# Optically Transparent Nano-Patterned Antennas: A Review and Future Directions
## 2 Transparent Antennas: A Review
### Nano Carbon
#### 2.1.2 Carbon Nanotubes  
Carbon nanotubes (CNTs), discovered in 1991 [10], have been studied for various physical and chemical properties and applications. Types of CNTs include single wall carbon nanotubes (SWCNT), double wall carbon nanotubes (DWCNT), and multi wall carbon nanotubes (MWCNT). SWCNTs are more difficult to synthesize than MWCNT and have high utilization because they have excellent  
Figure 1: Real (**a**) and imaginary (**b**) parts of graphene conductivity versus chemical potential. Graphene parameters are \(\tau\) = 0.5 ps and T = 300 \({}^{\circ}\)K [5]. (With permission from IEEE).  
Figure 2: Analysis of (**a**) radiation efficiency of an ideal lossless antenna and real graphene-based patch antennas at microwaves [5] and (**b**) Input impedance \(Z_{\text{in}}\) of the proposed antennas. The arrows indicate the real impedance W.P. [7]. (With permission from IEEE and AIP).  
electronic characteristics. The high purity SWCNT required in the field of electronic equipment is currently produced at a small amount at a high cost. MWCNTs have already started to be commercially produced in high volumes and are now used in super capacitors, lithium ion batteries, etc.  
In Reference [11], onion-like carbon (OLC) and MWCNT dipole antennas operating at 2.4 GHz for a wireless local area network (WLAN) such as IEEE 802.11.b/g are studied. The OLC antenna is fabricated with annealing process at 1800 \({}^{\circ}\)C for 3 h while the MWCNT antenna is grown on ethylene vapor using an iron catalyst. The sample with 3.5 mm square dimensions features conductivity of 1.5 S/cm and 9.0 S/cm for OLC and MWCNT films, respectively. The dipole antenna is attached to borosilicate glass with a thickness of 1 mm and a relative permittivity of 4.6. As illustrated in Figure 3, while the copper film dipole antenna shows a peak return loss of \(-\)30 dB, both the OLC and MWCNT dipole antennas exhibit a peak return loss of below \(-\)10 dB. Moreover, both the OLC and MWCNT antennas exhibit peak gains of \(-\)1.48 dBi and \(-\)2.76 dBi, respectively, while the copper film antenna features a peak gain of 3.5 dBi at 2.4 GHz.  
In Reference [12], an MWCNT-based microstrip patch antenna operating at diverse frequencies is proposed. A conductive MWCNT ink-jet printing process is used to pattern patch antennas in two dimensions (10 mm \(\times\) 10 mm and 20 mm \(\times\) 20 mm) with three different substrates (RO3210, CCFR4, and TMM4). MWCNT ink features an average DC conductivity of 2.2 \(\times\) 10\({}^{4}\) S/m with four probe techniques, and complex relative permittivity is obtained from 6.5\(-\)10.9 to 4.5\(-\)90.1 at a frequency range from 300 kHz to 8.5 GHz. While the MWCNT antenna has low gain compared to a copper-based antenna owing to its lower conductivity, it shows a bandwidth increase compared to the corresponding copper antenna without critical degradation in gain and radiation patterns.  
As analyzed above, graphene and CNT classified as nano carbon features a much higher sheet resistance than that required for use as a transparent antenna. Graphene is expected to be more active in studying antenna operation at terahertz frequencies using the plasmonic mode. Most of the CNT film electrodes mass-produced at present show a sheet resistance (\(R_{\rm s}\)) of about 200-500 \(\Omega\)/sq and an optical transmittance (\(T\)) of 80%. So, to use CNT film as an antenna in microwave frequencies, it is necessary to study a composite material composed of two or more materials in order to compensate for the disadvantages while maintaining the advantages of one material [13; 14], such as spin coating base graphene/SWCNT (\(R_{\rm s}\) = 280 \(\Omega\)/sq, \(T\) = 90%, FoM = 12.4) [15]. Therefore, the development of transparent antennas using nano carbon is expected to be required for the electrical performance improvement of the aforementioned TCFs.  
Figure 3: (**a**) Onion-like carbon (OLC) antenna; (**b**) return loss versus frequency of OLC, carbon nanotubes (CNT), and copper antennas [11]. (With permission from AIP).