Ligand-Mediated Energy-Level Modification in PbS Quantum Dots

Article Cite This: J. Phys. Chem. C 2018, 122, 11570−11576

pubs.acs.org/JPCC

Ligand-Mediated Energy-Level Modification in PbS Quantum Dots as Probed by Density of States (DOS) Spectra Biswajit Kundu and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

J. Phys. Chem. C 2018.122:11570-11576. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/19/18. For personal use only.

S Supporting Information *

ABSTRACT: Ligands are known to passivate the surface of semiconductor nanocrystals and consequently alter their Fermi energy and band positions. In this work, we have utilized scanning tunneling spectroscopy (STS) to record differential tunnel conductance spectra (dI/dV) of PbS nanocrystals; since dI/dV has a correspondence to the semiconductors’ density of states, their band edges with respect to the Fermi energy could be located directly from the dI/dV spectra. With a series of ligands in PbS quantum dots (QDs), ligand-dependent shift in Fermi energy has been observed. From the location of Fermi energy relative to the conduction and valence band edges, the STS measurements allowed determination of the semiconductor type, which could be correlated to the functional groups of the ligands. While thiol-based ligands introduced excess sulfur, resulting in leadvacancies and thereby p-nature in PbS QDs, other ligands yielded ntype QDs due to their electron-donating nature. The p- and n-type nanocrystals, when cast sequentially to form pn- or nphomojunctions, resulted in current rectification due to type II band alignment at the interface.

■

INTRODUCTION Colloidal quantum dots (QDs) are considered as building blocks in a range of electronic and optoelectronic devices, since their energy levels can be compatibly tailored primarily by their size to suit a functionality of a device.1−4 The electronic properties of the QDs are also dictated by surface atoms, which are large in numbers in materials having low dimensions.5 These surface atoms are passivated during the growth process; long-chain ligands are generally used to retain dispersibility of the QDs in suitable solvents. While forming thin films of QDs, the surface ligands are often replaced with shorter ones to ease the conduction process.6−9 That is, films for device applications are typically formed by dip- or spin-coating a layer of QDs, followed by replacing the original long-chain insulating organic ligands with short-chain molecules to decrease the inter-QD separation in creating a conductive QD film.4,10,11 All of these ligand-exchange strategies passivate the QDs through minimizing the density of defect states, effectually resulting in bandengineered materials,12−14 which in turn affects device performance largely. For example, in solar cells based on QDs, the power conversion efficiency has been found to depend strongly on the surface ligands attached to the nanostructures.15,16 Scanning tunneling spectroscopy (STS) is a unique tool to determine conduction band and valence band (CB and VB, respectively) edges with respect to Fermi energy.17−19 A differential tunnel conductance (dI/dV) spectrum, which corresponds to density of states (DOS) of a semiconductor, provides band energies with respect to the Fermi energy. Since © 2018 American Chemical Society

STS provides both of the band-edge positions of an individual QD due to the localized mode of measurement, the technique would allow one to study the effect of passivation on their energy levels. This is in contrast to other modes of measurement, involving optical spectroscopy, which considers band gap of QDs in ensembles. For example, Chuang et al. used ultraviolet photoelectron spectroscopy to determine the VB edge of PbS QDs; the CB edge was then calculated by adding the optical gap obtained from optical absorption spectroscopy.8 In this work, we took STS as a direct approach for simultaneous determination of CB and VB edges of PbS QDs having a range of surface ligands. The dI/dV spectroscopy and therefore the DOS of the semiconductors allowed us to study ligand-mediated energy-level modification in these QDs.

■

EXPERIMENTAL SECTION

Materials. For the growth of PbS nanoparticles, lead oxide (PbO), oleic acid (OA), 1-octadecene (ODE), and hexamethyldisilathiane (TMS) (synthesis grade) were purchased from Sigma-Aldrich Chemical Co. Tetrabutylammonium iodide (TBAI), 1,2-ethanedithiol (EDT), and 3-mercaptopropionic acid (MPA) were also purchased from Sigma-Aldrich, and ethylenediamine (EDA) was procured from Merck India. The materials were used without further purification.

