Single walled carbon nanotube (SWCNT) and alkaline metal oxide have been identified as potential materials for management of CO2 emission. Yet the underlying operating mechanism is still not well understood, while an in-depth understanding would possibly lead to development of superior CO2 monitoring, capture, and storage devices. Here we present ab initio density functional theory calculations to provide a comprehensive description of CO2 gas interaction with SWCNT and CaO surface. In particular, our results revealed that CO2 is chemisorbed on CaO surface with negligible effect on electronic properties of the absorbent, while CO2 interaction with SWCNT can be categorized as physisorption interaction a process that can be easily reversed using thermal treating of the tube at 150 °C. Thus CaO is found to be ideal for long term storage of CO2 while SWCNT reported superior performance in CO2 sensing and capture. This work may guide the development of better devices based on CaO and SWCNT for CO2 sensing, capture, and storage.

Single-photon emitters in hexagonal boron nitride have attracted great attention over the last few years due to their excellent optoelectronical properties. Despite the vast range of results reported in the literature, studies on substitutional impurities belonging to the 13th and 15th groups have not been reported yet. Here, through theoretical modeling, we provide direct evidence that hexagonal boron nitride can be opportunely modified by introducing impurity atoms such as aluminum or phosphorus that may work as color centers for single-photon emission. By means of density functional theory, we focus on determining the structural stability, induced strain, and charge states of such defects and discuss their electronic properties. Nitrogen substitutions with heteroatoms of group 15 are shown to provide attractive features (e.g. deep defect levels and localized defect states) for single-photon emission. These results may open up new possibilities for employing innovative quantum emitters based on hexagonal boron nitride for emerging applications in nanophotonics and nanoscale sensing devices.

Single walled carbon nanotubes has been identified as a potential material for CO2 gas sensing, capture and storage, however, comprehensive understanding of adsorption/desorption mechanisms that drives these application is still lacking yet such knowledge is essential for mainstream application of SWCNT in the identified areas. In this work, we use Density Functional Theory to study CO2 storage and sensing on SWCNTs with the aim of unraveling how such applications can be enhanced with the introduction of dopants with emphasis on Al, B, N and S as potential dopants. It is observed that doping SWCNT with N and B can be easily achieved compared to Al and S, which reported high and positive formation energies thus, can only be achieved under non-equilibrium condition. N doping improves SWCNT interaction with CO2 molecules and when subjected to thermal treatment the adsorbed CO2 is release to the atmosphere at 423 K thus a reusable sensing element can be achieved. It was further observed that the diffusion of molecular CO2 within the proximity of Al and S dopants in SWCNT matrix is not favored, while N and B doped SWCNT tend to have lower barrier energies to CO2, thus can offer better control of carbon storage. Our finding can assist in the design and optimization of SWCNT for energy and environmental applications.

ZnO nanowires have been proposed as potential photo-anode materials for photo-electrochemical water splitting due to their low toxicity, simple synthesis and easy modification routes. However, ZnO suffers from low PEC activity and photo-corrosion eff ;ects, and therefore, application of ZnO nanowires in PEC water splitting still awaits development of effective design and synthesis strategies to improve its PEC efficiencies to commercially viable levels. Here, we present ab initio Density Functional Theory calculations considering 3d transition metal doping as a potential route towards attainment of ZnO nanowires with superior PEC activity. Our results show that the stability of 3d transition metal dopants in ZnO NWs is dependent on the d character of the transition metal dopant as well as their concentration and doping site, with most transition metal atoms being energetically most favorable at the Zn substitutional site both in O-rich and Zn-rich conditions considered. Specifically, we find all 3d transition metal dopants in ZnO NW under O-rich conditions as well as Sc, Ti and V under Zn-rich conditions have negative formation energies at the considered dopant concentrations of 1−6 atm. %, indicating that these dopants can readily be incorporated into ZnO NWs at thermodynamic equilibrium conditions. The electronic properties of Ti and V at 2% and 4% dopant concentration, respectively, yield a staggered band-structure configuration, while Sc, Cr, Mn, Co, Ni, and Cu dopants in ZnO NWs induce band-edge states. In addition, 3d TM dopants induces significant red-shift of the absorption edge of ZnO NW due to reduction in band gap, and are projected to improve visual light harvesting capabilities. Finally, the band alignment relative to the redox potential of water revealed that the valence band maximum of Sc, V, Ni and Cu doped ZnO NWs remains strongly positive above the oxidation potential of O2/H2O, while their reduction potential remain negative below the reduction potential of H+/H2, favouring PEC applications.

Niobium carbides and nitrides have been proposed as potential candidates for hardness and related applications, however, comprehensive studies are still needed for better understanding that may pave way for their re-engineering for the ultra hard industry. Here we present ab initio Density Functional Theory calculations that provide a comprehensive description of various hardness characterization parameters. Our results show that NbC in rocksalt (RS) had a higher shear modulus, Young's modulus, and Voigt-Reuss-Hill shear modulus compared to other phases of NbC and NbN considered in this work. Further, it was noted that NbC in RS had a higher value of Vickers hardness amongst the various phases NbC and NbN studied, thus identified as a potential candidate for hardness and related application. Finally, we showed that compounds with Vickers hardness (Hv) > 20 GPa were found to be brittle while those with Hv < 20 GPa were ductile.

