Process and device characteristics of self-assembled metal nano-crystal EEPROM

Metal nanocrystals self-assembled on gate tunneling oxide can be used to replace the conventional Si/Ge nanocrystals as the floating gate in EEPROM cells. We have demonstrated the successful use of Au and W with their respective process dependence and self-assembly characteristics. The new material options can potentially enhance the applicability and functionality of the nanocrystal EEPROM device. Implications on process integration, in particular the control oxide growth and overall thermal budget, are examined by microscopy, gate current injection and channel mobility monitoring. Charging by hot-carrier injection and control gate tunneling have both been observed by shifts in I–V characteristics. The electrostatic behavior of metal nanocrystals is similar to that of Si nanocrystals in terms of the asymmetrical threshold voltage on source–drain reversal after hot-carrier injection and the Coulomb blockade effects. The electrodynamic behavior is expected to be quite different due to the density of states, but further study is required for quantitative analysis.     

锐钛矿二氧化钛超细纳米晶的三声子互作用

本文通过对粒径为2.2-25.5nm的锐钛矿二氧化钛超细纳米晶在83-293 K温度范围内的变温拉曼光谱的研究,得到了三声子互作用对拉曼频率和线宽的贡献随粒径的变化关系。结果表明锐钛矿二氧化钛超细纳米晶的三声子相互作用随粒径减小而加强。     

Preparation of pure CdSe nanocrystals through mechanical alloying

Pure CdSe nanocrystals have been successfully synthesized by mechanical alloying Cd and Se elemental powders. XRD results show that pure CdSe compound in wurtzite structure has been fabricated after mechanical alloying the elemental powders for 130 min. All the diffraction peaks from elemental Cd and Se disappeared completely in those XRD patterns of as-milled CdSe nanocrystals for more than 3 h. When the mechanical alloying process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Structural evolution of CdSe nanocrystal with ball milling time has been discussed in detail. A phase transformation from wurtzite to zinc blende structure took place when the mechanical alloying process prolonged to 40 h. HRTEM images of the individual as-milled CdSe nanocrystals confirmed such phase transformation. The grain size of the as-milled CdSe nanocrystals ranges from 2 to 30 nm, with majority being distributed within the range from 2 to 8 nm.     

Analysis of structural defects in the CdSe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript> nanocrystals

Spectral dependence of the absorption coefficient of the CdSe _x S_{1 – x} nanocrystals in silicate glass with different perfections of crystalline lattice is experimentally studied. A dependence of the concentration of defects on the time of thermal processing is obtained. It is demonstrated that the defect elimination results from thermal activation process.     

Optical properties and ferromagnetism of ternary Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te nanocrystals

Cd_1−x Mn _x Te (x = 0, 0.1, 0.2) nanocrystals have been synthesized by mechanical alloying (MA) Cd, Mn, and Se elemental powders. XRD patterns and HRTEM images confirmed the formation of cubic Cd_1−x Mn _x Te nanocrystals. All the diffraction peaks from elemental Cd, Mn, and Te powders disappeared completely in those XRD patterns of as-milled Cd_1−x Mn _x Te nanocrystals for more than 20 h. When the MA process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Subsequently, capping the surface of as-milled Cd_1−x Mn _x Te nanocrystals with long chain trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NA) molecules has achieved colorful dispersion solution, which shows similar optical properties to those CdTe nanocrystals prepared by wet chemical process. The grain size is within the range of 2–8 nm for the capped Cd_1−x Mn _x Te nanocrystals being ball milled for 40 h. The PL excitation peak red shifts to longer wavelength side with increasing Mn concentration. Pure CdTe nanocrystals show ferromagnetism behavior at room temperature, the saturation magnetization value and magnetic hysteresis loop increase with the content of substituting Mn ions within the Cd_1−x Mn _x Te nanocrystals.     

