Gated-Controlled Rectification of a Self-Assembled Monolayer-Based

Article pubs.acs.org/JPCC

Gated-Controlled Rectification of a Self-Assembled Monolayer-Based Transistor Elad D. Mentovich,†,‡ Natalie Rosenberg-Shraga,† Itsik Kalifa,‡ Michael Gozin,† Vladimiro Mujica,§,∥,⊥ Thorsten Hansen,# and Shachar Richter*,†,‡ †

School of Chemistry and ‡University Center for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel § Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States ∥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ⊥ Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States # Department of Chemical Physics, Lund University, Box 124, SE-21000 Lund, Sweden S Supporting Information *

ABSTRACT: A vertical gate symmetrical molecular transistor is demonstrated. It includes self-assembled monolayer of ferrocene molecules chemically bonded to be a flat Au source and Au nanoparticles drain electrodes while gated with the central gate electrode. Using this configuration, we show that negative differential resistance, symmetrical behavior, and rectification effects can be tuned by controlling the gate voltage. The I−V curves shift from symmetric to strongly rectifying over a gate voltage range of a few tenths of volts around a threshold value where the junction behaves symmetrically. This is due to charging of the nanoparticle contact, which modifies the spatial profile of the voltage across the junction, a fact that we have included in a simple theoretical model that explains our experimental results quite well. Our device design affords a new way to fine-tune the rectification of molecular devices in a way that does not necessarily involve the Coulomb charging of the wire.

1. INTRODUCTION

Since the I−V of MJs are determined by the molecular electron transfer processes, the contribution of the contact− molecule interface by means of charge and spin transfer should not be neglected.7,13,18 Thus, it is important to explore to what extent the I−V in MJ reveals intrinsic molecular electric “fingerprints” and attempt to discriminate it from the other interfacial phenomena. Of special interest is the rectification phenomenon (i.e., the appearance of I−V characteristics that are asymmetric with respect to the polarity of the applied bias voltage).11,18 Though it was suggested more than 30 years ago that molecules can be used as rectifiers, both theory and experiment concur in that the rectification phenomenon involves both the contacts and the interfacial properties of the MJ and that “pure” molecular rectification is very rare and hard to achieve.11,19 As example, Nijhuis et al. work on Ferrocene (Fc) selfassembled monolayers (SAM) derivatives and their rectification properties in two-terminal device showed that rectification depended on the position of the Fc in the device and was absent when Fc was not present. Moreover, it was concluded

The basic structure of an ideal molecular junction (MJ) consists of a single molecule or molecular film sandwiched between a source and a drain metallic electrodes.1−3 The introduction of a third gate electrode essentially transforms a MJ into a transistor and allows not only a better control of the electric behavior of a MJ but also the exploration of a number of physical phenomena related to charge accumulation that are not generally present in two electrode junctions, e.g., Coulomb blockade.4−8 Even though the number of components structuring the MJ is small, understanding the nature of electron transport (ET) in such systems is very challenging since it is determined by several parameters, some of which are not well controlled experimentally, e.g., geometry of the molecular bonding at the molecule−electrode interface and the number of molecules in the MJ and the electrode’s structure.9−13 In this context, it should be noted that even small differences between the MJ’s contacts, caused by either the specific materials used or the geometry of the junction, might influence its ET properties.9−11 To complicate things further, both electron−phonon and electron−electron coupling influence the ET properties of the molecule, and this in turn may strongly modify the current−voltage characteristics (I−V) of MJs.14−17 © 2013 American Chemical Society

Received: December 3, 2012 Revised: March 19, 2013 Published: March 27, 2013 8468

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molecules to both the Au source contact and gold nanoparticles placed in between the Pd drain contact of the device (see Figure 2) . During the past few years Fc derivatives have been

that in these systems the charge transport is governed by the SAMs and not by the contacts.20−28Although complete separation between the molecular and interfacial parameters is not possible, as exemplified in the latter case, one can simplify the problem by choosing an appropriate type of MJ in which the contribution of the various components of the MJ may be evaluated. For this task we consider here the use of a specific system based on the following four basic properties: (i) the use of a symmetrical molecular compound which should exhibit a unique electric molecular marker. In our case this will be a molecular negative differential resistance (NDR) “fingerprint” as explained below (ii), the use of a symmetrical molecular linker to the two contacts, and (iii) the application of a gate voltage VG. As mentioned above, the design and control of symmetric MJs are very challenging since even small interfacial effects may affect significantly the symmetry of the junction. Thus, application of an appropriate VG to a semisymmetric MJ introduces substantial changes in the electrostatic potential and the occupation of the molecular level and could, in principle, allow the fine-tuning of the voltage profile across the MJ which can then be used as a control parameter for the current. As we will show below, we have been able to cover an ample range of I−V spectra ranging from a symmetrical characteristics to a rectifying junction. The molecular marker is an essential ingredient in the experiment, since it helps distinguishing between the contact and the molecular contribution to the I/V spectra. Moreover, it is necessary to explore the relationships between the symmetry of the marker and the applied potential as indicated by Tao et al.19,29,30 In addition, the use of a gate voltage also permits the investigation of several potential landscapes within the transistor that might show different behavior in each voltage regime.17,31−34

