Semiconductor Nanocrystals Functionalized with Antimony Telluride Zintl Ions for Nanostructured Thermoelectrics

Recent advances in lanthanide-doped up-conversion probes for theranostics

Up-conversion (or anti-Stokes) luminescence refers to the phenomenon whereby materials emit high energy, short-wavelength light upon excitation at longer wavelengths. Lanthanide-doped up-conversion nanoparticles (Ln-UCNPs) are widely used in biomedicine due to their excellent physical and chemical properties such as high penetration depth, low damage threshold and light conversion ability. Here, the latest developments in the synthesis and application of Ln-UCNPs are reviewed. First, methods used to synthesize Ln-UCNPs are introduced, and four strategies for enhancing up-conversion luminescence are analyzed, followed by an overview of the applications in phototherapy, bioimaging and biosensing. Finally, the challenges and future prospects of Ln-UCNPs are summarized.

Solution-Processed Inorganic Thermoelectric Materials: Opportunities and Challenges

Thermoelectric technology requires synthesizing complex materials where not only the crystal structure but also other structural features such as defects, grain size and orientation, and interfaces must be controlled. To date, conventional solid-state techniques are unable to provide this level of control. Herein, we present a synthetic approach in which dense inorganic thermoelectric materials are produced by the consolidation of well-defined nanoparticle powders. The idea is that controlling the characteristics of the powder allows the chemical transformations that take place during consolidation to be guided, ultimately yielding inorganic solids with targeted features. Different from conventional methods, syntheses in solution can produce particles with unprecedented control over their size, shape, crystal structure, composition, and surface chemistry. However, to date, most works have focused only on the low-cost benefits of this strategy. In this perspective, we first cover the opportunities that solution processing of the powder offers, emphasizing the potential structural features that can be controlled by precisely engineering the inorganic core of the particle, the surface, and the organization of the particles before consolidation. We then discuss the challenges of this synthetic approach and more practical matters related to solution processing. Finally, we suggest some good practices for adequate knowledge transfer and improving reproducibility among different laboratories.

Effects of combining some theoretical models in the cross-section calculations of some alpha-induced reactions for natSb

In cases where it is not possible to obtain the cross-section values experimentally due to various factors, the importance of obtaining them with theoretical models has been explained in many studies available in the literature. In this context, the comparison of the cross-section values obtained by using the theoretical models with the experimental data will also be very beneficial for updating and developing these models. Existing studies, which also serve this purpose, have given inspiration to this study and it is aimed to examine the effects of the simultaneous use of the alpha optical model potentials and the level density models on the cross-section calculations for some alpha-particle-induced reactions on natural antimony. The effects of theoretical models on the cross-section calculations were investigated by comparing the obtained calculation results with the experimental data taken from the literature. The TALYS code, which is frequently preferred in the literature, was used in all calculations within the scope of this study. For the comparison of the calculated results with the experimental data, not only a visual analysis by graphing the outcomes, but also a mean-weighted-deviation calculation was used, and the findings were interpreted by accounting for both of them.

Stacking-Dependent Electrical Transport in a Colloidal CdSe Nanoplatelet Thin-Film Transistor

Here, we report an exceptional feature of the one-dimensional threadlike assemblies of a four-monolayer colloidal CdSe nanoplatelet (NPL)-based thin-film transistor. A series of different lengths of threads (200-1200 nm) was used as an active n channel in thin-film transistors (TFTs) to understand the change in mobility with the length of the threads. The film with the longest threads shows the highest conductivity of ∼12 S/cm and electron mobility of ∼14.3 cm² V^-1 s^-1 for an applied gate voltage of 2 V. The mobility trends with the length seem to be driven mostly by the lower defects in threads, where the loss of electron hopping is less. Furthermore, our results show the mobility trends in stacking-dependent CdSe NPL threads and provide a new insight into fabricating high-mobility TFTs with the use of colloidal CdSe NPLs.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

