J. Phys. Chem. C 2010, 114, 1655–1662
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Manipulating I-V Characteristics of a Molecular Switch with Chemical Modifications Julio L. Palma,†,‡ Chao Cao,†,§ X.-G. Zhang,| Predrag S. Krstic´,⊥ Jeffrey L. Krause,†,‡ and Hai-Ping Cheng*,†,§ Quantum Theory Project, UniVersity of Florida, GainesVille, Florida 32611-8435, Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200, Department of Physics, UniVersity of Florida, GainesVille, Florida 32611-8440, and Center for Nanophase Materials Sciences and Computer Science, Mathematics DiVision, and Physics DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6372 ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
We present a study of the effects of chemical modifications on the electron transport properties of the azobenzene molecule, which has been proposed as a component of a light-driven molecular switch. This molecule has two stable conformations (cis and trans) in the electronic ground state, with considerable differences in conductance. The electron transport properties were calculated using first-principles methods combining nonequilibrium Green’s function techniques with density functional theory. Chemical modifications of the azobenzene consist of incorporation of electron-donating and electron-withdrawing groups in meta and ortho positions with respect to the azo group. The results show that the transport properties in electronic devices at the molecular level can be manipulated, enhanced, or suppressed by a careful consideration of the effects of chemical modification, and such modifications become crucial in optimizing the electron transport properties of chemical structures. 1. Introduction Future generations of electronic devices will be on the molecular level, and the ability to control the transport properties of single molecules will have a major impact on this promising technology. In recent years, important breakthroughs in advanced microfabrication and self-assembly techniques have been reported.1-4 Research on electron conduction measurements by scanning probe microscopy,5 micromachined silicon nanopores,6 and proximal probe techniques7,8 have shown substantial and significant progress. These experiments enable the study of electron transport in molecular-scale systems. The development and application of nanomaterials consisting of molecular electronic devices have accentuated the importance of a theoretical understanding of the scattering process that occurs in these systems.9,10 The azobenzene molecule (Figure 1A) has been proposed as a component of a light-driven molecular switch.11 Recent applications of azobenzene include its use as photoswitchable molecular glue for DNA to control biological functions12 and as an ionotropic glutamate receptor to control ion channels in cells.13 Azobenzene has two stable conformations, cis and trans, in its ground state, which makes it a promising component for molecular devices.14-16 Azobenzene can be converted from one conformation to the other by photoexcitation, with the structural change taking place in an electronic excited state. A beam of light with a wavelength of 365 nm isomerizes the trans conformation to the cis conformation, and a second beam of light with a wavelength of 420 nm reverses the isomerization * To whom correspondence should be addressed. E-mail: cheng@ qtp.ufl.edu. † Quantum Theory Project, University of Florida. ‡ Department of Chemistry, University of Florida. § Department of Physics, University of Florida. | Center for Nanophase Materials Sciences and Computer Science, and Mathematics Division, Oak Ridge National Laboratory. ⊥ Physics Division, Oak Ridge National Laboratory.
Figure 1. (A) Azobenzene molecule. (B) Isomerization of the azobeznene by photoexcitation. (C) Schematic of the lead-azobenzenelead system.
(Figure 1B).16 The trans conformation is more stable by ∼0.6 eV,17 and the energy barrier between the conformations is about 1.6 eV;18 therefore, in the electronic ground state, thermal fluctuation causes the cis conformation to overcome the energy barrier and relax to the most stable trans conformation. Previous calculations showed that the trans conformation has a conductance considerably higher than that of the cis conformation at equilibrium (zero bias),11 enabling the use of the azobenzene and possibly its derivatives as a single-molecule light-driven molecular switch with ON and OFF states repre-
10.1021/jp9062466 2010 American Chemical Society Published on Web 01/05/2010
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J. Phys. Chem. C, Vol. 114, No. 3, 2010
Palma et al.
sented by the trans and cis conformations, respectively. In this work, we study the effects of chemical modifications on the electron transport properties of the azobenzene molecule. Experiments have shown that small differences in molecular structure can lead to great variations in conductance and current-voltage characteristics. For example, a comparison of benzenedithiol and benzenedimethanethiol shows that the former has a conductance of 0.011 G0 while the latter has a conductance of 0.0006 G0 (where G0 ) 2e2/h).19 A theoretical study of the effects of different substituents is necessary to predict structures that may have optimized properties by varying chemical structures for a particular application. This study is inspired by recent calculations that showed dramatic changes in the excited state dynamics of azobenzene derivatives20 and a subsequent impact on the isomerization mechanism. Experiments on a related series of molecules were performed by Venkatarama et al.,21 in which conductance was measured in substituted benzenediamine molecules with electrondonating and electron-withdrawing groups. The experimental data show that for the benzenediamine molecule connected to gold electrodes, the electron-donating groups increase the conductance, while electron-withdrawing groups decrease it. Huang et al.22 presented one of the first attempts at a theoretical investigation of chemical substituents in a potential molecular switch. They studied diarylethene derivatives using a nonequilibrium Green’s function formalism (NEGF) with Density Functional Theory (DFT). They found that substituting F or S with H or O led to a remarkable change in the switching behavior under a bias. To provide a switch function, we connect the azobenzene and its derivatives via two linker SCH2 groups to semi-infinite aluminum leads (Figure 1C). Electron-donating groups and electron-withdrawing groups are included in meta and ortho positions with respect to the azo group. 2. Method and Calculation Details We use the nonequilibrium Green’s function (NEGF) formalism for quantum transport and Density Functional Theory26 (DFT) to calculate the electronic structure. This method (NEGF/DFT) was first established by Brandbyge et al.27 and Xue et al.28 In particular we use the SMEAGOL29,30 code which incorporates the NEGF method, and DFT, based on the SIESTA package,31 to calculate the single-particle Hamiltonian. The system is constructed in the usual three-subsystem segmentation, i.e., “left” electrode, “right” electrode, and the central part between the electrodes, which is the region where the electron scattering process occurs. Transport properties are calculated with SMEAGOL using an electronic structure calculation for the leads, which are treated as semi-infinite periodic nanowires. The electron transport calculation uses the NEGF/DFT method, in which the semi-infinite leads are connected to the device region. For the device region, only the gamma point is necessary. Density functional theory is used to optimize the structures and construct the Hamiltonian matrices,
(1)
and the overlap matrices Rβ SJK ) 〈ψJRβ |ψKRβ〉
GC+(E) ) [+SC - HC - ΣL+(E) - ΣR+(E)]
(2)
where J and K can be L, R, or C, symbolizing the left lead, right lead, and central part, respectively, R and β represent orbital indices, and F denotes the electron density. This
(3)
where the + sign indicates that an infinitesimal positive imaginary part has been added to the energy term (E + iδ) and the ∑+ functions are the self-energy terms of the left and right leads. These are found as,
ΣL+(E) ) (+SCL - HCL)GL+(E)(+SLM - HLM)
(4)
ΣR+(E) ) (+SCR - HCR)GR+(E)(+SRM - HRM)
(5)
and
The most important physical term is the electron density and is calculated from the following expression:
F)
1 2πi
∫ dEGC< (E)
(6)
where the lesser Green’s function G
