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Synthesis of bi-dimensional Prussian blue analogue using an inverted Langmuir-Schaefer method ANDREAS ROSSOS, Régis Y.N. Gengler, Daniel Salvatore Badali, and R. J. Dwayne Miller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02445 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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Langmuir

Synthesis

of

bi-dimensional

Prussian

blue

analogue using an inverted Langmuir-Schaefer method ‡Andreas K. Rossosa, ‡ Régis Y. N. Genglera, Daniel S. Badalia and R. J. Dwayne Miller*a,b a

Max Planck Institute for the Structure and Dynamics of Matter, Center for Free Electron

Laser Science, The Hamburg Centre for Ultrafast Imaging (CUI), Universität Hamburg Luruper Chaussee 149, 22761 Hamburg, Germany b

Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto,

Ontario M5S 3H6, Canada ‡ these authors contributed equally to this work ABSTRACT. One of the aspects of modern materials science that has been captivating scientific interest for the last decade is low-dimensional systems. This stems from the fact that the physical, chemical, and biological properties of such systems are often vastly different from their bulk counterparts. Additionally, low-dimensionality structures frequently serve as a convenient platform for device applications. However, such materials are typically constructed from building blocks that are inherently three-dimensional, and so, from a morphological point of view, these can still be categorized as bulk powders or crystals. To push the boundaries of reduced dimensionality, we synthesized truly two-dimensional films of Prussian blue analogues (mixed valence tetracyanides) by combining an air-water interface reaction and a novel inverted Langmuir-Schaefer technique. The methodology introduced in this study offers control and tailoring over the Prussian blue analogues’ film characteristics, which is an important step towards their incorporation into tangible applications. Standard ACS Paragon Plus Environment

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isotherms were collected as a function of the initial reactant volume and a number of characterization techniques such as X-Ray Photoelectron Spectroscopy (XPS), UV-Visible Spectroscopy (UV-Vis), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) and Atomic Force Microscopy (AFM) were performed on films transferred on various substrates. The results indicated a collection of single crystalline and polycrystalline flakes possessing different thicknesses and having a structural coherence length of 11 +/- 3 nm. INTRODUCTION Prussian Blue Analogues (PBAs) are coordination materials of the form DnAp[B(CN)6]q, where n, p, and q are stoichiometric numbers, D is an alkali cation, and A and B are transition metals ions. They exhibit a wide range of exotic properties which are tunable by the selection of the appropriate combination of transition metals. PBAs’ palette of magnetic attributes includes high Tc magnetism1,2, photo-magnetism3,4, magnetic pole inversion5,6, super paramagnetism7, and spin glass behavior8. Additionally, other properties such as pressure induced cyanide linkage isomerization7, negative and zero thermal expansion7, hydrogen storage9,

electrochromic behaviour10,11, and photophysical behaviour12 have been

demonstrated. These exciting characteristics have led PBAs to find use in many applications; biosensors13,14, battery cells15 and “smart” windows16 are just a few of the many areas in which PBAs have contributed significantly. In recent years, these areas, among others, have been revolutionized by the discovery and use of two-dimensional materials. This work strives to combine the merits of both the low dimensional world and functional coordination compounds by purposefully constructing a genuinely two-dimensional PBA (2D-PBA). As will be outlined below, this was accomplished by the concerted variation of the chemistry and synthesis of PBA. ACS Paragon Plus Environment

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Traditional PBA forms three-dimensional building blocks arranged in a face-centered cubic lattice structure, with [B(CN)6] occupying the corners and faces of the cube, and the metal A present at all the octahedral sites17. The synthesis of PBA simply involves the slow mixing of two solutions; more precisely, it follows a classic Lewis acid-base reaction:

BCN

Lewis base + A Lewis acid → A BCN ⋅ H O(solid) Eq. (1)

To reduce the dimensionality of PBA and bring it into the two-dimensional world, PBA's crystal structure and the reaction in Eq. (1) were carefully analyzed. This led to the realization that the inherent three-dimensional structure is imposed by the coordination level of the hexacyanide precursor. Therefore, it was hypothesized that using a precursor with a tetracyanide group would force the PBAs to form two-dimensional monolayers. To explore this hypothesis, we performed a synthesis analogous to Eq.(1), with the selected metals being copper for A and platinum for B, in the precursor forms of copper(II) chloride (CuCl2) and potassium tetracyanoplatinate(II) (K2Pt(CN)4), respectively. The latter was chosen based on the limited availability of appropriate tetracyanides. This led to the final reaction:

CuCl2 solid→Cu + 2Cl K2 PtCN4 →2K + PtCN4

II,III PtII PtCN4

CN4 +Cu →Cu

Eq. 2 although we expect that this final reaction should hold in the more general form

BCN4

+A →A BCN ⋅ H O(solid) .

In a naive model of the lattice structure, the forces on each metal atom within the plane will be identical in the two-dimensional and three-dimensional cases. Therefore, it is expected that

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the in-plane structure of the 2D-PBA formed in this reaction will maintain the same square arrangement as three-dimensional PBA. While it is possible for there to be some out-of-plane buckling, the planar square crystal maximizes the A-A distance, consistent with the repulsive Coulombic forces between the charged metal ions. Additionally, it is a well-established fact that tetracyanide compounds, in particular Pt(CN)4, take planar geometries in crystal structures18,19. In light of this and restriction of the coordination level of the Pt center, the structure of the 2D-PBA can be anticipated, the result of which is shown further on. While the traditional synthesis of Prussian blue and its analogues follows a particularly elementary procedure (see the reaction in Eq. (1)), various more sophisticated approaches such as electrochemical deposition20-23, casting from colloidal solution24, dip coating25, and single crystal growth26 exist with the aim of creating application friendly deposition methods. Unfortunately, these methods suffer from a lack of control over important parameters, such as film thickness and composition, which are vital for creating useful functional materials. In light of this, recent studies27 using a Langmuir-Blodgett/Schaefer (LB/LS) technique are extremely promising, and have shown great potential towards fabricating device quality PBAs. The power of this technique lies in its precise control over the film thickness (number of layers deposited) with molecular level accuracy. Here we utilize the control offered by the LS method to facilitate the creation of truly twodimensional crystals of Prussian blue analogues following the reaction in Eq. (2). It is important to emphasize that while previous studies27-29,31 have developed composite thin films containing lower dimensionality PBA, the aspect of restricted dimensionality was created through a self-limited on-surface reaction. However, the material deposited in such a way remains partially three-dimensional as imposed by the coordination level of the precursor. Like Makiura et al.30 did for metal-organic frameworks, our approach also uses a modified LS technique to synthesize PBA from precursors through an on-surface reaction.


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Figure 1. Schematic illustration of the two-dimensional film formation using the inverted Langmuir-Schaefer technique; the “gentle deposition” of potassium tetracyanoplatinate(II) (K2Pt(CN)4), facilitated by an inclined piece of glass, allows it to react slowly with the copper(II) chloride (CuCl2) surface to produce the two-dimensional Prussian blue analogue (Step 1). Finally, the formation of a continuous film is accomplished by compressing the barriers (Step 2). Although the water-air interface of traditional LS already provides a two-dimensional surface, in order to ensure that truly two-dimensional material is created at the interface we employed a novel technique which we deem an “inverted” Langmuir method. In a classic LS experiment where amphiphilic molecules are used as surface active agents, the packing goes from a two-dimensional gaseous, liquid, and finally solid state as the surface area is decreased. This is accompanied by discrete stepwise pressure rises. In our approach (illustrated in Fig. 1), the barriers are initially closed, providing a restricted area on which the reaction in Eq. (1) can take place. The creation of the material then occurs by gently depositing an aqueous solution of potassium tetracyanoplatinate(II) at the interface between air and a CuCl2 aqueous subphase. As solid PBA is formed at the interface, the barriers are free to move and slowly open to compensate for the increased surface pressure (which results


