Pinhole-free 2D Ruddlesden-Popper perovskite layer with close packed large crystalline grains, suitable for optoelectronic applications.

Here, we achieved pinhole-free 2D Ruddlesden-Popper Perovskite (RPP) BA2PbI4 layers with close packed crystalline grains with dimension of about 30 × 30 µm2, which have been demonstrated to be favorable for optoelectronic applications, such as fast response RPP-based metal/semiconductor/metal photodetectors. We explored affecting parameters in hot casting of BA2PbI4 layers, and proved that oxygen plasma treatment prior to hot casting plays a significant role to achieve high quality close packed polycrystalline RPP layers at lower hot cast temperatures. Moreover, we demonstrate that crystal growth of 2D BA2PbI4 can be dominantly controlled by the rate of solvent evaporation through substrate temperature or rotational speed, while molarity of the prepared RPP/DMF precursor is the dominant factor that determines the RPP layer thickness, and can affect the spectral response of the realized photodetector. Benefiting from the high light absorption and inherent chemical stability of 2D RPP layers, we achieved high responsivity and stability, and fast response photodetection from perovskite active layer. We achieved a fast photoresponse with rise and fall times of 189 µs and 300 µs, and the maximum responsivity of 119 mA/W and detectivity of 2.15 × 108 Jones in response to illumination wavelength of 450 nm. The presented polycrystalline RPP-based photodetector benefits from a simple and low-cost fabrication process, suitable for large area production on glass substrate, a good stability and responsivity, and a promising fast photoresponse, even around that of exfoliated single crystal RPP-based counterparts. However, it is well known that exfoliation methods suffer from poor repeatability and scalability, which make them incompatible with mass production and large area applications.

Plasmonic vortices for tunable manipulation of target particles, using arrays of elliptical holes in a gold layer.

Here, we numerically prove that light with linear polarization can be coupled to surface plasmon polaritons at an elliptical hole perforated in a gold layer to generate plasmonic vortex (PV). Benefiting from the smooth variation of the minor to major ellipse axes, a gradual variation in the phase profile of the generated PV is achieved. Regarding this, three types of independent arrays of elliptical holes are presented, which can produce uniform and high quality PVs with different topological charges at the center of the arrays. The first array can produce PV with topological charges of + 1 and - 1, depending on the polarization orientation of the incident light. In the second one, the topological charge of the PV can be switched between 0 and + 2, by switching the polarization direction of the incident light. In the third array, a robust PV with topological charge of + 1 is generated independent of possible tolerances in the polarization orientation. In order to use the generated PVs for plasmonic tweezing application, there are side fringes around the central vortex of the arrays that should be eliminated. To produce a single vortex, we propose metal-insulator-metal (MIM) structures, screening excessive fringes and allowing the central PVs to leak out. It is also demonstrated by simulation that target particles, such as gold and polystyrene spheres of subwavelength dimensions, can be efficiently manipulated by our MIM designs, suitable for different applications including local mixing, and applying switchable torque or force to target particles to explore their complete elastic characteristics.

Semiempirical modeling of the effects of the intrinsic and extrinsic optical phonons on the performance of the graphene-based devices.

Surface plasmons in graphene have mainly been affected by intrinsic optical phonons due to the vibrations of the carbon atoms and surface polar optical phonons (S-POPs) of the underlying dielectric surface. This plasmon hybridization dramatically changes the features of the plasmonic devices. However, a complete theoretical model for the graphene impedance to consider the optical phonons effects is yet remained to be developed. Here, we show how to derive a model for graphene impedance to include such impacts on graphene surface plasmons. Earlier models suffer from two limitations-i.e., the inability to show (i) the transformation of a single pure plasmonic mode into multiple hybrid plasmon-phonon excitations and (ii) the damping effect for energies beyond that of the intrinsic optical phonons due to the phonon emission. Our new model overcomes these two limitations. Then, we calculate the extinction spectra for a one-dimensional periodic array of graphene ribbons obtained through the impedance boundary condition method, addressing these obstacles. These spectra are directly related to graphene impedance, modeled using the dielectric function we developed in our earlier work. The extinction spectra show the presented model overcoming the limitations, firmly fitting the experimental data reported by others. Furthermore, we introduce our developed model for graphene to the CST Studio software to verify the accuracy of our extinction relation and impedance model. This study can be a step forward correctly predicting the behavior of graphene-based plasmonic devices.

Simulating a graphene-based acousto-plasmonic biosensor to eliminate the interference of surrounding medium.

