Formation of Electronic Junctions on Molecularly Modified Surfaces by

Feb 2, 2009 - It is based on the lift of predefined contacts of silver paste on a water surface and their pickup and float on a soft surface by capill...
0 downloads 0 Views 539KB Size
Download PDF
Langmuir 2009, 25, 3305-3309

3305

Formation of Electronic Junctions on Molecularly Modified Surfaces by Lift-and-Float Electrical Contacts I. Mohamed Ikram, M. K. Rabinal,* M. N. Kalasad, and B. G. Mulimani† Department of Physics, Karnatak UniVersity, Dharwad-580003, Karnataka State, India ReceiVed October 24, 2008. ReVised Manuscript ReceiVed December 27, 2008 Here, we report a simple method of forming electrical contacts on soft surfaces of organic monolayers and organically capped nanoparticles. It is based on the lift of predefined contacts of silver paste on a water surface and their pickup and float on a soft surface by capillary force. Three different surfaces of silicon—hydrogen terminated, covalently bonded organic molecules, and a thin film of organically capped CdSe nanoparticles—were used to constitute electronic junctions by lift and float of individual contacts. Charge transport measurements clearly demonstrate that these junctions are free from shorting and wrinkling of the top contact and damage of molecular films. Hence, the method is simple, effective, nondestructive, and economical to form electronic junctions on molecular surfaces.

1. Introduction Organic molecule(s) carefully connected between two external electrodes constitutes a molecular junction,1-3 which can be divided into three active components: a molecular core and two interfaces that exist between molecules and electrodes.4 Experimental evidence shows that the interfaces are as important as the molecular core in transporting electronic charge under an applied electric field, hence the physics/chemistry at the interface is a critical element in the design of molecular junctions.4-7 The major challenge here is to establish tailor-made and reliable electrical contacts on organic molecules (monolayer) supported on metal/semiconductor surfaces.8 Unlike inorganic counterparts, organic monolayers are fragile and are quite sensitive to processing conditions that occur while forming metal contacts.9-11 Most commonly, top contact is formed by thermal/electron-beam/ sputtering evaporation of metals under vacuum, and metal vapors at elevated temperature cause serious problems such as penetration, damage, shorting and sometimes permanent change in the chemical composition of the monolayer.12-14 Further, metal penetration leads to a wider interface (band bending/depletion width) that is impossible to accommodate in molecular lengths. By trial and error, thermal evaporation is still a popular choice * Corresponding author. Email: [email protected]. † Present Address: Gulbarga University, Gulbarga, Karnataka State, India.

(1) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950–957. (2) McCreery, R. L. Chem. Mater. 2004, 16, 4477–4496. (3) Bumm, L. A. ACS Nano 2008, 2, 403–407. (4) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Matter. 1999, 11, 605–625. (5) Cahen, D.; Kahn, A.; Umbach, E. Mater. Today 2005, 8, 32–41. (6) Crispin, X.; Geskin, V.; Cripsin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Bredas, J.-L. J. Am. Chem. Soc. 2002, 124, 8131–8141. (7) Kushmerick, J. G. Mater. Today 2005, 8, 26–30. (8) (a) Ratner, M. Nature 2000, 404, 137–138. (b) Tredgold, R. H.; Vickers, A. J.; Allen, R. A. J. Phys. D 1984, 17, L5–L8. (c) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836–1847. (9) Xue, Y. Q.; Datta, S.; Ratner, M. A. J. Chem. Phys. 2001, 115, 4292–4299. (10) Lioubtchenko, D. V.; Markov, I. A.; Briantserva, T. A. Appl. Surf. Sci. 2003, 211, 335–340. (11) Ohgi, T.; Sheng, H.-Y.; Dong, Z.-C.; Nejoh, H. Surf. Sci. 1999, 442, 277–282. (12) Fauple, F.; Willecke, R.; Thran, A. Mater. Sci. Eng. R 1998, 22, 1–55. (13) Birgerson, J.; Fahlman, M.; Broms, P.; Salaneek, W. R. Synth. Met. 1996, 80, 125–130. (14) (a) Herdt, G. C.; Czanderna, A. W. J. Vac. Sci. Technol. A 1994, 12, 2410–2414. (b) Fisher, G. L.; Walker, A. V.; Hooper, A. E.; Tighe, T. B.; Bahnck, K. B.; Skriba, H. T.; Reinard, M. D.; Haynie, B. C.; Opila, R. L.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 2002, 124, 5528–5541.

