Mahdi Sasar's Publications

(with a short commentary)

2023

Spinristor: A Spin-Filtering Memristor

Adam Jaroš, Mahdi Sasar, Lucie Tučková, Esmaeil Farajpour Bonab, Zahra Badri, Michal Straka, and Cina Foroutan-Nejad.

Advanced Electronic Materials 9, no. 8 (2023): 2300360.

• Summary:

  In this study, we presented an in silico proof of concept of a new type of molecular electronic component that combines functionalities of a spin-filter, molecular switch, and a diode in a single molecule, which we called a spinristor.

  This study was done in a collaboration between one team led by Dr. Michal Straka of IOCB Prague and another team led by Dr. Cina Foroutan-Nejad of IOCB Prague and IOC PAS. Our candidate for a spinristor was a single molecule of Ti@C70 connected to electrodes in an electric circuit.

  We showed that with a low external voltage bias, the Ti@C70 molecule not only rectifies the current (showing a diode-like behavior), but it also shows different resistivities to different spin states of the electrons flowing through the molecule. Furthermore, we showed that a higher applied bias voltage at Ti@C70 causes relocation of the titanium atom, thus switching the whole system into a different set of rectification and spin-filtering characteristics. This device doesn't have a macroscopic counterpart in electronics and it promises to find new applications in novel computer architectures, memristors and memristive computation.

2020

Enhancement of Responsivity and Sensitivity of P-Silicon/n-Zinc Oxide-Based Photodetector Using Titanium Dioxide Nanoparticles

Mahta Monshipouri, Shahrzad Molavi, Amirhossein Mosaddegh, Mahdi Sasar, and Yaser Abdi.

IEEE Transactions on Nanotechnology 19 (2020): 744-48.

• Summary:

  In this paper, we fabricated a UV photodetector based on a direct p-n junction formed between a p-type Si wafer and n-type ZnO nanorod thin film. Such a junction was reported in literature before, but it suffered from low performance. I.e. the responsivity and sensitivity of the device was reported to be low. Because of the need for improvement in metal-oxide/Si p-n junctions, we investigated a novel method to improve their performance. We first found that vertically aligned ZnO nanorods can be a better substitute for a ZnO thin film in such a junction.

  To further improve the performance of a ZnO/Si photodetector, we found that by coating the ZnO nanorods with a 20 nm layer of titanium dioxide (TiO2) using open chemical vapor deposition (CVD) method, we can dramatically reduce the reverse dark current of the device. We showed that the TiO2 coating does not affect the device in the forward bias regime, and it only decrease the dark current in the reverse bias regime. We showed that this decrease directly translates into higher relative photocurrent change in the reverse bias regime, and therefore, significantly increases the responsivity in the reverse bias regime. We showed that the responsivity in the reverse bias regime increases from 0.04 (A/W) (without TiO2 coating) to 0.18 (A/W) (with TiO2 coating). The wavelength of the UV radiation was 365 nm. We showed that this sensor clearly detects UV intensities as low as 80 (µW/cm2).

Detection of Glycated Albumin Using a Novel Field Effect Aptasensor

Mahdi Sasar, Azad Farzadfard, Yaser Abdi, and Mehran Habibi-Rezaei.

IEEE Sensors Journal 20, no. 18 (2020): 10387-92.

• Summary:

  Diabetes is a prevalent metabolic disorder that results in a dangerous series of health complications for patients, especially if it remains unaddressed and undiagnosed. It has been estimated that over 500 million people had type 2 diabetes in 2018 worldwide. This fact has generated significant incentive in scientific and medical research for the investigation of fast and efficient glucose sensors, making glucose one of the most focused bio-analytes in scientific literature to date.

  However, medical research has found that for monitoring the glycemic control of the body, measurement of glycated albumin (GA) in the blood is the best approach and it reveals a better overview for the detection of or the onset of diabetes. High accuracy detection of GA is a relatively modern field and because of the need for accurate and fast GA sensors, in this study, we investigated the possibility of fabricating a novel biosensor for GA using the standard tools of nanotechnology.

