Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr_0.55(Ca_1 − ySr_y)_0.45MnO₃ (y = 0.25), SrTiO₃, and Ti_1 − xCo_xO₂ (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.     

Study on preparation and characterization of MOF based lanthanide doped luminescent coordination polymers

Coordination polymers (metal–organic frameworks or MOFs) offer the opportunity for fine-tuning the luminescence behavior because of the possibility to entrap in the network pores molecules that can influence the lanthanide (Ln) emission. In this study, Zn (II) and polycarboxylate based MOFs were first pre-formed by solvothermal method, then considered as host-matrix for in situ doping of low-input concentration of Eu³⁺ and Tb³⁺ (two most commonly used lanthanides in life science assays), and afterwards lanthanide doped luminescent materials were synthesized. Different characterizations (X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS)) were carried out to confirm accordingly MOF's crystallinity, the structure and chemical composition. The study on luminescent properties of the material has revealed an efficient energy transfer from the ligand excited states to the Eu³⁺ and Tb³⁺ f-excited states. With quite low input concentrations (8–15%) of doped rare earth ions, these complexes displayed intense emissions at room temperature and proved to be good candidates for red and green emitter luminescent materials. Generally, this design concept can be extended for the preparation of other rare earth coordination polymers.     

一种用于飞行时间质谱的激光光电子枪

基于光电效应原理.我们设计了一个新型脉冲激光光电子枪,它可以作为飞行时间质谱的电子碰撞电离的电离源。考察了这个电子枪产生电子束的密度与激光能量的关系,发现控制激光的能量,可以在4个量级范围内调节电子束的强度。这个电子束的脉宽与激光的脉竞相当,电子束的电流比一般热阴极电子束高几个量级,因此电高效率很高,是飞行时间质谱仪的一个理想的电离手段。最后介绍了这个电子束在惰性气体高价离子的产生方面及混合气体的分析方面的应用。     

Photoemission Study of Perovskite-Type Manganites with Stripe Ordering

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E _F) as functions of temperature and carrier concentration in Pr_1−x Ca _x MnO₃ (PCMO) and Nd_1−x Sr _x MnO₃ (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the E _F of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at E _F even in the paramagnetic insulating phase above the CO phase for x≳0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.     

X-ray Photoelectron Spectroscopy characterization of native and RCA-treated Si (111) substrates and their influence on surface chemistry of copper phthalocyanine thin films

In this paper native and RCA-treated n- and p-doped Si(111) substrates and ultra-thin 16-nm copper phthalocyanine (CuPc) layers deposited thereon were investigated using X-ray Photoemission Spectroscopy and Angle-Resolved X-ray Photoemission Spectroscopy. The oxide layer thickness was determined to be 1.3 nm on the RCA-treated substrates and 0.8 nm on the native ones. The analysis of substrate carbon contamination showed the existence of C-H, C-OH and COOH components on all substrates. The RCA clean removes more readily the carbon components with the OH group from the n-type Si and causes the segregation of the contaminants. The initial carbon species propagate in the evaporated CuPc layer up to a thickness of about 5 nm affecting the shape of the C1s peak. Additionally, the behavior of the binding energy difference between N1s and Si2p peaks upon the CuPc growth shows that there may occur various CuPc molecule adsorption modes on investigated Si substrates. It could be a useful information, from the technological point of view, especially for low dimensional electronic device preparation.     

Characterization of bimetallic Au-Pt(111) surfaces

Structural and electronic properties of ultrathin Au films deposited on Pt(111) and annealed at different temperatures have been studied by ultraviolet photoelectron spectroscopy (UPS), photoemission of adsorbed xenon (PAX) and low energy electron diffraction (LEED). The LEED measurements indicate an initial pseudomorphic growth of the Au films. The UPS and PAX experiments show a strong temperature dependence of the surface morphology. The surface covered with Au at 150 K is quite rough but smoothens significantly above room temperature. At a temperature of 750 K intermixing and the formation of an Au-Pt surface alloy start at the interface. The electronic properties of this surface alloy seem to be nearly independent from the originally deposited Au amount in the investigated range of 1-10 monolayers. The removal of Au from the surface regions has also been verified by scanning tunneling microscopy. Adsorption experiments with CO as a titration agent show a significantly lower affinity of the Au-Pt surface alloy in comparison with the clean Pt surface.     

