The Surface Chemistry of Colloidal HgSe Nanocrystals, toward

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Article Cite This: Chem. Mater. 2018, 30, 7637−7647

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The Surface Chemistry of Colloidal HgSe Nanocrystals, toward Stoichiometric Quantum Dots by Design Valeriia Grigel,†,‡,⊥ Laxmi Kishore Sagar,†,‡,⊥ Kim De Nolf,†,‡ Qiang Zhao,¶ Andre ́ Vantomme,¶ Jonathan De Roo,†,§ Ivan Infante,|| and Zeger Hens*,†,‡ †

Physics and Chemistry of Nanostructures and ‡Center for Nano and Biophotonics, Ghent University, 9000 Gent, Belgium Instituut voor Kern-en Stralingsfysica, KU Leuven, 3001 Leuven, Belgium § Department of Chemistry, Columbia University, New York, New York 10027, United States || Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, 1081 HZ Amsterdam, The Netherlands

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S Supporting Information *

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 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.

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PbSe,16 or InP17 were found to be terminated by an excess of metal cations in combination with so-called 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 not only the replacement of the X-type moiety1,18 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 successfully 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

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 us to specifically probe NCbound 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 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 © 2018 American Chemical Society

Received: July 10, 2018 Revised: October 8, 2018 Published: October 22, 2018 7637

DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647

Article

Chemistry of Materials

that CH3NH3Cl 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 byproduct of the precursor conversion and (2) it makes a more stable surface bond than HgCl2, the surface group that would make HgSe NCs nonstoichiometric 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.

Table 1. Overview of the Different Nanocrystal-Ligand Binding Motifs, Including a Schematic Representation, a Short Hand Notation and Experimental Examples for Each Motifa

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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, 2propanol, 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. Afterward, 24.6 mg (0.2 mmol) of selenourea dissolved in 2 mL of OlNH2 was injected at 110 °C. After 6 min, 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/antisolvent 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 min. 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 min 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 4He+ 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 Z2 dependence of the backscattering intensity into account. RBS experiments were done on the 5.8- and 4.4-nm-sized HgSe/OlNH2 and HgSe/DDT NCs, after confirming their purity with NMR analysis. RBS samples were prepared by spin-coating 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

a

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.

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 assynthesized 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 nonzero 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(CH3NH3Cl)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 structure. Moreover, DFT calculations indicate 7638

DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647

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Figure 1. Characterization of as-synthesized 5.8 nm HgSe NCs. (a) Absorbance spectrum of a HgSe NC dispersion in tetrachloroethylene. The trace is composed of two separately recorded 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. aliquots or stock solutions of HgSe NCs dispersed in toluene kept in a nitrogen-filled glovebox, drying these dispersions under nitrogen atmosphere and redispersing the obtained powder in tetracholoroethylene (TCE). The analysis was done using a PerkinElmer 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. 1H nuclear magnetic resonance (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 1H frequency of 500.13 MHz. One-dimensional (1D) 1H and 2D NOESY (nuclear Overhauser effect spectroscopy) spectra were acquired using standard pulse sequences from the Bruker library. For the quantitative 1D 1H 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 13C 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 diffusion 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 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 HgSe nanocrystals were carried out at the density functional level of theory (DFT)32 using the Perdew−Burke−Ernzerhof (PBE) exchange correlation functional33 within the CP2K program package.34 Calculations are carried out using a dual basis of Double-Zeta-Valence-Polarization (DZVP) localized Gaussians and plane-waves with a 400 Ry plane-wave cutoff.35 Goedecker-Teter-Hutter pseudopotentials were employed for core electrons.35 All relaxations were performed as nonperiodic 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.

