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ABSTRACT: We report the growth, structural and electrical characterization of single-crystalline iron pyrite (FeS2) nanorods, nanobelts and nanoplates synthesized via sulfidation reaction with iron dichloride (FeCl2) and iron dibromide (FeBr2). The as-synthesized products were confirmed to be single-crystal phase pure cubic iron pyrite using powder X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. An intermediate reaction temperature of 425 oC or a high sulfur vapor pressure under high temperatures were found to be critical for the formation of phase pure pyrite. Field effect transport measurements showed that these pyrite nanostructures appear to behave as a moderately p-doped semiconductors with an average resistivity of 2.19 ± 1.21 Ω cm, an improved hole mobility of 0.2 cm2 V-1 s-1 and a lower carrier concentrations on the order of 1018-1019 cm-3 compared with previous reported pyrite nanowires. Temperature dependent electrical transport measurements reveal Mott variable range hopping transport in the temperature range of 40-220 K and transport via thermal activation of carriers with an activation energy of 100 meV above room temperature (300-400 K). Most importantly, the transport properties of the pyrite nanodevices do not change if highly pure (99.999%) precursors are utilized, suggesting that the electrical transport is dominated by intrinsic defects in pyrite. These single-crystal pyrite nanostructures are nice platforms to further study the carrier conduction mechanisms, semiconductor defect physics, and surface properties in depth, towards improving the physical properties of pyrite for efficient solar energy conversion.
ACS Nano 01/2013; · 10.77 Impact Factor
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ABSTRACT: Quantum dot nanoscale semiconductor heterostructures (QDHs) are a class of materials potentially useful for integration into solar energy conversion devices. However, realizing the potential of these heterostructured systems requires the ability to identify and synthesize heterostructures with suitably designed materials, controlled size and morphology of each component, and structural control over their shared interface. In this review, we will present the case for the utility and advantages of chemically synthesized QDHs for solar energy conversion, beginning with an overview of various methods of heterostructured material synthesis and a survey of heretofore reported materials systems. The fundamental charge transfer properties of the resulting materials combinations and their basic design principles will be outlined. Finally, we will discuss representative solar photovoltaic and photoelectrochemical devices employing QDHs (including quantum dot sensitized solar cells, or QDSSCs) and examine how QDH synthesis and design impacts their performance.
Chemical Society Reviews 12/2012; · 28.76 Impact Factor
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ABSTRACT: The increasing demands from large-scale energy applications call for the development of lithium-ion battery (LIB) electrode materials with high energy density. Earth-abundant conversion cathode material iron trifluoride (FeF(3)) has a high theoretical capacity (712 mAh g(-1)) and the potential to double the energy density of the current cathode material based on lithium cobalt oxide. Such promise has not been fulfilled due to the non-optimal material properties and poor kinetics of the electrochemical conversion reactions. Here, we report for the first time a high-capacity LIB cathode that is based on networks of FeF(3) nanowires (NWs) made via an inexpensive and scalable synthesis. The FeF(3) NW cathode yielded a discharge capacity as high as 543 mAh g(-1) at the first cycle and retained a capacity of 223 mAh g(-1) after 50 cycles at room temperature under the current of 50 mA g(-1). Moreover, high-resolution transmission electron microscopy revealed the existence of continuous networks of Fe in the lithiated FeF3 NWs after discharging, which is likely an important factor for the observed improved electrochemical performance. The loss of active material (FeF(3)) caused by the increasingly ineffective reconversion process during charging was found to be a major factor responsible for the capacity loss upon cycling. With the advantages of low cost, large quantity, and ease of processing, these FeF(3) NWs are not only promising battery cathode materials, but also provide a convenient platform for fundamental studies and further improving conversion cathodes in general.
Nano Letters 10/2012; · 13.20 Impact Factor
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ABSTRACT: We report the synthesis of CdS and CdSe nanowires (NWs) and nanoribbons (NRs) with gold catalysts by H(2)-assisted chemical vapor deposition. Nanopods and nanocones were obtained without catalysts at higher system pressure. Transmission electron microscopy (TEM) studies, including two-beam TEM and displaced-aperture dark-field TEM characterization, were used to investigate the NW growth mechanism. Dislocation contrast and twist contours have been routinely observed within the synthesized one-dimensional (1D) CdS and CdSe NWs, suggesting the operation of the dislocation-driven NW growth mechanism under our experimental conditions. The Burgers vectors of dislocations and the associated Eshelby twists were measured and quantified. We hypothesize that gold nanoparticles provide nucleation sites to initiate the growth of CdS/CdSe NWs and lead to the formation of dislocations that continue to drive and sustain 1D growth at a low supersaturation level. Our study suggests that the dislocation-driven mechanism may also contribute to the growth of other 1D nanomaterials that are commonly considered to grow via the vapor-liquid-solid mechanism.
