WO2016094384A2

WO2016094384A2 - Nanoelement assembly at a polar-nonpolar solvent interface

Info

Publication number: WO2016094384A2
Application number: PCT/US2015/064459
Authority: WO
Inventors: Asli Sirman; Aditi HALDER; Ahmed Busnaina
Original assignee: Northeastern University
Priority date: 2014-12-08
Filing date: 2015-12-08
Publication date: 2016-06-16
Also published as: WO2016094384A3

Abstract

A technique for rapid large-scale assembly of monolayers and multilayers of nanoelements on a variety of different substrates is provided. The technique is based on self-assembly of nanoelements suspended at the interface between a polar solvent and a nonpolar solvent. The layer of nanoelements is collected onto a substrate at a shallow angle, forming a continuous monolayer or multilayer of nanoparticles which can be optionally patterned or can be transferred to other substrates to form components of nanoelectronics, optical devices, and sensors.

Description

TITLE OF THE INVENTION Nanoelement Assembly at a Polar-Nonpolar Solvent Interface

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was developed with financial support from Grants 0425826 and 0832785 from the National Science Foundation. The U.S. Government has certain rights in the invention.

BACKGROUND

Large-scale assembly of nanoelements has gained wide-spread interest due to their applications in the fields of nanoelectronics, optical devices, single electron transistors, and sensors, as well in fundamental research. Existing methods for nanoelement assembly include solvent evaporation, template-assisted self- assembly, use of charge-stabilized nanoparticles, and Langmuir-Blodgett or Langmuir-Schaefer deposition methods in which a mono-layer of nanoparticles is transferred onto a substrate. However, the existing methods have certain disadvantages. For example, transfer of nanoparticle monolayers onto different substrates by the Langmuir-Blodgett method often leads to breakdown of the monolayers due to mechanical forces, leading to formation of fractal stacks or folded bilayers. In the solvent evaporation technique, nanoparticle assembly at the liquid surface relies on solvent evaporation, while a substrate is placed under the liquid surface. Following evaporation, the substrate is gently lifted up to collect the nanoparticle film; however, this method produces discontinuities and cracks in the film during the transfer process.

Previous methods are not suitable for high rate nanomanufacturing because the resulting yield and reproducibility are inadequate. Hence, there is a need to

l develop a reproducible assembly method to assemble monolayers of nanoparticles rapidly onto various substrates.

SUMMARY OF THE INVENTION

The invention provides a novel technique for rapid large-scale assembly of monolayers and multilayers of nanoelement assemblies on different substrates. The technique is facile, based on self-assembly, and utilizes nanoparticles or other nanoelements suspended in a nonpolar solvent that is poured slowly on top of a polar solvent to form a layered two-phase liquid system including an interface between the phases. Due to their immiscibility, the nanoelements form a monolayer and/or multilayers at the solvent interface.

The method of the present invention enables assembly of nanoparticles and other types of nanoelements into extended monolayers with length scales from about one micron to several millimeters with few or no defects. The method makes possible the high-rate fabrication of large-scale nanoelement coatings. When suitably controlled, the assembly process of the invention allows the assembly process to be increased by two orders of magnitude over previous techniques, and enables nanoelement coatings to be deposited on flat surfaces as well as patterned surfaces.

One aspect of the invention is a method of assembling nanoelements at a polar/nonpolar solvent interface. The method includes the steps of: (a) providing a polar solvent and a suspension of nanoelements in a nonpolar solvent, and (b) moving a substrate through the interface while the substrate is maintained at an angle Θ to the interface. The polar and nonpolar solvents form a two-phase system having an interface between the nonpolar and polar solvents. The two-phase system contains a layer of nanoelements disposed at the interface. The angle of withdrawing the substrate is such that 10° < Θ < 60°. As the substrate is moved through the interface, a layer or film containing the nanoelements is assembled on the substrate. In a preferred embodiment of the method, Θ is about 30°. In certain embodiments, the step of moving is performed at a rate of 0.5 to 3 mm/min of linear motion through the interface; in a preferred embodiment, the rate is about 3 mm/min. In certain embodiments, the nonpolar solvent is disposed above the polar solvent in the two-phase system; in other embodiments, polar solvent is disposed above the nonpolar solvent in the two-phase system. In certain embodiments, the polar solvent is selected from the group consisting of water, aqueous solutions, methanol, ethanol, and mixtures thereof. In certain embodiments, the nonpolar solvent is selected from the group consisting of hexane, toluene, chloroform, and mixtures thereof. In certain embodiments, the nanoelements are selected from the group consisting of nanoparticles, nanotubes, nanocrystals, quantum dots, macromolecules, and mixtures thereof.

