Metal Interface

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C: Energy Conversion and Storage; Energy and Charge Transport

Controlling the Electronic Contact at the Terpyridine/Metal Interface Max Mennicken, Sophia Katharina Peter, Corinna Kaulen, Ulrich Simon, and Silvia Karthaeuser J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05865 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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The Journal of Physical Chemistry

Controlling the Electronic Contact at the Terpyridine/Metal Interface Max Mennicken1,2, Sophia Katharina Peter3, Corinna Kaulen3,4*, Ulrich Simon3, Silvia Karthäuser1* 1Peter

Grünberg Institut (PGI-7) and JARA-FIT, Forschungszentrum Jülich GmbH, Jülich 52425,

Germany. *E-Mail: [email protected] 2RWTH 31.

Aachen University, Aachen 52062, Germany

JARA – FIT and 2. Institute of Inorganic Chemistry, RWTH Aachen University, Aachen 52074,

Germany 4Faculty

of Medical Engineering and Applied Mathematics, FH Aachen, University of Applied Science,

52428 Jülich, Germany *E-Mail: [email protected]

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1. Abstract Terpyridine derivatives reveal a rich coordination chemistry and are frequently used to construct reliable metallo-supramolecular wires which are promising candidates for optoelectronic or nanoelectronic devices. Here, we examine especially the terpyridine/electrode interface which is a critical point in these organic/inorganic hybrid architectures and of utmost importance with respect to the device performance. We use the approach to assemble nanodevices by immobilization of single terpyridine-functionalized gold nanoparticles with a diameter of 13 nm in between nanoelectrodes with a separation of about 10 nm. Conductance measurements on the formed double barrier tunnel junctions reveal several discrete conductance values in the range of 10-9 S to 10-7 S. They can be attributed to distinct terpyridine/electrode contact geometries by comparison with conductance values estimated based on the Landauer formula. We could clearly deduce that the respective terpyridine/metal contact determines the length of the tunneling path through the molecule and thus, the measured device conductance. Furthermore, the formation of a distinct terpyridine/electrode contact geometry correlates with the chemical pretreatment of the terpyridine ligand shell of the gold nanoparticles with an alkaline solution. By applying infrared reflection absorption spectroscopy (IRRAS) we found that only a chemical treatment with concentrated ammonia solution results in an effective deprotonation of the terpyridine anchor group. This enables the electrical contact to the middle pyridyl ring and thus, a short tunneling path through the molecule corresponding to a high conductance value. These findings indicate a way to control the contact geometry at the terpyridine/metal interface which is a prerequisite for reliable nanodevices based on this class of molecules.


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2. Introduction In the last decade, metallo-supramolecular wires attracted considerable interest as promising materials with a variety of possible applications in the fields of nanoelectronics,1 luminochromic materials,2,3 biotechnologies,4 catalysis,5 or solar energy conversion.6,7 They can be synthesized with different transition metal centers and versatile chelating ligands in order to fine tune their molecular electronic properties and to design distinct functionalities. Thus, highly conductive, long molecular wires which are able to bridge large-gap electrodes,8 redox-switches suitable as memory devices,9 and tunable rectifiers10 are available. However, the integration of these functional metallo-supramolecular wires, that is, to build up a solid state device by self-assembly, remains a challenge. Usually, the self-assembly of metallo-supramolecular wires on metallic electrodes starts with the chemisorption of an anchor molecule equipped with a metal-binding site. Subsequently, transition metal ions and bridging ligands are alternately provided to enable wire growth. Among different bridging ligands, ligands with one or more 2,2’:6’,2”-terpyridine (TP) motives have proven particularly popular since they form reliable metallo-supramolecular wires.10-12 TPs are easily accessed synthetically and have a rich coordination chemistry.13,14 High binding constants to different transition metals make them especially useful for the incorporation of multiple metal centers into molecular wires. Using TP derivatives as bridging ligands, the last step of the metallo-supramolecular wire assembly between two nanoelectrodes is often the formation of the electrical contact between one TP group and the counter electrode. To-date, this step is not under control. In fact, very recently it has been reported that a bis-TP derivative (1,4-bis(3,2’:6’,3”-terpyridine-4’yl)benzene) can contact two electrodes theoretically in 61 possible single and multi-terminal configurations through its 6 pyridyl groups.15 Single molecule conductance measurements on this bis-TP derivative using the STM break junction method revealed four distinct conductance states in the range of 10-2 Go to 10-7 Go depending on the length of the resulting conduction path through the molecule. Consequently, the undefined contact geometry at the end of a TP based metallo-supramolecular wire might cover desired effects of functional metallic complexes within the wire. 3 ACS Paragon Plus Environment


