Zeolite-templated carbons – three-dimensional microporous graphene frameworks

Following the development of many novel two-dimensional (2D) materials, investigations of van der Waals heterojunctions (vdWHs) have attracted significant attention due to their excellent properties such as smooth heterointerface, highly gate-tunable bandgap, and ultrafast carrier transport. Benefits from the atom-scale thickness, physical and chemical properties and ease of manipulation of the heterojunctions formulated by weak vdW forces were demonstrated to indicate their outstanding potential in electronic and optoelectronic applications, including photodetection and energy harvesting, and the possibility of integrating them with the existing semiconductor technology for the next-generation electronic and sensing devices. In this review, we summarized the recent developments of vdWHs and emphasized their applications. Basically, we introduced the physical properties and some newly discovered phenomena in vdWHs. Then, we emphatically presented four classical vdWHs and some novel heterostructures formed by vdW forces. Based on their unique physical properties and structures, we highlighted the applications of vdWHs including in photodiodes, phototransistors, tunneling devices, and memory devices. Finally, we provided a conclusion on the recent advances in vdWHs and outlined our perspectives. We aim for this review to serve as a solid foundation in this field and to pave the way for future research on vdW-based materials and their heterostructures.

Recent progress in van der Waals heterojunctions

Nanoscale science is the study of objects and phenomena at a very small scale and it has been an emerging, interdisciplinary science involving Physics, Biology, Chemistry, Engineering, Material Science, Computer Science and other areas The main interest in studying in the nanoscale is related to how nanosized particles have different properties than large particles of the same substance Nanoscale science allow us to learn more about the nature of matter, develop new theories, discover new questions and answers in many areas including health care, energy and technology and also discover how to make new technologies and products that can improve people’s life Although nanoscale science is a recent development in the scientific community, the development of its main concepts happened over a long period of time and the emergence of nanotechnology is related to experimental advances such as the invention of the Scanning Probe Microscope (SPM), a branch of microscopy that captures surface imagery using physical probes that scan the specimen

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Measurement of the Nanoscale Roughness by Atomic Force Microscopy: Basic Principles and Applications

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 A appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 A peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At T=2 K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapolation of the temperature dependence of the free-to-bound peak to T=0 K, after correction for the temperature dependence of the exciton energy gap, leads to the value EA(B)−EX=628±1 meV, where EA(B) is the ionization energy of the deep boron center and EX is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.

Photoluminescence and transport studies of boron in 4H SiC

Low-voltage p-channel and n-channel organic transistors with channel lengths down to 0.5 μm using four small-molecule semiconductors and ultra-thin dielectrics based on two different phosphonic acid monolayers are fabricated on plastic substrates and studied in terms of effective mobility, intrinsic mobility and contact resistance. For the optimum materials combination, flexible complementary circuits have signal delays of 3.1 μs at 5 V.

Flexible Low-Voltage Organic Complementary Circuits: Finding the Optimum Combination of Semiconductors and Monolayer Gate Dielectrics

Nowadays, ca. 176,640 tons/year of silicon (Si) (>4N) is manufactured for Si wafers used for semiconductor industry. The production of the highly pure Si wafers inevitably includes very high-temperature steps at 1400-2000 °C, which is energy-consuming and environmentally unfriendly. Inefficiently, ca. 45-55% of such costly Si is lost simply as sawdust in the cutting process. In this work, we develop a cost-effective way to recycle Si sawdust as a high-performance anode material for lithium-ion batteries. By a beads-milling process, nanoflakes with extremely small thickness (15-17 nm) and large diameter (0.2-1 μm) are obtained. The nanoflake framework is transformed into a high-performance porous structure, named wrinkled structure, through a self-organization induced by lithiation/delithiation cycling. Under capacity restriction up to 1200 mAh g-1, the best sample can retain the constant capacity over 800 cycles with a reasonably high coulombic efficiency (98-99.8%).

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Beads-Milling of Waste Si Sawdust into High-Performance Nanoflakes for Lithium-Ion Batteries

Fabrication of Si nanoparticles from Si swarf and application to solar cells

An ultrathin silicon dioxide (SiO2) layer of 1.2–1.4 nm thickness has been formed by immersion of Si wafers in nitric acid (HNO3) aqueous solutions, and its electrical characteristics and physical properties are investigated as a function of the HNO3 concentration. Measurements of transverse optical and longitudinal optical phonons of Si–O–Si asymmetric stretching vibrational mode for SiO2 indicate that the atomic density of the SiO2 layer increases with the HNO3 concentration. X-ray photoelectron spectroscopy measurements show that the valence band discontinuity energy at the SiO2/Si interface also increases and the concentration of suboxide species decreases with the HNO3 concentration. The leakage current density of the ⟨Al/SiO2/Si(100)⟩ metal-oxide-semiconductor (MOS) diodes with the SiO2 layer formed in HNO3 aqueous solutions decreases with the HNO3 concentration and also decreases by postmetallization annealing (PMA) treatment at 250 °C in 5 vol % hydrogen atmosphere. For the MOS diodes with the SiO...

Ultrathin SiO2 layer with an extremely low leakage current density formed in high concentration nitric acid

A high-frequency pulsed electron paramagnetic resonance electron-nuclear double-resonance (ENDOR) study on a {sup 13}C-enriched 6H-SiC single crystal is reported. This type of spectroscopy, owing to its superior spectral resolution, allows us to obtain detailed information about the electronic structure of the shallow boron acceptor centers in 6H-SiC. It is concluded that around 40{percent} of the spin density is localized in the p{sub z} orbital of a carbon which is nearest to boron. The p{sub z} orbital is directed along the C-B connection line and is parallel to the c axis for the hexagonal site and 70{degree} away from the c axis for the two quasicubic sites. We conclude that the C-B bond is a dangling bond, that boron is neutral, and that there is no direct spin density on boron. There is a relaxation of the carbon atom, carrying the spin density, and the boron atom away from each other. From a {sup 29}Si and {sup 13}C ENDOR study it is further concluded that around 60{percent} of the spin density is distributed in the crystal with a Bohr radius of 2.2 {Angstrom}. {copyright} {ital 1997} {ital The American Physical Society}

Electronic structure of the shallow boron acceptor in 6H-SiC: A pulsed EPR/ENDOR study at 95 GHz

We have developed a method of formation of atomically smooth Si∕SiO2 interfaces by oxidation of atomically flat Si(111) surfaces by use of azeotropic nitric acid (HNO3) aqueous solutions (i.e., 68wt% HNO3 at 121°C). For the SiO2 layer on the atomically smooth Si substrates, the concentration of suboxide species, Si2+, is ∼50% of that on the rough Si substrates, and the valence band discontinuity is higher by ∼0.1eV. In this case, the leakage current flowing through the ∼1.2nm SiO2 is low, and further decreased by postmetallization annealing at 250°C in hydrogen (e.g., 0.5A∕cm2 at VG=1V).

Taketoshi Matsumoto

Papers

Beads-Milling of Waste Si Sawdust into High-Performance Nanoflakes for Lithium-Ion Batteries

Fabrication of Si nanoparticles from Si swarf and application to solar cells

Ultrathin SiO2 layer with an extremely low leakage current density formed in high concentration nitric acid

Electronic structure of the shallow boron acceptor in 6H-SiC: A pulsed EPR/ENDOR study at 95 GHz

Ultrathin SiO2 layer on atomically flat Si(111) surfaces with excellent electrical characteristics formed by nitric acid oxidation method