Chemical Physics Letters 479 (2009) 120–124

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Chemical Physics Letters

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Transport properties through diarylethene derivatives between carbonnanotube electrodes: A theoretical study

Jing Huang a, Qunxiang Li b,*, Haibin Su c, Jinlong Yang b

a School of Materials and Chemical Engineering, Anhui University of Architecture, Hefei, Anhui 230022, People’s Republic of Chinab Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of Chinac Division of Materials Science, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

a r t i c l e i n f o

Article history:Received 1 May 2009In final form 3 August 2009Available online 6 August 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.08.006

* Corresponding author. Fax: +86 551 3602969.E-mail address: liqun@ustc.edu.cn (Q. Li).

a b s t r a c t

We report first-principles calculations of the electronic and transport properties of diarylethene mole-cules sandwiched between metallic armchair (5,5) carbon nanotubes with open and closed configura-tions. The calculated transmission spectra of two configurations are distinctively different near theFermi level, and their profiles can be tuned by the gate bias voltage. The on–off ratio of currents betweenthe closed and open configurations under a gate bias voltage is predicted to be more than two orders ofmagnitude, which reproduces the essential features of the experimental results. Moreover, we find thatthe switching property of diarylethene molecular junctions is very robust to the anchoring configuration,substituent of cyclopentene, and choice of electrodes.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Using molecules as electronic transport elements have led tothe notion of molecular electronic which has attracted consider-able experimental and theoretical attention [1]. Although manysignificant progresses have been achieved in the past years, severalchallenges still hinder the development of molecular electronics,for example, the selection of suitable molecules, especially the fab-rication and control of electrode–molecule interface [2–4]. Thetypical molecular junctions are constructed by conjugated mole-cules connecting to gold electrodes through sulfur linkage atoms.Other atoms, such as nitrogen and phosphorus, have also beenused as end linkers [5–7]. In general, the exact contact geometricstructures between molecule and metal electrodes are hardly con-trolled in experiments, which leads to very difficult of reproducingthe junctions and control the transport properties. Recently, due tothe inherent advantages such as robust structure and richelectronic properties, carbon nanotube (CNT) has driven theresearch interesting [8–10]. Both experimental and theoreticalinvestigations have proved that CNT is an very promising candi-date for nanoelectronics. Conjugated molecules, such as DNA andphotochromic molecules, have been attached to single-walledCNT with amide anchoring groups [8–10]. This strategy allowsthe precisely control of molecule–electrode contacts. Usingfirst-principles calculations, Ke et al. [11,12] revealed that the goodcontact can be achieved by the connection of five-member rings to(5,5) armchair CNT.

ll rights reserved.

Many interesting properties, such as negative differential resis-tance, rectifying and spin filtering, have been observed in molecu-lar junctions [13–15]. Among various molecular devices, molecularswitch has attracted research interesting due to its application inmodern design of memory [16]. Diarylethene derivations haveopen (non-conjugated) and closed (conjugated) configurations.Extensive experimental and theoretical studies have been carriedout to explore the switching mechanism [17–22]. Using mechani-cally controllable break-junction, self-assembled monolayer ongold surface and gold nanoparticles, experimentalists successfullyconvert two states reversibly in diarylethene molecular junctions[18]. On the theoretical side, the switching mechanism of severaldiarylethene molecules combined with gold electrodes has beeninvestigated based on density functional theory (DFT) calculations[20,23–25].

Recently, Whalley et al. [26] measured switching properties ofdiarylethene molecules connected to CNT electrodes. Since no the-oretical investigation is available on transport properties andswitching mechanism of diarylethene molecules coupled to CNTelectrodes, in this Letter, we employ non-equilibrium Green’s func-tion technique (NEGF) combined with DFT to simulate quantumtransport properties and to explore the switching mechanism ofthe molecular junction constructed with diarylethene moleculessandwiched between two CNT electrodes with closed and openconfigurations. Our theoretical results show that the transmissionspectra of two configurations are distinctively different near theFermi level. The on–off ratio of currents between the closed andopen configurations is predicted to be more than two orders ofmagnitude, which reproduces the essential features of theexperimental results [26]. In addition, we find that the switching

J. Huang et al. / Chemical Physics Letters 479 (2009) 120–124 121

character of diarylethene molecules is very robust to the anchoringconfiguration, substituent of cyclopentene, and choice ofelectrodes.

