Layer-by-layer thinning of MoS2 by thermal annealing.

By thermal annealing, few-layer MoS2 flakes can be thinned down. In one hour, the upper layer is peeled off due to sublimation. Eventually, monolayer MoS2 is achieved. We have characterized the process by optical contrast, Raman spectroscopy and atomic force microscopy (AFM), and observed a mixture of surfaces of N and N- 1 layers.

Solution phase van der Waals epitaxy of ZnO wire arrays.

As an incommensurate epitaxy, van der Waals epitaxy allows defect-free crystals to grow on substrates even with a large lattice mismatch. Furthermore, van der Waals epitaxy is proposed as a universal platform where heteroepitaxy can be achieved irrespective of the nature of the overlayer material and the method of crystallization. Here we demonstrate van der Waals epitaxy in solution phase synthesis for seedless and catalyst-free growth of ZnO wire arrays on phlogopite mica at low temperature. A unique incommensurate interface is observed even with the incomplete initial wetting of ZnO onto the substrate. Interestingly, the imperfect contacting layer does not affect the crystalline and optical properties of other parts of the wires. In addition, we present patterned growth of a well-ordered array with hexagonal facets and in-plane alignment. We expect our seedless and catalyst-free solution phase van der Waals epitaxy synthesis to be widely applicable in other materials and structures.

Recent developments and future directions in the growth of nanostructures by van der Waals epitaxy.

Here we review the characteristics of "van der Waals epitaxy" (vdWE) as an alternative epitaxy mechanism that has been demonstrated as a viable method for circumventing the lattice matching requirements for epitaxial growth. Particular focus is given on the application of vdWE for nonplanar nanostructures. We highlight our works on the vdWE growth of nanowire arrays, tripods, and tetrapods from various semiconductors (ZnO, ZnTe, CdS, CdSe, CdSxSe1-x, CdTe, and PbS) on muscovite mica substrates, irrespective of the ensuing lattice mismatch. We then address the controllability of the synthesis and the growth mechanism of ZnO nanowires from catalyst-free vdWE in vapor transport growth. As exemplified herein with optical characterizations of ZnO and CdSe nanowires, we show that samples from vdWE may possess properties that are as excellent as those from conventional epitaxy. With our works, we aim to advocate vdWE as a prospective universal growth strategy for nonplanar epitaxial nanostructures.

Highly enhanced exciton recombination rate by strong electron-phonon coupling in single ZnTe nanobelt.

Electron-phonon coupling plays a key role in a variety of elemental excitations and their interactions in semiconductor nanostructures. Here we demonstrate that the relaxation rate of free excitons in a single ZnTe nanobelt (NB) is considerably enhanced via a nonthermalized hot-exciton emission process as a result of an ultrastrong electron-phonon coupling. Using time-resolved photoluminescence (PL) spectroscopy and resonant Raman spectroscopy (RRS), we present a comprehensive study on the identification and the dynamics of free/bound exciton recombination and the electron-phonon interactions in crystalline ZnTe NBs. Up to tenth-order longitudinal optical (LO) phonons are observed in Raman spectroscopy, indicating an ultrastrong electron-phonon coupling strength. Temperature-dependent PL and RRS spectra suggest that electron-phonon coupling is mainly contributed from Light hole (LH) free excitons. With the presence of hot-exciton emission, two time constants (∼80 and ∼18 ps) are found in photoluminescence decay curves, which are much faster than those in many typical semiconductor nanostructures. Finally we prove that under high excitation power amplified spontaneous emission (ASE) originating from the electron-hole plasma occurs, thereby opening another radiative decay channel with an ultrashort lifetime of few picoseconds.

Polarity assignment in ZnTe, GaAs, ZnO, and GaN-AlN nanowires from direct dumbbell analysis.

Aberration corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and the newly developed annular bright field (ABF) imaging are used to define a new guideline for the polarity determination of semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i) when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case where one of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoretical fundaments of these procedures allow us to overcome the main challenge in the identification of dumbbell polarity. It resides in the separation and identification of the constituent atoms in the dumbbells. The proposed experimental via opens new routes for the fine characterization of nanostructures, e.g., in electronic and optoelectronic fields, where the polarity is crucial for the understanding of their physical properties (optical and electronic) as well as their growth mechanisms.

Incommensurate van der Waals epitaxy of nanowire arrays: a case study with ZnO on muscovite mica substrates.