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from material formation). The film is finally compressed to facilitate continuity in the film. The term “inverted” comes from the fact that the solid state material is formed during the spreading of the reactants as opposed to during compression. The appeal of this approach is that the surface area only changes when necessary to compensate for the addition of newly formed film, which encourages an on-surface reaction over an in-solution reaction. EXPERIMENTAL SECTION Absorption Spectroscopy. Ultraviolet–visible spectra were measured at room temperature with a Shimadzu UV-2600 spectrophotometer. The material was deposited on quartz microscope slides, which were prepared with a chloroform-ethanol treatment for 30 minutes in an ultrasonic bath prior to their use. X-Ray Photoelectron Spectroscopy (XPS). XPS spectra were collected at room temperature with a SSX-100 (Surface Science Instruments) spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and operating at a base pressure of 3×10-10 mbar. The energy resolution was set to ~1.7 eV and the photoelectron take-off angle was 37°. Transmission Electron Microscopy (TEM). TEM measurements were taken at room temperature with a Philips CM12 operating at 80 kV (λ = 0.0418 Å) and at a base pressure of 10-6 mbar. Calibration was performed with a polycrystalline gold sample. The 2D-PBA film was deposited on untreated amorphous silicon nitride (Si3N4) wafers with 5 nm thin windows (Plano GmbH). The background from the silicon nitride window was measured from a substrate without deposition, and was subtracted from the angularly averaged diffraction patterns. The remaining diffuse background due to inelastic scattering was estimated using the Sonneveld-Visser algorithm42 and subtracted. Atomic Force microscopy (AFM) was performed using a Molecular Imaging Pico LE AFM. The instrument was used in tapping mode at a resonance frequency of 372 kHz using


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Tap300Al-G tips (force constant 40 N/m) form budget sensor. The samples investigated by AFM were “single dip” deposited on a silicon substrate. The Langmuir-Schaefer (LS) film formation and isotherm measurements were performed on a Nima Technology Ltd trough (Model 312D). The materials that were used were copper(II) chloride (CuCl2x2H2O) and potassium tetracyanoplatinate(II) hydrate (K2Pt(CN)4xH2O), which

were purchased from Sigma

Aldrich. Ultra-pure demineralized and deionized water (18.2 MΩ·cm) was used for all subphase preparation. Pure ethanol purchased from Sigma-Aldrich was used for the preparation of the tetracyanide solution. Chloroform from Carl Roth Ltd was used for the cleaning of the LS trough and for the glass substrates. All chemicals were used as-received. For the film preparation, the LS trough was cleaned thoroughly using chloroform and filled with the subphase, which would be an aqueous CuCl2 solution. The tetracyanide compound was deposited with an initial concentration of 0.25 mg/ml in a water-ethanol solution (1:3). Ethanol was used to assist the floating of the deposited material, so that it would not sink and react below the surface. Before depositing the tetracyanide, a piece of quartz glass was submerged partially in the trough approximately forming a 50o angle with the horizontal axis. The gentle addition of the tetracyanide compound was facilitated by depositing the material on the inclined glass slide which encourages the reaction to occur slowly (the tetracyanide was deposited on the inclined glass slide at a rate 10 µl/min), which also mediates the formation of a two-dimensional film at the interface. Different deposition rates were investigated varying between 5 µl/min-20 µl/min with 10 µl/min being the optimal condition although no distinguishable effects on the isotherms were observed. The produced films were deposited on silicon nitride windows before and after compression, in order to examine samples with TEM. Films were transferred onto the hydrophobic substrates by horizontal dipping (LS technique). Each time the substrate was lowered into the LS trough and was ACS Paragon Plus Environment

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allowed to touch the air-water interface in a very gentle dip of maximum 0.5 mm below the water level. RESULTS AND DISCUSSION One of the most facile and powerful ways to characterize the air-liquid interface of an LS trough is by isotherm measurements, where the surface tension is monitored while the barriers are compressed at a fixed bath temperature. The presence of any material at the interface and its nature (area on surface per volume of deposited material) will cause an increase in the observed surface tension. Although isotherm curves are often used to identify various phases of the material during compression, such analysis is not applicable to the inverted LS technique. Based on the discussion above, we expect the barrier movement to not induce any further phase transitions (unlike in classic LS), but instead push the patches of solid material together until the formation of a close-packed film.

Figure 2. Isotherm curves of the two-dimensional Prussian blue analogue for different volumes of the on glass-deposited potassium tetracyanoplatinate(II) using the inverted Langmuir-Schaefer technique.