Another aspect of the invention is a nanoelement assembly produced by the method described above.

Still another aspect of the invention is a device for assembling monolayer or multilayer films of nanoelements on a substrate. The device includes: a solvent- resistant container for liquids; a polar solvent and a nonpolar solvent within the container, forming a two-phase solvent system, and having an interface between the solvents; a substrate at least partially disposed within the container and arranged at an angle Θ to the interface, wherein 10° < Θ < 60°; and a substrate lifting mechanism capable of withdrawing the substrate disposed within from the container while at said angle.

The invention is further summarized by the following list of items:

1 . A method of assembling nanoelements at a polar/nonpolar solvent interface, the method comprising the steps of:

(a) providing a polar solvent and a suspension of nanoelements in a nonpolar solvent, the polar and nonpolar solvents forming a two-phase system having an interface between the nonpolar and polar solvents, wherein the two-phase system comprises a layer of nanoelements disposed at the interface; and

(b) moving a substrate through the interface while the substrate is maintained at an angle Θ to the interface, wherein 10° < Θ < 60°, whereby a layer of the nanoelements is assembled on the substrate

2. The method of item 1 , wherein Θ is about 30°.

3. The method of item 1 or item 2, wherein the step of moving is performed at a rate of 0.1 to 10 mm/min of linear motion through the interface.

4. The method of any of the preceding items, wherein the nonpolar solvent is disposed above the polar solvent in the two-phase system.

5. The method of any of items 1 -3, wherein the polar solvent is disposed above the nonpolar solvent in the two-phase system. 6. The method of any of the preceding items, wherein the polar solvent is selected from the group consisting of water, aqueous solutions, short chain alcohols, acetone, dimethylsulfoxide, and mixtures thereof.

7. The method of any of the preceding items, wherein the nonpolar solvent is selected from the group consisting of hexane, toluene, chloroform, and mixtures thereof.

8. The method of any of the preceding items, wherein the nanoelements are selected from the group consisting of nanoparticles, nanotubes, nanocrystals, quantum dots, macromolecules, and mixtures thereof.

9. The method of any of the preceding items, wherein the nanoelements are substantially monodispersed in the nonpolar solvent prior to forming the two-phase system.

10. The method of any of the preceding items, wherein the two-phase system is formed from the solvents alone, and the nanoelements are introduced into the nonpolar solvent after the formation of the two-phase system.

1 1 . The method of any of the preceding items, wherein the nanoelements are dispersed in the nonpolar solvent at a concentration from about 0.1 wt% to about 10 wt% prior to forming the two-phase system.

12. The method of any of the preceding items, wherein a monolayer of said nanoelements is formed on the substrate.

13. The method of item 12, wherein said monolayer is essentially continuous and free of gaps.

14. The method of any of items 1 -1 1 , wherein a multilayer of said nanoelements is formed on the substrate.

15. The method of any of the preceding items, wherein the substrate comprises a pattern of voids, and the nanoelements are selectively assembled in the voids.

16. The method of any of the preceding items, wherein the substrate comprises a smooth portion, and a continuous layer of nanoelements is assembled to cover the smooth portion.

17. The method of any of the preceding items, wherein the substrate is selected from the group consisting of silicon, silicon dioxide, metal oxides, and organic polymers. 18. The method of any of the preceding items, wherein the substrate is rigid and comprises a substantially planar surface upon which the nanoelements are assembled.

19. The method of any of items 1 -17, wherein the substrate is flexible and comprises a substantially flat or curved planar surface upon which the nanoelements are assembled.

20. The method of any of the preceding items, wherein the step of moving is continuous until the substrate, or a desired portion thereof, has been coated with assembled nanoelements.

21 . The method of any of the preceding items, wherein the step of moving comprises cyclic dipping of the substrate to deposit two or more layers of

nanoelements.

