Direct Observation of Vibronic Coupling between Excitonic States of

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C: Physical Processes in Nanomaterials and Nanostructures

Direct Observation of Electronic Mixing Excitonic States of Nanocrystals and Their Passivating Ligands Timothy G. Mack, Lakshay Jethi, Mark P. Andrews, and Patanjali Kambhampati J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11098 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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The Journal of Physical Chemistry

Direct Observation of Vibronic Coupling Between Excitonic States of CdSe Nanocrystals and their Passivating Ligands

Timothy G. Mack, Lakshay Jethi, Mark Andrews, Patanjali Kambhampati* Department of Chemistry, McGill University, Montreal, Quebec, H3A 0B8, Canada ABSTRACT: Here we report Resonance Raman spectra of CdSe colloidal nanocrystals (NCs) passivated with organic ligands. In addition to the well known Longitudinal Optical (LO) phonons we observe ligand vibrations. The ligand vibrations are shown to be resonantly enhanced through electronic mixing with the states of the NC. These measurements were enabled by substituting the native ligands with thiophenol. Thiophenol serves as an ideal probe for exciton-ligand coupling as it is a widely employed Raman molecular tag and quenches background luminescence in CdSe. The ligand vibrations are shown to be resonantly enhanced through exciting NC transitions. We show that coupling is observable in CdSe with diameters from 2-6 nm, and for both phosphonic acid and amine native ligands. The coupling is evidenced both by asymmetric mode enhancement through Herzberg-Teller and symmetric modes via Franck-Condon or Herzberg-Teller coupling. The ligand exchange quenching strategy may be generally applicable to study exciton-ligand interactions in a variety of semiconductor NC materials, and reveal information on electronic and vibrational structure of the NC surface.

Introduction:

in NCs has recently been suggested in the literature.9 Experimental evidence for it stems from μ-PL measurements on individual NCs reveal side bands significantly shifted from the zero phonon line, although so far no comprehensive study has been published. More recently, certain types of ligands that delocalize electron or holes have been shown to control the energetics of absorption and emission via the excitonic core states.10-12 At present there is a dearth of direct spectroscopic evidence of ligand vibrations coupled to the electronic states of the NCs. Ensemble resonance Raman measurements provide a direct measure of such coupling, although there are conflicting reports in the literature of whether resonance Raman enhancement of ligands bound to semiconductor NCs can be observed.13-16 Here we show that ligands are directly coupled to the excitonic states of CdSe NCs via their resonance enhancement in Raman spectra. This paper reports the first measurement of exciton-ligand coupling via resonance Raman signal enhancement for the case of the comprehensively studied system of colloidal CdSe NCs. We demonstrate that Raman scattering of covalently bound surface ligands is enhanced

Colloidal semiconductor nanocrystals (NCs) are passivated with covalently bound organic ligands.1 Ligands act as surfactants to enable stable colloidal dispersion with a controllable size and narrow size distribution.2-3 Their secondary role is to optimize the system for light emission by coordinating to surface atoms that may act as nonradiative trap states.4-5 In both cases, ligands serve a passive function and are not considered to be directly coupled with excitons generated in the inorganic core of these NCs. Due to bottlenecks of device efficiency in NC based devices, for instance in terms of Auger recombination, the extent to which ligands affect the exciton of the NC is now being reconsidered.6 In the past decade there has been increasing evidence that these surface ligands perform an actual functional role in controlling the behavior of excitons in NCs. Indirect evidence has initially suggested that these ligands play a role in controlling exciton cooling processes.7-8 The phenomenon of exciton-to-ligand charge-transfer coupling

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solutions (200 μL in the case of the amine capped NN labs dots, 400 μL in the case of the as synthesized phosphonic acid capped dots were transferred into a small glass vial. 100 μL of thiophenol (97%) was added, and the vial was immediately capped and vortexed for 10 seconds. The mixture was then left to sit in the fume hood under aluminum foil for 30 minutes. The mixture was then decanted into a centrifuge tube, and 2 mL of antisolvent (EtOH or MeOH) was added. The mixture was then centrifuged (10000 rpm, 10 minutes). The supernatant was discarded, and 200 μL of toluene was then added. The NCs were resuspended through vertexing to yield a concentrated suspension.

