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Dive into the research topics where Ramin Banan Sadeghian is active.

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Featured researches published by Ramin Banan Sadeghian.


Nature Materials | 2011

Ultralow-voltage field-ionization discharge on whiskered silicon nanowires for gas-sensing applications

Ramin Banan Sadeghian; M. Saif Islam

Several hundred million volts per centimetre of electric-field strength are required to field-ionize gas species. Such fields are produced on sharp metallic tips under a bias of a few kilovolts. Here, we show that field ionization is possible at dramatically lower fields on semiconductor nanomaterials containing surface states, particularly with metal-catalysed whiskers grown on silicon nanowires. The low-voltage field-ionization phenomena observed here cannot be explained solely on the basis of the large field-amplification effect of suspended gold nanoparticles present on the whisker tips. We postulate that field penetration causes upward band-bending at the surface of exposed silicon containing surface states in the vicinity of the catalyst. Band-bending enables the valence electron to tunnel into the surface states at reduced fields. This work provides a basis for development of low-voltage ionization sensors. Although demonstrated on silicon, low-voltage field ionization can be detected on any sharp semiconductor tip containing proper surface states.


IEEE Sensors Journal | 2008

A Novel Gas Sensor Based on Tunneling-Field-Ionization on Whisker-Covered Gold Nanowires

Ramin Banan Sadeghian; Mojtaba Kahrizi

Typical gas ionization sensors (GISs) work by fingerprinting the ionization breakdown voltages of the gases to be identified. In this work, we developed a GIS that operates by field-ionizing the unknown gas at exceptionally low voltages. The resultant field-ion current-voltage (I-V) characteristic was then used to identify the gas. Freestanding gold nanowires (AuNW), terminated with nanoscale whisker-like features, were employed as field-amplifiers to reduce the field ionization threshold voltages. Synthesis of the AuNWs was carried out by the template-assisted technique accompanied by two alterations: 1) polystyrene (PS) microspheres were incorporated to reduce the compactness of the pores, thus prevent the nanostructures from collapse, and 2) the template was impregnated by HAuCl4 to form gold nanowhiskers during the electrochemical nucleation of AuNWs. The sensor was tested in three elemental gases: Ar, N2 and He, in a pressure range of 0.01 < P < 100 torr. Each gas demonstrated a distinctive I-V curve, particularly in the field-limited regime. The threshold ionization voltages ranged from 1 to 10 V, almost three orders of magnitude lower than the voltages used in field-ion-microscopy. The low-voltage field ionization was attributed to the field-amplifying nanoscale whiskers on the AuNW tips, as well as the presence of residual amorphous alumina with semiconducting characteristics, due to incomplete removal of the porous anodized alumina (PAA) template.


ieee sensors | 2007

A Low Voltage Gas Ionization Sensor based on Sparse Gold Nanorods

Ramin Banan Sadeghian; Mojtaba Kahrizi

We report fabrication and characterization of a gas ionization sensor using a sparse array of vertically aligned gold nanorods (AuNR). Our device displayed improved sensitivity compared to the carbon nanotube (CNT) film counterpart, since the room temperature breakdown voltages (Vb) of the tested gases were further reduced. With AuNRs configured as the cathode, Vb was unaffected by gas pressure within a wide range (10-5 < P < 0.1 torr), suggesting that the breakdown process does not follow Townsends criterion. At higher and lower pressures, Vh increased. The field-emission (FE) and field-ionization (FI) properties of the AuNRs were also studied, and the latter showed measurable pressure dependency in a certain voltage range. The lowering of Vb as well as enhanced FE and FI actions, were attributed to the high field-amplification factor of atomically sharp features on AuNR tips covered by a thin alumina scale.


Applied Physics Letters | 2011

Miniaturized concentration cells for small-scale energy harvesting based on reverse electrodialysis

Ramin Banan Sadeghian; Oxana Pantchenko; Daniel Scott Tate; Ali Shakouri

We describe experimental and theoretical results that demonstrate the feasibility of power generation using concentration cells based on ionic concentration gradients and reverse electrodialysis. A peak power density of 0.2 (0.7) μW cm−2 and a maximum energy density of 0.4 (0.4) mJ cm−3 delivered in 3 h to a 2 (5) kΩ resistor were recorded using a microfiltration (anion exchange) membrane, respectively. A comprehensive model is developed to predict the evolution of the output voltage with time in relation to the solute concentration in each cell and derive the power density and efficiency limits.


international symposium on industrial electronics | 2007

A Low Pressure Gas Ionization Sensor Using Freestanding Gold Nanowires

Ramin Banan Sadeghian; Mojtaba Kahrizi

Freestanding gold nanowires (AuNWs) were grown by electrochemically filling the nanoscale pores of anodic aluminum oxide (AAO) membranes, and removing the membrane selectively. The array of vertically-aligned AuNWs was used at one of the electrodes of a capacitor-like gas ionizing cell, while the counter electrode was a polished silicon wafer coated with aluminum. The field enhancement property of high aspect-ratio AuNWs was employed to reduce the gaseous breakdown voltage (Vb) at room temperature. The device was operated in low-pressure gas to study the effect of AuNWs on pre-breakdown current, and tested in sub-torr argon, where it demonstrated considerable reduction in the breakdown voltage compared to uniform field conditions. The effect of nanowire polarity on Vb and the pre-breakdown discharge current was also studied.


