Spectroscopy Of Quantum Dots Research Articles

On-off QD switch that memorizes past recovery from quenching by diazonium salts

The understanding of the interaction of CdSe/ZnS semiconductor quantum dots (QD) with their chemical environment is fundamental, yet far from being fully understood. p-Methylphenyldiazonium tetrafluoroborate has been used to get some insight into the effect of diazonium salts on the spectroscopy of QD. Our study reveals that the surface of CdSe/ZnS quantum dots can be modified by diazonium salts (although not functionalized), showing and on-off fluorescence behaviour that memorizes past quenching recoveries. Facile modification of the surface confers protection against quenching by new molecules of diazonium salt and other known quenchers such as 4-amino-TEMPO. The reaction mechanism has been explored in detail by using different spectroscopic techniques. At the first time after addition of diazonium salt over QD the fluorescent is turned off with Stern-Volmer behaviour; the fluorescence recovers following irradiation. Subsequent additions of diazonium salts do not cause the same degree of quenching. We have noted that the third addition (following two cycles of addition and irradiation) is unable to quench the fluorescence. Monitoring the process using NMR techniques reveals the formation of p-difluoroborane toluene as a result of the irradiation of diazonium-treated QD; the treatment leads to the fluorination of the QD surface.

K-Space Image Correlation Spectroscopy of Quantum dot Labeled T Cell Receptors Characterizes their Nanoscale Clustering in Living Cells

Changes in distribution of membrane receptor organization are used by cells to modulate the dynamic range of their responses to environmental cues. Consequently, it is important to develop experimental methods that can accurately measure receptor transport and aggregation. We used quantum dot (QD) labeling of T cell receptors (TCR) and a recently developed technique, k-space image correlation spectroscopy (kICS) to characterize TCR state as a function of cell differentiation. We developed kICS to measure transport coefficients of fluorescently labeled membrane proteins while taking into account nanoparticle emission blinking. We use kICS to measure T cell receptor (TCR) aggregation in live cells by characterizing quantum dot (QD) blinking and distribution on the cell surface. 2C TCR transgenic cells in culture were observed from the naive state to 12 days after activation by antigen. Cells were labeled with biotin-MHC/peptide to bind the TCR with relatively high avidity, followed by streptavidin-quantum dots. They were then imaged on an emCCD-equipped microscope and kICS analysis was applied. Spatial intensity fluctuations in an image measured the clustering of receptors on the 100s of nm length scale. Changes in the intensity correlation function of the blinking QDs characterized clustering on the 10s of nm length scale. We also used kICS to measure changes in TCR diffusive transport. When T cells exhibited maximum activity 3-4 days after exposure to antigen, the degree of their TCR aggregation on both length scales was significantly higher than that of naive cells, while TCR diffusion was a minimum. This new technology has powerful applications as it can be applied to just a few cells and we will show that it is able to detect changes in receptor organization of cells in vivo.

Cryogenic spectroscopy of ultra-low density colloidal lead chalcogenide quantum dots on chip-scale optical cavities towards single quantum dot near-infrared cavity QED

We present evidence of cavity quantum electrodynamics from a sparse density of strongly quantum-confined Pb-chalcogenide nanocrystals (between 1 and 10) approaching single-dot levels on moderately high-Q mesoscopic silicon optical cavities. Operating at important near-infrared (1500-nm) wavelengths, large enhancements are observed from devices and strong modifications of the QD emission are achieved. Saturation spectroscopy of coupled QDs is observed at 77K, highlighting the modified nanocrystal dynamics for quantum information processing.

Superconducting tunneling spectroscopy of a carbon nanotube quantum dot

We report results on superconducting tunneling spectroscopy of a carbon nanotube quantum dot. Using a three-probe technique that includes a superconducting tunnel probe, we map out changes in conductance due to band structure, excited states, and end-to-end bias. The superconducting probe allows us to observe enhanced spectroscopic features, such as robust signals of both elastic and inelastic cotunneling. We also see evidence of inelastic scattering processes inside the quantum dot.

