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PERSPECTIVE

Cite this: DOI: 10.1039/c3dt51636a

Received 20th June 2013,Accepted 16th August 2013

DOI: 10.1039/c3dt51636a

www.rsc.org/dalton

Sequential growth at the sub-10 nm scale of cyanidebridged coordination networks on inorganic surfaces

Simon Tricard,a Fabrice Charrab and Talal Mallah*a

The elaboration of coordination networks’ nano-objects on surfaces can be realized by sequential

growth in solution (SGS). This bottom-up strategy gives the possibility to control the size, the isolation

and the organization of the objects with a precision going up to the molecular scale. Detailed descrip-

tions of the growth of the nickel(II)–iron(II) Prussian blue analog and of the copper–molybdenum

cyanide-bridged coordination network are reported to give insight about the mechanisms of the growth.

Then a comparative XPS analysis has been performed to explain the different reactivity of the precursors

of the growth of the nickel(II)–iron(II) and nickel(II)–chromium(III) Prussian blue analogs. This perspective

article proves that SGS can be optimized for each coordination system to build molecular superstructures

on surfaces, with interesting physical properties towards chemical devices.

Introduction

The use of coordination complexes to assemble three dimen-sional networks in order to elaborate materials with targetedphysical or chemical properties that can provide new functionsis one of the most intensive research fields in molecular chem-istry. One of the motivations during the nineties was thequest for molecular magnets with high critical temperatures.

Cyanide bridged networks turned out to be among the mostfertile systems since, apart from the high Curie temperaturesdiscovered in some Prussian blue analogs,1–3 photomag-netism, spin transition and other physical properties werediscovered.4–12 The next step consisted in processing thesecoordination networks at the nanoscale in order to create thinfilms,13–21 or nanocrystals,22–26 with the objective of usingtheir exceptional properties in devices.

In this perspective, we address the growth of sub-10 nmthin layers and isolated nano-objects of two model bimetalliccyanide-bridged networks, one based on Ni(II)-hexacyano-ferrate(II) (NiFe),17,20 and the other on Cu(II)-octacyanomoly-bdate(IV) (CuMo).21 Their growth mechanisms will be compared

Simon Tricard

Simon Tricard received his PhDin 2009 under the supervision ofTalal Mallah at Université Paris-Sud, France. He then worked fortwo years in the George White-sides group at Harvard Univer-sity, USA. He came back toFrance in 2012 and worked inAzzedine Bousseksou’s team atCNRS, Toulouse, France for oneyear. Since 2013, he has been apostdoctoral fellow at LPCNO, Uni-versité de Toulouse and has beenworking with Bruno Chaudret.

His current subjects of interest are self-assembly, coordinationchemistry, nano-particle synthesis and characterization, chargetransport measurements, and magnetism.

Fabrice Charra

Fabrice Charra conductsresearch in the field of nano-scale control of light-moleculeinteractions at the French agencyfor atomic energy and renewableenergies (CEA). As a directorof research, he heads the Nano-photonics Laboratory since 2002.He has also done research aboutphotonic and electronic pro-cesses in organic materials andtheir device applications, at theCEA division for Advanced Tech-nology (Leti), where he was suc-

cessively a research associate (1990) and the head of thelaboratory for organic devices (1994).

aInstitut de Chimie Moléculaire et des Matériaux d’Orsay, UMR CNRS 8182,

Université Paris-Sud, 91405 Orsay, France. E-mail: talal.mallah@u-psud.frbLaboratory for Organic Electronics and Nanophotonics, SPCSI, IRAMIS, CEA,

91191 Gif-sur-Yvette, France

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to that of Ni(II)-hexacyanochromate(III) (NiCr).19 The networkscontaining chromium and iron possess the face centred cubic(fcc) structure of Prussian blue,27 while the one containingmolybdenum has a tetragonal arrangement.6 Despite the iden-tical structure of the two Prussian blue analogs (NiFe andNiCr), their growth mechanism is completely different and isthought to be controlled by the basicity of the nitrogen atomof the cyanide. For the NiFe network, the growth can be con-trolled and predicted, i.e. the thickness of the nano-layer andthe height of the nano-objects correspond to a molecule-by-molecule growth. This is due to the large stability of the veryfirst objects formed on the substrate, while the less stableNi–NC–Cr(CN)5 bond does not lead to a robust first layer onwhich the growth can be performed. In the case of CuMo, thepresence of the labile Cu(II) ion and the possible formation ofcharged polynuclear species in the water solution are respon-sible for the fragility of the first CuMo layer and thus renderthe growth difficult to control. It is worth noting that the samegrowth behaviour was observed on the Si(100) substrates func-tionalized by an alkyl chain ending up in a mononuclear Ni(II)complex and on the bare platinum surfaces, showing that inthe CuMo case the nature of the surface has little effect on thegrowth process.

