Nanosubstrate with conductive zone and method for its selective preparation
The present invention provides novel nanostructure composed of at least one elongated structure element, an elongated structure element of said nanostructure bearing an electrically conductive zone selectively grown onto the elongated structure element. The present invention further provides a selective method for forming in a liquid medium, such nanostructures.
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This invention relates to the field of treatment of semiconductor nanostructures.
LIST OF REFERENCESThe following references are considered to be pertinent for the purpose of understanding the background of the present invention:
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The above references will be acknowledged in the text below by indicating their numbers [in brackets] from the above list.
BACKGROUND OF THE INVENTIONAnisotropic growth of nanomaterials has led to the development of complex and diverse nano-structures such as rods, tetrapods, prisms, cubes and additional shapes. These architectures display new properties and enrich the selection of nano-building blocks for electrical, optical and sensorial device construction. Even greater complexity and new function is introduced into the nanostructure by anisotropic growth with compositional variations. This has been elegantly realized by growing semiconductor heterostructures such as p-n junctions and material junctions in nanowires [1, 2], and in the case of colloidal nanocrystals, in growth of a dot-rod of two different semiconductors [3] and in complex branched growth. In these examples, anisotropic growth was performed with the same material type (semiconductor).
A process for the preparation of nanocrystalline semiconductors, having rod-like shape of controlled dimensions is described in U.S. Pat. No. 5,505,928 [13] and in WO 03/097904 [4] for especially Group III-V semiconductors, [4]. Nanocrystal particles having core with first crystal structure, and at least one arm with second crystal structure are described in WO 03/054953 [5].
Recently there have been several reports relating to connectivity formation for micron-long quasi-one-dimensional structures such as nanotubes and nanowires [6, 7, 8]. However, wiring in solution, of shorter semiconductor nanoparticles such as rods and tetrapods, with arm lengths of less then 100 nm is a difficult open problem.
SUMMARY OF THE INVENTIONThere is a need in the art for new nanostructures having selective, well-defined anchor points (preferably conductive anchor points) grown upon them for use in self-assembly in solution and onto substrates. Such nanostructures and method for their manufacture are not available to date.
Examples of desired nanostructures would be metal dots grown onto the tips of nanoparticles, in a controllable and repeatable manner that would also provide an electrical contact point. The conductive zones grown onto the tips of nanoparticles would provide well-defined anchor points onto which selective chemistries could be used to generate self-assembled structures of controlled arrangements.
The present invention thus provides in a first aspect new nanoscale materials in which a metal tip (conductive zone) is present on the edges of a nanostructure. The novel materials of the invention are nanostructures having an elongated shape such as rod, bipod, tripod and tetrapod. Excluded from the scope of the present invention are nanotubes and nanowires bearing electrodes formed by evaporation, such as those described in references [6-8] above.
The nanostructures of the invention are composed of at least one elongated structure element and comprise a first material, where an elongated structure element of the nanostructures bears an electrically conductive zone made of a second material.
The first material mentioned above is selected from semiconductor material, insulating material, metallic material and mixtures thereof. More preferably, the first material is a semiconductor material selected from Group II-VI semiconductors, Group III-V semiconductors, Group IV-VI semiconductors, Group IV semiconductors, alloys made of these semiconductors, combinations of the semiconductors in composite structures and core/shell structures of the above semiconductors. Even more preferably, the nanostructures are made from Group II-VI semiconductors, alloys made from Group II-VI semiconductors and core/shell structures made from Group II-VI semiconductors. Specific examples of Group II-VI semiconductors are CdSe, CdS, CdTe, alloys thereof, combinations thereof and core/shell layered-structures thereof.
The second material mentioned above is a metal or metal alloy. Preferably, the metal is a transition metal. Specific examples of such transition metals are Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe and Ti. In a preferred embodiment, the first material is different than the second material.
