Ultrathin Molecular Layer Junctions Based on Cyclometalated

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C: Physical Processes in Nanomaterials and Nanostructures

Ultrathin Molecular Layer Junctions Based on Cyclometalated Ruthenium Complexes Quyen Van Nguyen, Frederic Lafolet, Pascal Martin, and Jean-Christophe Lacroix J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10766 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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The Journal of Physical Chemistry

Ultrathin Molecular Layer Junctions Based on Cyclometalated Ruthenium Complexes Quyen Van Nguyen,a,b Frederic Lafolet*,a Pascal Martina and Jean Christophe Lacroix* a a

Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue JeanAntoine de Baïf, 75205 Paris Cedex 13, France.

b

Department of Advanced Materials Science and Nanotechnology, University of Science and

Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam.

AUTHOR INFORMATION Corresponding Author e-mail: [email protected]. [email protected]

ACS Paragon Plus Environment

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ABSTRACT

A cyclometalated ruthenium complex has been designed to be immobilized on a surface by diazonium electroreduction. Modified surfaces have been obtained and fully characterized by XPS, electrochemistry and AFM. Molecular junctions consisting of Ru(bpy)2(ppy) oligomer using direct top-coat evaporation are presented.

INTRODUCTION Metal complexes that display photophysical and redox properties represent a great interest due to the number of applications in the fields of energy conversion1 and memory devices,2-5 or molecular electronics.6-15 Ruthenium complexes are well known to be very attractive for their tunable electronic and redox properties, especially those containing polypyridinic ligands,9, 12-13 such as [Ru(bpy)3]2+.6 They have been recently incorporated in molecular junctions (MJs) and have shown interesting transport properties ranging from rectification,15-16 light emission6 and memories.17 In the last few years, renewed attention has been devoted to the cyclometalated analogues due to their possible application in dye-sensitized solar cells (DSCs).18 Such ruthenium complexes present appealing characteristics due to the ruthenium-carbon bond, which enhances electronic coupling between the metal and the ligands.19 As a consequence, significant changes in its redox potential are observed compared to [Ru(bpy)3] derivatives. The electrochemical gap is reduced and the HOMO is raised, which may impact transport properties in MJs. In the present work, a new cyclometalated Ru(II) complex, [Ru(bpy)2ppyNH2]+,PF6- , involving an amino-substituted phenyl-pyridine-based ligand has been synthesized. Generation


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of diazonium salts and electrochemical reduction has been used to obtain surfaces modified by the metallic complex. They have been fully characterized by AFM, XPS and electrochemistry. Pinhole-free monolayers and ultrathin multilayers have been easily generated on gold and allow the fabrication of MJs based on the cyclometalated ruthenium complex of various thicknesses (from 2 to 8 nm) using direct top-coat evaporation in high yield. Transport properties of such MJs are compared with those observed in MJs based on [Ru(bpy)3] derivatives.

RESULTS AND DISCUSSION A novel phenylpyridine derivative bearing a phenylamine group was first designed to enable in situ formation of a diazonium cation. The target ligand was prepared in four steps (Scheme 1). In a first step, p-nitrobenzaldehyde is coupled with pyruvic acid to lead to the intermediate enone. N-Phenacylpyridinium iodide is then added in a Hantzsch condensation to yield 2-phenyl-4-(4-nitrophenyl)pyridine-6-carboxylic acid, which is thermally decarboxylated. The

nitro

derivative

is

then

reduced

by

hydrazine

to

give

the

2-phenyl-4-(4-

aminophenyl)pyridine ligand in good yield. Then cis-[Ru(bpy)2Cl2] was reacted with the ligand in the presence of silver hexafluorophosphate (AgPF6) to give the cyclometalated ruthenium complex [Ru(bpy)2ppyNH2]+,PF6-. The 1H NMR spectra of the ligand and the complex are consistent with those reported in the literature.20


