Directed Immobilization of Janus-AuNP in Heterometallic Nanogaps: a

Article pubs.acs.org/JPCC

Directed Immobilization of Janus-AuNP in Heterometallic Nanogaps: a Key Step Toward Integration of Functional Molecular Units in Nanoelectronics Ninet Babajani,† Corinna Kaulen,‡ Melanie Homberger,‡ Max Mennicken,† Rainer Waser,† Ulrich Simon,*,‡ and Silvia Karthaü ser*,† †

Peter Grünberg Institut (PGI-7) and JARA- Fundamentals of Future Information Technologies, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany ‡ Institute of Inorganic Chemistry and JARA − Fundamentals of Future Information Technologies, RWTH Aachen University, D-52074 Aachen, Germany ABSTRACT: Forming reliable and reproducible molecule−nanoelectrode contacts is one of the key issues for the implementation of nanoparticles as functional units into nanoscale devices. Utilizing heterometallic electrodes and Janus-type nanoparticles equipped with molecules allowing selective binding to a distinct electrode material represents a promising approach to achieve this goal. Here, the directed immobilization of individual Janus-type gold nanoparticles (AuNP) between heterometallic electrodes leading to the formation of asymmetric contacts in a highly controllable way is presented. The Janus-AuNP are stabilized by two types of ligands with different terminal groups on opposite hemispheres. The heterometallic nanoelectrode gaps are formed by electron beam lithography in combination with a self-alignment procedure and are adjusted to the size of the Janus-AuNP. Thus, by choosing adequate molecular end group/metal combinations, the immobilization direction of the Janus-AuNP is highly controllable. These results demonstrate the striking potential of this approach for the building-up of novel nanoscale organic/inorganic hybrid architectures.

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INTRODUCTION The current strategy for miniaturization in the fields of electronics, energy conversion, and sensing caused great interest in ligand stabilized gold nanoparticles (AuNP) as building blocks for nanoscale devices.1 This interest is propelled by the high stability and the versatile surface chemistry of AuNP, which allows us to adapt the particle properties for respective applications.2−5 Several examples have demonstrated that AuNP can be used to assemble organic/inorganic hybrid architectures, which are suited for sensor applications, drug delivery systems, biological labels, optical detection, or nanoelectronic devices, such as single electron transistors.1,3 However, one of the main challenges in integrating molecules or ligand stabilized AuNP in nanoelectronic devices is the reproducible and reliable contact formation with solid-state electrodes in order to enable exact control over the charge transfer at the molecule−metal junction and to allow the selective addressing of intrinsic molecular functionalities.6−11 If these challenges could be met, more complex, molecule-defined device functionalities, such as rectifiers or memristors that go beyond the properties of single electron transistors, would come into reach. Recently, an increasing interest has risen in Janus-type materials that exhibit directional and hence anisotropic physical and chemical properties.12 If this concept would be applied to © 2014 American Chemical Society

AuNP, nanoelectronic devices could be realized that intrinsically have asymmetric charge transport characteristics. This would significantly expand the opportunities to design molecular−electronic devices, such as those with the functionality of a nanoscale rectifier. Moreover, if different molecule−nanoelectrode contacts with graded strength are employed on opposite hemispheres of one AuNP, like aspired in this work, it will be possible to control the rectification direction, which represents a crucial step in molecular electronics. In principle, two approaches can be followed to fabricate nanodevices with an asymmetric current/voltage (I/U) characteristic. The first concept is to immobilize a molecule or a nanoscale object with intrinsic rectifier properties between homometallic nanoelectrodes.13 However, in this approach, a predefinition of the forward direction of the rectifier is not feasible, due to the lack of directionality in binding or immobilization. The second approach is to exploit asymmetric contacts at two interfaces formed by an isotropic nanoelement in contact with different metals.14−16 Consequently, the unequal potential barriers at the two molecule−metal junctions Received: August 22, 2014 Revised: October 24, 2014 Published: October 27, 2014 27142

dx.doi.org/10.1021/jp5085179 | J. Phys. Chem. C 2014, 118, 27142−27149

The Journal of Physical Chemistry C

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Figure 1. Preparation scheme of Janus-type AuNPs. AuNP capping layers: Cit− (sodium citrate), MOA (red, 1,8-mercaptooctanoic acid), and MPA (blue, 4-mercaptophenylamine).

