Carbon nanotube scanning tunneling microscopy tips for chemically selective imaging

Carboxyl-terminated single-walled carbon nanotubes (SWNTs) were successfully immobilized from solution phases onto the apexes of gold tips for scanning tunneling microscopy (STM). Gold STM tips were first modified with self-assembled monolayers of 4-mercaptobenzoic acid, and its carboxyl groups were used to anchor carboxylated SWNTs through Zn2+ ion-bridged coordination. These SWNT tips gave high-resolution STM images of a diether monolayer formed on the graphite surface. In addition and more importantly, the ether oxygens of the sample molecules were selectively observed as bright spots with the SWNT tips with significantly high reproducibility, which is due to the facilitation of electron tunneling through hydrogen bond interactions between the ether oxygens and carboxyl groups at the end of the SWNT tips.     

Microscopic and spectroscopic study of self-ordering in poly(3-hexylthiophene)/carbon nanotubes nanocomposites

Poly(3-hexylthiophene)-single-walled carbon nanotubes (SWNTs) composites were studied using UV-visible absorbance and Raman spectroscopy, scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Monolayers of regioregular poly(3-hexyl thiophene) (rrP3HT) adsorbed on SWNTs have been imaged using scanning tunneling microscopy (STM) to obtain measurements of the chiral angles at which the thiophene polymer chains wrap around individual carbon nanotubes (41-48 degrees with respect to nanotube axis) and polymer interchain spacings (1.68 angstroms). The rrP3HT interchain distance is greater for rrP3HT monolayers adsorbed onto the curved surfaces of SWNTs than on the flat surfaces of highly ordered pyrolytic graphite samples (1.4 angstroms). UV-vis spectroscopic data provided strong evidence for increased interchain interactions in composites of rrP3HT and SWNTs compared to the pure polymer. The STM local-probe studies of the native polymer and the composites further confirmed that the rrP3HT interacts with carbon nanotubes to produce a highly ordered material at the molecular level.     

The atomic structure of nanotubes synthesized from a carbon mix of high reaction ability

The surface of carbon nanotubes was studied on an atomic resolution level in a scanning tunneling microscope (STM) operating under atmospheric conditions. The nanotubes were synthesized from a carbon mix of high reaction ability. Factors determining distortion of the STM image are analyzed and the main parameters (chiral angle and diameter) of nanotubes are calculated.     

Microscope studies of the morphology and structure of carbon nanotubes

We have studied the morphologies and structures of carbon nanotubes (bucky-tubes) and carbon nanoparticles (buckyonions) using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning tunnelling microscopy (STM). By SEM the carbon nanotubes are observed with features similar to those of some fibrous whiskers grown from pyrolytic graphite. This growth feature is supported by transmission electron microscopy (TEM) observations. The TEM results show also that the graphitic sheets can be bent into curved shapes to cap the nanotubes or form the onions. In the curved graphitic sheets elastic strains induced by layer mismatches and dislocations are revealed. The STM observations on the nanotubes show a bundle-like morphology of the carbon nanotubes, and by atomic resolution images the zigzag and armchair atomic configurations may be identified. The results also show structural distortions which may be produced by folding the graphite sheets to create the nanotubes and are responsible for the lattice mismatch.     

A comparison of different carbon filaments on the nanometer and atomic scales by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed as a tool to elucidate the structural differences between carbon nanotubes (CNTs) and carbon nanofibers (CNFs) at the nanometer and atomic scales. As opposed to the seamless, virtually defect-free structure of CNTs that was visualized by STM at both scales, CNFs displayed a fragmented graphitic conformation (nanographites), where the nanographitic domains were below 2-3 nm in lateral size. Thus, STM is proposed as a suitable technique to discriminate these two types of carbon nanostructures.     

