Intrinsic Physical Mechanism Research Articles

Analysis of azimuthal electron current driving by rotating magnetic field in field-reversed configuration electric propulsion

Field-reversed configuration electric propulsion is an advanced space electromagnetic propulsion technology with significant application prospects but poor thrust performance. To delve into the intrinsic physical mechanisms, a model of the rotating magnetic field penetrating into the plasma and azimuthal electron current driving was developed. The simulation results show that rotating magnetic field strength, frequency, and initial seed plasma density are the key factors that exist as an optimal threshold. Specifically, the rotating magnetic field feed current (i.e., magnetic field strength) was not less than 1000 A, the rotating magnetic field frequency was ∼200–300 kHz, and the plasma density was approximately 1 × 1018 m−3 order of magnitude.

Theoretical study of tip-enhanced photoluminescence of single molecule in nanocavity formed by apex protrusion and substrate

Tip-enhanced photoluminescence techniques, characterized by ultra-high detection sensitivity and spatial resolution, have exhibited unprecedented capabilities in exploring optoelectronic phenomena and unraveling intrinsic physical mechanism at the single-molecule level. To suppress the quenching of molecular luminescence and achieve high-resolution optical imaging, a rational design of the tip-substrate configuration is required. In this paper, we systematically investigate the tip-enhanced photoluminescence characteristics of single molecules located within the plasmonic nanocavity formed by the tip protrusion and the substrate from a theoretical perspective. The impacts of electron tunneling and electron-hole pair creation on localized electric field and quantum yield are discussed using the quantum correction model and nonlocal dielectric theory, respectively. The crucial roles of atomic-scale protrusion at the tip apex and the decoupling layer on the substrate in enhancing vertical and horizontal electric fields as well as quantum yields have been emphasized. The results indicate that in smaller sub-nanometer cavities, electron tunneling and nonlocal dielectric effects greatly weaken the electric field intensity and fluorescence quantum yield. Meanwhile, due to the sharp lightning rod effect, the presence of atomic-level protrusion at the tip apex can provide a strong and highly localized plasmonic field. We find that the added NaCl layer not only blocks charge transfer between molecules and the substrate but also effectively prevents fluorescence quenching. The maximum fluorescence and Raman enhancement factors near the tip apex approach up to 5 and 12 orders of magnitude, respectively, achieving sub-nanometer spatial resolution. Finally, compared to vertical electric field and dipole, sharp protrusion can excite a stronger horizontal electric field and couple with horizontal dipoles to yield a higher quantum yield, resulting in a three orders of magnitude higher in the fluorescence enhancement of horizontal dipole compared to the tip without protrusion. Our theoretical research not only confirms the experimental results of spectral enhancement with the proximity of the tip to the molecule in tip-enhanced photoluminescence, but also provides effective guidance for a deeper understanding of the mechanism of tip-enhanced fluorescence and Raman spectroscopy, as well as optimizing the experimental system.

A strategy for boosting photovoltaic performance based on a two-dimensional ZrSSe/HfSSe van der Waals heterostructure.

Identifying high-efficiency solar photovoltaic systems with two-dimensional (2D) materials is still an urgent challenge to meet modern energy requirements. Very recently, a 2D heterostructure with type-II band alignment has been confirmed to be more favorable for application in photoelectric conversion. However, the staggered band offset of 2D type-II heterostructures cannot always be guaranteed, nor the intrinsic hindrance mechanism of carrier recombination being clear. In this study, taking the emerging ZrSSe/HfSSe van der Waals heterostructure (vdWH) as a generic example, a boosting strategy for improving the photoelectric performances of 2D vdWHs is proposed. Through a series of in-depth systematic research studies based on first-principles, we demonstrate that via applying a vertical strain, an anticipated band alignment transition from type-I to favorable type-II of this ZrSSe/HfSSe vdWH can be induced due to the interfacial charge redistribution, during which a corresponding enlarged photocurrent can be detected from the latter based device compared to the former. Essentially, such enhanced photocurrent at the incident photon energy (Eph) around the band gap is attributed to the suppressed recombination rate of photoexcited carriers. Moreover, when Eph is increased into the visible light region, the photoelectric conversion performances can be further controlled by vertical strain. These generalized findings not only provide an effective manipulation strategy for enhancing the performances of 2D solar photovoltaic systems, but the intrinsic physical mechanism can also be extended to the next practical design and regulation of other 2D photovoltaic devices.

