First Principle Study on the Rectification of Molecular Junctions Based on the Thiol-ended Oligosilane①

ZHANG Rong-FngYANG ELI YiLIN Li-XingLING Qi-Dn②(College of Mterils Science & Engineering, Fujin Norml University, Fuzhou 350007, Chin)(College of Chemistry, Fuzhou University, Fuzhou 350116, Chin)

First Principle Study on the Rectification of Molecular Junctions Based on the Thiol-ended Oligosilane①

ZHANG Rong-FangaYANG EaLI YibLIN Li-XiangaLING Qi-Dana②a(College of Materials Science & Engineering, Fujian Normal University, Fuzhou 350007, China)b(College of Chemistry, Fuzhou University, Fuzhou 350116, China)

The electron transport properties of various molecular junctions based on the thiol-ended oligosilane are investigated through density functional theory combined with non-equilibrium Green’s function formalism.Our calculations show that oligosilanes doped by the phenyl and -C10H6groups demonstrate better rectifying effect and their rectification ratios are up to 15.41 and 65.13 for their molecular junctions.The current-voltage (I-V) curves of all the Au/ modified oligosilane/Au systems in this work are illustrated by frontier molecular orbitals, transmission spectra and density of states under zero bias.And their rectifying behaviors are analyzed through transmission spectra.

first principle, thiol-ended oligosilane, rectification;

1 INTRODUCTION

In the past ten years, the field of silicon-based 1D nanowires has attracted more and more attention among researchers all over the world.With wellcharacterized silicon nanowires having been produced, they have been utilized in chemical or biological sensors[1,2]field-effect transistors[3], solar cells[4], and lithium battery anodes[5].And siliconbased 1D nanomaterials show some interesting conducting properties under doping conditions[6-9].One of the most promising candidates for research is oligosilane in molecular devices[10-14], such as nanodiodes and nanosensors[10,11].

Molecule-level rectification plays an important role in molecular electronics.In 1974, Aviram and Ratner[15]proposed a donor-σ-bridge-acceptor (D-σ-A) model and opened up a new area of designing molecular rectifier.The σ-bridge provides a tunneling barrier for the electron transport between the donor and acceptor in a p-n junction semiconductor device.Based on D-σ-A proposal, Zhang and coworkers[13]designed and studied transport property of the p-n junction oligosilane nanowire caused by boron-doping and phosphorus-doping.It was found that the undoped oligosilane chain has no rectification character and the p-n junction oligosilane nanowire showed a satisfying rectification.Motivated by Aviram and Ratner’ model, more efforts have been devoted to search for other kinds of rectifiers[16-19].One research interest has been focused on the D-π-A molecules[17].Recently, Li andcoworkers have reported that the modification of bridging group makes it possible to improve the performance and obtain new functions in a single cross-conjugated molecular junction[20].However, the rectification of oligosilane chains modified by bridge groups still lack report and the experiment report is also few.The semiconducting silicon nanowires used in electronic devices are strongly conditioned by the presence of doping impurities, which are able to displace the Fermi level close to the band edges[21,22].So, we design to introduce different σ-bridge groups and π-bridge groups into the oligosilane chains, such as -CH2, -CH2CH2, -CH=CH, -C≡C, -C6H4and -C10H6.Such successful bridge-doping upon the silicon nanowires to create rectifier would open up exciting opportunities in nanoscience and technology.

Here, we investigate the transport properties of pure and doped oligosilanes in a systematic comparison way by using density functional theory (DFT) combined with non-equilibrium Green’s function (NEGF) approach.Our results show that the doped oligosilane exhibits higher rectification ratio by introducing phenyl and -C10H6groups.

2 COMPUTATIONAL DETAILS

Fig.1 illustrates the models of molecular junctions a～g with metal/molecule/metal structures.In each so-called two probe system, a thiolate-ended molecule based on oligosilane is sandwiched between two gold electrodes.The thiol end group is employed widely in the field of molecular devices.Molecules containing thiol end groups can be self-assembled on the Au substrate because the hydrogen atom in the thiol group will be dissociated and strong Au–S covalent bonds will form when the thiol group interacts with Au surface.The two Au(111)-(3×3) surfaces (i.e., each layer consisting of nine gold atoms) with periodic boundary conditions were used to model the left and right electrodes[23,24].The molecule in the central region of system a is a pure dithiolate-terminated oligosilane.-CH2and -CH2CH2groups are introduced into the oligosilane as σ-bridge in system b, thus forming two (SiH2)6-σ-(SiH2)5molecules.The groups of -CH=CH, -C≡C, -C6H4and -C10H6are introduced into the oligosilane as π-bridge in systems c～g, thus forming five (SiH2)6-π-(SiH2)5molecules.

