Molecular Ligands Control Superlattice Structure and Crystallite

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Molecular Ligands Control Superlattice Structure and Crystallite Orientation in Colloidal Quantum Dot Solids Pralay K. Santra, Axel F. Palmstrom, Christopher J. Tassone, and Stacey F. Bent Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03076 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

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Chemistry of Materials

Molecular Ligands Control Superlattice Structure and Crystallite Orientation in Colloidal Quantum Dot Solids

Pralay K Santra,1,†,* Axel F. Palmstrom,1 Christopher J. Tassone,2 Stacey F. Bent 1*

1

Department of Chemical Engineering, Stanford University, Stanford, California 94305,

United States 2

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory,

Menlo Park, CA 94025, United States

Email: [email protected]; [email protected] †

Current address: Department of Physics and Astronomy, Molecular and Condensed

Matter Physics, Uppsala University, Box 516, 751 20 Uppsala, Sweden

1

ACS Paragon Plus Environment


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Abstract Colloidal quantum dot solids represent a new materials platform that has garnered interest for a variety of electronic, optoelectronic and photovoltaic applications. In such solids, individual quantum dots must be coupled with each other to facilitate charge transport through the solid. Past improvements on charge transport of colloidal quantum dot solids have been achieved primarily through the control of the interparticle spacing. However, the role of morphological ordering of the crystalline facets of individual quantum dots on the charge transport of the quantum dot solid is unknown. Here, we show for the first time that small passivating ligand molecules around the quantum dots can control the arrangement of different facets of quantum dots within the quantum dot solid. The insights from this study provide important directions for future enhancement in orientation of quantum dots in colloidal quantum dot solids.

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Introduction Colloidal quantum dots (QDs) are a propitious group of materials for optoelectronic applications including photovoltaics,1-4 light emitting diodes,5 and photodetectors.6 The promise of quantum dot materials in this field arises from the solution processability of these materials with fine control of size,7 shape and composition,8,

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allowing precise tuning of their optoelectronic

properties. Lead chalcogenides (PbX, X=S, Se, and Te) are one class of quantum dots that have gained interest in the photovoltaics community as an emerging candidate for large area, low cost and high efficiency photovoltaic devices. In less than a decade, the photovoltaic efficiencies of these devices have progressed nearly ten-fold from sub 1 % to 10.6 % through band engineering of colloidal QDs.3, 10, 11

In order to employ the quantum dot solids in electronic devices, individual quantum dots must be electronically coupled with each other to facilitate charge transport in the solid. Charge transport in quantum dot solids with low carrier mobilities, arises predominantly via a hopping through localized quantum confined states.12,

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As a result, the charge carrier mobility is greatly

influenced by interparticle spacing,14, 15 size and size distribution,16 surface chemistry,17, 18 and morphological order of the nanocrystals.19 Interparticle spacing has been the most explored of the above parameters.14, 16 As synthesized quantum dots are passivated with long alkyl chain organic molecules after solution-based chemical wet synthesis. The long chain alkyl molecules prevent the coalescence of quantum dots during synthesis; however, they increase the interparticle distance within the quantum dot solid, thus creating a tunnel barrier.

Early studies showed that the charge transport of quantum dot solids could be improved by exchanging the long alkyl molecules with short and conducting passivating ligands, e.g.,

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hydrazine,20 1,2 ethanedithiol,21,

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or 3-mercaptopropionic acid (MPA).23 The best colloidal

quantum dot solar cells employ small molecule organic ligands to achieve effective mobilities of ~ 10-3 – 10-2 cm2V-1s-1.2,

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The use of inorganic passivating ligands, e.g., Sn2S62-,24,

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or

ammonium thiocyanate ligands,26 can yield impressive mobilities (~10 - 30 cm2V-1s-1) of the quantum dot solid while retaining the quantum confinement of the individual particles. The high carrier mobility suggests band-like charge transport in such coupled quantum dot solids.17

While the body of literature clearly shows that ligand exchange can increase carrier mobility,14, 16, 27, 28

there has been only a small number of studies on charge transport that explore the

details of the morphological order within the quantum dot solid, e.g. the relative orientation between particle facets, as well as the shape of the nanoparticles themselves. A few theoretical studies predict that charge transport occur through minibands that formed in ‘ideally ordered’ QD solids.

