Zhong-Yi Wu

Contact to ZnO and intrinsic resistances of individual ZnO nanowires with a circular cross section

Single crystalline ZnO nanowires (NWs) with a circular cross section and ∼40nm in diameter have been synthesized and utilized to fabricate two-contact ZnO NW devices. The electrical properties of the NW devices can be categorized into two classes according to the magnitude of their room-temperature resistances. I-V curves of low-resistance devices exhibit downward bending features and their temperature dependent resistances demonstrate thermal activation transport in the ZnO NWs. The high-resistance NW devices can be modeled as back-to-back Schottky contacts and the electron transport through the contacts reveals a variable-range-hopping mechanism.

Single crystalline ZnS nanotubes and their structural degradation under electron beam irradiation

ZnS nanotubes were synthesized using wet-chemistry method. These nanotubes appear to be extremely unstable under electron beam irradiation. Time dependent transmission electron diffraction patterns disclose the appearance of additional diffraction spots that belong to ZnO, with the prolonged e-beam irradiation duration. The experimental results suggest that displacement damage followed by oxidation is mainly responsible for the structural degradation of these ZnS nanotubes.

Cross-sectional shape modulation of physical properties in ZnO and Zn1−xCoxO nanowires

We have prepared comparable-diameter ZnO and Zn1- xCoxO nanowires with both circular and hexagonal cross-sections. The average diameters are ~113 and ~134 nm for cylindrical and hexagonal nanowires, respectively. The as-grown nanowires have been characterized via structure, electrical conductivity and photoluminescence (PL) spectrum measurements. Pure ZnO nanowires were Co-ion implanted to make magnetic Zn1- xCoxO nanowires for magnetization studies. Bumpier edge surfaces on a nanometre scale, higher densities of stacking faults and a bending feature along the growth direction have been found in cylindrical ZnO nanowires. As compared with hexagonal nanowires, we have observed relatively higher conductivities in cylindrical nanowires, which implied large numbers of shallow donors existing in the latter nanowires. The cylindrical ZnO nanowires also displayed intensified green defect emission and considerably more stacking faults in the crystalline structure. In addition, we have found increased magnetization and stronger ferromagnetic ordering in cylindrical than in hexagonal Zn1-xCoxO nanowires, and have experimentally identified that the point defects of either Zn interstitials or O vacancies played governing roles in ferromagnetism. We conclude that the cross-sectional shape effect originating a varied point defect density can profoundly modulate the structural, electrical, optical as well as magnetic properties of ZnO and Zn1-xCoxO nanowires.

Orbital susceptibilities of PbSe quantum dots.

Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size.

Nanocontact resistance and structural disorder induced resistivity variation in metallic metal-oxide nanowires

Several systems of metallic metal-oxide nanowires (NWs), including pure RuO2 and as-implanted and annealed Ru(0.98)Cu(0.02)O2 and Ru(0.93)Cu(0.07)O2 NWs, have been employed in two-probe electrical characterizations by using a transmission electron microscope-scanning tunneling microscope technique with a gold tip. Thermal, mechanical, and electron beam exposing treatments are consecutively applied to reduce the electrical contact resistance, generated from the interface between the NW and the gold tip, so as to evaluate the intrinsic NW resistance. It is found that the residual contact resistance cannot be entirely removed. For each system of metallic metal-oxide NWs, several tens of NWs are applied to electrical characterizations and the total resistances unveil a linear dependence on the ratio of the length to the area of the NWs. As a result, the average resistivity and the contact resistance of the metallic metal-oxide NWs could be evaluated at room temperatures. The average resistivities of pure RuO2 NWs agree well with the results obtained from standard two- and four-probe electrical-transport measurements. In addition, the as-implanted Cu-RuO2 NWs reveal disordered crystalline structures in high-resolution TEM images and give higher resistivities in comparison with that of pure RuO2 NWs. The residual contact resistances of all kinds of metallic metal-oxide NWs unveil, more surprisingly, an approximation value of several kilohms, even though the average resistivities of these NWs change by more than one order of magnitude. It is argued that the ductile gold tip makes one or more soft contacts on the stiff metal-oxide NWs with nanometer roughness and the nanocontacts on the NWs contribute to the electrical contact resistance.

Characterization of Room-Temperature Ferromagnetic Zn1-xCoxO Nanowires

The manipulation and detection of an electrons charge and, simultaneously, its spin orientation in electronic devices have been developed to be a new emerging field of spintronics (or magnetoelectronics) (Prinz, 98; Wolf et al., 2001). At present, the most notable spintronic applications could be the hard disk read heads and the magnetic random access memory which are based on metal magnetic materials and are assorted into metallic spintronic devices. The establishment of metallic spintronics might be ascribed to a discovery of giant magnetoresistance (Baibich et al., 1988; Binasch et al., 1989) and, subsequently, understanding and exercise of a spin-valve scheme (Moodera et al., 1995), and tunnelling magnetoresistance (Dieny et al., 1991) in ferromagnetic multilayers. On the other hand, in order to integrate with the modern industrial technology, new semiconductor materials such as diluted magnetic semiconductors (DMSs) (Furdyna, 1988), also known as ferromagnetic semiconductors (Ohno, 1998), have been searched for a supply of a spinpolarized carrier source. Those devices building on a transport of spin current in semiconductors are categorized into semiconductor spintronics. Spin injection, maintenance of a spin coherence, spin detection, and a spin carrier source in semiconductors are all important issues for semiconductor spintronics. The DMSs, based on host materials of II-VI and IV-VI semiconductors, have been studied for several decades. Although the indirect exchange mechanisms between 3d transition metal dopants in these semiconductors have been inspected experimentally and discussed theoretically (Story et al., 1986; Sawicki et al., 1986; Furdyna, 1988) for a long time, the Curie temperature (TC), below which a spontaneous magnetization and a spin-polarized current in the DMSs arise, was too low to be capable of employment. Until recent advance in III-V DMSs of (In,Mn)As and (Ga,Mn)As (Ohno et al., 1996), TC’s of some new DMSs such as (Ga,Mn)As have been raised up to ~110 K. These new III-V DMS materials were exploited to demonstrate tunneling magnetoresistance in (Ga,Mn)As ultrathin heterostructures (Hayashi et al., 1999), electrical spin injection in a ferromagnetic semiconductor heterostructure (Ohno et al., 1999), electric-field control of ferromagnetism (Ohno et al., 2000}, electrical manipulation of magnetization reversal (Chiba et al., 2003), and current-induced domainwall switching (Yamanouchi et al., 2004). On the other hand, the other approach of spincurrent injection into semiconductors from ferromagnetic metals has recently been achieved, so as to realize semiconductor spintronics at room temperature.

Determination of contact and intrinsic nanowire resistivity in two-contact ZnO nanowire devices

Cylindrical ZnO nanowires were synthesized to fabricate two-contact ZnO nanowire devices with the same separation distance between the two contact electrodes. Electrical properties including temperature dependence of resistance and I-V curves were recorded. According to distinct electrical behaviors and room-temperature resistance, ZnO nanowire devices can be categorized into three different types exhibiting either contact or intrinsic NW attributes.