Fig.1.Schematic view of the single pure and modified oligosilane molecular junctions.“+” is replaced by -SiH2, -CH2, -CH2CH2, -CH=CH, -C≡C, -C6H4and -C10H6, corresponding to a～g, respectively.These thiolate-ended molecules self-assemble on the Au(111)-(3×3) surface, and consist of the two-probe Au/molecule/Au systems with right and left semi-infinite electrode and the scattering region

The whole computation is composed of two procedures.First, the geometry optimizations and electronic structures of isolated molecules in the central region in Fig.1 are performed using the Gaussian03 program[25]at the hybrid DFT/B3LYP[26,27]level of theory with the 6-31G(d,p) basis set.The next procedure is the transport computation after the above geometry optimizations.The geometries of isolated molecules are extracted from the optimized extended molecules and then translated into the central region between the two gold electrodes, as illustrated in Fig.1.The two Au(111)-(3×3) sur-faces with periodic boundary conditions are used to model the left and right electrodes.The Au/molecule/Au configuration is divided into three parts: left electrode, right electrode, and central scatting region.In our models, there are three gold layers in each left and right electrode unit cells.The scattering region is composed of isolated molecules together with the respective three gold layers on the left and right sides.The distance between the Au(111) surface and the terminal S atom was 2.28 ?, which is in the range from 1.90 to 2.39 ? used by most studies[28].The electron-transport properties of the metal/molecule/metal systems were investigated using ab initio software package, Atomistix ToolKit (ATK)[29,30], which is based on density functional theory (DFT) combined with the first-principles non-equilibrium Green’s function(NEGF).In this work, a double-x polarization (DZP) basis set is used for all atoms of molecule with the exception of H, and a single-x with polarization (SZP) basis set is used for Au and H atoms.The exchange-correlation potential is described by the Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA)[31,32].The convergence criterion is set to 1×10-5for grid integration to obtain accurate results.A k-point sampling of 1 × 1 × 100 is used for the metal-electrode models.On a realspace grid, a mesh cutoff energy of the charge density and potentials is set to150 Ry.

In these molecular junctions, the current-voltage (I?V) characteristics are obtained from the Landauer-Büttiker formula[33].

where 2e2/h = G0is the quantum unit of conductance, e expresses the elementary charge and h shows the Planck’s constant.f is the Fermi function, μRand μLare respectively for the electrochemical potentials of the right and left electrodes: μR(Vb) = Ef+ eVb/2 and μL(Vb) = Ef? eVb/2, where Efrepresents the Fermi energy of the electrode, and [μL(Vb), μR(Vb)] shows the current integral, known as the energy region or the bias window.T(E, Vb) is the transmission function for an incident electron with energy E at a bias voltage Vb.

3 RESULTS AND DISCUSSION

3.1 Electronic structures of the isolated molecules

The molecular electronic structure can affect the conductance of the molecular transport junction.As suggested by Cohen et al.[34], the density distribution of frontier molecular orbital is intrinsic to the molecule rather than to the junction.It is an important factor determining the conductance of molecular transport junction.Therefore, the electronic structures of isolated molecules were investigated before the electron-transport calculations are performed.Fig.2 shows the frontier molecular orbital diagrams of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO).The orbital density distributions of LUMO for all the molecules are delocalized almost completely.However, the cases of HOMO of these molecules are not fully delocalized and exhibit some obvious differences.For pristine oligosilane (a), the HOMO density distributions are nearly fully delocalized except one -SH group, because there is no orbital density distribution on it.The HOMO density distributions are similar for the -CH2and -CH2CH2groups modified oligosilane molecules (b and c).The HOMOs of molecule d are localized strongly on the Si5H10CH=CH part.The delocalized extent of HOMOs for molecule e is little stronger than that of system d.Though molecules f and g exhibit similar arch structures, the HOMO density distributions of f are almost delocalized except the -SiH2SH group, and the HOMOs of molecule g are nearly localized on the functionalized group -C10H6.

The energy levels of the frontier molecular orbital and the related HOMO-LUMO gaps (HGLs) are shown in Fig.3.The change tendency of HOMO energy level for all the molecules is different from that of LUMO energy level.Different modifiedgroups in the oligosilane affect the energy levels obviously.In addition, introducing different groups into oligosilane leads to a rise of LUMO energy levels and has a few effects on the occupied molecular orbital (HOMO and HOMO-1) energy levels, except for the introduction of -C10H6group.When the -C10H6group is introduced into oligosilane by replacing the -SiH2group, the occupied molecular orbitals (HOMO and HOMO-1) and unoccupied molecular orbitals (LUMO and LUMO+1) energy levels are all reduced.As a result, the HGL of molecule g is reduced obviously.

