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Surprising electronic–magnetic properties of close-packed organized organic layers
Chemical Physics 281 (2002) 305–309 www.elsevier.com/locate/chemphys
Surprising electronic–magnetic properties of close-packed organized organic layers Zeev Vager a, Ron Naaman b,* a b
Department of Particle Physics, Weizmann Institute of Science, 76100 Rehovot, Israel Department of Chemical Physics, Weizmann Institute of Science, 76100 Rehovot, Israel Received 30 August 2001
Abstract Close-packed organized organic monolayers are finding their place in many applications and specifically in molecular electronic related functions. As a result, the relation between the properties of the single molecule and of the molecule imbedded within the monolayer is of importance. Here it is shown that upon formation of the layer the molecular electric dipole is reduced and is independent of the free molecules’ electric dipole. Its upper bound value is derived from the thin film property of dielectric breakdown. Moreover, when the molecules in the layer are homochiral, the monolayer may have appreciable magnetic properties. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Close-packed, organized organic layers are the focus of substantial studies in recent years, due to their abilities to modify electronic properties of substrates [1,2], metals or semiconductors [3,4], and to serve as elements in modern optical [5] and electronic devices [6–9], light emitting diodes [10], solar cells [11], sensors [12], etc. It is usually assumed that the electronic properties of the adsorbed molecules are similar to that of the isolated molecule or of the molecule embedded in an isotropic medium. The weak coupling between the molecules in a monolayer seems to support this
*
Corresponding author. Tel.: +972-8-9343409; fax: +972-89344123. E-mail address: [email protected] (R. Naaman).
notion. This is taken as a justification to use molecular based calculations for predicting the properties of the monolayer [13,14]. In what follows we show that this assumption is generally not justified and that properties of molecules can vary significantly upon adsorption to a close-packed layer. It is known that adsorbed layers can affect the work function of the substrates by acting as a dipole layer in which a force is exerted on the electrons while passing through it. Hence, the energy required to remove an electron from the substrate (that is, the work function) depends on the layer’s dipole density. When atoms are adsorbed on the surface, the dipole layer arises from either charge transfer between the substrate and the adsorbate layer or an induced polarization of the atom. It has been known for quite some time that for adsorbed at-
0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 2 ) 0 0 3 7 4 - 9
306
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
oms, the size of the dipole of the layer is limited due to the dipole–dipole interaction between the adsorbed species. Clearly the atomic adsorbates are modified and their electronic structure differs from that of the isolates atoms [15]. The dipole properties result directly from the adsorption process. The situation when molecules are the adsorbates is different, since the free molecules may carry their own dipole moment. A simple picture is that the properties of the dipole layer result solely from the molecular properties. Indeed, several studies [16,17] find correlations between the dipole moment of the isolated molecule and the dipole density of the adsorbed layer. Furthermore, this approach looks as a good way to achieve a very high dipole density. The rationalization is as follows. Strong chemical binding between the adsorbed molecule and the substrate provide enough energy to overcome the dipole–dipole repulsion, keeping negative the total change in the free energy of the adsorption process. Hence, the size of the dipole of the adsorbed molecules seems to be limited only by the energy that the bond to the surface can supply for overcoming the dipole– dipole repulsion within the layer. However, it has been already recognized that when molecules are assembled into a dense packed layer, their electronic structure vary. The surface potential theory of thin films on water or metal substrates is probably best described by Taylor and Bays [18]. They used a perturbative approach to deal with the effects caused by the layer formation. Similar approach has been taken in more recent publications that show that indeed the dielectric constant of an organized molecular monolayer varies with the layer density [19,20]. The theoretical approach taken by Taylor and Bays [18] deals with an effective molecular dipole M which is modified from the free molecular dipole, l, by the self-consistent electric field in which the dipoles are immersed. The modification occurs through the Stark effect on the free molecular states and is calculated by first order perturbation theory as ~ ¼~ M l þ a~ E; where a is the electric molecular polarizability.
