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Hollow Platinum Nanoshell Tube Arrays: Fabrication and Characterization Chang Chen,*,†,‡ Josine Loo,† Meng Deng,† Ronald Kox,†,§ Roeland Huys,†,§ Carmen Bartic,†,| Guido Maes,‡ and Gustaaf Borghs†,| IMEC Vzw., Kapeldreef 75, 3001 LeuVen, Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 F, 3001 LeuVen, Department of Electrical Engineering, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 10, 3001 LeuVen, and Department of Physics, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 D, 3001 LeuVen, Belgium ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: January 30, 2009
Thin hollow platinum tubes are nanostructures with interesting properties for applications in the fields of catalysis, electrochemistry, biosensors, and fuel cells. In this paper, we describe a facile and fast nanofabrication method to directly prepare hollow platinum nanoshell tube arrays on a wafer through a galvanic replacement reaction (GRR). A silicon wafer containing an array of copper electrodes and a porous silicon oxide layer is used as a template. In the presence of a gold seed layer, platinum particles were deposited homogeneously on the walls of the submicrometer pores of the template structures to form tubular Pt structures by using copper as the reductant. The reaction parameters, the mechanism, and the properties of the platinum micro/ nanotubes are discussed. This method can also be applied to other materials such as gold and silver in order to fabricate nanoshell tube arrays or even bimetal structures. The method is CMOS compatible and thus can be applied onto fully processed CMOS circuits in order to create fully integrated nanosensors or actuators. Introduction Nanoscaled platinum materials are well-known, performing much better than bulk platinum in catalysis and electrochemistry.1 In the past decade, Pt nanoparticles have attracted much attention from researchers and businessmen due to the simple fabrication process. However, the agglomeration of these small particles caused by the higher surface energy and the lower melting point is still a serious problem in applications such as catalysis. Due to the zero-dimension morphology, there is an enormous interface resistance between spherical particles, which makes these nanoparticles work poorly as micro/nano electrodes.2,3 One-dimensional (1D) Pt nanomaterials, in particular, Pt nanotubes, are considered as a fascinating alternative solution for nanoplatinum applications in the fields of catalysts,4 fuel cells,5,6 and microelectrodes.7,8 A platinum-coated carbon nanotube was incorporated into the membrane electrode of the fuel cells, resulting in a higher power density and an increased catalyst usage.6 Amerometric glucose sensor using Pt nanowires as the electrodes obtained a detection limit of 0.05 µM glucose, as the 1D morphology of Pt nanostructures improved the signalto-noise ratio.8 Nanotubes not only make particles harder to be agglomerated, but also make the surface area larger than that of nanoparticles with similar sizes. Compared with the surface area of a nanoparticle with the same diameter, the surface area of a nanotube is several times larger, as it depends on the thickness of the wall. A simple calculation is shown in the Supporting Information. Different from various preparation methods of Pt nanoparticles, almost all fabrication methods of Pt nanotubes use templates, including negative templates and positive templates. * Corresponding author. Tel: +32 16287794, Fax: +32 16281097, Email:
[email protected]. † IMEC vzw. ‡ Department of Chemistry, KULeuven. § Department of Electrical Engineering, KULeuven. | Department of Physic, KULeuven.
