Kian Ping Loh

@article{ISI:001629007700001,
title = {Structure-Dependent Electronic Relaxation Dynamics of Two-Dimensional Silver Monolayers},
author = {Matthew W -J Liu and Kanchan Ajit Ulman and Boyang Zheng and Arpit Jain and Daniel J Heintzelman and Ke Wang and Wen He and Chengye Dong and Li-Syuan Lu and Vincent H Crespi and Su Ying Quek and Joshua A Robinson and Kenneth Knappenberger L Jr},
doi = {10.1021/acs.nanolett.5c04723},
times_cited = {0},
issn = {1530-6984},
year = {2025},
date = {2025-12-01},
journal = {NANO LETTERS},
volume = {25},
number = {49},
pages = {17145-17151},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag-(1) and Ag-(2), differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant tau(1) is <400 fs for both phases. In contrast, carrier-phonon scattering exhibited lattice-specific and excitation wavelength-independent relaxation time constants; tau(2) similar to 2 ps for Ag-(1) and similar to 1 ps for Ag-(2). The shorter tau(2) in Ag-(2) is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000844406700001,
title = {Interface-Mediated Resonant Raman Enhancement for Shear Modes in a 2D Polar Metal},
author = {Wen He and Maxwell T Wetherington and Kanchan Ajit Ulman and Joshua A Robinson and Su Ying Quek},
doi = {10.1021/acs.jpcc.2c04433},
times_cited = {3},
issn = {1932-7447},
year = {2022},
date = {2022-08-18},
journal = {JOURNAL OF PHYSICAL CHEMISTRY C},
volume = {126},
number = {34},
pages = {14581-14589},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {2D polar metals synthesized by confinement heteroepitaxy at the SiC/graphene interface are covalently bound to the SiC substrate. In this work, we elucidate the importance of the SiC substrate, and specifically the Ga/Si interface, on the low frequency resonant Raman spectra of 2D Ga on SiC. The low-frequency Raman modes are dominated by in-plane shear modes in 2D Ga. We show that the frequency of these shear modes is modified by the presence of the substrate for few-layer Ga and that these shear modes couple strongly to the electronic states corresponding to the interface Ga and Si atoms. Consequently, resonant Raman enhancement occurs at laser incident energies that are resonant with the interband optical transitions involving these interface Ga and Si states. This resonant Raman enhancement is observed in laser-energy dependent measurements, an experimental signature of the strong electron-phonon coupling present in these 2D polar metals.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000803568300007,
title = {Shear Modes in a 2D Polar Metal},
author = {Wen He and Maxwell T Wetherington and Kanchan Ajit Ulman and Jennifer L Gray and Joshua A Robinson and Su Ying Quek},
doi = {10.1021/acs.jpclett.2c00719},
times_cited = {4},
issn = {1948-7185},
year = {2022},
date = {2022-05-12},
journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS},
volume = {13},
number = {18},
pages = {4015-4020},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {Low-frequency shear and breathing modes are important Raman signatures of two-dimensional (2D) materials, providing information on the number of layers and insights into interlayer interactions. We elucidate the nature of low-frequency modes in a 2D polar metal-2D Ga covalently bonded to a SiC substrate, using a first-principles Green's function-based approach. The low-frequency Raman modes are dominated by surface resonance modes, consisting primarily of out-of-phase shear modes in Ga, coupled to SiC phonons. Breathing modes are strongly coupled to the substrate and do not give rise to peaks in the phonon spectra. The highest-frequency shear mode blue-shifts significantly with increasing thickness, reflecting both an increase in the number of Ga layers and an increase in the effective interlayer force constant. The surface resonance modes evolve into localized 2D Ga modes as the phonon momentum increases. The predicted low-frequency modes are consistent with Raman measurements on 2D polar Ga.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000747115200058,
title = {Atomic-Level Structure Determines Electron-Phonon Scattering Rates in 2-D Polar Metal Heterostructures},
