RSC Advances View Article Online PAPER View Journal | View Issue Single wall carbon nanotube based optical rectenna Cite this: RSC Adv.,2021,11,24116 Lina Tizani, ac Yawar Abbas, b Ahmed Mahdy Yassin,ac Baker Mohammadac and Moh’d Rezeq*bc We present an optical rectenna by engineering a rectifying diode at the interface between a metal probe of an atomic force microscope (AFM) and a single wall carbon nanotube (SWCNT) that acts as a nano-antenna. Individual SWCNT electrical and optical characteristics have been investigated using a conductive AFM nano-probe in contact with two device structures, one with a SWCNT placed on a CuO/Cu substrate and the other one with a SWCNT on a SiO2/Si substrate. The I–V measurements performed for both designs have exhibited an explicit rectification behavior and the sensitivity of carbon nanotube (CNT)- based rectenna to light. The measured output current at a set voltage value demonstrates the significant effect of the light irradiation on the current signal generated between the Au nano-probe and CNT interface. This effect is more prominent in the case of the CuO/Cu substrate. Detailed analysis of the Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Received 30th May 2021 system, including the energy band diagram, materials characterization and finite element simulation, is Accepted 29th June 2021 included to explain the experimental observations. This work will pave the way for more investigations DOI: 10.1039/d1ra04186j and potential applications of CNTs as nano-rectennas in optical communication and energy harvesting rsc.li/rsc-advances systems. Introduction Also, CNTs make exemplary antenna elements as they absorb electromagnetic energy in a broad spectrum.15,16 Since the 1D Optical antennas represent an optical detector similar to growth technique is rapidly developing, CNTs will not only This article is licensed under a radio-frequency antennas but operating in the optical regime. substitute materials in many areas, but also will introduce new Nowadays, optical antennas are attracting a lot of attention devices with improved performances. The nano-scale size will due to their potential for wide applications in nano-scale introduce unique properties for CNT-based photo-detectors Open Access Article. Published on 08 July 2021. Downloaded 9/28/2021 7:22:36 PM. optical microscopy, spectroscopy, solar energy conversion, that can reduce dark current due to the small size of CNTs.17 integrated optical nano-circuitry, molecular sensing, etc.1This In addition, the high surface to volume ratio of the CNT can increased focus on nano-antennas is due to the progress in improve the sensitivity of the photo-detectors. More interest- nano-manufacturing technologies, which makes it possible to ingly, in addition to their high conductivity single wall carbon utilize sub-wavelength plasmonic structures in different nanotubes (SWCNTs) which can be obtained by geometrically shapes and sizes.2 One essential synthesis process required for rolling a graphene sheet, have showed a band gap structure with the realization of nano-antennas at a small scale (nano scale) a band gap that varies from 0 to 2 eV.18 Several major techniques is utilizing a nano-probe that relies on ultra-sharp tips3,4 in have been used to synthesize CNT's: arc discharge, laser abla- making physical contact with individual CNTs. Moreover, this tion, and chemical vapor deposition (CVD).19 has been integrated with a new approach used for modeling Classical rectenna is formed from an antenna and a diode and fabrication of a tunneling-based nano-Schottky diode connected. The rectenna at optical frequency encounter several rectier.5 fundamental challenges related to the rectieroperatingathigh CNTs' outstanding electrical, mechanical, and physical frequency as well as the antenna having a low RC time constant. properties make them ideal candidates for many potential Several types of diodes have been explored Metal/Insulator/Metal applications including transistors,6–8 gas sensors,9,10 optical (MIM) and Metal/Insulator/Insulator/Metal (MIIM) diodes, sensors,11 light emitters12,13 and for MEMS/NEMS applications.14 diodes that combine multiple dielectrics show better performance than single insulator diodes through more control of electron tunneling.20 A new approach has been demonstrated using CNT in aDepartment of Electrical Engineering and Computer Science, Khalifa University, Abu the rectenna design. This has been demonstrated and fabricated Dhabi, 127788, UAE for the rst time using multi wall CNT vertically aligned as metal bDepartment of Physics, Khalifa University, Abu Dhabi, 127788, UAE. E-mail: mohd. electrode in MIM diode. These rectenna devices show increased [email protected] nonlinearity when illuminated.21 Another rectenna using multi cSystem on Chip Center, Khalifa University, Abu Dhabi, 127788, UAE 24116 | RSC Adv.,2021,11, 24116–24124 