Research Area Summary

Quantum dots are a new form of matter that can be considered as "artificial atoms." They have linear discrete absorption spectra (like atoms) and photoluminescence that is tunable (by changing the dot size) over a wide range, from far infrared to deep ultraviolet. They can be moved around for different purposes:

  • to form quantum-dot "molecules"
  • to form three-dimensional "meta-crystals" that form new materials having tailored lattice constants, tailored crystal symmetry, and tailored band structure
  • to act as dopants in other materials
  • to be joined with a larger molecule to form a super molecule.

In this way, instead of 109 elements we have at our disposal, in principle, an unlimited number of atomic "elements."


Electrons and Excitons in Quantum Dots

Figure 1. The density of electron spin precession modes in an ensemble of singly charged QDs is modified by the nuclei. The black curve shows the density of modes before the nuclear induced frequency focusing due to the ensemble dispersion of electron g-factor and nuclear polarization fluctuations. The train of short optical pulses reduces the continuous density of electron spin precession modes to just three frequencies (blue).
Figure 1. The density of electron spin precession modes in an ensemble of singly charged QDs is modified by the nuclei. The black curve shows the density of modes before the nuclear induced frequency focusing due to the ensemble dispersion of electron g-factor and nuclear polarization fluctuations. The train of short optical pulses reduces the continuous density of electron spin precession modes to just three frequencies (blue).

Our research objective is to understand the physical properties of quantum dots (QD) and the processes controlling their fabrication for two areas of application:

  • Optoelectronics. Desirable properties include broad photoluminescence (PL) tunability, high PL quantum efficiency, and enhanced nonlinear optical properties. Applications include new generation of highly efficient solar cells, tunable infrared-ultraviolet lasers and LEDs, display luminophores, optical electro-modulation, switches, memory storage, optical limiting, DNA site markers, and efficient sensors of explosives and toxic materials.
  • Quantum Information Processing. Coherent manipulation of electron spin in single QDs and in quantum-dot ensembles, and creation of QDs with very long spin-coherence times. Applications include spin-transistors, nanosize magnets, quantum computing, and electron spin-based memory.

An electron spin localized in a quantum dot is a quantum bit, the basic unit for solid-state-based quantum computing and quantum information processing. The spin replaces a classical digital bit, which can take on two values, usually labeled 0 and 1. The electron spin can also take on two values. However, since it is a quantum object, it can take also all values in between. Obviously, such a quantum unit can hold much more information than a classical one and, even more importantly, the use of such quantum bits makes certain computer calculations exponentially more efficient than those using a standard computer. That is why scientists are trying to find an efficient way to control and manipulate the electron spin in a quantum dot in order to enable new quantum devises using magnetic and electric fields.

Until now, the major problem with using charged quantum dots in such devices is that the electron spins in different quantum dots are never identical. The electron spin precession frequencies in an external magnetic field are different from each other due to small variations of the quantum dot shape and size. In addition, the electron spin precession frequency has a contribution from the random hyperfine field of the nuclear spins in the quantum dot. This makes a coherent control and manipulation of electron spins in an ensemble of quantum dots impossible and pushes researchers to work with individual spins, and to develop single spin manipulation techniques - which are much more complicated than an ensemble manipulation technique.

Together with researchers at the University of Dortmund and the University of Bochum we have taken a significant step toward solving this problem by suggesting a new technique that would allow coherent manipulations of an ensemble of electron spins. Two years we demonstrated a method whereby a tailored periodic illumination with a pulsed laser can drive a large fraction of electron spins (up to 30%) in an ensemble of quantum dots into synchronized motion. More recently we showed that almost the whole ensemble of electron spins (90%) precesses coherently under periodic resonant excitation. It turns out that the nuclear contribution to the electron spin precession acts constructively by focusing the electron spin precession in different quantum dots to a few precession modes controlled by the laser excitation protocol, instead of acting as a random perturbation of electron spins as was thought previously. The modification of the laser protocol should allow scientists to reach a situation in which all electron spins have the same precession frequency, in other words, to make all spins identical.

Future efforts involving the use of these identical electron spins will focus on demonstrating all coherent single q-bit operations using an ensemble of charged quantum dots. Another important use of such ensembles for quantum computing will be the demonstration of a quantum-dot gate operation. The macroscopic coherent precession of the electron spin ensemble will allow scientists to study coherent optical phenomena such as electromagnetically induced transparency and slow light.
Principal Investigator: Alexander Efros


Doping Semiconductor Nanocrystals

Figure 2. Nanocrystals of zinc selenide can be controllably doped with atoms of manganese, which selectively adsorb on certain crystal facets before being incorporated.
Figure 2. Nanocrystals of zinc selenide can be controllably doped with atoms of manganese, which selectively adsorb on certain crystal facets before being incorporated.

Nanocrystals are tiny semiconductor particles just a few millionths of a millimeter across. Due to their small size, they exhibit unique electronic, optical, and magnetic properties that can be utilized in a variety of technologies. To move toward this end, chemical methods have been optimized over the last 20 years to synthesize extremely pure nanocrystals. More problematic, however, has been the goal of controllably incorporating selected impurities into these particles. Conventional semiconductor devices, such as the transistor, would not operate without such impurities. Moreover, theory predicts that dopants should have even greater impact on semiconductor nanocrystals. Thus, doping is a critical step for tailoring their properties for specific applications.

A long-standing mystery has been why impurities could not be incorporated into some types of semiconductor nanocrystals. Together with David Norris and Lijun Zu of the University of Minnesota's (UMN) Department of Chemical Engineering and Materials Science, we have recently established the underlying reasons for these difficulties, and have provided a rational foundation for resolving them in a wide variety of nanocrystal systems. The key lies in the nanocrystal's surface: If an impurity atom can stick, or 'adsorb,' to the surface strongly enough, it can eventually be incorporated into the nanocrystal as it grows. If the impurity binds to the nanocrystal surface too weakly, or if the strongly binding surfaces are only a small fraction of the total, then doping will be difficult. From calculations based on this central idea, we predicted what conditions would be favorable for doping. Experiments at UMN then confirmed these predictions, including the incorporation of impurities into nanocrystals that were previously believed to be undopable. Thus, a variety of new doped nanocrystals may now be possible, an important advance toward future nanotechnologies.

An exciting aspect of these results is that they overturn a common belief that nanocrystals are intrinsically difficult to dope because they somehow 'self-purify' by expelling impurities from their interior. According to this view, the same mechanisms that made it possible to grow very pure nanocrystals also made it extremely difficult to dope them. We have shown that doping difficulties are not intrinsic, and indeed are amenable to systematic optimization using straightforward methods from physical chemistry.

Future efforts will focus on incorporating impurities which are chosen for specific applications. For example, solar cells and lasers could benefit from impurities that add an additional electrical charge to the nanocrystal. In addition, impurities will be chosen to explore the use of nanocrystals in spin electronics (or "spintronics"). Spintronic devices utilize the fact that electrons not only possess charge, but also a quantum mechanical spin. The spin provides an additional degree of freedom that can be exploited in devices to realize a host of new spintronic technologies, from nonvolatile "instant-on" computers to so-called "reconfigurable logic" elements whose underlying circuitry can be changed on-the-fly.
Principal Investigator: Steven Erwin