ð1Þ
Regardless of perturbation theory, a relative permeability (dielectric constant), e, is defined by e¼
l : M
ð2Þ
Then, the condition for the first order perturbation to be valid is je 1j 1:
ð3Þ
It is shown here that for packing of molecules with substantial dipoles into a monolayer, the quantity jej, defined by Eq. (2), is always much larger than one. Furthermore, the effective dipoles within monolayers diminish drastically in a non-pertubative manner. Most importantly, as in adsorbed atoms, the electronic states of the adsorbed molecules differ considerably from the states of the free molecule. This electronic rearrangement by adsorption to a monolayer, must be accompanied by a current and plays a substantial role in defining the properties of the adsorbed layer. It alters significantly the electronic properties of the molecule imbedded in the monolayer, as compared to the isolated molecule or a molecule in an isotropic medium. Following this part, it will be shown that when the molecules are homochiral, a splitting between their spin states occurs which may result in a magnetic layer.
2. Substantial charge rearrangements We start by following the electrostatic formulation as in [18], except that e will be treated empirically. Denote by q the effective charge separation in the molecule before adsorption and w the effective distance of the charge separation, such that l ¼ qw is the electric dipole of the unadsorbed molecule. Idealize the layer as an electrostatic bilayer with smoothly distributed opposite charges on each side of the adsorbed layer. This idealization is applicable when the effective distance of the charge separation is larger than the distance between neighboring molecules. The displacement vector D of a bilayer is confined to the inner volume of the layer and it is zero elsewhere. Therefore, by Gauss law (esu), it is given by
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
D¼
4pq ¼ eE; r
307
ð4Þ
where r is the effective area occupied by a single adsorbed molecule and E is the physical electric field, also confined within the layer. A typical bilayer potential jump of V ¼ wE ¼
4pl er
ð5Þ
occurs along the width w of the layer. This potential jump is compared with a fictitious bilayer of effective electric dipole moments M such that V ¼
4pM : r
ð6Þ
Thus the effective electric dipole moment of an adsorbed molecule is consistent with Eq. (2) with no reference to first order perturbation theory. The conversion of Eqs. (4) and (6) to practical , and eV) results in units (Debye, A Þ ¼ E ðV=A
V ðVÞ ¼
38M ðDebyeÞ ; 3 Þ v ðA
38M ðDebyeÞ ; 2 Þ r ðA
ð7Þ
ð8Þ
where v ¼ wr is the effective molecular volume. Already here, the validity of first order pertubation theory for e is suspected, since for e 1 (Eq. (3)) unrealistically large electric fields are obtained. The real obstacle for reaching large electric fields in thin films (as well as bulk material) is the dielectric breakdown. For example, highest breakdown electric field in a very thin film of polypropylene [21] is Þ: EDB ¼ 0:06 ðV=A
ð9Þ
Using Eq. (7) with this limitation, an upper bound is found for the effective dipole in monolayers 3 Þ: M ðDebyeÞ 6 1:6 103 v ðA
ð10Þ
The upper bound for the dipole moment obtained from this consideration is not related to the initial electric dipole moment of the molecule. As is pointed out in [22], the breakdown mechanism may be related to the wave-like character of the electrons when laterally confined and therefore related to a
Fig. 1. The relative dielectric constant (solid line) and the electric field within a monolayer (dashed line) as a function of the dipole moment of the molecules, assuming that each mol2 and its length is 20 A . ecule occupies area of 20 A
universal phenomenon. In general, this upper bound is much smaller than the unadsorbed molecular dipole moment, hence by Eq. (2), jej 1. Fig. 1 presents the calculated effective dielectric constant and the effective electric field within a layer as a function of the dipole moment of the adsorbed molecules. It is assumed that each molecule occu2 and that the thickness of the pies an area of 20 A . Since the electric field cannot exlayer is of 20 A ceed the breakdown limit, the effective dielectric constant of the layer must increase linearly with the free molecule dipole moment. As a result of the above discussion, it is clear that intensive charge rearrangement occurs within the molecules of the layer. The rearrangement happens with the aid of the surroundings. This surrounding can be a substrate, in the case of a layer adsorbed on solid, or a solution in the case of Langmuir films or natural membranes. A formal way to express charge rearrangement is by strongly mixing the ground state either with excited electronic states of the molecule itself or of the surrounding. This process allows a finite probability that the molecules will be left with unpaired electrons.
