Pre-Prints
[1]
Phonon-Mediated Chirality Transfer from Organic Cation to Inorganic lattice in Hybrid Perovskites
Sankaran Ramesh, Prasenjit Mandal, Rafael Araujo, Jakob Thyr, Pratik Bhagwat, Yong Li, Tomas Edvinsson, Tönu Pullerits, Dmitry Baranov
arXiv Preprint arXiv:2507.10036 (2025)
Peer-Reviewed Publications
[16]
Dimensionality-dependent electronic and vibrational dynamics in low-dimensional organic-inorganic tin halides.
Yanmei He, Xinyi Cai, Rafael B. Araujo, Yibo Wang, Sankaran Ramesh, Junsheng Chen, Muyi Zhang, Tomas Edvinsson, Feng Gao, Tönu Pullerits
Nature Communications 17, 758 (2026)
Reducing dimensionality from 2D to 1D in organic-inorganic tin halides enhances exciton-phonon coupling by over tenfold, switching emission from free excitons to self-trapped states.
[15]
Coherent Phonons, Localization and Slow Polaron Formation in Lead-free Gold Perovskite
Sankaran Ramesh, Yonghong Wang, Pavel Chabera, Rafael Araujo, Mustafa Aboulsaad, Tomas Edvinsson, Feng Gao, Tönu Pullerits
Advanced Optical Materials, 13, 10, 2402882 (2025)
In caesium gold bromide, band-edge charge-transfer states are found to be strongly coupled to Au–Br stretching phonon modes whose coherent oscillations modulate the absorption frequency; polaron formation proceeds over ~20 ps via lattice relaxation along these modes
[14]
Enhancing Two-Dimensional Electronic Spectroscopy for Layered Halide Perovskites
Sankaran Ramesh, Minjun Feng, Tomoki Furuhashi and Tze Chien Sum
ACS Photonics, 10, 12, p. 4456 - 4464 (2023)
A hollow-core fibre compressor generating 6 fs broadband pulses spanning 500–950 nm overcomes the spectral coverage limitation of conventional 2D Electronic Spectroscopy setups, enabling simultaneous probing of all interacting excitonic states in layered perovskites
[13]
Unveiling the Impact of Organic Spacer Cations on Auger Recombination in Layered Halide Perovskites
Tomoki Furuhashi, Anil Kanwat, Sankaran Ramesh, Nripan Mathews, Tze Chien Sum
Advanced Optical Materials, 12, 8, 2301230 (2023)
Systematic study of phenyl-alkyl-ammonium spacer cations in 2D halide perovskites reveals that strong exciton-phonon coupling is the dominant factor suppressing Auger recombination rates, providing clear structure-function design rules for blue LEDs.
[12]
Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites
Jianhui Fu, Sankaran Ramesh, Jia Wei Melvin Lim, Tze Chien Sum
Chemical Reviews, 123, 13, p. 8154-8231 (2023)
A comprehensive review that constructs a unified photophysical picture of halide perovskites by examining the complex interplay of charge carriers, polarons, phonons & collective quasi-particle excitations across multiple temporal orders that underpin their exceptional light-harvesting and light-emitting performance.
[11]
Spacer cation engineering in Ruddlesden-Popper perovskites for efficient red light-emitting diodes with recommendation 2020 color coordinates
Jian Qing, Sankaran Ramesh, Xiao-Ke Liu, Heyong Wang, Hongling Yu, Chaoyang Kuang, Lintao Hou, Wenjing Zhang, Tze Chien Sum, Feng Gao
Applied Surface Science, 616, 156454 (2023)
Replacing conventional spacer cations with POEA in Ruddlesden-Popper perovskites produces narrow phase distributions, efficient energy funnelling, and low trap densities, yielding red perovskite LEDs with a peak EQE of 10.3% and display-standard Rec. 2020 colour coordinates.
[10]
Defect Passivation Using a Phosphonic Acid Surface Modifier for Efficient RP Perovskite Blue-Light-Emitting Diodes
Jayanta Kumar Mishra, Natalia Yantara, Anil Kanwat, Tomoki Furuhashi, Sankaran Ramesh, Teddy Salim, Nur Fadilah Jamaludin, Benny Febriansyah,
Zi En Ooi, Subodh Mhaisalkar, Tze Chien Sum, Kedar Hippalgaonkar, Nripan Mathews
ACS Applied Materials Interfaces, 14, 30, pp. 34238 (2022)
Octyl phosphonic acid simultaneously acts as a crystallisation modifier and defect passivator in Ruddlesden-Popper perovskite films by coordinating uncoordinated Pb²⁺ sites, controlling the n-domain distribution and suppressing nonradiative recombination to improve blue perovskite LED efficiency.
[9]
Tailoring the Energy Manifold of Quasi-Two-Dimensional Perovskites for Efficient Carrier Extraction
Sankaran Ramesh, David Giovanni, Marcello Righetto, Senyun Ye, Elisa Fresch, Yue Wang, Elisabetta Collini, Nripan Mathews, Tze Chien Sum
Advanced Energy Materials, 12, 10, 2103556 (2022)
Engineering the quantum-well energy cascade in quasi-2D Ruddlesden-Popper perovskites through organic spacer tuning slows excess carrier energy loss by up to two orders of magnitude compared to 3D perovskites, enabling efficient hot-carrier extraction beyond the Shockley-Queisser limit.
