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Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands Gerard Michael Carroll, Rens Limpens, and Nathan R. Neale Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00680 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands Gerard M. Carroll, Rens Limpens, and Nathan R. Neale* Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Email: [email protected] Abstract. The optical properties of silicon nanocrystals (Si NCs) are a subject of intense study and continued debate. In particular, Si NC photoluminescence (PL) properties are known to depend strongly on the surface chemistry, resulting in electron-hole recombination pathways derived from the Si NC band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. In this Letter, we perform a comparison of three different saturated surface functional groups—alkyls, amides, and alkoxides—on nonthermal plasma-synthesized Si NCs. We find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Time-resolved photoluminescence and transient absorption spectroscopies reveal no change in the excited-state dynamics between Si NCs functionalized with alkyl, amide, or alkoxide ligands, showing for the first time that saturated ligands—not only surface-derived charge transfer states or hybridization between NC and low-lying ligand orbitals—are responsible for tuning the Si NC optical properties. To explain these PL shifts we propose that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement. These results reveal a potentially broadly applicable correlation between the optoelectronic properties of Si NCs and related quantum-confined structures based on the interaction between NC surfaces and the ligand binding group. Keywords: Silicon nanocrystals; quantum confinement; photoluminescence; surface chemistry; lifetime; recombination. The size-tunable optoelectronic properties and solution processability of colloidal semiconductor nanocrystals (NCs) make them highly attractive candidates for disruptive solar, photocatalytic, thermoelectric, light-emitting, and photodetection technologies.1 Because of its dominance in the microelectronics and solar industries as well as its vast earthly abundance, silicon (Si) is of particular interest for nanoscale technologies. Perhaps the most widely investigated trait of semiconductor NCs are their tunable photoluminescence (PL) properties. Within the quantum confinement regime ( *Si–OR, but not for Si–NHR (which should be the most blue-shifted according to its σ value). The deviation of *Si–NHR from this trend could arise from poor orbital overlap between *Si and –NHR due to restricted rotational degrees of freedom at the Si NC surface.

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Figure 1. FTIR spectra of (a) Si–Hx stretching region and (b) the full FTIR spectra for d = 6.5 nm Si NC as-synthesized, H-terminated powder (black dashed line, red fill), and following ligation with alkyl *Si–CH2R (from 1-dodecene, black, solid), amide *Si–NHR (from 1-dodecylamine, blue), and alkoxide *Si–OR (from 1-dodecanol, red) groups. The spectra are normalized to the peak of their *SiHx absorptions, and offset in 1b for clarity.

Oxidation at the surface of Si NCs invariably accompanies synthesis or functionalization—or both—with varying impacts on the optical properties.3, 5 From the full FTIR spectrum plotted in Figure 1b, Si NC surface oxidation products such as *Si–O–*Si and (O)*SiHx are identifiable in all spectra with distinct IR absorption features at ν ~1100 cm–1 and 2240 cm–1, respectively.18, 35 With the exception of *Si–OR, however, the absorption intensity of the *Si–O–*Si feature at 1100 cm–1 is comparable for both *Si–CH2R and *Si–NHR, indicating a similar degree of oxidation between these two samples. We attribute the small absorption in Figure 1a for *Si–OR at 2210 cm–1 to a silicon hydride where the silicon is also bound to an oxide and an alkoxide, (RO)(O)*SiHx. The absorption feature at 3380 cm–1 in the *Si–NHR spectrum is assigned to the N–H stretch of the amide group bound to *Si. The N–H stretch of such secondary amines in FTIR display a characteristic sharp absorption feature with a single peak at 3300–3400 cm–1, where primary amines display two peaks in the same region. For comparison, this spectrum is plotted against neat 1-dodecylamine (Figure S1) that clearly shows our sample is a secondary amine (silyl amide, *Si–NHR). The presence of this single peak implies that a sizeable fraction of dodecylamide groups are singly bound to the Si NC surface (*Si–NHR) and do not bridge two adjacent Si surface sites (*Si–(R)N–*Si) or form a double bond to Si (silylimine, *Si=NR). Figure 2a displays the steady-state photoluminescence (PL) spectra from λ = 405 nm photoexcitation for six Si NC sizes with each of the three ligands. As determined by Gaussian fits to each spectrum, the peak PL energies of the alkyl-terminated (*Si–CH2R) NCs decreases from 1.58 eV to 1.26 eV as the nominal size increases, consistent with relaxing quantum confinement. Silicon NCs with alkyl surface functionalization display excitonic band-edge recombination that, in the absence of surface defects, is a reliable “reference point” to which the effects of other ligands can be compared.3, 10, 36, 37 We previously detailed an empirical relationship between the PL peak energy and the alkyl-functionalized Si NC diameter.25 Whereas NC size calculations from the effective mass approximation (EMA) can be influenced by sizedependent changes in the PLQY,9 this empirical formula for alkylated Si NCs was derived from TEM and XRD sizing analysis of numerous Si NC samples. This method thus does not rely on knowledge of the PLQY for each NC size and serves to identify the average size of the ensemble. Using this relationship to calculate the NC size, the *Si–CH2R NCs from Figure 2 range in diameter from 3.5–6.9 nm, all within the quantum confinement regime. The same batches of Si NC powder functionalized with the two non-alkyl head group ligands (*Si–NHR and *Si–OR) display markedly different PL peak energies than that of the corresponding *Si–CH2R NCs (Figure 2). The PL peak energies for the same Si NCs with *Si–

