WO2008030110A2 - Methods of forming nanoparticles - Google Patents

Abstract

The present invention provides a method for preparing nanoparticles of group IV elements, particularly nanoparticles of Si, Ge and Sn, and binary and ternary alloys of these elements. The method comprises the solution-phase decomposition of one or more group IV metal precursors at elevated temperature and under an inert atmosphere at atmospheric pressure, using a decomposition-promoting reagent. A surface-bonding agent is added to the reaction mixture to form an organic layer surrounding the nanoparticles and prevent aggregation.

Description

METHODS OF FORMING NANOPARTICLES

FIELD OF THE INVENTION

The present invention relates to methods for preparing nanoparticles of group IV elements. It relates particularly to the preparation of nanoparticles of Si, Ge and Sn, and binary and ternary alloys of these elements.

BACKGROUND TO THE INVENTION

The invention relates to quantum dots, also known as nanoparticles or nanocrystals.

The term "nanoparticle" is generally invoked to refer to particles that have an average diameter between about 1 run and about 100 nm. Nanoparticles have a size intermediate between individual atoms and macroscopic bulk solids. Nanoparticles that have diameters smaller or comparable to the Bohr exciton radius of the material can exhibit quantum confinement effects. Such effects can alter the optical, electronic, catalytic, optoelectronic, thermal and magnetic properties of the material.

Many nanoparticles exhibit photoluminescence effects that are significantly greater than the photoluminescence effects observed for macroscopic crystals having the same composition. Additionally, these quantum confinement effects may vary as the size and surface chemistry of the nanoparticle is varied. For example, size-dependent discrete optical and electronic transitions exist for nanoparticles of group II- VI semiconductors (e.g., CdSe) and group III- V semiconductors (e.g., InP).

The gas phase synthesis of group IV nanoparticles through processes such as metal organic chemical vapour deposition (MOCVD) is well-known. However, such approaches produce low yields and are highly capital intensive. The solution-phase synthesis techniques used to synthesise group II- VI and IH-V semiconductors have not been readily applied to group IV materials, largely due to the high temperatures required to produce highly crystalline nanoparticles in a high yield. The strong covalent bonding of amorphous Si and Ge means that synthesis temperatures significantly higher than those used for the group II- VI materials are required in order to achieve highly crystalline cores at commercially viable production rates, hi addition, the temperature required to thermally degrade many liquid phase group IV precursors exceeds the boiling points of many typical solvents at atmospheric pressure. There have been a few reports of the solution-phase reduction of group IV salts and of aerosol methods. However, the nanoparticles produced by these methods often have extremely broad size distributions and poor visible luminescence efficiencies.

In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a method of making nanoparticles that provides an alternative to those currently available; and/or to provide a method of making nanoparticles that produces nanoparticles of good mono-dispersity and/or of high luminescence.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method of preparing nanoparticles of one or more group IV metals or alloys thereof comprising the steps of: reacting, under an inert atmosphere, at atmospheric pressure and with heating, one or more group IV metal precursors with a decomposition-promoting reagent in a liquid reaction medium comprising a high temperature surfactant; adding a surface-bonding agent; and recovering the nanoparticles.

In one embodiment, the liquid reaction medium may comprise a high temperature solvent and a high temperature surfactant.

In a preferred embodiment, the group IV metal precursor comprises a compound of the general formula: G(Ar)_x Y_4-x; wherein G is the group IV metal, Ar is aryl, Y is halo and x takes a value that is at least 0 and no greater than 4.

In an alternative embodiment, the group IV metal precursor comprises a compound of the general formula: G(Ar)_yY_2-y wherein G is the group IV metal, Ar is aryl, Y is halo and y takes a value that is at least 0 and no greater than 2.

Preferably, the decomposition-promoting reagent is selected from one of: a) a strong reducing agent; or b) S, Se₅ Te, P or As or a compound comprising one or more of these elements in a zero valence state.

In one embodiment, the method of the invention includes a further step of adding a quenching agent prior to adding the surface-bonding agent. Preferably, the step of adding a quenching agent is prior to adding the surface-bonding agent but after adding the decomposition-promoting reagent.

In a preferred embodiment, the decomposition-promoting agent is selected from the group consisting of: S; Se; Te; P; As; and compounds comprising one or more of these elements in a zero valence state, and the method includes the step of adding a quenching agent.

In one embodiment, the surface-bonding agent may also act as the quenching agent.

Preferably, the step of adding the surface-bonding agent is effective to prevent aggregation of the nanoparticles. Preferably, the surface-bonding agent interacts with the nanoparticles to provide an organic layer surrounding the nanoparticles.

In a preferred embodiment, the surface-bonding agent is a carboxylic acid, aldehyde, amide or alcohol. More preferably, the surface-bonding agent is a carboxylic acid and, therefore, the resulting nanoparticles are "acid-terminated".

In another preferred embodiment, the surface-bonding agent comprises an alkenyl or alkynyl moiety.

Accordingly, one preferred embodiment of the invention comprises preparing acid-terminated nanoparticles of one or more group IV metals or alloys thereof; preferably acid-terminated nanoparticles of Ge, Si or Sn, or a binary or ternary alloy thereof.

Preferably, the step of reacting includes heating to a temperature between about 100°C and about 400⁰C; more preferably to between about 200⁰C and about 400⁰C; more preferably to about 300⁰C.

Preferably, the method of the invention is complete in less than about 30 minutes. More preferably, the method is complete in less than about 20 minutes.

Preferably, the method includes a further step of purifying the nanoparticles. Preferably, the method produces nanoparticles with size in the range about 1 nm to about 20 nm, more preferably about 1 nm to about 10 nm.

Preferably, the method produces a monodisperse nanoparticle size distribution such that the nanoparticle diameter has a standard deviation of less than 20% of the mean diameter. More preferably, the method produces a monodisperse nanoparticle size distribution such that the nanoparticle diameter has a standard deviation of less than 5% of the mean diameter.

Preferably, the method produces a solution of nanoparticles having a concentration >1 gl^'1; more preferably >1 O gI^"1.

Preferably, the method produces nanoparticles with a chemical reaction yield >50%; more preferably >60%.

Preferably, the method produces nanoparticles that produce luminescence in response to optical excitation with a quantum efficiency in excess of 1%. More preferably, the nanoparticles produce luminescence in response to optical excitation with a quantum efficiency in excess of 20%.

