Ultrahigh Vacuum Transmission Electron Microscope Research Articles

Diffusion Activation Energy and Desorption Activation Energy for Astrochemically Relevant Species on Water Ice Show No Clear Relation

The activation energies for desorption (E des) and for surface diffusion (E sd) of adsorbed molecules on dust grains are two of the most important parameters for the chemistry in the interstellar medium. Although E des is often measured by laboratory experiments, the measurement of E sd is sparse. Due to the lack of data, astrochemical models usually assume a simple scaling relation, E sd = f E des, where f is a constant, irrespective of the adsorbed species. Here, we experimentally measure E sd for CH4, H2S, OCS, CH3OH, and CH3CN on water-ice surfaces using an ultrahigh-vacuum transmission electron microscope. Compiling the measured E sd values and E des values from the literature, we find that the value of f ranges from ∼0.2 to ∼0.7, depending on the species. Unless f (or E sd) for the majority of species is available, a natural alternative approach for astrochemical models is running multiple simulations, varying f for each species randomly. In this approach, ranges of molecular abundances predicted by multiple simulations, rather than abundances predicted by each simulation, are important. We here run 10,000 simulations of astrochemical models of molecular clouds and protostellar envelopes, randomly assigning a value of f for each species. In the former case, we identify several key species whose E sd most strongly affects the uncertainties of the model predictions; E sd for those species should be investigated in future laboratory and quantum chemical studies. In the latter case, uncertainties in the E sd of many species contribute to the uncertainties in the model predictions.

Ultrahigh strength and shear-assisted separation of sliding nanocontacts studied in situ

The behavior of materials in sliding contact is challenging to determine since the interface is normally hidden from view. Using a custom microfabricated device, we conduct in situ, ultrahigh vacuum transmission electron microscope measurements of crystalline silver nanocontacts under combined tension and shear, permitting simultaneous observation of contact forces and contact width. While silver classically exhibits substantial sliding-induced plastic junction growth, the nanocontacts exhibit only limited plastic deformation despite high applied stresses. This difference arises from the nanocontacts’ high strength, as we find the von Mises stresses at yield points approach the ideal strength of silver. We attribute this to the nanocontacts’ nearly defect-free nature and small size. The contacts also separate unstably, with pull-off forces well below classical predictions for rupture under pure tension. This strongly indicates that shearing reduces nanoscale pull-off forces, predicted theoretically at the continuum level, but not directly observed before.

Structural changes in metallic nanoislands on 2D materials during in situ annealing in ultra-high vacuum TEM

Structural changes in metallic nanoislands on 2D materials during in situ annealing in ultra-high vacuum TEM

Electric current distribution of a multiwall carbon nanotube

The electric current distribution in a multiwall carbon nanotube (MWCNT) was studied by in situ measuring the electric potential along an individual MWCNT in the ultra-high vacuum transmission electron microscope (TEM). The current induced voltage drop along each section of a side-bonded MWCNT was measured by a potentiometric probe in TEM. We have quantitatively derived that the current on the outermost shell depends on the applied current and the shell diameter. More proportion of the total electronic carriers hop into the inner shells when the applied current is increased. The larger a MWCNT’s diameter is, the easier the electronic carriers can hop into the inner shells. We observed that, for an 8 nm MWCNT with 10 μA current applied, 99% of the total current was distributed on the outer two shells.

A solid-state cation exchange reaction to form multiple metal oxide heterostructure nanowires.

Metal oxide nanostructures have been investigated extensively due to their wide range of physical properties; zinc oxide is one of the most promising materials. It exhibits fascinating functional properties and various types of morphologies. In particular, ZnO heterostructures have attracted great attention because their performance can be modified and further improved by the addition of other materials. In this study, we successfully transformed ZnO nanowires (NWs) into multiple ZnO/Al2O3 heterostructure NWs via a solid-state cation exchange reaction. The experiment was carried out in situ via an ultrahigh vacuum transmission electron microscope (UHV-TEM), which was equipped with a video recorder. Moreover, we analyzed the structure and composition of the heterostructure NWs by Cs-corrected STEM equipped with EDS. Based on these experimental results, we inferred a cation exchange reaction ion path model. Additionally, we investigated the defects that appeared after the cation reaction, which resulted from the remaining zinc ions. These multiple heterostructure ZnO/Al2O3 NWs exhibited excellent UV sensing sensitivity and efficiency.