Negative templates, including porous anodic aluminum oxide membranes,4,9 silicondioxidefilms,10,11 andpolymermembranes,8,12 can be precisely designed and integrated into microelectronic or biochips. Usually, chemical modification of the surface or deposition of metal nanoparticle coatings on the walls of the templates (i.e., quasi seed layers) are necessary for the formation of the tubular structure by means of electrochemical plating or chemical reduction plating.4,9,12-14 The template wetting method developed by Go¨sele et al. is another universal nanotube fabrication method for various materials.10,11,15 This method used a soft Pt precursor, a liquid with low surface energy, spreading easily over substances with high surface energy to form a monolayer on the wall of the template in a short time. Pt nanotubes were generated after the degradation of the precursor layer by heating. The methods based on the positive template are coating processes on nanofibers or nanowires by sputtering platinum or by galvanic replacement reactions (GRR). Carbon nanofibers or semiconductor nanowires were widely used as templates for Pt nanotube creation by sputtering with efficient catalysis and conductivity.6,16,17 Silver or other nanowires with low reduction potential were used as self-sacrificed templates in GRR for the preparation of Pt nanotubes.18-20 As the fabrication of silver nanowires is also based on chemical reaction,21,22 this method shows enormous potential for the mass production furthering commercial applications. However, the alignment and the integration of these Pt nanotubes into CMOS circuits are still problematic. In this paper, we describe a simple and fast method that combines the negative template and GRR to produce hollow Pt nanoshell tube arrays (as shown in Figure 1). Pt nanoshell tubes were formed by the galvanic replacement between the platinum precursors and the copper plugs present underneath the silicon oxide template. The porous SiO2 layer on the wafer acts as the negative template to confine the tubular structure. The process can be completed in dozens of seconds or several minutes. The
10.1021/jp9001065 CCC: $40.75 2009 American Chemical Society Published on Web 03/18/2009
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Figure 1. (A) Schematic illustration of the Pt nanoshell tube arrays fabrication. (B) Schematic illustration of the Pt hollow nanotubes formation. Cu atoms act as the reductant to release electrons. The electrons migrate along the Au seed layer to reduce chloroplatinic ions to platinum atoms, and then these atoms deposit on the surface of the Au layer to form a nanoshell tube.
morphology of the Pt tubes can be well-controlled by adjusting the reaction parameters and the template parameters. Scanning electron microscopy (SEM), focused ion beams (FIB), and energy-dispersive X-ray (EDX) spectroscopes were used for the characterization of the realized structures. Experimental Section Chemicals and Materials. A pattern of Cu electrodes has been deposited onto a silicon wafer using the Cu damascene method in the IMEC p-line. This pattern consists of Cu lines with a thickness of 400 nm embedded in a SiO2 layer. The Cu lines have been coated with a SiO2 layer with a thickness of 1000 nm. Subsequently, holes with different diameters have been plasma-etched in the SiO2 layer to define the template. The main chemical for GRR, hexachloroplatinic acid hydrate (H2PtCl6), was obtained from Sigma-Aldrich and was used without any further treatment. The fabrication process is outlined in Figure 1A. There are three main steps. All processes were operated at room temperature. 1. Deposition of Seed Layers. A gold seed layer was deposited by sputtering on the wafer treated by a diluted HCl solution. The thickness of the Au layer was 20 or 30 nm in our experiments. Unwanted Au deposition on the surface of the SiO2 layer was removed by a mechanical polishing process. 2. Fabrication of Pt Nanoshell Tube Arrays. For the galvanic replacement reaction, the clean wafer was immersed into the H2PtCl6 solution with a concentration of 40 mM. The reaction time was varied between 30 and 180 s. Strong magnetic stirring was applied in every experiment at the same rate. The prepared wafers were rinsed by DI-water and then dried with a nitrogen gun. 3. Templates RemoWal. The SiO2 layer was etched in a buffered HF solution (BHF, 7:1) for 4-5 min to expose the Pt nanoshell tube arrays. Another DI-water rinsing was needed, but without nitrogen drying, since this could destroy the tubes. Characterization. The morphologies of the samples were analyzed with SEM (Philips, XL30) at an acceleration voltage of 5 kV. Focused ion beam SEM (FIB-SEM) (FEI, Strata 400 STEM) was used to make the cross section of the Pt tubes at an acceleration voltage of 5 kV. Before FIB, an ion-Pt layer was deposited in the selected area to protect the morphology of the Pt nanotubes during the FIB milling process. The EDX was taken with JEOL 5600LV SEM equipped with an EDX detector at an acceleration voltage of 20 kV and a distance of
