Maskless Arbitrary Writing of Molecular Tunnel Junctions - ACS

Subscriber access provided by LAURENTIAN UNIV

Article

Maskless Arbitrary Writing of Molecular Tunnel Junctions Seo Eun Byeon, Miso Kim, and Hyo Jae Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14347 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Maskless Arbitrary Writing of Molecular Tunnel Junctions Seo Eun Byeon,† Miso Kim,† and Hyo Jae Yoon*

Department of Chemistry, Korea University, Seoul, 02841, Korea KEYWORDS tunnel junction patterning, makless arbitrary writing, untethered junction, tunneling, charge transport

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

ABSTRACT

Since fabricating geometrically well-defined, noninvasive, and compliant electrical contacts over molecular monolayers is difficult, creating molecular-scale electronic devices that function in high yield with good reproducibility is challenging. Moreover, none of the previously reported methods to form organic–electrode contacts at the nanometer and micrometer scales have resulted in directly addressable contacts in an untethered form under ambient conditions without the use of cumbersome equipment and nanolithography. Here we show that in situ encapsulation of a liquid metal (eutectic Ga–In alloy) microelectrode, which is used for junction formation, with a convenient photocurable polymeric scaffold enables untethering of the electrode and direct writing of arbitrary arrays of high-yielding molecular junctions under ambient conditions in a maskless fashion. The formed junctions function in quantitative yields and can afford tunneling currents with high reproducibility; they also function at low temperatures and under bent. The results reported here promise a massively parallel printing technology to construct integrated circuits based on molecular junctions with soft top contacts.


Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Envisioned as extremely small-sized electrical components designed to overcome the limit predicted by Moore’s law, molecular-scale electronic devices promise engineering of the electronic function of devices through changes in atomic-level details of the structure of molecules incorporated into them.1-2 Thus, they have been exploited for not only a fundamental understanding of charge transport by quantum mechanical tunneling across extremely thin organic films3-11 but also various applications such as diodes,2, 12-16 memories,17 transistors,18-19 negative differential resistors,20-21 photoswitches,22-24 and flexible devices.7,

22-23

Currently

available methods for building molecular-scale electronic devices usually rely on singly formed electrode–molecule(s)–electrode junctions at the nanometer scale.10, 25-32 These methods require cumbersome equipment and inevitably yield devices comprising electrodes with a tethered form. Otherwise, nanolithography processes are needed to achieve geometrically defined organic– electrode contacts at the nanometer scale; metals are directly deposited onto delicate organic surfaces, which results in low production yields of working devices and irreproducible electrical behaviors. Given that commercialization of molecular-scale electronic devices mainly depends on a high-throughput technique for junction formation across a large-area substrate, ensuring direct addressability in forming electrically compliant, noninvasive, and geometrically welldefined top contacts in an untethered form onto ultrathin organic films without using bulky equipment or high-energy lithography is a formidable challenge. Electrode–molecule interfaces play a critical role in determining the performance of molecular-scale electronic devices because they govern the effective contact area participating in charge transport,5 the mechanism of charge injection between an electrode and molecule,32 and the interfacial dipole.6 Indeed, different top contacts result in tunneling currents that differ by



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

several orders of magnitude for identical molecules.5, 33-34 Therefore, intensive efforts have been devoted to fabricate a robust, reproducible top contact on a delicate organic surface and elucidate the role of interfacial properties in device performance. For junctions with large areas, introducing top contacts via the conventional metal evaporation process damages organic surfaces, leading to a high device failure rate.31, 35 To circumvent this problem, researchers have inserted a conducting polymer (e.g., PEDOT:PSS)36-37 or graphene film30, 38 as a protective layer between electrodes and delicate organic surfaces. In other cases, researchers have placed electrodes compatible with low-energy environments, such as liquid metals, on an organic surface as a top contact.25-26,

39

These previous methods contributed, at least in part, to the

improvement of the yield of functional devices. However, none of these approaches offer the ability to directly “write” in a geometrically defined and conformal manner onto subnanometerthick organic films under ambient conditions. To overcome this challenge, we have used a convenient photocurable polymeric scaffold to encapsulate in situ a liquid metal (eutectic Ga–In alloy4, 25)-based conical microelectrode in contact with a self-assembled monolayer (SAM)40 and then untether it without damaging the organic surface (Figure 1a and b). Our systematic study shows that the electrical characteristics at the SAM–liquid-metal top interface do not vary significantly before and after encapsulation. The resulting untethered junctions exhibit remarkably high yield, reproducible tunneling characteristics. Our method could lead to the development of soft top contacts relevant to massively parallel printing in quantum tunneling electronics based on individual molecules.


Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Eutectic Ga–In (EGaIn) is a non-Newtonian liquid metal that forms a self-passivating oxide (Ga2O3)41 layer upon exposure to air. Because of this oxide layer, EGaIn can be used to form a geometrically defined microelectrode such as a conical tip (Figure 1a). The simplicity of junction formation and measurements using this microelectrode makes it possible to obtain sufficient junction data to draw statistically meaningful structure–tunneling relations.3,

5, 7, 25, 33

The

photocurable polymer we used is available from a commercial vendor. It is sensitive to long wavelength ultraviolet (UV) light from 320 to 380 nm. See the Supporting Information for details about the photocurable polymer. In a typical experiment, a compliant Ga2O3/EGaIn conical tip was formed and brought to the surface of an SAM for top contact following previously reported procedures ((i) in Figure 1b).3, 5 A drop (~2 µg) of photocurable polymer was placed near the AgTS/S(CH2)nCO2H//Ga2O3/EGaIn junction ((ii) in Figure 1b). Subsequently, as the polymer overspread the junction ((iii) in Figure 1b), as confirmed by optical microscopy, we flooded it with UV light for a few seconds (~2 – 5 s) using a handheld lamp ((iv) in Figure 1b). The conical tip was tightly bound by the cured polymeric scaffold, and the well-defined Ga2O3/EGaIn top contact (~102 µm2) in untethered form was generated by lifting the syringe using a micromanipulator ((v) in Figure 1b). Figure 1b presents a schematic of the junction structure and snapshots for each step that are taken by an optical microscope. To determine whether the treatment of the polymer influences the electrical characteristics of the Ga2O3/EGaIn top contact, we prepared an SAM of mercaptododecanoic acid (HS(CH2)n−1CO2H, where n = 12; denoted as C12) on an ultrafast silver substrate formed by the template-stripping method.39 The carboxylic acid terminal unit increased the surface wettability of the photocurable polymer we used. Note that the wettability of photocurable polymer over



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

SAM is important for accomplishing our untethering technique. The photocurable polymer used here (NOA 081; see the Supporting Information for details) does not well wet n-alkanethiolate SAMs, and the yield for fabricating stable untethered junctions decreases. Indeed, contact angle of NOA 081 droplet for n-alkanethiolate SAM is higher than those for polar unit-terminated SAMs (Figure S1 in the Supporting Information). The EGaIn conical tip was successfully encapsulated by the polymer and subsequently untethered (as shown in Figure 1c). This method is not limited to conical tips: a spherical hanging drop of EGaIn electrode was also untethered by our method (Figure S2 in the Supporting Information). Upon encapsulation by the polymer, the geometrical contact area (as measured by optical microscopy) of the EGaIn conical tip over the substrate did not change significantly. We confirmed this by comparing the geometrical contact areas before and after untethering of the EGaIn conical tip on glass (Figure S3 in the Supporting Information). To determine whether the tunneling current density (J, A/cm2) across the EGaIn junction was influenced by the polymer encapsulation, current density–applied bias (J–V) measurements were conducted in the course of fabricating untethered junctions. No significant change was observed in J before and after the polymer encapsulation (Figure 1d and Figure S5 in the Supporting Information). This finding indicates that the change in interfacial environment from air to the polymer does not affect the tunneling characteristics of the EGaIn-based junction. The influence of change of environment on tunneling current density in large-area junction was previously reported. Barber et al.4 showed that J(V) for EGaIn-based junction formed with ndodecanethiolate SAM does not significantly vary when the atmosphere changes from air to oxygen, nitrogen, argon and ammonia. When acetic acid was vaporized into air, approximately an order of magnitude decrease in current density was observed, and this was attributed to the chemisorbed acetic acid onto the EGaIn tip. They also found that changing the relative humidity


Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from 20-60% does not influence junction measurements. Insignificant change in J(V) after untethering in our experiment also implies that the untethering process does not significantly change the effective contact area. Simeone et al.5 emphasized that for large-area junctions based on EGaIn conical tips effective contact area that participates in charge tunneling is approximately three orders of magnitude smaller than geometrical contact area (measured by optical microscopy). This results from rough surface of Ga2O3 in EGaIn conical tip. Our untethered junction showed fairly good consistency in J over repeated junction measurements. We cycled junction measurements 1000 times for untethered junction of SC11COOH SAM, and found no significant change in J (Figure 1e). The J-V trace did not change even after 72 h-aging in air (Figure 1f). The flux of UV light we used corresponds to ~2.7 × 10-15 W/m2, and our untethering process takes about 1-2 min. In this timescale and under the energy of light, aliphatic and aromatic SAMs did not show significant deterioration of J indicating no significant degradation of SAMs. When we shed the UV light to AgTS/SC11COOH//Ga2O3/EGaIn untethered junction for much longer time (2 h), J did not vary significantly (Figure S6 in the Supporting Information). For aromatic SAM, we tested terphenylthiolate SAM, and found the corresponding untethered junction shorted after 2 h UV irradiation (Figure S6 in the Supporting Information). This is probably due to not the degradation of conjugated moiety by UV absorption but UV light-facilitated airoxidation of sulfur on SAM.42-44 Our method is distinguished from other junction techniques by its ability to form arbitrary patterns on a large-area substrate (in the range from micrometers to centimeters) in a maskless fashion. This was evaluated by writing the letters “KU” in bitmap font (Figure 2a). The sketch of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 29

the letters was placed underneath 2.5 cm × 2.5 cm indium tin oxide (ITO), as shown in (Figure S7 in the Supporting Information), and an untethered EGaIn microelectrode was patterned thereon. The generated pattern comprised 23 junctions (Figure 2a and b) that contained an ITO//Ga2O3/EGaIn van der Waals interface. In this initial proof of concept, we compared electrical characteristics for the junctions at ±0.5 V. None of the junctions shorted; the values of J(V) for all the junctions were indistinguishable: the histogram of J(V) constructed from all of the junction data showed a mean value of log|J| (log|J|mean) of ~3.4 and a standard deviation (σlog|J|) of 0.2 (Figure 2c and Figure S8 in the Supporting Information). Non-shorting junction is defined as the junction that does not result in high current corresponding to the upper detection limit (~100 mA) of sourcemeter. The narrow dispersion of the tunneling data and the excellent yield in junction measurements indicate that our untethering method is reproducible and robust. It also enables direct writing of molecular junctions. Figure 3a shows the photo of 3 × 3 patterned junctions formed with HS(CH2)n−1CO2H, where n = 8 and denoted as C8, SAM on AgTS. All of the junctions functioned properly; the histogram of log|J| for the untethered junctions yielded values of log|J| that were indistinguishable from each other (Figure 3b) as well as from that of the analogous tethered junctions (Figure S9 in the Supporting Information). To establish the tunneling platform based on an untethered EGaIn top contact, we examined the dependence of tunneling current density on the width of the tunneling barrier, as described by the simplified Simmons model,45-46 J = J0 × exp(−βd). Here, J0 (A/cm2) is the tunneling injection current density and β is the tunneling decay coefficient (per carbon, C−1). From a plot of log|J| against molecular length (number of carbons), we can extract values of β (the slope) and J0 (the y intercept). Values of β and J0 are dictated by the electronic structure of the molecular backbone in the SAM and the molecule–electrode interfaces, respectively. These values can be 8 ACS Paragon Plus Environment

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


used to compare junctions containing identical molecules formed by different methods. For this study, mercaptoalkanoic acids (HS(CH2)n−1CO2H, where n = 8, 10, 12, and 16; denoted as C8, C10, C12, and C16, respectively) of different chain lengths were used to form SAMs (Figure 4a). The junctions of untethered EGaIn top contacts exhibited almost quantitative yields of working junctions and a narrow distribution of J(V) (σJ < 0.4; see Table S1 in the Supporting Information). Figure 4b shows a plot of J(±0.5 V) as a function of the alkyl chain length. (See Figure S9-S12 in the Supporting Information for the corresponding histograms of log|J|.) The value of log|J| decreased linearly with increasing length of the n-alkyl backbone. The simplified Simmons model accounts for the linear relation. The values of β and J0 were ~1.08 C−1 and ~104.4 A/cm2, respectively, consistent with those (β ≈ 1.08 C−1 and J0 ≈ 104.4 A/cm2) estimated in the analogous junctions of a tethered EGaIn top contact.47 These results indicate that the untethered top contact functions properly in the quantum tunneling regime and that the interfacial electrical characteristics for tethered and untethered junctions are indistinguishable. Carlotti et al. 48 demonstrated that the change of environment in large-area junction makes little influence on the electronic structure of SAM for aromatic and n-alkyl SAMs (oligophenylenethynylenes and nalkanethiols). When O2 content in atmosphere decreased from ~21% (typical O2 content in dry air) up to ~3%, tunneling decay constant (β) remained constant for both types of molecules. No changes in β indicate the change in environment did not lead to change of transport energy level of SAMs enough to cause detectable change in J(V). The performance of the untethered EGaIn top contact was further gauged in the context of repeatability in electrical contact. Figure 4c shows the J–V characteristics of the C8, C10, C12, and C16 molecular junctions; the J values were maintained upon repeated contact and de-contact at a single junction, indicating that no significant deterioration of J occurred under repeated voltage and top contact stress.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

