|Title of Invention||
A NANOTECHNOLOGICAL STRUCTURE FOR A SCANNING PROBE MICROSCOPE
|Abstract||A nanotechnological structure for a scanning probe microscope, comprising a tip member (4), upstanding from a support member (2), and a nanowhisker (16) grown on and projecting from a free end (6) of the tip member (4), characterized in that the nanowhisker (16) comprises a degenerately doped large band-gap semiconductor material that in use creates free electrons in the conduction band of the semiconductor material in order to provide a narrow energy distribution of electrons flowing through the nanowhisker (16).|
|Full Text||The present invention relates to a nanotechnological sttucture for a scanning probe microscope.
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No. 60/485,104 filed July 8,2003, which is incorporated herein by reference.
Field of the Invention
The present invention relates to structures, incorporating one-dimensional nanoelements and which are suitable for use in scanning probe microscopy, current injection applications, and other applications. " One-dimensional nanoelements" are structures, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and which are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc. More specifically, but not exclusively, the invention is concerned with structures incorporating nanowhiskers, related production methods, and to methods of forming nanowhiskers.
The basic process of whisker formation on substrates, by the so-called VLS (Vapour-Liquid-Solid) mechanism is well known. A particle or mass of catalytic material, usually gold, is heated on a substrate in the presence of certain gases. The gases are absorbed by the catalytic mass to form an alloy. The alloy supersaturates, and a pillar of solidified material forms under the mass, and the mass rises up on top of the pillar. The result is a whisker of a desired material with the catalytic mass positioned on top. (See E.I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978.) The dimensions of such whiskers were in the micrometer range.
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapour-Liquid-Solid process), it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and
presumably in the solid state. Under such conditions, the growth material, e.g., atoms
that are added to the tip of the whisker as it grows, may be able to diffuse through a
the body of a solid catalytic particle or may even diffuse along the surface of the solid
catalytic particle to the growing tip of the whisker at the growing temperature.
Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by
the catalytic particle, whatever the exact mechanism may be under particular
circumstances of temperature, catalytic particle composition, intended composition of
the whisker, or other conditions relevant to whisker growth. For purposes of this
application, the term "VLS process", or VLS mechanism, or equivalent terminology,
is intended to include all such catalyzed procedures wherein nanowhisker growth is
catalyzed by a particle, liquid or solid, in contact with the growing tip of the
International Application Publication No. WO 01/84238 discloses in Figures
15 and 16 a method of forming nano whiskers wherein nanometer sized particles from
an aerosol are deposited on a substrate and these particles are used as seeds to create
filaments or nanowhiskers.
For the purposes of this specification, the term nanowhiskers is intended to
mean one-dimensional nanoelements with a diameter or cross-dimension of
nanometer dimensions, preferably 500 nm or less.
Since the development of the Scanning Tunnelling Microscope in the 1980s
there has been intense research in examining and processing surfaces at atomic
dimensions by means of a tip of nanometer dimensions brought into close proximity
or contact with the surface. The STM operates on a principle of a tunnelling current
flowing between the dp and the sample surface, while moving the tip across the
surface. Various other microscopes have been developed which operate on somewhat
different principles for examining surfaces at the atomic level. These include, for
example, the Atomic Force Microscope which relies on sensing of the electronic
force of repulsion of the surface by means of a tip mounted on a flexible cantilever
beam, microscopes which measure a magnetic force of attraction or repulsion by
means of a magnetic tip, and microscopes which detect the heat generated by a
sample surface, (see www.nanoworld.org'). All of these microscopes fall into a
generic class known as Scanning Probe Microscopes (SPM). For the purposes of this
specification, the term Scanning Probe Microscope will be understood to include the
Scanning TunneDing Microscope, Atomic Force Microscope, and other microscopes
which include a very fine tip moved over the surface of a specimen for determining
characteristics of the surface on a nanometer or atomic scale.
The original form of STM comprised a tip mounted on a piezoelectric tube.
The tunnelling current to a specimen surface was monitored, and the distance
between the tip and the surface was adjusted to maintain the tunnelling current
constant. Nowadays, the tip of such an STM commonly comprises a wire of Pt/Ir, the
tip being formed by cutting and drawing the wire with cutters and pliers. Another
common form of STM tip is a wire of Tungsten, whose end is etched. Both forms of
tip have free ends with dimensions in the nanometer range.
A known construction of AFM uses a micromachined flexible cantilever beam
of silicon with an integral silicon tip upstanding from the free end of the beam, the
degree of flexure of the beam being measured as the tip is moved over the surface
(see, for example, the McGraw Hill Encyclopaedia for Science and Technology 7th
Edition). The end of the tip commonly has dimensions in the nanometer range.
