Effect of Surface Functionalization of Metal Wire on Electrophysical

Article pubs.acs.org/Langmuir

Effect of Surface Functionalization of Metal Wire on Electrophysical Properties of Inductive Elements Denis V. Voronin,*,†,‡ Dimitriya Borisova,‡ Valentina Belova,‡ Dmitry A. Gorin,† and Dmitry G. Shchukin‡ †

Faculty of Nano- and Biomedical Technologies, Saratov State University, Saratov 410012, Russia Department of Interfaces, Max-Planck Institute of Colloids and Interfaces, Potsdam D14476, Germany

‡

S Supporting Information *

ABSTRACT: The development of the microelectronics industry requires a new element basis with reduced size and increased functionality. The most important components in modern microelectronic integrated circuits are passive elements. One of the key challenges in order to improve the functionality of integrated circuits is to increase the quality of passive elements composing them. In this paper we suggest a novel approach to increase the quality factor Q of inductors by the surface modification and functionalization of the metal components. Ultrasound induced surface modification of metal wires led to the formation of a porous surface structure, which further can be functionalized with magnetite nanoparticles using layer-by-layer assembly technique. The surface modification and deposition of magnetite nanoparticles was investigated with SEM, XRD, and contact angle measurements. Additionally, inductance and resistance measurements, as the main parameters determining the Q-factor of inductors, were carried out. Samples with high number of magnetic nanoparticle−polyelectrolyte bilayers demonstrate a significant increase in inductance and a slight decrease in resistance in comparison to uncoated ones. The combination of these factors led to enhancement the Q-factor of the investigated inductive elements.

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the whole circuit.1 In general, inductor Q-factor is determined by the following relation1

INTRODUCTION The fast growing wireless market has created an urgent demand for small and cheap handsets with enhanced functionality and performance. One of the key elements there, especially in the silicon radiofrequency (RF) integrated circuits field, is the design of high-quality passive elements including integrated inductors. Inductors are very important elements of resonant circuits (e.g., inductor-capacitor (LC) parallel tank circuit), which define the parameters and characteristics of such important RF blocks as low noise amplifiers (LNA), voltage controlled oscillators (VCO), RF filters, etc.1 The quality of a resonant circuit is measured by its quality factor Q. The Q-factor is a characteristic of the oscillatory system, which determines the resonance bandwidth. This factor is defined as the ratio of the energy stored in the system to the energy lost during one oscillation period. The Q-factor is inversely related to the rate of decay of the natural oscillations in the system. Hence, the higher the Q-factor of the oscillating system, the less energy loss per period and the slower oscillation decay. In other words, the Q-factor indicates how long a signal could exist in a resonant circuit, until it completely dies out.2 In common, the performance of RF circuits depends on the Q-factor of the LC tank circuit. This is because inductors are characterized by significantly greater losses than capacitors and thus inductor quality determines the quality of © 2012 American Chemical Society

Q=

ωL R

(1)

where ω is the angular current frequency, L is the inductance, and R is the resistance. According to this formula, the Q-factor of an inductor can be increased by increasing its inductance or by decreasing its resistance. Currently, most of the inductors used in circuits as passive components have a microscopic size and are fabricated with a planar technology.1,3−5 One of the main activities in this field is the design of thin film and multilayer inductors of RF high integration level circuits with a high Q-factor. The general approach here is the incorporation of magnetic composite thin film layers in the structure of passive elements, e.g. inductors,3−5 electromagnetic noise suppressors,6−8 sensors, and others mainly in the RF range, at 0.8−6 GHz.1 Here, we propose a new way to increase inductance of existing inductors using two simple preparation steps: (1) ultrasonic surface modification and (2) functionalization of Received: February 29, 2012 Revised: July 19, 2012 Published: July 19, 2012 12275