Received: March 30, 2018 Revised: May 10, 2018 Published: May 10, 2018 11570

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C

Figure 1. Optical absorption spectra of PbS nanocrystals with (a) different surface ligands and (b) different diameters.

Table 1. IPUAC Name and Structural Formula of the Ligands used in Passivating PbS Nanocrystals

Growth of PbS Nanoparticles. PbS nanoparticles were grown using a standard reported procedure.20 In brief, 446 mg (2 mmol) of PbO was dissolved in a mixture of 2 mL of oleic acid and 20 mL of octadecene in a three-neck reaction flask. The mixed solution, when heated to 150 °C for 1 h under nitrogen atmosphere, formed lead oleate. The temperature of the flask was then reduced to 90 °C. A mixture of 0.2 mL of TMS and 10 mL of ODE was injected into the lead oleate solution under vigorous stirring condition. The transparent solution immediately turned dark brown, indicating the formation of PbS nanocrystals. The reaction was stopped after 2 min by allowing the solution to cool to room temperature. The nanocrystals were isolated by precipitation through addition of excess acetone, followed by centrifugation at 8000 rpm. They were washed in acetone repeatedly before redispersing in toluene. To control and tune the size of the nanocrystals, we varied the content of oleic acid while retaining the total volume of the precursors (oleic acid and octadecene). In this work, we have formed four different diameters of PbS nanocrystals. Ligand Exchange during the Formation of PbS Thin Films. In this work, we aimed to study the effect of different ligands on the DOS of PbS nanocrystals. To do so, we formed an ultrathin layer of the PbS nanocrystals on arsenic-doped (ntype) silicon wafers having a resistivity of 5−10 mΩ cm. A layer of oleic acid-capped PbS nanocrystals was first spun at 2500 rpm for 30 s from their dispersed solution in toluene (25 mg/ mL). A TBAI solution (10 mg/mL in methanol) was then applied to the nanoparticle layer for 30 s, followed by three rinse−spin steps with methanol. These steps allowed replacement of oleic acid with TBAI as the surface ligands of the PbS nanocrystal layer.21 The oleic acid ligands were

replaced also by EDT, MPA, and EDA following the similar protocol. The ligand concentrations and solvents used in this work were representative of well-characterized ligand-exchange conditions from the literature;22 EDT, MPA, and EDA had a concentration of 1.7, 115, and 1000 mM, respectively, in methanol. Characterization of the Nanocrystals and STS Measurements. The PbS nanocrystals, dispersed in toluene and also in thin films, were characterized with a UV−vis−NIR spectrophotometer (Cary 5000, Agilent Technologies). To determine the diameter of the nanocrystals, they were characterized by a transmission electron microscope (Jeol JEM-2100F TEM). X-ray diffraction (XRD) studies and X-ray photoelectron spectroscopy (XPS) of the materials were conducted by a Bruker D8 Advance X-ray powder diffractometer (Cu Kα radiation, λ = 1.54 Å) and an XPS instrument (Omicron: Serial no. 0571), respectively. To record differential tunnel conductance (dI/dV) of the semiconductors, ultrathin films of the QDs were formed on highly doped silicon wafers (n-type, As-dopant), which acted as an excellent base electrode for STS studies. The ultrathin films, each containing QDs of a desired diameter and with one type of surface ligands, were probed in an ultrahigh vacuum scanning tunneling microscope (UHV-STM). Pt/Ir (80:20%) wires having a diameter of 0.25 mm were mechanically cut to form tips for the microscope. The base pressure of the microscope chamber was 2.5 × 10−10 Torr. Measurements were carried out at room temperature. For approach of the tip, a bias of 2.0 V was applied to achieve a set current of 100 pA. dI/dV versus voltage characteristics were recorded using a lock-in amplifier (20 mV root mean square, 997 Hz). Since the direct current bias was applied to the substrate with respect to the tip, the tip injected electrons at a 11571