3. Chiral thin films In most molecular layers, the electric field is just below the breakdown field (due to previously ex-
308
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
isting dipole moments). This symmetry breaking huge field cannot split the spin states of the electrons, unless another intrinsic symmetry breaking element exists in the layer. Specifically, consider a state of a homochiral molecular layer and its mirror image. Though the Hamiltonian is invariant under parity transformation, the states are characterized by a handedness c, which changes sign upon a mirror reflection. Thus, the handedness c is an isoscalar quantity. For simplicity, choose a mirror plane parallel to the field E (normal to the surface of the layer). Such a transformation does not change E but does change c. Therefore, one can define another operator B ¼ cE which is an axial vector and can interact with the spin of electrons in the same way as angular momentum does in atoms or as a magnetic field does in a macroscopic system. Different handedness are expressed by c ¼ 1 or c ¼ 1 and their corresponding B points either parallel or anti-parallel to E. As a result, the expectation values of cs E can be readily estimated for uncoupled spins s in homochiral dipolar monolayers. Using the upper bound for the electric field given by Eq. (9), B ¼ cE corresponds to a value equivalent to a magnetic field of up to 20,000 G. This value is larger than the fields found in common ferrites. From the above considerations one can conclude that when chiral molecules form closepacked layers, due to the charge rearrangement, the layer may gain magnetic properties [23]. The precise nature of these properties depends on the detailed exchange interaction between the unpaired electrons. The new type of magnetic properties suggested here may be of relevance in biological membranes where homochiral molecules with relatively large dipole moments are packed together.
4. Summary From the deliberation above, it is clear that molecules possessing appreciable electric dipole moment must discharge into an almost zero dipole upon formation of a close-packed layer. This discharging, if measured, may provide a direct indication for adsorption [24]. The change in the
electronic properties of the molecules upon formation of a layer could modify dramatically the way one has to treat theoretically those molecules and can provide new properties, not found in the molecules when they are isolated or embedded in an isotropic medium. Adsorbed molecules in general and closepacked monolayers in particular are considered to be important building blocks in molecular electronics devices. The cooperative effects described here hint on two important factors that have to be taken into account when using monolayers for this purpose. First, care should be taken when assuming that the properties of the molecule in the layer are similar to that of the free species. Second and perhaps the most applicable conclusion is that since the system has 2D properties, one can transfer information (for example by the electric potential) over a large distance. High and nonlinear dielectric materials are very useful in the field of optics. It has been found that films made from highly polar molecules indeed show strong non-linear response. The arguments presented here provide new aspect to the experimental observations. Based on the above discussion is clear that the parameter e, of molecular thin films, must behave in a highly non-linear fashion to electric fields because the electric potential on the layer is close to the dielectric breakdown point. Therefore, in spite of the small effective electric dipole per molecule, such thin layers should show high non-linear effects and could be of practical interest. Despite the relative weak two-body interaction between the molecules on a monolayer, many of its properties are cooperative and stem from the macroscopic two dimensionality of the layer. For example, the potential change is felt at distances comparable to the sample size. Another important example is the potential magnetism of biological membranes. This last property has been observed before without a clue to its source [25–29].
Acknowledgements RN acknowledges the support of the Israel Science Foundation.
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
References [1] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [2] S.N. Yaliraki, A.E. Roitberg, C. Gonzalez, V. Mujica, M.A. Ratner, J. Chem. Phys. 111 (1999) 6997. [3] Vilan, A. Shanzer, D. Cahen, Nature 404 (2000) 166. [4] I.H. Campbell, J.D. Kress, R.L. Martin, D.L. Smith, N.N. Barashkov, J.P. Ferraris, Appl. Phys. Lett. 71 (1997) 3528. [5] T. Verbiest, S. Van Elshocht, M. Kauranen, L. Hellemans, J. Snauwaert, C. Nuckolls, T.J. Katz, A. Persoon, Science 282 (1998) 913. [6] C.P. Collier, E.W. Wong, M. Belohradsky, F.M. Raymo, J.F. Stoddart, P.J. Kuekes, R.S. Williams, J.R. Heath, Science 285 (1999) 391. [7] J. Chen, M.A. Reed, A.M. Rawlett, J.M. Tour, Science 286 (1999) 1550. [8] Vuillaume, B. Chen, R.M. Metzger, Langmuir 15 (1999) 4011. [9] E. Punkka, R.F. Rubner, J. Electron. Mater. 21 (1992) 1057. [10] See for example: A. Yamamori, S. Hayashi, T. Koyama, Y. Taniguchi, App. Phys. Lett. 78 (2001) 3343; B.K. Crone, P.S. Davids, I.H. Campbell, D.L. Smith, J. Appl. Phys. 87 (2000) 1974. [11] See for example: P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2650; U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weiss€ ortel, J. Salbeck, H. Spreitzer, M. Gr€atzel, Nature 395 (1998) 583. [12] D.G. Wu, D. Cahen, P. Graf, R. Naaman, A. Nitzan, D. Shvarts, Chem. Eur. J. 7 (2001) 1743. [13] A. Nitzan, I. Benjamin, Acc. Chem. Res. 32 (1999) 854.
