Switchable Schottky contacts: Simultaneously enhanced output

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Switchable Schottky contacts: Simultaneously enhanced output current and reduced leakage current Guirong Su, Sha Yang, Shuang Li, Christopher J. Butch, Sergey N. Filimonov, Ji-Chang Ren, and Wei Liu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11459 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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Journal of the American Chemical Society

Switchable Schottky contacts: Simultaneously enhanced output current and reduced leakage current Guirong Su,† Sha Yang,† Shuang Li,† Christopher J. Butch,‡ Sergey N. Filimonov,¶ Ji-Chang Ren,*,† Wei Liu*,† †Nano and Heterogeneous Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China ‡Department of Chemistry, Emory University, Atlanta, Georgia; Blue Marble Space Institute of Science, Seattle, Washington; Earth Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan ¶Department of Physics, Tomsk State University, 634050 Tomsk, Russia KEYWORDS: Schottky contacts, bistable state, Schottky barrier height, reverse leakage current, van der Waals forces

ABSTRACT: Metal-semiconductor contacts are key components of nanoelectronics and atomic-scale integrated circuits. In these components Schottky diodes provide a low forward voltage and a very fast switching rate but suffer the drawback of a high reverse leakage current. Improvement of the reverse bias characteristics without degrading performance of the diode at positive voltages is deemed physically impossible for conventional silicon-based Schottky diodes. However, in this work we propose that this design challenge can be overcome in the organic-based diodes by utilizing reversible transitions between distinct adsorption states of organic molecules on metal surfaces. Motivated by previous experimental observations of controllable adsorption conformations of anthradithiophene on Cu(111), herein we use density functional theory simulations to demonstrate the distinct Schottky barrier heights of the two adsorption states. The higher Schottky barrier of the reverse bias induced by chemisorbed state results in low leakage current; while the lower barrier of the forward bias induced by physisorbed state yields a larger output current. The rectifying behaviors are further supported by nonequilibrium Greens function transport calculations.

■ INTRODUCTION Diodes are indispensable components in integrated circuits due to their unique amplitude-limiting and rectification properties.1,2 However, traditional p-n junction diodes made from semiconductor materials cannot satisfy the increased technical demands of integrated circuits with respect to low power consumption, high current and ultra-high speed.3-6 Instead, these heightened technical requirements can be fulfilled by utilizing metal-semiconductor contacts, also known as Schottky diodes, which exhibit extremely short reverse recovery time, low turn-on voltage, and high-frequency response characteristics.7-14 For example, in the absence of charge carrier depletion at the junction, a typical Schottky diode reverse recovery time is in the range of ten to twenty picoseconds, significantly shorter than that of p-n diodes which are normally several hundred picoseconds.15,16 Early Schottky contacts were formed from the junction of a metal anode (Mo, Cr, W, and Pt), with an n-type silicon cathode.17 However, continuous miniaturization of electronic devices has created a demand for alternatives to silicon-based devices, which, due to physical limits, become sensitive and unstable.1,18,19 One possible route of eliminating silicon in these diodes is by replacement with organic components,20,21 especially -conjugated molecules, which comparatively have more varied chemistry, higher adsorption coefficients, and fewer intrinsic defects.22,23 The organic component of a Schottky diode is then selected to maximize output current

while minimizing reverse leakage. However, there is a long established tradeoff between these two design criteria, as any decrease to the Schottky barrier of the system allows increased current in both directions.24,25 This principle has been demonstrated concisely in the PDI8-CN21/Au system: as the thickness of the organic layer of this system is decreased, the Schottky barrier decreases and both the output current and the leakage current increase.26 Consequently, there is high demand for new materials for the design of Schottky diodes which exhibit thermal stability, low forward barrier height and low leakage current. The metal-molecule interface can have either a physisorbed or chemisorbed nature of bonding.27 For smaller adsorption distance, electron orbital interactions between the metal and the molecule are stronger, which may lead to a lower barrier height, and vice versa. A low Schottky barrier height would result in a large interfacial current while a high barrier height may suppress the current in the circuit. Based upon these observations, we hypothesized that a metal-molecule interface capable of both a chemisorbed and physisorbed state in a single system may allow both a high forward and low reverse current

