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A bias-selectable photodetector, which can sense the wavelength of interest by tuning the polarity of applied bias, is useful for target discrimination and identification applications. So far, those detectors are generally based on the back-to-back photodiode configuration via exploiting epitaxial semiconductors as optoelectronic materials, which inevitably lead to high fabrication costs and complex device architectures. Here, we demonstrate that our band-engineered van der Waals heterostructures can be applied as bias-selectable photodetectors. Our first prototypical device is mainly composed of black phosphorus (BP) and MoTe

Two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (OIRPPs), which consist of naturally formed "multiple quantum well (MQW)-like" structure, have received considerable interest in optoelectronic applications, owing to their outstanding optical properties and tailorable functionalities. While the quantum-confined electrons and holes at an MQW structure are under an applied electric field, the tilt of the energy bands may cause a significant influence on their optical properties. This work demonstrates the presence of internal built-in electric fields (BIEFs) at the as-synthesized 2D OIRPP single crystals. Spontaneous Franz-Keldysh oscillations, which usually act as the fingerprint to account for the presence of BIEFs in the MQW-like structures, are observed at 2D OIRPPs by the highly sensitive differential technique of modulated thermoreflectance spectroscopy. The strength of BIEFs at 2D OIRPP single crystals reduces with increased

Semiconducting layered group-IV monochalcogenides such as black phosphorous and germanium monosulfide with an anisotropic puckered crystalline structure in each layer have recently attracted much attention due to their unique optical and electronic properties. However, exciton-phonon interactions were only superficially elucidated, although they tremendously affect the opto-electronic operation principles and performance. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. The resonant Raman scattering leads to the observation of dark first- and second-order optical phonon modes whose symmetries and energies are calculated by means of a density functional perturbation theory. We reveal a double-resonance mechanism activating the Raman forbidden (dark) longitudinal-optical scattering processes: For (quasi)-resonantly exciting excitons in the GeS flakes the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our experiments demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multi-order scattering in optical processes, and outline that layered GeS as direct band-gap semiconductor provides a promising material system for opto-electronic applications.

Covalent organic frameworks (COFs) are an emergent class of two-dimensional (2D) crystalline organic materials that exhibit unique electronic, optical, and transport properties. In this study, we employ density functional theory (DFT) and the multiparticle Holstein formalism (MHF) to investigate the electronic structure and two-dimensional coherence of polarons in donor-acceptor COFs as a function of interlayer stacking arrangement. We show that simple modifications in the interlayer stacking arrangement have a profound impact on the transport properties, which can range from metallic behavior with vanishing band gap to highly localized states having completely flat bands. The extent of charge delocalization is found to be sensitive to the type of stacking arrangement and the precise arrangement of the donor and acceptor fragments within the COF structure. The results from the DFT calculations are consistent with MHF-based simulations, demonstrating that stacking-induced interlayer interac- tions facilitate better in-plane charge delocalization. As a consequence, we find that interlayer interactions help circumvent defect-induced trap states to enhance overall charge delocalization. Based on these analyses, we conclude that interlayer stacking can be exploited to guide the design of new 2D layered COF structures with potential applications in organic electronics.

Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. The...

Rhenium disulfide belongs to group VII transition metal dichalcogenides (TMDs) with attractive properties such as exceptionally high refractive index and remarkable oscillator strength, large in-plane birefringence, and good chemical stability. Unlike most other TMDs, the peculiar optical properties of rhenium disulfide persist from bulk to the monolayer, making this material potentially suitable for applications in optical devices. In this work, we demonstrate with unprecedented clarity the strong coupling between cavity modes and excited states, which results in a strong polariton interaction, showing the interest of these materials as a solid-state counterpart of Rydberg atomic systems. Moreover, we definitively clarify the nature of important spectral features, shedding light on some controversial aspects or incomplete interpretations and demonstrating that their origin is due to the interesting combination of the very high refractive index and the large oscillator strength expressed by these TMDs.

Band-edge excitons of few-layer nickel phosphorous trisulfide (NiPS3) are characterized via micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Prominent μTR features of the A exciton series and B are simultaneously detected near the band edge of NiPS3. The A exciton series contains two sharp A1 and A2 levels and one threshold-energy-related transition (direct gap, E∞), which are simultaneously detected at the lower energy side of NiPS3. In addition, one broadened B feature is present at the higher energy side of few-layer NiPS3. The A series excitons may correlate with majorly d-to-d transition in the Rydberg series with threshold energy of E∞ ≅ 1.511 eV at 10 K. The binding energy of A1 is about 36 meV, and the transition energy is A1 ≅ 1.366 eV at 300 K. The transition energy of B measured by μTR is about 1.894 eV at 10 K. The excitonic series A may directly transit from the top of valence band to the conduction band of NiPS3, while the B feature might originate from the spin-split-off valence band to the conduction band edge. The direct optical gap of NiPS3 is ~1.402 eV at 300 K, which is confirmed by μTR and transmittance experiments.

