Electroluminescence from a current-emitting nanostructured silicon device

A three-dimensional silicon based nanodevice mainly consisting of two conductive silicon cantilevers was fabricated out of silicon-on-insulator material by electron beam lithography, reactive ion etching, and fluoride based wet chemical etching. One of the cantilevers is bent and sticks to the silicon substrate while the other one is freely suspended. We observed electroluminescence in the visible range when a voltage of any polarity is dropped across both levers. The measured spectra covered the range 400–950 nm peaking at about 650 nm. The current applied to the device could tune the intensity of the electroluminescence spectrum. Light powers ranging from 160 fW to some pW were measured at frequencies up to 17 kHz. The origin of the electroluminescence is discussed in comparison to porous silicon and spark-processed silicon.     

Optical modes in nanoscale one-dimensional spin chains

The spin chain systems with one-dimensional magnetic ordering are promising candidates for quantum optical devices. This paper shows how the optical excitation can induce various phonon modes in an ideal Cu-O chain at various lengths. The calculation was carried out at different level theories including configuration interaction singles for excited states, density functional theory and second-order Möller-Plesset perturbation. In general, the number of modes increases with chain length due to growing asymmetry of atomic positions when chain exceeds 5 nm. There were, however, only two basic modes: one is associated with the symmetric oscillation of oxygen and another with the asymmetric motion of the same along the chain. At the length below 4.3 nm, the Raman activity of the symmetric mode (440 cm⁻¹) dominates. From analysis of density of states, this mode may be associated with the excitation across the lowest LUMO bands with changing in spin state.     

Anisotropic growth of indium antimonide nanostructures

InSb nanostructures have been synthesized by the use of gas aggregation process. Nanoparticles with different shapes are obtained by controlling the growth and deposition temperature of the InSb nanoclusters. Triangular nanocrystals are commonly observed when the clusters are extracted from the condensation chamber of the source and deposited on the room temperature substrate at high vacuum. When the deposition is performed inside the condensation chamber at high temperature near the melting point of bulk InSb, nanoparticles formed on the substrate surface show several kinds of 3-dimensional morphologies, such as triangular or rectangular prisms, as well as hexagonal tablets. Keeping the same conditions for the cluster source operation and deposition, after long time growth, nanorods with hexagonal and quadrangular cross sections are formed through vapor-liquid-solid (VLS) process. The origin of the difference on the morphologies and shapes of the nanostructures is attributed to the anisotropic growth of InSb, which is temperature dependent.     

Self-assembly of Mo<Subscript> 6</Subscript>S<Subscript> 8</Subscript> clusters on the Au(111) surface

The preferred adsorption sites and the propensity for a self-organised growth of the molybdenum sulfide cluster Mo₆S₈ on the Au(111) surface are investigated by density-functional band-structure calculations with pseudopotentials and a plane wave basis set. The quasi-cubic cluster preferentially adsorbs via a face and remains structurally intact. It experiences a strong, mostly non-ionic attraction to the surface at several quasi-isoenergetic adsorption positions. A scan of the potential energy surface exhibits only small barriers between adjacent strong adsorption sites. Hence, the cluster may move in a potential well with degenerate local energy minima at room temperature. The analysis of the electronic structure reveals a negligible electron transfer and S-Au hybridised states, which indicate that the cluster-surface interaction is dominated by S-Au bonds, with minor contributions from the Mo atom in the surface vicinity. All results indicate that Mo₆S₈ clusters on the Au(111) surface can undergo a template-mediated self-assembly to an ordered inorganic monolayer, which is still redox active and may be employed as surface-active agent in the integration of noble metal and ionic or biological components within nano-devices. Therefore, a classical potential model was developed on the basis of the DFT data, which allows to study larger cluster assemblies on the Au(111).     