will be the origin of the asymmetric I/U-characteristic of the resulting device and in addition, the forward direction is defined by the geometry of the nanoelectrodes. For this reason, the key issue for the fabrication of a rectifier or any organic/inorganic hybrid architecture is the immobilization of an anisotropic molecular building block into a solid state device in a directional manner. Facing these challenges, we designed Janus-AuNP with terminal groups suited for the immobilization in a heterometallic nanogap. In order to fabricate nanoelectrodes in a CMOS compatible way lithographic techniques are employed advantageously.17 Using electron beam lithography (EBL) we succeeded recently in fabricating homometallic nanoelectrodes with a separation of only 3 nm.18 Heterometallic nanoelectrodes with slightly larger separations in between can be obtained by electrodeposition of a second metal on a lithographically preformed electrode,19,20 a molecular lithography technique,21 or by applying our recently developed process employing a self-alignment procedure22 combined with EBL.23 Here, the utilized, highly reproducible EBL-process is based on an elaborated nanoelectrode design and a two-layer resist system in combination with an adopted developer system, that allow the control of the nanoelectrode pattern.18 The self-alignment procedure relies on an aluminumlayer deposited on top of the nanoelectrodes defined in a first EBL-step. The thickness of this aluminum layer determines the overhang of the resulting Al2O3 after oxidation. Next, the Al2O3-layer is used as a shadow mask in the second EBL-step to define the size of the gap between the already existing and the newly defined nanoelectrode.23 Thus, heterometallic nanoelectrodes with 5 nm spacing are available. Here, we present a method to build up a prototypical nanoscale device with an asymmetric transport characterized by directed immobilization of individual Janus-type AuNP. In detail, we prepare Janus-type AuNP with one hemisphere capped by 1,8-mercaptooctanoic acid (MOA) and the other hemisphere by 4-mercaptophenylamine (MPA). The particles are characterized by UV−vis, DLS, ζ-potential, SEM, and XPS. Furthermore, the pH-dependent properties of the carboxylterminated particles are determined in order to define the best trapping conditions. The Janus-AuNP as well as respective isotropically functionalized MOA-AuNP, which are applied as reference materials, are immobilized by dielectrophoretic trapping in between Pt and AuPd electrodes. The resulting nanoparticle devices are characterized by cyclic current−voltage measurements (I/U), transition voltage spectroscopy (TVS), and scanning electron microscopy (SEM). Our results demonstrate for the first time that it is indeed possible to form a nanoscale device with asymmetric electronic transport

characteristics in a reproducible and robust manner by applying the concept of directed immobilization of Janus-AuNP.

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EXPERIMENTAL SECTION Synthesis of MOA-AuNP and Janus-AuNP. Isotropically functionalized, MOA stabilized AuNP were prepared straightforward by ligand exchange reaction upon addition of an ethanolic solution (2 mM) of MOA to an aqueous solution of citrate stabilized AuNP (Cit-AuNP).24,25 The loosely bound citrate ligand is easily displaced by the thiol end-group of MOA due to the considerably larger binding energy (BE) of the S− Au bond (1.73 eV) compared to the COO−−Au bond (0.09 eV).26 The AuNP solution was allowed to stand overnight. After that the MOA functionalized particles were purified three times by centrifugation and redispersion in water. After the last centrifugation step, the particles were redispersed in 1 mL of buffer solution at different pH values (5 and 7) and characterized by UV−vis, DLS, and ζ-potential measurements. SEM measurements performed in transmission mode revealed a mean particle diameter of 13.5 ± 0.8 nm (evaluated from more than 150 particles). Janus-type AuNP were synthesized by a solid phase support similar to that described by Shumaker-Parry27 for the preparation of nanoparticle dimers. In the first step citrate stabilized AuNP were physisorbed on aminosilanized glass substrates from aqueous solution (Figure 1). The glass beads were incubated overnight, then the liquid phase was removed, and the beads were rinsed 3 times with water. Successful immobilization of the Cit-AuNP was evident by a color change of the glass beads from colorless (before immobilization) to red (after immobilization). Upon addition of 1 mL of water and 50 μL of MOA (2 mM solution in ethanol, 7.3 fold excess ligand with respect to the initial Cit-AuNP concentration) a partial ligand exchange on the immobilized AuNP occurred. Excess MOA and free citrate were removed by washing the glass beads three times with 2 mL of water. Then 1 mL of water, 50 μL of MPA (2 mM solution in ethanol, 10.2 fold excess ligand with respect to the initial Cit-AuNP concentration), and 50 μL of NaOH (0.1 M) were added, and subsequent sonication released the particles from the solid support, being obvious by reddening of the overlaying aqueous phase. The reaction mixture was allowed to stand for 2 h to ensure complete formation of a MPA coat on the particles surface. The particles were further purified two times by centrifugation and redispersion in water. After the last centrifugation step, the particles were redispersed from HEPES buffer as well as from phosphate buffer solution at different pH values (3, 5, 7, and 9) and characterized by UV−vis, DLS, and ζ-potential measure27143