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PEI is a positively charged polyelectrolyte which can interact with surfaces not only due to its charge but also through hydrogen bonding and hydrophobic interactions.21 Mesoporous aluminum wire samples were dipped into aqueous PEI solution with a concentration of 2 mg/ mL for 30 s. PEI adsorbs on the Al mesoporous surface from aqueous medium due to a hydrophobic interactions between the Al surface and PEI. Afterward, the samples were rinsed in deionized water and dried under nitrogen flow for about 1 min. Deposition of MNPs was made from the initial (undiluted) colloid with concentration of 5.6 mg/mL obtained from Sigma-Aldrich: MNPs are stabilized by oleic acid and dispersed in heptane. The immersion time of substrates in the magnetite colloid was 15 min. Adsorption of MNPs is due to interaction between an amino group of PEI and the oleic acid at the particle interface. This leads to the formation of a complex between the first layer of PEI on the Al surface and oleic acid at the MNPs interface (the formation of hydrogen bonds). The uncharged hydrocarbon tails of oleic acid are oriented into dispersion medium and further participate in MNPs adsorption through hydrogen bonding between two uncharged tails of the oleic acid of MNPs and of the previously formed complex with PEI. During the rinsing procedure in water the disruption of hydrogen bonds between PEI and the oleic acid complex is possible. However, this process affects only the upper layer of adsorbed MNPs. Then, the samples were rinsed in deionized water and dried under nitrogen again. Further, PEI and MNPs layers were alternated in order to achieve the appropriate number of PEI/MNPs bilayers. SEM Measurements. SEM measurements were performed with a Gemini Leo 1550 microscope at an operating voltage of 3 keV. All samples were attached to a carbon tab glued to an aluminum sample holder and then sputtered with gold in a vacuum. XRD Analysis. XRD analysis was carried out with an Enraf Nonius FR 590 diffractometer with a Cu Kα irradiation source with a wavelength λ = 1.540598 Å. Operating current and voltage were 40 mA and 50 kV, respectively. Each sample was measured for 1 h. Contact Angle Measurements. Wettability of the samples surface was investigated using a CA meter (Software DSA 1, Krüss GmbH, Hamburg). Drops of deonized Millipore Milli-Q water (V = 20 μL) were placed on the sample surface, and apparent CAs were measured. Resistance and Inductance Measurements. In order to conduct the resistance measurements, the samples were introduced in the DC circuit and then the voltage drop at the samples was measured according to Ohm’s law for circuit section. An EA-3048B unit by EA Elektro-Automatik was used as a power supply. Digital multimeters VC 220 and 95 types by Voltcraft were introduced into the circuit as ammeter and voltmeter, respectively. The resistance was measured for two cases. In the first case, an external magnetic field was applied perpendicularly to the samples. In the second case, the resistance was measured without an external magnetic field. The sample inductance was investigated indirectly by measuring the magnetic field induced by samples under direct current. The magnetic field was perpendicularly measured using a Voltcraft GM 100 magnetic field meter as seen in Scheme 1.

modified surfaces with magnetic nanoparticles using layer-bylayer (LbL) deposition. This approach allows us to obtain new surface nanostructures with elevated functionalities. Ultrasonic treatment of materials results in the formation of a homogeneous porous layer on the metal surface with an average depth of 300 nm.9 So far sonication has been employed to create porous surfaces on aluminum resulting in a specific surface chemistry for strong adhesion as well as catalysis.10,11 One of the simplest ways to fill the ultrasonically formed porous layers is to apply the LbL deposition technique.12 This technique is based on the alternate adsorption of oppositely charged polyelectrolytes due to the surface charge overcompensation from one layer to another. The main benefits of LbL are the low application costs and simplicity, which enables the use of a wide range of layer components (e.g., polyelectrolytes and charged nanoparticles). At the moment, there are many publications reporting on magnetic composite multilayer films with magnetite nanoparticles obtained using the LbL technique.13−17 Initially, LbL involved the creation of composite coatings by alternate adsorption of oppositely charged polyelectrolyte molecules,18 on the other hand it was shown that the alternate adsorption of substances interacting by hydrogen, covalent, and covalent−ionic bonding is also possible.12 The surface morphology obtained after sonication significantly increases the surface area, and as a result it allows to increase the volume fraction of magnetite nanoparticles in the coating obtained by LbL. Also, the LbL assembly method has been already successfully applied to planar technology for fabrication of integrated circuits. For example, the research group leaded by Prof. F. Caruso demonstrated the advanced fabrication of nanostructured polyelectrolyte/gold nanoparticlebased memory devices with tailored performance by using the LbL method.19 Furthermore, the LbL method is found to be compatible with the lithographic processes which are used in planar technology for fabrication of integrated circuits.20 Our novel approach can significantly improve the electrophysical properties of the macroscopic substrates with a complex shape and could contribute to already existing technology processes of passive elements fabrication. The Qfactor of the formed inductive elements will be evaluated by inductance and resistance measurements.

