Figure 1

From single atoms to linear chains of twenty atoms. (a) Individual Au atoms adsorbed on NiAl(110) appear as protrusions in constant current STM topographic images. (b) By lateral manipulation, atoms are brought together to form linear chains along the [001] direction. A schematic shows an individual Au atom and a six-atom chain. (c) STM topographic images from a single Au atom to a linear chain of twenty atoms, arranged in single atom increments from left to right. The images are cut from twenty separate scans, each taken with a sample bias voltage between 2.0 and 2.5 V and a tunneling current between 1.0 and 1.5 nA. Each chain has an apparent height between 2.4 and 2.7 Å. The chains between twelve and twenty atoms long were constructed and imaged with a tip different than the tip used to construct and image the other chains.Reuse & Permissions

Figure 2

$dI/dV$ images for a single atom and six-, eleven-, fifteen-, and twenty-atom chains reveal electronic density oscillations. The inset shows a scanning tunneling spectrum taken with the tip positioned directly above a single Au atom showing a peak at 1.95 eV above the Fermi energy. The spectrum was acquired as follows: with the tip positioned directly above a single Au atom and the distance between the tip and the atom fixed, $dI/dV$ was measured using a lock-in technique as a function of the sample bias voltage. The contribution of thermal, instrumental and modulation broadening to the peak width is quite small, about 20 meV. To obtain a $dI/dV$ image, the tip was scanned across the imaging area and $dI/dV$ was measured at regular intervals with the sample bias voltage fixed at a chosen value. A topographic image was acquired simultaneously. The $dI/dV$ image of the atom was taken with a sample bias voltage of 1.95 V. The $dI/dV$ images for the six-, fifteen-, and twenty-atom chains were taken with one tip (tip A) while the $dI/dV$ image for the eleven-atom chain was taken with another tip (tip B). Spectra taken with tip A showed a $-0.2\mathrm{e}\mathrm{V}$ shift for all features relative to those in spectra taken with tip B. As a result of this difference, attributed to an electric field effect, the $dI/dV$ image of the eleven-atom chain was acquired with a greater sample bias voltage (2.0 V) than that used to acquire the $dI/dV$ images of the other chains (1.8 V). There were no other prominent tip-dependent effects.Reuse & Permissions

Figure 3

Sixteen $dI/dV$ images of the eleven-atom chain taken at 0.1 V intervals between 1.0 and 2.5 V. To compensate for the edge effects discussed in the text, each $dI/dV$ image in Figs. 2, 3 is an average of two scans, one reading $dI/dV$ while the tip is scanned forward and the other while the tip is scanned backward.Reuse & Permissions

Figure 4

Electronic density oscillations for eleven-atom chain. Each curve in (a) is an average of five parallel cross sections of one $dI/dV$ image in Fig. 3 taken along the length of the chain. The cross sections are in order of increasing sample bias voltage from bottom to top and have been offset from one another to facilitate comparison. Individual eigenstates dominate the oscillations at 1.3–1.4, 1.7, and 2.5 V (black curves). Wavelengths for these states were measured from the cross sections in (a). For the second excited state, the wavelength was taken to be the average of the wavelengths measured in the 1.3 and 1.4 V cross sections and the energy of the state was taken to be 1.35 eV. The dispersion relation for the eleven-atom chain is shown in (b). A parabolic fit (solid gray line) yields an effective mass of 0.4 times the mass of a free electron. The fit predicts the energies of the ground state and first excited state to be 0.8 and 1.0 eV (solid, black diamonds). In (c), each state is modeled as a normal curve with a full-width-half-maximum equal to that of the resonance of an individual Au atom, 0.40 eV. The second, third, and fourth excited states dominate at 1.35, 1.7, and 2.5 eV. At lower energies, more than one state contribute to the total $dI/dV$ signal.Reuse & Permissions