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Integration of Nanostructured Light Emitting Devices
Nano-optics will play a central role in many foreseeable technologies of interest to the nation's defense, such as sensitive chemical and biological detection, quantum computing for secure cryptography, integrated optics, and diagnostics of nanoelectronic circuitry. To implement these concepts, it is key that light sources be available that can be easily fabricated and flexibly configured. Several avenues for realizing nanoscale light emitters are currently pursued worldwide, each with specific advantages and challenges. For example, schemes using semiconducting nanofilaments or epitaxially grown quantum-sized islands show great promise but are limited in the range of accessible configurations. To achieve a broad configurability, this nanoscience project aims to develop a nanoscale light-emitting diode, or NanoLED, that can be easily fabricated through chemically directed self-assembly.
The core of the approach involves the use of light-emitting nanocrystals (NCs) linked to electrically conductive molecular materials. The NCs, averaging in diameter from 1 to 10 nm, are also termed colloidal quantum dots (QDs): "colloidal" because they are conveniently synthesized as colloids suspended in solution and "quantum" because the NC's minute size confers useful and controllable quantum characteristics, analogous to a very large atom. Consider, for example, a semiconducting QD. In bulk form, the semiconductor from which the QD is made emits a characteristic color like the familiar red glow from an LED display, but when the semiconductor is reduced to the size of a QD, the wavelength of emission can be tuned from red to green to blue by tailoring the size of the dot. This quantum phenomenon permits controlling the color, through the synthesis-controlled size of the QDs, without altering the chemical processes used to manipulate the dots, an important advantage for the self-assembly of a NanoLED.
Colloidal QDs are amenable to self-assembly because they are inherently decorated with a shell of organic molecules. These molecules can be tailored in turn to control the dot's interaction with its environment, such as linking to a substrate or to each other. To date, such control has focused on using insulating molecules, but this project will implement the use of molecules with extended electron delocalization to boost the carrier transport between the QD and the respective electrodes. By manipulating the specific molecules that are decorating the QD, and those chemically attached to the carrier-injecting electrodes, we will promote the self-assembly of nanostructures such as depicted in Fig. 1. Here a QD is linked to an electrode, a conducting substrate, via a molecular "wire." The tip of a scanning probe microscope (SPM, Fig. 2) serves as the counter electrode. To ensure the isolation of single QDs, a pre-formed self-assembled monolayer (SAM) of insulating molecules is employed that has defects, i.e. openings, to receive the molecular wires. If the quantum energy levels of the molecular wires and QDs are properly engineered, electrical bias on the SPM tip will inject electrons and "holes" into the QD. Radiative recombination of the electron and hole will then lead to light emission from the NC in a process known as electroluminescence.
Figures 3a and b illustrate the ability of the SPM, here operating as a scanning tunneling microscope, to locate and topologically characterize nanoparticles dispersed on a SAM structure. In this example, the NCs are 5-nm gold clusters attached to an insulating SAM of alkanedithiol molecules. The synthetic route here is not aimed at isolated NCs because conduction between particles is desired and, indeed, the SPM finds NCs are organized as rafts (Fig. 3a), although individual NCs are also present (Fig. 3b).
In a NanoLED structure, the electrical current that will eventually be converted to light must be efficiently injected and transported from the electrodes to the molecular wires and the QD. Hence it is vitally important to engineer the electronic states of all involved components in order to minimize any energy barriers to the flow between components. Our approach entails four coupled fronts to be carried out by a team of chemists, physicists, and material scientists: (i) advanced quantumchemical computations of the electronic energy levels of the molecular components in pertinent non-equilibrium conditions; (ii) experimental interrogation of the actual energy levels through both optical and electron spectroscopy (see Fig. 4); (iii) optical and electrical characterization of the assembled device; and (iv) optimizing the device efficiency by tailoring the linking molecules and QDs in response to the theoretical and experimental results in (i)-(iii).
The invention and optimization of NanoLEDs can be expected to spawn new technologies of importance to the Navy and the Marine Corps. For example, figure 5 illustrates a hypothetical chemical sensor that exploits the color shift that is anticipated when analyte molecules bond to a QD. High sensitivity is possible, in principle, because the small size of a QD makes it susceptible to perturbation by only a few analyte molecules.
Contact the Principal Investigator, James Long, for more information