to C−OH/C−O−C bonds.34 As discussed above, the high-resolution Si 2p

spectrum (Figure 2e) exhibits a peak associated with C−Si bond, in agreement with the C 1s spectrum. Along with this Si−C bond, the spectrum also reveal two other peaks corresponding to Si−N and Si−O bonds.29,34,35 Finally, the Si 2s spectrum shown in Figure 2f shows the obvious presence of Si−Si and Si−O, along with a third peak corresponding to sodium silicate (Na2SiO3·5H2O), which is a byproduct of the reaction between APTES and sodium ascorbate.36,37 We see that all of the XPS features of the prepared samples match very well with previous reports dealing with various kinds of Si surfaces and NPs with hardly any discernable shift in the peak positions. The XPS study also expounds the findings of the Fourier transform infrared (FTIR) spectroscopy (Figure S3, Supporting Information) that is presented and discussed (see Section S3, Supporting Information) in the Supporting Information. The final sample therefore consists of standard chemical states, which are derived primarily from redox reactions and hydrolysis of APTES. Since the overall quality of the glass is largely determined by the presence of these bonds/species, the identification of each specie followed by a quantitative estimate would allow tailoring specific functionalities in the glasses by tuning a particular component.38,39 For example, the hydrophobic or hydrophilic nature may be controlled by the Si−N bonds as their presence has been known to increase hydrophilicity.40 On the basis of the structure and surface composition of the Si NP-embedded gel glass, we outline a possible mechanism involved in the formation of such a system. The alkoxysilane coupling reagents, the APTES molecules, are hydrophilic and can easily participate in simultaneous redox and hydrolysis reactions, as illustrated in Figure 3.41 It has been reported that reduction of APTES by a standard reducing agent, ascorbic acid, can be achieved at room temperature under continuous stirring.42 We believe that nucleation of Si NPs begins very early in this agitated solution, as dynamic light scattering (DLS) signals obtained from the solution at an early stage shows distribution of particles with average size ca. 1−2 nm D


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Figure 4. (a) TG, DTG, and DTA thermographs of the gel glass sample. (b) DSC curve for the gel glass sample.

Figure 5. (a) Load vs depth plots of gel glass (inset: scanning probe microscopy image of post-indentation footprint of the NP-embedded gel glass) and (b) variation of nanohardness and Young’s modulus of gel glass as a function of indentation load.

solution, suggesting that nucleation and growth of the NPs precede the formation of solid gel glass. Although the Si NP-embedded solid gel glass is formed at low temperature, it remains a solid at room temperature and up to 55−60 °C. To investigate the thermal stability of this material, TG−DTA and DSC measurements were performed. Figure 4a shows the typical TG−derivative thermogravimetry (DTG) and DTA thermographs of the glass sample. Initial weight loss indicated in the TGA profile from room temperature to ∼200 °C is due to the removal of free water and structural water present in the material. An endothermic peak in the DTA curve at around 185 °C can be attributed to this water loss. TGA shows almost a steady state from ∼200 to 366 °C. A weight loss after 366 °C could be noticed in the TGA thermograph, and this is due to the dehydroxylation of the structural OH. The peak around 465 °C in the DTA curve corresponds to this weight loss. An exothermic peak at 453 °C is possibly due to the oxidation of the gel glass constituents. Carbonization of the hydrocarbonated compounds present in the sample leads to a rapid weight loss observed after 550 °C, and the endothermic peak in the DTA profile at ∼557 °C could be assigned to this event. From the DTA profile, the glass-transition temperature (Tg) is estimated to be around 65 °C. The DSC curve for the gel glass sample shown in Figure 4b has a close similarity with TG−DTA data. The DSC curve indicates a glass transition at around 66 °C, in agreement with the DTA result. A sharp endothermic peak at 200 °C is attributed to the removal of water from the sample as observed in TG−DTA, corresponding to the first mass loss. The exothermic peaks at 421 and 464 °C correspond to the oxidation and degradation processes of the glass constituents.

