Synthesis of crystalline and amorphous germanium nanorods

Article abstract of DOI:10.1016/S0025-5408(02)00887-5

Crystalline and amorphous germanium nanorods have been synthesized by PVD of germanium powders in flowing Ar/H₂ atmospheres. TEM images show diameters of the nanorods ranging from 20 to 200 nm and length up to 5 μm. Selected area electron diffraction indicates that the crystalline nanorods have tetragonal structure. Their growth process has been considered as VLS mechanism. Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0025-5408(02)00887-5

Materials Research Bulletin 37 (2002) 2179±2183
Synthesis of crystalline and amorphous
germanium nanorods
Xingcai Wu*, Yourong Tao
Department of Chemistry, Nanjing University, Nanjing 210093, PR China
(Refereed)
Received 4 January 2002; accepted 19 July 2002
Abstract
Crystalline and amorphous germanium nanorods have been synthesized by PVD of
germanium powders in ¯owing Ar/H2 atmospheres. TEM images show diameters of the
nanorods ranging from 20 to 200 nm and length up to 5 mm. Selected area electron diffraction
indicates that the crystalline nanorods have tetragonal structure. Their growth process has
been considered as VLS mechanism.
Published by Elsevier Science Ltd.
Keywords: A. Nanostructures; A. Semiconductors; B. Vapor deposition; C. X-ray diffraction; C.
Electron diffraction
1. Introduction
Nanoscale materials have unique properties in comparison to the bulk material
owing to quantum con®nement effects, for example, as the diameter of the Si nanowire
approaches the de Broglie wavelength, visible photoluminescence is induced [1,2];
silica nanowires at a diameter of about 15 nm emit stable and intense blue light at 2.65
and 3.9 eV. The intensity of the emission is two orders of magnitude higher than that of
porous silicon [3]. Lately special attention has been paid to one-dimensional structures,
such as nanotubes and nanowires owing to their contribution to understanding
fundamental concepts in mesophysics, and potential applications in nanoelectronic
devices [4,5]. For instance, semiconductor CdS nanowires have been used in laser
* Corresponding author. Tel.: ‡86-25-3592304; fax: ‡86-25-3302728.
E-mail address: wuxingcai@hotmail.com (X. Wu).
0025-5408/02/$ ± see front matter. Published by Elsevier Science Ltd.
PII: S 0 0 25 - 5 4 0 8 ( 0 2 ) 0 0 8 8 7 - 5

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X. Wu, Y. Tao / Materials Research Bulletin 37 (2002) 2179±2183
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 ˆ 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 ˆ 0:593 nm, c ˆ 0:698 nm
(ASTM card: 18±549), and a cubic unit cell with a ˆ 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

X. Wu, Y. Tao / Materials Research Bulletin 37 (2002) 2179±2183
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Fig. 1. XRD pattern of Ge nanorods.
(SAED) pattern of the single nanorod, shown as an inset in Fig. 3(a), can be indexed
ꢀ
on the basis of a tetragonal structure of Ge along (1 0 1) direction, which is consistent
with the XRD results. This further con®rms that the nanorods growth direction is
(1 1 1). HREM image in Fig. 3(b) reveals the (20 1) lattice fringe of the single
crystalline nanorods, whose spacing is 0.275 nm. Fig. 4 shows amorphous Ge
nanorods, which was also con®rmed by SAED (inset) and EDX quantitative micro-
anlysis. However, the lattice fringe image of the amorphous nanorod cannot yet be
observed by HREM. As shown in Fig. 3(a), crystalline and amorphous heterojunc-
tions appear on the nanorod. There seems to be a eutectic droplet at the end of the
Fig. 2. TEM image of Ge nanorods.

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X. Wu, Y. Tao / Materials Research Bulletin 37 (2002) 2179±2183
ꢀ
Fig. 3. (a) A magni®ed and SAED pattern (inset) along (1 0 1) direction; (b) lattice fringe image
(HREM) of single crystal Ge (high-pressure phase) nanorods.
Fig. 4. TEM image and SAED (inset) pattern of a single amorphous Ge nanorod.
nanorod (in Fig. 3a), indicating that the growth process follows vapor±liquid±solid
(VLS) mechanism [16].
4. Conclusions
Crystalline (high-pressure phase) and amorphous Ge nanorods have been success-
fully synthesized by physical-vapor-deposition (PVD) of Ge. One-dimensional
heterojunctions are formed during the growth process of the nanorods, which may
®nd its applications in optoelectronic nanodevices.
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