Carbon Nanotube Probes Research Articles

Study on ringing artefacts of carbon nanotube probes of atomic force microscopy

CNTs have been regarded as ideal AFM tips for their outstanding advantages, but due to their high aspect ratio, ringing artefacts often occur when they detect steep samples. Experimental results paired with a mathematical model are used to capture the true sample morphology when such ring artefacts are observed. The scan direction was found to have a prominent influence on the artefacts’ consistency and cycle, and the distance of the gaps between the lowest points of the artefacts and the trench obeys a certain rule. By establishing mathematical model, formulae were given to calculate the gaps. Finally, the theoretic morphology can be acquired, and it is in good agreement with the no-artefact images.

Picoampere Resistive Switching Characteristics Realized with Vertically Contacted Carbon Nanotube Atomic Force Microscope Probe

The resistive switching characteristics of a TiO2/Ti structure have been investigated using a conductive atomic force microscopy (AFM) system with 5-nm-diameter carbon nanotube (CNT) probes. The resistive switching showed bipolar resistive random access memory (ReRAM) behaviors with extremely low switching currents in the order of Picoamperes when voltages were applied. From transmission electron microscopy (TEM) observation, we confirmed that filament-like nanocrystals, having a diameter of about 10 nm, existed in TiO2 films at resistive switching areas after not only set operation but also reset operation. Moreover, photoemission electron microscopy (PEEM) analysis showed that the anatase-type TiO2 structure did not change after set and reset operations. From these results, we suggested that the Picoampere resistive switching occurred at the interface between the TiO2 dielectric and conductive nanocrystal without any structural changes in the TiO2 film and nanocrystal. The resistive switching mechanism we suggested is highly promising to realize extremely low-power-consumption ReRAMs with vertically contacted CNT electrodes.

Measurement of adhesion and bonding strength studies in 3D interconnect structures using atomic force microscopy

The wafer bonding process has become a flexible approach to material and device integration. The bonding strength in 3-dimensional processes is a crucial factor in various interface bonding processes such as silicon to silicon, silicon to metal, and oxide to adhesive intermediates. A method for measurement of bonding strength is proposed utilizing an ‘atomic force microscopy (AFM) applied carbon nanotube (CNT) probe tip’ which requires relatively simple preparation of sample and is able to measure bond strength regardless of film type. The bonding strength of the SiO2-Si surfaces cleaned with SPFM was 0.089 J/m2, while the bonding strength of surfaces cleaned with RCA 1 (NH4OH:H2O:H2O2) was 0.044 J/m2. This work verified the possibility that the new method is capable of accurately measuring bonding strength. It was also confirmed that more effective bonding is possible after cleaning with SPFM.

Frequency Response of Carbon Nanotube Probes during Tapping Mode of Atomic Force Microscopy

The dynamic behaviors of carbon nanotube probes applied in Atomic Force Microscope measurement are of interest in advanced nanoscalar topography. In this paper, we developed the characteristic equations and applied the model analysis to solve the eigenvalues of the microcantilever and the carbon nanotube. The eigenvalues were then used in the tapping mode system to predict the frequency responses against the tip-sample separations. It was found that the frequency drop steeply if the separation was less than certain distances. This instability of frequency is deduced from the jump of microcantilever or the jump of the carbon nanotube. Various lengths and binding angles of the carbon nanotube were considered, and the results indicated that the binding angle dominated the frequency responses and jumps.

Ultrahigh Currents in Dielectric-Coated Carbon Nanotube Probes

Carbon nanotubes used as conductive atomic force microscopy probes are expected to withstand extremely high currents. However, in existing prototypes, significant self-heating results in rapid degradation of the nanotube probe. Here, we investigate an alternative probe design, fabricated by dielectric encapsulation of multiwalled carbon nanotubes, which can support unexpectedly high currents with extreme stability. We show that the dielectric coating acts as a reservoir for Joule heat removal, and as a chemical barrier against thermal oxidation, greatly enhancing transport properties. In contact with Au surfaces, these probes can carry currents of 0.12 mA at a power of 1.5 mW and show no measurable change in resistance at current densities of 10(12) A/m(2) over a time scale of 10(3) s. Our observations are in good agreement with theoretical modeling and exact numerical calculations, demonstrating that the enhanced transport characteristics of such probes are governed by their more effective heat removal mechanisms.

