The energy level alignment that occurs at the interfaces in planar-hetero structured perovskite photovoltaic devices strongly influences the charge transport across the interface, and thus plays a crucial role in overall device performance. To directly observe the energy level alignment requires pristine homogeneous surfaces that are free of contamination including adventitious carbon. Co-evaporation offers the ability to grow perovskite thin films in-situ, and the method involves thermally evaporating the perovskite precursors such as PbI2 and CH3NH3I. Early reports have shown that the perovskite film formation and stoichiometry are problematic at ultralow coverages. In particular, it was reported that there was excessive PbI2 and a deficiency in CH3NH3I. Using photoemission spectroscopy, we investigated the perovskite precursor PbI2 on gold and highly oriented pyrolytic graphite (HOPG) surfaces. Results show that the nature of the surface and the deposition conditions can strongly influence the film formation. Excessive iodine observed in the initial evaporation stages appears to be substrate dependent, and this may influence the overall energy level alignment.

Cesium lead iodide perovskite (CsPbI3) with excellent optical and electrical properties have attracted numerous academic attentions. Specifically, the black cubic phase CsPbI3 with a direct band gap of 1.74 eV has been most appropriate materials for various optoelectronic applications, especially for photovoltaic (PV), Light-Emitting Diodes (LED) and photodetector applications1. However, the preferred cubic phase of bulk CsPbI3 (α-CsPbI3) is usually only stable at high temperatures and it will undergo an immediate phase transformation to orthorhombic phase (δ-CsPbI3) after fabrication at room temperature. In this work, we have discovered a convenient CVD method to investigate the growth behavior of the cubic α-CsPbI3 film on the porous alumina substrate. The lead iodide and cesium iodide were used as the precursors for the deposition of CsPbI3. The porous alumina with high surface area and large pore volume was used as growth substrate. It was shown that the porous alumina promoted the growth of CsPbI3 film by absorbing the precursor and increasing the nucleation density. The prepared CsPbI3 film emitted strong and stable red light under ultraviolet light excitation at room temperature and ambient atmosphere. The lead iodide was absorbed on the surface of the porous alumina firstly then reacted with cesiumiodide to form the CsPbI3. The successful preparation of the CsPbI3 by the direct CVD method paves the way for its large scale growth and application in optoelectronic devices.

Pristine poly(butylene adipate-co-terephthalate) (PBAT) is a biodegradable aliphatic-aromatic copolyester that shows no photoluminescent features. The radio induction of photoluminescence (PL) properties in PBAT, after exposure to high doses of gamma radiation, was firstly reported in 2014. These new PL properties have great potential for applications at “in vitro” imaging of human cancer, bio-imaging devices and radiation dosimetry. In this paper, we report the radio induction of photoluminescence (PL) properties in PBAT, after exposure to low energy UV irradiation. In this investigation, films of PBAT, produced by the wire-bar coating technique, were exposed to UV radiation for periods of time ranging from 50 to 500 hours. The PL emission analysis revealed that UV irradiation performed under O2 rich air atmosphere enhances the PL output when compared to irradiations performed in the air. FTIR data confirm that the mechanism behind the UV photo-induction of PL features are similar to the mechanism suggested for gamma radio-induction PL emission. The PL intensity x Spectral Irradiance relationship and RGB color components, the green (G) and blue (B) components, can be used to perform 2D UV dosimetry. The high quantum yield emission of UV-induced PL near 500 nm observed in PBAT is a very interesting finding because it involves the development of a new cheap biodegradable photoluminescent polymer that finds application in drug delivery, bio-imaging devices and also in 2D dosimetry.

Silicon nanowires (SiNWs) show unique optoelectronic properties such as band gap, radiative and nonradiative relaxations. In this research, the optoelectronic properties of <211> SiNW are calculated by combining time-dependent density matrix methodology. Description of photo-excited dynamics processes is enabled by computing “on–the–fly” nonadiabatic couplings (NAC) between electronic and nuclear degrees of freedom using density functional theory (DFT). The dynamics of electronic degrees of freedom is propagated by the reduced density matrix with Redfield equation of motion. Oscillator strengths are used to compute radiative relaxation and to generate time resolved photoluminescence (PL) spectrum. Analysis of the simulated nonradiative decay shows that high-energy photoexcitation relaxes to the band gap edge on the order of 1 ps. We also simulate time-resolved emission spectra of the <211> SiNW that reveals optical emissions above the optical band gap. These emission features are attributed to the interband transitions. The results of this study can be useful for the material choice for optoelectronic applications.

