Nanoscale Variable-Area Electronic Devices: Contact Mechanics and

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Functional Nanostructured Materials (including low-D carbon)

Nanoscale variable-area electronic devices: contact mechanics and hyper-sensitive pressure application Leandro Merces, Rafael Furlan de Oliveira, and Carlos César Bof Bufon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12212 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Applied Materials & Interfaces

Nanoscale Variable-Area Electronic Devices: Contact Mechanics and Hyper-Sensitive Pressure Application Leandro Merces†, Rafael Furlan de Oliveira†, and Carlos César Bof Bufon†,* †

Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for

Research in Energy and Materials (CNPEM), 13083-970, Campinas, SP, Brazil *corresponding author: [email protected]

ABSTRACT: Nanomembranes (NMs) are freestanding structures with few-nanometer thicknesses and lateral dimensions up to the microscale. In nanoelectronics, NMs have been used to promote reliable electrical contacts with distinct nanomaterials – such as molecules, quantum dots, and nanowires – as well as to support the comprehension of the condensed matter down to the nanoscale. Here, we propose a tunable device architecture that is capable of deterministically changing both the contact geometry and the current injection in nanoscale electronic junctions. The tunable device is based on a hybrid arrangement that joins metallic NMs and molecular ensembles, resulting in a versatile, mechanically compliant element. Such a feature allows the devices to accommodate a mechanical stimulus applied over the top electrodes, enlarging the junctions’ active area without compromising the molecules. A model derived from the Hertzian mechanics is employed to correlate the contact dynamics with the electronic transport in these novel devices denominated as variable-area transport junctions (VATJs). As a proof-of-concept, we propose a direct application of the VATJs as compression gauges envisioning the development of hyper-sensitive pressure pixels. Regarding sensitivity (~ 480 kPa-1), the VATJ-based transducers constitute a breakthrough in nanoelectronics, with the prospect of carrying its sister-field of molecular electronics out of the laboratory via integrative, hybrid organic/inorganic nanotechnology.

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KEYWORDS: Nanomembranes, Molecular Ensembles, Electronic Transport, Contact Mechanics, Injection Area, Compression Gauges.


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1.

INTRODUCTION The complete manipulation of nano- and molecular materials requires both the

control of individual components and the ability to modify/assemble them according to the envisioned application. Such requirements go along with the establishment of nanotechnology in the past half-century, with focus on exploiting the effects resulting from the nanoscale.1 More than two decades separate Feynman’s first predictions for nanotechnology from its early practical demonstrations with the development of the scanning tunneling microscope (STM).2 Dated from these events, intensive research confirmed that controlling nanostructures requires differentiated approaches, as the physical insights at the macroscale substantially differ from the nanoscopic ones.3–5 Phenomena involving nano-objects are strongly dependent on thermal/statistical ﬂuctuations, many-body interactions, and quantum effects.6 Thus, beyond miniaturization, the effective handling of nanomaterials demands the harmonization of strategies and interactions at the nanoscale.7 Based on such premises, a novel concept named nanoarchitectonics has emerged, combining elements from the so-called top-down manufacturing technologies with bottom-up approaches used in nano- fabrication.8 Within this context, nanomaterials are arranged via consolidated methodologies to create integrated and unexpected functionalities.9 The main appeal of nanoarchitectonics relies on the use of sophisticated nanotechnologies in daily life, replacing high-cost techniques by nano-integrated high-tech-driven strategies.7–11 A well-established route in nanoarchitectonics used to bridge the macroscopic world and the nanoscale involves the handling of self-organized nanomembranes (NMs).12 NMs are freestanding, nanometer thick, surface-like structures with typical lateral dimensions in the microscale.12–14 They distinguish from thin films due to their occurrence in a self-sustained form.13 The reduced thickness of NMs leads to flexural rigidities that are more than ten orders of magnitude smaller than those of bulk materials,15 allowing the attainment of a variety of geometries with notable options for multilayer integration.16 The successful use of NMs often implicates the manipulation of several micro- and nanomaterials, by enveloping, conforming to or accommodating them.13,14 For instance, Kiefer et al. have reported on the thickness-dependence of both the NM rigidity and the elastic energy release involved in NM delaminating processes.17 Deneke et al. unraveled 3 ACS Paragon Plus Environment