2. Theoretical formula and model

Our theoretical calculations are performed by a fully first-principles scheme combining DFT with NEGF method. The geomet-ric optimizations and electronic structures are calculated using theSpanish initiative for electronic simulations with thousands ofatoms (SIESTA) code [27], which employs Troullier–Martins non-local pseudopotential and linear combinations of atomic orbitalsas basis set. Double-zeta plus polarization basis sets are used fornon-Au atoms and only single-zeta orbitals are employed for Auatoms. The grid integration is defined by a energy cutoff of150 Ry, while the exchange and correlation functional is treatedat the level of Ceperley–Alder local-density approximation. Beforeperforming transport calculations, all atoms of free diarylethenemolecules are fully relaxed, and the optimized structures areshown in Fig. 1a. The molecular lengths for the relaxed open andclosed configurations are 18.32 and 17.09 Å, respectively, whichare close to the experimental measurements [26]. Test calculationswith larger basis set and cutoff energy do not change the calculatedresults computed with the above setup.

Our transport calculations are carried out by using the ATK pack-age [28,29], which interfaces SIESTA and NEGF method. This pack-age has been successfully used to explain many experimentalresults [20,25]. The molecular junction is modeled by diarylethenemolecules with open and closed configurations bridged betweentwo (5,5) armchair CNT electrodes, as shown in Fig. 1b. The molec-ular junction consists of left and right CNT electrodes, and one cen-tral scattering region. The left and right electrodes are described bya double unit cell of (5,5) armchair CNT along the z direction. Thebox labeled with solid blue lines in Fig. 1b stands for the scatteringregion, including the central diarylethene molecule and several

Fig. 1. (a) Diarylethene molecules with closed and open configurations. (b) A schemaarmchair (5,5) CNT electrodes.

layers of left and right CNT electrodes, where all the screening ef-fects are included into the contact region and charge distribution inthe electrodes is the same as that of the bulk phase. Following theproposed structures by Whalley et al., the diarylethene moleculeconnects to CNTs with amide anchoring groups (CONH). Beforeperforming transportation calculations, all atoms of the scatteringregion are fully relaxed to obtain the energetically favorablestructures.

In our calculations, the transmission function is calculated by

TðE;VÞ ¼ Tr½CLðE;VÞGðE;VÞCRðE;VÞGþðE;VÞ�; ð1Þ

where CLðRÞ stands for the coupling matrix between the left(right)electrode and scattering region, GðE;VÞ is the retarded Green’s func-tion of scattering region. The current through the molecular junc-tion is computed by

IðVÞ ¼Z

TðE;VÞ½f ðE� lLÞ � f ðE� lRÞ�dE; ð2Þ

here, f ðE� lLðRÞÞ and lLðRÞ is the Fermi function and chemical poten-tial of left(right) electrodes, respectively.

3. Results and discussions

We first estimate the modification on the electronic structuresof diarylethene molecules due to the presence of CNT electrodes.To obtain these modified molecular orbitals, we adopt the molecu-lar projected self-consistent Hamiltonian (MPSH) technique in ourcalculations [29]. Namely, the self-consistent Hamiltonian of themolecular junction is projected onto the central diarylethene mol-ecule, and then we obtain the perturbed molecular orbitals bydiagonalizing the MPSH matrix. The calculated eigenvalues of iso-lated molecules and the MPSHs are labeled with the empty andfilled upside-down triangles in Fig. 2, respectively. It is clear thatthe eigenvalues of MPSH correspond very well with these eigen-values of isolated molecules. However, the eigenvalue separation

tic of the switching junction. Diarylethene molecules are sandwich between two

Fig. 2. The zero-bias voltage transmission spectra vs the energy E� EF ofdiarylethene molecular junctions with the closed (the top panel) and open (thebottom panel) configurations. Here, the average electrode Fermi energy is set tozero for clarity, the empty and filled upside-down triangles stand for the energies ofthe MPSH and isolated molecules, respectively, and insets in the right panels standfor the spatial distribution of the perturbed HOMO and LUMO.

Fig. 3. (a) The transmission spectra of diarylethene molecular junctions, here,VG ¼ 2:0 V. (b) The corresponding calculated I–V curves. Here, the investigatedjunction is illustrated as inset, the filled (red) and empty (black) lines stand for openand closed forms, respectively. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

122 J. Huang et al. / Chemical Physics Letters 479 (2009) 120–124

is tuned obviously by CNT electrodes. The relative shift of the per-turbed molecular orbitals of diarylethene molecule with closedconfiguration is less than that of open case. For example, comparedto isolated molecule, the positions of the perturbed highest occu-pied molecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO) do not move obviously due to the existence ofCNT electrodes for the closed form, while the perturbed HOMO–LUMO gap increases about 0.2 eV for open one.

To explore the transport properties through the diarylethenemolecules with open and closed configurations, the energy depen-dence of total zero-bias voltage transmission spectra are self-con-sistently calculated and shown in Fig. 2. Here, the averagedelectrode Fermi energy are all set to zero for clarity. The spatial dis-tribution of the perturbed HOMO and LUMO for two consideredconfigurations are plotted in the right panels of Fig. 2. Obviouslydifferent features appear in two calculated transmission spectra.The perturbed HOMO and LUMO of diarylethene molecule withopen configuration are localized orbitals, which are not good con-ductive channels for electrons passing through the molecular junc-tion. As a result, there is no any significant transmission channellocating at the energy range from �0.7 to 1.8 eV, as shown in thetop panel of Fig. 2.

