Magnetic MoS₂ Interface Monolayer on a CdS Nanowire by Cation Exchange

MoS₂ atomic layers have recently attracted much interest because of their two-dimensional structure as well as tunable optical, electrical, and mechanical properties for next-generation electronic and electro-optical devices. Here we have achieved facile fabrication of MoS₂ thin films on CdS nanowires by cation exchange in solution at room temperature and importantly observed their extraordinary magnetic properties. We establish the atomic structure of the MoS₂/CdS heterostructure by taking atomic images of the MoS₂/CdS interface as well as performing first-principles density functional geometry optimizations and scanning transmission electron microscopy annular dark field image simulations. Furthermore, our first-principles density functional calculations for the MoS₂/CdS heterostructure reveal that the magnetism in the MoS₂/CdS heterostructure stems from the ferromagnetic MoS₂ monolayer next to the MoS₂/CdS interface. The ferromagnetism is attributed to the partial occupation of the Mo d_{<i>x</i>²–<i>y</i>²}/d_<i>xy</i> conduction band in the interfacial MoS₂ monolayer caused by the mixed covalent–ionic bonding among the MoS₂ and CdS monolayers near the MoS₂/CdS interface. These findings of the ferromagnetic MoS₂ monolayer with large spin polarization at the MoS₂/semiconductor interface suggest a new route for fabrication of the transition metal dichalcogenide-based magnetic semiconductor multilayers for applications in spintronic devices

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

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

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 Mn₅Ge₃/Ge/Mn₅Ge₃ 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 Mn₅Ge₃ and Ge with a relatively small lattice mismatch of 10.6% indicates that Mn₅Ge₃ is a high-quality ferromagnetic contact to Ge. Temperature-dependent <i>I</i>–<i>V</i> measurements on the Mn₅Ge₃/Ge/Mn₅Ge₃ nanowire heterostructure reveal a Schottky barrier height of 0.25 eV for the Mn₅Ge₃ contact to <i>p</i>-type Ge. The Ge nanowire field-effect transistors built on the Mn₅Ge₃/Ge/Mn₅Ge₃ heterostructure exhibit a high-performance <i>p</i>-type behavior with a current on/off ratio close to 10⁵, and a hole mobility of 150–200 cm²/(V s). Temperature-dependent resistance of a fully germanided Mn₅Ge₃ nanowire shows a clear transition behavior near the Curie temperature of Mn₅Ge₃ at about 300 K. Our findings of the high-quality room-temperature ferromagnetic Mn₅Ge₃ 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

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 Mn₅Ge₃ nanowire were investigated by temperature-dependent magneto-transport measurements. The ferromagnetic Mn₅Ge₃ 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 Mn₅Ge₃ 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 Mn₅Ge₃ 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 Mn₅Ge₃ lattice. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric, and stepwise axial magnetoresistance of the Mn₅Ge₃ nanowire. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn₅Ge₃ 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