Elastic Modulus, Hardness, and Fracture Toughness of Li$_{6.4}$La$_{3}$Zr$_{1.4}$Ta$_{0.6}$O$_{12}$ Solid Electrolyte

Organic crystalline semiconductors, which consist of conjugated molecules assembled via weak van der Waals (vdW) forces, have attracted increasing interest in low-cost large-area organic electronics and optoelectronics.^[1–3] Among organic crystalline semiconductors, two-dimensional (2D) molecular films with high crystallinity long-range structural order and chemical purity are representative materials for high-performance organic field-effect transistors (OFETs) and high-sensitivity sensors, enabling in-depth research on film growth mechanisms, carrier transport dynamics, charge injection physics, and interfacial properties.^[4–9] With continuous improvements in electrical performance of OFETs, more sophisticated applications, including organic complementary metal oxide semiconductor (CMOS) circuits and heterojunction devices, require both p-type and n-type semiconductors with solution processability, high mobility, and thermal stability.^[10–12]

Many p-type 2D organic semiconductors (OSCs), such as DTBDT-C9, C₈-BTBT, ph-BTBT-C10, and C₁₀-DNTT films, have recently been fabricated using dip-coating, bar-coating, solution-shearing, and the floating-coffee-ring-driven assembly methods, demonstrating inherent film crystallinity, high hole mobilities exceeding 10 cm²⋅V⁻¹⋅s⁻¹, and excellent charge injection. These p-type 2D OSCs are fascinating for direct exploration of the relationship between charge accumulation and transport behaviors with semiconductor/insulator interface, energy disorder, and dielectric polarizations, which promoted the developments of the 2D organic semiconductors.^{[5,8,13–15]} However, the development of high-quality n-type OSCs still lags behind their p-type counterparts due to the inferior stability under oxygen, moisture, and high-temperature exposure, hindering the study of interfacial transport physics in n-type semiconductors.^[16,17] Therefore, developing an efficient strategy to fabricate high-quality n-type 2D OSCs in large areas with excellent thermal stability and electrical performance is desirable.

The extended π-electron framework between molecules and energy levels of the lowest unoccupied molecular orbitals (LUMO) lower than –4.0 eV are characteristics of high-quality n-type 2D OSCs that can boost carrier transport and shield the material from oxidation by ambient O₂ and H₂O.^[16,18] N,N′-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN₂) as the n-type OSC has demonstrated the single-crystal feature, thermal stability, and high electron mobility exceeding 1 cm²⋅V⁻¹⋅s⁻¹ for solution-grown single-crystalline transistors, which will be a promising candidate for fabricating large-area thermally stable monolayer n-type molecular semiconductors.^[19,20]

Here, we successfully demonstrate the fabrication of monolayer PDIF-CN₂ films with millimeter-scale coverage and uniform morphologies using an antisolvent-confined spin-coating technique. The antisolvent not only speeds up the nucleation of the films but also induces a downstream Marangoni flow, improving the film morphologies. The monolayer PDIF-CN₂ films can tolerate annealing at a high temperature of 120 °C, which enhances the crystallinity of the films. The OFETs with monolayer PDIF-CN₂ films exhibit average and maximum electron mobilities of 0.28 and 0.5 cm²⋅V⁻¹6⋅s⁻¹. Degradations induced by ambient O₂ and H₂O after prolonged exposure to air can be recovered through a post-annealing treatment. Furthermore, monolayer-based OFETs show linear output characteristics. The results of ultraviolet photoelectron spectroscopy (UPS) indicate that the reduction in film thickness causes a considerable change in energy levels, which lowers the injection barrier between the LUMO of monolayer PDIF-CN₂ films and Au electrodes. This work reveals the process of antisolvent-confined molecular assembly and the charge injection mechanism at the molecular level via stable and high-quality monolayer PDIF-CN₂ films, deepening the understanding of n-type organic semiconductors and advancing the development of organic electronics.