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Scheme 1. Synthesis of 2-phenyl-4-(4-aminophenyl)pyridine and [Ru(bpy)2ppyNH2]+ complex. Conditions: (i) Na[MeC(O)CO2], KOH, EtOH-H2O 4:1, 0 °C, 6 h; (ii)NH4OAc (aq.), 100 °C, 6 h; (iii) 280 °C, under vacuum; (iv) hydrazine, EtOH-THF, reflux, 1 h; (v) cis-[Ru(bpy)2Cl2], AgPF6, CH2Cl2, reflux, 2 h. Next, the electrochemical behavior of a millimolar solution of the [Ru(bpy)2ppyNH2]+ complex was studied by cyclic voltammetry (CV) in acetonitrile containing tetra-nbutylammonium hexafluoro-phosphate (TBAPF6, 0.1 M) as supporting electrolyte (Figure 1a). In the anodic part, the CV presents a reversible peak at 0.44 V/SCE attributed to the reversible metal-centered oxidation process and an irreversible peak at 1.10 V/SCE attributed to irreversible amine oxidation. In the cathodic part, the CV presents two reversible peaks at -1.59 V and at -1.84 V/SCE, attributed to the ligand-based reduction processes. Compared to the electrochemical properties of [Ru(bpy)3]2+,6 these results clearly indicate that the HOMO of


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[Ru(bpy)2ppyNH2]+ is 0.6 eV above that of [Ru(bpy)3]2+ while the LUMO is not affected. The diazonium cation was generated in situ by addition of 15 eq. of tert-butyl nitrite following an established procedure.6, 16, 21 Successive cycles of electrochemical reduction between 0 and -0.4 V/SCE were run on the resulting solution. In the first reduction cycle an irreversible peak at ca. 0.25 V/SCE is observed and corresponds to diazonium reduction (see Figure 1b). In the following cycles, the current intensity decreases, which suggests that a partially insulating film is being grafted onto the surface. The electrochemical behavior of the modified electrode was studied by CV (Figure 1a, blue curves). Reversible oxidation of the ruthenium core occurs at E = 0.52 V/SCE with a shape typical of surface-bonded electroactive groups (ΔEp close to zero). The region of negative potential displays two reversible waves corresponding to the reduction of the ligands at E1/2 = -1.47 V/SCE and E1/2 = -1.73 V/SCE. These results clearly confirm that [Ru(bpy)2ppy] moieties can be easily grafted using diazonium electroreduction.

(a)

0

2.5µA

(b)

-2

Current (µA)

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


1µA

-4

10th

-6

-8

-10

1st

-12

-2.1 -1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0

0.3

0.6

0.9

1.2

-0.4

E (V vs. SCE)

Figure 1. (a) Cyclic voltammetry of a [Ru(bpy)2ppyNH2]+

-0.3

-0.2

-0.1

0.0

E vs. SCE (V)

millimolar solution (MeCN,

TBAPF6 0.1 M ) (red curves) and layer grafted on GC electrode (blue curves), scan rate 0.1 V.s-1. (b) Electroreduction of the [Ru(bpy)2ppyN2+]+ diazonium derivative on a carbon electrode in solution of 5 × 10−4 mol·L−1 [Ru(bpy)2ppyNH2]+, scan rate: 0.1 V·s−1.


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X-Ray photoelectron spectroscopy (XPS) was used to characterize the film deposited on gold electrode (Figure 2). The survey spectra show all the expected signals including C, N, P, F and Ru. A strong Au4f signal is also observed which indicates that the grafted film is thinner than the penetration depth of the X-rays (below 20 nm). A characteristic peak at 280 eV can be assigned to Ru(II) 3d5/2. The corresponding Ru(II) 3d3/2 peak at 283.9 eV is masked by the C1s signal (Figure 2b). There is a lower-binding-energy contribution visible at 281.5 eV as a weak peak, compared to the C1s peak, which can be attributed to the ruthenium-bonded carbon atoms.22 On top of the main C1s peak at 284.8 eV, several other signals can be attributed to bipyridine ligand CN bonds at 285.7 eV, CO bonds at 286.9 eV and a –* satellite due to aromatic cores at 292.4 eV. Experimental atomic ratios for C, N and Ru reproduce fairly well the expected composition of the layer. Indeed, experimental C/N, N/Ru, P/Ru and F/P are 5.4, 6, 0.7 and 6 and are in fairly good agreement with theoretical values of 5.4, 5, 1 and 6, respectively (Figure 2d). The N1s signal in the high-resolution spectra (Figure 2c) consists of a single peak at 400.1 eV, assigned to the nitrogen atoms of the bpy and ppy units. This observation is similar to that in [Ru(bpy)3] grafted layers which exhibit a single narrow N1s peak at 400.1 eV.6, 16