In Samuelson et ah, Physica Scripta. vol. T42, pages 149-152, (1992),
entitled 'Tunnel-Induced Photon Emission in Semiconductors Using an STM", there
is shown in Figure 6 an STM with a triangular semiconductor tip of gallium
phosphide. Various types of tip material are proposed, as shown in Figure 5, to
permit tunnelling current of P-type or N-type carriers for achieving photon emission
in the semiconductor surface. This is done by providing a tunnelling current formed
of a narrow band of low energy electrons, that may be injected resonantly with
specific electronic state features (e.g. bandgap) of the semiconductor surface that is to
be probed by this device.
Carbon nanotubes have been proposed for the tips of SPM, as by gluing a
carbon nanotube to the end of the cantilever beam. However, adhesive may fail,
particularly when the SPM is immersed in fluid. Furthermore, such SPM-tips will, in
principle, suffer from the same limitation as a conventional metallic SPM-tip, with
the simultaneous injection from a very broad band of electron states from the tip.
The use of nanotechnology in magnetic applications is well known. See, for
example, US-A-5,997,832 and WO 97/31139 to Lieber, which describe nanorods of
various materials, some of which are magnetic. The use of nanotechnology to develop
thin films for data storage applications is described in Shouheng Sun et al, Science
Vol. 287, 17 March 2000, entitled "Monodisperse FePt Nanoparticles and
Ferromagnetic FePt Nanocrystal Super-lattices". In the area of Spintronics, problems
arise in the efficient injection of spin-polarised electrons into the Spintronics device.
It has been proposed to use an SPM with ferromagnetic tip for such injections by a
vacuum tunnelling process. (Wolf et al,, Science Vol.294, pages 1488-1495,
16 November 2001, at page 1491.) See also Orgassa et al., Nanotechnology 12 ,
pages 281-284, (2001).)
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides a nanotechnological structure
for use in a scanning probe microscope, comprising a tip member, and a nanowhisker
projecting from a free end of the tip member, and being integral therewith.
There is thus provided a structure which may be used as a probe for a
Scanning Tunnelling Microscope (STM), AFM, and other forms of SPM, with
resulting technical advantages as set forth below. The tip member may be of any
desired shape, for example tubular, conical or triangular. In a common form of STM,
the tip member constitutes the end region of a metallic wire, and the nanowhisker
may be formed on a prepared region at the wire end. Alternatively, the tip member
may be formed as a separate member mounted on a substrate, or other appropriate
support, depending on the intended application. Both the tip member and
nanowhisker will usually be formed of conductive or semiconductive material, to
permit current flow, but there may be circumstances where insulative material is
employed, depending on the physical parameter used as a metric.
Measurements with STM are usually at the atomic scale for examining surface
features in extreme detail. Measurements with AFM, on the other hand, are more
commonly on a larger nanometer scale for examining engineered nanostructures.
Where, as is commonly the case, the probe structure is intended for atomic force
measurements, a tip support member may comprise a flexible elongate member or
beam of predetermined dimensions and mechanical characteristics, in particular
elasticity. The probe structure is then suitable for use in an Atomic Force Microscope
(AFM). The tip member may be integral with the beam, where the beam is of a
suitable material, e.g. silicon. Other forms of tip support member may be used, for
example V-shaped support members.
More specifically therefore, the invention provides a nanotechnological
structure, comprising a flexible support member, the support member having an
upstanding tip member at or adjacent a free end of the support member, and a
nanowhisker projecting from a free end of the tip member, and being integral
In a second aspect, the invention provides a method of forming a
nanotechnological structure for a scanning probe , comprising:
providing a tip member; and
forming a nanowhisker projecting from the tip member.
Li a preferred embodiment, the formation of the nanowhisker includes:
providing at the free end of the tip member a mass of catalytic material of
predetermined volume; and
heating the mass and exposing the mass to gases of predetermined type under
conditions such as to form, by the VLS process, a nanowhisker upstanding from the
It is possible and in accordance with the invention to have more than one
nanowhisker formed at the end of the tip member. More than one tip member may be
provided, each tip member having one or more nanowhiskers formed thereon. Such
tip members may be mounted on a single support, or may be independently mounted
for independent movement.
In at least one preferred embodiment of the invention, the tip member is
mounted on a cantilever beam of silicon or other conductive or semiconductive
material and has predetermined dimensions, usually in the micrometer range. The
beam has predetermined mechanical characteristics, in particular a predetermined
resilience in response to forces exerted on the end of the beam. The beam is formed
with an upstanding tip member at its free end. Where the beam is of a suitable
material such as Si, the tip member is formed integrally with the beam by a suitable
process such as micro-machining.
A nanowhisker is formed at the extreme end of the tip member and is
preferably grown by the process described in our copending US patent application
Serial No. 10/613,071 filed July 7, 2003 and International Application No.
