Two dimensional (2D) materials have attracted significant attention due to their unique electronic and optical properties. These properties arise from strong quantum confinement effects and reduced dielectric screening due to their atomic-layer thickness, distinguishing them from conventional bulk materials [1,2,3,4,5]. Thanks to these characteristics, 2D materials show distinct properties compared to bulk structures of the same materials, such as optical sensitivity, bandgap tunability, mechanical flexibility, and high transparency. In addition, recently, significant technological advancements have been made that precisely control optical responses at the nanoscale, including electric field and strain induced modulation of excitonic resonances, integration with plasmonic and photonic nanostructures for enhanced light matter coupling, and the creation of moiré superlattices to engineer spatially varying potentials. These approaches have enabled dynamic tuning of exciton and trion energies, lifetimes, and emission profiles, opening the door to tailored optoelectronic functionalities in 2D-material-based devices [6,7,8,9,10,11,12,13,14,15,16]. Specifically, transition metal dichalcogenides (TMDs) have stable excitons at room temperature [17,18,19,20,21,22,23,24,25,26,27,28,29], with bright photoluminescence (PL) properties owing to direct-gap band-structures. In addition, trions in 2D TMDs, which have relatively short PL lifetimes, allow to tune their properties by electrical control [30,31,32,33,34,35,36,37,38,39,40,41]. However, trions are highly sensitive to external environmental factors, such as fluctuations in charge carrier density, variations in the surrounding dielectric environment, and interactions with defects or impurities, posing challenges for their integration into practical optoelectronic devices. For practical device applications, it is important to selectively generate and sustain desired state of excitons or trions. For instance, excitons with high absorption efficiency can be advantageous for photodetectors and solar cells, whereas trions, which exhibit longer coherence times, are more suitable for quantum information processing.

Consequently, the precise control of exciton-trion interconversion in TMD-based optoelectronic devices has emerged as a key research topic. Extensive studies are being actively conducted to understand and optimize this process [42,43,44,45,46,47,48,49,50]. However, several challenges remain for systematic manipulation of the exciton-trion interconversion. Exciton diffusion at high temperatures reduces the conversion efficiency, while the limited electron supply at room temperature hinders effective trion formation, making precise control difficult. To overcome these issues, novel strategies are required to minimize losses during the exciton-trion interconversion process and enhance overall conversion efficiency.

Conventional static plasmonic structures serve as an effective strategy for enhancing exciton-trion interconversion [51,52,53,54,55,56,57,58,59,60]. However, their spatially fixed nature imposes limitations on dynamically controlling the conversion process. Recently, tip-induced manipulation (TIM) has been demonstrated as a promising approach for enabling dynamic control of exciton-trion interconversion. TIM is based on a near-field spectroscopy technique that harnesses plasmonic properties under an atomic force microscopy (AFM) apparatus, allowing real-time nanoscale monitoring and manipulation of the PL properties of various quantum emitters, such as single molecules [63, 64] and quantum dots [65,66,67], as well as comprehensive studies of their dynamic behavior [68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86].

Nevertheless, research on TIM-based exciton-trion interconversion in emerging systems, such as heterostructures and moiré superlattices, remains in its early stages, and most advanced modulation techniques have yet to be developed. Therefore, future studies should focus on investigating novel materials and hybrid systems, with expanding the controllable parameters of TIM to further enhance exciton-trion interconversion control.

This review comprehensively addresses the latest methods for controlling exciton–trion interconversion, including far-field spectroscopy, plasmonic enhancement, and tip-based techniques, as shown in Fig. 1. Specifically, we classify the text into three sections: i) exciton-trion interconversion controlled by various external factors in the far-field regime; ii) exciton-trion interconversion control through plasmonic enhancement via various nanostructures; iii) exciton-trion interconversion control using tip-based methodologies involving plasmonic nanocavities and high-speed AC gate voltage. By summarizing recent advances in exciton-trion interconversion control and future research directions, this review offers insights into the potential of transforming photonic quasiparticles for next-generation optoelectronic and quantum technologies.

