Strong Band Edge Research Articles

Atomic-like UV emission generated in hexagonal boron nitride single crystals by thermal annealing

A series of atomic-like photoluminescence (PL) emission peaks in UV region near 4.0 eV were created by thermal annealing hexagonal boron nitride (h-BN) single crystals in air. The pristine h-BN did not have these peaks, emitting strong phonon-assisted band edge PL with peaks at 5.78 and 5.89 eV. After annealing the h-BN crystals in ambient air, a new atomic-like sharp emission in UV region at 4.09 eV with a line width of 0.2 nm appeared along with its phonon replicas at 3.89 and 3.69 eV in the low temperature (8 K) PL measurement. Further testing demonstrated that annealing the h-BN samples in the temperature window of 700–950 °C for 60 min generated the atomic-like emission. The peak position of the emission line is stable with the temperature and PL excitation power. Our study also suggests that the defect responsible for the atomic-like emission resides in the surface region.

Surface crystallization effects on tellurium oxide thin films for low-power complementary logic circuit applications

Van-der-Waals-structured tellurium (IV) has demonstrated its potential as a promising building block for complementary logic device due to its superior electrical and optoelectronic features. Here, we demonstrated the novel approach for developing high-performance tellurium oxide (TeOX) thin film through TeOX formation and surface crystallization (SC). The crystalline structures and chemical bindings of the TeOX surfaces were carefully engineered by time- and temperature-dependent UV-ozone treatment and ALD-grown-Al2O3-assisted crystallization. The huge enhancement of device performance was achieved in the optimized SC-TeOX transistor, especially high on–off current ratio of 2.19 × 106, which can be ascribed to the synergistic effects of TeOX formation and SC process resulting in the enlarged bandgap, strong valence band edge localization, and the improved crystallinity. The performance enhancement mechanism was systematically studied by constructing SC-TeOX transistor with different TeOX region, where the TeOX creation in both Te channel and source/drain electrode regions dramatically improved performance owing to the decreased contact resistance and enhanced charge transport. Consequently, we achieved the high-performance low-power complementary logic circuits with SC-TeOX transistors, even reaching the maximum gain of 67.9 and low static power of 0.5 nW. Thus, this work demonstrates the great potentials of SC-TeOX transistor for developing advanced low-power complementary logic circuits.

Sensitive Thermochromic Behavior of InSeI, a Highly Anisotropic and Tubular 1D van der Waals Crystal.

Thermochromism, the change in color of a material with temperature, is the fundamental basis of optical thermometry. A longstanding challenge in realizing sensitive optical thermometers for widespread use is identifying materials with pronounced thermometric optical performance in the visible range. Herein, it is demonstrated that single crystals of indium selenium iodide (InSeI), a 1D van der Waals (vdW) solid consisting of weakly bound helical chains, exhibit considerable visible range thermochromism. A strong temperature-dependent optical band edge absorption shift ranging from 450 to 530 nm (2.8 to 2.3 eV) over a 380 K temperature range with an experimental (dEg/dT)max value extracted to be 1.26 × 10-3 eV K-1 is shown. This value lies appreciably above most dense conventional semiconductors in the visible range and is comparable to soft lattice solids. The authors further seek to understand the origin of this unusually sensitive thermochromic behavior and find that it arises from strong electron-phonon interactions and anharmonic phonons that significantly broaden band edges and lower the Eg with increasing temperature. The identification of structural signatures resulting in sensitive thermochromism in 1D vdW crystals opens avenues in discovering low-dimensional solids with strong temperature-dependent optical responses across broad spectral windows, dimensionalities, and size regimes.

Simultaneous Growth Strategy of High-Optical-Efficiency GaN NWs on a Wide Range of Substrates by Pulsed Laser Deposition.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

Effect of Co-Doping on the Photoelectric Properties of the Novel Two-Dimensional Material Borophene

