Intra-dot Auger relaxation in quantum dots

We report on calculations of relaxation in InAs/GaAs self assembled dots. We point out that the intra-dot Auger relaxation is extremely fast in these structures. This suggests that energy relaxation in InAs/GaAs self organized quantum dots is dominated by capture effects.     

Atomic structure of InAs quantum dots on GaAs

In recent years, the self-assembled growth of semiconductor nanostructures, that show quantum size effects, has been of considerable interest. Laser devices operating with self-assembled InAs quantum dots (QDs) embedded in GaAs have been demonstrated. Here, we report on the InAs/GaAs system and raise the question of how the shape of the QDs changes with the orientation of the GaAs substrate. The growth of the InAs QDs is understood in terms of the Stranski–Krastanow growth mode. For modeling the growth process, the shape and atomic structure of the QDs have to be known. This is a difficult task for such embedded entities.

In our approach, InAs is grown by molecular beam epitaxy on GaAs until self-assembled QDs are formed. At this point the growth is interrupted and atomically resolved scanning tunneling microscopy (STM) images are acquired. We used preparation parameters known from the numerous publications on InAs/GaAs. In order to learn more about the self-assemblage process we studied QD formation on different GaAs(0 0 1), (1 1 3)A, and ( )B substrates. From the atomically resolved STM images we could determine the shape of the QDs. The quantum “dots” are generally rather flat entities better characterized as “lenses”. In order to achieve this flatness, the QDs are terminated by high-index bounding facets on low-index substrates and vice versa. Our results will be summarized in comparison with the existing literature.     

Nanocrystalline TiO2 solar cells sensitized with InAs quantum dots

We report nanocrystalline TiO2 solar cells sensitized with InAs quantum dots. InAs quantum dots of different sizes were synthesized and incorporated in solar cell devices. Efficient charge transfer from InAs quantum dots to TiO2 particles was achieved without deliberate modification of the quantum dot capping layer. A power conversion efficiency of about 1.7% under 5 mW/cm2 was achieved; this is relatively high for a nanocrystalline metal oxide solar cell sensitized with presynthesized quantum dots, but this efficiency could only be achieved at low light intensity. At one sun, the efficiency decreased to 0.3%. The devices are stable for at least weeks under room light in air.     

Observation of molecules adsorbed on III-V semiconductor quantum dots by surface-enhanced Raman scattering

I report for the first time surface-enhanced Raman scattering (SERS) from molecules adsorbed on InAs/GaAs quantum dots. This result is very interesting because previous SERS experiments have been essentially restricted to molecules adsorbed on metallic surfaces. Raman scattering from pyridine molecules adsorbed on these III-V quantum dots structures is strongly enhanced relative to the same molecules in solution. The most interesting feature in the SERS spectrum is the appearance of a new vibrational band. I suggest that this line should be attributed to the chemisorbed pyridine that is formed by coordination of its lone pair electrons of the N atom to the semiconductor surface. This work provides unambiguous experimental evidence for SERS on III-V semiconductor quantum dots. Nanostructures are currently considered as potential building blocks for nanodevices. The performance and reliability of these devices strongly depend on the surface and interfacial properties of the constituent nanomaterials. Therefore, this work illustrates the considerable potential of SERS spectroscopy as a powerful new tool in nanoscience.     

InxGa1-xAs盖层InAs量子点的发光性能研究

用分子束外延系统在GaAs(001)衬底上生长InAs量子点,在InAs量子点上插入3 nm的In_0.4Ga_0.6As层,可将量子点发射波长调谐到1 300 nm附近.对样品进行氢等离子处理,研究处理前后样品的InAs量子点光致发光(PL)强度的变化.结果表明,在InAs量子点与相邻层的界面上以及GaAs层中存在界面缺陷,采用氢等离子处理可有效地抑制界面缺陷,大幅度地提高发光效率.     