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Table 1. Plasmon Peak Maximum, λmax, and the Full Width at Half Maximum (FWHM) from UV−Vis Absorption Spectra, the Hydrodynamic Diameter (Dh) and the ζ-Potential, Received from DLS Measurements of Janus-AuNP in HEPES Buffer Solution (Adopted to the Respective pH by Addition of HCl or NaOH) at pH 3, 5, 7, and 9 λmax/nm fwhm/nm Dh/nm ζ-pot./mV

pH 3

pH 5

pH 7

pH 9

592 184 997 20.8 ± 4.8

575 132 271 −28.3 ± 8.4

524 66 75 −32.5 ± 8.7

544 138 94 −39.0 ± 8.7

stabilization of single MOA-AuNP. From these results we conclude that the pH of the immobilization solution in the dielectrophoretic trapping experiments should be adjusted to values ≥5. Indeed, best results to immobilize single MOA-AuNP were achieved by using an AC electrical field while dissolving MOAAuNP in phosphate buffer solution at pH 5. Applying these conditions the yield of AC-DEPT raised up to 30%, that is 30% of the heterometallic nanogaps were filled with individual or a few MOA-AuNP after applying the DEPT procedure. HEPES or phosphate buffer solutions of Janus-type AuNP are found to show a broadening of the plasmon peak maximum λmax ranging from 500 to 700 nm at pH 3, 5, and 9 indicating agglomeration (Table1). Accordingly, the corresponding hydrodynamic diameter determined by DLS is increased and confirm the formation of agglomerates. However, at pH 7 a smaller value for the hydrodynamic diameter (Dh = 75 nm) is found. Under this condition, a distinct surface plasmon resonance (λmax = 524 nm) in combination with a negative ζpotential is observed. These data suggest that pH 7 is the best condition to be applied for DEPT. The intrinsic affinity of these Janus-type AuNP to agglomerate can be attributed to the property of the head groups used on the two opposite hemispheres, amine and carboxylate, respectively, to form hydrogen bonds. Of course, it is not possible to avoid these hydrogen bonds completely, but they can be reduced further by the use of solutions with higher ionic strength. Therefore, we preferred for our experiments phosphate buffer solutions at pH 5 with an ionic strength of 0.01 mol/L or HEPES/NaCl buffer at pH 7 with an ionic strength of 0.025 mol/L. Successful DEPT was achieved from these solutions utilizing an AC electrical field. XPS of MOA- and Janus-AuNP. In order to verify the Janus character and the adsorption behavior of our target AuNP, we performed XPS of Janus- and MOA-AuNP adsorbed on Pt surfaces. Furthermore, these results were compared to investigations of MPA-AuNP.22 XPS is a surface sensitive method and a reliable tool to identify the nature of organic compounds as well as to provide information on the relative quantity of the elements. XP spectra were recorded of MPA-, MOA-, and Janus-AuNP deposited on Pt surfaces as well as of Pt surfaces treated with the same organic solutions as used in the AuNP deposition procedure for reference purposes. Survey scans as well as core level spectra of Pt 4f, Au 4f, O 1s, N 1s, and C 1s were obtained. Here, we focus on the most interesting features that can be identified in the core level spectra of Au 4f and C 1s of the differently functionalized AuNP. One of the peculiarities in the Au 4f core level spectra (Figure 2a) is the large deviation in intensity of the Au 4f peak at 84.2 eV that corresponds to the AuNP core. Only very small signals are recorded in the case of MOA-AuNPs while a strong correlation between the intensity of the Au 4f peak and the coverage (deduced from SEM images) is obtained for MPA-

