Pronounced Photoluminescence Research Articles

Programming sp3 Quantum Defects along Carbon Nanotubes with Halogenated DNA.

Atomic defect color centers in solid-state systems hold immense potential to advance various quantum technologies. However, the fabrication of high-quality, densely packed defects presents a significant challenge. Herein we introduce a DNA-programmable photochemical approach for creating organic color-center quantum defects on semiconducting single-walled carbon nanotubes (SWCNTs). Key to this precision defect chemistry is the strategic substitution of thymine with halogenated uracil in DNA strands that are orderly wrapped around the nanotube. Photochemical activation of the reactive uracil initiates the formation of sp3 defects along the nanotube as deep exciton traps, with a pronounced photoluminescence shift from the nanotube band gap emission (by 191 meV for (6,5)-SWCNTs). Furthermore, by altering the DNA spacers, we achieve systematic control over the defect placements along the nanotube. This method, bridging advanced molecular chemistry with quantum materials science, marks a crucial step in crafting quantum defects for critical applications in quantum information science, imaging, and sensing.

Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films

Cuprous oxide (Cu2O) crystals and films of 10–65 nm thickness are investigated via electron diffraction, scanning tunneling microscopy (STM), and photoluminescence (PL) spectroscopy at temperature between 100 to 300 K. While the chemical composition and surface morphology of both systems are identical, large differences are found in the optical response. Bulk Cu2O shows pronounced PL peaks at 620, 730, and 920 nm, compatible with the radiative decay of free and bound excitons, whereas broad and asymmetric peaks at 775 and 850 nm are found for Cu2O films grown on Au(111) and Pt(111) supports. The latter represent the PL signature of VO2+ and VO+ defects, being inserted with substrate‐dependent concentrations due to the oxygen‐poor preparation of the Cu2O films. Despite the strong VO signature in PL, all Cu2O samples show p‐type conductance behavior in STM spectroscopy, indicating an abundance of Cu defects in the lattice. The fact that O vacancies still govern the thin‐film PL is explained by a more efficient recombination via VO‐ than Vcu‐emission channels, as the latter requires exciton formation first. Herein, the high sensitivity of low‐temperature PL to probe the defect landscape of dielectrics, being neither reached by photoelectron spectroscopy nor scanning probe techniques is demonstrated.

Luminescent iron disilicide film growth by metal–organic chemical vapor deposition

Semiconducting iron disilicide (β-FeSi2) films were epitaxially grown on the Ag-layer pre-coated Si(111) substrates with an initial β-FeSi2 layer by the metal–organic chemical vapor deposition method. These β-FeSi2 films had (101)-preferred orientations, and their crystal quality was improved as the growth temperature increased from 893 to 1093 K. The photoluminescence (PL) intensity of the (101)-oriented β-FeSi2 films grown at 973 K was larger than those of β-FeSi2 films at the other deposition temperatures, which indicates the decrease of the density of nonradiative recombination centers in β-FeSi2. A clear A-band emission originated from β-FeSi2 was observed for these films up to 285 K. This pronounced PL intensity enhancement from β-FeSi2 is attributed not only to the crystallinity evaluated by XRD measurement but also to the decrease in density of thermal equilibrium Si vacancy in β-FeSi2.

Synthesis and Characterization of Pt(II) and Pd(II) Complexes with Planar Aromatic Oximes

