Nanocrystal Monolayers Research Articles

Surface-Enhanced Infrared Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays

We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir–Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the three-dimensional electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 μm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted ...

In situ study of the formation mechanism oftwo-dimensional superlattices from PbSenanocrystals.

Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as one misaligned attachment would disrupt the 2D order beyond repair. Here, we demonstrate the formation mechanism of 2D PbSe superstructures with square geometry by using in situ grazing-incidence X-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. We observed nanocrystal adsorption at the liquid/gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liquid substrate, which maximize facet-to-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.

High Hole Mobility in Long‐Range Ordered 2D Lead Sulfide Nanocrystal Monolayer Films

Coupling between colloidal semiconductor nanocrystals (NCs) with long‐range order is critical for designing advanced nanostructures with controlled energy flow and charge carrier transport. Herein, under the premise of keeping long‐range order in 2D NC monolayer, its native organic ligands are exchanged with halogen ions in situ at the liquid–air interface to enhance the coupling between NCs. Further treatments on the films with dimethyl sulfoxide, methanol, or their mixture effectively improve carrier mobility of the devices. The devices show repeatable enhanced p‐type transport behavior with hole mobility of up to 0.224 ± 0.069 cm2 V−1 s−1, the highest value reported for lead sulfide NC solids without annealing treatment. Thanks to accurate control over the surface of NCs as well as the structure of NC film, the ordered NC monolayer film of high hole mobility suggests great potentials for making reliable high performance devices.

Multiexciton generation assisted highly photosensitive CdHgTe nanocrystal skins

Multiexciton Generation (MEG) enabled by the photogeneration of more than one electron-hole pairs upon the absorption of a single photon observed in colloidal semiconductor nanocrystals (NCs) is an essential key to high efficiency when operating in large enough photon energy regimes. Here, we report a newly designed class of solution-processed highly sensitive MEG-assisted photosensors of CdHgTe NCs, in which the charge accumulation is dramatically enhanced for photon energies greater than two times the bandgap of the employed NCs. We fabricated and comparatively studied five types of devices based on different NC monolayers of selected quantum-confined bandgaps resulting in different levels of photovoltage buildup readouts. Among these photosensitive platforms, MEG is distinctly observed for CdHgTe NCs, as the number of electrons trapped inside these NCs and the number of holes accumulating into the interfacing metal electrode were increased beyond a single exciton per absorbed photon. Furthermore, we conducted time-resolved fluorescence measurements and confirmed the occurrence of MEG in the CdHgTe NC monolayer of the photosensor. These findings pave the way for engineering of multiexciton kinetics in high-efficiency NC-based photosensors and photovoltaics.

Exciton and Trion Energy Transfer from Giant Semiconductor Nanocrystals to MoS2 Monolayers

We investigate nonradiative energy transfer (NRET) between CdSe/CdS core/shell “giant” nanocrystal quantum dots (gNQDs) and monolayer domains of molybdenum disulfide (MoS2) grown by chemical vapor deposition. We employ three sets of gNQDs with varied core/shell parameters that exhibit radiative emission from neutral and charged excitons (trions) at different spectral positions from 590 to 660 nm as confirmed by photon statistics of individual nanocrystals. Strong photoluminescence (PL) emission quenching is observed for the donor gNQDs placed on MoS2 domains, indicative of the efficient NRET. Analysis of the double-component PL decays reveals NRET from both neutral excitons and charged trions with the same efficiency. Applying a macroscopic electrodynamics model for the decay of electric-dipole emitters in the vicinity of an ultrathin semiconducting layer with a strong in-plane excitonic polarizability, we confirm high NRET efficiencies from >95% to 85% for dots with diameters from 10 to 20 nm. This demon...

Resonant surface-enhanced Raman scattering by optical phonons in a monolayer of CdSe nanocrystals on Au nanocluster arrays

Here we present the results on an investigation of resonant Stokes and anti- Stokes surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on arrays of Au nanoclusters using the Langmuir–Blodgett technology. The thickness of deposited NCs, determined by transmission and scanning electron microscopy, amounts to approximately 1 monolayer. Special attention is paid to the determination of the localized surface plasmon resonance (LSPR) energy in the arrays of Au nanoclusters as a function of the nanocluster size by means of micro-ellipsometry. SERS by optical phonons in CdSe NCs shows a significant enhancement factor with a maximal value of 2×103 which depends resonantly on the Au nanocluster size and thus on the LSPR energy. The deposition of CdSe NCs on the arrays of Au nanocluster dimers enabled us to study the polarization dependence of SERS. It was found that a maximal SERS signal is observed for the light polarization along the dimer axis. Finally, SERS by optical phonons was observed for CdSe NCs deposited on the structures with a single Au dimer. A difference of the LO phonon energy is observed for CdSe NCs on different single dimers. This effect is explained as the confinement-induced shift which depends on the CdSe nanocrystal size and indicates quasi-single NC Raman spectra being obtained.

