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°.  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.  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.  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 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.  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.  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.  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.  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.  US 9,981,240 Bl  10  9 