FeMo sub-nanoclusters/single atoms for neutral ammonia electrosynthesis
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99 GENERAL AND MISCELLANEOUS↗
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Enhanced thermoelectric performance of PbSe-graphene nanocomposite manufactured with acoustic cavitation induced defects
An order of magnitude rise in the thermoelectric (TE) performance of the PbSe, a scalable and easy-to-manufacture TE material, has been achieved by incorporating reduced graphene oxide (Gr) nanoplatelets in a PbSe/PbSeO 3 heterostructure formed by acoustic cavitation-assisted oxidation. The fabricated Gr/PbSe/PbSeO 3 nanocomposites exhibit high TE performance with an exceptionally high Seebeck coefficient coupled with low thermal conductivity. The variation in the Seebeck coefficient has been attributed to a reduction in charge carrier mobility due to the ferroelectric polarization effect. Furthermore, the increase in electrical resistivity is minimized by adding graphene. At an optimal weight fraction (0.2 wt%), graphene nano-inclusions lead to superior Seebeck coefficient values as high as ~2000 μV/K at ~500 K, providing high overall TE performance. This study shows substantial changes in the TE properties of PbSe through the incorporation of graphene and PbSeO 3 . The understanding and methodology developed in this study can be exploited for the scalable manufacturing of high-performance TE materials.
Structure-property correlations↗
Highly microporous Parinari Curatellifolia carbon nanomaterials for supercapacitors
High surface area nanostructured electrodes are essential for understanding charge storage mechanisms in high capacitance supercapacitors. Additionally, a growing number of studies provide credible evidence that the highest capacitance in carbon nanomaterials is attainable only when the pore size is in micropore range. In this study therefore, micropore-dominatedcarbon materials derived from Parinari curatellifolia (PC) waste seeds are prepared via pyrolysis and subsequent chemical activation with KOH. SEM, EDAX, TEM, Raman, and XRD studies show that the microstructure and composition of the as-prepared microporous carbon materials are influenced by adjusting the KOH to carbon mass ratio. Sorption studies demonstrate a very high BET surface area of 1898 m 2 g –1 , type I isotherm, and 99% micropore content in all the samples. The BET surface area, S micro , V micro , and total volume increased with increasing KOH content during activation. The fabricated electrodes exhibit high specific capacitances of 423 F g –1 at 5 mV s –1 and cyclic stability of 98% after 50000 cycles both of which are indicative of our carbon materials huge potential in making high performance supercapacitors. The specific capacitance increased with the micropore content in the samples signifying that the pores enhanced diffusion and hence the charge storage. The PC carbon materials deliver high specific capacitance due to the enhancement of diffusion and charge storage stemming from the synergetic interplay of the aforementioned textural parameters. As a result, the PC carbon materials therefore provide good prospects for supercapacitor applications.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Concentric core-shell tracks and spectroscopic properties of SrTiO 3 under intense electronic excitation
A deeper understanding of the intrinsic link between the irradiation-induced microstructures and the corresponding spectroscopic properties is becoming increasingly attractive for the research fields of materials science, physics, and information technology. In this work, the structural damage response, spectroscopic features, and associated physical mechanisms of SrTiO 3 single crystals under swift heavy ion irradiation were comparatively analyzed using a combination of experimental and theoretical approaches. Corresponding to 0.11–5.00 MeV/u ion irradiation with electronic energy loss ranging from 4.0 to 29.3 keV/nm, the inelastic thermal spike calculations combined with molecular dynamics simulations are compared with the experimental observations, revealing the track fine structures (individual spherical defects with a disordered region, and discontinuous and continuous tracks consisting of an amorphous core and a disordered outer shell) and demonstrating the dominant effects of the deposition energy and lattice temperature on track damage formation and evolution; thus, two essential thresholds for defect formation of ~0.60 eV/atom and amorphous region formation of ~1.81 eV/atom were identified to better describe the concentric core-shell track structure. The enhanced nanohillock formation in the surface region is attributed to the combined action of kinetic and potential energy depositions, and the fluence dependence in regulating the hillock dimensions is also presented. The measured refractive index profiles along different crystal axes further indicate the anisotropy of irradiation-induced lattice expansion. With increasing ion fluence and damage level, the intrinsic bandgap gradually decreased, and the concentration of additional radiative recombination centers (1.81 eV, 2.31 eV, and 2.42 eV) accordingly increased in the SrTiO 3 crystal, providing the possibility of regulating related defects to achieve tunable/selective photon emission for more critical technological applications.