Nanocrystal Arrays Research Articles

Peculiarities of CdS nanocrystal formation at annealing of a Langmuir‐Blodgett matrix

AbstractFormation and assembling of CdS nanocrystals (NC) on highly‐ordered pyrolytic graphite (HOPG) and on oxidized silicon substrates have been investigated by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). NC were initially formed within a Langmuir‐Blodgett (LB) matrix of cadmium behenate. The LB matrix was then removed by annealing in an ammonia atmosphere. It is shown that the NC self‐assembling is mainly determined by the LB matrix wetting properties. In case of wettable HOPG substrate homogeneous matrix evaporation occurs that leads to coarsening of the NC arrays as the main self‐assembly process. Otherwise, NC prepared on non‐wettable SiO2‐Si substrate demonstrate arrays formation determined by the LB matrix dewetting process. For the both cases, the influence of kinetic limitations connected with matrix evaporation rate on the self‐assembly process is observed. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Three Dimensional Dynamics and Fluctuations of DNA-Nanogold Dimers by Individual-Particle Electron Tomography

Organic-inorganic hybridized nanocrystals have been proven as an important additional tool for addressing many emerging challenges biological and material sciences. Nanogold and quantum dot conjugates have been used extensively as biomolecular markers, while DNA base pairing has directed the self-assembly of discrete groupings and arrays of organic and inorganic nanocrystals in formation of a network solid for electronic devices and memory components. The control of DNA-mediated assembly depends crucially upon the ability to exposure three-dimensional (3D) structure of DNA-nanocrystals hybridized building blocks. Current techniques limit in structure determination of flexible and heterogeneous sample. Here, we employed the electron tomography and negative-staining techniques to investigate the 3D structure of DNA-nanogold dimers that were self-assembled from a mixture of an 84-base-pair double-stranded DNA (dsDNA) conjugated with two 5-nm nanogold particles. We reconstructed a total of 14 electron density maps at a resolution of ∼2 nm from each individual dsDNA-nanogold particle using the individual-particle electron tomography (IPET) reconstruction method. Using these 3D density maps as constrain, we projected a standard flexible DNA model onto the observed EM density maps and derived 14 conformations of dsDNA by molecular dynamics simulations. By aligning these structures from one distal end nanogold, the distribution showed the DNA fluctuation and flexibility between nanogolds, which providing a blueprint research for studying of DNA-mediated assembly of complex inorganic nanostructures.

Omnidirectional enhancement of photocatalytic hydrogen evolution over hierarchical “cauline leaf” nanoarchitectures

The scrupulous design and integration of multiple active materials into hierarchical nanoarchitectures is essential for the creation of photocatalytic hydrogen evolution reaction (HER) system that can mimic natural photosynthesis. Here we report the design and preparation of a “cauline leaf”-like structure for highly efficient HER, by decorating TiO2 nanofibers with vertical arrays of atomically-thin MoS2 nanosheets and CdS nanocrystals. The unique integrated “cauline leaf” design can promote light trapping and absorption for highly efficient light harvesting and photocarrier generation, and offer unblocked electron transport pathway for rapid charge separation/transport to suppress charge recombination, as well as high surface area and high density of active sites for highly efficient utilization of photo-generated carriers for productive HER. Structural characterizations by transmission electron microscopy show well-integrated nanoarchitectures. Significantly, photocatalytic studies demonstrate rapid HER rates as high as 12.3 or 6.2mmolh−1g−1 under simulated solar light or visible light irradiation, with apparent quantum efficiencies of 70.5% at 365nm or 57.6% at 420nm, and excellent long term stability, representing one of the best reported MoS2 hybrid HER photocatalysts. The study could open new opportunities for the rational design of nanoscale architectures for HER or other application.

Miniband Calculation of 3-D Nanostructure Array for Solar Cell Applications

Within the envelop-function framework, we propose a more efficient finite-element method to calculate the miniband structure and density of states in an idealistic nanocrystal array with realistic geometrical parameters. This method clearly reveals the miniband formation and accurately calculates the energy dispersion relationship. The calculated result agrees well with the analytical Kronig-Penney method. More importantly, this method surmounts the theoretical approximations of the multidimensional Kronig-Penney method, and provides significant information for 3-D quantum dots solar cell design by simulating an in-plane germanium nanodisk array in bulk silicon.

