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Impact of Disorder Dynamics and Multi-Domain Kinetics on the Sliding Ferroelectricity of CVD-Grown 3R-WSe2 Bilayers
Authors:
Sourav Paul,
Prasenjit Ghosh,
Krishna Prasad Maity,
Vineet Pandey,
Abhijith M. B.,
Premananda Chatterjee,
Kenji Watanabe,
Takashi Taniguchi,
Nicholas R. Glavin,
Ajit K. Roy,
Atindra Nath Pal,
Vidya Kochat
Abstract:
Sliding ferroelectricity in van der Waals (vdW) layered systems has emerged as a promising route toward non-volatile nanoscale devices, where interlayer displacement in non-centrosymmetric bilayers generates an out-of-plane polarization. In particular, 3R-stacked bilayer transition metal dichalcogenides (TMDs) grown via chemical vapor deposition (CVD) have been shown to host such polarization due…
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Sliding ferroelectricity in van der Waals (vdW) layered systems has emerged as a promising route toward non-volatile nanoscale devices, where interlayer displacement in non-centrosymmetric bilayers generates an out-of-plane polarization. In particular, 3R-stacked bilayer transition metal dichalcogenides (TMDs) grown via chemical vapor deposition (CVD) have been shown to host such polarization due to broken inversion symmetry. However, a detailed investigation of the 2D ferroelectric (FE) properties of CVD-grown 2D films, particularly the role of intrinsic disorder, such as structural defects and domain structure, remains poorly understood. Here, we investigate the FE switching characteristics of CVD-grown 3R-stacked WSe2 using a graphene-based ferroelectric field-effect transistor (graphene-FE-FET) architecture, where graphene serves as a highly sensitive probe of induced charge modulation due to polarization switching of FEs. We show that the growth-induced structural disorder significantly impacts polarization switching, while multi-domain kinetics governs the evolution of the FE response. These findings provide important insights into the design and optimization of FE devices based on vdW materials.
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Submitted 30 May, 2026;
originally announced June 2026.
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Interface Fluctuations in a Turbulent Binary Fluid using Data-Driven Methods
Authors:
Samuel Z Khiangte,
Triparna Sanyal,
Sumantra Sarkar,
Nairita Pal
Abstract:
Interfacial fluctuations in a two-phase binary fluid mixture reveal signatures of underlying physical processes that occur within each phase and on a range of spatial and temporal scales. In this study, we investigate a model binary fluid system consisting of a single droplet of one phase moving in the background of the second phase. The binary fluid system is subjected to turbulent forcing. We pe…
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Interfacial fluctuations in a two-phase binary fluid mixture reveal signatures of underlying physical processes that occur within each phase and on a range of spatial and temporal scales. In this study, we investigate a model binary fluid system consisting of a single droplet of one phase moving in the background of the second phase. The binary fluid system is subjected to turbulent forcing. We perform extensive direct numerical simulations of the turbulent system to examine how quantities such as interfacial dynamics and droplet acceleration can be systematically decoded. Extensive simulations of binary fluid systems are computationally expensive and time-consuming. In contrast, data-driven models have shown promise in recent times in reducing computational cost. In this work, we build and compare the performances of four interpretable data-driven models, i.e., dynamic mode decomposition (DMD), Hankel DMD, sparse identification of nonlinear dynamics (SINDy), and Stochastic Langevin regression (SLR), each using dimensionality reduction via proper orthogonal decomposition, to identify simplified dynamical equations governing interfacial dynamics and center-of-mass acceleration. We show how these learned models can be generalized to encode physical properties, such as the interfacial surface tension and droplet size. In particular, we show that SLR predicts the underlying dynamical equations of the binary-fluid system with the greatest accuracy over a wide range of interfacial tension values and droplet sizes. In addition, SLR requires fewer terms compared to SINDy to capture the underlying dynamics, and is thus computationally the most efficient among the four methods. These data-driven techniques can be used in many practical applications, such as the dynamics of biological cell membranes, thin films, and other industrial applications.
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Submitted 2 March, 2026;
originally announced March 2026.