Printing thermoelectric inks toward next-generation energy and thermal devices

The ability of thermoelectric (TE) materials to convert thermal energy to electricity and vice versa highlights them as a promising candidate for sustainable energy applications. Despite considerable increases in the figure of merit zT of thermoelectric materials in the past two decades, there is still a prominent need to develop scalable synthesis and flexible manufacturing processes to convert high-efficiency materials into high-performance devices. Scalable printing techniques provide a versatile solution to not only fabricate both inorganic and organic TE materials with fine control over the compositions and microstructures, but also manufacture thermoelectric devices with optimized geometric and structural designs that lead to improved efficiency and system-level performances. In this review, we aim to provide a comprehensive framework of printing thermoelectric materials and devices by including recent breakthroughs and relevant discussions on TE materials chemistry, ink formulation, flexible or conformable device design, and processing strategies, with an emphasis on additive manufacturing techniques. In addition, we review recent innovations in the flexible, conformal, and stretchable device architectures and highlight state-of-the-art applications of these TE devices in energy harvesting and thermal management. Perspectives of emerging research opportunities and future directions are also discussed. While this review centers on thermoelectrics, the fundamental ink chemistry and printing processes possess the potential for applications to a broad range of energy, thermal and electronic devices.

Annual performance evaluation of thermoelectric generator-assisted building-integrated photovoltaic system with phase change material

Owing to the economic recession due to the Coronavirus disease (COVID-19) pandemic, energy-efficient building retrofitting has been considered as an integrated solution to recover the economy and maintain global greenhouse gas reduction. As part of retrofitting existing building-integrated photovoltaic systems during building renovations, this study evaluated the energy generation potential of a thermoelectric generator-assisted building-integrated photovoltaic system with a phase change material. The combination of a thermoelectric generator and phase change material with photovoltaic systems results in solar cell temperature reduction and additional electricity output owing to the Seebeck effect, increasing the total generated energy from the system. Simulations of the proposed system were performed using MATLAB R2020a, based on transient energy balance equations. The appropriate melting temperature and thickness of the phase change material were derived to maximize the annual electricity generation of the proposed system from simulations of 12 design days in each month. The proposed system with the selected phase change material conditions exhibited a 1.09% annual increase in generation output and 0.91%, -1.32%, 2.25%, and 3.16% generation improvements from spring to winter, compared with the building-integrated photovoltaic system alone. Theoretically, the proposed system is expected to generate 4.47% more energy by minimizing the thermal resistance of the system and improving thermoelectric generator performance.

Enhancing Carrier Diffusion Length and Quantum Efficiency through Photoinduced Charge Transfer in Layered Graphene-Semiconducting Quantum Dot Devices

Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, d_s, the carrier diffusion length, L_D, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and L_D could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.

Effect of metal ion solubility on the oxidative assembly of metal sulfide quantum dots

The versatility of the oxidative assembly method for the creation of 2D and 3D quantum dot (QD) architectures represents both an opportunity and a challenge as a method enabling controlled placement of chemically distinct QDs in multicomponent systems. The opportunity lies in the ability to independently tune the kinetics of the different components so that they are similar (leading to well-mixed systems) or different (enabling gradient or phase-segregated composites) using a wide range of variables; the challenge lies in understanding those variables and how their interplay affects the overall kinetics. Here, we show that the identity of the cation in the sulfide matrix (M = Cd²⁺ vs Zn²⁺) plays a large role in the kinetics of assembly of mass spectrometry QDs, attributed to differences in solubility. Time resolved dynamic light scattering is used to monitor the hydrodynamic radius, R¯_h. ZnS shows an exponential growth associated with reaction-limited cluster aggregation (RLCA), whereas CdS demonstrates a significant induction period (10-75 min) followed by a growth step that cannot be distinguished between RLCA and diffusion limited cluster aggregation. These data correlate with relative solubilities of the nanoparticles, as probed by free-cation concentration. Data also confirm prior studies showing that cubic-closest-packed (ccp) lattices are kinetically slow relative to hexagonally closest-packed (hcp); using the slope of the ln R¯_h vs time plot for the rate constant, the values of 0.510 s^-1 and 3.92 s^-1 are obtained for ccp ZnS and hcp ZnS, respectively. Thus, both the structure and the solubility are effective levers for adjusting the relative reactivity of QDs toward oxidative assembly.

