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Single-Electron Charging Effects in Hybrid Organic/ Inorganic Nanomembrane-Based Junctions Rafael Furlan de Oliveira, Leandro Merces, Felipe Marques, Erico TeixeiraNeto, Davi Henrique Starnini de Camargo, and Carlos César Bof Bufon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00233 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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The Journal of Physical Chemistry

Single-electron charging effects in hybrid organic/inorganic nanomembrane-based junctions

Rafael Furlan de Oliveira†, Leandro Merces†,§, Felipe Marques†,Ɨ, Érico Teixeira-Neto†, Davi Henrique Starnini de Camargo†,¥ and Carlos César Bof Bufon†,§,‡,* †

Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for

Research in Energy and Materials (CNPEM), 13083-970 Campinas, São Paulo, Brazil §

Institute of Physics “Gleb Wataghin” (IFGW), University of Campinas (UNICAMP),

13083-859 Campinas, São Paulo, Brazil Ɨ

Department of Materials Engineering, University of São Paulo (USP), 13568-250 São

Carlos, São Paulo, Brazil ‡

Department of Physical Chemistry, Institute of Chemistry (IQ), University of

Campinas (UNICAMP), 13084-862 Campinas, São Paulo, Brazil ¥

Postgraduate Program in Materials Science and Technology (POSMAT), São Paulo

State University (UNESP), 17033-360 Bauru, São Paulo, Brazil * corresponding author: [email protected] ABSTRACT The controllable transfer of a single electron in devices (SEDs) is one of the viable trends for a new generation of technology. For this purpose, fundamental studies and numerous applications on the manipulation of individual charge carriers in different nanostructures have been carried out. However, novel applications demand innovative strategies to fabricate and evaluate SEDs. Here, we report a hybrid organic/inorganic SED that combines an ensemble of physisorbed, semiconducting molecular layers (SMLs) and Au nanoclusters embedded in the transport channel by in-situ, field-

1 ACS Paragon Plus Environment

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induced metal migration. The SED is fabricated using an integrative platform based on rolled-up nanomembranes (rNM) to connect ultrathin SMLs from the top, forming large-area tunnel junctions. The combination of high electric fields (1-4 MV/cm), electrode point contacts, low temperatures (10K) and ultrathin molecular layers (< 10 nm) lead to field-induced migration of Au electrode nanoparticles inwards the SML of the junction channel. This phenomenon can be either observed in as-prepared rNM junctions or intentionally induced by the application of high electric fields (> 1 MV/cm). The propelled electrode particles become trapped in the soft molecular material, acting as Coulomb islands positioned in between a double-barrier tunnel junction. As a result, the hybrid organic/inorganic rNM junctions present single-charge effects, namely Coulomb blockade and Coulomb staircase. Such in situ, field-induced metal migration process opens possibilities to create novel and complex SEDs by using different molecular materials. From another perspective, the reported metal diffusion in such nanoscale junctions deserves attention as it can occasionally mask moleculedependent responses.

KEYWORDS:

Molecular

electronics,

Coulomb

blockade,

metal

diffusion,

nanomembranes, charge transport.

TOC


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1. INTRODUCTION One of the main questions of modern science and technology is how to explore the properties of the matter down to the quantum limits.1 Regarding electronic properties, one possibility lies in the controlled transfer of individual electrons in a nanoscale device or a piece of matter (viz. nanostructure).2–4 From this opportunity, different applications such as supersensitive electrometers, amplifiers, sensors, and memories have been envisioned2,3 and developed,5–7 paving the way towards a new generation of technology. Another perspective is that single-electron processes are also relevant in fundamental research, for example, to investigate equilibrium states of charged nanostructures8 and heat transfer at the nanoscale.9,10 The transfer of electrons one-by-one to a particular system is only possible by tunnel effect because of the discreteness of such transport mechanism.2,3 In practice, one can engineer a tunnel junction, i.e., a device having two metallic electrodes separated by a nanometric insulating barrier, so that electrons can transverse it by tunnelling.2,3 The precise control of individual electrons is achieved by a series combination of two electrically isolated tunnel junctions having a conductive nanostructure (island) in between.2,3 The addition of a single electron in the island increases its charging energy, preventing the injection of a further electron in the system unless the island potential is reduced by the external bias. Such electrostatic repulsion of confined electronic charges is known as Coulomb blockade (CB) and accounts for controlling the flow of electrons individually in the device.2,3 By coupling a third electrode (gate) to the island, singleelectron transistors can be obtained.4–6 The practical exploitation of single-electron processes depends, among other things, on the proper choice of materials and their manipulation. Most single-electron devices (SEDs) utilize inorganic structures (viz. metallic and semiconducting quantum