In order to perform the growth of the films and the nano-objects, we use the “Sequential Growth in Solution” approach,abbreviated as SGS. It consists of a sequence of coordinationreactions performed on the substrate,28 which can be con-trolled by different parameters as detailed below.

The paper is divided into five sections. The first one isdevoted to the NiFe systems and presents the growth processof bidimensional layers and isolated nano-objects on functio-nalized Si(100) substrate and the growth of the nano-objectson platinum dots pre-organized on SiO2. The second sectionconcerns the CuMo system and its growth on the two types ofsubstrates used for the NiFe network, plus bare platinum. Thethird section discusses the origin of the difference in growth

behaviour between two Prussian blue analogs. The fourthsection presents briefly an approach to grow nano-pillars ongraphite surfaces already “patterned” using molecular self-assembly. And the last section gives a general conclusion.

The nickel–iron network, a model system

We first focused on the nickel–iron Prussian blue analog.Prussian blue analogs have been studied for almost thirtyyears in the bulk phase, and more recently at the nano-scale.22–24,26,29–32 Physical properties of Prussian blue ana-logues – and more generally cyanide-bridged systems – showvarious physical properties, particularly magnetic and photo-magnetic ones.1,2,4–6,8,12 The nickel(II)–iron(II) network weused is paramagnetic; it is a very good candidate as a modelsystem of SGS because its growth is efficient and easy tocharacterize.17,19,20

Prior to SGS on silicon, the substrate was functionalized asfollows:17,33 a Si(100) substrate was hydrogenated by fluoridricacid. We then grafted undecenoic acid by hydrosilylation andperformed a peptidic coupling with the ligand N,N-bis-[(pyridin-2-yl)methyl]propane-1,3-diamine that gave a triden-tate ligand pending on the surface. This ligand was metalizedby nickel by immersion in a solution of nickel chloride.

Control of the SGS

The SGS consists of alternate immersions of a functionalizedsubstrate in Lewis base and Lewis acid solutions, followed by athorough rinsing to remove the excess of molecules. We termstep the sequence of one immersion – either in the Lewis acidor in the Lewis base solution – and one rinsing, and we termcycle the sequence of two steps: the successive grafting of theLewis base and the Lewis acid. The SGS of the nickel–ironnetwork was performed with K4[Fe(CN)]6 and [Ni(H2O)6]Cl2solutions. A functionalization of the silicon substrates wasnecessary to get anchoring points for the SGS; we used NiII

complexes (Fig. 1) and called this pre-SGS step “step Ni0”. Ferro-cyanides could then be grafted on the nickel complexes atthe step Fe1. After rinsing with the solvent of assembly (water),the substrate was immersed in a solution of [Ni(H2O)6]

2+ thatbound to the pending cyanides during the step Ni1. The SGScould be continued to as many cycles as desired.

Before presenting the results for all the steps, it is impor-tant to analyze the nature of the network for the NiFe bilayerwhen the Ni0 layer was immersed for the first time in theK4[Fe(CN)6] solution (step Fe1). Indeed, the nature of this firstbilayer impacts the growth during the subsequent steps. Firstof all, the energy of the asymmetric cyanide vibration wasfound to be equal to 2060 cm−1 by infrared (IR) spectroscopy,shifted towards high energy by about 10 cm−1 in comparisonto [Fe(CN)6]

4− (2050 cm−1), which is a signature of the pres-ence of bridging cyanides and thus of the occurrence of achemical bond between the ferrocyanide molecules and the Nicomplexes of the surface (Fig. 2a). It is interesting to note thatfor the Prussian blue analog Ni2Fe(CN)6 where all the cyanides

Talal Mallah

Talal Mallah was appointed Pro-fessor of Inorganic Chemistry atthe Université Paris Sud in 1997.He is the head of the InorganicChemistry Laboratory since 2005and a member of the InstitutUniversitaire de France (IUF)since 2012. His research, in thearea of molecular magnetism,focuses on the study of the effectof the size, scale and electronicstructure of molecular systemson their physical properties.Talal Mallah and his group use

coordination chemistry, surface chemistry, nano-chemistry and selfassembly as tools for the design of new functional (nano)materials.

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Fig. 1 Principle of sequential growth in solution (SGS); example of the SGS on functionalized silicon.

Fig. 2 Characterization at the step Fe1 of SGS of the nickel–iron network on fully functionalized silicon: (a) IR spectrum of the cyanide stretching band; (b) XPSspectra at the Fe2p3/2 edge (left), at the Ni2p3/2 edge (middle) and at the N1s edge (right; with deconvolution of the signal of the ligand at high energy and of thecyanides at low energy); (c) AFM image, with height profile.