Interface interactions between two distinct layered materials of vertically stacked van der Waals (vdW) heterostructures play a vital role in the modulation of their electrical/optoelectronic prope...

Two-dimensional (2D) van der Waals heterostructure has drawn much attention due to its native built-in barrier benefiting the applications to electronic and optoelectronic devices. There are severa...

SnS and SnSe are renowned energy materials that are applied for photoelectric and thermoelectric conversions owing to their suitable band gap, close to 1 eV, and superior figure of merit (ZT), larg...

Monolayers of transition-metal dichalcogenides with direct band gap located at the binary [Formula: see text] points of the Brillouin zone are promising materials for applications in opto- and spin-electronics due to strongly enhanced Coulomb interactions and specific spin-valley properties. They furthermore represent a unique platform to study electron-electron and electron-phonon interactions in diverse exciton complexes. Here, we demonstrate processes in which the neutral biexciton and two negative trions, namely the spin-triplet and spin-singlet trions, upconvert light into a bright intravalley exciton in an hBN-encapsulated WS[Formula: see text] monolayer. We propose that the energy gains required in the polarized upconversion photoluminescence originate from different interactions including resonant optical phonons, a cooling of resident electrons and a non-local and an anisotropic electron-hole exchange, respectively. The temperature dependence (7-120 K) of the excitonic upconversion intensity obtained at excitation energies corresponding to the biexciton and trions provides insight into an increasing phonon population as well as a thermally enhanced electron scattering. Our study sheds new light on the understanding of excitonic spin and valley properties of van der Waals heterostructures and improves the understanding of photonic upconversion mechanisms in two-dimensional quantum materials.

Modern infrared (IR) microscopy, communication, and sensing systems demand control of the spectral characteristics and polarization states of light. Typically, these systems require the cascading of multiple filters, polarization optics and rotating components to manipulate light, inevitably increasing their sizes and complexities. Here, we report two-terminal mid-infrared (mid-IR) emitters with electrically controllable spectral and polarization properties. Our devices are composed of two back-to-back p-n junctions formed by stacking anisotropic light-emitting materials, black phosphorus and black arsenic-phosphorus with MoS2. By controlling the crystallographic orientations and engineering the band profile of heterostructures, the emissions of two junctions exhibit distinct spectral ranges and polarization directions; more importantly, these two electroluminescence (EL) units can be independently activated, depending on the polarity of the applied bias. Furthermore, we show that when operating our emitter under the polarity-switched pulse mode, its EL exhibits the characteristics of broad spectral coverage, encompassing the entire first mid-IR atmospheric window (λ: 3–5 µm), and electrically tuneable spectral shapes. Our results provide the basis for developing groundbreaking technology in the field of light emitters.

As scaling down the size of metal oxide semiconductor field-effect transistors (FETs), power dissipation has become a major challenge. Lowering down the sub-threshold swing (SS) is known as an effective technique to decrease the operating voltage of FETs and hence lower down the power consumption. However, the Boltzmann distribution of electrons (so-called 'Boltzmann tyranny') implements a physical limit to the SS value. Use of negative capacitance (NC) effect has enabled a new path to achieve a low SS below the Boltzmann limit (60 mV dec

A two-state version of the Schrodinger–Langevin equation is developed to describe dissipative effects on electronic nonadiabatic dynamics. This equation is obtained by including the dissipative potential on each potential surface into the coupled time-dependent Schrodinger equations for a two-state system in the diabatic representation. The hydrodynamic equations of motion for dissipative Bohmian trajectories evolving on each potential surface are derived using the quantum trajectory formulation. Several important properties associated with the two-state Schrodinger–Langevin equation are derived as well. The frictional effects on nonadiabatic transitions are studied for two model systems. Computational results are presented and analyzed, including the evolution of the probability densities, the dynamics of quantum trajectories, and the evolution of population transfer between two surfaces. This study demonstrates that the two-state Schrodinger–Langevin equation provides a phenomenological description for dissipative electronic nonadiabatic dynamics.

Charge transport across molecular junctions can be described by G = Gcontactexp(−βL), envisioned as sequential propagation through electrode-molecule contacts (Gcontact) and the molecular backbone ...