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Fluid learning: Mimicking brain computing with neuromorphic nanofluidic devices
Relentlessly rising energy demands in computing call for rethinking hardware paradigms with energy efficiency in mind. Nature’s example—the brain—raises the question: How can these natural computers achieve remarkable feats with minimal energy compared to supercomputers? Neuromorphic computing mimics the brain’s principles, but current neuromorphic concepts using electronic components face scalability and their own power consumption challenges. A potentially revolutionary approach is emerging: computing with ion transport in water through nanochannels. This field offers energy-efficient possibilities by imitating brain-like information processing with different types of ions as carriers. Finally, the goal is to converge advanced nanoscale architectures with brain-inspired efficiency, heralding a new era of computing.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Catalyzed oxidation of IG-110 nuclear graphite by simulated fission products Ag and Pd nanoparticles
To evaluate the stability of nuclear materials in high temperature gas reactors under air ingress conditions, catalytic oxidation of IG-110 graphite by two simulated fission products, metallic Pd and Ag, was studied in oxidative atmosphere and at temperatures up to 1000°C using an integrated furnace, mass spectroscopy and infrared spectroscopy system. Transmission electron microscopy and X-ray diffraction studies show that Pd and Ag nanoparticles were successfully introduced onto powdery IG-110 graphite through an impregnation and subsequent heat-treatment process. The combined mass spectroscopy and infrared spectroscopy methods allow simultaneous analysis of two gaseous products, CO and CO 2 , and separate measurements of activation energy for their formation reactions. It was found that the introduction of Pd or Ag to IG-110 graphite substantially catalyzed the oxidation of graphite, characteristic of decreased onset temperatures for the oxidation of graphite. Moreover, the catalytic effects by Pd and Ag are considerably different based on measured concentration ratios of CO 2 to CO as a function of oxidation temperatures. Ag makes the graphite oxidation commence at approximately 400°C with CO 2 being the dominant product. In contrast, Pd significantly increases the concentration ratio of CO 2 to CO at temperatures higher than approximately 690°C, although it decreases the onset temperature for the oxidation reaction to around 525°C. To understand the catalytic difference, the mechanism of the graphite oxidation was discussed based on the changes of surface oxygen species on Ag and Pd.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
The microstructure and He + ion irradiation behavior of novel low-activation W-Ta-Cr-V refractory high entropy alloy for nuclear applications
Microstructure and nanohardness of a nearly equimolar W-Ta-Cr-V high entropy alloy (HEA), as well as its irradiation response under He + irradiation, were investigated. The single-phase body-centered cubic nanostructured alloy with a 1 µm thick layer was fabricated on a silicon substrate using a magnetron sputtering method. The HEA film has a complex microstructure consisting of micrometric domains that exhibit internal nanostructure controlled by their crystal orientation. The measured nanohardness of the W-Ta-Cr-V alloy is 13 ± 2 GPa, which significantly exceeds the hardness of nanocrystalline tungsten as a result of the high solid-solution strengthening effect. In order to evaluate the irradiation resistance of the HEA film, the material was irradiated with 200 keV He + ions at room temperature, with two different ion fluences: 1 × 10 16 and 5 × 10 16 ions/cm 2 . Using transmission electron microscopy, a high density of extremely fine He bubbles is observed that were uniformly distributed in the matrix. The increase of He + ion fluence increased the density of bubbles, whereas their size remained at a similar level, which indicates that the damage proceeds by the nucleation of additional He bubbles, not by their growth.
