ABSTRACT
This chapter addresses the information-coding and retrieval technology by using photoluminescent (PL) semiconductor quantum dots (QDOTs) synthesized via wet-chemistry approaches. QDOTs with different wavelengths and intensities span a 2D coding space. Secure information is coded in such a group of QDOTs, which is then applied to various substrates, such as ink, paint, and labels. When an exciting light beam shines at these information carriers, their emitting spectral features, that is, wavelengths and intensities, provide the encoded information. As the wavelength of the photoluminescence can be precisely controlled when designing the code, these QDOTs are able to be made to emit at Fraunhofer line positions, namely, black lines in the solar spectrum; thus, the retrieval system can even extract useful information in sunshine-covered areas. Furthermore, multiphoton excitation (MPE) technology enables the retrieval
system to perform multilayer information extraction, which greatly increases the dimensions of coding space. Anticipated applications include security, military, and law enforcement, for example, coding and retrieval information from military helmets, vehicles, and even fingernails. In addition, QDOT-based security information can be easily destroyed by preset expiration in the presence of timing agents, which results in the expiration of this information carrier. 13.1 IntroductionThe most important issues in security technologies are information storage and retrieval. The prevailing technology used nowadays for object identification is the use of a one-dimensional (1D) or twodimensional (2D) barcode. The visibility of the printed pattern of a barcode is vulnerable to counterfeiting; therefore, a barcode is definitely not qualified in many security applications. In addition, as the barcode reader has to scan the 1D bar sequence or to register the 2D dot image, these procedures make the system bulky and complicated. Regarding high-level security, a hidden information carrier, which is invisible to the human eye and tiny in size, is mandatory; moreover, the invariance under the changes of position and rotation of the information encoded in the carrier is critical to simplify the information retrieval procedure with enhanced reliability. Multiplexed spectral coding technology that makes use of multiple wavelengths and multiple intensities as the coding space meets the security and invariant requirements to a certain degree. Organic dyes and metal complexes are commonly used as fluorescence-sensing materials in various applications. Basically, they seem to be suitable in the multiplexed spectral coding technology; however, their intrinsic optical properties limit them to be the ideal candidates. For example, different dye molecules require different excitation wavelengths; furthermore, it is different to retrieve information from a mixture of these fluorescent molecules due to their broad emission bandwidth and asymmetry. The limitation may also be related to the certain reaction of different dye molecules or the immiscibility of some of the dye molecules in a common matrix material. From a technical point of view, an ideal set of luminescent substances should have the following properties in order to meet the prerequisite for multiplexed encoding and decoding:
(1) A single light source for all materials to emit at different wavelengths; (2) Each emission independent of the excitation wavelength; (3) Stable emission with narrow bandwidth from each luminescent material; (4) No chemical interaction among the fluorescent molecules; (5) No emission from matrix materials; and (6) Good miscibility of all luminescent materials in the selected matrix materials. Recently, colloidal photoluminescent (PL) semiconductor quantum dots (QDOTs) have demonstrated many of the above-mentioned characteristics. For example, QDOTs exhibit emission with narrower bandwidth, broader absorption, and better photostability compared to traditional luminescent materials. Furthermore, they are excellent with multiphoton excitation due to large multiphoton action cross sections; accordingly, they are magnificent candidates for multilayer information extraction. Moreover, their bandgap emission and absorption peak positions can be easily and accurately tuned via the control of their size, structure, and composition. Due to their intrinsic optical properties, QDOTs are suitable in the multiplexed optical coding technology with potential in security and defense.1-5 Basically, QDOTs are ultrasmall nanocrystals and are spherical in shape, usually in the range of 1-10 nm. Elements made up of QDOTs are often from group IIB and group VIA in the periodic table. QDOTs can be binary compounds, such as cadmium selenide (CdSe) and zinc sulfide (ZnS); ternary compounds, such as CdTeSe and ZnCdS; or layered structures, such as core-shell CdSe/ ZnS and CdTe/CdSe/CdTe.6-12 When these nanocrystals dispersed in a transparent solution are excited, they provide coding information based on their emission position and intensity; such a solution with the secure information encoded is the so-called info-ink. A mixture of various QDOTs with different emission positions can be dedicatedly designed to feature a special code with a set of data consisting of the emission positions and intensity. Such coding information is hidden in a fluorescence spectrum; accordingly a spectroscopic device is needed rather than a scanner or a camera to decode the encoded information. It is necessary to point out that such QDOT-based information carriers are miniature in size and invisible to human eyes, demonstrating their potential in security applications.