Optical properties of cadmium sulfide nanocrystals obtained by the sol-gel method in gelatin

Based on analysis of optical absorption data for CdS nanocrystals obtained by sol-gel technology in gelatin, we have studied the effect of technological factors (reagent concentrations, gelatin concentration) on the growth process and size distribution of the synthesized nanocrystals. Depending on the reagent concentration, we synthesized CdS nanocrystals with mean radii in the range . We have shown that for a low gelatin content (1%), nanocrystals of different sizes are formed (1.7 nm and 2.6 nm). With an increase in the gelatin concentration, the size dispersion decreases and nanocrystals of a single mean radius (2.3 nm) are formed. We have established a correlation between the size dispersion and the shape of the photoluminescence spectrum of the CdS nanocrystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 556–562, July–August, 2008.     

Multiple grains in nanocrystals: effect of initial shape and size on transformed structures under pressure

Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)     

Dependence of electronic structure on the grain size in Mn<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals

. The dependence of electron structure on the grain size in Mn₂O₃ nanocrystals has been investigated using X-ray photoelectron spectroscopy. It is found that the electron structure is sensitively dependent on the average grain size. The experimental results indicate that the O–1s core-level spectra are hardly shifted with decreasing average grain size; however, for a sample with a small grain size (e.g. a 9-nm sample), the shoulder peak evidently appears on the higher-binding-energy side. The main peak width of the Mn–2p core-level spectra becomes wide for samples with decreasing average grain size. The remarkable aspect of the Mn–3s core-level spectra is that the peak is multiply split, with a complex peak structure. The exchange interaction of the 3s and 3d electrons in the Mn³⁺ ion can be used to explain this behavior. PACS 71.23.-k; 73.22.-f     

X-ray photoelectron spectroscopy investigations of Si in non-stoichiometric SiNx LPCVD multilayered coatings

Si nanocrystals were formed in the non-stoichiometric Si-enriched SiN_x low-pressure chemical vapor deposited (LPCVD) coatings on Si wafers treated by various modes. The coating structure as a function of technological conditions was investigated by ellipsometry and X-ray photoelectron spectroscopy (XPS) depth profiling. It was found that nanocomposites on base of SiN_x films enriched by Si have a complex multilayered structure varying in dependence of deposition and annealing parameters. Analysis of the XPS spectra and Si 2s peaks shows the existence and quantity of four chemical structures corresponding to the Si–O, Si–N states, nanocrystalline and amorphous Si. The XPS results show evolution of the chemical structure of silicon nitride and formation of Si nanocrystals. It was found:

• The LPCVD technology of nanocrystals formation allows to get enough high concentration of Si nanocrystals on different depths from the sample surface.

• The volume fraction of nanocrystalline and amorphous Si is changed with depth; this relation depends from SiN_x composition and annealing parameters.

• XPS detects these two phase compositions of Si nanoparticles in SiN_x and SiO₂ layers. The ellipsometry, HR-TEM, and XPS results are in good agreement.

Decay dynamics of ultraviolet photoluminescence in ZnO nanocrystals

Time resolved photoluminescence (PL) measurements at low temperature are performed on colloidal ZnO nanocrystals dispersed in t-butanol. Considering the particle size dependence of the decay times we conclude that the luminescence is composed of two trap related emissions one of which undergoes lifetime shortening due to a non-radiative process. Initial fast shift of the spectrum within 30 ps is observed and it is interpreted as a fast hole cooling just after the excitation.     

Structural analysis of cobalt titanate nanoparticles obtained by sol–gel process

In this work we report the synthesis of nanocomposites based on nanoparticles of cobalt titanate and titanium dioxide in their anatase crystalline phase by a sol–gel process. The synthesized nanoparticles of titanate vary from 1 to 6 nm in size. They are embedded in the anatase matrix, and they were obtained from TiO₂ monoliths doped with Co²⁺. The formation of cobalt titanate nanoparticles showed a linear dependence on the cobalt concentration. The cobalt titanate nanocrystals are very stable even at temperatures higher than 1000 °C. The crystalline structures of the samples were examined using high-resolution transmission electron microscopy and X-ray diffraction. Molecular simulation methods were utilized for a better understanding and for improving the analytical data interpretation of the experimental results. PACS 61.16.Bg; 79.60.Jv; 61.46.+w; 61.50.Ah     

