WO2008028521A1 - A probe, a raman spectrometer and a method of manufacturing a probe - Google Patents

Abstract

Silicon nanowires grown by e.g. the vapor-liquid-solid (VLS) mechanism with gold as the catalyst show gold caps atop -20 nm - 500 nm in diameter with an almost ideal half- spherical shape. These gold caps are extremely well suited to exploit the tip- or surface enhanced Raman effects. Attaching a nanowire with gold cap to an AFM-tip the signal enhancement by the gold nanoparticle can be used to spatially resolve a Raman-signal. Using an ensemble of nanowires as a SERS- template, that grow self- organized, bottom-up on a silicon substrate, highly sensitive signal enhanced Raman spectroscopy is feasible of all materials that show a characteristic Raman signature. A combination of a nanowire-based TERS-Probe and a nanowire-based SERS-substrate promises optimized signal enhancement so that the detection of even single molecules (e.g. of explosives, poisonous gases,...) or single bacteria, DNA strands, and other soft matter is in reach. Potential applications of this novel nanowire based technical SERS- and/or TERS solution are widespread and lie in the fields of bio-medical and life-sciences as well as security and in the field of solid state research e.g. in silicon technology where the detection of materials composition, doping, orientation and lattice strain can be probed by Raman spectroscopy, now using TERS with the spatial resolution of the nanowire based AFM-tip.

Description

A probe, a Raman spectrometer and a method of manufacturing a probe

The present invention relates to a probe, a Raman spectrometer and a method of manufacturing a probe.

Raman spectroscopy is a versatile tool which has been in use for decades to perform materials research in a variety of different fields, such as molecular chemistry, life sciences, archeology, and solid-state physics. However, the lateral resolution of conventional micro-Raman spectroscopy is limited by the diameter of the focused laser spot on or in the probed sample and is usually at best in the range of ~ 1 μm. To overcome this resolution limit, recently attempts were made to make use of near field optical information during Raman spectroscopy. As a result the near-field scanning optical microscopy (NSOM) -Raman or the so-called tip enhanced Raman spectroscopy (TERS) technique arose. This technique is, for example, described in the papers by W.X. Sun and Z.X. Chen in Mater. Phys. Mech. 4, 17 (2001), by B. Hecht, B. Sick, U.P. Wild, V. Deckert, R. Zenobi, O.J.F. Martin and D.W. Pohl in J. Chem. Phys. 112, 7761 (2000) by W.X. Sun and Z.X. Chen in J. Raman Spec. 34, 668 (2003) and by M.S. Anderson, in Appl. Phys. Lett. 76, 3130 (2000)

The TERS technique provides spatially selective enhancement of a Raman signal using the surface enhanced Raman scattering (SERS) effect as mentioned in the above referenced paper by M.S. Anderson and in the paper by B. Pettinger, B. Ren, G. Picardi, R. Schuster and G. Ertl in Phys. Rev. Lett. 92, 096101(2004). The SERS effect exploits a property of nanometer sized metal particles or surface grains and relies on the geometry of the metal particles as described in the papers by R.G. Milner and D. Richards, in the Journal of Microscopy 202, 66 (2001), by M. Fleischmann, P. J. Hendra, and A. J. McQillan in Chem. Phys. Lett. 26, 163 (1974) and by D. Zeisel, V. Deckert, R. Zenobi and T. Vo-Dinh in Chem. Phys. Lett. 283, 381 (1998).

Incident laser photons are absorbed into the metal particles through oscillations of surface electron charge density (plasmons). The plasmon radiation can couple e.g. with molecules or crystals in close proximity and provide an efficient pathway to transfer energy to the molecular vibrational modes, and generate Raman photons that can be detected. The enhancement is maximized when the metal grains are smaller than the incident laser wavelength, the metal has the optical properties to generate surface plasmons and an optimized geometry is available as described by M. Mi- cic, N. Klymyshyn, Yung Doug Suh and H. P. Lu in J. Phys. Chem. B 107, 1574 (2003).

The greatest enhancements are observed with silver, gold, and copper with grain diameters between 10 and 200 nm. In addition to an electromagnetic field enhancement, that contributes the most to the signal enhancement, there is an additional chemical enhancement that occurs when a molecule coordinates with the metal particle surface and forms charge transfer states with the energy levels of the metal. This results in a charge transfer transition in the visible wavelength region and a surface localized resonance Raman enhancement. Recently, enhancement factors of 10⁸- 10¹⁴ have been reported with single molecule detection of molecules absorbed on a silver substrate as described for example by K. Kneipp, H. Kneipp, I, Itzkan, R. R. Dasari, M. S. FeId and M. S. Dresselhaus in Topics Appl. Phys. 82, 227 (2002).

The object underlying the present invention is to provide an improved probe and an improved Raman spectrometer as well as improved methods of making probes which provide improved probes and performance of Raman spectrometers incorporating them and also make it possible to broaden the field of applicability of Raman spectroscopy in at least the following two directions:

(i) enhance the sensitivity for the detection of single molecules, bacteria, DNA, viruses, explosives and other biological agents through optimizing the SERS effect and

(ii) to provide for lateral resolution on the nanometer scale through applying TERS.

In order to satisfy these objects there is provided a probe having at least one working tip and a support for the working tip, the working tip projecting from the support and comprising an elongate semiconductor shaft having cross-sectional dimensions in the nanometer range and attached at a first end to said support and having a second free end, and a droplet of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as silicon, GaAs and others provided at said second end.

Probes in accordance with the present invention can either have a single working tip, or a plurality of working tips, with a probe with a single working tip either being used in juxtaposition with a surface or with a probe having a plurality of working tips. Furthermore, probes are possible hav- ing opposed working tips as will be later explained in more detail. Preferred embodiments of probes are set forth in the subordinate claims.

Furthermore, there is provided a Raman spectrometer comprising a sample holder, a laser for generating a laser beam for projection onto a sample in said sample holder, a laser for generating a laser beam for projection onto a sample in said sample holder, a detector for a Raman spectrum from the sample and a probe having a working tip as described above, as well as a system for producing relative movement in three dimensions between the working tip of said probe and the sample holder.

In addition there is provided a method of manufacturing a probe comprising the steps of:

- providing at least one droplet of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as silicon, GaAs and others, on a support in air, in an inert atmosphere and/ or in an evacuated chamber, if necessary to preclude undesired reactions such as the oxidization of aluminum,

- placing the support in an evacuated chamber or retaining it in the evacuated chamber from the previous step, if applicable,

- providing a material comprising a semiconductor in the chamber,

- carrying out a PVD (physical vapor deposition) or CVD (chemical vapor deposition) process to enable growth of said semiconductor material in said chamber on said support at an interface with the or each said droplet and

- maintaining a temperature in the chamber at which the or each said droplets of material is liquid or solid in the presence of said semiconductor material and the semiconductor material grows into one or more nanowires, each being attached at one end to said substrate and having a respective droplet attached to a free end thereof.

It is noted that the method specified above can be carried out as a so- called VLS process (vapor-lquid- solid) or as a VSS process (vapor-solid- solid) or as an SLS process (solid-liquid-solid).

There are at least two different ways of carrying out this method. In the first the support is a part of a probe, e.g. a cantilever, with a said droplet provided thereon and said nanowire grows directly from a droplet provided on said part of said probe, e.g. on the cantilever.

