Unidirectional Current in Layered Metal Hexacyanometallate Thin Films: Implication for Alternative Wet-Processed Electronic Materials

Rectifying behavior of alternative electronic materials is demonstrated with layered structures of a crystalline coordination network whose mixed ionic and electronic conductivity can be manipulated by switching the redox state of coordinated transition-metal ions. The coordinated transition-metal ions can convey additional functionality such as (redox)catalysis or electrochromism. In order to obtain rectifying behavior and charge trapping, layered films of such materials are explored. Specifically, layered films of iron hexacyanoruthenate (Fe-HCR) and nickel hexacyanoferrate (Ni-HCF) were formed by the combination of different deposition procedures. They comprise electrodeposition during voltammetric cycles for Fe-HCR and Ni-HCF, layer-by-layer deposition of Ni-HCF without redox chemistry, and drop casting of presynthesized Ni-HCF nanoparticles. The obtained materials were structurally characterized by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy for nanoparticles, and scanning force microscopy (SFM). Voltammetry in 1 mol L–1 KCl and current–voltage curves (I–V curves) recorded between a conductive SFM tip and the back electrode outside of an electrolyte solution demonstrated charge trapping and rectifying behavior based on the different formal potentials of the redox centers in the films.

Ni-HCF was electrochemically deposited during 15 potential cycles in the potential window 0.0 to 0.75 V with a scan rate of v = 40 mV s -1 in an aqueous solution of 1 mmol L -1 NiCl2 + 0.5 mmol L -1 K3[Fe(CN)6] + 500 mmol L -1 KCl (Figure S1b).An increase in peak current in the cyclic voltammograms of Figure S1a-c indicates an incremental growth of the films on the electrode surface in each potential cycle.
The mixed material of Fe-HCR and Ni-HCF was obtained by executing one potential cycle in the range -0.2 to 0.6 V with a scan rate v = 40 mV s -1 in an aqueous solution of 1 mmol L -1 K4[Ru(CN)6] + 1 mmol L -1 FeCl3 + 70 mmol L -1 KCl.The substrate was then removed from the solution, rinsed and transferred to aqueous 1 mmol L -1 NiCl2 + 0.5 mmol L -1 K3[Fe(CN)6] + 500 mmol L - 1 KCl, in which one potential cycle was executed in the range 0.0 to 0.75 V with a scan rate of v = 40 mV s -1 .After emersing and rinsing the sample, the sequence was repeated 14 times.Figure S1c shows the deposition cycles and the incremental growth of the deposited materials.Curves 1 (in blue) are the deposition of Fe-HCR and the curves 2 (in black) are the deposition cycles for Ni-HCF.The formal potential E°' was calculated as the mean of anodic and cathodic peak potentials: For Ni-HCF, the marked peak pair in Figure 4b was selected.According to literature, 1,2 it corresponds to the redox process: KNi II [Fe III (CN)6] + e + K +  K2Ni II [Fe II (CN)6] (S2) The exact peak positions are listed in Tables S1 to S4 Table S1.Anodic and cathodic peak potentials of Fe-HCR (Figure 4a) Peak E / V Epa 0.31 Epc 0.21 Table S2.Anodic and cathodic peak potentials of Ni-HCF (marked in Figure 4b) Peak E / V Epa 0.72 Epc 0.63 Table S3.Anodic and cathodic peak potentials of the layered material with Fe-HCR as inner layer and Ni-HCF as outer layer (Figure 4c) The dependence of the peak current on scan rate v was investigated for Fe-HCR|Ni-HCF layered films (Figure S2).The plot of the peak current vs. the square root of the scan rate is almost linear (Figure S2) indicated the control by a diffusion process (diffusion of charge balancing K + ).The plot of peak potentials vs. scan rate shows a trend for increasing the peak separation with scan rate without changing the overall qualitative picture.
Please note, that the data on Figure S2 were acquired on a different sample and with a different capillary in an in-house built droplet cell than Figure 4c of the main manuscript.
Therefore, the area of the sample wetted by the droplet is different and the voltammetric currents in Figure S2 vs. Figure 4c are not comparable.We also noted that the addressed amount of material may differ even for different landing sites on the same sample and with the same capillary.The different amount of material addressed also causes different resistivity effects in the sample material that causes slight shifts in peak potentials.The Fe 2p3/2 signal can be fitted with a multiplet as theoretically calculated by Gupta and Sen 3 using Hartree-Fock approximation for free ions with the same number of unpaired 3d electrons as in considered emitter atom.This is isolated Fe 2+ for Fe 2+ high-spin and isolated Fe 3+ for Fe 3+ high-spin and Cr 5+ for Fe 3+ low-spin.
The Fe 2p3/2 spectrum of Fe-HCR (Figure 3a) indicates the presence of high-spin Fe 2+ with three components at EB = {709.5,710.8, 711.9} in an intensity ratio of 78 : 100 : 40, a surface peak and satellite peak (assignment in Table S7). 4This assignment is in agreement with previous findings that the N-coordinated transition metal (i.e., Fe in Fe-HCR, Figure 3a) is preferentially in the high-spin configuration, 5,6 while the C-coordinated metal ion is in the lowspin configuration as found for Fe 2+ in Ni-HCF (Figure 3c, e).Since all six Fe 3d electrons are paired, a single Fe 2p3/2 component at EB = 708.5 eV is recorded without multiplet splitting. 7e expected valence state of iron is Fe 2+ in the mixed material because the electrochemical deposition cycle was stopped after reduction (-0.2 V vs Ag|AgCl|3 mol L -1 KCl).Note, that the expected Fe 2+ high-spin from Fe-HCR in Figure 3c is only visible as a small signal.Several reasons might be responsible for this.The current in the electrodeposition experiments indicates that the Ni-HCF films grow more rapidly than the Fe-HCR film.This could lead to rapid coverage of the material so that only a few high-spin Fe 2+ ions are located within the information depth of XPS.Furthermore, there might be a change from high-spin to low-spin configuration in the mixed materials in which the coordination environment is less defined than in a pure material.
The signal for Fe 2+ low-spin of the mixed material is also found for a pure Ni-HCF sample (Figure S5).In addition, the Fe 2p3/2 spectrum in Figure S5 contains a multiplet indicative of additional Fe 3+ low-spin in the Ni-HCF sample.
In the layered material there are two different iron centers, in Ni-HCF and in the covered film of Fe-HCR.The spectrum in (Figure 3e) shows only the signal for Fe in Ni-HCF.This can be concluded from the complete absence of the signal for Ru 3d5/2 at EB = 281.0 in Figure 3f.This conclusion is in agreement with the film thickness of the Ni-HCF film (ca.110 nm), which is greater than the information depth of XPS.The peak at EB = 710.2eV and associated multiplet components in Figure 3e likely originate from Fe 3+ species in the material, which may originate from a partial oxidation during preparation in a lengthy layer-by-layer procedure.Due to the low intensity of the high EB components, a safe distinction between Fe 3+ and high-spin Fe 2+ cannot be made with certainty for this overlaid signal.The Fe 2p3/2 signal in Figure 3e contains a single component from low-spin Fe 2+ (also found in Figure 3c).S5-S7).Two doublets can very clearly be seen in Figure 3d (mixed material) with K 2p3/2 binding energies of 293.0 eV and 293.9 eV.In Figure 3b and 3f only one doublet can be identified clearly although the slightly broader K 2p3/2 peak in Figure 3b (full width at half maximum (FWHM) of 1.49 eV compared to 1.05 eV in Figure 3f) may suggest the existence of two different species, whose photoemission peaks could not be resolved spectroscopically.The value for EB is 293.8

