In-situ Scanning Tunneling Microscopy Study of Bismuth Electrodeposition on Au(100) and Au(111)S.H. Zhenga, C.A. Jeffreya,b, D.A. Harringtonb E. Bohannanc, S.G. Chena and S. Morina

aDepartment of Chemistry,York University, Toronto, ON and bDepartment of Chemistry, University of Victoria, Victoria, BC.and cDepartment of Materials Science and Engineering and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO, USA.

This work was financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canadian Foundation for Innovation, the Ontario Innovation Trust and York University. S. M. also acknowledges the financial support from the Canada Research Chair Program.

Single crystalline bismuth thin films and nanostructures display very large magnetoresistance (MR) [1-4] . While UHV-based deposition methods are usually used to produce high quality Bi thin films, electrodeposition was shown recently to be a suitable method to obtain Bi thin films and nanostructures with interesting magnetic properties [3,4] .

In this study the early stages of Bi OPD and Bi bulk deposition on Au(100) in 0.1 M HClO4 are investigated by in-situ STM and XRD. We observed that the morphology of Bi OPD on Au(100) substrate was similar as that of Bi OPD on Au(111)[5,6], films have similar morphology which develops from needles (anisotropic) and two-dimensional isotropic growth modes. Our electrodeposited Bi films were crystalline on Au(100) substrates, and they were (012) oriented based on a hexagonal unit cell. Epitaxial orientations of the Bi(012) film were confirmed using x-ray pole figure analysis.

Useful properties:• Bismuth thin films and nanostructures have very large positive magneto- resistance (MR)

• Bismuth oxides are semiconductors with 3-4 eV band gaps

Applications:• Solid electrolytes for fuel cells

(δ-phase Bi2O3 has the highest known oxide-ion mobility) • Oxygen sensors and oxygen pumps• Electrochromic devices• Electronic/magnetic materials

• At the onset of Bi OPD, Bi growth starts at step edges and terraces through the formation of mono- and bi-layer islands.

• The morphology of Bi OPD on Au(100) substrate proceeds in a similar way to that of Bi OPD on Au(111), island nucleation is followed by isotropic and anisotrophic growth mode. The former mode follows specific crystallographic directions on the substrate.

• The electrodeposited Bi films are crystalline and the domains are (012) oriented based on the square unit cell of the Au(100) surface.

• The Bi(202) diffraction pole figure yield four peaks due to the orientation of the Bi needle along the [001] directions of the Au(100) surface.

Figure 7: Epitaxy relationship between Bi(012) plane and the Au(100)(left) and Au(111)(right) surfaces. Note that for clarity only one Bi layer corresponding to the bulk film structure is shown. A single atomic arrangement accounts for the peaks in the pole figures (Fig.4&6). Note that the Bi atoms are shaded differently to emphasize the slight modulation (0.11Å) of Bi atoms above and below the (012) plane.

• The Bi(012) surface unit cell is rectangular. The spacing corresponding to the interatomic distances (center-to-center in Å) are shown by line diagrams at the bottom of the figure.

• The dotted line vectors represent one corner of a coincidence lattice with the lengths of the arrows indicating the relative lengths of the sides. The coincidence lattice is 36.63 Å × 56.98 Å on Au(100) and 31.9 Å × 95 Å on Au(111), much larger than the figure.

• The early stages of Bi electrodeposition were observed for films up to 6 monolayers in thickness (Fig. 1 & 2).

• At the onset of Bi OPD, Bi growth starts at step edges and terraces through the formation of mono- and bi- layers islands Fig. 1d & 2.1).

• Two growth modes are identifiable, namely needles (anisotropic) and two-dimensional isotropic, both propagate as mono- and bi- layers.