Ultrafast measurement technology provides essential contributions to our understanding of the properties and functions of solids and nanostructures. Atomic-scale vistas with ever-growing spatial and temporal resolution are offered by methods based on short pulses of x-rays and electrons. Time-resolved electron diffraction and microscopy are among the most powerful approaches to investigate nonequilibrium structural dynamics. In this article, we discuss recent advances in ultrafast electron imaging enabled by significant improvements in the coherence of pulsed electron beams. Specifically, we review the development and first application of ultrafast low-energy electron diffraction for the study of structural dynamics at surfaces, and discuss novel opportunities for ultrafast transmission electron microscopy facilitated by laser-triggered field-emission sources. These and further developments will render coherent electron beams an essential component in future ultrafast nanoscale imaging.

A novel luminogen-functionalized SBA-15, denoted as SNT, was developed by incorporating tris(4-bromophenyl)amine (TBPA) into SBA-15 via a “fixation-induced emission” strategy. The emission of TBPA on the matrix of SBA-15 was greatly enhanced, making the SNT possible as a fluorescence sensor. Cefalexin, a typical antibiotic, was chosen as the model analyte to be assayed and sensitive detection performance was achieved. This is the first time for cefalexin to be detected by a fluorescent method. Moreover, the SNT can be recycled by simply washing with proper solvents then used for next detection. This work provides a strategy to greatly improve the emission characteristics of fluorophores, even if a mediocre small fluorophore. It can be extended to design practical fluorescent sensors with high performance and recyclability by this strategy.

Vanadium dioxide thin films are considered as smart functional coatings for thermal shielding, and are attractive as a passive thermal shield for spacecrafts. In space they would, however, be subjected to bombardment by interstellar dust particles and electromagnetic radiation. Materials subjected to irradiation will suffer damages induced by the displacement cascades initiated by nuclear reaction. Such cosmic radiation can severely impact the structure and function of materials. To study this effect in the laboratory, we have deposited VO2 films on silicon wafers and exposed them to γ-radiation of doses up to 100 kGy by using a 60Co nuklid source with 1.17 and 1.33 MeV photon energy. We anticipate that the γ-radiation causes local structural perturbations which can amount to defects with a corresponding change in electronic structure and thermal shielding property. We report on the photo emission spectroscopy of gamma irradiated VO2 thin films.

The unique electronic structure of diamond and its excellent thermal properties allow a broad range of possible applications; from electron sources to RF electronics. However, knowledge of the surface energy levels is essential to produce efficient, high-quality devices. We investigate the valence band position and resulting negative electron affinity for hydrogen terminated diamond under ambient, low vacuum and ultra-high vacuum (UHV) conditions. There was a -0.5 eV change in valence band position causing a negative electron affinity shift from -1.1 eV under UHV to -0.6 eV in ambient pressure. We compare the photoemission current under each environment to predict the ability of the sample to be used as an electron source. The maximum emission was observed when the sample displayed the largest negative affinity. A scanning photoemission measurement is demonstrated to highlight the superior photoemission yield from the hydrogen terminated diamond surface compared to the stainless steel contact. A scanning Kelvin probe measurement is shown to illustrate a method of analyzing the contact potential difference across the diamond surface. Within high-power RF electronics, devices are likely to be operating at increased temperatures so knowledge of the impact of temperature on the energy levels is important. We study the valence band and Fermi level positions for hydrogen terminated diamond from room temperature (300K) to 425K under low and UHV conditions. The Fermi level moved below the valence band edge at increased temperature, illustrating the effect of the 2D hole gas at the surface. We also analyzed the photoemission characteristics and found an increase in yield with increasing temperature. The measurement techniques used to evaluate the energy levels of diamond: photoemission spectroscopy and Kelvin probe measurements, in ambient and vacuum, allow analysis to be completed in minutes. This offers an initial analysis alternative to elucidate more information and predict performance prior to the more time-consuming full device manufacture and characterization.

Electron emission represents the key mechanism enabling the development of devices that have revolutionized modern science and technology. Today, science still relies on advanced electron-emission devices for imaging, electronics, sensing, and high-energy physics. New generations of emission devices are continuously being improved based on innovative materials and the introduction of novel physical concepts. Recent advances are highlighted by emerging low-work-function and low-dimensional materials with unusual electronic and thermal properties. Nanotubes, nanowires, graphene, and electron-emission models are discussed in this issue, as well as original mechanisms, such as the thermoelectronic effect, thermionic emission, and heat trap processes. Advances in electron-emission materials and physics are driving a renaissance in the field, both opening up new applications, such as energy conversion and ultrafast electronics, as well as improving traditional applications in electron imaging and high-energy science.

