Because the surface-to-volume ratio of quasi-two-dimensional materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layered semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, here we show that the surface of high-quality synthesized molybdenum disulfide (MoS₂) is a major n-doping source. The surface electron concentration of MoS₂is nearly four orders of magnitude higher than that of its inner bulk. Substantial thickness-dependent conductivity in MoS₂nanoflakes was observed. The transfer length method suggested the current transport in MoS₂following a two-dimensional behavior rather than the conventional three-dimensional mode. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements confirmed the presence of surface electron accumulation in this layered material. Notably, the in situ-cleaved surface exhibited a nearly intrinsic state without electron accumulation.

Designing highly conducting metal–organic frameworks (MOFs) is currently a subject of great interest for their potential applications in diverse areas encompassing energy storage and generation. Herein, a strategic design in which a metal–sulfur plane is integrated within a MOF to achieve high electrical conductivity, is successfully demonstrated. The MOF {[Cu₂(6-Hmna)(6-mn)]·NH₄}_n (1, 6-Hmna = 6-mercaptonicotinic acid, 6-mn = 6-mercaptonicotinate), consisting of a two dimensional (–Cu–S–)_n plane, is synthesized from the reaction of Cu(NO₃)₂, and 6,6′-dithiodinicotinic acid via the in situ cleavage of an S–S bond under hydrothermal conditions. A single crystal of the MOF is found to have a low activation energy (6 meV), small bandgap (1.34 eV) and a highest electrical conductivity (10.96 S cm⁻¹) among MOFs for single crystal measurements. This approach provides an ideal roadmap for producing highly conductive MOFs with great potential for applications in batteries, thermoelectric, supercapacitors and related areas.

The electronic transport properties of two-dimensional (2D) niobium diselenide (NbSe₂) layer materials with two-hexagonal single-crystalline structures grown by chemical vapor transport were investigated. Those NbSe₂ nanostructures isolated simply using mechanical exfoliation were found to exhibit lower conductivity and semiconducting properties, compared with their bulk metallic counterparts. Benefiting from lower dark conductivity, NbSe₂ nanoflakes exhibit a remarkable photoresponse under different wavelengths and intensity excitations. The photocurrent responsivity and photoconductive gain can reach 3.8 A W⁻¹ and 300, respectively; these values are higher than those of graphene and MoS₂ monolayers and are comparable with those of GaS and GaSe nanosheets. The presence of electron trap states at the surface was proposed as an explanation for the reduced dark conductivity and enhanced photoconductivity in the 2D NbSe₂ nanostructures. This work identifies another possibility for the application of a metallic layer material as an optoelectronic component in addition to an ultrathin transparent conducting material.