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Carbon Nanotube Thin-Film Antennas Ivan Puchades, Jamie E. Rossi, Cory D. Cress, Eric Naglich, and Brian J. Landi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05146 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016
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Carbon Nanotube Thin-Film Antennas Ivan Puchades,1,2 Jamie E. Rossi,1,2 Cory D. Cress,3 Eric Naglich,3 Brian J. Landi1,2,* 1
Department of Chemical Engineering, Rochester Institute of Technology, Rochester, NY, 14623 2
NanoPower Research Laboratories, Rochester Institute of Technology, Rochester, NY, 14623 3
Electronics Science and Technology Division, United States Naval Research Laboratory, Washington, D.C. 20375 *Corresponding author: Phone: 585 475-4726. Email:
[email protected] Abstract Multi-walled carbon nanotube (MWCNT) and single-wall carbon nanotube (SWCNT) dipole antennas have been successfully designed, fabricated, and tested. Antennas of varying length were fabricated using flexible bulk MWCNT sheet material and evaluated to confirm the validity of a full wave antenna design equation.
The ~20x improvement in electrical conductivity
provided by chemically doped SWCNT thin-films over MWCNT sheets presents an opportunity for the fabrication of thin-film antennas leading to potentially simplified system integration and optical transparency. The resonance characteristics of a fabricated chlorosulphonic acid-doped SWCNT thin-film antenna demonstrate the feasibility of the technology and indicate that when the sheet resistance of the thin-film is >40 ohm/sq no power is absorbed by the antenna, and that a sheet resistance of 100°C) results in an increase in sheet resistance to 8.2 ohm/sq, and S11 is decreased to -15.2 dB indicating further impedance mismatch. The sudden increase in sheet resistance observed after this thermal oxidation step may be explained by water evaporating from the SWCNT thin-film. As the temperature of the thermal oxidation is increased even further beyond 150°C, a second decomposition event takes place, which coincides with the boiling point of CSA (151 - 152oC),23 at which point the sheet resistance increases to 12.5 ohm/sq and the S11 decreases stepwise to -11.6 dB at 250°C . Figure 2c and 2d show that a third decomposition event takes place at 400°C, where the antenna resonates with a low quality factor and increased reflected power. The sheet resistance increases to 44.6 ohm/sq after 425°C indicating the electrical conductivity enhancement provided by the CSA dopant has been completely removed. A second SWCNT thin-film antenna was fabricated to further investigate the effect of the impedance mismatch on the performance of this type of antenna as it relates to changes in the sheet resistance. Figure S2a shows the value of S11 at resonance and the measured DC sheet resistance (presented in this case in a linear scale) while Figure S2b shows the measured real and imaginary impedance at resonance of the thin-film antenna after each of the thermal oxidation steps without any matching. The changes in measured input impedance follow a similar trend to that of the sheet resistance. Both the real and the imaginary components of the impedance show a slight decrease in magnitude from 56.82 – j10.24 ohm at room temperature to 62.96 –j20.93 ohm after the 375 °C thermal oxidation step. This correlated to a decrease of S11 and from -19.9 to 13.0 dB and an increase of sheet resistance from 6.8 to 20.5 ohm/sq. In this particular antenna, when the thermal oxidation temperature is increase to 425 °C the antenna is essentially turned off as the S11 approaches 0dB and the input impedance is measured as 4.48 - j73.45 ohm. This 8 ACS Paragon Plus Environment