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dots), with several interesting applications being developed using such materials.4–7,11 A plenty of room in SEDs exists for organic compounds, such as molecular quantum dots,12–16 and eventually for hybrid organic/inorganic structures.17,18 The advantages of molecular systems for SEDs are numerous. The control of molecular structures from single-molecules to ensembles via bottom-up fabrication, the broad diversity of existing molecules with different properties, and the flexibility to synthesize new compounds with specific characteristics are the main advantages. The attractiveness of hybrid systems, in turn, arises from the possibility of combining materials of entirely different nature (e.g., oxides, metals, or polymers) having

diverse

properties

(e.g.,

conducting,

superconducting,

insulating,

or

semiconductor) to obtain complementary characteristics. Additionally, as hybrid materials contain an organic moiety, they benefit from the same appealing features of molecular systems. This endow limitless possibilities of combinations and functions to be explored in SEDs based on hybrid materials. For example, Yamamoto et al. recently demonstrated the operation of single-electron transistors combining a gold nanoparticle (AuNP) and a few dye molecules. The AuNP acts as the Coulomb island and the molecules as photoreactive floating gates which reversibly change the island potential upon light irradiation.17 In this way, different charging states of the molecular compound can be induced and studied. The materials involved not only determine the functionalities of the device but also the methods employed for its fabrication and evaluation. Typically, inorganic SEDs use electron-beam lithography in their manufacturing.4–7 Purely organic systems frequently utilize self-assembly for the fabrication of the molecular layer or attachment of single-molecules to the electrodes.16 Mechanically-controlled break junctions,16 electromigration break junctions,19 and scanning tunneling microscopy (STM)18 are the


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most used techniques for the evaluation of the single-electron transfer in both inorganic and molecular systems. Most hybrid organic/inorganic SEDs combine inorganic nanoparticles with self-assembled monolayers (SAMs) as support20 and/or capping in a core-shell configuration.18 The evaluation of the single-electron transfer in hybrid SEDs is also commonly achieved by STM.18 This, however, prevents the development of hybrid systems in an integrated, practical device architecture. To develop novel applications, the controllable transfer of single electrons demands innovative strategies to fabricate and evaluate SEDs. In this sense, we report here a hybrid organic/inorganic device that combines an ensemble of physisorbed molecular semiconductors and Au nanoclusters embedded in the junction by in situ, field-induced migration of particles from the electrodes’ interfaces. The hybrid SED is fabricated using an integrative platform based on rolled-up nanomembranes (rNM), and it is compatible with different molecular materials (i.e. tunability). The device fabrication and evaluation do not need sophisticated apparatus or techniques, such as ebeam lithography or STM. The rNM strategy allows the electrical connection of sufficiently thin molecular ensembles (ca. 5-6 molecules along the transport channel) to ensure charge tunneling through the device over large contact areas (µm2). The rNM preparation relies on the controlled release of a strained metallic thinfilm from a sacrificial layer, which rolls up and lands softly on top of molecules deposited onto the bottom electrode. The method is an alternative to the incompatible thermal electrode deposition or liquid metal electrodes to connect ultrathin molecular layers from the top.21 The rNM technology has been successfully employed for the electrical characterization of SAMs,22 arrays of nanoparticles23 and organic semiconducting layers.24–27 Here, rNM SEDs are formed using SMLs of copper (II) phthalocyanine (CuPc) and Au nanoclusters propelled from the contacts by the



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controlled action of the applied voltage bias. Because of the ultrathin nature of the junction, electric fields of 1- 4 MV/cm - consistent with the formation of Au agglomerates in the electrodes of nanoscale transport junctions28 - are easily reached. Such agglomerates, when detached from the contacts, may be treated as Au islands embedded within the SML, leading to single-electron charging effects such as CB and Coulomb staircase (CS). We verified that signatures of CB/CS could be either observed in as-prepared junctions or intentionally induced by increasing the applied electric field > 1 MV/cm. The electrical response of such hybrid devices is unique and differs from that of the isolated materials.23,25 Also, the possibility to systematically promote the Au particle migration allows the creation of novel, hybrid and complex junction compositions exploiting different molecular materials. From another perspective, such an in-situ, electrode particle migration deserves attention as the response of the hybrid junction can be erroneously attributed to the molecular layer24, or even mask electrical signatures of other phenomena only observable at ultrathin thicknesses. 2. EXPERIMENTAL SECTION We prepared the rolled-up nanomembrane (rNM) junctions on Si (100) substrates covered by a 2 µm thick SiO2 coating, following similar descriptions in the literature.24,25 A summary of the fabrication steps with experimental details is provided in the Supplementary Information (Figure S1). Here, vertical transport junctions are formed by a patterned Au bottom (finger) electrode, the SML (5 nm) and the Au top rNM electrode. CuPc was selected as the material of interest due to its thermal and chemical stability, vast literature about its structural, morphological and transport characteristics, and wide applicability in the fields of molecular and organic electronics.29–31 Thus, we chose Au as the bottom and top contacts to ensure a negligible injection barriers with the CuPc layer during the electrical measurements.32,33


