Molecules that mimic Schottky diodes

Abstract

Self-assembled monolayers of cationic donor–(π-bridge)–acceptor dyes coupled with anionic donors exhibit asymmetric current–voltage (I–V) characteristics when contacted by Au or PtIr probes. Rectification ratios of 3000 at ±1 V are obtained from Au–S–C₁₀H₂₀–A⁺–π–D|D^–|Au structures in which the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counterion is copper phthalocyanine-3,4′,4″,4‴-tetrasulfonate(SAM 1). Similar behaviour, with a high rectification ratio of 700–900 at ±1 V, is also obtained for the CuPc(SO₃^–)₄ salt of 4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium (SAM 2). The properties are dependent upon the D–π–A⁺ moieties which, for these highly rectifying salts, have sterically locked non-planar structures causing the conjugation to be effectively broken. Its effect on the electrical asymmetry is less spectacular when the cationic species is sterically unhindered: the rectification ratio decreases to 15–70 at ±1 V for films of the 4-[2-(4-dimethylaminophenyl)-vinyl]-pyridinium salt (SAM 3), which has single-ring substituents on opposite sides of the –CH=CH– bridge and an almost planar D–π–A⁺ structure. Rectification ratios from the sterically hindered structures are on a par with electrical asymmetries from metal–insulator–metal (MIM) devices where oxide-induced Schottky barriers dominate the behaviour.

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Introduction

Molecular electronics concerns organic counterparts of electro-active components (diodes¹ and switches²) as well as connectors (molecular wires³) but their characteristics fall short of those of macroscopic inorganic devices. For example, when diode-like molecules are aligned by self-assembly^4–9 or Langmuir–Blodgett (LB) deposition^7a,10–13 and contacted by non-oxidizable electrodes, rectification ratios of 2–30 at ±1 V are typical but ratios in excess of 100 have been reported.^14,15 The molecular origin of the electrical asymmetry has been verified from an orientation induced dependence of the bias for rectification^7b,10a and from chemically induced switching of the rectifying behaviour as the donor/acceptor combination is disrupted and then restored.^5–7a,14 There are also many unsubstantiated claims of molecule-induced rectification where the contacts are oxidizable, for example, titanium,^16–18aluminium^19–21 or lead:²² the electrical asymmetry arising from the organic layer is overwhelmed by oxide-induced Schottky barrier effects and assignment of the properties is ambiguous. Nonetheless, early reports of molecular rectification from LB films of C₁₆H₃₃–Q3CNQ, Z-β-(N-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide, contacted by silver-coated magnesium,²³ have since been verified by sandwiching the films between gold electrodes¹² and by scanning tunnelling spectroscopy.^7a Rectification has also been established from STS studies on self-assembled Au–S–C_nH_2n–Q3CNQ films contacted by alkanethiolate-coated gold probes, which locate the active D–π–A moiety midway between the electrodes and, as far as possible, provide symmetrical coupling to each.^7b The rectifying behaviour is independent of the length of the flexible linking group for 3 ≤ n ≤ 12 but is dependent upon the orientation, the higher current being observed in opposite quadrants of the I–V plot when the SAM is attached to the gold-coated substrate electrode (acceptor up) or the probe (acceptor down). The rectifying behaviour is also affected by protonation which disrupts the donor/acceptor combination, the I–V characteristics being symmetrical when the films are exposed to acidic vapour and exhibiting electrical asymmetry once more when exposed to base.^7a

Intrinsic rectification from organic materials arises when a donor/acceptor sequence is aligned between non-oxidizable electrodes. As long ago as 1964, Meinhard²² reported organic rectifying junctions formed from adjacent layers of donors and acceptors but the study was compromised by use of oxidizable contacts. A decade later, Aviram and Ratner¹ proposed rectification from donor and acceptor moieties linked via a σ-bridge but there are only three examples to date: two TCNQ–σ–donor derivatives aligned by the LB technique on platinum and contacted by silver-coated magnesium electrodes^24,25 and a TTF–σ–acceptor derivative contacted by alkanethiolate-coated mercury drop electrodes.²⁶ The rectifying behaviour of the former has an unexpected bias for rectification, which suggests that LB alignment is difficult to control with the flexible σ-bridge permitting scorpion-like arrangements with the acceptor over the back of the donor. Consequently, our work focuses upon π-bridged diodes and, from a systematic investigation of several types of molecule, we note that rectification is dependent upon the molecule being sterically hindered to effectively break the conjugation by enforcing a non-planar D–π–A structure.^4,6,14 Integrity of the donor and acceptor on opposite sides of the π-bridge is then maintained and, at forward bias, electrons tunnel from the cathode to lowest unoccupied molecular orbital of the acceptor on one side of the device and from the highest occupied molecular orbital of the donor to anode on the other.