Results and Discussion. PDIF-CN₂ is an n-type molecular semiconducting material with the π-conjugated core and N-fluorocarbon functionalization. PDIF-CN₂ films exhibit excellent electrical performance and thermal stability.^[19,20] We first dissolved PDIF-CN₂ in the solvent of anisole with a concentration of 0.1 wt%. The solution (40 μL) was drop-cast onto a SiO₂/Si substrate. Then, the antisolvent of N,N-dimethylformamide (DMF) (40 μL) was further drop-cast onto the PDIF-CN₂ solution for the spin-coating process [Fig. 1(a) and see details in the Experimental Methods in the Supplementary Material]. During the spin-coating procedure, monolayer PDIF-CN₂ films with millimeter-sized coverage and good morphological uniformity can be deposited on the substrate as the solution was pulled by the centrifugal force [Fig. 1(c)]. Finally, the monolayer films were annealed at the temperature of 120 °C for 15 min in the N₂ glove box. The annealed films show no signs of film degradation and maintain their outstanding quality [Figs. 1(d) and 1(e)]. Atomic force microscopy (AFM) images of the monolayer films reveal that the film thicknesses are 1.1 and 1.5 nm before and after annealing, respectively. These are less than the c-axis unit cell parameter of PDIF-CN₂ single crystals (2.28 nm), validating the monolayer property of the depositing films [Figs. 2(a) and 2(c) and Fig. S1 in the Supplementary Material].^[21] The smaller thickness of monolayer films results from the tilted molecular arrangement on the substrate, which is induced by the strong vdW interaction between the molecules and the substrate (Fig. S1). Additionally, the rms roughness of the AFM images before and after annealing is 2.6 and 2.0 Å, indicating the atomic smoothness of the monolayer PDIF-CN₂ films [Figs. 2(b) and 2(d)]. Furthermore, the crystallinity of the PDIF-CN₂ films before and after annealing was examined by the grazing incidence x-ray diffraction (GIXRD) (Fig. S2 and Note S1 in the Supplementary Material). The GIXRD results indicate that the PDIF-CN₂ films exhibit a crystalline property. As a result, the antisolvent-confined spin-coating method is efficient for the growth of high-quality monolayer PDIF-CN₂ films.

Fig. Fig. 1.  Large-area PDIF-CN₂ films using the antisolvent-confined spin-coating method. (a) Schematic diagram of the antisolvent-confined spin-coating deposition of PDIF-CN₂ thin films. For step 1, the solution of PDIF-CN₂ in anisole is drop-cast onto a SiO₂/Si substrate. For step 2, the upper solvent of DMF entirely covers the solvent of the anisole. For step 3, crystalline sheets assemble into monolayer films at the anisole/DMF interface during the spin-coating process. (b) Simulation of the Marangoni flow at the Meniscus line. (c) The optical microscopic image of the deposited large-area monolayer PDIF-CN₂ films. The inset is the molecular structure of PDIF-CN₂. Scale bar, 200 μm. (d) and (e) The optical microscopic images of monolayer PDIF-CN₂ films before and after annealing at 120 °C. Scale bar, 100 μm (d), 50 μm (e).

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Fig. Fig. 2.  The effect of the spin-coating speed and thermal annealing on the crystallinity of PDIF-CN₂ films. (a) and (b) AFM images of monolayer PDIF-CN₂ films before annealing, with scale bar 2 μm. The inset in (b) is the schematic diagram of molecular packing of PDIF-CN₂ films on SiO₂ before annealing. (c) and (d) AFM images of monolayer PDIF-CN₂ films after annealing, with scale bar 2 μm. The inset in (d) is the schematic diagram of molecular packing of PDIF-CN₂ films on SiO₂ after annealing. (e) Coverage and rms roughness of spin-coated monolayer PDIF-CN₂ films on SiO₂/Si substrate at various speeds of spin coating. (f) Raman spectrum of monolayer PDIF-CN₂ films on SiO₂ before and after annealing.

