Research

Semiconductor Quantum Dots

Semiconductor nanoparticles (NP) or nanocrystals (NC) have been investigated for many decades, with particular acceleration in interest upon the discovery of quantum confinement effects thereby yielding quantum dots (QD) from certain well-grown NC. The term NP is commonly used in the case of metal and wide gap semiconductor nanocrystals. The term NC is commonly used in semiconductor nanocrystals, whether covalent II-VI or ionic perovskites, that are colloidally grown. And the term QD applies to select semiconductor nanocrystals for whom their size is on the order of the excitonic Bohr diameter. In the case of colloidal particles on the nm length scale, these terms are often used carelessly and interchangeably. The words have specific meaning in relationship to specific physical effects which give rise to specific key processes that can be measured. Here, we provide a Perspective upon the ways in which size confers function across different families of NP. In this way we aim to find ways to identify their similarities and differences by providing the correct semantics for discussion of the salient processesOur group has done pioneering work on unraveling electron / hole pair (exciton) relaxation dynamics in Semiconductor Quantum Dots as model systems that show quantum size effects as noted below.

Topic 0: Overview of excitonics of Quantum Dots.

The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an “artificial atom” because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies.

Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the “excitonics”, confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have developed a state-resolved spectroscopic approach which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials.

We focus on four main topics of longstanding interest and controversy: (i) hot exciton relaxation dynamics, (ii) multiexcitons, (iii) optical gain, and (iv) exciton−phonon coupling. Using this state-resolved approach, we reconcile long standing controversies related to phenomena such as exciton cooling and exciton−phonon coupling and make surprising new observations related to optical gain and multiexcitons.

Selected publications:

“Unraveling the Structure and Dynamics of Excitons in Semiconductor Quantum Dots”, P. Kambhampati, Acc. Chem. Res. 44, 1 (2011). DOI: 10.1021/ar1000428 - Invited

Topic 1: Multiexcitons in Semiconductor Nanocrystals: A Platform for Optoelectronics at High Carrier Concentration

It is well-known that the shape, size, and composition of semiconductor nanocrystals give rise to a quantized manifold of electronic states, that is, excitons. In addition, these nanocrystals can support multiple excitations per particle under relatively modest conditions. Beyond a laboratory curiosity, these multiexcitons dictate a wide variety of optoelectronic properties of semiconductor nanocrystals including those from lasers, light-emitting diodes, photon sources, and possibly photovoltaics. Whereas their existence has been known for some time, observation of the structure and dynamics of multiexcitons has remained elusive due to their ultrafast lifetimes. In this Perspective, we discuss the first glimpses of the structural dynamics of multiexcitons in CdSe semiconductor nanocrystals as revealed by excitonic state-resolved femtosecond pump/probe spectroscopy. These measurements of multiexciton formation, cooling, and recombination are related to the optical gain performance of nanocrystals. In particular, we show that the gain threshold, bandwidths, and dynamics are dictated by the previously unobserved structural dynamics of multiexcitons.

Selected publications:

“ Multiexcitons in Semiconductor Nanocrystals: A Platform for Optoelectronics at High Carrier Concentration”, P. Kambhampati, J. Phys. Chem. Lett. 3, 1182 (2012). DOI: 10.1021/jz300239j

Topic 2: Hot exciton relaxation dynamics in quantum dots: Radiationless Transitions on the Nanoscale.

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the various excitonic states of the dot and the optical and acoustic phonons. We show that the divergence of prior results can be reconciled by adopting this multichannel picture of hot exciton relaxation dynamics. This picture establishes a framework for designing materials with relaxation properties targeted for specific applications. We conclude with connection to hot exciton surface trapping. The process of surface trapping is the key step in creation of the photoproduct which can obscure measurements of optical gain, multiexciton recombination, multiple exciton generation, and single dot blinking. We show that hot exciton surface trapping can effectively compete with hot exciton relaxation, thereby obfuscating these processes.

Selected publications:

“Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale”, P. Kambhampati, J. Phys. Chem. C 115, 22089 (2011). DOI: 10.1021/jp2058673

Topic 3: Optical Gain and Light Emission in Quantum Dots

Semiconductor nanocrystals have been extensively studied for optoelectronic applications including light emission, the focus of this review. Historically, the core of the nanocrystal was the main aspect of the system as it gives rise to the confined excitons and multiexcitons which absorb and emit light. In addition to the core, the surface or interface of these nanocrystals is also important by virtue of their small size. Yet, our understanding of the surface is in its early stages in terms of both chemical and electronic structure. Here, we review the ways in which the interface can control the excitonics, which gives rise to optoelectronic function. We focus our discussion on the ways in which the interface can control optical gain for nanocrystal lasers and white light generation for nanocrystal based light emitting diodes. These processes are connected to different interfacial structures. Finally, we discuss two new applications based on surface electronic structure control: optical switching and optical thermometry. The work here suggests that the interface of nanocrystals should be a profitable route for chemical control of the electronic structure which can yield both performance enhancements as well as qualitatively different functional behavior.

Topic 4: On the nature of the Surface of Nanocrystal Quantum Dots

The surface of semiconductor nanocrystals is one of their defining features. The kinetics of surface trapping can be monitored by pump/probe spectroscopy. The thermodynamics of surface trapping is revealed by photoluminescence spectroscopy. We produce the first microscopic picture of how excitons are coupled to the surface. We discuss the possibility of surface excitons in nanocrystals.

The surface of semiconductor nanocrystals is one of their defining features by virtue of their nanometer size. Yet the surface is presently among the most poorly understood aspects of nanocrystal science. This perspective provides an overview of spectroscopic work that has revealed the first insights into the nature of the surface, focusing upon CdSe nanocrystals. We focus on two aspects of surface processes in nanocrystals: the kinetics of surface trapping and the thermodynamics of core/surface equilibria. We describe femtosecond pump/probe spectroscopic experiments which reveal the signatures of carrier trapping at the surface. We also describe temperature dependent steady-state photoluminescence experiments which reveal new aspects of the surface. This work suggest that the surface emission is largely driven by homogeneous broadening via phonon progressions. The implications are that the surface electronic state bears similarity to the quantized excitonic core of the nanocrystal.

Selected publications:

“On the Kinetics and Thermodynamics of Excitons at the Surface of Semiconductor Nanocrystals: Are There Surface Excitons?”, P. Kambhampati, Chem. Phys. 446, 92 (2015). DOI: 10.1016/j.chemphys.2014.11.008.