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Figure 2. Characterization of [Ru(bpy)2ppy]+,PF6- functionalized gold electrodes: (a) Survey XPS. (b) C1s-Ru3d high-resolution XPS spectra. (c) N1s high-resolution XPS spectra. (d) Atomic ratio calculated from experimental data and theory Gold microelectrodes (ca. 20 μm x 2 cm and 50 nm-thick on Si/SiO2) were functionalized with [Ru(bpy)2ppy]+,PF6- using the same electrografting conditions described above. The presence of the molecular layer was confirmed by AFM (Figure 3). Surface topography reveals a rms roughness of ca. 0.8 nm, of the same order of magnitude as that observed for bare electrodes (ca. 0.6 nm). This shows that the molecular layer is quite densely packed and homogeneous. The thicknesses of the molecular layers were measured using the cross-sections of the modified and


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bare gold steps. An important result is that 2 nm-thick layers were easily and reproducibly deposited. This thickness is close to that of a monolayer, which confirms that steric hindrance in organometallic-based diazoniums promotes monolayer grafting, as observed with other systems.23 Thicker films (8 nm) were also deposited by changing the electrochemical conditions (different scan ranges for film deposition). A histogram of the height data is fitted by two separate Gaussian functions, with the height expressed as the difference between the centers of the two functions and the uncertainty given by the quadrature addition of the two best-fits values (Fig. 3c). It gives an average value of 8.3 nm for the thickness of the active layer.

Figure 3. (a) AFM image of [Ru(bpy)2ppy]+,PF6- functionalized gold electrodes (8nm thickness), (b) cross section of the modified nanostructured electrode corresponding to the thickness measurement, (c) AFM statistical analysis of the thickness, fitted with Gaussian.


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Next, large-area molecular junctions were fabricated. The top-contact electrode was obtained using photo-lithography by direct evaporation of Ti (2 nm) and Au (50 nm) onto organometallic layers of two thicknesses.16, 21, 24 Current density vs. applied voltage (JV) curves are presented in Figure 4. Using 2 nm-thick layers a high yield of operating junctions is reached (80%, 24 out of 30 junctions fabricated) showing that the monolayer is robust enough to enable MJ creation despite the fact that the top electrode is directly e-beam evaporated. The fabrication yield increases to above 90% when the film thickness is 8 nm. Figures 4a and 4b show the average JV curve (on 24 different MJs) and standard deviation for 2 nm and 8 nm-thick [Ru(bpy)2ppy]+,PF6- MJs, respectively. It clearly shows that the 24 devices generated with the same thickness behave in a similar way; this underlines the reproducibility of the fabrication process. High current densities of 0.25 A/cm2 are reached with a small bias of only 1 V in 2 nmthick MJs, and the JV curves are almost symmetrical with no preferred current direction flow at any thickness, despite the use of contacts with different work functions. This is in marked contrast with results obtained with bisthienylbenzene (BTB)-based molecular junctions, naphthalene diimine (NDI) or Ru-NDI based MJs using the same contacts, which show pronounced rectification properties.16,

24

Figure 4c overlays representative JV curves for two

thicknesses of the [Ru(bpy)2ppy]+,PF6- layer. In addition, we observe a strong influence of the thickness on the electrical properties of the MJs. For example, ongoing from 2 to 8 nm, the measured current density at 1 V decreases 50 times. This ‘thickness’ signature is usually observed in MJs.6-7, 21, 25-30


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Figure 4. JV curves of several [Ru(bpy)2ppy]+,PF6- MJs of different thicknesses (a) 2nm; (b) 8nm; (c) Overlay of JV curves of [Ru(bpy)2ppy]+,PF6- ((ii): 8 nm and (iv): 2 nm) and [Ru(bpy)3]2+,2PF6- ((i): 7 nm , (iii): 2 nm) MJs with two different thicknesses; (d) Attenuation plot for [Ru(bpy)2ppy]+,PF6- and [Ru(bpy)3]2+,2PF6- MJs for V = 1V. These results have to be compared with current densities flowing through a [Ru(bpy)3]2+,2PF6based MJs, as depicted in Figure 4c. At 2 nm layer thickness [Ru(bpy)2ppy]+,PF6- MJs and [Ru(bpy)3]2+,2PF6- show similar JV curves and transport does not seem to be influenced by the