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Shake-down spectroscopy as state- and site-specific probe of ultrafast chemical dynamics
Authors:
Henry J. Thompson,
Matteo Bonanomi,
Jacob Pedersen,
Oksana Plekan,
Nitish Pal,
Cesare Grazioli,
Kevin C. Prince,
Bruno N. C. Tenorio,
Michele Devetta,
Davide Faccialà,
Caterina Vozzi,
Paolo Piseri,
Miltcho B. Danailov,
Alexander Demidovich,
Alexander D. Brynes,
Alberto Simoncig,
Marco Zangrando,
Marcello Coreno,
Raimund Feifel,
Richard J. Squibb,
David M. P. Holland,
Felix Allum,
Daniel Rolles,
Piero Decleva,
Michael S. Schuurman
, et al. (5 additional authors not shown)
Abstract:
Tracking the multifarious ultrafast electronic and structural changes occurring in a molecule during a photochemical transformation is a challenging endeavor that benefits from recent experimental and computational progress in time-resolved techniques. Measurements of valence electronic states, which provide a global picture of the bonding structure of the molecule, and core electronic states, whi…
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Tracking the multifarious ultrafast electronic and structural changes occurring in a molecule during a photochemical transformation is a challenging endeavor that benefits from recent experimental and computational progress in time-resolved techniques. Measurements of valence electronic states, which provide a global picture of the bonding structure of the molecule, and core electronic states, which provide insight into the local environment, traditionally require different approaches and are often studied separately. Here, we demonstrate that X-ray pulses from a seeded free-electron laser (FEL) enable the measurement of high-resolution, time-resolved X-ray photoelectron spectra (XPS) that capture weak satellite states resulting from shake-down processes in a valence-excited molecule. This approach effectively combines the advantages of both valence- and core-state investigations. We applied this method to investigate photoexcited CS$_2$ molecules, where the role of internal conversion (IC) and intersystem crossing (ISC) in determining the pre-dissociation dynamics is controversial. We present XPS spectra from photoexcited CS$_2$, obtained at the FERMI FEL. High-resolution measurements, compared to the corresponding spectra obtained from accurate multireference quantum chemical calculations, reveal that shake-down satellite channels are highly sensitive to both valence electronic and geometric changes. Previous studies of the pre-dissociation dynamics have led to uncertain assignments of the branching between singlet and triplet excited states. We derive a propensity rule that demonstrates the spin-selectivity of the shake-downs. This selectivity allows us to unequivocally assign contributions from the bright and dark singlet excited states, with populations tracked along the pre-dissociation dynamic pathway.
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Submitted 4 August, 2025; v1 submitted 17 June, 2025;
originally announced June 2025.
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H2-roaming dynamics in the formation of H3+ following two-photon double ionization of ethanol and aminoethanol
Authors:
Aaron Ngai,
Sebastian Hartweg,
Jakob D. Asmussen,
Björn Bastian,
Matteo Bonanomi,
Carlo Callegari,
Miltcho Danailov,
Michele di Fraia,
Raimund Feifel,
Sarang Dev Ganeshamandiram,
Sivarama Krishnan,
Aaron LaForge,
Friedemann Landmesser,
Ltaief Ben Ltaief,
Moritz Michelbach,
Nitish Pal,
Oksana Plekan,
Nicolas Rendler,
Lorenzo Raimondi,
Fabian Richter,
Audrey Scognamiglio,
Tobias Sixt,
Richard J. Squibb,
Katrin Dulitz,
Frank Stienkemeier
, et al. (1 additional authors not shown)
Abstract:
Roaming reactions involving a neutral fragment of a molecule that transiently wanders around another fragment before forming a new bond are intriguing and peculiar pathways for molecular rearrangement. Such reactions can occur for example upon double ionization of small organic molecules, and have recently sparked much scientific interest. We have studied the dynamics of the H$_2$-roaming reaction…
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Roaming reactions involving a neutral fragment of a molecule that transiently wanders around another fragment before forming a new bond are intriguing and peculiar pathways for molecular rearrangement. Such reactions can occur for example upon double ionization of small organic molecules, and have recently sparked much scientific interest. We have studied the dynamics of the H$_2$-roaming reaction leading to the formation of H$_3^+$ after two-photon double ionization of ethanol and 2-aminoethanol, using an XUV-UV pump-probe scheme. For ethanol, we find dynamics similar to previous studies employing different pump-probe schemes, indicating the independence of the observed dynamics from the method of ionization and the photon energy of the disruptive probe pulse. Surprisingly, we do not observe a kinetic isotope effect in ethanol-D$_6$, in contrast to previous experiments on methanol where such an effect was observed. This distinction indicates fundamental differences in the energetics of the reaction pathways as compared to the methanol molecule. The larger number of possible roaming pathways compared to methanol complicates the analysis considerably. In contrast to previous studies, we additionally analyze a broad range of dissociative ionization products, which feature distinct dynamics from that of H$_{3}^{+}$ and allow initial insight into the action of the disruptive UV-probe pulse.
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Submitted 31 March, 2025;
originally announced March 2025.
