Influence of Halides on the Optical Properties of Silicon Quantum Dots

DOI: 10.1021/acs.chemmater.5b00115. Publication Date (Web): February 4, 2015 ... in Crossref's Cited-by Linking service. For a more comprehensive list...
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Influence of Halides on the Optical Properties of Silicon Quantum Dots Mita Dasog,*,† Kathrin Bader, and Jonathan G. C. Veinot* Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta T6G2G2, Canada S Supporting Information *

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Scheme 1. Halogenation and alkylation of hydride terminated Si-QDs

ilicon quantum dots (Si-QDs) have received substantial attention over the last two decades owing to their abundance, biocompatibility, and optical properties.1−8 Similar to their group II−VI and III−V quantum dot counterparts, hydride-terminated Si-QDs with diameters smaller than 5 nm can exhibit size-dependent photoluminescence (PL) in the visible spectral region.9−13 However, the hydride surface is highly susceptible to oxidation and requires further modification to passivate it and render the QDs soluble in common solvents.14,15 Thermal hydrosilylation is among the most widely used procedures for attaching alkenes/alkynes to silicon surfaces. Unfortunately, lower reactivity of gaseous alkenes and alkynes limits effective passivation of Si-NC surfaces with shorter alkyl groups,16,17 and when long chain alkyl groups are attached they form an electrically insulating barrier that can limit the performance of various semiconductor QDs devices (e.g., photovoltaics).18−20 Photochemical hydrosilylation may appear as an attractive alternative; however, it suffers from particle size-dependent reactivity and does not allow for effective functionalization of larger Si-QDs (i.e., d > 5 nm).21 Halogenation/alkylation provides a reasonable approach toward attaching short chain alkyl moieties to the Si surfaces.22 Commonly, halogen terminated Si-QDs (i.e., X-Si-QDs) are prepared in situ by reacting Zintl salts (i.e., ASix, A = Na, K, Mg) with SiCl4/Br2 or reduction of silicon halides with strong reducing agents such as sodium naphthalide or sodium metal.23−25 Often X-Si-QDs are not isolated; rather, they are directly reacted with Grignard or akyllithium reagents to yield alkyl terminated Si-QDs. This general approach often yields polydisperse QDs (standard deviation >2 nm) that show blue photoluminescence, and the reaction procedure cannot be readily scaled to afford large quantities of material.25 Recently, we demonstrated that chloride termination of Si-QDs can be achieved upon treating hydride-terminated particles with phosphorus pentachloride (PCl5).26 This reactive platform was subsequently used to prepare near-monodispersed amine,27 sugar, and amino acid surface-terminated Si-QDs.28 Particles prepared in this way show blue luminescence irrespective of particle size; however, its origin remains the subject of study. Further, no reports exist to date that describe bromination or iodination of hydride-terminated Si-QDs. Herein, we report the chlorination, bromination, and iodination of hydride-terminated Si-QDs using PCl5, Br2, and I2, respectively (Scheme 1). The influence of reaction time and temperature on particle size was also investigated. Alkylation of X-Si-QDs was achieved using Grignard reagents. The presence of trace halide impurities was found to significantly alter the optical properties of the resulting alkyl-terminated Si-QDs. © XXXX American Chemical Society

Oxide-embedded Si-QDs (diameters ca. 3 and 9 nm) were synthesized via the disproportionation of hydrogen silsesquioxane at 1100 and 1300 °C, respectively.29 Freestanding hydrideterminated Si-QDs were obtained by treating oxide embedded Si-QDs with HF. Chlorination of bulk Si(111) surfaces is typically achieved by treating them with PCl5 at temperatures higher than 90 °C in the presence of benzoyl peroxide.30 These conditions lead to rapid etching and complete dissolution of SiQDs (Figure 1A). We have found chlorination of hydrideterminated Si-QDs proceeded at room temperature without the need for a radical initiator. The etching rate was slower at lower temperatures and changes in NC sizes depended linearly with reaction time at the temperatures investigated (Figure 1A). Bromination of hydride-terminated Si-QDs using Br2 was performed at ambient temperature under inert conditions. The etch rate was faster than that observed for PCl5 and resulted in anisotropic etching of the QDs as shown in the TEM images (Figure 1B). No detectable etching was observed for the reaction of I2 with hydride-terminated Si-QDs. Despite longer reaction times, complete iodination could not be achieved, as is evidenced by the presence of a residual Si−H stretch at ca. 2100 cm−1 (Supporting Information Figure S1).29 Because Received: January 12, 2015 Revised: February 4, 2015

