From GaN to ZnGa₂O₄ through a Low-Temperature Process: Nanotube and Heterostructure Arrays

We demonstrate a method to synthesize GaN–ZnGa₂O₄ core–shell nanowire and ZnGa₂O₄ nanotube arrays by a low-temperature hydrothermal process using GaN nanowires as templates. Transmission electron microscopy and X-ray photoelectron spectroscopy results show that a ZnGa₂O₄ shell forms on the surface of GaN nanowires and that the shell thickness is controlled by the time of the hydrothermal process and thus the concentration of Zn ions in the solution. Furthermore, ZnGa₂O₄ nanotube arrays were obtained by depleting the GaN core from GaN–ZnGa₂O₄ core–shell nanowire arrays during the reaction and subsequent etching with HCl. The GaN–ZnGa₂O₄ core–shell nanowires exhibit photoluminescence peaks centered at 2.60 and 2.90 eV attributed to the ZnGa₂O₄ shell, as well as peaks centered at 3.35 and 3.50 eV corresponding to the GaN core. We also demonstrate the synthesis of GaN–ZnGa₂O₄ heterojunction nanowires by a selective formation process as a simple route toward development of heterojunction nanodevices for optoelectronic applications

Dynamic Visualization of Axial p–n Junctions in Single Gallium Nitride Nanorods under Electrical Bias

We demonstrate a direct visualization method based on secondary electron (SE) imaging in scanning electron microscopy for mapping electrostatic potentials across axial semiconductor nanorod p–n junctions. It is found that the SE doping contrast can be directly related to the spatial distribution of electrostatic potential across the axial nanorod p–n junction. In contrast to the conventional SE doping contrast achieved for planar p–n junctions, the quasi-one-dimensional geometry of nanorods allows for high-resolution, versatile SE imaging under high accelerating voltage, long working distance conditions. Furthermore, we are able to delineate the electric field profiles across the axial nanorod p–n junction as well as depletion widths at different reverse biases. By using standard p<i>–</i>n junction theory and secondary ion mass spectroscopy, the carrier concentrations of p- and n-regions can be further extracted from the depletion widths under reverse biasing conditions. This direct imaging method enables determination of electrostatic potential variation of p–n junctions in semiconductor nanorod and nanowire devices with a spatial resolution better than 10 nm

The Growth and Optical Properties of ZnO Nanowalls

Nanowalls are novel nanostructures whose networked morphology holds potential for applications such as solar cells and gas sensors. The realization of such nanowall-based devices depends directly on a comprehensive understanding of the nanowall growth, namely, its competition with nanowire growth and the role of seed particles. We induced a morphological evolution from nanowires to nanowalls by increasing source flux during vapor transport and condensation growth. Nanowall growth kinetics indicates that their morphological dominance was driven by a time-dependent curvature of the nanowall growth facet. Nanowalls have excellent crystalline quality and strong near-band-edge luminescence and were found to grow by a combination of Au- and nonassisted mechanisms, resulting in Au nanoparticles within 300 nm of the substrate whose positions were associated with the origin of green luminescence. These results imply that the growth mechanism causes nanoscale structural variations, which in turn locally affect the optical properties of nanowalls

Optoelectronic Properties of Single-Crystalline Zn₂GeO₄ Nanowires

In this work, Zn₂GeO₄ nanowires (NWs) were successfully synthesized on Si(100) substrates through carbon thermal reduction and a vapor–liquid–solid method. The NWs were of around 100 nm diameter and high aspect ratio (AR > 150). High-resolution transmission electron microscopy studies indicate that the NWs are single-crystalline with [110] growth direction. Moreover, the atomic resolution high-angle annular dark-field and bright-field images of scanning transmission electron microscopy have distinguished the different elements. They also further identified the structure of Zn₂GeO₄ and located the positions of the elements. Additionally, we have fabricated devices and measured the electrical properties of a single NW. It is remarkable that individual Zn₂GeO₄ NW devices exhibited excellent optoelectronic properties with fast switching speed under 254 nm UV illuminations. Furthermore, with short wavelength UV illumination, as we soaked Zn₂GeO₄ NWs in methyl orange solution, the methyl orange was degraded. Therefore, Zn₂GeO₄ NWs have potential applications in UV photodetectors and degradation of organic pollutants

