First principles study of titanium-coated carbon nanotubes as sensors for carbon monoxide molecules

In this work, the electronic properties of the system composed by the CO molecules adsorbed on Ti-coated single-wall carbon nanotubes (SWNTs) are studied through first principles calculations. The changes in the electronic properties of the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT are analyzed for different CO concentrations. A strong interaction between CO molecules and the SWCT/Ti system is observed, which decreases when the concentration of CO molecules increases. The resulting system are shown to be very sensitive to the CO concentration adsorbed on the tube/Ti system, making that the SWNT, which is originally semiconductor and becomes metallic after Ti covering, to recover the semiconductor behavior again when enough high concentrations of CO molecules is adsorbed on the SWNT/Ti system. These three distinct steps (semiconductor/metallic/semiconductor) constitute the basis for a feasible, flexible and efficient sensor device for CO molecule recognition.     

Micropatterned vertically aligned carbon-nanotube growth on a Si surface or inside trenches

The good field-emission properties of carbon nanotubes coupled with their high mechanical strength, chemical stability, and high aspect ratio, make them ideal candidates for the construction of efficient and inexpensive field-emission electronic devices. The fabrication process reported here has considerable potential for use in the development of integrated radio-frequency amplifiers or field-emission-controllable cold-electron guns for field-emission displays. This fabrication process is compatible with currently used semiconductor-processing technologies. Micropatterned vertically aligned carbon nanotubes were grown on a planar Si surface or inside trenches, using chemical vapor deposition, photolithography, pulsed-laser deposition, reactive ion etching, and the lift-off method. This carbon-nanotube fabrication process can be widely applied for the development of electronic devices using carbon-nanotube field emitters as cold cathodes and could revolutionize the area of field-emitting electronic devices. Received: 30 August 2001 / Accepted: 3 September 2001 / Published online: 20 December 2001     

Role of the curvature of atomic layers in electron field emission from graphitic nanostructured carbon

Layers of oriented carbon nanotubes and nanometer-size plate-shaped graphite crystallites are obtained by chemical vapor deposition in a glow-discharge plasma. A structural-morphological investigation of a carbon material consisting of nanotubes and nanocrystallites is performed, and the field-emission properties of the material are also investigated. It is shown that electron field emission is observed in an electric field with average intensity equal to or greater than 1.5 V/μm. The low fields giving rise to electron emission can be explained by a decrease in the electronic work function as a result of the curvature of the atomic layers of graphitic carbon. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 381–386 (10 March 1999)     

非掺杂半导体型碳纳米管低温电导的非线性特征

依据相空间玻耳兹曼输运方程和碳纳米管的电子结构,对非掺杂单个半导体型单层碳纳米管（SWNTs）的低温电导特性进行数值计算,结果表明：SWNTs电导是量子化的,并呈现明显的非线性特征。     

Carbon systems of polymerized nanotubes: Crystal and electronic structures

Molecular dynamic calculations are carried out for the (P, T) phase diagram of a covalent compound of cross-linked carbon single-wall nanotubes (SWNT) and for the structures and electronic spectra of the novel crystals of polymerized carbon nanotubes. It is shown that the transformation of covalently bonded nanotubes in a close-packed conducting structure cardinally modifies their electronic properties. The P-SWNT crystal becomes semiconducting and, upon complete transformation of sp ²-hybridized carbon atoms into sp ³-hybridized ones, it becomes an insulator.     

Magnetoadsorptive Particles Enabling the Centrifugation‐Free,Preparative‐Scale Separation,and Sorting of Single‐Walled Carbon Nanotubes

A novel magnetic composition of the high surface area particles with amide chemical functionality, Sephacryl S‐200, is reported enabling the preparative‐scale (1 L, ≈5–10 mg) separation of metallic and semiconducting single‐walled carbon nanotubes (SWNT) from completely unpurified and uncentrifuged nanotube stocks. Sephacryl S‐200 has previously been utilized in separating semiconducting SWNT (s‐SWNT) on the laboratory scale. Significantly, use of these magnetic derivative particles in absorptive separation of SWNT allows the unprecedented and industrially scalable purification of both metallic SWNT (m‐SWNT) as well as s‐SWNT directly from uncentrifuged, ultrasonicated surfactant‐based SWNT solutions by simple and scalable magnetic separation. These particles also allowed for the systematic study on the effect of SWNT–polymer interaction time on the resulting SWNT “payloads.” Ultimately, high ‐purity m‐SWNT and s‐SWNT products are independently achieved by controlling the SWNT–polymer interaction time and relative concentrations, as well as SWNT sonication conditions. Furthermore, by controlling these factors, single‐chirality (6,5) s‐SWNT can be isolated with 92% purity directly from unpurified stocks. Thermogravimetric analysis indicates a total process SWNT yield of 1.2% and 1.7% for m‐SWNT and s‐SWNT, respectively. These results demonstrate the potential for a preparative method for separating carbon nanotubes based on electronic properties.     

