On the Electron‐Transfer Mechanism in the Contact‐Electrification Effect

A long debate on the charge identity and the associated mechanisms occurring in contact‐electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real‐time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti–SiO₂ and Ti–Al₂O₃. Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron‐cloud–potential‐well model is proposed based on the electron‐emission‐dominatedcharge‐transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.     

In situ RHEED control of self-organized Ge quantum dots

In situ registration of high-energy electron diffraction patterns was used for constructing the diagram of structural and morphological states of the Ge film on the Si(100) surface. The following regions identified in the diagram: two-dimensional (2D)-growth, ‘hut’- and ‘dome’-clusters, ‘dome’-clusters with misfit dislocations at the interface. Variations in the lattice constants of the Ge film during the MBE growth on the Si(100) surface were determined. An increase in the lattice constant at the (100) surface was attributed to the elastic deformation at the stage of 2D growth and formation of ‘hut’-clusters and to the plastic relaxation for the ‘dome’-clusters. As a result, epitaxial silicon structures with germanium quantum dots of 15 nm base size at the density of 3×10¹¹ cm⁻² were synthesized. The total electron structure of the hole spectrum of Ge quantum dots in Si was established.     

Band-offset driven efficiency of the doping of SiGe core-shell nanowires

Impurity doping of semiconducting nanowires has been predicted to become increasingly inefficient as the wire diameter is reduced, because impurity states get deeper due to quantum and dielectric confinement. We show that efficient n- and p-type doping can be achieved in SiGe core-shell nanowires as thin as 2 nm, taking advantage of the band offset at the Si/Ge interface. A one-dimensional electron (hole) gas is created at the band-edge and the carrier density is uniquely controlled by the impurity concentration with no need of thermal activation. Additionally, SiGe core-shell nanowires provide naturally the separation between the different types of carriers, electron and holes, and are ideally suited for photovoltaic applications.     

Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid–Solid Interface

As a well-known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the mechanism for CE between liquid and solid remains controversial. The CE process between different liquids and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid–liquid interface. The CE between deionized water and PTFE can produce a surface charges density in the scale of 1 nC cm⁻², which is ten times higher than the calculation based on the pure ion-transfer model. Hence, electron transfer is likely the dominating effect for this liquid–solid electrification process. Meanwhile, as ion concentration increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amount. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liquid and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liquids and solids, which directly impacts the traditional understanding of the formation of an electric double layer (EDL) at a liquid–solid interface in physical chemistry.     

Room-temperature solution route to free-standing SiO2-capped Si nanocrystals with green luminescence

We report a new solution route for the preparation of SiO₂-capped silicon nanocrystals (Si NCs). The Si NCs terminated with SiO₂ are fully characterized by transmission electron microscopy, X-ray diffraction, UV–vis absorption, photoluminescence decay and Fourier transform infrared spectra. The photoluminescence spectra reveal that the Si NCs solution emits green luminescence at 535 and 578 nm excited at 490 nm. The origin of the green luminescence of Si NCs is studied. Our theoretical calculations reveal that the green emission is due to the surface related localized states of self-trapped excitons rather than the purely quantum-confined states, which agrees well with the experimental results. A self-trapped exciton model is proposed to take into account the stepwise localization of electron and hole at the Si–SiO₂ interface. From the localization energies the effective Bohr radii of the localized electrons and holes are estimated to be about 1.71 and 1.57 nm, respectively.     

单晶硅低能电子束辐照效应

利用Monter-Carlo方法,模拟低能一维平面电子束在本征、不同掺杂类型、掺杂浓度下的单晶Si中能量沉积分布情况。采用电子顺磁共振(EPR)技术测量了(111)晶向、两种掺杂类型下的掺杂浓度分别为1×1015cm-3、1×1017cm-3的单晶硅片在一定电子注量下辐照前后缺陷顺磁吸收谱,比较了样品辐照前后缺陷顺磁中心强度的变化,并用X光电子能谱(XPS)对Si-SiO2系统原子化学态的变化进行分析。结果表明,相比于P型Si,N型Si、特别是高掺杂的N型Si,在低能电子一定注量下,界面区内易引起辐射感生缺陷,主要来自于键合于磷的非桥联氧对空穴的诱捕作用,表现为POHC中心明显的变化,P2P芯能级谱突变。并根据理论和实验结果,对电子能量沉积、电离缺陷和辐照效应间的相互关系进行了分析。     

Combined Experimental and Computational Methods Reveal the Evolution of Buried Interfaces during Synthesis of Ferroelectric Thin Films