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are bridging, the energy of the asymmetric cyanide vibration is2097 cm−1. In a first approximation where the cyanide band isthe convolution of two components that overlap each other –

one from the bridging and one from the non-bridging cyanide,the distance of the maximum of the peak from the peaks ofthe two reference compounds gives the relative proportion ofbridging and non-bridging cyanides. Here the main peak is at10 cm−1 from [Fe(CN)6]

4− and at 40 cm−1 from Ni2Fe(CN)6; thisobservation leads to the conclusion that mainly less than twoCN molecules out of the six belonging to [Fe(CN)6]

4− are brid-ging. The X-ray Photoemission Spectroscopy (XPS) study on theFe2p and Ni2p edges confirms the presence of ferrocyanideand leads to a Fe : Ni atomic ratio equal to 1.2, which corres-ponds to 1.2 ferrocyanide molecules for each Ni atom(Fig. 2b). At the N1s edge (Fig. 2b), the two peaks at bindingenergies equal to 398.6 and 400.9 eV correspond to the nitro-gen atoms of the cyanide (NCN) and of the organic ligand (NL)respectively. The deconvolution of the spectrum leads to aratio of NCN : NL = 1.23, which corresponds to around 0.8 mole-cule of ferrocyanide for each Ni atom. From these results onecan reasonably deduce that the Fe to Ni ratio n is close to1. The results from XPS and IR studies are consistent with thefact that most of the entities on the substrate correspond toN3Ni–NC–Fe(CN)5 binuclear units and that very few if any poly-nuclear FeNin units are present. Another possibility would be

the formation of extended FenNin entities with a Fe to Ni ratioclose to 1, but this possibility can be excluded on the basis ofthe IR results. In addition, the binding energy at the N1s edgefor the NiFe bilayer is very close to that of [Fe(CN)6]

4− (nobridging cyanides) and is significantly different from that ofK2{Ni3[Fe(CN)6]2} (all cyanides are bridged, see below). Finally,the atomic force microscopy (AFM) investigation with an ultrasharp tip (2 nm) reveals a rather homogenous surface withobjects with an average height of 1.3 nm (Fig. 2c). The formationof a relatively robust molecular bilayer allows growing the NiFenetwork with a good control of the height as developed below. Itis worth noting that, when [Fe(CN)6]

4− is replaced by [Mo(CN)8]4−

or [Cr(CN)6]3− and when exactly the same procedure is used, the

IR and XPS spectra show that no or a few molecules have beenadsorbed on the surface revealing the difference in reactivityand/or in the stability of the entities on the surface as a functionof the nature of the polycyanide molecule used. This aspect willbe presented below.

One advantage of the cyanide-bridged networks is theircharacteristic IR signature of the cyanide stretching vibration.This peak was particularly intense with the nickel(II)–iron(II)network, so that it was easy to follow the evolution of this IRband and prove the efficiency of the SGS (Fig. 3a).20 At the stepFe1, the peak was centered at 2060 cm−1, which is consistentwith the presence of very few bridging cyanides (see above). At

Fig. 3 (a) IR following (IR spectra and peak area at each step) of the cyanide stretching band during the SGS of the nickel–iron network on fully functionalizedsilicon; (b) XPS spectra at the Fe2p3/2 edge for reference compounds and at the steps Fe1, Ni4 and Ni6 of SGS (left), and at the N1s edge for the reference com-pounds, and at the steps Ni0, Fe1, Ni4 and Ni6 of SGS (right).

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the step Ni1 a shift from 2060 cm−1 towards 2069 cm−1

