Conduction switching behaviors of a small molecular deviceM. Qiu, Z. H. Zhang, X. Q. Deng, and K. Q. Chen

Citation: Journal of Applied Physics 107, 063704 (2010); doi: 10.1063/1.3331928 View online: http://dx.doi.org/10.1063/1.3331928 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Negative differential resistance and rectifying behaviors in phenalenyl molecular device with different contactgeometries Appl. Phys. Lett. 96, 053509 (2010); 10.1063/1.3309708 Ni @ B 80 : A single molecular magnetic switch Appl. Phys. Lett. 95, 133115 (2009); 10.1063/1.3242362 Asymmetric electrical bistable behavior of an eicosanoic acid/zirconium oxide bilayer system with rectifying effect Appl. Phys. Lett. 92, 123302 (2008); 10.1063/1.2892654 Analysis on the contribution of molecular orbitals to the conductance of molecular electronic devices J. Chem. Phys. 125, 194113 (2006); 10.1063/1.2388272 Possible performance improvement in [2]catenane molecular electronic switches Appl. Phys. Lett. 88, 163112 (2006); 10.1063/1.2195087

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21

Conduction switching behaviors of a small molecular deviceM. Qiu,1 Z. H. Zhang,1,a� X. Q. Deng,1 and K. Q. Chen2,b�

1School of Physics and Electronic Science, Changsha University of Science and Technology, Changsha410004, People’s Republic of China2Department of Applied Physics and Key Laboratory for Micro-Nano Optoelectronic Devices of Ministryof Education, Hunan University, Changsha 410082, China

�Received 3 December 2009; accepted 30 January 2010; published online 16 March 2010�

We calculate the current-voltage properties for a small organic molecule system based on the localatomic orbital density-functional theory. It has been found that our system has a distinctiveconduction switching behavior with the “on/off” ratio on the order of 102 at a bias of 0.8 V and thenup to more than the order of 103 in a bias range from 0.8 to 1.8 V, and its explicit steady state andmetastable state can be converted to each other by thermal activation. These findings suggest thatthis small molecular system has obvious potential advantages for the realization of the miniaturizedmolecular switch. © 2010 American Institute of Physics. �doi:10.1063/1.3331928�

I. INTRODUCTION

Rapid and drastic changes are taking place in the infor-mation technology field around the world in last decade. Anovel concept of electronic devices for the solutions ofMoore’s Law’s crisis has been brought forward to stimulatethe research into the atomic scale1–7 and a lot of molecularelectronic devices, such as the molecular rectifier,8,9 molecu-lar diode and transistor,10–13 molecular current switch,14–16

etc., have been proposed for potential applications. In Ref.10 Di Ventra et al. first used a first-principle method to cal-culate the transport properties of a benzene-1,4-dithiolatemolecule under the gate voltages. They found that the resis-tance decreased notably and the �� antibonding orbitalsshifted under Fermi level with the gate voltages increasing. Itis very interesting and important to the molecular transistordesign.

The molecular switches refer to the physical and chemi-cal molecular systems with obvious “on/off” behaviors thatcan be converted from on-state to off-state and vice versa byan external stimulus such as light, electricity, or a chemicalreaction. It plays a crucial role for the development of themolecular electronics. But the key questions ahead of us con-cerning a new conceptual molecular switch are the selectionsand architectures of various molecules to demonstrate a sig-nificant on/off ratio and the corresponding reversible twostates: metastable state and steady state.

Recently, the molecular switching behaviors have beenextensively studied and the intrinsic origins inducing the evi-dently change currents have been excavated and emergedgradually. Moresco et al.17 talked about the single molecularswitch induced by the comformational changes on theCu�211� surface and manipulated the on-off states with scan-ning tunneling microscopy tip experimentally and theoreti-cally. Martins, et al.18 studied theoretically that a benzenemolecule assembled between single-walled carbon nanotubes�SWCNT� electrodes works similar to a highly efficient

switch mediated by micromechanical operations where itson/off ratio was up to more than 300. Li et al.19 reported theswitching behaviors of the photochromic molecule whichcan convert with the photoexcitation and the on/off ratio isnear 102 that is agreement with experiment. van der Molen etal.20 presented a nanoscale light-controlled and reversibleswitch devices based on photochromic diarylethene mol-ecules. However, how to design a smart and small molecularswitch with a high on/off ratio is still a pursuing goal forresearchers.

