Preparation And Device Properties Of Two-dimensional SnSe_x Atomic Crystal Materials

Since the discovery of the first two dimensional?2D?material-graphene,in the past decade,people have been continually exploring the new kinds of 2D materials,and extended the 2D materials from the elementary substance to binary compounds,almost involving the one fourth of the first six cycls elements in the periodic table and showing various structures and divers properties.Among the 2D binary compounds,there is few reports about the main metal chalcogenide,especially the group IVA metal chalcogenides.Research on these compounds will enrich the 2D material family and achieve a comprehensive understanding.2D layered Sn-Se binary compounds,including SnSe and SnSe₂,are two typical group IVA metal chalcogenides and both of them are semiconductor materials.SnSe has the orthorhombic structure similar as black phosphorus and is p type.The mobility of bulk SnSe reaches up to 150 cm2V-1s-1 at room temperature,which is largest among all the group IVA metal chalcogenides.SnSe has stable chemical properties and low toxicity.Because of the giant phonon anharmonicity,the thermal conductivity is very low in SnSe,leading to the excellent thermoelectric property.On the contrary,SnSe₂ is n type semiconductor with graphene-liked hexagonal structure.A bandgap of 0.97 e V make SnSe₂ feasible to form good bandgap alignment with SnSe.With above advantages,we choose 2D layered SnSe and SnSe₂ as research topics.The main results are as follows:1.We have successfully epitaxilly grow SnSe flakes on flexible organic siliconePDMS by CVD method.The as-prepared SnSe shows thickness dependent Raman with thickness above 10 nm,which is different from that of Mo S2 and can be attributed to the stronger interlayer interaction.As the temperature increases,Raman spectra shows more softening and anharmonicity features along c axis.Moreover,with the contact resonance AFM technique,we investigated the Young's module of SnSe along a axis,which reaches to 54±2 GPa.2.The field effect transistors?FETs?based on SnSe flakes are investigated.Because the shallow acceptor level?52 me V?,the hole density of SnSe flakes is high 1018 cm-3.The high hole density makes SnSe can't be turned off but still can be tuned by the back gate.The mobility of SnSe FETs ranges from 2 15 cm2V-1s-1,which is higher than the other reports but still lower than the bulk value.Because anisotropy of optical phonon,field-effect mobility also shows in-plane anisotropy and is higher along c axis at high temperature above 240 K.3.The thermoelectric property of SnSe flakes with thickness of 15 nm is studied.With decreasing temperatures,the gate tunability of Seebeck coefficient of SnSe flakes will increase.Both the DC and AC tests give the similar result on Seebeck coefficients ranges from 100 to 400 mV/K at room temperature,comparable with the hole-doped bulk SnSe.For this thickness? 15 nm?,there isn't obvious quantum confinement effect.At the same time,the decay of thermoelectric power factor suggests that the defects formed during CVD process seriously limit the thermoelectric performance of SnSe flakes.4.2D atomic crystal SnSe₂ is fabricated by mechanical exfoliation.It shows n type degenerate doped characteristic.Through optimizing the device configuration of SnSe₂ transistor,high current on/off ratio?104?is achieved by coupling the capacitances of the 70 nm HfO back gate insulator and the top capping layer of polymer electrolyte.Then,the instability of Li+ intercalation at high positive bias is also investigated.