Solution Synthesis and Optical Properties of Transition-Metal-Doped

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Solution Synthesis and Optical Properties of Doped Silicon Nanocrystals Benjamin McVey, Justinas Butkus, Jonathan E. Halpert, Justin M Hodgkiss, and Richard D. Tilley J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00589 • Publication Date (Web): 10 Apr 2015 Downloaded from http://pubs.acs.org on April 13, 2015

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The Journal of Physical Chemistry Letters

Solution Synthesis and Optical Properties of Transition Metal Doped Silicon Nanocrystals Benjamin F. P. McVey†‡, Justinas Butkus†, Jonathan E. Halpert†, Justin M. Hodgkiss†, Richard D. Tilley†‡* †

School of Chemical and Physical Sciences and the MacDiarmid Institute for Advanced

Materials and Nanotechnology, Victoria University of Wellington, Wellington 6012, New Zealand ‡

School of Chemistry and Electron Microscopy Unit of the Mark Wainwright Analytical

Centre, Chemical Sciences Building Kensington, University of New South Wales Sydney, New South Wales 2052, Australia AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ABSTRACT A new synthetic method was developed to produce a range of transition metal (Mn, Ni and Cu) doped Silicon Nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by Transmission Electron Microscopy (TEM). Dopant composition was confirmed by EDX (Energy Dispersive X-Ray Spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady state and transient) and photoluminescence spectroscopy. Doped Si 1 ACS Paragon Plus Environment


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NCs demonstrated unique dopant dependent optical properties compared to undoped Si NCs such as enhanced sub-gap absorption, and 40 nm shifts in the emission. Transient Absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs.

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KEYWORDS “Silicon, Nanocrystal, Quantum Dot, Optical Properties, Solution Synthesis”. Silicon Nanocrystals (Si NCs) are an interesting class of semiconductor NCs due to their unique optical properties, high natural abundance, and proven low toxicity.1-2 Their size and surface dependent optical properties makes them highly suitable for LEDs and solar cells.3-7 Furthermore, their low toxicity, biocompatibility, and strong resistance to photobleaching make Si NCs favorable candidates for bioimaging.8-10 Doping semiconductor NCs unlocks a host of new and unique properties including a wider range of emission tunability, enhanced electrical transport in thin films, and magnetism.11-12 Studies of doped NCs have focused primarily on the well-studied II-VI system.11-12 Using CdSe, ZnSe, and their sulfur analogues as model NC systems, a range of dopants (Ag+, Mn2+, Cu2+) have been successfully incorporated into these materials giving a high level of control over dopant incorporation and position within the NC.13-17 2 ACS Paragon Plus Environment

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Doping of Si NCs with metal dopants in particular has been relatively unexplored.18-19 Kauzlarich and co-workers reported a multi step process to synthesize Fe and Mn doped Si NCs through use of doped alkali silicides.18-19 Fe and Mn doped Si NCs demonstrated emission and paramagnetism making them suitable candidates for multimodal contrast agents.18-20 There is great scope to expand the range of metal dopants available to Si NCs. Reduction of halide salts is a versatile synthetic method to produce Si NCs with well defined sizes and optical properties.21-23 Using reduction of halide salts as a synthetic platform we have developed a generalized method to dope Si NCs with several different transition metals (Mn, Ni, and Cu). The optical properties of doped NCs were fully characterized using a range of techniques, including UV-Visible absorption, photoluminescence and transient absorption spectroscopy. Doped NCs displayed unique dopant dependent optical properties including emission and absorption redshifts, as well as fundamentally different excited state dynamics.

Scheme 1. Schematic diagram of the synthesis of metal doped Si NCs To synthesize doped Si NCs, silicon tetrachloride and metal salt (MCl2 M = Mn, Ni, Cu) are co-reduced by hydride reducing agents in the presence of a quaternary amine surfactant seen in Scheme 1. Dopant levels were controlled by the molar ratio of metal salt to silicon precursor. Surface modification was employed to protect doped Si NCs from oxygen and moisture, using UV assisted hydrosilylation to create a strong, covalent Si – C bond.24

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Figure 1. (a – d) Contain TEM images of undoped and doped Si NCs (scale bar = 10 nm) Figure 1a, b, c, and d shows transmission electron microscope (TEM) images of undoped and Mn, Ni, Cu doped Si NCs. Undoped Si NCs (Figure 1a) were spherical and relatively monodisperse with an average size of 2.1 ± 0.3 nm (SI Figure 2a). TEM images of doped NCs can be seen in figure 1b, c, and d. Doped NCs were also spherical and relatively monodisperse, with average sizes of 2.4 ± 0.4 nm (Mn), 2.5 ± 0.4 nm (Ni), and 2.5 ± 0.4 nm (Cu) (SI Figure 2b, c and d). The size of undoped Si NCs matches well with previous reports that used a similar synthetic approach.10 Importantly the sizes of doped Si NCs are comparable to undoped Si NCs enabling comparison of the optical properties of undoped and doped Si NCs. The atomic composition of undoped and doped Si NCs was determined by EDX. EDX of undoped Si NCs confirmed the presence of Si. EDX of doped Si NCs gave NC stoichiometries of Mn0.006Si, Ni0.006Si, and Cu0.007Si. The average number of Si atoms in a 2.3-2.4 nm NC is 360 atoms, giving an average of 2 – 3 dopant atoms per NC.


