Electron-beam induced synthesis of nanostructures: a review.

As the success of nanostructures grows in modern society so does the importance of our ability to control their synthesis in precise manners, often with atomic precision as this can directly affect the final properties of the nanostructures. Hence it is crucial to have both deep insight, ideally with real-time temporal resolution, and precise control during the fabrication of nanomaterials. Transmission electron microscopy offers these attributes potentially providing atomic resolution with near real time temporal resolution. In addition, one can fabricate nanostructures in situ in a TEM. This can be achieved with the use of environmental electron microscopes and/or specialized specimen holders. A rather simpler and rapidly growing approach is to take advantage of the imaging electron beam as a tool for in situ reactions. This is possible because there is a wealth of electron specimen interactions, which, when implemented under controlled conditions, enable different approaches to fabricate nanostructures. Moreover, when using the electron beam to drive reactions no specialized specimen holders or peripheral equipment is required. This review is dedicated to explore the body of work available on electron-beam induced synthesis techniques with in situ capabilities. Particular emphasis is placed on the electron beam-induced synthesis of nanostructures conducted inside a TEM, viz. the e-beam is the sole (or primary) agent triggering and driving the synthesis process.

Chemical vapor deposition of twisted bilayer and few-layer MoSe2 over SiO(x) substrates.

The chemical vapor deposition of monolayer and few-layer transition metal dichalcogenides is a rapidly developing area of materials science due to the exciting electrical, optical, thermal and mechanical properties of transition metal dichalcogenides in their layered form. These properties make these innovative materials potentially relevant to wide-ranging commercial applications. One of these promising materials is MoSe2; however, just recently, a few research groups have been able to demonstrate its synthesis via chemical vapor deposition. Moreover, only oriented few-layer MoSe2 has been exhibited by synthetically formed material using chemical vapor deposition thus far. Here, we confirm twisted-layer MoSe2 can also form during chemical vapor deposition. Twisted-layer transition metal dichalcogenides alter their properties as compared to their oriented counterparts. Therefore, twisted-layer structures are of interest because they can tune their properties.

High-yield photolytic generation of brominated single-walled carbon nanotubes and their application for gas sensing.

We present a facile and efficient photobromination technique for the covalent sidewall functionalization of SWNT using N-bromosuccinamide as the bromine source. The modified bromine functionalized SWNTs are used as active agents in a resistance measuring electrode system for sensing and discrimination of analyte vapors.

Retro-fitting an older (S)TEM with two Cs aberration correctors for 80 kV and 60 kV operation.

For the characterization of light materials using transmission electron microscopy, a low electron acceleration voltage of 80 kV or even 60 kV is attractive due to reduced beam damage to the specimen. The concomitant reduction in resolving power of the microscope can be restored when using spherical aberration (C(s) ) correctors, which for the most part are only available in the latest and most expensive microscopes. Here, we show that upgrading of existing TEMs is an attractive and cost-effective alternative. We report on the low-voltage performance on graphitic material of a JEOL JEM-2010F built in the early 1990s and retro-fitted with a conventional imaging C(s) corrector and a probe C(s) corrector. The performance data show C(s) retro-fitted instruments can compete very favourably against more modern state-of-the-art instruments in both conventional imaging (TEM) and scanning (STEM) modes.

Graphene at high bias: cracking, layer by layer sublimation, and fusing.

Graphene and few-layer graphene at high bias expose a wealth of phenomena due to the high temperatures reached. With in situ transmission electron microscopy, we observe directly how the current modifies the structure, and vice versa. In some samples, cracks propagate from the edges of the flakes, leading to the formation of narrow constrictions or to nanometer spaced gaps after breakdown. In other samples, we find layer-by-layer evaporation of few-layer graphene, which could be exploited for the controlled production of single layer graphene from multilayered samples. Surprisingly, we even find that two pieces of graphene that overlap can heal out at high bias and form one continuous sheet. These findings open up new avenues to structure graphene for specific device applications.

Understanding the metal-carbon interface in FePt catalyzed carbon nanotubes.

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon.

Synthesis of carbon-encapsulated iron nanoparticles by pyrolysis of iron citrate and poly(vinyl alcohol): a critical evaluation of yield and selectivity.