To facilitate the modulation of exciton-trion interconversion, it is crucial to understand how various external factors influence this dynamic process. Exciton-trion interconversion has been modulated through various control parameters, such as the gate voltage [42, 44], the laser irradiation time [46], the temperature [45], and the orbital angular momentum (OAM) of light [44], as for potential applications in quantum optics and optoelectronic device engineering. Therefore, in this section, we discuss the role of external factors in effectively controlling exciton-trion interconversion, as well as the fundamental approaches. Z. Wang et al. experimentally confirmed exciton-trion interconversion by adjusting the gate voltage (V_g) [42]. As shown in Fig. 2a, they measured the PL spectra at room temperature while varying V_g. Through this measurement, they found that the injected carrier concentration was very low at the threshold near V_g = 0.5 V. Accordingly, the number of trions was also limited. In this condition, PL suppression caused by nonradiative decay into trions was not observed. In contrast, when a higher gate voltage was applied, more carriers were injected, leading to a decrease in overall PL intensity and a redshift of the PL peak.

Exciton-trion interconversion can also be influenced by laser excitation power. In n-type monolayer TMDs with abundant electron carrier concentrations, the ratio of the PL intensity of excitons to trions varies with laser excitation power.

This can be understood by the fact that as the laser excitation power increases, more electrons are excited, increasing the probability of trion formation through the combination of excitons and excess electron carriers [44]. Additionally, C. Yang et al. demonstrated the thermal energy-dependence of interconversion equilibrium properties of excitons and trions through the temperature-dependent PL spectra measurement of a monolayer WS₂ treated with water and acetonitrile [45]. Under the cooling condition of 4 K, the PL signal of neutral exciton emission nearly disappears, while trion emission significantly increases. An extra PL shoulder also appears below 150 K, attributed to defect-bound localized excitons. These localized states are thermally unstable due to their low binding energy of ~20 meV and can easily dissociate because of thermal fluctuations, even at room temperature. This indicates that exciton-trion interconversion can also be controlled by temperature modulation [45]. As shown in Fig. 2b, the relative proportion of exciton PL gradually decreases as the laser irradiation time increases, while the proportion of trion PL increases. Although excitons convert into trions more rapidly during the initial stage, the overall interconversion occurs over several seconds and gradually stabilizes [46]. This behavior arises from photo-induced carrier doping and molecular adsorption effects, and represents a quasi-static modulation, in contrast to the ultrafast dynamic control techniques emphasized in this review. Furthermore, R. Kesarwani et al. demonstrated that the OAM characteristics of light can be used to regulate exciton-trion interconversion at room temperature [44]. Following the increase of OAM, the PL intensity of trions decreases while the PL intensity of excitons increases. The underlying principle of this phenomenon is that the OAM of light applies torque to the electrons in the monolayer WS₂, reducing the number of unbound electrons in the conduction band and lowering the probability of trion formation.

In this section, we have summarized that various control parameters, such as gate voltage, laser power, laser excitation time, temperature, and the OAM of light, can control exciton-trion interconversion in 2D semiconductors. These findings provide valuable insights into how the conversion between excitons and trions can be controlled by external factors. By carefully adjusting and expanding these external factors, we can gain a deeper understanding of the interconversion dynamics of excitons and trions, as well as insights into advanced excitonic devices.

In the previous section, external factors affecting exciton-trion interconversion were discussed. However, there are still several challenges in applying the knowledge of exciton-trion interconversion control technologies from existing studies to devices. At high temperatures, exciton diffusion reduces the exciton-trion interconversion efficiency, while limited electron availability at room temperature hinders effective trion formation, making precise control of the interconversion process difficult. To overcome these challenges, an approach utilizing plasmonic structures has been introduced since 2009 to reduce losses during the exciton-trion interconversion process. By focusing on these innovative strategies, the conversion efficiency of excitons and trions has been successfully enhanced, and research has been conducted that allows for a deeper understanding of the dynamics of excitons and trions in various systems, providing insights into their manipulation for future applications.