Borophene is a novel two-dimensional material with abundant crystal structure and photoelectric properties. We focus on the effect of co-doping on the electronic structure and optical properties of borophene using the first-principles method. The results show that the structure of Al and Ga co-doped borophene is obviously distorted because Al and Ga atoms have formed bonds with a bond length of 2.378 Å, and the two B atoms that bond together with Al and Ga are no longer formed bonds. However, it is also a two-dimensional planar structure after co-doping. After co-doping, the band gap width of the borophene system is narrowed from 1.409 eV to 1.376 eV, and the band gap is narrowed by 0.033 eV. Mulliken population analysis shows an obvious charge transfer between Al-B and Ga-B atoms in the co-doped borophene. The calculation of optical properties shows that the static dielectric constant ε1 (0) increases from 5.08 to 7.01, and ε2 (ω) is larger than that of the undoped sample in the low-energy range. Thus, the co-doping of Al and Ga can enhance the electromagnetic energy storage capacity and the visible light absorption ability. Although the reflectance of borophene is reduced by co-doping (the peak of the reflectivity can be decreased from 71% to 61% at E = 2.94 eV), it still presents metallic reflection characteristics. The static refractive index n0 can be increased from 2.25 to 2.65 by co-doping. The extinction coefficient shows strong band edge absorption at the low-energy range with an absorption edge of 0.85 eV. The light loss is limited to a very narrow energy range of approximately 7.30 eV, which indicates that borophene co-doped with Al and Ga can also be used as a light storage material. The optical conductivity reaches its maximum at E = 1.78 eV and 2.52 eV, which correspond to the light irradiation with a wavelength of 698 nm (red light) and 492 nm (cyan light), respectively. The results show that the Al-Ga co-doped borophene is sensitive to cyan light and red light, so it can be used to make photosensitive devices. The results can hopefully fill the gap in the application of borophene in semiconductor photoelectric devices and provide a theoretical basis for its application.

Synthesis and Characterization of Sol–Gel Screen Printed Zn:CdO (ZCO) Film for Optoelectronic Technologies

In present report on Zn doped (5%) CdO thick film is synthesized on to the glass substrate (pre-cleaned) via sol–gel screen printing process. The synthesized sintered Zn:CdO film are then characterized via XRD, SEM, EDX, UV–vis spectroscopy, PL spectroscopy, and dark conductivity measurement examinations. The XRD examination of sintered film informs the polycrystalline nature and authenticates that Zn:CdO film holds the cubic crystalline structure. SEM examination dispenses the information on morphology of Zn:CdO film. The EDX examination supports the existence of Cd, O, and Zn in the sintered film. The UV–vis spectroscopy via reflectance mode substantiates the direct transition of band gap with a value of 2.60 eV. PL spectra of the sintered film manifest a strong near band edge emission located around 500 nm. The Arrhenius plot (log σDC vs 1000/T) attests the semiconducting nature for the sintered film.

Photoluminescent and photocatalytic behaviour of zinc oxide nanostructures synthesized through the room temperature ultrasonication method.

In the present article we report the synthesis of zinc oxide (ZnO) nanostructures at room temperature using an ultrasonication technique to study their photoluminescent and photocatalytic behaviour. Synthesized nanomaterial showed a strong near band edge ultraviolet (UV) light emission and red emission, thereby finding its use in photoluminescent materials. We developed a UV/ZnO/O2 /H2 O2 system for the photodegradation of organic pollutants in an aqueous system. We used synthesized nanostructures to photodegrade phenol red (PR) dye to check their photocatalytic activity. The ZnO nanostructures photodegraded more than 90% of the PR dye under UV light irradiation in which photonic energy is converted to chemical energy (photocatalytic energy conversion), thereby exploitable for water purification applications. Synthesized ZnO nanostructures were characterized using powder X-ray diffraction, Fourier transform infrared spectroscopy, UV-visible light spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy to investigate their structural, optical, morphological, and compositional properties, respectively.

Photoluminescence spectroscopy of excitonic emission in CsPbCl3 perovskite single crystals

We analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals. Strong near band edge excitonic emission is observed at 420 nm and a series of excitonic states was observed. Temperature-dependent PL measurements on cesium lead chloride over a wide temperature range from 13 K to 295 K revealed significant electron phonon coupling with a Huang-Rhys parameter ranging from 1.34 to 3.38. A primary experimental finding is that the near band edge PL emission shows existence of multiple sharp features at low temperatures that are attributable to free excitons, bound excitons, and their phonon replicas. The PL also shows evidence of band tailing effects due to disorder. We investigated the recombination mechanism of photogenerated charge carriers by means of time-resolved PL (TR-PL) spectroscopy at low temperatures and various excitation powers and observed competing mono- and bimolecular recombination processes. A dynamic model is proposed to describe the TR-PL. We observed both long-lived and short-lived recombination where the short-lived time decay is attributed to free exciton recombination, whereas the long-lived decay to bound exciton recombination involving traps due to intrinsic disorder.

