n-type colloidal semiconductor nanocrystals

Electrochemiluminescence aptasensor of TiO2/CdS:Mn hybrids for ultrasensitive detection of cytochrome c

A novel electrochemiluminescence (ECL) aptasensor was proposed for ultrasensitive detection of cytochrome c (cyt c) using CdS:Mn quantum dot-modified TiO2 nanowires (NWs) as electrode. The Mn-doped CdS was deposited on the TiO2 NWs by successive ion layer adsorption and reaction (SILAR) as ECL emitter, on which thiol-modified aptamer of cyt c was attached via Cd-S bond. Due to the high photo-electrical transfer efficiency, the as-prepared aptasensor shows high selectivity and sensitivity towards cyt c with a detection limit of 9.5fM and a linear range from 50fM to 125pM.

Direct observation of narrow mid-infrared plasmon linewidths of single metal oxide nanocrystals

Infrared-responsive doped metal oxide nanocrystals are an emerging class of plasmonic materials whose localized surface plasmon resonances (LSPR) can be resonant with molecular vibrations. This presents a distinctive opportunity to manipulate light–matter interactions to redirect chemical or spectroscopic outcomes through the strong local electric fields they generate. Here we report a technique for measuring single nanocrystal absorption spectra of doped metal oxide nanocrystals, revealing significant spectral inhomogeneity in their mid-infrared LSPRs. Our analysis suggests dopant incorporation is heterogeneous beyond expectation based on a statistical distribution of dopants. The broad ensemble linewidths typically observed in these materials result primarily from sample heterogeneity and not from strong electronic damping associated with lossy plasmonic materials. In fact, single nanocrystal spectra reveal linewidths as narrow as 600 cm⁻¹ in aluminium-doped zinc oxide, a value less than half the ensemble linewidth and markedly less than homogeneous linewidths of gold nanospheres.

Establishing the cause of inhomogeneous broadening would help to produce narrow ensemble localized surface plasmon resonance peaks, favourable for sensing applications. Here, Johns et al. use near field optics for enhancing signal contrast, enabling the measurement of mid-infrared spectra of single nanocrystals.

Exploiting the colloidal nanocrystal library to construct electronic devices

Synthetic methods produce libraries of colloidal nanocrystals with tunable physical properties by tailoring the nanocrystal size, shape, and composition. Here, we exploit colloidal nanocrystal diversity and design the materials, interfaces, and processes to construct all-nanocrystal electronic devices using solution-based processes. Metallic silver and semiconducting cadmium selenide nanocrystals are deposited to form high-conductivity and high-mobility thin-film electrodes and channel layers of field-effect transistors. Insulating aluminum oxide nanocrystals are assembled layer by layer with polyelectrolytes to form high-dielectric constant gate insulator layers for low-voltage device operation. Metallic indium nanocrystals are codispersed with silver nanocrystals to integrate an indium supply in the deposited electrodes that serves to passivate and dope the cadmium selenide nanocrystal channel layer. We fabricate all-nanocrystal field-effect transistors on flexible plastics with electron mobilities of 21.7 square centimeters per volt-second.

Potentiometric Measurements of Semiconductor Nanocrystal Redox Potentials

A potentiometric method for measuring redox potentials of colloidal semiconductor nanocrystals (NCs) is described. Fermi levels of colloidal ZnO NCs are measured in situ during photodoping, allowing correlation of NC redox potentials and reduction levels. Excellent agreement is found between electrochemical and optical redox-indicator methods. Potentiometry is also reported for colloidal CdSe NCs, which show more negative conduction-band-edge potentials than in ZnO. This difference is highlighted by spontaneous electron transfer from reduced CdSe NCs to ZnO NCs in solution, with potentiometry providing a measure of the inter-NC electron-transfer driving force. Future applications of NC potentiometry are briefly discussed.

