ARTICLE pubs.acs.org/Langmuir
Autonomic Self-Healing Lipid Monolayer: A New Class of Ultrathin Dielectric Carine Dumas, Racha El Zein, Herve Dallaporta, and Anne M. Charrier* Centre Interdisciplinaire de Nanoscience de Marseille, CINaM, UPR CNRS 3118, Aix-Marseille Universite, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France ABSTRACT: The electrical performance of stabilized lipid monolayers on H-terminated silicon is reported for the first time. We show that these 2.7 nm thick only ultrathin layers present extremely low current leakage at high electric field and high breakdown voltage that both compare favorably with the best data reported on organic thin film dielectrics. We demonstrate a very unique property of autonomic selfhealing of the layer at room temperature with the total recovery of its performance after electrical breakdown. The mechanisms involved in breakdown and self-healing are described.
1. INTRODUCTION The functionalization of inorganic substrates with monolayers of self-assembled molecules (SAMs) of long alkyl chains was a breakthrough in the 1980s. Their properties are still largely being exploited in a large range of applications, their functional group driving surface properties such as surface charge,1,2 wettability,3,4 or serving as a linker to bind other molecules.5,6 Several reasons have led researchers to investigate their use in electronic devices.7 9 In field effect transistors (FETs), the substitution of commonly used oxide gate dielectrics by an organic dielectric of SAMs was first motivated by the necessity to decrease the large operating voltage using thinner gate dielectric layers.10 SAMs with thicknesses of 2.5 nm were found to exhibit very large energy barriers to carriers tunneling of typically 4.5 eV, therefore bringing a solution to gate current leakage in smaller electronic devices. The fabrication of these layers at low temperature is also a major asset for flexible electronic devices that do not handle the high temperatures required in inorganic oxide layer manufacturing methods.11 Some recent works have also shown an interest in using such layers with specific headgroups as the semiconducting channel of the transistor.12 Phospholipids constitute another class of molecule that can be used to functionalize inorganic substrates as an alternative to SAMs of long alkyl chains or polymers. They are made of two hydrophobic alkyl chains and a hydrophilic headgroup. As phospholipids are the main components of biological membranes, they have attracted considerable interest among the scientific community, especially in the biomedical field. These bilayers, often used as model systems to study membrane proteins13 and ion channel formation,14 possess unique properties that are being exploited. For example, because of their amphiphilic nature, they naturally form vesicles in solution that can be used to encapsulate drugs or particles for drug delivery or in vivo targeting.15,16 Supported phospholipid monolayers and bilayers can also easily be formed on flat substrates r 2011 American Chemical Society
by direct vesicle fusion17 or Langmuir Blodgett transfer leading to very homogeneous flat surfaces.18 If such lipid layers have widely been used in biomedical applications, their inherent instability in air has long been a limitation to further use in other industrial applications. Our recent results on the immobilization and stabilization of a highly dense supported lipid monolayer in air directly at the surface of silicon19 overcome this difficulty and open a new world of investigations of such layers. In this paper, we investigate the electrical properties of these monolayers as an alternative to inorganic oxide dielectrics to be used in electronic devices such as field effect transistors. A few studies have reported on the electrical properties of supported lipid bilayers,20,21 but all of them were measured in solution, on nonpolymerized layers, unstable in air, such as black lipid membranes, for example, to understand processes involved in ion channel formation and functioning in cell membrane. In our system, the monolayer is stabilized by a polymerization process of the lipids in the plane of the surface and can be regarded as a two-dimensional ultrathin polymer of a few nanometers thickness. The performances of these monolayers are evaluated through current voltage (I V) electrical measurements in air. In addition to showing their high performance in terms of current leakage, breakdown voltage, and stability in temperature, we demonstrate a very unique property of autonomic self-healing after dielectric breakdown.
