Nanocrystal Interface Research Articles

Effect of the interface on UV–vis–IR photodetection performance of PbS/ZnO nanocomposite photocatalysts

Two PbS/ZnO nanocomposites with the same Pb/Zn molar ratio of 1:12 and different PbS/ZnO interface were prepared by depositing PbS nanocrystals on nano ZnO with the reaction between Pb(NO3)2 and Na2S through changing the sequence of adding Pb(NO3)2 and Na2S to the nano ZnO suspension: (A) first adding Pb(NO3)2 followed by adding Na2S (denoted PbS/ZnO-A); (B) first adding Na2S followed by adding Pb(NO3)2 (denoted PbS/ZnO-B). The PbS/ZnO nanocomposites are characterized by XRD, BET, Raman, TEM, XPS, and UV–vis–IR. The characterizations indicate that PbS/ZnO-A has an interface of PbS nanocrystal closely contacted to ZnO nanocrystal while PbS/ZnO-B has an interface of a disordered layer between PbS nanocrystal and ZnO nanocrystal. It is found for the first time that the PbS/ZnO interface plays an important role in their photodetection performance. PbS/ZnO-A exhibits much higher photoresponse current and lower rise and recovery time than both PbS/ZnO-B and a mixture of nano PbS and ZnO with the same Pb/Zn molar ratio as PbS/ZnO-A for visible and near-infrared photodetection. PL and the impedance measurement in dark and irradiation reveal that the superior photodetection performance of PbS/ZnO-A over PbS/ZnO-B is attributed to its lower e–h recombination and migration resistance under the irradiation of visible and infrared light due to its very good PbS/ZnO interface of PbS nanocrystals closely attached ZnO nanocrystals, through which photogenerated electrons inject efficiently from the conduction band of PbS to that of ZnO. In contrast, the defect sites in the disordered layer between PbS nanocrystal and ZnO nanocrystal for PbS/ZnO-B act as e–h recombination centers, significantly decreasing the e–h separation efficiency.

Formation of size-controlled and luminescent Si nanocrystals from SiOxNy/Si3N4 hetero-superlattices

Silicon nanocrystals formed in the annealed SiNx/Si3N4 superlattices are attractive for research due to the smaller band offsets of Si3N4 matrix to Si in comparison with commonly used SiOx/SiO2 superlattices. However, the annealed SiNx/Si3N4 structures contain an increased number of nanocrystal interface defects, which completely suppress nanocrystal emission spectrum. In this work, we study a novel SiOxNy/Si3N4 hetero multilayer combination, which compromises the major issues of SiOx/SiO2 and SiNx/Si3N4 superlattices. The annealed SiOxNy/Si3N4 superlattices are investigated by TEM, demonstrating a precise sublayer thicknesses control. The PL spectra of the annealed SiOxNy/Si3N4 superlattices are centered at 845–950 nm with an expected PL peak shift for silicon nanocrystals of different sizes albeit the PL intensity is drastically reduced as compared to SiO2 separation barriers. The comparison of PL spectra of annealed SiOxNy/Si3N4 superlattice with those of SiOxNy/SiO2 superlattice enables the analysis of the interface quality of silicon nanocrystals. Using the literature data, the number of the interface defects and their distribution on the nanocrystal facets are estimated. Finally, it is shown that the increase of the Si3N4 barrier thickness leads to the increased energy transfer from the Si nanocrystals into the Si3N4 matrix, which explains an additional drop of the nanocrystal PL intensity.

Study of surfactant-free lead sulfide nanocrystals-P3HT hybrid polymer solar cells

Surfactant free nanocrystals (NCs) of lead sulfide (PbS) prepared using Continuous Spray Pyrolysis (CoSP) technique had been dispersed in poly (3-hexylthiophene) (P3HT) and used in hybrid polymer solar cells (HSCs) to study the role of surfactant free NCs in these devices. Absence of surfactant on PbS NCs improves the polymer–NCs interface and is responsible for an efficient electronic interaction between the two materials. FTIR results show a shift in the characteristics band for the blends confirming the proper bonding between the polymer and NCs. Hybrid polymer solar cells with configuration ITO/PEDOT:PSS/P3HT:PbS/Al have been fabricated with different PbS loading. With the increase in PbS NC content, the device efficiency increases first and then decreases, the device having 30wt% of PbS NCs shows the best performance with an energy conversion efficiency of 1.6%, an open circuit voltage of 0.42V, a short-circuit current density of 9.90mA/cm2, and a fill factor of 0.40 under Air Mass 1.5G illumination with the irradiation of 100mW/cm2. Structural and morphological investigations show that better performance is achieved with the optimum amount of PbS NCs due to good distribution and interconnection in the polymer layer.