Spin-Related Spectroscopy of CdTe-Based Quantum Dots

Spin-Related Spectroscopy of CdTe-Based Quantum Dots

Nonlocal bias spectroscopy of the self-consistent bound state in quantum point contacts near pinch off

We perform nonlocal bias spectroscopy of the self-consistent bound state (BS) in quantum point contacts (QPCs), determining the lever arm (γ) that governs the gate-voltage induced shift in its energy. The value of γ allows us to infer an enhanced g factor, and large remnant spin splitting, for the BS. Our results show many similarities with bias spectroscopy of quantum dots and are reproduced by calculations that assume a discrete BS coupled to a reservoir. This study therefore provides independent evidence in support of the notion of BS formation in QPCs.

Near-field Optical Wavefunction Mapping and Its Application to Single Quantum Dot Spectroscopy

Currently, near-field scanning optical microscopy offers a spatial resolution down to 10−30 nm. High resolution imaging spectroscopy enables us to visualize the exciton wavefunctions confined in semiconductor quantum structures. In this article, the principle of near-field optical wavefunction mapping is described. Then as an application, we performed mapping out of exciton and biexciton wavefunctions in single GaAs quantum dots. Significant displacement of the center of emission profiles was found, in contrast to the usual difference in the emission profiles of an exciton and a biexciton. By conducting a numerical calculation, such a displacement could be reproduced by introducing a shallow potential dip, which causes a significant difference in the penetration of the wavefunctions into the barrier. Precise mapping of exciton and biexciton wavefunctions of quantum-confined structures will provide a good probe for weakly localized states due to local strain and disorder.

Quantum Dot Spectroscopy Using Cavity Quantum Electrodynamics

We show how cavity quantum electrodynamics using a tunable photonic crystal nanocavity in the strong-coupling regime can be used for single quantum dot spectroscopy. From the distinctive avoided crossings observed in the strongly coupled system we can identify the neutral and single positively charged exciton as well as the biexciton transitions. Moreover we are able to investigate the fine structure of those transitions and to identify a novel cavity mediated mixing of bright and dark exciton states, where the hyperfine interactions with lattice nuclei presumably play a key role. These results are enabled by a deterministic coupling scheme which allowed us to achieve unprecedented coupling strengths in excess of 150 microeV.

Light‐Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy

AbstractSpectra of individual silicon nanocrystals within porous Si grains are studied by the wide‐field imaging microspectroscopy and their ON–OFF blinking is detected by the confocal single‐photon‐counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties of SiO2 passivated SiNCs are found, namely the quantum efficiency (QE) of about 10–20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.

Controlling the energy transfer between near-field optically coupled ZnO quantum dots

We performed time-resolved spectroscopy of ZnO quantum dots (QD), and observed exciton energy transfer and dissipation between QD via an optical near-field interaction. Two different sizes of ZnO QD with resonant energy levels were mixed to test the energy transfer and dissipation using time-resolved photoluminescence spectroscopy. The estimated energy transfer time was 144 ps. Furthermore, we demonstrated that the ratio of energy transfer between the resonant energy states could be controlled.

Polarization sensitive spectroscopy of charged quantum dots

AbstractWe present an experimental and theoretical study of the polarized PL spectrum of single semiconductor quantum dots in various charge states. We compare our polarization sensitive spectral measurements with a novel many‐carrier theoretical model. The model considers both the isotropic and anisotropic exchange interactions between all participating electron‐hole pairs. We demonstrate that lines emanating from evenly charged states are linearly polarized. Their measured polarization direction does not necessarily coincide with the traditional crystallographic direction. It depends on the shells of the single carriers which participate in the recombination process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scanning Tunneling Spectroscopy of Individual PbSe Quantum Dots and Molecular Aggregates Stabilized in an Inert Nanocrystal Matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a two-dimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates.

