Figure 12. Magnetization versus applied field hysteresis (M vs. H) loops of cobalt NPs of different sizes. Reprinted with permission from [401], C. B. Murray et al., MRS Bull. 26, 985 (2001). © 2001, Materials Research Society.

Suslick et al. have introduced the sonochemical reactions of volatile organometallic compounds as a general approach to the synthesis of nanophase materials [404]. The chemical effects of ultrasound result from acoustic cavitation, the formation, growth, and implosive collapse of bubbles within a liquid. The collapsing bubbles generate localized hot spots (T ~ 5000 K) where molecular species decompose, and the resulting metal atoms agglomerate to form nanostructured metals that are often amorphous. A variety of metals, alloys and carbides have been produced using this approach [405]. A scanning electron micrograph of amorphous nano-structured iron obtained by the sonication of iron pentacar-bonyl (Fig. 13) shows agglomerated nanometric particles (ca. 20 nm), which were shown by transmission electron microscopy to consist of smaller (4-6 nm) nanocrystals.

Thin films of Fe [406], Ru [407], and Os [407] have been formed by thermal decomposition of metal pentacarbonyls. Since pentacarbonyls are difficult to handle because of their air-, light-, and temperature-sensitive nature, Shore et al. have used trinuclear precursors M3(CO)12 (M = Fe, Ru, Os) to deposit metallic films; however, a dynamic vacuum of 10-5 torr was necessary to transport the metal carbonyls to the heated substrate [408]. An exhaustive account of the CVD of a variety of metals, including the basic principles of precursor design and a classification of the precursors, is available in the book "The Chemistry of Metal CVD," authored by Kodas and Hampden-Smith [386].

The technological importance of films of single metals, multilayers, and alloys, together with the increasing demand of materials with improved performance, has led to a great deal of interest in nanostructured device-quality films. High-purity, dense metal films with controlled crystallite size and smooth surfaces deposited at relatively low temperatures with high deposition rates are required for microelectronic applications [409]. Precursors for CVD of metals (Table 1) can be broadly classified into three categories: (i) inorganic precursors, which do not contain carbon such as metal halides; (ii) metal-organic precursors, which contain organic

Figure 13. Scanning electron micrograph of nanostructured iron. Reprinted with permission from [403], K. S. Suslick et al., Chem. Mater. 8, 2172 (1996). © 1996, American Chemical Society.

ligands but do not possess metal-carbon bonds, such as metal alkoxides, metal amides, and metal hydrides; and (iii) organometallic precursors that contain organic ligands and metal-carbon bonds, for example, metal carbonyls, metal alkyls, and metallocenes [52, 387].

Inorganic precursors such as metal halides typically need high temperatures (>600 °C) and a reducing agent such as H2 to produce pure metal films. Furthermore, they are inexpensive, commercially available precursors that can easily be purified. However, most of them are solids, and homogeneous precursor delivery into the CVD reactor is difficult. In contrast, the metal-organic or organometallic molecular precursors decompose at lower temperatures and possess high vapor pressures, and most of them are liquids or can be obtained as liquids by changing the ligands. The major disadvantage is the incorporation of heteroelements (C, O, F, N, P), which is difficult to control, particularly in the case of reactive metals. However, there are many examples where high-purity metal films have been deposited from single-source molecular precursors without coreactant like reactive (reducing) gas. This requires a decomposition mechanism where the organic ligands are removed intact from the reaction chamber or in which a reaction pathway exists to form volatile species that desorb easily from the surface [52, 387]. Disproportionation of Cu(I) compounds yields elemental Cu and a volatile Cu(II) species [410], and the metal-carbon bond is cleaved by ^-hydrogen elimination in Al( 'Bu)3 [411], homolytic scission of Au-C bonds in (CH3)Au(PMe3) [412] and the use of precursors, which do not contain metal-carbon bonds such as AlH3-NR3 [413]. The chemical transformations of these precursors are displayed in Eqs. (9)-(12).

In the case of metal CVD, precursors devoid of a metal-carbon bond or a low metal-carbon bond order are preferred because of the low carbon contamination in the deposits. The possibility of carbon incorporation is increased because of the increased stength of M-C bonding. For example, CVD of Fe using Fe(CO)5 produces realtively pure films when compared with the decomposition of ferrocene (Fe(C5H5)2), in the absence of any reducing agent [406].