3.1 Exciton-trion interconversion induced by conventional static plasmonic structures

A common approach for fabricating plasmonic hetero-structures is the vertical stacking of different materials. Recently, plasmonic structure-induced exciton-trion interconversion has been studied using Ag nanowires and Ag films, as shown in Fig. 3a [58]. The regions labeled "in-cavity"(green circle) and "off-cavity"(red circle) in Fig. 3b indicate where PL signals were measured. The in-cavity position corresponds to the WS₂-plasmonic nanocavity hybrid structure, while the off-cavity position represents the bare monolayer WS₂ region. In the cavity region, two dominant emission peaks are observed at 1.96 eV and 2.01 eV [24], corresponding to the trions and excitons, respectively. In contrast, only the exciton peak is detected in the off-cavity region, as shown in Fig. 3c. The exciton PL intensity in the in-cavity region is ~1.7 times greater than that in the off-cavity region, while the trion peak intensity is enhanced by a factor of 10.8. To further investigate the trion characteristics, power-dependent PL analysis was performed, as shown in Fig. 3d and e [25, 50]. At lower excitation powers, the exciton peak dominates, but as the excitation power increases, exciton emission is suppressed, evidenced by the sublinear slope of 0.64 for the exciton peak [26], while the trion peak exhibits a slope of ~1. In this system, the slightly n-doped monolayer WS₂ contributes to the trion peak, which is attributed to trion emission [30]. Previous studies have shown that increasing excitation power enhances trion emission in 2D materials [25, 26]. However, achieving strong trion emission by only increasing optical power requires much higher excitation levels. In contrast, the plasmonic nanocavity allows for more efficient enhancement of trion emission due to the intensified electromagnetic field. Time-resolved photoluminescence (TRPL) measurements were performed to investigate the carrier dynamics of excitons and trions in both the in-cavity and off-cavity regions, as shown in Fig. 3f. The TRPL spectra were fitted with a bi-exponential function, comprising a fast decay component (τ₁) and a slow decay component (τ₂). The τ₁ is associated with exciton radiative recombination, while the τ₂ corresponds to trion radiative recombination [66]. For monolayer WS₂ at the in-cavity region, the lifetimes (τ₁= 42 ps and τ₂ = 280 ps) are shorter than the corresponding lifetimes in the off-cavity region (τ₁ = 50 ps and τ₂ = 350 ps). This indicates that the plasmonic nanocavity enhances the radiative recombination processes of both excitons and trions in WS₂ monolayers. Exciton-trion interconversion induced by plasmonic structures is being investigated across various configurations. For example, S. Moon et al. fabricated a TMD-metal nanostructure array by depositing an exfoliated monolayer WS₂ onto a template-stripped Ag nanohole (AgNH) array to explore its physical properties. Figure 3g presents an optical microscopy (OM) image, AFM image, and PL mapping image of the WS₂/AgNH sample [59]. Regions of higher PL intensity correspond to suspended WS₂, whereas areas of lower PL intensity indicate direct contact between monolayer WS₂ and the Ag surface. This reduction in PL intensity is attributed to charge transfer at the WS₂/Ag interface, in agreement with previous reports [68]. Figure 3i displays the PL spectra of monolayer WS₂ flakes on both AgNH and a flat Ag surface. The PL peak positions confirm that the WS₂ flakes are monolayers, in agreement with previous studies [27, 28].

The PL intensity of WS₂/AgNH is ~4 times greater than that of WS₂/Flat-Ag, and the trion peak is significantly stronger in WS₂/AgNH, indicating a higher free electron density induced by the plasmonic structure. Figure 3j shows a log–log plot of the exciton and trion emission intensities in WS₂/AgNH as a function of excitation laser power. The trion emission follows a power-law dependence with an exponent of ~1.5, while the exciton intensity exhibits an exponent of ~1.1. The steeper slope of the trion emission is attributed to enhanced spontaneous emission [51], resulting from local field enhancement in the WS₂/AgNH structure [56].