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CuPc SMLs were deposited onto the Au finger electrode by thermal sublimation at room temperature. The thickness of the CuPc layer was determined by a calibrated quartz crystal microbalance in the deposition chamber and confirmed afterwards by atomic force microscopy (AFM). The topographies of the CuPc SML and the electrodes are shown in Figure S2. After the CuPc deposition, the roll-up process is carried out to form Au/CuPc/Au junctions. This procedure consists in the controlled release of the strained nanomembrane from a sacrificial layer (here GeOx), leading to the formation of a tubular structure (typical diameter of 8 µm) with an external Au coating. After rolling up, the Au-coated rNM lands onto the finger electrode (5 µm wide), contacting the SML from the top. The contact areas with the finger and rNM electrodes are expected to be about a few square micrometers,23 while the charge injection areas have been reported to be much smaller (viz. a few nm2).25 A sketch in Figure 1a shows the CuPc molecular structure and the roll-up procedure before and after the junction formation. A scanning electron microscopy (SEM) image of the junction is shown in Figure 1b (FEI Helios Nanolab 660 equipment). Finally, the electrical characteristics of the CuPc rNM junctions were evaluated by current(I)– voltage(V) measurements in the dark, at 10K, and at high vacuum (10-5 Torr) conditions. The rNM electrode is biased having the finger terminal grounded. The voltage bias was limited to provide a maximum electric field of 1 MV/cm to the junction unless otherwise stated. Before the electrical measurements the samples were kept in vacuum for 24h. The electrical signatures reported here result from the systematic evaluation of multiple junctions, excluding devices presenting either a short- or open-circuit response, or even defects during manufacturing. For a reliable interpretation, however, we considered only working junctions from the same chip, i.e. same fabrication batch. Quantitative data presented



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refer to ca. 10 identical junctions, which is statistically meaningful considering the response of large-area molecular devices.21

Figure 1. a) Nanomembrane junction layout before (left) and after (right) the roll-up process, and the molecular structure of copper (II) phthalocyanine (CuPc). The controlled removal of the sacrificial layer allows the strained nanomembrane to roll up and land on top of the CuPc SML, forming Au/CuPc/Au junctions. b) SEM image of the rNM junction. (c) Current(I)–voltage(V) characteristics at 10K (semi-log plot, y arbitrary units) showing the two typical responses (types I and II) observed for a junction comprising a 5 nm thick CuPc layer. Inset: Sketch of the junction cross section.

3. RESULTS AND DISCUSSION The selective removal of the GeOx sacrificial layer (here GeOx), allows the rNM to gently touch the CuPc SML from the top, providing a robust and self-adjustable electrical connection22–27 (Figures 1a and 1b). Albeit the room-temperature electrical characteristics of such junctions are relatively featureless,25 their low-temperature


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response is rich. Figure 1c shows the typical I-V traces obtained for 5 nm thick Au/CuPc/Au junctions at 10K. From a minimum set of 10 devices on the same chip, the observed responses have been categorized as types I and II (black and blue curves, respectively). The current is exhibited in arbitrary units to highlight the significant differences in the electrical responses, which refer to the presence of steeper steps in the type II curve. While type I response is a typical non-linear I-V trace observed for thin insulating layers,34 type II resembles the electrical characteristics found for single, conjugated organometallic molecules (other than CuPc) corresponding to resonant tunneling (RT) through delocalized molecular orbitals (MOs) weakly coupled to the contacts.35,36 Here, if we consider that 5-6 CuPc molecules are integrated along the vertical transport channel for a 5 nm thick layer (see SI for details), thus RT through MOs of individual CuPc molecules cannot be disregarded. In ca. 50% of all working 5 nm thick CuPc rNM junctions, the type II response is present. Thus, type I and II responses can be interpreted, at first glance, as variations of the junction electrical behavior when collective or individual CuPc molecules, respectively, dominate the overall I-V characteristics. Finally, the observed asymmetry with respect to the applied bias on both curves explains the different nature of the related electrode interfaces (Aufinger/CuPc and Au-rNM/CuPc).24–26 No apparent reason exists for higher rectifications in type II than in type I response. However, other junctions in the literature containing phthalocyanine layers under the same bias configuration exhibit slightly higher currents at positive voltages.24–26 From the I-V responses, we have calculated the respective numerical differential conductance (dI/dV), as shown in Figure 2. The I-V traces, now in linear scale, allow the direct comparison between the current and the conductance characteristics. The schemes



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of the idealized charge transport mechanisms across the rNM junction and their respective band diagrams are also presented.

Figure 2. Schematics of the charge transport across ultrathin (5 nm) CuPc rNM junctions and their respective low-temperature (10K) current (I-V) and differential conductance (dI/dV-V) plots for a) type I and b) type II responses. Inset: corresponding band diagrams.