Figure 2. Transistor structure. The transistor consists of highly doped silicon (1), SiO2 separating layer (2), Au bottom source electrode (3, source), dielectric Si3N4 layer (4), gate Ti, (5) TiO2 (6) electrode, SAM (7), Au-NP (8), and top Pd drain electrode (9). Transistor operation: upon application of VSD between the source and drain electrodes (3, 9) current flows thorough the SAM and the Au-NP (7, 8) and controlled by the gate voltage applied using the back-gate Si contact (1) to the gate electrode (5, 6).

also investigated in solid state-based system by various techniques such as electrochemical STM, break junctions, vertical two terminals devices, and STM.29,35−37 We and others have indicated the existence of a typical sharp peak at the I/V characteristics of an Fc-based MJ. This peak termed as NDR is associated with the redox center of the Fc. This NDR peak is used as a “molecular indicator” of the Fc compound in the junction.19,29,38 2.2. Transistor Fabrication. The transistor’s structure is presented in Figure 2. The full procedure is described elsewhere.32,39. In short, the device is termed a “central-gate molecular vertical transistor” (C-Gate MolVeT, Figure 2) comprising a central or side Ti/TiO2 gate electrode that is used to activate a SAM sandwiched between source and drain metal leads. This transistor,32,39,40 exhibits low operating voltages, ambipolar behavior, and high gate sensitivity, and it has been successfully used to investigate several systems such as molecular quantum dots and proteins.40,41 Using this transistor configuration, we have previously demonstrated, using various types of MJs rectification or NDR. In the latter case a correlation between NDR appearance and the presence of the redox center was shown. Nevertheless, due to the asymmetrical nature of the source− drain contacts in previously demonstrated transistors (e.g., Pd− SAM−Au structure), one cannot differentiate between the contribution of contact asymmetry and other effects such as charging, molecular asymmetry, and NDR formation.1,14−16,42 In our case, a symmetric Fc-derivative compound attached to two Au contacts was used attached to two Au contacts, thus comprising a “semi”-symmetrical junction. However, the existence of a dithiol moiety in the molecule may lead to the formation of an unwanted bridging conformation in the MJ.43 To this end, we have adopted the use “layer exchange process” in which a preadsorbed layer is used to reduce the bridging-type adsorption (Figure 3). The exposed SH group of the SAM layer

2. EXPERIMENTAL SECTION 2.1. Ferrocene Derivatives. Fc, a highly stable redox molecule,29,33 is considered to be one of the well-investigated markers in electrochemistry. Figure 1 shows the synthesis route of a symmetric molecule which is composed of a Fc- based structure (compound 4). This molecule is comprised of the Fc core centered between two saturated alkane chains ended with thiol (SH) moieties. The thiol groups are used to anchor the

Figure 1. The synthesis scheme of the molecule: synthetic pathway for preparation of compound 1. Reagents and conditions: (i) imidazole, tert-butyldimethylsilyl chloride, DMF, 0 °C; (ii) ferrocene, nbutyllithium, tetramethylethylenediamine (TMEDA), hexanes, tetrahydrofuran (THF), reflux; (iii) THF, tetrabutylammonium fluoride, 0 °C; (iv) PPh3, THF, diisopropyl azodicarboxylate (DIAD), thiolacetic acid, 0 °C; (v) NaOCH3, methanol, THF, NH4Cl(aq), room temperature (RT) (for detailed synthesis route see Supporting Information). 8469

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Figure 3. The layer exchange process. NH3 was preadsorbed on the Au surface to reduce possible molecular bridging effects. Upon formation of the SAM, Au−NP introduced forming a layer on top if the SAM via thiol−Au bond.

