Modeling Selective Single Molecule Sensors for Transition Metal Ions

Aug 13, 2009 - JadaVpur 700032, Kolkata, India. ReceiVed: May 26 ... Voltage dependent electronic transport studies show changes in current and negati...
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J. Phys. Chem. C 2009, 113, 16203–16209

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Modeling Selective Single Molecule Sensors for Transition Metal Ions Bidisa Das* Center for AdVanced Materials (CAM), Indian Association for the CultiVation of Science, JadaVpur 700032, Kolkata, India ReceiVed: May 26, 2009; ReVised Manuscript ReceiVed: July 20, 2009

A bis(2-pyridylmethyl)amine (BPA) based model sensor is studied which can selectively detect Zn, Fe, Ni, and Cu ions by measurable changes in conductance when coupled to a gold two-probe junction. The nature and the energies of the molecular orbitals change drastically after complexation to the target, forming the basis of selective sensing. Voltage dependent electronic transport studies show changes in current and negative differential resistance (NDR) characteristic of various target ions present in the system. Introduction Organometallic molecules, with a wide range of exciting electronic, magnetic and optical properties are potential candidates for designing single molecule electronic devices. Properties of such molecules are primarily determined by the metal ion present and can be fine-tuned by varying the metal centers. Recent studies involving individual organometallic molecules as active components of molecular devices have shown many interesting results.1-4 In addition to excellent electronic or magnetic properties, these molecules can also recognize other molecules through specific binding interactions, so they can suitably be used to design single molecular sensors. In this context, organic molecules capable of binding reversibly to different metal ions offer a great potential as the electronic transport properties are likely to be modified by the presence of metal centers. Design and fabrication of single molecule sensors which change conductance in the presence of specific targets have been an area of active research5-10 in the recent past. Such molecular sensors offer ultimate sensitivity by detecting one single molecule of the target. This paper reports a theoretical study of a bis(2-pyridylmethyl)amine (BPA) based single molecular sensor, which can selectively detect metal ions (targets) like zinc, iron, copper, and nickel by sharp changes in electrical conductance. BPA is known to recognize the abovementioned metal ions in solution phase.11 In this study, BPA is functionalized by thiol bonds at the two ends so that it can be covalently bonded to two gold electrodes, to form a stable two probe molecular junction. Fabrication of a two probe setup is necessary for the electrical transport studies of any molecular conductor. It is observed that the conductance of the sensor bound to different transition metal ions shows strong changes, forming the basis of selective sensing. All BPA based molecular junctions studied here except the copper complex show negative differential resistance (NDR) which can be explained in terms of bias dependent changes in transport. Interestingly, NDR peaks appear at different applied voltages for different targets, so that it can be used as a selectivity criterion, as well. Theoretical Methods The main aspect of this study is to understand how the electronic and transport properties of dithiolated BPA (DTBPA; * To whom correspondence should be addressed. E-mail: cambd@ iacs.res.in.

Figure 1. Optimized structures of DTBPA and [M(DTBPA)Cln] complexes.

for the structure see Figure 1) change in the presence of a target metal chloride unit and then compare between the bound and unbound cases for all targets. Electronic structure methods based on density functional theory (DFT) are employed to study the properties of free and complexed DTBPA. Unconstrained structures of free DTBPA and the corresponding metal chloride complexes are modeled using DFT as implemented in Gaussian 0312 software employing restricted and unrestricted methods as required. In this study, DFT hybrid functional B3LYP13,14 is used along with 6-31G** and 6-311G** basis-sets for all atoms except Au. For Au, Lanl2DZ15 basis-set and effective core potential is used as 6-31G** (or 6-311G**) is unavailable for gold. The hybrid functional B3LYP, has been heavily used for transition metal containing organometallic molecules.16 However to compare the results, geometries of all complexes have also been optimized using GGA-PBE17 methods as implemented in ATK200818 software, with DZP basis-set for all atoms. It is found that the geometries (details in the Supporting Information) remain virtually the same upon changing the basis-sets and functionals. In this study, the changes in molecular orbitals of DTBPA after complexation with a specific target are studied to understand the changes in conductivity. To probe potential conductance channels, the nature of the frontier molecular orbitals (FMO), especially the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of free DTBPA and [M(DTBPA)Cln], (n ) 2 and 3) are analyzed. The energy gap between HOMO and LUMO orbital (HLG) is an important factor controlling the tunnelling current

