Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy

The transport properties of single InAs quantum dots (QDs) grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current flows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4.2 K.     

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs.     

Electrical conduction properties of Si δ-doped GaAs grown by MBE

The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25–300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures.     

Growth of InGaAs-capped InAs quantum dots characterized by Atomic Force Microscope and Scanning Electron Microscope

Atomic force microscopy (AFM) is typically used to measure the quantum dot shape and density formed by lattice mismatched epitaxial growth such as InAs on GaAs. However, AFM images are distorted when two dots are situated in juxtaposition with a distance less than the AFM tip width. Scanning electron Microscope (SEM) is much better in distinguishing the dot density but not the dot height. Through these measurements of the growth of In_xGa_1-xAs cap layer on InAs quantum dots, it was observed that the InGaAs layer neither covered the InAs quantum dots and wetting layer uniformly nor 100% phase separates into InAs and GaAs grown on InAs quantum dots and wetting layer, respectively.     

Properties of Si δ-layers embedded in GaAs

An ultrathin, a few monolayers (MLs) thick Si δ-layer, has been embedded in GaAs. The Si δ-layer properties have been electrically and structurally characterized. A conductivity transition has been observed, when going from free carrier transport in thin (1 ML) ordered δ-layers to the disordered conditions for Si δ-layers thicker than 4 MLs. Two novel emission bands are observed in photoluminescence (PL) for Si δ-layers in the width range 1–4 MLs, but solely at below bandgap excitation. The pronounced 2D properties of these δ-layers have been clearly demonstrated by an observed blue shift of the PL characterization as the thickness of the Si δ-layer is reduced. The so-derived results on the transition energies and the electronic structure are in good agreement with theoretical predictions obtained by a self-consistent approach.     

Electron transport and shell structures of single InAs quantum dots probed by nanogap electrodes

We have investigated electron transport and electron filling in single InAs quantum dots (QDs) using nanogap electrodes. Elliptic InAs QDs with diameter of 60/80 nm exhibited clear shell filling up to 12 electrons. Shell-dependent charging energies and level quantization energies for the s, p, and d states were determined from the addition energy spectra. Furthermore, it is found that the charging energies and the tunneling conductances strongly depend on the shell, reflecting that the electron wave functions for higher shells are more extended in space.     

Improving the photoluminescence properties of self-assembled InAs surface quantum dots by incorporation of antimony

This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing.     

Controlled stacking growth of uniform InAs quantum dots by molecular beam epitaxy

Double-stacked InAs quantum dots (QDs) were grown by molecular beam epitaxy via Stranski–Krastanov growth mode. Transition of the facet formation from {1 3 6} plane to {1 1 0} plane was observed during the stacking growth of InAs QDs by reflection high-energy electron-beam diffraction. The enhanced growth rate and the different facet formation in the stacking growth were caused by tensile strain of the GaAs underlying layer. Low arsenic pressure and low growth rate conditions played an important role for a perfect coupling and uniformity in the size of the stacked QDs. The narrow photoluminescence line width of 17.6 meV was successfully obtained from the stacked InAs QDs.     

STEM-study of 1.3 μm InAs/InGaAs quantum dot structures

We investigated InAs-Dots-in-a-well structures emitting near 1.3 μm by bright field and Z-contrast mode in a scanning transmission electron microscope. The chemically sensitive Z-contrast mode is found to give direct information on the actual position of the InAs-Dots inside the embedding well, while the bright field mode monitors the strain fields. Comparing a series of structures, we found that the most symmetric design is realized by an nominally asymmetric growth. These symmetric structures exhibit the best performance with respect to photoluminescence spectra and laser threshold current density.     

Study of δ-doped GaAs layers by micro-Raman spectroscopy on bevelled samples

Si δ-doped GaAs layers were studied by micro-Raman spectroscopy. The spectra were recorded along the bevelled structure using the light of Ar⁺-ion laser (514.5 nm line) with high power density. The observed changes in the Raman spectra are discussed in the sense of coupling present between LO phonon and photoexcited electron–hole plasma and plasma of electrons arising from ionised Si atoms. Plasmon-LO-Phonon modes of the coupling of photoexcited plasma in δ-doped GaAs layers were observed for the first time. The minimal thickness of cap layer was estimated in the range of 10–19 nm depending on the doping concentration.     

