Linearly polarized emission from colloidal semiconductor quantum rods

Colloidal quantum rods of cadmium selenide (CdSe) exhibit linearly polarized emission. Empirical pseudopotential calculations predict that slightly elongated CdSe nanocrystals have polarized emission along the long axis, unlike spherical dots, which emit plane-polarized light. Single-molecule luminescence spectroscopy measurements on CdSe quantum rods with an aspect ratio between 1 and 30 confirm a sharp transition from nonpolarized to purely linearly polarized emission at an aspect ratio of 2. Linearly polarized luminescent chromophores are highly desirable in a variety of applications.     

Ultrafast dynamics at semiconductor and metal surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.     

Spatially resolved visible luminescence of self-assembled semiconductor quantum dots

Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of InxAI(1-x)As-AIyGa(1-y)As.     

Quantization of multiparticle auger rates in semiconductor quantum dots

We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.     

Coherent manipulation of coupled electron spins in semiconductor quantum dots

We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of approximately 10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.     

Tuning the quantum stability and superconductivity of ultrathin metal alloys

Quantum confinement of itinerant electrons in atomically smooth ultrathin lead films produces strong oscillations in the thickness-dependent film energy. By adding extra electrons via bismuth alloying, we showed that both the structural stability and the superconducting properties of such films can be tuned. The phase boundary (upper critical field) between the superconducting vortex state and the normal state indicates an anomalous suppression of superconducting order just below the critical temperature, Tc. This suppression varies systematically with the film thickness and the bismuth content and can be parametrized in terms of a characteristic temperature, Tc* (less than Tc), that is inversely proportional to the scattering mean free path. The results indicate that the isotropic nature of the superconductive pairing in bulk lead-bismuth alloys is altered in the quantum regime.     

Confinement of electrons to quantum corrals on a metal surface

A method for confining electrons to artificial structures at the nanometer lengthscale is presented. Surface state electrons on a copper(111) surface were confined to closed structures (corrals) defined by barriers built from iron adatoms. The barriers were assembled by individually positioning iron adatoms with the tip of a 4-kelvin scanning tunneling microscope (STM). A circular corral of radius 71.3 A was constructed in this way out of 48 iron adatoms. Tunneling spectroscopy performed inside of the corral revealed a series of discrete resonances, providing evidence for size quantization. STM images show that the corral's interior local density of states is dominated by the eigenstate density expected for an electron trapped in a round two-dimensional box.     

Crystal growth of silicon and germanium in metal films

Amorphous silicon in contact with silver films and amorphous germanium in contact with aluminum films form crystalline precipitates when heated to temperatures well below those at which any liquid phase is present. Crystallization occurs by an initial dissolution of the semiconductor into the metal filmsolvent followed by the growth of crystals out of the solvent.     

金属抗性促生菌对玉米生长和锌积累的影响

【目的】探讨金属抗性促生菌对锌污染土壤中玉米生长和Zn吸收的影响,为利用微生物缓解重金属对农作物胁迫和减少重金属吸收提供理论依据.【方法】通过盆栽试验研究接种促生菌Ⅱ2R3和Ⅳ8R3是否能够缓减Zn对玉米的毒害作用,减少玉米对Zn的吸收和积累.【结果和结论】促生菌Ⅱ2R3、Ⅳ8R3不仅有很强的抗Zn胁迫能力,而且具有较强地吸附、积累Zn的能力.在中、轻度Zn污染(200和400 mg·kg-1)土壤上,单独接种促生菌Ⅳ8R3促进了玉米的生长;当土壤Zn污染水平达到800 mg·kg-1时,同时接种促生菌Ⅳ8R3+Ⅱ2R3显著促进玉米的生长.单独或同时接种促生菌Ⅳ8R3、Ⅱ2R3使不同Zn污染水平土壤中的有效Zn含量均显著降低.当土壤Zn含量达到800mg·kg-1时,单独接种菌株Ⅱ2R3或同时接种Ⅱ2R3+Ⅳ8R3均显著抑制玉米对Zn的吸收和向地上部转运,而在中、轻度Zn污染(Zn≤400 mg·kg-1)土壤上,3种接种处理对玉米吸收Zn均没有显著影响.表明在高锌污染(800mg·kg-1)土壤中,接种促生菌Ⅳ8R3、Ⅱ2R3能显著抑制玉米对Zn的吸收和向地上部转运;菌株Ⅳ8R3、Ⅱ2R3对玉米生长的影响与土壤Zn污染水平和菌株的性质有关.     

