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light-emitting diodes and optical devices based on nonlinear properties [6,7]. Zinc
oxide nanowires grown on sapphire substrates have been fabricated into nanolasers [8].
Ge has especially interesting properties because the direct gap is about 741 meV
above the indirect gap [9]. Nanocrystallite Ge embedded in a SiO2 matrix shows
broadband photoluminescence (PL) spectra in the visible wavelength range at room
temperature [10]. The synthesis of crystalline semiconductor nanowires, such as Si
and Ge holds considerable technological promise for device applications, and for
improving the optical properties of these indirect gap materials but has been dif®cult
to achieve. Ge whiskers [11,12] were once synthesized, but their diameter was larger,
and their structure was not analyzed. Single crystal Ge quantum wires synthesized by
organic solvothermal method [13] and laser ablation [14] were of cubic structure
(with lattice constant: a 0:5656 nm).
In this communication, we reported the synthesis of crystalline and amorphous Ge
nanorods. The crystalline nanorods reveal a tetragonal structure, which is the high-
pressure phase of germanium at 308C under 25 kbar [15]. Amorphous Ge nanorods
have not been reported so far. Crystalline and amorphous heterojunction in the
nanorod was observed, which may be used in optoelectronic nanodevice.
2. Experimental
High-pure GeO2 powders (Speciality products, Johnson Matthey Chemicals Ltd.,
England) were placed at the center of conventional horizontal furnace with a quartz
tube and reduced into Ge powders at 7008C for 2h in ¯owing hydrogen atmosphere.
The Ge powders were placed in alumina crucible, and then an alumina ceramic plate
was placed on top of the crucible. Finally, the crucible was pushed into the hot zone
inside the quartz tube. As Ge powders were calcined at 10508C in ¯owing Ar
(40 ml minÀ1)/H2 (80 ml minÀ1) atmospheres for 2.5 h, a black product was depos-
ited on the alumina plate. The sample structure was analyzed by X-ray diffractometer
(XRD, Philips PW 1700) with Ni-®ltered Cu Ka radiation, transmission electronic
microscopy (TEM, JEM-200CX), and high-resolution electronic microscopy
(HREM, JEOL-2010). The chemical components of the nanorods were estimated
by OXFORD-6498 energy dispersive X-ray spectrometer attached HREM.
3. Results and discussion
Fig. 1 shows the XRD pattern of the products. It was identi®ed as a mixture of
tetragonal lattice unit cell dimensions of constants: a 0:593 nm, c 0:698 nm
(ASTM card: 18±549), and a cubic unit cell with a 0:5656 nm (ASTM card: 4±545)
Ge. The tetragonal structure reveals (0 0 3) and (0 0 2) peaks, compared with the bulk
material. Fig. 2 indicates the coexistence of nanorods and nanocrystallites. The
nanorods have a diameter ranging from 20 to 200 nm and lengths up to 5 mm. Fig. 3(a)
shows the morphology of single nanorods. Selected area electronic diffraction