We describe a photogating effect in mono-and few-layer MoS2, which allows the control of the charge carrier density by almost two orders of magnitude without electrical contacts. Our Raman studies are consistent with physisorbed environmental molecules, that effectively deplete the intrinsically n-doped charge carrier system via charge transfer, and which can be gradually removed by the exposure to light. This photogating process is reversible and precisely tunable by the light intensity. The photogating efficiency is quantified by comparison with measurements on electrostatically gated MoS2.Two-dimensional layered van-der Waals materials and their heterostructures, particularly the transition metal dichalcogenides (TMDCs) family such as MoS 2 , are of great interest for fundamental research as well as for novel device concepts in the areas of electronic [1,2], optoelectronic [1,[3][4][5], valley-and spintronic [6] as well as solar energy conversion [7] and sensing applications [8]. It has been demonstrated that MoS 2 undergoes a transition from an indirect to a direct bandgap semiconductor by thinning it down to one single layer [3]. The direct gap at the K-point with E gap = 1.9 eV [3] remains almost unaffected by the number of layers. The indirect gap existing in vicinity of the Γ-point decreases in energy for increasing number of layers [3]. In particular, a high absorption of the monolayer of 5-10% in the visible regime [9] makes MoS 2 a promising material for optoelectronic devices such as phototransistors and solar cells. For atomistic thin materials, knowledge about the interaction with the dielectric environment is of great importance. The impact of substrates [10,11], the gaseous environment as well as of adsorbates [12][13][14] on the optoelectronic properties of 2D materials have been reported. The atomistic interfaces offer the opportunity of novel route for interfacial engineering of electronic, optical and optoelectronic properties [10,12,15,16].In this letter, we study the effect of photogating in MoS 2 mono-and few-layer flakes by means of Raman spectroscopy. Our observations are consistent with physisorbed environmental molecules acting as molecular gates. The molecules can be gradually removed from the MoS 2 surface by the exposure to light. We find that the photogating effect facilitates the control of the charge carrier density in MoS 2 by almost two orders of magnitude in pristine flakes without the need for electrical contacts. In general, Raman spectroscopy provides access to various properties of 2D layered materials [17] such as to the number of layers [18,19], to strain [20,21] to the local temperature [22] and to the doping level [23]. The Raman spectrum of MoS 2 includes two prominent firstorder phonon modes, the E 1 2g and the A 1g mode [17]. The E 1 2g mode is an in-plane mode, i.e. the atoms are oscillating parallel to the basal plane of the van-der Waals coupled crystal layers. The A 1g mode is an out-of plane oscillation, where the sulfur atoms are moving in opposite directions...