茎尖菜用甘薯筛选及速冻保鲜工艺研究

筛选茎尖菜用甘薯品种,并对其速冻保鲜工艺进行了研究.试验结果表明:蒲薯53是优良的茎尖菜用甘薯品种,食用品质佳;以碳酸氢钠为护色剂和在95 ℃漂烫40 s工艺条件下,可以显著地抑制多酚氧化酶的活性,能较好地保存茎尖的感官品质;在-30～-35 ℃的温度下速冻处理15 ～20 min,保鲜效果较好.     

应用蚕豆根尖微核技术研究3种重金属的遗传毒性

运用蚕豆根尖细胞微核技术,以微核率和有丝分裂指数为指标,研究不同浓度Pb2+、Cr3+和Cd2+对蚕豆根尖细胞的遗传毒性效应.结果表明,Pb2+、Cr3+和Cd2+在不同浓度下均可诱导微核的产生,其遗传毒性依次为:Cd2+>>Pb2+>Cr3+.在一定浓度范围内,微核率随重金属浓度的升高而增加,达到一定浓度后,微核率反而有下降的趋势.在一定低浓度条件下,蚕豆根尖细胞有丝分裂指数增高,高浓度下有丝分裂指数降低,意味着低浓度可促进细胞有丝分裂,高浓度则会抑制细胞有丝分裂.     

弱光胁迫对不同砧木西瓜嫁接苗生长及生理指标的影响

[目的]分析不同砧木西瓜嫁接苗在弱光胁迫下的生长及生理指标差异,为筛选耐弱光西瓜嫁接砧木提供参考依据.[方法]选择4份葫芦(编号分别为H07、H08、H09和H10)和1份野生西瓜(编号为X02)为砧木,以黑公子西瓜为接穗,在正常光照(8000 1x)和两种弱光(4000和20001x)条件下,测定接穗自根嫁接苗(XZ)和砧木嫁接苗的接穗高度、茎粗、叶面积、叶绿素含量、叶片过氧化物酶(POD)活性及叶片丙二醛(MDA)含量等生长、生理指标,用隶属函数法和聚类分析法对不同砧木嫁接苗的耐弱光性进行综合评价.[结果]弱光处理后14 d,不同砧木西瓜嫁接苗的生长、生理指标表现不同,其中H07和H10嫁接苗生长较健壮,叶片POD活性和MDA含量显著高于其他砧木嫁接苗和XZ(P<0.05,下同),叶片质膜氧化程度较低;X02的嫁接苗生长较弱,叶片POD活性低、MDA含量高.在4000 1x弱光条件下,不同砧木西瓜嫁接苗的耐弱光性排序为H 10>H07>H08>H09>X02;在2000 1x弱光条件下,不同砧木西瓜嫁接苗的耐弱光性排序为H07>H10>H09>H08>X02.[结论]不同砧木西瓜嫁接苗的耐弱光性差异明显,葫芦砧木嫁接苗的耐弱光性优于野生西瓜砧木,其中H10和H07因其嫁接苗的耐弱光性表现最佳,可作为西瓜耐弱光砧木或耐弱光瓜类砧木育种种质资源使用.     

金属离子对褐黄孢链霉菌生长和纳他霉素生物合成的影响

【目的】研究不同金属离子对褐黄孢链霉菌菌体生长和纳他霉素生物合成的影响。【方法】用单因素摇瓶发酵试验方法,研究不同浓度Na+,Fe2+,Mg2+,K+,Ca2+,Cu2+,Mn2+,Zn2+和Co2+对褐黄孢链霉菌生长和纳他霉素生物合成的影响。【结果】在发酵培养基中添加Mg2+和Ca2+可促进褐黄孢链霉菌的菌体生长,添加Fe2+、Cu2+、Mn2+和Co2+可抑制菌体生长,而添加Na+、K+和Zn2+对菌体生长无显著影响;在发酵培养基中添加低浓度K+、Ca2+和Co2+可促进纳他霉素的生物合成,添加Na+、Fe2+、Cu2+和Zn2+抑制纳他霉素的生物合成,而添加Mg2+和Mn2+对纳他霉素的生物合成无显著影响。【结论】不同种类、不同浓度的金属离子,对褐黄孢链霉菌菌体生长和纳他霉素生物合成的影响差别较大;在培养基中添加适当浓度和种类的金属离子,有助于提高纳他霉素的产量。     