[8]
Spacer Cation Alloying in Ruddlesden–Popper Perovskites for Efficient Red Light-Emitting Diodes with Precisely Tunable Wavelengths
Jian Qing, Sankaran Ramesh, Qiang Xu, Xiao‐Ke Liu, Heyong Wang, Zhongcheng Yuan, Zhan Chen, Lintao Hou, Tze Chien Sum, Feng Gao
Advanced Materials, 33, 49, 2104381 (2021)
Alloying two organic spacer cations in iodide-based Ruddlesden-Popper perovskites allows the quantum-well thickness distribution to be precisely controlled without degrading charge transport or energy funnelling, producing red-to-deep-red perovskite LEDs with continuously tunable emission wavelengths and improved external quantum efficiencies
[7]
Electronic States Modulation by Coherent Optical Phonons in 2D Halide Perovskites
Jianhui Fu, Mingjie Li, Ankur Solanki, Qiang Xu, Yulia Lekina, Sankaran Ramesh, Ze Xiang Shen, Tze Chien Sum
Advanced Materials, 33, 11, 2006233 (2021)
Transient absorption spectroscopy reveals that the exciton fine structure in 2D halide perovskites is coherently modulated by optical phonons arising from PbI₆ octahedral vibrations and the phonon frequency and amplitude strongly dependent on the identity of the organic spacer cation — demonstrating that the organic layer actively tunes the inorganic quantum-well electronic states through lattice dynamics.
[6]
The Physics of Interlayer Exciton Delocalization in Ruddlesden–Popper Lead Halide Perovskites
David Giovanni*, Sankaran Ramesh*, Marcello Righetto, Jia Wei Melvin Lim, Qiannan Zhang, Yue Wang, Senyun Ye, Qiang Xu, Nripan Mathews, Tze Chien Sum
Nano Letters, 21, 1, 405-413 (2021) [* indicates equal contribution]
Ultrafast spectroscopy combined with a superlattice model demonstrates that excitons in Ruddlesden-Popper perovskites are not confined to individual quantum wells but delocalize across adjacent inorganic layers via interlayer coupling, with this delocalization identified as the physical origin of the ultrafast energy-funnelling cascade from low-n to high-n phases that underpins the exceptional optoelectronic properties of these materials.
[5]
Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping
David Giovanni, Marcello Righetto, Qiannan Zhang, Jia Wei Melvin Lim, Sankaran Ramesh, Tze Chien Sum
Light: Science & Applications, 10, 2 (2020)
Direct imaging of exciton transport in MAPbBr₃ nanocrystal films reveals diffusion lengths exceeding 1 μm and exciton mobilities several times higher than in bulk 3D perovskites, with ultralong transport arising primarily from inter-nanocrystal Förster hopping and a smaller but significant photon-recycling contribution — the dominant mechanism tunable by organic ligand choice.
[4]
Hot carriers perspective on the nature of traps in perovskites
Marcello Righetto, Swee Sien Lim, David Giovanni, Jia Wei Melvin Lim, Qiannan Zhang, Sankaran Ramesh, Yong Kang Eugene Tay, Tze Chien Sum
Nature Communications, 11, 2712 (2020)
Contrary to the widely accepted defect-tolerance of lead halide perovskites, optical spectroscopy and Marcus-theory modelling reveal that higher-energy hot carriers trap at shallow defects and ligand sites at dramatically faster rates — overturning the benign-defect consensus and showing that phosphine oxide passivation can suppress this hot-carrier trapping pathway.
[3]
Excitons in 2D perovskites for ultrafast terahertz photonic devices
Abhishek Kumar, Ankur Solanki, Manukumara Manjappa, Sankaran Ramesh, Yogesh Kumar Srivastava, Piyush Agarwal, Tze Chien Sum, Ranjan Singh
Science Advances, 6, 8, eaax8821 (2020)
The self-assembled quantum-well structure of 2D Ruddlesden-Popper perovskites confines free carriers into excitons within 20 ps; integrating these films with metamaterial resonators on rigid & flexible substrates produces THz field modulation at 50 GHz, the fastest switching speed demonstrated for any solution-processed semiconductor photonic device.
[2]
Inducing formation of a corrugated, white-light emitting 2D lead-bromide perovskite via subtle changes in templating cation
Benny Febriansyah, David Giovanni, Sankaran Ramesh, Teck Ming Koh, Yongxin Li, Tze Chien Sum, Nripan Mathews, Jason England
Journal of Materials Chemistry C, 8, 3, 889-893 (2020)
Subtle changes in the geometry of the organic templating cation in a 2D lead-bromide perovskite are shown to induce corrugation of the inorganic framework, switching the emission character from narrow excitonic emission to broadband white-light emission through structural distortion-induced self-trapping.
[1]
Thermodynamic stability of LuRhO₃ in a photoelectrochemical cell
KT Jacob, Sankaran Ramesh
Journal of Alloys and Compounds, 695, 1891-1899 (2017)
The thermodynamic properties & long-term electrochemical stability of lutetium rhodite (LuRhO₃) as a photoelectrode for water splitting are determined between 875 and 1300 K using a solid-state electrochemical cell with calcia-stabilised zirconia as the electrolyte, establishing its standard Gibbs energy of formation from its constituent binary oxides.