20 mm. I-V curves of the samples were measured with a HP Parameter Analyzer with two probes. Results and Discussion A conductive seed layer is crucial in the negative template method. In the preparation of the seed layers, both the surface modification chemistry and the metal deposition processes (sputtering or evaporation) are widely used.4,14 Here, we selected Au sputtering to form the seed layer, since it is hard to form a continuous seed layer on the vertical side wall through the evaporation process. The thickness of the Au layer can be controlled by the sputtering time (20 and 30 nm in this study). A polishing process was used to remove the Au deposition on the surface of the wafers, leaving the Au seed layer inside the pores. The thickness of the Au layer on the wall of the pores (seed layer) was somewhat thinner than the one on the surface of the template. As the latter was the reference parameter in the sputtering process, it is not surprising that the real thickness of a seed layer in 20 nm sputtering was only about 12 nm and the one in 30 nm sputtering was about 20 nm (as shown in Figure 2A,B). It must be noted that such a thin layer is not mechanically strong enough to remain on the wafer without the template support. The thickness of the seed layer determines the thickness of the resultant Pt tubes. The hollow Pt tube array shown in Figure 1C was obtained by immersing wafers with a 20 nm Au seed layer into the H2PtCl6 solution for 30 s and then immersing into a BHF solution for 4 min to release the tubes. As shown in the SEM image, the resultant Pt tubes with the wall average thickness of 75 nm are mechanically strong enough to stand vertically on the wafer. Another similar array in Figure 2D was obtained by using a 30 nm Au seed layer in the same solution for 30 s, resulting in Pt tubes with a thicker wall (about 90 nm). The thickness of the tube wall is almost 2 times that of the seed layer, in agreement with the intuitive estimation. The seed layer plays an important role in the formation of hollow tubes. As a matter of fact, there was almost no tubular product formed by GRR in the absence of the seed layer (Figure 2E). After 30 s immersion in the H2PtCl6 solution, only a thin ring was obtained, which might have arisen from the interface reaction between the Cu and the solution. In a normal GRR, the ion diffusion and the quantity of the active surface spots determine the growth speed of the Pt tubes. Upon addition of H2SO4 (1.1 M) to improve the surface activity of Cu and
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Figure 2. The role of the seed layer in the preparation of the Pt hollow tubes. (A) 20 nm Au seed layers in the holes; (B) 30 nm Au seed layers in the holes; (C) Pt hollow tubes formed in the presence of 20 nm Au seed layers through GRR for 30 s; (D) Pt hollow tubes formed in the presence of 30 nm Au seed layers through GRR for 30 s. (E,F) No tubes formed without a seed layer: (E) 30 s GRR reaction, (F) 60 s GRR reaction in the presence of H2SO4. Magnified images of the inserted SEM images can be found in Supporting Information Figure S4.
prolonging the reaction to 60 s, Pt started to deposit largely on top of the Cu substrate by GRR, but without forming any tubular structures (Figure 2F). The wall thickness of the hollow Pt tubes mainly depends on the reaction time. The wall thickness of tubes obtained by GRR over 30, 60, 120, and 180 s are shown in Figure 3. The thickness of these four samples is controlled by the reaction time. The calculated correlation coefficient is 0.956, indicating that the thickness is almost linearly increasing with the reaction time. The slope of the curve is 0.926, meaning that the growth speed of the wall of the tubes is about 1 nm/s. Generally, the rate of electroplating in the presence of a seed layer is too fast to be well controlled, but the GRR method shows enormous potential for the manufacture of hollow tubes in a controllable way. This is meaningful and helpful in the control of nanofabrication processes. As the amount of Pt precursors inside the holes was not large enough to form Pt tubes, ion diffusion from the solution toward the template holes (GRR area) was necessary and has affected the morphology of the Pt nanoshell tubes. This influence of the ion diffusion is illustrated by the deposition in templates with
Figure 3. The influence of reaction time on the thickness of the Pt hollow tubes formed by GRR in the presence of 20 nm Au seed layers: (A) 30 s; (B) 60 s; (C) 120 s; (D) 180 s. Magnified images of the inserted SEM images can be found in Supporting Information Figure S5.
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Figure 4. The influence of size of the template holes on the wall thickness of the Pt hollow tubes. The tube thickness increases linearly with increasing size of the template holes (solid black squares). The inner diameters of Pt hollow tubes also change by using a different size of the template holes (open blue circles).