of researchers in constituting molecular junctions.15 Special methods such as scanning tunneling microscopy,16 conductive force microscopy,17 electrical/mechanical assisted nanojunctions,18,19 and Hg-droplets20 are also being used to characterize these junctions. Simple and easily scalable approaches such as lift-off float-on (LOFO),21 soft-lithographic methods: nanotransfer printing (nTP),22 and soft-contact laminations (ScL),23 are becoming increasingly popular.24 Although these are highly promising, they suffer from wrinkling and tearing of contacts during the process of contact formation. Recently Shimizu et al. in their pioneering work, have suggested the polymer-assisted lift-off (PALO) method to avoid wrinkling and tearing of gold contacts.25 To lift contacts, the method requires chemical etching in 45% KOH and acetic acid solutions. For gold this may be a mild treatment, but certainly it is too harsh for other metals, such as Ag, Al, Cr, Ni, Cu, which are essential for low-cost fabrication of molecular junctions. Then these metals react vigorously with (15) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075–5085. (16) (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323– 1325. (b) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571–574. (c) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705–1707. (d) Klein, D. L.; McEuen, P. L. Appl. Phys. Lett. 1995, 66, 2478– 2480. (17) (a) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549–5556. (b) Rawlett, A. M.; Hopson, T. J.; Nagahara, L. A.; Tsui, R. K.; Ramachandran, G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81, 3043–3045. (18) (a) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. W.; Mayor, M.; Lo¨hneyson, H. V. Phys. ReV. Lett. 2002, 88, 176804-1–176804-4. (b) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L. Nature 2000, 407, 57–60. (19) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252–254. (20) (a) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257–7261. (b) Hiremath, R. K.; Mulimani1, B. G.; Rabinal, M. K.; Khazi, I. M. J. Phys.: Condens. Matter 2007, 19, 446003. (1-12). . (21) (a) Vilan, A.; Cahen, D. AdV. Funct. Mater. 2002, 12, 795–807. (b) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166–168. (22) (a) Loo, Y. L.; Willet, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654–7655. (b) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562–564. (23) Loo, Y.-L.; Someya, T.; Baldwin, K. W.; Bao, Z.; Ho, P.; Dodabalapur, A.; Katz, H. E.; Rogers, J. A. Proc. Natl. Acad. Sci. USA 2002, 99, 10252–10256. (24) Hsu, J. W. Mater. Today 2005, 8, 42–54. (25) Shimizu, K. T.; Fabbri, J. D.; Jelincic, J. J.; Melosh, N. A. AdV. Mater. 2006, 18, 1499–1504.

10.1021/la8035488 CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

3306 Langmuir, Vol. 25, No. 5, 2009

Ikram et al.

alkalis and acids to form metal-hydroxides and halides;26 such contamination forbids electronic contact to molecules and becomes unsuitable. Further, the PALO method is developed for extended contacts (strips-assembly) and is not suitable to form individual contacts (dots). To overcome these difficulties, herein we report the extension of the PALO method to form contacts using silver paste; thermal evaporation of various metals can also be used. The method combined with capillary force helps to pick up individual contacts and float them on soft surfaces with a high success rate.