Achieving Out-of-Plane Thermoelectric Figure of Merit ZT = 1.44 in a p-Type Bi2Te3/Bi0.5Sb1.5Te3 Superlattice Film with Low Interfacial Resistance

Recently, low-dimensional superlattice films have attracted significant attention because of their low dimensionality and anisotropic thermoelectric (TE) properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. For these superlattice structures, both electrons and phonons show highly anisotropic behavior and exhibit much stronger interface scattering in the out-of-plane direction of the films compared to the in-plane direction. However, no detailed information is available in the literature for the out-of-plane TE properties of the superlattice-based films. In this report, we present the out-of-plane Seebeck coefficient, thermal conductivity, and electrical properties of p-type Bi₂Te₃/Bi_0.5Sb_1.5Te₃ (bismuth telluride/bismuth antimony telluride, BT/BST) superlattice films in the temperature range of 77-500 K. Because of the synergistic combination of the energy filtering effect and low interfacial resistance of the superlattice structure, an impressively high ZT of 1.44 was achieved at 400 K for the 200 nm-thick p-type BT/BST superlattice film, corresponding to a 43% ZT enhancement compared to the pristine p-BST films with the same thickness.

Ink Processing for Thermoelectric Materials and Power-Generating Devices

The growing concern over the depletion of hydrocarbon resources, and the adverse environmental effects associated with their use, has increased the demand for renewable energy sources. Thermoelectric (TE) power generation from waste heat has emerged as a renewable energy source that does not generate any pollutants. Recently, ink-based processing for the preparation of TE materials has attracted tremendous attention because of the simplicity in design of power generators and the possibility of cost-effective manufacturing. In this progress report, recent advances in the development of TE inks, processing techniques, and ink-fabricated devices are reviewed. A summary of typical formulations of TE materials as inks is included, as well as a discussion on various ink-based fabrication methods, with several examples of newly designed devices fabricated using these techniques. Finally, the prospects of this field with respect to the industrialization of TE power generation technology are presented.

Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications

Metal chalcogenide materials are current mainstream thermoelectric materials with high conversion efficiency. This review provides an overview of the scalable solution-based methods for controllable synthesis of various nanostructured and thin-film metal chalcogenides, as well as their properties for thermoelectric applications. Furthermore, the state-of-art ink-based processing method for fabrication of thermoelectric generators based on metal chalcogenides is briefly introduced. Finally, the perspective on this field with regard to material production and device development is also commented upon.

Electrophilic substitution reaction as a facile and general approach for reactive removal of native ligands from nanocrystals surface

Surface property that strongly affects physical and chemical performances of inorganic nanocrystals (NCs) is a key enabler for NCs applications. Here, we report a facile, versatile and general strategy for reactive removal of NCs surface ligands based on electrophilic substitution reaction, in which an electrophile directly reacts with the electron-rich coordinating headgroup of surface-tethered ligands to form a non-coordinating product. This process leads to the break of NC-ligand bond, thereby achieving reactive removal of surface ligands. Based on this strategy, various hydrophobic NCs with different compositions and morphologies can be transferred into polar and hydrophilic media while preserving their size and shape. More importantly, the treated NCs present a great improvement in catalytic and biological performances in comparison with the untreated counterparts. This work not only provides a versatile ligand removal strategy for NCs surface modification but also opens up more opportunities for applications in the fields of electronics, catalysis and biotechnology.

Nanoscale self-assembly of thermoelectric materials: a review of chemistry-based approaches

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

Synthesis of thermo-responsive nanocomposites of superparamagnetic cobalt nanoparticles/poly(N-isopropylacrylamide)

Novel nanocomposites of superparamagnetic cobalt nanoparticles (Co NPs) and poly(N-isopropylacrylamide) (PNIPAM) were fabricated through surface-initiated atom-transfer radical polymerization (SI-ATRP). We firstly synthesized a functional ATRP initiator, containing an amine (as anchoring group) and a 2-bromopropionate group (SI-ATRP initiator). Oleic acid- and trioctylphosphine oxide-coated Co NPs were then modified with the initiator via ligand exchange. The process is facile and rapid for efficient surface functionalization and afterwards the Co NPs can be dispersed into polar solvent DMF without aggregation. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and dynamic light scattering measurements confirmed the success of ligand exchange. The following polymerization of NIPAM was conducted on the surface of Co NPs. Temperature-dependent dynamic light scattering study showed the responsive behavior of PNIPAM-coated Co NPs. The combination of superparamagnetic and thermo-responsive properties in these hybrid nanoparticles is promising for future applications e.g. in biomedicine.

