W. M. Lau

Blocking reactions between indium-tin oxide and poly (3,4-ethylene dioxythiophene):poly(styrene sulphonate) with a self-assembly monolayer

In the fabrication of polymeric electroluminescent devices with indium-tin oxide (ITO) as anode, indium contamination of the polymers can greatly degrade the device performance. In the present study, we have used x-ray photoelectron spectroscopy to measure indium incorporation in poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate), referred to as PEDOT:PSS, which were spincast on bare ITO and encapsulated ITO. We found that the deposition of a self-assembled monolayer of alkylsiloxanes on ITO prior to spincasting PEDOT:PSS was effective and practical in blocking the reactions between ITO and PEDOT:PSS.

Improved Crystallization of Perovskite Films by Optimized Solvent Annealing for High Efficiency Solar Cell

Organic-inorganic halide perovskite-based thin film solar cells show excellent light-to-power conversion efficiency. The high performance for the devices requires the preparation of well-crystallized perovskite absorbers. In this paper, we used the postannealing process to treat the perovskite films under different solvent vapors and observed that the solvent vapors have a strong effect on the film growth. A model regarding the perovskite film growth was proposed as well. Intensive characterizations including scanning electron microscopy, electrochemical impedance spectroscopy, and admittance spectroscopy allowed us to attribute the improved performance to reduced recombination loss and defect density. Solar cell based on the DMSO-treated films delivered a power conversion efficiency of over 13% with negligible photocurrent hysteresis.

High efficiency low operating voltage polymer light-emitting diodes with aluminum cathode

By blending poly(ethylene glycol) (PEG) into an electroluminescence (EL) polymer, significantly enhanced EL efficiency in a polymer light-emitting diode (PLED) with aluminum electrode was achieved. An orange-color-emitting PLED with 10 wt % PEG blending achieved device efficiencies exceeding 2.6 cd/A for a wide range of bias voltage, which is more than two orders of magnitude higher than that of a similar PLED without the PEG blending. The enhanced efficiency was a result of the reduction of electron injection barrier height at the cathode–polymer interface. It is believed that interfacial interaction that is specific to Al plays an important role in the enhancement mechanism.

First-Principles Study of Phosphorene and Graphene Heterostructure as Anode Materials for Rechargeable Li Batteries

There is a great desire to develop the high-efficient anodes materials for Li batteries, which require not only large capacity but also high stability and mobility. In this work, the phosphorene/graphene heterostructure (P/G) was carefully explored based on first-principles calculations. The binding energy of Li on the pristine phosphorene is relatively weak (within 1.9 eV), whereas the phosphorene/graphene heterostructure (P/G) can greatly improve the binding energy (2.6 eV) without affecting the high mobility of Li within the layers. The electronic structures show that the large Li adsorption energy and fast diffusion ability of the P/G origin from the interfacial synergy effect. Interestingly, the P/G also displays ultrahigh stiffness (Cac = 350 N/m, Czz = 464 N/m), which can effectively avoid the distortion of the pristine phosphorene after the insertion of lithium. Thus, P/G can greatly enhance the cycle life of the battery. Owing to the high capacity, good conductivity, excellent Li mobility, and ultrahigh stiffness, P/G is a very promising anode material in Li-ion batteries (LIBs).

An x‐ray photoelectron spectroscopy study on ozone treated GaAs surfaces

The formation of sacrificial oxides on GaAs surfaces has become the normal procedure in the preparation of in situ cleaned substrates for molecular‐beam epitaxial growth. In this paper we show that sacrificial oxides can be formed on InP by a UV/ozone treatment, yielding more controlled and reproducible properties than thermal and air formed oxides. Results from x‐ray photoelectron spectroscopy studies on acid/DI water/air and UV/ozone treatments are compared. It is shown that thermal desorption of the former leaves residues containing carbon which can seriously degrade epitaxial growth, whereas UV/ozone formed sacrificial oxides could be removed at 494 °C leaving atomically clean, stoichiometric surfaces.

Modification of surface band bending of diamond by low energy argon and carbon ion bombardment

Argon and carbon ion bombardment of p‐diamond at 500–5000 eV in ultrahigh vacuum were studied by in situ x‐ray photoelectron spectroscopy (XPS) and low energy electron diffraction analysis. Both argon and carbon ion bombardment at room temperature in the present energy range created a defective surface layer. The radiation damage was manifested by the introduction of a distinct C 1s peak (referred to as the ‘‘defect’’ peak later) with a binding energy about 1 eV less than that of the bulklike diamond peak, and by the introduction of some additional filled states (referred to as the ‘‘filled states’’) near the valence band edge of diamond. It was found that in comparison to argon bombardment, carbon bombardment was more efficient in producing the filled states but less efficient in raising the C 1s defect peak. While the filled states disappeared by annealing at about 500 °C, the C 1s defect peak did not change much even with a 1000 °C anneal. These results suggest that the C 1s defect peak, which has also...