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the strain sharing in InAs quantum dots acting as nanostressors onto compliant NM-based Si substrates.18 Band splitting,19 charge carrier mobility,20 electronic confinement,21 and electro-optical phenomena22 are some other examples of fundamental research performed employing NMs. NMs have also shown excellence in electronics.13,23 Their main appeal arises from the possibility of providing for devices properties that cannot be addressed by any other bulk, thin film, or nanoscale material.14 Flexible and stretchable electronics have benefited from mechanical layouts and 3D nanoarchitectures based on NMs,24,25 which allow devices to bend, fold or even twist without damaging the components.18,26 Bioelectronic devices,27 solar cells,28 transformers,29 and field-effect transistors30,31 are some other successful demonstrations of the NM engineering for electronics. One of the most outstanding applications, however, lies in the use of NMs to bridge nano- and molecular electronics.23,32–35 Specifically, NMs have been used to promote reliable electrical contacts with distinct materials, such as molecular ensembles or hybrid organic/inorganic nanostructures, forming NM-based transport junctions,23,32 diodes,36,37 and ultra-compact capacitors.34,38 Such devices have potential to carry molecular electronics out of the laboratory, by means of integrated architectures for the establishment of electrical connections with nanoscale materials, without the limitations imposed by thermal deposition of electrodes (viz. metal diffusion),39 and with no need of sophisticated techniques (e.g. STM, AFM, and liquid metal contact methods).40 The NM-based transport junction was proposed by Bof Bufon et al. in 2011,35 and became henceforth a pivotal device architecture for integrated nano-/molecular electronics. Its structure comprises a microfabricated NM that rolls up and assumes a cylindrical shape. The rolled-up NM is arranged to land on top of the nanomaterial of interest, initially positioned onto a bottom electrode, forming a sandwich-like two-electrode structure.23 Such a device architecture has been demonstrated robust and reliable for connecting semiconducting molecular ensembles (SMEs),23,32,33 arrays of nanoparticles (NPs),36,37,41 and self-assembled monolayers (SAMs).35 For these materials, NM-based junctions have been proved useful to elucidate the conduction mechanisms as a function of voltage bias, temperature, and transport distance. A persistent issue in nanoscale junctions, however, is the misleading correlations between the geometrical and the effective device contact 4 ACS Paragon Plus Environment

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areas.40 Accordingly, measurements of electronic transport at the nano-/molecular scale have shown plenty of contradictions.42 Such a difficult is related to the effective contact mainly occurring via the asperities distributed over the surfaces of the nearest nanoobjects.32 Consequently, a small fraction of the physically contacted region is in fact involved in the charge transport measurements.32 Estimations of the effective contact obtained from measurements of surface adhesion/friction indicate discrepancies up to ~1,000,000% between the geometrical and the effective contacts. Such a difference depends on the hardness of the involved materials, their topographies, and the external loads applied to them.42 Hence, the electrode roughness becomes critical for both the active material uniformity and the measured transport properties.23 For the referred NM-based junctions, which exhibit transport distances of few nanometers, such contact issues persist.23,32,33 Here, we demonstrate that the inherent flexibility and mechanical robustness of NMs to connect molecular ensembles may provide an additional and unexploited advantage for molecular electronics: the top NM-electrode can be controllably deformed by applying an external compression. Consequently, the interface between the NM electrode and the material of interest in the junction (viz. SME) can be manipulated to precisely control the current injection areas at the nanoscale. Such a strategy allowed us to design the first realistic variable-area transport junction (VATJ). Following the Hertzian theory for the contact mechanics,43 the VATJs exhibited injection areas ranging from units to hundreds of nm². Additionally, the VATJs have been proved capable of reversibly operating over NM compressions varying from tens to thousands of nanometers. Such elastic compressions have direct implications on the devices’ electric current. Last, but not least, we have demonstrated a proof of concept application of the VATJs as compression gauges with pressure hyper-sensitivity. The designed device has shown ability to convert the applied load in pressures that vary from tenths to tens of Pa, with the sensitivity of 900 kPa-1 (for the 10-1 Pa range) – a record in the literature considering organic sensitive pixels. Our report on the fabrication, operation, and application of VATJs consolidates NMs as a reliable strategy in nanoarchitectonics to bring together nano- and molecular electronics in a fully-integrated scalable manner.



2.

