Mona Rajabali - Academia.edu (original) (raw)
Papers by Mona Rajabali
arXiv (Cornell University), Jan 10, 2023
Mutual synchronization of N serially connected spintronic nano-oscillators increases their cohere... more Mutual synchronization of N serially connected spintronic nano-oscillators increases their coherence by a factor N and their output power by N 2. Increasing the number of mutually synchronized nano-oscillators in chains is hence of great importance for better signal quality and also for emerging applications such as oscillator-based neuromorphic computing and Ising machines where larger N can tackle larger problems. Here we fabricate spin Hall nano-oscillator chains of up to 50 serially connected nano-constrictions in W/NiFe, W/CoFeB/MgO, and NiFe/Pt stacks and demonstrate robust and complete mutual synchronization of up to 21 nano-constrictions, reaching linewidths of below 200 kHz and quality factors beyond 79,000, while operating at 10 GHz. We also find a square increase in the peak power with the increasing number of mutually synchronized oscillators, resulting in a factor of 400 higher peak power in long chains compared to individual nano-constrictions. Although chains longer than 21 nano-constrictions also show complete mutual synchronization, it is not as robust and their signal quality does not improve as much as they prefer to break up into partially synchronized states. The low current and low field operation of these oscillators along with their wide frequency tunability (2-28 GHz) with both current and magnetic fields, make them ideal candidates for on-chip GHz-range applications and neuromorphic computing.
Electronics
Spin-Hall-effect nano-oscillators are promising beyond the CMOS devices currently available, and ... more Spin-Hall-effect nano-oscillators are promising beyond the CMOS devices currently available, and can potentially be used to emulate the functioning of neurons in computational neuromorphic systems. As they oscillate in the 4–20 GHz range, they could potentially be used for building highly accelerated neural hardware platforms. However, due to their extremely low signal level and high impedance at their output, as well as their microwave-range operating frequency, discerning whether the SHNO is oscillating or not carries a great challenge when its state read-out circuit is implemented using CMOS technologies. This paper presents the first CMOS front-end read-out circuitry, implemented in 180 nm, working at a SHNO oscillation frequency up to 4.7 GHz, managing to discern SHNO amplitudes of 100 µV even for an impedance as large as 300 Ω and a noise figure of 5.3 dB300 Ω. A design flow of this front end is presented, as well as the architecture of each of its blocks. The study of the low...
GOQ-08 Injection locking of edge, bullet, and interior spin wave modes in nano-constriction spin Hall nano-oscillators
Injection locking of spin Hall nano-oscillators (SHNOs) via a microwave current has been investig... more Injection locking of spin Hall nano-oscillators (SHNOs) via a microwave current has been investigated for applications in novel communication and computation schemes, such as phase shift keying, and neuromorphic computing [1-3]. However, no detailed study has been dedicated to investigate how the different spin wave (SW) modes in nano-constriction SHNOs lock to an external signal as previously done for nano-gap SHNOs [4]. Here, we study the injection locking behavior of three distinctly different SW modes in SHNOs: field-localized edge mode, internal modes, and the self-localized SW bullet, in a Pt(50 Å)/Hf(5Å)/NiFe(50Å) SHNO.<br/>The power spectral density (PSD) vs. drive current for a 150nm wide SHNO in two qualitatively different operating regimes has been shown in Fig. 1(a) and (b). As can be seen, three distinct SW modes in the same device are observed merely by changing the field conditions. For an external magnetic field of H<sub>ext</sub>=7.6 KOe and applie...
Physical Review Applied, 2022
Nanoscale, 2022
Schematic of nano-gate voltage controlled spin Hall nano-oscillator and significantly large frequ... more Schematic of nano-gate voltage controlled spin Hall nano-oscillator and significantly large frequency tunability with applied voltage and negligible leak current.
