Nguyen Tuan Hung

Low-dimensional materials and functional material systems have become increasingly important across environmental remediation, sustainable materials development, energy conversion, biomedical technologies, catalysis, protective coatings, and nanoelectronics

Native lattice defects strongly govern the optoelectronic properties of ‐, an emerging van der Waals material. At room temperature, its photoluminescence (PL) spectrum features a band‐edge excitonic “A” peak at 1.36 eV and a defect‐related sub‐bandgap “B” peak at 1.3 eV. While the B peak emission brightens significantly below 150 K, its exact origin and its connection to the low‐temperature phase transition of have remained unclear. Here, we demonstrate that the B peak emission originates from excitons trapped at Se vacancies. Upon cooling below 150 K, ‐ undergoes a structural transformation to the phase, which increases the localized electron density at Se vacancies and drives the brightening of the B peak emission. X‐ray photoelectron spectroscopy and a negative Seebeck coefficient confirm the presence of Se vacancies, which introduce electron doping and form a surface two‐dimensional electron gas observed using angle‐resolved photoemission spectroscopy. The trapped excitons exhibit strong electron–phonon coupling, with a PL lifetime of 28 ns and a large Huang–Rhys factor of 14. These results offer a framework for the deterministic control of defect‐bound excitons in ‐ for optoelectronics applications.

Low-dimensional and nano-scaled materials have attracted sustained interest because their reduced dimensionality, large surface-to-volume ratios, tunable electronic structures, and distinctive mechanical, optical, and chemical properties provide new opportunities for microsystems and nanosystems

Rapid and reliable identification of pharmaceutical residues is important for safeguarding public health, ensuring food safety, and enabling practical Raman-based screening. In this study, we propose HyMLRaman, a hybrid Raman spectroscopy framework that combines deep spectral feature extraction, generative models, and classical machine-learning classifiers to identify six pharmaceutical compounds, including amoxicillin, chloramphenicol, ciprofloxacin, tetracycline, ibuprofen, and paracetamol. Raman spectra are converted into spectral images and encoded with several deep neural-network backbones, among which EfficientNet-B3 yields the most effective representation. The resulting 1536-dimensional embeddings are then used to train downstream classifiers, including SVM, KNN, logistic regression, random forest, XGBoost, and ANN, using stratified 10-fold cross-validation. The hybrid EfficientNet-B3-SVM configuration achieves the strongest baseline performance, reaching 96.31% accuracy and a macro-F1 score of 96.36%, outperforming the standalone CNN baseline. To address limited-data conditions, a generative model, a DDPM-based feature augmentation, is introduced in a PCA-reduced EfficientNet-B3 latent space. The low-data ablation results show that DDPM augmentation provides selective benefits, particularly for KNN with reduced training fractions, and that its effect remains classifier-dependent. Finally, an application-level Raman Pharmaceutical Analyzer demonstrates the feasibility of embedding the trained model into an interactive Raman analysis workflow. These results suggest that HyMLRaman provides a practical and interpretable route for rapid Raman-based pharmaceutical screening.

Raman enhancement has been widely observed for molecules adsorbed on van der Waals (vdW) materials including graphene, MoS2, hexagonal boron nitride, and transition-metal dichalcogenides. This enhancement is often dominated by a chemical mechanism. However, how the Raman response evolves when a molecule is sandwiched between two vdW layers remains unclear. Here, using copper phthalocyanine (CuPc) as a probe, we compare three sandwich structures: graphene/CuPc/TiS2, graphene/CuPc/TiSe2, and graphene/CuPc/graphene, spanning asymmetric to near-symmetric dual-interface geometries. We find that adding a second interface in contact with the molecule can suppress, moderate, or enhance the Raman response of CuPc depending on the contacting material pair and vibrational symmetry. The graphene/CuPc/graphene sandwich shows stronger enhancement than single-interface CuPc/graphene, with an excitation wavelength dependence. These results show that the dual-interface Raman response is influenced by interface-dependent charge-transfer effects and increased nonradiative damping. This work provides an experimental framework for understanding Raman enhancement in molecule-sandwiched structures.