Vertical MSM-type CsPbBr<sub>3</sub> thin film photodetectors with fast response speed and low dark current

Halide perovskites exhibit excellent electrical and optical properties and are ideal candidates for active layers in optoelectronic devices, especially in the field of high-performance photodetection, where they demonstrate a competitive advantage in terms of development prospects. Among them, the all-inorganic perovskite CsPbBr<sub>3</sub> has received widespread attention due to its better environmental stability. It is demonstrated in this work that a vertical MSM-type CsPbBr<sub>3</sub> thin-film photodetector has characteristics of fast response time and ultra-low dark current. The use of a vertical structure can reduce the transit distance of photo carriers, enabling the device to achieve a fast response time of 63 μs, which is two orders of magnitude higher than the traditional planar MSM-type photodetectors with a response time of 10 ms. Then, by spinning a charge transport layer between the p-type CsPbBr<sub>3</sub> and Ag electrodes, effective separation of photocarriers at the interface is realized and physical passivation between the perovskite and metal electrodes is also achieved. Due to the superior surface quality of the spun TiO<sub>2</sub> film compared with the NiO<sub><i>x</i></sub> film, and through Sentaurus TCAD simulations and bandgap analyses, with TiO<sub>2</sub> serving as the electron transport layer, it effectively inhibits the transmission of excess holes in p-type CsPbBr<sub>3</sub>. Consequently, the electron transport layer TiO<sub>2</sub> is more effective in reducing dark current than the hole transport layer NiO<sub><i>x</i></sub>, with a dark current magnitude of only –4.81×10<sup>–12</sup> A at a –1 V bias. Furthermore, this vertical MSM-type CsPbBr<sub>3</sub> thin-film photodetector also has a large linear dynamic range (122 dB), high detectivity (1.16×10<sup>12</sup> Jones), and good photo-stability. Through Sentaurus TCAD simulation, it is found that the charge transport layer selectively blocks carrier transmission, thereby reducing dark current. The simulation results are in good agreement with experimental data, providing theoretical guidance for a more in-depth understanding of the intrinsic physical mechanisms.

Origin of thermal deformation induced crystallization and microstructure formation in additive manufactured FCC, BCC, HCP metals and its alloys

The additive manufacturing (AM) technology has received the widespread attention in the industrial application because of its unique ability to achieve the complex shape and ideal performance. Unfortunately, the impact of the material natural characteristics, such as the crystal structure and alloying, on the microstructure formation at the nanoscale has never been revealed in AM metals. Here, we use large-scale molecular-dynamics simulations to study the rapid thermal deformation processes of additively-manufactured face-centered-cubic (FCC), body-centered-cubic (BCC), hexagonal close-packed (HCP) metals and their alloys (FCC Cu, CuAl0.1, BCC Fe, FeAl0.1, and HCP Mg, MgAl0.1) in the molten pool, in an attempt to elucidate the intrinsic physical mechanisms controlling the crystal nucleation and growth under the complex thermal stress. The results show that the arcuate solid-liquid interface migrates towards the liquid phase until the complete crystallization, in good agreement with the experimental observation. The solidification rate follows a unified slow-to-fast law in the three types of metals, due to the competition between the compressive thermal stress and fast atomic motion. It is noteworthy that the critical radius of the crystal nucleus in the FCC and BCC metals is smaller than that in the HCP metal. The FCC metal exhibits the weak nucleation ability and growth rate, and yet the HCP metal shows the fast growth rate and stable columnar crystal nucleation. Especially, due to the dual regulation of the steep thermal gradient and solute redistribution on the growth driving force, the alloying increases the constitutive supercooling of the solidification front and inhibits the growth of columnar crystals. This trend is particularly prominent in FCC alloys, which is manifested by the obvious amorphous phase and discrete nucleation point in the upper part of the melt pool. The nucleation barrier free energy descends in order of FCC, BCC, and HCP pure metals, but the alloying would increase this energy in the corresponding alloys. The current work gives an insight into the atomic mechanism of the nucleation and growth in AM FCC, BCC, and HCP metals.