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Fig. 2 displays isotherm curves of the 2D-PBA films on the air-liquid interface. A pressure rise observed as a consequence of the close packing is described above. The area of 300 cm2 corresponds to fully opened barriers whereas 60 cm2 is reached when the barriers are at the closed position. The final pressure reached for fully closed barriers depends on the volume of deposited reactant; for volumes lower than 300 µl (at 0.25 mg/ml) little to no change in surface tension was observed during compression meaning that the total area of created solid material was less than 60 cm2. Larger deposited volumes lead to increased final pressure. A maximum surface pressure is observed at a pressure 40mN/m, and saturation at this pressure implies that an irreversible collapse or overlapping of material has occurred at the interface. Large patches of thin solid material are expected to be formed at the interface (see Fig. 1), as a consequence, the barrier movement does not induce any further phase transition (like in classic LS), but pushes the patch together until formation of a close-packed film. As expected, the larger the deposited volume of the tetracyanide is the bigger (in terms of occupying space) the produced PBA patches will be as a result of the pre-mentioned reaction (this practically means that the number of PBA molecules on the bath is increased with the reactant volume) with the latter leading to higher values of final pressure for a fixed final barrier position. In a control experiment potassium tetracyanoplatinate(II) was deposited on a pure water subphase. No surface tension rise was observed, meaning that one of the reactants alone does not affect the surface properties, and therefore no material can be created at the interface. Only the combination of a tetracyanide with an appropriate subphase can react so as to lead to the creation of material. The synthesized 2D-PBA was transferred horizontally (the LS paradigm) onto various substrates (Langmuir-Schaefer depositions of PBA on Si3N4 for the TEM and SAED, on Boron-doped Si-wafer for the XPS and AFM measurements as well as on quartz glass for the UV-Vis) at pressure 30mN/m for further analysis. In order to confirm the chemical composition of the product floating on the aqueous CuCl2 subphase, X-Ray Photoelectron ACS Paragon Plus Environment

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Spectroscopy (XPS) was performed. XPS is a quantitative elemental analysis of surfaces (probing depth of 1-5 nm), and so is well suited to study the 2D-PBA films. A wide survey of the sample (not shown) revealed the presence of various elements: Si and B from the substrate, and Pt, C, N, and Cu from the 2D-PBA layer. Notably, chlorine and potassium are not observed, which leads to the conclusion that the measured sample is the product of an interfacial reaction and not just precursor compounds floating at the interface. In-depth analyses of the nitrogen, copper and platinum regions were performed in order to address the chemical nature of the bond formed. The nitrogen 1s region (not shown) revealed a single peak at 398.2 eV, which is in accordance with the expected peak position for cyano-type bonds31. Fig. 3 shows a detailed scan of the copper (left) and platinum (right) photoemission peaks. For the platinum, the doublet feature (4f7/2 and 4f5/2) appeared at a binding energy of 72.8 eV and 76.2 eV respectively32. This high binding energy (as compared to Pt(0) testifies for the occurrence of Pt(II). Samples were checked for uniformity in composition analyzing multiple independent regions and such a Pt photoemission peak (as displayed) was observed throughout the sample. On the contrary, the copper content was rather un-uniform and showed large composition variation. Fig. 3 (left) shows the copper 2p3/2 region. One can clearly see several components within the photoemission line, at 932.5 eV, 935.7 eV and above 940 eV. We assert these peaks correspond to Cu(II), Cu(III) and a Cu(II,III) satellite. While the peak at 932.5 eV and its satellite above 940 eV are very common and frequently referred to in the literature and handbook33, XPS investigations of Cu(III) are more rare, although several studies of metal-organics hybrid compounds (often cyanide based) have similarly revealed the presence of Cu(II) without/or rather exclusively Cu(III)34,35. While we have observed clear variation of the Cu(III)/Cu(II) ratio, it seems that compositions with a high Cu(III)/Cu(II) content (case A in Fig. 3) are statistically (over the many samples observed) dominant. This variation in composition will need to be addressed in further studies.


