-
Journal of The Electrochemical Society 01/2012; 159:A222-A227. · 2.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Focused ion beam (FIB) instruments have proven to be an invaluable tool for transmission electron microscopy (TEM) sample preparation. FIBs enable relatively easy and site-specific cross-sectioning of different classes of materials. However, damage mechanisms due to ion bombardment and possible beam heating effects in materials limit the usefulness of FIBs. Materials with adequate heat conductivity do not suffer from beam heating during FIB preparation, and artifacts in materials such as metals and ceramics are primarily limited to defect generation and Ga implantation. However, in materials such as polymers or biological structures, where heat conductivity is low, beam heating can also be a problem. In order to examine FIB damage in polymers we have undertaken a systematic study by exposing sections of a PS-b-PMMA block copolymer to the ion beam at varying beam currents and sample temperatures. The sections were then examined by TEM and scanning electron microscopy (SEM) and analyzed using electron energy loss spectroscopy (EELS). Our empirical results show beam heating in polymers due to FIB preparation can be limited by maintaining a low beam current (≤100pA) during milling.
Ultramicroscopy 02/2011; 111(3):191-9. · 2.47 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The intriguing possibility of frictionless gliding of one solid surface on another has been predicted for certain incommensurate interfaces in crystals, based on Aubry's solution to the Frenkel-Kontorova model of a harmonic chain in a periodic potential field. Here we test this prediction for grain boundaries by comparing atomistic simulations with direct experimental observations on the structure and load-deformation behavior of gold nanopillars containing a root-two incommensurate grain boundary. The simulations show supergliding at this boundary limited by finite-size effects which cause edges to act as defects of the incommensurate structure. Structural relaxation at the edges generates stacking faults, dislocations, and asymmetric surface steps. These features as well as the related load-displacement behavior are replicated by experimental observations on the compression of nanopillars using a quantitative nanoindentation device inside a transmission electron microscope. The good agreement between the observed and predicted behavior suggests that incommensurate interfaces could play an important role in the deformation of polycrystalline materials.
Nano Letters 02/2010; 10(2):695-700. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An electric-current-assisted method was used to mineralize dense hydrogels and create hydroxyapatite/hydrogel composites with unique hierarchical structures. The microstructure of the final material can be controlled by the mineralization technique and the chemistry of the organic matrix. A hydroxyapatite/hydrogel composite was obtained with a large inorganic content (approximately 60% of the weight of the organics). After being heated to 1050 degrees C, the sintered inorganic phase has a very uniformly distributed porosity and its Brunauer-Emmett-Teller (BET) surface area is 0.68 m(2)/g.
Journal of the American Chemical Society 08/2009; 131(29):9937-9. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have directly observed motion of inorganic nanoparticles during fluid evaporation using a transmission electron microscope. Tracking real-time diffusion of both spherical (5-15 nm) and rod-shaped (5 x 10 nm) gold nanocrystals in a thin film of water-15% glycerol reveals complex movements, such as rolling motions coupled to large-step movements and macroscopic violations of the Stokes-Einstein relation for diffusion. As drying patches form during the final stages of evaporation, particle motion is dominated by the nearby retracting liquid front.
Nano Letters 06/2009; 9(6):2460-5. · 13.20 Impact Factor
-
Enrique D Gomez,
Ashoutosh Panday,
Edward H Feng,
Vincent Chen,
Gregory M Stone, Andrew M Minor,
Christian Kisielowski,
Kenneth H Downing,
Oleg Borodin,
Grant D Smith,
Nitash P Balsara
[show abstract]
[hide abstract]
ABSTRACT: Energy-filtered transmission electron microscopy (EFTEM) was used to determine the distribution of lithium ions in solid polymer electrolytes for lithium batteries. The electrolytes of interest are mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene-block-ethylene oxide) copolymers (SEO). In contrast to current solid and liquid electrolytes, the conductivity of SEO/salt mixtures increases with increasing molecular weight of the copolymers. EFTEM results show that the salt is increasingly localized in the middle of the poly(ethylene oxide) (PEO) lamellae as the molecular weight of the copolymers is increased. Calculations of the inhomogeneous local stress field in block copolymer microdomains, modeled using self-consistent field theory, provide a quantitative explanation for this observation. These stresses, which increase with increasing molecular weight, interfere with the ability of PEO chains to coordinate with lithium cations near the walls of the PEO channels where ion mobility is expected to be low.
