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We report on a photon (â¼3.08 eV equivalent to 402 nm) controlled optical emission from ZnO (101¯0). Under below band gap excitation (â¼2.33 eV equivalent to â¼532 nm), significant photoluminescence (PL) overlapped with Raman response is observed. The broad PL consists of three bands (629 (A), 690 (B), and 751 (C) nm) attributed to the defects arising due to excess zinc and charged oxygen vacancy. By employing asecondexcitation source at 402 nm, we demonstrate about 50% reduction in the overall PL. We utilize the doubly positive oxygen vacancy state to control the PL emission while transiently reducing its density.
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Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.
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We report a study on the stability, optical absorption and modulated electronic properties of the ZnO(0001-) and MoS2Van der Waals heterostructure using density functional theory. We employed a supercell of ZnO/MoS2hybrid and specifically explored the effects of creating an interface with the O-terminated face of ZnO while considering the interlayer interaction. We observed an increase in the band gap opening of MoS2within the hybrid structure (1.37 eV) is primarily attributed to in-plane strain, with minimal contribution from the identified charge transfer occurring from MoS2to ZnO. Notably, the hybrid structure exhibits enhanced photo absorption in the visible and near-infrared regions, highlighting their significance for optoelectronic applications.
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Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defects or dopant levels, creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extent of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2015, 91, 235313]. The transient dopants exhibit a multi-step decay ranging from hundreds of picoseconds to 77 µs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we have described previously [Gierster et al., Nat. Commun., 2021, 12, 978]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to µs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in the literature [Madel et al., J. Appl. Phys., 2017, 121, 124301]. In agreement with the theory on PPC [Lany and Zunger, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 035215], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties, such as the electrical conductivity, from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects.
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Molecular adsorption at the surface of a two-dimensional material poses numerous questions regarding the modification to the band structure and interfacial states, which of course deserve full attention. In line with this, first principles density functional theory is employed on a graphene/ammonia system. We identify the effects on the band structure due to strain, charge transfer and presence of molecular orbitals (MOs) of NH3 for six adsorption configurations. Induced-strain upon ammonia-adsorption opens the band gap (E g) of graphene due to the breaking of translational symmetry. The charge transfer/MOs of NH3 shifts the equilibrium Fermi energy (E F). The E g and E F values and charge density distribution are dependent on the adsorption configuration, where the MO structure of NH3 plays a crucial role. The presence of MOs of N or H-originated pushes the unoccupied states of graphene towards E F. NH3 forms an interfacial occupied state originating from N2p below the E F within â¼1.6 to 2.2 eV for all configurations. These findings enhance fundamental understanding of graphene/NH3 system.
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The conversion of optical and electrical energies in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2',7'-ditertbutyl-9,9'-spirobifluorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron and optical spectroscopies. We find that, after initial ultrafast relaxation on femtosecond and picosecond time scales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (rDex = 1.3 nm) and exhibits a long decay time of 0.1 s. Because of this long lifetime, a photostationary state with average T-T distances below 2 nm is established at excitation densities in the 1020 cm-2 s-1 range. This large density enables decay by T-T annihilation (TTA) mediating autoionization despite an extremely low TTA rate of kTTA = 4.5 â 10-26 m3 s-1. The large external quantum efficiency of the autoionization process (up to 15%) and photocurrent densities in the mA cm-2 range offer great potential for light-harvesting applications.
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Graphene oxide (GO) was prepared by a solvothermal synthesis method using sodium and ethanol. A sequence of pyrolysis, washing and purification steps was developed for the total removal of all by-products. The first pyrolysis step is essential to obtain graphitic forms of carbon while a washing and a second pyrolysis step further improved the graphenic structures obtained via the reduction of OH/COOH and C-O groups and the attendant increase in C[double bond, length as m-dash]C bonding (sp2 hybridization). Two purification processes were employed to remove sodium carbonate (by-product), i.e. vacuum filtration and centrifugation, but the latter produced a more stable GO product, typically with a few-layer (ca. 3 nm) stack and relatively long platelets (up to ca. 1.3 µm). The functionality of this GO was demonstrated by preparing composites of it with poly(ε-caprolactone) (PCL). Some of the GO was arranged in flower-like domains dispersed in the PCL matrix. The crystalline content of PCL decreased on addition of GO, though the dynamic modulus of PCL increased and an electrical percolation at 0.5 vol% GO was obtained, manifest by a â¼104 increase in electrical conductivity (in an overall increase of â¼105 achieved at >1 vol%), more than sufficient for anti-static applications.