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Figure 3. X-Ray Photoelectron Spectroscopy (XPS) spectrum of the two-dimensional Prussian blue analogue. On the left, the copper 2p3/2 region probed on various single layer samples (labelled A, B and C), testifies for the presence of copper (II,III) with variable intensity ratios. On the right, a typical platinum 4f region for the probed sample, testifies for the presence of Pt(II) . The shoulder at high binding energy is due to the overlap with Cu2p contributions, an aspect which was taken into account in the fitting. The examined sample was prepared using one deposition cycle on boron-doped silicon substrate at 30 mN/m.

A multilayer structure was formed by consecutively depositing monolayers of 2D-PBA on a quartz plate and subsequently analyzed by ultraviolet-visible absorption spectroscopy. Selected absorption spectra for various numbers depositions are shown in Fig. 4. One can see that the mono- and multi-layer samples absorb mostly at the low wavelength side (250 - 400 nm). This behavior is similar to what has been observed for nano-sized PBA assemblies, such as nanoparticules43. The fact that the shape of the spectra is independent of the number of depositions advocates that the monolayers in the stacked structure retain their twodimensional nature. The dependence of the absorbance on the number of depositions of 2DPBA at a certain wavelength is displayed in Fig. 4, revealing a linear relationship. This


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suggests that the same amount of material is transferred after each deposition. Both of these results prove that sequential deposition of 2D-PBA is possible, and offers precise control over the film thickness.

Figure 4. Ultraviolet-visible absorption spectra of multiple depositions of Prussian blue analogue on glass (right) and the dependence of the absorbance (at 250nm) on the number of depositions (left) after 1 (PBA1), 20 (PBA20), 40 (PBA40) and 80 (PBA80) sequential cycles of deposition on glass respectively at 30mN/m.

Figure 5. Selected area electron diffraction patterns of the two-dimensional Prussian blue analogue deposited on a silicon nitride window (one single deposition at 30mN/m) revealing polycrystalline (A) and single crystal (C) domains. The widths of the peaks in the polycrystalline diffraction patterns where analyzed in a Williamson-Hall plot (B) to infer information about structural imperfections in the crystals.


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The morphology and crystallinity of the created material was analyzed using transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A typical SAED pattern of a monolayer of 2D-PBA deposited on a silicon nitride window is shown in Fig. 5 A. This is a representative pattern formed by diffracting from many flakes (such as those shown in the bright-field images in Fig. 6 A). The broad, diffuse rings are due to scattering from the silicon nitride window. The sharpness of the peaks suggests that each flake is crystalline, although with a random azimuthal angle relative to its neighbors, such that a collection of flakes results in a polycrystalline pattern.

The widths of the diffraction peaks contain information about structural imperfections in the crystals. For the majority of materials, the most significant contributing factors are the finite size of the crystalline domains and the strain leading to distortions from a perfect crystal structure. These two effects can be identified by their different dependencies on the scattering angle36,37: the broadening of a diffraction peak due to finite size effects is proportional to

cos ! " , whereas the broadening due to strain is proportional to tan ". Accounting for the contributions &2" =

() * +,- .

from

both

of

these

effects

results

in

the

expression36,37

+ 2/ tan ", where & is the total integral width of a diffraction peak, 0 ≈ 0.94 is

a geometric parameter, 5 is the average “size” of the nanocrystals, and / is the width of the strain distribution. By analyzing the widths of the polycrystalline diffraction peaks with a Williamson-Hall plot38 (Fig. 5 B), this relation takes the form of a straight line, and so the fitting parameters can be used to estimate the size and the strain of the crystals. By doing so it was determined that the strain is 7.5 ± 2.1 %, and the average domain size is 10.5 ± 2.6 nm. It is important to emphasize that the size determined by such an analysis cannot be interpreted as the true size of the flakes, but rather corresponds to an effective coherence length of the crystalline domains. It represents a lower bound on the particle size, as is evident from the fact that it is significantly smaller than the flake size observed in the bright-field images (see ACS Paragon Plus Environment

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Fig. 6 A). This size is similar to what has been measured in three-dimensional PBAs39. The measured strain is quite large relative to traditional PBAs, which is typically around 1 %39. This is likely due to two contributions: first, there may have been some inaccuracy in determining the peak widths because of their weak intensity and the significant background from the silicon nitride window. The second reason is more interesting as it alludes to the structure of the PBAs: while the face-centered cubic structure of three-dimensional PBAs has bonds stabilizing it along the vertical axis, these forces are absent in the proposed twodimensional structure, allowing for out-of-plane buckling and bending. This distorts the perfect crystalline structure, leading to the high strain.