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances Fig. 1 (a) Steps for the dispersion of CNT powder and sample preparation, (b) schematic of the first device using Si as a substrate with a thin SiO2 layer of 2.31 nm, (c) schematic of the second device using Cu sheet as the substrate with a thin CuO layer of 3.69 nm. insulator along with CNT as metal electrode has been fabricated solution was sonicated by tip for 2.0 minutes. The resulting and used in order to model the photon assisted tunneling mech- mixture was bath sonicated for 4 h followed by 1 minute tip anism. This rectenna shows excellent rectication behavior in the sonication. This sonication process is considered the most optical regime.22 essential step in the dispersion process where most nanotubes In this work, we have investigated the opto-electrical de-bundle.33 A small volume was drop-casted on a clean Creative Commons Attribution-NonCommercial 3.0 Unported Licence. behavior of SWCNTs as nano-rectennas. However, isolating an substrate at room temperature and dried in a vacuum oven at individual carbon nanotube is an essential requirement for 120 C for 4 hours as shown Fig. 1(a). The substrate was cleaned testing light detection characteristics of SWCNT-based nano- using a bath sonication in acetone followed by isopropanol and rectenna. Pristine CNTs as synthesized are bundled. They are distilled water. Two substrates were used in our experiments: hard to disperse due to the strong van der Waals forces and the rst substrate is n-Si with 2.31 nm SiO2 layer on the top. their high aspect ratios. Various solvents have been reported to Fig. 1(b) shows the schematic of SiO2/n-Si substrate and the successfully debundle CNTs as well as obtaining stable and thickness of SiO2 was measured using spectroscopic ellipsom- dispersed nanotubes.4,23–33 N-Methylpyrrolidone (NMP) has etry. The second substrate is a copper sheet with a thickness of been a highly effective solvent for dispersing and debundling 625 mm. The substrate is cleaned using a bath sonication in This article is licensed under a SWCNTs.23–26 acetone followed by isopropanol and distilled water. Oxidation Here we show that an efficient optical rectenna can be con- at temperatures lower than 400 C leads to the growth of a thin structed by using individually dispersed SWCNT on metal oxide copper oxide layer (CuO) over the Cu surface.35 For our device, Open Access Article. Published on 08 July 2021. Downloaded 9/28/2021 7:22:36 PM. or silicon oxide substrates. The optical characteristics of rec- a thin layer of copper oxide of 3.69 nm, measured by spectro- tenna are demonstrated using conductive mode atomic force scopic ellipsometry, is formed by thermal oxidation of Cu by microscope (C-AFM). This demonstration is based on the eld heating at 150 C for 1 h prior to the deposition of CNT, as À enhancement at the carbon nanotube interface with the metal shown Fig. 1c. In our work, the concentration of 0.0093 mg ml 1 nano-probe of C-AFM. Using C-AFM equipped with a white light of SWCNTs was used to prepare our samples, since no aggre- À source, we measured the electrical characteristics of the nano- gates have been reported to be present below 0.02 mg ml 1 that diode and the effect of the adsorption of the optical light by is considered as nanotube dispersion limit in NMP.26 the CNT. The results agree with the rectenna theory21,34 where our devices show a clear eld rectication from the antenna. Measurement methodology and CNT characterization Conductive mode atomic force microscope (C-AFM) has been Experimental measurements used as a powerful tool to investigate the photo detection and rectication behavior at the nano-scale metal-semiconductor Dispersing single wall CNTs (SWCNTs) (MS) junctions.36–39 Using C-AFM we can determine the topog- Materials used in this experiment: SWCNTs powder with the raphy and the electrical properties simultaneously with the help chirality (7,6), $77% carbon as SWCNT and 2–3 nm diameter in of a conductive tip, while a bias voltage is applied on the sample average from Sigma-Aldrich. N-Methyl-2-pyrrolidone (NMP) and the tip is kept at ground as shown in Fig. 2(a). During the used for dispersion of these SWCNTs from the powder. 1.0 mg acquisition of the I–V curves, the tip is brought into contact with of SWCNT powder was mixed with 15 ml of NMP, the resulting the CNT of interest. To assess the light and dark condition of mixture of SWCNT was tip sonicated for 2.0 minutes. This CNT nano-rectenna, the I–V measurements were performed initial dispersion was serially diluted in order to produce using the AFM system that is placed in a closed chamber by a range of solutions with different concentrations from switching the light source on and off with 0.1 Hz (10 s), which is À À 0.066 mg ml 1 to 0.0093 mg ml 1.Aer each dilution, the guided via and optical ber to the junction between the probe © 2021 The Author(s).