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NHR ligation is higher in energy (1.71–1.27 eV) than the *Si–CH2R analogue (cf. 1.58–1.26 eV). Furthermore, the magnitude of the difference in PL peak energies between *Si–NHR and *Si–CH2R for a given NC size increases as the NC size decreases. Similarly, the *Si–OR NCs also exhibit size-dependent blue-shifted PL peak maxima relative to the *Si–CH2R NCs, but to a lesser degree than *Si–NHR.

PLQY 3.5 nm Si 6.5 nm Si

*Si–CH2R 9% 9%

*Si–NHR 11% 9%

*Si–OR 8% 10%

Figure 2. (a) Photoluminescence spectra of d = 3.5 (purple), 3.8 (blue), 4.5 (green), 5.3 (yellow), 6.3 (red), and 6.9 nm (black) colloidal Si NCs with alkyl *Si–CH2R, amide *Si–NHR, and alkoxide *Si–OR surface functionalization using a λexe = 405 nm photoexcitation source. All

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spectra are normalized at their maximum intensities and offset vertically for clarity. The dashed lines of each color indicate the maximum PL energy of the alkyl *Si–CH2R NCs for a particular size as the reference value as described in the text. (b) Scatter plot of the PL peak energies for the Si NCs with three different surface ligands versus Si NC diameter. The peak energies were determined by fitting the PL spectra with a Gaussian function. The size of the Si NCs were determined by the PL maximum of the alkyl-functionalized Si NCs. The inset shows the difference in peak PL energies between *Si–CH2R and *Si–NHR (blue), or *Si–OR (red) for the same Si NCs. The table lists the photoluminescence quantum yield for two Si NC sizes with all ligands. PLQY values have been rounded to the nearest percent.

The width of the PL spectra also are dependent on the ligand binding group and NC size (Figure S2, Table S1). The full width at half-maximum (FWHM) of the d = 3.5 nm *Si– CH2R NC PL spectrum is 0.26 eV (comparable to the 0.27 eV found for oxide-embedded Si NCs),38 whereas the *Si–NHR NCs is broadened to FWHM = 0.33 eV. The difference in FWHM between *Si–CH2R and *Si–NHR NCs decreases in magnitude as the size of the Si NCs increase; for the largest Si NCs (d = 6.9 nm), *Si–CH2R NC PL FWHM is 0.20 eV, and similarly, the *Si–NHR NC PL FWHM is 0.21 eV. Broadening FWHM values with decreasing size relative to the alkyl-functionalized NCs also are observed in the *Si–OR NC samples, but similar to the PL peak energy shift, to a lesser degree than *Si–NHR NCs. Reports of single particle Si NC PL measurements at room temperature reveal FWHM on the order of 0.10–0.15 eV,39,40 and polydispersity in Si NC size is also expected to contribute to the PL energy breadth,23,25 but an increasing FWHM with surface functionalization is not expected for supposedly “innocent” saturated ligands. Indeed, these results provide strong evidence that the steady-state PL characteristics of these Si NCs depend strongly on the ligand binding atom. To visualize these changes in peak PL energies for the different Si NC surface chemistries more clearly, we plot the peak PL energies for each sample as a function of the NC size in Figure 2b. From this plot, the surface-ligand-dependent PL characteristics are apparent: both *Si–NHR and (to a lesser degree) *Si–OR NC PL peak energies deviate from those of *Si– CH2R, and the magnitude of the differences increases as Si NC size decreases. Given that PL spectra can be skewed by differences in PLQY between NC sizes within the same polydisperse sample,6,9,41 we determined the PLQY of both large (6.5 nm) and small (3.5 nm) Si NC samples with all three ligand binding groups. The PLQY values were very similar across all different ligands and NC sizes (8–11%; Table in Figure 2) and consistent with our previous report.25 Thus, we conclude that PLQY is not responsible for the altered in PL spectra between *Si–CH2R, *Si– NHR, and *Si–OR. Next, we considered the impact of the radiative electron-hole recombination pathway, which is known to play a large role in determining the PL energies in colloidal Si NCs. Radiative decay is observed from both band-edge exciton relaxation as well as non-excitonic surface-state pathways for a variety of surface ligands.3,5 Of particular interest to this study are oxide and oxynitride surface-states as well as nitrogen-based ligand states that bring about supra-band gap surface-state mediated PL.8,10,11,14,15 Hallmark signatures of these relaxation channels are

