Showing papers by "Sanjay K. Banerjee published in 2020"

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Abstract: Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2 . Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump-probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. The findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2 , mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

Lithium-ion electrolytic substrates for sub-1V high-performance transition metal dichalcogenide transistors and amplifiers.

Abstract: Electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), leading to the accumulation of high surface charge carrier densities, has been often exploited in 2D devices However, the intrinsic liquid nature of ILs, their sensitivity to humidity, and the stress induced in frozen liquids inhibit ILs from constituting an ideal platform for electrostatic gating Here we report a lithium-ion solid electrolyte substrate, demonstrating its application in high-performance back-gated n-type MoS2 and p-type WSe2 transistors with sub-threshold values approaching the ideal limit of 60 mV/dec and complementary inverter amplifier gain of 34, the highest among comparable amplifiers Remarkably, these outstanding values were obtained under 1 V power supply Microscopic studies of the transistor channel using microwave impedance microscopy reveal a homogeneous channel formation, indicative of a smooth interface between the TMD and underlying electrolytic substrate These results establish lithium-ion substrates as a promising alternative to ILs for advanced thin-film devices Electrostatic gating of 2D transistors with ionic liquids presents intrinsic limitations Here, the authors demonstrate n-type MoS2 and p-type WSe2 transistors on a lithium-ion solid electrolyte substrate, displaying sub-threshold values approaching the ideal limit of 60 mV/dec and complementary amplifier gain of 34 with 1 V supply

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Abstract: Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

Progress in Materials Development for the Rapid Efficiency Advancement of Perovskite Solar Cells

Abstract: The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single-junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single-junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

Rational design principles for giant spin Hall effect in 5d-transition metal oxides.

Abstract: Spin Hall effect (SHE), a mechanism by which materials convert a charge current into a spin current, invokes interesting physics and promises to empower transformative, energy-efficient memory technology. However, fundamental questions remain about the essential factors that determine SHE. Here, we solve this open problem, presenting a comprehensive theory of five rational design principles for achieving giant intrinsic SHE in transition metal oxides. Arising from our key insight regarding the inherently geometric nature of SHE, we demonstrate that two of these design principles are weak crystal fields and the presence of structural distortions. Moreover, we discover that antiperovskites are a highly promising class of materials for achieving giant SHE, reaching SHE values an order of magnitude larger than that reported for any oxide. Additionally, we derive three other design principles for enhancing SHE. Our findings bring deeper insight into the physics driving SHE and could help enhance and externally control SHE values.

Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies.

Abstract: We investigate the role of growth temperature and metal/chalcogen flux in atmospheric pressure chemical vapor deposition (APCVD) growth of MoSe2 and WSe2 on Si/SiO2 substrates. Using scanning elect...

The microscopic origin of DMI in magnetic bilayers and prediction of giant DMI in new bilayers

Abstract: Skyrmions are widely regarded as promising candidates for emergent spintronic devices. Dzyaloshinskii–Moriya interaction (DMI) is often critical to the generation and manipulation of skyrmions. However, there is a fundamental lack of understanding of the origin of DMI or the mechanism by which DMI generates skyrmions in magnetic bilayers. Very little is known of the material parameters that determine the value of DMI. This knowledge is vital for rational design of skyrmion materials and further development of skyrmion technology. To address this important problem, we investigate DMI in magnetic bilayers using first principles. We present a new theoretical model that explains the microscopic origin of DMI in magnetic bilayers. We demonstrate that DMI depends on two parameters, interfacial hybridization and orbital contributions of the heavy metal. Using these parameters, we explain the trend of DMI observed. We also report four new materials systems with giant DMI and new designs for magnetic multilayers that are expected to outperform the best materials known so far. Our results present a notably new understanding of DMI, uncover highly promising materials and put forth pathways for the controlled generation of skyrmions.

Contact Engineering of Layered MoS2 via Chemically Dipping Treatments

Abstract: This work was supported by NSF Grants DMR 1207213, DMR1400432, and EFRI-2DARE 1433490, and by LEAST-STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA and by SRC NRI SWAN. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Nos. NRF-2018R1C1B5085644 and NRF-2019R1A2B5B02070657).

