NANOPARTICLES WITH GRAFTED ORGANIC MOLECULES

BACKGROUND OF THE INVENTION The present invention relates to grafting of molecules onto nanoparticles. More specifically, the present invention relates to grafting of organic molecules onto Group IV nanoparticles.

Nanoparticles have recently attracted significant attention from researchers in a variety of disciplines, due to a wide array of potential applications in the fabrication of nanostructured materials and devices. Semiconductor nanoparticles, such as silicon nanoparticles, are of special interest due to their potential uses in photoluminescence-based devices, doped electroluminescent light emitters, photovoltaic devices, memory devices and other microelectronic devices, such as diodes and transistors.

In many instances, it is desirable to graft molecules onto the surface of nanoparticles. The grafting of molecules to the surface is sometimes called "passivation," but may also impart new properties to the nanoparticles. In one example prior art approach to grafting molecules, nanoparticles are collected as a powder and then transferred into mixtures of solvents and ligands molecules and reacted for several minutes to hours. This process can be used to attach the ligands to the surface of the nanoparticles. The ligand molecules should have a sufficient length such that they are able to overcome the van der Walls forces between particles to push agglomerated nanoparticles apart. Another example prior art technique is the thermal gas phase method. In the thermal gas phase method, the molecules are electrically neutral and no charge prevents them from agglomerating. In order to passivate these particles while avoiding agglomeration, the thermal method is limited to relatively small particle concentrations such that the particles remain sufficiently separated and do not agglomerate.

SUMMARY OF THE INVENTION

An apparatus for producing grafted Group IV nanoparticles is provided and includes a source of Group IV nanoparticles. A chamber is configured to carry the nanoparticles in a gas phase and has an inlet and an exit. The inlet configured to couple to an organic molecule source which is configured to provide organic molecules to the chamber. A mixture of particles and organic molecules is formed in the gas phase. A plasma source is arranged to generate a plasma. The plasma causes the organic molecules to break down and/or become activated in the chamber and bond to the nanoparticles. A method of producing grafted Group IV nanoparticles is also provided and includes receiving Group IV nanoparticles in a gas phase, creating a plasma with the nanoparticles, and allowing the organic molecules to break down and/or activate in the plasma and bond with the nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified diagram showing a two stage plasma reactor in accordance with one example embodiment of the present invention. Figure 2 is a simplified diagram showing a single stage plasma reactor in accordance with another example embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention provides an apparatus and method for grafting organic molecules onto Group IV nanoparticles. The grafting is performed with the nanoparticles in a gas phase. An organic molecule source is provided and the organic molecules break down and/or are activated in a plasma to enable their reaction with the nanoparticle surfaces.

The grafting of molecules to the surface of nanoparticles can be called

"passivation", but it also imparts new properties to the nanoparticles. For example, the grafting can also make the particles soluble in various organic solvents. This is important if one wants to make a nanoparticle ink which requires, isolated, non-agglomerated particles in solution. Applications of such

inks are in printable electronics and solution processable solar cells. However, the invention is not limited to this implementation.

In prior approaches to grafting molecules to particles, particles were typically collected as a powder, transferred into mixtures of solvents and ligand molecules and reacted for a period of from several minutes to hours to attach the ligands to the surface. In one example of the present invention, nanoparticles are synthesized in a first plasma in a few milliseconds and next transferred into a second plasma, in which the ligands are attached within a few hundreds of milliseconds to one second. This provides an advantage in terms of time and processing ease. Moreover, this "in-flight" method allows attaching shorter molecules than is typically possible in the liquid phase. In prior liquid phase approaches, one starts with a powder of agglomerated particles. The ligand molecules need to have a sufficient length such that they can overcome the van der Walls forces between particles to push agglomerated nanoparticles apart. In the plasma method of the invention, the surface grafting is performed on individual particles suspended in a gas stream. It is thus possible to attach very short ligands, which may be desirable for some applications such as in printed electronics.

In-flight passivation of particles can be achieved by thermal gas- phase methods, for instance by flowing the mixture of particles and organic molecules through a furnace. The plasma method of this invention differs from the thermal gas phase method in a number of aspects. In the plasma, particles are negatively charged, which significantly reduces agglomeration of individual particles. In the thermal gas-phase method, particles are electrically neutral and are not prevented from agglomerating. In order to passivate particles while avoiding agglomeration, the thermal gas-phase method is typically limited to very small particle concentrations, such that the particle agglomeration rate is lower than the reaction rate between the particles and the organic molecules. The plasma approach allows for much higher particle concentrations, as the electric repulsion between particles significantly lowers the agglomeration rate.