different diameters. Fick’s first law is commonly used to describe the diffusion rate of the ions: J ) -D × ∇C (here, J is the flux of ions, D is the diffusion coefficient, and ∇C is the concentration gradient between bulk solution and template hole).23 The chloroplatinic ion reduction is so fast that the concentration gradient approximates a constant. The total amount of Pt ions (Ndiffu) from the solution into the holes is Ndiffu ) J × πR2 (here, R is the radius of the template hole). Considering only the Pt deposition on the side wall (it can be observed directly by SEM) and assuming the deposition rate on the side wall (Adepos) to be a constant not related to the size of the hole, the amount of Pt atoms (Ndepos) deposited on the sidewall of the hole is Ndepos ) Adepos × (2πRh) (where h is the depth of the hole). The thickness of the Pt tube can be calculated from
TPtNT )
Ndiffu
/Ndepos ) J/Adepos ×
πR
/(2h)
(1)
So far as we know, there is no report on the deposition rate (Adepos) of Pt on the Au layer through GRR using Cu as the reductant. The quantitative relationship between TPtNT and R is still not clear. However, eq 1 shows a qualitative relationship in which the TPtNT increases proportionately to R. This result is in agreement with the data shown in Figure 4 (solid black squares): the wall thickness of the Pt tubes increases with the size increase of the template holes. The calculated correlation coefficient is 0.997, indicating that the thickness is almost linearly related to the size of the holes. The slope of the curve indicates that the increasing coefficient of the thickness is about 6.33 nm/100 nm size. It is noticed that these template holes with different sizes were fabricated in the same wafer to minimize the influence of other reaction parameters. The inner diameters of the resultant hollow Pt nanoshell tubes ranged from tens to hundreds of nanometers (open blue circles). FIB-SEM and EDX were used to study the morphology and the composition of the hollow Pt nanoshell tube arrays. Figure 5A displays the SEM cross section image through a tube obtained by FIB sculpting. It exhibits the cross section of a Pt tube prepared by a 30 s typical GRR using a 20 nm Au seed layer. The voids inside a Cu layer indicate the replacement between copper atoms and platinum precursor ions. The platinum atoms were deposited on the surface of
the copper layer and the gold seed layer. The Au seed layer is indistinguishable in the SEM image of the Pt tube. The half-ball structures are the ion-Pt deposits formed by ion-Pt sputtering during the FIB sculpting, which is used to protect the Pt tubes from possible damage during the sample preparation. EDX was also used to demonstrate the presence of Pt in the produced structures. Compared with the reference (an unreacted sample) curve, a Pt peak appeared at about 2.13 keV in the reacted sample. Considering the limited amount of Pt in the probe volume (a sphere with a diameter of about 1 µm), it is easy to understand the small intensity of this Pt peak. The mechanism of the galvanic replacement reaction is similar to the one reported by Xia’s group,24 but influenced more by the distribution of the reactive ions. In the GRR process developed by Xia’s group, silver nanocubes were used as the self-sacrificed template to reduce the gold precursor ions and obtained Au/Ag alloy nanoshells.24 In their process, the reaction temperature and the concentration should be well-controlled to avoid silver chloride precipitation. In our approach, the precipitation problem could be easily solved by using Cu instead of Ag. Furthermore, an additional advantage is that copper is a much more widely used material in the semiconductor industry than silver (for contamination reasons, Ag has to be avoided in standard CMOS processing). In our work, the morphology of the Pt nanoshell tubes mainly depends on the parameters of GRR, such as the deposition rate and the ion diffusion rate. A plausible mechanism is schematically shown in Figure 1B. Cu is considered as the reductant and the reaction follows the mechanism of a galvanic replacement reaction. We believe that the Cu layer releases electrons in the GRR process due to the low reduction potential of the Cu2+/Cu redox pair (E0 ) 0.34 V). These electrons migrate in the Au layer, which acts as a wire in this small “galvanic cell”. The Pt precursor ions near the Au seed layer receive electrons due to the higher reduction potential of the (PtCl6)2-/Pt redox pair (E0 ) 1.41 V) and thus become atoms. Such platinum atoms assemble and deposit on the surface of the Au layer to form nanoshell tubes. The chemical reaction can be described by eq 2. Cu acts as the selfsacrificed cathode and the Au layer as the anode for Pt deposition. According to this principle, metal precursors with higher reduction potential than copper can also be used here to make tubular structures. Several examples of Au and Ag tubes can be found in the Supporting Information (Figure S2). Bimetal nanostructures such as Au or Pt caps on top of Cu needles (Supporting Information Figure S3) can also easily be prepared by GRR. Au
Cu(s) - 2e- 98 Cu2+(aq) Au
PtCl62-(aq) + 4e- 98 Pt(s) Au
2Cu(s) + PtCl62-(aq) 98 2Cu2+(aq) + Pt(s) + 6Cl-(aq) (2)
GRR kinetically guarantees the feasibility of the reaction, but the morphology of the products relies on the recipe based on the ion diffusion. Compared with the slow diffusion rate of ions in solution, the migration rate of electrons inside the gold layer is fast enough to reduce all chloroplatinic ions near the Au layer simultaneously. In this GRR, the rate controlling step is not the chloroplatinic ion diffusion rate inside holes like in Lee’s
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Figure 5. (A) The cross section SEM image of Pt tubes made by FIB, showing the uniform thickness of the tube. (B) The EDX spectrum of Pt tubes, indicating that Pt deposited on the sidewall after GRR.