The chemical vapor transport method was used in this research to synthesize MoS2 bulk. Through mechanical exfoliation, we limited the thickness of MoS2 flakes from 1 to 3 μm. In order to fabricate a p–n homogeneous junction, we used oxygen plasma treatment to transform the MoS2 characteristics from n-type to p-type to fabricate a p–n homogenous junction and demonstrate the charge neutrality point shift from −80 to +102 V successfully using FET measurement. The MoS2 p–n homogeneous junction diode showed an excellent p-n characteristic curve during the measurements and performed great rectifying behavior with 1–10 Vpp in the half-wave rectification experiment. This work demonstrated that MoS2 flake had great potential for p-n diodes that feature significant p–n characteristics and rectifying behavior.

High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO2/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped single-layer MoS2 stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS2/hBN heterostructure without an hBN cap we resolve two trion peaks, T1 and T2, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (T S and T T ) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS2 has a dark excitonic ground state, despite having a 'bright' single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of -2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS2 structures, the excitonic g-factor varies from -1.15 to -1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time.

Photoconductivities (PCs) with high responsivity in two-dimensional (2D) diindium triselenide (In

Direct band edge is a crucial factor for a functional chalcogenide to be applied in luminescence devices, photodetectors, and solar-energy devices. In this work, the room-temperature band-edge emission of III–VI Ga2Se3 has been first observed by micro-photoluminescence (μPL) measurement. The emission peak is at 1.85 eV, which matches well with the band-edge transition that is measured by micro-thermoreflectance (μTR) and micro-transmittance (μTransmittance) for verification of the direct band edge of Ga2Se3. The temperature-dependent μTR spectra of Ga2Se3 show a general semiconductor behavior with its temperature-energy shift following Varshni-type variation. With the well-evident direct band edge, the peak responsivities of photovoltaic response (∼6.2 mV/μW) and photocurrent (∼2.25 μA/μW at f = 30 Hz) of defect zincblende Ga2Se3 can be, respectively, detected at ∼2.22 and ∼1.92 eV from a Cu/Ga2Se3 Schottky solar cell and a Ga2Se3 photoconductor. On the basis of experimental analysis, the optical band edge and photoresponsivity properties of a III–VI Ga2Se3 defect semiconductor are thus realized.

Layered Mo1–xCrxSe2 (0 ≤ x ≤ 0.2) and Cr2Se3 single crystals are grown by a chemical vapor transport method using I2 as a transport agent. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses show that the Cr-doped MoSe2 is maintained in a two-layer hexagonal (2H) structure like that of undoped MoSe2, whereas Cr2Se3 is crystallized in the rhombohedral (hexagonal) phase. Lattice constants of the layered Mo1–xCrxSe2 (0 ≤ x ≤ 0.2) series are increased as the Cr content is increased. The direct and indirect bandgaps are also decreased with increasing Cr composition in Mo1–xCrxSe2 (0 ≤ x ≤ 0.2). X-ray photoemission spectroscopy (XPS) revealed the Mo site (valence Mo4+) substituted by Cr in the Mo1–xCrxSe2 (0 < x ≤ 0.2) series mainly causes a Cr6+ valence state and produces n-type conductivity. For Cr2Se3, the main state is Cr3+. Resistivity, Hall, and hot-probe measurements show high p-type conductivity of Cr2Se3. Temperature variations of magnetic susceptibility show that an antiferroma...

Excitonic emission is a key factor for a two-dimensional (2D) semiconductor to be fabricated in light-emitting device for optoelectronics application. Especially when the 2D material has a specific in-plane optical anisotropy, its excitons would become polarized dipole emitters. In this study, the whole series excitonic emissions of few-layer ReS2 and ReSe2 have been detailed characterized using polarized micro-photoluminescence (μPL) measurement. There are totally five excitonic emissions denoted as E1ex, E2ex, E3ex, ES1ex and ES2ex can be detected by the μPL experiment for both the few-layer ReS2 and ReSe2. The angular dependencies of the polarized excitonic emissions with respect to b axis (Re4 diamond-chain direction) are determined. The emission feature E1ex is the band-edge exciton while it shows orthogonal polarization with respect to both E2ex and E3ex in layered ReX2 (X = S, Se). The formation of Re4 diamond chains (DCs) in the 1 T′-phase ReS2 and ReSe2 will also assist the development of in-plane built-in electric field and forming dipoles around the DC regions. The built-in electric field effect greatly enhances the higher ES1ex and ES2ex excitonic features as evident in the photo-modulated reflectance spectra of the layered ReX2 with in-plane electrical field modulation. The binding energy and transition origin of all the excitons are also analyzed and discussed.