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C

Figure 2. Typical DOS spectra of oleic acid-capped PbS QDs having different diameters (as shown in legends). The solid and broken vertical arrows point to the peaks closest to 0 V locating the position of VB and CB edges, respectively, with respect to the Fermi energy, which was set to 0 V. The plots in the insets represent histogram of VB and CB edges of the respective QDs.

positive substrate voltage; in the dI/dV versus sample voltage plots, the peak at a positive bias closest to 0 V hence denoted the CB of the nanocrystals. Similarly, the first peak at the negative voltage implying withdrawal of electrons provided the VB energy of the semiconductors. It may be stated that under equilibrium condition Fermi energy of a semiconductor is considered to be aligned to 0 V.

We then proceeded to record dI/dV spectra of their ultrathin films. For each diameter, a typical differential tunnel conductance spectrum is presented in Figure 2. The peaks closest to 0 V at positive and negative biases correspond to the CB and VB of the nanocrystals, respectively. Irrespective of the QD diameter, the Fermi energy, which is aligned at 0 V, could be seen to be slightly closer to VB edge than the CB energy. The results hence inferred a mild p-type nature of the nanocrystals that presumably appeared due to surface states of the QDs. The peaks are boarder than the discrete energy levels of QDs due to the single-electron charging effects prevalent in the tunneling spectroscopy of ligand-associated QDs; such an effect would lead to Coulomb blockade and thereby increase the differential resistance.23 Since STS is an extremely localized mode of measurement, we have recorded dI/dV spectra at many points on the ultrathin films, each of which contained QDs having a particular diameter. While each dI/dV spectrum provided CB and VB edges at the point of measurement, the energies from a large number of STS measurements for each diameter of PbS QDs were summed in the form of histograms of CB and VB edges. The histograms have been added in the inset of the representative dI/dV spectrum (Figure 2). The peak of the histograms allowed us to obtain the CB and VB energies of the QDs and also the diameter dependence of band energies. The histogram of CB and VB edges inferred a mild p-type nature of the nanocrystals, since the Fermi energy could be found to be located closer to the VB edge than to the CB edge. The separation between the band edges, the transport gap, corresponded to the diameter of the QDs in the usual manner. That is, the gap shrunk with an increase in the diameter of the QDs.

■

RESULTS AND DISCUSSION PbS nanocrystals, which were grown through a reported colloidal synthesis route,20 were characterized through optical absorption spectroscopy along with Tauc plots, TEM, and energy-dispersive X-ray spectroscopy (EDS), XRD studies, and XPS analysis. Absorption spectra are shown in Figure 1, and other results are presented in the Supporting Information (Figures S1−S6). The characterization results inferred the formation of nanocrystals. Structural formulae of the series of ligands used during the growth of PbS nanocrystals are listed in Table 1, and optical absorption spectra of PbS nanoparticles capped with the ligands are shown in Figure 1a. The spectra provided the signature of PbS nanocrystals with a peak in the near-infrared region. The ligands, however, did not affect the optical absorption spectrum of the nanocrystals in their dispersed solution. Tauc plots of the materials inferred invariance of optical gap upon ligand exchange (Figure S2); the gap turned out to be 1.2−1.3 eV, which is expected from PbS QDs having a diameter of 3.0 nm. Optical absorption spectra responded to the diameter due to quantum confinement effect (Figure 1b). The diameters of the nanocrystals were determined from histograms of particle size by analyzing TEM images with ImageJ software (Figure S3). 11572

