Direct Band Gap Nature Research Articles

Synthetic Control of Water-Stable Hybrid Perovskitoid Semiconductors.

Hybrid metal-halide perovskites and their derived materials have emerged as the next-generation semiconductors with a wide range of applications, including photovoltaics, light-emitting devices, and other optoelectronics. Over the past decade, numerous single-crystalline perovskite derivatives have been synthesized and developed. However, the synthetic methods for these derivatives mainly rely on acidic crystallization conditions. This approach leads to crystals comprising metal halide building blocks, which show problematic stability when directly exposed to water. In this study, a methodology is developed for synthesizing hybrid metal-halide compounds using lead iodide and the zwitterionic bifunctional molecule cysteamine (CYS), to form various perovskitoid structures under a broad pH range. Interestingly, the different pH conditions alter the coordination environment of lead halides, leading to lead-sulfide and lead-nitride covalent bond formation. This modification significantly enhances their stability when in direct contact with water, lasting for months. Photoluminescence measurements and first principal density functional theory (DFT) calculations reveal that the perovskitoids synthesized under basic and acidic pH conditions exhibit a direct bandgap nature, while those synthesized under neutral conditions display an indirect bandgap. This approach opens new avenues for manipulating synthetic methods to develop water-stable hybrid semiconductors suitable for a wide range of applications, such as solid-state light emitters.

Indirect-to-direct bandgap transition in GaP semiconductors through quantum shell formation on ZnS nanocrystals

Although GaP, a III-V compound semiconductor, has been extensively utilized in the optoelectronic industry for decades as a traditional material, the inherent indirect bandgap nature of GaP limits its efficiency. Here, we demonstrate an indirect-to-direct bandgap transition of GaP through the formation of quantum shells on the surface of ZnS nanocrystals. The ZnS/GaP quantum shell with a reverse-type I heterojunction, consisting of a monolayer-thin GaP shell grown atop a ZnS core, exhibits a record-high photoluminescence quantum yield of 45.4% in the violet emission range (wavelength = 409 nm), validating its direct bandgap nature. Density functional theory calculations further reveal that ZnS nanocrystals, as the growth platform for GaP quantum shells, play a crucial role in the direct bandgap formation through hybridization of electronic states with GaP. These findings suggest potential for achieving direct bandgaps in compounds that are constrained by their inherent indirect energy gaps, offering a strategy for tailoring energy structures to significantly improve efficiencies in optoelectronics and photovoltaics.

First-principle study of the ground-state crystal structure and optoelectronic response of CsPbX3 (X= Cl, Br, I) for photovoltaic applications

The optoelectronic response and ground state crystal structure of tetragonal (P4/mbm), orthorhombic (Pmnb) and monoclinic (P21/m) phases of CsPbX3 (X = Cl, Br, I) are studied for optoelectronic applications using first principle simulations. Electric field gradient (EFG) and magnetic hyperfine interactions are investigated to explore the charge distributions surrounding the nucleus and local magnetic environment of the atomic nucleus. The orthorhombic phase of these compounds is more stable having lowest ground-state energy compared to tetragonal and monoclinic phases. The understudied halide perovskites have direct band gap nature and spin orbit coupling (SOC) effect splits Pb-p and X-p orbitals at the edges of conduction and valence band (CB and VB) respectively. At the edge of VB, the strongly hybridized X-p and Pb-p orbital and direct band gap nature of CsPbX3 is excellent for the light absorption and photovoltaic (PV) applications. The n-i-p configuration (FTO/ETL/CsPbX3/HTL/Au) of the simulated structure shows that CsPbI3 has improved power conversion efficiency (PCE) of 22.48 % and 22.32 % and quantum efficiency (QE) about ∼80 % in tetragonal and monoclinic phases respectively. The Pb and Cs values of EFG tensor in z-direction (Vzz) are close to each other, which show that the distribution of electronic charge density is similar near these nuclei. Moreover, Cs-s orbital has negligible contribution and Pb-p and I-p orbitals have higher contribution in the hyperfine field (HFF). The negative HFF indicates the opposite HFF contribution.