In a second alternative, the support is a substrate having a plurality of droplets provided thereon and the method includes the further steps of

- detaching at least some of said nanowires from said substrate and

- bringing a part of a probe, e.g. a cantilever, into the vicinity of a selected nanowire and

-attaching an end portion of the probe body to a free end of a nanowire remote from the attached droplet to form said working tip.

In this method at least one of said part of the probe and said substrate is conveniently incorporated in a handling device for relative coordinate movement in at least two dimensions, e.g. in a handling device constructed as an atomic force microscope or a handling device in the form of piezoelectric actuators operative to move the said part of the probe and the substrate relative to one another in at least two different coordinate directions. The detaching step then comprises effecting relative movement of said part of the probe and said substrate by said handling device to break said nanowires at a position intermediate said substrate and said droplet. The handling device can then be used to maneuver the part of the probe relative to said substrate into a position close to a free end of one of the broken off nanowires that is in an accessible position and has the desired dimensions and to subsequently attach the free end of the nanowire to the said part of the probe to form the working tip.

The attaching step conveniently comprises forming an initial weaker attachment between an end portion of said part of the probe and said free end of said nanowire, for example by the operation of Van der Waals forces or adhesive forces, and subsequently forming a stronger attachment by a welding or reinforcing process, for example by using an electron or ion beam directed close to the point of attachment and simultaneously either directing a beam of material to the point of attachment to react with the ion or electron beam and deposit material on the structure at the point of attachment or by using the ion or electron beam to cause reactions with, e.g., carbonaceous species present in the atmosphere of the machine .

The substrate can be configured to support an array of nanowires each having a said droplet at its end remote from the support.

For example, the substrate can be prepared, e.g. by lithography to define a trench having opposing sides, which optionally diverge in a direction going from a base of the trench to a free surface thereof and wherein said droplets are formed on said opposing sides of said trench and said nanowires are subsequently grown from said opposing sides to produce at least one pair of generally oppositely disposed droplets positioned in immediate proximity to each other. This pair of generally oppositely disposed droplets can then be used in a Raman spectrometer for TERS and SERS. In an alternative, the array of nanowires can be grown on a planar or otherwise configured surface of the substrate and can be used with a probe having a working tip comprising a nanowire with a droplet provided at the end thereof which is moved, by relative movement between the probe and the substrate, into juxtaposition with a selected nanowire and droplet provided on said substrate. In such an arrangement the working tip is preferably a working tip in an atomic force microscope, e.g. formed as the working tip of a cantilever in an atomic force microscope. This arrangement has the advantage that well established designs of atomic force microscopes can be used to ensure reliable positioning and movement of the working tip and the associated readout.

In the methods in which an array or plurality of nanowires are manufactured, this can be done by forming the droplets either by depositing a layer of the respective material on said support and heating the support to cause the layer to split into droplets or by lithographically treating said layer to provide islands of said material and subsequently heating said support to cause said islands to melt and form said droplets. If only a single droplet is required, e.g. at the free end of a cantilever, then either a local deposit of the material of the droplet can be effected or a larger area deposit can be provided which is then lithographically restricted, e.g. by suitable masking and etching to the desired area. The desirable rounded form of the droplet then results by heating of the support and droplet.

Thus, the solution of the object set out above is achieved by making use of the so called vapor liquid solid (VLS) mechanism, i.e. on the demonstration, that a nanowire can be grown, for example, from a gold catalyst. The VLS mechanism per se is described in the papers by S. Wagner and W. C. Ellis in Appl. Phys. Lett. 4, 89 (1964), by E. I. Givargizov in J. Cryst. Growth 31, 20 (1975).

The recognition that such nanowires with a metallic or quasi-metallic droplet at the end are ideally suited in terms of size and shape to provide spatially selective and strong enhancement of a Raman signal using the SERS effect is an important recognition of the present teaching.

As noted above it has previously been demonstrated that gold or silver coated atomic force microscopy (AFM)- or scanning force microscopy (STM)-tips show the TERS-effect. However, these tips have to be metal coated prior to their use, the shape of the tips is unpredictable and rarely as sharp as only 10 nm or a few 10's of nm in diameter.

In contrast, the present teaching makes use of the self-organized growth of a silicon nanowire by the VLS mechanism from a gold catalyst to serve as a perfect TERS-probe with an ideal spherically shaped gold (Au)-cap at the tip of the nanowire and diameters usually in the range of 10 nm - 500 nm, which therefore lie within the sensitive range for field enhancement.

One aspect of the present concept consists of providing a modified standard AFM -cantilever with a solidified Au-cap of a silicon nanowire at the very tip. The nanowire can either be grown directly on the cantilever by in- place, bottom-up VLS growth on a positioned gold catalyst particle or a nanowire can be grown by the VLS mechanism on a substrate and can be removed from there and can be 'post-growth' welded onto an AFM-tip using, for example, an electron beam in a scanning electron microscope (SEM) to weld with carbon species that naturally reside within the SEM or using a focused ion beam (FIB) machine to weld e.g. with WC3, platinum, or gold materials available for deposition in these ion beam machines. One can then use the 'nanowire-AFM-tip' in a standard AFM, which is optically coupled with a micro-Raman spectrometer to perform TERS.

The present invention will now be described in more detail by way of example and with reference to the accompanying drawings in which are shown:

Fig. 1 an SEM micrograph of a part of a probe in the form of a cantilever of an AFM having a working tip in accordance with the present invention,

Fig. 2 an SEM micrograph showing statistically distributed gold droplets on a Si(I I l) substrate generated during the course of manufacturing a probe in accordance with the present invention,

Fig. 3A an assembly of nanowires grown by electron beam evaporation (EBE - a PVD technique) at 650⁰C heater temperature at 8OmA evaporation current and 1.2nm sputtered gold on a pSi(l l l) substrate,

Fig. 3B as an alternative to Fig. 3A, a dense coverage of thin nanowires on a (111) Si substrate produced by a CVD process using silane,

Fig. 4 an SEM micrograph of part of the surface of the SEM micrograph of Fig. 3 to an enlarged scale, Fig. 5 an SEM micrograph illustrating nanomanipulation of an AFM tip to break off some of the nanowires on the surface of the substrate of Fig. 4,

Fig. 6 an SEM micrograph similar to Fig. 5 but showing the AFM tip being brought into contact with a free end of a broken off nanowire by nanomanipulation,

Fig. 7 an SEM micrograph subsequent to Fig. 6 showing the nanowire being moved into the desired position by nanomanipulation of the AFM tip,

Fig. 8 an SEM micrograph showing the subsequent welding of the nanowire to the AFM tip,

Fig. 9 a Raman spectrometer configured in accordance with the present invention,

Fig 1OA a schematic diagram of an ultra-sensitive SERS-TERS set-up that allows spatial resolution in Raman spectroscopy on the nanometer scale,

Fig. 1OB an enlarged view of part of Fig. 1OA,

Fig. HA an SEM micrograph of a lithographically realized trench of 600 to 800 nm width in an SOI wafer after reactive ion etching using CHF3,

Fig. HB an SEM micrograph showing nanowire growth from adjacent surfaces to the middle of the trench of Fig. HA after gold deposition and subsequent annealing, the nanowire growth being effected by electron beam evaporation of silicon, Fig. 12 a schematic diagram of a working tip of an alternative probe in accordance with the present invention and

Fig.13 a schematic diagram illustrating one possible use in accordance with the invention of a probe in accordance with the present invention.

