Electronic excitation transfer from an organic matrix to CdS nanocrystals produced by the Langmuir–Blodgett method

The absorption, photoluminescence, and photoluminescence excitation spectra of CdS nanocrystals formed by the Langmuir–Blodgett method are explored. Features of the absorption and photoluminescence excitation spectra defined by optical transitions in the matrix and nanocrystals are identified. The efficiency of electronic excitation transfer from an organic matrix to nanocrystals is studied. It is shown that charge carriers efficiently transfer from the matrix to electron and hole size-quantization levels in nanocrystals and to acceptor defect levels in the band gap of nanocrystals. A large Stokes shift defined by fine exciton structure (bright and dark excitons) is observed. The shift is in the range 140–220 meV for nanocrystals 2.4 and 2.0 nm in radius.     

Intraband Cooling in All‐Inorganic and Hybrid Organic–Inorganic Perovskite Nanocrystals

Intraband relaxation in all‐inorganic cesium lead tribromide (CsPbBr₃) and hybrid organic–inorganic formamidinium lead tribromide (FAPbBr₃) nanocrystals is experimentally investigated for a range of particle sizes, excitation energies, sample temperatures, and excitation fluences. Hot carriers in CsPbBr₃ nanocrystals consistently exhibit slower cooling than FAPbBr₃ nanocrystals in the single electron–hole pair per nanocrystal regime. In both compositions, long‐lived hot carriers (>3 ps) are only observed at excitation densities corresponding to production of multiple electron–hole pairs per nanocrystal—and concomitant Auger recombination. These presented results are distinct from previous reports in bulk hybrid perovskite materials that convey persistent hot carriers at low excitation fluences. Time‐resolved photoluminescence confirms the rapid cooling of carriers in the low‐fluence (single electron–hole pair per nanocrystal) regime. Intraband relaxation processes, as a function of excitation energy, size, and temperature are broadly consistent with other nanocrystalline semiconductor materials.     

Tunneling transport through passivated CdS nanocrystal arrays grown by the Langmuir-Blodgett method

Tunneling electron transport through CdS nanocrystal arrays fabricated by the Langmuir-Blodgett method are studied by scanning electron spectroscopy. The effect of the matrix-annealing atmosphere on tunneling transport through the nanocrystal arrays is studied. Electron capture at traps in the case of nanocrystals annealed in vacuum is detected by tunneling current-voltage characteristics analyzed using a model relating the data of tunneling spectroscopy, photoluminescence, and quantum-mechanical calculation. Analysis shows that the nanocrystal surface is passivated by an ammonia monolayer upon annealing in an ammonia atmosphere. It is found that the substrate and surrounding non-passivated nanocrystals have an effect on the electron polarization energy.     

InAs Nanocrystals with Robust p-Type Doping

Robust control over the carrier type is fundamental for the fabrication of nanocrystal-based optoelectronic devices, such as the p–n homojunction, but effective incorporation of impurities in semiconductor nanocrystals and its characterization is highly challenging due to their small size. Herein, InAs nanocrystals (NCs), post-synthetically doped with Cd, serve as a model system for successful p-type doping of originally n-type InAs nanocrystals, as demonstrated in field effect transistors (FETs). Advanced structural analysis, using atomic resolution electron microscopy and synchrotron X-ray absorption fine structure spectroscopy reveal that Cd impurities reside near and on the nanocrystal surface acting as substitutional p-dopants replacing Indium. Commensurately, Cd-doped InAs FETs exhibit remarkable stability of their hole conduction, mobility, and hysteretic behavior over time when exposed to air, while intrinsic InAs NCs FETs are easily oxidized and their performance quickly declines. Therefore, Cd plays a dual role acting as a p-type dopant, and also protects the nanocrystals from oxidation, as evidenced directly by X-ray photoelectron spectroscopy measurements of air exposed samples of intrinsic and Cd-doped InAs NCs films. This study demonstrates robust p-type doping of InAs nanocrystals, setting the stage for implementation of such doped nanocrystal systems in printed electronic devices.     