through charge transfer coupling when excitation wavelengths resonant with the NC electronic transition are employed. Specific modes are enhanced according to their vibrational symmetry, consistent with previously studied systems, and discussed in terms of Frank-Condon and Herzberg-Teller coupling motifs. The methodology described here is proposed as a strategy that can be adapted for future Raman studies on exciton-ligand coupling in several NC systems, including recently discovered lead halide perovskite NCs.17 Methods: Chemicals: All chemicals and solvents were purchased from Sigma Aldrich unless otherwise specified. Thiophenol (PhSH 97%), CdO (>99.99%), Selenium power (100 mesh, 99.99%), Octadecylphosphonic acid (ODPA, 97%), Trioctylphosphine oxide (TOPO, 99%) and trioctylphosphine (TOP, 97%) were used as received without further purification. PhSH can be deprotonated to form thiophenolate (PhS) or dimerize as a disulfide (PhS)2. These forms are shown in Figure S1 of the Supporting Information.

Sample preparation for Raman experiments: Samples were prepared for Raman measurement by depositing 50 μL of the PhS ligand exchanged NCs suspension onto clean p-doped Si wafer and dried under ambient conditions. Raman measurements: Raman measurements were obtained using a Horiba LabRam HR800 microscope in conjunction with an external diode-pumped solid-state 473nm laser (Ciel, Laser Quantum) or 532 nm laser (Excel, Laser Quantum). The external line was directed into a microscope through a series of pinholes and mirrors and directed into the microscope turret using a superholographic notch filter (Kaiser optical systems). The power at the sample was measured to be 1.3 mW. A 100x objective (Olympus N Plan N, NA=0.9) was used, in conjunction with a grating 1800 lines/mm and the spot size was approximately ~1 um, and thus the irradiance is approximately 1.65x105 W/cm2. Full spectra were acquired with stitching of 590 cm-1 windows in which the total acquisition time was 1.6 minutes (10s, 10 accumulations), the overall acquisition time from 150 cm-1 – 3200 cm-1 was approximately 45 minutes. No sample damage was apparent after laser exposure. Raman baseline correction was performed using the Goldindec20 algorithm in MATLAB 2016®.

Synthesis of Phosphonic Acid (PA) capped CdSe NCs: The synthesis of CdSe NCs follows the protocol described elsewhere2-3 with the following modifications. ODPA (0.03 g), CdO (0.06 g) and TOPO (3.0 g) were added to a 50 mL round bottom flask, on a Schlenk line setup and heated to 150 °C under vacuum for 1 h. The vacuum was removed, and the flask was filled with Argon and then heated to 340 °C, at which point the contents of the flask became colourless. In a glovebox under nitrogen atmosphere, a vial contained Se (0.06 g) in TOP (0.360 g) was prepared and mixed vigorously under gently external heating until a clear solution had formed. The solution was injected into the flask and left to stir for 120 seconds. The heating mantle was then removed, and the flask allowed to cool to room temperature. The contents of the flask were then added to a 50 mL centrifuge vial, and approximately 35 mL of methyl acetate was added to precipitate the NCs. The mixture was centrifuged (10 mins, 10k rpm) and resuspended in 8 mL of toluene.

Fluorescence and Absorbance characterization: Fluorescence spectra were acquired on a Cary Eclipse Spectrometer. Absorbance data were acquired on a Cary 300 UV-VIS spectrometer. Infrared absorbance measurements were performed on a Perkin Elmer Spectrum Two ATR-FTIR spectrometer. All measurements were done in ambient conditions. The data were scaled by Jacobian correction factors for the conversion between wavelength and energy. Synthesized NC sizes were obtained by through an empirical sizing curve.21 Commercially purchased NCs sizes are the reported supplier values.