An untethered EGaIn top contact is relevant to achieving molecular rectification. 2,2′Bipyridine-terminated n-alkanethiol (HSC11BIPY) causes an asymmetric response of tunneling currents to applied biases of opposite polarities, that is, a high and reproducible rectification ratio (r = |J(+V)|/|J(−V)|; log|r| = 1.9 ± 0.2) has been reported.14 As shown in Figure 5, we fabricated a 3 × 3 array of untethered junctions of HSC11BIPY SAM on template-stripped gold (AuTS). All of the formed junctions yielded reproducible r values (log|r| = 1.8 ± 0.4) that were consistent with those of the analogous tethered junctions (log|r| = 1.9 ± 0.3) (Figure 5 and Figure S13 in the Supporting Information). The simplicity and direct addressability of untethered Ga2O3/EGaIn top contacts enabled us to build practical devices suitable for low-temperature experiments. Tunneling data at low temperatures can be used to discern whether transport involves a thermally activated hopping process.32 The AgTS/SCnCO2H//Ga2O3/EGaIn junctions of untethered form were loaded into a cryogenic probe station, and temperature-variable J–V measurements were performed at low temperatures (from 300 to 100 K) under vacuum (~10−4 torr). The J–V characteristics of the junctions were temperature-independent; J–V traces did not vary within the tested temperature range (Figure S14 in the Supporting Information), and no changes in β and J0 values were observed (Figure 6a). The Arrhenius plot shown in Figure S15 in the Supporting Information indicates that hopping does not occur during transport and that the transport is dominated by pure tunneling.47 The charge transport across untethered junctions occurs in the absence of hopping and shows the molecular length-dependence of tunneling current density. Therefore, the charge transport across the Cn (n=8, 10, 12, 16) SAMs in untethred junctions is assumed as throughbond tunneling.49-51 It has been well established that transport in SAMs of n-alkyl backbone occurs in a pure tunneling regime; highest occupied molecular orbital (HOMO) and lowest 10 ACS Paragon Plus Environment

Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unoccupied molecular orbital (LUMO) levels of n-alkyl molecules are not close to the Fermi level of electrode, and the barrier height created by the molecules is large. For example, density functional theory (DFT) calculation suggests that SC11COOH (C12) on silver clusters shows HOMO and LUMO levels of -4.90 eV and -2.93 eV, respectively; values of work function for silver and EGaIn are -4.5 eV and -4.3 eV, respectively, at 0V.44 We further studied the relevance of untethered EGaIn top contacts under bending conditions. With respect to the fabrication of flexible electronic devices, hard and brittle metals inevitably suffer from stress-induced cracks when mechanical stress is applied.52 In contrast, the soft and fluidic nature of a liquid metal could lead to electrical compliance of electrodes to external stress. The electrical performance of the untethered EGaIn top contact under bent conditions was demonstrated using untethered junctions constructed on template-stripped silver formed on a thin, flexible polymer film (~1.5 × 102 µm in thickness) (see the Supporting Information for the detailed procedure). Figure 6b shows a diagram defining the bending radius (R). Figure 6c presents a plot of the J values at ±0.5 V for the C8 and C12 junctions over various bending radii. When the substrates were gradually bent from a radius of ∞ (flat) to 2.2 mm, the J values remained constant (Figure 6c and Figure S17 in the Supporting Information). Upon repeated bending and unbending cycles of the substrates, the J(V) values did not change significantly (Figure 6d). The stability of the C8 molecular junction based on an untethered EGaIn top contact was tested under flat and bent conditions for 1 × 103 s with short time intervals (∆t = 10 s); degradation was not observed (Figure 6e). These results indicate the uniform and stable electrical characteristics of the untethered EGaIn top contact under various bending conditions. This finding is not surprising because our bent conditions are not on a molecular scale. Although study about deformation of molecular monolayer on a substrate (similar to the SAM system in 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

this work) is rarely available in literature, we could find the relation between stress and structural deformation at molecular level in other research fields. The community of carbon nanotubes has extensively studied deformation of nanotubes, and found that buckling behavior of nanotubes takes place at nanometer scale.53 For research of lipid layers, the bending radius of a few tens of micrometers has been reported, and the self-assembled molecular structure was retained under the bent condition.54 Our bent conditions probably result in stress on the photocured polymer scaffold rather than the structure of individual molecules comprising SAM. The bending experiments using untethered junctions demonstrate that our untethering technique could be promising for achieving flexible electronics based on molecules as an active component.