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EXPERIMENTAL SECTION 2.1. Fabrication of the NM-based junctions. SiO2-coated (2 m thick) Si (100)

substrates were employed for the device fabrication. The substrates were cleaned in an ultrasonic bath with VLSI acetone (5 min) and isopropanol (5 min), rinsed in water (10 s), and dried with N2. The substrate mesa structure (Figure 1a) was patterned on the SiO2 surface by wet etching using hydrogen fluoride (HF) aqueous solution (10% v/v in deionized water) for 3 min. The mesa structure is then coated with Cr/Au (5 nm/10 nm) to act as the device bottom (finger) electrode and contact pad (Figure 1b). It is worth mentioning that the metal layers used in the junction fabrication were deposited by highvacuum (~10-7 Torr) electron-beam evaporation, at room temperature. During evaporation, a quartz crystal microbalance inserted in the chamber was used to monitor the respective film thicknesses and deposition rates (< 2 Ås-1). As it follows, a 20 nm thick Ge film was deposited and oxidized in air, on a hot plate at 80°C for ~10 h, to form the GeOx sacrificial layer (Figure 1c). After the GeOx formation, Cr/Au anchorage structures for the coming NMs were made, and the strained layer – Au (10 nm)/Ti (20 nm)/Cr (28 nm) – is deposited as shown in Figure 1d. The tube-connecting pads were sequentially deposited, as depicted in Figure 1e. Finally, the device active layer, namely copper (II) phthalocyanine (CuPc), was sublimed (powder, 576.08 g/mol, Alfa Aesar) over the finger electrode. The CuPc deposition was carried out in high vacuum (10-6 Torr), at a rate of 0.2 nm/s, while keeping the substrate at room temperature. To complete the device architecture, the sample was immersed in 0.5% v/v H2O2 aqueous solution for the dissolution of the GeOx sacrificial layer, allowing the Au/Ti/Cr strained film to release spontaneously from the substrate. The elastic relaxation of the strained structure curls the quasi-2D object into a cylindrical shape,44 forming the rolled-up, NM-based electrode that contacts the CuPc SME from the top (tube electrode in Figure 1f). For the devices presented here, the tube electrode exhibited a typical radius of R = 3.5 ± 0.5 m. 2.2. Active materials for the NM-based junctions. We have employed mainly 60 nm thick CuPc SMEs as the junction active layer due to their high chemical stability, and versatility in both organic45 and molecular electronics,32 with well-known electrical characteristics.33,38 The CuPc molecular structure is illustrated in Figure 2b (inset). For 6 ACS Paragon Plus Environment

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60 nm thick CuPc SMEs at room temperature, activated hopping is recognized as the dominant electronic transport mechanism.32 Details on the hopping conduction formalism is given as Supporting Information. This transport mechanism allows the VATJs to operate in a conducting regime that exhibits a linear relationship between the injection area and the measured electric current.33 To reduce the VATJs power consumption, we have also employed thermally deposited, 50 nm thick ensembles of dinaphtho[2,3-b:2′,3′f]thieno[3,2-b]thiophene (DNTT) as organic layer for the devices. The deposition parameters were the same employed for CuPc. The measurements involving the DNTTVATJs are provided as Supporting Information. For both the active materials (CuPc and DNTT), we used Au as top (tube) and bottom (finger) electrode-covering to ensure an optimized charge carrier injection in the devices.46

Figure 1. Optical microscopy images of the junction fabrication stages: (a) definition of the SiO2 mesa structure; (b) deposition of the Cr/Au finger structures (electrode and pad); (c) deposition of the GeOx sacrificial layer; (d) prior deposition of anchorage-structures and deposition of the Au/Ti/Cr strained layer; (e) deposition of the tube connecting pads. The selective removal of the GeOx sacrificial layer causes the strained layer relaxation, resulting in the rolled-up tube electrode. (f) NM-based vertical junction (finger/molecular ensemble/tube). 2.3. Electromechanical evaluations. The junction current-voltage (I-V) characteristics were evaluated by using a Keithley 4200 SCS coupled with a MPS150 Cascade Microtech probe station positioned into a Faraday cage. To precisely control the tube diameter compressions (), we have used a calibrated piezoelectric actuator (NanoCube® XYZ Piezo Stage). The actuator can vary z-positions with 1 nm of precision, from zero to ~5 m of amplitude. To elucidate the distribution of strain energy in the NMbased junction, finite-element simulations on the device mechanical compliance were 7 ACS Paragon Plus Environment


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performed using COMSOL Multiphysics®. Details on the simulation parameters are given as Supporting Information.


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3.