There has been reluctance to accept the concept of molecular rectification but extensive studies now show it to be the rule rather than the exception for aligned assemblies of sterically hindered D–π–A molecules. The applied bias for rectification is dependent upon the molecular orientation^7b,10a and the electrical asymmetry may be switched off and on by protonating the molecular species.^5–7a,14 The challenge has now shifted to fabricating molecular systems with improved electrical asymmetries as typical rectification ratios are far too low to have any practical significance. In this work, we now report a method of self-organization that yields radically improved behaviour with ratios in excess of 3000 at ±1 V for Au–S–C₁₀H₂₀–A⁺–π–D|D^– structures where the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the anionic donor is copper(II) phthalocyanine-3,4′,4″,4‴-tetrasulfonate (Fig. 1). The rectification ratio is on a par with electrical asymmetries induced by metal oxide Schottky barriers, for example, 800 at ±2 V for Ti/TiO₂²⁷ and 1000 at ±2 V for Al/Al₂O₃,²⁸ and these ultra-thin organic films mimic conventional metal oxide diodes.

Fig. 1 Molecular structure of SAM 1: top, the copper(II) phthalocyanine-3,4′,4″,4‴-tetrasulfonate salt of the self-assembled cationic dye; bottom, the copper(II) phthalocyanine-3,4′,4″,4‴-tetrasulfonate counterion.

Results and discussion

SAM 1

Films of the rectifying device were obtained by molecular self-assembly of bis-[N-(10-decyl)-5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydroisoquinolinium]-disulfide diiodide and metathesis with the tetrasodium salt of copper(II) phthalocyanine-3,4′,4″,4‴-tetrasulfonate. Initial attempts to deposit the film focused upon a two-stage process but ionic exchange with CuPc(SO₃^–)₄ was unsuccessful as the iodide probably locates between the chemisorbed molecules and is inaccessible. Ionically coupled structures were instead obtained by immersing gold-coated substrates in an aqueous methanol solution of the component cationic and anionic dyes (4 : 1 mole ratio), the deposited films then being rinsed with solvent to remove any physisorbed material and washed with copious volumes of water to remove sodium iodide. Evidence of metathesis is provided by X-ray photoelectron spectroscopy (XPS ): peaks at 162 and 167 eV are distinctive of the binding energies of the two types of sulfur (S 2p, Fig. 2) present in the gold thiolate link and sulfonate groups of the cationic and anionic species, respectively, and areas under the curves are in a ratio of ca. 1 : 1 consistent with the molecular structure. Peaks that correspond to the Na⁺ and I^– ions are either weak or not evident from spectra obtained whereas those at 533 eV (O 1s) and 935 eV (Cu 2p) are unique to the CuPc(SO₃^–)₄ counterion that replaces the iodide.

Fig. 2 XPS data for SAM 1 showing peaks at ca. 162 and 167 eV (S 2p) that correspond to the binding energies of the gold thiolate link and sulfonate groups, respectively of the component ions.

Verification of metathesis is also provided by the UV/visible spectrum of the SAM on a 10 nm thick platinum coating on a glass substrate, the platinum being preferable to gold as it has a simple absorption profile that may be subtracted to provide the spectrum of the SAM alone (Fig. 3). The spectrum exhibits a transition at 675 nm that is characteristic of the Q band of CuPc(SO₃^–)₄, another at 470 nm that corresponds to the charge-transfer band of the D–π–A⁺chromophore and a higher energy transition at 350 nm that probably relates to both ions. The middle band may be compared with LB film spectra of long tailed alkyl derivatives of the D–π–A⁺ dye, which exhibit maxima at 415–450 nm and are transparent above ca. 540 nm when the counterion is non-absorbing.²⁹ The shift in the absorption maximum is not unexpected and probably results from different molecular tilts and overlaps in the SAM and LB film.