Effects of catalysts on the internal structures of carbon nanotubes and corresponding electron field-emission properties

Using a chemical vapor deposition (CVD) method, multi-walled carbon nanotubes with uniform diameters of approximately 10 nm were synthesized on silicon substrates by the decomposition of acetylene using Fe, Co and Ni as the catalysts. Catalyst effects on the internal structures of the carbon nanotubes were evident in the Fe, Co and Ni catalyzed nanotubes. Although these nanotubes demonstrated similar morphologies, due to the variety of internal structures, the nanotubes synthesized from different catalysts demonstrated various electron field-emission characteristics including turn-on field, threshold field and field enhancement factor. Compared with carbon nanotubes from Ni catalyst, nanotubes from Fe and Co with the same diameters have better field-emission properties. Graphite layers in nanotubes from Fe and Co are much straighter and more parallel to the tube axis with fewer defects. For instance, the turn-on field and threshold field for nanotubes from Ni are 5 V/m and 9 V/m, respectively. These electric fields are much higher than those for nanotubes from Fe, which are 0.35 V/m and 2.8 V/m, respectively. This could be due to the effect of catalysts on the work function of nanotubes, since the catalyst particle usually terminates the free end of the nanotube, and the influence of internal structure on electron transportation along the nanotube axis. Therefore, this study suggests that besides a small diameter, good graphitization (crystallization) is an important prerequisite for a good carbon nanotube emitter. PACS 79.70.+q; 68.37.Lp; 81.07.De     

Selection of isolated and individual single-walled carbon nanotube from a film and its usage in nanodevices

Individual and isolated single-walled carbon nanotubes (SWNTs) are important for fabricating relevant nanode- vices and studying the properties of the SWNT devices. In this work, we demonstrate that individual and isolated SWNT can be selected and obtained from a film containing a huge number of SWNTs. By using both the polymethyl-methacrylate (PMMA) as a negative resist and the electron beam lithography, the selected SWNT can be fixed on a substrate, while the other SWNTs in the film can lift off. The selected SWNT can be used to fabricate nanodevice and a gas sensor of oxygen is demonstrated in this work.     

Chemically induced conductance switching in carbon nanotube circuits

The chemical reactivity of carbon nanotubes in H2SO4 is investigated using individual, single-walled carbon nanotubes (SWNTs) incorporated into electronic devices. Exploiting the device conductance as a sensitive indicator of chemical reactions, discrete oxidation and reduction events can be clearly observed. During oxidation, a SWNT opens circuits to a nanometer-scale tunnel junction with residual conduction similar to Frenkel-Poole charge emission. When electrochemically reduced, a SWNT returns to its original conductance. This redox cycle can be repeated many times, suggesting a novel chemical method of reversibly switching SWNT conductivity.     

Effect of electrode material on impedance spectra of metal-polyethylene structures with carbon nanotubes

The effect of an electrode material on electrical properties of a composite material based on super-high-molecular polyethylene (SHMPE) filled with carbon nanotubes has been studied using impedance spectroscopy. Using the method of replacing the sample by an equivalent electric circuit, it has been found that, depending on the electrode material, a blocking barrier with high active resistance and a space charge region adjacent to it arise in the interface region. It has been shown that the barrier height is controlled by surface electronic states of SHMPE and weakly depends on the electron work function of metal electrodes (Bardeen barrier). The characteristic times of electrical relaxation characterizing bulk and interface regions of the composite under study have been determined.     