Understanding interfaces between dissimilar materials is crucial to the development of modern technologies, for example, semiconductor–dielectric and thermoelectric–semiconductor interfaces in emerging electronic devices. However, the structural characterization of buried interfaces is challenging because many measurement techniques are surface sensitive by design. When interested in interface evolution during synthesis, the experimental challenges multiply and often necessitate in situ techniques. For solution‐derived lead zirconate titanate (PZT) ferroelectric thin films, the evolution of buried interfaces during synthesis (including dielectric–metal and metal–metal) is thought to dramatically influence the resultant dielectric and ferroelectric properties. In the present work, multiple experimental and computational methods are combined to characterize interface evolution during synthesis of ferroelectric PZT films on platinized Si wafers—including in situ X‐ray diffraction during thermal treatment, aberration‐corrected scanning transmission electron microscopy of samples quenched from various synthesis states, and calculations using density functional theory. Substantial interactions at buried interfaces in the PZT/Pt/Ti/SiO _x /Si heterostructure are observed and discussed relative to their role(s) in the synthesis process. The results prove that perovskite PZT nucleates directly from the platinum (111)‐oriented bottom electrode and reveal the roles of Pb and O diffusion and intermetallic Pt₃Pb and Pt₃Ti phases.     

The tribovoltaic effect

Contact electrification (or triboelectrification) (CE) is a universal phenomenon between any two materials or two phases of materials. But a contact between two different materials may results in different output. When a p-type semiconductor sliding on a n-type semiconductor surface, the current flowing between the two electrodes on the top of the p-type and the bottom of the n-type is a direct current. This phenomenon is called tribovoltaic effect discovered in the last few years. The mechanism of the tribovoltaic effect is resulted from the electron-hole pairs generated at the PN junction due to the energy released by the formation of the newly formed chemical bonds at the interface due to mechanical sliding, and the inner field built at the PN junction separates the electrons from the holes, resulting in a DC output. The energy released by forming a chemical bond is called “bindington”, which serves as the exciton for exciting the electron-hole pairs, in analogy to the photovoltaic effect. Here, we first review the recent works on the tribovoltaic effect observed at different interfaces. Then, the mechanism of the tribovoltaic effect is presented. The surface chemical methods for regulating the tribovoltaic effect are discussed. Finally, a technique of hybrid tribovoltaic nanogenerator based on the tribovoltaic effect and its potential applications are elaborated.     

Excitonic Mechanism of Local Phase Transformations by Optical Pumping

Transformations of cooperative electronic states by impacts of optical pumping and/or electrostatic doping is a new mainstream in physics of correlated systems. Here we present a semi-phenomenological modeling of spatio-temporal effects in a system where the light absorption goes through a channel creating the excitons—intra-molecular ones or bound electron–hole pairs—and finally the condensate of optical excitons feeds and stimulates phase transformations. Interacting with a near-critical order parameter and deformations, the excitons are subject to self-trapping. That locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The model can be used e.g. as a simplified version of optically induced neutral-ionic transitions in organic chain compounds.     

Effect of deep-trap level on transverse acoustoelectric voltage measurements

The effect of trapped charges on the transverse acoustoelectric voltage (TAV) is investigated with the aim of extending the use of TAV measurements to the study of semiconductors with high defect density. Even if surface acoustic wave frequencies are as high as 100 MHz, charge trapping can influence the TAV. This has been verified by two particular experiments performed on Si/SiO(2) structures with high density of interface states. A theoretical model is proposed to explain the effects of the presence of deep-trap levels on the TAV. Novel boundary conditions for the acoustoelectric equations are introduced and an approximate solution for the TAV amplitude is presented. The model is used to define a novel procedure for the determination of interface-states' density using TAV versus bias voltage measurements.     

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p‐phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap‐free space‐charge‐limited conduction, with the mobility depending on the electric field and charge‐carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm^?3. The trapped electrons recombine with free holes via a non‐radiative trap‐assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap‐assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge‐carrier mobilities and amount of trapping centers known from charge‐transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non‐radiative trap‐assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single‐layer polymer LEDs.     

Properties of Evaporated Si_(1-x)Sn_x and Si_(1_x)Sn_x:H Amorphous Alloys

Amorphous Si_(1-x)Sn_x alloys have been prepared by co-evaporation onto substrates maintained atliquid nitrogen temperature. Their atomic structure is investigated using density measurements,scanning high-energy electron diffraction and Mossbauer spectroscopy. The optical and electricalproperties are reported. Then, a method to hydrogenate the films during the evaporation process isdescribed and applied to the preparation of amorphous semiconductors from pure silicon to pure tin.Finally, multilayers of type Si / Si:H / ... or Si:H / Si:D / ... are studied. The modulation of hydrogen isshown by low-angle neutron scattering and measurements of hydrogen diffusivity are presented.     

Electronic properties of the Ge/RbF/GaAs system: the effect of the RbF intralayer

The electronic structure and layer resolved density of states have been calculated for the Ge/RbF/GaAs slab using the linear combination of atomic orbitals procedure. In this epitaxial system, the RbF layer is used as an insulating intralyer, and the aim of the work is to study the electronic states introduced by this intralayer. Two crystallographic orientations of the slab were investigated: (100) and (111). It has been found that the RbF layer ensures the insulation of the two semiconductors, so their electronic structures do not influence each other. Besides, an inert interface, which preserves the electronic structure of a clean, unreconstructed semiconductor surface and does not introduce any interface states, can be obtained when two polar (as GaAs/RbF(111)) or two non-polar (as Ge/RbF(100)) surfaces are in contact.     