Growth of high-quality GaN nanowires on p-Si (1 1 1) and their performance in solid state heterojunction solar cells

We report on the optimized growth of catalyst-freeGaN nanowires (NWs)/p-Si by the vapor–solid (V-S) method using chemical vapor deposition (CVD). The effect of NH3 gas flow rate on the morphology and photovoltaic behavior of the material has been investigated. The length and the diameter of the NWs decrease as the NH3 flow rate increases. Raman and X-ray diffraction (XRD) analyses reveal lower internal stress in the prepared NWs. The photoluminescence (PL) spectra indicate strong near band-edge (NBE) peaks extending from 365 to 368 nm and their intensity varied significantly with the NH3 flow rate. The assembled n-GaN NWs/p-Si solar cell devices reveal a maximum conversion efficiency of ∼7.87% under AM 1.5G illumination. This study shows that the morphology, optical, and performance of the fabricated n-GaN NWs on p-Si are strongly affected by the gas flow rate.

Complex features of the photoluminescence from ZnO nanorods grown by vapor-phase transport method

We present photoluminescence (PL) studies of ZnO nanorods synthesized by high-temperature vapor-phase transport method under pulse laser excitation. The PL spectrum contains a strong near band edge (NBE) emission located at around 387 nm and no emission bands in the visible range were observed. Our studies show that an increase in power density of pulse UV laser results in appearance of a so-called P-band centered at 389 nm and it overlaps the NBE emission. The variations in excitation intensity and detection angles revealed that the band at 389 nm can be deconvoluted into Lorentzian components at ~382 and ~395 nm. Excitation intensity dependence of the band at 389 nm exhibits a clear threshold behavior while the intensity of the band at ~395 nm grows almost linearly with the increase in the excitation intensity. It is possible that the emission band at 395 nm is associated with emission centers caused by zinc vacancy defects or their complexes in ZnO nanorods.

Leakage current reduction in n-GaN/p-Si (100) heterojunction solar cells

We report on the growth of n-GaN/p-Si heterojunction solar cells via thermal chemical vapor deposition on Si (100) substrates at different growth temperatures (900, 950, and 1000 °C). The influence of growth temperature on the morphological, structural, optical, and electrical properties of GaN films has been elucidated. Increasing the growth temperature was found to reduce the internal stress and improve the material's crystallinity as confirmed via x-ray diffraction and Raman spectroscopy analyses. The photoluminescence spectra exhibit strong near band edge peaks in the range between 375 and 366 nm, with the peak intensity increasing with increasing the growth temperature. The current–voltage (J–V) characteristics of the assembled heterojunction solar cells showed the reverse leakage current to decrease with increasing the growth temperature. Consequently, the solar cell fabricated using the films grown at 1000 °C exhibits higher conversion efficiency (8.17%) than those grown at 950 °C (5.15%) and 900 °C (2.48%), respectively. This work shows that the structural, optical, and photovoltaic properties of the grown n-GaN/p-Si heterojunction solar cell structures are strongly influenced by the growth temperature.

Electrical and optical properties in O-polar and Zn-polar ZnO films grown by pulsed laser deposition

O-polar and Zn-polar ZnO films were grown on c-sapphire by pulsed laser deposition. Positron annihilation spectroscopy study reveals that the VZn-related defects in the ZnO films with different polarities are different in structure and their thermal evolution is different. Hall effect measurement and luminescence spectroscopy reveal that the electrical and optical properties and their corresponding thermal evolution are strongly dependent on the polarity of the film. The luminescence spectra of the as-grown Zn-polar ZnO film is signified by a negligible green defect emission (at ~ 2.4 eV) and strong near band edge emission as compared with the O-polar film. The as-grown Zn-polar film exhibited a lower electron concentration (2 × 1018 cm−3) than that of the O-polar film (6 × 1018 cm−3); this difference is attributed to their different H concentrations. For the O-polar film, the electron concentration decreased with annealing temperature Tanneal, reaching a minimum at 700°C and then increased to 4 × 1018 cm−3 at Tanneal = 900 °C. In comparison, the electron concentration of the Zn-polar ZnO film monotonically decreased with Tanneal attaining a value of ~1 × 1017 cm−3 at Tanneal = 900 °C, 40 times smaller than that of the O-polar film. The cause for the differences in the optical and electrical properties for the O-polar and Zn-polar films is explained by the presence of different defects in these films.

