Sub-band Gap Research Articles

Consequence of Mn and Ni doping on structural, optical and magnetic characteristics of ZnO nanopowders: the Williamson–Hall method, the Kramers–Kronig approach and magnetic interactions

The crucial role of Ni and Mn dopants on the structural, optical, and magnetic characteristics of the zinc oxide (ZnO) nanopowders have been investigated in detail using XRD, diffuse reflectance spectroscopy (DRS), Kramers–Kronig (KK), and vibrating sample magnetometer (VSM). The structural analysis indicates that all patterns are mostly assigned to the hexagonal wurtzite structure. Also, XRD analysis shows that the Ni2+ and Mn2+ ions replace Zn2+ ions which causes lattice strain. Other than the Debye–Scherrer method, the Williamson–Hall method has also been applied to study the influence of strain in the calculation of the crystallite size. Based on DRS data, the band gap of the nanoparticles (NPs) is calculated by Tauc relation in two different ways. The band gap of the synthesized undoped ZnO NPs decreases from 3.25 to 3.23 and 3.15 eV through Ni and Mn doping, respectively. The band gap investigation shows a red-shift with Ni and Mn doping. The width of the localized states or sub-band gap absorption edge (often termed Urbach energy) of the samples was determined by the curves of Ln α versus photon energy hυ. The optical parameters such as complex refractive index, complex dielectric function, and absorption coefficient of the samples were evaluated by applying KK method. Investigating on magnetic properties showed a room temperature ferromagnetic (RTFM) behavior. Also, the interactions and reasons of the observed RTFM mechanisms are examined in detail.

Physical mechanisms for the optical properties of the femtosecond-laser-treated black diamond

Vacancies in diamond can introduce defect bands in the band gap of diamond and lead to the sub-band gap absorption in the visible and infrared regions. At sufficiently high concentrations of vacancy, the defect bands overlap each other to form a single partially filled intermediate band (IB) in the band gap and the sub-band gap absorption in the infrared region is especially strong. Along with the decreasing of the vacancy concentrations, the IB splits into two separate bands and the sub-band gap absorption decreases sharply. The computed results are important for thoroughly understanding of the optical properties of black diamond.

CdSe Quantum Dot Sensitized Molecular Photon Upconversion Solar Cells

Incorporating photon upconversion, via triplet–triplet annihilation (TTA-UC), directly into a solar cell is an intriguing strategy for harnessing sub-band gap photons and surpassing the Shockley–Queisser limit. A majority of TTA-UC solar cells to date rely on difficult to synthesize and expensive platinum and/or palladium porphyrin sensitizers. Here, we present, as far as we know, the first TTA-UC solar cell that integrates quantum dot (QD) sensitizers directly into the photocurrent generation mechanism. The photoanodes are composed of a nanocrystalline TiO2 substrate, 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (A) as the annihilator molecule, and CdSe QDs as the sensitizer in an inorganic–organic-inorganic layered architecture (TiO2-A-QD). The TiO2-A-QD devices generate a photocurrent that is more than 1.4 times the sum of its parts and does so via a TTA-UC mechanism as demonstrated by intensity dependence, IPCE, and spectroscopic measurements. The maximum efficiency onset threshold (...

High quantum yields generated by a multi-band quantum dot photocell

Abstract We perform the quantum yields in a multi-band quantum dot (QD) photocell via doping an intermediate band (IB) between the conduction band (CB) and valence band (VB). Under two different sub-band gap layouts, the output power has a prominent enhancement than the single-band gap photocell and the achieved peak photo-to-charge efficiency reaches to 74.9% as compared to the limit efficiency of 63.2% via the IB approach in the theoretical solar cell prototype. The achieved quantum yields reveal the potential to improve efficiency by some effective theoretical approaches in the QD-IB photocell.

Broadband White Emission in Cs2AgIn1-xBixCl6 Phosphors.