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to be obtained. Bistable adsorption systems of this type, such as tetrachloropyrazine on Pt(111) and anthradithiophene (ADT) on Cu(111), have recently been proposed based on theoretical investigations and fabricated in recent experimental studies.28-30 In these systems, charge carriers injected from a scanning tunneling microscope (STM) tip cause ADT to switch between two stable geometries, one physisorbed and one chemisorbed, in a fully controlled manner.29,30 In this contribution, we demonstrate that the bistable ADT/Cu(111) interface is capable of bypassing this intrinsic Schottky diode design trade-off, allowing creation of a diode with both high output current and low leakage current. We show that this system possesses two distinct Schottky barrier heights and reverse leakage currents corresponding to the difference between the interface dipoles in the two states. Specifically, the reverse leakage current of the chemisorbed state is much reduced from the physisorbed state, while the physisorbed molecule exhibits a larger output current. Thus, this study demonstrates this type of molecular switching as an attractive new design paradigm for the higher performing Schottky diodes necessary for more exquisite integrated circuits. S S

S trans-ADT

S cis-ADT

Chart 1. The two isomers of anthradithiophene (ADT), transADT and cis-ADT, which are pentacene analogue with two terminal benzene groups replaced by thiophene rings that can be arranged in two ways resulting in a cis- and a transconfiguration.

■ COMPUTATIONAL METHODS Calculations have been performed within the density functional theory (DFT) framework using two well-known codes: the Fritz-Haber-Institute ab initio molecular simulations (FHI-aims)31 and the Vienna ab initio simulation package (VASP).32 The geometric relaxations, the molecular electron affinities and the electronic density differences were performed by the FHI-aims code. We used the “tight” settings for numerical atom-centered orbitals basis sets. The “tier2” standard basis was utilized for H and C, and “tier1” for S, O, Se and Cu. All atomic positions were relaxed until the maximal force on each atom was smaller than 10-2 eV/Å. The convergence criteria of 10-5 electrons per unit volume for the charge density and 10-6 eV for the total energy of the systems were used. The Cu(111) surface was modelled with a 4-layer slab taking into account the dipole correction which is demonstrated to be large enough to obtain converged results (see Supporting Information Table S1 and Figure S1 for details). A 251 Monkhorst-Pack33 mesh for the (84) surface unit cell was used. For geometry relaxations, the molecule and the three uppermost metal layers were fully relaxed while the remaining layers were constrained in their bulk positions. Different slabs were separated by 90 Å of vacuum. The work functions, molecular dipole and bond dipole potentials were calculated with the VASP code. Based on the optimized structures, we calculated the electron potential

energy of trans-ADT on Cu(111) within the PBE functional. We treated explicitly the H(1s), C(2s, 2p), O(2s, 2p), S(3s, 3p) Se(3d, 4s, 4p) and Cu(3p, 3d, 4s) electrons as valence electrons and their wave-functions were expanded in plane-waves with a cut-off energy of 450 eV. A Monkhorst-Pack grid with 2×5×1 k-point sampling was used. The energy convergence criterion for the electronic self-consistency is set at 10-4 eV/atom. For the description of van der Waals (vdW) interactions between the molecule and metal substrate, the PBE+vdWsurf method was applied, which considers the collective effect of electron correlations at the metal surface.34-38 The PBE+vdWsurf method has shown high accuracy and efficiency for the prediction of energetics and structures of both physisorbed and chemisorbed systems.39 This method was selected over the many-body dispersion (MBD) method which goes beyond the vdWsurf method by non-perturbative treatment of the whole molecule-metal system.40-44 Our recent studies have demonstrated that PBE+MBD always underestimates the adsorption energy, and the expensive hybrid HSE functional should be used for accurate results.45 Consequently, PBE+vdWsurf was selected as a less demanding high accuracy method. The electronic transport calculations for trans-ADT/Cu(111) were performed using the nonequilibrium Greens function (NEGF) method combined with DFT implemented with the Nanodcal.46,47 Our models were constructed to calculate the current and voltage characteristic of the two states, which can be partitioned into three regions: the central region containing the relaxed slab model mentioned above, and the right and left electrodes that extend to the infinity in either direction. For the two states, the distance between the tip and the substrate is fixed at the same height. The valence electrons of Cu were treated with double-ζ polarized basis sets. The PBE method was used as the exchange-correlation functional. The I-V curves were performed with a 1×1×1 k-point mesh for the central region, and a 1×1×100 k-point mesh for the two electrodes.

