Design and analysis of functional multiwalled carbon nanotubes for infrared sensors
Introduction
Carbon nanotubes (CNTs) have been known as the exceptionally promising material for infrared (IR) sensors. One dimensional quantum nature and large surface to volume ratio of CNTs makes their intrinsic properties highly sensitive to small external perturbations [1], [2], [3], [4], [5], [6], [7]. Infrared photodectors are highly desirable for various applications including remote sensing and biological imaging [8]. Multiwalled carbon nanotubes (MWCNTs) based infrared detectors have received much attention due to their moderate band gap of 0.4–6.0 eV and high absorption efficiency in IR band [9]. The extremely small specific heat of a carbon nanotube gives a bolometric (change in resistance under heating) thermal detector with a very fast response time and good sensitivity [10], [11], [12].
CNTs infrared detectors were developed based on thermal and photo effects. In thermal effect like Bolometers, the output signal (resistance, current or voltage) was produced by temperature change due to infrared illumination [13], [14], [15]. Moreover in photo detector electron–hole pairs are generated and dissociated by photon absorption of nanotubes leading the photo current or photo voltage in the device [16]. Bolometer devices were fabricated using single or multiple tube arrays of single-walled and multiwalled carbon nanotubes in the form of pristine or composites as active material for infrared detection [15], [17], [18], [19], [20], [21]. For instance, the Mikhail et al. reported the photo response of suspended SWCNTs network under vacuum on infrared illumination. The results showed the Bolometric effect due to ultrafast relaxation of photo excited carriers under IR illumination and transferring radiation energy to crystal lattice which leads to change in resistance by rising the temperature of samples [13]. Gohier et al. reported the Bolometric effect using MWCNTs deposited on flexible polyimide substrate. The response to IR radiation had a significant resistance drop of 0.35% after 10 s of illumination at room temperature [9].
Regardless of these remarkable results, scientists are still trying to improve the integrity, detectivity, responsivity, and signal to noise ratio of the CNTs-based Bolometers. Alternatively, buckypapers of CNTs have provided a potential approach to enhance the integrity and sensitivity of infrared sensors [22].
CNTs with no intrinsic defects are chemically inert due to sp2 hybridization in C
C bonding. Defects were created by acid treatment which causes the functional group to attach on defect sites along side walls of nanotubes [23], [24], [25]. The functional groups induces the localized impurity states near the Fermi level within the band gap region along the radial directions on the side walls of nanotubes and forms the local sp3 rehybridization of CC bonding [26]. These impurity states dislocates the conducting π and π* states. The scattering centers were formed due to functionalization which disturbs the ballistic conducting properties of nanotubes and leads the considerable change in electronic states as well as in the electrical properties of nanotubes [27]. Furthermore, the electrodes on the outermost shells of MWCNTs, which may be either semiconducting or metallic, plays the dominant role in electrical transport via photon absorption which causes the phonons generation and exciton dissociation in nanotubes [12]. The semiconductor band gap of MWCNTs decreases inversely with the tube diameter making nanotubes the excellent candidate for infrared sensing. Motivated by these advantages, we have investigated the infrared photoresponse of unsuspended buckypapers of nanotubes.
In this work, buckypapers of chemically side-wall surface modified semiconducting CNTs by attaching different functional groups: carboxylic acid (COOH) and thiol (SH) were utilized as an important step for sensing infrared radiations. Buckypapers of functionalized CNTs were obtained by covalent cross-linking of nanotubes. The cross-linking between multiwall nanotubes was achieved by controlled chemical treatments which enhances the mechanical strength as well as to characterize the electrical conduction behavior as compared to those which are linked by only weak interactions. Vacuum filtration process was used for buckypaper formation which provides the control of homogeneous thickness of buckypaper. The density and thickness of buckypaper has been maintained by observing the same weight measurements in each chemical reaction of reactants. The CNTs in random network have been interconnected through electrodes for continuous electrical paths between electrodes. It revealed the nonlinear current–voltage behavior and showed the semiconducting transport in network. As functionalized CNTs produces the outer wall defects in nanotubes, so the incident radiation on buckypaper absorb energy and excitations and relaxation of valance electrons occurred with the creation of vibrational modes when the frequency of infrared was same as the vibrational frequency of attached bonds. This led to increase in thermal energy with electronic excitations and relaxation and hence the sensitivity via resistance change was observed which may be due to the charge carrier density.