Numerical simulation of all inorganic CsPbIBr2 perovskite solar cells with diverse charge transport layers using DFT and SCAPS-1D frameworks

All inorganic CsPbX3 perovskites (X = Br and I) are excellent candidates for stable and efficient perovskite solar cells (PSCs). Among them, CsPbIBr2 demonstrated the most balanced characteristics in terms of band gap and stability. Nevertheless, the power conversion efficiency (PCE) of CsPbIBr2-based solar cells is still far from that of Hybrid PSCs, and more research is required in this aspect. Herein, DFT and SCAPS-1D frameworks are employed to explore the optimized device configurations of CsPbIBr2 PSCs. DFT is used to explore the structural and optoelectronic characteristics of CsPbIBr2, while SCAPS-1D is employed to examine various device structures of CsPbIBr2-based PSCs. The band structure demonstrated the direct band gap nature of CsPbIBr2 with a band gap of 2.12 eV. Moreover, we have used TiO2, SnO2, ZnO, WS2, IGZO, CeO2, In2S3, and CdS as ETLs, and Cu2O, CuI, MoO3, NiO, CuSCN, CuSbS2, CBTS, CFTS, and CuO as HTLs for identifying the best ETL/CsPbIBr2/HTL configurations. Among 72 device combinations, eight sets of PSCs are identified as the most efficient configurations. In addition, the influence of various parameters like the thickness of various layers, doping concentration, perovskite defect density, ETLs and interfaces, series resistances, shunt resistances, and temperature on device performance have been comprehensively studied. The results demonstrate Cu2O as the best HTL for CsPbIBr2 with each ETL, and PSC with device structure ITO/WS2/CsPbIBr2/Cu2O/C exhibited the highest PCE of 16.53%. This comprehensive investigation will provide new path for the development of highly efficient all-inorganic CsPbIBr2 solar cells.

An efficient way to grow large size single crystal of imidazole by modified Czochralski technique and study of its optical and mechanical properties

Large-sized single crystal of imidazole was successfully grown for the first time by cost-effective modified Czochralski technique for lower melting point materials. Its monoclinic crystal system and cell parameters a = 7.7251 Å, b = 5.4450 Å, c = 9.2599 Å, and β = 110.556° were determined through single crystal and powder XRD analysis. Hisrshfeld surface analysis and 2D fingerprint plot of imidazole revealed that H···H, C···H/H···C and H···N/N···H has 49.7 %, 18.7 % and 28.9 % of overall intermolecular interactions present in the crystal. Direct band gap nature, high transmittance and lower absorption cutoff at 236 nm was found in UV–Vis spectroscopy. Thermal studies of the crystal were carried out using TGA and DSC analysis and sharp endothermic peak showed high crystallinity and thermal stability up to its melting point. Fundamental functional groups of the crystals were identified by FTIR technique. Mechanical strength of imidazole single crystal was analyzed by Vickers microhardness at different loads, and it was observed that after 20 g load crystals showed indentation size effect.

A DFT study to explore structural, elastic, mechanical, electronic and optical properties of inverse perovskite SbPMg3 for solar cell applications

This study presents the structural, mechanical and optoelectronic properties of inverse perovskite SbPMg3 for optoelectronic applications. The structural optimization is carried out by employing Generalized Gradient Approximation of Wu–Cohen, Perdew–Burke–Ernzerhof and PBE revised for solids. The optimized lattice constant and bulk modulus of SbPMg3 is 4.85 Å and 54.20 GPa with WC-GGA scheme. The positive phonon frequencies are indicating that SbPMg3 is dynamically stable. The mechanical properties were calculated by obtaining elastic constants C11, C12 and C44. The structural stability is evaluated according to Born–Huang stability criteria. The band gap, static dielectric constant and static refractive index is 1.70 eV, 8.23 and 2.87 calculated with WC-mBJ exchange and correlation functional. The band structures show that inverse perovskite SbPMg3 possesses direct band gap nature. The absorption peaks in visible and ultraviolet energy region anticipates that SbPMg3 is promising novel material for optoelectronic applications.