found that 60% of the devices met these criteria. Most of the nonworking devices exhibited Ohmic behavior (25%), and the rest did not show NDR at low voltages. It can be seen that the I−V shape can be tuned via VG (Figure 4a−d) ranging from symmetrical (low VG, Figure 4d) characteristics to full rectification behavior (high VG, Figure 4b,c). In addition, notable NDR features are observed. High-resolution measurements reveal several distinct NDRsymmetry relations. Figure 4b shows a symmetrical I−V while a single NDR peak is observed at 1.2 V. Application of different VG results in current amplification in the negative source- drain voltage (VSD) regime leading to a clear rectification effect. At a well-defined value of VG a fully symmetrical junction with two mirror-like NDR peaks is obtained. Figure 4d (inset) shows the same measurement taken for different devices under the same conditions. It can be seen that while the overall source-drain current (ISD) is of different magnitude, the structures of the spectra are similar. The difference in the ISD magnitude can be attributed to difference in the number of molecules measured in the various devices. The overall I−V characteristics obtained for this MJ system also obey the general properties of the previously demonstrated nonredox vertical molecular transistor: The lack of saturation current which is a typical in the other cases of very shortchannel-length transistors, extremely highly sensitivity to the gate voltage, low operating voltages,and low gate-source leakage currents. We now address the main aspect of our investigation: the dependency of the NDR and rectification effects on VG. First, we consider the case of low VG, where a distinct NDR peak

allows the introduction of Au nanoparticles (Au−NP) that are used as the first layer of the top electrode (Figure 2). This procedure results in the formation of Au−SAM−Au (NP)−Pd junction in which compound 4 is covalently attached to the source and drain Au leads. We notice that even though the top and bottom electrodes are composed of the same material, different interfacial effects may take place near the contacts, which can be compensated by the gate voltage (see model suggested). All measurements were performed at room temperature using “Desert-Cryogenics” probe station equipped with a “Keithley 4200” semiconductor parameter analyzer. 2.3. Theoretical Model and Computational Details. We use a simple model to substantiate our suggestion that the observed gate effects are the results of gold nanoparticle charging. The response of the charged interface to the gate voltage, and its impact on the measured current is modeled by a gate voltage dependence of the voltage division factor: η(VG).

3. RESULTS AND DISCUSSION Figure 4a shows representative room temperature I−V characteristics of the transistor over large span of VGs. Each device has been measured for 10 cycles. NDR reproducibility has been obtained in the relevant regimes, indicating the stability of the device and the lack of filament formation.44 Generally, we defined two criteria for working devices: (i) currents should not exceed 10−3 A (since I−V curves with larger currents showed pure Ohmic behavior) and (ii) existence of NDR peaks (molecular indicators) at low gate voltages. We 8470

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Figure 4. Transfer characteristics of the Fc transistor at different gate voltages: (a) ISD/VSD as a function of VG at a broad range of gate voltages (from −0.8 to 0.8 V). (b) I/V curve for VG = −0.4 V. (c) Transfer characteristics at VG = 0.1 V. (d) Symmetric curve obtained at VG = −0.25 V. Inset-I−V characteristics obtained for different device under the same conditions.

that switching voltages are fixed around ±1.2 V, showing no VG dependency. This finding indicates on a possible pinning of molecular energy levels that is associated with the NDR phenomenon. This type of molecular protection through the linkers has been observed in other contexts45−49 and is probably due to the strong chemical bonding and coupling between compound 4 and the top and bottom Au contacts.50,51 If the molecular levels are strongly pinned relative to the Fermi level of a lead, then effectively the molecule should be screened from the gate potential, and therefore it seems reasonable to expect a weak gate effect. Such a strong molecule−lead coupling would strongly constrain dynamic polaron formation, which requires a weak gate coupling. A remarkable effect is that the gate voltage needed to tune the current−voltage curve to be either symmetric or rectifying is of the order of a few tenths of an electronvolt with respect to the value of −0.25 V associated with the symmetric curve. This is too small an energy scale for a molecular charging event. It does, however, suffices to induce the charging of one or more of the Au−NP that constitute the top electrode, since they are larger and thus have a smaller effective capacitance. In fact, charging of gold nanoparticles in the context of transport measurements has been reported in the literature.36−38 Further indication of this effect can be found in the lack of variation in the NDR location. Charging a gold nanoparticle will cause a surface charge density that changes the NPs double layer.13,52−54 This will effectively shift the work function of the nanoparticle. In fact, we can think of charged gold as a different material than gold; thus, we can consider the junction to be semisymetric. As the circuit is formed, charge will rearrange

appears with larger probability at positive VSD. Figure 5 presents the switching voltage (the value of VSD which corresponds to the largest current in an NDR peak) measured in a representative transistor at several VG values. One can see