Carbon-encapsulated iron nanoparticles were synthesized by pyrolysis at 1000 °C of two solid precursors: poly(vinyl alcohol) and iron citrate. The weight ratio between the precursors controlled the reaction yield, crystallinity, morphological features and magnetic properties of the products. The encapsulation yield of iron nanoparticles in carbon shells was strongly influenced by the iron-to-carbon ratio and depended on the iron citrate content in the initial reactant mixtures. Despite the inherent simplicity of the process and the use of low cost starting materials the demonstrated route possesses limited selectivity, especially at high iron-to-carbon ratios. At these experimental conditions the as-obtained products contained non-encapsulated Fe particles and graphite in addition to magnetic carbon encapsulates. These by-products were effectively removed by a one-pot purification procedure that included acid treatment.

Dispersion and diameter separation of multi-wall carbon nanotubes in aqueous solutions.

Comparative studies on dispersing of multi-wall carbon nanotubes (CNTs) using two anionic surfactants (sodium dodecyl sulphate, SDS, and sodium dodecyl benzenosulfonate, SDBS) are presented. The studies were conducted on the surfactant concentrations that were close to the critical micelle concentration (CMC). The stability of CNTs suspensions obtained for surfactant solutions at concentrations lower than the CMC was investigated. It was also found that the surfactant structure has an influence on the diameter distribution of dispersed CNTs.

Screening the missing electron: nanochemistry in action.

The excitement of nano-test-tube chemistry in a single-walled carbon nanotube is exemplified in our study on electron doping in carbon nanotubes. Electron doping through the 1D van Hove singularity of single-walled carbon nanotubes is realized via a chemical reaction of an encapsulated organocerium compound, CeCp3. The decomposition of CeCp3 inside the carbon nanotubes increases the doping level and greatly enhances the density of conduction electrons. The transition of the cerium encapsulating semiconducting tubes to metallic results in enhanced screening of the photoexcited core hole potential. This fact illustrates the importance of many body effects in understanding core-level excitation process in carbon nanotubes.

Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene.

We have measured a strictly linear pi plasmon dispersion along the axis of individualized single-wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single-wall carbon nanotubes. Comparative ab initio studies on graphene-based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects cause a mixing of electronic transitions, including the "Dirac cone," resulting in the observed linear dispersion.

Single-walled carbon nanotubes synthesis: a direct comparison of laser ablation and carbon arc routes.

Carbon arc and chemical vapor deposition are at present the most efficient methods for mass production of single-walled carbon nanotubes. However, laser ablation is renowned for high quality nanotubes with narrow diameter distributions and hence is also of great interest. The aim of this work was to compare both the carbon arc and laser ablation techniques with respect to the quality--and relative yield of the produced SWCNTs. For this comparative study we used Fe as the catalyst, which is known not to be very active in laser ablation. However, we show this is not the case when H2 is included in the reaction. The reactions for both synthesis routes were carried out in a N2-H2 (95-5% vol.) atmosphere. The same homogenous carbon rods with different iron contents, between 1 and 5 at.% were used as the carbon feedstock and catalyst supply in both synthesis routes. Additionally, two types of carbon rods containing 1 at.% Fe with different graphitization degrees were also investigated. In the arc-discharge case, the low-graphitized electrode produced a web-like product rich in SWCNTs, while the high-graphitized carbon rods yielded soot containing carbon-encapsulated iron nanocrystallites, amorphous carbon nanoparticles, and surprisingly a small fraction of SWCNTs. With laser ablation synthesis, the Fe content and the reactor temperature significantly influenced the SWCNTs yield. Carbon arc plasma diagnostics were also performed. By using optical emission and Absorption spectroscopy the plasma temperature, C2 and CN radical content in the arc zone were determined.

Effects of the reaction atmosphere composition on the synthesis of single and multiwalled nitrogen-doped nanotubes.

Single and multiwalled nitrogen-doped carbon nanotubes were grown by chemical vapor deposition varying the feedstock composition between pure acetonitrile and ethanol/acetonitrile mixtures. The advantage of using CN sources that develop close vapor pressure values has been used in order to elucidate the effects of the reaction atmosphere in the synthesis of N-doped nanotubes. Our findings show that the morphology of the nanotube material depends strongly on the composition of the reaction atmosphere. When carrying out the experiments in an atmosphere solely determined by the vapor pressure of the feedstock components, improved homogeneity is achieved with pure CN sources or low concentration of the foreign solute. Under these conditions the temperature has strong influence in the diameter distribution.

Microwave sputtering of conductors and insulators for optical emission and mass spectrometry.

A microwave-powered slab-line cavity was used to excite a discharge in low pressure argon or neon and to demonstrate the sputtering of conducting and non-conducting samples by a microwave excited discharge. Both optical emission spectroscopy and mass spectrometry were used as detection systems. The dependence of the signals on gas pressure and net microwave power was investigated.