GaSb nanowires

The simple pyrolysis of GaSb-adducts of type R3GaSbR'3 in sealed glass ampoules in the temperature range 350-500 °C yields GaSb crystallites.[1] Their carbon content is below the detection limit of the electron energy loss spectroscopy (EELS), while oxygen is present as a surface contamination.

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Figure 1: REM picture of GaSb dendrites as-obtained from thermolysis reactions of (t-Bu)3Ga–Sb(t-Bu)3 in a closed glass ampule at 450 °C.

Dendritic-like growth was observed at high decomposition temperatures while isolated GaSb nanowires were rather obtained at lower temperatures (250 - 350 °C) as well as from other adducts such as [t-Bu3Ga]2[Sb2Et4]. The nanowires were characterized in detail by EDX, EELS (Sb-mapping) and high-resolution transmission-electron-microscopy (HR-TEM).[2]

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Figure 2: SEM images showing GaSb dendrites as-obtained from (t-Bu)3Ga–Sb(i-Pr)3 at Tdec. = 275 °C (A, D), 300 °C (B, E), and 325 °C (C, F).

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Figure 3. SEM and TEM images of GaSb nanowires obtained from [t-Bu3Ga]2[Sb2Et4] at Tdec. = 250 °C (top) and 400 °C (bottom). D shows the formation of GaSb nanowires with high aspect ratio.

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Figure 4. SAED pattern with simulation as an overlay and HRTEM image of of GaSb nanowires obtained from [t-Bu3Ga]2[Sb2Et4] at Tdec. = 250 °C in <110> zone axis. The indicated distances of 353 and 305 pm correspond to {111} and {200} lattice fringes, respectively.

In addition, a rather controlled self-catalyzed growth of GaSb nanowires using 90 nm-sized Ga droplets, which were pre-deposited on Si(100) substrates by thermal decomposition of t-Bu3Ga, was achieved with Sb2Et4 at 250 °C.[2] Ga droplets have been shown in the past to initiate nanowire growth of a variety of materials such as Si and Ge.[3] The pyrolysis experiments clearly showed that Ga droplets act as an initiator (catalyst) for the GaSb nanowire growth and the growth of the nanowires, which occured according to the so-called VLS mechanism (vapor-liquid-solid),[4] showed a strong dependency on the pyrolysis temperature.

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Figure 5. SEM images showing tapered GaSb nanowires formed at Tsubstrate = 250 °C with different deposition times (tdep.(Sb2Et4) = 90 min A, C; 120 min B, D).

The decomposition of t-Bu3Ga forms a Ga-droplet on the substrate, which catalyse the decomposition of the Sb-precursor, resulting in an enrichment of the Ga-droplet with elemental Sb. After exceeding the saturated solution level, the crystallization starts at the liquid-solid interface and the nanowire is formed and the Ga-droplet is preserved on the top of the whisker.

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Figure 6. Formation of nanowires by the VLS mechanism

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Figure 7. Nanowires of crystalline GaSb, at the top is the (amorphous) Ga-droplet visible.

Bismuth nanowires

Bismuth is a semimetal, promising very large quantum size effects due to the small effective mass of charge carriers and very low carrier density.[5] Bi-nanowires, were predicted to undergo a transition from semimetal to semiconductor with diameters smaller than 50 nm (77 K), or <10 nm (25 °C). In addition, Bi-nanowires are discussed as interesting thermoelectric materials and different procedures for the production of nanowires and nanocrystals have been developed.[6] However, typical procedures such as hydrothermal methods,[7] particularly electrochemical deposition in porous aluminium oxide templates and others,[8,9] typically bares problems related to the low melting point of Bi (271.3 °C).

In the course of our studies we focused our investigations on the deposition of Bi-nanowires by MOCVD process. Bi nanowires were obtained with trialkylbismuthanes as well as dibismuthanes under very mild reaction conditions, below 200 °C using Bi2Et4.

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Figure 8. SEM images of Bi-nanowires


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