Under the radiation of ultraviolet light (about 365 nm) in exper-iments, diarylethene molecule can be photocyclized from an open,non-conjugated form to a closed, conjugated form, and the photo-reversion occurs with visible light (>500 nm) [26]. The bottom pa-nel of Fig. 2 presents the calculated zero-bias transmission curvefor diarylethene molecule with closed configuration. Clearly, thereare two significant transmission peaks locating at the left and rightsides of the Fermi level. The perturbed HOMO and LUMO of diary-lethene molecule with closed configuration are delocalized p-con-jugated orbitals, which provide good conductive channels. It meansthat the narrow and broad transmission peaks located at about�0.08 and 0.8 eV are contributed by the perturbed HOMO andLUMO, respectively. It is well known that the transport propertiesthrough molecule under small bias voltage is determined by thetransmission spectra near the Fermi level. That is to say, the tailof the perturbed HOMO induced transmission peak at �0.08 eV

determines the transport behavior of diarylethene molecular junc-tion with closed form. According to the theoretical results, as seenin Fig. 2, when the molecule in the junction changes from open toclosed configuration, the diarylethene molecular wire is predictedto switch from the off (high resistance) state to the on (low resis-tance) state, which qualitatively agrees with the experimentalobservation [26].

Similar to the conventional field-effect transistor (FET), the FETbehavior in single molecules can be experimentally observed byplacing the gate electrode close to the molecule [26]. Previous the-oretical calculations indicated that the transport propertiesthrough single molecule can be controlled with a gate electrode[30]. We then turn to explore the gate effect. In our calculations,the applied gate bias voltage simply shifts the MPSH energy levelsthrough inducing an electrostatic potential localized to the molec-ular region. Namely, the gate is not considered as a physical elec-trode and the gate voltage does not change the coupling betweendiarylethene molecule and CNT electrodes. The energy dependenceof transmission spectra of diarylethene molecule with open andclosed configurations under gate bias voltage ðVG ¼ 2:0 VÞ areshown in Fig. 3a. The shape of transmission spectra and positionof peaks are obviously tuned by the gate bias voltage, comparedto Fig. 2. For both closed and open configurations, there are severaltransmission peaks above the Fermi level, and no conducting chan-

J. Huang et al. / Chemical Physics Letters 479 (2009) 120–124 123

nel from �2.0 to 0.0 eV (except for the small peak at �0.8 eV com-ing from the perturbed HOMO for the closed case). For the closedform, there are three broaden and large transmission peaks locat-ing in 0.1, 0.8 and 1.2 eV, respectively. As a delocalized p-orbital,the perturbed LUMO leads to the transmission peak near the Fermilevel (0.1 eV), which mainly determines the conductance of diary-lethene molecule with closed configuration under small source-drain bias voltage. Clearly, the gate bias voltage can be used toeffectively tune the transport properties of diarylethene molecules.Nonetheless, the switching feature is undoubtedly retained, theclosed configuration shows good conductivity while the open formhas very low conductivity.

The current through diarylethene molecules with open andclosed configurations under a gate bias voltage ðVG ¼ 2:0 VÞ inthe bias voltage range [0,0.8 V] is self-consistently calculated bythe Landauer–Bütiker formula. We present the calculated I–Vcurves in Fig. 3b and observe the obvious switching behavior.The I–V curve for the junction with open structure appears ratherflat characteristics, while the current through closed form monot-onously increases in the bias voltage range from 0 to 0.8 V. Clearly,the diarylethene molecule with closed form is predicted to be onstate, while the open configuration corresponds to off state. Thecurrent enhancement can be quantified by the on–off ratio of cur-rent defined as RðVÞ ¼ IclosedðVÞ=IopenðVÞ. When the applied source-drain bias voltage is 0.8 V, the current through the closed and openform is about 4.49 and 0.042 lA, respectively. The on–off ratio ofcurrent is about 100. This predicted on–off ratio at 0.8 V is about4 times larger than the experimental value which is 25 [26].