Application of Aqueous-Based Covalent Crosslinking Strategies to the Formation of Metal Chalcogenide Gels and Aerogels

An aqueous-based metal ion crosslinking approach for assembly of metal chalcogenide nanoparticles (NPs) into robust gels is reported. Short chalcogenide ligands (S2−) undergo crosslinking with metal salts (Sn4+) to form a gel [NP/S2−/Sn4+]n (NP=PbTe, PbS, CdS, CdSe). The corresponding aerogel networks retain the crystallinity and quantum confinement effects of the native building blocks while achieving excellent porosity [Brunauer–Emmett–Teller (BET) surface areas of 160–238 m2/g]. Treatment of sulfide-capped PbTe nanoparticles with an excess of Sn4+ leads to ion exchange and formation of an amorphous “SnTe” gel.

When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome

This study demonstrates that ligand exchange of nanocrystals (NCs) is not always an innocuous process, but can lead to facile (room temperature) ion exchange, depending on the surface crystal faceting. Rock salt PbTe NCs prepared as cubes with neutral facets undergo room-temperature ligand exchange with sulfide ions, whereas cuboctahedron-shaped particles with neutral {100} and polar {111} facets are transformed to PbS, driven by ion exchange along the ⟨111⟩ direction. Likewise, cation exchange (with Ag⁺) occurs rapidly for cuboctahedra, whereas cubes remain inert. This dramatic difference is attributed to the relative surface area of {111} facets that promote rapid ion exchange and shows how facet engineering is a powerful knob for the control of reaction pathways in nanoparticles.

Silicon Nanocrystal Superlattice Nucleation and Growth

Colloidal dodecene-passivated silicon (Si) nanocrystals were dispersed in hexane or chloroform and deposited onto substrates as face-centered cubic superlattices by slowly evaporating the solvent. The uniformity of the nanocrystals enables extended order; however, the solvent and the evaporation protocol significantly influence the self-assembly process, determining the morphology of the films, the extent of order, and the superlattice orientation on the substrate. Chloroform yielded superlattices with step-flow growth morphologies and (111)_SL, (100)_SL, and (110)_SL orientations. Hexane led to mostly island morphologies when evaporated at room temperature with exclusively (111)_SL orientations. Higher evaporation temperatures led to more extensive step-flow deposition. A model for the surface diffusion of nanocrystals adsorbed on the superlattice surface is developed.

ZnO Nanocrystal Networks Near the Insulator-Metal Transition: Tuning Contact Radius and Electron Density with Intense Pulsed Light

Networks of ligand-free semiconductor nanocrystals (NCs) offer a valuable combination of high carrier mobility and optoelectronic properties tunable via quantum confinement. In principle, maximizing carrier mobility entails crossing the insulator-metal transition (IMT), where carriers become delocalized. A recent theoretical study predicted that this transition occurs at nρ³ ≈ 0.3, where n is the carrier density and ρ is the interparticle contact radius. In this work, we satisfy this criterion in networks of plasma-synthesized ZnO NCs by using intense pulsed light (IPL) annealing to tune n and ρ independently. IPL applied to as-deposited NCs increases ρ by inducing sintering, and IPL applied after the NCs are coated with Al₂O₃ by atomic layer deposition increases n by removing electron-trapping surface hydroxyls. This procedure does not substantially alter NC size or composition and is potentially applicable to a wide variety of nanomaterials. As we increase nρ³ to at least twice the predicted critical value, we observe conductivity scaling consistent with arrival at the critical region of a continuous quantum phase transition. This allows us to determine the critical behavior of the dielectric constant and electron localization length at the IMT. However, our samples remain on the insulating side of the critical region, which suggests that the critical value of nρ³ may in fact be significantly higher than 0.3.

