Intra-bandgap States Research Articles

Infrared active narrow bandgap Ni doped LaFeO3 nanoparticles for desalination and decontamination of water leveraging interfacial solar steam generation

Photothermal Ni-doped LaFeO3 (NLFO) (LaFe1-xNixO3, x = 0, 0.2, 0.3, 0.4, and 0.5) microspheres composed of nanoparticles synthesized by hydrothermal method are utilized for interfacial solar steam generation (ISSG) of salty and contaminated water. The orthorhombic (pnma) to rhombohedral (R3c¯) phase transition of LaFeO3 (LFO) at morphotropic phase boundary (MPB) flattens the free energy profile, and high absorbance in the 800–2000 nm Vis-NIR region arises due to the creation of intra-band gap states are accountable for superior activity towards the ISSG for desalination. The La, Ni, and Fe possess the oxidation states of 3+, 2+, and 3+/4+, respectively, showing successful doping of Ni2+ at the Fe3+ sites that produce lattice distortion at La/FeO6 octahedra. LaFe0.5Ni0.5O3 (NLFO5) sample exhibits surface temperature of 50.4 °C due to heat localization and produces evaporation flux of 2.89 kg/m2h under IR illumination at the air-water interface. Importantly, NLFO5 loaded cellulose paper shows good repeatability and cyclic stability for 10 consecutive cycles under IR illumination and equivalent evaporation flux of 2.4 kg/m2h under direct sunlight illumination. Moreover, 3.5 wt% saline water shows a drastic decrement in ion concentration after ISSG, as confirmed by atomic absorption spectroscopy. Furthermore, NLFO5 possesses good evaporation flux of 2.27 and 2.20 kg/m2h for water contaminated with RhB and MB organic dye. Our results propose the NLFO as distinguished photothermal material for ISSG application and wastewater purification by means of evaporation.

Quantitative Determination the Role of the Intrabandgap States in Water Photooxidation over Hematite Electrodes.

The intrabandgap states on the hematite (α-Fe2O3) electrodes are believed to play an important role in water photooxidation. Yet, it is not fully understood how the intrabandgap states are involved in the reaction. In this work, the intraband-gap states in water photooxidation on α-Fe2O3 electrodes are investigated by a combination of multiple (photo-) electrochemical techniques and operando spectroscopic methods. Two kinds of surface states are observed on the electrodes during water photooxidation, and their roles are quantitatively determined by the correlation with the steady-state photocurrent. It is demonstrated that the intrinsic electronic surface state close to the conduction band can act only as the recombination center for the photocarriers. However, the photogenerated surface state closer to the valence band is revealed to be the reactant in the rate-determining step in oxygen evolution reaction. These findings may be beneficial to elucidate the actual function of the surface states and provide insights into the kinetic and mechanism studies of water photooxidation on the α-Fe2O3 electrodes.

Spectroscopic Evidence of Intraband Gap States in α‐SnWO4 Photoanodes Introduced by Interface Oxidation

α‐SnWO4 is an emerging photoelectrode material for photoelectrochemical water splitting, with several promising properties such as the favorable bandgap of 1.9 eV and suitable positions of the valence and conduction band. However, a major challenge remains: unprotected α‐SnWO4 undergoes surface passivation that blocks further charge transfer, and α‐SnWO4 electrodes that are covered with a protection/catalytic overlayer (e.g., NiOx, CoOx) show limited photovoltage. Earlier studies reveal that interfacial oxidation occurs due to the deposition of the overlayer. This negatively impacts the photovoltage that can be extracted, which is attributed to Fermi‐level pinning at the interface. The exact origin of this Fermi‐level pinning mechanism, however, remains unclear. In the present study, a combination of surface photovoltage analysis and hard X‐ray photoelectron spectroscopy is used to elucidate the electronic structure of the α‐SnWO4/oxide interface. Both techniques offer compelling and consistent evidence for the presence of a defect state that is energetically located within the bandgap energy of α‐SnWO4 and is likely responsible for the Fermi‐level pinning.

Comparative visible-light driven selective oxidation to aldehydes of phenylmethanol (benzyl alcohol) and 4-pyridinylmethanol (4-pyridinecarbinol) on N-TiO2 and some commercial TiO2 samples.