The presence of species other than the target biomolecules in the fluidic analyte used in the refractive index biosensor based on the surface plasmon resonances (SPRs) can lead to measurement ambiguity. Using graphene-based acousto-plasmonic biosensors, we propose two methods to eliminate any possible ambiguity in interpreting the measured results. First, we take advantage of the dynamic tunability of graphene SPRs in the acousto-plasmonic biosensor with a surface acoustic wave (SAW) induced uniform grating, performing measurements at different applied voltages. Second, a single measurement employing a similar biosensor but with SAW-induced dual-segment gratings. The numerical results show the capability of both methods in decoupling the effect of the target analyte from the other species in the fluid, enabling interpreting the measurement results with no ambiguity. We also report the results of our numerical investigation on the effect of measuring parameters like the target layer effective refractive index and thickness, and the fluid effective refractive index, in addition to the controlling parameters of the proposed acousto-plasmonic biosensor, including graphene Fermi energy and electrical signaling on the sensing characteristics. Both types of proposed biosensors show promising features for developing the next generation lab-on-a-chip biosensors with minimal cross-sensitivities to non-target biomolecules.

Far-field position-tunable trapping of dielectric particles using a graphene-based plasmonic lens.

In this report, a graphene-based plasmonic lens is designed for far-field position-tunable trapping of dielectric particles at a wavelength of 1550 nm, in which target particles can be floated at a variable z-position, using a variable gate voltage applied to the graphene ribbons. Preventing proximity of the trapped particle and the metallic lens structure, we can diminish general thermal issues in plasmonic tweezers, while realizing higher degrees of freedom in studying target characteristics of the particles by achieving position-tunable 3D trapping. These advantageous aspects are impossible in conventional plasmonic tweezers, because of the highly evanescent nature of the plasmonic field at the metal interface. The proposed structure is comprised of two concentric circular slit-sets (S1, S2), each capable of sending a directive beam, which can lead to a constructive interference, and forming a subwavelength focal spot in the far-field. Taking advantage of the epsilon-near-zero (ENZ) behavior of graphene, each of the radiating slit-sets can be switched ON/OFF, with a radiation switching ratio of about 49, by applying a small electric pulse of 80 meV to change the Fermi energy of the corresponding graphene ribbon from 0.535 eV to 0.615 eV. Hence, inverting the radiation state of the designed lens, from (S1:ON, S2:OFF) to (S1:OFF, S2:ON), we can change the z-position of the focal trapping site from 5000 nm to 9800 nm. This configuration can be proposed as a new generation of long-range, electrostatically tunable 3D plasmonic tweezing, without the need for any external bulky optomechanical equipment.

A numerical study on the closed packed array of gold discs as an efficient dual mode plasmonic tweezers.

In this report, we propose the closed pack array of gold discs on glass, as a dual mode plasmonic tweezers that benefits from two trapping modes. The first trapping mode is based on leaky surface plasmon mode (LSPM) on the gold discs with a longer penetration depth in the water and a longer spatial trapping range, so that target nanoparticles with a radius of 100 nm can be attracted toward the gold surface from a vertical distance of about 2 µm. This trapping mode can help to overcome the inherent short range trapping challenge in the plasmonic tweezers. The second trapping mode is based on the dimer surface plasmonic mode (DSPM) in the nano-slits between the neighboring gold discs, leading to isolated and strong trapping sites for nanoparticles smaller than 34 nm. The proposed plasmonic tweezers can be excited in both LSPM and DSPM modes by switching the incident wavelength, resulting in promising and complementary functionalities. In the proposed plasmonic tweezers, we can attract the target particles towards the gold surface by LSPM gradient force, and trap them within a wide half width half maximum (HWHM) that allows studying the interactions between the trapped particles, due to their spatial proximity. Then, by switching to the DSPM trapping mode, we can rearrange the particles in a periodic pattern of isolated and stiff traps. The proposed plasmonic structure and the presented study opens a new insight for realizing efficient, dual-mode tweezers with complementary characteristics, suitable for manipulation of nanoparticles. Our thermal simulations demonstrate that the thermal-induced forces does not interefe with the proposed plasmonic tweezing.

Thermophoresis suppression by graphene layer in tunable plasmonic tweezers based on hexagonal arrays of gold triangles: numerical study.

Taking advantage of highly confined evanescent fields to overcome the free-space diffraction limit, we show plasmonic tweezers enable efficient trapping and manipulation of nanometric particles by low optical powers. In typical plasmonic tweezers, trapping/releasing particles is carried out by turning the laser power on and off, which cannot be achieved quickly and repeatedly during the experiment. We introduce hybrid gold-graphene plasmonic tweezers in which the trap stiffness is varied electrostatically by applying suitable voltages to a graphene layer. We show how the graphene layer absorbs the plasmonic field around the gold nanostructures in particular chemical potentials, allowing us to modulate the plasmonic force components and the trapping potential. We show graphene monolayer (bilayer) with excellent thermal properties enables more efficient heat transfer throughout the plasmonic tweezers, reducing the magnitude of thermophoretic force by about 23 (36) times. This thermophoresis suppression eliminates the risk of photothermal damage to the target sample. Our proposed plasmonic tweezers open up possibilities to develop tunable plasmonic tweezers with high-speed and versatile force-switching functionality and more efficient thermal performance.