Amine capped CdSe NCs: Octadecylamine (ODA) capped CdSe NCs suspended in toluene were purchased from NN laboratories (CSE 480-10, diameter = 2.2 nm 5 mg/mL, CSE 500-10, diameter = 2.3, 5 mg/mL) and used as received. Synthesis of Cd-ODPA: The synthesis of Cd-ODPA was performed as described by Peng.3 Synthesis of Cd(PhS)2: The synthesis of Cd(PhS)2 was performed as described by Dance.18

TEM/EDX: TEM/EDX measurements were taken on a Jeol JEM-2100F, (200 keV). Samples were deposited form toluene onto copper grids (Ted Pella, 400 mesh, Formvar coated).

Ligand exchange: The ligand exchange protocol was inspired by the work of Aruda et al.19 Briefly, stock


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1H NMR: 1H NMR measurements in CDCl were acquired 3 on a Varian-Mercury 400 MHz spectrometer.

Density Function Theory (DFT) calculations: DFT calculations were performed using ORCA 4.0.1.22-23 Avogadro 1.2 was used as a molecular builder.24 Raman spectra were simulated using B3LYP exchange-correlation, and polarizability property-optimized Karlsruhe basis sets, def2-SVPD.25

Results and Discussion: In 2005, Guyot-Sionnest first proposed that the ligands provided a non-trivial electronic function. That work provided results that suggested that ligands contributed to electron relaxation in NCs.7 Subsequently our group suggested that ligands are coupled to hole dynamics albeit with a different coupling scheme.8 These ligand effects upon relaxation dynamics have been seen by different groups in a variety of ways. Ligands such as thiols or dithiocarbamates can significantly delocalize excitonic states.10-12, 19, 26-28 Most recently our group has provided evidence that ligands broaden electronic transitions.29 Throughout all these works, what is missing is direct spectroscopic evidence of ligand coupling to electronic states. In all cases, the ligand coupling had been inferred. Hence what is needed is a vibrationally resolved electronic spectrum as made possible through resonance Raman spectroscopy. Extensive background fluorescence has typically prohibited resonance Raman measurements of II-VI semiconductors NCs. Two approaches to mitigate its impact are commonly employed. The first is to select an excitation wavelength much lower wavelength than the corresponding first exciton absorption feature, while the second more commonly employed approach is to ligandexchange with a ligand that can act as an electron/hole trap of the NC to quench the photoluminescence.30 The former approach has the advantage of retaining the original native surface passivation of the dot, whereas the second approach alters the surface. Using the latter approach with a hole trapping thiol, resonance Raman studies measured phonon modes of various sizes of CdSe NCs.31-34 There are several experimental considerations and limitations associated with thiol and dithiocarbamate ligands. Recent studies looking at ligand exchange of CdSe NCs with thiols or dithiocarbamates have demonstrated that heating the sulfur ligands to elevated temperatures results in the growth of surface CdS shell.14 CdSe/CdS is a type I structure, confining the excitonic wavefunction in the core. Shelling mitigates the extent that excitons couple to the surrounding ligand shell, necessitating low temperature ligand exchanges on CdSe NCs for our experiments. In summary, several experimental parameters above must be taken into consideration to successfully observe ligand signals via resonant excitation of the NC.

Figure 1. Ligand exchange enables Resonance Raman observation of ligand vibrations. (a) Linear absorption and emission spectra of CdSe NCs (X1= 1.97 eV, 6.1 nm diameter) passivated with native phosphonic acid (PA) and PhS ligands. The blue arrow corresponds to the Raman excitation energy (2.62 eV). (b) Raman spectra of the PhS exchanged CdSe NCs, showing both LO phonon modes (ωLO = 208 cm-1) and overtones, as well as ligand modes.

