Increase In Carrier Mobility Research Articles

Monodispersed semiconducting SWNTs significantly enhanced the thermoelectric performance of regioregular poly(3-dodecylthiophene) films

The improved thermoelectric (TE) properties of conducting polymers are generally achieved through the use of single-walled carbon nanotubes (SWNTs) as additives. However, their aggregation and aggregation-induced over-loading of SWNTs generally result in high thermal and low electrical conductivity. Herein, sc-SWNTs were selectively captured by regioregular poly(3-dodecylthiophene)(rr-P3DDT) in a toluene solution containing a semiconducting and metallic carbon nanotubes mixture (mix-SWNTs) to achieve rr-P3DDT films with monodispersed semiconducting SWNTs (rr-P3DDT/sc-SWNT). The single sc-SWNTs were tightly wrapped by a rr-P3DDT layer with a thickness of several nanometers, which was orderly aligned at the micrometer scale. The large number of rr-P3DDT/sc-SWNT nanointerfaces present in composite films contributed to increasing carrier mobility through π–π conjugation while reducing the thermal conductivity via phonon scattering. Thus, the rr-P3DDT/sc-SWNT films had significantly higher electrical conductivity and lower thermal than the rr-P3DDT/mix-SWNT composite films. Both the electrical conductivity and Seebeck coefficient of the rr-P3DDT/sc-SWNT composite films were much greater than the calculated values using the parallel connected mixture model. The maximum ZT value of the rr-P3DDT/sc-SWNT composite film was 0.1, approximately 18 times greater compared to the rr-P3DDT/mix-SWNT films and among the highest for the polymer/carbon nanotube composites. This work provides a convenient but efficient method to maximize the TE properties of conducting polymers.

Largely Enhanced Thermoelectric Power Factor of Flexible Cu2-xS Film by Doping Mn.

Copper-sulfide-based materials have attracted noteworthy attention as thermoelectric materials due to rich elemental reserves, non-toxicity, low thermal conductivity, and adjustable electrical properties. However, research on the flexible thermoelectrics of copper sulfide has not yet been reported. In this work, we developed a facile method to prepare flexible Mn-doped Cu2-xS films on nylon membranes. First, nano to submicron powders with nominal compositions of Cu2-xMnyS (y = 0, 0.01, 0.03, 0.05, 0.07) were synthesized by a hydrothermal method. Then, the powders were vacuum-filtrated on nylon membranes and finally hot-pressed. Phase composition and microstructure analysis revealed that the films contained both Cu2S and Cu1.96S, and the size of the grains was ~20-300 nm. By Mn doping, there was an increase in carrier concentration and mobility, and ultimately, the electrical properties of Cu2-xS were improved. Eventually, the Cu2-xMn0.05S film showed a maximum power factor of 113.3 μW m-1 K-2 and good flexibility at room temperature. Moreover, an assembled four-leg flexible thermoelectric generator produced a maximum power of 249.48 nW (corresponding power density ~1.23 W m-2) at a temperature difference of 30.1 K, and had good potential for powering low-power-consumption wearable electronics.

Effect of drying behavior-induced self-organization process on the morphology and electronic properties of conjugated polymer films

In this study, the morphology and electronic properties of semiconducting polymer films were investigated in relation to a different drying behavior induced by the spinning time variation. Morphology changes related to the decreased spinning time were evaluated for p-type (P3HT, PQT-12) and n-type (N2200, PDBPyBT) polymer films fabricated at different preparation conditions. Based on the improved morphology, OFET devices were fabricated showing a 2.5 times carrier mobility increase (0.044 ± 0.012 cm2V−1s−1 for 3 s spin to 0.018 ± 0.011 cm2V−1s−1 for 30 s spin) for P3HT films, and 3 times improvement (0.088 ± 0.023 cm2V−1s−1 for 50 s to 0.28 ± 0.017 cm2V−1s−1 for 5 s) for N2200 films. For short spinning times, the slow drying of residual solvent allowed for the self-organization of polymer chains improving the crystallinity of polymer films. The presented approach synergizes the benefits of the spin coating method, like highly controlled film uniformity and thickness, with improved film morphology provided by other methods, like drop casting, or dip-coating.