309
[14] G.R. Hutchison, M.A. Ratner, T.J. Marks, R. Naaman, J. Phys. Chem. B. 105 (2001) 2881. [15] See for example: W. Zhao, G. Kerner, M. Asscher, X.M. Wilde, K. Al-Shamery, H.-J. Freund, V. Staemmler, M. Wieszbowska, Phys. Rev. B 62 (2000) 7527. [16] M. Bruening, E. Moons, D. Cahen, A. Shanzer, J. Phys. Chem. 99 (1995) 8368. [17] S. Liao, Y. Shnidman, A. Ulman, J. Am. Chem. Soc. 122 (2000) 3688. [18] D.M. Taylor, G.F. Bays, Phys. Rev. E 49 (1994) 1439. [19] M. Iwamoto, Y. Mitzutani, A. Sugimura, Phys. Rev. B 54 (1996) 8186. [20] C.-X. Wu, M. Iwamoto, Phys. Rev. B 55 (1997) 10922. [21] D. Liufu, X.S. Wang, D.M. Tu, K.C. Kao, J. Appl. Phys. 83 (1998) 2209. [22] E. Miranda, J. Sune, Appl. Phys. Lett. 78 (2001) 225. [23] I. Carmeli, V. Skakalova, R. Naaman, Z. Vager, Angew. Chem. Int. Ed. 41 (2002) 761. [24] Recently this discharging current was measured, see: Dm. Shvarts, A. Haran, R. Benshafrut, D. Cahen, R. Naaman, Chem. Phys. Lett. 354 (2002) 350. [25] N. Chalazonitis, R. Changeux, A. Arvanitai, C. R. Acad. Sci. Paris, Ser. D 271 (1970) 130. [26] N.E. Geacintov, F. Van Nostrand, M. Pope, J.B. Tinkel, Biochim. Biophys. Acta 226 (1972) 486; N.E. Geacintov, F. Van Nostrand, J.F. Becker, J.B. Tinkel, Biochim. Biophys. Acta 267 (1972) 65. [27] J.D. Clement-Metral, FEBS Lett. 50 (1975) 257. [28] D.-Ch. Neugebauer, A.E. Blaurock, D.L. Worcester, FEBS Lett. 78 (1977) 31. [29] S. Ozeki, H. Kurashi, M. Miyanaga, C. Nozawa, Langmuir 16 (2000) 1478.
Surprising electronic–magnetic properties of close-packed organized organic layers Zeev Vager a, Ron Naaman b,* a b
Department of Particle Physics, Weizmann Institute of Science, 76100 Rehovot, Israel Department of Chemical Physics, Weizmann Institute of Science, 76100 Rehovot, Israel Received 30 August 2001
Abstract Close-packed organized organic monolayers are finding their place in many applications and specifically in molecular electronic related functions. As a result, the relation between the properties of the single molecule and of the molecule imbedded within the monolayer is of importance. Here it is shown that upon formation of the layer the molecular electric dipole is reduced and is independent of the free molecules’ electric dipole. Its upper bound value is derived from the thin film property of dielectric breakdown. Moreover, when the molecules in the layer are homochiral, the monolayer may have appreciable magnetic properties. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Close-packed, organized organic layers are the focus of substantial studies in recent years, due to their abilities to modify electronic properties of substrates [1,2], metals or semiconductors [3,4], and to serve as elements in modern optical [5] and electronic devices [6–9], light emitting diodes [10], solar cells [11], sensors [12], etc. It is usually assumed that the electronic properties of the adsorbed molecules are similar to that of the isolated molecule or of the molecule embedded in an isotropic medium. The weak coupling between the molecules in a monolayer seems to support this
*
Corresponding author. Tel.: +972-8-9343409; fax: +972-89344123. E-mail address: [email protected] (R. Naaman).