The present invention provides, in another of its aspects, a method for forming such an electrically conductive zone on a nanostructure having at least one elongated structure portion. The method of the present invention is carried out in liquid medium and it comprises: contacting a solution comprising nanostructures with a solution comprising a metal or metal alloy source, to obtain upon isolation, nanostructures bearing an electrically conductive zone on said at least one elongated portion thereof. The reaction is carried out at a temperature between about −40° C. to about 350° C., preferably between about 10° C. to about 80° C., more preferably between about 20° C. to about 30° C. and even more preferably at room temperature.
According to a preferred embodiment the reaction is carried out in the presence of at least one of the following agents in addition to said nanostructures and metal source: electron donor, surfactant and stabilizer.
The nanostructures used in the method of the invention have an elongated shape, for example of rods, wires, tubes, or in branched form. More preferably the nanostructures have an elongated shape such as for example nanorods and branched shape such tripods, tetrapods and the like. The term “nanorod” or “rod” as used herein is meant to describe a nanoparticle with extended growth along the first axis while maintaining the very small dimensions of the other two axes, resulting in the growth of a rod-like shaped nanocrystal of very small diameter in the range of about 1 nm to about 100 nm, where the dimensions along the first axis may range from about several nm to about 1 micrometer. The term “tetrapod” is meant to describe a shape having a core from which four arms are protruding at tetrahedral angles. In the case of nanorods, the resulting structures after treating them with the metal or metal alloy source are shaped as “nano-dumbbells”.
The nanostructures have an elongated shape or even a branched shape and serve as a template at the nanometer level for the deposition of a conducting material, and as it will be described and exemplified herein below, the deposition is accomplished in a controllable manner on the edges of the elongated portions of the nanostructures.
The nanostructures are made of a material comprising semiconductor material, insulating material, metallic material or mixtures thereof. Preferably, the nanostructures are made of semiconductor material selected from Group II-VI semiconductors, such as for example CdS, CdTe, ZnS, ZnSe, ZnO and alloys (e.g. CdZnSe); Group III-V semiconductors such as InAs, InP, GaAs, GaP, InN, GaN, InSb, GaSb and alloys (eg. InAsP); Group IV-VI semiconductors such as PbSe and PbS and alloys; and Group IV semiconductors such as Si and Ge and alloys. Additionally, combinations of the above in composite structures consisting of sections with different semiconductor materials, for example CdSe/CdS or any other combinations, as well as core/shell structures of different semiconductors such as for example CdSe/ZnS core/shell nanorods, are also within the scope of the present invention.
The nanostructures may also be made of an insulating material such as for example oxides and organic materials or, alternatively the nanostructures are made of metals. Examples of oxides are silicon oxide, titanium dioxide, zirconia. Metals include Au, Ag, Cu, Pt, Co, Ni, Mn and the like, and various combinations and alloys thereof. Organic materials suitable for use in the nanostructures are for example polymers.
The metal or metal alloy source used in the method of the present invention preferably comprises a transition metal or mixture of such metals. A variety of metals may be used. This includes noble metals such as Cu, Ag, Au, or other transition metal elements such as Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe, Ti and the like. The metal growth procedure is done by using a proper metal salt source, for example AuCl3 for Au growth, Ag(CH3COO) for silver growth, Cu(CH3COO)2 for Cu growth, PtCl4 or Pt(acetylacetonate) for Pt growth, Ni(cyclooctadiene)2 for Ni growth, Co2(CO)8 or CoCl2 for Co growth, and Pd(NO3)2 for Pd growth.
The metal salts are dissolved in a proper organic solvent such as hydrocarbons, e.g. hexanes, cyclohexanes, etc., aromatic solvents e.g. toluene, etc., using a proper surfactant and/or stabilizer that stabilizes the nanostructures and the metal salt by preventing aggregation. The organic solvent used in the method of the present invention is one capable to solubilize both the nanostructure and the metal source.
Examples of surfactants are cationic surfactants such as ammonium salts, alkyl pyridinium and quaternary ammonium salts. More specific examples are tetrabutylammonium borohydride (TBAB), dodecyldimethylamonium bromide (DDAB), cetyltrimethylammonium bromide (CTAB), and salts of quaternary ammonium with acetate group ions such as acetate group ions, pivalate, glycolate, lactate and the like.