Impact of mobility and effective mass on the thermoelectric performance of Ni doped Cu2Se

Cu2Se, a liquid-like thermoelectric material, has shown tremendous promise in recent years due to its strong electrical properties and intrinsic low thermal conductivity, which might be optimized for future applications by engineering the performance of Cu ions. In the present study, the thermoelectric features of hydrothermally grown Cu2Se are modified by tuning the presence of nickel in the crystalline matrix. The approach of modifying electronic band structures has been explored to optimize carrier transports by balancing the competitive relationship between carrier mobility and effective mass in order to discover improvements in thermoelectric materials beyond nanostructures. Its exceptional thermoelectric performance stems from ultrahigh carrier mobility over the whole working temperature range, which is achieved by effectively modifying the transport parameters through Ni doping. The inclusion of nickel in the Cu2Se lattice at either substitutional or interstitial sites has a substantial influence on its thermal and electrical transport properties. A band sharpening caused by the presence of Ni in the lattice of Cu2Se tailored the carrier's effective mass, resulting in increased carrier mobility without compromising the carrier concentration. A simultaneous enhancement in the carrier concentration and carrier mobility is achieved due to the presence of Ni in the interstitial sites. A maximum power factor of 614 µW/mK2 is observed for the sample doped at 1.5 wt% Ni at 573 K. The lattice distortion and point defects caused by Ni substitution generated additional phonon scattering centers, resulting in a substantial decrease in thermal conductivity. The sample doped with 2 wt% Ni shows a maximum ZT value of 0.535 at 573 K, which is around 3 times higher than that of pristine Cu2Se. These findings show that nickel is a novel efficient dopant that may significantly increase the thermoelectric performance of Cu2Se via defect and band engineering.

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues.

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications.

Grain boundary engineering in metavalent SnTe: A simplified approach

The metavalently bonded SnTe has gathered enormous attention recently as an environmentally friendly alternative to PbTe. While some of the issues with SnTe, including its small bandgap and large valence band offset, have largely been resolved, its relatively high lattice thermal conductivity (κl) has remained a matter of concern. Here, we show that the temperature-induced vacancy migration in SnTe results in the growth of SnTe nanoparticles in the intergrain region. The enhanced grain boundary scattering due to this led to a highly reduced κl and increased carrier mobility, enhancing the zT of our SnTe by almost 70% over the zT of various SnTe ingot samples from this and several previous studies. The validity of this approach was further confirmed for a 3% Ag-doped SnTe sample, a composition well-investigated in the past. The simplicity and effectiveness of our approach enhance the prospect of SnTe for practical applications.

Optimized photoelectric performance of MoS2/graphene heterostructure device induced by swift heavy ion irradiation

Two-dimensional materials with exceptional electronic and photoelectric properties are expected to develop a new-generation of ultrathin, high efficiency, broadband, and flexible photodetectors. For the potential application towards outer space exploration, the harsh irradiation environment must be taken into consideration. In this work, irradiation effects of MoS2/Graphene heterostructure field effect transistors (MoS2/G FETs) induced by swift heavy ions (SHIs) were explored. After irradiation, a new Raman peak denoted as D peak emerged, which demonstrated that SHI irradiation induced damage in graphene. Photoluminescence investigation of MoS2 indicated that SHI irradiation activated the excitation conversion of trion A− to excitation A0. Latent tracks were observed on MoS2/G/SiO2 by using atomic force microscope. The decreased resistance and increased carrier mobilities were detected at fluence lower than 5 × 1010 ions/cm2, which could be ascribed to the localized annealing of graphene during irradiation. The increased photocurrent and responsivity at fluence lower than 1 × 1010 ions/cm2 could be interpreted as the enhanced photoinduced gate voltage resulting from the SHI irradiation induced defects in MoS2/G FET. The devices were deteriorated at fluence of 1 × 1011 ions/cm2. The optimized and degraded properties of the devices could be ascribed to competition among doping, local annealing and defect scattering.

Influence of annealing on the surface structure evolution of intrinsic p-type HgCdTe films.

