Size-, shape-, and assembly-controlled synthesis of Cu_2-xSe nanocrystals via a non-injection phosphine-free colloidal method

In this paper, we report a facile, "green", phosphine-free, low-cost, and non-injection method to obtain size-, shape-, and assembly-controllable Cu_2-xSe nanocrystals which can be used as uniform building blocks. Different sizes of monodispersed Cu_2-xSe nanocrystals were synthesized successfully by simply controlling the reaction temperature. The highly uniform Cu_2-xSe nanocrystals can be well controlled from 4 to 20 nm when the temperature was set from 120 to 200 °C. By changing the ratios of CuSt₂, oleic acid (OA), oleylamine (OAM), and selenium-octadecene (Se-ODE) precursor, Cu_2-xSe nanocrystals with different shapes (hexagonal, elongated hexagonal bipyramid-shaped, trigonal pyramidal-shaped) and assembly behaviors (hexagonal Cu_2-xSe nanodiscs with 1D, 2D, and 3D columnar self-assembly, elongated hexagonal bipyramid-shaped Cu_2-xSe with 2D and 3D self-assembly, trigonal pyramidal-shaped Cu_2-xSe nanocrystals with different arrays) were obtained indeed. Even though both OA and OAM were used as stabilizers to synthesize different shaped Cu_2-xSe nanocrystals, FTIR results indicated that the surface of the as-synthesized nanocrystals was only capped by OAM. XRD studies confirmed that three different shapes of Cu_2-xSe nanocrystals prepared with this non-injection method are all cubic berzelianite and well-crystallized. Current-voltage (I-V) behaviors of different shaped Cu_2-xSe nanocrystals were measured and all found to have low resistivities.