Self-organization of colloidal PbS quantum dots ... - ACS Publications

Subscriber access provided by ERCIYES UNIV

Article

Self-organization of colloidal PbS quantum dots into highly-ordered superlattices Alexander V. Baranov, Elena V. Ushakova, Valery V. Golubkov, Aleksandr P. Litvin, Peter S. Parfenov, Anatoly V. Fedorov, and Kevin Berwick Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503913z • Publication Date (Web): 16 Dec 2014 Downloaded from http://pubs.acs.org on December 29, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Self-organization of colloidal PbS quantum dots into highly-ordered superlattices Alexander V. Baranov†, Elena V. Ushakova†*, Valery V. Golubkov‡, Aleksandr P. Litvin†, Peter S. Parfenov†, Anatoly V. Fedorov†, Kevin Berwick†† †

ITMO University, 49 Kronverkskiy pr., Saint-Petersburg, 197101, Russia;

‡

Institute of Silicate Chemistry of Russian Academy of Sciences, 2 Adm. Makarova emb.,

Saint-Petersburg, 199034, Russia ††

School of Electronic and Communications Engineering, Dublin Institute of Technology,

Kevin Street, Dublin 8, Ireland

KEYWORDS nanocrystals, lead sulfide, self-organization, superlattices, QDSLs

ABSTRACT: X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSL’s period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an inter-dot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.

1.

INTRODUCTION

The formation of mono- and multi-component structures can be achieved in several ways: epitaxial growth,[1, 2] material assembly on macromolecule patterns and masks,[3-5] and particle self-assembly on substrates or phase boundaries.[6-23] This latter method is particularly attractive because of its simplicity and the ease of obtaining ordered structures, known as superlattices (SLs). This method also allows selection from a wide array of building block components and the substrate types. At present, SLs are obtained from nanoparticles (NPs) of materials such as semiconductors,[6-8, 12-18, 21-24] metals,[25] magnetic NPs,[26] their mixtures,[2729]

as well as from complexes of NPs with large organic molecules such as proteins.[30] For the

formation of novel SL based materials, semiconductor nanoparticles or quantum dots (QDs) can be used as building blocks. QDSL materials, consisting of superlattices made from QDs, will either display the optical properties of QDs, modified by confinement, or the “collective” properties of an ordered array of nanoparticles.[6, 7, 11, 12, 19, 20, 24]


2

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Colloidal lead-sulfate QDs have emerged as a promising class of semiconducting materials, suitable for a variety of applications, since they possess desirable properties, including large molar absorption coefficients, a large overlap of the absorption spectrum with the solar radiation spectrum, small effective masses of electrons and holes and the possibility of multiexciton generation and hot carrier extraction.[31, 32] Manipulation of the properties of PbS QDSLs is key to a variety of novel applications in electronics, optics and photovoltaics.[1, 2, 4-6, 12, 33-35]

For near-IR photovoltaic applications, close-packed PbS quantum dot solids

combining the advantages of tunable, QD quantum-confined energy levels with fast charge transport due to enhanced electronic coupling between QDs, can be obtained via controlled ligand displacement.[2, 33, 36] The short distance necessary for efficient charge transfer between QDs can also be achieved by replacement or removal of pristine ligands.[37] Despite intensive research, practical applications based on QDSL based materials remain a future ambition, rather than a present reality. Development of technologies for the formation of nanostructured materials based on QDSLs is constrained by our limited ability to control lattice parameters, such as the distance between the QDs and the symmetry of the ensembles of ordered QDs. QD self-assembly depends on many factors including solvent evaporation rate, [38] the size of the NPs and the interactions between them, the type of matrix or substrate [39]

and the number and type of surface ligands. [40, 41] Understanding the mechanisms that

govern the process of QD self-organization into mono- and multi-component SLs is crucial for the design of optoelectronic devices such as photodetectors,[42] solar cells,[43, 44] and LEDs[45]. However, our knowledge of QDs self-assembly is incomplete and detailed studies are necessary in order to clarify and control aspects of the self-assembly process.


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

In this work we report on the morphology and optical properties of QD superlattices deposited on glass. The QDSLs are obtained by self-organization of PbS quantum dots of various sizes and covered with surface ligands. 2.

EXPERIMENTAL SECTION

Colloidal PbS QDs were synthesized using the standard procedure described in detail in Ref.[46] QDs varying in from 2.8 to 8.9 nm were obtained by this method. In addition, stock solutions of synthesized quantum dots in tetrachlormethane (TCM) with a concentration of ~ 10-6 M were prepared. Absorption (ABS) and photoluminescence (PL) spectra of the PbS QDs in TCM obtained are shown in Figure 1. The values of absorption and PL peak positions of PbS QDs of different diameters are shown in Table 1.

Figure 1. Absorption (blue dashed lines) and PL (red solid lines) spectra of stock solutions of PbS QDs of various sizes in TCM. The QD diameter in nm is shown beside the curves.


