Supporting Information for

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1 Supporting Information for Dipolar Molecular Capping in Quantum Dot-Sensitized Oxides: Fermi Level Pinning Precludes Tuning Donor-Acceptor Energetics Hai I. Wang, 1,2 Hao Lu, 1 Yuki Nagata, 1 Mischa Bonn, 1 and Enrique Cánovas 1,* 1 Max Planck Institute for Polymer Research, Ackermannweg 10, D Mainz, Germany; 2 Graduate School of Material Science in Mainz, University of Mainz, Staudingerweg 9, Mainz, Germany; Current address: Institute of Physics, Johannes Gutenberg-University Mainz, Staudingerweg 7, Mainz, Germany * Correspondence to: Canovas@mpip-mainz.mpg.de Content: 1 Optical pump-terahertz probe (OPTP) spectroscopy 2. OPTP reproducibility test 3. Impact of QD dipolar molecular capping on QD/oxide backward ET rate 4 First Principle Calculations 5. Ultraviolet Photoelectron Spectroscopy (UPS)

2 1. Details of Optical Pump-Terahertz Probe (OPTP) Spectroscopy A detailed layout of the OPTP is shown in Figure S1. A Ti:sapphire amplified laser system (Spitfire ACE by Spectra-Physics) producing ultra-short laser pulses of ~40 fs duration at 800 nm at 1 khz repetition rate was used to drive our OPTP set-up. About ~ 900 mw energy is used to run the optical pump-thz probe spectrometer setup. For the THz generation and detection, 10% of the incoming laser beam is used (90 mw). THz radiation is generated in a phase-matched manner by optical rectification in a ZnTe crystal (<110> orientation, 10x10x1 mm thickness, purchased from MaTeck). The ZnTe generation crystal is pumped with a slightly focused beam (~ 3 mm diameter) of 800 nm light (80 mw power, 50 fs FWHM). The THz light exits the ZnTe generation crystal slightly divergent and is first collimated and subsequently focused on the sample using a pair of off-axis parabolic mirrors. The transmitted THz pulses are recollimated and focused on a second ZnTe detection crystal by another pair of parabolic mirrors, where the instantaneous THz field strength is detected through electrooptical sampling. Figure S1. Schematic layout of the optical pump-thz probe spectrometer set-up.

3 Using optical pump-terahertz probe spectroscopy for monitoring ET at QD-oxide interfaces, is based on exciting QD donors by a femtosecond optical laser pulse and subsequently probing the induced transient terahertz (THz) wave absorption with sub-picosecond time resolution. 2. Reproducibility Tests The OPTP signal response reproducibility was tested for all samples by measuring signals for a given sample at different sample s spots (5-10 points). As an example, Figure S2 illustrates that for a 2 SILAR cycle PbS QDs on SnO 2 before passivation, the ET dynamics of 5 different points A-E (a schematic illustration of sample s spots shown in the inset) provide a mean square error of ~3 %. For most cases, the variation of signal at plateau ranges from 3 % and 8 % from sample to sample. Figure S2. The reproducibility test of ET dynamics on 2 SILAR cycle PbS QDs on SnO 2. The inset illustrates how we measure and estimate the variation of signal by keeping measuring at 5 different points of the same spot.

4 3. Impact of QD Dipolar Molecular Capping on QD/Oxide Backward ET Rate Figure S3. (A-D) Comparison of the normalized OPTP dynamics before and after QD molecular capping in 1200 ps time window. (E) The ratio of OPTP responses at 1200 ps pump-probe delay before and after QD molecular capping. Figure S3 summarizes our findings regarding back electron transfer (BET) dynamics for the PbS/SnO 2 samples as a function of QD dipolar molecular capping. As evident from the plots, BET dynamics after QD molecular treatments are independent of the sign and magnitude of the dipolar capping agent. BET rates are speeded up when comparing dynamics before and after molecular treatments; this observation can supports the view of an effective QD molecular capping treatment. We tentatively associate this observation with an enhancement in wavefunction coupling between electrons populating the oxide CB and holes in the HOMO states. Faster BET lifetimes induced by enhanced coupling between oxide CB and QD HOMO states could be triggered e.g. by the enhanced density of states for the dipole/qd hybrid system when compared with bare QD. 1,2 In this line of reasoning, it is worth noting that several works have concluded that the effect of thiol based ligands interacting with QD surfaces promote hole delocalization from the QD core to the molecular shell. 3-5 Such effect has

5 been reported to depend primarily on the nature of the molecular head group rather than the specific dipole moment of the molecule First-Principle Calculations To examine the biding energy and packing conformations of the molecules attached on the PbS surface as well as the molecular dipole moment for the isolated molecules, we performed the firstprinciples calculations. Figure S4. The top views of the optimized molecular structures. (A) Two, (B) four, (C) eight, and (D) sixteen Ph-NO 2 molecules are bonded to the PbS(111) surface, while (E) two molecules are adsorbed on the PbS(100) surface. H, C, N, O, S, and Pb atoms are denoted in white, sky blue, red, blue, yellow, and brown colors, respectively. The dipole moments of the isolated molecules in gas phase listed in Figure 1 in the main text were calculated with the GAMESS package. 6 We used the cc-pvtz basis set and the B3LYP functionals for the optimization of the molecular structure as well as for computing the dipole moments.

