50 research outputs found
Nanomechanics and intermolecular forces of amyloid revealed by four-dimensional electron microscopy
The amyloid state of polypeptides is a stable, highly organized
structural form consisting of laterally associated ÎČ-sheet protofilaments
that may be adopted as an alternative to the functional,
native state. Identifying the balance of forces stabilizing amyloid is
fundamental to understanding the wide accessibility of this state
to peptides and proteins with unrelated primary sequences, various
chain lengths, and widely differing native structures. Here, we
use four-dimensional electron microscopy to demonstrate that the
forces acting to stabilize amyloid at the atomic level are highly
anisotropic, that an optimized interbackbone hydrogen-bonding
network within ÎČ-sheets confers 20 times more rigidity on the
structure than sequence-specific sidechain interactions between
sheets, and that electrostatic attraction of protofilaments is only
slightly stronger than these weak amphiphilic interactions. The
potential biological relevance of the deposition of such a highly
anisotropic biomaterial in vivo is discussed
Transient Structures and Possible Limits of Data Recording in Phase-Change Materials
Phase-change materials (PCMs) represent the leading candidates for universal data storage devices, which exploit the large difference in the physical properties of their transitional lattice structures. On a nanoscale, it is fundamental to determine their performance, which is ultimately controlled by the speed limit of transformation among the different structures involved. Here, we report observation with atomic-scale resolution of transient structures of nanofilms of crystalline germanium telluride, a prototypical PCM, using ultrafast electron crystallography. A nonthermal transformation from the initial rhombohedral phase to the cubic structure was found to occur in 12 ps. On a much longer time scale, hundreds of picoseconds, equilibrium heating of the nanofilm is reached, driving the system toward amorphization, provided that high excitation energy is invoked. These results elucidate the elementary steps defining the structural pathway in the transformation of crystalline-to-amorphous phase transitions and describe the essential atomic motions involved when driven by an ultrafast excitation. The establishment of the time scales of the different transient structures, as reported here, permits determination of the possible limit of performance, which is crucial for high-speed recording applications of PCMs
Diffraction of Quantum Dots Reveals Nanoscale Ultrafast Energy Localization
Unlike in bulk materials, energy transport in low-dimensional and nanoscale systems may be governed by a coherent âballisticâ behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In order to study this coherent limit, both the spatial and temporal resolutions must be sufficient for the length-time scales involved. Here, we report observation of the lattice dynamics in nanoscale quantum dots of gallium arsenide using ultrafast electron diffraction. By varying the dot size from h = 11 to 46 nm, the length scale effect was examined, together with the temporal change. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes that travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. Because ultrafast diffraction provides the atomic-scale resolution and a sufficiently high time resolution, other nanostructured materials can be studied similarly to elucidate the nature of dynamical energy localization
Observing (non)linear lattice dynamics in graphite by ultrafast Kikuchi diffraction
In materials, the nature of the strainâstress relationship, which is fundamental to their properties, is determined by both the linear and nonlinear elastic responses. Whereas the linear response can be measured by various techniques, the nonlinear behavior is nontrivial to probe and to reveal its nature. Here, we report the methodology of time-resolved Kikuchi diffraction for mapping the (non)linear elastic response of nanoscale graphite following an ultrafast, impulsive strain excitation. It is found that the longitudinal wave propagating along the c-axis exhibits echoes with a frequency of 9.1 GHz, which indicates the reflections of strain between the two surfaces of the material with a speed of âŒ4 km/s. Because Kikuchi diffraction enables the probing of strain in the transverse direction, we also observed a higher-frequency mode at 75.5 GHz, which has a relatively long lifetime, on the order of milliseconds. The fluence dependence and the polarization properties of this nonlinear mode are entirely different from those of the linear, longitudinal mode, and here we suggest a localized breather motion in the a-b plane as the origin of the nonlinear shear dynamics. The approach presented in this contribution has the potential for a wide range of applications because most crystalline materials exhibit Kikuchi diffraction
Ultrafast atomic-scale visualization of acoustic phonons generated by optically excited quantum dots
Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dotsâgrown by Droplet Epitaxy on AlGaAsâwith sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible
4D Cryo-Electron Microscopy of Proteins
Cryo-electron microscopy is a form of
transmission electron microscopy that has been used to
determine the 3D structure of biological specimens in the
hydrated state and with high resolution. We report the
development of 4D cryo-electron microscopy by integrating
the fourth dimension, time, into this powerful
technique. From time-resolved diffraction of amyloid
fibrils in a thin layer of vitrified water at cryogenic
temperatures, we were able to detect picometer movements
of protein molecules on a nanosecond time scale.