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The type I current response leads to a parabolic shaped dI/dV-V curve consistent with different tunneling mechanisms.37 The analysis of the junction current at low voltages (< 0.15 V) reveals an ohmic I-V relationship (see Figure S3), in agreement with Simmons’ model for direct tunneling (DT).38 As previously reported for a similar junction architecture and experimental settings, the charge transport in CuPc in such conditions is not thermally activated.24,25 Furthermore, the current decays exponentially with distance.25 These characteristics endorse DT as the dominant transport mechanism for such ultrathin CuPc layers at low temperatures. Thus, type I response is a result of electrons tunneling from one electrode to the other, having the CuPc SML in between, as illustrated in the scheme (top) and the band diagram (inset) of Figure 2a. The dI/dV-V plot for the type II response, on the other hand, presents peaks at voltages where a current step is observed, in addition to a conductance gap at low bias (< 0.1 V). These peaks are typical features of quantized charges traveling through the junction, similar to the characteristics observed for single molecules,16,36 molecular quantum dots,14,15 and other low-dimensional materials.23 As previously discussed, one possibility for the observation of conductance peaks in ultrathin CuPc junctions is the occurrence of RT into MOs of a single, isolated CuPc molecule in the junction. In this case, conductance peaks at well-defined voltages correspond to either the CuPc HOMO or LUMO (lowest unoccupied molecular orbital).14,15 The peak position, though, is dependent on the junction’s characteristics, such as the work function of the electrodes and the voltage drop profile at the involved interfaces.14,15,39 Figure 2b exhibits two conductance peaks, centered at approximately +0.15 V, and +0.31 V. The best practice to index conductance peaks to the molecular levels is the calculation of the electron transmission function of the molecule.40 Here, a simplified rationalization considering the Fermi level (EF) of the Au contacts (~5.1 eV)41 and the



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CuPc HOMO (~5.0 eV) and LUMO (3.3 eV) levels42,43 is satisfactory though. From these values, and considering a negligible voltage drop at the electrodes (as indicated by small current rectification shown in Figure 1c), RT of electrons to the CuPc HOMO or LUMO levels cannot explain such low-voltage, multiple conductance peaks.40,44,45 Other factors disregarding the RT mechanism are: (i) identical junctions present a variable number of conductance peaks at distinct energy positions (voltages) – see Figures 2b, 3 and 5 – and (ii) changes in the junction conductance characteristics occur during the voltage sweep. Thus, the conductance peaks showed in Figure 2b are likely to arise from extrinsic factors rather than from the electronic properties of the CuPc molecular ensemble. Here, we advocate that electrode nanoparticles (dots) migrate under the action of the electric field. Once detached from the electrodes, the metal dots become trapped in SML in the junction channel. The trapped metal dots act as sites for electron tunneling, as illustrated in the top and inset of Figure 2b. The observed multiple, low-voltage conductance peaks correspond, therefore, to signatures of CB/CS of electrons tunneling in and out the embedded metal particles, as discussed hereafter in details. In the following, we present experimental proofs of such a field-driven electrode metal diffusion, the possible reasons for this phenomenon, and a controlled manner to induce this effect to create hybrid organic/inorganic SEDs. The referred changes in the junction conductance during the measurements are shown in Figure 3. Here two different bias-dependent phenomena have been identified, namely (a) peak shifting and (b) peak formation. Conductance peaks shifting to lower voltages occur during consecutive bias scans within the limited voltage window of 0.4 V (Figure 3a). During the scans, some other peaks remained in their original position (highlighted by the dashed lines). This fact points out to two independent contributions to the measured conductance, with one of them being affected by the


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electrical stimulus during the measurements. Another observation is the emergence of conductance peaks. In Figure 3b, the I-V curve exhibits the formation of two peaks, centered at approximately +0.2 V and +0.4 V (inset), in the backward scan during a reverse sweep up to higher voltages (2 V), i.e., E ~ 4 MV/cm. Also, in this case the observed conductance peaks result from effects related to the electrical stimulus during the junction measurement.



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Figure 3. Junction responses are susceptible to the electrical stimulus. a) Three consecutive scans (vertical offset for clarity) showing the dI/dV peak shifting to lower voltages in a Au/CuPc (5 nm)/Au rNM junction. Peaks under the dashed lines remained immobile during the electrical measurements. The peak shift effect is related to a size increase of the Au nanoclusters within the junction (see sketch on top). b) I-V characteristics (forward and backward scans) at 10K for a similar junction measured up to 2 V (4 MV/cm). The dashed rectangle indicates the region (0 V to 0.5 V) where dI/dV was calculated. Inset: dI/dV-V plot for the 0 V - 0.5 V window showing the formation of conductance peaks during the reverse voltage sweep. Top: schematics of the propulsion of Au electrode particles inward the junction as the current increases at the positions (i), (ii) and (iii) during the voltage sweep.