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EXPERIMENTAL SECTION

Materials. Magnetite (Fe3O4) nanoparticles (MNPs) were purchased from Aldrich. The average particles size in a range of 20− 40 nm. MNPs are stabilized by oleic acid and dispersed in heptane. Polyethyleneimine (PEI) was obtained from Fluka. Initial PEI was a 50 wt % aqueous solution, with molecular weight about 600.000−1 000.000. The aluminum wire used as a substrate was obtained from Alfa Aesar (1 mm diameter, annealed, 99+ %). Aluminum alloy AA2024-T3 plates were used as planar substrates in order to analyze the surface modification with X-ray diffraction (XRD) and to investigate the surface wettability with contact angle (CA) measurements. Deionized Millipore Milli-Q water was used as a liquid medium during all set of experiments. Ultrasonic Treatment. All ultrasound experiments were carried out with an ultrasonic processor UIP1000 hd (Hielscher Ultrasonics GmbH Teltow, Germany) equipment. The samples were placed in a beaker with 350 mL of deionized H2O at a distance of about 5−7 cm from the tip of the sonotrode (tip area 3.14 cm2). The ultrasonic treatment was conducted at 20 kHz frequency and 57 W/cm2 power density for 10 min. The temperature of the liquid medium was kept at 50 °C. LbL Functionalization. In order to increase the nanoparticle adsorption, the aluminum surface was first functionalized with the PEI.

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RESULTS AND DISCUSSION Scheme 2 shows the experimental routes used in our work for the formation and functionalization of the homogeneous mesoporous layer on the surface of the aluminum wires. The first experimental step was the preparation of aluminum wire coils (Scheme 2a). Wires were wound by hand using a template with metric thread as a basis for wounding. This allowed us to obtain quite uniform spirally folded aluminum wires with fixed length and coil pitch with the following parameters: wire thickness, about 1 mm; number of turns, 17; turn diameter, 10 mm; distance between turns, 1 mm. Ultrasonic irradiation was applied in order to fabricate a mesoporous upper layers of the coils (Scheme 2b,c). After that, a surface functionalization with 12276

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Scheme 1. Measurements with Magnetic Field Induced by Direct Currenta

Figure 1. SEM images of Al wire surfaces before (a) and after 10 min sonication (b).

a The direction of magnetic lines and current flow are shown with arrows. n is the normal vector to the cross-sectional area S of the samples. The probe of the magnetic field tester is indicated by a blue arrow.

Figure 2. Photos of obtained samples: spirally folded initial aluminum wire (a) and sonicated wires coated with 1 (b), 5 (c), 10 (d), and 15 (e) PEI/MNPs bilayers.

the LbL technique resulting in functionalized porous morphology of the aluminum wire was applied (Scheme 2d,e). Figure 1 shows the SEM micrographs of the initial surface of an aluminum wire before sonication and the mesoporous structure, obtained on the wire surface after 10 min of sonication. The rough porous structure clearly seen on the surface of the sonicated samples provides additional surface area for further surface functionalization. According to the SEM images, the average size of the obtained pores is in a range of 200 nm, and they are big enough to adsorb the MNPs with an average size of 40 nm. Figure 2 illustrates the visual surface modifications of the samples with 1, 5, 10, and 15 bilayers of PEI/MNPs with

following structures: Al/PEI/MNPs/PEI, Al/(PEI/MNPs)5/ PEI, Al/(PEI/MNPs)10/PEI, and Al/(PEI/MNPs)15/PEI, respectively. The surface color of the samples became darker with increasing the number of PEI/MNPs bilayers as can be seen in Figure 2. The SEM images of the aluminum wire surfaces coated with different number of PEI/MNPs bilayers are shown in Figure 3. By comparing these images with the initial one (Figure 1b), we can see that with increasing the number of PEI/MNPs bilayers the surface became more uniform. Furthermore, comparing