Stabilizer compounds used in the method of the invention are such compounds capable to coordinate to the nanostructure surface and/or the metal particle surface and hence prevent aggregation of the nanostructures. Examples of stabilizers are aliphatic amines, e.g. hexadecylamine, dodecylamine, octylamine, alkylthiols, e.g. hexane thiol, decylthiol, dodecylthiol, etc. and carboxylic stabilizers such as myristic acid, palmitic acid and citrate.
The metal or metal alloy salt is first dissolved in an organic solvent comprising a surfactant and a stabilizer to give a solution which is subsequently added in a controllable manner and a suitable temperature to the nanostructures solution.
When an electron source is desired in the method of the invention, an electron donor compound may be used. Examples of electron donors are organic compounds, such as aliphatic amines, hydrides such as sodium borohydride and the like, ascorbic acid and other reducing agents. According to another example the electron source is obtained from an electron beam device. Alternatively, one may use electromagnetic radiation in order to excite the nanoparticles or the metal source.
More specifically, the present invention provides a method for forming in solution medium an electrically conductive zone on a nanostructure having at least one elongated structure, the method comprising: contacting an organic solution comprising semiconductor nanostructures with an organic solution comprising a metal or metal alloy source, a stabilizer and/or surfactant to obtain upon precipitation semiconductor nanostructures bearing at least one electrically conductive zone on said at least one elongated structure thereof. Preferably, the nanostructures used in the method of the invention are in the form of nanorods, tetrapods or any other branched structure and are made of elements of Group II-VI, alloys of such elements or core-shell layered structures thereof.
The method of the present invention provides new functionalities to the nanostructures, the most important of which is the formation of anchor points for directed self assembly. The selective tip growth of metal contacts provides the route to an effective wiring scheme for soluble and chemically processable nanostructures with branched shapes. This would allow to fully realize the potential of miniaturization of devices using such nano-building blocks, while employing the powerful principles of self-assembly to connect them to the ‘outside’ world.
BRIEF DESCRIPTION OF THE DRAWINGSIn order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
The method is exemplified hereinbelow with reference to selective growth of metal tips onto semiconductor nanorods and tetrapods.
In a method for selective growth of contacts made of gold, AuCl3 was dissolved in toluene by use of dodecyldimethylamonium bromide (DDAB) and dodecylamine, and the resulting solution was added to a toluene solution comprising of colloidal grown nanorods or tetrapods. The method is exemplified for the prototypical CdSe nanocrystal system that is highly developed synthetically and widely studied for its size and shape dependent properties.
CdSe rods and tetrapods of different dimensions (see below), were prepared by high temperature pyrolisys of suitable precursors, in a coordinating solvent containing a mixture of trioctylphosphineoxide (TOPO), and of phosphonic acids [9]. In a typical Au growth reaction, a gold solution was prepared containing 12 mg AuCl3 (0.04 mmol), 40 mg of DDAB (0.08 mmol) and 70 mg (0.37 mmol) of dodecylamine in 3 ml of toluene and sonnicated for 5 minutes at room temperature. The solution changed color from dark orange to light yellow. 20 mg of CdSe quantum rods of the required dimensions were dissolved in 4 ml toluene in a three neck flask under argon. The gold solution was added drop-wise over a period of three minutes. During the addition, carried out at room temperature, the color gradually changed to dark brown. Following the reaction, the rods were precipitated by addition of methanol and separated by centrifugation. The purified product could then be redissolved in toluene for further studies.
1NC—nanocrystals
2HDA—hexadecylamine
3DDAB—dodecyldimethylamonium bromide
An additional observation from the analysis of ˜200 particles per sample is that the overall rod length becomes shorter upon Au growth, and there is also a decrease in the average diameter of the rods, (Table 1 and
Several structural and chemical characterization methods have been carried out in order to verify the material content and structure of the gold on the rod tips.
Further evidence for Au growth onto single rods, is provided by HRTEM (high resolution TEM) studies of the nano-dumbbells.