Fig.2.Frontier molecular orbital shapes of thiol-ended oligosilane and its derivatives

Fig.3.Molecular energy levels HOMO-1, HOMO, LUMO and LUMO+1 of the isolated molecules and their HOMO-LUMO Gaps (HLGs)

3.2 I-V characteristics and rectification

The computed current-voltage (I-V) curves of the seven two-probe systems a～g are plotted in Fig.4.For the oligosilane and its functionalized molecules studied here, the I-V curves of molecule systems a～ e are nearly linear, and exhibit similar trends.The current of system a is consistent with the literature[13].Although the current change trends of systems a～e functionalized by -CH2, -CH2CH2, -CH=CH, and -C≡C groups are similar to the pureoligosilane, the currents of systems c, d and e decrease significantly compared with that of system a, and the current of system b is smaller than that of system a, lager than that of systems c, d and e.However, the I-V curves of molecule systems f and g are nonlinear.Under positive bias, the current of model f tends to be zero.The current value increases slowly about 0 to –1.58 V, then increases rapidly when the bias is above 1.58 V under negative bias.Thus, it is suggested that the doped groups -CH2, -CH2CH2, -CH=CH and -C≡C, and phenyl have obvious effects in declining the electron transport of oligosilane.For oligosilane derivative molecule doped by -C10H6, the I-V curve tends to be zero around the zero bias (about 0 to ±1.5 V).The current of model g increases rapidly when the bias is above about ±1.5 V.When the current value comes to a maximum, it begins to decrease at 1.83 V under positive bias voltage.This phenomenon is so-called negative differential resistance (NDR).The result shows that the -C10H6group may be more beneficial to the electronic transport of oligosilane at high bias voltage than others studied here.

Fig.4.I-V curves of systems a～g in the bias range from–2.0 to +2.0.The positive current means that the current flows from the left electrode to the right electrode and vice versa.The inset shows the rectification ratio (R) as a function of applied voltage for systems a, f, and g

As shown in Fig.3, the HOMO level of each molecule is aligned nearer to the work function of gold (approximate –5.1 eV) than its LUMO level.However, there are large energy level gaps (over 1.0 eV) between the HOMO level of molecular systems and the Fermi level of gold.And the HGLs of isolated molecules all exceed 4.0 eV.Consequently, no current value goes beyond 200 nA.As discussed in section 3.1, the HOMO density distributions of systems a～g exhibit the relative sequence a > b > c > f > e > d > g.The current flows increase in accord with the sequence a > b > f > c > e > g > d around the zero bias (about 0 to ±1.5 V).It seems that the equilibrium conductances of these oligosilane families are in good agreement with their density distributions of HOMOs of isolated molecules around the zero bias (about 0 to ±1.5 V), except molecular systems f and g.It may be related to the arch structures of systems f and g.

Furthermore, the transmission coefficients and DOS of a～g two-probe systems under zero bias have been analyzed and give insights into the states contributing to conductivity.Fig.5 plots the DOS of a～g two-probe systems under zero bias with redlines.In the pure system at zero bias, two resonance peaks are observed at about –0.4 and –1.0 eV, only originating from the HOMO states.Introducing groups of -CH2, -CH2CH2, -CH=CH, -C≡C, and phenyl into the oligosilane nanowires have no obvious effect on the resonance peaks of DOS under zero bias, except introducing the -C10H6group.For the g system, there are three resonance peaks at about –0.4, –0.9 and –1.0 eV.However, the transmission spectra under the zero-bias of b～g are changed as introducing the groups into system a, as shown in Fig.5 with black lines.For the pure system, there lies a broad resonance peak between –0.3 and–1.1 eV, which has good correlation with one resonance peak –1.0 eV of its corresponding DOS.The high transmission coefficient lies in the energy position where one resonance peak of its DOS appears.Therefore, the transmission peak is located at –1.0 eV for the pure oligosilane two-probe system.Note that the broad resonant peak becomes smaller than that of the pure system when introducing the groups of -CH2, -CH2CH2, -CH=CH, -C≡C, and phenyl into oligosilanes as bridges.Although the transmission peaks of the d and f systems are dropped, they are still located at the same energy positions with the pure system.The broad resonant peak becomes narrower and higher at –0.9 eV for the g system, where the transmission peak forms.According to the current depending on their transmission peaks and coefficients, we can speculate that the current flows of a, d, f and g systems will become larger as the applied bias voltage exceeds the forward threshold voltage, in line with the I-V curves.The transmission coefficient of the c～e systems drops to about 1.5×10-4, zero and 1.0×10-4, respectively.Therefore, we can speculate that the current flows of the c～e systems will be nearly zero when applying the bias voltage, in line with the I-V curves.

