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A major challenge with the use of quantum dots (QDs) for cellular imaging and biomolecular delivery is the attainment of QDs freely dispersed inside the cells. Conventional methods such as endocytosis, lipids based delivery and electroporation are associated with delivery of QDs in vesicles and/or as aggregates that are not monodispersed. In this study, we demonstrate a new technique for reversible permeabilization of cells to enable the introduction of freely dispersed QDs within the cytoplasm. Our approach combines osmosis driven fluid transport into cells achieved by creating a hypotonic environment and reversible permeabilization using low concentrations of cell permeabilization agents like Saponin. Our results confirm that highly efficient endocytosis-free intracellular delivery of QDs can be accomplished using this method. The best results were obtained when the cells were treated with 50 μg ml-1 Saponin in a hypotonic buffer at a 3:2 physiological buffer:DI water ratio for 5 min at 4 °C. [ABSTRACT FROM AUTHOR]

Reports the injection of spin-polarized electrons from ferromagnetic contacts into multi-walled carbon nanotubes, finding direct evidence for coherent transport of electron spins. Observation of a hysteric magnetoresistance in several nanotubes with a maximum resistance change of 9%; Outlook for the development of practical nanotube spin-electronic devices.

Nonlinear coupling of a piezoelectric cantilever to an external magnetic force increases the power harvested from a broadband vibration source for relatively large acceleration vibrations. A threshold exists in the minimum acceleration needed for enhancement of the cantilever response. Below this threshold the nonlinear coupling damps the cantilever motion, and the power output is reduced compared to the noncoupled cantilever. By reducing the size of the coupling magnet, the acceleration required to scavenge usable power can be greatly reduced. The smaller diameter magnet decreases the spatial extent of the magnetic force, reducing the acceleration required to surmount the local potential barrier. The experiments result in a six-fold decrease in the acceleration threshold, to levels comparable to those observed in ambient environmental vibrations. The results are in good agreement with numerical calculations. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) has attracted great attention as a structural material for advanced microelectromechanical systems (MEMS), thanks to its excellent ensemble of electrical and mechanical properties. Here, we report on studying the effects of ion radiation-induced displacement damage on resonant MEMS made of GaN film grown on aluminum nitride (AlN) buffer layer. We design and fabricate GaN/AlN heterostructure doubly-clamped microstring resonators with length $L \,\, =100$ , 200, 300, 400, 500, 600, and 700 $\mu \text{m}$ , and irradiate the devices with 440-keV Ar $^{+}$ ions, to probe the effects of ion radiation-induced displacement damage on the resonance behavior. The ion energy and range have been selected so that the ions would stop within the GaN layer, thus allowing diagnostics to show the effects of damage in the structure. The multimode resonance frequencies of the devices decrease significantly ($\vert \Delta f\vert /f >50$ %) with the increase of fluence beyond 1014 cm−2. According to scanning electron microscopy (SEM) imaging, the irradiated GaN/AlN resonators are visibly deformed at high fluence, where the amount of curvature increases monotonically with the fluence. The deformation of the structures can be ascribed to the change in both Young’s modulus and built-in stress resulting from ion radiation-induced displacement damage. The results extend the understanding of radiation-induced damage mechanisms in resonant MEMS. [ABSTRACT FROM AUTHOR]

We report on the first experimental characterization and analysis of the thermal response and temperature coefficient of resonance frequency (TC ƒ) of gallium nitride/aluminum nitride (GaN/AlN) heterostructure micro string resonators, in a wide temperature range from −10 °C up to 325 °C. Thanks to its excellent electrical and mechanical properties and chemical inertness, GaN has recently stimulated growing interests in GaN microelectromechanical systems (MEMS) for emerging high-power, high-temperature, and harsh-environment applications. GaN films on Si wafers often require AlN buffer layers, thus the residual tensile stress profile in the GaN epilayers and GaN/AlN hetero-layers can play a key role in affecting the MEMS specifications and performance. Here we design and fabricate GaN/AlN heterostructure micro string resonators with length L = 100, 200 and 300 μm to probe the stress and thermal effects on resonance behavior. All out-of-plane flexural modes show clear string behavior, and the multimode resonance frequencies downshift almost linearly with increasing temperature up to 325 °C. The linear temperature dependence and TC ƒ values of GaN/AlN heterostructure resonators can be directly employed for thermal sensing. Comparison among different devices indicates that higher tensile stress levels contribute to smaller TC ƒ values, suggesting strain engineering may be exploited for intentionally regulating the TC ƒ. [ABSTRACT FROM AUTHOR]

