wileyonlinelibrary.comlayers-is selectively etched, leaving behind oxygen, hydroxyl, and fl uoride surfaceterminating groups (T). [ 9,16 ] In appropriate solvents, the layers can be subsequently delaminated to form colloidal solutions of suspended M n +1 X n T x , or "MXene," nanosheets. [ 17 ] This work studies Ti 3 C 2 , one representative of more than a dozen 2D MXenes already discovered, with a vast family of possible compositions yet to be explored. [ 9-11 , 18 ] MXene, particularly Ti 3 C 2 , has demonstrated outstanding performance as an electrode material for supercapacitors with volumetric capacitance as high as 900 F cm −3 [ 16 ] and Li battery electrodes with capacities of 450 mA h g −1 . [ 17 ] In contrast to the existing applications of MXene, optical materials require nanometer-scale control over sample thickness and uniformity over wide areas. Preliminary optical studies of MXenes were previously possible only by employing physical vapor deposition. [ 19 ] In the present study, spincasting was used to fabricate continuous, opticalquality, conductive, thin fi lms making it possible to measure the fundamental optical and electrical properties of solutionprocessed, substrate-aligned Ti 3 C 2 T x fi lms. Separate percolative and bulk-like transport regimes are identifi ed as a function of fi lm thickness and the bulk in-plane conductivity is determined to be 6500 ± 800 S cm −1 , a value that is substantially larger than previous studies of Ti 3 C 2 T x . Hall effect measurements are used to determine the fundamental electronic properties of mobility (0.9 cm 2 V −1 s −1 ) and charge-carrier density (3.1 × 10 22 cm −3 ) for fi lms of this composition and structure. The complex optical dielectric function is measured, identifying free-plasma oscillations in the near-IR, suggesting the utility of these materials for plasmonics and making it possible to compare local, optical conductivity with long-range, DC conductivity. Finally, with an eye toward future optoelectronic applications, various fi gures of merit are calculated and the stability of the fi lms to atmospheric gases is measured. MXenes comprise a new class of solution-dispersable, 2D nanomaterialsformed from transition metal carbides and nitrides such as Ti 3 C 2 . Here, it is shown that 2D Ti 3 C 2 can be assembled from aqueous solutions into optical quality, nanometer thin fi lms that, at 6500 S cm −1 , surpass the conductivity of other solution-processed 2D materials, while simultaneously transmitting >97% of visible light per-nanometer thickness. It is shown that this high conductivity is due to a metal-like free-electron density as well as a high degree of coplanar alignment of individual nanosheets achieved through spincasting. Consequently, the spincast fi lms exhibit conductivity over a macroscopic scale that is comparable to the intrinsic conductivity of the constituent 2D sheets. Additionally, optical characterization over the ultraviolet-to-nearinfrared range reveals the onset of free-electron plasma oscillations above 1130 nm. Ti 3 C...

The perovskite phase of cesium lead iodide (α-CsPbI or "black" phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional "yellow" phase (δ-CsPbI). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of this phase.

Synthesis and purification of CsPbX3 nanocrystals: The synthesis followed the procedure of Protesescu et al. 1 with only slight modifications. 0.204 g of Cs2CO3 (Aldrich, 99.9%), 10 mL 1-octadecene (ODE, Sigma-Aldrich, 90%) and 0.63 mL of Oleic acid (OA, Sigma-Aldrich, 90%) were charged in a 3-neck flask and degassed at 120 °C for 1 h. The mixture temperature was raised to 150 °C under a N2 atmosphere and held there until the formation of Cs-oleate was observed by the complete dissolution of the solid. The solution temperature was maintained at 100 °C to avoid any precipitation. To synthesize the nanocrystals, 0.75 mmol of PbX2 (PbI2: 0.346 g, Strem Chemicals, 99.999%; PbCl2: 0.209 g, Aldrich, 99.999%), along with 20 ml of ODE, were combined in a 3-neck flask and degassed for 1 h at 120 °C. 4 mL of dried OA and 4 mL of dried Oleylamine (OLA, Aldrich, 70%) were injected at 120 °C under N2. For the PbCl2, 4 mL of tri-n-octylphosphine (TOP, Strem, 97%) was also added. Once PbX2 dissolved completely the mixture temperature was increased to 165 °C and 1.6 mL of Cs-oleate solution was injected rapidly. After a short wait (10 s), the synthesis product was quenched by an ice-water bath. After the synthesis, the reaction mixture was treated with an anti-solvent to flocculate the nanocrystals,

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Herein, we spincast aqueous colloidal Ti 2 CT x (MXene) solutions into conductive, transparent films with figures of merit (FOM), that are as good as Ti 3 C 2 T x or un-doped chemically vapor-deposited graphene. When normalized by the number of transition metal atoms, the FOM is the highest ever reported for a MXene film. At about 2.7 × 10 5 cm -1 the absorbance coefficient of Ti 2 CT x is quite comparable to that of Ti 3 C 2 T x . Quantitative relationships between film properties-conductance and transparency-and colloidal solution concentration and spin speeds are developed providing a road map for future work. IMPACT STATEMENTIn a first, we spincast aqueous colloidal Ti 2 CT x (MXene) solutions into conductive, transparent films with figures of merit-5-that are as good as Ti 3 C 2 T x or un-doped CVD graphene. ARTICLE HISTORY