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| 2. |
- Håkanson, Ulf, et al.
(author)
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Electric field effects in single semiconductor quantum dots observed by scanning tunneling luminescence
- 2003
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In: Journal of Vacuum Science and Technology B. - : American Vacuum Society. - 1520-8567. ; 21:6, s. 2344-2347
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Journal article (peer-reviewed)abstract
- Scanning tunneling microscopy (STM) and scanning tunneling luminescence were used to correlate the topography with the emission spectra from individual self-assembled, InP quantum dots (QDs). We have investigated in detail how the electric field induced by the STM tip affects the emission from the QDs. This was done when exciting a QD, by altering the bias for constant current, by altering the current for constant bias, or by changing the tip position. An increased bias (increased electric field) leads to Stark shift of the QD emission, whereas a larger tunneling current results in state filling of the emission. Furthermore, when exciting the QD, the position of the STM tip is shown to have large effects on the QD luminescence.
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| 3. |
- Håkanson, Ulf, et al.
(author)
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Photon mapping of quantum dots using a scanning tunneling microscope
- 2002
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:23, s. 4443-4445
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Journal article (peer-reviewed)abstract
- Scanning tunneling microscopy (STM) and scanning tunneling luminescence (STL) have been used to investigate the geometric and optical properties of individual self-assembled InP quantum dots overgrown with a thin layer of GaInP. STL spectra and monochromatic photon maps were used to correlate the surface topography with the optical properties of single quantum dots. We find a spatial resolution of about 10 nm in the photon maps. Theoretical emission spectra were calculated by six-band k.p theory using a realistic shape of the dot as well as of the cap layer. The calculated emission spectrum of a single dot is in good agreement with the experimental findings. (C) 2002 American Institute of Physics.
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| 4. |
- Håkanson, Ulf, et al.
(author)
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Photon mapping of single quantum dots by scanning tunneling microscopy induced luminescence spectroscopy
- 2002
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In: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Conference paper (peer-reviewed)abstract
- Scanning tunneling microscopy induced luminescence (STML) has been used to investigate individual self-assembled InP quantum dots overgrown with GaInP. We will present results correlating the surface morphology with the optical properties of single dots. In particular, the strain induced energy-shift of the dot emission with increasing cap layer thickness and its relation to the overgrowth will be discussed. Effects of the dots on the properties of the overgrown GaInP will also be treated. STML spectra and monochromatic photon maps are compared with results from photoluminescence and transmission electron microscopy measurements. Furthermore, a comparison with theoretical calculations is made
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| 5. |
- Håkanson, Ulf, et al.
(author)
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Quantum-dot-induced ordering in GaxIn1-xP/InP islands
- 2002
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:23
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Journal article (peer-reviewed)abstract
- Thin layers of GaxIn1-xP grown on top of self-assembled InP quantum dots has been studied using transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and low-temperature scanning tunneling luminescence (STL). STM reveals that the overgrowth is highly uneven, in which elongated GaxIn1-xP islands covering the dots are formed. TEM and high-spatial-resolution STL show that the quantum dots locally induce domains with higher degree of ordering in the islands. The luminescence from these domains is observed as a strong GaxIn1-xP peak at an energy below the emission from the GaxIn1-xP barrier material.
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| 6. |
- Håkanson, Ulf, et al.
(author)
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Single InP/GaInP quantum dots studied by scanning tunneling microscopy and scanning tunneling microscopy induced luminescence
- 2002
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:3, s. 494-496
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Journal article (peer-reviewed)abstract
- We have studied the optical and structural properties of single, self-assembled InP quantum dots (QDs) overgrown with nominally 5 nm of GaInP, using an ultrahigh-vacuum scanning tunneling microscope (STM) operating at low temperatures. The STM is combined with an optical detection system, which allows us to detect the emission from individual quantum dots with high spatial resolution. We find that the InP QDs act as nucleation points for the GaInP overgrowth, where the strain induced by the overlayer give rise to a QD emission around 1.46 eV. (C) 2002 American Institute of Physics.
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| 7. |
- Johansson, Mikael, et al.
(author)
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Correlation between overgrowth morphology and optical properties of single self-assembled InP quantum dots
- 2003
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 68:12
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Journal article (peer-reviewed)abstract
- We have studied the early stages of GaInP overgrowth on InP quantum dots (QD's) experimentally and theoretically. A direct correlation between the surface morphology and the optical properties of individual InP QD's is made using scanning tunneling microscopy (STM) and scanning tunneling luminescence. The geometric structure of the islands is further investigated using cross-sectional transmission electron microscopy (TEM). The overgrowth occurs in three stages; initially the InP QD's act as seeding points for the overgrowth, where the GaInP grows laterally from the side facets of the QD. The growth occurs preferentially in the [110] direction and elongated GaInP/InP islands are formed. As the overgrowth continues the islands increase laterally in size and GaInP also starts to grow between the islands, but not covering the top of the InP QD's. The growth of GaInP on top of the QD's commences once the islands have begun to coalesce. Using a model based on the STM and TEM results the electronic structures of the QD's have been calculated by eight-band k.p theory. The calculations are in good agreement with the experimental results. Our findings unravel the details of the strain induced energy shift of the QD luminescence previously reported [Pistol , Appl. Phys. Lett. 67, 1438 (1995)].
