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Aminophosphines are a class of inexpensive, environmentally benign phosphorus precursors that have provided routes to various metal phosphides. In this work, we use the aminophosphine tris(diethyl)aminophosphine to synthesize tin phosphide nanocrystals.

While HgTe nanocrystals (NCs) in the mid-infrared region have reached a high level of maturity, their far-infrared counterparts remain far less studied, raising the need for an in-depth investigation of the material before efficient device integration can be considered. Here, we explore the effect of temperature and pressure on the structural, spectroscopic, and transport properties of HgTe NCs displaying an intraband absorption at 10 THz. The temperature leads to a very weak modulation of the spectrum as opposed to what was observed for strongly confined HgTe NCs. HgTe NC films present ambipolar conduction with a clear prevalence of electron conduction as confirmed by transistor and thermoelectric measurements. Under the application of pressure, the material undergoes phase transitions from the zinc blende to cinnabar phase and later to the rock salt phase which we reveal using joint X-ray diffraction and infrared spectroscopy measurements. We discuss how the pressure existence domain of each phase is affected by the particle size.

We propose a true solid-state alternative to the vacuum photomultiplier tube using amorphous selenium (a-Se) as the bulk avalanche i-layer. A-Se is a unique photosensing material in which carrier t...

Magnetic anisotropy critically determines the utility of magnetic nanocrystals (NCs) in new nanomagnetism technologies. Using angular-dependent electron magnetic resonance (EMR), we observe magnetic anisotropy in isotropically arranged NCs of a nonmagnetic material. We show that the shape of the EMR angular variation can be well described by a simple model that considers magnetic dipole–dipole interactions between dipoles randomly located in the NCs, most likely due to surface dangling bonds. The magnetic anisotropy results from the fact that the energy term arising from the magnetic dipole–dipole interactions between all magnetic moments in the system is dominated by only a few dipole pairs, which always have an anisotropic geometric arrangement. Our work shows that magnetic anisotropy may be a general feature of NC systems containing randomly distributed magnetic dipoles., ACS Omega, 5 (20), ISSN:2470-1343

Metal chalcogenide nanoparticles offer vast control over their optoelectronic properties via size, shape, composition, and morphology which has led to their use across fields including optoelectronics, energy storage, and catalysis. While cadmium and lead-based nanocrystals are prevalent in applications, concerns over their toxicity have motivated researchers to explore alternate classes of nanomaterials based on environmentally benign metals such as zinc and tin. The goal of this research is to identify material systems that offer comparable performance to existing metal chalcogenide systems from abundant, recyclable, and environmentally benign materials. With band gaps that span the visible through the infrared, II–V direct band gap semiconductors such as tetragonal zinc phosphide (α-Zn3P2) are promising candidates for optoelectronics. To date, syntheses of α-Zn3P2 nanoparticles have been hindered because of the toxicity of zinc and phosphorus precursors, surface oxidation, and defect states leading to carrier trapping and low photoluminescence quantum yield. This work reports a colloidal synthesis of quantum confined α-Zn3P2 nanoparticles from common phosphorus precursor tris(trimethylsilyl)phosphine and environmentally benign zinc carboxylates. Shelling of the nanoparticles with zinc sulfide is shown as a method of preventing oxidation and improving the optical properties of the nanoparticles. These results show a route to stabilizing α-Zn3P2 nanoparticles for optoelectronic device applications.

Over the past decade, Ag2Se has attracted increasing attention due to its potentially excellent thermoelectric (TE) performance as an n-type semiconductor. It has been considered a promising alternative to Bi–Te alloys and other commonly used yet toxic and/or expensive TE materials. To optimize the TE performance of Ag2Se, recent research has focused on fabricating nanosized Ag2Se. However, synthesizing Ag2Se nanoparticles involves energy-intensive and time-consuming techniques with poor yield of final product. In this work, we report a low-cost, solution-processed approach that enables the formation of Ag2Se thin films from Cu2−xSe template films via cation exchange at room temperature. Our simple two-step method involves fabricating Cu2−xSe thin films by the thiol-amine dissolution of bulk Cu2Se, followed by soaking Cu2−xSe films in AgNO3 solution and annealing to form Ag2Se. We report an average power factor (PF) of 617 ± 82 μW m−1 K−2 and a corresponding ZT value of 0.35 at room temperature. We obtained a maximum PF of 825 μW m−1 K−2 and a ZT value of 0.46 at room temperature for our best-performing Ag2Se thin-film after soaking for 5 minutes. These high PFs have been achieved via full solution processing without hot-pressing.