A carbon nanotube metal semiconductor field effect transistor-based biosensor for detection of amyloid-beta in human serum

We have developed a carbon nanotube (CNT) film-based biosensor with a metal semiconductor field effect transistor structure (MESFET). A gold top gate was deposited on the middle of the CNT channel and probe antibodies were immobilized on the gold top gate with an antibody-binding protein, protein G or Escherichia coli outer membrane (OM) with autodisplayed Z-domains of protein A. These CNT-MESFET biosensors exhibited a higher sensitivity than the CNT-FET biosensor with probe antibodies immobilized using a chemical linker, since the orientation of immobilized antibodies was controlled by the antibody-binding proteins. In addition, nonspecific binding was effectively inhibited by E. coli OM. Using the CNT-MESFET biosensors with E. coli OM containing Z domain, we detected amyloid-β (Aβ) in human serum, one of the biomarkers for early diagnosis of Alzheimer's disease. Aβ at the level of 1pg/mL in human serum could be measured in real-time and without labeling, which was lower than a limit of detection for plasma Aβ using an enzyme-linked immune sorbent assay. These results suggested that our CNT-MESFET biosensors might be applicable for an early diagnosis of Alzheimer's disease.

Intracellular Neural Recording with Pure Carbon Nanotube Probes

The computational complexity of the brain depends in part on a neuron’s capacity to integrate electrochemical information from vast numbers of synaptic inputs. The measurements of synaptic activity that are crucial for mechanistic understanding of brain function are also challenging, because they require intracellular recording methods to detect and resolve millivolt- scale synaptic potentials. Although glass electrodes are widely used for intracellular recordings, novel electrodes with superior mechanical and electrical properties are desirable, because they could extend intracellular recording methods to challenging environments, including long term recordings in freely behaving animals. Carbon nanotubes (CNTs) can theoretically deliver this advance, but the difficulty of assembling CNTs has limited their application to a coating layer or assembly on a planar substrate, resulting in electrodes that are more suitable for in vivo extracellular recording or extracellular recording from isolated cells. Here we show that a novel, yet remarkably simple, millimeter-long electrode with a sub-micron tip, fabricated from self-entangled pure CNTs can be used to obtain intracellular and extracellular recordings from vertebrate neurons in vitro and in vivo. This fabrication technology provides a new method for assembling intracellular electrodes from CNTs, affording a promising opportunity to harness nanotechnology for neuroscience applications.

Mechanical Tomography of Human Corneocytes with a Nanoneedle

Atomic force microscopy (AFM) can image biological samples and characterize their mechanical properties. However, the low aspect ratio of standard AFM probes typically limits these measurements to surface properties. Here, the intracellular mechanical behavior of human corneocytes is determined using "nanoneedle" AFM probes. The method evaluates the forces experienced by a nanoneedle as it is pushed into and then retracted from the cell. Indentation loops yield the stiffness profile and information on the elastic and nonelastic mechanical properties at a specific depth below the surface of the corneocytes. A clear difference between the softer ∼50-nm-thick external layer and the more rigid internal structure of corneocytes is apparent, which is consistent with the current understanding of the structure of these cells. There are also significant variations in the mechanical properties of corneocytes from different volunteers. The small diameter of the nanoneedle allows this "mechanical tomography" to be performed with high spatial resolution, potentially offering an opportunity to detect biomechanical changes in corneocytes because of, e.g., environmental factors, aging, or dermatological pathologies.

Carbon nanotube thermal probe for quantitative temperature sensing

Quantitative temperature sensing at the nanoscale point contact is developed using a platinum hot film sensor with a carbon nanotube (CNT) as a thermal probe. High spatial resolution and robustness is achieved because of the small tip radius and high stiffness of the CNT. The quantitative local temperature at the CNT probe contact point is determined by bringing the probe in and out of contact and controlling the amount of heat of the Pt hot film in high vacuum environment. Using this method, we overcome the problems of thermal contact resistance (TCR) between the CNT and sample surface. Sensor sensitivity for TCR and thermal conductivity measurement of a CNT is analyzed and the sensor configuration is optimized.