PCT/GB03/002929, filed 8 July, 2003, the contents of which are incorporated herein
by reference. An area of gold or other catalytic material is provided on the end of the
tip member, as by a lithographic process, for example nanoimprint lithography (NIL),
or by the deposition of a gold nanoparticle. When heated in epitaxy apparatus, the
gold area coalesces and forms a catalytic melt. Gases introduced into the growth
system are absorbed by the melt, and form a eutectic alloy. Upon supersaturation, a
solidified material of desired composition, for example gallium arsenide, is deposited
at the interface between the melt and the semiconductor crystal underneath. In this
way a column is formed, and this column is termed a nanowhisker or nanowire.
A scanning probe microscope according to the invention has the feature that a
very narrow energy distribution of injected carriers may be provided. A very accurate
and sensitive tooJ for examination of a sample surface is therefore provided. This
narrow energy distribution may be obtained by the use of a degenerately doped large
band-gap semiconductor nanowire material (e.g. GaP, GaN, ZnO) that creates free
electrons in the conduction band of the semiconductor, with an energy range of about
lOmev - this is essentially independent of the specific material. Alternatively, an
even smaller energy distribution of about 1 mev may be obtained by the use of a
designed resonant tunnelling structure in the nanowire, for example. A resonant
tunnelling structure, consisting of a series of heterojunctions within the nanowire
between materials of different bandgap, is fully described in our copending US patent
application Serial No. 10/613,071 filed July 7, 2003 and International Application
PCT/GB03/002929, filed 8 July, 2003, the contents of which are herein incorporated
by reference, and is essentially formed by the process described above, but that the
gas constituents are rapidly switched during the growth of the nanowire to produce
segments of different material.
In either case, the nanowhisker may have a constant diameter cross-section
along its length, or, as preferred, a tapering/conical shape. The desired shape is
created by appropriate adjustment of growth conditions, principally temperature, as
described in our copending US patent application Serial No. 10/613,071 filed July 7,
2003 and International Application PCT/GB03/002929, filed 8 July, 2003.
The nanowhisker may be made of very precise dimensions, particularly in
diameter where it can be accurately dimensioned to a dimension of just a few
nanometers, that is less than lOnm. In general, the diameter of the nanowhisker may
be predetermined preferably within the range 5-50 nm. Its length may typically be
chosen to be anything between about 100 nm to several micrometers. The
nanowhisker thus formed constitutes an element of precise dimensions and
predetermined characteristics in the probe tip structure. When it is formed integrally
(rnonolithically) with the cantilever beam by the above process, it is very secure and
reliable in use, and further has a perfect, continuous and impedance-less electrical
coupling to the rest of the probe structure. This is in contrast to, for example,
arrangements employing carbon nanotubes glued onto a beam where there is a risk of
losing the tip, particularly when immersed in fluid, and further where a significant
electrical impedance may exist between the nanotube and the SPM.
A melt of catalytic material, remaining at the top of the nanowhisker, may in
some circumstances be undesirable; for example, it may affect the energy distribution
of a stream of electrons passing through the nanowhisker, and the shape of the
whisker end may not be especially well-defined. La accordance with a further aspect
of the invention, therefore, the melt may be removed. In a preferred embodiment,
using the techniques described hi our copending US patent application Serial No.
10/613,071 filed July 7,2003 and International Application PCT/GB03/002929, filed
8 July, 2003, the growth of the nanowhisker may be completed, by appropriate
change in growth conditions and substituting different gases in the reaction chamber,
to terminate the growth with a short segment of a "sacrificial" segment of a material
which is different from the major or adjacent part of the nanowhisker. For example,
the sacrificial material may be InAs where the whisker is GaAs, or GaAs where the
whisker is InAs. This sacrificial material may be later removed by a selective
etching, hence removing the catalytic (e.g., gold) particle and forming a fresh surface
which terminates the whisker. Further, the etching may produce a whisker end which
is sharply rounded or pointed, for further precision.
In a further aspect, the invention provides a process of forming a
providing a mass of catalytic material, and exposing the mass to one or more
gases under predetermined operating conditions to form by the VLS process a
terminating the growth of the nanowhisker by changing at least one operating
condition to provide at the end of the nanowhisker a segment of a different material
from that of the remainder or at least an adjacent portion of the nanowhisker, and
after formation of the nanowhisker, selectively etching the different material
so as to remove the different material and the mass of catalytic material there above.
