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Electrical Properties of Hybrid Nanomembrane/Nanoparticle Heterojunctions: The Role of Inhomogeneous Arrays Maria Bendova, Carlos César Bof Bufon, Vladimir M. Fomin, Sandeep Gorantla, Mark H. Rümmeli, and Oliver G. Schmidt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01036 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 13, 2016

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The Journal of Physical Chemistry

Electrical Properties of Hybrid Nanomembrane/Nanoparticle Heterojunctions: The Role of Inhomogeneous Arrays Maria Bendova,†,# Carlos C. Bof Bufon †,§,* Vladimir M. Fomin,† Sandeep Gorantla,‡,# Mark H. Rümmeli,‡,# Oliver G. Schmidt†,$ †

Institute for Integrative Nanosciences, ‡Institute for Solid State Research, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany

§

Brazilian Nanotechnology National Laboratory, CNPEM, PO Box 619, 13083-970, Campinas, SP, Brazil $

Material Systems for Nanoelectronics, TU Chemnitz, Reichenhainer Strasse 70, 09107, Chemnitz, Germany

* Address correspondence to [email protected], phone: +55 (0) 19 3517 5098.

#

Present addresses: MB: Central European Institute of Technology (CEITEC), Brno University

of Technology, Technicka 10, 616 00 Brno, Czech Republic; SG: Department of Physics, University of Oslo, Blindern, P.O. Box 1048, 0316 Oslo, Norway; MHR: Institute for Complex Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany.

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ABSTRACT

Investigation of charge transport mechanisms across inhomogeneous nanoparticle (NP) layers in heterojunctions is one of the key technological challenges nowadays for developing novel hybrid nanostructured functional elements. Here, we successfully demonstrate for the first time the fabrication and characterization of a novel hybrid organic/inorganic heterojunction, which combines free-standing metallic nanomembranes with self-assembled mono- and sub-bilayers of commercially available colloidal NPs with no more than ~105 NPs. The low-temperature conductance-voltage spectra exhibit Coulomb features that correlate with various interface’s configurations, including the presence of inhomogeneities at the nano- and micrometer scale owing to the NP size, the micrometer-sized voids, and the thickness of the layers. The charge transport features observed can be explained by a superposition of conductance characteristics of each individual type of NPs. The procedure adopted to fabricate the heterojunctions as well as the theoretical approach employed to study the charge transport mechanisms across the NP layers, may be of interest for investigating different types of NPs and commonly obtained inhomogeneous layers. In addition, the combination of metallic nanomembranes with selfassembled layers of NPs makes such hybrid organic/inorganic heterostructure an interesting platform and building block for future nanoelectronics, especially after intentional tuning of its electronic behavior.

INTRODUCTION A combination of nano-scale structures of different material classes (oxides, metals, semiconductors, or organics) and dimensionalities offers a unique opportunity for creating novel


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device concepts, which may be used in a wide range of multidisciplinary fields and applications. Colloidal nanoparticles (NPs), working as active islands, are among the potential candidates1,2 towards single-electron electronics.3 Nowadays, several experimental arrangements are adopted to explore the electronic properties of both single and few NPs,1,4 large NP assemblies in a film,5–8 or dispersed NPs in a solution.9 While scanning probe methods may access the information about the electronic properties of single NPs,4,10,11 it is clear that their use is not suited for solid-state devices. Towards integrated elements, methods based on nanogap electrodes have been widely employed. However, they usually require complicated fabrication techniques and often rely on precise and controllable assembly of NPs between the electrodes.1,12–17 Electroanalytical methods, in spite of their simplicity, require monodisperse NPs and lead to transport properties that are modified by the presence of solvent and ions and by the rate of NP diffusion onto the electrode.9,18 Moreover, an investigation of charge transport mechanisms at temperatures below the solvent melting point is not possible. Alternatively, in order to explore transverse electron transport properties of a NP assembly arranged in a two-dimensional film, top electrodes have been fabricated by both direct metal deposition5 and metallic droplets.6–8 While the former method usually leads to an unpredictable degradation of the capping ligand layer and requires a very well compacted layer of NPs in order to avoid short-circuit formation, the latter represents a drawback for lowtemperature characterization, integration, and device purposes.6–8 As for arrays of NPs (either homogeneous or inhomogeneous), in-plane and transverse configurations can be employed for charge transport. The in-plane (transverse) arrangement involves charge transport within (across) a layer. Nowadays, it is clear from literature that the inplane arrangement of NP arrays19–21 has been investigated more often than the transverse one.


