Photoluminescence Emission Energy Research Articles

Observation of band gap bowing effect vanishing in graded-composition monolayer Mo1−xWxS2 alloy

Over the past decade, tremendous effort has been put into developing 2D semiconductor materials with a tunable bandgap by alloying different individual components. However, the bandgap bowing effect has hindered the ability to arbitrary control the emission of these alloys. In this study, we report the chemical vapor deposition growth of a graded-composition Mo1−xWxS2 monolayer alloy, in which the photoluminescence emission energy exhibits nearly linear variation in the bandgap, indicating the vanishing of the bandgap bowing effect. Polarized Raman measurements show that the polarization is composition dependent, and a large symmetry breaking occurs at the point where the bandgap bowing effect vanishes. This suggests that the vanishing of the bowing effect may be attributed to the symmetry breaking induced by compressive strain. Our findings demonstrate a significant advancement in the synthesis of alloys for future use.

Toward Red Light Emitters Based on InGaN-Containing Short-Period Superlattices with InGaN Buffers

In order to shift the light emission of nitride quantum structures towards the red color, the technological problem of low In incorporation in InGaN−based heterostructures has to be solved. To overcome this problem, we consider superlattices grown on InGaN buffers with different In content. Based on the comparison of the calculated ab initio superlattice band gaps with the photoluminescence emission energies obtained from the measurements on the specially designed samples grown by metal-organic vapor phase epitaxy, it is shown that by changing the superlattice parameters and the composition of the buffer structures, the light emission can be shifted to lower energies by about 167 nm (0.72 eV) in comparison to the case of a similar type of superlattices grown on GaN substrate. The importance of using superlattices to achieve red emission and the critical role of the InGaN buffer are demonstrated.

Photoluminescence wavelength from stacking fault with complicated structure in 4H-SiC epitaxial layer

We investigated the photoluminescence wavelength emitted at room temperature from novel stacking faults with a complicated stacking sequence in the epitaxial layer on p-type 4H-SiC substrate. From analysis of photoluminescence imaging and synchrotron X-ray topography, we consider that these complicated stacking faults originate from p-type substrates. We investigated the relationship between photoluminescence energy and stacking sequence of various stacking faults, and confirmed that the maximum number of layers in complicated stacking faults determines the photoluminescence emission energy. This relationship, which was previously only reported for 3C-type stacking faults, follows simple quantum-well theory. We extended this theory to stacking faults with complex structures, focusing on the maximum number of layers.

Altering the Photoluminescence Emission Quenching and Energy Transfer for the Photocatalytic Behaviour of ZnO Nanostructures

Synthesis of nanoparticles (NPs) is gaining attention as a cost-effective and environmentally acceptable alternative to remove the pollutant by facile photocatalysis process. Role of hydrothermal treatment on Zinc-oxide (ZnO) nanostructures were investigated using non-ionic surfactant diethanolamine (DEA). Further, in order to investigate the effect of DEA on morphological variation different concentration of DEA was used. The samples were thoroughly characterized by XRD, Rietveld analysis, FESEM and TEM to get insight idea about the ZnO structural and morphological properties. Moreover, XPS spectra reveal the variation of surface oxygen defects as hydrothermal treatment induced more defects to ZnO material. BET measurement reveals the alteration of surface area and pore size of ZnO sample. The surface defect-states (mostly oxygen vacancies) of the catalyst nanoparticles can influence the photocatalytic degradation of MB dye activated by ZnO nanoflowers via a non-radiative energy transfer pathway. A steady-state photoluminescence analysis validated the photoinduced electron transport from ZnO to MB dye. Steady state photoluminesence emission spectra established one to one correlation between the defects and colour emission from ZnO. Spectral overlap between donor (ZnO) to acceptor (MB dye) also enhanced greatly after hydrothermal treatment ascribing more Förster resonance energy transfer (FRET) which accelerates photocatalytic degradation efficiency of methylene blue (MB) dye under UV light irradiation. The defect-engineered ZnO nanoparticles synthesized through facile hydrothermal treatment led to an efficient decolourization of MB dye which was strengthened by FRET based on a correlation of photocatalytic degradation and defect mediated colour emission.