Investigation of structural, morphological, optical and electronic properties of Cu-doped PbS thin films: a comparative experimental and theoretical study

Thin film technology has emerged as a cornerstone in optoelectronics, enabling the fabrication of compact, lightweight devices with enhanced performance and efficiency through precise control of the nanoscale thicknesses of functional materials. The current study explores the impact of copper (Cu) doping (3.125%, 6.25%, and 12.5%) on lead (Pb) sites in PbS to examine the structural, morphological, electronic, optical, and thermoelectric characteristics, employing both experimental and theoretical approaches. Polycrystalline thin films of PbS are deposited by spin coating technique on glass substrates. The XRD study discloses the cubic crystal structure of pristine and Cu-doped PbS with nominal variation in d-spacing. Surface morphological investigations reveal that Cu-doping transforms the coffee beans like grains to nanoplates that significantly affect the surface homogeneity and porosity. The tuning of band structure in the visible range, 1.64–2.21 eV is witnessed in the band structure analysis. Moreover, the experimental results are complemented by a theoretical study using WIEN2k software. Theoretical study exhibits the direct bandgap nature and with the incorporation of Cu, it increases from 0.89 to 2.11 eV. The density of states spectra for Cu-doped PbS exhibits strong hybridization between p-states of Pb and S, and d-states of Cu. Optical findings demonstrate significant variations in the absorption spectrum, which result in modifications in the optical energy band gap and peculiar optical parameters of doped samples. At room temperature, the increase in electrical conductivity (σ/τ) from 0.2 × 1020 (Ω.m.s)−1 for PbS to 0.3 × 1020, 3.1 × 1020 and 7.8 × 1020 (Ω.m.s)−1, thermal conductivity from 0.25 × 1014 W m.K.s−1 to 0.30 × 1014, 2.4 × 1014 and 5.2 × 1014 W m.K.s−1 and decrease in Seebeck coefficient from 72 to 35, 13 and 8 μV/K with the inclusion of Cu up to 3.125, 6.25 and 12.5% offer the potential for advancing thermoelectric technology. This could lead to improved efficiency and practical utilization in energy harvesting and waste heat recovery.

Ab initio Investigation of quasi-one-dimensional ternary chalcogenides Sr2ZnX3 (X = S, Se, Te) for efficient photovoltaic and thermoelectric applications

The quest for sustainable and renewable energy sources has become a vital global mission. Transition metal chalcogenide materials have garnered significant attention due to their unique low-dimensional physical properties. In this work, we explore the impact of ionic radius in modulating the physical properties of ternary chalcogenides by calculating structural, electronic, optical, and thermoelectric properties of Sr2ZnX3 (X = S, Se, Te) materials using first-principles density functional theory (DFT) as implemented in the WIEN2k program. To evaluate the effect of ionic radius, two novel semiconductors Sr2ZnSe3 and Sr2ZnTe3, were constructed using similar isostructural material Sr2ZnS3. Further optimized structure reveals that the corner-sharing ZnX4 tetrahedra extended along the b-axis confirms the quasi-one-dimensional (Q1D) nature of Sr2ZnX3 materials. Band structure analysis revealed a wide direct bandgap nature with the values of Sr2ZnS3 (3.4 eV), Sr2ZnSe3 (2.8 eV) and Sr2ZnTe3 (1.75 eV), respectively. The lower effective mass of electrons and holes suggests higher carrier mobility and charge recombination rates, vital for photovoltaic applications. The optical properties of the materials are studied in the energy range of 0–10 eV (IR–Vis–UV region). We found that Sr2ZnTe3 material exhibits higher values of dielectric function, absorption coefficient (α≈106 cm−1), refractive index, and lower reflectivity, which indicates its suitability for photovoltaic application. Thermoelectric properties are crucial for assessing material utility in thermoelectric applications, and the study revealed substantial values of the Seebeck coefficient, electrical conductivity, figure of merit (ZT) greater than 2, power factor, and lower electronic thermal conductivity. The studied properties signify Sr2ZnX3 materials as potential candidates for light-emitting semiconductors and thermoelectric applications.

