The Surface Chemistry of Colloidal HgSe Nanocrystals, toward

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The Surface Chemistry of Colloidal HgSe Nanocrystals, Towards Stoichiometric Quantum Dots by Design Valeriia Grigel, Laxmi Kishore Sagar, Kim De Nolf, Qiang Zhao, André Vantomme, Jonathan De Roo, Ivan Infante, and Zeger Hens Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02908 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Chemistry of Materials

The Surface Chemistry of Colloidal HgSe Nanocrystals, Towards Stoichiometric Quantum Dots by Design Valeriia Grigel,†,‡,⊥ Laxmi Kishore Sagar,†,‡,⊥ Kim De Nolf,†,‡ Qiang Zhao,¶ Andr´e Vantomme,¶ Jonathan De Roo,†,§ Ivan Infante,k and Zeger Hens∗,†,‡ Physics and Chemistry of Nanostructures, Ghent University, Ghent, Belgium, Center for Nano and Biophotonics, Ghent University, Belgium, Instituut voor Kern-en Stralingsfysica, KU Leuven, Leuven, Belgium, Department of Chemistry, Columbia University, New York, United States, and Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, The Netherlands E-mail: [email protected]

Abstract The prevalent structure of binary semiconductor nanocrystals is a crystallite enriched in metal cations and terminated by anionic surface species that are classified as X-type ligands. Here, we demonstrate that HgSe NCs synthesized from selenourea and mercury chloride in oleylamine have a stoichiometry close to that of bulk HgSe and ∗

To whom correspondence should be addressed Physics and Chemistry of Nanostructures, Ghent University, Ghent, Belgium ‡ Center for Nano and Biophotonics, Ghent University, Belgium ¶ Instituut voor Kern-en Stralingsfysica, KU Leuven, Leuven, Belgium § Department of Chemistry, Columbia University, New York, United States k Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, The Netherlands ⊥ Contributed equally to this work †

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feature a surface terminated by oleylammonium chloride; a combination of composition and ligand capping that preserves charge neutrality. We demonstrate that oleylammonium chloride can be formed as a side product of the formation of HgSe in the particular reaction mixture used. We complement the experimental work with a detailed investigation by density functional theory of a [HgSe]55 model system. This analysis confirms that the combination of a stoichiometric HgSe nanocrystal and alkylammonium chloride ligands forms a stable structure. Moreover, DFT predicts that stoichiometric HgSe nanocrystals prefer binding of methylammonium chloride over binding of mercury chloride. We thus conclude that HgSe NCs are retrieved with this unconventional surface termination because it is (1) possible by synthesis and (2) preferred by thermodynamics. Finally, we argue that the identification of surface ligands as acids or bases provides a convenient alternative to the covalent bond classification for describing NC-ligand binding motifs involving ion-pairs.

Introduction Over the last 5-10 years, considerable progress has been made in the understanding of how surface active species bind and pack as ligands on the surface of colloidal nanocrystals (NCs). 1–5 A number of powerful experimental approaches have been developed that allow to specifically probe NC-bound ligands or nanocrystal surface atoms. A key step was the introduction of solution nuclear magnetic resonance (NMR) spectroscopy, which provides a selective view on bound ligands. 6–8 This enabled bound ligands to be identified and quantified, but it also proved a key step in the preparation of purified dispersions that only contain NCs, solvent and bound ligands. Starting from such dispersions, ligands and ligand-surface interactions could be further investigated using approaches complementary to solution NMR, including solid-state NMR, infrared spectroscopy or x-ray photoelectron spectroscopy. 9–11 In the case of semiconductor NCs or quantum dots (QDs) made of binary metal chalcogenides or pnictides, which include the II-VI, IV-VI and III-V semiconductors, several studies

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Table 1: Overview of the different nanocrystal-ligand binding motifs, including a schematic representations, a short hand notation and experimental examples for each motif. Green dots indicate surface cations, red dots surface anions. The amount of surface cations and anions depicted reflects the chemical composition (cation-rich or stoichiometric) of the nanocrystals.

led to a consolidated picture of ligand binding within the framework of the covalent bond classification (CBC) scheme. 12 NCs of materials such as CdSe, 13 CdTe, 14 PbS, 15 PbSe, 16 or InP 17 were found to be terminated by an excess of metal cations in combination with socalled X-type ligands like carboxylates or phosphonates. This binding motif was translated into a generic nanocrystal class, 13 which we abbreviate as [NC](MX2 ) (see Table 1). Here, the core nanocrystal is placed between square brackets, whereas round brackets are used to specify the surface active group. This focus on the binding motif, which stresses the chemical properties of the surface ligand, offers direct insight in possible ligand exchange reactions. In the case of the [NC](MX2 ) class, examples of such reactions involve the replacement of the X-type moiety 1,18 but also the entire displacement of the MX2 complex, for example, driven by complex formation with deliberately added L-type ligands. 13 The approach of classifying NCs according to the NC-ligand binding motif was success-

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fully applied to other nanocrystal materials, beyond the aforementioned semiconductors. 19 However, this led to a marked proliferation of the relevant binding motifs. In the case of metal oxide NCs, for example, it was found that carboxylic acids do not bind as carboxylates to excess metal cations. Rather, they dissociate upon binding such that the carboxylate interacts with acidic (metal) and the proton with basic (oxide) surface sites. 20 This results in the effective binding of two different X-type ligands in a binding motif that was described as [NC](X)2 . The same binding motif was later retrieved for CsPbBr3 NCs, which were found to be stabilized after synthesis by oleylammonium bromide. 21 Very recently, CdSe NCs stabilized through a similar binding motif, described in terms of ion pairs rather than 2 X-type ligands, could be prepared through judicious ligand exchange reactions. 22,23 In certain cases of stoichiometric NCs, L-type ligands stabilize the NC surface with a binding motif of [NC](L). 22,24–26 Clearly, such findings raise the question as to how general the prevailing [NC](MX2 ) binding motif is for as-synthesized II-VI and IV-VI NCs. Here, we take this question as a starting point to analyze the surface chemistry of HgSe NCs. Bulk HgSe is a zero-gap semiconductor, yet in the case of HgSe NCs, size quantization results in a non-zero gap that can be tuned across the entire infrared spectrum by reducing the NC size down to a few nanometers. 27 This makes HgSe NCs especially interesting for infrared photodetection, 27,28 and high quality methods to form HgSe NCs from selenourea and HgCl2 dissolved in oleylamine have been recently developed for this purpose. 27 By combining elemental analysis and solution NMR spectroscopy, we show that as-synthesized HgSe NCs are nearly stoichiometric, and feature a surface terminated by pairs of oleylammonium and chloride ions that ensure charge neutrality. This particular binding motif accounts for the ligand exchange characteristics of HgSe NCs, where exposure to dodecanethiol leads to the complete displacement of the ion pair from the HgSe surface, whereas carboxylic acids show little affinity for HgSe NCs. Using density functional theory (DFT) calculations on a [HgSe]55 (CH3 NH3 Cl)13 model NC, we show that the combination of a stoichiometric HgSe NC and a surface termination by alkylammonium chloride results in a stable nanocrystal

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structure. Moreover, DFT calculations indicate that CH3 NH3 Cl binds as a single moiety to the HgSe surface, where the chloride interacts with surface Hg atoms and the protons with surface selenium. Finally, we show that HgSe NCs are obtained with this particular stoichiometry and binding motif because (1) oleylammonium chloride can be formed as a by-product of the precursor conversion and (2) it makes a more stable surface bond than HgCl2 , the surface group that would make HgSe NCs non-stoichiometric nanocrystals terminated by X-type ligands. This finding highlights the close interplay between precursor chemistry, surface affinity for ligands and surface termination, and shows how nanocrystals with a close-to-bulk stoichiometry can be directly obtained from synthesis.