To integrate exciton-trion interconversion structures with plasmonic materials for practical devices, it is essential to have a direction-controllable platform. H. Lee et al. demonstrated this by transferring a monolayer MoS₂ onto a metal–insulator-metal (MIM) nanogap structure, which induced strain and activated SPP modes. This approach significantly enhanced the efficiency of exciton-trion interconversion through plasmon-induced hot electron injection [60]. Figure 3k shows plasmonic structure-induced exciton to trion conversion process in MIM nanogap structure. The TRPL data were analyzed using a bi-exponential function model that includes both fast (τ₁) and slow (τ₂) decay components, as shown in Fig. 3k. In contrast to previous plasmon-coupled platforms, which typically show significant reductions in decay times [57], both decay components from the lateral MIM waveguide exhibit minimal changes. The strain gradient geometry in this system facilitates electron funneling, enhancing the efficiency of exciton-trion interconversion. Consequently, a high electron density and enhanced trion emission are induced; however, they remain weakly coupled to the plasmon. To verify that this phenomenon is attributable to the plasmonic structure, the activation and deactivation of the SPP modes were controlled by adjusting the polarization of the excitation laser. Spatially dependent PL responses were measured under three distinct excitation polarizations, as shown in Fig. 3m. When the waveguide was deactivated, no spectral changes were observed in the SPP mode. However, upon partial activation of the waveguide, trion emission emerged within the SPP mode, accompanied by a reduction in exciton emission. Full activation of the waveguide leads to high purity trion emission, with negligible exciton emission. The corresponding spectra are shown in Fig. 3n. These experimental results confirm that plasmonic structures enable the generation of high purity trions and that trion formation can be modulated by adjusting the polarization of the incident light. This highlights the potential for actively controlling exciton-trion interconversion through the implementation of various control parameters.

The integration of plasmonic structures with 2D materials presents significant technological challenges, particularly due to the limitations of existing dynamic control techniques in effectively modulating exciton-trion interconversion. To address these constraints, researchers have investigated alternative control parameters to enhance conversion efficiency. Strategies involving valley polarization, temperature modulation, and adaptive control techniques are being actively used. The interplay between these control factors offers new opportunities to maximize exciton-trion interconversion rate, thereby refining the integration of 2D materials with plasmonic structures and advancing their potential applications.

Several approaches have been explored to maximize exciton-trion interconversion mediated by plasmonic structures, among which the utilization of the valley polarization properties of 2D materials has been identified as a promising strategy. To explore the effect of the plasmonic nanocavity on valley polarization in 2D materials, circularly polarized light was used for excitation. According to X. Xu et al., right-handed circularly polarized light (σ⁺) was used for excitation, and the emission was analyzed for both right-handed (σ⁺) and left-handed (σ⁻) circular polarization components [62, 63]. As shown in Fig. 4a, the integration of a monolayer WS₂ with a plasmonic nanocavity significantly enhances valley polarization, reaching ~17%, while the valley polarization outside the nanocavity remains near zero. The degree of valley polarization is directly associated with the lifetimes of quasi-particles, and valley states through their interdependent dynamics, given by P = P₀/(1 + τ_E/τ_S), where, P₀, τ_S, and τ_E represent the initial polarization, valley relaxation time, and quasi-particle lifetime, respectively. Degree of polarization can be maximized either by increasing τ_S or by decreasing τ_E. It is well established that the lifetime of 2D materials integrated with plasmonic structures is shorter than that of their uncoupled 2D materials [57]. Consequently, trion emission exhibits an enhanced degree of valley polarization. They explored the valley polarization characteristics of 2D materials integrated with plasmonic structures. By illuminating circularly polarized light, they enhanced exciton-trion interconversion efficiency and systematically analyzed the underlying physical mechanisms. These findings highlight the potential of plasmonic-coupled 2D materials for advanced optoelectronic and valleytronic applications.