2. Experimental Details The glass plates were cleaned with a piranha solution in an ultrasonic bath and rinsed thoroughly with deionized water. Dilute solution of starch (2 g in 50 mL of deionized water) was spin-coated (1500 rpm) on a glass plate, and later it was annealed at 40 °C for half an hour in an oven to remove moisture. Surface morphology of these films was studied by scanning electron microscopy (SEM) using a JEOL JSM-840A. Commercial conducting silver paste is used to form circular contacts (1.5 mm in diameter) on starch film by screen-printing. A regular pattern of circular holes was created in a thin strip (2 cm width, 6 cm length and 40 µm thick); this was tightly fixed on the starch surface by firm fixing of its ends by cellophane tape. Silver paste was spread with a blade edge to get a uniform coating and was dried in room light; finally the strip was removed carefully. To lift individual contacts, the above glass plate was immersed in a beaker containing deionized water, after 5-10 min contact float on the water surface. Individual contacts were picked up by a capillary tube to transfer them to fresh water, and this process was repeated thrice to remove starch contamination. Finally, each contact was lifted and floated on silicon surfaces having different surface conditions. Commercial silicon (single crystal, (100) orientation, p+-type, 0.001 Ω-cm resistivity, boron doped, and one side mirror polished, supplied by Semiconductor Wafer, Inc., Taiwan) was used as the starting material. A piece of silicon was degreased in hot trichloroethylene (AR grade). It was etched in a dilute solution of hydrofluoric acid (HF) for 3 min in order to remove native oxide and to get hydrogen termination (H-Si), and was rinsed thoroughly with copious amount of deionized water. On polished surfaces, silver contacts were floated to form metal-silicon junctions. The H-Si was used as a starting material to bind organic molecules and to deposit CdSe nanoparticles. Propargyl chloride (70 wt% solution in toluene) was procured from Sigma-Aldrich. Thermal attachment of organic molecules was carried out under inert atmosphere using our home-built setup at 55 °C for 2 h.20b Then silicon was thoroughly washed with ethanol (AR grade) to remove physically adsorbed organic molecules, then silver contacts were immediately floated to form metal-molecules-silicon junctions. Binding of organic molecules on silicon surfaces was confirmed by X-ray photoelectron spectroscopy (XPS) measurements using a VG Scientific, ESCALAB MK-IV spectrometer with an Al KR source (1486.6 eV) at the takeoff angle of 35° under 5 × 10-9 torr vacuum. Thickness of the monolayer was measured by ellipsometery using a SENTECH SE850 at an incident angle of 60° with λ ) 670 nm, and the refractive index of the monolayer was taken as 1.42.20b Estimated thickness was 0.46 nm, which is close to the expected thickness of molecules when they stand up vertically on a silicon surface. An aqueous route is used to synthesize CdSe nanoparticles (2-3 nm) capped by 3-mercaptoproponic acid,27 then extracted particles were redispersed in deionized water (2 mg of CdSe powder in 2 mL of water). It was spin coated (1000 rpm) on a silicon surface and annealed at 40 °C to remove moisture. Silver contacts were immediately floated to constitute metal-nanoparticles-silicon (26) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced Inorganic Chemistry; Wiley Interscience Publications: Singapore, 1999. (27) Rogach, A. L.; Kornowski, A.; Gao, M.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 1999, 103, 3065–3069.

Figure 1. The method of making float contacts on molecularly modified/ soft surfaces: (a) cleaned glass plate as starting substrate; (b) glass plate spin-coated with a thin layer of starch solution as a sacrificial layer; (c) metal contacts (dots of 1.5 mm diameter) are formed by screen printing of conductive silver paste, and similar contacts of aluminum were also deposited by thermal evaporation under vacuum using shadow mask; (d) dissolution of sacrificial layer in water, lifting of metal contacts on water surface, and capillary pickup of these contacts by capillary force; (e) smooth release of metal contacts on bare, molecularly modified and nanoparticle (CdSe)-coated silicon surfaces to constitute electronic junctions; (f) series of electronic junctions; and (g) photograph of molecular junctions on silicon surface formed by float contacts of silver dots.

junctions. Ellipsometric estimation of thickness is 20 ( 2 nm. Current-voltage measurements on different electronic junctions were measured using Keithley 617 and 6512 electrometers. For all the junctions, forward bias means that the bottom silicon is positive with respect to the top metal dot. All the current-voltage measurements were carried out in our home-built conductivity setup that has a provision to make electrical contact to a top metal dot of a junction by lowering a fused palladium tip through a fine screw mounted on a cantilever. Further, to avoid damage to the top contact, a thin indium pad was attached to the palladium tip using silver paint.