High-performance large-area WS2-based transistors by a novel tin-oxide assisted liquid-phase exfoliation: doping adjustment by plasma treatment
2D Materials, 2021
We report a novel strategy to stitch small WS2 flakes to form larger features in liquid-phase exf... more We report a novel strategy to stitch small WS2 flakes to form larger features in liquid-phase exfoliation. Owing to the initial granular size of the bulk WS2, which, is around 2 µm, the use of conventional liquid-based exfoliation techniques leads to small area nanoflakes with sizes around 1 μm. However, by joining smaller flakes through tin-oxide quantum dot (SnO2 QD) decoration during probe sonication, sheets up to 20 μm are achieved. The neighboring flakes could stitch together to form larger features through the Sn–S and Sn–O bonds at their lateral sides of their flakes and to realize mosaic arrangement of arbitrary shaped larger sheets. While the original WS2 flakes show n-type behavior, an interesting conversion into a p-channel behavior is observed upon addition of SnO2 QDs followed by additional SF6 plasma treatment process. Additionally, we have investigated the use of SF6 plasma treatment to enhance the carrier mobility and on-off ratio of the field effect transistors, yie...
Field Effect Transistors: High‐Performance Phosphorene‐Based Transistors Using a Novel Exfoliation‐Free Direct Crystallization on Silicon Substrates (Adv. Mater. Interfaces 17/2020)
Advanced Materials Interfaces, 2020
High‐Performance Phosphorene‐Based Transistors Using a Novel Exfoliation‐Free Direct Crystallization on Silicon Substrates
Advanced Materials Interfaces, 2020
Phosphorene is one of the most promising elemental 2D materials. So far, mechanical exfoliation i... more Phosphorene is one of the most promising elemental 2D materials. So far, mechanical exfoliation is the main route toward the formation of phosphorene sheets. Although field‐effect transistors are realized on phosphorene, their arrangement and size are dictated by the randomness of the exfoliation and transfer methods. Here, the evolution of highly crystalline and large‐area phosphorene sheets with a laser‐assisted phase transition from red phosphorus directly on silicon substrates in desired locations is reported. The original red phosphorus can be patterned prior to laser‐assisted crystallization to achieve the desired shape, location, and layer thickness (either mono or few). A 1064 nm laser is employed as the source to impart energy onto the red phosphorus layer and to achieve allotrope transformation. A combination of argon–oxygen (Ar:O2) and helium–hydrogen (He:H2) gases is exploited in a sequential manner to lead to phosphorene sheets. Various techniques are used to examine the physical properties of phosphorene sheets. Field‐effect transistors are made at different phases of crystallization and a high field‐effect mobility of 1450 cm2 V−1 s−1 and on/off ratio of 103 are achieved. Moreover, the optoelectrical characteristics of the phosphorene‐based phototransistors under different illumination powers and back‐gate voltages are investigated, showing superior performance.
Highly Crystalline Nickel Silicon Sheets on Silicon Substrates Using Hydrogen Plasma Treatment
physica status solidi (RRL) – Rapid Research Letters, 2020
Herein, the evolution of nickel silicide and silicon nanosheets on silicon substrates using CH4 p... more Herein, the evolution of nickel silicide and silicon nanosheets on silicon substrates using CH4 plasma followed by hydrogen plasma treatment is reported. A 2–5 nm nickel layer is deposited on (100) silicon substrates using e‐beam evaporation at 150 °C. Hydrogen plasma treatment is the most critical step for the oriented diffusion of nickel atoms through [111] planes of silicon. Moreover, the incorporation of trace values of carbon on silicon is found to be crucial to achieve the oriented diffusion of nickel and formation of nanosheets. The presence of carbon is important to induce strain on [111] planes of silicon and expedite the nickel diffusion between such planes. Although for (100) silicon wafers, an in‐depth diffusion of nickel is observed, for (111) substrates, a planar sheet is formed. Unique nanochannels with a depth of 500 nm and width of 50–100 nm are realized on (100) silicon wafers and their length is extended to 100 μm. A thorough examination of the sheets using a transmission electron microscope reveals the presence of quantum dots of nickel, placed within silicon nanostructures. These films are investigated using scanning and transmission electron microscopy, X‐ray diffraction, atomic force microscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy.