notion. This is taken as a justification to use molecular based calculations for predicting the properties of the monolayer [13,14]. In what follows we show that this assumption is generally not justified and that properties of molecules can vary significantly upon adsorption to a close-packed layer. It is known that adsorbed layers can affect the work function of the substrates by acting as a dipole layer in which a force is exerted on the electrons while passing through it. Hence, the energy required to remove an electron from the substrate (that is, the work function) depends on the layer’s dipole density. When atoms are adsorbed on the surface, the dipole layer arises from either charge transfer between the substrate and the adsorbate layer or an induced polarization of the atom. It has been known for quite some time that for adsorbed at-
0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 2 ) 0 0 3 7 4 - 9
306
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
oms, the size of the dipole of the layer is limited due to the dipole–dipole interaction between the adsorbed species. Clearly the atomic adsorbates are modified and their electronic structure differs from that of the isolates atoms [15]. The dipole properties result directly from the adsorption process. The situation when molecules are the adsorbates is different, since the free molecules may carry their own dipole moment. A simple picture is that the properties of the dipole layer result solely from the molecular properties. Indeed, several studies [16,17] find correlations between the dipole moment of the isolated molecule and the dipole density of the adsorbed layer. Furthermore, this approach looks as a good way to achieve a very high dipole density. The rationalization is as follows. Strong chemical binding between the adsorbed molecule and the substrate provide enough energy to overcome the dipole–dipole repulsion, keeping negative the total change in the free energy of the adsorption process. Hence, the size of the dipole of the adsorbed molecules seems to be limited only by the energy that the bond to the surface can supply for overcoming the dipole– dipole repulsion within the layer. However, it has been already recognized that when molecules are assembled into a dense packed layer, their electronic structure vary. The surface potential theory of thin films on water or metal substrates is probably best described by Taylor and Bays [18]. They used a perturbative approach to deal with the effects caused by the layer formation. Similar approach has been taken in more recent publications that show that indeed the dielectric constant of an organized molecular monolayer varies with the layer density [19,20]. The theoretical approach taken by Taylor and Bays [18] deals with an effective molecular dipole M which is modified from the free molecular dipole, l, by the self-consistent electric field in which the dipoles are immersed. The modification occurs through the Stark effect on the free molecular states and is calculated by first order perturbation theory as ~ ¼~ M l þ a~ E; where a is the electric molecular polarizability.
ð1Þ
Regardless of perturbation theory, a relative permeability (dielectric constant), e, is defined by e¼
l : M
ð2Þ
Then, the condition for the first order perturbation to be valid is je 1j 1:
ð3Þ
It is shown here that for packing of molecules with substantial dipoles into a monolayer, the quantity jej, defined by Eq. (2), is always much larger than one. Furthermore, the effective dipoles within monolayers diminish drastically in a non-pertubative manner. Most importantly, as in adsorbed atoms, the electronic states of the adsorbed molecules differ considerably from the states of the free molecule. This electronic rearrangement by adsorption to a monolayer, must be accompanied by a current and plays a substantial role in defining the properties of the adsorbed layer. It alters significantly the electronic properties of the molecule imbedded in the monolayer, as compared to the isolated molecule or a molecule in an isotropic medium. Following this part, it will be shown that when the molecules are homochiral, a splitting between their spin states occurs which may result in a magnetic layer.
