  Trapped materials at the interfaces of two-dimensional heterostructures (HS)
lead to reduced coupling between the layers, resulting in degraded
optoelectronic performance and device variability. Further, nanobubbles can
form at the interface during transfer or after annealing. The question of what
is inside a nanobubble, i.e. the trapped material, remains unanswered, limiting
the studies and applications of these nanobubble systems. In this work, we
report two key advances. Firstly, we quantify the interface quality using
RAW-format optical imaging, and distinguish between ideal and non-ideal
interfaces. The HS-substrate ratio value is calculated using a transfer matrix
model, and is able to detect the presence of trapped layers. The second key
advance is identification of water as the trapped material inside a nanobubble.
To the best of our knowledge, this is the first study to show that optical
imaging alone can quantify interface quality, and find the type of trapped
material inside spontaneously formed nanobubbles. We also define a quality
index parameter to quantify the interface quality of HS. Quantitative
measurement of the interface will help answer the question whether annealing is
necessary during HS preparation, and will enable creation of complex HS with
small twist angles. Identification of the trapped materials will pave the way
towards using nanobubbles for novel optical and engineering applications.