In our papers, we make use of a small organic moleculeto design a simple molecular switch and perform its current-voltage �I-V� properties simulations based on density-functional theory �DFT�. It has been found that our switchhas a distinctive conduction switching behavior with the on/off ratio on the order of 102 at a bias of 0.8 V and then up tomore than the order of 103 in a bias range from 0.8 to 1.8 V.Calculations show that an obvious increase in delocalizationof the atomic orbital induced by the rotation of a mid-benzene ring around the x- or y-axes is the intrinsic origin ofdistinctive conducting switch behaviors. Such a switch hasthe explicit steady state and metastable state that can be con-verted to each other by an energy stimulus or micromechani-cal operations. These findings suggest that this small molecu-lar switch has obvious potential advantages for therealization of the miniaturized molecular electronic devices.

II. MODEL AND METHOD

The molecular switches we structured are shown in Fig.1. In Fig. 1�a�, the two-probe systems are composed of the�-stacked benzene rings and two Au electrodes. This�-stacked structure is derived from the beta sheets formed inthe genetically polypeptides evolution.21 The mid-benzenering in such a configuration rotates 90° about x-, y-, andz-axes, respectively, to form other device models in Figs.1�b�–1�d�. These structures in Figs. 1�a� and 1�b� are re-ferred, respectively, as to model A, X, Y, and Z. In our cal-culations, this two-probe system is divided into three areas:the left electrode area, the right electrode area, and the cen-

a�Author to whom correspondence should be addressed. Electronic mail:cscuzzh@163.com.

b�Electronic mail: keqiuchen@hnu.cn.

JOURNAL OF APPLIED PHYSICS 107, 063704 �2010�

0021-8979/2010/107�6�/063704/5/$30.00 © 2010 American Institute of Physics107, 063704-1

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21

tral scattering area �also called “device area”�. The two layeratoms on each electrode within the frame are included in thescattering area and can screen the potential effect of an em-bedded molecule to the electrodes. The used electrode sur-faces are Au�111� and each layer of the electrodes contains3�3 nine atoms.

Initially, three benzene rings are placed spatial sym-metrically in between of both electrodes and make the near-est ring-electrode and farthest ring-ring distances equal to 2.0and 10.6 Å, respectively. The finally optimized geometries ofthese models are obtained by using quasi Newton optimiza-tion algorithm until the remnants of atomic force is less than0.05 eV/Å. The transport characteristics of these systems arecalculated by the DFT. The details of such a theoreticalmethod have been described elsewhere.1,2 We employ norm-conserving pseudopotentials22 to represent the atomic coreand linear combinations of atomic orbitals to expand the va-lence states. The basis sets include double-� plus double po-larized orbital for C, and a single-� plus polarized orbital forH and Au both in performing the structural relaxations andthe transport calculations. We use the local density approxi-mation to compute the exchange and correlation energy. Thesizes of the unit cell are long enough along x and y to avoidoverlaps and strong electrostatic interactions with molecularimages. The current through this molecular device is com-puted by the Landauer–Büttiker formula

I�V� = 2e

h� T�E,V��f�E − �L� − f�E − �R��dE , �1�

where f�E� is the Fermi function, �L and �R are the electro-chemical potentials of the left and right gold electrodes, andT�E ,V� is the transmission probability of electrons for agiven energy E at an applied bias voltage V. The bias voltageenters the calculation via shifts in the electrostatic potentialenergy in the unperturbed left electrodes by the amount ofeV/2 and the right electrode by �eV/2, which forms theboundary condition for studying the charge and potential re-sponse of the device region. Under a bias voltage, the �L and�R shift each other and always satisfy �L−�R=eV. Thetransmission probability is given by T�E ,V�=Tr��LG†�RG�,where �L,R is the broadening function due to electrode cou-pling and G, the retarded Green’s function of the system, isevaluated

G = �HL + �L VL 0

VL† HC VR

0 VR† HR + �R

�−1

, �2�

where HR, HL, and HC are the Hamiltonian matrices in theright electrode area, the left electrode area, and the centralscattering area, respectively, and VL�VR� is the interactionbetween the left �right� electrode area and central area. Basedon the Eigenchannels decomposition of the conductance, thistotal transmission T�E� can be decomposed into nonmixingeigenchannels Tn�E� �Ref. 23� as T�E�=�nTn�E�. Thek-points we choose in current and transmission spectrum cal-culation is 8�8.

III. RESULTS AND DISCUSSION

The I-V characteristics for models A, X, Y, and Z in thebias range from 0 to 3.0 V are presented in the Fig. 2. Withan increase in the applied bias, currents in the models X andY show a distinct growth but the models A and Z are nearlynonconducting, just with very small leakage current �lessthan 0.3 �A�. So the currents in model X �Y� are muchbigger than that in model A �Z�, especially in a high-biasregion. Calculations show that the on/off ratio of our switch,taking model Y as “on” state and model A �Z� as “off” state,

FIG. 2. �Color online� The I-V curves of models A, X, Y, and Z. The curvesof models X and Y have a distinct growth but the models A and Z are nearlynonconducting.

