Atomic-scale Thickness Research Articles

An intrinsic growth instability in isotropic materials leads to quasi-two-dimensional nanoplatelets.

Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. Despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remains unclear, and even counterintuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead-halide perovskites). Here we show that an intrinsic instability in growth kinetics can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Negative Poisson's ratio in rippled graphene.

In this work, we perform molecular dynamics (MD) simulations to study the effect of rippling on the Poisson's ratio of graphene. Due to the atomic scale thickness of graphene, out-of-plane ripples are generated in free standing graphene with topological defects (e.g. heptagons and pentagons) to release the in-plane deformation energy. Through MD simulations, we have found that the Poisson's ratio of rippled graphene decreases upon increasing its aspect ratio η (amplitude over wavelength). For the rippled graphene sheet η = 0.188, a negative Poisson's ratio of -0.38 is observed for a tensile strain up to 8%, while the Poisson's ratio for η = 0.066 is almost zero. During uniaxial tension, the ripples gradually become flat, thus the Poisson's ratio of rippled graphene is determined by the competing factors of the intrinsic positive Poisson's ratio of graphene and the negative Poisson's ratio due to the de-wrinkling effect. Besides, the rippled graphene exhibits excellent fracture strength and toughness. With the combination of its auxetic and excellent mechanical properties, rippled graphene may possess potential for application in nano-devices and nanomaterials.

Atomic Layer Deposition of Silicon Nitride Thin Films: A Review of Recent Progress, Challenges, and Outlooks.

With the continued miniaturization of devices in the semiconductor industry, atomic layer deposition (ALD) of silicon nitride thin films (SiNx) has attracted great interest due to the inherent benefits of this process compared to other silicon nitride thin film deposition techniques. These benefits include not only high conformality and atomic-scale thickness control, but also low deposition temperatures. Over the past 20 years, recognition of the remarkable features of SiNx ALD, reinforced by experimental and theoretical investigations of the underlying surface reaction mechanism, has contributed to the development and widespread use of ALD SiNx thin films in both laboratory studies and industrial applications. Such recognition has spurred ever-increasing opportunities for the applications of the SiNx ALD technique in various arenas. Nevertheless, this technique still faces a number of challenges, which should be addressed through a collaborative effort between academia and industry. It is expected that the SiNx ALD will be further perceived as an indispensable technique for scaling next-generation ultra-large-scale integration (ULSI) technology. In this review, the authors examine the current research progress, challenges and future prospects of the SiNx ALD technique.

Directional-dependent thickness and bending rigidity of phosphorene

The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional based potential, we directly uncover the flexibility of individual $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene.

