COLE, Mr Ian Richard LEWIS, Councillor Andrew Ms LEY Councillor Holmes Councillor Mitchell HOLMES, Councillor John Ms LIVERMORE. UNLP (Universidad Nacional de La Plata) – ONG Nuevo Ambiente – La Agencia Ambiental La Plata – Ley Aliados estratégicos. 2s Ley da Annali & Istoriedi Corn. Tacito, tradotte Filli di Sciio di Bonarelli, cum Jig. corio turcico, filiis deaur. zs od.. ib. S
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With the end of Moore’s law in sight, researchers are in search of an alternative approach to manipulate information.
Spintronics or spin-based electronics, which uses the spin state of electrons to store, process and 1392 information, offers exciting opportunities to sustain the current growth in the information industry. For example, the discovery of the giant magneto resistance GMR effect, which provides the foundation behind modern high density data storage devices, is an important success story of spintronics; GMR-based sensors have wide applications, ranging from automotive industry to biology.
In recent years, with the tremendous progress in nanotechnology, spintronics has crossed the boundary of conventional, all metallic, solid state multi-layered structures to reach a new frontier, where nanostructures provide a pathway for the spin-carriers. Different materials such as organic and inorganic nanostructures are explored for possible applications in spintronics. Lsy this short review, we focus on the boron nitride nanotube BNNTwhich has recently been explored for possible applications in spintronics.
Unlike many organic materials, BNNTs offer higher thermal stability and higher resistance to oxidation. It has lsy reported that the metal-free fluorinated BNNT exhibits long range ferromagnetic spin ordering, which is stable at a temperature much higher than room temperature. Due to their large band lwy, BNNTs are also explored as a tunnel magneto resistance device. The purpose of this review is to highlight these recent progresses so that a concerted effort by both experimentalists and theorists can be carried out in the future to realize the true potential of BNNT-based spintronics.
The boron nitride nanotube BNNT has a one dimensional tubular structure. Its existence was first predicted in [ 12 ]. Soon after 15392 prediction, it was successfully synthesized by the arc discharge method [ 3 31592. BNNTs can have single-walled or multi-walled structures, as observed for CNTs, but in 135922 multi-walled BNNT structure, the chirality of the inner shell matches that of the outer shell [ 4 ].
Unlike the electronic properties of CNTs, which depend upon the chiral indices m, nthe electronic properties of BNNTs are found to be independent of chirality [ 5 ].
Boron Nitride Nanotubes for Spintronics
This structurally insensitive electronic property of BNNTs is advantageous for their practical applications, because separating these tubular structures based on their chirality is prohibitively difficult, though some new potential techniques to achieve this have recently emerged [ 67 ]. In addition, BNNT is bio-compatible, as it has been found to have no adverse effects on living cells [ 8 ]. It also offers a higher resistance to oxidation [ 910 ] and a higher thermal stability [ 11 ] in comparison to CNTs.
This observed large band gap and the lack of experimental control in 13952 BNNTs initially dissuaded researchers from working in this field. However, the development of viable synthesis techniques [ 12 — 20 ], together with various band gap modulating methods [ 21 — 29 ] have rekindled the hope in recent years and brought BNNTs into the forefront of material science research.
Currently, several approaches, such as electric fields as an external stimulus [ 13592222 ] functionalization [ 2425 ] doping [ oey27 ], and filling [ 2829 ], have been used extensively for oey the band gap of BNNTs. For example, using a transverse electric field of 0.
In addition to the band gap modulation, functionalization of BNNTs with different molecules have been shown to disentangle and unbundle the multi-walled BNNTs [ 31 — 34 ], as observed in CNTs [ 35 — 38 133592 and graphene [ 39 ]; this is an important first step toward their practical application.
The chiral or the circumferential vector C le terms of two unit vectors a 1 and a 2 describes how to roll up the 2D hexagonal BN sheet to form a BN nanotube.
The chiral indices m, n denote the number of unit vectors along two directions in the honeycomb 2D hexagonal BN lattice. Furthermore, functionalization and doping have been shown to induce d -electron free sp -electron magnetism in BNNTs [ 40 — 44 ], which offers new opportunities for their usage in spin-based electronics spintronics.