4

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table 1. Absorption and PL peaks position of PbS QDs of different diameters, D. D, nm

2.8

Abs, eV PL, eV

3.2

3.5

3.8

4.2

4.9

5.6

6.4

8.4

8.9

1.56 1.46

1.30

1.24

1.22

1.00

0.98

0.81

0.71

0.65

1.30 1.28

1.17

1.13

1.13

0.97

0.94

0.8

0.7

0.65

The quantum dot ensemble samples studied were prepared by dripping QD stock solution on to glass substrates. This was done from one to three times and then the droplets were dried at room temperature in an ambient atmosphere, forming a sufficiently concentrated film. The influence of the surface ligands on the process of self-organization was investigated using a sample consisting of quantum dots of diameters 4.3 and 6.6 nm, deposited on a glass substrate. As a result of this synthesis procedure, the surface of the QDs and the stock solutions contain molecules of oleic acid and octadecene-1, which may be selectively removed by washing with acetone and isopropanol, respectively. For this purpose, a standard solutionphase technique was used.[32] In brief, 100 µl of acetone or isopropanol was added to 50 µl of a QDs stock solution. This mixture was stirred in an ultrasound bath and then centrifuged to precipitate the nanocrystals. The precipitated quantum dots were redispersed in 50 µl of TCM and stirred in the ultrasonic bath for 5-10 minutes. Then, 15 µl of the QD solution obtained was dripped onto the glass substrate and dried in an ambient atmosphere at room temperature. For measurements of the absorption and luminescence spectra, as well as the luminescence decay times, the residual 35 µl of this solution was added to 2 ml of TCM. Reference samples (referred to as “initial” QDs) were obtained by dripping 15 µl of the QD stock solutions onto the glass surface. For all samples, the glass substrates were washed with isopropanol, dried, washed with distillate and dried again.


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

To determine the QD diameter and to investigate the relative arrangement of the nanoparticles in the QDSLs on the substrate, X-ray diffraction structural analysis techniques viz., WideAngle X-ray Scattering (WAXS) and Small Angle X-ray Scattering (SAXS) were used.[10, 4749]

PbS QD diameters were determined by WAXS using a Rigaku Ultima IV diffractometer

and FeKα radiation at a wavelength of 1.5405 Å. The SAXS technique was used for the investigation of the ordering of the QDSLs obtained by nanoparticle self-assembly on the substrates. The angular dependencies of the X-ray intensity in the range 6 to 250 angular minutes were obtained using a purpose-built setup with a slit collimation geometry defining a beam of effectively infinite length, CuKα radiation at a wavelength of 1.54 Å and a Ni filter. The angular resolution is about 1 arc minute. The size and composition of the PbS QDs were determined using Scanning Electron Microscopy using a Zeiss Merlin microscope operating in SEM mode at 20 kV. Optical microscopy images of the QD superlattices were obtained with a Research Grade Leica DM2500 optical microscope equipped with an objective with a numerical aperture of NA=0.75. To measure the absorption spectra of the samples, a UV-3600 Shimadzu spectrophotometer, equipped with an integrating sphere was used. For steady-state spectral PL analysis, a homebuilt fluorimeter with a spectral range of 0.7−2 µm, similar to that described in Ref.[50] was used. PL spectra were corrected by taking into account the spectral sensitivity of the fluorimeter, which was obtained using a blackbody spectrum.[51] For transient PL analysis, a custom setup described in Ref.[52] was used, allowing measurements of PL decay in the spectral range 0.8−1.7 µm and times ranging from 20 ns to 10 µs. 3.

RESULTS AND DISCUSSION


6

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

3.1. WAXS analysis. QD sizes were determined by the WAXS technique. Typical WAXS intensities as a function of double diffraction angle for the PbS QD samples are shown in Figure 2.

Figure 2. Typical WAXS patterns of PbS quantum dots of different sizes (3.5, 5.6, and 8.9 nm) deposited on the cover slide. Red lines intersected with the x-axis show diffraction peak positions for bulk PbS. The diameter of the PbS quantum dots was calculated using the Scherrer formula with the position and widths of the (111), (200), and (220) peaks obtained from the WAXS patterns:[47]

D=

k ×λ cos Θ × (∆ 2Θ) ,

(1)

where λ is the X-ray radiation wavelength [nm], θ and ∆(2θ) are the peak position and full width at half maximum [radians], respectively, k is the particle shape factor ≈ 0.94 for


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

spherical particles. Quantum dot diameters calculated using Equation 1 are in good agreement with those obtained from optical spectra as described in Ref.[53]. 3.2. SAXS analysis of QDs ensembles. A comparison between the SAXS patterns of 8.9 nm QDs in TCM solution and on the cover slide is shown in Figure 3.

Figure 3. SAXS patterns of 8.9 nm PbS QDs in TCM solution (black circles) and on the cover slide (red squares). Dashed and solid lines are the guides for eyes. For isolated QDs in a colloidal solution the scattered radiation intensity diminishes with an increase in scattering angle. For example, a typical SAXS pattern for isolated 8.9 nm PbS QDs in a dilute TCM solution is shown in Figure 3 as black circles. This angular dependence is

q= determined by the form factor F(q), where

4π ⋅ sin Θ

λ

, depending on the shape and size[47]


8

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

of QDs as well as on their concentration. Aggregation of the QDs into more ordered structures has a significant impact on the angular dependence of the SAXS intensity, a result of interference of the scattered radiation from the ordered particles. The appearance of distinctive maxima in the SAXS pattern, with the position of the maxima determined by the superlattice period,[10] is an indication of QD spatial ordering. This is shown in Figure 3, where the SAXS pattern for 8.9 nm QD SLs demonstrates a scattered X-ray intensity maximum at a scattering angle, 2θ, of about 2 degrees. Similar SAXS patterns exhibiting interference peaks have been obtained for all samples studied. Typical SAXS patterns for QDs of various sizes deposited on cover slides, shown in Figure 4, indicate the formation of PbS QDSLs.