Lately discovered silver selenide (Ag2Se) colloidal quantum dots with tetragonal crystal structure exhibit promising optical properties in the mid-wavelength infrared. Although colloidal synthesis of uniform sizes and shapes as well as detailed phase transformation and photoluminescence properties have been studied recently, investigations of their optoelectronic properties as an active layer in photodetector devices remain scarce. Herein, we present the fabrication and characterization of Ag2Se colloidal quantum dot-based photoconductive photodetectors. We investigate the effect of ligand exchange as well as temperature and spectral-dependent photoresponses. Our results suggest that further enhancement in performance could be achieved through accurate control of carrier concentration. With this improvement, Ag2Se colloidal quantum dots may serve as a promising mid-wavelength infrared absorber for the development of thermal infrared sensors and imagers with low size, weight, power consumption, and cost.

International audience; Extending the mechanical lifetime of NBR (i.e., poly(acrylonitrile-co-butadiene)), a large-volume synthetic rubber, requires a better understanding of its structure–property relationships. We demonstrate that industrial-grade and uncross-linked NBR can be mechanically rejuvenated and physically aged due to an inhomogeneous distribution of monomers along the polymer chains. As opposed to its nonpolar SBR counterpart (i.e., poly(styrene-co-butadiene)), NBR experiences thermodynamic driving forces for microphase separation and kinetic barriers for processing like those of block copolymers. Extruding NBR at high temperature and shear results in a weakly microphase-separated nanostructure of low relaxation time and resistance to flow, whereas physically aging NBR leads to lamellar nanodomains, a more solid-like material, and delayed stress relaxation. This effect of rejuvenation and aging on the nanostructure and rheological properties of NBR has important consequences on processing and storage conditions, such as the formation of defect-free interfaces in multilayered parts by polymer interdiffusion.

The p-n junction is one of the fundamental requirements for a practical semiconductor-based electronic device. Designing a heterojunction comprising of dissimilar p-type and n-type semiconductors calls for careful energy level considerations, both when selecting the semiconductor materials as well as the metal contacts. A homojunction based on a single semiconductor simplifies this task, as energy levels of the p-type and n-type materials are already fairly similar, allowing for easier selection of contacts. Traditionally, homojunctions rely on doping of a bulk semiconductor to achieve p- and n-type transport through controlled addition of aliovalent dopants via energy-intensive processes such as ion implantation or thermal annealing. Exact control of doping in nanocrystalline semiconductors is significantly more challenging, due to self-purification effects. However, owing to their large surface areas, surface moieties can be utilized to both dope the nanostructures as well as tune their energy levels. In this report, we present a facile technique based on an isoelectronic surface dopant in order to achieve p- and n-type materials based on the same semiconductor. We show that thin p-type colloidal Bi2Te3 nanowires can be switched to n-type through surface functionalization, thus increasing the availability of new nanocrystalline solution-processable p-n homojunctions.

Metal chalcogenide nanoparticles offer vast control over their optoelectronic properties via size, shape, composition, and morphology which has led to their use across fields including optoelectronics, energy storage, and catalysis. While cadmium and lead-based nanocrystals are prevalent in applications, concerns over their toxicity have motivated researchers to explore alternate classes of nanomaterials based on environmentally benign metals such as zinc and tin. The goal of this research is to identify material systems that offer comparable performance to existing metal chalcogenide systems from abundant, recyclable, and environmentally benign materials. With band gaps that span the visible through the infrared, II-V direct band gap semiconductors such as tetragonal zinc phosphide (α-Zn3P2) are promising candidates for optoelectronics. To date, syntheses of α-Zn3P2 nanoparticles have been hindered because of the toxicity of zinc and phosphorus precursors, surface oxidation, and defect states leading to carrier trapping and low photoluminescence quantum yield. This work reports a colloidal synthesis of quantum confined α-Zn3P2 nanoparticles from common phosphorus precursor tris(trimethylsilyl)phosphine and environmentally benign zinc carboxylates. Shelling of the nanoparticles with zinc sulfide is shown as a method of preventing oxidation and improving the optical properties of the nanoparticles. These results show a route to stabilizing α-Zn3P2 nanoparticles for optoelectronic device applications.