physica status solidi (RRL) – Rapid Research Letters, 2019
physica status solidi (RRL) – Rapid Research Letters, 2019
Tungsten disulfide is an emerging 2D material with unique electrical and optical properties. Alth... more Tungsten disulfide is an emerging 2D material with unique electrical and optical properties. Although chemical vapor deposition and mechanical exfoliation lead to large-area layers, sonication solvent-based exfoliation is a convenient method to realize mono-and few-layer flakes. Herein, a mixture of dimethyl-sulfoxide (DMSO) and water as a safe, facile, and user-friendly exfoliation solvent for the synthesis of large-scale WS 2 sheets is investigated. While water molecules weaken the Van der Waals force between the stacked layers, DMSO molecules facilitate the exfoliation process. To improve exfoliation and achieve large-area sheets, plasma pretreatment is exploited on bulk WS 2 prior to the exfoliation process. For this purpose, O 2 and H 2 gases are used to carry out the plasma pretreatment. The oxygen plasma treatment is shown to be an effective strategy, leading to large-area features. Moreover, inert gases as He and Ar are used to obtain small sheets. Transmission electron microscopy, atomic force microscopy, Raman spectroscopy, and dynamic light scattering analyses are used to understand the mechanism of exfoliation and study the effects of various parameters as water temperature, plasma power, and duration.
physica status solidi (RRL) - Rapid Research Letters, 2019
Acta Materialia, 2006
h i g h l i g h t s Low fraction of GO could significantly reduce the content of chloride diffusi... more h i g h l i g h t s Low fraction of GO could significantly reduce the content of chloride diffusion. The improved barrier properties are due to the refined microstructure by GO. Simulation showed the migration rate of ions and water is reduced by GO. Simulation results are consistent with the experimental findings.
arXiv (Cornell University), Jan 10, 2023
Mutual synchronization of N serially connected spintronic nano-oscillators increases their cohere... more Mutual synchronization of N serially connected spintronic nano-oscillators increases their coherence by a factor N and their output power by N 2. Increasing the number of mutually synchronized nano-oscillators in chains is hence of great importance for better signal quality and also for emerging applications such as oscillator-based neuromorphic computing and Ising machines where larger N can tackle larger problems. Here we fabricate spin Hall nano-oscillator chains of up to 50 serially connected nano-constrictions in W/NiFe, W/CoFeB/MgO, and NiFe/Pt stacks and demonstrate robust and complete mutual synchronization of up to 21 nano-constrictions, reaching linewidths of below 200 kHz and quality factors beyond 79,000, while operating at 10 GHz. We also find a square increase in the peak power with the increasing number of mutually synchronized oscillators, resulting in a factor of 400 higher peak power in long chains compared to individual nano-constrictions. Although chains longer than 21 nano-constrictions also show complete mutual synchronization, it is not as robust and their signal quality does not improve as much as they prefer to break up into partially synchronized states. The low current and low field operation of these oscillators along with their wide frequency tunability (2-28 GHz) with both current and magnetic fields, make them ideal candidates for on-chip GHz-range applications and neuromorphic computing.
Electronics
Spin-Hall-effect nano-oscillators are promising beyond the CMOS devices currently available, and ... more Spin-Hall-effect nano-oscillators are promising beyond the CMOS devices currently available, and can potentially be used to emulate the functioning of neurons in computational neuromorphic systems. As they oscillate in the 4–20 GHz range, they could potentially be used for building highly accelerated neural hardware platforms. However, due to their extremely low signal level and high impedance at their output, as well as their microwave-range operating frequency, discerning whether the SHNO is oscillating or not carries a great challenge when its state read-out circuit is implemented using CMOS technologies. This paper presents the first CMOS front-end read-out circuitry, implemented in 180 nm, working at a SHNO oscillation frequency up to 4.7 GHz, managing to discern SHNO amplitudes of 100 µV even for an impedance as large as 300 Ω and a noise figure of 5.3 dB300 Ω. A design flow of this front end is presented, as well as the architecture of each of its blocks. The study of the low...