Nanotubes in a gradient electric field as revealed by STM TEM technique

We have investigated the behavior of two nanotube systems, carbon and boron nitride, under controlled applied voltages in a high-resolution transmission electron microscope (TEM) equipped with a scanning tunneling microscope (STM) unit. Individual nanotubes (or thin bundles) were positioned between a piezomovable gold electrode and a biased (up to ±140 V) STM tip inside the pole-piece of the microscope. The structures studied include double-and multi-walled carbon nanotubes (the latter having diverse morphologies due to the various synthetic procedures utilized), few-layered boron nitride nanotube bundles and multi-walled boron nitride nanotubes (with or without functionalized surfaces). The electrical breakdown, physical failure, and electrostatic interactions are documented for each system. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Local temperature measurements on nanoscale materials using a movable nanothermocouple assembled in a transmission electron microscope

A nanoscale thermocouple consisting of merged Cu and Cu-Ni tips is developed for local temperature measurements on advanced nanomaterials by using a probing technique in a high-resolution transmission electron microscope (TEM) equipped with a double probe scanning tunneling microcopy (STM) unit. The fabricated nanothermocouple works as the so-called T-type thermocouple and displays a quick response and high spatial and thermal resolutions. A generated thermoelectromotive force which reflects rapid temperature changes controlled by electron beam intensity alternations on a metal nanoelectrode proves the technique's usefulness for high-precision local temperature measurements. The developed method demonstrates the effectiveness while also measuring temperature changes in Joule heated multi-walled carbon nanotubes (CNTs) and in a modeled electrical conductive composite nanosystem.     

Nitrogen doping in carbon nanotubes

Nitrogen doping of single and multi-walled carbon nanotubes is of great interest both fundamentally, to explore the effect of dopants on quasi-1D electrical conductors, and for applications such as field emission tips, lithium storage, composites and nanoelectronic devices. We present an extensive review of the current state of the art in nitrogen doping of carbon nanotubes, including synthesis techniques, and comparison with nitrogen doped carbon thin films and azofullerenes. Nitrogen doping significantly alters nanotube morphology, leading to compartmentalised 'bamboo' nanotube structures. We review spectroscopic studies of nitrogen dopants using techniques such as X-ray photoemission spectroscopy, electron energy loss spectroscopy and Raman studies, and associated theoretical models. We discuss the role of nanotube curvature and chirality (notably whether the nanotubes are metallic or semiconducting), and the effect of doping on nanotube surface chemistry. Finally we review the effect of nitrogen on the transport properties of carbon nanotubes, notably its ability to induce negative differential resistance in semiconducting tubes.     

Room temperature growth of single-wall coiled carbon nanotubes and Y-branches

Straight carbon nanotubes, carbon nanotube “knees,” Y-branches of carbon nanotubes and coiled carbon nanotubes were grown on a graphite substrate held at room temperature by the decomposition of fullerene under moderate heating (450 °C) in the presence of 200-nm Ni particles. The grown structures were investigated without any further manipulation by STM. The growth and the chemical stability of the carbon nanostructures containing nonhexagonal rings are discussed.     

Polypyrrole-modified tips for functional group recognition in scanning tunneling microscopy

Tailored chemical modification of scanning tunneling microscopy (STM) tips is a promising method for the recognition of specific chemical species and functional groups in STM images. The present study shows for the first time that tips modified with polypyrrole can be used to measure STM images with molecular resolution. A high conductivity of the polypyrrole film was found to be important for the observation of STM images, while the thickness of the polymer film did not affect the images significantly. Furthermore, it was shown that recognition of functional groups in STM images is possible with tips coated with conductive polypyrroles. 1-Octadecanol and 1-octadecanoic acid monolayers with polypyrrole-modified tips gave high-resolution STM images in which aligned OH and COOH residues were represented by easily recognizable elevated bands. These selective contrast enhancements resemble those observed by us previously with gold tips modified with self-assembled monolayers (SAMs) and seem to be due to hydrogen bond interactions between functional groups of the tip-modifying molecules and the sample. The reproducibility of contrast enhancements in this study was significantly higher than for SAM-modified tips, suggesting that polymer modification of STM tips is particularly promising for specific functional group recognition with chemically modified STM tips.     