As an alternative to gold catalytic material, the catalytic material may
comprise a group-III-metai such as Ga or In, which metal is comprised in the material
from which it is intended to form the nanowhisker. The nanowhisker may be fonned
simply of the group-III-metal alone, or the metal alloyed with a group-V-materia] to
form a semiconductor compound. In either case, the catalytic melt which remains at
the free end of the nanowhisker after the nanowhisker is formed is the same material
as that of the remainder of the nanowhisker, and this may be of advantage in some
The present invention envisages use of probe structures in bio sensing
applications. A bio sensing technique may be regarded as any sensor method which
utilises bio molecules such as, inter alia, nucleic acid, proteins or antibodies or
fragments, binding or amplification interactions being typical. A nanowhisker
incorporated into an SPM tip, in accordance with the invention, may have a coating
for binding predetermined molecules thereto, or the coating including biologically
A nanowhisker incorporated into an SPM tip in accordance with this aspect of
the invention is particularly adapted as a highly localised sensor for sensing
parameters of biological molecules, e.g. DNA. For example, such molecules may be
positioned on a substrate, and an AFM may be arranged to scan over the surface of
the substrate, and map properties of the DNA. Further, the nanowhisker incorporated
into the SPM tip may be formed of silicon or other oxidisable material. The
nanowhisker is oxidised to form a surrounding layer of oxide along its length, but
with the gold or other catalytic seed particle melt at the free end of the nanowhisker
remaining free of oxide. This therefore provides a highly accurate probe for
examining biological surfaces, where the interaction occurs within a precisely defined
region. This permits mapping of molecules in a height direction, as well as planar
directions, thus enabling a three dimensional XYZ mapping.
Further, and in accordance with the invention, a nanowhisker incorporated in
an SPM tip may have a series of segments of different material along its length, such
as to create between heterojunctions a light emitting diode of very small dimensions,
for example, as small as 20nm3. The wavelength of such a diode may be
predetermined to a desired value by appropriate choice of materials and dimensions.
Such diode, when appropriately energised, can be arranged to emit a single photon as
and when required, and this can be employed to irradiate a biological sample (e.g.
tissue, cell or molecule). The irradiation of biological samples with electromagnetic
radiation is an extremely sensitive tool for determining optical absorbance of
molecules, phosphorescence, luminescence, etc.
As regards magnetic applications, in the present invention, a probe tip
structure having a nanowhisker is of use for current injection purposes into an
electrical circuit, where the electrons forming the electric current should have
precisely determined parameters of spin. For example, where the nanowhisker is
formed of a magnetic material such as MnlnAs, MnGaAs, MnAs, or a semimagnetic
material, spin polarised electrons may be emitted from the tip of the whisker (a
semimagnetic material is a semiconductor compound containing a dilute
concentration of magnetic ions, e.g. Mn). Whilst the tip structure may be provided on
any suitable support member, e.g. a rigid substrate or metal wire, it is preferred to use
a cantilever beam construction, as the resilience of the beam gives a reliable contact,
and the dimensions of the beam and tip structure are compatible with the dimensions
of the circuit into which the electrons are injected.
As an alternative, the cantilever beam and tip member are formed of
ferromagnetic material for polarising and alignment of the electron spins prior to the
electrons entering the nanowhisker. The nanowhisker may then act as a conduit for
the spin polarised electron stream. This may be an advantage where it is inconvenient
to form the nanowhisker of a ferromagnetic material.
A further aspect of the invention is based on an array of nanowires or
nanowhiskers formed of an appropriate magnetic material and employed as a data
storage medium, wherein each nanowire may be selectively magnetised in a spin-up
or spin-down condition to represent a "1" or "0" bit.
With regard to ferromagnetic properties, nanowhiskers may present a
possibility for retaining ferromagnetism in very small regions. There is much interest
in magnetic memory devices employing very small, typically single-domain,
magnetic particles, or similar structures, as memory elements. However, it is known
that as the size of a ferromagnetic single domain is reduced a limit is reached below
which the ferromagnetic state cannot exist, and the domain, e.g., the single particle,
assumes the superparamagnetic state in which the magnetic moments of all the atoms
still line up to form the collective huge magnetic moment as in a ferromagnet, but
where the orientation of this huge spin is no longer locked into a defined direction as
it is in a ferromagnet,. This limit is typically about 50nm for a spherical magnetic
particle. However, when a magnetic domain, e.g., a ferromagnetic domain, is
incorporated into a nanowhisker, the diameter at which the domain ceases to be
ferromagnetic and undergoes a transition to the superparamagnetic state can be
reduced, because the substantially one-dimensional character of the nanowhisker
tends to restrict the possible reorientation of the magnetic moment of the ions (or
atoms) of the magnetic material. The material of the whisker can be made of iron,
cobalt, manganese, or an alloy thereof. Other possible materials include manganese
arsenide (ferromagnetic). Accordingly, it is possible to reduce the size of a
ferromagnetic domain formed in a nanowhisker to less than the conventional lower
limit for a particular material. Thus, ferromagnetic properties may be retained, at
least for some magnetic materials, at transverse dimensions of lOnm or less by
forming them into nanowhiskers having a diameter of 10 nm or less. Such very small
ferromagnetic elements have evident uses in the field of magnetic memory devices.