Experimental Section Chemicals. Mercury chloride (HgCl2 , 99.5%), selenourea (98%), dodecane thiol (DDT, 98%), tetracholoroethylene (TCE, 99.0%, ACS reagent), oleic acid (OAc, 90%), undecenoic acid (UDAc, 90%), and deuterated toluene (toluene-d8 , 99% atom D) were purchased from Sigma-Aldrich. Oleylamine (OlNH2 , 80-90%) was purchased from Acros Organics. All the solvents, such as toluene, methanol, 2-propanol, and acetonitrile were purchased from VWR.

HgSe synthesis. HgSe NCs with an average diameter of 5.8 nm were synthesized following Deng et al. 27 In a three-neck flask, 54 mg of HgCl2 (0.2 mmol) was dissolved in 8 mL of oleylamine (OlNH2 ). The reaction mixture was flushed under nitrogen and heated to 110 ◦ C for 1 h to remove dissolved water and oxygen. Afterwards, 24.6 mg (0.2 mmol) of selenourea dissolved in 2 mL of OlNH2 was injected at 110 ◦ C. After 6 minutes, the black reaction mixture was quenched with 5 mL of toluene, and the synthesized NCs were precipitated by addition of 15 mL of methanol to the reaction flask. After centrifugation, the supernatant was discarded, and the obtained NCs were purified a first time by dissolution in 3 mL toluene, precipitation with 2.5 mL methanol, and centrifugation. Two additional purification steps followed, using equal amounts (2.5 mL) of toluene/acetonitrile as solvent/anti-solvent 5



combination. HgSe NCs with an average diameter of 4.4 nm size were synthesized by injecting the selenourea-OlNH2 solution at 80 ◦ C, with a growth time of 5 minutes. Ligand Exchange Reaction. Dodecanethiol (DDT) capped HgSe NCs were prepared by a ligand exchange reaction, starting from purified, as synthesized HgSe NCs. As such, HgSe NCs (≈ 40 mg/mL) stabilized by OlNH2 (4.43 µmol) were loaded in a 10 mL vial and 720 µL of DDT (3 mmol) was added. The resulting solution was thoroughly stirred for 30 minutes at room temperature to facilitate the ligand exchange process. The NCs were purified twice with toluene/methanol and twice with toluene/acetonitrile to obtain dispersions of dodecane capped HgSe NCs. As purified NCs were used for NMR analysis.

Structural and Elemental Characterization. XRD samples were prepared by drop casting a dispersion of HgSe NCs in a hexane:heptane (80:20) solution to form a coating on a glass substrate. Measurements were performed on an ARL XTRA Powder Diffractrometer. Transmission electron microscopy (TEM) images were taken with an aberration corrected JEOL 2200-FS operated at 200 kV. The average composition of the NCs was determined by Rutherford backscattering spectrometry using a 1.57 MeV 4 He+ beam. The energy of the backscattered ions was measured with a standard PIPS detector positioned at a scattering angle of 166◦ . From the integrated intensity of the S, Cl, Se and Hg signals, the relative composition of the film – and thus the NCs – was determined taking the Z 2 dependence of the backscattering intensity into account. RBS experiments were done on the 5.8 nm and 4.4 nm sized HgSe/OlNH2 and HgSe/DDT NCs, after confirming their purity with NMR analysis. RBS samples were prepared by spincoating a thin layer of HgSe NCs on a magnesium oxide (MgO) substrate.

Optical characterization. Samples for UV-Vis absorption spectroscopy were prepared by taking known amounts of reaction aliquots or stock solutions of HgSe NCs dispersed in toluene and kept in a nitrogen-filled glovebox, drying these dispersions under nitrogen

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atmosphere and redispersing the obtained powder in tetracholoroethylene (TCE). The analysis was done using a Perkin-Elmer Lambda 950 UV-Vis-NIR spectrophotometer. The absorbance beyond 3 µm (0.41 eV) was measured using a Nicolet 6700 Fourier Transform Infrared Spectrometer. The volume fraction of HgSe was obtained from a measured absorbance using the intrinsic absorption coefficient µi,400 = 4.63 105 cm−1 of spherical HgSe, nanocrystals at 400 nm. This value was calculated using the optical constants of HgSe at this wavelength (n = 2.49, k = 1.75) , 31 and assuming a refractive index of the solvent of 1.5. By means of the diameter of HgSe NCs as determined by TEM, the HgSe NC concentration in a given samples was calculated from the HgSe volume fraction.

Solution Nuclear Magnetic Resonance Spectroscopy.

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H Nuclear magnetic reso-

nance (NMR) experiments were performed by evaporating HgSe NC dispersions to dryness under a continuous nitrogen flow and redispersing the obtained NC powder in 500 µL of dry toluene-d8 . NMR measurements were done on a Bruker Avance III spectrometer equipped with a BBI-Z probe and operating at a 1 H frequency of 500.13 MHz. One dimensional (1D) 1

H and 2D NOESY (nuclear Overhauser effect spectroscopy) spectra were acquired using

standard pulse sequences from the Bruker library. For the quantitative 1D 1 H measurements, 64k data points were sampled with the spectral width set to 16 ppm and a relaxation delay of 30 s. A J-modulated spin-echo method was used for the multiplicity editing of 1D

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C spectrum, 64k data points were sampled with the spectral width set to 240 ppm

and a relaxation delay of 2 s. The NOESY mixing time was set to 300 ms and 2048 data points in the direct dimension for 512 data points in the indirect dimension were typically sampled, with the spectral width set to 11.5 ppm. Diffusion measurements (2D DOSY) were performed using a double stimulated echo sequence for convection compensation and with monopolar gradient pulses. 29 Smoothed rectangle gradient pulse shapes were used throughout. The gradient strength was varied linearly from 2 to 95% of the maximum value within reach (calibrated at 50.2 G/cm) in 32 or 64 steps, with the gradient pulse duration and diffu-

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sion delay optimized to ensure a final attenuation of the signal in the final increment of less than 20% relative to the first increment. For 2D processing, the spectra were zero filled to a 4096 × 2048 real data matrix. Before Fourier transformation, the 2D spectra were multiplied with a squared cosine bell function in both dimensions, and the 1D spectra were multiplied with an exponential window function. Concentrations of compounds were obtained using the Digital ERETIC tool in Topspin 3.5 that is based on the PULCON method. For bound species, these concentrations were combined with the HgSe NC concentration in the NMR sample analyzed to obtain the surface concentration of ligands. For this calculation, the HgSe NCs were seen as spherical objects. The diffusion coefficients were obtained by fitting the Stejskal-Tanner equation to the signal intensity decay. 30

Theoretical Simulations. Full geometry optimization and electronic structure calculations of HgTe nanocrystals were carried out at the density functional level of theory (DFT) 32 using the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional 33 within the CP2K program package. 34 Calculations are carried out using a dual basis of Double-Zeta-ValencePolarization (DZVP) localized Gaussians and plane-waves with a 400Ry plane-wave cutoff. 35 Goedecker-Teter-Hutter pseudopotentials were employed for core electrons. 35 All relaxations were performed as non-periodic using a simulation box of 3.2 × 3.2 × 3.2 nm3 . Details on how the nanocrystal models were built are provided in the text.

