Changxi Zheng

Two-Dimensional CH3NH3PbI3 Perovskite: Synthesis and Optoelectronic Application

Hybrid organic-inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic-inorganic perovskite (i.e., CH3NH3PbI3) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW(-1) with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications.

Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Moving Toward the Recyclable Use of Sapphire Substrates

Two-dimensional layered transition metal dichalcogenides (TMDs) show intriguing potential for optoelectronic devices due to their exotic electronic and optical properties. Only a few efforts have been dedicated to large-area growth of TMDs. Practical applications will require improving the efficiency and reducing the cost of production, through (1) new growth methods to produce large size TMD monolayer with less-stringent conditions, and (2) nondestructive transfer techniques that enable multiple reuse of growth substrate. In this work, we report to employ atmospheric pressure chemical vapor deposition (APCVD) for the synthesis of large size (>100 μm) single crystals of atomically thin tungsten disulfide (WS2), a member of TMD family, on sapphire substrate. More importantly, we demonstrate a polystyrene (PS) mediated delamination process via capillary force in water which reduces the etching time in base solution and imposes only minor damage to the sapphire substrate. The transferred WS2 flakes are of excellent continuity and exhibit comparable electron mobility after several growth cycles on the reused sapphire substrate. Interestingly, the photoluminescence emission from WS2 grown on the recycled sapphire is much higher than that on fresh sapphire, possibly due to p-type doping of monolayer WS2 flakes by a thin layer of water intercalated at the atomic steps of the recycled sapphire substrate. The growth and transfer techniques described here are expected to be applicable to other atomically thin TMD materials.

Quantification of ZnO Nanoparticle Uptake, Distribution, and Dissolution within Individual Human Macrophages

The usefulness of zinc oxide (ZnO) nanoparticles has led to their wide distribution in consumer products, despite only a limited understanding of how this nanomaterial behaves within biological systems. From a nanotoxicological viewpoint the interaction(s) of ZnO nanoparticles with cells of the immune system is of specific interest, as these nanostructures are readily phagocytosed. In this study, rapid scanning X-ray fluorescence microscopy was used to assay the number ZnO nanoparticles associated with ∼1000 individual THP-1 monocyte-derived human macrophages. These data showed that nanoparticle-treated cells endured a 400% elevation in total Zn levels, 13-fold greater than the increase observed when incubated in the presence of an equitoxic concentration of ZnCl2. Even after excluding the contribution of internalized nanoparticles, Zn levels in nanoparticle treated cells were raised ∼200% above basal levels. As dissolution of ZnO nanoparticles is critical to their cytotoxic response, we utilized a strategy combining ion beam milling, X-ray fluorescence and scanning electron microscopy to directly probe the distribution and composition of ZnO nanoparticles throughout the cellular interior. This study demonstrated that correlative photon and ion beam imaging techniques can provide both high-resolution and statistically powerful information on the biology of metal oxide nanoparticles at the single-cell level. Our approach promises ready application to broader studies of phenomena at the interface of nanotechnology and biology.

Reversible Structural Swell–Shrink and Recoverable Optical Properties in Hybrid Inorganic–Organic Perovskite

Ion migration in hybrid organic-inorganic perovskites has been suggested to be an important factor for many unusual behaviors in perovskite-based optoelectronics, such as current-voltage hysteresis, low-frequency giant dielectric response, and the switchable photovoltaic effect. However, the role played by ion migration in the photoelectric conversion process of perovskites is still unclear. In this work, we provide microscale insights into the influence of ion migration on the microstructure, stability, and light-matter interaction in perovskite micro/nanowires by using spatially resolved optical characterization techniques. We observed that ion migration, especially the migration of MA(+) ions, will induce a reversible structural swell-shrink in perovskites and recoverably affect the reflective index, quantum efficiency, light-harvesting, and photoelectric properties. The maximum ion migration quantity in perovskites was as high as approximately 30%, resulting in lattice swell or shrink of approximately 4.4%. Meanwhile, the evidence shows that ion migration in perovskites could gradually accelerate the aging of perovskites because of lattice distortion in the reversible structural swell-shrink process. Knowledge regarding reversible structural swell-shrink and recoverable optical properties may shed light on the development of optoelectronic and converse piezoelectric devices based on perovskites.