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DOI: 10.1021/acs.chemmater.5b00115 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

Bright-field TEM images provide average particle diameters of 3.0 ± 0.4, 3.4 ± 0.5 and 3.4 ± 0.4 nm for Si-QDs derived from chloride, bromide and iodide surfaces, respectively (Figure 2). Despite the statistically equivalent particle sizes, the PL

Figure 2. TEM images and particle size distribution of hexyl functionalized Si-QDs derived from (A) chloride, (B) bromide, and (C) iodide surfaces.

response of n-hexyl functionalized Si-QDs varied significantly (Figure 3). The n-hexyl Si-QDs derived from bromide surfaces

Figure 1. (A) Si-NC size vs time upon reacting hydride terminated particles with PCl5 at various temperatures. The particle sizes were determined from respective TEM images. (B) TEM images of Si-QDs reacted with Br2 for indicated reaction times at ambient temperature (Scale bar = 50 nm).

chlorination and bromination of Si-QDs resulted in a decrease in particle size, d = 9 nm Si-QDs were used in these procedures. Iodination was performed using d = 3 nm Si-QDs. Raman spectra of chloride-, bromide-, and iodide-terminated Si-QDs show characteristic Si−Cl, Si−Br, and Si−I features at ca. 408, 310, and 290 cm−1, respectively (Supporting Information Figure S2).31 A feature at ca. 520 cm−1 corresponding to a Si− Si stretch was also noted. Halogenation was further confirmed by signature emissions in high-resolution X-ray photoelectron (HR XP) spectra (Supporting Information Figure S3).32−34 As expected, halide terminated Si-QDs did not exhibit detectable PL in the visible region under UV illumination. However, partial air oxidation induces blue PL from chloride-terminated Si-QDs as well as yellow-orange PL for both the bromide- and iodide-terminated systems (Supporting Information Figure S4). It is reasonable that the blue emission arises from oxychloride defects, while the yellow-orange PL likely originates from oxide defects.27 Surface alkylation was achieved by treating X-Si-QDs with alkylmagnesium halide reagents for 5 days at 80 °C. Shorter reaction times resulted in incomplete surface passivation and substantial oxidation. Methyl, n-butyl, n-hexyl, and n-dodecyl functionalized Si-QDs were prepared using this method. For convenience, the following discussion will focus on the n-hexyl functionalized Si-QDs that are representative of all alkyl terminated particles prepared here. FT-IR spectra of hexyl functionalized Si-QDs showed C−H (ca. 2850 cm−1) and Si− O−Si (ca. 1100 cm−1) stretching features consistent with alkylation (Supporting Information Figure S5).27 QDs derived from iodide surface showed the highest degree of oxidation, precipitated from toluene after 14 days under ambient conditions, and could not be redispersed. Alkyl-functionalized Si-QDs derived from chloride and bromide surfaces remained dispersed in toluene for extended periods (i.e., >6 months).

Figure 3. Photoluminescence spectra of hexyl functionalized Si-QDs in toluene derived from (A) chloride, (B) iodide, and (C) bromide surface. (λex = 325 nm).

showed red PL consistent with effective mass approximation (EMA) that provides a predicted PL maximum of ∼750 nm.27 However, the alkyl QDs derived from chloride and iodide surfaces showed blue and yellow emission. The yellow emission is believed to originate from oxide defects and exhibited shortlived photoluminescence excited state with lifetimes averaging around 5.4 ns (Supporting Information Figure S6). The blue emission was independent of particle size whereas the alkylterminated Si-QDs derived from the bromide surfaces showed size-dependent PL (Supporting Information Figures S7 and S8). The excited state lifetimes for blue-emitting Si-QDs was 6.1 ns (Supporting Information Figure S9) in agreement with previous reports.35,36 Red-emitting particles showed excited state lifetimes of 13 μs (Supporting Information Figure S10) consistent with band gap-based emission.27 No microsecond lifetime component was observed for blue- and yellow-emitting Si-QDs. The blue-emitting Si-QDs showed red-shift of the PL maximum with increasing excitation wavelength; no shift was B