to minimize the influence of the interface contacting resistance between the probes and the Pt tubes. In the insert image of Figure 6, the black squares were areas with a tube pitch of 2 µm, standing on the Cu wires (yellow areas). When probes were placed on top of the Pt tubes lying on different connected Cu electrodes, a current flowing longitudinally through the Pt tubes could be measured. Calculated from the I-V curve shown in Figure 6, the total resistance of two Pt tube arrays contacting with two probes is 21.3 Ω. It is well-known that platinum is an excellent catalyst for several oxidation reactions. The hollow Pt tube arrays described in this paper could also be used as novel electrode/catalyst complexes applied in fuel cells,4 biosensors,25 or other electrochemical devices.26
Figure 6. I-V curves of Pt tube arrays integrated on the wafer as the electrodes. The resistance is 21.3 Ω, indicating the potential application as electrochemical electrodes. The insert image is the I-V measurement of the Pt tube arrays by a two-probe station.
case,14 but the Cu ion diffusion rate from the voids into the solution through nanogaps in the Au layer. This mechanism is depicted in Figure 1B. The slow Cu ion diffusion allows these ions to congregate near the bottom of the hole to form a positive electric field. The transitory field attracts the chloroplatinic ions migrating from the top to the bottom of the hole, making the ion distribution inside the hole more uniform. The more uniform the ion distribution is, the more uniform the thickness of the resultant nanoshell will be. Additionally, as mentioned above, the ion diffusion from the solution into the template holes also influences the morphology of the nanoshell tube, which can be represented by the reaction time and the size of the template holes. Pt hollow nanoshell tube arrays prepared by this method exhibit an acceptable electrical conductivity, which allows potential applications as electrochemical electrodes in biosensors or fuel cells. I-V curve measurements were performed before removing the SiO2 template layer. Without the template layer, when probes touched on the free-standing Pt tubes, both the adhesion strength (between Pt tubes and Cu layers) and the mechanical strength of the Pt tubes were not strong enough to obtain reproducible results. It is also better to operate I-V curve measurements on the Pt tube arrays with the highest tube density
Conclusions In summary, a simple and fast method combining negative templates and GRR was investigated to produce hollow Pt nanoshell tube arrays. Such Pt tube arrays were formed by the galvanic replacement between platinum precursors and copper electrodes deposited on silicon wafers, in the presence of a porous SiO2 layer as the negative template and a thin Au layer on the sidewall as the seed layer. The tube formation process was completed in only dozens of seconds to several minutes. The morphology of the Pt tubes could be well controlled by adjusting the reaction time and the template hole sizes. The low resistance of these Pt tube arrays makes them suitable for applications such as electrode/catalyst complexes in biosensor and fuel cell related fields. Furthermore, this method was also extended to other materials such as gold or silver nanoshell tube arrays (shown in Supporting Information Figure S2), as well as bimetal structures such as Au or Pt caps on top of Cu needles (shown in Supporting Information Figure S3). Acknowledgment. We thank Hugo Bender for performing the FIB preparation and measurements and Robert Mu¨ell for some useful suggestions. Chang Chen acknowledges his scholarship, offered via the Selective Bilateral Agreements (SBA China) of KU Leuven. Supporting Information Available: Information regarding the surface area comparison of nanotubes and nanoparticles, the application examples of this method on the fabrication of
