Figure 1

(Color online) (a) UHV-STM image ($-2\text{V}$, 100 pA) of a SWNT interfaced with a partially depassivated $\text{Si}\left(100\right)\text{−}\left(2\times 1\right):\text{H}$ substrate. Darker (brighter) areas on the substrate indicate hydrogen passivation (depassivation), with the SWNT seen as a horizontal white feature. (b) STM constant-current height profile along the SWNT [thick horizontal line in (a)] revealing a sub-$0.5\text{Å}$ depression. Spectral data were subsequently acquired along the thick horizontal line in (c). The residual mechanical instability of the nanotube on H-Si to the right of the second vertical line manifests as a broadening of the apparent width of the SWNT. (d) Semilogarithmic differential conductance profile [acquired on the SWNT over the $\text{Si}\left(100\right)\text{−}\left(2\times 1\right):\text{H}$ area] indicates the SWNT is semiconducting; see Ref. 4. The $n$-type alignment of the substrate $\text{d}I/\text{d}V$ is also evident. (e) Absolute tunneling current plot along the axis of the SWNT [thick horizontal line in Fig. 1c. Spatial resolution: $2.5\text{Å}$]. To the right of the vertical dashed white line the tube sits on the depassivated stripe. The white traces in the profile signal a current of 1 pA, (close to the edge of the valence and conduction bands). Horizontal dotted lines serve as guide to the eye. The midgap energy (also shown) depicts a slight $n$ doping of the SWNT in the section on top of the depassivated substrate.Reuse & Permissions

Figure 2

(Color online) Top: Ball-and-stick model of the (8,4) SWNT on a partially depassivated ($10\text{Å}$ wide) $n$ doped $\text{Si}\left(100\right)\text{−}\left(2\times 1\right):\text{H}$ substrate. The red line indicates the direction of the Si dimmer rows, and it forms a $45°$ angle with respect to the SWNT axis. Only the lower half of the SWNT is shown for clarity. Side: The side view of the system, showing in red the location of the phosporous ($n$ type) dopant. Horizontal dashed lines serve to indicate distortion in the SWNT. Maximum distortion in SWNT occurs at $x_1$ and $x_2$. Inset: the (8,4) SWNT was contracted by 3.8% (blue vertical line) in order to be placed on top of the Si substrate. This implied a 30% reduction of its gap and a 0.75% increase in diameter to compensate longitudinal compression.Reuse & Permissions

Figure 3

(a) Band structures and PDOS of systems shown in Fig. 2. Flat bands around the Fermi level arise from localized states in the uppermost, unpassivated Si atoms. The highlighted PDOS of C atoms, shift downwards in energy in the $n$-doped plot, along with the bands in the substrate; compare to the C PDOS when no dopants are present. (b) The doping effect observed in Fig. 1e is independently confirmed from our calculations: both C band edges go down by 0.11 eV once the depassivated stripe is present in the substrate. (c) The band structure of a 2 unit-cell long (8,4) SWNT in the absence of compression, and the corresponding band structure of a SWNT with the parametric distortion as in Eq. (1). An ever slight reduction of the semiconducting gap is seen, as well as the breaking of the degeneracy in the points highlighted by horizontal arrows, also present in (a) when doping is present. (d) Schematic view of the parametric distortion [the distortion seen is larger than that in Eq. (1) for clarity].Reuse & Permissions