Figure 4. (a) Typical SAXS patterns for PbS QDs of different diameters (D), deposited on the cover slides. (b) and (c) are the SEM images of the QDSLs formed from QDs with sizes of 6.2 and 8.9 nm, respectively. Scale bars of 20 nm are shown in (b) and (c). From Figure 4, one can see that the X-ray scattering maxima position depends on the QD size and shifts to smaller angles with increasing dot size. This observed dependence is typical of


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

SLs composed from close-packed spheres. The period of these structures and, therefore, the angular position of the SAXS maximum, are determined by the sphere size. In the simplest approximation, the distance between the scattering centers in superlattices can be calculated

d= from the diffraction patterns by Bragg’s law:

m× λ sin Θ , where λ is the X-ray wavelength

(1.54 Å), θ is the angular position of the interference peak, and m is a numerical coefficient. However, this formula is not well suited to the description of QD ordered structures. This is due to the fact that scattering from each individual nanoparticle also affects the position of the SAXS peak, as described in Ref. [10] for the ordered arrays of CdSe quantum dots. To estimate the period of the self-organized QDSLs, a more advanced formula, derived in Ref. [47]

, is used:

L=

5.395 ⋅ 10 −3

ϕm

,

(2)

where φm is the angular peak position in angle minutes, and L is the distance between centers of the nanoparticles in Å. The formation of PbS QDSLs was additionally verified by the SEM images of QDSLs with dot sizes of 6.2 and 8.9 nm which are shown in Figures 4 b and 4 c, respectively. Reasonable agreement between the values of inter-dot distance, calculated by both methods, was obtained. Presence of only one peak in the SAXS patterns in analyzed scattering angle range contrasting to the set of peaks, typical for three-dimensional particles packaging,[54] and visual analysis of the SEM images argue that formed dense packed QD ensembles are, most likely, two-dimensional hexagonal superlattices.[11] At the same time the information about the homogeneity of the self-assembled structure can be obtained from the


10

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

width of the diffraction peak. For instance, the superlattice disorder and hence the dispersion of inter-dot distance leads to a broadening of the diffraction peak. In Figure 5, the QDSLs period, i.e. the distance between the QD centers, calculated from SAXS patterns using Equation 2, is compared with the diameter of quantum dots, obtained from WAXS analysis.

Figure 5. Comparison of the calculated QDSLs period (L) with QD diameters obtained from WAXS analysis. Closed dots are experimental data. Linear fit of the experimental data (y=1.28x +1.1) is shown by the dashed line. A black solid line with a slope equal to 1 and the red dashed-dotted line, described by y=x+1.8, are shown for comparison (see text). From Figure 5, L, in the structures obtained depends linearly on dot diameter. This indicates that self-organization of PbS quantum dots into densely packed ordered structures occurs on the glass substrates. The period of structural ordering is similar to the QD diameter. The slope for the dependence shown in the Figure 5 by a dashed line is 1.28 ± 1.1, while for the closepacked NPs this value should be equal to 1, as shown by the black solid line in Figure 5. A


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

slight difference in slope is most likely caused by the fact that there is a layer of ligands on the QD surface. The thickness of this layer increases with the QD diameter, preventing direct contact between nanoparticles. In Figure 5, a red dash-dotted line (y=x+1.8) shows the expected dependence of the layer thickness of ligand with ~ 1.8 nm, which corresponds to the chain length of oleic acid. The spacing between the QDs increases from ~1.4 nm for smallest dots to ~ 3.4 nm for the larger dots. This suggests the presence of the octadecene-1 on the surface of the large QDs and indicates that there is an excess of oleic acid molecules on these dots. We also should take into account a potential interdigitation of oleic acids molecules from neighboring nanoparticles. The interdigitation could affect the superlattice formation and control the interdot distance.[54,

55]

Indeed for the smallest QDs, the interparticle distance

corresponds to full interdigitation of the oleic acid chains while for the largest QDs with 3.4 nm interdot spacing the interdigitation is improbable. It was reported earlier that the interparticle distance is a crucial parameter for the fabrication of electronic devices based on charge carrier transfer in densely packed QD structures. For instance, thin films of PbTe QDs have shown an increase in conductance of ~10-12 orders of magnitude with decrease in the interparticle distance from 1.8 to 0.3 nm.[56] It was also shown that PbS QDSLs with interparticle spacings of over 1 nm are unsuitable for use as photovoltaic devices. [57] Clearly, development of QDSLs with interparticle distances of less than 1 nm is technologically important. 3.3. Optical properties of QDs superlattices. Absorption and photoluminescence spectra as well as PL decay times of QDSLs on glass have been measured and compared with spectra from QD in TCM. It was found that differences between the ABS and PL spectra of QDs in solution and in QDSLs are small. For the largest QDs the positions of the ABS and PL bands


12

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

are almost the same and only a small ~20% broadening of the ABS and PL bands is observed for QDSLs. With a reduction of the QD diameter to 2.8 nm, the PL red shift increases to 25 nm. This PL red shift originates in a nonradiative energy transfer between nearest-neighbor nanocrystals in the QD ensemble and is an indirect indication of the formation of densely packed QD structures. The observed changes in the red shift with QD size are most likely due to a reduction of the energy transfer efficiency in QDSLs formed by the larger dots due to an increase in the inter-dot spacing, as mentioned in the previous Section. In Figure 6, the ABS and PL spectra of QDs with diameters of 6.6 nm (6a) and 4.3 nm (6b) in the TCM solution and in the QDSLs, self-organized on the glass surface, are shown as an illustration of this trend. (a)

(b)

Figure 6. Absorption (black) and PL (red) spectra of PbS quantum dots with diameter of (a) 6.6 nm and (b) 4.3 nm in TCM (dashed lines) and in the superlattices (solid lines). The insets show the corresponding PL decay curves for QDs in TCM (dashed line) and self-organized into QDSLs (solid line).