2. EXPERIMENTAL SECTION 2.1. Materials. 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC), Figure 1, was purchased from Avanti Polar Lipids, Received: June 21, 2011 Revised: August 25, 2011 Published: October 03, 2011 13643
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Figure 2. Current density vs applied electric field for a lipid monolayer (black) and a 2.2-nm-thick silicon oxide layer (gray).
Figure 1. Bottom: Scheme of the electrical measurement setup. Soft top contact is taken directly on the lipid layer using a gold electrode with low spring constant. The bottom contact is taken on top of silicon using eutectic InGa.Top: Scheme showing the polymerization of diacetylenic species. Alabaster, AL. A stock solution was prepared in chloroform at a concentration of 1 mg/mL (0.1%) and stored at 20 °C. Polymerization initiator (2,2-azobis(2-methylpropionamidine)dihydrochloride, AAPH), chloroform, methanol, sulfuric acid, 96% (H2SO4), hydrofluoric acid, 49% (HF) and hydrogen peroxide, 30% (H2O2) were purchased from Sigma Aldrich. Cleaning and etching were all clean-room grade. Deionized water (DI water, 18 MΩ) was used for all experiments. Surface Characterization. Thickness Measurements. The average thickness of lipid layers and silicon oxide were estimated using a Sentech model SE-400adv single wavelength (632.8 nm) ellipsometer at an angle of incidence of 70°. The thickness was obtained using a two-layer model with n = 1.46 as the refactive index of the lipid layer and silicon oxide and silicon substrate was described by n = 3.85 and k = 0.02. Surface Imaging. Tapping-mode atomic force microscopy (AFM, NTEGRA from NT-MDT) was used to image the surface of the samples in air. All images were obtained at a frequency of 1 Hz. Electrical Measurements. Electrical measurements were measured in quasi static mode by applying voltage sweeps between substrate and lipid layer, without load resistance, using Keithley 236. Electrical contact on H-terminated silicon substrate was taken using eutectic InGa (2 mm in diameter), and mechanical contact was done directly on the lipid layer using a gold top electrode with low spring constant. 2.2. Silicon Surface Preparation. Silicon wafers ((100), F ∼ 5 Ω.cm, boron doped) were cleaned in piranha solution (2:1 H2SO4/ H2O2) at 130 °C for 30 min, then rinsed with DI water. H-Terminated silicon surface is obtained after etching the native silicon oxide layer of the crystal with HF (2 min in 2%) followed by a quick rinse in DI water. A typical thickness of 0.5 Å is obtained. 2.3. DCPC Layer Formation. DCPC Vesicles Preparation. A total of 70 μL of lipid from the stock solution is slowly heated at 40 °C to evaporate the chloroform. An amount of 200 μL of DI water in then added to the vial to form a 0.03% lipid solution. Unilamellar vesicles are then formed by 15 min of sonication. Small unilamellar vesicles are finally obtained by extrusion of the lipid solution using 100 nm pore size membrane. Supported DCPC Monolayer Formation. Lipid monolayers were obtained by fusion of the vesicles directly at the surface of H-terminated silicon below its liquid gel phase transition temperature (43 °C).