ments. SEM investigations performed in transmission mode revealed a mean particle diameter of 14.7 ± 1.1 nm (evaluated from more than 150 particles). Particle Characterization. UV−vis absorption spectra were recorded with a JASCO V-630 spectrophotometer, the position of the plasmon peak λmax was observed to determine the degree of particle dispersion. Dynamic light scattering (DLS) measurements and ζ-potential measurements were performed with a Malvern Zetasizer Nano S, He−Ne−laser λ = 633 nm, P = 4 mW, θ = 173° in order to determine the hydrodynamic radii (z-average) of single AuNP and AuNP agglomerates, respectively. Scanning electron microscopy (SEM) in transmission mode was conducted with a high resolution field emission scanning electron microscope (FE-SEM, LEO/ZEISS Supra 35 VP, Oberkochen, Germany). After electrical characterization of the single MOA-AuNP and Janus AuNP in between heterometallic nanoelectrodes SEM images were received with a SU8000 Series UHR Cold-Field Emission FE-SEM, at 10 kV acceleration voltage. X-ray photoelectron spectroscopy (XPS) was performed with a PHI5000 VersaProbe II with monochromatic Al Kα radiation in large area mode (1.4 mm × 200 μm, 100 W, 20 kV). Survey scans as well as core level spectra of Pt 4f, Au 4f, O 1s, N 1s, and C 1s were recorded. Quantification of the survey scans was performed with the help of MULTIPAK Software. Dielectrophoretic Trapping. In order to build up a prototypical nanoscale rectifier individual Janus-AuNP and for reference purposes MOA-AuNP were immobilized in between Pt- and AuPd-nanoelectrode pairs by dielectrophoretic trapping (DEPT).28−30 For MOA-AuNP dissolved in HEPES-buffer solution at pH 7.5 and phosphate buffer solution at pH 5 both, a DC-voltage and an AC-voltage (UAC = 0.7 to 1 eV for t = 7 to 10 s, v = 10 Hz), have been tested. As before in case of MOA-AuNP, we used Janus-type AuNP dissolved in HEPES-buffer solution at pH 7 as well as in phosphate buffer solution at pH 5 for the DEPT procedure. Subsequently, electrical characterization was performed using a Keithley 6430 subfemtoampere remote source meter at room temperature under vacuum or ambient.

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RESULTS AND DISCUSSION pH-Dependent Properties of MOA- and Janus-AuNP. Investigations concerning the pH-dependent agglomeration behavior of MOA-AuNP were performed to enable immobilization of single MOA-AuNP in between the nanogaps. Both the distinct surface plasmon resonance (λmax = 528 nm) and the small value of the hydrodynamic diameter (Dh = 25.3 nm) determined by DLS indicate that mostly individual MOAAuNP exist at pH ≥5.31 The negative ζ-potentials of −37.2 ± 10.7 mV at pH 5 and −40 ± 8.6 mV at pH 7 point to a negatively charged ligand shell, which results in an electrostatic repulsion between the nanoparticles and, accordingly, in a 27144



