Directional Carrier Transfer in Strongly Coupled Binary Nanocrystal

Article pubs.acs.org/JPCC

Directional Carrier Transfer in Strongly Coupled Binary Nanocrystal Superlattice Films Formed by Assembly and in Situ Ligand Exchange at a Liquid−Air Interface Yaoting Wu,⊥,† Siming Li,§,† Natalie Gogotsi,∥ Tianshuo Zhao,∥ Blaise Fleury,⊥ Cherie R. Kagan,⊥,∥,‡ Christopher B. Murray,*,⊥,∥ and Jason B. Baxter*,§ ⊥

Department of Chemistry, ∥Department of Materials Science and Engineering, and ‡Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States § Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Two species of monodisperse nanocrystals (NCs) can self-assemble into a variety of complex 2D and 3D periodic structures, or binary NC superlattice (BNSL) films, based on the relative number and size of the NCs. BNSL films offer great promise for both fundamental scientific studies and optoelectronic applications; however, the utility of as-assembled structures has been limited by the insulating ligands that originate from the synthesis of NCs. Here we report the application of an in situ ligand exchange strategy at a liquid−air interface to replace the long synthesis ligands with short ligands while preserving the long-range order of BNSL films. This approach is demonstrated for BNSL structures consisting of PbSe NCs of different size combinations and ligands of interest for photovoltaic devices, infrared detectors, and light-emitting diodes. To confirm enhanced coupling introduced by ligand exchange, we show ultrafast (∼1 ps) directional carrier transfer across the type-I heterojunction formed by NCs of different sizes within ligand-exchanged BNSL films. This approach shows the potential promise of functional BNSL films, where the local and long-range energy landscape and electronic coupling can be adjusted by tuning NC composition, size, and interparticle spacing.

1. INTRODUCTION Semiconductor NCs,1 also known as quantum dots, are being explored extensively as building blocks for applications such as solar cells,2−4 photodetectors,5−8 light-emitting diodes,9−11 and transistors,12−14 where their size-tunable band gap and utility in low-cost solution processing make them particularly appealing.12,15,16 Binary nanocrystal superlattice (BNSL) films have the potential to expand the palette of achievable physical and electronic landscapes by combining two kinds of NCs into twoor three-dimensional crystal structures with long-range order and precise control over their components and chemical composition.17−23 So far a variety of BNSL crystal structures have already been demonstrated, including MgZn2,24,25 NiAs,26 and AB618 that provide bicontinuous heterojunctions. In a typical BNSL film, the interpenetration of two kinds of NCs gives a much larger contact area than a traditional planar heterojunction and more precise control over the local structure than can be achieved with a randomly mixed NC film,27−30 while coupling between neighboring NCs of different band gaps can induce directional charge or energy transfer across the heterojunction interface.31−34 Additionally, in bicontinuous structures, connected chains of each NC species provide pathways for long-range charge transport. The © 2017 American Chemical Society

collective optical and electrical properties of the NC building blocks make BNSL films desirable for potential optoelectronic applications. Both interfacial charge or energy transfer across a heterojunction and long-range transport within a single subgroup of NCs require strong interparticle coupling that is limited by the long, insulating ligands originating from the synthesis of NCs. Ligand exchange is widely applied as a postsynthesis treatment to remove the original ligand, reduce interparticle spacing, and modify the surface chemistry of NCs.35−41 In the last 20 years, a series of ligands has been studied to meet the functional requirements of NC devices.39,42−44 The original ligands can be exchanged either in solution or after film deposition. The solution-based exchange has often been favored because it results in NCs that can then be directly processed into functional films using a variety of deposition methods.6,39,43,45,46 However, solutionbased ligand exchange is limited to only a small number of ligands. In most studies, NCs modified with short ligands have Received: December 7, 2016 Revised: February 5, 2017 Published: February 5, 2017 4146

