Direct observation of single-walled carbon nanotube growth at the atomistic scale

The growth dynamics of a single-walled carbon nanotube (SWNT) is observed in real-time using an in situ ultrahigh vacuum transmission electron microscope at 650 degrees C. SWNTs preferentially grow on smaller sized catalyst particles (diameter 相似文献   

Direct atomic scale imaging of III-V nanowire surfaces

We have succeeded in direct atomic scale imaging of the exterior surfaces of III-V nanowires by scanning tunneling microscopy (STM). By using atomic hydrogen, we expose the crystalline surfaces of InAs nanowires with regular InP segments in vacuum while retaining the wire morphology. We show images with atomic resolution of the two major types of InAs wurtzite nanowire surface facets and scanning tunneling spectroscopy (STS) data. Ab initio calculations of the lowest energy surface structures and simulated STM images, agree very well with experiments.     

Predictive modeling of atomic layer deposition on the feature scale

A feature scale simulator for atomic layer deposition (ALD) is presented that combines a Boltzmann equation transport model with chemistry models. A simple but instructive chemistry is considered; one reactant species adsorbs onto the surface, and a second reactant reacts with it from the gas phase (Eley–Rideal). This work includes potential desorption of the adsorbed species during purge steps, which may or may not play a role in any given ALD system. Three sets (cases) of rate parameters are chosen to compare chemical rates with transport rates. The duration of the ALD pulses and the geometry of the representative feature are the same for each case. Simulation results are presented for all four steps in one ALD cycle, adsorption, post-adsorption purge, reaction, and post-reaction purge. The results are extended to multiple ALD cycles, and the monolayers per cycle are estimated. We highlight the potential trade-off between pulse durations and deposition rate (wafer throughput); e.g. the time penalty required to increase the amount adsorbed during the adsorption step. The simulation methodology we present can be used to determine the pulse durations that maximize throughput for a given chemistry and chemical rate parameters. One overall observation is that transport is fast relative to chemical reactions, for reasonable kinetic parameters.     

Direct observation of local atomic order in a metallic glass

The determination of the atomic configuration of metallic glasses is a long-standing problem in materials science and solid-state physics. So far, only average structural information derived from diffraction and spectroscopic methods has been obtained. Although various atomic models have been proposed in the past fifty years, a direct observation of the local atomic structure in disordered materials has not been achieved. Here we report local atomic configurations of a metallic glass investigated by nanobeam electron diffraction combined with ab initio molecular dynamics simulation. Distinct diffraction patterns from individual atomic clusters and their assemblies, which have been theoretically predicted as short- and medium-range order, can be experimentally observed. This study provides compelling evidence of the local atomic order in the disordered material and has important implications in understanding the atomic mechanisms of metallic-glass formation and properties.     

Direct observation of isolated ultrananodimensional diamond clusters using atomic force microscopy

Isolated ultrananodimensional diamond (UND) particles obtained by means of detonation synthesis have been studied using atomic force microscopy (AFM). The UND particles were deposited onto the surface of highly oriented pyrolytic graphite from a suspension based on organic compounds. The deposited UND particles were deaggregated using a two-stage treatment with ultrasound and high-dynamic-pressure pulses. The isolated UND particles were stabilized in suspension by a benzene additive. AFM images of individual UND particles have been obtained, and the phenomenon of their alignment along atomic steps on the substrate surface has been observed.     

Direct observation of epitaxial alignment of Au on MoS2 at atomic resolution

The morphology and structural stability of metal/2D semiconductor interfaces strongly affect the performance of 2D electronic devices and synergistic catalysis. However, the structural evolution at the interfaces has not been well explored particularly at atomic resolution. In this work, we study the structural evolution of Au nanoparticles (NPs) on few-layer MoS₂ by high resolution transmission electron microscope (HRTEM) and quantitative high-angle annular dark field scanning TEM. It is found that in the transition of Au from nanoparticles to dendrites, a dynamically epitaxial alignment between Au and MoS₂ lattices is formed, and Moiré patterns can be directly observed in HRTEM images due to the mismatch between Au and MoS₂ lattices. This epitaxial alignment can occur in ambient conditions, and can also be accelerated by the irradiation of high-energy electron beam. In situ observation clearly reveals the rotation of Au NPs, the atom migration inside Au NPs, and the transfer of Au atoms between neighboring Au NPs, finally leading to the formation of epitaxially aligned Au dendrites on MoS₂. The structural evolution of metal/2D semiconductor interfaces at atomic scale can provide valuable information for the design and fabrication of the metal/2D semiconductor nano-devices with desired physical and chemical performances.