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The upper layer solvent of DMF strongly influences the morphology of the depositing monolayer films. Due to the immiscibility of anisole and DMF, the antisolvent of DMF shows vertical solvent separation with anisole.^[22] The antisolvent can speed up the nucleation and aggregation of PDIF-CN₂ molecules at the interface of anisole and DMF, promoting the growth of monolayer crystalline sheets.^[23] When the edge of the solution moves on the substrate by the centrifugal force during the spin-coating process, monolayer crystalline sheets can migrate at the anisole/DMF interface and assemble into large-area and high-quality monolayer films [Fig. 1(a)]. The fluid dynamics simulation was performed to further study the influence of the additive solvent of DMF on the film morphologies. When the solution was pulled by the centrifugal force, a meniscus line can be formed at the solution edge, controlling the solution flow and film deposition.^[24] In particular, the Marangoni flow, which results from the surface-tension gradient, may act as a vital fluid process. The Marangoni flow causes the fluid to flow from regions of low surface tension to regions of high surface tension and attributes to the molecule migration along the meniscus line.^[25]

Improvement in film morphologies requires a downstream Marangoni flow to supply sufficient solutes to the meniscus line for monolayer film depositions using the spin-coating process. In the heterogeneous solvent system of DMF and anisole with vertical phase separation, DMF could be regarded as an additive solvent for the primary solvent of anisole. DMF can modify the surface tension gradient along the meniscus to adjust the Marangoni flow. As the solution was pulled by the centrifugal force, the upper solvent of DMF significantly would diffuse into the primary solvent of anisole, leading to a downward increase in the concentration gradient of DMF along the meniscus line (Fig. S3). Since the surface tension of DMF was larger than that of anisole (Table 1), a downstream Marangoni flow could be induced by the surface tension gradient, which would improve the film morphologies.^[25] The CFD simulation using a confined geometry filled with 99% anisole and 1% additive DMF also demonstrates that DMF can induce Marangoni flow along the meniscus line in a downstream flow direction [Fig. 1(b), Fig. S3 and Note S2 in the Supplementary Material]. Therefore, the antisolvent-confined spin-coating method offers a downstream Marangoni flow to supply sufficient organic molecules to the contact line to achieve supersaturation, which yields highly uniform PDIF-CN₂ films.

  Table 1.  Properties of anisole and DMF.

	Boiling point	Density	Surface tension
	(°C)	(g/cm3)	(mN/m)
Anisole	154	0.995	33.97
DMF	153	0.950	36.50

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We further study the effects of spin-coating speed on the morphologies of the films. The spin-coating speeds are tuned from 1000 to 4000 rpm [Fig. 2(e)]. Figure S4 shows the optical images and the corresponding AFM images of PDIF-CN₂ films at different spin-coating speeds after annealing. When the spin coating process was performed at a low speed of 1000 rpm, discontinuous film morphologies were observed with the film coverage of only 61% on the substrate. The corresponding AFM images also exhibited defects and grain boundaries with a high rms roughness of 4.2 nm [Figs. S4(a) and S4(e)]. The morphological continuity of the films can be improved by raising the spin-coating speed to 2000 rpm. The film coverage and the surface roughness are 80% and 3.6 Å, respectively [Figs. S4(b) and S4(f)]. When the spin-coating process was at an optimized speed of 3000 rpm, we found that the substrate was almost entirely covered by a monolayer film with a low rms roughness of 2.1 Å, exhibiting atomic smoothness [Figs. S4(c) and S4(g)]. We further increased the spin-coating speed to 4000 rpm, and the films became discontinuous with obvious grain boundaries [Figs. S4(d) and S4(h)]. Therefore, the spin-coating speed of 3000 rpm was the optimized value to achieve a monolayer PDIF-CN₂ film with a large area and a uniform morphology. We believe that antisolvent-confined spin coating is a simple and efficient way to deposit high-quality monolayer molecular films.

