WO2009000285A1

WO2009000285A1 - Devices for and methods of handling nanowires

Info

Publication number: WO2009000285A1
Application number: PCT/EP2007/005546
Authority: WO
Inventors: Hendrik Ulbricht; Markus Arndt; Nikolaus Gotsche
Original assignee: Universität Wien
Priority date: 2007-06-22
Filing date: 2007-06-22
Publication date: 2008-12-31

Abstract

An apparatus for handling nanowires, the apparatus comprising an electromagnetic radiation source adapted for irradiating a body comprising nanowires with an electromagnetic radiation beam to thereby evaporate the body so that the nanowires are released from the body, and a fluidic stream generation unit adapted for generating a fluidic stream for carrying along the released nanowires with the fluidic stream. An apparatus and a method for sorting particles of high polarizability anisotropy - such as for instance carbon nanotubes - in the gas phase according to this property, using a three-grating deflectometer. A method for switching molecular beams of high polarizability using a three-grating deflectometer.

Description

Devices for and methods of handling nanowires

. The invention relates to an apparatus for handling nanowires. The invention further relates to a method of handling nanowires.

Moreover, the invention relates to a propagating fluidic stream. Beyond this, the invention relates to an apparatus for sorting nanowires. The invention further relates to a method of sorting nanowires.

Carbon nanotubes (CNTs) are allotropes of carbon. Carbon nanorubes may include single walled tubes and multi walled tubes. A single walled carbon nanotube is a one-atom thick sheet of graphite (called graphene) rolled up into a seamless cylinder with diameter of the order of a nanometer. This results in a nanostructure where the length-to-diameter ratio may be very large. Such cylindrical carbon molecules have properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of current and heat. Inorganic nanotubes have also been synthesized. US 6,974,926 discloses the sorting of single walled carbon nanotubes in fluids using optical dipole forces.

US 5,062,935 discloses laser desorption of molecules.However, investigations of chemically prepared carbon nanotubes, their sorting in the gas phase, deposition and alignment on a substrate may be difficult according to US 6,974,926

It is an object of the invention to provide a system for properly handling nanowires.

In order to achieve the object defined above, an apparatus for handling nanowires, a method of handling nanowires, a propagating fluidic stream (particularly a jet of CNTs in a gaseous and/or liquid stream propagating in vacuum), an apparatus for sorting nanowires, and a method of sorting nanowires according to the independent claims are provided. According to an exemplary embodiment of the invention, an apparatus for handling nanowires (such as carbon nanotubes) is provided. The apparatus comprises an electromagnetic radiation source (such as a laser) adapted for irradiating a ( for instance frozen or solid) body comprising nanowires to thereby evaporate (particularly for transferring into a gaseous phase) at least a part of the body so that at least a part of the nanowires are released from the body. The apparatus further comprises a fluidic stream generation unit adapted for generating a fluidic stream (such as a gas stream or a vapour stream or gas stream with suspended droplets) for carrying along the released (that is released from the solid matrix of the frozen body) nanowires with the fluidic stream.

According to another exemplary embodiment of the invention, a method of handling nanowires is provided. The method comprises irradiating a body comprising nanowires with an electromagnetic radiation beam to thereby evaporate at least a part of the body so that at least a part of the nanowires are released from the body. The method further comprises a fluidic stream for carrying along the released nanowires with the fluidic stream.

According to still another exemplary embodiment of the invention, a propagating (that is moving along a predetermined trajectory, particularly as a beam having a defined diameter and moving along an essentially straight direction in space) fluidic stream is provided comprising a carrier fluid (such as an inert gas or steam) and nanowires (such as carbon nanotubes) carried with the carrier fluid.

According to yet another exemplary embodiment of the invention, an apparatus for sorting nanowires is provided, the apparatus comprising a nanowire beam generation unit (for instance an apparatus for handling nanowires having the above mentioned features) adapted for generating a fluidic stream comprising nanowires, and a three-grating deflectometer adapted for sorting the nanowires into different fractions of nanowires. Particularly, an apparatus for sorting particles of high polarizability anisotropy - such as for instance carbon nanotubes - in the gas phase according to this property, using a three-grating deflectometer, may be provided. According to still another exemplary embodiment of the invention, a method of sorting nanowires is provided, the method comprising generating a fluidic stream comprising nanowires, and sorting the nanowires into different fractions of nanowires by a three-grating deflectometer. Particularly, a method for switching molecular beams of high polarizability using a three- grating deflectometer may be provided.

The term "nanowire" may denote a wire-like structure of dimensions in the order of magnitude of several to several hundreds of nanometers (and may also cover larger or smaller dimensions). Many different types of nanowires may be used for embodiments of the invention, including semiconducting nanowires (for instance made of silicon, germanium, InP, GaN, etc.), metallic nanowires (for instance nickel, platinum, gold), and nanotubes, particularly carbon nanotubes (intrinsic or doped). The nanowire may also be an isolating nanowire (in case the nanowire is covered by an isolation layer). The term "electromagnetic radiation" may particularly denote photons of an appropriate wavelength. Although embodiments of the invention are based on an infrared irradiation with photons in a wavelength range of about one micrometer, embodiments of the invention may also use microwaves, far-infrared, near-infrared, optical, UV, or even X-rays for evaporation. The term "substrate" may be used to define generally the elements for layers that underlie and/or overlie a layer or portions of interest. Also, the substrate may be any other base on which substances for forming the frozen body are deposited or on which an ordered structure of nanowires is formed after deposition, for example a copper plate or a semiconductor wafer such as a silicon wafer or silicon chip.

