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WIREs Comput Mol Sci
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One molecule, two states: Single molecular switch on metallic electrodes

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Abstract The state‐of‐the‐art density functional theory (DFT) has become an essential tool for the investigation and development of molecular electronics at the electronic and atomic level. In this review paper, we show several typical examples to demonstrate that the DFT approaches, combined with nonequilibrium Green's function method, are able to design many prominent molecular switches—the most fundamental component in molecular electronics that can be utilized in information storage and logic gates. We mainly review the progress and important features of four remarkable switches with distinct transition mechanisms: (a) azobenzene‐like switches based on the cis–trans isomerization; (b) diarylethene‐like switches based on open‐closed transition; (c) porphyrin‐like switches based on tautomerizations; and (d) benzene‐like switches based on the physisorbed state and chemisorbed state. Special attentions have been paid on the molecular configuration, switching mechanism, and the role of van der Waals forces between the molecules and the metallic electrodes. We also summarize the avenues to effectively tailor the bistability, reversibility, and transport properties of these systems. This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory
Energy profiles of the switching process of azobenzene in gas phase and on metal surfaces. (a,b The rotation and inversion switching pathway, respectively. w and α are the rotation angles of NN double bond and inversion angles of the NC bond, respectively. Z‐Ab and E‐Ab denote the cis and trans isomers of azobenzene, respectively. (Reprinted with permission from Reference 114. Copyright 2012 Wiley)
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(a) Schematic illustration of the isomerization of azobenzene molecule between trans and cis isomers. The switching of the azobenzene molecule can take place along either rotation or inversion pathways in gas phase and solution. (b) Theoretically proposed azobenzene molecular switches by bridging azobenzene between two gold electrodes. The sulfur atoms were used as the anchoring groups in each side of the molecule junction. (Reprinted with permission from Reference 24. Copyright 2004 APS) (c) Isomerization of azobenzene molecule on Au(111) surface by injecting tunneling electrons into the molecule from the STM tip. (d) Tunneling currents for azobenzene molecular switch at trans (solid line) and cis (dashed line) states. The panels represent the STM images of trans (left) and cis states (right). (Reprinted with permission from Reference 111. Copyright 2006 APS)
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(a) Adsorption geometries for cis‐ and trans‐ADT on the Cu(111) surface. The heights of the central C and side S atoms are indicated in the plot, which were calculated relative to the position of the unrelaxed topmost metal layer. (b) Simulated STM images for cis‐ and trans‐ADT on Cu(111) at the chemisorbed and physisorbed states, obtained using the Tersoff−Hamann approximation, are in good agreement with the experimental observations.158 (Adapted with permission from Reference 160. Copyright 2018 American Physical Society)
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Left panel: Adsorption energy −Ead as a function of the adsorption height d for benzene on Pt(111) from the PBE (blue curve) and PBE + vdWsurf methods (red curve). The dashed line represents the vdW interactions part from PBE + vdWsurf. The yellow intervals represent the experimental adsorption distances and energies.156,157 Right panel: Adsorption energy (x axis) as a function of the adsorption height d (y axis) and simulated STM images for tetrachloropyrazine (C4N2Cl4) on Pt(111) at the physisorbed and chemisorbed states. (Adapted with permission from References 41, 153, and 78. Copyright 2019 Elsevier; Copyright 2012 American Physical Society; Copyright 2013 Springer Nature)
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(a) Schematics of the tautomerization process between the two cis‐1 states. (b) Potential energy diagram of the reaction pathways for the porphycene tautomerization. (c) Plots of the barrier energy against the difference of the charge transfer between different states from HOMO−2 orbitals on Ag, Cu, Pd, and Pt(110) surfaces. (d) Schematic of the relative position of the HOMO−3, HOMO−2, HOMO−1, HOMO, and LUMO molecular orbitals for the isolated gas phase porphycene molecule in cis and trans configurations. (Reprinted with permission from Reference 138. Copyright 2019 APS)
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(a) STM images and adsorption structures of porphycene molecules on Cu(110) surface. (b) Schematic potential energy surface of the tautomerization of porphycene on Cu(110) surface. The two energy minima correspond to the different cis‐1 states. E is the calculated interaction energies, ∆E is the difference in barrier heights for the “high” and “low” states. (Reprinted with permission from Reference 31. Copyright 2014 Springer Nature)
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(a,b) Calculated stepwise and concerted minimum energy paths (MEPs) of tautomerization for porphycene on Ag(110) surface, respectively. Red crosses represent the zero‐point corrected energy for HH‐porphycene while blue crosses represent blue DD‐porphycene. TS and SS represents the first‐order transition state and second‐order saddle point, respectively. Inset in (b) describes the structure of cis state on Ag(110) surface. (c,d) MEPs for the stepwise and concerted switching on Cu(110) surface, respectively. (Reprinted with permission from Reference 148. Copyright 2017 American Chemical Society)
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(a) The calculated optimal structures for trans, cis‐1, and cis‐2 configurations on Cu(110) surface. (Reprinted with permission from Reference 31. Copyright 2014 Springer Nature). (b) (Left) Current‐trace obtained with the STM tip positioned at the molecule during a voltage pulse of 300 mV. (Right) Schematics of the porphycene tautomerization process on Cu(110) surface. (Reprinted with permission from Reference 145. Copyright 2013 APS)
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(a) (Left) Current‐trace obtained with the STM tip positioned at the molecule (red dot in STM images). (Right) STM image of high current or low current state. (b) The STM images and DFT calculation of LUMO and LUMO+1 for the molecule. (Reprinted with permission from Reference 30. Copyright 2007 Science)
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(a) Schematic illustration of the switching based on molecules. (b) Current of molecular junctions at the open and closed states. (c,d) Transmission and spatial distribution of the p‐HOMO and p‐LUMO of molecule at open and closed states, respectively. The red downward triangles represent the energies of the molecular projected self‐consistent Hamiltonian (MPSH) spectra. The red and blue rectangles denote the transmission peaks of hybridized HOMO and LUMO. These results were obtained by the local density approximation functional. (Reprinted with permission from Reference 132. Copyright 2016 Science)
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(a) Schematic illustration of the molecular switching in circuits. The diarylethene‐based molecule is bridged between two graphene electrodes. (b) Molecular structure of primitive molecule (1), molecule after fluorination (2), and molecule after both fluorination and adding CH2 functional group (3). (c–e) Molecular orbital and energy alignment between the molecule orbitals and Fermi level of graphene of these three molecules. ECB represents the Fermi level of electrode. (Reprinted with permission from Reference 131. Copyright 2013 Wiley)
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(a) Closed and open forms of diarylethene‐based molecular switch. In solution, the molecule can be reversibly switched between closed and open states under light with wave length of 300–400 and 500–700 nm, respectively. When coupled to the gold electrode, the switching process of open‐to‐closed vanishes due to the quenching effect. (b) Potential energy profile calculated by AM1 semi empirical quantum method. The solid line denotes the potential curve at ground state. The vacuum level is defined as the ionization potential of the molecule. TO and TC represent the first excited state of the open and closed forms under light stimuli, respectively. (Adapted with permission from Reference 128. Copyright 2003 APS)
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(a) Schematic illustration of the reversible switching process under light stimulus. (b) Electric field‐controlled stability for the molecular switch. −∆E0 represents the energy difference between the cis and trans form at zero bias voltage: −∆E0 = Et − Ec. (c) Transmission spectrum for the molecular junction under cis and trans states using the NEGF method. The p‐HOMO and p‐LUMO peaks are denoted in the dash‐line rectangles. (Reprinted with permission from Reference 118. Copyright 2019 Nature)
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