Matthew J. Bierman

Dislocation-Driven Nanowire Growth and Eshelby Twist

Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 〈110〉 directions having an estimated magnitude of 6 ± 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials.

Evaluation of Pt, Ni, and Ni–Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes

The dark electrocatalytic and light photocathodic hydrogen evolution properties of Ni, Ni–Mo alloys, and Pt on Si electrodes have been measured, to assess the viability of earth-abundant electrocatalysts for integrated, semiconductor coupled fuel formation. In the dark, the activities of these catalysts deposited on degenerately doped p+-Si electrodes increased in the order Ni < Ni–Mo ≤ Pt. Ni–Mo deposited on degenerately doped Si microwires exhibited activity that was very similar to that of Pt deposited by metal evaporation on planar Si electrodes. Under 100 mW cm−2 of Air Mass 1.5 solar simulation, the energy conversion efficiencies of p-type Si/catalyst photoelectrodes ranged from 0.2–1%, and increased in the order Ni ≈ Ni–Mo < Pt, due to somewhat lower photovoltages and photocurrents for p-Si/Ni–Mo relative to p-Si/Ni and p-Si/Pt photoelectrodes. Deposition of the catalysts onto microwire arrays resulted in higher apparent catalytic activities and similar photoelectrode efficiencies than were observed on planar p-Si photocathodes, despite lower light absorption by p-Si in the microwire structures.

Potential applications of hierarchical branching nanowires in solar energy conversion

Nanoscience and nanotechnology can provide many benefits to photovoltaic and photoelectrochemical applications by combining novel nanoscale properties with processability and low cost. Taking advantage of high quality, high efficiency, yet low cost nanomaterials could potentially provide the new and transformative approaches to enable the proposed generation-III solar technologies. Nanowires are interesting because they have a long axis to absorb incident sunlight yet with a short radial distance to separate the photogenerated carriers. In this perspective, we further suggest that more “complex” nanostructures, both in the form of hierarchically branching/hyperbranching nanowire structures and in the form of multi-component nanowire heterostructures of diverse materials, are potentially even more interesting for solar energy harvesting and conversion. The common bottom-up synthetic techniques to induce branching in nanowires to form hierarchical nanowire structures are reviewed. Several potential strategies for their incorporation into solar conversion devices are discussed and some fundamental issues and future directions are identified.

Mechanism and Kinetics of Spontaneous Nanotube Growth Driven by Screw Dislocations

Nanosynthesis Without a Twist The synthesis of many nanoscale materials occurs under conditions of changing saturation because generation of product decreases the concentration of reactants. Morin et al. (p. 476) used a flow reactor to maintain conditions of low supersaturation during the growth of zinc oxide nanotubes and nanowires. Under these conditions, growth of the tubes was controlled by the release of stress, which prevented the torquing of the crystals along their axis. Since growth at different saturation conditions matched predictions, this looks like a promising method to develop rational and controlled synthesis of nanomaterials at large scale and low cost. Low supersaturated conditions help control the growth of zinc oxide nanowires and nanotubes from defect sites. 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.

Screw dislocation-driven growth of two-dimensional nanoplates.

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.

Epitaxial growth of hierarchical PbS nanowires

Growth of extensively aligned hierarchical lead sulfide (PbS) nanowires with hyperbranched morphology has been achieved by synthesizing nanowires epitaxially on single crystal NaCl, rutile TiO2 (001), and muscovite mica in a chemical vapor deposition process. The morphology of as-grown PbS nanowires has been examined using scanning electron microscopy. Epitaxial match with the (100) plane of PbS has been observed on all substrates, and epitaxial match with PbS (111) was also observed on mica. In addition, the preferred orientation of nanowires led to particularly strong (200) reflections from PbS in powder X-ray diffraction. The potential epitaxial relationship and lattice match are proposed and discussed. PbS nanowires of the pine tree morphology can only be formed non-epitaxially in the presence of epitaxial hyperbranched clusters. The difficulty of forming epitaxial nanowire pine trees suggested that epitaxial growth might not be conducive to the creation of dislocations that drive the formation of pine tree nanowires. Electrical properties of PbS nanowires have also been investigated.

Photoelectrochemical Behavior of n‑Type Si(111) Electrodes Coated With a Single Layer of Graphene

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K3[Fe(CN)6]/K4[Fe(CN)6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm(-2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si-H electrodes yielded a nearly complete decay of the current density within ~100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact.

Photoanodic behavior of vapor-liquid-solid–grown, lightly doped, crystalline Si microwire arrays

Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me2Fc+/0)–CH3OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 1013 cm−3. When measured as photoanodes in contact with a Me2Fc+/0–CH3OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ∼0.44 V, carrier-collection efficiencies exceeding ∼0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells.

Element-specific magnetometry of EuS nanocrystals.

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.

Racemic iron(III) and cobalt(III) complexes containing a new pentadentate “helmet” phthalocyaninato ligand

Solvothermal reactions of iron(II) acetate tetrahydrate and cobalt(II) acetate tetrahydrate with 1,2-dicyanobenzene in methanol solution result in the formation of racemic six-coordinate iron(III) and cobalt(III) complexes, respectively, with a new bicyclic pentadentate 14,28-[1,3-diiminoisoindolinato]phthalocyaninato ligand.