The term "body" may particularly denote a structure comprising a liquid solution in which nanowires are included, wherein the solution is brought to a temperature at which it becomes very viscous or even solid. Thus, the body may be a droplet. For instance, the carrier solution may be water, oil, or a buffer. More generally, the body may be a frozen body, a solid body, a liquid body, a gel body, a (highly) viscous body, or a partially defrosted body. The body may be an ordered body in the condensed matter phase.

The term "releasing" nanowires from the frozen body may particularly denote the fact that evaporating the frozen body may evaporate an (aqueous) carrier solution, wherein the nanowires may be included in a gaseous phase spatially above, below or laterally of the remaining portion of the frozen body.

The term "fluidic stream" may particularly denote a propagating stream comprising a gas and/or a liquid, wherein also solid components may be included additionally in such a gaseous and/or liquid stream. According to an exemplary embodiment of the invention, an evaporated beam of nanowires, particularly of carbon nanotubes, may be provided which may serve as a proper basis for a subsequent treatment of the nanowires, for instance for separating nanowires regarding different physical/chemical properties (such as electric charge, polarizability, mass, or ratios between these or other physical or chemical parameters). Under the influence of forces (such as electric forces, magnetic forces or mechanical forces), different fractions of nanotubes may then be separated. Consequently, an enrichment of species of nanowires having similar properties may be achieved. This may form the basis for a subsequent deposition of such nanowires on a surface to form ordered or other useful structures. This may be particularly obtained by a combination of laser desorption of a frozen nanowire matrix and the generation of a beam in the gas phase.

According to an exemplary embodiment of the invention, the implantation of nanowires into a gaseous stream may be made possible. Furthermore, sorting of carbon nanotubes (particularly separation of metallic und semi conductive tubes) in the gas phase, particularly using a Moire-3 -grating-technique, may be made possible.

According to an exemplary embodiment of the invention, carbon nanotubes may be pre-selected/purified chemically in a liquid, and may be subsequently brought in a gas phase/liquid micro jet state. Such a procedure may be performed in vacuum. Without wishing to be bound to a specific theory, it is presently believed that the nanotubes (or more generally nanowires) are kept at a relatively low temperature.

Thus according to an exemplary embodiment of the invention, a method for generating a gas phase beam of carbon nanotubes using laser desorption of a frozen nanotube matrix may be provided. Such a beam of carbon nanotubes may be used, for instance, for molecular interferometry.

Starting from a frozen matrix of a solvent, beams of carbon nanotubes may be generated, for instance as a basis for a manipulation procedure for manipulating the carbon nanotubes. Thus, a molecular beam of carbon nanotubes may be produced using laser desorption of an iced matrix of solvent.

Nanotubes can be isolated, separated or individualized in a combined mechanical/chemical method (for instance ultrasonic sound and solution), and may be isolated in micelles. These can be frozen directly within the solvent. The cold matrix obtained in such a manner may be transferred into a vacuum apparatus and may be locally evaporated by laser pulses having, for instance, a duration of nanoseconds. A pulsed supersonic beam of inert gas (for instance nitrogen or noble gas), streaming over the desorption region may carry the desorbed material in vacuum for further use

There are different methods of generating carbon nanotubes and to deposit the structures onto substrates. However, in contrast to conventional approaches, embodiments of the invention may be capable of generating properly sorted tubes, wherein the sorting may be performed regarding criteria such as length, diameter, chirality of the lattice structure, etc. On the basis of a mechanically/chemically prepared carbon nanotube sample, embodiments of the invention generate molecular beams. This may be important for the controlled deposition on highly pure surfaces.

In the gas phase, carbon nanotubes may have an improved purity (lower contamination with solvents) as compared to a solution. A deposition method is compatible with a manufacturing method in vacuum, and also in ultra-high vacuum, for instance adding a differential pump state. Thus, chemically prepared (for instance pre-sorted, shorted, etc.) nanotube samples may be deposited on highly pure substrates such as a reconstructed silicon surface Si 111 (7x7). A molecular beam of nanotubes may be used as a proper basis for further manipulation methods. Such manipulation methods may involve sorting/enrichment of metallic or semi conductive nanotubes, for instance using Stark defiectometry in the gas phase. It is also possible to perform an electric switch for molecular beams using the combination of Stark defiectometry with alternating fields on a molecular beam that passes a three grating interferometer or a three grating Moire setup.

A technique is provided to generate a beam of carbon nanotubes, and particularly of single walled carbon nanotubes, in a high vacuum environment. Such a method may be based on laser desorption of individualized nanotubes, trapped in an iced solvent (or more generally a viscous) matrix. Many gas phase manipulation techniques and surface deposition methods for carbon nanotubes may profit from the availability of a directed nanotube beam in a high vacuum environment.

Next, further exemplary embodiments of the apparatus will be explained.

However, these embodiments also apply to the method and to the propagating fluidic beam.

The electromagnetic radiation source may be adapted for irradiating the frozen body with a pulsed electromagnetic radiation beam. Thus, pulses particularly with a time duration in the nanosecond regime, for instance of 5 ns, may be generated which may expose or release pulses of nanowires from the frozen body. However, alternatively, the pulses may have other durations particularly in a range between essentially 1 ps (or shorter) and essentially 1 μs (or longer).

The electromagnetic radiation beam may be a laser. Using a laser may allow to obtain a spatially well-defined evaporated beam, therefore ensuring a high accuracy of the generated propagating fluidic stream. By a laser, short pulses may be generated which have a sufficiently high energy to evaporate efficiently a well- defined amount of solution of an aqueous carrier.

Particularly, the fluidic stream generation unit may be adapted for generating a gas stream. When such a gas stream comprising nanotubes is mixed with the fluidic stream generated by the fluidic stream generation unit, a beam in the gas phase may be provided which may serve as a basis for a subsequent handling of the nanowires.

