Reversible, Tunable, Electric-Field Driven Assembly of Silver

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Letter pubs.acs.org/NanoLett

Reversible, Tunable, Electric-Field Driven Assembly of Silver Nanocrystal Superlattices Yixuan Yu,† Dian Yu,‡ and Christine A. Orme*,† †

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States University of California Los Angeles, Los Angeles, California 90095, United States

‡

S Supporting Information *

ABSTRACT: Nanocrystal superlattices are typically fabricated by either solvent evaporation or destabilization methods that require long time periods to generate highly ordered structures. In this paper, we report for the first time the use of electric fields to reversibly drive nanocrystal assembly into superlattices without changing solvent volume or composition, and show that this method only takes 20 min to produce polyhedral colloidal crystals, which would otherwise need days or weeks. This method offers a way to control the lattice constants and degree of preferential orientation for superlattices and can suppress the uniaxial superlattice contraction associated with solvent evaporation. In situ small-angle X-ray scattering experiments indicated that nanocrystal superlattices were formed while solvated, not during drying. KEYWORDS: Superlattice, colloidal crystals, field-driven assembly, grazing incidence small-angle X-ray scattering, Ag nanocrystals

O

runs into a dilemma where fast fabrication results in disordered or poorly ordered structures12,24 and highly ordered superlattices require fairly slow growth.25 For example, it usually take a few days or weeks for solvent evaporation or destabilization methods to generate faceted three-dimensional superlattices (also known as colloidal crystals or supercrystals).12,25−28 Collective properties, for example, electrical/thermal conductivity, of nanocrystal superlattices are functions of their lattice constants.19 The lattice constant of superlattices made by solvent evaporation or destabilization methods depends on complex parameters, such as superlattice formation rate and drying conditions, and therefore can not be readily and systematically controlled.29 In this paper, we demonstrate the first use of electric fields to reversibly drive nanocrystal superlattice self-assembly. This method can rapidly generate highly ordered colloidal crystals (within 20 min) with lattice constants and degree of preferential orientation controlled by tuning the electric field strength. This method can also suppress the uniaxial lattice contraction toward substrates, which is frequently observed in superlattices produced via solvent evaporation methods.30,31 The model material used in this work is silver (Ag) nanocrystals synthesized through a modified two phase method.32 These Ag nanocrystals with an average diameter of 6.6 nm (Figure S2) are capped with 1-dodecanethiol and dispersed in toluene at a concentration of 4 mg/mL prior to superlattice fabrication. Electric fields are generated by applying a voltage between two parallel double-sided polished, single-sided gold

rdered ensembles, or superlattices, of ligand-stabilized nanocrystals are emerging materials that can find applications in solar cells,1,2 photodetectors,3,4 light-emitting devices,5 field-effect transitors,6 memory devices,7 and beyond. Although nanocrystal superlattices have been intensively studied for more than two decades, their fabrication methods are still limited to manipulating the solvents in which they are dispersed.8 One can either let solvents evaporate to increase the concentration of nanocrystals until the entropy driven self-assembly, also known as the Kirkwood−Alder transition, takes place9−11 or alter the composition of the solvent to decrease the solubility of nanocrystals, leading to aggregation or self-assembly.12 External fields, such as electric, magnetic, and electromagnetic fields have been explored as means to modify the self-assembly process, for example, tilting the orientation of anisotropic particles,13−15 but never, to the best of our knowledge, used to drive the formation of nanocrystal superlattices. For instance, Ryan and Alivisatos13 used an electric field to assist the self-assembly of vertically aligned CdS nanorod superlattices, the driving force of which was still solvent evaporation. Jia and Herman et al.16 used an electric field to deposit thick CdSe nanocrystal films; however, these were disordered. Krejci et al.17 investigated the assembly of iron oxide nanocrystals in electric fields with in situ X-ray scattering and concluded that the electric field did not drive nanocrystal ordering in their system. The degree of order in nanocrystal superlattices is crucial for charge carrier delocalization18 and miniband formation,19 which in turn plays important roles in enhancing the efficiency of nanocrystal devices.20,21 Although ordered thin films can be quickly generated,22,23 fabrication of superlattices with larger dimensions, such as thick films and 3D colloidal crystals often © 2017 American Chemical Society