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These minibands broaden with the decrease in interparticle separation and

affects the charge transport. The arrangement of quantum dots in a 2D quantum dot solid affects the orbital coupling strength on a local scale.31 In addition, recent theoretical calculations have shown the electron and hole coupling strength varies with different facets of quantum dots and that the arrangement of those facets within the quantum dot solid plays an important role in charge transport by spatial orientation of the wave function.32 It is experimentally shown that oriented attachment of QDs with in the quantum dot solid improves interdot coupling while maintaining the quantum confinement.33-37 Recently, Hanrath et al. reported delocalization of electrons is possible in such ordered and epitaxially fused quantum dot solids thus improving the charge transport in such solids.38 These recent theoretical and experimental observations suggest that both the crystalline arrangement and interdot coupling of quantum dots within the quantum dot solid play an important role in the overall charge transport.

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In this work, we report the structural ordering and arrangement of different facets of quantum dots within a thin film of quantum dot solid using synchrotron-based grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXRD). We find that as-synthesized oleic acid passivated PbS QDs form a body centered cubic (BCC) superlattice arrangement along with particular orientations of the facets of the individual QDs within the quantum dot solid. We further show that it is possible to alter the facet arrangements among the nearest neighbor quantum dots through the treatment of MPA ligand, with the superlattice arrangement moving from BCC to body centered tetragonal (BCT) and finally entering a disordered isotropic phase where the relative orientation of the facets become random. These findings provide insight into choosing a ligand molecule and its concentration to best optimize the charge transport in quantum dot solids and can be used eventually to examine the validity of theoretical calculations regarding the differential carrier mobilities between electron and hole for different quantum dot facets.32

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Results and Discussion Superlattice arrangement of oleic acid (OA) passivated quantum dots. A typical transmission electron microscopy (TEM) image of as-synthesized PbS QD passivated with OA is shown in Figure 1a. The average particle diameter ( dQD ) is found to be 3.21 ± 0.23 nm (a size distribution of 7 %). Another TEM image of PbS QDs passivated with OA synthesized from a different batch, having an average size of 3.2 ± 0.5 nm, is shown in Figure SI-1a, indicating the reproducibility of the synthesis process. Close inspections of the TEM images suggest that the PbS QDs are in ordered arrangements in some places as highlighted by the colored squares within Figure 1a, and clearly evident in Figure SI-1a. We have shown the processed Fast Fourier Transform (FFT) image of the area enclosed within the red square in the inset of Figure 1a. The FFT pattern shows primarily a diffuse ring with a few high intensity spots with an angular separation of ~ 60° between spots on the same diffuse ring. This diffuse ring corresponds to the particle size and the spot pattern represents the hexagonal arrangement among the particles. The average particle size is calculated from the FFT pattern by integrating the 2D FFT image over 360° into a 1D line profile and is found to be 3.18 nm, which is in close agreement with the average of the multiple individual measurements of QD size from the TEM images.

The TEM images shown here are from monolayer QDs dispersed on a carbon film, hence 3D structural information of the multilayer PbS QDs films cannot be extracted from these TEM images. We probed the structural properties of the multilayer PbS QDs thin films using grazing incidence small angle x-ray scattering (GISAXS). A typical GISAXS pattern from spin coated OA passivated PbS QD on Si wafer (after methanol wash) is shown in Figure 1b. Well-defined scattering peaks at particular angles indicate that the QDs assembled in a particular

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superstructure in the multilayer film. To determine the superstructure, we converted the GISAXS pattern to a 2D sector graph (as shown in Figure SI-2a in supporting information), where the scattering intensity is plotted against different scattering vectors ( q ) for each azimuthal angle ( φ ). Peaks originating from the same set of planes of the superstructure appear at different φ for the same q as marked by the vertical dashed lines. In order to calculate the interplanar distances of the superstructure or superlattice, we integrated the sector graph over the azimuthal angle with the results presented in Figure SI-2b in supporting information. By analyzing the peak positions and angle between the peaks, we can identify the superlattice to be a body centered cubic (BCC, Space group Im-3m; Number 229) system with a lattice parameter ( aSL ) of 6.31 ± 0.09 nm. The detailed analysis and results are described in Table SI1 in the supporting information. All the scattering peaks can be labeled as marked in Figure 1b. The scattering peak corresponding to 011 and 022 planes appears at

q( x,y) = 0 Å-1 in

Figure 1b and at φ = 90° in Figure SI-2a indicating that the direction of the 011 planes of the superlattice is perpendicular to the substrate. In a BCC system, the arrangement on the {011} plane is a distorted hexagon. A close inspection of the ordered particles in the TEM image shows a slight distortion from a perfect hexagonal pattern as highlighted in the Figure SI-1a, confirming a BCC {011} surface. A few earlier reports39-41 have shown that OA passivated PbS QDs can arrange either in a face centered cubic (FCC) or BCC superlattice depending on the size and surface properties of the QDs.

