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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Optical Absorbance Enhancement in PbS QD/Cinnamate Ligand Complexes Daniel Kroupa, Márton Vörös, Nicholas P. Brawand, Noah D Bronstein, Brett W. McNichols, Chloe V. Castaneda, Arthur J Nozik, Alan Sellinger, Giulia Galli, and Matthew C. Beard J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01451 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry Letters

Optical Absorbance Enhancement in PbS QD/Cinnamate Ligand Complexes Daniel M. Kroupa,1,2 Márton Vörös,3,4 Nicholas P. Brawand,4 Noah Bronstein,1 Brett W. McNichols,5 Chloe V. Castaneda,1 Arthur J. Nozik,1,2 Alan Sellinger,1,5 Giulia Galli,3,4* and Matthew C. Beard1* 1. Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States. 2. Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States. 3. Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States 4. Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States 5. Department of Chemistry and Materials Science Program, Colorado School of Mines, Golden, Colorado 80401, United States. Corresponding Authors ([email protected], [email protected]) Abstract We studied the optical absorption enhancement in colloidal suspensions of PbS quantum dots (QD) upon ligand exchange from oleate to a series of cinnamate ligands. By combining experiments and ab initio simulations, we elucidate physical parameters that govern the optical absorption enhancement. We find that within the cinnamate/PbS QD system the optical absorption enhancement scales linearly with the electronic gap of the ligand, indicating that the ligand/QD coupling occurs equally efficient between the QD and ligand HOMO and their respective LUMO levels. Disruption of the conjugation that connects the aromatic ring and its substituents to the QD core causes a reduction of the electronic coupling. Our results further support the notion that the ligand/QD complex should be considered as a distinct chemical system with emergent behavior rather than a QD core with ligands whose sole purpose is to passivate surface dangling bonds and prevent agglomeration. TOC Graphic

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Introduction Common synthetic procedures to prepare colloidal semiconductor nanocrystals, specifically quantum dots (QDs), result in the termination of the nanocrystalline lattice with aliphatic organic molecules such as alkyl carboxylates, alkyl thiolates, alkyl amines, and/or alkyl phosphonates.1-7 These surface coordinated molecules, also known as ligands, control QD nucleation kinetics during synthesis and impart colloidal stability of the product, as well as passivate unsaturated dangling bonds at the QD surface. The post-synthetic removal and replacement of these insulating, long-chain ligands is often necessary for the fabrication of electronically conductive QD films for optoelectronic applications, which has spurred intensive research examining the impacts of surface chemistry on QD optical and electronic properties.8-9 Recently, Giansante,10-11 Weiss12 and co-workers found that specific classes of ligands, specifically, short, aromatic, dipolar molecules, can enhance the optical absorption and/or shift the 1S exciton absorption energy of QDs, and that these changes are directly related to ligand/QD electronic interactions. We recently introduced a class of functionalized cinnamic acid ligands that can replace native, aliphatic oleate (OA-) ligands through a well-defined, 1:1 solution-phase X-type ligand exchange procedure.13 The advantages of the cinnamic acid based ligands are to preserve the carboxylate surface coordinating environment and core Pb:S stoichiometric ratio upon ligand exchange.14 The dipole moment and optical gap of the ligands are widely tunable through functionalization of the aromatic ring, and the vinyl linkage allows for electronic coupling of the functionalized aromatic ring to the QD core while ensuring good colloidal stability. The exchanges are performed in solution and not in the solid state; therefore, allowing us to cleanly monitor and correlate ligand properties with the resulting properties of ligand/QD organic/inorganic hybrid material systems. Here we study how the ligand properties impact the optical absorption. Our starting QD sample consisted of stoichiometric 3.2 nm diameter PbS QD cores passivated by Pb(oleate)2 ligands, resulting in an overall lead-rich QD structure with a 1S exciton absorbance peak around 1.3 eV. The as-prepared QD sample has a size distribution between 56%, as approximated by the half-width at half-maximum of the first exciton absorbance feature.15 We employed a synthetic protocol developed by Owen and co-workers4 that ensures a well-defined QD surface. We studied eight functionalized cinnamic acids (R-CAHs, Fig. 1a) that are classified by their functional group (Fig. 1a, right).14 We computed ligand properties using time-dependent density functional theory (TDDFT) with our recently developed screened exchange constant (SX) functional.16-17 The SX functional was shown to give accurate quasiparticle and optical properties of organic molecules in the Thiel’s test set.18 In addition, we found good agreement between energies computed at the GW level (G0W0@PBE) and SX single particle energies and gaps. Table S1 contains all the computed R-CAH HOMO energies, LUMO energies, and band gaps. We find that the calculated ligand dipole moments vary linearly as a 2 ACS Paragon Plus Environment

