Using nonequilibrium Green’s function formalism combined first-principles density functional theory, we analyze the transport properties of (Z)-4-aminopent-3-en-2-one and its 3 derivatives in R2 position (R2= H, CH3, and C2H5) molecular switches. The title compounds can convert between keto and enol forms. Three kinds of adsorption sites including the hollow, bridge and top sites are studied. We predict a 2–5 times conduction enhancement for the keto form over the enol form. The I–V characteristics, differential conductance, on-off ratio, electronic transmission coefficients, HOMO–LUMO gaps and effect of electrode materials Y (Y=Au, Ag, and Pt) on electronic transport corresponding to the keto and enol forms through the molecular devices are discussed in detail. It can be concluded that the best efficiency of the switching behavior occurs in the gold electrode. Based on the results as soon as possible the enol form transforms to the keto form, there is a switch from on state to the off state (low resistance switches to high resistance).
Keywords: Nano-electronic device, Electronic transport, Non-equilibrium Green’s function, ?-ketoenamines.
PACS: 85.65.+h; 31.15.E-; 73.23.-b; 71.15.Mb
With the getting smaller of traditional electron devices and the advancement of nanotechnology and microfabrication, using molecules as components in atomic-scale circuits has become an attractive field and nanomaterials have become one of the most dazzling stars in the twenty-first century 1,2.
Because of its inimitable applications and physical properties, various nanostructures have been synthesized 3,4, and these structuresdisplaysignificant promise as logic functionality 5, solar cells 6-8, switches 9-13, radio frequency communication circuitry 14, Photocatalyst 15, field-effect transistors 16,17, quantum dots18,19, nonvolatile memory devices 20, biological sensors 8, as well as developing opticaland photonic detectors 6,14,light-emitting diodes 7,8,6, and so on 21,22.Among these devices, molecular switches, have drawn considerable attention in recent years because they are considered to play an essential role in future logic and memory 23-26.
?,?-unsaturated-?-ketoenamines, are capable to exist in three different tautomers at equilibrium, i.e., ketoimine, aminoketone, and iminoenol forms, see Fig. 1. The aminoketone and iminoenol forms of ?-Enaminones are more stable than ketoimine form. Therefore, we investigate the behavior of I-V in these two more stable forms. The aminoketone and iminoenol tautomers are engaged in a six-member ring by intramolecular hydrogen bonded system, with the N-H…O and O-H…N systems, respectively. The calculated electronic energies of aminoketone forms are usually less than those of iminoenol forms. This is in agreement with the reported studies, which are shown the aminoketone forms are as a major form in ?-Enaminones 27-29.
Some of the effective factors such as temperature, Polarity of solvent, and type of ? and ? substitutions can change the mentioned tautomerization equilibrium. These effective factors were studied by theoretical and experimental methods such as quantum-mechanical calculation, IR, Raman, microwave, UV, and NMR spectroscopies, X-ray, electron and neutron diffraction measurements 12,13,27-32.
In this project, Non-equilibrium Green’s function (NEGF) formalism merged with first-principles density functional theory (DFT) were used to study the behavior of molecular switches conductance of the title compounds with three typical adsorption sites, namely the hollow, top and bridge sites. It is well-known that the transmission does not only depend on the properties of the molecule but also depends on the electrode’s nature. As the complicated interaction between the molecule and the electrodes plays a critical role in electronic transport property, the effect of the chemical potential difference in fabricated molecular devices should be well considered. We investigate the transport properties of the title molecule with different interface anchors for Au, Ag, and Pt electrodes.
Model and computational methods
The geometry optimization of the title compounds with two SH group as the linker is performed by Gaussian 03 program 33 with B3LYP type exchange-correlation functional 34 and 6-311++G(d,p) basis set. All calculated vibrational wave numbers are positive.
It is cleared that hydrogen atoms are unzipped upon adsorption to metal surfaces35,36. Therefore we build a two-probe system in which the two terminal hydrogen atoms that bonded to the sulfur atom are deleted from the optimized structure, and the total molecule is placed to between two parallel metal shells. It’s both enol and keto forms have two S-linker and are linked to the metals junction with (1 1 1) surfaces37,38,which is schematically illustrated in Fig. 2. The nano device is based on the three-region partition of the device of interest: the left electrode(L), the right electrode (R) and central region (C) and central extended molecule region (EM), which includes a few atomic layers of the electrodes.
The most common Y (1 1 1) surface is used with (4×4) periodic boundary conditions. The 4×4 super cell is vast satisfactory to prevent any interplay with molecules in another super cell 39,40. There are three possible adsorption sites of the S atom on the Au (1 1 1) surfaces, i.e., top, bridge, and hollow sites 41-44. We consider these three typical molecule–metal adsorption sites with the corresponding distances between S atom and adsorption sites being 1.9, 2.07, and 2.42 A? for hollow, top, and bridge sites, respectively 36,43,45.
Since transport properties are going to be calculated after the structural relaxations, only atoms in the scattering region are fully relaxed within a force convergence criterion of 0.02 eV/?A, while all the electrode atoms are considered fixed. The transport calculations were carried out using TRANSIESTA-C package based on SIESTA 46. The Perdew–Burke–Ernzerh of (PBE) exchange–correlation functional is adopted for the generated gradient approximation (GGA)47. The Y atoms are characterized by a single-? plus single polarization (SZP) basis set whereas for the other atoms a double-? plus single polarization (DZP) basis set is used. The k-grid sampling of 2×2×100 by Monkhorst-Pack to describe the Brillouin zone48, we employed simultaneously with the mesh cutoff of 150 Ry for the grid integration and select 10-5 as the convergence criterion for the total energy. The bias voltage is scanned from 0.0 to 2.0 V at 300 K. The current through the device is computed using the Landauer–Büttiker formula in the TranSIESTA package46.