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Due to its wide band gap and high carrier mobility, ZnO is, among other transparent conductive oxides, an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the nonequilibrium dynamics after photoexcitation: time-resolved photoluminescence, excited state transmission, and electronic sum-frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties. Noteworthy, a similar competition of intrinsic and defect-related dynamics could likely also be utilized in other oxides with marked defect density as, for instance, TiO2 or SrTiO3.
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We report negative photoresponse or increase of resistance in nanocomposites of n-type ZnO nanoparticles dispersed in a p-type polymer (PEDOT:PSS) under UV and visible light excitation, contrary to that of planar heterojunctions of the constituents. The underlying mechanism of charge transport, specifically negative photoresponse, is explored using spectroscopic and opto-electrical characterisation. Systemic variability in conductance, photoresponse sensitivity and rate with fractional nanoparticle loading in the nanocomposite is demonstrated. Here, photogenerated electrons in ZnO nanoparticles, trapped by the unbiased interfacial barrier, are understood to localize holes in the PEDOT:PSS conduction channel thereby increasing the overall nanocomposite resistance. Reversibility of the negative PR although with a slow decay rate bears testament to the proposed photogating mechanism as opposed to photocatalytic activity. Replacement of the p-type polymer with an electron transport matrix turns the negative photoresponse positive accentuating the role of the interfacial barrier in tuning the optoelectronic response of the composites. These hybrid materials and their unusual behaviour provide alternative strategies for building devices with novel photogating effects, exploiting the properties of their nanostructured forms.
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We have juxtaposed the structural, vibrational and emission properties of graphene oxide (GO) with various degrees of reduction with and without a model dispersant, unveiling a strong associative behavior between GO sheets and the influence of H-bonds. The interlayer spacings are â¼0.84 and 0.78 nm for the as prepared and reduced samples. -OH groups are predominantly effected by the photo-thermal reduction. Also we note some regeneration of [double bond splayed left]C[double bond, length as m-dash]O and -COOH groups in reduced samples. Clear changes to the phonon density of states indicated the doping effects due to H-bonds via the oxygeneous groups. Importantly, the defect related Raman bands are rather prone to the effect of dispersant, unveiling their intrinsic nature. In the context of fluorescence, internal vibration relaxation mediated by CC stretch vibrations emphasized the localized nature of sp2 domains of relatively smaller size. Fluorescence consists of 6 components, where the higher energy components are more influenced due to H-bonds than those of the lower energy regime, attributed to their associative behavior and chemical functionality, respectively. Excitation dependent fluorescence measurements indicated a range of optical gaps from â¼3.5 to 2 eV. The associative behavior of GO and rGO with and without a dispersant provides crucial insights into the fundamental understanding of various molecular processes.
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In ref. [Nat. Nanotechnol., 2012, 7, 465-471] interesting optoelectronic properties of ZnO/graphene oxide (GO) composite were presented. Essentially, in the luminescence spectrum indirect optical transitions were identified to be from the epoxy group of GO (GOepoxy) to the valance band (Ev) of ZnO. Viz. 406 nm, L1: (LUMO+2)GOepoxyâEv and 436 nm, L2: (LUMO)GOepoxyâEv. Furthermore, the emission peak at â¼550 nm was attributed to zinc interstitials (Znis) or oxygen vacancies (VOs) and shown to span from 350-650 nm (equivalent to a width of â¼0.8 eV). In this report we accentuate two vital though largely ignored concerns as itemized in the following. (i) By considering the growth mechanism of ZnO in the composite, there is a certain possibility that these two bands (L1 and L2) may originate from intrinsic defects of ZnO such as Znis and extended Znis (ex-Znis). Or L1 and L2 might be intrinsic to GO. (ii) The 550 nm emission involves VOs and consists of two components with a typical width of â¼0.3 eV. Here we present the results of a thorough investigation confirming the presence of Znis, ex-Znis and intrinsic emission from GO. We also note that during the synthesis the presence of dimethyl formamide significantly affected the emission from GO in addition to some chemical modifications. Apart from these, we have discussed other crucial factors which require deeper attention in the context of luminescence from complex systems such as those present.