attested to the formation of more cyanide bridges. A globalshift of the peaks towards the high wave numbers during theSGS was coherent with the formation of the nickel–ironnetwork. At the step Ni6, the energy of the band was equal to2096 cm−1, which is the expected value for the bulk compoundNi2Fe(CN)6 (2097 cm−1).20 The formation of the coordinationnetwork during the SGS was confirmed by XPS spectroscopy.First we compared the peak energies at the Fe2p3/2 edgebetween reference compounds (K4[Fe(CN)6] and K2{Ni3-[Fe(CN)6]2}) and the layers formed at different steps of the SGS(Fe1, Ni4 and Ni6) (Fig. 3b). We noted that the peak energy atthe step Fe1 (709.3 eV) corresponded to the one of K4[Fe(CN)6](709.2 eV) – the precision of a peak energy was equal to 0.2 eV.This result is realistic as a single bridging cyanide wasassumed to be formed during the step Fe1, and thus fivepending cyanides remained for each ferrocyanide. The systemat the step Fe1 is thus chemically close to free ferrocyanides. Incontrast, the peak energies at the steps Ni4 and Ni6 wereslightly – but still significantly – lower (708.8 eV and 708.7 eV,respectively) and very close to the peak energy of the K2{Ni3-[Fe(CN)6]2} bulk compound (708.8 eV). The more nickel we addfor one iron, the more the XPS 2p3/2 peak shifts towards thelow energy. As a consequence, the bridging of nickel ions tothe pending cyanides increased the local electronic density onthe iron atoms. The Fe/Ni ratios were equal to 0.7 at the stepsNi4 and Ni6, which is in very good agreement with the ratiocorresponding to the K2{Ni3[Fe(CN)6]2} bulk compound (Fe/Ni= 0.7). One advantage of the cyanide compounds is their signa-ture at the N1s edge. They indeed have a specific signal at lowenergy (<399 eV), which we can easily distinguish from amineor pyridine signals, as for example the ones of the graftingligand, at higher energy (400.5 eV; Fig. 3b). At the N1s edge,the binding energy shifts towards low energy when the nitro-gen atoms are coordinated to Ni from 398.6 eV in K4[Fe(CN)6]to 398.2 eV for K2{Ni3[Fe(CN)6]2}, which surprisingly corres-ponds to an increase of the electronic density on the nitrogenatoms upon its coordination to NiII as observed for the Fe2pbinding energy. The energy of the N1s peak at the step Fe1(398.6 eV) was comparable to the one of the ferrocyanide, andthe energies of the peaks at the steps Ni4 and Ni6 (398.2 eVand 398.1 eV, respectively) were comparable to the ones of thebulk coordination networks. The XPS spectroscopy thus gave asupplementary chemical proof of the formation of the coordi-nation network during the SGS.

The progressive increase of the global intensity of the IRspectra is consistent with a progressive increase of the quantityof matter on the surface. Atomic Force Microscopy (AFM)images proved the uniformity of the SGS, with homogeneoussurfaces after six cycles (Fig. 4).20 The roughness of the surfaceincreased from 0.28 nm before the SGS to 0.74 nm after sixcycles. The increase of the roughness was coherent with thegrafting of the coordination network and the presence ofinherent defects. Nonetheless this roughness is significantlylow and proves that the SGS allows the formation of homo-geneous and flat nanolayers. Upon building up the nanolayer

on the surface, we observed that the roughness after the stepFe1 (Fig. 2c) was larger (1.3 nm) than that of the step Ni6(0.74 nm). This is consistent with the fact that the NiFe binuc-lear units are well separated for the step Fe1 and that thegrowth of these primary nuclei occurs vertically and laterallywith an effect of a flattening of the surface during the firstcycles of SGS. This effect will be more apparent in the study onthe isolated objects as described in the following section. X-rayreflectivity measurements proved that the thickness of thenanolayer deposited during the SGS was equal to 6.7 nm.20

The lattice parameter of the face cubic centered (fcc) structureof the Prussian blue analog is 1 nm; thus one cycle – consist-ing of the addition of one ferrocyanide ion and one nickel(II)ion – would theoretically rise the thickness of the nano-layerfrom 1 nm. Thus the increase of 6.7 nm of the nanolayerduring six steps is in good agreement with a control of the SGSat the molecular scale, i.e. with a quantitative chemisorptionof the molecular components and a negligible physisorptionof interfering molecules, which would indubitably lead toparasite growth and thus to layers with larger thicknesses. Thegrowth of the NiII–FeII(CN)6

4− coordination network can thusbe controlled and more importantly leads to layers with a pre-dictable thickness mainly because the primary nuclei on thesubstrate possess a large thermodynamic stability.

SGS of isolated objects

The formation of robust NiFe binuclear units and the successof the sequential growth of the NiFe coordination networkprompted us to investigate the growth of isolated objectson the functionalized Si substrate. Two challenges wereaddressed. The first is the control of the growth of well-isolatednano-objects with predictable size and shape. The second con-sists of proving the growth mechanism that is responsible forthe smoothing up of the layer during the first steps of SGS.The growth of well isolated objects can be carried out by dilut-ing the anchoring point on the surface.17 To prove thisconcept, we performed exactly the same SGS on a surfacewhere the anchoring groups were diluted by alkyl chains, andobserved the growth of the isolated objects. AFM imagesshowed that the height of these objects was equal to3.6 ± 0.9 nm after 3 cycles and 6.1 ± 1.1 nm after 6 cycles(Fig. 5a), and scanning electron microscopy (SEM) images

Fig. 4 AFM image before (left, roughness: 0.28 nm) and after six cycles (right,roughness: 0.74 nm) of SGS of the nickel–iron network on fully functionalizedsilicon.