Organic and Inorganic Blocking Layers for Solution‐Processed Colloidal PbSe Nanocrystal Infrared Photodetectors

A PbSe solution‐processed nanocrystal‐based infrared photodetector incorporating carrier blocking layers is demonstrated, and significant reduction of dark current is achieved. Detectivity values close to 10¹² Jones are achieved by using poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] (TFB) and ZnO nanocrystals (NC) as the electron blocker and hole blocker, respectively. An improvement in lifetime is also observed in the devices with the ZnO NCs hole blocking layer.     

CdS0.1Se0.9纳米晶体的电光性质

对嵌埋于玻璃基体中的ＣｄＳ０．１Ｓｃ０．９纳米晶体进行了室温电调制透射谱测量，观察到较强的共振和非共振电光响应倚号（１０－４～１０－３）．共振电光响应具有如下特征：同一组分的纳米晶体其倩号的峰位随着尺寸减小向高能方向移动；同一样品谱线形状不随外电场强度而变；信号幅度与外电场强度的平方成线性关系，并且随调制频率的增加而减小；共振电光效应的物理机制是量子受限的Ｓｔａｒｋ效应．非共振电光响应信号呈与波长有关的振荡线形；外电场强度增加，非共振电光响应信号幅度也增加；用介电受限的局域场增强理论解释了非共振电光效应的物理机制．     

Molecular‐Level Switching of Polymer/Nanocrystal Non‐Covalent Interactions and Application in Hybrid Solar Cells

Hybrid composites obtained upon blending conjugated polymers and colloidal semiconductor nanocrystals are regarded as attractive photo­active materials for optoelectronic applications. Here it is demonstrated that tailoring nanocrystal surface chemistry permits to control non‐covalent and electronic interactions between organic and inorganic components. The pending moieties of organic ligands at the nanocrystal surface are shown to not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of this approach to photovoltaic applications is demonstrated for composites based on poly(3‐hexylthiophene) and lead sulfide nanocrystals, considered as inadequate until this report, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3%. By investigating (quasi)steady‐state and time‐resolved photo‐induced processes in the nanocomposites and their constituents, it is ascertained that electron transfer occurs at the hybrid interface yielding long‐lived separated charge carriers, whereas interfacial hole transfer appears hindered. Here a reliable alternative aiming to gain control over macroscopic optoelectronic properties of polymer/nanocrystal composites by mediating their non‐covalent interactions via ligands' pending moieties is provided, thus opening new possibilities towards efficient solution‐processed hybrid solar cells.     

A novel optical bistable multiple-quantum-well phase modulator

The authors present a new type of optically bistable phase modulator utilizing a self electrooptic effect device (SEED) integrated with an electrooptic wavelength modulator. An electrically bistable SEED, operating on the principle of the quantum-confined Stark effect, controls the bias voltage across an electrooptic waveguide phase modulator to produce optical bistability. A control signal at 0.848 μm, corresponding to the first electron to heavy hole exciton transition in GaAs/AlGaAs multiple-quantum-well is used to switch 1.152 μm light propagating through a waveguide in a direction normal to the control beam     

纳米晶浮栅存储器的模拟、制备和电学特性

介绍了在纳米晶浮栅存储器数据保持特性方面的研究工作,重点介绍了纳米晶材料的选择与制备和遂穿介质层工程。研究证明,金属纳米晶浮栅存储器比半导体纳米晶浮栅存储器具有更好的电荷保持特性。并且金属纳米晶制备方法简单,通过电子束蒸发热退火的方法就能够得到质量较好的金属纳米晶,密度约4×1011cm-2,纳米晶尺寸约6～7nm。实验证明,高介电常数隧穿介质能够明显改善浮栅存储器的电荷保持特性,所以在引入金属纳米晶和高介电常数遂穿介质之后,纳米晶浮栅存储器可能成为下一代非挥发性存储器的候选者。     

Electron-electron interaction and spin-orbit coupling in InAs/AlSb heterostructures with a two-dimensional electron gas

The effect of electron-electron interaction on the spectrum of two-dimensional electron states in InAs/AlSb (001) heterostructures with a GaSb cap layer with one filled size-quantization subband. The energy spectrum of two-dimensional electrons is calculated in the Hartree and Hartree-Fock approximations. It is shown that the exchange interaction decreasing the electron energy in subbands increases the energy gap between subbands and the spin-orbit splitting of the spectrum in the entire region of electron concentrations, at which only the lower size-quantization band is filled. The nonlinear dependence of the Rashba splitting constant at the Fermi wave vector on the concentration of two-dimensional electrons is demonstrated.     