  As the first step towards this goal, we synthesized single stranded DNA aptamers (ss-DNA aptamers or aptamers for short) that were strictly designed to attach to GA. Once synthesized, aptamers are cheap to produce at scale, they have very high binding affinity to their target molecule and are chemically more stable than antibodies that are sometimes used in biosensors to capture target molecules. Furthermore, we synthesized our aptamers and thiol-modified them, so that they can form a strong bond (with almost covalent bond strength) from their thiol group to gold surfaces. Next, as a platform to immobilize the aptamers, we used a layer of dense randomly oriented ZnO nanorods coated with 40 nm of Au. We chose ZnO nanorods for their ease of synthesis and for the fact that they are chemically stable in normal blood pH and biocompatible: they do not pose toxicity to biological organisms. We argued that this makes ZnO a perfect scaffolding for immobilizing the thiol-modified aptamers that we synthesized that bind only to GA.

  To fabricate the sensor, we used a novel device architecture that can best be described as a modified ISFET (ion-sensitive field effect transistor) configuration: whereas an ISFET requires the source-drain channels of the transistor to have the form of n-p-n (by n-type doping in the contact regions on a p-type wafer) or p-n-p (by p-type doping in the contact regions on an n-type wafer), we introduced an ISFET with no doping and used a pure p-type Si wafer as the source-drain channel of our ISFET. Our innovation in device architecture removes the need for the cumbersome wafer doping process in the contact regions (which also necessitates photolithography to isolate the contact points) and allows the application of an as-purchased Si wafer as the platform for device fabrication. The rest of our modified ISFET configuration is similar to an ordinary ISFET: the source-drain channel is under a gate insulator layer (in our study 100 nm SiO2), and the gate insulator is covered with a layer of active nanostructures (in our study Au coated ZnO nanorods with aptamers immobilized on them). The gate insulator layer is covered in the test solution and the introduction of the target molecule can have measurable effects on the conductivity of the p-type source-drain channel below the insulator by the application of electrostatic fields. A gate electrode is also placed inside the solution via an Ag/AgCl reference electrode, and changing the gate voltage can further allow us to control the source-drain channel current.

  In our device, we showed that the addition of glycated albumin to the test solution causes dramatic changes in the conductivity of the source-drain channel. We showed that we can quantify the change of the source-drain channel conductivity into accurate determination of GA concentrations in the test solution. We showed that GA concentrations from 77 (µg/ml) to 343 (µg/ml) were detectable in our configuration. Furthermore, we showed that detection time for our innovative sensor was in the order of a few minutes, which is a remarkable improvement over similar studies that report detection times in the order of hours. We must also note that the GA molecules have a negative net charge and as they bond to the aptamers near the gate insulator, they trap positively charged charge carriers near the gate insulator in the source-drain channel and lower the conductivity.

  The sensor we presented in this paper can be used as a fast and reliable sensor for determination of GA. The novel sensor architecture we used was quite novel: it was a modified ISFET, where the Si wafer did not require any doping steps. This significantly decreases the costs of manufacturing of our device and adds to its appeal. Finally, we note that our device is suitable for miniaturization, and it consumes very low power, which makes it suitable for wearable battery powered applications (e.g. smart watches, etc.).

Invasion front dynamics in disordered environments

Youness Azimzade, Mahdi Sasar, and Iraj Maleki.

Scientific Reports 10.1 (2020): 18231.

• Summary:

  Invasion (e.g. growth of cancer cells in a new medium) occurs in environments that are normally spatially disordered, however, the effect of such a randomness on the dynamics of the invasion front has remained less understood. In this paper, we studied Fisher’s equation in disordered environments both analytically and numerically. Using the Effective Medium Approximation, we showed that disorder slows down invasion velocity. Additionally, disorder imposes fluctuations on the invasion front. Using a perturbative approach, we showed that these fluctuations are Brownian. These findings were approved by numerical analysis.