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

We measured the PL decay time for QD samples of different diameters in the form of superlattices and in TCM solutions at room temperature. The insets in Figure 6a and 6b present PL decay curves for QDs with a diameter of 6.6 nm (6a) and 4.3 nm (6b) dissolved in TCM and self-organized into QDSLs on the glass substrates. The PL decay curves for the QDs in TCM are described well by a single-exponential dependence, while for the QDs in the superlattices the PL decay curve is best fit by biexponential. The average lifetimes τ for the 2 2 biexponential decay have been calculated as follows: τ = ( I1 ⋅ τ 1 + I 2 ⋅ τ 2 ) ( I1 ⋅ τ 1 + I 2 ⋅ τ 2 ) ,

where Ii and τi are the amplitude and decay time of the i-th component, respectively. Transient PL analysis of the data obtained shows that the QD PL decay time is sizedependent. As QD diameter increases the PL lifetime increases, as observed in Ref. [53] for PbS QDs in TCM solution. The formation of densely packed structures of QDs leads to a sharp decrease in the average QD PL lifetime, which also depends on the QD size. The average PL lifetime of QDs in ensembles decreases from 780 to 35 ns with increases in the QD diameter from 2.8 to 8.4 nm. This is due to the appearance of additional nonradiative channels, which dissipate photoexcited electrons and holes, depending on the conditions of QDSL formation. These dependencies correlate well with those observed for ordered QD ensembles in porous matrix,[58] where a 5-fold reduction of the PL lifetime for QD solids, as compared with that for QD in the liquid solutions was observed. 3.4. Dependence of parameters of self-organization on ligand type and quantity. The influence of ligand molecules on the QD surface on the morphology and optical parameters of the structures formed was analyzed on sample QDSLs formed on a glass substrate from QDs with diameters of 6.6 and 4.3 nm. Molecules of oleic acid and octadecene-1 were selectively


14

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

removed by washing with acetone and isopropanol, respectively. Figures 7 and 8 show the SAXS patterns of these QDSLs. As we can see, a removal of the capping agents does not affect on the crystalline structure of the QDSLs since their SAXS patterns contain one diffraction peak.

Figure 7. (upper panel) SAXS patterns of QDSLs formed from 6.6 nm QDs: initial (squares), washed by acetone (circles) and isopropanol (triangles). The inset shows PL decay curves for as-prepared and treated QDs. (lower panel). Optical microscopy images of the corresponding superlattices. Scale bars of 50 microns are shown. A comparison of the SAXS patterns, presented in Figure 7, shows that removal of the surface ligands results in a shift of the SAXS peak position to larger angles, corresponding to a reduction in the inter-dot distance in the QDSLs. Simultaneously, peak widths become


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

smaller, indicating a reduction in the inter-dot distance dispersion, i.e. a higher ordering of the superstructure. Thus, the initial QDSL is characterized by a relatively broad (~0.4 deg.) SAXS peak at ~0.9 deg., corresponding to an average inter-dot distance, L, of 8.8 nm with a dispersion of ±1.8 nm. These QDSL parameters are more suitable for domain structures similar to the structure shown in Figure 7 (lower panel “initial”). Optical microscopy images of QDSLs formed from QDs treated by acetone or isopropanol show that the fabricated superstructures look more homogeneous than those left untreated (see Figure 7 (lower panel)). This is supported by the SAXS data. Indeed, removal of the oleic acid results in a shift of the SAXS peak to 1.2 deg. and its narrowing to 0.3 deg., corresponding to an average inter-dot distance L = 7.4±1.1 nm. Removal of the octadecene-1 molecules with isopropanol leads to the formation of QDSLs exhibiting a SAXS peak at 1.27 deg. and with a width of 0.18 deg., i.e. with L = 7.0±0.7 nm. As a result, the average interdot distance is reduced down to ~ 0.4 nm. Since, as it was mentioned above, the length of the fully extended molecule of oleic acid is ∼1.8 nm we can exclude the full interdigitation of oleic acid molecules from neighboring nanoparticles in studied systems. At the same time, the attempt to remove ligands from the QD surfaces did not allow us to form close-packed QDSLs with interparticle distance equal to the dot diameter. The distance between the surfaces of the treated QDs varied from 0.4 to 0.8 nm indicating only a partial removal of ligands from the surface. Nevertheless, there are distances at which the effective transfer of photoexcited charge carriers through the QDSLs becomes possible, allowing them to be used for the fabrication of photovoltaic devices.[56] Removal of the ligands from the surface of the 6.6 nm QDs did not change the steady state ABS and PL spectra of the QDSL as compared with the “initial” QDs. The decay time of the


