Supporting Information for Surface-energy induced formation of single crystalline. bismuth nanowires over vanadium thin film at room temperature

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1 Supporting Information for Surface-energy induced formation of single crystalline bismuth nanowires over vanadium thin film at room temperature Mingzhao Liu,* 1 Jing Tao, 2 Chang-Yong Nam, 1 Kim Kisslinger, 1 Lihua Zhang, 1 and Dong Su 1 1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, United States 2 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, United States TABLE OF CONTENTS 1. Experimental Methods Vapor deposition of bismuth nanowires Electron microscopy analysis Energy dispersive X-ray (EDX) spectrum of the nanowire Figure S Yield of the bismuth nanowires... 4 Figure S Bismuth film deposited without vanadium.. 5 Figure S Temperature dependence of bismuth deposition over vanadium thin film... 6 Figure S Additional TEM images and SAED patterns of bismuth nanowires 7 Figure S Heating of the substrate by bismuth vapor. 8 Table S Motion of bismuth atoms in vacuum chamber Keissig fringe in the X-ray reflectivity from vanadium film Figure S Bismuth film deposited on bulk vanadium foil Figure S The surface-to-volume ratio of Bi incorporated in V film Figure S Phase diagram of bismuth. 15 Figure S Binary Phase diagram of bismuth-vanadium system Figure S

2 1. Experimental Methods Vapor deposition of bismuth nanowires In a typical experiment, glass or silicon substrates are cleaned in oxygen plasma before being introduced into the physical vapor deposition chamber, which is then pumped to a base pressure below 10-6 Torr. A vanadium thin film is first deposited by DC magnetron sputtering under argon plasma to achieve a nominal thickness of nm, at a deposition rate around 0.03 nm/s. Immediately after the deposition of vanadium and returning the chamber to its base pressure, bismuth is deposited over the same substrate by thermal evaporation to a nominal thickness of 50 nm, at a deposition rate of nm/s. Nominal thicknesses of vanadium and bismuth layers are monitored in situ with a calibrated quartz crystal microbalance. The substrate temperature is maintained at a desired value during the course of the deposition using a Peltier cooler/heater in contact with the back of the substrate. Electron microscopy analysis High resolution TEM and STEM images are respectively taken by a JEOL 2100F transmission electron microscope and a Hitachi HD2700C scanning transmission electron microscope. The specimen for elemental mapping is a 40-nm-thick cross-sectional slice of the bismuth/vanadium film mounted on a copper grid using the focused ion beam milling and lift-out technique. 1 The EDX mappings and HAADF images are collected by a JEOL JEM-ARM200F atomic resolution analytical electron microscope. 2

3 2. Energy dispersive X-ray (EDX) spectrum of the nanowire The EDX spectrum is collected in a JEOL 2100F TEM at 200 kv acceleration voltage. The sample is prepared by transferring as-grown nanowires to a carbon-film-coated copper grid using dry-transfer method. The TEM image of a typical nanowire and its EDX spectrum are shown in Figure S1. The EDX spectrum is indexed and shows clear signal from bismuth but nothing for vanadium. The presence of signal from copper is due to the copper grid. The tiny peak at 5.41 kev corresponds to chromium. In our experiment both the vanadium source and bismuth source have high purity and contain < 10 ppm chromium. The chromium contamination is most likely from the stainless steel tweezers used for handling the TEM grid, which is frequently encountered during EDX studies. It is therefore confirmed that the nanowires are purely bismuth. Figure S1. (a) The TEM image of a typical bismuth nanowire and (b) its EDX spectrum. The expected peak position for vanadium (4.95 kev) is labeled by a red arrow although there is no signal detected. 3

4 3. Yield of the bismuth nanowires We define the yield of the nanowires as the ratio of the total volume of bismuth nanowires to the total volume of bismuth deposits on the substrate. For the experiment presented in Figure 1(b), the bismuth deposits reach a nominal thickness of 50 nm, according to our calibrated quartz crystal thickness monitor. The nanowires have diameters nm and lengths 6-8 m. The packing density of nanowires is determined from top-view SEM images as ~0.6/ m 2 (Figure S2). Over an area of 1 m 2 the average total volume of bismuth nanowires is 0.36 m 3, while the total volume of bismuth deposits is 0.50 m 3. The apparent yield is therefore 72%. Figure S2. A top-view SEM image of the as-grown bismuth nanowires, taken from the same sample shown for Figure 1(b). 4

5 4. Bismuth film deposited without vanadium Figure S3. Bismuth forms a continuous but grainy thin film when deposited over the substrate without a layer of vanadium deposited first. The SEM image is taken at a vertical cross section of the substrate, showing the edge of the bismuth film. 5