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This finding on TP based molecules is in line with other results obtained recently on a quaterthiophene based molecular wire junction16 and on an OPE derivative with a central pyrimidine ring,17 respectively. In both cases, the same approach employing the STM break junction technique was used and an electrode contact to different anchors within one molecule was possible. Thus, distinct single molecule conductance states were obtained which could be attributed to the corresponding effective tunneling length through the respective molecule. These results strongly suggest that a TP group with three potential anchors, that is, the three pyridyl groups, is also a source of inexactness with respect to a defined electronic contact between molecule and electrode. Since the electrical characteristics of organic/inorganic hybrid architectures and especially molecular electronic devices are strongly affected by the metal/molecule interface, we focus here on this point. Our approach is to build up nanoscale devices using ligand stabilized gold nanoparticles (AuNPs) assembled between metallic nanoelectrodes. Nanoelectronic devices based on selfassembled arrays of AuNPs functionalized with organic ligands have been successfully employed to develop light-switching devices with memristive properties, diodes, sensors, or resistors.18,19 However, we use the concept to immobilize a single AuNP with a size of about 13 nm between metallic nanoelectrodes and determine the conductance of the resulting device, which is well-proven to reflect the electrical properties of the molecular shell and the metallic contact in a device geometry.20-22 Here, we investigate AuNPs that are functionalized with 4’-(4-mercaptophenyl)-2,2’:6’2”-terpyridin (MPTP), that is, the TP group is used to form the electrical contact to the nanoelectrodes. With this set-up we aim to reveal the role of the three potential anchors, the pyridyl groups forming the TP moiety, and show the impact of the respectively formed anchor group/metal contact on the device conductance. In order to obtain the required MPTP-AuNP with a size of approx. 13 nm we developed a synthesis route starting from citrate stabilized AuNP. Estimations of the tunneling current through nanodevices consisting of a single MPTP-AuNP in between nanoelectrodes are performed based on the Landauer formula and compared with the experimental findings. We will show that different electrical pathways through the MPTP ligand and 4 ACS Paragon Plus Environment

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thus, different combinations of anchor points are disclosed by conductance measurements. Furthermore, distinct electrical pathways can be predefined by a suitable chemical pretreatment of the TP group.

3. Experimental Methods 3.1. Materials. The following chemicals were purchased from Sigma-Aldrich Chemie GmbH and used as received: hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O), trisodium citrate dihydrate (C6H5Na3O7 ·2H2O), methylthiobenzaldehyde (C8H8OS), 2-acetylpyridine (C7H7NO), sodium, potassium hydroxide, propanethiol. Sodium carbonate anhydrous (Na2CO3) was purchased from Fluka, Sodium hydrogencarbonate (NaHCO3) from Grüssing, ammonia solution (25%) from Th. Geyer and used as received. All glassware was cleaned with aqua regia and rinsed with copious amount of water prior to use. Ultrapure water with a conductivity 10 TΩ (corresponding to a conductance of < 0.1 pS) were accepted for further experiments. Thereafter, single MPTP-AuNPs were immobilized between the nanoelectrodes to build functional devices. We obtained 16 functional single MPTP-AuNP nanometer sized devices from 8 samples, each with 12 nanoelectrode gaps. This corresponds to a yield of 6 % with respect to 264 investigated nanoelectrode gaps. The discarded 12 ACS Paragon Plus Environment

