By performing first-principles electronic structure calculations in frames of density functional theory we study the dependence of the valence band shape on the thickness of few-layer III-VI crystals ...(GaS, GaSe, and InSe). We estimate the critical thickness of transition from the bulklike parabolic to the ring-shaped valence band. Direct supercell calculations show that the ring-shaped extremum of the valence band appears in beta -GaS and beta -GaSe at a thickness below 6 tetralayers (~ 4.6nm) and 8 tetralayers (~ 6.4ran), respectively. Zone-folding calculations estimate the beta -InSe critical thickness to be equal to 28 tetralayers (~ 24.0nm). The origin of the ring-shaped valence band maximum can be understood in terms of k times p theory, which provides a link between the curvature of the energy bands and the distance between them. We explain the dependence of the band shape on the thickness, as well as the transition between two types of extremes, by the k-dependent orbital composition of the topmost valence band. We show that in the vicinity of critical thickness the effective mass of holes in III-VI compounds depends strongly on the number of tetralayers.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Controlling chirality in growth of single-walled carbon nanotubes (SWNTs) is important for exploiting their practical applications. For long it has been conceptually conceived that the structural ...control of SWNTs is potentially achievable by fabricating nanoparticle catalysts with proper structures on crystalline substrates via epitaxial growth techniques. Here, we have accomplished epitaxial formation of monometallic Co nanoparticles with well-defined crystal structure, and its use as a catalyst in the selective growth of SWNTs. Dynamics of Co nanoparticles formation and SWNT growth inside an atomic-resolution environmental transmission electron microscope at a low CO pressure was recorded. We achieved highly preferential growth of semiconducting SWNTs (~90%) with an exceptionally large population of (6, 5) tubes (53%) in an ambient CO atmosphere. Particularly, we also demonstrated high enrichment in (7, 6) and (9, 4) at a low growth temperature. These findings open new perspectives both for structural control of SWNTs and for elucidating the growth mechanisms.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Plasmonic interferometry is a rapidly growing area of research with a huge potential for applications in the terahertz frequency range. In this Letter, we explore a plasmonic interferometer based on ...graphene field effect transistor connected to specially designed antennas. As a key result, we observe helicity- and phase-sensitive conversion of circularly polarized radiation into dc photovoltage caused by the plasmon-interference mechanism: two plasma waves, excited at the source and drain part of the transistor, interfere inside the channel. The helicity-sensitive phase shift between these waves is achieved by using an asymmetric antenna configuration. The dc signal changes sign with inversion of the helicity. A suggested plasmonic interferometer is capable of measuring the phase difference between two arbitrary phase-shifted optical signals. The observed effect opens a wide avenue for phase-sensitive probing of plasma wave excitations in two-dimensional materials.
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IJS, KILJ, NUK, PNG, UL, UM
MgO supported monometallic Fe (FeMgO) and bimetallic FeMn (FeMnMgO) catalysts were developed for growing carbon nanotubes using CO as carbon source. Characterizations on the FeMgO catalyst revealed ...that Fe cations were well-dispersed in the porous MgO support, forming a solid solution. Since most Fe cations in the solid solution are difficult to reduce, FeMgO can only catalyze the growth of single-walled carbon nanotubes (SWCNTs) at temperatures of 700 °C and above. While Fe cations in FeMnMgO could be reduced at lower temperatures with the assistance of Mn, catalyzing the subsequent growth of SWCNTs. Compared with most SWCNTs grown on components like Co or Ni at ambient reaction pressure, the Fe-catalyzed SWCNTs demonstrate rather narrow chirality distributions. Particularly, preferential (6, 5) tube growth on FeMnMgO was achieved at 600 °C. The narrow SWCNT chirality distribution could be inherently related to the high carbon solubility of Fe nanoparticles, favoring the nucleation of SWCNTs by a perpendicular mode. The present studies not only offer essential insights into SWCNT growth mechanisms, but can guide the design of novel catalysts for chirality-controlled growth of SWCNTs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Compared to well-studied single-wall carbon nanotubes, narrow graphene nanoribbons (GNRs) are in shadow due to difficulties in their synthesis (necessity of ultra-high vacuum, noble-metal substrates, ...and tiny output yield). Here, we report a new approach to the bottom-up technique based on chemical vapor deposition to facilitate the synthesis of 7-armchair GNRs on a large scale under low vacuum on common Ni foil. The structure and high quality of produced GNRs are confirmed by multi-wavelength Raman. We demonstrate the characteristic optical absorption features of produced graphene nanoribbons at 2.05 and 2.22 eV associated with the E11 and E22 excitonic transitions of 7-armchair GNRs. The produced GNRs demonstrate bright photoluminescence (PL) in the red spectral range with the main characteristic feature at 2.04 eV associated with E11 exciton transition. We also measured the PL excitation spectral map that contains a diversity of spectrum features associated with 7-armchair GNRs. The presented results can be beneficial for further study of atomically precise GNRs, especially in nonlinear optics, and can be promising for applications of GNRs.