The products were characterized by using X-ray powder diffraction (XRD, X’Pert PRO Difractometer, PANalytical) with Cu Kα radiation (λ = 1.5418 Å) and Fourier transform infrared (FT-IR) spectrophotometer Nexus 670 Thermo Nicolet in a Potassium Bromide (KBr) tablet. Field emission scanning electron microscopy (FESEM) images were taken on JSM JEOL 7401 with ultra high resolution at 3 kV. Transmission electron microscopy (TEM) images were taken on a JEOL, JEM-2010 microscope (200 kV) with EDX attachment. Raman spectroscopy of samples was taken on Lab RAM HR800. Electrical measurements were taken using optical microscope unit FS-70Z with keithley source meter model 2636A using two-probe method. Sensor measurements were made using keithley 6½ digital multimeter 2100.
Section snippets
Chemicals
2-Mercaptoethanol (HSCH2CH2OH) 99% pure 4,4′-diaminobenzophenone (C13H12N2O), nitric acid (HNO3), hydrogen per oxide (H2O2), dimethylformamide (DMF) 99% pure, and all organic solvents were purchased from Sigma Aldrich and were used as received without further purification.
Fabrication and sensor setup
MWCNTs were purchased from Beijing DK Nano technology, China. The diameter of MWCNTs used was about 25–60 nm and length was 10–20 μm. The MWCNTs were purified by reflux and sonication in concentrated hydrochloric acid for 3 h at
Results and discussion
Fig. 2 shows the X-ray diffraction analysis of pristine carbon nanotubes. The XRD patterns were recorded in angle range (2θ) 20°–80°, where the angle 2θ is between the incident and scattered beams. The XRD pattern of CNTs samples revealed the presences of three peaks at 26.1576°, 44.3053° and 51.6300° corresponding to d0 0 2, d1 0 1 and d1 0 2 reflections of hexagonal graphite structure of carbon atoms. The structure has p63/mmc space group with lattice constant a = b = 2.47 Å and c = 6.8 Å. The metal
Conclusions
We have successfully fabricated the infrared sensor device using functionalized multiwall carbon nanotubes buckypapers by varying the cross-linker reagent ratio in the buckypapers. The carbon nanotubes were functionalized by carboxyl and thiol group. FT-IR and Raman spectroscopic analysis confirmed the attachment of the functional groups. SEM analysis showed the morphology and agglomeration of nanotubes in the buckypapers. TEM analysis showed the outer wall defects produced by chemical
Acknowledgements
The authors are grateful to Prof. Fei WEI, Department of Chemical Engineering, Tsinghua University for characterization tools HRTEM, FESEM, FT-IR, Raman spectroscopy and electrical measurements unit FS-70Z with keithley source meter. We are also thankful to Prof. Dr. Tajammul Hussain (Late) NCP and Higher Education Commission (HEC) Pakistan for the financial support through “National Research Program for Universities” and COMSATS Institute of Information Technology.
Mrs. R. Afrin is a PhD scholar (Physics, Micro and Optoelectronics) at COMSATS Institute of Information Technology, Islamabad, Pakistan. Her research topic is Fabrication and characterization of carbon nanostructures for infrared and chemical sensor.
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Mrs. R. Afrin is a PhD scholar (Physics, Micro and Optoelectronics) at COMSATS Institute of Information Technology, Islamabad, Pakistan. Her research topic is Fabrication and characterization of carbon nanostructures for infrared and chemical sensor.
Dr. N.A. Shah completed his PhD in Quaid-i-Azam University Islamabad, Pakistan. Currently he is an Associate Professor of Physics at Department of Physics, COMSATS Institute of Information Technology, Islamabad and Principal Investigator, HEC Pakistan. He is a material scientist and currently working on the thin films solar cells for renewable energies.
Mr. M. Abbas is a PhD scholar (Materials Science and Engineering) at the Department of physics, COMSATS. His research interest is nanostructures for sensing applications and nano solar cells.
M. Amin is a PhD scholar (Physics, Micro and Optoelectronics) at COMSATS Institute of Information Technology, Islamabad, Pakistan. His research topic is Oxide semiconductor Nanostructures for Chemical Sensors.
Prof. Dr. A.S. Bhatti completed his PhD in University of Cambridge, Cambridge, UK. At present he is a professor and a Dean, Faculty of Science at Department of Physics, COMSATS Institute of Information Technology, Islamabad. He is a semiconductor physicist by training and recently working in synthesis and application of nanostructures for sensor applications. He established state of the art device fabrication facilities in this institute.