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devices were either not filled with a single MPTP-AuNP, destroyed by electrostatic discharges or showed too large gap sizes. After immobilization, the functional devices were subjected to a chemical treatment with an alkaline buffer solution in order to remove protons possibly ligated to the terpyridine group of the MPTP ligand due to the synthesis in acidic solution. More precisely, 12 devices were treated with a carbonate buffer solution (pH = 9) while 4 devices were subjected to a more intense chemical treatment with a concentrated ammonia solution (pH = 12). In Figure 4e an example of a device consisting of a single MPTP-AuNP immobilized between an AuPd and a Pt nanoelectrode and treated with the carbonate buffer solution is shown.

Figure 4. Nanoelectrode device fabrication (all scale bars = 20 nm). a) and b) SEM top and side view of a nanoelectrode pair before stripping the Al2O3 hard mask. c) and d) SEM top and side view of a nanoelectrode pair after stripping the Al2O3 hard mask. e) A single MPTP-AuNP immobilized between an AuPd (darker grey) and a Pt nanoelectrode (lighter grey) indicated by a yellow arrow. 4.3. Conductance Measurements on Single MPTP-AuNP Devices. Each functional single MPTPAuNP device was electrically characterized by current-voltage sweeps in the voltage range between 1.2 and +1.2 V, usually for 10 h - 30 h resulting in 100 - 300 I-V cycles. Figure 5 shows the resulting I-V curves of the devices treated with the carbonate buffer (CB-dev) or the ammonia solution (A-dev), 13 ACS Paragon Plus Environment


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respectively. The data are displayed in a heat map and in a corresponding histogram of the current values at ±1 V. The heat map of the CB-dev (Figure 5a) shows two clearly distinguishable accumulations of I-V curves, which are transformed into peaks in the histogram. The respective values for the conductivity (at ±1 V) are derived from peak fits: G1 = 1 nS and G2 = 2.2 nS. Thus, two well defined conductivity values are identified for a single MPTP-AuNP treated with carbonate buffer solution. This property has been observed repeatedly in the course of the experiments and will be discussed later in detail. Figure 5b shows the data of a device treated with the ammonia solution. The conductivity of this A-dev is determined to G = 47 nS and thus, it is considerably higher than in the case of the CB-dev. This is not an accidental finding but rather systematic, as verified by the plot of the conductivities of all studied single MPTP-AuNP devices. In Figure 6 the conductivities of the CB-dev and the A-dev at ±1 V are given in ascending order, respectively. While the conductivity values of the CB-dev accumulate around 1.4 nS (median), the A-dev mostly exhibit values around 34 nS (median). This differing electronic behavior can be clearly traced back to the treatment with the particular buffer solution which obviously affects the conductivity through the single MPTP-AuNP device. At this point it is worth to mention that we can exclude ligand desorption or exchange reactions leading to an alteration of the device conductivity.


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Figure 5. I-V-measurements through single MPTP-AuNP devices displayed as heat maps (x-bin = 10 mV, y-bin = 0.04 log(I)) and corresponding current histograms extracted at ±1V. a) CB-dev b) A-dev.

Figure 6. Conductance values of single MPTP-AuNP devices in ascending order. The reported values represent averages of 100 - 300 I-V curves taken from independently prepared devices. a) CB-dev, the green bar corresponds to the conductance range 1 nS to 13 nS. b) A-dev, the blue bar corresponds to the conductance range 17 nS to 70 nS. The TP group of our MPTP ligand provides three potential pyridyl (Pyr) anchor groups and each of them can form individually an electrical contact to the electrode. Like shown recently,15 it is reasonable that such a scenario leads to a variety of contact configurations at the TP-electrode interface (Figure 7). Each of the possible contact geometries, involving one to three Pyr anchor groups, will lead to a distinct conductance state of the device depending on the length of the resulting tunneling path through the MPTP. In our special case, we have an immobilized MPTP-AuNP between two nanoelectrodes and look at it from the perspective of possible electrical pathways. In general, our system corresponds to a double barrier tunnel junction, that is, AuPd-MPTP1/AuNP/MPTP2-Pt, and we have to consider tunneling through the MPTP ligand shell on both sides of the AuNP (Figure 7a). While the sulfur group of MPTP forms a very stable chemical bond with the AuNP, the terpyridine group is located at the outer face of the ligand shell and thus, a suitable anchor group to form an electrode contact (see Figure 7b).