Scheme 2. Sketch of the Experimental Procedure Used for the Modification of the Aluminium Wiresa

a

(a) Initial spirally folded aluminium wire. (b) Formation of the mesoporous layer by ultrasound treatment in water. (c) Mesoporous layer on the aluminium surface. (d) Functionalization of the mesoporous layer using the LbL technique. Here the first step is deposition of positively charged PEI, the second is rinsing in water, the third is deposition of MNPs, and the fourth is again rinsing in water. (e) Resultant magnetically functionalized spiral aluminium element. 12277

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adsorption at the aluminum surface. This was confirmed by calculation of the particles size using the Debye−Scherrer equation D=

Kλ β cos θ

(2)

where K is the shape factor (usually 0.9), λ the X-ray wavelength, β the peak full width at half-maximum, and θ the Bragg diffraction angle. The particle size was calculated for the (311) magnetite peak. Calculation was provided for composites containing 5, 10, and 15 PEI/MNPs bilayers, and then the obtained results were averaged. Using eq 2 the average particle size of the MNPs adsorbed at the aluminum surface was found to be about 20 nm. Additionally, the MNPs size distribution from TEM images using ImageJ Software was estimated (Supporting Information). According to TEM images, the size of MNPs in suspension is about 15 nm. By DLS (Supporting Information) the average MNPs size is higher (about 35 ± 10 nm). This difference can be explained by two factors: (1) by DLS we analyze significantly higher particle volume fraction, hence we observe a very broad particles distribution; (2) DLS analysis estimates the hydrodynamic diameter of MNPs in suspension, which is larger than that for dry MNPs. The preferred adsorption of nanoparticles of small sizes was shown in a number of publications.28,29 This phenomenon was explained by the fact that particles of smaller sizes have higher adhesion energy. As a result, smaller particles cannot be easily removed from a surface than bigger ones. Moreover, smaller particles are less affected by the Stokes frictional force Fd = 6πμRvs during the rinsing step. Here μ is the dynamic viscosity, R is the radius of the particles, and vs is the velocity of liquid flow. The good adsorption and increase of the MNPs volume fraction at the aluminum surface used in our study was indicated in the XRD patterns by the increase of the MNPs peak intensity with higher PEI/MNPs bilayer numbers. Using hydrophobic MNPs allows us to confirm the adsorption of the following layers on the aluminum surface by measuring the contact angle of water droplets placed on the sample surface (Figure 5). The untreated aluminum plate cleaned with ethanol has a CA of 80°. After sonication the aluminum surface became hydrophilic, and the CA decreased up to 11°.22 The adsorption of the hydrophilic PEI layer led to

Figure 3. SEM images of the composite coatings with 1 (a), 5 (b), 10 (c), and 15 (d) PEI/MNPs bilayers, respectively.

Figures 1b, 3a, and 3b, we can see that adsorption of magnetite nanoparticles occurred exactly in pores, exhibiting a nonuniform layer on the surface. As a result, the first PEI layer did not cover the pores completely. This confirms our assumption, that mesoporous surface could be used further as a host for additional MPNs adsorption after polyelectrolyte prelayer deposition. Figure 4 shows XRD patterns of the initial aluminum alloy surface and samples with different numbers of PEI/MNPs

Figure 4. XRD patterns of the aluminum mesoporous surface layer (a) and aluminum mesoporous surface functionalized with 1 (b), 5 (c), 10 (d), and 15 (e) PEI/MNPs bilayers.

bilayers. The obtained high intensity diffraction peaks at 2θ = 39°, 45°, 65°, 78°, and 82° correspond to the (111), (200), (220), (311), and (222) crystal planes of Al, respectively.22,23 The small narrow peaks at 2θ = 33°, 43°, 76°, and 80° near the main aluminum peaks can be assigned to α-Al2O3 and γ-Al2O3 oxides,24,25 which we suppose were formed during sonication. The wide diffraction peaks detected at 2θ = 30°, 36°, 54°, 57°, and 63° correspond to the (220), (311), (511), (333), and (440) crystal planes of Fe3O4, which are typical peaks of magnetite in cubic phase.26,27 The broad width of these peaks confirms the presence of small sized MNPs. As result the MNPs did not aggregate during

Figure 5. Contact angles revealing the alternate change of the surface wettability after surface treatment with ultrasound and subsequent adsorption of PEI and MNPs. 12278