氮肥类型对东南景天生长及重金属积累的影响

【目的】研究氮肥类型对东南景天套种玉米模式下生长特性及重金属锌、镉积累的影响,为植物修复实践过程中合理使用氮肥、减少修复成本和环境污染奠定基础。【方法】通过大田试验,在重金属污染的矿区采用东南景天套种玉米模式,施用同氮素水平下有机肥（鸡粪）、硫酸铵和尿素3种类型氮肥,测定土壤pH、有机质含量、重金属有效态、东南景天和玉米地上部生物量及重金属锌、镉含量的变化。【结果】施用有机肥增加土壤有机质含量,提高土壤镉的生物有效性,对土壤pH和锌的生物有效性则无影响;施用硫酸铵和尿素均降低土壤pH和土壤镉的生物有效性,提高了土壤有机质含量和土壤锌的生物有效性。氮肥类型对东南景天中锌、镉以及玉米镉积累影响不显著,但两种化学氮肥均显著增加了玉米锌的累积量;不同类型氮肥对东南景天和玉米生长均有显著促进作用,有机肥处理两种作物的地上部生物量均显著高于其他处理（P〈0．05）。【结论】有机肥对东南景天修复土壤重金属污染效率的促进作用优于硫酸铵和尿素。     

大气降尘污染对典型农作物生长发育及重金属含量的影响

为揭示大气降尘对作物生长和农产品品质的影响,采用不同来源和不同程度的降尘,模拟开展降尘-农作物盆栽试验,分析青菜和水稻幼苗光合及呼吸速率、叶绿素含量、地上部干物质质量以及重金属含量的变化。结果表明,大气降尘显著抑制了2种农作物的光合速率,降低了叶片中叶绿素含量,同时提高了呼吸速率,导致青菜和水稻幼苗地上部干物质质量分别下降了11.8%~51.3%和12.7%~35.5%。大气降尘使青菜和水稻幼苗植株中部分重金属元素含量显著增加,农作物安全品质降低。     

根域水分调控对不同砧木苹果幼树生长和叶片光合特性的影响

以3年生盆栽不同砧木红富士苹果幼树为试材,研究了土壤水分对树体生长和叶片光合特性的影响。结果表明:土壤含水量对苹果新梢生长速率、2次生长的发生和生长时间起到了调控作用。在土壤相对含水量为(75±5)%、(60±5)%处理下,3种砧木苹果幼树新梢均有2次生长,且随着土壤含水量的增多2次生长提早。3种砧木苹果幼树的新梢生长量随着土壤含水量的降低显著降低,其中JM7砧木苹果幼树的降幅最小。不同处理下3种砧木苹果幼树叶绿素含量与对照相比没有下降,而SH40砧木苹果幼树叶绿素含量升幅在春梢停长期与秋梢生长期显著高于其他砧木苹果幼树。随着土壤含水量的降低,3种砧木苹果幼树叶片净光合速率、蒸腾速率、气孔导度均呈下降趋势,但不同砧木苹果幼树对不同水分处理的响应不同。JM7砧木苹果幼树在不同处理下与对照相比净光合速率降幅较小,而蒸腾速率降幅较大,所以其瞬时水分利用效率升幅最大。本试验中,在土壤相对含水量为(60±5)%处理下,树体可以正常生长,提高水分利用效率。     

3种重金属离子对牟氏角毛藻生长的影响

采用f/2培养基,研究了不同浓度的Cu(Ⅱ)、Zn(Ⅱ)和Pb(Ⅱ))对牟氏角毛藻Chaetoceros muelleri生长的影响。结果表明:在特定浓度条件下,3种重金属离子对牟氏角毛藻的生长具有显著影响。当Cu(Ⅱ)浓度为0.1 mg/L、Zn(Ⅱ)和Pb(Ⅱ)浓度为0.1¹.0 mg/L时,能促进牟氏角毛藻的生长;Zn(Ⅱ)和Pb(Ⅱ)浓度为101.0 mg/L时,能促进牟氏角毛藻的生长;Zn(Ⅱ)和Pb(Ⅱ)浓度为10⁵0 mg/L时,牟氏角毛藻能维持一定的速率生长;Cu(Ⅱ)浓度为150 mg/L时,牟氏角毛藻能维持一定的速率生长;Cu(Ⅱ)浓度为1⁵0 mg/L时,牟氏角毛藻的生长受到显著抑制。     