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accelerated recombination dynamics from ~103–104 s–1 for Si NC band-edge emission to ~108– 109 s–1 for the surface-state mediated pathway.3,8,10,14 To examine the possible presence of these non-excitonic decay channels affecting the PL peak energies in our samples, we measured the excited-state dynamics in time domains ranging from the ps- to the ms-regime with timeresolved photoluminescence (TRPL) and transient absorption (TA) measurements for Si NCs with the largest ligand-dependent PL deviations (d = 3.5 nm). In Figure 3a we plot the PL decay dynamics of colloidal d = 3.5 nm alkyl *Si–CH2R, amide *Si–NHR, and alkoxy *Si–OR group-bound Si NCs. All TRPL traces exhibit PL extending longer than 500 µs after the excitation pulse. The data are fit to double-exponential decay function with the longer time component exhibiting stretched exponential decay behavior, characteristic of a large distribution of single channel modes.40, 42 From the fits, both fast (τ1) and slow (τ2) time components are nearly identical for *Si–NHR, *Si–CH2R, and *Si–OR at τ1 = 13, 13, and 11 µs, and τ2 = 133, 119, and 104 µs, respectively. The τ2 timescales are consistent with phonon-mediated, band-edge recombination in Si NCs.3 The slight elongation of τ2 in the *Si– NHR relative to *Si–CH2R and *Si–OR NCs may be the result of a reduced concentration of non-radiative recombination pathways at the *Si–NHR NC surface, as indicated by the slightly higher PLQY for the amide versus alkyl and alkoxy samples (Table in Figure 2).

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Figure 3. (a) Normalized photoluminescence decay dynamics for d = 3.5 nm colloidal Si NCs with alkyl (*Si–CH2R, black), amide (*Si–NHR, blue), and alkoxide (*Si–OR, red) ligation. These data were fit to double exponential decay functions represented by the solid red line (*Si–NHR, τ1 = 13 µs, τ2 = 133 µs), the dashed red line (*Si–CH2R, τ1 = 13 µs, τ2 = 119 µs), and the dotted black line (*Si–OR, τ1 = 11 µs, τ2 = 104 µs). (b) Transient absorption data collected on the same Si NCs from 3a. The Si NCs were excited with λpump = 500 nm and probed with a white light continuum. The traces show absorption averaged from λprobe = 1000–1200 nm.