Among elemental semiconductors, germanium crystals, wires, and films of nanoscopic dimensions are receiving increasing attention because of their novel electronic and optical properties that are intrinsically associated with the low dimensionality and quantum confinement effect in semiconductor materials. The quantum confinement effect enables indirect band gap semiconductors like Si and Ge to become more efficient light emitters. Compared with Si, Ge nanostructures are of particular interest, since the Bohr exciton radius is larger in Ge than in Si, which consequently should lead to more prominent quantum confinement effects. Ge nanocrystals have been synthesized by different physical, chemical transport [414-416], CVD [70], and sol-gel [417] methods. Yang et al. have used chemical vapor transport for rational synthesis of random and aligned growth of 1-D structures [414-416, 418, 419]. In a typical synthesis, a mixture of Ge powder and I2 and a gold-coated Si wafer were placed at the high-temperature (1000 ° C) and low-temperature (800 ° C) ends, respectively. Ge reacts with I2 to form GeI2 vapor at the hot end, which is then transported to the cold end. GeI2 then decomposes into Ge and I2 vapor to form Ge nanowires on the Au-coated Si substrate (Scheme 14). Although the starting material is a mixture of two different chemical species, the precursor to Ge nanowires is the single compound germanium diiodide, which is formed in the gas phase as a single source.


Figure 14. SEM image of (a) as-obtained Ge nanowires and (b) nanocrystalline Ge islands obtained by the CVD of Ge(C5H5)2.


Figure 14. SEM image of (a) as-obtained Ge nanowires and (b) nanocrystalline Ge islands obtained by the CVD of Ge(C5H5)2.

1000 oC 800 oC

Scheme 14.

The molecular precursor strategy is less investigated for Ge-based nanostructures, largely because of the unavailability of suitable precursors. Chisholm et al. have used germanium bis(trimethyl)silylamide to deposit metallic Ge; however, the films were found to contain small amounts of N and O incorporated because of inefficient decomposition of precursor and partial oxidation of the films, respectively [47]. The most commonly used germanium precursor is tetrahydrogermane, GeH4. Since the germane precursor is pyrophoric in nature, the replacement of one of the hydrogen atoms in GeH4 by appropriate organic groups with leaving group characteristics should lead to alternative germanium precursors. Jutzi et al. have synthesized the cyclopentadienyl germanes (RGeH3; R = substituted cyclopentadienyl groups) as new liquid precursors for the deposition of thin germanium films [70]. These precursors are easy to handle and decompose at low temperatures (<400 ° C) to allow the deposition of Ge films under mild conditions; however, the films contain a significant amount of carbon (8 at.%). The quality of the films is comparable to the quality of those obtained from the decomposition of GeEt4 above 500 ° C [420]. The high organic contamination in Ge deposits (Ge: 92 at.%; C: 8 at.%) obtained from substituted cyclopentadienyl precursors was due to the high stability of the ligands and their fragments. We have used a simple germanium bis-cyclopentadienylide (Ge(C5H5)2) as a SSP to obtain high-purity germanium nanowires (Fig. 14a) as well as nanostructured films (Fig. 14b) at the lowest temperature (<350 ° C) reported so far for the CVD of Ge [421]. Ge(C5H5)2 is different from GeH4 or cyclopentadienyl germanes because of the weak dipolar interactions of the cyclopentadienyl groups with the metal centers, which results in the easy formation of metallic nanostructures. The micro-EDX analysis showed the Ge content to be more than 99 at.%, and carbon and oxygen contents were negligible. The thermal decomposition of Ge(C5H5)2 proceeds possibly by the cleavage of the Ge-Cp bond. This is supported by the observation of C5H+, (C5H5)+, and Ge+ species in the mass spectral analysis of the gaseous products formed during the thermolysis of Ge(C5H5)2 [421].

A high-resolution transmission electron microscope (HR-TEM) image (Fig. 15) shows the wires to be homogeneous and straight, which indicates that they are free of defects. The average diameter of the wires was found to be 15 nm. The selective area diffraction pattern of these structures exhibits sharp spots corresponding to their highly crystalline nature. The preferred growth direction was found to be [112].

Molecular compounds, together with standard nano-structure fabrication methods, can be used for obtaining patterned nanowires or dots. For instance, Himpsel et al. have obtained self-assembled Fe nanowires by pyrol-ysis (nitrogen laser) of ferrocene (Fe(C5H5)2), selectively adsorbed between CaF2 stripes on Si(111) [422]. Ferrocene is a suitable precursor because each molecule contains one Fe atom sandwiched between two cyclopentadienyl rings,

Figure 15. HR-TEM image showing the lattice fringes of a single-crystal Ge wire.

and, owing to the nonreactive nature of the precursor, the ferrocene-covered substrate is passive unless it is exposed to ultraviolet photons. The Fe nanowires were fabricated in three steps: (1) preparation of a Si template by annealing vicinal Si(111), (2) creation of continuous CaF2 stripes on a CaF1/Si(111) surface by the growth of one or two monolayers of CaF2, and (3) selective adsorption of ferrocene molecules in CaF1 trenches between the CaF2 stripes. Figure 16 shows the initial and final stages during Fe nanowire growth.

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