DOI: 10.1021/acs.jpcc.6b12327 J. Phys. Chem. C 2017, 121, 4146−4157

Article

The Journal of Physical Chemistry C

interface17 in a glovebox that maintains oxygen- and moisturefree conditions. To prepare a typical MgZn2-type BNSL, 4.5 and 6.0 nm PbSe NCs were separately dispersed in hexane at 10 mg/mL. The two samples were mixed 1:1 by volume to give the right number ratio of particles and form a high-quality MgZn2-type BNSL film. Then 10 μL of the mixture was dropcasted onto the surface of diethylene glycol in a square Teflon well. A glass slide was placed to cover the well and reduce the evaporation rate of hexane.50 After 30 min, a solid film was obtained on the liquid−air interface. To prepare a MgZn2-type BNSL film with 3.2 and 4.5 nm PbSe NCs, each size NC was dispersed in hexane at 10 mg/mL and then the two samples were mixed at a volume ratio of 9.5:10. Typical film thickness was ∼100 nm. Ligand Exchange. Liquid−air interface ligand exchange was performed by injecting a solution of short ligands into the Teflon well under the BNSL film, following the procedure described by Dong et al.47 For the primary work in this paper, the solution of short ligands was prepared by dissolving MPA in acetonitrile (0.1 M). 400 μL of the solution was injected into the diethylene glycol phase. Because of the low density of acetonitrile, the free ligand rises to the liquid−air interface with the floating film to initiate ligand exchange. The displaced oleic acid can dissolve in acetonitrile and diethylene glycol, facilitating its removal from the NC surface. After 10 min, the film was transferred to a solid substrate by stamping51 and dried in vacuum overnight for characterization. In addition to MPA, ligand exchange with tetrabutylammonium iodide (TBAI), ammonium iodide (NH4SCN), and formic acid (FA) were studied. Methods of ligand exchange with above three ligands are the same as with MPA. Characterization. Transmission electron microscopy (TEM) images were collected with a JEOL 1400 TEM operating at 120 kV. UV−vis spectra were collected using a Cary 5000 spectrophotometer. PbSe NCs were dispersed in tetrachloroethylene to collect solution phase spectra. The BNSL films were loaded on quartz chips to measure the assembled films. FT-IR spectra were collected using a Thermo-Fisher FT-IR spectrometer (Model 6700). The BNSL films were loaded on double-side polished silicon wafers for transmittance measurements. Grazing incidence small-angle X-ray scattering (GISAXS) was measured at the Advanced Photon Source Sector 8-ID-E (Argonne National Lab). The beam size was 50 μm tall and 100 μm wide. Beam power was 7.35 keV. The detector distance was 1514.26 mm for BNSLs formed with 3.2 and 4.5 nm PbSe NCs, and 1311.61 mm for BNSL formed with 4.5 and 6.0 nm PbSe NCs. We introduce the X-ray coherence length to describe the distance over which the supercrystal is diffracting coherently. The in-plane coherence length (S) can be linked to an average in-plane grain size in the BNSL film. S is defined by 2π S = FWHM , where FWHMy is the full width at half-maximum

proven difficult to self-assemble into superlattices (SLs) with long-range order because of instability of the NC suspension. Instability arises in nonpolar solvents for NCs capped with short organic ligands due to ineffective steric repulsion and in polar solvents for NCs capped with short inorganic ligands due to interaction of surface charges. Ligand exchange of a SL film on a solid substrate is applicable to a much wider range of compact ligands.33,35 However, the shrinkage of the interparticle spacing typically introduces severe cracks and disrupts the long-range order of the SL. Alternatively, Dong et al.47 have demonstrated that cracking of the films can be significantly reduced by carrying out the ligand exchange on a film floating at a liquid−air interface, allowing contraction of the SL unit cell without inducing any strain from a substrate. In this work, we extend the use of in situ ligand exchange at a liquid−air interface to create strongly coupled BNSLs, and we interrogate the ultrafast transfer of photoexcited carriers from donor to acceptor NCs that this strong coupling enables. The crystal structure and long-range order of the oleic acid capped NCs in BNSLs can be maintained after ligand exchange with selected ligands such as 3-mercaptonpropionic acid (MPA), allowing the surface-to-surface interparticle spacing to be significantly reduced from 1.9 to 0.9 nm to enhance electronic coupling. Directional ultrafast carrier transfer across a type-I heterojunction between neighboring MPA-capped PbSe NCs of different sizes in MgZn2-type BNSL films occurred within ∼1 ps, as determined by near-infrared transient absorption spectroscopy (NIR-TA). This result reveals that this assembly and ligand exchange strategy is an efficient tool to prepare strongly coupled BNSL films. We demonstrate the generality and limitations of the approach using multiple short ligands and size combinations of PbSe NCs. The directional carrier transfer within the bicontinuous BNSL indicates the feasibility of manipulating the flow of charge carriers or excitons within the ordered crystalline structure and suggests the potential for new optical and electrical functionalities in strongly coupled BNSL films.

2. EXPERIMENTAL METHODS Synthesis of NCs. PbSe NCs were synthesized by following reported recipes48 to create three sizes of PbSe NCs for use in BNSL films of MgZn2-type structure:24,49 3.2, 4.5, and 6.0 nm. To synthesize 4.5 nm PbSe NCs, a mixture of 0.895 g of PbO, 3 mL of oleic acid (OAc), and 20 mL of 1-octadecene was degassed at 120 °C for 1.5 h, and then the solution was further heated to 150 °C in a nitrogen atmosphere. Then 20 μL of diphenyl phosphine was separately added to 8 mL of trioctylphosphine (TOP)−Se solution (1 M), and the mixture was quickly injected into the reaction flask. The reaction was maintained at 150 °C for 5 min, at which time an ice bath was used to quench the reaction. The NCs were purified with 2propanol three times and then stored in hexane. The size of PbSe NCs was tuned by changing the growth time and injection temperature. The 3.2 nm PbSe NCs were synthesized by injecting TOP-Se at 120 °C and reacting for 5 min. The 6.0 nm PbSe NCs were synthesized by injecting TOP-Se at 180 °C and reacting for 10 min. Transmission electron microscopy (TEM) images and UV−vis spectroscopy, Figure S1, confirmed the monodispersity in size and shape of each NC sample to be 3.2 ± 0.3, 4.5 ± 0.3, and 6.0 ± 0.4 nm. Self-Assembly of BNSL Films. BNSLs with two combinations of NC sizes were formed: 4.5 and 6.0 nm NCs and 3.2 and 4.5 nm NCs. BNSLs were formed at the liquid−air