Charge Transfer Dynamics from Photoexcited Semiconductor Quantum Dots

Understanding photoinduced charge transfer from nanomaterials is essential to the many applications of these materials. This review summarizes recent progress in understanding charge transfer from quantum dots (QDs), an ideal model system for investigating fundamental charge transfer properties of low-dimensional quantum-confined nanomaterials. We first discuss charge transfer from QDs to weakly coupled acceptors within the framework of Marcus nonadiabatic electron transfer (ET) theory, focusing on the dependence of ET rates on reorganization energy, electronic coupling, and driving force. Because of the strong electron-hole interaction, we show that ET from QDs should be described by the Auger-assisted ET model, which is significantly different from ET between molecules or from bulk semiconductor electrodes. For strongly quantum-confined QDs on semiconductor surfaces, the coupling can fall within the strong coupling limit, in which case the donor-acceptor interaction and ET properties can be described by the Newns-Anderson model of chemisorption. We also briefly discuss recent progress in controlling charge transfer properties in quantum-confined nanoheterostructures through wavefunction engineering and multiple exciton dissociation. Finally, we identify a few key areas for further research.

Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures

Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals.

The Influence of Doping on the Optoelectronic Properties of PbS Colloidal Quantum Dot Solids

We report on an extensive spectroscopic investigation of the impact of substitutional doping on the optoelectronic properties of PbS colloidal quantum dot (CQD) solids. N-doping is provided by Bi incorporation during CQD synthesis as well as post-synthetically via cation exchange reactions. The spectroscopic data indicate a systematic quenching of the excitonic absorption and luminescence and the appearance of two dopant-induced contributions at lower energies to the CQD free exciton. Temperature-dependent photoluminescence indicates the presence of temperature-activated detrapping and trapping processes of photoexcitations for the films doped during and after synthesis, respectively. The data are consistent with a preferential incorporation of the dopants at the QDs surface in the case of the cation-exchange treated films versus a more uniform doping profile in the case of in-situ Bi incorporation during synthesis. Time-resolved experiments indicate the presence of fast dopant- and excitation-dependent recombination channels attributed to Auger recombination of negatively charged excitons, formed due to excess of dopant electrons. The data indicate that apart from dopant compensation and filling of dopant induced trap states, a fraction of the Bi ionized electrons feeds the QD core states resulting in n-doping of the semiconductor, confirming reported work on devices based on such doped CQD material.

From Impurity Doping to Metallic Growth in Diffusion Doping: Properties and Structure of Silver-Doped InAs Nanocrystals

Tuning of the electronic properties of presynthesized colloidal semiconductor nanocrystals (NCs) by doping plays a key role in the prospect of implementing them in printed electronics devices such as transistors and photodetectors. While such impurity doping reactions have already been introduced, the understanding of the doping process, the nature of interaction between the impurity and host atoms, and the conditions affecting the solubility limit of impurities in nanocrystals are still unclear. Here, we used a postsynthesis diffusion-based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy and analytical inductively coupled plasma mass spectroscopy (ICP-MS) were used to present a two-stage doping model consisting of a "doping region" and a "growth region", depending on the impurity to NC ratio in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impurity location and correlate between the structural and electronic properties for different sizes of InAs NCs and dopant concentrations. The resulting structural model describes a heterogeneous system where the impurities initially dope the NC, by substituting for In atoms near the surface of the NC, until the "solubility limit" is reached, after which the rapid growth and formation of metallic structures are identified.

Single and multistep energy transfer processes within doped polymer nanoparticles

Herein, we demonstrate the design of multiple fluorophores Coumarin 153 (C153) and Nile Red (NR) encapsulated in semiconducting poly[N-vinylcarbazole] (PVK) polymer nanoparticles (50-70 nm in diameter) by a simple re-precipitation technique, and elucidate their photophysical properties by steady-state and picosecond (ps) time resolved emission spectroscopy. It is interesting to note that multistep cascaded energy transfer occurs from the excited host PVK molecules to NR dye molecules through C153. The energy transfer time constants are found to be 180 ps for PVK→C153, 360 ps for PVK→NR, and 140 ps for the overall energy transfer process from PVK to NR through C153 dye molecules. The multistep energy transfer allows tuning of the wide range emission from 350 nm to 700 nm by changing the relative concentrations of the encapsulated dye molecules. Bright, stable, and white light emission of the dye doped polymer nanoparticles with a quantum yield of 14% is achieved at a particular concentration ratio of the C153 : NR dye. The generation of "cool" white emission in suspension and in the solid state film opens up new possibilities to obtain white light OLEDs based on single nanoparticles.

