Shallow Energy Research Articles

Ionizing radiation tolerance of stacked Si3N4-SiO2 gate insulators for power MOSFETs

The total ionizing dose radiation response of metal-nitride-oxide-semiconductor capacitors are examined. Capacitors with different oxide and nitride thickness combinations are fabricated and irradiated using a Co-60 gamma source. Electrical characterization showed all samples with oxide/nitride stack gate insulators exhibited significantly higher tolerance to irradiation when compared to metal-oxide-semiconductor capacitors. Thick oxide-nitride layers can be used as gate insulators in power metal-oxide-semiconductor field effect transistors. Technology computer-aided design simulations are performed to characterize the radiation effects mechanisms. Spatially distributed interface traps at shallow energy level were present in all the oxide/nitride stacks. These interface traps decrease as total dose increases. This result may be due to radiation induced hydrogen passivation.

Ohmic contact formation mechanism of Ge-doped 6H-SiC

Abstract We present a comprehensive investigation of mechanism of ohmic contact characteristic in Ge-doped 6H-SiC single crystals. Raman mapping and Rutherford backscattering spectrometry reveal high crystalline quality of Ge-doped 6H-SiC and the Ge as a substitutional impurity. Deep-level transient spectroscopy (DLTS) spectrum was measured. Its Arrhenius fitting shows two shallow energy levels. They are below the conductive band bottom 0.298 eV and 0.323 eV respectively. The first-principles calculations indicate that the no new levels are in ( Ge ) Si configuration, but ( Ge ) C can induce deep levels in 6H-SiC. Thus, the shallow energy levels are caused by native defects which arise in a process of crystal growth. The two characteristics shallow energy levels, the small trap cross sections and positions near to conductive band bottom, can emit electron under effect of an applied field. In addition, the Ge Si bonds are relatively easily to break for generating electrons to jump into conductive band bottom though the shallow energy levels.

Theoretical investigation on SiC(1 1 1)/Al4C3(0 0 0 1) interface using density functional theory calculations

Al4C3 is likely to generate on the interface of SiC reinforced aluminum- and magnesium-matrix composites. In order to clarify the interfacial properties and heterogeneous nucleation of Al4C3 on SiC(1 1 1) substrate, the interfacial adhesion, electronic structure, and bonding nature of SiC(1 1 1)/Al4C3(0 0 0 1) are investigated by using density functional theory (DFT) calculations. Considering different stacking sites (top, center, and hollow sites) and different surface terminations (Si- and C-terminated SiC(1 1 1); Al- and C-terminated Al4C3(0 0 0 1)), totally twelve different interface models are examined in the present work. The calculated interfacial adhesion work suggests top-site stacked Si/C- and C/Al-terminated models are more stable. By comparing different cleaving planes, the interfacial weak point is confirmed on the Al4C3(0 0 0 1) side for the both cases. The valence electron density and partial density of states (PDOS) are also analyzed. The interfacial bonding mainly comes from p orbital hybridizations of C-Si and Al-C atom pairs on shallow energy level (−6 eV to Fermi level), and on the deeper energy level (less than −6 eV), the s electrons will partly contribute to the bonding. The epitaxial stacking style is confirmed on the both interfaces, and epitaxial nucleation and growth of Al4C3 on SiC(1 1 1) substrate could also be identified.

Experimental feasibility study of a new attached hydronic loop design for geothermal heating of bridge decks

Geothermally heating of bridges is a green and sustainable alternative to deicing chemicals, which are energy-intensive, corrosive to the bridges, and dangerous to the environment. Geothermal bridges harness shallow earth geothermal energy, a readily available all-year-round renewable energy source. However, the existing geothermal heated bridge technology was developed based on embedded hydronic loops, which can be only deployed during the construction of new bridges. As there are more existing bridges in the critical need of deicing, a new attached hydronic loop design was developed in this study for geothermal heating of existing bridges. The hydronic loops are attached to the bottom of a bridge deck and are encapsulated inside an insulation material geofoam. The developed hydronic loop was tested on a lab-scale concrete deck with supplied warm water to evaluate its feasibility for field application. This heated deck system was fabricated and instrumented for the heating performance tests in an environmental chamber. It was first tested under various room temperatures higher than 4.4 °C, the lowest temperature permitted by the environmental chamber. Based on the test results, an empirical prediction equation is presented to estimate deck temperature at given ambient and supplied heating fluid temperatures. The applicability of this equation to freezing temperature was validated in localized freezing tests by placing a cooling box on the test deck. All test results show that this heated bridge deck allows efficient heat transfer, approximately 60% of the supplied heat, to the bridge deck surface. The supplied heat flux to the deck surface ranges from 120 to 270 W/m2 depending on the ambient and heating fluid temperature, which is comparable to the embedded hydronic system. The new hydronic loop design is feasible to be implemented on the existing bridges for deicing and snow removal.