Relating to the interface at the Au—CdSe, it is suggested that Au—Se bonds are formed, analogous to the known AuSe material [10]. This means that the interface is formed with covalent chemical bonds between the metal and the semiconductor and hence can be expected to provide good electrical connectivity.
The method for selective Au growth could be easily expanded and applied to rods of arbitrary dimensions, and to tetrapods, as well as to growth of other metals and to rods made of various semiconductor materials.
In another example, CdTe nanostructures served as the template for growing various metals on its edges. The synthesis of the CdTe in different shapes is known [14]. In a typical synthesis of CdTe rods, a mixture of 1 mmol of CdO dissolved in 1.125 gr oleic acid and 2.5 gr of 1-octadecene is heated in three neck flask to 300° C. to obtain a clear colorless solution. In the glove box, a solution of Te (0.5 mmol of Te is dissolved in 1 ml of TOP) is prepared and brought out in a vial sealed with septum to the injection. After the injection of the Te solution into the mixture in the three necked flask, the mixture is cooled to 260° C. for growth. Modification of this procedure in terms of the temperature or precursor concentration results in size and shape changes. The oleic acid is used as a ligand and it dissolves the CdO in the octadecene.
Another specific semiconductor material that may be used is CdS, which is controllable in size and shape. The synthesis is based on the same principle which is injection precursor to hot solution, the Cd and S precursor in this case is Cd(S2CNEt)2 that could be synthesized according to known literature method [12]. In typical synthesis of CdS nanorods, a warm solution of Cd(S2CNEt)2 (50 mg dissolved in about 0.3 g of hexadecylamine (HDA) at about 70° C.) is injected into hot solution of HDA and after 1 hr is cooled to 70° C. and treated with ethanol and separated by centrifuging. Controlling the shape of the nanocrystals is done by changing the growth temperature of the synthesis from 300° C. (rods) to 120° C. (tetrapods).
Metal tips by the method described above have also been grown onto CdSe/ZnS core/shell nanorods (29×4 nm rods with 2 monolayer ZnS shell) with initial emission quantum yield of 2% [15]. Treatment of these rods with DDAB and dodecylamine without Au led to an increased quantum yield of 4%, likely because of the effect of the excess amine. Several Au sizes were grown from about 2 nm to about 4.5 nm Au at the tips of the rods.
The metalized structures (in the case of Au growth the formed structures are termed herein “goldenized structures”) exhibit new and different electronic, electrical and optical properties as compared to the original rods, due to the strong effect of the metal on the semiconductor properties. Absorption and photoluminescence (PL) measurements were carried out to study the effect of Au growth on the rod optical properties as shown in
The significant coupling of the Au is also observed for the PL (
The selective tip growth of Au onto the rods and tetrapods not only provides important developments for enabling electrical connectivity and new paths for self-assembly for such nanostructures. It is also an interesting and novel chemical reaction route with clear selectivity and anisotropic character. The reaction mechanism for the gold growth entails a reduction of Au. Examining by TEM the Au solution with DDAB and dodecylamine, already reveals the formation of Au particles. When the reaction is carried out without dodecylamine, considerable aggregation of the CdSe rods was seen (
One of the benefits of the method of the present invention is its specificity leading to selective tip growth. This results from the preferential adsorption of the metal, e.g. Au complex formed in the Au solution by adding Au salt to DDAB and dodecylamine onto the nanostructures edges. The tips are more reactive due to the increased surface energy and also possibly due to imperfect passivation of the ligands on these faces, which also leads to preferential growth along the <001>axis of CdSe rods. Once Au nucleates on the edge, it is preferential for additional Au to adhere and grow on that seed. This gains support from controlling the extent of Au growth on the rod tips by using increased concentration of Au in the gold solution as was shown in
It is important to note that in some cases Au growth was identified on branching and defect points, but at slower rate compared to the distinctive tip growth discussed above. This can be seen in
The method may easily be expanded to additional semiconductor nanocrystal systems and to additional metals, to tailor the metal tip contacts as desired and the semiconductor element as well.