Synthesis of Atomically Thin Pt Monolayer on HOPG

High specific surface area is essential for Pt electrocatalyst. However, nanoparticles become less stable as the particle size decreases. Two-dimensional nanosheets are expected to have comparable or ever larger specific surface area than nanoparticles. Furthermore, nanosheets should be more stable than nanoparticles due to the two-dimensional bonding. Synthesis of Pt nanosheets with 1 nm thickness has previously been reported1). However, non-reactive Pt atoms still exist in the core of nanosheets, thus the utilization of Pt may be low. The purpose of this research is the synthesis of Pt nanosheets with atomic-scale thickness (nanosheets with 1 or 2 Pt monolayers). Highly oriented pyrolytic graphite (HOPG) was freshly cleaved using adhesive tape and then immersed into a diluted RuO2 nanosheet colloid for 2 minutes to coat RuO2 nanosheets on HOPG (RuO2 nanosheet/HOPG). Atomically thin Pt monolayer was deposited on the surface of RuO2 nanosheet/HOPG by vacuum evaporation2). Cyclic voltammetry was conducted to dissolve the RuO2nanosheets. The morphology and thickness were characterized by atomic force microscopy (AFM), and the chemical state of the nanosheets on HOPG was determined by X-ray photoelectron spectroscopy (XPS). Redox peaks at ~0.1, 0.65, 0.8 vs. RHE, which are characteristic of RuO2 nanosheets, disappeared after potential cycling between 0.05 and 1.55 V vs. RHE for 50 cycles, suggesting the dissolution of RuO2 nanosheets. AFM observation showed that the thickness of RuO2 nanosheets were 1.11±0.18 nm and the thickness of the as-deposited Pt on RuO2 nanosheets were 0.34±0.25 nm. Taking into account of the diameter of Pt atom as 0.28 nm, 1-2 monolayer of Pt was deposited on RuO2 nanosheets. The thickness of the nanosheet after cycling decreased to 0.40±0.08 nm, agreeing with the dissolution of the RuO2 support. The chemical state of Pt and Ru before and after electrochemical dissolution was studied by XPS. Pt 4f5/2 and Pt 4f7/2 peaks were observed before and after cycling, while Ru 3p1/2, Ru 3p3/2 peaks disappeared after cycling. Based on CV, AFM and XPS analysis, it is concluded that a Pt monolayer with atomic thickness was successfully prepared on HOPG by using RuO2 nanosheets as a structure directing scaffold and subsequent dissolution of RuO2nanosheets. 1) A. Funatsu, H. Tateishi, K. Hatakeyama, Y. Fukunaga, T. Taniguchi, M. Koinuma, H. Matsuura, and Y. Matsumoto, Chem. Commun., 50, 8503 (2014). 2) Q. Liu, C. Chauvin, and W. Sugimoto, J. Electrochem. Soc., 161, F360 (2014). Figure 1

(Invited) Core-Shell Nanosheet Catalysts: Enhanced Activity and Durability for Oxygen Reduction and Hydrogen Oxidation Reactions

Development of Pt-based catalysts with higher activity and durability is essential for implementation of cost-competitive polymer electrolyte membrane fuel cells. Nanoparticle catalysts with core-shell, skeleton, or skin-type structures have shown to outperform Pt nanoparticles. However, complications arise when such catalysts are exposed to harsh operation conditions of the fuel cell where Pt and the alloying metal undergoes consecutive oxidation/reduction and dissolution, de-activating the catalyst. Compared to nanoparticles, extended surfaces (bulk electrodes and nanostructured thin films) are more stable, but suffers from low surface area. In this study, we describe the concept of “nanosheet catalysts” with surface area, activity, and durability that are higher than conventional core-shell nanoparticles for the oxygen reduction reaction as well as the hydrogen oxidation of reformate fuel. Our approach utilizes metallic Ru nanosheets [1] with atomic-scale thickness prepared via thermal reduction of exfoliated RuO2nanosheets [2,3]. The metallic Ru nanosheet is used as a core for the synthesis of two-dimensional Ru-core@Pt-shell catalysts. Sub to a few monolayer Pt shell was successively formed on metallic Ru nanosheets with monoatomic thickness via galvanic displacement reaction between Cu and Pt2+. The electrochemically active Pt surface area of Ru-core@Pt-shell nanosheets with 1.5-4.5 monolayer Pt-shell was 112-151 m2 (g-Pt)‒ 1. This corresponds to Pt nanoparticles with 1-1.5 nm of diameter. A carbon supported core-shell nanosheet catalyst with a 3.5 monolayer Pt-shell (Ru@Pt-3.5ML(ns)/C) showed 4.5 times higher activity than the benchmark Pt/C catalyst for the oxygen reduction reaction with a slower degradation rate even at high potentials. For the anode reactions, Ru@Pt-1.5ML(ns)/C had 2 times higher hydrogen oxidation activity in pure H2 as well as 300 ppm CO containing H2, and better stability against potential cycling. The developed nanosheet catalysts should provide the high utilization of Pt towards catalytic reactions and concurrently the stability of extended surfaces, offering a practical solution to the trade-off issue. This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan. [1] K. Fukuda and K. Kumagai, e-Journal Surf. Sci. Nanotechnol., 12, 97 (2014). [2] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003). [3] K. Fukuda, H. Kato, J. Sato, W. Sugimoto and Y. Takasu, J. Solid State Chem., 182, 2997 (2009). Figure 1