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allowed determining their width that was found to be equal to6.4 ± 1.4 nm after 3 cycles and 12.4 ± 1.6 nm after 6 cycles(Fig. 5b).

We propose a geometrical model for the growth of thenano-objects when, for each step, all the physisorbed mole-cules are removed and all the possibilities of chemisorptionare fulfilled (Fig. 5c). The starting point of the growth is an iso-lated anchoring complex on the surface (step Ni0). OctahedralNiII complexes have six coordination positions: for Ni0, threeare occupied by the nitrogen atoms of the grafted ligand andthree by water molecules. Because of the large dilution on thesubstrate, we assume that two Ni complexes cannot be bridgedby a Fe(CN)6 molecule and that only one ferrocyanide moleculecan bind during the first step of the SGS – the remaining posi-tions are likely filled by water molecules, as SGS was not per-formed under anhydrous conditions. As a consequence, thenickel–iron dinuclear complexes at the step Fe1 are well separ-ated on the surface. Then, the system is dipped in a solutionof [Ni(H2O)6]

2+. There is sufficient space for binding five nickelions on the five cyanides that remain free on the ferrocyanidemolecules (step Ni1). The added molecules at the step Fe2 cantake place on the scaffold following the fcc structure of thePrussian-Blue analogues.34 Then, the growth can go on (steps

Ni2, Fe3, Ni3, etc.). Within this growth model, it is easy tocompute that the objects should be 3 nm high and 6 nm wideat the step Ni3 and 6 nm high and 12 nm wide at the step Ni6.The experimental values are in excellent agreement with theproposed model, which validates a clean predictable controlledgrowth. Unfortunately no direct proof of the pyramidal shapewas obtained by microscopy because of the small size of theobjects. Besides, there is no certainty to have a perfect pyrami-dal shape under the present conditions; defects of Fe(CN)6species are indeed probable to preserve the electroneutrality ofthe system. From these results, one can reasonably suggestthat the growth mechanism for the case where the density ofthe anchoring points is maximal (no dilution) proceeds bymerging the pyramidal-like objects during the growth processand thus smoothing up the nanolayer.

Organization of the objects

Another challenging issue for applications is the organizationof the coordination nano-objects. This can be achieved byusing metallic dots as platforms on which the objects aregrown by SGS. We proved this concept with a system of plati-num dots pre-organized in a hexagonal array on SiO2.

21,35,36

The SGS of the nickel–iron network was performed directly onthe platinum dots under the same conditions as previouslydescribed, followed by AFM and SEM (Fig. 6a and 6b). Theheight of the dots increased from 5.9 ± 0.8 nm to 8.9 ± 1.5 nmafter three cycles of growth and their width increased from17.1 ± 2.7 nm to 23.7 ± 3.1 nm. The morphology of the dotschanged drastically after the SGS. We first observed by AFMnew protuberances after the SGS, which corresponded tothe overlap of discrete coordination nano-objects. We thenexplained the more diffuse delimitation of the objects on SEMimages after the SGS by the presence of the isolating coordi-nation network nanolayer on top of the metallic platinumdots. The increase of the height of the objects of 3 nm and oftheir width of 6 nm after three cycles of growth is once againconsistent with a SGS controlled at the molecular scale.Indeed, three cycles of SGS following the crystal lattice of thePrussian blue analog would increase the height of the dots by3 nm and their width twice more (6 nm) if the growth alsooccurred laterally (Fig. 6c). It is worth noting that the SGS ofthe nickel–iron network was effective on the bare metal,without pre-functionalization of the surface. Indeed the ferro-cyanide molecules can coordinate to the atoms of the plati-num surface with their nitrogen atoms; bare platinum canthus be a starting point of the SGS. Our experience proved thatno difference of SGS was observed using either a bare platinumsurface or a surface functionalized by mercapto-pyridine.

In brief, we showed that SGS of the nickel–iron Prussianblue analogue can lead to objects with a defined size, isolationand organization on surfaces. The interpretation of the topo-logical results, with simple geometrical models, is coherentwith a control of the growth at the molecular scale, whichmeans an absence of physisorption and an effective chemi-sorption at every step of the SGS.

Fig. 5 (a) AFM images, with height distributions, and (b) TEM images, withwidth distributions, after three cycles (left) and after six cycles (right) of SGS ofthe nickel–iron network on diluted anchoring points on silicon; (c) geometricalmodel of the growth.