The advantage of using a d -electron free magnetic entity is that it would exhibit magnetism at higher temperature [ 4546 ], which is a highly desirable property for application in spintronics. Spintronics, which requires controlled transport of spin polarized carriers, has been explored in low dimensional materials such as organic molecules [ 47 — 53 ], graphene [ 5455 ], and CNTs [ 56 — 59 ]. However, spintronics in the BNNT is relatively a new concept. Though the effect of the hyperfine interaction [ 47 ] arising from the interaction of nuclear and electronic spins cannot be ignored in spin dephasing, the weak spin-orbit interaction in BNNT provides an important advantage for its application in spintronics; the spin-orbit interactions [ 47 ] scale as Z 4where Z is the atomic number, so low Z materials like B and N have weaker spin-orbit interactions.
There are several excellent reviews available on the synthesis, characterizations, and applications of BNNTs [ 560 — 67 ]. In this short review article, our focus is to explore the possible applications of BNNT in spintronics. The rest of this article is organized as follows: Finally, we conclude this review with a brief outlook. The lowest conduction band is found to be parabolic in both cases and thus behaves as free electron-like [ 2 ].
The magnitude of the band gap in BNNTs is strongly affected by the nature of B-N hybridization [ 12 ]; the curvature induced hybridization change leads to smaller band gap with the decrease in diameter. When the tube diameter increases beyond The large band gap of the BNNT limits in some instances its direct application in electronics.
The range of applications of BNNTs would substantially increase if the band gap can be tuned to a desirable value in a controlled manner. Several methods for tuning the band gap of BNNTs have been successfully demonstrated [ 21 — 29 ]. In the following section, we briefly discuss these techniques. Band structures are calculated by using density functional theory; the generalized gradient approximation GGA with the PW91 functional for the exchange-correlation is used.
The projected augmented wave PAW approach is used to describe the valence-core interaction. One of the possible ways of tuning the band gap of BNNTs is by using a transverse electric field, which breaks the symmetry of the electronic states in the direction of applied field and mixes the nearby sub-bands in the conduction band complex and the valance band complex separately [ 21 ].
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As a result, the bottom of the conduction band moves down and top of the valance band moves up, causing a reduction in the band gap of BNNTs [ 21 ]. This effect is diameter dependent; a bigger diameter BNNT exhibits a stronger response to the transverse electric field in comparison to a smaller diameter tube [ 21 ].
For example, the band gap of a BNNT with diameter In the case of a Guided by the above theoretical predictions, Ishigami et al. They used the tip of a scanning tunneling microscope to apply transverse electric fields and study the influence of these electric fields on the electronic properties of BNNTs [ 22 ]. Band gap modulation in semiconducting CNTs by applying transverse electric fields has also been observed [ 68 — 71 ].
A comparative study of the electronic structure of CNTs and BNNTs under the effect of applied transverse electric field shows a monotonic decrease in band gap with the increase of transverse electric field in the case of BNNTs [ 68 ].
The different responses of BNNTs and semiconducting CNTs to the transverse electric field are attributed to their different bonding features. Due to the semiconducting nature of BNNT, it is also expected to be a suitable candidate for optoelectronic applications [ 72 — 77 ]; tuning of optical properties of BNNTs upon applying the transverse electric field has also been reported [ 78 — 80 ].
The effect of external strain depends upon the elastic properties of the materials. BNNTs and CNTs are rigid along the tube axis, but they are highly flexible along the perpendicular direction of the tube axis and have very high Young’s modulus values [ 238586 ].
As a result, they can sustain remarkable deformation along the perpendicular direction. Significant modification in their electronic structure is expected under a strong deformation within the elastic limit. Experimentally, radial deformation can be employed in NTs by pressing them between the AFM tip and the substrate, as shown in the Figure 3 [ 87 ]. Very recently, Ghassemi et al. On the theoretical front, several groups [ 2389 — 91 ] have studied the effect of strain on the electronic structure of BNNTs.
For example, using a first-principles approach, Kim and colleagues have studied the electronic structure modulation for both zigzag 9, 0 and armchair 5, 5 BNNTs under the radial deformation [ 23 ].
A similar effect of strain on band gap in BNNTs has also been observed by other groups [ 89 — 91 ]. It should be noted that band gap modulation and conductivity enhancement upon application of radial strain are being reported in semiconducting CNTs [ 92 — 95 ].
The piezoelectric effect, which is a process of generating electrical charge using mechanical force, has also been demonstrated both theoretically [ 96 — 99 ] and experimentally [ ] in BNNTs for possible applications in nanomechanical sensors and actuators. An alternative approach for tuning the electronic property of BNNTs is surface functionalization with different atoms, molecules, and nanoparticles [ 6164 ].