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Hydrogen is an attractive alternative to fossil fuels in terms of environmental and other advantages. Of the various production methods for H2, photocatalysis requires further development so that it can be applied economically on an industrial scale. One- and two-dimensional nanostructures in both pristine and modified forms have shown great potential as catalysts in the generation of H2. We review here recent developments in these nanostructure catalysts and their efficiency in the generation of H2 under UV/visible/simulated solar light. Despite much research effort, many photocatalysts do not yet meet the practical requirements for the generation of H2, such as visible light activity. H2 production is dependent on a variety of parameters and factors. To meet future energy demands, several challenges in H2 production still need to be solved. We address here the factors that influence the efficiency of H2 production and suggest alternatives. The nanostructures are classified based on their morphology and their efficiency is considered with respect to the influencing parameters. We suggest effective ways of engineering catalyst combinations to overcome the current performance barriers.
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2D crystals such as graphene and its oxide counterpart have sought good research attention for their application as well as fundamental interest. Especially graphene oxide (GO) is quite interesting because of its versatility and diverse application potential. However the mechanism of fluorescence from GO is under severe discussion. To explain the emission in general two interpretations were suggested, viz localization of sp(2) clusters and involvement of oxygeneous functional groups. Despite this disagreement, it should be acknowledged that the heterogeneous atomic structure, synthesis dependent and uncontrollable implantation of oxygen functional groups on the basal plane make such explanations more difficult. Nevertheless, a suitable explanation enhances the applicability of the material which also enables the design of novel materials. At this juncture we believe that given the complexity in understanding the emission mechanism it would be very useful to review the literature. In this perspective we juxtapose various results related to fluorescence and influencing factors so that a conclusive interpretation may be unveiled. Apparently, the existing interpretations have largely ignored the factors such as self-rolling, byproduct formation etc. Vis-a-vis previous reviews did not discuss the interfacial charge transfer across heterostructures and the implication on the optical properties of GO or reduced graphene oxide (rGO). Such analysis would be very insightful to determine the energetic location of sub band gap states. Moreover, ionic and π-π type interactions are also considered for their influence on emission properties. Apart from these, quantum dots, covalent modifications and nonlinear optical properties of GO and rGO were discussed for completeness. Finally we made concluding remarks with outlook.
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Oxygen vacancies (V(O)s) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zn(i)) and V(O)s on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Zn(i)s were predominant in ZnO quantum dots (QDs), while the presence of V(O)s was negligible. As the ALD cycle number increased, V(O)s were introduced, whereas the density of Zn(i) remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Zn(i)s and V(O)s. A logical comparison of the relative defect densities of Zn(i)s and V(O)s suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.
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Heterojunctions are a well-studied material combination in photocatalysis studies, the majority of which aim to improve the efficacy of the catalysts. Developing novel catalysts begs the question of which photo-generated charge carrier is more efficient in the process of catalysis and the associated mechanism. To address this issue we have fabricated core-shell heterojunction (CSHJ) nanofibers from ZnO and TiO2 in two combinations where only the 'shell' part of the heterojunction is exposed to the environment to participate in the photocatalysis. Core and shell structures were fabricated via electrospinning and atomic layer deposition, respectively which were then subjected to calcination. These CSHJs were characterized and studied for photocatalytic activity (PCA). These two combinations expose electrons or holes selectively to the environment. Under suitable illumination of the ZnO-TiO2 CSHJ, e/h pairs are created mainly in TiO2 and the electrons take part in catalysis (i.e. reduce the organic dye) at the conduction band or oxygen vacancy sites of the 'shell', while holes migrate to the core of the structure. Conversely, holes take part in catalysis and electrons diffuse to the core in the case of a TiO2-ZnO CSHJ. The results further revealed that the TiO2-ZnO CSHJ shows â¼1.6 times faster PCA when compared to the ZnO-TiO2 CSHJ because of efficient hole capture by oxygen vacancies, and the lower mobility of holes.
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Zinc oxide is synthesised at low temperature (80°C) in nanosheet geometry using a substrate-free, single-step, wet-chemical method and is found to act as a blue-white fluorophore. Investigation by atomic force microscopy, electron microscopy, and X-ray diffraction confirms zinc oxide material of nanosheet morphology where the individual nanosheets are polycrystalline in nature with the crystalline structure being of wurtzite character. Raman spectroscopy indicates the presence of various defects, while photoluminescence measurements show intense green (centre wavelength approximately 515 nm) blue (approximately 450 nm), and less dominant red (approximately 640 nm) emissions due to a variety of vacancy and interstitial defects, mostly associated with surfaces or grain boundaries. The resulting colour coordinate on the CIE-1931 standard is (0.23, 0.33), demonstrating potential for use as a blue-white fluorescent coating in conjunction with ultraviolet emitting LEDs. Although the defects are often treated as draw-backs of ZnO, here we demonstrate useful broadband visible fluorescence properties in as-prepared ZnO.