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Thermal diffusivity (α) of hydrogenated amorphous carbon (a-C:H) thin-films was characterized using the picosecond thermoreflectance measurement technique in the present study. The samples were fabricated in a plasma enhanced chemical vapor deposition system with varying radio frequency (RF) discharge powers from 15 to 70 W. Chemical bonding properties of the a-C:H films were determined from the Fourier transform infrared spectra and Raman spectroscopy. Results show that α is enhanced from 0.07 to 0.20 mm2 s-1 as the RF power increases. It attributes to the dehydrogenation of film resulting from the bombardment of high-energy ion species that breaks C-H bonds at the lower power regime. Particularly at the highest RF power, however, the increase of α is more influenced by the restructuring of bonding configuration associated with the breaking up of sp2 clusters into smaller ones. [ABSTRACT FROM AUTHOR]

Luminescence and power-gated spectral holeburning studies have been performed on Eu-doped MgS nanoparticles. These particles are atomically tailored to produce and control the relative concentration of Eu2+ and Eu3+, which is necessary for power-gated holeburning. The spectral holes are permanent at low temperatures. Optical studies show that the electron-phonon coupling is stronger in nanoparticles than in thin films or microparticles of the same material. This is the reason for inherently broader spectral holes in nanoparticles as compared to microparticle or thin-film samples. Temperature broadening of spectral holes in nanoparticles follows a T2.4 behavior, a faster rate than thin films or microparticles. This behavior can be attributed to the glassy nature of the particles produced. [ABSTRACT FROM AUTHOR]

We report on the significant performance enhancement of SnO2 thin film ultraviolet (UV) photodetectors (PDs) through incorporation of CuO/SnO2 p-n nanoscale heterojunctions. The nanoheterojunctions are self-assembled by sputtering Cu clusters that oxidize in ambient to form CuO. We attribute the performance improvements to enhanced UV absorption, demonstrated both experimentally and using optical simulations, and electron transfer facilitated by the nanoheterojunctions. The peak responsivity of the PDs at a bias of 0.2V improved from 1.9A/W in a SnO2-only device to 10.3 A/W after CuO deposition. The wavelength-dependent photocurrent-to-dark current ratio was estimated to be ~592 for the CuO/SnO2 PD at 290 nm. The morphology, distribution of nanoparticles, and optical properties of the CuO/SnO2 heterostructured thin films are also investigated. [ABSTRACT FROM AUTHOR]

Resonant absorbers based on plasmonic materials, metamaterials, and thin films enable spectrally selective absorption filters, where absorption is maximized at the resonance wavelength. By controlling the geometrical parameters of nano/microstructures and materials' refractive indices, resonant absorbers are designed to operate at wide range of wavelengths for applications including absorption filters, thermal emitters, thermophotovoltaic devices, and sensors. However, once resonant absorbers are fabricated, it is rather challenging to control and tune the spectral absorption response. Here, we propose and demonstrate thermally tunable infrared resonant absorbers using hybrid gold-vanadium dioxide (VO2) nanostructure arrays. Absorption intensity is tuned from 90% to 20% and 96% to 32% using hybrid gold-VO2 nanowire and nanodisc arrays, respectively, by heating up the absorbers above the phase transition temperature of VO2 (68 °C). Phase change materials such as VO2 deliver useful means of altering optical properties as a function of temperature. Absorbers with tunable spectral response can find applications in sensor and detector applications, in which external stimulus such as heat, electrical signal, or light results in a change in the absorption spectrum and intensity. [ABSTRACT FROM AUTHOR]

Carefully conducted friction tests of a nano-lubricant in the thin film elasto-hydrodynamic lubrication regime showed that the presence of nanoparticles reduces friction. By using surface analyses techniques and molecular dynamics simulations, we explored the effectiveness of different interactions in the system, namely the interactions between nanoparticles with the lubricant or surfaces. Based on the results, the friction reduction mechanism was found to be that the nanoparticles induce an obstructed flow (plug flow) in the thin film between lubricated surfaces. This reduces friction by forcing only a few layers of lubricant molecules to slide on each other. [ABSTRACT FROM AUTHOR]

Silicon nanoclusters exhibit light emission with direct-like ns–µs time dynamics; however, they show variable synthesis and structure, optical, and electronic characteristics. The widely adopted model is a core–shell in which the core is an indirect tetrahedral absorbing Si phase, while the shell is a network of re-structured direct-like H–Si–Si–H molecular emitting phases, with the two connected via back Si–Si tetrahedral bonds, exhibiting a potential barrier, which significantly hinders emission. We carried out first-principles atomistic computations of a 1-nm Si nanoparticle to discern the variabilities. Enlarging the network reduces the potential barrier monotonically to a finite limit not sufficient for strong emission to proceed while inducing a path to quenching of emission via a conical crossing between the excited and ground states. However, enlarging the network is found to induce strain and structural instability, which causes structural relaxation that creates a direct path for emission without crossing the barrier. Following emission, the particle relaxes back to the indirect ground structure, which completes the cycle. The results also confirm the pivotal role of HF/H2O2 etching in synthesizing the core–shells and affording control over the molecular network. Measurements using synchrotron and laboratory UV excitation of thin films of 1-nm Si particles show good agreement with the simulation results. It is plausible that the relaxation is behind the stimulated emission, gain, or microscopic laser action, reported earlier in macroscopic distributions of 1- and 3-nm Si nanoparticles. [ABSTRACT FROM AUTHOR]

We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayer graphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses. [ABSTRACT FROM AUTHOR]

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the density of bulk trap states in the oxide due to partial incorporation of nitrogen in the oxide. Both of these low temperature deposition and annealing processes are compatible with integration into the upper levels of metallization for high density circuits.© 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]