4.C IRG1: Collective Properties of Nanostructure Arrays
Faculty: Bumm, Johnson, McCann, Mullen, Shi, (OU); Bellaiche, Fu, Gea-Banacloche, Manasreh, Peng,
Salamo, Tian, and Xiao (UA); 6 postdocs, 6 graduate students; Partners: Humboldt University,
(Germany), University of Central Florida, Université de Franche-Comte (France), University of
Arkansas-Pine Bluff, see sections 4.E.2 and 4.E.4 for descriptions of other collaborations.
Focus: Based on the theme that all material properties depend upon the arrangement of identical units
(atoms or molecules), the two overarching goals for this IRG are to:
• Refine and apply a host of fabrication techniques to synthesize ordered arrays of nanoscale synthetic
units (dots, wires, rings, shells, etc.) for a range of material systems that will allow exploitation of their
coherent and collaborative behavior.
• Combine these techniques with modeling capabilities to understand, predict and tailor properties, such
as excited state lifetimes, refractive index, ferroelectric coefficients etc. for technological applications.
Motivation: The last decade has seen great advances in our ability to create semiconductor structures on
the submicron scale. This has been driven in a large part by the desire for increased chip performance, and
memory density. Periodic nanostructure arrays are desired for their uniformity, which greatly improves
their efficiency as lasers, detectors, and nonlinear optical devices. In addition, the collective properties of
these arrays may allow us to create systems with optical and electronic properties not found in Nature.
Since traditional lithography fails at this length scale, other techniques are needed.
Proposed Research: Center researchers have demonstrated success in a number of complementary
techniques of self-assembly [83-92], and in the optical, electronic, and crystallographic understanding of
the resulting nanostructures [93-104]. Building on this developed expertise, we have begun to explore
both the fabrication and collective behavior of organized arrays of nanostructures. Such structures are
defined by complex collective interactions between individual units, potentially exposing new insights
and novel material properties. Highly ordered arrays of III-V, IV-VI and II-VI semiconductors and
ferroelectric materials will be fabricated making possible a number of remarkable properties: geometry
dependent excited state lifetimes and fluorescence yield; improved nanosize uniformity; tailored index of
refraction and dispersion; enhanced nonlinear optical coefficients; and variable dielectric and
piezoelectric coefficients. Such material structures appear promising for the development of light-emitting
devices, waveguides, detectors, memory, and applications in nonlinear optics and nanophotonics.
For these goals an interdisciplinary team is essential, from modeling synthetic units (Bellaiche,
Fu, and Mullen) to their growth (Bumm, Johnson, McCann, Peng, Salamo, Shi, and Tian) to
characterization and exploration (Bumm, Gea-Banacloche, Johnson, Manasreh, Salamo, and Xiao). Based
on our previous work, our synthetic units can be quantum dots, wires, rings or shells in a variety of
traditional and exotic materials. Below we give three examples and discuss the research challenges and
exciting opportunities that 2D and 3D arrays can provide.
Quantum Dot Arrays: We will create QD arrays ranging from clusters of several dots
to periodic structures with thousands of units. To explore their collective properties
we will obtain fine control of the lateral and vertical spatial position of QD units. To
accomplish this we plan three related directions: MBE self-assembly, colloidal
growth, and patterned self-assembly.
Under the current IRG1, Salamo demonstrated exceptional control of QD
arrays grown via MBE self-assembly (see adjacent) [105-108]. In this new effort
Salamo, Shi, and McCann will use stacking techniques to exploit the amazing
organizational ability resulting from strain, to produce new structures i.e. a square Plan-view AFM image
x-sectional TEM
lattice of QDs or an array of QD pairs, triplets, or clusters in III-V and IV-VI and
image of QD chains
materials. Guided by TEM (Johnson) this team will explore different growth
conditions, and index surfaces. Their goal is to better understand the roles that diffusion, strain,
composition, temperature, and over-pressure play in determining the lowest energy state of the surface,
the knowledge of which can provide control for the precise positioning of our nanostructures.