For reasons discussed above, the native PA passivated NC must be exchanged with a quenching ligand such as PhS. Figure 1 (a) shows an absorption spectrum of a representative CdSe NC. The band-edge exciton absorbance energy is denoted as X1. The diameter is approximately 6.1 nm (X1 = 1.97 eV) phosphonic acid capped CdSe NC (black), along with its thiophenolate (PhS) ligand exchanged form (red). The excitonic features present in both curves are similar, and the fluorescence intensity is quenched upon exchange with the hole-trapping thiophenol. The 2.62 eV line corresponds to the Raman excitation energy used in this study. Figure 1 (b) shows the resonance Raman spectrum of the ligand exchanged NC. Two salient regions are apparent. In the spectral region below 1000 cm-1, the LO phonon of CdSe (ωLO = 208 cm-1), and several overtones are observed. This observation is consistent with previous Resonance Raman studies on CdSe NCs.31 In the region



above 1000 cm-1 several Raman signals are apparent. These signals correspond to a combination of alkyl stretching modes from the native phosphonic acid (PA), as well as aromatic ring stretching modes from the substituted PhS. To our knowledge, it is the first direct observation of ligands under resonant excitation of CdSe NCs.

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to molecular lowest unoccupied molecular orbital (LUMO) transition, and to the Herzberg-Teller coupling element between the conduction band and LUMO final states. The valence band level of the 6.0 nm CdSe was estimated using the experimentally fitted expressions found through photoelectron spectroscopy.41-42 The conduction band and molecular LUMO states were obtained by simply adding in the experimentally measured UV-VIS optical transition energy (1.97 eV) to these energies. The optical transition energies of thiophenol are obtained from the literature.43-45 Symmetries are assigned based on the assignments provided in the literature, and independently verified through ORCA calculations (Figure S6, Supporting Information).46 The band energies in the schematic suggest that the LUMO orbitals of PhS are comparable in energy to the NC conduction band. The energetics reveal that the Raman spectrum of PhS can not be not be resonantly enhanced by 2.62 eV excitation in the absence of coupling to the NC in some manner.

Figure 2. Ligand systems and relevant electronic structure. (a) Schematic of Ligand exchange. (b) Depiction of X-type ligand exchange according to the covalent Bond Classification. Electronic energy levels for 6 nm CdSe NCs and Thiophenol.

To comprehensively study exciton-ligand coupling native alkyl-based ligands are substituted with a practical Raman tag. Nonetheless, ligand exchange is well known to not be complete due to equilibrium.19, 35-36 Figure 2 (a) schematically illustrates the process of ligand exchange from PA to PhS.1, 30, 37-38 Ligand exchange is described within the covalent bond classification method developed by Green et al.1, 37 Within this framework, ligands can be classified in terms of X-type (anionic), L-type (dative) or Z-type (Lewis acids). In the case depicted above, PhS displaces phosphonic acids in a 1:1 manner as both are anionic ligands (X-type). Both native PA and exchanged PhS signals are observed in the Raman data. Since resonance Raman is an electronic spectroscopy it is important to discuss the relevant electronic levels for the NC as well as the ligand. Figure 2 (b) shows the energy level diagram illustrates the relative energy spacings of a 6.0 nm CdSe NC and energy spacings of a thiophenol molecule. The symbols μex, μCT and hck are adopted from the notation of Lombardi.39-40 They correspond to the electronic transition dipole moment of the semiconductor valence to conduction band, the charge-transfer dipole moment from the valence

Figure 3. Ligand effects are more pronounced in smaller NCs. a) Linear absorption and emission spectra of CdSe NCs. of the commercial (X1 = 2.58 eV, Diameter 2.2 nm) passivated with for native amine (PA) and PhS ligands. The blue arrow corresponds to the Raman excitation energy (2.62 eV). (b) Raman spectra of the PhS exchanged CdSe NCs, showing both LO phonon modes (ωLO = 208 cm-1) and overtones, as well as ligand modes.