• Bi growth on Au(111) proceeded in a very similar way. For comparison, STM images of the morphology of a Bi film formed on Au(111) is also shown in Fig. 2.

a b c

e

25 min

f

31 min

d

9 min

0.26 V

-0.10 V-0.09 V-0.08 V

0.05 V0.15 V

Figure 1: STM images for Bi early stage OPD on Au(100) single-crystal in 120 µM Bi3+ / 0.1 M HClO4. Scan size was 500 × 500 nm2. Sample

potential was (a) 0.26 V; (b) 0.15 V; (c) 0.05 V; (d) -0.08 V; (e) -0.09 V; (f) –1.0 V. The time was counted from the first scan in the experiment to the end of the certain scan. The tip bias is 0.055 V. The tunneling current is 0.3 nA (a-c), 0.248 nA (d,e) and 0.203 nA (f).

1 5min 3 15min

5 25min

10min

4 20min

2

Figure 2: STM images for Bi early stage OPD on Au(111) single-crystal in 1mM Bi3+ / 0.1M HClO4. 1-5 scanned over the same area. Sample potential was –0.08 V. Tip bias was 0.06 V. The tunneling current was 0.20 nA. Scan size were 500 × 500 nm2(1-3), 300 × 300 nm2 (4,5). The time was counted at the onset of Bi growth i.e. when the potential reached a value of –0.08 V. Arrows indicate scan direction.

1. Liu et al., Appl. Phys. Lett., 1998, 73, 1436.2. Yang et al., Appl. Phys. Lett., 1998, 73, 1436.3. Yang et al., Appl. Phys. Rev. Lett., 1999, 82, 3328.4. Yang et al., Science, 1999, 284, 1335.5. C.A. Jeffrey, D.A. Harrington and S. Morin, Surf. Sci. (2002) 512, L3676. C.A. Jeffrey, S.H. Zheng, E. Bohanna, D.A. Harrington and S. Morin, Surf. Sci. (2006) 600, 95-1057. Vereecken et al., J. Electrochem. Soc., 2003, 150, C131 and Appl. Phys. Lett., 2000, 88, 6529.

[01-4]

[2-10]

Figure 8: Stereographic representation of Bi(012) (full circles) on Au(100) substrate (left) and Au(111) substrate (middle). The 2D unit cell of Bi(012) (right).

[-210]

[2-10]

[01-4]

(-111)

(101)(01-1)

(111)

(11-1) (1-11)

(-111)[-210]

[2-10]

[01-4]

(011)

(-111)[2-10] (101)

(01-1)

[01-4]

[-210]

(101)

(110)

Au(100) planes Au(111) planes

Bi(012) planes

• Figs. 3 and 5 clearly indicate that the bismuth films are crystalline and that it is (012) oriented based on an (1×1) unit cell. The film’s second and third order reflections are also visible in the scan. A strong contribution from the underlying Au surfaces is also observed.

• A crystalline Bismuth film oriented (012) has also been observed on p-GaAs substrates, but only after the deposit was annealed [7].

• Epitaxial orientation of the Bi(012) film on the Au(100) and Au(111) substrates is confirmed using x-ray pole figure analysis.

• Four peaks and twelve peaks are observed in the Bi(202) pole figure on Au(100) and Au(111) substrates, respectively. These support the presence of needles (domains) that are oriented at 90 and 120 degree from each other on Au(100) and Au(111), respectively.

Bi(012)

Bi(036)

Au(100)

Bi(024)

Figure 3: Bragg-Brentano (2θ) Scan of a thick Bi film on Au(100) ca. 50-100 layers.

Figure 4: a) Bi (202) pole figure; b) Au(222) pole figure and c) 2.5 D plot of Fig. 4 a).

a b c

Figure 5: Bragg-Brentano (2θ) Scan of a thick Bi film on Au(111) ca. 50-100 layers.

Figure 6: a) Bi (202) pole figure; b) Au(220) pole figure and c) 2.5 D plot of Fig. 6 a).

a b cAu(111)

Bi(024)

Bi(012)

Au(222)

Bi(036)