Note that thermal annealing can improve the crystalline properties of organic crystals. We then study the effect of thermal annealing on the crystal morphology of PDIF-CN₂ thin films at the monolayer limit by AFM and Raman spectroscopy characterizations. As shown in Figs. 2(a)–2(d), thermal annealing of monolayer PDIF-CN₂ films at 120 °C for 15 min in the N₂ glove box can increase the film thickness from 1.1 nm to 1.5 nm and reduce the rms roughness of films from 2.6 Å to 2.0 Å. The alterations in molecular packing show that thermal annealing can effectively reorganize the molecules to have more consistent morphologies and orient them upright [Figs. 2(c) and 2(d)]. The characteristic signals of the Raman spectrum of monolayer PDIF-CN₂ films can be measured at 1264 cm⁻¹, 1360 cm⁻¹, 1460 cm⁻¹, 1560 cm⁻¹, 1601 cm⁻¹, and 1701 cm⁻¹, which are close to the Raman spectrum of PDI [Fig. S5(b)].^[26,27] Notably, the Raman peak intensity of the monolayer film at 1360 cm⁻¹ after annealing is three times higher than its pre-annealing value, and the full width at half maximum (FWHM) of Raman peaks is smaller than the pre-annealing films, both supporting the significant improvement of the film crystallinity and reduction of defects by annealing [Fig. 2(f)].^[28] The GIXRD results also indicate that thermal annealing can effectively improve the crystallinity (Fig. S2). The insets in Figs. 2(b) and 2(d) show schematic depictions of molecular arrangement before and after annealing, illuminating how annealing may promote molecular reorganization and enhance the crystallinity of monolayer films. In addition, thermal annealing can also eliminate residual solvent, oxygen, and moisture in the films, improving film quality and stability.

OFETs were fabricated utilizing monolayer annealed PDIF-CN₂ films with bottom-gate top-contact structures to examine the electrical properties [inset in Fig. 3(a) and Fig. S6]. Au pattern stripes were transferred onto PDIF-CN₂ films as the source and drain electrodes, which feature a physical contact interface between the semiconducting films and electrodes.^[14,29] Figure 3(a) shows typical transfer characteristics of monolayer-based transistors with a pristine silicon oxide dielectric layer, yielding average and maximum field-effect mobilities (μ_FET) of 0.28 and 0.50 cm²⋅V⁻¹⋅s⁻¹, respectively [Fig. 3(b)]. A slight decrease in carrier mobility can be observed at high gate voltage (V_G), which could result from the interface polarization coupling of the dielectric or Coulomb interactions between electrons in the monolayer films (Fig. S6).^[30,31] Furthermore, the output characteristics exhibit a linear increase at small V_DS from the gate voltage of 0 V to 30 V [Figs. 3(c) and 3(d)]. Near-zero threshold voltage (V_TH) and negligible hysteresis of transfer curves are demonstrated at V_DS of 20 V, indicating that the interface between PDIF-CN₂ films and dielectric of SiO₂ has an ultralow density of trap states [Fig. 3(a)]. It is also shown that the V_TH is nearly unchanged after 15-cycle transfer scanning, further revealing the outstanding electrical stability of the monolayer-based transistor.^[32] The mobilities of monolayer-based transistors are lower than those of transistors with few-layer crystalline PDIF-CN₂ films.^[19] The absence of surface treatments is an important reason for the reduction of mobility in monolayer thin films. In addition, the molecules of monolayer (1 L) films are more tilted on the substrate than 2 L molecules due to the stronger molecule–substrate interactions, which lead to weaker π–π stacking between molecules, resulting in the reduction of mobility in monolayer films [Fig. S1(b)].

Fig. Fig. 3.  Electrical performance of transistors based on monolayer PDIF-CN₂ films. (a) Transfer characteristics at a drain voltage of 20 V of monolayer-based transistors with 15-cycle measurements. The inset is a typical diagram of a monolayer-based transistor. (b) The electron mobility histogram of monolayer-based OFETs. The green line is the Gaussian fitting curve. (c) Output characteristics at gate voltage from 0 to 30 V of the monolayer-based transistor. (d) Linear output curves at small V_DS corresponding to output characteristic of (c).

