Characterization of Ultrathin Polymer Films Using p-Polarized ATR-FTIR and Its Comparison with XPS.

We report on the chemical analysis of ultrathin (10 nm) polymer films using the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) technique based on p-polarized infrared light and two types of enhancing substrates, that is, metallic (Au) and dielectric (Si). We selected low-temperature plasma-treated ∼10 nm thick polystyrene films as a test case for demonstrating the capability of the p-polarized ATR-FTIR, whose performance was further compared with the conventional X-ray photoelectron spectroscopy (XPS) techniques. Although ATR-FTIR cannot be used for quantitatively determining elemental compositions in polymers at which XPS excels, it is able to be operated under nonvacuum conditions and allows the study of hydrogen-containing moieties. By correcting the contact condition between the polymer surface and the ATR prism, the relative concentration of the chemical bonds from different samples can be compared. Because ATR-FTIR and XPS provide complementary information on chemical bonds, their combination provides a powerful approach for studying the chemical composition of polymers.

Biaxially stretchable supercapacitors based on the buckled hybrid fiber electrode array.

In order to meet the growing need for smart bionic devices and epidermal electronic systems, biaxial stretchability is essential for energy storage units. Based on porous single-walled carbon nanotube/poly(3,4-ethylenedioxythiophene) (SWCNT/PEDOT) hybrid fiber, we designed and fabricated a biaxially stretchable supercapacitor, which possesses a unique configuration of the parallel buckled hybrid fiber array. Owing to the reticulate SWCNT film and the improved fabrication technique, the hybrid fiber retained its porous architecture both outwardly and inwardly, manifesting a superior capacity of 215 F g(-1). H3PO4-polyvinyl alcohol gel with an optimized component ratio was introduced as both binder and stretchable electrolyte, which contributed to the regularity and stability of the buckled fiber array. The buckled structure and the quasi one-dimensional character of the fibers endow the supercapacitor with 100% stretchability along all directions. In addition, the supercapacitor exhibited good transparency, as well as excellent electrochemical properties and stability after being stretched 5000 times.

Polystyrene as a model system to probe the impact of ambient gas chemistry on polymer surface modifications using remote atmospheric pressure plasma under well-controlled conditions.

An atmospheric pressure plasma jet (APPJ) was used to treat polystyrene (PS) films under remote conditions where neither the plume nor visible afterglow interacts with the film surface. Carefully controlled conditions were achieved by mounting the APPJ inside a vacuum chamber interfaced to a UHV surface analysis system. PS was chosen as a model system as it contains neither oxygen nor nitrogen, has been extensively studied, and provides insight into how the aromatic structures widespread in biological systems are modified by atmospheric plasma. These remote treatments cause negligible etching and surface roughening, which is promising for treatment of sensitive materials. The surface chemistry was measured by X-ray photoelectron spectroscopy to evaluate how ambient chemistry, feed gas chemistry, and plasma-ambient interaction impact the formation of specific moieties. A variety of oxidized carbon species and low concentrations of NOx species were measured after APPJ treatment. In the remote conditions used in this work, modifications are not attributed to short-lived species, e.g., O atoms. It was found that O3 does not correlate with modifications, suggesting that other long-lived species such as singlet delta oxygen or NOx are important. Indeed, surface-bound NO3 was observed after treatment, which must originate from gas phase NOx as neither N nor O are found in the pristine film. By varying the ambient and feed gas chemistry to produce O-rich and O-poor conditions, a possible correlation between the oxygen and nitrogen composition was established. When oxygen is present in the feed gas or ambient, high levels of oxidation with low concentrations of NO3 on the surface were observed. For O-poor conditions, NO and NO2 were measured, suggesting that these species contribute to the oxidation process, but are easily oxidized when oxygen is present. That is, surface oxidation limits and competes with surface nitridation. Overall, surface oxidation takes place easily, but nitridation only occurs under specific conditions with the overall nitrogen content never exceeding 3%. Possible mechanisms for these processes are discussed. This work demonstrates the need to control plasma-ambient interactions and indicates a potential to take advantage of plasma-ambient interactions to fine-tune the reactive species output of APP sources, which is required for specialized applications, including polymer surface modifications and plasma medicine.