Received: March 29, 2017 Revised: April 27, 2017 Published: May 16, 2017 3862

DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869

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Figure 1. (a) GISAXS pattern of Ag nanocrystal superlattices formed on the cathode under an electric field of 400 V/cm, showing diffraction spots indexed to an fcc superlattice with a lattice constant of 11.9 nm, oriented with (111) plane parallel to the substrate. R(h,k,l) corresponds to (h,k,l) diffraction spot of the reflected beam. (b) Left: optical microscope image of superlattices made on the cathode with a field of 400 V/cm, showing faceted colloidal crystals; right: optical microscope image of a superlattice sample immersed in solution for 30 min, showing no colloidal crystals. (c) GISAXS pattern of drop cast Ag nanocrystal superlattices, showing fcc superlattices with a lower degree of order. (d) Horizontal line plots for (002) diffraction spots of EPD (blue curve) and drop cast (black curve) samples with dashed lines highlighting the calculated peak positions. (e) Vertical line plots of (−111) and (002) diffraction spots of EPD (blue curve) and drop cast (black curve) samples. Red solid line highlights the peak position of Yoneda line. Dashed lines represent the calculated (002) peak positions.

coated p-type silicon wafer eletrodes. Such an experimental setup, illustrated in Figure S1, has been used in the electrophoretic deposition (EPD) of colloidal nanocrystal solutions.16,33 We would like to refer to our deposition method as EPD to emphasize the similarity in setup between our method and EPD, despite their different deposition mechanisms. Figure 1a shows a grazing incidence small-angle X-ray scattering (GISAXS)34,35 pattern of Ag nanocrystal superlattices formed on the cathode (negatively charged electrode) under a field strength of 400 V/ cm for 30 min. The GISAXS pattern exhibits distinct diffraction spots that can be indexed to a face-centered cubic (fcc, a = b = c, α = β = γ = 90°) superlattice with a lattice constant of 11.9 nm, oriented with its (111) plane parallel to the substrate. These superlattices have faceted polyhedral shapes, as shown in the optical microscope image in the left column of Figure 1b. Turning off the electric field while the electrodes are immersed in solution causes the superlattices to dissolve (Figure 1b, right column). When the field is turned back on, the superlattices regrow. A control sample made by dip coating the same electrode in the same solution for same amount of time (30 min) created a nanocrystal monolayer (Figures S3 and S4), suggesting that the electric field played a critical role in the superlattice formation. The GISAXS pattern of a drop cast sample (Figure 1b) can also be indexed to a (111) oriented fcc superlattice with a lattice

constant of 12.0 nm, the diffraction spots of which are broader, indicating a lower degree of order. Nanocrystal superlattices produced through solution evaporation on a substrate often exhibit a uniaxial contraction toward the substrate.28,30 In this case, the contraction is along the [111] direction of fcc, which distorts fcc and transforms it to a face-centered rhombohedral (a = b = c, α = β = γ ≠ 90°) structure with the angle α corresponding to the degree of contraction. The superlattice distortion will cause the diffraction spots in GISAXS pattern to shift both vertically and horizontally. The horizontal (qx) and vertical (qz) peak position of (h,k,l) diffraction spot of a facecentered rhombohedral superlattice oriented on a substate with (111) plane can be calculated using the following equations36 qx = q(h , k , l) ×

1 − cos2 θ

qz = q(h , k , l) × cos θ

(1) (2)

q(h,k,l) is the magnitude of wave vector difference between the (h,k,l) diffraction beam and incident beam and could be expressed as q(h , k , l)

3863

⎛h⎞ ⎜ ⎟ = 2π ( h k l ) × G × ⎜ k ⎟ ⎜ ⎟ ⎝l⎠

(3) DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869

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Figure 2. (a) SEM image of Ag nanocrystal superlattices formed on the cathode at 400 V/cm, showing faceted colloidal crystals. SEM images of Ag nanocrystal colloidal crystals with (b) triangular prism and (c) truncated triangular prism shapes. (d) High-resolution SEM image showing the preferential orientation of superlattices and a grain boundary highlighted with white dashed lines.

