  The emerging Au-assisted exfoliation technique provides a wealth of
large-area and high-quality ultrathin two-dimensional (2D) materials compared
with traditional tape-based exfoliation. Fast, damage-free, and reliable
determination of the layer number of such 2D films is essential to study
layer-dependent physics and promote device applications. Here, an optical
method has been developed for simple, high throughput, and accurate
determination of the layer number for Au-assisted exfoliated MoS$_2$ and WS$_2$
films in a broad thickness range. The method is based on quantitative analysis
of layer-dependent white light reflection spectra, revealing that the
reflection peak intensity can be used as a clear indicator for determining the
layer number. The simple yet robust method will facilitate the fundamental
study on layer-dependent optical, electrical, and thermal properties and device
applications of 2D materials. The technique can also be readily combined with
photoluminescence and Raman spectroscopies to study other layer-dependent
physical properties of 2D materials.
