  Visible transparent but infrared reflective materials are ideal candidates
for both transparent conductive films and low-emissivity glass, which are
highly desired in a broad variety of areas such as touchscreens and displays,
photovoltaics, smart windows, and antistatic coatings. Ultrathin Ti3C2Tx MXene
films are emerging as promising low-emissivity transparent candidates. However,
the fundamental IR properties of Ti3C2Tx has not been revealed experimentally
due to daunting challenges in the preparation of continuous, large-area, and
ultrathin films of optical quality on flat substrates. Herein, we proposed a
tape-free transfer method that can help prepare centimeter-size and ultrathin
(down to 8 nm) Ti3C2Tx films on diverse optical substrates. Benefitting from
this method, the refractive index and permittivity for Ti3C2Tx were
successfully measured. Ti3C2Tx films exhibit large in-plane permittivity in the
IR region, yielding maximum IR reflectance of 88% for bulk films.
Interestingly, three anomalies were found in ultrathin Ti3C2Tx films: strong
dispersion in the permittivity, interlayer space-dependent optical properties,
and abnormally high IR absorption for a 15-nm-thick film. These anomalies are
important guidelines in the design of Ti3C2Tx-based low-emissivity transparent
films and other related devices, and may inspire other intriguing applications
such as ultrathin IR absorption coatings and tunable IR optical devices.