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IJS, KILJ, NUK, PNG, UL, UM
The suitability of the NiMgO catalyst as a catalyst in chiral-selective growth of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition has been assessed. It reveals that catalyst ...calcination temperature plays an important role in affecting the catalyst performances. Using CO as the carbon precursor and a chemical vapor deposition reaction temperature of 600 °C, NiMgO pre-calcined at 600 °C demonstrates the best performances in catalyzing the growth of SWNTs with predominant (6, 5) species. Systematic characterizations on catalysts calcinated at different temperatures indicate that Ni2+ ions diffuse towards the interior of MgO matrix upon annealing. DFT-based calculations reveal that the binding energy between Ni2+ and adjacent Ni(0) is larger than that between Mg2+ and Ni (0), while Ni2+ situated deep inside MgO has weak interactions with surface Ni atoms. This work highlights the importance of subsurface Ni2+ in anchoring reduced surface Ni atom, which inhibits the aggregation of Ni particles and therefore, facilitates the growth of SWNTs with a narrow chirality distribution.
A Ni-incorporated MgO catalyst was developed for predominant synthesis of (6, 5) single walled carbon nanotubes. Density functional theory-based calculations revealed that the unreduced subsurface Ni stabilized reduced Ni atoms on the surface, facilitating the growth of carbon nanotubes with a narrow chirality distribution. Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Chirality-controlled growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD) is one of the most challenging tasks in carbon nanotube synthesis field. During CVD growth, ...the catalyst plays crucial roles in governing SWCNT nucleation thermodynamics as well as growth kinetics. However, the performances of catalyst are generally sensitive to the metal loading amount in the catalyst and the reaction conditions, like the partial pressure of carbon source and the reaction time. In this work, we have systematically investigated a robust CoxMg1-xO solid solution, which can predominantly yield (6, 5) SWCNTs in a wide range of Co concentration, with a diversity of CO concentrations or a broad-ranging reaction time. Besides, the effect of reaction temperature on SWCNT chirality distribution is demonstrated, the mechanism of which is clarified with the assistance of environmental transmission electron microscopy. Finally, the chirality distribution of SWCNTs grown using CH4 as the carbon source is presented. The effects of carbon sources are discussed in view of SWCNT growth mode.
A CoxMg1-xO solid solution facilitates the epitaxial formation of Co nanoparticles, yielding single-walled carbon nanotubes with dominant (6, 5) species regardless of Co content in catalyst, CO concentration and reaction time. Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Here the authors present the investigation of controllable growth of large area high quality graphene from different solid carbon sources. A home‐made cold‐wall chamber is used for chemical vapor ...deposition realization to grow graphene on copper foils. It is supplied with tools for in situ monitoring the temperature along the entire sample. Monolayer graphene films are synthesized using solid poly(methyl methacrylate) (PMMA) and paraffin precursors at a growth temperature of about 850 °C and under low pressure conditions. The area of 1 × 1 cm2 appears to be covered by graphene. Raman spectroscopy and scanning electron microscopy are used to estimate the quality of graphene films fabricated and the graphene layer number.
The amount of hydrogen during synthesis significantly affects the quality of graphene synthesized from poly(methyl methacrylate) (PMMA) and paraffin. High quality graphene can be easily, quickly, and safely synthesized without the use of explosive gases. The authors find unique features of synthesis with resistive heating compared to the standard chemical vapor deposition (CVD) synthesis in the furnace. The obtained Raman spectra of the synthesized graphene films from solid carbon sources are identical to those of gas CVD film spectra.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Narrow graphene nanoribbons (GNRs) with atomically precise structure are interesting nanomaterials promising in various applications like optoelectronics, nanoelectronics, and photonics. The GNRs ...optical properties are barely studied due to the difficulty of their production. Herein, a new bottom‐up method based on chemical vapor deposition to scale up the production of 3‐armchair GNRs (3‐AGNRs) under low vacuum on common Ni foil is reported. The Raman study confirms the structure and high quality of the produced GNRs. The wide optical bandgap of 3‐AGNRs is demonstrated based on UV‐Vis‐IR absorption and photoluminescence (PL). Bright PL is observed in the blue spectral range with the characteristic features of 3‐AGNRs located at 430 and 456 nm. The PL excitation spectral maps are presented, containing multiple spectrum features associated with GNRs. The demonstrated results can be helpful for unveiling the optical properties of atomically precise GNRs and can be beneficial for applications of GNRs.
Herein, the chemical vapor deposition method for synthesis of the atomically precise 3‐armchair graphene nanoribbon (3‐AGNR) films on Ni foil under low vacuum is presented. The structure of the produced graphene nanoribbons is confirmed by measuring the Raman fingerprints of 3‐AGNRs. The graphene nanoribbons obtained in large quantities allow to detect bright photoluminescence and to register 3‐AGNR optical transitions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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