Dimensional Semiconductor Research Articles

Quantum MEP Hydrodynamical Model for Charge Transport

A well known procedure to get quantum hydrodynamical models for charge transport is to resort to the Wigner equations and deduce the hierarchy of the moment equations as in the semiclassical approach. If one truncates the moment hierarchy to a finite order, the resulting set of balance equations requires some closure assumption because the number of unknowns exceed the number of equations. In the classical and semiclassical kinetic theory a sound approach to get the desired closure relations is that based on the Maximum Entropy Principle (MEP) (Jaynes in Phys Rev 106:620–630, 1957) [see Camiola et al. (Charge transport in low dimensional semiconductor structures, the maximum entropy approach. Springer, Cham, 2020) for charge transport in semiconductors]. In Romano (J Math Phys 48:123504, 2007) a quantum MEP hydrodynamical model has been devised for charge transport in the parabolic band approximation by introducing quantum correction based on the equilibrium Wigner function (Wigner in Phys Rev 40:749–749, 1932). An extension to electron moving in pristine graphene has been obtained in Luca and Romano (in: Atti della Accademia Peloritana dei Pericolanti—Classe di Scienze Fisiche, Matematiche e Naturali, [S.l.], p. A5, 2018, https://doi.org/10.1478/AAPP.96S1A5). Here we present a quantum hydrodynamical model which is valid for a general energy band considering a closure of the moment system deduced by the Wigner equation resorting to a quantum version of MEP. Explicit formulas for quantum correction at order ħ2 are obtained with the aid of the Moyal calculus for silicon and graphene removing the limitation that the quantum corrections are based on the equilibrium Wigner function as in Romano (J Math Phys 48:123504, 2007), Luca and Romano (in: Atti della Accademia Peloritana dei Pericolanti—Classe di Scienze Fisiche, Matematiche e Naturali, [S.l.], p. A5, 2018, https://doi.org/10.1478/AAPP.96S1A5). As an application, quantum correction to the mobilities are deduced.

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

AbstractSolid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC=1.2 %, out of 50 %), low threshold power density (Ith=110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (<500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~6×1010 s−1) interfacial Dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

Hiding in plain sight: The prevalence and impact of trions and Fermi polarons in transient absorption spectroscopy experiments of 2D semiconductors.

Transient absorption (TA) spectroscopy is one of the most popular experimental methods to measure the excited state lifetimes and charge carrier recombination mechanisms in two dimensional (2D) semiconductors. This fundamental information is essential for designing and optimizing the next generation of ultrathin and lightweight 2D semiconductor-based optoelectronic devices. However, the interpretation of TA spectroscopy data varies across the community. The community lacks a unifying physical explanation for how and why experimental variables such as incident light intensity, sample-substrate interactions, and/or applied bias affect TA spectral data. This Perspective (1) compares the physical chemistry TA literature to nanomaterial physics literature from a historical perspective, (2) reviews multiple physical explanations that the TA community developed to explain spectral features and experimental trends, (3) provides a unifying explanation for how and why trions-and, more generally, Fermi polarons-contribute to TA spectra, and (4) quantifies the extent to which various physical interpretations and data analysis procedures yield different timescales and mechanisms for the same set of experimental results. We highlight the importance of considering trions/Fermi polarons in TA measurements and their implications for advancing our understanding of 2D material properties.

Direct coupling of light to valley current

The coupling of circularly polarized light to local band structure extrema ("valleys”) in two dimensional semiconductors promises a new electronics based on the valley degree of freedom. Such pulses, however, couple only to valley charge and not to the valley current, precluding lightwave manipulation of this second vital element of valleytronic devices. Contradicting this established wisdom, we show that the few cycle limit of circularly polarized light is imbued with an emergent vectorial character that allows direct coupling to the valley current. The underlying physical mechanism involves the emergence of a momentum space valley dipole, the orientation and magnitude of which allows complete control over the direction and magnitude of the valley current. We demonstrate this effect via minimal tight-binding models both for the visible spectrum gaps of the transition metal dichalcogenides (generation time ~ 1 fs) as well as the infrared gaps of biased bilayer graphene ( ~ 14 fs); we further verify our findings with state-of-the-art time-dependent density functional theory incorporating transient excitonic effects. Our findings both mark a striking example of emergent physics in the ultrafast limit of light-matter coupling, as well as allowing the creation of valley currents on time scales that challenge quantum decoherence in matter.