The fluidic stream generation unit may be adapted for generating a pulsed fluidic stream. Thus, also the fluidic stream may be pulsed, so that pulses of nanowires with the fluidic carrier may be supplied for further analysis. This may, for instance, provide portions of nanowires of an accurate size or amount for subsequent deposition on a dedicated surface portion.

The fluidic stream generation unit may be adapted for generating a supersonic fluidic stream. Supersonic velocities of the fluidic stream may be highly advantageous since they may keep the flight time short before deposition, preventing an undesired broadening of a fluidic pulse during the flight phase.

The fluidic stream generation unit may be adapted for generating a fluidic stream of an inert material. Using an inert material, particularly an inert gas such as nitrogen or argon, may allow to keep the sensitive nanotubes free from any disturbing interaction. This may allow to enrich fractions of the carbon nanotubes having very similar or essentially identical properties.

The apparatus may comprise a vacuum chamber in which (several or all of) the components of the apparatus are accommodated. By performing at least a portion, preferably the entire handling procedure, within a vacuum chamber, for instance in ultra-high vacuum, impurities and undesired interaction between an atmosphere and the nanowire beam may be avoided. Furthermore, the propagating pulses of fluidic packets may be free from interactions with the surrounding gas in a vacuum chamber. Such a vacuum may be generated, for instance, using one or more pumps. However, alternatively, the method of generating a nanowire jet may also performed in an atmosphere which is not a vacuum.

The apparatus may comprise a mounting substrate for mounting the body. In other words, the body may be provided on a surface portion of the mounting substrate, for instance a (specially shaped, including grooves, arrays of grooves, etc.) copper substrate, so that the body may be exposed at a surface of the mounting substrate for interaction with the laser beam. The body may be a frozen body and the apparatus may comprise a cooling reservoir thermally coupled to the mounting substrate for cooling the frozen body when being mounted on the mounting substrate. For instance, a cool finger may thermally connect a copper substrate with a liquid nitrogen or a liquid helium reservoir, thereby allowing to bring the frozen body to a desired temperature. The use of nitrogen may be advantageous due to the simplicity of such a corresponding handling.

The apparatus may comprise a sorting unit adapted for sorting the nanowires in the fluidic stream regarding at least one sorting criteria. Such sorting criteria may comprise the electric charge, the mass, the dimension, the electrical conductivity, the polarizability, or a combination of these and other parameters.

Particularly, it is possible to separate different components using a Stark deflectometer, which may separate the components regarding a ratio α/m, wherein α is the polarizability and m is the mass of the nanowire. Electric fields, magnetic fields, or mechanical forces, etc., may be used to generate forces which effect nanowires of different fractions in a different manner. Particularly, this may allow to separate nanowires of different length. An example would be the separation of semiconducting nanotubes and metallic nanotubes.

The apparatus may comprise a fluidic stream manipulation unit adapted for manipulating a trajectory of the fluidic stream with the nanowires. For instance, such a fluidic stream manipulation unit may comprise a system of electrodes and/or laser fields for generating an electric force field (in particular a homogeneous field gradient) which acts on the (mostly neutral) nanowires. By any component which generates a force acting on the fluidic stream, it may be possible to direct the fluidic stream onto a dedicated surface portion of a deposition substrate and/or to align the molecules in the fluidic stream

The apparatus may comprise a deposition substrate carrier adapted for carrying a deposition substrate on which the nanowires of the fluidic stream are depositable. For example, it may be desired to form nanowires for nanoelectronic applications in a surface portion of a silicon substrate. Then, the silicon substrate (such as a silicon wafer or a silicon chip) may be mounted on the deposition substrate carrier, and the stream of nanowires may be directed on the various surface portions of the deposition substrate. To promote such a deposition, Moire methods may be applied, particularly for mapping nanowires onto a substrate via two gratings. The apparatus may comprise a preparation unit adapted for preparing the frozen body comprising nanowires in the apparatus. Such a preparation unit may allow to prepare a liquid matrix (for instance an aqueous matrix) in which the nanowires such as carbon nanotubes are embedded. This preparation and the corresponding freezing procedure to generate the frozen body from such a solution may be carried out within the apparatus, thereby preventing that impurities are introduced in the process.

The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to these examples of embodiment.

The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Fig. 1 illustrates an apparatus for handling nanowires according to an exemplary embodiment of the invention. Fig. 2 illustrates an absorption spectrum of a single walled carbon nanotube solution.

Fig. 3 illustrates a laser desorption setup according to an exemplary embodiment of the invention.

Fig. 4 illustrates Raman spectra from three different points on a substrate which was coated by nanotubes from the beam.

Fig. 5 illustrates high resolution transmission electron microscope images of small bundles of single walled carbon nanotubes on top of a special carbon substrate for electron microscopy.

Fig. 6 illustrates a three grating deflection setup according to an exemplary embodiment of the invention. Fig. 7 illustrates a longitudinal polarizability versus length and diameter of single walled carbon nanotubes.

Fig. 8 illustrates a predicted fringe pattern for semiconducting and metallic carbon nanotubes.

The illustration in the drawing is schematically. In different drawings, similar or identical elements are provided with the same reference signs.

In the following, referring to Fig. 1, an apparatus 100 for handling nano wires 101 according to an exemplary embodiment of the invention will be explained. The apparatus 100 comprises a laser 102 as an electromagnetic radiation source adapted for irradiating a frozen body 103 comprising a frozen aqueous solution 104 and individual carbon nanotubes 101 dissolved therein with a pulsed laser beam 105. Upon impinging of the laser beam 105 onto a surface of the frozen body 103, material of the frozen body 103 may evaporate so that nano wires 101 are brought above the surface of the frozen body 103.

