Thermoelectric Properties Of Nitrogenated Holey Graphene Nanostructures

With the successful fabrication of graphene,the research on two-dimensional nanomaterials has attracted widespread interest.Compared with traditional bulk materials,two-dimensional nanomaterials exhibit many excellent properties,especially in thermoelectric field.As an effective way to solve the energy crisis,thermoelectric materials can directly transform temperature gradients into electric voltage and vice versa which is measured by ZT value.Therefore,it is of great significance to study the thermoelectric properties of two-dimensional nanomaterials.Recently,a new 2D graphene variant-nitrogenated holey graphene(C2N)has been proposed and synthesized successfully.The room temperature thermal conductivity of C2N is much lower than that for suspended graphene.In view of its low thermal conductance and semiconductor characteristic,C2N might be a promising candidate for excellent thermoelectric material.In this paper,we systematically investigate the ballistic thermoelectric properties of C2N nanostructures by using the nonequilibrium Green's function(NEGF)method.The primary coverage of this thesis is as follows:1.The thermoelectric properties of C2N nanoribbons are investigated by NEGF method.In case of zigzag edged C2N nanoribbons(Z-NHGNRs),the thermoelectric performance is insensitive to the width of nanoribbon(ZT is almost constant,0.75 for electron and 0.2 for hole transport).Moreover,the ZT for electron transport is larger than that of hole transport.This characteristic is attributed to lower growth rate of electronic transmission for hole transport compared to that of electron transport.For armchair edged C2N nanoribbons(A-NHGNRs),the ZT of electron presents an oscillatory behavior due to the irregular growth rate of electronic transmission and conductance.Nevertheless the hole's ZT decreases gradually with the width.The quasi-stable ZT value of A-NHGNRs is about 0.6 and 0.2 for electron and hole transport.One can conclude that compared to graphene nanoribbions(the ZT is about 0.05 at room temperature),the C2N nanoribbons is more suitable for the carbon-based thermoelectric materials especially for the Z-NHGNRs.2.For the purpose of achieving better thermoelectric performance,we also investigate the thermoelectric properties of chevron-type Z-NHGNRs.The ZT values of chevron-type Z-NHGNRs are always larger than that of pure ones.Particularly,the maximum ZT value of chevron-type Z-NHGNRs for electron transport is 1.5 which is about two times larger than that of pristine Z-NHGNRs.Such enhancement could be attributed to the combined effect of increased Seebeck coefficient and reduced phononic and electronic thermal conductance.Therefore an appropriate chevron-type structure could be helpful to boost the ZT values.3.We also investigate the thermoelectric properties of Z-NHGNRs imported with a rhomboid quantum dot.The maximum When the length of the system is fixed,the ZT value for electron transport is insensitive to the width,while that for hole transport decreases with width increasing.Moreover,no matter electron and hole transport,the quantum dot with short length always possesses better thermoelectric performance.Based on the system calculated in this paper,the maximum ZT value in the case of rhomboid quantum dot is 1.4 for electron and 0.6 for hole.This result indicates that importing with a rhomboid quantum dot is another effective way to enhance the thermoelectric properties of C2N nanoribbons.