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| 9. |
- Larsson, Birgitta, et al.
(author)
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Extracts of ECL-cell granules/vesicles and of isolated ECL cells from rat oxyntic mucosa evoke a Ca2+ second messenger response in osteoblastic cells
- 2001
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In: Regulatory Peptides. ; 97:2-3, s. 153-161
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Journal article (peer-reviewed)abstract
- Surgical removal of the acid-producing part of the stomach (oxyntic mucosa) reduces bone mass through mechanisms not yet fully understood. The existence of an osteotropic hormone produced by the so-called ECL cells has been suggested. These cells, which are numerous in the oxyntic mucosa, operate under the control of circulating gastrin. Both gastrin and an extract of the oxyntic mucosa decrease blood calcium and stimulate Ca2+ uptake into bone. Conceivably, gastrin lowers blood calcium indirectly by releasing a hypothetical hormone from the ECL cells. The present study investigated, by means of fura-2 fluorometry, the effect of extracts of preparations enriched in ECL cell granules/vesicles from rat oxyntic mucosa on mobilization of intracellular Ca2+ in three osteoblast-like cell lines, UMR-106.01, MC3T3-E1 and Saos-2, and of extracts of isolated ECL cells in UMR-106.01 cells. The extracts were found to induce a dose-related rapid increase in intracellular Ca2+ concentrations in the osteoblast-like cells. The response was not due to histamine or pancreastatin, known ECL cell constituents, and could be abolished by pre-digesting the extracts with exo-aminopeptidase. The results show that the increase in [Ca2+](i) reflects a mobilization of Ca2+ from the endoplasmic reticulum. The observation of an increase in [Ca2+](i) also in murine embryonic fibroblasts show that the response is not limited to osteoblastic cells. The finding that the extracts evoked a typical Ca2+ -mediated second messenger response in osteoblastic cells provides evidence for the existence of a novel osteotropic peptide hormone (gastrocalcin), produced in the ECL cells, and supports the view that gastrectomy-induced osteopathy may reflect a lack of this hormone.
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| 10. |
- Lindström, Erik, et al.
(author)
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Gastrin and the neuropeptide PACAP evoke secretion from rat stomach histamine-containing (ECL) cells by stimulating influx of Ca2+ through different Ca2+ channels
- 2001
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In: Journal of Physiology. - 1469-7793. ; 535:3, s. 663-677
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Journal article (peer-reviewed)abstract
- Gastrin and PACAP stimulate secretion of histamine and pancreastatin from isolated rat stomach ECL cells. We have examined whether or not secretion depends on the free cytosolic Ca2+ concentration ([Ca2+i) and the pathways by which gastrin and PACAP elevate [Ca2+i. Secretion was monitored by radioimmunoassay of pancreastatin and changes in [Ca2+i by video imaging. The patch clamp technique was used to record whole-cell currents and membrane capacitance (reflecting exocytosis). In the presence of 2 mM extracellular Ca2+, gastrin and PACAP induced secretion and raised [Ca2+i. Without extracellular Ca2+ (or in the presence of La3+) no secretion occurred. The extracellular Ca2+ concentration required to stimulate secretion was 10 times higher for gastrin than for PACAP. Depletion of intracellular Ca2+ pools by thapsigargin had no effect on the capacity of gastrin and PACAP to stimulate secretion. Gastrin-evoked secretion was inhibited 60-80 by L-type channel blockers and 40 by the N-type channel blocker -conotoxin GVIA. Combining L-type and N-type channel blockers did not result in greater inhibition than L-type channel blockers alone. Whole-cell patch clamp measurements confirmed that the ECL cells are equipped with voltage-dependent inward Ca2+ currents. A 500 ms depolarising pulse from -60 mV to +10 mV which maximally opened these channels resulted in an increase in membrane capacitance of 100 fF reflecting exocytosis of secretory vesicles. PACAP-evoked secretion was reduced 40 by L-type channel blockers but was not influenced by inhibition of N-type channels. SKF 96365, a blocker of both L-type and receptor-operated Ca2+ channels, inhibited PACAP-evoked secretion by 85 . Combining L-type channel blockade with SKF 96365 abolished PACAP-evoked secretion. The results indicate that gastrin- and PACAP-evoked secretion depends on Ca2+ entry and not on mobilisation of intracellular Ca2+. While gastrin stimulates secretion via voltage-dependent L-type and N-type Ca2+ channels, PACAP acts via L-type and receptor-operated Ca2+ channels.
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