author = {Megan A Steves and Siavash Rajabpour and Ke Wang and Chengye Dong and Wen He and Su Ying Quek and Joshua A Robinson and Kenneth L Knappenberger},
doi = {10.1021/acsnano.1c05944},
times_cited = {8},
issn = {1936-0851},
year = {2021},
date = {2021-11-23},
journal = {ACS NANO},
volume = {15},
number = {11},
pages = {17780-17789},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {The electron dynamics of atomically thin 2-D polar metal heterostructures, which consisted of a few crystalline metal atomic layers intercalated between hexagonal silicon carbide and graphene grown from the silicon carbide, were studied using nearly degenerate transient absorption spectroscopy. Optical pumping created charge carriers in both the 2-D metals and graphene components. Wavelength-dependent probing suggests that graphene-to-metal carrier transfer occurred on a sub-picosecond time scale. Following rapid (<300 fs) carrier-carrier scattering, charge carriers monitored through the metal interband transition relaxed through several consecutive cooling mechanisms that included sub-picosecond carrier-phonon scattering and dissipation to the silicon carbide substrate over tens of picoseconds. By studying 2-D In, 2-D Ga, and a Ga/In alloy, we resolved accelerated electron-phonon scattering rates upon alloy formation as well as structural influences on the excitation of in-plane phonon shear modes. More rapid cooling in alloys is attributed to increased lattice disorder, which was observed through correlative polarization-resolved second harmonic generation and electron microscopy. This connection between the electronic relaxation rates, far-field optical responses, and metal lattice disorder is made possible by the intimate relation between nonlinear optical properties and atomic-level structure in these materials. These studies provided insights into electronic carrier dynamics in 2-D crystalline elemental metals, including resolving contributions from specific components of a 2-D metal-containing heterojunction. The correlative ultrafast spectroscopy and nonlinear microscopy results suggest that the energy dissipation rates can be tuned through atomic-level structures.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000693328500001,
title = {Tunable 2D Group-III Metal Alloys},
author = {Siavash Rajabpour and Alexander Vera and Wen He and Benjamin N Katz and Roland J Koch and Margaux Lassauniere and Xuegang Chen and Cequn Li and Katharina Nisi and Hesham El-Sherif and Maxwell T Wetherington and Chengye Dong and Aaron Bostwick and Chris Jozwiak and Adri C T van Duin and Nabil Bassim and Jun Zhu and Gwo-Ching Wang and Ursula Wurstbauer and Eli Rotenberg and Vincent Crespi and Su Ying Quek and Joshua A Robinson},
doi = {10.1002/adma.202104265},
times_cited = {16},
issn = {0935-9648},
year = {2021},
date = {2021-09-04},
journal = {ADVANCED MATERIALS},
volume = {33},
number = {44},
publisher = {WILEY-V C H VERLAG GMBH},
address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY},
abstract = {Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-InxGa1-x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000578331800001,
title = {Light-Matter Interaction in Quantum Confined 2D Polar Metals},
author = {Katharina Nisi and Shruti Subramanian and Wen He and Kanchan Ajit Ulman and Hesham El-Sherif and Florian Sigger and Margaux Lassauniere and Maxwell T Wetherington and Natalie Briggs and Jennifer Gray and Alexander W Holleitner and Nabil Bassim and Su Ying Quek and Joshua A Robinson and Ursula Wurstbauer},
doi = {10.1002/adfm.202005977},
times_cited = {22},
issn = {1616-301X},
year = {2020},
date = {2020-10-15},
journal = {ADVANCED FUNCTIONAL MATERIALS},
volume = {31},
number = {4},
publisher = {WILEY-V C H VERLAG GMBH},
address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY},
abstract = {This work is a systematic experimental and theoretical study of the in-plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type.k-space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near-zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model-based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light-matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum-)plasmonics and nano-photonics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}