4.4. Comparison of the Experimental Device Conductance with Conductance Estimations. In order to evaluate the influence of the length of the tunneling path upon the device conductance we make a 15 ACS Paragon Plus Environment


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rough estimate of the tunneling current based on the single channel Landauer formula and suppose that the device geometry is represented by the scheme given in Figure 7. This includes that we assume only one tunneling path, that is, tunneling through one MPTP on each side of the AuNP. In principle, more molecules could be involved. However, the number is very limited, since the curvature of the AuNP has to be considered and the anchor group needs to approach the electrode to form a reliable electrical contact. In addition, a vacuum gap of 0.1 nm connected in series to the molecule reduces the conductance by a factor of 10. Taking this into consideration, former estimations for a device based on mercaptophenylamine capped nanoparticles (which have a considerably smaller footprint than MPTP) lead to a maximum conductance which corresponds to the conductance of 9 molecules.21 However, the so far experimentally observed conductances of single AuNP-devices can be correlated best to the single channel values. 20-22 Applying the single channel Landauer formula the conductance GMol through a molecule contacted to two electrodes can be described by:15,22,36 GMol = G0*exp(-ßd)*TL*TR

(1)

Here, G0 = 77.5 µS is the quantum conductance,  the decay constant of the molecule, d the length of the tunneling path, and TL and TR are the transmission coefficients of the molecule-metal interface to the left and right electrode, respectively. We apply the following values deduced from experimental single molecule conductivities of TP derivatives given in literature: TAu-SPhen = 0.4, TAu-Pyr = TPt-Pyr = 0.37, and  = 6.9 nm-1.15,37 The transmission coefficients correspond to the binding situation between MPTP and the respective metals and the unusual high decay constant for an aromatic compound reflects the tilt between the phenyl and Pyr group characteristic for the configuration of MPTP. In consequence, this tilt reduces the electron transport along the molecular backbone. If bridging MPTP exhibit other configurations (increased tilt or no tilt), this can lead to considerably changed transport properties. Since the values of the transmission coefficients, corresponding to the Pyr group in contact with the different metals used in our device, are comparable, no differences in the conductance through MPTP is expected due to the electrode material on the left or right side of 16 ACS Paragon Plus Environment

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the device. However, due to varying contact geometries involving different numbers and positions of Pyr groups (Figure 7) the length of the tunneling path can vary considerably. Thus, the conductance through the double barrier tunnel junction has to be calculated with respect to the distinct tunneling length through MPTP1 and MPTP2 on both sides of the AuNP, d1 and d2, as follows: 1/Gdev = 1/GMPTP1 + 1/GMPTP2

(2)

Figure 7. a) Schematic displaying an example of a possible MPTP-AuNP device structure forming a double barrier tunnel junction, AuPd-MPTP1/AuNP/MPTP2-Pt with d1, d2 ϵ {dout, dout2, dmid}. b) Possible contact configurations at the TP-electrode interface and resulting tunneling path, d. From left to right: contact with one outer (dout), two outer (dout2) or three (two outer and the middle  dmid) Pyr anchor groups indicated by green or blue crosses. The length of the tunneling path through MPTP amounts to dout = 1.165 nm, if only one outer Pyr forms an electrode contact, and reduces to dmid = 0.74 nm, if the middle Pyr binds also to the electrode. In the latter case a significant increase in conductance through the MPTP molecule results according to equation (1), GMPTPout = 3.7 nS and GMPTPmid = 70 nS. Remarkably, the increase in molecular conductance is not a result of the stronger electrode contact, three Pyr groups instead of only one, but a result of the exponential dependence of the conductance on the reduced tunneling length. In principle all possible combinations of contact geometries might appear, for example TP1-AuPd interface contact with both outer Pyr groups (dout2) and TP2-Pt interface contact with one Pyr group (dout). This combination would result in Gdev2_1 = 2.5 nS (see Table 2). In real terms we have to consider the three different binding configurations (with one, two or three Pyr groups given in Figure 7b) for the TP1-AuPd and TP2-Pt interface, respectively. The resulting conductance values through the MPTP17 ACS Paragon Plus Environment