Figure 3a and 3b corroborate that the junction’s type II response is a result of extrinsic factors rather than an effect related to the CuPc electronic properties. As previously discussed, such a dynamic behavior is incompatible with the RT mechanism. Other phenomena that could bear with changes in the conductance characteristics of the molecular system during measurements is the oxidation or reduction of metal centers of organometallic molecules.35 However, this process is incompatible with the progressive peak shift showed in Figure 3a. The I-V and conductance characteristics suggest instead that the junction undergoes structural modifications during the electrical measurements. The peak formation exhibited in Figure 3b well matches with the CB/CS explanation, where the electrode metal particles generate conducting sites embedded in the organic barrier. Thus, when electrons tunnel into such a metal site (island), a conductance peak arises. A sketch of the island formation in the junction as the current increases is shown at the top of Figure 3b. The peak shift indicated in Figure 3a also agrees with the CB/CS mechanism, suggesting that embedded metal particles can grow in size during the electrical measurements (schematics depicted in the top of Figure 3a). This is because larger islands lead to smaller Coulomb gaps, i.e., conductance peaks at lower voltages. 14 ACS Paragon Plus Environment

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In this scenario, the immobile peaks are metal particles that do not increase in size during the measurements, for example, for being farther away from the contacts. Finally, the effects showed in Figure 3a,b also indicate that such electrode particle migration can be intentionally induced in the rNM junctions by increasing the electric field or performing multiple scans. Therefore, junctions initially presenting type I response can be converted into devices that exhibit type II characteristics (SEDs). This opens up the possibility to combine different molecular materials with Au nanoclusters in complex, hybrid organic/inorganic devices to explore single-electron charging effects. The field-induced electrode particle migration in the junction occurs by a combination of effects that include the presence of electrode point contacts (Figure S2), soft SMLs, and the application of high electric fields (≥ 1 MV/cm).21,46,47 Although the use of rNM top electrodes eliminates the need for thermal electrode deposition to connect molecular layers, excluding respective diffusion problems,48 it does not prevent electrode particles to diffuse due to the action of high electric fields. The reason is because Au is very malleable and, under bias, Au atoms are likely to migrate.21,47 Metzger et al,28 for example, reported on the formation of Au “stalagmites” at the bottom electrode surface of transport junctions containing SAMs. Because of the thin nature of the junction (2.2 nm thick layer), electric fields of 2.7 - 4.5 MV/cm are applied, leading to high electric currents and rectification.28 Field-assisted electrode diffusion has also been reported for lower electric fields (0.1 MV/cm) and under lowcurrent regimes.49 Field-induced metal migration is also a common cause of failure in OLEDs.46 Here, the applied electric fields are ≥ 1 MV/cm, superior to those commonly reported for the devices poisoned by metal diffusion.28,49 Additionally, both finger and tube electrodes are relatively rough: root-mean-square/peak-to-valley roughness of



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1 nm/3.2 nm and 3 nm/8.5 nm, respectively (Figure S2), which can increase the local electric field by the proximity of electrode point contacts.25 Rough electrodes can also suffer from an additional bias-induced roughening, which can create sharper or new protrusions in the electrodes.50 The characteristics of the molecular film (ultrathin ~ 5 nm, mechanically soft and pin hole-compliant) make the diffusion of electrode particles into the junction even more probable. To support the existence of embedded electrode particles in the junction’s channel, we performed SEM analysis of the CuPc SML combining information from secondary (SE) and backscattered (BSE) electrons. The image acquired from secondary electrons (SE), with average low kinetic energies (< 50 eV) and a very small mean free path into the organic layer, reveals contrasts that reflect mostly the topography of the scanned region. On the other hand, the contrast in the BSE image depends on the average atomic number of the scanned material. Most BSE electrons have energies comparable to that of the the incident beam, resulting in a longer mean free path into the sample. Then, the distinction between the organic matter (CuPc SML) and metallic electrode particles embedded in the junction can be achieved. Here, the rNM footprint region, i.e., the CuPc-coated (10 nm) Au finger electrode, was scanned after removing the top electrode of a junction stressed to the failure. We employed a thicker CuPc layer to prevent the predominance of the electronic signal arising from the background (Au finger electrode). The SEM images were recorded in an FEI Helios Nanolab 660 equipment operating at 2 kV, providing an electron beam with insufficient energy to damage the molecular layer. The schematics of the experiment, the acquired SEM images and a sketch of the CuPc layer structure presenting the embedded metal clusters are shown in Figure 4a-c, respectively.