How fast can vanadium dioxide neuron-mimicking devices oscillate? Physical mechanisms limiting the frequency of vanadium dioxide oscillators

The frequency of vanadium dioxide (VO2) oscillators is a fundamental figure of merit for the realization of neuromorphic circuits called oscillatory neural networks (ONNs) since the high frequency of oscillators ensures low-power consuming, real-time computing ONNs. In this study, we perform electrothermal 3D technology computer-aided design (TCAD) simulations of a VO2 relaxation oscillator. We find that there exists an upper limit to its operating frequency, where such a limit is not predicted from a purely circuital model of the VO2 oscillator. We investigate the intrinsic physical mechanisms that give rise to this upper limit. Our TCAD simulations show that it arises a dependence on the frequency of the points of the curve current versus voltage across the VO2 device corresponding to the insulator-to-metal transition (IMT) and metal-to-insulator transition (MIT) during oscillation, below some threshold values of . This implies that the condition for the self-oscillatory regime may be satisfied by a given load-line in the low-frequency range but no longer at higher frequencies, with consequent suppression of oscillations. We note that this variation of the IMT/MIT points below some threshold values of is due to a combination of different factors: intermediate resistive states achievable by VO2 channel and the interplay between frequency and heat transfer rate. Although the upper limit on the frequency that we extract is linked to the specific VO2 device we simulate, our findings apply qualitatively to any VO2 oscillator. Overall, our study elucidates the link between electrical and thermal behavior in VO2 devices that sets a constraint on the upper values of the operating frequency of any VO2 oscillator.

Enhanced resolution and sensitivity acoustic-resolution photoacoustic microscopy with semi/unsupervised GANs

Acoustic-resolution photoacoustic microscopy (AR-PAM) enables visualization of biological tissues at depths of several millimeters with superior optical absorption contrast. However, the lateral resolution and sensitivity of AR-PAM are generally lower than those of optical-resolution PAM (OR-PAM) owing to the intrinsic physical acoustic focusing mechanism. Here, we demonstrate a computational strategy with two generative adversarial networks (GANs) to perform semi/unsupervised reconstruction with high resolution and sensitivity in AR-PAM by maintaining its imaging capability at enhanced depths. The b-scan PAM images were prepared as paired (for semi-supervised conditional GAN) and unpaired (for unsupervised CycleGAN) groups for label-free reconstructed AR-PAM b-scan image generation and training. The semi/unsupervised GANs successfully improved resolution and sensitivity in a phantom and in vivo mouse ear test with ground truth. We also confirmed that GANs could enhance resolution and sensitivity of deep tissues without the ground truth.

Optically transparent conformal ultra-broadband metamaterial absorber based on ITO conductive film

An optically transparent metamaterial absorber with polarization insensitivity and wide-angle absorption is proposed. The absorber, which consists of an indium tin oxide resistive film and a low-loss substrate, is optically transparent and conformal. By tuning the reflection response of the frequency-selective surface and the thickness of the spacer layer, the whole structure achieves an absorption of more than 97% in the range of 6.54–18.66 GHz. Numerical simulation results show that the absorber can still maintain an absorption rate of more than 85% in a wide range of oblique incidence angles from 0° to 60°. In addition, the intrinsic physical mechanism of the absorber is elucidated using the impedance matching theory and the distribution of surface currents. The ratio of dielectric loss and ohmic loss is also quantitatively analyzed. Finally, the reflection and transmission coefficients of the sample were measured in a microwave anechoic chamber, which showed a good agreement with the simulation results. This design uses a low-resistivity resistive film as a frequency-selective surface, which was rarely involved in previous studies, and provides a new idea for future design and application.

Deep sub-60 mV/dec subthreshold swing independent of gate bias sweep direction in an in situ SiN/Al0.6Ga0.4N/GaN-on-Si metal-insulator high electron mobility transistor

In this Letter, we demonstrated deep sub-60 mV/dec subthreshold swings (SS) independent of gate bias sweep direction in GaN-based metal–insulator–semiconductor high electron mobility transistors (MISHEMTs) with an Al0.6Ga0.4N/GaN heterostructure and in situ SiN as gate dielectric and surface passivation. Average SS values of 22 and 29 mV/dec over 3–4 orders of drain current (ID) swing were measured during forward and reverse sweeps, respectively, in a 75-nm gate length (LG) device. Sub-60 mV/dec SS was also observed in the GaN MISHEMTs with longer LG up to 350 nm. The intrinsic physical mechanisms of deep sub-60 mV/dec SS were comprehensively studied. The observed negative differential resistance in the gate current, the kinks in the output ID-VD curve, and the capacitance spike in the depletion region of the C–V curves suggest that the capture and emission of electrons in the traps are the dominant physical mechanisms responsible for the small SS.

Emergent superconductivity in TaO3 at high pressures.

