NEW Contributo HIGHLIGHT per Report Istituto 2012
TITLE: Physical properties of elongated inorganic nanoparticles.
Journal: Physics Reports.
Ref.: Vol. 501 (2011) 75–221.
Author: Morello Giovanni, giovanni.morello@unisalento.it
Collaborators: Roman Krahne, Albert Figuerola , Chandramohan George, Sasanka Deka, Liberato
Manna
Abstract
Inorganic nanoparticles are among the most investigated nano-objects nowadays, both in
fundamental science and in various technological applications. In this review, the authors describe
the physical properties of nanoparticles when their shape is elongated, i.e. rod-like or wire-like. In
most cases, they analyze the transition in the physical properties when the shape of the
nanoparticles evolves from spherical to rod-like.
The review is divided in sections.
Section 1: Introduction
A brief description of the structure of the work is given, together with the motivations which driven
the authors for its drawing up.
Section 2: Optical properties of elongated semiconductor nanoparticles
The optical properties of nanocrystals are dictated by their electronic structure, and the section starts
with a short overview on the main quantum effects in confined systems. The authors describe the
optical peculiarities of semiconductor nanorods by highlighting the main characteristics that
distinguish them from the more traditional quantum dots.
Section 3: Electrical properties of elongated nanoparticles
In this section the electrical properties of elongated nanoparticles (nanorods) both on a single
particle level and for thin films consisting of densely aggregated nanorods are discussed. It deals
only with semiconductor nanorods, and it focuses mostly on the CdSe material system as an
example to explain the physical properties, since CdSe nanorods have been studied in great detail.
Section 4: Optical properties of elongated metal nanoparticles
The shape of metal nanoparticles has perhaps the most striking influence on their optical properties.
As an example, in rod-shaped nanoparticles the plasmon mode is split into two modes, a
longitudinal one and a transverse one. The discussion is general and covers all metal nanoparticles,
but in some occasions it focuses on gold nanoparticles, for several reasons: first of all gold
nanoparticles do not undergo oxidation processes, hence their theoretical description does not need
to take into account the presence of a surface oxide layer (which complicates considerably the
theoretical treatment); secondly, gold nanoparticles are now fabricated in a variety of sizes and
shapes, hence the testing of the various computational approaches on real samples is directly
feasible; lastly, gold nanoparticles have been employed and/or proposed in a variety of applications.
A brief overview of the optical properties of metal nanoparticles in various size regimes is given,
and the attention is focused on the effect of the shape, with particular emphasis on elongated
nanoparticles.
Section 5: Magnetic properties of elongated nanoparticles
One of the main breakthroughs in the application of magnetic materials offered by nanostructured
systems is found in life sciences and biomedicine. Due to their small sizes and their zero net
magnetization at zero fields, they can be safely exploited in biomedicine, for instance as
nanovectors for specific targets that are not accessible via other conventional approaches. Nanorods
or other one-dimensional nanosystems could also widen the temperature range of application of a
certain magnetic material compared to its bulk form (as will be shown later). The uniaxial shape
anisotropy of metallic and oxide nano-objects will probably become a key factor for the
development of improved devices. The section mainly focuses on the description of the structural
and magnetic properties of magnetic nanoparticles. Since the assembly of anisotropic nanoparticles
is crucial for the exploitation of their properties, special attention is paid to works in which some
degree of organization of elongated nanoparticles has been achieved.
Section 6: Catalytic properties of elongated inorganic nanoparticles
The specificity of a catalyst depends largely on catalyst’s surface, on the type and distribution of
surface active sites, onto which reactive species are adsorbed and undergo the following steps of
dissociative adsorption and desorption, to yield the final or the targeted products. Nowadays, there
is a huge demand for novel catalytic materials, in terms of catalytic efficiency, cost of production,
specificity, durability and environmental sustainability. This demand is constantly driving research
towards the exploitation of new nanoscale materials, in which the individual components have
tailored size, shape, exposure of specific reactive surfaces, and suitable combination of materials.
Section 7: Mechanical properties of elongated nanoparticles and melting studies
In the last decade, pure metals and alloys with at least one dimension in the micro- and nanoscale
range have been investigated, thanks to advances in the fabrication of new generations of samples
suitable for mechanical testing (for example micropillars prepared by focused ion beam) and in
various techniques for studying their stress and deformation properties, under both tensile and
compressive deformation, the latter for instance via micro/nanoindentation. All these studies have
revealed a marked deviation in the mechanical properties of samples from bulk-like behavior
already when their size is of the order of a few micrometers, which is comparable to the length scale
of many plasticity mechanisms based on dislocation nucleation and propagation.