Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Size-dependent valence and conduction band-edge energies of semiconductor nanocrystals.

Acenes can be thought of as one-dimensional strips of graphene and they have the potential to be used in the next generation of electronic devices. However, because acenes larger than pentacene have been found to be unstable, it was generally accepted that they would not be particularly useful materials under normal conditions. Here, we show that, by using a physical vapour-transport method, platelet-shaped crystals of hexacene can be prepared from a monoketone precursor. These crystals are stable in the dark for a long period of time under ambient conditions. In the crystal, the molecules are arranged in herringbone arrays, quite similar to that observed for pentacene. A field-effect transistor made using a single crystal of hexacene displayed a hole mobility significantly higher than that of pentacene. This result suggests that it might be instructive to further explore the potential of other higher acenes.

The synthesis, crystal structure and charge-transport properties of hexacene

The electrical performance of organic semiconducting polymers in field-effect transistor devices is now sufficient for initial low complexity circuit applications. To achieve high performance, either operation in an inert atmosphere or a hydrophobic surface treatment and annealing step is typically required. In this communication we report a strategy to prepare fully air stable, amorphous semiconducting polymers which can achieve charge carrier mobilities in the range of 0.04 cm2/(V s), remaining over a period of over 3 months in ambient conditions

Systematic improvement in charge carrier mobility of air stable triarylamine copolymers.

The conduction band offset (CBO) of SnS as the light absorbing layer and Zn1−xMgxO as the buffer layer in SnS thin film solar cells has been optimized to improve the solar cell conversion efficiency. We controlled the CBO experimentally by varying the Mg content (x) of the Zn1−xMgxO layer. The optimum CBO value range for improved solar cell performance was determined to be from −0.1 to 0 eV. A SnS thin film solar cell sample with the optimum CBO value exhibited conversion efficiency of approximately 2.1%.

SnS thin film solar cells with Zn1−xMgxO buffer layers

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics.

Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy

A new type of analyzer for detecting low-energy electrons emitted from solid surfaces in an arbitrary gas atmosphere was constructed. It was equipped with two grids for the external quenching of the counter discharge and for the suppression of a positive-ion bombardment. The work functions of metals and semi-conductors, the oxide film thickness on Si, and exoelectrons from CaF2 were successfully measured for the first time in air at atmospheric pressure by use of the open counter for low-energy electron detection.

https://iopscience.iop.org/article/10.7567/JJAPS.24S4.284/pdf

M.Uda M.Uda

Papers

Open Counter for Low Energy Electron Detection