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Figure 4. a) Schematics for the SEM imaging of the rNM footprint combining information from SE and BSE. b) Corresponding SE and BSE images for 10 nm thick CuPc SML in a junction without the rNM top electrode. The highlighted regions refer, respectively, to (1) metallic and (2) organic debris on the CuPc film surface and (3) metallic clusters embedded within the organic layer (gray region). c) Sketch of the CuPc layer structure containing embedded metal clusters (not to scale). The CuPc molecules (lateral view) are oriented according with the description of Peisert et al51.

Figure 4b shows the SEM images of the rNM footprint obtained over the finger electrode area. In both images, the large gray region corresponds to the CuPc SML covering the Au finger electrode. The image acquired from SE reveals the presence of particles with defined contours on the surface, highlighted as (1) and (2). The image obtained from BSE (1) is shown as a bright spot, while (2) is much darker. Consequently, (1) has a higher atomic number than (2), suggesting they are, respectively, metal and organic debris on the surface. Such debris may be a result of the removal of the rNM electrode for the acquisition of the SEM images. The region (3) exhibits a collection of particles (clusters) as shown in the SE (gray) and BSE (bright) images. Region (3) has a high atomic number, being metal particles embedded within the CuPc SML. Thus, (3) corresponds to metal clusters propelled from the electrodes, as



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hypothesized from the electrical measurements. Such clusters are the main reason for the charge transport features (viz. CB/CS) observed in the rNM junctions. The embedded clusters exhibited in Figure 4b have varied sizes, with radii ranging from ~7 nm to ~30 nm. These, however, correspond to clusters formed in a junction that was stressed to the failure (short-circuit) to guarantee particles large enough to be observable with SEM. From the electrical measurements of 5 nm thick junctions, and considering the CB formalism, smaller sizes (≤ 5.4 nm) have been identified (see below). This is in agreement with other junctions presenting embedded Au particles and similar conductance characteristics.23,52–54 Finally, as illustrated by the SE image, the height of the clusters is smaller than the CuPc layer thickness (10 nm), which suggests the particles adopt a disk-like configuration within the organic matter. CuPc films have been reported to be compliant with the diffusion of Au atoms, leading to the formation of Au clusters into the film structure.55 Such cluster formation is related to the high cohesive energy of noble metals and their weak interaction with organic materials.56,57 For CuPc SMLs deposited on polycrystalline Au, the first 1 nm of organic material is known to comprise 2-3 molecules in a “lying down” configuration.51 As the thickness increases, the CuPc molecules adopt a “standing up” orientation. For a 5 nm thick layer, ca. 5-6 molecules are likely to be present along the junction transport channel.51 Vacancies originated from such change of molecular conformation may accommodate metal clusters propelled from the electrodes, as illustrated in Figure 4c. The electrode metal nanoparticles embedded in the CuPc layers act then as small capacitors in a double-barrier tunnel junction formed between the finger and the rNM electrodes. The Coulomb charging of such particles dominates the junction response, resulting in a conductance gap and multiple peaks. Similar conductance characteristics have been reported in electromigration break junctions presenting metal debris in the


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gap formed during the thining and rupture of metallic nanowires.52,58 Equally to our results, a different number of conductance peaks with variable-position has been verified, which are related to the different particle configurations and dimensions within the gap.52 Our conductance characteristics are also comparable to those found in rNM junctions comprising metal nanoparticles intentionally introduced in the gap by Langmuir−Schaefer method.23 We emphasize though the reported conductance characteristics are unique and differ from the response of the individual materials.23,25 The ultrathin junction containing the metal nanoclusters surrounded by the CuPc SML can be considered as a double-barrier tunnel junction. To observe CB in such junctions, additional experimental conditions have to be satisfied though.13 First, the junction tunneling resistance (RT) must be significantly higher than the resistance quantum (RQ = h/e2), where h is the Planck’s constant and e is the elementary charge. Second, the charging energy (EC) of the cluster, where the electron tunnels to must be larger than the energy of thermal fluctuations (kBT), i.e., EC > kBT, where kB is the Boltzmann constant and T the absolute temperature. To check the validity of these conditions, we present in Figure 5 I-V and conductance characteristics of a rNM CuPc junction during a stable response, i.e., no peak shifting or formation during a reverse voltage sweep.



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Figure 5. a) Stable I-V and dI/dV-V characteristics of an Au/CuPc (5 nm)/Au rNM junction measured at 10K for E ≤ 1 MV/cm. The current and conductance responses considerably retrace (see arrows for scan direction), and no effects such as peak shifting or peak formation are observed. b) dI/dV-V corresponding to the data in (a). Two sets of periodical peaks (A and B) are identified after deconvoluting the experimental data (white circles). The error bars correspond to the standard deviation values during the reversible voltage sweep (forward and backward scans). The solid blue line corresponds to the sum of the deconvoluted peaks with the background response considering the Simmons’ formalism for DT.