Route to the Smallest Doped Semiconductor: Mn(2+)-Doped (CdSe)13 Clusters

Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn(2+)-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure.

Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation

Electronically doped colloidal semiconductor nanocrystals offer valuable opportunities to probe the new physical and chemical properties imparted by their excess charge carriers. Photodoping is a powerful approach to introducing and controlling free carrier densities within free-standing colloidal semiconductor nanocrystals. Photoreduced (n-type) colloidal ZnO nanocrystals possessing delocalized conduction-band (CB) electrons can be formed by photochemical oxidation of EtOH. Previous studies of this chemistry have demonstrated photochemical electron accumulation, in some cases reaching as many as >100 electrons per ZnO nanocrystal, but in every case examined to date this chemistry maximizes at a well-defined average electron density of ⟨Nmax⟩ ≈ (1.4 ± 0.4) × 10(20) cm(-3). The origins of this maximum have never been identified. Here, we use a solvated redox indicator for in situ determination of reduced ZnO nanocrystal redox potentials. The Fermi levels of various photodoped ZnO nanocrystals possessing on average just one excess CB electron show quantum-confinement effects, as expected, but are >600 meV lower than those of the same ZnO nanocrystals reduced chemically using Cp*2Co, reflecting important differences between their charge-compensating cations. Upon photochemical electron accumulation, the Fermi levels become independent of nanocrystal volume at ⟨N⟩ above ∼2 × 10(19) cm(-3), and maximize at ⟨Nmax⟩ ≈ (1.6 ± 0.3) × 10(20) cm(-3). This maximum is proposed to arise from Fermi-level pinning by the two-electron/two-proton hydrogenation of acetaldehyde, which reverses the EtOH photooxidation reaction.

Colloidal Nanoparticles for Intermediate Band Solar Cells

The Intermediate Band (IB) solar cell concept is a promising idea to transcend the Shockley-Queisser limit. Using the results of first-principles calculations, we propose that colloidal nanoparticles (CNPs) are a viable and efficient platform for the implementation of the IB solar cell concept. We focused on CdSe CNPs and we showed that intragap states present in the isolated CNPs with reconstructed surfaces combine to form an IB in arrays of CNPs, which is well separated from the valence and conduction band edges. We demonstrated that optical transitions to and from the IB are active. We also showed that the IB can be electron doped in a solution, e.g., by decamethylcobaltocene, thus activating an IB-induced absorption process. Our results, together with the recent report of a nearly 10% efficient CNP solar cell, indicate that colloidal nanoparticle intermediate band solar cells are a promising platform to overcome the Shockley-Queisser limit.

Electronic doping and redox-potential tuning in colloidal semiconductor nanocrystals

