Suspended Nanotubes Research Articles

Fabrication and characterisation of suspended carbon nanotube devices in liquid

Suspended carbon nanotube devices represent a promising platform for future bio-electronic applications. Suspended carbon nanotube transistors have been previously fabricated in air; however all previous attempts to bring them into liquid failed. We analyse forces acting on the suspended nanotube devices during immersion into liquids and during device operation and show that surface tension forces acting on the suspended nanotubes during transfer into the liquid phase are responsible for the nanotube damage. We have developed a new strategy that circumvents these limitations by coating suspended nanotubes with a rigid inorganic shell in the gas phase. The coating reinforces the nanotubes and allows them to survive transfer through the interface. Subsequent removal of the coating in the solution phase restores pristine suspended nanotubes. We demonstrate that devices fabricated using this technique preserve their original electrical characteristics.

Electrically driven light emission from hot single-walled carbon nanotubes at various temperatures and ambient pressures

Electroluminescence of individual single-walled carbon nanotubes down to ∼15K is measured. We observe electrically driven light emission from suspended quasimetallic nanotubes in vacuum down to ∼15K and under different gas pressures at room temperature. Light emission is found to originate from hot electrons in the presence of electrically driven nonequilibrium optical phonons. Reduced light emission is observed in exponential manner as electron and optical phonon temperatures in the nanotube are lowered by lower ambient temperature or higher gas pressure. The results reveal over wide ambient conditions, light emission in a suspended tube is from thermally excited electron-hole recombination.

Modelling suspended carbon nanotube resonators

AbstractWe study the bending mode vibration in suspended carbon nanotubes. Based on the theory of continuum mechanics, we have developed a model for flexural oscillations of suspended nanotubes. A detailed analysis of the electrostatic force, the scaling behavior of the model and the gate tuning is given. The model is used to fit experimental data and to reconstruct the gate dependence of the tension and strain in the nanotube. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Suspended carbon nanotube double quantum dots

AbstractWe present fabrication of and the first measurements on partially suspended double quantum dots in a carbon nanotube. Our objective is to enhance the energy resolution for spectroscopy of the vibronic modes of a suspended nanotube. When fabricating our devices, nanotubes grown on‐chip are localized by AFM scanning, and subsequently contacted and gated with customized electrode geometries. First low‐temperature transport measurement results are discussed, where the double quantum dot characteristics are clearly recognizable in the measured current. In addition, we address current technological challenges and future plans. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultra-High Oxidation Resistance of Suspended Single-Wall Carbon Nanotube Bundles Grown by an "All-Laser" Process

Single-wall carbon nanotubes (SWCNTs) were laterally grown on SiO2/Si substrates by means of an "all-laser" growth process. Our "all-laser" process stands out by its exclusive use of the same pulsed UV laser, first, to deposit the CoNi nanocatalyst and, second, to grow SWCNTs through the laser ablation of a pure graphite target. The "all-laser" grown SWCNTs generally self-assemble into bundles (5-15 nm-diam.) sprouting from the CoNi nanocatalyst and laterally bridging the 2 microm gap separating adjacent catalysed electrodes (in either "suspended" or "on-substrate" geometries). A comparative study of the oxidation resistance of both suspended and on-substrate SWCNTs was achieved. The "all-laser" grown SWCNTs were subjected to annealing under flowing oxygen at temperatures ranging from 200 to 1100 degrees C. Systematic scanning electron microscopy observations combined with micro-Raman analyses revealed that more than 20% of suspended nanotubes were still stable at temperatures as high as 900 degrees C under flowing O2 while the on-substrate counterpart were completely burnt out at this temperature. Accordingly, the activation energy, as deduced from the Arrhenius plot, of the suspended SWCNTs is found to be as high as approximately 180 kJ mol(-1) (approximately 9 times higher than that of the on-substrate ones). The high quality (almost defect-free) of the nanotubes synthesized by the "all-laser" approach, their protected tips into the embedded CoNi catalyst nanolayer together with their suspended geometry are thought to be responsible for their unprecedented ultra-high oxidation resistance. This opens up new prospects for the use of these suspended nanotubes into nanodevices that have to operate under highly oxidizing environments.

A Carbon Nanotube Field Effect Transistor with a Suspended Nanotube Gate

We investigate theoretically field effect transistors based on single-walled carbon nanotubes (CNTFET) and explore two device geometries with suspended multiwalled carbon nanotubes (MWNT) functioning as gate electrodes. In the two geometries, a doubly or singly clamped MWNT is electrostatically deflected toward the transistor channel, allowing for a variable gate coupling and leading to, for instance, a superior subthreshold slope. We suggest that the proposed designs can be used as nanoelectromechanical switches and as detectors of mechanical motion on the nanoscale.

