Excitons in boron nitride nanotubes: dimensionality effects

The excitonic effects in single and double-walled boron nitride nanotubes

The electronic structures and excitonic optical properties of single- and double-walled armchair boron nitride nanotubes (BNNTs) [e.g., (5,5) and (10,10), and (5,5)@(10,10)] are investigated within many-body Green's function and Bethe-Salpeter equation formalism. The first absorption peak of the double-walled nanotube has almost no shift compared with the single-walled (5,5) tube due to the strong optical transition in the double-walled tube that occurs within the inner (5,5) one. Dark and semi-dark excitonic states are detected in the lower energy region, stemming from the charge transfer between inner and outer tubes in the double-walled structure. Most interestingly, the charge transfer makes the electron and the hole reside in different tubes. Moreover, the excited electrons in the double-walled BNNT are able to transfer from the outer tube to the inner one, opposite to that which has been observed in double-walled carbon nanotubes.

Strong photoluminescence enhancement of MoS(2) through defect engineering and oxygen bonding

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.

Theoretical calculations of low-field electroreflectance of ultra-thin hexagonal BN films at the fundamental absorption edge

A theory is developed to describe the electric field effect on the reflectivity of ultra-thin films of hexagonal boron nitride (h-BN) at the fundamental absorption edge. The formulation of the theory is based on the original nonlinear optical approach to electroreflectance (ER) worked out by Aspnes and Rowe (1972 Phys. Rev. B 5 4022). Within the framework of the approach, the electric-field-induced change in the reflectivity in the low-field regime is expressed in terms of the third derivative of the linear optical dielectric function of the system. An explicit closed-form expression for this function is derived within the independent-layer approximation using the tight-binding representation for the π-electron energy bands of h-BN atomic layers. Incorporating this result with the general formalism of Aspnes and Rowe enables the electro-optical response function to be obtained in an explicit analytic form convenient to further numerical analysis for any particular set of input empirical parameters. The results of such an analysis suitable for the ER effect in few-layer h-BN films are presented and discussed in the context of important information they are able to provide for band structure parameters of this material. Our findings (e.g. distinct resonant field-invariant spectral features in the ER near the fundamental bandgap of the material under study) suggest that the ER technique can be used as a sensitive tool to help characterize the electronic structure of atomic hexagonal layers built from boron and nitrogen.

Many-body effects in semiconducting single-wall silicon nanotubes

The electronic and optical properties of semiconducting silicon nanotubes (SiNTs) are studied by means of the many-body Green's function method, i.e., GW approximation and Bethe-Salpeter equation. In these studied structures, i.e., (4,4), (6,6) and (10,0) SiNTs, self-energy effects are enhanced giving rise to large quasi-particle (QP) band gaps due to the confinement effect. The strong electron-electron (e-e) correlations broaden the band gaps of the studied SiNTs from 0.65, 0.28 and 0.05 eV at DFT level to 1.9, 1.22 and 0.79 eV at GW level. The Coulomb electron-hole (e-h) interactions significantly modify optical absorption properties obtained at noninteracting-particle level with the formation of bound excitons with considerable binding energies (of the order of 1 eV) assigned: the binding energies of the armchair (4,4), (6,6) and zigzag (10,0) SiNTs are 0.92, 1.1 and 0.6 eV, respectively. Results in this work are useful for understanding the physics and applications in silicon-based nanoscale device components.

Tunable band gap photoluminescence from atomically thin transition-metal dichalcogenide alloys

Band gap engineering of atomically thin two-dimensional (2D) materials is the key to their applications in nanoelectronics, optoelectronics, and photonics. Here, for the first time, we demonstrate that in the 2D system, by alloying two materials with different band gaps (MoS2 and WS2), tunable band gap can be obtained in the 2D alloys (Mo(1-x)W(x)S(2) monolayers, x = 0-1). Atomic-resolution scanning transmission electron microscopy has revealed random arrangement of Mo and W atoms in the Mo(1-x)W(x)S(2) monolayer alloys. Photoluminescence characterization has shown tunable band gap emission continuously tuned from 1.82 eV (reached at x = 0.20) to 1.99 eV (reached at x = 1). Further, density functional theory calculations have been carried out to understand the composition-dependent electronic structures of Mo(1-x)W(x)S(2) monolayer alloys.

Tunable and sizable band gap in silicene by surface adsorption

Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The ab initio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 10(8). Therefore, a way is paved for silicene as the channel of a high-performance FET.

Tunable and sizable band gap in silicene by surface adsorption

Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The ab initio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 10(8). Therefore, a way is paved for silicene as the channel of a high-performance FET.