Abnormal temperature-dependent photoluminescence characteristics of ReS2 nanowalls

Two samples with [001] orientated rhenium disulfide (ReS2) nanowalls (NWs) grown above and in front of precursor (NH4ReO4) by chemical vapor deposition were investigated. The temperature-dependent photoluminescence (PL) indicated that the PL peak exhibited linear blue-shift at a rate of ∼0.24 meV K−1 with increasing the temperature from 10 to 300 K, while the linewidth monotonically increased due to the exciton–phonon interaction. This abnormal blue-shift of PL emission energy, which is explained by a competition between the band gap shrinkage and the energy level degeneracy with respect to the increase of temperature and lattice constant, enables ReS2 NWs to possess great potential for development of thermal sensors. In addition, exciton localization effect in the ReS2 NWs from abundant edges and weak interlayer interaction was also observed to be related to the height and density of ReS2 NWs. These results not only enrich the understanding for exciton dynamics in ReS2 NWs, but also help to exploit ReS2 NWs for device applications.

Growth-related photoluminescence properties of InSb/GaAs self-assembled quantum dots grown on (001) Ge substrates

Various photoluminescence (PL) spectroscopies are applied to reveal the growth-related properties of self-assembled InSb/GaAs quantum dots (QDs) grown on (001) Ge substrate. Obtained QDs have rectangular-based shape and mainly locate near GaAs antiphase domain boundaries (APBs), which are formed during the growth of GaAs on (001) Ge substrate. The morphology and structural properties of InSb QDs can be varied by the change of controlled growth conditions. Optical phonons scatterings of the embedded InSb QDs are revealed by Raman spectroscopy. The low-temperature PL emission energy of InSb/GaAs QDs is in the range of 1.18–1.29 eV. The power-dependent PL spectroscopy confirms the type-II band alignment in this system. The thermal activation energies (22.7 meV and 146.5 meV) are extracted from the temperature-dependent PL spectroscopy (10–140 K). The polarization-dependent PL spectroscopy is performed and exhibits the polarization degree of ~ 18%, which is attributed to the shape anisotropy of buried InSb QDs.

Band gap tuning in InxGa1-xN/InyGa1-yN short period superlattices

To overcome the technological problem of the low In incorporation in InGaN based heterostructures we consider superlattices of the type: InxGa1-xN/InyGa1-yN (x > y). It is shown that the structures of the proposed type allow for a more precise tuning of the emission energy and shift of the light emission to lower energies by about 400 meV (50–60 nm) compared to the conventional InxGa1-xN/GaN SLs with the same concentration x. The above conclusions were drawn on the basis of comparing the calculated ab-initio band gaps with the photoluminescence emission energies obtained from the measurements performed on the specially designed samples grown by metal-organic vapor phase epitaxy.

Tuning PL emission energy and bandgap with Ni dopant of MgO thin films

In this study, for the first time, the effect of Nickel (Ni) additive on Magnesium oxide (MgO) thin films produced by using successive ionic layer adsorption and reaction technique (SILAR) was investigated. Absorption, photoluminescence (PL), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) measurements were executed to examine how the optical, structural and morphological properties of the samples were affected by the addition of Ni. In the absorption analysis, it was noted that the band gaps of the MgO samples decreased from 4 eV to 3.5 eV with the increase of Ni dopant concentrations. Also, the transmittance values of MgO nanostructures decreases with the increase of Ni contribution, and in the same way, the reflection measurements show that the reflection of MgO decreases with the increase of Ni doping. PL measurements revealed that the fabricated structures radiate around 410 nm and 730 nm. According to XRD measurements, besides the cubic structure of the samples, NiO formations were detected inside the MgO thin film samples due to the increase in Ni dopant. XPS measurements have proven the presence of Ni doping in MgO. SEM measurements showed that all samples exhibited nanowall structure. All these results demonstrate that Ni doping on MgO thin films can be achieved by using SILAR deposition technique.