Fig.5.Transmission spectra (black lines) and DOS (red lines) spectra of the a～g two-probe systems under zero bias

As a whole, the total transmission coefficient is almost zero in a region near the Fermi level in the pure system.The result of the pure system agrees with the report by Zhang and his coworkers[13].Like the pure system, all the bridge-doped systems have the same characteristics that their total transmission coefficients are almost zero in a region near the Fermi level.At the zero bias, the electron transmission mainly depends on the size of the transmission coefficient near the Fermi level.This means the electrons can not permeate effectively through the oligosilane chain, which indicates that bridgedopings with -CH2, -CH2CH2, -CH=CH, -C≡C, phenyl, and -C10H6groups do not improve the conductivity of such short oligosilane wire.On the basis of the resonances in the transmission spectra and the states of DOS spectra under zero bias, we can conclude that the HOMO states will mainly contribute to the current of the systems, and the bridge-dopings with -CH2, -CH2CH2, -CH=CH, -C≡C, phenyl, and -C10H6groups do not improve the conductivity of such short oligosilane wire.And thetransmission wave is very weak for all the molecular two-probe systems.This is also in good agreement with the weak currents in the I-V curves.

In Fig.4, it is evident that the I-V curves of molecular systems f and g are obviously asymmetric at about zero bias.In order to reveal the features of the asymmetry in detail, the rectification ratios of systems f and g were computed.The rectification ratio is defined as

By definition, R(V) = 1 means no rectification.R(V) > 1 shows that the current is larger in the negative direction than in the positive direction, and vice versa.

According to our calculation, models f and g have obvious rectification effects.Their rectification values are up to 15.41 and 65.13, respectively, as shown in the inset in Fig.4.Rectification ratios of other models are close to 1 in the applied bias ranges.It comes out into that the modified groups -CH2, -CH2CH2, -CH=CH, and -C≡C have few effects on the oligosilane’ rectification.

In this work, the rectifying for the molecular systems is interpreted by analyzing the transmission spectra.The current through a molecule system is determined by the transmission spectra within the bias window (L(Vb),R(Vb)).The region of the bias window is actually (–Vb/2, + Vb/2) if the Fermi level is set to zero.Theoretically, the transmission is determined by the molecular electronic structure modified by the applied bias and the coupling between molecule and electrode, etc.Fig.6 illustrates the transmission spectra of the two-probe systems a～g in the energy range from –1.5 to +1.5 eV at their bias voltages of the highest rectification ratios.

Fig.6.Transmission spectra of two-probe systems a～g at special bias of each highest rectification ratio.Black, red and black dashed lines indicate positive bias voltage, negative bias voltage and bias windows at each bias voltage, respectively

It is noted that transmission spectra of systems a～e exhibit a very small difference at the positive and negative bias voltages.The transmission resonance peaks within each bias windows of systems a～e at the negative bias are a bit broader and higher than those at the positive bias.Therefore, each rectification ratio of systems a～e all exhibits R > 1 slightly at bias ±2 V.In system f, there is a completetransmission peak within bias window under the negative bias.However, the peak almost disappears when the positive bias voltage is applied to the two-probe system.Consequently, the current of system f at the positive bias +1.96 V is nearly zero, then the rectification ratio of system f exhibits R = 15.41 at bias ±1.96 V.In system g, there is a bit transmission wave within bias window under the positive bias, and the transmission peak partly enters into the bias window when the negative bias voltage is applied.Therefore, the current of system g at the positive bias +1.58 V is very small, then the rectification ratio of system g exhibits R = 65.13 at bias ±1.58 V.

In summary, our calculations show that the introducing groups of -CH2, -CH2CH2, -CH=CH, -C≡C, and phenyl into the oligosilane nanowires have no obvious effect on the resonance peaks of DOS under zero bias, the transmission spectra under the zero-bias of b～g are changed as introducing the groups into pure oligosilane nanowires, the bridgedopings with -CH2, -CH2CH2, -CH=CH, -C≡C, phenyl, and -C10H6groups do not improve the conductivity of such short oligosilane wire, the oligosilanes doped by phenyl and -C10H6groups demonstrate rectifying effect better than that of the pure oligosilane, and their rectification ratios are up to 15.41 and 65.13 for the corresponding Au/doped oligosilane/Au molecular junction.

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10.14102/j.cnki.0254-5861.2011-0590

1 December 2014; accepted 2 March 2015

①This work was supported by National Natural Science Foundation of China (21401023 and 21203027), Cultivating Fund for Excellent Young Scholar of Fujian Normal University (FJSDJK2012063), and Program for Innovative Research Team in Science and Technology in Fujian Province University (IRTSTFJ)

②Corresponding author.E-mail: qdling@fjnu.edu.cn