Results Characteristics of As-Synthesized HgSe NCs We synthesized HgSe NCs by reacting HgCl2 and selenourea in oleylamine (OlNH2 ) at 110 ◦ C for 6 minutes, according to the procedure introduced by Deng et al. 27 (see Experimental Section for a detailed description). This resulted in dispersions of HgSe NCs that featured an absorbance spectrum consisting of a pronounced mid infrared absorption line peaking at

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Figure 1: Characterization of as-synthesized 5.8 nm HgSe NCs. (a) Absorbance spectrum of HgSe NCs dispersion in tetrachloroethylene. The trace is composed of two separately measured UV-Vis and IR spectra that were normalized relative to the absorbance measured at 400 nm. The inset shows the level diagram used to interpret the spectra, where (dashed lines) H-P indicates the second interband transition and (dotted lines) S-P the intraband transition. (b) (red) x-ray diffractogram of HgSe NCs and (black bars) bulk zinc blende HgSe. (c,d) (top) bright field TEM images and (bottom) size histogram of HgSe NCs. (e) (red) Rutherford backscattering spectrum and (blue) integrated backscattering intensities of a HgSe NC thin film spin-coated on a MgO substrate. The inset shows the signal corresponding to backscattering on chlorine atoms. ∼ 0.32 eV, a zero absorbance gap, and a continuous higher energy absorption band starting at ∼ 0.85 eV (see Figure 1a). These absorption characteristics are well-known for HgSe NCs, and they have been attributed before to the accumulation of electrons in the lowest bandedge state of HgSe NCs. As illustrated in the inset of Figure 1a, this makes that the mid infrared absorption can be assigned to an intraband transition, whereas the first high energy feature is assigned to electron excitations to the second conduction-band state. By means of x-ray diffraction and bright-field transmission electron microscopy imaging, we confirmed that the synthesis method used resulted in HgSe NCs with a zinc blende crystal structure, an average diameter of 5.8 nm and a size dispersions of ∼ 5% (see Figure 1b-d). The elemental composition of the HgSe NCs studied here was analyzed using Rutherford backscattering spectrometry (RBS). For this purpose, a 1-2 monolayer thick coating of as-synthesized 5.8 nm HgSe NCs was spun on a MgO substrate and the energy of backscattered 1.57 MeV He+ ions was analyzed under an angle of 166◦ . As shown in Figure 1e, this resulted in two narrow backscattering signals at energies of 1.450 and 1.288 MeV; signals 9



Table 2: Elemental composition of HgSe NCs as obtained from RBS spectra. Error margins are estimated to be 1-2% for the Se:Hg ratio and 30% for the Cl:Hg and S:Hg ratios (‡ sulfur signal too weak to be quantified). See Supporting Information S1 for a representation of all RBS spectra analyzed here. Sample 5.8 nm HgSe/OlNH2 5.8 nm HgSe/DDT 4.4 nm HgSe/OlNH2 4.4 nm HgSe/DDT

Se:Hg Cl:Hg S:Hg 1.04 0.08 – 1.05 – ‡ 1.00 0.2 – 1.00 – 0.1

that were assigned to backscattering from Hg and Se, respectively. In addition, a low intensity signal could be discerned at 1.00 MeV, which corresponds to backscattering from Cl. As shown in Supporting Information S1, such RBS spectra feature separate Hg and Se backscattering signals amid a nearly noise-free background. This enabled us to accurately determine the composition of as-synthesized HgSe NCs from the integrated intensity of the respective backscattering signals. Importantly, apart from weighing intensities by 1/Z 2 , with Z the atomic number of the respective element, this approach does not require any sensitivity factors or matrix corrections as is the case with methods based on photo-electron or x-ray fluorescence spectroscopy. In the case of the RBS spectrum shown in Figure 1e, this resulted in a Se:Hg ratio of 1.04 (see Table 2, sample 5.8 nm HgSe/OlNH2 ). Hence, opposite from other metal selenide nanocrystals such as CdSe 2,13,36 and PbSe, 37 we find that the HgSe NCs studied here can be slightly anion rich. We extended this composition analysis to 4.4 nm HgSe NCs, in which case we obtained a Se:Hg ratio of 1.00 for the as-synthesized NCs (see Table 2, sample 4.4 nm HgSe/OlNH2 ). This confirms that HgSe NCs are close to stoichiometric. In addition, we estimated the Cl:Hg ratio for both samples to be 0.08 and 0.2, respectively, which would correspond to a chlorine or chloride surface concentration of 1.3 and 2.5 nm−2 for these respective samples.

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Figure 2: (a)1D 1 H NMR spectrum of (red) OlNH2 and (black) a purified, 20 µM dispersion of as-synthesized 5.8 nm HgSe NCs in toluene-d8 . The resonances of OlNH2 are assigned to the different protons of OlNH2 as indicated. The # denotes solvent resonances. (b) Normalized resonance intensity as a function of the square of the field gradient plotted for (blue) the CH3 resonance 5, (red) the coinciding CH2 resonances 2 and (black) the alkene proton resonance 4. The vertical axes have a logarithmic scale and extend over 1 decade. The full lines are obtained from a global fit of the three decay traces to a double exponential, enforcing the same two decay constants for all traces. Both components are represented using (fast component) dotted and (slow component) dashed lines. The solvodynamice diameters determined thereof are indicated in the middle trace. (c) Two-dimensional NOESY spectrum of the same dispersion.

Surface Chemistry of As-Synthesized HgSe NCs To investigate the ligand capping of as-synthesized HgSe NCs, we used the solution NMR toolbox, including 1D, diffusion ordered and nuclear Overhauser effect 1 H NMR spectroscopy. 6 Since oleylamine is the only coordinating moiety used in the HgSe NC synthesis, Figure 2a represents the 1D 1 H spectrum of oleylamine and of a dispersion of 5.8 nm HgSe NCs purified using the toluene/acetonitrile solvent/non-solvent combination (see Experimental Section). At first sight, the spectrum of the NC dispersion shows the well-known set of broadened and shifted oleyl resonances that were assigned according to literature. 15,24 A closer inspection, however, reveals some intriguing inconsistencies. Most striking is the appearance of a wellresolved α−CH2 resonance 1 at ∼ 2.9 ppm together with a broad background extending up to ∼ 4 ppm. Whereas a resolved α−CH2 resonance is typical for oleyl moieties that exhibit

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a rapid exchange between a bound and a free state – as exemplified by the case of OlNH2 capped PbS NCs 15 – the extreme broadening of that resonance is typical for tightly bound ligands. Monitoring the diffusion behavior through pulsed field gradient spectroscopy provided insight into the joint presence of signals from bound and free ligands. As shown in Figure 2b, we found that the decay of the signal intensity as a function of the square of the field gradient of neither the CH3 resonance 5, nor the bulk of the CH2 resonances 2, nor the alkene resonance 4 fits to a single exponential, as would be expected for a species with a single diffusion coefficient. On the other hand, these decay traces could be fitted to double exponentials with the same two decay constants in all cases. These correspond to diffusion coefficients of 108 and 477 µm2 /s, which yield a solvodynamic diameter of 6.8 and 1.6 nm, respectively. This result indicates that 2 pools of OlNH2 are present in dispersions of assynthesized HgSe NCs. As the HgSe NCs have a diameter of 5.8 nm, we conclude that the slowly diffusing pool corresponds to tightly bound OlNH2 . On the other hand, since the solvodynamic diamter of OlNH2 is only 0.75 nm (see Supporting Information S2), we assigned the rapidly diffusing pool to OlNH2 in rapid exchange between a bound and a free state. Such a situation would indeed result in an effective diffusion coefficient that is a population-weighted average of that of bound and free OlNH2 . We confirmed the hypothesis that both pools of OlNH2 interact with the HgSe NCs through nuclear Overhauser effect spectroscopy (NOESY). As shown in Figure 2c, all OlNH2 related resonances feature strongly negative nOe crosspeaks, including the relatively narrow α−CH2 resonance 1 of the exchanging pool, a clear signature of ligand-nanocrystal interaction. 2 A quantitative analysis of the surface concentration of OlNH2 is difficult, since the contributions of both OlNH2 species are hard to separate. An estimate can be obtained from the difference between the intensity of the alkene resonance, which measures all OlNH2 species in the sample, and the resolved α−CH2 resonance, which only measures the amount of rapidly exchanging OlNH2 . This yields a surface concentration of rapidly exchanging OlNH2