A first-order Mott transition in LixCoO2

Despite many years of experimental searches for a first-order Mott transition in crystalline-doped semiconductors, none have been found. Extensive experimental work has characterized a first-order metal-insulator transition in Li(x)CoO(2), the classic material for rechargeable Li batteries, with a metallic state for x < 0.75 and insulating for x > 0.95. Using density functional theory calculations on large supercells, we identify the mechanism of this hereto anomalous metal-insulator transition as a Mott transition of impurities. Density functional theory demonstrates that for dilute Li-vacancy concentrations, the vacancy binds a hole and forms impurity states yielding a Mott insulator. The unique feature of Li(x)CoO(2) as compared with traditional doped semiconductors, such as Si:P, is the high mobility of the Li vacancies, which allows them to rearrange into two distinct phases at the temperature of the metal-insulator transition.     

Amorphous semiconducting Si:H

Glow discharge decomposition of silane produces amorphous semiconductors which can be doped to be eithern-type orp-type. Since these amorphous silicon films contain between 10–20 atomic per cent hydrogen, the density of undesired defects and localized gap states is very low. As a consequence of the high semiconductor quality of the a-Si:H films, space charge layers adjacent to the surface and substrate interface play an important role in the transport properties. We describe the effect of illumination, thermal treatments and surface layers on the conductance and discuss some difficulties in interpreting the field effect in terms of the bulk density of gap states. The preparation and some characteristic properties of the silicon-hydrogen alloys are described.     

Electronic density of states at the interface between silicon and lead borosilicate glass

The density-of-states distribution in the band gap of Si at the interface between Si and lead borosilicate (SiO₂-PbO-B₂O₂-Al₂O₃-Ta₂O₅) glass was assessed byC-V measurements. It is shown that reducing the temperature at which the passivating glass coating is applied decreases the interfacial density of states to a level comparable with the density of surface states on thermally oxidized Si.     

Spatial and Temporal Self-organization in Semiconductor–Electrolyte Systems

The processes taking place on the surface of semiconductors in electrolytes are discussed in terms of spatial and temporal self-organization. The capacitance and conductance of the semiconductor surface are shown to change abruptly at a critical electrode potential. Concurrently, the density of surface states at the interface decreases, which is characteristic of the formation of ordered crystalline structures. Experimental data are presented for Ge, CdHgTe, and HgTe in electrolyte solutions with glucose, ammonia, EDTA, and thiourea additions.     

Exciplex kinetics in nanocrystal organic light-emitting diodes

We studied the exciplex kinetics in nanocrystal organic light-emitting diodes (NC-OLEDs) where the emissive layer of inorganic nanocrystal quantum dots is sandwiched between the electron and hole transport layers of organic semiconductors at the organic–organic interface. We modeled exciplex generation, diffusion, recombination, and capture by nanocrystals via the Förster mechanism in NC-OLEDs. The exciplex kinetics determines the NC-OLED operation characterized by the quantum yield efficiency and emission intensity and it can be optimized by controlling the nanocrystal separation in the NC-OLED.     

Controllable Synthesis of [11−2−2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high‐quality InN samples. Here, high‐quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single‐crystals. The as‐prepared InN nanopyramids have well‐defined exposed facets of [0001], [11?2?2], [1?212], and [?2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First‐principles density functional calculations reveal that the nonpolar [11?2?2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n–n junction is naturally formed at the InN/ZnO interface. The formation of the n–n junction and the built‐in electric field is ascribed to the efficient separation of the photogenerated electron–hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm^?2 h^?1, which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.     

2D Molecular Crystal Bilayer p–n Junctions: A General Route toward High‐Performance and Well‐Balanced Ambipolar Organic Field‐Effect Transistors

Ambipolar organic field‐effect transistors (OFETs) are vital for the construction of high‐performance all‐organic digital circuits. The bilayer p–n junction structure, which is composed of separate layers of p‐ and n‐type organic semiconductors, is considered a promising way to realize well‐balanced ambipolar charge transport. However, this approach suffers from severely reduced mobility due to the rough interface between the polycrystalline thin films of p‐ and n‐type organic semiconductors. Herein, 2D molecular crystal (2DMC) bilayer p–n junctions are proposed to construct high‐performance and well‐balanced ambipolar OFETs. The molecular‐scale thickness of the 2DMC ensures high injection efficiency and the atomically flat surface of the 2DMC leads to high‐quality p‐ and n‐layer interfaces. Moreover, by controlling the layer numbers of the p‐ and n‐type 2DMCs, the electron and hole mobilities are tuned and well‐balanced ambipolar transport is accomplished. The hole and electron mobilities reach up to 0.87 and 0.82 cm² V^?1 s^?1, respectively, which are the highest values among organic single‐crystalline double‐channel OFETs measured in ambient air. This work provides a general route to construct high‐performance and well‐balanced ambipolar OFETs based on available unipolar materials.