Interesting quantum effects occur when electrons are confined in small regions, such as in nanostructures, examples of which include quantum dots (“QD”), nanocrystals (“NCs”), and nanotubes (“NTs”). Quantum effects in nanostructures typically occur as a result of the quantization of electronic charges and spins and due to the interactions between electrons and the local environment [1] (Reference No. 1 listed below). Some of these effects (e.g., Coulomb blockade, size-dependent fluorescence of nanocrystals, and nanocrystal blinking) have only recently been observed. Furthermore, research efforts in the study of NCs have already led to new applications. The first prototypes of useful nanocrystal (“NC”) devices based on quantum effects have been realized. Examples include a single-NC transistor [2], color-selective LEDs [3] and lasers [4,5], and nanoscale fluorescent markers [6,7].
As electronic circuitry shrinks, the dimensions of the circuitry will soon reach the NC size scales of a few nanometers where quantum effects tend to dominate electronic transport. Several properties of NCs that make them advantageous for use in quantum devices include, for example, small size to increase circuit density, room temperature quantum effects for ambient device operation, control of NC size, shape and composition, availability of large-scale synthetic techniques for preparing commercially viable amounts of NCs, and the ability of NCs to self-assemble.
NCs self-pack in either glassy arrays or ordered arrays, depending on the solvent composition and the drying parameters [16]. Although Brus and co-workers [24] have analyzed the drying-mediated self-assembly of nanoparticles using models of homogeneous and non-homogeneous solvent evaporation, the assembly process of nanostructures such as NCs is not fully understood.
The field of nanostructures is quite young. For example, researchers are currently working to discover, understand, and control mesoscopic effects that are observed in NC ordered arrays. Examples of mesoscopic effects include, inter alia, coulomb interactions, electron tunneling, charge ordering [8] and charge fluctuations. And because controlling such mesoscopic effects appears to be important for preparing electronic and optoelectronic devices, there is a continuing need to prepare nanostructure assemblies that are useful in preparing a wide variety of nanoscale electronic and optoelectronic devices. Examples of such electronic and optoelectronic devices include, inter alia, field-effect transistors (“FETs”), memory elements, photodiodes, sensors, and photovoltaic cells.
An array of coupled quantum dots has been proposed for quantum information processing (“QIP”) [9,10]. Researchers have demonstrated coherent spin transfer between two CdSe NCs using optical means [11]. Large 3D arrays of more than a million disordered CdSe NCs have revealed the NC ‘ensemble averages’ of electronic transport [12-16]. Compared to the well-established quantum dots in two-dimensional electron gases (“2DEGs”) [1], the NC QDs have not been electrically as controllable. Nanocrystal quantum dots are at least ten times smaller than 2DEG QDs and show quantum effects even at room temperature. However, small arrays of NC QDs, for example, a system of two NC QDs with independently controllable QDs, has not been achieved yet. Accordingly, there is presently a need to controllably assemble and configure small numbers (i.e., fewer than about 1000) of nanocrystal quantum dots and other nanostructures for preparing nanoscale electronic and optoelectronic devices.