Sreetosh Goswami
Email Address
physree@nus.edu.sg
Organizational Units
SCIENCE
faculty
SPECIALTY RESEARCH INST/CTRS
faculty
PHYSICS
dept
9 results
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Now showing 1 - 9 of 9
Publication Charge disproportionate molecular redox for discrete memristive and memcapacitive switching(Nature Research, 2020-03-23) Goswami, Sreetosh; Rath, Santi P.; Thompson, Damien; Hedstrom, Svante; Annamalai, Meenakshi; Pramanick, Rajib; Ilic, B. Robert; Sarkar, Soumya; Hooda, Sonu; Nijhuis, Christian A.; Martin, Jens; Williams, R. Stanley; Goswami, Sreebrata; Venkatesan, T.; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; NUS NANOSCIENCE & NANOTECH INITIATIVEElectronic symmetry breaking by charge disproportionation results in multifaceted changes in the electronic, magnetic and optical properties of a material, triggering ferroelectricity, metal/insulator transition and colossal magnetoresistance. Yet, charge disproportionation lacks technological relevance because it occurs only under specific physical conditions of high or low temperature or high pressure. Here we demonstrate a voltage-triggered charge disproportionation in thin molecular films of a metal–organic complex occurring in ambient conditions. This provides a technologically relevant molecular route for simultaneous realization of a ternary memristor and a binary memcapacitor, scalable down to a device area of 60 nm2. Supported by mathematical modelling, our results establish that multiple memristive states can be functionally non-volatile, yet discrete—a combination perceived as theoretically prohibited. Our device could be used as a binary or ternary memristor, a binary memcapacitor or both concomitantly, and unlike the existing ‘continuous state’ memristors, its discrete states are optimal for high-density, ultra-low-energy digital computing. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.Publication Size-selective Pt siderophores based on redox active azo-aromatic ligands(Royal Society of Chemistry, 2020-08-20) Debabrata Sengupta; Sreetosh Goswami; Rajdeep Banerjee; Matthew J. Guberman-Pfeffer; Abhijeet Patra; Anirban Dutta; Rajib Pramanick; Shobhana Narasimhan; Narayan Pradhan; Victor Batista; T. Venkatesan; Sreebrata Goswami; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEWe demonstrate a strategy inspired by natural siderophores for the dissolution of platinum nanoparticles that could enable their size-selective synthesis, toxicological assessment, and the recycling of this precious metal. From the fabrication of electronics to biomedical diagnosis and therapy, PtNPs find increasing use. Mitigating concerns over potential human toxicity and the need to recover precious metal from industrial debris motivates the study of bio-friendly reagents to replace traditional harsh etchants. Herein, we report a family of redox-active siderophore-viz. ã-acceptor azo aromatic ligands (L) that spontaneously ionize and chelate Pt atoms selectively from nanoparticles of size ó6 nm. The reaction produces a monometallic diradical complex, PtII(L??)2, isolated as a pure crystalline compound. Density functional theory provides fundamental insights on the size dependent PtNP chemical reactivity. The reported findings reveal a generalized platform for designing ã-acceptor ligands to adjust the size threshold for dissolution of Pt or other noble metals NPs. Our approach may, for example, be used for the generation of Pt-based therapeutics or for reclamation of Pt nano debris formed in catalytic converters or electronic fabrication industries.Publication Polaronic Trions at the MoS2SrTiO3 Interface(WILEY, 2019-08-26) Soumya Sarkar; Sreetosh Goswami; Maxim Trushin; Surajit Saha; Majid Panahandeh-Fard; Saurav Prakash; Sherman Jun Rong Tan; Mary Scott; Kian Ping Loh; Shaffique Adam; Sinu Mathew; Thirumalai Venkatesan; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; YALE-NUS COLLEGE; NUS NANOSCIENCE & NANOTECH INITIATIVEThe reduced electrical screening in 2D materials provides an ideal platform for realization of exotic quasiparticles, that are robust and whose functionalities can be exploited for future electronic, optoelectronic, and valleytronic applications. Recent examples include an interlayer exciton, where an electron from one layer binds with a hole from another, and a Holstein polaron, formed by an electron dressed by a sea of phonons. Here, a new quasiparticle is reported, “polaronic trion” in a heterostructure of MoS2/SrTiO3 (STO). This emerges as the Fröhlich bound state of the trion in the atomically thin