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The Journal of Physical Chemistry 2840-3000 cm-1 are alkyl C-H stretches associated with native phosphonic acid or alkyl amine. In both regions, the relative intensities are qualitatively consistent with the same modes present, significant differences in total intensities relative to the LO phonon. To better understand possibility of ligand desorption in our NC-PhS films, we examined the chemical stability of neat PhSH. Under ambient conditions a drop of thiophenol on a silicon wafer leads to the formation of white crystals after a few hours (Figure S2, Supporting Information). This product was subsequently identified as phenyl disulfide (PhS)2, based on high resolution mass spectroscopy (Figure S3). This was consistent with the disappearance of the characteristic S-H signal in NMR (3.5 ppm, Figure S4, Supporting Information) and the S-H stretching frequency band at 2530 cm-1 in Raman. Moreover, the disulfide formation is advantageous because (PhS)2 possesses a νS-S stretch in it’s Raman spectrum at 540 cm-1. (Figures S7, S8 Supporting Information).49-50 From this one can spectroscopically distinguish between PhS which is covalently bound (chemisorbed) to the surface cadmium of the NC and unbound (physisorbed) (PhS)2 formed through the oxidation of desorbed PhS or excess PhSH. In principle, a distribution of these three species can exist in NC solutions and possess distinct Raman signatures that may be discriminated in future studies. As in the case of prior studies on PhS passivated Au samples, the lack of the band apparent at 540 cm-1 indicates that the enhanced Raman signal stems from chemisorbed PhS rather than physisorbed (PhS)2.49

The properties of NCs are typically ascribed to large surface to volume ratios. Smaller NCs with larger ratios show more pronounced surface effects. In addition, the 2.62 eV Raman excitation wavelength could be made resonant with the X1 (1se - 1sh) transition in smaller NCs. To explore size effects and the scope of the exchange method we compared the smaller NC as well as different native ligand passivation. Fig 3 shows Raman spectra for smaller NCs. Commercially available NN labs NCs with X1 absorbance energies of 2.58 eV, (2.2 nm) and 2.48 eV (2.3 nm) were employed for reproducibility. The key difference between these commercially available NCs and NCs synthesized by our research group are in terms of the passivating ligand. NN labs NCs are amine passivated (Ltype) whereas the dots in Figure 1 are capped by phosphonic acids (X-type). This leads to significant differences in the ligand exchange mechanism. The These data show that smaller NC show more pronounced resonance Raman signals and that the ligand exchange procedure is valid for amine passivated NCs. Figure 3 (a) shows the absorption spectrum of 2.2 nm (X1 = 2.58 eV) the amine capped NCs (black) and PhS exchanged (red). A blue shift of approximately 100 meV is observed, corresponding to etching of the surface of the NC. We associate the spectral blue shift with surface etching of the NCs, which is related to the acid/base nature of the PhS/amine ligand exchange. The exchange from basic amines to acid thiophenol, is depicted in Figure S5. Our group already studied this phenomenon in terms of the case of amine addition to PA capped NCs (Z-type removal via Ltype addition of ligands).1, 47 Figure 3 (b) shows the resonance Raman spectrum obtained upon ligand exchange of the 2.2 nm amine passivated NC. Ligands accounts for a greater volume fraction per NC in smaller NCs than in larger ones. Both phonon and ligand features are apparent, although the normalized intensity of the ligand is significantly increased in comparison with the phonon intensities. This observation is consistent with the fact that the Raman excitation energy nearly matches the optical bandgap of the 2.2 nm NC and is thus resonant with ligand charge transfer. Previously in the case of the 6.1 nm dot, the excitation energy is nearly 0.6 eV greater, and this excess energy is efficiently converted to NC phonons, as evidenced by the strong overtones of the LO phonon mode. The possibility of ligand-phonon vibrational coupling has been observed in CdSe NCs48. This may account for the overall energy conservation. In order to better visualize the relative energies of the phonon and vibrational modes in terms the excitation energy, Figure 3 (b) is also plotted in units of meV in the Supporting Information (Figure S14). The differences in size and native ligands discussed above are compared in Figure 4. Figure 4 (a) shows the normalized resonance Raman spectra of the PhS exchanged 2.2 nm and 6.1 nm NCs, in the region of 970-1600 cm-1. Raman signal originates in both X1 and continuum-based excitations, as well as in both native amine and phosphonic acid capped samples. Panel (b) shows the 2800-3200 cm-1 region. The peak at 3060 cm-1 is indicative of the aromatic C-H stretch of PhS. The broad feature containing peaks from