Elaboration of cellulose based nanobiocomposite: Effect of cellulose nanocrystals surface treatment and interface “melting”

The present work deals with the elaboration of nanobiocomposite composed of a cellulose derivative matrix reinforced by cellulose nanocrystals. First, thermoplastic cellulose matrix was obtained by cellulose esterification with lauroyl chloride. Cellulose nanocrystals (NCC), as reinforcement, were obtained by sulfuric acid hydrolysis of cellulose and characterized by AFM. Surface of cellulose nanocrystals (NCC) was then modified by lauroyl chloride in toluene (non swelling solvent), before being used as reinforcing elements in nanobiocomposite. Both cellulose modification and NCC surface modification were proved by different characterizations techniques: FT-IR, contact angle measurements, elemental analysis and thermogravimetric analysis (TGA). Successful surface modification has been shown with rather high degree of substitution (DS). Structural changes of NCC were checked by X-ray Diffraction analyses. Nanocomposites with surface modified and unmodified cellulose nanocrystals as reinforcements were elaborated by casting process. Thermal post-treatment has been performed to “melt” the cellulose derivatives interface. Homogenous nanocomposite films were obtained by film casting process when NCC was grafted, whereas unmodified NCC did not yield the same result. Thermal and mechanical properties were performed with DMA. When NCCs were grafted and thermopressed with the matrix, positive results were obtained proving the efficiency of this new compatibilization process by interface “melting”.

A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells.

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials. Here we perform a systematic study on PbS nanocrystal-based diodes using temperature-dependent current–voltage characterization and thermal admittance spectroscopy to develop a model for charge transport that is applicable to different nanocrystal-solids and device architectures. Our analysis confirms that charge transport occurs in states that derive from the quantum-confined electronic levels of the individual nanocrystals and is governed by diffusion-controlled trap-assisted recombination. The current is limited not by the Schottky effect, but by Fermi-level pinning because of trap states that is independent of the electrode–nanocrystal interface. Our model successfully explains the non-trivial trends in charge transport as a function of nanocrystal size and the origins of the trade-offs facing the optimization of nanocrystal-based solar cells. We use the insights from our charge transport model to formulate design guidelines for engineering higher-performance nanocrystal-based devices.

Reversible conductance switching characteristics in a polymer-In2O3 nanocrystals junction

A transparent polymer-based resistive switching device containing In2O3 nanocrystals (NCs) is fabricated, and its nonvolatile memory characteristics are evaluated. Very clear reversible counter-clockwise bipolar-type resistive switching phenomena are observed. Stable retention is demonstrated. An Analysis of the temperature dependence of the bistable resistance states reveals additional features, not reported in previous studies, that the observed resistance switching is due to oxygen ions drift-induced redox reactions at the polymer/In2O3 NCs interface. The RESET and SET switching times (τRESET and τSET), which are defined as pulse widths extrapolated by the steepest slopes in the transition region, are τRESET ∼ 550 nsec and τSET ∼ 900 nsec. The authors propose that microscopic potential modification occurring near the polymer/In2O3 NCs boundaries plays a key role in determining resistive switching properties.

Structural analysis of the interface of silicon nanocrystals embedded in a Si3N4 matrix

The structure and interface states of thin nanocomposite layers containing Si nanocrystals embedded in an amorphous nitride matrix have been analyzed by Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The Si 2p core-level spectrum of the nanocomposite layer was deconvoluted by using five Gaussian–Lorentzian contributions corresponding to the different nitride states of Si. It was shown that the Si-ncs/Si3N4 interfaces formed during annealing are composed of a high density of Si2+ subnitrides. These subnitrides are more likely to be responsible for the shoulder at 494 cm−1 on the Raman spectra. Considering the proportion of Si2+ subnitrides with respect to Si0 states, an abrupt transition from the crystalline to the amorphous phase due to Si2=N–Si bridge bonds is suggested.