The interparticle distance ( LIP ) within the BCC superlattice is related to its lattice parameter as

LIP =

3aSL and is found from GISAXS data analysis to be 5.46 ± 0.08 nm, which is in close 2

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agreement with the TEM results (5.48 ± 0.45 nm) obtained from Figure SI-1b. The average ligand length ( llig ) can be estimated from the interparticle distance ( LIP ) and particle diameter (

dQD ) as 2llig = LIP − dQD and found to be 1.13 ± 0.24 nm, which is smaller than the length of OA. This is most likely due to the intercalation of the OA molecules between two neighboring PbS QDs, as reported earlier.39

Figure 1. (a) TEM image of OA passivated PbS QDs. Dashed squares highlight ordered arrangement of QDs. Inset shows the processed FFT pattern of the red squared area of the TEM image. (b) GISAXS pattern collected at an incident angle of 0.2° for an OA passivated PbS QD multilayer thin-film. The low intensity vertical strip is due to the beam blocker used during the experiments. All observed diffraction spots can be indexed to the Im-3m space group, with an orientation of the plane perpendicular to the substrate. (c) GIXRD pattern of OA passivated PbS QD multilayer. These scattering peaks, originating from the atomic planes of the QDs, are labeled. (d) Schematic model of a truncated octahedron PbS QD showing the orientation of different atomic planes ( – blue, – orange and – magenta) and the angle between these planes. The model shows the

planes parallel to the substrate.

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We also explored the orientation of the QDs within the superlattice using grazing incidence x-ray diffraction (GIXRD). The 2D diffraction pattern is shown in Figure 1c. The anisotropic nature of the intensity distribution as a function of the azimuthal angle ( φ ) indicates that the atomic lattice planes of the QDs are oriented with respect to the substrate. The integrated diffraction pattern over all azimuthal angles is shown in Figure SI-3 in the supporting information. The diffraction peaks can be indexed to the rock salt phase of PbS (Space group Fm-3m, Number 225), and are identified as the 111 appearance of the 022

QD

QD

, 200

peak at

QD

, 022

QD

, 311

QD

and 222

QD

diffraction peaks. The

q( x,y) = 0 Å-1 ( φ = 90°) in Figure 1c indicates that the

individual crystalline QDs are oriented with their 022

QD

atomic planes perpendicular to the

substrate, in agreement with earlier observations of PbS when ordered in a BCC superlattice.40 A schematic model of the truncated octahedron QD is shown in Figure 1d. For any cubic system, the {011}QD and

{022}QD planes are parallel to each other. The {011}QD plane is

drawn along the center of the QD in blue and shown parallel to the substrate. The appearance of 111

QD

and 200

QD

at an azimuthal angle φ = 54° and 45°, respectively, is consistent with

their expected angle with respect to the substrate as shown in Figure 1d.

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Effect of small molecule ligands on the superlattice. As described above, as-synthesized PbS QDs are passivated with OA, an 18-carbon chain long insulating molecule. Thin films of such multilayer QDs are highly resistive and cannot be used in colloidal QD solar cells or other electronic devices. To improve the charge transport in such QD solids, it is essential to replace the long, insulating molecules by small, conducting molecules. One commonly used small passivating ligand for lead chalcogenide based colloidal QD solar cells is 3-mercaptopropionic acid (MPA), a bidentate molecule.42 The ligand replacement around the QDs is performed during the layer-by-layer spin coating process by treating the thin multilayer film of PbS QDs passivated with OA with a certain concentration of MPA in methanol, as shown schematically in Figure SI-4 in supporting information. Different values for the optimal concentration of MPA needed to make efficient colloidal QD solar cells have been reported in the literature.1, 43 In this work, we investigated the effect of MPA concentration on the arrangement of the superlattice and orientation of individual PbS-QD because MPA concentration plays an important role in the overall photovoltaic performance of colloidal QD solar cells. We report the nominal concentration of MPA in methanol as the volume ratio and vary it from 1:10,000 to 1:5. To further characterize the surface of PbS QDs, FTIR spectroscopy was carried out on films of PbS QDs passivated with oleic acid and different concentrations of MPA as shown in Figure SI-5 in supporting information. Distinct aliphatic ν(C−H) stretching frequencies between 2800 and 3000 cm−1 were observed for oleic acid-passivated PbS QDs. The decrease in intensity of these aliphatic stretching frequencies, qualitatively, indicate replacement of native passivating ligand, oleic acid upon the treatment of MPA.

