Solution-liquid-solid growth of semiconductor nanowires

The serendipitously discovered solution-liquid-solid (SLS) mechanism has been refined into a nearly general synthetic method for semiconductor nanowires. Purposeful control of diameters and diameter distributions is achieved. The synthesis proceeds by a solution-based catalyzed-growth mechanism in which nanometer-scale metallic droplets catalyze the decomposition of metallo-organic precursors and crystalline nanowire growth. Related growth methods proceeding by the analogous vapor-liquid-solid (VLS) and supercritical fluid-liquid-solid (SFLS) mechanisms are known, and the relative attributes of the methods are compared. In short, the VLS method is most general and appears to afford nanowires of the best crystalline quality. The SLS method appears to be advantageous for producing the smallest nanowire diameters and for variation and control of surface ligation. The SFLS method may represent an ideal compromise. Recent results for SLS growth are summarized.     

Synthesis and characterization of dimensionally ordered semiconductor nanowires within mesoporous silica.

Semiconductor nanowires of silicon have been synthesized within the pores of mesoporous silica using a novel supercritical fluid solution-phase approach. Mesoporous silica, formed by the hydrolysis of tetramethoxysilane (TMOS) in the presence of a triblock copolymer surfactant, was employed for the nucleation and growth of quantum-confined nanowires. The filling of the silica mesopores with crystalline silicon and the anchoring of these nanowires to the sides of the pores were confirmed by several techniques including electron microscopy, powder X-ray diffraction, 29Si magic angle spinning nuclear magnetic resonance, infrared spectroscopy, and X-ray fluorescence. Effectively, the silica matrix provides a means of producing a high density of stable, well-ordered arrays of semiconductor nanowires in a low dielectric medium. The ordered arrays of silicon nanowires also exhibited discrete electronic and photoluminescence transitions that could be exploited in a number of applications, including nanodevices and interconnects.     

Catalytic growth and characterization of gallium nitride nanowires.

The preparation of high-purity and -quality gallium nitride nanowires is accomplished by a catalytic growth using gallium and ammonium. A series of catalysts and different reaction parameters were applied to systematically optimize and control the vapor-liquid-solid (VLS) growth of the nanowires. The resulting nanowires show predominantly wurtzite phase; they were up to several micrometers in length, typically with diameters of 10-50 nm. A minimum nanowire diameter of 6 nm has been achieved. Temperature dependence of photoluminescence spectra of the nanowires revealed that the emission mainly comes from wurtzite GaN with little contribution from the cubic phase. Moreover, the thermal quenching of photoluminescence was much reduced in the GaN nanowires. The Raman spectra showed five first-order phonon modes. The frequencies of these peaks were close to those of the bulk GaN, but the modes were significantly broadened, which is indicative of the phonon confinement effects associated with the nanoscale dimensions of the system. Additional Raman modes, not observed in the bulk GaN, were found in the nanowires. The field emission study showing notable emission current with low turn-on field suggests potential of the GaN nanowires in field emission applications. This work opens a wide route toward detailed studies of the fundamental properties and potential applications of semiconductor nanowires.     

Inorganic semiconductor nanowires: rational growth, assembly, and novel properties

Rationally controlled growth of inorganic semiconductor nanowires is important for their applications in nanoscale electronics and photonics. In this article, we discuss the rational growth, physical properties, and integration of nanowires based on the results from the authors' laboratory. The composition, diameter, growth position, and orientation of the nanowires are controlled based on the vapor-solid-liquid (VLS) crystal growth mechanism. The thermal stability and optical properties of these semiconductor nanowires are investigated. Particularly, ZnO nanowires with well-defined end surfaces can function as room-temperature ultraviolet nanolasers. In addition, a novel microfluidic-assisted nanowire integration (MANI) process was developed for the hierarchical assembly of nanowire building blocks into functional devices and systems.     

Synthesis, characterization, and optical properties of In2O3 semiconductor nanowires

In2O3 semiconductor nanowires were synthesized by the chemical vapor deposition method through carbon thermal reduction at 900 degrees C with 95% Ar and 5% O2 gas flow. The In2O3 nanowires were characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence spectroscopy (PL). For the first time, we observed the formation of corundum-type h-In2O3 nanowires and branched In2O3 nanowires. The PL spectra of In2O3 nanowires show strong visible red emission at 1.85 eV (670 nm) at low temperature, possibly caused by a small amount of oxygen vacancies in the nanowire crystal structure.     