Broadband second-harmonic and sum-frequency generation in KH2PO4 crystals doped with anatase nanocrystals

Broadband second-harmonic (SH) and sum-frequency generation was studied in KH2PO4 crystals with incorporated anatase modification of titania nanocrystals. A tiny amount (10−5 wt.%) of titania nanoparticles in the anatase phase exhibits a pronounced (up to 15%) increase of the SH efficiency in comparison with the nominally pure potassium dihydrogen phosphate (KDP) crystal. Higher anatase fraction (10−4 wt.%) results in a reduction of the SH conversion efficiency. The rise of the SH signal is attributed to resonant excitation of the defect states at the nanocrystal interface, and its subsequent decrease at higher concentration is likely to be related to a light scattering in the crystal bulk.

Fabrication, microstructure and microwave absorption of multi-walled carbon nanotube decorated with CdS nanocrystal

In this work, CdS–MWCNT heterostructure was achieved via a one-step approach. CdS–MWCNT composites with wax matrix were prepared. The composites show high permittivity and excellent microwave absorption. Compared to the MWCNT composites, microwave absorption of the CdS–MWCNT composites with the same mass ratio increases to about 1–1.6 times, and the increment of bandwidth is over 1GHz. The enhanced attenuation capacity arises from CdS nanocrystal and heterostructure interface. Cooperation of interface polarization and conductivity inside the composites increases the microwave attenuation of the composites. Furthermore, light weight is achieved due to the decreasing thickness of the CdS–MWCNT composites. The results suggest that the CdS–MWCNT composite is a highly promising candidate for microwave attenuation in many application fields.

Hybrid material design for energy applications: impact of graphene and carbon nanotubes

Abstract This article reviews the origin and progress of inorganic/nanocarbon hybrid material research in my Ph.D. career. Building chemical bonds between inorganic active materials and nanocarbon substrates is the key to controlled hybrid material synthesis that allows for controlling the size and morphology of the materials and enhancing electron transport within the electrodes. Our inorganic/nanocarbon hybrid materials manifest superior electrochemical performance for asymmetrical supercapacitors, ultrafast nickel-iron batteries, lithium ion batteries, lithium-sulfur batteries, and electrocatalysis of oxygen reduction and evolution reactions. X-ray absorption near edge structure (XANES) spectroscopy has been utilized to characterize the chemical bonding and charge transfer at the interface of inorganic nanocrystals and nanocarbon substrates. Probing the physical and chemical states of the hybrid materials under electrochemical reaction conditions is an important future direction in this area.

Highly efficient hybrid solar cells with tunable dipole at the donor-acceptor interface.

Effects of molecular dipole at the conjugated polymer-nanocrystal interface on the energy level alignment, the exciton dissociation process, and consequently the photovoltaic performance of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):CdSe quantum dot bulk heterojunction hybrid solar cells are systemically studied. Power conversion efficiency up to 4.0% is achieved when 4-fluorobenzenethiol is used for ligand exchange.

Achieving high sensitivity in hybrid photodetectors based on an organic single crystal and an inorganic nanocrystal array

We demonstrate an approach to enhance the photosensitivity of an organic single-crystal photodetector by combining it with a nanocrystal (NC) array. A systematic study of the dependence of the photodetector performance on illumination wavelength and light power together with the charge carrier mobility reveals that charge separation at the single-crystal/CdSe NC interface and subsequent electron trapping in the CdSe NCs generates effective photoconductive gain by hole circulation through the high-mobility single crystal. As a result, the responsivity and specific detectivity reached values up to 50 A W−1 and 2 × 109 cm Hz1/2 W−1, respectively.