FIG. 1. �Color online� The sketch of our models in studies, �a� models A,consisting of three �-stacked benzene rings and two Au electrodes, �b�Model X, �c� model Y, and �d� model Z, formed from rotations of themid-benzene ring in model A by 90° about x-, y-, and z-axes, respectively.

063704-2 Qiu et al. J. Appl. Phys. 107, 063704 �2010�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21

may reach the order of 102 in the bias range from 0 to 0.8 Vand then up to more than the order of 103 at the bias largerthan 0.8 V. It indicates the distinctive conducting switch be-havior.

For identifying the transport properties of our molecularswitch, we plot transmission spectrums of models, A and Z ata bias of 2.4 V, and models X and Y at a bias of 2.2 V. Biaseshere correspond to voltages at which electron resonant tun-neling occurs for these four models. In the bias range westudied the frontier highest occupied molecular orbital�HOMO� which has �-bonding character and is distributedalong the �-conjugation is responsible for the resonant tun-neling clearly and the lowest unoccupied molecular orbitallevel which has ��-antibonding character and is also distrib-uted along the �-conjugation has no contribution to the cal-culated currents. The transmission coefficient indicates a

strong or weak delocalization of a molecular orbital and re-flects the hybridization degree of delocalization wave func-tions between a molecular and electrodes. Therefore it can beregarded as a direct representation of the coupling between amolecular core and the electrodes. Thinking about these fea-tures, we can understand the coupling relationship between amolecule and electrodes by the transmission curve upon bias.In Fig. 3, for model A �Z�, the peak value of transmissioncoefficient at the HOMO level �EHOMO=−1.22 eV� is 0.1766�0.1722�. While for model X �Y�, the transmission spectrumpeaks have a significant broadening and enhancement, andthe HOMO level shifts more closely to Fermi energy �heresetting the energy origin as the Fermi energy� and lies at�1.14 eV, corresponding transmission coefficient is 0.5167�0.5719�. More transmission peaks appear in model X �Y�than in model A �Z�. Especially, the HOMO level of model Ysplits to form two levels and thus induces the stronger reso-nant tunnelling. Therefore, it is reasonable to expect a sig-nificantly different result between the I-V curves �see Fig. 2�for models X, Y, and A �Z�.

In order to investigate the transport properties of reso-nant tunneling thoroughly, we compute the distribution oftransmission coefficient in the two-dimensional �2D� Bril-louin zone. The transmission coefficients can be written asTkA,kB�E ,V�, and the E and V are the orbital energy and theapplied bias, respectively. Figures 4�a�–4�d� reveal the trans-mission coefficient distributions at the HOMO levels for fourmodels in the 2D Brillouin zone at a bias of 2.2 V �or 2.4 V�.It is evident that the high peaks occur at the edges and thediagonal zone from �1, 0� to �0, 1� and the transmission co-efficients for models X and Y are much larger than that formodel A and Z. This is also a direct verification for models Xand Y having much stronger currents in the bias range of ourcalculations.

Figure 5 shows the transmission eigenstates of the

FIG. 3. �Color online� The transmission spectrums of models A and Z at abias of 2.4 V, and models X and Y at a bias of 2.2 V. Biases here correspondto voltages at which electron resonant tunneling occurs for these four mod-els. The dotted lines indicate a bias window.

FIG. 4. �Color online� The distribution of transmission coefficient in the 2D Brillouin zone for the HOMO level of four models, �a� for model A �EHOMO

=−1.22 eV� at a bias of 2.4 V, �b� for model X �EHOMO=−1.14 eV� at a bias of 2.2 V, �c� for model Y �EHOMO=−1.14 eV� at a bias of 2.2 V, and �d� for modelZ �EHOMO=−1.22 eV� at a bias of 2.4 V.