The photoluminescent properties of the lead-free double perovskite solid solution Cs2AgIn1-xBixCl6 have been investigated. The In3+ end member, Cs2AgInCl6, is a direct gap semiconductor that absorbs UV light (λ < 350 nm) and shows little to no photoluminescence. Incorporation of Bi3+ leads to a strong sub-band gap absorption that peaks in the near UV (∼360 nm) and extends into the visible. This absorption, which is thought to originate from localized 6s2 → 6s1p1 transitions on Bi3+ ions, is split by a Jahn-Teller distortion of the excited state. In-rich samples show strong photoluminescence that is attributed to radiative decay of self-trapped excitons, with a broad emission peak of significant intensity from 450 to 750 nm. The color of the emitted light is best described as yellow-white (λmax ≈ 625 nm), due to the extreme breadth of the emission peak (fwhm ≈ 217(2) nm). The excitation spectrum extends out to 450 nm for samples near x = 0.25, while the photoluminescent quantum yield (PLQY) reaches a maximum of 39 ± 3% in the x = 0.167 sample. The emission characteristics, which include a correlated color temperature (CCT) of 3119 K and a color rendering index (CRI) of 85, coupled with an excitation spectrum that can be driven by visible photons emitted from a Ga1-xInxN LED, make Cs2AgIn1-xBixCl6 phosphors promising for use in solid state white lighting applications.

Optical constants and band gap of wurtzite Al1−xScxN/Al2O3 prepared by magnetron sputter epitaxy for scandium concentrations up to x = 0.41

Wurtzite Al1−xScxN thin films with scandium Sc concentrations up to x = 0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 °C and 400 °C on Al2O3 substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al1−xScxN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al1−xScxN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap Eg. For x &amp;lt; 0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x = 0.41, indicating a high phase purity in line with X-ray diffraction data.

Sub-Band Gap Photodetection from the Titanium Nitride/Germanium Heterostructure.

Photoexcited hot carriers through nonradiative decay offer new opportunities for harnessing longer wavelength light. Here, we have demonstrated a hot-carrier-mediated sub-band gap photodetection in germanium-based planar heterojunction devices. The planar samples that form in situ germanium/titanium nitride (Ge/TiN) interfaces are fabricated by the dc sputtering technique, and the generation of photocurrent by near-infrared (NIR) light illumination is confirmed up to 2600 nm, well exceeding the absorption limit of Ge. The photocurrent obtained with nickel contacts is 3 orders larger than that obtained without metal contacts or with gold contacts in similar structures. The specific detectivity ( D*) value for the TiN/Ge photodetector is obtained to be 6.32 × 105 Jones at the sub-band gap excitation wavelength of 2000 nm without applying any bias. The superior performances of our device are attributed to the broad absorption of the TiN, the plasmonic hot carrier transfer from the TiN to Ge, and built-in potential of the TiN/Ge non-Ohmic junction, which allows efficient separation of photoexcited electron-hole pairs. Our results further support the use of TiN, which is robust and cost-effective, as an alternative to metals for NIR photodetection and photovoltaics when it forms a heterostructure with Ge.

Anomalous increase of sub-band gap photoluminescence from InPBi layers grown by liquid phase epitaxy

The luminescence properties of InPBi layers, grown by liquid phase epitaxy from an In-P melt containing 4 and 18 wt% Bi are investigated. As-grown layers, as well as layers subjected to a high temperature anneal, are studied by atomic force microscopy (AFM), energy dispersive x rays (EDX), high resolution x ray diffraction (HRXRD) and low temperature photoluminescence (PL) spectroscopy. AFM shows formation of microscopic granular structure over the surface of the layer grown from melt containing 18 wt% Bi after a high temperature furnace anneal at 700 °C for 1 h. Lattice contraction and a decrease of Bi content is observed for both kinds of samples after anneal though it is only marginal in case of the layer grown from melt containing 4 wt% Bi but quite appreciable in case of the other sample. From EDX and XRD results, the observed decrease in Bi content as a result of high temperature anneal is suggested to be due to diffusion of Bi atoms from the layer to InP grains formed over the layer surface. 10 K PL measurements showed the expected band edge luminescence peak at 1.38 eV. A second peak at 1.33–1.34 eV is observed which shows a large post anneal enhancement for materials grown from melts containing higher amounts of Bi. It is suggested that this peak is due to luminescence from defects formed by complexes of Bi with phosphorous vacancies. Photoluminescence also revealed a broad peak between 1–1.22 eV whose intensity increased anomalously with temperature up to 104 K. The anomalous increase up to 28 K is found to be associated with a low activation energy of 0.35 meV and is speculated to be due to the transfer of holes in the localized states to the valence band. Above 28 K, PL increases with an activation energy of 9.66 meV and may be explained on the basis of similar observation made by a previous group [Chen et al Appl. Phys. Lett. 110, 051 903 (2017)] who explained the phenomenon by assuming thermal hopping of some electrons available for recombination with holes to some non-radiative states .