Preferably, the method produces nanoparticles with a high degree of crystallinity. hi a preferred embodiment, wherein G is germanium, the crystal structure is substantially that of diamond.

In a further aspect, the present invention provides nanoparticles of a group IV metal or a group, IV metal alloy prepared substantially according to the method of the invention.

In a yet further aspect, the present invention provides a method of preparing chemically functionalised nanoparticles of one or more group IV metals or alloys thereof comprising the steps of: reacting hydrogen-terminated nanoparticles of the group IV metals or alloys thereof with a compound of the formula L-R-N; wherein R is alkyl, alkenyl or aryl, L is a group having the desired functionality and N is a functional group capable of bonding to the hydrogen- terminated nanoparticle surface; and recovering the chemically functionalised nanoparticles. Suitable N groups include, but are not limited to: -NH₂; -COOH; -CONH₂; -CONH₂; -OH; - CHO; -SO₃H; -PO₃H₂; -PH₂; -SH; -CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

Preferred chemically functionalised nanoparticles include water soluble nanoparticles prepared by reacting hydrogen-terminated nanoparticles with a compound of the formula L-R-N; wherein L is a polar functional group. An alternative embodiment provides biochemically functionalised nanoparticles reacting hydrogen-terminated nanoparticles with a compound of the formula L-R-N; wherein L is a functional group capable of binding to a biological antibody and/or biologically active molecule. Suitable L groups include, but are not limited to: -NH₂; -COOH; -CONH₂; -OH; -CHO; -SO₃H; -PO₃H₂; -PH₂; -SH; -CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

hi another aspect, the present invention provides chemically functionalised nanoparticles of one or more group IV metals or alloys thereof prepared according to the method of the invention.

In another aspect, the present invention provides a method of preparing hydrogen-terminated nanoparticles of one or more group IV metals or alloys thereof; preferably hydrogen-terminated nanoparticles of Ge, Si or Sn, or a binary or ternary alloy thereof; comprising reacting acid-, aldehyde-, alcohol- or amide-terminated nanoparticles of the invention with a hydride reducing agent in the absence of water and oxygen; and recovering the hydrogen-terminated nanoparticles.

hi a preferred embodiment, the hydrogen-terminated nanoparticles are prepared from acid-terminated nanoparticles of the invention.

hi another aspect, the present invention provides hydrogen-terminated nanoparticles of one or more group IV metals or alloys thereof prepared substantially according to the method f the invention.

Nanoparticles of the invention are suitable for incorporation into a matrix of a second material such as a polymeric, ceramic, metallic or other material. They are also suitable for the preparation of devices such as optoelectronic devices, including photovoltaic devices, and biochemical imaging devices.

Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

As used herein, the term "aryl" is intended to include optionally substituted aromatic radicals including, but not limited to: phenyl; naphthyl; indanyl; biphenyl; and the like; and optionally substituted heteroaromatic radicals including, but not limited to: pyrimidinyl; pyridyl; pyrrolyl; furyl; oxazolyl; thiophenyl; and the like. As used herein, the term "alkyl" is intended to include optionally substituted straight chain, branched chain and cyclic saturated hydrocarbon groups.

As used herein, the term "alkenyl" is intended to include optionally substituted straight chain, branched chain and cyclic mono-unsaturated hydrocarbon groups.

As used herein, the term "alkynyl" is intended to include optionally substituted straight chain and branched chain hydrocarbon groups that include a -G≡C- moiety.

As used herein, the term "halo" refers to an iodo, bromo, chloro or fluoro group.

Where a particular compound includes more than one alkyl, alkenyl, alkynyl, aryl or halo group, each of such groups may be independently selected.

As used herein, the term "optionally substituted" is intended to mean that one or more hydrogen atoms in the group is replaced with one or more independently selected suitable substituents, provided that the normal valency of each atom to which the optional substituent/s are attached is not exceeded.

As used herein, the terms "nanoparticle", "nanocrystal" and "quantum dot" refer to any particle less than 100 nanometres in size.

Although a "nanocrystal" may have a higher degree of crystallinity than a nanoparticle, references to nanoparticles in this specification should be understood by one skilled in the art to also include nanocrystals and quantum dots.

As used herein, the "size" of a nanoparticle refers to the diameter of the core of the nanoparticle. A nanoparticle will typically comprise a core of one or more first materials and can optionally be surrounded by a shell of a second material.

A nanoparticle of the invention will typically comprise a "core" of a group IV metal (such as silicon, germanium or tin) or an alloy thereof and can be optionally surrounded by a "shell" of a second material. The term "core" refers to the central region of the nanoparticle. A core can substantially include a single homogeneous material. A core may be crystalline, polyatomic or amorphous. While a core may be referred to as crystalline, it is understood that the surface of the core may be amorphous or polycrystalline and that this non-crystalline surface layer may extend to a finite depth into the core. The "shell" of a nanoparticle may comprise a layer of either organic or inorganic material or a bi-layer comprising both an inner inorganic layer and an outer organic layer, or vice versa. The shell material may be selected to minimise the number of "dangling bonds" at the surface of the nanoparticle core. The shell material in the nanoparticles of the present invention is generally dictated by the surface-bonding agent employed in the method of the invention.

As used herein, the term "photoluminescence" of nanoparticles refers to the emission of light of a first wavelength (or range of wavelengths) by the nanoparticles following irradiation with light of a second wavelength (or range of wavelengths). The first wavelength (or range of wavelengths) is longer than the second wavelength (or range of wavelengths).

As used herein, the term "quantum efficiency" of the nanoparticles refers to the ratio of the number of photons emitted by the nanoparticles to the number of photons absorbed by the nanoparticles.

As used herein, the term "monodisperse", with respect to nanoparticles, refers to a population of nanoparticles wherein at least 75% and preferably 100% of the population, (or an integer or non- integer there between) falls within a specified particle size range. A population of

'monodispersed' particles has a standard deviation of less than 20% of the mean diameter and a 'highly monodispersed' population has a standard deviation of less than 5% of the mean diameter. '

As used herein, the term "surfactant molecule" refers to a molecule containing a non-polar end comprising an alkyl, alkenyl or aryl group or combination thereof and a polar end containing one or more groups selected from: various acids, such as carboxylic, sulfinic, sulfonic, phosphinic and phosphonic acids, and their salts; primary, secondary, ternary or quaternary amines; halides; oxides; sulfides; thiols; phosphines; phosphides; phosphates; and glycols.