Realizing both short- and long-term memory within a single magnetic tunnel junction based synapse

Abstract: Synaptic plasticity forms the basis of memory retention in the human brain. Whereas a low “rehearsal” rate causes short-term changes in the synaptic connections such that the synapse soon “forgets,” a high rehearsal rate ensures long-term retention of memory in the brain. In this paper, we propose an artificial short- and long-term memory magnetic tunnel junction (SALT-MTJ) synapse. Changes in the synaptic strength are mapped to the SALT-MTJ conductance, which is varied stochastically via spin-transfer torque resulting from input current stimuli. A meta-stable intermediate magnetic state of the SALT-MTJ synapse provides short-term synaptic plasticity and the associated forgetting behavior as in a biological synapse. Repeated spin-current stimulations, while the SALT-MTJ remains in the short-term state, then can cause a near-permanent change in the magnetic state and associated conductance to provide long-term potentiation. The synaptic weight sensitivity to the input stimulus and the forgetting behavior of these short- and long-term states can be controlled via shape engineering of the artificial synapse.

Structural and Magnetic Properties of Molecular Beam Epitaxy Grown Chromium Selenide Thin Films

Abstract: Chromium selenide thin films were grown epitaxially on Al${_2}$O${_3}$(0001) and Si(111)-(7${\times}$7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 A. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a Neel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

Monte Carlo Study of Si, Ge, and In 0.53 Ga 0.47 As n-Channel FinFET Scaling: Channel Orientation, Quantum Confinement, Doping, and Contacts

Abstract: The effects of channel scaling in silicon (Si) and Si-alternative germanium (Ge) and In0.53Ga0.47As (InGaAs) n-channel devices toward the end of the CMOS roadmap are addressed theoretically. The devices are simulated using a quantum-corrected semiclassical Monte Carlo method. The results are discussed with an emphasis on the underlying physics. Multiple effects of quantum conf inement within the channel, far-from equilibrium-degenerate statistics, saddle/slot contact geometries with appropriate material-dependent specific contact resistivities, and appropriate material-dependent source and drain doping concentrations are considered, with the exception of the contact resistivity of Ge, which is idealized to reflect what may be possible if the currently prohibitive contact challenge can be overcome.

Structural and magnetic properties of molecular beam epitaxy grown chromium selenide thin films

Abstract: Chromium selenide thin films were grown epitaxially on $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}(0001)$ and Si(111)-(7 \ifmmode\times\else\texttimes\fi{} 7) substrates using molecular beam epitaxy. Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate flat smooth film growth along the $c$ axis, which is very similar to that from a hexagonal surface. The x-ray diffraction pattern confirms the growth along the $c$ axis with a $c$-axis lattice constant of 17.39 \AA{}. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature-dependent resistivity. The antiferromagnetic nature of the film with a N\'eel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

Two-dimensional transport model of spin-polarized tunneling in a topological-insulator/tunnel-barrier/ferromagnetic-metal heterostructure

Abstract: Spin-polarized electrons injected from a ferromagnet (FM) onto the surface of a topological insulator (TI) tend to produce a charge current transverse to the direction of the spin polarization because of the spin-momentum helical locking of the TI surface states. The charge current can be measured as an open-circuit voltage that will change in sign if the magnetization direction of the FM is reversed. Here, we model the two-dimensional transport on the TI surface coupled to a FM through a tunnel barrier (TB). The transport equations are solved analytically for two different boundary conditions on the TI surface, and the effectiveness of the TI-TB-FM junction for determining such voltage change upon FM magnetization reversal has been derived for different device dimensions. Such measurement can be used to study the spin-momentum helical locking of the TI surface states as well as for reading the FM magnetization direction in memory and logic devices based on TI-TB-FM heterostructures.

Large spin Hall effect in 5d-transition metal anti-perovskites

Abstract: The spin Hall effect (SHE) is highly promising for spintronic applications, and the design of materials with large SHE can enable ultra-low power memory technology. Recently, 5d-transition metal oxides have been shown to demonstrate a large SHE. Here we report large values of SHE in four 5d-transition metal anti-perovskites which makes these anti-perovskites promising spintronic materials. We demonstrate that these effects originate in the mixing of dx2-y2 and dxy orbitals caused by spin orbit coupling.