Solution processing of chalcogenide materials using thiol–amine “alkahest” solvent systems

We highlight recent studies utilizing thiol/amine mixtures to dissolve bulk inorganic materials for facile solution processing of functional thin films.

Interface control of electronic and optical properties in IV–VI and II–VI core/shell colloidal quantum dots: a review

Core/shell heterostructures provide controlled optical properties, tuneable electronic structure, and chemical stability due to an appropriate interface design.

High-performance shape-engineerable thermoelectric painting

Output power of thermoelectric generators depends on device engineering minimizing heat loss as well as inherent material properties. However, the device engineering has been largely neglected due to the limited flat or angular shape of devices. Considering that the surface of most heat sources where these planar devices are attached is curved, a considerable amount of heat loss is inevitable. To address this issue, here, we present the shape-engineerable thermoelectric painting, geometrically compatible to surfaces of any shape. We prepared Bi₂Te₃-based inorganic paints using the molecular Sb₂Te₃ chalcogenidometalate as a sintering aid for thermoelectric particles, with ZT values of 0.67 for n-type and 1.21 for p-type painted materials that compete the bulk values. Devices directly brush-painted onto curved surfaces produced the high output power of 4.0 mW cm^-2. This approach paves the way to designing materials and devices that can be easily transferred to other applications.

Magnetic and thermoelectric properties of the ternary pseudo-hollandite BaxCr5Se8 (0.5 < x < 0.55) solid solution

The structure of Ba0.5Cr5Se8 has been recently resolved, and its thermoelectric and magnetic properties have been studied. A ZT of 0.12 was found at around 800 K. Here, we report a study on the pseudo-hollandite BaxCr5Se8 solid-solution with 0.5 ≤ x ≤ 0.55 and its thermoelectric and magnetic properties. There is no significant impact either on the cell parameters depending on the cation content or on the magnetic properties. However, thermoelectric properties are radically changed depending on x content. While the low thermal conductivity, around 0.8 W m(-1) K(-1), remains similar for all samples, a respective increase and decrease of the resistivity and the Seebeck coefficient are observed with increasing Ba content. The maximum Seebeck coefficient is found with Ba0.5Cr5Se8 at around 635 K with 315 μV K(-1), and the Seebeck coefficient then decreases and is correlated with an activation of minority charge carriers confirmed by Hall measurements. A similar but steeper behavior is observed for the Ba0.55Cr5Se8 temperature dependence plot at around 573 K. Finally, the best thermoelectric performances are found using the lowest content of Ba, unlike when x tends to 0.55, ZT approaches a tenth of the initial best value. BaxCr5Se8 compounds are antiferromagnetic with TN = 58 K. A large peak in thermal conductivity is observed around the antiferromagnetic transition for all stoichiometry.

Water-soluble, heterometallic chalcogenide oligomers as building blocks for functional films

Highly water-soluble, discrete, heterometallic, chalcogenide oligomers displaying various valences in a single metal chalcogenide oligomer are proposed for functional films.

Host-guest chemistry for tuning colloidal solubility, self-organization and photoconductivity of inorganic-capped nanocrystals

Colloidal inorganic nanocrystals (NCs), functionalized with inorganic capping ligands, such as metal chalcogenide complexes (MCCs), have recently emerged as versatile optoelectronic materials. As-prepared, highly charged MCC-capped NCs are dispersible only in highly polar solvents, and lack the ability to form long-range ordered NC superlattices. Here we report a simple and general methodology, based on host–guest coordination of MCC-capped NCs with macrocyclic ethers (crown ethers and cryptands), enabling the solubilization of inorganic-capped NCs in solvents of any polarity and improving the ability to form NC superlattices. The corona of organic molecules can also serve as a convenient knob for the fine adjustment of charge transport and photoconductivity in films of NCs. In particular, high-infrared-photon detectivities of up to 3.3 × 10¹¹ Jones with a fast response (3 dB cut-off at 3 kHz) at the wavelength of 1,200 nm were obtained with films of PbS/K₃AsS₄/decyl-18-crown-6 NCs.

