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We report a straightforward chemical methodology for controlling the thickness of black phosphorus flakes down to the monolayer limit by layer-by-layer oxidation and thinning, using water as solubilizing agent. Moreover, the oxidation process can be stopped at will by two different passivation procedures, namely the non-covalent functionalization with perylene diimide chromophores, which prevents the photooxidation, or by using a protective ionic liquid layer. The obtained flakes preserve their electronic properties as demonstrated by fabricating a BP field-effect transistor (FET). This work paves the way for the preparation of BP devices with controlled thickness

To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.