As used herein, the term "decomposition-promoting reagent" refers to a compound or material that accelerates a chemical reaction involving a group IV metal precursor compound at a given temperature, such as to yield one or more group IV metals.

As used herein, the term "and/or" means "and" or "or", or both.

As used herein, "(s)" following a noun means the plural and/or singular forms of the noun. The term "comprising" as used in this specification means "consisting at least in part of. When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement or claim, all need to be present, but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

It is intended that reference to a range of numbers disclosed herein (by way of example only, with respect to the size of nanoparticles, 1 run to 10 nm) also incorporates reference to all integers and non-integers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of integers and non-integers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7),

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and with reference to the drawings in which:

Figure 1 : is a generalised flow diagram of the method of the invention;

Figure 2: is a flow diagram of one embodiment of the method of the invention;

Figure 3 : is a flow diagram of an alternative embodiment of the method of the invention;

Figure 4a: is a transmission electron micrograph of germanium nanoparticles prepared according to the invention;

Figure 4b: is another transmission electron micrograph of germanium nanoparticles prepared according to the invention;

Figure 5: is the X-ray diffraction data for germanium nanoparticles prepared according to the invention; Figure 6: shows electron diffraction by germanium nanoparticles prepared according to the invention;

Figure 7: shows the luminescence spectra of germanium nanoparticles prepared according to the invention;

Figure 8: is a diagram of typical glassware apparatus useful in a method of the invention;

Figure 9a: represents a chemically functionalised nanoparticle, having a surface organic layer, prepared according to the invention;

Figure 9b: represents a hydrogen-terminated nanoparticle prepared according to the invention; and

Figure 9c: shows a carboxylic acid group bound to the surface of an acid-terminated nanoparticle prepared according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of forming nanoparticles comprising one or more group IV elements. In a preferred embodiment, the method comprises forming nanoparticles that include a semiconductor material consisting of one or more of the elements Si, Ge and Sn.

The method comprises reacting, in a liquid reaction medium at atmospheric pressure and under an inert atmosphere, one or more group IV metal precursors, with a decomposition-promoting reagent. The reaction is carried out by heating the reaction mixture. In a preferred embodiment, the group IV metal precursors are sources of semiconductor material consisting of one or more of Si, Ge and Sn.

Li general terms, the decomposition-promoting reagent may be a reducing agent, a polymerising agent, or other suitable substance. The method also includes the use of a high temperature surfactant in the reaction mixture. The method includes the addition of a strongly-interacting surface-bonding agent to the reaction mixture. This surface-bonding agent forms an organic coating on the nanoparticle, preventing aggregation of the nanoparticles during subsequent crystal growth.

The general method of preparing nanoparticles is illustrated in Figure 1. As shown in Figure 1, the method of the invention comprises four main steps:

(a) heating, under an inert gas at atmospheric pressure, a solution comprising a liquid reaction medium and one or more group IV metal precursors;

(b) introducing a decomposition-promoting reagent to accelerate the decomposition of the group IV metal precursors;

(c) optionally introducing a quenching agent to remove excess decomposition- promoting reagent. This optional step is not necessary if, for example, there is no excess decomposition-promoting reagent, and may not be necessary if the decomposition-promoting reagent is a reductant (as discussed below); and (d) introducing a surface-bonding agent to prevent aggregation of the resultant nanoparticles.

Each of these steps will be discussed in detail below.

Group IV metal precursor

A number of group IV metal precursors are suitable for use in a method of the invention. These include organometallic compounds containing a silicon, germanium or tin atom. Preferably, these organometallic compounds containing the silicon, germanium or tin atoms are tetravalently bonded to a combination of aryl groups and halides following the molecular formula: G(Ar)_xY_4-X where G is the group IV metal, Ar is aryl, Y is halo and x takes a value that is at least 0 and no greater than 4. ^;- ^{■ ■}

Such compounds include, but are not limited to: tetraphenylgermane; triphenylchlorogermane; diphenyldichlorogermane; phenyltrichlorogermane; tetraphenylsilane; triphenylchlorosilane; diphenyldichlorosilane; phenyltrichlorosilane; tetraphenylstannane; triphenylchlorostannane; diphenyldichlorostannane; phenyltrichlorostannane; as well as their bromo-, iodo- and fluoro- analogues.

Alternatively, the group IV metal precursor species includes a group IV atom divalently bonded to a combination of aryl groups and halides following the molecular formula: G(Ar)_yY_2-y where G is the group IV metal, Ar is aryl, Y is halo and y takes a value that is at least 0 and no greater than 2.

In preferred embodiments, Ar is optionally substituted phenyl; more preferably phenyl. In a preferred embodiment, the group IV precursor is the phenyl-substituted precursor G(Ph)_xY_4-x. This precursor is particularly preferred when the decomposition-promoting reagent is selected from the group consisting of: S; Se; Te; P; As; and compounds comprising one or more of these elements in a zero valence state (see below).

The group IV precursor may be a mixture of different group IV metal precursors, including those having different group IV metals. This can result in alloy or interphase formation in the final nanoparticles.

Liquid reaction medium

The primary role of the liquid reaction medium is to solvate the group IV metal precursors and the various reagents to ensure a homogenous liquid phase reaction. The liquid reaction medium also provides a barrier to nanoparticle aggregation through the interaction of a high temperature surfactant with the nanoparticle surface. These measures can be effective in producing a high degree of monodispersity of the size of the nanoparticles.

Those persons skilled in the art will appreciate that the liquid reaction medium should be thermally stable at the reaction temperature.

In one embodiment, the liquid reaction medium comprises a high temperature surfactant and a high temperature solvent. Alternatively, the high temperature surfactant also acts as the high temperature solvent.

As used herein, the term "high temperature" means that the surfactant or solvent should be thermally stable under an inert atmosphere at the reaction temperature and have a boiling point in excess of the reaction temperature.

The reaction temperature is preferably between about 100°C and about 400⁰C; more preferably between about 200⁰C and about 400°C; more preferably about 300⁰C.

The surfactant possesses a molecular structure containing a functional group capable of interacting with the nanoparticle surface. This acts to form a thin layer of solvent material that is weakly bound to the nanoparticle surface. In such a way aggregation of the nanoparticles can be inhibited. Preferred surfactants do not take part in parasitic side-reactions with the reagents or precursors. Suitable surfactants that meet the above criteria may include, but are not limited to compounds of the formulae: R-NH₂; R-PH₂; R₃N; R₃P; R₂NH; R₂PH; R₄N⁺; and RE_y; wherein R is alkyl, alkenyl or aryl, E is an ethylene glycol group and N, P and H take their common IUPAC meaning.