10.1021/jp904894w CCC: $40.75  2009 American Chemical Society Published on Web 08/13/2009

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through the molecule and comparison of the values for different complexes gives an idea about the ease with which their electron densities can be modified. HLGs calculated using the B3LYP method are reported here as they are more reliable19 than nonDFT or LSDA based methods. However it is known that the energies of HOMO, LUMO, and the gap are predicted with varying accuracies for different theoretical methods.20 All of the stable geometries studied are characterized by vibrational frequency calculations. As some of the transition metal complexes reported are magnetic, Mulliken spin densities are reported. The sum of Mulliken spin densities of all atoms of a molecule equals the total spin of the system. Mulliken spin densities of transition metal complexes obtained from hybrid density functional methods are close to experimental values.21 The binding energies for metal complex formation of DTBPA, calculated using the following scheme: Ebinding ) (EMCln + EDTBPA) - E[M(DTBPA)Cln] are reported for all cases. The transport properties of DTBPA and [M(DTBPA)Cln] are calculated with ATK200818 software, which combines the nonequilibrium Green function (NEGF) formalism and density functional theory. The DFT implementation uses a numerical atomic basis set to solve the Kohn-Sham equations.22-24 DZP basis-set was used for all atoms except Au (SZP basis-set, to save computational resources) with the GGA-PBE17 functional for the exchange correlation method for this study. The nonlinear current at an applied bias voltage Vb through the contact is calculated using the Landauer formula25,26

I(Vb) ) G0

∫µµ

R

L

T(E, Vb) dE

where G0 ) 2e2/h is the quantum unit of conductance, µL/R is the electrochemical potentials of the left and right electrodes, and h is Planck’s constant. The zero-bias conductance is given by transmission at the Fermi energy. The left and the right electrodes in this study are modeled by two Au(111) (3 × 3) surfaces. The unit cell for the study contains two Au(111) layers from left and right electrodes each along with the central DTBPA molecule in the scattering region. Although it is known that screening approximation holds accurately only when many layers of electrodes are included in the contact region, here only a modest number of layers could be included to keep computational times reasonable. All of the results reported here are comparative so this issue may not be very important. DTBPA is bonded to the electrodes with thiol (-S-Au) bonds in fcc hollow sites of Au(111) which is a energy minimum on the surface. This creates two identical molecular junctions on both ends of DTBPA. The structure of the central molecule is pre optimized (B3LYP/6-31G**) in free conditions and Au(111)DTBPA-Au(111) has not been relaxed to save time. Though the DFT schemes used for the optimization of free molecules and the transport studies are different, there is actually not much error included in the transport results because the molecular parameters are minimally affected by the changes in methodologies (see the Supporting Information). In general the calculated transmission spectra are all very sharp, resulting possibly due to a scatterer which is not fully conjugated. Finally, for all cases the transmission spectra with 3 × 3 k-points and current-voltage (I-V) characteristics are calculated and analyzed. In all of the transmission spectra reported, the energy is relative to the average Fermi level of the two-probe system, i.e., (µL + µR)/2, where µL and µR are the electrochemical potentials of the left and right electrodes (both are gold in this study), respectively. Transmission spectrum using 3 × 3 k-point sampling is