RESULTS AND DISCUSSION 3.1. Operating the NM-based junctions as electronic devices. A picture of a 28-

device microchip placed on a forefinger – an evidence of integration – is exhibited in Figure 2a. A scanning electron microscopy (SEM) image of an as-fabricated junction, digitally colored for better visualization of the device components, is shown in Figure 2b. The bias configuration is exhibited inset. The raw SEM image is depicted in the Supporting Information (Figure S1). The voltage bias was limited to V ≤ 6 V (i.e., electric fields E ≤ 1 MVcm-1) to prevent the junction damage by heating or diffusion of electrode particles during the electrical measurements.23,32 Figure 2c shows room-temperature I-V curves acquired as a function of : before (open circles) and under (full circles) an applied vertical compression of  ~ 1 m. The NM-based tube electrode is vertically compressed by a BeCu circular probe tip (diameter of 12 m) positioned over the vertical junction region by means of the piezoelectric stage. The BeCu tip is expected to contact the tube electrode along a contact area more extensive than the junction’s injecting ones due to the probe tip larger proportions in comparison to the device components. A cross-sectional view of the NM-based junction is illustrated inset. Notice the 60 nm thick SME acts as a spacer between the bottom (finger) and the top (NM) electrodes. Hence, the applied potential decreases along the molecular bridge (~ 60 nm) and the amount of current in the device is dependent on the applied vertical compression, as shown in Figure 2c.

Figure 2. (a) Picture of an as-fabricated microchip. (b) False-color SEM image of a NMdevice showing the CuPc SME sandwiched in-between the electrodes (tube and finger). 9 ACS Paragon Plus Environment


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The voltage bias configuration and the CuPc molecular structure are illustrated inset. (c) The electrical response of a 60 nm thick junction before (open circles) and under (full circles) a vertical compression (~1 m) of the tube electrode, for E ≤ 0.5 MVcm-1 at 300 K. A lateral view of the NM-based device is illustrated inset, exhibiting both the applied voltage bias and the applied vertical compression. 3.2. Mechanical and electronic analysis. The electrical characteristics presented in Figure 2c agree with the room-temperature response of a 60 nm thick CuPc ensemble,32 where hopping is referred to govern the charge transport. We observed, however, that a controlled vertical compression of the junction’s NM electrode produces a substantial current raise, while the I(V)-shape is preserved. For the whole applied voltage range, gains in current were observed, reaching increments of up to 8.5 times at 3 V. Such increments may be related to SME modifications, or to a change of interactions between the SME and the compressed NM electrode, e.g. by enlarging the injection area or shortening the conductive channel. Thus, to determine the underlying mechanism responsible for the current gains in the junctions under compression, one must elucidate the contact mechanics involved at the NM/SME interface. For this purpose, we have performed finite-element calculations, and simulated the mechanical energies in the involved structures. The simulation considers the 2D coordinate system (z, x) with symmetry along the y-direction. The model geometry consists of a metallic tube electrode (60 nm thick), a SME (60 nm thick), and a metallic finger electrode (bulk), preserving the real device architecture. As materials for the tube electrode, the molecular ensemble, and the finger electrode, we selected, respectively, chromium (the main component of the NM), an elastomer (to mimic the softness of the molecular ensemble), and isotropic silicon (the main component of the mesa structure shown in Figure 1a). The structural parameters of the materials involved, such as Young’s modulus and Poisson’s ratio, were considered for the calculation. To mimic the external stimulus of the larger BeCu probe tip over the system, the vertical load of 1 Pa was distributed over the top surface of the tube electrode as illustrates the Figure 3a. Notice that the pressure of 1 Pa is within the range in which the VATJs operate reversibly according to the Hertzian model discussed hereafter. More details on the finite-element calculations are provided as Supporting Information.


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The Figures 3a-e exhibit the simulation results, regarding the strain energy density (SED). The SED provides information on the elastic energy stored in the system due to the deformation states of each compressed/stretched portion. The geometric shapes (electrodes and molecular ensemble) of the as-prepared junctions are shown in Figure 3a (gray shades). Under an external mechanical pressure of 1 Pa, the tubular structure bends to accommodate the overall mechanical stimulus. The colored shades presented in Figures 3a-d represent the deformed structure according to the scale bar shown in Figure 3e (normalized SED). Notice that the strain energy is distributed along the bent shape. The NM mechanical compliance accommodates the significant portion of the applied pressure, which is distributed over its parts (viz. upper, lateral and bottom regions), as Figures 3b-d depict. The SED over the upper electrode region (Figure 3b) shows the applied strength is mainly stored by the tube as a tensile strain at the inner wall, as confirmed by the strain distribution exhibited in the Supporting Information (Figure S3a). A compressive strain, on the other hand, is achieved at the outer edge. Such strain distribution is inverted on the sides of the tube electrode (Figures 3c and S3b). At the tube bottom region, the strain configuration remains the same as the one observed at the upper part, except for the higher SED (Figure 3d and S3c). From the SED (Figures 3a-d), we find the NM bottom region holds the major fraction of the applied pressure, in comparison with other tube parts.

