Zhizhong Chen

Epitaxial Halide Perovskite Lateral Double Heterostructure

Epitaxial III-V semiconductor heterostructures are key components in modern microelectronics, electro-optics, and optoelectronics. With superior semiconducting properties, halide perovskite materials are rising as promising candidates for coherent heterostructure devices. In this report, spinodal decomposition is proposed and experimentally implemented to produce epitaxial double heterostructures in halide perovskite system. Pristine epitaxial mixed halide perovskites rods and films were synthesized via van der Waals epitaxy by chemical vapor deposition method. At room temperature, photon was applied as a knob to regulate the kinetics of spinodal decomposition and classic coarsening. By this approach, halide perovskite double heterostructures were created carrying epitaxial interfaces and outstanding optical properties. Reduced Fröhlich electron-phonon coupling was discovered in coherent halide double heterostructure, which is hypothetically attributed to the classic phonon confinement effect widely existing in III-V double heterostructures. As a proof-of-concept, our results suggest that halide perovskite-based epitaxial heterostructures may be promising for high-performance and low-cost optoelectronics, electro-optics, and microelectronics. Thus, ultimately, for practical device applications, it may be worthy to pursue these heterostructures via conventional vapor phase epitaxy approaches widely practised in III-V field.

Self-heating–induced healing of lithium dendrites

Healing away the dendrites The formation of lithium dendrites during charge-discharge cycles limits the development of lithium metal batteries, because the dendrites can cause electrical shorting of the cells. A number of tricks have been used to try to prevent dendrite formation. Li et al. took the opposite approach (see the Perspective by Mukhopadhyay and Jangid). They operated their cells at higher current densities, under which one would expect dendrites to form owing to the higher nucleation rates. However, under these conditions, the dendrites that started to form heated up and annealed, leading to their disappearance. Science, this issue p. 1513; see also p. 1463 Lithium metal dendrites can be healed in situ by Joule self-heating of the dendritic particles. Lithium (Li) metal electrodes are not deployable in rechargeable batteries because electrochemical plating and stripping invariably leads to growth of dendrites that reduce coulombic efficiency and eventually short the battery. It is generally accepted that the dendrite problem is exacerbated at high current densities. Here, we report a regime for dendrite evolution in which the reverse is true. In our experiments, we found that when the plating and stripping current density is raised above ~9 milliamperes per square centimeter, there is substantial self-heating of the dendrites, which triggers extensive surface migration of Li. This surface diffusion heals the dendrites and smoothens the Li metal surface. We show that repeated doses of high-current-density healing treatment enables the safe cycling of Li-sulfur batteries with high coulombic efficiency.

High-Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Decoupling interface effect on the phase stability of CdS thin films by van der Waals heteroepitaxy

Wurtzite (W) and zinc-blende (ZB) polytypism has long been observed in epitaxial CdS thin films. The present work, based on van der Waals epitaxial CdS thin films, is an attempt to explain which crystal modification, W or ZB, is favored under different growth conditions. In this van der Waals epitaxy system where the substrate influence is considered weak, it is found that the substrate temperature plays a crucial role in determining the crystal modification of CdS, that is, W and ZB CdS are more stable at low and high ends of substrate temperature, respectively. We attribute this temperature effect to the entropy difference (SW < SZB), a conclusion well supported by the thermodynamic hard sphere model formulation of the entropy difference between hexagonal close-packed and face-centered cubic structures. By summarizing other works, we find that the entropy difference model can also be applied to large mismatched (≳3%) CdS-substrate chemical epitaxy systems but not for small mismatched (≲3%) ones. In the ...

Defect-engineered epitaxial VO2±δ in strain engineering of heterogeneous soft crystals

Heterogeneous strain engineering of soft crystals is realized via phase transition nanocrystals. The success of strain engineering has made a step further for the enhancement of material properties and the introduction of new physics, especially with the discovery of the critical roles of strain in the heterogeneous interface between two dissimilar materials (for example, FeSe/SrTiO3). On the other hand, the strain manipulation has been limited to chemical epitaxy and nanocomposites that, to a large extent, limit the possible material systems that can be explored. By defect engineering, we obtained, for the first time, dense three-dimensional strongly correlated VO2±δ epitaxial nanoforest arrays that can be used as a novel “substrate” for dynamic strain engineering, due to its metal-insulator transition. The highly dense nanoforest is promising for the possible realization of bulk strain similar to the effect of nanocomposites. By growing single-crystalline halide perovskite CsPbBr3, a mechanically soft and emerging semiconducting material, onto the VO2±δ, a heterogeneous interface is created that can entail a ~1% strain transfer upon the metal-insulator transition of VO2±δ. This strain is large enough to trigger a structural phase transition featured by PbX6 octahedral tilting along with a modification of the photoluminescence energy landscape in halide perovskite. Our findings suggest a promising strategy of dynamic strain engineering in a heterogeneous interface carrying soft and strain-sensitive semiconductors that can happen at a larger volumetric value surpassing the conventional critical thickness limit.

