It was revealed that sp3-sp2 rehybridization of carbon atoms on the facets of nanocrystalline paricles of detonation diamond under thermal treatment results in weakening the hardness of its primary ...agglomerates. That discloses a secret of the unusual high strength of industrial diamond agglomerates and allows obtaining hydrosol of individual diamond nanoparticles ranging of 4–5 nm. Details of sp3-sp2 rehybridization have been studied by number of complementary methods including TEM, AFM, XPS, Raman scattering, X-ray diffraction and dynamic light scattering. A model for complete explanation of the processes during the annealing and obtaining stable hydrosols of 4–5 nm DND particles with different ζ-potentials is suggested. The model reveals the new ways for obtaining nanodiamond sols in other solvents.
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Here, the synthesis of holey carbonylated (C-ny) graphene derivative and its application for gas sensing is demonstrated. The carbonylation of graphene oxide leads to the 3-fold increase in the ...concentration of carbonyl groups’ up to 9 at.% with a substantial elimination of other oxygen functionalities. Such a chemical modification is accompanied by the perforation of the graphene layer with the appearance of matrices of nanoscale holes, leading to corrugation of the layer and its sectioning into localized domains of the π-conjugated network. Combined with the predominant presence of carbonyls, granting the specificity in gas molecules adsorption, these features result in the enhanced gas sensing properties of C-ny graphene at room temperature with a selective response to NH3. Opposite chemiresistive response towards ammonia when compared to other analytes, such as ethanol, acetone, CO2, is demonstrated for the C-ny graphene layer both in humid and dry air background. Moreover, a selective discrimination of all of the studied analytes is further approached by employing a vector signal generated by C-ny multielectrode chip. Comparing the experimental results with the calculations performed in framework of density functional theory, we clarify the effect of partial charge transfer caused by water and ammonia adsorption on the chemiresistive response.
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•GO can be converted to the carbonylated graphene containing up to 9 at.% of carbonyls.•Carbonylation leads to holey and corrugated structure of graphene layer.•Holey-matrixed structure and carbonyls enhance graphene gas sensing properties.•Carbonylated graphene exhibits an exceptional selectivity regarding the NH3 detection.
Crystalline properties of (1–2)-μm-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The ...best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780°C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10–15) x-ray rocking curve peaks of 469 and 1025arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7×108cm−2 and 5.9×109cm−2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures.
► High temperature MEE AlN nucleation layer is the best for PA MBE of AlN buffer layers. ► The initial lateral grain size influences on the TD densities in the AlN buffer layers. ► Incomplete stress relaxation was revealed in MEE AlN layers by XRD measurements.
A round-robin characterization is reported on the sputter depth profiling of 60×(3.0nm Mo/0.3nm B4C/3.7nm Si) and 60×(3.5nm Mo/3.5nm Si) stacks deposited on Si(111). Two different commercial ...secondary ion mass spectrometers with time-of-flight and magnetic-sector analyzers, a pulsed radio frequency glow discharge optical emission spectrometer, and a home-built time-of-flight low-energy ion scattering and quadrupole-based secondary ion mass spectrometer were used. The influence of the experimental conditions, especially the type and energy of sputter ions, on the depth profiles of Mo/Si nanostructures with and without B4C barrier layers is discussed in terms of depth resolution, modulation factor and rapidity of analysis. The pros and cons of each instrumental approach are summarized.
•Sputter depth profiling of thin 60 layer periods Mo/Si and Mo/B4C/Si X-ray mirrors•Different commercial and home-built instruments were used.•0.3nm B4C layers suppress the profile degradation for glow discharge spectrometer.
The molecular-dynamics simulation of the formation of dimers for the case of the low-temperature (001) reconstruction of a GaAs surface terminated in Ga or As atoms is employed using the analytical ...potential, which takes into account σ- and π-bonds between atoms. The values of the decrease in the potential energy of atoms during the formation of the surface of an isolated dimer are found, and it is discovered that the potential energy of an atom in the As dimer is lower by several tenths of eV than that in the Ga dimer. In the temperature range of 25–40 K, the kinetics of the initial stages of the formation of Ga dimers is studied, and it is obtained that the characteristic energy of thermal activation of the formation of single isolated Ga dimers is ~29 meV and lower than the same value for As dimers (~38 meV). In the temperature range of 28–37 K, the time constants characterizing the average rate of the conversion of a single dimer into a chain of two dimers are estimated. For double Ga and As dimers, the reciprocal values of these times turn out to be in the ranges of 10
11
–10
12
and 10
9
–10
10
s
–1
, while similar parameters for the formation of single dimers lie in the range of 4 × 10
6
–10
8
and 1.4 × 10
6
–7.4 × 10
7
s
–1
.
