Identifying individual single-walled and double-walled carbon nanotubes by atomic force microscopy

We show that the number of concentric graphene cylinders forming a carbon nanotube can be found by squeezing the tube between an atomic force microscope tip and a silicon substrate. The compressed height of a single-walled nanotube (double-walled nanotube) is approximately two (four) times the interlayer spacing of graphite. Measured compression forces are consistent with the predicted bending modulus of graphene and provide a mechanical signature for identifying individual single-walled and double-walled nanotubes.     

Ultrasharp and high aspect ratio carbon nanotube atomic force microscopy probes for enhanced surface potential imaging

The resolution of scanning surface potential microscopy (SSPM) is mainly limited by non-local electrostatic interactions due to the finite probe size. Here we present high resolution surface potential imaging with ultrasharp and high aspect ratio carbon nanotube (CNT) atomic force microscopy (AFM) probes fabricated via dielectrophoresis. Enhancement of surface potential contrast by several factors is reported for integrated circuit structures and purple membrane fragments for these CNT AFM probes as compared to conventional probes. In particular, ultrahigh lateral resolution (～2?nm) surface potential images of self-assembled bacteriorhodopsin proteins are reported at ambient conditions, with the implication of label-free protein detection by SSPM techniques.     

Carbon nanotube air-bubble interactions studied by atomic force microscopy

Interaction forces between a multi-walled carbon nanotube (MWCNT) and an air-bubble in pure deionized water and methyl isobutyl carbinol (MIBC) solutions were measured by atomic force microscopy (AFM). The MWCNT terminated probe was brought into contact with the bubble at controlled applied forces. The repulsive steps followed by attractive jumps recorded in the approach force curves correspond to changes in the MWCNT diameter along its length, an observation confirmed by transmission electron microscopy (TEM) data. By processing the retraction part of the force curves obtained in pure water it is possible to estimate the end diameter of the carbon nanotube with nanometer resolution using a capillary force model.     

Tribological properties of single-walled carbon nanotube solids

Binder-free single-walled carbon nanotube (SWCNT) solids were evaluated for solid lubrication applications. The steady-state friction coefficients (mu) for the SWCNT solids were found to reach values as low as 0.22-0.24, according to unidirectional sliding friction tests using Si3N4 counterparts in air. The values were slightly higher than that of bulk graphite material (mu = 0.20). SEM and Raman analyses showed that most SWCNTs that existed in the friction surface transformed into SWCNT-derived transferred film made up of amorphous carbon during sliding. The resultant friction behavior may be related to the smearing of transferred film over the contact area, which was expected to permit easy shear and then help to achieve a lubricating effect during sliding.     

Electronic sensitivity of a single-walled carbon nanotube to internal electrolyte composition

Carbon nanotubes (CNTs) are well known as materials for nanoelectronics and show great potential to be used as the sensing elements in chemical and biological sensors. Recently, CNTs have been shown to be effective nanofluidic channels and the transport of substances through small diameter CNTs is intrinsically fast, selective, and operates at the single molecule level. It has been shown that the transport characteristics of semiconducting single-walled CNT (SWCNT) field effect transistors (FETs) are sensitive to internal water wetting. We report here that the characteristics of semiconducting SWCNT FETs are also sensitive to the concentration, pH and ion type of the ionic solution when the electrolyte is inside the CNT. Such sensitivity is not observed at the outside surface of a semiconducting SWCNT. This opens a new avenue for building new types of CNT sensor devices in which the SWCNT concurrently functions as a nanochannel and an electronic detector.     

Adhesion forces measured by atomic force microscopy in humid air

Adhesive forces measured with an atomic force microscope under ambient conditions are generally regarded to be dominated by non-surface-specific capillary force. In this study, the nature of the "pull-off" force on a variety of surfaces was investigated as a function of relative humidity. The results indicate that even under the condition where capillary condensation occurs there is chemical specificity in the measured pull-off force. Issues such as tip-surface contact time and surface roughness were ruled out as possible artifacts. A mathematical model of pull-off force as a function of relative humidity is proposed in which the chemical specificity is explained.     

Effects of tip-nanotube interactions on atomic force microscopy imaging of carbon nanotubes

We examine the effect of van der Waals (vdW) interactions between atomic force microscope tips and individual carbon nanotubes (CNTs) supported on SiO₂. Molecular dynamics (MD) simulations reveal how CNTs deform during atomic force microscopy (AFM) measurement, irrespective of the AFM tip material. The apparent height of a single- (double-) walled CNT can be used to estimate its diameter up to ～2 nm (～3 nm), but for larger diameters the CNT cross-section is no longer circular. Our simulations were compared against CNT dimensions obtained from AFM measurements and resonant Raman spectroscopy, with good agreement for the smaller CNT diameters. In general, AFM measurements of large-diameter CNTs must be interpreted with care, but the reliability of the approach is improved if knowledge of the number of CNT walls is available, or if additional verification (e.g., by optical techniques) can be obtained.