■ RESULTS AND DISCUSSIONS By employing the PBE+vdWsurf method, we calculated the adsorption configurations of the bistable system of trans-ADT adsorbed on Cu(111). The molecule of interest in this work, ADT, has two isomers: trans and cis (see Chart 1). Herein, we focus on the former since the physisorption of cis-ADT on Cu(111) is unstable. As shown in Figure 1a, in contrast to conventional adsorption systems, there are two stable configurations (physisorption and chemisorption) possible in the same system. The stabilities of these bistable adsorption states have been studied in our recent work48 by calculating the potential energy surfaces of adsorption conformations. As depicted in Figure 1, the physisorbed trans-ADT is slightly deformed, while the chemisorbed molecule is severely deformed. These calculated relaxed geometries are supported by experimental STM images29: physisorbed trans-ADT has a rod-like appearance under STM, while the chemisorbed state exhibits a dumbbell-like appearance. This difference is reflected in the adsorption height determined by the average distance between the carbon atoms and the first layer of Cu(111). Physisorbed trans-ADT is positioned 2.88 Å above the Cu surface with the central ring in a planar conformation. In contrast, the chemisorbed trans-ADT is located 0.57 Å closer to the surface with the two center carbons adopting a stronger


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Journal of the American Chemical Society hybridization with the surface. This reflects the dominance of vdW interactions in the physisorbed state, while the chemisorbed state has contributions of both vdW forces and covalent bonding.28

Figure 1. Bistable states of trans-ADT adsorbed on Cu(111). (a) Side view of the electron density difference of transADT/Cu(111) at equilibrium distances. We obtain the plots by subtracting the electron density of an isolated molecule and that of the clean surface from the electron density of the entire adsorption systems. The same isosurface value of 0.001 e Å−3 was used for the upper and lower plots. The red and blue indicate the electron gain and loss, respectively. (b) Adsorption induced electron density redistribution integrated over the x-y plane () (left panel) and the resulting total charge transfer (Q) (right panel) for the trans-ADT/Cu(111) systems as a function of z-coordinate. Q(z) is obtained by integrating (z) from a point on the z-axis in the vacuum, where  has decayed to zero to a certain position z. The structure of the system is shown in the background as a guide. Zero represents the position of the first Cu layer. To obtain more insight into the adsorption geometries, we performed electron density difference calculations by subtracting the electron density of the isolated molecule and that of the clean metal surface from that of the adsorption systems. As shown in Figure 1a, physisorption leads to some charge rearrangement: the electron density between the adsorbate and substrate is depleted in the region near the trans-ADT side, while accumulated in the region close to the metal side. This charge rearrangement is more obvious in the chemisorbed system due to the formation of the CCu bonds. These differences in charge density are further illustrated in Figure 1b which shows the charge density differences averaged over the x-y plane along the z-coordinate. The variation of the charge density curve in chemisorption is more obvious in contrast to physisorption, indicating a stronger hybridization between the molecule and surface wave functions. Such difference is also confirmed by comparing the total transferred electrons between the two states: 0.39 electrons are transferred through physisorption while 0.43 electrons are transferred through chemisorption (see dashed line in Figure 1b). In the previous experimental investigations, it was demonstrated that the transition between the two states can be controlled using a pulsed voltage between the STM tip and substrate surface.29 Thus, for practical application these switchable diodes require active directional priming as illustrated in Figure 2d, wherein a pulse voltage and a constant