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Hence, the plasma approach will allow much larger processing rates than the thermal gas-phase method.

The invention provides other advantages such as the ability to operate with a larger range of molecules than the thermal method. Non-thermal plasma selectively heats the electrons in the plasma but leaves the background gas cold. The energetic electrons can dissociate and/or activate ligand molecules and produce reactive radicals which attach to the nanoparticle surfaces. In contrast, the thermal gas-phase method relies on thermally activating certain bonds to initiate the reaction, requiring heating of the entire gas stream to achieve a heating of the particles. Further, the thermal method requires that the ligands have certain excitable bonds, such as unsaturated C=C bonds. The plasma approach also functions with saturated molecules such as alkanes which do not have any unsaturated bonds. The techniques can be used with Group IV nanoparticles including germanium and silicon nanoparticles. Figure 1 is a simplified diagram showing a two stage reactor 100 in accordance with one example embodiment of the present invention. Reactor 100 includes a first plasma chamber 102 and a second plasma chamber 104. Plasma chamber 102 receives a gas from gas source 106 and includes an RF power supply connected to electrodes 110. The RF power supply 108 drives electrodes 110 with an RF signal and creates a non-thermal plasma in the gas from gas source 106 between electrodes 110. This causes nanoparticles to form from the gas. For example, if the gas is Ar-SiH ₄ -H ₂ , silicon nanoparticles will be formed in plasma chamber 102. The nanoparticles then enter second stage plasma (or passivation) chamber 104. Plasma chamber 104 includes a second RF power supply 116 which couples between electrode 118 and electrical ground 120. Plasma chamber 104 receives organic molecules in accordance with the present invention. In the configuration shown in Figure 1, the organic molecules are from an organic molecule source 130 which comprises a liquid precursor 132 through which a gas from gas source 134 is bubbled. For example, gas source 134 can comprise argon. This provides a flow of evaporated reactant which is

injected into plasma chamber 104 through inlet 136. The plasma chamber 104 receives the nanoparticles from chamber 102 through inlet 140. The pressure and temperature of the bubbler and the flow rate from gas source 134 can be adjusted to control the organic molecule density in the second stage chamber 104. Plasma chamber 104 comprises a parallel plate reactor configured as a continuous flow reactor, with particles being carried past the lower electrode plate 118 and into an exhaust blind 150 which carries a filter 152.

The first plasma chamber 102 can form nanoparticles in accordance with any appropriate technique. One example plasma based technique is shown and described in co-pending application Serial No. 11/155,340, entitled, PROCESS AND APPARATUS FOR FORMING NANOPARTICLES USING RADIO FREQUENCY PLASMAS, by Kortshagen et al. which is incorporated herein by reference in its entirety. As discussed below, the nanoparticles can be formed using any appropriate technique and are not limited to formation using a plasma. As discussed above, the nanoparticles are received in the second stage 104. For example, second stage 104 can comprise a five inch diameter parallel plate reactor having a two inch plate gap. The nanoparticles can be received directly into chamber 104 and a separate orifice is not required. In this example, particles are nucleated and grown in the first stage 102 and the gas flow brings them into the second stage through inlet 140. The organic molecules are received into the second stage chamber through inlet 136 and can be adjusted to have a desired density. The second RF power supply 116 causes the molecular gas to break down and/or activate and bond with the nanoparticles. The flow continues through exhaust 150 and the nanoparticles, with the bonded organic molecules, are collected in a filter 152. The filter can comprise any appropriate filter, for example, a wire mesh filter. In one configuration, a pressure gauge 170 is provided in the exhaust 150 and can be used to monitor the pressure in the chamber.

The organic molecules can be selected as desired. Examples of organic molecules are shown below in Table 1. Various different bonds are formed

between the carbon molecules and the nanoparticle and those are also shown in Table 1.

TABLE l Here G4SA refers to Group IV surface atoms including Si, Ge, C or any other Group IV element.

The list set forth in Table 1 is not exhaustive and other materials and compounds may be used such as Decyl aldehyde (Decanal). In general, the following compounds can be delivered into the plasma and reacted with the particles:

Compounds with saturated carbon bonds, i.e. alkanes.

Compounds with one or more aromatic rings (benzene, toluene, mesitylene, pentacene etc.). Compounds with unsaturated terminal functional group, i.e. alkenes and alkynes. Examples are 1-dodecene, 1-octadecene, 1-exene, 1-pentene, propene, butene, styrene, ethylene, acetylene, 1-pentyne etc.

