Growth Of Single Crystalline 2H-NbS₂ And The Effect Of Stoichiometry And Thickness On Its Superconducting Transition Temperature

High quality single crystals of 2H-NbS₂are prepared by chemical vapor transport method under varying conditions.The residual resistivity ratio?RRR?of the as-grown single crystals is observed to change from 10 to 65,while the superconducting transition temperatures?T_c?remain around 6.2 K.Changes in stoichiometry were obtained by annealing in different conditions,resulting in sulfur deficient 2H-NbS_2-ysingle crystals.Both T_cand RRR of the annealed samples are found to drop drastically when y increases from 0.05 to 0.10,which is argued to be the effect of en-hanced electron scattering caused by the decreased interlayer coupling evidenced by the increase of the c-axis lattice constant after annealing,possibly due to Nb intercalation between layers.2H-NbS₂nano-flakes are exfoliated from single crystals.By combining optical contrast de-termined via optical microscope with actual thickness determined by atomic force microscopy,we learn a colorcode to identify the thickness of nano-flakes immediately with only optical mi-croscopy.To measure the electrical and magnetic properties using four-probe method,the flakes are fabricated into devices.When the thickness decreases,we find the superconducting transition temperature decreases from 6.2 K of bulk to 4.9 K of 20 nm and 3.4 K of 10 nm,which is possible driven by the change of the electron–phonon coupling in 2H-NbS₂nano-flakes.