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C

the Fermi energy to be near the VB edge; the EDT- and MPAcapped nanocrystals had therefore a p-nature. The STS results hence provide a direct inference that the ligands can alter the type of nanocrystals to p- or n-type. The band gaps of the nanocrystals, as derived from the difference between the energy of histogram peaks, however remained unaltered in the four ligand cases. This invariance of band gap was also reflected in the optical absorption spectra and Tauc plots. The ligand-dependent shift in Fermi energy needs to be analyzed further. As such, Lewis base ligands, when bound to a central metal atom (lead, in this case) in forming a coordination complex, involve a formal donation of one or more of the ligand’s electron pairs. Accordingly, the EDA-capped nanocrystals, for example, had a higher electron density, prompting Fermi energy to shift toward the CB edge; the QDs hence turned to an n-type semiconductor. With TBAI, each counterion (iodide) capped the surface cation and hence acted as a source of an electron; the halide anions passivated the nanocrystals and thereby resulted in an n-type semiconducting nature of the ultrathin films. The EDA-capped nanocrystals were more n-type semiconductors due to the lone pair of electrons in nitrogen as the source of free electrons in the nanocrystals. With thiol capping, such as EDT and MPA, the thiols were bound to Pb2+, while substituting the long oleic acid ligands on the surface; the ligands hence acted as a source of sulfur and thereby yielded sulfur-rich PbS nanocrystals compared to the oleic acid-capped ones. The sulfur-rich nanoparticles accordingly behaved as a p-type semiconductor. When energies of EDT- and MPA-capped nanocrystals were compared, Fermi energy in the former ones could be seen to have moved more toward the VB edge due to its sulfur pairs and hence the ability to turn the nanocrystals more lead-deficient. The results hence provided a direct evidence of energy-level modification in nanocrystals upon passivation through surface ligands. Apart from monitoring the shift in Fermi energy, the STS studies in addition allowed a direct and simultaneous determination of CB and VB edges of the nanostructures with respect to their Fermi energy. The shift of Fermi energy to the CB and VB edges upon a suitable ligand exchange allowed us to form a homojunction. A junction between a p-type and an n-type PbS layer would have a type II band alignment at the interface. Therefore, the homojunction may act as a current rectifier due to the energylevel diagram at the junction. We hence formed junctions between EDT-capped PbS and TBAI-capped PbS QDs in both sequences in obtaining pn- and np-junctions. I−V characteristics of the two opposing junctions allowed us to exclude the effect of other interfaces in resulting rectification and confirm the role of the homojunctions in the characteristics. In Figure 5a, we presented the tunneling current versus voltage characteristics of the two homojunctions with p-type EDT@ PbS and n-type TBAI@PbS formed in both sequences. Due to the localized nature of measurement with an STM tip, I−V characteristics were recorded at many different points in each of the junctions. We presented a number of I−V characteristics along with their average one in the figures. The results first of all show reproducibility of the measurements. Both the homojunctions returned rectifying I−V characteristics; they exhibited current rectification in the opposite bias modes. That is, with voltage being applied to the substrate, the EDT@PbS/ TBAI@PbS pn-junction yielded a higher current in the positive-voltage region; on the contrary, the np-junction

The histograms show some extent of distribution in energy that was symmetric in nature. The distribution in energy histograms in general occurs due to the presence of defect or disordered states in compounds formed through colloidal synthesis method. A distribution in the energy histograms would occur also due to a diameter variation of the QDs in an ultrathin film. The full width at half-maximum (FWHM) of the histograms in the energy scale was the same (0.18−0.19 eV) for the CB and VB edges; the FWHMs did not respond to the average diameter of the QDs either (Table S1). The results hence infer invariance in the density of defect or disordered states in the nanocrystals grown through the colloidal synthesis method. The histograms can be summarized further to a plot of CB and VB energies as a function of the diameter of QDs (Figure 3). The band gap can be seen to shrink with an increase in the

Figure 3. CB and VB edges of PbS nanocrystals having different diameters. Fermi energy is aligned to 0 eV.