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function of the tabulated Hammet coefficients19 for the substituents (Fig. S3). Therefore, we use the calculated dipole moment as an indicator of the electron withdrawing/donating character. In Fig. 1a, we compare the computed ligand HOMO/LUMO energies to energies extracted from experimental data, which was gathered using a combination of optical absorbance spectroscopy (Fig. S1) and cyclic voltammetry (CV) measurements (CV data is compiled in supporting information, Fig. S2). In particular, CV measures the reduction potential/electron affinity of the ligands (i.e. energy of the LUMO), and the optical gap is subtracted from this value to obtain a quantity that approximates the ionization/oxidation potential (i.e. energy of the HOMO). The dashed gray lines in Fig. 1a represent a ∙ calculation of 3.2 nm PbS QD 1Se and 1Sh energy levels20-21 to approximate how the electronic gaps align prior to QD/ligand electronic coupling. Both the energy of the HOMO and the LUMO of the ligand increase with increasing dipole, thus, ligands with negative dipoles have HOMO levels that align well with the conduction band of the QDs while ligands with large positive dipoles have LUMO levels that align well with the valence band. The HOMO-LUMO gap is inversely proportional to the absolute value of the dipole (Fig. 1a), i.e., the smallest HOMO-LUMO gap occurs for ligands with both the largest electron withdrawing, 4-(CN)2, and electron donating, 4-N(CH3)2, substituents.

Figure 1. The model ligand/QD system used in this study. (a, left) Experimentally determined redox potential (open squares) compared to the calculated HOMO level (open diamonds) and redox potential



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minus the optical gap (closed squares) compared to calculated LUMO (closed diamonds) energy levels (see text) versus the calculated ligand dipole moment. Dashed gray lines represent calculated PbS QD 1Se and 1Sh energy levels. Labels correspond to the aromatic functionalization group(s) of the ligand structures displayed in (a, right) and are used throughout this work. 4-(CN)2-CAH = 4-(2,2dicyanovinyl)cinnamic acid; 4-CN-CAH = 4-cyanocinnamic acid; 4-CF3-CAH = 4trifluoromethylcinnamic acid; 3,5-F-CAH = 3,5-difluorocinnamic acid; 4-H-CAH = cinnamic acid; 2,6-FCAH = 2,6-difluorocinnamic acid; 4-OCH3-CAH = 4-methoxycinnamic acid; 4-N(CH3)2-CAH = 4dimethylaminocinnamic acid. (b) X-type ligand exchange in which surface bound oleate- is displaced by functionalized cinnamic acid molecules. The ligand exchange reaction is characterized by a surface coverage, , dependent equilibrium constant, .