Geometrical control of the interatomic coulombic decay process in quantum dots for infrared photodetectors

In electron dynamics calculations the interatomic Coulombic decay (ICD) process has recently been shown to take place in two vertically‐aligned quantum dots (QDs). Energy emitted during the relaxation of one electron in one QD is converted into kinetic energy of another electron ejected from a neighboring QD. As the electronic structure of QDs can be controlled by their geometries, we prove here in thorough scans of the transversal and vertical QD confinement potentials’ widths that geometries are likewise control parameters for ICD. Such a comprehensive investigation has been enabled by a significant development of the calculations in terms of speed achieved among others by optimization of the grid and Coulomb interaction operator representations. As key result of this study we propose two cigar‐shaped singly‐charged GaAs QDs vertically aligned in the direction of their long side for a most efficient QD ICD realization useful for an infrared photodetector. © 2016 Wiley Periodicals, Inc.     

InAs量子点光致发光效率的研究

利用分子束外延技术制得InAs量子点样品,采用分光光度法对样品的光致发光效率进行研究.发现在InAs层和GaAs覆盖层之间插入隧道阻挡层,当激发功率密度为60W/cm²时,InAs量子点发射强度的增加量超过一个数量级.这种光复合效率的增强是由于浸润层中的非辐射跃迁受到了抑制所致.     

Synthesis of cadmium telluride quantum wires and the similarity of their effective band gaps to those of equidiameter cadmium telluride quantum dots

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.     

Laser effects in excitons in a quantum wire

We investigate the effects of laser field intensity over the ground state binding energy of light and heavy hole excitons confined in GaAs/Ga_1?xAl_x As cylindrical quantum wire. We have applied the variational method using 1s‐hydrogenic wave functions, in the framework of the single band effective mass approximation with the spatial dielectric function. The polaronic effects are included in the calculation to compute the exciton binding energy as a function of the wire radius for different field of laser intensity. The valence‐band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The dressed laser donor binding energies are calculated and compared with the results of binding energy of excitons. The results show that (i) the binding energy is found to increase with decrease with the wire radius, and decrease with increase with the value of laser field amplitude, (ii) the heavy‐hole exciton in a cylindrical quantum wire is more strongly bound than the light‐hole exciton, (iii) the values of ground state binding energy for the laser field amplitude α₀ = 10 Å resemble with the values of heavy hole exciton binding energy, and (iv) the binding energy of the impurity for the narrow well wire is more sensitive to the laser field amplitude. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Interdependence of ICD rates in paired quantum dots on geometry

Using state‐of‐the‐art antisymmetrized multiconfiguration time‐dependent Hartree (MCTDH) electron dynamics calculations we study the interdependence of the intermolecular Coulombic decay (ICD) process on the geometric parameters of a doubly‐charged paired quantum dot (PQD) model system in the framework of the effective mass approximation (EMA). We find that ICD displays a maximum rate for a certain geometry of the electron‐emitting quantum dot, which is simultaneously dependent on both the distance between the quantum dots as well as the photon‐absorbing quantum dot's geometry. The rate maximum is shown to be caused by the competing effects of polarization of electron density and Coulomb repulsion. The ICD rate‐maximized PQD geometry in GaAs QDs yields a decay time of 102.39 ps. It is given by two vertically‐aligned cylindrical QDs with radii of 14.42 nm separated by 86.62 nm. The photon absorbing QD then has a height of 46.59 nm and the electron emitting QD a height of 16.33 nm. © 2017 Wiley Periodicals, Inc.     

Electronic excitations in narrow quantum wells via intersubband Raman scattering: Theoretical considerations

In this work a generalized self-consistent field theory was applied to investigate the elementary excitations of two-dimensional electron gas formed from narrow quantum wells via resonant intersubband Raman scattering. The developed model considers the existence of equally coupled and degenerated excitations of the electron gas and allows to observe that in extreme resonance regime the plasma oscillations splits into two contributions: a set of renormalized collective excitations (plasmons) and unrenormalized electronic transitions (single-particle excitations). Our results show that the asymmetries which appear in the Raman profile of doped narrow quantum wells can be interpreted as the entrance or exit of resonance of collective modes overlapped with single-particle transitions.     