Solid State Reaction to Form Metal Oxide Heterostructure Nanowire

ZnO nanostructure has been investigated extensively due to its fascinating functional properties. ZnO heterostructure especially attracts a lot attention owing to various kinds of applications such as light-emitting device, resistive random-access memory, and solar cell. In this study, we deposited few nanometer of Al onto ZnO nanowire by electron beam evaporator, and successfully transformed ZnO nanowires into Al2O3/ZnO axial heterostructure. This reaction was occurred through solid state reaction at 700˚C in ultrahigh vacuum transmission electron microscope (UHV-TEM). We directly observed diffusion process during heating; in addition, the analysis of the kinetic behavior was carried out through the in situ video. The structure and composition were also identified by energy dispersive spectrometry (EDS) and Cs-corrected STEM. This research provide a method to fabricate a new type of one dimensional Al2O3/ZnO axial heterostructure, which may be applied in opto-electrical device and nanosensors.

Growth dynamics and gas transport mechanism of nanobubbles in graphene liquid cells.

Formation, evolution and vanishing of bubbles are common phenomena in nature, which can be easily observed in boiling or falling water, carbonated drinks, gas-forming electrochemical reactions and so on. However, the morphology and the growth dynamics of the bubbles at nanoscale have not been fully investigated owing to the lack of proper imaging tools that can visualize nanoscale objects in the liquid phase. Here, we demonstrate for the first time that the nanobubbles in water encapsulated by graphene membrane can be visualized by in-situ ultra-high vacuum transmission electron microscopy. Our microscopic results indicate two distinct growth mechanisms of merging nanobubbles and the existence of a critical radius of nanobubbles that determines the unusually long stability of nanobubbles. Interestingly, the gas transport through ultrathin water membranes at nanobubble interface is free from dissolution, which is clearly different from conventional gas transport that includes condensation, transmission and evaporation.

In situ tuning and probing the ambipolar field effect on multiwall carbon nanotubes

We report a method of fabricating ultra-clean and hysteresis-free multiwall carbon nanotube field-effect transistors (CNFETs) inside the ultra-high vacuum transmission electron microscope equipped with a movable gold tip as a local gate. By tailoring the shell structure of the nanotube and varying the drain-source voltage (Vds), we can tune the electronic characteristic of a multiwall CNFET in situ. We have also found that the Schottky barriers of a multiwall CNFET are generated within the nanotube, but not at the nanotube/electrode contacts, and the barrier height has been derived. We have subsequently demonstrated the ambipolar characteristics of the CNFET with concurrent high-resolution imaging and local gating.

Characterization of the cleaning process on a transferred graphene

Intrinsic graphene possesses many outstanding physical and chemical properties, but their full explorations are often hindered by the effects of substrate and/or contamination. The authors employ the ultrahigh vacuum transmission electron microscopy equipped with a residual gas analyzer to in-situ characterize an effective decontamination process on a suspended graphene. Raman spectroscopic spectra further verify the cleanness of the resultant graphene membrane. The authors also present two contrasting growth morphologies of copper nanoparticles obtained on both clean and unclean graphene surfaces and show that the intrinsic growth dynamics can only manifest on the surface without contaminations.

Real Time Observation of the Formation of Hollow Nanostructures through Solid State Reactions

We demonstrate the formation of hollow nickel germanide nanostructures of Ni-Ge core-shell nanoparticles by solid state reactions. The structural evolutions of nickel germanide hollow nanostructures have been investigated in real-time ultrahigh vacuum transmission electron microscopy (UHV-TEM). Annealed above 450 °C, the nonequilibrium interdiffusion of core and shell species occurred at the interface; thus, Ni germanide hollow nanostructures were formed by solid state reactions involving the Kirkendall effect. In addition, the different hollow nanostructures formed from different core diameters of Ni-Ge core-shell nanoparticles have been studied. Also, we propose the mechanism with effects of the size and annealing duration on the solid state reactions based on the Kirkendall effect.

Growth of the Manganese Silicide/Silicon Nanowire Heterostructures and Their Physical Properties

Metal silicide nanowires (NWs) are of great interesting materials with diverse physical properties. Among these silicides, manganese silicides nanostructures have been attracted wide attention due to their several potential applications, including microelectronics, optoelectronics, spintronics and thermoelectric devices. In this work, we exhibited the formation of pure manganese silicide and manganese silicide/silicon nanowire heterostructures through solid state reaction with line contacts between manganese pads and silicon NWs. The growth process, structure and composition analysis of manganese silicide NWs have been investigated by in-situ ultrahigh vacuum transmission electron microscopy (UHV-TEM) and energy dispersive X-ray spectroscopy (EDX), respectively. The growth rates and the formation of various manganese silicide phases under thermal effect were systematically studied. We also characterized the dynamic diffusion of manganese atoms in the growth process and discussed the formation mechanism. Furthermore, we have investigated the electrical transport properties and fabricated field effect transistors (FETs) of the manganese silicide/silicon/manganese silicide nanowire heterostructures based devices. In addition to fundamental science, the significance of the study will be helpful for future processing techniques in nanotechnology and related applications.