Strain-mediated unusual bandgap bowing in continuous composition tuned monolayer Mo1−xWxS2 ternary alloys

Band gap engineering via 2D alloying is a vital strategy for three-atom-thick transition metal dichalcogenides based optoelectronics, valleytronics and nanophotonics. Here we demonstrate the growth of Mo1−xWxS2 ternary alloy monolayers and precise compositional tuning for the entire range of x from 0 to 1 using the gas-phase precursor approach. By means of Raman spectroscopy we show that W alloying in MoS2 lattice can lead to a tensile strain of ∼0.8%. The alloying-induced tensile strain plays a key role in observing redshift in optical absorption and photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large as 185 meV. Our optical spectroscopy results indicate three different phase-regions for the Mo1−xWxS2 alloy system. For x < 0.37, the alloys exhibit MoS2-like nature, whereas, WS2-like behavior is observed for x > 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.

Temperature dependence of photoluminescence spectra from a suspended single-walled carbon nanotube with water adsorption layer

On a single-walled carbon nanotube (SWCNT) surface, water forms a peculiar adsorption layer comprising two monolayers, and the physical properties of this adsorption layer remain unclear. We studied the changes that occurred in this water adsorption layer from room temperature down to 140 K using photoluminescence (PL) spectroscopy of suspended SWCNTs. The PL emission energy exhibited complex changes with temperature depending on the chirality of the SWCNTs. These changes were described quantitatively on the basis of changes in the bandgap, the dielectric constant of the adsorption layer, and the strain imposed by the adsorption layer. The results suggested that the adsorption layer might be a two-dimensional disordered solid rather than a liquid.

Interaction of gases with monolayer WS2: an in situ spectroscopy study.

The optical and electronic properties of two-dimensional (2D) materials can be tuned through physical and chemical adsorption of gases. They are also ideal sensor platforms, where charge transfer from the adsorbate can induce a measurable change in the electrical resistance within a device configuration. While 2D materials-based gas sensors exhibit high sensitivity, questions exist regarding the direction of charge transfer and the role of lattice defects during sensing. Here we measured the dynamics of adsorption of NO2 and NH3 on monolayer WS2 using in situ photoluminescence (PL) and resonance Raman spectroscopy. Experiments were conducted across a temperature range of 25-250 °C and gas concentrations between 5-650 ppm. The PL emission energies blue- and red-shifted when exposed to NO2 and NH3, respectively, and the magnitude of the shift depended on the gas concentration as well as the temperature down to the lowest concentration of 5 ppm. Analysis of the adsorption kinetics revealed an exponential increase in the intensities of the trion peaks with temperature, with apparent activation energies similar to barriers for migration of sulfur vacancies in the WS2 lattice. The corresponding Resonance Raman spectra allowed the simultaneous measurement of the defect-induced LA mode. A positive correlation between the defect densities and the shifts in the PL emission energies establish lattice defects such as sulfur vacancies as the preferential sites for gas adsorption. Moreover, an increase in defect densities with temperature in the presence of NO2 and NH3 suggests that these gases may also play a role in the creation of lattice defects. Our study provides key mechanistic insights into gas adsorption on monolayer WS2, and highlights the potential for future development of spectroscopy-based gas sensors based on 2D materials.

Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits

The theoretical maximum efficiency of a solar cell is typically characterized by a detailed balance of optical absorption and emission for a semiconductor in the limit of unity radiative efficiency and an ideal step-function response for the density of states and absorbance at the semiconductor band edges, known as the Shockley-Queisser limit. However, real materials have non-abrupt band edges, which are typically characterized by an exponential distribution of states, known as an Urbach tail. We develop here a modified detailed balance limit of solar cells with imperfect band edges, using optoelectronic reciprocity relations. We find that for semiconductors whose band edges are broader than the thermal energy, kT, there is an effective renormalized bandgap given by the quasi-Fermi level splitting within the solar cell. This renormalized bandgap creates a Stokes shift between the onset of the absorption and photoluminescence emission energies, which significantly reduces the maximum achievable efficiency. The abruptness of the band edge density of states therefore has important implications for the maximum achievable photovoltaic efficiency.

