Hongwu Zhao

Photogenerated Intrinsic Free Carriers in Small-molecule Organic Semiconductors Visualized by Ultrafast Spectroscopy.

Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π−π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly.

Electroforming and endurance behavior of Al/Pr0.7Ca0.3MnO3/Pt devices

We have investigated the electroforming (EF) and resistive switching (RS) of Al/Pr0.7Ca0.3MnO3 (PCMO)/Pt devices by using high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy combined with transport measurement. The device prefers EF with positive bias with respect to Pt electrode and their endurance is enhanced with the chemically reactive Al electrode. The presence of an Al2O3−δ layer in Al/PCMO junction indicates that the oxidization and reduction near the Al/PCMO interface play a key role in the RS.

Asymmetry of coercivity dependence on temperature in exchange-biased FeMn/Co bilayers

The temperature dependence of the coercivity in biased FeMn/Co bilayers is investigated. An asymmetric behavior of the left and right coercivity with varying temperature is observed. The asymmetry can be understood by taking into account the variation of the spin arrangements in the antiferromagnetic layer. The calculated results are in agreement with the experimental ones qualitatively. All these results suggest that the magnetization reversal mechanisms are different for increasing and decreasing fields.

Characteristic of interface effect in Cu–C60 granular films

In this letter, Cu–C60 granular film is prepared with coevaporation method at room temperature. The conductance of the film is measured by in situ method, and its microstructure is characterized by transmission electron microscopy. The charge transfer from Cu to C60 is investigated with Raman spectroscopy. The results indicate that the sample has the uniformly granular microstructure. The interaction between C60 and Cu at the Cu–C60 interfaces, which significantly affects the orientational order–disorder phase transition of C60 and induces the phase transition of C60 in the temperature range from 219 to 248 K. The mechanism of the characteristic of such phase transition is discussed.

Effect of TaOx thickness on the resistive switching of Ta/Pr0.7Ca0.3MnO3/Pt films

The influence of interfacial structure on the resistance switching behavior of Ta/Pr0.7Ca0.3MnO3/Pt films was investigated by varying the reactive Ta electrode thickness. Structure and component analyses revealed that a TaOx layer formed at the interface and its thickness increased with the Ta thickness in the thin region while staying the same in the thick region. The similar thickness dependences of the negative differential resistance and resistance switching characteristics were observed and interpreted by the TaOx thickness dependent oxidization and reduction reaction across the interfacial region. This study demonstrates that the resistance switching characteristics could be improved by suitable interfacial engineering.

Element-specific study of epitaxial NiO/Ag/CoO/Fe films grown on vicinal Ag(001) using photoemission electron microscopy

NiO/Ag/CoO/Fe single crystalline films are grown epitaxially on a vicinal Ag(001) substrate using molecular beam epitaxy and investigated by photoemission electron microscopy. We find that after zero-field cooling, the in-plane Fe magnetization switches from parallel to perpendicular direction of the atomic steps of the vicinal surface at thinner CoO thickness but remains in its original direction parallel to the steps at thicker CoO thickness. CoO and NiO domain imaging result shows that both CoO/Fe and NiO/CoO spins are perpendicularly coupled, suggesting that the Fe magnetization switching may be associated with the rotatable-frozen spin transition of the CoO film.

Low-Frequency Noise Characteristics of Zinc-Oxide-Film-Based Photoconductive Detectors

Low-frequency current noise measurements were performed on ZnO-based photoconductive detectors, and 1/f noise characteristics were observed both in the dark and under ultraviolet illumination. The noise measured under illumination is approximately one order of magnitude higher than the dark noise. In the higher voltage range, a knee-like voltage dependence of the photocurrent noise was observed. The enhanced photocurrent noise is attributed to the random trapping-detrapping processes of the oxygen-related hole-trap states at the grain boundary interface. The anomalous voltage dependence of the photocurrent noise in the higher voltage range indicated the presence of the inhomogeneous distribution of the oxygen-related hole-trap states

Impact of interfacial resistance switching on thermoelectric effect of Nb-doped SrTiO3 single crystalline

The thermoelectric properties of the bistable resistance states in Nb doped SrTiO3 single crystal have been investigated. The Seebeck coefficients for both low and high resistance states change linearly with temperature. The three-terminals contrast measurement demonstrates that a large fraction of the voltage drop is applied at the tiny volume near the bottom interface between the electrode and the oxide bulk. Therefore, the metallic oxide bulk plays a dominant role in the temperature dependence of Seebeck coefficients. The thermoelectric properties of new resistance switching (RS) devices with minimized non-RS volume could be exploited for the RS mechanism and novel applications.

Filamentary resistance switching in phthalocyanine thin films observed by electroluminescence

Metal phthalocyanine heterojunctions with highly stable resistance switching characteristics have been prepared and the switching mechanism has been further investigated by electroluminescence (EL). The point-like EL emission has been observed during switching, demonstrating that the filamentary conduction gives rise to resistance switching. Furthermore, the high correlativity between EL emission sites and domain boundaries of phthalocyanine films has been established, which indicates that the switching process is favorable to occur within the disordered region of organic films. A band-based model has been proposed to describe the EL associated electrical switching mechanism.

In situ observation of conducting filament in NiO memristive devices by electroluminescence

By synchronously measuring the electroluminescence (EL) of Pt/NiO/Pt unipolar resistive switching (RS) devices during switching, we have nondestructively observed the dynamic evolution of conducting filaments (CFs) across the device. We demonstrate that both the random distribution and the subtle change in the chemical composition of CFs contribute to fluctuation of switching parameters. The EL emission during RS is attributed to the radiative transition through oxygen vacancy related defect levels of NiO rather than from the thermal radiation. Our findings offer a method to investigate the RS mechanism which further can be utilized to improve the stability and scalability of RS devices.