Towards pick-and-place assembly of nanostructures

Single GaAs nanowire based photodetector fabricated by dielectrophoresis

Mechanical manipulation of nanowires (NWs) for their integration in electronics is still problematic because of their reduced dimensions, risking to produce mechanical damage to the NW structure and electronic properties during the assembly process. In this regard, contactless NW manipulation based methods using non-uniform electric fields, like dielectrophoresis (DEP) are usually much softer than mechanical methods, offering a less destructive alternative for integrating nanostructures in electronic devices. Here, we report a feasible and reproducible dielectrophoretic method to assemble single GaAs NWs (with radius 35-50 nm, and lengths 3-5 μm) on conductive electrodes layout with assembly yields above 90% per site, and alignment yields of 95%. The electrical characteristics of the dielectrophoretic contact formed between a GaAs NW and conductive electrodes have been measured, observing Schottky barrier like contacts. Our results also show the fast fabrication of diodes with rectifying characteristics due to the formation of a low-resistance contact between the Ga catalytic droplet at the tip of the NW when using Al doped ZnO as electrode. The current-voltage characteristics of a single Ga-terminated GaAs NW measured in dark and under illumination exhibit a strong sensitivity to visible light under forward bias conditions (around two orders of magnitude), mainly produced by a change on the series resistance of the device.

Recent advances in nanorobotic manipulation inside scanning electron microscopes

A scanning electron microscope (SEM) provides real-time imaging with nanometer resolution and a large scanning area, which enables the development and integration of robotic nanomanipulation systems inside a vacuum chamber to realize simultaneous imaging and direct interactions with nanoscaled samples. Emerging techniques for nanorobotic manipulation during SEM imaging enable the characterization of nanomaterials and nanostructures and the prototyping/assembly of nanodevices. This paper presents a comprehensive survey of recent advances in nanorobotic manipulation, including the development of nanomanipulation platforms, tools, changeable toolboxes, sensing units, control strategies, electron beam-induced deposition approaches, automation techniques, and nanomanipulation-enabled applications and discoveries. The limitations of the existing technologies and prospects for new technologies are also discussed.

Localized temperature and chemical reaction control in nanoscale space by nanowire array

We introduce a novel method for chemical reaction control with nanoscale spatial resolution based on localized heating by using a well-aligned nanowire array. Numerical and experimental analysis shows that each individual nanowire could be selectively and rapidly Joule heated for local and ultrafast temperature modulation in nanoscale space (e.g., maximum temperature gradient 2.2 K/nm at the nanowire edge; heating/cooling time < 2 μs). By taking advantage of this capability, several nanoscale chemical reactions such as polymer decomposition/cross-linking and direct and localized hydrothermal synthesis of metal oxide nanowires were demonstrated.

Manipulating InAs nanowires with submicrometer precision

InAs nanowires are grown epitaxially by catalyst-free metal organic vapor phase epitaxy and are subsequently positioned with a lateral accuracy of less than 1 μm using simple adhesion forces between the nanowires and an indium tip. The technique, requiring only an optical microscope, is used to place individual nanowires onto the corner of a cleaved-edge wafer as well as across predefined holes in Si(3)N(4) membranes. The precision of the method is limited by the stability of the micromanipulators and the precision of the optical microscope.

Top-down fabricated silicon nanowire sensors for real-time chemical detection

Silicon nanowire (SiNW) sensors have been developed by using top-down fabrication that is CMOS (complementary metal-oxide-semiconductor) compatible for resistive chemical detection with fast response and high sensitivity. Top-down fabrication by electron beam lithography and reactive ion etching of a silicon on insulator (SOI) substrate enables compatibility with the CMOS fabrication process, accurate alignment with other electrical components, flexible design of the nanowire geometry and good control of the electrical characteristics. The SiNW sensors showed a large operation range for pH detection (pH = 4-10) with an average sensitivity of (Delta R/R)/pH = 2.6%/pH and a rise time of 8 s. A small pH level difference (Delta pH = 0.2) near neutral pH conditions (pH = 7) could be resolved with the SiNW sensors. The sensor response to the presence of alkali metal ions and the long term drifting effects were also investigated.