Important breakthroughs in far-field imaging techniques have been made since the first demonstrations of stimulated emission depletion (STED) microscopy. To date, the most straightforward and widespread deployment of STED microscopy has used continuous wave (CW) laser beams for both the excitation and depletion of fluorescence emission. A major drawback of the CW STED imaging technique has been photobleaching effects due to the high optical power needed in the depletion beam to reach sub-diffraction resolution. To overcome this hurdle, we have applied a synchronous detection approach based on modulating the excitation laser beam, while keeping the depletion beam at CW operation, and frequency filtering the collected signal with a lock-in amplifier to record solely the super-resolved fluorescence emission. We demonstrate here that such approach allows an important reduction in the optical power of both laser beams that leads to measurable decreases in photobleaching effects in STED microscopy. We report super-resolution images with relatively low powers for both the excitation and depletion beams. In addition, typical unwanted scattering effects and background signal generated from the depletion beam, which invariably arises from mismatches in refractive index in the material composing the sample, are largely reduced by using the modulated STED approach. The capability of acquiring super-resolution images with relatively low power is quite relevant for studying a variety of samples, but particularly important for biological species as exemplified in this work. [ABSTRACT FROM AUTHOR]

The damping effect of various gas environments on a silicon, lateral microresonator implemented with piezoresistive detection is investigated in this study. The resonant frequency of the cantilever shifts due to viscous damping by an amount that is directly determined by the molar mass of the gas, thereby providing a method to determine the composition of the gas environment. In addition, the microresonator demonstrates the ability to perform CO2 composition analysis using this nonreaction based detection method. The advantages of this gas analysis method are that it is simple, repeatable, reversible and not limited to reactive gases. [ABSTRACT FROM AUTHOR]

Lighter doping, pretreatment (exposure to hydrogen in a steam bath), and lower dose rate are each found to exacerbate 10-keV X-ray-induced negative frequency shifts and increase in resistivity measured in T-shaped, asymmetric, piezoresistive, micromachined, and resonating cantilevers. All devices recover to levels close to preirradiation after several hours of postirradiation annealing. The effects are attributed to dose-rate-dependent efficiencies of the depassivation of dopants by hydrogen and surface charging effects, which alter the effective Young’s modulus and shear elastic constants. Increases in resistivity are attributed to increases in scattering causing decreases in mobility. Results indicate the need to consider dopant type and concentration when choosing microelectromechanical systems for use in radiation environments in addition to accounting for low-dose-rate effects. [ABSTRACT FROM AUTHOR]

Total-ionizing-dose-induced resonance frequency shifts in piezoresistive micromachined cantilevers are experimentally shown to be dose-rate dependent. Devices were irradiated to 1 Mrad(SiO2) at rates from 5.4 to 30.3 krad(SiO2)/min, with lower rate exposures producing up to four-times more negative frequency shifts than higher rate exposures. Devices that were hydrogenated in a steam bath for 1 h showed shifts similar to those of control (not hydrogenated) devices at higher dose rates, and larger shifts than control devices at lower rates. All devices recovered to levels close to preirradiation after several hours of post-irradiation annealing. The dose-rate dependence is attributed to differences in carrier concentration caused by varying efficiencies of the depassivation of boron by hydrogen at higher and lower dose rates and/or surface charging effects, and the subsequent differences in Young’s modulus that occur as a result. Many of these processes are similar to effects that lead to ELDRS in linear bipolar transistors, emphasizing the need to include low-dose-rate testing of microelectromechanical systems devices when considering them for use in space systems. [ABSTRACT FROM PUBLISHER]

The response of silicon-based microelectromechanical systems resonators to proton irradiation is determined by the combined effects of displacement damage and total ionizing dose (TID). Displacement damage (DD) can lead to carrier removal, which tends to decrease the carrier concentration, and TID leads to dopant activation and/or surface charging effects, which tend to increase the carrier concentration. These competing effects lead to changes in carrier concentration that alter Young’s modulus, and consequently the resonance frequency. For higher flux 2-MeV proton irradiation to 1014/cm2, TID effects dominate at low fluence, leading to a decrease in resonance frequency, which is offset by displacement damage effects at higher fluence. Fast recovery is observed as TID effects anneal out and DD effects remain. For lower-flux 0.8-MeV proton irradiation to 7 \times 10^12 /cm2, DD effects are relatively more significant than for 2-MeV proton irradiation, and an increase in resonance frequency is observed at all fluences. Stopping and range of ions in matter calculations reinforce these conclusions. [ABSTRACT FROM PUBLISHER]