A series of four Werner-type complexes of Pd(II) and Pt(II) with planar, isomeric conjugated aromatic naphtoquinone oximes were synthesized for the first time. These ligands were 1-oxime-2-naphtoquinone (HL1) and 2-oxime-1-napthoquinone (HL2). Compounds were characterized using thermal analysis, spectroscopic methods, and X-ray analysis. TG/DSC data were collected for pure starting organic ligands, their complexes, and indicated vigorous exothermic decomposition with at ~155 °C for starting HL and ~350 °C for transition metal complexes. Crystal structures for two Pt compounds with 2-oxime-1-quinone were determined and revealed the formation of the cis-geometry complexes and incorporation of molecules of stoichiometric solvents in the lattice: acetonitrile and nitrobenzene. Both solvents of crystallization displayed attractive interactions between their C-H groups and the oxygen atoms of the nitroso groups in complexes, leading to short distances in those fragments. Despite the presence of solvents of inclusion, the overall structure motifs in both compounds represent 1D columnar coordination polymer, in which the PtL2 units are held together via metallophilic interactions, thereby forming ‘Pt-wires’. The Hirshfield surface analysis was performed for both crystallographically characterized complexes. The results showed intermolecular π–π stacking and Pt–Pt interactions among the planar units of both complexes. In addition, the analysis also verified the presence of hydrogen bonding interactions between the platinum unit and solvent molecules. Solid bulk powdery samples of both PtL12 and PtL22 demonstrated pronounced photoluminescence in the near infrared region of spectrum at ~980 nm, being excited in the range of 750–800 nm. The NIR emission was observed only for Pt-complexes and not for pure starting organic ligands or Pd-complexes. Additionally, synthesized Pt-naphtoquinone oximes do not show luminescence in solutions, which suggests the importance of a 1D ‘metal wire’ structure for this process.

Synthesis and structural properties of thin YAG:Ce-=SUP=-3+-=/SUP=- films produced by pulsed laser deposition

Thin YAG:Ce3+ films were synthesized by pulsed laser deposition. For the first time, the dependences of the thickness, phase composition, and photoluminescence of films on the substrate temperature during synthesis were studied. It is shown that, regardless of the synthesis temperature in the range of 100-400oC, the resulting films had a polycrystalline structure. With an increase in the substrate temperature, an increase in the film growth rate was observed. Despite the polycrystalline structure of the films, after annealing at 900oC, YAG:Ce3+ thin films exhibited pronounced photoluminescence, which opens up the possibility of their application as modifying coatings for photoconverters, as well as for X-ray detectors. Keywords: pulsed laser deposition, YAG:Ce3+, functional coatings, x-ray diffraction, luminescence.

Синтез и структурные свойства тонких пленок YAG:Ce-=SUP=-3+-=/SUP=-, полученных методом импульсного лазерного напыления

Thin YAG:Ce3+ films were synthesized by pulsed laser deposition. For the first time, the dependences of the thickness, phase composition, and photoluminescence of films on the substrate temperature during synthesis were studied. It is shown that, regardless of the synthesis temperature in the range of 100-400 °C, the resulting films had a polycrystalline structure. With an increase in the substrate temperature, an increase in the film growth rate was observed. Despite the polycrystalline structure of the films, after annealing at 900 °C, YAG:Ce3+ thin films exhibited pronounced photoluminescence, which opens up the possibility of their application as modifying coatings for photoconverters, as well as for X-ray detectors.

Bi3+/Mn4+ co‐doped dual‐emission phosphors for potential plant lighting

AbstractDeveloping environment‐friendly dual‐emission phosphors of both blue–cyan and deep‐red lights is desirable for the utilized indoor plant lighting research. Notably, the naked 6s and 6p Bi3+ ions are sensitive to the lattice sites, which emit from Ultraviolet (UV) to red lights in various crystal compounds. Meanwhile, the 2E → 4A2g transition of Mn4+ ions promises its deep‐red light emissions, which satisfies the demand for specific wavelength lights for plants growth. Hence, a Bi3+/Mn4+ co‐doped Sr2LaGaO5: Bi3+, Mn4+ (SLGO:Bi3+:Mn4+) phosphor was finally synthesized. The phase, micromorphology and luminescent properties were systematically evaluated. Upon excitation at 350 nm light, dual emissions of both blue–cyan (470 nm) and deep‐red (718 nm) lights were observed. Besides, due to the pronounced photoluminescence (PL) spectral overlap between Bi3+ and Mn4+ ions, a potential energy transfer process from Bi3+ to Mn4+ ions was confirmed. The relative PL intensities between Bi3+ and Mn4+ ions can be tuned just by adjusting the Mn4+ ion concentration. Besides, Li+ co‐doping has been evidenced to improve the deep‐red emissions (718 nm) of SLGO:0.005Mn4+ due to charge compensation and rationally designed lattice distortion, together with the improved thermal stability. Finally, the emissions of SLGO:Bi3+, Mn4+, Li+ phosphor suit properly with the absorption of the four fundamental pigments for plant growth, indicating that the prepared phosphorescent materials may have a prospect in plant light‐emitting diodes lighting.

