Switching Behavior of the Composite Low Dimensional Structural Hybrids of Carbon After UV Exposure

Low dimensional multi structural components of carbon are of great interest currently owing to their applications in developing flexible plastic electronics. In this work, we discuss emergence of different structural hybrids of carbon from as-prepared HiPCO SWCNTs subjecting them to oxidative acid purification and covalent functionalization process. Transmission electron microscopy (TEM) investigations reveal single wall carbon nanotubes (SWCNTs) serve as a building block to obtain multi wall carbon nanotubes (MWCNTs), graphene sheets (GS), carbon nano scrolls (CNS) structures which coexist in the sample. These structures when grafted with polymer binder and spray coated on Si substrate provide highly sustainable thin film coatings that are stable even in adverse space conditions [1]. These CNT and CNS based composite coatings are promising candidates for stray light control space applications exhibiting a low reflectance of the order of 2-3% in the visible spectral range [1]. Electrical switching behaviour of these films were investigated by capturing current(time) (I(t)) response, displaying promising switching character with distinct ON-OFF cycle. These coatings offer opportunities for the development of facile, cost-effective carbon-based devices without going into the nontrivial task of separating different the structures.


Introduction
The discovery of carbon nanotubes (CNTs) by S. Iijima stands as a pivotal event that revolutionized nanomaterials research on both fundamental and practical fronts [2].In 1991, S. Iijima made a groundbreaking observation of multi-walled carbon nanotubes (MWCNTs) while conducting transmission electron microscopy (TEM) studies [3].He characterized these structures as carbon allotropes with a distinctive hollow cylindrical-tube shape.In 1993, Iijima and Bethune independently identified singlewalled carbon nanotubes (SWCNTs) [2].Subsequently, the development of high-pressure carbon monoxide (HiPCO) SWCNTs introduced a novel synthesis method, spearheaded by Richard Smalley and his team at Rice University in 2000 [4].This innovative approach represented a major advancement in the production of high-quality SWCNTs, known for their exceptional quality and structural consistency with fewer defects and a narrower diameter distribution compared to alternative synthesis techniques [5].These remarkable properties, encompassing high electrical and thermal conductivity, coupled with impressive mechanical strength, have positioned HiPCO SWCNTs as highly coveted materials for applications in materials science, electronics, and nanotechnology [6], [7], [8].Although production costs may be relatively higher, persistent research and development initiatives determinedly explore HiPCO SWCNTs' potential for next-generation materials and technologies, reinforcing their substantial impact in the realm of advanced materials.Recently, carbon nanomaterials have shown promising photo responsive behaviour using white light (WL) as a stimuli which is readily available and is a spontaneous choice for many applications owing to its wide spectral range.Photoactive structural forms of carbon, for example functionalized SWCNTs and graphene with azobenzene can induce switching of electrical conductivity when irradiated by UV and visible light, showcasing potential utility in sensors, memory and energy storage devices [9], [10], [11], [12], [13], [14], [15].
In this work, we explore the current switching characteristics of modified low-dimensional derivatives of carbon -polymer composite thin films under WL before and after treating with UV-Ozone exposure.We demonstrate light induced electrical switching behaviour of these films, an effect that is more prominent in 1300 (2024) 012029 IOP Publishing doi:10.1088/1757-899X/1300/1/012029 2 the UV-Ozone exposed samples compared to the untreated ones.This research holds significant importance as it extends the application of these high-absorber thin film coatings as electrical switch with a well demonstrated light ON-OFF cycle, documenting their capability beyond stray light control applications in the realm of space domain reported by us earlier [1].Further, this hybrid composite films were fabricated using facile, cost-effective methods of post-synthesis purification, functionalization and coating process along with the flexibility of choosing substrate as per requirement which make them robust for any practical implementations.The as-prepared HiPCO SWCNTs required purification and functionalization before subjecting them for any applications.We employed an oxidative acid purification process to remove the embedded Fe catalyst particles and amorphous carbon contaminations from the initial HiPCO SWCNT samples.The purification procedure consisted of three primary stages, which included wet oxidation at 300°C, followed by