基于碳纳米管分子封装的一维电子波函数扫描

利用第一性原理,设计并研究了一类基于单臂碳纳米管的分子封装的分子体系．计算表明,半环葫芦脲类化合物可有效封装碳纳米管,引入微弱的分子间相互作用,对碳纳米管的电子态能级结构分布 仅带来微弱影响．半环葫芦脲分子与碳纳米管在管径方向的一维电子态波函数充分耦合,进而有效改变了一些前沿分子轨道的波函数在管径两头的分布以及相应的电子布居浓度．基于电子输运的模拟,发现半环葫芦脲分子在碳纳米管一维方向滑动时的某个电压下的电导变化可准确反映电子态波函数在相应分子导电通道上的一维分布信息．     

Biophysical and electrochemical properties of Self-assembled noncovalent SWNT/DNA hybrid and electroactive nanostructure

DNA self-assembled hybrid nanostructures are widely used in recent research in nanobiotechnology. Combination of DNA with carbon based nanoparticles such as single-walled carbon nanotube (SWNT), multi-walled carbon nanotube (MWNT) and carbon quantum dot were applied in important biological applications. Many examples of biosensors, nanowires and nanoelectronic devices, nanomachine and drug delivery systems are fabricated by these hybrid nanostructures. In this study, a new hybrid nanostructure has been fabricated by noncovalent interactions between single or double stranded DNA and SWNT nanoparticles and biophysical properties of these structures were studied comparatively. Biophysical properties of hybrid nanostructures studied by circular dichroism, UV–vis and fluorescence spectroscopy techniques. Also, electrochemical properties studied by cyclic voltammetry, linear sweep voltammetry, square wave voltammetry, choronoamperometry and impedance spectroscopy (EIS). Results revealed that the biophysical and electrochemical properties of SWNT/DNA hybrid nanostructures were different compare to ss-DNA, ds-DNA and SWNT singly. Circular dichroism results showed that ss-DNA wrapped around the nanotubes through π-π stacking interactions. The results indicated that after adding SWNT to ss-DNA and ds-DNA intensity of CD and UV–vis spectrum peaks were decreased. Electrochemical experiments indicated that the modification of single-walled carbon nanotubes by ss-DNA improves the electron transfer rate of hybrid nanostructures. It was demonstrated SWNT/DNA hybrid nanostructures should be a good electroactive nanostructure that can be used for electrochemical detection or sensing.     

Functionalization of silicon-doped single walled carbon nanotubes at the doping site: An ab initio study

We performed ab initio calculations on the cytosine-functionalized silicon-doped single walled carbon nanotubes (SWNT). The results show that silicon substitutional doping to SWNT can dramatically change the atomic and electronic structures of the SWNT. And more importantly, it may provide an efficient pathway for further sidewall functionalization to synthesize more complicated SWNT based complex materials, for example, our previously proposed base-functionalized SWNTs, because the doping silicon atom can improve the reaction activity of the tube at the doping site due to its preference to form sp3 hybridization bonding.     

Effect of PVA functionalization on hydrophilicity of Y-junction single wall carbon nanotubes

Hydrophilic surface of carbon nanotubes (CNTs) are of great interest for various applications including chemical and biological sensing. Surface functionalization of single wall carbon nanotubes (SWNTs) mats with a biocompatible polymer polyvinyl alcohol (PVA) was studied. PVA modification induced a drastic change in water wettability of the SWNT surface transforming it from hydrophobic to highly hydrophilic. These PVA modified SWNTs mats have also demonstrated increasing impedance variation in relative humidity compared to the pristine nanotubes. An appreciable change in conductivity of Y-junction SWNT mats as a function of relative humidity indicates its potential application as humidity sensor. This higher sensitivity for humidity variation shown in Y-junction SWNT mats could be attributed to the greater portion of semiconducting nanotubes in these mats revealed by Raman analysis. A possible conductance changing mechanism of surface modified SWNTs mats is discussed.     

