# Exotic Photothermal Response in Ti-Based MXene Optoelectronic Devices

Stefano Ippolito, Francesca Urban, Paolo Samorì, Jonathan E Spanier, Yury Gogotsi

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Stefano Ippolito, Francesca Urban, Paolo Samorì, Jonathan E Spanier, Yury Gogotsi. Exotic Photothermal Response in Ti-Based MXene Optoelectronic Devices. Advanced Electronic Materials, 2025, 10.1002/aelm.202500017. hal-05079239

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# Exotic Photothermal Response in Ti-Based MXene Optoelectronic Devices

Stefano Ippolito, Francesca Urban, Paolo Samorì, Jonathan E. Spanier, and Yury Gogotsi*

MXenes represent one-of-a-kind materials to devise radically novel technologies and achieve breakthroughs in optoelectronics. To exploit their full potential, precise control over the influence of stoichiometry on optical and thermal properties, as well as device performance, must be achieved. Here, the characteristics of optoelectronic devices based on $Ti_3C_2T_x$ and $Ti_2CT_x$ thin films are uncovered, highlighting the striking difference in their photothermal responses to laser irradiation under different experimental conditions. Even though their absorption coefficients at 450 nm are comparable, the thermal excitation and relaxation phenomena display markedly different kinetics: $Ti_2CT_x$ devices show a strong asymmetry during the heating-cooling cycle, with the heat dissipation kinetics being three orders of magnitude slower than $Ti_3C_2T_x$ and strongly influenced by environmental conditions. The findings are expected to stimulate fundamental investigations into the photothermal response of MXenes and open exciting prospects for their use in printed and wearable optoelectronics, including memory devices and neuromorphic computing.

## 1. Introduction

2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, are the focus of an ever-growing interest in the materials science community,[1,2] stemming from a wide range of unique and tunable physicochemical properties that are pivotal in numerous applications,[3] including (opto)electronics.[4–6]

S. Ippolito, F. Urban, Y. Gogotsi

A. J. Drexel Nanomaterials Institute

Department of Materials Science and Engineering

Drexel University

3141 Chestnut St, Philadelphia, PA 19104, USA

E-mail: gogotsi@drexel.edu

F. Urban, P. Samorì

University of Strasbourg, CNRS

ISIS UMR 7006, 8 allée Gaspard Monge, Strasbourg F-67000, France

J. E. Spanier

Department of Mechanical Engineering and Mechanics

Drexel University

3141 Chestnut St, Philadelphia, PA 19104, USA

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202500017

© 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aelm.202500017

Their metallic electrical conductivity and strong interaction with a broad spectrum of electromagnetic waves have already demonstrated a great potential to define and shape the future of communication, information processing, energy harvesting, and sensing.[7–11] Nevertheless, little is known about the synergistic effects of electrical, optical, and photothermal properties on device performance,[12] thus causing a significant knowledge deficit from both a fundamental and application standpoint.[13] Notably, most works reported in the literature deal with $Ti_3C_2T_x$, and much less is known about other MXenes and their inherent properties.[13] This imbalance is a major limitation to advancing MXenes in optoelectronics, limiting access to state-of-the-art device performance. In particular, $Ti_2CT_x$ has long been neglected due to its lower environmental

stability and higher tendency to oxidize in aqueous dispersions than $Ti_3C_2T_x$. Because of that, just a handful of references can be found in the literature about the physicochemical properties and applications of $Ti_2CT_x$, none of which address its use as an active material in electronic devices. However, recent works have demonstrated that the quality and shelf life of Ti-based MXenes can be dramatically improved through specific synthesis protocols, which optimize stoichiometry and minimize the concentration of defects, as well as appropriate storage conditions.[14,15] Most of the studies reported so far refer to changes in electric current following irradiation as photoresponse.[16] However, this could be misleading, given the strong habit of using such a term for semiconducting materials. MXenes are metallic conductors, with semiconducting structures only theoretically predicted.[17] Therefore, it would be more accurate to refer to current changes after irradiation as bolometric effects or, more specifically, as photothermal response due to the variation of local temperature (viz., resistance) following photon absorption.