GOQ-08 Injection locking of edge, bullet, and interior spin wave modes in nano-constriction spin Hall nano-oscillators
Injection locking of spin Hall nano-oscillators (SHNOs) via a microwave current has been investig... more Injection locking of spin Hall nano-oscillators (SHNOs) via a microwave current has been investigated for applications in novel communication and computation schemes, such as phase shift keying, and neuromorphic computing [1-3]. However, no detailed study has been dedicated to investigate how the different spin wave (SW) modes in nano-constriction SHNOs lock to an external signal as previously done for nano-gap SHNOs [4]. Here, we study the injection locking behavior of three distinctly different SW modes in SHNOs: field-localized edge mode, internal modes, and the self-localized SW bullet, in a Pt(50 Å)/Hf(5Å)/NiFe(50Å) SHNO.<br/>The power spectral density (PSD) vs. drive current for a 150nm wide SHNO in two qualitatively different operating regimes has been shown in Fig. 1(a) and (b). As can be seen, three distinct SW modes in the same device are observed merely by changing the field conditions. For an external magnetic field of H<sub>ext</sub>=7.6 KOe and applie...
Physical Review Applied, 2022
Nanoscale, 2022
Schematic of nano-gate voltage controlled spin Hall nano-oscillator and significantly large frequ... more Schematic of nano-gate voltage controlled spin Hall nano-oscillator and significantly large frequency tunability with applied voltage and negligible leak current.
High-performance large-area WS2-based transistors by a novel tin-oxide assisted liquid-phase exfoliation: doping adjustment by plasma treatment
2D Materials, 2021
We report a novel strategy to stitch small WS2 flakes to form larger features in liquid-phase exf... more We report a novel strategy to stitch small WS2 flakes to form larger features in liquid-phase exfoliation. Owing to the initial granular size of the bulk WS2, which, is around 2 µm, the use of conventional liquid-based exfoliation techniques leads to small area nanoflakes with sizes around 1 μm. However, by joining smaller flakes through tin-oxide quantum dot (SnO2 QD) decoration during probe sonication, sheets up to 20 μm are achieved. The neighboring flakes could stitch together to form larger features through the Sn–S and Sn–O bonds at their lateral sides of their flakes and to realize mosaic arrangement of arbitrary shaped larger sheets. While the original WS2 flakes show n-type behavior, an interesting conversion into a p-channel behavior is observed upon addition of SnO2 QDs followed by additional SF6 plasma treatment process. Additionally, we have investigated the use of SF6 plasma treatment to enhance the carrier mobility and on-off ratio of the field effect transistors, yie...
Field Effect Transistors: High‐Performance Phosphorene‐Based Transistors Using a Novel Exfoliation‐Free Direct Crystallization on Silicon Substrates (Adv. Mater. Interfaces 17/2020)
Advanced Materials Interfaces, 2020
High‐Performance Phosphorene‐Based Transistors Using a Novel Exfoliation‐Free Direct Crystallization on Silicon Substrates
Advanced Materials Interfaces, 2020
Phosphorene is one of the most promising elemental 2D materials. So far, mechanical exfoliation i... more Phosphorene is one of the most promising elemental 2D materials. So far, mechanical exfoliation is the main route toward the formation of phosphorene sheets. Although field‐effect transistors are realized on phosphorene, their arrangement and size are dictated by the randomness of the exfoliation and transfer methods. Here, the evolution of highly crystalline and large‐area phosphorene sheets with a laser‐assisted phase transition from red phosphorus directly on silicon substrates in desired locations is reported. The original red phosphorus can be patterned prior to laser‐assisted crystallization to achieve the desired shape, location, and layer thickness (either mono or few). A 1064 nm laser is employed as the source to impart energy onto the red phosphorus layer and to achieve allotrope transformation. A combination of argon–oxygen (Ar:O2) and helium–hydrogen (He:H2) gases is exploited in a sequential manner to lead to phosphorene sheets. Various techniques are used to examine the physical properties of phosphorene sheets. Field‐effect transistors are made at different phases of crystallization and a high field‐effect mobility of 1450 cm2 V−1 s−1 and on/off ratio of 103 are achieved. Moreover, the optoelectrical characteristics of the phosphorene‐based phototransistors under different illumination powers and back‐gate voltages are investigated, showing superior performance.