Exact dispersion relations for the hybrid plasmon-phonon modes in graphene on dielectric substrates with polar optical phonons.

Intrinsic optical phonons and extrinsic polar optical phonons (POPs) strongly affect the graphene surface plasmons. Specifically, extraneous POPs present on the surface of an underlying substrate change the behavior of the graphene's surface plasmons sharply due to the plasmon-phonon hybridization. Here, we report modeling of exact dispersion relations for graphene's surface plasmons affected by intrinsic optical phonons and extrinsic POPs of the surface of polar dielectric substrates with one or more vibrational frequencies. In doing so, we have employed random phase approximation with modified two-dimensional polarizability (2D-Π0). The adapted Π0 addresses limitations of the previously derived plasmons dispersion, obtained using classical two-dimensional polarizability. We show the new model overcomes the unsatisfying behavior of the plasmonic dispersion relation obtained by the classical 2D-Π0 at high-wavenumbers and its inability to indicate the starting point of the mode damping. Our new simple model eliminates the complexity of the other presented models in describing the surface plasmons' behavior, specifically at high wavenumbers. Besides, we use our dispersion model to learn about the plasmon content of the hybrid modes, which is a vital value to compute output current in plasmonic graphene-based devices. The coupled-mode lifetime due to the hybrid nature depends on both plasmon and phonon lifetimes. We capture this value here. There is an excellent agreement between our theoretical results and the experimental data reported earlier. They pave the way for the exact modeling of graphene plasmons on common polar substrates and bring in the closeness of the theoretical approaches and experimental results.

Gold cauldrons as efficient candidates for plasmonic tweezers.

In this report, gold cauldrons are proposed and proved as efficient candidates for plasmonic tweezers. Gold cauldrons benefit from high field localization in the vicinity of their apertures, leading to particle trapping by a reasonably low power source. The plasmonic trapping capability of a single gold cauldron and a cauldrons cluster are studied by investigating the plasmon-induced variations of the optical trap stiffness in a conventional optical tweezers configuration. This study shows that the localized plasmonic fields and the consequent plasmonic forces lead to enhanced trap stiffness in the vicinity of the cauldrons. This observation is pronounced for the cauldrons cluster, due to the additive plasmonic fields of the neighboring cauldrons. Strong direct plasmonic tweezing by the gold cauldrons cluster is also investigated and confirmed by our simulations and experimental results. In addition to the presented plasmonic trapping behavior, gold cauldrons benefit from a low cost and simple fabrication process with acceptable controllability over the structural average dimensions and plasmonic behavior, making them attractive for emerging lab-on-a-chip optophoresis applications.

A modeling concept on removal of VOCs in wire-tube non-thermal plasma, considering electrical and structural factors.

In this study, benzene was selected as an indicator of VOCs, and a modeling procedure was carried out on benzene removal (outflow concentration of benzene, C/inflow concentration of benzene, C0), in DC and AC non-thermal plasma systems. Different diameters (18, 23, and 36 mm) of wire-tube plasma reactors were prepared, and models were raised based on the results of experiments with influencing factors of the used voltage, gap size inside the reactor, current density, and specific energy. The results showed correlation between the factors and benzene removal in both DC and AC discharge non-thermal plasma. The applied voltage as an electrical factor had negative correlation with C/C0, and the correlation was stronger than for gap size which was positively correlated with C/C0. Current density and specific energy were affected by the voltage and gap size of the reactor; the lowest C/C0 values were obtained in the highest values of specific energy and current density. Regarding the raised models, multi-factor exponential model was the most reliable one with the results.

Tunable plasmonic force switch based on graphene nano-ring resonator for nanomanipulation.

Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is -15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm2 and ≥22 nm with 29.33 mW/µm2.

Hexagonal arrays of gold triangles as plasmonic tweezers.

We present theoretical and experimental studies of the plasmonic properties of hexagonal arrays of gold triangles, fabricated by angle-resolved nanosphere lithography method. Our numerical and experimental results both show that a change in the angle of gold deposition affects the size and the distance between the triangles, leading to a controlled shift in their absorption and scattering spectra. We calculate the force exerted on the polystyrene particles of 650 nm radii numerically while passing above the hexagonal arrays. Simulation results show that the presented hexagonal arrays of gold triangles can operate as efficient plasmonic tweezers with a controllable operating wavelength and high trap strength, owing to the additive interaction of the neighboring triangles. Moreover, we apply the realized plasmonic nanostructures in a conventional optical tweezers configuration and show that the optical tweezers stiffness can be effectively modulated by the plasmonic forces, at the IR wavelength of 1064 nm.