van der Waals epitaxial ZnTe thin film on single-crystalline graphene

Graphene template has long been promoted as a promising host to support van der Waals flexible electronics. However, van der Waals epitaxial growth of conventional semiconductors in planar thin film form on transferred graphene sheets is challenging because the nucleation rate of film species on graphene is significantly low due to the passive surface of graphene. In this work, we demonstrate the epitaxy of zinc-blende ZnTe thin film on single-crystalline graphene supported by an amorphous glass substrate. Given the amorphous nature and no obvious remote epitaxy effect of the glass substrate, this study clearly proves the van der Waals epitaxy of a 3D semiconductor thin film on graphene. X-ray pole figure analysis reveals the existence of two ZnTe epitaxial orientational domains on graphene, a strong X-ray intensity observed from the ZnTe [ 1 ¯ 1 ¯2] ǁ graphene [10] orientation domain, and a weaker intensity from the ZnTe [ 1 ¯ 1 ¯2] ǁ graphene [11] orientation domain. Furthermore, this study systematical...

Effect of strain on the Curie temperature and band structure of low-dimensional SbSI

Photoferroelectric materials show great promise for developing alternative photovoltaics and photovoltaic-type non-volatile memories. However, the localized nature of the d orbital and large bandgap of most natural photoferroelectric materials lead to low electron/hole mobility and limit the realization of technologically practical devices. Antimony sulpho-iodide (SbSI) is a photoferroelectric material which is expected to have high electron/hole mobility in the ferroelectric state due to its non-local band dispersion and narrow bandgap. However, SbSI exhibits the paraelectric state close to room temperature. In this report, as a proof of concept, we explore the possibility to stabilize the SbSI ferroelectric phase above room temperature via mechanical strain engineering. We synthesized thin low-dimensional crystals of SbSI by chemical vapor deposition, confirmed its crystal structure with electron diffraction, studied its optical properties via photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy, and probed its phase transition using temperature-dependent steady-state photoluminescence spectroscopy. We found that introducing external mechanical strain to these low-dimensional crystals may lead to an increase in their Curie temperature (by ∼60 K), derived by the strain-modified optical phase transition in SbSI and quantified by Kern formulation and Landau theory. The study suggests that strain engineering could be an effective way to stabilize the ferroelectric phase of SbSI at above room temperature, providing a solution enabling its application for technologically useful photoferroelectric devices.

Merits and Challenges of Ruddlesden–Popper Soft Halide Perovskites in Electro‐Optics and Optoelectronics

Following the rejuvenation of 3D organic-inorganic hybrid perovskites, like CH3 NH3 PbI3 , (quasi)-2D Ruddlesden-Popper soft halide perovskites R2 An -1 Pbn X3 n +1 have recently become another focus in the optoelectronic and photovoltaic device community. Although quasi-2D perovskites were first introduced to stabilize optoelectronic/photovoltaic devices against moisture, more interesting properties and device applications, such as solar cells, light-emitting diodes, white-light emitters, lasers, and polaritonic emission, have followed. While delicate engineering design has pushed the performance of various devices forward remarkably, understanding of the fundamental properties, especially the charge-transfer process, electron-phonon interactions, and the growth mechanism in (quasi)-2D halide perovskites, remains limited and even controversial. Here, after reviewing the current understanding and the nexus between optoelectronic/photovoltaic properties of 2D and 3D halide perovskites, the growth mechanisms, charge-transfer processes, vibrational properties, and electron-phonon interactions of soft halide perovskites, mainly in quasi-2D systems, are discussed. It is suggested that single-crystal-based studies are needed to deepen the understanding of the aforementioned fundamental properties, and will eventually contribute to device performance.

Single-Crystal Graphene-Directed van der Waals Epitaxial Resistive Switching

Graphene has been broadcasted as a promising choice of electrode and substrate for flexible electronics. To be truly useful in this regime, graphene has to prove its capability in ordering the growth of overlayers at an atomic scale, commonly known as epitaxy. Meanwhile, graphene as a diffusion barrier against atoms and ions has been shown in some metal-graphene-dielectric configurations for integrated circuits. Guided by these two points, this work explores a new direction of using graphene as a bifunctional material in an electrochemical metallization memory, where graphene is shown to (i) order the growth of a low-ionicity semiconductor ZnS single-crystalline film and (ii) regulate the ion migration in the resistive switching device made of Cu/ZnS/graphene/Cu structures. The ZnS film is confirmed to be van der Waals epitaxially grown on single-crystal graphene with X-ray structural analysis and Raman spectroscopy. Charge transport studies with controlled kinetic parameters reveal superior ion regulating characteristic of graphene in this ZnS-based resistive switching device. The demonstration of the first graphene-directed epitaxial wide band gap semiconductor resistive switching suggests a possible and promising route toward flexible memristors.

Correction to Epitaxial Halide Perovskite Lateral Double Heterostructure

Lateral Double Heterostructure Yiping Wang,† Zhizhong Chen,† Felix Deschler, Xin Sun,‡ Toh-Ming Lu,‡ Esther A. Wertz,‡ Jia-Mian Hu, and Jian Shi*,† †Department of Materials Science and Engineering and ‡Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States