θ is the angle between (h,k,l) and (111) planes, which can be calculated with

cos θ =

samples with their best match calculated peak positions highlighted with dashed lines. The degree of uniaxial contraction along the [111] direction is calculated using

⎛1⎞ ( h k l ) × G × ⎜⎜1⎟⎟ ⎝1⎠ ⎛h⎞ ⎜ ⎟ (h k l ) × G × ⎜k ⎟ × ⎜ ⎟ ⎝l ⎠

contraction[111] = 1 −

⎛1⎞ (1 1 1) × G × ⎜⎜1⎟⎟ ⎝1⎠

=1− (4)

G is the reciprocal metric tensor for rhombohedral lattices 1 G= 2 a (1 − 3 cos2 α + 2 cos3 α) ⎛ sin 2 α cos2 α − cos α cos2 α − cos α ⎞ ⎟ ⎜ ⎜ cos2 α − cos α sin 2 α cos2 α − cos α ⎟ ⎟ ⎜ 2 ⎠ ⎝ cos α − cos α cos 2 α − cos α sin 2 α

d spacing rhombohedral(111) d spacing fcc(111) 3 ⎛1⎞ a (1 1 1) × G × ⎜⎜1⎟⎟ ⎝1⎠

(6)

The drop cast superlattices have an angle, α, of 93.2°, corresponding to 5.8% contraction toward the substrate, while the EPD sample has an angle, α, of 90.4°, corresponding to 0.7% contraction. It is worthwhile to mention that we do not use the most intense (−111) diffraction spot to determined the degree of contraction because this diffraction spot coincides with the Yoneda line (highlighted with a solid red line in Figure 1e) that distorts the vertical scattering intensity profile.37 Note that despite slight distortions (0.7%) to the fcc superlattice we will

(5)

Figure 1d,e shows horizontal line plots and vertical line plots of (002) diffraction spots, respectively, for EPD and drop cast 3864

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Figure 3. GISAXS pattern of Ag nanocrystal superlattices formed on the cathode by using electric fields of (a) 120 and (b) 200 V/cm. (c) Radial integration of GISAXS data within the white dashed lines in panels a and b, showing that diffraction peaks of the sample made at 120 V/cm are at lower q-values and narrower than those of the sample made at 200 V/cm. (d) Azimuthal angle plots of GISAXS patterns along red dashed curves in panels a and b.

index the following GISAXS patterns in the paper with undistorted conventional fcc structure. Figure 2 shows scanning electron microscopy (SEM) images of the sample shown in Figure 1, displaying colloidal crystals with mostly triangular prism (Figure 2b) or truncated triangular prism (Figure 2c) shapes. The superlattices are preferentially oriented with (111) planes parallel to the substrate, as clearly shown in Figure 2d. More SEM images showing the preferential orientation are in the Supporting Information (Figure S5 and S6). Defects are present in these superlattices. For instance, Figure 2d shows a superlattice grain boundary (highlighted with a white dashed line) that might be responsible for the colloidal crystals with irregular shapes, like those highlighted with dashed circles in Figure 2a. Electric field driven assembly of superlattices allows one to control the lattice constant and degree of preferential orientation by simply tuning the field strength. Figure 3a,b shows GISAXS patterns of superlattices formed on cathodes under fields of 120 and 200 V/cm, indexed to fcc superlattices with lattice constants of 12.4 and 11.9 nm, respectively. Such a lattice constant difference is clearly revealed by the radial integration of GISAXS patterns (Figure 3c). Diffraction peaks of the superlattice made at 120 V/cm are narrower than these of the samples made at 200 or 400 V/cm, indicating larger superlattice grain size (Figure S10). The degree of preferential orientation of superlattices is characterized by the full width at half-maximum (fwhm) of peaks in the azimuthal angle plot of GISAXS patterns, which display angular distribution of diffraction intensity for a crystal plane family. In extreme situations, the azimuthal angle plot of a structure with perfect orientation would exhibit delta functions, assuming no instrumental broadening, while that of a sample

