Electrolyte gating using ionic crystals: demonstration of iontronics with ionic crystals

We perform electrolyte gating using ionic crystals instead of conventional ionic liquids and gels. By applying a gate voltage and heating the ionic crystal to a liquid state, Fermi level tuning of a carbon nanotube (CNT) film was achieved. Subsequent resolidification at room temperature ensured a fixed ion distribution in the electric double layer at the tuned state. The CNT film maintained the tuned Fermi level for over 30 days, even after the gate electrode was removed. This addresses the challenges associated with handling conventional ionic liquids and is poised to revolutionize the field of electrolyte gating for nanomaterial devices.

We perform electrolyte gating using ionic crystals instead of conventional ionic liquids and gels.By applying a gate voltage and heating the ionic crystal to a liquid state, Fermi level tuning of a carbon nanotube (CNT) film was achieved.Subsequent resolidification at room temperature ensured a fixed ion distribution in the electric double layer at the tuned state.The CNT film maintained the tuned Fermi level for over 30 days, even after the gate electrode was removed.This addresses the challenges associated with handling conventional ionic liquids and is poised to revolutionize the field of electrolyte gating for nanomaterial devices.© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd L ow-dimensional materials, such as dots, tubes and sheets have garnered significant attention due to their unique physical properties resulting from quantum confinement effects.][3][4][5] Nanocarbon materials, which are originally lowdimensional, such as fullerenes, carbon nanotubes (CNTs) and graphene, were first discovered in the late 1900s.][16] Despite their appeal, nanomaterials cannot be processed using conventional techniques employed in bulk semiconductors, necessitating the development of novel technologies.For example, in the case of 3D nanostructures, such as CNT films and nanoporous graphene, controlling the Fermi level of the entire material through standard field-effect techniques through top and/or back-gate electrodes is challenging due to the significant decrease in the electric field within the 3D nanostructure with thickness.][19][20][21][22] In electrolyte gating, an ionic liquid comprising anions and cations serves as the charged medium instead of an insulating layer.The Fermi level of the nanomaterial can be controlled by electrophoresing ions and forming an electric double layer on the material surface through the application of a gate voltage.][25][26][27][28][29][30] By tuning the Fermi level of CNT films through electrolyte gating, we successfully identified plasmon resonance absorption in the terahertz band. 30)ectrolyte gating has significantly contributed to studies on materials and physical property evaluations.][33][34][35][36] However, the application of this technique in the development of devices is challenging.The utilization of ionic liquids, which are in liquid state at room temperature and prone to spillage, poses difficulties in integrating them into electronic devices.][39][40][41] In addition, all-solid-state electric double-layer transistors using solid electrolytes, such as a Y-stabilized ZrO 2 , that operate at high temperatures of 500 K have been reported. 42,43)Despite these advancements, the problem of device packaging compatibility remains unresolved.
To address this challenge, we propose the utilization of ionic crystals.These crystals remain in the solid state at room temperature.Our study focuses on controlling the Fermi level of CNT films through electrolyte gating by utilizing ionic crystals.Utilizing ionic crystals as charged media, we eliminate the risk of spills associated with ionic liquids.In addition, the Fermi level could be maintained without applying a gate voltage by solidifying the electric double layer in some ionic crystals.This presents a significant advantage for device applications that rely on electrolyte gating.
The concept of electrolyte gating using ionic crystals is shown in Fig. 1.This configuration is similar to that utilized in conventional electrolyte gating.A CNT film with source/ drain electrodes (sample) and one with a gate electrode (counter material) were connected through an ionic crystal.Unlike conventional methods, we manipulated the liquid and solid phases through temperature control.As shown in Fig. 1, we utilized 1-Ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide as the ionic crystal, with a melting point of 88 °C.This crystal melted when heated from 27 °C (room temperature) to 130 °C [Fig.1(b)].When a gate voltage was applied to the sample, an electric double layer formed on the CNT film due to anion and cation electrophoresis.This enabled the Fermi level of the CNT film to be uniquely tuned using the gate voltage.Upon cooling to room temperature while maintaining the gate voltage, the ions solidified, effectively immobilizing the anions and cations, and possibly forming an electric double layer.That is, the ion distribution in the electric double layer (Fermi level) remained unchanged regardless of the gate voltage.Therefore, even after the gate electrode was removed, the tuned Fermi level of the CNT film was maintained after solidification.
The schematics and photographs of the experimental setup are shown in Figs.2(a) and 2(b).An as-grown single-walled CNT sample, including metallic and semiconducting species, synthesized using an enhanced direct-injection pyrolytic synthesis method 44,45) was utilized in this study.A film of CNTs was prepared by filtering their (solvent) solution and later transferred onto a polyimide substrate.The length, width and thickness of the CNT film were adjusted to 5, 1 and 300 nm, respectively.The source and drain electrodes, made of K-type grounded thermocouples, were connected to the CNT film using silver paste and encapsulated with silicone sealant to prevent chemical reactions with ions.A chemical equivalent CNT film served as the counter electrode (gate electrode) of the sample, with the gate electrode area being at least twice the size of the sample to avoid hindering the electric double-layer formation on the sample surface.The CNT film, gate electrode and ionic crystals were placed in a bath of silicone sealant on a glass substrate, with the device temperature controlled by utilizing an external temperature controller.To prevent degradation of the ionic crystal resulting from exposure to oxygen and moisture, all experiments were conducted in a nitrogen gas-filled glove box.The thermoelectric power (ΔV ) and temperature difference (ΔT) were measured using the source and drain electrodes (grounded thermocouples).The Seebeck coefficient S, conductivity σ and power factor (PF) were calculated based on ΔV and ΔT.A graph of the physical properties of the CNT film versus gate voltage at 130 °C (liquid state) is shown in Fig. 2(c).When a positive gate voltage was applied, cations predominantly accumulated on the surface of the CNT film connected to the thermocouples, forming an electric double layer.This process induced electron carriers in the CNT film, changing them into ntype carriers.Experimental results indicating trends in the Seebeck coefficient, conductivity and PF in Fig. 2(c) support this estimation.Consequently, electrolyte gating using ionic crystals can be performed similarly to conventional ionic liquids and gels when the ionic crystal melts into a liquid state.However, a key difference lies in the behavior upon returning to room temperature.At room temperature, the ionic compound that forms an electric double layer solidifies, maintaining the electric field on the CNT film even without the application of a gate voltage [Figs.3(a

