Researchers have investigated the electronic and optical properties of ultrathin molybdenum disulfide (MoS₂) crystals with N=1 to 6 layers. Using optical absorption, photoluminescence, and photoconductivity spectroscopy, they observed that as the thickness decreases, the indirect band gap shifts upward by more than 0.6 eV, leading to a crossover to a direct-gap material in the monolayer limit. Unlike bulk MoS₂, which is an indirect-gap semiconductor with a band gap of 1.29 eV, the monolayer MoS₂ emits light strongly and has a luminescence quantum efficiency more than 1000 times higher than the bulk. This is attributed to the direct-gap nature of the monolayer, which allows for efficient light emission. The study also shows that the band gap of MoS₂ decreases with increasing layer number, and the indirect gap energy increases significantly. The monolayer MoS₂ is found to be a direct-gap semiconductor, while few-layer samples are indirect-gap. The results demonstrate that ultrathin MoS₂ can be an effective light emitter, with potential applications in photostable markers, sensors, and photocatalytic materials. The findings also show that the unique electronic properties of ultrathin layered materials are not limited to graphene but extend to a broader group of van-der-Waals bonded solids.Researchers have investigated the electronic and optical properties of ultrathin molybdenum disulfide (MoS₂) crystals with N=1 to 6 layers. Using optical absorption, photoluminescence, and photoconductivity spectroscopy, they observed that as the thickness decreases, the indirect band gap shifts upward by more than 0.6 eV, leading to a crossover to a direct-gap material in the monolayer limit. Unlike bulk MoS₂, which is an indirect-gap semiconductor with a band gap of 1.29 eV, the monolayer MoS₂ emits light strongly and has a luminescence quantum efficiency more than 1000 times higher than the bulk. This is attributed to the direct-gap nature of the monolayer, which allows for efficient light emission. The study also shows that the band gap of MoS₂ decreases with increasing layer number, and the indirect gap energy increases significantly. The monolayer MoS₂ is found to be a direct-gap semiconductor, while few-layer samples are indirect-gap. The results demonstrate that ultrathin MoS₂ can be an effective light emitter, with potential applications in photostable markers, sensors, and photocatalytic materials. The findings also show that the unique electronic properties of ultrathin layered materials are not limited to graphene but extend to a broader group of van-der-Waals bonded solids.