3. Results and Discussion 3.1. Formation of Electrical Contacts by the Lift-and-Float Method. The process begins with preparation of a thin sacrificial layer of starch on a clean glass plate (Figure 1b). A dilute solution of starch was spin coated and annealed at 40 °C to remove stress due to solvent evaporation. A sacrificial layer is critically important in obtaining better electrical contacts, and it must possess certain qualities: (a) a smooth and homogeneous surface, (b) be chemically inert to metal contacts, and (c) quick dissolution and better solubility in a given solvent. Starch meets all these requirements; in addition, it is an abundant, low-cost, and watersoluble material. Hence a wide number of metals can be chosen as electrical contacts to form molecular junctions by lift and float. The surface morphology of starch film annealed at 40 °C was studied by SEM, which is shown in Figure 2. It is smooth and hence can be a better supporting layer to obtain flat contacts.

Lift-and-Float Based Formation of Electronic Junctions

Langmuir, Vol. 25, No. 5, 2009 3307

has a small quantity of water in it. While lifting the contact, its bottom surface in contact with the liquid experiences a downward pull.30 Hence, the resultant lift force (Flift) is given by30,31

Flift ) 2πσrt cos φ - 2πR√Flgσ

Figure 2. SEM image of an annealed starch-coated glass plate.

Next, metal dots (circular) of conducting silver paste were deposited on starch film by screen-printing and were dried under a table lamp (Figure 1c). Similar contacts of aluminum were also deposited by thermal evaporation under vacuum (10-6 Torr) on starch film and were successfully lifted on the water surface to float on a given surface. Here the results obtained with silver paste will be discussed as the low-cost fabrication of molecular junctions. The next step is a smooth detachment of contacts, for which glass with contacts was immersed in a beaker containing deionized water. After 5-10 min, the starch layer is dissolved in water, and, as a result, the metal dots lift and float on water surface (Figure 1d). The measured thickness of the dots is 40 µm, and they float easily on the water surface. In the earlier work on the lift-and-float kind of contacts, attention was paid to restrict the thickness of contacts below 20-30 nm for easy lift and float in a liquid.21,25 However, such small thickness creates associated problems such as tearing and wrinkling of contacts while forming molecular junctions. A simple argument helps to find the critical thickness of a contact that can float on a liquid surface. It is well-known that a body partially submerged in a liquid experiences a net upward force (Fup), composed of force due to surface tension and buoyancy,28 which is opposite to the force due to the body’s own weight (Fdown). If Fup g Fdown, the body floats; otherwise it sinks. In the case of small bodies, the vertical component of surface tension force, created at the perimeter of body in contact with liquid, is a deciding factor for floating.29 A metallic contact (dot) having a radius R and thickness t, the condition for floating is mg ) 2πRσ sin θ (here buoyancy force is assumed to be negligible), where m is the mass of contact, g is the gravitational constant, 2πR is the perimeter of contact, σ is the surface tension of liquid, and σ sin θ is the vertical component of surface tension, for dot θ is close to 45°. In terms of critical thickness (tc) tc ) 2σ/RFg, where F is density of metal. For typical values of various parameters for silver as a contact and water as a liquid, we get tc) 1.486 mm, and similarly for gold-water it is 0.717 mm. These values obviously suggest that contacts much thicker than a nanometer can float on water surface, hence the contacts tearing and wrinkling can be avoided easily. The important and crucial step is the transfer of lift contacts on molecularly modified/soft surfaces to constitute electronic junctions. Here we adopted easy pickup of lift contact by capillary force using a small glass tube (1 mm inner diameter and 0.2 mm wall thickness). By placing the capillary on the contact surface with a gentle touch and immediately closing of upper end with a finger, the contact can be lifted from liquid (Figure 1d). In this process it is observed that liquid covers the top surface of the contact and makes a physical touch with the capillary and tries to rise upward. It is always noticed that after lift the capillary (28) (a) Hu, D. L.; Bush, J. W. M. Nature 2005, 437, 733–736. (b) Gao, X.; Jiang, L. Nature 2004, 432, 36–36. (29) Mansfield, E. H.; Sepangi, H. R.; Eastwood, E. A. Phil. Trans. R. Soc. London A 1997, 355, 869–919.