y

in the qy direction, corrected for the instrumental resolution. The instrumental resolution was 4 × 10−4 Å−1, leading to a coherence length resolution of 1.3 μm. The BNSL films were transferred from liquid−air interface to polished silicon wafer for measurement. NIR-TA spectroscopy was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory. A Spectra-Physics Spitfire Ti:sapphire laser that emits pulses with center wavelength of 800 nm, duration of 110 fs, and repetition 4147


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Figure 1. (a) Schematic of the formation of a BNSL film and the ligand exchange process on liquid−air interface. (b, c) Photographs of a typical film on top of diethylene glycol (b) before and (c) after ligand exchange with MPA. The scale bar represents 10 mm. (d) FT-IR spectra of BNSL film self-assembled with 4.5 and 6.0 nm PbSe nanocrystals before and after ligand exchange with MPA.

bottom layer of NCs was not pinned to a substrate, no strain developed, and the film remained intact. FT-IR spectroscopy was applied to characterize the replacement of ligands on the same BNSL film discussed above. Figure 1d shows the FT-IR spectra before and after ligand exchange, normalized to the first excitonic absorption peak of 6.0 nm NC at 5419 cm−1 (0.67 eV). Because of the predominant replacement of carboxylatebonded oleic acid (C17H32COOH) by mercaptan-bonded MPA (HSC2H4COOH), the intensity of C−H stretching (2852− 2954 cm−1) and asymmetric COO− stretching (1539 cm−1) were reduced by more than 60%, while the O−H stretching mode from the free carboxyl group of MPA appeared at ∼3369 cm−1.52,53 The red-shift in the PbSe excitonic absorption upon ligand exchange indicates stronger electronic coupling between NCs. The excitonic absorption by the 4.5 nm PbSe NCs at ∼7241 cm−1 cannot be observed due to the high absorbance of the silicon substrate within that spectral range. In panels a and b of Figure 2, real space TEM and Fourier transform images compare the local structure of corresponding BNSL films before and after ligand exchange, through the [001] projection of the unit cell (Figure 2a, inset). A perspective view of the MgZn2 unit cell is shown in Figure 2c. The TEM and Fourier transform images confirm that local order was maintained throughout the ligand exchange process with the liquid−air interface method. Local defects introduced by ligand exchange at the liquid−air interface were less frequent than in the film treated with the same ligand on a solid substrate. (Figure S2). To obtain a macroscopic view of long-range order, GISAXS data were collected from BNSL films before and after ligand exchange. Figure 2d shows the GISAXS pattern of the original MgZn2-type BNSL film composed of 4.5 and 6.0 nm PbSe NCs capped with oleic acid. Simulation of the diffraction pattern with space group P63/mmc along the [001] direction gives lattice parameters of a = b = 14.5 nm and c = 26.8 nm, which indicates uniaxial lattice expansion along the c axis compared to the standard MgZn2 unit cell. Just the opposite is reported for BNSL films prepared on solid substrates, where out-of-plane lattice contraction is caused by the evaporation of residual

rate of 1 kHz was coupled to an optical parametric amplifier (Light Conversion, TOPAS-C) to control the pump photon energy and to a TA spectrometer (Ultrafast Systems, Helios). Samples were pumped with 3.18 eV photons and typical pulse energies of ∼18 μJ/cm2. Spectral response was monitored from 0.76 to 1.38 eV using a white light continuum probe generated by focusing the 800 nm seed pulse into a sapphire crystal and detected by an InGaAs multichannel detector. Differential absorbance was probed over the first 3 ns after photoexcitation. The delay time was controlled by an optical delay line. Because of the air sensitivity of samples, films were deposited on a quartz window and sealed with another quartz window and Oring before removal from the glovebox. Cyclic voltammetry measurements were conducted using an electrochemistry workstation (Epsilon, C-3 cell stand) mounted in a nitrogen-filled glovebox. Films of each NC size were spin coated on Pd-coated Si wafers to serve as the working electrode. Then 0.1 M MPA and acetonitrile solution was applied for ligand exchange. Measurements were performed using tetrabutylammonium hexafluorophosphate in acetonitrile (10 mM) as the electrolyte, Ag/AgNO3 in the electrolyte solution (10 mM) as the reference electrode, and a Pt wire as the auxiliary electrode. The potential was scanned from zero to negative potentials and then to positive potentials at a rate of 20 mV/s and calibrated by a ferrocene/ferrocenium redox couple. The film thickness was measured by an atomic force microscope (AFM) (MFP-3D-BIO, Asylum Research) with a silicon cantilever (AC240TS, Olympus).