Nano Letters 03/2009; 9(3):1212-6. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The mechanical behavior of nanoporous gold was investigated during in situ nanoindentation in the transmission electron microscope. Thin films of nanoporous gold, with ligaments and pores of the order of 10-nm diameter, offer a highly constrained geometry for deformation and thus provide an opportunity to study the role of defects such as dislocations in the plasticity of nanomaterials. Films ranging in thickness from 75 to 300 nm were indented, while the motion of dislocations and deformation of ligaments were observed in situ. Dislocations were generated and moved along ligament axes, after which they interacted with other dislocations in the nodes of the porous network. For thicker films, the load-displacement curves exhibited load drops at regular intervals. The question of whether the spacing of these load drops was related to the collapse of pores in the nanoporous films or due to bursts of plasticity within the ligaments was investigated. Additionally, the effect of the indenter displacement rate on the mechanical response of these gold films with nanoscale porosity was investigated. Indentation rates were varied from 1.5 to 30 nm/s. There appears to be a kinetic factor related to dislocation nucleation, where slower displacement rates cause load drops to occur at shorter distance intervals and over longer time intervals.
Microscopy Research and Technique 02/2009; 72(3):232-41. · 1.79 Impact Factor
-
Advanced Materials 01/2009; 21(2):203 - 208. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Solid-state structural transformation coupled with an electronic property change is an important mechanism for nonvolatile information storage technologies, such as phase-change memories. Here we exploit phase-change GeTe single-nanowire devices combined with ex situ and in situ transmission electron microscopy to correlate directly nanoscale structural transformations with electrical switching and discover surprising results. Instead of crystalline-amorphous transformation, the dominant switching mechanism during multiple cycling appears to be the opening and closing of voids in the nanowires due to material migration, which offers a new mechanism for memory. During switching, composition change and the formation of banded structural defects are observed in addition to the expected crystal-amorphous transformation. Our method and results are important to phase-change memories specifically, but also to any device whose operation relies on a small scale structural transformation.
Nano Letters 01/2009; 8(12):4562-7. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Deformation twinning in nanocrystalline and ultrafine-grained materials has attracted much attention in recent years due to
the ability of a high density of twin boundaries to dramatically improve mechanical properties such as yield strength and
ductility. Various processing conditions such as ball milling, cryomilling, electrodeposition, and equi-channel angular extrusion
have been used to form deformation twins in metals. Most techniques for estimating the shear stress needed to form deformation
twins are based indirectly on the processing conditions. Here, a new method to directly measure the shear stress needed to
form twin boundaries through in-situ transmission electron microscopy nanocompression testing will be described.
JOM: the journal of the Minerals, Metals & Materials Society 08/2008; 60(9):66-70. · 1.42 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The fundamental processes that govern plasticity and determine strength in crystalline materials at small length scales have been studied for over fifty years. Recent studies of single-crystal metallic pillars with diameters of a few tens of micrometres or less have clearly demonstrated that the strengths of these pillars increase as their diameters decrease, leading to attempts to augment existing ideas about pronounced size effects with new models and simulations. Through in situ nanocompression experiments inside a transmission electron microscope we can directly observe the deformation of these pillar structures and correlate the measured stress values with discrete plastic events. Our experiments show that submicrometre nickel crystals microfabricated into pillar structures contain a high density of initial defects after processing but can be made dislocation free by applying purely mechanical stress. This phenomenon, termed 'mechanical annealing', leads to clear evidence of source-limited deformation where atypical hardening occurs through the progressive activation and exhaustion of dislocation sources.
Nature Material 03/2008; 7(2):115-9. · 32.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We establish a new systematic methodology for controlling the water retention of polymer electrolyte membranes. Block copolymer membranes comprising hydrophilic phases with widths ranging from 2 to 5 nm become wetter as the temperature of the surrounding air is increased at constant relative humidity. The widths of the moist hydrophilic phases were measured by cryogenic electron microscopy experiments performed on humid membranes. Simple calculations suggest that capillary condensation is important at these length scales. The correlation between moisture content and proton conductivity of the membranes is demonstrated.
Nano Letters 12/2007; 7(11):3547-52. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Thin films of nanoporous noble metals exhibit an interconnected, porous structure with ligament widths and pores on the order
of 10 nm or higher. In this study, thin film stress measurements and in-situ nanoindentation in a transmission-electron microscope
were performed to investigate the effects of nanoscale geometric confinement on the mechanical properties of metals and on
dislocation-mediated plasticity. Although some films exhibit macroscopic cracking, the deformation of individual ligaments
is completely ductile and clearly involves dislocation activity, even in 10 nm wide ligaments. The stresses generated in these
films during thermal cycling correspond to bulk stresses that approach the theoretical strength of the metal. Film stress
exhibits a dependence on film thickness, even though the ligament width is much smaller and would presumably govern deformation.
