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

Herein, we demonstrate the fabrication of Ti 3 C 2 T x MXene films using electrophoretic deposition (EPD). A systematic study under constant voltage conditions from aqueous and propylene carbonate-based suspensions was performed to investigate the effects of the EPD process parameters on film morphology, flake orientation, and functional properties. From measured suspension properties, the kinetics of deposition from both suspension media were successfully described by the well-established Sarkar model of EPD. Remarkably, EPD-processed films have electrical conductivities of 7400 S/cm, on par with the highest values reported in the literature for Ti 3 C 2 T x MXene. When employed as electrochemical capacitor electrodes in 1 M H 2 SO 4 , the capacitances were comparable to literature values. Given the process scalability and the morphological control that is possible, these results bode well for EPD as a fast, high-throughput method for making MXene films. Electrophoretic deposition (EPD) is a process wherein an electric field is applied to a stable colloidal suspension in between two electrodes.1 As a consequence, two distinct processes occur, as shown in Figure 1a: (1) charged particles in the colloid move toward the electrode of opposite polarity to that of the particles' surface charge (electrophoresis) and, (2) a solid deposit is formed by coagulation at the suspension-electrode interface (deposition). This technique is well-studied and successfully implemented for high-throughput production of dense, void-free ceramic structures ranging from thin coatings to bulk layers several centimeters thick.2 Because of the flexibility and scalability of the deposition apparatus (e.g. power sources and size/shape of deposition electrodes and cell), fast deposition rates, and excellent microstructural control compared to other processing methods, EPD has been used for commercial-scale production of various engineering and traditional ceramics. [2][3][4] In recent years, EPD has also become a versatile technique for making monolithic films and nanocomposites of two-dimensional (2D) materials -such as graphene, transition metal dichalcogenides, and layered transition metal oxides 5-10 -due to advances in colloidal processing. Recently, we discovered a new family of two-dimensional (2D) early transition metal carbides and nitrides, that we labeled MXene, because they are derived from the MAX phases and share properties similar to graphene. 11,12 The MAX phase composition is M n+1 AX n , where M is an early transition metal, A is an A-group element (mostly group 13 and 14 elements), X is carbon and/or nitrogen, and n = 1 to 3. The exfoliation process is carried out by selectively etching away the 'A' layers by hydrofluoric acid (HF) alone or by hydrochloric acid (HCl) combined with pre-dissolved fluoride salts. [13][14][15][16] The resulting material consists of loosely bonded layers of M n+1 X n T x , where T represents surface functionalization by -OH, -F and/or -O groups, [17][18][19] that can subsequently be delaminated...

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For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of these particles are increased, favoring deposition. In situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insight into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass-transport-limited to reaction-rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering, from stable, well-dispersed colloidal starting materials.