In our solution-phase ligand exchange procedure, the native OA- ligands are efficiently replaced by R-CAHs to form R-CA- passivated PbS QDs through an X-type ligand exchange (Fig. 1b).22 Incoming free R-CAHs transfer a proton to a surface bound OA- to form free OAH and surface bound R-CA-. Since there is no other source of protons in the system the exchange is 1:1, and adding excess of R-CAH drives the exchange towards completion.13 To ensure that the reactions go to completion we monitor the exchange reactions in-situ by observing the absorption enhancement when adding R-CAH ligands. Doing so requires the identification of common solvents for the OA-/QD, R-CAH, and R-CA-/QD species. In some cases, solvent combinations were required in order to ensure compatibility. Solvent compositions for the various ligand exchanges are described in Table 1 of the methods section. Similar to the observations of Giansante et al. for benzenethiolate ligands,10-11 we find that the addition of a solution containing R-CAH ligand to a solution of OA- terminated QDs results in: (1) a sharpening of the first exciton absorbance feature (insets to Fig. 2a) and (2) a broadband absorbance enhancement. Increasing the amount of added ligands results in further absorbance enhancement (cyan-to-magenta traces, Fig. 2a) until saturation, at which point the QDs are fully exchanged (red-trace, Fig. 2a). We verified that upon addition of OAH to OA/QDs the absorption does not change (see Fig. S4). Raw spectrophotometric absorbance titration data for the other R-CA- ligands is compiled in the supporting information (Fig. S5). The optical absorbance for all of the fully R-CA- exchanged QDs can be found in Fig. 2b where in each case the starting absorption spectra of the OA-/QDs is the same. Complete exchanges were verified using FTIR and NMR spectroscopic analysis and reported in Ref. 14. We find large enhancement of the absorption relative to that of the QDs terminated with oleates, therefore, in agreement with Giansante et al., we conclude that the optical properties of colloidal QDs are not simply determined by the inorganic QD core size and composition. We find that even at high photon energy, where the QD optical absorption was often considered to be simply proportional to the QD volume,10, 23 the properties of the ligands substantially affect QD optical properties.24 To quantify the absorbance enhancement, we compute ∆ / , where ∆ is ( − ), is the integration of the as-synthesized OA-/QD absorbance spectrum (Fig. 2b, black-trace), and

is the integration of the fully saturated R-CA-/QD spectrum (Fig. 2b, colored-traces). We integrate from 1.0 to 2.5 eV, starting below the QD 1S-exciton and ending prior to any R-CA-/R4 ACS Paragon Plus Environment

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CAH ligand absorbance feature (see Fig. S1). We find ∆ / has a linear dependence on the HOMO-LUMO optical gap (Fig. 2c) but not on the electron withdrawing/donating character (ligand dipole moment). However, we note that the HOMO/LUMO gap is correlated to the electron donating/withdrawing character: cinnamate ligands with the smallest gap are those with the largest electron withdrawing/donating character (Fig. S6).

Figure 2. (a) Absorbance spectra of OA- passivated (blue), fully 4-H-CA- exchanged (red) PbS QDs, and pure 4-H-CAH (gray) in dichloromethane. The cyan-to-magenta spectra are those of partially exchanged 4-H-CA-/QDs. The inset shows the first exciton absorbance feature. (b) Absorbance spectra for completely ligand exchanged R-CA-/QD solutions (colored) relative to the starting OA-/QD solutions (black). (c) Enhanced absorbance parameter plotted against ligand optical gap (dashed line is drawn in). (d-k) Joint density of states (JDOS) computed for R-CA-/QD models, the JDOS is resolved for transitions with mainly QDQD (dark shading) and either QDligand or ligandQD character (lighter shading).

To investigate the underlying cause of the enhanced absorption, we performed DFT calculations on three model QD/ligand complexes with various core faceting, ligand binding orientation, and number of bound ligands (see methods section and Fig. S7 for details). For computational reasons, the model QDs are smaller than the measured QDs. We built charge neutral QD models with Pb excess to satisfy charge orbital balance: the formal charge of the PbS QD core is balanced by the formal and opposite charge of the R-CA- ligands bound on the 5 ACS Paragon Plus Environment


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surface of the QD. The optical absorption of the ligand/QD models was computed in the independent particle approximation, which is expected to capture the trend as a function of the aromatic functional group of the ligands but does not include excitonic effects. The photoabsorption cross section is proportional to: 2 (1) |〈 || 〉| ( − ) 3 ∈

