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...

La0.3 Sr0.7 FeO3-δ films undergo dramatic changes in electronic and optical properties due to reversible oxygen loss induced by low-temperature heating. This mechanism to control the functional properties may serve as a platform for new devices or sensors in which external stimuli are used to dynamically control the composition of complex oxide heterostructures.

The electronic phase diagram of epitaxial La1−xSrxFeO3 films is presented. The films were grown on SrTiO3 using molecular beam epitaxy with post-growth annealing to minimize oxygen vacancies. Insulating behavior is observed from x = 0–0.9, with metallic conduction only present for x = 1.0. While the La-rich compounds exhibit polaron conduction over all temperatures measured, the Sr-rich films exhibit an electronic phase transition within the compositional window of x = 0.49–0.9 as revealed by temperature-dependent resistivity measurements. The transition temperatures are found to decrease with increasing Sr content. The constructed phase diagram is discussed in the context of other 3d eg perovskite systems including manganites and cobaltites.

The electronic properties of LaFeO3/LaMnO3 epitaxial heterojunctions are investigated to determine the valence and conduction band offsets and the nominal Mn and Fe valence states at the interface. Studying a systematic series of (LaFeO3)n/(LaMnO3)m bilayers (m ≈ 50) epitaxially grown in the (001) orientation using molecular beam epitaxy, layer‐resolved electron energy loss spectroscopy reveals a lack of significant interfacial charge transfer, with a nominal 3+ valence state observed for both Mn and Fe across the interface. Through a combination of variable angle spectroscopic ellipsometry and hard X‐ray photoelectron spectroscopy, type I energy level alignments are obtained at the LaFeO3/LaMnO3 interface with positive valence and conduction band offsets of (1.20 ± 0.07) eV and (0.5–0.7 ± 0.3) eV, respectively, with minimal band bending. Variable temperature resistivity measurements reveal that the bilayers remain insulating and that the presence of the heterojunction does not result in a conducting interface.

We have investigated the coating of an inner surface of superconducting radio frequency cavities with a magnesium diboride thin film by hybrid physical-chemical vapor deposition (HPCVD). To simulate a 6 GHz rf cavity, a straight stainless steel tube of 1.5-inch inner diameter and a dummy stainless steel cavity were employed, on which small sapphire and metal substrates were mounted at different locations. The MgB 2 films on these substrates showed uniformly good superconducting properties including T c of 37-40 K, residual resistivity ratio of up to 14, and root-mean-square roughness R q of 20-30 nm. This work demonstrates the feasibility of coating the interior of cylindrical and curved objects with MgB 2 by the HPCVD technique, an important step towards superconducting rf cavities with MgB 2 coating.