Here, we fabricated Ti-based MXene devices for optical sensing and storage and investigated their photothermal properties under different experimental conditions. More specifically, we compared the electrical response of $Ti_3C_2T_x$ and $Ti_2CT_x$ spray-coated films upon laser irradiation and explored their photothermal performance by modifying environmental conditions (i.e., pressure and temperature). We demonstrated that $Ti_2CT_x$ devices exhibit an exotic photothermal response, strikingly
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Optical and electrical properties of Ti₃C₂Tₓ and Ti₂CTₓ. a) Illustration of a typical Ti-based MXene device on a glass substrate. The measurement setup is also displayed, where a 450 nm laser line is used as the light source for the photothermal measurements. b) UV-vis spectra of Ti₃C₂Tₓ (blue) and Ti₂CTₓ (red), along with their corresponding crystal structures. The spectra were recorded from aqueous dispersions (0.1 mg mL⁻¹) of both MXenes. c) Resistance versus Temperature curves obtained for Ti₃C₂Tₓ and Ti₂CTₓ devices (dark conditions) in a temperature range from 300 to 77 K. The voltage bias applied for the measurements is 10 mV.</figcaption></figure>

different from Ti₃C₂Tₓ analogs. In particular, the former displays an asymmetric heating-cooling cycle, with thermal relaxation kinetics being three orders of magnitude slower and strongly affected by environmental conditions, such as air versus vacuum and low versus high temperature. As a proof of concept, these unique features are exploited for optical data storage, making our work the first of its kind. This behavior of Ti₂CTₓ has never been reported thus far, especially for all-MXene devices (i.e., no hybrid systems),[18,19] despite possessing great relevance since MXenes are the largest family of 2D materials and include thousands of different compositions when accounting for surface terminations and solid solutions. Considering the plethora of possible properties available for MXene systems, some rationalization must be provided based on specific variables, such as stoichiometry, chemical composition, structure, and/or morphology. In this Article, we prove the critical influence of stoichiometry on the photothermal properties of optoelectronic devices, showing the potential of MXenes to advance optoelectronics and provide guidelines to achieve state-of-the-art technologies.

## 2. Results and Discussion

Figure 1a portrays a typical MXene device and the experimental setup employed to investigate its photothermal properties. Each device, fabricated by photolithography, consists of a Ti-based MXene thin film obtained by spray coating (thickness = 125 ± 25 nm) on a glass substrate pre-patterned with metal electrodes (5 nm Ti, 45 nm Au) deposited via electron beam evaporation (see Experimental Section). The device area (2 × 1 mm²) is suitably designed to match the spot size of the irradiating laser line (λ = 450 nm, power = 50 mW), assuming a flat-top beam profile for ease of discussion. Before delving into the photothermal analysis, we explored the optical and electrical properties of Ti₃C₂Tₓ and Ti₂CTₓ. Figure 1b displays the normalized UV-vis absorption spectra for the systems under investigation, recorded from 200 to 1000 nm.

The two characteristic absorption peaks at ≈760–780 and 510–550 nm, depending on surface terminations, are observed for Ti₃C₂Tₓ and Ti₂CTₓ, respectively.[20] As the nature of these absorption bands is still under debate and could be ascribed to plasmonic resonance or interband transitions, we do not address
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Photothermal response of Ti₃C₂Tₓ and Ti₂CTₓ in air. Typical changes in device current following a single laser pulse irradiation (λ = 450 nm) in air for Ti₃C₂Tₓ and Ti₂CTₓ devices at a,b) 300 K and c,d) 350 K. The voltage bias applied for the measurements is 10 mV. ON and OFF refer to turning the laser beam on and off.</figcaption></figure>

such an open question here. However, from the two UV-vis spectra, we identified the wavelength regions where the two Ti-based MXenes show comparable absorption coefficients (viz., the points where the two curves intersect) and selected the laser line for the photothermal experiments accordingly. Excluding wavelengths outside the visible range (for ease of operation), two options remain, 435 and 620 nm. Thus, to take advantage of the highest absorption coefficient under such conditions, irradiation ≈435 nm represents the best choice.

Figure 1c displays the resistance (R) change as a function of temperature (T) for the two Ti-based MXene devices in a range from 300 to 77 K (dark conditions). We observed opposite trends for the two systems: Ti₃C₂Tₓ shows the typical metallic behavior with $dR/dT > 0$, while Ti₂CTₓ exhibits a $dR/dT < 0$ despite its metallic properties. This has already been reported for MXene thin films, and it is not ascribed to a bandgap opening but rather to the consequence of trapped intercalant species and/or changes in the temperature-dependent inter-flake electron hopping mechanism.[21–23] Although beyond the scope of our work, this phenomenon deserves further theoretical and experimental investigations. Figure 2 shows the typical photothermal response for Ti₃C₂Tₓ and Ti₂CTₓ devices in air at two different temperatures, 300 and 350 K. The first obvious difference comes from their opposite response to irradiation: while Ti₃C₂Tₓ shows a negative photoresponse (viz., current decreases following laser irradiation), Ti₂CTₓ exhibits a positive photoresponse (viz., current increases upon irradiation). As displayed in Figure S6 (Supporting Information), when the device is hit by the laser beam, its