The high polarity of colloidal inorganic-ligand-functionalized nanocrystals can be problematic for their processing, limiting their optoelectronic applications. Here, by complexation with macrocycles, the authors enabled broad amphiphilicity of such nanocrystals and processing from a variety of solvents.

Solvothermal Synthesis of Tetrahedrite: Speeding Up the Process of Thermoelectric Material Generation

Derivatives of synthetic tetrahedrite, Cu12Sb4S13, are receiving increasing attention in the thermoelectric community due to their exploitation of plentiful, relatively nontoxic elements, combined with a thermoelectric performance that rivals that of PbTe-based compounds. However, traditional synthetic methods require weeks of annealing at high temperatures (450-600 °C) and periodic regrinding of the samples. Here we report a solvothermal method to produce tetrahedrite that requires only 1 day of heating at a relatively low temperature (155 °C). This allows preparation of multiple samples at once and is potentially scalable. The solvothermal material described herein demonstrates a dimensionless figure of merit (ZT) vs temperature curve comparable to that of solid-state tetrahedrite, achieving the same ZT of 0.63 at ∼720 K. As with the materials from solid-state synthesis, products from this rapid solvothermal synthesis can be improved by mixing in a 1:1 molar ratio with the Zn-containing natural mineral, tennantite, to achieve 0.9 mol equiv of Zn. This leads to a 36% increase in ZT at ∼720 K for solvothermal tetrahedrite, to 0.85.

Harvesting Nanocatalytic Heat Localized in Nanoalloy Catalyst as a Heat Source in a Nanocomposite Thin Film Thermoelectric Device

This report describes findings of an investigation of harvesting nanocatalytic heat localized in a nanoalloy catalyst layer as a heat source in a nanocomposite thin film thermoelectric device for thermoelectric energy conversion. This device couples a heterostructured copper-zinc sulfide nanocomposite for thermoelectrics and low-temperature combustion of methanol fuels over a platinum-cobalt nanoalloy catalyst for producing heat localized in the nanocatalyst layer. The possibility of tuning nanocatalytic heat in the nanocatalyst and thin film thermoelectric properties by compositions points to a promising pathway in thermoelectric energy conversion.

Thermoelectric Properties of Solution Synthesized Nanostructured Materials

Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2(Te,Se)3, PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

Capping nanoparticles with graphene quantum dots for enhanced thermoelectric performance

Graphene quantum dots (GQDs) are shown to serve as phase transfer agents to transfer various types of nanoparticles (NPs) from non-polar to polar solvents. Thorough characterization of the NPs proves complete native ligand exchange. Pellets of this GQD-NP composite show that the GQDs limit the crystal size during spark plasma sintering, yielding enhanced thermoelectric performance compared with NPs exchanged with inorganic ions. A photoluminescence study of the GQD-NP composite also suggests energy transfer from GQDs to NPs.

Electron doping in bottom-up engineered thermoelectric nanomaterials through HCl-mediated ligand displacement

A simple and effective method to introduce precise amounts of doping in nanomaterials produced from the bottom-up assembly of colloidal nanoparticles (NPs) is described. The procedure takes advantage of a ligand displacement step to incorporate controlled concentrations of halide ions while removing carboxylic acids from the NP surface. Upon consolidation of the NPs into dense pellets, halide ions diffuse within the crystal structure, doping the anion sublattice and achieving n-type electrical doping. Through the characterization of the thermoelectric properties of nanocrystalline PbS, we demonstrate this strategy to be effective to control charge transport properties on thermoelectric nanomaterials assembled from NP building blocks. This approach is subsequently extended to PbTe(x)Se(1-x)@PbS core-shell NPs, where a significant enhancement of the thermoelectric figure of merit is achieved.

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.

