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Yi Zheng

Publications and source records attributed to Yi Zheng.

215 records · Page 12Linked to original sources

Graphene-Ferroelectric Hybrid Structure for Flexible Transparent Electrodes

Graphene has exceptional optical, mechanical and electrical properties, making it an emerging material for novel optoelectronics, photonics and for flexible transparent electrode applications. However, the relatively high sheet resistance of graphene is a major constrain for many of these applications. Here we propose a new approach to achieve low sheet resistance in large-scale CVD monolayer graphene using non-volatile ferroelectric polymer gating. In this hybrid structure, large-scale graphene is heavily doped up to 3{\times}1013 cm-2 by non-volatile ferroelectric dipoles, yielding a low sheet resistance of 120 Ω{\Box} at ambient conditions. The graphene-ferroelectric transparent conductors (GFeTCs) exhibit more than 95% transmittance from the visible to the near infrared range owing to the highly transparent nature of the ferroelectric polymer. Together with its excellent mechanical flexibility, chemical inertness and the simple fabrication process of ferroelectric polymers, the proposed GFeTCs represent a new route towards large-scale graphene based transparent electrodes and optoelectronics.

cond-mat.mes-hall↗

A Green's function formalism of energy and momentum transfer in fluctuational electrodynamics

Radiative energy and momentum transfer due to fluctuations of electromagnetic fields arising due to temperature difference between objects is described in terms of the cross-spectral densities of the electromagnetic fields. We derive relations between thermal non-equilibrium contributions to energy and momentum transfer and surface integrals of tangential components of the dyadic Green's functions of the vector Helmholtz equation. The expressions derived here are applicable to objects of arbitrary shapes, dielectric functions, as well as magnetic permeabilities. For the case of radiative transfer, we derive expressions for the generalized transmissivity and generalized conductance that are shown to obey reciprocity and agree with theory of black body radiative transfer in the appropriate limit.

math-ph↗

Entropy density and entropy flux in near-field thermal radiation

We propose a method to evaluate the entropy density and entropy flux in a vacuum gap between two half-spaces that takes into account influence of near-field effects, i.e., interference, diffraction, and tunneling of waves. The method developed is used to determine the maximum work that can be extracted through near-field radiative transfer between two half-spaces at different temperatures.

cond-mat.mes-hall↗

Analysis of Dual-beam Asymmetrical Torsional Bi-Material Cantilever for Temperature Sensing Applications

An extremely sensitive temperature measurement MEMS device is developed based on the principle of structural deflection in a bi-material cantilever caused by a difference in thermal expansion coefficients. A dual-beam asymmetrical geometry is used to produce a torsional response from the device. An analytical model is developed to predict the performance and optimize the free parameters of the device. In this work, it is performed to analyze the flexural and torsional eigenfrequencies as well as confirm the theoretical predictions of DC and AC response. Lastly, a procedure is developed to allow fabrication of the device using equipment available in the Columbia University clean room.

physics.ins-det↗

Calculation of Droplet Size and Formation Time in Electrohydrodynamic Based Pulsatile Drug Delivery System

Electrohydrodynamic (EHD) generation, a commonly used method in BioMEMS, plays a significant role in the pulsed-release drug delivery system for a decade. In this paper, an EHD based drug delivery system is well designed, which can be used to generate a single drug droplet as small as 2.83 nL in 8.5 ms with a total device of 2x2x3 mm^3, and an external supplied voltage of 1500 V. Theoretically, we derive the expressions for the size and the formation time of a droplet generated by EHD method, while taking into account the drug supply rate, properties of liquid, gap between electrodes, nozzle size, and charged droplet neutralization. This work proves a repeatable, stable and controllable droplet generation and delivery system based on EHD method.