A few selected GISAXS patterns of the PbS-QD multilayer thin films treated with different concentrations of MPA are shown in Figure 2a-c. The complete set of GISAXS patterns for other concentrations of MPA is shown in Figure SI-6 in the supporting information. With the treatment 10


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Figure 2. GISAXS patterns of PbS QDs multilayer thin-film treated with (a) 1:10,000, (b) 1:5000 and (c) 1:100 MPA concentration. Only half of the symmetric GISAXS pattern is shown in these figures. The blue diamond symbols represent the position of the respective scattering peaks for OA passivated PbS QD films and the black dashed box represent the new broad peak appeared along axis. (d) Variation of azimuthal angle-integrated scattering intensity with scattering vector for PbS QDs multilayers treated with different concentrations of MPA.

of even very low concentration MPA (1:10,000), the GISAXS pattern (in Figure 2a) changed from the original pattern of the BCC superlattice of OA-passivated PbS QDs as shown earlier in Figure 1b. The first set of scattering peaks, corresponding to 011

SL

planes of the modified

superlattice, are at the same positions in the GISAXS image of the untreated OA-passivated PbS-QD thin film. However, the higher order scattering peaks are not visible. Instead, a new and broad low intensity scattering peak along qz (highlighted by the black dashed box) appears below the 020 peak (highlighted with a blue diamond symbol). With a further increase in MPA

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concentration to 1:5000, the 011 scattering intensity around 020

SL

SL

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scattering peaks has decreased in intensity and the

has become stronger and appears elongated primarily along

the qz axis (with some contribution of

q( x,y) ) indicating a broad distribution in particle ordering

length along the perpendicular direction of the substrate. With further increase in the concentration of MPA, no strong scattering peaks can be observed; rather, a low intensity, broad scattering peak uniformly spread out over all the azimuth angle range is observed, as shown in Figure 2c for an MPA concentration of 1:100. The isotropic intensity variation over all azimuthal angles indicates a disordered arrangement of QDs within the thin film of the multilayer QDs. The azimuthal angle-integrated 1D GISAXS patterns for untreated and MPA treated PbSQD multilayers are shown in Figure 2d. It is evident from this figure that the MPA treatment at very low concentrations (1:10,000 and 1:5000) strongly affects the 011

SL

peak and a broad

peak originates at higher q as a shoulder to the main peak. With further increase in concentration of MPA, the broad peak shifts slightly towards higher q . These GISAXS results indicate that with an increase in the concentration of MPA, a structural phase transition occurs within the superlattice of the multilayer QDs. Starting from a BCC system for OA passivated PbS-QDs, the superlattice transforms through a different structural phase and eventually reaches a disordered and isotropic phase, with an average interparticle separation of 3.17 ± 0.39 nm as determined from the broad scattering peak position.

A quantitative determination of the structural phases of the PbS-QDs thin films treated with MPA at low concentrations cannot be extracted from the GISAXS data presented in Figure 2, due to the fact that the horizon of the substrate obscures the diffraction spots with only in-plane momentum transfer (i.e. I = 0), making unambiguous structure solution impossible. To identify

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precisely the unknown superlattice phase, we measure diffraction in the horizontal-plane by collecting GISAXS data at a low incident angle of 0.1° and extended the analysis for all the samples. A typical GISAXS pattern collected at such low incident angle for the untreated PbSQDs thin film is shown in Figure 3a. The low angle GISAXS patterns for other samples are shown in Figure SI-7 in supporting information. In an attempt to determine the ordering in the horizontal plane, we have measured the purely in-plane momentum transfer by taking a line cut at the Yoneda peak as shown by the horizontal dashed line in Figure 3b. The line profile analysis of the scattered intensity at the Yoneda angle for different samples are shown in Figure 3c. Based on the variations in their position and shape of the scattering peaks, the samples can be categorized in three distinct structural regimes; (a) ordered and mixed superlattice, (b) distorted superlattice and (c) disordered system.