,∈"#

where is the energy of the transition: = − , where is the energy of the Kohn-Sham states involved in the transition; and 〈 || 〉 is the dipole matrix element between occupied (i) and unoccupied (j) Kohn-Sham wave functions.25-28 All three models show similar trends as a function of the ligand identity/dipole, indicating that the results are not dependent on ligand orientation, facet binding, or coverage. In agreement with the experimental data, we find that the integrated absorption is proportional to the HOMO-LUMO gap of the isolated protonated ligands (see Fig. S8). Similar to the findings of Giansante et al.,11 we attribute the absorption enhancement observed in our experiments to optical transitions to/from states that have significant ligand character. This is further corroborated by a detailed joint density of states (JDOS) analysis; and provides a histogram of available optical transitions at given energy, .

%&'(() =

∈)** ,∈+,)**

( − ) = - &'( (′ − )&'("# (′)/′

(2)

We decompose the JDOS into contributions from two spatial regions defined by the QD core and the ligands. Figure 2(d-k) shows that the JDOS for QD→QD transitions (i.e. involving only QD electronic states) is constant across the ligand library (dark shaded regions), while the JDOS for the QD→ligand and ligand→QD transitions change (light shaded regions). The band alignment between the ligand and QD core states determines whether QD→ligand or ligand→QD JDOS are enhanced (Fig. S9): there are more available ligand→QD transitions when the occupied ligand states are high in energy relative to the QD core states (d-f); on the other hand, the QD→ligand JDOS increases when the unoccupied ligand states are low in energy relative to the QD core states (g-k). The relative band alignment is likely altered from that shown in Fig. 1a for the bound QD/ligands due to electronic coupling and the exact nature is likely not fully captured by our simple models. However, irrespective of the exact band alignment, the sum of the CB/LUMO and VB/HOMO contributions is proportional to the ligand HOMO-LUMO gap as long as the electronic coupling is equally efficient for the QD-ligand HOMO levels as they are for QD-ligand LUMO levels. Experimentally we can test whether this simple result can be generalized to a larger class of ligands. Functionalized benzenethiolates (4-R-S-) are another well-known class of short aromatic ligands10-11 whose properties can be modified by the substituent groups along the benzene ring. They differ from the cinnamic acid primarily through the QD coordinating groups (R-COO- versus R-S-). We performed quantitative spectrophotometric titrations (Fig. S11) for three functionalized benzenethiolates (see methods section for chemical details relating to the 6 ACS Paragon Plus Environment

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exchanges) and observed enhanced PbS QD broadband absorbance with increased ligand addition and an eventual absorbance saturation similar to that of the R-CA- ligands. Additionally, we observed a gradual red shift of the first exciton feature with increased 4-R-Sligand addition, similar to the observations of Giansante et al.10,11 A plot of the maximum absorbance enhancement for each ligand versus the corresponding ligand absorbance onset/optical gap (Fig. 3b) shows that, consistent with the R-CAH ligands, the experimental enhanced absorbance of the ligand/QD complex is greatest when the absorbance onset/optical gap of the ligands is the lowest (Fig. 3d). The overall magnitude of the 4-R-S-/QD absorbance enhancement is greater than that of the R-CA-/QD, likely because of stronger coupling between the ligand S- binding group frontier orbitals and QD VB S states compared to the ligand COOfrontier orbitals. Thus, the linear dependence seems valid, but only within a specific class of ligands.

Figure 3. Enhanced ligand/QD absorbance using 4-R-SHs. Color coding in all of the panels follow the legend in panel (b). Labeling of the Benzenethiolates is 4-NH2-SH (4-Aminobenzenethiol), 4-CH3-SH (4methylbenzenethiol), and 4-CF3-SH (4-trifluoromethylbenzenethiol). (a) Absorbance spectra of the fully exchanged 4-R-S-/QDs (colored) compared to the stock OA-/QDs (black). (b) Absorbance spectra of pure ligands in DCM normalized at the first absorbance peak feature. (c) ∆0/01 as a function of ligand equivalents added per PbS QD. The dashed black vertical line indicates 100 OA- ligands per QD. The solid colored lines are guides to the eye. (d) The linear relationship between ∆0/01 and ligand optical gap.