First-principles study on the electronic structure and photocatalytic properties of novel two-dimensional Janus CrXCN4 (X = Si, Ge)

Abundant studies demonstrate the significant role of Janus-structured two dimensional semiconductors as photocatalytic materials, highlighting their substantial advantages and importance in photocatalysis. In this work, CrXCN4 (X = Si, Ge) Janus monolayers were constructed based on CrC2N4, and the thermal stability, thermodynamic stability, mechanical stability, electronic properties, and optical properties of the monolayers were systematically investigated. Furthermore, an investigation was conducted to examine the impact of biaxial strain on their electronic and light absorption properties. The findings reveal that both monolayers exhibit direct band gap characteristics, with high absorption coefficients for visible light owing to their appropriate band gaps (1.44 eV for CrSiCN4 and 1.15 eV for CrGeCN4, respectively). At compressive strains exceeding 3%, the CrSiCN4 monolayer demonstrates an optimal band edge position, suggesting its potential as a photocatalyst for overall water splitting. Furthermore, as the compressive strain increases, the absorption spectra have blue-shifted and the absorption coefficient becomes higher, exceeding 2 × 105 cm−1 under a −3% compressive strain. Our study highlights the potential applications of CrXCN4 monolayers in the field of optoelectronic device, particularly emphasizing the promising candidacy of CrSiCN4 as an efficient photocatalyst.

Facet-selective growth of halide perovskite/2D semiconductor van der Waals heterostructures for improved optical gain and lasing

The tunable properties of halide perovskite/two dimensional (2D) semiconductor mixed-dimensional van der Waals heterostructures offer high flexibility for innovating optoelectronic and photonic devices. However, the general and robust growth of high-quality monocrystalline halide perovskite/2D semiconductor heterostructures with attractive optical properties has remained challenging. Here, we demonstrate a universal van der Waals heteroepitaxy strategy to synthesize a library of facet-specific single-crystalline halide perovskite/2D semiconductor (multi)heterostructures. The obtained heterostructures can be broadly tailored by selecting the coupling layer of interest, and can include perovskites varying from all-inorganic to organic-inorganic hybrid counterparts, individual transition metal dichalcogenides or 2D heterojunctions. The CsPbI2Br/WSe2 heterostructures demonstrate ultrahigh optical gain coefficient, reduced gain threshold and prolonged gain lifetime, which are attributed to the reduced energetic disorder. Accordingly, the self-organized halide perovskite/2D semiconductor heterostructure lasers show highly reproducible single-mode lasing with largely reduced lasing threshold and improved stability. Our findings provide a high-quality and versatile material platform for probing unique optoelectronic and photonic physics and developing further electrically driven on-chip lasers, nanophotonic devices and electronic-photonic integrated systems.

First principles calculation of interface interactions and photoelectric properties of ZnSe/SnSe heterostructure.

Heterostructure engineering is an effective technology to improve photo-electronic properties of two dimensional layered semiconductors. In this paper, based on first principles method, we studied the structure, stability, energy band, and optical properties of ZnSe/SnSe heterostructure change with film layer. Results show that all heterostructures are the type-II band arrangement, and the interlayer interaction is characterized by van der Waals. The electron concentration and charge density difference implies the electron (holes) transition from SnSe to monolayer ZnSe. By increasing the layer of SnSe films, the quantum effects are weakened leading to the band gap reduced, and eventually show metal properties. The optical properties also have obvious change, the excellent absorption ability of ZnSe/SnSe heterostructures mainly near the infrared spectroscopy. These works suggest that ZnSe/SnSe heterostructure has significant potential for future optoelectronic applications.