In this study, scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and the Hall effect were used to characterize the microscopic morphology, structural features and electrical properties of a HgCdTe film surface before and after annealing. The effect of annealing on the surface quality of the films was studied in detail. The results showed that the annealing treatment reduced the roughness, improve the denseness and uniformity and ameliorate the surface crystallinity. It also eliminated the defects, such as stress and stacking layer errors, caused by lattice distortion during the growth of the HgCdTe films. An increase in surface carrier concentration and mobility after annealing and improved electrical properties of the intrinsic p-type HgCdTe materials were also observed. It is therefore shown that annealing had a significant effect on improving the surface quality of HgCdTe films.

Magnetic ordering boost excellent thermoelectric performance of flexible films

The poor electrical transport performance of flexible thermoelectric (TE) films is a major bottleneck for the development of in-plane heat dissipation technology based on Peltier effect. Herein, ordered Fe nanoparticles (Fe-NPs) and in-situ Fe3O4 nanoparticles (Fe3O4-NPs) as magnetic functional elements (MFEs) were embedded into Bi0·5Sb1·5Te3 (BST)/epoxy flexible films in order to enhance the TE performance. The remarkable increases in electrical conductivity of BST/MFEs/BST/epoxy flexible films have been confirmed to originate from the carrier mobility increases. The retention of large Seebeck coefficient is attributed to the negative magnetoresistance (MR−) induced by spin-dependent scattering and weak localization effect from MFEs. As a result, the maximum power factor of BST/MFEs/BST/epoxy flexible films increased by 45%, and the maximum cooling temperature difference enhanced by 1.6 times. This work reveals that the ordered MFEs can significantly boost the electro-thermal conversion performance of flexible TE films and the magnetoresistance theory can reveal the scattering mechanism of charge carriers in thermo-electromagnetic flexible films.

Study on the Targeted Improvement Mechanism of the Carrier Concentration and Mobility of BiCuSeO Ceramics.

BiCuSeO has great application prospects in thermoelectric power generation and thermoelectric catalysis, but it is limited by its lower thermoelectric performance. Herein, BiCuSeO bulk materials were prepared using a solid-phase reaction method and a ball-milling method combined with spark plasma sintering, and then the thermoelectric properties were improved by synergistically increasing carrier concentration and mobility. Al was adopted to dope into the BiCuSeO matrix, aiming to adjust the carrier mobility through energy band adjustment. The results show that Al doping would widen the bandgap and enhance the carrier mobility of BiCuSeO. After Al doping, the thermoelectric properties of the material are improved in the middle- and high-temperature range. Based on Al doping, Pb is adopted as the doping element to dope BiCuSeO to modify the carrier concentration. The results show that Al/Pb dual doping in the BiCuSeO matrix can increase the carrier concentration under the premise of increasing carrier mobility. Therefore, the electrical conductivity of BiCuSeO can be improved while maintaining a large Seebeck coefficient. The power factor of Al/Pb doping reached ~7.67 μWcm-1K-2 at 873 K. At the same time, the thermal conductivity of all doped samples within the test temperature range maintained a low level (<1.2 Wm-1K-1). Finally, the ZT value of the Al/Pb-doped BiCuSeO reached ~1.14 at 873 K, which is ~2.72 times that of the pure phase, and the thermoelectric properties of the matrix were effectively improved.

Study of the effect of the substitution of Fe by Ti on the microstructure and the physical properties of the perovskite system La0.67Ca0.2Ba0.13Fe1-xTixO3 with x = 0 and 0.03 at low temperatures.

La0.67Ca0.2Ba0.13Fe1-xTixO3 samples (x = 0 and 0.03) were synthesized by the auto-combustion method. Analysis of XRD diffractograms revealed that these compounds crystallize in the cubic system with the space group Pm3̄m. The dielectric properties have been studied in the 102-106 frequency range and the 120-280 K temperature range. Analysis of AC conductivity shows that the conduction mechanisms are of polaronic origin and that they are co-dominated by the NSPT and OLPT models. The monotonic increase in conductivity with increasing temperature results from the reduction of defect centers and the increase in charge carrier mobility. Such variation is consistent with impedance variation at different frequencies and temperatures indicating semiconductor behavior. Nyquist diagrams are characterized by the appearance of semi-circular arcs. These spectra are modeled in terms of equivalent electrical circuits confirming the contribution of grains (Rg//CPEg) and grain boundaries (Rgb//CPEgb). The dielectric analysis showed an evolution of the dielectric constant in accordance with Koop's theory and the phenomenological model of Maxwell-Wagner. The low conductivity and the high values of the real permittivity at low frequency make our compounds potential candidates for energy storage and applications for electronic devices and microwaves.