Visible light (λ > 420nm) selective photooxidation of phenylmethanol and 4-pyridinylmethanol in CH3CN to the corresponding aldehydes on N-TiO2 is compared with homemade undoped TiO2 (U-TiO2) and commercial undoped anatase specimens (such as PC105, PC500). Significant differences observed between N-TiO2 and undoped TiO2 are neither directly related to the surface area nor to the adsorbed amount of alcohol in the dark by surface area unit. FTIR and EPR spectroscopies are used to study the surface of TiO2 samples and to deeply understand the phenomena intervening in the visible-light photocatalytic activation of the doped vs the undoped oxides. In particular, it is shown that on N-TiO2 (and also on undoped PC105) strong Lewis acid sites (LAS) exist. The favorable role of LAS on the photocatalytic activity is illustrated by the higher photooxidation of 4-pyridinylmethanol vs phenylmethanol over N-TiO2 and PC105 in contrast to the other undoped samples, whose visible light sensitivity originates from a charge transfer between the alcohol and the solid. EPR spectra of N-TiO2 point out the presence of paramagnetic centers related to nitrogen that disappear when the photocatalyst is irradiated with visible light in the presence of alcohol, which starts its oxidative process. On the basis of presented results, we propose that doping with N introduces new intraband gap states that not only contribute to LAS and adsorption of alcohol but also are directly involved in the photochemical process occurring under visible light irradiation.

Mixed Tin-Titanium Oxides by Atomic Layer Deposition on Planar Substrates: Physical and Electronic Structure

Ternary atomic layer deposition of Sn-doped TiOx (Sn:TiOx) and Ti-doped SnOx (Ti:SnOx) was performed using tetrakisdimethylamidotitanium (IV) and tin(IV) with water at 150 °C on (100) Si and quartz substrates. The physical structure of the films was investigated using scanning electron, atomic force, and Kelvin probe force microscopies as well as spectroscopic ellipsometry, Raman spectroscopy, and X-ray diffraction. All as-deposited films are amorphous and possessed flat and conformal topographies with low surface roughness (<1 nm). Annealed Sn:TiOx and Ti:SnOx crystallize into the tetragonal anatase TiO2 and cassiterite SnO2 structures, respectively, without large variations to the film topography. The chemical composition of the as-deposited and annealed films was investigated through X-ray photoelectron spectroscopy. While dopants are homogenously mixed, the ternary ALD to form Sn:TiOx and Ti:SnOx surprisingly does not follow the rule of mixtures, despite similarities in both the growth rates of binary SnOx and TiOx and the precursors used. This finding is also supported by spectroscopic ellipsometry. We postulate the deviation from the rule of mixtures is due to non-ideal surface chemistry of the binary ALD of SnOx. The optical bandgap of all films was quantified by the Tauc analysis and the valence band position was investigated by valence band X-ray photoelectron spectroscopy. Intra-bandgap states above the valence band maximum were observed for as-deposited Ti:SnOx and Sn:TiOx and ascribed to 5 s states from Sn2+ ions. The presence of these intra-bandgap states induces enhanced visible absorption and bathochromic shifts to the apparent bandgap for all as-deposited films. Annealing the films removes the intra-bandgap states and increases the optical bandgap. Simplified band diagrams created using the data available demonstrated that the conduction and valence band positions did not vary between the positions of pure TiO2 or SnO2 with monotonic increases to the dopant incorporation.

Spectroscopic insight of low energy electron emission from diamond surfaces

Low work function materials are desirable in many applications such as electron emission and photocatalysis. We have studied low energy electron emission from low work function hydrogen terminated diamond surfaces via electron spectroscopy to gain insight into the mechanisms involved during electron excitation and emission. Electron emission was found to be dominated by electrons within the band gap energy region, allowed due to the negative electron affinity diamond surface, while sub-bandgap illumination was able to significantly increase emission current. Substantial upward surface band bending greater than 2 eV was observed for the diamond samples, which affect electron accumulation at the surfaces. Intra-bandgap states are shown to strongly influence electron emission behavior, which can have great implications for various energy conversion devices.

Understanding the Role of Surface States on Mesoporous NiO Films.

Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I–/I3– as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3– species. The intra-bandgap states here act as catalysts to assist I3– reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

X-ray absorption spectroscopy and Eu3+-emission characteristics in GaAs/SnO2 heterostructure

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are used for investigating heterostructure samples of GaAs/SnO2. XANES data are used for analyzing the local organization around Eu in the heterostructure formed by GaAs and Eu-doped SnO2. The differences between the XANES data for these samples and data obtained for Eu-doped SnO2 thin films, deposited on glass substrate, are assumed as responsible for the differences in the photoluminescence (PL) spectra concerning the Eu3+ emission, since films deposited on glass substrate do not present Eu3+ PL transitions until the annealing temperature is rather high. Eu3+ emission is explored using two different excitation sources: 350 nm from a Kr+ laser (above SnO2 energy bandgap) and 488 nm from an Ar+ laser (below SnO2 bandgap energy). The existence of more organized regions around the Eu3+ site observed for the heterostructure surface may be associated with the Eu3+ luminescent emission. The main and secondary features in the XANES show that there are differences in the average local Eu environment for the SnO2:Eu isolated thin films and heterostructures, being more organized in the latter. Electrical characterization evidences that the portion of the resistivity reduction that corresponds to photo-ionized intrabandgap states is responsible for the persistent photoconductivity phenomenon in the heterostructures.