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Unraveling the relaxation dynamics of Uracil: insights from time-resolved X-ray photoelectron spectroscopy
Authors:
Davide Faccialà,
Matteo Bonanomi,
Bruno Nunes Cabral Tenorio,
Lorenzo Avaldi,
Paola Bolognesi,
Carlo Callegari,
Marcello Coreno,
Sonia Coriani,
Piero Decleva,
Michele Devetta,
Nađa Došlić,
Alberto De Fanis,
Michele Di Fraia,
Fabiano Lever,
Tommaso Mazza,
Michael Meyer,
Terry Mullins,
Yevheniy Ovcharenko,
Nitish Pal,
Maria Novella Piancastelli,
Robert Richter,
Daniel E. Rivas,
Marin Sapunar,
Björn Senfftleben,
Sergey Usenko
, et al. (4 additional authors not shown)
Abstract:
We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase, using time-resolved core level photoelectron spectroscopy together with high level calculations. The dynamics was investigated by trajectory surface-hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excite…
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We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase, using time-resolved core level photoelectron spectroscopy together with high level calculations. The dynamics was investigated by trajectory surface-hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics was probed on the oxygen, nitrogen and carbon site by core electron spectroscopy. Assuming a particular model, we find that the initially excited $S_2(ππ^*)$ state of uracil decays with a time constant of 17 $\pm$ 4 fs to the ground state directly, or to the $S_1(nπ^*)$ state via internal conversion. We find no evidence that the $S_1(nπ^*)$ state decays to the ground state by internal conversion; instead it decays to triplet states with a time constant of 1.6 $\pm$ 0.4 ps. Oscillations of the $S_1(nπ^*)$ state O 1s intensity as a function of time correlate with those of calculated C4=O8 and C5=C6 bond lengths, which undergo a sudden expansion following the initial $π\to π^*$ excitation. We also observe oscillations in the mean energy of the main line (core ionized ionic state), which we tentatively assign to dynamics of the hot ground state. Our calculations support our interpretation of the data, and provide detailed insight into the relaxation processes of uracil.
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Submitted 24 March, 2025;
originally announced March 2025.
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Relaxation dynamics in excited helium nanodroplets probed with high resolution, time-resolved photoelectron spectroscopy
Authors:
A. C. LaForge,
J. D. Asmussen,
B. Bastian,
M. Bonanomi,
C. Callegari S. De,
M. Di Fraia,
L. Gorman,
S. Hartweg,
S. R. Krishnan,
M. F. Kling,
D. Mishra,
S. Mandal,
A. Ngai,
N. Pal,
O. Plekan,
K. C. Prince,
P. Rosenberger,
E. Aguirre Serrata,
F. Stienkemeier,
N. Berrah,
M. Mudrich
Abstract:
Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of…
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Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of nanoscopic bubbles or excimers, and autoionization channels generating low-energy free electrons. Here, we discuss the full spectrum of ultrafast relaxation processes observed when helium nanodroplets are electronically excited. In particular, we perform an in-depth study of the relaxation dynamics occurring in the lowest 1s2s and 1s2p droplet bands using high resolution, time-resolved photoelectron spectroscopy. The simplified excitation scheme and improved resolution allow us to identify the relaxation into metastable triplet and excimer states even when exciting below the droplets' autoionization threshold, unobserved in previous studies.
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Submitted 13 March, 2025;
originally announced March 2025.
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Strong-field quantum control in the extreme ultraviolet using pulse shaping
Authors:
Fabian Richter,
Ulf Saalmann,
Enrico Allaria,
Matthias Wollenhaupt,
Benedetto Ardini,
Alexander Brynes,
Carlo Callegari,
Giulio Cerullo,
Miltcho Danailov,
Alexander Demidovich,
Katrin Dulitz,
Raimund Feifel,
Michele Di Fraia,
Sarang Dev Ganeshamandiram,
Luca Giannessi,
Nicolai Gölz,
Sebastian Hartweg,
Bernd von Issendorff,
Tim Laarmann,
Friedemann Landmesser,
Yilin Li,
Michele Manfredda,
Cristian Manzoni,
Moritz Michelbach,
Arne Morlok
, et al. (18 additional authors not shown)
Abstract:
Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with s…
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Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.
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Submitted 17 October, 2024; v1 submitted 4 March, 2024;
originally announced March 2024.