36 MATERIALS SCIENCE↗
Comparison of carrier localization effects between InAs quantum dashes and quantum dots in a DWELL (dashes- or dots-in-a-well) configuration
The optical properties of InAs quantum dashes (QDashes) grown on InP and InAs quantum dots (QDots) grown on GaAs in a dashes- or dots-in-a-well (DWELL) configuration are comparatively investigated using temperature-dependent photoluminescence (PL) measurements. The trends in PL characteristics such as exciton energy, spectral bandwidth and integrated intensity with respect to temperature are found to be distinctly dissimilar between the two systems. A rate-equation model involving exciton recombination and thermal transfer in a localized-state ensemble is used to quantitively interpret the experimental data. The results of this study suggest that QDashes in this configuration exhibit PL properties more consistent with a lower degree of carrier localization compared to QDots. A preliminary structural analysis highlighting the shape/size differences between the two nanostructures is also presented.
carrier localization↗
Cs absorption capacity and selectivity of crystalline and amorphous Hf and Zr phosphates
Removal of radioactive Cs from sodium-rich solutions is a technical challenge that goes back to post World War II nuclear waste storage and treatment; and interest in this topic was reinvigorated by the Fukushima-Daiichi nuclear power plant disaster, 10 years ago. Since the 1960's there has been considerable focus on layered Zr phosphates as robust inorganic sorbents for separation of radionuclides such as Cs. Here we present synthesis and characterization, and direct comparison of Cs sorption capacity and selectivity of four related materials: 1) crystalline α-Zr phosphate and α-Hf phosphate, and 2) amorphous analogues of these. Powder X-ray diffraction, thermogravimetry, solid-state 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy, and compositional analysis (inductively coupled plasma optical emission spectroscopy and mass spectroscopy, ICP OES and ICP MS) provided formulae; respectively M(HPO4)2∙1H2O and M(HPO4)2∙4H2O (M = Hf, Zr) for crystalline and amorphous analogues. Maximum Cs loading, competitive Cs-Na selectivity and maximum Cs-Na loading followed by the above characterizations plus 133Cs MAS-NMR spectroscopy revealed that amorphous analogues are considerably better Cs-sorbents (based on maximum Cs-loading and selectivity over Na) than the well-studied crystalline Zr-analogue. Additionally, crystalline α-Hf phosphate is better Cs-sorbent than crystalline α-Zr phosphate. All these studies consistently show that Hf phosphate is less crystallize than Zr phosphate, when obtained under similar or identical synthesis conditions. We attribute this to lower solubility of Hf phosphate compared to Zr phosphate, preventing ‘defect healing’ during the synthesis process.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Current challenges and potential directions towards precision microscale additive manufacturing – Part III: Energy induced deposition and hybrid electrochemical processes
The Part III of the four-part series of articles discusses the challenges and opportunities in microscale additive manufacturing processes, specifically focusing on energy-induced deposition and electrochemical processes. Compared to the direct ink write (DIW) and laser-based processes, the energy-induced deposition methods can fabricate high-resolution, high aspect ratio and complex parts, while the hybrid electrochemical process can be used to fabricate complex parts using a wide range of conductive and photoactive materials. However, the volumetric throughput of these processes is lower than their DIW and laser-based counterparts. The processes that have been explored in this process are Focused-ion Beam Induced Deposition (FIBID), Laser Chemical Vapor Deposition (LCVD), Menicus-confined Electrodeposition (MCED) and Laser-Enabled Electrochemical Printing (LECP). The range of processable materials, feature-size resolution, geometry and volumetric throughput are used as factors to evaluate the current state-of-the-art for these processes. Finally, novel approaches have been proposed in the article to address these challenges associated with microscale AM processes.