Anatase–Rutile Transition and Photo-Induced Conductivity of Highly Yb-Doped TiO2 Films Deposited by Acid Sol–Gel Dip-Coating Method

A method to prepare Yb-doped TiO2 is proposed, using titanium isopropoxide and ytterbium oxide. Thin films and powders are produced through the sol–gel technique where the Yb concentrations are 0.01 at.%, 0.02 at.%, 1 at.%, 2 at.% 4 at.% and 8 at.%. The increase in the doping concentration intensifies gradually the sample transparency for the whole observed spectrum and reaches the transparency degree of the undoped sample with the highest concentration of ytterbium. By means of x-ray diffraction (XRD) we find that the solution’s pH leads to the formation of a rutile structure which is compensated for by lanthanide doping, inhibiting the anatase–rutile phase transformation. Then, the XRD profile depends on the Yb concentration, being mostly anatase for undoped and highly (8 at.%) doped samples, with mixed ratios of these two phases in other concentrations. Maintaining an acid solution leads to fewer intergrain defects in the rutile structure contributing to higher conductivity. Photo-induced investigation of 8 at.% Yb-doped thin film, using several light sources leads to distinct behavior concerning the excitation of intra-bandgap states and/or electron–hole pairs. This result helps to understand the electronic transport in Yb-doped TiO2 and shows that devices based on its conductivity may have improved performance by irradiation with controlled light energy.

Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes.

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.

Electrochemical Semiconductor Analysis By Square Wave Voltammetry

Studies in the past 20 years have utilized cyclic voltammetry (CV) for determining various electronic properties (e.g. density of states, electron carrier concentrations, trap state capacitances) of semiconducting titanium oxide films in photoelectrocatalytic systems such as dye-sensitized solar cells [1-4]. Though primarily used for redox sensing, this work presents square wave voltammetry (SWV) as an alternative to determining said properties. As an electrochemical characterization technique, SWV can provide significantly less background current and greater sensitivity than CV [5]. This is beneficial for the accurate measurement and calculation of electronic properties from electrochemical semiconductor current-voltage data. By varying the applied frequency as well as step and/or pulse sizes of the square wave, one can probe electronic processes of various time scales, in contrast to CV which only allows control of the potential scanning rate across the same potential window [5, 6]. This work presents a comparative study of electronic properties as determined by SWV versus CV for various semiconducting electrodes such as silicon, germanium, titanium oxide, and doped diamond films. This technique was also extended to semiconductor heterostructures - such as noble metal-decorated titanium oxide - for observation of intraband gap states and their contribution to the overall electronic properties of the electrode as determined by SWV.[1] Liu, B. et. al., “Intrinsic intermediate gape states of TiO2 materials and their roles in charge carrier kinetics”, J. Photochem. and Photobio. C: Photochem. Rev., 39 (2019) 1-57[2] Zare, M., Mortezaali, A., and Shafiekhani, A., “Photoelectrochemical determination of shallow and deep trap states of platinum-decorated TiO2 nanotube arrays for photocatalytic applications”, J. Phys. Chem. C, 120 (2016) 9017-9027[3] Fabregat-Santiago, F. et al., “High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping”, J. Am. Chem. Soc., 130 (2008), 11312-11316[4] Abayev, I. et al., “Properties of the electronic density of states in TiO2 nanoparticles surrounded with aqueous electrolyte”, J. Solid State Electrochem, 11 (2007) 647-653[5] Mirceski, V. et al., ”Square-wave voltammetry: A review on the recent progress”, Electroanalysis, 25 (2013) 2411-2422[6] Bard, A. J. and Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications, 2nd ed. Hoboken, NJ: John Wiley & Sons, Inc., 2001.

