Superradiant dissipative tunneling in a double p–i–n semiconductor heterostructure with thermal injection of electrons

We propose a semiconductor device with two p–i–n junctions maintained at two different temperatures. When the current injected in the device due to this temperature difference exceeds a threshold value, a superradiant field is created in the first gate that induces an additional current in the second gate. The injection current is amplified by this reaction loop. In this way, the heat flow between the two junctions is partially transformed in superradiant power.     

Growth and characterization of ZnSe/CdSe/ZnSe quantum dots fabricated by using an alternate molecular beam supplying method

ZnSe/CdSe/ZnSe structures inserted CdSe thin layer are fabricated using an alternate molecular beam supply (ALS). Examining the PL peak energy dependence on beam irradiation time in ALS cycle, we studied the initial stage of CdSe growth. When CdSe below the critical thickness is supplied on ZnSe grown on GaAs (1 0 0), two kinds of 2D islands (platelets) appear. We confirmed the alloying of 2D-CdSe islands and 3D-CdSe islands (dots) is prominent under Cd beam irradiation in ALS growth.     

Fluorescence modulation of single CdSe nanowires by charge injection through the tip of an atomic-force microscope

We demonstrate a direct correlation between the charge state and photoluminescence (PL) intensity of individual CdSe nanowires by actively charging them and performing electrostatic force microscopy and PL measurements simultaneously. While the injection of positive charges leads to an immediate PL quenching, a small amount of injected electrons can lead to an increase of the PL intensity. We directly observed the migration of excess charges into the substrate, which leads to a recovery of the PL. Further, we show that the PL of individual NWs can be actively switched between on and off states by charging with the atomic-force microscope tip. We propose a model based on charge trapping and migration into the substrate to explain our results.     

Optical emission of CdSe nanocrystals induced by the electrical current of a scanning tunneling microscope

Electroluminescence from a single CdSe nanocrystal (NC) excited by the tunneling current of a scanning tunneling microscope (STM) is measured. Such samples produce no plasmon emissions. This allows us to measure pure signals from nanocrystals. The time dependence of electroluminescence differs from the photoluminescence of an identical nanocrystal because of different physical processes of excitation.     

Giant fullerenes formed on C60 films irradiated with electrons field-emitted from scanning tunneling microscope tips

It has been found that spherical large clusters of carbon atoms are formed by irradiation of crystalline C₆₀ films grown on Si(1 1 1)-(7 × 7) surfaces with electrons field-emitted from a scanning tunneling microscope probe tip. The size distribution of the clusters deduced from surface profile measurements suggests that the dominant clusters were not necessarily C_60n (n = 2-4) expected from the simple fusion of C₆₀ molecules. It was proposed that electronic excitations of C₆₀ molecules caused the fragment and coalescence of the molecules to form the giant fullerenes as in the photo-induced similar effects.     

CdSe/ZnSe/ZnS多壳层结构量子点的制备与表征

展示了一种简捷的多壳层量子点合成路线。在含有过量Se源的CdSe体系中直接注入Zn源,"一步法"合成了CdSe/ZnSe量子点;进一步以CdSe/ZnSe为"核",表面外延生长ZnS壳层制备了核/壳/壳结构CdSe/ZnSe/ZnS量子点。相对于以往报道的多壳层结构量子点的制备方法,该方法通过减少壳层的生长步骤有效地简化了实验操作,缩短了实验周期,同时减少对原料的损耗。对量子点进行高温退火处理,能够大幅提高CdSe/ZnSe/ZnS量子点的发光量子产率。透射电镜、XRD以及光谱研究表明:所制备的量子点接近球形,核与壳层纳米晶均为闪锌矿结构,最终获得的CdSe/ZnSe/ZnS量子点的光致发光量子产率达到53％。为了实现量子点的表面生物功能化,通过巯基酸进行了表面配体交换修饰,使量子点表面具有水溶性的羧基功能团,并且能够维持较高的光致发光量子产率。     

CdSe/ZnSe异质结构中Zn1－xCdxSe量子岛(点)的显微荧光光谱和显微拉曼光谱研究

利用显微荧光和显微拉曼光谱,研究了分子束外延生长的CdSe/ZnSe异质结构中,由两种不同机理形成的两类具有不同尺寸和组分的Zn1-xCdxSe量子岛.4.2 K时的显微荧光光谱表明,当CdSe淀积厚度由1.8 ML增加到2.3ML时,Zn1-xCdxSe量子岛的激子荧光峰有166 meV的较大红移,这是量子岛尺寸改变引起的量子限制势能变化所不能完全解释的.经过对样品的显微荧光和显微拉曼光谱的对比分析,发现还存在另外两种引起量子岛荧光峰较大红移的机理：一方面是因为随着CdSe淀积厚度的增加,具有更低能态的大岛密度的增加,并逐渐取代小岛而主导Zni-xCdxSe量子岛的荧光性质;另一方面是由于CdSe/ZnSe量子结构中的两类量子岛的Cd组分浓度也会随CdSe淀积厚度的增加而增加,从而引起量子岛荧光峰的较大红移.     