Electronic doping is one of the most important experimental capabilities in all of semiconductor research and technology. Through electronic doping, insulating materials can be made conductive, opening doors to the formation of p-n junctions and other workhorses of modern semiconductor electronics. Recent interest in exploiting the unique physical and photophysical properties of colloidal semiconductor nanocrystals for revolutionary new device technologies has stimulated efforts to prepare electronically doped colloidal semiconductor nanocrystals with the same control as available in the corresponding bulk materials. Despite the impact that success in this endeavor would have, the development of general and reliable methods for electronic doping of colloidal semiconductor nanocrystals remains a long-standing challenge. In this Account, we review recent progress in the development and characterization of electronically doped colloidal semiconductor nanocrystals. Several successful methods for introducing excess band-like charge carriers are illustrated and discussed, including photodoping, outer-sphere electron transfer, defect doping, and electrochemical oxidation or reduction. A distinction is made between methods that yield excess band-like carriers at thermal equilibrium and those that inject excess charge carriers under thermal nonequilibrium conditions (steady state). Spectroscopic signatures of such excess carriers, accessible by both equilibrium and nonequilibrium methods, are reviewed and illustrated. A distinction is also proposed between the phenomena of electronic doping and redox-potential shifting. Electronically doped semiconductor nanocrystals possess excess band-like charge carriers at thermal equilibrium, whereas redox-potential shifting affects the potentials at which charge carriers are injected under nonequilibrium conditions, without necessarily introducing band-like charge carriers at equilibrium. Detection of the key spectroscopic signatures of band-like carriers allows distinction between these two regimes. Both electronic doping and redox-potential shifting can be powerful tools for tuning the performance of nanocrystals in electronic devices. Finally, key chemical challenges associated with nanocrystal electronic doping are briefly discussed. These challenges are centered largely on the availability of charge-carrier reservoirs with suitable redox potentials and on the relatively poor control over nanocrystal surface traps. In most cases, the Fermi levels of colloidal nanocrystals are defined by the redox properties of their surface traps. Control over nanocrystal surface chemistries is therefore essential to the development of general and reliable strategies for electronically doping colloidal semiconductor nanocrystals. Overall, recent progress in this area portends exciting future advances in controlling nanocrystal compositions, surface chemistries, redox potentials, and charge states to yield new classes of electronic nanomaterials with attractive physical properties and the potential to stimulate unprecedented new semiconductor technologies.

Measuring Ligand-Dependent Transport in Nanopatterned PbS Colloidal Quantum Dot Arrays Using Charge Sensing

Colloidal quantum dot arrays with long organic ligands have better packing order than those with short ligands but are highly resistive, making low-bias conductance measurements impossible with conventional two-probe techniques. We use an integrated charge sensor to study transport in weakly coupled arrays in the low-bias regime, and we nanopattern the arrays to minimize packing disorder. We present the temperature and field dependence of the resistance for nanopatterned oleic-acid and n-butylamine-capped PbS arrays, measuring resistances as high as 10(18) Ω. We find that the conduction mechanism changes from nearest neighbor hopping in oleic-acid-capped PbS dots to Mott's variable range hopping in n-butylamine capped PbS dots. Our results can be understood in terms of a change in the interdot coupling strength or a change in density of trap states and highlight the importance of the capping ligand on charge transport through colloidal quantum dot arrays.

Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes

Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the ‘reversed oxygen-sensing’ capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

Colloidal quantum wells have great potential as solution-processed light sources. Here, Lorenzon et al. demonstrate that such colloidal quantum wells can also be exploited as luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response.

Tuning carrier mobilities and polarity of charge transport in films of CuInSe(x)S(2-x) quantum dots

CuInSe(x)S(2-x) quantum dot field-effect transistors show p-type, n-type, and ambipolar behaviors with carrier mobilities up to 0.03 cm(2) V(-1) s(-1). Although some design rules from studies of cadmium and lead containing quantum dots can be applied, remarkable differences are observed including a strong gating effect in as-synthesized nanocyrstals with long ligands.

Capping agent-free gold nanostars show greatly increased versatility and sensitivity for biosensing

We report the first assessment of the plasmonic biosensing capabilities of capping agent-free gold nanostars. Capping agent removal was carried out using aqueous solutions of sodium borohydride, which yielded a refractive index sensitivity of 474 nm/RIU for the polyvinylpyrrolidone (PVP)-free nanostars compared with 98 nm/RIU for PVP-coated gold nanostars. Following PVP removal, biotinylated thiol and streptavidin protein were added to the nanostars, which resulted in red shifts as large as 51 nm and a limit of detection as low as 0.1 pM. Refractive index-based sensing of prostate specific antigen (PSA) both in buffer and serum was then carried out and was shown to yield shifts as large as 127 nm and have a limit of detection of 100 pM in serum. Last, a sandwich assay involving PSA was developed to aggregate the nanostars together for greater sensitivity. The sandwich assay did, indeed, give shifts close to 200 nm and was capable of detecting 10(-17) M PSA in serum. The greatly increased sensitivity and amenability to functionalization of PVP-free gold nanostars should prove useful in applications ranging from catalysis to drug delivery.

