Surface engineering of zinc phthalocyanine organic thin-film transistors results in part-per-billion sensitivity towards cannabinoid vapor

Phthalocyanines (Pcs) and their metal complexes (MPcs) are macrocyclic organic compounds known for their stability, color, and electronic properties, leading to their use in various applications including dyes, pigments, organic thin-film transistors (OTFTs), and organic photovoltaics (OPVs). Pc-based OTFTs have shown potential as sensors for various analytes, including cannabinoids. Accurate and cost-effective detection of Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD) is crucial for quality control and regulation in the cannabis industry, as current techniques like HPLC and GC-MS can be impractical for many users. Previous research from the authors showed a correlation between OTFT sensing characteristics and analyte-induced physical thin-film effects, emphasizing the importance of material selection. Pcs offer versatility through modifications to their central metal and peripheral substituents, enabling tuning of electronic, colorimetric, and solubility properties. Peripheral fluorination, for example, alters thin-film band structures and crystal morphologies, enhancing solubility and n-type behavior in OTFTs. Deposition methods, such as physical vapor deposition (PVD), influence thin-film morphology, impacting charge transport characteristics like mobility (µ), voltage threshold (V_T), hysteresis, on/off current ratio, and defect density (N). Post-deposition annealing can further refine these characteristics. Zinc phthalocyanine (ZnPc) stands out as a highly tunable and sensitive material, with analyte exposure inducing structural changes that affect charge transport. Therefore, the hypothesis is that different ZnPc film structures will exhibit varied sensing responses. This study investigates the effects of THC vapor on ZnPc, F₄-ZnPc, and F₁₆-ZnPc OTFTs, comparing their performance to CuPc and F₁₆-CuPc. The impact of deposition conditions, crystal morphology, and film thickness on THC vapor sensitivity is investigated using various characterization techniques, including XRD, GIWAXS, AFM, and SEM.

The literature extensively covers phthalocyanines' applications in various fields due to their unique spectral and electronic properties. Their use as dyes and pigments dates back to 1907. Research highlights their role as charge transport layers in OTFTs and OPVs. Previous work has demonstrated the use of Pc-based OTFTs for liquid and gas sensing, including the authors' own work on ratiometric detection and differentiation of THC and CBD. The importance of accurate cannabinoid quantification is emphasized due to the different pharmacological effects of THC and CBD. Commercial methods like HPLC and GC-MS are available, but they are often expensive and inaccessible. Studies focusing on peripheral fluorination of Pcs show its influence on thin-film properties and OTFT performance. The impact of deposition conditions and annealing techniques on charge transport in Pc-based OTFTs is well-documented. ZnPc is identified in the literature as a highly tunable and sensitive material for various sensing applications, and previous work by the authors established the relationship between analyte exposure, physical film changes, and resulting OTFT responses.

Five different phthalocyanine materials (CuPc, F₁₆-CuPc, ZnPc, F₄-ZnPc, F₁₆-ZnPc) were deposited as 400 Å thin films using PVD at 25 °C with a rate of 0.2 Å/s onto OTS-treated Si/SiO₂ substrates, some with pre-patterned gold electrodes. GIWAXS and XRD were used to characterize the films' crystallinity before THC exposure. OTFT performance was evaluated before and after exposure to 4 ppm THC vapor for 90 seconds, measuring %Δµ, ΔV_T, ΔN, %ΔHys, and ΔOn/Off. For ZnPc, the effects of deposition conditions (PVD rate, substrate temperature, p-sexiphenyl patterning agent) on thin-film crystallinity and OTFT performance were investigated. Films with varying degrees of α-crystallinity were prepared and exposed to 400 ppb THC vapor for 90 seconds. AFM was used to analyze grain sizes and surface area. The impact of film thickness (200 Å and 800 Å) was assessed using low-crystallinity α-ZnPc films exposed to 40 ppb THC vapor. Real-time electrical characterization during vapor exposure was conducted at fixed saturation biases to observe current changes. β-ZnPc thin films were prepared by toluene vapor treatment of α-ZnPc films and characterized before and after 400 ppb THC vapor exposure using XRD, GIWAXS, SEM, and OTFT measurements. All GIWAXS data was calibrated and analyzed using GIXSGUI software. AFM and SEM were used for surface morphology analysis. OTFT mobility, defect density, and threshold voltage were calculated from transfer curve data. XRD was used to analyze changes in crystallinity.

Among the initially tested materials, non-fluorinated ZnPc OTFTs exhibited the highest sensitivity to THC vapor. Peripheral fluorination limited analyte-induced structural changes, while the central metal played a role in voltage threshold shifts. Varying the crystallinity of α-ZnPc films showed that less crystalline films underwent more significant physical changes upon analyte exposure and had larger OTFT electrical changes. Film thickness affected sensitivity; 200 Å low-crystallinity films showed sensitivity to 40 ppb THC. Real-time measurements on α-ZnPc OTFTs revealed an immediate, reversible hole-trapping effect with THC vapor onset, followed by a sustained, irreversible current decrease due to film restructuring. Highly crystalline β-ZnPc films underwent significant structural changes (transitioning from large crystals to sheet-like structures) when exposed to THC vapor, resulting in a turn-on OTFT response. Film engineering resulted in a 100x increase in sensitivity compared to previous CuPc-based devices. The optimal conditions for high sensitivity were achieved with low-crystallinity α-ZnPc thin films of 200 Å thickness.

The findings directly address the research question by demonstrating the significant impact of thin-film engineering on the sensitivity of ZnPc-based OTFTs to THC vapor. The 100x increase in sensitivity shows the potential of these devices for practical cannabinoid detection. The observation that less crystalline films are more sensitive suggests that increased surface area and more accessible sites for THC interaction contribute to higher sensitivity. The real-time measurements provide insights into the mechanisms of THC interaction with the ZnPc film, involving initial hole-trapping followed by irreversible structural changes. The different responses of α- and β-ZnPc highlight the importance of polymorph selection. The results are significant to the field as they demonstrate a substantial improvement in the sensitivity of a previously developed sensing platform, paving the way for more sensitive and practical cannabinoid detection methods. The observed selectivity towards THC over other compounds in cigarette smoke demonstrates potential for specific cannabinoid detection.

This study demonstrates a 100x increase in the sensitivity of ZnPc-based OTFTs for THC detection through thin-film engineering. The optimal sensor design involves low-crystallinity α-ZnPc films of 200 Å thickness. Future research could focus on exploring other Pc derivatives and optimizing film processing techniques to further enhance sensitivity and selectivity. Investigating the long-term stability and reproducibility of these sensors under various environmental conditions is also warranted.

The study primarily focused on THC detection. Further research is needed to assess the sensitivity and selectivity for other cannabinoids, such as CBD, and other volatile organic compounds. The real-time measurements were conducted under specific bias conditions, and the sensor response might vary under different operating parameters. The long-term stability and reusability of the sensors still require further investigation.