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Copper–molybdenum network, a functionalsystem

The copper–molybdenum cyanide bridged coordinationnetwork shows photo-magnetic properties, i.e. the possibilityto switch its magnetization with light.9 Studies by others andus have shown that this bistable behavior can be preserved atthe nanoscale.24,25,31

A less controlled – but still effective – SGS

When we followed the same procedure as that for the SGS ofthe nickel–iron network with solutions of K4[Mo(CN)8] and[Cu(H2O)6]Cl2, no effective growth was observed (Fig. 7a).21

Two peaks were present, one at 2120 cm−1 corresponding tothe non-bridging cyanides and one at 2165 cm−1 corres-ponding to the bridging cyanides, but no significant increaseof the global intensity of the cyanide peak was observed,proving that no supplementary material was added after thefirst cycle. Yet, we were able to obtain an effective SGS, byrinsing the step Mon with pure methanol instead of thesolvent of assembly (pure water). We then observed a signifi-cant increase of the global intensity of the cyanide peaks ateach cycle and a global shift of the main peak from 2120 cm−1

Fig. 7 IR following (IR spectra and peak area at each step) of the cyanidestretching band during the SGS of the copper–molybdenum network: (a) withregular rinsing on functionalized silicon; (b) with methanol rinsing of the stepMon on functionalized silicon; (c) with methanol rinsing of the step Mon onplatinum; (d) with methanol rinsing of the step Mon on platinum but with aconcentration of the precursors in solution of 10−2 mol L−1 instead of 10−3 molL−1 – we kept the same scale as for (c) to facilitate comparison; (e) IR follow-ing (IR spectra and peak area) of immersions of the surface at the step Cu4 onplatinum in pure water and in solutions with different concentrations of K4Mo-(CN)8.

Fig. 6 (a) AFM images, with height distributions, and (b) TEM images, withwidth distributions, before (left) and after three cycles (right) of SGS of thenickel–iron network on platinum dots; (c) geometrical model of the growth.

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to 2165 cm−1 at the step Cun, depicting the apparition of brid-ging cyanides and thus the formation of the copper–molybdenum coordination network (Fig. 7b). XPS measure-ments confirmed the chemical composition of the copper–molybdenum coordination network on the surfaces.21 Metha-nol is a bad solvent for octacyanomolybdate, so more[Mo(CN)8]

4− molecules remained attached to the surface aftermethanol rinsing than after water rinsing at the step Mon.More molecules are then available to form the coordinationnetwork by reacting with the [Cu(H2O)6]

2+ cation at the stepCun.

The SGS with methanol rinsing at the step Mon was similaron functionalized silicon and on bare platinum, with compar-able infrared signatures of the SGS (Fig. 7c), which exclude therole of the underlying substrate in the growth process. The lowersignal to noise ratio on platinum compared to the experimentson silicon comes from the use of different techniques ofmeasurement. IR experiments were carried out with multiplereflections (∼70) inside the silicon substrate, whereas only onereflection was possible on platinum surfaces. Here too, nofunctionalization of the platinum surface was necessary to getan effective SGS. If we look more closely at the IR spectra fol-lowing of the SGS, we can notice that at the step Mon, theband at 2120 cm−1 – which corresponds to non-bridging cya-nides of the octacyanomolybdate – increased, but at the sametime the band at 2165 cm−1 – which corresponded to the brid-ging cyanides – decreased compared to the step Cu(n − 1).This observation means that a part of the coordinationnetwork formed at the step Cu(n − 1) was removed during thestep Mon, even after a methanol rinsing. When we increasedthe concentration of the precursors in solution from 10−3 molL−1 to 10−2 mol L−1, the SGS was not effective anymore andthe infrared signal remained constant and weak (Fig. 7d). Anincrease in concentration of the precursors in solution is there-fore not reflected by an increase of the amount of materialgrafted on the surface, but rather by a desorption of the mole-cules. Thus, the concentration of the precursors in solutionhas a significant effect and plays a role in the partial removalof the network at the step Mon. We thus focused our attentionon the effect of the [Mo(CN)8]

4− concentration on a pre-formedlayer. We took a substrate at the step Cu4 and immersed it forten minutes in pure water, which is the solvent of assembly ofthe octacyanomolybdate. We did not observed any significantchange in the IR spectrum, confirming that the decrease insignal intensity of the bridging cyanides is not an only solventeffect (Fig. 7e). Then we immersed the substrate in a solutionof K4[Mo(CN)8] at 10

−5 mol L−1, and we rinsed it with metha-nol; we observed a very slight decrease of the bridging cyanidecontribution in the IR spectrum. Then the sample wasimmersed in a solution of K4[Mo(CN)8] at 10−4 mol L−1; aslight decrease of the bridging cyanide band was observed, butthis time accompanied by a slight increase of the non-bridgingcyanide band. Finally, we immersed the substrate in a solutionof K4[Mo(CN)8] at 10