Das considered sufficient because increasing k-points to 6 × 6 (reported in the Supporting Information) gives nearly the same spectrum. Results and Discussions Free DTBPA and [M(DTBPA)Cln] Complexes. Optimized structures of free DTBPA, [M(DTBPA)Cln], (n ) 2 and 3) are shown in Figure 1, and detailed bond-lengths are given in the Supporting Information. It is found that DTBPA is nonplanar, and it has two pyridine rings arranged at 111° from the central -N-CH2 bond on either side. Molecular orbitals of DTBPA as shown in Figure 3 are not spread uniformly over the entire length of the molecule due to its nonplanar geometry. It is found that in [Zn(DTBPA)Cl2] Zn(II) is penta-coordinated, and it shows a trigonal bipyramidal coordination. In [Fe(DTBPA)Cl3], the Fe(III) center is in a distorted octahedral environment consisting of two pyridine nitrogens and an amine nitrogen from the tridentate ligand and three chloride ions, with each chloride being trans to a nitrogen atom. The X-ray diffraction data27 of [Fe(BPA)Cl3] give a geometry which is close to the calculated one. [Fe(DTBPA)Cl3] is a high spin metal complex28 with a spin multiplicity value of six. Mulliken spin densities on atoms show the following values: Fe, 4.20; Cl, 0.19; and N, 0.07 with almost zero spin density on C atoms. The lower value (from expected value 5) of spin-density on an Fe atom may result from spin transfer to Cl or N atoms. The geometry of the [Cu(DTBPA)Cl2] (doublet) complex is distorted square pyramidal with structural parameters very similar to the reported values for the [Cu(BPA)Cl2] complex in literature.29,30 Mulliken atomic spin density on Cu is 0.7 with some spin density on Cl and N atoms. [Ni(DTBPA)Cl2] (triplet) is similar to [Cu(DTBPA)Cl2] with identical Ni-Cl distances. In this case, the spin density on Ni is 1.68 and modest densities present on Cl and N atoms. BPA is known to bind to a large number of metal ions by means of coordinate bond formation.11 In this context, the stabilities of the various complexes are important and the metal ions having higher stability are likely to form molecular junctions more often. The calculated binding energies of MCln units to DTBPA are 54.4 kcal/mol for ZnCl2, 73.7 kcal/mol for NiCl2, 82.6 kcal/mol for CuCl2, and 54.4 kcal/mol for FeCl3, which says that Cu2+ would form the most stable complex with DTBPA. [Ni(DTBPA)Cl2] has intermediate stability, whereas Zn2+ and Fe3+ would actually compete to form complexes with DTBPA if both of the ions are present in the sample. A comparative presentation of the calculated (B3LYP/631G**) orbital energies of DTBPA and [M(DTBPA)Cln] are given in Figure 2. HLG for free sensor (DTBPA) is 5.05 eV which changes to 4.71 eV for [Zn(DTBPA)Cl2], 3.51 eV for [Fe(DTBPA)Cl3](down-spin),3.26eVfor[Cu(DTBPA)Cl2](downspin), and 3.85 eV for [Ni(DTBPA)Cl2](down-spin). There is clearly a decrease in HLG after complex formation, which is small for Zn2+ but large for Fe3+, Cu2+, and Ni2+ complexes. This qualitatively indicates a strong change in electronic conductance values for free DTBPA after metal complex formation. Analysis of the FMOs of DTBPA and [M(DTBPA)Cln] shows that the LUMO and the orbitals near the LUMO are partially or fully conjugated over the molecule with appreciable electron densities on the sulfur atoms (Figures 3-6). However, the nature of the HOMO changes when DTBPA forms a complex with metals. The HOMO which is localized on the central part of DTBPA in the unbound case is found to be localized on the metal chloride unit in [M(DTBPA)Cln] (Figure 4) with no electron density on sulfur. However there are molecular orbitals close to HOMO which are extended over

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Figure 2. Comparative study of the energies of the molecular orbitals of DTBPA and [M(DTBPA)Cln] complexes (up spin, dash; down spin, triangle).

Figure 4. Frontier molecular orbitals with energies in eV for free [Zn(DTBPA)Cl2] and [Zn(DTBPA)Cl2] with Au contacts on both sides. Numbers written in the parentheses are the degeneracies of the corresponding orbital.