063704-3 Qiu et al. J. Appl. Phys. 107, 063704 �2010�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21

HOMO levels for the four models at a bias of electron reso-nant tunneling in the real space. Each eigenstate has foureigenchannels, numbered as No. 1,2,3,4. �A-1� to �A-4�,�X-1� to �X-4�, �Y-1� to �Y-4�, and �Z-1� to �Z-4� correspondto models A, X, Y, and Z, respectively. The transmissioneigenstate of eigenchannels of the whole system is consid-ered to be a scattering state, which is a Bloch wavelike inreal space and shows the path of the electrons as it is trans-mitted through the device area. If a molecular orbital is de-localized, the electron that has the same energy will have ahigher possibility T�E ,V� reaching the other electrode via thedevice area. That is the way they clearly reflect the contribu-tions of each component in a molecular device to the elec-tronic transmission. Obviously, a mid-benzene ring in modelA or Z has little contributions to the resonant tunneling. Itindicates that the atomic orbitals of the mid-benzene ringhave a strong localization and very little hybridizations withthe benzene rings at its both sides, and this makes the elec-trical conduction of such devices very weak. But a mid-benzene ring in model X or Y has distinctive contributions tothe resonant tunneling, which reveals that the atomic orbitalsof the mid-benzene ring have a strong delocalization andmore hybridizations with the other two benzene rings. Thebig �-bond formed with the six �-orbital in the mid-benzeneconstructs a molecular bridge to connect the side ones mak-ing the transport properties of our switch like a nanowire. Soit improves the electrical conduction of a device signifi-cantly. This shows that an obvious increase in delocalizationof the atomic orbital induced by the 90° rotation of a mid-benzene ring about the x- or y-axes is the intrinsic origin ofdistinctive switching behaviors.

Figure 6 presents the calculated curves of the energyincrement as a function of the rotating angle of the mid-

benzene ring in model A around x-, y-, and z-axes from 0° to180°. The energy has almost no change when the mid-benzene ring rotates around the z-axis. While for the mid-benzene ring rotating by 90° around the x- and y-axes �cor-responding to models X and Y�, there exists a metastablestate corresponding to a minimum in the energy and a poten-tial barrier in a region of 0° to 90° �also 90° to 180°�. Obvi-ously, if choosing model A and Z to be the off state, andmodel X and Y to be the on state, then a simple molecularswitch will be constructed. Of course, considering a largeron/off ratio for this molecular switch is achieved, it is betterto set model A as the off state and model Y as the on state.Therefore, we can obtain a molecular switch characterizedwith reversible two stable states and rather larger on/off ra-tio. However, the key problem we should solve is how toexperimentally make reversible orientation transitions be-

FIG. 5. �Color online� The transmission eigenstates of the HOMO levels for the four models at a bias of electron resonant tunneling in the real space. Eacheigenstate has four eigenchannels, numbered as No. 1,2,3,4. �A-1� to �A-4�, �X-1� to �X-4�, �Y-1� to �Y-4�, and �Z-1� to �Z-4� correspond to models A, X, Y,and Z, respectively.

FIG. 6. �Color online� The calculated curves of the energy increment as afunction of the rotating angle of the mid-benzene ring from 0° to 180° inmodel A, X, Y, and Z.

063704-4 Qiu et al. J. Appl. Phys. 107, 063704 �2010�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21

tween the off state and the on state accessible, we considerthat such a manipulation may be a thermal activation, micro-mechanical operations, or neighboring molecules as dis-cussed in Refs. 18 or 24. This conclusion bases on a fact thata barrier of the conformation transition as seen in Fig. 6 isvery small �25 meV� since a coupling between three ringsis weak as compared with that between the left �right� ringand electrodes leading to having a relatively free rotationsfor the mid-ring. For a thermal activation, we can perform asimple estimation, at a room temperature T=300 K, thenkT24 meV. Thus, there is a sufficiently large probabilityfor a thermal fluctuation to drive a molecular orientationtransition and result in a conduction switch.

IV. CONCLUSION

We have studied the transport characteristics of a simpleorganic molecular switch consisting of three benzene ringsconnecting to Au electrodes based on the first principlessimulations. The calculations show that our switch reaches aon/off ratio on the order of 102 at a bias of 0.8 V and then upto more than the order of 103 at the bias larger than 0.8 V.Meanwhile, we find that such a switch has both explicitlystable state and metastable state that can be converted fromeach other by an energy stimulus or micromechanical opera-tions. The analysis on the transmission spectrums, transmis-sion coefficients in the 2D Brillouin zone, and the transmis-sion eigenstate of each eigenchannel at the HOMO levelsdemonstrates that an obvious increase in delocalization ofthe atomic orbital induced by the rotation of a mid-benzenering about the x- or y-axes is the intrinsic origin of distinctiveconducting switch behaviors. These findings suggest thatsuch a small molecular switch has obvious potential advan-tages and brings up great possibility for the molecular engi-neering to design an ultrasmall nanodevice.