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We presume that some of the main reasons for such prevalence are the aforementioned contacting challenges and/or layer homogeneity requirements.5–8 The approach employed here to contact inhomogeneous layers of NPs allows for fabrication of solid-state devices in the transverse configuration and their subsequent electrical characterization, without damaging or modifying the layer of NPs. Here we combine two different self-assembled nano-scaled systems to create hybrid organic/inorganic solid-state devices, aiming at a new approach for investigating the effects of inhomogeneous and imperfect layers of commercially available NPs on the electronic properties of the heterojunction. To this end, rolled-up metallic nanomembranes22,23 are used to top-contact a relatively small array of mono- and sub-bilayers of NPs (up to ~105), which allows for a systematic investigation of a transverse electrical transport in devices with a current injection area of a few µm2, without the need of sophisticated nano-lithographic methods,16 like electron beam lithography, for their manufacture. Rolling-up nanomembranes has been used in the past years as an efficient approach to produce nanostructured devices ranging from ultra-compact three-dimensional elements24–26 to soft, robust, and reproducible small-area top electrodes for contacting ultra-thin (< 10 nm) hybrid organic/inorganic layers,22,23,27 including self-assembled monolayers,22 molecular semiconductors,23 and organic nanocrystals.27 In the present work, inhomogeneous layers of colloidal metal NPs are incorporated between a bottom planar and a top rolled-up nanomembrane-based electrode, to form a hybrid heterojunction that enables transverse electrical charge transport across a NP layer. At low temperatures, the transport measurements exhibit extremely rich conductance-voltage spectra, showing irregular Coulomb features that can clearly be correlated with a variety of possible current pathways between the two electrodes across the films of NPs. Owing to a small


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current injection area (few µm2), we observe typical signatures of both a single NP system and large NP assemblies in the spectra. In the following, we report these relations in detail and provide an insight into possible applications of the nanomembrane-based contacting method for novel device development and as a platform for investigation of charge transport phenomena through thin inhomogeneous layers.

EXPERIMENTAL SECTION Device Fabrication. The fabrication of the rolled-up structure (finger and nanomembrane electrodes shown in Figure 1) used to incorporate the NPs between two electrodes was described in detail elsewhere.22,23 Few differences, however, are present in this study. Briefly, prepatterned structures of a Au finger electrode and a strained metallic layer (Au/Ti/Cr/AlOx, thicknesses 10/15/20/5 nm, Au being in the bottom) were exposed to warm deionized water (60 °C) in order to trigger rolling-up of the strained metallic layer to form the tubular nanomembrane electrode. The releasing of the nanomembrane occurs by selective removal of a sacrificial layer (Ge/GeOx, total thickness 20 nm). To allow the incorporation of the NPs and promote the formation of the junction without damaging the layers, the releasing of the nanomembranes was performed until 1/2 to 2/3 of the strained layer was rolled-up. Then, the sample was rinsed in isopropanol and dried in air, followed by the deposition of a NP layer from a water droplet surface by the Langmuir-Schaefer technique28: the chip with partially rolled nanomembranes was gently attached to the top of a water droplet containing a layer of NPs for ~1 s. Further, the sample was placed once again into warm water for additional ~30 min to complete the nanomembrane rolling. The rolling was finished when the tubular nanomembrane


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electrodes were lying on the finger electrodes (see Figure 1c). The sample was then washed in fresh deionized water, to remove possible contaminants, and dried in air and vacuum.

Figure 1. (a) Sketch of the heterojunction structure before (left) and after (right) the rolling process. (b) Sketch of a monolayer of NPs sandwiched between the finger and nanomembrane electrodes (ideal arrangement), showing also an equivalent electrical circuit. (c) Color-enhanced optical-microscope (OM) image of Device 1, showing the difference between the NP layers (darker parts) and empty surface (brighter parts). Different color enhancement is needed for the


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SiOx and Au surfaces to visualize the NP layers; details of the Au finger electrodes are shown in the left inserts (doubled magnification). The micrometer-sized holes are clearly visible.