Turning first to Fig. 1 there can be seen a conventional AFM tip 10 in the form of a cantilever 12 having a vestigial working tip 14 projecting from it. However, instead of the vestigial working tip 14 ending in a more or less sharp point in this case it ends - within the circle 16 - in a shaft 18 in the form of a nanowire having a hemispherical droplet of gold 20 at its free end 24. The area within the circle is difficult to see with the scale of the micrograph in Fig. 1, however it can clearly be seen in the micrograph of Fig. 8 where the relevant reference numerals have also been entered.

Thus, the probe has a working tip 14 and a support 12 for the working tip. The working tip 14 projects from the support 12 and comprises an elongate semiconductor shaft 18 having cross- sectional dimensions in the nanometer range and attached at a first end 22 (Fig.8) to said support and having a second free end 24 at which a droplet of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as silicon, GaAs and others is provided.

In this case the shaft 18 is a substantially cylindrical shaft in the form of an Si-nanowire. The droplet 20 has a hemispherical shape with a circular base, said circular base being attached to the shaft 18 at its second free end 24. The cross-sectional dimensions of the shaft 18 are in the range from 10 nm to 500 nm and preferably from 20 to 100 nm. The nanowire does not have to be of silicon it could also be a nanowire of another semiconducting material, e.g. selected from the group comprising silicon, SiC, germanium, alloys of elements belonging to groups III and IV of the periodic table of elements and group Ill-nitrides and II-IV semiconductors.

The droplet 20 is a self-organised crystal structure of hemispherical shape and the working tip has an aspect ratio of shaft length to shaft diameter of preferably less than 100 and especially less than 10. These dimensions relate to the shaft extending from the joint 22 to the remainder of the working tip 14.

Before proceeding to a description of the use of the above probe in a Raman spectrometer the methods by which it can be manufactured will now be explained in some detail.

The silicon nanowire used for the shaft 18 was selected from a plurality of silicon nanowires grown by electron beam evaporation (EBE) following the vapor- liquid- solid (VLS) growth mechanism. We note that the VLS growth mechanism is described in the document "Growth peculiarities during vapor-liquid-solid growth of silicon nanowhiskers by electron-beam evaporation" published in "Applied Physics A: Materials Science and Processing" which can be viewed at "http://dx.doi.org/ 10, 1007/s00339-006-3675-0". The EBE system used in this work has a commercial pumping system (RIBER, France). The cylindrical main chamber (volume ca. 200 1) is a bell- jar system with a small loadlock chamber (volume ca. 30 1) attached to it. The samples are transferred via a loadlock at a pressure of 4x10^~6 Torr. In EBE, a PVD process, the nanowires grow from atomic silicon species that are generated by evaporation using an electron beam from a high resistiv- ity silicon target. For evaporation a beam current of e.g. 80 mA is used. However, a whole range of currents, e.g. between 35mA and 100 mA, lead to VLS growth of nanowires. The silicon atoms from the vapor are incorporated in the liquid Au-Sϊ-eutectic alloy on the substrate surface. At super- saturation, the silicon crystallizes and the nanowires start to grow. During growth, the Au-caps stay on top of the nanowires and during solidification, perfect Au half-spheres form, i.e. the droplets such as 20 in Fig. 8. The diameter of the nanowires is determined by the size of the Au-cap which itself depends on statistics and on the thickness of the sputtered Au-starting layer. This process can be used with any semiconductor material and any suitable metal or quasi-metal that forms a eutectic alloy with the semiconductor material.

The nanowires used for the SERS-TERS-experiments were grown within 1 hour at 650 ⁰C and at a chamber pressure of 2-5 x 10-⁷ Torr. The growth rates were between 20-50 nm/min. For the EBE deposition of nanowires substrates in the form of pieces (25 mm x 25 mm) of p-type (l l l)-Silicon wafers were used. Initially, a continuous Au-layer is deposited on the substrate by sputtering at a thickness e.g. of 0.5 nm - 3 nm, 1.2nm has been found to work very well, but other thicknesses can be used as well. This Au-layer disintegrates upon annealing (here 650⁰C is used) into Au-caps distributed in size with an average diameter of the order of a few tens of nanometers. These Au-caps are statistically distributed on the substrate surface as shown in Fig. 2 and are found to have the shape of a lens.

The SEM micrograph of Fig. 3 shows an assembly of nanowires grown under the above-mentioned conditions by EBE. Fig. 4 shows an enlarged view of part of Fig. 3 from which two nanowires next to each other can be clearly seen. One is particularly small and one is particularly large in diameter. The almost ideal half-spherical shape of the Au-caps for SERS and TERS is visible. The Au-half- sphere of the small nanowire has a diameter of ~ 100 nm and the diameter of the large nanowire is ~ 350 nm. The length of both nanowires is about 1 μm and depends mainly on growth time, however, with the thinner nanowire growing a little faster, as expected. The deep troughs around the nanowires and the pronounced faceting of the nanowires occur at growth temperatures above 650⁰C, which were used for the growth run leading to Figs. 3 and 4. It is preferable, when using gold droplets and silicon as the semiconductor, to grow the nanowires at around 650⁰C or below where this trough formation is far less pronounced. It is noted that all temperatures recited here are "block temperatures", i.e. temperatures measured for the block on which the wafer or substrate sits and are around 150⁰C higher than the real surface temperature of the wafer. Although 650⁰C has been found useful, block temperatures above 400°C and preferably below 900°C are also readily possible. This may be of importance in order to not give rise to trapping in these troughs of the species of interest for spectroscopic analysis, which can be a consideration when realizing the probe as an array of working tips as will be described later with reference to Figs. 1OA and 1OB and Figs. HA and HB.

The basic method of manufacturing a probe suitable for the present teaching comprises the steps of:

- providing at least one droplet of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as silicon, GaAs and others on a support in air, in an inert atmosphere and /or in an evacuated chamber, if necessary to preclude undesired reactions such as the oxidization of aluminum,

- placing the support in an evacuated chamber or retaining it in the evacuated chamber from the previous step, if applicable, - providing a material comprising a semiconductor in the chamber,

- carrying out a PVD (physical vapor deposition) or CVD (chemical vapor deposition) in the evacuated chamber to enable growth of said semiconductor material in said chamber on said support at an interface with the or each said droplet, and

- maintaining a temperature in the chamber at which the or each said droplet of material is liquid or solid in the presence of said semiconductor material and the semiconductor material grows into one or more nanowires each being attached at one end to said substrate and having a respective droplet attached to a free end thereof.

There are two basic ways of carrying out the method to manufacture a probe with a working tip. In the first the support is a part of a probe, e.g. a cantilever such as 12 in Fig. 1. For direct growth of the shaft as a nanowire on a vestigial working tip 14 of a cantilever a single droplet 20 of a relevant material can be deposited thereon prior to initiating nanowire growth. Growth of the nanowire is then effected in the aforementioned vacuum chamber using silicon containing vapor provided in the vacuum chamber. The silicon containing vapor results in growth of the nanowire on the cantilever, i.e. on the working tip at the location of the gold droplet, so that the nanowire is grown directly on said stylus at the growth temperature (ca. 650⁰C in the case of silicon and gold).