CONCLUSION In summary, to our knowledge, this is the first demonstration of a direct writing approach for arbitrarily patterned arrays of molecular tunnel junctions in the context of a mask-free system. It offers substantial advantages in terms of readiness, scalability, and significantly decreased complexity compared with many previous approaches.10, 26-32 Therefore, our method can be a practical platform for studying organic and molecular electronics, and it should be possible to expand our method into high-throughput printed electronics technology by exploiting the advantages of liquid metal microelectrodes for massively parallel patterning of quantum tunneling devices.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 12 ACS Paragon Plus Environment

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental details, data of junction measurements and minor discussions (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF2017M3A7B8064518) and Korea University Future Research Grant.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

REFERENCES (1) Mirkin, C. A.; Ratner, M. A., Molecular Electronics. Annu. Rev. Phys. Chem. 1992, 43, 719-754. (2) Aviram, A.; Ratner, M. A., Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277-283. (3) Yoon, H. J.; Bowers, C. M.; Baghbanzadeh, M.; Whitesides, G. M., The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn Junctions. J. Am. Chem. Soc. 2014, 136, 16-19. (4) Barber, J. R.; Yoon, H. J.; Bowers, C. M.; Thuo, M. M.; Breiten, B.; Gooding, D. M.; Whitesides, G. M., Influence of Environment on the Measurement of Rates of Charge Transport across AgTS/SAM//Ga2O3/EGaIn Junctions. Chem. Mater. 2014, 26, 3938–3947. (5) Simeone, F. C.; Yoon, H. J.; Thuo, M. M.; Barber, J. R.; Smith, B.; Whitesides, G. M., Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions. J. Am. Chem. Soc. 2013, 135, 18131–18144. (6) Fracasso, D.; Muglali, M. I.; Rohwerder, M.; Terfort, A.; Chiechi, R. C., Influence of an Atom in EGaIn/Ga2O3 Tunneling Junctions Comprising Self-Assembled Monolayers. J. Phys. Chem. C 2013, 117, 11367-11376. (7) Park, S.; Wang, G.; Cho, B.; Kim, Y.; Song, S.; Ji, Y.; Yoon, M.-H.; Lee, T., Flexible molecular-scale electronic devices. Nat. Nanotech. 2012, 7, 438-442. (8) Chen, X.; Braunschweig, A. B.; Wiester, M. J.; Yeganeh, S.; Ratner, M. A.; Mirkin, C. A., Spectroscopic Tracking of Molecular Transport Junctions Generated by Using Click Chemistry. Angew. Chem. Int. Ed. 2009, 48, 5178-5181. 14 ACS Paragon Plus Environment

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Bowers, C. M.; Liao, K.-C.; Zaba, T.; Rappoport, D.; Baghbanzadeh, M.; Breiten, B.; Krzykawska, A.; Cyganik, P.; Whitesides, G. M., Characterizing the Metal–SAM Interface in Tunneling Junctions. ACS Nano 2015, 9, 1471–1477. (10) Wan, A.; Jian, L.; Sangeeth, C. S. S.; Nijhuis, C. A., Reversible Soft Top-Contacts to Yield Molecular Junctions with Precise and Reproducible Electrical Characteristics. Adv. Func. Mater. 2014, 24, 4442-4456. (11) Liao, K.-C.; Bowers, C. M.; Yoon, H. J.; Whitesides, G. M., Fluorination, and Tunneling across Molecular Junctions. J. Am. Chem. Soc. 2015, 137, 3852–3858. (12) Christian A. Nijhuis, W. F. R. a. G. M. W., Molecular Rectification in Metal-SAM-Metal Oxide-Metal Junctions. J. Am. Chem. Soc. 2009, 131, 17814–17827. (13) Metzger, R. M., Unimolecular Electrical Rectifiers. Chem. Rev. 2003, 103, 3803–3834. (14) Yoon, H. J.; Liao, K.-C.; Lockett, M. R.; Kwok, S. W.; Baghbanzadeh, M.; Whitesides, G. M., Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n-Alkanethiolates. J. Am.