In a parallel effort colloidal QDs will also be linked into clusters and
extended 3D structures. The synthetic expertise developed in CSPIN (Peng) in
the past five years provides us with one of the broadest spectra of colloidal
nanocrystals available, with a variety of sizes, shapes, compositions, and ligand
modifications [109,110]. Tian—a new Center member and a pioneer on
hierarchical assembly of colloidal nanostructures—will explore sophisticated
nanostructures in solution or on substrates. The state-of-the-art, mostly
developed by his group, is limited to building blocks bigger than several tens of
Self-assembled 3D clusters
nanometers [111]. Coupled with Peng’s expertise on synthesis and manipulation
of colloidal SiO2 particles.
of nanocrystals and Johnson’s TEM ability, a combined effort will extend this
down to smaller and more exciting size range.
In patterned self-assembly we alter a substrate to control subsequent
growth. Johnson has used anodized aluminum oxide (AAO) templates with
ordered 10 to 100 nm pores [92] as masks for both etching and deposition.
Johnson, Salamo, and Peng will use these structures to do in situ electron or ion
beam “milling” of substrates to control new growth and to guide colloidal
structures into periodic patterns. Bumm will exploit AAO templates as mold
masters for nano-imprinting.[112] A complementary approach by Xiao and
Salamo will use optical interference and e-beam lithography (both in house and
with Humboldt Univ.) to pattern substrates. We also propose to explore nano SEM view of Ni rings on Si
substrate.
textured QD arrays made possible by using our AAO templates to deposit
materials on top of quantum wells (Johnson). In principle this technique allows us to deposit periodic
arrays of any material we wish atop a quantum well, perturbing the bandgap, spin field or screening of the
electron gas below.
The IRG1 team will work to integrate our QD arrays into a host material that mediates the
coupling between the dots and to understand, predict, and tailor their properties. A collaboration of
Salamo, Xiao, and Gea-Banacloche will use their experience in coherent pulse propagation and quantum
optics to design and measure dot arrays with tailored absorption and dispersion properties and even
negative dispersion [113]. A very large change in refractive index can slow down the propagation of light
to enhance the matter-light interaction and greatly increase optical nonlinearities at low light levels. A
negative dispersion slope will be designed through the dispersion relations of coupled oscillators
producing a group velocity that is faster than the speed of light [114]. Collaborating with the Universities
of Central Florida and de Franche-Comte, they will design an enhanced nonlinear optical response using
either solitonic waveguides [115] or electromagnetically induced transparency (EIT), a phenomena in
which Xiao and Gea-Banacloche are experts [116,117]. EIT is based on creating interference between
electronic transitions that reduce absorption and take advantage of the strong nonlinearity near resonance.
Arrays present the interesting possibility of studying EIT using an induced collective coherence to
enhance and tailor linear and nonlinear absorption and dispersion.
Another exciting opportunity is the manipulation of temporal and spatial dispersion by organizing
oscillator phases to control optical behavior. For example, cooperative emission from QD ordered arrays
can be lengthened or shortened. Guided by TEM (Johnson) Xiao, Salamo, and Manasreh will rely on
their experience with the atomic and solid state to measure, design (with Mullen and Fu) and control the
radiative decay time (T1) and the dephasing time (T2) by varying the array geometry [118,119].
Controlling the electronic coupling between quantum dots presents a greater set of challenges.
We have already demonstrated the growth of small dot clusters that show extended states that cross
between the dots; however the inter-dot tunneling amplitude is extremely sensitive to the dot geometry
and materials. As we increase this amplitude we move through a series of regimes in physics: thermally
activated hopping, to extended single-particle states to, at the strongest coupling, many-body states. These
regimes will have different electronic transport characteristics. Since it is easier to grow arrays where the
tunneling amplitude between dots will be weak, Mullen and Fu will start with a tight binding model. At
integer filling (one electron/dot) we expect the system to be a Hubbard insulator. Near this point we can
have correlated electron states, just as in the Hubbard model. However experimentally we (Salamo) can
alter the spin coherence between dots, their dephasing time and a range of other properties that determine
how correlations will form. As we increase the tunneling amplitude we develop a new laboratory for
many-body physics.