Thus it is possible in accordance with the invention to prepare smaller
magnetic memory elements that can be selectively magnetized and produce a
magnetic flux that can be sensed. The reduced symmetry in the nanowire (or
nanowhisker) geometry may make possible a higher Curie temperature for magnetic
semiconductor materials. Furthermore, the freedom in combining materials (inside a
whisker) having different lattice constants may enhance the use of new magnetic
semiconductors for these applications, such as MnGaP and MnGaN, which may have
Curie-temperatures above room-temperature. Alternatively, metallic ferromagnetic
materials including elements such as Fe, Co, Ni may be employed.
In general, the invention may be practised with a ferromagnetic material, a
semimagnetic material (a dilute solution of magnetic ions in a semiconductor matrix),
or other appropriate magnetic material, such as ferrimagnetic.
In a further aspect therefore, the present invention provides a nanowhisker
comprising magnetic material, the diameter of the nanowhisker being such that a
single ferromagnetic domain exists within the nanowhisker. Preferably the diameter
of the nanowhisker is not greater than about 25 nm, preferably not greater than about
The nanowhiskers produced in accordance with the inyention may be
essentially cylindrical and have a constant diameter, or may have a slightly tapered
form, depending on the precise nanowhisker growth conditions. Where the diameter
is not strictly constant along the length of the nanowhisker, the diameter of the
nanowhisker is to be regarded as an average value.
In a further aspect, the present invention provides a data storage medium
comprising an array of nanoelements, preferably nanowhiskers, each including
magnetic material, and read/write structure for selectively magnetising each
nanowhisker in either of first and second magnetised directions and sensing the
magnetised direction of each nanowhisker.
The sensing device preferably comprises an SPM type arrangement, with a
cantilever support provided with a dp member and nanowhisker for providing a
stream of spin-polarised electrons, as described above. Such tip structure (tip
member and nanowhisker) may be moved across the array to scan the nanoelements,
and may be selectively positioned in alignment with an element, in order to sense the
direction of magnetisation. The impedance of the element to current flow provides an
indication of magnetisation direction. The device for writing magnetisation direction
may comprise the device for sensing, but wherein the magnitude of the spin polarised
current is greatly increased to force the nanoelement into a desired magnetisation
direction. Alternatively, a separate writing head may be provided which comprises,
merely by way of example, a tip which can be strongly magnetised to selectively
magnetise, by means of its magnetic field, the nanoelements.
In a further aspect, the invention provides a method forming a data storage
forming volumes of catalytic material at predetermined sites on a substrate;
growing at each site, a nanowhisker of magnetic material and of such
dimensions that only a single ferromagnetic domain exists within the nanowhisker.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described with reference
to the accompanying drawings wherein:
Figures 1-lf show steps in the process of formation of a tip for an atomic
force microscope (AFM), forming a first embodiment of the invention;
Figures 2a and 2b show a second embodiment of the invention comprising a
tip for a scanning tunnelling microscope (STM),
Figure 3 shows a third embodiment of the invention adapted for determining
properties of biological samples;
Figure 4 shows a fourth embodiment of the invention comprising a
nanostructure that forms a mechanism for current injection of spin polarised electrons
into a Spintronics circuit;
Figures 5a-5c show a fifth embodiment of the invention comprising an array
of nanowhiskers of magnetic material forming a data storage medium; and
Figures 6a-6e show a process for forming the nanowhisker array.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Figure la, a tip for an AFM comprises a beam of silicon 2
which is micro-machined (for example by etching) to form a rectangular elongate bar
of a length, for example between 100 and 500 pm, and having a rectangular crosssection
50 x 5 fun. This provides a bar with a predetermined resilience to flexure.
This resilience makes the structure suitable for use in an AFM. At one end of the
beam 2, a conical tip 4 is formed integrally with the beam, with a base 10 \an wide
and a height of 20 um. The extreme end 6 of tip 4 has a dimension about 20nm
As shown in Figure Ib, a volume 10 of gold is affixed to the end 6 of the tip.
A variety of techniques may be employed for carrying out this step. For example, the
gold 10 may be electrolytically plated by immersing the end 6 in a solution
containing gold ions and employing the tip as one electrode of a pair of electrodes,
with a voltage applied between the electrodes. Alternatively, a beam of molecules
may be directed at the end 6, in molecular beam apparatus. The molecules are of
organometallic type containing gold ions. Under appropriate operating conditions,
the incident molecules fragment at the end 6, with the gold ions bonding to the end 6.
As a further alternative, an aerosol droplet of gold may be affixed to the end of the tip
by exposing the tip to such aerosol. Desirably a voltage is applied to the tip, to attract
droplets via the electric field in the region of end 6. None of these techniques is
illustrated, since their implementation would be straightforward for a person skilled
in the art.