AuNP device, Gdev, considering all possible binding combinations are given in Table 1. However, due to the invariant transmission coefficients and the rotational symmetry of the MPTP molecule and the MPTP-AuNP device not all possible anchoring combinations lead to a different conductance through the double barrier tunnel junction AuPd-MPTP1/AuNP/MPTP2-Pt. Table 2: Calculated conductance values of the MPTP-AuNP device, Gdev, for the possible binding configurations (one, two or three Pyr groups) at the TP-AuPd and the TP-Pt interface, based on Equations (1) and (2). TP-Pt

TP-AuPd

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Pyr

1

2

3

1

1.9 nS

2.5 nS

3.5 nS

2

2.5 nS

3.7 nS

6.7 nS

3

3.5 nS

6.7 nS

35 nS

Low conductance values in the range of 1.9 nS to 6.7 nS (green in Table 2) are calculated for MPTP-AuNP devices if at minimum one TP-electrode interface is determined by outer Pyr groups forming the contact. That means, at minimum one tunneling path through a MPTP molecule is long (dout or dout2) leading to a low conductance. Since in a double barrier tunnel junction the longer tunneling distance determines the conductivity of the device,22 the resulting MPTP-AuNP device conductance is low as well. Remarkably, this range of low conductivities calculated for our device corresponds very nicely to the measured device conductivities in case of the carbonate buffer treated devices, CB-dev, shown in Figure 6a. Due to the error limit of the calculated as well as of the experimental device conductivities, it is not expected that individual, experimentally obtained Gdev can be correlated with unique TP-electrode interface contacts. The Gdev values corresponding to different possible combinations of the TP-electrode interface contacts are too close for this purpose (Table 2). However, a valid explanation for the observed change of the device conductance from G1 = 1 nS to G2 = 2.2 nS during I-V measurement (Figure 5a) is the change in TP-electrode interface contacts, for example, from Gdev1_1 = 1.9 nS to Gdev2_1 = 2.5 nS (Table 2). More general, we assume that the repeatedly observed distinct increase in device conductance for CB-dev during measurement can be attributed to 18 ACS Paragon Plus Environment

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a change in the TP-electrode interface contact. That is, during the I-V characterization additional Pyr groups form a contact with an electrode resulting in a higher Gdev. The highest conductance value of Gdev = 35 nS (green in Table 2) is only calculated, if the middle Pyr groups form the electronic contact at the TP-AuPd and the TP-Pt interface and thus, the tunneling path through MPTP1 and MPTP2 in the double barrier tunnel junction is short. This value corresponds well to the experimentally obtained conductance value of MPTP-AuNP devices treated with ammonia solution (A-dev in Figure 6). For these devices usually no distinct increase in Gdev is observed during the I-V characterization and they exhibit an extraordinary stability, that is, measurements over 30 h could be performed. This finding is in line with two robust TP-electrode interface contacts each based on three Pyr groups. 4.5. IRRAS Measurements. In order to shine more light on the conductance change of MPTPAuNP devices as a result of the respective chemical pretreatment we performed IRRAS on MPTP-AuNP as prepared from acidic solution and immobilized on a Au surface. Subsequently, spectra were taken from these MPTP-AuNP rinsed with either carbonate buffer (CB) or ammonia solution (A). The IRRAS spectra of the MPTP-AuNP immobilized from acidic solution differ distinctly from the spectra of the CB- and A-treated nanoparticles in the region 1550 cm-1 to 1650 cm-1 (Figure 8). Here, the as prepared MPTP-AuNP show one strong band at 1596 cm-1 with shoulders at 1586 and 1568 cm-1. After rinsing with CB or A this strong band splits up into a triplet with vibrational bands at 1604, 1587 and 1568 cm-1. The shoulders in the spectrum of protonated MPTP-AuNP increase in intensity to form distinct bands in the spectra of CB- and A-treated MPTP-AuNP, while the strong band at 1596 cm-1 decreases to a weaker band at 1604 cm-1. These particular vibrations are assigned to C=C and C=N stretching vibrations corresponding to different regions in the molecule. Those bands are altered upon the configurational change taking place at the site of the outer Pyr groups accompanying the deprotonation.38,39 For details see the displacement vectors of the protonated and deprotonated MPTP (Supporting Information, Figure S1). In the as prepared, protonated TP group all Pyr moieties are rather fixed due to hydrogen bridging between all available N-atoms. In contrast, in the deprotonated state 19 ACS Paragon Plus Environment