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Figure 3. (a) GISAXS pattern of an OA-passivated PbS QDs multilayer collected at an incident angle of 0.1°. (b) The GISAXS pattern zoomed near the Yoneda angle as highlighted by the dashed line. (c) Variation of the scattering intensity along the Yoneda angle for PbS QD multilayers treated with different concentrations of MPA.

(a) Ordered and mixed superlattice system: The first regime contains the untreated and very low concentration of MPA (1:10000 and 1:5000) treated PbS QD thin films, and this regime exhibit two distinct scattering peaks that correspond to 011

SL

and 200

SL

. For the untreated PbS-

QD thin film, the scattering peaks appear at 0.1386 Å-1 and 0.1975 Å-1. As the first peak corresponds to the 011

SL

set of planes, it is possible independently to calculate the lattice 011

parameter “ b ” and “ c ” of the superlattice ( b = c = 2d011 = 2 2π / q( x,y) ) from this peak position.

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The lattice parameter “ a ” can be similarly determined from the

200

SL

peak (

a = 2d200 = 4π / q(200 x,y) ). For the untreated PbS-QDs thin-film, the lattice parameters are 6.41 nm and 6.36 nm calculated from 011

SL

and 200

SL

, respectively, which match well with the

lattice parameter obtained from the complete GISAXS pattern as discussed earlier in Figure 1b.

With the treatment of a very low concentration of MPA, the overall intensity of the 011 decreases significantly. A new peak appears at higher

SL

peak

q( x,y) = 0.1461 Å-1 (lower d) as a shoulder

of the original peak as highlighted by the dashed lines. The 200

SL

peak shifts to lower

0.1950 Å-1 (higher d) value and this does not show any shoulder peak, unlike the 011

SL

q( x,y) = peak.

The non-uniform change in different lattice parameters ( a = 6.44 nm, b = c = 6.08 nm) suggests that with treatment of MPA, the superlattice of the QDs has transformed to a body centered tetragonal (BCT, Space group I4/mmm, Number 139) system from the body centered cubic (BCC). In turn, the observation of a non-uniform change in the lattice parameters is a probable indication of preferential replacement of OA by the significantly smaller MPA ligand from specific crystal facets of PbS QDs, as suggested earlier from DFT calculations.40 We will discuss the change in superlattice with the help of a schematic diagram of the QD orientation later in the manuscript.

(b) Distorted superlattice system: By increasing the MPA concentration to 1:1000 or higher, the scattering pattern of the PbS-QD thin film changes significantly. In this second regime, the original 011

011

SL

SL

has moved towards higher

q ( x,y) = 0.1591 Å-1. The absence of the original sharp

peak suggests that MPA with 1:1000 concentration has reacted with QDs throughout

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the entire thickness of the thin-film. The 011

SL

peak continues to shift towards higher

with further increase in MPA concentration. The 200 to

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SL

q ( x,y)

scattering peak only decreases slightly

q ( x,y) = 0.1926 Å-1 with the higher concentration of MPA. These scattering peaks are related

to the interplanar distance along that particular plane of the superlattice and the increase in

q ( x,y) of the peak position ascertains the decrease in that particular interplanar distance.

(c) Disordered system: Eventually with treatment of higher MPA concentration (1:50 and greater), the scattering along the horizontal plane leads to one broad peak and no individual scattering peaks can be identified. This isotropic behavior suggests all the particles are randomly oriented with an average interparticle spacing of 3.24 ± 0.48 nm.

Figure 4. GIXRD pattern of PbS QD multilayers treated with (a) 1:10,000 and (b) 1:5000 MPA. (c) Schematic model of PbS QD with different atomic planes showing

parallel to the substrate.

Orientation of individual QDs in the superlattice after exchange with small molecule ligand. To explore the possible reasons for the structural changes in the thin film, we also monitored the orientation of the individual QDs in the thin films treated with different MPA concentration using GIXRD. Two representative GIXRD patterns for multilayer QD thin films treated with low concentrations of MPA are presented in Figure 4a and 4b for MPA 16


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concentrations of 1:10,000 and 1:5000, respectively. The GIXRD patterns for other samples are shown in Figure SI-8 in supporting information. Similar to the untreated thin film, the diffracted peaks show anisotropy with respect to the intensity distribution as a function of the azimuthal angle; however, this angular dependence has changed from the untreated films, suggesting a new orientation distribution. The azimuthal angle-integrated XRD patterns for all the different thin films (Figure SI-3 in supporting information) suggest that the treatments of MPA of different concentrations do not alter either the crystallinity or the size of the QDs, but rather systematically alter the orientation of the QDs within the superlattice. The 2D GIXRD pattern for a PbS-QD multilayer thin film treated with the lowest concentration of MPA (1:10000) shows that the diffraction peaks corresponding to 111