Figure 3. (a) Cross-sectional view of the device vertically compressed by 1 Pa. Both SED and device deformed shape were calculated using COMSOL Multiphysics®. The SED is 11 ACS Paragon Plus Environment


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exhibited in detail for the junction (b) upper area, (c) side region, and (d) tube/SME-contact region. The SED intensities follow the scale bar exhibited in panel (e). The SED along the z-axis, depicted in (d) by the dotted line, is shown in (e). The mechanical contact between the NM and the SME surface determines the contact area, which may vary according to . The first point of contact between these structures undoubtedly reaches the maximum strain energy. Consequently, the applied pressure is not uniformly distributed over the NM/SME interface, as shown in Figures 3d,e. Because of the NM mechanical compliance, just a fraction of the strain is transferred to the SME. The Figure 3e exhibits the SED profile obtained along the z-direction at the contact region. For the tube bottom region, a SED of ~120 µJm-2 is reached, while the calculated value is ~4 µJm-2 for the SME. Thus, only ~ 3% of the maximum strain energy is transferred to the molecular layer. Additionally, such a small transference is limited to the SME upper interface, being promptly reduced to zero along the overall ensemble thickness (viz. 60 nm). Therefore, we can claim that the SME structure is preserved during the tube compression. Consequently, the observed current increments are likely to be caused by an increase of contact area provided by the compressed electrode. 3.3. Insights on the device contacting areas. The enlargement of the contact area in the junction leads to an increase in the device injection area (Ainj) and, therefore, to higher electric currents. Envisioning a complete description of the VATJ characteristics, the correlation between Ainj and the measured current must be found. During the junction formation, the initial Ainj is given by the intrinsic tube diameter compression (  which is due to the accommodation of the tube over the SME-coated finger structure. In our previous work,32  was estimated approximately as the SME thickness because no compression along the SME occurs for as-prepared junctions.32,33 However, such an estimation is not sufficient here as the NM electrode is intentionally pressed against the SME during the VATJ operation. Nevertheless, as supported by the simulations (Figures 3a-e), the 60 nm thick CuPc ensemble has its dimensions preserved under vertical tube compressions. Therefore, the combination of a constant SME thickness and a fixed applied voltage results in the same E for both loaded and unloaded devices. In this scenario, the relationship between the measured current and the junction area can be determined once 12 ACS Paragon Plus Environment

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the electronic transport in the junction, at a given E, is known. For E ≤ 106 Vcm-1, at room temperature, the current through the CuPc SME is well described by the hopping conduction mechanism (see the Supporting Information), which predicts a linear correlation between I and Ainj.32,33 Thus any current increment can be directly associated with an Ainj increase. To quantitatively evaluate the I-Ainj relationship, we have fabricated devices in which the no-load situation corresponds to the imminent electrical contact between the CuPc ensemble and tube electrode. Next, a set of I-values are collected for the junctions at a fixed bias (V = 4 V) under two distinct situations: (i) with no load applied on the NM electrode and, (ii) with a pressure high enough to cause a deformation of  ≈ R = 3.5 m. Figure 4a exhibits the distribution of the corresponding electric currents for the described situations (illustrated inset).

Figure 4. (a) Discrete distributions (~1,200 counts) of I for i) ≈ zero and ii) ≈ R = 3.5 m. The situations (i) and (ii) are depicted inset. The vertical arrow in (ii) represents the direction of the mechanical compression. b) Reproducibility assay performed by switching  from 0 to 3.5 μm, continuously and repeatedly. (c) 3D-projection 13 ACS Paragon Plus Environment