Tantalum (Ta) is an interesting transition metal that exhibits superconductivity in its elemental states. Additionally, several Ta chalcogenides (S and Se) have also demonstrated superconducting properties. In this work, we propose the existence of five high-pressure metallic Ta-O compounds (e.g., TaO3, TaO2, TaO, Ta2O, and Ta3O), composed of polyhedra centered on Ta/O atoms. These compounds exhibit distinct characteristics compared to the well-known semiconducting Ta2O5. One particularly interesting finding is that TaO3 shows an estimated superconducting transition temperature (Tc) of 3.87 K at 200 GPa. This superconductivity is primarily driven by the coupling between the low-frequency phonons derived from Ta and the O 2p and Ta 5d electrons. Remarkably, its dynamically stabilized pressure can be as low as 50 GPa, resulting in an enhanced electron-phonon coupling and a higher Tc of up to 9.02 K. When compared to the superconductivity of isomorphic TaX3 (X = O, S, and Se) compounds, the highest Tc in TaO3 is associated with the highest NEF and phonon vibrational frequency. These characteristics arise from the strong electronegativity and small atomic mass of the O atom. Consequently, our findings offer valuable insights into the intrinsic physical mechanisms of high-pressure behaviors in Ta-O compounds.

A Deep-Ultraviolet Photodetector of a Hybrid Organic-Inorganic p-CoPc/n-Ga2O3 Heterostructure Highlighting Ultra-Sensitive

A practical method for organic–inorganic hybrid heterojunction photodetector by spin-coating the cobalt phthalocyanine (CoPc) solution onto the β-gallium oxide (β-Ga2O3) film is available to realize the sensitive detection of ultraviolet light signals. We have carefully measured and calculated the optoelectronic performance of the prepared device. The device demonstrates excellent rectification characteristics under different light intensities, and the rectification ratio reaches 144.96 under 900 μW/cm2 at ±5 V. Benefiting from the construction of CoPc/Ga2O3 heterojunction, the device shows an extremely low dark current of 5.73 fA, a high detectivity of 1.92 × 1017 Jones, a responsivity of 18.4 mA/W, and a high light-to-dark current ratio of 3.76 × 106. In addition, the intrinsic physical mechanism of the device is investigated through the energy band diagram under different conditions. The device is equipped with the possibility to work under self-powered mode and has good stability in the air environment.

A deep-ultraviolet photodetector of a β-Ga2O3/CuBiI4 heterojunction highlighting ultra-high sensitivity and responsivity

• A β-Ga 2 O 3 /CuBiI 4 heterojunction deep-ultraviolet photodetector was constructed. • The fabricated device exhibited responsivity of 20.68 A/W and detectivity of 3.72 × 1015 Jones. • The detector could display self-powered operation. Herein, a β-Ga 2 O 3 /CuBiI 4 heterojunction deep-ultraviolet (DUV) photodetector was constructed based on metal-organic chemical vapor deposition grown β-Ga 2 O 3 film and spin-coated CuBiI 4 film. In which, we prepared the pure and well-crystallized CuBiI 4 film from environment-friendly materials under optimized experimental conditions. The fabricated device exhibited a large responsivity of 20.68 A/W, a high specific detectivity of 3.72 × 10 15 Jones, a high photo-to-dark current ratio of 5.62 × 10 6 , a low dark current of 0.385 pA, and an ultrahigh UV/visible rejection ratio ( R 250 nm / R 400 nm ) of 1.14 × 10 4 under 254 nm UV light illumination at 5 V. In all, the device can sensitively monitor a wide range of UV light (254 nm) intensities from 0.1 to 2000 μW/cm 2 . Moreover, the potential of the device operating under self-powered mode has been verified, and the intrinsic physical mechanism of the heterojunction photodetector was analyzed via energy-band alignment. Overall, this work would present a promising route to realize high-performance and cost-effective DUV photodetector.

Current bifurcation, reversals and multiple mobility transitions of dipole in alternating electric fields

Anomalous transports of dipole in alternating electric fields are investigated by means of numerical calculation of its average angular velocity (or current). Our results show that the alternating electric fields can make the dipole exhibit many interesting transport behaviors. There exist current bifurcation and multiple current reversal phenomena about frequency of the alternating electric fields in the system in the absence of constant bias force, while many platforms appear in the curve of its average angular velocity vs. the force, i.e., multiple mobility transitions phenomenon in the presence of the constant force, dependent on frequencies of the alternating electric fields. Further investigation indicates that the multiple mobility transitions are attributed to the traveling forces on the dipole. Intrinsic physical mechanism and conditions for the characteristic dynamical behaviors to occur are also discussed in detail. These findings will possess crucial significance for optimizing heating control in the alternating electric fields.