Figure 3. Frontier molecular orbitals with energies in eV for free DTBPA and DTBPA with Au contacts on both sides. Numbers written in the parentheses are the degeneracies of the corresponding orbital.

the whole molecule and these orbitals serve as the pathway for the conduction electrons. To understand the changes taking place in the FMOs after it is connected to electrodes DTBPA, [Zn(DTBPA)Cl2] and [Fe(DTBPA)Cl3] are studied with model Au contacts on either side of the molecule. Three Au atoms arranged as a fcc hollow site of Au(111) are used to model electrode contacts in both ends of DTBPA. The central molecule is relaxed fully (keeping contact atoms fixed in space) with and without applied electric field and the molecular orbitals are studied. Due to the metallic contacts on either side, the molecular orbitals are modified and HLG is decreased. The decrease in HLG is mainly due to new unoccupied orbitals which arise due to the contacts and would not contribute to conductance significantly. In the case of DTBPA (see Figure 3), it is found that HOMO is degenerate and electron density is mainly on Au atoms, but HOMO-2 is located centrally on DTBPA. Being close in energy there is a fair chance for these orbitals to mix and form a conducting channel when connected to infinite gold electrodes. HOMO-3 is degenerate and located partially on the contacts and on DTBPA.

For the studies with applied electric field, one must fix the reference frame in which the system is defined. Here, the sulfur atom of DTBPA is fixed as the origin and Z axis is along the length of DTBPA with YZ plane placed perpendicular to the molecular plane. In this frame the dipole moment of DTBPA is 3.58 D (X ) -2.76, Y ) 2.28, Z ) -0.01) which points perpendicular to Z axis. Static electric fields of varying strengths are applied along Z axis to see structural and electronic changes of DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3]. It is found that, since the dipole moment points perpendicular to the field direction, molecular parameters do not change much for small applied fields (approximately 1 V/nm). When electric field is applied to DTBPA, the degeneracy of HOMO is lifted and HOMO-1 is located centrally on the molecule with some electron density on the contact. Also the energies HOMO-2, HOMO-3 become closer to HOMO-1 so that they can mix to give better conductance. Lowest unoccupied orbitals are also degenerate and mainly on contacts. LUMO+4 and few orbitals close to it are good overlapping orbitals. In case of [Zn(DTBPA)Cl2] a degenerate HOMO is located mostly on the contacts with a modest electron density on DTBPA. HOMO-2 is mostly on central portion of DTBPA with small density on the contacts. HOMO-3 and HOMO-4 are localized on halide atoms only. First few unoccupied orbitals are localized on contacts, but are delocalized for LUMO+3 (deg.). For [Fe(DTBPA)Cl3] with Au contacts (Figures 5 and 6), HOMO is degenerate for up- and down-spin cases and are mainly concentrated on the contacts with small density in the central part of the molecule. HOMO-3 and HOMO-4 are somewhat delocalized and can conduct for both spins. Among unoccupied orbitals, LUMO+4 for downspin case is a delocalized, conducting orbital. On application of electric field, degeneracy in the HOMO is lifted and also energy gaps between orbitals reduce, allowing mixing of orbitals. However, on application of electric field there are no major changes in the nature of the molecular orbitals.31 Conductance Studies. Transmission Spectra. Let us consider a two-probe setup consisting of a molecule coupled to two metal

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Figure 5. Frontier molecular orbitals (up-spin) with energies in eV for free [Fe(DTBPA)Cl3] and [Fe(DTBPA)Cl3] with Au contacts on both sides.