This method which is simple and fast allows the fabrication of perfectly homogeneous layers with unlimited substrate size.19 Polymerization of the DCPC Monolayer. The polymerization process of diacetylenic species using free radicals or UV exposure is well-known and has already been described in several papers. The spatial proximity required for polymerization has been shown to yield significant interactions between neighboring, electron-rich diacetylene units giving rise to lateral association within the monolayer similar to that observed in systems showing hydrogen bonding, π-stacking, or dipole coupling.22 In our experiment, the polymerization is activated by addition of AAPH (free radical) after the formation of a dense layer and heating of the sample at 40 °C. A scheme of the polymerization of diacetylenic species is shown in Figure 1. The sample is finally rinsed with DI water and methanol to remove eventual overlayers. The quality of such prepared polymerized monolayers was investigated by atomic force microscopy (AFM), and topographic images showed a flat and homogeneous layer over the entire surface. The thickness of the layer was also measured by ellipsometry, and an average value (7 different samples) of 2.7 ( 0.3 nm was obtained smaller than the expected thickness of 4.0 nm for this type of lipid. This is actually in good agreement with the expected thickness of polymerized diacetylenic moieties for which a tilt angle of 45° from the surface plane is required to polymerize.22
3. RESULTS AND DISCUSSION 3.1. Electrical Measurements: I(V) Characteristic. The typical evolutions of current density versus applied electrical field (J E curve) measured at room temperature in air on a lipid monolayer and on a 2.2-nm-thick silicon wet oxide layer for comparison are reported in Figure 2. For the two dielectrics, these J E characteristics present similar behavior with symmetric curves but show, however, large differences in current densities of 2 orders of magnitude. Typically, these increase with applied electrical field from 4 10 9 A/cm2 at 0.5 MV/cm to 8 10 7 A/cm2 at 5 MV/cm for the lipid monolayer and from 4 10 7 A/cm2 at 0.5 MV/cm to 2 10 5 A/cm2 at 5 MV/cm for the silicon oxide layer. These later measurements on silicon oxide are in very good agreement with other data in the literature.23,24 To our knowledge, these current density measurements on polymerized lipid monolayers are a first and compare favorably with the best data obtained in organic thin film dielectrics, for which current density falls in the range of 10 6 to 10 8 A/cm2 at high electric field.25,26 The averaged lipid layer resistance, deduced from several J E characteristics recorded on different samples, is found to be equal to 300 MΩ at 1 MV/cm. The thermal stability of the layer was tested in a range of temperature going from 20 to 90 °C. The corresponding J V curves were 13644
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Figure 3. (a) Electrical breakdown obtained on 5 different lipid monolayers. (b) Current measured just after the breakdown with respect to the breakdown voltage.
Figure 4. (a) I(V) curves measured at different time after electrical breakdown. (b) Evolution of the monolayer resistance extracted from the I V curves in (a) at 1 MV/cm.
similar to the one performed at room temperature and did not show any dependence. 3.2. Electrical Breakdown. In dielectrics, breakdown voltage is a critical parameter that defines the maximum electric field that can be applied across the material before the insulator collapses and conducts. This parameter therefore defines the maximum working potential that can be applied to the insulator. It is generally characterized by a local increase in conductance and material degradation. Figure 3a shows the evolution of current with increasing voltage applied through five different lipid monolayers. Electrical breakdown is characterized by an abrupt increase of the current at electric fields in the range 6 10 MV/cm and an average value of 7.5 MV/cm. These results are extremely good regarding other measurements realized on supported lipid bilayers or black lipid membranes for which breakdowns were reported below 2 MV/cm.27,28 The main difference relies on the polymerization state of the lipid membrane. In these previous studies, the lipids were not polymerized, and like in biological membranes, they were highly mobile within the membrane, with diffusion rates that can be reasonably high depending on the temperature and on the lipid type. When