3. RESULTS AND DISCUSSION Figure 1a shows a schematic of the BNSL film formation and in situ ligand exchange process. Panels b and c of Figure 1 show typical photographs of a MgZn2-type BNSL film assembled from 4.5 and 6.0 nm PbSe NCs before and after ligand exchange with MPA. Because of the nanoscopic reduction of interparticle spacing, the area of the film shrank by approximately 29 ± 2% on the liquid−air interface, as determined by comparing images of 10 BNSL films before and after ligand exchange under the same conditions. Since the 4148


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Figure 2. TEM and Fourier transform images of BNSL films self-assembled with 4.5 and 6.0 nm PbSe NCs (a) before and (b) after ligand exchange with 0.1 M 3-MPA in acetonitrile. Scale bars represent 100 nm. (c) Structural model of MgZn2-type BNSL. The 4.5 nm PbSe NCs occupy two different sites, represented by light and dark blue spheres, while the 6.0 nm PbSe NCs are represented by the red spheres. (d, e) Corresponding GISAXS diagram of BNSL films in panels a and b. (f) Line cut along the dashed line on GISAXS patterns in panels d and e.

preservation of the long-range order by ligand exchange at the liquid−air interface. All the lattice constants, interparticle distances, and coherence lengths of BNSLs discussed in this paper are collected in Table 1.

solvent during the formation of the SL while the NCs are pinned to the substrate in the plane.54−57 The in-plane coherence length (S) was used to quantify and compare the degree of order of BNSL films. The calculated S was 220 nm for the (100) peak in the original BNSL film. After ligand exchange, the high intensity and distinct scattering pattern of Figure 2e confirm the preservation of long-range order of the BNSL film, while the shifting of scattering peaks indicates the change in interparticle spacing. The linecuts in Figure 2f compare the positions of scattering peaks along the dashed lines in Figure 2, panels d and e. The increase in qy corresponds to the reduction of spacing between planes. Simulation of the scattering pattern (Figure 2e) showed that the lattice constants were reduced to a = b = 13.2 nm and c = 20.2 nm, which equates to a contraction of 9% in a and b and 25% in c. Because the electronic coupling between semiconductor NCs is significantly affected by interparticle spacing, this reduction can also be considered in the context of the surface-to-surface distance. In this MgZn2-type unit cell, the 4.5 nm PbSe NCs occupy two different sites, represented by light and dark blue spheres in Figure 2c, while the 6.0 nm PbSe NCs are represented by the red spheres. Coordination of NCs in single unit cell can be found in Figure S3 and Table S1. The shortest interparticle distance is 6.3 nm, between the 6.0 nm NC (sphere A in Figure S3) and the 4.5 nm NC (sphere B in Figure S3). Before ligand exchange, the center-to-center distance between A and B was 7.0 nm, so the total thickness of the ligand shells was 1.8 nm. After ligand exchange with MPA, this interparticle distance was reduced to 1.1 nm, resulting in stronger electronic coupling. The in-plane coherence length remained at 220 nm, which confirms the

Table 1. Lattice Constants, Shortest Inter-Particle Spacing, and In-Plane Coherence Length of MgZn2-Type BNSL Film Formed with Multiple Size Combinations of PbSe QDs and Capped with Different Ligandsa NC size (nm) 4.5 and 6.0 4.5 and 6.0 4.5 and 6.0 3.2 and 4.5 3.2 and 4.5

ligand oleic acid MPA TBAI oleic acid MPA

a, b (nm)

c (nm)

shortest interparticle spacing (nm)

in-plane coherence length (nm)

14.5

26.8

1.8

220

13.2 12.8 11.8

20.2 20.2 23.5

1.1 0.9 1.9

220 350 57

9.9

17.1

0.9

35

a

The lattice constant is simulated from GISAXS patterns of corresponding samples. The calculation of shortest inter-particle spacing can be found in Figure S3.

Having established that ligand exchange with MPA maintains the long-range order of the BNSL structure formed with 4.5 and 6.0 nm PbSe NCs, we now discuss the generality and limitations of the approach for other ligands and particle sizes. First, we consider the same NC sizes with different ligands and surface treatments that are of technological interest for NC devices. Because of distinct chemical properties, the ligands have a strong effect on the structure of the BNSL film. The 4149


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Figure 3. (a−c) TEM and Fourier transform images and (d−f) GISAXS patterns of BNSL films self-assembled with (a, d) 4.5 and 6.0 nm NCs and ligand exchanged with TBAI, (b, e) 3.2 and 4.5 nm NCs before ligand exchange, and (c, f) 3.2 and 4.5 nm NCs ligand exchanged with MPA. Scale bars represent 50 nm.