Photoinduced charge dissociation and transport at P3HT/ITO interfaces: studied by modulated surface spectroscopy

Results of a temperature dependence study of photoinduced charge separation across P3HT nanocrystals at P3HT/ITO interfaces have been investigated by modulated surface photovoltage (SPV) spectroscopy in a fixed capacitor arrangement. The SPV measurements were correlated with the crystalline sizes of P3HT nanocrystals determined by grazing incidence X-ray diffraction (GIXRD). The crystalline sizes of P3HT nanocrystals were varied systematically by progressive heating/cooling cycles identical for SPV and GIXRD measurements. Photovoltage signals, indication of photoinduced charge dissociation in space, at the P3HT/ITO interface were collected, and electrons were separated across the first monolayer of P3HT nanocrystals at the P3HT/ITO interface due to band bending. The activation energies for quenching of the in-phase and phase-shifted by 90° SPV signals were 0.7 and 0.6 eV, respectively. Thermal activation of the formation of P3HT nanocrystals was of the same order as the enthalpy of fusion of ideal crystals from regioregular P3HT. A schematic drawing of photoinduced charge separation at the P3HT/ITO is proposed.

Flexible and fragmentable tandem photosensitive nanocrystal skins.

We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm(-2) at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.

Combination of surface- and interference-enhanced Raman scattering by CuS nanocrystals on nanopatterned Au structures.

We present the results of a Raman study of optical phonons in CuS nanocrystals (NCs) with a low areal density fabricated through the Langmuir–Blodgett technology on nanopatterned Au nanocluster arrays using a combination of surface- and interference-enhanced Raman scattering (SERS and IERS, respectively). Micro-Raman spectra of one monolayer of CuS NCs deposited on a bare Si substrate reveal only features corresponding to crystalline Si. However, a new relatively strong peak occurs in the Raman spectrum of CuS NCs on Au nanocluster arrays at 474 cm−1. This feature is related to the optical phonon mode in CuS NCs and manifests the SERS effect. For CuS NCs deposited on a SiO2 layer this phonon mode is also observed due to the IERS effect. Its intensity changes periodically with increasing SiO2 layer thickness for different laser excitation lines and is enhanced by a factor of about 30. CuS NCs formed on Au nanocluster arrays fabricated on IERS substrates combine the advantages of SERS and IERS and demonstrate stronger SERS enhancement allowing for the observation of Raman signals from CuS NCs with an ultra-low areal density.

Monolayer of close-packed Pt nanocrystals on a reduced graphene oxide (RGO) nanosheet and its enhanced catalytic performance towards methanol electrooxidation

A close-packed monolayer composed of (111)-orientated Pt nanocrystals was fabricated on reduced graphene oxide, exhibiting excellent electrocatalytic activity and stability towards methanol oxidation, ~3 times better mass activity than the commercial Pt/C.

Nanofabrication of uniform and stabilizer-free self-assembled platinum monolayers as counter electrodes for dye-sensitized solar cells

We report a two-step dip-coating approach for the fabrication of self-assembled monolayers of platinum nanocrystals (SAM-Pt) with a particle size of ∼3 nm and that are uniformly deposited on a transparent conducting oxide (TCO) surface to serve as a counter electrode (CE) for dye-sensitized solar cells (DSSCs). In the first step, we prepared a polyol solution containing H2PtCl6 and ethylene glycol at 110 °C, in which the reduction kinetics were controlled by adding various proportions of NaOH. In the second step, we immersed a thiol-modified TCO substrate into the polyol solution with monodispersed Pt nanoparticles prepared at pH 3.7 at approximately 295 K to complete the nanofabrication. The DSSC devices using Z907 dye as a photosensitizer and the CE prepared using this SAM-Pt approach attained notable photovoltaic performance (η=9.2%) comparable with those fabricated using a conventional thermal decomposition method (η=9.1%) or a cyclic electrodeposition method (η=9.3%) under the same experimental conditions. We emphasize that the SAM-Pt films feature a clean surface, uniform morphology, narrow size distribution, small Pt loading and great catalytic activity; the present approach is hence not only suitable for DSSCs but also applicable for many other energy-related applications that require platinum as an efficient catalyst. Eric Wei-Guang Diau and colleagues at National Chiao Tung University in Taiwan have developed a technique to fabricate uniform platinum layers as electrodes for solar cells. Platinum is a catalyst that plays an important role in many electrochemical applications such as water splitting, fuel cells and solar cells. However, the thick films of platinum that are used add cost to these devices. The researchers have reduced this cost by developing a self-assembly method to deposit monolayers of platinum nanoparticles with a mean size of 3 nanometers at room temperature. The method involves functionalizing an electrode substrate with molecules that specifically attach to the platinum nanoparticles. Hence, when the electrode is dipped into a solution of platinum nanoparticles well prepared by controlling the pH values, a uniformly covered surface forms. In initial tests, these electrodes show very good photovoltaic properties, making the new fabrication technique even more attractive. A novel two-step approach is presented for the fabrication of self-assembled monolayers of platinum nanocrystals (SAM-Pt) uniformly deposited on a transparent conducting oxide (TCO) surface to serve as a counter electrode (CE) for dye-sensitized solar cells (DSSCs). A true self-assembled Pt nanocrystalline monolayer with a mean particle size of ∼3 nm at facet {111} was unambiguously observed in the high-resolution TEM images. We emphasize that the SAM-Pt films feature a clean surface, uniform morphology, narrow size distribution, small Pt loading and great catalytic activity; the present approach is hence not only suitable for DSSCs but is also promising for many other energy-related applications that require platinum as an efficient catalyst to expedite the oxygen reduction reaction (ORR).

