Prospects of Colloidal Nanocrystals for Electronic - Computer Science

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390 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. Dmitri V. Talapin is an Assistant Professor in the Department of Chemistry at the University of Chicago. He received his doctorate degree from the University of Hamburg, Germany, in 2002 under the supervision of Horst Weller. In 2003, he joined the IBM Research Division at the T. J. Watson Research Center as a postdoctoral fellow to work with Chris Murray on the synthesis and electronic properties of semiconductor nanostructures. In 2005, he moved to the Lawrence Berkeley National Laboratory as a staff scientist at the Molecular Foundry, newly founded DOE Center for Nanoscience and Nanotechnology. In 2007, he accepted a faculty position at the University of Chicago. His research interests revolve around colloidal inorganic nanomaterials, spanning from synthetic methodology to device fabrication, with the desire of turning colloidal nanostructures into useful electronic materials. Jong-Soo Lee received a Ph.D. in the Department of Metallurgical Engineering from Chonbuk National University in South Korea in 1999. From 2001 to 2004, he worked in the Department of Electrical Engineering at Korea University in South Korea as a research assistant professor. In 2006-2007, he worked as a postdoctoral researcher at the Molecular Foundry in the Lawrence Berkeley National Laboratory with Dmitri V. Talapin and A. Paul Alivisatos. In 2007, he joined the Talapin group as a senior research technologist. His research interests include synthesis and charge transport studies in colloidal inorganic NCs and multifunctional nanostructures for electronic, photovoltaic, and thermoelectronic applications. of over hundreds to thousands atoms is necessary to develop the band structure of metals, semiconductors, and magnetic materials. 1-4 In this regime, the electronic structure, optical, and magnetic properties of materials can be tuned by varying the physical size of the crystal, leading to new phenomena, such as superparamagnetism of magnetic NCs, surface plasmon resonance in Au and Ag nanoparticles, 1 and the sizedependent band gap of semiconductor NCs. 2,3 These open interesting opportunities for device applications, for example, tailoring the electronic structure without introducing any changes in the chemical composition. Formerly, lithographic techniques aggressively invaded the nanoworld; nowadays, electronic circuits with features as small as 45 nm are used for commercial applications, with good prospects for 32 and Maksym V. Kovalenko received his doctorate degree from the University of Linz (Austria) in 2007 under the supervision of Prof. Wolfgang Heiss. His doctoral thesis was focused on the synthesis and device applications of infrared-active colloidal semiconductor nanocrystals. In 2008, he joined the group of Prof. Dmitri Talapin at the University of Chicago as a postdoctoral scholar. He is currently working on the functionalization of inorganic nanocrystals with inorganic ligands and integration of these novel materials into electronic, optoelectronic, and thermoelectric devices. His other scientific interests include magnetism and self-assembly of nanocrystals. Elena V. Shevchenko received her first degree in chemistry at the Belorussian State University and her Ph.D. from the University of Hamburg in 2003 with Horst Weller. From 2003 to 2005, she was a joint postdoctoral fellow between Columbia University (with Stephen O’Brien) and the T. J. Watson Research Center IBM (with Christopher Murray). In 2005, she moved to the Molecular Foundry, Lawrence Berkeley National Laboratory, as a staff scientist. Since 2007, she is a scientist at the Center for Nanoscale Materials at Argonne National Laboratory. Her primary research interests are in the design of nanoscale functional materials and understanding their fundamental properties. 22 nm technologies that are on the way from laboratories to fabrication facilities. 5 Because of similarities in feature size, the knowledge obtained from electronic studies of nanoscale materials should help in understanding the design rules for next generations of electronic circuits. Inorganic semiconductors have a proven track record in electronics and optoelectronics; they offer superior carrier mobilities, light absorption, photo, and thermal stability. 5 However, they are difficult to form by low cost processes. Crystallization of inorganic semiconductors requires high temperature treatments, which force trade-offs between device performance, fabrication costs, and compatibility with flexible plastic substrates. The development of applications ranging from photovoltaics and light-emitting devices to “smart cards”, radio frequency tags, and sensors could be accelerated by introducing lower cost alternatives to conventional technologies that rely on single crystals. 5,6 Here,
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 391 the nanometer size crystals can find commercial use as the building blocks for inexpensive manufacturing of low cost and large area devices. Solution-based processes such as spin coating, dip coating, or inkjet printing offer substantial cost reductions for the fabrication of electronic and optoelectronic devices when combined with novel materials like organic semiconductors, 6,7 carbon nanotubes, 8 nanowires, 9 or hybrid organic-inorganic films. 10 Properly designed NCs can ideally fit the requirements for solution-processed electronic and optoelectronic devices because they form thermodynamically stable and easy-to-handle colloidal solutions. Moreover, NCs can self-assemble from colloidal solutions into ordered structures called “superlattices” or “NC solids”, which will be an important subject of this review. Tunable electronic structure combined with small exciton binding energy, high luminescence efficiency, 11 and very low thermal conductivity 12 make NC solids especially attractive for photovoltaic, lighting, and thermoelectric applications. Efforts to harness the quantum tunability of semiconductor NCs have led to many successes in optical and optoelectronic applications, such as light emitting devices, 13-16 luminescent tags, 13 and lasers. 