6 5. Temperature dependence of bismuth deposition over vanadium thin film Figure S4. Morphology of bismuth deposits as the substrate is held at different temperatures: 348 K (a), 323 K (b), 285 K (c), and 273 K (d). Panels a, b, and c indicates that the nanowires grow longer and thicker at higher substrate temperature. However, bismuth deposits as a flat but grainy film over the vanadium film at the lowest substrate temperature (Panel d, 273 K). 6

7 6. Additional TEM images and SAED patterns of bismuth nanowires Figure S5. (a) Annular dark-field scanning TEM image of the bismuth nanowire shown in Figure 1(d), showing the base of the nanowire. The nanowire was deposited using a lacey carbon TEM grid as growth substrate. (b-d) Transmission electron microscopy images of bismuth nanowires with growth orientations respectively along (b) and (c)(d) directions. The insets respectively show the corresponding electron diffraction patterns. 7

8 7. Heating of the substrate by bismuth vapor To achieve the 0.2 nm/s deposition rate at the substrate, the bismuth source is heated to 1006 ± 2 K, which is determined by a type-k thermocouple in direct contact with molten bismuth. The heating source has a diameter of 1 cm and is 50 cm away from the substrate. The substrate is heated both by the source radiation and by the hot vapor. By assuming the source as a blackbody, the radiation power at the source surface is W m -2 through the Stefan Boltzmann law. At the substrate the radiation power becomes 23.2 W m -2 following the R -2 relation. The deposition rate of 0.2 nm s -1 translates to the deposition of 9.4 mol Bi per square meter per second. Using the thermodynamic constants of Bi listed in Table S1, we estimate that when the Bi vapor condenses over the substrate and cools to 298 K, a heating power of 1.7 W m -2 is released. Altogether, we find the heat received by the substrate is about 25 W m -2, or equivalently 2.5 mw cm -2. The power is too small to lead to a significant temperature difference between the substrate and the substrate holder, which serves as a heat bath. In case of a 1-mm ITO glass slide being used as deposition substrate (thermal conductance ~ 1 W m -1 K -1 ), at steady state the temperature difference between its top and bottom surfaces is merely 25 mk. Table S1. Thermodynamic constants of Bismuth Heat of vaporization kj mol -1 Heat of fusion kj mol -1 Molar Heat capacity (liquid, 545 K) J mol -1 K -1 Molar Heat capacity (solid, 298 K) J mol -1 K -1 8

9 8. Motion of bismuth atoms in vacuum chamber. The mean free path of bismuth vapor within the deposition chamber writes where k B is the Boltzmann constant and m is the Van der Waals diameter of bismuth atom. Using P = 10-6 Torr = Pa and T = 1006 K, we find m. Considering that in our deposition apparatus the distance between bismuth evaporation source and the deposition substrate is only 0.5 m, it is safe to say that the bismuth atoms reaches the substrate in a ballistic fashion, following line-of-sight. 9

10 9. Keissig fringe in the X-ray reflectivity from vanadium film The X-ray reflectivity data shown in Figure 3(c) displays Kiessig fringe that arises from the interference between reflection beams from the top and bottom surface of the vanadium film. At large incident angle q, i.e., Q >> Q c, it is known that the film thickness D can be conveniently estimated by the relation D 2 / Q, where Q is the fringe periodicity. However, in our case the Keissig fringe is only observed at smaller Q, as the film roughness damps the fringe at larger incident angle. Thus, we will use the following section to discuss how the film thickness is interpreted from the Kiessig fringe. As illustrated in Figure S6(a), X-ray beams reflected from the top and bottom surfaces of a thin film has different optical path lengths, which leads to constructive and destructive interference at different incident angles, thus the Keissig fringe. The film has a thickness of D and refractive index n 2 whereas the top medium has refractive index n 1. The incident angle of X- ray is 1. The optical path difference thus writes L 2Dsin, where D is the film thickness 2 and 2 is the angle of refraction that is given by Snell s law n2 cos 2 n1 cos. The phase 1 difference between the two beams is 2 n2 L 4 n2dsin 2, where is the X-ray wavelength in vacuum. Therefore, between two neighboring reflectivity maxima (or minima) that are associated with incident angles 1 and 1, thus refraction angles 2 and 2, the following relation is established 4 n Dsin ' 4 n Dsin 2, (S1) which explicitly gives the value of D through a simple reconfiguration: D 2 n (sin sin ). (S2)

