Free carrier-mediated ferromagnetism in nonmagnetic ion (Bi-Li) codoped ZnO nanowires.

Non-magnetic dopants and p-type materials are attractive choices to explore the mechanism and origin of room-temperature defect-based ferromagnetism in metal oxide-based DMSs. In this study, we performed comprehensive transport, magnetic, structural, optical, and compositional as well as DFT studies of pristine, Li-doped, and Bi-Li codoped vertically aligned ZnO NW films to explore the mechanism and origin of ferromagnetism. We used a simple solution process to synthesize a wurtzite structure and vertically aligned ZnO NWs on a Si substrate. The doping, high crystallinity, and vertical alignment along the 002 planes were evidenced through HRTEM, FESEM, and XRD measurements. The XPS analysis confirmed the +1 and +3 states of Li and Bi, respectively. Moreover, Raman analysis also depicted the characteristic peaks of ZnO NWs at 98.31 cm-1 and 437.71 cm-1. The PL studies of doped NWs showed a typical NBE peak of ZnO at ∼395 nm along with a sub-gap defect-related broad peak at ∼504 nm indicating the presence of defects due to doping. The pure ZnO NW samples showed negligible saturation magnetization (Ms) at room temperature while the saturation magnetization was observed to increase with Li-doping and reduced with Bi-Li codoping. According to the Hall studies the pure ZnO NW film showed n-type conductivity, while all doped and codoped samples showed p-type conductivity. The hole concentration was observed to increase with Li-doping and decrease with Bi-Li codoping showing similar behavior to that of the Ms value, thereby suggesting a direct correlation between Ms and carrier concentration. The I-V properties showed a similar trend to that of carrier concentration and Ms. Our DFT studies showed that magnetization increased by Li doping and reduced by Li-Bi codoping in defective ZnO crystals by replacing Zn with Li and Bi atoms at the Zn site. Overall, our studies highlight the immense potential of hole-mediated Bi-Li codoped ZnO NW devices which are expected to play a pivotal role in developing spintronic devices.

Trap density probing on top-gate MoS2 nanosheet field-effect transistors by photo-excited charge collection spectroscopy.

Two-dimensional (2D) molybdenum disulfide (MoS2) field-effect transistors (FETs) have been extensively studied, but most of the FETs with gate insulators have displayed negative threshold voltage values, which indicates the presence of interfacial traps both shallow and deep in energy level. Despite such interface trap issues, reports on trap densities in MoS2 are quite limited. Here, we probed top-gate MoS2 FETs with two- (2L), three- (3L), and four-layer (4L) MoS2/dielectric interfaces to quantify deep-level interface trap densities by photo-excited charge collection spectroscopy (PECCS), and reported the result that deep-level trap densities over 10(12) cm(-2) may exist in the interface and bulk MoS2 near the interface. Transfer curve hysteresis and PECCS measurements show that shallow traps and deep traps are not that different in density order from each other. We conclude that our PECCS analysis distinguishably provides valuable information on deep level interface/bulk trap densities in 2D-based FETs.

Non-classical logic inverter coupling a ZnO nanowire-based Schottky barrier transistor and adjacent Schottky diode.

On a single ZnO nanowire (NW), we fabricated an inverter-type device comprising a Schottky diode (SD) and field-effect transistor (FET), aiming at 1-dimensional (1D) electronic circuits with low power consumption. The SD and adjacent FET worked respectively as the load and driver, so that voltage signals could be easily extracted as the output. In addition, NW FET with a transparent conducting oxide as top gate turned out to be very photosensitive, although ZnO NW SD was blind to visible light. Based on this, we could achieve an array of photo-inverter cells on one NW. Our non-classical inverter is regarded as quite practical for both logic and photo-sensing due to its performance as well as simple device configuration.

A ZnO nanowire-based photo-inverter with pulse-induced fast recovery.

We demonstrate a fast response photo-inverter comprised of one transparent gated ZnO nanowire field-effect transistor (FET) and one opaque FET respectively as the driver and load. Under ultraviolet (UV) light the transfer curve of the transparent gate FET shifts to the negative side and so does the voltage transfer curve (VTC) of the inverter. After termination of UV exposure the recovery of photo-induced current takes a long time in general. This persistent photoconductivity (PPC) is due to hole trapping on the surface of ZnO NWs. Here, we used a positive voltage short pulse after UV exposure, for the first time resolving the PPC issue in nanowire-based photo-detectors by accumulating electrons at the ZnO/dielectric interface. We found that a pulse duration as small as 200 ns was sufficient to reach a full recovery to the dark state from the UV induced state, realizing a fast UV detector with a voltage output.

Photoelectric probing of the interfacial trap density-of-states in ZnO nanowire field-effect transistors.

We have fabricated transparent top-gate ZnO nanowire (NW) field effect transistors (FETs) on glass and measured their trap density-of-states (DOS) at the dielectric/ZnO NW interface with monochromatic photon beams during their operation. Our photon-probe method showed clear signatures of charge trap DOS at the interface, located near 2.3, 2.7, and 2.9 eV below the conduction band edge. The DOS information was utilized for the photo-detecting application of our transparent NW-FETs, which demonstrated fast and sensitive photo-detection of visible lights.