a concentrated hydrochloric acid (HCl) wash, and concluded with annealing at 900°C in an inert atmosphere [16].Functionalization process is conducted by combining concentrated H2SO4 and HNO3 (3:1 volume ratio) and weightage amount 100 mg of purified sample in a beaker and stirring the mixture with a magnetic stirrer for 8 hours at 70 ℃.Subsequently, the mixture was allowed to cool down to room temperature before rinsing with deionized water until a neutral pH is reached.The final functionalized product was obtained by filtering the solution with Whatman filter paper and drying at room temperature under vacuum [1].To make the coated film, functionalized powder was blended with organic polyurethane binder and solvent, and subjected to ultrasonication for 60 minutes using a Q500 ultrasonic probe sonicator from Q-Sonica.This ensures creation of a well-dispersed solution, free from agglomerations.The resulting solution was employed for hand spray coating onto pre-cleaned Si substrate with a 2 μm SiO2 layer.This whole process of purification, functionalization and coating is discussed elsewhere [1].A visual representation of the experimental process is shown in Figure 1.The samples were investigated using transmission electron microscopy (TEM) by drop-casting the samples on Cu grids in an FEI-Titan Themis instrument operated at 300 kV, and a two-probe probe station was used to acquire the current-time (I-t) response under white light (WL) with an Agilent B2912A source/measure unit.Two-terminal electrodes of gold (Au) were deposited on the sample surface in a thermal evaporation chamber using shadow mask technique.Figure 2 shows high resolution TEM (HRTEM) micrographs obtained from the as-prepared, purified, functionalized and polymer grafted samples [12].The embedded FeNPs in the initial HiPCO SWCNTs can be seen as dark spots in Figure 2(a).Figure 2(b) reveals huge reduction of the FeNPs after the purification process, though, there are some traces of the Fe particles still evident in the purified data.We observe emergence of various low dimensional structural forms of carbon like MWCNTs [Figure 2(c, d)], attachment of functional groups [Figure 2(d)], graphene sheet (GS) and carbon nanoscrolls (CNS) [Figure 2(e, f)] [15], [17] in the TEM investigations as a result of purification and functionalization process.Rolling of suspended graphene layers to form CNS can be clearly seen in Figure 2(f).During purification, FeNPs undergo oxidation and are converted into soluble ferric salts that can be washed away [18].However, removal of these impurities is associated by generating localized stress, leading to the rupture of the domains around the FeNPs.Also, the FeNPs that are present in the purified sample, albeit in a small amount can act as a catalytic scissor and help in tearing or cutting the SWCNT bundles [Figure 2 This unzipping or tearing process [17], [18] in turn, is further intensified when functional groups are introduced, causing separation of SWCNT bundles via ion intercalation [7], [19].Consequently, various functionalized low-dimensional structures of carbon emerge due to exfoliation and merging of the initial CNT networks.Functionalization adds on various functional groups such as carboxyl (-COOH), hydroxyl (-OH), carbonyl (-C-O-C-) etc. which we have reported elsewhere [1], [16].Figure 2(g) shows a twisted structure in the sample, while Figure 2(h) is an HRTEM image of a polymer grafted MWCNT with a polymer layer of thickness around ~5 nm.We studied the electrical switching behaviour of these functionalized multi-structural all-carbon coated films under WL exposure by performing I(t) measurements and looking into its On-OFF states.These results are shown in Figure 3. Figure 3(a) is the schematic of the two probe measurement, while Figure 3(b) is the SEM micrograph obtained from the spin coated film on the Si substrate.I(t) behaviour under WL before and after irradiation with UV-Ozone exposure are shown in Figure 3 (c) and (d) respectively.These I(t) characteristics were acquired for a Voltage of 12 V.We observe a drastic improvement in the switching characteristic of the current with proper On-OFF cycle when the sample is treated with UV-Ozone radiation.While, CNTs are metallic or semiconducting in nature, the electrical photo-response behaviour of this conjugated multi-structural derivatives of carbon and polymer composite remains unreported.The study becomes particularly intriguing when after UV treatment, light induced electrical switching of the films show promising outcome.This experiment comprehends us about two key aspects that could be responsible for the observed results.Firstly, in the case of CNTs, there is a transition from π to π* under UV exposure [20], [21].When WL illuminates the composite film, it assists these excited electrons in transitioning to the conduction band more swiftly compared to the untreated sample without UV-Ozone treatment [20].