6 For exploring the binding energy of the molecules on the PbS surface and packing conformations, we calculated the total energy of the system for molecules attached on the PbS surface (E mol+pbs ), the energy of the bare PbS surface (E PbS ), and the energy of the isolated molecules (E mol ) with the SIESTA code package. 7 We prepared the PbS (100) and (111) surfaces within the slab model. The PbS slab model was composed of three PbS layers, where the structure of the topmost 1 layer along the z- direction were optimized, while the atom positions for the rest of the two layers were fixed. Note that the topmost layer in this study means that the layer where the molecules were attached. The size of the cell was Å Å 35 Å for the (100) surface and Å Å 35 Å for the (111) surface with periodic boundary conditions. The x- and y-directions of the cell size was set to the experimentally measured lattice constant. We used the Troullier-Martins norm-conserving pseudopotential 8 with the Kleinman-Bylander nonlocal projector 9 for describing the core electrons. We used the PBE functions and DZP basis set. The k-points were sampled by Note that the same calculation conditions were used for all the systems of molecules absorbed on the PbS surface, isolated molecules, and bare PbS surface. For the cases of the molecules absorbed on the PbS surface, the H atoms next to S atoms of the thiol molecules were dissociated from the molecules and were placed on the PbS surface where the molecules were not attached. The binding energy of a molecule when m molecules are added onto the PbS surface where n molecules are attached ( ) can be computed from =,,, where E n,pbs is the conformation energy of the system of n molecules attached on the PbS surface, E mol is the energy of a single isolated molecule, and E PbS is the energy of the bare PbS surface. The molecular geometries of the Ph-NO 2 molecules absorbed on the PbS (111) surface as well as PbS (100) surface are displayed in Figure S4. The calculated bonding is plotted in Figure 4(A) in the main text. The angles of the molecules absorbed on the PbS surface and surface normal were calculated by using the optimized structure for two molecules absorbed on the PbS (111) surface. The data are given in Figure 4(B) in the main text.

7 5. Ultraviolet Photoelectron Spectroscopy (UPS) Figure S5. (A) Basic principles and schematic illustration of the energy diagram in UPS measurement. (B) Exampled UPS spectra of the SnO 2 /PbS QDs with Ph-COOH ligand capping. UPS has been used as reliable approach to determine the Fermi level (E Fermi ) and the valence band maximum (E VBM ) with respect to vacuum level (E vac ). The basic principles of UPS and one representative UPS spectrum of the SnO 2 /PbS QD sample under current study are shown in Figure S5 (A) and (B), respectively. For a photoelectron to escape the sample surface and to be collected by the detector, it has to gain sufficient energy to surpass the sum of the binding energy of its initial level (with respect to E Fermi ) and the work function (φ), where φ = E vac E Fermi. Therefore, for a definite excitation energy (21.2 ev) in UPS, the secondary electron cut-off at high binding energy edge (e.g. in Figure S5(B) represents photoelectrons with zero kinetic energy (E k ) when they escape the sample surface, and their initial energy level is marked as the dotted line in Figure S5(A). The work function φ is determined by the difference between the incident photon energy (21.2 ev) and the binding energy of the secondary electron cut-off. For example, Figure S5(B) shows one representative UPS spectra SnO 2 /PbS QDs with Ph-NO 2 ligand capping. As shown, the secondary electron cut-off locates at binding energy of 17.1 ev as determined by the intersection of the linear portion of the spectrum and the baseline. The work function of this sample is thus φ = =4.1 ev, i.e., E Fermi is 4.1 ev with respect to E vac.

8 References 1. Giansante, C.; Infante, I.; Fabiano, E.; Grisorio, R.; Suranna, G. P.; Gigli, G. "Darker-than- Black" PbS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands. J. Am. Chem. Soc. 2015, 137, Azpiroz, J. M.; De Angelis, F. Ligand Induced Spectral Changes in CdSe Quantum Dots. ACS. Appl. Mater. Inter. 2015, 7, Wuister, S. F.; Donega, C. D.; Meijerink, A. Influence of Thiol Capping on the Exciton Luminescence and Decay Kinetics of CdTe and CdSe Quantum. J. Phys. Chem. B 2004, 108, Frederick, M. T.; Amin, V. A.; Weiss, E. A. Optical Properties of Strongly Coupled Quantum Dot-Ligand Systems. J. Phys. Chem. Lett. 2013, 4, Frederick, M. T.; Amin, V. A.; Swenson, N. K.; Ho, A. Y.; Weiss, E. A. Control of Exciton Confinement in Quantum Dot-Organic Complexes through Energetic Alignment of Interfacial Orbitals. Nano Lett. 2013, 13, Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J. Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic-Structure System. J. Comput. Chem. 1993, 14, Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.-Condens. Mat. 2002, 14, Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev. B 1991, 43, Kleinman, L.; Bylander, D. M. Efficacious Form for Model Pseudopotentials. Phys. Rev. Lett. 1982, 48,