Journal of Electronic Materials | 2012

Calculation of Nonlinear Thermoelectric Coefficients of InAs1−xSbx Using Monte Carlo Method

Ramin Banan Sadeghian; Je-Hyeong Bahk; Zhixi Bian; Ali Shakouri

It was found that the nonlinear Peltier effect could take place and increase the cooling power density when a lightly doped thermoelectric material is under a large electrical field. This effect is due to the Seebeck coefficient enhancement from an electron distribution far from equilibrium. In the nonequilibrium transport regime, the solution of the Boltzmann transport equation in the relaxation-time approximation ceases to apply. The Monte Carlo method, on the other hand, proves to be a capable tool for simulation of semiconductor devices at small scales as well as thermoelectric effects with local nonequilibrium charge distribution. InAs1−xSbx is a favorable thermoelectric material for nonlinear operation owing to its high mobility inherited from the binary compounds InSb and InAs. In this work we report simulation results on the nonlinear Peltier power of InAs1−xSbx at low doping levels, at room temperature and at low temperatures. The thermoelectric power factor in nonlinear operation is compared with the maximum value that can be achieved with optimal doping in the linear transport regime.


Applied Physics Letters | 2007

Study of the structural evolution within polystyrene and polystyrene-gold composite colloidal crystals by atomic force microscopy and scanning electron microscopy

Simona Badilescu; Ahmad-Reza Hajiaboli; Nader Seirafianpour; Ramin Banan Sadeghian; Mojtaba Kahrizi; Vo-Van Truong

The simultaneous presence of hexagonal and quadratic arrangements of polystyrene microspheres in the top layers of polystyrene and gold-polystyrene composite colloidal crystals has been evidenced by both atomic force microscopy and scanning electron microscopy. It is thought that the formation of layers with the two different packing modes is in agreement with the phase transformations found in systems with confined geometries. The results of this study are relevant to the fabrication of high quality photonic crystals.


ieee eurocon | 2009

A silver nanowire based gas ionization sensor EUROCON2009

Nika Azmoodeh; Nicoleta Chivu; Ramin Banan Sadeghian; Mojtaba Kahrizi

We report fabrication and successful testing of a gas ionization sensor made of silver nanowires (AgNWs) sandwiched between two parallel plates. The device was tested in low pressure argon gas (0.1≤ P ≤10−4 torr) where the breakdown voltages (Vb) were recorded against pressure. The new device displayed considerably lower Vb compared to its gold nanowire (AuNW)-based counterparts. The reduction of Vb was justified based on the increase in the secondary electron emission factor on AgNWs due to the lower workfunction of silver. We have also developed a simulation tool to model the device. Our model is based on combined Particle-In-Cell and Monte-Carlo-Collision (PIC-MCC) approaches which provides a platform for further development and optimization of the device.


international conference of the ieee engineering in medicine and biology society | 2013

An e-nose concept based on semiconductor-assisted field ionization and gaseous discharge on arrays of whiskered nanowires

Ramin Banan Sadeghian

Electronic nose systems employ an array of gas detectors each tailored to respond differently to a range of odors. In the presence of a particular gaseous compound, the responses of sensors form a signature that is analyzed in the signal transduction process. The gas sensing cells operate by a variety of different mechanisms, but generally, catalyst based sensors suffer from issues such as poor specificity, slow response time, and irreversibility. This work introduces a novel approach towards instantaneous and selective discrimination of gases based on their unique ionization properties: electric breakdown voltage and field ionization current-voltage characteristic. Synthesized gold and silicon nanowires, covered with sharp nanoscale whiskers, exhibited anomalously strong discharge and field ionization characteristics at very low bias voltages. The anomalous field ionization phenomenon was attributed to the combination of geometric field enhancement and the presence of localized surface states at the surface of the emitters.


Sensors and Actuators A-physical | 2007

A novel miniature gas ionization sensor based on freestanding gold nanowires

Ramin Banan Sadeghian; Mojtaba Kahrizi

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M. Saif Islam

University of California

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Zhixi Bian

University of California

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Aaron M. Katzenmeyer

Sandia National Laboratories

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