bias are applied to the circuit, with the former used to control the transition between the two states while the latter is for the working current. When such a pulse of positive voltage is applied, the lowest unoccupied molecular orbital (LUMO) of the molecule is populated and leads to strong hybridization with the metal substrate, triggering the conformational transition to the chemisorbed state. Conversely, when a pulsed negative voltage is applied the LUMO is depopulated, and the surface hybridization is destabilized inducing the transition to the physisorbed state. These differences in adsorption geometries and electronic properties of the two states may then naturally give rise to distinct transport behaviors in rectifying circuits. To confirm these differences, we calculated the Schottky barrier height by the alignment of energy levels between the Fermi surface of the metal side of the diode and the LUMO of the organic side.49,50 In an ideal case of weakly interacting materials, the Schottky barrier height (B) can be calculated using the Schottky-Mott rule (Equation 1). B = MEA

(1)

where M is the metal work function, and EA denotes the electron affinity of the organic molecule. In contrast to the Schottky-Mott rule, in our metal-organic interfaces this vacuum level alignment rule is broken due to the formation of an interfacial dipole, inter, that offsets the electronic structure of the two materials and changes the Schottky barrier as follows:50 B = MEAinter

(2)

where  equals to the difference between the work function modification induced by adsorption and the molecular dipole51. For the real metal-organic Schottky diode, the molecule is sandwiched between the two surfaces of electrode (Figure 2d), and the transport properties of the diodes are affected by two Schottky barrier heights in the chemisorbed system (L1, L2) and physisorbed system (L3, L4), respectively. For simplicity, we fixed the distance between the electrodes to be twice of physisorption distance, i.e. L3 = L4. To obtain the Schottky barrier height of the two systems, the metal work function was calculated from the electron potential energy in the direction normal to the surface. These calculations yielded 4.81 eV for the work function of the clean Cu(111) surface, which is in a good agreement with the experimental results.52 With transADT adsorbed on Cu(111), as shown in Figure 2a, an obvious energy drop can be observed from the metal side to the organic side of the diode. Moreover, the changes of work functions for different adsorption distances are distinct: 0.92 (L1), 0.33 (L2), and 0.84 (L3) eV. These different work functions of the two systems stem from two aspects: (1) the molecular dipole that is induced by structural deformation; and (2) the bond dipole caused by the interfacial charge rearrangement. The individual contributions of the molecular and bond dipoles to the altered metal work function were also calculated. The molecular dipole is the lesser of these two factors and can be calculated from the difference in electron potential energy of the adsorbed trans-ADT molecules, as compared to the potential energy calculated without the metal present. As shown in Figure 2a, in the chemisorbed state the electron potential energy of trans-ADT is decreased on the side facing away from the metal surface, while the opposite trend can be seen in the physisorbed case (Figures 2b and 2c). The opposite signs of these molecular dipoles are ascribed to the opposite bending directions of trans-ADT in the two states (Figure 1a). inter



Figure 2. (a) (b) (c) top: Work function changes of physisorbed and chemisorbed systems of trans-ADT on Cu(111). Plane averaged electron potential energy of the L1, L2 and L3 side at the two systems, respectively (solid line) and the plane average potential energy of the metal slab alone (dotted line) as a function of z-coordinate (normal to the surface). M represents the work function changes induced by adsorption. The dashed line represents the position of the first Cu layer, and the dashed line next to it denotes the average position of carbon atoms of the molecule. (a) (b) (c) bottom: Schematic plane averaged electron potential energy of the trans-ADT molecule in the geometry of the two systems at L1, L2 and L3 side without the metal present, respectively. Zero represents the average position of carbon atoms of the molecule along the z-coordinate. (d) Schematic illustration of the controllable transition between the physisorption and chemisorption in a real integrated diode. Firstly, the transition between physisorption and chemisorption states can be realized by adding a pulse voltage (step 1) between the right lead and substrate surface. Then, a constant bias voltage (step 2) is applied for the working current in the circuit. The distances between molecule and the surfaces of electrode are represented as L1, L2, L3, L4. (e) Energy level alignments in the physisorbed and chemisorbed states between trans-ADT and Cu(111) surfaces. Evac is the vacuum level of the metal substrate. EF denotes the Fermi level of the metal substrate. (L2)inter and (L4)inter denote the key interface dipole potentials of chemisorbed and physisorbed systems, respectively. The larger contribution to the altered metal work function is the bond dipole. A large difference in the bond dipole potentials of the two adsorption states results in a shift of the vacuum level of the molecule, and consequently, changes the values of the Schottky barrier height. This principle has been demonstrated by Heimel et al.,53 showing that for thiols adsorbed on a gold surface the bond dipole results in a shift of the highest occupied molecular orbital (HOMO) relative to the metal work function. Consistent with this work, our results demonstrate that the bond dipole is a significant factor in the energy level alignment between the metal and the organic molecule. One final dipole, the metal surface dipole, contributes to the interfacial dipole of the system.54 To obtain the surface dipole, we calculate the electrostatic potential of the metal substrate without the molecule present, which is shown by the dotted line