Among the various factors influencing exciton-trion interconversion in 2D materials integrated with plasmonic structures, temperature represents a key tunable parameter. J. Cuadra et al. investigated the control of degree of coupling between the plasmonic structure and trion through temperature adjustment [61]. They fabricated a sample consisting of Ag nano-prism and WS₂, as shown in Fig. 4b and c, and confirmed the coupling between the plasmonic structure and exciton/trion through dark-field (DF) spectroscopy. Scanning electron microscopy confirms that the system consists of a single Ag nano-prism (inset of Fig. 4b). At 300 K, only the dip corresponding to the exciton resonance is observed, whereas as the system is cooled, a trion feature begins to emerge in the PL spectrum, indicating a redistribution of oscillator strength between the exciton and trion resonances. This redistribution influences the coupling between the plasmonic nano-prism and the 2D material. As the temperature decreases, two dips appear in the DF scattering spectra (Fig. 4b). Additionally, the cooling process is accompanied by a blue shift in both the exciton and trion resonances. With a spectral detuning of ~40 meV between the exciton and trion resonances, both can interact with the plasmonic cavity, as the plasmon linewidth is much broader than the exciton-trion detuning. As a result, the hybrid system transitions from plasmon-exciton interactions at room temperature to more complex plasmon-exciton-trion interactions at lower temperatures, as schematically illustrated in Fig. 4c. This study demonstrates that temperature control effectively enhances exciton-trion interconversion efficiency in 2D materials integrated with plasmonic structures. Furthermore, a detailed investigation of the underlying mechanism reveals that the degree of coupling between the plasmonic structure and the 2D material varies as a function of temperature.

3.2 Exciton-trion interconversion induced by optimized static plasmonic structures

In the previous section, we discussed a strategy for controlling exciton-trion interconversion by activating SPP modes through the integration of nanogap plasmonic structures with 2D materials.

However, in conventional static plasmonic structures, the fixed nature of SPP modes imposes limitations on optimizing exciton-trion interconversion efficiency.

To achieve higher adaptability and functionality, precise, tunable control of the SPP mode is required. This dynamic control was made possible through adaptive wavefront shaping using a spatial light modulator (SLM), as shown in Fig. 4d. The optical signal was collected from a region with reduced SPP intensity, and a feedback algorithm was implemented to optimize the wavefront, thereby maximizing the PL intensity [63, 84]. Figure 4e shows the evolution of SPP intensity during the wavefront shaping process. The SPP intensity progressively increases, reaching a peak enhancement of ~210% with the optimized phase mask, as shown in Fig. 4f. This result demonstrates that the SPP mode can be spatially controlled at specific locations, enabling the spatial modulation of the exciton-trion interconversion region. Figure 4g compares the PL spectra obtained with and without the optimized phase mask. Upon applying the optimized phase mask, trion emission is significantly enhanced, driven by the additional electrons generated through plasmon-induced hot electron excitation. In contrast, in the absence of the phase mask, the PL spectrum is primarily dominated by exciton emission due to the insufficient electron density. Interestingly, the increase in trion PL intensity exceeds the reduction in exciton PL intensity with the phase mask, which is attributed to the SPP-induced excitation of additional exciton that is subsequently converted to trion. This fully optical process enables non-invasive modulation with great repeatability, allowing precise control of exciton-trion interconversion at specific locations and dynamic switching between exciton and trion emissions, as shown in Fig. 4h. Conventional static plasmonic waveguides inherently exhibit limitations in dynamically controlling the spatial distribution of the exciton-trion interconversion region due to their fixed SPP modes. By employing an SLM, adaptive wavefront shaping enables precise spatial modulation of the SPP mode, thereby optimizing the exciton-trion interconversion efficiency in plasmonic structures. 2D materials have garnered considerable attention due to their unique electronic and optical properties. Unlike charge neutral excitons, trions exhibit charged characteristics, making them more favorable for practical applications in optoelectronics. Consequently, enhancing exciton-trion interconversion is crucial for advancing the functionality of 2D material-based devices.