absent of preferential orientation (e.g., powder diffraction) should be a flat line. Figure 3d shows azimuthal angle plots for {311} superlattice planes. fwhm of azimuthal angle peaks of the superlattices made at 120 V/cm are much narrower than those of the sample made at 200 or 400 V/cm, indicating a much higher degree of preferential orientation. Electric fields drive Ag nanocrystals in a neat solution to form superlattices on the cathode but not on the anode. Regardless of the electric field strength, only nanocrystal monolayers could be found on the anodes,36 which is consistent with dip coating (Figure S11). The fact that Ag nanocrystals migrate toward the negatively charged electrode suggests they are positively charged. The charging mechanism remains unclear, as thiolate Ag clusters are often reported to be negatively charged.38,39 One of the possible routes to positively charge an alkyl thiol-capped Ag nanocrystal dispersed in toluene is through electron transfer from surface thiolate (a weak base) to toluene (a weak acid).40 Ion adsorption can neutralize or flip the charge of nanocrystals, leading to no deposition or deposition on the opposite electrode.37 Bromide ions (Br−) are well-known to attach to the surface of Ag nanocrystals,41−43 Au nanocrystals,44 and PbS quantum dots.45 Adding 0.7 mM of tetraoctylammonium bromide (TOAB)46 to Ag nanocrystal solution (number ratio Br−/ AgNC = 160) neutralized the surface charge of Ag nanocrystals and resulted in no superlattice formation on either electrode (Figure S12 and S13). Continuing to add TOAB flipped the charge of Ag nanocrystals and led to superlattice formation on the positively charged electrode (anode). Figure 4a shows a GISAXS pattern of superlattices formed on the anode at a field strength of 200 V/cm when approximately 1 mM of TOAB was added to the solution (number ratio Br−/AgNC = 240). 3865

DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869

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Figure 4. (a) GISAXS pattern of Ag nanocrystal superlattices formed on the anode with a field of 200 V/cm and ∼1 mM of TOAB added to the solution, showing continuous diffraction rings. The diffraction rings of the incident X-ray beam are highlighted with white dashed curves and those of the reflected X-ray beam are highlighted with gray dashed curves. (b) SEM images of Ag nanocrystal superlattices formed on the anode, showing no preferential orientation. Inset shows the overall look of a superlattice on the anode with a scale bar of 5 μm. (c) The {111} diffraction peak of Ag nanocrystal superlattices formed on the anode under various field strengths, showing larger lattice constants for those made at weaker fields. (d) Plot of lattice constant as a function of field strength with various TOAB concentrations.