42 ENGINEERING↗
Quantitative analysis of Cu XANES spectra using linear combination fitting of binary mixtures simulated by FEFF9
We report x-ray absorption near edge structure (XANES) is a powerful tool to probe the fingerprint of local structures, and when coupled with X-ray microscopy, the small spot size enables one to probe very specific regions of interest in a device or material, e.g. interfaces/bulk, different grains, good/bad electrical areas. In this work, we investigate the use of linear combination fitting (LCF) of XANES spectra for the particular case of Cu doping in CdTe. We show that the experimental data seem to be accurately represented by standards of Cu 2 Te and its substoichiometric counterpart, Cu 1.43 Te. We use Cu in CdTe as a case study to evaluate the accuracy of linear combination fitting using simulated standards, given that experimental standards for certain phases (e.g. Cu 1.43 Te) or defect structures (e.g. vacancies) cannot be readily obtained. We discuss how spectral features of the FEFF9-simulated standards, fitting ranges, and noise levels all dictate the accuracy of this type of analysis. We show that the greater the spectral difference between the two standards, the better the LCF is able to differentiate between the two structures and to tolerate experimental noise. Finally, we estimate the error of the fitted weights for different spectral features and noise levels and propose a framework to study local structures semi-quantitatively by using binary mixtures of FEFF9-simulated standards.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
In situ study of the electronic structure of polar-to-polar SrTiO 3 /($000\bar1$)ZnO heterointerface
The SrTiO 3 (STO)/ZnO heterointerface, which is widely used in the fabrication of novel optoelectronic devices, is a classical system combining functional perovskite oxides and wurtzite-structure semiconductor materials. The electronic structure of the heterointerface often plays a significant role in controlling the functions of novel devices. In this study, the electronic structure was explored using in situ photoemission spectroscopy and X-ray absorption spectroscopy. X-ray diffraction results showed the coexistence of (111) STO and (011) STO orientations for the STO film deposited on the ZnO-($000\bar1$) substrate via pulsed laser deposition. High-resolution transmission electron microscopic results revealed two types of polar interfaces: [$11\bar2$][$10\bar1$](111) STO //[$1\bar210$][$10\bar10$]($000\bar1$) ZnO and [111][$2\bar1\bar1$](011) STO //[$10\bar21$][$10\bar10$]($000\bar1$)ZnO. In situ photoemission spectroscopic results revealed downward band bending and the transformation of the valence states of Ti from 4+ to 3+, with extra electrons transferring to the hybridization states between O 2p and Ti t 2g orbitals at the polar-to-polar STO/ZnO interface. We propose that the polar discontinuity drives the electron transfer to the STO/ZnO interface during the growth process. This study provides insight into the electronic structure of the STO/($000\bar1$)ZnO heterointerface.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Switching limits of top-gated carbon nanotube field-effect transistors
The performance limits of carbon nanotube field-effect transistors (CNFETs) based on a recently reported process are studied by computational techniques at temperatures between 300 K and 4 K. Here, the impact of band-to-band tunneling (BTBT) and source-to-drain tunneling (SDT) is examined for devices with varying gate length through the use of simulations based on the non-equilibrium Green’s function (NEGF) formalism, and calibrated to measurements. Additionally, the case of junctionless chemical doping profiles is analyzed in contrast to electrostatic doping recently reported for test structures. The switching limits of CNFETs are further explored for devices based on carbon nanotubes (CNTs) with more favorable electronic structures.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
H 2 Pc and pentacene on Cu(110)-(2×1)O: A combined STM and nc-AFM study
Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) with a CuO tip is used to investigate adsorption of metal-free phthalocyanine (H 2 Pc) and pentacene on the Cu(110)-(2×1)O striped-phase reconstructed surface. We show that the combination of STM and nc-AFM is necessary to reveal the detailed adsorption geometry, necessary for interpreting the observed STM contrast. Comparison of the nc-AFM images with simulations shows that some of the H 2 Pc molecules are deformed out-of-plane on this surface, while pentacene retains its molecular geometry regardless of adsorption site. Our research highlights how the stiffness of CuO tips makes it possible to probe the structure of organic semiconductor / metal interfaces.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Investigation of coverage dependence of the stretching frequency of CO adsorbed on Pd surfaces at low coverage limits
