Quantum Dots For Imaging Research Articles

From analog to digital: exploring cell dynamics with single quantum dots

Semiconductor quantum dots (QDs) have emerged as new fluorescent probes for biology. When combined with ultrasensitive optical techniques, they allow motions of individual biomolecules to be tracked in live cells with high signal-to-noise and over unprecedented durations. Single QD imaging readily offers a powerful tool to investigate the organization in cell membranes. Altogether QDs will contribute to more advanced biological imaging and enable new studies on the dynamics of cellular processes.

Targeting quantum dots to surface proteins in living cells with biotin ligase

Escherichia coli biotin ligase site-specifically biotinylates a lysine side chain within a 15-amino acid acceptor peptide (AP) sequence. We show that mammalian cell surface proteins tagged with AP can be biotinylated by biotin ligase added to the medium, while endogenous proteins remain unmodified. The biotin group then serves as a handle for targeting streptavidin-conjugated quantum dots (QDs). This labeling method helps to address the two major deficiencies of antibody-based labeling, which is currently the most common method for targeting QDs to cells: the size of the QD conjugate after antibody attachment and the instability of many antibody-antigen interactions. To demonstrate the versatility of our method, we targeted QDs to cell surface cyan fluorescent protein and epidermal growth factor receptor in HeLa cells and to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in neurons. Labeling requires only 2 min, is extremely specific for the AP-tagged protein, and is highly sensitive. We performed time-lapse imaging of single QDs bound to AMPA receptors in neurons, and we compared the trafficking of different AMPA receptor subunits by using two-color pulse-chase labeling.

Growth mechanism of InAs quantum dots on GaAs by metal-organic chemical-vapor deposition

The growth parameters affecting the deposition of InAs quantum dots (QDs) by metal-organic chemical-vapor deposition are reported. Experiments with arsine pause, gas switching, and hydrogen shroud flow show that a low V∕III ratio is the key to obtaining three-dimensional InAs island formation with high density and uniformity. Based on atomic force microscopy images of InAs QDs deposited under different growth conditions, a physical model for the epitaxial growth of three-dimensional islands is proposed. In this model, the InAs QD growth is governed by two types of arsenic sources at the growth surface: free arsenic atoms arriving at the boundary layer and dangling arsenic bonds available at the GaAs wafer surface. At high V∕III ratio, free arsenic atoms arriving at the boundary layer are the dominant hydride species and produce a low density of InAs islands with irregular shape and polycrystalline defects. At low V∕III ratio arsenic bonds on the GaAs surface are the main sites for indium atoms to attach to, thus producing high island densities and small coherent island sizes.

Nanofabrication of cylindrical STEM specimen of InGaAs/GaAs quantum dots for 3D-STEM observation

We demonstrate the multiazimuth observation (360° in principle) of InGaAs/GaAs quantum dots (QDs) by means of a 300kV scanning transmission electron microscope (STEM), where both cross-sectional and plan-view observations are performed on a single STEM specimen for the first time. A cylindrical specimen with a diameter of 200–300nm including the QD layer inside along the rotation axis was fabricated by the focused ion beam (FIB) technique, with the application of a newly developed mesa-cutting method to adjust the position and angle of the QD layer precisely. The 360° STEM observation is realized by mounting the cylindrical specimen on a holder equipped with a specimen-rotation mechanism. High potential of 3D-STEM observation is briefly presented by showing high contrast images of QDs, dark field images, and moire fringes with various incident angles.

Imaging and probing electronic properties of self-assembled InAs quantum dots by atomic force microscopy with conductive tip

Atomic force microscopy with a conductive probe has been used to study both the topography and the electronic properties of 10-nm-scale self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on n-type GaAs. The current flowing through the conductive probe normal to the sample surface is measured for imaging local conductance, while the deflection of cantilever is optically detected for disclosing geometrical structure. The conductance on InAs QDs is found to be much larger than that on the wetting layer, allowing imaging of QDs through measurements of local current. We attribute this change in conductance to the local modification of surface band bending associated with surface states on InAs QD surface. Mechanisms of electron transport through QDs are discussed based on current–voltage characteristics measured on QDs of various sizes.

Luminescence Spectroscopy on Individual Nanostructures and Impurity Atoms Using Stm and Sem

AbstractIn the exploration of the nano-world of semiconductors there is a strong focus on low-dimensional structures and ultra-small devices. Two fundamental problems, which challenge progress in this field are: (i) large ensembles of nano-objects, like Quantum Dots (QDs), do not have identical geometrical shapes and electronic properties, and, (ii) the properties of a low-dimensional structure can be dominated by a few impurity atoms, whereas the properties of a macroscopic structure is determined by the quasi-continuous background of dopant impurities. To allow QDs and discrete impurities to be studied, novel experimental techniques are required. In this paper we describe how local luminescence has been excited from single QDs using electrons injected from a Scanning Electron Microscope (SEM), from the tip of a Scanning Tunneling Microscope (STM) or using highly focused photons for excitation. We present images of QDs as well as characteristic spectra of individual QDs. We finally show how the local character of the excitation enables us to excite and image individual impurities in low-dimensional structures, including the measurement of characteristic emission spectra from a single impurity atom in GaAs.