After formation of the gold volume 10 on end 6, the beam 2 is then moved
into a Chemical Beam Epitaxy (CBE) apparatus 14, Figure Ic. The beam is heated to
a temperature of around 400°C so that the gold melts and coalesces into a particle 12.
A beam of organic molecules containing gallium, TMGa (trimethylgallium) or TEGa
(triethylgallium) is then injected into the source chamber 14, and a gas containing
arsenide ions, for example TBAs (tributylarsine) or AsHa, is introduced into the
chamber. The TBAs material is decomposed by the high temperature employed
whereas the group III molecules, TMGa or TEGa are broken down at the sample
surface. In any event gallium and arsenic atoms are absorbed by the gold catalytic
particle 6 to form a eutectic alloy. Upon further absorption, the eutectic alloy
supersaturates and gallium arsenide is deposited between the particle 12 and the
surface of the tip free end, whereby to form a nanowhisker column 16. This process
is more fully described in our International Application PCT/GB03/002929, filed 8
July, 2003. Depending on the temperature employed, the nanowhisker may be
perfectly cylindrical, or, as preferred, it may be formed conically. The diameter of
the nanowhisker depends on the initial area of the gold 10 and the resultant diameter
of the particle 12. The resultant AFM tip is shown in Figure Id.
There is thus formed, as shown schematically in Figure Id, a tip for an atomic
force microscope or other microscopic instrument with the novel property that a very
narrow energy distribution of injected carriers may be designed and controlled. This
narrow energy distribution may be obtained by the use of a degenerately doped large
band-gap semiconductor nanowire material (e.g. GaP, GaN, ZnO). that creates free
electrons in the conduction band of the semiconductor, with an energy range of about
lOmev - this is essentially independent of the specific material. Alternatively, an
even smaller energy distribution of about 1 mev may be obtained by the use of a
designed resonant tunnelling structure in the nanowire. A resonant tunnelling
structure, consisting of a series of heterojunctions within the nanowire between
materials of different bandgap, is fully described in our Copending United States
Application 10/613,071 and International Application PCT/GB03/002929, filed 8
July, 2003, the contents of which are herein incorporated by reference, and is
essentially formed by the process described above, but that the gas constituents are
rapidly switched during the growth of the nanowire to produce segments of different
material. This is shown schematically in Figure le, where the nanowhisker 16
comprises segments 17 of wide band gap material bounding a conductive segment 18
of low band gap material in order to form a resonant tunnelling diode (RTD).
In an alternative construction, the material of the segment 18, and its width
along the length of the nanowhisker, are selected in order to produce a light emitting
diode of a particular wavelength, as more fully described with reference to Figures 15
and 16 of International Application PCT/GB03/002929, filed 8 July, 2003. The diode
may be so small (20nm3) that it may be regarded as a point source, and the diode may
be accurately controlled so as to be capable of emitting single photons "on demand".
This may be of use in mapping and scanning biological molecules, as described
In an alternative construction, as shown in Figure If, a short segment 20 of a
sacrificial material such as InAs is formed at the end of a GaAs nanowhisker, by
rapidly switching the constituents of the gas in the CBE chamber. A subsequent
etching process with a suitable acid removes the segment 20, and the gold particle
melt 12. The remaining nanowhisker 16 is of the same material throughout in this
example (although it may include portions or segments of different materials), and
has a well-defined end, the etching process producing a pointed or sharply rounded
end 22. The diameter of the wire at its end may be between 5 and 25 nm. Whilst the
whisker could in principle be made of smaller diameter, it has been found that this
range is suitable for the intended applications of an AFM. This construction is of
advantage where it is necessary to have a well-defined stream of electrons flowing
through the nanowhisker.
Although as described above, the AFM. tip has a flexible cantilever beam, this
is not strictly necessary for other applications, and a rigid substrate or other support
member may replace the beam.
Figures 2 and 2b show a probe for an STM according to a second embodiment
of the invention. In Figure 2a, a support 24 mounts an STM dp structure comprising
a metallic wire tip member 26 held in a holder 28. The end of the wire 26, as shown
in Figure 2b, is tapered as at 30. A nanowhisker 34 is formed at the end, in
accordance with the processes described above with reference to Figures Ib to Ig.
Since STM applications usually require measurements of an atomic scale, the
nanowhisker may have a very small diameter, at least at its tip, say lOnm or less, or
even less than 5nm.
Referring now to Figure 3, a third embodiment is shown comprising a tip
structure of an AFM, with integral nanowhisker, where similar parts to those of
Figure 1 are denoted by the same reference numerals. A nanowhisker 36 is formed by
the method described above. The whisker is formed of silicon and has a gold particle
melt 12 at one end. Subsequent to formation of the whisker, the whisker is exposed
to an atmosphere at a suitable temperature for oxidation of the silicon. This forms an
outer shell 38 of silicon dioxide surrounding the whisker and extending along its
length. The gold particle melt 38 remains in an unoxidised condition.
