arXiv subjects

Huu Thoai Ngo

Publications and source records attributed to Huu Thoai Ngo.

4 recordsLinked to original sources

arXiv2026

Graphene-capped bismuthene on SiC as a platform for correlated quantum spin Hall edge states

Epitaxial bismuthene on SiC(0001) hosts symmetry-protected metallic edge states within a large bulk band gap, establishing it as a promising two-dimensional topological insulator for hightemperature quantum spin Hall (QSH) transport. Here we realize bismuthene islands by intercalating Bi beneath zero-layer graphene on SiC(0001) followed by hydrogen treatment, yielding well-defined edges with controlled terminations. Spectroscopic measurements demonstrate that the edge states reside inside the bulk band gap and remain charge neutral. The graphene overlayer interacts only weakly with the bismuthene, preserving its topological character while providing environmental protection. Notably, the one-dimensional edge channels exhibit signatures of enhanced electronic correlations relative to freestanding bismuthene, suggesting proximity-induced modification of the QSH edge physics. These results establish graphene-capped bismuthene as a robust and tunable platform for correlated quantum spin Hall states.

cond-mat.mes-hall↗

arXiv2025

Intercalant-induced Kekule ordering and gap opening in quasi-free-standing graphene

We present a comprehensive investigation of the structural and electronic properties of Sn intercalated buffer layers on SiC(0001) using low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS), spot-profile analysis low-energy electron diffraction (SPA-LEED), and density functional theory (DFT) calculations. Sn intercalation effectively decouples the buffer layer, yielding quasi-free-standing monolayer graphene (QFMLG) while introducing local lattice distortions. Bias-dependent STM imaging revealed the coexistence of conventional and Kekule-ordered graphene domains, governed by the underlying Sn(1x1) reconstruction at the SiC interface. The measured STS spectra exhibit good agreement with DFT results. However, achieving homogeneous Sn(1x1) domains remains challenging, apparently, due to strain within the Sn monolayer, which drives the emergence of Kekule distortions and the associated electronic band-gap opening omogeneously in graphene. These findings highlight the crucial role of intercalant homogeneity and strain in tuning graphene`s structural and electronic properties.

cond-mat.mes-hall↗

arXiv2025

Monolayer-Defined Flat Colloidal PbSe Quantum Dots in Extreme Confinement

Colloidal two-dimensional lead chalcogenide nanocrystals represent an intriguing new class of materials that push the boundaries of quantum confinement by combining a crystal thickness down to the monolayer with confinement in the lateral dimension. In particular flat PbSe quantum dots exhibit efficient telecommunication band-friendly photoluminescence (1.43 - 0.83 eV with up to 61% quantum yield) that is highly interesting for fiber-optics information processing. By using cryogenic scanning tunneling microscopy and spectroscopy, we probe distinct single layer-defined PbSe quantum dot populations down to a monolayer with in-gap state free quantum dot-like density of states, in agreement with theoretical tight binding calculations. Cryogenic ensemble photoluminescence spectra reveal mono-, bi-, and trilayer contribution, confirming the structural, electronic and theoretical results. From larger timescale shifts and ratio changes in the optical spectra we infer Ostwald ripening in solution and fusing in deposited samples of thinner flat PbSe quantum dots, which can be slowed down by surface passivation with PbI2. By uncovering the interplay between thickness, lateral size and density of states, as well as the synthetic conditions and post-synthetic handling, our findings enable the target-oriented synthesis of two-dimensional PbSe quantum dots with precisely tailored optical properties at telecom wavelengths.

physics.chem-ph↗

arXiv2024

Sub-meV Linewidths in Polarized Low-Temperature Photoluminescence of 2D PbS Nanoplatelets

Colloidal semiconductor nanocrystals are promising materials for classical and quantum light sources due to their versatile chemistry and efficient photoluminescence (PL) properties. While visible emitters are well-established, the pursuit of excellent (near-)infrared sources continues. One notable candidate in this regard are photoluminescent two-dimensional (2D) PbS nanoplatelets (NPLs) exhibiting excitonic emission at 720 nm (1.7 eV) directly tying to the typical emission range limit of CdSe NPLs. Here, we present the first comprehensive analysis of low-temperature PL from this material class. Ultrathin 2D PbS NPLs exhibit high crystallinity confirmed by scanning transmission electron microscopy, and revealing Moire patterns in overlapping structures. At 4K, we observe unique PL features in single PbS NPLs, including narrow zero-phonon lines with line widths down to 0.6 meV and a linear degree of polarization up to 90%. Time-resolved measurements identify trions as the dominant emission source with a 2.3 ns decay time. Sub-meV spectral diffusion and no immanent blinking over minutes is observed, as well as discrete spectral jumps without memory effects. These findings advance the understanding and underpin the potential of colloidal PbS NPLs for optical and quantum technologies.

physics.optics↗