Morphology transformation of InGaN nanowires grown on Si substrate by PA-MBE

The influence of the growth time on the structural properties of InGaN nanowires grown on Si substrate by plasma-assisted molecular beam epitaxy are studied. Under appropriate other growth conditions, the growth for 2h leads to the formation of separated nanowires, whereas the growth for 2h 30min and 3h leads to the formation of nanostructures such as nano-umbrellas. The separated NWs exhibit a photoluminescence spectrum with maxima at about 590 nm, whereas the nano-umbrellas show two pronounced photoluminescence lines at 421 and 619 nm.

Si-based light emitters synthesized with Ge+ ion bombardment

The photoluminescence (PL) of Ge/Si nanostructures synthesized by using Ge+ ion bombardment is studied. The structure represents a Si substrate with GeSi nanoclusters created by 80 keV Ge implantation with a fluence of ∼1015 ions/cm2 and subsequent thermal annealing. The PL measurements confirm the advantage of Ge/Si structures synthesized using Ge+ ion bombardment over the usual epitaxial structures with GeSi quantum dots. The presence of defects produced by Ge implantation results in pronounced PL at telecom wavelengths up to room temperature. The results provide a basis for creating efficient light emitters compatible with the existing Si technology.

Anomalous Room-Temperature Photoluminescence from Nanostrained MoSe2 Monolayers

Strain-engineering is an effective strategy for manipulating the optical properties of low-dimensional materials. The integration of two-dimensional (2D) transition metal dichalcogenides (TMDs) with other low-dimensional materials can yield nonplanar assemblies that manifest a diverse range of interfaces and strain characteristics. Here we identify anomalous photoluminescence from nonplanar assemblies of 2D MoSe2 monolayers with silicon nanowires (SiNWs). Near-field scanning optical microscopy identifies pronounced photoluminescence (PL) at 1.38 eV that emanates from the nonplanar region between the 2D monolayer and SiNW. Notably, this anomalous emission is distinct (by nearly 200 meV) from the characteristic A exciton emission of monolayer MoSe2. Scanning transmission electron microscopy reveals an apparent straining of the MoSe2 unit cell across a 3–5 nm wide linear region coincident with the nonplanar boundary. The unusual room-temperature PL may be ascribed to a new excitonic state localized within this nanostrained region of the 2D MoSe2 monolayer. This work highlights the importance of nanoscale manipulation of strain in low-dimensional materials to elicit desired control over excitonic and other properties.

Photoluminescence of Fully Inorganic Colloidal Gold Nanocluster and Their Manipulation Using Surface Charge Effects.

Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal-organic ligand-capped counterparts. As their synthesis by chemical methods is challenging, details about their optical properties remain widely unknown. In this work, laser fragmentation in liquids is performed to produce fully inorganic and size-controlled colloidal gold NCs with monomodal particle size distributions and an fcc-like structure. Results reveal that these NCs exhibit highly pronounced photoluminescence with quantum yields of 2%. The emission behavior of small (2-2.5nm) and ultrasmall (<1nm) NCs is significantly different and dominated by either core- or surface-based emission states. It is further verified that emission intensities are a function of the surface charge density, which is easily controllable by the pH of the surrounding medium. This experimentally observed correlation between surface charge and photoluminescence emission intensity is confirmed by density functional theoretical simulations, demonstrating that fully inorganic NCs provide an appropriate material to bridge the gap between experimental and computational studies of NCs. The presented study deepens the understanding of electronic structures in fully inorganic colloidal gold NCs and how to systematically tune their optical properties via surface charge density and particle size.