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indicates that not only the input impedance indicates that the antenna is not well matched, but also that the resistance of the thin film has also become so large that the antenna is essentially off. The sequential removal of CSA from SWCNT thin-films during thermal oxidation, leading to the complete dopant removal at 400°C, has been shown previously using spectroscopic analysis (Raman and optical absorbance spectroscopy).20 Thermogravimetric analysis (TGA) was performed on a representative sample to confirm the decomposition events observed in the sheet resistance and S11 measurements. Figure S3 shows that after an initial weight loss at around 100°C, there are two additional decomposition events at around 200°C and 400°C that correspond to the stepwise degradation observed in sheet resistance, impedance mismatch, and S11. The fact that very little power is delivered to the antenna at a sheet resistance value above ~40 ohm/sq indicates that a critical resistance value is needed to reduce the resistive losses due to the conductivity of the material. Previously published reports indicate that the SWCNT thin-film morphology does not appear to change during thermal oxidation as demonstrated by SEM analysis.20 However, this technique does not provide enough resolution to investigate possible changes at the nanoscale level, which could indeed account for the changes observed in the impedance measurements, and specifically in the measured increase in capacitance, as the CSA dopant is removed from the SWCNTs. Carrier transport in SWCNTs is complex and its mechanism is still being investigated in terms of tube-tube and intra-tube contact resistance as well as the interactions with dopants.24 Our investigation does not aim to answer these questions and only looks at the effect that bulk material properties, such as sheet resistance, has on the properties of the presented unbalanced antenna. 9 ACS Paragon Plus Environment
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Figure 2. (a) Picture of SWCNT thin-film antenna on a glass substrate. (b) 3D profilometry on the end of the radiating arm of a SWCNT thin-film antenna, the inset shows the SWCNT bundle morphology. (c) Plot summarizing measured progression of S11 of a dipole SWCNT thin-film antenna after thermal oxidation at different temperatures. (d) |S11| and sheet resistance (Rs), in a logarithmic scale, of a SWCNT thin-film antenna as a function of the thermal oxidation temperatures. Comparison to other thin-film antennas 10 ACS Paragon Plus Environment
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Additional CNT dipole antennas were fabricated in order to ascertain whether antenna performance is dependent on CNT-type. Figure 3 summarizes the performance of several thinfilm and CNT thin-film antennas, in terms of return loss (|S11|) as a function of sheet resistance. Included in Figure 3 is the data for the CSA-doped SWCNT thin-film antenna at the different thermal oxidation conditions, as presented above, (labeled as SWCNT-LV-CSA Thermal Ox) and the MWCNT antenna with L=20 mm, fabricated from bulk Nanocomp sheet material (labeled as MWCNT). The effect of KAuBr4 chemical doping on antenna performance was evaluated by fabricating additional thin-film antennas with LV SWCNT material using the same CSA-based process described above. The resulting SWCNT thin-film antenna was purified via thermal oxidation, and doped with a 5 mM KAuBr4 solution,20 as described in the supporting information (labeled as SWCNT-LV, -CSA, and -KAuBr4 in Figure 3). SWCNT thin-film antennas were also fabricated using electronic-type-separated SWCNTs synthesized via an arcdischarge (ARC) method to assess antenna performance with respect to SWCNT synthesis method, electronic-type, and effects of doping. Electronic-type separated SWCNTs were purchased from NanoIntegris, Inc, where they are separated using density gradient ultracentrifugation. Purified, CSA doped, and KAuBr4 doped SWCNT thin-film antennas were fabricated with unsorted arc-discharge SWCNTs (SWCNT-ARC-CSA, KAuBr4), with metallic SWCNTs (SWCNT-ARC-Metallic, KAuBr4), and with semiconducting SWCNTs (SWCNTARC-Semi-CSA, KAuBr4). The value of |S11| at resonance has been extracted from recently reported CNT-based and thinfilm antennas and included in Figure 3 to further compare the performance of the fabricated CNT antennas, and to determine whether the material type (metal, metal oxide, CNT-based composites) has a strong influence on the resonance behavior. All these surveyed antennas were 11 ACS Paragon Plus Environment