Highly Crystalline Nickel Silicon Sheets on Silicon Substrates Using Hydrogen Plasma Treatment
physica status solidi (RRL) – Rapid Research Letters, 2020
Herein, the evolution of nickel silicide and silicon nanosheets on silicon substrates using CH4 p... more Herein, the evolution of nickel silicide and silicon nanosheets on silicon substrates using CH4 plasma followed by hydrogen plasma treatment is reported. A 2–5 nm nickel layer is deposited on (100) silicon substrates using e‐beam evaporation at 150 °C. Hydrogen plasma treatment is the most critical step for the oriented diffusion of nickel atoms through [111] planes of silicon. Moreover, the incorporation of trace values of carbon on silicon is found to be crucial to achieve the oriented diffusion of nickel and formation of nanosheets. The presence of carbon is important to induce strain on [111] planes of silicon and expedite the nickel diffusion between such planes. Although for (100) silicon wafers, an in‐depth diffusion of nickel is observed, for (111) substrates, a planar sheet is formed. Unique nanochannels with a depth of 500 nm and width of 50–100 nm are realized on (100) silicon wafers and their length is extended to 100 μm. A thorough examination of the sheets using a transmission electron microscope reveals the presence of quantum dots of nickel, placed within silicon nanostructures. These films are investigated using scanning and transmission electron microscopy, X‐ray diffraction, atomic force microscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy.
physica status solidi (RRL) – Rapid Research Letters, 2019
physica status solidi (RRL) – Rapid Research Letters, 2019
Tungsten disulfide is an emerging 2D material with unique electrical and optical properties. Alth... more Tungsten disulfide is an emerging 2D material with unique electrical and optical properties. Although chemical vapor deposition and mechanical exfoliation lead to large-area layers, sonication solvent-based exfoliation is a convenient method to realize mono-and few-layer flakes. Herein, a mixture of dimethyl-sulfoxide (DMSO) and water as a safe, facile, and user-friendly exfoliation solvent for the synthesis of large-scale WS 2 sheets is investigated. While water molecules weaken the Van der Waals force between the stacked layers, DMSO molecules facilitate the exfoliation process. To improve exfoliation and achieve large-area sheets, plasma pretreatment is exploited on bulk WS 2 prior to the exfoliation process. For this purpose, O 2 and H 2 gases are used to carry out the plasma pretreatment. The oxygen plasma treatment is shown to be an effective strategy, leading to large-area features. Moreover, inert gases as He and Ar are used to obtain small sheets. Transmission electron microscopy, atomic force microscopy, Raman spectroscopy, and dynamic light scattering analyses are used to understand the mechanism of exfoliation and study the effects of various parameters as water temperature, plasma power, and duration.
physica status solidi (RRL) - Rapid Research Letters, 2019
Acta Materialia, 2006
h i g h l i g h t s Low fraction of GO could significantly reduce the content of chloride diffusi... more h i g h l i g h t s Low fraction of GO could significantly reduce the content of chloride diffusion. The improved barrier properties are due to the refined microstructure by GO. Simulation showed the migration rate of ions and water is reduced by GO. Simulation results are consistent with the experimental findings.