an electric field is applied, electroporation occurs due to electrostatic repulsion and pores are formed within the membrane, therefore increasing its permeability to ions and charges.27,28 In the present case, the lipids are polymerized, their entropy is strongly reduced, and their mobility is limited to small fluctuations in their surrounding. In this case, the lipid layer has to be considered as an ultrathin two-dimensional polymer. In ultrathin polymers, the voltage at which breakdown occurs and therefore the electron energy are too low to consider molecule ionization and bonds breaking in the polymer. It has been shown that, in this case, electrical breakdown is not a critical event that occurs at a certain electricfield intensity characteristic of the considered polymer. It is a
kinetic process that develops in time, which is characterized by the damage accumulation rate and its inverse value, the lifetime of the polymer in the electric field; the measured electrical strength will be higher for higher increasing rate of the electric field.29,30 Some models have been developed to explain such breakdown, and it has been suggested that it is a consequence of a multistage process that leads to the breaking of macromolecules into free radicals. The initial injection of charges into the polymer is initiated by enhancement of the electric field at some asperities present on the electrode surface. These charges recombine with traps present in the polymer to build up a homospace charge that modifies the internal field distribution and causes local field enhancement in small volumes of the polymer. The resulting released energy of the charge recombination, which is approximately equal to the trap depth, is transferred to another electron to end up in the formation of hot electrons. Interactions of the macromolecules with these hot electrons lead to the homolytic dissociation of polymer macromolecules into free radicals and to the formation of pores in the layer in which electron can tunnel through.30,31 It was shown by Murashov and Green that, in diacetylenic polymers, dissociation occurs at a typical energy of ∼5 eV.32,33 In Figure 3b, the current after breakdown is reported versus the breakdown voltage. All the reported data were measured at the same increasing rate of the electric field. The variability of the results can only be explained by different lipid densities in the different samples for which it is very likely that pores develop more rapidly in low-density areas where fewer bonds are required to break. For a given type of polymer, it is therefore expected that the electrical strength shall be dependent on the molecular density of the polymer. A very noticeable point is that the current after breakdown increases linearly with the breakdown voltage with a resistance of ∼25 kΩ that corresponds to the expected 13645
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initial electrical properties without the requirement of any external processes. All these results are extremely promising and suggest that lipid layers will certainly constitute a new class of ultrathin dielectric to be used in electrical devices.
’ AUTHOR INFORMATION Corresponding Author
*
[email protected], +33(0)491418916.
Figure 5. (a) Recovery time (closed circles, left) and resistance of the lipid layer at 1 MV/cm (open squares, right), after successive electrical breakdowns measured on the same sample area.
value of the silicon substrate. From this observation, one can relate this current to the size of the pore which is formed, and we can conclude that it is directly proportional to the voltage. Lower breakdown voltage implies smaller lipid density and total pore area. 3.3. Autonomic Self-Healing. After breakdown, the sample was left at room temperature to recover. I(V) measurements realized at different times after electric breakdown clearly reavealed a decrease of the current with time (Figure 4a). The corresponding resistances are shown in Figure 4b, and surprisingly, the initial averaged resistance (∼300 MΩ, at 1 MV/cm) is recovered after typically one hour, indicating that the layer is not irreversibly damaged by the breakdrown and that it can restore without the application of any external input. Similar measurements were realized on native and chemical oxide layers and showed no recovery after breakdown for several hours, therefore excluding that the recovery could be due to the formation of native oxide. In fact, self-healing of polymer was already reported after soft breakdown,34,35 