Temperature dependent Raman spectra of isolated suspended single-walled carbon nanotubes.

Ultralong isolated single-walled carbon nanotubes (SWCNTs) were synthesized on a SiO2/Mo mesh to fabricate long individual suspended SWCNT samples. We detected a delicately temperature dependent Raman RBM, G and G' band frequency shift and linewidth increase of these SWCNTs in the range of 240-600 K. Softening of force constants induced redshift and the mechanism of corresponding spectra linewidth evolution with temperature was analysed and compared with previously reported theories and experimental results. Temperature coefficient differences between G(-) and G(+) were not observed, but the linewidth broadening tendency differences of G(-) and G(+) are obvious. In addition, temperature-dependent G' band evolution, including both frequency red-shifting and FWHM increase has been reported here first. These results will facilitate understanding of the second order Raman scattering process in SWCNTs. Different temperature coefficients measured on suspended and supported parts of the same SWCNT verifies the substrate's influence.

Substrate-induced effects on the optical properties of individual ZnO nanorods with different diameters.

We present the influence of a substrate on the properties of well-dispersed individual ZnO nanorods (NRs) with different diameters, especially on the photoluminescence (PL) properties. The studied ZnO NRs were partially supported by the quartz substrate and partially suspended in air. Continuous redshift and intensity decrease of the near band-edge emission (NBE) were observed along the suspended segment of the ZnO NRs due to the increasing temperature under UV laser excitation, suggesting that the presence of the substrate can effectively enhance the heat-sinking capability of ZnO NRs. Based on the PL measurements on individual suspended ZnO NRs with diameters from 86 nm to 2.35 µm, the redshift of NBE along the suspended segment was more obvious for ZnO NRs with a smaller diameter, indicating that the thermal conductive ability increases as diameter increases. Additionally, by combining the experimental and simulation results, we found that the presence of the substrate also quenched the whispering gallery modes (WGMs) of the ZnO NRs with a diameter above about 350 nm due to the symmetry breaking induced by the quartz substrate which has a larger refractive index compared with air. Our studies confirm that the substrate significantly influences the properties of ZnO NRs. To fully utilize the potential properties of nanomaterials for applications in nanodevices, the substrate-induced effects should be considered thoughtfully.

Super-stretchable, transparent carbon nanotube-based capacitive strain sensors for human motion detection.

Realization of advanced bio-interactive electronic devices requires mechanically compliant sensors with the ability to detect extremely large strain. Here, we design a new multifunctional carbon nanotube (CNT) based capacitive strain sensors which can detect strains up to 300% with excellent durability even after thousands of cycles. The CNT-based strain gauge devices exhibit deterministic and linear capacitive response throughout the whole strain range with a gauge factor very close to the predicted value (strictly 1), representing the highest sensitivity value. The strain tests reveal the presented strain gauge with excellent dynamic sensing ability without overshoot or relaxation, and ultrafast response at sub-second scale. Coupling these superior sensing capabilities to the high transparency, physical robustness and flexibility, we believe the designed stretchable multifunctional CNT-based strain gauge may have various potential applications in human friendly and wearable smart electronics, subsequently demonstrated by our prototypical data glove and respiration monitor.

Surface modification effect on photoluminescence of individual ZnO nanorods with different diameters.

Optical properties of Al2O3 coated individual ZnO nanorods (NRs) with different diameters were studied by confocal micro-photoluminescence spectroscopy. The one-dimensional ZnO/Al2O3 core-shell NRs showed enhanced near band-edge emission compared with the same ZnO NRs before Al2O3 coating at room temperature. Besides, the relative intensity of the deep-level emission with respect to the near band-edge emission was reduced. A model was proposed to explain these spectral changes. For ZnO NRs with diameters above 360 nm, a multi-mode behavior resulting from whispering gallery resonance was observed. In addition, selective enhancement or quenching of different whispering gallery modes in ultraviolet (UV) emission was observed after Al2O3 coating at room temperature, which is due to the larger refractive index of Al2O3 compared with air. We proposed a model to explain these spectral changes as well. By comparing the optical properties before and after surface coating, our results suggest that surface coating of an Al2O3 layer is an effective way to tailor the optical properties of ZnO NRs.