One application for the metal tips is in serving as electrical contact points. The role to be played by the Au tips as contact points for wiring the rods is exemplified by conductive atomic force microscopy (C-AFM) measurements carried out on goldenized 60×6 nm rods. Rods were deposited onto a conducting highly ordered pyrolitic graphite substrate, and embedded in a thin layer of poly methyl methacrylate (PMMA) to avoid dragging by the tip as reported earlier for regular rods [16]. The current image of a single rod measured by this method reveals that already at a bias of 1.5-2 V, small tunneling current is flowing through the tips which are composed of Au, while the central part of the rod consisting of the semiconductor is non-conductive at these conditions (see
Several strategies can be employed to realize such contacts. It is possible to form the metallized nanorods or other branched structures onto a substrate, identify their position, and then write by electron-beam lithography electrodes to overlap with the Au tips. In a different approach, it is also possible to deposit the metalized rods onto pre-existing electrode structures, with or without electrostatic trapping by an applied electric field. Since the metal tipped nanostructures enable the connectivity to electrode structures, this clearly opens the path for using them as transistors, in sensing applications, andin light emitting or light detecting devices.
The metal edges can also impart the rods with advantageous and novel optical properties. They exhibit enhanced linear and non-linear optical properties. The polarizibility of such a structure may obviously be significantly increased compared with that of the regular rods. For example, enhancement in second harmonic generation and also the observation of novel plasmon resonances related to highly controlled distances that could be tailored for the metal tips on rods.
Additionally, is possible to apply the powerful approach of self assembly by using for example, biological templates e.g. DNA, for creating the connections to the metal tips of nanorods or of branched structures, or bifunctional ligands such as dithiols or diamines for binding preferentially to the Au tips. In such applications the metal tips serve as selective anchor points for ligands and chemistries preferential for the Au surface. Such self assembly could for example be done in solution or onto surfaces. In solution, examples include formation of AAAA chains where A represents rods of one type. This is done by adding bifunctional ligands such as dithiols, for example hexane dithiol, to a solution with goldenized nanorods. The preferential binding of thiols to the Au tips leads to chain formation as can be seen in
The same chemistries can be used to self-assemble rods and tetrapods with Au tips onto patterned or non-patterned substrates. For example, a gold or silicon substrate is used together with a bifunctional ligand that binds with one function to the substrate and with the second function to the Au tip on the nanostructure.
Metal tipped structures also provide selective metal growth points for additional materials via a seeded growth solution-liquid-solid mechanism.
Claims
1. Nanostructure composed of at least one elongated structure element and comprising a first material, an elongated structure element of said nanostructure bearing an electrically conductive zone made of a second material.
2. The nanostructure of claim 1, wherein said first material is selected from semiconductor material, insulating material, metallic material and mixtures thereof.
3. The nanostructure of claim 2 wherein said first material is a semiconductor material.
4. The nanostructure of claim 3 wherein said semiconductor material is selected from Group II-VI semiconductors, Group III-V semiconductors, Group IV-VI semiconductors, Group IV semiconductors, alloys made of these semiconductors, combinations of the semiconductors in composite structures and core/shell structures of the above semiconductors.
5. The nanostructure of claim 4 wherein said nanostructures are made from Group II-VI semiconductors, alloys made from Group II-VI semiconductors and core/shell structures made from Group II-VI semiconductors.
6. The nanostructure of claim 1 wherein said second material is selected from metal and metal alloy.
7. The nanostructure of claim 6 wherein said metal is a transition metal.
8. The nanostructure of claim 7 wherein said transition metal is selected from Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe and Ti.
9. The nanostructure of claim 1 having an elongated shape selected from rod, bipod, tripod and tetrapod.
10. A method for forming in a liquid medium, an electrically conductive zone on a nanostructure having at least one elongated structure element, the method comprising: contacting a solution comprising nanostructures with a solution comprising a metal or metal alloy source, to obtain upon isolation nanostructures bearing at least one electrically conductive zone on said at least one elongated structure thereof.