ACS Nano 04/2012; 6(5):4461-8. · 10.77 Impact Factor
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ABSTRACT: We report the growth and structural, electrical, and optical characterization of vertically oriented single-crystalline iron pyrite (FeS(2)) nanowires synthesized via thermal sulfidation of steel foil for the first time. The pyrite nanowires have diameters of 4-10 nm and lengths greater than 2 μm. Their crystal phase was identified as cubic iron pyrite using high-resolution transmission electron microscopy, Raman spectroscopy, and powder X-ray diffraction. Electrical transport measurements showed the pyrite nanowires to be highly p-doped, with an average resistivity of 0.18 ± 0.09 Ω cm and carrier concentrations on the order of 10(21) cm(-3). These pyrite nanowires could provide a platform to further study and improve the physical properties of pyrite nanostructures toward solar energy conversion.
Nano Letters 03/2012; 12(4):1977-82. · 13.20 Impact Factor
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ABSTRACT: ZnO nanowires were investigated at high pressures of up to 27 GPa in situ in a diamond anvil cell using synchrotron X-ray diffraction. Upon compression, a wurtzite-to-rocksalt phase transformation was observed, but both the onset and the completion pressures of this transformation were enhanced compared with all previously studied morphologies of ZnO, including nanocrystals and their bulk counterparts. Upon decompression, the rocksalt phase was found to sustain at near ambient pressure and could be recovered in a significant amount. Moreover, the pressure-volume equations of state for both the wurtzite and the rocksalt phases indicate that their bulk moduli are significantly higher than those of bulk ZnO and nanocrystals. The SEM images of the ZnO nanowires both before and after the compression suggest the pressure-induced morphology modifications, corroborating the understanding of other structure and property evolutions with pressure. Finally, possible pressure-induced phase transition mechanisms were explored by examining the cell parameters and the internal structural parameter with pressures.
The Journal of Physical Chemistry C 01/2012; 116(3):2102. · 4.80 Impact Factor
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ABSTRACT: We report for the first time the facile solution growth of α-FeF(3)·3H(2)O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe(2)O(3) (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF(3)·3H(2)O NWs (typically <100 nm in diameter) are converted to single-crystal α-Fe(2)O(3) NWs with internal pores, while thick ones (typically >100 nm in diameter) become polycrystalline porous α-Fe(2)O(3) NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ~400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm(2) at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm(2), pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.
Nano Letters 01/2012; 12(2):724-31. · 13.20 Impact Factor
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ABSTRACT: We report a green synthesis of Cu(2)O nanowires and nanotubes in aqueous solution by reducing Cu(2+) to Cu(+) with glucose or fructose via Fehling's reaction. The screw dislocation-driven growth of Cu(2)O nanowires and nanotubes is confirmed by imaging the dislocation contrast, the Eshelby twist associated with dislocations and the spontaneously formed hollow nanotubes.
Chemical Communications 12/2011; 48(8):1174-6. · 6.17 Impact Factor
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ABSTRACT: Copper (Cu) nanowires (NWs) are inexpensive conducting nanomaterials intensively explored for transparent conducting electrodes and other applications. However, the mechanism for solution growth of Cu NWs remains elusive so far. Here we show that the one-dimensional anisotropic growth of Cu NWs and nanotubes (NTs) in solution is driven by axial screw dislocations. All three types of evidence for dislocation-driven growth have been conclusively observed using transmission electron microscopy (TEM) techniques: rigorous two-beam TEM analysis that conclusively characterizes the dislocations in the NWs to be pure screw dislocations along <110> direction, twist contour analysis that confirms the presence of Eshelby twist associated with the dislocation, and the observation of spontaneously formed hollow NTs. The reduction-oxidation (redox) electrochemical reaction forming the Cu NWs presents new chemistry for controlling supersaturation to promote dislocation-driven NW growth. Using this understanding to intentionally manipulate the supersaturation, we have further improved the NW growth by using a continuous flow reactor to yield longer Cu NWs under much milder chemical conditions. The rational synthesis of Cu NWs with control over size and geometry will facilitate their applications.
Nano Letters 12/2011; 12(1):234-9. · 13.20 Impact Factor
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ABSTRACT: We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.