Investigation on structural, morphological, optical and electrical features of nanocrystalline Zn1−xGdxO films synthesized by unconventional sol–gel screen-printing process

Nanocrystalline pure and gadolinium (Gd) doped zinc oxide (Zn1−xGdxO) films were synthesized on a glass substrate by unconventional sol–gel screen-printing process. The prepared samples were then fired (sintered) at 500 °C to get the desired stoichiometry. The samples thus prepared were analyzed for their structural, morphological, optical and electrical features by exploiting x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), UV–visible spectroscopy, photoluminescence (PL) spectroscopy and dark (DC) electrical resistivity appraised by two-probe unit. XRD analysis exposed that all the samples were polycrystalline in character exhibiting single-phase hexagonal wurtzite structure. Fiber-like surface morphology was exhibited by the samples. EDX technique indicates that the samples have the prerequisite constituent elements in stoichiometric ratio. The optical band gap (Eg) showed an increment from 3.24 to 3.45 eV on increasing the doping concentration of gadolinium from x = 0 to x = 0.05 illustrating a band gap tuning of 6.48%. PL spectra of prepared samples displayed a strong near band edge emission along with a weak green emission. Dark (DC) electrical resistivity was found to decrease with the increase in gadolinium doping concentration.

Ti-Alloying of BaZrS3 Chalcogenide Perovskite for Photovoltaics.

BaZrS3, a prototypical chalcogenide perovskite, has been shown to possess a direct band gap, an exceptionally strong near band edge light absorption, and good carrier transport. Coupled with its great stability, nontoxicity with earth-abundant elements, it is thus a promising candidate for thin film solar cells. However, its reported band gap in the range of 1.7–1.8 eV is larger than the optimal value required to reach the Shockley–Queisser limit of a single-junction solar cell. Here, we report the synthesis of Ba(Zr1–xTix)S3 perovskite compounds with a reduced band gap. It is found that Ti-alloying is extremely effective in band gap reduction of BaZrS3: a mere 4 atom % alloying decreases the band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power conversion efficiency of 32%. Higher Ti-alloying concentration is found to destabilize the distorted chalcogenide perovskite phase.

Strong Spin-Selective Optical Stark Effect in Lead Halide Perovskite Quantum Dots.

The strong spin-orbital coupling and intense optical transition in lead halide perovskites enable facile optical injection and manipulation of spin states potentially useful for spintronics, one manifestation of which is the optical Stark effect (OSE). A strong OSE would benefit from discrete energy levels and a strong band edge transition with concentrated oscillator strength, which can be realized in three-dimensionally confined quantum dots (QDs). This idea, however, has not been explored yet for perovskite materials. Here we report the observation of strong OSE in perovskite colloidal QDs using circularly polarized transient absorption spectroscopy. The large OSE shifts correspond to transition dipoles as large as 52 D, which compares favorably to those recently reported for higher-dimensional perovskites as well as transition metal dichalcogenide monolayers. Colloidal synthesis also allows for facile tuning of the OSE spectral ranges of perovskite QDs via sizes and compositions, and the impact of these parameters on the OSE is examined, providing fundamental insights into the band edge transition of lead halide perovskites.

Ag‐doped PbS thin films by nebulizer spray pyrolysis for solar cells

Silver (Ag)-doped PbS (PbS:Ag) thin films of 616 to 745 nm in thickness were prepared on glass substrates via cost-effective nebulizer spray method by adding different Ag levels from 2% to 8% at 200°C. For solar cell applications, the effect of Ag doping concentration on structural, morphological, optical, photoluminescence, and electrical chattels of PbS thin film has been studied. X-ray diffraction pattern confirmed the polycrystalline behavior of the prepared PbS:Ag films with cubic crystalline nature. The crystalline size and texture coefficient were increased by increasing Ag doping concentration. From the morphological studies by scanning electron microscope (SEM) and atomic force microscope (AFM), the grain size of the films and surface roughness values were increased for the increase in Ag doping concentration. EDS spectra confirmed the existence of Ag, Pb, and S elements in the select 6% Ag-doped PbS film. Peaks related to silver oxide started to emerge at 6% of Ag doping level. The optical direct band gap value was reduced from 1.51 to 1.17 eV for Ag doping from 2% to 6% and thereby slightly increased as 1.79 eV for 8% Ag doping level. For all PbS:Ag films, the photoluminescence spectrum emitted a strong near band edge (NBE) emission at approximately 580 nm, meaning better optical quality. Hall effect measurements evidenced that Ag doping provides enhancement on the characteristics of mobility, carrier concentration, and resistivity with p-type conducting nature. The observed high carrier concentration and low resistivity values were 4.32 × 1014 cm−3 and 80 Ωcm, for 6% Ag-doped PbS film. The FTO/CdS/PbS:Ag heterostructure solar cell was formed from 6% Ag-doped film.