Nanocorrugation of 2D crystals is an important phenomenon since it affects their electronic and mechanical properties. The corrugation may have various sources; one of them is flexural phonons that, ...in particular, are responsible for the thermal conductivity of graphene. A study of corrugation of just the suspended graphene can reveal much of valuable information on the physics of this complicated phenomenon. At the same time, the suspended crystal nanorelief can hardly be measured directly because of high flexibility of the 2D crystal. Moreover, the relief portion related to rapid out-of-plane oscillations (flexural phonons) is also inaccessible by such measurements. Here we present a technique for measuring the Fourier components of the height–height correlation function H(q) of suspended graphene which includes the effect of flexural phonons. The technique is based on the analysis of electron diffraction patterns. The H(q) is measured in the range of wavevectors q≈0.4–4.5nm−1. At the upper limit of this range H(q) does follow the T/κq4 law. So, we measured the value of suspended graphene bending rigidity κ=1.2±0.4eV at ambient temperature T≈300K. At intermediate wave vectors, H(q) follows a slightly weaker exponent than theoretically predicted q−3.15 but is closer to the results of the molecular dynamics simulation. At low wave vectors, the dependence becomes even weaker, which may be a sign of influence of charge carriers on the dynamics of undulations longer than 10nm. The technique presented can be used for studying physics of flexural phonons in other 2D materials.
•A technique for measuring free-standing 2D crystal corrugation is proposed.•The height-to-height correlation function of the suspended graphene corrugation is measured.•Various parameters of the intrinsic graphene properties are experimentally determined.
In this work we present a facile method for the synthesis of the carboxylated graphene derivate. The resulting material contains up to 10.9 at.% of carboxyl groups and with negligible content of ...other oxygen-containing groups. Moreover, formation of large round-shaped nanoscale holes is observed due to the applied synthesis process. The synthesized graphene derivative is of high interest for the sensing applications due to combination of its conductive nature and chemical reactivity provided by the attached carboxyl groups.
Using the molecular dynamics modelling the study of the process of As dimer formation on the GaAs (001) surface was carried out at different temperatures as a function of time. To describe GaAs ...properties the bond-order potential was used. It was found that the activation energy of formation of single dimer is about 38 meV, while the dimer dissociation energy is about 2 eV. The characteristic time of the single dimer generation lies in the range 10-6 - 10-8 s for temperatures from 28 to 37 K.
High-efficiency semiconductor lasers and light-emitting diodes operating in the 3–5 μm mid-infrared (mid-IR) spectral range are currently of great demand for a wide variety of applications, in ...particular, gas sensing, noninvasive medical tests, IR spectroscopy etc. III-V compounds with a lattice constant of about 6.1 Å are traditionally used for this spectral range. The attractive idea to fabricate such emitters on GaAs substrates by using In(Ga,Al)As compounds is restricted by either the minimum operating wavelength of ∼8 μm in case of pseudomorphic AlGaAs-based quantum cascade lasers or requires utilization of thick metamorphic InxAl1-xAs buffer layers (MBLs) playing a key role in reducing the density of threading dislocations (TDs) in an active region, which otherwise result in a strong decay of the quantum efficiency of such mid-IR emitters. In this review we present the results of careful investigations of employing the convex-graded InxAl1-xAs MBLs for fabrication by molecular beam epitaxy on GaAs (001) substrates of In(Ga,Al)As heterostructures with a combined type-II/type-I InSb/InAs/InGaAs quantum well (QW) for efficient mid-IR emitters (3–3.6 μm). The issues of strain relaxation, elastic stress balance, efficiency of radiative and non-radiative recombination at T = 10–300 K are discussed in relation to molecular beam epitaxy (MBE) growth conditions and designs of the structures. A wide complex of techniques including in-situ reflection high-energy electron diffraction, atomic force microscopy (AFM), scanning and transmission electron microscopies, X-ray diffractometry, reciprocal space mapping, selective area electron diffraction, as well as photoluminescence (PL) and Fourier-transformed infrared spectroscopy was used to study in detail structural and optical properties of the metamorphic QW structures. Optimization of the growth conditions (the substrate temperature, the As4/III ratio) and elastic strain profiles governed by variation of an inverse step in the In content profile between the MBL and the InAlAs virtual substrate results in decrease in the TD density (down to 3 × 107 cm−2), increase of the thickness of the low-TD-density near-surface MBL region to 250–300 nm, the extremely low surface roughness with the RMS value of 1.6–2.4 nm, measured by AFM, as well as rather high 3.5 μm-PL intensity at temperatures up to 300 K in such structures. The obtained results indicate that the metamorphic InSb/In(Ga,Al)As QW heterostructures of proper design, grown under the optimum MBE conditions, are very promising for fabricating the efficient mid-IR emitters on a GaAs platform.
Nanoindentation to a depth of 1 nm in (001) GaAs surfaces terminated by As simulated in the temperature range from 1 to 15 K using the molecular-dynamics method is simulated. It is shown that this is ...accompanied by surface reconstruction with the formation of stable dimers As (1 × 2), which do not disappear after indenter withdrawal from the surface.
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