Estimating the Clear-Sky Longwave Downward Radiation in the Arctic from FengYun-3D MERSI-2 Data

Surface longwave downward radiation (LWDR) plays a key role in determining the Arctic surface energy budget, especially in insolation-absent boreal winter. A reliable LWDR product is essential for understanding the intrinsic physical mechanisms of the rapid changes in the Arctic climate. The Medium-Resolution Spectral Imager (MERSI-2), a major payload of the Chinese second-generation polar-orbiting meteorological satellite, FengYun-3D (FY-3D), was designed similar to the NASA Moderate-Resolution Imaging Spectroradiometer (MODIS) in terms of the spectral bands. Although significant progress has been made in estimating clear-sky LWDR from MODIS observations using a variety of methods, few studies have focused on the retrieval of clear-sky LWDR from FY-3D MERSI-2 observations. In this study, we propose an advanced method to directly estimate the clear-sky LWDR in the Arctic from the FY-3D MERSI-2 thermal infrared (TIR) top-of-atmosphere (TOA) radiances and auxiliary information using the extremely randomized trees (ERT) machine learning algorithm. The retrieval accuracy of RMSE and bias, validated with the Baseline Surface Radiation Network (BSRN) in situ measurements, are 14.14 W/m2 and 4.36 W/m2, respectively, which is comparable and even better than previous studies. The scale effect in retrieval accuracy evaluation was further analyzed and showed that the validating window size could significantly influence the retrieval accuracy of the MERSI-2 clear-sky LWDR dataset. After aggregating to a spatial resolution of 9 km, the RMSE and bias of MERSI-2 retrievals can be reduced to 9.43 W/m2 and −0.14 W/m2, respectively. The retrieval accuracy of MERSI-2 clear-sky LWDR at the CERES SSF FOV spatial scale (approximately 20 km) can be further reduced to 8.64 W/m2, which is much higher than the reported accuracy of the CERES SSF products. This study demonstrates the feasibility of producing LWDR datasets from Chinese FY-3D MERSI-2 observations using machine learning methods.

Multiple sawtooth-shape current reversals of traveling wave system

In this paper, we investigate current reversal (CR) of overdamped traveling wave system subjected to an symmetric periodic driving. Numerical results shown that current takes the form of sawtooth-shape, and its direction can reverse more than ten times with traveling wave velocity, called multiple sawtooth-shape CR here. In virtue of transformation of reference frame, we reveal that occurrence of the sawtooth-shape CR comes from step amplitudes induced by the periodic driving in the new reference frame, and reversal time of current depends on the driving and the spatial potential. Moreover, direction of the current changes with the increment of noise intensity in the rocking system, i.e., noise-induced CR, which has not been reported in previous works on stochastic traveling system, and the intrinsic physical mechanisms responsible for the CR is analyzed in detail.

Simultaneous Seismic Deep Attribute Extraction and Attribute Fusion

Seismic attributes comprise an effective method for oil and gas reservoir characterization and prediction. Hundreds of seismic attributes have been introduced in the last 30 years. Among the seismic attributes targeting different reservoir features, the autoencoder (AE) receives a significant amount of attention, as it extracts deep attributes of seismic data, providing more details of seismic lateral features than other seismic waveform data and seismic attributes. However, data-driven deep attributes bring new challenges to interpretation as they lack the support of intrinsic physical mechanisms. Hence, a shared AE (S-AE) method is proposed in this article, which can extract seismic deep attributes and fuse traditional seismic attributes simultaneously. An S-AE is a revised version of an AE, which consists of an encoder and decoder. An S-AE takes the seismic waveform as the input of the encoder and obtains the deep attribute, and the decoder then transforms the deep attribute to reconstruct the seismic waveforms and attributes. In an S-AE, the network in front of the decoder is shared, while the networks after the decoder consist of independent layers. Such a network structure ensures the effect of reconstruction and associates seismic attributes with the extracted deep attribute, so as to achieve the purpose of attribute fusion and deep attribute extraction. The proposed S-AE method is compared with conventional seismic data fusion methods, such as RGB and principal component analysis, and the superiority of the S-AE is demonstrated in both synthetic and field applications.

