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.

Electrical probing of magnetic phase transition and domain wall motion in single-crystalline Mn5Ge3 nanowire.

In this Letter, the magnetic phase transition and domain wall motion in a single-crystalline Mn(5)Ge(3) nanowire were investigated by temperature-dependent magneto-transport measurements. The ferromagnetic Mn(5)Ge(3) nanowire was fabricated by fully germaniding a single-crystalline Ge nanowire through the solid-state reaction with Mn contacts upon thermal annealing at 450 °C. Temperature-dependent four-probe resistance measurements on the Mn(5)Ge(3) nanowire showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferromagnetism to paramagnetism. The transition temperature was able to be controlled by both axial and radial magnetic fields as the external magnetic field helped maintain the magnetization aligned in the Mn(5)Ge(3) nanowire. Near the magnetic phase transition, the critical behavior in the 1D system was characterized by a power-law relation with a critical exponent of α = 0.07 ± 0.01. Besides, another interesting feature was revealed as a cusp at about 67 K in the first-order derivative of the nanowire resistance, which was attributed to a possible magnetic transition between two noncollinear and collinear ferromagnetic states in the Mn(5)Ge(3) lattice. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric, and stepwise axial magnetoresistance of the Mn(5)Ge(3) nanowire. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn(5)Ge(3) nanowire and the annihilation of domain walls driven by the magnetic field. The Kurkijärvi model was used to describe the domain wall depinning as thermally assisted escape from a single energy barrier, and the fitting on the temperature-dependent depinning magnetic fields yielded an energy barrier of 0.166 eV.

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.

Ferromagnetic germanide in Ge nanowire transistors for spintronics application.

To explore spintronics applications for Ge nanowire heterostructures formed by thermal annealing, it is critical to develop a ferromagnetic germanide with high Curie temperature and take advantage of the high-quality interface between Ge and the formed ferromagnetic germanide. In this work, we report, for the first time, the formation and characterization of Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire transistors, in which the room-temperature ferromagnetic germanide was found through the solid-state reaction between a single-crystalline Ge nanowire and Mn contact pads upon thermal annealing. The atomically clean interface between Mn(5)Ge(3) and Ge with a relatively small lattice mismatch of 10.6% indicates that Mn(5)Ge(3) is a high-quality ferromagnetic contact to Ge. Temperature-dependent I-V measurements on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire heterostructure reveal a Schottky barrier height of 0.25 eV for the Mn(5)Ge(3) contact to p-type Ge. The Ge nanowire field-effect transistors built on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) heterostructure exhibit a high-performance p-type behavior with a current on/off ratio close to 10(5), and a hole mobility of 150-200 cm(2)/(V s). Temperature-dependent resistance of a fully germanided Mn(5)Ge(3) nanowire shows a clear transition behavior near the Curie temperature of Mn(5)Ge(3) at about 300 K. Our findings of the high-quality room-temperature ferromagnetic Mn(5)Ge(3) contact represent a promising step toward electrical spin injection into Ge nanowires and thus the realization of high-efficiency spintronic devices for room-temperature applications.