Thickness Identification of Thin InSe by Optical Microscopy Methods

Indium selenide (InSe), as a novel van der Waals‐layered semiconductor, has attracted large research interest, thanks to its excellent optical and electrical properties in the ultrathin limit. Herein, four different optical methods to quantitatively identify the thickness of thin InSe flakes on various substrates, such as SiO2/Si or transparent polymeric substrates, are discussed. In the case of thin InSe deposited on a transparent substrate, the transmittance of the flake in the blue region of the visible spectrum is used to estimate the thickness. For InSe supported by SiO2/Si, the thickness of the flakes is estimated by either assessing their apparent colors or accurately analyzed using a Fresnel law‐based fitting model of the optical contrast spectra. Finally, the thickness dependency of the InSe photoluminescence emission energy is also studied, which provides an additional tool to estimate the InSe thickness, and it works both for InSe deposited on SiO2/Si and on a transparent polymeric substrate.

Carrier Dynamics in Al‐Rich AlGaN/AlN Quantum Well Structures Governed by Carrier Localization

The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic “S”‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed.

Influence of quantum dot morphology on the optical properties of GaSb/GaAs multilayers

We examine the influence of quantum dot (QD) morphology on the optical properties of two-dimensional (2D) GaSb/GaAs multilayers, with and without three-dimensional nanostructures. Using nanostructure sizes from scanning transmission electron microscopy and local Sb compositions from local-electrode atom-probe tomography as input into self-consistent Schrödinger–Poisson simulations based on 8 × 8 k·p theory, we compute confinement energies for QDs, circular arrangements of smaller QDs, termed QD-rings, and 2D layers on GaAs substrates. The computed confinement energies and the measured photoluminescence emission energies increase from QDs to QD-rings to 2D layers, enabling direct association of nanostructure morphologies with the optical properties of the GaSb/GaAs multilayers. This work opens up opportunities for tailoring near to far infrared optoelectronic devices by varying the QD morphology.

High-quality nanodisk of InGaN/GaN MQWs fabricated by neutral-beam-etching and GaN regrowth: towards directional micro-LED in top-down structure

A nanodisk array of blue InGaN/GaN multiple quantum wells was made using neutral beam etching (NBE) followed by GaN regrowth. The NBE-fabricated nanodisk presented a vertical and highly smooth sidewall surface where the InGaN well layers were easily distinguished even with a scanning electron microscope. A high interface quality without any voids or obvious defects was obtained between the nanodisk and the regrown-GaN layer. The nanodisk after regrowth presented a smaller blueshift of photoluminescence emission energy (12 meV) and a substantially higher and almost constant internal quantum efficiency of ∼50% over three orders of magnitude of excitation laser power when compared to the nanodisk before regrowth. This study shows that the process of NBE nanodisk etching followed by GaN regrowth represents a promising step forward in the development of truncated cone-shaped directional micro-LEDs with a buried active region in a top-down structure.

Organic Cation Alloying on Intralayer A and Interlayer A' sites in 2D Hybrid Dion-Jacobson Lead Bromide Perovskites (A')(A)Pb2Br7.

