I received my Master’s Degree in Chemistry from the University of Bari (Italy) in 1996, discussing a thesis on crystallography of macromolecules, and received my Ph.D. degree from the same university in 2001. During my Ph.D. studies, and later as a postdoctoral fellow, I worked in the group of professor A. Paul Alivisatos, at the University of California, Berkeley, until 2003. My early research was focused on synthesizing nanostructures using chemical approaches. Coming from an undergraduate background in physical chemistry and crystallography, I was interested in the thermodynamics and kinetics of growth of colloidal nanocrystals, which are nanometer-size crystals synthesized in the solution phase. More specifically, I was fascinated by the processes occurring at the interface between the nanocrystals and the liquid solution, and by how these could be influenced by crystal symmetry and chemisorption of molecules.

When I moved to UC Berkeley (in early 1999), this field of nanostructures was simply exploding. I was triggered by the idea that chemists could give a significant contribution to this area of research. Nanostructures, and especially the semiconductor-based ones, have been in fact for a long time the unchallenged realm of physicists and engineers, who had developed sophisticated tools to grow them in various types of architectures, such as the so-called "Quantum Dots, quantum wells, and quantum wires."

“Being able to engineer the shape of a particular nanostructure, as well as the location of different materials on it at the same time, leads to nano-objects with tailored chemical and physical properties.”

In the early ’90s chemists started to move into this business by developing methods that yielded high-quality nanocrystals, and at the end of the ’90s that was still a young field. There were several breakthroughs on how to grow nanocrystals in the solution phase, which were characterized by a narrow distribution of sizes, an important pre-requisite for studying their properties and for practical applications. In particular, important contributions appeared on how to synthesize highly fluorescent nanocrystals, and this triggered the exploitation of these materials (especially CdSe nanocrystals) in light-emitting diodes and in biological studies. However, a method to control the shape of semiconductor nanocrystals (when they are grown in solution), another important parameter that regulates their physical properties, was still missing. The search for that particular method, at least for semiconductors, became the focus of my research in the Alivisatos group at UC Berkeley.

My work deals mainly with nanocrystals with controlled shapes and chemical composition. These now range from rods to tetrapods (a particular structure in which four rods branch out from a central region, at roughly tetrahedral angles) to nanocrystals with even more complex shapes (such as multiple-branched objects) and more recently to nanocrystals made of various sections of different materials connected together. Being able to engineer the shape of a particular nanostructure, as well as the location of different materials on it at the same time, leads to nano-objects with tailored chemical and physical properties. The additional advantage of preparing these nanostructures in a liquid is that we can easily coat their surface with several types of molecules, including biomolecules, and this expands the range of their possible applications.

In that paper we reported a method on how to grow CdSe nanocrystals with rod shapes. We found out that this was possible by synthesizing CdSe nanocrystals in the presence of small amounts of specific molecules, which "poison" some facets of the nanocrystals and hinder their growth, therefore promoting the growth of the nanocrystals only along some preferential directions. This results in the formation of nanorods. We also showed how the length and the diameter of such nanorods could be controlled tightly at the nanometer scale. The importance of the rod’s shape resides in the fact that, as opposed to "spherical" CdSe nanocrystals, CdSe nanorods emit linearly polarized light and show additional interesting optical properties. This was demonstrated by that work and confirmed by a series of subsequent reports. In general, shape control is important, as several physical properties of nanocrystals are strongly affected by this parameter.

The development of methods to control the shape of nanocrystals is a field that has flourished in the last few years. Several groups (including the one I started in Italy in 2003) have extended the early approach of the 2000 paper to the fabrication of other materials. This in turn has given the nanocrystals community access to a new generation of samples with novel physical properties, and the possibility to envision new applications. Also, the principles and the chemicals underlying the anisotropic growth of nanocrystals that were discussed in that 2000 paper have been useful not only for designing even more complex nanostructures, but have triggered research on less hazardous and environmentally friendlier methods for their preparation.

Besides the 2000 paper cited above, there are actually a few papers which I would like to mention. A 2003 paper that was published in Nature Materials (Manna L, et al., "Controlled growth of tetrapod-branched inorganic nanocrystals," Nature Materials 2[6]: 382-5, 2003), for instance, reported a method to grow tetrapod-branched inorganic nanostructures in solution. A tetrapod is an interesting nanostructure, as it self-aligns on a substrate, with three legs touching it and the fourth leg pointing upwards. This structure has been exploited by several groups in applications that range from photovoltaics to single-electron devices. This work was followed by another work (published in 2004 in Nature—Milliron DJ, et al., "Colloidal nanocrystal heterostructures with linear and branched topology," Nature 430[6996]: 190-5, 2004) that reported the synthesis of branched nanostructures with sections of different materials.

More recently, in the group that I have started in Italy, we have been interested (among various things) in the early stage of growth of nanocrystals in a liquid solution (Kudera S, et al., "Sequential growth of magic-size CdSe nanocrystals," Advanced Materials 19[4]: 548-52, 2007). We have found out that when crystals are aggregates of only a few tenths of atoms (therefore immediately after their formation), not all aggregates of atoms are stable. Some aggregates have exceptionally higher stabilities, and the growth of nanocrystals at this stages proceeds by "jumps" from a stable aggregate (a so-called "magic size") to the next larger and stable aggregate (the next large magic size). I believe that this work is important, as it focuses the attention on a growth regime that is rarely exploited by scientists in this field.

Nanocrystals with complex shapes and compositions are currently under investigation as potential candidates in various applications. CdSe nanorods and CdTe tetrapods, for instance, have been shown to increase the efficiency of polymer-based solar cells when they are included as additives in the polymer layer, and larger efficiencies are expected when more complex nanostructures are incorporated. Polarized displays based on aligned nanorod arrays are also under investigation by several groups. Some recent reports have also pointed at the potential advantages of semiconductor nanorods as fluorescence labels in biological applications. Interesting developments of these materials might also come from the fields of photonics and nanoelectronics.

There are, however, some key limitations that delay a deeper exploitation of most types of nanocrystals in nanotechnology. One is certainly the toxicity of many of the materials of which nanocrystals can be made. Therefore, several groups (including mine) are heading towards the development of non-toxic, or a least less toxic types of nanocrystals. Another limitation is that the fabrication of nanocrystals yields always samples with a certain distribution of sizes and shapes. Things get worse when the shape and/or composition of the nanocrystals becomes more complex. This is mainly due to the still "primitive" way in which these nanostructures are prepared. Therefore, one direction towards which I want to orient my group is the development of innovative concepts of chemical reactors that allow for a tighter control of the synthesis parameters, along with the development of computational tools that model the growth process.

Finally, I believe that for a breakthrough in the science and technology of nanocrystals the understanding on how to organize them into complex, controllable geometries over large areas using bottom-up self-assembly approaches is of tremendous importance. This is an essential part of implementing shape-controlled nanocrystals in diverse applications where controlled assembly is required. It is clear to me that only a self-assembly approach is going to be applicable to create complex systems with millions or even billions of nano-components, and this is where my group is trying to make a significant contribution. To this aim, I am actually coordinating a European Project, which involves eight research institutions across Europe in total, whose purpose is to progress in the understanding and controlling the self-assembly of shape-controlled nanocrystals.

Liberato Manna, Ph.D.
National Nanotechnology Lab
Istituto Nazionale per la Fisica della Materia
CNR-INFM
Lecce, Italy