Nano Letters 09/2011; 11(10):4431-7. · 13.20 Impact Factor
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ABSTRACT: We report the dislocation-driven growth of two-dimensional (2D) nanoplates. They are another type of dislocation-driven nanostructure and could find application in energy storage, catalysis, and nanoelectronics. We first focus on nanoplates of zinc hydroxy sulfate (3Zn(OH)(2)·ZnSO(4)·0.5H(2)O) synthesized from aqueous solutions. Both powder X-ray and electron diffraction confirm the zinc hydroxy sulfate (ZHS) crystal structure as well as their conversion to zinc oxide (ZnO). Scanning electron, atomic force, and transmission electron microscopy reveal the presence of screw dislocations in the ZHS nanoplates. We further demonstrate the generality of this mechanism through the growth of 2D nanoplates of α-Co(OH)(2), Ni(OH)(2), and gold that can also follow the dislocation-driven growth mechanism. Finally, we propose a unified scheme general to any crystalline material that explains the growth of nanoplates as well as different dislocation-driven nanomaterial morphologies previously observed through consideration of the relative crystal growth step velocities at the dislocation core versus the outer edges of the growth spiral under various supersaturations.
Nano Letters 09/2011; 11(10):4449-55. · 13.20 Impact Factor
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ABSTRACT: We report the rational synthesis of α-FeOOH (goethite) nanowires following a dislocation-driven mechanism by utilizing a continuous-flow reactor and chemical equilibria to maintain constant low supersaturations. The existence of axial screw dislocations and the associated Eshelby twist in the nanowire product were confirmed using bright-/dark-field transmission electron microscopy imaging and twist contour analysis. The α-FeOOH nanowires can be readily converted into semiconducting single-crystal but porous α-Fe(2)O(3) (hematite) nanowires via topotactic transformation. Our results indicate that, with proper experimental design, many more useful materials can be grown in one-dimensional morphologies in aqueous solutions via the dislocation-driven mechanism.
Journal of the American Chemical Society 06/2011; 133(22):8408-11. · 9.91 Impact Factor
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ABSTRACT: We report the synthesis, phase transformation, and electrical property measurement of single-crystal NiGe and ε-Ni(5)Ge(3) nanowires (NWs). NiGe NWs were spontaneously synthesized by chemical vapor deposition of GeH(4) onto a porous Ni substrate without the use of intentional catalysts. The as-grown NWs of the orthorhombic NiGe phase were transformed to the hexagonal ε-Ni(5)Ge(3) phase by thermal annealing induced Ni enrichment. This controllable conversion of germanide phases is desirable for phase-dependent property study and applications, and the observation of novel metastable ε-Ni(5)Ge(3) phase suggests the importance of kinetic factors in such nanophase transformations. Electrical studies reveal that NiGe NWs are highly conductive, with an average resistivity of 35 ± 15 μΩ·cm, while the resistivity of ε-Ni(5)Ge(3) NWs is more than 4 times that of the NiGe phase. NWs of nickel germanides, particularly NiGe, would be useful building blocks for germanium-based nanoelectronic devices.
ACS Nano 05/2011; 5(6):5006-14. · 10.77 Impact Factor
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ABSTRACT: We present the chemical vapor deposition (CVD) reactions of the single source precursor Fe(SiCl(3))(2)(CO)(4) over Si, Ge, CoSi(2)/Si, and CoSi/Si substrates to explore the growth and doping processes of silicide nanowires (NWs). Careful investigation of the composition and morphology of the NW products and the intruded silicide films from which they nucleate revealed that the group IV elements (Si, Ge) in the NW products originate from both the precursor and the substrate, while the metal elements incorporated into the NWs (Fe, Co) originate from vapor phase precursor delivery. The use of a Ge growth substrate enabled the successful synthesis of Fe(5)Si(2)Ge NWs, the first report of a metal silicide-germanide alloy NW. Further, investigation of the pyrolysis of the CoSiCl(3)(CO)(4) precursor revealed independent delivery of Co and Si species during CVD reactions. This understanding enabled a new, more robust two-precursor synthetic route to Fe(1-x)Co(x)Si alloy NWs using Fe(SiCl(3))(2)(CO)(4) and CoCl(2).
ACS Nano 03/2011; 5(4):3268-77. · 10.77 Impact Factor
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ABSTRACT: EuS nanocrystals (NCs) were doped with Gd resulting in an enhancement of their magnetic properties. New EuS and GdS single source precursors (SSPs) were synthesized, characterized, and employed to synthesize Eu(1-x)Gd(x)S NCs by decomposition in oleylamine and trioctylphosphine at 290 °C. The doped NCs were characterized using X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy, which support the uniform distribution of Gd dopants through electron energy loss spectroscopy (EELS) mapping. X-ray absorption spectroscopy (XAS) revealed the dopant ions in Eu(1-x)Gd(x)S NCs to be predominantly Gd(3+). NCs with a variety of doping ratios of Gd (0 ≤ x < 1) were systematically studied using vibrating sample magnetometry and the observed magnetic properties were correlated with the Gd doping levels (x) as quantified with ICP-AES. Enhancement of the Curie temperature (T(C)) was observed for samples with low Gd concentrations (x ≤ 10%) with a maximum T(C) of 29.4 K observed for NCs containing 5.3% Gd. Overall, the observed T(C), Weiss temperature (θ), and hysteretic behavior correspond directly to the doping level in Eu(1-x)Gd(x)S NCs and the trends qualitatively follow those previously reported for bulk and thin film samples.