Fig. 3 UV/visible spectrum of SAM 1 on platinum-coated glass corrected for the absorption of the substrate.

Deposition was monitored from the frequency change of gold-coated 10 MHz quartz crystals as a function of the immersion time in a solution of the component ions (Fig. 4a). The frequency saturates to a constant value after ca. 60 min and, when analysed by the Sauerbrey technique,³⁰ the data correspond to a limiting area of ca. 1.6 nm² molecule^–1 for [Au–S–C₁₀H₂₀–A⁺–π–D]₄[CuPc(SO₃^–)₄]. Comparison with the van der Waals cross-sections of the components indicates that the self-assembled and ionically coupled species form separate layers: there is insufficient space for coexistence in the same layer and the data at saturation correspond to areas of 0.40 and 1.6 nm², respectively. Thicknesses were obtained from surface plasmon resonance (SPR) studies of SAMs on gold-coated BK7 glass substrates index matched to BK7 glass prisms, the films being adsorbed for periods of ca. 2 h prior to investigation. Data were collected using an attenuated total reflection geometry,³¹ the angle of incidence of a p-polarized laser beam being varied and the SPR spectrum corrected for reflections at the entrance and exit faces using Fresnel reflection formulae (Fig. 4b). A two-layer analysis of the SPR data, with all parameters unrestrained, yielded a thickness of 2.8 ± 0.2 nm and dielectric permittivities of ε_r = 2.1 ± 0.1 and ε_i = 1.0 ± 0.1 for the organic layer where the high imaginary component of the dielectric permittivity arises from the intense absorption of the CuPc(SO₃^–)₄ anion at an excitatory wavelength of 632.8 nm. The thickness is consistent with the anion forming a separate layer and it contrasts with 1.9 nm obtained for SAMs of the iodide salt in which spherical anions probably locate between the D–π–A⁺ moieties. The area and thickness suggest a vertical arrangement of self-assembled cationic chromophores with the planar counterions being on edge and tilted towards the substrate. The dimensions yield a density of ca. 1.0 Mg m^–3, which is within the expected range for molecular close packing.

Fig. 4 Molecular dimensions in SAM 1. (a) Mean molecular area derived from the frequency change of a gold-coated 10 MHz quartz crystal versus the period of immersion in an aqueous methanol solution of the cationic (2 × 10^–4 M) and anionic (5 × 10^–5 M) dyes. (b) SPR spectra of glass|Au (squares, left) and glass|Au|monolayer (crosses, right) structures at an excitatory wavelength of 632.8 nm where the solid lines correspond to the following fitting parameters: Au, d = 47.9 nm, ε_r = –11.4 and ε_i = 1.43; SAM, d = 2.8 nm, ε_r = 2.1 and ε_i = 1.0.

A Nanoscope IV MultiMode scanning tunnelling microscope was used to obtain STM images of the ionically coupled Au–S–C₁₀H₂₀–A⁺–π–D|[CuPc(SO₃^–)₄]_1/4 films on gold-coated mica and gold-coated highly oriented pyrolytic graphite (HOPG), the images being obtained using a gold probe and a set point current of 0.1 nA at a sample bias of –1 V. All films exhibit “streaks” approximately 2 nm long, as shown in Fig. 5, that probably correspond to the van der Waals dimension of the edge of the phthalocyanine anion. This is consistent with surface located arrangements indicated by the thickness and area from SPR and quartz crystal studies, respectively. The STM images reveal a highly disordered surface arrangement, which is not unexpected as the positive charges of the underlying cationic lattice are not ideally placed to locate the four SO₃^– groups of CuPc(SO₃^–)₄. Consequently, STM images of the edge-on arrangement of anions compare less favourably than those previously reported for phthalocyanine molecules that adsorb face-down on Au(111) substrates.³²

Fig. 5 A 10 nm × 10 nm STM image of the highly disordered surface of SAM 1 on Au(111) obtained for a set point current of 0.1 nA and sample bias of –1 V. The Au–S–C₁₀H₂₀–A⁺–π–D|[CuPc(SO₃^–)₄]_1/4 structure, albeit disordered, exhibits surface features that are consistent with the van der Waals dimension of the phthalocyanine moiety viewed on edge. The arrangement probably arises from a non-ideal distribution of positive charges in the cationic layer that locates the SO₃^– groups of the phthalocyanine.

