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...

Plastically anisotropic/layered solids are ubiquitous in nature and understanding how they deform is crucial in geology, nuclear engineering, microelectronics, among other fields. Recently, a new defect termed a ripplocation–best described as an atomic scale ripple–was proposed to explain deformation in two-dimensional solids. Herein, we leverage atomistic simulations of graphite to extend the ripplocation idea to bulk layered solids, and confirm that it is essentially a buckling phenomenon. In contrast to dislocations, bulk ripplocations have no Burgers vector and no polarity. In graphite, ripplocations are attracted to other ripplocations, both within the same, and on adjacent layers, the latter resulting in kink boundaries. Furthermore, we present transmission electron microscopy evidence consistent with the existence of bulk ripplocations in Ti3SiC2. Ripplocations are a topological imperative, as they allow atomic layers to glide relative to each other without breaking the in-plane bonds. A more complete understanding of their mechanics and behavior is critically important, and could profoundly influence our current understanding of how graphite, layered silicates, the MAX phases, and many other plastically anisotropic/layered solids, deform and accommodate strain.

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Herein we present experimental and modeling evidence for a new deformation micromechanism operating in layered solids termed a ripplocation. Select Ti 3 SiC 2 grains were cyclically indented-either parallel or normal to the basal planes-with spherical tips with radii, R of 21 μm and 100 μm. When the load vs. displacement curves were converted to indentation stress vs. a/R curves, where a is the contact radius, fully and spontaneously reversible hysteresis loops were recorded. The energy dissipated per unit volume per cycle, W d , was found to be a function of basal plane orientation; W d was smaller when the basal planes were loaded edge on. Transmission electron microscope images of areas under the indentations revealed the existence of defects that previous worked confirmed have a strain component along the c-axis and for which no g b condition was found that resulted in their disappearance. These defects thus cannot be basal dislocations; their characteristics, however, are consistent with bulk ripplocations, BRs. It is the to-and-from movement of these BRs-and not basal dislocations as previously assumed-that is believed to be responsible for the fully and spontaneously reversible loops in Ti 3 SiC 2 and possibly in most other layered solids. Consistent with the need of the basal planes to expand upon the introduction of BRs, the initial friction stress needed to move them was found to be almost three times lower when the basal planes were loaded edge-on. Molecular dynamics simulations on graphite at 10 K faithfully reproduce many features observed below the indenter in this, and previous, work on Ti 3 SiC 2. The existence of BRs will require a revisiting and reassessment of our understanding of how layered solids-from geologic formations to 2D solids-deform.

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