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd layer formed in this solidified state retains the ion distribution from the liquid state.When solidified by applying a gate voltage of 0.2 V, the CNT film remains p-type, depending on the Fermilevel position tuned in the liquid state, and vice versa.Note that the position of fixing the Fermi level can be adjusted to any position depending on the gate voltage [Fig.3(b)].Once the electric double layer solidified at the desired Fermi level, the Fermi-level-tuned CNT film operated even after the gate electrode was removed, as shown in Fig. 3(c).Maintaining electric double-layer charges without external voltage has previously been reported by solidifying ionic liquids by lowtemperature cooling or UV curing. 18,35,36)The advantage of our technique over these previous techniques is that it can fix the Fermi level at room temperature and re-tune the Fermi level by reheating ionic crystals.Figures 3(d) and 3(e) present the stability and power-generation capacity after the gate electrode was removed.Evaluation of the temporal stability of the Seebeck coefficient after the removal of the gate electrode revealed that both p-and n-type electronic states were maintained for over 30 days [Fig.3(d)].Through the use of an ionic crystal, we successfully fine-tuned the electronic state with electrolyte gating, maintaining the electronic state for an extended period in a stand-alone state after separating the gate electrode.Furthermore, we validated the repeatability of the process.Even after solidification, the electronic levels could still be re-tuned by reheating and melting the ionic crystal.
As shown in Figs. 2 and 3, the Fermi level of the CNT film can be controlled and fixed through electrolyte gating using ionic crystals.However, these properties varied significantly depending on the type of ionic crystal utilized.An overview (a) .Furthermore, when an ionic crystal containing a highly reactive anion was heated for an extended period, it absorbed moisture and failed to solidify even after returning to room temperature (it remained in a liquid state at room temperature).Based on this comparison study, maintaining the electric double layer involves complex considerations of ion species and compatibility with target materials, whereas the Fermi level can be easily tuned by applying a gate voltage to move ions in the liquid state.In summary, we demonstrated Fermi-level tuning of a CNT film through electrolyte gating with ionic crystals.This technology involves controlling the liquid and solid states of the ionic crystal.By melting the ionic crystal, the Fermi level of the CNT film is tuned by applying a gate voltage, similar to that of conventional ionic liquids and gels.Notably, a key feature of this technology is the ability to fix the Fermi level through solidification, allowing for electronic level control with no external gate electrode.This addresses the limitations of conventional electrolyte gating, which can be challenging to manage due to its liquid nature.][39] These results are expected to broaden the scope of electrolyte gating in the field of nanomaterial device applications.The solidification characteristics of each ion type, along with the evaluation of the crystal structure, will be investigated in the future.071002-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd ) and 3(b)].However, the electric double

Fig. 2 .
Fig. 2. (a), (b) Schematics (a) and photographic images (b) of the experimental setup of the electrolyte gating with ionic crystal.(c) Physical properties of the CNT film versus the gate voltage at 130 °C (liquid state of the ionic crystal of 1-Ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide).

Fig. 3 .
Fig. 3. (a) Photographic image and (b) Seebeck coefficient of the CNT film versus the gate voltage at 40 °C (solid state of the ionic crystal).Fermi level could be fixed at any position depending on the gate voltage tuned in the liquid state at 130 °C.(c) Photographic image of the Fermi-level-tuned CNT film after removing the gate electrode.(d) Transient Seebeck coefficient of the Fermi-level-tuned CNT film at the stand-alone state under atmospheric conditions.(e) Generated voltage versus the temperature gradient of the Fermi-level-tuned CNT film.