The roughness of the finger electrode before the deposition of NPs was confirmed by atomic force microscopy (AFM) to be sufficiently low regarding the size of the used NPs: Rq and Ra values were determined as 0.40 and 0.30 nm having maximum peak value below 2 nm and full width at half maximum of 25–50 nm (see Figure S2 in Supporting Information). For the formation of the NP layers, commercial Au NPs protected by n-dodecanethiol with given diameter of 2–5 nm were used (Alfa Aesar, 2% (w/v) solution in toluene). The layers were prepared by self-assembly at the air-water interface:5 a droplet of deionized water was deposited onto a glass slide, having a diameter of ~1 cm, followed by careful dropping of 30 µl of diluted NP solution on top of it (dilution: 1 µl of the commercial NP solution in 1 ml toluene, resulting concentration ~1x1014 NPs ml−1). During toluene drying, layers of NPs formed on the watertoluene interface and moved to the water surface (see Figure 2a,b). Then they were transferred to the chip surface by the Langmuir-Schaefer technique28 after toluene has evaporated. No attempts to remove the excess ligand from the nanoparticle solution were made. Therefore, the presence of ligands in the pinholes of the NP layers may be non-negligible. However, as will be discussed further, no clear evidence of their presence was observed.


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Figure 2. (a,b) OM images of layers of NPs on top of a water droplet. The brighter phase represents the NPs, while the darker circles are the water surface. Mainly two different patterns can be observed on the water surfaces: layers containing circular voids in the micrometer-scale range (a) and almost compact layers (b). The insert in (a) has a double magnification. (c–e) TEM images of layers of NPs transferred from a water surface to a carbon grid. At low magnification, a border between a layer of NPs and the empty surface is clearly visible in the right part of the image (c). The brighter and darker regions here correspond to monolayers (d) and sub-bilayers (e) of NPs, respectively. The insert in (c) shows the size distribution of NPs obtained from the TEM images of the monolayers.

Solvents were used as received in VLSI quality (toluene in p.a. quality was filtered by polypropylene syringe filter with 200 nm pore size prior to preparation of diluted NP solutions).


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TEM Characterization. TEM characterization of the NP layers was carried out using low voltage aberration corrected transmission electron microscopy (LVTEM) on a JEOL 2010F TEM retrofitted with both image and probe spherical aberration (Cs) correctors by CEOS. The TEM investigations were carried out using an 80 kV accelerating voltage. The TEM specimens were prepared equivalently to the device fabrication but standard Cu TEM grids covered with an amorphous carbon support film were used instead of a chip with prefabricated devices. Electrical Characterization of Devices. The conductance between the finger (grounded) and nanomembrane (biased) electrodes was measured in a cryostat (Lakeshore probe station) using the standard lock-in technique (alternating-current modulation with 2 mVAC and 1.5 kHz by Agilent LCR meter E4980) at different temperatures from 4.3 K to 300 K in vacuum. Devices that were in short circuit had conductance between 100 µS to 100 mS, mostly 10–20 mS. Simulation of Conductance Spectra. A calculation program was developed to enable the simulation of measured G(V) spectra containing up to 10 individual Gi(V) curves, using the theory of correlated electron tunneling.28–30 The maximal possible capacitance values were preferred in order to fit maximum number of peaks present in the spectra with one Gi(V) curve, otherwise the simulation would be too arbitrary, as shown later for Device 3. All parameters used in the simulation of the Gi(V) curves (C1,i, C2,i, R1,i, R2,i, Q0,i) together with their graphical representation for Devices 1–3 are summarized in Supporting Information.

RESULTS AND DISCUSSION Junction Fabrication. The hybrid heterostructures comprising the layers of NPs are prepared by spontaneous stress-induced rolling-up32,33 of a metallic thin film during selective etching of an underlying sacrificial layer. After the rolling-up, the tube-shaped nanomembrane gently touches