Defect states of organic lead halide single crystals grown by inverse-temperature crystallization

We investigated the defect states of organic lead halide (CH3NH3PbX3: CH3NH3+) methylammonium (MA; X = Br− and I−) by using optical pulse-induced conductance transient spectroscopy. Organic lead halide single crystals having a bandgap ranging from 2.16 to 1.45 eV were grown by inverse temperature crystallization. The shallow trap energy level of MAPbBr2.5I0.5 was Ev + 0.06 eV. The deep level traps of MAPbBr3 and MAPbI3 were Ev + 1.24 and Ev + 0.84 eV, respectively. The deep level traps of organic lead halide may have decreased the reactivity between iodide and bromide ions in the crystal lattice. Varying the ratio of halides in the perovskite can change the defect species in the crystals and can increase the photoconversion efficiency by decreasing the density of deep level traps.

Correlation effects in B1s core-excited states of boronic-acid derivatives: An experimental and computational study.

We performed a theoretical investigation on the influence of electronic correlation effects on the B1s NEXAFS spectrum of boronic acid derivatives, namely, boric acid [B(OH)3], phenyl boronic acid (PBA), and 1,4-phenyl diboronic acid (PDBA), employing different computational schemes of increasing complexity, ranging from the purely one-electron scheme based on the transition potential method of density functional theory (DFT-TP), time-dependent DFT (TDDFT), and multiconfigurational self-consistent field (MCSCF). We also report experimental measurements of the B1s NEXAFS spectra of the aforementioned molecules together with the high-resolution C1s NEXAFS spectrum of PBA. We demonstrate that due to the shallow B1s core energy levels compared to C, O, and N, the inclusion of static correlation effects, which can be incorporated by using multireference approaches to excited states, assumes a decisive role in reconciling experiment and theory on B1s core-electron excitation energies and oscillator strengths to valence states. This claim is corroborated by the good agreement that we find between the DFT-TP calculated C1s NEXAFS spectrum and that experimentally measured for PBA and by the failure of both DFT-TP and TDDFT approaches with a selection of xc functionals kernels to properly describe the B1s NEXAFS spectrum of PBA and PDBA, at variance with the good agreement with the experiment that is found by employing the MCSCF wave function approach.

City-scale solutions for the energy use of shallow urban subsurface resources – Bridging the gap between theoretical and technical potentials

One solution for reducing the current consumption of fossil fuels is a more frequent use of shallow geothermal energy. However, particularly regarding urban subsurface resources, increased use conflicts are predictable. Consequently, reasonable exploitation of subsurface resources requires an assessment of technologically achievable energy potentials with scientific based tools. We present application-oriented management tools which target on deriving shallow subsurface energy potentials. 3D groundwater flow and heat-transport models are used to capture groundwater flow and heat-transport dynamics on the city- and quarter-scale, 2D box models are used to quantify technically feasible extraction rates of well doublets for groundwater heat pump systems.For Basel (Switzerland), prospective large theoretical energy potentials can be derived for areas with high advective heat flux and high temperature gradients. Likewise, single city quarters are suitable for ‘active’ thermal use with well doublets, whereas thermal power potentials reach 1.2 MW. Regarding ‘passive’ installations of energy absorbers in subsurface structures located within the groundwater, energy potentials amount to 4 and up to 40 W m−2.The assessment results can be integrated into urban energy plans and support architects, city planners and potential users to acquire initial site-specific information on the technical feasibility of shallow geothermal energy systems.

Mobility of electrostatically and sterically stabilized gold nanoparticles (AuNPs) in saturated porous media.