Structural, electronic and photochemical properties of cerium-doped zirconium titanate

Abstract Mixed solid system involving cerium and zirconium titanate (ZrTiO4) have been prepared using the sol-gel technique. Both X-ray diffraction and DFT calculations firmly indicate that, till a doping level of 10 mol%, cerium ions are dissolved in the titanate matrix (which has the scrutynite structure, analogous to those of the main TiO2 polymorphs) occupying the cationic sites and progressively altering its cell parameters. Cerium is hosted in the matrix both in the form of Ce4+ and Ce3+ ions (XPS results). The trivalent state seems to be favoured even though the state of the dopant depends on the treatment undergone by the material. DFT calculations describe the intra-band gap states formed in both cases and the strong localisation of the single electron in the case of Ce3+ (4f1). Differently from the case of Ce doped ZrO2, that shows photoactivity in the visible light because of the presence of cerium, the doped titanate is inactive in the same conditions. Under UV–vis illumination charge separation occurs (EPR results) and the low-loading doped systems (0.5%, 1%) form OH radicals, detected by spin trapping, more efficiently than the pristine matrix. The absence of photoactivity in the visible range is interpreted in terms of the detrimental role (charge recombination) played by both the occupied intra-band gap states associated to Ce3+ and the corresponding oxygen vacancies formed in the lattice by charge compensation.

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20.

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, the perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related two-dimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) hybrids both as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSCs. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both hole-extraction and electron-blocking properties. In fact, on one hand, intrinsic n-type doping-induced intraband gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to >3.2 eV for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of the conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional nature of RGO effectively plugs the pinholes of the MoS2 QD films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enables MAPbI3-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs.

Optical properties of anatase TiO2: synergy between transition metal doping and oxygen vacancies.

Charge carriers (electrons and holes) are generated on the TiO2 using UV radiation; this excitation energy can be reduced by modifying the material electronic structure, for example, by doping or creating oxygen vacancies. Here, the electronic structure of a transition metal-doped anatase, bulk and surface, and their interaction with oxygen vacancies are studied using density functional theory. The visible light response of metal-doped TiO2 (101) is also determined. Transition metals generate intra-band gap states, which reduce the excitation energy but may also act as charge recombination sites. Dopants Fe, Co, and Ni remarkably enhance the visible light response due to the states in the middle of the gap. However, Co and Ni create heavier charge carriers. Our results show that Pd and Pt-doped TiO2 generate states near the valence and conduction band with a "clean" band gap (without states in the middle of the gap). Moreover, Pt-doped TiO2 maintains the charge mobility because it presents a small charge carriers mass. Hence, Pt-doped TiO2 represents the best alternative to activate TiO2 under visible light. The optical response of transition metal-doped TiO2 follows the order 3d > 4d > 5d. The oxygen vacancies reduce the response of metal-doped TiO2 to visible light because the unpaired electrons generated occupy the empty states of metal-doping. Graphical Abstract Density of states of TiO2 (101) surface doped with transition metals and oxygen vacancies.

Detection of intra-band gap defects states in spin-coated sol-gel SnOx nanolayers by photoelectron spectroscopies

The presence of occupied intra-band gap states in oxygen-deficient tin dioxide (SnOx; 1 < x < 2) is crucial for efficient manufacturing of multipurpose electronic devices based on transparent conducting oxides. Former experimental determination of these states was conducted for well-defined, usually thick tin oxides obtained under highly controlled vacuum conditions. In this work, we present precise specification of gap defects states for ultra-thin SnOx layers prepared by sol-gel synthesis followed with spin-coat deposition. Post-deposition drying and annealing processing changed layers’ surface morphology and bulk crystalline structure as monitored by scanning electron microscopy, atomic force microscopy and x-ray diffraction. An x-ray photoemission spectroscopy (XPS) analysis of chemical composition revealed the presence of both Sn2+ and Sn4+ species in layers with and without post-drying annealing step. A stronger contribution of SnO was found for dried SnOx. In the valence band region, XPS studies revealed pronounced O 2p and hybridised Sn 5p/5s–O 2p states as well as deep, overlapping with the O 2p, band gap states resulting from Sn 5s orbitals. These states—attributed to defect states—indicated enhanced presence of Sn2+ cations, and were assigned to ‘bridging’ oxygen vacancies. Complementary photoemission yield spectroscopy (PYS) studies of the SnOx band gap region revealed an increased effective density of occupied electronic states below the Fermi level EF for annealed layers. The consequence was a work function reduction by 0.15 eV after the annealing process. PYS results allowed a precise detection of SnOx shallow band gap states close to EF. These states were attributed to surface oxygen vacancies, which was confirmed by computer modelling. Finally, the annealed layers exhibited higher calculated charge carrier concentration, hence the increased n-type character.

Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations.

We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.