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the top of a finger electrode22,23 covered by NPs (see Figure 1a,c). Thus, a junction is formed as a sandwich comprising the finger electrode, the layer(s) of NPs, and the free-standing metallic nanomembrane (Figure 1b). In such a configuration, the charge transport across the layer of NPs can be measured by applying a voltage bias between the finger and rolled-up nanomembranebased electrodes. In our experiments, we have set the finger electrode 4 µm broad and the tubeshaped nanomembrane with ~7 µm diameter. Therefore, the contact area between the electrodes is expected to be about a few square micrometers.22,23 The process is highly reproducible with a yield of ~90% of measurable devices, whereas about a hundred of devices are fabricated in parallel on a chip.22,23 The layers of NPs are prepared from the commercial 2–5 nm Au NPs, protected by ndodecanethiol, by self-assembly at air interface of a water droplet.5 Optical microscopy (OM) investigation of the surface of water droplets with as-prepared NP layers on top (Figure 2a,b) shows layers that are mostly not compact across the water surface and that are disrupted by empty circular micrometer-sized holes (Figure 2a). This is further confirmed by a TEM investigation of the NP layers transferred to a TEM carbon grid (Figure 2c), where the border of a NP layer is clearly visible. Furthermore, two types of layers can be identified: monolayers and sub-bilayers (see brighter and darker regions in the low magnification TEM image, Figure 2c, and their details in the high magnification TEM images, Figure 2d,e). The monolayers are neither hexagonally close-packed nor completely dense, exhibiting seldom-empty spaces of a size of up to a few NPs. The insert of Figure 2c shows a broad distribution of core sizes ranging from 1.5 nm to 3.5 nm with a maximum at 2.4 nm. The sub-bilayers have a NP density between an ideal monolayer and a bilayer. These layer imperfections can be explained by a broad size distribution of the commercial NPs. The non-homogeneities observed at the micrometer-scale


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(circular holes, Figure 2a) could originate partially from the access ligand often present in the commercial NP solutions, or the holes may be empty. Nevertheless, it is worth mentioning that, considering the non-conformal nature of the top nanomembrane electrode and its small contacting area,22,23 the presence of the large voids in the NP layers does not represent a substantial problem for transverse electrical contacting as demonstrated here. This particular feature makes the nanomembrane-based top electrode a powerful tool to evaluate and utilize such inhomogeneous systems. After the micro-structuring of the finger and nanomembrane electrodes, the NP layers are transferred from a water surface to a chip by the Langmuir-Schaefer technique.28 During the following rolling-up (releasing of the nanomembrane electrode from the sacrificial layer), the layers of NPs, and the NPs themselves, are not changed because they are not soluble in water and are bonded to the Au surface of the finger electrode by van der Waals interactions. Consequently, the removal of the NPs from the finger electrode during the rolling-up process is improbable as confirmed by OM (see Figure 1c and Figure S1 showing two devices with different density of NP layers) and by electrical characterization. Electrical Characterization of Junctions. The prepared junctions containing a layer of NPs are electrically characterized by measuring the conductance between the finger and nanomembrane electrodes. While symmetric and almost featureless conductance-voltage characteristics (G(V)) are observed at room temperature, extremely rich behaviors are obtained when cooling devices down. At 4 K, about 75% of the measurable structures are in short circuit meaning that the finger and the nanomembrane are in direct contact. This effect is connected with the low density of the prepared layers and the average roughness of both the finger and the nanomembrane electrodes. Nevertheless, the remaining working devices (called NP devices in


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the following) exhibit G(V) characteristics that are typically found for single-electron transport through metal colloidal NPs and allow for a detailed investigation of the system. Both Coulomb blockade and Coulomb peaks are clearly present, with their width, position, and height being strongly correlated with the NP array. However, periodical Coulomb staircases are not observed for these devices. Even though several devices can be measured, their electrical characteristics are not quantitatively identical. First, both finger and nanomembrane roughnesses are different (reaching up to 2 nm peak-to-peak); second, the existence of voids, NP bilayers, and monolayers, with a particular NP size distribution, have to be considered. Third, the contact area of the junction may vary, leading to distinct nominal values of G, for the same device class. However, we succeed in demonstrating that specific sets of G(V) traces have identical qualitative characteristics, which correlate with the NP arrangement in the heterojunction, as discussed in the following. Selected G(V) traces, taken at 4 K, and their respective I(V) curves (calculated by integration of the measured G(V) curves) are shown in Figure 3, presenting 4 typical representative features of the NP devices. Device 1 represents a class of junctions (20% of measured NP devices) with a seemingly periodical Coulomb staircase. Here, as distinct from the theory of correlated electron tunneling,29–31 a linear background is observed. Device 2 represents structures with a finite zerobias conductance G0. This value is typically found to be nearly 0 for the electron transport through NPs, in agreement with the aforementioned theory.29–31 For the NP devices in the present study, G0 is found to be between 0 and 20 µS, with an average of 2 µS. Device structure 3 exhibits a non-periodical position of Coulomb peaks in the G(V) traces. Devices 4 represent cases where the Coulomb features are not clearly distinguished, but rather smeared out (~40% of