16

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

PL from the QDSL formed from the treated QDs, however, becomes shorter, as shown on the Inset in Figure 6, where PL decay curves for QDSLs, formed on glass substrates from 6.6 nm QDs, with and without surface treatment are presented. The average PL lifetime is reduced from 40±2 ns for “initial” QDs down to 23±3 ns for QDSLs with shorter inter-dot spacings formed from QDs with a treated surface. This shortening of the PL lifetime indicates that a reduction in the number of ligands passivating the surface of the quantum dots leads to the appearance of additional centers of nonradiative relaxation for photoexcited electrons and, therefore, to a reduction of the PL decay time. Thus, removal of organic ligands from the surface of large quantum dots results in a decrease in the average distance between the QDs and, consequently, to an increase in the ordering and uniformity of the superlattices. The steady state absorption and luminescence spectra do not changed but a marked decrease of the PL lifetime occurs as a result of additional centers of nonradiative relaxation. This would also be expected to cause a reduction in the PL quantum yield which is undesirable in practical applications of QDSLs. Somewhat different results were obtained for the self-organization of smaller quantum dots with a diameter of 4.3 nm, the surface of which was cleared of ligands. In Figure 8 the SAXS patterns of three different QDSLs, formed from as-prepared and treated QDs, are shown.


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Figure 8. SAXS diffraction patterns of three different QDSLs formed from 4.3 nm PbS QDs: "initial" (squares, black line), washed with acetone (circles, red line), and isopropanol (triangles, blue line). Inset shows corresponding PL decay curves. For the QDSL formed from “initial” QDs, the SAXS peak is located at ~1.63 deg., corresponding to an average inter-dot distance of L=5.5±0.5 nm and to a 1.2 nm distance between dot surfaces. Unlike the 6.6 nm QD based QDSLs, treatment of the nanocrystal surface with acetone and isopropanol results in some degradation of the superlattices, with an increase in average inter-dot distance and dispersion. Treatment with isopropanol, i.e. removal of octadecene-1, results in a slight increase in L and its dispersion to 5.7 nm and ±0.7 nm, respectively. Removal of the oleic acid with acetone causes a strong degradation of the QDSL, resulting in an almost complete disappearance of the SAXS interference peak. These results indicate the importance of the presence of some oleic acid molecules on the QD surface to self-organization of the PbS nanocrystals into ordered superstructures, as already noted for PbSe QDs.[40] The presence of octadecene-1 on the QD surface does not appear to be as important.


18

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Removal of organic ligands from the surface of the 4.3 nm QDs does not lead to significant changes in the steady state ABS and PL spectra of the QDSLs. This is expected, since washing of the dot surface leads to a degradation of the QDSLs with a reduction of the interdot interaction. In contrast, transient PL analysis shows a strong dependence of the QD PL decay time on dot surface treatment. As in the QDSLs with a dot size of 6.6 nm, the QD PL lifetime decreases with the removal of ligands from the dot surface, as shown in Inset of Figure 8. With the ligands removal, the PL decay time decreases from 497 ns for the “initial” QDs to 337 ns (for isopropanol) and to only 96 ns (for acetone). As in the case of 6.6 nm QDs, this reduction is due to the opening of new channels of nonradiative annihilation of electron holes pairs. So, removal of organic ligands from the surface of PbS QDs in the QDSLs depends on QD size and ligand type. For large QDs, ligand removal leads to a strong reduction in the distance between QD surfaces in the QDSLs and a better ordering of their structure. For small QDs, e.g. QDs with diameters of 4.3 nm and smaller, ligand reduction, particularly the oleic acid, on the dot surface obstructs the self-organization process and impairs the ordering of the QDSLs. This comparison shows that there is an optimal surface concentration of oleic acids, which allows self-organization of the QDs in the QDSLs and a shortening of inter-dot distance to values allowing effective charge transfer in the densely packed PbS QD structures. Clearly, the influence of ligands on the properties of superstructures warrants further investigation. 4. CONCLUSIONS X-ray (WAXS and SAXS) analysis, together with steady-state and transient optical spectroscopy, has been used for studying the morphology and optical properties of QDSLs.


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

The QDSLs were fabricated by self-organization of PbS quantum dots of different sizes (2.88.9 nm) deposited on glass substrates. These quantum dots had ligands on the QD surface. It was found that the period of the QDSLs is proportional to the QD diameter. The slope of this dependence is larger than unity, which is expected for closely packed structures. This is caused by the presence of the ligand layer on the QD surface, which prevents direct contact between the nanoparticles. The ligand layer thickness increases from 1.1 to 3.4 nm with an increase in the QD diameter from 2.8 to 8.9 nm. QDSL formation does not significantly affect the absorption or PL spectra of the QDs although it does result in a sharp decrease in the average QD PL lifetime. This latter effect was attributed to the appearance of additional nonradiative channels of dissipation of photoexcited electrons and holes. The QD PL lifetime in the QDSLs depends on the QD size, decreasing from 780 to 35 ns with an increase in the QD diameter from 2.8 to 8.4 nm. It was shown that removal of organic ligands from the PbS QDs surface influences the QDSL morphology. For small QDs, a reduction in the amount of ligands on the dot surface, especially oleic acid, obstructs the self-organization process and impairs QDSL ordering. The inter-dot spacing remains almost unchanged at 1.2 nm. For large QDs, ligand removal leads to a strong reduction in the inter-dot distances in the QDSLs down to 0.4 nm and better ordering of the superlattice. These small inter-dot distances allow fast charge carrier transport through the QDSLs enabling potential applications such as photodetectors and efficient photovoltaic devices. Removal of organic ligands from the QD surface does not lead to detectable changes in the steady state ABS and PL spectra of the QDSLs, while the QD PL lifetime decreases with the QD surface treatment with acetone and isopropanol, opening new paths for nonradiative recombination of electron holes pairs.