In an alternative version of the method the support can be a substrate separate from the part of the probe. There is a plurality of droplets provided on the support, as shown in Fig 2 and the nanowires are grown on the substrate as described above with reference to Figs. 3 and 4.

The method then includes the further step of detaching at least some of said nanowires from said substrate which can be done by moving the AFM tip 10 of Fig. 1 and using the vestigial working tip part 14 to break off some of the nanowires of Figs. 3 and 4. The removed nanowires fall either away into the vacuum, onto the substrate or onto other nanowires.

With a nanomanipulation setup inside an SEM, as described, for example, in the paper by S. Hoffmann, I. Utke, B. Moser, J. Michler, S. Christiansen, V. Schmidt, S. Senz, P. Werner, U. Gόsele and C. Ballif in Nano Letters 6, 622 (2006) a suitable nanowire can be chosen and aligned with the AFM tip. Thus, the said part of the probe, i.e. the cantilever or AFM tip, is then brought - as shown in Figs. 5 and 6 - into the vicinity of a selected nanowire, selected so that it has the desired size (length and cross-sectional dimension (diameter) as described above) and so that its free end 22, i.e. the end remote from the droplet 20, is readily accessible. The nanowire can then be moved by movement of the cantilever relative to the substrate to bring the nanowire into the desired alignment relative to the cantilever as indicated in Fig. 7.

The end portion of the cantilever, i.e. the free end of the vestigial working tip 14, is then brought up to the free end 22 of the selected nanowire and becomes weakly attached thereto by Van der Waals forces.

The aligned nanowire can be attached to the AFM tip, for example by electron beam induced deposition of carbonaceous contaminants present on the sample's surface and in the residual gas of the SEM chamber. Alternatively a gas can be supplied via a syringe to the welding site and the welding effected by means of the electron beam. As a further alternative, the welding can be carried out in a focused ion beam machine by means of an ion beam combined with a respective gas inlet. Applied to a large enough area around the joint, this deposit provided a good adhesion and is stronger than the nanowire itself or the AFM tip. The nanowire is thus permanently attached at its free end 22 to the vestigial working tip 14 of the cantilever 10. This can also be done by utilizing a welding or reinforcing process to produce a stronger attachment. For example, an electron or ion beam can be directed close to the point of attachment and a beam of material can simultaneously be directed to the point of attachment to react with the ion or electron beam and deposit material on the structure at the point of attachment 22. The material can for example be WC3, gold, platinum or other materials. A suitable technique is described in the paper by I. Utke, A. Luisier, P. Hoffmann, D. Laub, P.A. Buffat, Appl. Phys. Lett. 81, 3245 (2002).

The movement required for the AFM tip 10 , to realize the method steps shown in the Figs. 5, 6, 7 and 8 can readily be achieved using the AFM tip incorporated in a handling device realized as an AFM microscope. This has the advantage that all movements of the tip required can be generated by the AFM microscope and that all readouts necessary to visualize the working tip and the substrate with nanowires are automatically included in the AFM microscope. The AFM microscope can for example include piezo- actuators for movement of the tip 10, 20 in three Cartesian coordinate directions in space or can include piezo-actuators for movements in two Cartesian coordinates, i.e. in a horizontal plane, coupled with a bimorph structure incorporated on the cantilever to produce controlled bending thereof by electrically induced differential thermal expansion and movement of the tip in a third Cartesian direction, e.g. vertically. Alternatively, the handling device can be a special actuator, e.g. incorporating three piezo-actuators to move the tip in three dimensions. Also, it is possible to move the substrate rather than the tip and to provide for movement of the substrate in say two coordinate directions and movement of the tip in a third coordinate direction or vice versa.

The present teaching also relates to an alternative type of probe as illustrated in Figs. 1OA and 1OB comprising a support configured to support an array of nanowires 18 each having a said droplet 20 at its end remote from the support. An array of this kind can be used with a probe having a single movable working tip, so that the working tip of the probe, i.e. the droplet 20 in Fig. 8 can be brought into juxtaposition with a selected one of the tips of the array as illustrated in Figs. 1OA and 1OB. This provides for a particularly enhanced Raman signal. For this embodiment the array of nanowires is preferably grown on a planar surface of the substrate 19.

Another method of preparing a probe in the form of an array of tips is illustrated in Figs. 1 IA and 1 IB. This method comprises the step of preparing a support 19, e.g. by lithography, to define a trench 60 having opposing sides 62, 64, which optionally diverge in a direction going from a base of the trench to a free surface thereof. The droplets 20 are then formed on the opposing sides of said trench and the nanowires 18 are subsequently grown from said opposing sides to produce at least one pair 66 of generally oppositely disposed droplets 20 positioned in immediate proximity to each other. This pair of generally oppositely disposed droplets can then be used in a Raman spectrometer to produce signal enhancement of a sample in the vicinity of the generally oppositely disposed droplets.

In the above methods the droplets are formed either by depositing a layer of the respective material on said support 19 and heating the support to cause the layer to split into droplets or by lithographically treating said layer to provide islands of said material and subsequently heating said support to cause said islands to melt and form said droplets.

It should be noted that it is not essential for the substrate to be a semiconductor substrate. It could also be an insulating substrate or semiconductor layer on an insulating substrate, e.g. an SOI substrate as described in connection with Figs. HA and HB or a conductive substrate. Also, if a semiconductor is used as a substrate it need not necessarily be the same semiconductor as is used for the growth of the nanowire. The explanation for this lies in the growth mechanism for the nanowire. With a misfitting substrate or an amorphous substrate the nanowire growth can be epitaxial. In the case of insulating often amorphous substrates, e.g. oxidized silicon wafers, nanowires can grow at random without the need for any crystallographic relationships. In the case of misfitting substrates of totally different crystallography than the semiconductor nanowires, random nanowire growth may also occur. The point is that the vaporized semiconductor material and the material of the droplet combine to form a two phase system such that an enhanced one dimensional growth is obtained, so that the interface of the nanowire/ droplet moves away from the substrate as the nanowire grows behind it.

Turning now to Fig. 9 there is shown a schematic drawing of a Raman spectrometer configured in accordance with the present teaching. The Raman spectrometer comprises a sample holder 30, a laser 32 for generating a laser beam 34 for projection onto a sample 36 in said sample holder, a detector 38 for a Raman spectrum from the sample and a probe 10 having a working tip 14, 20 as described above. There is also a system 40 for producing relative movement in three dimensions between the working tip 20 of said probe and the sample holder 30. The system 40 is preferably realized as an atomic force microscope incorporating the probe 10 and working tip 14, 20. It includes a coordinate movement system for producing movement of said tip 14, 20 relative to said sample holder 30, here in the directions x, y and z and thus movement of the tip 20 relative to a sample 36 provided in the sample holder 30.

As mentioned above, the relative movement could also be produced by movement of both the sample holder 30 and the tip 14, 20. E.g. the sample holder 30 could be moved in the x-y plane and the tip 14, 20 could be moved in the z direction or vice versa.

The spectrometer also includes a computer 42, which is connected by respective leads 44, 46, 48, 50 and 52 to the laser source, the coordinate movement system 40, a screen 54 and a keyboard associated with the computer and the detector/ spectrometer 38.