Thermal stability of Te-hyperdoped Si: Atomic-scale correlation of the structural, electrical, and optical properties

Si hyperdoped with chalcogens (S, Se, Te) is well-known to possess unique properties such as an insulator-to-metal transition and a room-temperature sub-bandgap absorption. These properties are expected to be sensitive to a post-synthesis thermal annealing, since hyperdoped Si is a thermodynamically metastable material. Thermal stability of the as-fabricated hyperdoped Si is of great importance for the device fabrication process involving temperature-dependent steps like ohmic contact formation. Here, we report on the thermal stability of the as-fabricated Te-hyperdoped Si subjected to isochronal furnace anneals from 250 {\deg}C to 1200 {\deg}C. We demonstrate that Te-hyperdoped Si exhibits thermal stability up to 400 {\deg}C with a duration of 10 minutes that even helps to further improve the crystalline quality, the electrical activation of Te dopants and the room-temperature sub-band gap absorption. At higher temperatures, however, Te atoms are found to move out from the substitutional sites with a migration energy of EM = 2.1+/-0.1 eV forming inactive clusters and precipitates that impair the structural, electrical and optical properties. These results provide further insight into the underlying physical state transformation of Te dopants in a metastable compositional regime caused by post-synthesis thermal annealing as well as pave the way for the fabrication of advanced hyperdoped Si-based devices.

Enhanced Photoelectrochemical Water Splitting through Bismuth Vanadate with a Photon Upconversion Luminescent Reflector.

As the performance of photoanodes for solar water splitting steadily improves, the extension of the absorption wavelength in the photoanodes is highly necessary to substantially improve the water splitting. We use a luminescent back reflector (LBR) capable of photon upconversion (UC) to improve the light harvesting capabilities of Mo:BiVO4 photoelectrodes. The LBR is prepared by dispersing the organic dye pair meso-tetraphenyltetrabenzoporphine palladium and perylene capable of triplet-triplet annhilation-based UC in a polymer film. The LBR converts the wavelengths of 600-650 nm corresponding to the sub-band gap of Mo:BiVO4 and the wavelengths of 350-450 nm that are not sufficiently absorbed in Mo:BiVO4 to a wavelength that can be absorbed by a Mo:BiVO4 photoelectrode. The LBR improves the water splitting reaction of Mo:BiVO4 photoelectrodes by 17 %, and consequently, the Mo:BiVO4 /LBR exhibits a photocurrent density of 5.25 mA cm-2 at 1.23 V versus the reversible hydrogen electrode. The Mo:BiVO4 /LBR exhibits hydrogen/oxygen evolution corresponding to the increased photocurrent density and long-term operational stability for the water splitting reaction.

One-dimensional topological metal

We propose a type of topological states of matter exhibiting topologically nontrivial edge states (ESs) within gapless bulk states (GBSs) protected by both spin rotational and reflection symmetries. A model presenting such states is simply comprised of a one-dimensional reflection symmetric superlattice in the presence of spin-orbit (SO) coupling containing an odd number of sublattices per unit cell. We show that the system has a rich phase diagram including a topological metal (TM) phase where nontrivial ESs coexist with nontrivial GBSs at Fermi level. Topologically distinct phases can be reached through a subband gap closing-reopening transition depending on the relative strength of inter- and intraunit cell SO couplings. Moreover, the topological class of the system is AI with an integer topological invariant called $\mathbb{Z}$ index. The stability of TM states is also analyzed against Zeeman magnetic fields and on-site potentials resulting in that the spin rotational symmetry around the lattice direction is a key requirement for the appearance of such states. Also, possible experimental realizations are discussed.