The stretching frequency of the C—O bond is a sensitive probe of the local environment of a surface-bound CO molecule, including the adorption site and density, i.e. surface coverage. In this work, we extend our analysis beyond the frequency shift due to differences in adsorption configurations. Using density functional theory (DFT) calculations, we directly explore the correlations between surface coverage and the stretching frequency of adsorbed CO on Pd surfaces. Here we also perform constant pressure infrared reflection absorption measurements of CO on Pd(111) and use existing relations between pressure and coverage to derive coverage dependency. Both results are compared to previously reported experimental data. Our derived correlations of peak frequency and area with surface coverage can help interpret experimental IR spectra in real time and extract time-dependent concentration data from transient kinetic experiments.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Hollow-cathode chemical vapor deposition of thick, low-stress diamond-like carbon films
A radio-frequency (RF), hollow-cathode plasma source with confining magnetic field is described for the chemical vapor deposition of thick ( > 10 µm), amorphous diamond-like carbon ablator films for inertial confinement fusion applications. Plasma is characterized by optical emission spectroscopy, while properties of the resultant films are measured by a combination of profilometry, Rutherford backscattering spectrometry, elastic recoil detection analysis, X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The dependence of the deposition rate, film density, elemental composition, self-bias and residual stress is reported as a function of RF power. Higher density films were found when using Ar plasma, than N 2 or H 2 plasma. The coatings produced are x-ray amorphous, exhibit low compressive stress ( ~ 100 MPa), high density ( < 1.7 g/cm 3 ), hydrogen content of ~ 30 at.%, and a low average roughness of 0.75 nm. Finally, applications of these films as tunable-density ablators for inertial confinement fusion experiments are discussed.
36 MATERIALS SCIENCE↗
Aggregated nanoparticles: Sample preparation and analysis by atom probe tomography
Atom probe tomography (APT) allows measurement of the three-dimensional structure and composition of materials, but specific sample preparation procedures are required for challenging materials such as aggregates of nanoparticles. Indeed, the presence of porosity within the specimen affects both the stability of the sample and the accuracy of the data. In this work, aggregates of nanoparticles were transferred onto a micromanipulator tip and embedded via electron-beam-assisted deposition of Pt. Successive FIB-millings and Pt-depositions are needed to create suitable APT tips. The 3D reconstruction reveals the presence of 15–20 nm nanoparticles, and massspectral analysis shows the absence of trace elements within the catalyst, thus serving as quality control for the synthesis of nanoparticles with specific compositions.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Origin and Suppression of Beam Damage-Induced Oxygen-K Edge Artifact from γ-Al 2 O 3 using Cryo-EELS
Gamma-alumina (γ-Al 2 O 3 ), like other low-Z oxides, is readily damaged when exposed to an electron beam. This typically results in the formation of a characteristic pre-edge peak in the oxygen-K edge of electron energy-loss spectra (EELS) acquired during or after the damage process. This artifact can mask the presence of intrinsic O-K edge fine structure that would reveal chemical properties of the material; therefore, its suppression is key. In this work, we systematically investigate the conditions that give rise to the damage-induced O-K pre-edge peak and show that it can be effectively suppressed by performing EELS experiments at cryogenic (cryo) temperatures. Prolonged exposure of γ-Al 2 O 3 to a focused electron beam results in a hole bored through the sample; this was used as a reproducible beam damage condition. O-K edge EELS spectra were collected from a single-crystal γ-Al 2 O 3 sample both during and after focused 2 electron beam hole drilling, and at room and cryo temperatures, using a monochromated scanning transmission electron microscope (STEM). Furthermore, the characteristic 531 eV pre-edge peak visible in the room temperature EELS spectra was completely suppressed in the cryo-EELS spectra, even in the presence of a visible drilled hole. We then correlated these experimental observations with multiple-scattering EELS simulations to determine the likely atomistic origin of the damage-induced O-K pre-edge peak. The findings indicate that the pre-edge peak is caused primarily by the presence of surface O dimer (O-O) bonds formed during beam damage, and that operating at cryo temperature suppresses the formation of surface O-O bonds, thus preventing formation of the O-K pre-edge peak. Additionally, Al-L 2,3 edge EELS spectra revealed Al loss primarily from tetrahedral sites during hole drilling.
77 NANOSCIENCE AND NANOTECHNOLOGY↗