Sequential Cation Exchange Generated Superlattice Nanowires Forming Multiple p–n Heterojunctions

Fabrication of superlattice nanowires (NWs) with precisely controlled segments normally requires sequential introduction of reagents to the growing wires at elevated temperatures and low pressure. Here we demonstrate the fabrication of superlattice NWs possessing multiple p–n heterojunctions by converting the initially formed CdS to Cu₂S NWs first and then to segmented Cu₂S–Ag₂S NWs through sequential cation exchange at low temperatures. In the formation of Cu₂S NWs, twin boundaries generated along the NWs act as the preferred sites to initiate the nucleation and growth of Ag₂S segments. Varying the immersion time of Cu₂S NWs in a AgNO₃ solution controls the Ag₂S segment length. Adjacent Cu₂S and Ag₂S segments in a NW were found to display the typical electrical behavior of a p–n junction

All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal–oxide–semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al₂O₃-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of “thresholdless” lasing for the blue plasmonic nanolaser

Complete Replacement of Metal in Metal Oxide Nanowires via Atomic Diffusion: In/ZnO Case Study

Atomic diffusion is a fundamental process that dictates material science and engineering. Direct visualization of atomic diffusion process in ultrahigh vacuum in situ TEM could comprehend the fundamental information about metal–semiconductor interface dynamics, phase transitions, and different nanostructure growth phenomenon. Here, we demonstrate the in situ TEM observations of the complete replacement of ZnO nanowire by indium with different growth directions. In situ TEM analyses reveal that the diffusion processes strongly depend and are dominated by the interface dynamics between indium and ZnO. The diffusion exhibited a distinct ledge migration by surface diffusion at [001]-ZnO while continuous migration with slight/no ledges by inner diffusion at [100]-ZnO. The process is explained based on thermodynamic evaluation and growth kinetics. The results present the potential possibilities to completely replace metal-oxide semiconductors with metal nanowires without oxidation and form crystalline metal nanowires with precise epitaxial metal–semiconductor atomic interface. Formation of such single crystalline metal nanowire without oxidation by diffusion to the metal oxide is unique and is crucial in nanodevice performances, which is rather challenging from a manufacturing perspective of 1D nanodevices

Aqueous Synthesis of Concave Rh Nanotetrahedra with Defect-Rich Surfaces: Insights into Growth‑, Defect‑, and Plasmon-Enhanced Catalytic Energy Conversion

The control of morphology in the synthesis of Rh nanocrystals can be used to precisely tailor the electronic surface structure; this in turn directly influences their performance in catalysis applications. Many works have brought attention to the development of Rh nanostructures with low-index surfaces, but limited effort has been devoted to the study of high-index and surface defect-enriched nanocrystals as they are not favored by thermodynamics because of the involvement of high-energy surfaces and increased surface-to-volume ratios. In this work, we demonstrate an aqueous synthesis of concave Rh nanotetrahedra (CTDs) serving as efficient catalysts for energy conversion reactions. CTDs are surface defect-rich structures that form through a slow growth rate and follow the four-step model of metallic nanoparticle growth. Via the tuning of the surfactant concentration, the morphology of Rh CTDs evolved into highly excavated nanotetrahedra (HETDs) and twinned nanoparticles (TWs). Unlike the CTD surfaces with abundant adatoms and vacancies, HETDs and TWs have more regular surfaces with layered terraces. Each nanocrystal type was evaluated for methanol electrooxidation and hydrogen evolution from hydrolysis of ammonia borane, and the CTDs significantly showed the best catalytic performance because of defect enrichment, which benefits the surface reactivity of adsorbates. In addition, both CTDs and HETDs have strong absorption near the visible light region (382 and 396 nm), for which they show plasmon-enhanced performance in photocatalytic hydrogen evolution under visible light illumination. CTDs are more photoactive than HETDs, likely because of more pronounced localized surface plasmon resonance hot spots. This facile aqueous synthesis of large-surface-area, defect-rich Rh nanotetrahedra is exciting for the fields of nanosynthesis and catalysis