Cu2Se thin films provide a promising route toward relatively safe, sustainable and solution processed thermoelectric (TE) modules in contrast to more expensive and toxic materials currently on the ...

Two-dimensional (2D) anisotropic ferroelectric nanoparticles are considered as potential candidates in polymer-based dielectric energy storage materials. We successfully synthesized a new class of Ag@NiFeMOF 2D ferroelectric hybrid nanoplates and added it to PVDF matrix to prepare Ag@NiFeMOF/PVDF composites (MOF = metal-organic framework). The morphology of composites reveals that Ag atoms are uniformly dispersed in the nanoplates as 14 nm clusters. The discharge energy storage density of composites shows a remarkable improvement of over 610% compared to the pure PVDF sample and reaches a value of 8.01 J/cm3 (η = 64%). We hypothesize that the bimetallic NiFeMOF exhibits a stronger electric dipole moment than the monometallic NiMOF under the strengthening effect of Ag particles, which improves the polarization ability of composites. In addition, under high electric field strength, the Ag-induced Coulomb blocking effect in NiFeMOF nanoplates inhibits the migration of electrons, improves the electric field breakdown strength, and thereby increases the energy storage ability of the composites.

Photodiodes using amorphous-selenium (a-Se) have gained significant attention in medical diagnostic imaging due to their avalanche phenomena. At high electric fields above 70 V/μm, holes in a-Se undergo impact ionization which increases the effective sensitivity of the photodetector. However, at the high electric fields required for avalanche gain, dark current due to charge injection from metallic electrodes can become a significant source of noise, limiting the dynamic range of the device. To maximize gain while maintaining low dark conductivity, and to achieve reliable and repeatable impact ionization without irreversible breakdown, we need a non-insulating n-type hole-blocking/electron-transporting layer with a high dielectric constant. Among previously reported hole blocking layers that include both polymers and crystalline inorganic materials, vacuum deposited bulk cerium dioxide (CeO2) has shown promising results and has been successfully utilized in n-i-p avalanche a-Se sensors. However, bulk CeO2 cannot be directly deposited on a-Se owing to the low crystallization temperature of a-Se. In this talk, I will present our results on colloidal CeO2 quantum dots as potential hole blocking layers for a-Se devices. These CeO2 quantum dots are electronically passivated by surfactants/ligands and spin coated directly on a-Se at room temperature without inducing crystallization. Owing to quantum confinement, the QDs have a larger bandgap than bulk CeO2 thus raising the effective hole energy barrier. Prototype devices based on these QDs as interface layers on a-Se achieved an ultra-low dark current of 30 pA/cm2 at 70 V/μm which is three orders of magnitude lower than any avalanche a-Se device previously reported and may be the lowest dark current ever recorded for an avalanche device operated at room temperature.

When a semiconductor is under photoexcitation, the voltage response to a temperature gradient is the photo-Seebeck effect. Here we study this effect, focusing on the contribution from transport of photo-excited carriers. We demonstrate that by combining photo-Seebeck with photoconductivity measurements, one can determine the ratio between electron and hole mobilities, and hence both of them when one is known. This is found for the case of defect-free samples, where no detail on the absorbance, carrier lifetime or recombination is necessary. Our method reported here does not require chemical doping, which could introduce defects and is often not feasible. It applies to both thin film and bulk samples. Experiment-wise, photo-Seebeck effect is relatively easy to implement, or added to existing systems. In a broader context, for semiconductors with significant influence from defects, our result suggests that the photo-Seebeck behavior can still be understood. In this case another photo-transport property is necessary, in order to identify the mobilities of carriers and information regarding the defects. This framework integrates the information from photoexcitation and thermal gradients to provide a general method to determine fundamental electronic properties of materials.