Impact of oxide substrate on electrical and optical properties of carbon nanotubedevices

We have studied suspended nanotube devices to investigate directlythe impact of oxide substrate on the properties of nanotubes. The1/f noiseamplitude is reduced by about one order of magnitude when the nanotube is suspended, suggesting thatthe 1/f noise is dominated by the trapped charges in the oxide. We have also utilized the enhancedRaman signal intensity in suspended nanotubes to correlate electrical transportproperties with Raman spectroscopy on the same individual nanotube, yieldingclose agreement in the tube diameter determined from both types of experiments.

All-carbon nanotube-based junction with virtual source and drain of carbon nanotubes by in situ one-step process for practical integrated nanoelectronics

The authors propose a suspended in situ lateral grown all-carbon nanotube-based junction and report on the dc carrying behaviors of the carbon nanotube junction, especially with and after UV exposure. Also, the release of carriers of the junctions was studied by capacitance (C) measurements with ac excitation. The designed diluted magnetic impurity doped oxide film was adopted as catalyst in the fabrication of the junction. The suspended nanotube channel showed ambipolar function and almost the same low barrier height for the holes and electrons, which was determined by I-T(K) measurements. Subsequent measurements on the channel treated by low intensity UV resulted in a highly conductive channel with high current carrying behavior. Both their junction structure and analogy between dc I-V and capacitance can be applied to develop a practical and accessible system for forming reproducible integrated nanoelectronic junctions as well as to accelerate the realization of all low dimensional molecular devices.

Direct wiring of carbon nanotubes for integration in nanoelectromechanical systems

The authors present the use of electron beam induced deposition (EBID) for direct wiring of carbon nanotubes (CNTs). Since this technique is a maskless, direct patterning method, EBID is applicable to suspended nanotubes potentially integrated in predefined nanoelectromechanical systems (NEMS). The authors developed a process including CNT adsorption from solution on prepatterned planar samples, evaluation and localization of CNTs by scanning force microscopy, preparation of GDSII layouts, and contacting CNTs by EBID of tungsten. EBID was performed in an integrated e-beam lithography and nanoengineering workstation using tungsten hexacarbonyl as precursor. With well controlled exposure conditions, a resistivity on the order of 10−2Ωcm has been achieved. Moreover, electrical measurements on CNTs contacted by EBID tungsten leads clearly show transistorlike behavior. First tests on EBID metal lead integration on alumina NEMS structures were carried out. Thus EBID can enable a top metallization of CNTs on suspended NEMS structures, which should enhance also the mechanical stability between the lead and the CNT.

PH-Dependent Electron-Transport Properties of Carbon Nanotubes

Carbon nanotube electrochemical transistors integrated with microfluidic channels are utilized to examine the effects of aqueous electrolyte solutions on the electron-transport properties of single isolated carbon nanotubes. In particular, pH and concentration of supporting inert electrolytes are examined. A systematic threshold voltage shift with pH is observed while the transconductance and subthreshold swing remain independent of pH and concentration. Decreasing pH leads to a negative shift of the threshold voltage, indicating that protonation does not lead to hole doping. Changing the type of contact metal does not alter the observed pH response. The pH-dependent charging of SiO2 substrate is ruled out as the origin based on measurements with suspended nanotube transistors. Increasing the ionic strength leads to reduced pH response. Contributions from possible surface chargeable chemical groups are considered.

Tube−Tube and Tube−Surface Interactions in Straight Suspended Carbon Nanotube Structures

An investigation concerning the tautness of suspended carbon nanotubes (CNTs) grown using the chemical vapor deposition (CVD) method is presented. The suspended nanotubes were analyzed with both a transmission electron microscope (TEM) and a high-resolution scanning electron microscope (HR-SEM). The HR-SEM and TEM investigations revealed that the interaction between CNTs among themselves as well as with the surface on which they are grown is a primary cause for the tautness of suspended tubes. Specifically, the tube-tube and tube-surface dynamics cause adjoining tubes to create a "zipper-effect", thereby straightening and tightening them. Suspended CNTs cling to each other and to as much of the surface as possible and thus minimize their total energy, creating taut, suspended structures. This effect can be so strong so as to force wide tubes to buckle, with no other external force involved. The implications of this study include all forms of alignment processes of nanotubes using the CVD method. The results presented here provide the groundwork for the capability of fine-tuning the control of CNT network formation using substrate mechanical features.

Real-time global Raman imaging and optical manipulation of suspended carbon nanotubes

We demonstrate confocal, direct Raman imaging of carbon nanotubes based on the detection of the $G$-band using an electron multiplying charge coupled detector. Individual carbon nanotubes and bundles of nanotubes are observed in real-time over a large area using global illumination. We use the technique to show that suspended nanotubes can be manipulated selectively with a focused laser beam and describe the relevant physical mechanisms.

Voltage-controlled high-frequency oscillations based on suspended semiconducting carbon nanotubes

A series of tunable negative differential conductance regions at room temperature is demonstrated in semiconducting carbon nanotubes suspended over a trench with a metallic gate at its bottom. The substrateless carbon nanotube is in fact a wide quantum well that experiences a series of negative differential conductance regions due to the very large difference between the effective masses in the well and the tunneling contacts. The positions of these regions can be tuned by modifying the gate voltage. Since the negative differential conductance is the key element of high-frequency oscillators, the suspended nanotube is a voltage-controlled oscillator able to work up to THz frequencies. This device could have important applications in high-frequency nanoelectronic devices and multivalued logic.