We investigated the ternary ReS2(1−x)Se2x alloys and fabricated few-layer ReS1.02Se0.98 nanoflakes to realize passive Q-switch and noise-like pulses operating at 1 μm wavelength. Stable Q-switching pulses can be achieved with a threshold pump power of 180 mW and a maximum pump power of 300 mW. The single pulse energy of 81.47 nJ and pulse width as narrow as 2.2 μs at 1035 nm center wavelength were achieved. By optimizing the parameters of the cavity, self-started noise-like pulses were realized, which are centered at 1031 nm. The average pulse width of the spike was 283 fs on the top of a broad pedestal and the fundamental repetition was 38.98 MHz. Our work demonstrates that ReS1.02Se0.98 has potential applications in optical modulators, and also opens up new application opportunities for these novel materials in nonlinear optics and ultrafast laser photonic technologies.

GaSe1-xSx mixed crystals have recently drawn worldwide attention owing to their wide bandgap, which can be tuned by varying the ratio between Se and S. We have addressed the scarcity of comprehensive studies on its dynamic properties by investigating the photoluminescence (PL) and time-resolved photoluminescence (TRPL) of GaSe1-xSx (x = 0, 0.1, 0.2, 0.5) mixed crystals at different temperatures (14 – 300 K) using 404 nm excitation from a Ti:Sapphire laser. This paper aims to show that the PL spectra and the lifetime of GaSe1-xSx could be used to determine the stacking type and composition of the sample. Three Gaussian peaks were deconvoluted from the obtained PL spectrum and were attributed to excitons from the direct and indirect bandgaps. As the sulfur composition increases, the color of the emission changes and the PL spectra exhibits a blueshift. Selenium-rich samples, which have a e-stacking, have an orange emission while sulfur-rich samples, which have a e-β mixed stacking, have a yellow to green emission. TRPL results reveal that the lifetime becomes longer as the sulfur composition increases. The stacking type influences the separation and transfer of the carriers. Hence, sulfur-rich samples have a significantly longer lifetime than selenium-rich samples due to their β- stacking. As the temperature increases, the lifetime also becomes longer due to the increase of the non-radiative recombination rate, which plays a dominant role in the PL emission. These results are useful for the development of GaSe1-xSx-based optoelectronic devices in the red to the blue visible region.

Two-dimensional (2D) semiconductors play a crucial role in high-efficiency photocatalysts because of their high surface-to-volume ratio. The surface property is a key part of photocatalysis. In this work, the enhanced photocatalytic behavior of the layered ReS2 with optical polarization along the Re4 nano-diamond-chain (DC) direction (b axis) has been demonstrated. The unpolarized photoconductivity (PC) response of ReS2 with an applied bias along the b axis is approximately 1 order higher than that of the applied bias perpendicular to the b axis. The polarization-dependent PC spectra of E ∥ b also reveal a higher photoresponsivity with respect to those measured along the E ⊥ b polarization for the layered ReS2. This result indicates that stronger polarization dipoles as well as a larger amount of photogenerated carriers and surface states can contribute to the C-plane ReS2 under the illumination of E ∥ b polarized photons. With the special axial effect, the layered ReS2 2D photocatalyst shows much faster ...

A wide gap semiconductor material has attracted attention as a heterophotocatalyst because of its light harvesting nature to be used in alternative energy production for the next generation. We, herein, grow and synthesize ZnS(1–x)Ox series compounds using the chemical vapor transport (CVT) method with I2 serving as the transport agent. Different crystals, such as undoped ZnS and oxygen-doped ZnS0.94O0.06 and ZnS0.88O0.12, revealed different bright palette emissions that were presented in photoluminescence spectra in our previous report. To study the electron–hole pair interaction of this sample series, the near-band-edge transitions of the sample series were characterized in detail by photoconductivity (PC) experiments. Additional results from surface photovoltage (SPV) spectra also detected the surface and defect-edge transitions from the higher oxygen-doped ZnS crystals. PC measurement results showed a red-shift in the bandgap with increasing incorporation of oxygen on ZnS. Consequently, the samples were subjected to photoirradiation by xenon lamp for the degradation of methylene blue (MNB) by acting as heterophotocatalysts. Undoped ZnS emerged as the best photocatalyst candidate with the fastest rate constant value of 0.0277 min–1. In cubic {111} ZnS [{111} c-ZnS], the polarized Zn+ → S– ions may play a vital role as a photocatalyst because of their strong electron–hole polarization, which leads to the mechanism for degradation of the MNB solution.