QD

and 022

QD

planes appear primarily at φ = 90°

and spread over a broad azimuth angle, compared to untreated OA-passivated PbS QD thin films. With increases in MPA concentration to 1:5000, the 111

QD

diffraction peak appears less

broad along the azimuth angle at φ = 90°. The orientation of the 200 quantitatively identical; however, the 022

QD

QD

peak remains

peak broadens over the azimuth angle. Overall,

the azimuthal peak broadens for all three diffraction peaks with increases in MPA concentration. These interesting results indicate that the MPA treatment oriented the individual QDs to have their 111

QD

planes perpendicular to the substrate as shown in the schematic in Figure 4c. We

quantitatively analyzed the orientation of the three different atomic planes 111 and 022

QD

of the PbS QDs by plotting the intensity of the 111

QD

, 200

QD

QD

, 200

, and 022

QD

,

QD

diffraction peaks as a function of the azimuthal angle for different exposure to MPA as shown in Figure 5.

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Figure 5. Variation in scattering peaks (a)

, (b)

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and (c)

with azimuthal

angle for PbS QDs multilayer thin films treated with different concentrations of MPA. The peaks were -1

-1

-1

-1

-1

-1

integrated from 1.725 Å – 1.975 Å , 1.975 Å – 2.250 Å and 2.860 Å – 3.140 Å for and

,

, respectively.

The variations in the azimuthal angle for the 111

QD

peak for thin films treated with different

MPA concentrations are shown in Figure 5a. For untreated PbS QD thin-films, the 111

QD

planes are preferentially oriented at an azimuthal angle of 55.9° (width ±14.3°) and 123.9° (width ±14.7°). Upon treatment of the lowest concentration of MPA (1:10000), the 111

QD

diffraction peaks appear at the original azimuthal angles as well as at a new azimuthal angle φ = 90°. At such low concentration of MPA, QDs are only partially affected within the thin film, further supporting the findings of mixed phases from the low incident angle GISAXS results, presented earlier in Figure 3. With further increase in the concentration of MPA, the intensities of the peaks originally at φ = 56° and 124° decrease significantly, with the primary peak appearing at φ = 90° and the average orientation of the 111

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QD

plane remaining unchanged.

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The 200

QD

peak appears at azimuthal angles of 46.3° and 133.4° for untreated PbS QD; upon

treatment by MPA, this angular dependence changes slightly, with the peaks appearing at azimuthal angles of 40.4° and 140.1°. Furthermore, the intensity dependence on azimuthal angle becomes weaker with increases in the concentration of MPA. Untreated PbS QD thin films exhibit strong orientation of the 022

QD

plane perpendicular to the substrate as evident from

the peak positions at 32.2°, 89.9° and 147.8° in Figure 5c. With even the lowest concentration of MPA treatment (1:10,000), new peaks appear at φ = 52.2° and 125.6° and remain unchanged with further increase in MPA concentration.

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Page 20 of 31

The results of the GISAXS experiments (Figure 1b) indicate that the QDs within the untreated PbS-QD thin film have arranged themselves to form a BCC superlattice as shown schematically in Figure 6a, consistent with literature reports.39-41 Earlier reports have also suggested that the PbS QDs synthesized using this chemical wet synthesis method are highly faceted.40 In the schematic diagram of Figure 6a, the

{100}QD and {111}QD crystal facets are highlighted in orange and green colors, respectively. The unit cell of the BCC superlattice is shown with black lines. As found from our GISAXS results, the

{011}SL plane is drawn by the blue translucent plane in the schematic. The GIXRD studies also suggested that the 011

QD

atomic planes are

arranged perpendicular to the substrate. The plan view of {011}SL of the BCC superlattice is shown in Figure 6b. This is a distorted hexagonal arrangement as BCC lattice system is not a Figure 6. Schematic 3D representations of PbS QD thin film relative to a substrate (blue slab) with (a) body centered cubic (BCC), (b) plan view orientation along

direction of

the BCC superlattice, (c) body centered tetragonal (BCT), (d) plan view orientation along

direction of the BCT superlattice

and (e) disordered arrangements. The and

facets are shown in

closed packed structure.