Next, TA measurements were conducted to probe the carrier dynamics in the ultrafast (ps-ns) timescale. The same functionalized Si NC dispersions from TRPL measurements were pumped with interband excitation at λpump = 500 nm and probed with a NIR continuum (absorbance averaged from 1000–1200 nm). Excitation was performed in the linear excitation regime to avoid Auger-induced recombination effects.5, 43 From the kinetic traces in Figure 3b the ultrafast dynamics between the alkyl *Si–CH2R, amide *Si–NHR, and alkoxide *Si–OR NCs are nearly superimposable, where no sample exhibits appreciable decay on the ps to ns timescale. Because no nanosecond decay channel is introduced with amide and alkoxide functionalization relative to alkyl functionalization, we conclusively rule out the commonly observed ns decay channels as the cause of our observed PL energy shifts. To the best of our knowledge, this is the first record of nitrogen-ligated colloidal Si NC PL with a slow (µs) recombination channel indicative of band edge emission. Thus, the observed high-energy PL shift in Si NCs with amide *Si–NHR and alkoxide *Si–OR versus alkyl *Si–CH2R ligation thus does not result from a change in recombination pathways and requires an alternative explanation. One possible explanation of our results is that p- or n-type doping—which gives rise to plasmonic intraband absorption features and blue-shifted PL in n-type relative to undoped Si NCs38—causes the observed PL shifts. p-Type doping was suggested from ammonium bromideand allylamine-ligated Si NCs based on shifts of the Fermi level toward the valence band measured by scanning tunneling spectroscopy.44 In our samples, intraband absorption features and accelerated recombination dynamics, both signatures of degenerate doping, are absent, ruling out this possiblity.45, 46 Another possible explanation of these PL shifts is that the density of surface ligands is different between the alkyl, amide, and alkoxide-functionalized Si NC samples, since a dependence on ligand density has been shown to affect band gap energies in colloidal semiconductor NCs.47 We conducted exhaustive studies that also rule out this possibility. 1H NMR and ICP-AES data demonstrate that the density of surface ligands between *Si–CH2R and *Si–NHR NCs, the samples with the largest PL deviations, are the same within experimental uncertainty at 3.8±0.9 nm–2 and 4.0±1.1 nm–2, respectively (see Figures S3, S4 and Table S2 and associated discussion for full details). Furthermore, from reaction time-dependent PL measurements using an identical batch of Si NCs functionalized with two different ligands, it is clear that *Si–CH2R and *Si–NHR NC PL peak energies diverge at the beginning of the ligation process and asymptotically approach their respective maxima (Figure 4). If indeed the ligand density were responsible for these optical changes, the expected PL shifts would follow

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the same energetic trajectory with increasing reaction time, instead of shifting in opposite directions. Alternatively, structural changes to Si NCs through surface oxidation,48, 49 or bond distortion50 have been reported in colloidal Si NCs. From powder X-ray diffraction measurements on three Si NC sample batches with all three ligands (Figure S5 and Table S3), no differences in the Si NC diamond-packing structure was observed, and only minimal differences (4.2±0.1 nm, 5.0±0.2 nm, 5.2±0.2 nm) in NC sizes were determined from Scherrer analysis between the three surface functionalizations.

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Figure 4. (a) Photoluminescence spectra of as-synthesized, H-terminated d = 3.9 nm Si NC powder during the radical initiated reaction with either 1-dodecylamine (blue) or 1-dodecene (black). The *Si–NHR PL spectra were recorded after 1 min (light blue) and 3 min (dark blue) under reaction conditions. The *Si–CH2R spectra were recorded after 30 min (grey) and 3 h (black). The dotted and dashed lines indicated the peak PL energy. (b) Peak photoluminescence of d = 5.1 nm Si NCs during the radical initialed reaction with either 1-dodecylamine (blue, circles) or 1-dodecene (black, squares) as a function of the reaction time. (c) Photoluminescence spectra of d = 3.9 nm *Si–NHR (blue), *Si–CH2R (black), and ligand-exchanged *Si–NHR to *Si–CR (purple, dashed). All PL spectra have been normalized at their maximum intensities.