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measured NP devices). Consequently, none of the measured devices exhibits the typical behavior10 observed for single-electron tunneling according to the well-established theories.29–31

Figure 3. Measured G(V) at 4.3 K (left ordinate) and its respective I(V) characteristics (right ordinate) of 4 typical devices containing layers of Au NPs. Each device represents a particular junction class. The red lines are simulated G(V) and I(V) characteristics obtained from the


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superposition of individual Gi(V) curves, corresponding to the transport through NPs of individual type, and the background conductance. The inserts represent schematic arrangements of the measured NP layers in individual devices (see text for explanation). “Tube” stands for the nanomembrane electrode.

Based on the measured G(V) curves, we conclude that all NP devices contain, at a certain level, a parasitic conductance in the form of a voltage-dependent background component or a finite-valued G0. As will be discussed below, these features are correlated with the inhomogeneities in the arrays of NPs, where charge tunneling across the empty sites may contribute to the overall charge transport at a level comparable to the active NP layers. Furthermore, the apparently randomly distributed Coulomb peaks originate from a superposition of contributions of individual NPs with slightly different sizes and/or geometries within the junction. Parasitic Conductance. Before we proceed to presenting the modeling and simulation of the G(V) spectra of the measured junctions, we discuss the possible origins of the parasitic conductance, starting with the voltage-dependent background conductance. One eventual reason for its presence is the suppression of the tunnel barrier by applied voltage, which was previously observed in transport measurements on single colloidal metallic NPs, and is often responsible for linear, parabolic, or exponential background conductance behaviour.12,34–38 However, this effect gains relevance only if the energy of barrier suppression is comparable to the charging energy of the NPs,39 which is not the case presented here (see Supporting Information for calculation). Based on the device’s structure, the most probable reason for the observed voltage-dependent background conductance is the presence of several slightly different parallel current pathways


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between the two electrodes, caused by NPs with a certain size distribution, and by their nonconstant separation from the nanomembrane electrode. Furthermore, the intrinsic roughness of the electrodes (see Figure 4a), the tunnel barrier height of the NP capping shell that can differ due to variable ligand density and orientation, as well as the surrounding NPs (their size, distance, and charge present on them)19,40 may be considered. Thus, a measured G(V) characteristic of a particular junction is a result of superposition of all individual Gi(V) curves of NPs placed between the finger and nanomembrane electrodes (see also the equivalent circuit in Figure 4b), leading to smearing-out of the Coulomb peaks of the individual NPs and contributing to the so-called “background” conductance. A typical example, and a support for this assumption, are the G(V) spectra of Devices 4 (Figure 3) without well-pronounced Coulomb peaks, which have, owing to the expected larger number of NPs sandwiched between the finger and nanomembrane electrodes, roughly one order of magnitude higher conductance as compared with the residual NP devices. This explanation is similar to the one used for the scaling law of electron transport within (not across) arrays of NPs, which is based on the presence of many different current pathways with variation of NP sizes, their interactions, and random background charges.19,20,41 Additionally, the data reported in literature for transverse electron transport across monolayers of NPs5,42 also show the presence of a background conductance (G ∝ V3.4 and G ∝ V2 in Ref. 5 and 42, respectively).


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Figure 4. (a) Sketch of an inhomogeneous monolayer of NPs sandwiched between the finger and nanomembrane electrodes considering the existence of different current pathways between finger and nanomembrane electrodes. (b) Its equivalent electrical circuit.

