suppressed, they may replace or augment silicon (Si) to overcome scaling and performance limitations of current semiconductor technologies. [5] However, few-and single-layer 2D materials are supremely sensitive to their dielectric environments, which must be controlled on the atomic level in a scalable manner to make full use of their intrinsic properties in optical and electronic device applications. [4] This represents a considerable challenge, since most conventional techniques used for dielectric or metal integration in semiconductor technology, such as high-temperature oxidation, sputtering, evaporation, and chemical vapor deposition (CVD), can degrade the 2D crystal as a consequence of high energy atoms impacting the surface [6] or gas-phase reactants interacting with the 2D material at elevated temperatures. In contrast, atomic layer deposition (ALD) relies on the sequential injection of gasphase reactants into a reactor chamber at moderate temperatures (<300 °C) and provides a delicate route to the controlled and large-scale deposition of thin films on sensitive substrates.Although ALD offers considerable advantages for 2D/3D materials integration, conformal deposition on 2D materials without the introduction of defects has proven to be challenging due to the lack of reactive sites on the fully coordinated basal planes of vdW materials. [7] For the realization of continuous and ultrathin coatings by ALD, seed layers on 2D nanosheets have been demonstrated to facilitate nucleation and yield uniform ALD coatings, [8] but their application is incompatible with large-scale manufacturing and may reduce interface stability during device operation. As an alternative, gas-phase surface activation prior to or during ALD, including (UV) ozone treatment [7a,9] or plasma-assisted approaches, [10] can be easily combined with the growth process. However, such treatments can be deleterious to the 2D crystal and introduce additional defects. [11] In this regard, there is a pressing need to understand the evolution of the optoelectronic characteristics of 2D materials during growth of films on their surfaces, and especially during the critical stage of nucleation when the basal plane is exposed to the reactive gas environment.Despite the importance of the initial growth regime in defining both interface and film, in situ investigations during Here, it is shown that in situ spectroscopic ellipsometry (SE) is a powerful method for probing the effects of reactant adsorption and film formation on the excitonic properties of 2D materials during atomic layer deposition (ALD), thus allowing optimization of both film growth and opto(electronic) characteristics in real time. Facilitated by in situ SE during ALD on monolayer MoS 2 , a low temperature (40 °C) process for encapsulation of the 2D material with a nanometer-thin alumina (AlO x ) layer is investigated, which results in a 2D/3D interface governed by van der Waals interactions rather than chemical bonding. Charge transfer doping of MoS 2 by AlO x is found to be an inte...