22. The method of any of the preceding items, wherein the step of moving is automated.

23. The method of any of the preceding items, wherein at least 4 mm² of a substantially defect-free film of the nanoelements is formed on the substrate in one minute.

24. The method of any of the preceding items, further comprising the step of:

(c) allowing residual solvent to evaporate from the substrate.

25. The method of any of the preceding items, wherein the method is performed at a temperature in the range from about 10°C to about 40°C.

26. The method of any of the preceding items, wherein the nanoelements are not functionalized.

27. A nanoelement assembly produced by the method of any of the preceding items.

28. The nanoelement assembly of item 27 comprising a monolayer of said nanoelements on the substrate.

29. The nanoelement assembly of item 28, wherein said monolayer is essentially continuous and free of gaps.

30. The nanoelement assembly of item 27 comprising a multilayer of said nanoelements on the substrate.

31 . The nanoelement assembly of any of items 27-30, wherein the substrate comprises a pattern of voids, and the nanoelements are selectively assembled in the voids. 32. The nanoelement assembly of any of items 27-31 , wherein the substrate comprises a smooth portion, and a continuous layer of nanoelements is assembled to cover the smooth portion.

33. The nanoelement assembly of any of items 27-32, wherein the substrate is selected from the group consisting of silicon and polymer materials.

34. The nanoelement assembly of any of items 27-33, wherein the substrate is rigid and comprises a substantially planar surface upon which the nanoelements are assembled.

35. The nanoelement assembly of any of items 27-33, wherein the substrate is flexible and comprises a substantially flat or curved planar surface upon which the nanoelements are assembled.

36. The nanoelement assembly of any of items 27-35, wherein the entire substrate, or a desired portion thereof, is completely coated with assembled nanoelements.

37. The nanoelement assembly of item 36, wherein the coated substrate or desired portion thereof has an area of at least about 10 mm².

38. The nanoelement assembly of item 37, wherein the coated substrate or desired portion thereof has an area of at least about 100 mm².

39. The nanoelement assembly of any of items 27-38, wherein the nanoelements are not functionalized.

40. A device for assembling monolayer or multilayer films of nanoelements on a substrate, the device comprising:

a solvent-resistant container for liquids;

a polar solvent and a nonpolar solvent within the container, forming a two- phase solvent system, and having an interface between the solvents;

a substrate at least partially disposed within the container and arranged at an angle Θ to the interface, wherein 10° < Θ < 60°; and

a substrate lifting mechanism capable of withdrawing the substrate disposed within from the container while at said angle.

41 . The device of item 40, wherein the angle is about 30°.

42. The device of item 40 or 41 , further comprising a plurality of nanoelements disposed at said interface.

43. The device of item 42, further comprising a plurality of said nanoelements disposed on a surface of the substrate BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A-1 C show prior art nanoelement assembly techniques (Fig. lA, Langmuir-Blodgett method; Fig. 1 B, Langmuir Schaefer method) compared with a method according to the present invention (Fig. 1 C).

Figure 2A shows a photograph of an experimental apparatus containing water overlayered with a dispersion of gold nanoparticles in hexane. Figure 2B shows the apparatus of Fig. 2A containing a submerged substrate being withdrawn at an angle through the water-hexane interface. As the substrate is withdrawn, a layer of the nanoparticles is deposited on the surface of the substrate. Fig. 2C shows a schematic illustration of the indicated area of Fig. 2B during assembly.

Figure 3A shows an SEM image of assembled 5 nm diameter silver nanoparticles, and Figure 3B shows an SEM image of assembled 5 nm diameter gold nanoparticles.

Figures 4A and 4B show the results of assembly using a pulling speed of 0.5 millimeters per minute. Figures 4C and 4D show the results of assembly using a faster pulling speed of 3.0 mm/min. Figures 4B and 4D are enlarged images of the assembled nanoparticles of Figs. 4A and 4C, respectively.

Figures 5A and 5B show the effect of nanoparticle concentration on the assembly process. Figure 5A shows the assembled film obtained using a concentration of 1 wt% of 5 nm silver nanoparticles in hexane, and Fig. 5B shows the assembled film obtained when the concentration was doubled to 2 wt%.

Figures 6A and 6B show nanoparticle monolayers assembled using nanoparticle suspensions in hexane (Fig. 6A) and toluene (Fig. 6B).