氮掺杂手性碳纳米管的电子结构和输运特性的理论研究

运用第一性原理的密度泛函理论,结合非平衡格林函数,研究了氮原子取代掺杂手性单壁(6,3)碳纳米管的电子结构和输运特性.计算结果表明:不同构形和不同数目的氮原子取代掺杂对手性碳管的输运性质有很复杂的影响.研究发现,氮原子掺杂明显改变了碳管的电子结构,使金属型手性碳管的输运性能降低,电流-电压曲线呈非线性变化,而且输运性能随着杂质原子间间距的变化而发生显著改变.在一定条件下,金属型碳管向半导体型转变. 关键词： 手性单壁碳纳米管 氮掺杂 电子结构 输运性能     

Humidity sensing properties of different single-walled carbon nanotube composite films fabricated by layer-by-layer self-assembly technique

Poly(diallyldimethylammonium chloride)/single-walled carbon nanotube (PDDA/SWNT) multilayered thin films were prepared on quartz crystal microbalance by layer-by-layer self-assembly technique, and their sensing properties to humidity were studied. The SWNTs were characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The composite films were observed by field-emission scanning electron microscope. Two types of SWNT humidity sensors were fabricated using SWNTs and carboxyl (COOH) modified SWNTs as sensitive material, respectively. The results showed that the sensitivity of the PDDA/SWNT?CCOOH humidity sensor was 20.23?% higher than that of the PDDA/SWNT sensor. In contrast, the latter had a much superior hysteresis property, and the reason to cause this phenomenon was discussed.     

Electron transport through SWNT/<Emphasis Type="Italic">trans</Emphasis>-PA/SWNT structure (the role of solitons): A t-matrix technique

Using a tight-binding model and a transfer-matrix technique, we numerically investigate the effects of the coupling strength and the role of solitons on the electronic transmission through a system in which trans-polyacetylene (trans-PA) molecule is sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT). We rely on Landauer formalism as the basis for studying the conductance properties of this system. Our calculations show that the solitons play an important role in the response of this system causing a large enhancement in the conductance. Also our results suggest that the conductance is sensitive to the CNT/molecule coupling strength.      

Optical transition energies for carbon nanotubes from resonant Raman spectroscopy: environment and temperature effects

This Letter reports the laser energy dependence of the Stokes and anti-Stokes Raman spectra of carbon nanotubes dispersed in aqueous solution and within solid bundles, in the energy range 1.52-2.71 eV. The electronic transition energies (E(ii)) and the radial breathing mode frequencies (omega(RBM)) are obtained for 46 different (18 metallic and 28 semiconducting) nanotubes, and the (n,m) assignment is discussed based on the observation of geometrical patterns for E(ii) versus omega(RBM) graphs. Only the low energy component of the E(M)(11) value is observed from each metallic nanotube. For a given nanotube, the resonant window is broadened and down-shifted for single wall carbon nanotube (SWNT) bundles compared to SWNTs in solution, while by increasing the temperature, the E(S)(22) energies are redshifted for S1 [(2n+m) mod 3=1] nanotubes and blueshifted for S2 [(2n+m) mod 3=2] nanotubes.     

Comparison of influence of incorporated 3d-, 4d- and 4f-metal chlorides on electronic properties of single-walled carbon nanotubes

In the present work, the channels of single-walled carbon nanotubes were filled with melts of ZnCl₂, CdCl₂, and TbCl₃ by a capillary method with subsequent slow cooling. The detailed study of electronic structure of filled nanotubes was performed using Raman, optical absorption, and X-ray photoelectron spectroscopy. The obtained data are in mutual agreement and it proves that the filling of carbon nanotube channels with all these salts leads to the charge transfer from nanotube walls to the incorporated compounds, thus acceptor doping of nanotubes takes place. It was found out that encapsulated terbium chloride has the largest influence on the electronic properties of carbon nanotubes.     

Transport properties of poly(GACT)-poly(CTGA) deoxyribonucleic acid: A ladder model approach

In this paper, based on the tight-binding Hamiltonian model and within the framework of a generalized Green’s function technique, the electronic conduction through the poly(GACT)-poly(CTGA) DNA molecule in SWNT/DNA/SWNT structure has been numerically investigated. In a ladder model, we consider DNA as a planar molecule containing M cells and four further sites (two base pair sites and two backbone sites) in each cell, sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT) as the electrodes. Having relied on Landauer formalism, we focussed on studying the current-voltage characteristics of DNA, the effect of the coupling strength of SWNT/DNA interface and the role of tube radius of nanotube contacts on the electronic transmission through the foregoing structure. Finally, a characteristic time was calculated for the electron transmission, which measures the delay caused by the tunnelling through the SWNT/DNA interface. The results clearly show that the calculated characteristic time and also the conductance of the system are sensitive to the coupling strength between DNA molecule and nanotube contacts.