20

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

AUTHOR INFORMATION Corresponding Author *e-mail [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by grant 14.В25.31.0002 and Government Assignment No. 3.109.2014/K of the Ministry of Education and Science of the Russian Federation. A.P.L. and E.V.U. thank the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (2013–2015). REFERENCES (1)

Selinsky, R. S.; Shin, S.; Lukowski, M. A.; Jin, S. Epitaxial Heterostructures of Lead

Selenide Quantum Dots on Hematite Nanowires. J. Phys. Chem. Lett. 2012, 3(12), 1649-1656. (2)

Lu, L. X.; Bharathi, M. S.; Zhang, Y. W. Self-assembly of ordered epitaxial

nanostructures on polygonal nanowires. Nano Lett. 2013, 13(2), 538-542.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

Page 22 of 30

Deng, Z.; Samanta, A.; Nangreave, J.; Yan, H.; Liu, Y. Robust DNA-Functionalized

Core/Shell Quantum Dots with Fluorescent Emission Spanning from UV–vis to Near-IR and Compatible with DNA-Directed Self-Assembly. J. Am. Chem. Soc. 2012, 134(42), 1742417427. (4)

Mark, S. S.; Bergkvist, M.; Yang, X.; Teixeira, L. M.; Bhatnagar, P.; Angert, E. R.;

Batt, C. A. Bionanofabrication of metallic and semiconductor nanoparticle arrays using Slayer protein lattices with different lateral spacings and geometries. Langmuir. 2006, 22(8), 3763-3774. (5)

Hellstrom, S. L.; Kim, Y.; Fakonas, J. S.; Senesi, A. J.; Macfarlane, R. J.; Mirkin, C.

A.; Atwater, H. A. Epitaxial growth of DNA-assembled nanoparticle superlattices on patterned substrates. Nano Lett. 2013, 13(12), 6084-6090. (6)

Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O'Brien, S.; Murray, C. B. Structural

diversity in binary nanoparticle superlattices. Nature. 2006, 439(7072), 55-59. (7)

Talapin, D. V.; Shevchenko, E.V.; Bodnarchuk, M. I.; Ye, X.; Chen, J.; Murray, C. B.

Quasicrystalline order in self-assembled binary nanoparticle superlattices. Nature. 2009, 461(7266), 964-967. (8)

Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and emerging applications of self-

assembled structures made from inorganic nanoparticles. Nat. Nanotechnol. 2009, 5(1), 15-25. (9)

Murray, C. B.; Sun, S. H.; Doyle, H.; Betley, T. Monodisperse 3d Transition-Metal

(Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices. Mrs Bull. 2001, 26, 985–991.


22

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(10)

Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and characterization of

monodisperse nanocrystals and close-packed nanocrystal assemblies. Annu. Rev. Mater. Sci. 2000, 30(1), 545-610. (11)

Corricelli, M.; Enrichi, F.; Altamura, D.; De Caro, L.; Giannini, C.; Falqui, A.;

Agostiano, A.; Curri, M. L.; Striccoli, M. Near Infrared Emission from Monomodal and Bimodal PbS Nanocrystal Superlattices. J. Phys. Chem. C. 2012, 116(10), 6143-6152. (12)

Quan, Z.; Fang, J. Superlattices with non-spherical building blocks. Nano Today.

2010, 5(5), 390-411. (13)

Goodfellow, B. W.; Korgel, B.A. Reversible solvent vapor-mediated phase changes in

nanocrystal superlattices. ACS Nano. 2011, 5(4), 2419-2424. (14)

Bian, K.; Choi, J. J.; Kaushik, A.; Clancy, P.; Smilgies, D. M.; Hanrath, T. Shape-

anisotropy driven symmetry transformations in nanocrystal superlattice polymorphs. ACS nano. 2011, 5(4), 2815-2823. (15)

Wang, Z.; Schliehe, C.; Bian, K.; Dale, D.; Bassett, W.A.; Hanrath, T.; Klinke, C.;

Weller, H. Correlating superlattice polymorphs to internanoparticle distance, packing density, and surface lattice in assemblies of pbs nanoparticles. Nano Lett. 2013, 13 (3), 1303–1311. (16)

Rinnerbauer, V.; Egelhaaf, H. J.; Hingerl, K.; Werner, S.; Warming, T.; Hoffmann, A.;

Kovalenko, M.; Heiss, W.; Hesser, G.; Schaffler, F. Energy transfer in close-packed PbS nanocrystal films. Phys. Rev. B. 2008, 77, 085322.


