We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant related features that fall into two classes, which we call As1 and As2. When imaged in occupied states the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography, and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D 0 ) charge state when imaged in filled states, but become positively charged (D + ) through electrostatic ionisation when imaged under empty state conditions, similar to previous observations of acceptors in GaAs. Density functional theory (DFT) calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled state images and an isotropic depression in empty state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.
Thermoelectric generation
capable of delivering reliable performance in the low-temperature
range (<150 °C) for large-scale deployment has been a challenge
mainly due to limited properties of thermoelectric materials. However,
realizing interdependence of topological insulators and thermoelectricity,
a new research dimension on tailoring and using the topological-insulator
boundary states for thermoelectric enhancement has emerged. Here,
we demonstrate a promising hybrid nanowire of topological bismuth
telluride (Bi2Te3) within the conductive poly(3,4-ethylenedioxythiophene):polystyrenesulfonate
(PEDOT:PSS) matrix using the in situ one-pot synthesis to be incorporated
into a three-dimensional network of self-assembled hybrid thermoelectric
nanofilms for the scalable thermoelectric application. Significantly,
the nanowire-incorporated film network exhibits simultaneous increase
in electrical conductivity and Seebeck coefficient as opposed to reduced
thermal conductivity, improving thermoelectric performance. Based
on comprehensive measurements for electronic transport of individual
nanowires revealing an interfacial conduction path along the Bi2Te3 core inside the encapsulating layer and that
the hybrid nanowire is n-type semiconducting, the enhanced thermoelectricity
is ascribed to increased hole mobility due to electron transfer from
Bi2Te3 to PEDOT:PSS and importantly charge transport
via the Bi2Te3–PEDOT:PSS interface. Scaling
up the nanostructured material to construct a thermoelectric generator
having the generic pipeline-insulator geometry, the device exhibits
a power factor and a figure of merit of 7.45 μW m–1 K–2 and 0.048, respectively, with an unprecedented
output power of 130 μW and 15 day operational stability at ΔT = 60 °C. Our findings not only encourage a new approach
to cost-effective thermoelectric generation, but they could also provide
a route for the enhancement of other applications based on the topological
nanowire.
Engineering crystal facets have been proved as one of the most promising strategies for promoting photocatalytic performance of titanium dioxide (TiO2). The earlier research in this field focused on trying to obtain as high ratio of the high energy {001} facet as possible, while later found that the co‐existence of facets is more beneficial. However, controlling crystals to expose suitable facet pairs and facet ratios remains challenging. In this work, we not only comprehensively match possible low‐index facets such as {101}‐{001} and {010}‐{001} facet pairs, but also systematically tune their ratios. Moreover, these faceted particles can be directly grown onto the transparent conductive substrate, which can be directly used as a photoanode. So, their intrinsic behaviors can be precisely evaluated without interference from other exogenous factors such as binders, additives, or assembly skills. Various characterization techniques reveal that both the types of facet pairs and the ratios of facets play crucial roles on photocatalytic behaviors, due to the different electron affinity and dissociative adsorption ability of water molecules on a particular facet. Charge transport and surface chemistry have been thoroughly investigated to identify the underlying mechanism. This work sheds light on a material design strategy considering a suitable match of facet pairs for optimizing photocatalytic performance for a wide variety of applications.
Control of dopants in silicon remains crucial to tailoring the properties of electronic materials for integrated circuits. Silicon is also finding new applications in coherent quantum devices, as a magnetically quiet environment for impurity orbitals. The ionization energies and shapes of the dopant orbitals depend on the surfaces and interfaces with which they interact. The location of the dopant and local environment effects will therefore determine the functionality of both future quantum information processors and next-generation semiconductor devices. Here we match observed dopant wave functions from scanning tunneling microscopy (STM) to images simulated from first-principles density functional theory (DFT) calculations and precisely determine the substitutional sites of neutral As dopants between 5 and 15Å below the Si(001):H surface. We gain a full understanding of the interaction of the donor state with the surface and the transition between the bulk dopant and the dopants in the surface layer.
A strategy to enhance the energy conversion performance of a cement-based triboelectric nanogenerator (TENG) has been proposed for large-scale energy harvesting from human footsteps. A cement-carbon black (CB) composite is fabricated by incorporating CB nanoparticles with a hydroxyethyl cellulose (HEC) admixture and used as a triboelectric material for TENG. The fabricated cement-CB@HEC TENG exhibits a 13fold enhancement of the electrical output with the highest power density of 2.38 W/m 2 . The tremendous improvement is ascribed to a good dispersion of conductive CB nanoparticles in the cement matrix, which is key for achieving a high dielectric constant that is required to intensify the triboelectric charge density of TENG. Moreover, it was found that the addition of the HEC admixture plays a crucial role in not only providing a good dispersion of CB for generating dielectric polarization but also facilitating the development of a microstructure and crystallization of cement hydration products to reduce the formation of air voids and micropores. These factors synergistically contribute to the improved dielectric constant and compressive strength of the cement-CB@HEC composite. This remarkable strategy presents great prospects for the development of smart cement materials for realizing applications in large-scale energy harvesting toward sustainable, clean, and renewable energy sources.
Introduction Lipid nanoparticles are used as an alternative to traditional carriers such as nanoemulsion, liposomes and polymeric nanoparticles for different routes of pulmonary 1 , oral 2 and dermal 3 administration. Their advantages include biocompatibility, possibility of large-scale production, organic solvent free synthesis and controlled drug release 4, 5. Over the last two decades, lipid nanoparticles have developed from first generation of solid lipid nanoparticles SLNs to second generation of nanostructured lipid carriers NLCs by adding liquid lipid into particles instead of using only solid lipid. Liquid lipid interrupts polymorphic transformation that is the main cause of drug expulsion during storage of SLNs. Therefore, oil contents in NLCs play a role in the polymorphic forms of lipid matrix and
Because
of a lack of electron donor/acceptor groups, TENGs fabricated
from natural rubber (NR) usually produce low electrical outputs. The
present study demonstrated that the electrical outputs of NR TENGs
could be enhanced by the presence of lignin. Lignin with an unanticipated
triboelectric property became a triboelectric active material upon
subjecting to ultrasound in a basic solution. Physically, lignin irradiated
with ultrasound would undergo particle size reduction from tens of
micrometers down to a micrometer in diameter, leading to an increase
of interfacial area for triboelectric charge generation. Accompanying
the size reduction was a partial depolymerization of lignin arising
from the chemical bond cleavage via the nucleophilic attack of hydroxide
ions. This event caused an increase of polar functional groups on
lignin that benefited the charge generation and transfer. TENGs from
the NR/lignin composite generated a power output density up to 500%
larger than that produced from NR TENG. However, NR/lignin TENGs suffered
from the aggregation of lignin leading to a compromise of the electrical
outputs. Therefore, NR was grafted with polyacrylamide (NR-g-PAM) to inhibit lignin aggregation as well as to promote
an even distribution of lignin particles in the NR matrix. As a result,
NR-g-PAM/lignin TENG was capable of enhancing the
power output density to 260 and 1300% compared with that of NR/lignin
and NR TENGs, respectively.
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