Constructing Functional Mesostructured Materials from Colloidal Nanocrystal Building Blocks

Through synthesizing colloidal nanocrystals (NCs) in the organic phase, chemists gain fine control over their composition, size, and shape. Strategies for arranging them into ordered superlattices have followed closely behind synthetic advances. Nonetheless, the same hydrophobic ligands that help their assembly also severely limit interactions between adjacent nanocrystals. As a result, examples of nanocrystal-based materials whose functionality derives from their mesoscale structure have lagged well behind advances in synthesis and assembly. In this Account, we describe how recent insights into NC surface chemistry have fueled dramatic progress in functional mesostructures. In these constructs, intimate contact between NCs as well as with heterogeneous components is key in determining macroscopic behavior. The simplest mesoscale assemblies we consider are networks of NCs constructed by in situ replacement of their bulky, insulating surface ligands with small molecules. Transistors are a test bed for understanding conductivity, setting the stage for new functionality. For instance, we demonstrated that by electrochemically charging and discharging networks of plasmonic metal oxide NCs, the transmittance of near infrared light can be strongly and reversibly modulated. When we assemble NCs with heterogeneous components, there is an even greater potential for generating complex functionality. Nanocomposites can exhibit favorable characteristics of their component materials, yet the interaction between components can also have a strong influence. Realizing such opportunities requires an intimate linking of embedded NCs to the surrounding matrix phase. We accomplish this link by coordinating inorganic anionic clusters directly to NC surfaces. By exploiting this connection, we found enhanced ionic conductivity in Ag2S-in-GeS2 nanocrystal-in-glass electrodes. In another example, we also found enhanced optical contrast when linking electrochromic niobium oxide to embedded tin-doped indium oxide (ITO) NCs. These dramatic effects emerge from reconstruction of the inorganic glass immediately adjacent to the NC interface. When co-assembling NCs with block copolymers, direct coordination of the polymer to NC surfaces again opens new opportunities for functional mesoscale constructs. We strip NCs of their native ligands and design block copolymers containing a NC tethering domain that bonds strongly, yet dynamically, to the resulting open coordination sites. This strategy enables their co-assembly at high volume fractions of NCs and leads to well-ordered mesoporous NC networks. We find these architectures to be exceptionally stable under chemical transformations driven by cation insertion, removal, and exchange. These developments offer a modular toolbox for arranging NCs deliberately with respect to heterogeneous elements and open space. We have control over metrics that define such architectures from the atomic scale (bonding and crystal structure) through the mesoscale (crystallite shapes and sizes and pore dimensions). By tuning these parameters and better understanding the interactions between components, we look forward to boundless opportunities to employ mesoscale structure, in tandem with composition, to develop functional materials.

ChemInform Abstract: Mechanisms for Charge Trapping in Single Semiconductor Nanocrystals Probed by Fluorescence Blinking

AbstractReview: tutorial review; 54 refs.

Enhancing the Efficiency of Solution-Processed Polymer:Colloidal Nanocrystal Hybrid Photovoltaic Cells Using Ethanedithiol Treatment

Advances in colloidal inorganic nanocrystal synthesis and processing have led to the demonstration of organic-inorganic hybrid photovoltaic (PV) cells using low-cost solution processes from blends of conjugated polymer and colloidal nanocrystals. However, the performance of such hybrid PV cells has been limited due to the lack of control at the complex interfaces between the organic and inorganic hybrid active materials. Here we show that the efficiency of hybrid PV devices can be significantly enhanced by engineering the polymer-nanocrystal interface with proper chemical treatment. Using two different conjugated polymers, poly(3-hexylthiophene) (P3HT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), we show that treating the polymer:nanocrystal hybrid film in an ethanedithiol-containing acetonitrile solution can increase the efficiency of the hybrid PV devices by 30-90%, and a maximum power conversion efficiency of 5.2 ± 0.3% was obtained in the PCPDTBT:CdSe devices at 0.2 sun (AM 1.5G), which was slightly reduced to 4.7 ± 0.3% at 1 sun. The ethanedithiol treatment did not result in significant changes in the morphology and UV-vis optical absorption of the hybrid thin films; however, infrared absorption, NMR, and X-ray photoelectron spectroscopies revealed the effective removal of organic ligands, especially the charged phosphonic acid ligands, from the CdSe nanorod surface after the treatment, accompanied by the possible monolayer passivation of nanorod surfaces with Cd-thiolates. We attribute the hybrid PV cell efficiency increase upon the ethanedithiol treatment to the reduction in charge and exciton recombination sites on the nanocrystal surface and the simultaneous increase in electron transport through the hybrid film.