electrodes. Under low bias voltages and at low temperatures, transport properties of such molecular junctions are successfully described by Landauer theory.25 The metal-molecule-metal junction mimics a finite potential well between two infinite metal electrodes. When a molecule bridges the interelectrode gap with a strong molecule-metal bond (strong molecule electrode coupling limit), appropriate orbitals may act as channels for coherent electronic transport. Since planarity causes conjugated molecular orbitals to overlap better, nonplanar DTBPA or [M(DTBPA)Cln] does not show very good conductance. The two probe setup for free DTBPA and [M(DTBPA)Cln] is shown in Figure 7. The whole metal-molecule-metal junction has not been optimized in this study. Only minor changes in geometry and recognition is expected after optimization, because the metal electrodes are away from the detection part. To support this idea geometry optimizations of DTBPA under electric field are done with model electrodes on both sides and have been described in previous section. Such preoptimized molecules incorporated in two-probe metallic junctions have been reported earlier.9,10 The central molecule between two electrodes is positioned symmetrically in the gap between the gold electrodes on fcc hollow sites on both electrodes. In the present study, only fcc-hollow site adsorbed molecules are considered, though it is known that electronic transport properties of two probe systems vary with the orientation and the adsorption geometry of the central molecule. However, the influence of molecule metal interface is considered to be approximately nullified as this study is comparative. Initially for DTBPA, Au-S bonds are 2.53 Å which decreases to accommodate the MCln unit in metal complexes. For [M(DTBPA)Cln] systems the Au-S distances are 2.40 Å (for ZnCl2), 2.37 Å(for FeCl3), 2.39 Å (for CuCl2), and 2.40 Å (for NiCl2) complexes. Au-S bonds in the range of 2.40-2.55 Å have been reported in literature.32-34 The central molecule is also assumed to be perpendicular to both the electrodes.

Figure 6. Frontier molecular orbitals (down-spin) with energies in eV for free [Fe(DTBPA)Cl3] and [Fe(DTBPA)Cl3] with Au contacts on both sides.

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Figure 7. Two probe setup for transport studies.

Figure 9. Transmission spectra (up-spin and down-spin) along with MPSH levels (vertical lines for up-spin and triangles for down spin) of [Cu(DTBPA)Cl2] (top) and [Ni(DTBPA)Cl2] (bottom) in two probe setup.

Figure 8. Transmission spectra along with MPSH eigenvalues of DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3] in two probe setup. For [Fe(DTBPA)Cl3], MPSH energies are plotted as vertical lines for up-spin and as triangles for down spin.

All of the transmission spectra shown in this study show sharp and narrow peaks indicating the lack of planar, fully conjugated conduction channels in these systems unlike oligo-phenylene ethenylene or oligo-phenylenevinylenes. Generally, the transmission peaks can be related to the molecular orbitals of the central molecule, which have been modified by the electrodes. These modified molecular orbitals can be obtained from the molecular projected self-consistent Hamiltonian (MPSH).35,36 To understand the origin of the transmission peaks, the eigenvalues of MPSH are also shown in corresponding transmission spectrum. The transmission spectra for free DTBPA, [Zn(DTBPA)Cl2], [Fe(DTBPA)Cl3] and the corresponding MPSH eigenvalues are shown in Figure 8. In the transmission spectrum of DTBPA the small peak close to zero of energy is due to transmission from HOMO and HOMO-1 (see Figure 3). HOMO-3 is degenerate and shows a double peak near -1.1 eV. First few unoccupied orbitals are not suitable for conduction; however, orbitals starting from LUMO+4 are highly conducting in nature showing transmission peaks near 3.2 eV and above. Comparison of the transmission spectra for [Zn(DTBPA)Cl2] and DTBPA shows a small decrease in the HLG. There is a transmission peak corresponding to the HOMO (2-fold degenerate) and the charge density is on molecule as well as contacts however, HOMO-2 is nonconducting. HOMO-3 and HOMO-4 are found to have tiny peaks in transmission spectrum, though the orbitals are mainly localized on chlorine atoms. HOMO-5 gives a well-defined transmission peak showing that it has orbitals suitable for electron transport. [Zn(DTBPA)Cl2] complex and free DTBPA, both are very low conducting (7.7 × 10-9 S). Comparison of the transmission spectra of [Fe(DTBPA)Cl3] and [Zn(DTBPA)Cl2], shows a much smaller HLG for

