W(110)表面反常STM图像的密度泛函理论研究

Density-functional theory study on the STM images of W(110) p(1×1) surface

采用密度泛函理论的第一性原理方法研究了W(110)p(1×1)表面的STM图像对于衬底偏压的依赖性.计算结果表明:在衬底负偏压条件下,W(110)p(1×1)表面的钨原子在STM图像中显示为暗点而非通常在其它过渡金属中观察到的亮斑,并且暗点随偏压绝对值的减小而逐渐弱化.计算还模拟了恒流模式的STM测量时针尖的起伏变化.当衬底偏压在0～100meV区间时,针尖起伏高度最为明显(～0.006nm).在更高的正偏压下,STM的针尖起伏随偏压改变而线性变化(0.0015～0.0035nm).这些结果说明了W(110)p(1×1)表面是非常平坦的.由于钨原子的价电子为5d态,和3d电子相比具有更为扩展的行为,表面态电子波函数交叠区间集中在原子周围,所以STM测量时亮点突起出现在原子的周围.

The dependence of STM images on bias voltage for reconstructed W（110） p（1×1） surface was investiga- ted by using the density-functional theory calculations. The calculated results showed that in negative bias voltage region the positions over the surface atoms are dark while the empty hollow sites are bright and the darkness of atom positions are smoothed when decreasing bias voltages. In constant-current mode, the topography of the STM images with several voltages was simulated in the same approach. The simulated STM tip corrugations were also presented in details： the maximum corrugation （-0. 006 nm） appeared within 0--100 meV and in higher positive bias voltages the corrugation varies linearly in the range of 0. 001 5--0. 003 5 nm. These observations indicated that the W（110） p（1×1） surface is almost flat even there exists the hollow sites in the sense of STM experiment. Since the 5d states of tungsten are more extended than that of 3d electrons, the overlap of surface electron wave function mainly occurs normally between neighboring atoms, which may explain why bright spots appear around the atomic sites in STM images of W（110） p（1×1）.