Lateral manipulation of single-walled carbon nanotubes on H-passivated Si(100) surfaces with an ultrahigh-vacuum scanning tunneling microscope

Ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM) can be used for the manipulation of individual atoms and molecules into complex arrangements for sensitive electrical and structural characterization. However, the systematic UHV STM manipulation of single-walled carbon nanotubes (SWNTs), high-aspect-ratio molecular wires derived from graphene that exist in both semiconducting and metallic forms, has yet to be reported. In this work, we demonstrate the room-temperature lateral manipulation of approximately 1-nm-diameter SWNTs on UHV-prepared, hydrogen-passivated Si(100) surfaces. We show the reproducible actuation of SWNTs having lengths as small as 13 nm, along with the partial division of a two-tube bundle. Moreover, UHV STM desorption of H at the SWNT/Si interface is introduced as a means of locally strengthening the interaction between the tube and the surface. The UHV STM manipulation scheme described here is potentially extensible to the orientational control of SWNTs interfaced with atomically clean semiconducting surfaces, such as InAs(110), GaAs(110), and unpassivated Si(100), for which first-principles calculations based on density functional theory have been reported recently in the literature.     

Scanning tunnelling microscopy of carbon nanotubes

This paper briefly reviews how scanning tunnelling microscopy (STM) and spectroscopy (STS) are used to analyse the atomic structure and the electronic properties of individual single-wall carbon nanotubes. In this area, the progress accomplished over the past several years has been spectacular. As this paper demonstrates, all the effects predicted by theory have been verified experimentally. Geometrical and electronic effects specific to carbon nanotubes are illustrated by analysing a series of STM images and STS spectra computed using a tight-binding theory. The simulations include a catalogue of images of 27 single-wall nanotubes, Stone-Wales defects in semiconducting nanotubes, and a symmetric Y-junction.     

流动催化法连续制备碳纳米管及其形态和结构的研究

以二茂铁作为催化剂来源、以苯作为碳源、氢气和氩气分别作为载气和稀释气体,在１１００℃连续地合成了碳纳米管．碳纳米管的生成分二个过程：催化生长和表面无定形碳的生成．所得到的碳纳米管的内径为３～６ｎｍ,而外径约为２０～７０ｎｍ碳纳米管的外径随气体流速的增加而变细,在较细的碳纳米管中观察到了由催化生长而成的具有光滑薄壁的原始碳纳米管．生成的碳纳米管的长度达数十微米、直径较均匀,其端部大多为圆形,但也观察到其他的形状的端部．     

Solvent-free derivatization of pristine multi-walled carbon nanotubes with dithiols

We report on direct solvent-free derivatization of pristine multi-walled carbon nanotubes (MWNTs) with aliphatic dithiols (1,4-butanedithiol, 1,6-hexanedithiol and 1,8-octanedithiol), by means of heating at 130–150 °C under reduced pressure. This method requires no additional chemical activation and about 2 h only for completion. Studies by high-resolution transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy showed that dithiol-derivatized MWNTs have a high affinity to ZnCl₂ in solution, which covers the nanotubes with a dense amorphous layer. According to PM3 semi-empirical calculations, employing a closed-cap zigzag (10,0) single-walled carbon nanotube (SWNT) model incorporating a Stone-Wales defect, site-specificity of the addition depends on the mutual position of pentagons. If the nanotube contains pyracylene units or Stone-Wales defect, the addition takes place on their 6,6 or 7,7 bonds, respectively, whereas for isolated pentagons, preferential reaction sites are their C–C bonds. Ideal graphene sheet sidewalls with cylindrical curvature are relatively inert (although one cannot discard the possibility to activate the reaction by heating). Dithiol groups introduced in the way proposed can be used as chemical linkers for anchoring metal complexes and nanoparticles to carbon nanotubes, attaching SWNTs to gold tips for atomic force and scanning tunneling microscopy, and potentially for adsorption and concentration of trace metal ions.     