Single crystals of Bi13I2S18 have been grown by chemical vapor transport method. The band-edge properties of Bi13I2S18 are evaluated using temperature-dependent optical absorption and thermal-modulated reflectance (TR) measurements in the temperature range between 30 and 300 K. Single-crystal X-ray diffraction of different orientations showed structural anisotropy and identified the crystalline phase of the as-grown Bi13I2S18 hexagonal micro pillars. According to transmission and TR measurements, the Bi13I2S18 hexagonal rod is confirmed to be an indirect semiconductor that simultaneously possesses indirect (Egind at ∼0.73 eV) and direct (Egd at ∼1.06 eV) optical gaps. The averaged value of direct and indirect gaps is close to 1 eV that may efficiently absorb the sunlight spectrum for solar energy application. Temperature-energy variation and line-width broadening behavior (TR) of the band-edge transition of Bi13I2S18 are also analyzed and discussed herein.

A photodetector using a two-dimensional (2D) low-direct band gap indium selenide (InSe) nanostructure fabricated by the focused ion beam (FIB) technique has been investigated. The FIB-fabricated InSe photodetectors with a low contact resistance exhibit record high responsivity and detectivity to the ultraviolet and visible lights. The optimal responsivity and detectivity up to 1.8 × 107 A W–1 and 1.1 × 1015 Jones, respectively, are much higher than those of the other 2D material-based photoconductors and phototransistors. Moreover, the inherent photoconductivity (PC) quantified by the value of normalized gain has also been discussed and compared. By excluding the contribution of artificial parameters, the InSe nanoflakes exhibit an ultrahigh normalized gain of 3.2 cm2 V–1, which is several orders of magnitude higher than those of MoS2, GaS, and other layer material nanostructures. A high electron mobility at room temperature reaching 450 cm2 V–1 s–1 has been confirmed to be one of the major causes of the ...

Flexible optoelectronics devices play an important role for technological applications of 2D materials because of their bendable, flexible and extended two-dimensional surfaces. In this work, light emission properties of layered gallium selenide (GaSe) crystals with different curvatures have been investigated using bending photoluminescence (BPL) experiments in the curvature range between R−1 = 0.00 m−1 (flat condition) and R−1 = 30.28 m−1. A bendable and rotated sample holder was designed to control the curvature (strain) of the layered sample under upward bending uniformly. The curvature-dependent BPL results clearly show that both bandgaps and BPL intensities of the GaSe are curvature dependent with respect to the bending-radius change. The main emission peak (bandgap) is 2.005 eV for flat GaSe, and is 1.986 eV for the bending GaSe with a curvature of 30.28 m−1 (the maximum bending conditions in this experiment). An obvious redshift (i.e. energy reduction) for the GaSe BPL peak was detected owing to the c-plane lattice expansion by upward bending. The intensities of the corresponding BPL peaks also show an increase with increasing curvature. The correlations between BPL peak intensity, shiny area and bond-angle widening of the bent GaSe under laser excitation have been discussed. The lattice constant versus emission energies of the bending GaSe was also analyzed. An estimated lattice constant vs. bandgap relation was present for further application of the layered GaSe in bendable flexible light-emission devices.

Exciton is a bound state that consisted of one electron and one hole attracted each other by electrostatic Columbic force in materials. Especially in a two-dimensional (2D) semiconductor, an exciton is generated by absorption of a photon with energy larger than band gap and it may survive stably with higher binding energy due to the reduced 2D dimension for weakening dielectric screening effect. Here we report Rydberg-series band-edge excitons of multilayered CuInP2S6 observed by micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Bulk CuInP2S6 is one of the renowned ferroelectric 2D materials with wider piezoelectric hysteresis loop for electrical memory use. The μTR result demonstrated that multi-layered CuInP2S6 of ferroelectric (FE) phase displays prominent band-edge excitonic series that consists of E1 = 2.958 eV (n = 1), E2 = 2.993 eV (n = 2) and E∞ = 3.004 eV (direct gap) at 10 K. The transition amplitude of the E1 feature is approximately 3 to 4 times larger than that of the E2 transition for verification of the intrinsic behavior of Rydberg series starting from n = 1. At 300 K, E2 is ionized and the observed transitions by μTR are E1 = 2.748 eV and E∞ = 2.794 eV, respectively. The spontaneous band-edge excitons observed in CuInP2S6 render it available for application in optoelectronics devices.