Enhanced Charge-Transport Behavior on PbS Nanocrystals Capped with Atomic Ligands

In order to enhance charge-transport nanocrystal (NC) arrays, long and insulating ligands have to be replaced by short and electronically transparent ligands. Traditionally, benzene dithiol or mercaptopropionic acid have been frequently used as such surface ligands to overcome low charge-carrier mobility issues that affect colloidal NCs. However, the resulting devices usually exhibit a limited enhanced mobility of ~0.01 cm2/Vs. In this study, to realize a high charge-carrier mobility over 0.1 cm2/Vs, we demonstrate the utilization of atomic ligands such as halide anions (X-, X : Cl, Br, I) as electrically transparent ligands for lead chalcogenide NCs. We discuss the origin of such improved charge-transport behavior, which results from the introduction of atomic ligands, as well as various characterization results.

Controlling shape and spatial organization of silver crystals by site-selective chemical growth method for improving surface enhanced Raman scattering activity

Abstract A novel approach for a controlled growth of pattern-directed organization of Ag nanocrystals (NCs) onto rigid substrate has been proposed to achieve large-scale functional materials. A soft nanoporous template was used to create first a hexagonal array of gold nanoparticles by a seed-mediated growth colloidal process. The size and morphology of the Ag NCs were then controlled by a site-selective heterogeneous nucleation and growth onto the Au precursors. While removing the nanoporous polymer mask before the growth process leads to the formation of Ag flower-shape crystals, keeping this thin polymer mask rather leads to the formation of sheet-like structures. The growth mechanism of the template-directed synthesis of Ag crystals arrays was investigated. In addition, the optical properties of Ag NCs arrays were discussed in relation to their morphology and organization. Finally, this facile and simple multistep approach is particularly attractive due to its environmentally gentle processing conditions and represents an open pathway to several technologically large-scale nanomaterials fabrication such as hierarchically ordered crystal architectures for sensor applications.

Formation of Monocrystalline 1D and 2D Architectures via Epitaxial Attachment: Bottom-Up Routes through Surfactant-Mediated Arrays of Oriented Nanocrystals.

Monocrystalline architectures with well-defined shapes were achieved by bottom-up routes through epitaxial attachment of Mn3O4 nanocrystals. The crystallographically continuous 1D chains elongated in the a axis and 2D panels having large a or c faces were obtained by removal of the organic mediator from surfactant-mediated 1D and 2D arrays of Mn3O4 nanocrystals, respectively. Our basal approach indicates that the epitaxial attachment through the surfactant-mediated arrays is utilized for fabrication of a wide variety of micrometric architectures from nanometric crystalline units.

Diffraction Studies of the Formation of Silicon Nanocrystals in the SiOx/Si Compounds after Carbon Ion Implantation

The films have been deposited on the silicon subtracts with the (111) and (100) orientations by thermal evaporation of SiO powder and carbon implanted with doses of 6 · 1016 to 1,2 · 1017 cm−2 followed by annealing in nitrogen at 1100 oC. Diffraction studies of these structures confirm the occurrence of a preferred orientation in the nanocrystals during high temperature thermal annealing, controlled by the substrate orientation. It was possible to detect the existence of two arrays of silicon nanocrystals in the dielectric matrix, with one having a smaller average size of 5—10 nm and a lattice parameter close to that of crystalline silicon, and the other one having a large size of 50—100 nm and a greater lattice parameter. We have estimated the carbon implantation doses for which the large size nanocrystals (> 50 nm) do not form. This dose is 6 · 1016 cm−2 for the (111) substrates and 9 · 1016 cm−2 for the (100) ones.

Solution-Processed Large-Area Nanocrystal Arrays of Metal-Organic Frameworks as Wearable, Ultrasensitive, Electronic Skin for Health Monitoring.

Pressure sensors based on solution-processed metal-organic frameworks nanowire arrays are fabricated with very low cost, flexibility, high sensitivity, and ease of integration into sensor arrays. Furthermore, the pressure sensors are suitable for monitoring and diagnosing biomedical signals such as radial artery pressure waveforms in real time.

Three-Dimensional Arrays of 1D MnO2 Nanocrystals for All-Solid-State Asymmetric Supercapacitors.