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have developed new methods for assembling nanoelements onto a substrate at high speed for use in nanomanufacturing. The methods enable the assembly of nanoparticles and other nanoelements as extended monolayers and multilayers with length scales from one micron to several millimeters with few or no defects. With the methods of the invention, the nanoelement assembly process can be sped up by hundreds of times, which makes the coating process suitable for mass production. Commercial applications include the production of nanoelectronics, optical devices, single electron transistors, and materials and devices for energy harvesting and energy storage.

With the present methods, only a small amount of a nanoelement suspension is required for continuous and large-scale film fabrication. The assembly process parameters such as pulling speed and angle can be controlled by a dipcoater or similar apparatus. Both flat and patterned surfaces made from a variety of substrate materials can be coated with nanoelements of various sizes, including small nanoparticles of 4 nm size or less. Functionalization of the nanoelements is not required for their assembly.

Methods of using a water-organic solvent interface for assembling nanoparticles on a substrate and then transferring them onto another substrate have been developed. Figures 1A and 1 B illustrate two such methods. The Langmuir- Blodgett method shown in Fig. 1A involves forming a monolayer of nanopartilces at an air-water interface, and pulling the mononlayer (or two monolayers) up using a vertically oriented movable substrate is lifted vertically through the monolayer while movable barriers maintain lateral pressure in the monolayer. Figure 1 B shows the related Langmuir-Schaefer technique, in which a horizontally oriented movable substrate is lifted vertically through the surface monolayer, which it captures on the substrate. In contrast, the method of the present invention, which is depicted schematically in Fig. 1 C, utilizes the withdrawal of the monolayer, which is positioned at a polar/nonpolar solvent interface, at a gentle angle onto the surface of a substrate drawn up through the interface while maintaining the angle.

These prior methods involve difficulties that are overcome by the present invention. For example, in both the Langmuir-Blodgett and Langmuir-Schaefer methods, abrupt changes in the angle of the monolayer, together with the force of gravity acting at steep angles, can lead to breaks or gaps in the monolayer deposited on the substrate. The process of forming a monolayer of nanoparticles on a substrate by evaporation of solvent can freeze lateral mobility of the nanoparticles after a certain point in the evaporation process. This leads to heterogeneity in the form of multilayer domains and microscopic voids, especially when assembling nanoelements of very small size. In contrast, the present invention utilizes continuous pulling of a monolayer or multilayer of nanoelements at a gentle angle, avoiding abrupt angles, and relying on the cohesiveness of the nanoelements and the physics of the polar/nonpolar solvent interface to provide continuous assembly of the nanoelements into a continuous layer on the substrate.

Any type of nanoelements can be utilized. For example, nanoelements for use in the invention can be nanoparticles, nanotubes, nanocrystals, quantum dots, or macromolecules. The term "nanoparticles" as used herein refers to small, compact solid materials having a diameter, or a largest dimension across, in the range from about 1 nm to about 999 nm, preferably from about 1 nm to about 100 nm. In different embodiments of the invention, the nanoparticles can be 4 nm or less, 5 nm or less, 6 nm or less, 7 nm or less, 8 nm or less, 10 nm or less, 15 nm or less, 20 nm or less, 50 nm or less, or 100 nm or less. As used herein, the term "nanoparticles" does not include extended structures such as nanotubes, needles, or fibers. In certain embodiments, the nanoparticles are essentially spherical, meaning that for a given particle all dimensions across the particle are within +/-10%, +/-20%, +/-30%, or +/-50% of the mean dimension across the particle. The nanoelement material can be crystalline or polycrystalline. The nanoelement material can be metallic, such as nanoparticles or other nanoelements comprising or consisting of gold, silver, platinum, cobalt, or other noble or non-noble metals, or mixtures thereof. The nanoelement material also can comprise or consist of a polymer material such as polystyrene, or an oxide material such as silica. The nanoelements can be any type of carbon nanotubes, such as single-walled carbon nanotubes, multi-walled carbon nanotubes, semiconducting carbon nanotubes, metallic carbon nanotubes, or a mixture thereof.