Fabrication of Nanostructures of Low-Resistivity Silicon Wafer with High-Aspect-Ratio Using Carbon Nanotube Probe of Scanning Tunneling Microscope

ABSTRACTVarious nanostructures with high-aspect-ratio formed in a low-resistivity silicon wafer by the nano-processing using a carbon nanotube (CNT) probe of a scanning tunneling microscope (STM) have been investigated. The multi-wall CNT probes were obtained with our original pulling-method from CNT dispersion liquid. Nanostructures of point configurations (pit and mound) and line configurations were obtained at the constant tunneling current of 0.1 nA by controlling the bias voltages up to 10 V, processing times up to 300 s and scanning speeds of probe up to 480 nm/s for a line configuration. The aspect-ratio of the pit configuration fabricated at the bias voltage of 3 V increased about 6 times in proportion to the increase in processing time. Remarkable influence of the bias voltage on the configurations indicated that there exists a threshold bias voltage for the transition from the pit configuration to the mound one between 3 V and 5 V, and the aspect ratio of all nanostructures fabricated by the CNT probe were larger than those by a conventional tungsten probe. Finally, cross-sectional TEM observations were also applied to clarify the difference in the formation mechanisms between the pit configuration and the mound configuration. The TEM image of the pit configuration showed neither dislocations nor remarkable strains existed, but in the case of the mound shape TEM analysis indicated the existence of single crystalline silicon region solidified with atomic defects under the mound configuration. Therefore the drastic change of the configurations was attributed to the changes of the atomic-scale microstructures by applying the bias voltages.

Effective lateral stiffness enhancement of high-aspect-ratio carbon nanotube probe

This article systematically studied the generation mechanism of image artefacts in the case of the high-aspect-ratio carbon nanotube (CNT) atomic force microscopy probe. The scanning electronic microscope online nanomanipulation technology was used to investigate the generation mechanism then the strategy of site-specified electron beam induced deposition of platinum was proposed to enhance the lateral stiffness of CNT probes without obvious morphology advantage degradation. This technology was found effective for avoiding the image artefacts and technical parameters including the deposition dose and deposition model were optimised. Also, the outstanding performance of property-enhanced CNT probes were used to scan the typical structures fabricated by focused ion beam milling technology, it was found the CNT probes could achieve the ideal gradient values which could be 20° larger than that of Si probe.

Electrochemical impedance measurement of a carbon nanotube probe electrode

We measured and analyzed the electrochemical impedance of carbon nanotube (CNT) probe electrodes fabricated through the physical separation of insulated CNT bridges. The fabricated CNT electrodes were free-standing CNTs that were completely covered with an insulator, except for their tips. Typical dimensions of the nanoelectrodes were 1–10 nm in CNT diameter, 80–300 nm in insulator diameter, 0.5–4 μm in exposed CNT length and 1–10 μm in probe length. The electrochemical impedance at frequencies ranging from 40 Hz to 1 MHz was measured in physiological saline. The measured impedance of the CNT electrode was constant at 32 MΩ at frequencies below 1 kHz and was inversely proportional to frequency at frequencies above 10 kHz. By means of comparison with the parasitic capacitive impedance of the insulator membrane, we confirmed that the electrode was sufficiently insulated such that the measured constant impedance was given by the exposed CNT tip. Consequently, we can use the CNT electrode for highly localized electrochemical impedance measurements below 1 kHz. Considering an equivalent circuit and the nanoscopic dimensions of the CNT electrode, we demonstrated that the constant impedance was governed by diffusion impedance, whereas the solution resistance, charge-transfer resistance and double-layer capacitance were negligible.

On the applicability of carbon nanotubes as nanomechanical probes and manipulators

Carbon nanotube (CNT) probes offer improved imaging resolution in atomic force microscopy (AFM) and nanomanipulating devices due to their excellent mechanical properties and high aspect ratios. The basis of ascertaining scanning image quality using CNT probes is often centered on whether axial buckling has occurred or not. Here we explore the mechanical behavior and applicability of CNTs in surface scanning using molecular dynamics simulations in which the influence of van der Waals interactions is accounted for. Our results indicate the possible deleterious effects from van der Waals interaction dominated buckling of the probe, which is exacerbated by surface corrugations at the atomic scale. Under the premise that these issues can be surmounted, a cantilever model developed under known requirements for the structural characteristics of CNT probes is shown to be able to assess imaging fidelity. This model offers an effective guide to the selection and design of CNT probes for AFM.

Assessment of atomic force microscopy for characterisation of nanofiltration membranes

Shortly after atomic force microscopy (AFM) became a common technique for characterisation of surfaces; it was applied to image surface morphologies of polymeric membranes. The main interest is in analysis of the nodule size, the pore size and the roughness. Because nanofiltration (NF) membranes comprise nanostructures with dimensions lying within the lateral resolution of the microscope, the interpretation of NF membrane features is challenging. In this paper, we describe and critically discuss the outcomes from experiments on NF membranes using AFM combined with carbon nanotube probes. This combination increases the microscope resolution. The surface roughness, the nodule size, and the pore size were analysed for a range of P84 co-polyimide integrally skinned asymmetric NF membranes formed by phase inversion, and compared with the experimentally measured separation performance. Furthermore, we demonstrate a new concept for membrane characterisation through the quantitative analysis of phase images to elucidate the macromolecular packing at the membrane surface, and correlate this with the membrane functional performance.