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of the VATJ electrode topographies derived from atomic force microscopy (AFM) micrographs. The topographies of both tube and finger electrodes are exhibited in panels (d) and (e), respectively. From Figure 4a, the junction currents are I(i) = 60 ± 10 fA for the no-load condition and I(ii) = 1.2 ± 0.1 nA for  ≈ 3.5 μm. In both cases, the measured current is superior to the experimental electric noise (~10-20 fA). The current density for the 60 nm thick CuPc ensembles submitted to V = 4 V (E ≈ 0.7 MVcm-1) at 300 K is JCuPc,60 nm ≈ 102 kAcm-2.32 Thus, I(i) implies Ainj(i) = I(i)/JCuPc,60 nm ≈ zero for no load applied on the tube. This condition represents the situation where point contacts dominate the devices’ charge injection.32 On the other hand, when the tube electrode is considerably deformed, Ainj(ii) = I(ii)/JCuPc,60 nm reaches approximately 1.2 nm2, increasing I by 4-5 orders of magnitude. Figure 4b exhibits the

repeatability

evaluation

of

the

junctions

upon

performing

several

compression/decompression cycles. A minimum of 150 compression/decompression cycles were achieved before the device response loses its reversibility. Supplementary data on the VATJ durability can be found as Supporting Information (Figure S4). During compression, the junction current rises rapidly, reaching steady-state values. After decompression the current decays to approximately the same values presented in the initial response for the unloaded situation, as expected for an elastic tube deformation. A detailed evaluation of the junction’s response time for a short, intermittent mechanical stimulus is discussed hereafter. The junction failure can be attributed to permanent damages on the NM electrode, as  = R implicates a severe deformation that distorts the tube electrode geometry. For  > R, irreversible tube distortions and short circuit issues are observed. The experiment regarding intermittent stimulus, however, was used to set up both the lower (i) and upper (ii) tube deformation limits to reproductively operate the VATJs. Within such limits, Ainj-variations can be well controlled and determined. 3.4. The correlation between electric current and applied load. A relevant feature of NM-based junctions is that Ainj can be determined from the geometrical characteristics of the electrodes, by using a Hertzian approach for the contact mechanics,43 and considering the electrode topographies. The Hertzian model used here considers a cylinder (tube electrode) placed in contact with a flat surface (molecular ensemble on the 14 ACS Paragon Plus Environment

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finger electrode), as two elastic half-spaces.43 Additionally, such contacting objects may exhibit low deformations in comparison to their initial shapes,43 guarantying the operation of the devices within the elastic limits of the materials. Finally, for the approximation, the involved surfaces are considered continuous, non-conforming, and frictionless.43 A detailed discussion regarding the Hertzian approach used to estimate Ainj is found in the Supporting Information. To account for the contribution of electrode roughness on Ainj, refining the Hertzian model proposed for the NM-based junction, we performed atomic force microscopy (AFM) imaging of the device electrodes. Figure 4c exhibits a 3Dprojection of the VATJ electrode topographies derived from the AFM micrographs for the as-fabricated device. The tube and finger topographies are shown in Figures 4d,e, respectively. The measured roughness (root-mean-square, rms) of the NM and finger electrodes are 3.0 ± 0.4 and 1.0 ± 0.2 nm, respectively, in agreement with our recent findings using a similar platform.23,32 It is worth mentioning that such small roughness values are crucial to precisely control the electric field across the junctions. From the AFM analysis, another primary parameter accounting for surface irregularities is the lateral correlation length, which provides information about the extension of topography uniformity around a point on the investigated surfaces. Here, the calculated correlation lengths, for both electrodes, are found between ~50 and ~90 nm. Such a range of values corresponds to a region of thousands of nm2, more extensive than the Ainj-values (hundreds of nm2). Thus, a uniform contact at the NM/SME interface guarantees a reliable I- relationship – i.e., I α 1/2 (see the Supporting Information).



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Figure 5. (a) Current and injection area (Ainj) as a function of  for E = 0.5 MVcm-1. The experimental points are shown as open circles. The solid line represents the theoretical prediction of Ainj considering the Hertzian contact mechanics. Dotted lines represent the model limits with experimental inputs. (b) Time-resolved measurement: device current response varies ~2400% within t < 100 ms for an external load equivalent to ~16 Pa. Optical microscopy images of the VATJ during the measurement are shown inset. (c) Device current response as a function of the compression. Both curves were obtained for E = 0.5 MVcm-1 (V = 3 V), where the device power consumption is within 60-165 nW. An illustration of the compression gauge for pressure sensing is shown inset. In Figure 5a, we present the estimated I- relationship (solid line) based on the Hertzian mechanical contact theory. The dotted lines represent the reliable limits for the model, considering deviations from the electrode topographies and the experimental tube diameter. The circles are the experimental points. The first point corresponds to 0, which is inherent to each junction under test. Since the device’s fabrication is entirely performed by micro-scale techniques, 0 can neither be previously designed nor predicted, but determined after the junction processing. The device current response as a function of  for  > was recorded applying V = 3 V (E = 0.5 MVcm-1) in atmosphere conditions and at 16 ACS Paragon Plus Environment