−3 mol L−1, the concentration usually usedfor SGS. In this case, there was a strong decrease of the brid-ging cyanide band, accompanied by a sharp increase of the

peak of the non-bridging cyanides. The spectrum showed thesame shape as the spectra at the steps Mon previouslydescribed. A longer immersion in the solution of K4[Mo(CN)8]at 10−3 mol L−1 did not affect the IR signal. Therefore, theintensity decrease of the bridging cyanide band at the stepMon, which reflects the partial removal of a part of thenetwork formed at the step Cu(n − 1), is due to the presence of[Mo(CN)8]

4− complexes in solution and is especially importantat high concentrations. A possible mechanism consistent withall these observations is a displacement of the equilibriumtowards the formation of a thermodynamically more stablespecies in solution upon the addition of [Mo(CN)8]

4−. Sincethe new species must have Cu–N bonds as the network on thesurface, their larger stability must be of entropic origin, asthey have more degrees of freedom in solution than immobi-lized on the surface. In addition to this entropic effect, acharge effect can enhance the removal of the network if thenew formed species are charged so that they are soluble inwater, while the grafted network is not.

Organization of the objects

The same strategy of SGS on bare platinum dots as with thenickel–iron network was also successful with the copper–molybdenum network (Fig. 8). Indeed, the height of the dotsincreased from 8.5 ± 1.2 nm to 10.6 ± 1.7 nm after three cyclesof SGS and a small roughness appeared on the dots, in accord-ance with the deposition of the copper–molybdenum coordi-nation network. The nanometric dots were clearly distinct andthe organization of the coordination system at the nano-scale,with a preferential deposition on the platinum dots, was con-firmed. The 2.1 nm increase of the height of the dots afterthree cycles of SGS confirmed that the height cannot be pre-dicted, as three cycles of growth under ideal conditions wouldhave led to a height increase of 3 nm – following the formationof the tetragonal structure of the coordination network. None-theless, the SGS of the copper–molybdenum network wasslower under the present conditions than an ideal SGS andthus still allow some control of the size. Actually having anSGS slower than an ideal SGS does not preclude a control ofthe thickness of the layer: to reach a given thickness, we justhave to perform more cycles than with an ideal SGS. However,the prediction of the size of the objects as for the NiFe

Fig. 8 AFM images, with height distributions, before (left) and after threecycles (right) of SGS of the copper–molybdenum network on platinum dots.

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network is not possible, and calibration pre-studies must beperformed.

Different coordination networks with SGSthat behave differently

Nickel–iron and copper–molybdenum networks grow withdifferent efficiency, due to different mechanisms, on inorganicsurfaces. On the one hand, the SGS of the Prussian blueanalog can be controlled at the molecular scale, with the graft-ing of one molecular layer at each step so that the thicknesscan be predicted. On the other hand, the SGS of the copper–molybdenum network is more complex because of the pres-ence of equilibrium between the grafted neutral network andcharged species that are stable in solution, so that the thick-ness cannot be controlled. However, modifying the solubilityof the precursors in the solvent of rinsing allows observing aneffective SGS and thus permits some control on the thicknessof the final layer.

A difference in reactivity was also found to occur for theSGS between two Prussian blue analogs, i.e. possessing thesame structure. The SGS of the nickel(II)–iron(II) network iswell controlled and can be predicted, and gives very uniformand homogeneous nano-layers, while for the nickel(II)–chromium(III) network, isolated objects formed on the surface,with a very low density, and no extension in two dimensionsoccurred.19 As we kept exactly the same operating procedurefor the two studies, the explanation for these different beha-viors should be looked for in the difference in reactivitybetween the ferrocyanide and the chromicyanide. Comparisonof XPS peak energies at the N1s edge of the cyanide nitrogencan give a first insight about this different reactivity (Fig. 9).First, we can notice a slightly higher binding energy forK3[Cr(CN)6] (399.0 eV) compared to K4[Fe(CN)6] (398.6 eV).This difference means that the electronic density is weaker onthe nitrogen atom of the chromicyanide than on the nitrogenatom of the ferrocyanide, and thus that the chromicyanide isa weaker Lewis base than the ferrocyanide. One can, thus,

conclude that the nickel–chromium network is more difficultto form than the nickel–iron network because the Ni–N bondin the former would be weaker than in the latter. This effect isconsistent with the fact that for [Fe(CN)6]

4−, the M → CN back-donation is stronger than for [Cr(CN)6]

3− because of the largernumber of t2g electrons for low spin FeII (6) than for CrIII (3).Thus, even though kinetic effects may play a role, the thermo-dynamic stability of the final network in the case of the Prus-sian blue analogs is a key parameter for the growth. In thecase of the CuMo network, the stability plays a role but thepresence of CuII, which is known to be a highly labile ion, isimportant in the formation of the layer of the surface. It isprobable that this high labile ion together with formation ofthermodynamically stable species in water is responsible forthe decreased stability of the copper–molybdenum nanolayer.