JOM: the journal of the Minerals, Metals & Materials Society 08/2007; 59(9):54-58. · 1.42 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In nanoscale contact experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal, defect-free lattice, a trend also observed in idealized molecular dynamics simulations. Here we report direct evidence that plasticity in a dislocation-free volume of polycrystalline aluminium can begin at very small forces, remarkably, even before the first sustained rise in repulsive force. However, the shear stresses associated with these very small forces do approach the theoretical shear strength of aluminium (approximately 2.2 GPa). Our observations entail correlating quantitative load-displacement measurements with individual video frames acquired during in situ nanoindentation experiments in a transmission electron microscope. We also report direct evidence that a submicrometre grain of aluminium plastically deformed by nanoindentation to a dislocation density of approximately 10(14) m(-2) is also capable of supporting shear stresses close to the theoretical shear strength. This result is contrary to earlier assumptions that a dislocation-free volume is necessary to achieve shear stresses near the theoretical shear strength of the material. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation.
Nature Material 10/2006; 5(9):697-702. · 32.84 Impact Factor
-
Eric A. Stach,
Tony Freeman, Andrew M. Minor,
Doug K. Owen,
John Cumings,
Mark A. Wall,
Tomas Chraska,
Robert Hull,
J.W. Morris,
A. Jr,
Ulrich Zettl
[show abstract]
[hide abstract]
ABSTRACT: In situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this article, we detail the development of a novel specimen goniometer that allows real-time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer-scale positioning of a sharp, conductive diamond tip onto the edge of an electron-transparent sample. This allows application of loads to nanometer-scale material volumes coupled with simultaneous imaging of the material's response. The emphasis in this report is qualitative and technique oriented, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.
Microscopy and Microanalysis 12/2001; 7(6):507-517. · 3.01 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on quantitative in situ transmission electron microscopy nanocompression tests used to study the deformation behavior of NiTi pillars on the nanometer scale. By recording the diffraction patterns in real time we have obtained direct evidence that the stress-induced B2 to B19′ (austenite to martensite) transformation exists in NiTi even when the sample size is below 200 nm. Correlation of the appearance of the B19′ phase in the diffraction pattern with our quantitative data showed that the transformation starts at approximately 1 GPa. We found that the transformation occurred through a multi-step process, and that the reverse transformation did not occur due to extensive deformation of the B19′ phase. Our results have direct implications for the application of the shape memory effect in nanoscale NiTi devices.
Acta Materialia.
-
[show abstract]
[hide abstract]
ABSTRACT: We have performed in-situ nanocompression testing in a transmission electron microscope (TEM) of copper pillars having dimensions of the same order of typical via and line structures used in the semiconductor industry. We show direct evidence that twin boundaries can withstand extensive plastic deformation and still retain their structure when compared to regular grain boundaries. Through real-time TEM observations we have verified the deformation mechanisms of twin boundaries predicted by molecular dynamic (MD) simulations. Our quantitative in-situ stress measurements are also in close agreement with those reported by MD and energetics based calculations.
Microelectronic Engineering.
-
[show abstract]
[hide abstract]
ABSTRACT: We present results from in situ transmission electron microscopy uniaxial nanocompression experiments of Mg and Mg–0.2 wt.% Ce pillars. We show examples of two common deformation mechanisms, basal plane sliding and extension twinning, for each sample. By directly measuring the critical stress for nucleation of either basal sliding or extension twinning we have established that there is a clear size effect for both deformation mechanisms, and that by alloying with Ce the critical stress for twinning was dramatically reduced.
Scripta Materialia. 64(3):292-295.
-
[show abstract]
[hide abstract]
ABSTRACT: As a tribute to the scientific work of Professor David Brandon, this paper delineates the possibilities of utilizing in situ transmission electron microscopy to unravel dislocation-grain boundary interactions. In particular, we have focused on the deformation characteristics of Al–Mg films. To this end, in situ nanoindentation experiments have been conducted in TEM on ultrafine-grained Al and Al–Mg films with varying Mg contents. The observed propagation of dislocations is markedly different between Al and Al–Mg films, i.e. the presence of solute Mg results in solute drag, evidenced by a jerky-type dislocation motion with a mean jump distance that compares well to earlier theoretical and experimental results. It is proposed that this solute drag accounts for the difference between the load-controlled indentation responses of Al and Al–Mg alloys. In contrast to Al–Mg alloys, several yield excursions are observed during initial indentation of pure Al, which are commonly attributed to the collective motion of dislocations nucleated under the indenter. Displacement-controlled indentation does not result in a qualitative difference between Al and Al–Mg, which can be explained by the specific feedback characteristics providing a more sensitive detection of plastic instabilities and allowing the natural process of load relaxation to occur. The in situ indentation measurements confirm grain boundary motion as an important deformation mechanism in ultrafine-grained Al when it is subjected to a highly inhomogeneous stress field as produced by a Berkovich indenter. It is found that solute Mg effectively pins high-angle grain boundaries during such deformation. The mobility of low-angle boundaries is not affected by the presence of Mg.