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Observation of interatomic Coulombic decay induced by double excitation of helium in nanodroplets
Authors:
B. Bastian,
J. D. Asmussen,
L. Ben Ltaief,
H. B. Pedersen,
K. Sishodia,
S. De,
S. R. Krishnan,
C. Medina,
N. Pal,
R. Richter,
N. Sisourat,
M. Mudrich
Abstract:
Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound complexes exposed to intense or high-energy radiation. So far, neutral or ionic atoms or molecules have been prepared in singly excited electron or hole states which can transfer energy to neighboring centers and cause ionization and radiation damage. Here we demonstrate that a doubly excited atom, despite its extremely short l…
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Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound complexes exposed to intense or high-energy radiation. So far, neutral or ionic atoms or molecules have been prepared in singly excited electron or hole states which can transfer energy to neighboring centers and cause ionization and radiation damage. Here we demonstrate that a doubly excited atom, despite its extremely short lifetime, can decay by ICD; evidenced by high-resolution photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find that ICD proceeds by relaxation into excited He$^*$He$^+$ atom-pair states, in agreement with calculations. The ability of inducing ICD by resonant excitation far above the single-ionization threshold opens opportunities for controlling radiation damage to a high degree of element specificity and spectral selectivity.
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Submitted 24 October, 2023;
originally announced October 2023.
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Spectroscopically resolved resonant interatomic Coulombic decay in photoexcited large He nanodroplets
Authors:
L. Ben Ltaief,
K. Sishodia,
R. Richter,
B. Bastian,
J. D. Asmussen,
S. Mandal,
N. Pal,
C. Medina,
S. R. Krishnan,
K. von Haeften,
M. Mudrich
Abstract:
Interatomic Coulombic decay (ICD) processes play a crucial role in weakly bound complexes exposed to intense or high-energy radiation. Using large helium nanodroplets, we demonstrate that ICD is efficient even when the droplets are irradiated by weak synchrotron radiation at relatively low photon energies. Below the ionization threshold, resonant excitation of multiple centers efficiently induces…
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Interatomic Coulombic decay (ICD) processes play a crucial role in weakly bound complexes exposed to intense or high-energy radiation. Using large helium nanodroplets, we demonstrate that ICD is efficient even when the droplets are irradiated by weak synchrotron radiation at relatively low photon energies. Below the ionization threshold, resonant excitation of multiple centers efficiently induces resonant ICD as previously observed for intense pulses [A. C. LaForge et al., PRX 11, 021011 (2021)]. More surprisingly, we observe ICD even above the ionization threshold due to recombination of photoelectrons and ions into excited states which subsequently decay by ICD. This demonstrates the importance of secondary processes, in particular electron scattering and recombination, in inducing ICD in extended condensed phase systems. In addition, we show that ICD can serve as a diagnostic tool for monitoring the relaxation dynamics of highly-excited and ionized weakly-bound nanosystems.
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Submitted 28 August, 2023;
originally announced August 2023.
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Electron energy loss and angular asymmetry induced by elastic scattering in helium droplets
Authors:
Jakob D. Asmussen,
Keshav Sishodia,
Björn Bastian,
Abdul R. Abid,
Ltaief Ben Ltaief,
Henrik B. Pedersen,
Subhendu De,
Christian Medina,
Nitish Pal,
Robert Richter,
Thomas Fennel,
Sivarama Krishnan,
Marcel Mudrich
Abstract:
Helium nanodroplets are ideal model systems to unravel the complex interaction of condensed matter with ionizing radiation. Here we study the effect of purely elastic electron scattering on angular and energy distributions of photoelectrons emitted from He nanodroplets of variable size ($10$-$10^9$ atoms per droplets). For large droplets, photoelectrons develop a pronounced anisotropy along the in…
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Helium nanodroplets are ideal model systems to unravel the complex interaction of condensed matter with ionizing radiation. Here we study the effect of purely elastic electron scattering on angular and energy distributions of photoelectrons emitted from He nanodroplets of variable size ($10$-$10^9$ atoms per droplets). For large droplets, photoelectrons develop a pronounced anisotropy along the incident light beam due to a shadowing effect within the droplets. In contrast, the detected photoelectron spectra are only weakly perturbed. This opens up possibilities for photoelectron spectroscopy of dopants embedded in droplets provided they are smaller than the penetration depth of the light and the trapping range of emitted electrons.
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Submitted 9 May, 2023;
originally announced May 2023.
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Efficient Indirect Interatomic Coulombic Decay Induced by Photoelectron Impact Excitation in Large He Nanodroplets
Authors:
L. Ben Ltaief,
K. Sishodia,
S. Mandal,
S. De,
S. R. Krishnan,
C. Medina,
N. Pal,
R. Richter,
T. Fennel,
M. Mudrich
Abstract:
Ionization of matter by energetic radiation generally causes complex secondary reactions which are hard to decipher. Using large helium nanodroplets irradiated by XUV photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently…
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Ionization of matter by energetic radiation generally causes complex secondary reactions which are hard to decipher. Using large helium nanodroplets irradiated by XUV photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently induces interatomic Coulombic decay (ICD) in the droplets. This type of indirect ICD even becomes the dominant process of electron emission in nearly the entire XUV range in large droplets with radius $\gtrsim40~$nm. Indirect ICD processes induced by electron scattering likely play an important role in other condensed phase systems exposed to ionizing radiation as well, including biological matter.
