Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires

Phase-engineered synthesis of atomically thin te single crystals with high on-state currents

Multiple structural phases of tellurium (Te) have opened up various opportunities for the development of two-dimensional (2D) electronics and optoelectronics. However, the phase-engineered synthesis of 2D Te at the atomic level remains a substantial challenge. Herein, we design an atomic cluster density and interface-guided multiple control strategy for phase- and thickness-controlled synthesis of α-Te nanosheets and β-Te nanoribbons (from monolayer to tens of μm) on WS2 substrates. As the thickness decreases, the α-Te nanosheets exhibit a transition from metallic to n-type semiconducting properties. On the other hand, the β-Te nanoribbons remain p-type semiconductors with an ON-state current density (ION) up to ~ 1527 μA μm-1 and a mobility as high as ~ 690.7 cm2 V-1 s-1 at room temperature. Both Te phases exhibit good air stability after several months. Furthermore, short-channel (down to 46 nm) β-Te nanoribbon transistors exhibit remarkable electrical properties (ION = ~ 1270 μA μm-1 and ON-state resistance down to 0.63 kΩ μm) at Vds = 1 V.

Metal-insulator transition in single crystalline ZnO nanowires

In this work, we report on the metal-insulator transition and electronic transport properties of single crystalline ZnO nanowires synthetized by means of Chemical Vapor Deposition. After evaluating the effect of adsorbed species on transport properties, the thermally activated conduction mechanism was investigated by temperature-dependent measurements in the range 81.7-250 K revealing that the electronic transport mechanism in these nanostructures is in good agreement with the presence of two thermally activated conduction channels. More importantly, it was observed that the electrical properties of ZnO NWs can be tuned from semiconducting to metallic-like as a function of temperature with a metal-to-insulator transition (MIT) observed at a critical temperature above room temperature (T _c ∼ 365 K). Charge density and mobility were investigated by means of field effect measurements in NW field-effect transistor configuration. Results evidenced that the peculiar electronic transport properties of ZnO NWs are related to the high intrinsic n-type doping of these nanostructures that is responsible, at room temperature, of a charge carrier density that lays just below the critical concentration for the MIT. This work shows that native defects, Coulomb interactions and surface states influenced by adsorbed species can significantly influence charge transport in NWs.

Long-Range Magnetic Order in Oxide Quantum Wells Hosting Two-Dimensional Electron Gases

To incorporate spintronics functionalities into two-dimensional devices, it is strongly desired to get two-dimensional electron gases (2DEGs) with high spin polarization. Unfortunately, the magnetic characteristics of the typical 2DEG at the LaAlO₃/SrTiO₃ interface are very weak due to the nonmagnetic character of SrTiO₃ and LaAlO₃. While most of the previous works focused on perovskite oxides, here, we extended the exploration for magnetic 2DEG beyond the scope of perovskite combinations, composing 2DEG with SrTiO₃ and NaCl-structured EuO that owns a large saturation magnetization and a fairly high Curie temperature. We obtained the 2DEGs that show long-range magnetic order and thus unusual behaviors marked by isotropic butterfly shaped magnetoresistance and remarkable anomalous Hall effect. We found evidence for the presence of more conductive domain walls than elsewhere in the oxide layer where the 2DEG resides. More than that, a relation between interfacial magnetism and carrier density is established. On this basis, the intermediate magnetic states between short-range and long-range ordered states can be achieved. The present work provides guidance for the design of high-performance magnetic 2DEGs.

Influence of free charge carrier density on the magnetic behavior of (Zn,Co)O thin film studied by Field Effect modulation of magnetotransport

The origin of (ferro)magnetic ordering in transition metal doped ZnO is a still open question. For applications it is fundamental to establish if it arises from magnetically ordered impurity clusters embedded into the semiconducting matrix or if it originates from ordering of magnetic ions dilute into the host lattice. In this latter case, a reciprocal effect of the magnetic exchange on the charge carriers is expected, offering many possibilities for spintronics applications. In this paper we report on the relationship between magnetic properties and free charge density investigated by using Zinc oxide based field effect transistors, in which the charge carrier density is modulated by more than 4 order of magnitude, from 1016 to 1020 e-/cm3. The magnetotransport properties are employed to probe the magnetic status of the channel both in pure and cobalt doped zinc oxide transistors. We find that it is widely possible to control the magnetic scattering rates by field effect. We believe that this finding is a consequence of the modulation of magnetization and carrier spin polarization by the electric field. The observed effects can be explained by the change in size of bound magnetic polarons that induces a percolation magnetic ordering in the sample.

Oleylamine-assisted and temperature-controlled synthesis of ZnO nanoparticles and their application in encryption

A temperature-controlled synthesis process for ZnO nanoparticles (NPs) with the assist of oleylamine (OAm) has been demonstrated, and the ZnO NPs show bright fluorescence under ultraviolet illumination. In this process, zinc nitrate was firstly converted to zinc nitrate hydroxide (Zn₅(OH)₈(NO₃)₂) sheets with the assist of OAm, then the Zn₅(OH)₈(NO₃)₂ was decomposed into fluorescent ZnO NPs by increasing the ambient temperature. Furthermore, information encryption has been realized based on this process. For encryption, the encrypted information cannot be observed, while the encrypted information appears when they are proceeded in the temperature of 120 °C for about one minute. The results shown in this work provide a controllable way for the synthesis of ZnO NPs by adjusting the reaction temperature, and this may inspire wide applications of ZnO in information encryption.

Tunable electron heating induced giant magnetoresistance in the high mobility GaAs/AlGaAs 2D electron system

Electron-heating induced by a tunable, supplementary dc-current (I_dc) helps to vary the observed magnetoresistance in the high mobility GaAs/AlGaAs 2D electron system. The magnetoresistance at B = 0.3 T is shown to progressively change from positive to negative with increasing I_dc, yielding negative giant-magnetoresistance at the lowest temperature and highest I_dc. A two-term Drude model successfully fits the data at all I_dc and T. The results indicate that carrier heating modifies a conductivity correction σ₁, which undergoes sign reversal from positive to negative with increasing I_dc, and this is responsible for the observed crossover from positive- to negative- magnetoresistance, respectively, at the highest B.

Magnetoresistance manipulation and sign reversal in Mn-doped ZnO nanowires

We report magnetoresistance (MR) manipulation and sign reversal induced by carrier concentration modulation in Mn-doped ZnO nanowires. At low temperatures positive magnetoresistance was initially observed. When the carrier concentration was increased through the application of a gate voltage, the magnetoresistance also increased and reached a maximum value. However, further increasing the carrier concentration caused the MR to decrease, and eventually an MR sign reversal from positive to negative was observed. An MR change from a maximum positive value of 25% to a minimum negative value of 7% was observed at 5 K and 50 KOe. The observed MR behavior was modeled by considering combined effects of quantum correction to carrier conductivity and bound magnetic polarons. This work could provide important insights into the mechanisms that govern magnetotransport in dilute magnetic oxides, and it also demonstrated an effective approach to manipulating magnetoresistance in these materials that have important spintronic applications.