diameter. The gap derived from STS studies could also be correlated to the optical gap obtained from optical absorption spectra and Tauc plots thereof (Figures 1b and S1). Both the band edges could be seen to move toward the Fermi energy in the same quantum. The diameter-dependent band gap along with their band edge energies hence infers the success of STS in determining DOS of the semiconductors in their lowerdimension form. We then took up PbS nanoparticles with different surface ligands. Here also, we measured differential tunnel conductance at many points on the ultrathin films. Each dI/dV spectrum provided locations of CB and VB energies in the form of peaks in the two bias directions. We took up four different surface ligands, namely, TBAI, EDT, MPA, and EDA, in forming ultrathin films of PbS nanocrystals with a diameter of 3.0 nm. Typical dI/dV spectra along with the histogram of band energies derived from the dI/dV spectra separately for the four cases are shown in Figure 4. The spectra, upon comparison, show that the band energies now depended on the ligands attached to the nanocrystals. The histograms also inferred a similar dependence. FWHMs of the band edges did not alter in the four ligand cases, since the diameter distribution in the QDs remained the same, that is, the ligands in the nanocrystals did not introduce any additional defects or disorders, which are known to give rise to a distribution in the energy histograms. The position of Fermi energy relative to the CB and VB edges could be seen to depend on the nature of ligands. For example, with EDA or TBAI ligands in the nanocrystals, Fermi energy was closer to the CB edge, inferring their n-type nature. On the contrary, the nanocrystals with EDT or MPA ligands evidenced 11573

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C

Figure 4. (a−e) Typical DOS spectra of PbS having different ligands. The solid and broken vertical arrows point to the peaks closest to 0 V, locating the position of VB and CB edges, respectively. The plots in the insets represent the histograms of VB and CB edges of the respective materials. (f) CB and VB edges of PbS nanocrystals having different ligands.

observation of current rectification in the homojunctions formed between PbS nanoparticles having different ligands and thereby due to a type II band alignment at the interface. Since current rectification originates due to a type II energy alignment at the interface, a change in energy levels of a component should affect the alignment and thereby the current rectification. With another p-type PbS (MPA-capped), we can hence expect that the rectification in MPA@PbS/TBAI@PbS (and its inverse one) will differ from that with junctions having EDT-capped QDs as one of the components. In Figure 5e, we presented I−V characteristics of MPA@PbS/TBAI@PbS junction and its reverse one. As expected, they evidenced rectification in opposing directions. The rectification ratio in the MPA@PbS/TBAI@PbS junction was less than that in the EDT@PbS/TBAI@PbS junction. The energy-level diagrams of EDT@PbS/TBAI@PbS and MPA@PbS/TBAI@PbS (insets of Figure 5a,e, respectively) indicate that the former junction may act as a better current rectifier. Such a presumption is due to the fact that EDT@PbS being a stronger p-type material, the

exhibited a higher current at the negative substrate voltage. The direction of rectification is in accordance with the energy levels of different QDs (inset of Figure 5a), which suggests a preferential flow of current from EDT@PbS to TBAI@PbS in EDT@PbS/TBAI@PbS junctions. The junction with a reverse sequence (TBAI@PbS/EDT@PbS) yielding rectification in the opposite direction reinforces that the rectification indeed appeared due to a type II band alignment at the interface. It should be stated here that the characteristics of a homojunction was not a mirror image of the other. This was expected since tunneling current was measured here; the I−V’s hence ought to pass through the set point (2.0 V, 0.1 nA) of tip approach. The set point is marked as a red circle in the figures. Both the junctions yielded a rectification ratio of 4 at 1.5 V. We also characterized the components of the junctions separately. I−V characteristics through the components, also recorded at many different points on each of the ultrathin films, are shown in Figure 5b−d. The characteristics were nonrectifying in nature. The results hence further support the 11574