Revealing the enhanced crystalline quality mechanism of perovskites produced by microwave-assisted synthesis: toward the fabrications in a fully ambient air environment

The conventional stirring method (CSM) in a nitrogen-controlled glovebox has been widely used to synthesize the Cs/MA/FA precursor solution. However, it involves a long preparation time and limits the fabrication conditions in an ambient environment, hindering their potential for high-end applications and environmental stability. Herein, we systematically developed a fast and efficient microwave-assisted synthesis (MAS) method to produce a high-quality and stable triple cation perovskite (Cs0.1(MA0.17FA0.83)0.9Pb(Br0.17I0.83)3) solution and fabricated a device in ambient conditions. The superiority of using the MAS compared to CSM is manifested by comparing the improvements in crystal nucleation, enhanced optical properties, and its excellent stability with time, owing to the homogeneous heating of the precursor solution during the synthesis method. The MAS-prepared perovskite films showed anomalous photoluminescence blue-shift and linewidth broadening with increasing temperature, emphasizing the unique properties of the triple cation perovskite fabricated. These results were later interpreted and fitted to discuss the induced lattice thermal expansion, structural phase transition, and electron–phonon interactions that highlighted the enhanced crystal quality from the MAS-perovskite films. Considering the substantial fabrication parameters and conditions, the structural and optical stability of the MAS-perovskite films were maintained over time during exposure to air and ultraviolet light, even without encapsulation. To further elucidate the crystal quality improvements from using the MAS-perovskite solution, the fabrication of perovskite transistors in a completely ambient environment resulted in a 10-fold increase in electron carrier mobility compared to the CSM-perovskite devices. The developed synthesis method has contributed significant advantages to enhance the structural and environmental stability of perovskites, making a promising approach for larger-scale and more efficient future optoelectronic device applications.

Sensitivity investigation of 100-MeV proton irradiation to SiGe HBT single event effect

The single event effect (SEE) sensitivity of silicon–germanium heterojunction bipolar transistor (SiGe HBT) irradiated by 100-MeV proton is investigated. The simulation results indicate that the most sensitive position of the SiGe HBT device is the emitter center, where the protons pass through the larger collector-substrate (CS) junction. Furthermore, in this work the experimental studies are also carried out by using 100-MeV proton. In order to consider the influence of temperature on SEE, both simulation and experiment are conducted at a temperature of 93 K. At a cryogenic temperature, the carrier mobility increases, which leads to higher transient current peaks, but the duration of the current decreases significantly. Notably, at the same proton flux, there is only one single event transient (SET) that occurs at 93 K. Thus, the radiation hard ability of the device increases at cryogenic temperatures. The simulation results are found to be qualitatively consistent with the experimental results of 100-MeV protons. To further evaluate the tolerance of the device, the influence of proton on SiGe HBT after gamma-ray (60Coγ) irradiation is investigated. As a result, as the cumulative dose increases, the introduction of traps results in a significant reduction in both the peak value and duration of the transient currents.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Stability, electronic, thermodynamic, and optical aspects of CsPbI3-xBrx(x=0, 1, 2, 3) compounds: An ab-initio study