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Submitted 26 March, 2023;
originally announced March 2023.
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High performance Broadband Photodetection Based on Graphene -- MoS$_{2x}$Se$_{2(1-x)}$ Alloy Engineered Phototransistors
Authors:
Shubhrasish Mukherjee,
Didhiti Bhattacharya,
Samit Kumar Ray,
Atindra Nath Pal
Abstract:
The concept of alloy engineering has emerged as a viable technique towards tuning the bandgap as well as engineering the defect levels in two-dimensional transition metal dichalcognides (TMDC). Possibility to synthesize these ultrathin TMDC materials through chemical route has opened realistic possibilities to fabricate hybrid multi-functional devices. By synthesizing nanosheets with different com…
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The concept of alloy engineering has emerged as a viable technique towards tuning the bandgap as well as engineering the defect levels in two-dimensional transition metal dichalcognides (TMDC). Possibility to synthesize these ultrathin TMDC materials through chemical route has opened realistic possibilities to fabricate hybrid multi-functional devices. By synthesizing nanosheets with different composites of MoS$_{2x}$Se$_{2(1-x)}$ (x = 0 to 1) using simple chemical methods, we systematically investigate the photo response properties of three terminal hybrid devices by decorating large area graphene with these nanosheets (x = 0, 0.5, 1) in 2D-2D configurations. Among them, graphene-MoSSe hybrid phototransistor exhibits superior optoelectronic properties than its binary counterparts. The device exhibits extremely high photoresponsivity (>10$^4$ A/W), low noise equivalent power (~10$^{-14}$ W/Hz$^{0.5}$), higher specific detectivity (~ 10$^{11}$ Jones) in the wide UV-NIR (365-810 nm) range with excellent gate tunability. The broadband light absorption of MoSSe, ultrafast charge transport in graphene, along with controllable defect engineering in MoSSe makes this device extremely attractive. Our work demonstrates the large area scalability with wafer-scale production of MoS$_{2x}$Se$_{2(1-x)}$ alloys, having important implication towards facile and scalable fabrication of high-performance optoelectronic devices and providing important insights into the fundamental interactions between van-der-Waals materials.
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Submitted 30 June, 2022;
originally announced June 2022.
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High Responsivity Gate Tunable UV-Visible Broadband Phototransistor Based on Graphene-WS2 Mixed Dimensional (2D-0D) Heterostructure
Authors:
Shubhrasish Mukherjee,
Didhiti Bhattacharya,
Sumanti Patra,
Sanjukta Paul,
Rajib Kumar Mitra,
Priya Mahadevan,
Atindra Nath Pal,
Samit Kumar Ray
Abstract:
Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition metal-dichalcogenides (TMDC) quantum dots (QDs) with their broadband absorption spectrum and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution based processing, we demonstrate a super large (~ 0.75 mm^2), UV-Vis broadband (365-633 nm), phototransi…
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Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition metal-dichalcogenides (TMDC) quantum dots (QDs) with their broadband absorption spectrum and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution based processing, we demonstrate a super large (~ 0.75 mm^2), UV-Vis broadband (365-633 nm), phototransistor made of WS_2 QDs decorated CVD graphene as active channel with extraordinary stability and durability in ambient condition (without any degradation of photocurrent till 4 months after fabrication). Here, colloidal 0D WS_2-QDs are used as the photo absorbing material and graphene acts as the conducting channel. A high photoresponsivity (3.1 x 10^2 A/W), higher detectivity (2.2 x 10^12 Jones) and low noise equivalent power (4 x 10^{-14} W/Hz^0.5) are obtained at a low bias voltage (V_{ds} = 1V) at an illumination of 365 nm with an optical power as low as 0.8 μW/cm^2, which can further be tuned by modulating the gate bias. While comparing the photocurrent between two different morphologies of WS_2 (QDs and 2D nanosheets), a significant enhancement of photocurrent is observed in case of QDs based device. Ab initio density functional theory based calculations further support our observation, revealing the role of quantum confinement for the enhanced photo response. Our work reveals a strategy towards making a scalable, cost-effective, highly performing hybrid two-dimensional (2D/0D) photo detector with graphene-WS_2 QDs, paving the way towards the next generation optoelectronic applications.
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Submitted 9 November, 2021;
originally announced November 2021.