Light-induced generation of free electrons is of interest for a wide variety of vacuum electronic devices and systems. The properties of nanomaterials, stemming from their geometry and the strong manifestation of quantum phenomena in them, have opened up new avenues for developing new cathodes and exploring and exploiting electron emission. This article presents the heat trap effect—efficient localized heating of carbon nanotube arrays using light, leading to electron emission through the thermionic mechanism. This process requires unexpectedly modest amounts of optical power—available from sources such as handheld lasers—and dramatically simplifies the electron emitter. Potential applications, including thermionic and thermoelectric conversion for solar-energy harvesting and simple electron-beam systems, are also highlighted.

A polyethyleneimine (PEI) interlayer has been applied on indium tin oxide (ITO) to improve electron injection in organic devices including inverted organic light-emitting diodes (OLEDs). To understand the improvement effect by PEI insertion, the energy level alignment at bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2)/PEI/ITO interfaces was investigated by UV photoemission spectroscopy (UPS). The deposition of a PEI layer was found to reduce the absolute work function of ITO by 1.4 eV. The vacuum level shifts at Bebq2/ITO and Bebq2/PEI interfaces were also determined as 0.3 eV and 0.1 eV in the direction to reduce the electron injection barrier, respectively. Thus the work function reduction by PEI and downward vacuum level shift at the Bebq2/PEI interface can contribute to the improvement effect. Kelvin probe measurement revealed the weak orientation polarization in Bebq2 film with the bottom side positively polarized. This polarization polarity is also advantageous for electron injection in inverted devices.

Electric field-modulated photoluminescence (PL) of perovskite solar cells is investigated to gain deeper insight about the role of the mobile ions in organometal halide perovskites. The PL intensity of perovskite solar cells show significant dependence on the polarity of the external electric field and the voltage scanning direction. This phenomenon is discussed in the framework of an ion migration mechanism, which has been widely accounted for the current density-voltage (J-V) hysteresis in perovskite solar cells. The result suggests that the mobile ions not only change the internal electric field of perovskite solar cells, but also have an effect on the recombination of photogenerated charge carriers.

The impact of different annealing temperatures on the crystal structure, emission and radiative defects in ZnO nanocrystals (NCs) has been investigated by means of the scanning electron microscopy (SEM), Energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. ZnO NCs were prepared by the anodization of zinc sheets in an electrolyte and thermal annealed at the various temperatures: 200, 240, 280, 320, 360 and 400°C for two hours in ambient air. The XRD study indicates that ZnO NCs are characterized by the hexagonal wurtzite structure.

The study of annealing temperature impact on the morphology of ZnO NCs has shown that the NC size enlarges and the film crystallinity improves with increasing annealing temperatures from 200°C up to 400°C. But in the temperature range of 360-400oC the dissolution of oxygen atoms raises essentially in ZnO NC films as it follows from EDS data. Simultaneously, the near band edge (NBE) emission intensity falls down, XRD parameters of ZnO NCs change and the intensity of defect related orange and green PL bands increases owing to increasing the defect concentrations. The optimal temperatures for the ZnO NC oxidation, together with keeping the high NBE emission intensity, are estimated as 360 °C. The nature of native defects responsible for orange and green emissions in ZnO NCs has been discussed.

Diamond is a promising wide band-gap semiconductor material for use in devices; therefore a thorough understanding of the surface electronic structure is important. The Kelvin Probe (KP), Surface Photovoltage / Surface Photovoltage Spectroscopy (SPV/SPS) and Ambient Pressure Photoemission Spectroscopy (APS) techniques are commonly applied to traditional and organic semiconductor materials. The application of these techniques to synthetic and natural diamond samples provides some challenges: surface charge on the samples and atypical capacitive interaction with the KP tip. In this study, measurements using a combination of KP, SPV/SPS and APS techniques are taken of samples of natural and synthetic diamond samples to investigate their surface electronic structure and compare their different properties. These techniques are all non-contact and non-destructive. The Fermi Level position of the diamond samples was found to vary, typically between 4.3 – 4.9 eV, depending on the light illumination. For example, when a natural diamond sample was illuminated with 400 nm light from a 150W Quartz Tungsten Halogen light source, there was a surface photovoltage response of ∼250 mV. The oxygen terminated synthetic diamond sample required near continuous illumination at low visible wavelengths in order to retain sufficient conductivity to allow measurement with the Kelvin Probe. By contrast, the natural diamond samples measured showed good conductivity in the layers underneath the top surface. In summary, the KP, SPV/SPS and APS measurement techniques provided some interesting information on the diamond samples and an initial investigation of their surface electronic states is performed.