Beyond this, the apparatus 100 comprises a fluidic stream generation unit 106 which generates a pulsed gas stream 107 of inert helium gas for carrying along the released nano wires 101 with the fluidic stream 107. The fluidic stream 107 travels, referring to Fig. 1 , from the left-hand side to the right-hand side, with supersonic velocity.

The components of the apparatus 100 are housed within a vacuum chamber 108 which can be evacuated by a pump 109 when a valve 110 is in a corresponding open state. Thus, the entire nano wire 101 handling procedure can be performed in a vacuum environment. The apparatus 100 comprises a copper substrate 111 for mounting the frozen body 103. A Dewar vessel 112 filled with liquid nitrogen 113 as a cooling agent is provided as a cooling reservoir coupled via a cool finger 124 to the thermally conductive copper substrate 111 to keep the frozen body 103 in a frozen state.

A capacitor formed by a first capacitor plate 114 and a second capacitor plate 115 may generate an electric field under the influence of which the charged particles within the fluidic packets 107 are deflected so as to be brought to a specific surface portion of a silicon substrate 116 on the surface of which carbon nanotubes 117 of a specific size are to be immobilized, for instance for a nanoelectronic application.

The present embodiment uses a capacitor 114, 115 for manipulating electrically charged nanotubes. However, other embodiments of the invention relate to the treatment of neutral nanowires (for instance comprising metallic nano wires and semiconductive nanowires). In such a scenario, the capacitor 114, 115 may be omitted. For separating electrically neutral particles, an electric field gradient may be needed for separating different fraction of the electrically neutral particles differing regarding a ratio of polarizability and mass. For such a purpose, electric field gradients, Moire methods, etc., may be employed.

Although not shown in Fig. 1, also the entire preparation of the frozen sample 103 can be performed optionally within the vacuum chamber 108 to avoid problems when transporting a sample from a separate preparation apparatus (not shown) to the apparatus 100.

Furthermore, Fig. 1 shows a control unit 118 such as a CPU or a computer which controls cooperation of the individual components of the apparatus 100, particularly of the fluidic stream generation unit 106, the laser 102, the capacitor plates 114, 115 and the pump 109 in combination with the valve 110. Beyond this, the control unit 118 may communicate bidirectionally with an input/output unit 120 via which a user may input commands for operating the apparatus 100. For example, the input/output unit 120 may comprise input elements such as buttons, a joystick, or a keypad. Furthermore, the input/output unit 120 may comprise output elements such as a display, for instance an LCD display or a cathode ray tube.

Next, a procedure according to an exemplary embodiment of the invention will be described in further detail.

Carbon nanotubes are carbon structures, which are of interest because of their high potential for nanotechnology applications. Their excellent mechanical and electrical properties make them ideal building blocks for nanomechanical devices, mechanical nanosensors and material reinforcement on the one hand as well as components for microelectronics, such as field effect transistors, field emitters in flat panel displays and diodes on the other hand. The large surface to volume ratio of carbon nanotubes also changes their electronic properties very sensitively in response to changes in the local environment, which is important for sensor applications.

Of particular interest are single walled carbon nanotubes which exhibit a strong dependence of their electronic properties (metallic, semiconducting) on the geometrical arrangement (chirality) of the carbon items in the rolled up graphene sheets which make up the tube structure. The sorting of nanotubes according to their chirality is still one of the biggest challenges for large scale applications of these materials.

Up to now, all separation, individualization, manipulation and sorting schemes with carbon nanotubes were using nanotube solutions.

One of the reasons for the lack of gas phase manipulation techniques for generating air-borne carbon nanotubes in the conventional art is the fact that nanotubes are large objects, typically more than 1 nm in diameter and often more than 1 μm in length, which bind tightly to each other and to any substrate.

Exemplary embodiments of the invention provide a method to carry carbon nanotubes from an iced matrix into the gas phase using laser desorption from a specially prepared frozen matrix.

Such a method can be distinguished into two main parts:

1. The chemical preparation

2. The laser desorption from an iced solvent matrix inside high vacuum. The combination of such components may allow for the volatilisation of solvated carbon nanotubes.

The chemical preparation of individualized tubes in solution may involve purchased HiPCO (High Pressure CO Synthesis) tubes from CNI, Houston/USA. The subsequent laser desorption method, an important aspect of exemplary embodiments of the invention, is expected to be independent on the specific carbon nanotube supplier and also of the tube synthesis method. The nanotubes are cleaned by the vendor with a purity of 75 % to contain only single walled nanotubes with a diameter of 0.8 nm to 1.2 ran and a length distribution between 100 run and 1000 ran.

Commercial carbon nanotubes are usually delivered as bundles, typically composed of 20 to 50 single nanotubes, bound together by their large van der Waals interaction. In order to individualize the tubes, the bundle solution may be exposed to intense ultrasound (for instance horn sonifier Branson 450, but any other one of similar power would do). The individual nanotubes are then stabilized in micelles by the solvent and additional surfactants. This prevents the nanotubes from rebundling when the ultrasound is switched off. After this procedure, the solution contains individual nanotubes and remaining bundles of different size.

These two fractions can be separated according to their density in an ultra centrifuge. This centrifugation step also significantly releases the content of amorphous carbon and other impurities left from the synthesis procedure.

One aspect of the chemical part is to experimentally find the best chemical synthetics for optimizing the output of individual nanotubes. For instance, it is possible to use NMP (N-methyl-2-pyrrolidinone, Sigma Aldrich) as a solvent and to sonify the samples at 80 W for 30 minutes for centrifuging the resulting samples for four hours at 30,000 imp (Sorvall OTD combi, swinging bucket rotor No. TH 641). The degree of individualization in the final solution can be monitored using absorption and photoluminescence spectroscopy.

Fig. 2 shows a diagram 200 having an abscissa 201 along which a wavelength is plotted. Along an ordinate 202, an optical density is plotted.