While the previous discussion concerned polycrystalline ring patterns, further measurements were performed to glean information on the crystal structure of the 2D-PBA. Using the smallest diffraction aperture available, SAED patterns were recorded from individual nanocrystals. A representative pattern is shown in Fig. 5 C, displaying evident single-crystal character. These measurements suggest that each of the flakes observed in the bright-field images (Fig. 6 A) are single-crystals of 2D-PBA.


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Figure 6. Brightfield transmission electron microscope images of the two-dimensional Prussian blue analogue deposited (one single deposition at 30mN/m) on a silicon nitride window (A) and the corresponding distribution of flake size (B). In the brightfield images, Prussian blue analogue flakes are the light grey (or darker) regions and the substrate is the light, almost white, background.

Fig. 6 A shows a representative high magnification TEM micrograph of the 2D-PBA. Many flakes are visible with varying sizes and apparent thicknesses. Based on the diffraction data discussed above, it can be concluded that these flakes correspond to nanocrystals of 2DPBA. The distribution of the nanocrystal areas is shown in Fig. 6 B. Although heavily ACS Paragon Plus Environment

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weighted towards smaller crystals, a significant fraction (over 15 %) of the crystals was quite large (over 10,000 nm2 in area). The average area was around 6,500 nm2, while the median was around 2,100 nm2. These roughly correspond to lateral sizes just over 80 nm and 45 nm, respectively, if the nanocrystals are considered to be approximately square. The sizes determined from the bright-field images are consistent with what has been measured in threedimensional PBAs39.

All bright-field images showed relatively low contrast; for instance, the difference in the image intensity between the bare substrate and the substrate containing 2D-PBA was only around 20 %. This result suggests that the 2D-PBA nanocrystals are very thin. In addition, some flakes are significantly darker or lighter than others, indicative of a variation in flake thickness. Because the mechanism behind bright-field imaging is amplitude (diffraction) contrast, this discussion can be quantified by comparing the intensity of the bright-field image of the bare substrate (6789 ) with that of the 2D-PBA film (6:;< ). The ratio of the two is given by:

=>?@ =ABC

= D E>?@F>?@

Eq. (3)

where µPBA and tPBA are the 2D-PBA's linear attenuation coefficient and thickness, respectively. To estimate the thickness of the 2D-PBA layer by using Eq. (3), µPBA must be known since 6:;< and 6789 are determined experimentally from the bright-field TEM images. Calculating µPBA involves the summation of the cross-section from each atom in the unit cell, and so requires some insight into the structure of the 2D-PBA. In light of the arguments based on coordination number restriction and the diffraction data, it will be assumed that the 2DPBA nanocrystals are stacks of m monolayers with a square unit cell of side length a and


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thickness LPBA. The total thickness is then tPBA=mLPBA. This assumption allows for the number of monolayers to be calculated as

G=∑

H

K JK LK

ln N

=ABC

=>?@

O

Eq. (4)

where PQ is the number of atoms of type R in the unit cell, and SQ is the corresponding total scattering cross section, which includes contributions from both elastic and inelastic scattering. In using Eq.(4), the elastic cross sections were calculated using the experimental values for the differential scattering cross sections40, and the inelastic cross sections were calculated using the Wentzel model for the screened atomic potential41.