CdSe/ZnSe自组装量子点中非线性系数随着温度的规律性变化(英文)

研究了在CdSe/ZnSe自组装量子点中CdSe量子点的发光随着激发光强度变化的特性。发现当激发强度(I)变化3个数量级的时候,量子点发光的峰位、峰形都没有发生明显的变化。通过公式L∝Ik(其中I是激发光强度,L是量子点发光强度,k是非线性系数)得到非线性系数k值。实验结果表明:在温度由21 K升高到300 K的过程中,k值随温度变化可以分为3个区域:当温度低于120 K时,k值接近于1;然后,随着温度升高,k值慢慢变小;最后,随温度进一步升高,k值由200 K时的0.946迅速减少到0.870。结合发光随温度变化的实验结果,确认在120 K以下发光主要来源于束缚激子复合。在温度由200 K升高到300 K的过程中,非线性系数的单调减小主要归因于随着温度的升高,发光部分来自于由自由电子或空穴到束缚态能级(FB)的复合。进一步通过分析量子点发光的积分强度随着温度的变化的实验结果,发现发光强度随温度升高而减弱的主要原因是材料中的缺陷或者位错等提供非辐射渠道。     

Photovoltaic characterization of WSe2 with the scanning tunneling microscope

In the present study we use the Scanning Tunneling Microscope (STM) as an instrument to investigate the photovoltaic properties of semiconducting materials. The surfaces of the layered semiconductor WSe₂ were optically illuminated during the tunneling process. The resulting photo-induced tunneling current (PITC) was measured as a function of the wavelength. Microscopic information on the energy dependent generation and recombination of the photo-electrons in the vicinity of the tunneling tip was obtained by this method without the necessity of covering the surface with a conducting electrode. The analysis of the wavelength dependence of the PITC points at the existence of excitonic excitations. Compared to the spectral response of conventional photosensitive heterodiodes our PITC spectra exhibit a more structured response and an increase of the signal in the UV region. Finally, PITC studies on differently treated WSe₂ surfaces show an improved photoactivity for WSe₂ crystals treated with NaI/I₂ solutions.     

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).     

Inelastic tunneling of electrons through a quantum dot with an embedded single molecular magnet

We report a theoretical analysis of electron transport through a quantum dot with an embedded biaxial single-molecule magnet (SMM) based on mapping of the many-body interaction-system onto a one-body problem by means of the non-equilibrium Green function technique. It is found that the conducting current exhibits a stepwise behavior and the nonlinear differential conductance displays additional peaks with variation of the sweeping speed and the magnitude of magnetic field. This observation can be interpreted by the interaction of electron-spin with the SMM and the quantum tunneling of magnetization. The inelastic conductance and the corresponding tunneling processes are investigated with normal as well as ferromagnetic electrodes. In the case of ferromagnetic configuration, the coupling to the SMM leads to an asymmetric tunneling magnetoresistance (TMR), which can be enhanced or suppressed greatly in certain regions. Moreover, a sudden TMR-switch with the variation of magnetic field is observed, which is seen to be caused by the inelastic tunneling.     

Giant injection magnetoresistance in the heterostructure gallium arsenide/granular film with cobalt nanoparticles

We have studied the electron transport and have observed new phenomenon—the positive injection magnetoresistance on heterostructures gallium arsenide/granular film SiO₂ with Co nanoparticles and gallium arsenide/granular film TiO₂ with Co island layers.     

Monte Carlo calculation of the interaction characteristics of low-energy electrons with a gas

Russian Physics Journal -     

Role of localized excitons in the stimulated emission in ultra-thin CdSe/ZnSe/ZnSSe single quantum-well structures

Photo-pumped lasing properties have been investigated in a CdSe/ZnSe/ZnSSE single quantum wells (SQWs) with the well-layer thickness (L_W) of 1, 2 and 3 monolayer (ML). At 20 K, the laser threshold for the SQW withL_W = 1 ML was the lowest in spite of the smallest active layer thickness. The carrier (exciton) sheet density at the threshold (n)_thwas estimated to be as low as 7 × 10¹⁰cm⁻², which is well below Mott's screening density. Time-resolved photoluminescence has revealed that the localized biexciton band, whose peak energy agrees with the lasing peak, appeared on the low-energy side of the main PL peak at this level of carrier concentrations. Theoretical calculation has also shown that the localized biexciton recombination has to be taken into account for the lasing process. On the contrary, then_thvalues of the SQWs with 2 and 3 ML are 1 order of magnitude larger than that of the SQW with 1 ML. This may be due to the smaller oscillator strength of both localized excitons and localized biexcitons because of the larger inhomogeneous broadening, resulting in an increased carrier density for achieving optical gain sufficient to overcome the reflection losses.     

Investigation of localized surface plasmons with the photon scanning tunneling microscope

A Photon Scanning Tunneling Microscope (PSTM) with probe-sample distance control by electron tunneling is used to probe the localized surface-plasmon fields of individual nanometric silver particles. As samples, conventional island films produced by thermal evaporation and regular particle arrays produced by an electron-beam-lithography-based technique, respectively, are used. In either case the strength and spatial localization of the surface-plasmon fields strongly depend on the excitation wavelength. The results are interpreted as different resonance frequencies of individual particles or of different sample areas. On regular arrays consisting of particles with a smallest diameter of 40 nm, the PSTM maps represent the plasmon field strength spatially resolved for individual particles.