(Invited) 2D Van Der Waals Crystals and Heterostructures for Smart Life

The experimental demonstration of graphene in 2004, a truly one-atom thick layer of carbon atoms, has opened up a window to the two-dimensional (2D) world of materials. This has subsequently triggered a surge of research activities on various 2D crystals including single layers of hexagonal-boron nitride (h-BN), several dichalcogenides (such as MoS2 and WSe2), and complex oxides. Atomic scale thicknesses (few Å/layer) of 2D semiconducting crystals and their controllable precise band gaps as a function of number of layers also enable the scaling of electronic devices without inducing performance variations. Moreover, seamless planar synthesis and stacking of various 2D crystals can be exploited to build novel lateral and vertical heterostructures, respectively. This talk will highlight and discuss the prospects of such 2D crystals for designing ultra-low power, low-loss, and ultra-energy-efficient active and passive devices targeted for designing next-generation green electronics needed to support the emerging paradigm of Internet of Things. More specifically, this talk will examine the genesis of the power dissipation challenge in conventional MOSFETs, and provide an overview of the recently demonstrated 2D-channel tunnel-FET from my group that overcomes this challenge and is a fundamentally different transistor employing several innovations. This talk will also bring forward some applications uniquely enabled by 2D crystals, including sensors and flexible high-frequency electronics for improving quality of life, and discuss related challenges and opportunities.

Nanoscale Structuring of Surfaces by Using Atomic Layer Deposition.

Controlled structuring of surfaces is interesting for a wide variety of areas, including microelectronic device fabrication, optical devices, bio(sensing), (electro-, photo)catalysis, batteries, solar cells, fuel cells, and sorption. A unique feature of atomic layer deposition (ALD) is the possibility to form conformal uniform coatings on arbitrarily shaped materials with controlled atomic-scale thickness. In this Minireview, we discuss the potential of ALD for the nanoscale structuring of surfaces, highlighting its versatile application to structuring both planar substrates and powder materials. Recent progress in the application of ALD to porous substrates has even made the nanoscale structuring of high-surface-area materials now feasible, thereby enabling novel applications, such as those in the fields of catalysis and alternative energy.

Electronic properties of transition-metal dichalcogenides

Abstract

Colloidal CdSe(1-x)S(x) Nanoplatelets with Narrow and Continuously-Tunable Electroluminescence.

Colloidal nanoplatelets, quasi-two-dimensional quantum wells, have recently been introduced as colloidal semiconductor materials with the narrowest known photoluminescence line width (∼10 nm). Unfortunately, these materials have not been shown to have continuously tunable emission but rather emit at discrete wavelengths that depend strictly on atomic-layer thickness. Herein, we report a new synthesis approach that overcomes this issue: by alloying CdSe colloidal nanoplatelets with CdS, we finely tune the emission spectrum while still leveraging atomic-scale thickness control. We proceed to demonstrate light-emitting diodes with sub-bandgap turn-on voltages (2.1 V for a device emitting at 2.4 eV) and the narrowest electroluminescence spectrum (FWHM ∼12.5 nm) reported for colloidal semiconductor LEDs.

이산화 티타늄 위에서의 원자층 증착법 백금의 성장 특성

Atomic layer deposition (ALD) is essential for the fabrication of nanoscale electronic devices because it has excellent conformality, atomic scale thickness control, and large area uniformity. Metal thin films are one of the important material components for electronic devices as a conductor. As the size of electronic devices shrinks, the thickness of metal thin films is decreased down to few nanometers, and the metal films become non-continuous due to inherent island growth of metal below a critical thickness. So, fabrication of continuous metal thin films by ALD is fundamentally and practically important. Since ALD films are grown through self-saturated reactions between precursors on surface, initial growth characteristics significantly depend on the surface properties and the selection of precursors. In this work, we investigated ALD Pt on TiO2 substrate by using trimethyl-methyl-cyclopentadienyl-Platinum (MeCpPtMe3) precursor and O3 reactant. By using O3 instead of O2, initial nucleation rate of ALD Pt was increased on TiO2 surface, resulting in formation of continuous thin Pt films. Morphologies of ALD Pt on TiO2 were characterized by using Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). Crystallinity of ALD Pt on TiO2 correlated with its growth characteristics was analyzed by X-Ray Diffraction (XRD).