Microstructural and electrical properties evaluation of lead doped tin sulfide thin films

A low cost and simple spray methodology with nebulizer was employed to fabricate lead doped tin sulfide (SnS:Pb) thin films. Different doping weight percentages (1, 3, 5, 7, and 9 wt%) were used to prepare SnS:Pb thin films on glass substrates with 350 °C substrate temperature, and we subsequently investigated Pb element influence on microstructural, electrical, and optical properties. Structural studies using X-ray diffraction confirmed orthorhombic crystal structure with (111) plane preferred orientation and atomic force micrographs identified significant variation due to the different Pb wt%. Photoluminescence showed a strong band edge emission peak at 761 nm, with optical band gaps at 1.90–1.60 eV over the Pb dopant concentrations. Hall effect showed low electrical resistivity (3.01 × 10−2 Ω cm), high carrier concentration (~1.01 × 1019 cm−3), and high Hall mobility (~20.5 cm2 V−1 s−1) for 7 wt%, which is suitable to fabricate solar cell devices. The p–n junction properties were analyzed under dark and illumination conditions by current–voltage characteristics using the FTO/n-CdS/p-SnS:Pb/Al structure.

Transition metal ion doping perovskite nanocrystals for high luminescence quantum yield

All-inorganic perovskite nanocrystals (NCs) have become a new generation of low-cost semiconductor luminescent materials for optoelectronic applications due to their excellent optical properties. Prepared perovskite NCs undergo different ion doping and surface modification, which makes the NCs photo-luminescence quantum yield (PLQY) basically uniform. However, stability of perovskite NCs solution in the environment has always been an important factor restricting its practical application. Here, we used melt-quenching method to prepare a series of different Ni2+ doped CsPbBr3 NCs glass. Through experimental characterization and DFT calculation it is found that the entry of Ni2+ into the NCs lattice can eliminate the structural defects of NCs and improve the order of the perovskite lattice. Ni2+ doped perovskite NCs have strong band edge emission and high quantum yield (up to 84.3%).

Facile synthesis of nanocrystalline β-SnWO4: as a photocatalyst, biosensor and anode for Li-ion battery

Here, we have successfully synthesised β-SnWO4 Nps via simple and low-cost co-precipitation method. XRD pattern confirms the wolframite cubic structure of β-SnWO4 Nps which is belonging to P213 or T4 space group with an average crystallite size of ~38 nm. FTIR spectrum of β-SnWO4 Nps shows the bands at 620–825 cm−1 is assigned to characteristical stretching and bending vibrational modes of WO 6 6 , W–W bonds and stretching vibration of W–O–W bridging bonds in β-SnWO4. UV-DRS spectrum shows the strong absorption band edge at 604 nm and the estimated bandgap of β-SnWO4 Nps is found to be 1.9 eV. The β-SnWO4 shows excellent photocatalytic activity against indigo carminedye under various conditions of β-SnWO4 Nps. In addition to this, the electrochemical senor performance toward the quantification of dopamine at nanomolar concentration and anode material for Li-ion batteries by showing high reversible discharge/charge capacity was explored.

Structural and optical properties of low temperature grown single crystalline GaN nanorods on flexible tungsten foil using laser molecular beam epitaxy

The single crystalline GaN nanorods (NRs) have been grown on flexible tungsten (W) foil using laser molecular beam epitaxy (LMBE) at low growth temperature of 600 °C. The field emission scanning electron microscopy showed the growth of GaN NRs having length, width and density of 400∼530 nm, 40–70 nm and 3.4 × 109 cm−2, respectively. The high-resolution transmission electron microscopy reveals the c-axis oriented growth of single crystalline GaN NRs on W foil. The photoluminescence spectroscopy shows that the GaN NRs possesses a strong near band edge luminescence at 3.41 eV with a line-width of ∼100 meV at room temperature which resembles the high optical quality of GaN nanorods grown on W foil. The possible mechanism of GaN nanords growth on W foil has been also discussed. The low temperature LMBE growth of single crystalline GaN NRs on flexible metal foil with high optical quality paves the way for the development of III-nitride based flexible opto-electronics devices.