In general, the transport properties of molecular junctionsdepend nontrivially on the interface structures between the elec-trodes and the molecule, which are very difficult to be character-ized clearly in experiments. Previous theoretical study reportedthat a near perfect contact transparency can be achieved throughall-carbon contacts via five-member rings [11]. Here, we considerthis kind of connections instead of the CONH anchoring groups.The top and bottom panels of Fig. 4 presents the calculated trans-mission spectra of diarylethene molecular junction with open andclosed configurations, respectively, and inset stands for the compu-tational model. Clearly, the switching behavior is observed again.For the open case, there is no any significant transmission channellocating at the energy range from �1.0 to 1.5 eV, while a broad res-onant peak locates at the Fermi level for the close configuration be-cause the parallel configuration between the plane of the closeddiarylethene molecule and the surfaces of the CNTs leads to very

Fig. 4. The transmission spectra of diarylethene molecules coupled to CNTelectrodes via five-member rings. For clarity, inset illustrates the investigatedjunction.

good CNT–molecule coupling, resulting in a good conductivechannel.

Previous studies revealed that the visible adsorption spectrachanged when two S atoms of the switching unit were substitutedby other atoms [31]. Now we turn to examine the substituting ef-fect on transport properties through the diarylethene derivations.According to the experimental measurements [26], we conductthe transport calculations of molecular junction where the S atomsin the central switching unit are replaced by NCH3 groups. Fig. 5presents the calculated zero-bias transmission spectra of diaryleth-ene molecules with the closed and open configurations under gatebias voltage VG ¼ 0:0 and 2.0 V, which is labeled with the black so-lid and red dotted lines, respectively. Clearly, compared to Figs. 2and Fig. 3a, the shape of transmission curves changes very slightlyand the positions of significant transmission peaks do not signifi-cantly shift. Clearly, this observed switching behavior does not de-pend sensitively on the NCH3 substituent, and the closed structureis undoubtedly more conductive regardless of the substituent ofcyclopentene with S atoms.

To understand the choice of electrodes on the switching processin diarylethene derivations, the transport properties of the diary-lethene molecules sandwiched between two Au(111) electrodeswith closed and open configurations (inset of Fig. 6) are also inves-tigated. As seen in Fig. 6, the obtained zero-bias transmission spec-tra without gate bias voltage obviously display differentcharacteristics. There are two broaden and large transmissionpeaks for closed configuration which come from the perturbedHOMO and LUMO, respectively, while it lacks any transport chan-nel in a wide energy window from �1.0 to 0.5 eV. That is to say, theclosed form presents the on state while the open form shows theoff state. This result indicates that the switching behavior is notsensitive to the choice of electrodes and originates from the intrin-sic molecular properties. This explored switching mechanism isclose to that of other photochromic diarylethene derivations[25,32].

Note that using CNT as electrodes, when the S atoms in the cen-tral switching unit are replaced by NCH3 groups in experiments[26], the diarylethene molecular switch is reversible between theopen and closed structures. Otherwise, the molecular junction cy-clizes only in one direction, namely, open to closed form (in con-trast to other diarylethene molecule–gold systems, one-wayswitching from the closed to open form [17]). To find the micro-scopic origin of such a difference, we calculate the electronic struc-

Fig. 5. The transmission spectra of diarylethene molecular junctions with NCH3

substitution. Here, the red dotted and black solid lines stand for closed and openconfigurations, respectively. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 6. The transmission spectra of diarylethene molecules sandwiched betweentwo Au(111) surfaces with closed and open structures.

124 J. Huang et al. / Chemical Physics Letters 479 (2009) 120–124

tures of free diarylethene molecules with two different S and NCH3

groups, and obtain the close energy barriers transiting from closedto open structures, 1.6 and 1.3 eV for S and NCH3 cases, respec-tively. Unfortunately, these results can not be used to explain theseexperimental observations clearly [26,17] and further theoreticalinvestigations on these diarylethene molecular junctions areneeded to unravel this subtle direction-dependent switching issue.

4. Summary

In summary, the electronic structures and transport propertiesof diarylethene molecules with closed and open configurationsare investigated by using the DFT calculations combined withNEGF technique. The transmission function of two different config-urations is obvious different, which can be effectively tuned by thegate bias voltage. The closed structure has two obvious transmis-sion peaks around the Fermi level, while the open form lacks anysignificant transmission peak within a wide energy window. Forclosed configuration, the transport properties through diarylethenemolecular junction with and without a gate bias voltage are mainlydetermined by the tail of the transmission peak contributed by theperturbed LUMO and HOMO, respectively. The calculated on–offratio of currents is about two orders of magnitude, which repro-duces the essential features of the experimental measurements.Moreover, we find that the observed switching behavior is very ro-bust to the anchoring configuration, substituent of cyclopentene,and choice of electrodes.

Acknowledgements

This work was partially supported by the National Natural Sci-ence Foundation of China under Grants 10674121, 20773112,20533030, and 50721091, by National Key Basic Research Programunder Grant No. 2006CB922004, by the Science and TechnologicalFund of Anhui Province for Outstanding Youth (No. 08040106833),by the USTC-HP HPC project, and by the SCCAS and ShanghaiSupercomputer Center. Work at NTU is supported in part by MOEAcRF-Tier-1 grant (No. M52070060).

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