Formation of gold nanocontacts in an ultrahigh vacuum transmission electron microscope: A kinetic Monte Carlo simulation

The effective method for the simulation of self-organization of metal nanocontacts produced in an ultrahigh vacuum transmission electron microscope has been designed. In the framework of this method the main stages of nanocontact formation have been determined. The basic diffusion events which are responsible for the shape of nanocontacts and the time of their formation have been outlined. The self-organization of nanocontacts with different orientation relative to the base fcc lattice has been studied. The results of our simulations are in a good agreement with experimental data.

Transient Oxidation of Cu-5at.%Ni(001): Temperature Dependent Sequential Oxide Formation

The transient oxidation stage of a model metal alloy thin film was characterized with in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and analytical high-resolution TEM. We observed the formations of nanosized NiO and Cu2O islands when Cu-5at.%Ni(001) was exposed to oxygen partial pressure, $$ {\text{pO}}_{ 2} = 1 \times 10^{ - 4} \,{\text{Torr}} $$ and various temperatures in situ. At 350 °C epitaxial Cu2O islands formed initially and then NiO islands appeared on the surface of the Cu2O island, whereas at 550 °C NiO appeared first. XPS and TEM revealed a sequential formation of NiO and then Cu2O islands at 550 °C. The temperature-dependent oxide selection may be due to an increase of the diffusivity of Ni in Cu with increasing temperature.

Endoepitaxial growth of hexagonal-Fe13Ge8 islands on Cubic-Ge(001)

The growth and shape evolution of epitaxial Fe13Ge8 (hexagonal lattice) islands on single crystal Ge(001) (cubic lattice) substrate was observed in real time using an in situ ultra-high vacuum transmission electron microscope (TEM). Post-deposition high-resolution TEM in conjunction with stereographic projection enabled the identification of the interface structure between the Fe13Ge8 islands and the Ge substrate. Only one low-energy coherent interface formed via Fe13Ge8 islands growing into the substrate along the inclined Ge(11̄1) plane. This indicates that minimization of net interfacial energy is the driving force for hexagonal Fe13Ge8 islands formation on Ge(001).

Study of ballistic gold conductor using ultra-high-vacuum transmission electron microscopy

Metal contacts are regarded as key elements of nanometer-scale electronics. Since gold contacts show quantized conductance even at room temperature, much effort has been devoted to understand their conductance behavior on the nanoscale. However, gold contacts do not always show quantized conductance steps during their thinning process, the reason for which has been an open question. Thus, it is necessary to investigate the relationship between the atomic structure and conductance of gold contacts. We developed a custom-made scanning tunneling microscope combined with an ultra-high vacuum transmission electron microscope to clarify the structural dependence of conductance quantization in gold contacts. We found that [111] and [001] gold contacts with a bottleneck shape showed a gradual decrease in conductance with elastic elongation and successive conductance jumps with periodic plastic deformation. In contrast, [110] gold contacts had a hexagonal prism shape (termed gold [110] nanowires). In the conductance histogram, peaks appeared nearly in steps of the quantum unit. We found that the prominent peaks corresponded to stable gold nanowires with a regular hexagonal cross-section. Following first-principles calculations, we confirmed that very thin gold [110] nanowires were ballistic conductors. The conductance behavior differed depending on the contact shape.

In situ ultra-high vacuum transmission electron microscopy studies of the transient oxidation stage of Cu and Cu alloy thin films

Because environmental stability is an essential property of most engineered materials, many theories exist to explain oxidation mechanisms. Yet, nearly all classical oxidation theories assume a uniform growing film, where structural changes were not considered because of the previous lack of experimental procedure to visualize this non-uniform growth in conditions that allowed for highly controlled surfaces and impurities. With the advent of vacuum technologies and advances in microcopy techniques, especially in situ, one can now see structural changes under controlled surface conditions. Here, we present a review of our systematic studies on the transient oxidation stages of a model metal system, Cu, and its alloys, Cu–Au and Cu–Ni, by in situ ultra-high vacuum transmission electron microscopy (UHV-TEM). The dependence of the oxidation behavior on the crystal orientation, oxygen pressure, temperature and alloying is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy and the interfacial strain energy. Heteroepitaxial concepts, developed to explain thin film formation on a dissimilar substrate material (e.g., Ge on Si), described well these initial oxidation stages.