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Figure 3: (a)(center, greyscale) zoom on the alkene region of the DOSY spectrum recorded on a dispersion of purified, as-synthesized HgSe NCs in toluene-d8 , featuring mostly the contribution of the rapidly exchanging OlNH2 species; (left, blue) the same, after addition of 10 equiv undecenoic acid (UDA) to the original HgSe NC dispersion; (right, red) the same, after addition of 10 equiv dodecanethiol (DDT) to the original HgSe NC dispersion. Dashed lines indicate the diffusion coefficient of (red) free OlNH2 and (blue) free UDA, respectively. (b) 1D 1 H NMR spectrum of 5.8 nm HgSe NCs after addition of 10 equiv of DDT. Equivalences are expressed with respect to the amount of OlNH2 originally present in the dispersion. of 2.2 nm−2 and of tightly bound OlNH2 of 1.5 nm−2 . Note that the latter is comparable to the surface concentration of chlorine we estimated from the RBS spectrum. The presence of tightly bound OlNH2 moieties on the surface of HgSe NCs is remarkable. In the case of comparable II-VI and IV-VI metal chalcogenides, such as CdSe, 7,22,38 CdTe 39 and PbS, 15 amines were always found to be in rapid exchange between a free and a bound state, a finding that is attributed to amines being relatively weak L-type ligands for these materials. 19 This may suggest that amines bind significantly stronger to HgSe NCs. As demonstrated by the example of oleate capped CdSe NCs exposed to an excess of oleic acid, 2 such a condition can explain the separate sets of slowly and rapidly diffusion OlNH2 species. Alternatively, it could imply that OlNH2 binds to HgSe NCs with two different binding motifs, an L-type motif similar to its interaction with CdSe, CdTe and PbS, and a second, as yet unknown motif that results in tightly bound OlNH2 moieties.

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Ligand Exchange to Carboxylic Acids and Alkane Thiols To further elucidate the HgSe-OlNH2 binding motif, we exposed purified dispersions of assynthesized HgSe NCs to either undecenoic acid (UDA) or dodecanethiol (DDT). For these experiments, undecenoic acid was chosen because of its distinct resonances in 1 H NMR, which strongly facilitate the subsequent analysis of 1D and DOSY spectra. 24 To assess the influence of UDA or DDT addition on the as-synthesized HgSe NCs, Figure 3a represents the alkene region of three DOSY spectra recorded on dispersions of 5.8 nm HgSe NCs. The middle spectrum corresponds to that of a purified dispersion of as-synthesized NCs, similar to the dispersions analyzed in the previous section. Note that the spectrum highlights in particular the contribution of the rapidly exchanging OlNH2 species. The DOSY spectrum in blue at the left in Figure 3a was recorded after addition of 10 equiv of UDA, relative to the amount of OlNH2 present, to a dispersion of as-synthesized HgSe NCs. In this case, the resonances of the two sets of alkene protons of UDA can be seen at around 5.8 and 5.0 ppm (see Supporting Information S3). Most remarkably, one sees that the diffusion coefficient of OlNH2 does not change upon addition of UDA, whereas the diffusion coefficient of the UDA resonances corresponds to that of free UDA. On the other hand, the NOESY spectrum of this dispersion shows that both OlNH2 and UDA resonances exhibit negative nOe crosspeaks (see Supporting Information S3). This indicates that only a minor fraction of UDA interacts at the time with the HgSe surface, without displacing measurable quantities of OlNH2 . This result is markedly different from what was found in the case of OlNH2 -capped PbS NCs, where an exposure of as-synthesized NCs to oleic acid resulted in the formation of an oleate ligand shell. 15 On the other hand, it resembles somewhat the case of HfO2 NCs, where carboxylic acids would only bind to the HfO2 surface upon addition of a Brønsted base. 20 We indeed found that upon further addition of OlNH2 , interaction between undecenoic acid and HgSe NCs does occur (see Supporting Information S4). The right DOSY spectrum in Figure 3a, represented in red, shows the diffusion characteristics of OlNH2 in a 5.8 nm HgSe NC dispersion after addition of 10 equiv of DDT. As 14


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Figure 4: (a) 1 H 1D NMR spectrum of dispersion of 5.8 nm HgSe NCs in toluene-d8 after addition of DDT and subsequent purification. The protons of DDT are denoted by the numbers as indicated. Solvent resonances are labeled by #. (b) DOSY spectrum recorded on the same dispersion of 5.8 nm HgSe NCs. (c) RBS spectra of (red) 4.4 nm HgSe/OlNH2 NCs and (blue) the same 4.4 nm HgSe NCs after DDT ligand exchange and purification. The inset shows a zoom on the light element region, indicating the presence of (red) chlorine for HgSe/OlNH2 NCs, and (blue) sulfur for HgSe/DDT NCs. compared to the central spectrum, it can be seen that DDT addition shifts the diffusion coefficient of OlNH2 more towards that of free OlNH2 . This points towards an increased fraction of free OlNH2 , most likely due to DDT displacing OlNH2 from the surface of HgSe NCs. Figure 3b shows the 1D 1 H NMR spectrum of 5.8 nm HgSe NCs after addition of 10 equiv of DDT. Focusing on the spectral range in between the aromatic and alkyl resonances, one indeed sees that the broad resonances of bound or exchanging OlNH2 have disappeared, whereas the more narrow resonances 1 and 4 typical of free OlNH2 have appeared. In addition, a broad resonance at around 6.5 ppm shows up, which has been attributed before to the N − H proton of an ammonium compound. This suggests that at least part of the OlNH2 − 20,21 is displaced from the HgSe NC surface as oleylammonium chloride (OlNH+ 3 Cl ).

Surface Chemistry of Thiol-Capped HgSe NCs After addition of DDT, HgSe NCs can be purified and redispersed using, for example, toluene/acetonitrile as the solvent/non-solvent combination (see Experimental Section). Figure 4a represents the 1 H 1D NMR spectrum of a 5.8 nm HgSe NC dispersion that we obtained

15



in this way. The absence of the characteristic alkene resonance confirms that after exposure to DDT and further workup, OlNH2 is effectively displaced from the HgSe surface and removed from the dispersion. On the other hand, the spectrum features broadened resonances characteristic of bound CH2 and CH3 protons. 1 These come with a diffusion coefficient of 73 ± 1 µm2 /s (Figure 4b), which correspond to a hydrodynamic diameter of 10.2 ± 0.2 nm. We thus conclude that after exposure to DDT, the mixture of rapidly exchanging and tightly bound OlNH2 ligands is replaced by a single set of tightly bound DDT ligands. To determine possible changes in composition after the exchange of OlNH2 by DDT, we analyzed the same HgSe NCs before and after ligand exchange using Rutherford backscattering spectrometry. Interestingly, we find that the Se:Hg ratio is hardly affected by the OlNH2 /DDT exchange. As shown in Table 2, as-synthesized 5.8 nm HgSe NCs (5.8 nm HgSe/OlNH2 ) and the same HgSe NCs after DDT exchange (5.8 nm HgSe/DDT) exhibit essentially the same Se:Hg ratio of 1.04 and 1.05, respectively. In addition, Figure 4c shows RBS spectra recorded on 4.4 nm HgSe NCs, where smaller NCs and somewhat thicker coatings were used to enhance the signal of light elements at the NC surface. As can be seen in Table 2, these 4.4 nm HgSe NCs feature the same Se:Hg ratio of 1.00 before and after DDT exposure. Moreover, although such a thicker film increases the background counts, the spectrum of the as-synthesized NCs again contains a clear signal from backscattering from chlorine, which confirms the assignments already made in Figure 2e. After DDT exposure, this chlorine signal disappears and a contribution from backscattering from sulfur appears on top of the background noise. This confirms the conclusion made above that DDT binds to the HgSe NCs and, in doing so, displaces all the OlNH2 and the chlorine originally present at the HgSe surface. The complete displacement of chloride from the HgSe surface by DDT addition enabled us to better quantify the equivalence between tightly bound OlNH2 and chloride in a sample of as-synthesized HgSe NCs. As outlined in Supporting Information S5, we set up an analysis in which bound OlNH2 was determined using solution NMR and chloride was analysed

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Figure 5: Schematic representation of the surface chemistry of HgSe NCs. (a) As-synthesized HgSe NCs are retrieved as stoichiometric nanocrystals stabilized by oleylammonium and chloride, and possibly oleylamine. (b) After exposure to dodecanethiol, stoichiometric HgSe NCs are obtained, stabilized solely by dodecanethiol that binds as an L-type ligand. photometrically after displacing all chloride from as-synthesized HgSe NCs by DDT addition. This yielded a ratio between both compounds in a dispersion of as-synthesized HgSe NCs of 1.0. Clearly, this result the indication we obtained from the RBS spectra that the surface concentration of tightly bound OlNH2 and chloride are comparable.