SI-2.2 K 2p
eV in Figure 3f and thus equal within the uncertainty range of the method and also equal to the high binding energy component in Figure 3d.Typical values reported in literature are approximately 293.9 eV. 8,9The second K 2p3/2 signal likely originates from Fe-HCR.The K 2p3/2 binding energy of a K4[Ru II (CN)6] × x H2O is found at 293.00 eV (Figure S4a).A further reason for slightly different binding energies of the charge balancing K + could be different hydration states of the material, especially in the vicinity of defects.Those defects might be much more common in the mixed material.Reference spectra were taken from powder samples of K4[Ru(CN)6] ×•xH2O (Figure S4).The high resolution N 1s spectrum also shows the component C at 402.4 eV in Figure S4b for surface-oxidized CN groups similar to features in Figure S3c and ref 12.The signals located at 397.8 eV can be associated with N-atoms of cyanide coordinated to Ru. 10 The origin of the very small component B is unclear at the moment.
The Ru 3d5/2 XP spectrum in Figure S4a can be fitted with a signal at 281.0 eV.This signal can be attributed to Ru 2+ -species in the material.This value is in good agreement to reference measurements of Cataldi et al. 15 , where the Ru 3d5/2 signal was found at 280.9 eV.configuration and produce a good fit. 3The remaining component at 711.9 eV is a satellite.S-14
Figure S6 shows the XRD patterns of Fe-HCR, Ni-HCF, the layered material and the mixed material.Both, Fe-HCR and Ni-HCF materials are known to have a cubic crystal structure. 16,17e main peaks can be assigned to the (200), ( 220) and ( 400