The {011}SL and

{200}SL facets can be seen and the respective SL

SL

interplanar distances of d011 and d200 are marked in the figure. Following MPA treatment, the GISAXS and the

orange and green color respectively.

GIXRD data (Figures 2-5) show that both the superlattice and the orientation of individual QDs within the QD multilayer change from that of the OA-terminated QDs. At very low concentrations

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of MPA, the QDs on the top of the thin film react with MPA and the ordering of this partially affected layer changes from BCC to a BCT structure, as shown schematically in Figure 6c. Along with the change in the superlattice pattern, the individual QDs also orient themselves to have the 111

QD

facets perpendicular to the substrate. This is illustrated in the plan view SL

schematic of {011}SL in Figure 6d. The interplanar distance d011 decreases significantly SL

whereas the interplanar distance d200 remains almost unchanged. With even higher MPA concentrations, the interparticle distance between QDs decreases and a random arrangement and orientation ensue, as shown in Figure 6e.

The variations in the structural ordering of the PbS QD thin film can be explained due to different interaction strengths between the QDs and the ligands. In general, the complex QD-ligand and ligand-ligand interactions play the most important role in assembling the thermodynamically stable, lowest-energy superlattice in QD solids.39 It is known that the QD-ligand interaction varies with the facets of the QD. Based on DFT calculations, Hanrath et al. have earlier provided important insights on binding and desorption of oleic acid from different facets of PbS QDs.40 Their calculations showed that oleate ligands bind more weakly to {100}QD facets compared to

{111}QD

facets of the PbS QD. This suggests that MPA will replace the OA from {100}QD prior

to removing them from {111}QD , as shown in the case of OA replacement with ammonium halide ligands.44 In the case of an OA-passivated PbS QD solid, the nearest-neighbor facets are

{111}QD ,

and the interparticle distance is determined by OA. Upon treatment with low

concentrations of MPA, we propose that MPA replaces the long OA ligands preferentially from the {100}QD facets and also orients the individual QDs within the superlattice. With this new

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Page 22 of 31

orientation, {111}QD facets are no longer the nearest-neighbor facets, but rather they are diagonal and parallel to the substrate as shown in Figure 6c, which would accommodate long OA chains in the BCT superlattice. In the new configuration, the {100}QD facets face the corners, thus decreasing the lattice parameters “ b ” and “ c ”. With further increase in MPA concentration, both

{100}QD

and

{111}QD

facets become passivated with MPA and this

decreases the interparticle distances. Probably due to the bidentate nature of MPA, the preferred orientation of the facets among the nearest neighbor quantum dot is largely lost.

Recent calculations32 by Kaushik et al.

have conclusively shown that the orientational

attachment of QDs significantly affects electron and hole coupling and charge transfer, due to the preferential overlap between HOMO and LUMO along a particular direction. They have shown strong electron coupling along {111}QD facets and hole coupling along {100}QD facets for octahedrally shaped QDs. Building upon those theoretical calculations, here we show that one can control crystallite orientation within the quantum dot solid by the passivating ligands around the QDs. It is ideal for QD based electronic devices to replace the long alkyl passivating molecules from the surface with small conducting molecules while maintaining the orientation of different facets. Combining different approaches like doping to make QDs n -type18, 45 or p type18 together with structural control exerted by the passivating ligands demonstrated in this work, one can improve the conductivity of the minority carriers in the colloidal quantum dot solids.

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Conclusions In this work, we show that variations in ligand interactions with different facets of QD crystallites can significantly influence the arrangement and the orientation of the QDs within quantum dot solids. PbS QDs passivated with long chain oleic acid assemble into a BCC superlattice with

011

SL

planes perpendicular to the substrate, along with a specific orientation of the atomic

planes of the QD. The superlattice undergoes a phase transition from BCC to BCT and finally to an isotropic disordered state upon replacement of OA ligands by MPA ligands. We suggest that different interaction strengths between {100}QD and {111}QD facets and oleic acid lead to preferential replacement of the native ligands from {100}QD facets of the QD upon treatment by MPA. A central finding in our investigation is that due to these different interaction strengths, MPA orients the QDs within the superlattice to have {111}QD facets parallel to each other, a configuration found in the BCT structure. However, application of higher concentrations of MPA decreases the ordered-arrangement of the QDs within the quantum dot solid and leads to disorder. The insights gained from this study provide guidance for the role of small molecule ligand concentration on the arrangement of QDs in quantum dot solids, which in turn is an important parameter for charge transport in QD thin film devices. We believe that the present findings offer a new way of thinking about colloidal quantum dot solids that can lead to novel approaches to improve charge transport in QD solid based electronic devices.