Hybrid organic/inorganic thermoelectric materials based on conducting polymers and inorganic nanostructures have been demonstrated to combine both the inherently low thermal conductivity of the polymer and the superior charge transport properties (high power factors) of the inorganic component. While their performance today still lags behind that of conventional inorganic thermoelectric materials, solution-processable hybrids have made rapid progress and also offer unique advantages not available to conventional rigid inorganic thermoelectrics, namely: (1) low cost fabrication on rigid and flexible substrates, as well as (2) engineering complex conformal geometries for energy harvesting/cooling. While the number of reports of new classes of viable hybrid thermoelectric materials is growing, no group has reported a general approach for bottom-up design of both p- and n-type materials from one common base. Thus, unfortunately, the literature comprises mostly of disconnected discoveries, which limits development and calls for a first-principles approach for property manipulation analogous to doping in traditional semiconductor thermoelectrics. Here, molecular engineering at the organic/inorganic interface and simple processing techniques are combined to demonstrate a modular approach enabling de novo design of complex hybrid thermoelectric systems. We chemically modify the surfaces of inorganic nanostructures and graft conductive polymers to yield robust solution processable p- and n-type inorganic/organic hybrid nanostructures. Our new modular approach not only offers researchers new tools to perform true bottom-up design of thermoelectric hybrids, but also strong performance advantages as well due to the quality of the designed interfaces. For example, we obtain enhanced power factors in existing (by up to 500% in Te/PEDOT:PSS) and novel (Bi2S3/PEDOT:PSS) p-type systems, and also generate water-processable and air-stable high performing n-type hybrid systems (Bi2Te3/PEDOT:PSS), thus highlighting the potency of our ex situ strategy in opening up new material options for thermoelectric applications. This strategy establishes a unique platform with broad handles for custom tailoring of thermal and electrical properties through hybrid material tunability and enables independent control over inorganic material chemistry, nanostructure geometry, and organic material properties, thus providing a robust pathway to major performance enhancements.

Charge trapping due to defects in semiconductor quantum dots (QDs) is expected to challenge the applicability of QDs in future technologies. The efficient elimination of defects from QDs demands an understanding of their origin and of their impact on (photo)electronic properties. Here, we identify the presence of two charge states of a defect in CdSe QDs using electron paramagnetic resonance (EPR), combined with electronic tuning of QDs via chemically induced Ag doping. From light-induced EPR, we show that these defects have a central role in Fermi level pinning in ensembles of intrinsic QDs. By analyzing the dependence of the EPR signal of the defects on the concentration of Ag dopants, we further demonstrate that the defects act as effective electron traps in the QDs. Our study also provides support to the proposed behavior of Ag dopants in CdSe QDs, according to which Ag atoms are n-type dopants at concentrations below 2 Ag atoms per QD and become p-type dopants for higher Ag concentrations. From tempe...

This work demonstrates the first method for controlled growth of heterostructures within hybrid organic/inorganic nanocomposite thermoelectrics. Using a facile, aqueous technique, semimetal–alloy nanointerfaces are patterned within a hybrid thermoelectric system consisting of tellurium (Te) nanowires and the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Specifically, this method is used to grow nanoscale islands of Cu1.75Te alloy subphases within hybrid PEDOT:PSS-Te nanowires. This technique is shown to provide tunability of thermoelectric and electronic properties, providing up to 22% enhancement of the system’s power factor in the low-doping regime, consistent with preferential scattering of low energy carriers. This work provides an exciting platform for rational design of multiphase nanocomposites and highlights the potential for engineering of carrier filtering within hybrid thermoelectrics via introduction of interfaces with controlled structural and energe...