Experimental and computational study of Tm-doped TiO2: The effect of Li+ on Vis-response photocatalysis and luminescence

The utilization of solar light to induce a photocatalytic reaction in the presence of wide band gap semiconductors requires the modification of their surface and optical properties. The Vis-to-UV up-conversion process (UCP) that occurs in lanthanide ions could be employed to induce TiO2. Thus, having in mind solar-forced photocatalysis, nanostructured TiO2 co-modified with Tm and Li was prepared via a hydrothermal method. In addition to the exhaustive surface characterization of the as-prepared samples, the partial density of states (PDOS) was computed to better understand the electronic structure and physicochemical properties of the prepared photocatalysts. It was demonstrated that Tm ions were beneficial for phenol degradation in the aqueous phase under visible irradiation, whereas the co-presence of Tm and Li ions was profitable for enhanced luminescence. Although action spectra revealed that excitation of Tm-TiO2 in the range from 400 to 490 nm led to the photodegradation of phenol and 2-propanol, the UCP was not responsible for the photocatalytic activity under Vis irradiation. Therefore, it is proposed that the formation of sub-band gap states from the lanthanide 4f level played a key role in the phenol degradation process.

Electrical characterization of ZnO-coated nanospring ensemble by impedance spectroscopy: probing the effect of thermal annealing

The effects of thermal annealing on the electrical properties of randomly oriented ZnO-coated nanospring ensembles were extensively investigated through AC impedance spectroscopy. Annealing the nanospring mats for an hour at 873 K in air showed significant change in ZnO morphology, reduced electrical conductivity due to the presence of grain boundaries, decreased apparent donor concentration, and faster decay of sub-band gap photocurrent. The role of the nanospring-nanospring junctions in the conduction of carriers in the ensemble was also examined, as well as evaluation of their responsiveness to thermal and optical stimulations. This work identifies the effects of heat treatment in the presence of air on the electrical properties of the nanospring ensembles, which are related to the mesoscopic morphology and interconnect within the ensemble and the properties of the ZnO coating.

Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites.

To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH3NH3PbI3), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH3NH3PbI3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2.

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO2 and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO2 deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al2O3) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.

Sub-Band Gap Absorption Mechanisms Involving Oxygen Vacancies in Hydroxyapatite

Hydroxyapatite (HAp) is a widely used biomaterial for the preparation of bone and dental implants. Despite the relevance of HAp in medicine, exciting applications involving this material as a bio-compatible photocatalyst, depend on how well we understand its fundamental properties. Experimental evidence suggests that oxygen vacancies play a critical role in the production of surface radicals upon exposure of HAp to ultra-violet (UV) light. However, very little is known about the underlying physical and chemical details. We present a hybrid density-functional study of the structural and electronic properties of oxygen vacancies in large HAp supercells within the plane-wave formalism. We find that under equilibrium conditions, vacancies occur either as a simple vacant oxygen site (in the neutral charge state), or as extended structures replacing several crystalline moieties (in the double plus charge state). Large atomic relaxations upon ionization make the oxygen vacancy a negative-$U$ defect, where the single plus charge state is metastable, being only accessible under UV excitation. From inspection of the transition levels, we find that electron promotion from the valence band top to the donor state of the vacancy, involves a zero-phonon transition of 3.6-3.9 eV. This mechanism is the most likely explanation to the 3.4-4.0 eV absorption onset for the observation of photocatalysis using HAp under persistent UV illumination.

Lead-Free Direct Band Gap Double-Perovskite Nanocrystals with Bright Dual-Color Emission.