physics.med-ph↗

Pulsatile Drug Delivery System Based on Electrohydrodynamic Method

Electrohydrodynamic (EHD) generation, a commonly used method in BioMEMS, plays a significant role in the pulsatile drug delivery system for a decade. In this paper, an EHD based drug delivery system is well designed, which can be used to generate a single drug droplet as small as 2.83 nL in 8.5 ms with a total device of 2\times2\times3 mm^3, and an external supplied voltage of 1500 V. Theoretically, we derive the expressions for the size and the formation time of a droplet generated by EHD method, while taking into account the drug supply rate, properties of liquid, gap between two electrodes, nozzle size, and charged droplet neutralization. This work proves a repeatable, stable and controllable droplet generation and delivery system based on EHD method experimentally as well as theoretically.

physics.flu-dyn↗

Wafer-scale graphene/ferroelectric hybrid devices for low-voltage electronics

Preparing graphene and its derivatives on functional substrates may open enormous opportunities for exploring the intrinsic electronic properties and new functionalities of graphene. However, efforts in replacing SiO$_{2}$ have been greatly hampered by a very low sample yield of the exfoliation and related transferring methods. Here, we report a new route in exploring new graphene physics and functionalities by transferring large-scale chemical vapor deposition single-layer and bilayer graphene to functional substrates. Using ferroelectric Pb(Zr$_{0.3}$Ti$_{0.7}$)O$_{3}$ (PZT), we demonstrate ultra-low voltage operation of graphene field effect transistors within $\pm1$ V with maximum doping exceeding $10^{13}\,\mathrm{cm^{-2}}$ and on-off ratios larger than 10 times. After polarizing PZT, switching of graphene field effect transistors are characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics.

cond-mat.mes-hall↗

Lifshitz theory of van der Waals pressure in dissipative media

We derive a first--principles method of determining the van der Waals or Casimir pressure in a dissipative and dispersive planar multilayered system by calculating the Maxwell stress tensor in a fictitious layer of vacuum, that is eventually made to vanish, introduced in the structure. This is illustrated by calculating the van der Waals pressure in a thin film with dissipative properties embedded between two semi--infinite media.

quant-ph↗

Graphene field effect transistors with ferroelectric gating

Recent experiments on ferroelectric gating have introduced a novel functionality, i.e. nonvolatility, in graphene field effect transistors. A comprehensive understanding in the non-linear, hysteretic ferroelectric gating and an effective way to control it are still absent. In this letter, we quantitatively characterize the hysteretic ferroelectric gating using the reference of an independent background doping (nBG) provided by normal dielectric gating. More importantly, we prove that nBG can be used to control the ferroelectric gating by unidirectionally shifting the hysteretic ferroelectric doping in graphene. Utilizing this electrostatic effect, we demonstrate symmetrical bit writing in graphene-ferroelectric FETs with resistance change over 500% and reproducible no-volatile switching over 10^5 cycles.

cond-mat.mes-hall↗

30 inch Roll-Based Production of High-Quality Graphene Films for Flexible Transparent Electrodes

We report that 30-inch scale multiple roll-to-roll transfer and wet chemical doping considerably enhance the electrical properties of the graphene films grown on roll-type Cu substrates by chemical vapor deposition. The resulting graphene films shows a sheet resistance as low as ~30 Ohm/sq at ~90 % transparency which is superior to commercial transparent electrodes such as indium tin oxides (ITO). The monolayer of graphene shows sheet resistances as low as ~125 Ohm/sq with 97.4% optical transmittance and half-integer quantum Hall effect, indicating the high-quality of these graphene films. As a practical application, we also fabricated a touch screen panel device based on the graphene transparent electrodes, showing extraordinary mechanical and electrical performances.

cond-mat.mtrl-sci↗

Temperature-dependent transition from injection-limited to space-charge-limited current in metal-organic diodes

Based on the assumption that the contact barrier height determines the current flow in organic semiconductor-based electronic devices, charge injection at metal-organic (MO) interfaces has been extensively investigated, while space-charge conduction in organic bulk is generally overlooked. Recent theoretical modeling and simulation have pointed out that such a simplification is questionable due to the hopping nature of charge injection and hopping-related space-charge conduction. Here we show experimentally that charge transport in metal-organic diodes is complex interplay between injection-limited current (ILC) and space-charge-limited current (SCLC). We report the experimental observation of ILC-to-SCLC transition in Ag/pentacene/Ag diodes as a function of temperature.