SI-4 SFM Measurements
SFM images in Figure S7a indicate that Fe-HCR forms a continuous film that is composed of small crystals.The image roughness (root mean-square roughness Rq on 10 µm × 10 µm) is 32 nm (Table 10).The film thickness is about 80 nm.
Figure S7b shows the images of the layered material of Fe-HCR as inner layer and Ni-HCF as outer layer.Ni-HCF grows in larger crystals compared to the Fe-HCR film, the roughness Rq is 21 nm on 10 µm × 10 µm.The film thickness of the layered sample is 190 nm.With the thickness of the layered material and the thickness of the Fe-HCR film, the approximate thickness of Ni-HCF nanoparticles can be determined as 190 nm -80 nm = 110 nm.
Figure S7c shows Ni-HCF nanoparticles on the Fe-HCR film.The analysis of their height as difference between z-coordinate on top of the particle vs the z-coordinate on the Fe-HCR film yields a mean value of 120 nm (N = 12).
SFM images of the mixed sample (Figure S7d) show a rougher film compared to the other samples of Rq = 45 nm on 10 µm × 10 µm (Table S10).The increased roughness can result from alternating electrochemical deposition with different average amounts of Fe-HCR or Ni-HCF being deposited in one cycle.

SI-5 TEM Measurements of Ni-HCF Nanoparticles
Ni-HCF nanoparticles were synthesized according to the procedure of Li et al. 18 Briefly, a 100 mL aqueous solution of 80 mmol L -1 NiCl2 × 6 H2O and an equal volume of 73.5 mmol L -1 K3[Fe(CN)6] were mixed by simultaneously dropwise addition (10 mL h -1 of each solution) to 200 mL of deionized water.After complete addition, the solution was stirred for 18 h.The nanoparticles were centrifuged four times for 15 min at 4200 rpm (Megafuge 16, Thermo Scientific, USA), and stored in aqueous solution in a fridge at 4 °C for further use.The procedure is a controlled precipitation synthesis which results in nanoparticles of cubic shape.
The nanoparticles on a Fe-HCR film serve as model system for the electronically communicating layered material, therefore no capping agent was used in the synthesis of the Ni-HCF nanoparticles.
Transmission electron microscopy (TEM, Zeiss EM 900N, Carl Zeiss AG, Oberkochen, Germany) was performed with an acceleration voltage of 80 kV. 8 µL of nanoparticle suspension was dropcasted on a copper grid with formvar film (300 mesh, Plano GmbH, Wetzlar, Germany).The mean diameter of the nanoparticles was calculated from 469 particles using ImageJ software. 19,20-HCF nanoparticles of cubic shape can be found with most of the nanoparticles having a size of between 170 to 250 nm (Figure S8).The I-V curves were recorded with conductive doped-diamond tips (CDT-FMR, Nano and More, Wetzlar, Germany, tip radius 100-200 nm according to supplier specifications) attached to a SFM stage (Enviroscope, Veeco Instruments Inc., Santa Barbara, CA, USA) controlled by a Nanoscope IIIa controller (Veeco).The application of the potential and the measurement of the current was carried out with the instrument controller.The measured current showed an offset, which was quantified by taking the current reading when the tips were placed on a microscope glass slide.This offset current was subtracted from all I-V curves shown in Figure 5 of the main text.
I-V curves were recorded on different regions of each sample.Figure 5 of the main text shows the individual I-V curves (in gray) and the average curve obtained from each sample as a thick solid line.
Table S11.Number of replicate I-V curves recorded over each sample and shown in Figure 5 of the main text.

SI-6.2 Details of I-V Curves
The overlayed curves in Figure 5    The currents in the I-V curves of a layer of Fe-HCR (Figure 5a of the main manuscript) are significantly lower than those of the layered Fe-HCR|Ni-HCR film (Figure 5d) although the same tips and experimental parameters were used.We assume that that this is a result of the different film morphologies.Electrodeposited Fe-HCR has a pronounced granular structure that is shown in Figure 2a.This structure is typical for electrodeposited films.The granular structure makes it likely, that the current is conducted through a limited number of grains as schematically depicted in Figure 11a, because gaps between grains and grain boundaries are likely to have a much higher resistance than the material itself.