However, the exciton-trion interconversion rate at room temperature is notably low, presenting a significant challenge for efficient device performance. To overcome this limitation, various strategies have been investigated, with plasmonic structure integration emerging as a promising approach. This review systematically examines different types of plasmonic structures and their underlying mechanisms that influence exciton-trion interconversion. Furthermore, to maximize conversion efficiency, we review not only conventional methods using static plasmonic structures but also advanced strategies incorporating dynamic control parameters, providing comprehensive insights into dynamic modulation techniques for optimized exciton-trion interconversion.

In the previous section, we discussed the exciton-trion interconversion induced by plasmonic structures in static environments. While these techniques are effective in enhancing the exciton-trion interconversion efficiency, their spatially fixed nature limits dynamic control over the conversion process. Therefore, this section focuses on recent studies that demonstrate the dynamic control of exciton-trion interconversion using TIM. TIM is a near-field spectroscopic technique that combines a plasmonic tip with an AFM [63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86]. When a nanoscale tip is illuminated, the electrostatic lightning rod effect and LSPR generate a strongly confined electric field at the tip apex [63, 65, 68, 70, 75, 79, 81, 85, 86]. The enhanced field, combined within the cavity formed between the tip and sample, produces an amplified optical signal, enabling nanoscale resolution comparable to the tip apex radius. This technique is highly effective for exciton physics in 2D materials, as it allows for the dynamic control of light matter interaction on the material surface by adjusting the tip-sample distance. Moreover, recent advancements have begun to explore the manipulation of excitons through the integration of additional degrees of freedom, such as externally applied electric fields [65, 79, 85] and strain engineering [68, 70, 75, 86], providing new opportunities for controlling excitonic properties and behaviors in 2D materials.

4.1 Tip-enhanced quantum tunneling effects

In addition to amplifying optical signals, there exists a quantum mechanical phenomenon that plays a crucial role in exciton-trion interconversion. Utilizing the quantum tunneling effect is one of the methods that enables precise control of exciton-trion interconversion. When a metallic tip is illuminated, LSPR generates hot electrons, which can tunnel into an adjacent 2D semiconductor and promote exciton-trion interconversion. Z. He et al. provided experimental evidence of tip-induced exciton-trion conversion in monolayer TMDs [82]. Tip enhanced photoluminescence (TEPL) experiments were conducted on monolayer WS₂/Au structure using an Ag tip. By precisely controlling the tip-sample distance at the picometer scale, the effect of the tip on trion formation was examined. In the picocavity regime, where tunneling is pronounced, the PL spectra show an increased trion peak intensity and a decreased exciton peak intensity compared to classical tip-sample distances. In TMDs, spatial inhomogeneities in substrate sample coupling can lead to variations in PL quenching behavior, complicating the interpretation of exciton physics [52, 53, 74]. Nonetheless, reducing the tip-sample distance within the quantum tunneling regime increases the exciton/trion peak intensity ratio, dominating the quenching.

4.2 Tip-enhanced control in hybrid plasmonic nanostructures

By combining TIM with nanoplasmonic structures and utilizing the electronic density characteristics generated by the plasmonic structures, this approach demonstrates that TIM can be effectively applied in plasmonic structures with various designs, enabling the control of exciton-trion interconversion Combining tip-enhanced spectroscopy with other plasmonic structures enables dynamic control of the unique modes arising from both structures, expanding its range of applications. In a recent study, M. Kang et al. utilized the TEPL approach to investigate the spatial distribution of exciton species in monolayer TMDs on an Au nanowire structure. (Fig. 5a) [83]. As shown in Fig. 5b, the field distribution along the nanowire exhibits a periodic standing wave resonance of SPP, leading to spatial variations in local charge density [54, 55]. TEPL imaging results presented in Fig. 5c revealed that excitons predominantly exist in regions with low electron density, whereas trions are more prevalent in regions with higher electron density. Figure 5d shows TEPL spectra for different regions along the nanowire, as well as for a region without the nanowire. The spectral transition from far-field to near-field demonstrates that hot electron injections from the nanowire promote trion formation, while reducing the tip-sample distance further enhances the exciton-trion interconversion ratio. By integrating tip-enhanced techniques with another plasmonic nanostructure, this study demonstrates a highly controllable approach to exciton–trion interconversion. The ability to manipulate exciton and trion populations at the nanoscale highlights the potential of exciton-trion interconversion for advanced techniques, such as applications in excitonic circuitry.