  Alongside this continuum model, we used the Stepping Stone Model to check how our findings change when we move from the continuum approach to a discrete approach. Our analysis suggested that individual-based models exhibit inherent fluctuations and the effect of environmental disorder becomes apparent for large disorder intensity and/or high carrying capacities.

2019

Fabrication and UV Sensitivity of a ZnO Decorated NiO Thin Film Field Effect Transistor

Mahdi Sasar, and Yaser Abdi.

IEEE Electron Device Letters 40, no. 11 (2019): 1764-67.

• Summary:

  In this paper, we developed a novel method for the fabrication of a thin film of nickel oxide (NiO) sparsely decorated with vertically aligned zinc oxide (ZnO) nanorods on a Si/SiO2 wafer. We found that this novel UV photodetector can be very easily fabricated by first coating a Si/SiO2 wafer (consisting of a Si wafer passivated with 100 nm of SiO2 layer that acts as a gate insulator in our field effect transistor) with 40 nm of Ni thin film. We found that this substrate can be used as a platform for a single step fabrication of a thin film field effect transistor, where the conductive channel is a NiO thin film decorated with vertically aligned ZnO nanorods. We showed that this can be done by placing the said substrate in an aqueous solution containing equimolar zinc nitrate and hexamethylenetetramine (HMTA) in 93 Celsius for 24 hours. The device was then taken off the heat, and left to cool to room temperature, and finally left at room temperature for another 24 hours.

  We showed that the rough surface of the deposited Ni thin film acts as nucleation points for ZnO nanocrystals (i.e. heterogeneous nucleation) and that these nucleation points act as crystal seeds for the growth of ZnO nanorods. One of the main advantages of the fabrication method we have introduced is that it is chemical, does not require special vacuum equipment and is done in low temperatures (below boiling point of water).

  After this fabrication step, we showed that the conductivity of the NiO/ZnO layer can be changed by the application of a gate voltage via the Si wafer. Furthermore, we showed that gate voltage can be used to control the photoconductivity of the NiO/ZnO thin film under UV light. We used UV radiation in the UV-A region with a wavelength of 365 nm. We showed that the device could easily detect UV intensities as low as 30 (µW/cm2) in room temperature. We found that gate voltage can be used to tune the quantum efficiency and the responsivity of the transistor: it shows a maximum of 55% quantum efficiency and a responsivity of 160 mA/W. We also found that the charge carrier mobility of the transistor is 59.6 (cm2/V.s) which is in the typical order of amorphous semiconductors.

  Both the mobility and the quantum efficiency are remarkable for a room temperature UV photodetector based on metal-oxide semiconductors. Furthermore, detection of UV intensities as low as 30 (µW/cm2) is a remarkable improvement over similar devices reported in the literature. In this paper, we also showed how the application of the gate voltage changes the depletion layer thickness in the ZnO/NiO junction, and how this can be effectively used to increase the sensitivity of the sensor. We showed that at a high enough positive gate voltages (above 2 V), the NiO layer is completely depleted of positive charge carriers. We showed that this results in a very low dark current, and a large relative photocurrent change, even at very low UV intensities.

  Our paper shows that an innovative thin film field effect transistor based on wide bandgap metal oxide semiconductors can indeed be used as a very low intensity UV photodetector. Furthermore, our device can even be used for the detection of higher energy radiation (UV-B, UV-C and far UV). Furthermore, our photodetector is visible light blind: it only detects UV radiation and higher energy radiation. Therefore, it does not require filters to eliminate ambient noise. Finally, we also showed that our device consumes very little power: from the data presented in our paper, we can calculate that it uses approximately 180 µW of power when illuminated with 1mW of radiation power. This is remarkable, and is suited in applications where the device is dependent on a charged battery.

Invasion front dynamics in disordered environments

Youness Azimzade, Mahdi Sasar, and Iraj Maleki.

