Low-Temperature Layer-by-Layer Growth of Semiconducting Few-Layer γ-Graphyne to Exploit Robust Biocompatibility

The sp-hybridized carbon network in single- or few-layer γ-graphyne (γ-GY) has a polarized electron distribution, which can be crucial in overcoming biosafety issues. Here, we report the low-temperature synthesis, electronic properties, and amyloid fibril nanostructures of electrostatic few-layer γ-GY. ABC stacked γ-GY is synthesized by layer-by-layer growth on a catalytic copper surface, exhibiting intrinsic p-type semiconducting properties in few-layer γ-GY. Thickness-dependent electronic properties of γ-GY elucidate interlayer interactions by electron doping between electrostatic layers and layer stacking-involved modulation of the band gap. Electrostatic few-layer γ-GY induces high electronic sensitivity and intense interaction with amyloid beta (i.e., Aβ40) peptides assembling into elongated mature Aβ40 fibrils. Two-dimensional biocompatible nanostructures of Aβ40 fibrils/few-layer γ-GY enable excellent cell viability and high neuronal differentiation of living cells without external stimulation.

Oscillatory Order-Disorder Transition during Layer-by-Layer Growth of Indium Selenide

It is important to understand the polymorph transition and crystal-amorphous phase transition in In₂Se₃ to tap the potential of this material for resistive memory storage. By monitoring layer-by-layer growth of β-In₂Se₃ during molecular beam epitaxy (MBE), we are able to identify a cyclical order-disorder transition characterized by a periodic alternation between a glassy-like metastable subunit cell film consisting of n < 5 sublayers (nth layers = the number of subunit cell layers), and a highly crystalline β-In₂Se₃ at n = 5 layers. The glassy phase shows an odd-even alternation between the indium-cluster layer (n = 1, 3) and an In-Se solid solution (n = 2, 4), which suggests the ability of In and Se atoms to diffuse, aggregate, and intermix. These dynamic natures of In and Se atoms contribute to a defect-driven memory resistive behavior in current-voltage sweeps that is different from the ferroelectric switching of α-In₂Se₃.

Regulated Dynamics with Two Monolayer Steps in Vapor-Solid-Solid Growth of Nanowires

The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor-solid-solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

Layer-by-Layer Growth of AA-Stacking MoS2 for Tunable Broadband Phototransistors

The stacking configuration has been considered as an important additional degree of freedom to tune the physical property of layered materials, such as superconductivity and interlayer excitons. However, the facile growth of highly uniform stacking configuration is still a challenge. Herein, the AA-stacking MoS₂ domains with a ratio up to 99.5% has been grown by using the modified chemical vapor deposition through introducing NaCl molecules in the confined space. By tuning the growth time, MoS₂ domains would transit from an AA-stacking bilayer to an AAAAA-stacking five-layer. The epitaxial growth mechanism has been insightfully studied, revealing that the critical nucleation size of the AA-stacking bilayer is 5.0 ± 3.0 μm. Through investigation of the photoluminescence, the photoemission, especially the indirect photoexcitation, is dependent on both the stacking fashion and layer number. Furthermore, by studying the gate-tuned MoS₂ phototransistors, we found a significant dependence on the stacking configuration of MoS₂ of the photoexcitation and a different gate tunable photoresponse. The AAA-stacking trilayer MoS₂ phototransistor delivers a photoresponse of 978.14 A W^-1 at 550 nm. By correction of the external quantum efficiency with external field and illumination power density, it has been found that the photoresponse tunability is dependent on the layer number due to the strong photogating effect. This strategy provides a general avenue for the epitaxial growth of van der Waals film which will further facilitate the applications in a tunable photodetector.

In Situ Precise Tuning of Bimetallic Electronic Effect for Boosting Oxygen Reduction Catalysis

Tuning intermediate adsorption energy by shifting the d-band center offers a powerful strategy to tailor the reactivity of metal catalysts. Here we report a potential sweep method to grow Pd layer-by-layer on Au with the capability to in situ measure the surface structure through an ethanol oxidation reaction. Spectroscopic characterizations reveal charge-transfer induced valence band restructuring in the Pd overlayer, which shifts the d-band center away from the Fermi level compared to bulk Pd. Precise overlayer control gives the optimal bimetallic surface of two monolayers (ML) Pd on Au, which exhibits more than 370-fold mass activity enhancement in oxygen reduction reaction (at 0.9 V vs. reversible hydrogen electrode) and 40 mV increase in half-wave potential compared to the Pt/C. Tested in a homemade Zn-air battery, the 2-ML-Pd/Au/C exhibits a maximum power density of 296 mW/cm² and specific activity of 804 mAh/g_Zn, much higher than Pt/C with the same catalyst loading amount.