S-22
In contrast, the Ni-HCF layer in the layered structure Fe-HCR|Ni-HCF was obtained by the layer-by-layer procedure and has a distinctly different morphology (Figure 2b).This may be caused by the different growth mechanisms.The smoother appearance of the film implies less grain boundaries.Usually the surface roughness decreases compared to the underlaying Fe-HCR film.As such the Ni-HCF may contact several grains of the underlaying Fe-HCR film and enables the distribution over the current over more grains than can be assessed by a direct contact of the conducting SFM tip (Figure 11b).Before assembly of the spectroelectrochemical cell, the prism was rinsed with water, ethanol and cleaned for 10 min in an ozone chamber (Bioforce Nanosciences, Ames, USA).The cell was filled with 1 mol L -1 KCl electrolyte solution and purged with Ar for 30 min to remove dissolved oxygen.In each experiment positive and negative going potential scans were recorded.At selected potential, the scan was interrupted, and 100 spectra were accumulated and averaged.

SI-7.2 Fe-HCR
Spectroelectrochemical PM IRRAS data for a thin Fe-HCR film are shown in Figure S12.
Fe-HCR is a solid solution of the 'soluble' and 'insoluble' structure with vacancies on the M 2 site, 21 they can cause different signals in PM IRRA spectra even for one material. 22In the PM IRRA spectra are dominated by a strong v(C≡N) absorption mode at 2080 cm -1 .This mode disappears in the potential range of 0.38 V -0.65 V in the forward scan and 0.65 -0.35 V in the back scan.Instead, a new v(C≡N) absorption mode appears at 2106 cm -1 .In accordance with literature of other metal hexacyanometallates with redox reactions at high-spin iron, the IR absorption mode at around 2080 cm -1 can be assigned to Fe 2+ -N≡C-Ru 2+ and the IR absorption mode at 2106 cm -1 can be attributed to Fe 3+ -N≡C-Ru 2+ . 14The PM IRRA spectra of the oxidized material in Figure S12 still contain a shoulder at wavenumbers attributed to the reduced form.The position of this signal was found as local minimum of the second derivative of the spectra at 2042 cm -1 and 2056 cm -1 (Figure S13a).The vibration mode can be attributed to defects in the structure, like vacancies, changes in binding motif to Ru-N≡C-Fe, but also terminal cyanides. 23,22e assignment of the signals for the oxidized and reduced form of Fe-HCR are compiled in Table S12.Additionally, a new, rather weak absorption mode appears at 2163 cm -1 at potentials of  E > 0.55 V.In accordance with literature, the strong IR absorption mode at around 2100 cm -1 can be assigned to Ni 2+ -N≡C-Fe 2+ .4][25] Beside the main features, the second derivative of the spectra (not shown) revealed further, very weak, absorption modes at 2050 cm -1 , 2065 cm -1 , 2125 cm -1 and 2140 cm -1 , which can be attributed to defects in the structure and / or also terminal cyanides. 23,22During the potential change, the intensities of the absorption modes at 2050 cm -1 and 2125 cm -1 change concomitantly.The same is observed for the modes at 2065 cm -1 and 2140 cm -1 .This allows their assignment to the oxidized and reduced forms in Table S13.In the negatively going potential scan, the absorption mode at around 2163 cm -1 assigned to Fe 3+ -C≡N-Ni 2+ disappears at potentials of E < 0.55 V.   HCl in a potential window of 0 to 1.2 V at v = 50 mV s -1 (Figure S17).Electrodeposited films of Zn-HCF does not show a uniform film morphology (Figure S18).

S-26
As depicted in Figure S17, the electrodeposition of Zn-HCF necessitates higher potentials than compatible with aqueous electrolyte solutions and gold electrodes.Au electrodes are preferred here because they are more suitable for subsequent XPS characterization.Electrodes from thin conducting oxides (TCO) may be suitable for the electrodeposition and electrochemical characterization of Zn-HCF, but the Sn 3p photoemission signal overlaps with the Fe 2p signals and makes a detailed XP analysis impossible.Furthermore, the oxide component of TCOs also with the O 1s signal in XPS.

Figure S1 .
Figure S1.Electrochemical deposition of a) Fe-HCR; b) Ni-HCF; c) and d) mixed material, (c) selected cycles of Fe-HCR deposition and (d) selected cycles of Ni-HCF deposition on ITO.Scan rate is 40 mV s -1 .The numbers indicate the potential cycle.The first and last potential cycles are indicated in color.
) reflections of the materials.It is noteworthy to mention that ITO patterns overlay the materials patterns, even though the intensities are negligible in the (200) and (220) crystal plane regions.Patterns from Fe-HCR occur at lower angles compared to those of Ni-HCF due to the slightly larger unit cell of Fe-HCR.