Comparative study of sodium and potassium compounds as promoters for growth of monolayer MoS2 with high crystal quality on SiO2/Si substrate

Monolayer MoS2 is promising candidate for fabrication of optoelectronic devices due to its direct bandgap nature and high carrier mobility. Alkali metal compounds have been demonstrated to be helpful promoters for the growth of large single crystal monolayer MoS2 on SiO2/Si substrate. However, the catalytic mechanism of alkali metal compounds is still under debate. Herein, we compared the surface morphology, optical properties, and electrical properties of monolayer MoS2 flakes grown on SiO2/Si substrate assisted by promoters containing potassium or sodium cations and halogen (chlorine) or non-halogen (hydroxide) anions, i.e. NaCl, NaOH, KCl and KOH. Based on the analysis of existing growth mechanism, we proposed that the alkali metal cation, plays a dominant role in promoting the lateral growth of monolayer MoS2 and obtaining high crystal quality. Furthermore, potassium has a greater promoting effect than sodium. By optimizing growth conditions, monolayer triangular MoS2 flakes with large lateral size over 160 μm were grown assisted by KCl promoter. Raman and PL spectra verified excellent crystal quality of the flakes, with typical electron mobilities of 2.98 and 20 cm2 V−1 s−1 for the back-gated filed effect transistors fabricated on as-grown and fresh SiO2/Si substrates, respectively.

First-principles calculation of structural, electronic, optical and thermoelectric properties of Li3TrAs2 (Tr = Al, Ga)

Computational analysis of the materials Li3TrAs2 (Tr = Al, Ga) through first principles DFT approach using WIEN2k code was done to evaluate the feasibility of the materials for several applications. The study comprises structural, electronic, optical and thermoelectric parameters. Structural parameters for the orthorhombic compounds with c/a variation in accordance to ground state energy is in well agreement with experimental data. From the electronic properties, direct bandgap semiconducting behavior of both compounds has been observed. Optical studies such as absorption coefficient and its direct bandgap nature implying the materials capability in utilizing in optoelectronic devices. Higher figure of merit (ZT) at ambient conditions implies the material to be good contender in thermoelectric devices such as waste heat recovery systems at room temperature. Based on the exciton binding energy from excitonic study reveals the weak or Mott-Wannier type exciton within both the material.

Lattice Engineering via Transition Metal Ions for Boosting Photoluminescence Quantum Yields of Lead-Free Layered Double Perovskite Nanocrystals.

Lead-free layered double perovskite nanocrystals (NCs), i.e., Cs4M(II)M(III)2Cl12, have recently attracted increasing attention for potential optoelectronic applications due to their low toxicity, direct bandgap nature, and high structural stability. However, the low photoluminescence quantum yield (PLQY, <1%) or even no observed emissions at room temperature have severely blocked the further development of this type of lead-free halide perovskites. Herein, two new layered perovskites, Cs4CoIn2Cl12 (CCoI) and Cs4ZnIn2Cl12 (CZnI), are successfully synthesized at the nanoscale based on previously reported Cs4CuIn2Cl12 (CCuI) NCs, by tuning the M(II) site with different transition metal ions for lattice tailoring. Benefiting from the formation of more self-trapped excitons (STEs) in the distorted lattices, CCoI and CZnI NCs exhibit significantly strengthened STE emissions toward white light compared to the case of almost non-emissive CCuI NCs, by achieving PLQYs of 4.3% and 11.4% respectively. The theoretical and experimental results hint that CCoI and CZnI NCs possess much lower lattice deformation energies than that of reference CCuI NCs, which are favorable for the recombination of as-formed STEs in a radiative way. This work proposes an effective strategy of lattice engineering to boost the photoluminescent properties of lead-free layered double perovskites for their future warm white light-emitting applications.

Exploring the multifaceted properties of novel oxide-based perovskites ABO3 (A=Nd and B[dbnd]Lr, Y): A DFT study

The full-potential linear augmented plane wave with local orbital (FP-LAPW) technique is now used in this approach to better understand the structural, electronic, optical and mechanical properties of simple cubic oxide perovskite. NdLrO3 and NdYO3 compounds are studied by employing the density functional theory (DFT) with CASTEP code for the first time. The lattice parameters are found 4.2462 and 4.2301 Å for NdLrO3 and NdYO3 respectively. Both the compounds have 1.9 eV and 1.4 eV band gap showing the direct band gap nature of both materials. The exchange-correlation (XC) energy has been selected during this investigation with the Local Density Approximation (LDA) approach. We optimize as well as clarify the elastic constants Cij, the bulk modulus B, elasticity modulus G, Young's modulus of stretch Y, and the Poisson ratio v. Every substance shows anisotropic behavior. NdLrO3 and NdYO3 compounds exhibit a direct band-gap nature, as revealed by the electronic band structure computations. These compounds were all classified as semiconductors. To quantify the number of localized electrons in various bands, partial density of states (PDOS) and total density of states (TDOS) were deployed. Both compounds are computed by fitting the dispersion relation of the imagined component. Optical properties showed that these compounds were excellent absorbers of incident radiation. Therefore, it may be assumed that these compounds could be employed in magnetic sensors because of anisotropic magnetic behavior and to capture the ultraviolet range of solar radiations.