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Electrical transport properties of oligothiophene-based molecular films studied by current sensing atomic force microscopy

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.     

Characterization of materials' nanomechanical properties by force modulation and phase imaging atomic force microscopy with soft cantilevers

Soft cantilevers, although having good force sensitivity, have found limited use for investigating materials' nanomechanical properties by conventional force modulation (FM) and intermittent contact (IC) atomic force microscopy. This is due to the low forces and small indentations that these cantilevers are able to exert on the surface, and to the high amplitudes required to overcome adhesion to the surface. In this paper, it is shown that imaging of local elastic properties of surface and subsurface layers can be carried out by employing electrostatic forcing of the cantilever. In addition, by mechanically exciting the higher vibration modes in contact with the surface and monitoring the phase of vibrations, the contrast due to local surface elasticity is obtained.     

Determination of nano-roughness of carbon fibers by atomic force microscopy

We present a novel approach to determine the surface roughness on varying scales using atomic force microscopy data. The key factor is to find a suitable background correction for the desired scale. Using the example of the surface of sized and unsized high-tenacity carbon fibers, we present an easy method to find backgrounds for widely varying scales and to evaluate respective topography and surface roughness with the same lateral resolution as the microscope itself. The analysis is done by subtracting a tunable background from the respective height data. By choosing an appropriate background to investigate the surface topography of a carbon fiber on a nm-scale, only small nano-structures with a width of around 20 nm remain after the background subtraction. Evaluating the mean roughness R _a of these nano-structures, sized carbon fibers show an overall value of around 0.1 nm while unsized carbon fibers a value of around 0.4 nm. Total background corrected height analysis shows an even distribution of these nano-structures along the fibrils of the unsized fibers, whereas for the sized fibers the nano-structures are not present. The presented method allows analysis and visualization of the distribution of nano-structures on a carbon fiber surface for the first time. This feature is used to visualize the distribution of the sizing and can further be used to investigate the influence of different production parameters on the fiber topography or to evaluate the contribution of mechanical interlocking to the interfacial strength.     

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (～1?aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.     

Ligand-induced structural changes in maltose binding proteins measured by atomic force microscopy

We use atomic force microscopy (AFM) based force-compression measurements to probe the ligand-induced functional conformational changes in surface-immobilized dicysteine-terminated maltose binding proteins (dicys-MBPs). The proteins are immobilized at well-defined locations directly on Au substrates using the previously reported technique of nanografting. By measuring the difference between the ligand-free and ligand-bound mechanical work performed by the AFM-tip during the protein compression, we determine the open-closed transition energy for dicys-MBPs to be DeltaE0 = (8 +/- 4) Kcal/mol. We also compare the binding kinetics of two different ligands (maltose and maltotriose) to dicys-MBPs by performing AFM-friction measurements. We show that our results are consistent with a simple model for the surface-immobilized dicys-MBPs: the protein consists of two rigid lateral lobes connected by a hinge-loaded spring.     

Charge trapping in carbon nanotube loops demonstrated by electrostatic force microscopy

Electronic devices made from carbon nanotubes (CNTs) can be greatly affected by substrate charges, which, for instance, induce strong hysteresis in CNT field effect transistors. In this work, electrostatic force microscopy (EFM) is employed to investigate single-walled nanotubes grown by chemical vapor deposition on SiO2 substrates. We demonstrate the use of this technique to gain quantitative information on the substrate charges. It is found that charge pools with densities around 10(-8) C/cm2 can be trapped inside nanotube loops for extended periods of time, showing that nanotubes can act as confining barriers for substrate charges. The trapped charges can be removed by scanning probe manipulation.     

Multimodal biomedical imaging with asymmetric single-walled carbon nanotube/iron oxide nanoparticle complexes

Magnetic iron oxide nanoparticles and near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWNT) form heterostructured complexes that can be utilized as multimodal bioimaging agents. Fe catalyst-grown SWNT were individually dispersed in aqueous solution via encapsulation by oligonucleotides with the sequence d(GT)15, and enriched using a 0.5 T magnetic array. The resulting nanotube complexes show distinct NIR fluorescence, Raman scattering, and visible/NIR absorbance features, corresponding to the various nanotube species. AFM and cryo-TEM images show DNA-encapsulated complexes composed of a approximately 3 nm particle attached to a carbon nanotube on one end. X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) measurements reveal that the nanoparticles are primarily Fe2O3 and superparamagnetic. The Fe2O3 particle-enriched nanotube solution has a magnetic particle content of approximately 35 wt %, a magnetization saturation of approximately 56 emu/g, and a magnetic relaxation time scale ratio (T1/T2) of approximately 12. These complexes have a longer spin-spin relaxation time (T2 approximately 164 ms) than typical ferromagnetic particles due to the smaller size of their magnetic component while still retaining SWNT optical signatures. Macrophage cells that engulf the DNA-wrapped complexes were imaged using magnetic resonance imaging (MRI) and NIR mapping, demonstrating that these multifunctional nanostructures could potentially be useful in multimodal biomedical imaging.     