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aliphatic carbon. Accordingly, XP spectra prove that the strongest bond, i.e., in this case the N−Pt bond, determines the adsorption behavior of the Janus-AuNP to the Pt substrate. Thus, coupling chemistry is suitable to immobilize a nanoparticle with a predefined functionality, like a potential rectifier, in a directed way between two electrodes composed of different metals. Isotropically Functionalized AuNP in Heterometallic Nanogaps. First, the goal was to verify that the tunneling barriers in “electrode-molecule/AuNP/molecule-electrode” devices are determined by molecular properties. Therefore, isotropically functionalized MOA-AuNP were used. Since the binding energy of the molecular headgroup (−COOH or −COO−) to the AuPd or the Pt electrode is comparable no asymmetry in the resulting I/U curves is expected. Successful immobilization of single or a countable number of MOA-AuNP between heterometallic nanoelectrodes was performed in 9 cases. These functional devices exhibited more than 10 I/U curves in a reproducible manner and the analysis of the measurements revealed molecular properties. The conductances of the 9 functional MOA-AuNP devices (for a source-drain voltage of USD = 1 V) cover values from 1 to 750 pS with a core area between 17 and 60 pS which is a rather small range. The statistics of the experimentally obtained conductance values of MOA-AuNP devices, together with a typical I/U measurement, a Fowler−Nordheim plot (TVS), and the SEM image of a single MOA-AuNP device, are depicted in Figure 3.

Figure 2. (a) Au 4f core level spectra of MPA-, MOA-, and JanusAuNP deposited on a Pt-substrate (MPA- and MOA-AuNP spectra are shifted by 40 and 20 kcounts, respectively, for clarity); (b) C 1s core level spectra: of a solvent treated Pt-substrate for reference and MPA-, MOA-, and Janus-AuNP deposited on a Pt substrate, respectively.

capped AuNP. This fact can be attributed to the increased attenuation of the Au 4f photoelectrons by the MOA molecules with a length of 1.25 nm compared to MPA molecules with a length of 0.78 nm. Obviously the increased molecular length of MOA is sufficient to suppress efficiently the Au 4f signal arising from the AuNP core. A comparison of the Au 4f core level spectrum of Janus-AuNP with MPA- and MOA-capped nanoparticles clearly reveals that the hemisphere available for spectroscopy is capped by MOA. In addition to the Au 4f spectra the C 1s core level spectra are given in Figure 2b. Examination of the spectrum obtained from the solvent treated Pt-substrate reveals peaks at binding energies of 284.7 and 286.0 eV which can be assigned to carbon and small amounts of carbon containing adsorbates, respectively. Next, the C 1s peak with highest intensity in the spectra of the differently functionalized AuNP is inspected. While it is obtained for MPA at a binding energy of 284.8 eV corresponding to aromatic carbon rings, it is significantly shifted for MOA to 285.1 eV corresponding to aliphatic carbon chains. The shift of 0.3 eV is characteristic for the difference between aromatic and aliphatic carbon.32 Comparing the C 1s spectrum of the Janus-AuNPs with the spectra of MPA and MOA functionalized AuNP (Figure 2b) discloses that the hemisphere facing the X-ray beam is capped by MOA, labeled by the C 1s peak corresponding to the binding energy of

Figure 3. MOA-AuNP device: (a) I/U-characteristic (average of 80 cycles, black) and exponential fit according to Simmons14,33 (dashed red). A positive voltage in the diagram corresponds to application of a positive bias voltage to the AuPd-electrode, Pt electrode grounded. Inset: corresponding SEM of the single MOA-AuNP device. (b) Fowler−Nordheim plot; (c) statistics of experimental conductance of 9 MOA-AuNP devices at USD = 1 V. The reported values represent averages of multiple measurements taken from independently prepared devices, the black line corresponds to the calculated Gdev,MOA = 0.12 nS (applying the Landauer formular) and the red bar to the conductance range 24 pS to 0.60 nS. 27145