Alcohols (-OH), thiols (-SH), amines (-NH ₂ ), aldehydes (-CH=O), carboxylic acids (-COOH).

Compounds of point 3 and 4, but with more than one of the same functional group present in the same molecule. Examples are molecules such as 1,3 butadiene, ethylene glycol (ethane- 1,2-diol), ethylene diamine, divinylbenzene.

Compounds of point 3 and 4, but with different functional groups present in the same molecule. Examples are allylamine, acrylic acid, mercaptopropionic acid. ^* Compounds containing any of the above mentioned functional groups but also containing one or more silicon atoms. Such compounds are usually referred to as organisilanes.

The molecular monomer to be grafted onto the particle surface may be solid, liquid or gaseous at room temperature. If gaseous, it may be delivered into the reactor from a compressed gas cylinder using any type of compatible flow meter; if liquid, a bubbler system may be used to saturate a carrier gas with the monomer; if solid, a sublimator may be used to saturate a carrier gas with the monomer. The carrier gas may be an inert gas (such as Argon, Helium etc.), a reactive gas (such as H ₂ ), or it might be an organic monomer precursor itself (for instance, ethylene may be bubbled through 1-dodecene to deliver monomers of

different length to the reactor, and to achieve mixed termination of the particle surface).

More than one monomer may be delivered into the reactor. This can be done by building a separate delivery line for each monomer, or (if both monomers are liquid) by mixing the two liquids and by using a bubbler to deliver the mixture to the reactor. More than one monomer can be delivered to the reactor to achieve mixed termination of the particle surface.

The organic molecule might contain one or more atoms of group in and V, such as P, As, B, Sb. Examples are triphenilphosphine, trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), tributylborane, etc.

In one example experiment, 1-dodecene was reacted and used to bond with silicon nanoparticles. For this gas phase reaction 30sccm of argon and 8sccm of SiH ₄ (5% in He) were provided in the first stage plasma reactor 102. Further, 15sccm of argon was provided through the bubbler 130 which was maintained at the same pressure as the reactor (1.4 Torr). A power of 60 watts was used for the RF power supply 108 of the first reactor. A relatively low power (10-15 watts) was used for the second reactor 104 to successfully passivate the silicon particle surfaces. The resultant passivated nanoparticles were collected in the filter 152 and appeared to have a compact brown layer covering a stainless steel mesh of the filter. When the filter 152 was dipped into a solvent (toluene), the deposited nanoparticles began to disperse in the solvent. This dispersing occurred even without any sonication in the solution. A clear colloidal dispersion of silicon nanoparticles was obtained.

Figure 2 is a simplified diagram of another configuration of the present invention in which a single stage plasma reactor 180 is provided. Elements in Figure 2 which are similar to those shown in Figure 1 have retained that numbering. In the single stage plasma reactor of Figure 2, the nanoparticles are nucleated and form nanocrystals as discussed above in the first plasma chamber 102. As the particles leave chamber 102, they are exposed to a plasma afterglow 190 in passivation chamber 192. Passivation chamber 192 is

configured to receive the gas from an organic molecule source 130. If the plasma afterglow is sufficiently energetic to break down and/or activate the bonds of the organic molecules in the gas, these molecules will bond with the nanoparticles as discussed above. As discussed above, the nanoparticles can be formed using any appropriate technique and are not required to be formed in a plasma chamber. However, if a plasma chamber is employed to form the nanoparticles, a single stage reactor may be possible in which the nanoparticles are passivated in the afterglow plasma from the production chamber. Although RF plasma chambers are discussed herein, other types of plasma chambers may also be used including DC, or microwave based plasma sources. Plasmas are desirable because they impart a negative charge to the nanoparticles and reduce the particle agglomeration rate.

The present application is applicable to Group IV nanoparticles which include pure Group IV elements as well as Group IV compounds which include other elements including doped particles. Specific examples of Group rv elements includes silicon and germanium. The nanoparticles may be produced just prior to their passivation, or may be produced separately, in time and/or location as desired. For example, the nanoparticles can be produced using a thermal process with a furnace and subsequently injected into a plasma chamber. The nanoparticles may be injected into the plasma chamber using any appropriate technique including injection in a liquid phase through an atomizer or the like, electrosprayed, etc. During the bonding, the Group IV nanoparticles are bonded with the organic molecules. Examples of bonds linking organic molecules to the Group IV nanoparticles include Si-carbon, Si-oxygen, Si- sulfur, and Si-Nitrogen bonds, or others based upon any Group IV element. The precursor used in passivation may be any appropriate gas, liquid or solid which may be injected into the chamber in a dispersed form. Although argon is discussed above as a carrier gas, the present invention is not limited to this configuration.