Direct observation of melting behaviors at the nanoscale under electron beam and heat to form hollow nanostructures

We report the melting behaviours of ZnO nanowire by heating ZnO-Al(2)O(3) core-shell heterostructures to form Al(2)O(3) nanotubes in an in situ ultrahigh vacuum transmission electron microscope (UHV-TEM). When the ZnO-Al(2)O(3) core-shell nanowire heterostructures were annealed at 600 °C under electron irradiation, the amorphous Al(2)O(3) shell became single crystalline and then the ZnO core melted. The average vanishing rate of the ZnO core was measured to be 4.2 nm s(-1). The thickness of the Al(2)O(3) nanotubes can be precisely controlled by the deposition process. Additionally, the inner geometry of nanotubes can be defined by the initial ZnO core. The result shows a promising method to obtain the biocompatible Al(2)O(3) nanotubes, which may be applied in drug delivery, biochemistry and resistive switching random access memory (ReRAM).

In situ observation of self-assembled Fe 13Ge 8 nanowires growth on anisotropic Ge (1 1 0) surface

Self-assembled iron germanide nanowires (NWs) were grown by directly depositing Fe onto a Ge (1 1 0) substrate, in an in situ ultra-high vacuum transmission electron microscope from 430 to 500 °C. All observed NWs had a similar length/width aspect ratio (∼8:1) at all deposition temperatures, as well as the same elongation orientation with respect to the underlying Ge (1 1 0) substrate. The growth dynamics was investigated by real time observations of NWs growth at elevated temperatures. It is elucidated that the formation of NWs in similar shape at all deposited temperatures is attributed to the similar activation energy barriers in length and width of NWs, which can result in the constant growth rate independent of growth temperatures. Furthermore, the difference in pre-exponential factor along the length and width of growing islands arose due to the anisotropic constraint of the Ge (1 1 0) substrate, leading to the unique elongation of NWs. This growth dynamics suggests the possibility of uniform control of the morphology of self-assembled NWs, as well as other morphologies of bottom-up fabricated devices, at different deposition temperatures.

Growth of CuInSe2 and In2Se3/CuInSe2 Nano-Heterostructures through Solid State Reactions

In(2)Se(3) is an essential phase change material and CuInSe(2) is the fundamental basis of the copper-indium-gallium-diselenide (CIGS) solar energy material. In this paper, we demonstrate the feasibility to transform the phase change material to the solar energy material via the solid state reaction. The In(2)Se(3) nanobelts (NBs) were synthesized via the vapor-liquid-solid mechanism. The chemical composition and the optical properties were investigated by energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and reflectance and photoluminescence spectra. In the in situ observation of the solid state reaction with Cu to form the CuInSe(2) NBs with ultrahigh vacuum transmission electron microscopy, we observed the In(2)Se(3)/CuInSe(2) transformation at atomic scale in real time. The progression of the atomic layer at the interface provided the pertinent information on the kinetic mechanism. In(2)Se(3)/CuInSe(2) nano-heterostructures were also obtained in the present investigation. The approach to the CIGS nanosolar cell was also proposed. This study shall be beneficial in the development of high-performance nanowire solar cells and nanodevices with In(2)Se(3)/CuInSe(2) nano-heterostructures.

In situ observations of endotaxial growth ofCoSi2 nanowires on Si(110) using ultrahigh vacuum transmission electron microscopy

We report in situ observations of the growth of endotaxialCoSi2 nanowires on Si(110) using an ultrahigh vacuum transmission electronmicroscope with a miniature electron-beam deposition system locatedabove the pole-piece of the objective lens. Metal deposition at 750–850 °C results in formation of coherently strained silicide nanowires with a fixedlength/width (L/W) aspect ratio that depends strongly on temperature. Both dimensions evolve with time asL,W ∼ t1/3. To explain this behavior, we propose a fixed-shape growth mode based on thermallyactivated facet-dependent reactions. A second growth mode is also observed at850 °C, with dimensionsthat evolve as L ∼ t and W ∼ constant. This mode is accompanied by formation of an array of dislocations. We expect that otherendotaxial nanowire systems will follow coherently strained growth modes with similargeometrical constraints, as well as dislocated growth modes with different growth kinetics.