Photosensitivity enhancement with TiO2 in semitransparent light-sensitive skins of nanocrystal monolayers.

We propose and demonstrate light-sensitive nanocrystal skins that exhibit broadband sensitivity enhancement based on electron transfer to a thin TiO2 film grown by atomic layer deposition. In these photosensors, which operate with no external bias, photogenerated electrons remain trapped inside the nanocrystals. These electrons generally recombine with the photogenerated holes that accumulate at the top interfacing contact, which leads to lower photovoltage buildup. Because favorable conduction band offset aids in transferring photoelectrons from CdTe nanocrystals to the TiO2 layer, which decreases the exciton recombination probability, TiO2 has been utilized as the electron-accepting material in these light-sensitive nanocrystal skins. A controlled interface thickness between the TiO2 layer and the monolayer of CdTe nanocrystals enables a photovoltage buildup enhancement in the proposed nanostructure platform. With TiO2 serving as the electron acceptor, we observed broadband sensitivity improvement across 350-475 nm, with an approximately 22% enhancement. Furthermore, time-resolved fluorescence measurements verified the electron transfer from the CdTe nanocrystals to the TiO2 layer in light-sensitive skins. These results could pave the way for engineering nanocrystal-based light-sensing platforms, such as smart transparent windows, light-sensitive walls, and large-area optical detection systems.

Nanocrystal Skins with Exciton Funneling for Photosensing

Highly photosensitive nanocrystal (NC) skins based on exciton funneling are proposed and demonstrated using a graded bandgap profile across which no external bias is applied in operation for light-sensing. Four types of gradient NC skin devices (GNS) made of NC monolayers of distinct sizes with photovoltage readout are fabricated and comparatively studied. In all structures, polyelectrolyte polymers separating CdTe NC monolayers set the interparticle distances between the monolayers of ligand-free NCs to <1 nm. In this photosensitive GNS platform, excitons funnel along the gradually decreasing bandgap gradient of cascaded NC monolayers, and are finally captured by the NC monolayer with the smallest bandgap interfacing the metal electrode. Time-resolved measurements of the cascaded NC skins are conducted at the donor and acceptor wavelengths, and the exciton transfer process is confirmed in these active structures. These findings are expected to enable large-area GNS-based photosensing with highly efficient full-spectrum conversion.

Fabrication of highly stable silica coated ZnCuInS nanocrystals monolayer via layer by layer deposition for LED application

In this study, non-toxic and highly stable silica coated ZnCuInS NCs were synthesized by a reverse microemulsion method. The single NCs were uniformly encapsulated in a silica shell with a diameter of ~30 nm. Although hydrolyzed TEOS caused a QY reduction, and a 12.5 nm red shift occurred after silica coating, the photo and thermal stabilities were extremely improved. For LED application, the silica coated ZnCuInS NCs phosphor layer was arrayed on the InGaN LED surface by layer-by-layer deposition utilizing electrostatic attraction. When the ZnCuInS/SiO2 NCs single monolayer was fabricated, 6.73% high color conversion efficiency was achieved.

The facile assembly of nanocrystals by optimizing humidity.

The ambient humidity and the nature of substrates are considered coordinately in the assembly of nano-sized crystals. The nanocrystal monolayers show large-area uniformity without any aggregates. Zeolite and hematite monolayers with thicknesses of 20-100 nm and excellent orientations are produced.