Surface functionalization not only modifies the band gap of BNNTs, but also makes them soluble in several solvents [ 24 ]. Functionalization even reduces the work function of BNNTs significantly, which facilitates the field electron emission from the tube surface [ ].
Nanoparticle-decorated BNNTs [ — ] have been explored for conductance enhancement, modification of field emission behavior [ ] and designing room temperature tunneling field effect transistors Figure 4 [ ]. Due to these advantages, functionalization of BNNTs has been the subject of intense research over the past few years [ 56164 ]. Functionalization of BNNTs can be done in two ways, namely covalent chemisorption and non-covalent physisorption functionalization [ 61 ].
Here, we will discuss them briefly. A Schematic diagram of a gold nanoparticle-decorated BNNT based room temperature tunneling field effect transistor.
When an adsorbate affects strongly the hybridization at the adsorption sites in a host, the functionalization is referred to as covalent functionalization or chemisorption [ 3261 ]; electronic structure modification is relatively stronger in such covalent functionalization.
For example, the chemisorption of F on BNNT surface changes the sp 2 hybridization at the adsorption site to sp 3 [ — ]. A strong charge transformation happens in between adsorbates and BNNTs during the chemisorption, which makes covalently functionalized BNNTs either a p-type or an n-type semiconductor, depending upon the electronegativity of the adsorbates .
There are numerous schemes being published for the functionalization of CNTs [ 35 — 38 ] and graphene [ 39 ]. Due to the very low inherent chemical reactivity of the BNNT surface, several techniques that are used for the functionalization of CNTs and graphene are found to be unsuitable for the BNNTs [ 6061, ]. Despite the challenges, several groups have successfully demonstrated functionalization of BNNTs in recent years [ 24— ]. For example, Bando and colleagues have demonstrated covalent functionalization of BNNTs with long alkyl chains.
The same research group has also functionalized BNNTs covalently with different molecular groups, namely amino and -COCl groups [ ]. They have observed a similar shift in the adsorption peak towards the smaller value upon the covalent functionalization as observed for the alkyl chains [ 24 ]; the theoretical study confirms that the shift in adsorption peak is due to the presence of new eigenstates around the Fermi energy arising from the adsorbed molecules [ ].
Later, using ammonia plasma irradiation, BNNT surface functionalization with amine groups has been reported [ ]. It has been shown theoretically that the chemical functionalization with amine groups reduces the band gap [ — ] and makes the functionalized BNNT a p-type semiconductor [ ].
Recently, using a simple scheme that involves strong oxidation of BNNT with nitric acid, followed by silanization of the surface using 3-aminopropyltriethoxysilane APTES molecules, researchers have successfully demonstrated chemical linking of amino groups to the BNNT surface Figure 5 [ ]. The adsorptions of these different atoms and molecules introduce impurity states within the band gap of BNNT [ 43, — ]. In addition, like CNTs [ — ], BNNTs have also been explored as potential materials for hydrogen storage [ — ]; BN nanotubes have been demonstrated to retain 1.
Electronic structure calculations show the hydrogen adsorption on BNNTs is curvature dependent [ ]. The functionalization in which hybridization at the adsorption site of the host remains unchanged upon functionalization is called non-covalent physisorption functionalization [ 3261 ]. Physisorption changes the electronic structure, while preserving the intrinsic desirable properties [ 32 ].
For example, Wang et al. Guided by the above experimental report, Gao et al. An important finding of this work [ ] is that although the interaction between BN layer and PTAS is of the van der Waals type, the band structure of the physisorbed system is different from the superposition of the band structures of the host BN layer and the PTAS adsorbate. Recently, different types of aromatic molecules have been used to functionalize BNNTs non-covalently  for possible applications in field effect transistors; thus, adsorption of an electrophilic nucleophilic molecule on BNNTs makes them p-type n-type semiconductors.
Using density functional theory, Peyghan et al. Interaction between different biological molecules such as amino acids [ ] Figure 7 and nucleic acid bases [ ] with BNNT surfaces have also been investigated using density functional theory; polar amino acids are found to exhibit a relatively stronger interaction with the BNNT surface in comparison to non-polar ones [ ]. Non-covalent functionalization of BNNT with polar and non-polar amino acids. Trp non-poarAsp polarand Arg polar stand for tryptophane, asparatic, and argenine, respectively.
The upper panel shows the charge transfer between the BNNT and amino acids, and the lower panel represents the iso-surface charge density of amino acids functionalized BNNT system.