Deep UV laser induced periodic surface structures on silicon formed by self-organization of nanoparticles

We have investigated the formation of laser-induced periodic surface structures (LIPSS or “ripples”) on silicon upon excitation with p-polarized excimer laser pulses in the deep ultraviolet region (λ = 193 nm, 20 ns). Well-pronounced ripples with a period close to the laser wavelength were observed for pulse numbers N ≥ 100, and the ripple period increased with the angle of incidence. While these results seem to be qualitatively consistent with the standard Sipe-theory, we observed a fundamentally different ripple formation mechanism and ripple morphology. At low pulse numbers, isolated nanoparticles with a size of a few tens of nanometers are observed at the silicon surface, which then start to agglomerate in 2D and self-organize to form ripples with a very shallow modulation depth as the pulse number increases. Employing a recently developed plasmonic model based on the propagation of a surface plasmon polariton on a rough surface, we demonstrate excellent quantitative agreement of the evolution of the ripple period with incidence angle. Finally, we show that surface regions exposed to lower laser fluence feature micro- and nanopores, which give rise to pronounced photoluminescence (PL) emission in the visible spectral region, as opposed to the nanoparticle-based ripples not showing PL.

One-Step Synthesis of TiO2/Graphene Nanocomposites by Laser Pyrolysis with Well-Controlled Properties and Application in Perovskite Solar Cells.

This work presents an original synthesis of TiO2/graphene nanocomposites using laser pyrolysis for the demonstration of efficient and improved perovskite solar cells. This is a one-step and continuous process known for nanoparticle production, and it enables here the elaboration of TiO2 nanoparticles with controlled properties (stoichiometry, morphology, and crystallinity) directly grown on graphene materials. Using this process, a high quality of the TiO2/graphene interface is achieved, leading to an intimate electronic contact between the two materials. This effect is exploited for the photovoltaic application, where TiO2/graphene is used as an electron-extracting layer in n–i–p mesoscopic perovskite solar cells based on the reference CH3NH3PbI3–xClx halide perovskite active layer. A significant and reproducible improvement of power conversion efficiencies under standard illumination is demonstrated, reaching 15.3% in average compared to 13.8% with a pure TiO2 electrode, mainly due to a drastic improvement in fill factor. This beneficial effect of graphene incorporation is revealed through pronounced photoluminescence quenching in the presence of graphene, which indicates better electron injection from the perovskite active layer. Considering that a reduction of device hysteresis is also observed by graphene addition, the laser pyrolysis technique, which is compatible with large-scale industrial developments, is therefore a powerful tool for the production of efficient optoelectronic devices based on a broad range of carbon nano-objects.

Study of the Structural and Emission Properties of Ge(Si) Quantum Dots Ordered on the Si(001) Surface

A method for creation of Ge/Si structures with space-arranged nanoislands by heteroepitaxy on the pre-patterned Si(001) substrates with a square grid of the etched pits is developed. The influence of depth and inter-pit spacing on the nucleation and growth of Ge(Si) nanoislands is studied. It is shown, that the nanoislands are formed either inside pits or at their periphery depending on the pit depth. It is found that the size of the nanoislands grown inside the pits goes up with the increase of the inter-pit distance from 1 to 4 μm. The pronounced photoluminescence signal related with the space-arranged arrays of quantum dots with a period of 1 μm is observed in the range of energies from 0.9 to 1.0 eV.

Исследование структурных и излучательных свойств Ge(Si) квантовых точек, упорядоченных на поверхности Si(001)

AbstractA method for creation of Ge/Si structures with space-arranged nanoislands by heteroepitaxy on the pre-patterned Si(001) substrates with a square grid of the etched pits is developed. The influence of depth and inter-pit spacing on the nucleation and growth of Ge(Si) nanoislands is studied. It is shown, that the nanoislands are formed either inside pits or at their periphery depending on the pit depth. It is found that the size of the nanoislands grown inside the pits goes up with the increase of the inter-pit distance from 1 to 4 μm. The pronounced photoluminescence signal related with the space-arranged arrays of quantum dots with a period of 1 μm is observed in the range of energies from 0.9 to 1.0 eV.