Fig. 2 shows an absorption spectrum of a single walled carbon nanotube solution, recorded by a Varian Cary 5G photo spectrometer. Then structural calculations allow the assignments of individual peaks to specific electronic excitations of distinct type of nanotubes. Absorption spectroscopy is therefore sensitive to different tube types in the solution.

Soft ultrasound may further decrease the mean length within the rather broad distribution of nanotubes. Such a method may be used to engineer the distribution of the tubes to 480 nm + 290 nm, for instance, identified by atomic force microscopy (AFM) of dried droplets of the solution on silicon substrates.

Next, laser desorption of individualized carbon nanotubes from an iced solvent will be explained in more detail. The solution of individualized single walled carbon nanotubes is frozen to ice on a copper target at a liquid nitrogen temperature (77 K). Cooling should be fast to avoid re-bundling of the individual nanotubes during the ice formation process. Ideally, the solvent ice crystal then contains fixed isolated nanotubes. The ice may be formed on a cold substrate in air within less than one second. It may then be rapidly transferred into a vacuum chamber. Alternatively, the preparation procedure may be performed within the vacuum chamber.

It may also be possible to avoid the slight contamination with residual dust particles. An in situ ice preparation state may be used for this purpose. The base pressure in the vacuum chamber during the experiments may be better than 10^"4 mbar and may be compatible with the free molecular beam over short distances.

A schematic picture of a laser desorption setup 300 is shown in Fig. 3. The cold copper target is attached to a linear drive and can be moved perpendicular to the direction of a supersonic expanding gas jet generated by a fast switching gas valve (valve 99, Parker, USA). The iced nanotubes are laser-desorbed from the top of the copper target. The target can be moved in contact to a cold finger to extend the lifetime of the ice matrix.

After evacuation of the chamber, the target may be locally superheated by a nanosecond laser pulse. For this purpose, a Nd:YAG-laser (Spectron) may be used with a pulse width of 5 ns, a wavelength of 1064 nm and about 20 mJ to 60 mJ pulse energy. The wavelength may be chosen to be close to a resonance line of NMP. The laser locally melts the ice and the emerging cloud of both free single walled carbon nanotubes and also remaining carbon nanotube bundles are carried away by a supersonic beam of noble gas atoms, for instance Argon items. The supersonic noble gas beam may be generated by a pulsed valve (Jordan Inc.), with a short opening time (300 μs) and a backing pressure of 3 bar. It may be important to avoid rebundling of the nanotubes. For this aim, the desorption time should be short, the pulse energy not too high and the freezing process fast.

Such a method has been demonstrated to work for very massive nanotubes desorbed from an NMP ice matrix. Thus, it has become possible to generate a beam of carbon nanotubes from individualized tubes in solution.

This has been demonstrated by depositing the free flying nanotube beam on a substrate 10 mm behind the desorption plume. The ideal matrix should evaporate essentially completely during laser heating, and only pure individual single walled carbon nanotubes remain in the gas phase. Fragments or intact NMP molecules which may also be deposited on the substrate surface may be expected to be sufficiently volatile to be desorbed by heating the substrate above the NMP boiling point of about 200°C after the deposition.

Next, experimental results will be explained.

Anumber of parameters have to be compared to characterize the nanotube beam. Those parameters are for instance the distribution of length, diameter, chirality as well as the number of defects of the tube, the remaining degree of bundling, the beam purity and also its charge.

To identify and characterize the deposited single walled carbon nanotubes, various complementary methods may be used. Some of them - as for instance atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and high resolution transmission electron microscopy (HRTEM) - are capable of imaging single molecules. Others - such as Raman spectroscopy, photoabsorption, photoluminescence spectroscopy and scanning electron microscopy (SEM) - provide integrated information obtained from larger sample areas. Raman spectroscopy is a sensitive method to explore the vibration spectrum of solid-state materials down to nanostructures and even single molecules. It has evolved into the most prominent technique for the identification and analysis of carbon nanotubes.

Fig. 4 is a diagram 400 having an abscissa 401 along which a Raman shift in cm^"1 is plotted. Along an ordinate 402, a intensity is plotted in arbitrary units. Fig. 4 therefore shows Raman spectra of deposited material. The presence of nanotubes is identified by the existence of the line associated with their radial breathing mode (RBM).

More particularly, Fig. 4 shows Raman spectrum from three different points on the substrate which was coated by nanotubes from the beam. They are shown in comparison with a bulk sample of HiPCO carbon nanotubes. The typical G-mode and the radial breathing mode (RBM) are shown, which indicate the existence of single walled carbon nanotubes. The excitation wavelengths are at 488 ran and 515 run. Information regarding the metallicity/chirality of the nanotubes are also available using this method. The influence of the number of defects, the effective rebundling and the nanotube length on electronic and vibration properties of single walled carbon nanotubes may be systematically investigated by samples prepared by gas phase technique. Also, possible aerodynamic effects on the tubes alignment are possible.

High resolution transmission electronic microscopy (HRTEM) can also be used to identify the tubes as being single walled instead of multi walled (see Fig. 5c) and microscopy also be performed to obtain some information on the length distribution and the alignment of the nanotubes on the surface. The HRTEM images still show many bundles but also some isolated nanotubes. Better de-bundling may be made possible by optimization of the chemical preparation step.

Fig. 5a to Fig. 5c show high resolution transmission electron microscope of small bundles of single walled carbon nanotubes on top of a special carbon substrate for electron microscopy. Different concentrations of small bundles of single walled carbon nanotubes can be seen in Fig. 5a to 5c.

Fig. 5a shows smaller clusters of carbon nanotube bundles.

Fig. 5b shows a larger field of view at a magnification of a factor of 1500.