monolayer of MoS2 and the very unique low energy soft phonon mode (≤7 meV, which is temperature and field tunable) in the quantum paraelectric substrate STO, arising below its structural antiferrodistortive (AFD) phase transition temperature. This dressing of the trion with soft phonons manifests in an anomalous temperature dependence of photoluminescence emission leading to a huge enhancement of the trion binding energy (≈70 meV). The soft phonons in STO are sensitive to electric field, which enables field control of the interfacial trion–phonon coupling and resultant polaronic trion binding energy. Polaronic trions could provide a platform to realize quasiparticle‐based tunable optoelectronic applications driven by many body effects.Publication Nanometer-Scale Uniform Conductance Switching in Molecular Memristors(WILEY, 2020-09-06) Sreetosh Goswami; Debalina Deb; Agnès Tempez; Marc Chaigneau; SANTI PRASAD RATH; Manohar Lal; Ariando; R. Stanley Williams; SREEBRATA GOSWAMI; Thirumalai Venkatesan; DEPT OF PHYSICS; ELECTRICAL AND COMPUTER ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEOne common challenge highlighted in almost every review article on organic resistive memory is the lack of areal switching uniformity. This, in fact, is a puzzle because a molecular switching mechanism should ideally be isotropic and produce homogeneous current switching free from electroforming. Such a demonstration, however, remains elusive to date. The reports attempting to characterize a nanoscopic picture of switching in molecular films show random current spikes, just opposite to the expectation. Here, this longstanding conundrum is resolved by demonstrating 100% spatially homogeneous current switching (driven by molecular redox) in memristors based on Ru-complexes of azo-aromatic ligands. Through a concurrent nanoscopic spatial mapping using conductive atomic force microscopy and in operando tip-enhanced Raman spectroscopy (both with resolutionPublication Robust resistive memory devices using solution-processable metal-coordinated azo aromatics(Nature Research, 2017-10-23) Sreetosh Goswami; Adam J. Matula; Santi P. Rath; Svante Hedström; Surajit Saha; Meenakshi Annamalai; Debabrata Sengupta; Abhijeet Patra; Siddhartha Ghosh; Hariom Jani; Soumya Sarkar; Mallikarjuna Rao Motapothula; Christian A. Nijhuis; Jens Martin; Sreebrata Goswami; Victor S. Batista; T. Venkatesan; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; NUS NANOSCIENCE & NANOTECH INITIATIVENon-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼1012 cycles), stability (>106 s) and scalability (down to ∼60 nm2). In situ Raman and ultraviolet–visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines the switching states of the device whereas the counterions control the hysteresis. This insight may accelerate the technological deployment of organic resistive memories.Publication Direct Growth of Wafer-Scale, Transparent, p‑Type Reduced-Graphene-Oxide-like Thin Films by Pulsed Laser Deposition(American Chemical Society, 2020-02-26) M. M. Juvaid; Soumya Sarkar; Pranjal Kumar Gogoi; Siddhartha Ghosh; Meenakshi Annamalai; Yung-Chang Lin; Saurav Prakash; Sreetosh Goswami; Changjian Li; Sonu Hooda; Hariom Jani; Mark B. H. Breese; Andrivo Rusydi; Stephen John Pennycook; Kazu Suenaga; M. S. Ramachandra Rao; Thirumalai Venkatesan; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEReduced graphene oxide (rGO) has attracted significant interest in an array of applications ranging from flexible optoelectronics, energy storage, sensing, and very recently as membranes for water purification. Many of these applications require a reproducible, scalable process for the growth of large-area films of high optical and electronic quality. In this work, we report a one-step scalable method for the growth of reduced-graphene-oxide-like (rGO-like) thin films via pulsed laser deposition (PLD) of sp2 carbon in an oxidizing environment. By deploying an appropriate laser beam scanning technique, we are able to deposit wafer-scale uniform rGO-like thin films with ultrasmooth surfaces (roughness <1 nm). Further, in situ control of the growth environment during the PLD process allows us to tailor its hybrid sp2–sp3 electronic structure. This enables us to control its intrinsic optoelectronic properties and helps us achieve some of the lowest extinction coefficients and refractive index values (0.358 and 1.715, respectively, at 2.236 eV) as compared to chemically grown rGO films. Additionally, the transparency and conductivity metrics of our PLD grown thin films are superior to other