Preferred high temperature surfactants include oleylamine, hexadecylamine, trioctylamine and trioctylphosphine. These surfactants act as co-ordinating solvents and so need no additional solvent.

In those embodiments wherein the liquid reaction medium comprises a high temperature surfactant and a high temperature solvent, suitable high boiling point organic solvents include, but are not limited to: alkanes, alkenes, aromatic hydrocarbons and other paraffins; mono- or poly-ethylene glycol ethers; crown ethers; and phenyl ethers.

The group IV metal precursor can be introduced into the liquid reaction medium either prior to or subsequent to heating of the liquid reaction medium to the reaction temperature. In a preferred embodiment, the liquid reaction medium is heated to the desired reaction temperature under a flow of a suitable inert gas to purge gaseous contaminants from the reaction vessel prior to the introduction of the group IV metal precursor and other reagents.

Suitable inert gases are known to those persons skilled in the art. Such gases include, but are not limited to: nitrogen; argon; and helium. Preferably_?,the inert gas is nitrogen.

The relative chemical stability of the precursors precludes the need for the reaction to be carried out in a controlled atmosphere glovebox, and simple Schlenk line or similar gas-purged glassware apparatus can be used. Figure 8 shows a typical benchtop glassware apparatus suitable for this synthesis. The liquid reaction medium 1 is contained in a three-necked round bottomed flask 2. The temperature is monitored using a thermometer probe 3 housed in a protective glass enclosure 4. The solution is stirred with a magnetic stir bar 5 and heat is applied 6, by a heating mantle, heat bath or similar apparatus. Inert gas is admitted into the flask 2 via the gas inlet 7 and leaves via the outlet 10. Reagents can be added to the flask 2 using a syringe temporarily, admitted at the gas inlet 7. A condenser 8 is also used to cool and condense vapour above the liquid reaction medium 1. The condenser can be water cooled through the water inlet 9 and outlet 9 to an outer glass sheath. This reaction mixture is usually heated to the reaction temperature during step a), although it is possible to heat the mixture to the reaction temperature during step b).

Decomposition-promoting reagent

After degassing, the decomposition-promoting reagent is added to the solution of the group IV metal precursor in the liquid reaction medium, typically under conditions of heat and an inert atmosphere.

The decomposition-promoting reagent may be injected into the reaction vessel as a solution or suspension, hi a preferred embodiment, the decomposition-promoting reagent is added as a solution in a carrier liquid, which may be the same as that comprising the liquid reaction medium.

The decomposition-promoting reagent accelerates the decomposition of the group IV metal precursor at the reaction temperature to the elemental form of the group IV metal — typically Ge or Si or Sn (or alloys of these) as nanoparticles. The generalised reaction scheme is as follows:

Ge, Si and/or Sn precursor/s + decomposition-promoting reagent ► group IV metals or alloy _(np) + by-products

wherein np indicates nanoparticle formation.

In one embodiment of the invention, illustrated in Figure 2, the decomposition-promoting agent is a strong reducing agent. The decomposition-promoting agent must be able to cleave the substituent-Ge, substituent-Si or substituent-Sn bond.

hi those embodiments wherein the group IV metal precursor is a phenylmetallic compound, the decomposition-promoting agent must be able to cleave one or more Si-Ph, Sn-Ph or Ge-Ph bonds. This strong covalent bond has previously been assumed to be stable to reductive attack, but the liquid phase method disclosed herein utilises a combination of high reaction temperatures and strong reducing agents to obtain elemental group IV metals from precursors containing Ge- Ph, Si-Ph or Sn-Ph bonds.

Example strong reducing agents include, but are not limited to, one or more of: sodium borohydride; lithium borohydride; potassium borohydride; sodium naphthalide; sodium anthracide; lithium naphthalide; lithium anthracide; potassium naphthalide; potassium anthracide; lithium aluminium hydride; sodium aluminium hydride; potassium aluminium hydride; sodium hydride; lithium hydride; potassium hydride; sodium; lithium; potassium; sodium sulfide; lithium sulfide; potassium sulfide; tin dichloride; tin dioleate; tin dibromide; tin di-iodide; sodium amide; sodium azide; and triphenyl phosphine.

In an alternative embodiment of the invention, illustrated in Figure 3, the decomposition- promoting reagent comprises one or more elements from group V or VI, or a compound comprising one or more of these elements in a zero valence state, wherein the group V or VI elements are selected from the group comprising: sulfur; selenium; tellurium; phosphorus; and arsenic.

In a preferred embodiment, the decomposition-promoting reagent comprises S or Se (or a compound comprising one of these elements in a zero valence state). A generalised reaction scheme for an example of this embodiment; -wherein the group IV metal precursor is (Ph)₃GeCl, is shown below:

260-360⁰C, 5-30 mins (Ph)₃GeCl + S or Se ► Ge_(np) + by-products

wherein np indicates nanoparticle formation.

In those embodiments wherein the decomposition-promoting reagent comprises S (or a compound- comprising S in a zero valence state), the by-products may include: Ph-S-Ph₅ Ph-Cl₅ Ph, Ph-Ph₅ Cl₂, S and GeS₂. ; .

Quenching agent

A quenching agent is typically used in the embodiment of the invention illustrated in Figure 3. However, a quenching agent may also be used in those embodiments wherein the decomposition- promoting reagent is a strong reducing agent.

The quenching agent acts to remove excess decomposition-promoting reagent from the reaction mixture, thus eliminating any inhibitory effects this may have on nanoparticle growth. In addition, the use of a quenching agent prevents the reaction of any excess decomposition- promoting reagent with the surface-bonding agent which, in some embodiments, is added to the reaction mixture after the quenching agent. When the decomposition-promoting reagent is selected from the group consisting of: S; Se; Te; P; As; and compounds comprising one or more of these elements in a zero valence state, a quenching agent is preferably added subsequent to precursor decomposition.

In a preferred embodiment, wherein the decomposition-promoting reagent is selected from one of: a) a strong reducing agent, or b) S, Se, Te, P or As or a compound comprising one or more of these elements in a zero valence state; the quenching reagent is a non-aqueous high boiling point acid. Suitable non-aqueous high boiling point acids include, but are not limited to: carboxylic acids; and compounds containing one or more carboxylic acid groups.