Design of Magnetic-Plasmonic Nanoparticle Assemblies via Interface Engineering of Plasmonic Shells for Targeted Cancer Cell Imaging and Separation

Magnetic-plasmonic nanoparticles have received considerable attention for widespread applications. These nanoparticles (NPs) exhibiting surface-enhanced Raman scattering (SERS) activities are developed due to their potential in bio-sensing applicable in non-destructive and sensitive analysis with target-specific separation. However, it is challenging to synthesize these NPs that simultaneously exhibit low remanence, maximized magnetic content, plasmonic coverage with abundant hotspots, and structural uniformity. Here, a method that involves the conjugation of a magnetic template with gold seeds via chemical binding and seed-mediated growth is proposed, with the objective of obtaining plasmonic nanostructures with abundant hotspots on a magnetic template. To obtain a clean surface for directly functionalizing ligands and enhancing the Raman intensity, an additional growth step of gold (Au) and/or silver (Ag) atoms is proposed after modifying the Raman molecules on the as-prepared magnetic-plasmonic nanoparticles. Importantly, one-sided silver growth occurred in an environment where gold facets are blocked by Raman molecules; otherwise, the gold growth is layer-by-layer. Moreover, simultaneous reduction by gold and silver ions allowed for the formation of a uniform bimetallic layer. The enhancement factor of the nanoparticles with a bimetallic layer is approximately 10⁷ . The SERS probes functionalized cyclic peptides are employed for targeted cancer-cell imaging and separation.

Molecular Beam Epitaxy Scalable Growth of Wafer-Scale Continuous Semiconducting Monolayer MoTe2 on Inert Amorphous Dielectrics

Monolayer MoTe₂ , with the narrowest direct bandgap of ≈1.1 eV among Mo- and W-based transition metal dichalcogenides, has attracted increasing attention as a promising candidate for applications in novel near-infrared electronics and optoelectronics. Realizing 2D lateral growth is an essential prerequisite for uniform thickness and property control over the large scale, while it is not successful yet. Here, layer-by-layer growth of 2 in. wafer-scale continuous monolayer 2H-MoTe₂ films on inert SiO₂ dielectrics by molecular beam epitaxy is reported. A single-step Mo-flux controlled nucleation and growth process is developed to suppress island growth. Atomically flat 2H-MoTe₂ with 100% monolayer coverage is successfully grown on inert 2 in. SiO₂ /Si wafer, which exhibits highly uniform in-plane structural continuity and excellent phonon-limited carrier transport behavior. The dynamics-controlled growth recipe is also extended to fabricate continuous monolayer 2H-MoTe₂ on atomic-layer-deposited Al₂ O₃ dielectric. With the breakthrough in growth of wafer-scale continuous 2H-MoTe₂ monolayers on device compatible dielectrics, batch fabrication of high-mobility monolayer 2H-MoTe₂ field-effect transistors and the three-level integration of vertically stacked monolayer 2H-MoTe₂ transistor arrays for 3D circuitry are successfully demonstrated. This work provides novel insights into the scalable synthesis of monolayer 2H-MoTe₂ films on universal substrates and paves the way for the ultimate miniaturization of electronics.

Atomic Mechanism in Layer-by-Layer Growth via Surface Reconstruction

Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO₂ (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO₂ experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO₂. A similar phenomenon was also observed in the MoO₂ dissolution procedure.

Layer-by-layer growth of CH₃NH₃PbI(3-x)Clx for highly efficient planar heterojunction perovskite solar cells

A layer-by-layer approach is developed to prepare uniform and compact CH₃NH₃PbI(3-x)Clx perovskite films for perovskite solar cells with a high efficiency up to 15.12% and an improved stability. Moreover, a record high efficiency of 12.25% is achieved for these flexible perovskite solar cells. This study represents an important step forward in developing high-performance and stable perovskite solar cells.

Smooth growth of organic semiconductor films on graphene for high-efficiency electronics

High-quality thin films of conjugated molecules with smooth interfaces are important to assist the advent of organic electronics. Here, we report on the layer-by-layer growth of the organic semiconductor molecule p-sexiphenyl (6P) on the transparent electrode material graphene. Low energy electron microscopy and micro low energy electron diffraction reveal the morphological and structural evolution of the thin film. The layer-by-layer growth of 6P on graphene proceeds by subsequent adding of {111} layers.