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Ephemeral Antibubbles: Spatiotemporal Evolution from Direct Numerical Simulations
Authors:
Nairita Pal,
Rashmi Ramadugu,
Prasad Perlekar,
Rahul Pandit
Abstract:
Antibubbles, which consist of a shell of a low-density fluid inside a high-density fluid, have several promising applications. We show, via extensive direct numerical simulations (DNSs), in both two and three dimensions (2D and 3D), that the spatiotemporal evolution of antibubbles can be described naturally by the coupled Cahn-Hilliard-Navier-Stokes (CHNS) equations for a binary fluid. Our DNSs ca…
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Antibubbles, which consist of a shell of a low-density fluid inside a high-density fluid, have several promising applications. We show, via extensive direct numerical simulations (DNSs), in both two and three dimensions (2D and 3D), that the spatiotemporal evolution of antibubbles can be described naturally by the coupled Cahn-Hilliard-Navier-Stokes (CHNS) equations for a binary fluid. Our DNSs capture elegantly the gravity-induced thinning and breakup of an antibubble via the time evolution of the Cahn-Hilliard scalar order parameter field $φ$, which varies continuously across interfaces, so we do not have to enforce complicated boundary conditions at the moving antibubble interfaces. To ensure that our results are robust, we supplement our CHNS simulations with sharp-interface Volume-of-Fluid (VoF) DNSs. We track the thickness of the antibubble and calculate the dependence of the lifetime of an antibubble on several parameters; we show that our DNS results agree with various experimental results; in particular, the velocity with which the arms of the antibubble retract after breakup scales as $σ^{1/2}$, where $σ$ is the surface tension.
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Submitted 30 March, 2021;
originally announced March 2021.
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Local Wave Number Model for Inhomogeneous Two-Fluid Mixing
Authors:
Nairita Pal,
Ismael Boureima,
Noah Braun,
Susan Kurien,
Praveen Ramaprabhu,
Andrew Lawrie
Abstract:
We present a study of a two-point spectral turbulence model (Local Wave-Number model or LWN model) for the Rayleigh-Taylor (RT) instability. The model outcomes are compared with statistical quantities extracted from three-dimensional simulation of the RT problem. These simulations are initialized with high wavenumber perturbations at the interface of a heavy fluid placed on top of a light fluid so…
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We present a study of a two-point spectral turbulence model (Local Wave-Number model or LWN model) for the Rayleigh-Taylor (RT) instability. The model outcomes are compared with statistical quantities extracted from three-dimensional simulation of the RT problem. These simulations are initialized with high wavenumber perturbations at the interface of a heavy fluid placed on top of a light fluid so that the density gradient is in the direction opposite to acceleration due to gravity. We consider flows of low to medium density contrast and compare the LWN model against simulation data using the mix-width evolution as the primary metric. The original model specified physically reasonable but largely \emph{ad hoc} terms to account for the inhomogeneous mechanisms involved in growing the mixing layer. We systematically assess the role of each of the terms in the LWN model equations by comparison with simulation. Two of these, the kinematic source term, introduced to maintain a finite covariance between density and specific volume, and a spectral distortion term, introduced as spectral modifications of the density-specific-volume covariance, both result in severely over-predicting the mix layer growth. A simplified model eliminating those two terms is shown to improve the capture of both mix-width evolution as well as the turbulent mass flux velocity profiles across the mix layer at different times. However, this simplification reveals that fidelity to other metrics such as the density-specific-volume covariance, and the turbulent kinetic energy are somewhat compromised. The implications of this outcome are discussed with respect to the physics of the RT problem, and we provide this study as a guide for the practical use of such a model.
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Submitted 16 August, 2021; v1 submitted 18 March, 2020;
originally announced March 2020.