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(17)

Page 24 of 30

Pileni, M. P. Supracrystals of inorganic nanocrystals: an open challenge for new

physical properties. Acc. Chem. Res. 2008, 41(12), 1799-1809. (18)

Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H. Y.; Gao, J.; Nozik, A. J.; Beard,

M.C. Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science. 2011, 334 (6062), 1530-1533. (19)

Baimuratov, A.S.; Rukhlenko, I.D.; Turkov, V.K.; Baranov, A.V.; Fedorov, A.V.

Quantum-dot supercrystals for future nanophotonics. Sci. Rep. 2013, 3, 1727. (20)

Baimuratov, A.S.; Rukhlenko, I.D.; Fedorov, A.V. Engineering band structure in

nanoscale quantum-dot supercrystals. Opt. Lett. 2013, 38(13), 2259. (21)

Sayle, D. C.; Seal, S.; Wang, Z.; Mangili, B. C.; Price, D. W.; Karakoti, A. S.;

Kuchibhatla, S. T. V. N.; Hao, Q.; Mobus, G.; Xu, X.; Sayle, T. X. T. Mapping nanostructure: a systematic enumeration of nanomaterials by assembling nanobuilding blocks at crystallographic positions. ACS Nano. 2008, 2(6), 1237-1251. (22)

Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and

Their Uses in Self-Assembly. Small. 2009, 5(14), 1600–1630. (23)

Lee, C. H.; Crosby, A. J.; Hayward, R. C.; Emrick, T. Patterning Nanoparticles into

Rings by “2-D Pickering Emulsions. ACS Appl. Mater. Interfaces. 2014, 6(7), 4850-4855. (24)

Luckyanova, M. N.; Johnson, J. A.; Maznev, A. A.; Garg, J.; Jandl, A.; Bulsara, M. T.;

Fitzgerald, E.A.; Nelson, K. A.; Chen, G. Anisotropy of the Thermal Conductivity in GaAs/AlAs Superlattices. Nano Lett. 2013, 13(9), 3973-3977.


24

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(25)

Goodfellow, B. W.; Rasch, M. R.; Hessel, C. M.; Patel, R. N.; Smilgies, D. M.;

Korgel, B. A. Ordered Structure Rearrangements in Heated Gold Nanocrystal Superlattices. Nano Lett. 2013, 13(11), 5710-5714. (26)

Behrens, S. Preparation of functional magnetic nanocomposites and hybrid materials:

recent progress and future directions. Nanoscale. 2011, 3(3), 877-892. (27)

Ye, X.; Chen, J.; Diroll, B. T.; Murray, C. B. Tunable Plasmonic Coupling in Self-

Assembled Binary Nanocrystal Superlattices Studied by Correlated Optical Microspectrophotometry and Electron Microscopy. Nano Lett. 2013, 13(3), 1291-1297. (28)

Evers, W. H.; Nijs, B. D.; Filion, L.; Castillo, S.; Dijkstra, M.; Vanmaekelbergh, D.

Entropy-driven formation of binary semiconductor-nanocrystal superlattices. Nano Lett. 2010, 10(10), 4235-4241. (29)

Boneschanscher, M. P.; Evers, W. H.; Qi, W.; Meeldijk, J. D.; Dijkstra, M.;

Vanmaekelbergh, D. Electron Tomography Resolves a Novel Crystal Structure in a Binary Nanocrystal Superlattice. Nano Lett. 2013, 13(3), 1312-1316. (30)

Miao, L.; Han, J.; Zhang, H.; Zhao, L.; Si, C.; Zhang, X.; Hou, C.; Xu, J.; Liu, J.

Quantum-Dot-Induced Self-Assembly of Cricoid Protein for Light Harvesting. ACS Nano. 2014, 8(4), 3743-3751. (31)

Rogach, A. L.; Eychmüller, A.; Hickey, S. G.; Kershaw, S. V. Infrared‐Emitting

Colloidal Nanocrystals: Synthesis, Assembly, Spectroscopy, and Applications. Small. 2007, 3(4), 536-557.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32)

Page 26 of 30

Giansante, C.; Carbone, L.; Giannini, C.; Altamura, D.; Ameer, Z.; Maruccio, G.;

Loiudice, A.; Belviso, M.R.; Cozzoli, P.D.; Rizzo, A.; Gigli, G. Colloidal arenethiolatecapped PbS quantum dots: optoelectronic properties, self-assembly, and application in solution-cast photovoltaics. J. Phys. Chem. C. 2013, 117(25), 13305-13317. (33)

Talapin, D. V.; Murray, C.B. PbSe Nanocrystal Solids for n- and p-Channel Thin Film

Field-Effect Transistors. Science. 2005, 310, 86−89. (34)

Tang, J.; Sargent, E.H. Infrared colloidal quantum dots for photovoltaics:

fundamentals and recent progress. Adv. Mater. 2011, 23 (1), 12−29. (35)

Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of colloidal

nanocrystals for electronic and optoelectronic applications. Chem. Rev. 2010, 110, 389−458. (36)

Baumgardner, W. J.; Whitham, K.; Hanrath, T. Confined-but-Connected Quantum

Solids via Controlled Ligand Displacement. Nano Lett. 2013, 13(7), 3225-3231. (37)

Lingley, Z.; Lu, S.; Madhukar, A. The dynamics of energy and charge transfer in lead

sulfide quantum dot solids. J. Appl. Phys. 2014, 115(8), 084302. (38)

Jiang, Z.; Lin, X. M.; Sprung, M.; Narayanan, S.; Wang, J. Capturing the crystalline

phase of two-dimensional nanocrystal superlattices in action. Nano Lett. 2010, 10(3), 799-803. (39)

Martín-García, B.; Velázquez, M. M. Nanoparticle Self-Assembly Assisted by

Polymers: The Role of Shear Stress in the Nanoparticle Arrangement of Langmuir and Langmuir–Blodgett Films. Langmuir. 2014, 30(2), 509-516.