Upconversion luminescence and magnetic properties of ligand-free monodisperse lanthanide doped BaGdF5 nanocrystals

Lanthanide (Ln=Yb³⁺, Er³⁺and Tm³⁺) doped monodisperse oleate-capped BaGdF₅ nanocrystals with a mean diameter of approximately 18 nm were prepared via liquid-solid-solution method. The cell parameter of the as-prepared cubic BaGdF₅ nanocrystals is 5.884 Å, which is different from the reported 6.023 Å (JCPDS 24-0098). When excited by a 980 nm laser, these Ln³⁺ doped nanocrystals exhibit multi-color up-conversion (UC) emissions including blue, yellow and white, by precisely adjusting the dopant concentration of Yb³⁺, Er³⁺ and Tm³⁺. The oleate ligands capped on the surface of the as-synthesized products, which can be conversed from hydrophobic to hydrophilic along with certain extent of weakening of UC intensity, can be moved by the acid treatment process. The measured field dependence of magnetization (M-H curves) of the BaGdF₅ nanocrystals shows excellent paramagnetism. At room temperature, the magnetization of BaGd_0.798Yb_0.2Tm_0.002F₅ nanocrystals is 0.9165 emu/g and the magnetic mass susceptibility reaches 6.11×10⁻⁵ emu g⁻¹ Oe⁻¹ at 15 kOe. Our results indicate that these bi-functional hydrophilic Ln³⁺ doped BaGdF₅ nanocrystals have potential applications in color displays, bioseparation and optical-magnetic dual modal nanoprobes in biomedical imaging.     

Influence of the anderson transition on the energy-level spectra of an ensemble of closely packed nanocrystals

The possibility of the Anderson transition in an ensemble of closely packed monodisperse nanocrystals is mathematically substantiated. Results of a statistical analysis of the dependence of the number of delocalized electronic states on the nanocrystal concentration are reported. Conclusions are drawn about the influence of the Anderson transition on the optical spectra of the given system. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 628–631, November–December, 1999.     

Surface energy and pressure of diamond and silicon nanocrystals

Based on the pair potential of interatomic interaction, we study the dependence of various properties of diamond and silicon nanocrystals with a free surface on size, surface shape, and temperature. A model nanocrystal has the form of a parallelepiped faceted by {100} planes with a square base. The number of atoms N in the nanocrystals is varied from 5 to infinity. The Debye temperature, Gruneisen parameter, specific surface energy, isochoric derivative of specific surface energy with respect to temperature, and surface pressure are calculated as a function of the size and shape of diamond and silicon nanocrystals at temperatures ranging from 20 K to the melting point. The surface pressure P _sf(N) ∼ N ^−1/3 is much lower than the pressure calculated by the Laplace formula for similar nanocrystals for given values of density, temperature, and number of atoms. As the temperature increases from 20 K to the melting point, the isotherm P _sf(N) lowers and changes the shape of the dependence on N; at high temperatures, it goes to the region of extension of small nanocrystals of diamond and silicon.     

Antimicrobial silver nanoparticles generated on cellulose nanocrystals

We describe a new approach to the formation of silver nanoparticles (Ag NPs) using cellulose nanocrystals. The process involves periodate oxidation of cellulose nanocrystals, generating aldehyde functions which, in turn, are used to reduce Ag⁺ into Ag⁰ in mild alkaline conditions. The nanoparticles were characterized using transmission electron microscopy (TEM) and ultraviolet–visible absorption spectroscopy. From the microscope studies (TEM) we observed that Ag NPs have spherical shape with a size distribution comprise between 20 and 45 nm. The antibacterial activity was assessed using the minimum inhibitory concentration. The antibacterial assays compare favourably with most of other experiments conducted with the same species.     