Understanding interface properties of heterojunction is an important step towards controllable and tunable interfaces for photocatalytic and photovoltaic based devices. To this aim, we propose a thorough study of a double heterostructure system consisting of two semiconductors with large band gap, namely, wurtzite ZnO and anatase TiO2. We demonstrate via first-principle calculations two stable configurations of ZnO/TiO2 interfaces. Our structural analysis provides a key information on the nature of the complex interface and lattice distortions occurring when combining these materials. The study of the electronic properties of the sandwich nanostructure TiO2/ZnO/TiO2 reveals that conduction band arises mainly from Ti3d orbitals, while valence band is maintained by O2p of ZnO, and that the trapped states within the gap region frequent in single heterostructure are substantially reduced in the double interface system. Moreover, our work explains the origin of certain optical transitions observed in the experimental studies. Unexpectedly, as a consequence of different bond distortions, the results on the band alignments expect different electron accumulation between the two interfaces. Such behavior provides more choice for the sensitization and functionalization of TiO2 surfaces.

The effect of nitrogen doping on the structural, electronic and optical properties of hexagonal and cubic Ge₂Sb₂Te₅ (GST) has been investigated from first principles. The nitrogen content was set to 10 at.% and 25 at.%, in which it was found that the hexagonal phase becomes more stable, whereas the cubic phase becomes more unstable with increasing nitrogen content. A difference in optical reflectivity of about 8% was calculated for pure hexagonal and cubic phases. Pure GST was found to possess a higher reflectivity contrast in the infrared spectral range, whereas doped GST had a higher reflectivity contrast, which increased with rising nitrogen content, in the visible and ultraviolet spectral range. The optical conductivity was found to fall with increasing nitrogen content, in agreement with experiment and other theoretical studies.

In this work, ZnO thin films were investigated to sense NO2, a gas exhausted by the most common combustion systems polluting the environment. To this end, ZnO thin films were grown by RF sputtering on properly designed and patterned substrates to allow the measurement of the electrical response of the material when exposed to different concentrations of the gas. X-ray diffraction was carried out to correlate the material’s electrical response to the morphological and microstructural features of the sensing materials. Electrical conductivity measurements showed that the transducer fabricated in this work exhibits the optimal performance when heated at 200 °C, and the detection of 0.1 ppm concentration of NO2 was possible. Ab initio modeling allowed the understanding of the sensing mechanism driven by the competitive adsorption of NO2 and atmospheric oxygen mediated by heat. The combined theoretical and experimental study here reported provides insights into the sensing mechanism which will aid the optimization of ZnO transducer design for the quantitative measurement of NO2 exhausted by combustion systems which will be used, ultimately, for the optimized adjustment of combustion resulting into a reduced pollutants and greenhouse gases emission.

Oxygen vacancies in ZnO crystals have significant impacts on its properties and applications. On the basis of ab initio results, we describe the oxygen vacancy distribution and diffusion paths away from the ZnO( 11¯00) surface, aiming to elucidate thermodynamics and kinetic stability of the vacancies and a possible control mechanism. In view of defect engineering and sensor applications, we propose efficient routes to chemically control the equilibrium concentration of the oxygen vacancies at ZnO surfaces by exposure to specific reactive gases: we show that the oxygen vacancy concentration can be increased using sulfur oxide as post-growth treatment, while under exposure to ozone, no significant amount of oxygen vacancies can be sustained on the surface.

Solid-state nanostructured gas sensors based on oxide materials play an important role in environmental monitoring, chemical process control, and personal safety. Yet, the underlying operating mechanism is still not well comprehended, while a deeper understanding would possibly lead to the engineering of sensing elements with enhanced sensitivities. Here we present ab initio density functional theory calculations that provide a comprehensive description of the ethanol sensing mechanism for ZnO nanowires: our results reveal that the competitive adsorption at the nanostructure surfaces between the analyte and the oxygen molecules present in the atmosphere induces a switching in surface conductance between semiconducting and conducting behavior that is related to the ethanol concentration and can be detected electronically, thus disclosing the sensing mechanism.

Nanowires made of materials with non-centrosymmetric crystal structures are expected to be ideal building blocks for self-powered nanodevices due to their piezoelectric properties, yet a controversial explanation of the effective operational mechanisms and size effects still delays their real exploitation. To solve this controversy, we propose a methodology based on DFT calculations of the response of nanostructures to external deformations that allows us to distinguish between the different (bulk and surface) contributions: we apply this scheme to evaluate the piezoelectric properties of ZnO [0001] nanowires, with a diameter up to 2.3 nm. Our results reveal that, while surface and confinement effects are negligible, effective strain energies, and thus the nanowire mechanical response, are dependent on size. Our unified approach allows for a proper definition of piezoelectric coefficients for nanostructures, and explains in a rigorous way the reason why nanowires are found to be more sensitive to mechanical deformation than the corresponding bulk material.

Bulk properties and stability of the entire series of group 4d transition metal carbides and nitrides are reported in this work. The theoretical calculations were carried out within Local Density Approximation and Generalized Gradient Approximation using the Perdew, Burke and Ernzerhof exchange correlation functional. The generalized gradient approximation predictions were found to be closer to experimental values than the local density approximation predictions. In particular, LDA predictions were found to overestimate bulk moduli properties by as much as 5.6–11.5% while equilibrium lattice constants were found to be underestimated by as much as 0.2–5% compared to experimental values. On the other hand, GGA calculations were found to overestimate the lattice parameters by 0.2–6.9%, while underestimating the bulk moduli by as much as 0.07–5%. Out of the carbides considered, TcC and RuC were found to have the highest values of bulk moduli while YC and CdC had the lowest. Similarly, out of the nitrides, MoN and TcN were found to exhibit the largest bulk moduli, indicating that they were the hardest, while CdN had the lowest value and hence relatively softer. Overall, the nitrides presented higher values of bulk moduli than the carbides, an observation that is well supported by their correspondingly shorter bondlengths. The cohesive and structural properties of the 4d transition metal carbides and nitrides are also reported.