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The raw reaction mixture contained a molar ratio of 1 % dopant salt relative to SiCl4, for all three dopants to have a final concentration of over 0.5 % indicates the effectiveness of the synthetic protocol to make doped Si NCs.

Figure 2. Contains normalized absorption and emission ( λexc= 360 nm) of undoped and doped Si NCs. The black dotted line is added to emphasize the emission shift between undoped and doped Si NCs. The inset of each contains color photographs of doped and undoped NCs dispersed in hexane under illumination by a 360 nm UV light Figure 2 shows the absorption and emission spectra of doped and undoped Si NCs. The absorption spectrum of undoped NCs (Figure 2) shows a sharp peak at 245 nm and a shoulder at 275 nm. The positions of the sharp peak and the shoulder are in good agreement with previous reports.10, 25 Mn, Ni, and Cu doped Si NCs show sharp peaks at 268 nm, 270 nm, and 266 nm, respectively. The sharp peaks around 270 nm in the doped Si NCs correspond 5 ACS Paragon Plus Environment


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well to the weak shoulder seen in the undoped Si NCs. Compared to undoped, doped Si NCs show enhanced absorption in the 260 nm – 270 nm region, particularly in the case of Cu doped Si NCs which shows a fourfold enhancement in the absorption at 268 nm compared to undoped Si NCs. The introduction of dopants is computationally predicted to cause unique changes to the electronic structure of Si NCs leading to dopant dependent modifications to the absorption spectra of Si NCs.26 The enhanced absorption in the 260 nm – 270 nm region seen in doped Si NCs is important as majority of Si NCs show broad weak absorbance compared to other semiconductor NCs (CdSe etc) hampering device applications.27 Increased absorption in the UV region particularly is useful to emerging Si NC based UV sensors.28 The emission spectra of undoped and doped Si NCs taken at an excitation wavelength of 360 nm are also shown in Figure 2. Undoped Si NCs show a peak at 445 nm with a FWHM of 85 nm. Full emission spectrum over a range of excitation wavelengths for undoped and doped Si NCs is shown in SI Figure 3. The position of the emission peak and the range of excitation wavelengths for undoped Si NCs agrees well with previous reports on alkyl capped Si NCs.10, 21

Mn, Ni, and Cu doped Si NCs show emission peaks at 483 nm, 489 nm, and 476 nm,

giving a redshift of over 40 nm relative to the undoped Si NCs. Mn, Ni, and Cu Doped Si NCs have FWHM of 80 nm, 76 nm, and 75 nm, similar to undoped Si NCs. The inset of figure 2 contains colour images of undoped and doped Si NCs dispersed in hexane excited by a UV light (360 nm), showing the difference from the blue of undoped Si NCs to the greener colour of doped Si NCs. Si NCs synthesized in the solution phase demonstrate blue luminesence which can lead to autoflouresence issues in bioimaging applications.20 The emission properties of doped Si NCs could offer disctinct advantages to undoped Si NCs in bioimaging applications by potentially minimising autofluorescence and UV induced damage.20 Doped Si NCs demonstrated 40 nm redshifts pushing the emission from blue towards green. Doped Si NCs also move away from


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the shorter excitation wavelengths seen in (280 nm – 380 nm) undoped Si NCs to longer excitation wavelengths (340 nm – 420 nm). Emission redshifts have been reported for metal doped Si NCs.18-19 Kauzlarich and coworkers have shown a dopant concentration dependence on the emission redshifts in Mn doped Si NCs with a redshift of over 100 nm (from 410 nm to 510 nm) for 4.5 nm 5 % doped NCs compared to a 35 nm redshift (from 410 nm to 445 nm) for 4.5 nm 1 % Mn doped NCs.19-20 Ab initio calculations predict emission redshifts from doped Si NCs due to the introduction of dopant states within the bandgap.26 When increasing the dopant concentration to 5 % the resultant emission spectra as seen in SI Figure 4 resembled a mixture of Si NCs and Mn doped Si NCs. Formation energies for dopants have been computationally predicted to be larger for smaller sized Si NCs, therefore at higher concentrations dopants are segregated from the Si NCs due to favoured Si-Si interactions.29 PL quantum yields of undoped and doped Si NCs in hexane were obtained through the comparative method of Williams et al.30 using quinine sulfate as a standard (SI Figure 5). Quantum yield (QY) of undoped NCs was found to be 13.5 % which matches previous reports of 2.0 nm alkyl terminated Si NCs.23 Mn, Ni, and Cu doped Si NCs had QYs of 15.5 %, 15.8 %, and 14.9 %. Although optically allowed transitions are associated with dopant states, the dominance of non-radiative relaxation for both doped and undoped Si NCs means that their QYs are similar. The QYs of doped Si NCs are similar to blue emitting Si NCs previously used in bioimaging applications.8 Undoped Si NCs showed long term photostability (over 1 year) in agreement with previous reports of alkyl capped Si NCs.22 The emission from Mn and Ni doped Si NCs degraded in a matter of weeks under ambient conditions, however they remained stable over 4-6 months if stored under a N2 atmosphere. Cu doped Si NCs had superior photostability compared to Mn or Ni doped Si NCs. SI Figure 6 contains emission spectrum of Cu doped Si NCs that had