(1)

The first term is due to surface tension, and second term is due to downward pull. Here rt is the radius of the capillary tube (0.5 × 10-3 m), φ is the contact angle of water with the glass, taken as zero, Fl is the density of water (998 Kg · m-3), and the other terms have their usual meanings. Using typical numbers, force due to surface tension is 226 µN, and force due to downward pull 93.6 µN. It shows that capillary action can easily lift small metal contacts from a liquid surface. Then individual contacts were lifted by capillary tube and brought close to the surface, where they were to be fixed, and then released by a gentle touch while lifting the top finger (Figure 1e). A desired number of contacts can be formed on a given sample (Figure 1f). Before measuring electrical conductivity, the formed junctions were allowed to dry and were then annealed at 40 °C for half an hour to remove the moisture. To test the present technique, it was applied on single crystals of silicon (p+) having different surface conditions: (a) hydrogen terminated, (b) covalently bonded molecules of propargyl chloride (C3H3Cl), and (c) thin film of CdSe nanoparticles (spin coated). Once these surfaces were created, immediately silver dots were floated, as described above, to constitute metal-silicon, metal-moleculessilicon and metal-nanoparticles-silicon junctions, respectively. A photograph of molecular junctions formed on p+ silicon with float contacts of silver is shown in Figure 1g. 3.2. Charge Transport Measurements. 3.2.1. Metal-Silicon Junction. Transport measurements were carried out at room temperature on all the junctions. Current-voltage (I-V) curves recorded on metal-silicon junctions are shown in Figure 3a and are found to be pseudo-ohmic in nature. Here, five different junctions were formed on the same piece of sample whose curves are plotted in Figure 3a to compare reproducibility. There is a slight variation in I-V curves from junction to junction, and all the curves are more or less symmetric with respect to voltage. High doping density of silicon leads to a very weak barrier at the interface between metal and silicon, and hence it can be treated as a metal-to-metal junction (contact), even if there is a weak barrier that will be highly transparent for charge tunneling. The small dispersion in the curves could be due to chemical variation of the silicon surface after HF etching. From previous studies, it is a well-known that such chemical treatment gives a Si-Hx surface, which is highly prone to atmospheric contamination that leads to the formation of Si-H, Si-H2, Si-H3, and SiOx species.32 3.2.2. Metal-Molecules-Silicon Junction. Current-voltage curves on metal-molecules-silicon junctions are shown in Figure 3b. It is interesting to note that the curves recorded on different dots are almost identical and exactly overlap, and this suggests that molecular passivation generates a chemically homogeneous and kinetically stable surface of silicon. This is not a new phenomenon; recently it is demonstrated that covalent grafting of alkenes and alkynes on a silicon surface by wet chemistry produces stable monolayers.33-37 This is a simple and most effective approach to control surface states of a semiconductor.38 Quite interestingly, these monolayers are highly robust under (30) Brown, B. General Properties of Matter; Butterworths: London, 1969. (31) Alberty, R. A.; Daniels, F. Physical Chemistry, 5th ed.; Wiley Eastern Ltd.: Mumbai, India, 1984. (32) Nemanick, E. J.; Hurley, P. T.; Webb, L. J.; Knapp, D. W.; Michalak, D. J.; Brunschwing, B. S.; Lewis, N. S. J. Phys. Chem. B 2006, 110, 14770– 14778.

3308 Langmuir, Vol. 25, No. 5, 2009

Ikram et al.