High-yield boron nitride nanosheets from 'chemical blowing': towards practical applications in polymer composites

An improved 'chemical blowing' route presuming atmospheric-pressure pre-treatment and moderate heating rate of designated precursors was developed to synthesize ultra-thin boron nitride (BN) nanosheets with high yield and large lateral dimensions. The yield reached as high as 40 wt% with respect to raw materials (ammonia borane). The strong oxygen-related ultraviolet luminescence together with a blue emission of these BN nanosheets was then documented and analyzed. This implies potential applications in solid-state lighting, ultraviolet lasing and full-color luminescence. Mechanical strength of different polymeric composites with a small fraction of BN nanosheet fillers was dramatically increased by tens of per cent, while high transparency of composite materials was still maintained in the visible optical range. The increased yield and reduced cost of BN nanosheets should promote their wide practical applications in various composites.

Artificially stacked atomic layers: toward new van der Waals solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.

Band gap opening of graphene by doping small boron nitride domains

Boron nitride (BN) domains are easily formed in the basal plane of graphene due to phase separation. With first-principles calculations, it is demonstrated theoretically that the band gap of graphene can be opened effectively around K (or K') points by introducing small BN domains. It is also found that random doping with boron or nitrogen is possible to open a small gap in the Dirac points, except for the modulation of the Fermi level. The surface charges which belong to the π states near Dirac points are found to be redistributed locally. The charge redistribution is attributed to the change of localized potential due to doping effects. The band opening induced by the doped BN domain is found to be due to the breaking of localized symmetry of the potential. Therefore, doping graphene with BN domains is an effective method to open a band gap for carbon-based next-generation microelectronic devices.

Tunable bandgap in silicene and germanene

By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.

First-principles simulations of chemical reactions in an HCl molecule embedded inside a C or BN nanotube induced by ultrafast laser pulses

We show by first-principles simulations that ultrafast laser pulses induce different chemical reactions in a molecule trapped inside a nanotube. A strong laser pulse polarized perpendicular to the tube axis induces a giant bond stretch of an encapsulated HCl molecule in semiconducting carbon nanotube or in a BN nanotube. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different: either complete disintegration or rebonding of the HCl molecule. Radial motion of the nanotube is always observed and a vacancy appears on the tube wall when the HCl is perpendicular to the tube axis. Those results are important to analyze confined nanochemistry and to manipulate molecules and nanostructures encapsulated in organic and inorganic nanotubes.

Large-Scale Fabrication of Boron Nitride Nanotubes via a Facile Chemical Vapor Reaction Route and Their Cathodoluminescence Properties

Cylinder- and bamboo-shaped boron nitride nanotubes (BNNTs) have been synthesized in large scale via a facile chemical vapor reaction route using ammonia borane as a precursor. The structure and chemical composition of the as-synthesized BNNTs are extensively characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and selected-area electron diffraction. The cylinder-shaped BNNTs have an average diameter of about 100 nm and length of hundreds of microns, while the bamboo-shaped BNNTs are 100-500 nm in diameter with length up to tens of microns. The formation mechanism of the BNNTs has been explored on the basis of our experimental observations and a growth model has been proposed accordingly. Ultraviolet-visible and cathodoluminescence spectroscopic analyses are performed on the BNNTs. Strong ultraviolet emissions are detected on both morphologies of BNNTs. The band gap of the BNNTs are around 5.82 eV and nearly unaffected by tube morphology. There exist two intermediate bands in the band gap of BNNTs, which could be distinguishably assigned to structural defects and chemical impurities.

Advances in computational studies of energy materials

We review recent developments and applications of computational modelling techniques in the field of materials for energy technologies including hydrogen production and storage, energy storage and conversion, and light absorption and emission. In addition, we present new work on an Sn2TiO4 photocatalyst containing an Sn(II) lone pair, new interatomic potential models for SrTiO3 and GaN, an exploration of defects in the kesterite/stannite-structured solar cell absorber Cu2ZnSnS4, and report details of the incorporation of hydrogen into Ag2O and Cu2O. Special attention is paid to the modelling of nanostructured systems, including ceria (CeO2, mixed Ce(x)O(y) and Ce2O3) and group 13 sesquioxides. We consider applications based on both interatomic potential and electronic structure methodologies; and we illustrate the increasingly quantitative and predictive nature of modelling in this field.

Many-body effects in the excitation spectrum of a defect in SiC

We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle band structure and excitonic effects. We consider the carbon vacancy with two possible excitation channels that involve conduction and valence bands. Corrections to the Kohn-Sham ionization levels strongly depend on the defect charge state. Excitonic effects introduce a redshift of 0.23 eV. The analysis reassigns excitation mechanism at the thresholds in photoinduced paramagnetic resonance measurements [J. Dashdorj, J. Appl. Phys. 104, 113707 (2008)].