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room temperature. We have chosen a device which displays response signals in the nArange under small applied loads to improve the dc signal detection. In this scenario, the device current density is expected to be J = 10 kAcm-2.32 We found that the Hertzian approach converges for  > 200 nm, as Figure 5a depicts. For significant tube deformations (hundreds of nanometers), I is well-explained by the model, bringing the conduction phenomenon and the mechanical contact theory at the nanoscale altogether. 3.5. VATJs as compression gauges for pressure-hyper-sensitivity. As a proof of concept on the applicability of VATJs, we demonstrate an application as compression gauges aiming the development of new pressure-sensitive pixels. By using a device where  ≈ 10 nm, we have determined the external pressure applied over the VATJ by univocally translating the device current into pressure values, following the refined Hertzian model discussed above. The VATJ sensing mechanism is governed by the electronic properties of the as-prepared junction. The pressure sensing characteristics are exhibited in Figures 5b,c. Its main figure of merits are presented in Table 1, alongside to the respective parameters of other pressure sensitive pixels reported in the literature. Figure 5b depicts the device time-resolved response to an external stimulus. Here, a step-like load corresponding to ~16 Pa was applied over the VATJ, while the device’s current measured as a function of time. The stimulus transient is faster than 50 ms and the device response time was found t < 100 ms. Therefore, the resulted response speed is of ~160 Pa s-1. Due to the reversible characteristics of the VATJ response, within the Hertzian limits (Figure 4b), the device recovery time is also < 100 ms for similar stimuli. For such compression limits, the corresponding pressure varies from 0.1 to 20 Pa, defining the device detection range. One may notice the low detection limit achieved by the VATJs (see Table 1). We attribute such a characteristic to three core aspects. The first one regards the fact that the electronic transport observed for the ensemble-molecular devices is dominated by hopping conduction. For this mechanism, at a fixed bias, the electrical current is proportional to Ainj as discussed in Section 3.3. The second point is that the device architecture is suitable as it allows significant increments of Ainj – e.g., from zero to 1.2 nm2, and from ~200 to ~500 nm2 in Figures 4b and 5a, respectively – depending on the external stimuli of compression. These Ainj-increments are directly translated, by the 17 ACS Paragon Plus Environment


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VATJ’s architecture, into electrical current intensification. The third aspect is related with the low rigidities exhibited by the NMs employed here as the device top electrode (Figure 3). Accordingly, a low-pressure stimulus (~1 Pa) may represent a central tubedistortion which results in a critical change in the injecting areas, implying the significative variation observed in the electrical current. The Figure 5b (inset) displays optical microscopy images of the VATJ-based gauge during the time-resolved experiment. A video displaying a series of top electrode vertical compressions is presented as Supporting Information. Another important feature of the VATJ-based compression gauge is its low operational voltages. Since the device response relies on the hopping conduction across a nanoscale transport channel (60 nm thick SME), we can operate the VATJ-based transducer within V = 1 – 5 V. Consequently, our devices display a power consumption of less than 275 nW. For instance, by using a voltage bias of 1 V, the power consumption of the measured device decreases to ~ 20-55 nW. Additionally, by using another active material with better electronic transport performance, viz. DNTT,32 we demonstrated that the VATJ could successfully operate at 0.1 V, reaching the unprecedented power consumption of 2 to 6 nW. Such values may represent an improvement of ~98% in efficiency when compared to the ultra-sensitive pressure sensor based on organic transistors, reported by Zang et al.47

TABLE 1. Sensing features of state-of-art, organic pressure-sensitive pixels. Sensing pixel

Sensitivity (kPa-1)

Response time (ms)

Detection limit (Pa)

Detection range (Pa)

Reference

Capacitivea

~ 15

–

~ 5 104

~ 105

(48)

OFETb

0.1 – 0.6

~ 10

~ 102

102 – 104

(49)

SGOFETc

192

~ 10

~ 5 10-1

102 – 103

(47)

Piezoelectrica

~ 0.06

~ 0.1

~ 103

104 – 105

(50)

Resistive

~8 – 42

< 50

~ 101

101 – 102

(51)

e-Skind (OFET)

< 8.4

< 10 ms

~ 5 10-2

10-2 – 101

(52)