To further check the generalities of our findings we measured the absorption enhancement for a slightly modified cinnamic acid ligand. We synthesized 4-dimethylamino-αcyanocinnamic acid (4-N(CH3)2-α-CN-CAH) for direct comparison with 4-N(CH3)2-CAH. The 7 ACS Paragon Plus Environment


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functionalization of cinnamic acids with an α-cyano group reduces its optical band gap.29 We performed spectrophotometric absorbance titration experiments on PbS QD solutions using both of the ligand candidates under identical experimental conditions (both ligands were dissolved in acetone), and the results are summarized in Fig. 4 (raw data is in Fig. S10). Despite having a higher optical gap (Fig. 4b) 4-N(CH3)2-CAH induces greater absorbance enhancement than does 4-N(CH3)2-α-CN-CAH suggesting a stronger ligand/QD core state mixing in the 4-N(CH3)2-CA/QD system. We suspect that the α-cyano group, a strong electron withdrawing group, contributes to a substantial weakening of the electronic coupling between the R-CA- backbone structure and QD core, decreasing the overall electronic coupling of QD-ligand orbitals, thus leading to a smaller optical absorbance enhancement.

Figure 4. (a) Absorption enhancement, ∆0/01 , saturation curves for 4-N(CH3)2-α-CN-CA-/QD (red) and 4-N(CH3)2-CA-/QD (blue). Dashed lines serve as a guide to the eye. (b) Ligand optical absorbance spectra (colored traces) and as-synthesized OA-/QD absorbance spectrum (black trace).

Optical absorption enhancement is not the only effect of the ligand exchange from oleate to cinnamate. We also find that the exchange of OA- with R-CA- results in shifting of the optical gap of the QD/ligand complexes. In our previous study of the cinnamic acid ligands on the same 3.2 nm PbS QDs studied here, we found that the band-edge energies could be shifted by over 2 eV.14 Here we find that the magnitude of the induced first exciton energy shift is proportional to the observed shift of the work function / ionization energy as measured in Ref. 14 (Fig. 5b and 5c). In particular, we find that the 1S exciton energy shift is ~12 meV for each 1 eV of shift in ionization energy. Such shifts suggest that the induced surface dipole formed by the ligand shell influences the electron and hole wave function delocalization. The observed shift of the optical gap is similar to the observations of Weiss and coworkers12 who reported shifts of the optical gap when the native ligands of CdSe QDs were exchanged with dithiocarbamates. However, they attributed the shifts to the HOMO ligand orbitals being resonant with the HOMO of the QD, and trends as a function of ligand dipole were not reported. The shift in the ligand HOMO was also considered to be the main reason why thiolates shift the first absorption peak of Pb chalcogenide QDs, as observed by us here and discussed before in the literature.10,11


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Figure 5. (a) First exciton absorbance peak for as-prepared, OA--passivated (black) and R-CA- passivated (colored) PbS QDs. Color coding is consistent with the legend in Figure 2. The peak energies change as a function of ligand electron donating (+) and withdrawing (-) character. Shift of 1S exciton transition relative to the OA-/QD complex as a function of R-CA-/QD film (b) ionization energy and (c) work function (black dashed lines are drawn in to highlight the linear relationship).