I–V characteristics of SAMs on gold-coated HOPG were investigated by landing the probe on targeted features from which data were collected by averaging multiple scans on the same spot and the reproducibility confirmed by varying the set point current and voltage for these scans. Specific sites across each of four films were investigated and, without exception, every one exhibited electrical asymmetry unparalleled by any other organic diode. They exhibit rectification ratios in excess of 3000 at ±1 V when contacted by either Au or PtIr probes (Fig. 6), there being a fairly close match between the work functions of the latter (5.5 eV)³³ and the gold substrate (5.1–5.3 eV).³⁴ The rectification ratio (RR) is almost three orders of magnitude higher than obtained from SAMs of the iodide salt of the same dye (RR ≈ 5 at ±1 V).^5a It is also 400 times higher than the upper limit reported for monolayer structures aligned by LB deposition (2–8 at ±1 V),^7a,10–13 the films in each case being contacted by non-oxidizable electrodes, and is more than 20 times higher than the upper limit so far reported for self-assembly (4–150 at ±1 V).^4–9,14 However, ratios as high as 450 at ±1 V have been obtained by ionically coupling cationic molecules on anionic surfaces: Au–S–C₃H₆–SO₃^–|A⁺–π–D.¹⁵ The much improved behaviour reported here is attributed to two key factors: (i) a non-planar D–π–A⁺ moiety induced by the CH₂CH₂CH₂ link between the heterocycle and π-bridge is vital as out-of-plane rotations of the donor and acceptor disrupt the conjugation and maintain the integrity of these electroactive end groups; (ii) location of the CuPc(SO₃^–)₄ anion at the surface increases the electron-donating capability of the electroactive device, which is represented here as Au|Au–S–C₁₀H₂₀–A⁺–π–D|D^–|Au.

Fig. 6 Molecular structure of SAM 1 and I–V characteristics obtained using a set point current of 0.8 nA at –40 mV. The bias is designated by the sign of the substrate electrode and the higher current in the negative quadrant corresponds to electron flow from the gold-coated substrate to the contacting tip. Similar behaviour is obtained when the set point current and voltage are varied: they affect the magnitude of the current by influencing the distance between probe and surface but have minimal effect on the rectification ratio.

SAM 2

Rectifying structures were also obtained by self-assembly and ionic coupling of 1-(10-acetylsulfanyldecyl)-4-{2-(4-dimeth-ylaminonaphthalen-1-yl)-vinyl}-quinolinium iodide and the tetrasodium salt of CuPc(SO₃^–)₄ from aqueous methanol solution to which two drops of ammonia solution were added to facilitate removal of the acetyl group from the former. Evidence of metathesis is provided by the UV/visible spectrum and XPS studies, the phthalocyanine Q-band being observed at ca. 670 nm and the distinctive binding energies of the two types of sulfur of the component ions occurring at 162 eV for the Au–S link of the cationic dye and at 167 eV for the SO₃^– groups of the counterion. The films exhibit high rectification ratios of 700–900 at ±1 V when contacted by Au or PtIr probes and characteristic behaviour is shown in Fig. 7. The direction of electron flow at forward bias, as also found for SAMs 1 and 3, is from substrate to probe: it corresponds to electron tunnelling from the cathode to lowest unoccupied molecular orbital of the heterocyclic acceptor on one side of the device and from the highest occupied molecular orbital of the phthalocyanine donor to anode on the opposite side. Rectification arises as a result of an energy mismatch between the Fermi and molecular levels at reverse bias and we note that the rectification ratio of SAM 2 shows significant enhancement compared with values of 50–150 at ±1 V recently obtained for the iodide salt of this cationic moiety.¹⁴

Fig. 7 Molecular structure of SAM 2 and I–V characteristics obtained using a set point current of 1 nA at –40 mV. The bias is designated by the sign of the substrate electrode and the higher current in the negative quadrant corresponds to electron flow from the gold-coated substrate to the contacting tip.