The passivated nanoparticles may be collected on a filter as discussed above, or collected using other collection techniques. One example includes bubbling the passivated nanoparticles through a liquid solvent.

Although the discussion above describes the passivation of a single type of nanoparticle, multiple types of nanoparticles may be passivated at the same time. Such a configuration, nanoparticles having desired concentrations are injected into the passivation chamber and bond with the injected organic molecules. Similarly, multiple types of organic molecules may be used in the bonding process. These bonding steps may be performed in a single chamber, or as the nanoparticles pass through multiple chambers. Any appropriate organic molecules may be used including those set forth above.

In one configuration, the first chamber can comprise any gas phase method used for the synthesis of nanoparticles such as, but not limited to, ultraviolet, thermal or plasma. Following the synthesis of the nanoparticles, the nanoparticles are then subjected to a passivation with organic molecules. In such a configuration, with the gas phase nanoparticles immediately followed by functionalization, problems associated with agglomeration are eliminated or significantly reduced and the resultant particles may be immediately processed in a solution without requiring additional steps. Electromagnetic radiation (including visible or ultraviolet) from the plasma can be used to assist in the bonding process.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. As used herein, the plasma as set forth above may be any type of plasma including, "a gaseous medium that is partly ionized by the applications of a DC (direct current) or AC (alternating current) electric field. The electric field can be applied in continuous wave (CW) mode or in a pulsed mode. In the most general case, plasmas can be produced under gas pressures ranging from less than 1 Pa to exceeding 10 ⁶ Pa. The densities of the free charge carriers

(electrons and ions) range from smaller than 10 ¹² m ^*3 to larger than 10 ²² m ^"3 . The temperatures of electrons may range from less than 0.1 electronvolt to more than 10 electronvolt. Temperatures of the heavy charge carriers (ions) may range from the temperature of the neutral gas (nonthermal plasma) up to temperatures equaling the electron temperature (thermal plasma)." Another definition of plasma, includes, " Gas pressures between 1 Pa to 10 ⁵ Pa, free charge carrier densities ranging from 10 ¹⁴ m ^"3 to larger than 10 ¹⁸ m ^"3 , electron temperatures between 0.1 electronvolt and 10 electronvolt, and ion temperatures between room temperature and up to -1000 Kelvin." Various flow rates of the materials through the reactor may be used. The flow rates through the functionalization plasma can range, depending on the size and throughput of the reactor to be designed, from a few seem (cubic centimeter per minute at standard reference conditions) to many slm (liter per minute at standard reference conditions). More relevant than the flow rates are the residence times of nanoparticles in the reaction zone, which reasonably can range from around a millisecond to several seconds. Further, the present invention is not limited to the specific plasma source as set forth above. In various embodiments, reactions may be conducted in both low pressure gas environment and at gas pressures on the order of atmospheric gas pressure. At low pressures (1 Pa- 1000 Pa) plasma can be produced through a number of plasma sources in both continuous and pulse discharged operation:

DC glow discharges

Radiofrequency (RF) capacitively coupled plasmas

Radiofrequency inductively coupled plasmas

^* Microwave produced plasmas in microwave resonators

Radiofrequency and microwave traveling wave produced plasmas such as, but not limited to, surface waves, helicon waves, electron cyclotron resonance discharges

Electron beam produced plasmas

^* DC and RF hollow cathode discharges

At pressures closer to atmospheric pressures (1000 Pa-IO ⁶ Pa), plasmas can be produced both in a continuos wave and a pulsed discharge operation by:

DC arc discharges

Radiofrequency (RF) capacitively coupled plasmas

^* Radiofrequency inductively coupled plasmas

Microwave produced plasmas in microwave resonators

Radiofrequency and microwave traveling wave produced plasmas such as, but not limited to, surface waves. Electron beam produced plasmas

^* DC and RF microhollow cathode discharges

Corona discharges

Spark discharges

Dielectric barrier discharges

Atmospheric pressure glow discharges

^* Gliding arc discharges

At both low pressures as well as close to atmospheric pressures, nanoparticles may be functionalized by either being directly exposed to the plasma zone containing the molecular precursors for surface functionalization, or by being exposed to the effluent (afterglow) of the plasma. The plasma breaks down (disassociates) and/or activates the molecules to achieve the desired reaction. The plasma can provide a source of reduction to assist in the process. Various nanoparticle sources include plasmas, thermal methods, laser pyrolysis, laser photolysis, laser ablation, liquid phase reactions, combustion processes and flame processes.