We find that the enhanced absorption observed experimentally results from ligand/QD state mixing and that ligands with the smallest optical gap yield ligand/QD complexes with the greatest enhanced absorbance within a ligand class. The enhancement depends on the ratio of the ligand to the QD core DOS. Since the former scales with the QD surface area while the latter with the QD volume, the impact of ligands on optical transitions is expected to decrease for larger QD cores, consistent with the experimental findings of Giansante et al.24 We observed a similar trend in absorbance enhancement with ligand HOMO-LUMO gap for the functionalized benzene thiolates, although the overall magnitude of absorbance enhancement is greater than that of the cinnamate ligand class. Presumably, this is due to greater state mixing between the thiolate binding group and the PbS QD HOMO compared with the carboxylate binding group of the functionalized cinnamic acids. Additionally, we found that a 4-R-α-CN-CA- ligand induces less absorbance enhancement than a similar 4-R-CA- ligand; this likely stems from a decreased ligand/QD state mixing, arising from reduced electronic coupling between the R-CA- backbone structure and QD core, as a result of the electron withdrawing α-CN group. Finally, we observe a small shift of the 1S exciton energy (30 meV) when the oleate ligands are exchanged for cinnamate ligands. Our results support the conclusion that QD-ligand interactions result in emergent behavior of the QD/ligand complex. Methods Materials All chemicals were used as received without further purification unless noted. Anhydrous octane (≥99%), anhydrous diethylene glycol dimethyl ether (diglyme, 99.5%), N,N’-diphenylthiourea (98%), anhydrous toluene (99.5%), anhydrous tetrachloroethylene (TCE, ≥99.9%), anhydrous methyl acetate (MeOAc, 99%), anhydrous hexane (≥99%), anhydrous dichloromethane (DCM, ≥99.8%), anhydrous acetonitrile (ACN, 99.8%), anhydrous isopropanol (IPA, 99.5%), Acetone (≥99.9%, degassed), transcinnamic acid (4-H-CAH, ≥99%), trans-2,6-difluorocinnamic acid (2,6-F-CAH, 99%), trans-3,5-



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difluorocinnamic acid (3,5-F-CAH, 99%), trans-4-(trifluoromethyl)cinnamic acid (4-CF3-CAH, 99%), 4methoxycinnamic acid, predominantly trans (4-OCH3-CAH, 99%), 4-(dimethylamino)cinnamic acid, predominantly trans (4-N(CH3)2-CAH, 99%), ferrocene (Cp2Fe, 98%), triethylamine (TEA, ≥99%), benzenethiol (4-H-SH, ≥98%), 4-aminobenzenethiol (4-NH2-SH, 97%), and 4-methylbenzenethiol (4CH3-SH, 98%) were obtained from Sigma Aldrich. 4-(trifluoromethyl)benzenethiol (4-CF3-SH, 97%) was obtained from Alfa Aesar. Synthesis of ~3.2 nm Diameter PbS QDs. Oleate capped PbS QDs were synthesized following the substituted thiourea protocol developed by Hendricks et al.29 First, Pb(oleate)2 was prepared and purified. In a nitrogen glove box, 8.81 g Pb(oleate)2 and 150 mL anhydrous octane were added to a 2-neck 250 mL Schlenk flask equipped with a magnetic stirbar and sealed using a glass stopcock and two rubber septa. Separately, 1.74 g of N,N’-diphenylthiourea and 5 mL of diglyme were mixed in a 20 mL scintillation vial and sealed with a rubber septa. After transferring to a Schlenk line, both vessels were brought to 95 °C in an oil bath under nitrogen and allowed to stir for approximately 30 minutes or until both solutions were clear. Subsequently, the N,N’-diphenylthiourea diglyme solution was quickly injected into the Pb(oleate)2 octane solution under vigorous stirring. After 60 seconds, the flask now containing a dark brown solution was removed from the oil bath and allowed to cool to room temperature. The septa were then removed under positive nitrogen pressure and replaced with glass stoppers so the volatiles could be removed from the flask under vacuum. The flask was transferred to a nitrogen filled glovebox and the sticky, brown reaction crude was dispersed in approximately 40 mL toluene and split between four 50 mL centrifuge tubes and centrifuged at 7000 RPM for 10 minutes. The brown nanocrystal solution was decanted into four new centrifuge tubes and the remaining dark pellets were discarded. To each centrifuge tube, approximately 30 mL of methyl acetate was added to precipitate the QDs and then centrifuged at 7000 RPM for 10 minutes. This cycle of precipitation and redissolution using toluene and methyl acetate was repeated a total of three times. We found that there were approximately 100 ligands per PbS QD, giving an estimated OA- surface grafting density of 3.1 ligands / nm2. The QD product was dried under vacuum and finally suspended in hexane for storage in a nitrogen-filled glove box. Due to the large yield (multi-gram scale) of the QD synthesis, we performed all experiments on the same stock QD sample, thereby eliminating the effects of sample-to-sample variations. Quantitative Spectrophotometric Titration with R-CAHs. Optical absorbance spectra were collected using a Cary 500 UV-Vis-NIR spectrometer. A stock solution of 5-15 µM PbS QDs in DCM, standardized from absorbance measurements taken in TCE, was prepared under ambient conditions. Separately, a stock ligand solution was prepared by dissolving a known amount of the ligand in a compatible solvent (see Table 1). The stock ligand solution was combined with neat ligand solvent in separate vials to make diluted ligand samples of varying ligand concentration. In a 2 mm path length cuvette, 0.1 mL of a diluted ligand solution was added to a 0.6 mL of the stock QD solution to always maintain a constant sample volume of 0.7 mL. The sample was thoroughly mixed, and an absorbance spectrum was immediately taken. This protocol was followed for diluted ligand samples with ligand content ranging from 0 – 1000 ligands per QD per addition. Solution measurement and mixing was performed with calibrated micropipettes.