Hollow Platinum Nanoshell Tube Arrays other metal tubular structures and bimetal structures, and the magnified images of the inserted SEM images in Figure 2 and Figure 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baker, L. A.; Jin, P.; Martin, C. R. Crit. ReV. Solid State Mater. Sci. 2005, 30, 183. (2) Maillard, F.; Schreier, S.; Hanzlik, M.; Savinova, E. R.; Weinkauf, S.; Stimming, U. Phys. Chem. Chem. Phys. 2005, 7, 385. (3) Ott, L. S.; Finke, R. G. Coord. Chem. ReV. 2007, 251, 1075. (4) Guo, Y. G.; Hu, J. S.; Zhang, H. M.; Liang, H. P.; Wan, L. J.; Bai, C. L. AdV. Mater. 2005, 17, 746. (5) Matsumoto, T.; Komatsu, T.; Arai, K.; Yamazaki, T.; Kijima, M.; Shimizu, H.; Takasawa, Y.; Nakamura, J. Chem. Commun. 2004, 840. (6) Caillard, A.; Charles, C.; Boswell, R.; Brault, P.; Coutanceau, C. Appl. Phys. Lett. 2007, 90. (7) Delvaux, M.; Moustier-Champagne, S. Biosens. Bioelectron. 2003, 18, 943. (8) Yang, M. H.; Qu, F. L.; Lu, Y. S.; He, Y.; Shen, G. L.; Yu, R. Q. Biomaterials 2006, 27, 5944. (9) Lee, W.; Alexe, M.; Nielsch, K.; Gosele, U. Chem. Mater. 2005, 17, 3325. (10) Luo, Y.; Lee, S. K.; Hofmeister, H.; Steinhart, M.; Gosele, U. Nano Lett. 2004, 4, 143. (11) Steinhart, M.; Wehrspohn, R. B.; Gosele, U.; Wendorff, J. H. Angew. Chem., Int. Ed. 2004, 43, 1334. (12) Wirtz, M.; Martin, C. R. AdV. Mater. 2003, 15, 455.
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5477 (13) Lahav, M.; Sehayek, T.; Vaskevich, A.; Rubinstein, I. Angew. Chem., Int. Ed. 2003, 42, 5575. (14) Lee, W.; Scholz, R.; Niesch, K.; Gosele, U. Angew. Chem., Int. Ed. 2005, 44, 6050. (15) Steinhart, M.; Wendorff, J. H.; Wehrspohne, R. B. ChemPhysChem 2003, 4, 1171. (16) Jang, S. G.; Yu, H. K.; Choi, D. G.; Yang, S. M. Chem. Mater. 2006, 18, 6103. (17) Alexe, M.; Hesse, D.; Schmidt, V.; Senz, S.; Fan, H. J.; Zacharias, M.; Gosele, U. Appl. Phys. Lett. 2006, 89, 172907. (18) Mayers, B.; Jiang, X. C.; Sunderland, D.; Cattle, B.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 13364. (19) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (20) Wiley, B.; Sun, Y. G.; Chen, J. Y.; Cang, H.; Li, Z. Y.; Li, X. D.; Xia, Y. N. MRS Bull. 2005, 30, 356. (21) Chen, C.; Wang, L.; Jiang, G. H.; Zhou, J. F.; Chen, X.; Yu, H. J.; Yang, Q. Nanotechnology 2006, 17, 3933. (22) Chen, C.; Wang, L.; Yu, H. J.; Wang, J. J.; Zhou, J. F.; Tan, Q. H.; Deng, L. B. Nanotechnology 2007, 18, 115612. (23) Brett, C. M. A.; Brett, A. M. O. Electrochemistry Principles, Methods, and Applications; Oxford University Press: Oxford, 1993. (24) Sun, Y. G.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 3892. (25) Yuan, J. H.; Wang, K.; Xia, X. H. AdV. Funct. Mater. 2005, 15, 803. (26) Huys, R.; Braeken, D.; Van Meerbergen, B.; Winters, K.; Eberle, W.; Loo, J.; Tsvetanova, D.; Chen, C.; Severi, S.; Yitzchaik, S.; Spira, M.; Shappir, J.; Callewaert, G.; Borghs, G.; Bartic, C. Solid-State Electron. 2008, 52, 533.
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