VATJe

480f

~ 80

~ 5 10-2

10-2 – 101

(this work) 18

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a. Some parameters were calculated from the data presented in the corresponding references. b. OFET: organic field-effect transistor. c. SGOFET: suspended gate organic field-effect transistor. d. e-Skin: electronic skin e. VATJ: variable-area transport junction. f. The value of 480 kPa-1 is an average; the maximum sensitivity was found as 900 kPa-1. The solid line in Figure 5c is the calibration curve for the VATJ-compression gauge. Such a curve represents the predicted pressure as a function of the device current within the Hertzian limits (dotted lines). The open circles are the experimental points. The error bars correspond to the uncertainty propagation from repeated current measurements at each applied load. From the measurements exhibited in Figure 5c, the VATJ-transducer can resolve pressure values of tenths of Pa. The curve slope varies from 33 to 14 nAPa-1, from lower to higher pressure values. By normalizing the current in order to eliminate possible device-to-device variations – once individual devices may present initial currents slightly different from each other – the parameter effectively measured to determine the difference of pressures becomes ΔI/I0. Hence, the secondary y-axis of Figure 5c exhibits the parameter I/I0 for the proof of concept compression gauge. The sensitivity S of our devices are defined as S = (I/I0)/P.47,49,51,52 By taking advantage of the VATJs loadcompliance, we found S = 480 kPa-1. Such a value is higher than the one previously reported for the device so-called ultra-sensitive pressure sensor.47 The absolute sensitivity, within the lower Hertzian limit exhibited in Figure 5c may reach values of 900 kPa-1. Hence, VATJ-compression gauges may stand as a viable strategy in pressure mapping for practical applications, with reasonable fast response times, hypersensitivity, low-pressure operation ranges, and low-power consumption.



4.

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CONCLUSION We reported on the use of NMs to investigate the contact mechanics in molecular

junctions, aiming the connection between micro- and nanotechnologies for practical applications (viz. pressure sensing). Hence, NM-based transport junctions composed of CuPc SMEs were analyzed during controlled mechanical compressions of the junctions’ top electrode (tube). Due to the mechanical features of the NMs, the junctions have exhibited high compliance and robustness against applied load stimuli. Finite-element calculations provided information on the strain distributions (viz. SED) along the electrodes and the molecular ensemble. The NM mechanical compliance allowed the tube distortions to be well approximated by pure diameter compressions. Then, during reversible compression assays, the compliant NM-electrode readjusts itself allowing to control the effective injection area of the junctions. A model derived from Hertzian mechanics was employed to elucidate the mechanical contact, and the first variable-area transport junction – the VATJ – was demonstrated. As a proof of concept, the VATJs were evaluated as compression gauges envisioning the development of novel hyper-sensitive pressure pixels. Therefore, by employing nano-integrated strategies to connect molecules in electronic junctions, our findings contribute to the further use of sophisticated nanotechnology in daily life, carrying together nano- and molecular electronics.


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ASSOCIATED CONTENT Supporting Information - Raw SEM-data, details on the finite-element calculations, theoretical considerations regarding both the charge transport and the mechanical contact formulation, measurements of the DNTT-VATJs (PDF); - A video displaying sequential tube vertical compressions (AVI).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Leandro Merces: 0000-0002-6202-9824 Rafael Furlan de Oliveira: 0000-0001-8980-3587 Carlos César Bof Bufon: 0000-0002-1493-8118 Author contributions L.M. fabricated the VATJs, performed the electrical measurements and data evaluation, discussed results, and wrote the manuscript. R.F.O. contributed to the electrical measurements, data interpretation, and manuscript writing. C.C.B.B. supervised the experiments, discussed the results, and wrote the manuscript as well. Notes The authors have no conflicts of interest to disclose.

ACKNOWLEDGMENTS We acknowledge CNPq (153282/2018-5), CAPES (88882.143389/2017-01), SISNANO, and FAPESP (processes 2014/25979-2 and 2013/22127-2) for the financial support. We also thank the collaborators from LNNano/CNPEM-Brazil: E. Teixeira-Neto and E.M. Lanzoni for the acquisition of SEM and AFM images, respectively; D.H.S. Camargo 21 ACS Paragon Plus Environment


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for the technical support; R.M.L. Silva, L.O.Z. Falsetti, and F. Marques for the fruitful discussions about the SED.

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Nanoscale Variable-Area Electronic Devices: Contact Mechanics and

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