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Table 1. Solvent compositions for the various ligand exchanges

1

Ligand ID 4-(CN)2 – CAH 4-CN – CAH 4-CF3 - CAH 3,5-F - CAH 4-H - CAH 2,6-F - CAH 4-OCH3 - CAH 4-N(CH3)2 - CAH

Ligand Solvent1,2 5:1 ACN:IPA 5:1 ACN:IPA Acetone MeOAc DCM DCM 5:1 ACN:IPA 2:1 IPA:DCM

Heating and sonication were sometimes necessary to solubilize ligand. Addition of neat ligand solvent had no significant effect on QD absorbance spectrum in 6:1 ratios of DCM:ligand solvent. Benzene thiolate ligand titrations. Quantitative spectrophotometric titrations followed a similar procedure as that described for the R-CAHs, except that all of the ligands were dissolved in DCM and included an equimolar amount of trimethylamine (TEA) to deprotonate the thiol group. Addition of [4CH3-S-][TEA+] and [4-CF3-S-][TEA+] resulted in stable colloidal solutions with ligand equivalents up to 400 at which point addition of more concentrated ligand solutions precipitated the QDs from the exchange solution. In the case of [4-NH2-S-][TEA+], addition of ligand equivalents above 100 resulted in precipitation of the PbS QDs; however, addition of pure 4-NH2-SH in DCM allowed for stable ligand exchanges at all ligand equivalents studied. One explanation is that 4-NH2-SH can more easily undergo proton transfer with OA- than the other thiols. Another possible explanation is that 4-NH2-SH undergoes self-deprotonation to form 4-(NH3) +-S- in solution to drive the exchange. 2