Surprisingly, such analysis yields a reasonably low value of barrier height, which is significantly small compared to the expected barrier from the energy band diagram of molecular junctions.17a,43 Therefore, we believe a combination of tunneling and thermionic emission would be more appropriate to describe charge transport. Further, in metal-molecules-silicon (p+), applied voltage appears across the molecules, hence the Schottky emission plays the role. Therefore, charge transport can best be described as41

[

J ) A*T2 exp[-βd] exp

Figure 3. Current-voltage (I-V) curves of different electronic junctions: (a) Hydrogen terminated metal-silicon junction, each curve belongs to different junctions created on the same silicon surface; and (b) semilog plot of metal-molecules-silicon junctions by covalent attachment of propargyl chloride molecules, again the different curves here belong to different molecular junctions formed on the same silicon surface. Inset in panel (b) is the plot of Schottky emission.

adverse conditions, such as boiling in water, organic solvents, acids, and bases, and are free from atmospheric oxidation.35 Our work on molecular passivation of porous silicon, prior to metal formation, gave highly stable electronic junctions.39 The comparison of metal-silicon junctions with molecular junctions shows that the latter are less conducting and are symmetric with respect to voltage. This shows that the molecular core of propargyl chloride is symmetric for injection of charges either from silicon or from silver contacts. Such behavior is often noticed in molecular junctions,7 such as alkanethiols between metal electrodes.17,40 We attempted to fit the data of molecular junctions to various mechanisms of charge transport, such as thermionic emission, direct tunneling, Fowler-Nordheim tunneling, and space charge limited Poole-Frenkel emission.41,42 It is observed that the fit is poor either to tunneling or to thermionic emission. In most of the earlier work on molecular junctions, it is assumed that tunneling is the sole mechanism of charge transport.7,17,25,40 (33) (a) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631–12632. (b) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (34) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513– 11515. (35) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudholter, E. J. R. AdV. Mater. 2000, 12, 1457–1460. (36) Sun, Q. U.; de Smet, L. C. P. M.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudholter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 1352–1355. (37) Chen, R.; Bent, S. F. Chem. Mater. 2006, 18, 3733–3741. (38) Hiremath, R. K.; Rabinal, M. K.; Mulimani, B. G.; Khazi, I. M. Langmuir 2008, 24, 11300–11306. (39) Rabinal, M. K.; Mullimani, B. G. New J. Phys. 2007, 9, 440-1–440-9. (40) Wang, W.; Lee, T.; Reed, M. A. Phys. ReV. B 2003, 68, 035416-1– 035416-7.

-q(ΦBO - √qE ⁄ 4πεi) kBT

]

(2)

where J is the current density, A* is the Richardson constant, T is temperature, ΦBO is the ideal barrier height for the Schottky emission, E is the electric field across the molecules, εi is the dielectric constant of organic molecules, and kB is the Boltzmann constant. The new term exp[-βd] accounts for the tunneling through molecules.44 The β term is the tunneling constant, and d is the effective thickness of the monolayer. The β significantly depends on the structure and conjugation of the molecule and also on the nature of the interface between molecules and electrodes.44 It ranges from 0.72 to 1.0 Å-1 for alkanethiols, 0.4 to 0.7 Å-1 for certain aromatic molecules, and 0.1 to 1.4 Å-1 for biomolecules.45 Then the effective barrier height becomes Φeff ) Φb + (kBT/q)βd. The plot of ln(J) as a function of E1/2 gives a straight line, as shown in Figure 3b as an inset; the intercept gives Φeff ) 0.72 ( 0.005 eV. It agrees well with reported values for metal-molecules-silicon (p+) junctions.46 The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the present molecule is estimated using Gaussian software that is equal to 7.2 eV; combining this with electrode work functions, a much higher barrier height is expected than that observed. Such significant reduction in the value of barrier height is a common phenomenon for varieties of molecular junctions. This has been attributed to chemical interaction between molecules and contacts (superexchange interaction).44 Even in case of molecules of a wider HOMOLUMO gap (∼9 eV) between gold and silicon exhibited lower barriers (between 0.74 to 0.87 eV).44,46 3.2.3. Metal-Nanoparticles-Silicon Junction. Finally to test our technique, charge transport is studied in the case of metal-nanoparticles-silicon junctions. Current-voltage curves of different junctions on the same sample were measured; a little dispersion for junction to junction is observed and is attributed to lateral inhomogeneity in film thickness. A typical curve for one of the junctions is shown in Figure 4a. The behavior suggests that the junction is a good Schottky with a high rectification ratio (more than 4 orders of magnitude even for a small current) and very low leakage current. The best values of work function of silver, p+-Si, and CdSe nanoparticles (3-4 nm clusters) are chosen to draw the energy band diagram for this junction,41,47 as shown in Figure 4b. It is obvious that p+-Si is a good hole injecting contact with negligible barrier for CdSe nanoparticles. But, there (41) Sze, S. M. Physics of Semiconductor DeVices, 2nd ed.; Wiley-InterScience: Singapore, 2005. (42) Rohdrick, E. H. Metal-Semiconductor Contacts, 2nd ed.; Clarendon: Oxford, 1988. (43) Cui, X. D.; Zarate, X.; Tomfohr, J.; Primak, A.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Sankey, O. F.; Lindsay, S. M. Nanotechnology 2002, 13, 5–14. (44) Selzer, Y.; Solomon, A.; Cahen, D. J. Phys. Chem. B 2002, 106, 10432– 10439. (45) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075–5085. (46) Scott, A.; Janes, D. B.; Risko, C.; Ratner, M. A. Appl. Phys. Lett. 2007, 91, 033508-1–033508-3. (47) Fendler, J. H. Chem. Mater. 2001, 13, 3196–3210.