Janus two-dimensional (2D) materials exhibiting Rashba spin splitting have recently attracted considerable attention owing to their potential applications in spintronic devices and photocatalytic water splitting. In this work, we investigate, using first-principles calculations, the structural, mechanical, electronic, optical, and photocatalytic properties of Janus InXPbP (X = S, Se, Te) monolayers that exhibit significant Rashba effects. Our results demonstrate that all three monolayers are energetically, dynamically, and mechanically stable, as evidenced by cohesive energy calculations, phonon dispersion analysis, and elastic constants. By varying the chalcogen atom (X = S, Se, Te), the Rashba effect in InXPbP can be effectively tuned. Rashba parameters of 0.16 and 0.20 eVÅ are obtained near the conduction-band minimum (CBM) for InSPbP and InSePbP, respectively, whereas InTePbP exhibits giant Rashba spin splitting near both the CBM and valence-band maximum (VBM), with corresponding Rashba parameters of 0.90 and 0.87 eVÅ. Furthermore, the Janus InXPbP monolayers exhibit suitable band gaps of 1.21, 1.27, and 0.76 eV for InSPbP, InSePbP, and InTePbP, respectively, which are favorable for photocatalytic applications. All three monolayers possess suitable band-edge alignments for overall water splitting, yielding solar-to-hydrogen (STH) conversion efficiencies of 21.67%, 26.03%, and 29.83% for InSPbP, InSePbP, and InTePbP, respectively. Our findings not only enrich the family of Janus materials but also suggest that the Janus InXPbP monolayers are promising candidates for spintronic devices and high-performance photocatalytic water-splitting applications.

We present temperature-dependent electrical transport and Raman spectroscopy measurements of 1T-SnSe₂ single crystals. Room-temperature in-plane optical absorption spectra through spectroscopic ellipsometry analysis reveal indirect and direct band gaps of 1.19 and 1.52 eV, respectively. The transport data exhibit a broad conductivity maximum near 100 K and can be described by a thermally activated semimetallic conduction channel over 28-300 K, coexisting with a two-dimensional variable-range hopping conductivity. Temperature-dependent Raman spectra resolve the first-order E_g and A_1g phonon modes at 109 and 183 cm^-1. These phonon modes are fitted using Breit-Wigner-Fano (BWF) and Voigt line shapes, respectively. Notably, the Fano asymmetry parameter and the Voigt broadening exhibit anomalies near 100 K that quantitatively track the transport signature, whereas the peak positions and spectral widths evolve monotonically from 10 to 300 K. First-principles calculations of the electronic band structure and phonon dispersion corroborate the extracted band gap energies and spectral assignments. These findings establish a consistent picture for the electronic band structure, charge transport, and lattice dynamics in 1T-SnSe₂.

Helicity-resolved Raman spectra are measured for silicon (100), (110), and (111) surfaces in a back-scattering geometry using a 532 nm laser and compared with linearly-polarized Raman spectra. The polar plots of the helicity-resolved Raman intensity for the (110) and ( 111) surfaces as a function of the rotating angle of the polarizer for the scattered light are quantitatively described by rotating the Raman tensor from the (100) surface. Notably, the polar patterns of the T 2g phonon mode at 520 cm -1 for the (110) surface exhibit a tilted angular dependence in the helicity-changing Raman spectra. For the (100) surface, a weak Raman peak is observed in the helicity-conserved spectra that can not be captured by the standard T 2g Raman tensor, indicating a resonance-induced renormalization of the Raman tensor. This effect is reproduced by introducing a small diagonal component to the Raman tensor, which is supported by the first-principles resonant Raman calculations. The calculations further show that the relative value of the diagonal to off-diagonal component increases with increasing laser excitation energy. Our findings establish helicity-resolved Raman spectroscopy as a non-destructive method for probing symmetry-dependent Raman selection rules and identifying surface crystallographic orientation in silicon. This approach provides direct information on the Raman tensor and is applicable to other semiconductor materials.