Fig. 5c shows a loose network of smaller bundles and clearly indicates single walled nanotubes. The nanotube solution was not centrifuged. The images of Fig. 5a to 5c were taken at 80 kV. Also the purification of deposited single walled carbon nanotubes is an important feature. The HRTEM images show a lot of circular particles in addition to the nanotubes. They should be suppressed.

The fraction images taken inside the HRTEM indicate that the observed structures are made of carbon, since the characteristic bundle length correspond to those of single (C-C) and double (C=C) carrier bonds.

A molecular beam of carbon nanotubes may have several advantages over tubes and solutions. It can be coupled to ultra-high vacuum preparation methods, using a second differential pumping stage. Any attempt to combine carbon nanotubes with clean reconstructed silicon surfaces such as the Si 111 (7x7) surface which only survives at ambient pressures below 10^"9 mbar, may profit from the method. Solvent- based methods necessarily contaminate any surface with the solvent. This is expected to become particularly relevant for molecular electronic applications. Such a system may also allow the characterization of the tube's optical and electronic properties in free flight, that is to say without any perturbing environment. It may permit the steering and switching of the molecular flux by electric fields in a new nano- patterning scheme. Molecular beams of nanotubes are also interesting for experiments within the realm of basic physics: new molecular cooling sheet schemes, trapping methods and quantum interference experiments are possible, based on the extreme polarizability and the large anisotropy of polarizability in carbon nanotubes.

The surface deposition of carbon nanotubes has many applications. Coating of thin carbon nanotube films or arrays of individualized nanotubes may be used for optical limiting (light protection systems), solar cells on top of windows, thin film heaters, conducting thin film against static surcharges, light emitting films, optical polarizers, catalysts (heterogen catalysis, polyacetylene), non-linear optics (high harmonic generation), information storage elements, linear optical elements (polarizes), semiconducting nanotubes (single nanotube field effect transistors, single molecule light emitting diodes, nanotube light sensors, fluorescence markers in cells or for pharmacological applications, thin film light emitting diodes), metallic nanotubes (spintronics with freely hanging single walled carbon nanotubes, metallic nanowires, wind screen heating, wiring of semiconducting carbon nanotubes, cantilevers in particle mass sensors, gas sensors).

The solvent-based preparation may be modified for unbundling/separation, shortening/cutting, or evaluation of the influence of the surfactant/solvent. The desorption process may be altered, for instance by coupling the nanotubes into the expending gas jet, by providing different desorption colours and intensities, and by providing the purity of the seed gas/clean gas system. The matrix may be improved by modifying cooling for the iced matrix (in situ preparation may reduce the possibility of contamination). The detection schemes may be extended. It may be made an attempt to F2 post ionisation and time of flight mass spectroscopy on very short tubes, deflectrometry on short tubes, quartz balance, nano-cantilevers, etc. Further, the dissolved tubes may be characterized, for instance regarding beam distributions, intensities and velocities. It may be detected how many defects are introduced, and how they can be healed, the length distribution, and the degree of unbundling. A detailed analysis of the beam parameters may be performed, for instance regarding velocity distribution, variations of tube types across the beam, absolute beam intensity, etc. Furthermore, the gas phase nanotubes may be oriented and manipulated using laser fields, using shallow deposition of nanomechanical gratings, using electrodes, etc. Next, gas phase sorting of nanoparticles will be explained.

Stark deflectometry may be applied on micro-modulated molecular beams for the enrichment of biomolecular isomers and single walled carbon nanotubes. The molecular sorting may be based on the species-dependent polarizability-to-mass ratio α/m. Such a device is compatible with a high molecular throughput, and the spatial micro-modulation of the beam permits to obtain a fine spatial resolution and a high sorting sensitivity.

The manipulation of large molecules in the gas phase is of interest. Since many large particles, among them biomolecules or carbon nanotubes, exist in various different isomers and conformations at equal mass, it is intriguing to provide sorting methods in the gas phase which select the particles according to their polarizability- to-mass ratio α /m.

Classical deflection experiments may employ the deflection of a well- collimated neutral beam in the presence of a static inhomogeneous electric field. In this case, it is possible to chose between a wide molecular ray of high throughput or a narrow beam with a lower total signal whose shift can be determined with higher precision.

According to an exemplary embodiment of the invention, a method for molecular beam sorting is provided which combines high transmission and high resolution. This can be achieved by imprinting a very fine spatial modulation onto the molecular beam.

Our starting point is a three-grating matter- wave interferometer 600 as shown in Fig. 6.

The interferometer 600 is composed of three micro-machined gratings 601 to 603, which prepare, sort and detect the molecules. The combination of the first two gratings 601 , 602 modulates the particle flux such as to generate a periodic particle density pattern in the plane of the third grating 603. All gratings 601 to 601 and also the molecular micromodulation have identical periods. The density pattern can therefore be revealed by scanning the third grating 603 while counting all transmitted molecules, as shown in Fig. 8. The device 600 is usually operated in a quantum mode, with molecular masses and velocities chosen such as to reveal fundamental quantum phenomena related to matterwave diffraction at the second grating 602. Deflection by a deflector 605 can be taken from an axis denoted with reference numeral 604.

However, the same device 600 can also be used in a Moire or shadow mode, where the molecules can be approximated by small classical particles. This situation applies in particular to fast and very massive molecules where quantum wave effects may be too small to be observed. It then still combines a high molecular flux with a fine spatial micro-modulation, which allows to increase the detector resolution in any beam-displacement measurements by several orders of magnitude over experiments without micro-imprint. A beam-displacement may for instance be caused by an inhomogeneous electric field acting on the molecular polarizability. In the setup of Fig. 6, a pair of electrodes close to the second grating may generate a constant force field F_x = α(E V )E_X, which shifts the molecular fringe pattern along the x-axis by As_x cc (a I m) • (EN)E_x I v_y ² . Here v_y is the beam velocity in the forward direction.