Organic nanowire sensor with seeing, smelling and heat sensation capabilities

Ultra-long high aspect ratio one dimensional (1D) organic semiconductor nanowire is extremely valuable but very challenging to prepare. Based on diacetylene-bridged perylene mono-imide small molecule, scalable 1D nanowire is prepared by simple method, lengths of wires are more than 3 mm and aspect ratios are higher up to ∼9000. Organic flexible conductive wires made devices exhibit good multi-functional sensibilities to light, organic gases and heat. Single wire photodetector shows 80 mA/W responsivity and 3.05×1012 Jones detectivity at 532 nm. Mass-wire sensors demonstrate excellent sensibility, (tetrahydrofuran 0.0166 Ω/ppm; acetone 0.0139 Ω/ppm; ethanol 0.0164 Ω/ppm), low detection limitation and good repeatability to various volatile compounds and sensitive temperature variation response (0.394 mA K−1). The nanowires provide flexible media of sensors for the next generation miniature devices.

Wide-direct-band-gap monolayer carbon nitride CN2: a potential metal-free photocatalyst for overall water splitting.

Two dimensional metal-free semiconductors with high work function have attracted extensive research interest in the field of photocatalytic water splitting. Herein, we have proposed a kind of highly stable monolayer carbon nitride CN2 with an anisotropic structure based on first principles density functional theory. The calculations of electronic structure properties, performed using the HSE06 functional, indicate that monolayer CN2 has a wide direct band gap of 2.836 eV and a high work function of 6.54 eV. And the suitable band edge alignment, high electron mobility (∼103 cm2 V-1 s-1) and visible-light optical absorption suggest that monolayer CN2 has potential on visible-light photocatalytic water splitting at pH ranging from 0 to 14. Moreover, we have observed that uniaxial strain can effectively control the electronic structure properties and optical absorption of monolayer CN2, which can further improve its solar to hydrogen efficiency from 9.6% to 16.02% under 5% uniaxial tension strain along the Y direction. Our calculations have not only proposed a new type of potential metal-free photocatalyst for water splitting but also provided a functional part with high work function for type-I and scheme-Z heterojunction applied in photocatalytic water splitting.

Emergence of promising n-type thermoelectric material through conductive network and strong phonon softening

The synergistic effect of conductive network coupled with strong optical phonon softening and avoided crossing phenomena helps to achieve high thermoelectric performance in a quasi-ternary system.

Electrochemical Intercalation and Exfoliation of CrSBr into Ferromagnetic Fibers and Nanoribbons.

Recent studies dedicated to layered van der Waals crystals have attracted significant attention to magnetic atomically thin crystals offering unprecedented opportunities for applications in innovative magnetoelectric, magneto-optic, and spintronic devices. The active search for original platforms for the low-dimensional magnetism study has emphasized the entirely new magnetic properties oftwo dimensional (2D) semiconductor CrSBr. Herein, for the first time, the electrochemical exfoliation of bulk CrSBr in a non-aqueous environment is demonstrated. Notably, crystal cleavage governed by the structural anisotropy occurred along two directions forming atomically thin and few-layered nanoribbons. The exfoliated material possesses an orthorhombic crystalline structure and strong optical anisotropy, showing the polarization dependencies of Raman signals. The antiferromagnetism exhibited by multilayered CrSBr gives precedence to ferromagnetic ordering in the revealed CrSBr nanostructures. Furthermore, the potential application of CrSBr nanoribbons is pioneered for electrochemical photodetector fabrication and demonstrates its responsivity up to 30µAcm-2 in the visible spectrum. Moreover, the CrSBr-based anode for lithium-ion batteries exhibited high performance and self-improving abilities. This anticipates that the results will pave the way toward the future study of CrSBr and practical applications in magneto- and optoelectronics.