Computational. Calculations on the ligands and the QD/ligand complexes were carried out as described in our previous paper.14 In short, density functional theory calculations were carried out with the plane wave basis set code Quantum-Espresso30 with the PBE parametrization of the generalized gradient approximation exchange-correlation functional31. A wave function cutoff of 80 and 60 Ry was used for structural optimizations and single point calculations, respectively. Electron-nuclei interaction was approximated by recently developed optimized norm-conserving pseudopotentials32-33 The functionalized cinnamic acid ligands were optimized in the trans conformation i.e., the acidic proton bound on either one of the carboxylic oxygen atoms. We built charge neutral QD models with Pb excess and by satisfying charge orbital balance: the formal charge of the PbS QD core is balanced by the formal and opposite charge of the R-CA- ligands bound on the surface of the QD. We started by cutting out isolated cubes from bulk PbS. Cubes with an odd number of layers were always off-stoichiometric while cubes with even number of layers were always stoichiometric. We generated three structural models with varying number of ligands. For model A with formula [Pb43S38][R-CA-]10 we first generated a five layer cube and cut off atoms to define a QD with small (111) and (110) facets and a near spherical shape. The surface was then passivated with 10 R-CA- ligands that were bound both on (111) and (100)-like facets. The ligands coordinated the surface Pb atoms in a chelating conformation. Model B was constructed by removing the eight corner atoms of a QD with six layers and attaching six R-CA- ligands to the (100) mini-facets in a bidentate manner and also passivating the corners with iodine atoms to keep chargeorbital balance. The final formula was [Pb62S55I8][R-CA-]6. Model C was made by removing selected atoms from the PbS core of model B in a way to have larger (111) facets. We then passivated the surface with four R-CA- ligands while making sure that all the ligands align along one cartesian direction. We made this choice for computational reasons: by having ligands point in only one direction, we only had to use larger cell size in one cartesian direction. To ensure charge balance, four iodine atoms were used to passivate the remaining four (111) facets. The formula of model C was [Pb44S40I4][R-CA-]4. In all three cases, we generated several different binding conformations, and we verified that the models discussed here are the most stable structures. Figure S7 shows the ball-and-stick structural model of the relaxed 4HCA- covered models. The large variety of these models sampling several possible shapes, facets, and



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binding moieties allowed us to draw robust conclusions in regards to optical absorption of the ligand/QD complexes. The ligand HOMO-LUMO gaps were also computed using higher level of theory. In particular, we used our recently developed dielectric dependent hybrid functional SX16-17 and single-shot GW@PBE. These calculations were performed with the WEST code.34-36 Here, eigenpotentials and eigenvalues of the dielectric matrix were calculated within the random phase approximation using the projective dielectric eigen decomposition algorithm.37-38 Quasiparticle energies were converged to within 0.2 eV with respect to the number of eigenpotentials for each system using 20 times the number of electrons with a cell size of 25 Å or larger. The SX functional is a global hybrid functional, where the mixing fraction �SX is computed by the average screening in the system as the ratio of the screened exchange and exact exchange total energies: ∑8 〈5 |6273 |5 〉

23 = 8 ∑ 〈5 |673 |5 〉 where 9 is the number of occupied states in the system, 5 is the : ;< Kohn-Sham state, 6273 is the screened exchange self energy and 673 is the exact exchange self energy. The non-local exchangecorrelation potential is then defined as: =23 = 23 673 + (1 − 23 )=@ + = where =@ and = are the PBE exchange and correlation potentials, respectively. In order to compute the optical gaps of the ligands we used adiabatic time-dependent density functional theory (TDDFT) as implemented in the turboTDDFT code,39 with the SX functional in the kernel (TDSX).We recently showed that this method yielded accurate optical gaps for the Thiel test set of organic molecules16 and we expect it to work equally well for R-CAHs. Acknowledgments This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. D.M.K., C.V.C., J.G., and M.C.B gratefully acknowledge support through the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, Office of Science within the US Department of Energy. N.P.B, A.J.N, and G.G. acknowledge support from the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. N.B. acknowledges support from a Director’s fellowship at NREL. M.V. was supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory. Work at Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. This research used computational resources of: the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231; and the Los Alamos National Laboratory Institutional Computing Program, which is supported by the U.S. Department of Energy National Nuclear Security Administration under Contract No. DE-AC52-06NA25396. A.S. was supported through Colorado School of Mines start-up funds. B.W.M. respectfully acknowledges the sponsorship and support of the United States Air Force Institute of Technology. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.


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Supporting Information Ligand absorbance spectra, cyclic voltammetry data, raw optical absorbance enhancement data, computational structural models/details/results, and ligand synthetic details.

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