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Dielectric/semiconductor interfacial doping to develop solution processed high performance 1 V ambipolar oxide-transistor and its application as CMOS inverter
Authors:
Nitesh K. Chourasia,
Anand Sharma,
Nila Pal,
Sajal Biring,
Bhola N. Pal
Abstract:
p-type doping from the dielectric/semiconductor interface of a SnO2 thin film transistor (TFT) has been utilized to develop high carrier mobility balanced ambipolar oxide-transistor. To introduce this interfacial-doping, bottom-gate top-contact TFTs have been fabricated by using two different ion-conducting oxide dielectrics which contain trivalent atoms. These ion-conducting dielectrics are LilnO…
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p-type doping from the dielectric/semiconductor interface of a SnO2 thin film transistor (TFT) has been utilized to develop high carrier mobility balanced ambipolar oxide-transistor. To introduce this interfacial-doping, bottom-gate top-contact TFTs have been fabricated by using two different ion-conducting oxide dielectrics which contain trivalent atoms. These ion-conducting dielectrics are LilnO2 and LiGaO2 respectively, containing mobile Li+ ion. During SnO2 thin film fabrication on top of the ionic dielectric, those trivalent atoms allow p- doping to the interfacial SnO2 layer to introduce the hole conduction in channel of TFT. To realize this interfacial doping phenomena, a reference TFT has been fabricated with Li2ZnO2 dielectric under the same condition that contains divalent zinc (Zn) atom. Our comparative electrical data indicates that TFTs with LilnO2 and LiGaO2 dielectric are ambipolar in nature whereas, TFT with Li2ZnO2 dielectric is a unipolar n-channel transistor which reveals the interfacial doping of SnO2. Most interestingly, by using LilnO2 dielectric, we are capable to fabricated 1.0 V balanced ambipolar TFT with a high electron and hole mobility values of 7 cm2 V-1 s-1and 8 cm2 V-1 s-1 respectively with an on/off ratio >102 for both operations which has been utilized for low-voltage CMOS inverter fabrication.
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Submitted 4 January, 2020;
originally announced January 2020.
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Preferential Sampling of Elastic Chains in Turbulent Flows
Authors:
Jason R. Picardo,
Dario Vincenzi,
Nairita Pal,
Samriddhi Sankar Ray
Abstract:
A string of tracers, interacting elastically, in a turbulent flow is shown to have a dramatically different behaviour when compared to the non-interacting case. In particular, such an elastic chain shows strong preferential sampling of the turbulent flow unlike the usual tracer limit: an elastic chain is trapped in the vortical regions and not the straining ones. The degree of preferential samplin…
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A string of tracers, interacting elastically, in a turbulent flow is shown to have a dramatically different behaviour when compared to the non-interacting case. In particular, such an elastic chain shows strong preferential sampling of the turbulent flow unlike the usual tracer limit: an elastic chain is trapped in the vortical regions and not the straining ones. The degree of preferential sampling and its dependence on the elasticity of the chain is quantified via the Okubo-Weiss parameter. The effect of modifying the deformability of the chain, via the number of links that form it, is also examined.
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Submitted 10 January, 2019; v1 submitted 2 September, 2018;
originally announced September 2018.
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Two-point spectral model for variable-density homogeneous turbulence
Authors:
Nairita Pal,
Susan Kurien,
Timothy Clark,
Denis Aslangil,
Daniel Livescu
Abstract:
We present a study of buoyancy-driven variable-density homogeneous turbulence, using a two-point spectral closure model. We compute the time-evolution of the spectral distribution in wavenumber $k$ of the correlation of density and specific-volume $b(k)$, the mass flux $\bm{a}(k)$, and the turbulent kinetic energy $E(k)$, using a set of coupled equations. Under the modeling assumptions, each dynam…
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We present a study of buoyancy-driven variable-density homogeneous turbulence, using a two-point spectral closure model. We compute the time-evolution of the spectral distribution in wavenumber $k$ of the correlation of density and specific-volume $b(k)$, the mass flux $\bm{a}(k)$, and the turbulent kinetic energy $E(k)$, using a set of coupled equations. Under the modeling assumptions, each dynamical variable has two coefficients governing spectral transfer among modes. In addition, the mass flux $\bm{a}(k)$ has two coefficients governing the drag between the two fluids. Using a prescribed initial condition for $b(k)$ and starting from a quiescent flow, we first evaluate the relative importance of the different coefficients used to model this system, and their impact on the statistical quantities. We next assess the accuracy of the model, relative to Direct Numerical simulation of the complete hydrodynamical equations, using $b$, ${\bm a}$ and $E$ as metrics. We show that the model is able to capture the spectral distribution and global means of all three statistical quantities at both low and high Atwood number for a set of optimized coefficients. The optimization procedure also permits us to discern a minimal set of four coefficients which are sufficient to yield reasonable results while pointing to the mechanisms that dominate the mixing process in this problem.
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Submitted 11 July, 2018;
originally announced July 2018.