From the junction I-V curve (Figure 5a), RT ~ 1 GΩ – see Figure S4a for low voltages – is considerably larger than RQ (25.8 kΩ), therefore satisfying the first 20 ACS Paragon Plus Environment

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condition to observe CB. The conductance gap in the dI/dV-V plot corresponds to EC ~100 meV, which exceeds the thermal energy at 10K (0.86 meV) and fulfills the second requirement as well. Finally, a substantial increase of the differential resistance (dV/dI) around zero voltage has also been observed for such devices (Figure S4b). All these characteristics corroborate the observed conductance peaks are in fact due to CB, caused by the field-induced “doping” of the junction by electrode Au nanoclusters. From the CB characteristics found we calculated the size of the embedded clusters from the cluster self-capacitance (C0) using the relationship EC = e2/2C0.13 From Figure 5, the conductance gap gives EC = 100 meV and, consequently, C0 = 8x10-19 F. Considering an ideal disk shape for the nanocluster, C0 is given by C0 = 8εrε0r, where εr is the relative permittivity of the cluster’s surrounding medium (viz. CuPc SML), ε0 is the vacuum permittivity, and r is the disk radius.13 From the literature, we find 7 > εr > 2.1 for polycrystalline CuPc films,59 which gives a cluster radius of 1.6 nm < r < 5.4 nm. For the in-plane CuPc dielectric constant (εr = 13)24,60 we find r = 0.8 nm. The calculated sizes agree with the dimensions Au nanoparticles reported in the literature for other junctions presenting CB.23,52–54 From the set of measured junctions, EC is within the range of ~100 - 600 meV, which gives r ≤ 5.4 nm and corresponds to disk areas of Adisk ≤ 90 nm2. As the metal clusters are formed from tiny pieces of Au that detach from the electrodes due to the electrical stimulus, the calculated disk areas agree with the effective injection areas previously reported for similar junctions.25 Finally, for thinner CuPc layers (3 nm), the occurrence of conductance peaks in rNM junctions are slightly higher (~60% of the tested junctions). For thicker layers, e.g., 15 and 20 nm, ~30% and ~11% of the devices presented peaks, respectively (Figure S5). The thinner the molecular layer, the shorter the barrier for electrons to tunnel through and reach a metal island within the junction. Additionally, thinner layers are expected to impose a less



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mechanical resistance to the diffusion of the electrode particles. Finally, if the temperature is raised to make kBT closer to EC, the transport becomes thermally activated,25 i.e., dominated by charge hopping. Consequently, the current increases and no dI/dV-V peaks are observed (Figure S6). The observed conductance characteristics are attributed to CB/CS due to the diffusion of electrode particles into the organic rNM junction. Here, each device has a variable number and distribution of electrode point contacts, which leads to different current paths for metal cluster formation, distribution, and measurement of the junction’s single-charge effects. From the conductance characteristics of Au/CuPc (5 nm)/Au junctions (Figure 5), we observe two sets of periodical peaks (A and B), characteristic of CS. These two peaks are better resolved when the dI/dV-V curve is deconvoluted (Figure 5b) and suggest that multiple, parallel tunnel events take place during measurements. Additionally, one can conclude that more than one cluster is probed in the junction at the same time. The curve shown in Figure 5b also highlights the contribution of DT from Simmon’s model,38 which accounts for electrons tunneling directly from the finger electrode to the top rNM contact, in parallel to the singleelectron charging events. Lastly, a contribution from Fowler-Nordheim field emission tunneling was also observed in such junctions (Figure S7). Finally, we validated our experimental paradigm by measuring the electrical characteristics of rNM junctions comprising ensembles of another semiconducting molecule, namely dinaphtho[2,3b:2′,3,-f]thieno[3,2-b]thiophene (DNTT) widely used in organic devices (Figure S8 and S9).61 The observed response agrees with that of CuPc hybrid SEDs, confirming the events here reported arise from the peculiar characteristics of the rNM junctions and not from the SML used. Similar to the CuPc junctions, one can induce such effect in structures that initially do not present single-electron charging responses by, for


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example, increasing the applied electric field (Figure S8b). This process can be considered a viable approach to create new and complex SEDs by using different molecular materials. From another perspective, the reported in-situ, field-induced electrode diffusion in such nanoscale junctions deserves attention as the response of the hybrid device can occasionally mask electrical signatures from other phenomena, e.g., molecule-dependent responses. 4. CONCLUSIONS In summary, we demonstrated that ultrathin (3-10 nm) junctions based on conjugated, physisorbed semiconducting molecules and rNMs present rich I-V and conductance features resembling those of single-electron devices (SEDs). The observed characteristics, however, cannot be attributed to electronic properties of the involved molecules, but to Coulomb blockade (CB) and Coulomb staircase (CS) caused by metallic nanoclusters formed within the junction by in-situ, field-driven electrode migration. Such metal diffusion is ascribed as a combination of high electric fields (14 MV/cm), electrode point contacts and the mechanically soft molecular layers. The ultrathin rNM junctions with embedded metal nanoclusters are regarded as doublebarrier tunnel junctions (or SEDs), with the SML acting as the insulating barrier and the metal cluster as the island. From the electrical characteristics, we find a cluster selfcapacitance of 8 x 10-19 F, which corresponds to cluster radius ranging from 1.6 to 5.4 nm. Embedded metal particles having larger radii (from 7 to 30 nm) have been observed by SEM, which suggests that the clusters can grow during the electrical measurements. We also found that metal migration could be intentionally induced into junctions that initially do not present single-charge effects by increasing the applied electric field above 1 MV/cm. This fact opens up possibilities to create novel, complex and hybrid organic/inorganic SEDs using rNMs and different molecular materials.