Reported is the synthesis of 3D hierarchical structures based on one-dimensional MnO2 nanobuilding blocks (nanorods, nanowires, and nanoneedles) by means of a facile and scalable coprecipitation method and their use as electrodes for the assembly of all-solid-state supercapacitors. Asymmetric devices were also assembled by using these nanostructured MnO2 materials as the positive electrode and reduced graphene oxide (rGO) as the negative electrode with a polymeric gel electrolyte. The asymmetric cells successfully extend the working voltage windows beyond 1.4 V and allowed for a maximum voltage of 1.8 V. An asymmetric device based on hierarchical nanoneedle-like MnO2 and rGO achieved a maximum specific capacitance of 99 F g-1 at a scan rate of 10 mV s-1 with a stable operational voltage of 1.8 V. This high value allowed for a large specific energy of 24.12 Wh kg-1 .

Fabrication and photoelectrochemical study of vertically oriented TiO2/Ag/SiNWs arrays

Ordered channeled and porous TiO2 and Ag modified silicon nanowires (TiO2/Ag/SiNWs) heterostructured nanocrystals arrays are synthesized by a two-step method based on an electrochemical etching procedure and a sol–gel process. The morphology and photoelectrochemical properties of the TiO2/Ag/SiNWs are studied. The TiO2/Ag/SiNWs photocatalysts possess ordered channels and a porous structure with large specific surface area. UV–visible diffuse reflectance spectroscopy and ultraviolet Raman scattering demonstrate that the incorporated Ag significantly enhances light absorption by the TiO2/SiNWs in the visible spectral range and improves the separation of photo-induced charge carriers in the TiO2/SiNWs. The photoelectrochemical properties of the TiO2/Ag/SiNWs are investigated by monitoring the degradation of pnitrophenol (PNP) and Ag enhances PNP photodegradation under UV–vis irradiation due to the Ag–TiO2 heterojunctions and surface texture. The photoelectrochemical properties of TiO2/Ag/SiNWs have promising applications in photoelectrochemical solar cells and other light-harvesting devices.

Local field corrections to the spontaneous emission in arrays of Si nanocrystals

We present a theory of the local field corrections to the spontaneous emission rate for the array of silicon nanocrystals in silicon dioxide. An analytical result for the Purcell factor is obtained. We demonstrate that the local-field corrections are sensitive to the volume fill factor of the nanocrystals in the sample and are suppressed for large values of the fill factor. The local-field corrections and the photonic density of states are shown to be described by two different effective permittivities: the harmonic mean between the nanocrystal and the matrix permittivities and the Maxwell–Garnett permittivity.

Self-assembled silicon nanocrystal arrays for photovoltaics

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of Si tandem solar cells since their bandgap exceeds that of bulk silicon and can be tuned by adjusting nanocrystal size. Due to this effect, size control is required to maintain a uniform bandgap throughout a Si NC film. This can be achieved by annealing superlattices of Si-rich and stoichiometric dielectrics. This paper reviews the progress that has been made using host matrices SiO2, Si3N4, and SiC. Si NCs in SiO2 show excellent NC size and shape control and strong quantum-confinement-related photoluminescence, however electrical conductivity is poor. Ordering and size control is also obtained in Si3N4, but conclusive evidence of quantum confinement is lacking. Preparing ordered but separated Si NCs in SiC is difficult, but the narrow parameter space in which this is possible has been elucidated, good electrical conductivity was obtained, and functioning single-junction and tandem cells have been produced. Si NC formation can now be well-controlled in all three materials, and the key weaknesses for photovoltaics have been identified to be the electrical conductivity of SiO2, and defect density for Si3N4 and SiC. Addressing these is expected to lead to competitive Si tandem solar cells.

Monte Carlo study of the Electron Transport Properties of an Array of Si Nanocrystals

We have studiedthe conductivity of an array of Si nanocrystals separated by energy barriers using the Monte Carlo technique. The energy band diagram was calculated by self-consistent solution of the Poisson equation. It is shown that the simulation data can be interpreted by an equivalent network of series resistors. A resistance can be attributed to the energy barrier that increases with increasing barrier width. The barrier resistance has been calculated as a function of the barrier width. Our resultscan be useful in studying the thermoelectric properties of composite structures in the presence of energy barriers. The assumption of an in-series resistor network commonly used to study the thermoelectric properties of nanocomposites has been confirmed by our simulations on 1d nanocomposites.

Template-assisted synthesis of CdS nanocrystal arrays in chemically inhomogeneous pores using a vapor–solid mechanism

Template of chemically inhomogeneous pores enables one to grow an ordered array of CdS multi-armed nanocrystals free of catalyst.