length of 57 nm was calculated from the (100) diffraction ring of the GISAXS pattern and is ∼4× smaller than that for BNSL films with larger NCs. After ligand exchange with MPA, the lattice parameters were reduced to a = b = 9.9 nm and c = 17.1 nm, which equates to a 16% and 27% reduction, respectively. As expected, the BNSL films composed of smaller particles had a larger fractional reduction of the lattice constant than BNSLs formed with large particles because the ligands occupy a larger fraction of the film volume. After ligand exchange, in-plane coherence length was reduced to 35 nm, or ∼4 unit cells. This reduction in order was also observed by TEM, in Figure S6. The increasingly severe shrinkage of the lattice with smaller particles causes strain that can be relieved by the creation of point and line defects and grain boundaries. The volume of the unit cell was reduced by 48% upon ligand exchange with the small NCs, while the change was only 36% for the larger NCs. The shortest interparticle spacing (center to center) was reduced from 5.7 to 4.7 nm, which corresponds to a reduction in distance between particle surfaces from 1.9 to 0.9 nm and enhanced electronic coupling. Ligand-exchanged BNSL films with well-defined physical arrangement of NCs and strong electronic coupling are ideal samples in which to study charge and energy transfer across the heterojunction interface between different types of NCs. Nearinfrared transient absorption spectroscopy was used to characterize the ultrafast carrier dynamics of a series of single-component SLs and BNSL films composed of 3.2 and 4.5 nm PbSe NCs. The pairing of 4.5 and 6.0 nm PbSe NCs resulted in BNSL films with a higher degree of long-range order, but the energy of the first excitonic absorption feature of the 6.0 nm PbSe NCs (0.67 eV) was below the range of our detector. Samples were photoexcited with an ultrafast (∼100 fs)

ligand exchange with MPA was performed with acetonitrile as solvent. Blank acetonitrile resulted in a slight reduction in lattice parameters as loosely bound ligands are removed (Figure S4), but the effect was small since most oleic acid ligands are still attached. (Figure S4c) Like MPA, TBAI is of interest for application in quantum dot solar cells because the halides can effectively remove the original ligand, passivate the surface of NCs, and improve their air stability.35,58 Panels a and d of Figure 3 show a TEM image and GISAXS pattern of BNSL films after ligand exchange with TBAI, which resulted in capping of the NCs with atomic iodide. The in-plane lattice constants a and b of a TBAI-treated BNSL film were further reduced compared to those for MPA, to 12.8 nm, while the outof-plane lattice constant was ∼20.2 nm. The smaller unit cell gives a shorter interparticle spacing of 0.9 nm. The in-plane coherence length of 350 nm indicates that the in-plane order was not negatively affected by TBAI. In contrast to TBAI and MPA which are commonly used in solar cells to increase carrier lifetime, NH4SCN and formic acid treatments are commonly used in transistors to achieve high carrier mobility.35,42,45 However, both of these treatments are quite aggressive, and we found that they destroyed the crystal structure of the BNSL by fusing neighboring NCs, as shown in Figure S5.59−61 The liquid−air interface ligand exchange was also applied to BNSL films with different NC sizes. For example, panels b, c, e, and f of Figure 3 show structural data for MgZn2-type BNSL films composed of 3.2 and 4.5 nm PbSe NCs. The TEM image clearly shows strong local ordering before ligand exchange. The GISAXS diagram shows similar diffraction pattern as with larger NCs in Figure 2d but with increased smearing into rings due to the polycrystalline SL structure. The simulated lattice constants are a = b = 11.8 nm and c = 23.5 nm. An in-plane coherence 4150


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Figure 4. (a) 2D plot of NIR-TA spectra of OAc-capped 4.5 nm NC SL film. (b) NIR-TA spectra of OAc-capped 4.5 nm NC SL film at several pump−probe delay times, along with the linear absorption spectrum. (c) Dynamics of the 1S bleach of 4.5 nm PbSe NCs under three conditions: dispersed in tetrachloroethylene, SL film before ligand exchange, and same film following ligand exchange. Samples were pumped at 3.2 eV with 12− 18 μJ/cm2. (d) Fluence-dependent dynamics of a SL film of MPA-capped 4.5 nm NCs (points) with global fitting by an Auger recombination model (lines). The fluence was 6.1 μJ/cm2 for N0 = 1.9 × 1018 cm−3.