Dimension-dependent phase transition and magnetic properties of VS2

Among dozens of layered transition metal dichalcogenides (TMDs), VS2 has attracted particular interest due to its intrinsic magnetism and potential applications as a high-performance functional nanomaterial. The phase stability and electronic properties of the typical crystal structures of both monolayer and bulk VS2 are carefully investigated based on first-principle calculations. The results reveal that the relative stability between different phases is greatly affected by the thickness of the layers and the temperature. Below room temperature, both bulk and monolayer VS2 prefer to exhibit the hexagonal (H) structure instead of the trigonal (T) structure. Interestingly, at room temperature, although the H monolayer VS2 remains more stable than the T-VS2, the bulk T-VS2 becomes more stable than H-VS2. These results reveal that a phase transition between H and T will occur on changing either the thickness of the slab or the temperature. Furthermore, the different crystal structures (H and T) exhibit significantly distinct magnetism: the bulk T-VS2 has the lowest magnetism (0.31 μB), while the monolayer H-VS2 has the largest magnetism (about 1.00 μB) among the structures. Most importantly, our results reveal that the magnetism will increase sharply on the exfoliation of monolayer VS2 from the bulk at room temperature because of the phase transition from T to H. The present results provide an efficient way to modulate the magnetic moment through controlling the crystal structure and the thickness of the VS2 nanosheets.

X-ray photoemission spectroscopy of nonmetallic materials: Electronic structures of boron and BxOy

Although an increasing volume of x-ray photoemission spectroscopic (XPS) data has been accumulated on boron and boron-rich compounds because of their unusual properties, including a unique three-center, two-electron bonding configuration, their common nonmetallic nature has been overlooked. Typically, the measured energy-state data are not clarified by surface Fermi level positions of these nonmetallic samples, which compromises the scientific contents of the data. In the present study, we revisited the XPS studies of sputter-cleaned β-rhombohedral boron (βr-B), the oxidized surface of βr-B, B6O pellet, and polished B2O3, to illustrate the impact and resolution of this scientific issue. These samples were chosen because βr-B is the most thermodynamically stable polytype of pure boron, B2O3 is its fully oxidized form, and B6O is the best known superhard family member of boron-rich compounds. From our XPS measurements, including those from a sputter-cleaned gold as a metal reference, we deduced that our βr-...

Low-temperature, solution processed metal sulfide as an electron transport layer for efficient planar perovskite solar cells

Organic–inorganic halide perovskites possess excellent chemical, optical, and electronic properties that make them attractive for next-generation solar cells. In this paper, we introduce all-low-temperature processed perovskite solar cells using metal sulfide as an electron transport layer. First, we evaluated the alignment of energy levels at the perovskite/metal sulfide layer interface. The properties of metal sulfide and the perovskite layer, as well as the corresponding device performance, were then investigated. Using a CdS layer as an electron transport layer, we have achieved a maximum power conversion efficiency of 11.2% under reverse scans. The successful use of the CdS layer in perovskite solar cells likely would create new pathways and opportunities for the advancement of device design.

Site-specific catalytic activity in exfoliated MoS2 single-layer polytypes for hydrogen evolution: basal plane and edges

We performed ab initio calculations on the basic set of MoS2 single-layer materials, namely the 1H, 1T and 1T′ polytypes, to lay a theoretical framework on the emerging breakthrough-discoveries of high activity towards the hydrogen evolution reaction (HER) in exfoliated MoS2 and related materials. Our calculations show that for exfoliated MoS2, 1T′ is the most HER active polytype, with active sites both on the basal plane and at the edges of the layered grains. In comparison, the basal planes of the 1H and 1T polytypes are HER inactive and their edge-sites are not as active as those of the 1T′ polytype. We also found that 1T-MoS2 is unstable and easily transforms into 1T′-MoS2, and the 1T′ phase is metastable with a considerable barrier >0.7 eV to bar its transformation into the most stable 1H phase. Further, unlike the case of exfoliated WS2, the HER activity of exfoliated MoS2 is not so critically affected by the lattice strain. In addition, in contrast to the 1T′-WS2, the 1T′-MoS2 is not metallic but has a very small bandgap of 0.1–0.2 eV. Hence, 1T′-MoS2 should have a high enough conductivity and other suitable properties for it to function as an electrochemical HER catalyst.