Acoustically-Driven Trion and Exciton Modulation in Piezoelectric Two-Dimensional MoS2

By exploiting the very recent discovery of the piezoelectricity in odd-numbered layers of two-dimensional molybdenum disulfide (MoS2), we show the possibility of reversibly tuning the photoluminescence of single and odd-numbered multilayered MoS2 using high frequency sound wave coupling. We observe a strong quenching in the photoluminescence associated with the dissociation and spatial separation of electrons-holes quasi-particles at low applied acoustic powers. At the same applied powers, we note a relative preference for ionization of trions into excitons. This work also constitutes the first visual presentation of the surface displacement in one-layered MoS2 using laser Doppler vibrometry. Such observations are associated with the acoustically generated electric field arising from the piezoelectric nature of MoS2 for odd-numbered layers. At larger applied powers, the thermal effect dominates the behavior of the two-dimensional flakes. Altogether, the work reveals several key fundamentals governing acousto-optic properties of odd-layered MoS2 that can be implemented in future optical and electronic systems.

Caustic imaging of gallium droplets using mirror electron microscopy

We discuss a new interpretation of mirror electron microscopy (MEM) images, whereby electric field distortions caused by surface topography and/or potential variations are sufficiently large to create caustics in the image contrast. Using a ray-based trajectory method, we consider how a family of rays overlaps to create caustics in the vicinity of the imaging plane of the magnetic objective lens. Such image caustics contain useful information on the surface topography and/or potential, and can be directly related to surface features. Specifically we show how a through-focus series of MEM images can be used to extract the contact angle of a Ga droplet on a GaAs (001) surface.

Profound Effect of Substrate Hydroxylation and Hydration on Electronic and Optical Properties of Monolayer MoS2

Atomic force microscopy, Kelvin probe force microscopy, and scanning photoluminescence spectroscopy image the progressive postgrowth hydroxylation and hydration of atomically flat Al2O3(0001) under monolayer MoS2, manifested in large work function shifts (100 mV) due to charge transfer (>10(13) cm(-2)) from the substrate and changes in PL intensity, energy, and peak width. In contrast, trapped water between exfoliated graphene and Al2O3(0001) causes surface potential and doping changes one and two orders of magnitude smaller, respectively, and MoS2 grown on hydrophobic hexagonal boron nitride is unaffected by water exposure.

Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS2 Heterostructures

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene-WS2 heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene-WS2 heterojunctions as-grown on sapphire and transferred to SiO2 with and without thermal annealing. Both p-n and n-n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS2, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene-WS2 than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene-WS2 junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.

Laplacian image contrast in mirror electron microscopy

We discuss an intuitive approach to interpreting mirror electron microscopy (MEM) images, whereby image contrast is primarily caused by the Laplacian of small height or potential variations across a sample surface. This variation is blurred slightly to account for the interaction of the electrons with the electrical potential away from the surface. The method is derived from the established geometrical theory of MEM contrast, and whilst it loses quantitative accuracy outside its domain of validity, it retains a simplicity that enables rapid interpretation of MEM images. A strong parallel exists between this method and out of focus contrast in transmission electron microscopy (TEM), which allows a number of extensions to be made, such as including the effects of spherical and chromatic aberration.

Atomically thin lateral p-n junction photodetector with large effective detection area

The widely used photodetector design based on atomically thin transition metal dichalcogenides (TMDs) has a lateral metal-TMD-metal junction with a fairly small, line shape photoresponsive active area at the TMD-electrode interface. Here, we report a highly efficient photodetector with extremely large photoresponsive active area based on a lateral junction of monolayer-bilayer WSe2. Impressively, the separation of the electron–hole pairs (excitons) extends onto the whole 1L–2L WSe2 junction surface. The responsivity of the WSe2 junction photodetector is over 3200 times higher than that of a monolayer WSe2 device and leads to a highest external quantum efficiency of 256% due to the efficient carrier extraction. Unlike the TMD p–n junctions modulated by dual gates or localized doping, which require complex fabrication procedures, our study establishes a simple, controllable, and scalable method to improve the photodetection performance by maximizing the active area for current generation.