APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 1 3 JULY 2000

Germanium Õcarbon core–sheath nanostructuresYiying Wu and Peidong Yanga)Department of Chemistry, University of California, Berkeley, California 94720

~Received 14 April 2000; accepted for publication 10 May 2000!

Germanium/carbon core–sheath nanostructures and junctions are produced when Ge nanowires aresubject to a thermal treatment in an organic vapor doped vacuum. The organic molecules pyrolyzeon the surface of the Ge nanowires and form a continuous graphitic coating. The carbon-sheathedGe nanowires undergo melting and evaporation at high temperature, which results in the formationof germanium/carbon junctions. These core–sheath nanostructures and junctions generally havediameters of 5–100 nm, sheath thickness of 1–5 nm, and lengths up to several micrometers. Thisprocess may prove to be an effective approach to prevent the nanowire surface oxidation and createnanowires with chemically inert surface. ©2000 American Institute of Physics.@S0003-6951~00!01127-X#

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One-dimensional nanoscale materials have drawn mattention recently due to their interesting physical properand potential device applications.1 Nanowires and nanorodof various compositions have been synthesized using chcal vapor deposition/transport and laser ablation methods2–6

Progress has also been made in fabricating nanowiresfilling the hollow cavities of the nanotubes with elementscompounds.7–9 More recently, concentric nanocables aheterojunctions have also been synthesized through caredesigned nanowire/tube growth processes.10–14 We demon-strate, in this letter, the formation of germanium/carb~Ge/C! core–sheath nanostructures and junctions througsimple thermal treatment carried out in an organic vadoped vacuum. This report involves two important achiements. First, creation of carbon sheath around semicondunanowire cores could be an effective methodology to prevsurface oxidation commonly associated with semiconduSi, Ge nanowires. Thus, the process itself should have geral implications for fabricating semiconductor nanostrutures with a chemically inert interface that is desired for ctain applications; Second, thein situ high temperaturetransmission electron microscope used here proves to bficient to unveil the formation mechanism of ondimensional nanostructures.

A two-step process was used to fabricate the germanicarbon nanostructures. Ge nanowires were first synthesvia the vapor–liquid–solid mechanism using gold nanoclters as catalysts in a sealed-tube chemical vapor transsystem.5 The wires on silicon substrate are sealed inevacuated quartz tube for the second step thermal treatmA small amount of organic vapor, from chemicals suchlight mineral oil, was introduced into the tube. The fininternal pressure was 20–50 mTorr. The sealed tubethen subject to a thermal treatment at 700–900 °C for 0.5h. After the treatment, the resultant materials were ultrascally dispersed in acetone and examined with a transmiselectron microscope~TEM, Philips CM-200! operated at 200keV.

The as-made Ge nanowires have a fluffy web-like

a!Electronic mail: pyang@cchem.berkeley.edu

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pearance and generally have diameters of 5–200 nm.5 Afterthe thermal treatment, the products on Si substrate apdark gray in color. Scanning electron microscopy studiesthe products show bundles of web-like nanowires entangtogether with lengths of;1 – 10mm. Detailed studies usingTEM show that they comprise of about 50% Ge nanowicarbon nanotube junctions and 50% carbon nanotube stures. Figure 1~a! shows an image of nanowire/nanotubjunctions. Both straight and tortuous structures have bobserved, and they normally have diameters of 5–100

FIG. 1. Typical TEM images of Ge/C core–sheath structures, junctions~a!and carbon nanotubes~b! formed after the thermal treatment of Ge nanwires.

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44 Appl. Phys. Lett., Vol. 77, No. 1, 3 July 2000 Y. Wu and P. Yang

Figure 1~b! shows a typical image of nearly pure nanotubMost of these nanotube structures are curved and hasimilar diameter distribution as that of nanowire/nanotujunctions.

High magnification TEM studies reveal that ea

FIG. 2. ~a! TEM image of a Ge/C core–sheath and junction structure.~b!High resolution TEM image of the Ge/C core–sheath interface.~c! Highresolution TEM image of the turbostratic carbon nanotube.