Radiative and nonradiative recombination of self-trapped exciton on silicon nanocrystal interface

The theory of the multiphonon and radiative recombination of a selftrapped exciton on the interface of a sil� icon nanocrystal in a SiO2 matrix is developed. Selftrapped excitons play a key role in the hot carrier dynam� ics in nanocrystals under photoexcitation. The ratio of the probabilities of the multiphonon and radiative recombination of the selftrapped exciton is estimated. The probabilities of exciton tunnel transition from the selftrapped state to a nanocrystal are calculated for na nocrystals of various sizes. The infrared range spec� trum of the luminescence of the selftrapped exciton is obtained.

Mechanisms for charge trapping in single semiconductor nanocrystals probed by fluorescence blinking

Semiconductor nanocrystal optical and charge transport properties are largely influenced by the trapping of charge carriers on the nanocrystal surface. Charge trapping increases the non-radiative exciton decay pathways, thus decreasing the fluorescence quantum yield, and it also impedes efficient charge transfer at the nanocrystal interface. On a single nanocrystal basis, charge trapping causes interruptions in the otherwise continuous fluorescence known as fluorescence intermittency or blinking. In this tutorial review we examine the relationship between charge trapping and fluorescence blinking. The fluorescence microscopy techniques and statistical analysis methods used to measure single nanocrystal blinking are introduced. The development of numerous blinking mechanisms is reviewed, as is the physical nature of charge trapping sites. An overview of blinking experiments used to probe specific mechanisms for charge carrier trapping is presented. Finally, a summary and outlook are offered. Although the detailed mechanism is not fully understood, blinking experiments are found to provide direct evidence for several charge trapping mechanisms and report on changes to the nature and distribution of charge trapping sites.

Defect engineering of Si nanocrystal interfaces

AbstractThe photoluminescence of Si nanocrystals is often assigned to exciton recombination confined within the nanocrystals. However, localized radiative or non‐radiative defects at the nanocrystal (NC) interface can drastically influence the NCs ensemble and reduce the luminescence output. In addition, size of the NCs as well as embedding host are playing a major role and have a significant influence on the optical properties. Here we summarize our work on size controlled Si NCs based on the SiOx/SiO2 superlattice approach. We will discuss how to distinguish between defects at the NC‐Si/SiO2 interface and defects in the matrix and demonstrate ways for effective NC surface passivation. The influence of different annealing ambient used for crystallization will be shown. Such perfectly defect passivated and size controlled samples will be used as a model system to discuss size dependent quantum properties such as the temperature dependent blue shift of the Si NC band gap.

Si Nanocrystals Embedded in SiO2 Produced by Reactive Sputtering for Light Emission

In the present work, we confectioned light emitting SiOx (x<2) samples using reactive sputtering. By controlling the oxygen concentration in the deposition chamber (range from 0.5% up to 10), an excess of silicon was created in the formed silicon oxide films. Only after a thermal annealing (temperatures from 1050ºC to 1100ºC), the samples showed the photoluminescence (PL) effect. Silicon nanocrystals formed during the thermal annealing were the responsible for the observed PL signal when excited with the 488nm line of argon laser. We also performed thermal annealing at 450ºC, using forming gas ambient to passivate the silicon nanocrystals interface. The result was a significant improvement in PL signal. Furthermore, the Rutherford Backscattering Spectrometry experiment showed that the deposition time is an important parameter to create the silicon excess and achieve light emission.

Light Emission from Nanocrystalline Si Inverse Opals and Controlled Passivation by Atomic Layer Deposited Al2O3

3D-nanostructured ensembles of silicon nanocrystals forming inverse opals, directly prepared by magnesioreduction, exhibited intense photoluminescence in a broadband range (visible and NIR). Light emission, which strongly diminished under photoexcitation, was preserved by engineering the nanocrystals interface with atomic layer deposition of alumina. The high accessibility of the nanocrystals throughout the self-supported structure was a key factor for efficient photoluminescence stabilization.