literature.65,66 Spoor et al. have shown that the band gap bleach arises from roughly equal contributions of electrons and holes for PbSe NCs,67 which is consistent with the PbSe band structure. We note that this is in contrast to CdSe and ZnSe NCs, where TA features are primarily ascribed to electrons because of the highly asymmetric degeneracy of the valence and conduction band states.68−70 The excitonic TA features in these PbSe NCs were red-shifted by ∼25 meV upon ligand exchange to MPA due to increased electronic coupling between NCs, which reduces carrier confinement, as shown in Figure S7 and also indicated in the FT-IR spectra in Figure 1d.71 The dynamics of the 1S bleach are shown in Figure 4c for the 4.5 nm NCs in solution and in SL films before and after ligand exchange. The dynamics of the oleic acid capped NCs are nearly identical in solution and in films, with ∼50% decay within ∼200 ps, followed by slow decay over many nanoseconds. We estimate that our typical fluence results in an average of ∼1 exciton generated per NC, see Table S3. With a distribution following Poisson statistics,72 58% of photoexcited NCs would initially contain multiple excitons. Oleic acidcapped PbSe NCs in solution were previously reported to have excitonic lifetimes of hundreds of nanoseconds under very low fluence.64 Therefore, the fast decay is primarily attributed to Auger recombination,73 although fast trapping may also contribute.59 The initial fast decay is greatly diminished at

pump pulse at 3.18 eV, which is significantly above the band gap of both 3.2 and 4.5 nm PbSe NCs and enables strong absorption needed to generate significant exciton density in the thin films that are only ∼100 nm thick. The pump photon energy was less than 3.7 times the band gap, so multiple exciton generation is not expected to significantly influence our conclusions.62 The fluence was typically 18 μJ/cm2 to provide sufficient signal to observe all features of interest while limiting dynamics arising from multiexciton interactions. By probing the spectral response over time, the dynamics of photoexcited carrier cooling, trapping, recombination, and transfer across the heterojunction could be observed. Figure 4a shows a 2D plot of the TA spectra as a function of both probe energy and pump−probe delay time for a singlecomponent SL of 4.5 nm PbSe NCs capped with oleic acid. A series of 1-D slices of the data at different pump−probe delay times displays the time evolution of the TA spectra more quantitatively in Figure 4b, which also shows the linear Vis/ NIR absorption spectra of the same sample. The primary TA features are the bleaching of the first excitonic peak (1Sh − 1Se) at 0.89 eV and a derivative feature at the second excitonic transition (1Ph−1Pe) at 1.12 eV.63,64 Additional positive photoinduced absorption features are observed within the first 2 ps at positions slightly shifted from the primary excitonic features, which is attributed to hot excitons as described in the 4151


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Figure 5. 2D plot of NIR-TA spectra as a function of both probe energy and pump−probe delay time of MPA-capped NC films for (a) 3.2 nm NC SL, (b) 4.5 nm NC SL, and (c) BNSL film. (d) Cartoon showing the transfer of photoexcited carriers from 3.2 nm PbSe NC (donor) to 4.5 nm PbSe NC (acceptor). (e) 1S bleach dynamics of 3.2 nm NCs in single-component SL and in BNSL probed at 1.18 and 1.14 eV. (f) 1S bleach dynamics of 4.5 nm NCs in single-component SL and BNSL, probed at 0.88 eV.

lower pump fluence (Figure S8), further supporting the assignment of Auger recombination. After ligand exchange with MPA, the decay shown in Figure 4c is faster for two reasons. First, replacement of oleic acid passivating ligands with MPA can introduce new surface states that lead to trapping and recombination.71,74 Second, the strong electronic coupling introduced by MPA ligand exchange enables charge or energy transport to nearby NCs. Carrier transfer to slightly larger NCs with smaller band gaps is energetically favored, resulting in additional Auger recombination.75,76 Fluence-dependent dynamics of the MPA ligandexchanged SL film were measured and globally fit with the rate dn equation dt = kAugn3, as shown in Figure 4d (for 4.5 nm NC SL) and in Figure S9 (for 3.2 nm NC SL). More general models that included defect-mediated recombination/trapping (first order) and radiative recombination (second order) were also tried, but those terms were negligibly small, indicating that Auger recombination alone is sufficient to describe the dynamics. The calculated time constants of Auger recombination (1/(no2k3)) are 45 and 100 ps for 4.5 and 3.2 nm NC SL, respectively, at our typical pump fluence of 18 μJ/cm2 as shown in Table S2. Auger recombination is slower in SLs formed from 3.2 nm NCs because they begin with a lower average number of excitons per NC due to their smaller size and smaller absorption coefficient at 3.18 eV. Additional details regarding these calculations can be found with Table S3. Although Auger recombination is undesirable and can obscure other dynamics, we will show that transfer of carriers from donor to acceptor occurs on much faster time scales. Panels a−c of Figure 5 show 2D colorplots of NIR-TA spectra of 3.2 and 4.5 nm PbSe NC SLs and the corresponding MgZn2-type BNSL films, all ligand-exchanged with MPA and