In an alternative embodiment, wherein the decomposition-promoting reagent is selected from the group consisting of: S; Se; Te; P; As; and compounds comprising one or more of these elements in a zero valence state, the quenching agent may be a hydride reducing agent such as sodium borohydride, lithium borohydride, potassium borohydride, lithium aluminium hydride, sodium aluminium hydride, potassium aluminium hydride, sodium hydride, lithium hydride or potassium hydride.

Surface-bonding agent

The surface-bonding agent is added to the reaction mixture to prevent aggregation of the nanoparticles. A surface-bonding agent can provide stronger interactions with the nanoparticle surface than those obtained from the high temperature surfactant within the liquid reaction medium. Typically, the surface-bonding agent is added in excess.

Advantageously, it is possible to avoid parasitic side-reactions of the surface-bonding agent with the decomposition-promoting reagent by adding the surface-bonding agent after the initial decomposition of the precursor. A second motivation for adding the strongly binding surface- bonding agent after the initial decomposition of the precursor is that very strong bonds between the nanoparticle and the surface-bonding agent may inhibit nanoparticle growth. Notwithstanding this point, it is possible to add the surface-bonding agent to the reaction mixture prior to, or at the same time as, the decomposition-promoting agent. Suitable surface-bonding agents that may be susceptible to reduction as a parasitic side-reaction in those embodiments wherein the decomposition-promoting reagent is a strong reducing agent include, but are not limited to: trioctylphosphine oxide; carboxylic acids; sulfonic acids; phosphonic acids; alcohols; thiols; and other proton-donating compounds.

Suitable surface-bonding agents that may be susceptible to parasitic side-reactions in those embodiments wherein the decomposition-promoting reagent is selected from the group consisting of: S; Se; Te; P; As; and compounds comprising one or more of these elements in a zero valence state, include, but are not limited to: primary, secondary and ternary phosphines.

Advantageously, in those embodiments wherein the surface-bonding agent comprises an alkenyl or alkynyl moiety, the surface-bonding agent may form stable bonds between the group FV metal and carbon at the nanoparticle surfaces

In an alternative embodiment, the surface-bonding agent comprises a compound of the formula R-N, wherein R is alkyl, alkenyl or aryl and N is a functional group capable of bonding to the surface of the group IV metal nanoparticle.

Alternatively, the surface-bonding agent may comprise a bi-functional compound of the formula L-R-N wherein R is alkyl, alkenyl or aryl, L is a group having the desired functionality and N is a functional group capable of bonding to the surface of the group IV metal nanoparticle.

hi a preferred embodiment, L and N are each independently selected from the group comprising: -NH₂; -COOH; -CONH₂; -OH; -CHO; -SO₃H; -PO₃H₂; -PH₂; -SH; -CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

In a preferred embodiment, the surface-bonding agent is the same as the quenching agent. In this embodiment, the excess surface-bonding agent acts to remove excess decomposition-promoting reagent from the reaction mixture.

The nanoparticles remain in suspension following the addition of the surface-bonding agent. The particles are typically subject to a purification/recovery step:

Nanoparticle recovery and purification

The nanoparticles are recovered and optionally purified following their preparation according to the methods disclosed above. Recovery and purification may proceed by a number of techniques known to those skilled in the art. Such techniques include, but are not limited to: nanoparticle flocculation and centrifugation; or two phase separation in an appropriate choice of immiscible solvents.

An example purification procedure, which is illustrative only, is: (a) addition of a flocculent to the reaction mixture;

(b) centrifugation of the flocculated mixture at a rotational acceleration above 100OG;

(c) removal and disposal of the supernatant;

(d) re-suspension in an organic solvent with a boiling point below about 120°C (a low boiling point solvent); and, optionally (e) repetition of steps (a) to (d) until the required purity is obtained.

Suitable flocculents include, but are not limited to: distilled or suitably purified water; alcohols including methanol, ethanol, propanol and butanol; methyl formamide; and acetone.

Suitable low boiling point solvents include, but are not limited to: toluene; tetrahydrofuran (THF); chloroform; dichloromethane; and liquid alkanes having fewer than 10 carbon atoms.

Characterisation of nanoparticles

The methods of the present invention lead to the formation of nanoparticles and/or nanocrystals of group IV elements or alloys thereof. The particles may be robust, chemically stable, crystalline, and may be coated by an organic or inorganic passivating layer.

Although the product of the method of the invention is described herein as a nanoparticle or nanocrystal, those persons skilled in the art will appreciate that the group IV metal nanoparticle has a surrounding layer, the composition of which is dependent on the surface-bonding agent. For example, when the surface-bonding agent is oleic acid, the oleic acid will be bound to the surface as illustrated in Figures 9a and 9c.

Figure 9a shows the group IV metal nanoparticle core 70 with the surface-bonding agent 71, which may be a carboxylic acid such as oleic acid, forming a surface organic layer. Figure 9b is a diagram of a hydrogen-terminated group IV metal nanoparticle produced according to one aspect of the present invention. Figure 9c shows a carboxylic acid group, such as that present in oleic acid, binding to the surface of the group IV metal nanoparticle. Other common examples of functional groups that may bind to the surface of the group IV metal nanoparticle include -NH₂, -COOH₅ -CONH₂, -OH, -CHO, -SO₃H, -PO₃H₂, -PH₂, -SH, -CH=CH₂, -OCH, -Cl, -F, -Br, -I and -H.

Further steps

Hydrogen- terminated nanoparticles

In many potential applications of group IV metal nanoparticles it is advantageous to be able to chemically functionalise the nanoparticles by the addition of specific molecules to the nanoparticle surface. The ability to obtain clean unfunctionalised crystal surfaces from which the funetionalisation reaction can proceed is, therefore, important. Of particular interest is the hydrogen-terminated crystal surface illustrated in Figure 9b.

A further aspect of this invention is the preparation of hydrogen-terminated group IV metal nanoparticles from acid-, aldehyde-, alcohol- or amide-terminated group IV metal nanoparticles grown outside of a glovebox environment.

In a preferred embodiment, the hydrogen-terminated nanoparticles are prepared from acid- terminated nanoparticles.