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Figure 7. The distribution of the contrast ratio between the two-dimensional Prussian blue analogue flakes (one single deposition on Si3N4 substrate at 30mN/m) and the bare substrate (A); using the model outlined in the main text, this can be converted to the distribution of the number of monolayers per flake (B), which shows a heavy bias toward true monolayers (B, inset). Fig. 7 A shows the contrast ratio 6789 − 6:;< ⁄6789 measured for many flakes; the distribution in these values results from the fact that the flakes have varying thicknesses. If we take V ≈ 10 Å, (which is typical for three-dimensional PBAs17 and is expected from the proposed structure) and use Eq. (3), the distribution in number of monolayers shown in Fig. 7 B is obtained. It is evident that the majority of the 2D-PBA flakes are very thin, with over 25 % of the observed flakes consisting of less than five monolayers, and around 20 % of the flakes being true monolayers (see Fig. 7 B, inset). However, the distribution is quite broad, spanning all the way to several hundreds of monolayers in thickness. Each thickness is only populated by a few percent of flakes, but taken together these thick films represent about half of the observed 2D-PBA crystals. The coexistence of true monolayers and thick flakes is puzzling, especially considering that such thickness variation was observed from flakes created from the same LS film. This observation supports the conclusion that the inverted LS method does successfully produce two-dimensional monolayers of PBA, but the deposition process during transfer to the substrate leads to stacking and layering of the film. Additionally, the smoothness of the LS isotherms in Fig. 2 is indicative of the successful formation of a continuous film. Regardless of the breadth of the distribution, it is encouraging that the majority of the PBA flakes are monolayers. This feature, as discussed above, comes from the expected two-dimensional nature of the films. Based on the experimental variability in measuring 6:;< and 6789 , it is estimated that there is a ~40 % uncertainty in the calculation of the number of monolayers. Additionally, it should


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be stressed that these results are rather sensitive to the choice of the unit cell size V. In fact, a 1 % change in V results in a 2 % change in the number of monolayers G. Therefore, because the value a≈10 Å is an estimate based on the traditional three-dimensional structure of PBA, there remains some uncertainty in these results in terms of the absolute value of m. However, this will not affect the determination of the fraction of crystals which are monolayers.

Figure 8. Atomic force microscopy topography (left) and phase (right) image of a single layer 2D-PBA flakes. The dimensions of the images are 1 µm x1 µm. To confirm the in-depth TEM analysis, AFM measurements were performed on compounds deposited on silicon wafer. Overview images (not shown) revealed a flat surface fully covered by flake like objects. From image Fig. 8 (left), which shows a 1x1 micron topology micrograph, one can observe flake like objects, sometimes with an overlayer (top left corner), sometimes with contaminant or aggregates (white object). One can also observe darker regions (Fig 8 left) corresponding to lighter region in the phase image (Fig 8 right). These correspond to areas which are not covered by any 2D-PBA. From these observations and quantitative analysis thereof, the 2D-PBA layer was found to be about 1.5 to 2.5 nm in height. This observation seems to indicate that more than a single layer are transferred upon deposition from the LB trough to the substrate, or that the recompression of the layer before deposition induces multi-layer formation (then transferred as multilayer during deposition). ACS Paragon Plus Environment

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CONCLUSIONS This work bridges the gap between two fields of contemporary materials science research of current great interest: two-dimensional systems and functional coordination compounds. The results presented here demonstrate, as proof-of-principle, that it is possible to synthesize monolayers of two-dimensional PBA at the air-water interface by a novel variation of the LS technique. XPS results demonstrated that a covalently bonded network was created using an on-surface reaction, and the observed product was indeed PBA with Cu(II,III), Pt(II) oxidation states. Absorption measurements were performed during sequential depositions, with a linear increase of the absorption as a function of the thickness observed, demonstrating the viability of multilayer deposition. Finally, TEM and AFM measurements showed crystalline nanometer thick flakes covering the substrate demonstrating the low dimensionality aspects of the formed 2D-PBAs. AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected] Author Contributions ‡These authors contributed equally. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS ACS Paragon Plus Environment

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This work was supported by the Max Planck Society and the Hamburg Centre for Ultrafast Imaging. D.S.B. acknowledges support from the Natural Sciences and Engineering Research Council of Canada. The authors acknowledge Prof. P. Rudolf for the use of the X-ray photoelectron spectroscopy (XPS) as well as the use of Atomic Force Microscopy (AFM) equipment at the Zernike Institute for Advanced Materials, Groningen, The Netherlands. Finally, we acknowledge Maria Katsiaflaka and Gaston Corthey for the scientific support during the experimental procedures and TEM measurements, respectively, as well as Djordje Gitaric for the adjustments and modifications on the Langmuir-Blodgett trough.

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