Deflection measurements in external fields then allow to derive precise values for the polarizability of the molecules. The operation of the deflectometer is performed in a classical Moire mode with biomolecules and carbon nanotubes.

To first illustrate the sorting idea for biomolecules we discuss and simulate the enrichment of a 50:50 mixture of the tripeptide Tryptophan-Glycin-Tyrosin (YGW) and its isomer YWG which differ only by the swapped position of Glycin and Tryptophan in the amino acid sequence. Their masses are equal (m=460 u) but their susceptibility differs between χ(YWG) = 100 A³ and χ(YGW) = 480 A³. The equation χ = α + <μ_z >/(k_BT) is used, where T is the molecule temperature and <μ_z > is the orientation averaged square of the projection of the electric dipole moment onto the direction of the external field. The α in the previous force field formula can then be simply replaced by χ. The molecular fringe shifts of both isomers will differ by a factor of five, if all other beam parameters are equal. Therefore, when the device 600 is designed for maximum fringe contrast, it may be possible to chose the electric field such that one molecule will be transmitted by the deflectometer 600 while its isomer will be deposited on the third grating 603. The transmitted beam will then reveal a significant enrichment of one particular isomer. To quantify the sorting process, it is possible to define the maximal enrichment of two mixed species Pl and P2

η= max|_x {S_P1(x)-Sp₂(x)}

where S(x) = S(X)Z[S_m3x(X)+ Smm(x)] is the respective normalized signal of each Moire curve (see Fig. 8), and x is the position of the third grating 603. For small polypeptides, such as those treated here, the combination of a pulsed beam source with a pulsed laser detection, may allow to select a mean velocity of v_y=300 m/s with a relative spread of Δv_y/v_y = 0.5%. Assuming a grating separation of L = 38.5 cm, a grating constant of 990 ran, and a grating opening fraction of f = 0.2, i.e gap openings which are about 200 nm wide, a relative enrichment for YWG as high as η = 0.97 is found. Here, the voltage has been optimized to (EV)E^ = 2.1 • 10¹³ V² 1 nv" in order to maximize the transmitted content of this isomer. This field can be generated between two convex electrodes at a difference potential of U= 15 kV, which are spatially separated by a minimum distance of 4mm. Also the preparation and selection of carbon nanotubes with a defined internal structure is a technological challenge. The deflectometer 600 differs from earlier conventional methods in that it is vacuum compatible and therefore better suited for a certain class of technological applications. hi the following, a free molecular beam of single walled carbon nanotubes (SWCNTs) with a length distribution between 50 nm and 150 nm, an arbitrary mixture of chiralities and diameters between 0.7 nm to 1.3 nm is assumed.

To simulate the Moire fringes for nanotubes, their α/m has to be determined. Their mass can be computed from the number of carbon atoms per unit cell. The static polarizability of nanotubes is extremely anisotropic and it should be considered separately both the transverse and the longitudinal value per carbon atom, i.e. the reduced polarizabilities. The reduced transverse static polarizability of a carbon nanotube is independent of its metallicity but proportional to its radius R. For SWCNTs it can be approximated by a_Lred « 1.3 A /atom, a value very similar to that of C60 or medium-sized alkali clusters. The longitudinal polarizability of semiconducting tubes a^_s depends on their band gap energy E_g according to α_||s ∞ (R / E²) At is possible to use α_||s « 8.2R² +

20.5 for R > 0.35 nm. Even for semiconducting SWCNTs the reduced longitudinal polarizability thus exceeds already the transverse value by about a factor often and the polarizability of medium-sized metal clusters by about a factor of two. This relation for a_p can not be applied to metallic tubes because of their vanishing band gap, E_g = 0. Therefore, short metallic tubes of length 1 may be approximated by perfectly conducting hollow cylinders, and it may be found for their axial polarizability

/³ .. 4/3 -ln(2), a,,_m = (1 + — ) l^|m 24(ln(// Λ) -l In(I / R) -V

This value exceeds that of equally long semiconducting SWCNTs by a factor between ten and one hundred. hi Fig. 7, the reduced polarizabilities for a range of different tube diameters and lengths is plotted. Fig. 7 shows a diagram 700 having a first abscissa 701 along which a SWCN length is plotted, having a second abscissa 702 along which a SWCN diameter is plotted, and having an ordinate 703 along which the longitudinal polarizability per atom is plotted. The clear separation between metallic tubes (reference numeral 704) and semiconducting tubes (reference numeral 705) in the diagram 700 indicates that mixtures of these species will be separable in a Moire deflection experiment.

The reduced longitudinal polarizability of semiconducting tubes does not scale with the tube's length, since both their mass and their polarizability grow linearly with it. It is believed that the separation process will therefore also work for nanotubes beyond the given parameter range of Fig. 7.

With the masses and polarizabilities at hand, it may be proceeded to simulate the Moire fringe pattern.

Fig. 8 shows a diagram 800 having an abscissa 801 along which a position of the third grating 603 is plotted. Along an ordinate 802, a signal is plotted in arbitrary units. Fig. 8 further shows a diagram 810 having an abscissa 801 along which a position of the third grating 603 is plotted. Along an ordinate 803, a signal is plotted in arbitrary units.