Solution-based growth and optical properties of self-assembled monocrystalline ZnO ellipsoids

Self-assembled unusual ZnO ellipsoids have been grown by a facile low-temperature (60 degrees C) solution process on a large scale. FESEM and TEM reveal that these ellipsoids have an average horizontal axis of 1.5 microm and a longitudinal axis of 0.6 microm. Experimental results obtained from the early growth stage demonstrate that the ZnO ellipsoidal structures are single crystals and formed from direct "oriented attachment" of two types of building blocks, that is, nanorods and nanoparticles. It is further found that the existence of poly(ethylene glycol) (PEG-10 000) is vital to the formation of the complex microparticles. Raman spectrum, room-temperature photoluminescence, and UV-vis absorption spectra are also discussed. This work presents a simple and effective route for large-scale fabrication of single-crystal ZnO ellipsoids with micrometer-scale sizes and 3D self-assembled structures.     

Growth modes in homo- and heteroepitaxial growth

Due to scientific and technological interest many studies are concerned with growth modes for the first monolayers of epitaxial growth. By combination of microscopy (like STM) and diffraction (like spot profile analysis of LEED) the important steps of film formation via deposition, migration, nucleation and film completion are studied. Temperature, deposition rate and defects of substrate and of the growing film influence drasticly the growth modes in homoepitaxial systems of metals and semiconductors. For heteroepitaxial growth the misfit has to be accommodated during growth. For different systems the processes are quite different. For metals on metals the film is more or less floating where only the orientation is provided by the substrate. For metals on semiconductors some influence on the lattice constant is seen, which depends on the temperature of deposition. For semiconductors on semiconductors a one-to-one correspondence of film atoms to substrate atoms requires a clear transition from pseudomorphic to relaxed growth of the film by dislocations at the interface.

The wide variety of growth modes may be well distinguished by a careful analysis of diffraction spot profiles, which should be recorded during deposition with varying scattering conditions.     

Band-gap engineering of semiconductor nanowires through composition modulation

Alloyed ternary CdS(1-x)Se(x) nanowires were synthesized by template-assisted electrodeposition, in which the ratio of S to Se in the nanowires was controlled by adjusting the relative amounts of the starting materials. Higher-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) showed that the alloyed ternary CdS(1-x)Se(x) nanowires are highly crystalline, and no phase-separated Cd was observed in these nanowires. Optical measurements indicated that the band-gap engineering can be realized in these CdS(1-x)Se(x) nanowires through modulating the composition of S and Se. With broadly tunable optical and electrical properties, these alloyed nanowires could be used in color-tuned nanolasers, biological labels, and nanoelectronics.     

Synthesis and structural metastability of CdTe nanowires

A new organometallic preparation method is described for CdTe nanowires with a high aspect ratio and a predominantly metastable wurtzite phase. The optical and morphological properties of the resulting nanowires were studied, as well as the influence of elevated temperatures on the crystallographic properties. A phase transition from wurtzite to sphalerite was observed at about 500 degrees C. The results show that the wurtzite phase is stabilized by the synthetic method and the surfactants.     

Synthesis and structural characterization of II–VI semiconductor nanocrystallites (quantum dots)

A methodology for the production of II–VI semiconductor nanocrystallites employing organometallic precursors has been developed. The rapid pyrolysis of reagents in a coordinating solvent provides temporally discrete nucleation. Subsequent controlled growth allows the production of macroscopic quantities of nanocrystallites with consistent structure, surface derivatization and a high degree of monodispersity. The samples produced are structurally characterized with a combination of X-ray and Electron Beam based techniques.     

Lyotropic self-assembly of high-aspect-ratio semiconductor nanowires of single-crystal ZnO

Lyotropic nanowire dispersions are attractive precursors for semiconductor device fabrication because they permit the alignment control of active nanomaterials. The reliable production of nanowire-based mesophases, however, is very challenging in practice. We show that appropriately functionalized high-aspect-ratio nanowires of single-crystal ZnO spontaneously form nematic phases in organic and aqueous media. These systems show isotropic, biphasic, and nematic phases on increasing concentration, in reasonable agreement with Onsager's theory for rigid rods interacting via excluded volume. Suspensions were readily processed to produce films with large-area monodomains of aligned nanowires. Imprints of the director field in quiescently dried films display a propensity for bend deformation in the organic mesophase versus splay deformation in the aqueous case, suggesting that system elasticity may be tuned via surface functionalization. These results provide critical insight for the utilization of semiconductor nanowires as novel mesogens and further enable the use of solution-based routes for fabricating optoelectronic devices.     