Binding Motifs in the Case of HgSe NCs The results presented here indicate that as-synthesized HgSe NCs are close to stoichiometric, while containing an excess of chlorine. In addition, the ligand shell of HgSe NCs consists of a combination of tightly bound OlNH2 moieties and OlNH2 moieties in rapid exchange, where the surface concentration of bound OlNH2 matches that of chlorine. Finally, an oleylammonium compound and chlorine are displaced from the HgSe surface upon exposure to DDT. All these features are in line with the structural model of HgSe NCs depicted in Figure 5a. The model represents a stoichiometric HgSe NC passivated by both oleylammonium and chloride, a combination that could be seen as a [HgSe](X)2 binding motif. In addition, we have included the possibility that oleylamine binds as an L-type ligand. As the negative charge on surface chloride is balanced by the positive charge on oleylammonium, this model reconciles the approximately 1:1:(0.1-0.2) Hg:Se:Cl stoichiometry and the need to have a charge neutral nanocrystal. Moreover, the binding motifs proposed can account for the joint presence of OlNH2 species in rapid and slow exchange. The rapidly exchanging

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OlNH2 pool could either involve free OlNH2 that exchanges by proton transfer with bound oleylammonium, a mechanism similar to what was observed with oleate-capped CdSe NCs exposed to an excess of oleic acid, or OlNH2 involved in a rapid L-type adsorption/desorption equilibrium. Exposure to DDT results in the complete removal of chlorine and OlNH2 from HgSe NC dispersion, whereas a resonance characteristic of NH+ 3 protons can be observed in the NMR spectra when this exchange is monitored in-situ. Both elements indicate that DDT displaces − OlNH+ 3 Cl from the HgSe surface, whereby it binds as an L-type ligand instead of an X-type

ligand (see Figure 5b). The latter interpretation is in line with the observation that exposure to DDT does not affect the Se:Hg ratio, which implies that HgSe NCs lack the cation excess to balance the negative charge on thiolate ligands. The same surface chemistry model also accounts for the lack of interaction of undecenoic acid with the HgSe surface. In principle, − such a carboxylic acid could displace entire OlNH+ 3 Cl (X)2 moieties by acting as an L-type

ligand. However, since carboxylic acids are very weak Lewis bases, this exchange reaction is expected to be highly unfavorable. Alternatively, undecenoic acid could exchange for chloride or bind as an ion-pair in combination with oleylammonium. The observation that increasing the amount of excess amine lowers the diffusion coefficient of both the exchanging amine and the carboxylic acid (see Supporting Information S4) indicates that the latter is the more favorable process. This would indeed imply that carboxylic acids can only bind provided a sufficient amount of amine is available.

− DFT Analysis of the HgSe(RNH+ 3 Cl ) Binding Motif

As highlighted in the previous section, HgSe NCs present an almost 1:1 stoichiometric pro− portion between Hg and Se atoms with a surface terminated by OlNH+ 3 Cl . This is in stark

contrast with the cation excess that is typically found in other II-VI colloidal NCs, which typically comes with a surface passivated by X-type ligands such as carboxylates or halides. − In Figure 5a, we have represented the binding of the OlNH+ ion-pair by means of a 3 Cl

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Figure 6: Representation of [HgSe]55 model NCs obtained at the DFT/PBE level of theory, featuring (yellow) Se, (dark gray) Hg, (green) Cl, (white) H, (blue) N, (light gray) C and (orange) S atoms in their relaxed position. (left) The original [HgSe]55 (HgCl2 )13 NC as obtained from cleaving a bulk crystal and passivating crystal facets by HgCl2 . (center) The − [HgSe]55 (MeNH+ 3 Cl )13 NC as obtained by progressively replacing HgCl2 by methylammo− + nium chloride (MeNH+ 3 Cl ). The insets provide a detailed view on the binding of MeNH3 and Cl− from two different perspectives. (right) The [HgSe]55 (MeSH)13 obtained by replacing − all MeNH+ 3 Cl by methane thiol (MeSH). The insets provide a detailed view on the binding of MeSH. dissociated species, with both the chloride and the oleylammonium binding to the HgSe surface. This description is in line with the [NC](X)2 binding motif as introduced in the case of − metal oxide NCs. On the other hand, one could argue that surface binding of OlNH+ 3 Cl to

HgSe NCs resembles the recently described surface termination of CdSe NCs by ammonium carboxylates. 22,23 Here, the carboxylate is described as the binding species that forms a first coordination sphere that binds through ion-pair formation with an alkylammonium in the second coordination sphere. − To elucidate the actual binding of OlNH+ 3 Cl to HgSe NC surfaces, we investigated a − [HgSe]55 model NC terminated by methylammonium chloride (MeNH+ 3 Cl ) by means of

density functional theory (DFT). Here, methylammonium chloride was used as a substitute − for OlNH+ to reduce the computational cost. The [HgSe]55 model was built starting 3 Cl

from a non-stoichiometric zinc blende HgSe NC of about 2 nm that was cleaved from the bulk HgSe crystal and passivated with X-type chlorine atoms to maintain charge balance; a model we denote as [HgSe]55 (HgCl2 )13 . We then selectively replaced HgCl2 surface groups by 19



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− MeNH+ 3 Cl . At each replacement, we performed a structural relaxation at the DFT/PBE

level of theory, and used the relaxed structure as a starting point for the next replacement. This ensured that the NC would be stable upon each ligand replacement and not break apart. We verified the structural integrity of the NC models by means of the integrated radial pairdistribution functions (pdf) of the Hg-Se, Hg-Hg and Se-Se atom pairs, computed for the relaxed HgSe NC structures. As shown in Supporting Information S6, this analysis indicates that stable structures are obtained in which Hg atoms prove to be more mobile at the surface than Se atoms, which stick more rigidly to bulk-like positions. − By means of the model NCs, we analyzed the behavior of both MeNH+ 3 Cl and methanethiol

(MeSH) as ligands for HgSe NCs. In the case of MeSH, we initially tested both an L-type binding motif, and a dissociated X2 binding motif with the methane thiolate binding to Hg2+ and the proton to Se2− surface sites. However, since the proton and the methane thiol spontaneously associated to form methanethiol during the initial surface reconstruction, we − did not consider the X2 binding motif futher. For the MeNH+ 3 Cl ion-pair, the outcome of

the DFT calculations is represented by the structural model shown in the middle of Figure 6. It follows that the ammonia groups sit on crystalline sites usually occupied by the Hg ions. Here, the ammonium headgroup forms two, presumably weak, hydrogen bonds with the nearby Se atoms and the third hydrogen points towards the Cl− anion, which simultaneously passivates one of the Hg2+ cations at the surface. It thus appears that both compounds of − the OlNH+ 3 Cl ion-pair interact with the HgSe surface, in line with the binding motif that

proposed by the structural model shown in Figure 5. Overall, the interaction energy of the − MeNH+ 3 Cl ion-pair with the surface is computed to be about -55 kcal/mol. As shown at

the right of Figure 6, MeSH binds to the HgSe surface by forming only one hydrogen bond with a surface Se atom, while the sulphur atom of the thiol group binds directly to the Hg atom. In the case of MeSH, the interaction with the HgSe surface is considerably smaller, amounting to only -17.4 kcal/mol.