Characterization and Properties of Nickel Aluminide Nanocrystals in an Alumina Layer for Nonvolatile Memory Applications

Intermetallic nanocrystal memory devices with nickel aluminide nanocrystals in the electron-trapping layer and an alumina layer as the blocking layer were prepared on the surface of oxidized silicon substrates by sputter-coating of Ni and Al₂O₃ in sequence, followed by an annealing procedure. Several aluminide nanocrystal memory devices are reported. The effect of annealing at 900°C on the memory properties was investigated. Intermetallic nanocrystals were identified by high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy as Ni₂Al₃ with sizes of 15–20 nm. The results showed that a sixfold increase (0.37 V to 2.34 V) in the memory window could be achieved after annealing for the optimal time of 3 min.     

The influence of irradiation and subsequent annealing on Si nanocrystals formed in SiO2 layers

Luminescent Si nanocrystals formed in SiO₂ layers were irradiated with electrons and He⁺ ions with energies of 400 and 25–130 keV, respectively. The effects of irradiation and subsequent annealing at 600–1000°C were studied by the methods of photoluminescence and electron microscopy. After irradiation with low doses (～1 displacement per nanocrystal), it was found that photoluminescence of nanocrystals was quenched but the number of them increased simultaneously. After irradiation with high doses (～10³ displacements per nanocrystal), amorphization was observed, which is not characteristic of bulk Si. The observed phenomena are explained in terms of the generation of point defects and their trapping by Si-SiO₂ interfaces. Photoluminescence of nanocrystals is recovered at annealing temperatures below 800°C; however, an annealing temperature of about 1000°C is required to crystallize the precipitates. An enhancement of photoluminescence observed after annealing is explained by the fact that the intensities of photoluminescence originated from initial nanocrystals and from nanocrystals formed as a result irradiation are summed.     

Self-assembly of metal nanocrystals on ultrathin oxide for nonvolatile memory applications

The self-assembly of metal nanocrystals including Au, Ag, and Pt on ultrathin oxide for nonvolatile memory applications are investigated. The self-assembly of nanocrystals consists of metal evaporation and selective rapid-thermal annealing (RTA). By controlling process parameters, such as the thickness of the deposited film, the post-deposition annealing temperatures, and the substrate doping concentration, metal nanocrystals with density of 2–4 × 10¹¹ cm⁻², diameter less than 8.1 nm, and diameter deviation less than 1.7 nm can be obtained. Observation by scanning-transmission electron microscopy (STEM) and convergent-beam electron diffraction (CBED) shows that nanocrystals embedded in the oxide are nearly spherical and crystalline. Metal contamination of the Si/SiO₂ interface is negligible, as monitored by STEM, energy dispersive x-ray spectroscopy (EDX), and capacitance-voltage (C-V) measurements. The electrical characteristics of metal, nanocrystal nonvolatile memories also show advantages over semiconductor counterparts. Large memory windows shown by metal nanocrystal devices in C-V measurements demonstrate that the work functions of metal nanocrystals are related to the charge-storage capacity and retention time because of the deeper potential well in comparison with Si nanocrystals.     

Using different work function nanocrystal materials to improve the retention characteristics of nonvolatile memory devices

Recently, nanocrystal nonvolatile memory (NVM) devices have attracted great research interest. Taking into account the effect of work function to account for the better retention characteristics for nanocrystals with larger work function, utilizing different work functions Au, W and Si as floating gates is proposed and comparatively studied in this paper. It was found that Au nanocrystals have better retention characteristic than W and Si. The good retention characteristic of the Au nanocrystal device is due to the larger work function and it is difficult for electrons captured by Au nanocrystal to escape from them. So, the retention characteristic of the device can be improved by using larger work function nanocrystal materials.     

