Quantum Confinement between Self-Organized Pt Nanowires on Ge(001)

The existence of one-dimensional (1D) electronic states between self-organized Pt nanowires spaced 1.6 or 2.4 nm apart on a Ge(001) surface is revealed by low-temperature scanning tunneling microscopy. These perfectly straight Pt nanowires act as barriers for a surface state (located just below the Fermi level) of the underlying terrace. The energy positions of the 1D electronic states are in good agreement with the energy levels of a quantum particle in a well. Spatial maps of the differential conductivity of the 1D electronic states conclusively reveal that these states are exclusively present in the troughs between the Pt nanowires.

Figure 1

Scanning tunneling microscopy image of an array of self-organizing Pt nanowires on Ge(001). Note the periodicity doubling of the Pt nanowires along the direction of elongation. The spacing between adjacent Pt nanowires is only 1.6 nm. Sample bias $-1.35\mathrm{V}$ and tunneling current 0.54 nA.

Figure 2

(a) Differential conductivity ($dI/dV$) of Pt-nanowires array at 300 K (diamonds) and 77 K (solid line). The Pt nanowires are 1.6 nm spaced apart. (b) Local density of states ($(dI/dV)/(I/V)$) of the bare $\beta$ terrace (lower curve), troughs between Pt nanowires that are 1.6 nm spaced apart (middle curve) and troughs between Pt nanowires that are 2.4 nm spaced apart. All spectra are recorded at 77 K. For $L=1.6\mathrm{nm}$ only one novel peak at 0.1 eV above the Fermi level is observed, whereas for $L=2.4\mathrm{nm}$ two novel peaks are found at 0.04 and 0.16 eV above the Fermi level, respectively.

Figure 3

Topography (a) and spatial map of the differential conductivity ($dI/dV$) (b) of a $8\times 8\mathrm{nm}$ area with several Pt nanowires recorded at $T=77\mathrm{K}$. The sample bias is 0.15 V and the tunnel current is 0.437 nA. The $dI/dV$ map is recorded with a modulation voltage of 10 mV and an oscillation frequency of 797 Hz (this frequency exceeds the bandwidth of the feedback loop). In the lower image a 3-dimensional representation of the topography (orange) and $dI/dV$ map (purple) is shown. The one-dimensional electronic state is exclusively located in the troughs of the Pt nanowires (black arrows refer to the position of the Pt nanowires). The yellow and green circles (ellipses) refer to defects in the Pt nanowire and underlying substrate, respectively. The confinement of the electronic state disappears near these defects, as expected.