temperature increases, and the resistance changes too. In this context, the negative and positive photoresponse can be explained by rationalizing the data in Figure 1c. The $dR/dT > 0$ slope observed for Ti₃C₂Tₓ implies that an increase in temperature causes an increase in resistance, corroborating the negative photoresponse recorded for such devices. Conversely, Ti₂CTₓ exhibits $dR/dT < 0$; thus, an increase in local temperature leads to a decrease in resistance, underpinning the observed positive photoresponse. The second main difference between the two Ti-based MXene systems lies in the drastically different kinetics and symmetry of the ON-OFF cycle (which can be equally considered a heating-cooling cycle).

In this regard, Ti₃C₂Tₓ shows a faster and symmetric response to laser irradiation, unlike Ti₂CTₓ, with a pronounced asymmetry during the drastically slower heating-cooling cycle (Section S4, Supporting Information). Finally, additional considerations can be made by looking at the results of the photothermal response at 350 K in air: Ti₃C₂Tₓ maintains a symmetric response to laser irradiation with slightly faster kinetics during thermal excitation and relaxation, whereas Ti₂CTₓ devices show minimal changes in the ON-step but dramatic variations considering the OFF-step, characterized by significantly faster kinetics (while retaining a strong asymmetry during the overall cycle).

On this basis, we explored the opportunity to take advantage of the photothermal characteristics of Ti₂CTₓ to fabricate devices for optical sensing and memory, exploring the influence of different environmental conditions, including temperature and pressure. Figure 3a shows the photothermal response for Ti₂CTₓ upon
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Optical sensing and storage for Ti₂CTₓ devices. a) Normalized Current versus Time curves showing the response to laser irradiation for different irradiation times. b) Number of distinct current levels as a function of the irradiation time at 300 K in air. The inset shows the same data reported on semi-logarithmic scale. The red line represents the best linear fit. c) Lifetime of each current level as a function of temperature in air, following a 5 s irradiation pulse. The red line represents the best exponential decay fit. d) Semi-log plot showing the current change when moving from level *n* to level *n*+1, recorded for a device exposed to 5 s laser pulses in air at 300 K. The voltage bias applied for the measurements is 10 mV. The red line highlights the threshold (0.5 nA) used to distinguish each current level. The inset recalls the opportunity to use such devices for 4-bit computing.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

laser irradiation with pulses of varying duration. We define the current level as the current value reached by the device upon each laser pulse, where two current levels (viz., *n* and *n*+1) result distinct if the difference of their current values (ΔCurrent) is above 0.5 nA. This value is selected because it is in the same order of magnitude as the typical resolution of the most common source measure units found in laboratories and operating in current ranges from 10s to 100s μA. Figure 3b displays the trend obtained by plotting the number of distinct levels reached as a function of the irradiation time at 300 K in air. As shown in the inset, reporting the same data in a semi-log plot (x in log-scale, y in linear scale), the number of possible levels achieved in a Ti₂CTₓ device is directly proportional to the logarithm of the irradiation time. From this investigation, we demonstrated the possibility of fine-tuning the maximum number of possible current levels available in the optoelectronic device by adjusting the duration of the laser pulse. In this regard, light has emerged as an attractive external stimulus to influence the device operation due to its noninvasive, remote, and digitally controllable nature, with high spatial and temporal resolution, avoiding more complex and time-consuming device engineering strategies.[24] As a next step, we investigated the stability of the current levels in air at different temperature conditions. Figure 3c shows the lifetime (viz., the time required to return to baseline) of the current levels achieved

in a Ti₂CTₓ device irradiated after 5 s laser pulses in a temperature range from 350 to 290 K. The level of stability is strongly influenced by temperature, with an exponential decrease in lifetime when temperature increases. Such behavior provides additional tuning in terms of device operation while maintaining the mild and easy-to-use nature of the external stimuli. The singular photothermal properties of Ti₂CTₓ offer the opportunity to be exploited for the fabrication of optical sensors and memories, with the latter being attractive for innovative and state-of-the-art technologies for neuromorphic applications.[25,26] In this framework, we demonstrated the successful use of the abovementioned systems for optical memory devices. Figure 3d shows the possibility of obtaining 18 distinct current levels using 5 s laser pulses, working at room temperature in air. This exceeds the 16 independent and distinct values required, for instance, in 4-bit computing, offering a chance to transfer our findings to real-life applications.