Chem. Soc. 2014, 136, 17155–17162.

(15) Kong, G. D.; Kim, M.; Cho, S. J.; Yoon, H. J., Gradients of Rectification: Tuning Molecular Electronic Devices by the Controlled Use of Different-Sized Diluents in Heterogeneous Self-Assembled Monolayers. Angew. Chem. Int. Ed. 2016, 55, 10307–10311. (16) Qiu, L.; Zhang, Y.; Krijger, T. L.; Qiu, X.; Hof, P. v. t.; Hummelen, J. C.; Chiechi, R. C., Rectification of Current Responds to Incorporation of Fullerenes into Mixed-Monolayers of Alkanethiolates in Tunneling Junctions. Chem. Sci. 2017, 8, 2365-2372. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

(17) Chao Li, W. F.; Lei, B.; Zhang, D.; Han, S.; Tang, T.; Liu, X.; Liu, Z.; Asano, S.; Meyyappan, M.; Han, J.; Zhou, C., Multilevel Memory based on Molecular Devices. Appl. Phys. Lett. 2004, 84, 1949-1951. (18) Liang, W.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H., Kondo Resonance in a Single-Molecule Transistor. Nature 2002, 417, 725-729. (19) Song, H.; Kim, Y.; Jang, Y. H.; Jeong, H.; Reed, M. A.; Lee, T., Observation of Molecular Orbital Gating. Nature 2009, 462, 1039-1043. (20) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M., Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device. Science 1999, 286, 1550-1552. (21) Perrin, M. L.; Frisenda, R.; Koole, M.; Seldenthuis, J. S.; Gil, J. A. C.; Valkenier, H.; Hummelen, J. C.; Renaud, N.; Grozema, F. C.; Thijssen, J. M.; Dulić, D.; Zant, H. S. J. v. d., Large Negative Differential Conductance in Single-molecule Break Junctions. Nat. Nanotech. 2014, 9, 830-834. (22)

Meng, F.; Hervault, Y.-M.; Shao, Q.; Hu, B.; Norel, L.; Rigaut, S.; Chen, X.,

Orthogonally Modulated Molecular Transport Junctions for Resettable Electronic Logic Gates. Nat. Commun. 2014, 5, 3023. (23) Seo, S.; Min, M.; Lee, S. M.; Lee, H., Photo-switchable Molecular Monolayer Anchored Between Highly Transparent and Flexible Graphene Electrodes. Nat. Commun. 2013, 4, 1920. (24) Kim, D.; Jeong, H.; Hwang, W.-T.; Jang, Y.; Sysoiev, D.; Scheer, E.; Huhn, T.; Min, M.; Lee, H.; Lee, T., Reversible Switching Phenomenon in Diarylethene Molecular Devices with


Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reduced Graphene Oxide Electrodes on Flexible Substrates. Adv. Func. Mater. 2015, 25, 59185923. (25) Chiechi, R. C.; Weiss, E. A.; Dickey, M. D.; Whitesides, G. M., Eutectic Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers. Angew. Chem. Int. Ed. 2008, 47, 142-144. (26) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M., Electron Transport through Thin Organic Films in MetalInsulator-Metal Junctions Based on Self-Assembled Monolayers. J. Am. Chem. Soc. 2001, 123, 5075-5085. (27) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D., Length-Dependent Transport in Molecular Junctions Based on SAMs of Alkanethiols and Alkanedithiols: Effect of Metal Work Function and Applied Bias on Tunneling Efficiency and Contact Resistance. J. Am. Chem. Soc. 2004, 126, 14287-14296. (28) Xu, B.; Tao, N. J., Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions. Science 2003, 301, 1221-1223. (29)

Pourhossein, P.; Chiechi, R. C., Directly Addressable Sub-3 nm Gold Nanogaps

Fabricated by Nanoskiving Using Self-Assembled Monolayers as Templates. ACS Nano 2012, 6, 5566-5573. (30) Wang, G.; Kim, Y.; Choe, M.; Kim, T.-W.; Lee, T., A New Approach for Molecular Electronic Junctions with a Multilayer Graphene Electrode. Adv. Mater. 2011, 23, 755-760.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