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no significant change of the CA. However, the subsequent adsorption of the magnetite nanoparticles made the sample surface hydrophobic as revealed by the significant increase of the CA up to 131°. The adsorption of the next PEI layer decreased the CA down to 102° due to its hydrophilicity. The alternate change of the aluminum surface wettability after adsorption of every single layer confirmed the LbL adsorption of PEI and magnetite nanoparticles takes place. Electrophysical measurements of the magnetic filed induced by samples with 10 and 15 magnetite layers under direct current were made following the procedure described elsewhere.30 The obtained results (Figure 6) show that the

Figure 7. Measured resistance (R) of spirally folded aluminum wire without surface functionalization and with one adsorbed PEI/MNPs bilayer as a function of applied voltage (U). The resistance was measured with and without an applied external magnetic field.

The obtained results are comparable with the results of D. Zhang et al.31 reported for MgO/Fe3O4 core−shell nanowires. These structures could be considered similar to functionalized aluminum wires obtained in our work. Zhang’s group also demonstrated that the resistance of these structures is increased with an applied external magnetic field. Resistance measurements results for the samples with 5, 10, and 15 PEI/MNPs bilayers are shown in Figure 8. Figure 8 shows that the resistance of the samples with 5 PEI/MNPs bilayers is higher than the resistance of uncoated one. On the other hand for the samples with 10 bilayers it is almost the same. Finally, for the samples with 15 PEI/MNPs bilayers the resistance is lower compared to that of the initial aluminum coils. Similar results were reported in works in which the electrical properties of composites materials containing different volume fractions of MNPs were investigated.32,33 The observed effect in our study could be explained by the theories of the effective medium. When the volume fraction of MNPs is low, the nanoparticles in the layer are not interconnected and no conductive paths are available. As a result, the resistance in this case of less MNPs is expected to be high. This situation is represented for the samples with a low number of PEI/MNPs bilayers in our case. In contrast, when the volume fraction of MNPs is high, the nanoparticles can interact between each other and some conductive paths could be formed. This is the case for the samples with high number of PEI/MNPs bilayers. Surface current flow could explain the difference in resistance for samples functionalized with different number of PEI/MNPs bilayers. The resistance of the samples with 5, 10, and 15 PEI/ MNPs bilayers measured with an applied external magnetic field is higher than the resistance measured with an applied external magnetic field, like in the case for samples with 1 PEI/ MNPs bilayer. In all resistance measurements the liner increase in resistance could be explained by the thermal effect of direct current. This effect could be estimated according to the following relation:

Figure 6. Magnetic field, B (black curves), induced by spirally folded aluminum wires without surface functionalization and with 10 and 15 adsorbed PEI/MNPs bilayers, and calculated inductance, L, for these samples (red curves) as a function of the applied voltage (U).

magnetic field generated by samples with PEI/MNPs bilayers adsorbed on the aluminum surface is higher than the magnetic field generated by nonfunctionalized samples. Using the obtained data, it is possible to calculate the inductance with the following equation BS cos α (3) I where B is the magnetic field, created by inductors under direct current I; S is the cross-sectional area of the inductor (6.36 mm2), and α is the angle between the vector B and the area S (0° in our case, see Scheme 1). The calculated results are shown in Figure 6. We also compared the resistance of the samples without adsorbed PEI/MNPs bilayers (aluminum wires treated only with ultrasound) and samples with composite coatings at direct current. Coils with one PEI/MNPs bilayer have a higher resistance than a coil without coating (Figure 7). The reason for this is that the magnetic field induced by the adsorbed magnetite nanoparticles could deflect the electrons which move through the conductor. This effect becomes more obvious when magnetic field is applied perpendicularly to the coil. We used the GX06 magnet from IBSMagnet. At a distance of 3 cm to the sample the magnetic field was about 950 μT. In this case, the charge carriers would be deflected by the Lorentz force. By comparing the results of the resistance measurements with and without applied external magnetic field, we found that the resistance for the first case is higher (Figure 7). L=

R=

ρ0 (1 + αt )l S

(4)

Here ρ0 is resistivity, α is the temperature coefficient of resistance, t is the temperature, l is the length of the conductor, 12279

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Figure 8. Resistance (R) of spirally folded aluminum wires without surface functionalization and with 5, 10, and 15 adsorbed PEI/MNPs bilayers as a function of applied voltage (U). The resistance was measured with (a) and without (b) an applied external magnetic field.