photoexcited with 3.18 eV photons. The spectra of the 4.5 nm NC SL exhibit the same features described in panels a and b of Figure 4, but only the first 30 ps are shown here. The spectra of the 3.2 nm NC SL show only the 1S transition at 1.18 eV because the 1P transition is above the spectral range of the detector. The BNSL spectra exhibit bleach features of both the 3.2 and 4.5 nm NCs, with partial overlap of the 1P feature of the 4.5 nm NCs and the 1S feature of the 3.2 nm NCs. Both types of NCs were photoexcited by the 3.18 eV photons in the pump pulse, so both initially contain excitons that affect the probe. Charge or energy transfer between strongly coupled NCs of different sizes within a BNSL film provides an additional decay (or excitation) channel for photoexcited carriers. Cyclic voltammetry, Figure S10, indicates that the 3.2 and 4.5 nm PbSe NCs form a type-I heterojunction with conduction band offset of 0.25 eV and valence band offset of 0.05 eV. This band alignment is shown schematically in Figure 5d. Although symmetric shifts might be expected given the similar effective masses of electrons and holes in bulk PbSe,77,78 our measured band alignment is consistent with a previous report that changing the PbSe NC size results in a more significant shift of the conduction band edge than of the valence band edge.79 We note that the type-I heterostructure results in a driving force for electrons, holes, and excitons all to transfer from the 3.2 nm NCs to the 4.5 nm NCs. We temporarily assume the terminology of carrier transfer and will later revisit the distinction between charge and energy transfer. Panels e and f of Figure 5 compare the dynamics of the 1S bleach of each NC size in the ligand-exchanged singlecomponent SLs with the corresponding NC size in the BNSL film. Probe energies are shown as horizontal lines in Figure 5a− c. The 1S bleach of the 3.2 nm (donor) NCs decays faster in 4152


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Figure 6. (a) 2D color plot of NIR-TA spectra as a function of both probe energy and pump−probe delay time of MPA ligand-exchanged BNSL film with background subtraction. (b) 1S bleach kinetics and exponential fitting of 3.2 nm (donor) and 4.5 nm (acceptor) PbSe NCs in MPA ligandexchanged BNSL film, probing at 0.88 and 1.18 eV respectively. (c) 2D color plot of NIR-TA spectra of MPA ligand-exchanged BNSL film, photoexcited at 0.92 eV.

the BNSL film than in the single-component film as carriers are transferred to the 4.5 nm (acceptor) NCs. Concurrently, the 1S bleach of the 4.5 nm NCs decays more slowly in the BNSL compared to the single component film because new carriers are injected from neighboring 3.2 nm NCs. Two different probe energies are shown in Figure 5e because the dynamics depend sensitively on the probe energy due to partial overlap of the 1P feature of the acceptor with the 1S feature of the donor. Nonetheless, the dynamics of the single-component SL and the BNSL are significantly different on picosecond time scales. In contrast, without ligand exchange, the dynamics of the singlecomponent SL and BNSL are similar over the first 30 ps, as shown in Figure S11. The acceptor dynamics of the two samples are nearly indistinguishable, indicating negligible carrier transfer over this time scale due to the large inter-NC spacing with the oleic acid ligand. Figure S11 also compares acceptor dynamics with and without ligand exchange over 300 ps, which does show some evidence of carrier transfer but to much less extent than in the case of the strongly coupled NCs with MPA ligands. While the dynamics of the ligand-exchanged BNSL clearly signify ultrafast charge transfer, they are partially obscured by the simultaneous photoexcitation of excitons in the acceptor NCs. In fact, 64% of the excitons generated with pump photon energy of 3.18 eV originate within the acceptor NCs, as calculated in Table S3.

To specifically focus on the dynamics related to donor− acceptor coupling, we developed a background subtraction procedure to effectively remove the contribution of the carriers directly photoexcited in the acceptor NCs. While the ideal experiment would selectively photoexcite only the donor NCs, that is not possible because the extinction coefficient of the 4.5 nm acceptor NCs is larger than that of the donor NCs at all photon energies. Instead, an appropriate fraction (0.47) of the 4.5 nm pure-component transient spectrum was subtracted from the BNSL transient spectrum to remove the contribution from excitons generated in the acceptor. This specific fraction was based on the ratio of 3.18 eV photons absorbed by 4.5 nm NCs in the two films, but the dynamics were not very sensitive to the chosen fraction over the range of 0.40−0.70. The detailed calculation of absorbed photon density in BNSL is shown in Table S3, with associated data from Figures S12−14. Following this subtraction procedure, panels a and b of Figure 6 show dynamics for only the carriers generated in the donor NCs. The ultrafast carrier transfer process is readily apparent in the instantaneous increase in the donor bleach followed by fast decay on the picosecond time scale. In concert, the acceptor bleach appears more slowly as carriers are injected from the donor NCs, following an exponential rise to a maximum with a time constant of 1.1 ps. This characteristic time for charge transfer is much faster than the Auger 4153


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significant in our BNSLs because the surface-to-surface distances are still relatively large (0.9 nm). While the mechanism of transfer has not been assigned with full confidence, it is clear that we have successfully produced strongly coupled BNSL films using the liquid−air interface method of ligand exchange.