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chemical composition of the layer used, suggesting that the HOMO level does not strongly affect transport. On the contrary when the thickness of the film is around 8 nm, [Ru(bpy)2ppy]+,PF6MJs are more conductive (roughly 10 times) than [Ru(bpy)3]2+,2PF6- based MJs, a result that underlines the effect of the lower oxidation redox potential (higher HOMO) of [Ru(bpy)2ppy]+,PF6-. These results are better seen in the plot of Ln J (at 1 V) versus the thickness of the [Ru(bpy)2ppy]+,PF6- and [Ru(bpy)3]2+,2PF6 layers (Figure 4(d)). For [Ru(bpy)3]2+,2PF6- layers, Ln J varies linearly with thickness which suggests that

applies on the studied distance range. From 2 to 4 nm, the attenuation value is ~ 2.1 nm-1 and has to be compared with several previous studies on conjugated molecules,25, 27-29, 31 showing similar attenuation plots with ranging from 2 to 3 nm-1 25, 27-29 and with 2.4 nm-1 obtained in all carbon [Ru(bpy)3]2+,2PF6- MJs.6 Such values generally indicate that transport occurs by direct nonresonant tunneling between the two electrodes which explains that [Ru(bpy)2ppy]+,PF6- and [Ru(bpy)3]2+,2PF6- 2 nm thick MJs show similar JV curves. When thicknesses increase variation of the slopes in the attenuation plot is observed with both compounds. Such change in value, indicates a critical distance where the dominant transport mechanism changes. In most cases above this distance activated redox “hopping”, intrachain electron transfer,6,

28

25, 27,30

field induced ionization associated to

or sequential multistep tunneling28,

30-31

occurs and dominates.

This is clearly observed in [Ru(bpy)3]2+,2PF6- if the layer is thicker than 3.8 nm, where  falls to 0.3 nm-1. It is also clearly observed with [Ru(bpy)2ppy]+,PF6- MJs but the critical distance where transport mechanism changes is lower and close to 2 nm that corresponds to a monolayer. Above this distance,  is also close to 0.3 nm-1 and as a consequence [Ru(bpy)2ppy]+,PF6- MJs are more conductive than [Ru(bpy)3]2+,2PF6- MJs.


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CONCLUSIONS To conclude, a cyclometalated ruthenium complex has been immobilized on a surface by the diazonium electroreduction to form well controlled layers of various thicknesses. Modified surfaces have been fully characterized by XPS, electrochemistry and AFM. Monolayers of Ru(bpy)2(ppy) can be easily grafted on carbon and gold. The chemical composition of the layers is close to that of Ru(bpy)2(ppy) oligomer. Films on gold characterized by cyclic voltammetry show stable redox states, with a redox switch attributed to the ruthenium center occurring at 0.52 V/SCE. Large-area molecular junctions incorporating Ru(bpy)2(ppy) oligomer, have been fabricated in high yield despite the use of direct metal evaporation to generate the top electrode. All MJs exhibit symmetric JV curves, and devices using monolayers show similar current densities to those observed in [Ru(bpy)3]2+,2PF6- MJs. On the contrary, Ru(bpy)2(ppy) devices are significantly more conductive when the layer thicknesses are In between 4 and 8 nm, thanks to a change of transport mechanism occuring at smaller thickness. These results underlines that the molecular signature on transport is best seen above the distance where direct tunneling between the contacts controls transport.

SUPPORTING INFORMATION : Experimental section (Instrumentation, AFM, XPS, Electrochemistry, Synthesis)

ACKNOWLEDGMENT We thank the Centre National de la Recherche Scientifique, and Agence Nationale de la Recherche (ANR-15-CE09 0001-01) for their financial support. We also warmly thank P. Decorse for XPS analyses and discussions. We thank Dr. John Lomas for editing our manuscript.


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 

-1 nm

-2 Ln J (Current Density A.cm )

TOC GRAPHIC

 

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

Page 18 of 18

-2 -4

 nm -1

  

-6 -8

   

0

 n -1 m

2 4 6 8 10 Molecular Thickness (nm)


18

Ultrathin Molecular Layer Junctions Based on Cyclometalated

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