Metal matrix-metal nanoparticle composites with tunable melting temperature and high thermal conductivity for phase-change thermal storage

Phase-change materials (PCMs) are of broad interest for thermal storage and management applications. For energy-dense storage with fast thermal charging/discharging rates, a PCM should have a suitable melting temperature, large enthalpy of fusion, and high thermal conductivity. To simultaneously accomplish these traits, we custom design nanocomposites consisting of phase-change Bi nanoparticles embedded in an Ag matrix. We precisely control nanoparticle size, shape, and volume fraction in the composite by separating the nanoparticle synthesis and nanocomposite formation steps. We demonstrate a 50-100% thermal energy density improvement relative to common organic PCMs with equivalent volume fraction. We also tune the melting temperature from 236-252 °C by varying nanoparticle diameter from 8.1-14.9 nm. Importantly, the silver matrix successfully prevents nanoparticle coalescence, and no melting changes are observed during 100 melt-freeze cycles. The nanocomposite's Ag matrix also leads to very high thermal conductivities. For example, the thermal conductivity of a composite with a 10% volume fraction of 13 nm Bi nanoparticles is 128 ± 23 W/m-K, which is several orders of magnitude higher than typical thermal storage materials. We complement these measurements with calculations using a modified effective medium approximation for nanoscale thermal transport. These calculations predict that the thermal conductivity of composites with 13 nm Bi nanoparticles varies from 142 to 47 W/m-K as the nanoparticle volume fraction changes from 10 to 35%. Larger nanoparticle diameters and/or smaller nanoparticle volume fractions lead to larger thermal conductivities.

Atomistic description of thiostannate-capped CdSe nanocrystals: retention of four-coordinate SnS4 motif and preservation of Cd-rich stoichiometry

Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-state materials. Inorganic surface functionalization, used to displace native organic capping ligands from NC surfaces, has been a major enabler of electronic solid-state devices based on colloidal NCs. At the same time, very little is known about the atomistic details of the organic-to-inorganic ligand exchange and binding motifs at the NC surface, severely limiting further progress in designing all-inorganic NCs and NC solids. Taking thiostannates (K4SnS4, K4Sn2S6, K6Sn2S7) as typical examples of chalcogenidometallate ligands and oleate-capped CdSe NCs as a model NC system, in this study we address these questions through the combined application of solution (1)H NMR spectroscopy, solution and solid-state (119)Sn NMR spectroscopy, far-infrared and X-ray absorption spectroscopies, elemental analysis, and by DFT modeling. We show that through the X-type oleate-to-thiostannate ligand exchange, CdSe NCs retain their Cd-rich stoichiometry, with a stoichiometric CdSe core and surface Cd adatoms serving as binding sites for terminal S atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y-2m)]shell (taking Sn2S7(6-) ligand as an example). Thiostannates SnS4(4-) and Sn2S7(6-) retain (distorted) tetrahedral SnS4 geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn2S6(4-) (and SnS3(2-)) in most solvents and its lower adaptability to the NC surface caused by rigid Sn2S2 rings.

New insights into the phosphine-free synthesis of ultrasmall Cu2−xSe nanocrystals at the liquid–liquid interface

A liquid–liquid interfacial strategy to prepare ultra small (<5 nm) colloidal Cu_2−​xSe NCs with blue-fluorescence, noncaustic and environmentally friendly NH₄SCN replaces the long-chain organic ligands for fabrication of NC-sensitized solar.

Inorganic nanoparticles engineered to attack bacteria

Antibiotics delivered to bacteria using engineered nanoparticles (NP), offer a powerful and efficient means to kill or control bacteria, especially those already resistant to antibiotics.

High infrared photoconductivity in films of arsenic-sulfide-encapsulated lead-sulfide nanocrystals

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2×10(13) Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

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.

Colloidal nanocrystals with inorganic halide, pseudohalide, and halometallate ligands

We investigate simple halides and pseudohalides as an important class of inorganic ligands for nanocrystals (NCs) in solution phase ligand exchange. These short, robust, and easy to model ligands bind to the NC surface and provide electrostatic stabilization of NC dispersions in N-methylformamide. The replacement of organic ligands on NCs with compact halide and pseudohalide ligands greatly facilitates electronic communication between NCs. For example, a high electron mobility of μ ≈ 12 cm(2) V(-1) s(-1) has been observed in thin films made of I(-)-capped CdSe NCs. We also studied charge transport properties of thin films based on the pseudohalide N3(-)-capped InAs NCs, suggesting the possibility of obtaining "all III-V" NC solids. In addition, we extend the surface chemistry of halometallates (e.g., CH3NH3PbI3), which can stabilize colloidal solutions of lead chalcogenide NCs. These halide, pseudohalide, and halometallate ligands enrich the current family of inorganic ligands and can open up more opportunities for applications of NCs in the fields of electronics, optoelectronics, and thermoelectrics.