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C

Figure 5. (a) Tunneling current versus sample voltage characteristics of an EDT@PbS/TBAI@PbS junction and its inverse one, TBAI@PbS/EDT@ PbS, formed between two layers of the semiconductor nanocrystals. (b−d) Tunneling current versus sample voltage characteristics of the components, namely, EDT@PbS and TBAI@PbS, and also of MPA@PbS monolayer. (e) Tunneling current versus sample voltage characteristics of MPA@PbS/TBAI@PbS and TBAI@PbS/MPA@PbS junctions. The insets of (a) and (e) show band diagrams of EDT@PbS/TBAI@PbS and MPA@PbS/TBAI@PbS homojunctions, respectively. The set point of tip approach is marked by a red circle in each figure.

band edges upon a suitable ligand exchange, homojunctions with a type II band alignment at the interface were formed, which acted as current rectifiers.

EDT@PbS/TBAI@PbS junction had a more effective type II band alignment, which is the key parameter in yielding current rectification. The results hence provide further evidence of energy-level modification in QDs upon passivation through surface ligands.

■

■

ASSOCIATED CONTENT

S Supporting Information *

CONCLUSIONS In conclusion, we have studied ligand-mediated energy-level modification in PbS QDs through STS. We have passivated the surface of PbS QDs with a range of ligands. Differential conductance tunneling spectra were used to determine the DOS of the semiconducting QDs that in turn provided their CB and VB edges. In EDA-capped nanocrystals, the Lewis base ligands donated one or more of the ligand’s electron pairs and accordingly produced n-type semiconductors as evidenced from a shift in Fermi energy toward the CB edge. With TBAI, each iodide counterion capped the surface cation and hence acted as a source of an electron. These QDs hence have an n-type nature in the thin film of PbS QDs. With EDT and MPA capping, the thiols acted as a source of sulfur and resulted in sulfur-rich PbS nanocrystals; their Fermi energy shifted toward the VB edge, and the QDs therefore acted as p-type semiconductors. With the shift of Fermi energy toward either

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b03022. Characterization section including Tauc plots (Figures S1 and S2), TEM images (Figure S3), energy-dispersive X-ray spectra (EDS) (Figure S4), XRD studies (Figure S5), XPS analysis (Figure S6), and FWHM of the histograms of the two band edges as a function of QD diameter (Table S1) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-33-24734971. Fax: +9133-24732805. ORCID

Amlan J. Pal: 0000-0002-7651-9779 11575

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576

Article

The Journal of Physical Chemistry C Notes

(16) Kim, B. S.; Hong, J.; Hou, B.; Cho, Y.; Sohn, J. I.; Cha, S.; Kim, J. M. Inorganic-Ligand Exchanging Time Effect in PbS Quantum Dot Solar Cell. Appl. Phys. Lett. 2016, 109, No. 063901. (17) Katz, D.; Wizansky, T.; Millo, O.; Rothenberg, E.; Mokari, T.; Banin, U. Size-Dependent Tunneling and Optical Spectroscopy of CdSe Quantum Rods. Phys. Rev. Lett. 2002, 89, No. 086801. (18) Maruccio, G.; Meyer, C.; Matsui, T.; Talapin, D. V.; Hickey, S. G.; Weller, H.; Wiesendanger, R. Wavefunction Mapping of Immobilized InP Semiconductor Nanocrystals. Small 2009, 5, 808− 812. (19) Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Heavily Doped Semiconductor Nanocrystal Quantum Dots. Science 2011, 332, 77−81. (20) Hines, M. A.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844−1849. (21) Zhitomirsky, D.; Furukawa, M.; Tang, J.; Stadler, P.; Hoogland, S.; Voznyy, O.; Liu, H.; Sargent, E. H. N-Type Colloidal-QuantumDot Solids for Photovoltaics. Adv. Mater. 2012, 24, 6181−6185. (22) Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulovic, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863−5872. (23) Wang, B.; Wang, K. D.; Lu, W.; Yang, J. L.; Hou, J. G. SizeDependent Tunneling Differential Conductance Spectra of Crystalline Pd Nanoparticles. Phys. Rev. B 2004, 70, No. 205411.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.J.P. acknowledges JC Bose Fellowship (SB/S2/JCB-001/ 2016) of SERB. The authors acknowledge financial support from SERIIUS project with grant number IUSSTF/JCERDCSERIIUS/2012.