We speculate about the possibility of fabricating an optical memory using light to write information and temperature to erase it while working in air under mild conditions. Also, such devices operate under low-power consumption, as highlighted by the small voltage bias, paving the way for the progress of low-power (opto)electronics. After demonstrating the possibility of achieving such results by controlling irradiation time and current level stability under ambient conditions, we tested the Ti₂CTₓ
<figure><img src="image_9.png" /><figcaption>**Figure 4.** Photothermal response of Ti₂CTₓ devices in air and high vacuum. Typical photothermal response for a Ti₂CTₓ device to a single laser pulse in a), air (60 s irradiation pulse, from 350 to 300 K) and b), high vacuum (180 s irradiation pulse, from 350 to 100 K). The insets depict the main mechanisms dominating the photothermal properties under those conditions.</figcaption></figure>

devices in high-vacuum and cryogenic conditions, performing similar experiments inside a suitable probe station. In particular, we explored the influence of temperature on device performance at low pressure, shedding light on the possible mechanisms behind such exotic behavior. **Figure 4** compares the Ti₂CTₓ photothermal response in air and vacuum, exploring a temperature range from 350 to 300 K and 350 to 100 K, respectively. Interestingly, the influence of temperature on the device photothermal response following a single laser pulse occurs oppositely, suggesting two different thermal relaxation mechanisms for the environmental conditions under analysis. As reported in Figure 3c and discussed above, an increase in temperature in air leads to faster thermal relaxation, with exponential change in the related kinetics (Figure 4a). This phenomenon can be attributed to the increase in phonon density as temperature increases, which leads to a faster thermal relaxation with the surrounding environment. This agrees with reports in the literature for other 2D systems, highlighting the crucial role that environmental conditions play (e.g., air molecules, gases) in facilitating heat dissipation.[27] Also, we would like to point out that our experiments in air are limited to 350 K to avoid possible material oxidation and/or changes in the surface terminations, which have a strong influence on the optical and electronic properties of MXenes.[28–30] Therefore, the temperature range considered in this study aligns well with the thermal stability of both Ti-based MXenes,[31] allowing for a more accurate and rigorous comparison. Conversely, as shown in Figure 4b, the photothermal response in vacuum follows an opposite trend when exploring the range from 350 to 100 K. Interestingly, the heat dissipation phenomenon becomes faster when decreasing temperature, pointing out a different relaxation mechanism. Considering the thermal conductivity of vacuum is negligible and close to zero, the devices cannot thermalize with the surrounding environment, favoring a different route. At this point, the alternative option is to exploit the lattice vibrations (viz., phonons) to dissipate heat. In such conditions, two aspects are crucial: i) flake-to-flake interfaces, where the lack of air in between flakes hinders the heat flow throughout the device and drastically slows down the relaxation phenomenon;[32] ii) higher phonon density (viz., increasing temperature) leads to dominant phonon scattering, further decreasing the process kinetics. Over-

all, such considerations support our data, where Ti₂CTₓ devices relax exponentially faster when temperature decreases from 350 to 100 K. Our findings highlight another feature to be exploited in real-life applications, such as the control over the photothermal response through environmental conditions; this characteristic could represent an added value when targeting devices operating under both cryogenic and ambient conditions.

### 3. Conclusion

We have demonstrated the fabrication of Ti-based optoelectronic devices, aiming to explore the influence of MXene stoichiometry on optical and electronic properties. Our results reveal that Ti₂CTₓ exhibits a strongly asymmetric photothermal response characterized by relaxation kinetics being three orders of magnitude slower than Ti₃C₂Tₓ, dramatically increasing its thermal energy storage. This behavior is strongly influenced by the irradiation time, temperature, and environmental conditions, paving the way for the fabrication of devices with on-demand tunability. Moreover, Ti₂CTₓ devices show access to both long-term and short-term synaptic plasticity, never reported for optoelectronic devices based on MXenes or other solution-processed 2D materials with such ease of fabrication and characterization.[33–35] Our findings highlight the great potential of harnessing thermal transport and management in Ti-based MXenes for optoelectronic applications and beyond, such as solar desalination, wearable heaters, etc.[36,37] The richness of surface chemistry and the versatility of photothermal properties in MXenes, combined with their light-to-heat conversion efficiency close to 100%,[38] will be a game changer in developing state-of-the-art technologies for optical data storage and neuromorphic applications.