All organic metal halide perovskite, due to tunable band gap and stability, has great potential for use in optoelectrical devices. So, we investigate the structural, mechanical, dynamic stability, and electronic, thermodynamic, and optical properties of CsPbI3−xBrx(x=0.1.2.3) compounds. All studied compounds are mechanically stable in the equilibrium temperature and pressure. The phonon dispersion curve of these compounds confirms their dynamical stability. Replacing the halide position with the Br atom is associated with a structural change from cubic to tetragonal, which in turn leads to an increase in stability, hardness, and the value of the direct band gap of CsPbI3−xBrx compounds. Moreover, this structural deformation gives rise to a significant increase of carrier mobility in the asymmetric direction z of the tetragonal structure compared to other symmetric x and y directions.Optical properties calculations show high absorption in the blue-purple frequency region of visible light. The optical conductivity of the deformed structures along the asymmetric z direction is higher than along the symmetric x or y direction, which is in agreement with the charge carrier mobility results. The optical absorption calculations show that the increase of Br concentration in the CsPbI3−xBrx compounds cause the main peak of the absorption to move to a larger amount of energy. The static refractive index of these compounds is comparable to the light-absorbing semiconductor materials in solar cells, and their first peak of refractive index is within the visible energy spectrum of the electromagnetic beam.The direct bandwidth behavior with structural, electronic, and optically adjustable properties makes these compounds attractive for use in optoelectronic devices.

Dilute Rhenium Doping and its Impact on Defects in MoS2.

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

Scrap Recovery of n‐Type Bismuth Telluride Ingot by Second Zone Melting and PbBr2 Doping

Bismuth telluride is the only commercially applied thermoelectric material. The n‐type material is usually prepared by the method of zone melting; however, the utilization ratio of the ingot is relatively low, owing to the bad thermoelectric performance of the two ends. Herein, the waste of n‐type bismuth telluride is recovered by the second zone melting and PbBr2 doping. It is shown that the second zone melting can improve the grain alignment, leading to the increase of carrier mobility. When PbBr2 doping is introduced, lattice thermal conductivity is largely reduced by the high dense dislocations. As a result, the peak power factor and maximum ZT reach 53 μW cm−1 K−2 and 1.15, respectively, which are even higher than the values of the original commercial material. This work provides an economic and applicable strategy to recover the waste of bismuth telluride thermoelectric materials.

Ultrasonication‐Assisted Seed Screening Enables Oriented and Efficient Low‐Dimensional Crystal‐Structural Thin‐Film Photovoltaics

AbstractUltrasonication‐assisted liquid exfoliation of layered materials is widely used to produce a large number of 2D nanosheets due to its simplicity, universality, and mass production. Here, the utility of ultrasonication is extended from liquid exfoliation to seed screening in the case of layered GeSe and GeS. It is found that the ultrasonic stripping force can also be used to selectively remove the lying seeds interacting with substrate through weak van der Waals (vdW) forces while retaining the standing seeds connected with substrate via tightly covalent bonds. The standing seeds then induce the growth of 2D crystal‐structural films with standing orientation. This thereby enables efficient carrier transport within covalently bonded layers, instead of poor transport between layers held together by vdW forces. The resulting standing‐oriented GeSe films exhibit a 21‐times increase in carrier mobility compared to lying‐oriented films. As a result, this study demonstrates the highest power conversion efficiencies of 6.1% and 10.5% under AM1.5G 1‐ and 0.01‐sun illumination reported for GeSe solar cells, respectively.

Neutron-irradiated effect on the thermoelectric properties of Bi2Te3-based thermoelectric leg

Thermoelectric (TE) materials working in radioisotope thermoelectric generators are irradiated by neutrons throughout its service; thus, investigating the neutron irradiation stability of TE devices is necessary. Herein, the influence of neutron irradiation with fluences of 4.56 × 1010 and 1 × 1013 n/cm2 by pulsed neutron reactor on the electrical and thermal transport properties of n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thermoelectric alloys prepared by cold-pressing and molding is investigated. After neutron irradiation, the properties of thermoelectric materials fluctuate, which is related to the material type and irradiation fluence. Different from p-type thermoelectric materials, neutron irradiation has a positive effect on n-type Bi2Te2.7Se0.3 materials. This result might be due to the increase of carrier mobility and the optimization of electrical conductivity. Afterward, the effects of p-type and n-type TE devices with different treatments on the output performance of TE devices are further discussed. The positive and negative effects caused by irradiation can cancel each other to a certain extent. For TE devices paired with p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thermoelectric legs, the generated power and conversion efficiency are stable after neutron irradiation.

Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices.

ConspectusColloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II-VI and IV-VI materials and are being developed as interest in III-V and I-III-VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.