arXiv subjects

Mingzu Liu

Publications and source records attributed to Mingzu Liu.

9 recordsLinked to original sources

arXiv2026

Anisotropic Dopant and Strain Architectures in WS$_2$ Nanocrystals Driven by Growth Kinetics

Dopant distribution in two-dimensional semiconductors is typically assumed to be stochastic, limiting deterministic defect engineering. Here, we show that non-equilibrium growth kinetics can be harnessed to define dopant-driven strain architectures in vanadium-doped WS$_2$ monolayers. Using synchrotron X-ray fluorescence, we identify preferential vanadium incorporation, anti-correlated with tungsten content, along crystallographic bisectors. An adsorption-growth-diffusion model with a single kinetic parameter quantitatively captures the dopant segregation arising from preferential corner adsorption and limited diffusion during chemical vapor deposition growth. Hyperspectral Raman imaging demonstrates mechanically induced vibrational responses, revealing localized tensile strain ($\varepsilon \approx0.70\%$) channels associated with the anisotropic dopant distribution. This regime is marked by the depletion of W-site-sensitive in-plane modes and the emergence of a localized $J2$ mode (210~cm$^{-1}$), which our ab-initio calculations attribute to antiphase V$-$V oscillations. These findings establish kinetic segregation as a route to deterministic chemical and strain architectures in 2D semiconductors, enabling programmable defect landscapes and strain engineering during synthesis.

cond-mat.mtrl-sci↗

arXiv2026

TCAP: Tri-Component Attention Profiling for Unsupervised Backdoor Detection in MLLM Fine-Tuning

Fine-Tuning-as-a-Service (FTaaS) facilitates the customization of Multimodal Large Language Models (MLLMs) but introduces critical backdoor risks via poisoned data. Existing defenses either rely on supervised signals or fail to generalize across diverse trigger types and modalities. In this work, we uncover a universal backdoor fingerprint-attention allocation divergence-where poisoned samples disrupt the balanced attention distribution across three functional components: system instructions, vision inputs, and user textual queries, regardless of trigger morphology. Motivated by this insight, we propose Tri-Component Attention Profiling (TCAP), an unsupervised defense framework to filter backdoor samples. TCAP decomposes cross-modal attention maps into the three components, identifies trigger-responsive attention heads via Gaussian Mixture Model (GMM) statistical profiling, and isolates poisoned samples through EM-based vote aggregation. Extensive experiments across diverse MLLM architectures and attack methods demonstrate that TCAP achieves consistently strong performance, establishing it as a robust and practical backdoor defense in MLLMs.

cs.AI↗

arXiv2024

Strong magneto-optical responses of an ensemble of defect-bound excitons in ambient exposed WS$_{2}$ and WSe$_{2}$ monolayers

Transition metal dichalcogenide (TMD) monolayers present a singular coupling in their spin and valley degrees of freedom. Moreover, by applying an external magnetic field it is possible to break the energy degeneracy between their K and $-$K valleys. This valley Zeeman effect opens the possibility of controlling and distinguishing the spin and valley characters of charge carriers in TMDs by their optical transition energies, making these materials promising for the next generation of spintronic and photonic devices. However, the free excitons of pristine TMD monolayers present a moderate valley Zeeman splitting of $\approx 0.23$ meV/T. Therefore, alternative excitonic states with higher magnetic responses are mandatory for application purposes. Here, we investigate the magneto-optical properties of ambient exposed WS$_2$ and WSe$_2$ monolayers by circularly polarized magneto-photoluminescence experiments at cryogenic temperatures. A broad lower energy photoluminescence emission related to an ensemble of defects is observed, presenting remarkable valley-related splittings of $\approx 1.45$ meV/T and $\approx 1.11$ meV/T for WS$_2$ and WSe$_2$ monolayers, respectively. In addition, we report a significant valley polarization of charge carriers in the defect mid-gap states induced by the external magnetic field. We explain this valley-polarized population and enhanced valley-related splitting in terms of imbalanced intervalley relaxations, leading to a magnetic field-dependent distribution of charge carriers in multiple defect levels. This effect, together with the individual Zeeman shiftings of the mid-gap states, explains the strong magneto-optical responses observed. Our work uncovers the singular potential of manipulating the light emission of ambient exposed TMD monolayers by an external magnetic field.

cond-mat.mes-hall↗

arXiv2024

Vanadium-Doped Molybdenum Disulfide Monolayers with Tunable Electronic and Magnetic Properties: Do Vanadium-Vacancy Pairs Matter?