(Figure S4, Supporting Information). The embryonic Si nuclei are then thermodynamically driven to form small nanocrystals. Progress of this reaction inhibits further nucleation and growth as concentration of APTES decreases, leading to the formation of clusters of Si NPs in solution. The as-formed Si NPs exhibit excellent aqueous dispersibility as the surfaces of the NPs are covered with numerous hydrophilic amino groups, as evident from the FTIR and XPS results. Availability of APTES for reduction is also limited by its consumption in the hydrolysis process (Figure S1, Supporting Information), which proceeds simultaneously, to form silanol groups. Following an earlier report that a solution temperature of ca. 50−60 °C provides a suitable hydrolysis rate,43 we raised the temperature to ∼50 °C and maintained it for several hours. With aging, these silanol groups create siloxane bridges by self-condensation or polymerization (Supporting Information).44 It seems that polymerization with silanols continues until the entire solution gels into a transparent, chemically formed gel glass. During this process, the Si NPs, which are produced simultaneously in the colloidal solution as a consequence of the redox reaction, get incorporated within the glass network. Gradual drying and cooling of the gel-like solution at ∼4 °C for 2−3 days leads to the formation of a transparent gel glass, where the large sodium ions are attached in the cavities resulting from the disruption of oxygen bridges. Drying of the gel-like colloidal dispersions is accompanied by the formation of bands associated with local shear strains.45 Potentially, these shear bands can also induce crystallization of Si.24,25 However, here it is difficult to find the contribution of shear banding in crystallizing Si as the Si NPs are found to be well distributed throughout the matrix instead of being concentrated at the shear band edges. Also, we find that the Si NPs start nucleating at an early stage in the colloidal E


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ACS Applied Materials & Interfaces It is important to note that the Tg of this gel glass sample is much lower than that of other silicate glasses reported in the literature. The widely studied sodium- and bismuth-based silicate glasses have reported Tg’s of 645 and 447 °C, respectively.46 The low glass-transition temperature of our sample is due to the presence of abundant trapped water molecules. Both TG−DTA and DSC confirm that water forms a significant constituent of the gel glass matrix, and this water in turn produces weak breaking points in the glass structure.47,48 However, a low-Tg glass, particularly a glass where the transition takes place at temperatures below 200 °C, has important applications in glass−metal sealing, integrated circuit packaging, photon conversion, etc.49,50 The Si NPembedded gel glass looks extremely promising for all such applications and particularly for photonic convertors, which can be of immediate and significant consequence. Additionally, the Tg of such a gel glass can be controlled by controlling the amount of retained water in the glass matrix. This was verified by noting that the Tg increases from 66 to 73 °C if the evaporation of the final solution at ca. 50−60 °C is carried out for 12 h instead of 5−6 h (Figure S6, Supporting Information). Mechanical stability of the prepared gel glass system was investigated by nanoindentation technique. Load versus depth plots of gel glass are shown in Figure 5a. The smooth nature of both the loading and unloading curves of the NP-embedded glass sample reveals that microscale cracking or damage does not occur during indentation. The post-indentation footprint of gel glass also supports this observation (Figure 5a, inset) as the indentation impression appears to be smooth without any imminent sign of disintegration or damage. The variation of nanohardness and Young’s modulus of the gel glass as a function of indentation load is presented in Figure 5b. Both nanohardness and Young’s modulus exhibit nearly loadinsensitive characteristics, indicating excellent mechanical stability along with strain-tolerant behavior of the prepared gel glass. The observed strain tolerance of the sample is plausibly due to the unique microstructure consisting of interspersed shear bands and well-dispersed Si NPs (Figure 1a). The load average nanohardness and Young’s modulus of the Si NP-embedded glass are 198 MPa and 2.7 GPa, respectively, which compare very well with the literature reported values of elastic moduli and hardness of silica xerogeland gel-based glasses.51,52 A striking feature of the prepared samples is that the colloidal suspensions of Si NPs and the Si NP-embedded gel glass exhibit intense room-temperature PL, which could be detected easily with the naked eye, as shown in Figure 6a. The UV−visible absorption and PL emission spectra of the colloidal Si NPs and the gel glass sample are shown in Figure 6b. The absorption spectrum of the Si NP colloid (red curve) is very similar to the absorption profiles of other colloidal Si NPs reported in the literature.53,54 There is an absorption onset at ∼500 nm, after which the absorbance increases steadily with increasing energy. An absorption shoulder appears at ∼253 nm, which is a typical feature of colloidal Si NPs and has been widely observed before. However, the exact origin of this feature is still not well understood.55 Lin et al.53 suggested that the weak absorption feature ∼260 nm could be due to the Γ−Γ direct band gap transitions, while Wu et al.35 attributed the origin of a shoulder at 270 nm to the direct band gap transition of L−L (4.4 eV). Even though the overall absorption profile of the glass sample (blue curve) is similar to the Si NP colloid, there are some significant differences. The