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The air stability of the monolayer-based devices is also investigated. As shown in Fig. S7(a), the carrier mobility of monolayer-based OFETs is stable, ranging from 0.32 to 0.20 cm²⋅V⁻¹⋅s⁻¹ under a vacuum environment for 300 h. Furthermore, the effects of the aqueous solution on PDIF-CN₂ films are studied. We submerged the sample in deionized water for 10 s and then dried it by airflow. Obvious mobility reduction from 0.20 to 0.025 cm²⋅V⁻¹⋅s⁻¹ can be observed. For the degraded device, a post-annealing treatment at 120 °C for 20 min under vacuum conditions could restore the electrical performance to approach the origin mobility values of 0.14 cm²⋅V⁻¹⋅s⁻¹ [Fig. S7(b)]. Therefore, the high-temperature annealing can effectively eliminate attached water and oxygen molecules, contributing to the performance recovery of destroyed films. The air stability of the transistors with multilayer PDIF-CN₂ films (∼ 3 L) was also investigated. The multilayer films were deposited by the inclined drop-cast method (Note S3 in the Supplementary Material). The carrier mobility of multilayer-based OFETs is more stable, ranging from 0.48 to 0.39 cm²⋅V⁻¹⋅s⁻¹, than that of 1 L-based ones (Fig. S7). Since the organic molecules in the monolayer film would directly interact with the water and oxygen in the air as well as interface polarization of the dielectric layer, the monolayer films would be more susceptible to damage than thicker films. However, the monolayer PDIF-CN₂ films still exhibit good stability among the reported n-type organic monolayer semiconductors.^[10,33]

Fig. Fig. 4.  Characteristics of the energy levels of PDIF-CN₂ films. (a) UPS spectra near the cutoff for secondary electron emission (left) and near the HOMO edge (right) for 1 L, ∼ 2 L, and ∼ 3 L PDIF-CN₂ films. (b) The photoluminescence spectra for 1 L, ∼ 2 L, and ∼ 3 L PDIF-CN₂ films. (c) The diagram of energy levels for 1 L, ∼ 2 L, and ∼ 3 L PDIF-CN₂ films as well as the work function of Au.

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Remarkable energy level shifts can be found when the films decrease to the monolayer limit. A small Schottky barrier of 0.04 eV exists between the Au electrode and the LUMO of monolayer PDIF-CN₂ films, while a large Schottky barrier of 0.31 eV is found between Au and ∼ 3 L films. When the film thickness is varied from ∼ 3 L to ∼ 2 L, the energy levels are almost unchanged. Hence, the decrease of the film thickness is not the reason for the energy level shifts. Such an abrupt transition for monolayer films may be attributed to strong modulation of the molecular packing by interfacial vdW interactions. The molecules of 1 L films are more tilted on the substrate than 2 L molecules due to the stronger molecule–substrate interactions, which leads to weaker π–π stacking between molecules in monolayer films.^[5] Different molecular packing has important influences on the molecular orbitals in 1 L and 2 L films, which could result in energy level shifts.^[5,14] The low injection Schottky barrier can enhance the carrier injection, leading to linear output characteristics. Additionally, a nonlinear increase of output curves can be found for transistors with multilayer PDIF-CN₂ films, which is consistent with the larger Schottky barrier of 0.31 eV for charge injections (Fig. S10). Therefore, the high-quality monolayer PDIF-CN₂ films not only enhance the electrical performance of OFETs but also offer unique physical characteristics of energy level changes for improved charge injections, which have great potential for high-performance heterojunction devices.

In summary, we have successfully demonstrated an antisolvent-confined spin-coating method for depositing large-area and high-quality n-type monolayer molecular films, which shows excellent thermal stability at an annealing temperature of 120 °C. Furthermore, the monolayer-based OFETs exhibit linear output characteristics and stabilities after 15 days of vacuum storage. Particularly, we find that the reduction in film thickness leads to a significant shift in energy levels, which lowers the injection barrier between the LUMO of monolayer PDIF-CN₂ films and Au electrodes. Therefore, this work can help to develop low-cost, large-area, and high-performance n-type monolayer molecular films, which are important for more complicated devices, such as heterojunctions and organic circuits.