Development and Application of Multiple‐Probe Scanning Probe Microscopes

In the research of advanced materials based on nanoscience and nanotechnology, it is often desirable to measure nanoscale local electrical conductivity at a designated position of a given sample. For this purpose, multiple‐probe scanning probe microscopes (MP‐SPMs), in which two, three or four scanning tunneling microscope (STM) or atomic force microscope (AFM) probes are operated independently, have been developed. Each probe in an MP‐SPM is used not only for observing high‐resolution STM or AFM images but also for forming an electrical contact enabling nanoscale local electrical conductivity measurement. The world's first double‐probe STM (DP‐STM) developed by the authors, which was subsequently modified to a triple‐probe STM (TP‐STM), has been used to measure the conductivities of one‐dimensional metal nanowires and carbon nanotubes and also two‐dimensional molecular films. A quadruple‐probe STM (QP‐STM) has also been developed and used to measure the conductivity of two‐dimensional molecular films without the ambiguity of contact resistance between the probe and sample. Moreover, a quadruple‐probe AFM (QP‐AFM) with four conductive tuning‐fork‐type self‐detection force sensing probes has been developed to measure the conductivity of a nanostructure on an insulating substrate. A general‐purpose computer software to control four probes at the same time has also been developed and used in the operation of the QP‐AFM. These developments and applications of MP‐SPMs are reviewed in this paper.     

STM investigation of carbon nanotubes connected by functional groups

Chemical functionalization of carbon nanotubes (CNTs) is essential for many applications. Attachment of functional groups to nanotubes can dramatically increase the solubility of the nanotube material. Sidewall functional groups should react with polymers and improve the mechanical properties of nanocomposites. Tubes interconnected by chemical bonds will have a reduced contact resistance and can be used for interconnection purposes in nanoscale circuits. Carbon nanotubes covered with functional groups attached to their exterior wall were analyzed using scanning tunneling microscopy (STM) and TEM. The functionalization was carried out in three steps: acid treatment in H₂SO₄/HNO₃ (3:1) mixture, reaction with SOCl₂ and reaction with diaminopropanol (DAP). The binding force between the nanotubes connected by functional groups and the mechanical stability of the connection was investigated.     

Probing quantized image-potential states at supported carbon nanotubes

Discrete image-potential states (ISs) are revealed at double-walled carbon nanotubes by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in the distance-voltage z(V) spectroscopy mode. The nanotubes are supported by flat Au(111) substrates. Due to the high sensitivity of the hot IS electrons to local variations of the surface potential, they can be considered as a sensitive probe to investigate interactions with the supporting substrate and impurities or defects at the nanotube surface. ISs provide information on the local electronic structure as well as on the electron dynamics at supported nanotubes.     

Measuring the work function of carbon nanotubes with thermionic method

The work function of carbon nanotubes might depend on their diameters and the number of walls, and be different for their tips and sidewalls. Here we report the work function measurement of single-walled, double-walled, and multiwalled carbon nanotubes by investigating the thermionic emission from the middle of their bundles. It is found that the sidewall work functions of the three kinds of carbon nanotubes are all around 4.7-4.9 eV; the diameter and the numbers of walls have no obvious influence on their work functions. For the carbon nanotube bundle with some tips appearing in the middle, the measured work function is smaller than without tips, indicating that the work function of tips is smaller than that of the sidewalls. This tip effect also results in a difference in the thermionic emission characteristic, implying non-uniform work function distribution along the bundle.     

碳纳米管场发射特性的研究

将CVD方法制成的碳纳米管沉积在钼针尖上 ,测试了这种材料的场发射特性。结果表明这种材料可作为一种新型高效的场发射体。同时还将其与纯钼针在场发射方面进行了比较     

Scanning tunneling microscopy with chemically modified gold tips: in situ reestablishment of chemical contrast

A method was developed for the reestablishment of chemical contrast in STM images obtained with chemically modified gold tips. Such tips display selective chemical contrast, which allows the selective imaging of specific species on the sample surface. Chemically modified STM tips can be fabricated by forming a self-assembled monolayer (SAM) on an electrochemically etched gold tip. One difficulty with this method thus far has been the relatively short lifetime of SAM-treated tips. The method described here utilizes the brief application of a high bias voltage between the sample and the tip to cause SAM molecules to reoccupy the tip apex, thereby allowing the tips to display selective chemical contrast in imaging. These treatments consist of applying a +1.9-V sample bias for 0.5-10 min under tunneling conditions. The usable lifetime of SAM-modified tips could be increased by more than 2 orders of magnitude, from hours to at least a month, dramatically increasing the efficiency of using SAM-modified gold tips. SAM molecules can also be removed from the tip apex by application of a negative sample bias (-2.0 V for 0.5-10 min) making it possible to alternate between conventional STM images and STM images with chemically enhanced contrasts.