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RESULTS We synthesized HgSe NCs by reacting HgCl2 and selenourea in oleylamine (OlNH2) at 110 °C for 6 min, 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 ∼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 band-edge state of HgSe NCs. As illustrated in the inset of Figure 1a, 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 assynthesized 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 that were assigned to backscattering from Hg and Se, respectively. In addition, a 7639

DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647

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used the solution NMR toolbox, including 1D, diffusion ordered and nuclear Overhauser effect 1H NMR spectroscopy.6 Since oleylamine is the only coordinating moiety used in the HgSe NC synthesis, Figure 2a represents the 1D 1H spectrum of oleylamine and of a dispersion of 5.8 nm HgSe NCs purified using the toluene/acetonitrile solvent/nonsolvent 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 the literature.15,24 A closer inspection, however, reveals some intriguing inconsistencies. Most striking is the appearance of a well-resolved 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 a rapid exchange between a bound and a free stateas exemplified by the case of OlNH2 capped PbS NCs15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 as-synthesized 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 fast exchange between a bound and a free state. Such a situation would indeed result in

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/Z2, 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 photoelectron 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 Table 2. Elemental Composition of HgSe NCs as Obtained from RBS Spectraa Sample 5.8 nm 5.8 nm 4.4 nm 4.4 nm

HgSe/OlNH2 HgSe/DDT HgSe/OlNH2 HgSe/DDT

Se:Hg

Cl:Hg

S:Hg

1.04 1.05 1.00 1.00

0.08 − 0.2 −

− ‡ − 0.1

a 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.

other metal selenide nanocrystals such as CdSe2,13,36 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. Surface Chemistry of As-Synthesized HgSe NCs. To investigate the ligand capping of as-synthesized HgSe NCs, we

Figure 2. (a) 1D 1H 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, (green) the coinciding CH2 resonances 2, and (red) 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 solvodynamic diameters determined thereof are indicated in the middle trace. (c) Two-dimensional NOESY spectrum of the same dispersion. 7640

DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647

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Figure 3. (a) (center, grayscale) 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 1H 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.

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 assynthesized 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 compared to the central spectrum, it can be seen that DDT addition shifts the diffusion coefficient of OlNH2 more toward that of free OlNH2. This points toward an increased fraction of free OlNH2, most likely due to DDT displacing OlNH2 from the surface of HgSe NCs. Figure 3b shows the 1D 1H 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

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 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 more strongly 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 OlNH 2 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. Ligand Exchange to Carboxylic Acids and Alkane Thiols. To further elucidate the HgSe-OlNH2 binding motif, we exposed purified dispersions of as-synthesized HgSe NCs to either undecenoic acid (UDA) or dodecanethiol (DDT). For these experiments, undecenoic acid was chosen because of its distinct resonances in 1H 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 7641

DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647

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Chemistry of Materials

Figure 4. (a) 1H 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.

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 analyzed photometrically after displacing all chloride from assynthesized 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 confirms 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 OlNH 2 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

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 is displaced from the HgSe NC surface as oleylammonium chloride (OlNH3+Cl−).20,21 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/ nonsolvent combination (see Experimental Section). Figure 4a represents the 1H 1D NMR spectrum of a 5.8 nm HgSe NC dispersion that we obtained 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

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.

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) 7642

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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(MeNH3+Cl−)13 NC as obtained by progressively replacing HgCl2 by methylammonium chloride (MeNH3+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 MeNH3+Cl− by methane thiol (MeSH). The insets provide a detailed view on the binding of MeSH.

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 OlNH3+Cl− to HgSe NC surfaces, we investigated a [HgSe]55 model NC terminated by methylammonium chloride (MeNH3+Cl−) by means of density functional theory (DFT). Here, methylammonium chloride was used as a substitute for OlNH3+Cl− to reduce the computational cost. The [HgSe]55 model was built starting from a nonstoichiometric 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 MeNH3+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 pair-distribution 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 MeNH3+Cl− and methanethiol (MeSH) as ligands for HgSe NCs. In the case of MeSH, we initially tested both an Ltype 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 further. For the MeNH3+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 toward the Cl− anion, which simultaneously passivates one of the Hg2+ cations at the surface. It thus appears that both compounds of the OlNH3+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

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 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 NH3+ protons can be observed in the NMR spectra when this exchange is monitored in situ. Both elements indicate that DDT displaces OlNH3+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 OlNH3+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(RNH3+Cl−) Binding Motif. As highlighted in the previous section, HgSe NCs present an almost 1:1 stoichiometric proportion between Hg and Se atoms with a surface terminated by OlNH3+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 OlNH3+Cl− ion-pair by means of a 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 OlNH3+Cl− to HgSe NCs resembles the recently described surface termination of 7643

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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) 13C 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 guanidine resonance 4 is retrieved in the spectrum.