Theoretical study of small silicon clusters on a graphite layer

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si₅. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model. Received 30 November 2000     

Coalescence process of monodispersed Co cluster assemblies

Cluster-cluster coalescence process of monodispersed Co clusters with mean diameter d = 8.5 and 13 nm deposited a plasma-gas-condensation-type cluster beam deposition system was investigated by in situ electrical conductivity measurements and ex situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and analyzed by percolation concept. The electrical conductivity measurement and TEM observation indicated that, below temperature T≈ 100^°C, the Co clusters in the assemblies maintain their original structure as deposited at room temperature, while that the inter-cluster coalescence takes place at T > 100^°C, although the size distribution and the interface morphology of the clusters showed no marked change at substrate temperatures T _s≤200^°C. Received 29 November 2000     

Atomic motion in Se nanoparticles embedded into a porous glass matrix

By neutron diffraction it was shown that nanostructured Se confined within a porous glass matrix exists in a crystalline as well as in an amorphous state. The spontaneous crystallization of crystalline Se from confined amorphous phase was observed. The root-mean-square amplitudes of the atomic motions in the bulk as well as in confinement are found to be essentially different in a basal plane and in the perpendicular direction along the hexagonal axis. The atomic motions in the confined Se differ from the atomic motions in the bulk at low temperatures. The results shows an unusual “freezing" of the atomic motion along the chains, while the atomic motions in the perpendicular plane still keep. This “freezing" is accompanied by the deformation of nanoparticles and the appearance of inner stresses. This effect is attributed to the interaction of confined nanoparticle with the cavity walls.     

Band structures of transverse waves in nanoscale multilayered phononic crystals with nonlocal interface imperfections by using the radial basis function method

A radial basis function collocation method based on the nonlocal elastic continuum theory is developed to com-pute the band structures of nanoscale multilayered phononic crystals. The effects of nonlocal imperfect interfaces on band structures of transverse waves propagating obliquely or verti-cally in the system are studied. The correctness of the present method is verified by comparing the numerical results with those obtained by applying the transfer matrix method in the case of nonlocal perfect interface. Furthermore, the influ-ences of the nanoscale size, the impedance ratio and the incident angle on the cut-off frequency and band structures are investigated and discussed in detail. Numerical results show that the nonlocal interface imperfections have signif-icant effects on the band structures in the macroscopic and microscopic scale.     

Nanoscale Local Contacts Enable Inverted Inorganic Perovskite Solar Cells with 20.8 % Efficiency

Inorganic perovskite solar cells (IPSCs) have gained significant attention due to their excellent thermal stability and suitable band gap (~1.7 eV) for tandem solar cell applications. However, the defect-induced non-radiative recombination losses, low charge extraction efficiency, energy level mismatches, and so on render the fabrication of high-efficiency inverted IPSCs remains challenging. Here, the use of 3-amino-5-bromopyridine-2-formamide (ABF) in methanol was dynamically spin-coated on the surface of CsPbI_2.85Br_0.15 film, which facilitates the limited etching of defect-rich subsurface layer, resulting in the formation of vertical PbI₂ nanosheet structures. This enabled localized contacts between the perovskite film and the electron transport layer, suppress the recombination of electron-hole and beneficial to electron extraction. Additionally, the C=O and C=N groups in ABF effectively passivated the undercoordinated Pb²⁺ at grain boundaries and on the surface of CsPbI_2.85Br_0.15 film. Eventually, we achieved a champion efficiency of 20.80 % (certified efficiency of 20.02 %) for inverted IPSCs with enhanced stability, which is the highest value ever reported to date. Furthermore, we successfully prepared p-i-n type monolithic inorganic perovskite/silicon tandem solar cells (IPSTSCs) with an efficiency of 26.26 %. This strategy provided both fast extraction and efficient passivation at the electron-selective interface.     

Nano- and Pico-Scale Transport Phenomena in Fluids

A new theoretical approach to describe pre-hydrodynamic stages of evolution in nonequilibrium fluids is presented. The local density, velocity, and temperature fields are expressed as integrals over Green's functions that depend on initial-state ensemble averages of dynamical quantities. For systems in which the initial states are nonuniform in only one spatial direction, explicit expressions for the Green's functions are derived in terms of initial-state ensemble averages of moments of particle displacements and products of particle velocities and particle displacements.