Memristor effect on bundles of vertically aligned carbon nanotubes tested by scanning tunnel microscopy

We report on the results of experimental study of an array of vertically aligned carbon nanotubes (VA CNTs) by scanning tunnel microscopy (STM). It is shown that upon the application of an external electric field to the STM probe/VA CNT system, individual VA CNTs are combined into bundles whose diameter depends on the radius of the tip of the STM probe. The memristor effect in VA CNTs is detected. For the VA CNT array under investigation, the resistivity ratio in the low- and high-resistance states at a voltage of 180 mV is 28. The results can be used in the development of structures and technological processes for designing nanoelectronics devices based on VA CNT arrays, including elements of ultrahigh-access memory cells for vacuum microelectronics devices.     

Position-selective growth of vertically aligned carbon nanotubes for application of electronic-measuring nanoprobes

Position-selective growth of carbon nanotubes (CNTs) and vertically aligned CNTs (VACNTs) on patterned metal electrodes have been prepared by thermal chemical vapor deposition (TCVD) and DC plasma enhanced chemical vapor deposition (PECVD). We propose newly a position-controlling method of CNTs by controlling not only a position of Ni as catalysts but also the morphology of Mo as underlayers for the catalysts. The position-selective growth of CNTs was achieved at the edges of the patterned metal by TCVD. The morphologies of the Mo underlayer at the selected area were rough and porous. No CNTs grew on smooth Mo surfaces. The minimum width of selectively grown CNTs, ca. 2.6 μm, was approximately one-eightieth of the patterned metal, 200 μm. VACNTs were synthesized by a PECVD method, however, the VACNTs grew up all over the patterned metal. The Ni catalysts formed into fine particles on rough surfaces of the Mo underlayer. Then the selective growth was achieved by Ni fine particles formed only at the edges of the metal pattern. The results of PECVD suggest that the plasma promoted the Ni catalysts to become fine particles on smooth surfaces of Mo. Conclusively a position-controlling method of CNTs was demonstrated in the optimum conditions of the TCVD.     

Hydrophobic vertically aligned carbon nanotubes on Corning glass for self cleaning applications

Vertically aligned carbon nanotubes were prepared by Plasma enhanced chemical vapor deposition (PECVD) on inexpensive Corning glass substrates using different under layers. The samples were functionalised by a simple 1H,1H-2H,2H perfluorodecyl-trichlorosilane (FDTS) and hexane mixture. The surface roughness of the CNTs and protective FDTS coating provides an ideal hydrophobic surface of around 141°. Auger spectroscopy analysis was performed to confirm fluorination of the sample. It was also found titanium provides a suitable under layer support for Ni catalyst due to the wetability of the two elements.     

The effects of hydrogen plasma pretreatment on the formation of vertically aligned carbon nanotubes

The effects of H₂ plasma pretreatment on the growth of vertically aligned carbon nanotubes (CNTs) by varying the flow rate of the precursor gas mixture during microwave plasma chemical vapor deposition (MPCVD) have been investigated in this study. Gas mixture of H₂ and CH₄ with a ratio of 9:1 was used as the precursor for synthesizing CNTs on Ni-coated TiN/Si(1 0 0) substrates. The structure and composition of Ni catalyst nanoparticles were investigated by using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (XTEM). Results indicated that, by manipulating the morphology and density of the Ni catalyst nanoparticles via changing the flow rate of the precursor gas mixture, the vertically aligned CNTs could be effectively controlled. The Raman results also indicated that the intensity ratio of the G and D bands (I_D/I_G) is decreased with increasing gas flow rate. TEM results suggest H₂ plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles and, thus, is playing a crucial role in modifying the obtained CNTs structures.     

Synthesis of aligned carbon nanotubes

Carbon nanomaterials seem to be most attractive because of their fascinating features. Carbon nanotubes emerged recently as unique nanostructures with remarkable mechanical and electronic properties. Future applications will require a fabrication method capable of producing uniform carbon nanotubes with well-defined and controllable reproducibility of their properties. In this review, recent results addressing rational and efficient methods to obtain aligned arrays of these one-dimensional carbon nanomaterials will be discussed. Received: 3 November 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Influence of the nitrogen content on the electrochemical capacitor characteristics of vertically aligned carbon nanotubes

In this study, the electrochemical capacitor characteristics of vertically aligned carbon nanotubes (VACNTs) with different nitrogen content were investigated. The pristine VACNTs and nitrogen-doped VACNTs were synthesized by thermal chemical vapor deposition (CVD) with a mixture of C₂H₂ and NH₃ gases. The content of nitrogen doped into VACNTs played an important role in enhancing the capacitance characteristics. With increasing the NH₃ pressure, the nitrogen content of VACNTs increased. However, the capacitance did not increase with the increase in nitrogen content of VACNTs entirely. There was an optimal NH₃ pressure for synthesizing the nitrogen-doped VACNTs to obtain the maximum capacitance. Furthermore, for the real practice of supercapacitor, the stability of capacitance was tested for 500 cycles. The results revealed that the nitrogen-doped VACNT was a promising supercapacitance device material.     