This therefore provides a structure highly suitable for precise examination of
biological samples, since the region of interaction with the biological sample is very
precisely defined. The nanowhisker 36, 38, 12 may be used, for example, to map
properties of biological tissue in three directions of movement of the tip structure,
As an alternative, the whisker 36 may be exposed to an atmosphere of a
suitable material for forming a high band gap material as an alternative to the
oxidation layer 38. The gold particle melt 12 may in either case be coated with an
enzyme material or other biologically active material, in order to create desired
reactions with biological samples.
In an alternative construction for three dimensional mapping and
characterisation of biological tissue, a light emitting diode is formed within a
nanowhisker 16, 17, 18, as described above with reference to Figure le. The
interaction of light with biological tissue provides a highly sensitive tool for
characterising the tissue, particularly where the diode is so small (20nm3) that it may
be regarded as a point source, and where the diode is capable of emitting single
photons "on demand".
Referring now to Figure 4, a fourth embodiment of the invention is shown for
use in the field of Spintronics. Spintronics is a technical field where the properties of
electronic devices rely on the transport of electron spin through the device. In
Figure 4 similar parts to those of Figure 1 are denoted by similar reference numerals.
A whisker 40, formed at the end of the tip member 4, by the process described above,
is of a magnetic material (MnlnAs, MnGaAs, MnAs) or semimagnetic material,
containing a dilute concentration of Mn. Under an applied voltage V, spin polarised
electrons 44 are emitted from the tip of the whisker, which makes electrical contact
with an electrical contact 46 disposed on a substrate 48. The spin polarised electrons
44 are injected by means of a tunnelling process into contact 46 and are then used for
a desired function, such as reading the state of a magnetic memory element, such as
nanopillar 49 disposed on substrate 48 and electrically connected by means of lower
and upper electrical conductors diagrammatically shown at 50L and 50U respectively.
In a fifth embodiment, as shown in Figure 5a, a regular array of nanowhiskers
50 is formed on a substrate 52. Only a small part of a practical array is shown in
Figure 5a, and, for clarity, only the sites of many of the nanowhiskers are indicated.
Each nanowhisker 54 is of a diameter 20 nm and is formed of a magnetic material
(e.g. Fe, Co, Mn, MnAs, MnGaAs, MnlhAs) which consists of a single ferromagnetic
domain and may be in spin-up condition as shown in Figure 5b or a spin-down
condition as shown in Figure 5c. When incorporated in a nanowhisker, in accordance
with the invention, the domain, diameter can be reduced because of the reduced
possibilities for geometrical symmetrical alignment in a one-dimensional system,
which makes it more difficult for the ions of the material to have more than one
orientation. The material of the whisker can include iron, cobalt, manganese, or an
The array 50 is arranged as a square matrix with rows and columns 56, 58.
Each nanowhisker is 20mn in diameter, and is spaced by a distance of lOnm from
adjacent nanowhiskers in row and column directions. In general, the spacing between
adjacent nanowhiskers should be less than twice their diameter. This value represents
a compromise between the requirement for the nanowhiskers to be as closely packed
as possible, and a requirement that the nanowhiskers be sufficiently well spaced that
they may be individually monitored. Instead of a rectangular matrix, the
nanowhiskers may be arranged in any desirable configuration, such as a hexagonal
lattice configuration (hexagonal close packed), or even a linear arrangement. A
cantilever & tip arrangement 2, 4, 40, similar to that of Figure 4, is employed as a
read/write head which is movable over the array to scan the array in row and column
directions X, Y. The head movement is controlled by conventional SPM techniques
for selective positioning directly overhead in alignment with each nanowhisker.
In a read or sensing mode, the head 2, 4, 40 emits a weak current of spinpolarised
electrons into the adjacent nanowhisker. The impedance of the
nanowhisker to current flow provides an indication of magnetisation direction.
In a write mode, the magnitude of the current of spin polarised electrons
emitted from the head is greatly increased and is sufficient, when flowing through the
nanowhisker, to force the nanowbisker into a desired direction of magnetisation.
As regards the process of forming the array of nanowhiskers, gold catalytic
areas are formed on substrate 52 by a NIL process at the desired sites of the
nanowhiskers 54. This is shown in Figures 6a-e, which are sectional views of part of
a row of sites. In Figure 6a, substrate 52 has formed on its upper surface a layer of
deformable polymer 60. The polymer has been deformed by a rigid stamp (not
shown) to form rectangular depressions at the intended sites 62 of the nanowhiskers.
The polymer is then etched, so as to remove the polymer in the site depressions 62,
and a layer of gold 64 is applied. The result is shown in Figure 6b, where the gold 64
makes contact with the substrate at the sites, and is elsewhere disposed on top of the
remaining polymer 60. Finally, as shown in Figure 6c, a further etching step removes
the remaining polymer areas, to leave gold regions 66 at the nanowhisker sites 62.