Nevertheless, these considerations do not account for the often present finite-valued zero-bias conductance at 4 K (G0(4K), as shown by Devices 2 and 4b in Figure 3). The expected G0(4K) value for the junctions under consideration here is smaller than 1 nS (see Supporting Information for calculation). However, only a fraction (about 10%) of the NP devices exhibit G0(4K) ≲1 nS, whereas most of them show G0(4K) = 2 µS in average. Our explanation for the high G0(4K) is based on the presence of micrometer-sized holes in the NP layers (see Figure 2a). As their size is of the order of the finger electrode, substantial parts of the finger might be not covered by NPs. Thus, the situation depicted in Figure 4a (right part), where the finger and nanomembrane electrodes are separated only by an air gap (eventually by a self-assembled layer of ligand from the NP solution) instead of a layer of NPs, is the most probable. Consequently, another transport pathway is opened via direct tunneling across these air gaps. In this case, G(V) is expected to follow the Simmons’ equation at a low bias, G(V) ≅ G0(1 + CV2),43 which accounts for the high observed values of G0(4K). The direct tunneling might also explain the voltage-dependent background conductance for those NP devices exhibiting finitevalued G0(4K). However, the contribution from the direct tunneling to the voltage-dependent background conductance is calculated to be ~100 times smaller in average than the one obtained from the measured G(V) curves (see Supporting Information). Consequently, it is considered negligible at this stage.


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The presence of micrometer-sized holes in the NP layers is also one of the main reasons for the short circuits observed in ~75% of the studied junctions. Therefore, we favor air gaps instead of the ligand in the circular voids of the NP layers to be the origin of zero-bias conductance. Simulation of G(V) Spectra. At this point, the G(V) spectra can be simulated taking into account the parasitic conductance. Because of a large number of NPs measured in parallel (~105), an “ensemble Coulomb staircase”44 is expected. Therefore, we simulate the G(V) spectra of Devices 1–3 by superposing the parasitic conductance with the individual Gi(V) curves, according to the equivalent electrical circuit in Figure 4b. Each Gi(V) curve corresponds to a set of double-barrier tunnel junctions of one kind, based on the theory of correlated electron tunneling.30,31 This approach is adequate because no (or very weak) quantum coupling between the neighboring NPs is expected, since the distances between the surfaces of the NPs are sufficiently large (>1.5 nm).19,36,45–47 If there were appreciable quantum coupling between neighboring NPs, no Coulomb effects would be observed because electrons would be delocalized over more than one NP, and, consequently, no single-electron transport could take place.19 Our calculations are performed under the following assumptions. (a) A monolayer of NPs is situated in the junctions of Devices 1–3. (b) The largest possible capacitances (of biggest NPs) are accounted because the biggest NPs have the shortest distance to the nanomembrane electrode (see Figure 4a) and, hence, possess the lowest RN,i and provide the highest contribution to G(V). (c) The theoretically estimated capacitance between an Au NP and a planar electrode (0.3 aF for the biggest NPs, see Supporting Information) increases in the presence of surrounding NPs due to the dipole-dipole interactions.19 As expected, more than one Gi(V) curve are necessary to fit the measured G(V) spectra (see Figure 3, where red lines represent the simulated G(V) and I(V) characteristics). Each Gi(V)


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curve is obtained using 5 fitting parameters, namely CF,i, CN,i, RF,i, RN,i, Q0,i;30,31 where the indices F and N correspond to the NP-electrode junction facing the finger or the nanomembrane electrode, respectively (Figure 1b). From these parameters, we highlight the importance of C1,i (the capacitance of the more resistive junction) and the excess charge Q0,i, because they directly influence the position of the main Coulomb peaks. Notice that because it is not clear whether CF,i or CN,i is dominating, we call it C1,i, whereas C1,i × R1,i >> C2,i × R2,i, R1,i being the resistance of the more resistive junction; C2,i and R2,i are the respective parameters of the less resistive junction. The other 3 fitting parameters (C2,i, R2,i, and R1,i) determine the peak height and width. Consequently, due to the nature of the interfaces, they are less reliable when comparing simulation and experiments. The Gi(V) curves with the highest capacitances enable us also to fit the largest number of peaks present in the spectra. A summary of the most relevant fitting parameters C1,i and Q0,i for Devices 1–3, as well as the traces of simulated Gi(V) and G(V) characteristics with all parameters, and few notes about the simulation systematics are given in Supporting Information. The capacitances obtained for the more resistive junction (C1,i) of Devices 1 and 2 are within a relatively narrow range (0.35 to 0.47 aF and 0.25 to 0.47 aF, respectively), being in a good agreement with the NP size when the capacitance increase due to the dipole-dipole interactions with surrounding NPs is taken into account (see Supporting Information).19 However, the values of C1,i for Device 3 are between 0.16 and 0.5 aF; the curves with C1,i of 0.16 aF are the most prominent ones. Besides that, they are less reliable when compared with those for Devices 1 and 2, because only 2 peaks are correlated within one Gi(V) in the measured voltage range. Nevertheless, the obtained C1,i values are definitely much lower than the respective values for Devices 1 and 2. This is an indication that either smaller NPs are measured in Device 3, or there