Hybrid layered halide perovskites have achieved impressive performance in optoelectronics. New structural types in the two-dimensional (2D) halide system such as the Dion-Jacobson phases have attracted wide research attention due to the short interlayer distance and unique layer orientation that facilitate better charge-transport and higher stability in optoelectronic devices. Here, we report the first solid solution series incorporating both A and A' cations in the 2D Dion-Jacobson family, with the general formula (A')(A)Pb2Br7 ((A' = 3-(aminomethyl)piperidinium (3AMP) and 4-(aminomethyl)piperidinium) (4AMP); A = methylammonium (MA) and formamidinium (FA)). Mixing the spacing A' cations and perovskitizer A cations generates the new (3AMP)a(4AMP)1-a(FA)b(MA)1-bPb2Br7 perovskites. The crystallographically refined crystal structures using single-crystal X-ray diffraction data reveal that the distortion of the inorganic framework is heavily influenced by the degree of A' and A alloying. A rising fraction of 4AMP in the structure, decreases the Pb-Br-Pb angles, making the framework more distorted. On the contrary, higher FA fractions increase the Pb-Br-Pb angles. This structural evolution fine-tunes the optical properties where the larger the Pb-Br-Pb angle, the narrower the band gap. The photoluminescence emission energy mirrors this trend. Raman spectroscopy reveals a highly dynamical lattice similar to MAPbBr3 and consistent with the local distortion environment of the [Pb2Br7] framework. Density functional theory (DFT) calculations of the electronic structures reveal the same trend as the experimental results where (3AMP)(FA)Pb2Br7 has the smallest band gap while (4AMP)(MA)Pb2Br7 has the largest band gap. The structural effects from solely the organic cations in the 2D system highlight the importance of understanding the high sensitivity of the optoelectronic properties on the structural tuning in this broad class of materials.

Effect of indium accumulation on the growth and properties of ultrathin In(Ga)N/GaN quantum wells

In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitor the growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface accumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly shows that, even at low growth temperatures of ~550 °C, not all of the supplied indium is incorporated into the quantum well (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of the QW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior. The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. The PL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at ~423 nm for >2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to explain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs.

Green synthesis of ZnS quantum dot/biopolymer photoluminescent nanoprobes for bioimaging brain cancer cells

Semiconductor quantum dots (QDs) are one of the most interesting photoluminescent nanomaterials with very promising applications in cancer nanomedicine. In this work, ZnS fluorescent quantum dots (ZnS-QDs) were synthesized and stabilized by carboxymethylcellulose (CMC) as a pH-sensitive biopolymer using a facile one-step green aqueous colloidal process at distinct pH conditions (acidic, neutral and alkaline) and chemical proportions of precursors (Zn2+, S2−). The optical properties of these nanoconjugates (ZnS@CMC) were characterized by UV–visible and photoluminescence spectroscopy. The morphological features and physicochemical properties were evaluated by TEM, FTIR spectroscopy, zeta potential, and dynamic light scattering (DLS) analyses. The cytocompatibility in vitro of ZnS@CMC was assessed by MTT assay using normal and malignant glioma cells. The UV–Vis results indicated that ZnS-QDs were effectively produced with different bandgap energies (from 4.5 to 3.8 eV) blue-shifted from bulk (Ebulk = 3.61 eV), and sizes (typically from 3.3 to 4.5 nm), dependent on the pH and concentration ratio of precursors during the synthesis. Analogously, the changes of synthesis parameters significantly altered the photoluminescence emission energies and intensities within the visible range of spectra (PL maxima from λ = 400–430 nm, at pH = 3.5, [Zn:S] ratio = 1:2). The cell viability results in vitro (>90%) demonstrated no cytotoxicity of ZnS@CMC nanohybrids towards both cell types. Importantly, these ZnS@CMC nanoconjugates behaved as active fluorescent nanoprobes for bioimaging malignant glioma cells proving the high potential for applications in cancer nanomedicine.

Preparation, structure, and properties of a novel chain-like terbium–mercury complex

By means of hydrothermal reactions, a novel terbium–mercury compound, {[Tb(IA)3(H3O)2]2 n [2n(HgCl4)][n(HgCl5)]}·3nH3O·nH2O (1) (HIA = isonicotinc acid) was prepared and structurally characterized by single-crystal X-ray diffraction. It exhibits a one-dimensional (1D) chain-like structure. Solid-state photoluminescence measurement shows that it display green light emission. The emission bands originate from the characteristic emissions of the 4f electrons intrashell transitions of 5 D 4 → 7 FJ (J = 6, 5, 4, and 3) of the terbium(III) ions in 1. The photoluminescence emission energy transfer mechanism is elucidated by the energy level diagrams of the terbium(III) and isonicotinic acid ligand. Complex 1 has CIE chromaticity coordinates of (0.3177, 0.4440). Solid-state UV/Vis diffuse reflectance spectra reveal that it has a wide optical band gap of 3.03 eV.