Growth of vertically aligned single crystal ZnO nanotubes by plasma-molecular beam epitaxy

Highly oriented and vertically aligned single crystalline ZnO nanotubes were fabricated on Al₂O₃ (0001) substrates by plasma-molecular beam epitaxy without employing any external metal catalysts or templates. Field emission scanning electron microscope images indicate that the regularly aligned ZnO nanotubes with uniform size distribution were obtained. The chimney-like single crystal ZnO nanotube was confirmed by the transmission electron microscope and selected area electron diffraction pattern of the single nanotube. The formation mechanism of the nanotubes was also described briefly.     

Stimulation of the local growth of aligned carbon nanotubes by pulse laser exposure of the substrate

The local stimulation of carbon nanotubes (CNT) growth at the laser-modified sites that have been obtained by excimer laser irradiation at 248 nm causing a local surface modification has been investigated by two different processing methods. The influence of the laser processing parameters on the CNT growth is compared for the irradiation of thin spin-coated iron nitrate films on silicon substrates and the backside irradiation of a fused silica substrate being in contact with an iron nitrate solution. Both techniques cause the formation of catalytic surface sites either by decomposition of the film or by deposition from the solution. For both laser modification approaches the local growth of vertical aligned nanotubes has been observed. In the case of spin-coated film the laser irradiation conditions have only a small influence on the CNT growth whereas at backside modification by means of a solution a strong dependence on the laser processing parameters has been found.     

离子轰击控制准直碳纳米管生长的研究

用CH₄,H₂和NH₃作为反应气体，利用等离子体增强热丝化学汽相沉积制备出准直碳纳米管，并用扫描电子显微镜研究了不同负偏压对准直碳纳米管生长的影响. 结果表明，随着负偏压的增大，准直碳纳米管的平均直径减小、平均长度增大. 由于辉光放电的产生，在衬底表面附近形成阴极鞘层，并在阴极鞘层内形成大量的离子和在衬底表面附近形成很强的电场. 离子在电场的作用下对衬底表面的强烈轰击将对准直碳纳米管的生长产生影响. 结合有关理论，分析和讨论了离子的轰击对准 关键词： 准直碳纳米管 离子轰击 负偏压     

Coating of carbon nanotubes on flexible substrate and its adhesion study

The primary goal of this project was to develop a flexible transparent conductor with 100 Ω/sq and 90% transmittance in the wavelength range of 400-700 nm on a flexible substrate. The best result achieved so far was 110 Ω/sq at 88% transmittance using purified single-walled carbon nanotubes (SWNTs) coated on a polyethylene naphthalate (PEN) substrate. The secondary goal was to simplify the overall coating procedure; we successfully reduced the process from five (prior art method) to three steps utilizing a sonication method. We also found that the use of metallic SWNTs significantly improved the conductivity and transmittance compared with the use of mixed SWNTs, i.e., unseparated SWNTs. Furthermore, a possible adhesion mechanism between SWNTs and the surface of PEN was studied; we concluded that a π-π stacking effect and a hydrophobic interaction are the major contributing factors for SWNTs to adhere to the surface of the substrate.     

Nonlocal discrete and continuous modeling of free vibration of stocky ensembles of vertically aligned single-walled carbon nanotubes

Free dynamic analysis of transverse motion of vertically aligned stocky ensembles of single-walled carbon nanotubes is of particular interest. A linear model is developed to take into account the van der Waals forces between adjacent SWCNTs because of their bidirectional transverse displacements. Using Hamilton's principle, the discrete equations of motion of free vibration of the nanostructure are obtained based on the nonlocal Rayleigh, Timoshenko, and higher-order beam theories. The application of such discrete models for frequency analysis of highly populated ensembles would be associated with so much computational effort. To overcome such a problem, some useful nonlocal continuous models are established. The obtained results reveal that the newly developed models can successfully capture the predicted fundamental frequencies of the discrete models. Through various numerical studies, the roles of slenderness ratio, radius of the SWCNT, small-scale parameter, population of the ensemble, and intertube distance on the fundamental flexural frequency of the nanostructure are examined and discussed. The capabilities of the proposed nonlocal continuous models in predicting flexural frequencies of the nanostructure are also addressed.     

Functionalization of vertically aligned carbon nanotubes with polystyrene via surface initiated reversible addition fragmentation chain transfer polymerization

Here we demonstrate the covalent attachment of vertically aligned (VA) acid treated single-walled carbon nanotubes (SWCNTs) onto a silicon substrate via dicyclohexylcarbodiimide (DCC) coupling chemistry. Subsequently, the pendant carboxyl moieties on the sidewalls of the VA-SWCNTs were derivatized to acyl chlorides, and then finally to bis(dithioester) moieties using a magnesium chloride dithiobenzoate salt. The bis(dithioester) moieties were then successfully shown to act as a chain transfer agent (CTA) in the reversible addition fragmentation chain transfer (RAFT) polymerization of styrene in a surface initiated “grafting-from” process from the VA-SWCNT surface. Atomic force microscopy (AFM) verified vertical alignment of the SWCNTs and the maintenance thereof throughout the synthesis process. Finally, Raman scattering spectroscopy and AFM confirmed polystyrene functionalization.     