26

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(40)

Evers, W. H.; Goris, B.; Bals, S.; Casavola, M.; de Graaf, J.; Roij, R. V.;

Vanmaekelbergh, D. Low-dimensional semiconductor superlattices formed by geometric control over nanocrystal attachment. Nano Lett. 2012, 13(6), 2317-2323. (41)

Dalmaschio, C. J.; da Silveira Firmiano, E. G.; Pinheiro, A. N.; Sobrinho, D. G.; de

Moura, A. F.; Leite, E. R. Nanocrystals self-assembled in superlattices directed by the solvent–organic capping interaction. Nanoscale. 2013, 5(12), 5602-5610. (42)

Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina,

L.; Sargent, E.H. Sensitive solution-processed Bi2S3 nanocrystalline photodetectors. Nature. 2006, 442(7099), 180-183. (43)

Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid Nanorod-Polymer Solar Cells.

Science. 2002, 295(5564), 2425-2427. (44)

Zhao, N.; Osedach, T. P.; Chang, L. Y.; Geyer, S. M.; Wanger, D.; Binda, M. T.;

Arango, A. C.; Bawendi, M. G.; Bulovic, V. Colloidal PbS quantum dot solar cells with high fill factor. ACS Nano. 2010, 4(7), 3743-3752. (45)

Coe, S.; Woo, W. K.; Bawendi, M.; Bulović, V. Electroluminescence from single

monolayers of nanocrystals in molecular organic devices. Nature. 2002, 420(6917), 800-803. (46)

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(21), 1844-1849.


27

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(47)

Page 28 of 30

Porod, G.; Glatter, O.; Kratky, O. Small Angle X-ray Scattering, Academic Press:

London, 1982; 515 p. (48)

Massa, W. Crystal structure determination, Springer-Verlag: New York, 2004; 213 p.

(49)

Guiner, A.; Fournet, G. Small-angle scattering of X-rays, John Willey-Champan: New-

York, 1955; 267 p. (50)

Parfenov, P. S.; Baranov, A. V.; Veniaminov, A. V.; Orlova, A. O. A complex for the

fluorescence analysis of macro-and microsamples in the near-infrared. J. Opt. Technol. 2011, 78, 120−123. (51)

Parfenov, P. S.; Litvin, A. P.; Baranov, A. V.; Veniaminov, A. V.; Ushakova, E. V.

Calibration of the spectral sensitivity of instruments for the near infrared region. J. Appl. Spectrosc. 2011, 78, 433−439. (52)

Parfenov, P. S.; Litvin, A. P.; Baranov, A. V.; Ushakova, E. V.; Fedorov, A. V.;

Prudnikau, A. V.; Artemyev, M. V. Measurement of the luminescence decay times of PbS quantum dots in the near-IR spectral range. Opt. Spectrosc. 2012, 112, 868−873. (53)

Ushakova, E. V.; Litvin, A. P.; Parfenov, P. S.; Fedorov, A. V.; Artemyev, M.;

Prudnikau, A. V.; Rukhlenko, I. D.; Baranov, A. V. Anomalous size-dependent decay of lowenergy luminescence from PbS quantum dots in colloidal solution.. ACS Nano. 2012, 6(10), 8913-8921. (54)

Lee, B.; Podsiadlo, P.; Rupich, S.; Talapin, D. V.; Rajh, T.; Shevchenko, E. V.

Comparison of structural behavior of nanocrystals in randomly packed films and long-range


28

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ordered superlattices by time-resolved small angle X-ray scattering. J. Am. Chem. Soc. 2009, 131(45), 16386-16388. (55)

Li, R.; Bian, K.; Hanrath, T.; Bassett, W. A.; Wang, Z. Decoding the Superlattice and

Interface Structure of Truncate PbS Nanocrystal-Assembled Supercrystal and Associated Interaction Forces. J. Am. Chem. Soc. 2014, 136(34), 12047-12055. (56)

Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Murray, C. B. Self-assembly of PbTe

quantum dots into nanocrystal superlattices and glassy films. J. Am. Chem. Soc. 2006, 128(10), 3248-3255. (57)

Scheele, M.; Hanifi, D.; Zherebetskyy, D.; Chourou, S.T.; Axnanda, S.; Rancatore,

B.J.; Alivisatos, A.P. PbS Nanoparticles Capped with Tetrathiafulvalenetetracarboxylate: Utilizing Energy Level Alignment for Efficient Carrier Transport. ACS nano. 2014, 8(3), 2532-2540. (58)

Litvin, A.P.; Parfenov, P.S.; Ushakova, E.V.; Fedorov, A.V.; Artemyev, M.V.;

Prudnikau, A.V.; Golubkov, V.V.; Baranov, A.V. PbS quantum dots in a porous matrix: Optical characterization. J. Phys. Chem. C. 2013, 117 (23), 12318–12324.

TOC


29

Langmuir

Page 30 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

30

Self-organization of colloidal PbS quantum dots ... - ACS Publications

Recommend Documents