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The Fowler−Nordheim plot is deduced from the I/U curve by plotting ln(I/U2) vs 1/U. The characteristic minimum of the resulting curve is named transition voltage (Utrans) and represents a transition in conduction behavior, e.g., from direct to Fowler−Nordheim tunneling. According to Beebe34 Utrans scales linearly with the tunneling barrier height, ϕB = EF − EHOMO in the case of hole transport, where EF is the Fermi energy and EHOMO is the energy of the highest occupied molecular orbital (HOMO). Within the coherent molecular transport model the minimum obtained from the Fowler− Nordheim plot (Figure 3) corresponds to the tunneling barrier height of MOA, ϕMOA,exp = 1.25 ± 0.1 eV. This value is in agreement with literature data for tunneling barrier heights of alkanes, with ϕalk,lit = 1.45 eV35 or rather 1.2 eV,36 and confirms tunneling through MOA molecules. Besides the analysis of the electron transport through the MOA-AuNP devices using a Fowler−Nordheim plot, the I/U curves of all functional devices were analyzed employing Simmons theory.14,33 The black curve in Figure 3a displays the experimental I/U curve (average of 80 cycles) of a single 13 nm MOA-AuNP immobilized in between a AuPd and a Ptnanoelectrode and the dashed red curve corresponds to the fit based on the Simmons formula as described earlier.23 The main outcome of the exponential fit according to the tunneling model is the mean decay parameter (βd) (β̅ = mean decay constant, d̅ = mean tunneling distance), which is characteristic for the respective device. We compare this mean decay parameter with a theoretical decay parameter (∑βidi) with i = alkane, carboxylate, and vacuum in the case of MOA molecules. This theoretical decay parameter is calculated employing the respective β values from literature as described23 and taking the device geometry into account. If (βd) resulting from the Simmons fit is considerably smaller than (∑βidi), another transport mechanism besides tunneling transport has to be assumed. On the contrary, if (βd) > (∑βidi), the tunneling path seems to be longer than assumed and there might be an additional vacuum gap, due to a not ideal molecule−electrode contact. Setting (βd) = (∑βidi) gives dvac, the length of a possible vacuum gap. Very importantly, the result of the exponential fit according to Simmons theory is the mean decay parameter, which includes all tunneling barriers along the tunneling path. On the contrary, the tunneling barrier height deduced from the Fowler−Nordheim plot corresponds to a certain tunneling barrier in the measured voltage range. Tunneling barriers resulting from vacuum gaps, i.e., with a barrier height corresponding to the work function of one electrode, are usually out of the possible voltage range for molecular devices and, thus, not seen in Fowler−Nordheim plots. In the case of MOA-AuNP devices the resulting exponential fits according to the tunneling model disclose that indeed the mean tunneling distance is determined by the molecular length of MOA and that, in addition, at most a tiny vacuum gap of 0.1 ± 0.1 nm remains between the molecular capping layer of the AuNP and one nanoelectrode. Finally, the SEM image clearly reveals the immobilization of a single MOA-AuNP in the nanogap. This result proves that charge transport through a nanoparticle device, e.g., an electrode1-molecule/AuNP/ molecule-electrode2 device forming a double barrier tunneling junction, is determined by the molecule properties. Concept of Directed Immobilization: Janus-AuNP in Heterometallic Nanogaps. Our strategy to build-up a prototypical nanoscale device by directed immobilization of

individual Janus-type AuNP, stabilized by MPA and MOA ligands on each hemisphere, respectively, into a heterometallic nanoelectrode gap is 2-fold. We (i) apply these ligands with elaborated end group functionalities in order to create different molecular/metal contacts (that is, Pt−N and COO−AuPd, respectively) with unequal potential barriers and (ii) adjust the electronic transport properties of the molecular backbones (aliphatic and aromatic, respectively) on the opposite sites of the Janus-AuNP (Figure 4). Furthermore, as already previously

Figure 4. Schematic of the “Pt-MPA/AuNP/MOA-AuPd” device; the Fermi energies (EF), tunneling barrier heights (ϕB), as well as the molecular lengths of alkane (dalk = 1.03 nm), carboxylate (dCOOH = 0.2 nm), and MPA (dMPA = 0.61 nm) are indicated.

reported in our work on isotropically functionalized AuNP and investigations on molecule/metal junctions, these selected molecular terminal group/metal combinations should allow us to control the directionality of Janus-AuNP immobilization.26,31 The resulting device Pt-MPA/AuNP/MOA-AuPd should ideally exhibit an asymmetry of the measured I/U characteristic. Furthermore, if directed immobilization is successful, i.e., MPA is always bound to the Pt electrode and MOA is always bound to the AuPd electrode, like suggested from XP-spectroscopy, the asymmetry of the I/U characteristic should be in the same direction for all devices. From 270 fabricated devices 37 functional ones were obtained, preferably from phosphate buffer solution at pH 5 (31), with conductance (for USD = 1 V) in the range from 1 pS to 100 nS. However, despite the fact that the conductance of these Janus-AuNP devices cover quite a large range, a statistical analysis (Figure 5, inset) reveals that the utmost number of devices exhibit conductance values in the range from 40 pS to 1 nS. These conductance values are significantly larger than the ones obtained for isotropically functionalized MOA-AuNP. In order to examine the possible influence of the molecular capping layer on the experimental conductance through the corresponding AuNP devices, we estimated for a first approximation the tunneling current through single molecules, MPA and MOA, respectively, connected to two electrodes applying the single-channel Landauer formula for conductance (Gmolecule):26,37,38 27146