The substrate is then transferred to a epitaxial growth reaction chamber, where
heat is applied to make the gold areas coalesce into particles 12, as indicated in
Figure 6d. Gases are introduced into the reaction chamber, and nanowires 54 are
grown by the VLS process, Figure 6e. The nanowires are precisely formed, and are
precisely located at the desired locations. If desired, a subsequent etching step may
remove the gold particles at the end of the nanowires as previously described.
1. A nanotechnological structure for a scanning probe microscope, comprising a tip member (4), upstanding from a support member (2), and a nanowhisker (16) grown on and projecting from a free end (6) of the tip member (4), characterized in that the nanowbisker (16) comprises a degenerately doped large band-gap semiconductor material that in use creates free electrons in the conduction band of the semiconductor material in order to provide a narrow energy distribution of electrons flowing through the nano whisker (16).
2. A structure as claimed in claim 1, wherein the support member (2) comprises a flexible member of predetermined dimensions and mechanical characteristics, the upstanding tip member (4) being located at or adjacent a free end of the flexible member.
3. A structure as claimed in claim 2, wherein the flexible member comprises an elongate beam.
4. A structure as claimed in claim 1, wherein the range of the narrow energy distribution of the electrons that is provided by the degenerately doped large band gap semiconductor material is about 10 meV.
5. A structure as claimed in claim 1, wherein the nanowhisker (16) comprises a heterojunction between segments (17, 18) of semiconductor materials of different band-gap, and one of said segments (17,18) comprises the degenerately doped large band gap semiconductor material.
6. A structure as claimed in claim 5, wherein the nanowhisker (16) comprises a resonant tunnelling diode structure including a sequence of segments of semiconductor material of different band gaps.
7. A structure according to claim 6, wherein the resonant tunnelling diode structure in use provides an energy distribution of electrons that has a range of about 1 meV.
8. A structure as claimed in claim 1, wherein the nanowhisker (16) comprises a light emitting diode structure including a sequence of segments of semiconductor material of different band gaps.
9. A structure as claimed in claim 1, wherein a coaxial layer of material that is inert to biological material is provided along a length of the nanowhisker.
10. A structure as claimed in claim 7, wherein the nanowhisker (16) is formed of silicon, and the coaxial layer is silicon dioxide.
11. A method of forming a nanotechnological structure for a scanning probe microscope, comprising: providing a tip member (4), upstanding from a support member (2); providing at a free end (6) of the tip member (4) a mass of catalytic material (10); and heating the mass and exposing the mass to gases of predetermined type under conditions such as to form, by a VLS process, a nanowhisker (16) comprising a degenerately doped large band-gap semiconductor material, the nanowhisker (16) being grown on a free end of the tip member (4) and upstanding from the tip member (4).
12. A method as claimed in claim 11, wherein the mass of catalytic material (10) is provided on the tip member free end (6) by an electrolytic process.
13. A method as claimed in claim 11, wherein the mass of catalytic material (10) is provided on the tip member free end (6) by depositing an aerosol particle thereon.
14. A method as claimed in claim 11, wherein the gases of predetermined type are switched during the growth to form a nanowhisker (16) that has segments (17, 18) of semiconductor material of different band gaps.
15. A method as claimed in claim 11, wherein the nanowhisker (16) is formed to have a light emitting diode structure having a sequence of segments of semiconductor material of different band gaps.
16. A method as claimed in claim 11, wherein the catalytic material is of a same material as the nanowhisker (16).
17. A method as claimed in claim 11, wherein the nanowhisker (16) is formed of an oxidisable material, and the method further comprises exposing the nanowhisker (16) to an oxidising environment so as to form a coaxial oxide layer along a length of the nanowhisker (16).
18. A method as claimed in claim 11, further comprising: terminating growth of the
nanowhisker (16) by changing at least one operating condition to provide at the end of the
nano whisker (16) a segment (20) of a different material from that of an adjacent portion of the
nanowhisker (16); and selectively etching the different material so as to remove the different
material and the catalytic material from the nanowhisker (16).
19. A method as claimed in claim 11, wherein the support member (2) comprises an
20. A method as claimed in claim 18, wherein the end of the nanowhisker is etched
to produce a sharply rounded or pointed end (22).
|Indian Patent Application Number||50/DELNP/2006|
|PG Journal Number||5/2010|
|Date of Filing||03-Jan-2006|
|Name of Patentee||QUNANO AB.,|
|Applicant Address||IDEON SCIENCE PARK SE-223 70 LUND, SWEDEN.|
|PCT International Classification Number||G12B 21/02|
|PCT International Application Number||PCT/GB2004/000066|
|PCT International Filing date||2004-01-07|