Perylene Diimide Aggregates on Sb-Doped SnO2: Charge Transfer Dynamics Relevant to Solar Fuel Generation

The deposition of perylene diimide-based aggregates (PDI) onto wide band gap n-type Sb-doped SnO2 (ATO) was investigated with the aim of finding efficient and versatile dye-sensitized platforms for photoelectrochemical solar fuel generation. These ATO-PDI photoanodes displayed hydrolytic stability in a wide range of pH (from 1 to 13) and revealed superior performances (up to 1 mA/cm2 net photocurrent at 1 V vs SCE) compared to both WO3-PDI and undoped SnO2-PDI when used in a photoelectrochemical setup for HBr splitting. Although ATO, SnO2, and WO3 are endowed with similar conduction band edge energetics, in ATO the presence of a significant density of intrabandgap states, whose occupancy varies with the applied potential, plays a substantial role in tuning the efficiency of photoinduced charge separation and collection. Furthermore, the investigation of the charge injection kinetics confirmed that, even in the absence of applied bias, ATO and WO3 are the best substrates for the oxidative quenching of poor...

Emission Properties Related to Distinct Phases of Sol-Gel Dip-Coating Titanium Dioxide, and Carrier Photo-Excitation in Different Energy Ranges

Titanium dioxide (TiO2) in the form of pellets (pressed powder) and thin films are investigated, revealing the presence of distinct phases: mainly anatase and rutile. Characterization of optical, structural and electrical properties were carried out on samples submitted to different sort of thermal annealing (TA), at distinct temperatures, 500 and 1000ºC, due to their influence on the obtained phases. TA temperature along with pressure application for sample conformation, determine the bands present in the photoluminescence (PL) spectra, being about 550nm, characteristic of anatase phase and 800nm, related to the presence of rutile phase. The bandgap of thin films is determined from optical absorbance data, yielding 3.4eV for anatase phase (indirect transition), and 2.9eV for rutile phase (direct transition). Besides, irradiation with monochromatic light strongly affects the thin film conductivity, but the energy range (above or below the bandgap energy) does not seem to affect the behavior, which is associated with the excitation of intrabandgap states or crystallites belonging to phases with distinct bandgaps.

Direct Femtosecond Observation of Charge Carrier Recombination in Ternary Semiconductor Nanocrystals: The Effect of Composition and Shelling

Heavy-metal free ternary semiconductor nanocrystals are emerging as key materials in photoactive applications. However, the relative abundance of intra-bandgap defect states and lack of understanding of their origins within this class of nanocrystals are major factors limiting their applicability. To remove these undesirable defect states which considerably shorten the lifetimes of photogenerated excited carriers, a detailed understanding about their origin and nature is required. In this report, we monitor the ultrafast charge carrier dynamics of CuInS2 (CIS), CuInSSe (CISSe), and CuInSe2 (CISe) nanocrystals, before and after ZnS shelling, using state-of-the-art time-resolved laser spectroscopy with broadband capabilities. The experimental results demonstrate the presence of both electron and hole trapping intra-bandgap states in the nanocrystals which can be removed significantly by ZnS shelling, and the carrier dynamics is slowed down. Another important observation remains the reduction of carrier lifetime in the presence of Se, and the shelling strategy is observed to be less effective at suppressing trap states. This study provides quantitative physical insights into the role of anion composition and shelling on the charge carrier dynamics in ternary CIS, CISSe, and CISe nanocrystals which are essential to improve their applicability for photovoltaics and optoelectronics.

Multiple Energy Exciton Shelves in Quantum-Dot–DNA Nanobioelectronics

Quantum dots (QDs) are semiconductor nanocrystallites with multiple size-dependent quantum-confined states that are being explored for utilizing broadband radiation. While DNA has been used for the self-assembly of nanocrystals, it has not been investigated for the formation of simultaneous conduction pathways for transporting multiple energy charges or excitons. These exciton shelves can be formed by coupling the conduction band, valence band, and hot-carrier states in QDs with different HOMO-LUMO levels of DNA nucleobases, resulting from varying degrees of conjugation in the nucleobases. Here we present studies on the electronic density of states in four naturally occurring nucleobases (guanine, thymine, cytosine, and adenine), which energetically couple to quantized states in semiconductor QDs. Using scanning tunneling spectroscopy of single nanoparticle-DNA constructs, we demonstrate composite DOS of chemically coupled DNA oligonucleotides and cadmium chalcogenide QDs (CdS, CdSe, CdTe). While perfectly aligned CdTe QD-DNA states lead to exciton shelves for multiple energy charge transport, mismatched energy levels in CdSe QD-DNA introduce intrabandgap states that can lead to charge trapping and recombination. Although further investigations are required to study the rates of charge transfer, recombination, and back-electron transfer, these results can have important implications for the development of a new class of nanobioelectronics and biological transducers.