高密度、长波长InGaAs量子点材料的制备与表征

利用MOCVD外延生长技术, 对InAs/GaAs量子点材料的生长参数进行调节, 获得了高密度(~5×1010 cm^-2)的InAs量子点. 室温荧光光谱表明, 覆盖厚度为5 nm的InGaAs(In组分的摩尔分数为12%)低应变层量子点材料的基态发光波长为1.346 μm, 光谱线宽为24 meV. 研究结果表明, 利用较低温度生长InAs量子点, 结合较高In组分的InGaAs低应变层量子点材料可以实现发光波长红移, 有效地改善材料的光学特性.     

Colloidal GaAs quantum wires: solution-liquid-solid synthesis and quantum-confinement studies

Colloidal GaAs quantum wires with diameters of 5-11 nm and narrow diameter distributions (standard deviation = 12-21% of the mean diameter) are grown by two methods based on the solution-liquid-solid (SLS) mechanism. Resolved excitonic absorption features arising from GaAs quantum wires are detected, allowing extraction of the size-dependent effective band gaps of the wires. The results allow the first systematic comparison of the size dependences of the effective band gaps in corresponding sets of semiconductor quantum wires and quantum wells. The GaAs quantum wire and well band gaps scale according to the prediction of a simple effective-mass-approximation, particle-in-a-box (EMA-PIB) model, which estimates the kinetic confinement energies of electron-hole pairs in quantum nanostructures of different shapes and confinement dimensionalities.     

High-resolution analytical transmission electron microscopy of semiconductor quantum structures

Analytical transmission electron microscopy was applied to characterize the size, shape, real structure, and, in particular, the composition of different semiconductor quantum structures. Its potential applicability is demonstrated for heterostructures of III-V semiconducting materials and II-VI ones, viz. (In,Ga)As quantum wires on InP and (In,Ga)As quantum dots on GaAs both grown by metal organic chemical vapor deposition, and CdSe quantum dots on ZnSe grown by molecular beam epitaxy. The investigations carried out show that the element distribution even of some atomic layers can be detected by energy-dispersive X-ray spectroscopy, however, exhibiting a smeared profile. Contrary to that, sub-nanometre resolution has been achieved by using energy-filtered transmission electron microscopy to image quantum dot structures.     

Engineering InAs(x)P(1-x)/InP/ZnSe III-V alloyed core/shell quantum dots for the near-infrared

Quantum dots with a core/shell/shell structure consisting of an alloyed core of InAs(x)P(1-x), an intermediate shell of InP, and an outer shell of ZnSe were developed. The InAs(x)P(1-x) alloyed core has a graded internal composition with increasing arsenic content from the center to the edge of the dots. This compositional gradient results from two apparent effects: (1) the faster reaction kinetics of the phosphorus precursor compared to the arsenic precursor, and (2) a post-growth arsenic-phosphorus exchange reaction that increases the arsenic content. The cores have a zinc blend structure for all compositions and show tunable emission in the near-infrared (NIR) region. A first shell of InP leads to a red-shift and an increase in quantum yield. The final shell of ZnSe serves to stabilize the dots for applications in aqueous environments, including NIR biomedical fluorescence imaging. These NIR-emitting core/shell/shell InAs(x)P(1-x)/InP/ZnSe were successfully used in a sentinel lymph node mapping experiment.     

Cadmium selenide quantum wires and the transition from 3D to 2D confinement

Soluble CdSe quantum wires are prepared by the solution-liquid-solid mechanism, using monodisperse bismith nanoparticles to catalyze wire growth. The quantum wires have micrometer lengths, diameters in the range of 5-20 nm, and diameter distributions of +/-10-20%. Spectroscopically determined wire band gaps compare closely to those calculated by the semiemipirical pseudopotential method, confirming 2D quantum confinement. The diameter dependence of the quantum wire band gaps is compared to that of CdSe quantum dots and rods. Quantum rod band gaps are shown to be delimited by the band gaps of dots and wires of like diameter, for short and long rods, respectively. The experimental data suggest that a length of ca. 30 nm is required for the third dimension of quantum confinement to fully vanish in CdSe rods. That length is about six times the bulk CdSe exciton Bohr radius.     