Lift-and-Float Based Formation of Electronic Junctions

Langmuir, Vol. 25, No. 5, 2009 3309

ΦB0, that is estimated to be equal to 0.683 eV ( 0.004. It is low compared to the value expected from the energy band diagram (1.1 eV), but such reduction is often observed for metal-semiconductor junctions.42,47 In the past, similar composite junctions of ITO-CdSe-PPV-Mg and ITO-PPV-CdSe-Mg were studied to understand the effect of metal work function on charge injection into CdSe nanoparticles.48 It is shown that metals of lower work function (Mg) acts as an efficient electron injector into nanoparticles. As a result, the latter junction has a considerably lower operating voltage than the former junction for light emission. From the reported data on these junctions, we estimated effective barrier heights using eq 3: for ITO-CdSe-PPV-Mg it is 0.9 eV, and for ITO-PPV-CdSe-Mg it is 0.44 eV.48 In the present measurements, Ag is put in the place of Mg, whose work function is higher by 0.5 eV; hence a higher barrier is expected for electron injection, and this is what is observed. Further, the present technique was also successfully extended to form strip contacts on molecular surfaces.

4. Conclusions

Figure 4. (a) Current-voltage (I-V) characteristics (semilog) of metal-nanoparticles (CdSe)-silicon junctions. This curve is representative of other molecular junctions formed on the same silicon surface. These junctions exhibit very low leakage current in the reverse region of voltage.

is a barrier of 1.1 eV for electron injection from the silver contact into nanoparticles. These junctions are expected to act like a Schottky barrier having thermionic emission as a process of charge transport,41 and its empirical equation42 is given by

(

J ) A*T2 exp -

) ( )[

( )]

qΦBO qV eV exp 1 - exp kBT nkBT kBT

(3)

in which the terms have their usual meaning. Plotting ln(J/[1 exp(-eV/kBT)]) versus V gives a straight line, from the intercept

In summary, a simple method of making float contacts on soft surfaces is suggested to constitute electronic junctions. Starch film coated on a glass plate serves as a sacrificial layer to lift the predefined silver contacts on a water surface, and these can be successfully picked up by capillary force to float on molecular/ soft surfaces. The usefulness of this technique is demonstrated by constituting electronic junctions on silicon surfaces that are terminated with hydrogen, organic molecules, and organically capped CdSe nanoparticles. Charge transport measurements clearly show that these junctions are free from shorting and wrinkling of the top contact and damage of molecular films. Hence, the method is a simple, effective, nondestructive, and economical way to form electronic junctions on soft surfaces. Acknowledgment. Authors thanks to the Department of Science and Technology, Government of India, for funding the project (under Contract No. SR/S2/CMP/-28/2002). I.M.I. gratefully acknowledges receipt of the Research Fellowship in Science for Meritorious Students from the University Grants Commission, Govt. of India. LA8035488 (48) Colvin, V. L.; Schlamp, M. C.; Allvisatos, A. P. Nature 1994, 370, 354– 357.