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nanowire of the nanowire/tube junction actually possessecore–sheath structure. The tube and the sheath are madisordered graphitic carbon. Figures 2~a!–2~c! show typicalhigh resolution TEM images of an individual wire/tube juntion and interface. It was found that the Ge single crystallnanowires are encapsulated within layers of disorderedphitic sheet@Fig. 2~b!#. An atomic resolution image of thecore area indicates that most of Ge wires grow along@111# axis. Occasionally@110# growth direction was also observed. The thickness of the carbon sheath is gener;1 – 5 nm. Figure 2~c! shows a high resolution image on thcarbon nanotube, where tens of disordered graphitic shwrapped into a porous tube structure. The lattice fringfrom the sheath indicate that the interlayer distance is ab0.34 nm, similar to the~002! spacing of turbostratic graphiticcarbon.

It is further noticed that the core/sheath interfacestructurally and compositionally sharp and no oxide laywas detected at the interface in contrast to many reportsthe oxidation layer formation on Si and Gnanostructures.2,4,6,12This is confirmed by the local composition analysis across the interface using an energy disperx-ray ~EDX! spectrum. Figure 3 shows a typical EDX spetrum recorded on the Ge/C interface with electron beprobe size of 10 nm. Only Ge and C signals were detectethe interface.

In our previous studies,5 we observed size reduction oGe nanowires after thermal treatment in vacuum whenorganic vapor is introduced. In the current process, howethe organic molecules absorb on the surface of the as-mGe nanowires, pyrolyze when the system temperatureramping to 800 °C, and form a turbostratic carbon sheoutside of the Ge nanowire. In fact, pure Ge/C core–shestructures were obtained when the thermal treatment wasried out at 500 °C. To study the formation mechanismthese nanostructures, we carried outin situ high temperatureTEM ~JEOL 200-CX! studies on the carbon coated Gnanowires. The carbon coated Ge nanowires were dispeon a Cu TEM grid covered with holey SiOx thin film. Duringthe experiment, the TEM samples were heated up to 900with a rate of 5–20 °C /min and then cooled down. Tmelting and evaporation behavior of the nanowires w

FIG. 3. Energy dispersive x-ray spectrum recorded on the Ge/C interfCu signal comes from the TEM grids.

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45Appl. Phys. Lett., Vol. 77, No. 1, 3 July 2000 Y. Wu and P. Yang

monitored and recorded. We found the Ge nanowires camelted at temperature significantly lower than that of bGe. In addition, supercooling of liquid Ge cores is routineobserved for these Ge nanowires.15 Meantime, two possiblemechanisms were observed for junction formation durthis thermal cycle. One occurs during the heating procwhere high temperature evaporation of Ge and solid–liqtransition induced volume shrinkage cause the partial evaating of the carbon sheath. Another occurs during the cooprocess as shown by two TEM images recorded duringcooling process of a 30 nm Ge nanowire liquid core@Figs.4~a! and 4~b!#. The liquid front was observed receding b

FIG. 4. In situ high temperature TEM images of a molten liquid Ge cowithin a carbon sheath. From~a! to ~b!, the liquid front is receding like ananoscale thermometer. The arrow indicates a reference point.

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150 nm within 37 s at 650 °C, resembling a nanoscale thmometer. A liquid droplet reservoir was generally observat one end of the wire. At temperatures between 450 and°C, the remaining Ge liquid core recrystallizes to form wirtube junctions. A pure carbon nanotube forms when all luid Ge escapes from the sheath.

In summary, nanoscale core–sheath and junction sttures of single crystalline germanium nanowire and turbtratic carbon were synthesized during the thermal treatmof Ge nanowires in the presence of a small amount of thmally pyrolyzible organic vapor.In situ high temperatureTEM studies give direct evidence of the formation mechnism of such nanostructures. This process could be usefuthe fabrication of Si, Ge nanowires and other nanoscale jutions with chemically inert interfaces for nanoelectronics adevice applications where surface oxidation is undesirab

This work was supported in part by a New FacuAward from the Dreyfus Foundation and start-up funds frothe University of California, Berkeley. One author~P.Y.!thanks the 3M company for an untenured faculty award aDr. E. Stach and D. C. Nelson for help with the TEM studieThe authors thank the National Center for Electron Microcopy for the use of their facilities.

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