Quantum Ring Arrays: Rings present their own set of novel dynamics, since
their “orbitals” can be tuned by ring geometry. Semiconductor ring structures as
small as 40 nm across will be fabricated via self-assembly, a GaIn droplet
technique (shown here by Salamo) and by sputtering and re-deposition through
AAO masks (Johnson, see figure previous page). Fu will calculate single particle
ring states for realistic ring materials and geometries. Rings also differ from dots
in that a classical point charge on a ring always has a dipole moment of fixed
magnitude, although of arbitrary direction. Theory indicates that nano-ring arrays AFM image (500 nm on a
side) of an array of
undergo a phase transition from isotropic to anti-ferroelectric ordering as their
GaAs/AlGaAs rings
interactions are increased. We will detect and study this phase transition in the
excitonic gap of these structures and in their collective electronic excitations (Johnson and Xiao). It may
also be possible to tune ring geometries and produce structures with anisotropic polarizabilities (Mullen).
Such materials will have applications as optical switches and ferroelectric memory. In addition, because
they are not simply connected, an electron confined to the ring will be sensitive to a threading magnetic
flux in a way not possible in a quantum dot. Consequently, when we measure the magneto-conductance
of ring arrays, if inter-ring tunneling is significant, we will find a rich structure due to interference effects.
Exciton transfer in close-packed arrays of quantum rings will be interesting to explore for similar reasons.
Ferroelectric Dot and Rod Arrays: Bulk ferroelectrics (FE) undergo a structural phase transition at low
temperatures into stable states characterized by a spontaneous polarization. Accessing these states by an
external electric-field is the key idea for devices such as nonvolatile ferroelectric random access
memories (NFRAM). Compared to bulk FEs, low-dimensional finite FE structures promise to increase
the storage density of NFRAM ten thousand fold. This anticipated benefit, however, hinges on whether
the phase transitions and multi-stable states still exist in low-dimensional structures. This question is the
subject of much contention and long-standing interest for understanding and revealing collective
interactions. In fact, previous studies have shown that phase transitions are impossible in 1D systems and
become increasingly less likely as dimensionality further decreases, thereby limiting the potential rewards
for NFRAM. Recently, however, Fu and Bellaiche have demonstrated state-of-the-art ab-initio
simulations [120-122] that led to the discovery that (1) phase transitions do exist in dot and rod FE
nanostructures; (2) these phase transitions profoundly differ from those occurring in bulk in that they
form a spontaneous solenoidal moment below a critical temperature; and (3) the resulting lowtemperature phases promise the generation of new devices with phenomenal capabilities [123]. Further,
the minimum diameter of the disks that display low-temperature structural bistability is 3.2 nm, enabling
a NFRAM density of 60 Terabits/inch—five orders of magnitude larger than currently available.
CSPIN is unique in its combination of theoretical and experimental expertise in this field.
Motivated by the above theoretical results, Salamo and Peng have begun growth of FE dots, rods, and
wires via MBE and colloidal suspension. The team will explore phase transitions in arrays of nanodots, to
see if they are sharp, as well as the nature of the spontaneous moment using a custom piezoelectric sensor
added to a scanning force microscope. The precise effects of the substrate, growth orientation, surface
termination, and thickness on the ferroelectric properties of nanodot arrays are not clear. We will
investigate if there is a critical thickness below which FE can not occur, and if it exists, how this critical
thickness depends on the boundary conditions (i.e., short-circuit versus open-circuit). A team of
Manasreh, Xiao, and collaborators from UA-Pine Bluff will measure linear and nonlinear optical and
piezoelectric character of quantum wells perturbed by a periodic FE dot array.
Summary: CSPIN brings together a rare and essential combination of techniques and talents to design,
grow, and study nanostructures. Together this group has the power to uncover the nature of collective
dynamics in quantum dot and ring arrays and nanoferroelectrics and begin an era for their application.