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of the TP group the outer Pyr rings are able to rotate freely due to the lack of hydrogen bridges. Therefore, the band intensities of CB-treated MPTP-AuNP lying in between the intensities of protonated and A-treated ones indicate, that the deprotonation in case of CB-treated MPTP-AuNP is not completed.

Figure 8. IRRAS spectra of MPTP-AuNP redispersed in 0.05 M aqueous HCl (red), washed with carbonate buffer (0.1 M, pH = 9, green), washed with ammonia solution (25 w%, pH = 12, blue).

From these results we conclude that the chemical treatment with either CB or A effects the deprotonation of the TP group in a distinct way. While CB obviously does not lead to a complete removal of protons from the TP group of the MPTP ligand covering the AuNPs, this is possible with an ammonia solution. In consequence the protonated TP will contact an electrode preferably only with the outer Pyr groups while the deprotonated TP is able to contact an electrode with three Pyr groups. Thus, a treatment of the MPTP-AuNP device with an ammonia solution results in robust TP-electrode contacts, short tunneling pathways through MPTP, and a high device conductivity. At the same time a treatment with carbonate buffer solution leads to longer tunneling pathways through the MPTP and a lower device conductivity by one order of magnitude. That is, distinct tunneling pathways through MPTP can be selected by the chemical treatment.

5. Conclusions 20 ACS Paragon Plus Environment

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We fabricated single nanoparticle devices by immobilization of 13 nm MPTP-AuNP in between heterogeneous nanoelectrodes with a gap size of about 10 nm. The electrical characterization of these devices revealed that distinct conductance values of the respective devices result as a consequence of a pretreatment with carbonate or ammonia solution. This behavior was explained by the removal of protons ligated to the TP group only by the ammonia solution leading to a robust TP-electrode interface and a short tunneling pathway through MPTP. Thus, device conductivities differing by an order of magnitude can be controlled by selected chemical treatments. This finding is generally applicable to all TP derivatives with TP groups foreseen to form the contact to an electrode. Our findings indicate that in order to design reliable nanometer scale devices it is mandatory to control the chemical environment in detail, so that the desired physical properties are obtained.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI : Displacement vectors from DFT calculations for MPTP

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +49 2461 614015 *E-mail: [email protected]. Tel: +49-241-600953895

Orcid Author Contributions C.K. and S.K.P. synthesized the ligand capped AuNP and performed the spectroscopic characterization. M.M. fabricated the nanoelectronic devices, performed the conductance measurements, and the conductance estimations. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 21 ACS Paragon Plus Environment


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The German Research Foundation (DFG Si 609/16-1, Ka 1819/7-1)) supported this work. The authors gratefully acknowledge the help of R. Borowski, S. Trellenkamp, S.K. Potts and thank A. Hoffmann for performing the DFT calculations. We furthermore thank the Paderborn Center for Parallel Computing, PC², for providing computing time on the High-Performance Computing (HPC) system OCuLUS as well as support.

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