The requirement of lattice matching between a material and its substrate for the growth of defect-free heteroepitaxial crystals can be circumvented with van der Waals epitaxy (vdWE). However, the utilization and characteristics of vdWE in nonlamellar/nonplanar nanoarchitectures are still not very well-documented. Here we establish the characteristics of vdWE in nanoarchitectures using a case study of ZnO nanowire (NW) array on muscovite mica substrate without any buffer/seed layer. With extensive characterizations involving electron microscopy, diffractometry, and the related analyses, we conclude that the NWs grown via vdWE exhibit an incommensurate epitaxy. The incommensurate vdWE allows a nearly complete lattice relaxation at the NW-substrate heterointerface without any defects, thus explaining the unnecessity of lattice matching for well-crystallized epitaxial NWs on muscovite mica. We then determine the polarity of the NW via a direct visualization of Zn-O dumbbells using the annular bright field scanning transmission electron miscroscopy (ABF-STEM) in order to identify which atoms are at the base of the NWs and responsible for the van der Waals interactions. The information from the ABF-STEM is then used to construct the proper atomic arrangement at the heterointerface with a 3D atomic modeling to corroborate the characteristics of the incommensurate vdWE. Our findings suggest that the vdWE might be extended for a wider varieties of compounds and epitaxial nanoarchitectures to serve as a universal epitaxy strategy.

Epitaxial II-VI tripod nanocrystals: a generalization of van der Waals epitaxy for nonplanar polytypic nanoarchitectures.

We report for the first time the synthesis of nonplanar epitaxial tripod nanocrystals of II-VI compounds (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) on muscovite mica substrate. With CdS as a case study, we conclude via Raman spectroscopy and electron microscopy studies that the tripods, which are found to be polytypic, followed a seeded growth mechanism. The epitaxy, manifested by the in-plane alignment of the legs of the tripods within a substrate, is attributed to the van der Waals interaction between the tripod bases and the mica surface, instead of to the covalent chemical bond which would require lattice matching between the epilayer and the substrate. The results demonstrated herein could have widespread immediate implications, including the potential of van der Waals epitaxy to be applicable in producing ordered arrays of more complex nanoarchitectures from various classes of compounds toward a broad range of technological applications.

Synthesis and optical properties of II-VI 1D nanostructures.

1D nanostructures from II-VI semiconductors have been demonstrated to exhibit outstanding optical properties with strong promise for novel optoelectronic devices with augmented performance and functionalities. Herein, we present a comprehensive review discussing important topics pertinent to the fundamental properties and applications of II-VI 1D nanostructures. With practical applications in mind, the considerations, principles and experimental techniques on the sample preparation of high quality 1D nanostructures are highlighted. Fundamentals on the optical properties of II-VI materials, along with relevant investigation techniques and recent progress in the field, are also extensively discussed. With the steady development of their synthesis, characterization and device fabrication, it is strongly expected that II-VI 1D nanostructures will assume a unique position in future technology.

Vertically aligned cadmium chalcogenide nanowire arrays on muscovite mica: a demonstration of epitaxial growth strategy.

We report a strategy for achieving epitaxial, vertically aligned cadmium chalcogenide (CdS, CdSe, and CdTe) nanowire arrays utilizing van der Waals epitaxy with (001) muscovite mica substrate. The nanowires, grown from a vapor transport process, exhibited diameter uniformity throughout their length, sharp interface to the substrate, and positive correlation between diameter and length with preferential growth direction of [0001] for the monocrystalline wurtzite CdS and CdSe nanowires, but of [111] for zinc blende CdTe nanowires, which also featured abundant twinning boundaries. Self-catalytic vapor-liquid-solid mechanism with hydrogen-assisted thermal evaporation is proposed to intepret the observations. Optical absorption from the as-grown CdSe nanowire arrays on mica at 10 K revealed intense first-order exciton absorption and its longitudinal optical phonon replica. A small Stokes shift (∼1.3 meV) was identified, suggesting the high quality of the nanowires. This study demonstrated the generality of van der Waals epitaxy for the growth of nanowire arrays and their potential applications in optical and energy related devices.

Heteroepitaxial decoration of Ag nanoparticles on Si nanowires: a case study on Raman scattering and mapping.

Metallic nanoparticle-decorated silicon nanowires showed considerable promise in a wide range of applications such as photocatalytic conversion, surface-enhanced Raman scattering, and surface plasmonics. However there is still insufficient amount of Raman scattering in Si nanowires with such decoration. Here we report the heteroepitaxial growth of Ag nanoparticles on Si nanowires by a surface reduction mechanism. The as-grown Ag nanoparticles exhibited highly single crystallinity with a most probable diameter of 25 nm. Raman scattering spectroscopy showed a new sideband feature at 495 cm(-1) below the first order Si transverse optical Raman peak due to HF etching. This new feature sustained after sequential surface treatments and rapid thermal annealing, therefore was attributed to polycrystalline defect at subsurface, which was confirmed by high-resolution transmission electron microscopy observations. Correlated atomic force microscopy and Raman mapping demonstrated that single Ag nanoparticle decoration significantly enhanced Raman signal of Si nanowire by a factor of 7, suggesting that it would be a promising approach to probe phonon confinement and radial breathing mode in individual nanowires down to sub-10 nm regime.