3种重金属离子对牟氏角毛藻生长的影响

采用f/2培养基,研究了不同浓度的Cu(Ⅱ)、Zn(Ⅱ)和Pb(Ⅱ))对牟氏角毛藻Chaetoceros muelleri生长的影响。结果表明:在特定浓度条件下,3种重金属离子对牟氏角毛藻的生长具有显著影响。当Cu(Ⅱ)浓度为0.1 mg/L、Zn(Ⅱ)和Pb(Ⅱ)浓度为0.1~1.0 mg/L时,能促进牟氏角毛藻的生长;Zn(Ⅱ)和Pb(Ⅱ)浓度为10~50 mg/L时,牟氏角毛藻能维持一定的速率生长;Cu(Ⅱ)浓度为1~50 mg/L时,牟氏角毛藻的生长受到显著抑制。     

施用沼液和接种丛枝菌根真菌对甘草生长及重金属累积的影响

甘草(Glycyrrhiza uralensis Fisch)是一种重要的补益中草药。由于野生甘草面临过度采撷和资源耗竭,人工种植甘草规模日益扩大,发展低耗高效的甘草种植技术成为迫切需要。通过盆栽试验考查了不同沼液添加水平(沼液/去离子水体积比:10/0、9/1、5/5、1/9及0/10)下,接种丛枝菌根(arbuscular mycorrhiza,AM)真菌Rhizophagus intraradices对甘草生长及重金属累积的影响。试验结果表明,AM真菌能够对甘草根系形成良好侵染,菌根侵染率在25.0%~48.4%之间。AM真菌对甘草表现出显著促生作用,接种处理使植株生物量、磷浓度及叶片叶绿素含量分别提高了171%~271%、64%~143%和98%~127%。施用沼液显著提高了植株磷浓度和叶片叶绿素含量,在提高土壤有机质含量和土壤全磷的同时,也增加了植物及土壤铬、铜和铅浓度。在沼液稀释比为10/0和9/1水平下,植物铬、铜和铅浓度超标,而接种AM真菌显著降低了植株重金属浓度至安全阈值以内。施用沼液同时接种AM真菌可在促进甘草生长的同时保障甘草品质,因而在甘草人工种植中具有潜在应用价值。     

不同钙镁比处理对三种超富集植物生长及吸收重金属的影响

通过水培实验研究了不同钙镁比(Ca/Mg=0.2、0.5、1.0、2.0、4.0、6.0)对Zn/Cd超富集植物东南景天(Sedum alfredii)、Zn/Cd/Ni超富集植物遏蓝菜(Noccaca caerulescens)和Ni超富集植物庭荠属(Alyssum murale)在20 mg Zn·L~(-1)、18 mg Ni·L~(-1)、10 mg Pb·L~(-1)和0.5 mg Cd·L~(-1)胁迫下生长和吸收重金属的影响。研究结果显示:A.murale具有很强的将Ni从地下部转移至地上部的能力(转移系数TF达5.98~11.19)和较高的地上部Ca/Mg比(2.45~16.74);N.caerulescens对Mg转移能力(TFMg为1.43~3.70)大于Ca(TFCa为0.77~1.38);S.alfredii对Mg的转移能力与Ca相似。N.caerulescens在正常营养液(Ca/Mg=2)条件下获得最好的生长和地上部最大Zn/Cd/Ni含量,增加Ca或Mg的供给都会产生抑制作用;A.murale在高Ca(Ca/Mg=6)供给时获得最大的生物量和Ni含量,增Ca有助于地上部Ni的累积,但增Mg降低了地上部Ni含量;增加Ca的供给会增加S.alfredii地上部和地下部的Zn含量,增加Ca或Mg的供给都会增加S.alfredii地下部Cd的含量,但S.alfredii在Ca/Mg=0.2时获得最大的生物量和最大的Zn和Cd含量。S.alfredii、A.murale和N.caerulescens地上部分别在Ca/Mg比为6、0.2、1.0时获得最大的Pb含量,其他Ca/Mg比对三种超富集植物地上部吸收Pb均没有产生显著影响。Ca/Mg比对超富集植物富集重金属能力的影响因植物种类和元素而异。