Bulk InAsSb with 0.1 eV bandgap on GaAs

We report on the growth of near-minimum bandgap (0.1 eV) bulk InAs0.54Sb0.46 on GaAs with pronounced photoluminescence. Combining strain-mediating techniques effectively manages the ∼10% lattice mismatch. An interfacial misfit (IMF) dislocation array allows a GaAs substrate and a GaSb buffer layer to act as a direct substitute for a conventional GaSb substrate. We further increase the lattice constant with a linearly graded metamorphic buffer layer of AlGaInSb, upon which we grow an AlInSb virtual substrate with the targeted lattice constant of the active InAsSb region. We observe that the graded buffer not only manages the mismatch between GaSb and InAsSb but also reduces the density of residual threading dislocations from the GaSb/GaAs IMF by a factor of ∼10×. Material characterization demonstrates the viability of using the coupled approach of metamorphic, graded buffers and IMF arrays to manage the large lattice mismatch between the substrate and the mixed group V ternary InAsSb.

Effect of annealing temperature on the optical property of high Cd content CdZnO films

CdZnO films with high Cd contents (59%) have been deposited on quartz substrate by radio-frequency (RF) magnetron sputtering. The as-deposited CdZnO films can hardly show detectable photoluminescence (PL). However, once subjected to suitable annealing temperature, the CdZnO films exhibit pronounced PL. Furthermore, when the annealing temperature at 300 °C and above, that the CdZnO are changed from the single phase of the rs structure to involving w, zb, and rs phases. Consequently, reliable formation and optical property improvement of the CdZnO layers are achieved through annealing temperature at 300 °C.

Probing the origin of excitonic states in monolayer WSe2.

Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications.

Influence of applied current density on the nanostructural and light emitting properties of n-type porous silicon

Effects of current density on nanostructure and light emitting properties of porous silicon (PS) samples were investigated by field emission scanning electron microscope (FE-SEM), gravimetric method, Raman and photoluminescence (PL) spectroscopy. FE-SEM images have shown that below 60 mA/cm2, macropore and mesopore arrays, exhibiting rough morphology, are formed together, whose pore diameter, pore depth and porosity are about 265–760 nm, 58–63 μ m and 44–61%, respectively. However, PS samples prepared above 60 mA/cm2display smooth and straight macropore arrays, with pore diameter ranging from 900–1250 nm, porosity of 61–80% and pore depth between 63–69 μ m . Raman analyses have shown that when the current density is increased from 10 mA/cm2to 100 mA/cm2, Raman peaks of PS samples shift to lower wavenumbers by comparison to crystalline silicon (c-Si). The highest Raman peak shift is found to be 3.2 cm-1for PS sample, prepared at 90 mA/cm2, which has the smallest nanocrystallite size, about 5.2 nm. This sample also shows a pronounced PL, with the highest blue shifting, of about 12 nm. Nanocrystalline silicon, with the smallest nanocrystallite size, confirmed by our Raman analyses using microcrystal model (MCM), should be responsible for both the highest Raman peak shift and PL blue shift due to quantum confinement effect (QCE).

Observation of Emission Enhancement Caused by Symmetric Carrier Depletion in III–V Nanomembrane Heterostructures

Semiconductor nanomembranes are promising systems for many applications, since the band structure of a given material can be tailored to achieve specific configurations, which are not feasible by conventional growth procedures on rigid substrates. Here we show that optically active III–V membranes containing InAs quantum dots exhibit a pronounced photoluminescence enhancement with respect to equivalent systems grown on top of flat substrates. The effect is explained by the formation of carrier depletion regions symmetrically located with respect to the optically active layer. This leads to the filling of excited states of the quantum dots and the overall spectra are enhanced at higher energies. Changes on the strain field that are expected to lead to a red-shift of the quantum dot emission play a reduced role in the final emission spectra in comparison with the depletion effects. These effects can be considered as another degree of freedom and a key ingredient for band engineering of extremely thin semiconductor membranes.