Boron nitride nanotubes and nanosheets

Hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing non-carbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogue-a monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as optoelectronic nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices. This review gives an introduction to the rich BN nanotube/nanosheet field, including the latest achievements in the synthesis, structural analyses, and property evaluations, and presents the purpose and significance of this direction in the light of the general nanotube/nanosheet developments.

Tunable excitons in biased bilayer graphene

Recent measurements have shown that a continuously tunable bandgap of up to 250 meV can be generated in biased bilayer graphene [ Zhang , Y. ; et al. Nature 2009, 459 , 820 ], opening up pathway for possible graphene-based nanoelectronic and nanophotonic devices operating at room temperature. Here, we show that the optical response of this system is dominated by bound excitons. The main feature of the optical absorbance spectrum is determined by a single symmetric peak arising from excitons, a profile that is markedly different from that of an interband transition picture. Under laboratory conditions, the binding energy of the excitons may be tuned with the external bias going from zero to several tens of millielectronvolts. These novel strong excitonic behaviors result from a peculiar, effective "one-dimensional" joint density of states and a continuously tunable bandgap in biased bilayer graphene. Moreover, we show that the electronic structure (level degeneracy, optical selection rules, etc.) of the bound excitons in a biased bilayer graphene is markedly different from that of a two-dimensional hydrogen atom because of the pseudospin physics.

Exciton-plasmon States in nanoscale materials: breakdown of the Tamm-Dancoff approximation

Within the Tamm-Dancoff approximation, ab initio approaches describe excitons as packets of electron-hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton-plasmon states where the electron-hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and the carbon nanotubes, the Tamm-Dancoff approximation yields errors larger than the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm-Dancoff approximation, correctly describes excitons, plasmons, and exciton-plasmon states, and provides a good agreement with experimental results.

Effective growth of boron nitride nanotubes by thermal chemical vapor deposition

Effective growth of multiwalled boron nitride nanotubes (BNNTs) has been obtained by thermal chemical vapor deposition (CVD). This is achieved by a growth vapor trapping approach as guided by the theory of nucleation. Our results enable the growth of BNNTs in a conventional horizontal tube furnace within an hour at 1200 °C. We found that these BNNTs have an absorption band edge of 5.9 eV, approaching that of single h-BN crystals, which are promising for future nanoscale deep-UV light emitting devices.

Optical gap measurements on individual boron nitride nanotubes by electron energy loss spectroscopy

Electromagnetic response of individual boron nitride nanotubes (BNNTs) has been studied by spatially resolved electron energy loss spectroscopy (EELS). We demonstrate how dedicated EELS methods using subnanometer electron probes permit the analysis of local dielectric properties of a material on a nanometer scale. The continuum dielectric model has been used to analyze the low-loss EEL spectra recorded from these tubes. Using this model, we demonstrate the weak influence of the out-of-plane contribution to the dielectric response of BNNTs. The optical gap, which can be deduced from the measurements, is found to be equal to 5.8 +/- 0.2 eV, which is close to that of the hexagonal boron nitride. This value is found to be independent of the nanotubes configuration (diameter, helicity, number of walls, and interaction between the different walls).

Isotope effect on band gap and radiative transitions properties of boron nitride nanotubes

We have carried out an isotope study on the band gap and radiative transition spectra of boron nitride nanotubes (BNNTs) using both experimental and theoretical approaches. The direct band gap of BNNTs was determined at 5.38 eV, independent of the nanotube size and isotope substitution, by cathodoluminescences (CL) spectra. At lower energies, several radiative transitions were observed, and an isotope effect was revealed. In particular, we confirmed that the rich CL spectra between 3.0 and 4.2 eV reflect a phonon-electron coupling mechanism, which is characterized by a radiative transition at 4.09 eV. The frequency red shift and peak broadening due to isotopic effect have been observed. Our Fourier transform infrared spectra and density functional theory calculations suggest that those radiative transitions in BNNTs could be generated by a replacement of some nitrogen atoms with oxygen.

A theoretical investigation of defects in a boron nitride monolayer

We have investigated, using first-principles calculations, the energetic stability and structural properties of antisites, vacancies and substitutional carbon defects in a boron nitride monolayer. We have found that the incorporation of a carbon atom substituting for one boron atom, in an N-rich growth condition, or a nitrogen atom, in a B-rich medium, lowers the formation energy, as compared to antisites and vacancy defects. We also verify that defects, inducing an excess of nitrogen or boron, such as N(B) and B(N), are more stable in its reverse atmosphere, i.e. N(B) is more stable in a B-rich growth medium, while B(N) is more stable in a N-rich condition. In addition we have found that the formation energy of a C(N), in a N-rich medium, and C(B) in a B-rich medium, present formation energies comparable to those of the vacancies, V(N) and V(B), respectively.