Prospects of nanoscience with nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

A first-principles study of the electronic and structural properties of Sb and F doped SnO2 nanocrystals

We examine the electronic properties of Sb and F doped SnO2 nanocrystals up to 2.4 nm in diameter. A real-space pseudopotential implementation of density functional theory is employed within the local density approximation. We calculate electron binding energies and dopant formation energies as function of nanocrystal size, dopant concentration, and dopant species. Structural changes for different dopant species are also investigated. Our study should provide useful information for the design of transparent conducting oxides at the nanoscale.

Fast doping of Cu into ZnSe NCs by hydrazine promoted cation exchange in aqueous solution at room temperature

Controllable doping is an effective way of tuning the properties of semiconductor nanocrystals (NCs). In this work, a simple strategy of fast doping Cu ions into ZnSe NCs under ambient conditions was proposed. The principle of doping is based on hydrazine (N2H4) promoted cation exchange reaction. By direct addition of Cu ion stock solution into the preformed ZnSe NCs, Cu doped ZnSe NCs can be obtained. Furthermore, the emission of doped NCs can be tuned by changing the amount of impurity ion addition. The cation exchange reaction is facilitated by three factors: 1) N2H4 addition, 2) fast impurity ions, and 3) partial stabilizer removal. The proposed cation exchange reaction in aqueous solution could be an alternate route for NC doping as well as synthesis of ionic NCs.

NaYF4:Yb,Er–MoS2: from synthesis and surface ligand stripping to negative infrared photoresponse

The synthesis, surface ligand stripping (SOCl₂/DMF treatment), and unusual negative infrared photoresponse of new NaYF₄:Yb,Er–MoS₂ sheet nanocomposites were reported.

Visible-light photocatalyzed cross-linking of diacetylene ligands by quantum dots to improve their aqueous colloidal stability

Ligand cross-linking is known to improve the colloidal stability of nanoparticles, particularly in aqueous solutions. However, most cross-linking is performed chemically, in which it is difficult to limit interparticle cross-linking, unless performed at low concentrations. Photochemical cross-linking is a promising approach but usually requires ultraviolet (UV) light to initiate. Using such high-energy photons can be harmful to systems in which the ligand–nanoparticle bond is fairly weak, as is the case for the commonly used semiconductor quantum dots (QDs). Here, we introduce a novel approach to cross-link thiolated ligands on QDs by utilizing the photocatalytic activity of QDs upon absorbing visible light. We show that using visible light leads to better ligand cross-linking by avoiding the problem of ligand dissociation that occurs upon UV light exposure. Once cross-linked, the ligands significantly enhance the colloidal stability of those same QDs that facilitated cross-linking.

Colloidal quantum dots intraband photodetectors

Photoconductivity is demonstrated with monodispersed HgSe colloidal quantum dots that are illuminated with radiation resonant with 1S(e)-1P(e) intraband electronic absorption, between 3 and 5 μm. A doping of two electrons per dot gives the lowest dark current, and a detectivity of 8.5 × 10(8) Jones is obtained at 80 K. Photoluminescence of the intraband transition is also observed. The detector properties are discussed in terms of the measured photoluminescence quantum yield, the electron mobility in the 1P(e) state, and the responsivity. The intraband photoresponse allows to fully harness the quantum confined states in colloidal nanostructures, extending the prior limited use of interband transition.

Continuous flow purification of nanocrystal quantum dots

Colloidal quantum dot (QD) purification is typically conducted via repeating precipitation-redispersion involving massive amounts of organic solvents and has been the main obstacle in mass production of QDs with dependable surface properties. Our results show that the electric field apparently affects the streamlining of QDs and that we could continuously collect stably dispersed QDs by the electrophoretic purification process. The purification yield increases as the electric potential difference increases or the flow rate decreases, but reaches an asymptotic value. The yield can be further improved by raising the absolute magnitude of the mobility of QDs with the addition of solvents with high dielectric constants. The continuous purification process sheds light on industrial production of colloidal nanomaterials.