Figure S6 .
Figure S6.Grazing Incidence X-ray diffraction (GI-XRD) patterns of a) ITO; b) Fe-HCR thin film; c) Ni-HCF nanoparticles; d) layered material of Fe-HCR and Ni-HCF; e) mixed material of Fe-HCR and Ni-HCF.Symbols indicate position of reflections according to literature.
of the main manuscript demonstrate the variability of I-V curves at different locations of the sample, but do not show characteristic features of individual curves with sufficient details.Therefore, representative curves from Figure 5c (mixed material) and Figure 5d (layered material) are shown below with expanded scale in Figure S9 and S10, respectively.The curves from the layered material in Figure S10 have similar shapes.The specific currents vary as expected from slightly different contact areas of the probe.The examples from the mixed materials in Figure S9 show also qualitative differences.Curves 1 and 2 in Figure S9a, b are close to the behavior of the layered material in Figure S10.In contrast, the curves 4 and especially curve 3 are more resembling an ohmic behavior.

Figure
Figure S9.a)-d) Selected I-V curves of the mixed material from Figure 5c of the main text.The numbers of the curves correspond to those in Figure 5c.

Figure
Figure S10.a)-d) Selected I-V curves of the layered material from Figure 5d of the main text.The numbers of the curves correspond to those in Figure 5d.

Figure S11 . 24 SI- 7
Figure S11.Schematic illustration of possible current path in I-V measurements in a) electrodeposited Fe-HCR and b) Ni-HCF layer prepared by layer-by-layer deposition on top of Fe-HCR.

Figure S12 .
Figure S12.PM IRRA spectra in 2225-2000 cm -1 region of Fe-HCR in positively and negatively potential scans.

Figure S13 .
Figure S13.Second derivative of the reduced and oxidized state of the PMIRRA spectra of a) Fe-HCR, b) Ni-HCF and c) the Fe-HCR | Ni-HCF layered material.

14 a
v.w.-very weak; v.s.-very strong SI-7.3Ni-HCF Spectroelectrochemical PM IRRAS data of a thin Ni-HCF film are shown in Figure S14.Electrochemical responses of the pure Ni-HCF thin film (CVs in Figure 4b and 4e of the main manuscript) indicate the presence of non-stoichiometric forms.The defects associated with the non-stoichiometric structure (solid solution of K2Ni[Fe(CN)6] and KNi1.5[Fe(CN)6]) may have an influence on the PM IRRA spectra.In the PM IRRA spectra a strong v(C≡N) absorption mode at 2100 cm -1 is present.It disappears completely in the potential range of E > 0.73 V.

Figure S14 .
Figure S14.PM IRRA spectra in 2225-2000 cm -1 region of Ni-HCF in positively and negatively potential scans.

Figure S15 .
Figure S15.PM IRRA spectra in 2225-2000 cm -1 region of Fe-HCR|Ni-HCF layered material in positive and negative potential scans.

Table S4 .
Anodic and cathodic peak potentials of the mixed material of Fe-HCR and Ni-HCF (Figure4d)

Table S6 .
Fitted peaks of high-resolution XP spectra of mixed material.

Table S7 .
Fitted peaks of high resolution XP spectra of Fe-HCR SI-2.4 Reference Spectra of K4[Ru(CN)6]

Table S8 .
3,7ted Reference Spectra of Ni-HCF Nanoparticles Reference spectra were taken from Ni-HCF nanoparticles.A drop of the nanoparticle suspension was applied to a gold substrate and allowed to dry under an Ar stream.The sample was then transferred to the ultrahigh vacuum and characterized by XPS.The binding energies of the components are listed in Tables S9.Of particular interest is the multiplet splitting on the Fe 2p3/2 spectrum of Ni-HCF nanoparticles in FigureS5.The signal at 708.7 eV can be assigned to low-spin Fe 2+ .The remaining signal components belong to a multiplet that is formed for photoemission of open shell transition metal ions, here either low-spin Fe 3+ or high-spin Fe 2+ .3,7Adistinction can be made by the analysis of the intensity ratios within the multiplet.The signals at 710.0 eV (100% intensity), 710.6 eV (58% intensity) and 711.5 eV (16% intensity) belong to low-spin Fe 3+ peaks of high resolution XP spectra of K4[Ru(CN)6]  xH2O.EB[eV]

Table S12 .
Wavenumber , intensity and assignment of the v(C≡N) stretching mode in Fe-HCR thin films deposited on a Au electrode surface.

Table S13 .
Wavenumber, intensity and assignment of the v(C≡N) stretching mode in Ni-HCF thin films deposited on a Au electrode surface.