     

Characterization of titanium dioxide atomic layer growth from titanium ethoxide and water

Atomic layer growth of titanium dioxide from titanium ethoxide and water was studied. Real-time quartz crystal microbalance measurements revealed that adsorption of titanium ethoxide is a self-limited process at substrate temperatures 100–250°C. A relatively small amount of precursor ligands was released during titanium ethoxide adsorption while most of them was exchanged during the following water pulse. At temperatures 100–150°C, incomplete reaction between surface intermediates and water hindered the film growth. Nevertheless, the deposition rate reached 0.06 nm per cycle at optimized precursor doses. At substrate temperatures above 250°C, the thermal decomposition of titanium ethoxide markedly influenced the growth process. The growth rate increased with the reactor temperature and titanium ethoxide pulse time but it insignificantly depended on the titanium ethoxide pressure. Therefore reproducible deposition of thin films with uniform thickness was still possible at substrate temperatures up to 350°C. The films grown at 100–150°C were amorphous while those grown at 180°C and higher substrate temperature, contained polycrystalline anatase. The refractive index of polycrystalline films reached 2.5 at the wavelength 580 nm.     

Preferential growth of ZnO thin films by the atomic layer deposition technique

Preferred orientation of ZnO thin films deposited by the atomic layer deposition (ALD) technique could be manipulated by deposition temperature. In this work, diethyl zinc (DEZn) and deionized water (H(2)O) were used as a zinc source and oxygen source, respectively. The results demonstrated that (10.0) dominant ZnO thin films were grown in the temperature range of 155-220?°C. The c-axis crystal growth of these films was greatly suppressed. Adhesion of anions (such as fragments of an ethyl group) on the (00.2) polar surface of the ZnO thin film was believed to be responsible for this suppression. In contrast, (00.2) dominant ZnO thin films were obtained between 220 and 300?°C. The preferred orientations of (10.0) and (00.2) of the ZnO thin films were examined by XRD texture analysis. The texture analysis results agreed well with the alignments of ZnO nanowires (NWs) which were grown from these ZnO thin films. In this case, the nanosized crystals of ZnO thin films acted as seeds for the growth of ZnO nanowires (NWs) by chemical vapor deposition (CVD) process. The highly (00.2) textured ZnO thin films deposited at high temperatures, such as 280?°C, contained polycrystals with the c?axis perpendicular to the substrate surface and provided a good template for the growth of vertically aligned ZnO NWs.     

Direct observation of a noncatalytic growth regime for GaAs nanowires

We identify a new noncatalytic growth regime for molecular beam epitaxially grown GaAs nanowires (NWs) that may provide a route toward axial heterostructures with discrete material boundaries and atomically sharp doping profiles. Upon increase of the As/Ga flux ratio, the growth mode of self-induced GaAs NWs on SiO(2)-masked Si(111) is found to exhibit a surprising discontinuous transition in morphology and aspect ratio. For effective As/Ga ratios <1, in situ reflection high-energy electron diffraction measurements reveal clear NW growth delay due to formation of liquid Ga droplets since the growth proceeds via the vapor-liquid-solid mechanism. In contrast, for effective As/Ga ratios >1 an immediate onset of NW growth is observed indicating a transition to droplet-free, facet-driven selective area growth with low vertical growth rates. Distinctly different microstructures, facet formation and either the presence or absence of Ga droplets at the apex of NWs, are further elucidated by transmission electron microscopy. The results show that the growth mode transition is caused by an abrupt change from As- to Ga-limited conditions at the (111)-oriented NW growth front, allowing precise tuning of the dominant growth mode.     

原子层沉积低温制备AZO薄膜

采用原子层沉积技术与改进的Al掺杂模式在石英玻璃基体上低温制备AZO薄膜,利用椭圆偏振仪、原子力显微镜、X射线衍射仪、X射线光电子能谱仪、Hall效应测试仪系统地对样品的生长速率、表面形貌、晶体结构、薄膜成分与电学性能进行了表征和分析。结果表明,采用原子层沉积在150℃下制备AZO薄膜,其为六方纤锌矿结构,Al掺杂对Zn O的(002)有明显的抑制作用,Al在基体中弥散分布,其部分替换Zn O晶格中的Zn,以Al—O的形式存在于晶体中,晶体中存在大量的氧空位,最佳铝锌循环比为1∶19,此条件下AZO薄膜电阻率为4.61×10-4Ω·cm。     

Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition

We report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH₃). At 185 °C, deposition rate saturated for TMA and NH₃ doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH₃ resulted in a constant growth rate of ~ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≥ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ~ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum.     

In-situ synchrotron X-ray scattering study of thin film growth by atomic layer deposition

We report an atomic layer deposition chamber for in-situ synchrotron X-ray scattering study of thin film growth. The chamber was designed for combined synchrotron X-ray reflectivity and two-dimensional grazing-incidence X-ray diffraction measurement to do a in-situ monitoring of ALD growth. We demonstrate ruthenium thermal ALD growth for the performance of the chamber. 10, 20, 30, 50, 70, 100, 150 and 250-cycled states are measured by X-ray scattering methods during ALD growth process. Growth rate is calculated from thickness values and the surface roughness of each state is estimated by X-ray reflectivity analysis. The crystal structure of initial growth state is observed by Grazing-incidence X-ray diffraction. These results indicate that in-situ X-ray scattering method is a promising analysis technique to investigate the initial physical morphology of ALD films.     