cond-mat.mtrl-sci↗

Gate-controlled non-volatile graphene-ferroelectric memory

In this letter, we demonstrate a non-volatile memory device in a graphene FET structure using ferroelectric gating. The binary information, i.e. "1" and "0", is represented by the high and low resistance states of the graphene working channels and is switched by controlling the polarization of the ferroelectric thin film using gate voltage sweep. A non-volatile resistance change exceeding 200% is achieved in our graphene-ferroelectric hybrid devices. The experimental observations are explained by the electrostatic doping of graphene by electric dipoles at the ferroelectric/graphene interface.

cond-mat.mes-hall↗

Can a Matter-Wave Interferometer Detect Translational Speed?

Based only on the Galilean addition of velocities and the de Broglie relation, it is deduced that in a matter-wave interferometer with slow-speed particles, a moving segment of deltaL with a velocity V contributes deltaPhi = (2Pi/vlamda)VdotdeltaL to the total phase difference of the interferometer, where v is the speed of the particles and lamda is the wavelength.. This expression is exactly the same as the generalized Sagnac effect for light waves found by experiments except that v is replaced by c. For a rotational motion, it leads to the Sagnac effect. Additionally, the scientific value of this relationship is also to explore the possibility of detecting translation speeds by a matter-wave interferometer. Two configurations of the experimental setup have been indicated and the key element is that the paths of the interfering beams constitute a loop with an opening. If the possibility is confirmed by experiments, the conclusions will be that there is a preferred reference frame for matter waves and a speedometer with a very high sensitivity is possible.

quant-ph↗

Test of the one-way speed of light and the first-order experiment of Special Relativity using phase-conjugate interferometers

With a Michelson interferometer using a phase-conjugate mirror (PCM) that reverses the uniform phase shift in a light path, we can conduct a first-order experiment of Special Relativity. Utilization of the PCM changes the basic concepts of an interference experiment. Placing a conventional partially reflecting mirror just in front of the PCM at the end of a light path, we can test the isotropy of the one-way speed of light in a system moving uniformly in a straight line and conduct the one-way Sagnac experiment. According to the reported phase-conjugate Sagnac experiment using a segment light path, we can expect that the phase shift is phi = 4pivL/clambda in the one-way Sagnac experiment with path length L and speed v, even with an increasingly larger radius of the rotation. Based on these and the experimental fact of the generalized Sagnac effect, it is very important to examine whether there is the same phase shift for the test of the one-way speed of light and the first-order experiment using the PCM in a system in straight-line uniform motion. The sensitivities of these experiments are very high.

physics.optics↗

Generalized Sagnac Effect

Experiments were conducted to study light propagation in a light waveguide loop consisting of linearly and circularly moving segments. We found that any segment of the loop contributes to the total phase difference between two counterpropagating light beams in the loop. The contribution is proportional to a product of the moving velocity v and the projection of the segment length Deltal on the moving direction, Deltaphi=4pivDeltal/clambda. It is independent of the type of motion and the refractive index of waveguides. The finding includes the Sagnac effect of rotation as a special case and suggests a new fiber optic sensor for measuring linear motion with nanoscale sensitivity.

physics.optics↗

Modified Sagnac experiment for measuring travel-time difference between counter-propagating light beams in a uniformly moving fiber

A fiber optic conveyor has been developed for investigating the travel-time difference between two counter-propagating light beams in uniformly moving fiber. Our finding is that there is a travel-time difference Deltat=2vDeltal/c^2 in a fiber segment of length Deltal moving with the source and detector at a speed v, whether the segment is moving uniformly or circularly.

physics.optics↗

Collective molecule formation in a degenerate Fermi gas via a Feshbach resonance

We model collisionless collective conversion of a degenerate Fermi gas into bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a classical field and seeding the pairing amplitudes with random phases. A dynamical instability of the Fermi sea against association into molecules initiates the conversion. The model qualitatively reproduces several experimental observations {[Regal et al., Nature {\bf 424}, 47 (2003)]}. We predict that the initial temperature of the Fermi gas sets the limit for the efficiency of atom-molecule conversion.

physics.atom-ph↗