DOI: 10.1021/acs.chemmater.5b00115 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

The alkyl functionalized Si-QDs derived from chloride and bromide surfaces were found to be more stable than those derived from iodide equivalents. We further demonstrated that the defects play an important role in the optical properties of Si-QDs. The oxychloride impurities resulted in blue PL in SiQDs irrespective of NC size, whereas bromide derived alkyl SiQDs yielded size-dependent PL properties. The PL from alkyl Si-QDs obtained from the iodide surface was dominated by the oxide defects and exhibited orange-yellow color under UV illumination.

observed in yellow- and red-emitting Si-QDs (Supporting Information Figure S11−S13). The relative emission quantum yields were found to be 13, 17, and 12% for blue-, yellow-, and red-emitting Si-QDs. To further investigate the impact of surface halide species on Si-NC properties, we employed Grignard reagents based upon different halides. Bromide terminated Si-QDs functionalized with RMgBr displayed size-dependent luminescence; however, those modified using RMgCl exhibited blue PL (Supporting Information Figure S14). We believe this difference arises because halide exchange can and does occur during the alkylation step (Scheme 2). This halide exchange proposed in

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 2. Proposed Halide Exchange during the Alkylation of Bromide Terminated Si-QDs

Experimental details, FTIR spectra, XPS spectra, PL spectra, and excited state lifetime measurements for halide and alkyl terminated Si-QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(J.G.C.V.) E-mail: [email protected]. Tel.: 1-(780)-4927206. *(M.D.) E-mail: [email protected]. Tel: 1-(780)-492-7206.

Scheme 2 was further supported by the presence of the Cl 2p peak in HR XP spectra (Figure 4). The high binding energy

Present Address †

(M.D.) Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge continued generous funding from the National Engineering Research Council of Canada (NSERC). M.D. thanks Alberta Innovates Technology Futures, Killam Trusts, and NSERC for scholarships. We also thank W. C. Moffat and G. Kaufmann for assistance with FTIR and Raman spectroscopy, respectively. The staff at the Alberta Centre for Surface Engineering and Sciences (ACSES) is thanked for XPS analysis. K. Cui at the National Institute of Nanotechnology (NINT) and T. Purkait are thanked for TEM analysis. G. de los Reyes is thanked for assistance with lifetime measurements. N. S. Lewis, R. Snitynsky, and N. Plymale are thanked for useful discussion.

Figure 4. HR XP spectra of Si 2p and Cl 2p region of hexyl functionalized Si-QDs obtained by reacting CH3(CH2)5MgCl with (A) chloride terminated Si-QDs and (B) bromide terminated Si-QDs.

emissions (i.e., ca. 204 eV) are consistent with oxychloride moieties on the Si-NC surfaces. No bromine or iodine emissions were observed at the sensitivity of the XPS technique. The Si 2p region shows two distinctive emissions corresponding to the Si core (ca. 99 eV) and surface oxide/ oxychloride (ca. 103 eV). On the basis of these observations we conclude the blue PL results from surface oxychloride impurities similar to oxynitride defect states.35,36 This observation is consistent with the literature reports of blue PL in Si-QDs synthesized in the presence of chlorine containing reagents.23−25 In conclusion, we have demonstrated chlorination and for the first time bromination and iodination of hydride-terminated Si-QDs. The halogenation reactions occur at room temperature, and PCl5 and Br2 can etch the Si-NC core. Iodination was incomplete, even after prolonged reaction times and high temperature (i.e., 100 °C) treatment. Alkylation was performed using alkylmagnesium halide reagents. The chloride ion in the Grignard can exchange with the bromide on the Si-NC surface.

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b00115 Chem. Mater. XXXX, XXX, XXX−XXX