Additional evidence that the PL differences arise from the nature of the ligand and not other effects is provided by an experiment in which amide *Si–NHR NCs were converted to alkyl *Si–CH2R NCs (Figure 4c). Recent reports have demonstrated successful covalent ligand substitution reactions at Si NCs surfaces via thermally-initiated alkyl for thiolate51 or hydroboration-initiated alkoxide for alkyl35 ligands. In the latter work, a slight red-shift in the PL peak energy (12 meV) for d = 3 nm Si NCs was reported when alkoxide ligands were exchanged for alkyl, but the observation was not explained.35 Figure 4c shows the PL spectra for d = 3.9 nm *Si–NHR, *Si–CH2R, and *Si–NHR to *Si–CH2R substituted NC samples. Consistent with Figure 2, this *Si–NHR sample displays a blue-shifted PL peak energy and a broadened PL spectrum relative to the *Si–CH2R NC from the identical Si NC batch. The same d = 3.9 nm *Si–NHR NCs were dried and re-dispersed in a mixture of neat 1-dodecene, ABCN radical initiator, followed by heating to 140 °C for 3 h. The dashed purple spectrum in Figure 4c shows that this ligand exchange reaction red-shifts the PL peak energy from 1.65 eV to 1.59 eV, and narrows the FWHM from 0.33 to 0.30 eV (cf. 1.50 eV and 0.26 eV for all-alkyl *Si–CH2R NCs as shown in the black spectrum). FTIR spectra of the Si–Hx stretching region (Figure S6) displays similar peak shifts; the low energy edge absorption of the Si–Hx envelope of *Si–NHR Si NCs is blue shifted by ~10 cm–1 from that of *Si–CH2R. Following reaction of *Si–NHR Si NCs with 1-dodecene, the Si–Hx absorption edge of the resulting NCs red-shifts and overlays with that of the *Si–CH2R NCs. We attribute these spectral shifts to an incomplete exchange reaction between the amide and alkyl ligands. Nonetheless, the partial reversibility of the PL blue-shift and FTIR Si–Hx stretching frequencies upon partial exchange of amide for alkyl ligation clearly demonstrates that the atomic identity of the ligand binding atom influences the radiative energy. The similar excited state dynamics between *Si–CH2R, *Si–NHR, and *Si–OR NCs show that the radiative mechanism in these samples is band-edge electron-hole recombination. The observed variation in PL peak energies thus implies a strong electronic interaction between the ligand binding atom and the Si NC surface. The Si NC concentration-corrected absorption spectra (Figure S7) displays an attenuated absorptivity of *Si–NHR and *Si–OR compared to the *Si–CH2R samples indicating that altered optical properties are present in both the ground-state and excited-state. To account for these results, we propose that the ligand binding atom modulates the Si NC quantum confinement. Covalent bond formation between surface *Si atoms and the heteroatom of amide and alkoxide ligands constricts the Si NCs wave function and blue-

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shifts the excitonic PL relative to Si NCs covalently bound to methylene groups of alkyl ligands. Based on this proposed hypothesis, the magnitude of the difference in PL peak energy between amide, oxide, and alkyl ligands should be Si NC size-dependent, and increase with decreasing NC size, as observed here.3 The origin of these changes in Si NC PL energies is intriguing. A recent computational analysis of Si NCs embedded in an amorphous Si matrix showed that the energetic difference between the Si NCs and the matrix highest occupied molecular orbitals (HOMOs) changes the degree of quantum confinement in the Si NC, termed “confinement softening”.52 Other computational analyses have found the amide and alkoxy termination at the surface of silicon clusters raises the semiconductor band edges relative to both alkyl and hydride termination.37 The formation of these *Si–N and *Si–O bonds may impart similar effects to those described in solid matrices and provide a strategy for manipulating the Si NC electronic structure using only the surface silicon atom *Si–ligand bond. To summarize, we extend our radical-initiated surface functionalization reaction of nonthermal plasma synthesized Si NCs beyond alkenes and demonstrate successful reactions with amines and alcohols. We characterize functionalized and GPC-purified Si NCs using FTIR, 1 H NMR and ICP-AES spectroscopies and find a surface ligand coverage of 3–4 ligands nm–2 irrespective of ligand type. The photoluminescence properties of these Si NCs are highly sensitive to the identity of the ligand binding atom where the amide- and alkoxide-bound dodecyl ligands shift the PL to higher energies relative to their alkylated counterparts. The magnitude of the PL blue-shift depends on the Si NC size and increases as the Si NC size decreases. Timeresolved photoluminescence and transient absorption spectroscopies reveal no difference in the dynamics between the alkyl-, amide-, and alkoxy-ligated Si NCs, suggesting the dominant radiative recombination pathway is excitonic in nature, and does not proceed via charge-transfer states as has been observed in all other Si NC systems with nitrogen-based and many oxygenbased ligands. We therefore propose that these PL shifts arise from changes to the NC quantum confinement dictated by the ligand chemistry that are not accounted for in current quantum confinement models. The ability to modify the electronic structure of Si NCs simply by changing the identity of the ligand binding atom may prove useful for developing advanced optoelectronic devices based on Si NCs and related quantum-confined structures.

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program under contract number DE-AC36-08GO28308 with the Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory (GMC and NRN). Ultrafast TA and PL spectroscopy was supported by NREL’s Director’s Fellowship Laboratory Directed Research & Development program (RL).

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