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fabricated without a balance circuit and were designed to resonate at a single frequency while changes in material parameters were quantified in terms of their return loss. As such, no attempt was made to correct for impedance mismatch and the sheet resistance of the material is used as a gauge of antenna performance. The result of a modeled 420 nm copper thin-film patch antenna is included as a reference of performance [Patil, 2014].25 Also included in Figure 3 are patch antennas fabricated using a mixed cellulose ester (MCE) based thin-film transfer method of MWCNTs to a PDMS substrate, and labeled as “1 µm MWCNT-PDMS-Gold [Tang, 2012]”.7 These MWCNT-PDMS composites were coated with gold in order to obtain a sheet resistance low enough (~10 ohms/sq) for efficient resonance (|S11| = 20dB). In addition, patch antennas fabricated with MWCNT sheets of different thickness, labeled as “0.5-5 µm MWCNT [Keller, 2014]”, are also shown in Figure 3.8 Due to the relatively low reported conductivity of 3×104 S/m, the thickness of these sheets was increased from 0.5 µm to 5 µm, which correlates to a 10x decrease in sheet resistance, in order to obtain an antenna with a better impedance match and an |S11| > 10dB at resonance. The |S11| value at resonance of MWCNT-ink-based patch antennas are also included in Figure 3 and labeled as “0.5 mm MWCNT Ink [Elwi, 2010]”.9 The MWCNTink was obtained by adding MWCNTs to a sodium cholate solution. The resultant thin-film has poor conductivity properties and a thickness of 0.5 mm was needed in order to obtain antenna with |S11| > 10dB and a sheet resistance of 0.1 ohm/sq. Figure 3 also includes the return loss value of monopole indium-tin-oxide thin-film antennas of varying sheet resistance (1.3 ohm/sq – 19.8 ohm/sq), labeled as “ITO [Guan, 2007]”,6 where their results indicate that the radiation efficiency of the antennas improves with thicker (lower sheet resistance) indium-tin-oxide films. Discussion
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The resistive losses of an antenna can be analyzed in terms of the ohmic resistance Rd= Rs*(L/W) for a simple antenna model, in which a network analyzer input impedance is a perfect 50 ohms and the ohmic loss has been modeled as a lumped element resistance for simplicity, the |S11| at resonance can be calculated as shown in Eq. (2).26 This equation also implies that the ohmic losses are going to be much larger than the dielectric losses, which is a reasonable assumption as the sheet resistance of the films explored in this work are much larger than that of the commonly used conductors for antennas The measured thin-film DC sheet resistance was used to calculate the ohmic resistance (Rd) based on the antenna dimensions presented in Figure 1, with W = 3 mm and L = 20 mm. The sheet resistance of a thin-film takes into account the material resistivity as well as its thickness. As such, it is a good indicator of the ohmic resistance of the structure, and provides insight on how thin an antenna can be made when a target resistance is required. The experimentally measured impedance Zin was included with an input resistance of 50 ohm, which is an average value for the range of values measured and presented in Figure S2 (40 to 60ohm), and with a reactance as it changes with the sheet resistance values as shown in Figure S2. For an unbalanced antenna, the reactance values are going to be a significant contributor to the input impedance at low sheet resistances. At higher sheet resistances the ohmic losses will dominate antenna performance. The resulting |S11| value is overlapped to the experimental and published data in Figure 3. Even though the equation is based on a particular antenna type (half-wave dipole), it follows the general trend of the presented data, which includes different antenna types, such as patch and monopole antennas.
| | = 20 log
! "#$% ) &) " ! "#$% )
(2)
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In addition, the radiation efficiency ('( ) of an antenna is defined as the ratio of the radiated power to the total power. In terms of resistance, it can also be defined as the ratio of the radiation resistance (Rr), a self-induced resistance created by the antenna’s own dynamic electric field, and the sum of the radiation and ohmic resistances (Rd), as shown in Eq. (3). The radiation efficiency is an indication of how much incident power is dissipated as heat in the antenna.27 In general, the low profile of thin-film antennas results in high ohmic resistances and highly inefficient antennas unless high conductivity materials are used, as is the case of advanced SWCNT films with the improved conductivity that is provided by chemical doping, enhanced purity and alignment.
'( =
)
)" !
