Highly Conductive Optical Quality Solution‐Processed Films of 2D Titanium Carbide

Abstract

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...

Transparent, conductive solution processed spincast 2D Ti2CTx (MXene) films

“…They were then stored in an N 2 glove box for 24 h, taken out of the glove box, and their τ measured. In our previous work on Ti 3 C 2 T x we showed that while exposure to the atmosphere affected the conductivities of the films, the effect on τ was minimal [32].…”

“…All electrical resistance measurements were carried out on the same films inside the glove box using a 4-point probe with a probe spacing of 2.3 mm (Keithley 2634B Sourcemeter) [32]. Sheet resistance was measured 5 times at different locations across the film and averaged.…”

“…Ag-nanowires [27] are a good example of non-uniform type. Examples of the former include, un-doped [28] and doped graphene [29,30], MXenes [8,9,31,32] and the industry standard indium tin oxide, ITO. While the combination of transparency and conductivity of ITO is in general difficult to surpass, the high cost and limited supply of indium have energized efforts, especially in the 2D community, to find alternatives.…”

“…If the films are thin enough, they are also transparent [8,9,31,32]. For quantitative comparison of transparent conductors, we use a common figure of merit (FOM), defined as the ratio of the DC conductivity, σ DC , to a quantity termed the 'optical conductivity', σ OP , derived from the optical transmittance [33].…”

“…To date all work in this area has focused on Ti 3 C 2 T x [8,9,31,32]. Herein, we start to probe the importance of MXene chemistry on the optoelectronic properties of spincast films from aqueous colloidal solutions.…”