Fundamental studies in nanosciences at the Institute of Electronics, Microelectronics, and Nanotechnology (IEMN)

This paper gives an overview over the fundamental research in nanosciences at the Institute of Electronics, Microelectronics and Nanotechnology (IEMN). We present some highlights from the numerical simulation of the electronic structure of nanowires and nanotubes, the charge spectroscopy of Si nanoparticles and C nanotubes, the scanning tunnelling spectroscopy of semiconductor quantum dots, to research in surface science for bio-screening.

Polarization sensitive spectroscopy of charged quantum dots

We present an experimental and theoretical study of the polarized photoluminescence spectrum of single semiconductor quantum dots in various charge states. We compare our high resolution polarization sensitive spectral measurements with a many-carrier model which we developed for this purpose. The model considers both the isotropic and anisotropic exchange interactions between all participating electron-hole pairs. With this addition, we calculate both the energies and polarizations of all optical transitions between collective, quantum dot confined charge-carrier states. We succeed in identifying most of the measured spectral lines. In particular, the lines resulting from singly, doubly, and triply negatively charged excitons and biexcitons. We demonstrate that lines emanating from evenly charged states are linearly polarized. Their polarization direction does not necessarily coincide with the traditional crystallographic direction. It depends on the shells of the single carriers, which participate in the recombination process.

Inelastic laser light scattering spectroscopy and functionalization of semiconductor quantum dots with peptides and integrins of cancer cells for biophotonic applications

Results of our study of structural properties of the nanoscale integrated semiconductor quantum dots such as CdS and ZnS-capped CdSe, conjugated with biomolecules such as short peptides and cells are presented. Nanoscale functionalization of semiconductor quantum dots with biomedical structures is promising for many applications and novel studies of intrinsic properties of both constituent systems. We study CdS semiconductor quantum dots functionalized with peptides composed of the following amino acid chains: CGGGRGDS, CGGGRVDS, CGGIKVAV, and CGGGLDV, where R is arginine, D is aspartic acid, S is serine, V is valine, K is lysine and L is Levine. As will be seen the cysteine (C) amino acid links to CdS semiconductor quantum dots via the thiol link. Furthermore, the GGG sequences of glycine (G) amino acids provide a spacer in the amino acid chain. At the same time the RGDS, RVDS, IKAV, and LDV sequences have selective bonding affinities to specialized transmembrane cellular structures known as integrins of neurons and MDA-MB-435 cancer cells, respectively. Since protein hydration is known to be a key factor affecting protein energy balance, we also studied a role that water and other bioenvironments may play in stability, surface properties, dynamical and structural characteristics of these systems. We found also the roles that the quantum confinement and functionalizing in the biomedical environments play in altering and determining the electronic, optical, and vibrational properties of these nanostructures as well as demonstrated the effectiveness to use the semiconductor quantum dots as integrin sensitive biotags.

Theory of nonlinear optical spectroscopy of electron spin coherence in quantum dots

We study in theory the generation and detection of electron spin coherence in nonlinear optical spectroscopy of semiconductor quantum dots doped with single electrons. In third-order differential transmission spectra, the inverse width of the ultranarrow peak at degenerate pump and probe frequencies gives the longitudinal spin relaxation time $({T}_{1})$, and that of the Stokes and anti-Stokes spin resonances gives the spin dephasing time including the inhomogeneous broadening $({T}_{2}^{*})$. The transverse spin relaxation time excluding the inhomogeneous broadening effect $({T}_{2})$ can be measured by the inverse width of ultranarrow hole-burning resonances in fifth-order differential transmission spectra.