(3)
The calculated antenna efficiency of the presented antenna, based on Eq. (3), is shown in the inset of Figure 3 as a function of sheet resistance. The calculated radiation efficiency does not take into account the impedance mismatch or the dielectric losses of the antennas. Figure 3 shows that there is a high return loss (most of the power is lost to reflection) for antennas with high Rs, where the radiation efficiency is also low and high thermal losses are expected. As the Rs of the thin-film material is reduced below 20 ohm/sq, the return loss decreases as the ohmic losses are reduced, the radiation efficiency increases, and thermal losses are reduced. At sheet resistances below 3 ohm/sq, the impedance mismatch, and not thermal losses, limits the return loss of the antenna. Based on the data presented in this paper and the observations made by other researchers, it can be concluded that a material with a sheet resistance of approximately 10 ohm/sq is needed for an unbalanced antenna with an acceptable return loss, taking |S11|>10dB as a benchmark, independent of the material used. Although the radiation efficiency may be low when compared 14 ACS Paragon Plus Environment
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to antennas fabricated with more conductive materials, the need for transparent films may justify its use. In the particular case of SWCNTs, the effect of chemical doping, synthesis method, and electronic-type have been analyzed, and it is expected that the SWCNT thin-film morphology (i.e., CNT alignment, density, length, bundle size) will also directly affect it’s the material conductivity,
28–30
and thus the antenna efficiency. Additionally, materials with lower sheet
resistances are needed in order to increase the radiation efficiency beyond 40% without significantly increasing their thickness and negatively affecting transparency. While others have cited skin depth effects as the cause for poor antenna performance in CNT devices,8 the likely cause is the poor film resistance preventing suitable antenna resonance. As such, it is imperative that high conductivity materials, such as chemically doped SWCNTs and other materials with low Rs, be used in order to realize the advantages of thin-film antennas. 35
1.0 0.8
30 η
e
0.6 0.4
25
0.2
11
|S | (dB)
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20
0.0
0.1
1
10
100
Rs (ohm/sq)
1000
15 10 5.0 0.0 0.1
1
10
100
1000
MWCNT SWCNT-LV-CSA Thermal Ox SWCNT-ARC SWCNT-ARC-CSA SWCNT-ARC-KAuBr4 SWCNT-ARC-Semi SWCNT-ARC-Semi-CSA SWCNT-ARC-Semi-KAuBr4 SWCNT-ARC-Metallic SWCNT-ARC-Metalllic-CSA SWCNT-ARC-Metalllic-KAuBr4 SWCNT-LV SWCNT-LV-CSA SWCNT-LV-KAuBr4 1 um MWCNT-PDMS-Gold [Tang, 2012] 0.5-5um MWCNT [Keller, 2014] 0.5mm MWCNT Ink [Elwi, 2010] ITO [Guan, 2007] |S11| Model - Eq. (3) Copper [Patil, 2014]
DC Sheet Resistance, Rs (ohm/sq)
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Figure 3. |S11| at resonance as a function of the sheet resistance for thin-film antennas fabricated with SWCNTs, MWCNTs and indium-tin-oxide (ITO). The dashed line is calculated |S11| based on Eq. 2. Inset: radiation efficiency as a function of sheet resistance as calculated using Eq. 3. Conclusions Dipole antennas resonating at 1.5 – 2 GHz have been successfully designed, fabricated, and tested with bulk MWCNT sheet material and SWCNT thin-films. The validity of the antenna design equation was demonstrated by fabricating MWCNT-sheet antennas of varying arm length and evaluating S11 performance. Doped SWCNT thin-film antennas were presented as an alternative to lower material usage due to their improved electrical conductivity. A correlation was found between the S11 at resonance, the sheet resistance, and the impedance mismatch of a CSA-doped SWCNT thin-film antenna. The results indicate that in an unmatched antenna, a minimum sheet resistance of 10 ohm/sq is required in order to achieve adequate antenna resonance (