Preparation of single-walled carbon nanotube/silicon composites and their lithium storage properties

Single-walled carbon nanotube (SWCNT)/silicon composites were produced from the purified SWCNTs and Si powder by high-energy ball-milling and then electrochemically inserted with Li using Li/(SWCNT/Si) cells. The highest reversible capacity and lowest irreversible capacity of the SWCNT/Si composites were measured to be 1845 and 474 mAh g(-1) after ball-milling for 60 min, respectively. During the charge/discharge process, most of the Li ions were inserted into the SWCNT/Si composites by alloying with Si particles below 0.2 V and extracted from the SWCNT/Si composites by dealloying with Si particles around 0.5 V. The enhanced capacity and cycle performance of the SWCNT/Si composites produced by high-energy ball-milling were due primarily to the fact that SWCNTs provided a flexible conductive matrix, which compensated for the dimensional changes of Si particles during Li insertion and avoided loosening of the interparticle contacts during the crumbling of Si particles. The ball-milling contributed to a decrease in the particle size of SWCNTs and Si particles and to an increase in the electrical contact between SWCNTs and Si particles in the SWCNT/Si composites.     

Real time atomic force microscopy imaging during nanogap formation by electromigration

We present real time atomic force microscopy imaging during nanogap fabrication by feedback controlled electromigration of a gold nanowire. The correlated measurements of electrical resistance and atomic force microscopy reveal that the major structural changes appear at the early stage of the process. Moreover, despite important morphological changes, the resistance of the nanowire shows a weak increase of just a few ohms. The detailed analysis of the atomic force microscopy images clearly shows that the electromigration process is strongly influenced by the initial microstructure of the nanowire.     

Studying electrical transport in carbon nanotubes by conductance atomic force microscopy

An introduction to conductance atomic force microscopy in the context of carbon nanotubes is provided where the main problems and performances of this technique are discussed. The conductance measured in SWNT as a function of the loading force applied by an AFM metallized tip is reported. These experiments allow us to study the process of the electrical contact formation between the tip and the nanotube. This will also lead to a study of the electromechanical properties of nanotubes for radial deformations.     

Amplitude response of single-wall carbon nanotube probes during tapping mode atomic force microscopy: Modeling and experiment

Imaging of surfaces with carbon nanotube probes in tapping mode results frequently in complex behavior in the amplitude-distance curves monitored. Using molecular mechanics simulations, we calculate the force exerted on a nanotube pressed against a smooth surface as it undergoes deformation and buckling. This nonlinear force is then used in a macroscopic equation, describing the response of a damped harmonic oscillator, to predict the amplitude response of a nanotube AFM probe. Similarities between the prediction and experiment suggest that the complex amplitude response seen in the experiment may be explained by the nonlinearity in the force exerted on the nanotube and thus must not necessarily be related to the structure of the surface.     

Photocurrent imaging of nanocrystal quantum dots on single-walled carbon nanotube device

Semiconductor nanocrystal quantum dots (NQDs) are considered an attractive candidate for use in optoelectronic applications due to the ease of band gap control provided by varying the particle size. To increase the efficiency of NQDs when practically applied in devices, researchers have introduced the concept of coupling of NQDs to one-dimensional nanostructures such as single-walled carbon nanotubes (SWCNTs), which have a ballistic conducting channel. In the present study, NQDs of CdSe core and CdSe/ZnS are used as light absorbing building blocks. SWCNTs and functionalized NQDs are non-covalently coupled using pyridine molecules in order to maintain their electronic structures. To measure the electrical signals from the device, a NQDs-SWCNT hybrid nanostructure is fabricated as a field-effect transistor (FET) using the dielectrophoresis (DEP) method. A confocal scanning microscope was used to scan the devices using a diffraction-limited laser spot and the photocurrent was recorded as a function of the position of the laser spot. To improve the performance of detecting small electronic signal with high signal-to-noise ratio we used a lock-in technique with an intensity-modulated laser. In this paper, we have demonstrated that detection of local photoconductivity provides an efficient means to resolve electronic structure modulations along NQDs-SWCNT hybrid nanostructures.     

Two-dimensional dopant profiling by electrostatic force microscopy using carbon nanotube modified cantilevers

A two-dimensional (2D) dopant profiling technique is demonstrated in this work. We apply a unique cantilever probe in electrostatic force microscopy (EFM) modified by the attachment of a multiwalled carbon nanotube (MWNT). Furthermore, the tip apex of the MWNT was trimmed to the sharpness of a single-walled carbon nanotube (SWNT). This ultra-sharp MWNT tip helps us to resolve dopant features to within 10?nm in air, which approaches the resolution achieved by ultra-high vacuum scanning tunnelling microscopy (UHV STM). In this study, the CNT-probed EFM is used to profile 2D buried dopant distribution under a nano-scale device structure and shows the feasibility of device characterization for sub-45?nm complementary metal-oxide-semiconductor (CMOS) field-effect transistors.