Figure 4. Raman spectroscopy of ligand modes for two different sizes of NCs. (a) and (b) Comparison of 1.97 eV (6.1 nm diameter) and 2.58 eV (2.2 nm diameter) NCs in the regions of 970-1700 cm-1 and 2800-3200 cm-1. Panel (a) is normalized



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to 1000 cm-1, and panel (b) is normalized to the mode at 2880 cm-1.

Figure 5. Comparison of Raman spectra of adsorbed ligands and free ligands. (a) and (b) Comparison of Raman spectra of ligand exchanged 1.97 eV NCs in the regions of 970-1700 cm-1 and 2800-3200 cm-1 with free ligands. (c) and (d) Comparison of Raman spectra of ligand exchanged 2.58 eV NCs in the regions of 970-1700 cm-1 and 2800-3200 cm-1 with free ligands.

The resonance Raman signal is composed of contributions from both native and substituted PhS ligands. To study their relative contributions, we measured the Raman spectra of free (protonated) ODPA and ODA ligands in addition to (PhS)2. Native phosphonic acid and amine NC ligands possess long alkyl chains for dispensability in organic solvent. The observed Raman peaks from the native ligands appear to stem exclusively from alkyl stretches and bonds, such as the 1295 cm-1 ( (-CH2-)n in plane twisting mode, as well as CH2 bending and scissoring modes at 1437 cm-1 and 1455 cm-1 respectively.51 Fig 5 shows the contributions of native ligands and substituted PhS to the overall resonance Raman spectra. Notably the native ligands are also resonantly enhanced as the substituted ligands. Figure 5 (a) shows that the contributions of ligands for the cases of the large 6.1 nm NCs capped with

phosphonic acids (panels a) and b)) and the Raman spectra of the amine capped 2.2 nm NCs (panels c) and d)). The Raman contribution from the native ligands appears to be lower in the smaller amine passivated NCs than in the case of the larger phosphonic acid NCs. Predictions from computational studies predict coupling to non-symmetrical modes of ligands containing a carboxylic acid headgroup.2627 The lower relative intensity of these alkyl stretching modes in the NN labs NCs may be simply due to a more efficient displacement of amines by PhS (L-type to X-type) and/or possibly related to differences in coupling efficiencies intrinsic to the functional group. Figure 6 shows the effect of charge transfer enhancement on the intensities of the Raman bands. We have assigned the modes according to their C2V symmetry based on the literature assignments.52-54 We normalize the data to the


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The Journal of Physical Chemistry revealed through comparison with Fourier-transformed infrared spectrum (FTIR). Figure 7 (a) and (b) compares the NC Raman with FTIR of (PhS)2 in the 970-1700 cm-1 and 2800-3200 cm-1 regions. These resonant Raman spectra reveal the presence of Raman active and IR active modes. As argued in the literature, there are different possible explanations for the observation of forbidden modes, including the reduction of molecular symmetry upon chemisorption to the surface, chemical bonding enhancement of silent Raman modes, and changes to the due to changes in rates of the local electric field.63-64 The first explanation is not likely in the present case because there is no inherent reduction of symmetry of PhS upon binding, and frequency shifts with respect to (PhS)2 are modest.

feature at 1000 cm-1. These data clearly demonstrate that the intensities of both totally-symmetric (a1) and nontotally-symmetric modes (a2, b1, b2) are changed with respect to the Raman spectrum of (PhS)2. Further control experiments validate this interpretation. The controls involve the non-resonance Raman of PhS capped 2.2 nm CdSe NCs using 2.33 eV excitation (Figure S10), as well as the Raman measurement of Cd(PhS)2 using 2.62 eV excitation (Figure S9). Neither of these control experiments could reproduce the spectrum obtained for the PhS capped NCs under resonant excitation. This is a clear indication that the resonance Raman data stems from NC excitation.

