Browsing by Subject "Interfaces"

Now showing 1 - 6 of 6

Open Access
Dispersion irrelevant wideband asymmetric transmission in dielectric photonic crystal gratings
(Optical Society of America, 2012-11-22) Serebryannikov, A. E.; Colak, E.; Cakmak, A. O.; Özbay, Ekmel
Wideband suppression of zero order and relevant strongly asymmetric transmission can be obtained in photonic crystal gratings that are made of linear isotropic materials and show the broken structural (axial) symmetry, even if zero diffraction order may be coupled to a Floquet-Bloch (FB) wave at the incidence and exit interfaces. The studied mechanism requires that the peculiar diffractions at the corrugated exit interface inspire strong energy transfer to higher orders, including those not coupled to an FB wave. At the opposite direction of incidence, transmission due to zero and some higher orders that may be coupled at the corrugated input interface can vanish. This leads to the alternative scenario of wideband unidirectional transmission, which itself does not need but can coexist with the other scenario based on the merging of asymmetric diffraction and dispersion of the FB mode.
Open Access
Growth of Ge nanoparticles on SiO2 / Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films
(Elsevier B.V., 2007) Foss, S.; Finstad, T. G.; Dana, A.; Aydınlı, Atilla
Multilayer germanosilicate (Ge:SiO2) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO2 layer is separated by a pure SiO2 layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO2 layer closest to the interface through a pure SiO2 layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.
Open Access
Microwave heating induced on-demand droplet generation in microfluidic systems
(American Chemical Society, 2021-12-29) Cui, W.; Yesiloz, Gurkan; Ren, C. L.
In this note, we report a simple, new method for droplet generation in microfluidic systems using integrated microwave heating. This method enables droplet generation on-demand by using microwave heating to induce Laplace pressure change at the interface of the two fluids. The distance between the interface and junction and microwave excitation power have been found to influence droplet generation. Although this method is limited in generating droplets with a high rate, the fact that it can be integrated with microwave sensing that can be used as the feedback to tune the supply flow of materials presents unique advantages for applications that require dynamic tuning of material properties in droplets.
Open Access
Nanotribological properties of the h-BN/Au(111) interface: a DFT study
(American Chemical Society, 2019) Baksi, M.; Toffoli, D.; Gülseren, Oğuz; Üstünel, H.
Understanding the quantum-mechanical origins of friction forces has become increasingly important in the past decades with the advent of nanotechnology. At the nanometer scale, the universal Amontons–Coulomb laws cease to be valid, and each interface requires individual scrutiny. Because of the well-known lubricating properties of two-dimensional materials, a significant amount of research has been performed in an effort to understand interfaces they form with one another. However, the interfaces between these two-dimensional materials and metals red from a tribological point of view, important for such applications as friction force microscopy, have yet to be thoroughly investigated. In the current work, we present a detailed density functional theory investigation of the hexagonal BN/Au(111) interface. Because of a good agreement between their characteristic lengths, a high level of commensurability is achieved in a suitably constructed model between the bulk surfaces of the two materials. As a result of our calculations, we find that the corrugation in the potential energy surface and the lateral forces in this interface are low compared to other similar interfaces. The friction coefficient falls rapidly with increasing load down to 0.005 for the largest loads considered. In contrast, Aun clusters (n = 1, 4, 13, and 19) sliding on the h-BN surface exhibit much larger lateral forces, indicating strong size and edge effects. The reduction of energy corrugation in going from the Au4 to the Au19 cluster may already indicate a decreasing trend with increasing size even at this very small scale.
Open Access
Probing the dynamics of non-faradaic processes in Ionic liquids at extended time and length scales using XPS with AC modulation
(American Chemical Society, 2021-05-06) Aydoğan Göktürk, Pınar; Süzer, Şefik
Charging dynamics of ionic liquid (IL) electrolytes play important roles in various aspects of electrochemical processes. However, the precise understanding of such processes at extended time and length scales is incomplete due to the experimental difficulties in probing the electrochemical potential and other relevant parameters. In principle, such shortcomings should not apply to theoretical/computational approaches; however, existing works have mostly concentrated on or around electrode/electrolyte interfaces and short timescales due mostly to prohibitive demands on computational efforts. To fill this gap, we have utilized X-ray photoelectron spectroscopy to study the charging dynamics of ILs in contact with two wire electrodes under AC square wave excitation, with frequencies ranging from hundreds of kHz down to the mHz region. Using the changes in the binding energy position of the IL-related core-level peaks, electrical potential profiles along the lines in between the electrodes and on the entire surface of the electrolyte have been investigated in situ. From these results, we identify two widely different time constants. The timescale of the fast process was shown to be on the order of RC time constant, while the slow process takes place on a timescale of seconds. Our method in the present study is expected to open up a new way for extracting novel dynamic information for gaining a better understanding of such processes and designing efficient IL-based electrochemical devices with a novel perspective on the charging.
Open Access
X-ray photoelectron spectroscopy with electrical modulation can be used to probe electrical properties of liquids and their interfaces at different stages
(American Chemical Society, 2019) Uzundal, Can Berk; Şahin, Özgür; Aydoğan-Göktürk, Pınar; Wu, H.; Mugele, F.; Ülgüt, Burak; Süzer, Şefik
Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.