in Figure 2a. The potential of the top-most and bottom-most layer in the two systems are equal, indicating no dipole formed across the metal substrate. Consequently, only the bond dipole contributes to the interfacial dipole potential (inter = bond). The interfacial dipole potentials of the two systems are illustrated in Figure 2e, which shows that larger distance between trans-ADT and the substrate would lead to smaller interface dipole. Finally, we obtained the last contributors to the Schottky barrier, the molecular electron affinity (EA = 3.19 eV), from calculations of the molecule in the absence of the metal substrate. According to Equation (2), despite the same electron affinity, the Schottky barrier heights are significantly different for the two systems. For the physisorbed system the Schottky barriers are equal on both sides of the molecule due to equal contributions from each interfacial dipole, while for the chemisorbed system the conformation is asymmetric, with L2 much larger than L1, and the smaller interfacial dipole on the L2 side being the controlling parameter of the Schottky barrier height. Since the interfacial dipoles of the L2 and L4 sides are distinct, controlled switching between the adsorption states may be used to tune the Schottky barrier heights in this system. To qualitatively estimate the performance of such a bistable diode with distinct barrier heights, it is worthwhile to discuss the transport mechanisms across the Schottky interface. Two mechanisms dominate the behavior of carriers at the atomic scale: (1) carrier tunneling; and (2) carrier diffusion. The carrier tunneling effect is described by Landauers theorem,55 which is based on the wave nature of electrons. The output current is the transmission probability of electronic density of states weighted by Fermi distribution. Here, the tunneling probability of carriers can be expressed as56 2wB (3) 2mΦB) PTB = exp( ћ where wB is the tunneling width, ћ is the reduced Plancks constant, and m is the mass of the free electron. For simplicity, a

comprehensive factor C = w2BΦB is proposed to estimate the tunneling probability. The larger C represents the smaller efficiency of electron injection. For the sandwich systems in Figure 2d, physisorption exhibits an overall smaller value of C (Table S2 for details), indicating low tunneling barrier and, consequently, high electronic current in the circuit. The distinct C factors in the two adsorption systems originate from changes of both B and wB. The two parameters exhibit similar trends between the physisorbed and chemisorbed states, with both contributing to the tunneling current. Diffusive carrier transport is described within the framework of thermionic emission theory,11 in which the transmission probability is always unit, but the carriers should absorb energy, either from phonons or photons, the so-called hot carrier, to overcome the Schottky barrier. The occupation number of hot carriers follows the Boltzmann distribution function, exhibiting a feature of exponential decay with increasing energy barrier. Therefore, for both transport mechanisms, the Schottky barrier has a scattering effect on the carriers and, consequently, tunable Schottky barriers in our system can be used to control the magnitude of the current. In fact, if starting from Landauers formula, by taking transmission to unit, e.g. in the hot carrier limit, the formulism of the thermionic emission theory can be recovered.57 Such theory indicates that the current decreases exponentially with the barrier height:


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Journal of the American Chemical Society I  exp(qΦB)[exp(qV)1]

