Pb on Mo(110) studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to investigate the growth behavior of ultrathin Pb films on the Mo(110) surface at room temperature. The analysis of STM measurements indicates that for a coverage $\theta <1\text{ML}$ two-dimensional growth of the first Pb monolayer (wetting layer) took place. Above $\theta >1\text{ML}$, the three-dimensional growth of the Pb islands with strongly preferred atomic scale “magic height” and flat top is observed. At coverages between $1\text{ML}<\theta \le 2\text{ML}$, only islands containing two atomic layers of Pb are observed. At coverages between 2 and 3 ML, islands containing two and four atomic layers of Pb are observed. At higher coverages $\theta >3\text{ML}$, the island height distribution shows peaks at relative heights corresponding to $N=\left(2,4,6,7,\text{and}9\right)$ of Pb atomic layers.

Figure 1

STM images of Pb deposited on Mo(110) at $T=300\text{K}$ at coverage $\theta \le 1.0\text{ML}$: (a) clean Mo(110) ($1000\times 1000{\text{Å}}^2$, $I_{\text{T}}=0.8\text{nA}$, and $U_{\text{bias}}=100\text{mV}$). The line scan shown in the inset evidences terraces approximately between 100 and $400\text{Å}$ wide, (b) 0.4 ML ($1140\times 1140{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=1\text{V}$), (c) 0.8 ML ($2159\times 2159{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=1\text{V}$), (d) 1.0 ML ($1951\times 1951{\text{Å}}^2$, $I_{\text{T}}=0.3\text{nA}$, and $U_{\text{bias}}=800\text{mV}$). The inset presents zoom $\left(242\times 242{\text{Å}}^2\right)$ in the area indicated by a square. Arrows indicate the places where the Pb atoms of the second layer start to nucleate at dislocations of the first Pb layer.

Figure 2

STM images of Pb deposited on Mo(110) at $T=300\text{K}$ at coverage $1\text{ML}<\theta \le 4\text{ML}$. The height of Pb islands is measured from the wetting layer: (a) 1.2 ML ($10000\times 10000{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=1\text{V}$), (b) Height profile along the line A from the image in (a) demonstrating that the height of the islands corresponds to the height of a double Pb step height, (c) 1.5 ML ($10000\times 10000{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=800\text{mV}$). Inset presents differentiated STM image to enhance the contrast. (d) 2.5 ML ($8563\times 8563{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=1.0\text{V}$). The height of the Pb islands corresponds to 2 and 4 atomic layers of Pb(111).

Figure 3

(a) STM image of Pb deposited on Mo(110) at $T=300\text{K}$ at coverage $\theta =5\text{ML}$ ($10000\times 10000{\text{Å}}^2$, $I_{\text{T}}=0.2\text{nA}$, and $U_{\text{bias}}=1\text{V}$). (b) Island height distribution (number of Pb islands with a given height) obtained from the image in (a), showing the strongly preferred heights of the Pb islands corresponding to two, four, and six atomic layers of Pb(111).