4.3 High-speed nanoscale control via tip-enhanced voltage modulation

The key advantage of tip-based cavity configurations is their ability to introduce additional modulation parameters while maintaining high spatial resolution. Recently, H. Lee et al.

designed a system enabling High-speed exciton–trion interconversion control using a tip-enhanced approach [85]. Figure 6a shows the TEPL experimental setup, where a monolayer MoS₂ is transferred onto an HfO₂-coated Au film.

The energy band diagram illustrates the underlying mechanism of exciton-trion interconversion in a monolayer MoS₂, driven by charge tunneling from an externally applied voltage between the tip and the sample. HfO₂ layer was introduced to enhance electron density in MoS₂ by facilitating oxygen vacancy formation at the interface and improve optical field confinement through dipole–dipole interactions within the gap. The strong confinement effect is evident in the simulation results presented in Fig. 6b. Similarly, in the quantum tunneling regime (d ≤ 2 nm), voltage-induced potential changes also remain confined within an FWHM of 10 nm, validating the ability to selectively manipulate carriers at the metal–semiconductor junction beneath the tip. As shown in Fig. 6c, TEPL spectra at different tip–sample distances reveal that reducing the distance enhances PL intensity while simultaneously increasing the trion fraction, consistent with previous studies on tip-induced exciton-trion interconversion. As shown in Fig. 6c, TEPL spectra at different tip–sample distances reveal that reducing the distance enhances PL intensity while simultaneously increasing the trion fraction, consistent with previous studies on tip-induced exciton-trion interconversion. Figure 6d presents TEPL spectra at a tip–sample distance of 3 nm, revealing that the applied voltage can significantly alter the exciton–trion ratio. Additionally, this study demonstrated that applying an AC voltage enabled exciton-trion interconversion at frequencies up to 100 MHz, highlighting its potential for high-speed switching applications.

These findings highlight a high-speed nanoscale exciton-trion interconversion method by combining a plasmonic tip with an external voltage. By independently adjusting the tip–sample distance and the applied voltage, the study achieved super-resolution control over both the PL intensity and the exciton-trion interconversion ratio. Integrating this tip-enhanced approach with other established control parameters, such as chemical doping and optical angular momentum control, unlocks new possibilities to spatially resolved, multidimensional control over exciton-trion interconversion processes.

4.4 Tip-enhanced control of interlayer excitons

While conventional exciton/trion control methods have achieved super-resolution and continuous measurement capability under various conditions by incorporating tip-based techniques, certain phenomena can only be facilitated through purely tip-enhanced approaches. One such case involves interlayer excitons in heterostructures of 2D materials. These excitons are formed by Coulomb interaction between electrons and holes in different layers, exhibiting long lifetimes and out-of-plane oriented dipoles [22, 87, 88]. Owing to these unique properties, interlayer excitons have garnered significant research interest in recent years. Recent investigation conducted by Y. Koo et al., interlayer exciton-interlayer trion interconversion control in a TMD heterostructure was demonstrated [86]. Tip-enhanced system was introduced to facilitate interlayer excitons by aligning their dipole orientation with the strong plasmonic field formed between the Au tip and Au substrate.

Additionally, hot electron injections from the tip further enhanced interlayer exciton emission, as depicted in Fig. 7a.

To verify tip-induced activation of interlayer excitons, tip–sample distance-dependent TEPL spectra were measured, as shown in Fig. 7b. As the tip approaches the sample, a shift from intralayer to interlayer exciton PL was observed. Figure 7c presents TEPL spectra obtained from a different region, revealing a peak with an energy distinct from that of interlayer excitons. Notably, this peak was identified as the interlayer trion component, as its intensity increases significantly when the tip approaches the sample. Given the strong dependence of interlayer excitons and interlayer trions on interlayer coupling strength, the tip was utilized to apply GPa-level pressure to the sample, further modulating their behavior. Pressing the heterojunction in contact reduced the interlayer distance, enhancing coupling while simultaneously inducing strain.