2. Substantial charge rearrangements We start by following the electrostatic formulation as in [18], except that e will be treated empirically. Denote by q the effective charge separation in the molecule before adsorption and w the effective distance of the charge separation, such that l ¼ qw is the electric dipole of the unadsorbed molecule. Idealize the layer as an electrostatic bilayer with smoothly distributed opposite charges on each side of the adsorbed layer. This idealization is applicable when the effective distance of the charge separation is larger than the distance between neighboring molecules. The displacement vector D of a bilayer is confined to the inner volume of the layer and it is zero elsewhere. Therefore, by Gauss law (esu), it is given by
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
D¼
4pq ¼ eE; r
307
ð4Þ
where r is the effective area occupied by a single adsorbed molecule and E is the physical electric field, also confined within the layer. A typical bilayer potential jump of V ¼ wE ¼
4pl er
ð5Þ
occurs along the width w of the layer. This potential jump is compared with a fictitious bilayer of effective electric dipole moments M such that V ¼
4pM : r
ð6Þ
Thus the effective electric dipole moment of an adsorbed molecule is consistent with Eq. (2) with no reference to first order perturbation theory. The conversion of Eqs. (4) and (6) to practical , and eV) results in units (Debye, A Þ ¼ E ðV=A
V ðVÞ ¼
38M ðDebyeÞ ; 3 Þ v ðA
38M ðDebyeÞ ; 2 Þ r ðA
ð7Þ
ð8Þ
where v ¼ wr is the effective molecular volume. Already here, the validity of first order pertubation theory for e is suspected, since for e 1 (Eq. (3)) unrealistically large electric fields are obtained. The real obstacle for reaching large electric fields in thin films (as well as bulk material) is the dielectric breakdown. For example, highest breakdown electric field in a very thin film of polypropylene [21] is Þ: EDB ¼ 0:06 ðV=A
ð9Þ
Using Eq. (7) with this limitation, an upper bound is found for the effective dipole in monolayers 3 Þ: M ðDebyeÞ 6 1:6 103 v ðA
ð10Þ
The upper bound for the dipole moment obtained from this consideration is not related to the initial electric dipole moment of the molecule. As is pointed out in [22], the breakdown mechanism may be related to the wave-like character of the electrons when laterally confined and therefore related to a
Fig. 1. The relative dielectric constant (solid line) and the electric field within a monolayer (dashed line) as a function of the dipole moment of the molecules, assuming that each mol2 and its length is 20 A . ecule occupies area of 20 A
universal phenomenon. In general, this upper bound is much smaller than the unadsorbed molecular dipole moment, hence by Eq. (2), jej 1. Fig. 1 presents the calculated effective dielectric constant and the effective electric field within a layer as a function of the dipole moment of the adsorbed molecules. It is assumed that each molecule occu2 and that the thickness of the pies an area of 20 A . Since the electric field cannot exlayer is of 20 A ceed the breakdown limit, the effective dielectric constant of the layer must increase linearly with the free molecule dipole moment. As a result of the above discussion, it is clear that intensive charge rearrangement occurs within the molecules of the layer. The rearrangement happens with the aid of the surroundings. This surrounding can be a substrate, in the case of a layer adsorbed on solid, or a solution in the case of Langmuir films or natural membranes. A formal way to express charge rearrangement is by strongly mixing the ground state either with excited electronic states of the molecule itself or of the surrounding. This process allows a finite probability that the molecules will be left with unpaired electrons.
3. Chiral thin films In most molecular layers, the electric field is just below the breakdown field (due to previously ex-
308
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
isting dipole moments). This symmetry breaking huge field cannot split the spin states of the electrons, unless another intrinsic symmetry breaking element exists in the layer. Specifically, consider a state of a homochiral molecular layer and its mirror image. Though the Hamiltonian is invariant under parity transformation, the states are characterized by a handedness c, which changes sign upon a mirror reflection. Thus, the handedness c is an isoscalar quantity. For simplicity, choose a mirror plane parallel to the field E (normal to the surface of the layer). Such a transformation does not change E but does change c. Therefore, one can define another operator B ¼ cE which is an axial vector and can interact with the spin of electrons in the same way as angular momentum does in atoms or as a magnetic field does in a macroscopic system. Different handedness are expressed by c ¼ 1 or c ¼ 1 and their corresponding B points either parallel or anti-parallel to E. As a result, the expectation values of cs E can be readily estimated for uncoupled spins s in homochiral dipolar monolayers. Using the upper bound for the electric field given by Eq. (9), B ¼ cE corresponds to a value equivalent to a magnetic field of up to 20,000 G. This value is larger than the fields found in common ferrites. From the above considerations one can conclude that when chiral molecules form closepacked layers, due to the charge rearrangement, the layer may gain magnetic properties [23]. The precise nature of these properties depends on the detailed exchange interaction between the unpaired electrons. The new type of magnetic properties suggested here may be of relevance in biological membranes where homochiral molecules with relatively large dipole moments are packed together.