11. The method according to claim 10 wherein said nanostructure is made of a material comprising semiconductor material, insulating material, metallic materialor mixtures thereof.
12. The method according to claim 10 wherein said nanostructure is made of semiconductor material.
13. The method according to claim 10 wherein said nanostructure has an elongated shape.
14. The method according to claim 13 wherein said elongated shape comprises a branched shape.
15. The method according to claim 14 wherein said branched shape comprises rod, bipod, tripod and tetrapod.
16. The method according to claim 12 wherein said nanostructure is made of a semiconductor material selected from Group II-VI semiconductors, Group III-V semiconductors, Group IV-VI semiconductors, Group IV semiconductors, alloys made of these semiconductors, combinations of the semiconductors in composite structures and core/shell structures of the above semiconductors.
17. The method according to claim 16, wherein said nanostructures are made from Group II-VI semiconductors, alloys made from Group II-VI semiconductors and core/shell structures made from Group II-VI semiconductors.
18. The method according to claim 11 wherein said nanostructure is made of an insulating material selected from oxides and organic polymers.
19. The method according to claim 10 wherein the metal or metal alloy source solution further comprises a surfactant and/or a stabilizer.
20. The method according to claim 19 wherein said surfactant is a cationic surfactant.
21. The method according to claim 19 wherein said stabilizer prevents aggregation of nanoparticles during the formation of an electrically conductive zone on a nanostructure.
22. The method according to claim 20 wherein said stabilizer is selected from ammonium salts, alkyl pyridinium alts and quaternary ammonium salts.
23. The method according to claim 10 wherein said metal or metal alloy source comprises a transition metal element.
24. The method according to claim 23 wherein said metal or metal alloy source is a salt of a transition metal or transition metal alloy.
25. The method according to claim 24 wherein said transition metal is selected from Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe and Ti.
26. The method according to claim 24 wherein said metal or metal alloy salt is first dissolved in an organic solvent comprising a surfactant and/or a stabilizer to give a mixture which is subsequently added in a controllable manner to the nanostructures solution.
27. The method according to claim 10 wherein said electron donor is an organic compound.
28. The method according to claim 27 wherein said electron donor is selected from aliphatic amine, hydride and ascorbic acid.
29. A method for forming in solution medium an electrically conductive zone on a nanostructure having at least one elongated structure element, the method comprising: contacting, an organic solution comprising semiconductor nanostructures with an organic solution comprising a metal or metal alloy source, a stabilizer and/or surfactant and/or electron donor to obtain upon precipitation semiconductor nanostructures bearing at least one electrically conductive zone on said at least one elongated structure thereof.
30. The method according to claim 29 wherein said nanostructures are in the form of nanorods, bipods, tripods or tetrapods.
31. The method according to claim 29 wherein said semiconductor nanostructures are made of a material comprising elements of Group II-VI, alloys of such elements or core-shall layered structures thereof.
32. The method according to claim 31 wherein said semiconductor nanoparticles are made of a material comprising CdSe, CdS, CdTe, alloys thereof, combinations thereof or core/shell layered-structures thereof.
33. The method according to claim 29 wherein said electrically conductive zone comprises a metal selected from Au, Ag, Cu, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe, Ti or mixtures of such metals.
34. Article of manufacture comprising the nanostructure of claim 1.
35. An electronic device comprising the nanostructure of claim 1, or into which the nanostructure of claim 1 is integrated.
36. An electrode comprising the nanostructure of claim 1.
37. An optical device comprising the nanostructure of claim 1, or into which the nanostructure of claim 1 is integrated.
38. Self assembled construct comprising a plurality of nanostructures according to claim 1, wherein each nanostructure is linked to another nanostructure in the construct through its conductive zone.
Type: Application
Filed: May 27, 2004
Publication Date: Aug 4, 2005
Applicant: YISSUM RESEARCH DEVELOPMENT COMPANY OF THE HEBREW UNIVERSITY OF JERUSALEM (Jerusalem)
Inventors: Uri Banin (Mevasseret Zion), Taleb Mokari (Jerusalem)
Application Number: 10/854,746