If the sample is a solid then it will normally be convenient to use just a probe 10 provided with a working tip 20 which can be moved relative to the surface of the sample. If the sample is a liquid or gas or a finely divided solid material or soft material (for example molecules, DNA or biological agents) which can be brought into the direct vicinity of an array of working tips as shown in Figs. 1OA and 1OB, then the tip 20 can be used with such an array, as illustrated in Fig. 9 to enhance the Raman signal. If an array in accordance with Figs. HA and HB is used then the tip 20 may no longer be essential although it can still be used to enhance the Raman signal locally.

For the sake of completeness two further working tips for probes are shown in Figs. 12 and 13. In the Fig. 12 embodiment a nanowire shaft 18 of e.g. silicon with a gold droplet or cap 20 at its end is grown on a substrate and separated therefrom as described with reference to Figs. 2 to 8.

The shaft of the nanowire with the gold cap is then coated by any suitable means, e.g. chemically or galvanically or by a PVD or CVD method, to form a coating 70. The coating 70 can endow the probe with the ability to further enhance the Raman signal. The coated shaft 18 forms a core and shell structure.

With such a setup the incident laser beam is focused onto the region close to the Au-cap on the silicon nanowire, where the field enhancement is realized. With these realizations it is possible to focus on two fields of active research and technological developments, providing information that is otherwise not accessible:

A) in semiconductor physics and technology, the nanowire based TERS- probes can be used to:

(i) locally measure on the nanometer scale strain/ stress distributions in wafers or in thin film structures such as multicrystalline thin film solar cells and especially in nanoscale devices such as transistors built onto or into these wafers. The measurement of stress/ strain by Raman spectroscopy is for example described in the paper by De Wolf in Semicond. Sci. Technol., 1 1, 139 (1996);

(ii) locally map doping profiles with nanometer scale resolution, utilizing the Raman- specific Fano-effect as described in the papers by U. Fano in Phys. Rev 124 (6), 1866, (1961) and N. H. Nickel, P. Lengsfeld and I. Sieber in Phys. Rev. B61, 15 558 (2000).

B) For applications in the bio-medical field such as e.g. molecular chemistry or life sciences a combination of a nanowire based TERS-probe and a nanowire based SERS-substrate (as illustrated in Figs. 1OA and 10B) can be used. This makes it possible to approach ultimate enhancement driving the sensitivity of the SERS-technique to the limits (detection of single molecules, bacteria, DNA sequences and others are in reach). This includes the availability of spatial resolution through the special nanowire-based AFM tip (Fig. l)if required. In this connection reference can be made to the paper by P. Rόsch, M. Harz, M. Schmitt, K. Peschke, O. Ronneberger, H. Burkhardt, H.Motzkus, M. Lankers, S. Hofer, H. Thiele and J. Popp in Applied and Environmental Microbiology 71, 1626 (2005). For this purpose a dipole configuration of two Au-caps can be provided between which the species of interest reside (as shown in Figs. 1OA and 10B). The 'SERS'- TERS '-setup of Figs. 1OA and 1OB allows spatial resolution in Raman spectroscopy on the nanometer scale. The spatial resolution is obtained through the nanowire-based TERS-tip. Signal enhancement is obtained when placing the material of interest on the nanowire- assembly based SERS substrate. A combination of nanowires on the template and at the AFM tip yields a dipole configuration of nanowires as shown in the close-up. Strong enhancement is only achieved if the polarization vector (dotted arrow) of the incident light possesses a large component along the dipole axis. With this configuration an ultra-high sensitivity can be expected.

The SERS- and TERS-capabilities of the Si-nanowires with the Au-caps have been demonstrated by simple SERS-experiments. Micro-Raman measurements were performed on nanowire arrays, which were coated with a molecular layer of malachite green havinga chemical structure as shown below and as described at the internet address http: / / de.wikipedia.org/wiki/ Malchitgr%C3%BCn

For such a molecular coverage the arrays of nanowires were rinsed in an aqueous solution of malachite green and subsequently rinsed in de- ionized water. For the micro-Raman measurements use was made of a Jo- bin Yvon LabramHR 800 spectrometer, equipped with a HeNe-Laser (633 nm) and an Ar-Ion-Laser (488 nm and 514 nm), operated with the HeNe- Laser for the SERS-measurements. The undamped laser power on the sample is ~ 10 mW. The laser is focused by a 10Ox objective lens to a spot size of ~ 1.5 μm in diameter. To avoid degradation of the molecules due to heating, the laser intensity was damped by a factor of 100 with a grey filter. With this procedure, the energy density within the focused laser spot is ~ 6 x 10³ W/ cm². Using a motorized x-y-stage it was possible to perform SERS-mappings and to determine SERS-active regions within a nanowire array or on single nanowires. A typical nanowire array or ensemble to serve as a SERS-substrate for SERS-mappings is shown in the SEM- micrograph in Fig. 3 where nanowires with Au-cap diameters within the range of about 50 nm - 500 nm are visible.

It was found that large enhancement occurs at positions where 2-3 Au- caps of nanowires are close to one another and form a cluster of nanowires. The spectra recorded at the cluster-like regions show a signifi- cant signal enhancement. Regions without any nanowires show the broad fluorescence band of malachite green and only a very faint Raman signal.

The Raman spectra recorded at the position of a single Au-cap and at a position close to the cap on the bare substrate surface shows that even a single nanowire Au-cap with a diameter of the order of 200 nm is capable of producing a significant enhancement of the Raman signal.

To compare the enhancement effect produced by a single nanowire gold cap with the enhancement achieved by standard SERS-measurements, SERS-measurements using the lens-shaped gold caps shown in Fig. 2 were made. These measurements showed little signal enhancement due to the lens-shaped gold caps or droplets, i.e. gold nanoparticles of less favorable shape than the hemispherical gold droplets at the end of a nanowire.

Within the mapped area of Fig. 3, regions of large signal enhancement were found as well as regions, where only small or no enhancement occurs although gold particles were present in these areas. The largest enhancement within the mapped area is comparable to the enhancement produced by the single nanowire gold cap.

The advantages of using the nanowire gold caps for SERS-measurements now become evident. In contrast to the lens-shaped gold particles from Fig. 2, a nanowire gold cap forms an almost perfect half sphere, which enables a larger signal enhancement per cap probably due to geometrical effects. In addition, in the surrounding of a single nanowire, there is a zone denuded of gold, i.e. essentially all the gold is incorporated in the nanowire gold cap. This enables spatially resolved SERS-measurements as the enhanced signal is solely produced by one gold cap. It is of utmost importance to have spectroscopic Raman capabilities with a probe size on the nanometer scale, especially for today's applications in the field of solid-state physics and technology. Focusing the exciting laser on the surface gives however at best probing spots of 600 nm - 1 μm depending on the specifics of the setup (laser, focusing optics, a.o.). Using the TERS probe, anticipating enhancement under the tip to be sufficiently high to distinguish a Raman signal from the entire probed volume and the volume under the TERS tip, it is be possible to get spectroscopic information with the resolution determined by the dimensions of the TERS tip. Taking a look at the experimentally realized nanowire-based TERS tip shown in Fig. 8, the following can be stated:

The tip has a silicon nanowire with a diameter of -150 nm and a hemispherical gold cap of the same diameter at the end. For current transistors of the 90nm technology node as described in the International Technology Roadmap for Semiconductors: http://public.itrs.net/ this type of tip is too big to spatially resolve and measure e.g. implant distributions or strain that is utilized in present and future transistor generations, see for example the papers by K. Rim, J. Chu, H. Chen, K. A. Jenkins, T. Kanarsky, K. Lee, A. Mocuta, H. Zhu, R. Roy, J. Newbury, J. Ott, K. Petrarca, P. Mooney, D. Lacey, S. Koester, K. Chan, D. Boyd, M. Ieong and H. -S. Wong in VLSI Symp. Tech. Dig. 2002, 98 (2002) and by J. L. Hoyt, H. M. Nayfeh, S. Eguchi, I. Aberg, G. Xia, T. Drake, E.A. Fitzgerald and D. A. Antoniadis in IEDM Tech. Dig. 2002, 23 (2002) as well as http : / / www. intel. com / technology / silicon / micron . htm# silicon . These descriptions make it clear that quality control based on strain determination on the nanometer scale is required where up to now no sufficiently suitable method has been identified. The nanowire based TERS probe (the AFM tip) proposed herein makes it possible to solve this problem, particularly by using nanowires in the diameter range 20 nm to 40nm. A combination of a nanowire based TERS-probe and a nanowire based SERS-substrate as shown in Figs. 1OA and 1OB is useful to utilize the large field enhancement in between the dipole of two Si-nanowire Au-caps. Chemical or biological substances are attached to either the nanowire SERS-substrate or the TERS probe. For the case of attachment to the SERS substrate, the nanowire based TERS-probe (the AFM tip) then approaches an Au-cap of the SERS-substrate, where the approaching process is controlled by an AFM-control unit. When the two Au-caps are in close proximity (a few nanometers apart), the Raman scattered signal of the investigated substance will be locally enhanced by the dipole configuration. A strong field enhancement can only be achieved, if the polarization vector of the incident light possesses a large component along the dipole axis, as is indicated in Figs. 1OA and 1OB by the dotted arrows. With the combined TERS-SERS measurements benefit will be obtained from both the high lateral resolution of the TERS-probe and the large field enhancement produced by the dipole configuration of the two Au-caps.

The arrangement of Fig. HB shows a SERS configuration with the promise of optimized enhancement. To utilize the large field enhancement in between a dipole of two Si-nanowire Au-caps nano-patterning and self- organized *bottom-up' nanowire growth is utilized in the manner shown in Figs. HA and HB. At first electron beam lithography or nanoimprint lithography are used to create nanopatterns (here lines) in a resist. The patterns are transferred in a second step by reactive ion etching, for example into a silicon or silicon-on-insulator (SOI) substrate. That way, trenches are realized in silicon that reside, e.g. parallel to < 110>-directions in silicon wafers with Si(IOO) surface normal. The trenches used here are 600 nm - 800 nm wide. Subsequent to etching, gold is evaporated onto the Si or SOI wafers, which reaches surfaces on the wafer and within the trenches. Upon annealing at growth temperature, the gold forms caps at the surfaces also within the trenches. Using, for example, PVD or CVD deposition nanowires grow from adjacent trench-surfaces to the middle where they can either meet when sufficiently long growth times are assumed and the geometrical pre-conditions are met or where gold caps can stay close without ever meeting. Some of those cases of impinging and not-impinging nanowires are shown in the SEM micrograph in Fig. HB. The nanowires that grow from both rims of the trench to the center either meet or pass each other in close vicinity just as statistics permit. Some wires also almost reach the rim on the other side where they can become close to gold particles that just started to grow or have not yet even started to grow. Using the "malachite technique" described earlier to prove the Raman signal enhancement concept, it is found that the signals are strongest where several gold caps of nanowires are in close vicinity. This occurs once close to the middle of the trench and once close to the rim of the trench where a nanowire is close to a gold cap of a wire that has just started to grow off the rim.

This trench guided nanowire configuration can be used as another type of SERS template and again a molecular layer of malachite green is used to study the signal enhancement induced by gold caps and configurations of two or more gold caps on nanowires in close vicinity. To obtain such a molecular coverage of malachite green, the samples were rinsed in an aqueous solution of malachite green and subsequently rinsed in de-ionized water. In most of the mapped area inside and outside of the trench, no Raman-intensity indicative of the presence of malachite green is visible. However, strong signal enhancement is visible in the detected signal from a location close to the middle of the trench where two or more nanowire gold caps approach each other closely. Another strong signal is obtained close to the rim of the trench where a nanowire with gold cap gets close to a gold cap of a wire that has just started to grow off the rim.

From the foregoing it is concluded that silicon nanowires grown by the vapor-liquid-solid mechanism from gold catalyst show gold caps atop with an almost ideal half-spherical shape to be used to exploit the tip- or surface enhanced Raman (TERS /SERS) effects. Attaching a nanowire with gold cap to an AFM-tip the signal enhancement by the gold nanoparticle can be used to spatially resolve a Raman- signal, i.e. to perform nano- Raman spectroscopy.

A combination of a nanowire-based TERS-probe and a nanowire-based SERS-substrate promises optimized signal enhancement so that the detection of even single molecules (e.g. of explosives, poisonous gases, etc) or of single bacteria, DNA strands and other soft matter is in reach. Applications of this novel nanowire based technical SERS- and/ or TERS solution are widespread and lie in the fields of bio-medical and life-sciences as well as security and in the field of solid state research e.g. in silicon technology where the detection of materials composition, doping, orientation and lattice strain can be probed by Raman spectroscopy, now using TERS with the spatial resolution of the nanowire based AFM-tip.

Fig 13 shows in drawing a) the cantilever tip of Fig. 1 and the active region of the tip is shown in drawing b) and corresponds to the drawing of Fig. 8. Beneath the drawing b) there are two further drawings c) and d). Drawing c) is a cross-section made using a transmission electron microscope of a known Intel MOSFET transistor of the 90 nm technology node. In the drawing the reference numeral 80 shows a bulk silicon substrate. Immediately above the substrate 80 there is a channel 82 which can only just be recognized and which extends between source and drain electrodes to the left and the right of the substrate, which can only just be seen as darker areas in drawing c). Above the channel 82 is an insulating gate region 84 resembling a cylindrical column of 50 nm diameter, which is provided with a gate electrode 86. The structure is covered by a dome- shaped insulation 88. The L-shaped region 90 is a cross-section through a top hat structure which acts as an insulating spacer.

Below the drawing c) there is a further drawing d) showing a nanowire probe tip 92 of approximately 15 nm diameter with a gold cap, which is in fact an inverted version of the tip shown in the drawing b). The tip 92 of drawing d) is used for Raman enhanced strain measurements on the transistor of drawing c) and enables quality control of individual densely packed transistors hitherto not attainable.

Preferred examples of the methods of the invention will now be described. In this method epitaxial semiconductor nanowires are desposited using the vapor-liquid-solid (VLS) growth mechanism from gold (Au) catalyst nano-particles. Chemical (CVD) and physical vapor deposition methods (here: electron beam evaporation, for short: EBE from pure silicon) are used to obtain nanowires with different diameters.