Alumina-coated Ag nanocrystal monolayers as surfaceenhanced Raman spectroscopy platforms for the direct spectroscopic detection of water splitting reaction intermediates

A novel Ag-alumina hybrid surface-enhanced Raman spectroscopy (SERS) platform has been designed for the spectroscopic detection of surface reactions in the steady state. Single crystalline and faceted silver (Ag) nanoparticles with strong light scattering were prepared in large quantity, which enables their reproducible self-assembly into large scale monolayers of Raman sensor arrays by the Langmuir-Blodgett technique. The close packed sensor film contains high density of sub-nm gaps between sharp edges of Ag nanoparticles, which created large local electromagnetic fields that serve as “hot spots” for SERS enhancement. The SERS substrate was then coated with a thin layer of alumina by atomic layer deposition to prevent charge transfer between Ag and the reaction system. The photocatalytic water splitting reaction on a monolayer of anatase TiO2 nanoplates decorated with Pt co-catalyst nanoparticles was employed as a model reaction system. Reaction intermediates of water photo-oxidation were observed at the TiO2/solution interface under UV irradiation. The surface-enhanced Raman vibrations corresponding to peroxo, hydroperoxo and hydroxo surface intermediate species were observed on the TiO2 surface, suggesting that the photo-oxidation of water on these anatase TiO2 nanosheets may be initiated by a nucleophilic attack mechanism.

Monolayer FePt nanocrystal self-assembly embedded into atomic-layer-deposited Al2O3 films for nonvolatile memory applications

A simple approach is developed to fabricate oxide/FePt nanocrystals/oxide composite films by a combination of chemically-synthesized FePt nanocrystals’ self-assembly and atomic layer deposition for ultrahigh-density nonvolatile memory applications. A hexagonally arranged monolayer of well-monodispersed FePt nanocrystals with a grain size of 4.5nm has been assembled onto atomic-layer-deposited Al2O3 oxide by solution-based dip-coating. The lattice constant of this two-dimensional pattern is about 8nm with the high density of 1.8×1012/cm2. A fraction of the Fe is oxidized during annealing at 500°C for 5min in O2 atmosphere, and the core–shell structure is formed with fcc-Fe0.75Pt nanocrystal core and amorphous Fe2O3 shell. The metal–oxide–Si capacitors with unannealed and annealed FePt nanocrystals embedded into Al2O3 films are electrically measured, and exhibit obvious memory effects with a hysteresis memory window of 4.1 and 8.1V at the sweeping gate voltage of±8V, respectively. The enhanced memory window of samples with annealed FePt nanocrystals can be attributed to the existence of the Fe2O3 shell, which introduces additional interface and provides more trap sites for charge trapping storage.

Electrical bistability in monolayers of cobalt-doped ZnO nanocrystals: Effect of alignment of their magnetic domains

We grow a range of cobalt-doped ZnO nanocrystals, form a monolayer through electrostatic assembly approach, and characterize the nanostructures with a scanning tunneling microscope tip. The scanning tunneling spectroscopy results show electrical bistability, that is, presence of a low- and a high-conducting state, in such monolayers under application of a voltage pulse. The bistability depends on the content of cobalt in the nanocrystals. With the use of such ferromagnetic nanocrystals, we align the magnetization vector of the nanocrystals in a monolayer in order to study the effect of alignment of their magnetic domains on the electrical bistability. We observe that the transport gap of the nanocrystals decreases upon conductance switching. The gap decreases also due to alignment of the magnetic domains of the cobalt-doped nanocrystals. When the magnetically aligned nanocrystals undergo a conductance switching, the transport gap further decreases evidencing a correlation between the electrical bistability and the (change in) transport gap.

Metal nanoparticle field-effect transistor

We demonstrate that by means of a local top-gate current oscillations can be observed in extended, monolayered films assembled from monodisperse metal nanocrystals—realizing transistor function. The oscillations in this metal-based system are due to the occurrence of a Coulomb energy gap in the nanocrystals which is tunable via the nanocrystal size. The nanocrystal assembly by the Langmuir-Blodgett method yields homogeneous monolayered films over vast areas. The dielectric oxide layer protects the metal nanocrystal field-effect transistors from oxidation and leads to stable function for months. The transistor function can be reached due to the high monodispersity of the nanocrystals and the high super-crystallinity of the assembled films. Due to the fact that the film consists of only one monolayer of nanocrystals and all nanocrystals are simultaneously in the state of Coulomb blockade the energy levels can be influenced efficiently (limited screening).

Mapping the Energy Band Structure of Nanocrystal Monolayers under Ambient Conditions

With increasing demand for scale minimization of integrated circuit, nanostructures are likely to become primary components of future electronic devices. Self-assembled hybrid organic molecule–nanocrystal devices propose a prospective route for achieving this goal. In this type of devices, the characterization of the energy band structure of the semiconductor nanocrystal layers is of major importance. Recent works used scanning tunneling spectroscopy to measure locally the single nanocrystal band structure. These measurements are expensive, complicated, and usually done at low temperatures. In this work, we map the energy band structure of a monolayer using a simple hybrid device under ambient conditions. The ensemble band structure is similar to results achieved by scanning tunneling microscopy. In most cases, the energy band edges are wider due to the size distribution in consistence with photoluminescence results.