We evaluate the response of T-shaped, asymmetric, piezoresistive, micromachined cantilevers fabricated on p-type Si to 10-keV X-ray irradiation. The resonant frequency decreases by 25 ppm at 2.1 Mrad(SiO2), and partially recovers during post-irradiation annealing. An explanation of the results is proposed that is based on radiation-induced acceptor depassivation. This occurs because radiation-generated holes release hydrogen from previously passivated acceptors, causing the carrier concentration to increase, especially near the surface. Increased carrier concentration decreases Young’s modulus, resulting in a decrease in the cantilever resonant frequency. Finite element simulations show that the effect of a decreasing Young’s modulus in the surface region is consistent with the measured decrease in resonant frequency in the irradiated devices. [ABSTRACT FROM PUBLISHER]

The addition of 400 micron scale rare-earth metal oxide particles to the TiO 2 nanoparticle acceptor layer of a dye-sensitized solar cell (DSSC) dramatically increases the efficiency of the solar cell by 10-30%. Oxidized neodymium powder, Nd 2 O 3 , combined with TiO 2 nanoparticles forms a composite photoanode containing a wide range of Nd 2 O 3 particle sizes. Based on absorbance measurements, the dye-uptake on the Nd 2 O 3 /TiO 2 anode increases substantially compared to both a standard anode and to one made from a mixture of large and small TiO 2 particles. Electrochemical impedance spectroscopy (EIS) shows a similar electron lifetime, but increased electron transport diffusion coefficient through the Nd 2 O 3 /TiO 2 composite as compared to TiO 2 only. This shows that the Nd 2 O 3 /TiO 2 composite has a lower internal resistance for charge transport. This decrease in resistance, possibly due to the filling of trap states by the Nd 2 O 3 valence or f-states, and increased dye uptake results in improved efficiency by simple incorporation of Nd 2 O 3 powder. [ABSTRACT FROM AUTHOR]

Highlights: [•] Adding Nd2O3 to PCBM increases open circuit voltage and short circuit current. [•] Photo induced absorption decay rate increases in Nd2O3/PCBM mixture. [•] Photo induced bleaching is observed in Nd2O3/PCBM mixture. [•] Shows photo induced charge transfer occurs between PCBM and Nd2O3. [•] Provides first example of a rare earth oxide/organic hybrid solar cell. [Copyright &y& Elsevier]

Abstract: Capacitive photocurrent spectroscopy is used to probe the electronic states of graphene-oxide, and reduced graphene-oxide. Three peaks are observed whose intensities scale with the oxygen coverage. The energy of these peaks correlate with the luminescence spectra reported for graphene-oxide. Using a fitting procedure, the density of states for graphene oxide is extracted from the data. It consists of the π/π∗ states along with a distribution of mid-gap states centered at three different energies near the Dirac point. X-ray photoelectron spectroscopy measurements are used to identify the oxygen functional groups corresponding to the observed state distribution. [Copyright &y& Elsevier]

Photon absorption by carbon nanotubes creates bound electron–hole pairs called excitons, which can exist in spin-polarized triplet or spin-unpolarized singlet configurations. Triplet excitons are optically inactive owing to the weak spin–orbit coupling in nanotubes. This prevents the optical injection of electron spin into nanotubes for spintronic applications and limits the efficiency of photocurrent generation. Here, we show that it is possible to optically excite the triplet exciton by using a ferromagnetic semiconductor as a spin filter to mix the singlet and triplet excitons. The triplet contribution to the photocurrent is detected, representing the first direct evidence of the triplet exciton in carbon nanotubes. [ABSTRACT FROM AUTHOR]

Abstract: Proteins required for translesion DNA synthesis localize in nuclear foci of cells with replication-blocking lesions. The dynamics of this process were examined in human cells with fluorescence-based biophysical techniques. Photobleaching recovery and raster image correlation spectroscopy experiments indicated that involvement in the nuclear foci reduced the movement of RAD18 from diffusion-controlled to virtual immobility. Examination of the mobility of REV1 indicated that it is similarly immobilized when it is observed in nuclear foci. Reducing the level of RAD18 greatly reduced the focal accumulation of REV1 and reduced UV mutagenesis to background frequencies. Fluorescence lifetime measurements indicated that RAD18 and RAD6A or polη only transferred resonance energy when these proteins colocalized in damage-induced nuclear foci, indicating a close physical association only within such foci. Our data support a model in which RAD18 within damage-induced nuclear foci is immobilized and is required for recruitment of Y-family DNA polymerases and subsequent mutagenesis. In the absence of damage these proteins are not physically associated within the nucleoplasm. [Copyright &y& Elsevier]