Density Functional Theory Study On The Structure And Properties Of Cu_mX_n(X=Se,Te;m+n?5) Clusters

Based on the density functional theory?DFT?in the B3LYP method with LANL2DZ pseudopotential basis set level,the geometric configurations of the Cu_mX_n?X=Se,Te;m+n?5?clusters are optimized.A series of stable structures are achieved,and the related physical and chemical properties of these ground state structures are also systematically studied.For the ground state structure and stability,with the decrease of the number m of Cu atoms,the ground state structures of Cu_mX_n?X=Se,Te;m+n?5?clusters transform from complex planar or steric configurations into simple planar or linear ones.Cu_mX_n largely show similar ground state structures and the same symmetries for the same m,assoiated with uniformly low spin multiplicities and high symmetries.The bond length of Cu-Te in Cu_mTe_n is slightly longer than that of Cu-Se in Cu_mSe_n,indicating the slightly weaker Cu-Te bonds in the former.The analysis of the average binding energy reveals that Cu₃Se₂ and Cu₃Te₂,Cu₂Se₂ and Cu₂Te₂ as well as Cu₂Se and Cu₂Te are the highest thermodynamically stable systems for selenium and tellurium clusters at m+n=5,4 and 3,respectively.The molecular electronegativity calculations show that Cu₂Se₃ and Cu₂Te₃,CuSe₃and CuTe₃ as well as CuSe₂ and CuTe₂ have the strongest attraction to bonding electrons at m+n=5,4 and 3,respectively.The analysis on energy gap reveals that Cu₂Se₃and Cu₃Te₂,Cu₂Se₂ and Cu₂Te₂ as well as CuSe₂ and CuTe₂ have the highest chemical stability at m+n=5,4 and 3,respectively.And Cu₂X₂?X=Se,Te?show the highest chemical stability among all the systems.In addition,CuX₄,Cu₄X,Cu₃X,Cu₂X?X=Se,Te?,CuTe₃ and CuTe₂ show smaller energy gaps than 3 eV,indicating that they are expected to become promising semiconductor nanomaterials.As regards the electronic structure,the shapes of the density of states?DOSs?are roughly the same for Cu_mSe_n and the corresponding Cu_mTe_n,which is attributed to the same main group of the periodic table and hence similar electronic properties for Se and Te.The distance between the peaks adjacent to the Fermi energy is closely related to the covalency of the system.The peaks of DOSs locate mainly in the range of-21.77 to5.44 eV,where the peaks of total density of states?TDOS?are concentrated at-0.3 a.u.?-8.16 eV?,and electrons are mainly distributed near the Fermi energy.The total magnetic moments of Cu_mX_n arise predominantly from the local magnetic moments of X atoms and are closely related to the symmetries.The atoms in the symmetric positions show the same local magnetic moments.The clusters exhibit both ferromagnetic and anti-ferromagnetic interactions.The subdivisions of the polarizability of the clusters are mainly concentrated on the XX,YY and ZZ prinipal axes.The strong atomic bonding interactions are found in Cu₄Se and CuTe₄,associated with significant nonlinear optical effects and high polarizibility under external electric fields.The largest average of the polarizability tensor per atom is<???>?37.52 a.u.for CuTe₄in tellurium clusters,with the relatively unstable electronic structure and significant delocalization effect.The dipole moments of the clusters are related to the polarities and symmetries of the systems.As for the spectroscopic properties,the IR and Raman spectra of Cu_mX_n have many vibrational peaks in the ranges of 40-280 and 60-300 cm^-1,respectively.The vibration modes of the strongest or secondary peaks are mainly ascribed to the stretching vibration of Cu or X atoms.The strongest absorption peaks of UV-Vis spectra locate mainly at the range of 200-400 nm.The intensities of UV-Vis peaks for larger m of copper atoms is generally higher than those for smaller m,roughly attributable to the X to Cu charge transfer transitions.The positions of the UV-Vis peaks show slight red shifts from X=Se to Te with the same m,indicating that the bonding strength and energy levels decrease slightly.