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A regularity criterion for solutions of the three-dimensional Cahn-Hilliard-Navier-Stokes equations and associated computations
Authors:
John D. Gibbon,
Nairita Pal,
Anupam Gupta,
Rahul Pandit
Abstract:
We consider the 3D Cahn-Hilliard equations coupled to, and driven by, the forced, incompressible 3D Navier-Stokes equations. The combination, known as the Cahn-Hilliard-Navier-Stokes (CHNS) equations, is used in statistical mechanics to model the motion of a binary fluid. The potential development of singularities (blow-up) in the contours of the order parameter $φ$ is an open problem. To address…
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We consider the 3D Cahn-Hilliard equations coupled to, and driven by, the forced, incompressible 3D Navier-Stokes equations. The combination, known as the Cahn-Hilliard-Navier-Stokes (CHNS) equations, is used in statistical mechanics to model the motion of a binary fluid. The potential development of singularities (blow-up) in the contours of the order parameter $φ$ is an open problem. To address this we have proved a theorem that closely mimics the Beale-Kato-Majda theorem for the $3D$ incompressible Euler equations [Beale et al. Commun. Math. Phys., Commun. Math. Phys., ${\rm 94}$, $ 61-66 ({\rm 1984})$]. By taking an $L^{\infty}$ norm of the energy of the full binary system, designated as $E_{\infty}$, we have shown that $\int_{0}^{t}E_{\infty}(τ)\,dτ$ governs the regularity of solutions of the full 3D system. Our direct numerical simulations (DNSs), of the 3D CHNS equations, for (a) a gravity-driven Rayleigh Taylor instability and (b) a constant-energy-injection forcing, with $128^3$ to $512^3$ collocation points and over the duration of our DNSs, confirm that $E_{\infty}$ remains bounded as far as our computations allow.
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Submitted 17 December, 2016; v1 submitted 18 August, 2016;
originally announced August 2016.
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Binary-Fluid Turbulence: Signatures of Multifractal Droplet Dynamics and Dissipation Reduction
Authors:
Nairita Pal,
Prasad Perlekar,
Anupam Gupta,
Rahul Pandit
Abstract:
We present an extensive direct numerical simulation of statistically steady, homogeneous, isotropic turbulence in two-dimensional, binary-fluid mixtures with air-drag-induced friction by using the Cahn-Hilliard-Navier-Stokes equations. We choose parameters, e.g., the surface tension, such that we have a droplet of the minority phase moving inside a turbulent background of the majority phase. We ch…
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We present an extensive direct numerical simulation of statistically steady, homogeneous, isotropic turbulence in two-dimensional, binary-fluid mixtures with air-drag-induced friction by using the Cahn-Hilliard-Navier-Stokes equations. We choose parameters, e.g., the surface tension, such that we have a droplet of the minority phase moving inside a turbulent background of the majority phase. We characterize the deformation of the droplet and show that it displays multifractal dynamics. The probability distribution functions of the components of the acceleration of the center of mass of the droplet exhibit wide, non-Gaussian tails. Our study reveals that the droplet enhances the energy spectrum $E(k)$ when the wavenumber $k$ is large; this enhancement leads to dissipation reduction.
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Submitted 14 November, 2016; v1 submitted 31 December, 2015;
originally announced December 2015.
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Two-dimensional Turbulence in Symmetric Binary-Fluid Mixtures: Coarsening Arrest by the Inverse Cascade
Authors:
Prasad Perlekar,
Nairita Pal,
Rahul Pandit
Abstract:
We study two-dimensional (2D) binary-fluid turbulence by carrying out an extensive direct numerical simulation (DNS) of the forced, statistically steady turbulence in the coupled Cahn-Hilliard and Navier-Stokes equations. In the absence of any coupling, we choose parameters that lead (a) to spinodal decomposition and domain growth, which is characterized by the spatiotemporal evolution of the Cahn…
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We study two-dimensional (2D) binary-fluid turbulence by carrying out an extensive direct numerical simulation (DNS) of the forced, statistically steady turbulence in the coupled Cahn-Hilliard and Navier-Stokes equations. In the absence of any coupling, we choose parameters that lead (a) to spinodal decomposition and domain growth, which is characterized by the spatiotemporal evolution of the Cahn-Hilliard order parameter $φ$, and (b) the formation of an inverse-energy-cascade regime in the energy spectrum $E(k)$, in which energy cascades towards wave numbers $k$ that are smaller than the energy-injection scale $k_{inj}$ in the turbulent fluid. We show that the Cahn-Hilliard-Navier-Stokes coupling leads to an arrest of phase separation at a length scale $L_c$, which we evaluate from $S(k)$, the spectrum of the fluctuations of $φ$. We demonstrate that (a) $L_c \sim L_H$, the Hinze scale that follows from balancing inertial and interfacial-tension forces, and (b) $L_c$ is independent, within error bars, of the diffusivity $D$. We elucidate how this coupling modifies $E(k)$ by blocking the inverse energy cascade at a wavenumber $k_c$, which we show is $\simeq 2π/L_c$. We compare our work with earlier studies of this problem.
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Submitted 29 June, 2015;
originally announced June 2015.