Absorption cross-section and related optical properties of colloidal InAs quantum dots

We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3.45 nm. We find excellent agreement between the measured results and calculated values based on a model of small-particle light absorption. The absorption cross-section per dot is 6.2 x 10(-16)R(3) cm(2) at 2.76 eV and 3.15 x 10(-16)R(1.28) cm(2) at the first-exciton absorption peak, with the dot radius R in nm. We find that the per-quantum-dot particle oscillator strength of the first-exciton transition is constant for all sizes studied. The radiative lifetime of the first exciton calculated from the oscillator strength increases with dot size and ranges from 4 ns for the smallest dots to 14 ns for the largest ones.     

Observation of individual microtubule motor steps in living cells with endocytosed quantum dots

We report the observation of individual steps taken by motor proteins in living cells by following movements of endocytic vesicles that contain quantum dots (QDs) with a fast camera. The brightness and photostability of quantum dots allow us to record motor displacement traces with 300 micros time resolution and 1.5 nm spatial precision. We observed individual 8 nm steps in active transport toward both the microtubule plus- and minus-ends, the directions of kinesin and dynein movements, respectively. In addition, we clearly resolved abrupt 16 nm steps in the plus-end direction and often consecutive 16 nm and occasional 24 nm steps in minus-end directed movements. This work demonstrates the ability of the QD assay to probe the operation of motor proteins at the molecular level in living cells under physiological conditions.     

Ternary I-III-VI quantum dots luminescent in the red to near-infrared

We report the synthesis of a size series of copper indium selenide quantum dots (QDs) of various stoichiometries exhibiting photoluminescence (PL) from the red to near-infrared (NIR). The synthetic method is modular, and we have extended it to the synthesis of luminescent silver indium diselenide QDs. Previous reports on QDs luminescent in the NIR region have been primarily restricted to binary semiconductor systems, such as InAs, PbS, and CdTe. This work seeks to expand the availability of luminescent QD materials to ternary I-III-VI semiconductor systems.     

pH-sensitive ligand for luminescent quantum dots

We developed a strategy to switch the luminescence of semiconductor quantum dots with chemical stimulations. It is based on the photoinduced transfer of either energy from CdSe-ZnS core-shell quantum dots to [1,3]oxazine ligands or electrons from the organic to the inorganic components. The organic ligands incorporate a dithiolane anchoring group, an electron-rich indole, and a 4-nitrophenylazophenoxy chromophore in their molecular skeleton. Their adsorption on the surface of the quantum dots results in partial luminescence quenching. Electron transfer from the indole fragment to the nanoparticles is mainly responsible for the decrease in luminescence intensity. Upon addition of base, the [1,3]oxazine ring of the ligands opens to generate a 4-nitrophenylazophenolate chromophore, which absorbs in the range of wavelengths where the quantum dots emit. This transformation activates an energy-transfer pathway from the excited nanoparticles to the ligands. In addition, the oxidation potential of the ligand shifts in the negative direction, improving the efficiency of electron transfer. The overall result is a decrease in the luminescence quantum yield of 83%. Addition of acid also opens the [1,3]oxazine ring of the ligands. However, the resulting 4-nitrophenylazophenol does not absorb in the visible region and cannot accept energy from the excited nanoparticles. Furthermore, the oxidation potential shifts in the positive direction, lowering the electron-transfer efficiency. In fact, the luminescence quantum yield increases by 33% as a result of this transformation. These changes are fully reversible and can be exploited to probe the pH of aqueous solutions from 3 to 11. Indeed, our sensitive quantum dots adjust their luminescence in response to variations in pH within this particular range of values. Thus, our general design strategy can eventually lead to the development of pH-sensitive luminescent probes for biomedical applications based on the unique photophysical properties of semiconductor quantum dots.