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Thus, a theoretical device conductance Gdev,Janus = 0.21 nS at USD = 1 V is deduced, which corresponds impressively to the experimentally obtained values (Figure 5, inset, Gexp,Janus = 0.04 to 1 nS). The conductance of MOA- and MPA-AuNP devices estimated in the same way applying formulas 1 and 2 amounts to Gdev,MOA = 0.12 nS (Gexp,MOA = 0.02 to 0.06 nS, Figure 3) and Gdev,MPA = 10.1 nS (Gexp,MPA = 5 to 30 nS),23 respectively. It is obvious that the observed transport behavior of the JanusAuNP device is mainly determined by the considerably higher and broader tunneling barrier formed by the alkane chain of the MOA molecule. Consequently, the theoretical Janus-AuNP device conductance is only about twice the conductance of the MOA device which is in astonishing agreement with the experimentally obtained values for both kinds of devices. However, the rough estimation of Gdev,Janus discloses that charge transport through this device can only hardly be affected by changes in the tunneling mechanism through the MPA molecule since it is of minor influence. Keeping these considerations in mind, we analyzed the experimental I/U curves characterizing charge transport through Janus-type AuNP immobilized between heterogeneous nanoelectrodes in detail. In Figure 5 representative I/U measurements (averaged over about 50 cycles) are shown, which are selected in such a way that the whole conductance range is covered. Obviously, all the I/U curves show a distinctly increased conductance for negative voltage values, that is, for applying a negative voltage to the AuPd electrode while the Pt electrode is grounded. Most remarkably, this increase of more than 10% in conductance (at USD ≈ 0.7 V) is observed in the same way for 80% of the Janus-AuNP devices from phosphatebuffer solution (for 25 out of 31). This strongly supports that we have indeed immobilized the Janus-AuNP in a directed way and that coupling chemistry can be used to control the integration of individual AuNP. Charge Transfer through an Asymmetric Double Tunneling Barrier. Even more details become apparent when examining I/U curves that correspond to the theoretical conductance value of Janus-AuNP devices, Gdev,Janus = 0.2 nS (Figure 6). The asymmetry of this curve is pronounced in the

Figure 5. I/U curves of 6 representative Janus-AuNP devices (violet: devices with conductance values in the range from 40 pS to 1 nS at USD = 1 V; gray: device with conductance >1 nS; black: device with conductance 1 V. Supplementary, a Fowler−Nordheim plot was used to analyze these results. Thus, for negative voltages two minima can be deduced corresponding to Utrans1 = 0.75 ± 0.1 V and Utrans2 = 1.2 ± 0.1 V while for positive voltages one minimum corresponding to Utrans3 = 0.95 ± 0.1 V can be identified.

(2) 27147



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Utrans1 is in good agreement with tunneling barrier heights obtained for isotropically functionalized MPA-AuNP (ϕMPA,exp = 0.85 ± 0.1 V)23 and phenylene or MPA in contact with Au electrodes.34,36,43 Utrans2 corresponds to MOA, ϕMOA,exp = 1.25 ± 0.1 eV. This observation clearly indicates that both molecules are involved in the charge transport through the device. Furthermore, the dependence of the current−voltage curves and Utrans on bias polarity can be attributed to the asymmetry of the metal−molecule contacts. The device properties can be explained within a double tunneling barrier model. Within this basic approach the dissimilar molecular headgroup−nanoelectrode couplings and the molecular level broadening caused by the coupling to the electrodes are neglected. We assume a strong coupling between the molecules and the AuNP (Au−S bonds), take the molecular energy levels, HOMO of phenyl and alkane chains, into account, and assume hole transport in accordance with the donor character of MPA and saturated alkane chains (Figure 7).