Surface functionalization of semiconductor and oxide nanocrystals with small inorganic oxoanions (PO4(3-), MoO4(2-)) and polyoxometalate ligands

In this work, we study the functionalization of the nanocrystal (NC) surface with inorganic oxo ligands, which bring a new set of functionalities to all-inorganic colloidal nanomaterials. We show that simple inorganic oxoanions, such as PO4(3-) and MoO4(2-), exhibit strong binding affinity to the surface of various II-VI and III-V semiconductor and metal oxide NCs. ζ-Potential titration offered a useful tool to differentiate the binding affinities of inorganic ligands toward different NCs. Direct comparison of the binding affinity of oxo and chalcogenidometallate ligands revealed that the former ligands form a stronger bond with oxide NCs (e.g., Fe2O3, ZnO, and TiO2), while the latter prefer binding to metal chalcogenide NCs (e.g., CdSe). The binding between NCs and oxo ligands strengthens when moving from small oxoanions to polyoxometallates (POMs). We also show that small oxo ligands and POMs make it possible to tailor NC properties. For example, we observed improved stability upon Li(+)-ion intercalation into the films of Fe2O3 hollow NCs when capped with MoO4(2-) ligands. We also observed lower overpotential and enhanced exchange current density for water oxidation using Fe2O3 NCs capped with [P2Mo18O62](6-) ligands and even more so for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] with POM as the capping ligand.

Air-stable n-type colloidal quantum dot solids

Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing and energy conversion. These applications rely on rectifying junctions that require the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure. Here we report high-performance, air-stable n-type CQD solids. Using density functional theory we identify inorganic passivants that bind strongly to the CQD surface and repel oxidative attack. A materials processing strategy that wards off strong protic attack by polar solvents enabled the synthesis of an air-stable n-type PbS CQD solid. This material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%. We also feature the n-type CQD solid in the rapid, sensitive, and specific detection of atmospheric NO2. This work paves the way for new families of electronic devices that leverage air-stable quantum-tuned materials.

Heterovalent cation substitutional doping for quantum dot homojunction solar cells

Colloidal quantum dots have emerged as a material platform for low-cost high-performance optoelectronics. At the heart of optoelectronic devices lies the formation of a junction, which requires the intimate contact of n-type and p-type semiconductors. Doping in bulk semiconductors has been largely deployed for many decades, yet electronically active doping in quantum dots has remained a challenge and the demonstration of robust functional optoelectronic devices had thus far been elusive. Here we report an optoelectronic device, a quantum dot homojunction solar cell, based on heterovalent cation substitution. We used PbS quantum dots as a reference material, which is a p-type semiconductor, and we employed Bi-doping to transform it into an n-type semiconductor. We then combined the two layers into a homojunction device operating as a solar cell robustly under ambient air conditions with power conversion efficiency of 2.7%.

To use colloidal quantum dots in applications such as p-n junction solar cells, doping of the quantum dots is needed. Here, Stavrinadis et al. achieve lead sulphide quantum dot p-n homojunctions by heterovalent cation substitution of lead using bismuth.

Air-Stable n-Doped Colloidal HgS Quantum Dots

HgS nanocrystals show a strong mid-infrared absorption and a bleach of the near-infrared band edge, both tunable in energy and reversibly controlled by exposure to solution ions under ambient conditions. The same effects are obtained by applying a reducing electrochemical potential, confirming that the mid-infrared absorption is the intraband transition of the quantum dot. This is the first time that stable carriers are present in the quantum state of strongly confined quantum dot in ambient conditions. The mechanism by which doping is achieved is attributed to the rigid shifts of the valence and conduction band with respect to the environment, similar to the sensitivity of the work function of surfaces to adsorbates.