p-type rGO films and conducting oxides. Unlike chemical methods, our growth technique is devoid of catalysts and is carried out at lower process temperatures. This would enable the integration of these thin films with a wide range of material heterostructures via direct growth.Publication An organic approach to low energy memory and brain inspired electronics(AIP Publishing, 2020-04-21) Sreetosh Goswami; Sreebrata Goswami; T. Venkatesan; DEPT OF PHYSICS; ELECTRICAL AND COMPUTER ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEBrain inspired electronics with organic memristors could offer a functionally promising and cost-effective platform for flexible, wearable, and personalized computing technologies. While there are different material approaches (viz. oxides, nitrides, 2D, organic) to realize memristors, organic materials are characteristically unique, as they could, in principle, offer spatially uniform switching, tunable molecular functionalities, and ultra-low switching energies approaching atto joules that are highly desirable but elusive with other material systems. However, despite a long-standing effort spanning almost 2 decades, the performance and mechanistic understanding in organic memristors are quite far from a translational stage and even a single suitable candidate is yet to emerge. Almost all the reported organic memristors lack reproducibility, endurance, stability, uniformity, scalability, and speed that are needed for an industrial application. In this review, we analyze the root cause of the prolonged failures of organic memory devices and discuss a new family of organic memristors, made of transition metal complexes of redox active organic ligands (RAL), that satisfy and go beyond the requirements specified in the 2015 ITRS roadmap for RRAM devices. These devices exhibit cyclability > 1012, retention of several months, on/off ratio > 103 , switching voltage approaching 100 mV, rise time less than 30 ns, and switching energyPublication Direct Bandgap-like Strong Photoluminescence from Twisted Multilayer MoS2 Grown on SrTiO3(American Chemical Society, 2020-12-07) Soumya Sarkar; Sinu Mathew; Sandhya Chintalapati; Ashutosh Rath; Majid Panahandeh-Fard; Surajit Saha; Sreetosh Goswami; Sherman Jun Rong Tan; Kian Ping Loh; Mary Scott; Thirumalai Venkatesan; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEWhile direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an important optoelectronic material due to strong light–matter interactions, their multilayer counterparts exhibit an indirect bandgap resulting in poor photon emission quantum yield. We report strong direct bandgap-like photoluminescence at ∼1.9 eV from multilayer MoS2 grown on SrTiO3, whose intensity is significantly higher than that observed in multilayer MoS2/SiO2. Using high-resolution electron microscopy we observe interlayer twist and >8% increase in the van der Waals gap, which leads to weaker interlayer coupling. This affects the evolution of the band structure in multilayer MoS2 as probed by transient absorption spectroscopy, causing higher photo carrier recombination at the direct gap. Our results provide a platform that could enable multilayer TMDs for robust optical device applications.Publication Evidence of Rotational Fröhlich Coupling in Polaronic Trions(American Physical Society (APS), 2019-11-19) Trushin, Maxim; Sarkar, Soumya; Mathew, Sinu; Goswami, Sreetosh; Sahoo, Prasana; Wang, Yan; Yang, Jieun; Li, Weiwei; MacManus-Driscoll, Judith L; Chhowalla, Manish; Adam, Shaffique; Venkatesan, T; Dr Trushin Maxim; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; YALE-NUS COLLEGE; NUS NANOSCIENCE & NANOTECH INITIATIVEElectrons commonly couple through Fr\"ohlich interactions with longitudinal optical phonons to form polarons. However, trions possess a finite angular momentum and should therefore couple instead to rotational optical phonons. This creates a polaronic trion whose binding energy is determined by the crystallographic orientation of the lattice. Here, we demonstrate theoretically within the Fr\"ohlich approach and experimentally by photoluminescence emission that the bare trion binding energy (20 meV) is significantly enhanced by the phonons at the interface between the two-dimensional semiconductor MoS$_2$ and the bulk transition metal oxide SrTiO$_3$. The low-temperature {binding energy} changes from 60 meV in [001]-oriented substrates to 90 meV for [111] orientation, as a result of the counter-intuitive interplay between the rotational axis of the MoS$_2$ trion and that of the SrTiO$_3$ phonon mode.