Lead-free double-perovskite nanocrystals (NCs), that is, Cs2AgIn xBi1- xCl6 ( x = 0, 0.25, 0.5, 0.75, and 0.9), that can be tuned from indirect band gap ( x = 0, 0.25, and 0.5) to direct band gap ( x = 0.75 and 0.9) are designed. Direct band gap NCs exhibit 3 times greater absorption cross section, lower sub-band gap trap states, and >5 times photoluminescence quantum efficiency (PLQE) compared to those observed for indirect band gap NCs (Cs2AgBiCl6). A PLQE of 36.6% for direct band gap NCs is comparable to those observed for lead perovskite NCs in the violet region. Besides the band edge violet emission, the direct band gap NCs exhibit bright orange (570 nm) emission. Density functional theory calculations suggesting forbidden transition is responsible for the orange emission, which is supported by time-resolved PL and PL excitation spectra. The successful design of lead-free direct band gap perovskite NCs with superior optical properties opens the door for high-performance lead-free perovskite optoelectronic devices.

Tunable mechanical, electronic and magnetic properties of monolayer C3N nanoribbons by external fields

Two-dimensional (2D) polyaniline with C3N stoichiometry, is a newly fabricated layered material that has been expected to possess fascinating electronic, thermal, mechanical and chemical properties. The nature of its counterpart nanoribbons offering even more tunability in properties because of the unique quantum confinement and edge effect, however, has not been revealed yet. Here we systemically study the mechanical, electronic and magnetic properties for various nanoribbons cutting from a monolayer C3N sheet along the typical crystallographic orientations. By the first-principles calculations we find that C3N nanoribbons exhibit sensitive responses to the externally applied electric field and strain. Specifically, the spin-selective half-metallicity depends on the external electric field or strain, as well as the ribbon width. For the asymmetric zigzag-edged ribbon, the spin-polarization rate approaches −100% at electric field strength −0.2 V/Å. Interestingly, an applied strain can transform a symmetric zigzag-carbon-edge ribbon from a magnetic semiconductor to a half-metal. And the half-metal property remains unchanged when the strain increases from 8 to 15%, while the spin-up subband gap further increases to 0.46 eV. These numerical results may be useful to engineer and design magnetic-field-free spintronic devices based on the 2D C3N.

Modelling of Intrinsic Loss Processes in the Intermediate Band Solar Cells

Abstract For the first time, the formalism of intrinsic loss processes such as below-band gap, thermalisation, angle mismatch, Carnot and emission loss in the intermediate band solar cells (SCs) is obtained by a theoretical framework based on the principle of detailed balance approach, and with the physical origins of losses discussed in terms of photon absorption and emission in the presence of intermediate energy band (IB). Then, the effect of IB position on each intrinsic loss in the intermediate band SCs is investigated. The results show that the introduction of IB reduces the below-band gap loss due to absorption of low energy photons by narrower sub-band gap and reduces the thermalisation loss due to absorption of high energy photons by the band gap of host semiconductor. Furthermore, the thermalisation and angle mismatch losses are dominant fractions of intrinsic loss, while the emission loss presents less than 2.2 % of intrinsic loss.

Tailoring sub-bandgap of CuGaS2 thin film via chromium doping by facile chemical spray pyrolysis technique

Tailoring sub-bandgaps using intermediate states or bands with wide bandgap semiconductors is a promising and novel technique for the application in high efficiency solar cells. Pure and chromium (Cr) doped chalcopyrite CuGaS2 (CGS) thin films were prepared by facile chemical spray pyrolysis technique and annealed in vacuum, nitrogen and argon atmospheres. Structural characterization confirmed that the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature. No secondary phases were present in both pure and Cr doped CGS thin films. Presence of Cr ions was confirmed by X-ray photoelectron spectroscopy and energy dispersive analysis of X-rays analyses. The optical direct and sub band gap of pristine and Cr doped CGS thin films were measured from UV absorption data. It was revealed that the pure CGS film has a band gap of 2.40 eV sub-band gap values were observed at 2.25 and 2.15 eV for 1 and 2 wt% of Cr doping, respectively. These gaps can be ascribed to the formation of intermediate bands due to hybridization of Cr d-states into the host electronic structure. Meanwhile, photoconductivity study demonstrated the photo-electric activity of the intermediate bands in the Cr doped thin films.