The stability of gold nanoparticles (AuNPs) stabilized electrostatically with citrate or (electro)sterically by commercially available amphiphilic block copolymers (PVP-VA or PVA-COOH) was studied under various physicochemical conditions. Subsequently, the mobility of the AuNPs in porous media (sand) was investigated in column studies under environmental relevant physicochemical conditions. Electrostatically stabilized AuNPs were unstable under most physicochemical conditions due to the compression of the electrical double layer. Consequently, aggregation and deposition rapidly immobilized the AuNPs. Sterically stabilized AuNPs showed significantly less sensitivity towards changes in the physicochemical conditions with high stability, high mobility with negligible retardation, and particle deposition rate coefficients ranging an order of magnitude (1.5 × 10-3 to 1.5 × 10-2 min-1) depending on the type and amount of stabilizer, and thereby the surface coverage and attachment affinity. The transport of sterically stabilized AuNPs is facilitated by reversible deposition in shallow energy minima with continuous reentrainment and blocking of available attachment sites by deposited AuNPs. The stability and mobility of NPs in the environment will thereby be highly dependent on the specific stabilizing agent and variations in the coverage on the NP. Under the given experimental conditions, transport distances of the most mobile AuNPs of up to 20 m is expected. Due to their size-specific plasmonic properties, the easily detectable AuNPs are proposed as potential model or tracer particles for studying transport of various stabilized NPs under environmental conditions.

Catalyst: Can Systems Chemistry Unravel the Mysteries of the Chemical Origins of Life?

Daniela Kroiss obtained her MSc degree in biotechnology at BOKU and is currently a PhD student in the biochemistry program at CUNY, where she works on a joint project between the Nanoscience Initiative and the University of Strathclyde. Gonen Ashkenasy is head of the Laboratory for Systems Chemistry at the Ben-Gurion University Chemistry Department. His research focuses on the design and analysis of synthetic replication networks. Adam Braunschweig earned his PhD in organic chemistry at the University of California and is an associate professor in the CUNY Nanoscience Initiative. His research focuses on the interface of organic chemistry with biology and material science. Tell Tuttle obtained his PhD in theoretical chemistry at Goteborg University and is a professor of theoretical chemistry at the University of Strathclyde. His research focuses on the directed discovery of peptide-based materials. Rein Ulijn gained his PhD in physical chemistry at the University of Strathclyde and is founding director of the Nanoscience Initiative at the CUNY Advanced Science Research Center. His research focuses on nanotechnology inspired by living systems.

Unbiased Discovery of Dynamic Peptide‐ATP Complexes

AbstractThe front cover artwork is provided by the groups of Prof. Rein V. Ulijn at CUNY Advanced Science Research Center and Prof. Tell Tuttle at the University of Strathclyde. The image shows adenosine triphosphate, life's ubiquitous energy source, interacting with a short peptide that was discovered using phage display. The formed complexes are dynamic and sample a shallow energy landscape. Read the full text of the Communication at 10.1002/syst.201900013.

Defect-Mediated Charge-Carrier Trapping and Nonradiative Recombination in WSe2 Monolayers.

Nonradiative charge-carrier recombination in transition-metal dichalcogenide (TMD) monolayers severely limits their use in solar energy conversion technologies. Because defects serve as recombination sites, developing a quantitative description of charge-carrier dynamics in defective TMD monolayers can shed light on recombination mechanisms. Herein we report a first-principles investigation of charge-carrier dynamics in pristine and defective WSe2 monolayers with three of the most probable defects, namely, Se vacancies, W vacancies, and SeW antisites. We predict that Se vacancies slow down recombination by nearly an order of magnitude relative to defect-free samples by breaking the monolayer's symmetry and thereby reducing the spectral intensity of the A1g phonon mode that promotes recombination in the pristine monolayer. By contrast, we find W vacancies accelerate recombination by more than an order of magnitude, with half of the recombination events bypassing charge traps. The subsequent dynamics feature both charge trapping and charge-trap-assisted recombination. Although SeW antisites also slightly accelerate recombination, the predicted mechanism is different from the W vacancy case. First, a shallow energy level traps a photoexcited electron. Then, both shallow- and deep-trap-assisted recombination can occur simultaneously. Accelerated recombination arises for W vacancies and SeW antisites because they introduce new phonon modes that strongly couple to electron and hole dynamics. This work thus provides a detailed understanding of the mechanisms behind charge-carrier recombination in WSe2 monolayers with distinct defects. Thus, materials engineering, particularly to avoid W vacancies, could advance this technology. The insights derived are important for future design of high-performance photoactive devices based on WSe2 monolayers.