This is achieved according to the following method, which is illustrative only:

(a) adding a hydride reducing agent, under an inert gas at atmospheric pressure, to a purified mixture of acid-, aldehyde-, alcohol- or amide-terminated group IV metal nanoparticles in a solvent at a temperature at or below the boiling point of the solvent;

(b) reacting the resulting mixture for a period of up to 48 hrs in the absence of water and oxygen; and

(c) quenching the reaction.

Suitable hydride reducing agents include, but are not limited to: lithium aluminium hydride; sodium aluminium hydride; potassium aluminium hydride; lithium triethylborohydride; sodium triethylborohydride; sodium borohydride; lithium borohydride; potassium borohydride; hydrogen gas; sodium hydride; potassium hydride; lithium hydride; borane-tetrahydrofuran complex; lithium tri-tert-butoxyaluminium hydride; sodium cyanoborohydride; and di-isobutylaluminum hydride. Suitable inert gases are known to those persons skilled in the art. Such gases include, but are not limited to: nitrogen; argon; and helium. Preferably, the inert gas is nitrogen.

In a preferred embodiment, the reaction is quenched with an alcohol. Suitable alcohols include, but are not limited to: methanol; ethanol; butanol; and propanol.

Chemical functionalisation

A further aspect of this invention is the preparation of chemically functionalised group IV metal nanoparticles by the reaction of a specific chemically active species with the surface of the hydrogen-terminated nanoparticles of this invention.

The chemically active species may be a compound of the formula L-R-N wherein R is alkyl, alkenyl or aryl, L is a group having the desired functionality and N is a functional group capable of bonding to the hydrogen-terminated nanoparticle surface. Such groups include, but are not limited to: -NH₂; -COOH; -CONH₂; -OH; -CHO; -Cl; -F; -Br; -I; -PO₂H; -PH₂; -SH; -SO₃H; -CH=CH₂; and -C≡CH.

In one specific embodiment of this invention, water soluble nanoparticles may be produced by reacting the hydrogen-terminated nanoparticles with a compound of the formula L-R-N, wherein L is a polar functional group. In an alternative embodiment, biochemically functionalised nanoparticles may be prepared a compound of the formula L-R-N, wherein L is a functional group capable of binding to a biological antibody and/or biologically active molecule. Suitable L groups include, but are not limited to: -NH₂; -COOH; -CONH₂; -OH; -CHO; -SO₃H; -PO₃H₂; -PH₂; -SH; -CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

Favourable Characteristics

In preferred embodiments, the methods of the present invention, and the resultant nanoparticles, incorporate a number of favourable characteristics.

Relative ease of preparation

Advantageously, the method of the present invention may be carried out in a fumehood or benchtop location. In contrast, many of the prior art methods require high pressures and/or the use of controlled atmosphere glove-box techniques or other protective environments. This is because the method of the present invention is carried out at atmospheric pressure and uses comparatively benign or non-toxic reactants when compared with many of the prior art methods.

Accordingly, preferred embodiments of the method of the present invention are carried out under atmospheric pressure utilising standard glassware apparatus purged with an inert gas. Such apparatus is illustrated in Figure 8.

High yields

The methods described herein are capable of achieving high reaction yields of nanoparticles exceeding 60%. Such yields are achieved through the use of non-hydride decomposition agents, thus obviating the formation of the volatile by-products silane, germane, stannane and derivatives thereof, hi this way, the group IV elements in the precursor species remain within the reaction vessel until completion of the reaction. The reaction terminates at solid phase nanoparticles, and the crystallisation of elemental particles from the reaction solution ensures that the decomposition reaction drives to completion.

Those persons skilled in the art will appreciate that the presence of species surface-bonded to the nanoparticles gives rise to nanoparticle yields that are difficult to ascertain with any accuracy. The yield figures given herein refer to the yields from syntheses in the absence of quenching agents, surfactants or surface active material.

Relatively high throughput

Preferred embodiments of the invention require short reaction times, typically less than one hour and more preferably less than 30 minutes. Li addition, the nanoparticles can be synthesised with solution concentrations in excess of 2 gl^"1 and, in preferred embodiments, with concentrations in excess of 10 gl^"1. Such production rates represent a significant improvement over many of the prior art processes.

Good monodispersity

Figures 4a and 4b show transmission electron microscope (TEM) images of germanium nanoparticles prepared according to the methods of the present invention. The scale bar in both photographs is 20 nm. High monodispersity is observed. This is achievable through the homogeneous solution-phase nature of the reaction. Figure 5 shows the X-ray diffraction data of germanium nanoparticles prepared according to the methods of the present invention and demonstrates the high crystallinity of the nanoparticles. The copper peaks observed in the X-ray diffraction data are due to sampling of the TEM grid bars. Preferred embodiments of the invention produce nanoparticles with a diameter standard deviation of less than 20% of mean diameter; more preferably less than 5% of mean diameter.

Figure 6 shows the electron diffraction pattern from 8 run germanium nanoparticles prepared according to the methods of the present invention.

Spectral properties

In one embodiment, under optical excitation the nanoparticles of the present invention emit coloured light at wavelengths within the visible spectrum, hi other embodiments, the nanoparticles emit light under optical excitation at near-infrared and infrared wavelengths less than the bandgap of the bulk crystalline material. The wavelength of the light emitted by the nanoparticles may be tuned through manipulation of the nanoparticle size, the chemical make-up of the passivating layer and the chemical composition of the group IV metal or alloy comprising the nanoparticle. hi one embodiment, the method disclosed herein provides a passivated germanium nanoparticle displaying discrete optical transitions and photoluminescence. Figure 7 shows the typical luminescence (as a function of excitation wavelength) of a solution of germanium nanoparticles. Preferred embodiments of the method of the present invention produce nanoparticles with luminescence having a quantum efficiency >1%, more preferably up to and in excess of 20%.

APPLICATIONS

The nanoparticles of the present invention have many possible applications, as would be understood by one skilled in the art.

The nanoparticles are observed to luminesce in the visible spectrum and may be utilised in the field of biomedical imaging or as a starting material for novel optoelectronic devices. Group IV nanoparticles are of particular interest in the field of biomedical imaging where the long lifetime, high stability and low toxicity of germanium, silicon and tin is highly attractive compared to current alternatives. The nanoparticles of the present invention may be incorporated into biochemical imaging markers. For such applications, the nanoparticles may have an average particle diameter of between about 1 and about 200 angstroms and will produce nearly monochromatic luminescence within the visible spectrum in response to optical excitation. The biochemical imaging system may include a linker attaching, a biochemically active molecule to the nanoparticle, enabling the tagging of specific chemical compounds. The system may contain more than one type of linker type, with each type of linker being attached to nanoparticles that luminesce at a different and nearly monochromatic, visible wavelength.