Boosting the photocatalytic activity of β-FeOOH catalyst for toluene oxidation by constructing internal electric field at 0D/1D homojunction interfaces

Photocatalytic degradation is considered as the most energy-efficient, environmentally benign, and effective method for treating low fraction organic contaminants. However, the photocatalysts still suffer from low utilization efficiency of visible-light and severe carrier recombination. Heterojunctions can resolve these two main problems in some extent but still be restrained by the low quality of hetero-interface. In this study, homojunction was constructed of β-FeOOH quantum dots and nanorods with the same lattice by a two-step precipitation method, to avoid the heterointerface with too many defects and possess good charge separation as a consequence. The catalysts were characterized by activity test, electron spin resonance, Mott-Schottky plots, photocurrent density tests and open-circuit potential measurements, etc. The results revealed that a strong internal electric fields (IEFs) was created at the interface of catalyst. Beneficently, the electron rearrangement leads to a more rational distribution of oxygen vacancies in the catalyst, resulting in more efficient dissociation of oxygen molecules and formation of active radicals, thus facilitating the efficient degradation of toluene. This study proposes a novel strategy to boosting the photocatalytic activity of low dimensional semiconductors via forming homojunction interfaces to improve their charge transfer.

Facile solvothermal synthesis of Exfoliated-Corrugated g-C3N4@BiOBr heterojunction for fast visible light Photocatalyst: A structural and optical study

A novel and versatile strategy to increase the photoreactivity (PR) of catalytic systems is to modulate the coupling between two dimensional (2D) semiconductors. Herein, a 2D heterojunction of graphitic carbon nitride (g-C3N4) and bismuth oxobromide (BiOBr), g-C3N4@BiOBr, was carefully synthesized by a solvothermal method at different molar ratios. The optical, morphological, structural, and electronic properties were characterized for the heterojunction and compared with pure 2D BiOBr and g-C3N4. The photocatalytic degradation kinetics of rhodamine B showed that the molar composition (1:2) of g-C3N4@BiOBr produces the largest activity under visible light. Radical scavenger studies showed that the (1:2) heterojunction in an oxygen-saturated environment induces superoxide and hole radicals that actively participate in the photodegradation, while the contribution of hydroxyl radicals is negligible. The optimized PR derives from the perfect coupling between (001) crystalline planes of BiOBr with (002) planes of g-C3N4as evidenced by HRTEM and XRD.To prove the photocatalytic efficiency, tests were carried out on the degradation of different dyes, their reuse cycles, and the degradation capacity of emerging contaminants, particularly for the antibiotic sulfadiazine, obtaining excellent promising results for the development of future commercial applications for the treatment of wastewater.

A universal and stable metasurface for photonic quasi bound state in continuum coupled with two dimensional semiconductors

The strong coupling of excitons to optical cavity modes is of immense importance when understanding the fundamental physics of quantum electrodynamics at the nanoscale as well as for practical applications in quantum information technologies. There have been several attempts at achieving strong coupling between excitons in two-dimensional semiconductors, such as transition metal dichalcogenides (TMDCs) and photonic quasi-bound states in the continuum (BICs). We identify two gaps in the platforms for achieving strong coupling between TMDC excitons and photonic quasi-BICs: firstly, in the studies so far, different cavity architectures have been employed for coupling to different TMDCs. This would mean that typically, the fabrication process flow for the cavities will need to be modified as one moves from one TMDC to the other, which can limit the technological progress in the field. Secondly, there has been no discussion of the impact of fabrication imperfections in the studies on the strong coupling of these subsystems so far. In this work, we address these two questions by optimizing a cavity with the same architecture, which can couple to the four typical TMDCs (MoS2, WS2, MoSe2, WSe2) and perform a detailed investigation on the fabrication tolerance of the associated photonic quasi-BICs and their impact on strong coupling.

Optical Properties of Magnetic Monopole Excitons

Here we consider theoretically an exciton-like dipole formed by a magnetic monopole and a magnetic antimonopole. This type of quasiparticles may be formed in a magnetic counterpart of a one dimensional semiconductor crystal. We use the familiar Lorentz driven damped harmonic oscillator model to find the eigenmodes of magnetic monopole dipoles strongly coupled to light. The proposed model allows predicting optical signatures of magnetic monopole excitons in crystals.