The substrate upon which the nanoelements are assembled can be any solid material capable of binding and supporting the nanoelements, and resistant to degradation in the two-phase solvent system. For applications in which the nanoelements are intended to serve as electrical conductors, the substrate is preferably nonconductive. Examples of nonconductive substrates include silicon, silicon dioxide, and organic polymer materials. For flexible electronics, the substrate can be, for example, a flexible polyimide, polyether ether ketone (PEEK), polyester (e.g., polyethylene terephthalate (PET)) film. The substrate preferably has at least one essentially planar surface that receives the nanoelements. The nanoelement receiving surface can be smooth or patterned. A smooth substrate surface can be used to prepare a planar and continuous film of nanoelements, whereas a patterned surface, such as one containing a pattern of nanoscale trenches or other structures, can be used to form electrically conductive pathways as part of a circuit, such as a nanocircuit, or other electrical components of a circuit, processor, memory, or energy storage device. The dimensions and form of the substrate can be selected according to its intended use, and it can be adapted for attachment to a dipcoater or other device used to assemble nanoelements on the substrate surface. The substrate surface that is intended to receive the nanoelements can optionally be functionalized, for example, to improve adhesion of the nanoelements to the surface. For example, the substrate can be coated with a material to render it more or less hydrophobic, hydrophilic, or charged, or it can be treated to increase the density of charged groups on the surface for nanoelement assembly. It also can be functionalized or patterned by lithography so as to establish a desired pattern for the localization of nanoelements during assembly.

In the present invention, a two-phase solvent system is used to concentrate the nanoelements at the interface between the solvents, forming a monolayer or multilayered film of the nanoelements between the two solvent phases, which can then be collected on the substrate. A large number of two-phase solvent systems can be used in the invention. Such two-phase systems are generally formed by layering a less dense solvent over a denser solvent with which it is immiscible by virtue of their different polarity, or other properties that render them immiscible. For example, a hydrophilic solvent can be overlayered with a less dense hydrophobic solvent. More complex mixtures of three or more solvents are also possible, provided that they form two stable phases, one floating over the other. Alternatively, the solvents can be mixed or partially mixed, whereupon they spontaneously form a two-phase system under the influence of gravity, with the denser phase below the less dense phase, and with the two phases separated by a visible interface. Each phase contains predominantly one solvent, but an equilibrium mixture forms, in which each phase may contain some amount of the other solvent as well.

A nonpolar solvent is selected which allows the nanoelements to be well dispersed in it, i.e., without precipitation, flotation, or aggregation of the nanoelements, but with the nanoelements stably suspended and preferably monodisperse (i.e., distributed within the solvent as free monomers). A polar solvent is selected which forms a two-phase solvent system with the nonpolar solvent. In preferred embodiments, the polar solvent is water or an aqueous solution, and the nonpolar solvent is an organic solvent. From such a combination, a two-phase system is formed having a predominantly aqueous phase below and a predominantly organic solvent phase above. However, if the organic solvent is denser than water, the positions of the phases is reversed (organic solvent below, water above). Preferably, the nonpolar solvent is less dense than the polar solvent and also somewhat volatile, so that residual nonpolar solvent remaining on the withdrawn substrate can readily evaporate. Preferred polar solvents include water; aqueous solutions containing electrolytes, buffers, or other solutes; short chain alcohols such as methanol, ethanol, propanol, isopropanol; short chain ketones such as acetone; dimethylsulfoxide (DMSO); and mixtures thereof. Preferred nonpolar solvents include hexane, or other C4-C20 alkanes (with a preference for the more volatile short-chain alkanes of C4-C10), either straight chain, branched, or cyclic, or their alcohols, ketones, acids, amines, alkenes, or alkynes; toluene and other aromatic hydrocarbons; chloroform and other halogenated hydrocarbons; and mixtures thereof.

The nanoelements are typically dispersed in the nonpolar solvent because they can be more readily dispersed in a nonpolar solvent than in a polar one, such as water. However, depending on the type of nanoelements, the method could be practiced differently, i.e., starting with the nanoelements dispersed in the polar solvent instead of the nonpolar solvent, or even in both the polar and nonpolar solvents, provided that the nanoelements concentrate at the interface between the polar and nonpolar solvents. Mixtures of nonpolar solvents can be employed to improve nanoelement dispersibility, or to improve performance of a given two-phase solvent system. Mixtures of polar solvents can be used for the same purpose, or electrolytes or pH buffers can be added to a polar solvent which is or which contains water.