Synthesis, electrochemistry and structural characterization of luminescent rhenium(I) monoynyl complexes and their homo- and hetero-metallic binuclear complexes

A series of luminescent rhenium(I) monoynyl complexes, [Re(N---N)(CO)₃(CC---R)] (N---N=bpy, ^tBu₂bpy; R=C₆H₅, C₆H₄---Cl-4, C₆H₄---OCH₃-4, C₆H₄---C₈H₁₇-4, C₆H₄---C₆H₅, C₈H₁₇, C₄H₃S, C₄H₂S---C₄H₃S, C₅H₄N), together with their homo- and hetero-metallic binuclear complexes, {Re(N---N)(CO)₃(CC---C₅H₄N)[M]} (N---N=bpy, ^tBu₂bpy; [M]=[Re{(CF₃)₂-bpy}(CO)₃]ClO₄, [Re(NO₂-phen)(CO)₃]ClO₄, W(CO)₅) have been synthesized and their electrochemical and photoluminescence behaviors determined. The structural characterization and electronic structures of selected complexes have also been studied. The luminescence origin of the rhenium(I) alkynyl complexes has been assigned as derived states of a [dπ(Re)→π*(N---N)] metal-to-ligand charge transfer (MLCT) origin mixed with a [π(CCR)→π*(N---N)] ligand-to-ligand charge transfer (LLCT) character. The assignments are further supported by extended Hückel molecular orbital (EHMO) calculations, which show that the LUMO mainly consists of π*(N---N) character while the HOMO is dominated by the antibonding character of the Re---CCR moiety resulted from the overlap of the dπ(Re) and π(CCR) orbitals.     

Synthesis and characterization of group 6-9 metal-rich homo- and hetero-metallaboranes

To isolate the metal-rich metallaboranes of group 6-9, we have performed the reaction of various reaction intermediates, generally synthesized from the low-temperature reactions of [Cp1WCl₄] (Cp1 ​= ​η⁵-C₅Me₅), [(Cp1RhCl₂)₂], or [(Cp1RuCl₂)₂] and [LiBH₄ THF] with different transition metal carbonyl compounds. For example, the thermolytic reaction of [Fe₂(CO)₉] with an in situ generated intermediate, produced from the reaction of [Cp1WCl₄] and [LiBH₄THF] afforded a trigonal bipyramidal cluster, [(μ₃-BH)₂H₂{Cp1W(CO)₂}{Cp1W(CO)}{Fe(CO)₃}], 1 which contains a triply-bridging bis-{hydrido(borylene)} ligand. Similarly, the reaction of [Co₂(CO)₈] with nido-[(RhCp1)₂(B₃H₇)] I at room temperature, yielded an octahedral cluster, [(Cp1Rh)₂B₂H₂Co₂(CO)₅(μ₃-CO)], 2. In this reaction, nido-I having (n+2) skeletal electron pairs (SEP) goes on for the formation of a closo-rhodaborane with (n+1) SEP. In addition, we have isolated a trinuclear bis(μ₃-oxo) metalla cluster [(Cp1Ru)₃(μ₃-OBF₃)₂(μ-H)], 3. Compound 3 can be considered as cluster having trigonal bipyramidal geometry with exo-BF₃ fragment. All these clusters were characterized by IR, mass spectrometry, NMR, and single-crystal X-ray crystallographic analysis.     

Synthesis and characterization of single-crystalline alkali titanate nanowires

A simple, one-step method has been utilized to synthesize functional ternary alkali titanate nanowires of single crystallinity. Nanostructural characterization shows that the nanowires are Na2Ti6O13 with [010] growth direction and KTi8O16.5 with [001] growth direction.     

One-step fabrication and characterization of silica-sheathed ITO nanowires

Novel In_1.94Sn_0.06O₃ (ITO)/amorphous SiO_x core-shell structures were successfully synthesized by simple thermal evaporation. Studies indicated that the core-shell structures typically consisted of a core of crystalline, ITO nanowires surrounded by a shell of amorphous, SiO_x tubular structures. We proposed a gold-catalyzed, vapor-liquid-solid process as the dominant mechanism for the growth of the core ITO nanowires, whereas SiO_x was grown in a tubular structure by a simultaneous and dynamic process. The possible reason for the preferential formation of the SiO_x shells on the outside of the core-shell structures, is discussed. In regard to the core/shell structures, three emission peaks of 2.73, 3.06, and 1.65 eV were observed in the room-temperature photoluminescence measurements, and were attributed to the SiO_x shell.     

Sol-gel template synthesis and photoluminescence of n- and p-type semiconductor oxide nanowires.

A sol-gel template technique has been put forward to synthesize single-crystalline semiconductor oxide nanowires, such as n-type SnO2 and p-type NiO. Scanning electron microscopy and transmission electron microscopy observations show that the oxide nanowires are single-crystal with average diameters in the range of 100-300 nm and lengths of over 10 microm. Photoluminescence (PL) spectra show a PL emission peak at 401 nm for n-type semiconductor SnO2, and a PL emission at 407 nm for p-type semiconductor NiO nanowires, respectively. Correspondingly, the observed violet-light emission at room temperature is attributed to near-band-edge emission for SnO2 nanowires and the 3d(7)4s-->3d8 transition of Ni2+ for NiO nanowires.