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CONCLUSION Here, we demonstrated a new approach to increase the Qfactor of inductive elements combining the formation of a developed mesoporous layer by ultrasonic surface modification and surface functionalization with MNPs by the LbL assembly technique. SEM imaging showes that the adsorption of the layers occurred following the mesoporous surface structure and the PEI and MNPs were deposited precisely in the pores. The layer-by-layer character of the adsorption was confirmed by the contact angle measurements revealing an alternating surface wettability after the deposition of each layer. The study of the electromagnetic properties showed a significant increase of the magnetic field created by the surface modified samples under direct current. The resistance of the samples with a low number of MNPs layers increased in contrast to the ones with a high number of MNPs layers. On the basis of the obtained data, the relative change in the Q-factor of the samples with and without surface modification could be evaluated. Thus, the Q-factor of the samples with 10 and 15 layers of MNPs increased 2.4 and 2.6 times, respectively. Therefore, it can be concluded that the surface modification technique presented in this work can be applied to increase the Q-factor of passive elements, which are of great importance in the RF front-end and radio transceiver sections of the wireless terminal. As a result, the approach demonstrated here could contribute to the already existing technology processes of passive elements. This study offers a new direction in fabrication of inductive elements. Here, the main focus was given to the process of surface modification. Further dedicated studies should thus pave the way for making use of ultrasound to modify the morphology and chemistry of the surfaces and LbL to functionalize those surfaces. This would be a major step for establishing new technology for broad use in material science for electronics. In addition, further research of the electrophysical properties of wires coated with soft magnetic materials is of interest because these structures could be promising magnetic conductors.

and S is the conductor cross-sectional area (Supporting Information). Also, it should be noted that, according to our XRD data and previous publications related to sonication of the aluminum,34 after sonication aluminum oxide and hydroxide are formed on the aluminum surfaces. This could lead to an increase of the resistance and should not affect the inductance of the aluminum wire samples (Supporting Information). By knowing the values of the inductance and the resistance of the samples, it is possible to estimate the Q-factor according to relation 1. The frequency can be excluded, if we calculate relative change in Q-factor by the following relation QN Q UN

=

LN RUN RN L UN

(5)

where QN, LN, RN are the Q-factor, inductance, and resistance of the samples with a coating containing N number of PEI/ MNPs bilayers, and QUN, LUN, RUN correspond to the initial uncoated samples respectively. For samples containing 10 PEI/ MNPs bilayers, the Q-factor was 2.4 times higher compared to that of the uncoated ones. Moreover, for samples containing 15 PEI/MNPs bilayers the Q-factor increased 2.6 times. An increased Q-factor means that the losses in inductive elements with functionalized surface are less compared to the nonfunctionalized ones. Therefore, oscillation circuits based on such passive elements will have a narrower resonance bandwidth, which is critical for systems with tunable resonance band like integrated resonance circuits. However, we should note that the demonstrated modification procedure is quite a laborious process; nevertheless, it significantly increases the mass-transfer of MNPs. For the macroscale model of the inductive elements it is necessary to deposit a large number of PEI/MNPs layers in order to observe a 2.6-fold increase of quality factor (Q). It is a laborious process because it includes a number of deposition steps. For microscale devices, the similar increase of Q-factor will be obtained using small number of deposition steps because the influence of one adsorbed particles layer on the electrophysical properties of the inductors would be more significant due to the higher volume fraction of MNPs per substrate area. Therefore, the LbL procedure will be less laborious than for macroscale devices.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of the magnetite nanoparticles used in this study and calculated with ImageJ software MNPs size distribution and MNPs size distribution according to DLS analysis are provided. Calculated resistance as a function of the samples temperature 12280

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is also provided. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by RFBR-project 11-08-12058-ofi-m-2011 and DAAD and Ministry of Education and Science of Russian Federation cooperation program “Michail Lomonosov” (project number A/10/75870).

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REFERENCES

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dx.doi.org/10.1021/la300870y | Langmuir 2012, 28, 12275−12281