the former (Figure 8). In [Fe(DTBPA)Cl3], HLG for down-spin case is smaller than the up-spin and conductance at zero applied bias is 9.3 × 10-8 S for the down spin case. There are no transmission peaks for the up-spin electrons corresponding to HOMO or nearby levels which is clear from their localized orbitals. For the down-spin case LUMO+4 (deg.) shows a very sharp peak in the transmission spectrum which is a delocalized, conducting orbital. Transmission spectra of [Cu(DTBPA)Cl2] and [Ni(DTBPA)Cl2] are shown in Figure 9 and the zero bias conductances (down spin) are 1.79 × 10-5 and 3.45 × 10-6 S, respectively. These higher values in comparison to others result from a better energetic alignment of the molecular orbitals with electrode Fermi levels, which results in a peak in transmission at the Fermi level of the electrode for the down-spin case for both the cases. The up-spin case however has a larger HLG in comparison to down-spin. I-V Characteristics and NDR. In order to obtain the current versus voltage characteristics self-consistent calculations at positive bias voltages until +1.0 V are done for DTBPA, [Zn(DTBPA)Cl2], [Fe(DTBPA)Cl3], and for remaining two cases current is calculated for still higher applied voltages (2.0 V). The I-V curves for free DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3] are shown in Figure 10a. The I-V plots for DTBPA, [Ni(DTBPA)Cl2], and [Cu(DTBPA)Cl2] are shown in Figure 10b for clarity. The I-V characteristics are non linear and current increases with increased applied bias voltage for all the systems studied. Current increases much rapidly for [Fe(DTBPA)Cl3] in comparison to free DTBPA. For [Zn(DTBPA)Cl2], [Cu(DTBPA)Cl2], and [Ni(DTBPA)Cl2] the increase in current is less compared to free DTBPA. Molecular junctions containing DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3] exhibit NDR within 1.0 V, whereas for [Ni(DTBPA)Cl2] and [Cu(DTBPA)Cl2] the current starts increasing steeply only at higher applied bias (shown in the inset of Figure 10b). Since the molecule is kept frozen in space, NDR due to conformational changes32 is ruled out. Analysis of molecular orbitals with model contacts suggest NDR is also not due to any accidental degeneracies in molecular orbitals as found in some other case.31 However, the transmission spectra for DTBPA and [M(DTBPA)Cln], have sharp resonances and they are close to Fermi energy of gold electrodes, which make them ideal for observing

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Figure 10. Calculated I-V curve for (a) DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3] and (b) DTBPA, [Cu(DTBPA)Cl2], and [Ni(DTBPA)Cl2] in twoprobe setup. Current at higher applied bias voltages is given in the inset for [Cu(DTBPA)Cl2] and [Ni(DTBPA)Cl2].

NDR.37,38 NDR of these systems are explained by the evolution of the transmission spectra under different bias voltages. In the bias dependent transport study for free DTBPA, initially there is a steep increase in current until 0.6 V after which there is a valley region. Current starts increasing once more at 0.8 V and peaks at 0.9 V, after which it falls. Looking into the evolution of the transmission spectra at different applied bias voltages (see Figure 11), one finds the relative shift in the sharp peak due to HOMO responsible for the onset of NDR. The LUMO and other conducting yet unfilled levels are much higher in energy to influence the current in this bias window. For [Zn(DTBPA)Cl2] the current peaks at 0.8 V with a small hump at 0.5 V. After 0.8 V the current decreases with increasing bias indicating NDR. The change in transmission at higher bias voltages show the peak due to HOMO to shrink and partially shift out of the bias window causing NDR. In the case of [Fe(DTBPA)Cl3], there are two current peaks in the bias window studied, one at 0.7 V and next at 1.0 V after which the current falls. The change in position of the transmission peak at 0.55 eV (see Figure 8) with respect to the bias window causes this NDR. [Cu(DTBPA)Cl2] and [Ni(DTBPA)Cl2] does not show any NDR in this bias window, so higher applied bias voltages were used to study them and the evolution of the transmission spectra with application of bias has been plotted (see Figure 12). It is seen that for [Cu(DTBPA)Cl2] there is only a very sharp increase after 1.8 V, and there is no NDR until 2.0 V. Though there are sharp transmission peaks near Fermi energy of Au, the onset of one peak is nullified by the depletion of another peak which levels off the current. For [Ni(DTBPA)Cl2] a dramatic increase in current occurs at 1.4 V with NDR at 1.8 V (see inset of Figure 10b). A structural and transport study has also been performed for [Fe(DTBPA)Cl2] to check out how the transport compares with [Fe(DTBPA)Cl3], but it is seen that the transmission and the I-V studies (until 1.0 V) are very similar to [Cu(DTBPA)Cl2] and [Ni(DTBPA)Cl2] and hence can not be identified selectively on the basis of transport and detailed results are not reported here.