Direct observation of X-ray induced atomic motion using scanning tunneling microscope combined with synchrotron radiation

X-ray induced atomic motion on a Ge(111)-c(2 x 8) clean surface at room temperature was directly observed with atomic resolution using a synchrotron radiation (SR)-based scanning tunneling microscope (STM) system under ultra high vacuum condition. The atomic motion was visualized as a tracking image by developing a method to merge the STM images before and after X-ray irradiation. Using the tracking image, the atomic mobility was found to be strongly affected by defects on the surface, but was not dependent on the incident X-ray energy, although it was clearly dependent on the photon density. The atomic motion can be attributed to surface diffusion, which might not be due to core-excitation accompanied with electronic transition, but a thermal effect by X-ray irradiation. The crystal surface structure was possible to break even at a lower photon density than the conventionally known barrier. These results can alert X-ray studies in the near future about sample damage during measurements, while suggesting the possibility of new applications. Also the obtained results show a new availability of the in-situ SR-STM system.     

Direct comparison between experiments and computations at the atomic length scale: a case study of graphene

This paper discusses a set of recent experimental results in which the mechanical properties of monolayer graphene molecules were determined. The results included the second-order elastic modulus which determines the linear elastic behavior and an estimate of the third-order elastic modulus which determines the non-linear elastic behavior. In addition, the distribution of the breaking force strongly suggested the graphene to be free of defects, so the measured breaking strength of the films represented the intrinsic breaking strength of the underlying carbon covalent bonds. The results of recent simulation efforts to predict the mechanical properties of graphene are discussed in light of the experiments. Finally, this paper contains a discussion of some of the extra challenges associated with experimental validation of multi-scale models.     

Simulation of growth dynamics in atomic layer deposition. Part I. Amorphous films

This first article in a series on simulation of growth of thin films by the ALD (= atomic layer deposition) technique deals with the theoretical background for growth and preparation of amorphous films. The growth dynamics of amorphous thin films have been simulated on the basis of growth on randomly positioned hemisphere-shaped seed objects. The objects were positioned in the simulation space according to rules which intend to create entirely random or guided random patterns. The simulations are able to reproduce the non-linear growth in the initial stages of ALD growth according to type-2 substrate-inhibited growth, and provide a mechanism for this based on purely geometrical concepts; also a self-driving mechanism toward flat surfaces is demonstrated. An initial stage of roughening of the films occurs before smoothening takes over.     

Direct observation of inhomogeneous solid electrolyte interphase on MnO anode with atomic force microscopy and spectroscopy

Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young's modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems.     

Temperature dependence of SiO2 film growth with plasma-enhanced atomic layer deposition

The growth per cycle (GPC) temperature dependence was investigated for SiO₂ films prepared by plasma-enhanced atomic layer deposition (PEALD). During preparation of PEALD-SiO₂ using bis-diethyl-amino-silane, the GPC was saturated via increasing the precursor dose time and flow rate. The saturated GPC decreased with increasing deposition temperature. GPC saturation curves as a function of precursor dose time were analyzed by a two-step adsorption model, where the amino-silane reversibly adsorbed (physisorption) during the first step, and then irreversibly adsorbed (chemisorption) on the SiO₂ surface upon reaction with surface OH absorbents. This model is in good quantitative agreement with the saturation curve. The GPC value was determined by the surface reaction of amino-silane with OH sites, whose surface density was decreased by increasing the deposition temperature. The GPC saturation became slower with increasing deposition temperature, because the desorption rate of the physisorbed precursor increased with increasing temperature.     

Effect of catalyst layer density and growth temperature in rapid atomic layer deposition of silica using tris(tert-pentoxy)silanol

Rapid atomic layer deposition (RALD) of SiO? thin films was achieved using trimethyl-aluminum and tris(tert-pentoxy)silanol (TPS) as the catalyst and Si precursor, respectively. A maximum growth rate as high as ～28 nm/cycle was obtained by optimizing the catalyst layer density, whereas the previous reports showed lower values of 12 to 17 nm/cycle [Hausmann et al. Science2002, 298, 402-406; Burton et al. Chem. Mater. 2008, 20, 7031-7043]. When the growth temperature was increased from 140 to 230 °C, the growth rate was not much reduced and the TPS pulse time showing a saturated growth rate became rather longer. Si-CH?, Si-OH, and Si-H bonds were not detected in infrared spectra from the RALD SiO? film grown at 230 °C. The film quality could be enhanced substantially by applying a higher growth temperature and an in situ post plasma treatment process.