■

REFERENCES

(1) McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics. Nat. Mater. 2005, 4, 138−142. (2) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Ultrasensitive Solution-Cast Quantum Dot Photodetectors. Nature 2006, 442, 180−183. (3) Bisri, S. Z.; Piliego, C.; Yarema, M.; Heiss, W. G.; Loi, M. A. Low Driving Voltage and High Mobility Ambipolar Field-Effect Transistors with PbS Colloidal Nanocrystals. Adv. Mater. 2013, 25, 4309−4314. (4) Hou, B.; Cho, Y.; Kim, B. S.; Hong, J.; Park, J. B.; Ahn, S. J.; Sohn, J. I.; Cha, S.; Kim, J. M. Highly Monodispersed PbS Quantum Dots for Outstanding Cascaded-Junction Solar Cells. ACS Energy Lett. 2016, 1, 834−839. (5) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (6) Kroupa, D. M.; Voros, M.; Brawand, N. P.; McNichols, B. W.; Miller, E. M.; Gu, J.; Nozik, A. J.; Sellinger, A.; Galli, G.; Beard, M. C. Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification. Nat. Commun. 2017, 8, No. 15257. (7) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X. H.; Debnath, R.; Cha, D. K.; et al. Colloidal-Quantum-Dot Photovoltaics using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765−771. (8) Chuang, C.-H. M.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796−801. (9) Crisp, R. W.; Kroupa, D. M.; Marshall, A. R.; Miller, E. M.; Zhang, J. B.; Beard, M. C.; Luther, J. M. Metal Halide Solid-State Surface Treatment for High Efficiency PbS and PbSe QD Solar Cells. Sci. Rep. 2015, 5, No. 9945. (10) Talgorn, E.; Gao, Y. N.; Aerts, M.; Kunneman, L. T.; Schins, J. M.; Savenije, T. J.; van Huis, M. A.; van der Zant, H. S. J.; Houtepen, A. J.; Siebbeles, L. D. A. Unity Quantum Yield of Photogenerated Charges and Band-Like Transport in Quantum-Dot Solids. Nat. Nanotechnol. 2011, 6, 733−739. (11) Balazs, D. M.; Dirin, D. N.; Fang, H. H.; Protesescu, L.; ten Brink, G. H.; Kooi, B. J.; Koyalenko, M. V.; Loi, M. A. CounterionMediated Ligand Exchange for PbS Colloidal Quantum Dot Super lattices. ACS Nano 2015, 9, 11951−11959. (12) Millo, O.; Katz, D.; Cao, Y. W.; Banin, U. Imaging and Spectroscopy of Artificial-Atom States in Core/Shell Nanocrystal Quantum Dots. Phys. Rev. Lett. 2001, 86, 5751−5754. (13) Steiner, D.; Dorfs, D.; Banin, U.; Della Sala, F.; Manna, L.; Millo, O. Determination of Band Offsets in Heterostructured Colloidal Nanorods using Scanning Tunneling Spectroscopy. Nano Lett. 2008, 8, 2954−2958. (14) Wang, T.; Vaxenburg, R.; Liu, W. Y.; Rupich, S. M.; Lifshitz, E.; Efros, A. L.; Talapin, D. V.; Sibener, S. J. Size-Dependent Energy Levels of InSb Quantum Dots Measured by Scanning Tunneling Spectroscopy. ACS Nano 2015, 9, 725−732. (15) Barkhouse, D. A. R.; Pattantyus-Abraham, A. G.; Levina, L.; Sargent, E. H. Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency. ACS Nano 2008, 2, 2356−2362. 11576

DOI: 10.1021/acs.jpcc.8b03022 J. Phys. Chem. C 2018, 122, 11570−11576