ACKNOWLEDGMENTS

This work was supported by the National Natural Sci-ence Foundation of China �Grant Nos. 60771059 and60871065�, the Scientific Research Fund of Hunan Provin-cial Education Department �Grant Nos. 08A005 and08C110�, the Ministry of Science and Technology of China

�No. 2006CB605105�, and the Construct Program of the KeyDiscipline in Changsha University of Science & Technology.

1V. M. García-Suárez and C. J. Lambert, Nanotechnology 19, 455203�2008�.

2J. Taylor, H. Guo, and J. Wang, Phys. Rev. B 63, 245407 �2001�.3Z. Yang, N. D. Lang, and M. Di Ventra, Appl. Phys. Lett. 82, 1938 �2003�;Z. Yang, A. Tackett, and M. Di Ventra, Phys. Rev. B 66, 041405�R��2002�; Z. Yang and M. Di Ventra, ibid. 67 161311�R� �2003�; Z. Yang, M.Chshiev, M. Zwolak, Y.-C. Chen, and M. Di Ventra, ibid. 71, 041402�R��2005�; Y.-C. Chen, M. Zwolak, and M. Di Ventra, Nano Lett. 3, 1691�2003�.

4C. Stampfer, J. Guttinger, C. Roman, A. Jungen, T. Helbling, and C.Hierold, Nano Lett. 7, 2747 �2007�.

5M. Q. Long, K. Q. Chen, and L. L. Wang, Appl. Phys. Lett. 91, 233512�2007�; L. M. Tang, L. L. Wang, and K. Q. Chen, ibid. 88, 163505 �2006�.

6L. Deretzis and A. La Magna, J. Chem. Phys. 128, 164706 �2008�.7Z. H. Zhang, Z. Yang, J. H. Yuan, H. Zhang, X. Q. Deng, and M. Qiu, J.Chem. Phys. 129, 094702 �2008�; Z. H. Zhang, M. Qiu, X. Q. Deng, K. H.Ding, and H. Zhang, ibid. 130, 184703 �2009�.

8A. Troisi and M. A. Ratner, J. Am. Chem. Soc. 124, 14528 �2002�.9R. McCreery, J. Dieringer, A. O. Solak, B. Snyder, A. M. Nowak, W. R.McGovern, and S. DuVall, J. Am. Chem. Soc. 125, 10748 �2003�.

10M. Di Ventra, S. T. Pantelides, and N. D. Lang, Appl. Phys. Lett. 76, 3448�2000�.

11M. A. H. Khalafalla, Y. Ono, K. Nishiguchi, and A. Fujiwara, Appl. Phys.Lett. 91, 263513 �2007�.

12K. Stokbro, J. Taylor, and M. Brandbyge, J. Am. Chem. Soc. 125, 3674�2003�.

13Y. Zhou, X. Zheng, Y. Xu, and Z. Y. Zeng, J. Chem. Phys. 125, 244701�2006�.

14D. Cheng, W. Y. Kim, S. K. Min, T. Nautiyal, and K. S. Kim, Phys. Rev.Lett. 96, 096104 �2006�.

15M. F. Ng, L. P. Zhou, S. W. Yang, L. Y. Sim, V. B. C. Tan, and P. Wu,Phys. Rev. B 76, 155435 �2007�.

16Z. C. Wang, T. K. Gu, T. Tada, and S. Watanabe, Appl. Phys. Lett. 93,152106 �2008�.

17F. Moresco, G. Meyer, and K. H. Rieder, Phys. Rev. Lett. 86, 672 �2001�.18T. B. Martins, A. Fazzino, and J. R. da Silva Antônio, Phys. Rev. B 79,

115413 �2009�.19J. Li, G. Speyer, and O. F. Sankey, Phys. Rev. Lett. 93, 248302 �2004�.20S. J. van der Molen, J. H. Liao, T. Kudernac, J. S. Agustsson, L. Bernard,

M. Calame, B. J. van Wees, B. L. Feringa, and C. Schonenberger, NanoLett. 9, 76 �2009�.

21X. Y. Liu, J. E. Reynolds, C. Wells, J. Welch, and T. S. Cale, J. Appl. Phys.98, 033712 �2005�.

22D. R. Hamann, M. Schlüter, and C. Chiang, Phys. Rev. Lett. 43, 1494�1979�.

23M. Brandbyge, M. R. Sørensen, and K. W. Jacobsen, Phys. Rev. B 56,14956 �1997�.

24J. Taylor, M. Brandbyge, and K. Stokbro, Phys. Rev. B 68, 121101 �2003�.

063704-5 Qiu et al. J. Appl. Phys. 107, 063704 �2010�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

82.139.7.251 On: Sun, 30 Mar 2014 13:46:21