(4)

where q is the electron charge, and V is the applied voltage. As demonstrated in the above equation, under positive bias the output current of physisorption is significantly larger than that for the chemisorption, due to the lower Schottky barrier height in the former case (at L3 side). Simultaneously, this also leads to the considerably smaller leakage current in the chemisorption case when the reverse bias voltage is applied to the adsorption systems (at L2 side). Consequently, our calculations clearly demonstrate that high output current and low leakage current can be achieved in a single contact via switching between the chemisorbed and physisorbed states. To further validate the benefits of switchable molecular diode states above, we conducted a first principles calculation with NEGF-DFT to determine the quantum transport properties of a prototypical trans-ADT/Cu(111) switchable Schottky diode. The determined I-V curves of the physisorbed and chemisorbed systems are shown in Figure 3. Both adsorption conformations show asymmetric transport properties, which are attributed to the interfacial dipole induced by the charge transfer between the substrate and the adsorbate. Moreover, in the whole bias range, the conductance of physisorbed system is much larger than that of chemisorbed system, which is consistent with the qualitative analysis previously discussed. Within the NEGF theory, the entire junction is calculated as a single unit where all the interactions (within the single electron approximation) have been considered self-consistently. Therefore, the interfacial Schottky barrier height plays the central role in the conductance of the circuit. Interestingly, due to the asymmetric interfacial characteristics, the I-V curves of both the physisorbed and chemisorbed states exhibit rectification properties. Smaller circuit current appears for negative bias voltage compared to positive bias, demonstrating that even the non-switching system can function as a Schottky diode. However, the diodes formed by either individual adsorption conformation would not be suitable for many applications given that the reverse leakage current has not been substantially reduced. However, experimentally, control of the adsorption conformations has been realized by applying a pulse voltage to inject hot carriers. Here we propose that, as shown in Figure 2d, chemisorption can be triggered with a positive pulse voltage, which significantly reduces the reverse leakage current. It is worth discussing the validity and accuracy of the method applied above. Firstly, for the systems studied here, the behaviors of tunneling current are well located in the coherent transport regime. The Kondo effect and Coulomb blockade, where strong electron correlation effect dominates the transport process, can be safely excluded based on the following evidences. On the one hand, the Kondo effect is a result of magnetic impurities in the conducting molecule. An example system which exhibits this effect is TCNQ adsorbed on a graphene/Ru(0001) surface.58 The Kondo phenomenon appears in the weak interfacial coupling regime, where both magnetic moments in TCNQ and surface resonance states contribute to Kondo transport behaviors. However, neither feature exists in our system. Moreover, even for physisorption, as demonstrated by the interfacial geometric distortion and charge transfer, the molecule-surface coupling is relatively strong compared to a typical Kondo system. On the other hand, Coulomb blockade is typified by low-bias current suppression,59 which is attributed to Coulomb on-site repulsion in molecule, resulting in a two-

step tunneling behavior.60 For trans-ADT physisorbed on Cu(111) system, no step-like feature of the tunneling current appeared at low-bias,29 indicating that the transport behavior is not in the Coulomb blockade regime. Therefore, based on the above analysis, the transport process in trans-ADT/Cu(111) system can be well described by coherent transport in the meanfield approximation.

Figure 3. First-principles simulation of transport properties of trans-ADT/Cu(111). The I-V curves for the proposed operation mode of the diode, involving controllable transitions from the physisorbed to chemisorbed state of the molecule, are shown by solid lines. The geometric structure employed to calculate the IV curves of the trans-ADT/Cu(111) interface is shown in insets. The NEGF-DFT technique is appropriate to describe coherent transport without strong electron-electron correlations, where the scattering mechanisms are treated effectively within the potentials used for the construction of the interaction Hamiltonian. Within the framework of DFT, the inherent limitations solely stem from exchange-correlation functional (or exchange-correlation potential). More sophisticated techniques, such as NEGF-GW61 and NEGF-DFT+Σ,62 could provide more accurate results, but the application of such methods on our systems is extremely demanding. Despite the methodological drawbacks, NEGF-DFT is qualitatively appropriate, with a systematic overestimation of tunneling current within a factor of ten, which is accurate enough for theoretical predictions.63,64 To further investigate the breadth of possible electronic properties for similarly constructed Schottky diodes, we determined the barrier heights for cis- and trans-anthradifuran (ADF), and trans-anthradiselenophene (ADS) molecules on Cu(111) surfaces (the oxygen and selenium analogues to ADT, respectively; chemical structures are shown in Figure 4). Each of these molecules exhibits bistable behaviors upon adsorption on the Cu(111) surface.48 As shown in Figure 4, these systems can be divided into two categories according to the computed Schottky barrier heights of the two states. The first category includes the trans-ADT, cis-ADF, and trans-ADS for which the Schottky barrier heights of the two systems sharply differ, and more importantly, the B value of the physisorbed system is smaller than that of the chemisorbed system. The second category includes trans-ADF/Cu(111), which has higher Schottky barrier height in the physisorbed state than that in the chemisorbed state. In this case, the physisorbed state should be utilized under negative voltage for achieving a low leakage current, while the chemisorbed state is used in the positive bias