but it always requires an external stimulus such as heating for a few hours, exposure to UV, or the addition of a polymerizing agent.36 38 Even more interesting, submission of the same polymer area to successive breakdowns always led to spontaneous self-healing. The resistance after recovery at 1 MV/cm and the time to recovery are both reported in Figure 5 for 11 successive breakdowns. Clearly, the resistance of the membrane always goes back to its initial value with some variability due to the measurement. One can see that, after the sixth breakdown, the recovery time increases from typically 1 h to several hours, therefore suggesting some aging of the polymer with breakdowns. An interesting point is that annealing the sample at only 36 °C for 30 min (after the 10th breakdown in Figure 5) led to the recovery of the initial healing time at room temperature, as if the polymer had been reinitialized. This property is remarkable and highly interesting for electron devices for which breakdowns are usually dreadful. In summary, we report the reliable and reproducible electrical performances of phospholipid layers as novel dielectric layer. Lipid layers were elaborated by a simple and fast method, consisting of direct fusion of lipid vesicles directly at the surface of hydrogen-terminated silicon substrate and subsequent polymerization. This method led to the creation of stable, homogeneous, dense, and ultrathin (2.7 nm) monolayers. We have demonstrated that this lipid layer exhibits excellent electrical performance with low leakage of current at high electric field and breakdown voltage of typically 7.5 MV/cm. The self-healing process after electrical breakdown is unique and remarkable. The layers can undergo several breakdowns, and always recover their
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’ ACKNOWLEDGMENT We thank the region Provence Alpes C^otes d’Azur (PACA) for its support and funding (contract no 2010_12146 DEB 101174). We thank Marie-Pierre Valignat from the Cell Adhesion and Inflammation Laboratory (LAI, INSERM UMR 600 - CNRS UMR 6212) for letting us use their ellipsometer. ’ REFERENCES (1) Chiu, C.-S.; Lee, H.-M.; Gwo, S. Langmuir 2010, 26, 2969–2974. (2) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashida, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431–6438. (3) Checco, A.; Schollmeyer, H.; Daillant, J.; Guenoun, P.; Boukherroub, R. Langmuir 2006, 22, 116–126. (4) Minari, T.; Kano, M.; Miyadera, T.; Wang, S. - D.; Aoyagi, Y.; Seto, M.; Nemoto, T.; Isoda, S.; Tsukagoshi, K. Appl. Phys. Lett. 2008, 92, 173301–173303. (5) Kira, A.; Okano, K.; Hosokawa, Y.; Naito, A.; Fuwa, K.; Yuyama, J.; Masuhara, H. Appl. Surf. Sci. 2009, 255, 7647–7651. (6) Whiting, G. L.; Snaith, H. J.; Khodakhsh, S.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M.; Greenham, N. C.; Friend, R. H.; Huck, T. S. Nano Lett. 2006, 6, 573–578. (7) Malmstadt, N.; Nash, M. A.; Purnell, R. F.; Schmidt, J. J. Nano Lett. 2006, 6, 1961–1965. (8) Sanii, B.; Smith, A. M.; Butti, R.; Brozell, A. M.; Parikh, A. N. Nano Lett. 2008, 8, 866–871. (9) Quinti, L.; Weissleder, R.; Tung, C.-H. Nano Lett. 2006, 6, 488–490. (10) Robert, G.; Derycke, V.; Goffman, M. F.; Lenfant, S.; Vuillaume, D.; Bourgoin, J.-P. Appl. Phys. Lett. 2009, 93, 143117–143120. (11) Sekitani, T.; Yokota, T.; Zschieschang, U.; Klauk, H.; Bauer, S.; Takeuchi, K.; Takamiya, M.; Sakurai, T.; Someya, T. Science 2009, 326, 1516–1519. (12) Gholamrezaie, F.; Mathijssen, S. G. J.; Smits, E. C. P.; Geuns, T. C. T.; van Hal, P. A.; Ponomarenko, S. A.; Flesch, H.-G.; Resel, R.; Cantatore, E.; Blom, P. W. M.; de Leeuw, D. M. Nano Lett. 2010, 10, 1998–2002. (13) Smith, A.-S.; Sengupta, K.; G€onnenwein, S.; Seifert, U.; Sackmann, E. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 6906–6911. (14) Meleleo, D.; Micelli, S.; Toma, K.; Haneda, K.; Gallucci, E. Peptides 2006, 27, 805–811. (15) Carr, R.; Weinstock, I. A.; Sivaprasadarao, A.; Muller, A.; Aksimentiev, A. Nano Lett. 2008, 8, 3916–3921. (16) Rosenkranz, T.; Katranidis, A.; Atta, D.; Gregor, I.; Enderlein, J.; Grzelakowski, M.; Rigler, P.; Meier, W.; Fitter, J. ChemBioChem. 2009, 10, 702–709. (17) Charrier, A.; Thibaudau, F. Biophys. J. 2005, 89, 1094–1101. (18) Fenz, S. F.; Merkel, R.; Sengupta, K. Langmuir 2009, 25, 1074–1085. (19) Charrier, A.; Mischki, T.; Lopinski, G. P. Langmuir 2010, 26, 2538–2543. (20) Kramar, P.; Miklavcic, D.; Lebar, A. M. IEEE Trans. NanoBiosci. 2009, 8, 132–138. 13646
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