The nanoparticles of the present invention may be incorporated into optoelectronic materials by a variety of methods, including the deposition of a film of the nanoparticles upon a substrate. The nanoparticles ₅may include, for example, silicon or germanium or an alloy thereof. For such applications, the nanoparticles may have an average particle diameter of between about 1 and about 200 angstroms. The deposited nanoparticles may be sintered at a temperature between about 300⁰C and 900°C to produce a film of material exhibiting optoelectronic properties, hi certain embodiments, the film may consist of a plurality of nanoparticles resulting in a polycrystalline material. This material may be further used to produce a variety of electronic devices, including photovoltaic devices, infrared emitting devices and light emitting devices.

Another embodiment of an optoelectronic material may include the incorporation of one or more nanoparticles into a matrix of one or more polymeric species whereby electrons can be transferred to and from the nanoparticles by means of external electronic contacts. The nanoparticles may include silicon or germanium or an alloy thereof. At least one of the nanoparticles may have an average particle diameter of between about 1 nm to about 20 nm. The said nanoparticle-incorporating matrix may be deposited upon a substrate or flexible film. This material may be further used to produce a variety of electronic devices including photovoltaic devices, infrared emitting devices and light emitting devices.

The nanoparticles of the present invention may be incorporated into photovoltaic devices for the generation of electrical power from optical and near-infrared radiation. Such devices may include a plurality of nanoparticles. The photovoltaic devices may include electrical contacts at their anode and cathode. The photovoltaic devices may include nanoparticles of a size optimised to provide maximum absorption of an incident solar spectrum. EXAMPLES

Various aspects of the invention will now be illustrated in non-limiting ways by reference to the following examples.

Example 1

1) Place 4.5g of hexadecylamine (HAD) or 7.5ml of oleylamine in a three necked round bottom flask

2) Add 0.05g triphenylcblorogerniane to flask

3) Place flask in a stirring heating mantle under a cold water condenser and nitrogen purge flow v-

4) Heat to 285-300⁰C whilst stirring

5) Inject a solution of 0.005g sulfur in 1.5ml trioctylamine

6) Wait approximately 5 minutes

7) Observe swift (<30 seconds) reaction as solution turns dark

8) Immediately add 0.2-5ml oleic acid. The solution will rapidly clear

9) Heat to a defined temperature between 285°C and 360⁰C for up to one hour

10) Cool to 15O⁰C and then quench with a 1 : 1 mixture of ethanol and toluene, added dropwise

11) Remove from flask. Add ethanol (for hexadecylamine) or methanol (for oleylamine) drop-wise until flocculation is observed

12) Centrifuge out flocculated sediment and remove and keep supernatant

13) Further dilute precipitate with flocculent and centrifuge. Repeat steps 9-11 until required purity is reached. Steps 9-11 may also be repeated on the supernatant to yield size selective precipitation of nanoparticles

14) Resuspend final precipitated material in toluene, hexane or ethanol Example 2

An alternative procedure omits steps (2) and (3) from the procedure described in Example 1 and instead utilises solutions of sulfur in oleylamine and triphenylchlorogermane in oleylamine which are injected into the heated solvent at a temperature between 26O⁰C and 36O°C. The procedure is then followed as from step (5) of Example 1.

Example 3

1) Place 4.5g of hexadecylamine or 7.5ml of oleylamine in a three-necked round bottom flask

2) Add 0.012g selenium to flask

3) Place flask in stirring heating mantle under a cold water condenser and nitrogen purge flow

4) Heat to 285-300°C whilst stirring

5) Inject solution of 0.05g triphenylchlorogermane in 1.5ml trioctylamine

6) React for 30 minutes

7) Cool to 150⁰C and then quench with a 1 : 1 mixture of ethanol and toluene, added dropwise

8) Remove from flask. Add ethanol (for hexadecylamine) or methanol (for oleylamine) drop-wise until flocculation is observed

9) Centrifuge out flocculated sediment and remove and keep supernatant

10) Further dilute precipitate with flocculent and centrifuge. Repeat steps 9-11 until required purity is reached. Steps 9-11 may also be repeated on the supernatant to yield size selective precipitation of nanoparticles

11) Resuspend the final precipitated material in toluene, hexane or ethanol Example 4

1) Place 6g of hexadecylamine or 1 OmI of oleylamine in a three-necked round bottom flask

2) Add 0.05g triphenylchlorogermane to flask

3) If tetraglyme is used in step 6 a defined quantity of trioctylphosphine may also be added to the flask at this point

4) Place flask in stirring heating mantle under a cold water condenser and nitrogen purge flow

5) Heat to 285°C (boiling point of triphenylchlorogermane) whilst stirring

6) Inject ImI of IM NaBH₄ solubilised in either tetraglyme or trioctylphosphine. The solution will rapidly clear

7) Heat to a defined temperature between 285°C and 360°C for up to one hour

8) Cool to 150°C and then quench with methanol, added dropwise

9) Remove from flask. Add ethanol (for hexadecylamine) or methanol (for oleylamine) drop- wise until flocculation is observed

10) Centrifuge out flocculated sediment and remove and keep supernatant

11) Further dilute precipitate with flocculent and centrifuge. Repeat steps 9-11 until required purity is reached. Steps 9-11 may also be repeated on the supernatant to yield size selective precipitation of nanoparticles

12) Resuspend final precipitated material in toluene, hexane or ethanol

Example 5

An alternative procedure omits steps (2) and (3) from the procedure described in Example 4 and instead utilises a solution of triphenylchlorogermane in oleylamine and trioctylphosphine, which is injected into the heated solvent at a temperature between 260°C and 360⁰C at the same time as step (6). The procedure is then followed as from step (6) of Example 4. It is apparent that a wide variety of reducing agents are capable of accelerating the decomposition of aryl-containing group IV metal precursors at temperatures in excess of 24O⁰C, even though a number of such reducing agents are not stable at these temperatures. For example, sodium napthalide breaks down at a temperature close to the boiling point of naphthalene (~220°C), but addition of naphthalene to a mixture containing triphenylchlorogermane and sodium metal at 275⁰C exhibits decomposition of the group IV metal precursor in the time required for the naphthalene to enter the solution, attain thermal equilibrium with its surroundings and then re- volatilise.