Mn离子注入InAs/GaAs量子点结构材料的光电性质研究

用高能离子注入(160keV)的方法对InAs/GaAs量子点结构进行掺杂，研究了不同退火工艺处理后量子点的光致发光和电学性能.相对于长时间退火，快速退火处理后的量子点发光通常较强.在相同的退火条件下，量子点发光峰位随着Mn注入剂量的增加，先是往高能量端快速移动，而后发光峰又往低能方向移动.后者可能是由于Mn原子进入InAs量子点，释放了InAs量子点中的应变所致.对于高注入剂量样品和长时间退火样品，变温电阻曲线在40 K附近会出现反常行为. 关键词： 离子注入 InAs/GaAs量子点 光致发光 团簇     

Photoluminescence in γ-irradiated α-quartz investigated by synchrotron radiation

We report an experimental investigation of the photoluminescence, under excitation by synchrotron radiation within the absorption band at 7.6 eV, induced in γ-irradiated α-quartz. Two emissions centered at 4.9 and 2.7 eV are observed at low temperature: the former decreases above 40 K, whereas the second band exhibits an initial slight increase and its quenching is effective above 100 K. Furthermore, the decay kinetics of both emissions occur in a time scale of nanoseconds: at T=17.5 K we measured a lifetime τ1.0 ns for the photoluminescence at 4.9 eV and τ3.6 ns for that at 2.7 eV. These results give new insight on the optical properties associated with defects peculiar of crystalline matrix, also on the basis of their comparison with previous studies on silica.     

SERS spectrum of the peptide thymosin‐β4 obtained with Ag nanorod substrate

Thymosin‐β4 is a peptide found at high concentrations in a wide range of mammalian cells. It has been shown to be elevated in metastatic cells. In this study, the surface‐enhanced Raman spectrum of thymosin‐β4 was obtained using a silver nanorod array surface‐enhanced Raman scattering substrate. Significant Raman peaks were found, and the corresponding vibrational modes were assigned based on previously published literature, with amide A–B, amide I, and amide III modes all identified. The majority of the remaining peaks were assigned modes based on the spectra of lysine and glutamic acid, the two most abundant amino acids in the thymosin‐β4 sequence. A standard Raman spectrum of thymosin‐β4 was also obtained and analyzed for comparison. Because thymosin‐β4 plays a significant role in regulating the formation of cellular cytoskeleton and other several biological processes, its Raman spectrum will be of use to researchers. Copyright © 2014 John Wiley & Sons, Ltd.     

Interaction of water‐soluble triphenylphosphines with β‐cyclodextrin: a quantum chemistry study

The interaction of β‐cyclodextrin (β‐CD) with meta‐trisulfonated triphenylphosphine derivatives bearing one or two methyl (or methoxy) groups on the aromatic rings has been investigated by PM3 calculations. The results show that phosphine molecules interact with β‐CD having either an unsubstituted sulfophenyl group or a substituted sulfophenyl group at the para and/or meta‐position. The presence of one methyl or methoxy group in the ortho‐position on each aromatic ring prevents the formation of an inclusion complex between meta‐trisulfonated triphenylphosphine derivatives and β‐CD. The deeply included phosphines in the β‐CD cavity show significant van der Waals interactions with β‐CD. These interactions are at the origin of the high association constants between these molecules and β‐CD. Phosphines exhibiting small association constants interact with β‐CD by forming H‐bonds and weak (or null) van der Waals interactions. Copyright © 2011 John Wiley & Sons, Ltd.     

Single InAs/InP quantum dot spectroscopy in 1.3–1.55 μm telecommunication band

We present an optical spectroscopy and photon correlation measurement at telecommunication wavelengths performed on single InAs/InP quantum dots. Two main approaches brought high optical quality: an application of a ‘double-cap’ growth method to metalorganic chemical vapor deposition, and fabrication of a small mesa structure using low-damage wet chemical etching. Sharp and discrete exciton transition lines have been observed on the single quantum dots, which widely cover the spectral range of 1.3–1.55 μm. Using a pulsed excitation source and gated single-photon detection modules, we observed a photon antibunching behavior for an isolated exciton emission line, indicating nonclassical light emission near the wavelength of 1.3 μm.     