Figure 6. Intense room-temperature emission from the Si NPembedded gel glass: (a) photograph showing the prepared gel glass under normal visible light (left) and under UV excitation (right); (b) co-plot of absorbance and PL emission spectra of Si NP colloid and the NP-embedded gel glass along with the time evolution of the PL spectra as the colloid gradually transforms to the gel glass.

gel glass exhibits absorbance for the entire UV−visible range (shown till 700 nm in Figure 6b)there is no clear absorption onset. The entire spectrum of the glass is red-shifted with respect to the colloidal suspension and shows two small absorption humps located at ∼245 and ∼320 nm, respectively. Shoulder-like features in the UV region (∼200 and 300 nm) have been observed in many glass samples and identified as charge-transfer UV bands arising due to the presence of unavoidable trace impurities like iron, which get incorporated during glass production.56 The presence of such impurities in our sample can be ruled out. Therefore, the feature at ∼320 nm observed here is probably due to enhanced absorption by the electron traps, the so-called E−3 and E−4 centers present in the oxide matrix.57 The feature at ∼245 nm appears to be a convolution of the effect of such centers along with the effect of Si NPs present in the matrix, which independently give rise to an absorption shoulder in this region of the absorption profile as discussed above. Continuous photoabsorption of the NP-embedded gel glass in the visible region indicates that the formation of a disordered glassy matrix is associated with the creation of a number of such defects/traps, which absorb through the entire visible region. Many researchers have studied the absorbance of silicate glasses after irradiating glass samples with different radiations and have reported absorption features in the visible region.57,58 The sample under investigation, even in the absence of any external treatment, exhibits reasonably good absorbance ranging from the UV region up to the red region of the electromagnetic spectrum. Figure 6b also shows the PL spectra of the colloidal Si NPs and the synthesized gel glass under excitation with 390 nm radiation. The excitation energy was determined from PL excitation studies. The PL peak of the as-synthesized colloidal sample is at ∼506 nm (red PL curve), whereas for the glass, F



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the peak is blue-shifted to around 456 nm (blue PL curve). We see that the PL peak slowly but consistently blue-shifts as the colloid gradually transforms to a gel glass. However, the timedependent blue shift dramatically slows down once the solid glass is formed. We recorded the PL spectra of the gel glass at regular intervals and found a negligible (∼3 nm) blue shift after 3 months. The origin of room-temperature PL in Si NPs has been an active area of research for nearly three decades. Yet the exact mechanism of luminescence from Si NPs is still not completely understood.16,55 We have shown in our previous works that quantum confinement of excitons and surface, interface, and/or defect states (primarily due to oxides) play a combined role in light emission from Si nanostructures.59−61 In this communication, we refrain from delving into the photophysics of Si NPs and simply point out that our sample is highly luminescent with a quantum yield (QY) (Section S6, Supporting Information) of around 35−40%. However, the observed blue shift of the PL peak of the glass sample provides interesting clue toward understanding the mechanism of light emission. It has been reported that in the absence of specific surface treatment, Si NPs undergo continuous atmospheric oxidation, resulting in the decrease in size of the crystallite Si core and consequently affecting a time-dependent blue shift of the PL peak.17 Therefore, a continuous time-dependent blue shift in PL peak position of the NP colloid is expected. This time-dependent blue shift drastically slows down after the formation of the gel glass as the NPs now get embedded and protected against ambient oxidation, albeit partially, by the glass matrix. We also see that the intensities of PL peaks consistently increase as the colloid transforms to the solid gel. As the colloid dries up, the density of the luminescent particles is expected to increase, affecting an increase in the PL intensity due to excitation of a larger number of NPs. Understandably, after the formation of the glass, the intensity remains more or less fixed as the density of luminescent NPs does not increase any further. However, a more detailed investigation of the mechanism and further tuning of the gross properties of this gel glass warrant separate investigations and open up the scope for further research.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17604. Hydrolysis and self-condensation reactions; additional HRTEM images of Si NPs; Fourier transform infrared (FTIR) spectroscopy; details of XPS peak assignments; size distribution of Si NPs measured by DLS; calculation of PL quantum yield (QY); DSC curves of two samples showing increase in glass-transition temperature; photon conversion in commercial LEDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Arjun Dey: 0000-0003-3727-3056 Mallar Ray: 0000-0001-8173-1857 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Niladri Banerjee of Loughborough University for a useful informal review of the manuscript and Professor Tarun Mondal, Professor Sugata Ray, and Indranath Bhowmik of IACS, Kolkata, for extending their support to record XPS data of our samples. They are also grateful to Dr. H.C. Barshilia from CSIR-NAL, Bengaluru, for providing the facility for carrying out nanoindentation tests under ISRO rate contract.