Numerical Investigation of Tunable Plasmonic Tweezers based on Graphene Stripes.

We are proposing tunable plasmonic tweezers, consisting two parallel graphene stripes, which can be utilized to effectively trap and sort nanoparticles. We show that by electrostatically tuning the chemical potential of a graphene stripe by about 100 meV (equivalent to ΔV G ≈ 4.4 V), the plasmonic force can be switched efficiently, without a need to switch the laser intensity. This enables high speed and low power switching with a large number of switching cycles. By applying two independent and appropriate gate bias voltages to the stripes, the direction of the plasmonic force can be reversed, which leads to separation of nanoparticles that satisfy the trapping conditions. Numerical simulations show that the potential depths obtained for polystyrene nanoparticles of refractive index n = 1.5717 and radii r ≥ 50 nm is deeper than -10 k B T , confirming the ability of the proposed system to effectively separate such nanoparticles. This capability holds for smaller nanoparticles with larger refractive indices. Finally, performing thermal simulations, we have demonstrated that the heat induced by the illumination increases the fluid temperature by at most 9 °C, having negligible effect on the trapping mechanism. The proposed system opens up new possibilities in developing tunable on-chip manipulation devices, suitable for biological applications.

Fabrication of sensitive glutamate biosensor based on vertically aligned CNT nanoelectrode array and investigating the effect of CNTs density on the electrode performance.

In this report, the fabrication of vertically aligned carbon nanotube nanoelectrode array (VACNT-NEA) by photolithography method is presented. Electrochemical impedance spectroscopy as well as cyclic voltammetry was performed to characterize the arrays with respect to different diffusion regimes. The fabricated array illustrated sigmoidal cyclic voltammogram with steady state current dominated by radial diffusion. The fabricated VACNT-NEA and high density VACNTs were employed as electrochemical glutamate biosensors. Glutamate dehydrogenase is covalently attached to the tip of CNTs. The voltammetric biosensor, based on high density VACNTs, exhibits a sensitivity of 0.976 mA mM(-1) cm(-2) in the range of 0.1-20 µM and 0.182 mA mM(-1) cm(-2) in the range of 20-300 µM glutamate with a low detection limit of 57 nM. Using the fabricated VACNT-NEA, the sensitivity increases approximately to a value of 2.2 Am M(-1) cm(-2) in the range of 0.01 to 20 µM and to 0.1 A mM(-1) cm(-2) in the range of 20-300 µM glutamate. Using this electrode, a record of low detection limit of 10 nM was achieved for glutamate. The results prove the efficacy of the fabricated NEA for low cost and highly sensitive enzymatic biosensor with high sensitivity well suited for voltammetric detection of a wide range of clinically important biomarkers.

Mediator-less highly sensitive voltammetric detection of glutamate using glutamate dehydrogenase/vertically aligned CNTs grown on silicon substrate.

A sensitive glutamate biosensor is prepared based on glutamate dehydrogenase/vertically aligned carbon nanotubes (GLDH, VACNTs). Vertically aligned carbon nanotubes were grown on a silicon substrate by direct current plasma enhanced chemical vapor deposition (DC-PECVD) method. The electrochemical behavior of the synthesized VACNTs was investigated by cyclic voltammetry and electrochemical impedance spectroscopic methods. Glutamate dehydrogenase covalently attached on tip of VACNTs. The electrochemical performance of the electrode for detection of glutamate was investigated by cyclic and differential pulse voltammetry. Differential pulse voltammetric determinations of glutamate are performed in mediator-less condition and also, in the presence of 1 and 5 µM thionine as electron mediator. The linear calibration curve of the concentration of glutamate versus peak current is investigated in a wide range of 0.1-500 µM. The mediator-less biosensor has a low detection limit of 57 nM and two linear ranges of 0.1-20 µM with a sensitivity of 0.976 mA mM(-1) cm(-2) and 20-300 µM with a sensitivity of 0.182 mA mM(-1) cm(-2). In the presence of 1 µM thionine as an electron mediator, the prepared biosensor shows a low detection limit of 68 nM and two linear ranges of 0.1-20 with a calibration sensitivity of 1.17 mA mM(-1) cm(-2) and 20-500 µM with a sensitivity of 0.153 mA mM(-1) cm(-2). The effects of the other biological compounds on the voltammetric behavior of the prepared biosensor and its response stability are investigated. The results are demonstrated that the GLDH/VACNTs electrode even without electron mediator is a suitable basic electrode for detection of glutamate.