Selective room-temperature dimethylformamide vapor sensing using MoSe2-rGO composite synthesized via facile hydrothermal method

Two dimensional semiconductors such as MoSe2 with large specific surface area, bandgap in visible region and high carrier mobilities have attracted a greater attention as a sensing platform. A composite of MoSe2 and reduced graphene oxide (rGO) has been successfully synthesized using a facile hydrothermal method and used as a gas sensing platform for the room-temperature detection of dimethylformamide (DMF). Structural analysis has revealed the formation of MoSe2/rGO composite along with the presence of MoO3 due to partial oxidation of MoSe2 into MoO3. In addition, the presence of J2 mode (150.6 cm−1) in Raman spectra and 3d doublet with lower spin-orbit splitting in XPS spectra indicated the presence of 1T-MoSe2 in addition to main 2H-MoSe2. Sensor displayed a sensitive and selective detection of DMF down to 2 ppm with response (recovery) time of 130 s (50 s) and a relative response of 3.6 %. Present work highlights the role of optimum weight ratio in determining the overall sensing performance of the sensing devices. The advantages of repeatability, selectivity and excellent stability, provide a great possibility for the use of MoSe2/rGO-based composites in sensing.

Structural and electronic properties of substitutionally doped SiAs monolayer

The substitutional single-atom doping is proven to be a significant method to alter the electronic properties of low dimensional materials. Based on the first-principles density functional theory, we present a systematic study on the structural and electronic properties of single-atom substitutionally doped SiAs monolayers. SiAs monolayer is a semiconductor with an indirect band gap of 1.6 eV. Through substitutional doping, the band gap (calculated at the PBE level) of the monolayer SiAs ranges from 0.032 eV to 1.625 eV. The Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) systems present magnetic ground states with a magnetic moment of 1.00 μB. Moreover, monolayer Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) have long range and local short range magnetic moments owing to the degree of hybridization between the dopant and its adjacent atoms. Metallic behavior can be found in Si31XAs32 (X = N, P, Sb), Si32As31Y (Y = O, S, Se) systems due to the contribution of doped atoms intersecting the Fermi level. The calculated results could provide a new approach for low dimensional doped SiAs-based photoelectronic and magnetic semiconductor devices.

Molecular layer modulation of two-dimensional organic ferroelectric transistors

Ferroelectric transistors hold great potential in low consumption devices. Due to the high film quality and clean system, two dimensional organic semiconductors are widely employed to fabricate high performance organic electronic devices and explore the modulation mechanism of the molecular packing on device performance. Here, we combine the ferroelectric hafnium oxide HfZrO x and two-dimensional molecular crystal 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2b]thiophene (C10-DNTT) with controllable layers to study the molecular layer modulation of ferroelectric organic thin-film transistors (OTFTs). The contact resistance, driving current and transconductance are directly affected by the additional access resistance across the upper molecular layers at the source/drain contact region. Simultaneously, the capacitance of Schottky junction related to the molecular layer thickness could effectively adjust the gate potential acting on the organic channel, further controlling the devices’ subthreshold swing and transconductance efficiency. This work would promote the development of low voltage and high performance OTFTs.

Highly Sensitive Transistor Sensor for Biochemical Sensing and Health Monitoring Applications: A Review

Transistors have gained tremendous attraction in biochemical sensing due to their outstanding performance with high sensitivity, flexibility, selectivity, real-time monitoring, ease of fabrication, and biocompatibility for various point-of-care biomedical and healthcare applications. Herein, critical sensing capability by different transistor classes of i) organic field-effect transistor, ii) organic electrochemical transistor, iii) low dimensional semiconductor field-effect transistor, and iv) hybrid transistors are summarized for biochemical (e.g., ions, glucose, antigen/antibody, DNA, bacteria, virus, cancer biomarkers) detection. Recent advancements in the transistor field for bioanalyte sensing, including a new class of a photoelectrochemical transistor, bioreceptor conjugation, strategies to improve sensitivity and overcoming the limit of detection are discussed in a focused manner. Furthermore, organic and inorganic active material-based transistors are compared to find the best fit for specific target bio-detection and healthcare applications.