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Prompted by the success in functionalizing 2D sheets of MoS2 and MoSe2 by their reaction with organic halides, we have investigated the functionalization of layered tungsten chalcogenides, WS2, WSe2, and WTe2, by their reaction with substituted iodobenzenes as well as fluorophores based on pyrene and coumarin. Carbon–chalcogenide bonds are formed readily in the case of metallic chalcogenides while there is need for the Pd(0) catalysts in the case of insulating chalcogenides. Thus, the formation of carbon–chalcogen (S, Se, and Te) bonds is accomplished with the metallic 1T as well as the semiconducting 2H phases of the tungsten chalcogenides. Functionalization in the case of substituted iodobenzenes is favored by the electron withdrawing groups in the benzene ring. Interestingly, functionalization has been successful with other insulating layered chalcogenides such as GaS and GaSe. Reaction with organic halides emerges as a general method for the covalent functionalization of chalcogenide nanosheets, suggesting the possible use of this strategy to functionalize various chalcogenide structures.

I. INTRODUCTION

Covalent functionalization of graphene and carbon nanotubes by several routes has been reported.1–6 Covalent functionalization bestows novel properties on these materials. Recently, surface-functionalized 2D transition metal dichalcogenides have emerged as potential candidates for biosensing applications due to their tunable optoelectronic characteristics.7,8 Covalent functionalization of nanosheets of metallic 1T-forms of MoS2 and MoSe2 has been carried out recently giving rise to major changes in the attributes of these 2D materials.9–11 In the procedures used, the basal plane defects as well as the sites at the edges of the nanosheets of MoS2 get functionalized by their reaction with organic halides. In the case of MoS2 and MoSe2 nanosheets, the 1T-metallic form generated by lithium intercalation and exfoliation enables electron transfer to organic halides causing the formation of covalent C–S or C–Se bonds. In view of the success of this method of functionalization in the case of MoS2 and MoSe2, we have extended the studies to the dichalcogenides of tungsten, WS2, WSe2, and WTe2. In these studies, we have used organic halides to functionalize both metallic and insulating phases of layered tungsten chalcogenides, the latter by using the Pd(0) catalyst to facilitate the formation of the carbon–chalcogen Contributing Editor: Venkatesan Renugopalakrishnan a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.224

(S, Se, and Te) bonds. The reagents used for functionalization are substituted iodobenzenes as well as pyrene and coumarin halides with fluorescing properties. The use of para-substituted benzenes has afforded a study of the effect of electron withdrawing and donating power of the benzene substituents in functionalization. Gallium chalcogenides, GaS and GaSe, which are insulators could be functionalized by the use of Pd(0) catalyst by the reaction with organic ha