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Figure 7: (a) Tentative mechanism that can account for the formation of oleylammonium chloride as a side reaction of the conversion of HgCl2 and selenourea into HgSe. (b) 13 C NMR spectrum of a blank reaction mixture diluted in THF-d8 . The blank reaction involved the injection of selenourea in oleylamine in the absence of HgCl2 . Next to resonances of the solvent and oleylamine, the guandine resonance 4 is retrieved in the spectrum. − Preferred Formation of the HgSe(RNH+ 3 Cl ) Binding Motif − The finding that stoichiometric HgSe NCs, terminated by OlNH+ 3 Cl , are obtained from

a synthesis where HgCl2 and selenourea react in oleylamine is puzzling. The formation of PbS out of PbCl2 and elemental sulfur in oleylamine, for example, results in PbCl2 termi− nated NCs that are dynamically stabilized by OlNH2 . 15 Termination of HgSe by OlNH+ 3 Cl − therefore raises the questions as to (1) how OlNH+ 3 Cl is formed since the initial reagents − seemingly lack a proton and (2) why the HgSe NCs preferentially bind OlNH+ 3 Cl instead

of, for example, HgCl2 . − We hypothesize that OlNH+ 3 Cl can be formed as a byproduct of the conversion of the

reagents into HgSe. As outlined in Figure 7a, selenourea could react with OlNH2 to form H2 Se and oleylguanidine. This reaction would create a pathway to produce HgSe and HCl – and − thus OlNH+ 3 Cl – out of the reaction between H2 Se and HgCl2 . We investigated the possible

occurrence of this reaction pathway by means of a blank reaction involving the injection of selenourea in plain OlNH2 . Figure 7b reproduces the

13

C spectrum of this blank reaction

mixture. Next to the resonances that characterize the oleyl chain, the spectrum contains the characteristic resonance of the guanidine carbon 4 at 157 ppm. 40 While this analysis

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Table 3: Total binding energy and its decomposition in a deformation and an interaction part as calculated at the DFT/PBE level of theory for a set of L-type and X2 -type ligands and the [HgSe](HgCl2 )12 model NCs. All energies are in kcal/mol. L-type ligands are characterized additionally by their acidity constant. AcH stands for acetate. Ligand

HgCl2 − MeNH+ 3 Cl MeNH2 MeSH AcH HCl

Type

Z X2 L L L L

pKa

Deformation Energy

[HgSe](HgCl2 )12 60.5 22.7 40 9.5 10-11 4.7 4-5 0.7 -7 0.7

Interaction Total Binding Energy Energy

Ligand Total 42.0 102.5 14.1 36.8 0.5 10.0 0.2 4.9 0.5 1.2 0.3 1.0

does not exclude other reaction pathways – note that some

13

-111.4 -54.5 -27.9 -17.3 -8.5 -4.8

-8.9 -17.7 -17.9 -12.4 -7.4 -3.8

C resonances in Figure 7b

remain as yet unidentified – it does show that the pathway outlined in Figure 7a occurs − and can account for the in-situ formation of OlNH+ 3 Cl during the reaction. We therefore − conclude that the termination of as-synthesized HgSe by OlNH+ 3 Cl is probably linked to

the particular combination of reagents used in the synthesis. Possibly, this implies that also other metal chalcogenide NCs can be synthesized as stoichiometric nanocrystals with an − OlNH+ 3 Cl surface termination, when using similar reaction conditions. − To understand why the in-situ formation of OlNH+ results in the termination of 3 Cl − the HgSe NC surface by OlNH+ 3 Cl , we extended the DFT simulations, and addressed the

binding of several ligands to the [HgSe]55 (HgCl2 )12 model NC. For convenience, we took the latter as a reference NC to compare all the computed binding interaction energies to the nanocrystal surface. To do so, the interaction energy is retrieved from the following reaction:

[HgSe]55 (HgCl2 )12 + L → [HgSe]55 (HgCl2 )12 (L)

(1)

Here, L denotes any possible ligand that can form an adduct with the HgSe NC; in practice, this will be an ordinary L-type ligand or an ion-pair.

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For comparison, we decomposed the interaction energy in terms of two contributions. The first, the deformation energy, measures the energy required to bring both reactants from their stable (isolated) geometrical conformations to the one they assume in the super adduct. The second, the instantaneous interaction energy is the interaction energy between the two reactants calculated in the configurations they have in the adduct. The sum of these two contributions represents the actual binding energy. For example, addition of a HgCl2 moiety to the [HgSe]55 (HgCl2 )12 model NC comes with a very large instantaneous interaction energy of -111.4 kcal/mol (see Table 3). This is expected because the deformed HgCl2 fits exactly the crystalline site still available at the HgSe surface. However, the deformation energy is very large, as both the isolated model NC and the HgCl2 ligand are markedly deformed in the eventual adduct. For example, the linear HgCl2 molecule ends up as a bent structure on the NC surface. The overall deformation energy cancels a large part of the instantaneous interaction energy, resulting in a rather weak total binding energy of -9.0 kcal/mol. − Binding between a MeNH+ 3 Cl ion pair and the HgSe model NC yields an instantaneous

interaction energy of -55 kcal/mol. In combination with the relatively small deformation energy of the two fragments, this results in a total binding energy of -17.7 kcal/mol. While − still not particularly large, it indicates that MeNH+ can indeed be a stronger ligand 3 Cl

than HgCl2 , which makes that HgSe NCs in the reaction mixture will preferentially bind − MeNH+ 3 Cl . Additionally, we looked at the total binding energy for several organic L-type

ligands, characterized by an increased acidity. As can be seen in Table 3, the notable feature of this study is that we retrieve an almost linear correlation between the binding energy and the acidity constant of a ligand, which indicates that more acidic species are indeed weaker L-type ligands. This analysis provides a few important outcomes that are confirmed by the experiments: − (1) the binding of the MeNH+ 3 Cl ion-pair is favoured over the the insertion of the Z-type − HgCl2 salt, explaining why the HgSe NC preferentially binds OlNH+ 3 Cl ion-pairs; (2) the

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interaction of carboxylic acid with the nanocrystal is rather weak and could explain why undecanoic acid does not bind as an L-type ligand to HgSe NCs; (3) the interaction with − MeSH, while weaker than MeNH+ 3 Cl , is still large enough to suggest that a displacement − of OlNH+ 3 Cl by DDT from the surface is feasible.

Discussion We have shown that HgSe NCs synthesized by reacting selenourea and HgCl2 in oleylamine − are stoichiometric and have a surface terminated by OlNH+ 3 Cl ion pairs. In Figure 5, we

have represented this surface termination by a structural model that builds on the CBC-based ligand binding motifs as listed in Table 1. However, one could argue that this intepretation of the binding motif is ambiguous. The structural model explicitly assigns charges to the chloride and the oleylammonium fragments – in line with their forming an ion-pair – whereas the CBC is solely based on neutral moieties. In addition, the DFT analysis shows that the − MeNH+ 3 Cl ion-pair, but also the MeSH L-type ligand, can form multiple interactions with

the HgSe surface. This suggests that apart from the intrinsic disregard of the CBC for the covalent or ionic character of the eventually formed bond, the classification of ligands as L, X, Z is also too crude to grasp the details of ligand/nanocrystal binding. 41 At this point, it is worth considering that the CBC was originally developed to describe ligand binding to a single central atom. However, nanocrystals can contain more than 1000 atoms and will in most cases expose facets with chemically distinct binding sites. Surfaces of metal chalcogenide NCs, for example, contain anions and cations, which may act as Lewis bases or Lewis acids, respectively. 19 Additional variety is linked to the different coordination of atoms present in the middle of facets, on edges or at vertices. 42–44 Finally, as mentioned above, the CBC is based on covalent interactions. It may therefore provide a counter-intuitive description of purely ionic interactions and will not include interactions through hydrogen bonds.