Predictive modeling of novel GaAgX2 (X = S, Se) chalcogenides: First-principles study of electronic, optical, and thermoelectric properties

The most prominent aspects of ternary-type chalcogenides are their tunable optical abilities and outstanding thermoelectric stability. Here the advanced density functional theory is used to examine the complicated interaction of the electronic structure, optical, thermodynamic, and thermoelectric properties of novel GaAgX2 (X = S, Se) chalcogenides. The total energies vs volume of the two materials were computed to formulate ground state characteristics. Both the materials confirmed to have direct band gap nature with an energy gap of 0.8 eV, and 2.2 eV, using TB-mBJ potential, respectively. As compared to GaAgS2, the crystal structure of GaAgSe2 have more covalent bonds that enlarged the covalency and results in wider energy gap of GaAgSe2 as compared to GaAgS2. For the negative value of ε1(ω), these materials display metallic nature, resulting to show a total reflection of the incoming photons. The GaAgS2 displays better absorption than GaAgSe2 materials with higher energy photons. With a few high peaks of refractive index in (2.8–5.5) eV energy range, suggesting these materials appropriate to be used in optical devices. These materials have positive Seebeck coefficients and are confirmed to have a p-type nature. At ambient temperature, GaAgS2 has lower thermal conductivity than GaAgSe2.

Optoelectronic and transport properties of Lead-Free halide based Na2AgTlZ6 (Z = Cl, Br, I) double perovskites for energy harvesting applications

The bandgap engineering and deliberate band edge manipulation of perovskites make them potential candidates for the photovoltaic industry. Here we theoretically investigate the structure, density of states, and photovoltaic and thermoelectric characteristics of Na2AgTlZ6 (Z = Cl, Br, I) using WIEN2k code. The incorporation of Br and I at the Cl site caused the increase in lattice constant which leads to the decrease in bulk modulus and Debye temperature. The computed values of Poisson (<0.26) and Pugh (<1.75) ratio verified their brittle nature. The bandgap analysis reveals direct bandgap nature and bandgap value lie in the visible region for parent composition and infrared and far infrared when Cl is replaced with Br and I. The density of state plots uncovers the shifting of energy states in valence and conduction bands towards the Fermi level with the increasing number of electrons in the halogens. The increase in the value of dielectric constant, refractive index, and shifting of peaks towards lower energy value is witnessed with atomic number of halogen atoms. The viability of these compositions for thermoelectric devices is further confirmed when electrical conductivity and Seebeck coefficient dependent Figure of merit and power factor are evaluated in the temperature range of 200–800 K. In addition, the anisotropic nature of studied compositions is explored through the computation of the modulus of elasticity including Young’s modulus, Shear modulus in addition to compressibility, and Poisson’s ratio.

High photovoltaic performance of lead-free Cs2AgInCl6-xBrx perovskite solar cell using DFT and SCAPS-1D simulations