4. Summary From the deliberation above, it is clear that molecules possessing appreciable electric dipole moment must discharge into an almost zero dipole upon formation of a close-packed layer. This discharging, if measured, may provide a direct indication for adsorption [24]. The change in the
electronic properties of the molecules upon formation of a layer could modify dramatically the way one has to treat theoretically those molecules and can provide new properties, not found in the molecules when they are isolated or embedded in an isotropic medium. Adsorbed molecules in general and closepacked monolayers in particular are considered to be important building blocks in molecular electronics devices. The cooperative effects described here hint on two important factors that have to be taken into account when using monolayers for this purpose. First, care should be taken when assuming that the properties of the molecule in the layer are similar to that of the free species. Second and perhaps the most applicable conclusion is that since the system has 2D properties, one can transfer information (for example by the electric potential) over a large distance. High and nonlinear dielectric materials are very useful in the field of optics. It has been found that films made from highly polar molecules indeed show strong non-linear response. The arguments presented here provide new aspect to the experimental observations. Based on the above discussion is clear that the parameter e, of molecular thin films, must behave in a highly non-linear fashion to electric fields because the electric potential on the layer is close to the dielectric breakdown point. Therefore, in spite of the small effective electric dipole per molecule, such thin layers should show high non-linear effects and could be of practical interest. Despite the relative weak two-body interaction between the molecules on a monolayer, many of its properties are cooperative and stem from the macroscopic two dimensionality of the layer. For example, the potential change is felt at distances comparable to the sample size. Another important example is the potential magnetism of biological membranes. This last property has been observed before without a clue to its source [25–29].
Acknowledgements RN acknowledges the support of the Israel Science Foundation.
Z. Vager, R. Naaman / Chemical Physics 281 (2002) 305–309
References [1] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [2] S.N. Yaliraki, A.E. Roitberg, C. Gonzalez, V. Mujica, M.A. Ratner, J. Chem. Phys. 111 (1999) 6997. [3] Vilan, A. Shanzer, D. Cahen, Nature 404 (2000) 166. [4] I.H. Campbell, J.D. Kress, R.L. Martin, D.L. Smith, N.N. Barashkov, J.P. Ferraris, Appl. Phys. Lett. 71 (1997) 3528. [5] T. Verbiest, S. Van Elshocht, M. Kauranen, L. Hellemans, J. Snauwaert, C. Nuckolls, T.J. Katz, A. Persoon, Science 282 (1998) 913. [6] C.P. Collier, E.W. Wong, M. Belohradsky, F.M. Raymo, J.F. Stoddart, P.J. Kuekes, R.S. Williams, J.R. Heath, Science 285 (1999) 391. [7] J. Chen, M.A. Reed, A.M. Rawlett, J.M. Tour, Science 286 (1999) 1550. [8] Vuillaume, B. Chen, R.M. Metzger, Langmuir 15 (1999) 4011. [9] E. Punkka, R.F. Rubner, J. Electron. Mater. 21 (1992) 1057. [10] See for example: A. Yamamori, S. Hayashi, T. Koyama, Y. Taniguchi, App. Phys. Lett. 78 (2001) 3343; B.K. Crone, P.S. Davids, I.H. Campbell, D.L. Smith, J. Appl. Phys. 87 (2000) 1974. [11] See for example: P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2650; U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weiss€ ortel, J. Salbeck, H. Spreitzer, M. Gr€atzel, Nature 395 (1998) 583. [12] D.G. Wu, D. Cahen, P. Graf, R. Naaman, A. Nitzan, D. Shvarts, Chem. Eur. J. 7 (2001) 1743. [13] A. Nitzan, I. Benjamin, Acc. Chem. Res. 32 (1999) 854.
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