Physics and applications of aligned carbon nanotubes

Ever since the discovery of carbon nanotubes (CNTs) by Iijima in 1991, there have been extensive research efforts on their synthesis, physics, electronics, chemistry, and applications due to the fact that CNTs were predicted to have extraordinary physical, mechanical, chemical, optical, and electronic properties. Among the various forms of CNTs, single-walled and multi-walled, random and aligned, semiconducting and metallic, aligned CNTs are especially important since fundamental physics studies and many important applications will not be possible without alignment. Even though there have been significant endeavors on growing CNTs in an aligned configuration since their discovery, little success had been realized before our first report on growing individually aligned CNTs on various substrates by plasma-enhanced chemical vapor deposition (PECVD) [Science 282 (1998) 1105–1108]. Our report spearheaded a new field on growth, characterization, physics, and applications of aligned CNTs. Up to now, there have been thousands of scientific publications on synthesizing, studying, and utilizing aligned CNTs in various aspects. In this communication, we review the current status of aligned CNTs, the physics for their alignment, their applications in field emission, optical antennas, subwavelength light transmission in CNT-based nanocoax structures, nanocoax arrays for novel solar cell structures, etc.

The focus of this review is to examine various aligned CNT systems, either as an individual or as an array, either the orientation is vertical, parallel, or at other angles to the substrate horizon, either the CNT core structures are mostly hollow channels or are composed of complex compartments. Major fabrication methods are illustrated in detail, particularly the most widely used PECVD growth technique on which various device integration schemes are based, followed by applications whereas current limitations and challenges will also be discussed to lay down the foundation for future developments.     

Enhancement of electron field emission by carbon coating on vertically aligned Si nanowires

Electron field emission properties of vertically aligned Si nanowires, synthesized by chemically etching p-type Si wafers with different etching times were investigated in detail. Fabrication of Si nanowires was confirmed by field emission scanning electron microscopic investigation. It was observed that a thin layer of amorphous carbon coating over the grown Si nanowires enhanced the field emission properties significantly.     

All-fiber pulsed lasers passively mode locked by transferable vertically aligned carbon nanotube film

An all-fiber passive laser mode locking is realized with a vertically aligned single-walled carbon nanotube film that can be transferred onto an arbitrary substrate using only hot water. A D-shaped fiber is employed as the substrate for the evanescent field interaction of propagating light with the nanotubes. The scheme highlights the efficient interaction achieved by the nanotube alignment as well as the dramatically simplified device preparation process. The demonstrated pulsed laser output has 2.9 nm of spectral full width at half-maximum and a 20.8 MHz repetition rate.     

Characterization of carbon nanotubes by scanning probe microscopy

Carbon nanotubes, fabricated by the Ebbesen-Ajayan method, were imaged using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in air and were compared to images obtained with high-resolution transmission electron microscopy (HRTEM). The HRTEM images revealed an abundance of elongated structures ranging in diameter from 3.0 to 30 nm, and with lengths of up to 0.8 μm. Many of the structures possessed several graphitic shells as if the tubes were nested one in the other. Reproducible images of the tubular structures, typically 20 nm in diameter and with a large variation in length, were obtained with both STM and AFM when the nanotubes were deposited on hydrogen-terminated Si(111), confirming that the nested structures observed with HRTEM do indeed have a tubular morphology. No single-walled, bare nanotubes or spherical fullerenes (typical of the Krätschmer-Huffman process) were observed.     

Polarized raman study of aligned multiwalled carbon nanotubes

Polarized Raman spectra of high purity aligned arrays of multiwalled carbon nanotubes, prepared on silica substrates from the thermal decomposition of a ferrocene-xylene mixture, show a strong dependence of the graphitelike G band and the disorder-induced D band on the polarization geometry employed in the experiments. The experimental G-band intensity exhibits a minimum at straight theta(m) = 55 degrees in the VV configuration, in good agreement with theoretical predictions of a characteristic minimum at 54.7 degrees for A(1g) modes in single wall nanotubes, where straight theta(m) denotes the angle between the polarization direction and the nanotube axis.     

Electric field aligned growth of single-walled carbon nanotubes

Electric field aligned, single-walled carbon nanotubes are grown between electrodes using thermal chemical vapour deposition (CVD) of methane. The growth occurs on a thin film layered catalyst of aluminium, iron and molybdenum patterned on top of electrodes. The nanotubes bridge 10 μm sized electrode gaps and have a typical diameter of less than 2 nm as measured by Raman spectroscopy and atomic force microscopy. We present electrical transport measurements on a directly grown nanotube which shows p-type semiconducting behaviour.     

Single-shell carbon nanotubes imaged by atomic force microscopy

Single-shell carbon nanotubes, approximately 1 nm in diameter, have been imaged for the first time by atomic force microscopy operating in both the contact and tapping modes. For the contact mode, the height of the imaged nanotubes has been calibrated using the atomic steps of the silicon substrate on which the nanotubes were deposited. For the tapping mode, the calibration was performed using an industry-standard grating. The paper discusses substrate and sample preparation methods for the characterization by scanning probe microscopy of nanotubes deposited on a substrate.