With a TOAB concentration of 1.6 mM, (number ratio Br−/ AgNC = 380) electric field strength of 160 V/cm, and deposition time of 20 min, we produced well-defined polyhedral colloidal crystals (Figure 5a). These colloidal crystals have an fcc structure (GISAXS shown in Figure S20) and tend to expose the lowest energy planes, that is, {111}. The most frequently observed shape is octahedron, but triangular prisms (tetrahedron with one corner truncated) are also found, as shown in Figure 5b. The polyhedral superlattices can fuse to form more complex structures. Figure 5c shows a triangular prism fused with an octahedron sitting on top of it. The packing of nanocrystals in these two colloidal crystals seems to adopt the same orientation. Additional SEM images of colloidal crystals showing growth edges are in the Supporting Information (Figure S21−S25). Control experiments using gold-coated glass slides as electrodes show no superlattice formation on the glass side, suggesting electric fields only drive nanocrystal superlattice to form on conducting surfaces (either semiconductors or metals) where charge transfer can occur. Figure 6 shows schemes illustrating the proposed superlattice formation mechanism. Upon applying electric fields, charged nanocrystals migrate toward the oppositely charged electrode, become neutralized, and then accumulate near the electrode, creating a concentration gradient that decays from the electrode surface to bulk solution. In equilibrium, the nanocrystal diffusion toward the bulk solution, which is proportional to the concentration gradient, would balance the field-driven nanocrystal drift toward the electrode, which is positively related to the field strength. A stronger electric field can therefore generate a higher nanocrystal concentration near the electrode. If the field strength is above a threshold, the

The GISAXS pattern displays continuous diffraction rings, indicating that there is no preferential orientation, which is confirmed with SEM (Figure 4b and Figures S15−S17). Figure 4c displays {111} diffraction peaks of superlattices formed on anodes with various field strengths, showing larger lattice constants for superlattices made with weaker fields. Figure 4d shows the lattice constant as a function of field strength with various TOAB concentrations. The cathode, with added TOAB, showed a nanocrystal monolayer with poor order (Figure S18). The lack of order is probably due to the deposition of organic species (TOA+ ions, Figure S19). The smaller lattice constant of superlattices made at a stronger field could possibly be due to (I) closer nanocrystal separation or (II) smaller nanocrystals in the superlattices. Our measurement cannot distinguish between these two mechanisms, however, superlattices grow faster under a stronger field, and thus the capping ligands have less time to rearrange and interdigitate. This would result in a larger nanocrystal separation for superlattices made at a stronger field, which is opposite to what we have observed. Regarding the second mechanism, smaller nanocrystals have larger diffusion constants and therefore are more likely to diffuse back into the bulk solution than the larger ones, increasing the average size of nanocrystals in the superlattice. Such an effect is less significant for faster superlattice growth. This could lead to a smaller average size for nanocrystals incorporated into the superlattice, and consequently a smaller lattice constant at a stronger field. When there is no TOAB, superlattices grow slower, which means the size-selection effect is more pronounced, leading to a more significant lattice constant shift (red dot in Figure 4d). 3866

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Figure 6. Schematic description of proposed mechanism for electric field-driven nanocrystal assembly, showing a three-step process: (I) charged Ag nanocrystals are driven toward the oppositely charged electrode by electrophoretic force and neutralized through charge transfer with the electrode; (II) neutralized nanocrystals accumulate near the electrode surface until their diffusion to bulk solution balances the field-driven drift to the electrode; (III) if the field strength is above a threshold, the nanocrystal concentration approaches the saturation point, leading to superlattice formation.

nanocrystals, the threshold field strength is ∼40 V/cm, below which no superlattice formation could be observed. Turning off the electric field terminates the nanocrystal flow toward the electrode, which decreases the nanocrystal concentration, and dissolves the superlattices. To provide experimental support for the proposed mechanism, we have performed in situ small-angle X-ray scattering experiments with a homemade EPD cell that allows us to probe nanocrystal superlattices formation in solution. Nanocrystal solution was loaded into a polyether ether ketone cell sealed with O-rings, and an electric field was generated between the top and bottom electrodes. After applying an electric field of 160 V/cm to an Ag nanocrystal solution with 1.6 mM of TOAB for 20 min, the solution showed a graded concentration with more nanocrystals found near the anode (bottom electrode), as shown in Figure 7a. The X-ray beam was then transmitted through the concentrated nanocrystal solution near the anode, and the radial integration of scattering data is shown in Figure 7b. X-ray scattering data clearly indicate that nanocrystals were assembled into fcc superlattices while solvated with a lattice constant of asolvated = 14.1 nm. This is 15.6% larger than that of dried superlattices, adry = 12.2 nm. The effective radius50,51 of nanocrystal is calculated as adry R eff = = 4.32 nm (7) 2 2