11 From the equations above it is also apparent that the Keissig fringe must be evenly spaced when plot against n 2 sin 2. When the X-ray absorption is neglected, the refractive index of X-ray in a medium writes n r, where r e = Å is the classical electron radius, e the electron e e 2 density, and the X-ray wavelength. Given the 22% porosity of vanadium film, its electron density e is found as 1.27 Å -3. Using Å for the Cu-K X-rays, we obtain n 2 = In Figure S6(b), the X-ray reflectivity presented in Figure 3(c) is plot against n 2 sin 2 and does show uniform period of x = (1.85 ± 0.05) 10-3 for each oscillation cycle. From Eq. S2 we find the vanadium film thickness as 42 ± 1 nm, which is consistent to the SEM image. Figure S6. (a) The formation of Kiessig fringe is due to the interference between X-ray beams reflected from the top and bottom surfaces of the thin film. (b) The X-ray reflectivity from the vanadium film is plot against n 2 sin 2, showing uniform spacing for the fringe. 11

12 10. Bismuth film deposited on bulk vanadium foil A bulk vanadium foil (99.7%) is obtained from Sigma-Aldrich and is polished to remove the natural oxide on the surface. After placing into the deposition chamber the surface was further cleaned by an Ar-ion mill. Bismuth is then deposited over the vanadium foil by thermal evaporation, with the foil held at room temperature. It turns out that bismuth form a continuous but grainy film over the flat vanadium foil that is not porous (Figure S7), which suggests that the pores are indeed critical for the formation of bismuth nanowires. Figure S7. Bismuth forms a continuous but grainy thin film when deposited over a bulk vanadium foil. 12

13 11. The surface-to-volume ratio of Bi incorporated in vanadium film The surface-to-volume ratio of the Bi incorporated in the vanadium film can be most easily estimated by the trench width between the vanadium islands. As the trench width is about 1 nm (Figure 3(b)), the surface-to-volume ratio S/V is ~2 nm -1 by treating the infiltrated bismuth as a thin slab. However, due to the large uncertainty in the measurement of trench width, a better estimation is required and can be obtained from the vanadium film s microscopic structural parameters. In Figure S8, the columnar structure within the film is idealized as an array of hexagonal cylinders over the substrate that has a surface area S 0. The hexagonal cylinders are characterized by side width a, height (also the film thickness) L, and packing density N. Based on these dimensions the porosity P of the vanadium film writes 2 Vcylinder 3 3a LNS0 2 2 P a N 2 V LS, total 0 where V total is the apparent volume of the thin film. As the porosity is determined experimentally as 22% and the density of vanadium cylinders is m -2, the side width a is found at nm, which agrees well with the TEM image shown in Figure 3(b). When the trenches between vanadium cylinders are completely filled with bismuth, the surface area of bismuth writes S Bi = 6aLNS 0 + 2PS 0. The surface-to-volume ratio thus writes S Bi /V Bi = (6aLNS 0 + 2PS 0 )/PV total = 6aN/P + 2/L, from which we obtain S Bi /V Bi at nm -1 using the film thickness 40 nm. 13

14 Figure S8. The columnar structure of the sputter-deposited vanadium film is idealized as a honeycomb array of hexagonal pillars of vanadium, each with height L and side width a. 14

15 12. Phase diagram of bismuth The phase diagram of bulk bismuth (Figure S9) is adapted from the work by Bundy, which shows the lowest bulk melting point of bismuth is 170ºC (443 K) and is achieved at kg/m 2 (2.13 GPa). 4 Figure S9. The phase diagram of bulk bismuth, showing the liquid phase (magenta) and solid phases (cyan), which include the rhombohedral phase (I) and several high-pressure phases. The diagram is reconstructed based on the work of Bundy. 4 The lowest melting point of bismuth (labeled by the red arrow) corresponds to the triple point between the liquid phase, soild phase II, and solid phase III. 15

16 13. Binary Phase diagram of bismuth-vanadium system The binary phase diagram of Bi-V system (Figure S10) was obtained from SpringerMaterials - The Landolt-Börnstein database (Ref. 16 in manuscript). According to the phase diagram, the mutual solubility of solid bismuth and vanadium is negligibly small. The dissolving of vanadium in liquid bismuth is only observed at temperature above 1200K, a temperature that is far above the nanowires growth temperature. Figure S10. The binary phase diagram of Bi-V system. 1. Giannuzzi LA, Kempshall BW, Schwarz SM, Lomness JK, Prenitzer BI, Stevie FA. FIB lift-out specimen preparation techniques. In: Giannuzzi L, Stevie F (eds). Introduction to focused ion beams. Springer US, 2005, pp Lide DR. CRC handbook of chemistry and physics. 89 ed. Boca Raton, FL: CRC Press; p Bell H, Hultgren R. Heat Capacity of Liquid Bismuth. Met. Trans. 1971, 2(11): Bundy FP. Phase diagram of bismuth to kg/cm 2, 500 C. Phys. Rev. 1958, 110(2):