Figure 6. Assignment of Raman modes according to C2V symmetry. Comparison of Raman signals of PhS (black), and PhS capped 2.58 eV and 2.48 eV CdSe.

The theory of Raman enhancement of modes of adsorbates on semiconductors has been described in detail elsewhere.39, 55-58 Broadly, the framework developed by Albrecht falls into terms commonly denoted as A (FranckCondon), B, C (Herzberg-Teller) terms. Totally-symmetric modes (Aʹ, a1) can be enhanced either through A or B/C terms, whereas asymmetric modes (Aʹʹ, b1, b2) can only be enhanced through B/C terms. A term enhancement occurs only when the Raman excitation energy is resonant with a possible optically allowed MLCT or LMCT. By contrast, the B/C term only is applicable if the selection rule criteria are met.39, 56-57, 59-61 More recently, there have also been efforts to achieve predictions to distinguish between these various mode enhancement computationally, calculating resonance Raman profiles using through density functional theory (DFT) of small CdSe or ZnSe clusters passivated with surface ligands.26-27, 62 These papers suggest that both Franck-Condon and Herzberg-Teller enhancements may be present. More work is needed to disentangle their respective contributions.

Figure 7. Raman spectra of adsorbed ligands reveal Raman and IR modes. (a) and (b) Comparison of Raman spectra and FTIR of free PhS with Raman signals of ligand exchanged 2.48 eV (2.3 nm) NCs in the regions of 970-1700 cm-1 and 2800-3200 cm-1.

Conclusion: This work provides the first experimental observation of ligands electronically coupled to the states of a colloidal CdSe NC. We have developed a simple strategy to obtain resonance Raman spectra on high PLQY NCs through a simple room temperature ligand exchange with PhS,

We briefly comment on the appearance of additional silent (forbidden) modes that become Raman active as



followed by subsequent washing with EtOH/MeOH. The resonance Raman of the NC-PhS films shows enhancement of covalently bound PhS species as evidenced by the disappearance of the 540 cm-1 band corresponding to the disulfide. In comparison with free (PhS)2, the NC-PhS signals obtained in the 970-1700 cm-1 window show Raman enhancements of a1 and b2 modes, that suggests that Herzberg-Teller coupling, and possibly Franck-Condon coupling are present. The results provided in this work can be compared to DFT calculations of small CdSe clusters.26-27, 62 These results confirm long standing theoretical predictions and have implications for the design of NCs.