The concentration of nanoelements dispersed in the nonpolar solvent is an important factor in providing a suitably dense array of nanoparticles at the solvent interface, which ultimately can determine whether a complete monolayer or multilayers are assembled on the substrate. Too low a concentration of nanoelements will result in formation of a monolayer with gaps on the substrate. Too high a concentration of nanoelements can result in regions of bilayer or multilayer structures formed on the substrate. A suitable concentration is in the range from about 0.1 to about 10 wt%, or about 0.5 to about 5 wt%, preferably from about 1 to about 2 wt%. Adjustment of the concentration may be necessary to achieve a desired structure, such as a complete monolayer, and may depend on the solvent system and type of nanoelements used.

The substrate is maintained at a constant angle with respect to the solvent interface during its withdrawal through the solvent interface. The angle, Θ, is selected such that 10° < Θ < 60°, or 15° < Θ < 55°, or 30° < Θ < 60°, or 20° < Θ < 50°, or 20° < Θ < 40°, or 25° < Θ < 35°. In preferred embodiments, Θ is a shallow angle of less than 45°, or less than 40°, or about 40°, about 35°, about 30°, about 25°, about 20°, or about 15°. Steep angles > 60°, > 50°, > 45°, or > 40° are to be avoided, as they can lead to breaks or gaps in the deposited film of nanoelements.

Withdrawal of the substrate through the nanoelement film at the solvent interface is preferably automated and performed by a motorized device with adjustable angle. The substrate can be withdrawn by a linear motion that maintains a constant angle with respect to the solvent interface. The motion should be constant and smooth, without interruption of abrupt change in speed. The speed of withdrawal is another important factor in determining the quality of nanoelement assembly on the substrate. The speed can be, for example, in the range from about 0.1 mm/min to 10 mm/min, or about 0.2 mm/min to about 8 mm/min, or about 0.3 mm/min to about 6 mm/min, or about 0.5 mm/min to about 5 mm/min, or about 0.5 mm/min to about 3 mm/min, or about 1 mm/min to about 10 mm/min, or about 1 mm/min to about 4 mm/min, or about 2 mm/min to about 4 mm/min, or about 3 mm/min to about 5 mm/min, or about 3 mm/min to about 4 mm/min. Preferably, the motion is about 3 mm/min, or about 4 mm/min. In alternative embodiments, the substrate is lifted upwards while maintaining a constant angle with respect to the interface, or a combination of upwards motion and linear retraction along the angle to the interface is used.

Figures 2A and 2B show an experimental apparatus for performing a method of the invention. In Fig. 2A, the apparatus contains water overlayered with a dispersion of gold nanoparticles in hexane. In Fig. 2B, a submerged substrate is being withdrawn at an angle through the water-hexane interface. As the substrate is withdrawn, a layer of the nanoparticles is deposited on the surface of the substrate in the region of the dashed rectangle.

Fig. 2C shows a schematic illustration of device 10 for carrying out a method of the present invention. Container 80 holds the two-phase solvent system, including polar solvent 20 which is overlayered with nonpolar solvent 30 (containing a dispersion of nanoelements, not shown). A region of air 40 is above the nonpolar solvent layer. Interface 25 separates the two immiscible solvent phases and contains an array or layer of nanoelements 60 suspended between the phases. Substrate 50 is withdrawn by linear motion in the direction of arrow 70 while maintaining angle Θ with respect to the interface. The substrate is mounted onto movable arm 80 of a motorized dipping device.

EXAMPLES Example 1 . Deposition of Nanoparticles

Silver and gold nanoparticles were deposited on a silicon substrate. A two- phase system of hexane, containing 1 wt% of the nanoparticles in homogeneous dispersion, layered over deionized water was established. After the solvent system was stabilized, a flat (unpatterned) silicon substrate attached to a dip coating apparatus was submerged into the solvent system, and the substrate was withdrawn from the solvent system in a computer-programmed, controlled manner. Withdrawal speed was 3 mm/min and the dip coater was set at 30° with respect to the horizontal solvent interface. The resulting nanoparticle films were viewed using scanning electron microscopy (SEM) and are shown in Fig. 3A (5 nm silver nanoparticles) and Fig. 3B (5 nm gold nanoparticles). The assembly process took less than 1 minute to assemble a 2 mm x 2 mm uniform layer.