(31) Kim, T.-W.; Wang, G.; Lee, H.; Lee, T., Statistical Analysis of Electronic Properties of Alkanethiols in Metal–Molecule–Metal Junctions. Nanotechnology 2007, 18, 315204. (32) Nijhuis, C. A.; Reus, W. F.; Barber, J. R.; Dickey, M. D.; Whitesides, G. M., Charge Transport and Rectification in Arrays of SAM-Based Tunneling Junctions. Nano Lett. 2010, 10, 3611–3619. (33) Nijhuis, C. A.; Reus, W. F.; Barber, J. R.; Whitesides, G. M., Comparison of SAM-Based Junctions with Ga2O3/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions. J. Phys. Chem. C 2012, 116, 14139−14150. (34) Xiang, D.; Wang, X.; Jia, C.; Lee, T.; Guo, X., Molecular-Scale Electronics: From Concept to Function. Chem. Rev. 2016, 116, 4318–4440. (35) Wang, G.; Kim, T.-W.; Lee, H.; Lee, T., Influence of Metal-Molecule Contacts on Decay Coefficients and Specific Contact Resistances in Molecular Junctions. Phys. Rev. B 2007, 76, 205320. (36) Akkerman, H. B.; Blom, P. W. M.; Leeuw, D. M. d.; Boer, B. d., Towards Molecular Electronics with Large-Area Molecular Junctions. Nature 2006, 441, 69-72. (37) Hal, P. A. V.; Smits, E. C. P.; Geuns, T. C. T.; Akkerman, H. B.; Brito, B. C. D.; Perissinotto, S.; Lanzani, G.; Kronemeijer, A. J.; Geskin, V.; Cornil, J.; Blom, P. W. M.; Boer, B. D.; Leeuw, D. M. D., Upscaling, Integration and Electrical Characterization of Molecular Junctions. Nat. Nanotechnol. 2008, 3, 749 - 754.


Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Seo, S.; Min, M.; Lee, J.; Lee, T.; Choi, S.-Y.; Lee, H., Solution-Processed Reduced Graphene Oxide Films as Electronic Contacts for Molecular Monolayer Junctions. Angew. Chem. Int. Ed. 2002, 51, 108-112. (39) Weiss, E. A.; Chiechi, R. C.; Kaufman, G. K.; Kriebel, J. K.; Li, Z.; Duati, M.; Rampi, M. A.; Whitesides, G. M., Influence of Defects on the Electrical Characteristics of MercuryDrop Junctions: Self-Assembled Monolayers of n-Alkanethiolates on Rough and Smooth Silver. J. Am. Chem. Soc. 2007, 129, 4336-4349. (40) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M., SelfAssembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103–1170. (41) Cademartiri, L.; Thuo, M. M.; Nijhuis, C. A.; Reus, W. F.; Tricard, S.; Barber, J. R.; Sodhi, R. N. S.; Brodersen, P.; Kim, C.; Chiechi, R. C.; Whitesides, G. M., Electrical Resistance of AgTS-S(CH2)n-1CH3//Ga2O3/EGaIn Tunneling Junctions. J. Phys. Chem. C 2012, 116, 10848– 10860. (42)

Hutt, D. A.; Leggett, G. J., Influence of Adsorbate Ordering on Rates of UV

Photooxidation of Self-Assembled Monolayers. J. Phys. Chem. 1996, 100, 6657-6662. (43) Schoenfisch, M. H.; Pemberton, J. E., Air Stability of Alkanethiol Self-Assembled Monolayers on Silver and Gold Surfaces. J. Am. Chem. Soc. 1998, 120, 4502-4513. (44) Kong, G. D.; Yoon, H. J., Influence of Air-Oxidation on Rectification in Thiol-Based Molecular Monolayers. J. Electrochem. Soc. 2016, 163, G115-G121.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(45)