Influence of pyridine molecule adsorption on concentrations of free carriers and paramagnetic centers in porous silicon layers

The influence of adsorption of donor pyridine (C₅H₅N) molecules on free-hole and defect concentrations in porous silicon layers differing in the morphology of composing nanocrystals and pores, as well as in the boron doping concentration, was studied using infrared and electron spin resonance spectroscopy. It was shown that the dependence of the hole concentration on the pyridine vapor pressure is controlled by the initial boron doping level of porous silicon, while the number of defects, i.e., dangling silicon bonds, is almost unchanged during adsorption for all sample types. For samples on substrates with a boron concentration of ～10²⁰ cm^?3, a decrease in the number of holes at low pyridine pressures is observed and is attributed to hole capture by surface states of adsorbed C₅H₅N molecules. At pyridine pressures close to the saturated vapor pressure, the hole concentration in porous silicon layers increases, which is associated with hole “trap” depopulation due to an increase in the permittivity of the silicon nanocrystal neighborhood under conditions of C₅H₅N vapor condensations in sample pores.     

Hole transfer from PbS nanocrystal quantum dots to polymers and efficient hybrid solar cells utilizing infrared photons

Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO₂, and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700 nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050 nm.     

Interface Modifications of InAs Quantum‐Dots Solids and their Effects on FET Performance

InAs nanocrystals field‐effect transistors with an ON/OFF ratio of 10⁵ are reported. By tailoring the interface regions in the active layer step‐by‐step, the evolution of the ON/OFF ratio can be followed from approximately 5 all the way to around 10⁵. The formation of a semiconducting solid from colloidal nanocrystals is achieved through targeted design of the nanocrystal–nanocrystal interaction. The manipulation characteristics of the nanocrystal interfaces include the matrix surrounding the inorganic core, the interparticle distance, and the order of nanocrystals in the 3D array. Through careful analysis of device characteristics following each treatment, the effect of each on the physical properties of the films are able to be verified. The enhanced performance is related to interparticle spacing, reduction in sub‐gap states, and better electronic order (lower σ parameter). Films with enhanced charge transport qualities retain their quantum‐confined characteristics throughout the procedure, thus making them useful for optoelectronic applications.     

Special features of the electrical conductivity in doped α-Si:H films with silicon nanocrystals

The electrical properties of undoped and phosphorus-doped α-Si:H films with Si nanocrystals are studied. The silicon nanocrystals are formed by a solid-solid phase transition resulting from the nanosecond effect of a XeCl excimer laser on an amorphous film. The formation of the nanocrystals in the undoped films is accompanied by an increase in the electrical conductivity by two to three orders of magnitude and a simultaneous decrease in the effective activation energy of the conductivity from 0.7 to 0.14 eV. The nanocrystal sizes range from 2 to 10 nm for various laser treatment modes and are determined from Raman scattering data and high-resolution electron microscopy. The temperature dependence of the Fermi level is obtained by calculating the energies of the localized states of electrons and holes in the nanocrystals. It is shown that, as the temperature decreases, the Fermi level tends to the energy of the states in the Si nanocrystals for a wide concentration range of the dopant. The Fermi level’s location close to the states in the nanocrystals is a consequence of the fact that these states are multicharged. It is found that phosphorus effectively transforms into an electrically active state during laser treatment of the doped amorphous Si films, which is an important consideration in the fabrication of shallow p-n junctions and contacts for amorphous Si films.     

Optoelectronic properties of n-type CdZnTe 2-DEGsingle-quantum-well heterostructures

Optoelectronic properties of asymmetrically strained II-VI CdZnTe single-quantum-well structures grown by molecular-beam epitaxy are reported. Indium doping CdZnTe n-type using a two-dimensional electron gas heterostructure achieved a carrier mobility of 5000 cm²·V^-1 s^-1 at 40 K. A shallow donor ionization energy of 14.5 meV was determined from Hall effect measurements. Fabrication of a large-area-mesa heterostructure device allowed us to investigate exciton absorption of the mixed type-I and type-II single quantum well. Control of the electron concentration in the quantum well allows optical absorption modulation using both the quantum-confined Stark effect (QCSE) and phase-space absorption quenching. Separation of electron and hole photocurrents in different layers is demonstrated and results in photogain. A heavy-hole red-shift of 9.9 meV/V due to the reverse QCSE is reported     

Energy-Efficient Gas Sensors Based on Nanocrystalline Indium Oxide

Semiconductors - The effect of the nanocrystal size on the sensitivity of nanocrystalline indium oxide In2O3 with an extremely small size of nanocrystals (from 7 to 40 nm) synthesized by the...