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Semiconductor Quantum Dots. Physical Review Letters 2007, 98, 177403. 9. Lifshitz, E., Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots. J Phys Chem Lett 2015, 6, 4336-47. 10. Amin, V. A.; Aruda, K. O.; Lau, B.; Rasmussen, A. M.; Edme, K.; Weiss, E. A., Dependence of the Band Gap of Cdse Quantum Dots on the Surface Coverage and Binding Mode of an Exciton-Delocalizing Ligand, Methylthiophenolate. Journal of Physical Chemistry C 2015, 119, 19423-19429. 11. Frederick, M. T.; Weiss, E. A., Relaxation of Exciton Confinement in Cdse Quantum Dots by Modification with a Conjugated Dithiocarbamate Ligand. ACS Nano 2010, 4, 3195-200. 12. Frederick, M. T.; Achtyl, J. L.; Knowles, K. E.; Weiss, E. A.; Geiger, F. M., Surface-Amplified Ligand Disorder in Cdse Quantum Dots Determined by Electron and Coherent Vibrational Spectroscopies. J Am Chem Soc 2011, 133, 7476-81. 13. Grenland, J. J.; Maddux, C. J. A.; Kelley, D. F.; Kelley, A. M., Charge Trapping Versus Exciton Delocalization in Cdse Quantum Dots. J Phys Chem Lett 2017, 8, 5113-5118. 14. Grenland, J. J.; Lin, C.; Gong, K.; Kelley, D. F.; Kelley, A. M., Resonance Raman Investigation of the Interaction between Aromatic Dithiocarbamate Ligands and Cdse Quantum Dots. Journal of Physical Chemistry C 2017, 121, 7056-7061. 15. Wang, Y. F.; Zhang, J. H.; Jia, H. Y.; Li, M. J.; Zeng, J. B.; Yang, B.; Zhao, B.; Xu, W. Q.; Lombardi, J. R., Mercaptopyridine SurfaceFunctionalized Cdte Quantum Dots with Enhanced Raman Scattering Properties. Journal of Physical Chemistry C 2008, 112, 996-1000. 16. Islam, S. K.; Lombardi, J. R., Enhancement of Surface Phonon Modes in the Raman Spectrum of Znse Nanoparticles on Adsorption of 4-Mercaptopyridine. J Chem Phys 2014, 140, 074701. 17. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide Perovskites (Cspbx(3), X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett 2015, 15, 3692-6. 18. Dance, I. G.; Garbutt, R. G.; Craig, D. C.; Scudder, M. L., The Different Nonmolecular Polyadamantanoid Crystal Structures of Cadmium Benzenethiolate and 4-Methylbenzenethiolate. Analogies with Microporous Aluminosilicate Frameworks. Inorganic Chemistry 1987, 26, 4057-4064. 19. Aruda, K. O.; Amin, V. A.; Thompson, C. M.; Lau, B.; Nepomnyashchii, A. B.; Weiss, E. A., Description of the Adsorption and Exciton Delocalizing Properties of P-Substituted Thiophenols on Cdse Quantum Dots. Langmuir 2016, 32, 3354-64. 20. Liu, J.; Sun, J.; Huang, X.; Li, G.; Liu, B., Goldindec: A Novel Algorithm for Raman Spectrum Baseline Correction. Appl Spectrosc 2015, 69, 834-42. 21. Jasieniak, J.; Smith, L.; van Embden, J.; Mulvaney, P.; Califano, M., Re-Examination of the Size-Dependent Absorption Properties of Cdse Quantum Dots. Journal of Physical Chemistry C 2009, 113, 19468-19474. 22. Neese, F., The Orca Program System. Wiley Interdisciplinary Reviews-Computational Molecular Science 2012, 2, 73-78. 23. Neese, F., Software Update: The Orca Program System, Version 4.0. Wiley Interdisciplinary Reviews-Computational Molecular Science 2018, 8, e1327. 24. Hanwell, M. D.; Curtis, D. E.; Lonie, D. C.; Vandermeersch, T.; Zurek, E.; Hutchison, G. R., Avogadro: An Advanced Semantic Chemical Editor, Visualization, and Analysis Platform. J Cheminform 2012, 4, 17. 25. Rappoport, D.; Furche, F., Property-Optimized Gaussian Basis Sets for Molecular Response Calculations. J Chem Phys 2010, 133, 134105. 26. Swenson, N. K.; Ratner, M. A.; Weiss, E. A., Computational Study of the Resonance Enhancement of Raman Signals of Ligands Adsorbed to Cdse Clusters through Photoexcitation of the Cluster. Journal of Physical Chemistry C 2016, 120, 20954-20960. 27. Swenson, N. K.; Ratner, M. A.; Weiss, E. A., Computational Study of the Influence of the Binding Geometries of Organic Ligands on the Photoluminescence Quantum Yield of Cdse Clusters. Journal of Physical Chemistry C 2016, 120, 6859-6868.

ASSOCIATED CONTENT Supporting Information. Details of DFT calculations, characterization of samples and additional Raman spectra are provided. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT TM thanks Hydro Quebec for a scholarship. NSERC and FQRNT are acknowledged for funding.

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Direct Observation of Vibronic Coupling between Excitonic States of

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