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REFERENCES

(1) Shelby, J. E. Introduction to Glass Science and Technology; Royal Society of Chemistry, 2005. (2) Cao, J.; Chen, W.; Xu, D.; Li, X.; Wei, R.; Chen, L.; Sun, X.; Guo, H. Transparent glass ceramics containing Lu6O5F8: Tb3+ nanocrystals: Enhanced photoluminescence and X-ray excited luminescence. J. Am. Ceram. Soc. 2018, 101, 1585−1591. (3) Baral, K.; Ching, W.-Y. Electronic structures and physical properties of Na2O doped silicate glass. J. Appl. Phys. 2017, 121, No. 245103. (4) Kassab, L.; Kumada, D.; da Silva, D.; Garcia, J. Enhanced infrared-to-visible frequency upconversion in Yb3+/Er3+ codoped Bi2O3−GeO2 glasses with embedded silver nanoparticles. J. Non-Cryst. Solids 2018, 498, 395−400. (5) Okamoto, S.; Kanemitsu, Y. Photoluminescence properties of surface-oxidized Ge nanocrystals: Surface localization of excitons. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16421. (6) Blanc, W.; Mauroy, V.; Nguyen, L.; Bhaktha, B. N. S.; Sebbah, P.; Pal, B. P.; Dussardier, B. Fabrication of rare earth-doped transparent glass ceramic optical fibers by modified chemical vapor deposition. J. Am. Ceram. Soc. 2011, 94, 2315−2318. (7) Sonal; Sharma, A.; Aggarwal, S. Optical investigation of soda lime glass with buried silver nanoparticles synthesised by ion implantation. J. Non-Cryst. Solids 2018, 485, 57−65. (8) Gu, T.; Gao, J.; Ostroumov, E. E.; Jeong, H.; Wu, F.; Fardel, R.; Yao, N.; Priestley, R. D.; Scholes, G. D.; Loo, Y.-L.; Arnold, C. B. Photoluminescence of functionalized germanium nanocrystals embedded in arsenic sulfide glass. ACS Appl. Mater. Interfaces 2017, 9, 18911−18917. (9) Zhou, S.; Jiang, N.; Wu, B.; Hao, J.; Qiu, J. Ligand-driven wavelength-tunable and ultra-broadband infrared luminescence in single-ion-doped transparent hybrid materials. Adv. Funct. Mater. 2009, 19, 2081−2088.

4. CONCLUSIONS In summary, we have demonstrated a simple, inexpensive, onepot method for the preparation of Si NP-embedded, highly photoluminescent sodium silicate gel glass. Simultaneous hydrolysis and reduction of APTES followed by drying and freezing results in the formation of a glassy matrix impregnated with nanocrystalline Si. The glassy backbone is found to be interspersed with shear bands, and the NPs are found to be well distributed throughout the matrix. Such a microstructure results in nearly load-independent Young’s modulus and nanohardness of the NP-embedded gel glass and thereby endows strain-tolerant behavior, indicating good mechanical stability. Most interestingly, the sample exhibits intense (quantum yield, ca. 35−40%) blue-green, room-temperature PL that is detectable with the unaided eye. A low glasstransition temperature (Tg ∼ 65 °C) makes this material an attractive candidate for photonic converters of solar cells. Moreover, the Tg can be tuned by reducing retained water in the glass matrix, thereby rendering this material important for a number of other applications. G


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One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon

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