### 4. Experimental Section

**Synthesis of Ti₃C₂Tₓ and Ti₂CTₓ:** Stoichiometric Ti₃AlC₂ MAX powder of 1 g (produced following our previously reported synthesis protocols)[14] was slowly added to a 20 mL mixture of concentrated HF (49 wt.%), concentrated HCl (36 wt.%), and deionized water, with a volumetric ratio of 1:6:3. The MAX phase was etched at 35 °C for 72 h. After the reaction was
completed, the etching product was washed with deionized water via multiple centrifugation steps (3500 rpm, 5 min) until it had a neutral pH. To delaminate the MXene sheets, the Ti₃C₂Tₓ multilayer product was stirred at 300 rpm in a 50 mL aqueous solution containing 1 g of LiCl at 35 °C for 18 h. Then, LiCl was removed from the solution by a single centrifugation step (3500 rpm, 5 min), discarding the clear supernatant. After that, Ti₃C₂Tₓ was redispersed in water via shaking and centrifuged for 15 min at 3500 rpm (to precipitate the multilayer MXene and unreacted MAX particles). Finally, the dark supernatant (containing single-layer MXene sheets) was collected. The synthesis of Ti₂CTₓ, starting from stoichiometric Ti₂AlC MAX powder, followed the very same protocol, except for the etching and delamination time, reduced to 20 and 6 h, respectively.

**Device Fabrication:** The optoelectronic devices were fabricated on glass substrates through photolithography (AZ1505 as photoresist and MIF326 as a developer, from MicroChemicals) using Heidelberg DWL 66+ laser writer. Metal electrodes (5 nm Ti, 45 nm Au) were deposited via e-beam evaporation using Lesker PVD75, followed by lift-off in warm (50 °C) acetone for 90 min. Finally, the devices were rinsed with acetone and 2-propanol to remove any residues. Each substrate contained 8 identical devices.

**Film Deposition:** Before use, the glass substrates were cleaned by ultrasonication in acetone and 2-propanol for 10 min each. Then, they were heated on a hot plate at 350 K, with their electrodes covered by blue masking tape. Ti₃C₂Tₓ and Ti₂CTₓ aqueous dispersions (concentration ≈0.5 mg mL⁻¹) were sprayed using a commercial airbrush gun with a needle and nozzle diameter equal to 0.3 mm. The distance between the nozzle tip and the substrate was kept at ≈20 cm.

**Electrical Measurements:** The resistance versus temperature curves were recorded in a two-probe configuration using a customized Jansky cryogenic probe station connected to a semiconductor parameter analyzer (Keithley 4200A SCS). The chamber pressure was kept at ≈10⁻⁵–10⁻⁶ mbar. A Sumitomo RDK-408D2 closed-cycle cryocooler was thermally anchored to the copper sample holder. The photothermal response of Ti₃C₂Tₓ and Ti₂CTₓ devices was evaluated under 450 nm light irradiation (power = 50 mW). The electrical characterization was performed in both ambient and vacuum conditions, in a temperature range from 340 to 100 K. To enable statistical analysis, nearly 40 identical devices were fabricated and tested.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

The authors acknowledge the Army Research Laboratory for supporting this work under Cooperative Agreements no. W911NF-19-2-0119 and W911NF-24-2-0100. The device fabrication was carried out at the Singh Center for Nanotechnology (University of Pennsylvania), a member of the National Nanotechnology Coordinated Infrastructure (NNCI) network, which is supported by the U.S. National Science Foundation (Grant NNCI-1542153). The authors also acknowledge the EU through the Horizon Europe research and innovation program HORIZON-CL4-2023-DIGITAL-EMERGING-01-Cnect project 2D-PRINTABLE (GA-101135196) and HORIZON-CL4-2022-RESILIENCE-01 project GREEN-CAP (GA-101091572) as well as the Agence Nationale de la Recherche through the Interdisciplinary Thematic Institute SysChem via the IdEx Unistra (ANR-10-IDEX-0002) within the program Investissement d'Avenir, the Foundation Jean-Marie Lehn and the Institut Universitaire de France (IUF). The authors thank Marley Downes and Kateryna Shevchuk for their help during XRD and Raman characterization, respectively.

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

S.I. conceived the project, synthesized the materials, and fabricated the optoelectronic devices. S.I. and F.U. performed electrical characterization and data analysis. P.S., J.E.S., and Y.G. supervised the project. All the authors discussed the results and wrote the manuscript.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

kinetic asymmetry, memory device, MXenes, optoelectronics, photothermal response

Received: January 31, 2025

Revised: March 11, 2025

Published online:

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