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is a sub-bilayer, instead of a monolayer, sandwiched between the finger and nanomembrane electrodes. Because there is no reason for Device 3 to contain mainly smaller NPs than those of the other devices, its G(V) spectrum originates most probably from a sub-bilayer of NPs. In order to confirm this assumption, a further study (e.g., a Monte Carlo simulation48) or an experimental proof (e.g., using cross-sectional transmission electron microscopy) would be needed. However, both methods are challenging for the measured devices and are out of the scope of the present work. Furthermore, in order to fit the peak shape closely, the effective temperature used for the simulations (50 K for Devices 1 and 2, 20 K for Device 3) has to differ from the measurement temperature 4.3 K. This difference between the effective and the measured temperature is invoked in order to mimic the additional broadening of individual peaks presumably due to inhomogeneities of the double-barrier tunnel junctions of each type. By combining the information obtained from the simulations of the G(V) spectra with the presence of a parasitic conductance, the NP layers sandwiched in Devices 1–4 are described qualitatively in the following way (see simplified schemes in inserts of Figure 3). Device 1 contains a monolayer, where relatively monodisperse NPs contribute to its conductance. Device 2 is quite similar to Device 1, but with a slightly broader NP size distribution and with an additional current pathway across air gaps, which originate from a micrometer-sized hole(s) present in the NP layer. Device 3 is assumed to be formed mainly by a sub-bilayer of NPs, instead of a monolayer, and Devices 4 have a large amount of NPs with variable sizes and surroundings, leading to the averaged and not clearly manifested Coulomb features. Therefore, the information beyond the NP size, namely, about the homogeneity and the character of the


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layers, and consequently their influence on the transport properties, is obtained by measuring NP layers sandwiched between the finger and nanomembrane electrodes. The precise calculation of the number of NPs present in the junction is not straightforward due to the micrometer-sized inhomogeneities in the layers of NPs. Nevertheless, we obtain an estimated number of NPs by considering a junction area of about 10 µm2, which leads to ~5×105 NPs at full monolayer coverage. Similarly, the magnitude of the measured conductance cannot be used for such an estimation because the distance between the NPs and the nanomembrane electrode may vary, leading to an undefined resistance RN for each NP type. Compared with the approaches previously reported in literature for vertically contacted monolayers of NPs,5–8 the junctions investigated here exhibit up to several orders of magnitude smaller contact area, but a higher conductance. The investigation of the correlation between the electrical transport properties and the Au NP shell thicknesses is important in order to give additional support to our conclusions and interpretation, and will be subject of a follow-up work. Such investigation may be of interest to evaluate the relevance of the nano- and micrometer-sized inhomogeneities in the layers on the conductance-voltage characteristics and the parasitic conductance. In addition, a decrease of device conductance by increasing the ligand chain length is expected. Also, a systematic change of the NPs shell thickness would vary the strength of the electronic coupling between the NPs. On the other hand, the fabricated heterostructures offer several advantages: they allow for lowtemperature measurements, bring mechanical stability to the junction, and enable contacting imperfect layers. These features lead to more reliable and reproducible measurements. Furthermore, several (~100) devices can be prepared on a chip, and, in spite of inhomogeneity of the NP layers, working devices can be obtained in a controlled manner using self-assembly,


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without sophisticated lithographic methods. Thus, the structures presented in the present work contribute to the efforts to fulfil an important investigation gap that exists between a single NP and a large assembly of NPs. As we have shown, this intermediate regime creates a common space, where a convolution of phenomena, which are usually observed either in single-particle devices (Coulomb features)1 or in large assemblies of NPs (background conductance due to averaging of many current pathways),5 takes place.