Monolayers of molybdenum disulfide (MoS2) are the most studied two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its exceptional optical, electronic, and opto-electronic properties. Recent studies have shown the possibility of incorporating a small amount of magnetic transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic semiconductor (2D-DMS). However, the origin of the observed ferromagnetism has remained elusive, due to the presence of randomly generated sulfur vacancies during synthesis that can pair with magnetic dopants to form complex dopant-vacancy configurations altering the magnetic order induced by the dopants. By combining high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with first-principles density functional theory (DFT) calculations and magnetometry data, we demonstrate the critical effects of sulfur vacancies and their pairings with vanadium atoms on the magnetic ordering in V-doped MoS2 (V-MoS2) monolayers. Additionally, we fabricated a series of field effect transistors on these V-MoS2 monolayers and observed the emergence of p-type behavior as the vanadium concentration increased. Our study sheds light on the origin of ferromagnetism in V-MoS2 monolayers and provides a foundation for future research on defect engineering to tune the electronic and magnetic properties of atomically thin TMD-based DMSs.

cond-mat.mtrl-sci↗

arXiv2024

Effects of Vanadium Doping on the Optical Response and Electronic Structure of WS$_{2}$ Monolayers

Two-dimensional dilute magnetic semiconductors has been recently reported in semiconducting transition metal dichalcogenides by the introduction of spin-polarized transition metal atoms as dopants. This is the case of vanadium-doped WS$_2$ and WSe$_2$ monolayers, which exhibits a ferromagnetic ordering even above room temperature. However, a broadband characterization of their electronic band structure and its dependence on vanadium concentration is still lacking. Therefore, here we perform power-dependent photoluminescence, resonant four-wave mixing, and differential reflectance spectroscopy to study the optical transitions close to the A exciton energy of vanadium-doped WS$_2$ monolayers with distinct concentrations. Instead of a single A exciton peak, vanadium-doped samples exhibit two photoluminescence peaks associated with transitions to occupied and unoccupied bands. Moreover, resonant Raman spectroscopy and resonant second-harmonic generation measurements revealed a blueshift in the B exciton but no energy change in the C exciton as vanadium is introduced in the monolayers. Density functional theory calculations showed that the band structure is sensitive to the Hubbard $U$ correction for vanadium and several scenarios are proposed to explain the two photoluminescence peaks around the A exciton energy region. Our work provides the first broadband optical characterization of these two-dimensional dilute magnetic semiconductors, shedding light on the novel electronic features of WS$_{2}$ monolayers which are tunable by the vanadium concentration.

cond-mat.mes-hall↗

arXiv2020

Light-controlled room temperature ferromagnetism in vanadium-doped tungsten diselenide semiconducting monolayers

Atomically thin transition metal dichalcogenide (TMD) semiconductors hold enormous potential for modern optoelectronic devices and quantum computing applications. By inducing long-range ferromagnetism (FM) in these semiconductors through the introduction of small amounts of a magnetic dopant, it is possible to extend their potential in emerging spintronic applications. Here, we demonstrate light-mediated, room temperature (RT) FM, in V-doped WS2 (V-WS2) monolayers. We probe this effect using the principle of magnetic LC resonance, which employs a soft ferromagnetic Co-based microwire coil driven near its resonance in the radio frequency (RF) regime. The combination of LC resonance with an extraordinary giant magneto-impedance effect, renders the coil highly sensitive to changes in the magnetic flux through its core. We then place the V-WS2 monolayer at the core of the coil where it is excited with a laser while its change in magnetic permeability is measured. Notably, the magnetic permeability of the monolayer is found to depend on the laser intensity, thus confirming light control of RT magnetism in this two-dimensional (2D) material. Guided by density functional calculations, we attribute this phenomenon to the presence of excess holes in the conduction and valence bands, as well as carriers trapped in the magnetic doping states, which in turn mediates the magnetization of the V-WS2 monolayer. These findings provide a unique route to exploit light-controlled ferromagnetism in low powered 2D spintronic devices capable of operating at RT.

cond-mat.mtrl-sci↗

arXiv2020

Monolayer Vanadium-doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor

Dilute magnetic semiconductors, achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here we describe room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic percent and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry and first-principles calculations. Room-temperature two-dimensional dilute magnetic semiconductors provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures them into the realm of practical application.

cond-mat.mtrl-sci↗

arXiv2020

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-doped Tungsten Diselenide Monolayers at Room Temperature

The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step towards modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, we show tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe2. As vanadium concentrations increase within the WSe2 monolayers the saturation magnetization increases, and it is optimal at ~4at.% vanadium; the highest doping/alloying level ever achieved for V-doped WSe2 monolayers. The TISF occurs at ~175 K and becomes more pronounced upon increasing the temperature towards room temperature. We demonstrate that TISF can be manipulated by changing the vanadium concentration within the WSe2 monolayers. We attribute TISF to the magnetic field and temperature dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory calculation. Our findings pave the way for the development of novel spintronic and valleytronic nanodevices based on atomically thin magnetic semiconductors and stimulate further studies in this rapidly expanding research field of 2D magnetism.

cond-mat.mtrl-sci↗

arXiv2019

Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure Deposited on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides (TMDCs) with strong many-body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, we explore the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles (AuNTs) and monolayer WS2. We observe a narrow Fano resonance when the hybrid system is surrounded by water, and we attribute the narrowing of the spectral Fano linewidth to the plasmon-enhanced decay of dark K-K excitons. Our results reveal that dark excitons in monolayer WS2 can strongly modify Fano resonances in hybrid plasmon-exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.

physics.optics↗