Characterization of hole traps in MOVPE-grown p-type GaN layers using low-frequency capacitance deep-level transient spectroscopy

Traps in MOVPE-grown Mg-doped GaN samples composed of p+/p−/n+ structures were investigated using low-frequency capacitance deep-level transient spectroscopy (DLTS). A drop-off in capacitance with decreasing temperature was observed. This is caused by the longer RC time constant of the diode with lower temperature, which is due to a decrease in the number of ionized Mg acceptors (which have a high ionization energy). This limits the use of lower temperatures in DLTS measurements. To extend DLTS to a lower temperature (105 K), DLTS using a capacitance measurement frequency of 1 kHz was applied. Thus, we can quantitatively discuss concentrations of traps with shallow energy levels. We obtained a nearly one-to-one relation between Ha (EV +0.29 eV) and Hd (EV +0.88 eV) in the p-type layer, which strongly supports the theoretical calculation that a carbon on a nitrogen site forms donor-like (Ha) and acceptor-like (Hd) states.

Designing a Perylene Diimide/Fullerene Hybrid as Effective Electron Transporting Material in Inverted Perovskite Solar Cells with Enhanced Efficiency and Stability

Electron transport materials (ETM) play an important role in the improvement of efficiency and stability for inverted perovskite solar cells (PSCs). This work reports an efficient ETM, named PDI-C60 , by the combination of perylene diimide (PDI) and fullerene. Compared to the traditional PCBM, this strategy endows PDI-C60 with slightly shallower energy level and higher electron mobility. As a result, the device based on PDI-C60 as electron transport layer (ETL) achieves high power conversion efficiency (PCE) of 18.6 %, which is significantly higher than those of the control devices of PCBM (16.6 %) and PDI (13.8 %). The high PCE of the PDI-C60 -based device can be attributed to the more matching energy level with the perovskite, more efficient charge extraction, transport, and reduced recombination rate. To the best of our knowledge, the PCE of 18.6 % is the highest value in the PSCs using PDI derivatives as ETLs. Moreover, the device with PDI-C60 as ETL exhibits better device stability due to the stronger hydrophobic properties of PDI-C60 . The strategy using the PDI/fullerene hybrid provides insights for future molecular design of the efficient ETM for the inverted PSCs.

Designing a Perylene Diimide/Fullerene Hybrid as Effective Electron Transporting Material in Inverted Perovskite Solar Cells with Enhanced Efficiency and Stability

AbstractElectron transport materials (ETM) play an important role in the improvement of efficiency and stability for inverted perovskite solar cells (PSCs). This work reports an efficient ETM, named PDI‐C60, by the combination of perylene diimide (PDI) and fullerene. Compared to the traditional PCBM, this strategy endows PDI‐C60 with slightly shallower energy level and higher electron mobility. As a result, the device based on PDI‐C60 as electron transport layer (ETL) achieves high power conversion efficiency (PCE) of 18.6 %, which is significantly higher than those of the control devices of PCBM (16.6 %) and PDI (13.8 %). The high PCE of the PDI‐C60‐based device can be attributed to the more matching energy level with the perovskite, more efficient charge extraction, transport, and reduced recombination rate. To the best of our knowledge, the PCE of 18.6 % is the highest value in the PSCs using PDI derivatives as ETLs. Moreover, the device with PDI‐C60 as ETL exhibits better device stability due to the stronger hydrophobic properties of PDI‐C60. The strategy using the PDI/fullerene hybrid provides insights for future molecular design of the efficient ETM for the inverted PSCs.

Emergence of shallow energy levels in B-doped Q-carbon: A high-temperature superconductor

We report the spectroscopic demonstration of the shallow-level energy states in the recently discovered B-doped Q-carbon Bardeen-Cooper-Schrieffer (BCS) high-temperature superconductor. The Q-carbon is synthesized by ultrafast melting and quenching, allowing for high B-doping concentrations which increase the superconducting transition temperature (Tc) to 36 K (compared to 4 K for B-doped diamond). The increase in Tc is attributed to the increased density of energy states near the Fermi level in B-doped Q-carbon, which give rise to superconducting states via strong electron-phonon coupling below Tc. These shallow-level energy states, however, are challenging to map due to limited spatial and energy resolution. Here, we use ultrahigh energy resolution monochromated electron energy-loss spectroscopy (EELS), to detect and visualize the newly formed shallow-level energy states (vibrational modes) near the Fermi level (ranging 30–100 meV) of the B-doped Q-carbon. With this study, we establish the significance of high-resolution EELS in understanding the superconducting behavior of BCS superconducting C-based materials, which demonstrate a phenomenal enhancement in the presence of shallow-level energy states.