CONCLUSIONS In the present study, we report on the preparation and characterization of hybrid organic/inorganic heterostructures based on a combination of self-assembled mono- and subbilayers of NPs with free-standing nanomembranes. The creation of the junctions allowed us to vertically contact inhomogeneous arrays of commercially available NPs, investigate effects of the inhomogeneities on the transverse transport properties of the devices, and verify a variety of transport features, which are usually observed either in single-particle systems or in large NParray devices; all using a relatively simple solid state device. The linear and parabolic background conductance in the low-temperature spectra occurs due to the disorder in NP arrays of numerous possible origins. Furthermore, the finite-valued zero-bias conductance reveals the existence of additional current pathways, namely, the tunneling through air gaps between the two electrodes, due to the micrometer-sized holes in the NP layers. The simulations of the G(V) spectra, rich of diverse Coulomb features, provide information about the size distribution of the NPs as well as the number of layers of NPs present in the junction (monoand sub-bilayers are distinguished). We have found four representative distinct junctions by evaluating the G(V) spectra, whose characterization is discussed in detail, providing rich


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information about the nano- and micrometer-scale inhomogeneities of the measured layers. This result is in good agreement with the TEM characterization and with the theoretical predictions. Comparing our results with the previously reported data for transverse transport, the electrode contacting area is set in a way that the number of NPs under investigation is between a single NP and an assembly of NPs (~105 NPs). Additionally, the junctions are mechanically stable, prepared in high yield and amount on a chip, do not require sophisticated lithographic methods for their fabrication, allow for a detailed investigation at low temperatures, and, especially, thanks to the non-conformal nature of the top nanomembrane electrode, enable an electrical characterization of highly inhomogeneous layers, without modifying/destroying them. The last characteristic still represents a challenge from the technological point of view. Finally, the fabrication process applied in this work to manufacture such a particular hybrid heterojunction can be further adjusted to allow for integration and investigation of different types of NPs. For simplicity, we concentrate our investigation on the prototypical Au NPs that are commercially available. However, there is no technical limitation to incorporate semiconducting or insulating NPs, as well as different metallic or core-shell NPs. In addition, since the formation of the junction relies on self-assembly due to van der Waals interactions rather than a covalent bonding between the capping layer of the NPs and the electrode, an alternation of the NP shell can be easily performed without compromising the junction formation. Alternatively, the heterojunctions may be formed using any combination of metallic electrodes, adjusting the electrode work function. The self-releasing method, used to prepare the top nanomembrane electrode, can be also employed to generate nanomembrane electrodes based on different material classes like semiconductors33,49,50 or magnetic materials,51 allowing for investigation of other physical phenomena. Consequently, with the fabrication platform developed here, we have


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a wide degree of freedom to vary not only the active layer (the NPs), but also the electrodes. Such features make our structures useful for both application-oriented research (developing devices with desired electronic behavior) as well as a test platform to investigate charge transport properties of novel NP systems.

ASSOCIATED CONTENT Supporting Information. Additional information regarding experimental part (OM image and roughness of the finger electrode), calculations to suppression of tunnel barrier by applied voltage, to zero-bias conductance, to cubic coefficient in Simmons’ equation, and to junction and self-capacitances of NPs, and solutions of Devices 1–3. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], phone: +55 (0) 19 3517 5098 (C.C.B.B.). Present Addresses # MB: Central European Institute of Technology (CEITEC), Brno University of Technology, Technicka 10, 616 00 Brno, Czech Republic; SG: Department of Physics, University of Oslo, Blindern, P.O. Box 1048, 0316 Oslo, Norway; MHR: Institute for Complex Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources CCBB acknowledges the support of the CNPq (Grant No. 483550/2013-2). Notes The authors declare no competing financial interest. ABBREVIATIONS NP, nanoparticle; OM, optical microscopy; TEM, transmission electron microscopy; AFM, atomic force microscopy. ACKNOWLEDGMENT We would like to thank Martin Bauer, Céline Vervacke, and Daniel Grimm for the fruitful discussions and technical support. Carlos César Bof Bufon is a productivity research fellow from CNPq and acknowledges the support of the CNPq (Grant No. 483550/2013-2). REFERENCES (1)

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(51) Müller, C.; Bof Bufon, C. C.; Navarro Fuentes, M. E.; Makarov, D.; Mosca, D. H.; Schmidt, O. G. Towards Compact Three-dimensional Magnetoelectronics—Magnetoresistance in Rolled-up Co/Cu Nanomembranes. Appl. Phys. Lett. 2012, 100, 022409(4).

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