Figure 5. (a) SEM image of Ag nanocrystal colloidal crystals formed on the anode with 1.6 mM TOAB added to the nanocrystal solution. (b) High-resolution SEM image for a triangular prism colloidal crystal. The inset shows the overall look of the colloidal crystal with a blue dashed rectangle highlighting the area where the high-resolution image was acquired. (c) High-resolution SEM image for a triangular prism colloidal crystal (bottom, closer to substrate) fused with an octahedral colloidal crystal (top). The yellow arrows illustrate the orientation of nanocrystal packing in the top octahedral colloidal crystal, which is the same as that of the bottom triangular prism colloidal crystal (white arrows). The inset shows an overall view of the fused colloidal crystals with a blue dashed rectangle highlighting the area where the high-resolution image was acquired.

which gives an effective volume fraction of nanocrystals in the solvated superlattices

nanocrystal concentration can approach the saturation point (0.495 volume fraction for hard spheres undergoing Kirkwood− Alder transition),47,48 triggering superlattice nucleation and growth. In the case of 4 mg/mL toluene dispersion of 6.6 nm Ag

ρ=4× 3867

4 3 πR eff 3

asolvated 3

= 0.48 (8) DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01323. Experimental details, TEM image and size histogram of Ag nanocrystals, additional SEM images of Ag nanocrystal monolayer and superlattices, GISAXS patterns of Ag nanocrystal monolayer and superlattices, data from control experiments, Zeta potential data, Scherrer analysis results, and details of X-ray refraction correction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yixuan Yu: 0000-0003-3265-3260 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Lawrence Livermore National Laboratory Directed Research and Development Program, 16-ERD-033. Work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. X-ray scattering experiments were performed at Advanced Light Source supported by the Office of Science, Office of Basic Energy Sciences, the U.S Department of Energy under contract no. DE-AC02-05CH11231.

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Figure 7. (a) (Top) In situ SAXS cell. One of the Kapton windows is removed to allow a view of the inside of the cell. (Bottom) Pictures of the in situ cell with Ag nanocrystal solution sealed in it, before (left) and after (right) applying a 160 V/cm electric field, showing a fieldgenerated nanocrystal concentration gradient. (b) Small angle X-ray scattering data of Ag nanocrystal superlattices in solution (top) and dried (bottom).

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This is close to the typical value reported for Kirkwood−Alder transitions (0.505).47−49 This in situ X-ray scattering experiment could not distinguish whether the superlattices were nucleated homogeneously in solution or heterogeneously on the substrate due to the millimeter-scale beam spot size.52 It is worthwhile to mention that the proposed mechanism may not be universally applied to all the electric-field driven nanocrystal assembly situations. For instance, our ongoing work on electric-field driven assembly of oleic acid capped nickel nanocrystals under weaker fields, ∼5−20 V/cm, shows superlattice thin film formation via a layer-by-layer growth mechanism. In conclusion, we have demonstrated that electric fields can be used to reversibly drive nanocrystal aggregation and superlattice formation. In contrast to current methods, field-driven assembly does not require evaporation or change of composition of the solvent. Above a threshold field strength, Ag nanocrystals formed colloidal crystals on the cathode, and if more than 0.7 mM TOAB is added to the solution Ag nanocrystals formed colloidal crystals on the anode. This method produces highly ordered colloidal crystals in shorter times than current methods, allows us to readily control superlattice lattice constants and degree of preferential orientation by tuning the electric field strength, and suppresses the solvent evaporation induced uniaxial contraction toward substrates. 3868

DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869

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DOI: 10.1021/acs.nanolett.7b01323 Nano Lett. 2017, 17, 3862−3869