Page 20 of 29

Simmons, J., Generalized Formula for the Electric Tunnel Effect between Similar

Electrodes Separated by a Thin Insulating Film. J. Appl. Phys. 1963, 34, 1793-1803. (46) Simmons, J. G., Low‐Voltage Current‐Voltage Relationship of Tunnel Junctions. J. Appl. Phys. 1963, 34, 238-239. (47) Bowers, C. M.; Liao, K.-C.; Yoon, H. J.; Rappoport, D.; Baghbanzadeh, M.; Simeone, F. C.; Whitesides, G. M., Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga2O3 TopInterface of AgTS/S(CH2)nT//Ga2O3/EGaIn Junctions. Nano Lett. 2014, 14, 3521–3526. (48) Carlotti, M.; Degen, M.; Zhang, Y.; Chiechi, R. C., Pronounced Environmental Effects on Injection Currents in EGaIn Tunneling Junctions Comprising Self-Assembled Monolayers. J. Phys. Chem. C 2016, 120, 20437–20445. (49) Wold, D. J.; Haag, R.; Rampi, M. A.; Frisbie, C. D., Distance Dependence of Electron Tunneling through Self-Assembled Monolayers Measured by Conducting Probe Atomic Force Microscopy: Unsaturated versus Saturated Molecular Junctions. J. Phys. Chem. B 2012, 106, 2813-2816. (50) Slowinski, K.; Chamberlain, R. V.; Miller, C. J.; Majda, M., Through-Bond and Chainto-Chain Coupling. Two Pathways in Electron Tunneling through Liquid Alkanethiol Monolayers on Mercury Electrodes. J. Am. Chem. Soc. 1997, 119, 11910–11919. (51) Cui, X. D.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Primak, A.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M., Making Electrical Contacts to Molecular Monolayers. Nanotechnology 2002, 13, 5-14. (52) Lewis, J., Material challenge for flexible organic devices. Mater. Today 2006, 9, 38-45. 20 ACS Paragon Plus Environment

Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53) Shima, H., Buckling of Carbon Nanotubes: A State of the Art Review. Materials 2012, 5, 47-84. (54) Zhao, Y.; Mahajan, N.; Fang, J., Bending and Radial Deformation of Lipid Tubules on Self-Assembled Thiol Monolayers. J. Phys. Chem. B 2006, 110, 22060–22063.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

Figure 1. (a) Schematic of the proposed method to form an untethered junction based on a liquid metal (eutectic gallium–indium, EGaIn, covered by a self-passivating oxide layer, Ga2O3) for a top contact. Treating a photocurable polymer (denoted as PP) at the junction enables a highyielding and rapid untethering process in a mask-free fashion while the tunneling characteristics of the junction are kept constant. (b) Procedures for the untethering process. The molecular junction of a Ga2O3/EGaIn conical microelectrode is encapsulated in situ with a photocurable polymer. Optical microscopy snapshots are presented for each step (scale bar: 400 µm). (c) Microscopic images of an untethered junction with the structure AgTS/SC11CO2H//Ga2O3/EGaIn (AgTS is the template-stripped silver substrate, SC11CO2H is denoted as C12). (d) Junction measurements at ±0.5 V during untethering. No change is observed in the J–V characteristics before and after untethering of the Ga2O3/EGaIn top electrode. (e) Stability of untethered junction over 1000 cycles and (f) aging in air (up to 72 h).


Page 23 of 29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

Figure 2. (a) Direct writing of arbitrary arrays of tunnel junctions via the untethering process over indium tin oxide (ITO). Computer-generated sketch of letters “KU” in bitmap font, and the corresponding pattern, where each dot corresponds to an untethered junction of the form ITO//Ga2O3/EGaIn (where “//” is the van der Waals contact), are presented. (b) Magnified top and side views for parts of the patterned image. (c) Histograms of J(+0.5 V) for the patterned untethered junctions and the analogous tethered junctions. No significant difference is observed in J(V) for the untethered and tethered junctions.


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) A 3 × 3 array of untethered junctions created over the SC7CO2H SAM formed on AgTS. (b) Histogram of J(+0.5 V) for these junctions. None of the junctions shorted, and the J(V) values were narrowly distributed.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

Figure 4. (a) Molecules used to examine the length-dependence of tunneling currents through untethered molecular junctions. (b) Dependence of log|J| on the length of molecules comprising an SAM. (c) J values plotted against the number of contact–de-contact cycles.


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Molecular rectification in an untethered junction of the form AuTS/SC11BIPY//Ga2O3/EGaIn (AuTS is the template-stripped gold; SC11BIPY is 2,2′-bipyridylterminated n-undecanethiolate). The untethered junction showed a high rectification ratio (r = |J(+V)|/|J(−V)|; log|r| = 1.8 ± 0.4), which was consistent with the analogous tethered junction (log|r| = 1.9 ± 0.3).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

Figure 6. (a) Current density J (on a log scale) versus the molecular length at various temperatures. Photos show untethered EGaIn junctions inside a cryogenic probe station. (b) Bending radius (R, mm) defined over a bent substrate. (c) Junction measurements over flat and bent conformations for AgTS/SAM//Ga2O3/EGaIn junctions, where the SAM comprises C8 or C12. (d) A plot of log|J| as a function of the number of bending cycles. (e) Stability of flat (R = ∞) and bent (R = 2.2 mm) substrate containing a molecular junction of C8.


Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ToC Graphic


Maskless Arbitrary Writing of Molecular Tunnel Junctions - ACS

Recommend Documents