Author(s): Gordon, MP; Haas, K; Zaia, E; Menon, AK; Yang, L; Bruefach, A; Galluzzo, MD; Scott, MC; Prasher, RS; Sahu, A; Urban, JJ | Abstract: Organic–inorganic hybrids offer great promise as solution-processable thermoelectric materials. However, they have struggled to surpass the performance of their rigid inorganic counterparts due, in part, to a lack of synthetic control and limited understanding of how inorganic nanostructure dimensions impact overall charge transport. While it has been hypothesized that length, diameter, and aspect ratio (AR) all impact electronic transport in hybrid nanowires, the field lacks clarity on the relative role of each. In this study, the experimental parameter of ligand molecular weight (MW) is investigated as a synthetic knob for modulating nanowire dimensions, as well as the deconvolution of nanowire length versus diameter impacts on electron transport. By increasing ligand MW, larger nanowire AR dispersions occur and an optimal power factor of ≈130 μWm−1 K−2 is achieved for a modest AR of 73. Power factors of this magnitude are thought to only be achievable in ultrahigh AR systems; representing a 183% increase in performance over literature reports with similar AR. Additionally, nanowire diameter is demonstrated to be a far more sensitive parameter for enhancing performance than modulating length. This study provides improved fundamental insight into rational synthetic design avenues for future enhancements in the performance of hybrid materials.

Silver selenide colloidal quantum dots exhibit distinct optical absorption in the mid-wavelength infrared spectral region, which arises from the intraband transition between the first and the second quantum-confined energy levels. The optical absorption coefficient, carrier mobility, and carrier lifetime, which are the three primary parameters that determine the ultimate performance of a photodetector, are expected to be heavily dependent on the surface capping ligands. Herein, we characterize these parameters on silver selenide colloidal quantum dots films chemically treated with ligands selected from literatures that report high-performance quantum dot-based solar cells, photodetectors, transistors, and thermoelectrics. We correlate these results with the mid-infrared responsivities measured from photoconductive photodetectors fabricated from respective ligand-exchanged films. The insights gained from this study may serve as a foundation for enabling future intraband CQD-based MWIR sensing and imaging technologies.

An emerging class of materials that are hybrid in nature is propelling a technological revolution in energy, touching many fundamental aspects of energy-generation, storage, and conservation. Hybrid materials combine classical inorganic and organic components to yield materials that manifest new functionalities unattainable in traditional composites or other related multicomponent materials, which have additive function only. This Research News article highlights the exciting materials design innovations that hybrid materials enable, with an eye toward energy-relevant applications involving charge, heat, and mass transport.

Self-assembly of semiconductor nanocrystals (NCs) into two-dimensional patterns or three-dimensional (2-3D) superstructures has emerged as a promising low-cost route to generate thin-film transistors and solar cells with superior charge transport because of enhanced electronic coupling between the NCs. Here, we show that lead sulfide (PbS) NCs solids featuring either short-range (disordered glassy solids, GSs) or long-range (superlattices, SLs) packing order are obtained solely by controlling deposition conditions of colloidal solution of NCs. In this study, we demonstrate the use of the evaporation-driven self-assembly method results in PbS NC SL structures that are observed over an area of 1 mm × 100 μm, with long-range translational order of up to 100 nm. A number of ordered domains appear to have nucleated simultaneously and grown together over the whole area, imparting a polycrystalline texture to the 3D SL films. By contrast, a conventional, optimized spin-coating deposition method results in PbS NC glassy films with no translational symmetry and much shorter-range packing order in agreement with state-of-the-art reports. Further, we investigate the electronic properties of both SL and GS films, using a field-effect transistor configuration as a test platform. The long-range ordering of the PbS NCs into SLs leads to semiconducting NC-based solids, the mobility (μ) of which is 3 orders of magnitude higher than that of the disordered GSs. Moreover, although spin-cast GSs of PbS NCs have weak ambipolar behavior with limited gate tunability, SLs of PbS NCs show a clear p-type behavior with significantly higher conductivities.