It is important that the reducing agent be homogeneously distributed within the solution. Preferably, the reducing agent should be fully soluble in the utilised solvent system such that crystallisation occurs from a completely homogeneous distribution of decomposed precursor molecules. However, this reqμirement may be relaxed, provided the heterogeneity of the distribution of the reducing agent is on a length scale that is comparable with the average diffusion length within the solution for a reduced group IV atom undergoing crystallisation. For example, the thermal breakdown of lithium aluminium hydride to LiH and AlH₃ at temperatures above 200⁰C is not a barrier to its utilisation in the methods of the present invention because the resulting highly disperse emulsion of LiH reacts completely with the precursor prior to aggregation of the insoluble hydride salt to form a homogeneous distribution of precursor decomposition products within the solution. Vigorous stirring can assist the production of a homogeneous distribution of reduced or partially-reduced precursor molecules.

Although the invention has been described by way of example and with reference to particular embodiments, it is to be understood that modifications and/or improvements may be made without departing from the scope of the invention as set out in the accompanying claims.

In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognise that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Claims

1. A method of preparing nanoparticles of one or more group IV metals or alloys thereof comprising the steps of: reacting, under an inert atmosphere, at atmospheric pressure and with heating, one or more group IV metal precursors with a decomposition-promoting reagent in a liquid reaction medium comprising a high temperature surfactant; adding a surface-bonding agent; and recovering the nanoparticles.

2. A method as claimed in claim 1, wherein the group IV metal is Si, Ge or Sn.

3. A method as claimed in claim 2, wherein the group IV metal is Ge.

4. A method as claimed in claim 1, wherein the group IV metal precursor comprises a compound of the general formula: G(Ar)_xY_4-x; wherein G is the group IV metal, Ar is aryl, Y is halo and x takes a value that is at least 0 and no greater than 4.

5. A method as claimed in claim 1, wherein the group IV metal precursor comprises a compound of the general formula: G(Ar)_yY_2-y wherein G is the group IV metal, Ar is aryl, Y is halo and y takes a value that is at least 0 and no greater than 2.

6. A method as claimed in claim 4 or 5, wherein Ar is optionally substituted phenyl.

7. A method as claimed in claim 6, wherein Ar is phenyl.

8. A method as claimed in claim 1, wherein the liquid reaction medium further comprises a high temperature solvent.

9. A method as claimed in claim 1, wherein the decomposition-promoting reagent is selected from one of: a) a strong reducing agent; or b) S, Se, Te, P or As or a compound comprising one or more of these elements in a zero valence state.

10. A method as claimed in claim 9, wherein the decomposition-promoting reagent is selected from S, Se, Te, P or As or a compound comprising one or more of these elements in a zero valence state.

11. A method as claimed in claim 9 or 10, wherein the decomposition-promoting reagent is selected from S, Se or a compound comprising one or both of these elements in a zero valence state.

12. A method as claimed in claim 1, further comprising the step of adding a quenching agent prior to adding the surface-bonding agent.

13. A method as claimed in claim 12, wherein the step of adding a quenching agent is prior to adding the surface-bonding agent but after adding the decomposition-promoting reagent.

14. A method as claimed in claim 1, wherein the surface-bonding agent is also the quenching agent.

15. A method as claimed in claim 1 , wherein the step of reacting comprises heating to a temperature between about 100°C and about 400°C.

16. A method as claimed in claim 15, wherein the step of reacting comprises heating to a temperature between about 200⁰C and about 400⁰C.

17. A method as claimed in claim 16, wherein the step of reacting comprises heating to about 300⁰C.

18. A method as claimed in claim I₅ wherein said nanoparticles have a monodisperse nanoparticle size distribution such that the nanoparticle diameter has a standard deviation of less than 20% of the mean diameter.

19. A method as claimed in claim 18, wherein the nanoparticle diameter has a standard deviation of less than 5% of the mean diameter.

20. A method as claimed in claim 1, wherein the nanoparticles produce luminescence in response to optical excitation with a quantum efficiency in excess of 1%.

21. A method as claimed in claim 20, wherein the quantum efficiency is in excess of 20%.

22. A method as claimed in claim 1, wherein the surface-bonding agent is a carboxylic acid, aldehyde, amide or alcohol.

23. A method as claimed in claim 22, wherein the surface-bonding agent is a carboxylic acid.

24. A method as claimed in claim 1, wherein the surface-bonding agent comprises an alkenyl or alkynyl moiety.

25. A method as claimed in claim 1, wherein the surface-bonding agent comprises a compound of the formula R-N, wherein R is alkyl, alkenyl or aryl and N is a functional group capable of bonding to the surface of the nanoparticles.

26. A method as claimed in claim 22 or 23, further comprising reacting the nanoparticles with a hydride reducing agent in the absence of water and oxygen, to provide hydrogen- terminated nanoparticles.

27. A method as claimed in claim 26, further comprising reacting the hydrogen-terminated nanoparticles with a compound of the formula L-R-N; wherein R represents an alkyl or aryl group, L is a group having the desired functionality and N is a functional group capable of bonding to the hydrogen-terminated nanoparticle surface; to provide chemically functionalised nanoparticles.

28. A method as claimed in claim 25 or 27, wherein N is selected from the group consisting of: -NH₂; -COOH; -CONH₂; -CONH₂; -OH; -CHO; -SO₃H; -PO₃H₂; -PH₂; -SH;

-CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

29. A method as claimed in claim 27, wherein L is a polar functional group.

30. A method as claimed in claim 27, wherein L is a functional group capable of binding to a biological antibody and/or biologically active molecule.

31. A method as claimed in claim 27, wherein L is selected from the group consisting of:

NH₂; -COOH; -CONH₂; -OH; -CHO; -SO₃H; -PO₃H₂; -PH₂; -SH; -CH=CH₂; -C≡CH; -Cl; -F; -Br; and -I.

32. Nanoparticles of a group IV metal or a group IV metal alloy prepared according to the method claimed in any one of claims 1 to 25.

33. Hydrogen-terminated nanoparticles prepared according to the method claimed in claim

26.

34. Chemically functionalised nanoparticles prepared according to the method claimed in any one of claims 27 to 31.

35. A device comprising nanoparticles as claimed in any one of claims 32 to 34.