The search of the lead-free highly efficient photovoltaic material is a key challenge for next generation solar cells. The structural, electronic, and optical properties of lead-free double perovskite Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) were investigated using density functional theory and its photovoltaic performance by using one-dimensional solar cell capacitance simulator SCAPS-1D. By increasing Br content in Cs2AgInCl6, the lattice parameter increases and the electronic band gap decreases without affecting the direct nature of band gap at Г- point. By mixing Br in Cs2AgInCl6 also provides an effective way to tune the energy band gap. In addition, increasing the Br content in mixed halide composition Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) reproduces more dispersive conduction band minima and valence band maxima result in lower effective masses of both charge carriers and consequently higher bipolar carrier mobility. The optical properties of Cs2AgInCl6-xBrx, such as absorption coefficient, reflectivity, dielectric constants, refractive index, and extinction coefficient were investigated with different Br concentrations. Among all compositions, the Cs2AgInClBr5 exhibits better optical properties and behaves as poor reflector and good absorber in the visible region. The photovoltaic performance of different absorber layers of Cs2AgInCl6-xBrx compositions in the perovskites solar cell module with configuration FTO/ETL/Cs2AgInCl6-xBrx /HTL/Au is analysed by using the SCAPS-1D. It is also observed that Cs2AgInClBr5 has high bipolar carrier mobility, relatively higher stability and direct band gap value of 1.75 eV with the PCE of 21.95% in the visible region indicate the suitability of the Cs2AgInClBr5 as a promising lead-free photovoltaic material.

First principles investigation on optoelectronic and thermoelectric response of nitrides cadmium novel zintl compounds AB2C2 (A=Mg, Ca, Sr, Ba; B = Cd and C= N)

Novel materials for high efficiency advance devices are critical to investigate. Zintl compounds are widely explored in the recent years because of their direct narrow band gaps, higher optical conductivity and figure of merit required for optimal-optoelectronic and thermoelectric response respectively. Thus, in the current study, the structural, electronic, optical and thermoelectric properties of nitrides cadmium novel Zintl compounds AB2C2 (A=Mg, Ca, Sr, Ba; B=Cd and C= N) having trigonal structure are studied by full potential linearized augmented plane wave method based on density functional and Boltzmann transport theory. The main objective of this project is to provide novel thermo electrical data on Cd-based Zintl compounds, such as nitride cadmium-based compounds. To get the desire results, the exchange correlations with generalized gradient approximation were used to obtain the ground state structure parameters in these compounds. The computed lattice parameters and c/a -ratio increase from the top to down while the bulk-modulus B(GPa) decreases from top to bottom. The modified versions of the Beck-Johnson potential (TB-mBJ) predict that CaCd2N2, SrCd2N2, BaCd2N2 and MgCd2N2 compounds are semiconductors with narrow band gap of 0.18 eV, 0.21 eV, 0.84 eV and 0.07 eV respectively. Based on the optical properties a deep analysis of dielectric constant, refractive index, reflectivity and optical conductivity were carried. Across the entire temperature range, all the compounds exhibit a positive Seebeck coefficient, indicating P-type conduction. The direct energy band gap nature of these compounds with high conductivity in the visible region shows their suitability for optical applications. Also, the figure of merit of these compounds is close to unity which also speaks about the high efficiency for thermoelectric applications.

First-principles study of electronic, optical, and thermoelectric properties of KMCuS3 (M = Th and Sm) quaternary chalcogenides.

Copper-based quaternary chalcogenides are considered as intriguing material systems in terms of their remarkable optoelectronic and thermoelectric properties. Here we investigated the light interaction and electronic transport properties of novel KMCuS3 (M = Th, Sm) materials. Advanced computations based on density functional theory were used for these calculations. The PBE-GGA scheme predicted band gaps for the KSmCuS3 and KmThCuS3 were 0.61, and 2.03 eV, respectively. While the TB-mBJ computed band gap values for KSmCuS3 and KThCuS3 were 0.91, and 2.39 eV, respectively. A direct band gap nature for both materials was confirmed by identifying the CBM and VBM at the same high symmetry gamma point. The Cu-d, Sm-f, Th-f, and S-p orbitals unified to form the valence band region at the BZ high symmetry point, while the Th-d and Sm-d orbitals formed the conduction band region. Furthermore, linear optical properties such as complex dielectric function components, along with other important optical parameters were computed and explained for possible employment in optoelectronic devices. The considerable thermoelectric characteristics were also predicted, and the incredible outcomes were described, implying that these compounds have potential for thermoelectric applications.