Structure and thermoelectric properties of spark plasma sintered ultrathin PbTe nanowires

Solution-synthesized thermoelectric nanostructured materials have the potential to have lower cost and higher performance than materials synthesized by solid-state methods. Herein we present the synthesis of ultrathin PbTe nanowires, which are compressed by spark plasma sintering at various temperatures in the range of 405-500 °C. The resulting discs possess grains with sizes of 5-30 μm as well as grains with sizes on the order of the original 12 nm diameter PbTe nanowires. This micro- and nanostructure leads to a significantly reduced thermal conductivity compared to bulk PbTe. Careful electron transport analysis shows suppressed electrical conductivity due to increased short-range and ionized defect scatterings, while the Seebeck coefficient remains comparable to the bulk value. The PbTe nanowire samples are found unintentionally p-type doped to hole concentrations of 2.16-2.59 × 10(18) cm(-3). The maximum figure of merit achieved in the unintentionally doped spark plasma sintered PbTe nanowires is 0.33 at 350 K, which is among the highest reported for unintentionally doped PbTe at low temperatures.

Ligand engineering of polymer nanocomposites: from the simple to the complex

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

Lead halide perovskites and other metal halide complexes as inorganic capping ligands for colloidal nanocrystals

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MX(n), where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV-VI, II-VI, and III-V semiconductor nanocrystals. In particular, we show that the Lewis acid-base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20-30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50-65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Highly bright water-soluble silica coated quantum dots with excellent stability

A facile Stöber method for the synthesis of isolated silica coated QDs with high PL efficiencies, tunable small size and excellent stability leads to the practical bioapplication as robust biomarkers.

Bi(1)-(x)Sb(x) alloy nanocrystals: colloidal synthesis, charge transport, and thermoelectric properties

Nanostructured Bi1-xSbx alloys constitute a convenient system to study charge transport in a nanostructured narrow-gap semiconductor with promising thermoelectric properties. In this work, we developed the colloidal synthesis of monodisperse sub-10 nm Bi1-xSbx alloy nanocrystals (NCs) with controllable size and compositions. The surface chemistry of Bi1-xSbx NCs was tailored with inorganic ligands to improve the interparticle charge transport as well as to control the carrier concentration. Temperature-dependent (10-300 K) electrical measurements were performed on the Bi1-xSbx NC based pellets to investigate the effect of surface chemistry and grain size (∼10-40 nm) on their charge transport properties. The Hall effect measurements revealed that the temperature dependence of carrier mobility and concentration strongly depended on the grain size and the surface chemistry, which was different from the reported bulk behavior. At low temperatures, electron mobility in nanostructured Bi1-xSbx was directly proportional to the average grain size, while the concentration of free carriers was inversely proportional to the grain size. We propose a model explaining such behavior. Preliminary measurements of thermoelectric properties showed a ZT value comparable to those of bulk Bi1-xSbx alloys at 300 K, suggesting a potential of Bi1-xSbx NCs for low-temperature thermoelectric applications.

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.

Thermoelectric performance of PbSe quantum dot films

The thermoelectric (TE) performance of films of colloidal lead selenide (PbSe) quantum dots (QDs) with metal-chalcogenide complex ligands is seen to change with QD size and temperature. Films of smaller QDs have higher Seebeck coefficient magnitudes, indicating stronger quantum confinement, and lower electrical and thermal conductivities. The thermoelectric figure of merit ZT is ∼0.5 at room temperature and increases with temperature to 1.0-1.37 at ∼400 K, where it is larger for smaller QD films. This is better than previous results for solution-prepared QD TE materials at these elevated temperatures.