Phase field simulation of the void destabilization and splitting processes in interconnects under electromigration induced surface diffusion

Interconnect lines of integrated circuits inevitably exist micro-damage, such as voids, inclusions or cracks. Under the effect of different intrinsic physical mechanisms as well as external driving forces, the micro-damage will have different morphological evolution and even destabilize and split, which can affect the various properties of the interconnect lines. Based on the theory of diffusion interface of microstructure evolution in solid materials, a phase field model is established to simulate the morphological evolution of micro-damage in the interconnect line under electromigration-induced surface diffusion. Unlike the previously published work, the bulk free energy density and the degenerate mobility used in the model are both constructed by quartic double-well potential function. The applicability of the model for the morphological evolution of intracrystalline voids is proved by asymptotic analysis. The governing equation of the phase field method is solved by finite element method. And, the validity of the method is confirmed by the agreement of the numerical solutions with the theoretical solutions of a small circular void. The effects of the relative electric field intensity χ, the line width and the initial aspect ratio β on the morphological evolution of voids are discussed in detail. The results indicate that the intracrystalline voids drift in the direction of the electric field, and there is a destabilization critical value χ cr for the morphological evolution. When χ ⩾ χ cr, there exist two splitting forms after destabilization for circular void, type I and type II, respectively. The value of χ cr decreases as decreases or β increases. The smaller or the larger β is more prone to cause destabilization of the void. The effect of the change of or β on χ cr is more significant as or β is relatively small. In particular, when or β is sufficiently large, there exists upper or lower limit for χ cr, respectively.

Synergistic Effect of Dielectric Property and Energy Transfer on Charge Separation in Non-Fullerene-Based Solar Cells.

In non-fullerene-based photovoltaic devices, it is unclear how excitons efficiently dissociate into charge carriers under small driving force. Here, we developed a modified method to estimate dielectric constants of PM6 donor and non-fullerene acceptors. Surprisingly, most non-fullerene acceptors and blend films showed higher dielectric constants. Moreover, they exhibited larger dielectric constants differences at the optical frequency. These results are likely bound to reduced exciton binding energy and bimolecular recombination. Besides, the overlap between the emission spectrum of donor and absorption spectra of non-fullerene acceptors allowed the energy transfer from donor to acceptors. Hence, based on the synergistic effect of dielectric property and energy transfer resulting in efficient charge separation, our finding paves an alternative path to elucidate the physical working mechanism in non-fullerene-based photovoltaic devices.

Physical mechanism of pulsed laser interaction with fused silica optics during CO2 laser mitigation process

The intrinsic physical mechanism of CO2 laser rapid ablation mitigation micro-defects on fused silica optics surface are studied through experiments and simulations. Based on the multi-physics coupling mathematical model involving the stages of phase transition, melting flow and evaporative ablation, the calculated depth of Gaussian ablation crater and protrusion height at the edge formed by single laser pulse are 5.05 μm and 25 nm respectively, which agrees well with the experiment. Then, the temperature distribution, heat affected zone, solid-liquid transition process and surface morphology evolution under the action of moving pulsed laser are analyzed. Through studies of the relationship between laser parameters and processed surface quality, it can be concluded that the periodic structural features at the bottom of surface morphology are formed by the superposition of residual height of linear laser processing. Laser with short pulse width is more likely to cause material redeposition on the surface of fused silica optics.

Many-particle induced band renormalization processes in few- and mono-layer MoS2

Band renormalization effects play a significant role for two-dimensional (2D) materials in designing a device structure and customizing their optoelectronic performance. However, the intrinsic physical mechanism about the influence of these effects cannot be revealed by general steady-state studies. Here, band renormalization effects in organic superacid treated monolayer MoS2, untreated monolayer MoS2 and few-layer MoS2 are quantitatively analyzed by using broadband femtosecond transient absorption spectroscopy. In comparison with the untreated monolayer, organic superacid treated monolayer MoS2 maintains a direct bandgap structure with two thirds of carriers populated at K valley, even when the initial exciton density is as high as 2.05 × 1014 cm−2 (under 400 nm excitations). While for untreated monolayer and few-layer MoS2, many-particle induced band renormalizations lead to a stronger imbalance for the carrier population between K and Q valleys in k space, and the former experiences a direct-to-indirect bandgap transition when the initial exciton density exceeds 5.0 × 1013 cm−2 (under 400 nm excitations). Those many-particle induced band renormalization processes further suggest a band-structure-controlling method in practical 2D devices.