recombination times observed in single component SLs, confirming efficient coupling between donor and acceptor. The decay of the donor bleach was fit with a biexponential model with time constants of 3.6 and 76 ps. The fast time constant is consistent with the carrier transfer time measured in the acceptor, while the slow time constant is consistent with the Auger recombination time, among other possibilities. The carrier transfer time of 1.1 ps is similar to characteristic times previously reported for electron transfer from PbSe NCs to TiO2 nanoparticles at room temperature80,81 and to methylene blue,67 as well as from CdSe NCs into TiO2 nanoparticles.82,83 The carrier transfer rate is also similar to the tunneling rate of (2 ps)−1 between specific neighboring NCs of the same type, as calculated by Gao et al.75 While cyclic voltammetry indicates a driving force of ∼0.25 eV for electrons and ∼0.05 eV for holes that might be expected to accelerate donor−acceptor carrier transfer compared to carrier transfer between homogeneous NCs, the presence of excitons or charges within the acceptor may slow additional carrier transfer. The donor excitonic bleach decays to less than 10% of its maximum value within 200 ps, which is an order of magnitude faster than in the pure component SL, Figure S15. Meanwhile, the acceptor excitonic bleach remains for nanoseconds. Together these dynamics indicate that most carriers generated in the donor NCs are transferred to the acceptor NCs and remain there until being trapped or recombining. This efficient capture of holes is somewhat surprising, given that the measured valence band offset is only ∼2kBT. To test this conclusion, we photoexcited the BNSL film with photon energy of 0.92 eV, which generates excitons in only the acceptor NCs. Figure 6c shows no measurable bleaching of the donor NCs in the BNSL, indicating that the holes are indeed confined to the acceptor, or at least that the hole concentration in the donors is below the limit of our instrument’s sensitivity. Coulomb attraction of holes to electrons that have deeper confining potential likely enhances the hole localization in the acceptor NCs. Directional transfer of carriers from donor to acceptor occurred on picosecond time scales, but the precise mechanism of transfer remains unclear. The type-I band alignment leads to driving forces for electrons, holes, and excitons all to transfer from 3.2 to 4.5 nm NCs. Because the exciton binding energy in PbSe NCs is negligibly small,84 carriers may transfer between NCs individually or as excitons. We could not further differentiate rates of electron, hole, and energy transfer because the excitonic bleach in PbSe is comprised of similar contributions from electrons and holes.85 While Spoor et al.67 reported visible features that could be assigned to electron and hole transitions between higher energy states, we did not observe such features in our films. We have described the donor−acceptor interactions in the context of carrier transfer because picosecond charge transfer has been reported for numerous NC systems80−83 However, we cannot completely exclude the possibility of energy transfer.86−91 Characteristic inter-NC energy transfer times as fast as 89 ps in MPA-capped PbS and 18 ps in octadecylphosphonic acid-capped CdSe NCs followed from the Förster mechanism in NC films,76,92 although dipole−dipole interactions are not likely to result in the donor−acceptor transfer in our films, which occurs another order of magnitude faster. In contrast, exciton transfer from pentacene-containing capping ligands to PbSe NCs occurred on subpicosecond time scales following the Dexter mechanism of electron exchange.93 The Dexter mechanism is unlikely to be

4. CONCLUSIONS Liquid−air interface ligand exchange was shown to be an efficient method to replace the original synthesis ligands with shorter capping groups in a series of BNSL films, enabling strong coupling between neighboring NCs while preserving the long-range order of the structure. Ligand-exchanged BNSLs provided well-controlled energy landscapes to study directional interparticle carrier transfer within BNSL films using ultrafast optical spectroscopy. MPA-capped PbSe NCs of 3.2 and 4.5 nm diameters in the MgZn2 structure created a bicontinuous heterostructure with type-I band alignment. Carriers were transferred from small donor NCs to large acceptor NCs with a characteristic time of 1.1 ps, while no carrier transfer was observed when only the large NCs in the assemblies are photoexcited. BNSL films present an exciting opportunity to tailor the energy landscape, and ligand-exchanged BNSL films offer new opportunities for functional NC optoelectronic devices. With control over both the physical structure and the local electronic structure through engineering NC size and composition, extensions of this work can lead to deeper understanding of the theory and mechanism of charge and energy transfer in NC composites. Such knowledge can accelerate the development of the promising optical and electrical properties of BNSL films for applications such as solar cells, sensors, and light emitting diodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12327. UV−vis spectra, TEM images, GISAXS patterns, NIRTA spectra, AFM images, and cyclic voltammetry curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.B.B.) E-mail: [email protected]. *(C.B.M.) E-mail: [email protected]. ORCID

Yaoting Wu: 0000-0002-4363-9870 Jason B. Baxter: 0000-0001-8702-3915 Author Contributions †

Y.W. and S.L. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation through collaborative awards CBET-1333649 and CBET-1335821 and NSF PIRE grant #1545884. Cyclic voltammetry measurements were supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of 4154


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Science, Basic Energy Sciences. The GISAXS measurement used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors thank Joseph Strzalka and Zhang Jiang for their help with experimental setup and data analysis. The near-infrared transient absorption measurement used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility at Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors thank Matthew Sfeir for help with experimental setup and data analysis.

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