Example 2. Role of Pulling Speed.

The effect of pulling speed of the dip coater apparatus on quality of nanoparticle assembly was investigated. When the pulling speed was increased from 0.5 mm/min (Figs. 4A and 4C) to 3 mm/min (Figs. 4B and 4D), the quality of assembly improved drastically, manifested as a reduction of gaps in the nanoparticle monolayer at the higher pulling speed. Similar results were obtained with both 4 nm silver and 5 nm gold nanoparticles assembled on a silicon substrate. The better performance at higher pulling speeds is advantageous for high rate nanomanufacturing. Example 3. Role of Nanoparticle Concentration.

The role of nanoparticle concentration on the quality of nanoparticle assembly was investigated. At a lower concentration (1 wt%) of 5 nm silver nanoparticles dispersed in hexane, over a base of water, a monolayer of nanoparticles was assembled on the silicon substrate (Fig. 5B). However, there were void areas of 20- 30 nm size visible across the total covered area of the substrate. Doubling the concentration (to 2 wt%) resulted in very good coverage having essentially no voids (Fig. 5B). However, a few bilayer regions were observed interspersed within the nanoparticle monolayer.

Example 4. Effect of Solvent on Nanoparticle Assembly.

Different organic solvents, including hexane, chloroform, and toluene, were compared for the dispersion of nanoparticles and impact on the assembly method. The choice of solvent was limited to those producing a good dispersion of the nanoparticles. If the nanoparticles were well dispersed in a particular solvent, then the assembly was good. The volatility and surface tension of the solvent played an important role in the quality of assembly. The homogeneous dispersion of nanoparticles in organic solvent was poured as a thin layer onto the water, and floated over the water. The immiscibility of organic solvent with water helped to spread the nanoparticle dispersion over water, ensuring better Langmuir film formation on the water surface, i.e., at the solvent interface. The self-organization of nanoparticles over the water surface promoted ordering of the nanoparticles and assembly over a large area of the substrate. After pulling the floating Langmuir film from the surface layer and transferring in to the silicon substrate, time had to be allowed for the complete evaporation of the organic solvent. Fig. 6A shows a monolayer of 5 nm silver nanoparticles formed from a hexane dispersion, and Fig. 6B shows a similar monolayer obtained from a toluene dispersion.

Example 5. Effect of Pulling Angle.

A series of experiments confirmed that a 30° angle gave the best assembly result. Vertical assembly showed the effects of stick-slip motion of the meniscus and formed a discontinuous film, and it produced gaps separated by assembled lines of nanoparticles. For angles of 60° or greater, agglomerations of nanoparticles and disordered assembly on the silicon substrate surface were observed. This application claims the priority of U.S. Provisional Application No. 62/088810 filed 8 December 2014 and entitled "Rapid Large Scale Assembly of Nanoparticles at Water/Organic Solvent Interface", the whole of which is hereby incorporated by reference.

As used herein, "consisting essentially of" allows the inclusion of materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term "comprising", particularly in a description of components of a composition or in a description of elements of a device, can be exchanged with "consisting essentially of" or "consisting of".

While the present invention has been described in conjunction with certain preferred embodiments, one of ordinary skill, after reading the foregoing

specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein.

Claims

(b) moving a substrate through the interface while the substrate is maintained at an angle Θ to the interface, wherein 10° < Θ < 60°, whereby a layer of the nanoelements is assembled on the substrate.

2. The method of claim 1 , wherein Θ is about 30°.

3. The method of claim 1 , wherein the step of moving is performed at a rate of 0.1 to 10 mm/min of linear motion through the interface.

4. The method of claim 1 , wherein the nonpolar solvent is disposed above the polar solvent in the two-phase system.

5. The method of claim 1 , wherein the polar solvent is disposed above the nonpolar solvent in the two-phase system.

6. The method of claim 1 , wherein the polar solvent is selected from the group consisting of water, aqueous solutions, short chain alcohols, acetone,

dimethylsulfoxide, and mixtures thereof.

7. The method of claim 1 , wherein the nonpolar solvent is selected from the group consisting of hexane, toluene, chloroform, and mixtures thereof.