Figure 5. Histogram of conductance switching threshold voltages observed for a typical transistor (inset). 8471

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within the junction leading to an equilibration of the two different chemical potentials of the electrode to one common zero-bias chemical potential. This equilibration is hard to calculate since it depends on the detailed geometry and electronic structure of the junction. One should notice that even in the case of asymmetric junction, hysteretic behavior is enabled and switching can be observed.15−17 A simple model that can be used to substantiate our suggestion that the observed gate effects are the results of gold nanoparticle charging, describes the behavior of the charged interface using a voltage division factor, η42 To include the effect of the increased amount of charge at the gold nanoparticles, induced by the gate voltage, we consider a linear dependence of the voltage division factor on the gate voltage: η(VG) = η0 + αVG

Figure 6. Current−voltage curves using eq 4. For parameter values we have chosen, Γ = 0.5 eV, μ − εs = 2 eV, and η ranging from 0.5 (blue) to 1 (yellow) in steps of 0.1.

(1)

where η0 = η(V = 0), and the coefficient α is a dimensionless adjustable parameter (with values between 0 and 1) of the model that determines the spatial voltage profile of the junction. For a symmetric junction α = 0.5. This parameter depends on a very complex way on the electronic structure of the entire junction, but under some simplifying assumptions it gives a reasonable description of the voltage asymmetry.9 The current through the junction at zero temperature is given in the Landauer formalism by I (V ) =

2e h

introduction of electron−electron interaction, an essential ingredient in understanding NDR that we have considered in a previous publication where the dependence of NDR on charging and localization is explicitly discussed for a Hubbard model.55

4. CONCLUSION To summarize, our experiments are, to our knowledge, the first to exhibit the unconventional gate effect described here. We primarily attribute the strong gate dependence to charging of the gold nanoparticles in the contacts, which in turn modifies the spatial profile of the voltage across the junction. We have also developed a simple theoretical model that supports this interpretation. Our fabrication and control techniques may provide a new handle on investigating gated molecular junctions. Thus, the increased sensibility of the I−V characteristics to VG may be attributed to the novel gate design, in which the molecules experience a much stronger coupling to the gate.

μ + η eV

∫μ−(1−η)eV dεT(ε)

(2)

where μ is the chemical potential of the junction. For tunneling through a single orbital at energy εs of width Γ, the transmission function is given as T(ε) =

Γ 2/4 (ε − εs)2 + Γ 2/4

(3)

With this expression for the transmission the current is evaluated as ⎛2 ⎞ eΓ ⎡ ⎢⎣arctan⎜⎝ (μ − εs + η eV)⎟⎠ h Γ ⎛2 ⎞⎤ − arctan⎜ (μ − εs − (1 − η) eV)⎟⎥ ⎝Γ ⎠⎦

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ASSOCIATED CONTENT

S Supporting Information *

I (V ) =

A detailed synthesis scheme of the Fc molecule. This material is available free of charge via the Internet at http://pubs.acs.org.

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(4)

Figure 6 shows a plot of the theoretical I−V curve predicted by eq 4, for values of parameters that are well within the range of a typical MJs. Despite the simplicity of this model, it captures the essential physics of the problem and the turnover region showing a clear transition from the symmetric to a strongly rectifying junction that is in fairly good agreement with the experimental I−V curve in Figure 4a. Finally, we consider the behavior of the junction at large VG, where the NDR peak, as shown in Figure 3, disappears. This result can be understood as the result of two balancing trends: increasing the gate voltage has the effect of reducing the orbital energy and increasing the localizing of the molecular level involved in NDR. This effect is counteracted by an increase on the Coulomb repulsion induced by the charging of the gold NP and the wire itself as a result of the strong coupling. The net result is that the differential resistance becomes positivean explanation that hinges on the assumption that NDR in the system we are considering is essentially Coulombic in nature as no vibronic effects are included here. The explicit inclusion of this effect in our theoretical model would require the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Joseph E. Subotnik and Professor Mark A. Ratner for fruitful discussions and Mrs Netta Hendler and Bogdan Belgorodsky for technical support. This work was partly supported by USAF (project No. 073003), James Frank, and Israel Science Foundation (SR).

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dx.doi.org/10.1021/jp311875g | J. Phys. Chem. C 2013, 117, 8468−8474