SAM 3: reduced steric hindrance

Previous studies have demonstrated the relationship between steric hindrance and the rectifying behaviour of D–π–A moieties^4,6,14 and we now focus on an unhindered cationic dye, 1-(10-acetylsulfanyldecyl)-4-{2-(4-dimethylamino-phenyl)-vinyl}-pyridinium, with single ring substituents on opposite sides of a –CH=CH– bridge. This induces an almost planar structure as confirmed by its X-ray crystal structure.³⁵ Films of its CuPc(SO₃^–)₄ salt were formed as before and a Sauerbrey analysis yielded an area of ca. 0.4 nm² molecule^–1 for [Au–S–C₁₀H₂₀–A⁺–π–D][CuPc(SO₃^–)₄]_1/4 structures, the metathesis being confirmed by XPS and UV/visible studies. The area confirms insufficient space for both components to occupy the same layer and it is assumed that the film structure comprises a chemisorbed cationic species with its anions located at the surface.

Planar π-bridged chromophores should not rectify as it is necessary to maintain the integrity of the electroactive ends and, when conjugated, it is probable that aromatic and quinoid forms [A⁺–π–D ⇔ A=π=D⁺] coexist with only a small energy gap between the ground and excited states. Consequently, SAMs of the iodide salt of this almost planar cationic dye exhibit symmetrical I–V curves but this does not apply to this ionically coupled structure with a phthalocyanine donor as the Au|Au–S–C₁₀H₂₀–A⁺–π–D|D^–|Au device is also a rectifying junction. The films exhibit electrical asymmetry, albeit with much lower rectification ratios of ca. 15–70 at ±1 V compared with those of the aforementioned films, and the induced behaviour, as shown in Fig. 8, is attributed to the rectifying junction formed between the component parts. The direction of electron flow at forward bias is consistent with the aligned cationic layer being electron accepting and the phthalocyanine layer being electron donating. This is consistent with the behaviour shown in Fig. 6 and 7 but the much higher rectification ratios obtained for SAMs 1 (>3000 at ±1 V) and 2 (700–900 at ±1 V) may be attributed to the combined effect of a cationic molecular diode and a rectifying junction formed between the cationic and anionic components.

Fig. 8 Molecular structure of SAM 3 and I–V characteristics obtained using a set point current of 0.8 nA at –40 mV. The bias is designated by the sign of the substrate electrode and the higher current in the negative quadrant corresponds to electron flow from the gold-coated substrate to tip.

Conclusion

Rectification ratios reported in this work are substantially higher than intrinsic ratios reported from other organic films and those of SAM 1 and 2 are on a par with characteristic ratios from MIM structures where the behaviour arises from oxide-induced Schottky contacts. For oxidizable contacts, McCreery et al.²⁷ obtained symmetrical I–V curves from Au|organic|Ti structures when fabricated and studied in ultra high vacuum as well as rectification ratios of 800 at ±2 V induced by traces of oxygen in the vacuum chamber. Similar data have also been reported by Ashwell, Bonham and Lyons²⁸ for films contacted by aluminium and investigated before and after exposure to air, the induced ratio this time being 1000 at ±2 V. In contrast, rectification ratios in excess of 3000 at ±1 V have now been obtained by locating an anionic donor adjacent to the electron-donating end of a cationic D–π–A⁺ dye, the latter being connected to and aligned by a gold substrate. Reproducibility of alignment and an unparalleled electrical asymmetry signify that the properties of molecular electronic devices with nano-scale dimensions are now beginning to compete with their bulk inorganic counterparts.

Materials and substrate preparation

Iodide salts of the three self-assembling cationic dyes were synthesized as previously described^5a,14 and gave satisfactory analytical data. The CuPc(SO₃^–)₄ salt was supplied by Sigma Aldrich and used without further purification.

Gold-coated 10 MHz quartz crystals (International Crystal Manufacturing Co.) and Au(111) coated substrates on mica (Molecular Imaging Corporation) were sequentially washed with chloroform, propan-2-ol and water. BK7 glass and HOPG (Structure Probe Inc.) substrates were coated with gold overlays at Cranfield using a BOC Edwards 306 evaporator, the former to an optimum thickness of 47 nm for SPR studies. The coated substrates were stored in the solvent used for self-assembly and then plasma cleaned just prior to use using a PlasmaPrep 2 (Gala Instrumente).

Acknowledgements

We thank the Engineering and Physical Sciences Research Council (EPSRC), Royal Society/Wolfson Foundation and Leverhulme Trust for financial support, the EPSRC National Mass Spectrometry Service for providing analytical facilities, and Ben Robinson, Abdul Mohib and Anne Whittam for technical assistance.

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