Plasmonic eigenmodes in individual and bow-tie graphene nanotriangles.

In classical electrodynamics, nanostructured graphene is commonly modeled by the computationally demanding problem of a three-dimensional conducting film of atomic-scale thickness. Here, we propose an efficient alternative two-dimensional electrostatic approach where all calculation procedures are restricted to the graphene sheet. Furthermore, to explore possible quantum effects, we perform tight-binding calculations, adopting a random-phase approximation. We investigate multiple plasmon modes in 20 nm equilateral triangles of graphene, treating the optical response classically as well as quantum mechanically. Compared to the classical plasmonic spectrum which is “blind” to the edge termination, we find that the quantum plasmon frequencies exhibit blueshifts in the case of armchair edge termination of the underlying atomic lattice, while redshifts are found for zigzag edges. Furthermore, we find spectral features in the zigzag case which are associated with electronic edge states not present for armchair termination. Merging pairs of triangles into dimers, plasmon hybridization leads to energy splitting that appears strongest in classical calculations while splitting is lower for armchair edges and even more reduced for zigzag edges. Our various results illustrate a surprising phenomenon: Even 20 nm large graphene structures clearly exhibit quantum plasmonic features due to atomic-scale details in the edge termination.

シリコン表面超構造における超伝導輸送現象

Silicon surface reconstructions with metal adatoms are two-dimensional systems with atomic-scale thickness. We have successfully measured the superconducting transition of a representative silicon surface reconstruction Si(111)-(√7×√3)-In by electron transport measurement. In this article, we report our results together with the experimental techniques and future prospect for surface superconductors.

Mitigating irreversible capacity losses from carbon agents via surface modification

Greatly improved cycling performance has been demonstrated with conformally coated lithium-ion electrodes by atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques. This paper reports the impact of coating on the electrode additives towards mitigating undesired parasitic reactions during cycling. The ALD and MLD coatings with conformality and atomic scale thickness control effectively stabilize the surface of the electrode components, and the current collector, resulting in the increase of coulombic efficiency throughout cycling. The organic fragment integrated into the recently developed MLD process allows the coating to possess excellent mechanical properties and enhanced ionic conductivity, which significantly reduces cell polarizations throughout cycling. This work validates the importance of ALD and MLD as surface modifiers and further demonstrates their versatility and compatibility with lithium-ion battery technology.

Can 2D-Nanocrystals Extend the Lifetime of Floating-Gate Transistor Based Nonvolatile Memory?

Conventional floating-gate (FG) transistors (made with Si/poly-Si) that form the building blocks of the widely employed nonvolatile flash memory technology face severe scaling challenges beyond the 12-nm node. In this paper, for the first time, a comprehensive evaluation of the FG transistor made from emerging nanocrystals in the form of 2-dimensional (2D) transition metal dichalcogenides (TMDs) and multilayer graphene (MLG) is presented. It is shown that TMD based 2D channel materials have excellent gate length scaling potential due to their atomic scale thicknesses. On the other hand, employing MLG as FG greatly reduces cell-to-cell interference and alleviates reliability concerns. Moreover, it is also revealed that TMD/MLG heterostructures enable new mechanism for improving charge retention, thereby allowing the effective oxide thickness of gate dielectrics to be scaled to a few nanometers. Thus, this work indicates that judiciously selected 2D-nanocrystals can significantly extend the lifetime of the FG-based memory cell.