Stability of Aqueous Suspensions of Titanate Nanotubes

The long-term stability of titanate nanotubes, which were produced by alkaline hydrothermal treatment of TiO2, was studied at room temperature in acidic, pure water, and basic aqueous suspensions. In pure water and basic (0.1 mol dm-3 NaOH) solutions, the nanotubes were stable and minimal morphological changes occurred. In 0.1 mol dm-3 H2SO4, suspended titanate nanotubes slowly transformed to rutile nanoparticles of ca. 3 nm size, which were agglomerated into ellipsoidal particles. The porosity, crystal structure, and morphology of protonated titanates and TiO2 have been studied for intermediate states during the transformation by nitrogen adsorption, XRD, Raman spectroscopy, SEM, and HRTEM. The rate of conversion of nanotubes to nanoparticles has been related to the concentration of soluble titanium(IV) in solution, which depends on the nature of the acid. Thermodynamic and kinetic aspects of the acid transformation are discussed.

Thermally and Molecularly Stimulated Relaxation of Hot Phonons in Suspended Carbon Nanotubes

The high-bias electrical transport properties of suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at various temperatures in vacuum, in various gases, and when coated with molecular solids. It is revealed that nonequilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules surrounding suspended SWNTs assist the relaxation of hot phonons and afford enhanced current flow along nanotubes. Molecular solids of carbon dioxide frozen onto suspended SWNTs quench the nonequilibrium phonon effect. The discovery of strong environmental effects on high current transport in nanotubes is important to high performance nanoelectronics applications of 1D nanowires in general.

Fabrication of suspended single-walled carbon nanotubesvia a direct lithographic route

We report a fabrication method of suspended SWNTs between SiO2 pillars via a direct lithographic route using a simple mixture of catalyst precursor and conventional electron beam resist. The catalytic electron beam resist (Cat-ER) plays roles as both catalyst and ER for growth and alignment of CNTs, respectively. Since the Cat-ER pattern can be utilized as an etch mask to fabricate the pillar structure, the suspended nanotubes are compatibly obtained with current lithographic processes. The suspended nanotubes are characterized to be single-walled by intensity mapping of a radial breathing mode in two dimensions with resonant Raman spectroscopy.

Electron–vibron coupling in suspended nanotubes

We consider electron–vibron coupling in suspended nanotube quantum dots. Modelling the tube as an elastic medium, we study the possible coupling mechanism for exciting the stretching mode in a single-electron-transistor setup. Both the forces due to the longitudinal and the transverse fields are included. The effect of the longitudinal field is found to be too small to be seen in experiment. In contrast, the transverse field which couple to the stretching mode via the bending of the tube can in some cases give sizeable Franck–Condon factors. However, the length dependence is not compatible with recent experiments (Sapmaz et al 2005 New J. Phys. 7 243).

Air Bridge and Vertical Carbon Nanotube Switches for High Performance Switching Applications

ABSTRACTCarbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our first NEM structure, the air-bridge switch, consists of suspended single-walled nanotubes (SWNTs) that lie above a sputtered Nb electrode. Electrical measurements of these air-bridge devices show well-defined ON and OFF states as a dc bias of a few volts is applied. The switches were measured to have switching times down to a few nanoseconds. Our second NEM structure, the vertical CNT switch, consists of nanotubes grown perpendicular to the substrate. Vertical multi-walled nanotubes (MWNTs) are grown directly on a heavily doped Si substrate, from 200 − 300 nm wide, ∼ 1 μm deep nano-pockets, with Nb metal electrodes to result in the formation of a vertical single-pole-double-throw CNT switch architecture.

Coupling between electronic transport and longitudinal phonons in suspended nanotubes

Current–voltage characteristics of suspended single-wall carbon nanotube (NT) quantum dots show a series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode in the NT. Agreement is found with a Franck–Condon-based model in which the phonon-assisted tunnelling process is modelled as a coupling of electronic levels to underdamped quantum harmonic oscillators. Comparison with this model indicates a rather strong electron–phonon coupling factor of order unity. We investigate different electron–phonon coupling mechanisms and give estimates of the coupling factor.

Electron-microscopic imaging of single-walled carbon nanotubes grown on silicon and silicon oxide substrates

Direct imaging of single-walled carbon nanotubes (SWNTs) suspended on pillar-patterned Si or SiO2 substrates is investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The suspended nanotubes are successfully observed by direct TEM imaging and it is seen that they have either individual or bundles of SWNTs. Low energy (< or =2 keV) SEM produces high contrast images of suspended SWNTs. On the contrary, when SWNTs contact a SiO2 substrate, they are imaged using electron-beam induced current. The image brightness depends on the length of SWNTs.