Consequently, Fig. 7d shows an increase in both interlayer exciton and trion peaks, along with a redshift in their peak positions due to strain. Tracking the intensity ratio between interlayer excitons and trions under pressure revealed that strain facilitates the conversion from interlayer excitons to interlayer trions. These results confirm that emissions from interlayer excitons, which are challenging to control under general conditions, can be efficiently manipulated using tip-enhanced techniques. Moreover, by utilizing unique control parameters of the tip system, interlayer exciton-trion interconversion was successfully controlled. Similarly, leveraging various phenomena induced by tip-enhanced systems could potentially unveil new possibilities for investigating specialized forms of exciton-trion interconversion [88], such as room-temperature dark exciton-dark trion interconversion [81], an uncharted frontier in excitonic physics.

This section reviewed studies on tip-induced exciton-trion interconversion, encompassing both its fundamental principles and key experimental demonstrations. Various tip-based exciton-trion interconversion methods were studied, ranging from basic approaches that manipulate tip–sample distance to advanced modulation techniques, e.g., applying an external voltage through the tip. As research field on exciton-trion interconversion is still in its early stages, introducing previously unexplored variables into tip-based techniques is expected to drive further advance the field.

This review discussed the advancements in controlling exciton-trion interconversion, focusing on the latest methods applied in 2D materials. The discussion includes three main approaches that provide precise control in nanoscale systems. First, we examine far-field regime exciton-trion interconversion control through various external factors, such as gate voltage, laser power, laser excitation time, temperature, and OAM. These factors can be finely tuned to influence conversion dynamics, providing valuable insights into the fundamental principles governing exciton-trion interconversion. Second, we emphasize the role of plasmonic enhancement in improving the efficiency and precision of exciton-trion interconversion.

By utilizing various plasmonic structures, researchers have demonstrated the ability to control light-matter interactions to optimize the conversion process. This has significant implications for the development of new optoelectronic devices.

Third, we focus on tip-based methods that enable high spatial resolution in achieving exciton-trion interconversion. With TIM, it is possible to dynamically control the interaction between light and excitons or trions at the nanoscale, enabling real-time manipulation of the conversion process. The combination of these methods with advanced modulation techniques, such as high-speed AC gate voltage, opens up the possibility of more precise and efficient control over exciton-trion interconversion.

Despite these advancements, challenges remain in integrating these control methods into practical devices. In particular, applying these methods to complex materials like TMD heterostructures and the lack of advanced modulation techniques are major obstacles. Nevertheless, these technologies are continuously developing and hold great potential for the future of nano-photonics and quantum technologies. Expanding the range of controllable parameters with exploring new materials and hybrid systems will further enhance the potential for controlling exciton-trion interconversion, opening new opportunities for the next generation of optoelectronic and quantum devices.

In conclusion, this review highlights the exciting progress in controlling exciton-trion interconversion, with a particular focus on the potential of tip-based methods. These techniques, combined with the ability to precisely control various external factors, provide unique tools for manipulating light-matter interactions in 2D materials. In particular, tip-based systems offer dynamic modulation of the plasmonic cavity mode volume enables precise control over exciton–trion interconversion, establishing an ideal experimental platform in the strong-coupling regime for investigating coherence phenomena in exciton-polaritons and trion-polaritons. Moreover, this approach plays a pivotal role in experimentally validating recent theories—such as the reinterpretation of trions as “attractive exciton-polaritons” arising from many-body interactions with a Fermi sea—thus advancing our deeper understanding of excitonic physics [89]. The insights presented here pave the way for the development of advanced optoelectronic devices with unprecedented capabilities, marking an important advancement in the fields of next-generation optoelectronic and quantum technologies.