Calculation of conduction-to-conduction and valence-to-valence transitions between bound states in(In,Ga)As∕GaAsquantum dots

We have calculated the conduction-to-conduction and valence-to-valence absorption spectrum of bound states in (In,Ga)As/GaAs quantum dots charged with up to three electrons or holes. Several features emerge: (i) In pure (non-alloyed) InAs/GaAs dots, the 1S-1P_1 and 1S-1P_2 conduction intraband transitions are fully in-plane polarized along [1\bar 10] and [110], respectively, while valence transitions are weakly polarized because the hole P states do not show any in-plane preferential orientation. (ii) In alloyed In_{0.6}Ga_{0.4}As/GaAs dots the [110] and [1\bar 10] polarization of the corresponding 1S-1P conduction intraband transitions is weakened since the two 1P states are mixed by alloy fluctuations. The polarization of valence intraband transitions is insensitive to changes in alloy fluctuations. (iii) For light polarized along [001], we find a strong valence-to-valence transition that involves a weakly confined hole state with predominant light-hole character. (iv) When charging the dots with a few electrons, the conduction intraband transitions display spectroscopic shifts of ~1-2 meV. These shifts are a result of correlation effects (captured by configuration-interaction) and not well described within the Hartree-Fock approximation. (v) When charging the dots with holes, valence intraband spectra are more complex than the conduction intraband spectra as hole states are strongly affected by spin-orbit coupling, and configuration mixing is more pronounced. Spectroscopic shifts can no longer be identified unambiguously. These predictions could be tested in single-dot spectroscopy of n-doped and p-doped quantum dots.

Preface: phys. stat. sol. (a) 204/2

AbstractThe first International Workshop on Modulation Spectroscopy of Semiconductor Structures (MS3) in Wrocław (in the southwest part of Poland) at the beginning of July 2004 gathered almost 40 participants, half of them from abroad. This fruitful meeting resulted in a special issue of physica status solidi (a), Vol. 202, No. 7 (2005).The second meeting of scientists involved in all aspects of modulation spectroscopy of semiconductors, semiconductor heterostructures and devices – the 2nd MS3 Workshop – was organized at the same place (Wrocław, Poland) and almost at the same time of the year in 2006.The MS3 Workshop became a forum of discussion on the advantages of photoreflectance, electroreflectance, contactless electroreflectance, thermoreflectance, differential reflectance and wavelength‐modulated surface photovoltage spectroscopy. Applications of the above methods to investigate transistor, diode and laser structures including VCSELs, low‐dimensional structures of both wings of the spectrum, i.e. wide band gap materials like GaN, AlGaN and low band gap materials such as GaInN(Sb)As, InGaAs were demonstrated. A lot of time was also devoted to modulation spectroscopy of quantum dot structures.It is our great pleasure to publish the most interesting MS3 workshop presentations again in this special issue of physica status solidi (a).The organizers acknowledge the financial support of the Workshop by Wrocław University of Technology and the Foundation for Polish Science. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum dot solar concentrator: Device optimisation using spectroscopic techniques

The quantum dot solar concentrator (QDSC) is a novel non-tracking solar concentrator comprising quantum dots (QDs) seeded in materials such as plastics and glasses, that concentrates both direct and diffuse solar energy on attached photovoltaic cells. Spectroscopic measurements have been undertaken for a range of different quantum dot (QD) types and transparent host materials. High transparency in the matrix material and QDs with high quantum efficiency are essential for an efficient QDSC. An optimum matrix material for a QDSC has been determined based on absorption characteristics and an optimum commercially available QD type has been chosen using steady-state absorption, photoluminescence and photoluminescence excitation spectroscopy of QDs in solution and solid matrices.

Optical Spectroscopy of Quantum Dots in High Magnetic Fields

Optical spectroscopy measurements of single InAs/GaAs self-assembled quantum dots have been performed. The multiexcitonic emission from the s-, p-, and d-shells of a single dot is observed and investigated in magnetic field up to 28 T. The Zeeman splitting of the s-shell excitons has been found to depend on the dot morphology. While the energy-splitting in flat (2 nm height) dots linearly increases with magnetic field, its significant non-linearity is observed for larger in size, tall (4 nm height) dots. The effect of magnetic field on the orbital motion of carriers in dots has also been investigated. It has been found that the modified Fock–Darwin pattern explains the observed evolution of the emission from highly-excited single tall quantum dot.