Facile synthesis of ultra-small PbSe nanorods for photovoltaic application.

Nanocrystal array solar cells based on lead chalcogenide quantum dots (QDs) have recently achieved a high power conversion efficiency of over 8%. The device performance is expected to further increase by using 1-dimensional nanorods (NRs), due to their improved carrier transport over zero-dimensional quantum dots. However, previously reported PbSe NRs have not been used in solar cells mainly because of their large diameters, resulting in a small bandgap unsuitable for photovoltaic application. In this work, we have demonstrated a new method for synthesizing monodisperse ultra-small PbSe NRs with the diameter approaching 2 nm (Eg > 1.2 eV), which can be attributed to the use of diphenylphosphine (DPP) and trans-2-octenoic acid (t-2-OA). The introduction of trace DPP can greatly lower the reaction temperature, leading to reduced diameters for the obtained PbSe NRs as well as largely increased yield. The use of short-chain t-2-OA together with oleic acid as capping ligands results in high monomer reactivity, fast nucleus diffusion and high growth rate, which realize the anisotropic growth of ultra-small PbSe NRs at low reaction temperatures. The PbSe NRs show n-type properties and high electron mobility as measured using field-effect transistors. The PbSe NRs with narrow diameters also demonstrate a suitable bandgap for photovoltaic application. They are used for the first time in solar cells and their improved efficiency is demonstrated when used together with QDs.

Spin filter effect in iron oxide nanocrystal arrays

Integrating nanocrystals (NCs) into magnetic tunnel structures is of considerable interest due to expectation of novel properties from their spin selective transport and single electron features. Superstructures by colloidal NCs having translational and orientational order and interesting collective magnetic properties can be prepd. by soln. casting through sensitive interparticle and particle-substrate interactions. In this work, we discuss the study on magnetic field induced assembly of mono-dispersed iron oxide NCs to obtain spin filter effect across the superlattice array, when sandwiched between gold electrodes. The deposition of mixed phase Fe3O4@γ-Fe2O3 NCs on SiO2/Au surface proceeds through slow solvent evapn. and are studied for controlled interparticle spacing. For specific NC concn., the ordering depends on the substrate chem. and the ligands passivating NC surface, which affects the concn. of cluster nuclei formed. In presence of a magnetic field, the tunnel structure exhibits enhanced pos. tunnel magnetoresistance at low temps., which could be related to their ferromagnetism and the attempts by electrons to percolate NC superlattice with preserved spin. A sign reversal for magnetoresistance is exhibited by the vertical tunnel junctions on raising the temp.

A graphene-directed assembly route to hierarchically porous Co–Nx/C catalysts for high-performance oxygen reduction

Graphene-directed assembly of ZIF nanocrystal arrays was developed to synthesize hierarchically nanoporous Co–Nx/C materials. The resulting materials exhibit high ORR catalytic activity and superior stability under both alkaline and acidic conditions.

Gold nanocrystal arrays as a macroscopic platform for molecular junction thermoelectrics

Efficiencies of bulk thermoelectric systems have been limited because the Seebeck coefficient and electrical conductivity are typically inversely correlated in traditional materials. Decoupling of these properties has been demonstrated in molecular junctions by capitalizing on the unique electronic transport at organic-inorganic interfaces. In this work, the thermoelectric properties of gold nanocrystal arrays with varying thiol-terminated ligands are compared to molecular junction experiments. The experimental results and supporting theory demonstrate that gold nanocrystal arrays are a valuable model system for mapping the applicability of molecular junction design rules to the design of macroscale organic-inorganic hybrid thermoelectric materials.

Charge Transport in Cu2S Nanocrystals Arrays: Effects of Crystallite Size and Ligand Length

Abstract Quantum confinement effects are observed in transport measurements of Cu2S nanocrystal based devices. Two nanocrystals sizes are studied, 3 nm being in the quantum-confinement regime, and 14 nm, lacking confinement. The effect of ligand length on the charge transport mechanism is studied via conductance temperature dependence measurements. While in the 14 nm nanocrystal based devices unique non-monotonic temperature dependence is observed, the 3 nm based devices show only thermally activated transport for all ligands. The difference is attributed to a cross-over from inter-particle hopping to intra-particle dominated transport as the ligand length increases. In the 3 nm devices the effect of ligand length on the charge-hopping activation energy is also discussed.