Na、O在n-GaAs(100)-(4×1)表面吸附以及共吸附的光电子谱研究

利用紫外光电子谱,对Na、O在n型GaAs(100)-(4×1)表面吸附及共吸咐作了详细研究。结果表明:随着GaAs(100)表面Na覆盖度的变化,其表面呈现不同的能带弯曲。当Na的覆盖度很低时(通常认为θ≤0.02ML).n-GaAs(100)表面能带向下弯曲量达0.3eV。但随着覆盖度的增加,最后费米能级钉扎在价带顶以上0.8eV处。另外,O在n-GaAs(100)-(4×1)表面吸附。暴露量达10~5L时,导致了向上0.2eV的能带弯曲。如果在吸氧的表面继续蒸Na,则其费米能级钉扎在价带顶以上1.0eV处。同时还可看到,由于Na存在,O_2在n型GaAs(100)-(4×1)表面有粘附系数将大大地提高。     

Energy level alignment and interactive spin polarization at organic/ferromagnetic metal interfaces for organic spintronics

Energy level alignment and spin polarization at tetracyanoquinodimethane/Fe and acridine orange base/Fe interfaces are investigated by means of photoelectron spectroscopy and X-ray magnetic circular dichroism (XMCD), respectively, to explore their potential application in organic spintronics. Interface dipoles are observed at both hybrid interfaces, and the work function of Fe is increased by 0.7 eV for the tetracyanoquinodimethane (TCNQ) case, while it is decreased by 1.2 eV for the acridine orange base (AOB) case. According to XMCD results, TCNQ molecule has little influence on the spin polarization of Fe surface. In contrast, AOB molecule reduces the interfacial spin polarization of Fe significantly. Induced spin polarization of the two organic molecules at the interfaces is not observed. The results reveal the necessity of investigating the magnetic property changes of both the OSC and the FM during the process of energy level alignment engineering.     

Energy band alignment at interfaces of semiconducting oxides: A review of experimental determination using photoelectron spectroscopy and comparison with theoretical predictions by the electron affinity rule, charge neutrality levels, and the common anion rule

The energy band alignment at interfaces of semiconducting oxides is of direct relevance for the electrical function of electronic devices made with such materials. The most important quantities of the interface determined by band alignment are the barrier heights for charge transport given by the Fermi level position at the interface and the band discontinuities. Different models for predicting energy band alignment are available in literature. These include the vacuum level alignment (electron affinity rule), branch point or charge neutrality level alignment governed by induced gap states, and an alignment based on the orbital contributions to the density of states (common anion rule). The energy band alignment at interfaces of conducting oxides, which have been experimentally determined using photoelectron spectroscopy with in situ sample preparation, are presented. The materials considered include transparent conducting oxides like In₂O₃, SnO₂, ZnO, and Cu₂O, dielectric and ferroelectric perovskites like (Ba,Sr)TiO₃ and Pb(Zr,Ti)O₃, and insulators like Al₂O₃. Interface formation with different contact partners including metals, conducting and insulating oxides are addressed. The discussion focuses on the energy band alignment between different oxides. A good estimate of the band alignment is derived by considering the density of states of the materials involved.     

Fractal pattern formation in thermal grooving at grain boundaries in Ag films on Si(111) surfaces

Growth of Ag films on Br- and H-passivated Si(111) surfaces and the annealing behaviour have been investigated by Rutherford backscattering spectrometry, scanning electron microscopy and photoemission electron microscopy techniques. Upon annealing the phenomenon of thermal grooving was observed in the Ag films. Depending on the annealing temperature, at an intermediate annealing time Ag depletion (evaporation) from the grain boundaries produces fractal patterns of Ag-depleted regions. Continued annealing eventually produces a percolated network of Ag-depleted regions (thermal grooves) along the grain boundaries and isolated Ag grains appear as the depth of the grooves reaches the substrate. For the fractal structures produced by thermal grooving, the fractal dimension has been estimated to be 1.60 ± 0.04. Observation of a fractal pattern in thermal grooving was not hitherto reported. A thorough analysis of the experimental results has been carried out in the context of current theories. These theories are inadequate to describe the experimental results.