hemispheres in order to allow a directed immobilization, (ii) with molecular backbones from phenylene or alkane chains, that can be distinguished using spectroscopic methods, and (iii) at the same time with stable capping layers built from organic molecules that likely do not interfuse. In order to increase the rectification ratio, one possible approach can be formulated according to a rectification mechanism in tunneling junctions reported by Whitesides.44 The second generation of Janus-AuNP should, for example, be equipped with organic molecules with an increased donor character and a higher resistance than MPA molecules. These conditions can be achieved by using long phenylene chains with a conjugated π system since the HOMO−LUMO gap decreases and the resistance increases in this case with molecular length. At the same time the resistance of the MOA molecules should be reduced by using a shorter alkane chain. Following this concept, forward bias (= the HOMO(phen) is accessible) will be characterized by tunneling through the alkane chain while the phenylene chain shows almost no resistance. In contrast reverse bias will be marked by tunneling through both the phenylene and the alkane chain, i.e., the high resistance state. Our current research is devoted to the development of a new generation of Janus-AuP that fulfill these requirements.

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CONCLUSION In summary, we performed the synthesis of Janus-type AuNP equipped with molecules bearing different intrinsic electronic properties and different functional terminal groups on the opposite hemispheres. The terminal groups and thereby the solution properties have been adjusted so that the immobilization between heterometallic nanoelectrodes can be performed in a directed way. The charge transport through the resulting device reveals the characteristic of both types of molecules, forming an asymmetric double tunneling barrier. Thus, we demonstrated that applying Janus-type AuNP in combination with heterometallic nanogaps enables the formation of highly controllable molecule/nanoelectrode hybrids, paving the way for implementation of such systems in nanoelectronic applications.

Figure 7. Schematic displaying the molecular energy level alignment of HOMO(MPA) (blue) and HOMO(MOA) (red), strongly coupled to the AuNP with respect to EF,Pt and EF,AuPd in case of (a) U = -|ϕMPA| < 0, (b) U = -|ϕMOA| ≪ 0, (c) U = |ϕMPA| > 0, and (d) U = |ϕMOA| ≫ 0.

For both molecules direct tunneling (= off-resonant tunneling) is the relevant transport mechanism at low bias voltages. If a negative voltage is applied to the AuPd electrode the HOMO(MPA) will align with the Fermi energy of the Pt electrode, EF,Pt, first. At this point direct tunneling turns into Fowler−Nordheim tunneling (= resonant tunneling). The remaining tunneling barrier is built mainly by MOA. If a higher negative bias voltage is applied, also HOMO(MOA) will come into the bias window between the Pt and the AuPd electrode enabling Fowler−Nordheim tunneling through the whole device. Thus, two minima in TVS are obtained at negative voltages corresponding to ϕMPA and ϕMOA, respectively. On the contrary, if a positive voltage is applied to the AuPd electrode, no effect on the charge transport will be observed before HOMO(MOA) is within the bias window between the Pt and the AuPd electrode. According to that, only one transition voltage is obtained when applying positive voltages to the AuPd electrode fully explaining the observations. These results indicate that we were able to control the directionality of the Janus-AuNP immobilization and thus, the rectification direction of the formed nanoscale device. However, the rectification ratio shown in the voltage range 0.4−0.8 V (Figure 6) is still rather small and should be increased in future approaches. In this first approach the Janus-AuNP were designed (i) with two different terminal groups on opposite

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AUTHOR INFORMATION

Corresponding Authors

*Phone: ++49 241 8094644. Fax: ++49 241 99003. E-mail: [email protected]. *Phone: ++49 2461 614015. Fax: ++49 2461 612550. E-mail: s. [email protected]. Author Contributions

S.K., C.K., H.M., and U.S. designed the study. N.B. and C.K. performed all experiments and contributed equally to the work. N.B., C.K., M.H., and S.K. analyzed the data. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge the help of A. Besmehn, R. Borowski, and S. Trellenkamp. This work was supported by the Excellence Initiative of the German federal and state government and by the Jülich Aachen Research Alliance (JARA). 27148



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Directed Immobilization of Janus-AuNP in Heterometallic Nanogaps: a

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