Electronically Induced Atom Motion in Engineered CoCu_n Nanostructures

Motion of a single atom within a larger nanostructure can be induced by using electron excitation mechanisms in the STM. Atom manipulation with the scanning tunneling microscope (STM) is accomplished using a tunable chemical bond between the adatom and the scanning tip and/or local electronic excitations via the tunneling electrons. The dynamics of atomic motion during such processes can be followed by analyzing the noise in the tunneling signal. Such atomic motion is ultimately controlled by both the energy landscape and the type of excitation and relaxation pathways the atoms encounter. A detailed investigation of the energy barrier for atom motion can be obtained by examining stimulated atom motion in a systematic series of nanostructures constructed via atom manipulation. Such studies are ideally suited for theoretical modeling where parameters can be varied in accord with the systematic variation in the experimental constructions.

In the present work we built CoCu_n linear chain nanostructures on a Cu(111) surface using STM atom manipulation techniques. We measured the quantum yield for exciting the motion of a single Co atom in CoCu_n linear molecules. The Co atom switched between two lattice positions during electron excitation from the tip of the scanning tunneling microscope (STM). The electron excitation and quantum yield were found to be spatially localized on an atomic scale. Above an electron energy threshold, the Co atom motion resulted from a predominantly single electron process. By systematically varying the molecular structure, atom motion within the molecule was shown to be dependent on molecular length and composition, owing to the corresponding variation in electronic structure.

Building CoCun Nanostructures

Figure 1. STM topographic images showing the sequence of assembly of the CoCu2 molecule, tunneling current 1 nA, sample bias 10 mV, T=4.3 K. (A) two individual Cu atoms and a Co atom, (B) Cu2 dimer and a Co atom, (C) CoCu2 molecule. STM topographic images of the CoCu2 molecule at different tunneling biases; (D) 3.5 mV, (E) 10 mV, (F) 20 mV. Schematic positions of the Co (blue) and Cu (gold) atoms are superimposed on the image in D. (G) A portion of the tunneling current vs. time trace obtained in the left vicinity of the Co atom in the CoCu2 molecule at 15.36 mV sample bias. Schematic model of the CoCu2 molecule on the Cu(111) substrate with the Co atom in the fcc site (H) and hcp site (I). (J) Distribution of residence times that the Co atom spends in the high current state from the data in G with a fit to an exponential decay, e-1/π (red line). The inset shows the distribution of the tunneling current from the time trace data in G. The high current state at 0.5 nA is associated with the Co atom in the fcc site (H), and the 0.3 nA peak with the Co atom in the hcp site (I). The STM images are shown in 3-D views with light shading.

Measuring Atom Switching Rates in CoCun Nanostructures

Figure 2. (A) Transfer rate for the Co atom out of the fcc site as a function of tunneling current obtained at a fixed sample bias of 40 mV for the CoCu2 molecule. The three curves were obtained at different locations near the Co atom, and are fit to, IN (red lines). The average of the three data sets yields, N=1.3±0.1(red lines). (B) fcc quantum yield as function of sample bias, at fixed tip-sample separation. Symbols correspond to different set-point currents for each measurement. The tip-sample separation varied 0.75 Å when changing the current set-point from 1 to 6 nA, at 40 mV sample bias. (C) Simultaneous spatial images of the quantum yield and STM topography (not shown) for the CoCu2 molecule; sample bias 40 mV, tunnel current 1 nA, T=4.3 K. Positions of the Co (blue) and Cu (gold) atoms are schematically superimposed on the quantum yield image.

Systematic Variation of Nanostructures

Figure 3. (A) Side view (from the hcp site) of the wavefunction isosurface for the highest occupied molecular orbital for three molecular lengths. This d state (red and green) has maximum amplitude behind the Co atom (blue) in this view and decreases with chain length. (B) Sequence of STM topographic images, all obtained at a sample bias of 10 mV, for CoCun molecules with n varying from 2 to 5. Tunneling current 1 nA, T= 4.3 K. (C) Density of states projected of the Co in the CoCu2 molecule.

Supported in part by the Office of Naval Research

Online: January 2007
Last Updated: February 2008

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