14-16 NCs have a variety of biological and biomedical applications. 17-21 Semiconductor NCs can serve as stable fluorescence probes. 18 Magnetic NCs can be used as efficient diagnostic tools in magnetic resonance imaging and magnetic separation of biological targets 20 and therapeutic agents for hyperthermic tumor treatments, 19 drug, and gene delivery. 21 Plasmonic properties of noble metal NCs are utilized in molecular-specific imaging and sensing, as well as in photodiagnostic and photothermal therapy. 17 At the same time, realizing solid-state electronic applications (e.g., field-effect devices) of these nanoscale building blocks has been more challenging. The confinement of carriers inside the NCs imparts their fascinating size tunable properties, but, until recently, frustrated the efforts to efficiently contact and integrate them into devices that switch with useful speeds. 22-25 Charge transport in NCs relies on the electrons traveling between individual particles and is, therefore, dependent on the electronic communication and collective phenomena in NC arrays. In this respect, NCs are different from their cousins, nanowires and nanotubes, which can be directly connected to the electrodes. 2. Solution-Phase Synthesis of Metallic, Semiconducting, Magnetic, and Multicomponent Nanoparticles Colloidal synthesis of inorganic nanostructures is developing into a new branch of synthetic chemistry. Starting with preparations of simple objects like monodisperse spherical nanoparticles, 26-29 the field is now moving toward more and more sophisticated structures where size, shape, and connectivity of multiple parts of a multicomponent structure can be tailored in an independent and predictable manner. 30-36 Many technologically important metals, semiconductors, and magnetic materials can be synthesized as uniform sub-20 nm crystals; multiple materials can be combined in the form of the core-shell, dumbbell, or more complex morphologies. 37 Although the general methodology of colloidal synthesis of multicomponent structures is much less developed as compared to the conventional synthesis of molecular compounds, impressive progress has been achieved in the past years, which introduced totally novel approaches to materials design. For example, the band gap of semiconductor nanostructures can be precisely tuned by size and shape control, electron and hole can be spatially separated within the NC by introducing heterostructures with staggered band offsets, 35 different confined regimes can be achieved for electrons and holes, 33 etc. Growing heterostructures built of semiconductor and plasmonic materials allow coupling excitons and surface plasmon resonance generated by semiconductor and metal components, respectively. 24,38 Below, we briefly describe the methodology of colloidal synthesis. For more detailed information, we should address the readers to numerous comprehensive reviews on the synthesis of inorganic nanomaterials. 28,37,39-45 2.1. Basics of Colloidal Synthesis: Nucleation and Growth Among the appeals of solution-based colloidal synthesis, we should mention the excellent control over size and shape of prepared nanostructures and applicability to a broad range of materials. Moreover, the use of relatively simple experimental equipment and chemicals allows one to obtain highquality materials and tailor their properties at surprisingly low cost. 46 Typically, colloidal nanomaterials are synthesized by reacting appropriate molecular precursors, that is, inorganic salts or organometallic compounds. The colloidal synthesis generally involves several consecutive stages: nucleation from initially homogeneous solution, growth of the preformed nuclei, isolation of particles reaching the desired size from the reaction mixture, postpreparative treatments, etc. As a rule, temporal separation of the nucleation event from the growth of the nuclei is required for narrow size distribution. 26,47,48 The so-called hot-injection technique, when the precursors are rapidly injected into a hot solvent with subsequent temperature drop, satisfies this requirement. 26,48,49 The separation of nucleation and growth stages can also be achieved upon steady heating of the reaction mixture. 46 Nucleation and growth of NCs occurs in the solution phase in the presence of organic surfactant molecules, which dynamically adhere to the surface of growing crystals. 30,46 Typical surfactants include long-chain carboxylic and phosphonic acids (e.g., oleic acid and n-octadecylphosphonic acid), alkanethiols (e.g., dodecanethiol), alkyl phosphines, alkylphosphine oxides (classical examples are trioctylphosphine, TOP, and trioctylphosphine oxide, TOPO), and alkylamines such as hexadecylmine. The surfactant molecules play the key role in tuning the kinetics of nucleation and growth, 39,47 which should be kinetically balanced because if the nanoparticle nucleation rate is either too slow or too fast with respect to the growth rate, the reaction will generate either bulk crystals or molecular clusters. Achieving proper balance of these intrinsically different processes is an important and sometimes challenging problem, which is usually addressed empirically by searching for a good combination of molecular precursors, surfactants, solvent, and the reaction conditions (temperature regime, etc.). Development of reproducible synthetic approaches leading to nanoparticles with uniform size, shape, composition, and surface morphology is extremely important for further progress in fundamental studies and practical applications. Figure 1 shows several examples of metal nanocrystals synthesized by colloidal chemistry. Metal nanoparticles can be synthesized by reducing the metal ions using reductant such as borohydride, amines, or 1,2-diols in the presence of stabilizing agents, typically long-chain alkanethiols (e.g.,
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