In Fig. 8, simulations for two 100 run long semiconducting (17,0) and metallic (9,0) nanotubes are shown flying at 100 m/s with a velocity spread of Δv_y/v_y = 1% through a setup with metallic gratings separated by L=38.5 cm. The grating period is set to g = 10 μm and the open fraction to f=0.2, which would permit a fringe contrast of 100%, if the interaction with the gratings were negligible. The semiconducting tube is computed to have a diameter D=I .33 nm, m=3.2 x 10^"22 kg, (X_x = 2.6 x 10⁴ A³ and α, = 3.8 x 10⁵ A³. The metallic tube has D=0.71 nm, m=1.7 * 10^~22 kg, a_L = 9.5 x 10³ A³ and α,, = 1.1 x 10⁷ A³. It may be assumed that all nanotubes are (maximally) aligned with respect to the external electric force field, i.e. along the x-axis. At a deflection field of (EV)E, = 1.4 • 10¹² V² 1 m* , the metallic tube's fringe shift of 5200 nm would largely surpass the 150 nm shift of the semiconducting molecules. And it is easily possible to find a voltage that will enrich the metallic tubes in the beam by shifting their fringe maxima until they fall onto the openings of the third grating, while the semiconducting tubes will be blocked by the grating bars. In this idealized picture the enrichment could reach almost 100% (diagram 800, dashed and dotted curves).

This simple model may be extended to include the attractive Casimir-Polder potential between the aligned molecules and the grating walls in its approximation for long distances r: U(r) = -(3 h cα/8πr⁴). In addition, any molecular beam of SWCNTs will be in a highly excited rotational state. Each orientation is associated with a different fringe shift and the full line in diagram 810 is an average of all Moire curves including a full rotational distribution function. The expected fringe visibility then still amounts to 77% for the semiconducting (17,0) tubes and to 31% for the metallic (9,0) ones. As can be seen from diagram 800 this allows a significant enrichment of the metallic tubes. The predicted enrichment reaches η(17, 0) = 0.4 for the semiconducting and η (9, 0) = 0.6 for the metallic nanotube. It is interesting to see that the reasoning still holds even if including other chiralities of metallic and semiconducting tubes.

Concluding, α/m-variations can be used to sensitively sort different neutral molecular species from a wide molecular beam. The simulations show that the enrichment may be get close to 100% in the sorting of biomolecular isomers and will still be significant (about 60%) for single walled carbon nanotubes.

It should be noted that the term "comprising" does not exclude other elements or features and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined.

It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

C l a i m s

1. An apparatus for handling nanowires, the apparatus comprising an electromagnetic radiation source adapted for irradiating a body comprising nanowires with an electromagnetic radiation beam to thereby evaporate at least a part of the body so that at least a part of the nanowires are released from the body; a fluidic stream generation unit adapted for generating a fluidic stream for carrying the released nanowires along with the fluidic stream.

2. The apparatus of claim 1, wherein the electromagnetic radiation source is adapted for irradiating the body with a pulsed electromagnetic radiation beam.

3. The apparatus of claim 1 or 2, wherein the electromagnetic radiation source is a laser.

4. The apparatus of any one of claims 1 to 3, wherein the fluidic stream generation unit is adapted for generating a gas stream.

5. The apparatus of any one of claims 1 to 4, wherein the fluidic stream generation unit is adapted for generating a pulsed fluidic stream.

6. The apparatus of any one of claims 1 to 5, wherein the fluidic stream generation unit is adapted for generating the fluidic stream having a supersonic velocity.

7. The apparatus of any one of claims 1 to 6, wherein the fluidic stream generation unit is adapted for generating a fluidic stream comprising or consisting of an inert substance.

8. The apparatus of any one of claims 1 to 7, comprising a vacuum chamber in which at least a part of the components of the apparatus are accommodated.

9. The apparatus of any one of claims 1 to 8, comprising a mounting substrate for mounting the body.

10. The apparatus of claim 9, comprising a cooling reservoir thermally coupled to the mounting substrate for cooling the body when being mounted on the mounting substrate.

11. The apparatus of any one of claims 1 to 10, comprising a sorting arrangement adapted for sorting the nanowires in the fluidic stream regarding at least one sorting criteria.

12. The apparatus of claim 11, wherein the at least one sorting criteria comprises at least one of the group consisting of an electric charge, a mass, a dimension, an electrical conductivity, a thermal conductivity, a polarizability, dielectric properties, a chirality, a molecular surface adsorption, a magnetic property, mechano-elastic properties, optical absorption and dispersion properties, surface specific reactivity in the fluidic stream, and a combination of these sorting criteria or any other physical, chemical and/or biological property.

13. The apparatus of claim 11 or 12, wherein the sorting arrangement is a Stark deflectometer.

14. The apparatus of any one of claims 1 to 13, comprising a fluidic stream manipulation unit adapted for manipulating a trajectory of the fluidic stream with the nanowires.

15. The apparatus of any one of claims 1 to 14, comprising a deposition substrate carrier unit adapted for carrying a deposition substrate on which the nanowires of the fluidic stream are depositable.

16. The apparatus of any one of claims 1 to 15, comprising a preparation unit adapted for preparing the body comprising nanowires in the apparatus.

17. A method of handling nanowires, the method comprising irradiating a body comprising nanowires with an electromagnetic radiation beam to thereby evaporate at least a part of the body so that at least a part of the nanowires are released from the body; generating a fluidic stream for carrying the released nanowires along with the fluidic stream.

18. The method of claim 17, wherein the body is one of the group consisting of a frozen body, a solid body, a liquid body, a gel body, a viscous body, and a partially defrosted body.

19. An apparatus for sorting nanowires, the apparatus comprising a nanowire beam generation unit adapted for generating a fluidic stream comprising nanowires; a three-grating defiectometer adapted for sorting the nanowires into different fractions of nanowires.

20. A method of sorting nanowires, the method comprising generating a fluidic stream comprising nanowires; sorting the nanowires into different fractions of nanowires by a three-grating defiectometer.

21. A propagating fluidic stream, comprising a carrier fluid and nanowires carried along with the carrier fluid.