The Eu3+ doped CaIn2O4 is a novel oxide phosphor useful for multifunctional applications such as display systems, lasers, energy-converters, photocatalysts, optical imaging, medical tools, and several others. Here, a natural aloe-vera gel is explored to obtain a precursor gel bridging Eu3+, Ca2+ and In3+ cations in a network so that it yields Eu3+:CaIn2O4 of small crystallites bonding over a grafted C-sp2 surface layer. The Eu3+ in varied from 0.1 to 2 mol% promptly promotes blue and red light-emissions in two major bands over 300-580 nm and 600-900 nm in 5DJ → 7FJ′ (J = 3, 2, 1 and J′ = 4 →1) and 5D0 → 7F1→ 4 transitions in the Eu3+ species, what is it is required in many optical and catalytic devices. The novel results are described in correlation to the light absorption and core-shell nanostructure.

We report on the establishment of a new lab-scale experimental capability for Spin and Angular Resolved Photoemission Spectroscopy (SARPES) for the study of valleytronics related materials. The ARPES capabilities of the system were demonstrated by measurement on gold [Au(111)] and molybdenum disulphide (MoS2) single crystals and the full functionality of the spin detector was also verified. Experimental results are compared with theoretical modeling by ab-initio band structure calculations. We discuss the potential scope of measurement that this experimental setup affords for investigating spin-related properties (e.g. spin-orbit coupling, valley transport, etc.) in layered materials.

Lead halide perovskites have proven their great power conversion efficiency (PCE) in the last few years and attracted more and more attentions. Evaporation is an important method to get high quality perovskite films, especially for surface and interface investigation, which is important for the solar cell performance. In this paper, we present our investigations on growing PbI2 and CH3NH3I films by evaporation, and then CH3NH3PbI3 films by co-evaporation. X-ray photoemisson spectroscopy (XPS) was used to characterize the films. The results showed that CH3NH3I film was not stable in vacuum. Both N and I decreased in vacuum with time elapsing. PbI2 and CH3NH3PbI3 films are quite stable. The atomic ratio of CH3NH3PbI3 films (C: N: Pb: I =1.29:1.07:1.00:2.94) is very close to the ideal CH3NH3PbI3, which indicates that evaporation is a good method to get high quality perovskite films with accurate atomic ratio.

We introduce a new set of multifunctional metal-coordinating polymers as ligands for the surface functionalization of three different inorganic nanocrystals: luminescent quantum dots (QDs), magnetic iron oxide nanocrystals and metal gold nanoparticles. The ligand design relies on the introduction of a large but controllable number of anchoring groups, hydrophilic moieties and reactive functionalities all in the same polymer chain, via a one-step nucleophilic addition reaction. Nanocrystals capped with these polymer ligands exhibit long-term stability over a broad range of biological conditions. Furthermore, when zwitterion groups are introduced as hydrophilic blocks, this yields a compact ligand coating that allows conjugation of biomolecules to the nanocrystals via metal-histidine self-assembly. The resulting hydrophilic nanocrystals have been used to develop a few specific sensing platforms targeting soluble iron ions and cysteine.

The electronic structure of PuB6, an actinide analog of SmB6 , was investigated making use of a combination of the density functional theory (DFT), and the exact diagonalization (ED) of an effective discrete Anderson impurity model. Intermediate valence ground state with the f-shell occupation n4f =5.5 for the Pu atom in PuB6 is calculated. The 5f-shell magnetic moment is completely compensated by the moment carried by the electrons in the conduction band. Already in DFT, PuB6 is an insulator with a small amount of holes near the X-point, and the indirect band gap of ≈60 meV. This band gap becomes direct in DFT+ED calculations supporting the idea of “topological Kondo insulator” in PuB6. Connection between the electronic structure of PuB6 and δ-Pu is established. We propose that these materials belong to a new class of intermediate valence “Racah” materials with the multi-orbital “Kondo-like” singlet ground-state.