Relative stabilities and molecular structures of the isomeric enol ethers and carboxylic esters derived from α‐acetyl‐γ‐butyrolactone and α‐acetyl‐δ‐valerolactone

Recently recorded ¹⁷O NMR spectra of compounds studied in a previous work (Taskinen E. Acta Chem. Scand. 1985; B39 : 489–494) dealing with the thermodynamics of isomerization of the enol ethers of α‐acetyl‐γ‐butyrolactone reveal an error in compound identification, caused by an unexpected isomerization reaction during the synthetic procedure. Thus, acid‐catalyzed treatment of the lactone with HC(OR)₃ in the respective alcohol ROH is shown to lead initially to the desired enol ethers which, however, are gradually isomerized to a mixture of the enol ethers and an ester of 2‐methyl‐4,5‐dihydrofuran‐3‐carboxylic acid. As a result, only one of the two isomeric compounds detected in the previous equilibration study was the expected enol ether (the thermodynamically more stable E isomer) of α‐acetyl‐γ‐butyrolactone, while the other, dominating species was the respective carboxylic ester. In the present work, the evidence provided by the ¹⁷O NMR spectra is presented, and the relative stabilities of the isomeric compounds are discussed on the basis of computational enthalpy data. The treatment is also extended to the respective isomeric compounds derived from α‐acetyl‐δ‐valerolactone. Copyright © 2007 John Wiley & Sons, Ltd.     

1.5 μm emission from InAs quantum dots with InGaAsSb strain-reducing layer grown on GaAs substrates

InGaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs from the GaAs is reduced due to the tensile strain induced by the InGaAsSb SRL, because the lattice constant of InGaAsSb is closer to InAs lattice constant than that of GaAs, resulting in a significant red shift of photoluminescence peaks of the InAs QDs. The emission wavelength from InAs QDs can be controlled by changing the Sb composition of the InGaAsSb SRL. The 1.5 μm band emissions were achieved in the sample with an InGaAsSb SRL whose Sb compositions were above 0.3. The calculation of the electron and the hole wave functions using the transfer matrix method indicates that the electron and the hole were localized around InAs QDs and InGaAsSb SRL.     

Volatile metal β-diketonates — new precursors for the sonochemical synthesis of nanosized materials — sonolysis of thorium(IV) β-diketonates

Ultrasonic irradiation (22 kHz, Ar atmosphere) of Th(IV) β-diketonates Th(HFAA)₄ and Th(DBM)₄, where HFAA and DBM are hexafluoroacetylacetone and dibenzoylmethane respectively, causes them to decompose in hexadecane solutions, forming solid thorium compounds. The first-order rate constants for Th(IV) β-diketonate degradation were found to be (9.3±0.8)×10⁻³ for Th(HFAA)₄ and (3.8±0.4)×10⁻³ min⁻¹ for Th(DBM)₄, (T=92°C, I=3 W cm⁻²). The rate of the sonochemical reaction increased with the rising β-diketonate volatility and decreased with the rising hydrocarbon solvent vapor pressure. Solid sonication products consisted of a mixture of thorium carbide ThC₂ and Th(IV) β-diketonate partial degradation products. The average ThC₂ particle size was estimated to be about 2 nm. ThC₂ formation was attributed to the high-temperature reaction occurring within the cavitating bubble. The thorium β-diketonate partial degradation products formed in the liquid reaction zones surrounding the cavitating bubbles.     

β‐Elimination of an aziridine to an allylic amine: a mechanistic study

The base‐induced rearrangement of aziridines has been examined using a combination of calculations and experiment. The calculations show that the substituent on nitrogen is a critical feature that greatly affects the favorability of both α‐deprotonation, and β‐elimination to form an allylic amine. Experiments were carried out to determine whether E2‐like rearrangement to the allylic amine with lithium diisopropyl amide (LDA) is possible. N‐tosyl aziridines were found to deprotonate on the tosyl group, preventing further reaction. A variety of N‐benzenesulfonyl aziridines having both α‐ and β‐protons decomposed when treated with LDA in either tetrahydrofuran or hexamethylphosphoramide. However, when α‐protons were not present, allylic amine was formed, presumably via β‐elimination. Copyright © 2011 John Wiley & Sons, Ltd.