Quasiparticle energies and band gaps in graphene nanoribbons

We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach within the GW approximation. Because of the quasi-one-dimensional nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5-3.0 eV for ribbons of width 2.4-0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.

Excitonic effects in the optical spectra of graphene nanoribbons

We present a first-principles calculation of the optical properties of armchair-edged graphene nanoribbons (AGNRs) with many-electron effects included. The reduced dimensionality of the AGNRs gives rise to an enhanced electron-hole binding energy for both bright and dark exciton states (0.8-1.4 eV for GNRs with width approximately 1.2 nm) and dramatically changes the optical spectra owing to a near complete transfer of oscillator strength to the exciton states from the continuum transitions. The characteristics of the excitons of the three distinct families of AGNRs are compared and discussed. The enhanced excitonic effects found here are expected to be of importance in optoelectronic applications of graphene-based nanostructures.

C-BN patterned single-walled nanotubes synthesized by laser vaporization

We report on the synthesis of C-BN single-walled nanotubes made of BN nanodomains embedded into a graphene layer. The synthesis process consists of vaporizing, by a continuous CO2 laser, a target made of carbon and boron mixed with a Co/Ni catalyst under N2 atmosphere. High-resolution transmission electron microscopy (HRTEM) and nanoelectron energy loss spectroscopy (nanoEELS) provide direct evidence that boron and nitrogen co-segregate with respect to carbon and form nanodomains within the hexagonal lattice of the graphene layer in a sequential manner. A growth model is proposed to account for the observed C-BN self-organization and to explain how kinetics and local energetics at intermediate states can tailor ultimate single layer BN-C heterojunctions.

Short, highly ordered, single-walled mixed-oxide nanotubes assemble from amorphous nanoparticles

Nanotubes are important "building block" materials for nanotechnology, but a synthesis process for short (sub-100-nm) solid-state nanotubes with structural order and monodisperse diameter has remained elusive. To achieve this goal, it is critical to possess a definitive mechanistic framework for control over nanotube dimensions and structure. Here we employ solution-phase and solid-state characterization tools to elucidate such a mechanism, particularly that governing the formation of short ( approximately 20 nm), ordered, monodisperse (3.3 nm diameter), aluminum-germanium-hydroxide ("aluminogermanate") nanotubes in aqueous solution. Dynamic light scattering (DLS), vibrational spectroscopy, and electron microscopy show that pH-control of chemical speciation in the aluminogermanate precursor solution is important for producing nanotubes. A combination of DLS, UV-vis spectroscopy, and synthesis variations is then used to study the nanotube growth process as a function of temperature and time, revealing the initial condensation of amorphous nanoparticles of size approximately 6 nm and their transformation into ordered aluminogermanate nanotubes. The main kinetic trends in the experimental data can be well reproduced by a two-step mathematical model. From these investigations, the central phenomena underlying the mechanism are enumerated as: (1) the generation (via pH control) of a precursor solution containing aluminate and germanate precursors chemically bonded to each other, (2) the formation of amorphous nanoscale ( approximately 6 nm) condensates via temperature control, and (3) the self-assembly of short nanotubes from the amorphous nanoscale condensates. This mechanism provides a model for controlled low-temperature (<373 K) assembly of short, monodisperse, structurally ordered nanotube objects.

Vibrational properties of hexagonal boron nitride: inelastic X-ray scattering and ab initio calculations

The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering.

From Si nanowires to porous silicon: the role of excitonic effects

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.

Defective structure of BN nanotubes: from single vacancies to dislocation lines

A combination of electron microscopy and theoretical calculations provides new insights into the structure, electronics, and energetics of point defects and vacancy lines in BN single-wall nanotubes (SWNT). We show that the point defects forming under electron irradiation in the BN SWNTs are primarily divacancies. Due to the partially ionic character of the BN bonding, divacancies behave like an associated Schottky pair, with a dissociation energy of around 8 eV. Clustering of multiple vacancies is energetically favorable and leads to extended defects which locally change the nanotube diameter and chirality. Nevertheless these defects do not alter significantly the band gap energy, and all of them have electronic structure similar to that of BN divacancies. We thus conclude that under irradiation BN SWNT may have a very stable alteration of its electronic and optical properties.

Excitons and many-electron effects in the optical response of single-walled boron nitride nanotubes

We report first-principles calculations of the effects of quasiparticle self-energy and electron-hole interaction on the optical properties of single-walled boron nitride nanotubes. Excitonic effects are shown to be even more important in BN nanotubes than in carbon nanotubes. Electron-hole interactions give rise to complexes of bright (and dark) excitons, which qualitatively alter the optical response. Excitons with a binding energy larger than 2 eV are found in the BN nanotubes. Moreover, unlike the carbon nanotubes, theory predicts that these exciton states are comprised of coherent supposition of transitions from several different subband pairs, giving rise to novel behaviors.