Grain Growth Kinetics of BaTiO3 Nanocrystals During Calcining Process

以醋酸钡和钛酸四丁酯为原料,采用溶胶-凝胶法合成了纳米BaTiO3粉体;运用差示/热重、X射线衍射及透射电镜对前驱体凝胶和产物进行了表征,并根据XRD结果,研究了纳米BaTiO3的晶格常数、晶格畸变度和晶粒尺寸随焙烧温度及时间的变化。结果表明,焙烧温度与时间对纳米BaTiO3晶格常数的影响不明显;随焙烧温度或时间的延长,纳米BaTiO3的晶格畸变度减小,晶粒尺寸增大,但晶格畸变度和晶粒尺寸更敏感于焙烧温度. 基于扩散控制机理的传统模型探讨了焙烧过程纳米BaTiO3晶粒生长动力学,得出其晶粒生长指数为7,晶粒生长活化能为75.49 kJ/mol. 将基于扩散与反应共同控制机理的新型等温模型应用于本研究中,结果表明,新型等温模型更能真实地反映纳米BaTiO3焙烧过程中的晶粒生长行为,说明纳米BaTiO3晶粒生长过程同时受溶质扩散和表面反应控制,其藕合晶粒生长活化能为27.23 kJ/mol.     

Dopant induced morphology changes in ZnO nanocrystals

Zinc oxide (ZnO) nanocrystals doped with different groups of impurities, e.g., Li, Na, Cu, Pr and Mg synthesized by solid-state reaction method under similar conditions exhibit different morphology. XRD showed monophasic wurtzite structure but change in lattice parameters and Zn-O bond length indicates incorporation of dopant ion in ZnO lattice. The morphology of ZnO nanocrystals exhibited striking dependence on type of dopant ion with the shape changing from nanorods, spherical to petal like particles. Photoluminescence (PL) shows pronounced UV emission and negligible visible emission for Li, Na and Cu doped ZnO nanocrystals with peak positions coinciding with that of undoped ZnO. Whereas signature emission of Pr³⁺ ion as well as broad visible emission from Mg doped ZnO revealed the role of intra gap metastable states formed by the dopant ion in the emission process.     

Combustion synthesis and photoluminescence of Eu, Dy-doped La2Zr2O7 nanocrystals

A facile and energy saving sol-gel combustion method has been used to prepare La₂Zr₂O₇ nanocrystallines. The pyrochlore La₂Zr₂O₇ nanocrystals have been obtained at a relatively low temperature with the grain size ranging from 45 to 70 nm. Eu³⁺ and Dy³⁺ have been introduced into the La₂Zr₂O₇ crystal structure, respectively, and the intense photoluminescence was observed. The relative intensity of electric dipole transition and magnetic dipole transition is considered for luminescence emission both of Eu³⁺ and Dy³⁺. The dependence of luminescence intensity on dopant concentration and the effect of Dy³⁺ co-doping on Eu³⁺ luminescence are also discussed.     

Disclination Mechanism of Plastic Deformation of Nanocrystalline Materials

A model describing mechanical behaviour of nanocrystalline materials (NC) obtained by crystallization from amorphous precursor is presented. In the framework of this model a structure of such NCs is represented as a composite consisting of amorphous matrix and absolutely rigid inclusions corresponding to crystalline phase. Dependencies of stress concentration coefficient and yield stress of NCs on the average grain size are obtained. It is shown that the dependence of the yield stress has a point of inflection at the critical grain size in the range of 20–25 nm and is inverse to the Hall-Petch relationship at grain sizes smaller than the critical one. The model predicts a formation of a superlattice from disclinations located in triple junctions of grains on the stage of NC plastic flow. A process of the plastic flow of NC's amorphous matrix and amorphous metallic alloys is described as a go-ahead mechanism of dislocation movement, which includes emission, absorption and reemission of dislocations by disclinations.