Defects controlled wrinkling and topological design in graphene

Due to its atomic scale thickness, the deformation energy in a free standing graphene sheet can be easily released through out-of-plane wrinkles which, if controllable, may be used to tune the electrical and mechanical properties of graphene. Here we adopt a generalized von Karman equation for a flexible solid membrane to describe graphene wrinkling induced by a prescribed distribution of topological defects such as disclinations (heptagons or pentagons) and dislocations (heptagon–pentagon dipoles). In this framework, a given distribution of topological defects in a graphene sheet is represented as an eigenstrain field which is determined from a Poisson equation and can be conveniently implemented in finite element (FEM) simulations. Comparison with atomistic simulations indicates that the proposed model, with only three parameters (i.e., bond length, stretching modulus and bending stiffness), is capable of accurately predicting the atomic scale wrinkles near disclination/dislocation cores while also capturing the large scale graphene configurations under specific defect distributions such as those leading to a sinusoidal surface ruga22The Latin word ruga is used to refer a large-amplitude state of a wrinkle, crease, ridge or fold (Diab et al., 2013). or a catenoid funnel.

Giant dielectric constant in TiO2/Al2O3 nanolaminates grown on doped silicon substrate by pulsed laser deposition

High quality amorphous nanolaminates by means of alternate Al2O3 and TiO2 oxide sublayers were grown with atomic scale thickness control by pulsed laser deposition. A giant dielectric constant (>10 000), strongly enhanced compared to the value of either Al2O3 or TiO2 or their solid solution, was observed. The dependence of the dielectric constant and the dielectric loss on the individual layer thickness of each of the constituting materials was investigated between 0.3 nm and 1 nm, in order to understand the prevailing mechanisms and allow for an optimization of the performances. An impedance study confirmed as the key source of the giant dielectric constant a Maxwell–Wagner type dielectric relaxation, caused by space charge polarization in the nanolaminate structure. The current work provides better insight of nanolaminates and their sublayer thickness engineering for potential applications.

Controlling Catalytic Selectivities during CO2 Electroreduction on Thin Cu Metal Overlayers

The catalytic activity and selectivity of the electrochemical CO2 reduction on Cu overlayers with varying atomic-scale thickness on Pt was investigated. Hydrogen, methane, and ethylene were the main products. Beyond an activity improvement with increasing copper layer thickness, we observed that the thickest 15 nm Cu layer behaved bulk-like and resulted in high relative faradaic selectivities for hydrocarbons. With decreasing Cu layer thickness, the formation of methane decreased much faster than that of ethylene. As a result, the relative faradaic selectivity of the technologically useful product ethylene increased sharply. The selectivity ratios between methane and ethylene were independent of electrode potential on a Cu monolayer. A combination of geometric tensile strain effects and electronic effects is believed to control the surface reactivity and product distribution on the copper surfaces. This study highlights the general strategy to tune product distributions on thin metal overlayers.

Nanoscratch properties of extremely thin diamond-like carbon films

Scratch properties of extremely thin (targeted at 0.03–5.0-nm-thick) diamond-like carbon (DLC) films deposited by the filtered cathodic vacuum arc (FCVA) and electron cyclotron resonance chemical vapor deposition (ECR-CVD) methods are investigated. The difference in profiles and friction coefficients of scratches in both the corn and edge directions are evaluated. The difference of scratch properties between deposition methods is clearly evaluated by the scratching in the corn direction. When scratching in the corn direction, the friction coefficient and wear depth of the FCVA-DLC film increased rapidly at critical load, whereas those of the ECR-CVD-DLC film increased gradually even beyond this critical load. These results are deduced to be caused by the differences in the hardness and brittleness of the films. The dependence of nanoscratch friction force and scratch profile on DLC film thickness can reveal differences in mechanical properties that correspond to atomic-scale thickness.

Atomic Scale Thickness Control of SOI Wafers for Fully Depleted Applications

Smart Cut® Technology has recently been moved forward to a performance that seemed impossible until just recently. FDSOI requirements demand a method to transfer an ultra-thin crystalline layer with near perfect uniformity. Silicon thickness variation across all UTBOX wafers has reached +/- 5 A all points all wafers, while at the same time roughness has been improved to reach 0.8 A 30x30 µm2 RMS. BOX is available over a wide range of thicknesses from 10 to 50nm, ready to satisfy all possible applications. Such development efforts have now passed process technology qualification [1] and provided a proven path for further node scaling [2]. This paper reports the latest achievements in state of the art thickness and roughness control, which is enabling very low variability for fully depleted devices today.