Journal of the American Chemical Society 10/2010; 132(45):15997-6005. · 9.91 Impact Factor
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ABSTRACT: In the current examples of dislocation-driven nanowire growth, the screw dislocations that propagate one-dimensional growth originate from spontaneously formed highly defective "seed" crystals. Here we intentionally utilize screw dislocations from defect-rich gallium nitride (GaN) thin films to propagate dislocation-driven growth, demonstrating epitaxial growth of zinc oxide (ZnO) nanowires directly from aqueous solution. Atomic force microscopy confirms screw dislocations are present on the native GaN surface and ZnO nanowires grow directly from dislocation etch pits of heavily etched GaN surfaces. Furthermore, transmission electron microscopy confirms the existence of axial dislocations. Eshelby twist in the resulting ZnO nanowires was confirmed using bright-/dark-field imaging and twist contour analysis. These results further confirm the connection between dislocation source and nanowire growth. This may eventually lead to defect engineering strategies for rationally designed catalyst-free dislocation-driven nanowire growth for specific applications.
Nano Letters 09/2010; 10(9):3459-63. · 13.20 Impact Factor
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ABSTRACT: We discuss a nanowire and nanotube formation mechanism in which axial screw dislocations provide self-perpetuating steps to enable one-dimensional (1D) crystal growth, unlike previously understood vapor−liquid−solid (VLS) or analogous metal-catalyzed growth. We initially found this mechanism in hierarchical pine tree PbS nanowires with helically rotating branches. We further applied it to ZnO, demonstrating that screw dislocations can drive the spontaneous formation of nanotubes, and used classical crystal growth theory to confirm that their anisotropic 1D growth is driven by dislocations. Dislocation-driven growth should be general to many materials grown in vapor or solution and is underappreciated. It will create a new dimension in the rational synthesis of nanomaterials. The resulting complex hierarchical nanostructures can be useful for solar energy conversion, and our understanding will allow large-scale synthesis of 1D nanomaterials for practical applications.
05/2010;
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ABSTRACT: Single-crystal nanotubes are commonly observed, but their formation is often not understood. We show that nanotube growth can be driven by axial screw dislocations: Self-perpetuating growth spirals enable anisotropic growth, and the dislocation strain energy overcomes the surface energy required for creating a new inner surface forming hollow tubes spontaneously. This was demonstrated through solution-grown zinc oxide nanotubes and nanowires by controlling supersaturation using a flow reactor and confirmed using microstructural characterization. The agreement between experimental growth kinetics and those predicted from fundamental crystal growth theories confirms that the growth of these nanotubes is driven by dislocations.
Science 04/2010; 328(5977):476-80. · 31.20 Impact Factor
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ABSTRACT: We report the synthesis, structural characterization, and magnetotransport of single-crystalline nanowires of manganese monosilicide, MnSi. Bulk MnSi has unusual magnetic orderings, helimagnetism, and skyrmions at ambient pressure, and high pressure studies have revealed partial magnetic ordering and non-Fermi liquid behavior. MnSi nanowires were synthesized using chemical vapor deposition of MnCl(2) onto silicon substrates. The morphology, structure, and composition of these nanowires were analyzed using electron microscopy and X-ray spectroscopy. The low-temperature magnetoresistance characteristics of MnSi nanowires reveal the first signature of helimagnetism in one-dimensional nanomaterials.
Nano Letters 04/2010; 10(5):1605-10. · 13.20 Impact Factor
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ABSTRACT: A soft x-ray absorption and x-ray magnetic circular dichroism (XMCD) study of the ferromagnetism in solution-grown EuS nanocrystals (NCs) is reported. The absorption edges of Eu M(5) and M(4), S K, O K, and P K were probed to determine elementally specific contributions to the magnetism of EuS NCs. Differential spin absorption was observed by XMCD at the Eu M(5,4) edges confirming the presence of a magnetic moment on the Eu(2+) 4f shell. No dichroic signal was observed for S, O, or P. Vibrating sample magnetometry of ensembles of NCs shows ferromagnetic properties consistent with the XMCD studies.
Applied Physics Letters 11/2009; 95(20):202501. · 3.84 Impact Factor