Al-rich AlGaN Alloys: Unique Materials for Deep UV LEDs

Energy bandgap of AlxGa1−xN alloys can be tuned systematically from ∼ 3.4 to 6.1 eV by changing the alloy composition (x) from 0 to 1 and the direct bandgap nature is maintained in the entire range of alloy compositions which make the AlGaN alloys suitable materials for the development of light emitting diodes (LEDs) covering the ultraviolet (UV) spectral region from 210 to 400 nm. For LEDs in the deep UV regions (λ &lt; 300 nm), Al-rich AlGaN alloys of Al content higher than 50% are required. Deep UV LEDs have applications in a wide range of fields including display, disinfection, medical, sensing, and communication. With recent progress in the material growth and electrical conductivity, Al-rich AlGaN alloys have emerged as unique wideband gap materials for the development of deep UV LEDs. In this review article, how the progress of Al-rich AlGaN alloys has made in terms of the material growth and electrical conductivity leading its emergence as deep UV materials have been reviewed. Challenges and prospects of the deep UV LEDs to improve the performance of the devices will also be discussed.

Hexagonal-Ge Nanostructures with Direct-Bandgap Emissions in a Si-Based Light-Emitting Metasurface.

Si-based emitters have been of great interest as an ideal light source for monolithic optical-electronic integrated circuits (MOEICs) on Si substrates. However, the general Si-based material is a diamond structure of cubic lattice with an indirect band gap, which cannot emit light efficiently. Here, hexagonal-Ge (H-Ge) nanostructures within a light-emitting metasurface consisting of a cubic-SiGe nanodisk array are reported. The H-Ge nanostructure is naturally formed within the cubic-Ge epitaxially grown on Si (001) substrates due to the strain-induced nanoscale crystal structure transformation assisted by far-from-equilibrium growth conditions. The direct-bandgap features of H-Ge nanostructures are observed and discussed, including a rather strong and linearly power-dependent photoluminescence (PL) peak around 1562 nm at room temperature and temperature-insensitive PL spectrum near room temperature. Given the direct-bandgap nature, the heterostructure of H-Ge/C-Ge, and the compatibility with the sophisticated Si technology, the H-Ge nanostructure has great potential for innovative light sources and other functional devices, particularly in Si-based MOEICs.

(Invited) Deformation of 2D Semiconductors By Interfaces with Organic Materials

2D semiconducting materials gather attention for realizing atomically thin optoelectronic devices and a platform for emerging quantum phenomena. Especially monolayer 2D materials are fascinating because of their ultrathin crystalline structure with various physicochemical characteristics such as direct-band gap and asymmetric crystalline structures. In this presentation, I will show our recent results related to the deformation of transition metal chalcogenides (TMDCs) morphology by interfaces with organic materials.The mechanical exfoliation method is a simple method to prepare TMDCs and has been widely applied in many research studies. However, the exfoliated TMDCs on substrates show a bunch of bulk flakes and very few target monolayers. In the worst case, the exfoliated monolayer is surrounded by bulk flakes and is difficult to apply for fabricating devices. We developed a method of selectively removing bulk flakes, furthermore, isolating monolayer TMDCs on substrates. The method uses ultrasonication in organic solvents with a consideration of surface chemistry. Surprisingly, bulk flakes are selectively removed, and the adjacent monolayers can be effectively isolated on substrates. [1]Monolayer TMDCs show emission properties because of their direct-band gap nature. The emission strength, e.g., photoluminescence (PL) intensity, is often low. The limitation factor includes various parameters, for example, the over-carrier concentration is one limiting factor in sulfur-based compounds (MoS2 and WS2). So far, p-type doping to reduce the electron concentration is demonstrated to enhance their PL intensity, however, the method is not applicable to selenium-based compounds, such as MoSe2 and WSe2. In addition, a universal method to enhance PL intensity for the selenium-based compounds is still few. We found a general PL enhancement method for both sulfur- and selenium-based compounds, MoS2, WS2, MoSe2, and WSe2, by applying a hydrocarbon, paraffin. The method simply uses paraffin on the surface of TMDCs. I would show the details of the results and methods in the presentation. [2]REF:[1] T. Nakamoto, et al., " Selective isolation of mono to quad layered 2D materials via sonication-based solution engineering", ChemRxiv, 2022, DOI: 10.26434/chemrxiv-2022-lgp8l[2] T. Nakahara, et al., " Spontaneous crystal fluctuation in hydrocarbon polymer–coated monolayer MoS2, MoSe2, WS2, and WSe2 with strong photoluminescence enhancement", ChemRxiv, 2023, DOI: 10.26434/chemrxiv-2023-gxzqw