Achieving over 16% efficiency for single-junction organic solar cells

To achieve high photovoltaic performance of bulk hetero-junction organic solar cells (OSCs), a range of critical factors including absorption profiles, energy level alignment, charge carrier mobility and miscibility of donor and acceptor materials should be carefully considered. For electron-donating materials, the deep highest occupied molecular orbital (HOMO) energy level that is beneficial for high open-circuit voltage is much appreciated. However, a new issue in charge transfer emerges when matching such a donor with an acceptor that has a shallower HOMO energy level. More to this point, the chemical strategies used to enhance the absorption coefficient of acceptors may lead to increased molecular crystallinity, and thus result in less controllable phase-separation of photoactive layer. Therefore, to realize balanced photovoltaic parameters, the donor-acceptor combinations should simultaneously address the absorption spectra, energy levels, and film morphologies. Here, we selected two non-fullerene acceptors, namely BTPT-4F and BTPTT-4F, to match with a wide-bandgap polymer donor P2F-EHp consisting of an imide-functionalized benzotriazole moiety, as these materials presented complementary absorption and well-matched energy levels. By delicately optimizing the blend film morphology, we demonstrated an unprecedented power conversion efficiency of over 16% for the device based on P2F-EHp:BTPTT-4F, suggesting the great promise of materials matching toward high-performance OSCs.

Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer.

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (∼4000 cm-1) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm-1) in addition to large displacements of low-frequency torsional motions (∼300-340 cm-1). Transient absorption spectroscopy reveals the emergence of distorted singlet (S1) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 μs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S1. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.

Experimental, theoretical, and numerical simulation of the performance of CuInxGa(1-x) S2-based solar cells

In this study, the absorber layers CuInGaS2 (CIGS) and CuInS2 (CIS) were deposited on glass substrates by spray pyrolysis after optimization of the operating parameters. The deposition is carried out during a spray duration of 30 min, and then the samples underwent a heat treatment at the annealing temperature of 370 °C for 20 min, the structural, morphological, optical and electrical properties of thin CIS and CIGS were examined by X-ray diffraction and scanning electron microscopy. All optical parameters, such as band gap energy, refractive index, extinction coefficient and dielectric constant were extracted using visible UV spectroscopy; the semiconductor aspect of the films is shown by Four Point Probe Method. The performance of the CIS and CIGS-based solar cells was evaluated numerically using the SCAPS simulator. The effects of structural and physical parameters, such as thickness, shallow acceptor density and gap energy of the CIGS absorber layer were studied. Simulation results have shown that it is not necessary to use an absorbent layer too wide to achieve the compromise between efficiency and cost for mass production. The effect of the operating temperature showed that the performance of the cell is affected at higher temperature. These numerical studies essentially contribute to understanding the performance of the CIS and CIGS solar cell and provide an appropriate path for efficient solar cell fabrication with precise control of process parameters.

Study on the mechanism of afterglow in CsI: Tl and the afterglow suppression in CsI: Tl, Eu

Although there are many new scintillators being developed recently, CsI: Tl is still a very efficient one among them. However it suffers from afterglow which limits application in high-speed x-ray imaging. In order to find out the mechanism of afterglow and the method to suppress afterglow in CsI: Tl, the theoretical model of perfect CsI, CsI: Tl and CsI: Tl, Eu are built by the software Material Studio. The total and partial density of states of these three models is calculated within the framework of generalized gradient approximation and ultra-soft pseudo-potential. The results indicate that the generation of afterglow in CsI: Tl is due to the impurity energy bands introduced by Tl. Moreover, co-doping CsI: Tl with Eu will introduce deeper impurity energy bands which can effectively scavenge the electrons trapped by the shallow impurity energy bands introduced by Tl, thus suppressing the afterglow in CsI: Tl.

Adsorption of hydrogen and oxygen on graphdiyne and its BN analog sheets: A density functional theory study

In this paper, the structural and electronic properties of atomic oxygen and hydrogen adsorption on graphdiyne (GDY) and its BN analog (BNdiyne) has been studied by the first-principles calculations. The calculated results show that the most stable adsorption positions of atomic hydrogen (oxygen) for GDY and BNdiyne monolayer are the top site (T2) and the hollow site (H2), respectively. The strong interaction (chemisorption) between atomic hydrogen and the monolayers leads to a decrease of 0.1 eV in the band gap of GDY, while an increase of 0.15 eV in BNdiyne. The shallow impurity energy level introduced by oxygen enhances the conductivity of GDY which could be indicated from the non-zero transmission coefficient at zero bias as well. In addition, a tiny magnetic has occurred in the oxygenated GDY while it has not occurred in the hydrogenated GDY which is different from the graphyne.