TECHNOLOGY FOCUS - IMAGING AND COMMUNICATION - Preferred Device: The light emitting member of located in the polymer matrix of (D) is a polymer matrix itself, or includes at least second nanocrystals encapsulated in the polymer matrix which emits light at, at least one pre-selected wavelength. The polymer matrix allows electronic transport of electrons and/or holes. (D) includes a conducting, semiconducting or insulating layer located between (L1) and the polymer matrix to regulate the transport of electrons and holes into the polymer matrix. (D1) comprises a first conducting, semiconducting or insulating layer (L3) located between (L1) and (L2), a second conducting, semiconducting or insulating layer (L3a) located between (L2) and (L2a). (L3) and (L3a) regulate the transport of electrons and holes into (L2) and (L2a). (L2a) comprises greater than or equal to2 layers of nanocrystals, each including nanocrystals of a pre-selected size for emitting light of wavelength different from the other layers of nanocrystals. The pre-selected wavelength range for detection is an infrared or visible spectral region, and the at least one pre-selected wavelength for emission is a visible wavelength at a higher wavelength than wavelengths in the visible spectral region. (D) and (D1) include a light source for illuminating the first nanocrystals, and (L2) and (L2a); respectively. The light source of (D) emits light at, at least one pre-selected wavelength for absorption by the first nanocrystals. The light source of (D1) emits light at, at least one pre-selected wavelength for selective absorption by the first and second nanocrystals. Preferred Components: The first nanocrystals are of material and size (preferably diameter) selected to absorb light in the pre-selected wavelength range. The second nanocrystals are substantially monodisperse and of a material and size (preferably diameter) selected to emit at the at least one pre-selected wavelength. The areal concentration (N1) of the first nanocrystals is such that it satisfies a condition the N1a2 is greater than 1 (where a is radius of the first nanocrystals). The areal concentration (N2) of the second nanocrystals is such that it satisfies a condition the N1b2 is greater than 1 (where b is radius of the second nanocrystals. The first and second nanocrystals are made of the same material.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The insulating layer of (D1) is selected from mixed organic-inorganic insulating dielectrics, inorganic insulating dielectrics, SiO2, SiO or SiNxOy.x and y=not defined.The first and second nanocrystals are selected from inorganic semiconductors (preferably group III - V and group IV - VI semiconductors, especially InAs, PbSe, CdS or CdSe, especially PbSe nanocrystals). At least one (preferably both) of the first and second nanocrystals have a core/shell structure comprising a core made of a pre-selected type of material and a shell material surrounding the core, made of a different pre-selected type of material (preferably ZnS(CdSe) nanocrystals). The nanocrystals have a size of (1 - 20, preferably 2 - 10) nm. (L2a) includes greater than or equal to2 types of nanocrystals, each having a material and size selected to emit light of pre-selected wavelengths in the visible and/or infrared region.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The insulating layer of (D1) is selected from mixed organic-inorganic insulating dielectrics, inorganic insulating dielectrics, SiO2, SiO or SiNxOy.x and y=not defined.The first and second nanocrystals are selected from inorganic semiconductors (preferably group III - V and group IV - VI semiconductors, especially InAs, PbSe, CdS or CdSe, especially PbSe nanocrystals). At least one (preferably both) of the first and second nanocrystals have a core/shell structure comprising a core made of a pre-selected type of material and a shell material surrounding the core, made of a different pre-selected type of material (preferably ZnS(CdSe) nanocrystals). The nanocrystals have a size of (1 - 20, preferably 2 - 10) nm. (L2a) includes greater than or equal to2 types of nanocrystals, each having a material and size selected to emit light of pre-selected wavelengths in the visible and/or infrared region.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The first and second nanocrystals have ligands attached to their surface, selected from oleate, hexylamine, octylamine, dodecylamine, octadecylamine, tri-octyl phosphine oxide, butylamine or pyridine (preferably octadecylamine).    TECHNOLOGY FOCUS - POLYMERS - Preferred components: The polymer matrix of (D) comprises semiconducting polymers selected from poly(2-methoxy-5-(2'-ethyl-hexyloxy)-para-phenylene-vinylene (MEH-PPV) or associated poly-phenylene-vinylene (PPV) derivatives, PFO or associated polyfluorine derivatives or poly-thiophenes (preferably P3HT or P3OT) (preferably poly-(para-phenylene-vinylene)). The semiconducting layer of (D1) is selected from MEH-PPV or associated PPV derivatives, polyfluorine (PFO) or associated polyfluorine derivatives, poly-thiophenes, poly(3-octyl-thiophene) (P3HT), poly(9,9-dioctylfluorene-co-bitiophene) (F8T2) (preferably poly-(para-phenylene-vinylene)). The insulating layer of (D1) is selected from poly-methyl-methacrylate (PMMA) or poly-(para-phenylene-vinylene) (PPV) (preferably PPV). The conducting layer of (D1) is selected from poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), doped MEH-PPV or doped (poly-phenylene-vinylene) (PPV) (preferably poly-(para-phenylene-vinylene)).