Electronic and optoelectronic devices fabricated from colloidal quantum dots (CQD) provide a promising path toward realizing low-cost devices with greatly simplified device fabrication procedure, owing to their solution-processability. The impact that CQD technology would bring is expected to be significant, especially in the mid-infrared application areas, which is currently dominated by epitaxial semiconductor technologies. In this work, we introduce a new generation of infrared CQDs, namely Ag2Se CQDs, which has been recently uncovered to show distinct optical absorption in the mid-infrared. We report on the fabrication of solution-processed photoconductive photodetectors and discuss our analyses on the electronic and optoelectronic characteristics of our devices. We also demonstrate the feasibility of mid-infrared photodetection with a measured peak responsivity of 0.16 mA/W at 4 μm under room temperature operation.

The structure of CdSe nanocrystals doped with 0.2%–2.5% Ag corresponding to 1.1–13.6 Ag atoms per nanocrystal is studied in detail by a combination of X-ray diffraction (XRD) and X-ray absorption spectroscopy at the Ag–K, Cd–K, and Se–K edges. X-ray absorption near-edge structure (XANES) data are compared with ab initio multiple scattering simulations. Extended X-ray absorption fine structure (EXAFS) spectra are analyzed by reverse Monte Carlo (RMC) simulations. The XANES data provide evidence that Ag is located inside the CdSe nanocrystals, and the EXAFS spectra show that the local structure of Ag can be described by tetrahedral interstitial sites in either wurtzite or zinc blende lattices similar to the coordination of Ag in Ag2Se.

Conventional solar cells exhibit limited efficiencies in part due to their inability to absorb the entire solar spectrum. Sub-band-gap photons are typically lost but could be captured if a material that performs up-conversion, which shifts photon energies higher, is coupled to the device. Recently, molecular chromophores that undergo triplet-triplet annihilation (TTA) have shown promise for efficient up-conversion at low irradiance, suitable for some types of solar cells. However, the molecular systems that have shown the highest up-conversion efficiency to date are ill suited to broadband light harvesting, reducing their applicability. Here we overcome this limitation by combining an organic TTA system with highly fluorescent CdSe semiconductor nanocrystals. Because of their broadband absorption and spectrally narrow, size-tunable fluorescence, the nanocrystals absorb the radiation lost by the TTA chromophores, returning this energy to the up-converter. The resulting nanocrystal-boosted system shows a doubled light-harvesting ability, which allows a green-to-blue conversion efficiency of μ12.5% under 0.5 suns of incoherent excitation. This record efficiency at subsolar irradiance demonstrates that boosting the TTA by light-emitting nanocrystals can potentially provide a general route for up-conversion for different photovoltaic and photocatalytic applications.

Silicon wafers are commonly etched in potassium hydroxide solutions to form highly symmetric surface structures. These arise when slow-etching {111} atomic planes are exposed on standard low-index surfaces. However, the ability of nonstandard high-index wafers to provide more complex structures by tilting the {111} planes has not been fully appreciated. We demonstrate the power of this approach by creating chiral surface structures and nanoparticles of a specific handedness from gold. When the nanoparticles are dispersed in liquids, gold colloids exhibiting record molar circular dichroism (>5 × 10 9 M-1 cm-1) at red wavelengths are obtained. The nanoparticles also present chiral pockets for binding. © 2014 American Chemical Society., This work was supported by the Swiss National Science Foundation under Award Number 200021-146747 and the Ramón y Cajal Fellowship Program by the Spanish Ministry of Science and Innovation.

We report static and time-resolved terahertz (THz) conductivity measurements of a high- performance thermoelectric material containing tellurium nanowires in a PEDOT:PSS matrix. Composites were made with and without sulfur passivation of the nanowires surfaces. The material with sulfur linkers (TeNW/PD-S) is less conductive but has a longer carrier lifetime than the formulation without (TeNW/PD). We find real conductivities at f = 1THz of {\sigma}(TeNW/PD) = 160 S/cm and {\sigma}(TeNW/PD-S) = 5.1 S/cm. These values are much larger than the corresponding DC conductivities, suggesting DC conductivity is limited by structural defects. The free-carrier lifetime in the nanowires is controlled by recombination and trapping at the nanowire surfaces. We find surface recombination velocities in bare tellurium nanowires (22m/s) and TeNW/PD-S (40m/s) that are comparable to evaporated tellurium thin films. The surface recombination velocity in TeNW/PD (509m/s) is much larger, indicating a higher interface trap density., Comment: 6 pages, 3 figures, MRS Fall 2014