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Materials and Methods Synthesis and Characterization. PbS QDs were synthesized according to methods outlined in earlier reports.46, 47 In a typical synthesis, the cationic precursor was prepared by dissolving and degassing 2 mmol of PbO in 10 ml of ODE and 1.6 ml of OA under vacuum at 100 ℃. Once the reaction mixture turned clear and transparent, the reaction temperature was increased to 110 °C. The anionic precursor was prepared by dissolving 1 mmol of bis-(trimethylsilyl)sulfide (TMS) and 42 μL of DPP in 2 ml of ODE inside a N2 filled glove box. The TMS mixture was transferred from the glove box using a luer lock syringe and injected swiftly to the reaction mixture. The reaction was continued for 30 s and was stopped by removing the heating mantle. After cooling down to room temperature, the QDs were precipitated from the reaction mixture using ethanol and washed three times by sequential precipitating and dissolving using methanol and hexane, respectively. Finally, the washed QDs passivated with OA were dispersed in hexane for further experiments. The TEM samples were prepared by drop casting the PbS QDs solution on carbon coated Cu grid and the TEM images were collected using FEI Tecnai G2 F20 X-TWIN transmission electron microscope operating at 200 kV. The background subtraction and Fast Fourier Transforms (FFT) of the TEM images were performed using TEM Imaging and Analysis (TIA) and ImageJ software.

Sample preparation and GISAXS and GIXRD measurements. Thin films of PbS QD were deposited on 1 cm × 1.5 cm substrates (Si or FTO) by spin coating a 19 mg/mL QD solution in hexane under ambient conditions. Prior to spin coating, the substrates were UV-ozone cleaned for 5 minutes. OA-passivated PbS QDs were deposited onto the substrate by spin coating at 2500 rpm for 25 s. The film was washed with methanol once. In this washing procedure, we dropped ~ 100 μl of methanol onto the QD film and started spinning the QD film within 1- 2 s. To

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exchange the passivating ligand molecules around the QDs film in a similar way as washing the film with methanol, different concentrations of MPA in methanol were dropped onto the QD film followed by a spin at 2500 rpm for 25 s. The films were washed with methanol to remove any excess MPA.

GISAXS and GIXRD data of PbS QD thin films were collected at the Stanford Synchrotron Radiation Lightsource (SSRL) at beamlines 1-5 and 11-3, respectively. For GISAXS, the X-ray energy was 13 keV, and the sample to detector distance was set to 1 m. Scattered X-rays were collected using a 2D Rayonix 165 CCD camera. The GISAXS data was averaged over a total of six frames collected for each sample at different incident angle. GISAXS data was averaged, processed and reduced from 2D to 1D using NIKA (Version 1.67) software.48 The structural indexing of the 3D superlattice was performed using MatLab based GIXSGUI.49 X-ray energy of 12.7 eV was used for GIXRD, the sample to detector distance was 200 mm. A MAR 345-image plate detector was used to collect the diffracted beams. A total of three frames of GIXRD data were collected for each sample at an incident angle of 0.2°. GIXRD data were averages and analyzed using WxDiff developed by S.C. Mannsfeld at Stanford Synchrotron Radiation Laboratory.50 The model and atomic planes of PbS QDs were visualized using VESTA.51

Supporting Information Available: TEM image, interparticle spacing measurements, FTIR, 2D GISAXS patterns, 2D GIXRD patterns, table with azimuthal peak positions for BCC lattice, integrated XRD pattern and schematic of spin coating, azimuth angle integrated scattering intensity with scattering vector for as-deposited and methanol washed oleic acid passivated PbS QD film. The material is available free of charge via the Internet at http://pubs.acs.org.

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Acknowledgement. The authors would like to thank C. Nivargi, A. J. M. Mackus, C. MacIssac and D. Bergsman for helpful discussions. This work was supported as part of the Center for Nanostructuring for Efficient Energy Conversion (CNEEC), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DESC0001060. X-ray scattering data was collected at beamlines 11-3 and 1-5 of the Stanford Synchrotron Radiation Lightsource. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

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Mannsfeld, S. C. Stanford Synchrotron Radiation Lightsource, 2009.

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Table of Contents Graphic:

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Molecular Ligands Control Superlattice Structure and Crystallite

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