preferably from about 1 nanometer to about micron. In some preferred embodiments, at least some of the particles are nanoparticles, which may be construed to mean particles having a particle size or at least one dimension below 1 micron. In certain preferred embodiments, all of the particles are nanoparticles, prior to array formation. Much of the remaining disclosure will assume that the particles are nanoparticles, it being understood that the principles of the invention may be applied to microparticles (particles having a particle size of 1 micron or higher) as well. 

The nanoparticles may be symmetric or asymmetric. If the nanoparticles are asymmetric, the long axes of individual nanoparticles are preferably aligned in the same direction with respect to one another in the arrays. "Aligned" in this sense means that the long axis of the nanoparticles has a full 15 width at half maximum angular distribution with respect to the array aligmnent direction of at most about –20°, more preferably at most about –10°, and most preferably at most about –5°. 

An "array" or equivalently "assembly" or "packed struc› ture" as intended herein, means a plurality of nanoparticles that are packed together and touching or near touching. In the array, the center-to-center distance between nanopar› ticles is preferably less than the width of two nanoparticles. More preferably, the center-to-center distance between nanoparticles is less than the width of 1.5 nanoparticles. 

The nanoparticles may form multilayer packed structures/ arrays in solution. "Multilayer" means at least 2 layers, preferably at least 4 layers, more preferably at least 10 layers, and even more preferably at least 25 layers. Each layer is counted along a continuous layer of nanoparticles. Although nanoparticles preferably assemble into layers within the array, it is also possible to assemble the nano› particles into a 3-dimensional array without distinct layers in one or more of the dimensions. 

The nanoparticles may comprise a fluoride, an oxide, a sulfide, or a combination thereof. In some preferred embodi› ments, the nanoparticles include or consist essentially of fluorides. The nanoparticles have an average isoelectric point that may be measured using a zeta potential measure› ment. The isoelectric point is the pH where the zeta potential is 0. Note that "oxides" also includes hydroxides and oxy› hydroxides. 

In some embodiments, the nanoparticle contains a core of a metal and a shell that is or includes an oxide, fluoride, sulfide, or combination thereof. In these or other embodi› ments, the nanoparticle contains a core of metal and a shell that is or includes an oxide, fluoride, sulfide, or combination thereof. In these or other embodiments, the nanoparticle contains a core of a ceramic ( or ceramic with metal) and a shell that is or includes an oxide, fluoride, sulfide, or combination thereof. 

The oxide, fluoride, or sulfide may be metal oxides, metal fluorides, metal sulfides, or another material ( e.g., polymer or ceramic) in oxide, fluoride, or sulfide form. The oxide, fluoride, or sulfide may be present as carbon-free O-con› taining groups, carbon-free F-containing groups, or carbon› free S-containing groups, respectively, chemically bonded to the nanoparticle core. 

The bulk solution contains a solvent which may be 60 selected from water, dimethyl formamide, dimethylsulfox› ide, isopropanol or another alcohol, acetone, tetrahydro› furan, or mixtures of these solvents. The solvent preferably contains water. 

An exemplary method schematic is shown in FIG. 1 and flowchart is shown in FIG. 2. Some embodiments will now be further described. 

Nanoparticles may be dispersed (i.e., dissolved and/or suspended as a colloid) in solution by adjusting the solution pH to increase the zeta potential above +20 mV or below -20 mV. In this disclosure, the notation "–20 mV" (for example) in reference to zeta potential means that the zeta potential is 20 m Vin magnitude ( absolute value) and may be either 20 mV or -20 mV; this does not refer to a range of values between -20 mV to 20 mV. 

The nanoparticles should be already formed ( as individual nanoparticles) and not a solution of precursors. (A precursor for example would be a metal salt that contains cations or anions, matching the intended composition of the nanopar› ticles.) In some embodiments, a seed of assembled nano- particles could be added to a dispersed solution of the same nanoparticles. 

A salt such as NaCl, preferably with cations or anions that are not common with the nanoparticle composition, may be added to change the screening of electrostatic charges in solution. Electrostatic repulsion prevents the particles from agglomerating and may be controlled with initial pH. Brownian motion, sonication, gas sparging, and/or bulk mixing (e.g., agitation or vessel rotation) may be used to keep the nanoparticles suspended in solution. 

In addition to the nanoparticles, one or more triggerable pH-control substances are added to the solution. The trig› gerable pH-control substance is preferably selected from thermal acid generators, thermal base generators, photoacid generators, photobase generators, or a combination of the foregoing. A "photoacid generator," or "photoacid," is a molecule which releases an acid, or becomes more acidic upon absorption of light. This may be due either to the release of strong acids upon photolysis, or to formation of a strongly acidic excited state. A "photobase generator," or "photobase," is a molecule which releases a base, or becomes more basic upon absorption of light. This may be due either to the release of strong bases upon photolysis, or to formation of a strongly basic excited state. In this dis› closure, a "strong acid" means an acid with a pKa value less than 4, such as less than 3, 2, 1, 0, -1, or -2. A strong base" means a base with a pKb value less than 4, such as less than 3, 2, 1, 0, -1, or -2. 

The triggerable pH-control substance may be capable of controllably generating a Lewis acid, a Bronsted acid, or an acid that is both a Lewis acid and a Bronsted acid. Similarly, the triggerable pH-control substance may be capable of controllably generating a Lewis base, a Bronsted base, or an acid that is both a Lewis base and a Bronsted base. 

As an example, ( 4-fluorophenyl)diphenylsulfonium tri› flate is a photoacid generator and creates protons (H+ ions) in response to UV light. As another example, sodium tetra› phenylborate is a photobase generator and creates hydroxide ions in response to UV light. Another example is 2-(9- oxoxanthen-2-yl)propionic acid 1,5,7-triazabicyclo[ 4.4.0] dec-5-ene which is a photobase generator that contains multiple functional groups. 

Urea is a thermal base generator and creates base (ammo› nia) from exposure to temperatures above 50° C. Sulfonate salts may be employed as thermal acid generators, creating protons (H+ ions) in response to heat. 

Note that some thermal acid/base generators are effective when temperature is reduced, rather than increased. For example, carbon dioxide solubility in solvent can be shifted, with temperature. Changing solubility will shift the carbon dioxide-carbonic acid equilibrium, resulting in a change in pH. In particular, lowering temperature will increase CO$_{2 }$ solubility and result in acidification (via carbonic acid) of an aqueous medium. 