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Finally, attention has to be paid to such an effect as the response of hybrid junctions presenting CB/CS can occasionally mask electrical signatures from other phenomena (e.g., molecule-dependent responses) only observable at ultrathin thicknesses.

ASSOCIATED CONTENT Supporting Information Experimental details about the junction fabrication and additional characterization. This material is available at http://pubs.acs.org AUTHOR INFORMATION Corresponding author *E-mail: [email protected] ORCID Rafael Furlan de Oliveira 0000-0001-8980-3587 Leandro Merces 0000-0002-6202-9824 Érico Teixeira-Neto 0000-0002-6529-8472 Carlos César Bof Bufon 0000-0002-1493-8118 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge CAPES, SisNANO, CNPq (Project 483550/2013-2) and FAPESP (Project 2014/25979-2) for the financial support, and the Laboratory for Surface Science and the Electron Microscopy Laboratory at CNPEM (Brazil) for the


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acquisition of AFM and SEM images, respectively. The authors also thank Adalberto Fazzio for the insightful discussions.

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Table of Contents (TOC) 71x28mm (300 x 300 DPI)

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Figure 1. a) Nanomembrane junction layout before (left) and after (right) the roll-up process, and the molecular structure of copper (II) phthalocyanine (CuPc). The controlled removal of the sacrificial layer allows the strained nanomembrane to rolls up and lands on top of the CuPc molecular layer, forming Au/CuPc/Au junctions. b) SEM image of the rNM junction. (c) Current(I)–voltage(V) characteristics at 10K (semi-log plot, arbitrary units), showing the two typical responses (types I and II) observed for a junction comprising a 5 nm thick CuPc layer. Inset: Junction’s cross-sectional view. 51x38mm (600 x 600 DPI)


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Figure 2. Schematics of the charge transport across ultrathin (5 nm) CuPc rNM junctions and their respective low-temperature (10K) current (I-V) and differential conductance (dI/dV-V) plots for a) type I and b) type II responses. Inset: corresponding band diagrams. 180x432mm (300 x 300 DPI)



Figure 3. Junction responses are susceptible to the electrical stimulus. Sketch of the involved phenomena in each case. a) Three consecutive scans (vertical offset for clarity) showing the dI/dV peak shifting to lower voltages. Peaks under the dashed lines remained immobile during the electrical measurements. The peak shift effect is related to a size increase of Au nanoclusters in the junction (top). b) I-V characteristics (forward and backward scans) at 10K for a rNM junction measured up to 2 V (4 MV/cm). The dashed rectangle indicates the region (0 V to 0.5 V) where dI/dV was calculated. Inset: dI/dV-V plot for the 0 V 0.5 V window showing the formation of conductance peaks during the reverse voltage sweep. Top: schematics of the propulsion of Au electrode particles inward the junction as the current increases at the positions (i), (ii) and (iii) during the measurement. 184x454mm (300 x 300 DPI)


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Figure 4. a) Schematics for the SEM imaging of the rNM footprint combining information from SE and BSE. b) Corresponding SE and BSE images for 10 nm thick CuPc layer in a junction without the rNM top electrode. The highlighted regions refer, respectively, to (1) metallic and (2) organic debris on the CuPc film surface and (3) metallic clusters embedded within the organic layer (gray region). c) Sketch of the CuPc layer structure according to Peisert et al51 containing embedded metal clusters (not to scale). 101x48mm (300 x 300 DPI)



Figure 5. a) Stable I-V and dI/dV-V characteristics of an Au/CuPc (5 nm)/Au rNM junction measured at 10K for E ≤ 1 MV/cm. The current and conductance responses considerably retrace (see arrows for scan direction), and no effects such as peak shifting or peak formation are observed. b) dI/dV-V corresponding to the data in (a). Two sets of periodical peaks (A and B) are identified after deconvoluting the experimental data (white circles). The error bars correspond to the standard deviation values during the reversible voltage sweep (forward and backward scans). The solid blue line corresponds to the sum of the deconvoluted peaks with the background response considering the Simmons’ formalism for DT. 131x229mm (300 x 300 DPI)


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