A multi-walled carbon nanotube-aluminum bimorph nanoactuator

A multi-walled carbon nanotube (MWNT) bimorph nanoactuator has been modeled, fabricated and characterized. A thin aluminum film was uniformly deposited on the sidewalls of MWNTs using a pulsed laser deposition method to create the bimorph nanostructure. For a temperature change from 290 to 690 K with measured dimensions of 100 +/- 20 nm for the MWNT diameter, 40 +/- 10 nm for the Al thickness, and 5.2 +/- 0.5 microm for the bimorph length, the measured deflection was 550 +/- 200 nm, which was in good agreement with the calculation. The actuation force was measured using a lateral force microscopy technique, and the measured values agreed well with the prediction based on the model. The nanoactuator generated a microN force at its tip.

On the suitability of carbon nanotube forests as non-stick surfaces for nanomanipulation

A carbon nanotube forest provides a unique non-stick surface for nanomanipulation, as the nanostructuring of the surface allows micro- and nanoscale objects to be easily removed after first being deposited via a liquid dispersion. A common problem for smooth surfaces is the strong initial stiction caused by adhesion forces after deposition onto the surface. In this work, carbon nanotube forests fabricated by plasma-enhanced chemical vapour deposition are compared to structures with a similar morphology, silicon nanograss, defined by anisotropic reactive ion-etching. While manipulation experiments with latex microbeads on structured as well as smooth surfaces (gold, silicon, silicon dioxide, Teflon, diamond-like carbon) showed a very low initial stiction for both carbon nanotube forests and silicon nanograss, a homogeneous distribution of particles was significantly easier to achieve on the carbon nanotube forests. Contact-angle measurements during gradual evaporation revealed that the silicon nanograss was superhydrophic with no contact-line pinning, while carbon nanotube forests in contrast showed strong contact-line pinning, as confirmed by environmental scanning electron microscopy of microdroplets. As a consequence, latex microbeads dispersed on the surface from an aqueous solution distributed evenly on carbon nanotube forests, but formed large agglomerates after evaporation on silicon nanograss. Lateral manipulation of latex microbeads with a microcantilever was found to be easier on carbon nanotube forests and silicon nanograss compared to smooth diamond-like carbon, due to a substantially lower initial stiction force on surfaces with nanoscale roughness. Nanomanipulation of bismuth nanowires, carbon nanotubes and organic nanofibres was demonstrated on carbon nanotube forests using a sharp tungsten tip. We find that the reason for the remarkable suitability of carbon nanotube forests as a non-stick surface for nanomanipulation is indeed the strong contact-line pinning in combination with the nanostructured surface, which allows homogeneous dispersion and easy manipulation of individual particles.

Pick-and-place nanomanipulation using microfabricated grippers

Microfabricated grippers could be useful for the manipulation of nanoscopic and microscopic samples. A survey is presented of the force requirements for a microgripper to complete pick-and-place nanomanipulation tasks. We then demonstrate in situ pick-and-place operations of nanowires inside a scanning electron microscope using microfabricated electrostatically actuated grippers, and compare the theoretically estimated force requirements with the results of experimental tests of picking up nanowires to evaluate how grippers and strategies for nanomanipulation can be optimized.

Single- and multiwalled carbon nanotube networks and bundles assembled on microelectrodes

Dielectrophoresis is a convenient and flexible method for manipulating carbon nanotubes by the use of alternating electric fields. The method can be used for self-assembly of nanotubes on a set of microelectrodes. This paper shows bundles and nets of single- and multiwalled carbon nanotubes assembled between microelectrodes of various geometries as well IV curves recorded on the assembled nanosystems. The nets of multiwalled carbon nanotubes are suspended, while the single-walled nanotubes are lying on a surface. Carbon nanotube nets exhibit a large current-carrying capacity up to almost 4 mA with resistance of 1 to 4 kΩ, indicating that reliable electrical contacts can be formed using dielectrophoresis. This conclusion is further strengthened by the fact that the nanotubes tend to fail in intermediate regions far away from the electrodes, when subjected to currents above the maximal current capacity.

A simple electron-beam lithography system

A large number of applications of electron-beam lithography (EBL) systems in nanotechnology have been demonstrated in recent years. In this paper we present a simple and general-purpose EBL system constructed by insertion of an electrostatic deflector plate system at the electron-beam exit of the column of a scanning electron microscope (SEM). The system can easily be mounted on most standard SEM systems. The tested setup allows an area of up to about 50 x 50 microm to be scanned, if the upper limit for acceptable reduction of the SEM resolution is set to 10 nm. We demonstrate how the EBL system can be used to write three-dimensional nanostructures by electron-beam deposition.