SGS of coordination molecular pillars

One advantage of coordination chemistry is the control of thedimensionality of the assemblies. This control has beencarried out in molecular chemistry by reacting complexes thatplay the role of Lewis acid and base and by controlling thenumber of coordination sites. For example, an octahedralcomplex possessing a tetradentate chelating ligand and twocyanides in trans position as the one depicted in Fig. 10 canreact with a similar complex possessing two ligands in transthat can be substituted by the nitrogen atoms of the cyanideand would, obviously, lead to a one dimensional linear chainstructure.

Towards the miniaturization of the systems up to the mole-cular scale and the elaboration, characterization and utiliz-ation of systems composed of a few molecules, one approachis to perform SGS using well-chosen precursors in order togrow molecular nano-pillars on the substrate (Fig. 11a).However, to do so on a substrate, the surface must be preparedwith molecules adequately organized. One elegant strategy isto pattern the surfaces at the molecular level, i.e. at sizes stillnot accessible by lithographic techniques. This can beachieved by organizing the anchoring groups on a surface atintervals of two to three nanometers using a bidimensionalmolecular self-organized matrix. We have completed the self-assembly at the liquid–solid interface of tristylbene (TBS) –

Fig. 9 XPS spectra at the N1s edge for reference compounds based on ferro-cyanides and chromicyanides.

Fig. 10 View of the structure of [FeIII(Salen)(CN)2]+ (yellow: Fe, black: C, blue:

N, red: O, pink: H).

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functionalized with alkyl chains – on highly ordered pyrolyticgraphite (HOPG).37–39 A honeycomb nanoporous matrixformed, with cavities that can host guest molecules (Fig. 11b).The size of the cavities can be adapted to the size of the guestby modifying the length of the alkyl chains of the TBS mole-cules (from C8 to C12). With this strategy, we were able tofunctionalize surfaces with Lewis bases: pyrenes functiona-lized with a carboxylic acid group were isolated in a matrix ofTBS-C8. For steric reasons, only the pyrene moiety can enterinto the cavity; that is why we guess the carboxylic acid armshould bend and hang over the surface and is thus welloriented to start a SGS. We were also able to trap Lewis acids(FeII phthalocyanines) in a matrix of TBS-C12. SGS of nano-pillars on these systems is currently under investigation.

Conclusion

We showed that the SGS can be a powerful strategy to controlthe deposition of a coordination network on inorganic surfacesfor thicknesses below 10 nm, where a strict control of thegrowth at the molecular level is demanded. This bottom-upstrategy is simple and cheap, and can easily be scaled up tolarge quantities; it is thus a potential tool for nano-fabricationtowards devices: sensors, memories, micro/nano electronics,biological chips, etc.40,41 SGS is versatile and can in principlebe adapted to any coordination network. Optimization mightnonetheless be necessary as the SGS is specific to each system,as we have shown for three cyanide-bridged coordination net-works: the nickel(II)–iron(II) Prussian blue analog SGS was

controlled at the molecular scale, the nickel(II)–chromium(III)Prussian blue analog SGS was less effective and continuousfilms could be obtained only if a layer of NiFe was alreadypresent on the surface, and the copper–molybdenum SGSrequired for adapting the chemical process in order toperform the growth. In summary, even though the predictionof the thickness is possible only for a few systems possessing alarge thermodynamic stability, its control is possible in manysystems once the growth mechanism is well understood.

Acknowledgements

We thank Karim Aissou and Thierry Baron for providing thesurfaces with Pt dots, André-Jean Attias and his team for pro-viding the TBS molecules forming the nanoporous matrix, andSerge Palacin and Frédéric Miserque for XPS studies. Wethank Yousuf Raza and Cédric Renauld for experimentalsupport. We thank the CNRS (Centre National de la RechercheScientifique) and the Nanoscience Competence Center ofRégion Ile-de-France C’Nano IdF.

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Fig. 11 (a) Principle of SGS of molecular pillars (red: Lewis acid; green: Lewis base). (b) STM images of organized anchoring groups (with molecular structures),hosted in a nanoporous molecular matrix of tristylbene (TBS) – the TBS molecules are the molecules with a three-fold symmetry and formed a honeycomb matrix,the anchoring groups are adsorbed in the nanopores of the matrix; left: matrix filled with COOH functionalized pyrene (Lewis base); right: matrix filled with ironphthalocyanine (Lewis acid).

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