Using EBE straight nanowires which are essentially free of extended defects and which are perpendicular to the sample surface of a Si(111) wafer are grown. In this EBE case, the nanowires assume diameters larger than 50 nm (up to -350 - 400 nm) and <l l l>-growth directions are assumed for essentially all wires. The growth rates are small enough so that nanowires with comparably small aspect ratios (wire length / wire width) of < 10 can be realized. Using CVD nanowires in various low index growth directions such as < 111>, < 110> and <112> are grown. Even when Si(H l) wafers are used, different growth directions not just the wafer surface normal are assumed. The nanowires are essentially free of extended defects and assume diameters usually not larger than 150 nm (usually between 30 nm and 100 nm). The growth rates are comparably large so that nanowires with aspect ratios (wire length / wire width) of < 10 which are ideally suited for the welding procedure can not as easily be realized. However, it is nevertheless possible to stabilize these small aspect ratios.

For the current disclosure it is important to realize reproducibly a hemispherical shape of gold droplets of different size. This can be done by physical and chemical vapor deposition methods. The difference of these methods is, that with EBE the nanowires are slightly larger in diameter and with CVD the diameters are usually smaller. But in both cases the nanowire diameters can be controlled. Secondly, the nanowires by EBE grow usually less dense and straight on the Si(H l) surface, so that they are easier to break away from the substrate surface for the welding on the AFM tip. The CVD nanowires grow usually denser so that the managing of the welding is a little more difficult, but possible.

The EBE system used in this work has a commercial pumping system (RIBER, France). The cylindrical main chamber (volume ~ 200 1) is a bell- jar system with a small loadlock chamber (volume ~ 30 1) attached to it. The samples are transferred via a loadlock at a pressure of 4* 1O^-6 Torr. To ensure a low deposition pressure and to minimize outgassing from the chamber, the walls of the main chamber are water cooled. The silicon is evaporated from a water-cooled copper crucible using an electron beam. The deposition velocity can be adjusted by varying the electron-beam current. We use a cylindrical high-resistivity silicon (1 D. /cm) ingot. Halogen lamps located at the ceiling of the main chamber heat the substrate radia- tively, in a fast and effective manner. The substrate temperature is controlled using a thermocouple. This temperature is referred to as the heater temperature, which deviates from the real temperature at the specimen surface by - 15O⁰C. 25*25 mm² silicon substrate pieces are used, cut from single- side-polished p-Si(l l l) wafers. The silicon samples are cleaned by rinsing in acetone for 5 min followed by ethanol for another 5 min. Native silicon dioxide layers are removed by etching with HF (40 %) for 30 s followed by a 2 % HF rinse for 3 min. Finally, the samples are rinsed with de-ionized water and are blow dried with nitrogen. This cleaning procedure yields hydrogen-terminated silicon surfaces (for a limited time of a few minutes), which allows for subsequent deposition on an essentially oxide free surface. To obtain Au nano-particles to catalyze the nanowire growth, initially, a 1-2.5 nm-thick Au film (EDWARDS Sputter Coater S 150 B) is sputtered on the wafers to create a continuous Au layer that disintegrates upon heating the Au droplets that catalyze nanowire nucleation and growth. Au does not form a suicide and the bulk Au-Si eutectic temperature is relatively low (~ 373°C). After the Au layer deposition, the samples are immediately placed into the loadlock chamber. The loadlock is then pumped down to 4x10-6 Torr, followed by the transfer of the samples into the main EBE chamber. Then samples are heated for 30 min to temperatures of 625, 650 and 750⁰C to produce the Au nano- particles from the Au films on the Si substrates. For the nanowire growth experiments these templates are exposed to an atomic silicon beam as generated by evaporation of silicon with an electron beam at a beam current of 35 mA - 8OmA. AU growth experiments are carried out for 4 h at 625⁰C and at a chamber pressure of 2-5* 10-⁷ Torr. The growth rates are in the range 1-15 nm/min. The CVD system used for this work was home built. 25x25 mm² silicon substrate pieces are used, cut from single-side-polished p-Si(l l l) wafers. The silicon samples are cleaned by rinsing in acetone for 5 min followed by ethanol for another 5 min. Native silicon dioxide layers are removed by etching with HF (40 %) for 30 s followed by a 2 % HF rinse for 3 min. Finally, the samples are rinsed with de-ionized water and are blow dried with nitrogen. This cleaning procedure yields hydrogen-terminated silicon surfaces (for a limited time of a few minutes), which allows for subsequent Au layer deposition on an essentially oxide free surface. To obtain Au nano-particles to catalyze the nanowire growth, initially, a 1-2.5 nm-thick Au film is sputtered or evaporated on the wafers to create a continuous Au layer that disintegrates upon heating the Au droplets that catalyze nanowire nucleation and growth. After cleaning, oxide removal, the samples with the Au droplets are immediately transferred into the reaction chamber, which is pumped down to 1 x lO"⁶ mbar, and the substrates are annealed at ~580°C for 10 min (to form the Au nano-particles). The temperature is then reduced to 530⁰C and a mixture of 20 seem Ar and 4 seem SiH4 is introduced in order to grow the nanowires for e.g. 20 min at a pressure of 5 mbar.

A nanowire based probe for tip enhanced Raman spectroscopy (TERS) or conventional AFM applicatons can in principle be produced in two different ways:

a. A silicon nanowire with a Au-half-sphere atop can directly be grown e.g. on a silicon AFM cantilever by the growth method described a- bove. This requires the precise in place positioning of a Au-droplet on the AFM cantilever to catalyze subsequent bottom up nanowire growth. b. A silicon nanowire can be welded onto an AFM tip. One method to do so is the electron beam induced contamination deposition. This method was used to produce the TERS probes described here. Other options are to use a focused ion beam (FIB) machine for welding or focused electron beam (FEB) induced deposition of e.g. WC3, gold, platinum or other materials.

The fabrication of a nanowire based TERS probe by method b) needs in general three steps:

(i) At first, a large number of silicon-nanowires have to be broken off the substrate using either a moving AFM-tip as a kind of lawnmower for na- nowires or by an ultrasonic treatment. The removed nanowires fall either away into the vacuum, onto the substrate or onto other nanowires.

(ii) With a nanomanipulation setup inside an SEM, a nanowire (NW) of acceptable shape and diameter can be chosen and aligned with the AFM tip.

(iii) The aligned nanowire can be attached to the AFM tip by electron beam induced deposition of carbonaceous contaminants present on the sample surface and in the residual gas of the SEM chamber. Applied to a large enough area around the joint, this deposit is stronger than the nanowire itself or the AFM tip.

The attaching of a nanowire onto an AFM-tip was performed in a combined AFM /SEM setup, as described below.

The AFM tip (AdvanceTEC, 0.2 N/m, 2.8 N/m N/45 N/m, Nanosensors, Neuchatel, Switzerland) was mounted on a piezoelectric slip-stick robot arm (MM3A, Kleindiek Nanotechnik, Reutlingen, Germany) with two rota- tional and one linear axis. The substrate with the NWs is mounted on a x,y,z piezo stage (P-620.2CD and P-62.ZCL, Physik Instrumente (PI), Karlsruhe, Germany) with 50 μm range and sub-nanometer resolution. The whole setup was mounted inside an SEM (Hitachi Science Systems, Japan, S-3600N) such that the NWs are at an angle of 60° with the scanning electron beam. With the SEM table, the NWs of interest were moved in the field of view. The coarse positioning of the AFM tip toward the sample was done with the robot arm, and the fine positioning as well as the positioning of the NWs was achieved by moving the sample with the x,y,z piezo stage.