Figure 11. Transmission spectra at different applied bias voltages for DTBPA, [Zn(DTBPA)Cl2], and [Fe(DTBPA)Cl3](down-spin). 0 V curve is shown as the thick line at the back, followed by thin lines in the front for 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, and 1.0 V, respectively.

Figure 12. Transmission spectra at different applied bias voltages for [Ni(DTBPA)Cl2] and [Cu(DTBPA)Cl2] (both up and down-spin). 0 V curve is shown as the thick line, followed by thin lines for 0.6, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 V.

Conclusion This work proposes a DTBPA based transition metal sensor which can selectively detect Zn2+, Fe3+, Cu2+, and Ni2+ ions. DTBPA forms complex with these metal ions, and the binding causes the electronic properties of the systems to change so that selective sensing probing conductance change is possible. DTBPA and [M(DTBPA)Cln], (n ) 2 and 3) show strong

Modeling Selective Single Molecule Sensors NDR which is explained by the changes in transmission spectra caused by energetic realignment of frontier molecular orbitals of the twoprobe system at applied bias. NDR is observed at different voltages for different targets so can be used to identify the targets by changing the applied voltage. As far as conductance data are considered, extra emphasis is on the changes in conductance rather than the individual absolute values. Here, the two-probe setup consists of only fcc site bound molecules in the central region with symmetric molecule electrode junctions which is a simplistic representation. Microscopic details of a metal molecule contact during device operation are also not captured in theoretical studies, hence an accurate theoretical estimate of current though the molecular junction is not possible. But, this study has the advantage of being relative, hence the changes in the conductance after complex formation is likely to be more reliable than individual conductance values. It is necessary to mention that BPA binds to a vast range of metal ions and here only a few of them are studied theoretically. Hence, to implement it in reality all other possible complexes and their relative binding energies must also be taken into account. Acknowledgment. The author acknowledges Professor D. D. Sarma for fruitful scientific discussions. This work is supported by funds (SR/WOS-A/CS-59/2006) from Department of Science and Technology, India. Supporting Information Available: The detailed structural parameters of all molecules obtained using different methods are tabulated. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Getty, S. A.; Engtrakul, C.; Wang, L.; Liu, R.; Ke, S.-H.; Baranger, H. U.; Yang, W.; Fuhrer, M. S.; Sita, L. R. Phys. ReV. B, Rapid Commun. 2005, 71, 241401R(1-4). (2) Maslyuk, V. V.; Bagrets, A.; Meded, V.; Arnold, A.; Evers, F.; Brandbyge, M.; Bredow, T.; Mertig, I.Phys. ReV. Lett. 2006, 97, 097201(1-4). (3) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abrun, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722–725. (4) Seo, K.; Konchenko, A. V.; Lee, J.; Bang, G. S.; Lee, H. J. Am. Chem. Soc. 2008, 130, 2553–2559. (5) Tang, J.; Wang, Y.; Klare, J. E.; Tulevski, G. S.; Wind, S. J.; Nuckolls, C. Angew. Chem., Int. Ed. 2007, 46, 3892–3895. (6) Koyama, E.; Ishida, T.; Tokuhisa, H.; Belaissaoui, A.; Nagawaa, Y.; Kanesatoa, M. Chem Commun. 2004, 1626–1627.

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