Scientific Reports 10.1 (2020): 18231.

• Summary:

  Invasion (e.g. growth of cancer cells in a new medium) occurs in environments that are normally spatially disordered, however, the effect of such a randomness on the dynamics of the invasion front has remained less understood. In this paper, we studied Fisher’s equation in disordered environments both analytically and numerically. Using the Effective Medium Approximation, we showed that disorder slows down invasion velocity. Additionally, disorder imposes fluctuations on the invasion front. Using a perturbative approach, we showed that these fluctuations are Brownian. These findings were approved by numerical analysis.

  Alongside this continuum model, we used the Stepping Stone Model to check how our findings change when we move from the continuum approach to a discrete approach. Our analysis suggested that individual-based models exhibit inherent fluctuations and the effect of environmental disorder becomes apparent for large disorder intensity and/or high carrying capacities.

2018

Light-induced oxygen sensing using ZnO/GO based gas sensor

Yousef Khosravi, Mahdi Sasar, and Yaser Abdi.

Materials Science in Semiconductor Processing 85 (2018): 9-14.

• Summary:

  Because of the commercial and industrial need for gas sensors, many different studies investigate novel material and novel semiconductor architectures for use as gas sensors. An important desirable feature in a gas sensor is that it should operate at room temperature. Gas sensing in room temperature, usually rules out wide-bandgap metal-oxide semiconductors such as zinc oxide (ZnO), nickel oxide (NiO), titanium dioxide (TiO2), etc. since it is only in high temperatures (500oC) that they show a measurable change in their electrical resistance when subjected to different gases. However, metal-oxide semiconductors are relatively cheap and their diverse nanostructures can be fabricated at low temperatures (via chemical methods) and often without using vacuum deposition methods. Because of the desirable features of these semiconductors, we studied the possibility of using metal-oxide semiconductors as room temperature gas sensors, and found that ZnO nanowires in contact with a graphene oxide monolayer, can indeed be used in room temperature to detect oxygen gas with high accuracy and in concentrations as low as 500 ppm. We developed a new method for monitoring the oxygen gas in the environment at room temperature, by measuring the photocurrent generated in the device under UV light irradiation (wavelength 365 nm), as opposed to older studies that simply rely on the change of the electrical resistance of the semiconductor to track the gas concentration.

  We successfully fabricated this oxygen gas sensor by demonstrating that the UV photocurrent of a thin film of high aspect ratio ZnO nanowires (with lengths of ~3 µm and diameters of ~50 nm) on gold interdigitated electrodes, in contact with a dip coated large surface area monolayer graphene oxide, radically changes in the presence of different concentrations of oxygen gas. ZnO nanowires are n-type semiconductors and the GO layer acts as a p-type semiconductor. ZnO nanowires were synthesized using open chemical vapor deposition and the GO layer was obtained using modified Hummers' method.

  In our paper, we found that the photoelectrochemical process of desorption of oxygen molecules on the surface of ZnO nanowires and in the graphene oxide monolayer under the UV light irradiation and the adsorption of the oxygen molecules when UV is turned off, can explain the remarkable increase in the UV photocurrent as the oxygen concentration of the medium increases. We further found that the p-n junction between the ZnO nanowires and the GO monolayer helps separate the photogenerated electrons and holes in the ZnO nanowires, due to the internal electric field in the junction and therefore, increases the electron lifetime in the ZnO nanowires by reducing charge carrier recombination. Furthermore, we found that stretched exponential functions are able to generate very suitable fits to the transient photocurrents. We found that the stretched exponential fits to the transient photocurrent of the device can be used to quantify and measure the oxygen content of the medium. We found that photocurrent increases faster in oxygen rich environments, while the photocurrent increases much slower when the oxygen in the medium is reduced. We finally found that our proposed sensor showed sensitivity to visible light spectra as well. This is a very important result, since wide bandgap semiconductors (ZnO) are only sensitive to the UV and higher energy radiation.