Nanocrystals are known to alter the relative stability of bulk solid phases. Here we test the limits of this effect on Ag2Se nanocrystals, a promising new electronic and infrared material. In the bulk, Ag2Se exhibits a solid–solid phase transition to a superionic conducting phase at moderate temperatures. We map this phase transition as a function of size, temperature, and surface treatment in Ag2Se core-only and core–shell nanocrystals. We show that the transition can be tuned not just below but also above the bulk phase-transition temperature. This phase flexibility has implications for applications in optoelectronic and phase-memory devices.

We prepare Ag(2)Se nanocrystals with average diameters between 2.7 and 10.4 nm that exhibit narrow optical absorption features in the near to mid infrared. We demonstrate that these features are broadly tunable due to quantum confinement. They provide the longest wavelength absorption peaks (6.5 μm) yet reported for colloidal nanocrystals.

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core–shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.

We report the size- and temperature-dependence of electron transport in thin films of PbSe nanocrystals. Upon increasing temperature over the range 28–200 K, the electron transport underwent a transition in mechanism from Efros-Shklovskii-variable-range-hopping (ES-VRH) to nearest-neighbor-hopping (NNH). The transition occurred at higher temperatures for films with smaller particles. The electron localization length, estimated from the ES-VRH model, was comparable to the nanocrystal size and scaled systematically with nanocrystal diameter. The activation energy from the NNH regime was also size-dependent, which is attributed both to size-dependent Coulomb effects and the size-distribution of nanocrystals.

Structurally well-defined and nearly monodisperse colloidal Ag2Se nanocrystals are synthesized from tri-n-octylphosphine solutions containing AgNO3, Se, oleic acid, 1-octadecylamine, and 1-octadecene (150 °C, about 5 min).

A general, one-pot, single-step method for producing colloidal silver chalcogenide (Ag(2)E; E = Se, S, Te) nanocrystals is presented, with an emphasis on Ag(2)Se. The method avoids exotic chemicals, high temperatures, and high pressures and requires only a few minutes of reaction time. While Ag(2)S and Ag(2)Te are formed in their low-temperature monoclinic phases, Ag(2)Se is obtained in a metastable tetragonal phase not observed in the bulk.

Electrical transport in films of CdSe nanocrystals with diameters varying from 2.9 to 5.1 nm was examined over 233-300 K by employing electrolyte gating to control the electron density. The transport parameters varied strongly and systematically with nanocrystal diameter. First, a strong correlation was observed between the device turn-on voltage and the size-dependent position of the lowest unoccupied electronic states of the nanocrystals. Second, the electron mobility increased with increasing particle diameter and reached a high value of 0.6 cm(2)/(V s) for films with 5.1 nm nanocrystals. Third, the charge transport could be described in terms of the nearest-neighbor-hopping mechanism with a size-dependent activation energy and a pre-exponential factor for mobility. The activation energy can be viewed as a size-dependent charging energy of an individual nanocrystal. Collectively, the combination of size- and temperature-dependent measurements provides a powerful approach to understanding electrical transport in nanocrystal films.

Al- and In-doped CdSe nanocrystals were synthesized using a three-part core-shell synthesis. CdSe core nanocrystals were first prepared, then allowed to react with dopant precursors in the presence of weakly binding ligands, and finally overcoated with an additional shell of CdSe. The addition of Al dopants quickened shell overgrowth and led to more monodisperse nanocrystals while the addition of In dopants produced more polydisperse particles, as seen by absorption spectroscopy. Elemental analysis combined with chemical etching revealed the dopants were inside the particles and solid state 27Al nuclear magnetic resonance (NMR) spectra indicated that the Al impurities were well dispersed. When the Al-doped nanocrystals were processed into thin-film transistors, enhanced n-type transport was observed with a rise in the Fermi level compared to undoped particles.