Outlook and emerging semiconducting materials for ambipolar transistors

Ambipolar or bipolar transistors are transistors in which both holes and electrons are mobile inside the conducting channel. This device allows switching among several states: the hole-dominated on-state, the off-state, and the electron-dominated on-state. In the past year, it has attracted great interest in exotic semiconductors, such as organic semiconductors, nanostructured materials, and carbon nanotubes. The ability to utilize both holes and electrons inside one device opens new possibilities for the development of more compact complementary metal-oxide semiconductor (CMOS) circuits, and new kinds of optoelectronic device, namely, ambipolar light-emitting transistors. This progress report highlights the recent progresses in the field of ambipolar transistors, both from the fundamental physics and application viewpoints. Attention is devoted to the challenges that should be faced for the realization of ambipolar transistors with different material systems, beginning with the understanding of the importance of interface modification, which heavily affects injections and trapping of both holes and electrons. The recent development of advanced gating applications, including ionic liquid gating, that open up more possibility to realize ambipolar transport in materials in which one type of charge carrier is highly dominant is highlighted. Between the possible applications of ambipolar field-effect transistors, we focus on ambipolar light-emitting transistors. We put this new device in the framework of its prospective for general lightings, embedded displays, current-driven laser, as well as for photonics-electronics interconnection.

Charge-tunable quantum plasmons in colloidal semiconductor nanocrystals

Nanomaterials exhibiting plasmonic optical responses are impacting sensing, information processing, catalysis, solar, and photonics technologies. Recent advances have expanded the portfolio of plasmonic nanostructures into doped semiconductor nanocrystals, which allow dynamic manipulation of carrier densities. Once interpreted as intraband single-electron transitions, the infrared absorption of doped semiconductor nanocrystals is now commonly attributed to localized surface plasmon resonances and analyzed using the classical Drude model to determine carrier densities. Here, we show that the experimental plasmon resonance energies of photodoped ZnO nanocrystals with controlled sizes and carrier densities diverge from classical Drude model predictions at small sizes, revealing quantum plasmons in these nanocrystals. A Lorentz oscillator model more adequately describes the data and illustrates a closer link between plasmon resonances and single-electron transitions in semiconductors than in metals, highlighting a fundamental contrast between these two classes of plasmonic materials.

Size dependence of negative trion Auger recombination in photodoped CdSe nanocrystals

We report a systematic investigation of the size dependence of negative trion (T(-)) Auger recombination rates in free-standing colloidal CdSe nanocrystals. Colloidal n-type CdSe nanocrystals of various radii have been prepared photochemically, and their trion decay dynamics have been measured using time-resolved photoluminescence spectroscopy. Trion Auger time constants spanning 3 orders of magnitude are observed, ranging from 57 ps (radius R = 1.4 nm) to 2.2 ns (R = 3.2 nm). The data reveal a substantially stronger size dependence than found for bi- or multiexciton Auger recombination in CdSe or other semiconductor nanocrystals, scaling in proportion to R(4.3).

Insight into the effects of surface oxidation and carbonization on the electronic properties of silicon quantum dots and silicon slabs: a density functional study

In this work, we investigated the effects of surface backbond-oxygen oxidation and surface substitute-carbon carbonization on carrier recombination and transportation of 10-, 12- and 14 Å Si quantum dots (QDs).

The role of ligands in determining the exciton relaxation dynamics in semiconductor quantum dots

This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.

Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through “doping;” however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.

25th anniversary article: Colloidal quantum dot materials and devices: a quarter-century of advances

Colloidal quantum dot (CQD) optoelectronics offers a compelling combination of low-cost, large-area solution processing, and spectral tunability through the quantum size effect. Since early reports of size-tunable light emission from solution-synthesized CQDs over 25 years ago, tremendous progress has been made in synthesis and assembly, optical and electrical properties, materials processing, and optoelectronic applications of these materials. Here some of the major developments in this field are reviewed, touching on key milestones as well as future opportunities.