8. The method of claim 1 wherein the nanoelements are selected from the group consisting of nanoparticles, nanotubes, nanocrystals, quantum dots,

macromolecules, and mixtures thereof.

9. The method of claim 1 , wherein the nanoelements are substantially

monodispersed in the nonpolar solvent prior to forming the two-phase system.

10. The method of claim 1 , wherein the two-phase system is formed from the solvents alone, and the nanoelements are introduced into the nonpolar solvent after the formation of the two-phase system.

1 1 . The method of claim 1 , wherein the nanoelements are dispersed in the nonpolar solvent at a concentration from about 0.1 wt% to about 10 wt% prior to forming the two-phase system.

12. The method of claim 1 , wherein a monolayer of said nanoelements is formed on the substrate.

13. The method of claim 12, wherein said monolayer is essentially continuous and free of gaps.

14. The method of claim 1 , wherein a multilayer of said nanoelements is formed on the substrate.

15. The method of claim 1 , wherein the substrate comprises a pattern of voids, and the nanoelements are selectively assembled in the voids.

16. The method of claim 1 , wherein the substrate comprises a smooth portion, and a continuous layer of nanoelements is assembled to cover the smooth portion.

17. The method of claim 1 , wherein the substrate is selected from the group consisting of silicon, silicon dioxide, metal oxides, and organic polymers.

18. The method of claim 1 , wherein the substrate is rigid and comprises a substantially planar surface upon which the nanoelements are assembled.

19. The method of claim 1 , wherein the substrate is flexible and comprises a substantially flat or curved planar surface upon which the nanoelements are assembled.

20. The method of claim 1 , wherein the step of moving is continuous until the substrate, or a desired portion thereof, has been coated with assembled

nanoelements.

21 . The method of claim 1 , wherein the step of moving comprises cyclic dipping of the substrate to deposit two or more layers of nanoelements.

22. The method of claim 1 , wherein the step of moving is automated.

23. The method of claim 1 , wherein at least 4 mm² of a substantially defect-free film of the nanoelements is formed on the substrate in one minute.

24. The method of claim 1 , further comprising the step of:

(c) allowing residual solvent to evaporate from the substrate.

25. The method of claim 1 , wherein the method is performed at a temperature in the range from about 10°C to about 40°C.

26. The method of claim 1 , wherein the nanoelements are not functionalized.

27. A nanoelement assembly produced by the method of claim 1 .

28. The nanoelement assembly of claim 27 comprising a monolayer of said nanoelements on the substrate.

29. The nanoelement assembly of claim 28, wherein said monolayer is essentially continuous and free of gaps.

30. The nanoelement assembly of claim 27 comprising a multilayer of said nanoelements on the substrate.

31 . The nanoelement assembly of claim 27, wherein the substrate comprises a pattern of voids, and the nanoelements are selectively assembled in the voids.

32. The nanoelement assembly of claim 27, wherein the substrate comprises a smooth portion, and a continuous layer of nanoelements is assembled to cover the smooth portion.

33. The nanoelement assembly of claim 27, wherein the substrate is selected from the group consisting of silicon and polymer materials.

34. The nanoelement assembly of claim 27, wherein the substrate is rigid and comprises a substantially planar surface upon which the nanoelements are assembled.

35. The nanoelement assembly of claim 27, wherein the substrate is flexible and comprises a substantially flat or curved planar surface upon which the nanoelements are assembled.

36. The nanoelement assembly of claim 27, wherein the entire substrate, or a desired portion thereof, is completely coated with assembled nanoelements.

37. The nanoelement assembly of claim 36, wherein the coated substrate or desired portion thereof has an area of at least about 10 mm².

38. The nanoelement assembly of claim 37, wherein the coated substrate or desired portion thereof has an area of at least about 100 mm².

39. The nanoelement assembly of claim 27, wherein the nanoelements are not functionalized.

a solvent-resistant container for liquids; a polar solvent and a nonpolar solvent within the container, forming a two- phase solvent system, and having an interface between the solvents;

41 . The device of claim 40, wherein the angle is about 30°.

42. The device of claim 40, further comprising a plurality of nanoelements disposed at said interface.

43. The device of claim 42, further comprising a plurality of said nanoelements disposed on a surface of the substrate.