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 NCsa Deformation Energy Ligand

Type

HgCl2 MeNH3+Cl− MeNH2 MeSH AcH HCl

Z X2 L L L L

pKa

[HgSe](HgCl2)12

Ligand

Total

Interaction Energy

Total Binding Energy

40 10−11 4−5 −7

60.5 22.7 9.5 4.7 0.7 0.7

42.0 14.1 0.5 0.2 0.5 0.3

102.5 36.8 10.0 4.9 1.2 1.0

−111.4 −54.5 −27.9 −17.3 −8.5 −4.8

−8.9 −17.7 −17.9 −12.4 −7.4 −3.8

a

All energies are in kcal/mol. L-type ligands are characterized additionally by their acidity constant. AcH stands for acetate.

MeNH3+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 sulfur 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. Preferred Formation of the HgSe(RNH3+Cl−) Binding Motif. The finding that stoichiometric HgSe NCs, terminated by OlNH3+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 terminated NCs that are dynamically stabilized by OlNH2.15 Termination of HgSe by OlNH3+Cl− therefore raises the questions as to (1) how OlNH3+Cl− is formed since the initial reagents seemingly lack a proton and (2) why the HgSe NCs preferentially bind OlNH3+Cl− instead of, for example, HgCl2. We hypothesize that OlNH3+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 H2Se and oleylguanidine. This reaction would create a pathway to produce HgSe and HCland thus OlNH3+Cl− out of the reaction between H2Se 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 13C 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 does not exclude other reaction pathwaysnote that some 13C

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 OlNH3+Cl− during the reaction. We therefore conclude that the termination of assynthesized HgSe by OlNH3+Cl− is probably linked to the particular combination of reagents used in the synthesis. Possibly, this implies that other metal chalcogenide NCs can also be synthesized as stoichiometric nanocrystals with an OlNH3+Cl− surface termination, when using similar reaction conditions. To understand why the in situ formation of OlNH3+Cl− results in the termination of the HgSe NC surface by OlNH3+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, a Z-type ligand or an ion-pair. 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 7644

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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 counterintuitive description of purely ionic interactions and will not include interactions through hydrogen bonds. 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

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 MeNH3+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 MeNH3+Cl− can indeed be a stronger ligand than HgCl2, which makes that HgSe NCs in the reaction mixture will preferentially bind MeNH3+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 MeNH3+Cl− ion-pair is favored over the insertion of the Ztype HgCl2 salt, explaining why the HgSe NC preferentially binds OlNH3+Cl− ion-pairs; (2) the 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 MeNH3+Cl−, is still large enough to suggest that a displacement of OlNH3+Cl− by DDT from the surface is feasible.

Figure 8. 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 nanocrystalligand 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.

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DISCUSSION We have shown that HgSe NCs synthesized by reacting selenourea and HgCl2 in oleylamine are stoichiometric and have a surface terminated by OlNH3+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 ionpairwhereas the CBC is solely based on neutral moieties. In addition, the DFT analysis shows that the MeNH3+Cl− ionpair, as well as 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

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 ionpair such as OlNH3+Cl−. An advantage of this representation is that charges can be introduced at will to better capture the actual (ionic) character of the nanocrystal-ligand 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(RNH3+Cl−). 7645

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CONCLUSION We demonstrated that colloidal HgSe nanocrystals synthesized by reacting 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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tional work was carried out on the Dutch national einfrastructure with the support of SURF Cooperative.

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(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 Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots. J. Am. Chem. Soc. 2010, 132, 10195−10201. (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 TwoComponent 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. Interfaces 2015, 7, 1720−1725. (12) Green, M. A new approach to the formal classification of covalent compounds of the elements. J. Organomet. Chem. 1995, 500, 127−148. (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. Size-Tunable, 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

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02908. 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 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jonathan De Roo: 0000-0002-1264-9312 Ivan Infante: 0000-0003-3467-9376 Zeger Hens: 0000-0002-7041-3375 Author Contributions ⊥

Valeriia Grigel and Laxmi Kishore Sagar contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 computa7646

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Chemistry of Materials

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DOI: 10.1021/acs.chemmater.8b02908 Chem. Mater. 2018, 30, 7637−7647