Results and discussion
Secondly, during HRTEM analysis, we observe a coating of polymer layer on the CNTs, which thins down when exposed to UV radiation.Given that polymers are generally insulator [22], this thinning of the polymer shield over the CNTs facilitates the movement of charge carriers in the localized domains of the polyurethane network within the film [22], manifesting a promising light induced electrical photo-response and switching characteristics.Jintoku et al. [9] reported on SWCNT, graphene hybrid films of having better absorbance by controlling the intensity and irradiation time of UV light and are switchable after visible light irradiation and thermal treatment, making them potential candidates for optoelectronic devices and sensors.

Conclusion
This study aims to unravel the photosensing and electrical switching properties of composite films containing low dimensional structures of carbon that are derived from HiPCO SWCNTs.Previously, we have conducted a comprehensive analysis of these coated samples, highlighting their low reflectance and remarkable resilience in harsh space conditions [1].Here, we show that subsequent to UV exposure, the coated films display rectified photosensing and switching behavior, introducing an intriguing additional feature.The enhanced switching characteristic is attributed to the electronic transitions and a thinner polymer coating of the UV-Ozone irradiated sample.It marks a notable step in customizing the optical and electronic properties of the carbon composite films without going into non-trivial process of separating individual structures, and enabling us to selectively modulate the properties by choosing and controlling the purification, functionalization, polymer grafting, and coating techniques.These developments hold significant importance in manufacturing photodetectors, black absorber coatings, and optoelectronic devices [5].This may herald a fresh era in the realm of nanotechnology research associated with carbon nanomaterials.

Acknowledgement
K. B. sincerely thanks DST Mobility Scheme for offering essential professional and personal stability, as well as funding for comprehensive support.

Fig. 1 .
Fig. 1.Flow chart of the experimental process adapted in this work.

Fig. 2 .
Fig. 2. (a) High-resolution TEM image of SWCNT bundles, revealing the presence of FeNP contaminants.(b) Purified SWCNTs with huge reduction in Fe catalyst particles.(c, d) MWCNTs in the specimen after purification and functionalization process.Attachment of functional groups can be seen in (d) indicated by arrows.(e, f) Presence of GS and CNS in the functionalized and coated sample.Rolling of GS to CNS is seen in (f).(g) Evidence of twisted structure.(h) TEM images of polymer-coated CNTs with a polymer thickness of around 5 nm.
Figure2shows high resolution TEM (HRTEM) micrographs obtained from the as-prepared, purified, functionalized and polymer grafted samples[12].The embedded FeNPs in the initial HiPCO SWCNTs can be seen as dark spots in Figure2(a).Figure2(b) reveals huge reduction of the FeNPs after the purification process, though, there are some traces of the Fe particles still evident in the purified data.We observe emergence of various low dimensional structural forms of carbon like MWCNTs [Figure2(c, d)], attachment of functional groups [Figure2(d)], graphene sheet (GS) and carbon nanoscrolls (CNS) [Figure2(e, f)][15],[17] in the TEM investigations as a result of purification and functionalization process.Rolling of suspended graphene layers to form CNS can be clearly seen in Figure2(f).During purification, FeNPs undergo oxidation and are converted into soluble ferric salts that can be washed away[18].However, removal of these impurities is associated by generating localized stress, leading to the rupture of the domains around the FeNPs.Also, the FeNPs that are present in the purified sample, albeit in a small amount can act as a catalytic scissor and help in tearing or cutting the SWCNT bundles [Figure2(b), shown as white arrow].

Fig. 3 .
Fig. 3. (a) A schematic of the electrical characterizations.(b) SEM micrograph of the coated film.(c) I(t) photoresponse of the composite film using WL with a ON-OFF cycle of 40s before, and (d) after UV-Ozone treatment.The composite film shows rectified ON-OFF switching behaviour of the UV-Ozone irradiated sample.