Claims

Patent Claims

1. A probe (10) having at least one working tip (20) and a support (19) for the working tip, the working tip projecting from the support and comprising an elongate semiconductor shaft (18) having cross- sectional dimensions in the nanometer range and attached at a first end (22) to said support and having a second free end (24), and a droplet (20) of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as silicon, GaAs and others provided at said second end.

2. A probe (10) in accordance with claim 1 wherein said shaft is a substantially cylindrical shaft (18) comprising one of a nanowire, a nanotube and a core and shell structure.

3. A probe (10) in accordance with claim 1 or claim 2 wherein said droplet (20) has a substantially hemispherical shape with a circular or polygonal base, said circular base being attached to said shaft.

4. A probe (10) in accordance with any one of the preceding claims wherein said cross-sectional dimensions are in the range from 10 nm to 500 nm and preferably from 20 to 100 nm.

5. A probe (10) in accordance with claim 2, wherein the shaft (18) comprises a nanowire of a semiconducting material selected from the group comprising silicon, SiC, germanium, alloys of elements belonging to groups III and IV of the periodic table of elements and group Ill-nitrides and II-IV semiconductors.

6. A probe (10) in accordance with any one of the preceding claims, wherein said droplet (20) is a self-organised particle of hemispherical shape.

7. A probe (10) in accordance with one of the preceding claims, wherein said working tip (14, 20) has an aspect ratio of shaft length to shaft diameter of preferably less than 100 and especially less than 10.

8. A probe (10) in accordance with claim 1 and comprising a plurality of working tips in the form of an ensemble or array of nanowires (18) each having a respective droplet (20) at its free end.

9. A probe (10) in accordance with claim 8 wherein said substrate is one of a planar substrate, with said nanowires (18) with droplets (20) projecting from a planar surface of the substrate (Fig. 3; Figs. 1OA and 10B) and a substrate (19) having a trench (60) with opposing sides (62, 64) with nanowires (18) each having a droplet (20) at a free end projecting towards each other from said opposite sides of said trench (60) or trench patterns such as cross-hatched patterns of trenches or multiple rows of trenches each forming adjacent surfaces from which nanowires can grow and, e.g. gold droplets can meet in close proximity with a large enough statistical probability that enough events (meetings of droplets) occur that give rise to signal enhancement.

10. A Raman spectrometer comprising a sample holder (30), a laser (32) for generating a laser beam for projection onto a sample in said sample holder, a detector (38) for a Raman spectrum from the sample and a probe (10) having a working tip (14, 20) in accordance with one of the preceding claims, as well as a system for producing relative movement in three dimensions between the working tip (14, 20) of said probe (10) and the sample holder (30).

11. A Raman spectrometer in accordance with claim 10 wherein said system comprises one of an atomic force microscope incorporating said working tip (14, 20) and a coordinate movement system (40) for producing movement of said tip (14, 20) relative to said sample holder (30) and thus relative to a sample (36) provided in said sample holder (30).

12. A method of manufacturing a probe in accordance with one of the preceding claims, comprising the steps of:

- providing at least one droplet (20) of a material selected from the group comprising gold, silver, copper, aluminum, platinum and also highly doped and thus quasi metallic semiconductors such as sili con, GaAs and others on a support (14, 19) in air, in an inert at mosphere and/ or in an evacuated chamber, if necessary to preclude undesired reactions such as the oxidization of aluminum,

- placing the support in an evacuated chamber or retaining it in the evacuated chamber from the previous step, if applicable,

- providing a material comprising a semiconductor in the chamber,

- carrying out a PVD (physical vapor deposition) or CVD (chemical vapor deposition) to enable growth of said semiconductor material in said chamber on said support (14, 19) at an interface with the or each said droplet,

- maintaining a temperature in the chamber at which the or each said droplet of material is either liquid in the presence of said semiconductor material and the semiconductor material grows into one or more nanowires each being attached at one end to said sup port (14, 19) and having a respective droplet attached to a free end thereof or solid (e.g. for the case of aluminum) where the solid drop let in the presence of an ultra-high vacuum also gives rise to the one dimensional nanowire growth with a respective droplet attached to the free end thereof.

13. A method in accordance with claim 12 wherein said support (14) is a part of a probe, e.g. a cantilever (12), with a said droplet (20) provided thereon and said nanowire (18) is grown directly on said shaft.

14. A method in accordance with claim 12 wherein said support (19) has a plurality of droplets (20) provided thereon and including the further steps of

- detaching at least some of said nanowires (18) from said substrate and

- bringing a part of a probe, e.g. a cantilever (12), into the vicinity of a selected nanowire and attaching an end portion (14) of said part of said probe to a free end of a nanowire (14) remote from the attached droplet (20) to form said working tip (14) attached thereto.

15. A method in accordance with claim 14 wherein at least one of said part (14) of said probe and said substrate (19) is incorporated in a handling device for relative coordinate movement in at least two dimensions, e.g. in a handling device constructed as an atomic force microscope or a handling device in the form of piezoelectric actuators operative to move said part (14) of said probe and the substrate (19) relative to one another in two different coordinate directions, and wherein said detaching step comprises effecting relative movement of the said part (14) of the probe and said substrate (19) by said handling device to break said nanowires (18), e.g. at a position between a junction with said support (19) and said droplets (20).

16. A method in accordance with claim 15 and comprising the further step of using said handling device to maneuver the said part (14) of the probe relative to the substrate (19) into a position close to a free end (22) of one of the broken off nanowires (18) that is in an accessible position and subsequently attaching said free end (22) of said nanowire (18) to said part (14) of said probe to form said working tip attached thereto.

17. A method in accordance with one of claims 14 to 16 wherein said attaching step comprises an initial weaker attachment between said end portion (14) of said probe body and said free end (22) of said nanowire (18), for example by the operation of Van der Waals forces or adhesive forces, and subsequently a stronger attachment by a welding or reinforcing process, for example by using an electron or ion beam directed close to the point of attachment and simultaneously directing a beam of material to the point of attachment to react with the ion or electron beam and deposit material on the structure at the point of attachment.

18. A method in accordance with claim 12 wherein said support (19) is configured to support an array or ensemble of nanowires (18) each having a said droplet (20) at its end remote from the support.

19. A method in accordance with claim 12 wherein said support is prepared, e.g. by lithography, to define a trench (60) or array of trenches or cross-hatch pattern of trenches or free surfaces generated by means of lithographic processing having opposing sides (62, 64), which optionally diverge in a direction going from a base of the trench (20) to a free surface thereof and wherein said droplets (20) are formed on said opposing sides of said trench and said nanowires (18) are subsequently grown from said opposing sides to produce at least one pair of generally oppositely disposed droplets (20) positioned in immediate proximity to each other.

20. A method in accordance with claim 18 wherein said array or ensemble of nanowires (18) is grown on a planar or otherwise configured surface of the substrate (19).

21. A method in accordance with any one of the claims 12 to 20 wherein said droplets (20) are formed either by depositing a layer of the respective material on said support (19) and heating the support (19) to cause the layer to split into droplets (20) or by lithographically treating said layer to provide islands of said material and subsequently heating said support to cause said islands to melt and form said droplets (20).