Orientation-dependent friction between a carbon nanotube and graphite

We performed experiments to measure the friction force against the relative orientation between the carbon nanotube probe and graphite substrate. As the sample stage revolved one full circle, six sharp peaks were registered and each separated by 60°. Furthermore, these six peaks formed into three pairs, exhibiting the threefold symmetry, which can be explained with contribution beyond the top layer of the graphite. Yet, due to the prominent layer structure of the graphite, the energy loss caused by friction was dissipated mostly through its top layer.

Interfacing resistances in conducting probe atomic force microscopy with carbon nanotubes functionalised tips

A comparative study for the evaluation of carbon nanotube (CNT) functionalised tips as an interface for the determination of electrical and electronic properties of the surface of materials and devices is presented. The results of current spectroscopy measurements, complemented with force–current testing, reveal a strong dependence of the establishment of tip–sample electrical contact with environmental conditions and procedure. Multiwalled CNT-mediated measurements are compared with the results by commonly used as-purchased probes, metal-coated and Si probes, upon an inert and low-resistance Au substrate. The introduction of CNTs in the tip vicinity represents a drastic advance in the conduction capability as compared with the silicon probes. Despite a certain reduction of conduction as compared with metal-coated probes, a significant improvement of tip apex durability, together with, non-invasive mechanical contact to the sample is demonstrated. The present multiwalled CNT probes are proposed as an ideal element for electronic studies at the nanometre scale by atomic force microscopy.

Atomic force microscope method for sidewall measurement through carbon nanotube probe deformation correction

To use atomic force microscope to measure narrow vertical features is challenging. Using carbon nanotube (CNT) probes is a possible remedy. However, even with its extremely high stiffness, van der Waals attractive force from steep sidewalls bends CNT probes. This probe deflection effect causes deformation (or "swelling") of the measured profile. When measuring 100-nm-high vertical sidewalls with a 27-nm-diameter and 265-nm-long CNT probe, the probe deflection at the bottom is estimated as large as 5.8 nm. This phenomenon is inevitable when using long and thin probes. We proposed a method to correct this probe deflection effect. Detecting torsional motion of the base cantilever of the CNT probe makes it possible to estimate the CNT probe deflection. Using this information, we have developed a technique for correcting the probe deformation effect from measured profiles. This technique, in combination with correction of the probe shape effect, enables vertical sidewall profile measurement with AFM.

Theoretical Considerations on Resolution of Magnetic Force Microscopy Using Carbon Nanotube Probe

Theoretical Considerations on Resolution of Magnetic Force Microscopy Using Carbon Nanotube Probe

The advantages of the magnetic structure in ferromagnetic-film-coated carbon nanotube probes

The magnetic structures of ferromagnetic-film-coated carbon nanotube (CNT) probes and conventional pyramidal probes for a magnetic force microscope (MFM) were simulated using three-dimensional micromagnetic simulation. The CNT-MFM probes with a total probe diameter less than 60 nm are almost uniformly magnetized along the longitudinal direction of the CNT, which is the ideal magnetic structure for MFM observations. On the other hand, the pyramidal probes had a vortex structure around the point tip, which suggests that they require a greater thickness of the ferromagnetic film because only part of the magnetic moment participates in the detection of the z-component of the stray field from samples. The advantages of the CNT-MFM probe are uniform magnetization along the longitudinal direction and magnetic imaging ability using a smaller coating thickness.

Molecular Dynamics Deformation Simulation of Carbon Nanotube Probes

Understanding of the prosperities of the carbon nanotubes (CNTs) probes is crucial when measuring surface using atomic force microscopy (AFM). In this paper, we investigate the deformation of CNTs by adding lateral forces based on molecular dynamics (MD) simulation. In the simulation, Tersoff many-body potential function is used to describe the interaction between atoms. The movement of CNTs is periodic vibration, which is different from traditional material. We analyzed the vibration of different CNTs including single-walled carbon nanotubes (SWCNTs) and sharpened CNTs. Similarities and differences between different CNTs during the deformation are illustrated. It is shown that sharpened CNTs have better stiffness without declining the resolution of AFM. By analyzing the results obtained from the MD simulation, it is found that the sharpened CNTs may be more suitable as AFM probes.