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been stored under ambient conditions at an excitation wavelength of 360 nm 6 months after synthesis. The emission intensity of Cu doped NCs dropped by 60 % over the 6 month period. The emission peak having the same position as the previously recorded emission spectra 6 months earlier suggests that recombination pathways offered by metal dopants still remain. Internally doped NCs are known for demonstrating superior photostability compared to surface doped NCs.11-12 Cu has the highest solubility of all transition metals in silicon, as well as a high diffusion co-efficient.31 The combination of both high solubility and high diffusion co-efficient likely causes the Cu dopants to be internalized within the Si NC core rather than on the NC surface like the Mn or Ni doped Si NCs giving the Cu doped Si NCs greater photostability.

Figure 3. a) TA spectra of doped and undoped Si NCs measured 1 ps after photoexcitation with 100 fs pulses centered at 280nm. Spectra were renormalized to undoped Si according to the absorbed excitation photon density. (b) Normalized TA kinetics for the same samples


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probed at 350 – 450 nm and 500-600 nm. The color of the each sample in the kinetic traces is the same as in Figure 3a. Ultrafast TA spectroscopy was performed to directly resolve the effect of doping on Si NC excited state dynamics.32 The TA spectra for undoped and doped Si NCs can be seen in Figure 3a. Undoped Si NCs produce strong excitonic photoinduced absorption around 350 nm, along with broad induced absorption throughout the visible, peaking around 475 nm. The visible peak has previously been attributed to photoinduced absorption associated with population of surface trap states in undoped Si NCs.33 Kinetic traces representing both wavelength regions can be seen in Figure 3b; both induced absorption peaks mostly decay within a few picoseconds, and a dispersive tail remains in the case of the visible peak as a result of the trap distribution. For doped Si NCs, the excitonic UV photoinduced absorption peak (Figure 3a) has already disappeared within the first picosecond of excitation. The UV kinetics in figure 3b reveals excitonic decay times within the ~100 fs instrument limit (illustrated by the solvent-only trace). Doped Si NCs show broad induced absorption peaks in the visible region, peaking at 589 nm, 590 nm, and 600 nm. These peaks are similar to the surface trap peak in the undoped Si NCs, but redshifted by over 100 nm relative to the undoped Si NCs. The observed redshifts correlate strongly with the redshifts seen in PL measurements (Figure 2), highlighting the role of the metal dopants in creating new sub-gap dopant states. The kinetic trace between 500 nm – 600 nm shows that the peak in the visible region for Mn, Ni and Cu doped Si NCs has a longer decay time compared to undoped Si NCs, with substantial intensity remaining beyond 10 ps. Similar behavior is resolved in the near IR region (SI Figure 7). The longer decay times seen for visible TA kinetics further distinguish the doped from the undoped Si NCs. The spectral shift and prolonged decay kinetics observed for doped Si NCs are consistent with relaxation to lower energy traps associated with the metal dopants, which 9 ACS Paragon Plus Environment


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is also reflected in the red-shifted PL. Thus, relaxation of photoexcitations likely proceeds through two steps - exciton transfer to Si traps, followed by population of lower energy dopant states. A general low temperature route to synthesize transition metal (Mn, Ni, and Cu) doped Si NCs was developed with controllable size and dopant composition as determined by TEM and EDX. Undoped and doped Si NCs had comparable sizes which allowed comparison of the optical properties. The optical properties were studied using absorption (steady state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique optical properties including increased sub-gap absorption, and emission redshifts of over 40 nm. TA measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs. This study represents the first general approach to synthesize a range of transition metal doped Si NCs. There is great opportunity to expand the synthesis to traditional dopants (P, B, Al) as well co-doped NCs, it is expected this will lead to unique and enhanced optical and electronic properties.

ASSOCIATED CONTENT Supporting Information Detailed experimental methods can be found in the supporting information, including SI figures 1-7. This material can be found free of charge via the internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author **E-mail: [email protected]


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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thank the MacDiarmid Institute and MBIE for funding through grant UOAX0911

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Scheme 1. Schematic diagram of the synthesis of metal doped Si NCs

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Solution Synthesis and Optical Properties of Transition-Metal-Doped

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