Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

Negative differential capacitance in n-GaN/p-Si heterojunctions

Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE). The NDC is observed at low frequencies 1 and 10 kilohertz (kHz) and disappeared at a higher testing frequency of 100 kHz. The NDC is also studied with temperature and found that it has disappeared above 323 ^°C. Current-Voltage (I-V) characteristics of n-GaN /p-Si heterojunction were measured at different temperatures and are attributed to the space-charge-limited current (SCLC). A simple model involving two quantum states is proposed to explain the observed NDC behavior.     

Surfaces and interfaces in polymer-based electronics

Research on electronics applications such as light-emitting devices for flat-panel displays, transistors, sensors and even solid state lasers based on conducting polymers is presently under way and in some cases has reached the stage of prototype production. The mechanisms for charge injection and conduction in these materials are being studied, as are the physics of luminescence and its quenching. Lately, research into controlling film morphology through self-organizing techniques also has gained interest. Though the present interest in conducting polymers mainly concerns the pristine semiconducting state, doped conducting polymers are also studied for potential use in many applications.

In this paper, we present an overview of some of the central issues in surface and interface science in the field, as well as provide our view on what may lie ahead in the future. Specifically, the importance of metal/polymer, polymer/metal and polymer/polymer interfaces is addressed. We illustrate these using polymer-based light-emitting devices, though the same type of issues appear in other polymer-based applications such as transistors and solar cells.     

Halide Perovskite Single Crystals and Nanocrystal Films as Electron Donor-Acceptor Heterojunctions

Halide perovskites are materials for future optical displays and solar cells. Electron donor-acceptor perovskite heterostructures with distinguishing halide compositions are promising for transporting and harvesting photogenerated charge carriers. Combined e-beam lithography and anion exchange are promising to develop such heterostructures but challenging to prepare multiple heterojunctions at desired locations in single crystals. We demonstrate swift laser trapping-assisted band gap engineering at the desired locations in MAPbBr₃ microrods, microplates, or nanocrystal thin films. The built-in donor-acceptor double and multi-heterojunction structures let us transport and trap photogenerated charge carriers from wide-band gap bromide to narrow-band gap iodide domains. We discuss the charge carrier transport and trapping mechanisms from the viewpoints of engineered bands and band continuity. This work offers a convenient method for designing single-, double- and multi-heterojunction donor-acceptor halide perovskites for photovoltaic, photonic, and electronic applications.     

Synergistic effect of Fe2O3/Ho2O3 Co-modified 2D-titanate heterojunctions on enhanced photocatalytic degradation

TiO₂-based nanosheets (TNSs) co-modified by Fe₂O₃ and Ho₂O₃ were synthesized by one-pot hydrothermal method using Fe(NO₃)₃ and Ho(NO₃)₃ as precursors compositing with TiO₂. The Fe₂O₃/Ho₂O₃-TNSs heterojunctions possessed a thickness of approximately 3–4 nm, large specific surface area of 210–310 cm²/g, with Fe₂O₃ and Ho₂O₃ nanoparticles highly dispersed over the surface of the nanosheets. The crystallization of the samples gradually increased with the amount of Fe₂O₃ nanoparticles, which was confirmed by the XRD, BET and Raman spectra, indicating that Ho₂O₃ and Fe₂O₃ influenced the crystallinity and structure evolution of the TNSs, besides, led to an improved the visible-light absorption. Surface photocurrent and fluorescence spectral studies revealed that the photo-generated charge carrier separation efficiency could be efficiently improved by an appropriate amount of modification. The Fe₂O₃/Ho₂O₃-TNSs exhibited synergistic effect on photocatalytic degradation of RhB as well as MO under visible light. The highest efficiency was obtained by 0.05%-Fe₂O₃/Ho₂O₃-TNSs (Fe:Ho:Ti = 0.05:1:100), which was 8.86 and 6.72 times than that of individual 1.0%-Ho₂O₃-TNSs (Ho:Ti = 1:100) and 0.05%-Fe₂O₃-TNSs (Fe:Ti = 0.05:100), respectively. The possible mechanism for enhanced visible-light-induced photocatalytic activity was proposed. Ho₂O₃ introduced in the photocatalysts may act as the hole capture while Fe₂O₃ may share the same Fermi levels with TNSs and serve as the electron capture center in the n-n-p system, which reduced the recombination rate of photo-induced electron-hole pairs.     