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Figure 8: The covalent bond classification compared to an ionic model. In the covalent model, the ligands are classified as L, X or Z, according to the number of electrons the ligand contributes to the nanocrystal-ligand bond. Ligands contributing one electron to either metal or nonmetal atoms are both classified as X-type ligand, potentially leading to confusion. However, the 4 binding situations can be easily represented in an ionic model where we consider neutral (Lewis) or charged acids or bases. In the field of organometallic compounds, the ionic description of metal-ligand interactions and the CBC are used in parallel. As reports on surface termination of nanocrystals by ion-pairs continue to increase, it may be helpful to adopt this dual approach to describe nanocrystal-ligand interactions as well (see Figure 8). In this respect, we could see surface ligands as acids (A) or bases (B) that bind to basic or acidic surface sites, respectively. Clearly, in the case of neutral acids and bases, the binding motifs [NC](A) and [NC](B) are identical to the [NC](Z) and [NC](L) motifs put forward in the CBC. In the case of carboxylate capped, cation rich nanocrystals, [NC](MX2 ), the anionic ligands are charged bases, coordinating to cationic nanocrystals and resulting in the equivalent notation [NC](MB2 ). On the other hand, the ambiguous [NC](X)2 motif would be replaced by [NC](AB). Here, A and B could still be neutral moieties, yet they may also represent the cation and the − anion of an ion-pair such as OlNH+ 3 Cl . An advantage of this representation is that charges

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can be introduced at will to better capture the actual (ionic) character of the nanocrystalligand bond. To give an example, the approach allows for a simple notation to differentiate the three isomers of the [NC](A+ B− ) nanocrystal, which are NC(A+ B− ), [NC(A)]+ B− and [NC(B)]− A+ , where in the first isomer both ions interact with the surface, in the second only the acidic cation interacts and in the last only the basic anion does. Although it is very difficult to distinguish the isomers via spectroscopic methods, the DFT analysis included in − this study suggests that the presently studied NCs are best described as HgSe(RNH+ 3 Cl ).

Conclusion We demonstrated that colloidal HgSe nanocrystals synthesized by reaction HgCl2 and selenourea in oleylamine combine a close to stoichiometric core with a surface termination of oleylammonium chloride ion pairs. This binding motif is unusual for metal sulfide or selenide nanocrystals. By combining experimental results and simulations at the DFT level, which take the experimental surface termination into account, we attribute the formation of this binding motif to the in-situ formation of oleylammonium chloride during synthesis and the preference of HgSe surfaces to bind this ion pair over HgCl2 . This particular binding motif accounts for specific ligand exchange reactions, where exposure to dodecanethiol results in fully thiol-capped nanocrystals while carboxylic acids show little affinity for HgSe NCs. These results highlight the interplay between the precursor chemistry and the eventual nanocrystal surface termination, where the in-situ formation of ion-pairs could provide a general pathway to form stoichiometric nanocrystals. In addition, we put forward a notation relying on acidic and basic surface moieties as an alternative to classify colloidal nanocrystals by means of ligand binding motifs that is particularly suited to describe surface termination of nanocrystals by ion-pairs.

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Acknowledgement We are thankful to Willem Walravens for useful discussions. Z.H. acknowledges support by the European Commission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCAITN-642656) and COMPASS (H2020-MSCA-RISE-691185), IWT-Vlaanderen (SBO-MIRIS), the FWO-Vlaanderen (FWO17/PRJ/380) and Ghent University (GOA 01G01513) for funding. I.I. would like to thank the Netherlands Organization of Scientific Research (NWO) for providing financial support within the Innovational Research Incentive (Vidi) Scheme (Grant 723.013.002). The computational work was carried out on the Dutch national einfrastructure with the support of SURF Cooperative.

Supporting Information Available The Supporting Information provides additional information on (S1) the RBS analysis, (S2) the determination of the diffusion coefficient of the different ligands in toluene, (S3) the addition of UDA to purified HgSe nanocrystals and (S4) to amine-rich HgSe dispersions, (S5) the analysis of the oleylammonium:chloride ratio and (S6) the radial pair-distribution function of model HgSe NCs.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

References 1. Owen, J. S.; Park, J.; Trudeau, P. E.; Alivisatos, A. P. Reaction Chemistry and Ligand Exchange at Cadmium-Selenide Nanocrystal Surfaces. J. Am. Chem. Soc. 2008, 130, 12279–12281. 2. Fritzinger, B.; Capek, R. K.; Lambert, K.; Martins, J. C.; Hens, Z. Utilizing SelfExchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots. J. Am. Chem. Soc. 2010, 132, 10195–10201.

27



3. Owen, J. The Coordination Chemistry of Nanocrystal Surfaces. Science 2015, 347, 615– 616. 4. Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nat. Mater. 2016, 15, 141–153. 5. De Nolf, K.; Cosseddu, S. M.; Jasieniak, J. J.; Drijvers, E.; Martins, J. C.; Infante, I.; Hens, Z. Binding and Packing in Two-Component Colloidal Quantum Dot Ligand Shells: Linear versus Branched Carboxylates. J. Am. Chem. Soc. 2017, 139, 3456–3464. 6. Hens, Z.; Martins, J. C. A Solution NMR Toolbox for Characterizing the Surface Chemistry of Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1211–1221. 7. Morris-Cohen, A. J.; Malicki, M.; Peterson, M. D.; Slavin, J. W. J.; Weiss, E. A. Chemical, Structural, and Quantitative Analysis of the Ligand Shells of Colloidal Quantum Dots. Chem. Mater. 2013, 25, 1155–1165. 8. Smith, A. M.; Johnston, K. A.; Crawford, S. E.; Marbella, L. E.; Millstone, J. E. Ligand Density Quantification on Colloidal Inorganic Nanoparticles. Analyst 2017, 142, 11–29. 9. Cros-Gagneux, A.; Delpech, F.; Nayral, C.; Cornejo, A.; Coppel, Y.; Chaudret, B. Surface Chemistry of InP Quantum Dots: A Comprehensive Study. J. Am. Chem. Soc. 2010, 132, 18147–18157. 10. Knauf, R. R.; Lennox, J. C.; Dempsey, J. L. Quantifying Ligand Exchange Reactions at CdSe Nanocrystal Surfaces. Chem. Mater. 2016, 28, 4762–4770. 11. Torelli, M. D.; Putans, R. A.; Tan, Y.; Lohse, S. E.; Murphy, C. J.; Hamers, R. J. Quantitative Determination of Ligand Densities on Nanomaterials by X-ray Photoelectron Spectroscopy. ACS Appl. Mater. Inter. 2015, 7, 1720–1725. 12. Green, M. A new approach to the formal classification of covalent compounds of the elements. J. Organomet. Chem. 1996, 500, 127–148. 28