for the high output current. This opposite behavior stems from the formation of opposite molecular dipoles compared to transADT leading to a smaller interfacial dipole in the physisorbed state. The behaviors of the distinct barrier heights in these four systems begin to elucidate controlling parameters for the design of switchable Schottky contacts.

which could bring new freedom for the design of controllable semiconductor nano-devices. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Bond dipole for the physisorption and chemisorption of trans-ADT on Cu(111); tunneling barrier of physisorbed and chemisorbed systems of trans-ADT on Cu(111) surfaces; Schottky barrier height for the physisorbed and chemisorbed systems of C4N2Cl4 on Pt(111) surfaces; plane averaged electron potential energy of the bistable systems for trans-ADT on Cu(111) surfaces including Tables S1−S3 and Figures S1 (PDF) ■ AUTHOR INFORMATION Corresponding Author

Figure 4. Schottky barrier heights at L3 and L2 side for different molecules physisorbed and chemisorbed on the Cu(111) surface. Chemical structures of trans-isomers of ADF, ADT, and ADS and cis-ADF molecules are also presented in the figure. Note that ADF is obtained by substituting S atom in ADT with O atom (Se atom). The different bias voltages applied to the two states are also indicated in the insets. To generalize our conclusion to other molecules, we further calculated the Schottky barriers for tetrachloropyrazine molecule (C4N2Cl4), whose structure is sharply different from the family of molecules discussed above, on the Pt(111) surface, since they also exhibit bistable phenomenon.28 Similar to transADT/Cu(111), the C4N2Cl4/Pt(111) systems also exhibit distinct Schottky barrier heights and work function modifications at the physisorbed and chemisorbed states (Table S3). These results clearly indicate that the asymmetric Schottky barriers are only dependent on the different adsorption conformations. Therefore, any semiconductor molecule possesses bistable states could be utilized to form the switchable Schottky contact. ■ CONCLUSIONS By using van der Waals-inclusive density functional theory calculations, we designed a Schottky diode using bistable molecular switching in the system of trans-ADT adsorbed on a Cu(111) surface. We confirmed that the adsorption geometries and electronic properties of the two states are remarkably different, leading to substantial differences in Schottky barrier heights. Due to the distinct barrier heights of the two adsorption states, switching between physisorbed and chemisorbed states in a single system may obtain high output voltage and low leakage current simultaneously. Further, by applying NEGF transport calculations, we demonstrated that the I-V curves of the controllable contact conformations exhibit obvious rectifying behaviors, which can be significantly improved by switching between physisorbed and chemisorbed states. Finally, we expect our work may stimulate the community of semiconductor device manufacturers to consider incorporation of bistable states or phase transitions at the contact regime,

*[email protected] *[email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT We acknowledge support from the NSF of China (51722102, 21773120, 51602155), the NSF of Jiangsu Province (BK20180448), and the Fundamental Research Funds for the Central Universities (30917011201). We are grateful for support from Jiangsu Key Laboratory of Advanced Micro&Nano Materials and Technology.

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Journal of the American Chemical Society


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Switchable Schottky contacts: Simultaneously enhanced output

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