Figure 1

(Color online) Constant current filled-state STM imaging of a methylstyrene/styrene heterostructure on the H:Si(100) surface. (a) Growth of the first line segment of methylstyrene (8-L exposure). The chemically reactive silicon radical which initiated the line growth (not shown) is now located at the end of the ${\mathrm{CH}}_3$-styrene segment (indicated by white arrow). This silicon radical will serve as the initiation site for the growth of the second line segment. (b) Following a 78-L exposure of styrene, the methylstyrene segment in panel (a) has been extended to form a molecular heterostructure. The white wedge indicates the location of the heterointerface. At $V_s=-3.0\mathrm{V}$, the methylstyrene line segment images higher than the styrene segment. (c) $V_s=-2.4\mathrm{V}$, the methylstyrene and styrene line segments image with similar height. (d) $V_s=-1.8\mathrm{V}$, the methylstyrene and styrene line segments reveal different molecular contrast. Inset: Blue and red arrows show locations of topographic cross sections presented in Figs. 2a, 2b, respectively. Images were acquired using a constant tunnel current of 60 pA. Image areas: $\sim 9\times 9{\mathrm{nm}}^2$.Reuse & Permissions

Figure 2

(Color online) Topographic cross sections extracted from STM image data of the methylstyrene/styrene heterostructure presented in Fig. 1. (a) Topographic cross sections taken along the chemical attachment points between the heterostructure and the underlying dimer row, along direction of blue (left most) arrow in Fig. 1d. (b) Topographic cross sections taken to the right of the trench separating the underlying dimer rows, along direction of red (right most) arrow in Fig. 1d. Cross sections for a range of sample biases $(-3.0\text{to}-1.8\mathrm{V})$ are shown in each panel. Black arrows in panels (a) and (b) indicate the position of the interfacial methylstyrene molecule.Reuse & Permissions

Figure 3

(Color online) Model structure of a heterojunction between methylstyrene and styrene chains (each of four molecules) on a hydrogen-passivated silicon cluster representing a dimerized H-terminated Si (001) surface. Carbon atoms are black, silicon is blue, hydrogen is white. Inset: Another view of the molecules and nearby substrate atoms.Reuse & Permissions

Figure 4

(Color online) Geometry of a methylstyrene molecule and styrene molecule relaxed using density-functional theory on a small dimerized Si (001) cluster. Notation as in Fig. 3. For significance of the numbering see text.Reuse & Permissions

Figure 5

Histogram of the silicon (a), styrene carbon (b), methylstyrene carbon (c), and hydrogen (d) content of the electronic eigenstates of the eight-molecule styrene/methylstyrene chain on the hydrogen passivated silicon cluster depicted in Fig. 3. The arrows labeled H and L indicate the positions of the tungsten Fermi level relative to the molecular energy spectrum at the negative substrate biases corresponding to current curves H and L, ${\mathrm{L}}^{\prime }$ in Fig. 6. Arrows ll, l, e, and h show the tungsten Fermi level at the positive substrate biases corresponding to current curves ll, l, e, and h in Fig. 8.Reuse & Permissions

Figure 6

Calculated current $I$ (arbitrary units) flowing between the tungsten STM tip and the styrene/methylstyrene molecular chain on silicon at some negative substrate biases vs STM tip position along the chain. The current for curve H has been scaled down by a factor 10. Dotted vertical lines labeled m (s) indicate the locations of the highest carbon atoms of the methylstyrene (styrene) molecules. The scans are at constant tip height, 2 Å (3 Å) higher than the highest carbon atom of the chain for curves H and L (curve ${\mathrm{L}}^{\prime }$). L and ${\mathrm{L}}^{\prime }$ are at a lower bias, H is at a higher bias. The tungsten Fermi level at each bias is indicated by an arrow in Fig. 5; see text.Reuse & Permissions

Figure 7

Calculated current between the tungsten STM tip and the styrene/methylstyrene molecular chain on silicon vs STM tip position scanned across the chain, in the direction perpendicular to that of the scans in Figs. 6, 8. Notation as in Fig. 6. The scans are at constant tip height of 2 Å higher than the highest carbon atom of the molecular chain. (a) Scan sL (mL) is over the third styrene (methylstyrene) molecule from the junction for the same bias as scan L in Fig. 6. (b) Scan se (me) is over the third styrene (methylstyrene) molecule from the junction for the same bias as scan e in Fig. 8. Each scan passes over the highest carbon atom of the respective styrene or methylstyrene molecule and crosses the locus of the corresponding scan of Figs. 6 or 8 at $\text{position}=0$.Reuse & Permissions

Figure 8

Calculated current between the tungsten STM tip and the styrene/methylstyrene molecular chain on silicon at some positive substrate biases vs STM tip position along the chain. Notation as in Fig. 6 but the scale is the same for all curves. The scans are at constant tip height of 2 Å higher than the highest carbon atom of the molecular chain. The magnitude of the bias voltage increases from curve ll to l to e to h (see text). The tungsten Fermi level at each bias is indicated by an arrow in Fig. 5.Reuse & Permissions

Figure 9

Calculated Landauer transmission probability $T\left(E\right)$ (per spin channel) through a molecular chain of eight styrene molecules on a hydrogen-terminated silicon cluster in the energy range of the molecular LUMO band (a) and of the molecular HOMO band (c). Transmission in the same energy ranges with all Hamiltonian matrix elements and overlaps between orbitals of different molecules switched off (so that transport between the terminal molecules must proceed via the substrate) is shown in plots (b) and (d), respectively.Reuse & Permissions

Figure 10

Calculated current $I$ (arbitrary units) flowing between the tungsten STM tip and a styrene molecular chain on silicon at some negative substrate biases vs STM tip position along the chain projected onto an axis parallel to the Si dimer rows of the substrate. The tungsten Fermi level at each bias (L or H) is indicated by an arrow in Fig. 5. The styrene molecule at the left end of the chain has been constrained to stand approximately upright in a geometry representative of the region near the center of a molecular chain while the other molecules of the chain have been allowed to relax so that the molecular geometries at right end of the chain are representative of the physical end of a molecular chain on silicon (see text). The currents for curves H has been scaled up by a factor 5. Dotted vertical lines labeled s indicate the lateral locations of the carbon atoms of the styrene molecules that are furthest from the carbon chains anchoring the molecules to the silicon. The dashed curves are for scans in which the tip passes over each styrene molecule at an equal vertical distance (3.497 Å) above this carbon atom. The solid curves are for scans in which the tip is kept at a constant height above the silicon substrate. Inset: The solid curve shows the Landauer transmission probability between tip and substrate vs electron energy in eV with the STM tip 3.497 Å above the styrene molecule at the right end of the chain for the same applied bias as curves H. Dashed curve: Same except that all molecules other than the molecule at the right end of the chain are replaced with H atoms passivating the Si substrate. Dot-dashed curve: Landauer transmission probability averaged over tip positions 3.497 Å above the five styrene molecules nearest the left end of the molecular chain.Reuse & Permissions

Figure 11

Calculated current $I$ flowing between the tungsten STM tip and a styrene molecular chain on silicon at some positive substrate biases vs STM tip position. The tungsten Fermi level at each bias (l, e, and h) is indicated by an arrow in Fig. 5. Notation as in Fig. 10.Reuse & Permissions