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S. Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding. J. Am. Chem. Soc. 2013, 135, 18536–48. 14. Kamal, J. S.; Omari, A.; Van Hoecke, K.; Zhao, Q.; Vantomme, A.; Vanhaecke, F.; Capek, R. K.; Hens, Z. Size-Dependent Optical Properties of Zinc Blende Cadmium Telluride Quantum Dots. J. Phys. Chem. C 2012, 116, 5049–5054. 15. Moreels, I.; Justo, Y.; De Geyter, B.; Haustraete, K.; Martins, J. C.; Hens, Z. SizeTunable, Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study. ACS Nano 2011, 5, 2004–2012. 16. Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. Surface Chemistry of Colloidal PbSe Nanocrystals. J. Am. Chem. Soc. 2008, 130, 15081–15086. 17. Tamang, S.; Lincheneau, C.; Hermans, Y.; Jeong, S.; Reiss, P. Chemistry of InP Nanocrystal Syntheses. Chem. Mater. 2016, 28, 2491–2506. 18. Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q.; Vantomme, A.; Martins, J. C.; Hens, Z. Binding of Phosphonic Acids to CdSe Quantum Dots: A Solution NMR Study. J. Phys. Chem. Lett. 2011, 2, 145–152. 19. De Roo, J.; De Keukeleere, K.; Hens, Z.; Van Driessche, I. From Ligands to Binding Motifs and Beyond; the Enhanced Versatility of Nanocrystal Surfaces. Dalton Trans. 2016, 45, 13277–13283. 20. De Roo, J.; Van Den Broeck, F.; De Keukeleere, K.; Martins, J. C.; Van Driessche, I.; Hens, Z. Unravelling the Surface Chemistry of Metal Oxide Nanocrystals, the Role of Acids and Bases. J. Am. Chem. Soc. 2014, 136, 9650–9657. 21. De Roo, J.; Ib´an ˜ez, M.; Geiregat, P.; Nedelcu, G.; Walravens, W.; Maes, J.; Martins, J. C.; Van Driessche, I.; Kovalenko, M. V.; Hens, Z. Highly Dynamic Ligand Bind29



ing and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals. ACS Nano 2016, 10, 2071–2081. 22. Chen, P. E.; Anderson, N. C.; Norman, Z. M.; Owen, J. S. Tight Binding of Carboxylate, Phosphonate, and Carbamate Anions to Stoichiometric CdSe Nanocrystals. J. Am. Chem. Soc. 2017, 139, 3227–3236. 23. Zhou, Y.; Buhro, W. E. Reversible Exchange of L-Type and Bound-Ion-Pair X-Type Ligation on Cadmium Selenide Quantum Belts. J. Am. Chem. Soc. 2017, 139, 12887– 12890. 24. Dierick, R.; Van den Broeck, F.; De Nolf, K.; Zhao, Q.; Vantomme, A.; Martins, J. C.; Hens, Z. Surface Chemistry of CuInS2 Colloidal Nanocrystals, Tight Binding of L-Type Ligands. Chem. Mater. 2014, 26, 5950–5957. 25. Valdez, C. N.; Schimpf, A. M.; Gamelin, D. R.; Mayer, J. M. Low Capping Group Surface Density on Zinc Oxide Nanocrystals. ACS Nano 2014, 8, 9463–9470. 26. Shavel, A.; Ibez, M.; Luo, Z.; De Roo, J.; Carrete, A.; Dimitrievska, M.; Gen, A.; Meyns, M.; Prez-Rodrguez, A.; Kovalenko, M. V. et al. Scalable Heating-Up Synthesis of Monodisperse Cu2 ZnSnS4 Nanocrystals. Chem. Mater. 2016, 28, 720–726. 27. Deng, Z.; Jeong, K. S.; Guyot-Sionnest, P. Colloidal Quantum Dots Intraband Photodetectors. ACS Nano 2014, 8, 11707–11714. 28. Lhuillier, E.; Scarafagio, M.; Hease, P.; Nadal, B.; Aubin, H.; Xu, X.; Lequeux, N.; Patriarche, G.; Ithurria, S.; Dubertret, B. Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz. Nano Lett. 2016, 16, 1282–1286. 29. Connell, M. A.; Bowyer, P. J.; Adam Bone, P.; Davis, A. L.; Swanson, A. G.; Nilsson, M.;

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Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Morris, G. A. Improving the Accuracy of Pulsed Field Gradient NMR Diffusion Experiments: Correction for Gradient Non-uniformity. J. Magn. Reon. 2009, 198, 121–131. 30. Sinnaeve, D. The StejskalTanner Equation Generalized for Any Gradient Shape: an Overview of Most Pulse Sequences Measuring Free Diffusion. Concept. Magn. Reson. A 2012, 40A, 39–65. 31. Adachi, S. Optical Constants of Crystalline and Amorphous Semiconductors; Springer Science: New York, 1999. 32. Parr, R. G.; Yang, W. Density-functional theory of atoms and molecules; Oxford University Press, 1989; p 333. 33. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. 34. Hutter, J.; Iannuzzi, M.; Schiffmann, F.; Vandevondele, J. Cp2k: Atomistic Simulations of Condensed Matter Systems. Wires. Comput. Mol. Sci. 2014, 4, 15–25. 35. VandeVondele, J.; Hutter, J. Gaussian Basis Sets for Accurate Calculations on Molecular Systems in Gas and Condensed Phases. J. Chem. Phys. 2007, 127, 114105. 36. Jasieniak, J.; Mulvaney, P. From Cd-rich to Se-rich - The Manipulation of CdSe Nanocrystal Surface Stoichiometry. J. Am. Chem. Soc. 2007, 129, 2841–2848. 37. Moreels, I.; Lambert, K.; De Muynck, D.; Vanhaecke, F.; Poelman, D.; Martins, J. C.; Allan, G.; Hens, Z. Composition and Size-Dependent Extinction Coefficient of Colloidal PbSe Quantum Dots. Chem. Mater. 2007, 19, 6101–6106. 38. Hassinen, A.; Moreels, I.; Donega, C. d. M.; Martins, J. C.; Hens, Z. Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines. J. Phys. Chem. Lett. 2010, 1, 2577–2581.

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39. Fritzinger, B.; Moreels, I.; Lommens, P.; Koole, R.; Hens, Z.; Martins, J. C. In Situ Observation of Rapid Ligand Exchange in Colloidal Nanocrystal Suspensions Using Transfer NOE Nuclear Magnetic Resonance Spectroscopy. J. Am. Chem. Soc. 2009, 131, 3024– 3032. 40. Olah, G.; Arwed, B.; Golam, R.; Mohammed, H.; Surya, P. G. K. 1 H, 13 C, 15 N NMR and Ab Initio/IGLO/GIAO-MP2 Study of Mono-, Di-, Tri-, and Tetraprotonated Guanidine. J. Am. Chem. Soc. 1997, 119, 12929–12933. 41. Giansante, C.; Infante, I. Surface Traps in Colloidal Quantum Dots: A Combined Experimental and Theoretical Perspective. J. Phys. Chem. Lett. 2017, 8, 5209–5215, PMID: 28972763. 42. Aruda, K. O.; Amin, V. A.; Thompson, C. M.; Lau, B.; Nepomnyashchii, A. B.; Weiss, E. A. Description of the Adsorption and Exciton Delocalizing Properties of pSubstituted Thiophenols on CdSe Quantum Dots. Langmuir 2016, 32, 3354–64. 43. Drijvers, E.; De Roo, J.; Martins, J. C.; Infante, I.; Hens, Z. Ligand Displacement Exposes Binding Site Heterogeneity on CdSe Nanocrystal Surfaces. Chem. Mater. 2018, 30, 1178–1186. 44. Saniepay, M.; Mi, C.; Liu, Z.; Abel, E. P.; Beaulac, R. Insights into the Structural Complexity of Colloidal CdSe Nanocrystal Surfaces: Correlating the Efficiency of Nonradiative Excited-State Processes to Specific Defects. J. Am. Chem. Soc. 2018, 140, 1725–1736.

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The Surface Chemistry of Colloidal HgSe Nanocrystals, toward

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