Potential future applications of 4D cryo-electron microscopy
are numerous, and some are discussed here
Size Evolution of Ordered SiGe Islands Grown by Surface Thermal Diffusion on Pit-Patterned Si(100) Surface
The ordered growth of self-assembled SiGe islands by surface thermal diffusion in ultra high vacuum from a lithographically etched Ge stripe on pit-patterned Si(100) surface has been experimentally investigated. The total surface coverage of Ge strongly depends on the distance from the source stripe, as quantitatively verified by Scanning Auger Microscopy. The size distribution of the islands as a function of the Ge coverage has been studied by coupling atomic force microscopy scans with Auger spectro-microscopy data. Our observations are consistent with a physical scenario where island positioning is essentially driven by energetic factors, which predominate with respect to the local kinetics of diffusion, and the growth evolution mainly depends on the local density of Ge atoms
Antimicrobial prophylaxis for transrectal ultrasound-guided prostate biopsy: fosfomycin trometamol, an attractive alternative
OBJECTIVE:
To compare fosfomycin trometamol (FT) and ciprofloxacin (CIP) for antibiotic prophylaxis in transrectal prostate biopsy (TR-PB).
PATIENTS AND METHODS:
Data for 1109 patients (mean age 66.7 \ub1 8.45) who underwent TR-PB between March to September 2015 in seven Italian urological institutions were retrospectively reviewed, of which 632 received FT (Group 1) and 477 received CIP (Group 2) for prophylaxis. We reviewed all urine culture results obtained after the procedure, all adverse drug reactions (ADRs) related to the drug and all febrile and/or symptomatic urinary tract infections (UTIs) occurring within 1 month after TR-PB. The rate of symptomatic UTIs and the rate of ADRs were considered the main outcome measures.
RESULTS:
In the total study population, 72/1109 (6.5 %) patients experienced symptomatic UTIs and among these 11 (0.9 % of total) had urosepsis. Out of 72, 53 (73.6 %) symptomatic UTIs were caused by fluoroquinolone-resistant strains. Out of 632, 10 (1.6 %) patients in Group 1 and 62/477 (12.9 %) patients in Group 2 had symptomatic UTIs (p < 0.001); in particular, 2/632 (0.3 %) patients in Group 1 and 9/477 (1.8 %) patients in Group 2 had urosepsis (p < 0.001). No differences were reported in terms of adverse events (0.6 vs 0.4 %; p = 0.70). A Charlson comorbidity index 641 and type of antimicrobial prophylaxis (FT) were found to be associated with a lower probability of symptomatic UTIs in the multivariate model.
CONCLUSIONS:
Antibiotic prophylaxis with FT for TR-PB had a lower rate of adverse events and a lower rate of symptomatic UTIs as compared with CIP. Fosfomycin trometamol appears as an attractive alternative prophylactic regimen in prostate biopsies
Origin of Axial and Radial Expansions in Carbon Nanotubes Revealed by Ultrafast Diffraction and Spectroscopy
The coupling between electronic and nuclear degrees of freedom in low-dimensional, nanoscale systems plays a fundamental role in shaping many of their properties. Here, we report the disentanglement of axial and radial expansions of carbon nanotubes, and the direct role of electronic and vibrational excitations in determining such expansions. With subpicosecond and subpicometer resolutions, structural dynamics were explored by monitoring changes of the electron diffraction following an ultrafast optical excitation, whereas the transient behavior of the charge distribution was probed by time-resolved, electron-energy-loss spectroscopy. Our experimental results, and supporting density functional theory calculations, indicate that a population of the excited carriers in the antibonding orbitals of the nanotube walls drives a transient axial deformation in âŒ1 ps; this deformation relaxes on a much longer time scale, 17 ps, by nonradiative decay. The electron-driven expansion is distinct from the phonon-driven dynamics observed along the radial direction, using the characteristic Bragg reflections; it occurs in 5 ps. These findings reveal the nonequilibrium distortion of the unit cell at early times and the role of the electron(phonon)-induced stress in the lattice dynamics of one-dimensional nanostructures