Band offsets at the epitaxial anatase TiO2/n-SrTiO3(0 0 1) interface

We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO₂(0 0 1)/n-SrTiO₃(0 0 1) heterojunction prepared by molecular beam epitaxy. The valence band offsets range between −0.06 ± 0.05 and +0.16 ± 0.05 eV for anatase thicknesses between 1 and 8 monolayers and three different methods of substrate surface preparation, with no systematic dependence on film thickness. The conduction band offset (CBO) varies over a comparable range by virtue of the fact that anatase and SrTiO₃ exhibit the same bandgap (∼3.2 eV). In contrast, density functional theory predicts the VBO to be +0.55 eV. The lack of agreement between theory and experiment suggests that either some unknown factor in the interface structure or composition excluded from the modeling is influencing the band offset, or that density functional theory cannot accurately calculate band offsets in these oxide materials. The small experimental band offsets have important implications for the use of this interface for fundamental investigations of surface photocatalysis. Neither electrons nor holes are likely to become trapped in the substrate and thus be unable to participate in surface photocatalytic processes.     

Self assembled nanoparticle Pb1−xFexSe/single crystal Si heterojunctions

Nanoparticle Pb_1−xFe_xSe (0.00  x  0.16) thin films have been deposited on quartz, glass and silicon substrates by chemical bath deposition technique. Structural and optical properties of the films with iron concentration 0.00  x  0.16 indicate that the films grow as single-phase Pb_1−xFe_xSe ternary alloys with rocksalt structure and with direct optical band gaps (E_g) that increase with decrease in grain size and have values larger than 0.28 eV of the bulk PbSe. Average grain size in films grown at fixed bath temperature T_b of 85 °C is observed to decrease from 72 to 22 nm whereas lattice parameter is observed to increase from 6.12 to 6.14 Å with increase in Fe concentration from x = 0.00 to x = 0.16. The observed blue shift in film materials originates from quantum confinement in the nanograins. Nanoparticle Pb_1−xFe_xSe/single crystal Si heterojunctions show rectifying behavior. On illumination of heterojunctions with visible light current is observed to increase in forward and reverse bias. This increase in current in the presence of visible light is considered to be due to carrier multiplication by Auger electron emission.     

CuPd interface charge and energy quantum entrapment: A tight-binding and XPS investigation

Materials at heterojunction interfaces demonstrate many physical and chemical properties that are indeed fascinating with mechanisms that need yet to be explored. We show herewith that the “interface charge and energy quantum entrapment due to bond order distortion and bond nature alteration” perturbs essentially the Hamiltonian and hence the binding energy of the CuPd alloy interface. Analyzing the X-ray photoelectron emission of the thermally induced evolution of the Cu 2p and Pd 3d core-level energies at the Cu-Pd interface before and after thermally alloying revealed that the Pd 3d and Cu 2p interfacial potential traps are 0.36 and 0.95 times deeper than the potential wells of the corresponding bulk constituents standing alone.     

Controllable synthesis of highly crystallized mesoporous TiO_2/WO_3 heterojunctions for acetone gas sensing

Mesoporous semiconducting metal oxides(SMOs) heterojunctions are appealing sensors for gas detecting.However,due to the different hydrolysis and condensation mechanism of every metal precursor and the contradiction between high crystallinity and high surface area,the synthesis of mesoporous SMOs heterojunctions with highly o rdered mesostructures,highly crystallized frameworks,and high surface area remains a huge challenge.In this work,we develop a novel "acid-base pair"adjusted solvent evaporation induced self-assembly(EISA) strategy to prepare highly crystallized ordered mesoporous TiO_2/WO_3(OM-TiO_2/WO_3) heterojunctions.The WCl_6 and titanium isopropoxide(TIPO) are used as the precursors,respectively,which function as the "acid-base pair",enabling the coassembly with the structure directing agent(PEO-b-PS) into highly ordered meso structures.In addition,PEO-b-PS can be converted to rigid carbon which can protect the meso structures from collapse during the crystallization process.The resultant OM-TiO_2/WO_3 heterojunctions possess primitive cubic mesostructures,large pore size(～21.1 nm),highly crystalline frameworks and surface area(～98 m~2/g).As a sensor for acetone,the obtained OM-TiO_2/WO_3 show excellent re sponse/recovery perfo rmance(3 s/5 s),good linear dependence,repeatability,selectivity,and long-term stability(35 days).     

Controlled modification of multiwalled carbon nanotubes with Zno nanostructures

Multiwalled carbon nanotubes (MWNTs) have been successfully modified with ZnO nanostructures by zinc-ammonitum complex ion covalently attached to the MWNTs through the C-N bonds. Flower-like ZnO on the tips of MWNTs and ZnO nanoparticles on the surface of MWNTs have been obtained, respectively, via adjusting the reaction time. The modified MWNTs have been characterized with X-ray diffraction, scanning electron and transmission electron microscopy. A growth mechanism has been proposed in which the soaking time plays a key role in controlling the size, morphology, and site of ZnO nanostructures. Photoluminescence properties of the as-synthesized products have also been investigated.